Technology

Capacitors are never going to have the volumetric density of chemical batteries. That kind of tech is not going to work for anything that needs to be small and mobile (which includes cars, which need to be small for the amount of energy they can store).

I know less about hydrogen fuel cells, but hydrogen also has very low energy storage capacity per unit volume (unless you can manage to keep it at very high pressure or very low temperature, which introduces its own challenges).

There will be applications that can use these types of things, but I doubt it will ever be a drop in replacement for chemical storage.

Hydrogen is actually technically very light. Just 1kg (about 2lbs) is about the equivalent of the battery in a lot of EVs. However the equipment to convert that energy into motion at the wheels tends to be quite heavy and expensive.

More than that, the storage tanks required to store an effective amount of hydrogen are insanely heavy and inefficient. A full tank might be only 6% hydrogen by weight, the rest being the weight of the tank itself.

The tanks are kept under extremely high pressures to achieve acceptable storage density, so safety is a concern as well. Unless this problem is solved I don't see fuel cells replacing batteries in transportation.

the Japanese manufacturers (Toyota/etc) seem to think it's the right way to go.

A big factor in this is that Japan's overall energy transition strategy is heavily focused on hydrogen, and has been since the 1970s. Back then hydrogen was considered one of the promising alternatives alongside biofuel and battery electric vehicles. Today battery electric has taken a clear lead and fuel cells are nowhere close, but Japanese industry is already heavily invested in hydrogen tech (and receives substantial government subsidy).

There is some potential for hydrogen still. It's probably the only feasible means of decarbonising heavy industries such as steel production. It's a potential option for grid-scale energy storage, given that it's fairly easily produced using surplus renewable energy.

Ok, so we're onto capacitors and fuel cells now. Here we go with my mental image of those.

On the pros side, capacitors can charge/discharge in an instant, have nearly perfect energy recovery (almost everything you put in comes back out), and have almost limitless charging cycles with no memory effects or any of that nonsense. On the cons side, even a supercapacitor can't match battery tech for energy storage density and they tend to be not so great for long-term electrical storage.

Fuel cells are sort of the opposite. Once you've sorted out the challenges in producing/storing/transporting hydrogen (these are all non-trivial but not necessarily deal-breakers either?), you're looking at essentially limitless storage duration since it's, well, a fuel. Like you can stockpile it for next year. The energy density is enviably high, though with pure hydrogen, you're doing better by weight than by volume. In any case though, it's looking pretty good compared to batteries.

But pulling the energy out of them in a timely manner is a major pain. You need either a chemical or thermal catalyst to speed it along for most applications. And the chemical of choice is platinum, which is not exactly abundant. If you wanted a fuel cell in every car, is there even enough on Earth for that? I'm not so sure.

Also, I have read impurities in the fuel can really mess up this type of fuel cell. The thermal type is purportedly more forgiving in this respect, though I picture thermal fuel cells as these hulking things that would work best as stationary power plants? Well, maybe they would be a good fit for large ships? It's hard to picture some giant container vessel plying the oceans on battery power, at any rate.