On the technical challenges affecting the performance of direct internal reforming biogas solid oxide fuel cells

Fuel cells (FCs) are electrochemical devices that can be used for the direct generation of electrical energy from the chemical energy in fuels with high efficiency, and low or no environmental impact. In particular, high temperature FCs such as solid oxide FCs (SOFCs) are also recognized for their cost-effectiveness and the fact that they can be fed with complex fuels such as methane or biogas which makes them ideal technologies for simultaneous waste treatment and energy generation. Directly fed SOFCs are known as direct internal reforming SOFCs (DIR SOFC) where the reforming of the biogas fuel occurs at the inlet of the cell producing carbon monoxide and hydrogen, which consequently react with oxygen ions along the anode side, and thereby producing electricity. DIR SOFCs integrated with biogas fermenter in wastewater treatment plants or other biological processes offer a window of opportunity for widespread utilization of SOFCs on commercial scale as a green energy source capable of producing several kWs to MWs. However, the performance degradation rate is one of the key issues that directly affects the long-term costs of SOFCs. In addition to the traditional problems, i.e. thermal cycling, oxidation cycling, and incompatibility of components, the DIR biogas SOFC is also affected by the thermal stresses resulting from endothermic reforming reactions and exothermic electrochemical oxidation reactions at the anode surface, the formation of carbon deposits, and the poisoning with impurities such as H2S, siloxanes, and halides. This review paper presents an up-to-date summary of the different processes and mechanisms leading to most of the issues encountered with the operation of DIR biogas SOFC, and advises on the methods and strategies that can be applied to mitigate these problems.