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There are arguments against the storage of hydrogen for use as a propellant in answers to this question.

Speaking of LOX - there isn't any on the ISS. In this answer it is explained that oxygen is brought to the ISS in very high pressure gaseous form, and stored as a high pressure gas. To develop significant thrust, it is usually necessary to deliver oxygen and hydrogen in liquid form to get enough mass per second into a small enough reaction volume. So a system to produce and store LOX would also be necessary.

There are arguments against the storage of hydrogen for use as a propellant in answers to this question.

Speaking of LOX - there isn't any on the ISS. In this answer it is explained that oxygen is brought to the ISS in very high pressure gaseous form, and stored as a high pressure gas. To develop significant thrust, it is usually necessary to deliver oxygen and hydrogen in liquid form to get enough mass per second into a small enough reaction volume. So a system to produce and store LOX would also be necessary.

Using LH₂ for regular burns to maintain ISS altitude would require a whole new set of engines that may not yet exist anywhere. While LH₂ is one of the main workhorses getting big things into space from a launchpad (e.g. Saturn V except 1st stage, Space Shuttle) it's not usually kept stored for long periods, and engines designed for long term usage and multiple starts over time without maintenance would be a technical challenge. Engines that use cryogenic fuels are substantially more complex to build, operate, and maintain, as they have components that require pre-ignition cool downs.

Using LH₂ for regular burns to maintain ISS altitude would require somehow adding a new set of engines to the ISS , or transferring the hydrogen (and LOX from somewhere) to a docked spacecraft which could be refueled.

Speaking of LOX - there isn't any on the ISS. In this answer it is explained that oxygen is brought to the ISS in very high pressure gaseous form, and stored as a high pressure gas. To develop significant thrust, it is usually necessary to deliver oxygen and hydrogen in liquid form to get enough mass per second into a small enough reaction volume. So a system to produce and store LOX would also be necessary.

Then you have to invent an engine that can effectively use the hydrogen, which now means you have to dip into your precious supply of liquid oxygen to run the engine. There is so much experience with the types of engines and propellants used now that it's hard to imagine starting again from scratch for a hydrogen powered engine, and even harder to imagine how it would be better.

Using LH₂ for regular burns to maintain ISS altitude would require a whole new set of engines that may not yet exist anywhere. While LH₂ is one of the main workhorses getting big things into space from a launchpad (e.g. Saturn V except 1st stage, Space Shuttle) it's not usually kept stored for long periods, and engines designed for long term usage and multiple starts over time without maintenance would be a technical challenge. Engines that use cryogenic fuels are substantially more complex to build, operate, and maintain, as they have components that require pre-ignition cool downs.