Jiang, Chaoran; (2019) Novel electrocatalysts for the electrochemical and photoelectrochemical water oxidation reaction. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Chemical energy storage by water splitting, a combination of oxygen evolution (OER) and hydrogen evolution reactions (HER) has emerged as a promising solution for the utilisation of solar energy. Both efficient electrocatalysts and photoelectrodes with long-term stability are indispensable to achieve economic feasibility of solar water splitting. Although the product of interest is hydrogen gas, the thermodynamic and kinetic requirements of the oxygen evolution reaction (OER) are the main limiting factors. The goal of this dissertation was to discover effective strategies for improving the performance of electrocatalytic and photoelectrochemical (PEC) water splitting. To address the issues for the slow PEC water oxidation kinetics, two typical surface electrocatalysts Co-Pi and Ni-B were firstly synthesised and tested on a reference ZnO photoelectrode. The Ni-B/ZnO exhibited a benchmark photocurrent density (1.22 mA cm−2 at 1.0 V vs RHE), resulting into two folds enhancement compared with the unmodified ZnO. The stable photocurrent over a 1 h test period further demonstrated the dual functionality of Ni-B as an efficient OER catalyst and robust surface-protection layer inhibiting photocorrosion, which is much better than Co-Pi. Following the successful strategy used to stabilise of ZnO photoanode, the Ni-B electrocatalyst was introduced onto a very promising p/n junction GaAs by in-situ photoassisted electrodeposition to solve the critical stability issue of GaAs. A monolithic layer of Ni-B/Ga (As)Ox was generated during the Ni-B deposition process, resulting in a Ni-B/Ga (As)Ox/GaAs photoanode structure. Such structure was optimised by varying the GaAs surface architecture, electrolyte pH value and Ni-B deposition time to achieve the optimal photocurrent and best stability. The optimised photoanode, Ni-B/Ga (As)Ox/shallow GaAs exhibited a nearly 22 hour stable photocurrent density of 20 mA/cm2, while the bare GaAs lost 40% activity in just three hours under identical condition. The remarkable performance in both photocurrent and stability directly addresses the current severe limitation in the application the classic GaAs photoanodes for solar fuel synthesis and could apply to other initially efficient but unstable photoelectrodes. Although the above studies demonstrated the multi-function of an oxygen evolution reaction (OER) catalyst on the surface of a semiconductor photoanode, e.g. increasing the photocurrent and lifetime, the OER catalysts are still not efficient enough. Therefore, the development of a cost-efficient and long-term stable catalyst for OER is still crucial to produce clean and sustainable H2 fuels from water. A cobalt vanadium oxide (CoVOx-300) catalyst with high contents of Co3+ sites that was manipulated by V4+, was firstly reported as an efficient and durable electrocatalyst. The CoVOx-300 with highest Co3+/Co2+ ratio of 1.4 and corresponding highest V4+/ V5+ ratio of 1.7 exhibited remarkable OER activity with an overpotential of 330 mV at a current density of 10 mA cm-2 (10), a small Tafel slope of only 46 mV dec-1 and a current density of 100 mA cm-2 at an overpotential of 380 mV vs RHE, which was 20 time higher than the active CoOx-300 and 1000 time higher than VOx-300. The catalyst also showed excellent stability for 10 hours in alkaline media and a 40 % reduced activation energy to the counterpart, CoOx-300. This study demonstrated that high contents of surface Co3+ and V4+ species played a crucial role in improving electrocatalytic properties and stability for the water oxidation reaction. To further improve the OER performance of cobalt-vanadium based electrocatalyst, a series of Cobalt-vanadium (Co-V) bimetallic catalysts were synthesised and tested for the OER. It was found that spinel Co2-xVO4 with Co3+/Co2+ ratio of 2.8 and a moderate crystallinity exhibited the lowest overpotential of 240 mV at 10 mA/cm2, a smallest Tafel slope of 45 mV dec-1 and a current density of 100 mA/cm2 at an overpotential of 280 mV, where the current density was about 600 times higher than that of CoOOH and 20 times higher than the benchmark commercial RuO2 electrocatalyst, and it was also much better than the CoOx-300. The cobalt-vanadium based low-crystalline Co2-xVO4 nanoplates with remarkable electrochemical OER performance could be further deposited onto the promising semiconductor photoanodes to enhance the overall OER performance and longevity of the PEC devices.

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