Improved electrode performance in microbial fuel cells and the enhanced visible light-induced photoelectrochemical behaviour of PtOx@M-TiO2 nanocomposites
Introduction
Clean energy, such as solar energy, has attracted considerable attention because of the increasingly serious energy and environmental related issues [1], [2], [3], [4], [5]. Metal oxide nanostructures, such as TiO2, have received significant interest because of their high activity, good stability, low cost, non-toxicity, wide commercial availability, and chemical inertness [1], [2], [3], [4], [5], [6], [7], [8], [9]. On the other hand, the wide band gap of TiO2 (Eg=3.2 eV) greatly limits its light responding activities because it can only absorb ultraviolet light [3], [6], [8], [10]. This seriously limits the absorption efficiency in the visible region. Therefore, expanding the visible light response of TiO2 nanostructures for practical applications is becoming an important issue [3], [6], [8], [10]. Many attempts have been made to overcome this problem by modifying TiO2 to enhance its absorption ability in the visible range. Surface sensitization techniques using quantum dots, doping with metals and nonmetals, narrow band gap semiconductor coupling, and polymer composite formation have been performed to develop visible-light-active metal oxide photocatalysts that can make use of radiation in the visible light region [1], [2], [3], [4], [5], [6]. On the other hand, the modification of TiO2 using the above materials results in thermal instability or an increase in the number of charge carrier recombination centers. Recently, Khan et al. [10], Dong et al. [11], Yang et al. [12], and Zhu et al. [13] reported that the formation of defects, such as Ti3+ states and/or oxygen vacancies, in TiO2 facilitates absorption in the visible region. The presence of defects, such as Ti3+ and/or oxygen vacancies, appears to be a promising approach to improving the visible light absorption of TiO2 [9], [10], [11], [12], [13]. This opens up a new way to utilize a larger part of the solar spectrum (~ 43% visible light) compared to UV light only (~ 5%) [1], [2], [3], [4], [5], [6], [7], [8].
Noble metals are attracting increasing attention as doping materials for the modification of metal oxide nanostructures. Noble metals nanoparticles (NPs), such as silver, gold and platinum, can absorb visible light strongly because of their surface plasmon resonance (SPR) characteristics [14], [15]. In addition, the stronger ability of electron capture by noble metals has been postulated to extend the lifetime of the electron-hole pairs and enhance the catalytic activity. These promising properties give rise to a new approach to obtaining efficient visible light activity, i.e. depositing noble metal NPs on the surface of a metal oxide to form a metal–metal oxide nanocomposites [14], [15].
Based on the above discussion, the fabrication of noble metal nanocomposites of TiO2 with abundant defects (Ti3+ and/or oxygen vacancies) using an environmentally benign process can be an effective way of fabricating materials with strong visible light activity [9], [10]. This is due to the synergistic effect between the defects that induce visible light absorption and metal nanoparticles that contribute further to the enhancement of visible light absorption because of their SPR characteristics [14], [15].
Microbial fuel cells (MFCs) are devices that utilize microorganisms as a catalyst for the conversion of chemical energy to electrical energy [16]. Microorganisms generate electricity directly from a variety of waste materials, and are attracting increasing attention as alternative and relatively inexpensive energy sources [17]. The controlling factor for power generation is the cathodic oxygen reduction rate, which limits the use of MFCs for practical applications [18]. Therefore, it is important to modify the cathode surface further and reduce the charge transfer resistance [18].
This paper reports visible light active PtOx@M-TiO2 and PtOx@P-TiO2 (x=0, 1 and 2) nanocomposites fabricated using an electrochemically active biofilm (EAB). The photoelectrochemical performance determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV) in the dark and under visible light irradiation support the enhanced visible light-induced photoelectrochemical behavior of the PtOx@M-TiO2 and PtOx@P-TiO2 nanocomposites. The PtOx@M-TiO2 and PtOx@P-TiO2 nanocomposites were also assessed as MFC cathode electrodes and showed much better performance than plain carbon paper.
Section snippets
Materials
TiO2 nanoparticles (Degussa TiO2) were acquired from Degussa and abbreviated as P-TiO2. Hydrogen hexachloroplatinate (IV) hydrate (H2PtCl6·nH2O; n=5.5) was purchased from Kojima Chemicals, Japan. Ethyl cellulose and α-terpineol (C10H18O) were supplied by KANTO Chemical Co., Japan. Sodium phosphate dibasic (Na2HPO4), sodium phosphate monobasic (NaH2PO4), sodium sulfate (Na2SO4), sodium acetate (CH3COONa) were obtained from Duksan Pure Chemicals Co. Ltd. South Korea. The fluorine-doped
Proposed preparation mechanism of the PtOx@M-TiO2 and PtOx@P-TiO2 nanocomposites
The PtOx@M-TiO2 and PtOx@P-TiO2 nanocomposites (x=0, 1 and 2) were synthesized using an EAB. The presented method is a cost-effective, surfactant-free, environment-friendly process [9]. Fig. 2 presents a schematic diagram of PtOx@M-TiO2 or PtOx@P-TiO2 nanocomposites synthesis. The EAB plays a crucial role in synthesis by providing an excess of electrons through the decomposition of sodium acetate under partially anaerobic conditions, which are responsible for the reduction/oxidation of platinum
Conclusions
PtOx@M-TiO2 and PtOx@P-TiO2 (x=0, 1 and 2) nanocomposites were synthesized successfully using an EAB. Pt0 (At%=16.44), Pt2+ (At%=68.23), and Pt4+ (At%=15) were observed on the surface of M-TiO2, whereas Pt0 (At%=35.64), Pt2+ (At%=36.45) and Pt4+ (At%=27.91) were observed on the surface of P-TiO2. The PtOx@M-TiO2 nanocomposite showed enhanced photoelectrochemical behavior compared to the PtOx@P-TiO2 nanocomposite. The presence of different Pt states on the surface of M-TiO2 and defects in M-TiO2
Acknowledgment
This study was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2014R1A6A1031189).
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