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Enhanced oxygen and hydroxide transport in cathode interface by efficiently antibacterial property of silver nanoparticle modified activated carbon cathode in microbial fuel cells Da Li, Youpeng Qu, Jia Liu, Guohong Liu, and Yujie Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06419 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 27, 2016

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ACS Applied Materials & Interfaces

Enhanced oxygen and hydroxide transport in cathode interface by efficiently antibacterial property of silver nanoparticle modified activated carbon cathode in microbial fuel cells Da Li,1 Youpeng Qu,2 Jia Liu,1* Guohong Liu,1Yujie Feng 1*

1

State Key Laboratoty of Uran Water Resource and Environment, Harbin Institute of Technology. No 73 Huanghe Road, Nangang District, Harbin 150090, China 2

School of Life Science and Technology, Harbin Institute of Technology. No. 2 Yikuang Street, Nangang District, Harbin 150080, China *Corresponding Author: E-mail: [email protected] *Co-Corresponding Author: E-mail: [email protected]

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ABSTRACT The growing biofilm on air cathode was responsible for the decreased performances of microbial fuel cells (MFCs). To minimize the undesired biofilm, silver nanoparticles were synthesized on activated carbon as cathodic catalyst (Ag/AC) in MFCs. Ag/AC enhanced maximum power density by 14.6% compared to bare activated carbon cathode (AC) due to the additional silver catalysis. After operating MFCs over 5 months, protein content on Ag/AC cathode was only 38.3% of that on AC cathode, which resulted in higher oxygen concentration diffused through Ag/AC cathode. In addition, lower pH increment (0.2 units) was obtained near Ag/AC catalyst surface after biofouling compared to 0.8 units of AC cathode, indicating less biofilm on Ag/AC cathode had a minor resistance on hydroxide transported from catalyst layer interfaces to bulk solution. Therefore, less decrements of Ag/AC activity and MFC performance were obtained. This result indicated acceleration transport of oxygen and hydroxide benefited from antibacterial property of cathode would be efficient to maintain higher cathode stability during long-term operation.

KEYWORDS: microbial fuel cell, silver nanoparticles, antibacterial activity, oxygen and hydroxide transport, cathode interface


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1 INTRODUCTION Microbial fuel cells (MFCs) as an alternative technology for wastewater treatment combined advantage of energy recovery by exoelectrogenic bacteria at the same time. 1-2

Up to now, air cathode using oxygen as the electron acceptor was the most popular

cathode structure due to the higher activity, and catalysts towards oxygen reduction reaction (ORR) have acquired widely research.

3-4

More and more studies have

demonstrated the feasibility of carbon based catalysts in MFCs,

5-9

among which

activated carbon acquired a comparable performance to Pt/C but with even lower cost. 10-11

Activated carbon cathodes were generally prepared by press method which

constructed porous structure inside the catalyst layer. However, just the porous structure provided large specific surface area for biofilm development which caused severe polarization loss as described in previous research. 12-13

Biofilm colonization on catalyst layer was inevitable due to direct exposure to bulk solution. However, the undesirable situation could be reduced by incorporating effectively antibacterial chemicals with cathode. Activated carbon mixed with quaternary ammonium effectively decreased protein content of cathode by 96.2%, which caused only 21% decrement of power density compared to 31% of control.

14

Vulcan XC-72R with a mixed binder of polyvinyl alcohol (PVA) and vanillin (4-hydroxy-3-methoxybenzaldehyde) as cathode generated the higher power density which was 2 times than that of cathode with only PVA binder after operation for 5 weeks, and this was mainly because vanillin inhibited biofilm formation.

15

Recently,

carbon cathode treated with 20 mg enrofloxacin declined the biomass content by



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60.2%, therefore caused a quite stable power output compared to a 22.5% decrement of bare carbon cathode. 16

Silver as a broad spectrum of antimicrobial agent was used for medicine, purification

18

and hazardous gaseous removal.

