Direct/indirect strategies on engineering extracellular electron transfer of anode electrodes in microbial electrolysis for hydrogen production and pollutants removal: A review

Anode electrodes are at the focal point of developing microbial electrochemical cells (MECs) for hydrogen production. The extracellular electron transfer (EET) of microorganisms at the interface of anodes and biofilm is the driving force of MECs. This signifies, the role of designing high-performance anodes to maximize the rate of EET and hydrogen production. In this paper, a thorough discussion on the adopted strategies over the past 15 years on anodes to improve biohydrogen production and pollutant removal was presented. Importantly, these strategies categorized as the direct and indirect methods. For the first scenario, the surface of anodes was subjected to various modification methods such as utilizing biochars, nanomaterials, polymers etc. while in the second scenario, the performance of electrodes improved by implementing indirect strategies such as regulating electrodes ratio/spacing, acclimation, electrodes arrangement, magnetic field etc. just to name a few. It was realized that, employing functional materials are at the spotlight of improving anode electrodes while methods such as applying electrical shocks or magnetic field are in early stages. The pros and cons and synergistic impacts on concurrent utilization of methods were also analyzed. Finally, current bottlenecks and future directions for implementing fundamental studies to tackle existing challenges were suggested. • State-of-the-art strategies adopted to improve performance of anode electrodes • Examining the pros and cons of all strategies for various configurations of MEC • Synergies of utilizing combined methods on anode electrodes were elucidated. • Extracting internal/external optimized design parameters boosting biofilm and EET • Presenting the current bottlenecks, challenges and future research directions