Author Archives: danielfp

Studying a WiSE based all-Fe chemistry using our flow battery kit

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All-Fe flow batteries are very promising due to iron’s high abundance, low toxicity and low cost. In these batteries, FeCl2 is used as the main active salt in solution. When charging Fe2+ gets reduced to Fe metal on the anode while Fe2+ gets oxidized to Fe3+ on the cathode. However, these batteries suffer from a fundamental problem that has made their large scale adoption very difficult up until now.

Cyclic voltammetry of 1M FeCl2 and 4.5M CaCl2 using an Ag/AgCl reference electrode and a glassy carbon working electrode.

The main limiting problem of these batteries is H2 evolution at the anode. Since Fe plating happens at a lower potential than H2 evolution, there is always some hydrogen generation when the battery is charged. This H2 evolution causes an increase in the pH of the battery which then causes iron hydroxides and oxides to precipitate out of solution. This causes passivation of Fe metal surfaces, loss of capacity and potentially clogging of the battery.

A few years ago, Tao Gao’s group in Utah discovered that CaCl2 and MgCl2 water-in-salt-electrolytes (WiSE) (read the paper here) could substantially improve Fe plating behavior. While they didn’t test this in flow batteries, they showed that very high Coulomb efficiencies with very low hydrogen evolution rates were possible when using these salts. This is because these salts bond with water and therefore make it substantially harder for water to get reduced at the anode. We are talking about very highly concentrated solutions though, around 4.5M CaCl2, which is around 660g/L of CaCl2.2H2O.

Plate/strip experiment using the 1M FeCl2, 4.5M CaCl2 electrolyte.

We recently obtained some of these reagents to test these chemistries in our flow battery kit. I first ran CV experiments of the 4.5M CaCl2 electrolyte with 1M FeCl2 and obtained the curves showed above. This reveals standard potentials of -0.71V and 0.55V for the two half reactions, which means our battery should have a max potential of around 1.26V. I then ran plate/strip experiments (using different plating times), which generated the second curve above. Using the slope of this plot we can extract the plating efficiency, which is >99% in this case. This means that very little hydrogen evolution is happening within this electrolyte, a result I had never seen with FeCl2 solutions and which is in agreement with the Tao Gao group paper.

After doing this I then ran a flow battery using this electrolyte and noticed some problems with capacity loss because of undissolved metallic Fe on the anode using a Daramic separator. It seems that the above electrolyte has some issues with the reversibility of the plating reaction. While little hydrogen is generated on plating, there still seem to be some important losses of capacity due to passivation of the Fe. To fix this problem I then added 1M NH4Cl into the electrolyte, which helped me fix this issue when dealing with Zn-I flow batteries.

Cycling of an all-Fe battery using 1M FeCl2, 4.5M CaCl2 and 1M NH4Cl. Cycling was done at 20mA/cm2 Charging was done to 4Ah/L and discharge was done to 0V.

I cycled the battery at 4Ah/L, which generated the results above. You can see that we get extremely good CE values (97.88%) which are even better than those we were obtaining with the Zn-I system, even though we’re using a microporous separator. The energy efficiency is lower than for Zn-I but still reaches a value of 53.27%. Given a concentration of 1M FeCl2 , 4Ah/L represents a state-of-charge (SOC) of ~30%. The cycling is stable, although some increase of the charging voltage is indeed happening through time (although this is also matched by an increase in the discharge voltage). The mean discharge potential is quite low though, at 0.7V, which means we have considerable ohmic losses at this current density.

I am now testing the electrolyte at higher SOC values and will continue testing MgCl2 and other modifications, such as additions of ascorbic acid and thiosulfate to remove oxygen from the initial solutions and improve the initial state of the battery (as some capacity is lost due to the presence of oxidized Fe in the initial solution). Make sure to follow our progress on this forum thread.

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.

Our v2 Open Source Flow Battery Small Scale Kit is now stable

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After a lot of work on our small scale flow battery kit, the kit has now reached a form factor that is stable and will only require minor cosmetic changes from now on (to do things like hide exposed cabling, etc). The instructions for assembly of the kit are available at fbrc.dev. The kit is the culmination of 2 years of hard work from our team at the Flow Battery Research Collective, especially Kirk Smith and Josh Hauser, who both worked tirelessly with me in the design and fabrication of the kit.

Final form factor of the flow battery kit v2.
The back of the kit now features a box for the storage of the Arduino, to minimize the possibility of any solutions damaging the board. In the future we will add some modifications to better route the wiring.

