modified stainless steel as anode in air-cathode microbial fuel cells

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Bioengineering

Characteristics of Poly(3,4-ethylenedioxythiophene) modified stainless steel as anode in air-cathode microbial fuel cells Qian Ma, Kai-Bo Pu, Wen-Fang Cai, Yun-hai Wang, Qingyun Chen, and Fu-Jun Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00563 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Industrial & Engineering Chemistry Research

Characteristics of Poly(3,4-ethylenedioxythiophene) modified stainless steel as anode in air-cathode microbial fuel cells Qian Maa, Kai-Bo Pua, Wen-Fang Caia, Yun-Hai Wang*a,b, Qing-Yun Chenc, Fu-Jun Lid (aDepartment of Environmental Science and Engineering and cState Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China. bGuangdong Xi’an Jiaotong University Academy, Foshan 528300, China. dKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China.*E-mail: [email protected])

Abstract Poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemically polymerized to in-situ modify stainless steel (SS) plate electrode to improve its microbial bioelectrocatalytic activity as high-performance anode in microbial fuel cells. After modification, the surface of the electrode became rougher and showed better wettability. The electrochemical characteristics of PEDOT modified SS (PEDOT/SS) and bare SS electrodes were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel corrosion polarization curves respectively. It has been demonstrated that PEDOT modification could increase electrode capacitance and reduce electron transfer resistances. Compared with untreated SS, PEDOT/SS electrode showed better anti-corrosion property as well. The modified anode produced a maximum power density of 608.6 mW/m2, which was about 6 times higher than 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

bare SS anode.These results indicate that PEDOT treatment is an efficient method for SS to improve its performance as anode in MFCs. Keywords Surface modification; Microbial fuel cells; Stainless steel plate; Scalable electrode; PEDOT modification

2


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1. Introduction Microbial fuel cells (MFCs) are novel bio-electrochemical systems that use anode microorganisms as biocatalysts to spontaneously convert the chemical energy from biodegradable organic substance to electric power.1 The potential applications of MFCsin simultaneous wastewater treatment and electricity capture have drawn a dramatically increasing attention during the past decades but there are still several limiting factors blocking its further engineering applications.2 As both the support of electroactive bacteria and current collector in the system, anode material remains one of the main obstacles for the development of this nascent technology.3, 4 Carbon-based materials, such as carbon felt, carbon brush and carbon cloth, are the mostly used anode materials in MFCs owing to their excellent microbial adhesion and chemical stability.4, 5 Unfortunately, the comparatively high price and the lack of mechanical strength restrain their usage at large scales. Metals were always considered to be antimicrobial, but it did not apply to electrochemically active bacteria which were able to colonize the surface of materials and form a highly active biofilm.6 Among all the metallic materials, stainless steel (SS) is the most promising candidate for MFC anode due to its suitable mechanical and physical properties as well as low costs.4, 7 However, the direct use of SS as anode material in bioelectrochemical system is relatively rare primarily because the passivating oxide layer on SS surface makes it less biocompatible and also hinders the electron transfer between electrode and 3



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bacteria.8 For this reason, various surface modifications of SS, including flame-oxidization,9, coating12,

13

10

flame-deposition,11 binder and binder-free carbon material

were taken to produce a more biocompatible electrode surface with

enhanced bioelectrocatalytic activity. Nevertheless, the oxidation or deposition methods under high temperature would result in high risk of corrosion under the MFC environment.14 The coatings of carbon using polymer binder would lead to high internal resistance and poor electrical performances15 while the binder-free methods would cause a weak carbon-metal adhesion and unstable current generation.13, 16 And all these carbon coating methods are too expensive or complicated to be applicable in industries. Therefore, more efficient methods are still required in SS electrode modification. Recently, conductive polymers (CP) have exerted a tremendous fascination on electrode modification in virtue of their high conductivity, environmental durability and simple synthesis.17,

18

Conductive polymers such as polypyrrole (PPy),

polyaniline (PANI), polythiophene (PTh) and their composites have been successfully applied in the modification of carbon materials in MFCs and the results were remarkbale.16,

