Manipulating d-Electronic States via Transition Metal Doping in MnO2 to Boost Direct Seawater Electrolysis

Abstract

With the increasing scarcity of fresh water, seawater electrolysis holds great potential for hydrogen production. However, the slow kinetics of anodic oxygen evolution reaction (OER) limits the Faraday efficiency, and the high concentration of chloride ions in seawater leads to the competitive reaction that produces highly corrosive byproducts such as chlorine or hypochlorite. MnO₂ has demonstrated exceptional stability under acidic conditions, remarkably outperforming state-of-the-art OER noble metal catalyst, RuO₂. Conversely, the OER activity of MnO₂ is far inferior to that of RuO₂. In this study, we have designed 13 transition metal-doped γ-MnO₂ catalysts (TM-MnO₂) and screened 6 potential catalysts for direct seawater electrolysis with high activity and selectivity based on density functional theory. We thoroughly investigated the origin of activity and the effect of pH on selectivity. Our work demonstrates that the dispersed TM in γ-MnO₂ enhances the high covalency of the TM–O bond, thereby triggering the lattice oxygen mechanism (LOM) instead of the traditional adsorbate evolution mechanism (AEM). Notably, there is a volcano-type relationship between integrated crystal orbital Hamilton population (ICOHP) of the TM–O bond and the TM d-band center (ε_d), unveiling that the doping strategy can manipulate the covalency of the TM–O bond through tuning the TM ε_d, thereby regulating the activity. Moreover, we determine the stability of the catalysts at a range of potential U and pH values through constructing the Pourbaix diagrams. Finally, we validate that γ-MnO₂ doped with Mo exhibits superior OER performance with an overpotential of 0.27 V and high selectivity in suppressing the chloride oxidation reaction (ClOR). This study provides theoretical insights into the design and development of advanced OER catalysts, which can simultaneously suppress ClOR for direct seawater electrolysis.