Electricity Generation and Pollutant Degradation Using a Novel

Article pubs.acs.org/est

Electricity Generation and Pollutant Degradation Using a Novel Biocathode Coupled Photoelectrochemical Cell Yue Du, Yujie Feng,* Youpeng Qu, Jia Liu, Nanqi Ren, and Hong Liu State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Harbin 150090, People’s Republic of China S Supporting Information *

ABSTRACT: The photoelectrochemical cell (PEC) is a promising tool for the degradation of organic pollutants and simultaneous electricity recovery, however, current cathode catalysts suffer from high costs and short service lives. Herein, we present a novel biocathode coupled PEC (Bio-PEC) integrating the advantages of photocatalytic anode and biocathode. Electrochemical anodized TiO2 nanotube arrays fabricated on Ti substrate were used as Bio-PEC anodes. Field-emission scanning electron microscope images revealed that the well-aligned TiO2 nanotubes had inner diameters of 60−100 nm and wall-thicknesses of about 5 nm. Linear sweep voltammetry presented the pronounced photocurrent output (325 μA/cm2) under xenon illumination, compared with that under dark conditions. Comparing studies were carried out between the Bio-PEC and PECs with Pt/C cathodes. The results showed that the performance of Pt/C cathodes was closely related with the structure and Pt/C loading amounts of cathodes, while the BioPEC achieved similar methyl orange (MO) decoloration rate (0.0120 min−1) and maximum power density (211.32 mW/m2) to the brush cathode PEC with 50 mg Pt/C loading (Brush-PEC, 50 mg). The fill factors of Bio-PEC and Brush-PEC (50 mg) were 39.87% and 43.06%, respectively. The charge transfer resistance of biocathode was 13.10 Ω, larger than the brush cathode with 50 mg Pt/C (10.68 Ω), but smaller than the brush cathode with 35 mg Pt/C (18.35 Ω), indicating the comparable catalytic activity with Pt/C catalyst. The biocathode was more dependent on the nutrient diffusion, such as nitrogen and inorganic carbon, thus resulting in relatively higher diffusion resistance compared to the brush cathode with 50 mg Pt/C loading that yielded similar MO removal and power output. Considering the performance and cost of PEC system, the biocathode was a promising alternative for the Pt/C catalyst.

1. INTRODUCTION Energy crisis and environmental pollution are two hurdles to achieving sustainable development of human society in the 21st century. The photoelectrochemical cell (PEC) is a promising technology to address both issues and has been utilized in environmental remediation,1 solar energy conversion,2 water splitting for hydrogen production,3 and so forth. A TiO2-based PEC system is attractive due to its unselective oxidation for almost all organic compounds. For example, medical components; organics in petroleum chemical engineering; dye molecular,4,5 biomass, or biorelated compounds;6−9 and aromatic amines10 have been proven to be well decomposed. The catalysts are crucial in a PEC cathode to achieve high pollutant removal efficiency and energy recovery. Currently, platinum is one of the most popular and crucial catalysts in the field of fuel cells,7,8 but suffers from high cost and short service life. Alternatively, transition metals, such as Co-, Fe-, and Mnbased catalysts, have also been investigated as redox reaction catalysts in the area electrocatalysis. Bashyam demonstrated that a Co−polypyrrole composite catalyst produced a power density of ∼0.15 W/cm2 in a H2O2 fuel cell and displayed stable performance for more than 100 h.11 FeN, FeC, and © 2014 American Chemical Society

