Photocatalytic Fuel Cell (PFC) and Dye Self-Photosensitization

Mar 9, 2013 - ... John Jay College and the Graduate Center, The City University of New York, ... photo fuel cell (DSPFC) when irradiated with visible ...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/est

Photocatalytic Fuel Cell (PFC) and Dye Self-Photosensitization Photocatalytic Fuel Cell (DSPFC) with BiOCl/Ti Photoanode under UV and Visible Light Irradiation Kan Li,† Yunlan Xu,‡ Yi He,§ Chen Yang,† Yalin Wang,† and Jinping Jia*,† †

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400050, P. R. China. § Department of Sciences, John Jay College and the Graduate Center, The City University of New York, New York, New York 10019, United States ‡

S Supporting Information *

ABSTRACT: A fuel cell that functioned as a photo fuel cell (PFC) when irradiated with UV light and as a dye self-photosensitization photo fuel cell (DSPFC) when irradiated with visible light was proposed and investigated in this study. The system included a BiOCl/ Ti plate photoanode and a Pt cathode, and dye solutions were employed as fuel. Electricity was generated at the same time as the dyes were degraded. 26.2% and 24.4% Coulombic efficiency were obtained when 20 mL of 10 mg·L−1 Rhodamine B solution was treated with UV for 2 h and visible light for 3 h, respectively. Irradiation with natural and artificial sunlight was also evaluated. UV and visible light could be utilized at the same time and the photogenerated current was observed. The mechanism of electricity generation in BiOCl/Ti PFC and DSPFC was studied through degradation of the colorless salicylic acid solution. Factors that affect the electricity generation and dye degradation performance, such as solution pH and cathode material, were also investigated and optimized.

■

INTRODUCTION Energy and environment quality are two major issues faced by countries in modern society. Recovering energy released from degradation of organic pollutants during waste treatment is a promising, yet challenging approach to link and address the environmental and energy problems. Microbial fuel cell (MFC) is an example that demonstrates direct generation of electricity through using bacteria to break down organic compounds.1−3 However, current MFC techniques are difficult to scale up and use in large wastewater treatment plants because only the biodegradable compounds, which only account for part of the target pollutants in wastewater, can be utilized by the bacteria and the bacteria are easily affected by the toxic chemicals in the influent water.3 Photocatalytic (PC) oxidation is an emerging technique in wastewater treatment in recent years because of the improved treatment performance and the use of stable and nontoxic materials. In a typical PC process, a large amount of electrons are excited from the valence band of the photocatalyst to the conduction band, generating separate holes and electrons. The holes are powerful oxidants for degradation of organic pollutants and the electrons can react with O2 to yield O2·− to further oxidize organic pollutants or are employed to reduce H+ to generate H2.4,5 However, based on the fact that the photogenerated electrons can be transferred spontaneously to a © 2013 American Chemical Society

cathode through an external circuit due to the existing potential difference between the photo anode and cathode and thus generate electricity, the concept of photo fuel cell (PFC)6−8 was proposed. Compared with solar cells and dye-sensitized solar cells (DSSC), a PFC system uses organic pollutants in water as fuel to generate electricity. High energy density was obtained and only light and photocatalyst were required. Additionally, a photocatalyst works at ambient conditions, has no selectivity, and can degrade nearly any organic substance. Developing photocatalysts that respond to both UV and visible light is critical for the success of the PFC technique. Because of the wide band gap (3.2 eV), the commonly used TiO2 photocatalyst can only be excited by UV light, which accounts for less than 5% of the solar energy. Although photocatalysts with narrower band gap, such as CdS and Cu2O, were prepared in the past few years, the toxicity of CdS and the stability of Cu2O are of application concern.9,10 Bismuth oxychloride (BiOCl) has drawn great attention in PC and other fields because of its unique and excellent optical, catalytic, electrical, magnetic, and luminescent properties and the Received: Revised: Accepted: Published: 3490

October 2, 2012 March 4, 2013 March 9, 2013 March 9, 2013 dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

Figure 1. (a) Direct photo-excitation pathway of BiOCl under UV light irradiation; (b) dye self-photosensitization pathway of BiOCl under visible light irradiation; (c) schematic diagram of BiOCl/Ti−Pt PFC and DSPFC.

