On the role of hydroxyapatite as a key component in cathode materials to drive the electroreduction of CO2 to CH4

In the ongoing efforts to recycle carbon dioxide (CO2) emissions on a large-scale, the production of methane from CO2 using electrochemical reduction systems and renewable energy represents a promising technology in a power-to-gas (P2G) concept. To fully exploit the potential of P2G, advances in process engineering and development of effective materials for electrodes are still required.1 Here, we present our recent results in the combination of chemical and microbial CO2 reduction processes, experimenting an innovative and cost-effective composite material, consisting of porous carbon (biochar) doped with copper (Cu) and hydroxyapatite (Ca5(PO4)3OH, HAP) nanoparticles. The composition of cathodes has been optimized by application of the D-optimal factorial Design of Experiments (DoE) with the final aim to maximize methane production. Electromethanogenesis tests in mesophilic (45°C) conditions were performed in double-chamber experimental set-up. Strains of hydrogenotrophic microorganisms of Metanobacteriaceae enriched from the inoculum collected from a biogas plant. High methane production was achieved on cathodes made of biochar doped with Cu 20 wt.% and HAP 10 wt.%, with a max. of 866±199 mmol m-2 d-1 (projected cathode area, coulombic efficiency of 64%). These tests revealed the synergistic action of composites and microorganisms, favoring the selective reduction of CO2 to CH4. The characterization of electrodes, before and after use, revealed that HAP plays a pivotal role in stabilizing the pH of the interface, in favoring the adhesion of microorganisms and in the adsorption and stabilization of reaction key-intermediates (e.g. carbonate and formate).