My adventures building a DIY Mn/Fe flow battery

Patrik L said:

A v2.0 version, nice, and the 200mA (not 200mV ) gives room for greater flexibility, so thanks for the information - wonderful . Will look it over.

I see, then by that fact, degassing dissolved oxygen (DO) also make sense since cyclability is greatly dependant upon a low amount of DO. Nitrogen is fairly easy to get a hold of, so a nitrogen rich environment or partial vacuum, shouldn't be too difficult either. But yes ofc I am thinking about the 1000 litre tanks already, even if its years away. There is always time to test and develop.. So thanks for the oxygen info, nice bit of info to have moving forward. One could make low O2 content or vacuum monitoring a feature of the anolyte to maintain the longest lifespan as possible.​

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Another good tip is that the normal form of Fe-EDDHA, which is (Fe(EDDHA))-1 is a deep red color while the reduced form, (Fe(EDDHA))-2 is colorless. Only a battery full or close to full will start to fade in red color. Sadly the color is so strong that it is not very useful to determine charge state directly, because even a very tiny amount of the oxidized form will make the solution ultra red.

However it is an excellent tool to determine if your solution is indeed isolated from oxygen.

Also, vaccum is not a good idea, because it is energy intensive and creates negative pressure which ensures any leaks will cause oxygen to come in. You need to have a good purge of the system with nitrogen first and then leave it under a slight positive nitrogen pressure, to ensure any leaks cause nitrogen to push out but not oxygen to come in.

1. It will be interesting to observe the colour differences and general behaviour. Can one trace the dissolved O2 content via pH values as an example ?

2. The thought behind vacuum degassing was to prepare the solvent before adding the chelate, while the holding tank itself would receive a blanket of N2. Then keep monitoring O2/N2 content and service the N2 blanket when needed. An initial O2 purge is a must like you point out and several sites mention that N2 purging as in contrast to vacuum degassing, is the most effective method. So, in the end, introducing N2 at the bottom of the tank such that N2 can be introduced and allowed to flow though the solvent and finally generate some pressure in the tank, looks to be a better solution than using vacuum. This reduces the system complexity as well as financial investment.

Also, need to find a supplier for the chelates who are selling to private individuals. I know you have your sources which I open to, but is good to have more than one source

Interesting. Was talking to a retailer who sell YaraTera Rexolin Q48 and Mn13 and he mentioned that these two products contain lime. Does your source contain lime as well ?

Patrik L said:

Interesting. Was talking to a retailer who sell YaraTera Rexolin Q48 and Mn13 and he mentioned that these two products contain lime. Does your source contain lime as well ?

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Not that I know of. However they might contain some small amount, I haven't done any extensive analysis of the chelates. Pure Mn-EDTA is 13% Mn by weight, so if it contains any lime, it is likely not much.

The Rexolin Mn13 is 12.8% Mn when I look at the specifications ... and again, its a challenge to get a hold of but looking into it or working on it.

You can buy small quantities to experiment with (both Mn-EDTA and Fe-EDDHA) from borms (https://www.borms.eu/)

Most excellent. Btw, got a price from the one retailer I talked to and both are at 20€/kg, so not an expensive substance. But for development, 250g should go a long way.

Patrik L said:

Most excellent. Btw, got a price from the one retailer I talked to and both are at 20€/kg, so not an expensive substance. But for development, 250g should go a long way.

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For a 10kWh battery you would need around 175kg of Fe-EDDHA and 130kg of Mn-EDTA. So those 20 EUR/kg would start to count!

However at those values it is probably easier to just buy them from china, where you could import them for a total of around ~9 USD/kg including importing fees. You would be buying a pallet of the stuff though. This is where using high molar mass compounds starts to hurt.

Also the cycling of the Fe-EDDHA/Mn-EDTA setup is quite pretty. The Mn-EDTA side goes from yellow to red and the Fe-EDDHA side goes from red to transparent. the change on the Mn-EDTA side is far easier to track, as it happens much more noticeably.

I'm gonna address a few things in this one post, its simpler that way.

