ABSTRACT
Abstract
Provided are electrochemical cells for hydrogen production and methods for hydrogen production. The electrochemical cell and methods use a mediator that may have a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Also, provided are systems for generating hydrogen and water from oxygen and generating water from oxygen and hydrogen.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Application No. 62/809,429, filed on Feb. 22, 2019, the disclosure of which is hereby incorporated by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
This invention was made with government support under contract no. 1760540 awarded by the National Science Foundation. The government has certain rights in the invention.
BACKGROUND OF THE DISCLOSURE
The implementation of electrolysis systems for electrochemical hydrogen production has continued to grow as the paradigm shift towards renewable energy and fuels progresses. However, issues regarding conventional polymer electrolyte membrane (PEM) electrolysis systems remain; the performance of PEM electrolyzers degrade if operated with intermittent energy sources, while the high cost of electricity continues to hinder large scale adoption of the technology. In order to make electrochemical hydrogen production more feasible and compete with methane reforming on a large scale, renewable energy sources need to be used along with new strategies electrochemical hydrogen production. Decoupled electrolysis systems have been studied to temporally and spatially separate the evolution of hydrogen and oxygen, making the electrolysis system safer, but these systems tend to reduce efficiency and increase electricity costs.
Water electrolyzers, i.e. devices used for hydrogen (H 2 ) production, can be operated in a schedulable fashion and thus provide flexibility to power grids (e.g., additional energy, power, and ancillary services) on sec-to-sec to seasonal timescales. To facilitate the cost-efficiency of these systems, electrolysis can be scheduled for periods with low electricity costs. Unlike other schedulable industry-scale processes (e.g., steel manufacturing), hydrogen itself can be used to store electricity and perform spatio-temporal energy arbitrage and provide ancillary services just like other energy storage (ES) technologies (e.g., batteries or pumped hydro).
The low levelized cost of storage (LCOS) for long-duration energy storage (LDES) applications strongly depends on the cost of the energy storage media and the efficiency of the system. While multiple electrochemical energy conversion devices exist, they exhibit trade-off between costs and performance that do not satisfy LDES requirements. Systems such as Li-ion batteries or Vanadium redox flow-batteries are optimized to tackle shorter timeframes of energy storage where the cost of the energy storage media is not as significant and there are more stringent requirements with respect to round-trip efficiency and cost of power components. On the other hand, systems such as regenerative fuel cells rely on a fairly inexpensive storage media (i.e. H 2 ) but suffer from low round-trip efficiency.
Increased efforts to curb global warming have led to a drastic surge in the deployment of renewable electricity sources, such as wind and solar power. However, as these sources form a larger fraction of the energy on the grid, their intermittency causes supply instability which can lead to large fluctuations in energy prices.
The implementation of electrolysis systems for electrochemical hydrogen production has continued to grow as the paradigm shift towards renewable energy and fuels progresses. However, issues regarding conventional polymer electrolyte membrane (PEM) electrolysis systems remain; the performance of PEM electrolyzers degrade if operated with intermittent energy sources, while the high cost of electricity continues to hinder large-scale adoption of the technology.
SUMMARY OF THE DISCLOSURE
The present disclosure provides methods for producing hydrogen and/or oxygen. The present disclosure also provides electrochemical cells and energy storage systems.
In an aspect, the present disclosure provides methods for producing hydrogen and/or oxygen. The methods may be based on use of a mediator (e.g., a redox intermediate) having a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Various mediators can be used. A mediator can be oxidized to provide an oxidized mediator. An oxidized mediator can be reduced to provide a reduced mediator. A mediator that has (at least) two different oxidation states, which oxidation states may be accessed by oxidation or reduction from one state to the other. A mediator can comprise various anions. It is desirable that the anion be soluble in aqueous and non-aqueous electrolytes.
The present disclosure provides the advantage that the hydrogen and oxygen generation steps may be separated in time and/or space. Thus, for example, hydrogen is generated with concomitant oxidation of a mediator (e.g., redox intermediate). Later, the same oxidized mediator (e.g., redox intermediate) is used in a method to generate oxygen. Either the oxygen generation step or the hydrogen generation step may be performed first, if the reactions are to be run separately.
The present disclosure provides methods for the generation of hydrogen from protons using a mediator, which is oxidized in the process. The methods comprise reducing protons to generate hydrogen and oxidizing a mediator (which may be a reduced mediator) to generate an oxidized mediator. In various examples, a method for the generation of hydrogen comprises oxidizing a mediator at a working electrode to yield an oxidized mediator and reducing protons at a counter electrode to yield hydrogen. The hydrogen generated may be collected, and, optionally, pressurized, for storage and/or future use.
The present disclosure provides methods for the electrochemical generation of oxygen. The methods comprise oxidizing water to generate oxygen and reducing a mediator (which may be an oxidized mediator) to generate a reduced mediator. The yield of oxygen, with reference to the amount of electrons passed through the system, is desirable. The methods may comprise providing and maintaining a potential across the working electrode and the counter electrode and/or the reference electrode, where present. After the reaction is complete, the reduced mediator may be recovered. The reduced mediator may be oxidized to obtain (oxidized) mediator, which can be used in further oxygen production methods. The reduced mediator may be oxidized using electrochemical techniques. In an example, the mediator is oxidized in a hydrogen production method. Thus, the recycling of the mediator may be linked to the production of a useful product such as, for example, hydrogen. The oxygen generated may be collected, and optionally pressurized, for storage and/or future use.
The present disclosure provides independent methods for the generation of hydrogen and the generation of oxygen. Each of these methods may be used together to produce both hydrogen and oxygen. In various examples, the methods of generating hydrogen and methods of generating oxygen are used to provide a method for the generation of hydrogen and oxygen.
In an aspect, the present disclosure provides electrochemical cells. The electrochemical cells can be used to produce hydrogen and/or oxygen. In various examples, a method for producing hydrogen and/or oxygen of the present disclosure is carried out using one or more of the electrochemical cell(s). The electrochemical cells can comprise aqueous and non-aqueous electrolytes. For example, the electrolyte is an ionic liquid.
