Investigation of Pd and Pd-CeO2 Based Carbon-Supported Electrocatalysts for the Electrooxidation of Borohydride in Direct Borohydride Fuel Cell

Abstract

Direct borohydride fuel cells (DBFC) are considered high-energy density generators. The ideal case for the anodic reaction of a DBFC is the direct and complete borohydride oxidation reaction (BOR) that releases 8 electrons. However, the BOR is a very complex reaction that involves numerous intermediate species and suffers from competition with the heterogeneous hydrolysis of BH4 - that leads to molecular hydrogen, making full utilization of the fuel challenging [1]. Despite many studies of the BOR on several electrocatalysts (especially Au and Pt), this reaction is still poorly understood on most practical electrocatalysts and in relevant conditions to DBFC operation, and no ideal electrocatalyst fulfilling both high activity and high faradaic efficiency at low potential has been isolated, yet. In this study, the BOR is investigated on Pd-based carbon-supported electrocatalysts. A set of four Pd/C electrocatalysts have been synthesized using Vulcan XC72 as substrate and loaded at 22, 33, 44 and 53 wt% Pd. These materials have been thoroughly characterized in terms of BOR activity in a rotating disk electrode setup. The experiments were performed for three NaBH4 concentrations (5, 50 and 500 mM) at 60°C. The electrochemical characterizations of Figure 1 (A) reveal that the onset potential (E i=0) (see Figure 1 (B)) is lower than 0 V vs. RHE on all the Pd electrocatalysts, meaning that BH4 - anions can be valorized directly, albeit with slow kinetics. In contrast, Pt/C first decomposes BH4 - and utilizes hydrogen species, above 0 V vs. RHE. However, the kinetic is very slow. Similar experiments have been performed with advanced Pd electrocatalysts with a composite support made of Vulcan XC72 carbon and cerium oxide, which enhance hydrogen oxidation reaction (HOR) kinetics in alkaline media [2]. Figure 1