1)

1. If we look at a 48V@10kWh Li-ion battery pack from LG Chem, then 7500€ is a fair price. Here we can expect roughly 3-5k cycles.
2. If we then look at similar LFP battery pack, then we land between 5500-7500€ but get substantially longer cycle life. LFP's are estimated to return 5000-10,000 cycles.
3. A 10kWh NiFe pack on the other hand will cost roughly 10,000€, but there, >10,000 cycles is no issue.
4. An all iron salt battery, the FeCl, is estimated to do more than 20,000 as in little to no losses are seen at those cycling's - but I currently do not have a cost projection here. So lets use NiFe levels.

Point being, if we consider the battery's above, then the MnFe has to be able to compete and its here that, at least I, is fumbling in the dark. I have no cyclability reference besides little to no degradation at 500 cycles. So before one worry about the cost for hundreds of kg, I would want the base knowledge to be known

2). Stumbled upon a paper where they used an anion-exchange membrane in a FeCl battery at pH of 2-3 (Tokuyama A901, 11 μm thickness) which was stable for at least 50 cycles.

Ttitle: ... A High Efficiency Iron-Chloride Redox Flow Battery for Large-Scale Energy Storage - DOI 10.1149/2.0161601jes ... curious about your thoughts here.

and here is a PDF with reference to the membrane: https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/amfc_050811_fukuta.pdf

also some info here http://www.astom-corp.jp/en/product/02.html


ASTOM & Tokuyama merger information: http://www.membrane-guide.com/membrane_separation/ion-exchange/japan_ion_exchange_membrane.htm

3) The colour shifts sounds mesmerising

Patrik L said:

I'm gonna address a few things in this one post, its simpler that way.

1)

1. If we look at a 48V@10kWh Li-ion battery pack from LG Chem, then 7500€ is a fair price. Here we can expect roughly 3-5k cycles.
2. If we then look at similar LFP battery pack, then we land between 5500-7500€ but get substantially longer cycle life. LFP's are estimated to return 5000-10,000 cycles.
3. A 10kWh NiFe pack on the other hand will cost roughly 10,000€, but there, >10,000 cycles is no issue.
4. An all iron salt battery, the FeCl, is estimated to do more than 20,000 as in little to no losses are seen at those cycling's - but I currently do not have a cost projection here. So lets use NiFe levels.

Point being, if we consider the battery's above, then the MnFe has to be able to compete and its here that, at least I, is fumbling in the dark. I have no cyclability reference besides little to no degradation at 500 cycles. So before one worry about the cost for hundreds of kg, I would want the base knowledge to be known

2). Stumbled upon a paper where they used an anion-exchange membrane in a FeCl battery at pH of 2-3 (Tokuyama A901, 11 μm thickness) which was stable for at least 50 cycles.

Ttitle: ... A High Efficiency Iron-Chloride Redox Flow Battery for Large-Scale Energy Storage - DOI 10.1149/2.0161601jes ... curious about your thoughts here.

and here is a PDF with reference to the membrane: https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/amfc_050811_fukuta.pdf

also some info here http://www.astom-corp.jp/en/product/02.html

View attachment 127601​

ASTOM & Tokuyama merger information: http://www.membrane-guide.com/membrane_separation/ion-exchange/japan_ion_exchange_membrane.htm

3) The colour shifts sounds mesmerising

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An important point is that both NiFe and FeCl3 flow batteries are very maintenance heavy.

I don't know if you have used NiFe batteries before, but periodic electrolyte changes - which involves moving and dumping large amounts of highly concentrated KOH+LiOH solutions - need to be done, often several times per year. This is especially true for some of the modern chinese versions of NiFe batteries, which are constructed much more cheaply than those that Edison made early on. While NiFe might sound great in some aspects, in practice it is a quite annoying battery chemistry to run and maintain. I have run these batteries before and I would say they are anything but care-free.

