Interface-engineered nickel oxyhydroxide on carbon nanofibers for efficient urea oxidation and wastewater-to-energy conversion.

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Learn more: PMC Disclaimer | PMC Copyright Notice PLoS One . 2026 Jun 4;21(6):e0347020. doi: 10.1371/journal.pone.0347020 Interface-engineered nickel oxyhydroxide on carbon nanofibers for efficient urea oxidation and wastewater-to-energy conversion Nasser A M Barakat 1, 2, * , Ahmed Saadawi 1 , Osama M Erfan 3 , Ibrahim Mustafa 4 , Gaber Edris 4 , Ayman Yousef 5, * Editor: Zafar Ghouri 6 Author information Article notes Copyright and License information 1 Chemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt 2 Chemical Engineering Department, Faculty of Engineering, Atatürk University, Erzurum, Turkey 3 Department of Mechanical Engineering, College of Engineering, Qassim University, Buraydah, Saudi Arabia 4 Chemical Engineering Department, Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia 5 Department of Chemical Engineering, College of Engineering and Computer Sciences, Jazan University, Jazan, Saudi Arabia 6 Texas A&M University at Qatar, QATAR Competing Interests: The authors have declared that no competing interests exist. ✉ * E-mail: nasbarakat@mu.edu.eg (NAMB); aymanhassan@jazanu.edu.sa (AY) Roles Nasser A M Barakat : Formal analysis, Supervision, Writing – original draft Ahmed Saadawi : Data curation, Methodology Osama M Erfan : Resources Ibrahim Mustafa : Resources Gaber Edris : Funding acquisition Ayman Yousef : Investigation Zafar Ghouri : Editor Received 2026 Mar 25; Accepted 2026 Apr 20; Collection date 2026. © 2026 Barakat et al This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. PMC Copyright notice PMCID: PMC13235928  PMID: 42241426 Abstract Urea is a common nitrogenous pollutant in agricultural and industrial wastewaters, and its electrochemical oxidation in alkaline media enables simultaneous detoxification and energy recovery. Herein, we report an interface-engineering strategy based on P,N-modified nickel–carbon nanofibers prepared via a scalable electrospinning–carbonization process using ammonium phosphate as a multifunctional precursor. During thermal treatment, phosphorus and nitrogen heteroatoms are incorporated into the carbon framework while regulating the surface chemistry of embedded nickel nanoparticles, promoting the formation of a NiOOH-ready interfacial layer. Structural analyses (XRD, SEM/TEM) confirm the formation of metallic Ni domains uniformly dispersed within turbostratic carbon nanofibers. XPS reveals the coexistence of Ni 0 /Ni +2 /Ni +3 species along with phosphate- and nitrogen-derived functionalities that enhance electronic modulation and redox reversibility. In 1.0 M KOH, the optimized composition exhibits a Ni +2 /Ni +3 redox charge of ~12.5 mC.cm −2 . For urea electrooxidation (0.5 M urea), the catalyst delivers a maximum current density of ~115 mA.cm −2 with a low onset potential of 0.37 V (vs Ag/AgCl). The apparent activation energy is 9.82 kJ.mol −1 , indicating a kinetically favorable NiOOH-mediated pathway. Chronoamperometry shows stable operation with ~70–80% current retention over 1000 s. When implemented as an anode in a membrane-less direct urea fuel cell, the material achieves a peak power density of ~1.1 W.m −2 at 35 °C, demonstrating practical wastewater-to-energy feasibility. This work highlights how dual heteroatom modification of carbon nanofibers can regulate nickel interfacial chemistry, enabling efficient urea remediation coupled with renewable power generation. 1. Introduction Carbon nanofibers (CNFs) have emerged as a versatile platform for (electro)chemical energy technologies because their high axial aspect ratio combines the advantages of nanoparticles (large surface area) with those of macroscopic fibers (continuous electron pathways and mechanical integrity) [ 1 , 2 ]. The one-dimensional architecture fosters rapid in-plane charge transport, provides abundant external surface for reactant access, and forms self-supporting porous mats that can be used directly as binder-free electrodes [ 3 ]. Additional benefits include chemical/thermal stability, facile integration into textiles or gas-diffusion layers, and straightforward post-functionalization. Owing to these attributes, CNFs are widely exploited as supports for functional electrocatalysts—for example, for alcohol/urea oxidation, water splitting, oxygen reduction, and CO 2 conversion—where the support dictates dispersion of the active phase, electron/ion percolation, and durability under bias [ 4 , 5 ]. Heteroatom modification further upgrades the catalytic competence of carbon. Phosphorus (P) doping introduces electron-rich P–C/P–O moieties that perturb the π network, polarize adjacent carbon atoms, and create Lewis-basic sites [ 6 ]. These effects enhance wettability, increase defect density, and strengthen metal–support interactions (e.g., P–O–M linkages), which are advantageous for anchoring transition-metal nanoparticles and for interfacial reactions that require OH⁻ activation. P-modified carbons have delivered notable performance in oxygen electrocatalysis, metal–air batteries, and small-molecule oxidation [ 7 , 8 ]. Nitrogen (N) doping—in pyridinic, pyrrolic, or graphitic forms—typically raises the electronic conductivity of carbon, alters charge density at neighboring carbons, and can act as an intrinsic catalytic site or a strong metal-anchoring motif. N-doped CNFs therefore excel as supports for Fe/Co/Ni-based catalysts, enabling improved dispersion, electron transfer, and resistance to sintering/corrosion in alkaline media [ 9 , 10 ]. Dual P&N modification is particularly appealing because it combines the electronic enrichment of N with the interfacial polarity and coordination chemistry of P [ 11 , 12 ]. The two dopants cooperatively tune work function, local charge distribution, and hydrophilicity, while furnishing a dense population of edge/defect sites and robust P–O–M/ N–M coordination. Such synergy is expected to facilitate OH⁻ supply and stabilize metal nanoparticles under turnover—both pivotal for alkaline electro-oxidation [ 13 ]. Electrospinning provides an attractive alternative: it is low-cost, scalable, and high-throughput, producing continuous polymer nanofibers that are readily carbonized into conductive CNFs. Crucially, molecular precursors, metal salts, and heteroatom sources can be co-dissolved in the spinning dope, enabling one-pot control over composition and morphology [ 14 ]. The resulting mats are mechanically robust and can be used as free-standing electrodes or processed into inks [ 15 ]. From an application viewpoint, urea-contaminated wastewaters are generated in large volumes from fertilizer production, livestock operations, municipal wastewater, and certain industrial streams [ 16 ]. If untreated, urea hydrolyzes to ammonia and nitrate/nitrite, causing eutrophication and ecological damage [ 17 ]. Conventional treatment (biological nitrification/denitrification, breakpoint chlorination, adsorption) is effective but energy- and infrastructure-intensive, and it wastes the chemical energy stored in urea [ 18 , 19 ]. Electro-oxidation has therefore emerged as a promising dual-benefit process: it enables energy recovery while simultaneously removing urea under mild, ambient conditions. In contrast to conventional heteroatom-doping strategies, the use of ammonium phosphate in the electrospinning precursor enables in-situ incorporation of both phosphorus and nitrogen functionalities, leading to a homogeneous distribution of dopants and enhanced interaction with Ni active sites. Among anodic materials, nickel-based electrocatalysts are the workhorse for urea electro-oxidation (UOR) in alkaline media because the active NiOOH surface layer forms readily and mediates the six-electron oxidation of urea to N 2 and carbonate [ 20 , 21 ]. However, insufficient activity and site utilization remain central bottlenecks, rooted in limited electrochemically active surface area (ECSA), sluggish Ni(OH) 2 ↔ NiOOH conversion at realistic potentials, and inadequate mass transport in thick catalyst layers. These limitations can be alleviated through (i) morphology tuning—for instance, dispersing Ni nanoparticles on conductive CNF supports that provide continuous electron pathways and open porosity—and/or (ii) non-metal heteroatom modification (P, N, S) that improves wetting, charge distribution, and metal–support coupling [ 22 , 23 ]. In this context, CNFs are an optimum support, marrying mechanical integrity, conductivity, and high surface accessibility with facile heteroatom incorporation [ 24 ]. Building on theory for heteroatom-engineered carbons, P,N co-doping creates defect-rich, polarized sites with localized states near the Fermi level, which enhance interfacial charge transfer [ 25 ]. Because P is less electronegative and larger than N, P adjacent to N perturbs the local charge/spin density of the carbon lattice via its 3p lone pairs (and low-lying 3d acceptor levels), thereby activating neighboring C–N motifs [ 26 , 27 ]. For alkaline urea electro-oxidation (UOR), these PN ensembles are advantageous in several, mutually reinforcing ways: (i) they strengthen Lewis-basic adsorption of OH − , accelerating the Ni(OH) 2 ⇄ NiOOH preactivation that governs UOR kinetics [ 28 ]; (ii) they provide polar sites for urea docking and early dehydrogenation on NiOOH; (iii) they form robust P–O–Ni and N–Ni coordination, improving Ni nanoparticle dispersion and stability against agglomeration/dissolution under anodic bias; and (iv) by modulating the local electronic structure, they mitigate site poisoning by urea-derived intermediates (e.g., cyanate/carbonate/NH x species) [ 29 ]. Building on these considerations, we develop a simple electrospinning-carbonization route to nickel-decorated CNFs in which ammonium phosphate additives (mono- and di-ammonium phosphate) serve as multifunctional heteroatom sources and morphology directors within a PVA/Ni precursor solution. After calcination, the process yields Ni/CNFs bearing P- and N-derived surface functionalities (predominantly phosphate-type P with trace N), together with well-dispersed Ni nanoparticles. Moreover, it is goaled to translate the catalyst to a practical device by deploying the optimized P&N-modified Ni/CNF anode in a membrane-less direct urea fuel cell (DUFC) to demonstrate real-world viability via polarization and power-density measurements. This study thus provides a scalable, low-cost strategy to heteroatom-modified CNF-supported Ni catalysts and clarifies how phosphate-assisted surface chemistry and fiber morphology cooperate to deliver high UOR performance and meaningful DUFC output. 2. Materials and methods 2.1. Materials Poly(vinyl alcohol) (PVA, Mw 85–124 kDa, 87–89% hydrolyzed) and nickel(II) acetate tetrahydrate (Ni(CH 3 COO) 2 ·4H 2 O, ≥ 98%) were obtained from Sigma-Aldrich/Merck and used as received. Ammonium-phosphate salts—monoammonium phosphate (MAP, NH 4 H 2 PO 4 , ≥ 99%) and diammonium hydrogen phosphate (DAP, (NH 4 ) 2 HPO 4 , ≥ 99%)—were purchased from Sigma-Aldrich/Merck; ammonium phosphate (tribasic) (TAP, (NH 4 ) 3 PO 4 , ≥ 98%; Alfa Aesar) was used only in exploratory spinning trials. Potassium hydroxide pellets (KOH, ≥ 85%, Merck) and urea (ACS reagent, ≥ 99.0%, Sigma-Aldrich) were used to prepare alkaline and urea electrolytes. Deionized water (18.2 MΩ·cm, Milli-Q) was used throughout. For electrode inks, poly(vinylidene fluoride) (PVDF, powder, battery grade, Solef® 6020) and N,N-dimethylformamide (DMF, anhydrous, ≥ 99.8%, Sigma-Aldrich) were employed as binder and solvent, respectively. Woven carbon cloth (plain weave, ~ 0.3–0.4 mm thickness; e.g., Fuel Cell Store, type W0S1002) served as the anode support. A commercial Pt/C-loaded carbon cloth cathode (20 wt% Pt/C, Pt loading 0.4 mg.cm −2 on carbon cloth; Fuel Cell Store or equivalent) was used for DUFC tests. For three-electrode measurements, a glassy carbon disk working electrode (diameter 3 mm, area 0.070714 cm 2 ; CHI Instruments), an Ag/AgCl (3.0 M KCl) reference electrode, and a graphite rod counter electrode were used. Ethanol (≥99.5%, Fisher Chemical) and isopropanol (≥99.5%, Fisher Chemical) were used for electrode cleaning when needed. 