There's been some research into high-temperature electrolysis.

The idea is that a lot of the energy to split the hydrogen and oxygen can be provided as heat; that means you need a lot less electrical energy. To a rough approximation, for every three units of heat energy produced by a nuclear plant, you get one unit of electricity, so the process could in theory considerably reduce the cost of zero-carbon hydrogen.

In practice, it seems like it's going to be very hard for a nuclear plant to beat an electrolyser that runs when near-free solar electricity is available.

As you say, if you get hot enough you can do away with electrolysis entirely. A little bit of casual reading suggests that the temperatures required by a naive approach are in excess of 2000 degrees Celsius, far, far beyond the point where any existing or near-term reactor design would turn into a puddle of very radioactive molten metal.

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.

And then what do you do with the hydrogen? Unless you plan on fueling your spaceship with it, you must burn it off to harvest the thermal energy... I'm sure you see the issue here.

Ammonia, Methanol, etc - Fertilisers and fuels for marine shipping in addition to being transportable "energy" that's less slippery than hydrogen and can travle further than an HVDC transmission line "extension cord".

Currently green hydrogen plants are expanding and ammonia | methanol marine fuel ships have been built and trialed - there are contracts signed and in the works to both build a 4,000 km HVDC "suncable" and to ship hydogen products longer distance.

You pipe it somewhere and burn it, in a heater, or an engine, or a smaller generator, the way natural gas is used today. Or fill a tank with it and power a vehicle.

However, I appreciate the point made about hydrogen escaping and causing embrittlement of materials.

You want to use thermal energy to make hydrogen, then burn that hydrogen to make thermal energy, possibly even to harvest that thermal energy to produce electricity...

The idea is that nuclear fission produces thermal energy, the production of hydrogen provides a means of storing and transporting that energy to its point of use. It might be useful to draw a diagram.

Electricity is really just another means to the same end. You can't use all of the energy generated by a power plant, at the plant site, so you transport the energy to places where it's needed.