used as a precursor for preparing BiOCl film. NaCl (Shanghai Chemical Reagent Co., China) was used to provide Cl− to prepare BiOCl film. Rhodamine B, methylene blue, and basic orange (Shanghai Jiaying Chemical Co., Ltd., Shanghai, China) were of commercial grade and were used as received. Na2SO4 (Shanghai Chemical Reagent Co., Ltd., Shanghai, China) was employed as the supporting electrolyte. All other chemicals were of analytical grade and were used as received. Instruments and Experimental Conditions. Preparation and the characteristic details of the BiOCl/Ti photoanode are shown in Supporting Information. The schematic diagram of BiOCl/Ti PFC and DSPFC is shown in Figure 1c. The size of the cell was 4.5 cm × 1.0 cm × 6.0 cm (length × width × height). The current−voltage (J−V) characteristics of the cell were measured by a two electrodes system using an Autolab 4.9 system (Metrohm, Switzerland). BiOCl/Ti photoanode (4 cm × 4 cm) was used as the working electrode and a Pt cathode with the same size was used as the auxiliary and reference electrode at the same time if not mentioned. This cell was named “BiOCl/Ti−Pt cell”. The distance between the two electrodes was 5 mm. An 11 W mercury lamp (Philips, 254 nm) and a 150 W xenon lamp (Shanghai Lansheng Co. Ltd., Shanghai, China) with a 400 nm cutoff filter were used as UV and visible light source. Sunlight and artificial sunlight (a 150 W xenon lamp without cutoff filter) were also used to investigate the electricity generation performance of the cell. 20 mL 10 mg·L−1 RB, MB, and BO dye solutions were used as the fuel during investigation. 20 mL of 10 mg·L−1 colorless salicylic acid (SA) and the mixture of 20 mL of 10 mg·L−1 SA and RB solutions were treated under the same conditions to investigate the working mechanism of the PFC and DSPFC. Na2SO4 (0.05 mol·L−1) was added into the solution as the supporting electrolyte. The initial pH of the solution was adjusted to 2.0 with 1 mol·L−1 H2SO4 if not mentioned. Prior to irradiation, the solution in the cell was stirred in the dark for 40 min to reach adsorption−desorption equilibrium. The concentrations of RB, MB, and BO were determined by measuring solution absorbance using a UV−vis spectrophotometer (UV-2102PCS, UNICO, Shanghai) at the wavelength of 563, 665, and 449 nm, respectively. The concentration of salicylic acid was determined using a HPLC system (Series 200, Perkin-Elmer, Inc., USA)

extremely low toxicity.11−14 Although the wide band gap of BiOCl does not allow it to be activated directly in the visible light region, it has been found that dyes (like rhodamine B and methylene green) can be adsorbed on the surface of BiOCl and further be decomposed under visible light irradiation due to the indirect dye self-photosensitization.15−17 Figure 1a,b illustrates these two processes. In Figure 1a, electron−hole pairs are generated under UV irradiation. The photogenerated holes oxidize organic compounds directly or oxidize H2O to form ·OH, which are responsible for the high UV activity of BiOCl in organic pollutants degradation.18−20 In Figure 1b, the dye adsorbed on the BiOCl surface is excited by the visible light, and then the electrons are injected from the excited dye to the conduction band of BiOCl and scavenged by O2 to yield O2·−. The conduction band of BiOCl (−1.1 eV)21,22 is more negative than E(O2/O2·−) (−0.046 eV),23 and the oxidation activity of O2·− is strong enough to further decompose dyes. Based on these facts, we proposed in this work a new system termed dye self-photosensitization PFC (DSPFC) through the use of BiOCl as photocatalyst working under visible light and dye as fuel to generate electricity (Figure 1c). This work is of great significance considering that more than 15% of the world dye production, or ca. 400 tones a day, is released into natural environment during synthesis, processing, and using, and most of the dyes are nonbiodegradable.24 If recovered, the energy obtained during dye wastewater treatment will be tremendous. In this study, a BiOCl film was synthesized and used as photoanode to construct a PFC and a DSPFC working under UV and visible light. Commonly used dyes, rhodamine B (RB), methylene blue (MB), and basic orange (BO), were employed as the fuel to evaluate the performance of electricity generation and dye degradation of the cells. Additionally, the mechanism of electricity generation of the proposed systems was also investigated and confirmed.

■

MATERIALS AND METHODS Materials and Reagents. Titanium sheets (99.6% purity, 4 cm × 4 cm; thickness 1.5 mm) were purchased from Shanghai Hongtai Metal Production Co. Ltd. (Shanghai, China) and were employed as the substrates for BiOCl film coating. Bi(NO3)3·5H2O (Shanghai Chemical Reagent Co., China) was 3491

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

Figure 2. Current−voltage (J−V) plots and current−power (J−JV) plots for BiOCl/Ti−Pt cell with different fuel under (a,b) UV irradiation, (c,d) visible light irradiation, and (e,f) sunlight and artificial sunlight irradiation. CRB = CMB = CBO = CSA = 10 mg·L−1, 0.05 mol·L−1 Na2SO4, pH 2.0.

with an Athena C18 reversed-phase column (5 μm, 4.6 mm × 250 mm) and a UV detector (300 nm). Chemical oxygen demand (COD) of the solution before and after treatment was measured following China National Standard Method GB11914−89. All solutions were prepared in double-distilled water and all the experiments were repeated 3 times.