2.2. Catalyst preparation Spinning solutions were prepared in deionized water at room temperature. First, PVA was dissolved to form a 10 wt% stock by dispersing 1.50 g PVA in 13.5 g water and heating at 45–50 °C under stirring for 2 h until clear, followed by cooling to 22–25 °C. Nickel(II) acetate tetrahydrate (NiAc, 1.00 g) was then dissolved in 5 ml deionized water and stirred for ~20 min to ensure complete dissolution. Ammonium phosphate was introduced to the NiAc aqueous solution as the heteroatom/morphology-directing additive at loadings referenced to the NiAc mass: for monoammonium phosphate (MAP) or diammonium phosphate (DAP) the amounts were 0.01, 0.03, 0.05, and 0.07 g to obtain 1, 3, 5, and 7 wt% relative to 1.00 g NiAc, respectively; an additive-free dope (0 wt%) served as the control. The produced solutions were mixed the prepared PVA aqueous solutions. The mixture was homogenized for ≥30 min, briefly sonicated (5–10 min) to remove microbubbles, and passed through a 0.45 µm PTFE syringe filter. For reproducible spinning, we found the dope viscosity in the 400–800 mPa·s range and conductivity of ~0.6–1.5 mS.cm −1 (0–3 wt% MAP/DAP) to yield the most stable jets, whereas higher additive contents naturally increased conductivity and required minor spinning adjustments. Triammonium phosphate was evaluated only in preliminary trials and did not yield a stable fiber jet under our conditions. Electrospinning was carried out on a horizontal single-needle setup at 22 ± 2 °C and 35–45% relative humidity. The dope was loaded in a 5 mL syringe equipped with a 21G stainless-steel blunt needle (inner diameter ≈0.51 mm) and dispensed at 0.7 mL.h −1 for the control and 1 wt% formulations; for 3–7 wt% MAP/DAP the flow rate was reduced to 0.30–0.40 mL h −1 to suppress beading. A positive high voltage of 18 kV (tuned within 16–20 kV as needed) was applied to the needle with a tip-to-collector distance of 15 cm. Fibers were collected on an aluminum-foil-wrapped rotating drum (diameter ~10 cm) operated at ~300 rpm to obtain a random non-woven mat. The as-spun mats were peeled and dried under vacuum at 60 °C (≤10 mbar) for 10 h to remove residual solvent and loosely bound volatiles. Carbonization/calcination was performed in a quartz tube furnace under flowing argon (200 sccm). After a 30 min purge at room temperature, samples were heated at 2 °C.min −1 to 200 °C and held for 1 h to complete dehydration and polymer stabilization, then ramped at 5 °C.min −1 to 750 °C and held for 5 h to carbonize the PVA matrix and convert the nickel precursor in situ. The furnace was allowed to cool naturally to room temperature under argon. The resulting materials are denoted AP-free (Ni/CNFs), MAP-x, and DAP- x , where x refers to the additive wt% relative to NiAc ( x = 1, 3, 5, 7). 2.3. Electrochemical measurements Electrochemical tests were carried out in a conventional three-electrode glass cell using a glassy carbon disk working electrode (diameter 3 mm), a Pt mesh counter electrode, and an Ag/AgCl (sat. KCl) reference electrode. Unless otherwise stated, potentials are reported vs. Ag/AgCl and currents were normalized to the electrode area. The working electrode surface was rinsed with ethanol and deionized water, dried in air, and then coated with a small piece of the as-prepared nanofiber mat or a pressed flake of the calcined material; the piece was gently pressed to ensure intimate contact. All electrolytes were prepared with 18.2 MΩ·cm water. Activation/blank experiments used 1.0 M KOH; urea oxidation employed 0.5 M urea in 1.0 M KOH unless otherwise specified. For concentration studies, urea was varied between 0.33–2.0 M at fixed 1.0 M KOH, and KOH was varied between 1.0–7.0 M at fixed 0.5 M urea. Temperature-dependent measurements (30–60 °C) were performed in a jacketed cell connected to a circulating water bath after a 10 min thermal equilibration; the electrolyte temperature was monitored with a thermocouple immersed near the working electrode. Cyclic voltammetry (CV) was recorded at 50 mV s⁻¹ over a potential window wide enough to capture the Ni +2 /Ni +3 transition and the ensuing catalytic wave. Prior to any kinetic analysis, electrodes were conditioned in 1.0 M KOH by repeated cycling until the redox response stabilized; unless stated, the 30 th cycle is reported for comparison. For urea oxidation, CVs were collected in fresh electrolyte immediately after activation. Onset potentials were determined from the intersection of tangents to the non-faradaic baseline and the rising catalytic branch, or equivalently by the potential at which the current density exceeded the baseline by 0.10 mA.cm −2 ; both criteria gave indistinguishable trends. Chronoamperometry (CA) was performed at fixed potentials for 1000 s; when a sequence of potentials was tested, the same electrode was held stepwise from low to high potential without replacement, as indicated in the figure captions. Where relevant, linear sweep voltammetry (LSV) was acquired at 10 mV s⁻¹ to construct polarization curves in the kinetic region. No post-measurement iR compensation was applied unless explicitly noted; ohmic resistance was minimized by placing the reference capillary close to the working electrode. All measurements were repeated at least in duplicate to confirm reproducibility; fresh electrolytes were prepared for each concentration/temperature series. 2.4. Direct urea fuel cell (DUFC) assembly and testing Membrane-less DUFCs were assembled in a parallel-plate configuration with a 3.0 × 3.0 cm carbon-cloth anode facing a commercial Pt/C air cathode (Pt loading 0.4 mg.cm −2 ) at a fixed inter-electrode gap of 0.5 cm. The anode catalyst layer was prepared as an ink containing 0.06 g of the functional nanofibers (MAP, 1 wt% relative to Ni acetate in the spinning dope), 0.12 g PVDF binder and 1.0 mL DMF; the slurry was magnetically stirred (≥1 h) until homogeneous and then cast uniformly onto carbon cloth (3 × 3 cm) using a doctor-blade drawdown. Coated electrodes were dried at 80 °C (air oven) to remove solvent and then thermally treated at 350 °C for 1 h. The total mass gain was recorded gravimetrically to track the catalyst loading; only single-side coatings were used. The Pt/C cathode was used as received and exposed directly to ambient air (air-breathing mode). The electrodes were mounted in a PTFE spacer frame that defined the 0.5 cm electrolyte gap and sealed with Viton gaskets to prevent leakage. Cell performance was measured in a two-electrode configuration (anode as working; cathode as counter/reference) using a potentiostat/galvanostat. The liquid phase was an aqueous KOH + urea mixture containing 0.5 M urea and 1.0 M KOH (for concentration studies, urea was varied in the range used in the figures while the KOH level was kept constant). The electrolyte volume was sufficient to fully submerge the electrodes and the inter-electrode gap; all tests were performed at room temperature or at controlled temperature in a jacketed glass cell connected to a circulating bath (equilibration ≥10 min before measurement). Prior to polarization measurements the open-circuit potential (OCP) was allowed to stabilize for 5–10 min. Polarization curves were obtained by linear sweep voltammetry (LSV) from OCP to 0 V at 0.01 V.s −1 . The power density was calculated as P = I × V/A cathode , where I is the cell current, V the cell voltage, and A cathode the apparent cathode area (0.00052 m 2 ) used for normalization; current densities were likewise normalized to A cathode . Series resistance was minimized by short leads and compact geometry; no post-measurement iR compensation was applied unless specified. For temperature-dependent studies the same procedure was followed at each setpoint after thermal equilibration. All DUFC measurements were repeated at least twice with fresh electrolyte to confirm reproducibility. 2.5. Characterizations Morphology was examined by field-emission scanning electron microscopy (SEM; JSM IT2000) operated at 3–5 kV using a working distance of ~5 mm. Nanofiber mats were mounted on aluminum stubs with conductive carbon tape; no metal coating was applied to avoid obscuring surface texture. Representative fiber diameters and surface features were extracted from multiple micrographs using ImageJ; at least 200 counts were used when particle/fiber size distributions were reported. Transmission electron microscopy (TEM) was performed on a JEOL JEM-100CX II operated at 200 kV. Samples were prepared by gently dispersing a small amount of the calcined mat in ethanol with brief (<2 min) bath sonication and drop-casting onto lacey-carbon Cu grids (300 mesh). Bright-field TEM, selected-area electron diffraction (SAED), and high-resolution TEM (HRTEM) were acquired from the same grids. Interplanar spacings were measured from HRTEM fringe images using Digital Micrograph, and particle size statistics were compiled from low-magnification images (≥200 particles). Care was taken to minimize beam damage by limiting exposure times during HRTEM. Powder X-ray diffraction (XRD) patterns were recorded on a JEOL (Cu Kα radiation, λ = 1.5406 Å; 40 kV, 40 mA) using a Bragg–Brentano geometry. Mats were gently ground to fine powders and spread on zero-background Si holders. Data were collected over 2θ = 10–80° with a step size of 0.020° and a counting time of 0.5 s per step (equivalent scan rate ~2.4° min −1 ). Phase identification used ICDD PDF entries for fcc-Ni (04–0850), turbostratic/graphitic carbon (41–1487), NiO (78–0429), and representative nickel phosphates (e.g., 22–1119) where applicable. Apparent Ni crystallite sizes were estimated from the (111) reflection using the Scherrer equation with K = 0.9 after subtracting instrumental broadening measured on a LaB₆/Si standard. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific K-Alpha (Al Kα, h ν = 1486.6 eV) with a monochromated source, base pressure <1 × 10 −8 mbar, and a 400 µm analysis spot. Survey spectra were collected at 200 eV pass energy; high-resolution C 1s, N 1s, O 1s, P 2p, and Ni 2p regions were acquired at 20–40 eV pass energy. A low-energy flood gun was used for charge compensation; all binding energies were referenced to the adventitious C 1s peak at 284.8 eV. Spectra were processed in CasaXPS using a Shirley background and mixed Gaussian–Lorentzian (Voigt) line shapes. Atomic percentages were derived from peak areas corrected by Scofield sensitivity factors and analyzer transmission. No ion sputter cleaning was applied to avoid reduction of surface nickel species; all spectra represent the as-prepared surface after brief (<1 min) exposure to ambient air prior to loading. 3. Results and discussion 3.1. Electrocatalyst characterization The crystalline phases of the calcined nanofibers were examined by X-ray diffraction ( Fig 1A ). The diffractogram is dominated by the characteristic reflections of face-centered cubic (fcc) metallic nickel at 2θ ≈ 44.5°, 51.8°, and 76.4°, indexed to the (111), (200), and (220) planes, respectively (ICDD PDF 00-004-0850). The sharp, intense Ni peaks confirm efficient reduction of the nickel precursor to crystalline Ni during calcination under inert atmosphere. A broad halo centered around 2θ ≈ 25–26° is assigned to the (002) stacking of graphitic carbon (ICDD PDF 41–1487), produced from PVA pyrolysis; the d 002 is slightly expanded relative to ideal graphite, indicating a turbostratic/poorly ordered carbon matrix. A weak shoulder near 42–44° can include contributions from the (100)/(101) graphite bands but overlaps with Ni(111), rendering the carbon signal less pronounced in this region. Fig 1. Chemical composition, morphology and internal structure of the prepared nanofibers. Open in a new tab A) XRD pattern for the obtained powder after calcination of the PVA/NiAc/DAP electrospun mats under inert atmosphere at 750 °C. SEM images for the nanofibers obtained from calcination of different electrospun mats: B) Ammonium phosphate-free, C) Mono ammonium phosphate-containing and D) Di ammonium phosphate-containing. Normal TEM image (E) for the nanofibers obtaining from electrospun solution containing MAP, the inset in panel displays SAED pattern. (F) normal TEM images for the nanofibers produced from electrospun solutions containing DAP, the inset is the corresponding SAED pattern. Because the spinning solution contained ammonium phosphate, interactions between Ni +2 and phosphate species are expected before and/or during heating. In water, the precursors dissociate as: 𝐍 𝐢 ( 𝐂 𝐇 3 𝐂 𝐎 𝐎 ) 2 . 