Accessing the proton storage in neutral buffer electrolytes using an electrodeposited molybdenum phosphate

Proton as charge carrier provides unique merits over metal cations in aqueous batteries. Yet, aqueous proton batteries (APBs) usually use strongly acidic electrolytes, which may cause hydrogen evolution and corrosion issues. Herein, we fabricate a molybdenum phosphate (MP) electrode that experiences pure proton storage in neutral buffer electrolytes for the first time, breaking the limit of acidic electrolytes for APBs. The aqueous neutral buffer electrolytes possess fast proton transport network and proton-donor effect, affording fast and sufficient supplement of H+ for the MP electrode. Benefiting from the mixed valence of Mo and enhanced surface electronegativity upon PO43− introduction, the MP electrode exhibited superior discharge capacities to its oxide counterpart (MoOx). The MP electrode exhibits good electrochemical performances, such as a high specific capacity of 125.6 mAh g−1 at 0.1 A g−1 (mass loading: 15.6 mg cm−2), which is just slightly lower than that (147 mAh g−1) tested in 1 M HCl electrolyte, a low cutoff potential of −1 V vs. SCE, and long cycle life with 87.8% capacity retention after 2500 charge-discharge cycles at 1.0 A g−1. Our findings provide new opportunities for high-performance neutral proton batteries.