Improved Performance of Microbial Fuel Cell Using Esterified Corncob

Oct 12, 2017 - (15) Herein, the degree of substitution of hexanoyl group on E-CNF was calculated to be 0.7. As shown in Figure 2f, the water contact a...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9614-9618

Improved Performance of Microbial Fuel Cell Using Esterified Corncob Cellulose Nanofibers To Fabricate Air-Cathode Gas Diffusion Layer Jiliang Ci,†,§,∇ Chun Cao,‡,§,∇ Shigenori Kuga,† Jianquan Shen,*,‡ Min Wu,*,† and Yong Huang† †

State Engineering Research Center of Engineering Plastics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, 2 Zhongguancun North 1st Street, Haidian District, Beijing 100190, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: A novel type esterified cellulose nanofiber (ECNF) gas diffusion layer (GDL) was prepared from corncob cellulose and fabricated for air-cathode (AC) of microbial fuel cell (MFC). Remarkably, the E-CNF based AC (ACE‑CNF) achieved higher current density and lower resistance than PTFE based AC (ACPTFE). Moreover, the output voltage and maximum output power density were 7.5% and 30.1% higher than those of ACPTFE. E-CNF based GDLs could be a new category of MFC cathode material.

KEYWORDS: Microbial fuel cell, Air-cathode, Gas diffusion layer, Cellulose nanofibers, Energy conversion

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INTRODUCTION Microbial fuel cell (MFC) has attracted much attention in energy conversion and wastewater treatment.1−4 However, its practical use has been hampered by insufficient performance. One important factor for these problems is the air-cathode (AC) using a noble metal catalyst and a new gas diffusion layer (GDL),5,6 making development of a new efficient cathode very important. Many low-cost catalysts have been developed recently,7,8 but research on gas diffusion layer has been rare.9 The role of the gas diffusion layer is to supply oxygen from air while preventing seeping out of electrolyte liquid (“flooding”).10 The most widely used material is a PTFE based coating layer, which is high-cost and requires hot sintering for fixation. Alternative materials have been proposed such as poly(dimethylsiloxane) (PDMS) and polyvinylidene fluoride (PVDF).11,12 We here propose the use of cellulose nanofiber, an abundant, inexpensive renewable substance, as GDL material. Recently, a facile method of preparing esterified cellulose nanofiber (ECNF) by mechanochemical treatment was developed by our group.13,14 Its application to corncob cellulose yielded especially thin nanofibers that can be used to form hydrophobic nanopaper.15 Thereupon we examined the possibility of this material as a gas diffusion layer, which can be easily formed as a thin sheet without high temperature processing. To the best of our knowledge, this is the first report of using sustainable ECNF extracted from agricultural residues as an alternative gas © 2017 American Chemical Society

diffusion layer in MFCs, which is of significance to the development of diverse gas diffusion layers, and meanwhile provide a potential application of E-CNF in some other new energy fields such as metal−air batteries. Moreover, the innovative fabrication of uniformly E-CNF based GDL is developed for the first time by using a direct and facile vacuum filtration method in this work, which could eliminate the complicate the brushing process compared to PTFE. The actual MFC was built by using this material and its performance was compared with MFC using the PTFE based GDL. The construction and operating principle of MFC are shown in Figure 1a. The single-chamber cubic air-cathode MFC includes air-cathode, anode and anode chamber (6.5 cm in length, 5 cm in width and 5 cm height, the available capacity was 27 cm3 with a 7 cm2 air-cathode). As demonstrated in Figure 1d, the roughness of surface of the E-CNF film was characterized by AFM, the cellulose fibers were tightly stacked without obvious aggregates. Besides, mesoporous were evenly dispersed in the surface of GDL, which were conducive to the transfer of oxygen to TPIs for ORR. Figure 2 shows the SEM images of air cathode coated by PTFE or E-CNF. Both could form continuous layers without holes even at high magnification (Figure 2a,c). The broken Received: June 17, 2017 Revised: September 25, 2017 Published: October 12, 2017 9614

DOI: 10.1021/acssuschemeng.7b01970 ACS Sustainable Chem. Eng. 2017, 5, 9614−9618

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Construction and working principle of MFC; photos of carbon cloth (b) before and (c) after coating with E-CNF, (d) AFM image of GDLE‑CNF.

