I'm no expert but I think they are by design quite large...

 

I don't know, but Toyota are developing solid state batteries that are alleged to be half the weight of equivalent capacity Li-ion, and they charge very fast. Not sure it is instantaneous though.

With flow batteries, I would imagine the refilling and removal of spent electrolyte could cause a few issues for public use? But I guess there would be ways around that.

 

There have been a few prototype cars around using these but it requires a distribution network like petrol etc.

 

These aren't passive fluids. Highly oxidising/reducing.

Would you decant a gallon of conc nitric acid into one bucket and a gallon of lithium hydroxide in the other? heh. Make sure you don't splash!!!

What sort of buckets are you going to use, even?!?!

So the reasons are
1) handling issues are very dangerous
2) storage and transportation of haz mat is then required (where will they get processed
3) as they are processed, emptied, filled, they may become contaminated and this will kill your electro-cell very quickly, who do you blame for that, the last person that used the fluid, the manufacturer, it was raining at the time?

It makes hydrogen look safer than baby powder.

In a closed regenerating system then these problems go away. So it makes sense for static, large scale sealed systems. It is my considered professional opinion that anyone peddling this as a means for quick car recharging are peddling snake oil, and I won't change that opinion until it's available to buy from a pump. It never will, not in my lifetime, I would put good money into that bet.

Forget it. People are out to swindle investors/Gov grants here, stay very very wide clear of getting involved in anyone claiming to be able to do this.

If there are any genuine researchers and business investors that think this comment is gratuitously wrong and they have a serious proposition, do PM me, if it makes sense I would be a very strong advocate. Very happy to have a technical discussion about it with anyone who takes this stuff seriously.

 

In a closed regenerating system then these problems go away. So it makes sense for static, large scale sealed systems. It is my considered professional opinion that anyone peddling this as a means for quick car recharging are peddling snake oil, and I won't change that opinion until it's available to buy from a pump. It never will, not in my lifetime, I would put good money into that bet.

Forget it. People are out to swindle investors/Gov grants here, stay very very wide clear of getting involved in anyone claiming to be able to do this.

If there are any genuine researchers and business investors that think this comment is gratuitously wrong and they have a serious proposition, do PM me, if it makes sense I would be a very strong advocate. Very happy to have a technical discussion about it with anyone who takes this stuff seriously.

Completely agree.

I've looked into them a little and flow batteries in EV's will never happen, for a few reasons:

1) Energy density both volume and mass is a long, long way behind even current Lithium Ion technology, let alone the promised, fabled solid state electrolyte designs that are (hopefully) on the horizon in the next few years, with no real way to increase the density. (How can you increase the density of a tank of fluid ?)

Including the tanks and the pumping system they're big and heavy for the amount of energy they can store. That reason alone makes them impractical on a mobile device like a car where energy density is everything.

2) Power density is even worse in relation to Lithium Ion. For a given Ah capacity the peak power discharge and charge rates are WAY lower than Lithium Ion. This is fundamental to the design of having only a small portion of the electrolyte in the reaction chamber at once - power output (and charge rate) is limited by the surface area between the electrodes, and if 90% of your Ah capacity is sitting in a tank at any given time and only 10% is in the reaction chamber then your power output in relation to Ah is only going to be 10%. On the other hand all of the capacity of a Lithium Ion battery (and most other kinds) is available at once as all the electrolyte is "active" and available between the electrodes at all times.

So even if you could stomach the weight, size and complexity (pumps, filtering etc) the peak power output/input for acceleration and rapid charging rates would be painfully disappointing. Power output is also linked to flow rate, so at higher power outputs/inputs you need to pump the electrolyte through faster so it isn't as responsive as a normal battery design to large changes in output, (like suddenly punching the throttle) and yet you don't want to be pumping it through at maximum speed all the time either, so it isn't well suited to rapidly changing loads.

So what's so great about flow batteries ? They have two main strong points:

1) They scale up well to large Ah capacities - instead of having to make more and more cells, you can scale up the Ah capacity simply by having larger electrolyte tanks. Tanks are very low tech compared to the complexity of making lots more individual Lithium Ion cells.

