Asteroid Mining - Gas Storage Question

Ok. Here's the scenario. Early in the lunar facility development,
we're creating a lot of oxygen as a by-product of steel making.

We take the oxygen and steel up to cislunar orbit. Using vacuum
vapor disposition we create a big tank and attach it to a framework
with an engine attached. We attach the oxygen tank as well. The
power source is a solar powered ion engine using the low ISP oxygen
as the propellent.

This vehicle makes its way to a NEA. Once there is starts melting
ice by using the solar array that powered the engine and storing the
water in the tank. Once full, the tanker electrolysizes the water to
extract the oxygen for the ion engine.

Is it possible to pump the hydrogen gas back into the water tank? Is
there a limit to how much hydrogen can co-exist with the water?

I know we could use the hydrogen for the engine, but I'm assuming
the engine is optimized for oxygen, I'm assuming the hydrogen is
more useful in LEO or on the moon, where it can be combined with the
Lunar oxygen by-product.

So, what's wrong with this scenario? I also know I could be using
nuclear power or solar sails, but for now I'm concerned with the
technical flaws of what I've proposed.

Anyone care to point out the flaws?