in our previous work.15 Herein, the degree of substitution of hexanoyl group on E-CNF was calculated to be 0.7. As shown in Figure 2f, the water contact angle of E-CNF’s was 80°. Though it was not as high as 136° of PTFE (Figure 2e), but significantly higher than 42° of the unmodified cellulose sheet.15 As shown, the level of wettability of E-CNF seems to be in the right range. The performances of MFCs with different E-CNF wettabilities would be further investigated in next work. As shown in Figure S1, oxygen diffusion into MFC was enhanced by the substitution of PTFE with E-CNF. The oxygen mass transfer coefficient of ACE‑CNF was 7.4 ± 0.1 × 10−5 cm s−1, which was much higher than that of ACPTFE (3.1 ± 0.1 × 10−5 cm s−1). The higher oxygen mass transfer coefficient would facilitate the transfer of oxygen to TPIs, which is beneficial to the performance of MFC. The performance of MFC was evaluated by linear-sweep voltammetry. Figure 3a shows that the current density of ACE‑CNF was higher than ACPTFE, indicating superiority of the ECNF as gas diffusion layer. The AC impedance (initial potential: 0 V, frequency: 0.1−1.0 × 106 Hz) could evaluate ohmic- (Rs) and charge transfer- (Rct) resistances by Nyquist graph analysist. Figure 3c shows that the total resistance was decreased to 17.5 Ω

edges of air-cathode (Figure 2b,d) show how the layers are supported by carbon cloth. The PTFE layer had thickness of 15−20 μm, whereas E-CNF had thickness of 18−25 μm. Its constituent nanofibers were visible only by TEM, with width of 10−50 nm (Figure 2c, inset). Attachment of E-CNF layer had advantage of reinforcement effect. The electron, proton and oxygen are essential to the oxygen reduction reaction at triple-phase interfaces (TPIs). Water flooding is a significant negative factor for cathode performance as it reduces oxygen transfer to the TPIs.16 On the other hand, the full hydrophobicity is not a good thing for MFC performance because the proton supply would be limited; this situation is called “dry”.16 Thus, a “balance” situation neither “flooding” nor “dry” is beneficial for the formation of more TPIs and transfer of proton and oxygen to TPIs,17 as illustrated in Figure 2g, which is favorable for MFCs performance. The ECNF layer was effective to contain electrode solution similarly to the PTFE layer, whereas the unmodified (hydrophilic) cellulose sheet would readily swell with water and let it out. Obviously, this feature resulted from the introduction of hexanoyl group by reactive ball milling. The hydrophobicity of E-CNF increased with the increase of ball-milling time as shown 9615

DOI: 10.1021/acssuschemeng.7b01970 ACS Sustainable Chem. Eng. 2017, 5, 9614−9618

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ACS Sustainable Chemistry & Engineering

Figure 2. SEM images of (a) surface and (b) cross section of GDLPTFE; (c) surface and (d) cross section of GDLE‑CNF (inset: TEM image of E-CNF); (e) contact angles of PTFE and (f) E-CNF; (g) effect of contact angles on the TPIs for MFCs.

for MFCE‑CNF, 39.9% lower than that of the MFCPTFE (29.1 Ω). The Rs were similar for ACPTFE and ACE‑CNF, which could be attributed to the use of same cell configuration and nutrient medium.12 However, the Rct of ACE‑CNF was much lower than that of ACPTFE. The notably lower resistances of ACE‑CNF indicate higher electron conductivity and faster charge/ proton/oxygen transfer within ACE‑CNF. These differences could be attributed to its proper wettability favorable to the formation of more TPIs and transfer of proton and oxygen to TPIs. Because the performance of MFC is strongly affected by its internal resistance and its distribution, the lower internal resistance of ACE‑CNF will lead to a higher maximum power density.18,19 The MFCs were operated for about 1 week. Figure 3e shows that both MFCs gave voltage plateaus soon after replenishing nutrients. The stable voltage of MFCE‑CNF was 574.4 ± 11.4 mV, which was 7.5% higher than 534.2 ± 5.6 mV of MFCPTFE. Both MFCPTFE and MFCE‑CNF remained steady voltage plateaus after 5 cycles, indicating long-term stability of the MFCs. Long-term performance of MFCE‑CNF would be further investigated in our future work to check its stability. Output power was controlled by regulating external resistance. Figure 3e shows that the output power density

gradually increased with the increase of current density until reaching maximum, then fell down. The maximum output power density of MFCE‑CNF was 1518.3 ± 39 mW m−2, 30.1% higher than that of the MFCPTFE (1166.7 ± 29 mW m−2). Water pressure resistance, an important parameter for gas diffusion layer, is critical for the scale up application of MFCs.20 As demonstrated in Figure S2, the 1.5 m height water pressure did not cause obvious changes on the E-CNF film, which indicates that the E-CNF possess high water pressure resistance. Durability of ACE‑CNF was examined by SEM observation of surface and cross section of the diffusion layer after operation. Figure 4 illustrates that there was no visible change in the morphology of the GDL after operation. ATR-FTIR spectra of E-CNF before and after use showed no difference (Figure 4c), indicating high stability of E-CNF in MFC operation. In conclusion, a new type of gas diffusion layer could be built by E-CNF. The MFCE‑CNF showed higher current density and lower resistance than MFCPTFE. The voltage and maximum output power density of MFCE‑CNF were 7.5% and 30.1% higher than the MFCPTFE, respectively. Further improvements can be expected by optimizing thickness and structure of E-CNF layer. 9616

DOI: 10.1021/acssuschemeng.7b01970 ACS Sustainable Chem. Eng. 2017, 5, 9614−9618

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ACS Sustainable Chemistry & Engineering

Figure 3. (a) Linear sweep voltammetry curves; (b) electrochemical impedance spectroscopy spectra; (c) resistance distribution (inset: characteristic equivalent electrocircuit); (d) voltages versus time curves (V−t) and (e) output power density curves of MFCs with ACPTFE and ACE‑CNF.

