Stabilising methane production in a series-stack of bioelectrochemical systems

Electromethanogenesis (EMG) is a power-to-gas technology that converts CO₂ into methane in bioelectrochemical systems (BES) using electricity. Scaling up requires the stacking of multiple BES cells, often through parallel and series connections. However, series configurations can lead to voltage imbalances due to biocatalysts variability and uneven feeding, compromising performance. This work presents a simple, cost-effective cell balance system (CBS), based on commercial diodes, to equilibrate voltage across a three-cell BES stack performing EMG (2.7 V stack voltage). Cells were hydraulically connected in parallel and electrically connected in series. The stack was operated under normal and uneven anolyte feeding, with and without the CBS. Without CBS, stopping the feed to one cell dropped stack voltage by 1.45 V and reduced methane production from 0.66 to 0.23 m³-CH₄ m⁻³ cathode d⁻¹, while energy efficiency fell from 86 % to 44 %. With the CBS, voltage imbalances stayed below 1.15 V; methane production and efficiency increased by 57 % and 49 %, respectively, in comparison with the absence of CBS. The CBS redistributed the current, allowing a rapid recovery, after reestablishing feeding conditions. In conclusion, a simple diode-based CBS provides a robust, scalable, low-cost means to manage voltage imbalances, facilitating reliable operation and industrial upscaling.