The Role of Undercoordinated Sites on Zinc Electrodes for CO₂ Reduction to CO

Abstract

The electrochemical CO2 reduction reaction (CO2RR) using renewable energies is a promising route toward global carbon neutrality. Recently, the use of copper catalysts and CO feedstocks, instead of CO2, has been shown to enhance the selectivity toward multicarbon products, leading to increased efforts in developing tandem electrocatalytic systems. State‐of‐the‐art CO2‐to‐CO electrocatalysts are mainly based on noble metals such as silver and gold. Earth‐abundant zinc, in contrast, displays poorer selectivity and activity. Herein, the use of porous dendritic oxidederived zinc (OD‐Zn) catalysts for CO2RR is reported. These catalysts can reduce CO2 to CO with a maximum Faradaic efficiency of 86% at −0.95 V versus reversible hydrogen electrode (RHE) and partial current density of −266 mA cm–2 at −1.00 V vs RHE. OD‐Zn is further found to have a higher amount of undercoordinated sites and exhibits higher CO2RR activity and CO selectivity than electrodeposited Zn metal. While oxygen vacancies have been previously implicated as active sites, detailed experiments and density functional theory calculations show that Zn sites with a high degree of undercoordination provide even higher activity, in view of their nearly optimal *COOH adsorption energies. These findings showcase Zn‐Oderived particles with plentiful undercoordinated sites as cost‐effective electrocatalysts for CO production.

Enhanced Charge Transfer Kinetics for the Electroreduction of Carbon Dioxide on Silver Electrodes Functionalized with Cationic Surfactants

“…We note that Zn D lies slightly out of the grey band in Figure 6, while Ag D falls outside the grey band in the inset. The former deviation is rationalized based on a previous work, which found that the most active Zn sites for CO 2 RR to CO are less coordinated than step-edge sites [9] and bind *COOH more strongly. The latter deviation is explained by considering that Ag defects are the most active sites for both CO and HCOO − production, but tend to be CO-selective.…”

“…To rationalize the effects, we will consider that the experimental current densities on all substrates were contributed by the closest-packed surfaces ((111) terraces for Ag and Au, (0001) terraces for Zn) and defects (step edges at Ag(211), Au(110), and Zn(105), see Section S3 (Supporting Information) and previous works [9,41,43] for justifications on these choices). For each substrate, the total current density is: i total i T i D j j j = + , where T and D stand for terraces and defects, and i = CO and HCOO − .…”

From Single Crystal to Single Atom Catalysts: Structural Factors Influencing the Performance of Metal Catalysts for CO2 Electroreduction

“…Stepped Au surfaces (e.g., Au(211) and Au(110)) can better stabilize the *COOH intermediate and are thus more active than the Au(100) and Au(111) facets . Similarly, the *COOH intermediate binding strength inversely scales with the coordination number of Zn surfaces explaining faster CO evolution on undercoordinated Zn . Note that strong evidence exists that, for the production of CO on Au and Ag, the rate limiting step is the formation of *CO 2 – and that the proton–electron transfer to form *COOH is actually decoupled .…”

“…72 Similarly, the *COOH intermediate binding strength inversely scales with the coordination number of Zn surfaces explaining faster CO evolution on undercoordinated Zn. 73 Note that strong evidence exists that, for the production of CO on Au and Ag, the rate limiting step is the formation of *CO 2 − and that the proton−electron transfer to form *COOH is actually decoupled. 74 However, modeling a decoupled proton−electron transfer step is challenging, and since the following formation of *COOH from *CO 2 − is anyhow a nonelectrochemical step and fast, *COOH is often used as a descriptor to predict relative rates instead.…”

Gaseous CO2 Electrolysis: Progress, Challenges, and Prospects

“…Transition metal-based electrocatalysts, such as metal alloys, single atoms, and compounds, also produce gaseous products during CO 2 electrolysis. In addition, Zn-based electrocatalysts generally produced CO. − Zuttel’s group fabricated porous Zn (P-Zn) on a Cu mesh via an electrodeposition method and evaluated the performance of the gaseous flow cell for CO production . During CO 2 electrolysis, the porous structure of P-Zn on the Cu mesh induced a high local pH, leading to HER suppression because of the increase in OH – concentration near the catalyst surface.…”