Confinement Effects of Hollow Structured Pt-Rh Electrocatalysts toward Complete Ethanol Electrooxidation.

Abstract

In the anodic ethanol oxidation reaction (EOR) for direct ethanol fuel cells, the coverage of hydroxide (OH_ads) is a major adsorbent competing with C-C bond cleavage, which is necessary for complete ethanol oxidation (C1-pathway) and durability. Beyond utilizing a less-alkaline electrolyte that causes ohmic losses, an alternative strategy to optimize OH_ads coverage is to intentionally exploit local pH changes near the electrocatalyst surface that are governed by a combination of released H⁺ during EOR and OH^- mass transport from the bulk solution. Here, we manipulate the local pH swing by fine-tuning the electrode porosity with Pt_1-<i>x</i>Rh_<i>x</i> hollow sphere electrocatalysts based on particle size (250 and 350 nm) and mass loading. With the smaller size of 250 nm, Pt_0.5Rh_0.5 (∼50 μg cm^-2) shows a high activity of 1629 A g_PtRh^-1 (2488 A g_Pt^-1) in a 0.5 M KOH-containing electrolyte, which is ∼50% higher than the most active binary catalysts to date. Moreover, a higher C1-pathway Faradaic efficiency (FE) of 38.3% and 80% longer durability are achieved with a 2-fold increase in mass loading. In the more porous electrodes, a local acidic environment created by hindered OH^- mass transport better optimizes OH_ads coverage, providing more active sites for the desired C1-pathway and a continuous EOR.