19

17

water

It was reported that silver

nanoparticles alone or mixed with Pt/C as catalyst could effectively inhibited heterotrophic microbes in MFCs,

20-22

which was beneficial to higher dissolved

oxygen (DO) and cathode performance. In addition, as one of product of ORR, hydroxide was accumulated within the catalyst layer, surface biofilm would impose additional resistance for hydroxide transport from reaction sites to bulk solution, this would cause more cathode overpotential which further depressed cathode reaction.

23

Therefore, it was necessary to investigate the effect of biofilm on hydroxide transport. Although transport resistance of hydroxide in Pt/C cathode was detected especially with biofilm,

24

the structure and property of activated carbon based cathode were

distinctly different from Pt/C cathode. Therefore, little was known about how the biofilm affected the rate of hydroxide transport from the interface of cathode to bulk solution.

In this work, silver nanoparticles were synthesized on activated carbon powders (denoted as Ag/AC). The morphologies and electrochemical activities of Ag/AC were characterized. By comparison the protein content of biofilm on Ag/AC and bare activated carbon (AC) cathodes, the antibacterial property of the catalysts was evaluated. The variation of oxygen concentration through cathodes, and the hydroxide transport from the cathode surface to the bulk solution were studied to evaluate the


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effect of antibacterial activity on cathode performances.

2 EXPERIMENTAL SECTION 2.1 Catalyst Synthesis and Cathode Preparation. Silver nanoparticles loaded on activated carbon powders (Ag/AC) were prepared by in situ reduction method as described. 25 Briefly, 6 g AC powders (Carbosino Material Co., Ltd, Shanghai, China) and 100 mL deionized water were dispersed homogeneously in 3-neck boiling flask with the ice/water bath, then 100 mL of 62 mmol L-1 AgNO3 solution and 100 mL of 62 mmol L-1 sodium citrate solution were added into flask with drastically stirring. Later, 50 mL of 713 mmol L-1 NaBH4 solution was added dropwise into the above solution to trigger the reduction reaction. After continuously stirred the suspension for 5 h and aged for 19 h at room temperature, the suspension was filtered, washed and dried at 80 oC. Air cathodes were prepared by rolling press method as described. 11 Briefly, Ag/AC catalysts mixed with PTFE binder (60 wt %, Hesen, Electrical Co., Ltd, Shanghai, China) at mass ratio of 6:1 were stirred thoroughly with ethanol and then rolled to be catalyst layer, followed by pressed on one side of stainless steel mesh (60-mesh). Diffusion layer was prepared by carbon black (Hesen Electrical Co., Ltd, Shanghai, China) and PTFE with mass ratio of 3:7, after calcining diffusion layer at 340 °C for 30 minutes, the diffusion layer was pressed on another side of stainless steel mesh. As control cathode, AC powders as catalyst were used to prepare cathode with the same process. 2.2 Physical Characterization



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The structure and morphology of Ag/AC catalyst were characterized by scanning electron microscope (SEM, S-4700, Hitachi Ltd. ), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, H-7650, Hitachi Ltd.). X-ray diffraction (XRD) analysis was used to characterize crystal structure and performed with an X-ray diffractometer (λ= 0.154 nm) using Cu Kα source (40 keV and 30 mA), the 2θ ranged from 10° to 90°. The specific surface area of catalyst sample was identified by nitrogen adsorption measurement at 77 K using Brunauer-Emmett-Teller (BET) model, and pore distribution was determined using the Barett-Joyner-Halenda (BJH) model. For catalyst layer, the element dispersion was evaluated by X-ray photoelectron spectroscopy (XPS) with employing a monochromatic Al Kα radiation at 1486.6 eV (XPS, PH1-5700 ESCA system, US). The silver content of catalyst was measured using Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, PerkinElmer, Optima 8300). 2.3 Electrochemical Characterization Rotating disc electrode (RDE) tests were conducted to evaluate the kinetic activity of the catalysts. A glass carbon tip was firstly polished with alumina slurry for use. Catalyst solution was prepared by mixing 10 mg catalyst samples with 60 µL of deionized water and 40 µL of Nafion (10 wt.%), followed by ultrasonic treatment for 15 min. Then the catalyst ink was dropped on the glassy carbon electrode at a loading 5 mg cm–2, and dried in air.