An electrochemical cell and/or system where the mediator is separated from the counter electrode side of the cell is desirable in that the mediator cannot interfere with the chemistries that are occurring at the counter electrode. The membrane (e.g., semipermeable membrane) prevents movement of the mediator, for example, from the working electrode side of the electrochemical cell (e.g., the working electrode electrolyte space) to the counter electrode side of the electrochemical cell (e.g., the counter electrode electrolyte space). The membrane permits movement of other ions, such as, for example, protons, from, for example, moving from the working electrode electrolyte space to the counter electrode electrolyte space, and vice versa. For example, the membrane is a cationic permeable membrane (e.g., a proton permeable membrane).
In an aspect, the present disclosure provides energy storage systems. The energy storage systems can be used to store energy in the form of hydrogen. In various examples, a charging subsystem based on a redox-mediated water-splitting device comprising one or more electrochemical cell(s) of the present disclosure stores energy in the form of hydrogen (H 2 ) and a discharging subsystem based on, for example, a polymer-electrolyte fuel cell (PEFC) that uses H 2 to generate electricity. In various examples, the present disclosure provides energy storage systems to enable the flexible use of water electrolyzers for power grid operations. The systems use electrochemical energy conversion technology based on a redox mediated hydrogen generation and energy storage (ES) device.
BRIEF DESCRIPTION OF THE FIGURES
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying figures.
FIG. 1 shows a synergistic hydrogen redox energy storage (SHRXES) system design. The charging systems stores electricity in the form H 2 , and involves a redox cycle, where redox species are reduced to RX r in stack I and oxidized to RX o in stack II. The discharging subsystem uses H 2 to generate electricity in a PEM fuel cell.
FIG. 2 is a graph of estimated LCOS for various RX ions.
FIG. 3 are graphs showing (a) performance of stack II operated with Ce 3+ ions at room temperature (baseline) and 50° C. The results show the current (CE), voltage (VE) and energy conversion efficiencies (EE) as a function of current density. (b) Preliminary hardware-in-the-loop operation of 5 cm 2 stacks I and II with Ce 3+ /Ce 4+ ions, show the ability of the SHRXES charging subsystem to follow optimized power-grid operation instructions from algorithms. Stack I was operated at a potential resulting in a positive electricity output, thus allowing the system to discharge. The time scale was normalized by the maximum time of the simulation (24 h) and the experiments (24 min). The experiments can be extended to longer times (e.g. >100 h).
FIG. 4 shows an example of a system of the present disclosure.
FIG. 5 shows an example of an electrochemical cell of the present disclosure.
FIG. 6 shows chronoamperometry at 1, 1.6, 1.7, 1.8, and 1.9 V using a cell of FIG. 5 . Platinized titanium mesh as cathode and anode. Anolyte: 80 mM Ce(III) carbonate in 2M MSA. Catholyte: 2M MSA. Room temperature.
FIG. 7 shows steady state current density, power density, and Faradaic efficiencies at different voltages using a cell of FIG. 5 . Platinized titanium mesh as cathode and anode. Anolyte: 80 mM Ce(III) carbonate in 2M MSA. Catholyte: 2M MSA. Room temperature.
FIG. 8 shows chronoamperometry at 1, 1.6, 1.7, 1.8, and 1.9 V using a cell of FIG. 5 . Platinum mesh as cathode and anode. Catholyte: 80 mM Ce(IV) sulfate in 4M MSA. Anolyte: 4M MSA. Room temperature.
FIG. 9 shows steady state current density and power density at different voltages using a cell of FIG. 5 . Platinum mesh as cathode and anode. Catholyte: 80 mM Ce(IV) sulfate in 4M MSA using a cell of FIG. 5 . Anolyte: 4M MSA. Room temperature.
FIG. 10 shows a diagram of an example of a system design. On the left is the discharging cell and the right is the charging cell.
FIG. 11 shows (A) Faradaic efficiency with charging current density for various concentrations. (B) Polarization curves for the charging cell for various concentrations. The graph shows partial current densities for Ce(III) oxidation. (C) Polarization and power curve for the Discharging cell for various concentrations. Charging cell and discharging cell run at 5 mL/min and room temperature.
FIG. 12 shows (A) Faradaic efficiency with charging current density for various flow rates. (B) Polarization curves for the charging cell for various flow rates in terms of the partial current density for Ce(III) oxidation. (C) Polarization and power curve for the discharging cell for various flow rates. Charging cell run with 0.6M Ce(III) at room temperature. Discharging cell run with 0.35M Ce(IV) at room temperature.
FIG. 13 shows (A) Faradaic efficiency with charging current density for various temperatures. (B) Polarization curves for the charging cell for various flow rates in terms of the partial current density for Ce(III) oxidation. (C) Polarization and power curve for the Discharging cell for various temperatures. Charging cell run with 0.6M Ce(III) at 5 mL/min. Discharging cell run with 0.35M Ce(IV) at 5 mL/min.
FIG. 14 shows an example of the hourly price of electricity over the first 30 days of the year for the Low VRE (top) and the High Solar (bottom) renewable energy scenarios.
FIG. 15 shows charging and discharging behavior of the system for a High Solar scenario for an arbitrary 140 hour prior of operation.
FIG. 16 shows an example of optimization of three parameters, charging current density (j Ch ), charging area to discharging area ratio (A Ch/Dis ), and volume to discharging area ratio (V Ce ). (A) 3-D optimization space with optimal point in white. (B) 2-D optimization plane for j Ch and A Ch/Dis . (C) 2-D optimization plane for j Ch and V Ce . (D) 2-D optimization plane for A Ch/Dis and V Ce . Empty parts of graph do not fit within the constraint that the concentrations of Ce(III) and Ce(IV) must be positive.
FIG. 17 shows boxplots of the five parameters used in the model. The line represents the median cost value. The edges of the box correspond to the values between the 25 th and 75 th percentile. The whiskers represent the values contained in 99.3% of the data. The outliers are not shown.
FIG. 18 shows an example of a comparison between the cost of electrolysis and that of a system of the present disclosure. Improved Efficiency means that the overpotential for both cells was reduced by half. Improved j Ch/Dis means the maximum current densities for each cell was increased by 67%.
FIG. 19 an example of a total charging cell polarization curve for the variation of Ce(III) concentration.
FIG. 20 shows an example of a total charging cell polarization curve for the variation of flow rate.
FIG. 21 shows an example of a total charging cell polarization curve for the variation of temperature.
FIG. 22 shows a diagram of an example of a system design during stationary and transportable operati
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Application No. 62/809,429, filed on Feb. 22, 2019, the disclosure of which is hereby incorporated by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
This invention was made with government support under contract no. 1760540 awarded by the National Science Foundation. The government has certain rights in the invention.
