Breaking the Scaling Relationship of Oxygen Evolution Reaction and Chlorine Oxidation Reaction via MnO₂ Polymorphic Engineering for Selective Seawater Electrolysis

Seawater seems to be a sustainable feed for hydrogen generation through electrolysis. Despite the thermodynamic propensity for the oxygen evolution reaction (OER) at the anode during seawater electrolysis, the kinetically fast and unfavorable chlorine oxidation reaction (COR) dominates. Thus, designing active and selective anodes for seawater electrolysis is challenging. Here, we are investigating the effect of MnO2 polymorphic structures as an anode material for simulated seawater electrolysis in a basic medium. Contrary to the belief that MnO2 is an OER catalyst, we discovered that only α- and β-MnO2 are preferentially OER catalysts, whereas γ- and δ-MnO2 are selective for COR. The experimental findings imply that discrete translational symmetry in distinct polymorphs promotes different reaction intermediates, disrupting the scaling relation between the OER and COR. We also studied the polymorphic impact of MnO2 on limiting Cl– ion transport over a conventional catalyst of IrO2 in an alkaline medium to scale up seawater electrolysis. The research found that γ-MnO2 is the most likely to impede the COR active sites over IrO2 among the four polymorphs studied (α-, β-, γ-, and δ-MnO2). We identified that γ-MnO2 functions as a Lewis acid layer, thereby augmenting the kinetics of the OER across the IrO2 surface and establishing a barrier against Cl– ions.