FeCN catalysts were also reported with high reaction activity for oxygen reduction in acidic medium.12,13 Nevertheless, some disadvantages seriously impede the applications of metal-based catalysts, such as potential pollution to environment and performance deterioration due to the loss, agglomeration, and poisoning during long-term operation.14 Chen et al. innovatively introduced a photocatalytic Cu2O/Cu cathode to a visible-light responsive PEC.15 On the basis of the Fermi-level mismatch between photoanode WO3/W and photocathode Cu2O/Cu, the photogenerated electrons at the photoanode were driven to transfer to the photocathode through an external circuit, endowing organics decomposition by the anode and hydrogen generation by the cathode. The microbial-catalyzed cathode (biocathode) is definitely a different way to perform cathode reactions without the problems mentioned above. In a biocathode-coupled system, electrochemically active microbial strains accumulate and attach to the cathode surface to uptake the electrons transferred from Received: July 30, 2013 Accepted: May 27, 2014 Published: May 27, 2014 7634

dx.doi.org/10.1021/es5011994 | Environ. Sci. Technol. 2014, 48, 7634−7641

Environmental Science & Technology

Article

transmission of UV light (Supporting Information (SI) Figure S2). The photoanode chamber and cathode chamber were both 4 cm long by 3 cm in diameter and separated by cation exchange membrane. A 150 W xenon lamp (GY-10A, Tuopu Co. Ltd., China) was used as a light source and installed 10 cm away from Bio-PEC reactor. Methyl orange (5 mg/L) was used as a model pollutant in the PEC anode chamber. Na2SO4 (0.05 mol/L) was used as supporting electrolyte. Well-cultured biocathode in a microbial fuel cell as described in the SI (SI Figure S1) was installed into a cathode chamber to construct the biocathode-coupled PEC. The cathode solution for all the PEC tests was composed of NaHCO3 (1.0 g/L), NH4Cl (0.3 g/L), phosphate buffered solution (KCl 0.13 g/L, NaH2PO4· 2H2O 3.32 g/L, and Na2HPO4·12H2O 10.32 g/L), vitamins and trace minerals. The cathode solution was aerated with a pump in all experiments. The cathode part of PEC was wrapped by aluminum foil to shield the UV−visible light. The external resistance was kept at 1000 Ω for all experiments. The anolyte was stirred by magnetic bar in the dark for 20 min, to ensure adsorption−desorption equilibrium prior to irradiation, and continuously stirred during the experiment. The Pt/C coated cathodes were used in the experiments for comparison. For conciseness, the PEC systems with different cathodes were denoted as follows: Bio-PEC, Brush-PEC (0 mg), Brush-PEC (35 mg), Brush-PEC (50 mg), Brush-PEC (75 mg), Brush-PEC (100 mg), and Cloth-PEC (35 mg), where “Bio” indicated the biocathode, “ Brush” indicated the carbon brush cathode, “Cloth” indicated the carbon cloth cathode, and the number in the parentheses indicated the amount of Pt/C catalyst. 2.4. Analysis and Calculations. The output voltages (V) of MFC and PECs were recorded with a data acquisition board every 1 min. The polarization curves of PECs were carried out by recording the current response to a linear potential decrease imposed to the PECs at a scanning rate 10 mV/s using an electrochemical workstation (Metrohm Autolab 85061). Power density was calculated by normalizing the power (P = IV) by the surface area of the anode. Electrons harvested through external circuit (Q) was determined by integrating the current over time (Q = ∫ I dt). The performance of PECs was evaluated in term of fill factor (FF), which was defined as the ratio of maximum obtainable power output to the product of open-circuit voltage and shortcircuit current as described in eq 1:

the anodes to complete the whole electrochemical reaction. The terminal electron acceptors in the biocathode system can be O2, NO3−, SO42−, MnO2, Fe3+, CO2, and so forth. Compared with abiotic cathodes, biocathodes are more environmentally friendly, cost-effective, and self-sustained. Moreover, the metabolism of the biofilm in biocathodes can be utilized for environmental remediation, carbon dioxide capture, or biofuel production. Even though the concept of biocathodes was first proposed by Lewis in early 1960s,18 most recent research has only focused on microbial fuel cells.16 Clauwaert reported for the first time a complete denitrification biocathode, independent of H2-formation and energy input in the MFC system.17 Puig demonstrated nitrate removal in drinking water with low ion strength under the action of an autotrophic denitrification biofilm inoculated in the cathode surface.18 Cao demonstrated successful CO2 sequestration using a photobiocathode in a completely anoxic microbial fuel cell, utilizing sunlight as the driving force.19 In our lab, we designed a microbial carbon capture cell (MCC) with an algae (Chlorella vulgaris) cathode, which was efficient to convert CO2 to biomass and oxygen through the photosynthesis process.20 The system was of significance to reduce CO2 emission in municipal wastewater treatment plants. Can we employ the advantages of biocathodes into PECs to enhance the performance and avoid the problems of abiotic cathode-coupled PECs? Herein, we report a new PEC system coupled with a biocathode for organic degradation and electricity generation. Comparative studies were carried out between the biocathode-coupled PEC (Bio-PEC) and PECs with Pt/C cathodes, both with carbon brush and carbon cloth as carriers.

2. MATERIALS AND METHODS 2.1. Preparation of Pt/C Coated Cathodes. The Pt/C coated cathodes were prepared by loading the commercial Pt/C catalysts (10 wt % Pt/C, Hesen Co. Ltd., China) to carbon cloth and carbon brushes. For the Pt/C coated carbon cloth, the Pt/C catalyst was mixed homogeneously with Nafion (0.67 uL/mg Pt/C) and isopropanol (0.33 uL/mg), as described by Cheng.21 The paste was then applied to one side of a wetproofed carbon cloth (30 wt %, E-Tek, U.S.A.). Each Pt/C coated carbon cloth had a projected surface area of 7 cm2 and total amount of Pt/C 35 mg. For the preparation of Pt/C coated carbon brushes, different amounts of Pt/C catalyst (35, 50, 75, and 100 mg) were accurately weighed and mixed with Nafion solution (3 uL/mg Pt/C) and isopropanol (5 mL). The whole mixture was thoroughly blended using a ultrasonic vibration generator for 30 min. The heat-treated carbon brush22 was then impregnated into the solution for 10 min and taken out to dry at room temperature. The impregnation and drying process were repeated until all the Pt/C adhered to the carbon brush. The Pt/C coated carbon brushes were placed at room temperature overnight to allow the complete evaporation of isopropanol. 2.2. Preparation of TiO2 Photoelectrodes. Titanium sheets with diameters of 2.5 cm were cleaned by sonication in chloroform, acetone, and distilled water in sequence for 15 min each. Electrochemical anodization of titanium sheets was carried out at a voltage of 20 V for 30 min by a DC power supply. After being dried in ambient temperature overnight, the TiO2 electrodes were annealed at 450 °C for 3 h. 2.3. Bio-PEC Configuration and Operation. Bio-PEC was made of plexiglass with a quartz window to allow the

FF =

VmaxImax VocIsc

(1)

where Vmax and Imax were the maximum voltage and current obtained for PEC, respectively; Voc and Isc the open-circuit voltage and short-circuit current of PEC, respectively. The photocurrent−voltage (I−V) characteristics of TiO2 electrode were studied using an electrochemical analyzer (Wonatech WMPG 1000, Korea) with the scanning rate 10 mV/s in a three-electrode system, using a Pt sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The surface morphology of TiO2 electrode was observed by a field-emission scanning electron microscope (SEM, FEI Quanta 200F). Electrochemical impedance spectroscopy (EIS) of cathodes was conducted at polarized potential (0.1 V vs SCE) in a threeelectrode system with TiO2 as counter electrode and SCE as reference electrode using an electrochemical workstation 7635



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Figure 1. (A) Top-view SEM image of TiO2 nanotube arrays growing on the Ti substrate; (B) linear sweep voltammetric curves of TiO2 electrodes at the scan rate 10 mV/s in three-electrode system with and without illumination.

