Carbon neutral electrochemical conversion of carbon dioxide mediated by [Mn+(cyclam)Cln] (M = Ni2+ and Co3+) on mercury free electrodes and ionic liquids as reaction media

Abstract

In this work, the electrochemical reduction of carbon dioxide using [Mⁿ⁺(cyclam)Cl_n] (M = Ni²⁺ and Co³⁺) as electrocatalysts has been studied in 1-butyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMImBF₄ and BMImNTf₂ respectively) ionic liquids as reaction media. Complexes were characterized electrochemically in these salts and relevant parameters, such as a heterogeneous electron transfer rate was calculated. Results indicate a faster M(II)/M(I) redox process in BMImBF₄ despite its higher viscosity compared to BMImNTF₂. Cyclic voltammetry experiments demonstrated that [Ni(cyclam)Cl₂] is the most active macrocycle, towards the reaction under survey. For this compound, potential controlled electrolysis was carried out at−1.4 V vs. Ag/AgCl in BMImBF₄ as a solvent, yielding only CO as a reaction product, with a turnover frequency (TOF) of 0.73 h⁻¹. NMR spectra for the ionic liquids, after electrolysis, show that the applied potential does not affect the chemical structure of the salts. FT-IR thin layer spectroelectrochemical experiments in CO₂ saturated solution in BMImBF₄ at −1.4 V vs. Ag/AgCl show the formation of [Ni(cyclam)CO]⁺ as precursor species, and also the most stable [Ni(CO)₄].