Fuel cells, like batteries, perform direct electrochemical conversion of chemical energy to electrical energy. No pesky steam.

The problem is chemical bonds are super low energy density.

The only way we know to achieve something resembling that level of energy density is fusion or fission, both of which typically make steam, but that's not the only way to use them.

Satellites use radioisotope thermoelectric generators for power, which have no moving parts and are about 30-40% efficient

In order to make something high powered you can strap onto your chest, you would need some pretty miraculous innovation in:

• radiation shielding, because duh
• efficiency AND heat shielding. I'd wager, without doing the math, even at 99% efficiency, the equipment in Iron Man's suit would start to boil him alive if he didn't have a magic heat management system. Less efficient would of course = more heat to dissipate

If you can't manage those, then you need a whole new form of energy conversion. Some sort of matter to energy fuel cell ala Project Hail Mary Solarphage, or a means of efficiently bending space time to cheat modern physics.