BACKGROUND OF THE DISCLOSURE
The implementation of electrolysis systems for electrochemical hydrogen production has continued to grow as the paradigm shift towards renewable energy and fuels progresses. However, issues regarding conventional polymer electrolyte membrane (PEM) electrolysis systems remain; the performance of PEM electrolyzers degrade if operated with intermittent energy sources, while the high cost of electricity continues to hinder large scale adoption of the technology. In order to make electrochemical hydrogen production more feasible and compete with methane reforming on a large scale, renewable energy sources need to be used along with new strategies electrochemical hydrogen production. Decoupled electrolysis systems have been studied to temporally and spatially separate the evolution of hydrogen and oxygen, making the electrolysis system safer, but these systems tend to reduce efficiency and increase electricity costs.
Water electrolyzers, i.e. devices used for hydrogen (H 2 ) production, can be operated in a schedulable fashion and thus provide flexibility to power grids (e.g., additional energy, power, and ancillary services) on sec-to-sec to seasonal timescales. To facilitate the cost-efficiency of these systems, electrolysis can be scheduled for periods with low electricity costs. Unlike other schedulable industry-scale processes (e.g., steel manufacturing), hydrogen itself can be used to store electricity and perform spatio-temporal energy arbitrage and provide ancillary services just like other energy storage (ES) technologies (e.g., batteries or pumped hydro).
The low levelized cost of storage (LCOS) for long-duration energy storage (LDES) applications strongly depends on the cost of the energy storage media and the efficiency of the system. While multiple electrochemical energy conversion devices exist, they exhibit trade-off between costs and performance that do not satisfy LDES requirements. Systems such as Li-ion batteries or Vanadium redox flow-batteries are optimized to tackle shorter timeframes of energy storage where the cost of the energy storage media is not as significant and there are more stringent requirements with respect to round-trip efficiency and cost of power components. On the other hand, systems such as regenerative fuel cells rely on a fairly inexpensive storage media (i.e. H 2 ) but suffer from low round-trip efficiency.
Increased efforts to curb global warming have led to a drastic surge in the deployment of renewable electricity sources, such as wind and solar power. However, as these sources form a larger fraction of the energy on the grid, their intermittency causes supply instability which can lead to large fluctuations in energy prices.
The implementation of electrolysis systems for electrochemical hydrogen production has continued to grow as the paradigm shift towards renewable energy and fuels progresses. However, issues regarding conventional polymer electrolyte membrane (PEM) electrolysis systems remain; the performance of PEM electrolyzers degrade if operated with intermittent energy sources, while the high cost of electricity continues to hinder large-scale adoption of the technology.
SUMMARY OF THE DISCLOSURE
The present disclosure provides methods for producing hydrogen and/or oxygen. The present disclosure also provides electrochemical cells and energy storage systems.
In an aspect, the present disclosure provides methods for producing hydrogen and/or oxygen. The methods may be based on use of a mediator (e.g., a redox intermediate) having a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Various mediators can be used. A mediator can be oxidized to provide an oxidized mediator. An oxidized mediator can be reduced to provide a reduced mediator. A mediator that has (at least) two different oxidation states, which oxidation states may be accessed by oxidation or reduction from one state to the other. A mediator can comprise various anions. It is desirable that the anion be soluble in aqueous and non-aqueous electrolytes.
The present disclosure provides the advantage that the hydrogen and oxygen generation steps may be separated in time and/or space. Thus, for example, hydrogen is generated with concomitant oxidation of a mediator (e.g., redox intermediate). Later, the same oxidized mediator (e.g., redox intermediate) is used in a method to generate oxygen. Either the oxygen generation step or the hydrogen generation step may be performed first, if the reactions are to be run separately.
The present disclosure provides methods for the generation of hydrogen from protons using a mediator, which is oxidized in the process. The methods comprise reducing protons to generate hydrogen and oxidizing a mediator (which may be a reduced mediator) to generate an oxidized mediator. In various examples, a method for the generation of hydrogen comprises oxidizing a mediator at a working electrode to yield an oxidized mediator and reducing protons at a counter electrode to yield hydrogen. The hydrogen generated may be collected, and, optionally, pressurized, for storage and/or future use.
The present disclosure provides methods for the electrochemical generation of oxygen. The methods comprise oxidizing water to generate oxygen and reducing a mediator (which may be an oxidized mediator) to generate a reduced mediator. The yield of oxygen, with reference to the amount of electrons passed through the system, is desirable. The methods may comprise providing and maintaining a potential across the working electrode and the counter electrode and/or the reference electrode, where present. After the reaction is complete, the reduced mediator may be recovered. The reduced mediator may be oxidized to obtain (oxidized) mediator, which can be used in further oxygen production methods. The reduced mediator may be oxidized using electrochemical techniques. In an example, the mediator is oxidized in a hydrogen production method. Thus, the recycling of the mediator may be linked to the production of a useful product such as, for example, hydrogen. The oxygen generated may be collected, and optionally pressurized, for storage and/or future use.
The present disclosure provides independent methods for the generation of hydrogen and the generation of oxygen. Each of these methods may be used together to produce both hydrogen and oxygen. In various examples, the methods of generating hydrogen and methods of generating oxygen are used to provide a method for the generation of hydrogen and oxygen.
In an aspect, the present disclosure provides electrochemical cells. The electrochemical cells can be used to produce hydrogen and/or oxygen. In various examples, a method for producing hydrogen and/or oxygen of the present disclosure is carried out using one or more of the electrochemical cell(s). The electrochemical cells can comprise aqueous and non-aqueous electrolytes. For example, the electrolyte is an ionic liquid.
An electrochemical cell and/or system where the mediator is separated from the counter electrode side of the cell is desirable in that the mediator cannot interfere with the chemistries that are occurring at the counter electrode. The membrane (e.g., semipermeable membrane) prevents movement of the mediator, for example, from the working electrode side of the electrochemical cell (e.g., the working electrode electrolyte space) to the counter electrode side of the electrochemical cell (e.g., the counter electrode electrolyte space). The membrane permits movement of other ions, such as, for example, protons, from, for example, moving from the working electrode electrolyte space to the counter electrode electrolyte space, and vice versa. For example, the membrane is a cationic permeable membrane (e.g., a proton permeable membrane).
