Electrochemical reduction of gaseous feeds such as CO₂, CO, and N₂ holds promise for sustainable energy and chemical production. Practical application of this technology is impeded by slow mass transport of the sparingly soluble gases to conventional planar electrodes. Gas diffusion electrodes (GDEs) maintain a high gas concentration in the vicinity of the catalyst and improve mass transport, thereby resulting in current densities higher by orders of magnitude. However, gaseous feeds cause changes to the GDE environment, and specific features are required to efficiently tune the product selectivity and improve reaction stability. Herein, with a comprehensive review of the challenges and advances in GDE development for various electrocatalytic reactions, we intend to complement the body of material-focused reviews. This review outlines GDE fundamentals and highlights key advantages of GDE over conventional electrodes. Through critical discussion about steps in GDE fabrication, and specific shortcomings and remedial strategies for various electrochemical applications, this review discusses connections, unique design criteria, and potential opportunities for gas-fed reactions and desired products. Finally, priorities for future studies are suggested, to support the advancement and scale-up of GDE-based electrochemical technologies.

Reduction in methane emissions to the Earth's atmosphere is a critical strategy for tackling climate change. It is well established that anaerobic oxidation of methane (AOM) associated with sulfate reduction functions as an important methane sink in marine sediments. However, recent findings show that AOM uses diverse electron acceptors across a range of habitats, prompting examination of the potential role of AOM in mitigation of methane emissions and global climate change in non-marine environments. Methane is also a valuable energy source, widely used for production of electricity. Recent studies suggest that AOM could be used to produce liquid fuels/chemicals. The potential involvement of CO₂ in product formation is particularly exciting as methane bioconversion could act as a net sink of CO₂. The discovery that AOM is able to transfer electrons to solid electron acceptors suggests that methane may be a suitable source of electrons for a bioelectrochemical, biosynthesis cell. In addition, AOM has been used for pollution control and environmental remediation, such as nitrogen removal from contaminated water. Herein, we review and discuss implications of the latest scientific discoveries in AOM for methane emissions from aquatic and terrestrial environments, and methane as a feedstock for various biotechnology platforms.

Summary: Anaerobic oxidation of methane (AOM) can be microbially mediated by the reduction of different terminal electron acceptors. AOM coupled to reduction of sulfate, manganese/iron oxides, humic substances, selenate, arsenic and other artificial extracellular electron acceptors are recognized as processes associated with microbial extracellular respiration. In these processes, methane‐oxidizing archaea transfer electrons to external electron acceptors or to interdependent microbial species, which are mechanistically dependent on versatile extracellular electron transfer (EET) pathways. This review compiles recent progress in the research of electromicrobiology of AOM based on the catalogue of different electron acceptors. Naturally distributed and artificially constructed EET‐mediated AOM is summarized, with the discussion of their environmental importance and application potentials. The diversity of responsible microorganisms involved in EET‐mediated AOM is discussed with both methane‐oxidizing archaea and their putative bacterial partners. More importantly, the review highlights progress and deficiencies in our understanding of EET pathways in EET‐mediated AOM, raising open research questions for future research.

Anaerobic methanotrophic archaea (ANME) belonging to the family Methanoperedenaceae are crucial for the global carbon cycle and different biogeochemical processes, owing to their metabolic versatility to couple anaerobic oxidation of methane (AOM) with different electron acceptors. A universal feature of Methanoperedenaceae is the abundant genes encoded in their genomes associated with extracellular electron transfer (EET) pathways. Candidatus. 'Methanoperedens manganicus', an archaeon belonging to the family Methanoperedenaceae, was recently enriched in a bioreactor performing AOM coupled with Mn (IV) reduction. Using this EET-capable ANME, we tested the hypothesis in this study that ANME can catalyse the humic-dependent AOM for growth. A two-year incubation showed that AOM activity can be sustained by Ca. 'M. manganicus' consortium in a bioreactor fed only with humic acids and methane. An isotopic mass balance batch test confirmed that the observed AOM was coupled to the reduction of humic acids. The increase of relative abundance of Ca. 'M. manganicus', and the total archaea population in the microbial community suggested that Ca. 'M. manganicus' can grow on methane and humic acids. The observation of humic-dependent AOM led to a subsequent hypothesis that humic acids could be used as the electron shuttle to mediate the EET in dissimilatory Mn (IV) reduction by Ca. 'M. manganicus'. We tested this hypothesis by adding humic acids to a Ca. 'M. manganicus' dominated-culture, which showed that the AOM rate was doubled by the addition of humic acids. X-ray photoelectron spectroscopy (XPS) showed that quinone moieties were consumed when humic acids worked as electron acceptors while remaining stable when functioning as a shuttle for electron transfer. The results of our study suggest that humic acids may serve as electron shuttles to allow ANME to access more electron acceptors through long-range EET.