A typical three-electrode system was used, with glassy carbon electrode as working electrode, a platinum sheet (1 cm2) as counter electrode and a Ag/AgCl (saturated KCl,


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+197 mV versus standard hydrogen electrode; SHE) reference electrode. Before the tests, 50 mM PBS was sparged with oxygen for 30 min, and cyclic voltammetry was operated firstly at 10 mV s–1 between 0.5 V and – 0.8 V vs Ag/AgCl until similar current output.

26

The RDE tests were then carried out at rotation rates of 100 ~ 2500

rpm with oxygen flow. The electron-transfer number and kinetic current for ORR were calculated based on Kouteckye-Levich (K-L) plot: 27 1 1 1 = + ⁄ 0.62 ⁄ ⁄ where ik is the kinetic current (A), F is Faradaic constant (96485 C mol–1), A is the effective projected area of the disk electrode (0.071 cm2), D is the diffusion coefficient of oxygen (2.7 × 10–5 cm2 s–1), v is the kinematic viscosity (8.08 × 10–3 cm2 s–1), C is the concentration of oxygen in the solution (2.3 × 10–7 mol cm3), and ω is the rotation rate of the electrode.

Tafel plot was recorded in 50 mM PBS with the three-electrode system by sweeping the overpotential (|η|) from 0 mV to 100 mV at 1 mV s–1, where η = 0 is the open circuit potential of the cathode versus Ag/AgCl reference electrode. 28 Exchange current density (i0, A cm–2) was calculated based on Tafel plots with overpotential of 80 ~ 100 mV, the y-axis intercept in Tafel plots was the logarithm of the exchange current density based on simplified Butler-Volmer equation.

29

EIS was conducted at

constant potential of – 0.1 V vs Ag/AgCl over a frequency range of 100 kHz to 10 mHz with the amplitude of 10 mV. 2.4 MFCs Setup and Electricity Output



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The cube-shaped single chamber MFCs were prepared by Plexiglas with cylindrical chamber (4 cm in length by 3 cm in diameter) as previously reported.

30

The anodes

were carbon fiber brush which was pretreated by heating 30 minutes at 450 °C, 31 the prepared rolling cathodes were placed vertically against anode with distance of 1 cm.

All MFC reactors were operated in fed-batch mode at 30 °C for 5 months. For different cathodes, at least duplicate reactors were operated and all reactors were fixed with external resistance of 1000 Ω except for specified. During the start-up, MFCs were inoculated with domestic wastewater (20%, v/v) collected from a municipal pipe network (Harbin, China) and medium which contained glucose (1 g L–1), 50 mM phosphate buffer solution, vitamins (5 mL L–1) and minerals (12.5 mL L–1). 32 2.5 Protein Content The protein content in biofilm of cathode surface was determined by the bicinchoninic acid (BCA) method against a bovine serum albumin standard in 0.1 N NaOH.

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The entire biofilm was first scraped from cathode surface, followed by

cutting cathode into pieces, then the scraped biofilm together with the cathode pieces were placed into tube contained with 10 mL of 0.2 N NaOH and stored at 4 °C for 60 minutes during which the tube was vibrated every 15 minutes for 1 minute. After collecting the extracted liquid, 10 mL of deionized water was used to rinse the electrode fragments to remove the residues. By mixing the two extracted liquid, the final concentration of 0.1 N NaOH was obtained. The liquid was extracted thoroughly via three freeze-thaw cycles (frozen: –20 °C, 2 h; thaw: 90 °C, 10 min). Finally, the samples of protein extract were evaluated by BCA method.


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2.6 Oxygen Concentration through Cathodes Dissolved oxygen (DO) concentration in the MFC reactor was monitored at external resistance of 300 Ω to investigate the effect of biofilm on oxygen transport. The DO concentration during a whole cycle was conducted using a non-consumptive dissolved oxygen probe (FOXY, Ocean Optics, Inc., Dunedin, FL).

