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)

Ai Lien Ong,John R Varcoe,Michael L Fox,Daniel K Whelligan

doi:10.1039/c3cp50556a

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 (Ptpc) 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 Ptpc 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.

Highlights

Alkaline anion-exchange membranes (AAEMs) are rapid developing core components in alkaline polymer electrolyte fuel cell (APEFC) technologies due to the perceived advantages of: (a) inhibiting CO2-derived precipitate formation during fuel cell operation; (b) the ability to use cheaper electrocatalysts; (c) improved oxygen reduction reaction (ORR) electrokinetics at the cathode;[1] (d) potential for minimised fuel cross-over; and (e) novel water management possibilities.[2]
The effect of 1 mmol dmÀ3 concentrations of KCl, TMA, BMI, BTMA, BAABCO, and BOTMHA in the aqueous KOH (1 mol dmÀ3) electrolyte on the electrochemical behaviour of the bare Ptpc disk electrode was initially studied using cyclic voltammetry: the results are presented in Fig. 2 and the raw CV data (n = 3 replicates) can be found in Fig. ESI1 (ESI†)
The anodic currents increased at potentials >+1.0 V vs. Reversible Hydrogen Electrodes (RHE) [above the levels for Pt-oxidation as measured in additive-free aqueous KOH (1 mol dmÀ3) solutions] in the CVs of the BMI- and BOTMHA-containing electrolytes, with even higher currents with the BTMA- and BAABCO-containing electrolytes

Summary

Introduction

Alkaline anion-exchange membranes (AAEMs) are rapid developing core components in alkaline polymer electrolyte fuel cell (APEFC) technologies due to the perceived advantages of: (a) inhibiting CO2-derived precipitate formation during fuel cell operation; (b) the ability to use cheaper electrocatalysts; (c) improved oxygen reduction reaction (ORR) electrokinetics at the cathode ( hydrogen oxidation reaction [HOR] electrokinetics at the anode are poorer at high pH);[1] (d) potential for minimised fuel cross-over (in non-H2 fuel cells); and (e) novel water management possibilities.[2]. The effect of 1 mmol dmÀ3 concentrations of KCl, TMA, BMI, BTMA, BAABCO, and BOTMHA in the aqueous KOH (1 mol dmÀ3) electrolyte on the electrochemical behaviour of the bare Ptpc disk electrode was initially studied using cyclic voltammetry: the results are presented in Fig. 2 and the raw CV data (n = 3 replicates) can be found in Fig. ESI1 (ESI†).

Results

Conclusion