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My adventures building a Zn-MnO2 battery

I ran this experiment for 43 cycles (Exp31b, results below), charging to 1.5mAh at 5mA/cm2. The CE remains high, EE remains high, capacity was 1.38mAh on average. Energy density at this capacity, given total device volume,, is 16Wh/L.

Exp31b.png

I have now done another single conditioning cycle, charging to 2.2V at 5mAh/cm, then setting the battery to 2.2V until the current dropped to 1mAh/cm2, then fully discharging. I am now cycling by charging to 2.5mAh at 5mA/cm2 and discharging to 0V.
 
So, I read an article that came out last year that went deep into the characterization of these Mn/Zn systems and the effect that mildly acidic conditions and pH buffers have on the formation of MnO2 oxides and their charging/discharging behavior (https://hal.archives-ouvertes.fr/hal-02566126/document).

Given this information, it seemed clear that I needed to use an acetate buffer instead of just acetic acid. Note that I used acetic acid because this is allegedly what was used in the first paper I followed about acetic acid used in Zn/MnO2 batteries. I therefore decided to prepare a buffer and test a battery with it in Exp32.

I prepared an electrolyte with 1m ZnSO4 + 0.5m MnSO4 in a 0.8M pH 5 buffer that I created beforehand by neutralizing vinegar with sodium hydroxide till I got a pH reading of 5 (using a freshly calibrated Apera PH60 meter).

The buffer makes a huge difference. Here are 4 cycles, running to only 0.5mAh at 5mA/cm2 but discharging only to 1V instead of 0V.

1639757283113.png

I am getting a CE > 92% by discharging to only 1V plus, the energy efficiency of the battery has now increased to >74%. Not only this, but my average discharge voltage has now increased to ~1.47V. The acetate buffer has made the chemistry way more reversible. Capacity and efficiency values were also stable for these few cycles.

I am now going to try higher capacity, see what we can get here ?
 
So far I've been able to cycle the battery 9 times at 1mAh. I discharged to 0.8V. The discharge capacity is up to 0.90mAh, with a discharge potential of 1.42V this gives me an energy density of around 13Wh/L. The CE, EE and capacity are all stable so far. The EE>70% and the CE>90%.

1639773754646.png
 
It is worth mentioning that many papers that work on Zn/MnO2 chemistry will do constant potential, instead of constant current charging. This means that they will hold the device at a given potential until a given mAh is accumulated and then discharge at constant current to a given threshold potential.

I have now implemented such charging within the python program I use to test the batteries, so I'll be able to cycle batteries this way and see what difference it makes. This mode of charging is also easier in practice, since just putting a battery across a fixed potential difference is easier than trying to control the exact current flowing through it.
 
I decided to try higher energy density and go with a much thinner separator. The fiberglass is too porous, so I still get shorts from stray carbon fibers from the carbon fiber papers going through. For this reason, I changed to 1 layer of W42 filter paper separator, which measures around 200 micrometers. This is 1/3rd the separator thickness previously used. I also changed the current density to 10mA/cm2. The electrolyte is 1m ZnSO4 + 1m MnSO4 in a pH5 0.8M sodium acetate buffer. This run is Exp33.

I am charging to 1mAh and discharging to 0.8V. The average discharge voltage is close to 1.5V and capacity is at close to 0.9mAh. So far the configuration is stable after 7 cycles. CE is > 89% with an EE > 76%.

1639932770015.png
Energy density under these conditions is 28Wh/L. If dendrites don't pose an issue, then this is likely to be a usable configuration in practical setups.
 
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Very impressive.

Would a switch from distilled water to deionized water help make sure the chemistry is what you expect?
 
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.

1675593855653.png
 
danielfp248 said:
The latest test charging to 1mAh is at 45 cycles. The energy density of this battery is at 15 Wh/L.
Click to expand...
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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