Sulfur dioxide depolarized electrolyzer and method for performance recovery

Abstract

An electrolyzer can include an anode, a cathode, a separator disposed between the anode and the cathode. A method for operating the electrolyzer can include electrochemically oxidizing anolyte (e.g., sulfur oxide) and reducing catholyte (e.g., water), determining performance metrics of the electrochemical reaction, and recovering performance of the electrochemical reaction.

We claim: 
     
         1 . An electrolyzer comprising:
 an anode;   a cathode;   a separator disposed between the anode and the cathode, wherein the separator comprises a proton conductivity greater than about 0.01 S/cm;   an anolyte inlet configured to introduce anolyte into an anolyte flow path, wherein the anolyte flow path is in fluid communication with the anode;   an anolyte outlet configured to release oxidized anolyte from the anolyte flow path, wherein the oxidized anolyte comprises sulfuric acid; and   a catholyte flow path in fluid communication with the cathode; and   a catholyte outlet configured to release reduced catholyte from the catholyte flow path, wherein the reduced catholyte comprises hydrogen;   
       wherein the electrolyzer is configured to operate in an electrolysis mode and a recovery mode, wherein the electrolyzer is only operable in a single mode of the electrolysis mode and the recovery mode at a given time; 
       wherein in the electrolysis mode:
 the anode is configured to oxidize the anolyte, wherein the anolyte comprises gaseous sulfur dioxide with a relative humidity that is between 0-100%; 
 the cathode is configured to reduce a catholyte, wherein the catholyte comprises water; and 
 
       wherein in the recovery mode:
 the cathode is configured to oxidize sulfur deposited on a surface of the cathode by sulfur dioxide that is reduced after crossing through the separator, wherein the catholyte comprises oxygen, and wherein the anolyte comprises hydrogen. 
 
     
     
         2 . The electrolyzer of  claim 1 , wherein the anode and the cathode are electrically connected to a power source, wherein in the electrolysis mode a positive terminal of the power source is directly electrically connected with the anode and a negative terminal of the power source is directly electrically connected with the cathode, wherein in the recovery mode the positive terminal of the power source is directly electrically connected with the cathode and the negative terminal of the power source is directly electrically connected with the anode. 
     
     
         3 . The electrolyzer of  claim 1 , wherein in the electrolysis mode, the electrolyzer is operated with a substantially constant current density. 
     
     
         4 . The electrolyzer of  claim 3 , wherein when an operating electric potential exceeds a threshold electric potential, the electrolyzer switches from the electrolysis mode to the recovery mode. 
     
     
         5 . The electrolyzer of  claim 1 , wherein in the recovery mode, the electrolyzer is operated with a substantially constant electric potential, wherein when a current density is less than a threshold current density the electrolyzer switches to the electrolysis mode. 
     
     
         6 . The electrolyzer of  claim 5 , wherein in the recovery mode, the substantially constant electric potential is pulsed on and off at a substantially constant frequency. 
     
     
         7 . The electrolyzer of  claim 1 , wherein in the recovery mode, the electrolyzer is operated with a varying electric potential sequentially rising and lowering between a minimum electric potential and a maximum electric potential. 
     
     
         8 . The electrolyzer of  claim 7 , wherein in the recovery mode, the varying electric potential is pulsed on and off at a substantially constant frequency. 
     
     
         9 . A method for operating an electrolyzer comprising:
 providing sulfur dioxide proximal an anode of the electrolyzer;   providing water proximal a cathode of the electrolyzer;   maintaining the anode and the cathode at an electrical potential such that the sulfur dioxide is oxidized to sulfuric acid and the water is reduced to hydrogen, wherein a current density is substantially constant, wherein the anode is electrically connected to a positive terminal of a power source and the cathode is electrically connected to a negative terminal of the power source; and   when the electrical potential exceeds a threshold electrical potential:
 changing the electrolyzer connection to the power supply so that the anode is electrically connected to the negative terminal of the power supply and the cathode is electrically connected to the negative terminal; 
 providing an oxidizing agent proximal the cathode; 
 providing a reducing agent proximal the anode; and 
 operating the electrolyzer to oxidize sulfur deposited on the cathode into sulfur dioxide. 
   
     
     
         10 . The method of  claim 9 , wherein the current density is between 0.4 and 1 A/cm 2 . 
     
     
         11 . The method of  claim 9 , wherein operating the electrolyzer to oxidize sulfur deposited on the cathode into sulfur dioxide comprises applying a second electric potential between −0.5 V and 1 V. 
     
     
         12 . The method of  claim 11 , wherein the second electric potential is substantially constant. 
     
     
         13 . The method of  claim 11 , wherein the second electric potential linearly varies between a lower electric potential and an upper electric potential, wherein the lower electric potential and the upper electric potential are each between −0.5 V and 1 V. 
     
     
         14 . The method of  claim 11 , wherein the second electric potential is pulsed on and off at a substantially constant frequency. 
     
     
         15 . The method of  claim 9 , further comprising: when the electrical potential exceeds a threshold electrical potential, heating the electrolyzer to above 115° C. before operating the electrolyzer to oxidize the sulfur deposited on the cathode into sulfur dioxide. 
     
     
         16 . The method of  claim 9 , wherein the oxidizing agent is selected from the group consisting of: oxygen, water, ozone, hydrogen peroxide, sodium hypochlorite, potassium permanganate, sodium persulfate, ammonium persulfate, chlorine, or combinations thereof. 
     
     
         17 . The method of  claim 9 , wherein the reducing agent is hydrogen. 
     
     
         18 . The method of  claim 9 , wherein maintaining the anode and the cathode at the electrical potential further results in heating the electrolyzer to a temperature between 60° C. and 90° C., wherein operating the electrolyzer to oxidize the sulfur deposited on the cathode is performed at a temperature between about 10 and 40° C. 
     
     
         19 . The method of  claim 9 , wherein the electric potential is between 0.4 V and 1.3 V. 
     
     
         20 . A method for operating an electrolyzer comprising:
 providing sulfur dioxide proximal an anode of the electrolyzer;   providing water proximal a cathode of the electrolyzer;   maintaining the anode and the cathode at an electrical potential such that the sulfur dioxide is oxidized to sulfuric acid and the water is reduced to hydrogen, wherein a current density is substantially constant, wherein the anode is electrically connected to a positive terminal of a power source and the cathode is electrically connected to a negative terminal of the power source; and   when the electrical potential exceeds a threshold electrical potential, operating the electrolyzer to reduce sulfur deposited on the cathode into dihydrogen sulfide.