Miniaturizing microbial fuel cells for potential portable power sources: Promises and challenges

Microscale microbial fuel cells (MFCs) are attractive, due to small size, light weight, and potentially low cost, suitable for applications demanding miniaturized carbon-neutral and renewable energy sources to power lowpower electronics and implantable medical devices. The power density of microscale MFCs has enhanced significantly in the past decade, yet the scaling effect on microscale MFCs has not been addressed effectively. This review offers how the scaling impacts the power density of microscale MFCs via mass transfer, reaction kinetics, surface area to volume ratio, and internal resistance. The power density, especially volumetric power density, increases as scaling down the characteristic length of MFCs due to fast mass transfer, fast reaction kinetics, and high surface area to volume ratio, suggesting that microscale MFCs have large potential to improve further. Yet several challenges, including high internal resistance, incompatibility with microfabrication and inefficient extracellular electron transfer due to oxygen leakage need to be adequately addressed. These challenges, along with potential mitigations are discussed in detail in this review. If these challenges are mitigated appropriately, microscale MFCs may become one of the attractive alternatives as miniaturized carbon-neutral renewable power sources.