■

ff =

JVmax Jsc Voc

(1)

where JVmax is the maximum power density of the BiOCl/Ti− Pt cell obtained from the J−JV plots. The fill factor shows the deviation of the actual maximum power density produced by the cell from the value of JscVoc, the product of the highest possible values of current density and voltage. The performance of a fuel cell is directly related to its fill factor, which should be optimized as high as possible.26 For a PFC, BiOCl is excited with UV irradiation to generate electron−hole pairs. The photogenerated electrons spontaneously move to Pt cathode through an external circuit because of the existing potential difference, and thus electricity is generated. Compared with the solution with no fuel, there was

RESULTS AND DISCUSSION

Current−Voltage (J−V) Characteristics of the Cell. The current−voltage (J−V) plots and the current−power (J−JV) plots obtained using different light sources are shown in Figure 2. The open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factors (ff) of the cell are listed in Table 1. The fill factor is calculated using following equation:25 3492

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

and 66 kΩ·cm −2 for DSPFC (Figure S4, Supporting Information). The great increase of Rs in DSPFC was most likely attributed to the contact of BiOCl and Ti substrate because BiOCl itself can hardly be excited by visible light to generate electrons, but only transferred electrons form excited dyes to the Ti substrate in DSPFC. Meanwhile electron accumulation at the interface between the adsorbed RB and BiOCl decreased the Rsh, turned the J−V plot from a convex shape in PFC to a concave shape in DSPFC, and caused a drastically smaller ff.29 The mechanism of dye self-photosensitization in DSPFC was investigated using 20 mL of 10 mg·L−1 colorless salicylic acid solution as fuel, and the experiments were conducted under the same condition with UV or visible light irradiation. The J−V plots obtained using salicylic acid solution are shown in Figure 2a,c (the green line) and the current−voltage characteristics are listed in Table 1. Compared with the J−V plot with no fuel (the black line), when salicylic acid was used as fuel and with UV irradiation, the Jsc and Voc values increased from 0.0038 mA·cm−2 and 0.590 V to 0.0053 mA·cm−2 and 0.664 V due to UV excitation. However the indirect dye self-photosensitization did not occur when using visible light because salicylic acid cannot be excited by visible light (colorless). Compared with the J−V plot of no fuel (0.0017 mA·cm−2 and 0.611 V), the values of Jsc and Voc were almost the same (0.0020 mA·cm−2 and 0.599 V). These results confirmed that, when dyes were used as fuel, this BiOCl/Ti−Pt cell functioned as a PFC under UV irradiation and as a DSPFC under visible light irradiation. The J−V characteristics of BiOCl/Ti−Pt cell were further investigated under sunlight. In the meantime, a 150 W xenon lamp (without the light filter) was used as the artificial sunlight source to investigate the combination effect of UV and visible light. As shown in Figure 2e and Table 1, current was generated under both sunlight and artificial sunlight, and the values of Voc and Jsc increased when MB, BO, and RB were used as fuel in the BiOCl/Ti−Pt cell. Compared with the electricity generated under natural sunlight, it was much larger when using artificial sunlight, which was mainly due to the high power of the incident light and the large amount of UV light eliminated by xenon lamp. The ff values of the cell were between 0.17 and 0.23 for the three dye solutions investigated under sunlight and artificial sunlight. Although electricity was generated and observed in the BiOCl/Ti−Pt PFC and DSPFC, the efficiency of converting photo-energy to electricity needs to be improved. More work has to be done to enhance the light utilization. Such work includes, for example, designing a new reactor to reduce the light loss caused by solution adsorption and surface reflection, changing the planar structure of the photoanode to wedge or pyramid structure to harvest more incident light on the same irradiation area,30,31 or modifying the photoanode and cathode to reduce the internal resistance. Additionally, the cell can be separated by a proton exchange membrane and use N2 and O2 instead of air in photoanode and cathode region, respectively, to reduce the loss of electrons on BiOCl photoanode and improve the electron utilization on the Pt cathode. Rhodamine B Degradation Efficiency Obtained by PFC and DSPFC. In a PFC system, as shown in Figure 3a, approximately 8% RB was removed by photodegradation using a Ti plate connected to a Pt cathode under UV irradiation. 80% and 93% of RB removal (evaluated by color) were obtained in 60 and 120 min, respectively, when BiOCl/Ti was used as

