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Enhanced power generation of oxygen-reducing biocathode with an alternating hydrophobic and hydrophilic surface Haiman Wang, Jia Liu, Weihua He, Youpeng Qu, Da Li, Qing Jiang, and Yujie Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10876 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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ACS Applied Materials & Interfaces

Enhanced power generation of oxygen-reducing biocathode with an alternating hydrophobic and hydrophilic surface Haiman Wanga†, Jia Liu a†, Weihua Hea,**,Youpeng Qua,b, Da Lia, Qing Jianga, Yujie Fenga,*

a

State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology. No 73 Huanghe Road, Nangang District, Harbin 150090, China b School of Life Science and Technology, Harbin Institute of Technology. No. 2 Yikuang Street, Nangang District, Harbin 150080, China † Both authors contributed equally to the work

*Corresponding Author: phone: (+86)451-86287017; Fax: (+86) 451-86287017; E-mail: [email protected] **Co-corresponding Author: E-mail: [email protected]

ABSTRACT 1

ACS Paragon Plus Environment


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Most oxygen-reducing biocathodes for microbial electrochemical systems (MESs) require energy-intensive aeration of the catholyte, which negates the energy-saving benefits of MESs. To avoid aeration and enhance oxygen-utilization efficiency, columnar activated carbon with half of its surface coated by polytetrafluoroethylene (PTFE-coated CAC) was fabricated as biocathode material and its performance was investigated using a tide-type biocathode MES (TBMES). The TBMES with PTFE-coated biocathode achieved a maximum power density of 8.2 ± 0.8 W m–3, which was 39% higher than that of the untreated control (CAC biocathode). The PTFE-coated biocathode was able to store a cumulative total charge (Qm) of 10.8 ± 0.2 × 104 C m–3 during one charge-discharge cycle, whereas the Qm of CAC biocathode was only 6.9 ± 0.1 × 104 C m–3, demonstrating that the oxygen entrapment capability of PTFE-coated biocathode was 54 ± 3.8% higher than that of the control. Internal resistance analysis under both oxygen sufficient and reoxygenation conditions suggested the oxygen entrapped by this surface-hydrophobic biocathode was basically sufficient for cathodic oxygen reduction reaction. The slight difference in cathodic microbial communities of the two biocathodes further indicated that the higher accessibility of oxygen due to the hydrophobic surface was the primary cause for the better performance of the PTFE-coated biocathode, while the higher biocatalytic activity of the cathodic biofilm was a minor factor. KEYWORDS:

Microbial

electrochemical

system; air-entrapped

columnar activated carbon; hydrophobic surface; air affinity. 1. INTRODUCTION 2


biocathode;

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Microbial electrochemical systems (MESs) have intensively been investigated in last decade, as a solution for water crisis with simultaneous wastewater treatment and bioelectricity production.1,2 However, challenges remain before successful translation of MESs into real-world application, such as persistent kinetic limitations for cathodic oxygen reduction reaction (ORR) , high capital cost of construction due to the use of noble metals as cathode catalysts and the complexity of substrates used.3–6 As an attractive

alternative

for

abiotic

chemical

cathode,

biocathodes

using

electrochemically active microorganisms catalyst have drawn considerable attentions due to the advantages in lower costs, scaling up applicability and long-term sustainability for wastewater treatment.7–9 In addition, the biocathodes present a potential approach for effective removal of pollutant due to the variety of in-situ accumulated electrochemically active microorganisms and terminal electron acceptors.10–13 The granular activated carbon (GAC) in packed-bed structure is one of the commonly applied biocathode materials in MESs, which has been employed by researches for building application-oriented large scale MESs with three-dimension electrodes.14 The GAC in columnar shaped has recently been used in a novel pilot-scale stacked MES with a total volume of 72 L, which achieved high power generation and COD removal due to the coupling effects of stack design and electrode configuration.15 The GAC has also been used as biocathode material in a constructed wetland MES, which performed well due to its large surface area and helpful capillary water absorption.16 3



