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

Abstract 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 sp ³ ‐ND to sp ² ‐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.