Vacuum

Letter pubs.acs.org/NanoLett

Architecture Engineering of Hierarchically Porous Chitosan/VacuumStripped Graphene Scaffold as Bioanode for High Performance Microbial Fuel Cell Ziming He,†,§ Jing Liu,†,§ Yan Qiao,‡ Chang Ming Li,*,†,‡ and Timothy Thatt Yang Tan*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore Institute for Clean Energy and Advanced Materials, Southwest University & Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies, Chongqing 400715, China

‡

S Supporting Information *

ABSTRACT: The bioanode is the defining feature of microbial fuel cell (MFC) technology and often limits its performance. In the current work, we report the engineering of a novel hierarchically porous architecture as an efficient bioanode, consisting of biocompatible chitosan and vacuum-stripped graphene (CHI/ VSG). With the hierarchical pores and unique VSG, an optimized bioanode delivered a remarkable maximum power density of 1530 mW m−2 in a mediator-less MFC, 78 times higher than a carbon cloth anode. KEYWORDS: Vacuum-stripped graphene, chitosan, ice segregation induced self-assembly, hierarchically porous, microbial fuel cell

M

sophisticated architecture.9−11 A multiwall carbon nanotube/ chitosan (MWCNT/CHI) scaffold prepared using the ISISA technique improves the power density of Geobacter sulf urreducens (G. sulf urreducens)-catalyzed MFC by 2-fold in comparison to a graphite rod anode at the same potential12 due to increased bacteria/anode contact area in the macropores. Graphenebased porous materials prepared using the ISISA technique, with improved properties such as large specific surface area, good biocompatibility, and high electronic conductivity, are expected to realize a more efficient MFC anode. Preparation of graphene−polymer solid foam with layered structure by ISISA technique has been reported, but it requires additional stabilizer (polystyrenesulfonate) for the chemically reduced 2D graphene and utilizes nonconductive poly(vinyl alcohol) as a support material.11 Anodes with multileveled porous structure and high conductivity are deemed favorable for MFC,13 and hence clever design of the anode structure and careful selection of the anode materials are necessary. In this work, we utilize the ISISA technique to prepare a novel 3D chitosan/vacuum-stripped graphene (CHI/VSG) scaffold with hierarchically porous structure composed of macropores formed by layered-branched architecture and meso/micropores from porous VSG embedded in the macropores. The macropores are suggested to facilitate bacteria colonization in the scaffold interior12−14 and enhance affinitive mechanical contact between the biocompatible VSG and bacteria, while the meso/micropores are suggested to increase

icrobial fuel cell (MFC) is capable of degrading organic waste and generating electricity simultaneously and hence has garnered strong interest as an alternative renewable power source.1−3 In MFC, the organic substrate is degraded by bacteria, and the electrons released during the metabolic process are transferred to the extracellular electrode, converting the chemical energy to electrical energy. However, the relatively low power density of MFC resulting from the sluggish electron transfer between bacteria cells and electrode limits its practical applications.4 To improve MFC performance, extensive studies have been devoted to exploring new anode materials, as the electron transfer between bacteria and anode directly determines the bioelectrocatalysis processes and thus the performance of MFC.5−7 It has been reported that graphene/polytetrafluoroethylene (PTFE) modified stainless steel mesh (GMS) anode significantly improves the power density of Escherichia. coli (E. coli)-catalyzed MFC, attributed to high specific surface area and good biocompatibility of the incorporated graphene for enhancing bacteria attachment on the anode.8 However, the compact GMS anode prepared by pressing graphene/PTFE paste on steel mesh only makes the graphene on the anode surface exposed to the bacteria suspension, while most of graphene in anode interior is not used. Therefore, assembling of graphene sheets into 3-dimensional (3D) material with a porous structure could be expected to fully use graphene for further improvement of the MFC performance. Ice segregation induced self-assembly (ISISA) as a freezecasting technique is a versatile and green bottom-up method to produce aligned macroporous or layered materials in a © 2012 American Chemical Society

