Strategies to overcome mass transfer limitations of hydrogen during anaerobic gaseous fermentations: A comprehensive review

[Display omitted] •Gas fermentation is a clean and economic alternative to valorize CO2 emissions.•Gas-liquid mass transport of H2 is a bottleneck for efficient gas fermentations.•Five promising strategies to maximize H2 conversion were identified and discussed.•Bioreactor design, pressurization, and direct gas transport enhance H2 conversion.•Use of nanobubbles and electrosynthesis promise to surpass H2 limitations. Fermentation of gaseous substrates such as carbon dioxide (CO2) has emerged as a sustainable approach for transforming greenhouse gas emissions into renewable fuels and biochemicals. CO2 fermentations are catalyzed by hydrogenotrophic methanogens and homoacetogens, these anaerobic microorganisms selectively reduce CO2 using hydrogen (H2) as electron donor. However, H2 possesses low solubility in liquid media leading to slow mass transport, limiting the reaction rates of CO2 reduction. Solving the problems of mass transport of H2 could boost the advance of technologies for valorizing industrial CO2-rich streams, like biogas or syngas. The application could further be extended to combustion flue gases or even atmospheric CO2. In this work, an overview of strategies for overcoming H2 mass transport limitations during methanogenic and acetogenic fermentation of H2 and CO2 is presented. The potential for using these strategies in future full-scale facilities and the knowledge gaps for these applications are discussed in detail.