Flowable Nickel-Loaded Activated Carbon Cathodes for Hydrogen Production in Microbial Electrolysis Cells

Microbial electrolysis cells (MECs) can electrochemically produce green hydrogen from waste streams. However, cathode materials have been a bottleneck for the practical application of MECs due to difficulties in scale-up and high costs. To overcome current drawbacks, we have examined a novel flowable cathode in MECs, where nickel-loaded activated carbon (Ni/AC) powders were suspended in a buffering solution as a cathode without electrode fabrication processes. The Ni/AC flow cathode with higher Ni content and minimum Ni/AC loading (4 Ni-atom% and 0.125 wt-AC.%, Ni₄/AC_0.125) demonstrated the highest catalytic activities (−0.86 V vs Ag/AgCl at −10 A/m²) among Ni/AC flow cathodes tested. This result indicates that pseudocapacitive behavior toward Faradaic reactions can be promoted by increasing Ni loadings on Ni/AC particles. The MEC with a Ni₄/AC_0.125 flow cathode produced comparable hydrogen production rates (1.62 ± 0.15 L-H₂/L_reactor-day) to the Pt control (1.64 ± 0.09 L-H₂/L-day) and 40% higher than the blank (only current collector without Ni/AC, 1.29 ± 0.02 L-H₂/L-day) at a 4 h cycle. The impacts of carbon black blending remain unclear; there was a 10% increase in hydrogen production rates with the lowest carbon black content (0.06 wt %) in the Ni/AC flow cathode, but hydrogen production rates were not further improved as carbon black content increased.