Flow Battery Research Collective

On my last post I wrote about the potential of using Fe/Mn in acidic solution to create an Fe/Mn flow battery. I cited a paper published a few years ago which shows that you can achieve reversible Mn³⁺ chemistry in a solution of sulfuric acid and hydrochloric acid, I then proceeded to confirm this reversibility using cyclic voltammetry of Mn²⁺ solutions in hydrochloric acid.

However, it quickly became clear from analysis of the paper that this was only at very low capacities. This is because Mn³⁺ becomes unstable as its concentration increases in solutions, turning into MnO₂ and Mn²⁺.

Given the very low volumetric densities that can be achieved with the acid setup, there’s no option but to revisit the use of more stable and reversible forms of manganese. The best candidate seems to be Mn-EDTA. This complex has already been shown to work in flow batteries at the 0.5M-1.0M range (see here).

I had already thought about using this complex and wrote several posts about its potential use in combination with Fe-EDTA or Fe-EDDHA (see here). However, there is a big problem with the pH compatibility of the Mn-EDTA with the Fe-EDTA or Fe-EDDHA. The issue being that Mn³⁺-EDTA is only stable under acidic pH conditions, where the solubility of both Fe-EDTA and Fe-EDDHA is limited to around 0.1M. These chelates are only highly soluble under basic pH conditions, which are fully incompatible with Mn-EDTA.

The question is whether there is any easily accessible Fe chelate that is both compatible with Mn-EDTA in solution (so that we can create a symmetric electrolyte) and that can create soluble solutions at >0.5M concentrations in a pH ~5-6 buffer. Note that I need both chelates to be dissolved at >0.5M at the same time since I want the electrolyte to be symmetric so that it can work using a microporous membrane.

The answer is Fe-DTPA. This chelate is highly soluble at acidic pH values and – best of all – it is soluble enough to actually be in >0.5M solution in the presence of Mn-EDTA at this high concentration. Above you can see a picture of the Fe-DTPA+Mn-EDTA solution. The solution also contains an acetate buffer, which should ensure pH stability on charge/discharge, which should prevent degradation of the Mn-EDTA.

The second image shows a CV of the Fe-EDTA/Mn-EDTA buffered solution, showing that both the Fe and Mn electrochemical reactions are reversible. The half wave potentials are -0.11V and 0.61V, giving us an expected potential for the flow battery of +720mV. This is close to what I had measured before for Fe-EDTA/Mn-EDTA. This proves that the DTPA does not change the electrochemical characteristics of the system very much. The above test also confirms there acetate buffer is stable to the generated Mn³⁺-EDTA.

The next step is to build a flow battery using the above solution and see what performance characteristics we can get. With the current solutions this system will be limited to around 8-9Wh/L. However I haven’t tested the solubility limits of the chelates in this buffer.

Our goal at the Flow Battery Research Collective (FBRC) during the past year has been to develop and manufacture a flow battery kit that can be used to study flow batteries at a small scale in a low cost yet reproducible manner. Today I want to discuss all the problems we have found when attempting long term cycling in both versions of our kit and how we have addressed them so far.

Our first kit design – which you can see above – was able to do long term cycling of an electrolyte composed of Zinc Chloride (1m) and Potassium Iodide (2m) in a potassium acetate/acetic acid buffer (pH=5.2). We were able to obtain capacities higher than 10Ah/L (on total volume of electrolyte — note previous values on this blog were based on catholyte only) without much capacity fade at the 25 cycle range. I didn’t try longer term cycling because of issues with pumps getting damaged by the tubing I was using leaking iodide (see how orange the right pump above looks, that’s not supposed to happen). At this point we were also testing several types of tubing, reason why you can see different colors of tubing in the setup.

However, this design had two big issues. The first is that the polypropylene bodies were getting over compressed at the top and under compressed at the center, so after a single long term cycling event (cell cycling for 5 days), the cell would never seal well again after being opened due to warping of the body. This made this design completely unfeasible and forced us to accelerate the development of v2.

The second cell design – showed in the second image above – has a square design with even compression and complete isolation of the cell body from the electrolyte by using a sealed polypropylene flow frame (you can read more about it in the FBRC website). However, despite using the exact same electrolyte and membrane, this design has been showing some increasing decay of the electrolyte as a function of time. The cycled capacity is a bit lower and after 28 cycles the testing had to be stopped due to obvious deterioration of the charging potential.

Flow batteries using microporous membranes can suffer from significant issues related to hydraulic pressure differences between anolyte and catholyte. As the battery is cycled, there is net water transfer between both sides and this can lead to the accumulation of polyiodide species in a Zn-I flow battery. This deteriorates the charging potential and reduces the SOC of the battery. You can ready more about this decay mechanism in this paper. When my testing was done, there was indeed an almost 50% less fluid in the anolyte vs the catholyte side, interestingly the completely opposite effect when compared to the paper. Changing the supporting electrolyte concentration or Zn salt used – to make the water changes less extreme – or adjusting electrolyte flow rates, can help eliminate these volume balance issues.

There are however other problems with the current tests. The first is that the chemistry is not stable to oxygen – both due to iodide and zinc reactivity – so working under atmospheric conditions without any inert gas purging of the electrolyte has likely made my tests more unstable, due to Zinc passivation and pH instabilities caused by the reactions of iodide with oxygen. Since the new flow frames are 3d printed – therefore not 100% free of pores – they might be letting more oxygen in, which might also be why the system is more unstable than v1 in this regard. Getting flow frames manufactured through polypropylene sintering or injection molding could get rid of this issue.

Another issue – shown below – is the use of graphite foil (grafoil) as our electrode material. Although grafoil does the job fine, it does seem to be porous to iodine, which crosses it and reacts with the underlying copper, increasing series resistance and creating a protuberance on the copper plate (which is readily visible in the image after long term cycling). I scratched the grafoil to then reveal a white powder, which is copper iodide, leading to a further loss of capacity. This problem could be solved by using a proper bipolar plate material or by sealing the grafoil in some manner. Thicker grafoil could also be enough to ameliorate the problem.

I also want to note that we have achieved much higher capacities – see above – although not in long term cycling, as these higher capacities generate much more concentrated electrolytes that have significant problems (such as the precipitation of insoluble solid elemental iodine and the corrosion of the pumps and copper current collectors).

As you can see, getting long term cycling results is not easy. There are many hurdles to overcome to be able to provide a kit that is able to cycle a chemistry like Zn-I long term, without any problems, in a reproducible manner. All these hurdles can be overcome though – as there are plenty of examples of people cycling these batteries long term – so it is a matter of arriving at a proper combination of materials and chemistry. Our work continues! Thanks a lot for all your support.

This flow battery kit work is being funded by the Financed by Nlnet’s NGI0 Entrust Fund. We are also collaborating with the FAIR Battery project.