This study gives information of new opportunity fuels having increasing importance is all future energy scenarios. It compares the basic thermodynamic performance of fuel cells with various fuels — ammonia, methanol, hydrogen, carbon monoxide and carbon(s). For both oxygen ion conducting and proton conducting fuel cell, where applicable, its performance as a function of utilization is considered. The fuel cell itself will be considered as a reversible electrochemical reactor, generating power and mixing substances, but without further restrictions on its design. The thermodynamic state and the excess air are further parameters of variation. The consequences of the use of air and oxygen are considered as well. The principal reversible combustion of the fuel is the base of the operation of any fuel cell. The utilisation of the fuel changes the gas concentrations on the anode and cathode side depending on the ionic transport mechanism. The reversible SOFC model was used to describe the influence of the fuel utilisation, the thermodynamic state, and the operational parameters for the fuel H2 on the local Nernst voltage in previous publications. This work has been expanded to proton conducting cells and different opportunity and hydrocarbon fuels. Ammonia is quite different and at lower utilizations appears to be a superior fuel. Methanol is superior to methane over a wide utilization range. Hydrocarbons like methane have a smaller voltage decrease during utilization than hydrogen and carbon monoxide. Excess air larger than two has a small impact on voltage loss. Direct utilization of hydrocarbon fuels without reforming is a key development path toward higher efficiency.

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