Optimisation of PtxNiyAlz ratios as thin film electrocatalysts for the oxygen evolution reaction (OER) in alkaline medium
The world is currently reliant on energy sources such as oil, coal and gas (all fossil fuels) for the supply of the global growing energy demand. However, these fossil fuels are diminishing and have drawbacks such as the constant emission of carbon dioxide (CO₂) to the atmosphere. Research seeking alternative energy technologies, based on renewable energy resources, has identified hydrogen gas (H₂) as a clean energy carrier. Alkaline water electrolysis (AWE) is one of the most preferred and advantageous techniques for clean hydrogen gas (H₂) production. With the efficiency of this technique linked to the activity of the oxygen evolution reaction (OER) at the anode, the electrochemical production of the hydrogen needs to be optimised due to the sluggish kinetics and large overpotentials of the OER. This has resulted in the search for OER electrocatalysts that allow for high electrocatalytic activity and stability under harsh conditions. IrO₂ and RuO₂ have been identified as the most active OER catalysts in acidic medium for water electrolysers as they produce high current densities at low overpotentials, however these noble metals are costly. Research seeking to identify an efficient and cost-effective AWE electrocatalyst that can operate at low overpotential with higher current densities and have low noble metal content has shown that Ni and Ni-Al (also known as Rayney Nickel) combinations exhibit excellent activity and stability in alkaline environment for the OER. In this study, combinations of PtxNiyAlz as thin film electrocatalysts were produced on Si/SiO₂-wafers and glassy carbon supports with magnetron sputtering (PVD) and screened using a high-throughput equipment for analysis. The 10 most promising PtxNiyAlz were further subjected to electrochemical testing in a three-electrode cell connected to a rotating disc electrode for electrocatalytic activity and short-term stability. Linear sweep voltammetry and chronopotentiometry are the two electrochemical techniques that were employed to determine the electrocatalytic activity and stability. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy was employed for physical and stoichiometric characterisation of the thin film electrocatalysts. It was found that Pt and Al alone compared to the ternary combinations PtxNiyAlz are poor electrocatalysts towards the OER, they suffered from stability issues due to the dissolution in the alkaline electrolyte. However, various PtxNiyAlz exhibited an increase in activity subsequent to the stability testing. The ratios with other sputtered electrocatalysts both before and after durability testing. It was concluded that aa combination of Pt, Ni and Al does increase the electroactivityof the catalysts towards the OER. The electrolytic activity of these electrocatalysts was attributed to the higher percentage of Ni content in the electrocatalyst and a lower percentage of Al (which has a leaching property that results in more electroactive surface area).