User:KhalidH/Sealing the battery

The evolution of hydrogen and oxygen gas is a problem to a greater or lesser extent in all batteries with an aqueous electrolyte. This includes lead acid, NiMH and nicad, but the problem is particularly bad in nickel iron due to the quantity of gas produced being higher and for some reason the chemistry apparently not being amenable to the cheaper approaches used in the other types, possibly including that hydrogen is evolved even when the battery is not being overcharged.

This problem can be dealt with in a number of ways to the extent the battery can be considered "sealed" in that there should be no need to add electrolyte over the course of the batteries intended lifetime, which includes:

-Reducing it to an acceptable level so low that the supply of electrolyte is not the limiting factor in battery life. Especially helps if the battery life is not that long. Depends on clever chemistry, making the iron electrode (where hydrogen is evolved on overcharge) extra large to prevent formation of hydrogen gas until well after the battery is fully charged (because the nickel electrode is fully charged well before the iron one is, preventing the battery from storing any more energy after that point).

-Recombining the gasses There are a wide range of approaches used to do this in batteries of various sizes and types, see the research page for links and patents and notes. In sealed lead acid batteries they can get the gases to recombine in the porous electrode separator by getting the oxygen evolved to the other electrode, where it promptly recombines with the hydrogen for some reason.

The other common method is catalytic combiner caps, also called "hydro caps" which catalyze the conversion of hydrogen to oxygen. The big problem with this is that they usually use platinum group metals which are extremely rare and expensive, albeit in small amounts. Research needs to be done to determine if the relevant metals might be recoverable in the very small amounts needed locally. Patents indicate it might be possible to use the cheaper more common metals like osmium.

Research also needs to be done to determine if different more readily available catalysts like Raney nickel or something might be made to work.

The design of such caps is also not entirely trivial, as the catalyst surface must be kept free of liquid water and any mist from the electrolyte. A lot of heat can also be produced, especially during overcharge conditions.

It might be possible to ignite the mixture in a controlled and safe way with a spark gap or heated nichrome element, thereby eliminating the catalyst metals.

Whatever method is chosen there is also the choice of shared gas space and non-shared. In shared gas space the cells share the same atmosphere and so the water that is produced by recombination needs to be distributed equitably back to the cells again, or water or electrolyte solutes will accumulate in one cell or another. And thermal gradient will also result in redistribution of the water by evaporation and condensation. Non-shared gas space is the simplest in this regard, with each cell having it's own atmosphere, but then you need a separate combiner mechanism for each cell (although of fractional capacity), which may be more complex to manufacture.

Combiner caps made for lead acid batteries might work for us for now, and are readily available. But there may or may not be some minor snags with e.g. access to the catalyst being blocked by the solutes in the electrolyte accumulating, whereas in lead acid the electrolyte solute is also a volatile liquid itself rather than a solid in NiFe. Purchasing ones made for NiFe specifically has the problem that they would likely not be readily available and the supply might dry up in the future if the manufacturer stops making them or puts the price up.

However the battery could be designed, with some compromises, to work in either sealed or non-sealed mode.

Also, sometimes the gasses are not produced in stoichiometric proportions, usually there is an excess of hydrogen because the oxygen reacts with other components in the cell or electrolyte instead of going to O2 gas. Obviously this can't continue forever or it would cause major problems with the battery chemistry and limit battery life. It usually only happens for a while until equilibrium is (nearly) achieved, with the ratio of evolved gasses getting closer and closer to stoichiometric for recombination to water as time goes on, either during the charge cycle or with time after the battery is built. Thus there still needs to be provisions to allow some gas escape, even aside from the safety issues.

If it occurs cyclically (not clear yet form information available) then if venting the excess occurs every cycle, it would cause some loss of electrolyte. Unless it is quite low this is bad. Instead the container will probably have to be made to stand these transient pressures and contain the gas until the other of the pair is also evolved and recombination can proceed.