Fuel cells do not use electrolyte; but I think you are confusing fuel cells (which take hydrogen and oxygen as fuel to make electricity and water) with an electrolyzer (often called a generator) which uses electricity to split water into hydrogen and oxygen.
Our electrolyzer designs, like the on-board ER HyZor, use lye (NaOH) as the electrolyte.
I’ve tested hundreds of electrolytes, electrolyte concentrations, frequencies, plate spacings, etc. I’m always looking for the most efficient electrolyzer design, electrolyte and power supply (more on that elsewhere). There is a difference between efficiency and practicality. In my commercial electrolyzer designs I use PRACTICALITY as my main design parameter, sacrificing some efficiency for user-friendlyness and lower cost.
So far, I’ve discovered that lye is the most practical electrolyte, because it produces BG cleanly, efficiently, is low cost, has low toxicity and is fairly easy to acquire. I’ve designed my electrolyzers to optimize the characteristics of lye so lye (NaOH) will produce 30% more BG than using KOH.
My electrolyzers are over 100% Faraday efficient if you consider both the BG production and the heat produced. ER1200 WaterTorch independent efficiency test results (without measuring heat energy produced).
A lot of electrolyzers promoted on the internet use baking soda as the catalyst. The reason baking soda works as an electrolyte is because a small portion of it turns into lye. The rest of the stuff in baking soda just clogs the system up with impurities and/or turns into poisons.
You can cleanly achieve the same result (inefficient electrolysis) by using a 5% mixture of lye (by weight). This makes the electrolyte about as benign as citrus juice (see below).