Hydrogen + oxygen fuel cells powered the week-long Apollo missions to the Moon and Shuttle missions to orbit, and there is a industry trying to build itself around hydrogen-fed fuel cell-based road vehicles.
Cryogenic methane + oxygen deep space vehicles are probably coming, and missions could go well beyond Mars, to moons of Jupiter or Saturn possibly at some point, where solar power is only a few percent as strong as it is circa 1 AU. (see How far from the Sun can solar power be used as a reliable energy source?) Just look at Juno's giant solar panels and its fear of eclipses and Rosetta's need to hibernate when furthest from the Sun for examples.
Question: There is hydrogen in methane, so there is some potential for a fuel cell, but I'd like to know if there are any designs or research into a methane + oxygen based fuel cell for electrical power.
Yes, there are also proposed internal combustion space-engines and one could just use a natural gas powered electrical generator, but for the purposes of this question let's stick to conversion from chemical to electrical power using fuel cell technology.