Our final design uses Kaomer KPK200 24B pumps, polypropylene reservoirs and what Kaomer calls “Tygon Chemical Tubing” which seems to be a PTFE lined BPT tubing material. These materials are all compatible with the Zn-I chemistry and are also expected to be compatible with chemistries like Vanadium, Fe-Zn, pure Fe and even non-aqueous chemistries. The 24V pumps are driven using PWM signals coming directly from the Arduino, they also send digital signals back to the arduino that provide us with RPM information. This allows us to control the speed of the pumps quantitatively and achieve more robust experimentation. The mystat software has also been modified to allow direct control of the pumps from the software as well as monitoring of these readings.

Box design for the flow cell educational kit

As you can see above, we have also created a box design for the flow battery kit, which we have titled “Flow Cell Educational Kit” as applications for it can go beyond flow batteries. We may start publicly selling kits sometime in the near future, if there is enough interest from people wanting a plug-and-play solution (especially for academic environments). If you’re an academic researcher, battery enthusiast or professor looking to provide low cost flow battery setups for your students, feel free to let us know is such a kit would interest you.

These are some recent results using the latest kit setup using Zn-I chemistry. This test was ran at 2M KI, 1M ZnCl2, 2M NH4Cl with a 5% Triethylene glycol addition at 60mA/cm2 for both charge and discharge current.

Above you can see some experimental results using the kit and Zn-I chemistry. We are able to reproducibly cycle Zn-I chemistries to very decent SOC values, even though the above tests have been carried out without any nitrogen purging of the system. This is a testament to the robustness of the system and the Zn-I chemistry we are using as a showcase for it.

With all that said, none of the above would have been possible without the support of our financial sponsor for this project, the nlNet foundation. We are ever thankful for all their support in this endeavor. Thanks to them, we are also building a large scale, fully open source kit, that we also hope to release later this year.

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.

Working on a large scale open source flow battery design and kit

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During the past couple of years we have been working on the design of a small flow battery kit for the study of flow batteries (you can read a previous post about it here). With the help of an NLNet grant, we have fully developed the first two versions of our small scale kit – with the second one achieving even longer scale tests – and are now working on the development of a larger surface area kit that can be used to study flow batteries or build flow battery applications at a relevant scale for practical power storage.

A flowframe being printed on our Prusa Core One on Polypropylene. Such a large flat area piece is very hard to print without warping in non-heated chambers.

This new large area setup – which we have discussed in our forum – is now moving out of the design stage thanks to the acquisition of new systems for both 3D printing (Prusa Core One) and vinyl cutting (Cricut Maker4) that are allowing us to move to their in-house fabrication and testing. The new cell is designed to be compatible with the area of 3D printing beds and features an active area of 13.5×13.5cm, which is 182.5cm2. In practice this means a single cell will be able to handle a capacity of nearly 22Wh when using the Zn-I chemistry used in our small scale design.

We borrow some of the features that worked great for our small scale kit – like using a polypropylene enclosed flow frame – and add features that are needed for a scalable design (like all entry/exit points on the same side, stacking compatible design, etc).

Finished flow frame held against light so that you can see the flow field design. This design is not final and will change as we do fluid testing.

The fabrication of these pieces is especially challenging due to their size and the complexity of the flow fields in them. The flow fields have to follow complex paths to prevent the creation of large shunt currents within the device. A really good 3D printer with a heated enclosure is required to be able to print this in polypropylene with no warping and good enough resolution. Even with such a printer, a lot of fine tuning is still required to achieve the low level of surface roughness and high level of water tightness that is needed for this particular application.

While we are thinking about the potential of using heat welding or even adhesives to be able to put the cell together, the easiest first approach will be to use the same silicon gaskets as we used for the fabrication of our initial cells. To cut our gaskets in house we are using a Cricut Maker4 vynil cutter, which is able to make this high precision cuts without any problems.

One of our 3D printed polypropylene flow frames next to a silicone gasket

With these materials now fabricated, we are now close to putting together our first tests of a larger scale flow battery cell. The idea of these initial tests will be to test the cell for leaks and make sure we can circulate water without problems before we try to run any active materials. We will be using thick wood as endplates for this initial test, as this is the easiest to source, hard material, that can be used. Thanks to our use of fully enclosed gaskets, the end plates will also have zero contact with any water or active materials.

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.

