Having this in mind, the current work focuses on the electrolysis of black liquor for energy recovery. The process has several economic and environmental advantages, as it simultaneously generates a clean fuel (hydrogen) at the cathode and a precipitated material with economic value (lignin) at the anode surface [1]. The generated hydrogen can be eventually fed to a fuel cell that partially powers the electrolysis plant, thereby decreasing the electricity costs.
Herein, platinum (Pt), nickel (Ni), and AISI 304 stainless steel bulk electrodes are tested for black liquor electrolysis, both as anodes and as cathodes. Voltammetric methods are used to study the lignin anodic oxidation at the Pt, Ni and AISI 304 electrodes at temperatures ranging from 25 to 55 ºC. Kinetic and diffusional parameters are calculated (e.g., charge transfer and diffusion coefficients, number of exchanged electrons). The hydrogen evolution reaction in the black liquor is also evaluated at the same electrodes, with Pt and Ni showing the best performance. Activation energies are calculated for both electrochemical processes.
Finally, a small-scale laboratory black liquor electrolyzer using Ni plates, both for anode and for cathode, is assembled and its operation parameters are evaluated, specifically the applied cell voltage and the electrolysis duration. The produced lignin is characterized by MIR and reveals a high purity compared with typical lyophilized and Klason lignin.
[1] H.R. Ghatak, S. Kumar, P.P. Kundu, Int. J. Hydrogen Energy 33 (2008) 2904-2911.