Energy storage materials for use as photocatalysts in the dark
Authored by Dr. Pailin Ngaotrakarnwiwat1,3
Revised by Dr. Jatuphorn Wootthikanokkhan 2,3
1) Department of Chemical Engineering, Burapha University, Thailand
2) School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
3) Nanotec-KMUTT Center of Excellence on Hybrid Nanomaterials for Alternative Energy (HyNAE), Thailand
Titanium dioxide (TiO2) has been widely used in our every daylife as a pollution abatement catalyst for various applications (Fig.1) including water treatment, air pollution treatment glass coating for anti-fogging and metal coating for anti-corrosion. Fundamentally, it has been known that photocatalytic activity of TiO2 rely on its capability of releasing photo-excited electrons and holes, those of which subsequently react with ambient oxygen and moisture to generate hydrogen peroxide and hydroxyl radicals. These species are capable of inducing a decomposition of many organic substances. Besides, it has been reported that electrons generated from the system can also be applied for anti-corrosion of metals via a cathodic protection mechanism .
Figure 1 Applications of TiO2
However, the uses of TiO2 as a photocatalyst still have some limitations, i.e., it relies on the UV irradiation to activate the formation of photo-excited electrons and holes. Some researchers attempted to modified TiO2 by doping it with both metallic and non-metallic substances in order to reduce band gap energy of the metal oxide and so its photocatalytic activity under the visible light can be obtained. [2,3] However, the use of TiO2 as a catalyst for pollution abatement in the absence of both UV and visible light, which can be applied for indoor air quality, is still challenging (Fig.2). In this regard, the concept of using TiO2 in combination with other lower band gap metal oxides, capable of functioning as a kind of energy storage substance, has been developed .
Figure 2 Limitation of the neat TiO2 for use as a catalyst in the dark
Working mechanism of the mixed metal oxides can be described as following. In the presence of light, TiO2 will be photo-excited and electrons/holes are generated. Some of the electrons can be transferred to the lower band gap metal oxide (such as WO3) and stored in a form of HxWO3 compound. Then, in the dark, electrons from the intermediate compound (HxWO3) will be released and transferred back to the system. In other word, stored electrons in the lower band gap metal oxide compound will serve as a source of electron for catalytic activity of the system in the dark (Fig.3).
Figure 3 Mechanism of the energy storage photocatalyst
In this regard, the energy storage catalyst might be applied for a variety of applications, including anti-corrosion coating for metals. Specifically, the metal is used as a cathode whereas the mixed metal oxides based on TiO2 serves as an anode. The anode can releases electron to the ambient moisture and oxygen. Consequently, the metal corrosion could be inhibited. Notably, the electrons in this system are also available in the dark. This means that the corrosion protection mechanism works at all time. Moreover, it is worth remembering that the electrons can also interact with oxygen in the ambient, leading to the formation of super oxide radical and hydrogen peroxide. This means that catalytic activities of the mixed metal oxides also work under versatile illumination conditions including UV irradiation, visible light illumination and in the dark.
Research work on the development of energy storage photcatalysts at HyNAE
Apart from WO3, other metal oxides capable of functioning as energy storage substance have been explored and developed. These include the use of phosphotungstic Acid (PWA)  and the TiO2-V2O5 . Nevertheless, further research and development work have yet to be carried out in order to enhance catalytic activities of the mixed metal oxides system. According to previous research studies by Ngaotrakanwiwat et al. on the energy storage ability of WO3 and charge–discharge behaviors of TiO2/WO3 [7,8], it was suggested that the performance of the hybrid metal oxides relies on the presence of UV light and a humid atmosphere. An intimate contact between TiO2 and WO3 (or the so-called active region) should also be created to ensure the charge transfer between the metal oxide particles. In this regard, the connectivity between isolated metal oxide particles and the charge transfer between phases might be enhanced by using a semiconducting polymer as a polymeric binder for the hybrid metal oxides. Our recent work  demonstrated that the catalytic activity of TiO2/(TiO2-V2O5) hybrid metal oxides in the dark can be enhanced by using an in situ polymerized polypyrrole as a conductive polymeric binder (Fig. 4). It was also found that the amount of conducting polymer introduced into the hybrid metal oxide systems is an important factor that affects the properties of the metal oxides/polymer composites.
Figure 4. A schematic draw illustrating a postulated working mechanism of the catalyst based on TiO2/(TiO2-V2O5)/PPy mixed metal oxides/polymer composite system (Adapted from )
The similar effect was also observed by our researcher team from an analogue mixed metal oxide/polymeric binder system based on TiO2/WO3/polythiophene . In this latter case, it was found that the amount of polymeric binder, which was in turn controlled by the molar ratio of metal oxides to monomers, play an important role with regard to efficacy of the material for inducing decomposition of methylene blue in the dark. The use of the mixed metal oxides/polymer composites for inducing decomposition of other volatile organic compounds such as formaldehyde and benzene have also been investigated and the similar effects were observed. Further works, however, have yet to be carried out in order to develop a technique for applying the mixed metal oxides/polymer composites onto various substrates. This is an aspect of our ongoing work.
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