Biochar based cathode enriched with hydroxyapatite and Cu nanoparticles boosting electromethanogenesis

Electromethanogenesis is an innovative technology addressing the need of storing renewable energy from unprogrammable sources. It allows for the electrochemical production of methane from CO2-rich wastes on microbial cathodes, in a logic of power-to-gas (BEP2G). The challenge of cost-effective and sustainable biocathodes enhancing the microorganism performance and yield of electromethanogenesis is approached in this work. For the first time, porous carbonaceous cathodes were functionalized with Cu nanoparticles and hydroxyapatite (HAP) and successfully experimented for supporting microbial CO2 reduction reaction (CO2RR) to methane. Tests were performed in a double chamber system under CO2 flow at 45 °C. Next Generation Sequencing of 16S RNA indicated that the microbial pool on the cathodes was mostly enriched in Metanobacteriaceae (hydrogenotrophic Archaea) and different fermenting bacteria, depending on the cathode type. High methane production on cathodes made of Cu 20%, HAP 10%, and carbon balance (20Cu/10HAP) was achieved, with a maximum of 866 ± 199 mmol m−2 d−1 (projected cathode area, Coulombic efficiency of 64%), corresponding to values comparable to the maximum in literature, but in shorter timespans (8 vs. 30 days). The documented effect of pH stabilization in the cathodic chamber by HAP was one of the main parameters that concurred to the selectivity of CO2RR towards methane.