Effect of heat-treatment atmosphere on the current generation of TiO2 nanotube array electrodes in microbial fuel cells

Electrochimica Acta 257 (2017) 203–209

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Effect of heat-treatment atmosphere on the current generation of TiO2 nanotube array electrodes in microbial fuel cells Lijie Huanga,b , Xueqing Zhangc , Dongsheng Shena,b , Na Lia,b , Zhipeng Gea,b , Yuyang Zhoua,b , Mengjiao Zhoua,b , Huajun Fenga,b,* , Kun Guoa,d,* a

School of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310012, PR China Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, Hangzhou 310012, PR China Advanced Water Management Centre, The University of Queensland, St Lucia, QLD 4072, Australia d Center for Microbial Ecology and Technology, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium b c

A R T I C L E I N F O

Article history: Received 17 August 2017 Received in revised form 4 October 2017 Accepted 10 October 2017 Available online 12 October 2017 Keywords: TiO2 nanotube arrays Gas atmosphere Current density Crystallinity Electron transfer rate

A B S T R A C T

We investigated the effects of the heat-treatment atmosphere on the current generation of Ti electrodes in microbial fuel cells (MFCs). The maximum current density was achieved for TiO2 nanotube array electrodes heated in H2 (TNA-H2) (22.69  0.18 A m 2). Physical analysis of the Ti electrode surfaces revealed that the specific surface area increased after electrochemical anodization. The crystallinities of the TiO2 nanotube array electrodes heated in H2 (81.66%), CH4 (74.48%), N2 (59.67%), and O2 (44.82%) showed a significant positive correlation with the corresponding current densities (P < 0.05, R2 = 0.9469). In addition, the highest electron transfer rate constant was achieved for TNA-H2 (2.38 s 1). The large specific surface area, good chemical stability, high electron transfer rate, and good biocompatibility of TNA-H2 suggest its great potential for application as an anode system in MFCs. © 2017 Published by Elsevier Ltd.

1. Introduction Microbial fuel cells (MFCs) are a recently developed microbial electrochemical technology that can be used to directly extract chemical energy from organic matter and convert it into electrical energy [1]. The electrodes of these devices play a key role in their practical application, determining the amount of current generated and the cost of the overall system [2]. Widely used carbon-based materials are not expected to be suitable for practical applications in MFCs because of their low specific conductivity and poor mechanical strength [3,4]. Metallic electrodes such as Pt, Au, Ag, Cu, Ti, and stainless steel possess high conductivity and scale-up potential, thereby representing promising candidates for MFC applications [3,5,6]. However, the high cost associated with Au and Pt, the relatively low corrosion resistance of stainless steel, and the natural antimicrobial properties of Ag and Cu limit the practical use of these materials [3,5,7]. Ti is widely used as a stable anode in the electrolysis industry because of its good corrosion resistance, mechanical properties, and electrical conductivity [8,9]. However, the current generation

* Corresponding authors. E-mail addresses: [email protected] (H. Feng), [email protected] (K. Guo). https://doi.org/10.1016/j.electacta.2017.10.068 0013-4686/© 2017 Published by Elsevier Ltd.

performance of Ti as a bio-anode in MFCs is poor, which has been attributed to the poor electron transfer activity between Ti and bacteria and the low specific surface area of Ti [10,11]. The hydrophilicity and biocompatibility of the electrode surface are also important factors affecting the anode current generation performance [7]. Ti electrodes with modified surfaces, such as Pt [12] and TiO2 coatings [13] and heat-treated surfaces [10], have been shown to possess relatively high current densities. However, the maximum current density, i.e., 0.476 A m 2 [10,12,13], remains much lower than that generated by commonly used graphite plates (6.0 A m 2) [11]. Our previous studies have demonstrated that electrochemical anodization can be used to increase the specific surface area of Tibased electrodes by forming a layer of TiO2 nanotube arrays (TNA) on the Ti surface [11]. The hydrophilicity and biocompatibility also improved significantly after the TiO2 nanotubes were formed. The current produced from our modified Ti electrode (12.7 A m 2) was twice that of a graphite-plate-based electrode. However, this current density remained inferior to that of carbon-black-modified electrodes (15.3 A m 2) [4] and modified metal electrodes (15– 19.2 A m 2) [6,14]. Considering the potential for the broader application of Ti electrodes, an optimized modification method is necessary to further improve both the current production and electron transfer rate.

