Greetings Everyone
There have been a few recent threads where the subject of overdischarge of NiZn cells (namely PowerGenix AAs) has been brought up. I have not done any overdischarge tests on these cells, and likely won't have time in the near future to do so. However, I was traveling today and used some idle time on the plane to look into the matter from a more fundamental perspective.
In order for any cell to be considered "overdischarged", the potential of one or both electrodes must change enough that detrimental side chemical reactions are activated. So let's look at the potentials of both the electrode charge/discharge reactions for the NiZn cells. All of the half-cell potentials given below are versus the standard hydrogen electrode (SHE):
Positive (nickel) electrode:
2 NiOOH + 2 H2O + 2 e → 2 Ni(OH)2 + 2 OH- E=0.45 V
Negative (zinc) electrode:
Zn + 2 OH- → Zn(OH)2 + 2 e E=-1.25 V
The difference between the positive and negative electrodes is 0.45-(-1.25)=1.7V. This is the open circuit cell potential.
The two side reactions that one want to avoid during discharge of a NiZn cell are hydrogen evolution on the nickel electrode and oxygen evolution on the zinc electrode. Those reactions are given below for an alkaline (pH=14) solution:
Hydrogen evolution:
2 H2O + 2 e → H2 + 2 OH- E=-0.83 V
Oxygen evolution:
2OH- → 1/2O2 + H2O + 2e E=0.40 V
So, the potential of the nickel electrode must decrease from 0.45V to -0.83V to generate hydrogen, and the zinc electrode must increase from -1.25V to 0.40V to generate oxygen.
I have not confirmed this, but it is almost certain that NiZn cells are cathode-limited, i.e. they have excess zinc. This means that on discharge, the nickel electrode runs out of capacity first and will begin to drop. Hydrogen gas will begin to be generated on the nickel electrode when that electrode drops to -0.83V, which will occur at a cell voltage of (-0.83V-(-1.25V)=0.42V!
So if the cell is discharged below 0.42V and current continues to flow through the cell, then hydrogen is generated at the nickel electrode and the zinc electrode continues to discharge as normal until the zinc capacity is depleted. Once that happens, the zinc potential will rise. If the cell is not in series with other cells, then the zinc potential will rise to -0.83V and the cell voltage will be 0V and nothing further will happen. But if the cell is in a series string and current continues to flow, then the zinc electrode will rise to 0.40V and oxygen will be generated at the zinc electrode simultaneously with hydrogen at the nickel electrode. This will occur at a voltage of -1.23V.
Let's summarize. There are three stages of discharge for a NiZn cell. Stage 1 is normal discharge. Stage 2 starts at a cell voltage of 0.42V when the nickel electrode is depleted and hydrogen evolution occurs. Stage 3 starts at a cell voltage of -1.23V when the zinc electrode is depleted and oxygen evolution occurs concurrently with hydrogen evolution.
I should point out that there are also three stages of discharge in a NiMH cell. Using a similar analysis to that given above, Stage 2 starts at -0.1V and Stage 3 starts at -1.23V (note that Stage 3 starts at the same voltage for both NiMH and NiZn because the same reactions are occurring). So for a NiMH cell, you must drive the cell to a negative voltage in order to initiate overdischarge.
Also, in a NiMH cell, the hydrogen generated in stage 2 is absorbed by the metal hydride alloy, so the internal pressure of the cell stays relatively low. There is no such internal mechanism to absorb hydrogen in a NiZn cell, so hydrogen generated on overdischarge results in an increase in internal pressure. Same for the oxygen generated in Stage 3.
Ok, with me so far? Now let's look at how much hydrogen and oxygen are generated.
The amount of hydrogen and oxygen generated is directly proportion to the current being passed through the cell. During Stage 2 discharge, hydrogen is being generated at a rate of 14ml per amp per minute. During Stage 3 discharge, oxygen is being generated at a rate of 7ml per amp per minute (in addition to the hydrogen generation). So if you are discharging your series string of NiZn cells at 1 amp, and one cell drops to <0.4V, hydrogen is being generated at a rate of 14ml/minute. If that cell is driven into reversal to -1.23V, you are generating 14ml H2 + 7ml O2 per minute (please note that I am assuming 25 degrees C and 1 atm for these gas volume calculations).
You can see that the internal pressure of the cell can rise rapidly if a NiZn cell is overdischarged. The vent may even activate, resulting in loss of water (both liquid electrolyte and as hydrogen and/or oxygen gas). You will note in the positive electrode reaction above that water is consumed in the discharge process, so if you lose water, you lose capacity.
I will now summarize this for those of you whose eyes glazed over my rambling above. Overdischarge of a NiZn cell should start with hydrogen evolution on the nickel electrode when the cell potential drops to <0.42V. This is Stage 2 discharge. If the cell is driven into reversal down to -1.23V, then both hydrogen and oxygen will be generated. This is Stage 3 discharge.
Take home message: keep those NiZn cells above 0.42V!
NiMH cells are more robust with respect to Stage 2 discharge damage because Stage 2 starts at a lower voltage (<-0.1V), and the pressure will not rise because hydrogen is being absorbed by the metal hydride alloy.
Please note that this "paper" exercise was conducted with no testing. These are only estimates of the actual voltages where overdischarge will occur. I have also made the assumption that there is excess zinc in the NiZn cells, and although I think that this is a good assumption, I could be mistaken.
That being said, I believe that the results of this exercise are consistent with the experiences of NiZn users that have been posted on various threads in this forum.
Questions? Discussion?
Hope this was all clear. I kind of did brain dump here.
Cheers,
Battery Guy
