Integrated pre-selection of nickel cathode-catholyte systems for hydrogen production in microbial electrolysis cells

Microbial electrolysis cells (MECs) couple microbial oxidation of organic matter at the anode with externally assisted hydrogen evolution at the cathode, enabling H₂ recovery from waste-derived substrates. Cathode configuration and catholyte chemistry, however, are commonly optimized separately, limiting identification of electrode-electrolyte combinations that remain effective under reactor conditions. Here, nickel foam and mesh were assessed through pH screening, catholyte-composition screening, two-chamber MEC validation, and post-operation assessment of catholyte and surface evolution. In 50 mM potassium phosphate, pH 5 gave the highest apparent exchange current densities for both nickel structures. At pH 5 and 100 mM, sodium acetate yielded the highest apparent j₀ values (0.809 ± 0.008 A m⁻² for foam; 0.719 ± 0.003 A m⁻² for mesh), whereas sodium citrate gave lower apparent Tafel slopes. During 15-day MEC operation, acetate buffer sustained broader current profiles and greater H₂ production than phosphate controls. Nickel foam-acetate provided the highest integrated reactor response, reaching 20.98 ± 1.7 mL H₂ Lcat⁻¹ d⁻¹, 55.0 ± 0.82 % cathodic efficiency and 124.7 ± 2.1 % energy efficiency. Post-operation changes in catholyte pH, conductivity, and surface elemental signals revealed that the cathodic environment evolved during operation. These results establish catholyte composition as a co-determining electrochemical design variable and provide a transferable framework in which electrochemical descriptors, reactor output, and interfacial evolution are evaluated jointly for MEC cathodic-system selection.