Study of the Mechanism and Charge-Transfer Processes in Water Oxidation Catalysis by Nickel Hydroxide and Silk Fibroin: A Spectroelectrochemical Approach
Authors: do Nascimento Eduardo Ruben , de Souza Michele Lemos , Brolo Alexandre , Alves Wendel Andrade 
Published: December 2024

Water electrolysis has emerged as a crucial electrochemical reaction on the generation of sustainable fuels, due to the ongoing climate crisis. To divert from expensive catalysts based on rare noble metals such as Ir and Ru, transition metal oxides seem to be promising candidates and several recent studies have deepened the investigation on the nickel oxide/hydroxide for water oxidation catalysis. It has been proposed that in the presence of elements such as iron and gold, a partial charge-transfer occurs between the nickel and the metal, facilitating the formation of superoxide species and nickel in higher oxidation states that enhance the oxygen evolution reaction (OER). 1–4

Rao et al (2022)5 developed a methodology combining UV-Vis spectroscopy and voltametric techniques to determine the number of these nickel species in higher oxidation levels during the OER for nickel oxides dopped with Zn, Fe, Co and Mn. From their results, they proposed a mechanism for the OER that involves two steps: 1) initial oxidation from NiII to NiIII and superoxide formation, which is limited by the potential and 2) coupling and desorption of the superoxide species, which is limited by rate. Next, they demonstrated that the different dopants modulated the bond strength between the nickel and the oxygen: too weak-bonding limited the ability to oxidize nickel and too strong-bonding limited the combination and desorption of the oxygen.
In this work, monolayers of nickel hydroxide were electrodeposited, in the presence and the absence of silk fibroin (SF), on top of electrodes containing gold nanoparticles and used surface-enhanced Raman scattering (SERS)and UV-Vis spectroeletrochemical techniques to study the influence of the gold and SF on the catalytic performance of nickel hydroxide. Our previous results6 showed that the silk protein was able to stabilize the α-Ni(OH)2 nanoparticles and improved its electrochemical activity. Here, we followed the Rao et al’s methodology and found analogous results for the mechanisms of water oxidation. Figure 1A and B shows the number oxidized species and the calculated turnover frequency (TOF) as function of the potential for the samples with 1, 3, 6 and 9 monolayers, containing or not SF. While the number of oxidized species remains comparable across samples with the same number of monolayers, those containing fibroin generally exhibit higher TOFs. A volcano-like graph (Figure 1C) was obtained when the TOFs at an overpotential of 0.3 V is plotted against the NiII-NiIII redox potential, indicating a transition from potential-limited to rate-limited regions as monolayer thickness increases. We propose that this behavior is caused by the charge-transfer effect between nickel and gold, which is strongest for one monolayer and decreases as the number of nickel sites on the bulk moves away the gold surface. Notably, as monolayer thickness increases, samples containing SF show less susceptibility to decrease in TOFs, implying an additional charge-transfer effect between the protein and nickel, which contributes for the balance of the nickel bond strength with oxygen and facilitates its coupling and desorption.
In summary, our study advances understanding of OER mechanisms and charge-transfer processes involving nickel hydroxide electrocatalysis, suggesting that proteins like SF can actively contribute to catalytic processes. However, further investigations are needed in order to elucidate the precise role of the protein and how to use them to optimize catalytic performance.

REFERENCES

1. Diaz-Morales, O., Ferrus-Suspedra, D. & M. Koper, M. T. Chem. Sci.
7, 2639–2645 (2016).
2. Yeo, B. S. & Bell, A. T. J. Phys. Chem. C
116, 8394–8400 (2012).
3. Li, C.-F. et al.
Angew. Chem. Int. Ed.
61, e202116934 (2022).
4. Trotochaud, L., Young, S. L., Ranney, J. K. & Boettcher, S. W. J. Am. Chem. Soc.
136, 6744–6753 (2014).
5. Rao, R. R. et al.
J. Am. Chem. Soc.
144, 7622–7633 (2022). 6. do Nascimento, E. R. et al.
ACS Appl. Electron. Mater.
4, 1214–1224 (2022).

Figure 1