Reaction in Zinc-Carbon-Ammonium Chloride battery

I made a dry cell battery from zinc (Zn), carbon (C) and ammonium chloride (NH4Cl) which produced a current of 0.4 amps and voltage of 1.1V. However when I could not find the redox reaction that was occurring. 


What is the chemical reaction happening in the cell? 




Surprisingly this battery uses Zn without any oxidant like $\ce{MnO\_2}$ and it produces a voltage as high as $1.1$ V. In order to establish its characteristics, the two compartments should be studied independently.


First, the anode, with zinc. $\ce{Zn}$ should not produce simple $\ce{Zn^{2+}}$ ions, as there are ammonia and ammonium chloride in the solution. So the anode reaction is probably : $$\ce{Zn + 4 NH\_4^+ -> [Zn(NH\_3)\_4]^{2+} + 4 H+ + 2 e-}\label{rxn:zn4}\tag{1}$$ The reduction potential for this anodic equation is not given in the tables. But it may be obtained from the next equation, whose standard reduction potential is known to be $\ce{E\_2° = - 1.03 V}$. $$\ce{Zn + 4 NH3 -> [Zn(NH\_3)\_4]^{2+} + 2 e-}\label{rxn:zn}\tag{2}$$ In a $1$ M solution of $\ce{NH4Cl}$, $\ce{[NH\_3]}$ = $2.4 ·10^{-5}$ M. So the corresponding reduction potential of ($2$) is : $$\ce{E\_2 = - 1.03 \mathrm{V} + 0.0296\mathrm{V}· log (2.4 ·10^{-5})^4} = - 1.03~ \mathrm{V} - 0.54~ \mathrm{V} = -1.57 ~\mathrm{V}\label{rxn:nh}\tag{3}$$


Now the cathodic reaction should be studied. Suppose the cathode reaction is : $$\ce{2 H2O + 2 e- -> 2 OH- + H2}\label{rxn:z}\tag{4}$$ with a redox potential equal to $- 0.83$ V. Couping these two expressions ($2$) and ($3$) are never sufficient to get $-1.1$ V as the overall voltage. So, as the ammonium chloride solution is acidic, the cathode reaction must rather be : $$\ce{2 H+ + 2 e- -> H2}\label{rxn:znn}\tag{5}$$ In a $1$ M $\ce{NH4Cl}$ solution, the pH is $4.62$, so the corresponding potential of the cathode is E$\ce{\_5}$ = - $0.059$ pH = - $0.059·4.62$ V = - $0.27$ V.


To summarize, the anode reaction is ($1$) or ($2$), at will, and the cathode rection is ($5$). As a consequence, the theoretical voltage of the cell is : $\ce{E\_5}$ - $\ce{E\_2}$ = - $0.27$ V + $1.57$ V = $1.3$ V, if the concentration of ammonium chloride is $1$ M.


Ref.: Standard electrode potentials have been taken from "Chemistry Data Book" from J. G. Stark and H. G. Wallace, John Murray, London, $1994$, Table $43$, p. $70$.


 3

[Per a source](https://en.wikipedia.org/wiki/Zinc%E2%80%93carbon_battery), the chemical reactions in a zinc–carbon battery can be detailed as follows, to quote:



> 
> In a zinc–carbon dry cell, the outer zinc container is the negatively charged terminal. The zinc is oxidised by the charge carrier, chloride (Cl−) via the following half reactions:
> 
> 
> Anode (oxidation reaction, marked −)
> 
> 
> $\ce{Zn + 2 Cl− → ZnCl2 + 2 e−}$
> 
> 
> Cathode (reduction reaction, marked +)
> 
> 
> $\ce{2 MnO2 + 2 NH4Cl + H2O + 2 e- → Mn2O3 + 2 NH4OH + 2 Cl−}$
> 
> 
> Other side reactions are possible, but the overall reaction in a zinc–carbon cell can be represented as
> 
> 
> $\ce{Zn + 2 MnO2 + 2 NH4Cl + H2O → ZnCl2 + Mn2O3 + 2 NH4OH}$
> 
> 
> 


The battery cell is claimed to have an electromotive force (e.m.f.) of about 1.5 V. Note, personally above, I would write NH4OH as NH3(aq).


 0

I just made this battery and then found this question here and wanted to add to the discussion.


My battery uses a stainless steel electrode rather than the carbon used by the original author.


The voltage and current both drop quickly in conjunction with the build up of hydrogen bubbles on the stainless steel electrode.
I found that voltage and current dropped when a small load was connected. The current and voltage output (while loaded) was increased dramatically if I added a small amount of hydrogen peroxide to the electrolyte.
This seems to depolarize the stainless steel stopping the formation of the hydrogen bubbles.
Cheers


**Edit Aug 8**


The battery has been connected to some LEDs for the last 3 days. The output drops after a day so H2O2 has been topped up each day by adding 2 ml of 30% H2O2.


What is this white precipitate that is forming? I am guessing ZnO? I was expecting the Zn and the Cl from the NH4Cl to react which would have been soluble and that the H2O2 would be depolarizing the stainless electrode by preventing H2 gas bubbles from forming. Now I don't have a clue what the reactions are in this battery.
Does anyone have any idea?[![Zn-Nh4Cl-H2O2 Battery](https://i.stack.imgur.com/RkBWj.jpg)](https://i.stack.imgur.com/RkBWj.jpg)


 0