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Before

measurement, two-point calibration was conducted with oxygen-consumed and oxygen-saturated water. The DO probe and temperature sensor were simultaneously fixed into MFC chamber next to the cathode after replacing new medium. 2.7 Microelectrode Measurement Gas-diffusion cell 35 was constructed with Plexiglass chamber (3 cm ×3 cm × 3 cm) which used air cathode as working electrode, Pt sheet (1 cm2) as counter electrode and Ag/AgCl reference electrode. Before each measurement, the microelectrode was calibrated using standard pH buffer solutions. The calibration was carried out under the same potential as that of working electrode to offset the effect of electric field on the sensor.

36

Once the microelectrode was positioned 2000 µm from the

electrode/biofilm surface, it was stepped down with increments of 5 µm until reaching to the cathode surface by a stepper motor. During the movement of measurement, the microelectrode was set to wait for 10 s and measure for 10 s and then move to the next point. Variations of pH with distance were tested when cathodes produced a stable current. 2.8 Calculations The voltages output was recorded every 30 minutes using a data acquisition board



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(PISO-813, ICP DAS CO., Ltd.) connected to a personal computer. Current density was calculated from I = U/RA, and power density obtained with P = U2/RA, where U (V) is the voltage across given external resistor, R (Ω) is the external resistance, and A (m2) is the projected surface area of the cathode. Power density and polarization curves were obtained by varying the external resistor over a range from 1000 to 50 Ω. Anode and cathode potentials were measured using saturated calomel reference electrode.

3 RESULT AND DISCUSSION 3.1 Structural and Morphological Characters of Ag/AC The crystalline structure of silver nanoparticle was examined by X-ray diffraction (XRD) in Figure S1. The diffraction peaks at 2θ of 38.07°, 44.26°, 64.42°, 77.34° were clearly observed. According to the JCPDS cards No. 04-0783, these major peaks were assigned to (111), (200), (220) and (311) lattice planes of metallic Ag with face-centered cubic symmetry. The morphology and dispersity of nanosilver were further characterized by SEM and TEM. Figure 1A depicted the rough surface of individual activated carbon particles with nanoparticles distribution. For amorphous activated carbon, not only the surface area but also inherent open pores provided extended sites for silver particles from higher magnification (Figure 1B). Except for a fraction of silver aggregation, majority of crystals were synthetized homogeneously. From TEM (Figure 1C), the silver nanoparticles size was ranged from 6 to 25 nm, this polydisperse sizes were likely due to the amorphous structure of carbon powder which affected the unbalanced growth of nanoparticles as described. 37 High-resolution TEM


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(HR-TEM) (Figure 1D) clearly showed the well-defined lattice fringes of the Ag (111) plane in Ag/AC, which also confirmed the highly dispersed metallic sliver nanoparticles on carbon powders.

38

Based on the ICP-AES, the amount of silver in

the Ag/AC catalyst was 9.67 wt%.

Pore structure of catalysts was investigated by N2 adsorption-desorption isotherms (Figure S2). According to IUPAC, the isotherm of AC catalysts suggested the carbon powders in this study were amorphous carbon with random pores distribution.

39

Compared to AC catalyst, the isotherm of Ag/AC slightly dropped down which meant decreased specific surface area. Based on BET result (Table S1), the surface area of Ag/AC was 1,799 m2 g-1, lowered than that of AC (1,841 m2 g-1), the reduced BET surface area were likely due to the partially blocked mesopores and macropores of carbon powders as a result of silver nanoparticles synthesized inside the pores. However, micropores area and volume of catalysts increased by 6.3% and 9.5% after nanosilver synthesis, the increased micropores structure would enlarged activated sites for oxygen reduction,

40

therefore, the ORR activity would be enhanced after

loading with silver nanoparticles.



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Figure 1. SEM of silver nanoparticles on carbon surface (A 30000 ×) and within pores (B 120000 ×), TEM (C) and SAED patterns of HR-TEM (D) images of silver nanoparticles. 3.2 Surface Composition of Catalyst Layer To further confirm the existence of Ag nanoparticles on cathode surface, compositions of catalyst layer with and without silver nanoparticles were evaluated by XPS. The XPS spectra of Ag/AC not only presented the characteristic peaks of oxygen, carbon and fluorine, but also silver (Figure 2A), which indicated the stability of silver nanoparticles during cathode preparation processes. The high-resolution C1s spectrum was deconvoluted to detect the type of oxygen function groups. From Figure 2B, the three main peaks at 284.6 eV, 285.9 eV and 287.2 eV represented the graphitic carbon (C-C) bond, hydroxyl (C-OH) or ether (C-O-C), and carbonyl group in ketones


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and quinines (C=O).

