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Precipitating (not plating) ZnO + KOH Back to Zn + KOH

Q. My question is really about how to NOT electroplate something.

A zinc-air fuel cell will start with a solution of 40% metallic zinc powder in potassium hydroxide. A powder is one way to maximize the surface area and hence the current produced. As a load is placed across the electrodes, the result will be a current as the zinc becomes zinc oxide in KOH. So after the reaction is completed, I end up with a solution of zinc oxide in potassium hydroxide. The zinc could be electroplated back out to metallic zinc, but then it would need to be ground or formed to maximize the surface area for the next cycle of fuel cell use. My question is whether it could be precipitated out directly into a metallic zinc powder again rather than plating it out. I had thought that by using stainless steel or graphite electrodes, that the plating would "fail" but the oxygen would still have been split away from the zinc leaving a fine powder of metallic zinc. Or is there some better electrode metal that would be non-reactive in KOH through this process ? Ideally, there would be no additives involved, just pure KOH and zinc. Otherwise the reversibility of the reaction would be impaired.

Will the above work to precipitate the zinc ? If so, how do I go about calculating the current, voltage, and time required to return all the ZnO to Zn ?

Kirk Ellis
- Walnut, California

A. Nice theory, but not practical. Zinc air batteries are very complex and once started they have only a limited useful life. However, you can get rechargeable zinc air batteries - they are quite popular in mobile phones. There are quite a few websites on zinc-air batteries, so check them out too.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Q. Thanks for the input, and what you say is true for batteries. But I think you are confusing Zinc-Air batteries and Zinc-Air fuel cells. (I've read everything I can find on the web and at the patent office on both.)

Most of what I've described is a well-established cycle for Zinc-Air fuel cells and is in development in several places -- including Lawrence Livermore Labs and a company called PowerZinc. The only novelty to what I'm trying to pin down is the method and efficiency of directly precipitating the Zinc back out of the solution. Existing processes focus on plating it out and then grinding the metal back to a powder or pellet form.

This method requires the ZnO and solution be pumped out to an external system which drastically reduces the convenience and energy density of the complete system. What I'm thinking of may not be efficient enough to be practical, but I don't know how to calculate the energy required and I hoped someone here would be able to tell me how to go about that or recommend a good reference source.

Kirk Ellis
- Walnut, California


A. Dear Kirk, The concept of a zinc-air fuel cell relies on the sequential oxidation and reduction etc of zinc and zinc oxide. The concept has been shown to be feasible and can produce energy densities of 200 W-hr/kg of zinc. However, there are a few drawbacks.

Firstly, the released power efficiency of the oxidative reaction is only between 30 and 50%; whilst this compares favourably with the hydrogen regenerative fuel cell that has an efficiency of between 20 and 40%, it is not as good as it could be.

Secondly, the potassium hydroxide has to be continuously pumped through the zinc pellets as the reaction between the zinc and hydroxide forms a solution of potassium zincate. This dissociates to form insoluble zinc oxide that is deposited on the active zinc and kills any further reaction.

Thirdly, the regenerative fuel cell requires an "electrolyser" to regenerate the raw materials and this is where you want to recover the zinc from the zincate solution. Usually electrolysers use an external power source, such as the mains. Whilst I am no expert on the electrochemistry of zinc, I believe zinc can be recovered from zincate by electrolytic reduction but it only has a typical efficiency of about 55%, so there is a lot of energy lost there. If you use zinc cyanide in the alkaline bath, the efficiency increases significantly, as it also does for acid zinc, but these are of no use to you.

There is also a problem with potassium hydroxide, as it absorbs carbon dioxide from the atmosphere to form potassium carbonate - this will reduce the efficiency of the fuel cell and any recovery process. I think you are also asking whether it is possible to get an electroless deposit from a zincate solution and I do not think this is possible.

You will need to use a more electronegative metal, such as aluminium to stand any chance and even then I would suspect the reaction will only produce zinc oxide. It will also contaminate the electrolyte with potassium aluminate and aluminium oxide/hydroxide. Even if you do succeed in getting metallic zinc from the zincate solution, you will need to get the correct structure in the deposit; the key to any electrochemical reaction is the availability of active surface area within a good electrical conductor. Give the "porous electrode theory" a good looking at, as this may help you determine the best target conditions for the zinc anode. (The PET is good fun, especially if you also like beating yourself with birch twigs or barbed wire!).

"Fuel Cell Systems Explained"

on AbeBooks

or Amazon

(affil links)

You also mention using stainless steel or graphite electrodes; these will influence the efficiency of the zinc deposition of the cathode as their hydrogen overvoltage (HOV) will come into play. The HOV is a quite complex subject, but suffice it to say that the surface finish is one important aspect of its value, so you may want to consider that also. If you also want to regenerate oxygen on these electrode materials, you will similarly have to consider the oxygen overvoltage on the anode.

Consequently, not only do you have to take account of inefficient electrochemical reactions, you also have to consider the energy consumption of pumps, blowers and process controllers. Finally, remember the Law of Conservation of Energy - if you can crack that, patent it and sell it to any of the petrochemical companies and you will never have to work again!

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


Q. Trevor, now that's what I call some useful information ! Thanks !

First, I know the overall efficiency isn't great, but the other advantages over hydrogen fuel cells seem to more than compensate -- at least for transportation applications.

Second, I've seen the approach of pumping electrolyte to wash the zinc oxide from the zinc and keep the metallic zinc exposed and reactive. I have another approach to accomplish that which seems more elegant to me, but it will definitely be taken care of.

