Electrode surface embedded manganese(iii)–pincer complexes: efficient electrocatalysts for the oxygen evolution reaction

Abstract

Highly active, robust, cheap, and environmentally benign electrocatalysts for the oxygen evolution reaction (OER) are critically important to develop a sustainable artificial energy cycle. Here, we report the in situ synthesis of mononuclear manganese(III) pincer complexes [Mn(OAc)(L)(cbpy)] (1) and [Mn(OAc)(L₁)(cbpy)] (2) on a gold electrode surface through a self-assembly process (where OAc = acetate, cbpy = 2,2′-bipyridine-4-carboxylic acid anion, L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate (2-) and L₁ = N,N′-(diphenyl)-2,6-pyridinedicarboxamidate (2-)) and explore their activity for the electrocatalytic OER (2H₂O = 4H⁺ + 4e⁻ + O₂) in neutral pH medium. The modified electrode was characterized by using microscopic (FE-SEM), spectroscopic (EDX, ATR-FTIR, SERS), and electrochemical (CV and EIS) methods. The newly developed heterogenized molecular catalysts 1 and 2 can catalyze the OER exceptionally well with a low overpotential of 260 mV at J = 3.2 mA cm⁻² and 275 mV at J = 2.46 mA cm⁻², respectively. The Tafel slopes were found to be 68 and 77 mV decade⁻¹ for the catalysts 1 and 2, respectively. These results support the higher OER activity of complex 1 than complex 2 under similar conditions and this may be due to more electron donating power of pincer ligand L than L₁, which in turn stabilizes the high valent manganese complex intermediates. Furthermore, after long-term controlled potential electrolysis (∼ 6 h), the surface-immobilized manganese complex does not decompose into manganese oxide and preserves the molecular identity as evident from electrochemical, spectroscopic and microscopic results.