Nanoconfinement and Mass Transport in Hollow Structured Pt-Rh Electrocatalysts Towards Efficient and Durable Ethanol Electrooxidation

A major challenge of ethanol utilization in fuel cells relates to full oxidation of ethanol to CO₂, which comprises a 12-electron transfer reaction, with the obtained extent of this reaction at room temperature significantly lower due to unique thermodynamic and kinetic challenges of ethanol’s C-C bond cleavage step. Consequently, in practice, the Faradaic efficiency (FE) of full oxidation of ethanol to CO₂ is often limited to &lt;20%, with the majority of byproducts being C2 intermediates in which C-C bonds persist: acetaldehyde (H₃C-COH) or acetic acid (H₃C-COOH), which generate only 2 and 4 electrons rather than 12 electrons.<br/>In the ethanol oxidation reaction (EOR) mechanism, the surface coverage of OH functional groups affects reaction selectivity of the C-C bond cleavage step owing to competitive adsorption on the active sites between OH functional groups and each of the two carbons in ethanol for successful C-C bond splitting. While the optimized surface coverage of OH for high EOR selectivity can be achieved by reducing the pH of the alkaline electrolyte, the low concentration of OH^- in bulk solution may lead to undesired decreases in ionic conductivity. Therefore, it is compelling to examine methodologies that allow for decoupling of near-surface vs. bulk electrolyte composition and properties. Hence, to manipulate the surface coverage of OH without modulating the pH of bulk solution, we exploited the high generation rate of H⁺ released during EOR, which can create a local acidic environment near the electrocatalyst surface.<br/>In this study, the critical role of the electrode architecture affecting the EOR selectivity was investigated by controlling the extent of local pH swing manipulated by the different degree of electrode porosity that governs the mass transport of OH^- near the electrocatalyst surface. To modulate the degree of electrode porosity, binary Pt_1-xRh_x electrocatalysts were synthesized into hollow sphere morphologies with different particle sizes (250 and 350 nm) and compositions (x=0.0, 0.4, 0.5, 0.6, and 0.66). The FE of full oxidation of ethanol to CO₂ was systematically compared in terms of different particle size and mass loading of hollow spheres in the electrodes by quantifying the EOR products using NMR and gas chromatography. The higher FE of CO₂ could be achieved by a more acidic environment of the more porous electrode structure with a smaller particle size and higher mass loading of electrocatalysts, suggesting the potential to control the EOR outcomes via electrode structural engineering.