As far as I know (and its not very far to be fair), the main way to generate hydrogen would be via electrolysis in water, which requires electricity. Which mean

As far as I know (and its not very far to be fair), the main way to generate hydrogen would be via electrolysis in water, which requires electricity. Which means you have to turn that heat into electricity first anyway. This is one of the big critiques of using hydrogen as an everyday fuel source. Takes more energy to produce that you can recover. Its super hard to store (requires very cold temps, or high pressure), and it has a habit of wiggling through any other material's atomic bonds and overall weakening the container material. But if there is another chemical method that used thermal energy in the process to produce hydrogen then there might be some possibilities in the idea.