Every extra cell you add in a Lithium Ion pack statistically increases your risk of a cell failure, and some battery packs designs do not cope with failed cells. If they are all in series for instance like they are in an i-Miev (or Leaf ?) a single cell failure makes the whole pack useless. You can work around this in a parallel/series design like Tesla use where you have groups of 12 individually fused cells in parallel, with each group then connected in series - which will survive individual cell failures, but at the expense of additional complexity. (Individual fuse wires etc)

A bigger flow battery tank is fundamentally not any less reliable or more likely to fail than a small flow battery tank. Of course if you need higher power output or charging rates, you need a larger reaction chamber as well, but many applications like grid storage can make do with a very high Ah storage capacity with a modest power input/output rate.

2) In theory a flow battery can last "forever" with no Ah capacity degradation at all. This is also useful in something like grid storage - if you use current tech Lithium Ion batteries in grid storage today gradually they will lose their usable capacity, and then what do you do with them when they reach end of life usable capacity ? Flow batteries solve that by theoretically not degrading at all. Whether this has been proven in real world use yet I'm not sure.

All of which means flow batteries are the perfect choice for grid storage to help buffer peaks and dips in demand and make renewable generation the majority generation in the country. For stationary storage it doesn't matter how big and heavy they are if they are exceptionally reliable and long lived, and an already functioning flow battery can even have its storage capacity upgraded on the fly without shutting it down simply by building another pair of tanks and connecting them to the flow.

Not suitable for EV use whatsoever though, regardless of whether you charge them or use electrolyte swapping - anyone proposing this simply hasn't studied their properties, and you don't have to be a battery expert with a university degree to understand why they're not suitable.

 

We need to know how much energy is required to prepare the electrolites for use, and the overall costs and efficiencies.

 

Thanks @donald - I'll pass on the decanting of various noxious substances out of, and into, my car if that's all-right. Waiting 40 minutes or so now seems to be a quite reasonable thing to do.

 

I've done some work on flow batteries at the R&D stage. It's too early yet to see whether they will get to mass market in any form. There is no need to remove and replace the liquid electrolytes. They are stored in (usually two) tanks external to the cell, rather than being left in the cell all the time as in a conventional battery. That means the capacity of the flow-cell is limited by the size of the tanks rather than the size of the battery, which is simply large enough to provide then needed power output. So power output and range are separate things, just like with an ICE.

To recharge, the flow cell is just operated in reverse, with the electrolytes being pumped in the opposite direction and electricity being pumped into the cell. So there would be no need to transfer electrolyte in and out of the cell at all, as some of you seem to think.

There are a number of electrolyte systems, some of which should be safe for use in a car. But I've no idea whether the energy density / mass ratio is high enough. Work I'm aware of has all been for static energy storage. Here's a random article with some relevant info: http://www.sciencemag.org/news/2015...g/news/2015/11/new-type-flow-battery-can-store-10-times-energy-next-best-device

The rough rule of thumb is 'electrochemistry is very hard'. You can't tell in advance what will work (technically AND economically) and what will not - for example, fuel cells!

 

Thanks for that @cDy - I have only vaguely grasped the flow concept of course but your explanation has only made me even more confused. I had gained the impression that the speed of re-charge was because spent electrolyte was removed and replaced by charged electrolyte so that in minutes the cell was operational again. This led me to think that offsite there was a facility busily charging up 'spare' electrolyte ready to be pumped into a battery cell that has arrived with spent electrolyte. Much like a petrol tank is waiting with energy to fill an ICE's tank.

You seem to now indicate that the two states of electrolyte remain in the unit and are just transferred internally back and forth. This makes no sense as at some stage the spent has to be charged which clearly will take longer than a minute or so.

The whole point in respect of a car is that re-filling has to take place rapidly, and that surely must mean that there has to be a static pool/tank of charged up electrolyte waiting to be injected into a car so that it can move off leaving its spent electrolyte behind. The spent can then be recharged at leisure and ready for the next car to come along.

Could you please clarify for me ?

 

The whole point in respect of a car is that re-filling has to take place rapidly, and that surely must mean that there has to be a static pool/tank of charged up electrolyte waiting to be injected into a car so that it can move off leaving its spent electrolyte behind. The spent can then be recharged at leisure and ready for the next car to come along.

Could you please clarify for me ?

I've already clarified why that is BS. @cDy speaks, correctly, of the current designed-in usage cycle. Those that speak of draining and refilling are in search of bogus grant money from somewhere. Flow cells are a legitimate technology when used as designed. It is the MO of snake oil peddlers to take a legitimate technology and then stretch it to something semi-believable. It's just what they do. They use (later damaging) the credibility of bona fide science to mislead people into parting with money.