25, 41

These oxygen-containing functional groups improved the

hydrophilicity of carbon particles and provided favorable nucleation sites for growth of nanoparticles. Two peaks were observed for Ag3d as showed in Figure 2C, the binding energy of 368.4 eV and 374.2 eV corresponded to Ag3d5/2 and Ag3d3/2, respectively, and the spin-orbit splitting of 3d doublet of 6 eV indicated the silver on cathode surface was in the metallic form

42

which was in accordance with the XRD

results. Based on the previous study, the silver nanoparticles would facilitate the ORR activity in pH-neutral and alkaline solutions. 43-44



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Figure 2. XPS survey spectra (A) of cathodes with AC and Ag/AC catalyst, high-resolution of C1s (B) and Ag3d (C) spectrum for Ag/AC cathode. 3.3 Kinetic Activity of Catalyst RDE tests were conducted to identify the catalytic activity of catalysts. Based on the results, the kinetics of Ag/AC catalyst increased. The kinetic current of the Ag/AC catalyst was 0.21 mA which increased by 61.5% than that of plain AC (0.13 mA) (Figure 3), demonstrating the presence of silver nanoparticles remarkably improved the dynamics activity. The number of electron transfer calculated according to K-L equation was 3.4 for Ag/AC depending on the potential of – 0.5 V, higher than that for AC catalyst (n = 2.8), indicating ORR proceeded on Ag/AC cathode predominantly via the 4e– reaction pathway. In contrast, 2e– pathway likely dominated the mechanism of AC cathode. The increased number of electron transfer of Ag/AC was not only a result of the pore structure, but also an effective activity of silver particles. 45


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Figure 3. RDE results of AC (A) and Ag/AC (B) catalysts in 50 mM phosphate buffer solution aqueous solution saturated with oxygen at different rotating speeds; and Koutecky-Levich plots (C) of AC and Ag/AC catalysts for oxygen reduction at -0.5 V. 3.4 Effect of Antibacterial Activity on Cathode Performances MFCs with Ag/AC cathode produced the maximum power density (Pmax) of 1.1 ± 0.01 W m-2 which was 14.6% higher than that of AC cathode (0.96 ± 0.02 W m-2) (Figure 4). Although slight release of Ag+ from cathode was detected during the initial seven cycles (Figure S3), anode performances of MFCs with AC and Ag/AC cathodes were similar based on electrode potential, indicating the released silver would not affect the activity of anode microbes. The efficient immobilization of silver was likely due to the porous carbon structure and PTFE binder.

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The increased performance

was mainly attributed to the better catalytic activity of Ag/AC which was further confirmed by exchange current density (i0). The i0 of Ag/AC cathode was 1.37 × 10-4


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A cm-2, which was 1.55 times higher than that of AC cathode (0.88 × 10-4 A cm-2) (Figure 5).

Figure 4. Power density (A) and corresponding electrode potentials (B) of MFCs with AC and Ag/AC cathodes.



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During the long-term operation, thicker biofilm colonized surface of AC catalyst layer, whereas only a thinner biofilm grew on Ag/AC cathode (Figure S4), this was mainly because silver nanoparticles could effectively inhibit bacterial growth.

19, 46

Based on the measurement, the protein content of Ag/AC cathode was 6.7 ± 0.4 mg, which was only 38.3% of that on AC cathode (17.5 ± 0.6 mg). The thinner biofilm on Ag/AC cathode was also accompanied by the higher i0. After continuously operating MFCs for over 5 months, the i0 of Ag/AC cathode was 1.26 × 10-4 A cm-2, which was 1.62 times higher than that of AC cathode (0.76 × 10-4 A cm-2). Although i0 decreased for both catalysts, the decrement of Ag/AC cathode was only 8.0% compared to 13.6% of AC cathode, indicating more stable activity of Ag/AC cathode. Therefore, it was inferred that biofilm contamination was likely the reason for the decreased cathode performance during long-term operation.

