Impact of 1 mmol dm−3 concentrations of small molecules containing nitrogen-based cationic groups on the oxygen reduction reaction on polycrystalline platinum in aqueous KOH (1 mol dm−3)

Abstract

Alkaline anion-exchange membranes (AAEMs) containing cationic head-groups (e.g. involving quaternary ammonium and imidazolium groups) are of interest with regard to application in alkaline polymer electrolyte fuel cells (APEFCs). This initial ex situ study evaluated the effect of 1 mmol dm⁻³ concentrations of model molecules containing (AAEM-relevant) cationic groups on the oxygen reduction reaction on a polycrystalline platinum disk (Pt_pc) electrode in aqueous KOH (1 mol dm⁻³). The cationic molecules studied were tetramethylammonium (TMA), benzyltrimethylammonium (BTMA), 1-benzyl-3-methylimidazolium (BMI), 1-benzyl-4-aza-1-azoniabicyclo[2.2.2]octane (BAABCO) and 6-(benzyloxy)-N,N,N-trimethylhexan-1-aminium (BOTMHA). Both cyclic and hydrodynamic linear sweep rotating disk electrode voltammetry techniques were used. The resulting voltammograms, derived estimates of apparent electrochemically active surface areas, Tafel slopes, apparent exchange-current densities and the number of electrons transferred (per O₂ molecule) were compared. The results strongly suggest that 1 mmol dm⁻³ concentrations of BTMA, BAABCO, and (especially) BMI seriously inhibit the catalytic activities of Pt_pc in an aqueous KOH electrolyte at 25 °C. The negative influence of (benzene-ring-free) TMA and Cl⁻ anions (KCl control experiment) appeared to be less severe. The separation of the trimethylammonium group from the benzene ring via a hexyloxy spacer chain (in BOTMHA) also produced a milder negative effect.