Scalable Synthesis of Earth-Abundant Catalysts for (Photo-)electrochemical Water Splitting Devices
Creators: name: Soo, Joshua Zheyan
Publisher: name: The Australian National University
Year: 2024

description: As the world gradually decouples from using fossil fuels, the proportion of solar and wind renewables in the energy mix is growing. However, their intermittent availability warrants the need for storable alternatives to ensure stable energy harvesting and delivery to the end-user. One such method is to produce green H2 via water splitting. The electrolytic water splitting process can be carried using solar energy to ensure a carbon-free energy harvesting process. However, current water splitting devices are generally too expensive and inefficient for commercialisation, the former largely due to the use of noble metal catalysts such as Pt and IrOx. Earth-abundant catalysts such as NiFe(Co) hydroxides and NiMo alloys, whilst cost-effective and having comparable water spitting catalytic performance; require scalable synthesis methods to be viable in large-scale green H2 production. This thesis aims to study scalable approaches to synthesise earth-abundant catalysts for water-splitting devices. While the scope of this thesis revolves mainly on electrolytic water splitting, a brief focus towards integrating these catalysts with III-V semiconductors for efficient photoelectrochemical (PEC) water splitting devices is included. First, a binder-free method of electrode fabrication is investigated in Chapter 4, whereby the catalyst material is synthesised by solution corrosion of metal-plated substrates. NiFe layered double hydroxide (LDH) synthesised in this approach exhibited good oxygen evolution reaction (OER) performance, with consistently low overpotentials and good stability on a range of substrates. The electrocatalyst is also regenerative, with its OER performance sustained after multiple etch and re-deposition cycles. When used as an electrolyser anode, a cell voltage of 1.82 V at 500 mA/cm2 is achieved with close to 150 hours of operational stability. To further improve the OER performance of NiFe LDH, a two-step approach built upon the initial solution corrosion process is studied in Chapter 5, whereby Co ternary metal is introduced. The obtained NiFeCo hydroxide delivered an OER overpotential of 195 mV at 10 mA/cm2, outperforming NiFe LDH (264 mV). Adding ternary metal drastically increases the turnover frequency by more than 14 times, while reducing the catalyst charge-transfer resistance. This showed the versatility of this fabrication approach, while simultaneously demonstrating excellent performance and stability. To enable this synthesis technique for hydrogen evolution reaction (HER), a method to modify metal hydroxides with plasma is investigated in Chapter 6. NiMo hydroxide synthesised in a similar solution corrosion technique is reduced and nitrided when modified under NH3 plasma. The modified NiMo hydroxide gives a HER overpotential of 95 mV at 10 mA/cm2, improved over unmodified NiMo hydroxide (213 mV). Enhanced HER kinetics are found upon this modification, due to similar improvements in double-layer capacitance and charge transfer resistance. With good material stability of close to 120 hours, this modification opens the use of this solution corrosion technique for both HER and OER catalysts. Lastly, the integration of earth-abundant catalysts with III-V semiconductor for PEC water splitting is investigated in Chapter 7. A decoupled photoelectrode design is developed, whereby the rear contacts of a single-junction GaAs photovoltaic cell is layered with electroplated Ni, followed by an in-situ deposition of solution-corroded NiFe LDH at the rear. This approach achieved an excellent half-cell efficiency of 11.7% at 0.77 V vs RHE, with 100-hour device stability at saturated photocurrent density of 26 mA/cm2. This combined photoelectrode fabrication and catalyst synthesis opens the door for III-V semiconductors to be used as high-performing solar-to-H2 water splitting devices.