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Previous studies have shown that highly crystalline materials exhibit high electron transfer rates and low electron transport losses [15]. Enhanced electron transfer was observed when the structure of TiO2 was changed from amorphous to crystalline [16]. The crystallinity of TiO2 can be altered after heating in H2, CH4, N2, and O2 atmospheres [17–20]. Hence, we hypothesized that it would be possible to improve the crystallinity of TiO2 and, in turn, the electron transfer rate by optimizing the annealing atmosphere. Therefore, in this work, we focused on TNA electrodes heated in H2, CH4, N2, and O2 to determine the effect of heat treatment in different atmospheres on the crystal structure of TiO2. We aimed to optimize our modification strategy to achieve high current density output from Ti electrodes in MFCs. 2. Experimental 2.1. Electrode preparation Ti plates (Guangzhou China; 0.5 mm  20 mm  10 mm) were used as substrates to prepare TNA electrodes. Before modification, the Ti plates were sequentially polished with 180, 1000, and 2000 mesh sandpaper. The TNA electrodes were prepared using electrochemical anodization, the details of which are presented elsewhere [11,21]. The electrolyte solution was prepared by dissolving NaF (0.5 wt.%) in a glycol and water solution (Vglycol: Vwater = 8:2). The Ti plates for the anode were immersed in the prepared solution at 55  C, and a voltage of 30 V was applied for 6 h using a constant-voltage instrument. The prepared electrodes were first placed in a tube furnace and heated to 600  C over 1 h and then maintained under isothermal conditions at a constant gas flow rate for 30 min. The experiments were performed under four atmospheres (N2, O2, H2, and CH4). The temperature of the tube furnace was reduced to less than 50  C for removal of the sample. The gas flow was maintained at 300 sccm throughout the process. The electrode was sealed with a hot-melt adhesive in addition to the active electrode area. The electrodes were connected with Ti wire. 2.2. MFC operation All the reactor configurations and operating temperatures were the same as those applied in our previous study [11]. The anolyte consisted of modified M9 medium [22] with 1 g/L sodium acetate as the electron donor and a 10-mL inoculation of fresh anodic effluent from an existing parent MFC reactor. The catholyte consisted of 20 g/L potassium ferricyanide. The output voltages (V) of the MFCs across external resistors were measured and recorded with a data acquisition system (34970A, Agilent, USA). 2.3. Bioelectrochemical tests All the bioelectrochemical tests were performed using an electrochemical workstation (Biologic VSP, Claix, France) in a three-electrode cell configuration with a Ti working electrode, Ag/ AgCl (3.5 M KCl) reference electrode, and graphite plate counter electrode. Electrochemical impedance spectroscopy (EIS) measurements were performed at open-circuit potential with an amplitude of 10 mV over the frequency range of 100 kHz to 1 Hz in modified M9 solution (0.1 g/L NH4Cl, 0.5 g/L NaCl, 4.4 g/L KH2PO4, 3.4 g/L K2HPO4, 0.1 g/L MgSO4, and 2 g/L NaHCO3) [23]. The data were then fitted using EC-LAB software. The output power was recorded using linear sweep voltammetry (LSV) in M9 solution on the electrochemical workstation in a three-electrode configuration; the anode served as the working electrode and the cathode was used as the counter and reference electrodes [24].

