Nafion equivalent permselectivity values using a DIY PVA/Cellulose cation exchange membrane

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During the past couple of weeks I have been working on cation exchange membranes using PVA/cellulose (see here, here, here). The idea is to create a membrane that can replace Nafion in a pH neutral flow battery built using an Fe anolyte and a Mn based catholyte. In this post I will share the first results that are up to par with those of a Nafion membrane.

My initial idea was to both crosslink and add anionic sites to the PVA by using phosphoric acid with urea as a catalyst, heating the membrane to >150C in order to perform the esterification process. This worked to a decent degree, achieving membranes with permselectivity values above 80% with sheet resistance values around 10x those of Nafion membranes.

Membranes annealed at 150C (left) and 100C (right)

However there were some obvious issues with this process. The first is that the membranes produced had some stability issues, their permselectivity would drop with time – due to lack of enough crosslinking – and the mechanical stability of the membranes also left a lot to be desired. Both of these issues were likely due to limited crosslinking of the membranes, as forming a double phosphoric acid ester is not a very favorable process, even in the presence of urea.

I would see Fe-EDDHA-1 leak across the membrane within around 24 hours of setting up my cells, with the transparent side turning a slight pink within that timeframe. The permselectivity would also drop from 80% to around 40% within that timeframe. It was obvious that these membranes had components that were still dissolving or at least creating cavities that allowed too much water to flow through.

Thinking about this, I searched for possible crosslinking agents to enhance the issue. I also wanted to avoid usage of anything toxic, like glutaraldehyde, as the space where I carry out these experiments has limited ventilation, plus I want to avoid exposing myself or my cats to harmful substances. Expensive substances were also out of the question, like sulfosuccinic acid.

Multiple results of membranes being prepared using this method (each one is around the diameter of a 25c US coin)

Reviewing papers on the subject, citric acid appeared to be a viable substance. It is a tricarboxylic acid, so it would be able to crosslink cellulose with PVA, PVA or cellulose with themselves and also keep some exposed anionic carboxylic groups to provide cation exchange capacity. Adding phosphoric acid would also catalyze the esterification reaction plus also provide some phosphorylated sites for enhanced permselectivity.

The process for preparing these membranes is as follows:

  1. Prepare a solution by adding 15g of PVA to 200mL of water (solution A).
  2. Place solution A in a fridge for 48 hours, with occasional stirring/shaking. Surprisingly, cold conditions are much better for dissolving PVA because they discourage agglomeration.
  3. Wait till solution A is fully homogeneous, keep longer in fridge and shake/stir as needed.
  4. Prepare another solution by using 0.5mL of phosphoric acid (81%), 0.5g of citric acid and 15mL of solution A. This solution is stirred until everything is completely homogeneous (solution B).
  5. Dip a filter paper in Solution B. I used Stony Lab 101 but other fine grain filter papers should work just as well. Make sure all excess has dripped off and tap with paper towels to remove any excess.
  6. Place on a hot plate at 80C for 3min
  7. Flip it to the other side for another 3 minute.
  8. Use a brush to paint solution B on the filter paper while on the hot place.
  9. Wait for 3 minutes.
  10. Flip the filter paper and paint the other side, wait another 3 minutes.
  11. Repeat steps 8-10 three times.
  12. Increase the temperature to 150C.
  13. Flip the membrane every 10 minutes for one hour or until the membranes appear fully black. Put a petri dish on top if needed to keep the membrane flat.
  14. Allow the membrane to cool to room temperature.
  15. Place the membrane in a solution with 10g/L of potassium or sodium carbonate to neutralize any remaining acid, they can be stored in a 0.5M NaCl solution.

The membranes that result from this process are black in nature. However they do not feel like charcoal and do not crumb easily. Instead, they have the feeling of a piece of plastic film, which is exactly what we are looking for. Several papers discussing citric acid crosslinking of different polymers do have resulting black films, so this isn’t necessarily a bad thing.

I was also very pleasantly surprised by the permselectivity measurement for these membranes. Measuring the potential across NaCl 0.1M | NaCl 0.5M using identical Ag/AgCl reference electrodes, the potential is 38-39mV, meaning that these membranes have permselectivity values >99%, which is equivalent to that of the best Nafion membranes. Adding 0.01g of NaFeEDDHA to the NaCl 0.5M side – which makes it dark red – I could see absolutely no crossover of FeEDDHA-1 to the other side of the half-cell experiment within 48 hours of testing. There were also no drops in the permselectivity which remains extremely high. The sheet resistance measurements are also very favorable, with in place sheet conductivity values now in the <50 ohm/cm2 range.

