Visible-Light Responsive Photocatalytic Fuel Cell Based on ... - NSFC

Sep 13, 2012 - the holes of WO3/W photoanode due to its high oxidation power (+3.1−3.2. VNHE) ... limited by the low efficiency due to its complicat...
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Visible-Light Responsive Photocatalytic Fuel Cell Based on WO3/W Photoanode and Cu2O/Cu Photocathode for Simultaneous Wastewater Treatment and Electricity Generation Quanpeng Chen,† Jinhua Li,† Xuejin Li,† Ke Huang,† Baoxue Zhou,*,†,‡ Weimin Cai,† and Wenfeng Shangguan† †

School of Environmental Science and Engineering, Shanghai Jiao Tong University No. 800 Dongchuan Rd, Shanghai 200240, China Key Laboratory of Thin Film and Microfabrication Technology, Ministry of Education, Shanghai 200240, China

‡

S Supporting Information *

ABSTRACT: A visible-light driven photocatalytic fuel cell (PFC) system comprised of WO3/W photoanode and Cu2O/Cu photocathode was established for organic compounds degradation with simultaneous electricity generation. The central idea for its operation is the mismatched Fermi levels between the two photoelectrodes. Under light illumination, the Fermi level of WO3/W photoanode is higher than that of Cu2O/Cu photocathode. An interior bias can be produced based on which the electrons of WO3/W photoanode can transfer from the external circuit to combine with the holes of Cu2O/Cu photocathode then generates the electricity. In this manner, the electron/hole pairs separations at two photoelectrodes are facilitated to release the holes of WO3/W photoanode and electrons of Cu2O/Cu photocathode. Organic compounds can be decomposed by the holes of WO3/W photoanode due to its high oxidation power (+3.1−3.2 VNHE). The results demonstrated that various model compounds including phenol, Rhodamine B, and Congo red can be successfully decomposed in this PFC system, with the degradation rate after 5 h operation were obtained to be 58%, 63%, and 74%, respectively. The consistent operation for continuous water treatment with the electricity generation at a long time scale was also confirmed from the result. The proposed PFC system provides a self-sustained and energy-saving way for simultaneous wastewater treatment and energy recovery.

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INTRODUCTION In the process of industrialization, the environmental pollution becomes more and more deteriorating due to the rapid increasing effluent discharge. Seeking efficient approaches for eliminating pollution to maintain a green environment for human beings is urgent. Microbial fuel cell (MFC) is a promising technology to achieve this goal by using bacteria as the catalysts to decompose organic wastes. At the same time, electrons are produced by bacteria from these substrates that can migrate to an external electrode to produce electricity.1−5 Organic wastes degradation and the electric production, which can be regarded as the recovery of the chemical bonds in those substrates6,7 will be achieved simultaneously in this manner. It has great promises to address the environmental pollution as well as the energy crisis. However, the development of MFC is limited by the low efficiency due to its complicated electron transfer process and the stringent working conditions such as selectivity, long start-up time, and the bacteria cultivation.8,9 In contrast, the metal-oxide-semiconductors based photocatalysis represents a fast and direct transportation process, which can generate the electron/hole pairs once under the light illumination.10−13 The holes with high oxidation power can degrade organic compounds. The electricity can be produced by employing suitable cathode to migrate the electrons from © 2012 American Chemical Society

the external circuit. As-established PFC system using TiO2 photoanode and an O2-reducing cathode provides an efficient way for simultaneous water cleaning and energy recovery.14−16 A TiO2-nanotube-array-based PFC system has been recently reported, which shows the significant enhanced performance.9,17 This system has a maximum power density output of 0.67 mW cm−2 in the presence of acetic acid which is significant higher than that reported in MFC systems (e.g., 0.1 mW cm−2 and 0.27 mW cm−2 in refs 3 and 4). Theoretically, because of the high powerful holes of TiO2 this system can be expected to deal with all the organic pollutants. Even so, there still are two main issues in such system. On the one hand, the TiO2 photoanode only can be driven by the UV light and its response to visible light needs the modification with narrow band gap semiconductors. On the other hand, the O2-reducing cathode of Pt-black/Pt, that is imperative to provide a redox potential to produce electricity by migrating the electrons of TiO2 photoanode, requires continuous introducing oxygen into Received: Revised: Accepted: Published: 11451

