Dissecting the influence of hydroxyapatite surface features on the bioelectromethonegesis activity of ternary hydroxyapatite/copper/biochar composites.

The bioelectrochemical power-to-gas (BEP2G) strategy combines the biological methanogenesis process with the electrosynthetical approach to convert carbon dioxide (CO2) emissions into methane [1]. To fully exploit the potential of BEP2G in sustainably producing useful fuels while mitigating environmental issues caused by CO2 emissions, advances in process engineering and design of effective materials for electrodes are still required. Recently, an innovative and cost-effective composite cathode material, consisting of porous carbon (biochar) doped with zerovalent copper (Cu) and hydroxyapatite (HAP, Ca10(PO4)6(OH)2) nanoparticles showed relevant performance in the synergistic chemical and microbial CO2 reduction [2]. In particular, the remarkable performance for CO2-to-CH4 conversion could be attributed to the presence of hydroxyapatite phase, a crystalline biomaterial. It is known that HAP is able to strongly adsorb CO2 thanks to interactions between its surface OH− and O2− basic sites. Moreover, when present in a bioelectrochemical system, HAP is expected to create a biointerface able to promote the adhesion of microorganisms and their differentiation. Herein, we report some of our studies on the surface characteristics of hydroxyapatite to bridge the electromethanogenesis activity of ternary composites with the peculiar surface properties of HAP. The bioelectrochemical reduction of CO2 takes place in a complex system, which involves microorganisms, electrode materials and the solution components. Consequently, multiple factors are entangled and need to be studied to unravel the active role of HAP in the studied ternary composites. The role of the HAPinterface is then examined in connection to its surface characteristics using a suite of phyisico-chemical techniques (scanning electronic microscopy, N2 adsorption/desorption isotherms, solid-liquid phase titrations of acid and basic surface sites, adsorption studies, infrared spectroscopy). Overall, this work aims to provide important structure–property relationships that can aid in advancing the design of composite cathode materials for CO2 reduction.