(Metrohm Autolab 85061). The frequency range was from 100 kHz to 10 mHz. EIS data were fitted and simulated by ZSimpWin3.10 software (Echem., U.S.A.) based on the equivalent electrical circuit.23 The absorption spectra and contents of MO solution were analyzed using UV−vis spectrophotometry (TU-1810). About 4 mL anode solution was quickly taken out at fixed reaction internals for analysis, and then injected back to the reactor shortly after the measurement. The intensity of the characteristic peak of MO centered at 464 nm was measured to evaluate the concentration variation. The change in the total organic carbon (TOC) of MO solution after degradation was tested using TOC analyzer (TOC 2500, Shimadou, Japan).

3. RESULTS 3.1. Characterization of TiO2 Electrodes. TiO2 electrodes prepared by an electrochemical anodization method displayed a well-aligned tube-like morphology, with inner diameters of 60−100 nm and wall-thickness of ∼5 nm (Figure 1A). The in situ formed TiO2 nanotube array on Ti substrate was ideal for increasing the mechanical stability and avoiding the loss of photocatalysts during long-term operation.24 A Schottky-type contact naturally formed between TiO2 nanotubes and the Ti matrix can also provide a unidirectional electric channel for photogenerated electron transport.25 LSV results showed the photoelectrochemical properties of TiO2 electrodes in terms of photocurrent response (Figure 1B). The current density was negligible in the absence of light irradiation. When the cell was irradiated under xenon light, the photocurrent evidently increased with the increase of external bias. A maximal current density of 325 μA/cm2 was achieved at an applied bias of 1.0 V, suggesting that positive bias facilitated the separation of photogenerated charges. The photocurrent response under irradiation suggested that the TiO2 electrodes possessed considerable electron transfer properties. 3.2. Performance of PEC Systems. 3.2.1. Degradation of Methyl Orange (MO). The UV−vis absorption spectra of MO solution were recorded under wavelengths from 200 to 800 nm during the PEC operation (Figure 2A). The absorption peak of MO at 464 nm decreased as a function of time, confirming the rapid degradation of MO. New absorbance peaks that occurred ranging from 210 to 250 nm indicated the formation of small molecular intermediate products.26 A comparative study on the photoelectrochemical and photocatalytic degradation of methyl orange (MO) in PEC systems with various cathodes was carried out (Figure 2B). The

Figure 2. (A) UV−vis spectra of a typical degradation process; (B) methyl orange degradation processes of Bio-PEC, Cloth-PEC, and Brush-PECs with different loadings of Pt/C.

linear relationship of ln(Ct/C0) vs t showed pseudo-first-order kinetics of MO degradation in different systems with normalized R2 ranging from 0.9203 to 0.9990 (Table 1). The photocatalytic degradation of MO under open-circuit conditions can be negligible, with a k of 0.0009 min−1 and an MO decoloration ratio of only 17.89% after 4 h. With closed circuit, the degradation rate was significantly enhanced, even with the utilization of brush cathode (0 mg) (k = 0.0025 min−1 and MO decoloration ratio of 35.33%). The MO decoloration of PECs with Pt/C cathodes was related with the Pt/C amounts and the configuration of carriers. With the same Pt/C loading, the Brush-PEC (35 mg) had higher MO decoloration rate (k = 0.0070 min−1) than that Cloth-PEC (35 mg) (k = 0.0058 7636



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Table 1. Pseudo-First-Order Kinetics Constants, Regression Coefficients and MO Decoloration Ratios of Bio-PEC, Cloth-PEC, and Brush-PECs with Different Loadings of Pt/ C PEC system open circuit Brush-PEC (0 mg) Cloth-PEC (35 mg) Brush-PEC (35 mg) Bio-PEC Brush-PEC (50 mg) Brush-PEC (75 mg) Brush-PEC (100 mg)

initial MO (mg/L)

k (min−1)

R2

decoloration ratio (%)

