Importance of Site Diversity and Connectivity in Electrochemical CO Reduction on Cu

Chansol Kim; Nitish Govindarajan; Sydney Hemenway; Junho Park; Anya Zoraster; Calton J Kong; Rajiv Ramanujam Prabhakar; Joel B Varley; Hee-Tae Jung; Christopher Hahn; Joel W Ager

doi:10.1021/acscatal.3c05904

Importance of Site Diversity and Connectivity in Electrochemical CO Reduction on Cu

ACS Catal. 2024 Feb 14;14(5):3128-3138. doi: 10.1021/acscatal.3c05904. eCollection 2024 Mar 1.

Authors

Chansol Kim^{1

2

3}, Nitish Govindarajan⁴, Sydney Hemenway^{1

5}, Junho Park^{1

5}, Anya Zoraster^{1

6}, Calton J Kong^{1

5}, Rajiv Ramanujam Prabhakar¹, Joel B Varley⁴, Hee-Tae Jung², Christopher Hahn⁴, Joel W Ager^{1

5

7}

Affiliations

¹ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
² Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, South Korea.
³ Clean Energy Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, South Korea.
⁴ Materials Science Division, Lawrence Livermore National Laboratory, Livermore, California 94550, United States.
⁵ Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
⁶ Department of Chemical and Biochemical Engineering, University of California, Berkeley, Berkeley, California 94720, United States.
⁷ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.

Abstract

Electrochemical CO₂ reduction on Cu is a promising approach to produce value-added chemicals using renewable feedstocks, yet various Cu preparations have led to differences in activity and selectivity toward single and multicarbon products. Here, we find, surprisingly, that the effective catalytic activity toward ethylene improves when there is a larger fraction of less active sites acting as reservoirs of *CO on the surface of Cu nanoparticle electrocatalysts. In an adaptation of chemical transient kinetics to electrocatalysis, we measure the dynamic response of a gas diffusion electrode (GDE) cell when the feed gas is abruptly switched between Ar (inert) and CO. When switching from Ar to CO, CO reduction (COR) begins promptly, but when switching from CO to Ar, COR can be maintained for several seconds (delay time) despite the absence of the CO reactant in the gas phase. A three-site microkinetic model captures the observed dynamic behavior and shows that Cu catalysts exhibiting delay times have a less active *CO reservoir that exhibits fast diffusion to active sites. The observed delay times and the estimated *CO reservoir sizes are affected by catalyst preparation, applied potential, and microenvironment (electrolyte cation identity, electrolyte pH, and CO partial pressure). Notably, we estimate that the *CO reservoir surface coverage can be as high as 88 ± 7% on oxide-derived Cu (OD-Cu) at high overpotentials (-1.52 V vs SHE) and this increases in reservoir coverage coincide with increased turnover frequencies to ethylene. We also estimate that *CO can travel substantial distances (up to 10s of nm) prior to desorption or reaction. It appears that active C-C coupling sites by themselves do not control selectivity to C₂₊ products in electrochemical COR; the supply of CO to those sites is also a crucial factor. More generally, the overall activity of Cu electrocatalysts cannot be approximated from linear combinations of individual site activities. Future designs must consider the diversity of the catalyst network and account for intersite transportation pathways.