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.

What is claimed is: 
     
         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.   
     
     
         2 . The method of  claim 1 , further comprising separating the anode electrolyte from the cathode electrolyte by an anion exchange membrane and migrating the hydroxide ions from the cathode electrolyte to the anode electrolyte. 
     
     
         3 . 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. 
     
     
         4 . 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− , HSO 2   − , SeO 3   2− , Cu 2 O, CrO 3   3− , and TeO 3   2− . 
     
     
         5 . The method of  claim 1 , wherein 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− . 
     
     
         6 . The method of  claim 1 , wherein the non-metal ion in the non-metal oxyanion is selected from the group consisting of halogen, carbon, sulfur, nitrogen, and phosphorus. 
     
     
         7 . 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   − . 
     
     
         8 . The method of  claim 1 , further comprising maintaining a steady-state pH differential of between about 1-6 between the anode electrolyte and the cathode electrolyte. 
     
     
         9 . 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. 
     
     
         10 . The method of  claim 1 , further comprising oxidizing hydroxide ions at the anode to form oxygen gas. 
     
     
         11 . The method of  claim 10 , further comprising
 operating the electrochemical cell at lower current density for the oxidation 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 for the oxidation of the non-metal oxyanion with the non-metal ion in the lower oxidation state to the non-metal oxyanion with the non-metal ion in the higher oxidation state at the anode; and   operating the electrochemical cell at higher current density for the oxidation of the hydroxide ions at the anode to form oxygen gas.   
     
     
         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 12 , wherein the thermal reaction is carried out in presence of the hydroxide ions; at a pH of more than 10; and/or in presence of a catalyst. 
     
     
         14 . 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;   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.   
     
     
         15 . The method of  claim 14 , further comprising migrating hydroxide ions from the second cathode electrolyte to a second anode electrolyte through an AEM in the second electrochemical cell and oxidizing the hydroxide ions at a second anode in the second electrochemical cell to form oxygen gas. 
     
     
         16 . 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. 
   
     
     
         17 . The system of  claim 16 , 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. 
     
     
         18 . The system of  claim 16 , further comprising an anion exchange membrane disposed between the anode electrolyte and the cathode electrolyte and configured to migrate the hydroxide ions from the cathode electrolyte to the anode electrolyte. 
     
     
         19 . The system of  claim 16 , 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. 
     
     
         20 . The system of  claim 16 , wherein the anode is further configured to oxidize the hydroxide ions to form oxygen gas.