Overall, I am pleased with this DIY membrane result. The crosslinking of PVA using citric acid and phosphoric acid on a cellulose matrix provides you with a very robust membrane that has some wonderful characteristics. This will be my base membrane for the construction of Fe-Mn flow batteries. This membrane is also very low cost.

Measuring and improving the performance of PVA/Cellulose cation exchange membranes

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In a previous post I described how to create a DIY cation exchange membrane using some easy to get materials. These membranes could achieve significant permselectivity values, but still far away from those required to create membranes for a robust flow battery. Additionally, the sheet resistance of these membranes – which I measured using a 4 contact electrode method – was quite bad, with values often greater than 6000 ohm/cm2. The through plane resistance was around 3x that, although my method for through-plane resistance measurement is not reliable yet.

Some of the last membranes I produced using a PVA solution with a pH in the 6-7 range. The membrane remains an off-white yellowish color, but does not oxidize as in my previous tests.

In this post, I want to talk about the advancements I have made to improve the fabrication of these membranes. First of all, I have lowered the preparation temperature to 150C, this avoids charring the membranes and improves reproducibility. I also added 80 minutes of additional time at these temperature once all the PVA coats have been put on, this improves crosslinking and drastically reduces the solubility of the membrane in water (to the point where it becomes fully insoluble).

I have also found out that decreasing the acidity by adding some potassium hydroxide also helps retain membranes structure, increase permselectivty and decrease sheet resistance. This matches some papers on cellulose phosphorylation using potassium phosphate and ammonium phosphate salts, with solutions that have much higher pH values than phosphoric acid. The higher pH helps preserve the structure of the cellulose and PVA, as a lack of acid reduces the changes of degradation of the cellulose and PVA. The phosphorylation still happens, thanks to the urea catalyst present.

With this in mind, the membranes can probably be made using monopotassium or monoammonium phosphates, much more readily and less dangerous chemicals compared to concentrated phosphoric acid and potassium hydroxide.

One of my last experiments to measure permselectivity. The cell to the right contains a very small amount of NaFe(EDDHA), which has a very deep red color. This makes it very easy to see membrane crossover.

The best values I have achieved so far are a permselectivity of 80% and a sheet resistance of 373 ohm/cm2. These are still much worse than those of commercially available membranes, but certainly better than the values I was achieving before.

From the parameters I have tested, the cross-linking temperature and pH seem to be the most important to the qualities of the membrane, so I will try to study these too with a bit more detail to find out if I can produce membranes with better qualities. Increasing the concentration of P at higher pH values with higher urea quantities might also help achieve better cross-linking.

A DIY cation exchange membrane with PVA and cellulose

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In previous posts (here and here), I have talked about my goal to create an Fe/Mn flow battery and how to do this I will need to create a cation exchange membrane to use instead of Nafion. In this post I will talk about what I have achieved so far, which is the first iteration of a PVA based cation exchange membrane.

Early on, it became clear that polyvinyl alcohol (PVA) was going to be the easiest polymer choice, as it is readily available and easily to functionalize. Phosphorylation also seemed as the easiest route towards functionalization, as highly concentrated phosphoric acid is easy to get and urea catalyzed phosphorylation reactions of PVA are already well known. The introduction of phosphoric acid esters provides the ability of the membrane to repel anions and allow only cation transport.

Experimental setup to measure the potential across one of the membranes created. The potential between the Ag/AgCl reference electrode and graphite electrode is measured with both electrodes on the same side, then the potential is measure again by placing the reference electrode on the opposite side. The membrane potential is calculated from the difference between same side and opposite side potentials. (a small amount of dye was added to the right side so that you can see how the membrane separates the solutions).

My first problem creating a membrane of this sort had to do with casting PVA films and being able to peel them off. These membranes are extremely sensitive and can easily stick to glass or to themselves, making the fabrication process difficult. I tried casting on glass petri dishes – with mold release – and was unable to remove them without breaking them. A friend suggested casting on Al foil instead, so I will be keeping this for a future experiment.