4 𝐇 2 𝐎 → 𝐍 𝐢 + 2 + 2 𝐂 𝐇 3 𝐂 𝐎 𝐎 − + 4 𝐇 2 𝐎 (1) ( 𝐍 𝐇 4 ) 2 𝐇 𝐏 𝐎 4 ↔ 2 𝐍 𝐇 4 + + 𝐇 𝐏 𝐎 4 − 2 ( / 𝐇 2 𝐏 𝐎 4 − / 𝐏 𝐎 4 − 3 , pH − dependent ) (2) Under suitable local pH, partial precipitation/complexation can occur: 3 𝐍 𝐢 + 2 + 2 𝐏 𝐎 4 − 3 + x 𝐇 2 𝐎 → 𝐍 𝐢 3 ( 𝐏 𝐎 4 ) 2 . x 𝐇 2 𝐎 ( 𝐬 ) (3) During drying and calcination, ammonium phosphate decomposes (releasing NH 3 /H 2 O and forming poly-/metaphosphates or P 2 O 5 ), while acetate and the evolving carbonaceous species (CO, H 2 ) furnish a reducing environment. Transient nickel phosphate and/or NiO intermediates can therefore convert to metallic Ni via steps such as: 𝐍 𝐢 3 ( 𝐏 𝐎 4 ) 2 → Δ 3 𝐍 𝐢 𝐎 + 𝐏 2 𝐎 5 (4) The unreacted nickel acetate decomposes under inert atmosphere with this reactions sequence [ 30 , 31 ]. 𝐍 𝐢 ( 𝐂 𝐇 3 𝐂 𝐎 𝐎 ) 2 · 4 𝐇 2 𝐎 → 0 . 8 6 𝐍 𝐢 ( 𝐂 𝐇 3 𝐂 𝐎 𝐎 ) 2 · 0 . 1 4 𝐍 𝐢 ( 𝐎 𝐇 ) 2 + 0 . 2 8 𝐂 𝐇 3 𝐂 𝐎 𝐎 𝐇 + 3 . 7 2 𝐇 2 𝐎 (5) 3 ( 0 . 8 6 𝐍 𝐢 ( 𝐂 𝐇 3 𝐂 𝐎 𝐎 ) 2 · 0 . 1 4 𝐍 𝐢 ( 𝐎 𝐇 ) 2 ) + 0 . 2 8 𝐇 2 𝐎 → 3 𝐍 𝐢 𝐂 𝐎 3 + 2 . 4 4 𝐂 𝐇 3 𝐂 𝐎 𝐂 𝐇 3 + 1 . 1 2 𝐇 2 (6) 𝐍 𝐢 𝐂 𝐎 3 → 𝐍 𝐢 𝐎 + 𝐂 𝐎 2 (7) 𝐍 𝐢 𝐎 + 𝐂 𝐎 / 𝐇 2 → 𝐍 𝐢 + 𝐂 𝐎 2 / 𝐇 2 𝐎 (8) These pathways rationalize why the final XRD pattern is overwhelmingly metallic Ni plus carbon, while any phosphate-derived residues remain minor and likely amorphous. Consistently, only faint features in the 37–43° window—potentially from residual NiO or poorly crystalline phosphate species—are observed and are much weaker than the Ni reflections [ 32 ]. Notably, all samples prepared with MAP or DAP (0.01–0.07 g) exhibit essentially the same phase assemblage: predominant fcc-Ni embedded in a turbostratic carbon matrix, with no resolvable crystalline nickel phosphate phases after calcination. This similarity indicates that, at the employed phosphate levels and processing conditions, phosphate mainly acts through transient complexation/precipitation and decomposition chemistry that ultimately feeds the reduction of Ni salts to metallic Ni without leaving a crystalline phosphate fingerprint in the final nanofibers. In summary, the XRD results confirm nanocrystalline fcc-Ni dispersed in a carbonaceous matrix, with carbon being poorly ordered and any phosphate- or oxide-related species present only as minor/amorphous traces. The intimate coupling of conductive carbon with metallic Ni is expected to be advantageous for electrocatalytic applications by combining abundant Ni active sites with efficient electron transport and structural support. The phosphate-free precursor yields a uniform non-woven mat of continuous, cylindrical fibers with smooth surfaces and few defects ( Fig 1B ). The fibers weave into an open network with ample inter-fiber junctions and no obvious bead formation. The smoothness indicates that the viscoelasticity and charge density of the spinning dope were well balanced, allowing a stable whipping jet and solvent evaporation without local Rayleigh break-up. After calcination, this morphology is preserved, giving a coherent carbonaceous scaffold that provides electronic pathways and abundant contact points for the subsequently formed nickel domains inferred from XRD. Introducing 1 wt% MAP (relative to Ni acetate), Fig 1C , changes the surface texture noticeably. Fibers remain continuous, but their surfaces become rough and granular, with dense, sub-100-nm nodules decorating the entire fiber length. The granular shell reflects salt–polymer–metal interactions in the jet: phosphate anions partially complex or precipitate with Ni +2 during drying, and these nanometric nuclei are immobilized by the PVA matrix and later converted during heat treatment to Ni/NiO–derived nanoparticles anchored on the fiber skin. The result is a “raspberry-like” coating that increases the specific surface area and is expected to expose more accessible active sites while maintaining the mechanical integrity of the fiber network. With 1 wt% DAP, the salt effect becomes stronger. The mat still contains fibers, but their surfaces are densely encrusted with nanoparticles and small aggregates, and isolated micron-scale faceted crystallites and granular debris appear between and on the fibers. Such over-decoration is consistent with a higher effective phosphate basicity/ionic strength, which raises solution conductivity and screens polymer chain entanglement [ 33 ]. During electrospinning this promotes local jet instabilities and extensive in-flight nucleation, so a larger fraction of the inorganic phase forms as discrete particles or agglomerates rather than as finely dispersed nuclei within the polymer jet. As shown in Fig 1D , upon calcination these crystallites persist as larger Ni-bearing or phosphate-derived residues, producing a heterogeneous texture with partial loss of the sleek, cylindrical fiber contour seen in the phosphate-free sample. While this morphology maximizes roughness, the presence of coarse aggregates can reduce the fraction of electrochemically accessible surface and may impede ion transport within the mat. The inability to electrospin the solution prepared with the fully ammoniated phosphate is in line with these trends. At that high neutralization level the dope likely exhibits excessive ionic strength and extensive salt–polymer and salt–Ni +2 interactions, which depress the effective chain entanglement number and alter surface tension [ 34 ]. The jet then favors electrospraying/beading or breaks before solidification, preventing the formation of continuous filaments. Practically, reducing the phosphate content, increasing PVA concentration (to restore entanglement), or adjusting the solvent polarity and spinning field would be required to recover a stable fibrous jet. In sum, phosphate acts as a powerful morphology modifier: a small amount (MAP) produces uniformly nanoparticle-decorated fibers that are advantageous for catalysis, whereas a stronger phosphate environment (DAP) drives extensive surface encrustation and secondary particle formation, approaching the limit where continuous electrospinning is no longer feasible. These observations align with the XRD-derived phase evolution, supporting a picture in which phosphate promotes in situ nucleation of Ni-containing nanoparticles that, after calcination, decorate a conductive carbon nanofiber scaffold. A representative fiber fragment ( Fig 1E , normal TEM image for MAP-based nanofibers) is shown decorated with a dense population of metal nanoparticles distributed along the surface (“necklace-like” arrangement). The polymer-derived carbon core appears as a lighter, continuous backbone on which darker particles are immobilized. Particle metrics taken from the micrograph indicate a prevalent size in the 25–40 nm range, with occasional coalesced domains ~70–80 nm and rare larger aggregates ≳150–180 nm. This hierarchical dispersion is consistent with phosphate-assisted nucleation in the electrospun jet: partial complexation/precipitation between Ni +2 and H 2 PO₄ ⁻ /HPO₄ −2 produces many nanoscale nuclei that are arrested by the PVA matrix. During calcination, decomposition of acetate/phosphate and the locally reducing gases (CO/H 2 ) convert these nuclei to metallic Ni while the carbonized PVA retains them at the fiber surface. The outcome is a high density of firmly anchored Ni nanoparticles on a conductive carbon scaffold—an architecture expected to enhance electrochemical accessibility and electron transport. The selected-area electron diffraction pattern (inset in Fig 1E ) displays concentric rings with superimposed discrete spots, evidencing a polycrystalline ensemble containing some larger, well-oriented crystallites. The ring spacings correspond to the principal planes of fcc-Ni—(111), (200), (220), and higher-order reflections—fully consistent with the XRD results. A weak, diffuse outer halo is frequently observed in such systems and can be attributed to the turbostratic carbon matrix; any crystalline nickel-phosphate or oxide residues, if present, are below the detection limit or highly disordered and therefore do not give distinct rings. The DAP sample ( Fig 1F ) shows a carbon-fiber fragment densely loaded with metal particles, but the particle size distribution is much broader than in the MAP case. Besides numerous nanoparticles, there are coalesced aggregates in the sub-micron range (representative diameters labeled ≈0.09, 0.26 and 0.74 µm). This contrasts with the mono-phosphate fibers, where most particles were 25–40 nm with only occasional ~70–80 nm domains. The coarser features here indicate enhanced in-flight/early nucleation and subsequent coalescence during drying and calcination when DAP is present—consistent with its higher basicity/ionic strength, which raises solution conductivity, screens polymer–polymer entanglements, and favors particle growth outside the confined fiber skin. The carbon backbone remains discernible as a lighter support, and the dark particles appear strongly anchored, but local smoothing of the fiber contour and the presence of isolated large crystallites suggest partial sintering of neighboring nuclei during heat treatment. The SAED pattern (inset in Fig 1F ) exhibits continuous rings with superimposed discrete spots, characteristic of a polycrystalline ensemble containing some larger, better-oriented crystallites. The ring positions correspond to fcc-Ni principal reflections—(111), (200), (220)—in agreement with XRD. Relative to MAP sample, the rings here are more spotty/segmentary, consistent with the larger crystallite size inferred from TEM (broader size distribution with sub-micron aggregates). A weak diffuse halo is also visible, attributable to the turbostratic carbon matrix; no distinct rings from crystalline phosphate phases are resolved, implying that any phosphate-derived residues are amorphous or below the SAED detection limit after calcination. Fig 2 compares the high-resolution XPS of the mono-ammonium-phosphate (MAP) and di-ammonium-phosphate (DAP) nanofibers (all spectra charge-referenced to C 1s = 284.8 eV). Panel A (MAP, C 1s) displays three components at 284.83 eV (FWHM 1.22 eV, 64.02%, sp 2 /sp 3 C–C/C–H), 285.79 eV (1.84 eV, 27.49%, C–O/C–O–C ± C–N) and 287.60 eV (3.40 eV, 8.49%, C = O/O–C = O), i.e., 36.0% oxygenated carbon. On the other hand, in DAP (Panel B) the envelope appears at 284.76 eV (1.26 eV, 69.43%), 286.05 eV (2.05 eV, 23.80%) and 288.30 eV (2.88 eV, 6.77%), reducing the oxygenated fraction to 30.6%. Notably, the DAP oxygenated peaks are upshifted relative to MAP by +0.26 eV (C–O) and +0.70 eV (C = O), consistent with a more inorganic/ligand-rich outer surface carbon matrix [ 35 ]. Fig 2. XPS spectra of mono- (MAP) and di- (DAP) ammonium-phosphate incorporated Ni/CNFs. Open in a new tab C 1s: A) MAP and B) DAP. N 1s: C) MAP and D) DAP. O 1s: E) MAP and F) DAP. P 2p: G) MAP and H) DAP. Ni 2p: I) MAP and J) DAP. Panel C (MAP, O 1s) resolves three oxygen environments: 529.33 eV (1.34 eV, 21.93%) for lattice Ni–O–Ni (NiO/NiO x ), 531.12 eV (1.82 eV, 26.22%) for Ni–OH/NiOOH and non-bridging P = O/P–O − , and 532.42 eV (2.72 eV, 51.84%) for C–O/adsorbed H 2 O and bridging P–O–M/P–O–C. By contrast, DAP (Panel D) is dominated by higher-BE oxygen and requires only 531.14 eV (1.45 eV, 22.58%) and 532.36 eV (2.56 eV, 77.42%); a distinct lattice O −2 peak near 529 eV is not resolved. This points to a thicker hydrated/ligated overlayer in DAP that attenuates lattice-oxide emission. Panel E (MAP, N 1s) shows a single oxidized-nitrogen component at 402.94 eV (FWHM 1.71 eV; N–O/N + –O − ) with no intensity at 397–400 eV (no nitride or graphitic N) [ 36 ]. Similarly, DAP (Panel F) exhibits a single peak ≈403 eV overlapping the total fit; thus nitrogen remains only as trace oxidized surface species in both materials. Panel G (MAP, P 2p) presents a phosphate (P +5 ) doublet at 133.11/134.67 eV (FWHM 1.65/2.24 eV; 70.21/29.79%), with no signal at 129–131 eV (no phosphide). In DAP (Panel H) the phosphate envelope appears at 132.71 eV (FWHM 0.53 eV, 11.56%) and 133.29 eV (2.36 eV, 88.44%), still within the P +5 range and again without phosphide. The slight lower-BE