4.955 ± 0.018 4.936 ± 0.028

0.0009 0.0025

0.9203 0.9854

17.89 ± 0.13 35.33 ± 0.41

4.942 ± 0.016

0.0058

0.9924

62.73 ± 0.22

4.864 ± 0.013

0.0070

0.9990

81.77 ± 0.38

4.812 ± 0.014 5.109 ± 0.022

0.0120 0.0124

0.9807 0.9884

88.31 ± 0.26 95.41 ± 0.19

5.113 ± 0.019

0.0160

0.9697

98.21 ± 0.33

5.098 ± 0.020

0.0160

0.9845

97.96 ± 0.21

min−1). When the Pt/C amounts of carbon brushes increased from 35 to 75 mg, the MO decoloration rate increased from 0.0070 to 0.0160 min−1, but further increase the Pt/C amount to 100 mg contributed less to the improvement of MO decoloration rate (k = 0.0016 min−1). The MO decoloration rate of Bio-PEC was 0.0120 min−1, which was comparable with that of Brush-PEC (50 mg). Since photoanodes for PECs were identical, demonstrated by LSV curves (SI Figure S3), the performance differences of PECs should be attributed to biocathode performances, which provided an electron migration pathway under internal bias between the photoanode and cathode. With the higher cathode potential, the electrons of the photoanode tended to separate with holes and travel to the cathode more easily, resulting in a higher efficiency of the MO degradation. 3.2.2. Power Generation of PECs. The voltage and power output of PECs showed similar tendency as MO decoloration (Figure 3). The PEC system with faster MO degradation rate also possessed higher energy recovery efficiency. The BrushPEC (0 mg) merely produced voltages of ∼50 mV and a maximum power density of 8.82 mW/m2, indicating poor catalytic activity of the carbon brush without the assistance of catalyst. The Brush-PEC (35 mg) yielded a higher voltage and electricity generation than the Cloth-PEC (35 mg) due to larger active surface area. The Pt/C amounts played an important role in the power output of PECs with brush cathodes. With the increase of Pt/C from 35 to 75 mg, the maximum power density increased from 187.55 to 238.63 mW/ m2. However, further increase in the Pt/C amount to 100 mg, the maximum power density slightly dropped to 228.45 mW/ m2. The voltage generation by Bio-PEC was 250−290 mV, which was about 10 mV lower than the Brush-PEC (50 mg) (see Figure 3A). The maximum power density of Bio-PEC was 211.32 mW/m2, also comparable to that of Brush-PEC (50 mg) (218.40 mW/m2). The time-dependent voltage outputs changed in a similar tendency for all the PECs, e.g., an initial rapid decrease in the first 1 h, followed by a gradual decrease, which is clarified in detail in section 3.4. We also calculated the fill factor (FF) of the three systems (Table 2). The FF of Bio-PEC (39.87%) was slightly lower than that of Brush-PEC (50 mg) (43.06%). In PEC systems, the potential difference between the photoanode and cathode promoted the separation of photo-

Figure 3. (A) Voltage generation, (B) power generation, and (C) polarization curves of Bio-PEC, Cloth-PEC, and Brush-PECs with different loadings of Pt/C.

generated hole/electron pairs and migration of electrons from the anode to the cathode. The degradation rate of MO, voltage, and power generation were all dependent on the activity of cathodes. With a higher cathode reaction rate, the separation efficiency was higher, resulting in a higher current through the external circuit and more holes or •OH radicals in the cathode. The performance studies of PECs in MO decoloration, power generation, and FF went to the same conclusion that the biocathode possessed a comparable catalytic activity with Pt/C cathode and fit well with the PEC system. 3.3. EIS Analysis. Electrochemical impedance spectroscopy (EIS) was considered as a powerful method to investigate chemical and physical processes in solutions as well as in solids. EIS analysis for cathodes was conducted in a three-electrode system. By fitting the Nyquist plots (Figure 4B) to a suitable equivalent electrical circuit (Figure 4A) using a nonlinear leastsquares procedure, the internal resistance (Rint) distribution of PEC systems was obtained (Table 3). The Nyquist plots of biocathode and Pt/C cathodes turned out similar patterns: a 7637