A low cost, open source, Cu/Mn rechargeable static battery

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Building a rechargeable battery is not an easy task. Although many great technologies are available (like LiFePO4 or even lead acid batteries), building these batteries isn’t trivial because of the technological hurdles, manufacturing requirements, chemical substances, knowledge and safety requirements. It would be ideal if we had access to an open source rechargeable battery technology that was easy to construct in practice with readily available materials, robust and at low cost.

This is a sample 0.5mL cell using a copper anode and a carbon felt cathode (0.3mm). A polypropylene felt separator is used between both. The cell is 1cmx1cmx0.5cm in volume.

In a previous post I talked about Cu/Mn batteries and how several different papers describe batteries using Cu sulfate and Mn sulfate along with sulfuric acid to create robust batteries with significant capacities, even above 40Ah/L. Such batteries would be close to ideal as they are easy to build, use earth abundant materials and – in theory – are very robust. However, my efforts to reproduce these batteries were plagued with failure as I was unable to reproduce both their reversibility and their capacities.

Furthermore, this Cu/Mn technology using sulfuric acid has been patented by a French company (years before the Chinese articles started sharing the chemistry in 2017). This means that this chemistry is not open source and significant battles could arise from the use of this technology at a wide scale. This also explains why the patent applications mentioned by some of the Chinese researchers in their papers cannot be found (probably the patents were denied because of the preceding French patent).

Experimental results of the Cu/Mn cell using methylsulfonic acid. The electrolyte was prepared with 0.05g of FeSO4.7H2O, 2g CuSO4.5H2O, 1.8g of MnSO4.4H2O, 2mL of 70% CH3SO3H and 8mL of RO water. Cycling was done at 5mA/cm2

To tackle my problems reproducing this chemistry, the hurdles with intellectual property and some issues dealing with the solubility limit of copper/manganese sulfate mixes, I have modified this technology to use methansulfonic acid (CH3SO3H) instead of sulfuric acid. Methanesulfonic acid is easier to get than sulfuric acid – because it has no regulatory restrictions – and the solubility of both copper and manganese mesylates is higher than that of their sulfates, meaning that even higher capacities than with sulfuric acid should be possible.

The above experimental results show cycling of the cell shown in the first picture. This chemistry achieves a CE above 90% with an EE above 65%, the cycling is also very stable with very reversible MnO2 formation in the highly acidic media. The electrolyte tested is roughly 0.8m Cu, 0.8m Mn and 1m CH3SO3H. I haven’t tried changing the acid concentration or preparing more highly concentrated electrolytes yet, as I am still fine tuning the cell fabrication process to enhance reproducibility. The cells right now can be charged to 20Ah/L, which is already an interesting level of capacity, although 40Ah/L should be possible.

Image of dendrites due to electric field abnormalities around the edges of the Cu anode.

A very important issue I’ve noticed is that dendrites tend to appear around the edges of my Cu anodes due to electric field instabilities, as the Cu prefers to grow in the free solution rather than through the polypropylene separator. This can cause the battery to short when charging to very high capacities). Cells without separators – with just the electrodes hanging parallel as in the case of some of the Chinese papers – could help alleviate the issue. I am also trying passivating the edges using nail polish, to see if this fully solves the issue.

While the Cu/Mn battery chemistry using H2SO4 is clearly patented, the innovation using CH3SO3H is not protected, neither covered by the scope of the current patent nor previously published anywhere else (my innovation to the best of my knowledge). The publication of this blog post should ensure that this technology will remain patent-free.

Update:

The results above show the cell being charged to 1.45V (This is likely the Nernst limit of the cell). I got a discharge capacity of 33.6Ah/L at 10mA/cm2. Electrolyte is identical to the one mentioned before. The CE and EE are still the same at higher capacity. Dendrites do not seem to get worse provided enough space is left between the edge of the copper electrode and the edge of the separator.

Long term cycling of our Flow Battery kit using a Zn-I chemistry

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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.

Long term cycling results with the first version of our kit. Around 5 days of cycling.

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.

Long term cycling results of second version of our kit. Cycling was also around 5 days.

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.

New flow frame design for the second version of our kit. You can see the flow channel has been colored red by the catholyte. The flow frame was 3d printed. Since 3d printing leaves some porosity (even at 100% infill) you can see some coloring even away from the flow channel.

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.

Catholyte side graphite foil, 0.5mm, after 5 days of cycling. You can see a bump in the foil caused by buildup of copper iodide material below the electrode.
A high capacity short cycle result using a Zn-I electrolyte. This is using total volume of electrolyte, so a capacity that would be in line with current Vanadium batteries.

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.