What is the cathode in NiMH batteries

Question

What is the cathode in $\ce{NiMH}$ batteries?

I’m not sure if it is $\ce{Ni(OH)2}$ or $\ce{NiOOH}$. I’m confused because I thought that these batteries are like $\ce{NiCd}$ batteries except better (more energy density, less toxic, etc.) and use a metal hydride instead of $\ce{Cd}$ as the anode (this is all info from my textbook). So I thought the cathode in both kinds of batteries was $\ce{NiOOH}$ because that was in my textbook (it said the cathode in a $\ce{NiCd}$ battery is $\ce{NiOOH}$ and mentioned nothing about the cathode for a $\ce{NiMH}$ battery).

Here are sources I've found:

$\ce{Ni(OH)2}$ as cathode:

• “The components of $\ce{NiMH}$ batteries include an anode of hydrogen absorbing alloys $\ce{(MH)}$, a cathode of nickel hydroxide $\ce{(Ni(OH)2)}$ and a potassium hydroxide $\ce{(KOH)}$ electrolyte.” https://www.sciencedirect.com/topics/materials-science/nickel-metal-hydride-battery

• "Cathode: Nickel oxyhydroxide $\ce{Ni(OH)2}$.” https://www.powerstream.com/BatteryFAQ-nickel-cadmium.html

$\ce{NiOOH}$ as the cathode:

• “The chemical reaction at the positive electrode is similar to that of the nickel–cadmium cell $\ce{(NiCd)}$, with both using nickel oxide hydroxide $\ce{(NiOOH)}$.” https://en.m.wikipedia.org/wiki/Nickel–metal_hydride_battery

Answer 1

The cathode is made of $\ce{NiOOH}$ when the cell is new. This can also be written $\ce{NiO(OH)}$,where the oxidation degree of $\ce{Ni(III)}$ is more evident. The cathode is made of $\ce{Ni(OH)2}$ when the cell is discharged. Whatever the nature of the anode (cadmium, hydrogen, or any other material), the cathode is working according to the equation $$\ce{NiOOH + H2O + e^- -> Ni(OH)2 + OH^-}$$ Usually the batery is built up with $\ce{Ni(OH)2}$ and it must first be charged by electrolysis according to the reverse reaction, which is often mentioned in the publications.

Answer 2

According to Nickel Metal Hydride (NiMH): Handbook and Application Manual of Energizer Nical Metal Hydride:

The nickel-metal hydride battery chemistry is a hybrid of the proven positive electrode chemistry of the sealed nickel-cadmium battery with the energy storage features of metal alloys developed for advanced hydrogen energy storage concepts. This heritage in a positive-limited battery design results in batteries providing enhanced capacities while retaining the well-characterized electrical and physical design features of the sealed nickel-cadmium battery design.

The only difference between two kinds of batteries are nickel-metal hydride battery does not use cadmium electrode. Instead, it uses am alloy-metal hydride. The eelectrochemistry as follows:

At charge state at the negative electrode: $$\ce{Alloy + H2O + e- <=> Alloy(H) + OH-}$$ The water in the electrolyte is decomposed into hydroxyl ion and hydrogen atom, which is absorbed into the alloy.

At positive electrode: $$\ce{Ni(OH)2 + OH- <=> NiOOH + H2O + e-}$$ As evident, the charge reaction is based on the oxidation of nickel hydroxide just as it is in the nickel-cadmium couple.

At discharge state at the negative electrode: $$\ce{Alloy(H) + OH- <=> Alloy + H2O + e-}$$ The hydrogen atom is desorbed and combines with a hydroxyl ion to form water while an electron is released, which is contributing an electron to the circuit.

At positive electrode: $$\ce{NiOOH + H2O + e- <=> Ni(OH)2 + OH- }$$ As evident, nickel oxyhydroxide is reduced to its lower valence state, nickel hydroxide.

For complete review of these batteries, read here and here.