In an aspect, the present disclosure provides energy storage systems. The energy storage systems can be used to store energy in the form of hydrogen. In various examples, a charging subsystem based on a redox-mediated water-splitting device comprising one or more electrochemical cell(s) of the present disclosure stores energy in the form of hydrogen (H 2 ) and a discharging subsystem based on, for example, a polymer-electrolyte fuel cell (PEFC) that uses H 2 to generate electricity. In various examples, the present disclosure provides energy storage systems to enable the flexible use of water electrolyzers for power grid operations. The systems use electrochemical energy conversion technology based on a redox mediated hydrogen generation and energy storage (ES) device.
BRIEF DESCRIPTION OF THE FIGURES
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying figures.
FIG. 1 shows a synergistic hydrogen redox energy storage (SHRXES) system design. The charging systems stores electricity in the form H 2 , and involves a redox cycle, where redox species are reduced to RX r in stack I and oxidized to RX o in stack II. The discharging subsystem uses H 2 to generate electricity in a PEM fuel cell.
FIG. 2 is a graph of estimated LCOS for various RX ions.
FIG. 3 are graphs showing (a) performance of stack II operated with Ce 3+ ions at room temperature (baseline) and 50° C. The results show the current (CE), voltage (VE) and energy conversion efficiencies (EE) as a function of current density. (b) Preliminary hardware-in-the-loop operation of 5 cm 2 stacks I and II with Ce 3+ /Ce 4+ ions, show the ability of the SHRXES charging subsystem to follow optimized power-grid operation instructions from algorithms. Stack I was operated at a potential resulting in a positive electricity output, thus allowing the system to discharge. The time scale was normalized by the maximum time of the simulation (24 h) and the experiments (24 min). The experiments can be extended to longer times (e.g. >100 h).
FIG. 4 shows an example of a system of the present disclosure.
FIG. 5 shows an example of an electrochemical cell of the present disclosure.
FIG. 6 shows chronoamperometry at 1, 1.6, 1.7, 1.8, and 1.9 V using a cell of FIG. 5 . Platinized titanium mesh as cathode and anode. Anolyte: 80 mM Ce(III) carbonate in 2M MSA. Catholyte: 2M MSA. Room temperature.
FIG. 7 shows steady state current density, power density, and Faradaic efficiencies at different voltages using a cell of FIG. 5 . Platinized titanium mesh as cathode and anode. Anolyte: 80 mM Ce(III) carbonate in 2M MSA. Catholyte: 2M MSA. Room temperature.
FIG. 8 shows chronoamperometry at 1, 1.6, 1.7, 1.8, and 1.9 V using a cell of FIG. 5 . Platinum mesh as cathode and anode. Catholyte: 80 mM Ce(IV) sulfate in 4M MSA. Anolyte: 4M MSA. Room temperature.
FIG. 9 shows steady state current density and power density at different voltages using a cell of FIG. 5 . Platinum mesh as cathode and anode. Catholyte: 80 mM Ce(IV) sulfate in 4M MSA using a cell of FIG. 5 . Anolyte: 4M MSA. Room temperature.
FIG. 10 shows a diagram of an example of a system design. On the left is the discharging cell and the right is the charging cell.
FIG. 11 shows (A) Faradaic efficiency with charging current density for various concentrations. (B) Polarization curves for the charging cell for various concentrations. The graph shows partial current densities for Ce(III) oxidation. (C) Polarization and power curve for the Discharging cell for various concentrations. Charging cell and discharging cell run at 5 mL/min and room temperature.
FIG. 12 shows (A) Faradaic efficiency with charging current density for various flow rates. (B) Polarization curves for the charging cell for various flow rates in terms of the partial current density for Ce(III) oxidation. (C) Polarization and power curve for the discharging cell for various flow rates. Charging cell run with 0.6M Ce(III) at room temperature. Discharging cell run with 0.35M Ce(IV) at room temperature.
FIG. 13 shows (A) Faradaic efficiency with charging current density for various temperatures. (B) Polarization curves for the charging cell for various flow rates in terms of the partial current density for Ce(III) oxidation. (C) Polarization and power curve for the Discharging cell for various temperatures. Charging cell run with 0.6M Ce(III) at 5 mL/min. Discharging cell run with 0.35M Ce(IV) at 5 mL/min.
FIG. 14 shows an example of the hourly price of electricity over the first 30 days of the year for the Low VRE (top) and the High Solar (bottom) renewable energy scenarios.
FIG. 15 shows charging and discharging behavior of the system for a High Solar scenario for an arbitrary 140 hour prior of operation.
FIG. 16 shows an example of optimization of three parameters, charging current density (j Ch ), charging area to discharging area ratio (A Ch/Dis ), and volume to discharging area ratio (V Ce ). (A) 3-D optimization space with optimal point in white. (B) 2-D optimization plane for j Ch and A Ch/Dis . (C) 2-D optimization plane for j Ch and V Ce . (D) 2-D optimization plane for A Ch/Dis and V Ce . Empty parts of graph do not fit within the constraint that the concentrations of Ce(III) and Ce(IV) must be positive.
FIG. 17 shows boxplots of the five parameters used in the model. The line represents the median cost value. The edges of the box correspond to the values between the 25 th and 75 th percentile. The whiskers represent the values contained in 99.3% of the data. The outliers are not shown.
FIG. 18 shows an example of a comparison between the cost of electrolysis and that of a system of the present disclosure. Improved Efficiency means that the overpotential for both cells was reduced by half. Improved j Ch/Dis means the maximum current densities for each cell was increased by 67%.
FIG. 19 an example of a total charging cell polarization curve for the variation of Ce(III) concentration.
FIG. 20 shows an example of a total charging cell polarization curve for the variation of flow rate.
FIG. 21 shows an example of a total charging cell polarization curve for the variation of temperature.
FIG. 22 shows a diagram of an example of a system design during stationary and transportable operations.
DETAILED DESCRIPTION OF THE DISCLOSURE
Although claimed subject matter will be described in terms of certain examples, other examples, including examples that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure.
Ranges of values are disclosed herein. The ranges set out a lower limit value and an upper limit value. Unless otherwise stated, the ranges include all values to the magnitude of the smallest value (either lower limit value or upper limit value) and ranges between the values of the stated range.
As used herein, unless otherwise stated, âsecâ refers to second(s), âminâ refers to minute(s), and âhâ refers to hour(s).
The articles âaâ and âanâ are used in this disclosure to refer to one or more than one (i.e., to at least one) of the grammatical object of the article. By way of example, âan elementâ means one element or more than one element.
