Contrasting Oxygen and Chlorine Evolution Selectivity in Tunnel and Layered MnO2 Electrocatalysts

Controlling competing oxygen and chlorine evolution is a key challenge in chloride-containing electrolysis, particularly under neutral saline conditions relevant to seawater electrolysis. Here, two phase-pure MnO2 polymorphs with contrasting crystallographic frameworks, α-MnO2 and δ-MnO2, were synthesized through surfactant-free routes and evaluated under comparable conditions. Tetragonal α-MnO2 with a 2 × 2 tunnel framework preferentially promoted oxygen evolution, whereas layered δ-MnO2 favored chlorine evolution. X-ray photoelectron spectroscopy revealed no detectable chlorine signal for α-MnO2 after chlorine evolution, while δ-MnO2 exhibited pronounced chloride-associated surface modification. Electron paramagnetic resonance spectroscopy and density functional theory calculations further revealed distinct defect characteristics and substantially different oxygen-vacancy formation tendencies between the two frameworks. These observations suggest that framework-dependent defect formation influences chloride-associated surface chemistry and contributes to the contrasting oxygen and chlorine evolution selectivity. Importantly, the selectivity trend was retained when the MnO2 phases were integrated onto iridium-ruthenium-based dimensionally stable anodes. These results demonstrate that contrasting crystallographic frameworks and their associated defect characteristics can markedly alter anodic selectivity and provide a structural basis for designing selective electrodes for chloride-containing electrochemical systems.