Polyoxometalate-derived composites for photoelectrochemical energy conversion

Abstract

To alleviate the global energy crises and environmental problems caused by the conventional fossil fuels, it is important to widespread develop renewable alternative energy schemes (e.g. solar- and wind-energies) to meet the growing energy demand of human society. Since the pioneering works on photo-electrocatalysis using TiO2 reported by Fujishima and Honda, photoelectrochemical (PEC) water splitting is regarded as one of the most promising ways to achieve solar energy conversion and storage into H2 molecular by splitting water into H2 and O2. Compared to the H2 evolution reaction (HER), the O2 evolution reaction (OER) is a kinetic bottleneck for overall water splitting, due to the complicated transfer of four electrons and four protons. Moreover, during the water oxidation reaction, some other possible oxidation reactions can occur (e.g. degradation of catalysts) in a harsh oxidizing environment. Therefore, one of the most important tasks for solar H2 production is the development of efficient and stable OER photoanodes for enhancing the solar energy water splitting conversion efficiency. Bismuth vanadate (BiVO4) is supposed to be one of the most promising photoanodes for solar water splitting due to its suitable bandgap, low valence band position, earth-abundance, stability in aqueous solution and low cost. However, the efficiency of solar energy conversion is still far below the theoretical value due to the sluggish carrier mobility, poor electrical conductivity and slow surface reaction kinetics. Therefore, the rapid progress mainly focuses on the modification of the BiVO4 photoanode materials to improve the PEC performance. One approach to this end is the use of polyoxometalates (POMs) which are a unique class of molecular metal oxide clusters that consist of early transition metals. Benefiting from the unique structure and reactivity, POMs play a key role associated with metal oxide light absorbers materials in photo-electrocatalytic system in terms of both photoanodes and photocathodes. Based on this background, this thesis summarizes investigations into various BiVO4 photoanode modification strategies for photoelectrochemical water splitting and current progress of POMs-based/derived BiVO4 photoanodes. After an introduction, the main research topics of this thesis are presented in two chapters, which aims at designing new and low-cost POMs-based/derived BiVO4 OER composite materials and innovative methods to promote the OER rate in overall water splitting configurations. For instance, doping with molybdenum or cobalt polyoxometalate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) in bulk of BiVO4 enhances the electrical conductivity and improves the charge separation efficiency respectively. Meanwhile, surface modification with CoPOM water oxidation co-catalyst enhances the hole extraction and improves the charge transfer efficiency, particularly significant at very low bias potentials it can partially compensate the detrimental effects of the electron polaronic states from doping. Besides, several innovative methods are also developed for energy electrode materials fabrication, such as ‘Double-Use’ strategy for CoPOM used both as a bulk doping agent as well as a surface-deposited water oxidation co-catalyst to develop high-performance BiVO4 photoanodes materials. Apart from the aboved material design, a variety of characterization techniques such as UV-Vis absorption spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were employed to acquire sufficient information on the optical properties, elemental composition, crystal structures and morphology for all materials before and after reaction involved in the thesis. Besides, PEC performances are conducted by potentiostat in a typical 3-electrode system under a Xe lamp equipped with an AM 1.5G filter and O2 production is directly recorded by the FireSting optical fiber O2 sensor for the evaluation of OER performance. Parts of the corresponding studies have already been published in academic journals.