Third, yes I am attempting to determine whether the electrolysis back to metallic zinc can be done in situ. It would make the system more flexible -- allowing electrical recharging as well as mechanical recharging via fluid replacement. (Those zinc-air fuel cells I've read about point to the advantages of being able to mechanically refuel the cell, but that begs the question of an infrastructure being in place. Having the option to "plug in" seems critical to near-term acceptance, to me.) So far it looks to me like 4.1 V at 400 A/square meter is typical for ZnO electrolytic reduction, but that would be influenced by the less-than-ideal pure KOH solution I need to maintain, as well as by my choice of electrodes. 50% is the number I've seen most often for the efficiency of this end of the cycle. That makes it more expensive, but might be worth it to achieve convenience and the energy density for transportation uses. The absorption of CO2 and formation of carbonates is one of those things I have no numbers for. Of course, the air could be scrubbed of CO2 prior to entering the cells, but that adds complexity/size/weight and it may also not be necessary. Depending on how fast it accumulates, it may be acceptable to introduce a cleaning cycle. I am not after an electroless deposit of the zinc. What I want is to electrolyse it out. As you mention, most electrode materials would pollute the solution. Plus zinc would plate to them. I was looking for electrode material which would *reject* plating and be unaffected by the KOH solution. Stainless steel and graphite are safe in KOH, and I thought virtually "unplateable".

This is why I posted to in the first place. My assumption (layman and very possibly wrong) was that during electrolysing, the oxygen would be stripped back off the ZnO by all the available electrons on the cathode surface. The metallic zinc would then attempt to plate to the cathode, but through a combination of agitation of the cathode and the choice of cathode material, I could prevent that from happening. The metallic zinc -- having nothing to stick to -- would form high-surface area crystals in powder form and precipitate out or just sit in solution but not recombine to ZnO. Basically I need to choose an electrode material that is:

1) Safe in KOH
2) Difficult to plate zinc to
3) Highly conductive
4) Now you've added the hydrogen and oxygen production issues.

I'd assumed oxygen would be produced during the electrolysis, but I didn't know the electrode material would so significantly affect the rate of production. In fact, one of my concerns was that I was trying to build a sealed system of electrolyte and zinc, and I knew I'd end up with losses due to splitting of the water from the KOH solution. I expected to lose hydrogen during the "discharge" of the cell and oxygen during the "recharge" of the cell. Can you point me to a good reference to quantify those losses ?

Finally, the efficiency of the complete cycle isn't as important as the advantages over a hydrogen fuel cell. The energy density is theoretically higher. The initial capital cost of manufacturing this type of Zinc-Air fuel cell should be much lower than the PEM designs for hydrogen fuel cells. Plus the ease of fuel containment, much smaller infrastructure requirements, safety, etc.

I'm not trying to break the law of conservation of energy. In fact, I expect to lose a lot of it along the way. But how efficient do you figure gasoline is when you add in the losses during the refining stage as well as the piddly 20% efficiency in an engine ? Convenience and price rule when it comes to transportation energy. And if you can do it without any emissions, so much the better.

Kirk Ellis
- Walnut, California

A. Kirk, Just a few points on your generous response. Firstly, the reduction of zinc oxide on the cathode must be undertaken with the zinc oxide in solution, so it will be in some form of (Zn(OH)4)2- or another soluble hydrated oxide.
Secondly, remember if you do succeed in reducing the oxide to zinc metal, it will be a chemically very active material and is bathed in a bath of strong KOH. The KOH will re-dissolve it as soon as it can! Thirdly, one thought that may be useful to you is to consider fluidised bed technology as the recovery process. I saw it some years ago, but do not know how far the technology has advanced.
Fourthly, one book I do like, if it is still available is Kortum - Treatise on Electrochemistry. It makes excellent bedtime reading for the insomniac, but is a wealth of information.
Fifthly, do I detect blind faith in the gasoline companies? They consider anything that may take their market as disruptive technology and will do almost anything to keep it out of their sector!

I am not aware of any petrol company that willingly entered into pollution control and environmental improvements without being pushed by the legislators, or am I being a bit too cynical? Sixthly, I agree that petrol engines are only 15% efficient, and considering that only about 15% of crude ends up as gasoline, I would expect the net efficiency of a petrol engine to be well below 1%, especially once extraction and refinery energies are considered. Perhaps someone should do an energy audit on this issue. Finally, I have no idea if your estimates of 4.1 V and 400 A/m2 is correct! Perhaps someone with more experience in zinc plating can add to this discussion. Good luck.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

March 15, 2009

Q. I have read about Zinc air batteries some days now. I have seen this formula:

Zn + 1/2 O2 + KOH -> KZnOOH -> ZnO + KOH

I have also read in scientists papers that "zinc oxide is dissolved in KOH solution to form a zincate-rich feed":

ZnO + 2KOH + H2O -> K2Zn(OH)4

So that means you never will get any ZnO? When all Zn is dissolved into K2Zn(OH)4 you have to recharge (they call it electrowinning):

K2Zn(OH)4 -> Zn + 2KOH + H20 + 1/2 O2

If you want to make a closed battery I think it doesn't matter if the zinc will be fixed back onto the thin plates. Anyway the zinc have to be somewhere. You may make a big area by packing Cu plates with something between them to make a safe distance.

I don't know how this "zincate-rich feed" will behave between the plates. Anyway there is no ZnO on the plates preventing recharging.

I have no practice - I don't know chemistry. I just repeat what I have read and what make sense. Pure empty open minded brain ;)

Good Luck

Pauli Terho
- Sweden

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