Continuous co-current electrochemical reduction of carbon dioxide

Abstract

In various embodiments, the invention provides electro-chemical processes for reduction of carbon dioxide, for example converting carbon dioxide to formate salts or formic acid. In selected embodiments, operation of a continuous reactor with a three dimensional cathode and a two-phase (gas/liquid) catholyte flow provides advantageous conditions for electro-reduction of carbon dioxide. In these embodiments, the continuous two-phase flow of catholyte solvent and carbon dioxide containing gas, in selected gas/liquid phase volume flow ratios, provides dynamic conditions that favour the electro-reduction of COs at relatively high effective superficial current densities and gas space velocities, with relatively low reactor (cell) voltages (<10 Volts). In some embodiments, relatively high internal gas hold-up in the cathode chamber (evident in an internal gas to liquid phase volume ratio >0.1) may provide greater than equilibrium CO 2 concentrations in the liquid phase, also facilitating relatively high effective superficial current densities. In some embodiments, these characteristics may for example be achieved at catholyte pH>7 and relatively low CO 2 partial pressures (<10 bar). In some embodiments, these characteristics may for example be achieved under near adiabatic conditions, with catholyte outlet temperature up to about 80° C.

1 . An electrochemical process for reducing carbon dioxide to produce formic acid or formate salts comprising:
 a) continuously feeding a gas stream comprising carbon dioxide gas and a liquid catholyte into an electrochemical reactor, said electrochemical reactor having a porous 3D cathode in a cathode chamber, an anode in an anode chamber, the anode chamber being separated from the cathode chamber by an electrochemical cell membrane, wherein said gas stream and said liquid stream are fed into said 3D cathode, traveling from a cathode inlet into said 3D cathode to a cathode outlet out of said 3D cathode;   b) feeding an anolyte through an anolyte inlet into said anode chamber, said anolyte travelling through said anode chamber to an anolyte outlet out of said anode chamber;   c) maintaining a gas to liquid ratio in the porous cathode, measured as a ratio of the volume of said gas in the porous 3D cathode to the volume of said liquid in the porous 3D cathode greater than about 0.1 to promote a super-equilibrium concentration of carbon dioxide dissolved in the liquid stream within the porous 3D cathode; and   d) passing an electric current between said porous 3D cathode and said anode, to reduce the dissolved carbon dioxide to a formate salt.   
     
     
         2 . The process of any one of  claim 1 , wherein the gas (corrected to STP) to liquid volumetric feed ratio to the porous 3D cathode is greater than about 1. 
     
     
         3 . The process of  claim 1 , wherein the cathode chamber is maintained at a cathode pressure and the cathode pressure is in the range of 1 Bar (100 kPa(abs)) to 10 Bar (1000 kPa(abs)). 
     
     
         4 . The process of  claim 1 , wherein the catholyte liquid comprises an aqueous solution comprising at least one of:
 a dissolved alkali metal bicarbonate or formate;   a dissolved ammonium bicarbonate or formate; and   ammonium cations.   
     
     
         5 . The process of  claim 4 , wherein the bulk pH of the catholyte liquid is in the range of 4 to 
     
     
         10 . 
     
     
         6 . The process of any one of  claim 4 , wherein the anolyte comprises at least:
 a) a dissolved alkali metal hydroxide;   b) an ammonium salt;   c) a dissolved acid, being H 2 SO 4 , HCl, or H 3 PO 4 ;   d) dissolved sulphuric acid and ammonium sulphate; or   e) dissolved sulphuric acid and sodium sulphate.   
     
     
         7 . The process of  claim 6 , wherein the anolyte comprises ammonium ions. 
     
     
         8 . The process of  claim 6 , further comprising the step of separating an anode product from the anolyte outlet stream from the anolyte chamber. 
     
     
         9 . The process of  claim 6 , wherein the electrochemical cell membrane permits selected ions to cross the membrane to balance the process stoichiometry and to maintain a desired pH in the bulk catholyte. 
     
     
         10 . The process of  claim 1 , further comprising reacting anolyte exiting through said anolyte outlet with said liquid exiting from said porous 3D cathode. 
     
