A fuel cell could periodically charge a rechargeable battery or supercapacitor , therfore neutralising each other's shortcomings. I have a feeling you will end up needing a lot of power. Suppose the rechargeble could stay charged for 10 years, then the fuel cell should be able to work 11 times. This seems doable to me. Or you could use a wet battery and just replace the electrolyte with fresh one periodically.
Maybe temperature gradient exploitation using peltier devices?
I was going to suggest a radioisotope generator, but I doubt you can get one.
Or , someone mentioned a turtle. Don't they live 100 years? How much food could a turtle possibly eat in 100 years?