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Oxygen-reducing biocathodes have been widely used due to the high redox potential, wide availability and sustainability of oxygen as a terminal electron acceptor.17 However, most oxygen-reducing biocathodes had been proved to be effective only when aeration of catholyte was supplied.14,18 As an energy-intensive process, aeration negates some of the energy-saving benefits of using biocathodes.19 It is estimated that aeration will consume 0.3 kWh m–3 of wastewater treated in an activated sludge-based aerobic process, which is one order of magnitude higher than the normalized energy produced by a MES treating domestic wastewater.2 In addition, due to the low solubility of oxygen in the aqueous cathode solution, the ORR rate on the cathode electrode is often constrained, which consequently restricts the overall MESs performance.20 Moreover, the limited oxygen transfer efficiency even exacerbates the cost of catalytic site modification for oxygen accessibility. In some attempts for alternative methods of aeration, oxygen was enriched by either deliberately spraying liquid at a high flow rate or passively diffusing from atmosphere, which required large affiliated reoxygenation device or resulted in cathodic current density restriction because of low oxygen transfer efficiency.19,21 Therefore, increasing the oxygen utilization efficiency of biocathodes with energy-saving reoxygenation methods is critical for the practical applications of oxygen-reducing biocathode MESs. The wettability of hydrophobic surfaces is widely applied in daily life, which demonstrates excellent capability in waterproofing, self-cleaning and water harvesting.22 Besides, affinity to air is also a typical feature of hydrophobic surface. 4


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When the hydrophobic surface, being in contact with air earlier, is immersed into an aqueous phase then air can be entrapped and stay attached to the surface.23 Moreover, air presence at the hydrophobic surface can facilitate rupture of the liquid film and formation of three phase contact during bubble collisions with hydrophobic surface.24 The polytetrafluoroethylene (PTFE) has long been used as a binding agent for fabricating gas diffusion layer (GDL) in the fuel cells, especially in the proton exchange membrane fuel cell (PEMFC).25,26 The GDL possesses the combined and balanced properties of hydrophobicity and hydrophilicity, which enable it to function as channels for gas diffusion and water permeation.27 In addition, the PTFE has also been applied to the MESs as diffusion layer of the air-cathode and the air-diffusion biocathode, which is used for waterproof and gas diffusion.28,29 These previous studies mainly focus on reducing mass transfer limitation by taking advantage of the porous microstructure channels of the GDL to facilitate gas/water transportation, while the utilization of hydrophobic surface for in-situ air entrapment due to its high air affinity has not been taken into consideration. In this study, columnar activated carbon with half of its surface coated by PTFE (PTFE-coated CAC) was fabricated as biocathode material, and its air-entrapment capability was investigated using a tide-type biocathode microbial electrochemical system (TBMES), which employed siphon principle for periodical water drainage and enabled intermittent air access for the biocathode by continuous feeding. The performances of PTFE-coated biocathode in terms of bioenergy generation, electrochemical performance and oxygen utilization efficiency were compared with 5



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that of an untreated CAC biocathode. Microbial community and biomass of the two biocathodes were analyzed to determine if surface hydrophobic treatment had an impact on community ecology. 2. EXPERIMENTAL SECTION 2.1 Reactor configuration and biocathode material preparation The tide-type biocathode microbial electrochemical system (TBMES) was manufactured from Plexiglas tube with an inner diameter of 11 cm (Fig. 1).30 The anode chamber (Φ: 11 cm × H: 3 cm) installed with five carbon fiber brushes (Φ: 4 cm × H: 3 cm, Toray, 3K carbon fiber) 31 was located at the bottom portion, while the cathode chamber (Φ: 11 cm × H: 4 cm) with packed-bed columnar activated carbon (CAC, Φ: 3-5 mm × H: 8-10 mm, bituminous coal based, Beijing Sanye Carbon Co. Ltd., China) biocathode was located on the top. A stainless steel mesh electron collector was formed into a basket shape and packed with CAC particles. An inverted U-shaped glass siphon (Φ: 3 mm, height: 3 cm), installed to the bottom edge of cathode chamber, was used as the outlet of the system. The utilization of the siphon for effluent drainage enabled the system to accomplish continuous feeding and periodical draining process. With the influent continuously flowed into the cathode chamber from the anode chamber, the liquid level of the cathode chamber gradually increased and reached the highest point, which was determined by the height of the siphon, it would then be quickly drained out from the siphon and the liquid level of the cathode chamber would decline to the lowest point. This continuous feeding and periodical draining process (feeding-draining process) enabled the intermittent 6


