HHO is unfortunate an energy carrier not a source. Takes more power to separate h2o into 2h2o2 commonly called hho or orthohydrogen. Would be neat if you could bottle the stuff and have wind power make it but you absolutely can’t bottle it unfortunately. The other option with it is a battery bank and you plug it in and is the batteries to make hh0 while you drive. But electric assist motors seem to be more effective in just burning less fuel. NOX is a result of high pressure combustion and or higher temperatures combustion. I don’t see a way around not having SCR NOX capturing equipment even if a vehicle ran just on hydrogen. The exhaust would be indeed carbon free. Nearly pure water but also Nitric oxides, nitrogen dioxide, nitrous oxide. All the reactive NOX that likes to suck up free O2. Meh who knows what we will have in 20-30 years