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Table 2. Performance Comparison of Bio-PEC, Cloth-PEC, and Brush-PECs with Different Loadings of Pt/C Pmax (mW/m2)

PEC system Brush-PEC (0 mg) Cloth-PEC (35 mg) Brush-PEC (35 mg) Bio-PEC Brush-PEC (50 mg) Brush-PEC (75 mg) Brush-PEC (100 mg)

8.82 146.16 187.55 211.32 218.40 238.63 228.45

± ± ± ± ± ± ±

Voc (V)

1.38 3.04 4.61 3.54 5.46 4.90 3.23

0.26 0.65 0.68 0.74 0.69 0.71 0.69

± ± ± ± ± ± ±

0.02 0.04 0.02 0.03 0.04 0.04 0.05

Isc (mA/m2)

FF (%)

± ± ± ± ± ± ±

28.17 39.78 39.57 39.87 43.06 44.93 44.68

120.41 565.31 697.00 716.33 735.21 748.02 741.73

21.93 34.62 25.33 36.70 31.22 41.10 36.21

Q (C) 0.75 3.28 3.50 3.84 3.82 4.25 4.30

± ± ± ± ± ± ±

0.03 0.07 0.03 0.08 0.04 0.06 0.07

Figure 4. (A) Equivalent electrical circuit for EIS analysis, where Rohm, Rc, and Rd were denoted as ohmic resistance, charge-transfer resistance, and diffusion resistance of the cathodes, respectively; Qc and Qd indicated the double-layer capacity related to charge transfer and mass diffusion process; (B) Nyquist plots of biocathode, carbon cloth cathode, and carbon brush cathodes with different Pt/C loadings (inset of B, the Nyquist plot of carbon brush cathode without Pt/C coating).

Ω, lower than the brush cathode with Pt/C of 35 mg (18.35 Ω) and slightly higher than the brush cathode with Pt/C of 50 mg (10.68 Ω), indicating the comparable catalytic activity with Pt/ C cathodes. The diffusion resistance (Rd) accounted for a large fraction of the internal resistance (Rint), suggesting the limitation of oxygen and proton diffusion to the cathode surface. The Rd of the biocathode was 45.77 Ω, larger than that of Pt/C coated brush cathodes, since the growth and activity of the biocathode were more dependent on the supply of substrates, such as NaHCO3, NH4Cl, O2, and so forth. Rd can be further decreased by enhancement of the mass transfer process. The ohmic resistance (Rohm) was accounted for a small fraction of the Rint. The EIS results confirmed that the biocathode had comparable catalytic activity with the Pt/C cathode, but relied on more closely to the substrate diffusion. 3.4. Long-Time Performance of Bio-PEC. To investigate the sustainability, the Bio-PEC was operated in batch mode for about 48 h, and the voltage, anode, and cathode potential (vs SCE) were recorded as a function of time (Figure 5). The continuous voltage generation was obtained. The refreshment of both anode and cathode solution resulted in the change of anode and cathode potential. The nutrients in the cathode solution were essential for the metabolic and catalytic activity of microorganisms. Soon after the sluggish time due to the replacement of cathode solution, the cathode potential increased and remained stable for hours, indicating that the biocathode was efficient and reproducible for fuel cells. Different from the cathode potential, the anode potential went up quickly in the first half hour after the replacement of the anode solution, followed by gradual increase with time. The rapid increase of the anode potential was probably due to the sharp decrease of the anode pH, which dropped from 6.86 to

Table 3. Internal Resistance Distribution of Biocathode, Carbon Cloth Cathode and Carbon Brush Cathodes with Different Pt/C Loadings cathode

Rohm (Ω)

Rc (Ω)

Rd (Ω)

Rint (Ω)

Brush, Pt/C 0 mg Cloth, Pt/C 35 mg Brush, Pt/C 35 mg Biocathode Brush, Pt/C 50 mg Brush, Pt/C 75 mg Brush, Pt/C 100 mg

