Metal oxide-based, nano-structured catalyst materials for water oxidation

Abstract

Hydrogen has the potential to revolutionalise the transportation fuels market towards a greener future. (Photo)electrocatalytic water splitting into oxygen and hydrogen, at present, is one of the most promising technologies that could render large-scale hydrogen production commercially viable. One of the main challenges that has hampered the widespread use of water electrolysis techniques is the large overpotential involved in the water oxidation (anode) reaction, that accompanies the hydrogen evolution (cathode reaction). Therefore, the development of efficient catalyst materials for water oxidation is of crucial importance. Nano-materials have been found to exhibit unique, improved properties over bulk materials when used as either water oxidation or reduction catalysts. This finding mirrors those in many other applications in different fields of science. This thesis investigates the effect of nanostructuring on two prominent catalyst materials, ZnO and MnOx. The fabrication of nanostructured electrodes was explored first by a well-established electrodeposition method, then screen printing was introduced as a novel deposition method for water splitting catalysts. The results indicate that, besides intrinsic material properties and electrode geometry, the method of deposition has a significant effect on catalytic activity. Screen-printing was found to be a particularly effective and versatile method for the preparation of highly active, uniform catalyst films. A series of electrodeposited zinc oxide photoanodes were prepared first. ZnO, a wide-bandgap semiconductor, had previously been found to be a highly active water oxidizing photoanode when illuminated by UV light. With an aim to optimize the performance of electrodeposited ZnO photoanodes, a series of ZnO nanorod arrays was prepared with a variation in the seeding layer deposition. The effect of the inclusion of small amounts of Al in the ZnO films was also tested with a view to improve charge transfer from the semiconductor/liquid junction to the conducting glass substrate. The seeding layer was found to greatly affect film morphology and, consequently, catalytic performance. The inclusion of Al had little effect on catalytic activity. A preliminary investigation of the effects of the electrolyte pH showed a strong influence on catalytic activity and stability. This finding led to the further exploration of these effects on a reproducible model nano-ZnO catalyst, with a focus on addressing photodecomposition of the electrode. Photoanodes, using commercial ZnO nanoparticles, were prepared via a screen-printing process. The catalyst films were highly uniform, which allowed a systematic assessment of the effects of the electrolyte pH on photocorrosion of ZnO in aqueous electrolytes. The pH range, where ZnO is least susceptible to photodecomposition, was proposed to lie between pH 9 - 12.5 based on thermodynamic considerations. The hypothesis was tested by long-term controlled potential electrolysis and was, in most aspects, verified. Using a pH 10.5 borate buffer, 75% of the initial activity was preserved after 12 hours, representing a more than ten-fold improvement over standard testing conditions. High photocatalytic activity by the screen-printed ZnO films was observed, especially at little or no applied potential. At pH 10.5, a light current of 0.6 mA/cm2 was measured under zero-bias conditions, which is one of the highest currents reported recently under similar conditions. The promising results observed when using screen-printed ZnO electrodes led to the adaptation of the screen-printing method to the preparation of other water oxidizing catalysts. Initially, a series of nano-structured Mn-Ce composite oxides was tested, as these materials had been previously shown to exhibit high activity in the oxidative breakdown of organic pollutants in wastewaters, mainly alcohols. The screening of a series of these composites, differing mainly in their Mn:Ce ratios, showed that, unlike in wastewater treatment, the presence of Ce was detrimental to catalytic activity. However, the pure MnOx catalyst showed promising activity as a ``dark electrocatalyst in water oxidation. The nano-MnOx catalyst film was characterized by electron microscopy, X-ray diffraction and electrochemical testing methods. The manganese oxide phase was identified as beta-MnO2 with one or two other phases being present in small concentrations. The nano-MnO2 film exhibited much higher catalytic activity than a beta-MnO2 commercial reference sample, especially in alkaline media. At pH 13.6, the onset of oxidation current was below 300 mV overpotential, and current density reached 10 mA/cm2 at 500 mV overpotential. The catalytic activity, verified by oxygen evolution measurements, matched that of the most active manganese oxide catalysts reported to date. Additionally, the ease, versatility and high throughput capability of the screen-printing technique could potentially contribute to rendering this process attractive for commercial up-scaling in the future.