Figure 5. Tafel plots of cathodes with AC and Ag/AC catalysts in 50 mM phosphate


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buffer solution with scanning rate of 1 mV s–1 and the linear fit of the Tafel plots at overpotential from 80 mV to 100 mV. 3.5 Effect of Antibacterial Property on Interfacial Oxygen Diffusion Variation of dissolved oxygen (DO) in MFC reactor during a batch cycle was monitored at external resistance of 300 Ω to investigate the effect of biofilm on oxygen transport. With this lower external resistance (compared to 1000 Ω during MFC running), higher current density was generated as a result of cathode reduction, this would lead to more oxygen consumption. With higher difference of oxygen concentration between cathode interfaces and ambient environment, oxygen diffusion would be accelerated. Therefore, this condition was benefit to investigate the effect of biofilm on oxygen transfer. For new prepared cathodes, DO concentration in MFCs reactor was mainly affected by catalytic performance of cathode. As depicted in Figure 6A, DO in MFCs with new Ag/AC and AC cathodes swiftly dropped from 5.4 mg L-1 to about 0.5 mg L-1 as soon as fresh medium was replaced. After then, DO gradually increased, but lower DO concentration was detected with Ag/AC cathode compared to AC cathode during the first 14 hours, this was mainly because the higher kinetic activity of Ag/AC consumed more oxygen which decreased DO concentration at solution-cathode interface. Subsequently, DO increased sharply and almost similar DO was obtained for both cathodes due to substrate consumption until the end of cycle.

In addition to ORR, oxygen diffused through cathode was also consumed by aerobic microorganisms,

20

therefore DO concentration for all MFCs declined after



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biofilm developed. Compared to AC cathodes, Ag/AC cathode showed higher DO value as depicted in Figure 6B. Stable DO concentration in MFCs was sustained during the first 16 hours, and the average concentration of Ag/AC cathode was 0.63 ± 0.14 mg L-1 which was higher than 0.48 ± 0.09 mg L-1 of AC cathode. Electrochemical impedance spectroscopy was further tested to explain the change of oxygen concentration. From Figure S5 and Table S2, Ag/AC cathode with less biofilm had lower ohmic resistances (Rohm) and charge transfer resistance (Rct), which was consistent with higher i0, indicating the higher kinetic activity. Compared to AC cathode, Ag/AC cathode had lower diffusion resistance of 5.97 Ω, which was only 57.9% of that for AC cathode (10.31 Ω). Ag/AC cathode with higher catalytic activity would consumed more oxygen to ORR, therefore, it was likely because the minor resistance formed by thinner biofilm that introduced more oxygen compared to that of AC cathode. As our previous results revealed that thicker biofilm on AC cathode would block pores in catalyst layer, therefore obstructed oxygen diffusion.

47

It was

described that decreasing resistance was important for air cathode to keep higher performance during long-term operation as reported,

48

with better antibacterial

activity, Ag/AC effectively restrained microbial colonization which was benefit to maintain higher oxygen.


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Figure 6. Dissolved oxygen concentration through cathode with AC and Ag/AC catalysts in MFCs before (A) and after biofouling (B). 3.6 Effect of Antibacterial Property on Interfacial Hydroxide Transfer Hydroxide was one of product of oxygen reduction in cathode, the accumulation of



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hydroxide significantly affected cathode performance due to large potential loss according to Nernst equation. 49 To detect the effect of biofilm on hydroxide transport through microenvironment of cathode during ORR, microelectrode was used to monitor the pH variation of catalyst layer interface of AC and Ag/AC cathodes at initial and biofouling stage. Before measurement, the cathode was firstly polarized at – 0.1 V vs Ag/AgCl by chronoamperometry until the current output was stable. This potential was chosen based on electrode potential curve, which showed that better catalytic activities of cathodes were obtained between 0 V to – 0.1 V (vs. Ag/AgCl).

