Evaluation of carbon-supported palladium electrocatalysts for the borohydride oxidation reaction in conditions relevant to fuel cell operation

Abstract

Direct borohydride fuel cells (DBFCs) take advantage of sodium borohydride (NaBH4) fuel to exhibit high theoretical energy density and a high theoretical cell voltage. Despite these promises, the borohydride electrooxidation reaction (BOR) at the anode is highly complex, has numerous byproducts, and remains poorly understood (and even poorly studied) on many electrocatalysts (in practice, only the behavior of Pt and Au is well characterized), especially at high NaBH4 concentrations and high temperature, i.e. in typical DBFC operating conditions. We study here how high fuel concentration and high temperature impact the BOR on carbon-supported Pd-based electrocatalysts. Four carbon-supported palladium electrocatalysts (Pd/C) with Pd weight fractions of 22–53% are studied for the BOR by cyclic voltammetry with a rotating disk electrode for three NaBH4 concentrations (5, 50 and 500 mM) in 1 M NaOH at T = 60 °C. To ensure accurate comparison of the electrocatalyst BOR activities, the electrochemical surface area of the Pd nanoparticles (NPs) are characterized by palladium oxide reduction, CO-adsorption and transmission electron microscopy. BOR activity markers are proposed to compare the various electrode materials, based on the activity obtained at three electrode potentials (0, 0.2 and 0.5 V vs. the reversible hydrogen electrode (RHE)) and enable clear comparison between them and state-of-the-art electrocatalyst Pt/C. In conditions close to DBFC operating conditions, the BOR can start at very low potentials on Pd/C electrocatalysts (−0.23 V vs. RHE), but the BOR kinetics is very slow for 5 and 50 mM NaBH4 in comparison to Pt/C, owing, at least in part, to poisoning of the catalytic surface by byproducts of the BOR.