June 30, 2012 September 11, 2012 September 13, 2012 September 13, 2012 dx.doi.org/10.1021/es302651q | Environ. Sci. Technol. 2012, 46, 11451−11458

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Figure 1. A: XRD patterns of preannealed tungsten foil (a), the unannealed WO3/W (b) and annealed WO3/W (c). (c-: cubic-, W: tungsten, o-:orthorhombic-, and m-: monoclinic); B: Low magnification SEM image of the unannealed WO3/W, the insert: the high magnification SEM image; C: SEM image of the annealed magnification SEM image, insert: the high magnification SEM image; D: SEM cross-sectional image of the annealed WO3/W.

generating high oxidation powerful holes to TiO2.18−22 Cu2O can also be activated by visible light due to its optimal bandgap of 2.0 eV, which has been proven to be an efficient photocathode.23−25 It can be rational employed to replace Ptblack/Pt to fabricate the low-cost PFC system. The central reason for the successful operation is that two photoelectrodes have two different Fermi levels. Under light illumination, the Fermi level of WO3/W photoanode is more negative than that of Cu2O/Cu photocathode. In this case, Cu2O/Cu photocathode can offer a positive bias for WO3/W photoanode, whereas WO3/W photoanode offer a negative bias for Cu2O/ Cu photocathode. The combination between them thus produces a suitable photovoltage to migrate the electrons of WO3/W photoanode to combine with the holes of Cu2O/Cu photocathode, which facilitates the electron/hole pairs separation at the two photoelectrodes. The holes of WO3/W photoanode with high oxidation power can be released for decomposing the organic matters, whereas the electrons of Cu2O/Cu photocathode for water reduction. The simultaneous water cleaning and electricity recovery under visible light illumination thus can be achieved in this system. The PFC in this article represents an effective way to establish more

the electrolyte. As a noble metal, the material and operation costs also limit its large scale application. To overcome the above-mentioned drawbacks, the combination of photoanode with photocathode that have different Fermi levels is a promising way. The photoanode should be the n-type semiconductor that has a positive located valence band (VB) edge for water oxidation. The photocathode should be the p-type semiconductor that has a negative located conduct band (CB) edge for water reduction. More importantly, the Fermi level of photoanode should be lower than that of photocathode. Only in this way can the interior bias be produced to migrate the electrons of photoanode through the external circuit to combine with the holes of photocathode. The holes of photoanode and electrons of photocathode thus can be released for organics degradation and water reduction. On the basis of this theory, a novel PFC is presented in this paper that can operate under visible light illumination for simultaneous wastewater treatment and electricity generation by using WO3/W photoanode and Cu2O/Cu photocathode. WO3 has been identified as a prototype photoanode material with smaller bandgap of 2.6−2.8 eV for visible light response and comparable valence band potential (+3.1−3.2 VNHE) for 11452

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efficient and low-cost system for simultaneous organics degradation and electricity production. Compared with the microbial fuel cell (MFC), this PFC possesses an obvious superiority due to its instant photoresponse, fast electron transfer, low equipment, and working conditions requirement. Compared with the PFC system based on TiO2 photoanode and an O2-reducing cathode, the reported PFC can respond to the visible light illumination, which improves solar energy utilization efficiency. The use of low-cost semiconductor as photocathode (Cu2O/Cu) also lowers the expense for managing over the large areas application.

organics compounds were phenol (20 mg L−1), Rhodamine B (20 mg L−1), and Congo red (20 mg L−1). Fifteen mL 0.1 M KH2PO4 solution adjusted to pH 7 with KOH was used as the electrolyte. The degradation rate of phenol was monitored by gas chromatogram (GC-14B) analysis. The degradation rate of Congo red and Rhodamine B was analyzed with a UV−vis spectrophotometer (UV2102 PCS, UNICO, Shanghai) at wavelength of 488 and 552 nm, respectively. The degradation efficiency was defined by the formula:

MATERIALS AND METHODS Synthesis of WO3/W Photoanode. The synthesis of WO3/W photoanode was performed according to the hydrothermal process recently reported26 with some modification. Tungsten sheets (0.25 mm thick, 99.9% purity), cut into samples of size 1 cm ×2 cm, were degreased by sonicating in 1:1 acetone and ethanol, followed by rinsing with DI water and dried in a stream of air. After that, it was annealed in air at 450 °C for 30 min to oxidize its surface. Twenty-five mL Na2WO4·2H2O solution (0.006 M) containing 1/10 (v/v) polyethylene glycol (PEG, 300) was used as the precursor along with HCl to adjust the solution pH to reach 1.5. The precursor was transferred to a 50 mL autoclave. The oxidized tungsten foil was then perpendicular placed into the precursor, sealed, and maintained at 180 °C for 4 h. Finally, the obtained product was calcinated at 550 °C for 3 h in a muffle furnace for dehydration with heating and cooling rates of 1 °C/min. Synthesis of Cu2O/Cu Photocathode. The nanowires Cu2O/Cu photocathode was synthesized by wet chemical method reported previously.27 The copper foil (99.99%) was sonicated with acetone, ethanol, and distilled water consecutively. Then it was immersed into the aqueous solution containing 2.5 M NaOH and 0.125 M (NH4)2S2O8 for 30 min. After that, it was rinsed with distilled water and dried in the air, finally annealed at 450 °C for 1 h in air. Characterization. The phase of the product was identified by X-ray powder diffraction (XRD, Shimadzu). SEM images were collected by using a field emission scanning electron microscopy (FESEM FEI-Sirion200). UV−visible absorption spectra of the samples were recorded on a UV−vis photospectrometer (TU-1901, Pgeneral, China). The photoelectrochemical characteristics of the WO3/W photoanode and Cu2O/Cu photocathode were carried out using a threeelectrode system with Ag/AgCl electrode as the reference, platinum foil as the auxiliary electrode and the samples as the working electrode. The working electrode potential and current were controlled by an electrochemical workstation (CHI 660c, CH Instruments Inc. USA). A 350 W Xe lamp (Shanghai Hualun Bulb Factory) was used for light irradiation with an AM1.5 filter. The photocatalytic fuel cell (PFC) was set up using two electrode configurations. The photoanode and photocathode were immersed in the solution and then illuminated respectively with the same light intensity on both of them as shown in Figure S1 of the Supporting Information. All runs were repeated at least three times at ambient temperature to check their reproducibility. Organics Compounds Degradation. The organic compounds degradation was carried out in a quartz container under AM1.5 illumination (light intensity, 100 mW cm−2). The illumination area of the WO3/W photoanode and Cu2O/Cu photocathode were 1 cm2 and 1.5 cm2, respectively. The testing

where c0 and ct corresponded to the initial concentration of organic contaminant and instant concentration of organic contaminant at sampling time.

