Systems and methods to make hydrogen gas using metal oxyanions or non-metal oxyanions

Abstract

Disclosed herein are methods and systems that relate to oxidizing a metal ion of a metal oxyanion or a non-metal ion of a non-metal oxyanion from a lower oxidation state to a higher oxidation state at an anode and generate hydrogen gas at the cathode. The metal oxyanion with the metal ion in the higher oxidation state or the non-metal oxyanion with the non-metal ion in the higher oxidation state may be then subjected to a thermal reaction or a second electrochemical reaction, to form oxygen gas as well as to regenerate the metal oxyanion with the metal ion in the lower oxidation state or the non-metal oxyanion with the non-metal ion in the lower oxidation state, respectively.

1 . A method to generate hydrogen gas, comprising:
 providing an anode and an anode electrolyte in an electrochemical cell,. wherein the anode electrolyte comprises a metal oxyanion with a metal ion in a lower oxidation state or a non-metal oxyanion with a non-metal ion in a lower oxidation state;   oxidizing the metal oxyanion with the metal ion in the lower oxidation state to a metal oxyanion with metal ion in a higher oxidation state or oxidizing the non-metal oxyanion with the non-metal ion in the lower oxidation state to a non-metal oxyanion with non-metal ion in a higher oxidation state at the anode; and   providing a cathode and a cathode electrolyte in the electrochemical cell and forming hydrogen gas and hydroxide ions at the cathode;   wherein a theoretical anode half-cell voltage of the electrochemical cell for the oxidizing of the metal oxyanion with the metal ion in the lower oxidation state to the metal oxyanion with the metal ion in the higher oxidation state or the oxidizing of the non-metal oxyanion with the non-metal ion in the lower oxidation state to the non-metal oxyanion with the non-metal in the higher oxidation state at the anode is higher than a comparative theoretical anode half-cell voltage at a comparative anode of a comparative electrochemical cell for formation of oxygen gas at the comparative anode, wherein the comparative electrochemical cell comprises the comparative anode, a comparative anode electrolyte that does not comprise the metal oxyanion with the metal ion or the non-metal oxyanion with the non-metal ion, a comparative cathode, and a comparative cathode electrolyte, wherein the comparative electrochemical cell forms hydrogen gas and hydroxide ions at the comparative cathode and oxygen gas at the comparative anode, and   wherein an operating voltage of the electrochemical cell is lower than a comparative operating voltage of the comparative electrochemical cell.   
     
     
         2 . The method of  claim 1 , wherein the lower operating voltage of the electrochemical cell is due to one or both of an over-potential of the electrochemical cell being lower than a comparative over-potential of the comparative electrochemical cell and a thermo-neutral voltage of the electrochemical cell being lower than a comparative thermo-neutral voltage of the comparative electrochemical cell. 
     
     
         3 . The method of  claim 1 , wherein the operating voltage of the electrochemical cell is from about 1.5 V to about 3 V. 
     
     
         4 . The method of  claim 1 , further comprising separating the anode electrolyte from the cathode electrolyte by a separator. 
     
     
         5 . The method of  claim 4 , wherein the separator comprises an anion exchange membrane, and wherein the method comprises migrating hydroxide ions from the cathode electrolyte to the anode electrolyte. 
     
     
         6 . The method of claim e, wherein the separator comprises a cation exchange membrane, and wherein the method comprises migrating a corresponding cation from the anode electrolyte to the cathode electrolyte. 
     
     
         7 . The method of  claim 1 , wherein the metal ion in the metal oxyanion is selected from the group consisting of manganese, iron, chromium, selenium, copper, tin, silver, cobalt, uranium, lead, mercury, vanadium, bismuth, titanium, ruthenium, osmium, europium, zinc, cadmium, gold, nickel, palladium, platinum, rhodium, iridium, technetium, rhenium, molybdenum, tungsten, niobium, tantalum, zirconium, hafnium, and combination thereof. 
     
     
         8 . The method of  claim 1 , wherein the metal oxyanion with the metal ion in the lower oxidation state is selected from the group consisting of MnO 4   2− , FeO 4   2− , RuO 4   2− , OsO 4   2− , HSnO 2   − , SeO 3   2− , Cu 2 O, CrO 3   3− , and TeO 3   2− , and/or the metal oxyanion with the metal ion in the higher oxidation state is selected from the group consisting of MnO 4   − , HFeO 2   − , RuO 4   − , OsO 5   2− , SnO 3   2− , SeO 4   2− , CuO 2   2− , CrO 4   2− , and TeO 4   2− . 
     
     
         9 . The method of  claim 1 , wherein the non-metal ion in the non-metal oxyanion is selected from the group consisting of a halogen, carbon, sulfur, nitrogen, and phosphorus. 
     