The present disclosure provides methods for producing hydrogen and/or oxygen. The present disclosure also provides electrochemical cells and energy storage systems.
In an aspect, the present disclosure provides methods for producing hydrogen and/or oxygen. The methods may be based on use of a mediator (e.g., redox intermediate) having a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Non-limiting examples of methods for producing hydrogen and oxygen are provided herein.
The present disclosure provides an advantage that the hydrogen and oxygen generation steps may be separated in time and/or space. Thus, for example, hydrogen is generated with concomitant oxidation of a mediator (e.g., redox intermediate). Later, the same oxidized mediator (e.g., redox intermediate) is used in a method to generate oxygen. Either the oxygen generation step or the hydrogen generation step may be performed first, if the reactions are to be run separately.
Various mediators can be used. A mediator can be oxidized to provide an oxidized mediator. An oxidized mediator can be reduced to provide a reduced mediator. A mediator has two or more (at least two) different oxidation states, which oxidation states may be accessed by oxidation or reduction from one state to the other. It is desirable that a mediator is thermally and oxidatively stable in both the oxidized form and the reduced form. It is also desirable that a mediator has minimal cross reactivity with other components within an electrochemical cell (e.g., the electrodes, other components of the electrolyte, and the like). The mediators comprise one or more metal ion(s). Non-limiting examples of suitable metal ions include Ce(III), Nd(III), Pr(IV), Pr(III), Pr(II), Co(III), Co(II), and the like, and combinations thereof.
A mediator can comprise various anions. It is desirable that the anion be soluble in aqueous and/or non-aqueous electrolytes. For example, it is desirable that the anion be soluble in an ionic liquid. As an illustrative example, a mediator comprises (e.g., is) a Ce(III) metal ion and a methane sulfonate anion.
The mediator(s) may be used at various concentrations. For example, mediator(s) are present at a concentration of 0.5 to 2.0 M, including all 0.1 M values and ranges there between. In another example, mediator(s) are present at a concentration (independently or collectively) of 0.1 M to 20 M (e.g., 0.1 M to 10 M), including all 0.1 values and ranges therebetween.
The present disclosure provides methods for the generation of hydrogen from protons using a mediator, which is oxidized in the process. The methods comprise reducing protons to generate hydrogen and oxidizing a mediator (which may be a reduced mediator) to generate an oxidized mediator. In various examples, a method for the generation of hydrogen comprises oxidizing a mediator at a working electrode to yield an oxidized mediator and reducing protons at a counter electrode to yield hydrogen. The yield of hydrogen, with reference to the amount of electrons passed through the system, may be desirable. The Faradaic efficiency of hydrogen may be at least 90%, at least 95%, or 100%. In an example, the mediator (in either reduced or oxidized form) is prevented from contacting the counter electrode. A semi-permeable membrane, such as, for example, described herein, may be provided for this purpose.
The methods may comprise providing and maintaining a potential across the working electrode and the counter electrode and/or the reference electrode, where present. In various examples, the potential applied between the working and counter electrodes is at most 2.0 V, is at most 1.5 V, is at most 1.3 V, is at most 1.2 V, or is at most 1.1 V. For the avoidance of doubt, in the hydrogen generation step, the working electrode is an anode and the counter electrode is the cathode.
In various examples, the electrolyte used in the electrochemical reaction (which may be an aqueous electrolyte) has a pH that is at most 6, at most 5, at most 4, at most 3, or at most 2. In various other examples, the electrolyte has a pH that is at least 0.1, at least 0.2, or at least 0.3. In various examples, the electrolyte has a pH that is in a range having upper and lower values selected from the values above.
The pH of the electrolyte solution may be maintained at a substantially constant level during the electrochemical reaction. Thus, in an example, the electrolyte is buffered. The mediator itself may fulfill this function, for example where the mediator is capable of donating and accepting protons. In various examples, the change in pH of the electrolyte during the hydrogen generation may be less than 1 unit, less than 0.5 units, less than 0.3 units, less than 0.2 units, or less than 0.1 units of pH.
After the reaction is complete, the oxidized mediator may be recovered. The oxidized mediator may be reduced to obtain (reduced) mediator, which can be used in further hydrogen production methods. The oxidized mediator may be reduced using electrochemical techniques. In an example, the mediator is reduced in an oxygen production method. Thus, the recycling of the mediator may be linked to the production of a useful product such as, for example, oxygen.
The hydrogen generated may be collected, and, optionally, pressurized, for storage and/or future use. Suitable containers (e.g., suitable containers for hydrogen collection and/or pressurization) are well known in the art. Hydrogen presence (e.g., content or concentration) and yields may be determined using standard analytical techniques.
The present disclosure provides methods for the electrochemical generation of oxygen. The methods comprise oxidizing water to generate oxygen and reducing a mediator (which may be an oxidized mediator) to generate a reduced mediator. The yield of oxygen, with reference to the amount of electrons passed through the system, is desirable. The Faradaic efficiency of oxygen may be at least 90%, at least 95% or substantially 100%. In an example, the mediator (in either reduced or oxidized form) is prevented from contacting the counter electrode. A semi-permeable membrane, such as, for example, described herein, may be provided for this purpose.
The methods may comprise providing and maintaining a potential across the working electrode and the counter electrode and/or the reference electrode, where present. For example, the potential applied between the working and counter electrodes is at most â2.0 V, is at most â1.5 V, is at most â1.3 V, is at most â1.2 V, or is at most â1.1 V. For the avoidance of doubt, in the oxygen generation step, the working electrode is the cathode and the counter electrode is the anode. Thus, by convention, the voltages are expressed in negative terms.
In various examples, the electrolyte used in the electrochemical reaction has a pH that is at most 6, at most 5, at most 4, at most 3, or at most 2. In various examples, the electrolyte has a pH that is at least 0.1, at least 0.2, or at least 0.3. In various examples, the electrolyte has a pH that is in a range having upper and lower values selected from the values above.
The pH of the electrolyte solution may be maintained at a substantially constant level during the electrochemical reaction. Thus, in an example, the electrolyte is buffered. The mediator itself may fulfil this function, for example where the mediator is capable of donating and accepting protons. In a various examples, the change in pH of the electrolyte during the hydrogen generation may be less than 1 unit, less than 0.5 units, less than 0.3 units, less than 0.2 units or less than 0.1 units of pH.