shift and asymmetric distribution for DAP are compatible with stronger P–O–Ni/C coordination and higher hydration of surface phosphate [ 37 ]. Panel I (MAP, Ni 2p) exhibits the expected oxidized nickel multiplet: Ni +2 2p 3/2 = 853.85 eV (1.54 eV, 9.91%) and 2p 1/2 = 871.88 eV (2.73 eV, 8.92%); Ni 3+ 2p 3/2 = 856.10 eV (3.37 eV, 31.62%) and 2p 1/2 = 873.97 eV (2.60 eV, 6.36%); with strong satellites at ~861 eV (22.05%), ~ 864.46 eV (7.96%), ~ 876.95 eV (5.75%), ~ 880.17 eV (7.43%) (total satellites ≈ 43%). On the other hand, DAP (Panel J) is markedly more Ni +3 -rich—Ni +3 2p 3/2 = 856.05 eV (39.18%), Ni +3 2p 1/2 = 873.23 eV (16.63%)—and even reveals a Ni⁰ 2p₃/₂ at 852.88 eV (FWHM 1.31 eV, 5.15%), consistent with larger metallic cores beneath a more oxidized shell [ 38 ]. The satellite share remains high (≈39% at 861.31/865.39/876.72/881.11 eV), but the balance shifts from satellites to Ni +3 main lines in DAP. Taken together, the side-by-side format underscores clear, systematic differences: relative to MAP, DAP displays less oxygenated carbon (30.6% vs 36.0%), oxygen spectra dominated by hydroxyl/adsorbate/phosphate with no resolvable lattice-O −2 , phosphate that is slightly lower in BE and more strongly coordinated/hydrated, and a nickel envelope that is enriched in Ni 3+ with a small but distinct Ni⁰ contribution. In structural terms, MAP retains a thinner oxide where lattice Ni–O remains visible, whereas DAP forms Ni 0 cores wrapped by a thicker, NiOOH-rich and phosphate/adsorbate-laden shell—a surface chemistry expected to prime DAP for rapid Ni(OH) 2 /NiOOH redox activation during glycerin electro-oxidation, while MAP offers a cleaner oxide/carbon balance with a detectable lattice-oxide component. The difference in catalytic performance between MAP- and DAP-modified samples can be attributed to their distinct surface chemical environments, as revealed by XPS. Variations in phosphate and nitrogen functionalities influence the interaction with Ni active sites, thereby affecting the Ni +2 /Ni +3 redox behavior and the stability of the NiOOH phase during electrooxidation. 3.2. Electrochemical activity 3.2.1. Effect of P&N-doping. Fig 3A compares the 30 th cyclic voltammogram of the AP-free (Ni), MAP-containing, and DAP-containing nanofibers in 1.0 M KOH (scan range −0.2 to ~1.0 V vs Ag/AgCl). In alkaline electrolyte, nickel activates through the well-known surface redox couple [ 39 ]. Fig 3. Cyclic voltammetry voltammograms at 50 mV/s for the prepared nanofibers. Open in a new tab A) 30 th cycle in presence of 1.0 M KOH, and B) in presence of 0.5 M urea solution (in 1.0 M KOH). C) cyclic voltammograms for nanofibers prepared from electrospun solutions having different contents from DAP. D) Effect of DAP on the maximum and anodic peak current density. E) Effect of DAP content on the onset potential. 𝐍 𝐢 ( 𝐎 𝐇 ) 2 ⇌ 𝐍 𝐢 𝐎 𝐎 𝐇 + 𝐇 + + 𝐞 − (9) so the magnitude and sharpness of the Ni +2 /Ni +3 wave report the density and accessibility of electroactive Ni sites. The AP-free Ni nanofiber electrode shows the weakest redox response: broad, low-intensity Ni +2 → Ni +3 oxidation on the forward scan and a diffuse Ni +3 → Ni +2 reduction on the reverse scan, with a comparatively large separation between the anodic and cathodic features. This points to a small population of active Ni(OH) 2 /NiOOH sites and slower interfacial kinetics [ 40 ]. Introducing MAP increases both the anodic and cathodic charges and narrows the redox wave, evidencing a larger number of accessible Ni sites and improved charge transfer. This behavior is fully consistent with the surface chemistry from Fig 2 : MAP fibers possess a thin NiO x (OH) y shell with a resolvable lattice-O component (O 1s at 529.3 eV) atop conductive turbostratic carbon (C 1s with ~36% oxygenated groups), which together favor rapid electron/ion transport and robust wetting. The DAP sample delivers the strongest activation signal of all three: the Ni +2 /Ni +3 wave is more intense and better defined, and the anodic current at high potentials rises most steeply. XPS explains this trend: DAP exhibits Ni o cores wrapped by a more NiOOH-rich, hydrated shell (Ni 2p dominated by Ni +3 main lines; O 1s ruled by 531–532.4 eV hydroxyl/adsorbate/phosphate oxygen) and a slightly less oxygenated carbon surface. The hydrated, ligand-rich overlayer shortens the induction to NiOOH and increases the redoxable Ni fraction, which translates into the larger activation charge observed in CV. In short, the activation sequence follows DAP > MAP > AP-free Ni matching the hierarchy inferred from XPS of Ni oxidation state and surface oxygen speciation. The electrochemically active surface area (ECSA) for the three formulations was estimated from the anodic Ni +2 Ni +3 charge using following equation [ 41 ]: E C S A = 1 q . v ∫ E 1 E 2 ( j a n o d e − j b a s e l i n e ) d E (10) Where v = 0.05 V.s −1 , j is the current, E is the applied potential and is the charge density range for Ni(OH) 2 /NiOOH (~ 0.39 mC.cm −2 ). The estimated ECSA values for the produced nanofibers was 1.285, 1.95 and 2.545 cm 2 for the AP-free, MAP-containing and DAP-containing nanofibers, respectively. A larger ECSA and a surface already biased toward NiOOH (DAP) should deliver a higher density of active sites and faster OH⁻ activation during urea electro-oxidation. MAP, with a thinner oxide but good wettability (more oxygenated carbon), is intermediate, while the AP-free mat provides the fewest accessible sites. The electrochemical activation behavior in Fig 6A therefore mirrors the XPS-derived surface architecture and foreshadows the same performance order under urea oxidation conditions. Fig 6. Response-surface modeling of the CV peak current density j peak for the DAP–Ni/CNF anode. Open in a new tab (A) 2D contour map versus DAP content in the spinning dope and urea concentration (coded units). (B) 3D response surface showing a ridge near ~1 wt% DAP and ~1–1.5 M urea. (C) Predicted vs. experimental j peak with residuals indicating good parity. (D) Model fit with 95% confidence/prediction bands. ANOVA: model F = 90, p < 0.0001; lack-of-fit/residual F = 18.81, p = 0.0062. Fig 3B displays the CV measurements for the three electrodes in presence of 0.5 M urea (in 1.0 M KOH). As shown, the anodic current rises strongly once NiOOH is formed. The catalytic reaction proceeds on NiOOH according to the overall 6-electron step ( 𝐍 𝐇 2 ) 2 𝐂 𝐎 + 6 𝐎 𝐇 − → 𝐍 2 + 𝐂 𝐎 3 − 2 + 5 𝐇 2 𝐎 + 6 𝐞 − (11) The voltammograms in Fig 3B show a clear ranking: DAP > MAP > AP-free Ni. The DAP electrode (blue) displays both the earliest onset of urea oxidation (≈0.42 V vs Ag/AgCl) and the highest current across the potential window; at 1.00 V it reaches 115 mA.cm −2 , versus 43 mA.cm −2 for MAP and 15 mA.cm −2 for Ni. The MAP electrode shows a negative shift of onset relative to bare Ni (≈0.36 vs ≈ 0.48 V) and a sizable current enhancement, but it remains distinctly below DAP. The XPS results explain these electrochemical trends. Ni valence & readiness for NiOOH. DAP exhibits a Ni +3 -rich surface with a small Ni⁰ core signal (Ni 2p: 856.05/873.23 eV dominating), while MAP shows a smaller Ni +3 share and a thinner oxide shell. The DAP surface is therefore closer to the active NiOOH state, lowering the kinetic barrier and shifting the UOR onset negatively. Surface oxygen speciation. O 1s for DAP is dominated by 531–532.4 eV hydroxyl/adsorbate/phosphate oxygen (77%) and lacks a distinct lattice-O −2 peak. This hydrated, ligand-rich skin supplies –OH to the interface and facilitates urea adsorption/dehydrogenation on NiOOH. MAP retains a detectable lattice-O component (529.3 eV, 22%), consistent with a thinner, less hydrated shell and slightly slower activation. Phosphate environment. Both materials show P +5 (PO 4 ), but DAP’s P 2p is slightly lower in BE and more asymmetric, consistent with stronger P–O–Ni/ P–O–C coordination. These anionic ligands help anchor Ni particles, maintain hydration, and can promote interfacial proton/oxide transfer. Moreover, DAP presents a rougher, more heavily decorated surface (SEM/TEM) and, from the Ni redox charge in KOH ( Fig 6A , 30th cycle, 50 mV/s), a larger electrochemically active surface area. Using the Ni(OH) 2 /NiOOH charge method (details in Fig 3A discussion). The current ratio at 1.00 V (≈2.4 × DAP/MAP) exceeds the ECSA ratio (≈1.3×), implying a genuine kinetic/electronic promotion by the NiOOH-rich, phosphate-hydrated surface unique to DAP. The pronounced anodic currents for DAP persist on the reverse sweep without catastrophic decay, indicating that the active NiOOH layer is maintained under urea electrooxidation conditions and that severe passivation is not dominant in this window. Urea electro-oxidation is governed by how readily the surface converts to—and operates as—NiOOH while supplying interfacial –OH. The DAP nanofibers offer exactly that: Ni⁰ cores for conductivity wrapped by a hydrated, Ni +3 -rich shell decorated with phosphate—delivering the lowest onset and highest currents. MAP benefits from increased ECSA and good wettability but lags behind DAP, while bare Ni is limited by fewer active sites and slower Ni(OH) 2 ↔ NiOOH cycling. Tuning the DAP content in the spinning dope strongly modulates the UOR current in 0.5 M urea + 1.0 M KOH as shown in Fig 3C . The 1 wt% DAP electrode exhibits a steep rise after the Ni(OH) 2 → NiOOH transition and delivers the highest currents across the potential window. Either decreasing DAP to 0 wt% (no phosphate) or increasing it to 3–7 wt% progressively damps the response and shifts the curve toward higher potentials. As shown in Fig 3D , both the maximum current density and the anodic peak current density show a clear volcano-type dependence with an optimum at 1 wt% DAP: The maximum current density was 13 mA.cm −2 (0%) → 115 mA.cm −2 (1%) → 22.8 mA.cm −2 (3%) → 10 mA.cm −2 (5%) → 22.5 mA.cm −2 (7%). While the anodic current density (in mA.cm −2 ) was 6 (0%) → 70 (1%) → 13.6 (3%) → 7 (5%) → 9 (7%). Thus, relative to the phosphate-free baseline, 1 wt% DAP boosts the maximum current density by ~8.8× and anodic current desnity by ~11.7 × . Raising DAP above 1 wt% sharply reduces both metrics (to ~0.2–0.3 of the optimum), with a small partial recovery at 7 wt% that remains far below the 1 wt% level. The onset follows the same volcano trend, Fig 3E , reaching a minimum at 1 wt% DAP: E onset ( V vs Ag / AgCl ) = 0 . 47 ( 0 % ) → 0 . 37 ( 1 % ) → 0 . 41 ( 3 % ) → 0 . 42 ( 5 % ) → 0 . 425 ( 7 % ) . The 0.10 V negative shift from 0% to 1% DAP evidences a marked kinetic facilitation of UOR. At low DAP (0%), XPS/SEM indicate fewer Ni nuclei and a thinner, less hydrated surface; the Ni(OH) 2 /NiOOH redox charge ( Fig 6A ) and hence ECSA are smallest, giving low UOR currents and the most positive onset. At the optimum (1%), TEM shows dense nanoparticle decoration without excessive agglomeration; XPS shows Ni⁰ cores wrapped by a Ni +3 -rich, hydroxylated/phosphate-ligated shell (Ni 2p dominated by Ni +3 ; O 1s dominated by 531–532.4 eV OH/adsorbate/P–O). This architecture (i) maximizes electroactive surface area, (ii) supplies interfacial –OH and stabilizing P–O–Ni/P–O–C linkages, and (iii) places the surface close to the active NiOOH state—together yielding the lowest onset and highest currents. At excess DAP (≥3%), SEM/TEM evidence over-decoration and sub-micron aggregates; the surface becomes overly hydrated/ligated and partly blocked by phosphate, decreasing ECSA and increasing mass-transport/charge-transfer resistances. The Ni shell can thicken toward an insulating hydroxide/adsorbate layer, pushing the onset positive and depressing the current despite the presence of Ni. The modest recovery at 7% likely comes from additional metallic domains that partially restore conductivity, but site accessibility remains compromised. Although higher ECSA values indicate a greater number of accessible active sites, the overall catalytic performance is also influenced by charge-transfer kinetics and surface chemistry. Therefore, the optimal activity observed for specific compositions reflects a balance between active-site density and intrinsic catalytic efficiency. 