 

It's the article that you linked to that is causing the confusion. It says:

"One of the biggest advantages of flow batteries is that they can be almost instantly recharged by replacing the electrolyte liquid, while simultaneously recovering the spent material for re-energization."

But no flow batteries that I know of do this. Who would want to pump noxious chemical into (and out of) tanks. [Especially explosive ones like petrol!]

Flow batteries are made to store chemicals in a 'charged' state, run them through the cell to generate electricity and then store the 'discharged' chemicals. The process is reversed (just as in a normal battery) to recharge the chemicals and put them back in the original tanks (you can end up with four tanks).

The article you linked to is trying to change the usage model to one where you need filling (and emptying) stations. Conceivable but not at all practical in my opinion.

Just recharge the electrolytes in situ, as intended.

 

I should mention, if there was really any need to, that these systems operate low voltage which is then converted with large weighty electrical equipment.

None of this is practical on a car because if you imagine having 200 teeny separate tanks in your car to accommodate these fluids for each cell, which is what would be needed for 400V, each with its own little pump, seals (against very corrosive stuff) and protective circuits, imagine the nightmare of that as an engineering prospect. Stick it all in a vibration test, and it'll just peel apart.

Can you just imagine the kit needed to drain off 200 little tanks of fluid, then refill them, at a speed you might associate with a petrol pump? Absolute and total madness. It would take as long to refill one than it would take to recharge it, all advantage lost.

I cannot exaggerate the contempt I have for the suggestion. Still, if someone show me the design, I will take a keen interest in seeing if I am an idiot-know-nothing with no imagination (definitely not a criticism ever leveled at me by anyone!!! I'm usually the one with the bonkers ideas!).

 

No need to get all bent out of shape @donald - At no stage has any snake oil salesman suggested that this tech could be fitted in a car. My first entry just asked why it couldn't be used, as to a layman it seemed ideal. Suck out old and insert new charge. I was just seeking an explanation. I didn't expect to be yelled at. The vociferous rejection of even asking the question is a bit over the top. In post #7 I fully accepted that it wasn't practicable.

The link I saw was from here. :- |http://energystorage.org/about

They seem to be an outfit focussed on storage of electrickery in many ways. Flow cells being just one. They never mentioned cars. I was just puzzled as to why an instant replacement of charge wasn't possible. You have told me.

 

At no stage has any snake oil salesman suggested that this tech could be fitted in a car.

Actually, there are some snake oil salesmen around (though I'm by no means accusing you of being one of them!):

nanoFlowcell -Automobile

 

It would seem that after all the discussion the 'Flow Battery' is simply another type of rechargeable storage battery, thus it has no inherent advantages over existing, and proven, types.
Yet another non solution for known problem, using increased complication to do it?

 

As an aside, a comparison with fuel cells is interesting. They have a tank of hydrogen and the atmosphere is a second 'tank' containing oxygen. These are passed through a cell, generating electricity and producing water as a by product. The water is, of course, discharged to the environment and doesn't have to be lugged around.

But the water could have been stored in a tank and then electrolysed (think of running the cell in reverse, but you wouldn't do that) producing hydrogen and oxygen again. Indeed, that is how hydrogen is produced from water, in a specialised electrochemical cell.

But for vehicles it is an advantage to only carry the hydrogen in a tank (even though its energy density is not high), get the oxygen from the air and discharge the water produced. You only need one tank for the hydrogen and don't need tanks for the oxygen or the resultant product (water).

Looked at this way, a fuel cell is 'just' another variant of a flow cell! And it can be recharged at a filling station for rapid recharge, just like an ICE. There is no spent noxious chemical to store (or unload) either.

Fuel cells are not in vogue right now but it is not clear that they will never reach the volume market in cars. The requirement is to reduce the amount of platinum needed below an economical threshold. This may never be achieved, but lots of research is going on. If that threshold is reached (your guess is as good as mine) then it will then make commercial sense to convert petrol filling stations to hydrogen filling stations. If it is not reached (and it hasn't yet been) then fuel cells will remain too expensive ...

 

It would seem that after all the discussion the 'Flow Battery' is simply another type of rechargeable storage battery, thus it has no inherent advantages over existing, and proven, types.
Yet another non solution for known problem, using increased complication to do it?

Flow cells have the big advantage, already referred to, that you can increase the amount of energy stored by having bigger tanks. They are not too complicated and may become a good (static) storage technology for the grid. But researchers are (were) having problems with the technology, including gradual degradation of the cells.

