Preparation of NiO/WO3 Photocathode via Ammonium Paratungstate for PEC Application

Fossil fuels as the current major energy source has promoted many environmental issues due to the large-scale emission of CO2 and other pollutants. Issues brought by fossil fuels include global warming, climate change, and sea level rise. Solar energy as a clean energy source has caught great interest globally, and is belived to be a promising solution to climate change and fossil fuel depletion. Photoelectrochemical water splitting (PEC) is an effective solar conversion technology to store the energy in hydrogen gas (H2).

The best PEC material is TiO2 based material. However, it exhibits the drawbacks of large optical band-gaps. Thus, NiO was doped to WO3 as NiO/WO3 heterojunction photocathode for the photoelectrochemical water splitting reaction. NiO/WO3 photocathode had been synthesized using ammonium paratungstate as tungsten source by hydrothermal method and sol-gel:

Nickel chloride hexahydrate (NiCl2·6H2O), and ammonium hydroxide (NH4OH) were used as the raw materials. The NiO nanostructures were fabricated on the average-size Indium-tin oxide (ITO) glass via a hydrothermal method. Firstly, 0.474 g of NiCl2·6H2O and 6 mL of NH4OH were dissolved in 19 mL of deionized water and then kept stirring for 20 min. Then, the solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave reaction and kept at 200 °C for 24 h. Finally, the obtained samples were washed by distilled water, and be placed in the drying oven of 60 °C to dry. Following, the as-prepared films were annealed with a ramp rate of 2 °C/min at 400 °C in a muffle furnace for 2 h, and allowed to cool to room temperature naturally.

Then, 5.31 g of ammonium paratungstate (APT) and 10 g of polyethylene glycol (H(OCH2CH2)nOH) were dissolved in 10 mL of deionized water and then the transparent solution was obtained after adding 25 mL ethylene glycol (C2H6O2), 15 mL isopropanol (C3H8O) under vigorous stirring at the room temperature. After that, 5 mL ethanol (C6H2OH) and 5 mL ammonium hydroxide (NH4OH) were added slowly, then the solution was stirred for 2 h. Subsequently, the WO3 films were printed on the NiO on ITO substrates by a dip-coating method and then were heated at 60 °C for 5 min. Repeat this cycle until the thickness of the film reaches the required thickness to stop the cycle. Then, the as-prepared films were annealed with a ramp rate of 2 °C/min at 500 °C in a muffle furnace for 2 h, and allowed to cool to room temperature.

In conclusion, the PEC property of NiO/WO3 shows a much higher current density (−23.39 μA/cm−2) at 1.20 V which is 3.94 times that of the unmodified NiO photoelectrode (−5.93 μA/cm−2). High separation efficiency and high transmission efficiency of the photogenerated electron-hole pairs and enhanced light absorption lead this PEC property of NiO/WO3 heterojunction photocathode.

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