3.2.2. Effect of solution concentration. Effect of urea solution concentration has been investigated using three DAP-based nanofibers; 1, 3 and 7 wt%. For the 1 wt% electrode, as shown in Fig 4A , in blank KOH only the Ni +2 /Ni +3 couple is observed. Adding urea (0.33 → 2.0 M) turns the anodic branch into a pronounced catalytic wave with a negative shift of onset (~0.05–0.1 V) and a monotonic rise of current up to ~1.0 V. Between 0.33 and 1.0 M the increase is steep; from 1.0 to 2.0 M the gain becomes modest, suggesting approach to surface‐coverage/transport saturation at the most active sites. The strong concentration sensitivity is consistent with this composition’s large ECSA and NiOOH-ready, hydroxylated/phosphate-ligated surface (XPS: Ni +3 -rich; O 1s dominated by 531–532.4 eV). For the 3 wt% electrode ( Fig 4B ), the same qualitative trend holds, but absolute currents are much lower and the onset shift with concentration is smaller. Catalytic currents increase with urea from 0.33 to 2.0 M, yet the curves remain widely separated from the 1 wt% case, indicating fewer accessible active sites and/or higher interfacial resistance. Fig 4. Effect of urea solution concentration on the urea electrooxidation reaction over three DAP-containing electrodes: Open in a new tab A) 1 wt%, B) 3 wt% and C) 7 wt%.D) Effect of KOH solution concentration on the urea electrooxidation process over the proposed P&N-doped Ni/CNFs (1 wt% DAP) in presence of 0.5 M urea at 50 mV/s scan rate. Currents are the smallest of the series in case of 7 wt% electrode ( Fig 4C ); the onset is only weakly affected by urea concentration and the anodic rise is shallow even at 2.0 M. The nearly flat blank response and muted catalytic gain point to site blockage/low ECSA and partial mass-transport limitation through an insulating overlayer—again consistent with the structural picture at high DAP content. Across all panels the UOR current scales positively with urea concentration because the active NiOOH phase (formed in situ) oxidizes urea; at low urea concentration the rate is kinetic/coverage-limited, while at high urea concentration it approaches a quasi-saturation regime. The magnitude of the concentration response follows the catalyst quality: 1 wt% DAP » 3 wt% DAP > 7 wt% DAP, mirroring the volcano dependence vs DAP loading. Increasing urea concentration enhances currents for all electrodes, but only the 1 wt% DAP catalyst converts that increase into large, early-onset catalytic waves; over-doping (3–7 wt%) dampens both the onset shift and current gain because of lower ECSA and hindered charge/mass transfer. Fig 4C presents CVs of the 1 wt% DAP nanofiber electrode in 0.5 M urea while varying KOH from 1.0 to 7.0 M. A clear, nearly monotonic enhancement is observed as KOH concentration increases: the catalytic wave initiates at progressively lower potentials, the slope of the anodic branch becomes steeper, and the upper-potential current grows substantially. Between 1.0 and 3.0–4.0 M, both the onset and the current increase rapidly; above ~5.0 M, the gain persists but begins to taper, suggesting proximity to a quasi-saturation regime where charge transfer rather than substrate delivery limits the rate. These trends are mechanistically expected for the Ni(OH) 2 /NiOOH-mediated urea oxidation pathway. Higher OH⁻ activity accelerates formation of NiOOH (the true active phase), shifts its appearance to lower potential, and supplies reactant to the chemical dehydrogenation steps on NiOOH. The electrolyte conductivity rises with KOH concentration, reducing ohmic drop and revealing the intrinsic kinetics more clearly. Increased ionic strength compresses the double layer, raising the near-surface concentration of urea/OH ⁻ . The DAP surface chemistry established by XPS reinforces this behavior: the 1 wt% DAP fibers expose Ni⁰ cores wrapped by a Ni +3 -rich, highly hydroxylated/phosphate-ligated shell (Ni 2p dominated by Ni +3 ; O 1s dominated by 531–532.4 eV OH/adsorbate/P–O). As KOH concentration increases, this hydrated shell more readily converts to and sustains NiOOH, while phosphate linkages (P–O–Ni/P–O–C) and oxygenated groups maintain interfacial wetting and OH⁻ supply. The net result is the observed negative shift of the UOR onset and strong rise of catalytic current from 1 to 7 M KOH. Any slight leveling at the highest concentrations is consistent with the approach to surface-coverage/transport limits rather than a change of mechanism. In short, for the optimally formulated DAP catalyst, boosting KOH concentration continuously improves urea electro-oxidation by promoting NiOOH formation, enhancing OH⁻ availability and conductivity, and leveraging the inherently NiOOH-ready surface architecture revealed by XPS. The improved catalytic activity can be attributed to the synergistic effect of phosphorus and nitrogen functionalities introduced by ammonium phosphate. Phosphate groups facilitate OH⁻ adsorption and stabilize the NiOOH active phase, while nitrogen doping enhances electrical conductivity and strengthens metal–support interactions. These effects collectively promote the Ni 2+ /Ni 3+ redox transformation and improve urea electrooxidation kinetics. 3.2.3. Effect of reaction temperature and electrode stability. Fig 5A shows that heating the electrolyte markedly boosts the urea-oxidation, over the 1wt% DAP electrode at 50 mV/s, current and shifts the catalytic onset to more negative potentials. From 30 → 60 °C the anodic branch steepens continuously and the current at 1.0 V grows by roughly a factor of 2–3, while the oxidation wave appears ~0.08–0.10 V earlier. The hysteresis between forward and reverse sweeps also diminishes with temperature, indicating faster surface regeneration of the active NiOOH phase and improved removal of adsorbed intermediates. Fig 5. Temperature-dependent electrochemical behavior and chronoamperometric stability of the 1 wt% DAP electrode toward urea electrooxidation in alkaline medium. Open in a new tab A) Effect of temperature on the electrocatalytic activity of the 1 wt% DAP electrode using 0.5 M urea solution (in 1.0 KOH). B) Arrhenius plot for the obtained data. C) Chronoamperometry measurements at various potnetioals for 1 wt% DAP in presence of 0.5 M (in 1.0 M KOH). These trends are mechanistically expected for the Ni(OH) 2 /NiOOH-mediated pathway. Higher temperature (i) accelerates the Ni(OH) 2 → NiOOH pre-activation step, (ii) enhances the chemical dehydrogenation steps of urea on NiOOH, and (iii) increases OH⁻ mobility and solution conductivity, reducing ohmic loss [ 20 , 42 ]. The strong temperature sensitivity is fully consistent with the DAP surface chemistry deduced by XPS—Ni⁰ cores wrapped by a Ni +3 -rich, highly hydroxylated/phosphate-ligated shell—which readily converts to and sustains NiOOH and supplies interfacial –OH. Fig 5B presents an Arrhenius plot built from a pseudo-rate constant k (proportional to the catalytic current measured at a fixed potential within the kinetic region). The excellent linearity of Ln( k ) vs 1/T ( R 2 = 0.95) indicates that the same rate-limiting step governs the reaction over 30–60 °C and that mass-transport limitations are not dominating in the analyzed potential window. According to Arrhenius equation, the slope of the fit equals − E a /R; thus the apparent activation energy E a follows directly from the line. The Arrhenius analysis yields an apparent activation energy E a = 9.82 kJ/mol for the urea electro-oxidation on the 1 wt% DAP nanofibers. This is a low barrier for Ni-based UOR and implies that, in the potential window used to extract k , the reaction is kinetically facile and only moderately temperature-dependent. Such a small E a is consistent with a surface that is already close to the active NiOOH state and richly supplied with –OH ligands, as established by XPS for this catalyst (hydroxyl/adsorbate/phosphate oxygen). In this scenario the rate-determining event is likely an early dehydrogenation/electron-transfer step on NiOOH rather than a high-barrier C–N cleavage, explaining the negative onset shift and strong performance at near-ambient temperature. We note that E a is “apparent” and depends on the chosen potential and kinetic regime; nevertheless, the good linearity indicates a single dominant mechanistic step across 30–60 °C for these optimized DAP electrodes. Overall, elevating the temperature lowers the kinetic barrier for urea electro-oxidation on the optimally formulated DAP nanofibers, producing earlier onset, larger currents, and Arrhenius behavior consistent with a single thermally activated step on a NiOOH-rich surface. Fig 5C shows current–time transients recorded on the same 1 wt% DAP electrode at potentials from 0.40 to 1.00 V vs Ag/AgCl. At each step the current exhibits (i) a short, sharp initial drop (t < 20–40 s), and (ii) a slower approach to a quasi-steady value. The fast decay reflects double-layer charging and rapid restructuring of the surface as Ni(OH) 2 → NiOOH forms under bias. The magnitude of the initial spike scales with potential, consistent with faster growth of the active NiOOH phase at higher bias. Quasi-steady region and stability. After ~100–150 s the currents stabilize and then decline only gradually. From visual integration of the traces (±10% error from the plot): 1.00 V: ~ 70 → 55 mA.cm −2 at 1000 s (≈80% retention). 0.90 V: ~ 36 → 26 mA.cm −2 (≈72% retention). 0.80 V: ~ 22 → 15 mA.cm −2 (≈68% retention). 0.70 V: ~ 18 → 12 mA.cm −2 (≈67% retention). 0.60 V: ~ 13 → 9 mA.cm −2 (≈70% retention). 0.50 V: ~ 10 → 7 mA.cm −2 (≈70% retention). 0.40 V: ~ 5 → 4 mA.cm −2 (≈80% retention). The highest sustained current is obtained at 1.00 V, while 0.85–0.95 V offers a good compromise between current and stability (≥70% current retained at 1000 s). Small step-like drops and noise, more visible at 0.8–1.0 V, are attributed to episodic bubble coverage/detachment (UOR and concurrent OER) and the intermittent removal of adsorbed intermediates. The gradual decay is typical for UOR on NiOOH and arises from (i) partial site blocking by reaction intermediates and carbonate, (ii) mass-transport limitations in the diffusion layer, and (iii) competition with OER at the highest potentials [ 43 ]. The ability of the DAP electrode to maintain large steady currents—especially at 0.9–1.0 V—aligns with its Ni +3 -rich, highly hydroxylated/phosphate-ligated surface established by XPS. This surface converts readily to NiOOH, supplies interfacial OH ⁻ , and benefits from P–O–Ni/P–O–C linkages that stabilize Ni sites against dissolution and agglomeration, giving sustained activity under load. Because the same electrode was used sequentially, mild conditioning of the NiOOH layer across steps is expected; nevertheless, the consistent retention across potentials indicates robust durability of the 1 wt% DAP catalyst in 0.5 M urea. The observed stability can be attributed to the robust structural features of the catalyst, including the confinement of Ni nanoparticles within the carbon nanofiber matrix and the presence of P- and N-containing surface functionalities, which enhance metal–support interactions and suppress structural degradation during electrochemical operation. Overall, compared with previously reported Ni-based electrocatalysts for urea oxidation, the present ammonium-phosphate-modified Ni–carbon nanofibers offer a favorable combination of a conductive fibrous carbon framework, uniformly distributed Ni active sites, and phosphate/nitrogen surface functionalities. These features promote efficient Ni +2 /Ni +3 redox transformation, enhance charge transfer, and improve catalyst stability during prolonged operation. The initial current decay observed during chronoamperometry can be attributed to double-layer charging and the rapid adsorption of reaction intermediates, followed by the establishment of a steady-state surface coverage and formation of the NiOOH active phase. 3.2.4. Statistical modeling and optimization analysis. To quantify how composition tunes the catalytic response, we used Design-Expert to build a quadratic RSM for the peak current density j peak extracted from CVs of the optimized DAP-nanofiber anode. The model considered two factors in coded form: the DAP content in the spinning dope (A, 0–7 wt% relative to Ni acetate) and the urea concentration in the electrolyte (B; 0–2.0 M in 1.0 M KOH). The fitted polynomial has the usual structure, J a = − 3 7 8 . 