Interface Properties of Hydroxyapatite in Ternary Composites Cathodes for Electromethanogenesis

The bioelectrochemical conversion of CO2 to CH4 (electromethanogenesis), using renewable energy, is a promising power-to-gas technology. However, maximizing its potential requires the development of cost-effective materials for biocathodes. A recent research achieved significant progress by employing a ternary composite material comprising porous carbon (biochar) modified with 20 wt% copper and 10 wt% hydroxyapatite (HAP). Notably, the addition of HAP led to a remarkable increase in methane production compared to undoped biochar and Cu-doped biochar, reaching a maximum of 0.87 mol m−2 d−1 with a coulombic efficiency of 64%. Based on these results, this study aims to elucidate the pivotal role of hydroxyapatite as a crucial modifier in biochar-based cathode materials. Employing a range of characterization techniques such as N2 adsorption/desorption isotherms, solid–liquid phase titrations with probe molecules, electron microscopy, and infrared spectroscopy, the research delved into the unique contributions of hydroxyapatite surface features to the electromethanogenesis process on the composite cathode. The results underscored the beneficial effects of HAP in enhancing reagent adsorption, providing a hydrophilic interface beneficial to microbial adhesion, and stabilizing the interface pH. This comprehensive investigation sheds light on the factors underlying the enhanced performance of bioelectrochemical systems facilitated by HAP-modified cathode materials.