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contract between air and packed CAC biocathode, which could entrap air bubble in cathode chamber while the liquid level rising up. The regular rising and falling of the liquid level in cathode chamber was similar to the process of ocean tide. Therefore, the biocathode was termed as “tide-type biocathode”. Prior to use, the CAC (350 g) was hydrophobic pretreated by applying 100 mL 60 wt.% PTFE on half surface of it, with an average loading rate of 0.09 ± 0.01 mL-PTFE cm –1-CAC (detailed calculations in the supporting information). The PTFE was used to endow the CAC with hydrophobic surface for in-situ air entrapment, while the content of 60 wt.% for PTFE solution was chosen according to the commonly used content in air-cathode of MESs.28 The CAC was first dispersed on a piece of cardboard, then gently painted with 60 wt.% PTFE solution using a paintbrush. A porous plastic plate was fixed on the cardboard and covered the CAC to prevent them sticking to the paintbrush. The PTFE-coated CAC was air-dried at room temperature for 2 h until the coating turned white and then heat treated in a pre-heated furnace at 370 °C for 15 min (The preparation procedures for PTFE-coated CAC was presented in Fig. S1).

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Fig. 1. Configuration of the tide-type biocathode microbial electrochemical system (TBMES) and air affinity features of the two biocathodes 2.2 Inoculation and operation The TBMES with PTFE-coated CAC biocathode was initially inoculated with a mixture consisting of 20% domestic wastewater which was used as both the anodic and cathodic inoculums, and 80% sucrose medium containing sucrose, 0.71 g L–1; KCl, 0.13 g L–1; NaH2PO4·2H2O, 3.32 g L–1; Na2HPO4·12 H2O, 10.32 g L–1; vitamins, 5 mL L–1 and trace minerals, 12.5 mL L–1.32 After stable voltage generation was obtained, the above mixture was changed to complete sucrose medium without domestic wastewater. The reactor was operated in up flow mode with the influent continuously flowed through the separator into cathode chamber and periodically drained out by siphon. The cathodic effluent was continuously recirculated back to cathode chamber at a flow rate of 8 mL min–1 to enhance the feeding-draining process, which was set at a cycle of 15 min to ensure oxygen access for biocathodes. The 8


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TBMES was operated at 30 °C and continuously fed at a hydraulic retention time of 12 h with an external resistance fixed at 1000 Ω. Another TBMES with untreated CAC as biocathode material was operated in parallel as a control. 2.3 Measurement and analysis The output voltage and electrode potential were recorded every 30 min by a data acquisition system (PISO-813, ICP DAS Co., Ltd.).33 Polarization and power curves were obtained by varying the external resistances from 1000 to 5 Ω (1000, 500, 200, 100, 50, 30, 20 to 5 Ω) at 1 h intervals for each resistance. The electrochemical measurements were performed using a workstation (Autolab PGSTAT128N, Metrohm Co., Swiss) in a three-electrode system with the cathode as working electrode, a platinum mesh (2 cm–2) as counter electrode and a saturated calomel electrode (SCE, +0.242 V vs. standard hydrogen electrode; SHE) as reference electrode. The charge-discharge measurements were conducted to investigate overall charge storage capacity of the two biocathodes,34 which also represented their air (oxygen) entrapment capability. The open circuit condition was used as the charging period for the biocathodes. During the discharging period, the biocathodes were operated at a set potential of 0.012 V, which was chosen according to the maximum power density point of control CAC biocathode, and values of current output were recorded. The charging period lasted for 30 min, which included two successive feeding-draining processes. The discharging period was 40 min, during which time the feeding-draining process was stopped. The charge-discharge cycles were performed with 5 consecutive repeats. The cumulative total charge (Qm, C m–3) was 9