     
         10 . The method of  claim 1 , wherein the non-metal oxyanion with the non-metal ion in the lower oxidation state is selected from the group consisting of NO 2   − , PO 3   3− , SO 3   2− , ClO − , ClO 2   − , ClO 3   − , BrO − , BrO 2   − , BrO 3   − , IO − , IO 2   − , and IO 3   −  and/or the non-metal oxyanion with the non-metal ion in the higher oxidation state is selected from the group consisting of NO 3   − , PO 4   3− , SO 4   2− , ClO 2   − , ClO 3   − , ClO 4   − , BrO 2   − , BrO 3   − , BrO 4   − , IO 2   − , IO 3   − , and IO 4   − . 
     
     
         11 . The method of  claim 1 , wherein no oxygen gas is formed at the anode or less than 25% of the Faradaic efficiency is for the oxygen evolution reaction at the anode. 
     
     
         12 . The method of  claim 1 , further comprising subjecting the anode electrolyte comprising metal oxyanion with metal ion in the higher oxidation state or the anode electrolyte comprising non-metal oxyanion with non-metal ion in the higher oxidation state to a thermal reaction to form oxygen gas and the metal oxyanion with the metal ion in the lower oxidation state or the non-metal oxyanion with the non-metal ion in the lower oxidation state, respectively. 
     
     
         13 . The method of  claim 1 , further comprising
 transferring at least a portion of the anode electrolyte comprising the metal oxyanion with the metal ion in the higher oxidation state or the non-metal oxyanion with the non-metal ion in the higher oxidation state outside the electrochemical cell to a second cathode electrolyte of a second electrochemical cell; and   reducing the metal oxyanion with the metal ion in the higher oxidation state to the lower oxidation state or reducing the non-metal oxyanion with the non-metal ion in the higher oxidation state to the lower oxidation state at a second cathode of the second electrochemical cell.   
     
     
         14 . A system to generate hydrogen gas, comprising:
 an electrochemical cell comprising;   an anode and an anode electrolyte comprising a metal oxyanion with a metal ion in a lower oxidation state or a non-metal oxyanion with a non-metal ion in a lower oxidation state, wherein the anode is configured to oxidize the metal oxyanion with the metal ion in the lower oxidation state to a metal oxyanion with metal ion in a higher oxidation state or to oxidize the non-metal oxyanion with the non-metal ion in the lower oxidation state to a non-metal oxyanion with non-metal ion in a higher oxidation state; and   a cathode and a cathode electrolyte comprising water wherein the cathode is configured to reduce water to form hydroxide ions and hydrogen gas;   wherein the electrochemical cell is configured to maintain a steady-state pH differential of between about 1-6 between the anode electrolyte and the cathode electrolyte.   
     
     
         15 . The system of  claim 14 , further comprising a thermal reactor operably connected to the electrochemical cell, wherein the thermal reactor is configured to receive at least a portion of the anode electrolyte comprising the metal oxyanion with the metal ion in the higher oxidation state or the non-metal oxyanion with the non-metal ion in the higher oxidation state and subject the portion of the anode electrolyte to a thermal reaction to form oxygen gas and the metal oxyanion with the metal ion in the lower oxidation state or the non-metal oxyanion with the non-metal ion in the lower oxidation state, respectively. 
     
     
         16 . The system of  claim 14 , further comprising a second electrochemical cell operably connected to the electrochemical cell, wherein the second electrochemical cell comprises:
 a second anode and a second anode electrolyte; and   a second cathode and a second cathode electrolyte, wherein the second cathode electrolyte is configured to receive at least a portion of the anode electrolyte of the electrochemical cell comprising the metal oxyanion with the metal ion in the higher oxidation state or the non-metal oxyanion with the non-metal ion in the higher oxidation state,   wherein the second cathode is configured to reduce the metal oxyanion with the metal ion in the higher oxidation state to form the metal oxyanion with the metal ion in the lower oxidation state or to reduce the non-metal oxyanion with the non-metal ion in the higher oxidation state to form the non-metal oxyanion with the non-metal ion in the lower oxidation state.   
     
     
         17 . The system of  claim 14 , further comprising a separator disposed between the anode electrolyte and the cathode electrolyte. 
     
     
         18 . The system of  claim 17 , wherein the separator comprises an anion exchange membrane configured to allow the hydroxide ions to migrate from the cathode electrolyte to the anode electrolyte. 
     
     
         19 . The system of  claim 17 , wherein the separator comprises a cation exchange membrane configured to allow a corresponding cation to migrate from the anode electrolyte to the cathode electrolyte. 
     
     
         20 . The system of  claim 14 , wherein the electrochemical cell is configured so that no oxygen gas is formed at the anode or so that less than 25% of the Faradaic efficiency is for the oxygen evolution reaction at the anode.