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 mA h 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 mA h 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 mA h 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 10 000 cycles at 20 A g⁻¹. Notably, it maintains 88.9 mA h 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.