To emphasise the point, standard batteries (such as the Lithium ones we are using in cars) store the electrolyte in the cell, so the energy storage and energy generation are intimately related. If you add more batteries you store more energy but also get the ability to generate more instantaneous power. Above a certain energy capacity you get more energy generation than you really need - and that costs. Who really needs to accelerate their Tesla to 60 mph in 6 seconds (or whatever it is)? And if the range is doubled, do you need to be able to get to 60 mph in 3 seconds?

 

None of this is practical on a car because if you imagine having 200 teeny separate tanks in your car to accommodate these fluids for each cell, which is what would be needed for 400V, each with its own little pump, seals (against very corrosive stuff) and protective circuits, imagine the nightmare of that as an engineering prospect. Stick it all in a vibration test, and it'll just peel apart.

Can you just imagine the kit needed to drain off 200 little tanks of fluid, then refill them, at a speed you might associate with a petrol pump? Absolute and total madness. It would take as long to refill one than it would take to recharge it, all advantage lost.

I totally agree that this is not practical in a car (unless a very high energy density set of electrolytes is identified). But electrochemical systems, such as flow cells and fuel cells, don't store the fluids in separate tanks for each cell. The electrolytes are stored in one tank for each 'charged' electrolyte and (usually) one for each 'discharged' electrolyte. Usually four in total. The individual cells are stacked in series, so that the voltage they produce adds up, as for a normal battery. And there are plenty of such electrochemical systems in use that work for their application, although many have electricity as an input and produce a product as an output.

The issues preventing use in cars are the maturity of the technology (lots of electrochemical systems never reach market due to contamination of catalysts, unexpected byproduct deposits, expensive catalysts, ...) and energy density (both the specific chemicals used and carrying around four lots of chemicals in separate tanks).

So I fully agree with your views on practicality, but not on the specific reasoning.

 

Well, yes and no.

The electrolyte in each voltage stage of the cell stack has to be galvanically isolated. This could be done in a static system relatively easily by an assortment of means, but in a moving, accelerating vibrating chassis I struggle to see how to do this without isolation of the fluids during operation.

Maybe it is that limited imagination of mine, as I say I'd prefer just to see the design of an automotive 'replaceable electrolyte' system before heaping criticism on it. Sometimes things don't even get to the point of being privileged enough to receive criticism!!!

 

Donald

Try this:

I've worked on electrochemical systems for years. Cells get stacked, with each having separate anode and cathode chambers. But each electrolyte is a single linked fluid. The electric field is down the stack. As there is no field across the stack, there can be no flow of ions across the stack due to a field. The fluid flow is just down to the pumps

You do just flow the electrolyte through channels. I repeat - there are TWO electrolytes. One only contacts anodes, the other only contacts cathodes. So your issue about galvanic isolation is not relevant.

I can't teach a course on electrochemistry on SpeakEV. If you really want to understand it then contact me via a conversation and I'll suggest some possible text books.

 

I am struggling to believe this represents a viable operating battery.

An ion leaving the right-most electrode (in the diagram you are showing, and during discharge) would simply want to flow back up into the distribution tube, against the flow of electrolyte, and on towards the left-most electrode, as that would be the direction of the electric field through the electrolyte.

All the ions that would normally cross 'the first' cell would simply skip that and go straight for the most negative electro-potential it could go to, which is the single electrode on the "-ve" terminal in the diagram.

It wouldn't work unless the fluid flow could be assured to be so dominant that the ion speed is lower by orders of magnitude.

But if the electrolte fluid was that fast then the ions wouldn't cross one of the cells either, it would get 'washed away' and you'd end up with electrically charged fluids all flowing back together.

Put it another way.... why do car lead acid batteries have separate cells? If what you are saying is true, why bother to have separate compartments in a lead acid battery?

I do not accept the diagram is drawn up to represent a real system that has been made, notwithstanding that I am hazy on whether it could at all be built if the design was such that the ion's electrophoretic speed and fluid flow rate somehow juggled to prevent conduction via the distribution manifold.

Can you say where this diagram comes from please, I have some emails to write to the authors of these?

 

An electrolyte is a liquid which prevents electrons from flowing, That's the job of an electrolyte. They conduct by positive ions.

So as electrons are liberated (into the wire that flows around the electrical load) at one electrode by oxidation, a positive ion is formed there in the cell which flows to the other electrode. The ion is then reduced by an electron that is returning from having been around the electrical load. So electrons flow 'one way' through the load and ions flow 'the other' through the cell. Net charge is preserved, but electrical work is done, leaving behind electrodes that are now changed by oxidation or reduction. it is the change of oxidation state in transition metals that provides the 'storage' of electrons.