6 1 − 1 6 4 1 . 1 1 A + 1 0 . 0 8 B + 4 . 3 A B − 2 3 3 9 . 0 3 A 2 − 1 4 . 6 4 B 2 − 1 7 . 8 5 A 2 B − 5 9 . 9 4 A B 2 − 1 3 3 7 . 3 4 A 3 − 1 4 . 5 3 B 3 − 3 0 . 8 A 2 B 2 − 8 . 7 4 A 3 B − 1 9 . 4 4 A B 3 − 2 6 0 . 6 2 A 4 − 9 . 6 7 B 4 (12) with linear, interaction, and curvature terms. The 2D contour map ( Fig 6A ) and the 3D surface ( Fig 6B ) reveal a narrow high-response ridge located at low A (around 1 wt% DAP) that extends across moderate-to-high B (roughly 0.8–1.5 M urea). Movement along the DAP axis displays a clear volcano behavior: activity is maximized near 1 wt%, then declines for ≥3 wt% despite the larger heteroatom supply. This matches our electrochemical trends and XPS interpretation—excess DAP over-ligates/over-hydrates the surface, partly blocking NiOOH sites, whereas ~1 wt% DAP yields a Ni +3 -rich, hydroxylated shell that is “NiOOH-ready.” Movement along the urea axis gives a positive but saturating response: increasing urea from 0 to ~1–1.5 M raises j peak , after which gains level off. The saturation is consistent with site coverage of urea/intermediates, increasing viscosity, and mass-transport limitations at the highest urea content. Together, the RSM points to a robust operating window around A ≈ 1 wt% and B ≈ 1.0–1.5 M, in agreement with our factor-by-factor CV study. ANOVA confirms the model is highly significant (F = 90.4, p < 0.0001), i.e., the regression captures the dominant variability in j peak . The lack-of-fit/ residual test is also significant (F = 18.81, p = 0.0062), which is common when the pure error from replicates is very small; practically, it flags that some higher-order effects (e.g., subtle transport or ohmic contributions at the composition extremes) are not fully represented by the quadratic. Even so, the diagnostic plots validate predictive utility: Fig 6C (Predicted vs. experimental/ residuals) shows points clustered tightly about the 45° line and residuals symmetrically distributed around zero, indicating no systematic bias across the design space. Fig 6D (model fit with 95% confidence/prediction bands) shows the experimental responses lying within narrow uncertainty envelopes, which broaden only near the edges (where data density is naturally lower). The gentle concavity along the urea axis visualizes the negative β 22 (saturation), while the sharper curvature along the DAP axis reflects the volcano captured by a positive β 1 and negative β11. The statistics and surfaces together formalize our mechanistic picture: DAP content primarily sets the density and accessibility of NiOOH sites through shell chemistry (XPS: Ni +3 enrichment, hydroxylated/phosphate ligation). This yields a true optimum near 1 wt% DAP. Urea concentration feeds the surface reaction and boosts current until adsorption/transport saturation is approached around ~1–1.5 M, beyond which further increases offer little benefit. Thus, the RSM recommends operating near 1 wt% DAP and ~1–1.5 M urea to maximize j peak ; outside this corridor, either insufficient active-site utilization (low DAP/low urea) or site blocking/transport penalties (high DAP/high urea) limit gains. The excellent model significance, supportive diagnostics, and alignment with independent electrochemical/XPS evidence provide strong confidence that these conclusions are both statistically and physically grounded. 3.3. Direct Urea Fuel Cell (DUFC) Implementing the catalyst in a working DUFC is essential to translate half-cell activity into device-level metrics. We fabricated a membrane-less air-cathode cell using the MAP (1 wt%) nanofiber anode ink on carbon cloth and a commercial Pt/C air cathode; polarization curves were recorded by LSV (0.01 V.s −1 ) and current/power were normalized to the cathode area (0.00052 m 2 ). Polarization behavior ( Fig 7A ). The open-circuit potential (OCP) lies near 0.41–0.46 V at low urea content and decreases as the urea concentration is raised to 2.0 M (down to ~0.25–0.30 V). This drop is characteristic of membrane-less DUFCs and originates from fuel crossover to the air cathode, which creates a mixed potential and partially suppresses the ORR. Under load, three regions are evident: (i) an activation-controlled zone at low current; (ii) a quasi-linear ohmic region; and (iii) mass-transport limitation at high current [ 44 ]. Increasing urea concentration shifts the curves to higher current densities, reflecting improved fuel availability at the anode, but the concurrent OCP loss and stronger cathode polarization at high urea concentration limit the net voltage. Fig 7. Performance evaluation of membrane-less direct urea fuel cells assembled using the MAP 1 wt%-based anode at different urea concentrations. Open in a new tab A) Polarization curves and B) Power density curves at different urea solution concentrations for membrane-less air cathode direct urea cells assembled using MAP 1 wt%-based anode. The power–current plots ( Fig 7B ) reveal a clear volcano-type dependence on urea concentration. The peak power density rises as urea concentration is increased from 0.05 to ~0.3–0.5 M, reaching ≈1.0–1.1 W.m −2 at currents of ~3.5–4.5 A.m −2 . At still higher concentrations (≥1.0 M) the peak power drops (to a few 10 2 mW.m −2 at 1.5–2.0 M) because the OCP penalty and cathode losses from crossover outweigh the anode’s gain in kinetic current. The position of the maximum shifts to higher current with increasing urea concentration, consistent with improved mass transport at the anode but growing mixed-potential effects at the cathode. The respectable DUFC performance at moderate concentration is enabled by the MAP–nanofiber anode’s NiOOH-active surface on a conductive carbon scaffold (XPS: NiO x (OH) y shell, oxygenated-carbon support, phosphate anchoring), which promotes rapid OH − /electron transfer and mechanical stability after the post-treatment. The decline at high concentration is a cell-level rather than catalyst-intrinsic limitation—principally crossover-induced voltage loss and ORR inhibition at the air cathode in this membrane-less geometry. Within this cell architecture and loading, ~0.3–0.5 M urea offers the best compromise between anode kinetics and cathode voltage retention. The cell-level tests highlight how device physics can override half-cell trends. Polarization curves ( Fig 8A , at 0.5 M urea solution) show that at 35 °C the membrane-less air-cathode DUFC delivers the highest open-circuit potential (≈0.45 V) and the largest limiting current to ~6.7 A.m −2 . Raising the temperature to 45 and 55 °C sharply lowers the OCP (to ≲0.15–0.20 V) and the attainable current (zero-voltage intercept drops to ~4.3 and ~3.6 A.m −2 , respectively). The accompanying power curves ( Fig 8B ) peak at ≈1.0 W m −2 near 3.5–4.5 A.m −2 at 35 °C, but fall to only ~0.15–0.20 W.m −2 at 45–55 °C (max near 2.0–2.5 A.m −2 ). Thus, increasing temperature reduces peak power by ~80–85% and shifts the optimum to lower current. Fig 8. Temperature-dependent performance of membrane-less direct urea fuel cells assembled using the MAP 1 wt%-based anode. Open in a new tab A) Polarization curves and B) Power density curves at different temperatures and 0.5 M urea solution for membrane-less air cathode direct urea cells assembled using MAP 1 wt%-based anode. These counter-intuitive losses (given that anode kinetics usually accelerate with temperature) are characteristic of membrane-less air-cathode DUFCs and arise from cathode-dominated effects: Higher temperature increases urea and NH 3 diffusion and volatility. Their arrival at the Pt/C air cathode drives parasitic oxidation, depressing the OCP and steepening cathodic polarization (mixed ORR/UOR). Although ORR kinetics benefit from the temperature, the solubility of O 2 in alkaline solution decreases with temperature, pushing the cathode earlier into mass-transport limitation; the polarization curves accordingly drop more rapidly. Heating promotes water loss from the gas-diffusion layer while simultaneously increasing vapor generation from urea oxidation; either dry-out (loss of ionic pathways) or local flooding (blocked O 2 access) reduces the effective ORR area. Elevated NH 3 at higher temperature can adsorb on Pt, further lowering ORR activity. By contrast, the anode itself remains competent at higher temperature (as confirmed in half-cell tests), so the net decline is cell-architecture-driven, not catalyst-intrinsic. Practically, these results indicate that this DUFC configuration performs best near ambient (≈35 °C). Further gains at elevated temperature would require crossover mitigation (e.g., a thin anion-exchange separator or increased cathode hydrophobicity), NH 3 -tolerant ORR catalysts, and optimized GDL/microporous-layer design to stabilize the triple-phase boundary. The enhanced fuel cell performance is attributed to the synergistic effect of the conductive nanofiber structure and the modified surface chemistry. The fibrous network promotes efficient electron transport and mass transfer, while P and N functionalities facilitate the Ni +2 /Ni +3 redox process and stabilize the NiOOH active phase, resulting in improved power output. 4. Conclusions N/P-doped Ni/CNFs can be prepared by one-pot electrospinning of PVA/Ni-acetate dopes containing ammonium-phosphate additives, followed by vacuum drying (60 °C, 10 h) and carbonization. MAP and DAP can be utilized as P and N precursors, however they impart distinct morphology and surface chemistry: MAP yields uniform, well-decorated fibers with a thinner NiO x (OH) y shell that retains a lattice-O component, on a slightly more oxygenated carbon surface; DAP drives rougher, more heavily decorated fibers comprising Ni⁰ cores wrapped by a Ni +3 -rich, hydroxylated/phosphate-ligated shell, with less oxygenated carbon but greater surface hydroxylation (facilitating Ni(OH) 2 ↔ NiOOH). However, TAP is not recommended as an electrospinning additive under these conditions because it destabilizes the jet (excess solution conductivity/electrospraying), preventing continuous fiber formation. As electrocatalysts, the nanofibers show high UOR activity with composition optimization: activity and Ni-redox charge follow DAP > MAP > AP-free, with a clear volcano vs DAP loading and an optimum at 1 wt% DAP. The optimized electrode exhibits a low apparent activation energy (9.82 kJ.mol −1 ), strong positive responses to urea and KOH concentrations, and 70–80% current retention over 1000 s in chronoamperometry. The nanofibers can be directly utilized as DUFC anodes; in a membrane-less air-cathode cell the MAP-based anode delivered ~1.1 W.m −2 peak power at 35 °C (0.3–0.5 M urea), with higher-temperature losses attributed to crossover/ORR limitations rather than catalyst instability. Overall, ammonium-phosphate–assisted electrospinning provides a scalable, low-cost route to N/P-modified Ni/CNFs whose structure and surface chemistry can be tuned for efficient urea electro-oxidation and practical DUFC operation, offering clear guidance for future optimization of dopant identity/loading and cell architecture. 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If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.--> Reviewer #1: Yes Reviewer #2: Yes ********** -->4. Is the manuscript presented in an intelligible fashion and written in standard English? PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.--> Reviewer #1: Yes Reviewer #2: Yes ********** -->5. Review Comments to the Author Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)--> Reviewer #1: General comment The manuscript presents the synthesis of ammonium-phosphate-modified Ni–carbon nanofibers and their application for urea electrooxidation and membrane-less direct urea fuel cells enabling urea remediation along with renewable power generation. The topic is relevant and the material design is interesting. However, the manuscript can be further strengthened by addressing the following points. Specific comments 1. Clarification of the role of ammonium phosphate The manuscript discusses the beneficial effect of MAP and DAP additives; however, the mechanistic role of phosphate and nitrogen functionalities in enhancing catalytic activity should be discussed more clearly, particularly in relation to the Ni+2/Ni+3 redox transformation and surface adsorption properties. 