In the case of FeCl3 flow batteries, a similar problem happens. Due to the H2 evolution at the anode, you will need to constantly adjust the pH of the anolyte so that you don't start seeing hydroxide precipitation issues. These pH adjustments would need to be done practically every 50 or so cycles of the battery. Thankfully this process is much more prone to automation, given that you just need to keep the pH at a low value.

Using an anion exchange membrane would be ideal, but considerably more expensive than the commonly used cation exchange membrane types. Conductivity of anions is also slower than protons, so this means you also get a penalty in terms of current density.

Another issue is that since you're plating a metal, your capacity does not scale solely with the volume of electrolyte but you also need to increase the physical volume of the anodes to get more capacity. This imo, is counter productive since the ideal of a flow battery is to fully decouple power density and energy density of the setup.

danielfp248 said:

An important point is that both NiFe and FeCl3 flow batteries are very maintenance heavy.

I don't know if you have used NiFe batteries before, but periodic electrolyte changes - which involves moving and dumping large amounts of highly concentrated KOH+LiOH solutions - need to be done, often several times per year. This is especially true for some of the modern chinese versions of NiFe batteries, which are constructed much more cheaply than those that Edison made early on. While NiFe might sound great in some aspects, in practice it is a quite annoying battery chemistry to run and maintain. I have run these batteries before and I would say they are anything but care-free.

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But the salts are non consumable, the only consumption is water due to hydrogen evolution or evaporation. That one has to replace the entire electrolyte on the NiFe that often is new to me... honestly, that is brand new info. In some cases when the battery has been sitting for a long time (many years), replenishing the electrolyte is required.

For clarification and resolution: Why exactly the high (NiFe) electrolyte replacement frequency ?

danielfp248 said:

In the case of FeCl3 flow batteries, a similar problem happens. Due to the H2 evolution at the anode, you will need to constantly adjust the pH of the anolyte so that you don't start seeing hydroxide precipitation issues. These pH adjustments would need to be done practically every 50 or so cycles of the battery. Thankfully this process is much more prone to automation, given that you just need to keep the pH at a low value.

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I was about to say that adjusting the pH due to water losses isn't very complicated and indeed, an automatic filling system can be used - some good news ... LOL. One can go one step further and use a "gas filter" section, an adaptation of the AGM battery, which uses nonwoven poly-propylene (PP) fabric separator to slow down the motion of the gas or trap the gas for long enough such that H2 and O2 species have enough time to recombine. PP is inert to many chemicals and these types of fabrics can be found via geotextile as an example and is dirt cheap.

So outgassing is the least of ones problem I would say. A redundant system with pH monitoring, "gas filter" and ventilation can relatively easy be set up.

I was actually going to experiment with a NiFe PP pouch cell battery as a means to produce a dirt cheap NiFe battery, but ran into the nickel salt problems.

danielfp248 said:

Using an anion exchange membrane would be ideal, but considerably more expensive than the commonly used cation exchange membrane types. Conductivity of anions is also slower than protons, so this means you also get a penalty in terms of current density.

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Right... well, lets say outgassing is minimized (see above), then a cation membrane would be fine, even the lower roundtrip efficiency.

danielfp248 said:

Another issue is that since you're plating a metal, your capacity does not scale solely with the volume of electrolyte but you also need to increase the physical volume of the anodes to get more capacity. This imo, is counter productive since the ideal of a flow battery is to fully decouple power density and energy density of the setup.

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I thought that was the case for any battery. As in; 1 cm^2 have X mA that it can support. For flow battery we need to account for the electrolyte's ability to give or take a charge which is connected to the interaction of the electrolyte and current collector and the size of the current collector. So the lower the values are in terms of current and/or current needs, the larger the area has to be.

Are you saying there is an advantage to the non plating MnFe cell where since you are not plating, picking up or giving off a charge is easier and therefor you are not bound to current density in the form of area alone, but its more of an area and flow combination ? .... (man oh man do I wish I was properly trained as a chemist such that I could explain things properly... LOL)

Patrik L said:

For clarification and resolution: Why exactly the high (NiFe) electrolyte replacement frequency ?