19-22

But to our best knowledge, only a limited number of reports

discussed the applications of conductive polymer modification on SS anode,23,

24

which is still far from enough. Poly(3,4-ethylenedioxythiophene), commonly known as PEDOT, is an important derivative of polythiophene possessing a better conductivity in neutral solution, compared with other CPs like PPy and PANI.25 The dioxyethylene bridging group 4


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across the 3- and 4- positions of the heterocycle prevents the possibility of α-β’ coupling, thereby ensuring its electrochemical stability.26 It’s also worth pointing out that the PEDOT backbone is positively charged, which may electrostatically interact with the negatively charged bacteria and thereby facilitating biofilm formation. These properties of PEDOT could possibly be exploited in MFCs to facilitate performance improvement. Here, PEDOT modified SS plates were firstly used as anodes in MFCs. Electrochemical polymerization is an oxidant-free, efficient and easily controlled method to synthesize polymers.25,

27

In this paper, PEDOT was

synthesized in-situ over stainless steel plate using electrochemical method in a neutral aqueous medium. The characteristics of PEDOT/SS electrode have been analyzed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and SEM etc. The enhancement of MFC performance was further tested and discussed. Moreover, possible mechanisms were also proposed to help understand the characteristics of the PEDOT modified anode. 2.Materials and Methods 2.1Electrode modification A 304 stainless steel plate with a thickness of 1mm (Oudifu Metal Material Co., Ltd, China) was cut into 2 cm × 2 cm pieces before modification. The SS plates were then polished with sand paper carefully and ultrasonically cleaned in acetone, ethanol and deionized (DI) water for 30 min respectively. 3,4-ethylenedioxythiophene

(EDOT)

monomer

(>90%,Shanghai

5


Macklin


Biochemical Co., Ltd, China) was distilled before use. Other chemicals mentioned were all analytical reagents and were used as received. For electrode fabrication, PEDOT was electrochemically polymerized in situ under potentiostatic conditions using electrochemical workstation (IviumStat, Ivium Technologies BV, The Netherlands). EDOT monomer solution of 0.01 M was added into 0.1 M sodium p-toluenesulfonate (TsONa) solution by stirring for 1 h at room temperature to make it completely dissolved. All solutions were deaerated by a dry nitrogen stream. Afterwards, electrochemical polymerization was performed in a one-compartment cell, in which SS plate acted as working electrode, titanium sheet (2 cm × 2 cm) as counter electrode and Ag/AgCl as reference electrode. The potential applied was 2.5 V and the working-counter electrode distance was 1 cm. The reaction lasted for 10 min and black colored and uniform PEDOT film was successfully coated on SS plate (PEDOT/SS). PEDOT/SS was then immersed in ethanol and distilled water sequentially to remove adsorbed electrolyte, monomer and the soluble oligomers formed during electrochemical reaction and then dried at room temperature. 2.2Surface characterization The morphologies of the electrodes were characterized using field emission scanning electron microscope (FE-SEM, JEOL7800E). The contact angle meter (Attension, Theta, Biolin Scientific) was employed to investigate the hydrophobic property of the electrode surface. X-ray diffraction (XRD, XRD-7000S, Shimadzu) patterns were performed at room temperature with Cu Kα radiation (λ = 1.5418 Å). The X-ray tube was operated at 40 kV and 40 mA. The diffractograms were recorded 6


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in the 2θ range of 10–80 ° with a step size of 0.01 ° and a counting time of 10 s per step. The transmission electron microscope (TEM, JEOL 1200CX) was used to characterize the PEDOT particles which were dispersed in ethanol by peeling off the polymer coating. 2.3Electrochemical characterizations The electrochemical measurements were performed at an electrochemical workstation (Ivium Technologies BV, The Netherlands) in a three-electrode system. Platinum foil acted as counter electrode and Ag/AgCl as reference electrode. After deoxygenation by purging with nitrogen, 50 mM phosphate buffer solution (pH 7.0) and 0.1 g/L acetate was used as electrolyte medium in each test. The measurement of the cyclic voltammetry (CV) and Tafel corrosion polarization curves were both performed at the scan rates of 20 mV/s. And the electrochemical impedance spectroscopy (EIS) measurement was conducted at open circuit potential (OCP) and the frequencies swept was from 10 kHz to 0.01 Hz with a sine wave of 5 mV amplitude. Electrodes with biofilm adherence were marked as SS-biotic and PEDOT/SS-biotic accordingly. All the experiments were performed under strictly anaerobic conditions. 2.4MFC setup All MFC experiments were performed using a single-chamber membrane-less MFC. In particular, the cell was fabricated from a cylindrical container made of polymethylmethacrylate (PMMA) with an effective volume of 30 mL. The SS anode, bare or modified, was placed in the center of the reactor while the air-cathode (10% 7