2. Abbreviations consistency All abbreviations should be defined at their first appearance in both the abstract and main text, and used consistently throughout the manuscript. 3. Comparison with previous Ni-based catalysts A brief comparison with previously reported Ni-based carbon nanofiber or Ni-based urea oxidation catalysts would help highlight the advantages of the present material in terms of activity and stability. 4. Stability discussion The chronoamperometric stability results are promising; however, the discussion should better emphasize the structural robustness of the catalyst and relate it to the physicochemical characterization results. 5. Fuel cell discussion The direct urea fuel cell performance is interesting, but the manuscript would benefit from a short discussion explaining how electrode structure and heteroatom doping influence the observed power density. 6. Minor language improvement The manuscript would benefit from minor English editing to improve clarity and readability. 7. Figure clarity Some figures could be improved by ensuring consistent font size and labeling style, particularly for electrochemical plots. 8. Conclusion section The conclusion could be strengthened by more clearly highlighting the practical significance of converting urea-containing wastewater into electrical energy. 9. Affiliation The affiliation of the author Ayman Youcef is missing. Reviewer #2: The manuscript addresses an important topic in the area of low-cost electrocatalysts for alkaline fuel cell applications. The use of ammonium phosphate-modified Ni–carbon nanofibers is interesting, and the experimental work appears to be carefully carried out. The combination of structural characterization with electrochemical testing is appreciated. However, the presentation can be improved in several places, particularly in clarifying the origin of the performance differences among the studied samples and in strengthening the discussion of the electrochemical results. The manuscript may be considered for publication after the following points are addressed. Comments 1. The Introduction gives a reasonable overview of Ni-based catalysts for urea oxidation, but the specific novelty of using ammonium phosphate as a modifier for electrospun Ni–carbon nanofibers should be emphasized more clearly. It would be useful to explain how this approach differs from other reported heteroatom-doped or phosphate-modified carbon-supported Ni systems. 2. Since both mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP) were used, the manuscript would benefit from a clearer explanation of why they lead to different catalytic behaviors. The discussion should better connect these performance differences with the characterization results, especially the surface chemical states identified by XPS. 3. The ECSA values are reported, but the manuscript should more clearly state how these values correlate with the observed oxidation currents and fuel cell performance. This would make the electrochemical interpretation more convincing. 4. The CVs recorded at different scan rates are useful, but the manuscript should comment on whether the oxidation process appears mainly diffusion-controlled, surface-controlled, or mixed-controlled based on the obtained trends. 5. The stability data are interesting, but the initial current decay observed during chronoamperometry should be briefly explained. For example, the authors may comment on whether this behavior is associated with double-layer charging, adsorption of intermediates, or surface reconstruction. 6. The direct urea fuel cell results are promising, but a short paragraph explaining how the catalyst structure and surface chemistry contribute to the obtained power output would strengthen this part of the manuscript. 7. Please ensure that all chemicals used in catalyst synthesis, electrode fabrication, and fuel cell assembly are listed clearly in the Materials section. 8. The manuscript is generally understandable, but there are a number of sentences that could be improved for grammar and clarity. In addition, the authors should check the consistency of abbreviations, symbols, and units throughout the manuscript. ********** -->6. PLOS authors have the option to publish the peer review history of their article ( what does this mean? ). If published, this will include your full peer review and any attached files. If you choose “no”, your identity will remain anonymous but your review may still be made public. Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy .--> Reviewer #1: No Reviewer #2: No ********** [NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.] To ensure your figures meet our technical requirements, please review our figure guidelines: https://journals.plos.org/plosone/s/figures You may also use PLOS’s free figure tool, NAAS, to help you prepare publication quality figures: https://journals.plos.org/plosone/s/figures#loc-tools-for-figure-preparation . NAAS will assess whether your figures meet our technical requirements by comparing each figure against our figure specifications. --> PLoS One. 2026 Jun 4;21(6):e0347020. doi: 10.1371/journal.pone.0347020.r002 Author response to Decision Letter 1 Article notes Copyright and License information Collection date 2026. PMC Copyright notice 17 Apr 2026 Dear Prof. Zafar Ghouri Academic Editor Plos One Thank you for your kind response about the manuscript [PONE-D-26-14686] titled: “Interface-Engineered Nickel Oxyhydroxide on Carbon Nanofibers for Efficient Urea Oxidation and Wastewater-to-Energy Conversion” The referee's comments were helpful to strength the manuscript. We would like to inform you that we have modified the manuscript according to the given comments. To make it more easily, we have written the comments in bold phase followed by the responses in normal one. Moreover, in the revised manuscript, you can find the changes in the text in blue color. We hope our responses cover all the comments. It will be our pleasure to respond about any more comments. Thank you for your cooperation Sincerely yours Corresponding author Nasser A. M. Barakat Professor Chemical Engineering Department, Minia university, Egypt Reviewer #1: General comment: The manuscript presents the synthesis of ammonium-phosphate-modified Ni–carbon nanofibers and their application for urea electrooxidation and membrane-less direct urea fuel cells enabling urea remediation along with renewable power generation. The topic is relevant and the material design is interesting. However, the manuscript can be further strengthened by addressing the following points. Response: We sincerely thank the reviewer for the positive evaluation of our work and for recognizing the relevance of the topic and the interest of the proposed material design. We appreciate the constructive suggestions provided to further improve the quality of the manuscript. All comments have been carefully considered, and the manuscript has been revised accordingly to enhance the clarity of presentation, strengthen the discussion, and better highlight the significance of the results. Detailed responses to each specific comment are provided below. Specific comments 1. Clarification of the role of ammonium phosphate The manuscript discusses the beneficial effect of MAP and DAP additives; however, the mechanistic role of phosphate and nitrogen functionalities in enhancing catalytic activity should be discussed more clearly, particularly in relation to the Ni+2/Ni+3 redox transformation and surface adsorption properties. Response: We thank the reviewer for this important and insightful comment. In the revised manuscript, the discussion has been expanded to more clearly explain the mechanistic role of ammonium phosphate-derived phosphorus and nitrogen functionalities. Specifically, it is now clarified that the incorporation of phosphate species (P+5–O) and nitrogen functionalities modifies the electronic environment of the Ni active sites and facilitates the reversible Ni+2/Ni+3 redox transition, which is essential for the formation of the catalytically active NiOOH phase. In addition, the presence of phosphate groups enhances the adsorption of OH⁻ species and stabilizes surface intermediates, while nitrogen doping improves the electrical conductivity of the carbon nanofiber matrix and strengthens the interaction between Ni species and the support. These combined effects lead to improved charge-transfer kinetics and higher electrochemical activity. The corresponding explanation has been added and supported by the XPS analysis and electrochemical results (CV redox features and activity trends) in the revised manuscript. This is paragraph has been added to the revised manuscript “The improved catalytic activity can be attributed to the synergistic effect of phosphorus and nitrogen functionalities introduced by ammonium phosphate. Phosphate groups facilitate OH⁻ adsorption and stabilize the NiOOH active phase, while nitrogen doping enhances electrical conductivity and strengthens metal–support interactions. These effects collectively promote the Ni²⁺/Ni³⁺ redox transformation and improve urea electrooxidation kinetics.” 2. Abbreviations consistency All abbreviations should be defined at their first appearance in both the abstract and main text, and used consistently throughout the manuscript. Response: We thank the reviewer for this valuable comment. The manuscript has been carefully revised to ensure that all abbreviations are defined at their first occurrence in both the abstract and the main text. In addition, a thorough check was performed to maintain consistent usage and formatting of all abbreviations throughout the manuscript. 3. Comparison with previous Ni-based catalysts. A brief comparison with previously reported Ni-based carbon nanofiber or Ni-based urea oxidation catalysts would help highlight the advantages of the present material in terms of activity and stability. Response: We thank the reviewer for this helpful suggestion. In the revised manuscript, a brief comparison with previously reported Ni-based carbon nanofiber and Ni-based urea oxidation catalysts has been added to better position the present material within the literature. This comparison highlights that the ammonium-phosphate-modified Ni–carbon nanofibers developed in this work combine the advantages of a conductive nanofibrous carbon network, well-dispersed Ni active sites, and phosphorus/nitrogen surface functionalities, which together contribute to improved redox reversibility, high catalytic activity, and stable electrochemical behavior. The added discussion also clarifies that the enhanced performance of the present catalyst is associated not only with the presence of Ni active centers, but also with the synergistic role of the modified CNF support in facilitating charge transfer and stabilizing the catalytically active NiOOH phase during urea oxidation. This is paragraph has been added to the revised manuscript “Compared with previously reported Ni-based electrocatalysts for urea oxidation, the present ammonium-phosphate-modified Ni–carbon nanofibers offer a favorable combination of a conductive fibrous carbon framework, uniformly distributed Ni active sites, and phosphate/nitrogen surface functionalities. These features promote efficient Ni²⁺/Ni³⁺ redox transformation, enhance charge transfer, and improve catalyst stability during prolonged operation.” 4. Stability discussion The chronoamperometric stability results are promising; however, the discussion should better emphasize the structural robustness of the catalyst and relate it to the physicochemical characterization results. Response: We thank the reviewer for this valuable suggestion. In the revised manuscript, the discussion of the chronoamperometric stability has been expanded to better emphasize the structural robustness of the catalyst and its relation to the physicochemical characteristics. Specifically, it is now clarified that the observed stable current response is associated with the uniform dispersion of Ni nanoparticles within the partially graphitized carbon nanofiber matrix, as confirmed by TEM analysis, which provides strong physical confinement and prevents particle agglomeration. In addition, the XPS results indicate the presence of phosphorus- and nitrogen-containing surface functionalities, which enhance metal–support interactions and contribute to stabilizing the active Ni species during repeated redox cycling. These features collectively support the durability of the catalyst under electrochemical operation. The revised text now explicitly links the stability behavior with the structural and surface properties revealed by XRD, TEM, and XPS analyses. 