Lignin oxidation reaction kinetics and its conversion into high-value chemicals over self-prepared electrode during lignin-assisted water electrolysis for hydrogen production

Electrochemical oxidation of lignin coupled with water electrolysis forms lignin-assisted water electrolysis (LAWE), which reduces the energy consumption of hydrogen production and achieves the high-value conversion of lignin. However, the kinetics of lignin oxidation reaction (LOR) is slow, and the correlation between operating parameters and LOR kinetics is rarely found. Herein, a vertical self-grown nickel-iron layered hydroxide nanosheet on a nickel foam substrate (Ni1Fe2-LDH@NF) was synthesized for improving LOR kinetics, and the influence of lignin concentration, NaOH concentration, and temperature on LOR kinetics was revealed. A mathematical model correlating LOR kinetics with operating parameters was developed, and based on variance analysis, NaOH concentration was found the biggest factor affecting LOR kinetics. The best LOR kinetics was reached at the lignin concentration of 10 g/L, the NaOH concentration of 0.8 mol/L, and the temperature of 50–80 °C. In addition, the LOR over Ni1Fe2-LDH@NF and hydrogen production presented excellent stability during LAWE process at high current density of 200 mA cm−2 and 65 °C. The lignin underwent large skeleton breakage and oxygen-containing functional groups consumption, and successfully converted into value-added chemicals such as 2,4-di-tert-butylphenol. This work highlights the importance of understanding LOR kinetics, providing opportunities for optimized lignin upgrading in LAWE process.