Preparation And Application Of The Layered Composite Semiconductor Cathode Materials For Water Splitting

Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. Both two should be achieved by hydrogen evolution cathode catalyzing. However, there are some problems existing in the electrodes for the water splitting. For electric catalysis, high overpotential, complicated preparation process and high cost are the main issues limiting the catalytic activity of these cathodes in hydrogen production, while lack of effective light absorbing materials, electrons and holes easily complex, and low conversion efficiency are key issues in PEC water splitting. Thus, the design and construction of highly efficient and stable cathode materials for water splitting to achieve industrial application is significant. Based on this goal, the paper targets building efficient and stable cathodes with tunable semiconductor nanomaterials to design and prepare layered composite electrodes by optimizating the production process and loading cocatalyst. The main contents are as follows:1. The preparation and applications of Ni-S/ITO electrode for water splittingIn order to improve the complicated preparation of metal sulfide, Ni-S electrode was prepared by one-step photo-assisted potentiodynamic deposition, and it was demonstrated to be active HER catalysts in aqueous media. The polarization curves of samples with different loadings were compared, where the one of the 6 cycles exhibited the best activity. A long-term bulk electrolysis of the Ni-S film exhibited steady current over 22 h without deactivation, demonstrating its superior stability in neutral water. In addition, the Ni-S/ITO electrode showed high hydrogen production activity for HER with 1 mA?cm-2 current density and nearly 100% faraday efficiency. The Ni-S/ITO electrode has such high activity for HER is because that the interface resistance between the electrode and the electrolyte is greatly reduced, and promote the H protons and electrons combinning to generate H2. The above excellent electrode was characterized by SEM, TEM, XRD, XPS and other analytical approaches, which proved that the active ingredient of the electrode was NiS/Ni（OH）2. The preparation method was carried at normal temperature and pressure with simple operation and strong controllability, which made it possible for the mass production.2. The preparation and applications of MoS₂/CdSe/NiO photocathode for water splittingCdSe layer with wide range of light absorption and good stability was made in situ grow on NiO surface to form CdSe/NiO photocathode by SILAR, and it indicated that 6 layers CdSe deposition is the best performance one by optimizing the deposition amount. The composite films were characterized by SEM, UV-vis and chronoamperometry. The deposition of CdSe on the NiO film enhanced light harvesting in the visible-light region and photoelectrochemical properties. Moreover, the CdSe/NiO electrode showed superior stability both in nitrogen-saturated and air-saturated neutral environments for 7 h. The factors affecting CdSe/NiO electrode for HER was explored. The results showed that the electrode we prepared was an excellent photocathode, and the light source, the applied voltage and the co-catalyst were necessary conditions for PEC water splitting. After MoS₂ modification, the CdSe/NiO photoelectrode exhibited higher catalytic activity with 0.52 μmol h-1 cm-2（only 0.07 μmol h-1 cm-2 for the CdSe/NiO electrode） and almost 100% faraday efficiency.3. The preparation and applications of photocathode based on g-C₃N₄ for water splittingAs a newly non-metallic photocatalytic material with great value, g-C₃N₄ has not been studied in photocathode applications as the photosensitizer. In this paper, a novel photocathode based on g-C₃N₄ was prepared by a simple thermal polymerization method, and the structure and morphology were characterized by SEM, TEM, XPS analytical methods. Under illumination, the prepared g-C₃N₄/NiO photoelectrode achieved the highest photocurrent density at a bias potential of 0 vs RHE, which was approximately ten times as that of the NiO photoelectrode and twenty times as that of the g-C₃N₄ photoelectrode. And the g-C₃N₄/NiO photoelectrode showed superior stability both in nitrogen-saturated and air-saturated neutral environments. Moreover, the g-C₃N₄/NiO photocathode has excellent activity for hydrogen production with nearly 100% faraday efficiency without external co-catalyst and buffer solution. Furthermore, the possible mechanism for enhancement of the PEC properties was investigated by photoluminescence（PL） analysis and electrochemical impedance spectroscopy（EIS）.