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Design, Fabrication, and Application of Nickel Oxide-Base Nanostructured Electrocatalysts for the Production of Oxygen And Hydrogen from Seawater

Hemmati, Khadijeh | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56709 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Moshfegh, Alireza; Moradlou, Omran
  7. Abstract:
  8. Hydrogen is emerging as a clean and ideal fuel. However, its dependency on freshwater resources will be a threat to a sustainable environment. Considering the abundance of seawater and the scarcity of freshwater resources, direct electrolysis of seawater to produce hydrogen fuel can be a promising and potential alternative to pure water electrolysis technologies. Seawater contains electrochemically active ions with the nature of competition with the oxidation reaction of water, thus affecting the catalytic activity and, subsequently, the selectivity. From the thermodynamic and kinetic aspect in the electrolysis of seawater, the unfavorable reaction related to the formation of chlorine complexes (CER, ClO-, HClO-) competes with the favorable oxygen evolution reaction (OER) in the anode electrode and, subsequently, the performance of hydrogen evolution reaction (HER) is affected in the cathode. Therefore, in addition to activity, stability in the chloride environment and selectivity are among the important features of the electrocatalysts for seawater electrolysis. From this point of view, the design of active and stable bifunctional electrocatalysts in the direction of (OER) and (HER) (not only for the selective oxidation of seawater into oxygen but also suitable for the (HER)) based on available materials, with simple and environmentally friendly fabrication methods, to split of seawater into the hydrogen and oxygen fuels and the complete suppression of competitive chloride oxidation reactions were considered in this research. The electrocatalysts based on nickel oxide nanostructures with iron metal additive were prepared by environmentally friendly scalable methods of hydrothermal and electrodeposition without using any binder such as Nafion. Firstly, using the hydrothermal method, nickel oxide-based array nanostructures with urchin-like and sheet morphologies were made, and then the physicochemical properties of the obtained structures were characterized using the different material analysis techniques, which indicates favored electrocatalytic activity, stunning stability, and high selectivity toward the splitting of seawater into oxygen and hydrogen. The optimized urchin-like structures synthesized based on nanorods with nickel oxide phase showed favorable electrocatalytic activity in both half-reactions (OER) and (HER) in simulated and natural seawater (1.0 M KOH+0.5M NaCl) showed that the overpotential was only 470 and 482 mV for half-reactions (OER) and (HER), respectively, to reach a current density of 500 mA cm-2, and the electrochemical active surface (ECSA) of this sample is obtained 24 times that of nickel foam. Also, the desirability of nickel oxide nanostructures in the direction of two competitive OER/CER half-reactions was studied using DFT density functional theory. In another study, by considering the significant electrocatalytic activity of two-dimensional materials towards HER and OER in seawater splitting, through different times of the electrodeposition process, both sides of the nanosheets are decorated with active nickel-iron (NiFe) nanoparticles and a porous hierarchical sandwich-like nanostructure is constructed (NiFe|NiO|NiFe). Based on the analysis of the results obtained from the investigation and comparison of electrocatalytic activities, it was found that the optimal sandwich-like structures based on nickel with iron metal additive have the best electrocatalytic activity toward both HER and OER in simulated and natural seawater. As it only requires overpotentials of (OER; 370 and 560 mV) and (HER; 482 and 711 mV) to reach the current density of (500 and 1000 mA cm-2), respectively, and it catalyzes full seawater splitting at 1.52 V to achieve 100 mA cm−2 in simulated seawater as well as the ECSA of this sample is obtained 40 times that of nickel foam. Ultimately, as a result of combining the seawater electrolyzer based on the urchin-like and sandwich-like structures with a commercial silicon solar cell, the solar to hydrogen efficiency was obtained for photolysis systems of 9.9% and 10.2%, respectively
  9. Keywords:
  10. Bifunctional Electrocatalyst ; Nickel Oxide Nanoparticles ; Hydrogen Producing ; Catalytic Activity ; Stability ; Selectivity ; Oxygen Evoloution Reaction (OER) ; Seawater Splitting ; Oxygen Production

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