7.51 4.37 4.16 3.59 4.90 4.90

26.09 18.35 13.10 10.68 7.69 8.70

69.20 42.60 45.77 23.54 18.40 21.50

205.30 102.80 65.32 63.03 37.81 30.99 35.10

semicircle arc at high frequency corresponding to the charge transfer resistance (Rc) dominant kinetics at the electrode interface, and a straight sloping line correlated to the diffusion process between the electrode and the electrolyte at low frequency. The diameter of the kinetic loop, Rc, is a fairly good indicator of the catalytic property of cathodes, such as catalyst surface area, catalytic loading, and catalyst utilization.27 The Rc of a two-dimensional carbon cloth cathode was 26.09 Ω, 42% larger than the brush cathode with the same Pt/C loading amount (18.35 Ω). With Pt/C amounts increasing from 35 to 75 mg, the Rc decreased from 18.35 to 7.69 Ω. By contrast, with a further increase of the Pt/C amount to 100 mg, the Rc slightly increased to 8.70 Ω. The activity deterioration of the brush cathode with 100 mg Pt/C was probably because the excess Pt/ C and binder influenced the dispersion of carbon fibers and thus resulted in a less active surface area. There was an optimized Pt/C amount for the cathode, considering the catalytic activity and reasonable cost, which was also reported by other researchers.28,29 The Rc of the biocathode was 13.10 7638



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photocatalytic degradation of MO, the •OH radicals or holes attacked the azo band in the MO molecule and broke down the N−N bond first, and then these molecules were degraded to smaller molecules and finally to CO2 and H2O.26 The total organic carbon (TOC) of MO solution after the degradation of Bio-PEC was 0.7809 ± 0.0267 mg/L and the mineralization of MO was 65.95 ± 1.25%. Acidic compounds, such as paminobenzenesulfonic acid, aliphatic acid, and carboxylic acid were produced during the MO decomposition.26 With the consumption of OH− by the holes and the production of acidic compounds, the proton concentration of the anode solution increased (SI Figure S5), thus the protons may transport to the cathode chamber through a cation exchange membrane and react with oxygen. Figure 5. Voltage, anode, and cathode potential (vs SCE) of Bio-PEC in a batch fed mode, showing the sustainable voltage generation (the vertical blue and red arrows indicate the replacement of the cathode and anode solutions, respectively).

+ TiO2 + hv → hvb + e−

(2)

+ + hvb + H 2O → •OH + H+ or hvb + OH− → •OH

(3)

C14 H14N3NaO3S + 38•OH → 14CO2 + 3NO−3 + Na +

4.12 in half an hour (SI Figure S5), since the protons in the anode solution reacted with photo-generated electrons and thus elevated the anode potential. MO in the anode, which captured the holes and •OH radicals, also caused an increase of the anode potential with a decrease in the MO concentration during the PEC operation. The voltage output of Bio-PEC indicated that the biocathode can contribute stable cathode potential to the whole cell and fit well as a cathode catalyst.

+ SO24 − + 52H+

4H+ + O2 + 4e− → 2H 2O

(4) (5)

nitrifying bacteria

2NH4 + + 4O2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2NO−3 + 2H 2O + 4H+

(6)