After the reaction is complete, the reduced mediator may be recovered. The reduced mediator may be oxidized to obtain (oxidized) mediator, which can be used in further oxygen production methods. The reduced mediator may be oxidized using electrochemical techniques. In an example, the mediator is oxidized in a hydrogen production method. Thus, the recycling of the mediator may be linked to the production of a useful product such as, for example, hydrogen.
The oxygen generated may be collected, and optionally pressurized, for storage and/or future use. Suitable containers (e.g., suitable containers for oxygen collection and/or pressurization) are well known in the art. Oxygen presence (e.g., content or concentration) and yields may be determined using standard analytical techniques, including, for example, gas chromatography, or the like.
The present disclosure provides independent methods for the generation of hydrogen and the generation of oxygen. Each of these methods may be used together to produce both hydrogen and oxygen. In various examples, the methods of generating hydrogen and methods of generating oxygen are used to provide a method for the generation of hydrogen and oxygen. The hydrogen production step may not be performed simultaneously as the oxygen production step. Thus, the hydrogen and oxygen steps may be referred to as decoupled. Thus, two smaller energy inputs are used to split water to give hydrogen and oxygen at different times, as opposed to a single energy input that produces hydrogen and oxygen simultaneously.
In various exemplary methods, a method of hydrogen generation according to the present disclosure is used in combination with a known oxygen generation method. In other exemplary methods, a method of oxygen generation of the disclosure is used in combination with a known hydrogen generation method. In these exemplary methods, the mediator is not necessarily recycled during the oxygen and/or hydrogen production process. Instead, the mediator may need to be recycled separately, if it is to be reused in a hydrogen or oxygen production step.
An advantage of running the hydrogen and oxygen generation steps independently (in time and/or space) is that the product gases may be collected independently, without any requirement for a separation step. For the avoidance of doubt, it is noted that the oxygen producing may be performed before or after the hydrogen evolving step. The initial step may be selected based on the availability of the mediator in a particular oxidation step, which may favor a reduction or oxidation reaction first.
The oxygen generation may be performed simultaneously with the hydrogen generation and, optionally, at a different current density. The oxygen generation may be performed non-simultaneously relative to the hydrogen generation and, optionally, at a different current density.
In an example, the hydrogen and oxygen producing are performed non-simultaneously (i.e. separately in time). Such a method follows inevitably from the use of the mediator as an electron and, optionally, proton donor. Only once the mediator is converted to its oxidized form it may be used as an electron and proton acceptor. Likewise, only once the mediator is converted to its reduced form may it be used as an electron and, optionally, proton donor. The mediator which is oxidized or reduced in one generation step may be used in the other generation step, where it is reduced or oxidized accordingly, to yield the original mediator species. Thus, the mediator is recycled rather than consumed in the overall process.
The hydrogen and oxygen generation may be performed using the same electrochemical cell. Thus, once an oxygen generating step is complete and the mediator is suitably reduced, the bias across the working and counter electrodes may be reversed, thereby producing hydrogen with concomitant oxidation of the mediator (which is now converted to its original form prior to the oxygen generating step). After this sequence is complete, the oxygen and the hydrogen evolving steps may be repeated. Recycling of components in this way is particularly suitable for the production of significant amounts of hydrogen and oxygen, and without complex adaptations to the electrochemical cell or system.
The oxygen and hydrogen generated may be collected, and, optionally, pressurized, for storage and/or future use. Suitable containers (e.g., suitable containers for hydrogen and/or oxygen collection and/or pressurization) are well known in the art.
In the methods for the generation of hydrogen and/or oxygen, the reactions may be based on the presence of an aqueous composition, which may be referred to as an electrolyte or a composition. This composition comprises one or more mediator(s). The methods may be conducted at ambient temperature (e.g., 18-25° C., including all 0.1° C. values and ranges therebetween) and/or at ambient pressure (about 1 atm).
It will be apparent to one of skill in the art that the methods of the disclosure may be conducted at higher temperatures or lower temperatures. Changes in temperature may be associated in higher electrochemical efficiencies and reaction yields. In an example, the methods are conducted at a temperature in the range 5-60° C., including all 0.1° C. values and ranges therebetween. In an example, the methods are conducted at a temperature in the range 10-40° C. In an example, the methods are conducted at a temperature in the range 10-35° C. Likewise changes in pressure may also be associated in higher yields, and may be useful to promote the evolution of hydrogen and/or oxygen from the composition. The use of higher pressures (e.g., pressures greater than ambient pressure (such as, for example, an ambient pressure of 1 atmosphere, which may depend on the altitude at which the method is carried out) that may be an exogenously created pressure) may also be advantageous as the gas produced would later need to be pressurized for storage purposes. Higher pressures are also associated with the formation of smaller gas bubbles, for example, smaller bubbles of oxygen, within the composition (electrolyte or mixture), which is associated with greater reaction efficiencies.
Prior to the initiation of a hydrogen and/or oxygen generating method, the composition (the mixture or the electrolyte) and/or container may be purged or evacuated in order to minimize or remove air within the system. The air may be replaced with an inert atmosphere such as, for example, argon, helium, nitrogen, or a combination thereof.
In an aspect, the present disclosure provides electrochemical cells. The electrochemical cells can be used to produce hydrogen and/or oxygen. In various examples, a method for producing hydrogen and/or oxygen of the present disclosure is carried out using one or more of the electrochemical cell(s). Non-limiting examples of electrochemical cells are provided herein.
The electrochemical cells can comprise one or more mediator(s) described herein. The mediator(s) may be used at various concentrations within an electrochemical cell. For example, mediator(s) are present at a concentration of 0.5 to 2.0 M, including all 0.1 M values and ranges therebetween.
The electrochemical cells can comprise aqueous, non-aqueous electrolytes, or a combination thereof. For example, the electrolyte comprises or is an ionic liquid.
Electrodes for use in generation of hydrogen and/or oxygen are well described in the art. Non-limiting examples of electrodes (e.g., working electrodes, counter electrodes, references electrodes, and the like) include those formed from, comprising, or consisting of platinum, platinum oxide, palladium, iridium, iridium oxide, indium-tin oxide and/or carbon, tungsten trioxide, and the like, and combinations thereof (which may be alloys, composites, or the like). Other electrodes are also suitable for use, although it is desirable that such electrodes should be resistant to strong acid. The choice of electrode may be dependent on the nature of the hydrogen or oxygen generation method. The various electrodes may be the same or different (e.g., in terms of composition). The electrodes can have various shapes.
