Haithar,
First, there is a benefit of mixing HHO into a carbon based fuel stream, particularly diesel fuel, as you can get real world 15-24% fuel economy increases diesel fuel savings, and you cut significantly into the emissions and particulates coming out of the exhaust, and extend engine life. The ROI is often less than a year and the savings can run into serious money.
The storage medium for hydrogen is the water, if you can make the hydrogen very cost effectively, make it onsite, and make it on demand fast enough to meet demand, your storage issue goes away.
The problem with fuel cells is their CAP EX is too high, their run time is too short, and if the hydrogen is matched to a long running quality hydrogen engine and genset, the comps are going to push the fuel cells to very narrow specialty applications.