|M.Sc Student||Fidelsky Vicky|
|Subject||Theoretical Characterization for Nickel Oxyhydroxide - One|
of the Best Water Oxidation Catalysts for Energy
|Department||Department of Energy||Supervisor||Professor Maytal Caspary-Toroker|
|Full Thesis text|
One of the most prominent targets for supplying inexpensive and clean methods for energy conversion and storage is seeking for proper materials, which can be utilized in solar water splitting cells. In these photo electrochemical cells, solar energy enables to split water by reducing (oxidizing) water to H2 (O2) molecules at the cathode (anode).
In this work, the emphasis is on a photoanode material, β-NiOOH (beta-Nickel oxyhydroxide), which is known as one of the best catalysts for the oxygen evolution reaction (OER) in alkaline conditions. A proper photoanode should have a small enough band gap, proper valence band maxima position, good electronic conductivity, high solar light absorption, an efficient catalytic activity, stability, environmentally friendly and economically affordable.
For investigation of β-NiOOH properties of interest, Density Functional Theory (DFT) calculations were performed. DFT incorporate many useful theoretical tools for studying the structural and electronic state properties of various photoanode materials, particularly oxide semiconductors, through quantum modeling. This method uses a convenient quantity, i.e. electron density found through iterative calculations.
During this work, many properties of β-NiOOH were studied with DFT. The first property is band edges positions. A promising material for catalysis should have appropriate band edge positions. Unfortunately, the band edge positions are unknown for β-NiOOH. The band edge positions were calculated for surfaces of interest, i.e. (001), (100), and (0-15) by using DFT, PBE0 functionals and G0W0 method. It is obtained that the thermodynamically stable (100) surface should be the most active in enabling the oxygen evolution reaction. This conclusion has a broader implication that chemical activity of polar materials can be controlled through facet selection .
In addition to band edges, the catalytic activity of β-NiOOH in the presence of vacancies was studied with DFT functional. Previous experiments show that during operating conditions, hydrogen content of NiOOH changes. Our calculations on this topic reveal that the defects destabilize the surface by changing local oxidation states and therefore reduce the overpotential and improve the catalytic efficiency.
Catalytic activity of β-NiOOH can be improved not only by the presence of vacancies but also by the addition of dopants. Fe is known as a good dopant for nickel oxyhydroxide for improving efficiency, but the explanation to this phenomena was unknown. Water oxidation reaction intermediates of Fe substitutional doped β-NiOOH were studied through DFT calculations. We find that the smaller electronegativity of the Fe dopant relative to Ni allows the dopant to have several possible oxidation states with less energy loss.
Finally, since the β-NiOOH phase was suggested to have hydrogen vacancies, electronic properties of the bulk with vacancies were investigated. DFT and PBE0 calculations were used to analyze the electronic structure of crystalline β-NiOOH. We find that hydrogen vacancies lower the band gap but do not create energetic states far inside the band gap that could capture charge carriers. Therefore, our material with hydrogen vacancies should retain exceptional charge carrier mobility that is crucial for electronic devices.