H2O2 hydrogen peroxide in a highly concentrated form referred to as HTP, was used as a fuel for torpedos mid-20th century (WWII era). The primary reason for this was that through decomposition with another catalyst, the torpedo could provide it's own oxygen by liberating the additional oxygen molecule, providing oxygen to power a turbine engine which would allow it to operate submersed under water & rendering it relatively difficult to detect. Like you, I did research on H2O2 in consideration of converting my own vehicles. I have since discarded it as unfeasible. From what I've found it would be unnecessarily more expensive and complicated to build & process H2O into H2O2, to purchase & store the decomposition medium, to construct or convert a viable engine, and to keep synthesising more fuel for continued operation. You might want to consider other options. Plus some of the decomposition substances used in the past are highly toxic if not deadly if inhaled (I know from military experience that Hydrazine is particularly nasty stuff). Another problem to contend with is the high heat generated in the H2O2 decomposition process & it's effect on the operation of an engine. Granted this was not so much of a consideration for a expendable torpedo or rocket with an expected short operating lifespan, yet I doubt one would want to take a the chance on a personal vehicle engine explosions.
I presume you are NOT looking for an alternate fuel for an underwater engine anyway. So if you are looking for a cheap replacement fuel (or a partial replacement alternate fuel/petrolum fuel mix to reduce your fuel costs), for say an internal combustion engine, you might be better off going the H2O hydrogen fuel cell route instead. Those types of cells use electrolysis to separate highly flammable hydrogen from H2O for use as fuel and hydrogen burns extremely clean (Whereas the decomposing compound for an H2O2 engine would probably add to pollution), and you would be much easier to procure fuel. There is more than enough oxygen in ambient air to burn the hydrogen and if you add baking soda to the water being electrolyzed (as opposed to other suggestions such as salt, acids, lye, or other substances), the by-product is oxygen. I should warn you that hydrogen gas is very volitile when mixed with oxygen gas and can self ignite. The electralysis process creates what is called Brown's gas which cannot be stored safely. However there are ways to to construct a system to keep the gases separated and route the pure hydrogen gas straight into internal combustion system so that it does not combine with the oxygen until just before entering the combustion chamber during the intake cycle.
Another big plus is burning hydrogen does not create carbon emmissions, the con to that pro however is that if smog checks are common in you locality, you could feasibly fail a smog test because if the engine is NOT producing enough pollution (i.e. the test decides that your vehicle must have a leaking exhaust system). So the ability to temporarily adjust, bypass, or shut down the system might be a simpler option to fighting city hall.
Regardless of how nay-sayers respond, a lot of research has gone into synthesizing hydrogen through electrolysis and it is sound physics. Tesla's experiments included such devices. I personally remember seeing similar experiments being conducted in high school science class. It takes less energy to operate one of these types of devices than a vehicle's headlight. Even should you notice the additional drain on the vehicle's charging system, this is easily corrected by installing a high performance alternator. I would also recommend replacing the voltage regulator, preferably with a high performance or adjustable version so you can tweak up the output. If you still notice a problem check your starter for excessive draw, especially with older "broken in" vehicles.
I doubt that the installation a single device is a stand-alone unit sufficient to totally eliminate a need for petroleum fuel. Or that you'll win any drag races with such a device. However, single unit efficiency claims range from 10-90%improvement depending upon the device used, hydrogen output, vehicle weight, etceteras. It may take some experimentation, additional units and/or control/metering/fuel mixing devices to achieve your desired efficiency/performance goal. These units are definitely not the so-called "water injection" devices marketed in the 1980's. These are basically the same type of systems used in hybid vehicles since the beginning of the millinium. The designer of one home-made unit, claimed he purchased a hybrid vehicle solely for the purpose of disassembling it to descern how it worked and was surprised how simple it was to construct a home-made version of an electrolytic fuel cell.
In designing your unit, high grade stainless steel is preferred in constructing the adnoide & cathoids as it is relatively inexpensive, easy to obtain and more resistant to corrosion. Do not use disimilar metals, we are not interested in electroplating or making a battery here. As for servicing intervals, according to sources; in consideration of average privately owned vehicles use, you should only need to reservice the water/baking soda solution about once per month. Fill & drain ports and/or a level reservoir should be considered in your design. The entire unit should be disassembled, inspected for servicability, and cleaned annually.
In an attempt to keep this short(er); for more information & examples of such systems or conceptual help in designing your own, I recommend doing internet searches using the following terms:
"Hotsabi E-cell"
"Joecell Free Energy"
"The Power Tube"
"Brown's gas"
"Stan Meyer" or "Stanley Meyer"
Hope you find this post helpful. By the way I am just a well-read, regular guy with an open mind & no axe to grind who has always loved science & tinkering with stuff. I don't own any stock in any oil companies, I am as disgusted with fossil fuel prices as everyone else, & I have no affiliation with any of the above sources in any way shape or form.
