Electrochemical lignin depolymerization - effects of electrolyte composition and electrode materials

<ns3:p>Background Lignin, a significant by-product of the pulping industry, accounts for up to a third of the biomass weight. Despite its potential as a renewable alternative to petroleum, it is primarily burned onsite. Targeting the valorization of future lignin streams, this study focuses on the optimization of electrochemical depolymerization of lignin from black liquor, examining the influence of electrolyte composition and electrode materials. Methods The effects of adding water, white liquor (an aqueous solution of sodium salts), and various alcohols are investigated, alongside the testing of different metals, including nickel (Ni), copper (Cu), lead (Pb), and stainless steel as electrode materials. The experiments were conducted in batch mode applying lab-scale undivided flow-through cells with an active electrode area of 3.1 cm<ns3:sup>2</ns3:sup>. The electrolytes were prepared by dissolving sodium salts in water, to which industrial Kraft lignin and alcohols were added. The experiments consisted of chronoamperometric test protocols spanning the voltage range of 1-3V. Selected samples were analyzed with respect to their molecular size distribution applying size exclusion chromatography. Results and conclusions The results indicate that diluting black liquor with water and white liquor effectively enhances the solution's properties, including conductivity, viscosity, and the formation of deposits. Additionally, the utilization of alcohols as capping agents prevents oxidative repolymerization of lignin. Under optimized conditions, nickel electrodes are applied, and the electrolyte consists of a mixture of intermediate black liquor, water, white liquor, and alcohol in a volumetric ratio of 20:7.5:7.5:5, resulting in stable operation. The addition of alcohols as well as activation of the Ni anode by growing a NiOOH layer does not improve the performance.</ns3:p>