Employing Octahedral Net Charge Descriptors for Designing High-Performance Aqueous Proton Batteries

Recently, aqueous proton batteries have shown promise for electrochemical energy storage using MXene electrodes. However, designing high-performance MXene proton batteries remains challenging due to the inevitable hydrogen evolution reaction (HER), the vast chemical composition space of MXene, and the unclear proton transport mechanism. To tackle these challenges, we established a general descriptor based on structural units of MXenes, termed the octahedral net charge descriptor (Q_oct). This descriptor correlates well with HER activity, capacity, and proton transport performance. Based on the descriptor, prediction reveals a dual-proton storage mechanism per site in N-functionalized MXene. Meanwhile, the accuracy of the descriptor is verified across a broader MXene chemical space. Additionally, the kinetic process shows the topological transformation energy barrier of the interfacial solution is profoundly influenced by Q_oct, thereby impacting the proton transfer performance. This universal descriptor originates from the different electron filling states on the molecular orbitals of the octahedron. Overall, this work provides an efficient strategy for designing MXene proton batteries and can be extended to other battery and catalysis fields.