Table 1. Current−Voltage Characteristics of the BiOCl/Ti− Pt Cell under UV, Visible Light (Vis), Sunlight (SL), and Artificial Sunlight (ASL) Irradiation with Different Fuela

a

light source

fuel

Jsc (mA·cm−2)

Voc (V)

ff

UV UV UV UV UV Vis Vis Vis Vis Vis SL SL SL ASL ASL ASL

No fuel Salicylic Acid Methylene Blue Basic Orange Rhodamine B No fuel Salicylic Acid Methylene Blue Basic Orange Rhodamine B Methylene Blue Basic Orange Rhodamine B Methylene Blue Basic Orange Rhodamine B

0.0038 0.0052 0.0107 0.0112 0.0116 0.0017 0.0020 0.0063 0.0056 0.0052 0.0050 0.0068 0.0055 0.0254 0.0262 0.0293

0.590 0.664 0.629 0.654 0.655 0.611 0.599 0.585 0.617 0.639 0.657 0.632 0.648 0.697 0.646 0.663

0.23 0.20 0.28 0.29 0.39 0.30 0.27 0.19 0.17 0.18 0.18 0.20 0.21 0.17 0.23 0.20

CRB = CMB = CBO = CSA = 10 mg·L−1, Na2SO4 0.05 mol·L−1, pH 2.0.

0.04 to 0.07 V enhancement of Voc when dye solutions are used, and the maximum Voc obtained was 0.655 V when using RB solution. Jsc was also increased significantly when using dye solution in PFC. The Jsc values were 0.0107, 0.0112, and 0.0116 mA·cm−2 for MB, BO, and RB solution, respectively, and they were much higher than 0.0038 mA·cm−2 obtained at the same condition when no fuel existed. Such an increase might be because the dye molecules were easier to oxidize than H2O so that more electrons were transferred to Pt cathode. The obtained ff value of the PFC was between 0.28 and 0.39 for different dye solutions. For a DSPFC irradiated by visible light, the dye adsorbed on the surface of the BiOCl film is excited and the electrons are injected from the excited dye to the conduction band of BiOCl, and the moving of electrons generated in this process via external circuit produces electricity. As shown in Figure 2c, Jsc increased from 0.0017 mA·cm−2 with the absence of fuel to 0.0063, 0.0056, and 0.0052 mA·cm−2, respectively, when MB, BO, and RB solution was used. Compared with PFC, the DSPFC Jsc increase was lower and the ff value decreased from 0.28 to 0.39 to 0.17−0.19 when the same dye solution was used as fuel. Because the charge-transfer resistance at the electrode/ electrolyte interface is related to the Jsc and ff value of the cell, the decrease of Jsc and ff value in DSPFC may suggest current leakage, which might occur because part of the electrons generated from visible light irradiation reacted with O2 to yield O2·− on the surface of the BiOCl/Ti photoanode, and therefore they were not transferred via external circuit to generate higher current. Similar to solar cell, parasitic resistance would contribute to the loss of electricity generation efficiency in PFC and DSPFC. Parasitic resistances include the following: (a) the series resistance (Rs) such as resistance of semiconductor and metal−semiconductor contact resistance; and (b) the shunt resistance (Rsh), which includes the resistance of alternate electrical pathways that do not contribute to the photocurrent. In an ideal cell, then, Rs is zero, and Rsh is infinite, and the values of Rs and Rsh can be derived from the J−V plot.27,28 In our case, when RB was used as fuel, the values of Rs and Rsh were 16 kΩ·cm−2 and 92 kΩ·cm−2 for PFC and 69 kΩ·cm−2 3493

