Carboxylated CNT–Vanadium Hexacyanoferrate Cathode Enabling High-Energy-Density Quasi-Solid-State H₂ Batteries with Enhanced Low-Temperature Performance

Abstract

Rechargeable hydrogen batteries based on reversible proton insertion are promising for large-scale energy storage; however, most reported cathodes remain limited to capacities below 60 mAh g-1. Here, we report a carboxylated carbon nanotube-vanadium hexacyanoferrate (CC-VHCF) cathode for quasi-solid-state H₂ gas batteries. The comparison with C-VHCF suggests that carboxyl functionalization in the CNT-containing composite is associated with improved proton transport and storage behavior, affording a high capacity of 160.9 mAh g-1 at 1 A g-1 and an energy density of 109.7 Wh kg-1. A full cell comprising this cathode, a hydrogen anode, and a bentonite-phosphoric acid (BP) proton-conducting quasi-solid-state electrolyte achieves 163.7 mAh g-1 at 0.5 A g-1 with an energy density of 128.5 Wh kg⁻¹, based on the mass of the cathode active material, ranking among the highest values reported on a cathode-active-material basis for hydrogen batteries. The device exhibits strong rate capability and long-term cycling stability, retaining ~66% of its capacity after 10000 cycles at 20 A g-1. Notably, it maintains 88.9 mAh g-1 at -40 °C with negligible decay and operates reliably under extreme climatic conditions down to -80 °C, representing one of the best low-temperature performances reported for hydrogen and proton batteries.