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the integral of the current curve from each charge-discharge cycle in the tests.35 The net electric charge stored (Qn, C m–3) was obtained by integrating the area from the peak of current above the stable baseline.34 Linear sweep voltammetry (LSV) was conducted to examine the electrochemical activity of the biocathodes before and after biofilm acclimation, which was performed at a scan rate of 1 mV s–1 from +0.4 to -0.3 V.36 Electrochemical impedance spectroscopy (EIS) was performed both under oxygen sufficient condition and reoxygenation condition to determine the internal resistances of the two biocathodes. It was carried out at cathode potential of 0.012 V, over a frequency range of 100 kHz to 1 mHz with the amplitude of 10 mV.33 The experimental data was fitted into an equivalent circuit (Zsimpwin software 3.10) as previously described.37 The oxygen sufficient condition was achieved by pumping oxygen-saturated catholyte to the cathode chamber at the same speed as that of feeding-draining process, while the reoxygenation process directly pumped the cathodic effluent back to the cathode chamber. Before LSV and EIS measurements, the reactors were stabilized under open circuit condition for 12 h with a feeding-draining cycle of 15 min. Both the LSV and EIS measurements were performed several times until the curve was completely coincident with the previous one. Dissolved oxygen (DO) recorded every 1 min was measured by a non-consumptive fiber optic DO probe (FOXY; Ocean Optics, Inc., Dunedin, FL). Surface images of the biocathodes before and after biofilm acclimation were captured by scanning electron microscope (S-3400N, Hitachi High-Technologies Corporation, Japan). 10


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Proteins were extracted from the cathode biofilm as previously described and measured using a Bicinchoninic Acid Assay (BCA Protein Assay Kit, Sangon Biotech) according to the manufacturer’s instructions.38,39 Total biomass of the cathode biofilm was quantified by assuming that protein comprised 55% of biomass by dry weight.40 2.4 Microbial community analysis The microbial communities of PTFE-coated and untreated control biocathodes were analyzed by pyrosequencing. Total genomic DNA was extracted from all samples using a Bacteria DNA Mini Kit (Watson Biotechnologies, Inc., Shanghai) according to the manufacturer’s instructions and assessed by electrophoresis in 1% agarose gels. The bacterial 16S rDNA PCR was performed using 338F and 806R Primers targeting the variable region V1–V3. Pyrosequencing of amplicons was performed by Majorbio Company using MiSeq instrument. Venn diagram with shared and unique OTUs was used to depict the similarity and difference between the two communities.41 3. RESULTS AND DISCUSSION 3.1 TBMES performance with PTFE-coated biocathode Voltage output of the TBMES with PTFE-coated biocathode increased gradually during the start-up period, and stabilized at 0.67 ± 0.04 V after an acclimation period of ~800 h, demonstrating a good biomass buildup and adaption (Fig. S2A).The TBMES with untreated control biocathode showed a comparable start-up time, however, its stabilized voltage was 0.15 ± 0.02 V lower than that of the TBMES with PTFE-coated biocathode. Anode potentials kept essentially the same (0.47 ± 0.01 V, vs. SCE) during the stable operation period (Fig. S2B), indicating that cathode was 11



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responsible for the different voltage output of the two TBMESs. A maximum power density of 8.2 ± 0.8 W m–3 was produced by PTFE-coated biocathode TBMES, which was 39% higher than that of the control (Fig. 2A). In polarization tests, the open circuit voltage (OCV) of TBMES with PTFE-coated biocathode was 0.72 ± 0.03 V, compared with the 0.58 ± 0.02 V for untreated control reactor. The open circuit potential (OCP) of the PTFE-coated biocathode (0.21 ± 0.02 V) was much higher than that of the untreated control (0.07 ± 0.02 V), demonstrating the PTFE-coated biocathode produced a higher potential that was closer to the thermodynamic equilibrium potential for cathodic oxygen reduction reaction (ORR). Polarization curves of electrodes suggested the differences in power generation of the two TBMESs were mainly caused by cathode as well, while insignificant change of anode potential was expressed (Fig. 2B). The measured potentials of the PTFE-coated biocathode were more positive than untreated control biocathode over the entire test range. At the maximum power output point, the potential of PTFE-coated biocathode was 69 mV higher than untreated control, which might result in its superior power output. The enhanced cathode performance might be attributed to the higher accessibility of air with PTFE-coated biocathode, which provided more reactant (oxygen) for cathodic ORR in TBMES. The long-term stability of PTFE-coated biocathode was evaluated during a stable operation period of more than four months after startup. Though the top layer of the PTFE coating was slightly peeled off due to hydraulic scouring in the initial 20 days of operation, it had little impact on the power output capacity of the PTFE-coated 12


