My adventures building a Zn-MnO2 battery

adrian1702 said:

Daniel, do you have any new results?

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No, I moved countries again. This time I moved from Colombia to Spain, so I had to leave all my battery experiment stuff in Colombia with family. I will hopefully get the electronic and electrode parts back when they come to Spain later this year.

I recently got a visit from a friend and was able to have her bring me my potentiostat . She couldn't bring me my battery materials though, so I won't be performing experiments in this direction yet.

However, I have obtained all the materials necessary to build a home cyclic voltammetry setup (https://chemisting.com/2022/11/04/a-home-setup-for-cyclic-voltammetry/), something that I have been wanting to do for a while.

I will be experimenting with Mn+2/Mn+3 chemistry in this setup. My goal is to measure the reversibility of the Mn2+/Mn3+ reactions in concentrated sulfuric acid solutions using different inorganic additives. This is towards a more elaborate flow battery project I want to carry out in the medium term.

I won't be posting updates about that here (as this is a forum for batteries) but check my blog for time to time for updated information on my progress.

Another person is coming to visit me and bringing me a good chunk of my battery materials next week. I will therefore be restarting the Zn/Mn battery research, while I gather the resources to continue my research into flow batteries. Stay tuned for some new experiments on this thread!

Test experiment running 1m ZnSO4 + 1m MnSO4 + 10% acetic acid pH 5 buffer (adjusted with from acetic acid with potassium carbonate). Anode is Zn metal foil, cathode is CC Carbon cloth. Separator is 3 layers of fiberglass filter. Charging to 0.5mAh at 5mA, discharging to 0.8V at the same current.

No problem after 57 cycles. I am now going to try cycling to 1mAh. Equipment seems to be in good working order

This is fascinating to me! What volume is your cell?

justgary said:

This is fascinating to me! What volume is your cell?

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Volume is 0.089mL, counting both electrodes and separators.

The latest test charging to 1mAh is at 45 cycles. The energy density of this battery is at 15 Wh/L. So far no problems have showed up:

danielfp248 said:

The latest test charging to 1mAh is at 45 cycles. The energy density of this battery is at 15 Wh/L.

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Is this now 1mAh charging at 5mA, so now a 12 minute charge? Or did you bump the charge current to 10mA and keep a 6 minute charge?

justgary said:

Is this now 1mAh charging at 5mA, so now a 12 minute charge? Or did you bump the charge current to 10mA and keep a 6 minute charge?

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I left the current at 5mA, so it's now taking longer per cycle.

After 107 cycles there are no signs of degradation (the small drop at cycle 65 was because my computer restarted and I had to restart the testing process). Both charge and discharge potentials have started to stabilize. The battery has a CE > 95% and EE > 80%.

After 123 cycles, everything still stable. Charge/discharge potentials continue to stabilize.

Wow!

That's already looking better than an average Lead Acid battery which has a CE ~85%, EE ~70%, and at 100% DoD capacity starts dropping after ~100 cycles. Wonder how the $/kWh and Wh/kg compare to LFP (as I recall, you said in the OP they were "low cost")?

svetz said:

Wow!

That's already looking better than an average Lead Acid battery which has a CE ~85%, EE ~70%, and at 100% DoD capacity starts dropping after ~100 cycles. Wonder how the $/kWh and Wh/kg compare to LFP (as I recall, you said in the OP they were "low cost")?

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Lead acid is at 80-90 Wh/L. This is at currently less than 1/4th of that. We will need to see if we can reach those types of capacities!

The cost relative to lead acid should be substantially less if we do get there.

I stopped the test at 137 cycles. I will now be doubling the capacity and trying to charge to 2mAh at 5mA and discharge to 0.8V at the same current. This would bring the energy density to ~30Wh/L.

This battery failed quite quickly in the 2mAh test. The test was also quite slow, so I increased the current to 10mA, which was likely too much, as potentials increased all the way to 2.4-2.6V on charging. Upon opening the battery I measured the thickness of the device, which ended up being 0.4mm. The zinc anode showed clear signs of corrosion.

To continue, I have started Exp34. This test used a carbon cloth cathode, 3 layers of fiberglass filter separator, zinc anode and a solution containing 1m ZnSO4 + 1m MnSO4 + 8m urea in a 10% acetic acid buffer at pH 5 (adjusted with potassium carbonate). The idea of the urea is to decrease the water activity to see if this helps with both dendrite formation and hydrogen evolution.

I have started cycling this battery to 1mAh, at 5mA, discharging to 0.8V. This puts the energy density at around 23Wh/L. The battery is behaving in a stable manner after 111 cycles. The urea did diminish the conductivity of the solution, which increased its series resistance and diminished both energy efficiency and the average discharge potential. I will cycle it 500 times (if it allows me to do so) and will then open it up to see if there is in fact corrosion of the zinc anode.



For the next experiment I will likely try decreasing the pH5 buffer content to get a more concentrated urea + Zn + Mn solution. But I will decide once this text is done. If the urea does nothing to prevent corrosion then I will try a test with higher ZnSO4 concentration.