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About why a change of the electrolyte in NiFe cells is needed with a higher frequency, the answer is that the KOH/LiOH electrolyte is unstable to CO2 in the atmosphere, so it captures CO2 very efficiently as a function of time. This CO2 lowers the conductivity and the carbonates formed allow for the formation of Ni and Fe carbonates which are not active in the battery and cause the battery to actually lose capacity as a function of time. Since the batteries cannot be sealed due to their production of H2, they are exposed to CO2 and gradually become poisoned.

In my practical experience with modern NiFe batteries, this process can happen relatively quickly. Fast enough so that you need to perform this change once per year.

This is one of the problems with any rechargeable high pH battery technology that is not airtight. The formation of carbonates kills the battery with time.

Are you saying there is an advantage to the non plating MnFe cell where since you are not plating, picking up or giving off a charge is easier and therefor you are not bound to current density in the form of area alone, but its more of an area and flow combination ? .... (man oh man do I wish I was properly trained as a chemist such that I could explain things properly... LOL)

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With a flow battery, the idea is that your cathode/anode are completely neutral (don't participate in the reaction) so all they do is provide you with surface area for the electrochemistry. This means that if you need more power - to produce more watts - all you need is to add more electrode surface area (more stacks). This can help you increase your voltage and current to whichever levels you require.

However, if you want more capacity (more kWh) but you don't need more power output, all you need is to add more tank volume. Since capacity depends only on the electrolyte volume.

The above is true for any RFB that does not plate anything on the electrodes. Such as Vanadium RFB.

When you plate something on the electrodes, the area of your electrodes also becomes tied into your capacity, since you need to deposit metal onto those electrodes and the mass you can put in those electrodes depends on their volume and active area. It is then not enough to just increase your tank volumes to increase capacity, you also need to increase your electrode area. This negates a big advantage of the classic RFB which is that you can endlessly expand capacity just by getting bigger tanks.

When I started my battery journey, I had no idea what it would involved or what the end station is. Both the former and later is evolving as we speak, which I am very... very happy for, and this is the entire reason for me being such involved... that and the prosperity for building myself and saving some bucks .. LOL.

I had no idea oxygen or carbon-dioxide was such a crucial part for the degradation of the NiFe battery operation. I guess it makes sense if one start looking at the chemistry or get the "aha" moment, like you provide me with now. I am starting to understand why Encell Technology Inc stopped production of their flooded NiFe battery in favour for a sealed and service free variant and I am grateful I am aware of the oxygen issues for battery's in general.

So we are fundamentally talking about two types of RFB: Plating and non-plating variants and you want to work on non-plating aka a Vanadium Redox replacement chemistry. Kewl. Come to think of it, this was mentioned early on in your blog... duh... he he

I also got a price quote for the Yara chelate products. The Mn variant is 14,82€/kg and the Fe variant is 18€/kg (incl. VAT). I guess that can be viewed as premium price, on the other hand, I would say I trust Yara more than a Chinese vendor. Some delivery's and purity of the products is questionable according to some who order from China, Alibaba etc . But lets see what happens. No matter, I will work with smaller quantity's to start with.

Q: Proper cycle tests has to be done on the open source flow cell you are planning to build later this year ?

So what are you doing now, just monitoring crossover in a static environment ?

Patrik L said:

Q: Proper cycle tests has to be done on the open source flow cell you are planning to build later this year ?

So what are you doing now, just monitoring crossover in a static environment ?

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Yes, cycling can only be done on a proper flow cell. At the current densities I can sustain without side reactions - due to the low surface area of my electrodes and the big space between them - I need like 1 week per cycle of a quite dilute solution (50mM). So cycling is not gonna happen here.

I am just ensuring the chemistry works - meaning I dont see any iron oxides or manganese oxides precipitate - and the potentials measured are as expected. I can also observe crossover of the Fe-EDDHA and measure pH changes on the discharged and charged states. I can also observe the color changes and just get an overall idea of what changes to expect when cycling a true flow cell.

Got it. Because the two electrolytes are of different metal species, is there a way to recover crossover contamination or does that simply become a permanent loss of potential ? ...

