Hierarchical porous yet dense phenolic resin-based carbons for enhanced volumetric capacitances in zinc-ion hybrid capacitors

Hierarchical porous carbons (HPC) are considered as promising electrode materials for electrochemical energy storage showing a synergistic effect of different length-scale pores. However, the porous structure with large aperture reduces the density of the material and thus the low volumetric performances, limiting their applications in compact energy storage. Here, we employ a simple surfactant-mediated crosslinking strategy during the phenolic resin sol-gel process to achieve hierarchical but dense porous carbon materials. Increasing the surfactant/resorcinol ratio helps to diminish the particle size of network building units, thus leading to the decrease network-originated nanopores. The screened high-density hierarchical porous carbon (HD-HPC) demonstrates downsized mesopores to 10 nm, apart from the tremendous micropores generated by oxygen-assisted carbonization. When used as cathode materials in zinc-ion hybrid capacitors, HD-HPC has a 3.2 times higher volumetric capacitance, as compared to low-density HPC with larger network pores of around 40 nm. Meanwhile, HD-HPC exhibits an excellent long cycle life of 8000 cycles at 10 A g⁻¹ with negligible capacity loss and the rate performance exceeds commercial microporous carbon YP-50. Considering the low cost and simplicity of the proposed process, this work may provide new avenues for the structural design and practical application of dense yet porous carbon materials towards compact energy storage.