Identifying local gradients in methane-producing biocathodes.

CH₄-producing bioelectrochemical systems (BES) are a promising alternative to convert CO₂ and electricity into CH₄. However, not much is known about the local conditions and possible gradients at CH₄-producing biocathodes, especially when using granular activated carbon (GAC) as the electrode material. Detecting local conditions at different depths and heights of this 3D material provides better insights on possibly existing limitations. Process conditions and reactor design can be changed to tackle limitations and improve rates and efficiencies. Here, H₂, pH, and oxidation reduction potentials (ORP) were measured locally within the biocathode. First, H₂ was detected locally at -0.63 V_{cathode potential} (versus Ag/AgCl), whereas no H₂ was detected in the outlet gas, suggesting efficient biological use of H₂ as an intermediate. Second, gradients in all three parameters were observed at different depths in the biocathode. Hence, to improve biological activity, it is critical to consider H₂ as a mediator and pH dead zones.