Protons (H⁺) serve a vital role as energy carriers in diverse electrochemical systems, thanks to their ubiquity and distinct characteristics arising from their small size and light weight. Notably, faradaic proton storage which involves the electrochemical reduction and oxidation of host materials, offers significantly higher capacities than non-faradaic systems[1]. Hence, it is imperative to explore solid-state materials with the capability for reversible proton intercalation and sufficient electrochemical performance to advance proton-related energy systems. However, the use of acidic electrolytes in proton systems requires acid-resistant active materials, imposing significant limitations on the availability of suitable proton hosts.

 In this study, we investigate the electrochemical proton (de)intercalation properties of bronze-type vanadium dioxide (VO₂(B)), which is widely recognized for its instability in acidic conditions. Despite the inherent instability, we effectively investigate its electrochemical behavior using a non-aqueous protic electrolyte (pseudo Protic Ionic Liquid (pPIL), composed of 1-Methyl imidazole (C₁Im) and Acetic acid (AcOH)), which was previously proposed by Umebayashi group[2]. In contrast to the severe capacity decay observed in the 12 M H₃PO₄ aqueous electrolyte, the VO2(B) electrode in the non-aqueous pPIL electrolyte exhibits a reversible electrochemical capacity of 260 mAh g^-1 at 150 mA g^-1 and retains 75% of initial capacity after 200 cycles at 500 mA g^-1. In situ XRD measurement confirms a reversible and topochemical electrochemical reaction, indicating the ability of VO₂(B) bulk for reversible proton intercalation. Additionally, atomic simulations based on Density Functional Theory (DFT) calculations reveal potential pathways for proton transfer in the VO₂(B) crystal through repetitive proton hopping among the oxygen atoms. The energy barrier of each pathway correlates with the distances between neighboring oxygen atoms, suggesting that efficient and rapid proton transfer is closely associated with shorter O–O distances.

 This study demonstrates that the use of a non-aqueous electrolyte expands the range of proton hosts by enabling the assessment of overlooked transition metal compounds. I addition, this work will provide valuable insights into identifying proton hosts that promote stable proton (de)intercalation, aiming to the construction of proton-based electrochemical systems that exhibit high-rate and high-capacity performance.