Iron salt have the advantage of being the same on both sides, crossover is not a contamination per say, just loss of potential during that cycle. Which I guess is one of the advantages.

Patrik L said:

Got it. Because the two electrolytes are of different metal species, is there a way to recover crossover contamination or does that simply become a permanent loss of potential ? ...

Iron salt have the advantage of being the same on both sides, crossover is not a contamination per say, just loss of potential during that cycle. Which I guess is one of the advantages.

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In the case of this battery crossover would lead to permanent loss of capacity. It doesn't poison the catholyte/anolyte though, you will just lose some capacity as a function of time. However, the fully intermixed electrolyte is still functional, so you can never lose more than 50% of the capacity in this way. With a decent membrane it should take a decade for you to lose even 1% in this manner though, since the electroactive species are so large.

It doesn't affect potential much though, just capacity.

Sounds promising. The self-discharge is different I will assume since there we are talking about the free motion of cations through the membrane vs capacity loss due to contamination.

Generally speaking: What is an accepted self-discharge level ?

Patrik L said:

Sounds promising. The self-discharge is different I will assume since there we are talking about the free motion of cations through the membrane vs capacity loss due to contamination.

Generally speaking: What is an accepted self-discharge level ?

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These should not self-discharge in the traditional sense, since your storage is put into tanks and the amount that is left within the stack is negligible. The expected self-discharge would be due to reactions of atmospheric oxygen with reduced Fe-EDDHA.

So basically, without active redox, as in an ohmic load discharge, the anolyte and catholyte will hold their state of charge quite well.. and yes, because not all of the electrolyte is present in the cellstack, that which are located in the tanks and if influence of O2 and CO2 is very low, should exhibit good values.

Btw, how does one recover the lytes from dissolved O2/CO2... by means of nitrogen purging like we talked about earlier ?

Patrik L said:

So basically, without active redox, as in an ohmic load discharge, the anolyte and catholyte will hold their state of charge quite well.. and yes, because not all of the electrolyte is present in the cellstack, that which are located in the tanks and if influence of O2 and CO2 is very low, should exhibit good values.

Btw, how does one recover the lytes from dissolved O2/CO2... by means of nitrogen purging like we talked about earlier ?

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The influence of CO2 should be minimal, since at neutral pH it is not taken up in a major way by solutions and no significant amount of carbonates are formed.

Oxygen will react right away with reduced Fe-EDDHA, so any you don't keep out will just discharge your battery. You have to keep it out by keeping the system under N2 positive pressure and having it well sealed, especially the anolyte tank. The catholyte won't react with the air at all.

I am looking forward to starting my own tests under your wings. Clocking performance and endurance is one of the more exciting aspects of battery's besides the chemistry - not that I have ever done that and everything is new. Until then, I'd best let you steer the ship and do the groundwork

When will this end, I have a new question for you ... LOL

In your ion exchange membrane research, did you consider the use of Regenerated Cellulose (MWCO of 6,000–8,000 g mol^-1) ? Bellow is a picture which use regenerated cellulose dialysis membrane as the ion exchange membrane as a more affordable NAFION replacement with aqueous electrolyte and sodium chloride as support salt.

I am only investigating since regenerated cellulose is a commercial available product/film that could (if the right type and price to performance, is located) make the construction of the cells a bit simpler with less steps involved. Mind you, this is not criticism of your membrane work, I very much appreciate that. I ask only to bring clarification.

I will share my source if you want or need it.

- edit -
I wonder if your PVA/Cellulose is too close of a solution as in rendering Regenerated Cellulose mute. We don't exactly need the membrane to be clear/transparent, but I will keep this post present as an alternative.

I had an epiphany and memory yesterday. I've always wanted to have or own a nitrogen selective membrane, but there wasn't a good time for one and then we had all these debates and suddenly there is a need for one :D. I just added: "build a nitrogen gas station" to the list of stuff I will need. While buying a N2 gas-tube might be cheaper atm, I will need a gas station later on anyway so might as well do the investment.

Have a wonderful Saturday.