Iron and Vanadium based electrolytes can both be used Redox flow batteries, both use tanks, pumps and cell stack, but they don't operate the same way. While Vanadium Redox Flow Batteries stay completely liquid in their operation, All Iron Hybrid Batteries don't. And the name itself tells you why- Hybrid. When you charge an All Iron battery, its positive side converts Fe2+ to Fe3+, and stays liquid, but the negative side converts Fe2+ to Fe0, or solid iron.
So why is solid iron a problem ? Well because -
1.The electrode that we have used i.e. graphitic carbon felt, is a conductive sponge, it is designed to absorb liquid and transfer electrical energy to it, it instantly clogs when solid iron is deposited on it.
2. Solid iron is not soft, nor does it grow evenly, it grows in the shape of needles or dendrites (if you have ever seen iron filings, that's solid iron).
3. Deposition of solid iron is also called electroplating, and current density in electroplating is 50 mA/cm2 MAX. For reference, a Vanadium battery charges at 100 mA/cm2.
4. Since iron is being deposited, it will need space, so you need empty space in the cell stack and the more of empty space you have in the stack, more energy you can store.
So if you compare an iron redox flow battery with a vanadium one, you will need 2x more cells (because power = voltage x current), you cannot use cheap carbon felt in the cell stack, and if you don't manage the cell stack design you Daramic membrane will be punctured.
Then there is hydrogen -
All redox flow batteries, even Vanadium, evolve hydrogen at the negative electrode, because when you put an electron in a water based solution, there is always a competition between water to split into hydrogen and the metal to react. It is a problem you have to deal with in all battery chemistries, Vanadium luckily has very very less competition with water. Vanadium majorly wins the race and gains electrons before water splits. So in Vanadium batteries the hydrogen is controllable.
But if you use any other chemistry, Zinc Bromine (very complex), Iron-Chromium, All Iron, Zinc-Iron or any other cheap metal based chemistry, then hydrogen accumulates very quickly and becomes uncontrollable. And why is hydrogen a problem ?
Well...... because your electrolyte is acid, the more hydrogen comes out, the more your solution loses H+ ions, the less acidic it becomes, at some point the solution becomes such a weak acid that metals cannot stay dissolved in it, so they precipitate out and don't react at all.
Remember how water competed with metal ? yeah that means you are wasting electricity to make hydrogen, a gas that's one of the most flammable substances in the world, kinda kills the fire safety argument.
Hydrogen is a problem that every battery company has to design for.
So being an engineer, I thought I could assemble a good enough team and somehow raise funds for precision manufacturing, design for hydrogen, make bigger battery stacks, and commercialize.
I'll be honest, I was inspired by
ESS Inc. The only company that has bet so big on all iron chemistry, so I spent the next 3 months, researching about them and trying to copy them, read their patents, learn how they handle hydrogen.
Until I saw the dealbreaker.
That is the topic for next post.
None of this is AI generated, so feel free to fact check me and looking forward to your views and help.
Mayank Jately
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