alot of space companies don't really like volatile materials as ballast next to their primary payload for a number of reasons. one of them is that it isn't good to have news reports saying that their multi-million/billion dollar rocket or sattelite was proudly taken down by something the size of a stack of post-it notes. This means that that developers have a number of hoops to jump through in terms of flight readiness. One restriction is usually that there is a ban on all propellants but this can be overcome by proving that you cannot hurt/damage/disrupt/annoy the primary payload. One specific type of fuel that is typically involved is an inert propellant. This basically means that it doesn't react much with other elements or molecules under normal circumstances, which makes it a good candidate for use in my engine since it is likely that launch providers will not be upset over its chemical potential. The first place to look to find suitable fuel is the periodic table of elements.
All elements on the far right column are inert (helium, argon, neon etc) most of these (to my knowledge) are used industrially which means that they may be commercially availible for purchase. Helium is used in MRI machines and as a temporary fun, floating gas which is used to entertain children. Argon is (can be mixed with carbon dioxide) used in welding to ensure that the inside of the join isn't oxidised. Also neon is used in fancy signs outside some shops. Because of their commercial uses it will make it eaiser to obtain.
Many other elements either need to gain or lose electrons in order to be stable which leads on to the second place to look for inert fuel; molecules. these have an advantage since they sometimes take less room as a product than as individual reactants. For example, pure water will take up a much larger volume in its gaseous state than its liquid state since the vibrations of individual molecules is very fast. however one downside with using water is that the oxygen by itself may interfere with the electrodes and end up binding with the metal causing it to rust. If this was to be used, there may have to be expiriments to determine what mixture/coating of metal will be necessary to stop the oxidisation from occuring since it may hinder the conductivity of the electrodes. If a molecule based propellant is to be used, the effects of lone elements has to be considered.
probably the most important part about the fuel is that it has to have a low breakdown voltage. this refers to how strong the electrc field has to be in order to break the atom into an Ion. the componds with the lowest breakdown voltage will be favoured over those with higher values since it will take more power to split them up.