In an example, working and counter electrodes define an electrochemical space in which an electrolyte is provided. The electrochemical space is divided by a semi-permeable membrane to provide a working electrode electrolyte space and a counter electrode electrolyte space. The mediator is provided in the working electrode electrolyte space. No mediator is provided in the counter electrode space. The semi-permeable membrane prevents movement of the mediator (in either the oxidized or reduced form) from moving from the working electrode electrolyte space to the counter electrode electrolyte space, thus the mediator is prevented from contacting the counter electrode surface. An electrochemical cell and/or system where the mediator is separated from the counter electrode side of the cell is desirable in that the mediator cannot interfere with the chemistries that are occurring at the counter electrode. The working and counter electrodes are electrically connected or connectable.
The membrane (e.g., semipermeable membrane) prevents movement of the mediator, for example, from the working electrode side of the electrochemical cell (e.g., the working electrode electrolyte space) to the counter electrode side of the electrochemical cell (e.g., the counter electrode electrolyte space). The membrane permits movement of other ions, such as, for example, protons, from, for example, moving from the working electrode electrolyte space to the counter electrode electrolyte space, and vice versa. For example, the membrane is a cationic permeable membrane (e.g., a proton permeable membrane).
The membrane is not particularly limited. It is desirable that the membrane is capable of preventing movement of the mediator therethrough, while permitting movement of cations, particularly protons, therethrough.
Non-limiting examples of membranes include membranes comprising or consisting of one or more sulfonated tetrafluoroethylene based fluoropolymer-copolymer(s). Nafion® membranes are non-limiting examples of commercially available membranes of this type. Other non-limiting examples of membranes include poly(styrene) sulfonic acid membranes.
In an example, the electrochemical cell further comprises a voltage supply (or power supply). The voltage supply is preferably adapted to supply a constant bias between the working electrode and the counter electrode or the reference electrode, where present. The voltage supply is adapted to supply a constant bias of, for example, up to 2.0 V. In an example, the voltage supply is adapted to supply a constant bias of about 1.0 V. The voltage supply may be reversible as required. The electrochemical cell may further comprise a detector for monitoring current. The electrochemical cell may further comprise a controller for controlling the voltage supply and timing of that supply.
In an aspect, the present disclosure provides energy storage systems. The energy storage systems can be used to store energy in the form of hydrogen. In various examples, a charging subsystem is based on a redox-mediated water-splitting device comprising one or more electrochemical cell(s) of the present disclosure or the like and stores energy in the form of hydrogen (H 2 ) and a discharging subsystem based on, for example, a polymer-electrolyte fuel cell (PEFC) that uses H 2 to generate electricity or the like. Non-limiting examples of energy storage systems are provided herein (for example in FIG. 4 ).
Relative to non-electrochemical energy storage units, an advantage of the present technology is that it does not have exogenous (climatic, geographical, policy, etc.) restrictions on its placement, unlike pumped storage hydropower (PSH) units. Furthermore, the instant technology has an advantage of relatively low operating cost due to a relatively high round-trip efficiency and the flexibility to widely adjust the output electrical power (note that PSH units are often restricted to maintain a fixed power output due to hydrological constraints on the penstock).
In various examples, the present disclosure provides energy storage systems to enable the flexible use of water electrolyzers for power grid operations. The systems use electrochemical energy conversion technology based on a redox mediated hydrogen generation and energy storage (ES) device. This technology includes two components. The first one is an ES component in the form of cyclable, energy-dense redox species (e.g., redox mediators as described herein such as, for example, Cerium(III)/Cerium(IV) ions, and the like) and the second one is water electrolysis.
The systems of the present disclosure may advert two significant challenges for the application of this technology in grid operations: (1) the round-trip efficiency of the ES component must be significantly improved and (2) the gas crossover of H 2 and O 2 must be suppressed to avoid the generation of an explosive gas mixture.
Application of an example of this technology is illustrated in FIG. 22 for two possible operational modes. First, this technology can be operated as a stationary resource at a H 2 -fuel generation facility. In this case, the technology provides grid support services at a single location and can discharge energy stored in the cyclable redox species into the power grid or charge energy from the power grid to regenerate these species. During the stationary operation, the benefits of the technology for the power grid operations are similar to those of other ES technologies. On the other hand, the technology does not cause capacity degradation that is typical for many battery ES technologies. Additionally, the technology will generate added value by simultaneously serving as a grid-scale ES unit and a H 2 -fuel production device. Second, this technology can be operated as a transportable resource and provide power grid support services at multiple locations as the power grid needs change on an hour-to-hour, day-to-day, and weeks-to-seasons basis. Since the technology uses H 2 energy carriers that can be transported via public transportation routes, it can be deployed at variable power grid locations, where other ES technologies are either technologically infeasible or their stationary installation is not economically justified. Another important distinction of the technology relative to alternatives is that its power and energy ratings depend on the amount of energy carriers stored and the speed of the chemical reaction. Therefore, the ES ratings can be changed based on the evolving needs of power grid operators. Furthermore, the transportability of the instant technology can be used for power grid maintenance and mitigation of contingencies. As a transportable resource, the instant technology will compete with portable diesel generators, which are costly to operate and cause noise/air pollution, and customer-owned electric vehicles providing vehicle-to-grid services, which are not directly controlled by power grid operators.
The technology can overcome one or both significant technological limitations of existing electrolyzers that prevent their use in power grid operations: (1) gas crossover will be suppressed by the physical decoupling of the O 2 and H 2 generation processes in two distinct cells, thus avoiding the production of explosive gas mixtures during idling periods, and (2) the round-trip energy efficiency of the ES component will be enhanced by the introduction of a redox mediator cycle. This cycle will be used to store energy in the form of energy-dense ions with a round-trip efficiency of, for example, greater than 80%. This technology can provide the required flexibility for power grids, to enable the transportation of energy carriers (H 2 ) to multiple power grid locations and, ultimately, to produce H 2 -fuel at a significantly lower cost. Unlike existing water electrolyzers and flow batteries, the technology of the present disclosure can serve as both ES and H 2 production units, leading to economic gains from the synergy of two functions. Advantages over regular electrolyzers may be one or more or all of the following. First, the gas crossover is reduced by physically separating the H 2 and O 2 evolution chambers. Second, the H 2 and O 2 evolution reactions (HER and OER, respectively) can take place at different current densities. This will increase efficiencies since the least efficient reaction (OER) can be carried at lower rates than the HER. Third, given the small HER energy losses (i.e. low overpotentials), this reaction may be operated at a variable rate to accommodate fast power grid fluctuations and to capture economic gains from producing at periods with low electricity costs. Relative to flow batteries, the present technology creates an added value, as it also produces H 2 . Table 1 compares the present technology to a potential combination of a flow battery and an electrolyzer. The present technology is expected to benefit the US electric power and transportation sectors, which are at the core of the US economy. The technology will increase the utilization of renewable resources and reduce hazardous emissions (e.g., CO 2 , NO x , SO x , and the like, and combinations thereof). Coupling H 2 production with grid operations will increase the economic viability of domestic alternative transportation fuels.