Furthermore, the few times I was able to successfully peel off films, the films then dissolved quite easily in water. Although I thought the phosphorylation of the PVA would provide some crosslinking, it definitely increases the solubility of the polymer in water, making things actually worse. Using things like aldehydes for crosslinking is not going to be work, but perhaps future experiments with boric acid or citric acid would help with this issue.

A breakthrough came when I realized that cellulose is also known to be phosphorylated with phosphoric acid plus urea and that it could therefore be cross-linked through a phosphoric acid ester with PVA. The cellulose could also provide a support, which would greatly enhance my ability to work with the PVA solids.

Final result of the process mentioned below

My fabrication process was as follows:

  1. To 15mL of ice cold distilled water add 1g of PVA, 1g of Urea and 1mL of 81-85% phosphoric acid. This is solution A.
  2. Place in a fridge for 48 hours, with occasional stirring/shaking. Surprisingly, cold conditions are much better for dissolving PVA because they discourage agglomeration.
  3. Wait till solution A is fully homogeneous, keep longer in fridge and shake/stir as needed.
  4. Dip a filter paper in Solution A. I used Stony Lab 101 but other fine grain filter papers should work just as well. Make sure all excess has dripped off and tap with paper towels to remove any excess.
  5. Place on a hot plate at 180C for 3min
  6. Flip it to the other side for another 3 minute.
  7. Repeat steps 4-6 once.
  8. Place on the hot plate with a petri dish on top (to keep it flat) for 1 hour.
  9. The result should be as shown in the image above.

The process seems to work. The resulting membrane is not soluble in water, is sturdy and easy to manipulate and loses the micro porosity of the filter paper. It is quite brown, which means some oxidation has happened, but reducing the temperature or time leads to membranes that are not properly crosslinked, and dissolve quite easily (leaving just a porous cellulose membrane behind).

To determine whether the above membrane is in fact a cation exchange membrane, I can measure its permselectivity. To do this, I measure the membrane potential between a 0.1M NaCl and a 0.5M NaCl solution (more details about this process on the first image in this post). The membranes produced in this way have permselectivity values between 0.5-0.7, which means that the membrane does in fact act as a cation exchange membrane. However, the membrane is nowhere as good as Nafion, which has a permselectivity >0.95 under these conditions.

I will now try changes in the composition of solution A and optimize the curing temperature to increase the permselectivity of the membrane. So far I think the fabrication process is quite straightforward which allows me to reproducibly fabricate the membranes described in this post.

Thinking about a membrane for my Fe/Mn flow battery

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To build an Fe/Mn flow battery we need a cation exchange membrane to separate the catholyte and anolyte chambers of the device. In this post I want to talk about my initial thoughts about how to create a DIY membrane for this purpose.

Chemical representation of PVA (Polyvinyl alcohol) not to be confused with polyvinyl acetate (what PVA glue is made of).

Commercial cation exchange membranes do exist. Nafion membranes are the most commonly used, but their cost is too high. Just a small 10cm x 10cm square of Nafion can cost upwards of 50 USD, depending on the type of Nafion used. Lower cost membranes (like SPEEK based membranes) have been tested in the literature, but I cannot find any place that actually sells these “lower cost” membranes at a truly lower cost than Nafion.

To be able to make a viable DIY flow battery we need a membrane that we can make, that is lower cost. The requirements of a cation exchange membrane for the Fe/Mn system would be as follows:

  1. Not dissolve in water at neutral pH.
  2. Made from readily available, low cost materials.
  3. Mechanically stable.
  4. No reaction with any of the redox species in solution.
  5. Contain anionic groups (which makes it selective to cations)
  6. Have high conductivity

I looked at potential materials to build this membrane and PVA has become the most prominent base material. It is a polymer with OH functional groups, which I can use to react with readily available chemicals to create a functionalized polymer. My first experiments will involve using phosphoric acid, urea and potassium silicate to create functionalized membranes.

I will prepare 10% w/w solutions of PVA in distilled water, then add different amounts of the above mentioned additives to determine which compositions cast best and have the best properties. I will be casting the films in petri dishes, as this seems to be the most common method in the PVA membrane literature. I will also possibly anneal the membranes by heating them at different temperatures after they have settled.