     
         11 . The process of  claim 9 , further comprising Joule heating of the anolyte to provide heated anolyte, wherein the hot anolyte is used to heat the liquid catholyte exiting from the 3D cathode to separate water and-or formic acid by evaporation. 
     
     
         12 . The process of  claim 1 , further comprising separating the formate salt or the formic acid from the liquid exiting from the 3D cathode. 
     
     
         13 . The process of  claim 9 , wherein the anolyte comprises dissolved sulphuric acid and ammonium sulphate. 
     
     
         14 . A process of  claim 12  wherein the formate salt separated from the liquid exiting the 3D cathode is ammonium formate. 
     
     
         15 . An electrochemical process for reducing carbon dioxide to produce formic acid or formate salts comprising:
 a) continuously feeding a gas stream comprising carbon dioxide gas and a liquid catholyte into an electrochemical reactor, said electrochemical reactor having a porous 3D cathode in a cathode chamber, an anode in an anode chamber, the anode chamber being separated from the cathode chamber by an electrochemical cell membrane, wherein said gas stream and said liquid stream are fed through said 3D cathode, traveling from a cathode inlet into said 3D cathode to a cathode outlet out of said 3D cathode; wherein the gas hourly space velocity (GHSV) in the cathode chamber, being the volumetric gas flow rate at STP divided by the cathode chamber volume, exceeds about 100 per hour;   b) feeding an anolyte through an anolyte inlet into said anode chamber, said anolyte travelling through said anode chamber to an anolyte outlet out of said anode chamber; and   c) passing an electric current between said porous 3D cathode and said anode, to reduce the dissolved carbon dioxide to a formate salt.   
     
     
         16 . The process of any one of  claim 15 , wherein the gas (corrected to STP) to liquid volumetric feed ratio to the porous 3D cathode is greater than about 1. 
     
     
         17 . The process of  claim 15 , wherein the cathode chamber is maintained at a cathode pressure and the cathode pressure is in the range of 1 Bar (100 kPa(abs)) to 10 Bar (1000 kPa(abs)). 
     
     
         18 . The process of  claim 15 , wherein the catholyte liquid comprises an aqueous solution comprising at least one of:
 a dissolved alkali metal bicarbonate or formate;   a dissolved ammonium bicarbonate or formate; and   ammonium cations.   
     
     
         19 . The process of  claim 18 , wherein the bulk pH of the catholyte liquid is in the range of 4 to 10. 
     
     
         20 . The process of any one of  claim 18 , wherein the anolyte comprises at least:
 a) a dissolved alkali metal hydroxide;   b) an ammonium salt;   c) a dissolved acid, being H 2 SO 4 , HCl, or H 3 PO 4 ;   d) dissolved sulphuric acid and ammonium sulphate; or   e) dissolved sulphuric acid and sodium sulphate.   
     
     
         21 . The process of  claim 20 , wherein the anolyte comprises ammonium ions. 
     
     
         22 . The process of  claim 20 , further comprising the step of separating an anode product from the anolyte outlet stream from the anolyte chamber. 
     
     
         23 . The process of  claim 20 , wherein the electrochemical cell membrane permits selected ions to cross the membrane to balance the process stoichiometry and to maintain a desired pH in the bulk catholyte. 
     
     
         24 . The process of  claim 15 , further comprising reacting anolyte exiting through said anolyte outlet with said liquid exiting from said porous 3D cathode. 
     
     
         25 . The process of  claim 23 , further comprising Joule heating of the anolyte to provide heated anolyte, wherein the hot anolyte is used to heat the liquid catholyte exiting from the 3D cathode to separate water and-or formic acid by evaporation. 
     
     
         26 . The process of  claim 15 , further comprising separating the formate salt or the formic acid from the liquid exiting from the 3D cathode. 
     
     
         27 . The process of  claim 23 , wherein the anolyte comprises dissolved sulphuric acid and ammonium sulphate. 
     
     
         28 . A process of  claim 26  wherein the formate salt separated from the liquid exiting the 3D cathode is ammonium formate.