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biocathode, as verified by no significant reduction in maximum power density was observed during the first three months, which maintained at 8.6 ± 0.2 W m–3 (Fig. S3A). The power production decreased slightly in Month 4, with a 5.8% reduction in maximum power density to 8.1 W m–3. Polarization curves revealed that the decrease was caused by performance deterioration of the biocathode (Fig. S3B), which was likely due to overgrowth of heterotrophs on the autotrophic biocathode community during long-term operation, consequently leading to a competition between heterotrophic bacteria and cathodic autotrophic bacteria for oxygen.19 However, this deterioration was much lower than that of Pt /C cathode, which usually suffered a decrease of 7-17 % in maximum power after two months operation.42,43

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Fig. 2. (A) Cell voltages and power densities, and (B) electrode potentials as a function of current density for TBMESs with PTFE-coated CAC and untreated control CAC biocathodes (Error bars were based on averages measured in duplicate) (V: voltage; P: power density; AP: anode potential; CP: cathode potential) 3.2 Air entrapment and capacitance behavior The charge-discharge measurements were performed to investigate the charge storage capacity of PTFE-coated and untreated control biocathodes. The external circuit was periodically disconnected at 30 min interval followed by 40 min of closed circuit operation. Repeatable peak current densities were obtained by TBMESs using both kinds of biocathodes immediately after circuit closure followed by a current decay slowly towards stable values (Fig. 3). The PTFE-coated biocathode showed a highly repeatable peak current density of 62 ± 1.7 A m–3 and a stable current density of 33 ± 2.5 A m–3 after 40 min discharging, while those of untreated control was 37 ± 2.2 A m–3 and 23 ± 2.4 A m–3 (Fig. 3B). During the charge-discharge process, the 14


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cumulative total charge (Qm) for PTFE-coated biocathode was 10.8 ± 0.2×104 C m–3 based on total empty bed volume of cathode chamber, which was 54 ± 3.8% higher than the data of 6.9 ± 0.1×104 C m–3 obtained by untreated control biocathode. The net electric charge (Qn) derived from PTFE-coated and untreated control biocathodes were 2.6 ± 0.1×104 and 1.1 ± 0.2×104 C m–3. In the cathode chamber of TBMES, oxygen was the only terminal electron acceptor in the electrolyte, and the current flowing across the TBMES was generated only based on the ORR at the cathode. Therefore, the Qm, representing the total coulombs of electrons transferred during one charge-discharge cycle, could also reveal the amount of oxygen reduced in discharging process, which was also the amount of oxygen entrapped in cathode chamber during one charge-discharge cycle (detailed calculations in the supporting information). Therefore, it could conclude that the oxygen entrapment capability of PTFE-coated CAC biocathode was 54 ± 3.8% higher than the untreated control. In the charging process, when the catholyte was drained out by siphon, the air occupied the space of liquid and contact with the biocathode. During the subsequent rising of liquid level, the biocathode was gradually immersed and a large amount of air could be entrapped by the biocathode electrode. The entrapped air (oxygen) could be stored in the space of packed CAC and cathodic biofilm in gaseous and dissolved states (Fig. S4). With high air affinity feature, the hydrophobic surface of PTFE-coated CAC could entrap larger amounts of air bubbles than hydrophilic untreated control CAC when the biocathode electrode was immersed by the rising 15



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liquid (Fig. S5). The entrapped oxygen could gradually be absorbed by cathodic microbes, and finally consumed in bio-catalyzed ORR as reserved electron acceptor in the discharging process. The great disparity of capacitance property between the two kinds of biocathodes clearly indicated the superiority of PTFE-coated biocathode in oxygen (air) entrapment.

