On the electrooxidation of kraft black liquor on boron-doped diamond

Black liquor (BL) is a highly alkaline byproduct from pulp mills. BL is rich in inorganic and organic compounds, with lignin (a natural polymer) being the most abundant. Following a waste biorefinery concept, the electrolysis of BL comprises lignin oxidation at the anode and hydrogen evolution at the cathode. These paired electrochemical processes show the promise to be carried out at lower cell voltage than that used in conventional alkaline water electrolyzers. Presently, new materials are required to improve the kinetics of the anodic reaction in the BL electrolyzer. Boron-doped diamond (BDD) can oxidize organic compounds at low overpotentials, making it a potential electrode material for BL oxidation. Herein, a BDD/Si electrode was produced, characterized by Raman spectroscopy and SEM, and employed for the oxidation of BL. The properties of the used kraft BL were determined, namely the pH (12.7), conductivity (470 mS cm⁻¹), organic/inorganic ratio (1.0), and Klason lignin content (42.2 g L^-1). Fourier-transform infrared spectroscopy was also used in the BL characterization. The BDD performance for BL oxidation was assessed by cyclic voltammetry, chronopotentiometry, and chronoamperometry. A number of exchanged electrons and a charge transfer coefficient of 3.0 and 0.8, respectively, were calculated. It was demonstrated that BDD presents a good activity for BL oxidation, comparable to that of platinum.