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

O2·− and further oxidize RB. However, without the connection of Pt cathode, part of the photogenerated electrons would recombine with holes and resulted in decreased RB degradation efficiency. When using visible light, as shown in Figure 3a, almost no RB was removed by photodegradation alone (Ti electrode), and 89% RB was removed in 180 min when BiOCl/Ti was used as the photoanode and connected to the Pt cathode. However, different from the result obtained under UV irradiation, the color removal, 86% in 180 min, was almost same as that obtained without the connection of photoanode to the Pt cathode. RB molecules were easily adsorbed on the surface of BiOClapproximately 5% of RB was adsorbed on photoanode surface before light was on (Figure 3a). Dye self-photosensitization occurred to decompose RB under visible light irradiation. Different from UV excitation, there was no recombination of photogenerated electrons and holes in the dye self-photosensitization process. When a Pt cathode was connected, the electrons that excited from RB transferred to the conduction band of BiOCl first, and then move to the Pt cathode via the external circuit to generate electricity. As in the PFC system, the electrons on the Pt cathode reacted with O2 to yield O2·− for further degradation of RB. However, in a DSPFC system, when the photoanode was not connected to the cathode, the electrons on the conduction band of BiOCl still reacted with O2 directly to yield O2·− and resulted an almost unchanged RB degradation efficiency. The degradation efficiency of 20 mL 10 mg·L−1 colorless salicylic acid was investigated in the BiOCl/Ti−Pt PFC and DSPFC, respectively (Figure 3b). Approximately 62% salicylic acid was degraded under UV light in the PFC in 120 min. However, when using visible light, the degradation of salicylic acid was not observed in the DSPFC because salicylic acid was hardly adsorbed on the surface of BiOCl and further excited by visible light. The result of salicylic acid degradation agreed with the result of J−V plots obtained by the BiOCl/Ti−Pt PFC and the DSPFC. However, when salicylic acid and RB existed in solution simultaneously, as shown in Figure 3c, there is almost no influence on salicylic acid and RB removal in PFC due to the high oxidation ability of photogenerated electron−hole pairs. However, different from solution containing salicylic acid alone, about 15% salicylic acid in the salicylic acid and RB mixture was removed under visible light irradiation in DSPFC due to the O2·− generated by dye self-photosensitization. The RB removal efficiency slightly decreased due to the existence of salicylic acid (from 89% to 81% in 3 h). Coulombic Efficiency of BiOCl/Ti−Pt PFC and DSPFC. How efficiently the chemical energy is converted to electrical energy is an important parameter to evaluate the performance of a fuel cell. The Coulombic efficiency, Ec, defined as the ratio of total Coulombs actually transferred to the cathode from the photoanode to maximum possible Coulombs if all substrate removal produced current,33,34 was used in this study as an energy conversion indicator. For dye wastewater, the chemical oxygen demand (COD) better describes how much of the available fuel is converted to electrical current than using color removal. Mathematically, the total Coulombs obtained is determined by integrating the current over time, and the Ec for the proposed PFC and DSPFC over a time period t can be calculated as

Figure 3. (a) Color removal efficiency of RB with and without the connection of Pt cathode in BiOCl/Ti−Pt PFC and DSPFC. (b) Degradation efficiency of SA and (c) degradation efficiency of SA and RB in mixture solution BiOCl/Ti−Pt PFC and DSPFC. CSA = CRB = 10 mg·L−1, Na2SO4 0.05 mol·L−1, pH 2.0.

photoanode and connected to Pt cathode. As a comparison, when the photoanode was not connected to the Pt cathode, 58% RB was removed in 60 min, and 87% in 120 min. BiOCl can be excited by UV light directly to generate electron−hole pairs and the photogenerated holes oxidize RB molecules directly or oxidize H2O to generate ·OH radicals to further oxidize RB. It is known that the noble metal can store electrons and restrict the recombination of photogenerated electrons and holes.32 When Pt was used as cathode, the photogenerated electrons spontaneously move to Pt cathode due to the existing potential difference, and therefore are separated from positive holes. The electrons on Pt cathode can react with O2 to yield 3494

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

Table 2. Effect of pH on J−V Characteristics of BiOCl/Ti− Pt PFC and DSPFCa

t

Ec =

M ∫ I dt 0

FbV ΔCOD

(2)

where M is the molecular weight of oxygen; F is Faraday’s constant; b equals 4 and is the number of electrons released per mole of oxygen; V is the volume of liquid in the reaction system; and ΔCOD is the change of COD concentration over time t. In this study, 20 mL 10 mg·L−1 RB was used as fuel and degraded in the BiOCl/Ti−Pt PFC and DSPFC for 2 and 3 h, respectively. The total Coulombs actually transferred from BiOCl/Ti photoanode to Pt cathode in PFC and DSPFC were calculated to be 1.260 and 0.853 C, respectively, by integrating the current−time curve shown in Figure 4. The COD values of

a

cell

pH

Jsc (mA·cm−2)

Voc (V)

ff

PFC PFC PFC PFC DSPFC DSPFC DSPFC DSPFC

2.0 4.0 6.0 8.0 2.0 4.0 6.0 8.0

0.0116 0.0101 0.0093 0.0087 0.0052 0.0053 0.0057 0.0028

0.655 0.651 0.640 0.610 0.639 0.638 0.642 0.531

0.39 0.33 0.30 0.26 0.18 0.18 0.20 0.27

CRB = 10 mg·L−1, Na2SO4 0.05 mol·L−1.

