A Co-tris coordination complex as a stable homogeneous electrocatalyst for aqueous water oxidation

Abstract

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%).