Pt/C catalyst, 0.5 mgPt/cm2)28 was arranged at one end. All MFCs were operated in batch-cycle mode at 30 ℃ in a temperature controlled cabinet. The voltage data was recorded with a data acquisition module (DAM-3059R, Beijing Art Technology Development Co. Ltd., China). For anode acclimation, the inoculum source was fresh anaerobic granular sludge supernatant from fruit waste treatment process. In the stage of inoculation, under an external resistance of 1000 Ω, the reactors were fed with a mixture (volume ratio of 1:1) of inoculum source solution and 50 mM phosphate buffer (consisting of 11.47 g Na2HPO4·12H2O, 2.75 g NaH2PHO4·2H2O, 0.31 g NH4Cl, 0.13 g KCl and 12.5 mL trace mineral solution per liter)29 containing 1 g/L sodium acetate as carbon source, to promote the adaptation and adhesion of electricigens. After two or three cycles, distinct and stable voltage platforms would appear. The medium was then replaced with 1 g/L sodium acetate dissolved in 50 mM phosphate buffer to maintain the normal running of MFC systems.30 Long term operation of the electrodes was conducted for 30 days after acclimation stage in the above-mentioned fed-batch mode with 1000 Ω external resistance, which was designed for evaluating the repeatability of MFC system31 and the influence of PEDOT coating towards the corrosion risk of electrodes. The power generation performances were presented by current density based on Ohm’s law32. Domestication operation was also conducted by using different resistors from 1000 Ω to 50 Ω as external resistance (six downward steps). At least 2-3 cycles were maintained for each resistance stage to ensure the stability of MFCs. Then the bio-anode was considered to 8


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be ready for subsequent EIS, CV and power density tests. 3.Results and Discussions 3.1Characteristics of the PEDOT film Surface morphologies of the electrodes were characterized by SEM (Figure 1).As shown in Figure 1A, the surface of untreated SS showed scratches caused by polishing but it was relatively clean and smooth. However, it could be observed that the morphology of modified SS has significantly changed to be rough from Figure 1B and C. The granular and clustered PEDOT firmly adhered to SS and made a rough and porous surface, providing better nutrient distribution and larger electrode surface area, which was proved to be beneficial for microorganism attachment and consequently lead to higher electrical output.15, 21, 33

Figure 1. SEM images of (A) SS electrode (500×); (B) PEDOT/SS electrode (1000×) and (C) PEDOT/SS electrode (10000×)

The microstructures of the electrodes were evaluated by XRD and TEM (Figure 2). It was shown that the PEDOT/SS electrode had almost the same profiles as bare SS, indicating that the original crystal structure of SS is not affected by PEDOT coating. The absence of PEDOT diffraction peaks illustrated that the polymer was with low crystallinity.18 The TEM image of PEDOT coating exhibited a undefined and noncrystal morphology with irregular wrinkles, which was consistent with the results 9



of XRD.

Figure 2. XRD of SS and PEDOT/SS electrode (left) and TEM image of PEDOT particles (right)

The hydrophilicity was evaluated by water contact angle measurements. The untreated SS electrode was quite hydrophobic as its water contact angles were higher than 90 ° (Figure 3A). Comparatively, the contact angles of the electrode reduced to a little smaller than 50 ° after PEDOT modification (Figure 3B). These results demonstrated that the hydrophilicity of SS surface has been improved obviously due to PEDOT coating. And electrodes with better hydrophilicity have been proved to provide more approachable sites for bacterial colonization.