Figure 4. Current−time plot of PFC and DSPFC under UV and visible light irradiation over 2 and 3 h, respectively. Inset is the COD removal of RB at the same condition. CRB = 10 mg·L−1, Na2SO4 0.05 mol·L−1, pH 2.0.

RB solution before and after treatment are shown in the inset figure of Figure 4, and the resulting Ec values are 26.2% and 24.4%, respectively. Compared with the photogenerate holes and ·OH in PFC, the oxidation ability of dye self-photosensitization in DSPFC is weaker. Although some chromophores in RB, such as -CN-, break easily, such functional groups as aromatic rings are relatively stable and account for the lower COD removal efficiency in DSPFC. Practically, Ec can be improved by increasing light intensity or using photoanode with larger surface area. These factors will be investigated in our future work. Effect of pH. The solution pH affects the performance of electricity generation and organic compound degradation. The effects of pH on J−V characteristics of BiOCl/Ti−Pt PFC and DSPFC are shown in Table 2. In BiOCl/Ti−Pt PFC, both Voc and Jsc decreased with the increase of solution pH. While in BiOCl/Ti−Pt DSPFC, they remained constant in the pH range of 2.0 to 6.0 and sharply decreased when pH increased to 8.0. The effects of pH on RB color removal in PFC and DSPFC are shown in Figure 5. The RB adsorption capacity of BiOCl was not affected much in the pH range of 2.0 to 6.0; however, RB was hardly adsorbed with further increase of pH to 8.0. In basic condition, the cationic form RB turned to zwitterion, which easily aggregated with each other to form larger molecules that were hard to adsorb.35 In PFC, RB color removal efficiency decreased with the increase of pH value and this phenomenon was also observed when TiO2 was used as photocatalyst under UV irradiation to treat RB or other organic compounds.36 In

Figure 5. Effect of pH on RB color removal efficiency in (a) BiOCl/ Ti−Pt PFC and (b) BiOCl/Ti−Pt DSPFC. CRB = 10 mg·L−1, Na2SO4 0.05 mol·L−1.

DSPFC, the dye self-photosensitization occurred in the pH range of 2.0 to 6.0 and the reaction was hindered at higher pH. This RB degradation result also explains the effect of pH on J− V characteristics of PFC and DSPFC. Effect of Cathode Materials. To investigate the effect of cathode materials on the electricity generation performance of the PFC and DSPFC, four different materials (Pt, graphite, Ti, and Cu) were used as cathode to treat 20 mL of 10 mg·L−1 RB solution, and the results are shown in Table 3 and Figures S5 and S6 in Supporting Information. The experimental results indicated that the cathode material contributed significantly to electricity generation due to the difference of potential between the BiOCl/Ti photoanode and the cathode. Although the Pt cathode exhibited the best performance among the investigated 3495

