(Invited) Understanding on the Fundamental Processes of Electrocatalytic Conversion of CO2 to Multi-Carbon Chemicals
Authors: Liu Di-Jia 
Published: December 2024

The electrochemical CO2 reduction reaction (CO2RR) for chemical production using renewable electricity offers attractive “carbon-neutral” and “carbon-negative” mitigation strategies for greenhouse emission. CO2RR to C2+ chemicals, the compounds containing two or more carbons, represent a particularly interesting topic from both fundamental research as well as practical application points of view. For example, ethanol, ethylene, propanol, etc. are among the most produced chemicals by the industry and are widely used for various applications. Distributive CO2RR by recycling locally emitted CO2 to manufacture these chemicals for the local consumption could substantially reduce the cost of CO2 storage and transport while producing economic income.

Electrocatalytic reduction of CO2 involves a fundamental reaction step – proton couple electron transfer (PCET). CO2RR to CO or formic acid requires only two PCET steps, rendering them highly effective with fast kinetics. CO2RR to C2+ chemicals are significantly more challenging due to the rapid escalation of required PCET steps (for example, 12 for ethanol and 18 for propanol), as well as the C-C bond coupling reaction. These increased PCET steps substantially complicate the electrochemical reaction coordinate, leading to a high probability of branching reactions, which lower the single product Faradaic efficiency (FE). [1]

At Argonne, we developed an amalgamated lithium metal (ALM) synthesis method of preparing highly selective and active electrocatalysts through collaboration with Northern Illinois U. For example, we derived a Cu-based CO2RR catalyst that yielded > 90% FE for conversion of CO2 to ethanol [2]. More recently, we expanded the approach to metal Sn and prepared a family of CO2RR catalysts with Sn size varying from single atom, ultra-small clusters to nano-crystallite. High single-product FEs and low onset potential of CO2 conversion to acetate (FE = 90 % @ -0.6 V), ethanol (FE = 92% @ - 0.4 V), and formate (FE = 91% @ -0.6 V) were achieved over the catalysts of different active site dimensions. [3]

In this presentation, we will discuss in detail on the CO2 conversion mechanism behind these highly selective, size-modulated p-block element (Sn) catalysts. We will elucidate the catalytic reaction path using first-principle computation, advanced material characterizations, together with kinetic isotope effect (KIE) study. To our surprise, we found that PCET reaction paths over these catalysts can be completely altered by switching from H2O to D2O in the electrolyte, an important experimental observation to support future theoretical exploration on the transition state theory of CO2RR.

Acknowledgement: This work is supported by U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy - Industrial Efficiency & Decarbonization Office and by Office of Science, U.S. Department of Energy under Contract DE-AC02-06CH11357.

[1] “Electrochemical conversion of CO2 to long-chain hydrocarbons”, Di-Jia Liu, Joule
2022 6, 1969–1980

[2] “Highly selective electrocatalytic CO2 reduction to ethanol by metallic clusters dynamically formed from atomically dispersed copper”, Haiping Xu, Dominic Rebollar, Haiying He, Lina Chong, Yuzi Liu, Cong Liu, Cheng-Jun Sun, Tao Li, John V. Muntean, Randall E. Winans, Di-Jia Liu and Tao Xu, 2020
Nature Energy, 5, 623–632

[3] “Modulating CO2 electrocatalytic conversion to organics pathway by the catalytic site dimension”, Haiping Xu, Haiying He, Jianxin Wang, Inhui Hwang, Yuzi Liu, Chengjun Sun, Tao Li, John V. Muntean, Tao Xu, Di-Jia Liu,* J. Am. Chem. Soc.
2024, 146, 10357−10366