2.4. Electrode surface characterization The surface topography of the electrodes was characterized using scanning electron microscopy (SEM, Quanta F250, FEI, USA), and the surface crystal structure and crystallinity were determined using X-ray diffraction (XRD, D8 Advance, Bruker, Germany) and analysis with the assistance of Jade 5.0 software. The surface chemical composition and electrochemical properties were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, USA). The XPS spectra were calibrated to the elemental Ti peak and analyzed with XPSPEAK41 software. The specific surface area was characterized using a surface capacitance method measured by electrochemical workstation (Biologic VSP, Claix, France) [11,25]. Contact angle measurements with Milli-Q water were performed with a contact angle goniometer (Dataphysic oca20, Germany) to estimate the surface hydrophobicity. Correlation analysis was performed using SPSS software and bivariate correlation analysis. 2.5. Biofilm characterization The electrochemical activities of the biofilms at the electrodes were determined using cyclic voltammetry (CV) in the potential window between 0.7 and 0.2 V (vs. Ag/AgCl) at a scan rate of 1 mV/s. The bacterial viability of one of the biofilms for each condition was assessed using a LIVE/DEAD BacLight bacterial viability staining kit (Molecular Probes). The stained biofilms were visualized, and z-stacks were obtained using confocal laser scanning microscopy (CLSM, Zeiss LSM 780, Carl Zeiss, Germany). The 3D biofilm images were reconstructed using the software ZEN 2010. A biofilm sample from another specimen for each condition was subjected to cell disruption, and the protein concentration was then measured using the Coomassie Brilliant Blue method [26]. To determine the number of adhered cells in the biofilms, a crystal violet staining method (OD570) was used, as described elsewhere [27]. Briefly, after 24 h of biofilm formation, the medium was aspirated, and non-adherent cells were removed by washing with 10 mL of PBS. The remaining biofilms were stained with 0.25% crystal violet solution. After drying at room temperature, ethanol was used to solubilize the crystal violet, and the absorbance was measured at 570 nm (Bio-Rad, USA). 3. Results and discussion 3.1. Current generation performance and biofilm characteristics 3.1.1. Current generation performance The maximal current density (22.69  0.18 A m 2) was achieved with the TNA electrode heat treated in H2 (TNA-H2), followed by the TNA electrodes heat treated in CH4 (TNA-CH4, 17.62  0.27 A m 2), N2 (TNA-N2, 13.70  0.05 A m 2), and O2 (TNA-O2, 0.55  0.15 A m 2), as illustrated in Fig. 1. The power outputs showed the same trend. Compared with the lowest power density (0.16  0.04 W m 2) produced by TNA-O2, the maximum power densities produced by the other electrodes represented increases of 21, 24, and 37 fold for TNA-N2 (3.37  0.05 W m 2), TNA-CH4 (3.97  0.05 W m 2), and TNA-H2 (5.94  0.46 W m 2), respectively (Fig. S1A). Notably, the maximal projected current densities of the unheated TNA electrode and pristine Ti substrates were 0.79  0.15 and 0.49  0.12 A m 2, respectively (Fig. S1). Both of these values were much lower than the current generated by the TNA electrodes heated in H2, N2, and CH4. These results indicate that the current density of the Ti electrodes could be considerably enhanced by heat treatment and that the enhancement depended on the heattreatment atmosphere. The TNA electrodes heated in H2, N2, and CH4 showed enhanced current output when applied in MFCs;

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however, heating in O2 was detrimental for the current generation (Fig. 1). Although heat treatment under H2, CH4, and N2 enhanced the current, the maximum current density was achieved under H2. The current densities of the unmodified Ti substrates heated under the same atmospheres approximately doubled compared with that of the unheated Ti substrate; however, the values remained low because of the low surface areas of the unmodified Ti substrates.

Fig. 1. Current output over time of MFCs with different anodes in the reactor one (A); reactor two (B).

