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I am trying to determine how long a system that constantly converts water into H and O and then back will last, as in roughly how many cycles. Now this system can either be made of say a reversible fuel cell or a non-reversible fuel cell combined with a dedicated electrolysis machine (if there is a difference in lifetime). As far as I can tell, platinum is used (almost) everywhere as catalyst and as far I can determine it seems to corrode somewhat throughout use, especially in terms of splitting the oxygen in the fuel cell. I am no professional though and I can only find limited information so I would appreciate it if someone could please tell me roughly how long a system like this will last and how.

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Fuel cell manufacturers quote a range of different lifetimes which depend on many factors. Backup cells tend to have lower lifetimes as the start-stop process degrades them compared to continuous operation. On the other hand, use as engines in vehicles requires a reasonably long life.

Impure fuel will damage the platinum (CO for example will poison the catalytic surface). But platinum is recyclable so either poisoning or other factors won't necessarily be a fundamental barrier to use (some companies take used cells back at the end of life and reuse the platinum). Membranes and electrodes also degrade mechanically depending on duty cycle and any mechanical stabilisation in the design.

Nedstack quotes lifetimes ranging from 2,000 hours for backup cells (which presumably means years of actual operation as they only get used when other power is cut which isn't a routine happening) and 8,000-16,000 hours for continuous operation (which is two years). Tests of fuel cells in vehicles have reported lifetimes of 7,500 hours (which presumably relates to several years of use in practice), though they seem to need more maintenance than diesel engines.

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Poisoning of the catalyst, e.g. by $\ce{CO}$, is only one aspect that reduces the lifetime of PEM fuel cells.

The Nafion polymer membrane can suffer due to mechanical stress as a result of rapid temperature changes or dehydration.

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Fuel cells don't convert water into H and O. The generation of hydorgen gas is done at some point prior to being introduced into the cell (as in the case of hydrogen combustion engines). The first stage of the fuel cell uses a catalyst, like platinum, to split hydrogen gas (H2) into atomic hydrogen. Since the catalyst is a conductor and is sandwiched next to a proton-permeable membrane (proton exchange membrane), the electrons from the hydrogen are also separated and conducted to the final stage of the cell.

If the fuel cell itself was generating hydrogen from H2O, the system as a whole would generate no net energy--all of the energy obtained from the hydrogen reaction at the end would need to go back into separating the H2O in the first place.

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    $\begingroup$ Welcome to chemistry.se! While these are all interesting and informative (and true) facts, they unfortunately do not provide an answer to the question. $\endgroup$ – Martin - マーチン May 30 '14 at 7:23
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I have actually purchased and run three fuel cell stacks. One was dead on arrival and never converted any hydrogen to electricity. If you ever buy one, you will notice no warranty, no return policy and no expected useful life. This led us to actually invent a way to probe PEMFC health without putting hydrogen in, and we got a patent on that. The second fuel cell ran for about 40 hours, but the performance degraded quickly over that time. The third one also degraded and died within 200 hours. These were commercial stacks used with extreme care in a research lab facility by expert engineers. Given that demonstration cars and buses also don't last very long, I think we know why fuel cells are not going anywhere. The nature of the degradation is such that there isn't much that can be done to make the PEMFC more robust. See publication by S. Page in Journal of IEEE.

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