Kinetics and the potential well in electrochemical hydrogen evolution by [Co(4-tolyl-tpy)2]2+

Abstract

The ligand 4-tolyl-tpy [4-tolyl-tpy = 2,6-di(pyridin-2-yl)-4-p-tolylpyridine] and the corresponding cobalt complexes like [Co(4-tolyl-tpy)2]Cl2 and [Co(4-tolyl-tpy)2]Br3 has been synthesized and characterized by different spectroscopic and electrochemical methods. The electrochemical studies were carried out in 95:5 (v/v) DMF/H2O using acetic acid (AcOH) as a proton source. The faradaic efficiency found to be f = (86 ± 4) % using potential range −1.4 V to −1.7 V vs. SCE for a period of 1 h. The ic/ip value found to be 57.9 for a scan rate of 100 mV s−1, and TOF was calculated to be 652 s−1. The electrocatalytic H2 production by [Co(4-tolyl-tpy)2]2+ complex was also employed in phosphate buffer pH 7, and it shows the Faradaic efficiency of (88 ± 4%) within the overpotential range from −1.2 V to −1.5 V vs. SCE. The TOF of the reaction was found to be 3900 mmol of H2 per mole of catalyst per hour. The compare of electrochemical proton reduction, as well as water reduction by [Co(4-tolyl-tpy)2]2+ with the basic scaffold [Co(tpy)2]2+, reveals that it shows higher catalytic activity and have lower overpotential of 150 mV for proton reduction and 100 mV for water reduction. Various kinetics and spectroscopic studies are utilized to predict the mechanism for proton reduction. The kinetic and mechanistic studies are depicted based on changes in absorbance with the addition of Co(Cp)2, followed by AcOH. The intermediates involved in the kinetic process was analysed through global fitting and SVD analysis. The formation of [CoI(L2)] → [CoIII(H)(L)(2L)] and [CoIII(L)(2L)S] → [CoII(L)2] are faster reactions in comparison to the formation of Co(III)-Hδ-⋅⋅⋅Hδ+ to evolve hydrogen (Where 2L is bidentate 4-toly-tpy and S = solvent). The pK value was calculated to be 16.5 for Co(III)–H species in DMF generated from CoI species i.e. CoI + H+ ⇆ Co(III)–H.