A critical analysis of different electron donors in methanogenic methanation: Thermodynamics, kinetics, and economic feasibility

The rising concerns about climate change highlight the need for integrated energy systems that reduce CO 2 emissions and produce renewable fuels. CO 2 biomethanation offers a promising pathway by converting captured CO 2 into energy dense biomethane; however, the relative performance of different electron donors in practice remains insufficiently understood. In particular, systematic comparisons of gaseous, liquid, and solid-phase electron donors with respect to energy efficiency, reaction kinetics, surface driven behaviors, and economic feasibility are lacking. This study addresses this gap through a comprehensive, phase-resolved evaluation of electron donors for CO 2 biomethanation under mesophilic conditions, integrating thermodynamic performance, kinetic accessibility, surface reactivity, and cost considerations. Among gaseous electron donors, hydrogen (H 2 ) proved most favorable, combining high exergetic efficiency 88.31% with effective gas-liquid mass transfer, whereas hydrogen sulfide (H 2 S) exhibited significant exergy losses. Liquid-phase electron donors (LPEDs) showed superior overall energetic performance such as acetate achieved the highest exergetic efficiency 93.49% and energy recovery, while formate demonstrated the most favorable kinetics and bioavailability under substrate-limited conditions. Although butyric acid yielded the highest CH 4 production, diffusional limitations reduced its overall effectiveness. Solid-phase electron donors (SPEDs) displayed distinct behavior, with CH 4 formation governed by surface-mediated H 2 release. Zerovalent iron (ZVI) exhibited the highest surface-normalized H 2 production and CH 4 potential, followed by zinc. Cost-normalized analysis revealed, acetate and formate as the most economically competitive donors, with pyrite and aluminum emerged as promising SPEDs for long term, low maintenance biomethanation systems. Overall, this study provides a rational, phase-integrated framework for scalable electron donor selection for CO 2 biomethanation.