High Index Non-Noble Metal Electrocatalysts for Electrochemical CO2 Reduction to C1 Products
Authors: Won Da Hye , Woo Seong Ihl , Kim Hyungjun , Hwang Yun Jeong , Min Byoung Koun 
Published: January 2022

Accumulation of CO2 in the atmosphere triggers abnormal weather phenomenon through global warming and greenhouse effect. To reduce the CO2 concentration and its emission, extensive researches for CO2 capture, storage, and utilization have been conducted. Among them, electrochemical CO2 conversion is highly promising due to its ambient reaction conditions, high energy efficiency, and facile combination with other renewable energy source. However, because CO2 is chemically stable, the principal obstacle is to develop suitable catalyst having good catalytic activity, selectivity, and stability. Among various metal candidates, transition and post-transition metals have attracted much attention due to its low-cost, low-toxicity, and intrinsic catalytic property for CO2 reduction. Since general electrochemical catalysis is highly correlated with mass diffusion, crystal orientation, surface area, and conductivity, manipulating the catalyst structure is an effective method to improve catalytic performance. Furthermore, collaborative studies integrating experimental and theoretical approaches have been conducted for the investigation of CO2 reduction mechanism and suggestion of a rational design of a CO2 reduction catalyst.
Herein, we developed hierarchical nanostructured Sn, Bi, and Zn electrodes as electrocatalysts for CO2 reduction to C1 products (e.g., formic acid/formate and carbon monoxide). Various structured catalysts such as Sn dendrite, Bi dendrite, and hexagonal Zn were fabricated by facile electrodeposition methods. The synthesized catalyst electrodes showed highly efficient CO2 reduction activity in terms of current density, Faradaic efficiency, and more importantly, stable performance during long-term operation. Sn dendrite and Bi dendrite electrodes exhibited a superior formate/formic acid production rates (Sn dendrite: 228.6 mmol h-1 cm-2 at -1.36 VRHE) and high Faradaic efficiency (Bi dendrite: 90% at -0.73 VRHE) without any considerable catalytic degradation during 18 h and 12 h of long-term operations, respectively. Furthermore, the hexagonal Zn catalyst showed a high CO selectivity up to ~95% during unprecedented long-time over 30 h. It is worth noted that their high selectivity towards CO2 reduction is attributed to their local (or chemical) structures. In case of Sn electrode, we found that the native O content on the Sn surface is strongly correlated with the stabilization of reaction intermediate and the formate selectivity. To understand in-depth the factors to affect the CO2 reduction, we further conducted the theoretical studies about the mechanism of CO2 conversion to formic acid on various Bi planes such as close-packed and high-index surfaces using density functional theory calculation (DFT). We demonstrated that the most energetically favorable pathway was a path through the formation of oxygen bidentate intermediate (*OCOH) among the three possible pathways for formic acid formation. In addition, it was also revealed that the high-index Bi surfaces exhibited the lower reduction potential than the closed-packed surface of (003) plane. Similarly, in electrochemical analysis using Zn electrodes, it was figured out that Zn (101) facet was favorable to CO formation whereas Zn (002) facet, most stable surface, favors the H2 evolution during CO2 electrolysis. Indeed, DFT calculations showed that Zn (101) facet lowers the reduction potential for CO2 to CO by more effectively stabilizing a *COOH intermediate than Zn (002) facet. Consequently, the coordinately unsaturated sites derived from the nanostructured non-noble metal catalysts can effectively stabilize the reaction intermediate by lowering the energy barrier for its binding to the site. These results may suggest a design principle for further developments in other advanced catalysts as well as in CO2 reduction.

Figure 1