Double chamber electrochemical cell I bought (haven’t received it yet)

I have also bought a double chamber electrochemical cell to perform experiments using these membranes. The idea is to measure if there is any crossover across the membranes and possibly also measure the ionic conductivity of the membrane.

To measure crossover of ions I can setup one side with the Fe salt and another with the Mn salt, then carry out cyclic voltammetry measurements on the Mn side as a function of time, to measure the appearance of the Fe peak (if there is any crossover). I can compare times between membranes as well. I can also test microporous membranes and non-functionalized PVA membranes, to obtain some baseline measurements. If I setup one side with just NaCl and the other with Fe, I can likely obtain more sensitive measurements (as I will have no current from reactions with Mn species).

Additionally if I use Fe-EDDHA I could sample the solution and measure the appearance of the Fe-EDDHA visible absorption peak near 500nm, which is highly sensitive given the chelate’s very high molar extinction coefficient. Although for this I would near to purchase a Uv-Vis spectrometer, which would cost me 500-1000 USD.

I can also measure ion diffusion by setting up distilled water on one side and a 3M NaCl on the other side and measuring conductivity as a function of time on the distilled water side. This will allow me to compare different membranes and see which ones transport ions faster. If I add Fe chelate to the NaCl I could perhaps measure both ion transport and selectivity simultaneously.

It will be a very interesting journey!

The best low cost Fe/Mn flow battery: Some perspectives about solubility and chelates

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I have previously discussed my project to create a DIY flow battery using Fe/Mn chemistry. On this post I want to expand on the potential limits of this chemistry and some modifications that should enhance our ability to increase its energy density and performance.

My first idea is to attempt to create a flow battery using an NaFe(EDTA) solution as anolyte and an Na2Mn(EDTA) solution as catholyte. This battery would have a potential of around 0.74V, as I measured by cyclic voltammetry (CV) of the species involved. I commented on how the limit of solubility of these chemicals – without any additives – is limited to at best around 0.5M, which limits the battery power density to around 10 Wh/L.

This image shows some NaFe(EDDHA)

However, it is interesting to note that the solubility of these EDTA salts increases aggressively with pH, such that both can be dissolved above 1M at a pH of 7. I confirmed that the solubility increases aggressively as a function of pH, being able to create a solution that was around 1M for both compounds with 3M NaCl supporting electrolyte. To do this I used potassium carbonate to increase the pH gradually to the 7-7.5 range. I also confirmed that the reversibility of the electrochemistry was unaffected through CV, although both standard half-cell potentials are shifted negatively by around 50mV.

This increase in solubility is interesting, as it increases the power density of the battery substantially. If the compounds can be dissolved at 2M, then it would give the battery a density closer to lead acid, at 40Wh/L.

Sadly there are no published studies that show the solubility of EDTA salts as a function of pH, however one of the few published studies of Mn-EDTA in flow batteries (here) shows that you can dissolve Na2MnEDTA at concentrations past 1M. I have bought some additional Mn-EDTA to perform my own solubility experiments, I will let you know what I find out.

Image from this study, using a Zn/Mn flow battery at slightly acidic pH.
Image from this study using Fe-EDDHA at a slightly basic pH.

Another interesting note is to look at other Fe chelate candidates. While EDTA is the lowest cost chelate, the Fe-EDDHA chelate is interesting, as it has a significantly more negative potential Vs Ag/AgCl (-0.6V instead of -0.1V for Fe-EDTA). Recent literature of Fe-EDDHA chelate characterization and its use in flow batteries already shows its practical application (here and here). This increases the potential of an Fe/Mn battery from 0.74V to around 1.2V, which is a decent potential to achieve within the stable window of water at pH 7.

This means that, if using Fe-EDDHA, we could potentially achieve a power density of up to 80Wh/L at a solubility of 2M. If the solubility limit is around 1M, then it should still allow us to get to 40 Wh/L. With this in mind, the Fe/Mn chemistry should match lead acid power density and be a strong competitor against Vanadium based chemistries. This is especially given the fact that Fe/Mn are super abundant and this battery is based on already commercially available chemicals in water, at a neutral pH.

As you can see above, the anolyte and catholyte I propose have been tested, so this is definitely a system that can be built in a rather straightforward manner.