Figure 4. SEM images of GDLE‑CNF after use: (a) surface and (b) cross section; (c) ATR-FTIR spectra of GDLE‑CNF before and after use.

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where the m1 is the dry weights of raw cellulose; m2 is the dry weight of E-CNF; 162 is the molar mass of anhydroglucose unite; 98 is the molar mass increment by esterification. The suspension was diluted to 0.1% (w/w) with DMF and tipsonicated to give a homogeneous suspension. 60 mL of the suspension was vacuum-filtered through a Nylon-6 membrane filter (0.45 μm pore) covered by a 40 mm wide circular piece of carbon cloth, forming a 3.18 mg cm−2 E-CNF layer on the surface. The sample was dried at 60 °C for 3 h. The PTFE based gas diffusion layer for comparison was prepared by a traditional brushing method.21 In brief, four PTFE diffusion layers containing PTFE solution (30 wt %, 5 μL cm−2) were coated on carbon cloth layer by layer, followed by air-drying and heating to 350 °C for 5 min. For the cathode catalyst, 40 mg of 10% Pt/C powder was dispersed in a mixed solution of water (0.83 μL mg−1), Nafion suspension (5%, 6.67 μL mg−1) and isopropyl alcohol (3.33 μL mg−1) by sonication for 30 min. The mixture was applied on the opposite side of carbon cloth

EXPERIMENTAL METHODS

Corncob cellulose (90.4%) was provided by Ji’nan Shengquan Group Co. and used without further purification. PTFE (30 wt %) from Shandong DongYue Co.; Pt/C catalyst (HPT020, 20 wt % Pt), carbon cloth (HCP331, 0.35 ± 0.02 mm thick, basis weight 19−21 mg cm−2), carbon felt (5.0 mm thick) and Nafion suspension (DuPont D520, 5 wt % solid, 45 wt % water and 50 wt % isopropyl alcohol) from Shanghai HeShen Co. . The esterified cellulose nanofiber was prepared by reactive ballmilling.15 Briefly, dry corncob cellulose powder was dispersed in DMF containing 2.0 mL hexanoyl chloride and ball-milled at 300 rpm. After 8 h of milling, the sample was rinsed with DMF for 3 times. The degree of substitution (DS) was calculated as follows in eq 1:

DS =

162 × (m2 − m1) 98 × m1

(1) 9617

DOI: 10.1021/acssuschemeng.7b01970 ACS Sustainable Chem. Eng. 2017, 5, 9614−9618

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ACS Sustainable Chemistry & Engineering by brushing. The cathode was dried at room temperature for 12 h. The two types of cathode prepared are denoted as ACE‑CNF and ACPTFE. TEM (JEOL E-2100) was used to characterize the morphology of ECNF. The surface and cross section of the electrodes were examined by SEM (JEOL JSM-4800). AFM was conducted on a Bruker Multimode 8 in scanasyst mode. ATR-FTIR spectra were taken by a Varian 3100. Water contact angle was measured by a Data-physics OCA-20. Linear sweep voltammetry and electrochemical impedance spectroscopy were performed by a CHI660d electrochemical workstation (Chenhua Inst. Corp., Shanghai). The MFC was inoculated with the mixed bacteria culture from another MFC, which had been cultured in our lab for more than 4 years.22 It was operated at 35 °C in batch feeding mode. The nutrient medium contained sucrose (1.0 g L−1), vitamin solution (5.0 mL L−1) and trace elements solution (12.5 mL L−1) in 50 mM pH 7.0 phosphate (PBS) buffer. The nutrient medium was refreshed when the voltage dropped below about 150 mV. The external resistor of 1 kΩ was loaded between electrodes. The voltage was measured at 1 min interval using a data acquisition system (HIOKI KR8431-30). The power density curve of MFC was obtained by varying the external resistor from 5 kΩ to 30 Ω.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01970. Effects of different GDLs on oxygen diffusion through the cathode, water pressure resistance was tested a self-made device (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Jianquan Shen. E-mail: [email protected]. Tel: +86-1061934539. *Min Wu. E-mail: [email protected]. Tel: +86-1082543500. ORCID

Chun Cao: 0000-0002-7332-073X Min Wu: 0000-0003-0542-4235 Author Contributions ∇

These authors contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (No. 51472253, 51373191) and the Chinese Academy of Sciences Visiting Professorships.

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DOI: 10.1021/acssuschemeng.7b01970 ACS Sustainable Chem. Eng. 2017, 5, 9614−9618