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 comprising:
 a) continuously passing a catholyte mixture through a cathode chamber of an electrochemical reactor, the catholyte mixture comprising carbon dioxide gas and a liquid catholyte solvent containing dissolved carbon dioxide;   b) maintaining a catholyte gas to liquid volumetric hold-up ratio, being the ratio of the volume of gas to the volume of the liquid catholyte solvent, in the cathode chamber, greater than about 0.1.   c) passing an electric current between a cathode in the cathode chamber and an anode, to reduce the dissolved carbon dioxide to form a desired product.   
     
     
         2 . The process of  claim 1 , wherein the gas (corrected to STP) to liquid volumetric feed ratio to the cathode chamber is greater than about 1. 
     
     
         3 . The process of  claim 1  or  2 , wherein the effective superficial current density at the cathode is greater than 1 kA/m 2    
     
     
         4 . The process of  claim 1 ,  2  or  3 , wherein the carbon dioxide gas partial pressure in the cathode chamber is less than 10 Bar. 
     
     
         5 . The process of any one of  claims 1  to  4 , wherein the electric current is a direct current driven by an electrochemical cell voltage. 
     
     
         6 . The process of  claim 5 , wherein the electrochemical cell voltage is less than 10 Volts. 
     
     
         7 . The process of any one of  claims 1  to  6 , wherein fluids in the cathode chamber are maintained at a cathode temperature above 20° C. 
     
     
         8 . The process of any one of  claims 1  to  7 , 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)). 
     
     
         9 . The process of any one of  claims 1  to  8 , wherein the catholyte solvent is an aqueous solvent. 
     
     
         10 . The process of  claim 9 , wherein the catholyte solvent comprises: a dissolved alkali metal bicarbonate or formate; or, a dissolved 
     
     
         11 . The process of  claim 9 , wherein the bulk pH of the catholyte solvent is in the range of 4 to 10. 
     
     
         12 . The process of  claim 9 , wherein the catholyte solvent comprises ammonium cations. 
     
     
         13 . The process of any one of  claims 1  to  12 , wherein the cathode is a three dimensional electrode that has a thickness in the dimension of current flow of from 0.5 to 10 mm. 
     
     
         14 . The process of  claim 13 , wherein the cathode has a porosity or voidage of from about 5% to about 95%. 
     
     
         15 . The process of any one of  claims 1  to  14 , wherein the cathode comprises tin or lead. 
     
     
         16 . The process of any one of  claims 1  to  15 , wherein the anode is in an anode chamber, and the anode chamber is separated from the cathode chamber by an electrochemical cell membrane. 
     
     
         17 . The process of  claim 16 , wherein the anode chamber comprises an anolyte. 
     
     
         18 . The process of  claim 17 , wherein the anolyte is an aqueous anolyte. 
     
     
         19 . The process of  claim 18 , wherein the anolyte comprises:
 a) a dissolved alkali metal hydroxide;   b) a dissolved alkali metal or ammonium salt;   c) a dissolved acid, being H2SO4, HCl, or H3PO4;   d) dissolved sulphuric acid and ammonium sulphate; or   
     
     
         20 . The process of  claim 18 , wherein the anolyte comprises an ammonium ions. 
     
     
         21 . The process of  claim 16 , wherein the electrochemical cell membrane is a cation permeable membrane. 
     
     
         22 . The process of  claim 16 , wherein the electrochemical cell membrane permits selected ions to cross the membrane to balance the process stoichiometry. 
     
     
         23 . The process of any one of  claims 17  to  22 , further comprising recycling at least a portion of the anolyte, the recycling anolyte, from an anolyte chamber outlet to an anolyte chamber inlet. 
     
     
         24 . The process of  claim 23 , further comprising the step of separating an anode co-product from the recycling anolyte. 
     
     
         25 . The process of any one of  claims 17  to  24 , further comprising Joule heating of the anolyte to provide heated anolyte. 
     
     
         26 . The process of  claim 25 , further comprising Joule heating of the anolyte to provide heated anolyte, wherein the heated anolyte is used to heat the recycling catholyte solvent to separate water or the desired product from the recycling catholyte solvent by evaporation. 
     
     
         27 . The process of any one of  claims 1  to  26 , wherein the desired product comprises a formate salt or formic acid. 
     
     
         28 . The process of  claim 27 , wherein the formate salt is ammonium formate. 
     
     
         29 . The process of any one of  claims 1  to  28 , further comprising separating the desired product from the catholyte solvent. 
     
     
         30 . The process of any one of  claims 1  to  28 , further comprising recycling at least a portion of the catholyte solvent, the recycling catholyte solvent, from a cathode chamber outlet to a cathode chamber inlet. 
     
     
         31 . The process of  claim 30 , further comprising the step of separating the desired product from the recycling catholyte solvent. 
     
     
         32 . The process of  claim 30 , further comprising reacting recycling catholyte, comprising formate, with the anolyte, to obtain the desired product by an acidolysis reaction. 
     
     
         33 . The process of  claim 32 , further comprising recycling at least a portion of the anolyte, the recycling anolyte, from an anolyte chamber outlet to an anolyte chamber inlet, and wherein the anolyte used to obtain the desired product is a portion of the recycling anolyte.