Ag Dendrites on W/C as Enhanced Active and Stable Electrocatalysts for Scalable Solar-Driven CO2rr
Authors: Oh Hyung-Suk , Lim Chulwan 
Published: November 2022

The electrochemical conversion of solar energy into competent chemicals is the most efficient technique to ensure clean and sustainable energy source for society in the future. In fact, solar cells can electrochemically produce electricity without pollution by converting CO2 into a variety of chemicals. The carbon dioxide reduction reaction (CO2RR) is an environmentally friendly approach to produce useful hydrocarbons such as carbon monoxide (CO), methane, ethylene, formates, etc and alcohols and remove exhausted CO2, which is a greenhouse gas. Conventionally, studies on the CO2RR have been conducted by using liquid-phase H-cells and a CO2-saturated electrolyte. However, it resulted in a low current density because the low solubility of CO2 in the liquid electrolyte occurs mass transfer limitation. To improve of the current density related to the CO2RR, two types of gas-fed CO2RR electrolyzers containing gas diffusion layer (GDL) electrodes have been proposed. The first type of CO2RR devices use a liquid electrolyte, the electrolyte increases CO2RR performance through controlling the cathode conditions, such as pH and anion concentration. The second type of CO2RR devices is zero-gap electrolyzers that use an anion exchange membrane (AEM) and humidified CO2 gas. AEM transmits carbonate and hydroxide ion, inducing favorable environmental for CO2RR. These zero-gap CO2 electrolyzers are attracting attention as the most promising system owing to several advantages such as extremely low ohmic resistance, scalable and stackable configuration, and commercial applicability, as affirmed by using a system consisting of a fuel cell and water electrolyzer.
Among the products of the CO2RR, CO is particularly attractive as aspect of its economic benefits and large demand. Ag-based materials exhibit the most electrochemical performance for producing CO. and high selectivity. To enhance the catalytic activity of Ag-based catalysts, various approaches have been studied to change the surface electronic structure of Ag such as alloy formation, anion-based modification, shape control, near-surface structure and engineering. However, catalyst designs that do not consider the gas phase reactions cannot enough utilize their catalytic activity although recent progress has made in the development of Ag-based catalysts for the CO2RR. Therefore, considerable efforts must be preoccupied with the development of highly active, stable, and gas-transferable structured catalysts for gas-fed CO2RR electrolyzers with incoporating a GDL.
Additionally, aming for the realization of clean technology for producing CO, we demonstrate the practicalbility of sunlight-driven CO2RR on a large scale using commercial silicon-based solar cells and zero-gap electrolyzers. Silicon-based solar cells are still the preferred commercially applicapable options for producing large amounts of electrical power from solar energy because of their scalability. In this study, we report a 3D silver dendrite on W/C (as denoted WC@AgD) catalyst with abundant nanograin boundaries that show enhanced CO2RR performance and stability. WC@AgD exhibited marked catalytic activity with a maximum CO partial current density of 400 mA cm-2 and durability for 100 h at 150 mA cm-2. We also fabricated an solar-to-CO (STC) conversion device combined with a silicon-based solar cell of 120 cm2 area and a zero-gap CO2 electrolyzer with an area of 10 cm2. The stand-alone photovoltaic-electrochemical system achieved a solar-to-CO efficiency (ηSTC) of 12.1 % at 1A under AM 1.5 G illumination and realistic outdoor conditions. The device design and electrode configuration extended viable route for implementing large-scale installations for solar-driven production of chemicals.

Figure 1