Fig. 3. (A) Theoretical discharge graph, and (B) Current density behavior for five cycles of charge-discharge experiment with 30 min of charging and 40 min of 16


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discharging (Qm: cumulative total charge; Qn: electric charge stored) 3.3 Electrochemical analysis Linear sweep voltammetry (LSV) further indicated that electrochemical behavior of PTFE-coated biocathode was significantly superior to untreated control biocathode both in open circuit potential (OCP) and maximum current density. The PTFE-coated biocathode displayed an OCP of 0.21 V, which was more positive than that of control (0.05 V) (Fig. 4A). Moreover, the maximum current density of -150 A m–3 was generated by the PTFE-coated biocathode at -0.3 V (vs. SCE), which was 1.2 times as high as the untreated control biocathode. These results suggested the enhanced ORR catalytic capability of PTFE-coated biocathode. The ORR catalytic capability of PTFE-coated abiotic cathode was ignorable compared with PTFE-coated biotic cathode, and was as low as that of untreated control abiotic (Fig. 4A), demonstrating microbes acted as the cathode catalyst and surface hydrophobic treatment had little impact on chemical catalytic ability of PTFE-coated CAC. Therefore, the performance enhancement of PTFE-coated CAC biocathode was most likely to be associated with the higher in-situ accessibility of oxygen due to the air affinity of its hydrophobic surface. The total internal resistances (Rtotal) of the two biocathodes were evaluated by electrochemical impedance spectroscopy (EIS), which composed of ohmic resistance (Rohm), charge transfer resistance (Rct) and diffusion resistance (Rd).44 Rohm of the two biocathodes under both conditions were similar (Fig. 4B) (Fig. S6). Though the PTFE is a high electrical resistivity polymer, it was observed little impact on the Rohm of the 17



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PTFE-coated biocathode. Since only half of the CAC surface was coated by PTFE polymer, the great specific surface area of CAC was high enough to maintain the electrical conductivity between adjacent particles. The good connection between CAC and the current collector and the high conductive catholyte might also limit the negative effect of the PTFE on the electric contact between the CAC particles. Under reoxygenation condition, the Rct and Rd of the PTFE-coated biocathode were 2.0 ± 0.1 and 1.0 ± 0.1 Ω (Table 1), which were much lower compared with those of the untreated control (8.2 ± 0.1 and 3.8 ± 0.1 Ω), indicating the hydrophobic coating could reduce the Rct and Rd of the PTFE-coated biocathode, which would be beneficial for the increase of power generation. Since both the PTFE-coated and untreated control biocathodes showed higher total internal resistances under reoxygenation condition compared with those under oxygen sufficient condition, it demonstrated that oxygen was the kinetic limitations for ORR when employing the reoxygenation method for air access. However, the Rtotal of the PTFE-coated biocathode under reoxygenation condition (3.7 ± 0.1 Ω) only increased by 30 ± 5% in comparison with that under oxygen sufficient condition (2.8 ± 0.1 Ω), while the untreated control biocathode exhibited larger increase (133 ± 5% ) in Rtotal when changed from oxygen sufficient condition (5.6 ± 0.1 Ω) to reoxygenation condition (13.1 ± 0.1 Ω). Therefore, the superior air entrapment ability, achieved by the hydrophobic treatment, was the most probable reason that contributed to the lower

Rtotal of the PTFE-coated biocathode under reoxygenation condition. Besides the superior air entrapment ability, the higher biocatalytic activity of the 18


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cathodic biofilm might also be a minor factor that contributed to the lower Rtotal of the PTFE-coated biocathode. This could be verified by the observed difference in Rtotal of the two biocathodes under oxygen sufficient condition, when oxygen mass transfer could not be the limiting factor. The higher biocatalytic activity of the PTFE-coated biocathode might be due to more aerobic bacteria presenting on it, which was caused by the higher air-entrapped capability of hydrophobic surface as well. However, the biocatalytic activity only played a minor role in decreasing the Rtotal of the PTFE-coated biocathode, since the decrease under oxygen sufficient condition (2.7 ± 0.1 Ω) was much lower than that under reoxygenation condition (9.3 ± 0.1 Ω).

