Neutralization reaction in synthesis of carbon materials for supercapacitors

We have prepared nitrogen and oxygen enriched hierarchically porous carbon (NOHPC) based on a concept of neutralization reaction. The reactant of KOH and the reaction product of KCl mixed in soybean flour acts as chemical activators and porogens during carbonization, which play different roles to produce the hierarchical porous structure containing macropores, mesopores and micropores. After the investigation of the effect of the reaction pH and carbonization temperature on the structure, the optimized NOHPC with reaction pH at 11.5 and carbonization temperature at 900 °C has a large area (2404.3 m² g⁻¹), with a high pore volume of 1.31 cm³ g⁻¹, a nitrogen content (0.42%) and an oxygen content (32.1%). When assembling supercapacitors, NOHPC has a extremely large specific capacitance (381 F g⁻¹ at 1 A g⁻¹) and large energy density (20.78–15.42 Wh kg⁻¹), and possesses good rate performance. Particularly, the symmetric supercapacitor assembled based on NOHPC-900-11.5 has a good cycle stability in the voltage range of 0–1.8 V in a two-electrode system with 1 M Na₂SO₄ as the electrolyte. It is worth noting that when Tetraethylazanium tetrafluoroborate (TEATFB) in propylene carbonate and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (EMIMNTF₂) are selected as electrolyte, high energy densities of 31.4 Wh kg⁻¹ and 71.6 Wh kg⁻¹ are achieved. The above results indicate that NOHPC prepared based on neutralization reaction is superior energy storage materials. This work has universal significance in producing high performance porous carbons from biomass.