The metal hydride materials are conductive. Cobalt is used for the purpose of conductivity improvement and both cobalt and zinc have the function of oxygen overvoltage adjustment and microstructure refinement.

when an NiMH battery is overcharged, hydrogen gas is formed.. Cobalt absorbs hydrogen whereas carbon black can't.. there are some papers on cobalt free NiMH batteries.

Jindui Hong Thank you for inviting me to see your question, although I am not an expert in this field. Nevertheless, I think both answers above from Mohammad Mohsen Loghavi and Sridhar Vadahanambi are reasonable to certain degrees. The NiMH electrode is sufficiently conductive and hence no carbon addive is needed. Instead, my understanding is that Co or Zn may replace Ni in the hydrogen storage phase and hence improve the lattice stability upon repeated hydrogenation and dehydrogenation cycles. Also, we actually mixed Ni powder with hydrogen storage alloys to make the negative electrode, partly because it does not compromise the conductivity, but also because Ni itself is active towards hydrogen.

Dear Hong,

I wish to add few comments. I also support the above given comments by respected researcher. In addition I have little experience in NiMH battery but based on my electrochemistry experience, I can say that despite the good conductivity of the Carbon black (CB), it has low wettability in aqueous media. But those are wettable much have high specific surface area for large amount of hydrogen storage and this under-deposited hydrogen is adsorbed on the surface of CB. However, in case of CoOOH, despite having relatively low overvoltage for hydrogen evolution compared to CB (electrochemically active), the formation of metastable metal hydride (intermediate step) is kinetically more favorable and reversible during charge/discharge process.