What transportation energy “currency” (methane, H2, electricity) would be most suitable for mobile machinery on the Martian surface?
Photovoltaic (PV) cells or nuclear energy will likely be the only energy sources on Mars, but both are low mobility. Electricity needs storage before it can be used for the transportation and heavy construction proposed for Mars colonization.
Direct storage of PV electricity in batteries or super capacitors is very attractive, but they have much lower energy density than chemical fuels which lowers the endurance of mobile machinery.
Current plans project large scale ISRU liquid Methane and Oxygen production for rocket propellant, so it is an obvious candidate for transportation and heavy machinery.
A practical problem with chemical fuels which use molecular Oxygen as the oxidizer is the high temperatures produced when they are mixed stoichiometrically. Methane has a 1 bar flame temperature of 1880 °C in air and 2750 °C in oxygen https://www.sciencedirect.com/topics/engineering/flame-temperature . A Carnot cycle engine (ICE, turbine) will have significantly higher pressures and temperatures, exceeding the melting point of many materials.
Could ICE engines (piston or turbine) be run with multistage combustion (like a Raptor) for net stoichiometric combustion with “conventional” temperatures? By running fuel/oxidizer mixture at non-1.0 equivalence, temperatures are significantly lower. The uncombusted fuel (or oxidizer) could be utilized in the next combustion step with the inverse equivalence ratio. https://en.wikipedia.org/wiki/Adiabatic_flame_temperature
Another option would be an ECE such as a Sterling Engine. NASA has a isotope-powered free-piston Sterling alternator which has been running at full power continuously since 2003 https://www.spaceflightinsider.com/space-centers/glenn-research-center/it-keeps-going-and-going-stirling-engine-test-sets-long-duration-record-at-nasa-glenn . Since it outputs electricity, it would pair nicely with PV and battery storage.
Perhaps the Holy Grail is a low-temperature Methane Fuel Cell. https://cen.acs.org/articles/93/i43/Best-Effort-Yet-Make-Direct.html