Research PaperA visible-light responsive micro photocatalytic fuel cell with laterally arranged electrodes
Introduction
Directly discharging wastewater into natural water body not only threatens the human health but also triggers unwanted ecological effects. Therefore, the environmental problem of water pollution has become one of the critical issues facing our planet [1], [2], [3]. For this reason, many methods have been developed, such as biodegradation [4], [5], adsorption [6], [7] and chemical oxidation [8]. However, traditional wastewater treatment methods are usually concerned with how to rapidly and efficiently degrade pollutants contained in the wastewater. Actually, these pollutants also contains plenty of chemical energy, which almost meets 1/3 demand of the global energy consumption per year [2]. Unfortunately, the above-mentioned technologies are unable to efficiently utilize plentiful chemical energy contained in the wastewater, resulting in the energy loss. In this case, it is urgent to seek for new and efficient methods to simultaneously remove pollutants and recover the chemical energy stored in the wastewater.
The photocatalytic fuel cell (PFC) is one of promising technologies to meet the above demand, in which the abundant semiconductors are employed to function as the photoanode. As a result, the capital cost of the PFC can be much lower than that of conventional fuel cell [9]. During the PFC working process, the electron/hole pairs are photo-excited in the semiconductors upon illumination [10], [11]. The photo-excited holes can oxidize most organic compounds [12], [13], [14], [15], while the photo-exited electrons go through the external circuit to the cathode to complete the oxygen reduction reaction. Since most organic compounds can be degraded photocatalytically, not only the wastewater can be efficiently purified but also the electricity can be generated simultaneously by using solar energy. Therefore, the PFC technology has been regarded as a promising method to treat wastewater and generate clear energy simultaneously [16], [17], [18].
In the past, the development of highly active photocatalysts has received numerous attention towards the improvement in the PFC performance [19], [20], [21]. For instance, Antoniadou et al. used CdS to functionalize commercial nanocrystalline titania [22]. Shu et al. developed BiOI-based photoanode to generate electricity by using organic compounds [23]. Li et al. developed a photocatalytic fuel cell with a BiOCl/Ti photoanode and a Pt cathode for the dye degradation and electricity generation [24]. Li et al. developed a dual-photoelectrode photocatalytic fuel cell with a TiO2/Ti photoanode and a Cu2O/Cu photocathode for hazardous organics treatment with simultaneous electricity generation [25]. In addition to the photocatalysts, the PFC performance is also dependent of its cell structure design due to its effect on the photon and mass transport. Currently, most existing PFCs are referred to as bulk reactors [8], [9], [10]. Besides, Seger et al. [26], [27] developed a photoelectrochemical cell with a membrane electrode assembly design. Tang et al. [28] proposed a photocatalytic fuel cell with an aqueous-film rotating disk for the generation of hydrogen and electricity. However, conventional PFCs usually have large dimensions, which is not beneficial for the photon and mass transport. For this reason, microfluidics that has the advantages of enhanced mass transfer due to high surface-to-volume ratio [29], [30], has been incorporated into the photocatalytic technologies to improve the performance, such as wastewater treatment [17] and CO2 photoreduction [31], [32]. Recently, Xia et al. [33] developed a micro membraneless and monolithic photocatalytic fuel cell with the bare TiO2 photoanode and an air-breathing cathode, in which two electrodes were laterally arranged at the same plane. In this design, not only the ion exchange membrane was eliminated to reduce the cost and the issues associated with the membranes, but also the oxygen transport could be enhanced to improve the cell performance as compared to the oxygen-dissolved cathode. Moreover, this design allowed it to be easily fabricated and integrated with other microdevices [34], [35]. Besides, the lateral arrangement of the electrodes makes it possible to use the photocathode without two-side illumination. However, bare TiO2 photoanode was employed in this work, which could only respond to the UV light. The UV light only accounts for 3–5% sunlight. Under such a circumstance, the solar energy could not be efficiently utilized. Therefore, the development of a visible-light responsive photoanode for this new type of PFC is needed, by which the cell performance and solar energy utilization efficiency can be further improved. Aiming at this target, a visible-light responsive micro photocatalytic fuel cell (μPFC) with the lateral arrangement of two electrodes at the same plane was developed in this study. The developed μPFC was assessed by using methanol as a representative organics pollutant in an alkaline environment.
Section snippets
Design and fabrication of μPFC with the laterally arranged electrodes
The design of the developed μPFC with the lateral arrangement was similar to the previous work, which was consisting of a cover, a photoanode, a cathode and a baseplate [33], as shown in Fig. 1a. The polydimethylsiloxane (PDMS) was used to fabricate the cover and the baseplate. The photoanode was made by coating TiO2 nanoparticles (Aeroxide P25, Acros, Belgium) on the fluorine-doped SnO2 (FTO) conducting glass (resistance 10 Ω per square, Xinyan Technology Co., China). Commercially-available
Photoanode characterization
Fig. 2 shows the SEM images of the bare TiO2 porous layer and the quantum-dots photosensitized TiO2 porous layer. As shown in the Fig. 2a, for the bare TiO2 porous layer, although the TiO2 nanoparticles were easily agglomerated, forming the clusters, they were still distributed uniformly on the FTO conducting glass. After the photosensitization, CdS-ZnS spread in and between TiO2 nanoparticles. The presence of CdS-ZnS was demonstrated by the formed large clusters, each of which consisted of
Conclusions
A micro photocatalytic fuel cell with a visible-light responsive photoanode and the lateral arrangement of the electrodes at the same plane was developed in this study for simultaneously degrading organics and generating electricity. Photosensitizing the CdS-ZnS to TiO2 allowed the photoanode to be responsive to the visible light. The evaluation of the developed μPFC was performed by using methanol as the representative organic compound in the wastewater under various conditions. The UV–Vis
Acknowledgments
The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (No. 51576021, No. 51620105011, No. 51506039 and No. 51776026), the National High-Tech R&D Program of China (No. 2015AA043503), the Program for Back-up Talent Development of Chongqing University (No. CQU2017HBRC1A01) and the Fundamental Research Funds for the Central Universities (No. 2018CDXYDL0001).
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