Electrochemical Reduction of CO₂ to Oxalic Acid: Experiments, Process Modeling, and Economics

Abstract

We performed H-cell and flow cell experiments to study the electrochemical reduction of CO2 to oxalic acid (OA) on a lead (Pb) cathode in various nonaqueous solvents. The effects of anolyte, catholyte, supporting electrolyte, temperature, water content, and cathode potential on the Faraday efficiency (FE), current density (CD), and product concentration were investigated. We show that a high FE for OA can be achieved (up to 90%) at a cathode potential of −2.5 V vs Ag/AgCl but at relatively low CDs (10–20 mA/cm2). The FE of OA decreases significantly with increasing water content of the catholyte, which causes byproduct formation (e.g., formate, glycolic acid, and glyoxylic acid). A process design and techno-economic evaluation of the electrochemical conversion of CO2 to OA is presented. The results show that the electrochemical route for OA production can compete with the fossil-fuel based route for the base case scenario (CD of 100 mA/cm2, OA FE of 80%, cell voltage of 4 V, electrolyzer CAPEX of $20000/m2, electricity price of $30/MWh, and OA price of $1000/ton). A sensitivity analysis shows that the market price of OA has a huge influence on the economics. A market price of at least $700/ton is required to have a positive net present value and a payback time of less than 10 years. The performance and economics of the process can be further improved by increasing the CD and FE of OA by using gas diffusion electrodes and eliminating water from the cathode, lowering the cell voltage by increasing the conductivity of the electrolyte solutions, and developing better OA separation methods.

A Quantitative Analysis of Electrochemical CO2 Reduction on Copper in Organic Amide and Nitrile-Based Electrolytes

“…The first configuration is similar to that used in the aqueous systems, where the catholyte and anolyte compartments are filled with the same electrolyte and the compartments are separated by a Selemion anion exchange membrane. , The second configuration is similar to that used more often in studies investigating non-aqueous systems, where the catholyte is a non-aqueous electrolyte and the anolyte is an (acidic) aqueous electrolyte, separated by a Nafion cation exchange membrane. ,,, Electrochemical experiments are initially performed with 0.1 M TBAPF 6 in DMF considering the second reduction potential (−1.9 V) as the onset potential for CO 2 reduction, as this is very close to the typical onset potential reported in the literature. ,, Figure S2 shows the faradaic efficiencies (FE) of all the gaseous products and the current densities obtained at applied potentials of −1.9, −2.1, and −2.3 V using the first experimental configuration. CO and H 2 are the main gaseous products obtained over this potential range with the FE toward CO peaking at 52% at −2.1 V and decreasing to 42% at −2.3 V. The FE toward H 2 decreased from 16% to 2% as the applied potential is increased from −1.9 to −2.3 V. The production of H 2 in non-aqueous electrolytes mainly stems from the reduction of residual water present in the system; however, it could also be produced from the reduction of the organic electrolyte or the electrolyte salt. , The liquid products typically expected in the case of non-aqueous electrolytes are oxalate and formate, where oxalate is mainly produced in the absence of water and formate is mainly produced in the presence of water. ,,, In the experiments performed, the FE toward oxalate is observed to be abnormally high, adding to more than 80%, leading to a total FE of 130–150% at all the three potentials. This strongly suggests that (a part of) the observed products are formed from the reduction or breakdown of the electrolyte and not from the supplied CO 2 . , Oxalate is also detected in the anolyte, which is due to the migration of the oxalate anions through the anion exchange membrane in the first configuration.…”

Carbonation in Low-Temperature CO2 Electrolyzers: Causes, Consequences, and Solutions

“…The required power of the electrolyzers was computed from P j = i j × A × V where P j is the power required to produce component j , i j is the partial CD for component j , A is the electrode area, and V is the cell voltage. The electrode area ( A ) required to convert 1 ton/h of CO 2 was estimated from A = n j × N CO 2 × F i t × FE j where N CO 2 is the mole flow of CO 2 , i t is the total CD, F is the Faraday constant, FE j is the Faraday efficiency for component j , and n j is the number of electrons involved in the CO2RR (2 for CO2R to CO).…”

“…The required power of the electrolyzers was computed from (2) where P j is the power required to produce component j, i j is the partial CD for component j, A is the electrode area, and V is the cell voltage. The electrode area (A) required to convert 1 ton/h of CO 2 was estimated from 123 (3)…”

Revisiting Electrocatalytic CO2 Reduction in Nonaqueous Media: Promoting CO2 Recycling in Organic Molecules by Controlling H2 Evolution

“…Recent studies have featured conducting CO 2 reduction experiments using near‐zero gap flow cells to increase CO 2 conversion and decrease the presence of protons at the cathode. One successful example is the work by V. Boor and co‐workers, [ 42 ] where they describe the use of GDL electrodes in filter press reactors to evaluate the electrocatalytic efficiency of Pb nanoparticles under flow cells. The GDL electrode architecture has allowed for larger electrically active areas, while percolation of the gas into propylene carbonate further increases the local CO 2 concentration.…”

“…Several recovery and purification technologies can be found to purify these products, such as nanofiltration, [79,80] reverse osmosis, [80] distillation, [81,82] and liquid-liquid extraction. [42,83,84] While the purification technologies are usually studied for aqueous solutions, they can also be applied in nonaqueous solutions, making minor changes and/or combining two or more processes. [85] The mixture can be avoided using a sacrificial anode (discussed in Section 3), and only liquid-liquid extraction is necessary to purify the CO 2 reduction product.…”