Received: June 9, 2012 Published: August 13, 2012 4738

dx.doi.org/10.1021/nl302175j | Nano Lett. 2012, 12, 4738−4741

Nano Letters

Letter

FESEM, and the images are displayed in Figure S3 for comparison. The layer spacing of all the scaffolds (except CHI/ VSG-70) only changes slightly, indicating that the determining factors for CHI layer spacing may be the dipping rate (into liquid nitrogen bath) and/or the concentration of CHI, instead of VSG concentration. On the contrary, the branched structure is determined by VSG concentration. For the scaffolds with low VSG loading, almost no branched structure is observed between layers, while the VSG-covered branched structure appears and then increases with increasing VSG loading, demonstrating that the amount of loaded VSG is critical for the formation of branched network within the architecture. Based on these results, a possible mechanism for engineering the scaffold architecture with layered-branched structures is proposed in Scheme S1b. Unidirectional ice generation induces the formation of layered structure. For branched structure, it is proposed that the VSG-covered CHI could not completely combine into the CHI layers, therefore inducing the formation of VSG-covered branches. When the concentration of VSG reaches 70 wt %, the scaffold (CHI/VSG-70) loses its layered structure, which may be due to the presence of excessive VSG affecting the unidirectional generation of ice template during the ISISA process and hence the formation of CHI layer. The specific surface area of the CHI/VSG scaffolds increases with increasing VSG concentration as shown in Figure 1c, which is obviously due to the high specific surface area of loaded VSG. The specific surface area of CHI/VSG-50 scaffold is 248 m2 g−1, much larger than that (0.1−10 m2 g−1) of widely used graphite-based MFC anode materials.17 The pore size of CHI/VSG-50 scaffold mainly centers at 4 nm (the inset of Figure 1c), which is the same with that of VSG powder, indicating that the incorporated VSG powder keeps its original microchanneled structure in the scaffolds. To emphasize the importance of the 3D structure of VSG with meso/micropores, CHI/reduced graphene oxide-50 (CHI/RGO-50) scaffold was also prepared using a similar procedure and compared with CHI/VSG-50 scaffold as shown in Figure S4. Both scaffolds depict a layered structure, while the CHI/VSG-50 scaffold shows a more extensive network of branched structure. Most noticeably, the manner of the two kinds of graphene material embedded on the CHI layer is different. Because of its planar structure, RGO sheets attach layer-by-layer onto the CHI surfaces (Figure S4e), resulting in exposure of only the surface layer of RGO to bacteria suspension and thus decreasing the available surface of graphene for bacteria adhesion. However, with its 3D structure, VSG powder embeds on the CHI layers randomly and thus increases the surface roughness of the layers, availing more graphene surface area for bacteria adhesion. In addition, because of its meso/micropores, a greater amount of internal surface area is available to endogenous mediators for electron transfer between bacteria and anode. The electrocatalytic behavior of all CHI/VSG scaffolds was evaluated by electrochemical impedance spectra (EIS) measured in Pseudomonas aeruginosa (P. aeruginosa) cell suspension without adding glucose (Figure 1d). P. aeruginosa was selected as the biocatalyst in this work due to its ability to produce endogenous mediator,18 which enables stable electric current generation in MFC. The EIS shows well-defined semicircles followed by straight lines. The diameter of the semicircles represents the charge-transfer resistance (Rct) at the electrode/electrolyte interface.19 In the absence of glucose in the electrolyte, the Rct should be ascribed to the direct electrochemistry of P. aeruginosa cells.18 Rct decreases with

the available surface area of graphene for electron transfer from redox-active molecules produced by biocatalyst. The synergistic effect of the hierarchical pores in the anode structure and the unique properties of the VSG are expected to significantly improve the MFC performance. The FESEM image of VSG powder in Figure 1a exhibits accordion-like 3D structure with pores being observed at the

Figure 1. FESEM images of VSG powder (a) and CHI/VSG-50 scaffold (b). The plot of specific surface area vs VSG concentration of the CHI/VSG scaffolds (c) and Nyquist plots of different CHI/VSG electrodes (d). The inset of (b) is the magnification of the red-framed part of (b). The inset of (c) is the BJH pore size distribution of CHI/ VSG-50 scaffold.

edge of the flakes, indicating the existence of microchannels formed by interconnected flakes. This observation is consistent with a previous report.15 The specific surface area of VSG is 435 m2 g−1 obtained from a N2 adsorption−desorption isotherm and its pore size centers at 4 nm as shown in Figure S1. VSG flakes are actually graphene nanosheets,15 which is confirmed by AFM image (Figure S2). The height profile of extensively ultrasonicated VSG shows an average thickness of 0.9 nm, a typical thickness of single layered graphene nanosheets. Chitosan (CHI), a polycationic polymer, was utilized as the support material in the current study because of its good biocompatibility and adsorption property, and it is an environmentally friendly material.10 VSG can uniformly disperse in CHI aqueous solution without using a stabilizer. The homogeneous CHI/VSG suspensions are unidirectionally frozen by immersion in liquid nitrogen and subsequently freeze-dried, giving rise to self-supported sponge-like 3D scaffolds, as shown in Figure S3a. The scaffolds can recover to their original shapes after being repeatedly squeezed (data not shown), indicating good elastic and structure memory properties.16 The microscopic structure of CHI/VSG-50 scaffold is presented in Figure 1b. The scaffold exhibits wellaligned layered structure, with layer spacing of 30−50 μm, which is large enough to allow bacteria infiltration into the scaffold interior. In between the layers, branched structure covered with VSG (evident in the inset of Figure 1b) is crosslinked into a network, which forms the macropores for bacterial adhesion and fully utilizes the space between the layers. To reveal the effect of VSG concentration on the scaffold architecture, all CHI/VSG scaffolds were observed using 4739

