Aqueous rocking chair batteries have attracted attention as highly safe and inexpensive secondary batteries. Lithium and sodium ions have mostly been used as the mobile ions, whereas proton systems with potentially the highest mobility have rarely been reported. Recently, research on proton batteries has been conducted using concentrated sulfuric acid solutions as the electrolyte that can assist high-rate performance.^[1] Stability under concentrated acidic conditions is required for the electrode materials, as well as the capability of proton accommodation. MoO₃ has been reported as a stable negative electrode material for aqueous proton batteries,^[2] while few positive electrode materials are known except for bulky organic^[3] and Prussian-blue materials^[4]. For achieving high energy density and long life, oxide positive electrode materials are desirable. Here we propose the application of MoO₃ as the positive electrode material by optimizing the operating composition range. The potential values shown below are all shown versus SHE (actually measured with Ag/AgCl).

MoO₃ showed a discharge profile at around 0.5 V with the maximum capacity of ca. 100 mAh g^-1, as shown in the figure. This potential is sufficiently more positive than the redox potential of protonated MoO₃ of around -0.3 V as the negative electrode. With the aid of operando X-ray diffraction analysis, it turned out that the discharge regions at 0.5 V and 0.4 V are respectively associated with a biphasic transition of MoO₃/phase I (ca. 0 < x < 0.3 in H_xMoO₃) and a single-phase reaction of phase I (ca. 0.3 < x < 0.5 in H_xMoO₃). Deep discharging beyond this range results in the coexistence of phase I and phase III (ca. 0.5 < x < 1.5 in H_xMoO₃) and the proton extraction from phase III leads to the formation of phase II or phase IIa with its discharging potential of 0.0 V. Structural calculation based on the density function theory is employed to clarify the origin of this irreversible phase transition behavior. Different proton sites between these phases seem to be responsible.

An aqueous proton battery with a 7 mol dm^–3 sulfuric acid electrolyte was constructed with H-inserted MoO₃ and MoO₃as the negative and positive electrodes, respectively, and was successfully discharged and charged repeatedly, with the operating voltage of ca. 0.6 V, indicating the launch of aqueous proton battery composed of oxide active materials.

References:

[1] J. Li, H. Yan, C. Xu, Y. Liu, X. Zhang, M. Xia, L. Zhang, J. Shu, Nano Energy, 89 (2021) 106400.

[2] X. Wang, Y. Xie, K. Tang, C. Wang, C. Yan, Angew. Chem. Int. Ed., 57 (2018) 11569.

[3] X. Wang, J. Zhou, W. Tang, Energy Storage Mater., 36 (2021) 1.

[4] X. Wu, J. J. Hong, W. Shin, L. Ma, T. Liu, X. Bi, Y. Yuan, Y. Qi, T. W. Surta, W. Huang, J. Neuefeind, T. Wu, P. A. Greaney, J. Lu, X. Ji, Nat. Energy, 4 (2019) 123.