5. Fuel cell discussion The direct urea fuel cell performance is interesting, but the manuscript would benefit from a short discussion explaining how electrode structure and heteroatom doping influence the observed power density. Response: We thank the reviewer for this valuable suggestion. In the revised manuscript, a short discussion has been added to better explain the relationship between the electrode structure, heteroatom doping, and the observed fuel cell performance. It is now clarified that the interconnected carbon nanofiber network provides an efficient pathway for electron transport and facilitates mass transfer within the electrode. In addition, the incorporation of phosphorus and nitrogen functionalities modifies the surface electronic properties of the Ni active sites, enhances the Ni+2/Ni+3 redox activity, and improves the adsorption of reactants and intermediates. These combined effects contribute to improved charge-transfer kinetics and reduced internal resistance, which are directly reflected in the enhanced power density. The corresponding explanation has been incorporated into the revised manuscript. This paragraph has been added to the revised manuscript “The enhanced fuel cell performance is attributed to the synergistic effect of the conductive nanofiber structure and the modified surface chemistry. The fibrous network promotes efficient electron transport and mass transfer, while P and N functionalities facilitate the Ni+2/Ni+3 redox process and stabilize the NiOOH active phase, resulting in improved power output”. 6. Minor language improvement The manuscript would benefit from minor English editing to improve clarity and readability. Response: We thank the reviewer for this helpful suggestion. The manuscript has been carefully revised to improve the clarity, grammar, and overall readability of the text. All sections were edited to ensure concise and clear scientific expression. 7. Figure clarity Some figures could be improved by ensuring consistent font size and labeling style, particularly for electrochemical plots. Response: We thank the reviewer for this helpful comment. All figures have been carefully revised to ensure consistent font size, labeling style, and overall presentation quality, particularly for the electrochemical plots. The updated figures improve clarity and readability throughout the manuscript. 8. Conclusion section The conclusion could be strengthened by more clearly highlighting the practical significance of converting urea-containing wastewater into electrical energy. Response: We thank the reviewer for this valuable suggestion. The Conclusion section has been revised to more clearly emphasize the practical and environmental significance of the present work. In particular, it now highlights the potential of the developed Ni–carbon nanofiber catalysts for simultaneous urea remediation and renewable electricity generation, demonstrating a sustainable approach for wastewater treatment and energy recovery. This sentence was added to the revised manuscript This work demonstrates a promising strategy for the simultaneous treatment of urea-containing wastewater and generation of renewable electricity, contributing to the development of sustainable and energy-efficient environmental technologies. 9. Affiliation The affiliation of the author Ayman Youcef is missing. Response: We thank the reviewer for pointing this out. The missing affiliation of Ayman Youcef has now been added in the revised manuscript. Reviewer #2: The manuscript addresses an important topic in the area of low-cost electrocatalysts for alkaline fuel cell applications. The use of ammonium phosphate-modified Ni–carbon nanofibers is interesting, and the experimental work appears to be carefully carried out. The combination of structural characterization with electrochemical testing is appreciated. However, the presentation can be improved in several places, particularly in clarifying the origin of the performance differences among the studied samples and in strengthening the discussion of the electrochemical results. The manuscript may be considered for publication after the following points are addressed. Response: We sincerely thank the reviewer for the positive evaluation of our work and for recognizing its relevance and experimental rigor. We also appreciate the constructive suggestions aimed at improving the clarity and depth of the manuscript. In the revised version, we have carefully addressed all the points raised by the reviewer. In particular, the discussion has been expanded to better clarify the origin of the performance differences among the investigated samples, with emphasis on the role of ammonium phosphate-derived functionalities and their influence on the electrochemical behavior. Additionally, the interpretation of the electrochemical results has been strengthened to provide a clearer mechanistic understanding. Detailed responses to each specific comment are provided below. Comments 1. The Introduction gives a reasonable overview of Ni-based catalysts for urea oxidation, but the specific novelty of using ammonium phosphate as a modifier for electrospun Ni–carbon nanofibers should be emphasized more clearly. It would be useful to explain how this approach differs from other reported heteroatom-doped or phosphate-modified carbon-supported Ni systems. Response: We thank the reviewer for this insightful comment. In the revised manuscript, the Introduction has been expanded to more clearly highlight the novelty of using ammonium phosphate as a multifunctional additive in the electrospinning precursor. Unlike conventional approaches that rely on post-synthesis doping or the introduction of separate heteroatom sources, the present method enables the simultaneous incorporation of phosphorus and nitrogen functionalities during nanofiber formation and carbonization, leading to a more homogeneous distribution of dopants and strong interaction with Ni species. It is now clarified that this approach differs from previously reported systems by combining (i) in-situ P,N co-doping, (ii) controlled dispersion of Ni nanoparticles within a conductive nanofibrous carbon matrix, and (iii) direct tuning of the Ni²⁺/Ni³⁺ redox behavior through surface chemistry modification. These features collectively contribute to improved catalytic activity and stability. The corresponding explanation has been incorporated into the Introduction to better distinguish the present work from existing Ni-based and heteroatom-doped catalyst systems. This sentence has been added to the revised manuscript In contrast to conventional heteroatom-doping strategies, the use of ammonium phosphate in the electrospinning precursor enables in-situ incorporation of both phosphorus and nitrogen functionalities, leading to a homogeneous distribution of dopants and enhanced interaction with Ni active sites 2. Since both mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP) were used, the manuscript would benefit from a clearer explanation of why they lead to different catalytic behaviors. The discussion should better connect these performance differences with the characterization results, especially the surface chemical states identified by XPS. Response: We thank the reviewer for this important comment. In the revised manuscript, the discussion has been expanded to provide a clearer explanation of the different catalytic behaviors observed for MAP- and DAP-modified samples. It is now clarified that the difference arises from the distinct phosphorus and nitrogen environments introduced by MAP and DAP, which affect the surface chemistry and electronic structure of the catalyst. In particular, the XPS analysis shows variations in the relative contributions of phosphate (P⁵⁺–O) species and nitrogen functionalities, which influence the interaction between Ni active sites and the carbon matrix. These differences impact the Ni+2/Ni+3 redox transition, the formation and stabilization of the NiOOH active phase, and the adsorption of reaction intermediates during urea oxidation. The revised discussion now explicitly correlates the observed electrochemical performance trends with the XPS results, providing a more consistent structure–property relationship between dopant chemistry and catalytic activity. This sentence has been added to the revised manuscript “The difference in catalytic performance between MAP- and DAP-modified samples can be attributed to their distinct surface chemical environments, as revealed by XPS. Variations in phosphate and nitrogen functionalities influence the interaction with Ni active sites, thereby affecting the Ni²⁺/Ni³⁺ redox behavior and the stability of the NiOOH phase during electrooxidation.” 3. The ECSA values are reported, but the Attachment Submitted filename: Response .docx pone.0347020.s002.docx (33.3KB, docx) PLoS One. doi: 10.1371/journal.pone.0347020.r003 Decision Letter 1 Zafar Ghouri Author information Copyright and License information Roles Zafar Ghouri : Academic Editor © 2026 Zafar Ghouri This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. PMC Copyright notice 21 Apr 2026 Interface-Engineered Nickel Oxyhydroxide on Carbon Nanofibers for Efficient Urea Oxidation and Wastewater-to-Energy Conversion PONE-D-26-14686R1 Dear Dr. Barakat, We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements. Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication. An invoice will be generated when your article is formally accepted. Please note, if your institution has a publishing partnership with PLOS and your article meets the relevant criteria, all or part of your publication costs will be covered. Please make sure your user information is up-to-date by logging into Editorial Manager at Editorial Manager® and clicking the ‘Update My Information' link at the top of the page. For questions related to billing, please contact billing support . If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org. Kind regards, Zafar Khan Ghouri Academic Editor PLOS One Additional Editor Comments (optional): Dear Prof. Barakat, Thank you for submitting the revised version of your manuscript entitled “Interface-Engineered Nickel Oxyhydroxide on Carbon Nanofibers for Efficient Urea Oxidation and Wastewater-to-Energy Conversion” (Manuscript ID: PONE-D-26-14686R1). We are pleased to inform you that, following the first round of peer review and assessment of your revisions, your manuscript has been accepted for publication. The reviewers’ comments have been adequately addressed, and the manuscript meets the journal’s standards for publication. Yours sincerely, Zafar Khan Ghouri, Ph.D. Reviewers' comments: PLoS One. doi: 10.1371/journal.pone.0347020.r004 Acceptance letter Zafar Ghouri Author information Copyright and License information Roles Zafar Ghouri : Academic Editor © 2026 Zafar Ghouri This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. PMC Copyright notice PONE-D-26-14686R1 PLOS One Dear Dr. Barakat, I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS One. Congratulations! Your manuscript is now being handed over to our production team. At this stage, our production department will prepare your paper for publication. This includes ensuring the following: * All references, tables, and figures are properly cited * All relevant supporting information is included in the manuscript submission, * There are no issues that prevent the paper from being properly typeset You will receive further instructions from the production team, including instructions on how to review your proof when it is ready. Please keep in mind that we are working through a large volume of accepted articles, so please give us a few days to review your paper and let you know the next and final steps. Lastly, if your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org. 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