η(%) = (c0 − ct)/c0

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RESULTS AND DISCUSSION XRD Characterization of WO3/W. Part A of Figure 1 shows X-ray diffraction (XRD) patterns of the preannealed tungsten foil (a), WO3/W before (b), and after annealing (c). The directly oxidized tungsten foil corresponds to the cubic WO3 (JCPDS 46−1096). The diffraction peaks of the unannealed WO3/W can be exclusively indexed by orthorhombic WO3·0.33H2O (JCPDS 54−1012). The annealed WO3/W is monoclinic WO3 (JCPDS 43−1035) according to the three characteristic diffraction peaks of (200), (020), and (002). The strongest reflection of (020) diffraction peak suggests that the WO3 crystal has the (010) orientation perpendicular to the substrate. SEM Characterization of WO3/W. In the hydrothermal process, polyethylene glycol (PEG) was employed as the structure-directing agent to confine the orientation of crystal growth. The PEG molecule has a uniform and ordered chain structure. It can adsorb on the growing crystallites and decrease its activity subsequently leading to the anisotropic growth of the nanocrystal.24 The scanning electron microscope (SEM) images of the WO3/W before and after annealing at low and high magnification are shown in parts B, C, and D of Figure 1. As can be seen from part B of Figure 1, the hydrothermal reaction results in the obvious crystal growth on the preannealed tungsten surface. The sample consists of lots of blocks with a flat surface as indicated in the high magnification SEM image (insert in part B of Figure 1). The annealed sample also exhibits the dense assembled blocks (part C of Figure 1). The thickness of WO3/W can be calculated to be 1.3 μm from the SEM cross-sectional image in the insert. However, a mesoporous morphology of the block is found from the high magnification SEM image in part D of Figure 1. It can be concluded that the annealing process leads to not only dehydration and phase transformation but also distortion of the original crystal network thus changes the morphology. The mesoporous structure possesses the advantage of large surface area for both photons absorption and charge transfer between the semiconductor−electrolyte interface.28 UV−vis Absorption Characterization of WO3/W. Part A of Figure 2 shows the spectra of WO3/W samples before and after annealing with the corresponding band gap plots. The annealed WO3/W can have an obvious strong absorption at the wavelength lower than 470 nm. The unannealed WO3/W only slightly absorbs the light at the wavelength lower than 420 nm. The bandgap values are determined with the following equation:29 11453

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photocurrent due to its poor photoabsorption ability of the hydrated crystal (data not shown). The WNA exhibits a continued increasing photogenerated current until the potential reached 1.0 V suggesting the unsaturated photoresponse. The onset potential is found to be 0.04 V. In contrast, the annealed WO3/W shows a cathodic shifted onset potential of −0.09 V and significantly enhanced photoresponse that saturates at relatively low anodic potential. The photocurrent sharply increases to 1.95 mA cm−2 at 0.6 V then keeps stable. It is much higher than that of WNA at the measured potential region even though a higher photocurrent of WNA can be expected at high potential. This result clearly demonstrates the superiority of as-prepared WO3/W to WNA. It indicates more fast photons absorption and facilitated electron/hole pairs separation on WO3/W that leads to the negative shift of the onset potential and enhances the photoresponse at the low potential region. This feature also makes the WO3/W a good candidate to establish the photocatalytic fuel cell (PFC) system which provides a relatively low photovoltage as stated in the following. Characteristics of Cu2O/Cu Photocathode. Part A of Figure 3 shows the SEM image of Cu2O/Cu photocathode with the dense nanowires on the Cu substrate. The average diameter is calculated to be around 300 nm. Part B of Figure 3 gives the LSV curves of the Cu2O/Cu photocathode in the dark and under AM1.5 illumination (light density: 100 mW cm−2) in 0.1 M KH2PO4 solution (pH 7). The efficient photon absorption and charge separation can be found. The XRD pattern is shown

Figure 2. A: UV−vis absorption spectra of annealed and unannealed WO3/W samples; insert: the calculation diagram of the band gaps; the linear sweep voltammogram (LSV) curves of the annealed WO3/W (B) and WNA (C) in the dark and under AM 1.5 (100 mW cm−2) illumination in 0.1 M KH2PO4 solution adjusted to pH 7 with KOH.

α = A((hv − Eg )n /2 /hv)

Where α, v, Eg, A, and n correspond to the absorption coefficient, incident light frequency, band gap, constant, and an integer, respectively. The value of n is determined to be 4 for the indirect band transition of WO320,30 The band gaps of the annealed and unannealed WO3/W are calculated to be 2.65 and 2.97 eV, respectively. The spread photoresponse and lower band gap at higher crystallization could attribute to the crystalline transformation from orthorhombic WO3·0.33H2O to monoclinic WO3. This result is in line with the previous reported WO3 photoanodes.31,32 Photoelectrochemical Properties of WO3/W. Part B of Figure 2 gives the linear sweep voltammogram (LSV) curves of the WO3/W in 0.1 M KH2PO4 solution adjusted to pH 7 with KOH under AM 1.5 (100 mW cm−2) illumination and in the dark. The performance of WO3 nanoflake arrays (WNA) prepared according to the previous report24 is also investigated for comparison (part C of Figure 2). The shadow regions represent the photocurrent density that has taken off the dark current. The unannealed WO3/W only has a relative small