Figure 3. Contact angles of different electrodes (A: SS; B: PEDOT/SS)

3.2Electrochemical analysis CVs were performed with scanning rate of 20 mV/s for SS and PEDOT/SS electrodes under abiotic and biotic conditions, respectively (Figure 4). Insert showed the enlarged CVs of SS samples. The absence of obvious redox peaks of PEDOT/SS 10


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electrodes indicated the successive multiple surface redox reactions leading to the

pseudo-capacitive

charge

storage

mechanism,

implying

high

pseudo-capacitive activity of conducting polymers.34 The PEDOT modified anodes also exhibited higher current over the bare SS anodes, indicating an enhanced electron transfer rate due to the improvement of conductivity and electroactivity35. Moreover, the integral areas among the CV curves were related to the capacitance of the electrode. The specific capacitance (Csp) of electrodes can be determined using the following equation (1):

= ∆

(1)

Where is the specific capacitance, is the integrated area of the CV curve, m is the mass of the active material, ∆ is the potential range, and v is the scanning rate of CV test. Apparently, there was a significant increase in CV integral areas after PEDOT modification both in freshly prepared and long term used samples, indicating an obvious improvement in electrode capacitance. As is reported, the capacitive electrode could enable the on-demand release of extracellular electrons through charge-discharge process and thus helped to lead a stable performance of MFCs.

11



Figure 4. CVs for SS and PEDOT/SS electrodes under abiotic and biotic condition

To further characterize the effect of PEDOT modification, EIS studies were conducted and equivalent circuit analysis was used to get the impedance parameters. Figure 5 shows the Nyquist plots of fresh-prepared and biofilm-attached PEDOT/SS and SS electrodes in a mixture of 0.1 g/L acetate and 50 mM PBS solution. Qualitatively, it could be easily observed that the arc diameter decreased obviously after modification, both in abiotic and biotic conditions. An equivalent circuit of Rs(RctCdl) (Figure 5 inset) was selected to fit the experimental data and the corresponding simulated data were also given in Table 1, in which Rs was the electrolyte solution resistance, Rct was the charge transfer resistance of electrode film and Cdl was the double layer capacitance of the electrode surface. It could be observed that Rct value decreased from 139.4 to 34.4 Ω and 1155 to 461.1 Ω after modification with and without biofilm, which indicated that the PEDOT coating could decrease electrode impedance and favor electron transfer effectively. Moreover, the modified electrode showed much higher Cdl value as well, implying that more electrochemically active sites existing on PEDOT/SS surface. In general, The EIS results demonstrated that PEDOT modified anodes could greatly accelerate 12


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extracellular electron transportation and have better electrochemical kinetics, which contributed to improve the system performance and boost the power density.

Figure 5. Nyquist plots for SS and PEDOT/SS electrodes under abiotic and biotic condition Table 1. Simulated parameters of electrodes from Nyquist plots Object

Rs(Ω)

Error%

Rct(Ω)

Error%

Cdl(F)

Error%

SS

11.7

1.7

139.4

3.7

3.60E-4

2.8

PEDOT/SS

12.5

1.3

34.4

2.2

0.30

1.0

SS-biotic

25.43

3.1

1155

7.0

1.45E-3

5.2

PEDOT/SS-biotic

26.47

4.2

461.1

6.6

2.56E-3

4.8

Tafel plots of the electrodes were shown in Figure 6 and the corrosion current density (icorr), corrosion potential (Ecorr), polarization resistance (Rp) and corrosion rate (C) calculated by extrapolation of the linear portions were listed in Table 2. The results showed more positive Ecorr and higher Rp of PEDOT/SS due to the presence of electrochemically generated PEDOT film. The most likely reason was that PEDOT on SS surface made a thin, dense and stable film that impeded the invasion of corrosive ions (like Cl- and CH3COO-) from electrolyte to the SS. Surprisingly, after one month of operation, the corrosion rate increased from 0.39 to 1.25 mm/y for bare SS indicating there was an evident deterioration in the anti-corrosive property. While the 13



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corrosion rate of those with PEDOT coatings showed no significant change. It may be caused by the condition changes brought by attached bacteria. Previous investigations36 had shown that some of the electricigens were capable of secreting extracellular polymeric substance (EPS), which often leads to highly localized changes in the concentration of the electrolyte constituents and pH levels. These chemical changes may weaken the oxide film of SS bases at specific locations, thereby facilitating localized attack.37 For modified SS electrode, the presence of PEDOT film blocks their access to SS surface. However, as for bare SS anodes, long-term direct contact of these mediators changed its external environment and possibly altered the corrosion dynamics of SS electrodes in a significant way and consequently resulted in a high risk of corrosion.

