Electrocatalytic hydrogen evolution by a copper(II) complex: influence of pH and mechanistic insight via density functional theory
摘要
In this study, we report the synthesis and electrocatalytic proton reduction activity of a copper(II) complex, [Cu(HL)]ClO4, 1, formed using a redox-active N, O-donor Schiff base ligand (H2L). The complex was thoroughly characterized by FTIR, 1H NMR, ESI–MS, UV–Vis, and EPR spectroscopy, confirming successful coordination to the CuII center. Electrochemical studies in phosphate buffer solution (PBS) at pH 2.5 revealed quasi-reversible CuII/CuI and CuI/Cu0 redox couples, along with a ligand-centered oxidation process. The electrocatalytic hydrogen evolution reaction (HER) activity of 1 was evaluated in PBS at pH 2.5, 4.6, and 7. The catalyst showed efficient proton reduction with onset overpotentials of 892 mV (pH 2.5), 678 mV (pH 4.6), and 517 mV (pH 7). Bulk electrolysis at − 1.5 V for 130 min at pH 2.5 produced 95 C of charge, equivalent to a theoretical H2 gas generation of 2.62 × 10–4 mol. At pH 2.5, the observed current density is 8.15 mA cm−2 at scan rate 100 mV s−1, with icat/ip ratio ~57. The observed rate constant (kobs) for HER was 1.93 × 103 s−1 and 5.6 × 102 s−1 at pH 2.5 and 4.6, indicating that kobs value is highly dependent on the availability of proton sources. Mechanistic studies revealed a pH-dependent behavior: at pH 4.6 and 7, catalysis proceeds via a CuI intermediate; at pH 2.5, Cu0 involvement indicates a mechanistic shift. UV–Vis, FTIR, and DFT analyses confirmed structural stability and supported the proposed catalytic pathway, demonstrating 1 as a robust and efficient HER catalyst in aqueous media.