Speaker
Description
In recent years NiO has come into focus as a low-cost, efficient material in the Oxygen Evolution Reaction (OER) via electrochemical water splitting. However, the detailed mechanism of the electrochemically induced reaction is not yet fully understood.[1] Hence, three types of experiments were performed. Firstly, as a model catalyst, NiO$_{x}$ thin films were put in contact with oxygen and subsequently water vapor at 0.5 mbar and elevated temperatures and characterized by in situ NAP-XPS, NAP-XAS and ex situ SEM. Heating and cooling in oxygen reveal the creation and replenishing of oxygen vacancies. Such vacancies manifest themselves mainly as distinct features on the low and high binding energy sides of the NiO main peak in the O1s region. The former can be directly related to vacancies while the latter represents OH chemisorbed at the vacancy site. Subsequent heating and cooling in water vapor suggests that these vacancies not only instigate H$_{2}$O dissociation but, by way of OH chemisorption, act as precursors to the formation of stable OH-bonds and may play an important role in OER. Secondly, crystalline NiO(100) thin film model catalysts were studied in a quasi in-situ electrochemical set up. The thin film catalysts were characterized using LEED, XPS, LEIS and AFM before and after the electrochemical experiments. For these samples we observe that the hydroxide layer found at the surface after electrochemical surface activation depends on the state of the surface before OER: when the NiO(100) surface is treated with an O$_{2}$ plasma before OER, less OH-bonds are formed during catalyst activation, and there is a slight degradation on the OER performance. This suggests that embedding oxygen into the NiO surface is detrimental for the activity of the catalyst. Lastly, operando electrochemical experiments were performed on electrodeposited NiOx particles. Cyclic voltammetry from open circuit potential up to 0.8 V reveals an oxidation peak at 0.38 V (vs Ag/AgCl) as well as a reduction peak at 0.27 V (vs Ag/AgCl). Above the oxidation peak the Ni L-edge spectrum NiOx transforms into a spectrum dominated by ϒ-NiOOH features and thus OER is observed. Simultaneously, the O K-edge reveals the formation of a pre-peak at 528.9 eV which is caused by adding holes in the band gap which leads to a transformation of the electronic structure of NiO$_{x}$ from insulator to charge transfer gap. Both, experimental and theoretical results suggest that in the reactions oxygen vacancies are formed and seemingly play a crucial role acting as intermediate states in OER.
References:
1 L.J. Falling, J.J. Velasco-Vélez, R.V. Mom, A. Knop-Gericke, R. Schlögl,
D. Teschner, T.E. Jones, Curr. Opin. Electrochem., 2021, 30, 100842
| Abstract Number (department-wise) | AC 2.2 |
|---|---|
| Department | AC (Schlögl) |