Figure 3. A: SEM image of the Cu2O/Cu photocathode; B: The linear sweep voltammogram (LSV) curves of Cu2O/Cu photocathode in the dark and under AM 1.5 (100 mW cm−2) illumination in 0.1 M KH2PO4 solution adjusted to pH 7 with KOH. 11454

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in part A of Figure S3 of the Supporting Information, which exhibits main phases of Cu2O with some CuO phases. The results are in line with the previous report.27 Construction and Operation Mechanism of PFC System. The photocatalytic fuel cell (PFC) is established using two electrode configuration including WO3/W photoanode and Cu2O/Cu photocathode (WO3/W−Cu2O/Cu) as shown in Figure 4. The schematic diagram of the PEC cell is

Figure 4. Energy level diagram of the PFC cell (WO3/W−Cu2O/Cu) for organic compounds degradation and electricity generation (Vp, photovoltage). Figure 5. A, the open-circuit voltage (Voc) of WO3/W photoanode and Cu2O/Cu photocathode in the dark and under AM 1.5 (100 mW cm−2) illumination; B, the open-circuit voltage (Voc) of PFC cell of WO3/W−Cu2O/Cu in the dark and under AM 1.5 (100 mW cm−2) illumination.

shown in Figure S1 of the Supporting Information. Under light illumination, both of the two photoelectrodes can generate electron/hole pairs. Because the Fermi level of WO3/W photoanode is more negative than that of Cu2O/Cu photocathode, an interior bias can be produced to migrate the electrons of WO3/W photoanode to combine with the holes of Cu2O/Cu photocathode from the external circuit. This is the key for the successful operation of such combination between n-type photoanode and p-type photocathode. Only in this case can a self-driven operation under light illumination be ensured. The electron/hole pairs separation at the two photoelectrodes can be facilitated cooperatively. The holes of WO3/W photoanode and the electrons of Cu2O/Cu photocathode can be released to make the simultaneous water cleaning and energy recovery possible. To confirm this mechanism, we first measured the photovoltage of WO3/W photoanode and Cu2O/Cu photocathode in the dark and under AM1.5 illumination (100 mW cm−2). As shown in part A of Figure 5, the photovoltage values of WO3/W photoanode and Cu2O/Cu photocathode in the dark are measured to be −0.13 and 0.01 V. The WO3/W photoanode exhibits a relatively slow photoresponse and slight photoinduced cathodic shift of photovoltage. In contrast, Cu2O/Cu photocathode showed a very fast photoresponse of which the photovoltage increases instantly to 0.21 V and keeps stable. That is to say, in the combination of them, WO3/W photoanode provides a negative bias for Cu2O/Cu photocathode, whereas Cu2O/Cu photocathode provides a positive bias for WO3/W photoanode. The electrons of WO3/W photoanode can be attracted to combine with the electrons of Cu2O/Cu photocathode to produce the electricity. The holes of WO3/W photoanode and electrons of Cu2O/Cu photocathode can be released for organic compounds degradation

and water reduction. The theoretical photovoltage of this PFC could be calculated to be 0.14 V in the dark and 0.33 V under visible-light illumination. Part B of Figure 5 corresponds to the photovoltage curves of PFC, which shows an instant increase once under the light irradiation. Both the two photovoltage values in the dark and under visible-light illumination are in line with the theoretical ones indicating that the photovoltage of this PFC originated from the different Fermi levels of the two photoelectrodes. This result confirms that an interior bias exists in the system, which enhanced with the light intensity. Such a PFC system can work once under the light illumination without additional energy to mismatch the Fermi levels of the photoelectrodes that was required in the reported system of WO3 photoanode and pGaInP2 photocathode.33 The superiority of Cu2O/Cu photocathode at the efficiency and cost can also be demonstrated from this result. Performance of PFC. Part A of Figure 6 shows the I−t curves of this PFC system collected in 0.1 M KH2PO4 solution (pH 7) at zero external bias in the dark and under AM1.5 illumination (100 mW cm−2) are collected to investigate its performance. Other cells based on WNA photoanode and Cu2O/Cu photocathode, WO3/W photoanode and Pt cathode, Pt anode, and Cu2O/Cu photocathode (Pt−Cu2O/Cu) are also established for control test. Both the cells of WO3/W−Pt and Pt−Cu2O/Cu show negligible performance with the current in the dark and under light illumination tended to zero indicating that the systems based on photoanode or photocathode with Pt counter electrode cannot be driven by 11455

