Two Faces of the Same Oxide: How Crystal Phase Defines the Nature of Proton Storage in MoO3

Abstract Proton storage in transition-metal oxides is often linked to fast proton transport, but how crystal phase governs structural evolution, electronic response, and kinetics remains insufficiently understood. Here, α-MoO3 and h-MoO3 are compared as model polymorphs to reveal phase-dependent proton-storage behavior. In situ XRD and state-resolved XPS show that α-MoO3 undergoes an initial irreversible structural reconstruction and then enters a reversible regime with lattice breathing and multivalent Mo redox, exhibiting battery-type behavior and favorable rate capability. In contrast, h-MoO3 rapidly loses long-range order after deep protonation, with poor structural recovery, limited Mo-valence evolution, and predominantly pseudocapacitive charge storage. Electrochemical impedance spectroscopy coupled with distribution of relaxation times analysis and first-principles calculations further indicate that α-MoO3 preserves electronic delocalization and stronger electron–proton coupling, whereas h-MoO3 shows weakened electronic coherence and kinetically dispersed charge compensation. These findings identify structural/electronic coherence as a key criterion for high-performance proton storage.