I am surprised Russia has not developed a fuel cell AIP system. They have most of the tech in their space program already.
The Russian system was more ambitious than western systems which just use stores of hydrogen and stores of oxygen to run their hydrogen fuel cells.
The Russian system was many times more powerful than existing hydrogen fuel cells which makes them more viable... the Japanese tested fuel cells on a sub and found the energy level was so low the sub was essentially useless while it was charging up the batteries which took much longer than running diesel generators to do so, so while it is quieter, it makes the sub useless for a much longer period.
After experience with an AIP sub the next sub they made removed the AIP and hydrogen and oxygen tanks and just replaced them with more lithium ion batteries.
The Russian AIP is very very interesting because apparently it extracts the hydrogen from diesel fuel, which is handy because a diesel electric sub is already carrying diesel fuel and every port on the planet already has equipment and infrastructure in place to provide diesel fuel to ships and subs.. whereas very few have the same infrastructure for hydrogen or oxygen. It was also reported that this AIP technology generates four or five times more power than existing hydrogen fuel cell AIP types which might actually make them a viable means to recharge and also operate a sub underwater without the loss of function while charging.
Another interesting aspect is that fuel cells don't use combustion so an AIP that uses diesel fuel will create a lot of solid carbon waste and that might be the problem... how to deal with it.
Ironically solid carbon is actually valuable and can be used to make carbon fibre based products as well as carbon nano tubes which offer enormous potential.
Being able to use diesel or petrol or kerosene without combusting it solves a number of problems... transporting and storing hydrogen is a problem because you either need extreme cold or enormous pressure or a mix of both... diesel and other fuel storage is not only easier it is already in place and the distribution networks already in place can remain... your car might lose the internal combustion engine and get a fuel cell, you buy your fuel as normal, but when you fill up you might hand over a large cube of carbon produced from the previous tank of fuel that you can sell back to the garage or take to a recycling place... or do anything you like with... if it isn't burning then it isn't creating green house gasses so it is fine... the exhaust pipe at the back of your car will be releasing water vapour and heat from the fuel cell process that is essentially oxidising hydrogen.
Of course new graphine based batteries might mean you charge up once and can use them for months...