Catalytic electrodes for the oxygen reduction reaction based on co-doped (B-N, Si-N, S-N) carbon quantum dots and anion exchange ionomer

Abstract

B-N-, Si-N, and S-N-co-doped carbon quantum dots (CQD) were prepared by hydrothermal synthesis from inexpensive and non-toxic precursors. They were analysed by Raman, FTIR, and XPS spectroscopies and used as oxygen reduction reaction (ORR) electrocatalysts. The electrodes were prepared by drop-casting a slurry containing poly(sulfone trimethylammonium) hydroxide (PSU-TMA) to enhance the hydroxide-ion transport near the catalytically active centers. Furthermore, polyaniline (PANI), reported to be catalytically active for the ORR, was added to some electrodes. A similar trend was observed for the electrode capacitance measured by cyclovoltammetry in the non-Faradaic region and impedance spectroscopy. The ORR kinetics studied by linear sweep voltammetry showed the lowest onset and half-wave potentials and the lowest Tafel slopes for the B-N-co-doped samples, although the B concentration is low (0.5%). Data with and without PANI are quite similar. The S-N- and Si-N co-doped samples are slightly less efficient. The stability test demonstrated a decrease by about 10% of the diffusion-limited current after 500 cycles, indicating a microstructural change impacting the mass transport conditions, but the onset potential is not modified. Altogether, the good electrocatalytic activity of B-N-co-doped CQD with a low boron concentration in presence of PSU-TMA indicated the high potential of these electrodes. The inexpensive synthesis of co-doped CQD from non-toxic precursors is an important advantage of these materials vs other carbon-doped electrocatalysts.