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

photoanode and O2-reducing cathode. Electrochem. Commun. 2006, 8 (2), 336−340. (8) Antoniadou, M.; Lianos, P. Production of electricity by photoelectrochemical oxidation of ethanol in a PhotoFuelCell. Appl. Catal., B 2010, 99 (1−2), 307−313. (9) Yang, W. W.; Li, Y.; Miao, A. J.; Yang, L. Y. Cd2+ toxicity as affected by bare TiO2 nanoparticles and their bulk counterpart. Ecotox. Environ. Safe 2012, 85 (1), 44−51. (10) Liu, X.; Li, Z.; Zhang, Q.; Li, F. Controllable synthesis and enhanced photocatalytic properties of Cu2O/Cu31S16 composites. Mater. Res. Bull. 2012, 47 (9), 2631−2637. (11) Geng, J.; Hou, W. H.; Lv, Y. N.; Zhu, J. J.; Chen, H. Y. OneDimensional BiPO4 Nanorods and Two-Dimensional BiOCl Lamellae: Fast Low-Temperature Sonochemical Synthesis, Characterization, and Growth Mechanism. Inorg. Chem. 2005, 44 (23), 8503−8509. (12) Kusainova, A. M.; Lightfoot, P.; Zhou, W.; Stefanovich, S. Y.; Mosunov, A. V.; Dolgikh, V. A. Ferroelectric Properties and Crystal Structure of the Layered Intergrowth Phase Bi3Pb2Nb2O11Cl. Chem. Mater. 2001, 13 (12), 4731−4737. (13) Lin, X. P.; Shan, Z. C.; Li, K. Q.; Wang, W. D.; Yang, J. H.; Huang, F. Q. Photocatalytic activity of a novel Bi-based oxychloride catalyst Na0.5Bi1.5O2Cl. Solid State Sci. 2007, 9 (10), 944−949. (14) Deng, Z. T.; Tang, F. Q.; Muscat, A. J. Strong blue photoluminescence from single-crystalline bismuth oxychloride nanoplates. Nanotechnology 2008, 19 (29), 295705. (15) Jiang, J.; Zhao, K.; Xiao, X. Y.; Zhang, L. Z. Synthesis and FacetDependent Photoreactivity of BiOCl Single-Crystalline Nanosheets. J. Am. Chem. Soc. 2012, 134 (10), 4473−4476. (16) Pare, B.; Sarwan, B.; Jonnalagadda, S. B. The characteristics and photocatalytic activities of BiOCl as highly efficient photocatalyst. J. Mol. Struct. 2012, 1007, 196−202. (17) Xiong, J. Y.; Cheng, G.; Li, G. F.; Qin, F.; Chen, R. Wellcrystallized square-like 2D BiOCl nanoplates: mannitol-assisted hydrothermal synthesis and improved visible-light-driven photocatalytic performance. RSC Adv. 2011, 1 (8), 1542−1553. (18) Lei, Y. Q.; Wang, G. H.; Song, S. Y.; Fan, W. Q.; Zhang, H. J. Synthesis, characterization and assembly of BiOCl nanostructure and their photocatalytic properties. CrystEngComm 2009, 11 (9), 1857− 1862. (19) Zhang, K. L.; Liu, C. M.; Huang, F. Q.; Zheng, C.; Wang, W. D. Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst. Appl. Catal., B 2006, 68 (3−4), 125−129. (20) Chen, F.; Liu, H. Q.; Bagwasi; Segomotso; Shen, X. X.; Zhang, J. L. Photocatalytic study of BiOCl for degradation of organic pollutants under UV irradiation. J. Photochem. Photobiol. A 2010, 215 (1), 76−80. (21) Chai, S. Y.; Kim, Y. J.; Jung, M. H.; Chakraborty, A. K.; Jung, D.; Lee, W. I. Heterojunctioned BiOCl/Bi2O3, a new visible light photocatalyst. J. Catal. 2009, 262 (1), 144−149. (22) Shamaila, S.; Sajjad, A. K. L.; Chen, F.; Zhang, J. WO3/BiOCl, a novel heterojunction as visible light photocatalyst. J. Colloid Interface Sci. 2011, 356 (2), 465−472. (23) Wu, T. X.; Liu, G. M.; Zhao, J. C.; Hidaka, H.; Serpone, N. Photoassisted Degradation of Dye Pollutants. V. Self-Photosensitized Oxidative Transformation of Rhodamine B under Visible Light Irradiation in Aqueous TiO2 Dispersions. J. Phys. Chem. B 1998, 102 (30), 5845−5851. (24) Garcia, J. C.; Oliveira, J. L.; Silva, A. E. C.; Oliveira, C. C.; Nozaki, J.; De Souza, N. E. Decomposition of dinitrotoluene isomers and 2,4,6-trinitrotoluene in spent acid from toluene nitration process by ozonation and photo-ozonation. J. Hazard. Mater. 2007, 147 (1− 2), 105−110. (25) O’Regan, B.; Grätzel, M. A low-cost, high-efficiency solar cell based on dye sensitized colloidal TiO2 films. Nature 1991, 353 (24), 737−740. (26) Gupta, D.; Mukhopadhyay, S.; Narayan, K. S. Fill factor in organic solar cells. Sol. Energy Mater. Sol. Cells 2010, 94 (8), 1309− 1313. (27) Yuhas, B. D.; Yang, P. Nanowire-Based All-Oxide Solar Cells. J. Am. Chem. Soc. 2009, 131 (10), 3756−3761.

Table 3. Effect of Cathode Materials (Pt, Graphite, Ti, and Cu) on J−V Characteristics of BiOCl/Ti PFC and DSPFCa

a

cell

cathode

Jsc (mA·cm−2)

Voc (V)

ff

PFC PFC PFC PFC DSPFC DSPFC DSPFC DSPFC

Pt Graphite Ti Cu Pt Graphite Ti Cu

0.0116 0.0040 0.0013 0.0034 0.0052 0.0020 0.0007 0.0005

0.655 0.414 0.199 0.063 0.639 0.364 0.142 0.024

0.39 0.32 0.37 0.30 0.18 0.39 0.41 0.28

CRB = 10 mg·L−1, Na2SO4 0.05 mol·L−1, pH 2.0.

materials, graphite is also a good choice when considering operating cost and deserves more investigation.