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Fig. 4. (A) Linear sweep voltammetry for the two biocathodes before and after biofilm acclimation at a scan rate of 1 mV s-1 from + 0.4 to -0.3 V(vs. SCE), and (B) Nyquist plots of electrochemical impedance spectroscopy at cathode potential of 0.012 V (vs. SCE) over a frequency range of 100 kHz to 1 mHz with the amplitude of 10 mV(the equivalent circuit was inserted in the graph) Table 1 EIS fitting results of the two biocathodes based on the equivalent circuit

PTFE-coated

Untreated control

Rohm (Ω)

Rct (Ω)

Rd (Ω)

Rtotal (Ω)

Oxygen sufficient condition

0.79 ± 0.05

1.5 ± 0.1

0.65±0.02

2.8 ± 0.1

Reoxygenation condition

0.82 ± 0.02

2.0 ± 0.1

1.0 ± 0.1

3.7 ± 0.1

Oxygen sufficient condition

1.03 ± 0.03

3.3 ± 0.1

1.3 ± 0.1

5.6 ± 0.1

Reoxygenation condition

1.09 ± 0.01

8.2 ± 0.1

3.8 ± 0.1

13.1 ± 0.1

3.4 Morphological characteristic and bacterial community of the biocathodes After applying 60 wt.% PTFE on the surface, the rough surface of CAC was well covered by hydrophobic PTFE layer, which exhibited a relatively smooth surface with wide distribution of pores (Fig. 5A and B). Obvious difference in bacteria attachment characteristics was observed between PTFE-coated and untreated control CAC 20


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surface (Fig. 5C and D). On the surface of CAC samples in stable operation period, a dense colonization of bacteria was observed. The thick attached biofilm on PTFE-coated CAC surface was also observed but mainly distributed on cracks of PTFE layer. This was probably caused by hydrophobic nature of PTFE-coated CAC, which might prevent the microbial from congregating.45 Nearly identical morphologies of biofilm were observed on PTFE-coated and untreated control CAC surface, where dense populations of mainly short, rod-shaped organisms attached with high cell density (Fig. 5E and F). Different from the gas diffusion layer (GDL) of fuel cells, the pores and cracks on the PTFE layer of CAC did not function as gas/water transportation channels,46 but served as habitat for cathodic bacteria attachment. Therefore, the cathodic bacteria could easily access to the air bubbles that entrapped by the hydrophobic CAC surface, which would be beneficial to enhance the cathodic ORR.

(A)

(B)

(C)

(D)

21



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(C)

(D)

(E)

(F)

Fig. 5. Scanning electron micrograph (SEM) of PTFE-coated CAC and untreated control CAC covered with or without microbial biofilm. (A) untreated control CAC-abiotic (5,000×), (B) PTFE-coated CAC-abiotic (5,000×), (C) untreated control CAC-biofilm (250×), (D) PTFE-coated CAC-biofilm (PTFE side) (250×), (E) untreated control CAC-biofilm (5,000×), (F) PTFE-coated CAC-biofilm (PTFE side) (5,000×) The biomass densities appeared homogeneous at the macroscopic level, which were 4.18 ± 0.11 mg g–1-CAC on PTFE-coated and 4.01 ± 0.07 mg g–1-CAC on untreated control CAC samples. The similar biomass densities on two kinds of biocathodes indicated that the growth of cathodic microbes was not significantly affected by PTFE-coating at macro level, despite the anti-bacteria adhesion feature of 22


