This is a pure speculation. You can assume whatever you want. The problem is to confirm your assumption experimentally.

Thank you Yurii V Geletii for your time.

We can check the effect of pH over the activity of catalyst, but it will not unveil the actual proton acceptor. Please make me clear about the following questions.

1. Experimentally, how can we confirm that Oxygen of NiOOH is actually behaving as Proton acceptor?

2. Since hydroxyl group in the electrolyte and Oxygen of NiOOH both can act as proton acceptor how can we make sure which one is contributing more in OER activity?

To establish the mechanism of a heterogeneous reaction is an extremely complex task. The mechanism you are talking about is hypothetical. Nobody knows how to study elementary reactions. The intermediates are not identified. I don't know the answer to your question.

Yurii V Geletii Thank you for your humble response.

Hi Muhammd,

I think the above answers hits a very important point. Getting mechanistic insights purely from experiment is very difficult and for solid systems often almost impossible. The reasons for this are manifold but range from the problem of measuring the nature of short lived intermediates to the lack of clean systems (dirt and impurities are almost unavoidable) and to most experiments not being done at well characterized, single crystalline surfaces. But this is precisely why epxeriments aiming at providing fundamental understanding only have limited use when they are not combined with computational modelling.

Coming back to your original question, I think quantum chemical/DFT modelling can come to your aid. Here, you define a model system such as a single crystaline NiOOH surface and explore different mechanistic scenarios such as proton/hydrogen transfer to an adjacent adsorbate versus a proton transfer to a solvent molecule (and many more reaction paths). Based on the results from the DFT calculations one can then select the most likely reaction route. So any result obtained from thus modelling is certainly far more than pure speculation.

But let me make things very clear in the beginning: Even mechanistic predictions from modelling are just models for a real situation. So if you select the wrong model for your reaction you will obtain results which diverge from experiments! (But these results are not wrong! They simply correspond to a situation not relevant for the experiment)

Now let me come to your question regarding the proton/hydrogen transfer at NiOOH. In fact we have looked at a similar system a few years ago. Experimentally we observed that pure NiOOH is a very poor catalyst (if you make sure to remove any Fe impurities from your electrolyte). But if Fe remains in the solution, it forms small FeOOH nanoparticles at the NiOOH terrace sites. These systems display a much higher activity than what can be expected for a mechanism proceeding through the classical *OOH intermediate. When looking at this system more closely through DFT modelling we could clearly show, that the only possiblity to explain the system's acitivty is through an instantaneous hydrogen transfer from *OOH to a hydrogen acceptor unit at NiOOH:

So the true reaction that happens for this system is:

*O + OH- + (Ni3)-O -> *O2 + (Ni3)-OH + e-

For details see DOI: 10.1021/acscentsci.9b00053

Having said this, NiOOH is clearly able to act as a hydrogen acceptor. So it is very likely that such a mechanism can also happen for pure NiOOH. In fact this mechanism has also been observed (thorugh modelling) for many other systems like Au nanoparticles on MnOx or CoOx (DOI: 10.1002/cctc.201402756, DOI: 10.1016/j.coelec.2018.06.007), doped RuO2 (DOI: 10.1039/C4CP00571F) and transition metal doped graphene (DOI: 10.1016/j.nanoen.2016.04.011 ). A similar mechanism is of course also possible for the formation of the *OH intermediate. However, this will not result in any gains in activity. Such a mechanism also has the benefit of not requiring a signficant rearrangement of the double layer and is likely also helped by rather strong hydrogen bonds to adjacent *O/*OH groups. But this is something that would need to be shown by more advanced DFT modelling.

So in a nutshell, from a modelling perspective, the intramolecular proton/hydrogen transfer is the most likely alternative.

In case you are thinking about a specific system, you might want to contact your local computational chemists. I am sure they will be able to help you.

Michael Busch Thank you for the detailed response. i will look into the literature that you have shared.