Decoding CO ₂ Reduction on Bismuth–Tin Bimetallic Catalyst: Insights from Rotating Disk Electrode and Microelectrode Studies

The electrodics of bismuth-tin (Bi-Sn) bimetallic alloy has been investigated toward the electrocatalytic reduction of CO₂ to formate (CO₂RR). Bi-Sn with varied compositions and morphologies is electrochemically deposited on a gold-substrate, including a gold rotating disk electrode (RDE) and gold ultramicroelectrode (UME). The powder X-ray diffraction and differential thermal calorimetry analyses confirmed the formation of a Bi-Sn bimetallic alloy with traces of their corresponding oxides. X-ray photoelectron spectroscopy analysis further corroborates the presence of bismuth oxide on the catalyst surface. The primary investigation of the electrocatalytic performance of Bi-Sn alloy toward CO₂RR has been done using cyclic and linear sweep voltammetry. The mechanistic aspects of charge transfer have been obtained from the Tafel analysis of the RDE and UME data. The extracted Tafel slope, transfer coefficient (α) and standard rate constant (k⁰) underline the two-electron transfer pathway involving the formation of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>C</mml:mi> <mml:msubsup><mml:mi>O</mml:mi> <mml:mn>2</mml:mn> <mml:mrow><mml:mo>.</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:mrow> <mml:annotation>$\text{C} \text{O}_{2}^{. -}$</mml:annotation></mml:semantics> </mml:math> anion radical as a key intermediate. The nuclear magnetic resonance spectroscopy of the electrolysis products confirms formate as the major reduction product, with a faradaic efficiency (FE) of 85.3 %. The high FE toward CO₂RR is attributed to the synergistic interaction between Bi and Sn within the porous alloy framework.