The pH gradient from bulk solution to catalyst layer surface increased for both initial AC and Ag/AC cathode (Figure 7A). The pH for AC and Ag/AC cathode increased from 7.7 ± 0.03, 7.8 ± 0.01 in the bulk solution to 8.3 ± 0.1 and 8.5 ± 0.1 at the catalyst layer surface, respectively, indicating the hydroxides were accumulated at cathode surface. Generally, the hydroxide concentration at reaction sites was higher -

than that of bulk solution at pH 7.0 due to ORR via 4e pathway, this concentration gradient would drive hydroxide transport from cathode to bulk solution. However, carbon powders were rich in micro- and mesopores which were inferred to be the main sites of ORR, the produced hydroxides were likely absorbed within these pores and caused more serious hydroxide accumulation. 50 In addition, the PTFE binder was poor to convey hydroxide ions because of its hydrophobic property, therefore hydroxide accumulation was further aggravated. Compared to AC cathode (3.1 mA), the higher current of 3.7 mA was produced at – 0.1 V from chronoamperometry (Figure 7B), which meant more protons consumption and hydroxides production.


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Therefore, higher pH was detected with Ag/AC cathode.

Hydroxides accumulation was aggravated after biofilm attachment as pH obviously increased for AC cathode but slightly for Ag/AC cathode. Compared to new cathode, the pH near the biofilm AC catalyst surface increased notably to 9.1 ± 0.1 from 8.3 ± 0.1(initial), and the net increment of pH was almost 0.8 units. Whereas it increased slightly from 8.5 ± 0.1 to 8.7 ± 0.1 for Ag/AC cathode with pH increment of 0.2 units. It was known that higher catalytic performance of Ag/AC cathode would increase higher pH due to more hydroxides production, therefore the lower increment of pH with Ag/AC cathode was mainly because the resistance of hydroxides transport from cathode interface to bulk solution brought by biofilm was alleviated due to less biofilm on cathode. Along with the increased pH, the current output for both Ag/AC and AC cathode declined respectively compared to that of initial stage according to the i-t curve (Figure 7B). However, Ag/AC still generated higher current of 3.5 mA compared to 2.6 mA of AC cathode. With better antibacterial property, the cathode polarized loss caused by hydroxides accumulation decreased which was beneficial to catalytic activity. Since the hydroxides accumulation was accompanied by continuous cathode reaction, cathode with antibacterial property was desired to maintain higher performance.



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Figure 7. Variations in pH (A) and current production (B) of AC and Ag/AC cathodes at the cathode potential of –100 mV (vs Ag/AgCl) by chronoamperometry before and after biofouling.

CONCLUSION


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In this study, we synthesized silver nanoparticles on activated carbon (Ag/AC) as cathode catalyst in MFCs. Uniform silver nanoparticles were distributed on the surface and in the pores of AC powders. Compared to the activated carbon cathode (AC) cathode, Ag/AC sufficiently inhibited the biofilm growth on cathode surface as protein content decreased by 61.7%. With less biofilm, higher dissolved oxygen was maintained. In addition, the minor resistance of biofilm on hydroxide transport from catalyst layer interface to bulk solution was determined. The higher mass transport of oxygen and hydroxide favored better catalytic activity toward oxygen reduction. Therefore, the decrement performance of Ag/AC cathode was much lower than AC cathode. This experiment confirmed activated carbon with antibacterial property could be developed to maintain cathode performance.

Supporting Information Experimental details about the XRD patterns of Ag/AC catalysts, N2 adsorption /desorption isotherms of AC and Ag/AC and digital biofilm on AC and Ag/AC cathode is provided as Supporting Information.

ACKNOWLEDGEMENTS This work was supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Grant No. 2015DX05) and by the National Natural Science Fund for Distinguished Young Scholars (Grant No. 51125033) and National Natural Science Fund of China (Grant No. 51209061, 51308171, 51408156). The authors also acknowledged the supports from the International Cooperating Project between China and European Countries (Grant No.



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2014DFE90110) and Project funded by China Postdoctoral Science Foundation (2014T70355).

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Enhanced Oxygen and Hydroxide Transport in a ... - ACS Publications

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