■

ASSOCIATED CONTENT

S Supporting Information *

Preparation and characterization details of the BiOCl/Ti photoanode; the analysis of series resistance (Rs) and shunt resistance (Rsh) in BiOCl/Ti−Pt PFC and DSPFC; the effect of cathode materials on J−V and J−JV plots. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86 21 5474 2817. E-mail address: [email protected] (J. P. Jia). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Project No. 20937003 and 50878126) and Ph.D. Program Foundation of Ministry of Education of China (Project No. 20090073110033).

■

REFERENCES

(1) Du, Z. W.; Li, H. R.; Gu, T. Y. A state of the art review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy. Biotechnol. Adv. 2007, 25 (5), 464−482. (2) Rinaldi, A.; Mecheri, B.; Garavaglia, V.; Licoccia, S.; Nardo, P. D.; Traversa, E. Engineering materials and biology to boost performance of microbial fuel cells: a critical review. Energy Environ. Sci. 2008, 1 (4), 417−429. (3) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schröder, U.; Keller, J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Rabaey, K. Microbial Fuel Cells: Methodology and Technology. Environ. Sci. Technol. 2006, 40 (17), 5181−5192. (4) Ueno, H.; Nemoto, J.; Ohnuki, K.; Horikawa, M.; Hoshino, M.; Kaneko, M. Photoelectrochemical reaction of biomass-related compounds in a biophotochemical cell comprising a nanoporous TiO2 film photoanode and an O2-reducing cathode. J. Appl. Electrochem. 2009, 39 (10), 1897−1905. (5) Daskalaki, V. M.; Kondarides, D. I. Efficient production of hydrogen by photo-induced reforming of glycerol at ambient conditions. Catal. Today 2009, 144 (1−2), 75−80. (6) Lianos, P. Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a photoelectrochemical cell The concept of the Photofuelcell: A review of a re-emerging research field. J. Hazard. Mater. 2011, 185 (2−3), 575−590. (7) Kaneko, M.; Nemoto, J.; Ueno, H.; Gokan, N.; Ohnuki, K.; Horikawa, M.; Saito, R.; Shibata, T. Photoelectrochemical reaction of biomass and bio-related compounds with nanoporous TiO2 film 3496

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497

Environmental Science & Technology

Article

(28) Yang, L.; Zhang, T.; Zhou, H.; Price, S. C.; Wiley, B. J.; You, W. Solution-Processed Flexible Polymer Solar Cells with Silver Nanowire Electrodes. ACS Appl. Mater. Interfaces 2011, 3 (10), 4075−4084. (29) Gupta, D.; Bag, M.; Narayan, K. S. Correlating reduced fill factor in polymer solar cells to contact effects. Appl. Phys. Lett. 2008, 92 (9), 093301. (30) Li, K.; He, Y.; Xu, Y. L.; Wang, Y. L.; Jia, J. P. Degradation of Rhodamine B Using an Unconventional graded Photoelectrode with Wedge Structure. Environ. Sci. Technol. 2011, 45 (17), 7401−7407. (31) Li, K.; Yang, C.; Wang, Y. L.; Jia, J. P.; Xu, Y. L.; He, Y. A highefficient rotating disk photoelectrocatalytic (PEC) reactor with macro light harvesting pyramid-surface electrode. AIChE J. 2012, 58 (8), 2448−2455. (32) Kamat, P. V.; Meisel, D. Nanoparticles in advanced oxidation processes. Curr. Opin. Colloid Interface Sci. 2002, 7 (5−6), 282−287. (33) Rabaey, K.; Clauwaert, P.; Aelterman, P.; Verstraete, W. Tubular Microbial Fuel Cells for Efficient Electricity Generation. Environ. Sci. Technol. 2005, 39 (20), 8077−8082. (34) Cheng, S.; Liu, H.; Logan, B. E. Increased Power Generation in a Continuous Flow MFC with Advective Flow through the Porous Anode and Reduced Electrode Spacing. Environ. Sci. Technol. 2006, 40 (7), 2426−2432. (35) Zamouche, M.; Hamdaoui, O. Sorption of Rhodamine B by cedar cone: effect of pH and ionic strength. Energy Procedia 2012, 18, 1228−1239. (36) Xu, Y. L.; He, Y.; Cao, X. D.; Zhong, D. J.; Jia, J. P. TiO2/Ti Rotating Disk Photoelectrocatalytic (PEC) Reactor: A Combination of Highly Effective Thin-Film PEC and Conventional PEC Processes on a Single Electrode. Environ. Sci. Technol. 2008, 42 (7), 2612−2617.

3497

dx.doi.org/10.1021/es303968n | Environ. Sci. Technol. 2013, 47, 3490−3497