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I was recently reading this article by BMW, that said

The exhaust air of a hydrogen car consists of pure water vapor. The fuel cell drive is therefore locally emission-free. This means it keeps the air in cities clean

I wanted to know as to what extent is this statement true. Does this mean that from the exhaust of a hydrogen powered car, one would expect to collect water vapour, potentially as pure as distilled water? Or are there any other impurities that might also be emitted from the exhaust in trace amounts with this water vapour?

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If the hydrogen burned in a hydrogen car is coming from a tank, the tank contains pure hydrogen, without any impurity, even in traces. The combustion produces pure water, without impurities.

If the hydrogen is used in a galvanic cell, it also produce water, but this water is emitted as a liquid. So the liquid may contain some solute coming from the electrolyte used in the cell, usually caustic soda. The amount of impurities depends on the quality of the membrane allowing water to escape.

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    $\begingroup$ As NASA discovered during the Apollo program, hydrogen fuel cells tend to emit hydrogen in the waste stream. They went through several designs of hydrogen separator before finding one that would keep the drinking water from being excessively fizzy. $\endgroup$
    – Mark
    Commented May 15 at 22:49
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    $\begingroup$ @TAR86 and if (flame-)burning H₂ in air (as we would in a car but not a rocket) , with N₂ available, we could expect some oxides of nitrogen to form as well. $\endgroup$
    – Chris H
    Commented May 16 at 8:39
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    $\begingroup$ @TAR86 Intuitively, I wouldn't expect any significant $\ce{H2O2}$ production in a hydrogen flame: not only does $\ce{H2O2}$ disproportionate to water and oxygen on contact with itself (especially when heated), but it's also a pretty strong oxidizer and quite reactive. If it doesn't find anything else to oxidize, I'd expect hot $\ce{H2O2}$ in contact with air to result in something like $\ce{H2O2 + N2 -> N2O + H2O}$. I could be wrong, though. $\endgroup$
    – Ilmari Karonen
    Commented May 16 at 10:51
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    $\begingroup$ I cannot find the note I took in my undergrad, but I have found pubs.acs.org/doi/abs/10.1021/ja01375a070, which reports that some $\ce{H2O2}$ is formed and detected when rapidly cooling the products of the combustion. $\endgroup$
    – TAR86
    Commented May 16 at 11:51
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    $\begingroup$ Hydrogen burns at something like 1800K. @ChrisH has a very good point, you're going to get quite some NOx formation. This is a real problem. BMW did actually build a hydrogen-burning car before this fuel cell car. And while normal ICE's have catalytic converters, those rely on CO (carbon monoxide) to reduce NOx. You don't have that when you're burning hydrogen. $\endgroup$
    – MSalters
    Commented May 16 at 16:00

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