Systems and methods to make hydrogen gas with a steady state pH differential
Disclosed herein are methods and systems that relate to electrochemically producing hydrogen gas by maintaining a steady-state pH differential of greater than 1 between an anode electrolyte and a cathode electrolyte in a hydrogen-gas generating electrochemical cell.
1. A method to generate hydrogen gas, comprising:
providing an anode and an anode electrolyte comprising water in an electrochemical cell;
providing a cathode and a cathode electrolyte comprising water in the electrochemical cell and forming hydrogen gas and hydroxide ions at the cathode;
separating the anode electrolyte from the cathode electrolyte by a separator;
migrating the hydroxide ions from the cathode electrolyte to the anode electrolyte through the separator; and
maintaining a stready-state pH differential of greater than 1 between the anode electrolyte and the cathode electrolyte by thermally removing water from the anode electrolyte and transferring the removed water to the cathode electrolyte to increase a pH of the anode electrolyte and to decrease the pH of the cathode electrolyte so that the pH of the cathode electrolyte is lower than the pH of the anode electrolyte, wherein the pH of the anode electrolyte is about 11 or more and the pH of the cathode electrolyte is about 10 or more.
2. A method according to claim 1 , further comprising operating the electrochemical cell at a theoretical voltage of less than 1.23 V.
3. A method according to claim 1 , wherein the pH of the anode electrolyte is from about 12 to about 15 and the pH of the cathode electrolyte is from about 11 to about 13.
4. A method according to claim 1 , further comprising adding a salt comprising a polyatomic anion to the cathode electrolyte.
5. A method according to claim 4 , wherein a concentration of the salt is from about 0.1 M to about 3 M.
6. A method according to claim 4 , wherein the salt is stable in the cathode electrolyte and does not migrate across the separator to the anode electrolyte.
7. A method according to claim 1 , further comprising reducing water at the cathode to form the hydrogen gas and the hydroxide ions.
8. A method according to claim 1 , wherein thermally removing water from the anode electrolyte comprises heating at least a portion of the anode electrolyte to vaporize water from the portion of the anode electrolyte.
9. A method according to claim 1 , further comprising oxidizing the hydroxide ions at the anode to form oxygen gas.
10. A method according to claim 1 , wherein the anode electrolyte further comprises a redox mediator.
11. A method according to claim 1 , wherein the anode electrolyte further comprises an alkali metal halide or an alkali earth metal halide.
12. A method according to claim 1 , wherein the anode electrolyte further comprises a redox organic compound.
13. A method according to claim 1 , wherein the steady-state pH differential is from about 2 to about 7.
14. A method according to claim 1 , wherein the steady-state pH differential is from about 3 to about 6.
15. A method according to claim 10 , wherein the redox mediator is selected from the group consisting of:
a redox metal oxyanion, a redox non-metal oxyanion, a redox metal salt, and a redox organic compound.
16. A method according to claim 10 , further comprising oxidizing the redox mediator at the anode from a lower oxidation state to a higher oxidation state.
17. A method according to claim 8 , wherein heating at least the portion of the anode electrolyte comprises heating at least the portion of the anode electrolyte to a temperature of about 100° C.