3.1.2. Biofilm characteristics The biofilm characteristics are important factors in evaluating the current density [28]. As observed in the CLSM biofilm images (Fig. 2), bacteria only attached to certain points of the electrode surface after the heat treatment in O2, and the thickness of the biofilm was 22 mm. However, the TNA electrodes had biofilm thicknesses of 100, 80, and 60 mm after heat treatment in H2, CH4, and N2, respectively. Similarly, the protein concentration of the TNA electrodes heated in H2, CH4, and N2 were approximately 709.2, 457.2, and 363.9 mg cm 2, respectively. These values were all higher than the protein concentration obtained for the electrode heat treated in O2 (84.0 mg cm 2). The variation of the maximum current density for the different electrodes showed a positive correlation with the amount of biomass (P < 0.05, R2 = 0.9527) (Fig. S2). To further investigate the biomass and current density differences, we analyzed the hydrophilicity, specific surface area, and biocompatibility of the electrodes. After electrochemical anodization, the smooth surface of the Ti substrate was modified by a layer of TiO2 nanotubes (diameters of 50–70 nm, Fig. 3B). Studies have shown that the increase in capacitance is consistent with the increase of the BET surface area [11,14]. Further measurements of the specific surface area revealed that the TNA electrodes heated in

Fig. 2. CLSM images of biofilms on electrodes were heat-treated in different atmospheres (A: O2; B: N2; C: CH4; D: H2). The biofilms were stained using the LIVE/DEAD BacLight Bacterial Viability Kit (live bacteria, green).

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Fig. 3. Photos (A) and SEM images (B) of Ti electrode after different modifications.

H2, CH4, and N2 exhibited higher surface areas (Table S1); however, we did not find a significant correlation (P > 0.05) between the specific surface areas and current densities of the various electrodes tested. The specific surface area of the unmodified Ti substrate was approximately 1/60 of that of the TNA electrodes heated under different atmospheres except O2. This difference in surface area may explain the improved electrode current density after modification. In addition, the specific surface area of the TNAO2 electrode was only 1/180 of that of the TNA heated in the other three atmospheres, which can be attributed to the formation of a

visible white oxide layer on the surface of the TNA-O2 electrode (Fig. 3B). The water contact angle of the unheated TNA electrode (approximately 10.8 ) was higher than those of the electrodes heated under different atmospheres. All the heated substrates were completely hydrophilic with contact angles of 0 (Table 1). Hence, the heat treatment in all the atmospheres improved the biocompatibility of the electrodes. These results confirm that no major differences in the hydrophilic properties of the substrates were detected after heating.

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Table 1 Water contact angle of Ti electrode after different modifications. Different Treatments of Titanium Substrate

Water contact angle( )

Titanium Substrate Untreated TNA TNA heat-treated in TNA heat-treated in TNA heat-treated in TNA heat-treated in

