Photoelectro-catalytic oxidation of monoethanolamine: From the study of electrooxidation mechanism to the development of high-efficient Ni-based photoelectro-catalysts

Abstract

Monoethanolamine (MEA) electrooxidation occurring on Ni catalyst has proven to be a viable tactics for simultaneous MEA degradation and energy recovery. However, the excessively large overpotential (1505 mV) on Ni catalyst either causes high electricity consumption when acquiring H2 from MEA-containing electrolyzer or leads to low energy output when extracting electricity from direct MEA fuel cell. In this context, the reaction mechanism of MEA electrooxidation on Ni-based catalyst is deciphered at first by employing a specially designed three-step electrochemical procedure and in-situ Raman spectroscopy. Electrochemical data deconvolutions indicate the indirect electrochemical-chemical reaction path, via Ni(OH)2/NiOOH mediator, occupies ∼2.5 % of the total catalytic current density at 0.55 V vs. Ag/AgCl and the direct oxidation of MEA ranks the rest ∼97.5 %. To decrease the onset potential of MEA oxidation, a photoelectric-catalytic system including high-efficient Ni-based photoelectric-catalyst, where Ni is intimately coated on TiO2 by in-situ photodeposition to achieve the shortest carrier transport channel between Ni(OH)2 and TiO2, is developed. Results show the overpotential of MEA electrooxidation under illumination significantly drops to 255 mV. Such a Ni@TiO2/FTO electrode also exerts excellent durability toward MEA oxidation.