Peter Johnstone of Wyndham asks :-
If hydrogen is used to run a car, could the resulting water be re-electrolysed to produce a continuous hydrogen cycle, and do the same gasses bubble up from both electrodes?
John Campbell, a physicist at the University of Canterbury, responded.
No, because there are loses. In electrolysing water into hydrogen and oxygen the negative electrode has an excess of negative electrical charge, ie electrons, so attracts positively charged ions (hydrogen ions) each of which picks up an extra electron and becomes a neutral (uncharged) atom. Two of these combine to form a stable hydrogen molecule which joins with others and bubbles to the surface. The opposite (positive) electrode attracts only negatively charged ions (oxygen ions) and only oxygen is released there.
This is a standard demonstration in chemistry. Collecting the gases released at the separate electrodes shows that twice the volume of hydrogen gas is released compared to oxygen, demonstrating that water is a molecule consisting of two atoms of hydrogen and one atom of oxygen.
It is a standard way for schools to generate oxygen (plunging a glowing stick into that container sees it flare up and burn) and/or hydrogen (plunge a glowing stick into it mixed with air causes an explosion).
It takes energy (usually electrical) to split water into hydrogen and oxygen. It is not an efficient process as some energy is lost in the process. However, where it is efficient is when there is a low demand for hydroelectricity and water from a full dam is being wasted flowing over the dam's level controller. Then the hydrogen can be produced and stored and used to run engines, and the oxygen can be stored to be used in industry, e.g. in acetylene welders or in making steel.
When hydrogen and oxygen are burnt to form water they give out energy but there are large losses. (e.g. the engine gets hot.) So we cannot have a closed cycle engine recycling the same water. We must put energy into the system.
Don't forget hydrogen is a dangerous commodity. Several early airships that used hydrogen for their lift caught fire due to accidental sparks or lightning.