75.5  0.8 10.8  0.8 0 0 0 0

O2 N2 H2 CH4

The adhesion abilities of the biofilms (quantified by OD570) were 2.8797  0.0049, 3.0953  0.0376, 2.5610  0.0947, and 1.0540  0.1448 on TNA-H2, TNA-CH4, TNA-N2, and TNA-O2, respectively (Fig. S3), and showed no significant positive correlation with the current density (P > 0.05). According to the biocompatibility results, TNA-CH4 exhibits more affinity for microorganisms; however, the best biofilm growth occurred for TNA-H2. Therefore, beside biocompatibility, there are other reasons to promote the growth of biofilms, thereby further improving the current generation performance. In summary, the biocompatibility, hydrophilicity, and specific surface area of the electrodes were not determining factors contributing to the current density differences among the TNA electrodes heated in H2, CH4, and N2. 3.2. Effect of chemical properties of electrode surface on current density The differences in the current performance among the electrodes could not be explained by the variations in their physical properties. Hence, we applied XPS and XRD to analyze the differences in the chemical properties of the TNA electrodes (Fig. S4). The XPS peaks at 454.2 eV (Ti0 2p3/2), 459.2 eV (Ti4+ 2p3/ 2), and 465.1 eV (Ti4+ 2p1/2) for the Ti substrate electrodes confirm that the Ti surface underwent partial oxidization to TiO2 after exposure to air for a long time [30]. After electrochemical oxidation, regardless of whether heat treatment was applied, the TNA electrodes exhibited only two peaks (Ti4+ 2p and Ti4+ 2p1/ 2) at binding energies of 458.7 and 464.6 eV, respectively, indicating that the Ti surface was completely oxidized [31]. Therefore, the TiO2 surface layer was stable to heat treatment in reducing atmospheres [32]. The XPS analysis eliminates the possibility of chemical element transformation of the electrodes contributing to the differences in the current output. A small diffraction peak at 25.3 appeared in the XRD patterns (Fig. S5), which can be attributed to the 101 planes of anatase TiO2 (JCPDS No. 21-1272). This result indicated that a layer of amorphous TiO2 formed on the TNA electrodes, which is consistent with the XPS results. The XRD patterns of the TNA electrodes heated in different atmospheres contained two sharp reflections at 25.3 and 37.8 , corresponding to the (101) and (112) diffraction peaks of anatase TiO2 (JCPDS No. 83-2243), respectively (Fig. 4(A– D)). The small diffraction peak at 27.4 is consistent with the 111 planes of rutile TiO2 (JCPDS No. 21-1276). Three main peaks were observed at 2u values corresponding to the (101), (102), and (103) planes of Ti (JCPDS No 65-3362) [33]. The XRD patterns indicated that rutile- and anatase-type TiO2 phases appeared in the electrodes after the heat treatment in the four atmospheres, i.e., a mixture of crystal phases was induced by heating. The corresponding crystallinities of the TNA electrodes heated in H2, CH4, N2, and O2 were 81.66%, 74.48%, 59.67%, and 44.82%, respectively, (Fig. 4). The crystallinity showed a significant positive correlation with the current density (P < 0.05, R2 = 0.9469) (Fig. S6). Highly crystalline TiO2 has been previously reported to exhibit a high electron transfer rate [15]. Based on Laviron’s theory [34], the

Illustration

apparent electron transfer rate constants (kapp) were determined to be 2.38, 1.92, 1.41, and 1.24 s 1 for TNA-H2, TNA-CH4, TNA-N2, and TNA-O2, respectively (Fig. 5). The electron transfer rate also positively correlated with the crystallinity (R < 0.05, R2 = 0.9550), indicating that the higher current density could be attributed to the improved crystallinity. Anatase-type TiO2 has a lower electron transport limit than rutile-type TiO2 [35]. Hence, a larger proportion of anatase-type TiO2 in the crystal structure benefits the electron transport rate. Moreover, maximum anatase-type TiO2 was obtained at 600  C heat treatment. If the temperature is above 600  C, rutile-type TiO2 will be formed, which is not conducive to electron transport [36]. Therefore, heat treatment at 600  C result in a higher electron transfer rate. The proportion of anatase-type TiO2 in the crystal mixtures showed the same trend as that observed for the crystallinity (Table 2). This result indicated that the highest electron transfer rate obtained from TNA-H2 was the result of a combination of a higher degree of crystallinity and a more favorable phase composition. Our work on improving the electron transfer chain between electrochemically active microbes and electrodes may aid in the development of MFCs with high current output. We also observed a denser biofilm on the electrode surfaces, which evolved with the degree of crystallinity (Fig. 2). More energy could be captured by the microorganisms because of the more efficient electron transfer chain. 3.3. Electrochemical characteristics EIS was further used to verify the electron transfer rate. The charge transfer process between the anode and biofilm was determined by EIS under open-circuit potential conditions. Fig. 6A shows that the charge-transfer resistances of the TNA-H2, TNACH4, TNA-N2, and TNA-O2 electrodes were approximately 5.861, 87.32, 139.6, and 4337 Ohm, respectively. Compared with the

Fig. 4. XRD patterns of TNA electrode heated in different atmospheres at 600  C (A) O2 atmosphere; (B) N2 atmosphere; (C) CH4 atmosphere; (D) H2 atmosphere.