TABLE 1
Comparative performance of the instant technology.
Instant
Parameter
Li-ion Battery
Electrolyzer
technology
Round-trip ES efficiency, %
80-90%
N/A
>80%
H 2 production efficiency, %
N/A
60-70%
>75%
Degradation
High
Low
Low
Capital Cost
The instant technology is expected to cost less than a
coupled battery + electrolyzer
The steps of the method described in the various examples disclosed herein are sufficient to carry out the methods of the present disclosure. Thus, in an example, a method consists essentially of a combination of the steps of the methods disclosed herein. In another example, a method consists of such steps.
The following Statements described various examples of methods, uses, electrochemical cells and systems of the present disclosure:
Statement 1. A method for producing hydrogen and oxygen comprising: (i) oxidizing a mediator (e.g., redox intermediate) at a working electrode to yield an oxidized mediator (e.g., redox intermediate) and reducing protons or water at a counter electrode to yield hydrogen; and (ii) reducing an oxidized mediator at a working electrode to yield a reduced mediator (e.g., redox intermediate), and oxidizing water at a counter electrode to yield oxygen, where the oxygen generation may be performed simultaneously or non-simultaneously relative to the hydrogen generation and at the same or different current density, and where the oxidized mediator of (i) may be used as the oxidized mediator of (ii), or the mediator of (ii) may be used as the mediator of (i), and where the mediator may have a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and/or the hydrogen evolution reaction (HER). The oxidizing and reducing can be carried out in any order.
Statement 2. A method according to Statement 1, where the mediator is a H + donor and/or acceptor.
Statement 3. A method according to Statements 1 or 2, where the mediator is chosen from ions such as, for example, Ce(III), Nd(III), Pr(IV), Pr(III), Pr(II), Co(III), Co(II), compounds comprising these ions, and the like, and combinations thereof.
Statement 4. A method according to any one of the preceding Statements, where the mediator has a reversible reduction potential lying in the range of +1.5 to +3.5 V vs SHE and/or a reversible oxidation potential lying in the range of +0.2 to +3.5 V vs SHE.
Statement 5. A method according to any one of the preceding Statements, where the mediator and the oxidized mediator are prevented from contacting the hydrogen and/or oxygen evolution electrode, respectively.
Statement 6. A method according to any one of the preceding Statements, where the mediator and/or the oxidized mediator are provided in an acidic, basic or buffered aqueous electrolyte.
Statement 7. A method according to any one of the preceding Statements, where the mediator and/or the oxidized mediator are provided in a non-aqueous electrolyte.
Statement 8. A method according to any one of the preceding Statements, where the mediator is provided in an electrolyte (e.g., an aqueous electrolyte), and the pH of the electrolyte remains substantially constant throughout (i) and/or (ii).
Statement 9. A method according to any one of the preceding Statements, where (ii) is performed first, followed by (i).
Statement 10. A method according to any one of the preceding Statements, further comprising collecting the produced hydrogen and/or oxygen.
Statement 11. A method according to any one of the preceding Statements, where (i) includes the recovery of the oxidized mediator and/or (ii) includes the recovery of the mediator.
Statement 12. An electrochemical cell comprising: a working electrode; a counter electrode; and optionally, a reference electrode, where the working electrode and the counter electrode may define an electrolyte space divided into a working electrode space and a counter electrode space by a permselective ion-conducting membrane, the cell further may further comprise an electrolyte (e.g., an aqueous electrolyte, a non-aqueous electrolyte, or a combination thereof) within the electrolyte space, where the aqueous electrolyte in the working electrode space of the electrolyte space contains a mediator, and the permselective ion-conducting membrane, if present, is at least or partly impermeable to the mediator and the mediator has a reversible redox wave lying outside the onset of the oxygen evolution reaction (OER) and/or the hydrogen evolution reaction (HER).
Statement 13. An electrochemical cell according to Statement 12, where the mediator is a H + donor and/or acceptor.
Statement 14. An electrochemical cell according to Statement 12 or 13, where the mediato
CLAIMS
Claims ( 8 )
The invention claimed is:
1. A method for producing hydrogen and oxygen comprising:
(i) oxidizing a mediator at a working electrode to yield an oxidized mediator and reducing protons or water at a counter electrode to yield hydrogen; and
(ii) reducing an oxidized mediator at a working electrode to yield the mediator, and oxidizing water at a counter electrode to yield oxygen,
wherein the oxygen generation is performed simultaneously or non-simultaneously relative to the hydrogen generation and at a different current density, and
wherein the oxidized mediator of (i) is used as the oxidized mediator of (ii), or the mediator of (ii) is used as the mediator of (i), and
wherein the mediator has a reversible redox potential lying outside the onset of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER).
2. The method of claim 1 , wherein the mediator is a H + donor and/or acceptor.
3. The method of claim 1 , wherein the mediator has a reversible reduction potential lying in the range of +1.5 to +3.5 V vs SHE and/or a reversible oxidation potential lying in the range of +0.2 to +3.5 V vs SHE.
4. The method claim 1 , wherein the mediator and the oxidized mediator are prevented from contacting the hydrogen or oxygen evolution electrode, respectively.
5. The method of claim 1 , wherein the mediator is provided in an electrolyte, and a change in the pH of the electrolyte is 1 unit or less throughout (i) and/or (ii).
6. The method of claim 1 , wherein (ii) is performed first, followed by (i).
7. The method of claim 1 , further comprising collecting the produced hydrogen and/or oxygen.
8. The method of claim 1 , wherein (i) includes the recovery of the oxidized mediator and/or (ii) includes the recovery of the mediator.
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