Using nickel-molybdenum cathode catalysts for efficient hydrogen gas production in microbial electrolysis cells

A low-cost cathode catalyst based on a NiMo alloy was examined here to replace noble metals for the hydrogen production in microbial electrolysis cells (MECs). Two NiMo catalysts were synthesized through either an electrochemical assisted (NiMo Elec) or a hydrothermal (NiMo Ht) approach. The NiMo Ht method enriched the electrocatalyst with Mo atoms compared to the NiMo Elec approach, producing a similar current density with a minimal overpotential of 50 mV compared to Pt. In MEC tests using the NiMo Ht catalyst, H2 was generated at a highest rate of 81 ± 3 LH2/L-d (current density of 44.4 ± 0.9 A/m2) at a cell voltage of −0.86 V, and a Coulombic efficiency of >97%. Modifying the closely stacked MEC design to include a reference electrode, and analysis of the electrode potentials using the electrode potential slope method, revealed a large contribution of the cathode resistance (5.3 ± 0.5 mΩ m2) compared to the anode (1.4 ± 0.2 mΩ m2) and ohmic resistance (0.83 mΩ m2) for a total internal resistance of 7.6 ± 0.5 mΩ m2. The high performance of the NiMo Ht catalyst coupled with its low cost provides an economically viable approach to advance the generation of biohydrogen in MECs. •NiMo catalyst for hydrogen evolution reaction was synthesized by different routes.•Hydrothermal route (NiMoHt) had superior performance than electrochemical synthesis.•Using NiMo Ht in an MEC enabled large current densities and H2 production rates.•A novel configuration enabled inclusion of a reference electrode.•Cathode resistance dominated (70%) over MEC internal resistance.