Towards the industrial application of microbial electrolysis cells for hydrogen production: a critical review on cell’s components

During the last 20 years, Bioelectrochemical Systems (BES) have been studied as an environmentally friendly technology. These systems exploit the ability of peculiar microorganisms to interact with solid state electrodes by exchanging electrons. A type of BES is represented by a Microbial Electrolysis Cell (MEC), mostly used for hydrogen production while abating organic waste. This review underlines the steps needed to reach industrial readiness of MEC technology. Bioanodes, responsible for the electricity generation, suffer from low conductivity of the biocompatible and low cost materials. Even if steps were made to enhance the performance and, reaching a stable current density of 12 A/m² using real wastewater, the scalability and stability of bioanodes remain issues. Unfortunately, there are biological limits that cannot be overcome. However, there is still a lot of space for improvements. Cathodes are fundamental for hydrogen production. Research on cathodes has focused on affordable and durable materials like stainless steel, which in some configurations can reach the performance of platinum without its susceptibility to poisoning. Various strategies have improved hydrogen capture efficiency and reduced contamination by methanogens. Separators are essential for compartmentalizing the system, but are usually expensive and, as reported, prone to operational issues such as biofouling and pH split. We report new alternatives, including porous membranes or cellophane, which may help reduce costs and enhance performance. In our view, developing materials and conducting standardized tests under realistic conditions are critical steps toward making MECs commercially viable for sustainable hydrogen production. The scaling up step is still missing, even if in recent years some attempts were made by a few research groups reaching volumes of 1 m³. Perhaps in the coming years MECs will support the circular economy, but at the current state of the art, they are still not cost-effective. On the other hand, we aim to help the scientific community remove the last barrier that separates MECs from industrial scale-up by reporting information and giving a rough but fair opinion on how this should be achieved.