The structure impact of lignin in pulping material on the energy storage performance of black liquor derived carbon cathodes for zinc ion hybrid capacitors

Cooking black liquors generated during the pulping process have been recognized as promising electrode materials which can be directly applied as carbon sources. This paper investigates the relationship between the microstructure of lignin and the electrochemical properties of carbon derived from black liquor obtained from various plants, including softwood, hardwood, and grass. It was found that eucalyptus black liquor, abundant in methoxy groups, has a notable impact on the performance of carbon materials compared to black liquor derived from Pinus sylvestris and bamboo. The abundant methoxy groups contribute to micropore formation and facilitate the incorporation of oxygen atoms from the lignin side chains into the carbon matrix. This process results in a porous carbon structure with a substantial specific surface area (1599 m²/g) and an oxygen content of 5.4 %, which facilitate charge transfer and reduce the adsorption energy barrier (from −0.17 eV to −0.36 eV). The specific capacitance of the prepared single electrode reaches 271 F g⁻¹. Additionally, a zinc ion hybrid capacitor utilizing a carbon cathode produced from eucalyptus black liquor achieves a maximum energy density of 71.8 Wh kg⁻¹ and a power density of 1.11 kW kg⁻¹. This work offers recommendations for selecting raw materials to optimize the industrial production of electrode materials for high energy storage devices.