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Figure 6. A: I−t curves recorded with no external bias from the PFC of WO3/W−Cu2O/Cu and other cells including WNA-Cu2O/Cu, WO3/W− Pt, and Pt−Cu2O/Cu 0.1 M KH2PO4 (pH 7.0) using two electrode configuration under AM1.5 illumination (light density, 100 mW cm−2), the areas of WO3/W and WNA photoanodes are 1 cm2 and Cu2O/Cu photocathode is 1.5 cm2; B, C, and D corresponds to the concentration changes of phenol, Rhodamine B and Congo red over photolysis, the treatment in PFC systems of WO3/W−Cu2O/Cu and WNA-Cu2O/Cu, respectively.

steady state photocurrents (95 μA cm−2) can be observed, they are distinct smaller than that generated once sequential illumination on Cu2O/Cu photocathode. This result clearly shows the successful demonstration of self-bias action based on photoanode and photocathode. Because of the mismatched Fermi levels of the photoelectrodes, a suitable photovoltage can be formed once under light illumination. The photogenerated electrons of WO3/W photoanode can migrate through the external circuit to combine with the holes of Cu2O/Cu photocathode. The PFC cell of WNA−Cu2O/Cu also exhibits the consistent photoresponse but with much lower transient photocurrent (500 μA cm−2) and stationary photocurrent (105 μA cm−2), which is about only half of that produced by WO3/ W−Cu2O/Cu. The superior performance of WO3/W−Cu2O/ Cu can attribute to its mesoporous morphology that facilitates photoabsorption and charge transfer at the interface of semiconductor/electrolyte as indicated by the linear sweep voltammogram (LSV) results. Since the photogenerated holes of WO3/W photoanode possess a high oxidation power (+3.1− 3.2 VNHE),17 this PFC can be rational to be used for the degradation of organic matters with simultaneous electricity generation. Three kinds of typical organic compounds, including phenol (20 mg L−1), Congo red (20 mg L−1), and Rhodamine B (20 mg L−1), are selected as model substrates in this work. Phenol is selected as a typical phenolic compound that is widely used in

solar light. The electrons of WO3 are incapable to reduce protons due to its relative positive conduction band (CB) level.19 Meanwhile, the holes of Cu2O/Cu is kinetic infeasible for water oxidation due to its negative located valence band (VB) edge.27 The successful operation of WO3/W−Pt and Pt− Cu2O/Cu therefore requires external bias to drive water reduction and oxidation. However, the PFC system of WO3/W−Cu2O/Cu generates an obvious photocurrent of 205 μA cm−2 with the negligible background current of 20 μA cm−2. The current increases sharply once under light illumination and then declines to attain the steady state current within less than 10 s. The transient photocurrent can reach 950 μA cm−2. The constant performances can be observed after six periods of experimental (10 min). This system exhibits the superior performance to the previous reported composite PFC system based on TiO2 photoanode and Pt-black/Pt cathode that operated under UV light illumination,34 which indicates the advantage of the combination between WO3/W photoanode and Cu2O/Cu photocathode. The photocurrent is also higher than that of the reported system employing photocathode of p-SiC coated with Pt islands and TiO2 photoanode (50 μA cm−2)35 and the recently reported system employing WO3 photoanode and GaInP2 photocathode (20 μA cm−2).33 The measurement at 50 s is conducted by solely illuminating the WO3/W photoanode. Although the sharply increased transient (130 μA cm−2) and 11456