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Fig. 5. CVs of biofilms on different heat-treated TNA electrodes in fresh M9 medium without: (A) TNA-H2; (B) TNA-CH4; (C) TNA-N2; (D) TNA-O2. The insets: peak potentials as a function of the logarithm of the scan rates.

Table 2 Crystalline phase composition and crystallinity of different modified Ti based on the XRD analysis. Different Treatments of Titanium Substrate

Crystallinity

Phase composition (%) Anatase-type TiO2

Rutile- type TiO2

TNA TNA TNA TNA

81.66% 74.48% 59.67% 44.82%

85.7% 93.5% 95.2% 96.9%

14.3% 6.5% 4.8% 3.1%

heat-treated heat-treated heat-treated heat-treated

in in in in

O2 N2 CH4 H2

Fig. 6. (A) Electrochemical impedance spectroscopy for the biotic TNA electrode after heat-treated in different atmospheres at open circuit potential, an amplitude of 10 mV, and a frequency range of 100 kHz to 1 Hz in fresh culture (B) Cyclic voltammetric response of the biotic anode in fresh culture. The CVs of TNA that treat different were obtained starting from the second cycle. The potential window was 0.7 V to 0.2 V and the scan rate was 1 mV s 1.

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charge-transfer resistance of our previously reported modified Ti electrode (580 V) [11], we observed a significant reduction in resistance for the TNA-H2, TNA-CH4, and TNA-N2 electrodes in this study. This finding confirmed that the thermal treatment was beneficial for electron transfer and that the electron transfer rate was affected by the treatment atmosphere. Together with our XRD results, these findings suggest that the electron transfer rate was affected by the crystallinity. We performed CV measurements of the biofilms in the same medium to study the electrochemical activity of the biofilms at the electrodes under acetate turnover conditions. Compared with the electrode CV measurements in bare M9 medium (Fig. S7), those of the electrodes in a live medium (Fig. 6B) exhibited a classic sigmoidal shape for anodic biofilms under turnover conditions [37]. This result confirmed that the current generation could be attributed to biofilm-related extracellular electrons [25]. In addition, we assessed the corrosion resistance of the different TNA anodes after biofilm growth using Tafel analysis (Fig. S8). The pitting corrosion potential after the surface heat treatment in H2 increased to 0.35 V (vs. Ag/AgCl), which is higher than that of stainless steel (0.2 V) [22]. This result confirmed that biocorrosion did not occur in the anodic environment and that TNA-H2 has sufficiently high corrosion resistance for use as an anode in MFCs. 4. Conclusion The effect of the heat-treatment atmosphere (H2, CH4, N2, and O2) on the current generation of TNA electrodes was investigated in this work. Heat treatment in H2 at 600  C resulted in the highest electrode current density. In addition, heat treatment improved the hydrophilicity of the Ti electrode regardless of the treatment atmosphere. We further showed that a higher degree of crystallinity of the TiO2 phase correlated to an increased rate of electron transfer. The improved current density was attributed to the higher crystallinity of the TiO2 electrode in the MFC system. Our findings indicate that heat treatment in H2 is effective for enhancing the electron transfer efficiency of Ti anodes for MFC applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (51478431), the Science and Technology Planning Project of Zhejiang Province (Project 2015C33025), and the Zhejiang Provincial Natural Science Foundation of China under Grant No. LQ17E080002. KG acknowledges the Research Foundation Flanders (FWO), Belgium for the postdoctoral fellowship (grant agreement n 12Q0615N LV). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.electacta.2017. 10.068.

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