Synthetic Generality of Atomically Dispersed Pt Catalysts and Their Electrochemical Kinetics for the Chlorine Evolution Reaction

Abstract

Electrochemical chlorine evolution reaction (CER) is a major anodic reaction for Cl2 production or on-site HClO generation. Although Ru-based mixed metal oxides have been used as commercial CER catalysts for the past half century, their oxide active sites also catalyze a parasitic oxygen evolution reaction (OER). Recently, we have developed a new class of CER catalysts based on atomically dispersed Pt–N4 sites on carbon nanotubes (Pt1/CNT). In this work, we demonstrated the synthetic generality of Pt–N4 sites for catalyzing CER and identified potential-dependent kinetics of Pt–N4 sites for CER. We prepared two types of Pt1/CNT catalysts using H2PtCl6∙6H2O and ionic liquid via a bottom-up approach [Pt1/CNT(Cl)] and using Pt-porphyrin via a top-down approach [Pt1/CNT(P)]. Both the two Pt1/CNT catalysts comprised Pt–N4 active sites and exhibited similarly high CER activity (10 mA cm–2 @ 1.4 V vs. RHE), and ~100 % CER selectivity in OER-predominating 0.1 M NaCl electrolyte (pH 1), demonstrating the general nature of Pt–N4 as an effective catalytic site for CER. The analyses of Tafel slope, reaction order, and in situ X-ray absorption near edge structure (XANES) revealed pre-adsorbed Cl– before equilibrium potential of CER (1.36 V vs. RHE), indicating a unique CER behavior that commenced with Pt–Cl species. At low overpotential, the Pt1/CNT catalysts showed a reaction order (R) of ~1.8 and an average oxidation number (Ox.) of ~2.8 for CER evidencing the dominance of Volmer adsorption as the rate-determining step (RDS), which was clearly distinguished from the oxide-based CER catalysts (R ~ 1). As overpotential increases, CER over Pt1/CNT catalysts proceeds with decreasing reaction order and Ox. of ~3.9, indicating a switch to the Heyrovský discharge as the RDS.