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Topic: Electrochemistry problem: Nickel-Iron Battery  (Read 3372 times)

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Offline Thermofox

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Electrochemistry problem: Nickel-Iron Battery
« on: September 16, 2024, 06:50:42 AM »
PROBLEM

A Nickel-iron battery is composed by a Fe anode and a NiO cathode, in solution with the electrolyte KOH. The battery has a standard cell potential of Δεcell°= 0.400 V and operates through the following half reactions, at T= 25°C:
Fe + 2OH-  :rarrow: Fe(OH)2 + 2e-
NiO + H2O + e-  :rarrow: Ni(OH)2 + OH-
Determine,
  • ΔG° of the reaction and Keq.
  • E.M.F. when [KOH]= 5.00 M
ATTEMPT

For the first point I know that ΔG°= -nΔεcell°F. From the half reactions I notice that n=2
=> ΔG°= -(2)(0.400 J/C)(96485 C/mol)= -7.72 104 J/mol.
From this I can then determine Keq.=[itex]e^{(\frac {-\Delta G°}{RT})}[/itex] = [itex]e^{(\frac {(7.72)(10^{4} J/mol)}{(8.31 J/(mol K))(298 K)})}[/itex]= 31.2.

For the second point E.M.F. should ≡ Δεcell = [itex]\Deltaε_{cell}° + \frac {RT}{nF}\ln Q[/itex]. What I don't understand is how to determine Q. I know that Q is the reaction quotient so I thought of analyzing the total reaction of the battery:
Fe + 2OH- + 2NiO + 2H2O + 2e- :rarrow: Fe(OH)2 + 2e- + 2Ni(OH)2 + 2OH-

But now if I simplify, I obtain: Fe + 2NiO + 2H2:rarrow: Fe(OH)2 + 2Ni(OH)2 and consequently log(Q)= 0 since Q= 1 because under the conditions of the problem all reactants and products have an activity of 1.

Offline Borek

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #1 on: September 16, 2024, 07:44:07 AM »
I am not saying that's the answer, but at first sight the most obvious conclusion is that Δεcell just doesn't depend on KOH concentration.
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Offline Thermofox

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #2 on: September 16, 2024, 08:10:13 AM »
I am not saying that's the answer, but at first sight the most obvious conclusion is that Δεcell just doesn't depend on KOH concentration.

If Δεcell doesn't depend on [KOH], then Δεcell=Δεcell°. Since nothing that has an impact on Δεcell is changing from standard conditions, right?

Offline Hunter2

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #3 on: September 16, 2024, 08:55:11 AM »
There is also a mistake in the equation with nickel.
NiO and Ni(OH)2 has same oxidation number for Ni +2.
The stoichiometry is also wrong

NiO + H2O + e- → Ni(OH)2 + OH-

The right half reaction is

NiO(OH) +  H2O +  e−  → Ni(OH)2   +  OH−

https://en.m.wikipedia.org/wiki/Nickel%E2%80%93iron_battery




« Last Edit: September 16, 2024, 09:08:37 AM by Hunter2 »

Offline Thermofox

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #4 on: September 16, 2024, 09:17:19 AM »
There is also a mistake in the equation with nickel.
NiO and Ni(OH)2 has same oxidation number for Ni +2.
The stoichiometry is also wrong

NiO + H2O + e- → Ni(OH)2 + OH-

The right half reaction is

NiO(OH) +  H2O +  e−  → Ni(OH)2   +  OH−

https://en.m.wikipedia.org/wiki/Nickel%E2%80%93iron_battery

In the problem that I was given there was written that first half reaction, so I mindlessly pasted it here. But now I realize that it even isn't a half reaction. Still the doubt I had persists. Does Δεcell depend on [KOH]? If I add the 2 equations together, OH- cancels out, even with the correct half reaction.

Offline Hunter2

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #5 on: September 16, 2024, 09:22:56 AM »
OH- is only spectator ion.

The reaction simplyfied is
Ni3+ + e- => Ni2+ and
Fe => Fe2+ + 2e-

Combined
2 Ni3+ + Fe => 2 Ni2+ + Fe2+

This has to put in the Nernst equation.


Offline Thermofox

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #6 on: September 16, 2024, 09:35:01 AM »
OH- is only spectator ion.

The reaction simplyfied is
Ni3+ + e- => Ni2+ and
Fe => Fe2+ + 2e-

Combined
2 Ni3+ + Fe => 2 Ni2+ + Fe2+

This has to put in the Nernst equation.
Then Q= ([Ni2+]2 [Fe2+])/([Ni3+]2). And that is how I can use the information that the problem gives me, [KOH]= 5.00M; To determine the concentrations of the reactants and products I can use the half reactions and [KOH]= 5.00M.

Offline Borek

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #7 on: September 16, 2024, 12:21:39 PM »
If the reacting compound is NiO(OH) there is no such thing as Ni3+ in the solution. Or, more precisely, it can exist only in equilibrium with OH- and H2O

NiO(OH) + H2O :lequil: Ni3+ + 3OH-

but then to do any calculations you would need to know K for this reaction. Perhaps Ksp for Ni(OH)3 will do.

Actually the same goes for Fe2+/Fe(OH)2, but in this case finding Ksp is trivial.
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Offline Hunter2

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #8 on: September 16, 2024, 01:17:55 PM »
I think in this battery beside KOH, we don't have much ions in solution, the reaction takes place on the surface of the electrodes, NiO, Ni(OH)2, NiOOH, Fe(OH)2 and Fe itself is not much dissolved in solution. Probably the Hydroxide built some complex with Ni or Fe to get some solubility.

https://www.google.com/search?q=iron%20nickel%20battery&ie=utf-8&oe=utf-8&client=firefox-b-m#vhid=E84Hb9XnnXZdMM&vssid=l

Offline Thermofox

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #9 on: September 17, 2024, 10:11:45 AM »
but then to do any calculations you would need to know K for this reaction. Perhaps Ksp for Ni(OH)3 will do.

I've searched the Ksp for Ni(OH)3 to no avail, but I've found that Ksp for Ni(OH)2=2,8x10-16​. Generally, for all other hydroxides, the one with OH3 has a considerably higher Ksp than the one with OH2. So, I think it safe to assume that Ni(OH)3 won't dissolve in water in a considerable amount.

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Re: Electrochemistry problem: Nickel-Iron Battery
« Reply #10 on: September 17, 2024, 10:33:45 AM »
I think it safe to assume that Ni(OH)3 won't dissolve in water in a considerable amount.

That's not what is important here.

You can calculate standard potential of a more complicated electrode like Ag/AgCl assuming you know potential for Ag/Ag+ and Ksp of AgCl. Same for even less soluble salts like AgBr or AgI.

If you have Ni electrode covered with NiO(OH) you could be able to calculate its potential from Ni/Ni3+ potential and Ksp of Ni(OH)3 (which is in a way equivalent to Ni(OH)3). I am not surprised Ksp for Ni(OH)3 isn't listed, but it is the closest thing I can think of that could be used here).
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