Electrode processes in black liquor electrolysis and their significance for hydrogen production

Cathodic hydrogen evolution reaction and the reactions at the anodic counter electrode have been studied for black liquor electrolysis using standard electrochemical methods. These are compared with alkaline water electrolysis as control. In black liquor electrolysis the hydrogen evolution is found to be kinetically facile with exchange current densities of 2.194 mA/cm2, and 9.856 mA/cm2 for wheat straw black liquor and eucalyptus black liquor, respectively, in comparison to 1.075 mA/cm2 for alkaline water. The cathodic Tafel slopes are −112.8, −116.9, and −135.3 mV/dec for alkaline water, wheat straw black liquor, and eucalyptus black liquor, respectively. The activation overpotential for hydrogen evolution is significantly lower for black liquor electrolysis. Black liquor electrolysis shows additional anodic electroactivity in a potential window of −0.2 to 0.2 V, well shifted from the region of oxygen evolution reaction. This reaction follows a sluggish one electron oxidative charge transfer as revealed by cyclic and square wave voltammograms. The anodic charge transfer coefficient is slightly higher than 0.5. Black liquor electrolysis could produce hydrogen with a much lower inter-electrode potential with the exclusion of gaseous products being formed at the anode. This can have significant bearing on the energy efficiency of the process.