I'm developing a system for educational purposes where electrolysis of water takes place. The aim is to practically show how electrical energy can be converted to mechanical, by electrolyzing water, igniting the mixture of $\ce{H2}$ and $\ce{O2}$ produced, and accelerating a mass upwards.
Currently I'm using citric acid in distilled water ($4\%$ in weight, $18~\mathrm L$ water), platinum-coated mesh electrodes, $500~\mathrm W$ max power supply ($0\!-\!65~\mathrm{V_{DC}}$, $0\!-\!10~\mathrm A$). I'm wondering if there's any better option that meets the following requirements:
- Not harmful to maintainers (that have to prepare and replace the liquid) and bystanders
- Stable (not producing precipitates, and in general that can minimize the need for replacing the liquid)
- Low/no interaction with the materials in the tank (Plexiglas, brass, platinum, silicone tubes, ABS)
- Maximizes conductivity (the current setup at $20~^\circ \mathrm C$ has a conductance of $250\!-\!300~\mathrm{mS}$)
- Not suitable for microorganism development (in the current setup algae develops in about two weeks)
If the last point can't be improved with just the electrolyte, I'd kindly ask for a suggestion for an additive that is not going to spoil the points above.