水酸化における安定な均一系電触媒としてのCo-トリス配位子錯体

Development of a true molecular homogeneous catalyst for electrochemical water oxidation without its decomposition into a metal-oxide-based heterogeneous catalyst is truly challenging. The commercially available, low-cost, and non-noble metal salt CoCl₂·6H₂O is well known for electrochemical water oxidation for a limited time by generating CoO_xin situ; it acts as a pre-catalyst and not a true molecular catalyst for electrocatalytic water oxidation. Recently, we have reported a preliminary result of effective complexation of the TrisH₃ (tris(hydroxymethyl)aminomethane) ligand with Co(II) in an aqueous solution of CoCl₂·6H₂O and NaOAc at pH 8.6, leading to the formation of a Co(II)-coordination complex, [Co^II(TrisH₂)(TrisH₃)(CH₃COO)(H₂O)] (CoT). In the present report, we provide strong evidence that the Co(II)-coordination complex (CoT) acts as a true molecular homogeneous electrocatalyst for water oxidation, by suppressing the formation of CoO_x during the electrolysis. The absence of electrodeposited material (i.e., CoO_x) during electrolysis was confirmed using multiple characterization techniques, including FESEM, EDX, XPS, PXRD, IR, Raman, and UV-visible diffuse reflectance spectroscopy. The electrochemical stability and solution homogeneity of the CoT complex were evaluated using a combination of electrochemical, and spectroscopic techniques, including ¹H NMR, ¹³C NMR, HRMS, and UV-visible spectroscopy. The electrocatalytic water oxidation performance of the CoT complex attains a low onset overpotential value of 461 mV with a faradaic efficiency value of 94.3% (±4.7%).