Preparation And Mechanism Of Water-splitting Systems Containing Nickel Compounds For Hydrogen Generation

In order to implement the strategy of sustainable development for our planet, renewable and clean energy has been developed and used. Hydrogen is known as a great potential and clean energy, because of its advantages with carbon-free emissions and non-pollution. Among the various hydrogen production technologies, water spiltting is the most environmental and friendly method. However, the best catalyst performance of hydrogen production is Pt, which is rare and expensive. Considering the cost and catalytic properties, efficient and cheap of nickel base compound catalysts have a good application prospect. The main research of the article are as follows:1. Preparation and hydrogen production of NiS/CdTe/NiO electrode for water splittingOne of the methods which made full use of sun light to split water is photoelectrochemistry（PEC）. There are few reports about photocathode in PEC. Compared with Si-based and Cu2O-based photocathodes, Ni O-based photocathode have better stability. The current report of NiO-based photocathode mainly used dye molecules as photosensitizer which has low utilization efficiency in sun light. In order to improve the utilization efficiency, especially in the visible light range, quantum dot was used as photosensitizer to prepare photocathode. Characterizated by morphology, it was found that MPA-CdTe was covered on the surface of NiO. The photoelectrochemical performance and stability of CdTe/NiO was investigated, and it was found that CdTe/NiO showed response in the visible light range and good stability. Based on this, NiS/CdTe/NiO photocathode was fabricated by adding Ni₂+ in the electrolyte. By investigated the factors affected on hydrogen production, we found that overpotential, visible light, MPA-CdTe and Ni₂+ are essential for this system. At the overpotential of 92 mV, the production amount of hydrogen was 10.38 μmol with nearly 100% faraday efficiency. By analyzed the working mechanism of NiS/CdTe/NiO, we found that in situ generated NiS can effectively promote the photo-generated charge separation.2. Preparation and hydrogen production of Ni-P/NF electrode for water splittingCompared with sulfide, phosphide has the wider range of application and better stability. However, the catalyst was connected electrode with the aid of onductive adhesive or coated, which was badly limited the performance of catalyst. Considering this, the self-supporting nickel phosphide electrode was prepared by a facile potentiodynamic electrodeposition which increased the stability of structure and promoted the rapid transfer of charge. The electrochemical performance of Ni-P/NF electrode was evaluated in different environments, and the results showed that the Ni-P/NF electrode required an overpotential of only-54 mV to reach current density of 10 mA·cm-2 in neutral environment, and an overpotential of only-235 mV to reach current density of 50 mA·cm-2 in alkaline environment. By examining the longstanding stability of Ni-P/NF electrode, the superior performance was showed in different environments. Investigated the work mechanism of Ni-P/NF electrode through the characterization of electrochemical impedance, we found that Ni-P/NF electrode reduced the resistance between the electrode and electrolyte interface, which enhanced conductivity between electrode surface and the electrolytic solution. Characterized by polarization curves, there is synergistic effect between Ni3（PO4）2 in nickel phosphide electrode and phosphate ions in solution, making the proton in solution transfer rapidly to the active site of electrode surface.3. Preparation and hydrogen production of g-C₃N₄/Ni₂ P for water splittingAs a metal-free semiconductor, g-C₃N₄ is used as a photocatalyst in hydrogen production for water splitting. Nevertheless, the efficiency of g-C₃N₄ is still limited because of its high recombination rate of photogenerated charge carriers. Due to superior catalytic performance and stability of nickel phosphide, we combined Ni₂ P with g-C₃N₄ nanosheet for photocatalytic water splitting. The g-C₃N₄ nanosheet was prepared by a simple thermal polymerization method and nanoparticle Ni₂ P was synthesized by solid calcination. The gC₃N₄/Ni₂ P photocatalyst was fabricated by calcined under Ar atmosphere. By characterized the morphology and structure of g-C₃N₄/Ni₂ P, we demonstrated that the photocatalyst gC₃N₄/Ni₂ P was successfully prepared. The stability of g-C₃N₄/Ni₂ P was respectively investigated under different pH environments and longtime photocatalytic reaction, and the result showed that g-C₃N₄/Ni₂ P maintained good stability, which has a certain practical value. The production amount of hydrogen was 27.75 mmol·g-1 after 48 h photocatalytic reaction and the activity of g-C₃N₄/Ni₂ P became stable. Furthermore, the possible mechanism for enhancement of the photocatalytic properties was investigated by photoluminescence（PL） analysis.