Anyhow, back to your post @DBMandrake , yes, exactly so, you are considering what is wrong with these diagrams but just that it is positive ions (lithium, in the case of Li ion cells, of course, but in these cases they will be metal complexes) that will conduct through the electrolyte and 'short circuit' it.

....and it IS a short circuit. Electrolytes have very low resistances. They are milliohms in good cells,as conductive as the wires themselves that carry the current away, they have to be, the electrical flow (current) is the same at any point in the circuit, and that includes in the cell itself.

 

Look guys - basic electricity. Electrons and ions flow along the electric field. In electrochemical cells, the cells are thin cells stacked on each other and the field is at right angles to the plane of the electrolyte. So there is NO electric field in the plane of the electrolyte for the ions to be driven along to get to an outlet or inlet. They either react at the anode or cathode as appropriate or get carried out of the cell back to the tank. From the perspective of ion flow under an electric field the cells are isolated. That IS how it works. The world is full of systems working like that: chlorine generation, fuel cells, flow batteries, hydrogen generation ...

And, once again, there are TWO electrolytes. No direct conductive path in one electrolyte between the anode and the cathode.

It takes a couple of weeks of university courses to cover this stuff. What I've said is correct. Go do some studying if you want to understand it fully. I can only hope to give a feel for it here.

 

Look guys - basic electricity. Electrons and ions flow along the electric field. In electrochemical cells, the cells are thin cells stacked on each other and the field is at right angles to the plane of the electrolyte. So there is NO electric field in the plane of the electrolyte for the ions to be driven along to get to an outlet or inlet. They either react at the anode or cathode as appropriate or get carried out of the cell back to the tank.

This is wrong. There would clearly be an electric field generated down the manifold of the diagram you are showing there between the cells. Sure, the electric field from a surface conductor is indeed at right angles, but only right next to it In the channel, the electric field path will head off to the highest +ve or -ve potential (polarity dependent).

So, yes, I agree a very fine channel could make this work, but microfluidic channels would reduce the overall power performance of the stack, I would be interested to see a practical implementation and the current capacity it is capable of.

From the perspective of ion flow under an electric field the cells are isolated. That IS how it works. The world is full of systems working like that: chlorine generation, fuel cells, flow batteries, hydrogen generation ...

And, once again, there are TWO electrolytes. No direct conductive path in one electrolyte between the anode and the cathode.

It takes a couple of weeks of university courses to cover this stuff. What I've said is correct. Go do some studying if you want to understand it fully. I can only hope to give a feel for it here.

University courses don't cover this sort of thing, and the cells in your diagram clearly are not isolated, there is a conduction path through the manifold, it is plainly evident to anyone who pauses to view that diagram.

Again I ask, if what you are saying is correct, why are there separate cell compartments in a 12V lead acid battery? It is totally obvious why;- if there was just one common electrolyte it would simply be one big cell with some extra metal bits dunked into it!! The only electro-active parts would be the end electrodes.

 

Seriously, electrons can't flow in an electrolyte. Conduction in an electrolyte is by positive ions flowing in the opposite direction to what you have put there, which is otherwise correct.

If electrons could flow through the electrolyte then they'd simply flow across the cell, not around the external circuit. Think about it!

 

Seriously, electrons can't flow in an electrolyte. Conduction in an electrolyte is by positive ions flowing in the opposite direction to what you have put there, which is otherwise correct.

What you are describing is an idealised electrolyte. An ideal electrolyte would be something that has 100% ionic conduction and 0% electronic conduction. Many materials that are conductive and classed as electrolytes are a bit of both and are not perfectly one or the other:

Mixed Conductors, Determination of Electronic and Ionic Conductivity (Transport Numbers) - Springer

Conduction in Bi2O3-based oxide ion conductor under low oxygen pressure. II. Determination of the partial electronic conductivity

Appraisal of Ce1−yGdyO2−y/2 electrolytes for IT-SOFC operation at 500°C - ScienceDirect

Granted, in a quick Google search I could only find reference to solid electrolytes that had a small portion of electronic conductivity, so I may be wrong about this being a possibility in an aqueous electrolyte. On the other hand I'm sure I've read a research article on rechargable batteries that describes the "electronic conductivity" of the electrolyte being one of the primary contributors of self discharge in batteries that contain aqueous electrolyte in typical paste format as you might find in NiMH/Litihum Ion etc, and that batteries with lower electronic conductivity of the electrolyte usually have lower self discharge rates.