In the cathode of PECs, oxygen as terminal electron acceptors was reduced to H2O by accepting the electrons from cathode with assistance of either Pt/C or microorganism (eq 5). In the Bio-PEC (Figure 6), extracellular electron transfer was involved in cathode reaction. The C-type cytochrome of selected electrochemical active bacteria in biocathodes can uptake electrons from a solid electrode and pass electrons to more-positive electron acceptors (O2 in our work) within the periplasm and inner membrane.30 Moreover, nitrification of ammonia nitrogen occurred simultaneously in the cathode chamber (eq 6). Within 4 h of Bio-PEC operation, almost 34% of the NH4+N in the medium (78 mg/L) was oxidized to NO3−N (26.9 mg/L) (SI Figure S4). The nitrification reaction and O2 reduction reaction in biocathode chamber showed synergistic and competition interaction as well. The catalytic activity of oxygen reduction was strongly related to the microbial activity: with suitable culture condition, such as nutrients and oxygen, the metabolism and multiplication of bacteria were faster, thus resulting in higher catalytic efficiency. Besides, the oxygen reduction of the cathode in turn helped to maintain the proper pH value and alkalinity that were required by the nitrification reaction (data not shown here). The competition for oxygen also existed in the biocathode, but with the DO concentration ca. 5 mg/L in our work, oxygen was sufficient for both the cathodic reaction and nitrification. As biocathode microbes can also obtain electrons from NH4+ oxidation apart from the external circuit, there might be a “co-metabolism” of electrons in Bio-PEC, which can be described as electron-consuming and ammoniaoxidizing under oxygen atmosphere in a biocathode. The process of the biocathode was very attractive because the biocathode was no longer a passive electron consumer, but also converted ammonium nitrogen existing in water. Yet in the cathode chambers of the abiotic cathodes, there was no NO3−N detected during PECs operation. In this study, by coupling a microbial cathode with the photocatalytic anode, a novel biocathode coupled PEC (Bio-

4. DISCUSSION The system of Bio-PEC involving the photocatalytic process in the anode chamber and a biological conversion in the cathode chamber is shown in Figure 6. The half reactions in the

Figure 6. Schematics showing the working principles of Bio-PEC system.

photoanode chamber are illustrated as follows: With xenon illumination, the photogenerated electrons of TiO2 electrodes were excited from the valence band to the conduction band, yielding positive holes in the valence band and negative electrons in the conduction band (eq 2). Photogenerated electrons traveled through the interface of the TiO2 electrode to the cathode through an external circuit, while the holes were revolved in the oxidation reaction of the organic substrates (eq 4), either directly or via •OH radicals formed by the reaction with OH− and/or H2O in the solution (eq 3). During the 7639



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PEC) was constructed and evaluated in terms of power generation and pollutant degradation. The results demonstrated that the biocathode was a promising alternative for PEC catalysts: (1) the biocathode was cost-effective to construct and operate; (2) the biocathode boosted the separation of photogenerated hole/electron pairs by the voltage difference between anode and cathode; and (3) efficient nitrification took place in the biocathode chamber, endowing excess function compared with noble abiotic catalysts. Actually, this work was the first attempt for Bio-PEC application, and further study is required to optimize the system. To achieve effective transmission of UV light through wastewater to the surface of photocatalytic materials, the system should be optimized, such as the intensity and distance of light source, the method of irradiation, and the reactor configuration. The photoanode, which was adept at the treatment of toxic and recalcitrant compounds, and biocathode, more suitable for biocompatibility matters, should be connected together. The effluent of the anode should be further treated by biocathode, and the nitrate in the biocathode should also be denitrified to nitrogen, so the function of the biocathode in the aspect of simultaneous carbon removal, nitrification, and denitrification should be further explored.

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ASSOCIATED CONTENT

S Supporting Information *

The culture process of the biocathode in the microbial fuel cell, digital photographs of MFC and PEC reactors, LSV curves of TiO2 electrodes, and changes of nitrate concentration in the biocathode chamber and pH values of the anode solution during Bio-PEC operation as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 86-451-86287017; fax: 86-451-86287017; e-mail: yujief@ hit.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Grant No. 2013DX08), the National Natural Science Fund for Distinguished Young Scholars (Grant No. 51125033), and National Natural Science Fund of China (Grant No. 51209061). The authors also acknowledge support from the Creative Research Groups of China (Grant no. 51121062). The authors acknowledge discussion with Dr. Xin Wang and Dr. Chengyan Xu, and thank Mr. Baoyou Zhang for assistance with the SEM operation.

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REFERENCES

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