Copper and silver gas diffusion electrodes performing CO2 reduction studied through operando X-ray absorption spectroscopy

Abstract

X-ray absorption spectroscopy (XAS) offers the unique possibility to study metal electrocatalysts such as silver and copper while they are performing electrochemical carbon dioxide (CO₂) reduction. In this work, we present an approach to perform operando XAS experiments on an electrochemical cell performing CO₂ reduction with a gas diffusion electrode (GDE) as cathode. The experimental set-up, advantages and drawbacks, XAS data analysis, and XAS theory are discussed. Results on copper and silver GDEs obtained through the presented procedures are then presented and discussed. Structural and compositional catalyst data acquired under operando conditions can help further density functional theory calculations, and catalytic, and systems studies on CO₂ reduction. Structural and compositional data including crystallite size were obtained while performing high current density (up to 200 mA cm⁻²) CO₂ reduction. On the silver catalysts at higher than 100 mA cm⁻² applied current density, a Ag–X contribution was found and is ascribed to Ag–O. For both silver and copper, the XAS experiments revealed that the crystallite size of the ex situ samples is smaller than the samples during CO₂ reduction. Furthermore, metal particle size polydispersity was found in the silver catalysts by comparing the obtained coordination numbers with theoretical values. The operando EXAFS data was of such high quality (k: 3–14 Å⁻¹) that four shells could be fitted. The value of combining ex situ material characterisation and electrocatalyst performance data with operando XAS experiments is discussed and found to be of great importance to further CO₂ reduction research.