Progress of oxygen and hydrogen evolution reactions in parallel with chlorine evolution on manganese single-atom catalysts based on perfect and defective porphyrin, corrole, and phthalocyanine

Abstract

During the seawater electrolysis, besides the hydrogen evolution reaction (HER) at the cathode, there is always debate on the simultaneous occurrence of chlorine evolution reaction (CER) and oxygen evolution reaction (OER) at the anode. In this context, the selectivity toward anodic reactions is a challenge and could be adjusted through the variation of the electronic structure of the electrocatalyst. The present study aims to investigate the electrocatalytic behavior of four coordinated square-planar N4 templates toward parallel evolution of oxygen and chlorine at the anode as well as hydrogen evolution at the cathode. To this end, our focus will be on porphyrin, corrole, and phthalocyanine substrates incorporating Mn single atom highlighting the impact of perturbations around the Mn–N4 active site on the catalytic activity of these substrates. Accordingly, the reactivity and selectivity of 21 perfect and defective substrates toward the OER, CER, and HER have been evaluated from the thermodynamic point of view. Activity analysis indicates that in most cases the defective substrates outperform their perfect parent. However, in some cases, the vacancy defect leads to more favorable adsorption of intermediates which in turn hinders the fast discharge of O2, Cl2, or H2 and reduces the catalytic efficiency. Mechanistic investigations reveal that the chlorine gas evolution via the OCl intermediate (Volmer-Heyrovsky) is potentially more favored than the direct adsorption of Cl− (Volmer-Heyrovsky and Volmer-Tafel). Although, the O2 evolution was slightly hindered by OCl formation on the same OER active sites, however, it is shown that the water oxidation can be easily enhanced with small increases in pH value. We have also screened the catalytic activity of the examined substrates toward HER which provides the opportunity to design cost-effective bifunctional electrocatalysts in turn avoiding using different catalysts.