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 format 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 advantages 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 stream containing dissolved carbon dioxide 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 volumetric hold-up ratio, being the ratio of the volume of said gas to the volume of said liquid in the 3D cathode, between 1 and 2 to promote a super-equilibrium concentration of aqueous carbon dioxide in the liquid stream within the 3D cathode;   d) passing an electric current between said 3D cathode in the cathode chamber and said anode, to reduce the dissolved carbon dioxide to form either a formate salt or formic acid.   
     
     
         2 . The process of  claim 1 , wherein the gas (corrected to STP) to liquid volumetric feed ratio to the 3D cathode is greater than about 10 and less than 100. 
     
     
         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 solvent is an aqueous solvent 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 solvent is in the range of 4 to 10. 
     
     
         6 . The process of  claim 1 , wherein the anolyte comprises at least:
 a) a dissolved alkali metal hydroxide;   b) ammonium salt;   c) a dissolved acid, being H2SO4, HCl, or H3PO4;   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 7 , wherein the electrochemical cell membrane permits selected ions to cross the membrane to balance the process stoichiometry. 
     
     
         9 . The process of  claim 1 , further comprising reacting anolyte exiting through said anolyte outlet with said liquid exiting from said 3D cathode. 
     
     
         10 . The process of  claim 9 , further comprising Joule heating of the anolyte to provide heated anolyte. 
     
     
         11 . The process of  claim 1 , further comprising separating the formate salt or formic acid from the liquid exiting from the 3D cathode. 
     
     
         12 . The process of  claim 9 , wherein the anolyte comprises dissolved sulphuric acid and ammonium sulphate.