Iron‐Mediated Engineering of Thermally‐Conductive Hybrid Interfaces for 1.3 V Aqueous Symmetric Supercapacitors

Aqueous symmetric supercapacitors attract research interest for their intrinsic safety and environmental compatibility, yet their practical deployment is limited by a narrow voltage window imposed by water electrolysis, restricting achievable energy density. In this study, a carbon-based electrode material (Fe/ND-1200) with thermally-conductive hybrid interfaces through iron-mediated high-temperature welding of nanodiamond (ND) was designed, successfully suppressing water-splitting side reactions and broadening the operating voltage of aqueous supercapacitors. During the high-temperature treatment, iron catalyzes the phase transition of sp³-ND to sp²-graphite while forming an iron-carbon solid solution at ND-ND interfaces, thereby enhancing interfacial contact and thermal transport. The Fe/ND-1200 electrode enables an extended operating voltage of 1.3 V while maintaining stable cycling over 30 000 cycles at 2 A g^-1. In situ scanning electrochemical microscopy and surface-enhanced Raman spectroscopy analyses confirm that Fe/ND-1200 can effectively inhibit hydrogen evolution reactions and the generation of water-splitting intermediates. The above experimental results suggest that rapid heat dissipation through the thermally-conductive interfaces reduces localized temperatures at adsorption sites, thereby elevating the overpotential for water splitting and enabling high-voltage operation. This study provides a novel interfacial thermal regulation strategy to overcome voltage limitations and enhance the energy density of aqueous supercapacitors.