Figure 6.Tafel plots for SS and PEDOT/SS electrodes under abiotic and biotic condition Table 2. Corrosion parameters of SS and PEDOT/SS electrodes obtained from Tafel plots Material

Ecorr (V)

icorr(A/cm2)

Rp(Ω)

C (mm/y)

SS

-0.44

4.47E-5

292.6

0.39

PEDOT/SS

-0.23

4.36E-5

412.4

0.38

SS-biotic

-0.61

1.44E-4

162.0

1.25

PEDOT/SS-biotic

-0.11

4.42E-5

402.1

0.38

3.3Performance of MFCs 14


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As stated above, with rougher surface, better hydrophilicity and more conducive for anodic biofilm formation, PEDOT modified SS anode would greatly improve the power density output over the SS anode in MFCs. To verify this, the performance of the anodes was recorded and compared. Figure 7 showed the bioelectrocatalytic current generation over time at the SS and PEDOT/SS electrodes in fed-batch mode with external resistance of 1000 Ω. It was observed that the PEDOT/SS anode reached a plateau of ~1150 mA/m2 (at voltage of ~0.46 V), which was almost 2.6 times higher than the output of the untreated ones (~0.45 A/m2, ~0.18 V). Notably, the PEDOT/SS anode could generate a much obvious and stable voltage platform under the same experimental conditions. It might be attributed to the superior capacitance property and microporous structure of the PEDOT coating. As shown in Figure 8, the maximum power density (608.6 mW/m2) of MFC with PEDOT coating was approximately 6 times higher than that with bare SS anode (101.9 mW/m2). It certainly demonstrated that the PEDOT modification exhibited more beneficial effects on SS anode as mentioned above and efficiently increased the power output of MFCs. It was also worthy to note that the output power density of the PEDOT/SS anode was comparable to previously reported MFC with different anodes under similar experimental conditions. Thus, the PEDOT/SS electrode could be a very promising candidate as high-performance MFCs anode.

15



Figure 7. Power generation in MFCs with an external resistance of 1000 Ω

Figure 8. Power density and anode polarization curves of SS and PEDOT/SS anode in MFCs

4.Conclusions This study used PEDOT to modify SS and made it a high performance and stable anode in microbial fuel cells. PEDOT polymerized on SS plate produced a dense and conductive film which not only improved extracellular electron transportation but also protected the SS base from chemical and biogenic corrosion. This layer also provided a larger and more porous surface area and hence increased the electrode capacity. As a result, the power output of MFCs with PEDOT/SS anodes reached 608.6 mW/m2, nearly 6 times higher than that with bare SS anodes (101.9mW/m2). The results 16


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indicated that PEODT modified SS plate would be a promising anode material for high performance and scaling-up MFCs.

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Figures and Tables captions

Figure 1. SEM images of (A) SS electrode (500×); (B) PEDOT/SS electrode (1000×) and (C) PEDOT/SS electrode (10000×) Figure 2. XRD of SS and PEDOT/SS electrode (left) and TEM image of PEDOT particles (right) Figure 3. Contact angles of different electrodes (A: SS; B: PEDOT/SS) Figure 4. CVs for SS and PEDOT/SS electrodes under abiotic and biotic condition Figure 5. Nyquist plots for SS and PEDOT/SS electrodes under abiotic and biotic condition Figure 6. Tafel plots for SS and PEDOT/SS electrodes under abiotic and biotic condition Figure 7. Power generation in MFCs with an external resistance of 1000 Ω Figure 8. Power density and anode polarization curves of SS and PEDOT/SS anode in MFCs

Table 1. Simulated parameters of electrodes from Nyquist plots Table 2. Corrosion parameters of SS and PEDOT/SS electrodes obtained from Tafel plots

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179x140mm (96 x 96 DPI)


modified stainless steel as anode in air-cathode microbial fuel cells

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