In other words the electrolyte not only has a series equivalent resistance dictated by ion conductivity, it also has a parallel equivalent resistance dictated by electronic conductivity of the electrolyte - parasitic and undesired conductivity for the purposes of a battery as it would allow electrons to take a path other than through the external load and thus self discharge.

If I can find something along these lines later I'll post a link.

 

You are talking about self-discharging processes, which is fair enough, but it is not the process by which multiple cells sharing a common electrolyte wouldn't work. As we discuss very often, you can leave an EV for weeks if not months with barely any discharge.

The common flow in any electro-chemical cell is by positive ions. If electrolyte is shared, then it serves only one potential gradient (i.e. across one cell). One electrolyte volume cannot support multiple electric gradients. I'd have thought it was obvious.... but I guess not. In the gaps of limited understanding do the peddlers of snake oil operate.

 

I've been told that I'm wrong so I'll bow to your greater knowledge and leave you to theorise. In the gaps in their knowledge bullies seek to operate.

Here's the structure of a typical flow cell, to be built into a stack:

https://www.researchgate.net/figure...g2_Figure-2-Redox-flow-battery-test-cell-a-Steel-plate-b-Isolation-plate-c-Flow

Here's a paper on some flow cell research. Figure 2 shows an exploded view of a flow-cell stack:

Design of a miniature flow cell for in situ x-ray imaging of redox flow batteries - IOPscience

Here's an example system that works in the way I've described:

Fuel Cell Energy, Redox Flow Battery, Energy Storage Tech

Here is somewhere selling the components for fuel cells, which have the same architecture:

Fuel Cell Stacks

Of course, none of this is real, as can be proved by spurious arguments in social media.

 

I've been told that I'm wrong so I'll bow to your greater knowledge and leave you to theorise. In the gaps in their knowledge bullies seek to operate.

Here's the structure of a typical flow cell, to be built into a stack:

https://www.researchgate.net/figure...g2_Figure-2-Redox-flow-battery-test-cell-a-Steel-plate-b-Isolation-plate-c-Flow

We know what cells look like....

Here's an example system that works in the way I've described:

Fuel Cell Energy, Redox Flow Battery, Energy Storage Tech

I can't tell if it does or not, it just shows a diagram of one cell, and some equipment in a lab. How can you tell it is as you describe?

Here is somewhere selling the components for fuel cells, which have the same architecture:

Fuel Cell Stacks

I have no idea why you keep mentioning fuel cells. Completely different technology when you are using gases. They don't employ electrophoretic ions.

So these are not relevant to the discussion.

Here's a paper on some flow cell research. Figure 2 shows an exploded view of a flow-cell stack:

Design of a miniature flow cell for in situ x-ray imaging of redox flow batteries - IOPscience

This I don't understand. The diagram clearly shows bipolar plates, multiple ionic membranes and microfluidic channels connected to singular flow ports. This is relevant, and I will have to look closer at this. It doesn't make sense to me and I want to understand it.

I can only believe at this stage that using microfluidic channels means that ionic motion through the manifold is effectively limited in some way. I would have not expected that to be possible for a high power stack.

 

Well done for finding that photo. I was looking for the same type of thing on the internet but could not find it last night in the time available. Much of the material I have to hand is confidential and I could not share.

It does take some time to get your mind round this and, as I discovered, is difficult to explain in a few words. My apologies for not being as clear as I would like.

Each cell is, in practice, ionically isolated, with all the electric field running down the length of the stack. The ions respond to the local environment - there is no way they can get to the next cell or across the membrane. Many membranes are proton exchange membranes (PEMs) which are clever things that I don't fully understand, but which only transport protons (and 'no' electrons) across them. They are not just 'filters'.

That's been a long aside really, but in the end a constructive debate. I appreciate you putting in the time to understand it.

The main conclusion, that we've always agreed on, is that flow-cells are unlikely to appear in (practical) vehicles and that exchanging electrolyte to recharge them quickly is just a very daft idea.

By the way, you were quite correct in observing that fuel cells use gases and that this would avoid any ionic flow in a liquid, if that were an issue. I linked them in because the topology is identical and much of the hardware is similar. I suspect the photo you have included may actually be from a fuel cell, but it could well be from a flow cell.