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reduction, respectively. The operation mechanism and performance are investigated. The degradation experiments for various typical organic compounds confirm its successfully application. The result also demonstrates the good stability for long time scale application. The PFC system present here could open up new opportunities for fabricating self-driven devices for energy conversion and environmental protection. The improved performace for wastewater treatment and electricity generation is expected by modifying the photoelectrodes with catalysis or seeking more effective photocahtode materials. Related researches are right in progress.

preservatives, herbicides, and pesticides. It is highly toxic, persistent, and biorecalcitrant and can directly threat the health of ecosystems and humans by discharging into the ground and surface water. Rhodamine B is selected as the class of xanthene dyes, which is dangerous by causing irritation to the skin, eyes, and respiratory tract. Congo red is selected as a typical azo dye that has high toxicity and possible accumulation in the environment. Due to the extensive use of these dyes in textile industries, the industrial wastewater has become an integral part of pollution. As shown in parts B, C, and D of Figure 6, the phenol molecules are hard to be decomposed by photolysis, whereas Rhodamine B and Congo red exhibit the obvious remove with the degradation efficiency after 5 h calculated to be 19% and 24%, respectively. However, in the PFC system of WO3/W−Cu2O/Cu, the significantly enhanced degradation efficiency can be found. After 5 h operation, the degradation rates for phenol, Rhodamine B, and Congo red are calculated to be 58%, 66%, and 73%, respectively. The pH values of the electrolyte can remain constant in the presence of organics compounds as well as in the process of organics compounds degradation. The possible reason can attribute to the buffer action provided by H 2 PO 4 1−/ HPO 4 2− . The different degradation efficiencies between the various organic matters possibly attribute to their different molecular structures.17,36,37 The prolonged investigations show that the completely remove of phenol, Rhodamine B, and Congo red will spend 13, 11, and 10 h, respectively. This result confirms the successful application of this system for water cleaning with simultaneous production of electricity. The performance of PFC system of WNA−Cu2O/Cu is also presented for comparison. Although the obvious enhanced degradation rates can be obtained compared with the photolysis, they are much lower than that of the PFC system of WO3/W−Cu2O/Cu. The possible reason can attribute to the more effective WO3/W photoanode synthesized in this paper that shows the much more fast photons absorption and electron/hole pairs separation. This result is also in line with the I−t characteristics analyzed in part A of Figure 6. It can be concluded that more holes can be produced on the WO3/W photoanode surface that lead to the faster degradation rate. All the organic matters degradation measurements were repeated for 3 times to get more credible data. Take phenol as example, the degradation rate after 5 h operation for three times was 60, 55, and 59%. The photocurrents do not show the obvious decrease in the degradation process and have good reproducibility after three times replication as shown in Figure S2 of the Supporting Information. This result suggests that this system has a good stability for durable application. Parts B and C of Figure S3 of the Supporting Information show the used WO3/W photoanode and Cu2O/Cu photocathode, which retain their original crystal phase. Therefore, the consistent operation can be assured for continuous water treatment with the electricity generation at a long time scale. In summary, a visible light responsive photocatalytic fuel cell (PFC) system based on the mismatched Fermi levels between WO3/W photoanode and Cu2O/Cu photocathode is presented. It can work for organic compounds degradation with simultaneous electric production. An interior bias can be generated once under light illumination to migrate the electrons of WO3/W photoanode to combine with holes of Cu2O/Cu photocathode through the external circuit. The holes of WO3/W photoanode and electrons of Cu2O/Cu photocathode are released for organics degradation and water

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ASSOCIATED CONTENT

S Supporting Information *

Further information including schematic diagram of PFC system, the I−t curves for phenol degradation, and the XRD patterns for the used WO3/W photoanode, the freshly prepared and used Cu2O/Cu photocathodes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 21 5474 7351, e-mail: [email protected] (B.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the National Nature Science Foundation of China (No. 21207088, No.21177085), Doctoral Program of the Ministry of Education of China (No.20110073110029), and the Shanghai Basic Research Key Project (11JC1406200) for financial support, and Instrumental analysis center of Shanghai Jiao Tong University for materials characterization.

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REFERENCES

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dx.doi.org/10.1021/es302651q | Environ. Sci. Technol. 2012, 46, 11451−11458