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hydrophobic surface. However, the oxygen entrapped on hydrophobic surface provided in-situ accessible oxygen to cathodic microbial communities, which might have resulted in more active aerobic bacteria growth on the cracks or edges of PTFE layer and the uncoated side. The microbial communities were further characterized by pyrosequencing. Up to 34,840 qualified sequencing reads were yielded by PTFE-coated biocathode, which were clustered to 270 operational taxonomic units (OTUs) based on a threshold of 97%. Similarly, 31,360 qualified sequencing reads (279 OTUs) were obtained by untreated control. The total observed OTUs were 287, while up to 262 OTUs (91.3%) were in common between two kinds of biocathodes. Phylogenetic diversity of bacterial communities on two kinds of biocathodes based on phyla and family level further demonstrated the high similarity between them. At the phylum level, the bacterial communities of PTFE-coated and untreated control CAC were possessed of nearly the same structure with the predominance of Proteobacteria (48.8%, 47.2%), Bacteroidetes (10.9%, 12.8%), Actinobacteria (11.6%, 8.9%) and TM7 (6.8%, 7.0%) (Fig. 6A). These four dominant phyla accounted for up to 78.2% and 76.0% of the total sequencing reads on PTFE-coated and untreated control CAC. At the family level, eighteen families belonging to the four domain phyla were detected on both two biocathodes, most of which were also observed with similar abundance (Fig. 6B). These clustering results suggested that PTFE-coated and the untreated control biocathodes shared similar composition of bacterial communities. However, the PTFE-coated treatment caused decrease in the 23



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abundance of anaerobic bacteria and increase in the aerobic bacteria by providing more in-situ accessible oxygen to biofilm. Some anaerobic respiration bacteria, such as Geobacteraceae was much more abundant on untreated control CAC biocathode (7.6%) compared with that of PTFE-coated biocathode (0.23%), demonstrating a more anaerobic niche in the untreated CAC biocathode chamber. Moreover, the relatively higher abundance of Xanthomonadaceae (most genera of this family belonging to aerobic bacteria) on PTFE-coated biocathode (14.2%) also indicated oxygen concentration in its cathode chamber was higher than that in untreated CAC biocathode chamber.47 Though the functions and mechanisms of the cathodic oxygen-reducing bacteria were not as clearly as that of anodic electrochemically active microorganisms, it is certain that a wide range of oxygen-reducing bacteria, such as Acidithiobacillus ferrooxidans, Pseudomonas aeruginosa and Micrococcus luteus belong to aerobic bacteria.48–50 Therefore, the slight increase in the abundance of aerobic bacteria on the PTFE-coated biocathode might contribute to its higher biocatalytic activity. However, this factor played a minor role in the performance improvement compared with the air entrapment effect of the hydrophobic surface, as verified by the larger decrease in Rtotal under reoxygenation condition compared with the decrease under oxygen sufficient condition (Fig. 4B).

24


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Fig. 6. Relative abundance of bacterial reads retrieved from the PTFE-coated and untreated control biocathodes classified at the (a) phylum (b) family level (Others refer to the phylum and family with a relative abundance less than 1%) CONCLUSIONS In this study, polytetrafluoroethylene coated columnar activated carbon (PTFE-coated CAC) was fabricated as biocathode material and applied in a tide-type biocathode microbial electrochemical system (TBMES), which proved to be capable of avoiding aeration and enhancing oxygen-utilization efficiency of cathodic ORR. Due to the 25



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Page 26 of 35

higher air affinity of hydrophobic surface, the PTFE-coated biocathode exhibited superior air entrapment capability compared with the untreated control biocathode, which

could

therefore

provide

sufficient in-situ

oxygen

source

for

the

electrochemically active bacteria on biocathode. The increased in-situ oxygen accessibility resulted in the reduction of overpotentials and resistance for cathodic ORR, consequently a relatively higher maximum power density of 8.2 ± 0.8 W m–3 was obtained by the TBMES with PTFE-coated biocathode compared with that using untreated control biocathode (5.9 ± 0.2 W m–3). Based on these, the surface hydrophobic treatment of biocathode material shows great promise in improving the energy sustainability and promoting the biocathode MESs towards practical scale. Supporting Information Experimental details about the preparation procedures for PTFE-coated CAC, cell voltages and electrode potentials, long-term operation performance, dissolved oxygen concentration, photograph of bubbles on the biocathode surface, electrochemical impedance spectroscopy data and calculations on PTFE loading rate and oxygen entrapment are provided as Supporting Information. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Fund for Distinguished Young Scholars (Grant No. 51125033) and National Natural Science Fund for Young Scholars(Grant No.51408156). The author also acknowledgment the supports by the International Cooperating Project between China and European Union (Grant No. 2014DFE90110), supports from the State Key Lab of Urban Water Resource and 26


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