Oxygen evolution reaction kinetics and mechanisms on pristine carbon nanotubes: Effect of pH

Carbon nanotubes (CNTs), which exhibit stable surfaces under acidic, neutral and alkaline solutions, provide an ideal platform for studying the nature of oxygen evolution reaction (OER) mechanisms at different pH. Here, OER on four types of pristine carbon nanotubes (CNTs) with walls from single-walled, double-walled, three-walled to multi-walled were studied in acid, neutral and alkaline conditions. Mechanism and kinetics study reveals that the OER on all CNTs is constrained by the water deprotonation in acid and neutral conditions,resulting in high Tafel slopes close to 240 mV dec- 1, high overpotentials (0.7-1 V), and approaching zeroth-reaction-order to H+/OH-. The kinetics and mechanism shift at pH -9-10 due to competition discharge between H2O and OH-. The Tafel slopes decrease, and the reaction orders increase with the increase of OH- concentration. The higher kinetics and enhanced activities for CNTs with 2-4 walls support the proposed tunneling effect, highlighting the favourable electron transfer pathway between the outer wall and inner tubes. The finding will provide a new direction for designing highly efficient OER catalysts.