Tailoring Acid‐Salt Hybrid Electrolyte Structure for Stable Proton Storage at Ultralow Temperature

Abstract The critical challenges in developing ultralow‐temperature proton‐based energy storage systems are enhancing the diffusion kinetics of charge carriers and inhibiting water‐triggered interfacial side reactions between electrolytes and electrodes. Here an acid‐salt hybrid electrolyte with a stable anion−cation−H 2 O solvation structure that realizes unconventional proton transport at ultralow temperature is shown, which is crucial for electrodes and devices to achieve high rate‐capacity and stable interface compatibility with electrodes. Through multiscale simulations and experimental investigations in the electrolyte employing ZnCl 2 introduced into 0.2 M H 2 SO 4 solution, it is discovered that unique anion−cation−H 2 O solvation structure endows the electrolyte with low‐temperature‐adaptive feature and favorable water network channels for rapid proton transport. In situ XRD and multiple spectroscopic techniques further reveal that the stable 3D network structure inhibits free water‐triggered deleterious electrode structure distortion by immobilizing free water molecules to achieve outstanding cycling stability. Hence, VHCF//α‐MoO 3 hybrid proton capacitors deliver an unexpected capacity of 39.8 mAh g −1 at a high current density of 1 A g −1 (−80 °C) and steady power supply under ultralow temperatures (96% capacity retention after 1500 cycles at −80 °C). The anti‐freezing hybrid electrolyte design provides an effective strategy to improve the application of energy storage devices in ultralow temperatures.