The pH and Potential Dependence of Pb‐Catalyzed Electrochemical CO₂ Reduction to Methyl Formate in a Dual Methanol/Water Electrolyte

Abstract

The conversion of waste CO2 to value‐added chemicals through electrochemical reduction is a promising technology for mitigating climate change while simultaneously providing economic opportunities. The use of non‐aqueous solvents like methanol allows for higher CO2 availability and novel products. In this work, the electrochemistry of CO2 reduction in acidic methanol catholyte at a Pb working electrode was investigated while using a separate aqueous anolyte to promote a sustainable water oxidation half‐reaction. The selectivity among methyl formate (a product unique to reduction of CO2 in methanol), formic acid, and formate was critically dependent on the catholyte pH, with higher pH conditions leading to formate and low pH favoring methyl formate. The potential dependence of the product distribution in acidic catholyte was also investigated, with a faradaic efficiency for methyl formate as high as 75 % measured at −2.0 V vs. Ag/AgCl.

Comparative Technoeconomic Analysis of Pathways for Electrochemical Reduction of CO2 with Methanol to Produce Methyl Formate

“…An operating cell voltage of 3.8 V is consistent with early demonstrations of methyl formate production in a dual methanol/water cell and a higher current condition for a state-of-the-art aqueous CO 2 -to-formic acid flow electrolyzer. 26,37 An aqueous flow cell operating current density equal to or greater than 200 mA cm −2 has been achieved at that potential, and the model assumes that a similarly optimized methanol-based flow cell could be produced. A single-pass CO 2 conversion of 50% was assumed in all cases.…”

“…For this system, the base case cathode methyl formate (HCOOCH 3 ) faradaic efficiency was 60%, which was marginally lower than the peak value reported in an equivalent Pb-catalyzed system. 26 The balance of the cathode faradaic efficiency was assigned to hydrogen, formic acid, and carbon monoxide in a 17:2:1 ratio. Even with a methanol barrier layer composite membrane, some methanol crossover is bound to occur in a dual methanol−catholyte/water−anolyte system, and a base case anodic methanol oxidation faradaic efficiency of 5% was assumed with the balance assigned to water oxidation.…”

“…The methyl formate selectivity in the reactor was determined by assuming an equilibrium conversion dependent on the fraction of water in the methanol, as determined from a linear fit of previously reported data for mixtures of formic acid and methanol at pH ∼ 2. 26 Electricity. Electrical power consumption costs for the electrolyzer and separation units are a significant contributor to the plant operating expenses.…”

“…This dual CH 3 OH/H 2 O electrolysis has been demonstrated with a methyl formate faradaic efficiency (FE) as high as 75% using a Pb cathode and low pH (<2.5) catholyte to promote the esterification of formic acid and methanol. 26 In System 2, the power plant flue gas is directly utilized in the dual CH 3 OH/H 2 O electrolyzer without CO 2 capture, bypassing the capital expense and energy required for CO 2 purification. 31 System 3 uses capture technology to provide purified CO 2 to an all methanol solvent-based electrolyzer.…”

Recent advances in CO2 capture and reduction

“…Recent advances in surface chemistry and spectroscopy also offer new opportunities to probe reaction mechanisms of CO 2 RR at the interfaces and, in turn, guide the design of such interfaces for catalysis enhancement. [122][123][124][125] Electrochemical systems that can be used to optimize gas transport, [126][127][128][129] electrolyte functions, [130][131][132][133] intermediate detection, [134][135][136][137][138][139] and reaction pathway engineering, [140][141][142] have been rapidly developed to improve the overall catalytic performance. In this section, we highlight how interfacial engineering can be applied to optimize electrolyte and catalyst surface ligand effects to enhance CO 2 RR catalysis.…”