Sustainable H2 energy sources

H2 is the most abundant gas in the universe, so we can pretty much find it anywhere. With hydrogen and oxygen burned together comes energy.

The only byproduct of Hydrogen/Ox is water

Technically, the fuel can be re-used

Can be used for various different types of energy

Burns at a relatively low temperature

Rockets use Hydrogen and Oxygen propellant

Many rockets, including United Launch Alliance's Atlas V and Vulcan Centaur, Blue Origin's New Shepard and New Glenn Rocket, and many more, all use Hydrogen as their primary fuel. Let's not forget that the Space Shuttle also relied on Hydrogen Fuel to get to orbit successfully. In the future, humanity will be living among the stars, and we'll have plenty of Hydrogen. So, while we're already using it as energy to alter the position of the spacecraft when needed, we might as well use some of it for electricity as well.

In a controlled burn, you'd have infinite propellant

If things are done currently and you are burning your propellant (Hydrogen and Oxygen) in a controlled manner, you could have infinite propellant, as you could keep re-using that same propellant which you just burned. Pure Hydrogen and Oxygen burn together, it produces water vapor (gaseous water/H2O) as a byproduct, you let that gaseous water float around in a sealed environment, you convert the heat generated into electricity via a thermoelectric generator, the water vapor condenses eventually, you use electrolysis to split the water back into Hydrogen and Oxygen, and now you have your propellant back, ready to be burned again.

Rocket Propellant and an electricity source

So, you can use Hydrogen & Oxygen as a propellant for rockets (as humanity will eventually be living among the stars in spaceships A.K.A rockets), and electricity. This makes Hydrogen/Oxygen a great choice for an energy source, however, solar panels still remain extremely simple and reliable. In fact, they're so simple and reliable that most spacecraft use solar panels to constantly recharge their batteries, so that they do not die. Pretty much the only big disadvantage of using Hydrogen as a fuel is, Hydrogen gas (or liquid) is extremely un-dense. This means that Hydrogen will literally try to squeeze through tiny cracks holes, or bolts that aren't screwed in too well. You'd need to have a very thick and well-inspected tank to store Hydrogen, to ensure that no leaks are possible at any point in the tank. Other than that, Hydrogen is an awesome fuel :)