What are membranes ? Why every flow battery company cares about them ?

I may have mentioned membranes many times in my past posts, but I may have not mentioned what exactly does it do, but it is one of the most important parts in a flow battery, fuel cell, electrolyzer or any other electrochemical system.

Let's understand why -

Ion Exchange Membrane-

When most people talk about membranes in terms of electrochemical systems, they are mostly talking about a sheet of special materials that let one some atoms pass through and stop the others, think of it like a filter or sieve. If you have ever used an RO you know what a membrane is, the membrane in the RO or filter system lets mostly water pass through, not the salts dissolved in it, so what comes on one end of the RO is pure water and what remains is impure water.

The RO membranes are pretty cheap, because you can replace them every couple of years, but if you want a membrane that lasts 20 or maybe even 10 years, you enter a different league.

Types of Membranes-

There are many types of membranes in research and trust me there are highly accomplished researchers that have PhDs in membrane engineering. But if you are a 22 year old, bootstrapping a flow battery startup, your options are limited to commercially available membranes. So let's see what you can actually buy in real life -

1. Cation Exchange Membranes (CEMs) - 

These are one of the most commonly used membranes in electrochemical systems, these membranes (as their name suggests) allow only positively charged ions (Cation) to pass through, For Ex. in a salt (NaCl) only Na+ ions can pass, this is one of the most commercially used (and EXPENSIVE) membranes, it is used in chlor-alkali process.
If you want to buy CEMs in the market, the first one you'll see Nafion, one of the most expensive membranes in the market and a very pricey component of any electrochemical system. Nafion has maintained a monopoly over the industry for years. But it has been recently broken by Fumasep Membranes. They are as good as Nafion, but slightly cheaper. That makes it a little better.

Nafion Membrane- Source


2. Anion Exchange Membranes (AEMs) -

These have been developed more recently than CEMs, these membranes do the opposite work of CEMs, they allow negative ions (Cl- ion of NaCl example) to pass through while stopping the positive ions from going through. Fumasep membranes have won the race in this one, most widely available AEMs in the market are FumaTech's Fumasep membranes. 
Fumasep AEM - Source

The performance of a battery or fuel cell changes drastically on the basis of not only which membrane is used but also which grade of the membrane is used.

I learnt this the hard way, I had ordered a Fumasep FAA 3-50 membrane, only to find out that it was an Anion exchange membrane, and recently researchers published that AEMs don't work well for Fe-Cr Redox flow batteries. Rather they worked for All- Iron Hybrid Redox Flow batteries, and we already know the problems with that one. 

So which one of these membranes do I actually use now ? Well.... neither, because I can't afford to spend Rs 4000 ($40) on testing different membranes. 
So what do I use ? 

That's the topic for the 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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