dx.doi.org/10.1021/nl302175j | Nano Lett. 2012, 12, 4738−4741

Nano Letters

Letter

performance results are presented in Figure S7 for comparison. As shown in Figure 3a, the output current density of the MFC

increasing VSG loading in the CHI/VSG electrodes, indicating that the charge transfer rate of the electrode is mainly determined by VSG. When VSG loading is beyond 50 wt %, the Rct remains almost unchanged, which may be due to the CHI surface saturated with VSG coverage. The Rct of CHI/ VSG-50 is determined to be 150 Ω, much lower than the widely used graphite-based anode materials (1−5 kΩ).2,14,20 The EIS of CHI/VSG-50 electrode (electrode with optimal VSG loading) was also tested in bacteria suspension with glucose as shown in Figure S5, demonstrating an enhanced charge transfer rate, which can be attributed to the fast glucose oxidation under bioelectrocatalysis of P. aeruginosa on the CHI/VSG-50 electrode.21 The biocompatibility and bacteria-hosting capability of an anode are crucial for high performance MFC. The SEM images of the CHI/VSG-50 scaffold after incubation with P. aeruginosa (Figure 2a,b) show that the bacterial cells adhere in the

Figure 3. Constant-load discharge curve (a) and power density and polarization curves (b) of the MFC based on the CHI/VSG-50 anode. Arrows of (a) indicate the time of glucose feeding.

Figure 2. SEM images of CHI/VSG-50 (a, b) and CHI/RGO-50 (c, d) scaffolds after incubation with bacteria at low (a, c) and high (b, d) magnification.

with CHI/VSG-50 anode increases very slowly in the initial 70 h, a period of bacteria cells activation and colonization on the CHI/VSG anode, followed by a quick increase to a plateau of 2450 mA m−2 for 50 h, and then decreases sharply due to the depletion of glucose. With further glucose supplement, the current density quickly recovers to a value as high as 2550 mA m−2, demonstrating the excellent stability of P. aeruginosa-CHI/ VSG anode for electricity harvesting in MFC. The average steady-state current density of the MFC based on CHI/VSG-50 anode is 16 times higher than that based on carbon cloth anode (158 mA m−2). As reported in our previous work,22 graphene is capable of stimulating P. aeruginosa to excrete more endogenous phenazine mediators for fast mediated electron transfer. Herein, the normalized concentration of endogenous mediators in MFC using CHI/VSG-50 anode is 54.2 mg mL−1 cm−2, much higher than that of MFC using graphene/carbon cloth anode (5.7 mg mL−1 cm−2) and plain carbon cloth anode (1.9 mg mL−1 cm−2). Therefore, the significant improvement of current density is attributed to (1) high specific surface area resulted from the hierarchically porous structure of the anode, (2) enhanced affinitive contact between the multilayered biofilm and the anode due to the biocompatible VSG and the accessible macropores, (3) increased endogenous mediator production from bacteria due to the stimulation effect of VSG, and (4) low polarization from enhanced charge transfer rate due to the incorporation of high conductive VSG. The power density and polarization curves (Figure 3b) were determined by

macroporous architecture of the scaffold to form a thick multilayered biofilm network (further evident in Figure S6), indicating its excellent biocompatibility and bacteria-hosting capability. However, for the CHI/RGO-50 scaffold, only a single layer of biofilm was formed as observed in Figure 2c,d. This is attributed to the greater surface roughness, and more available surface area provided by the randomly assembled 3D VSG compared to the planar RGO as illustrated in Scheme S1. Furthermore, with sufficient spacing between the CHI layers, thick biofilm can be formed without clogging the macropores, thus favoring mass transport and bacteria infiltration into the interior of the anode. The CHI/VSG-50 scaffold was subsequently utilized as the anode in a P. aeruginosa-catalyzed mediator-less MFC with dual-chamber configuration for performance evaluation. As the performance of MFC can be affected by many factors, including cathodic reaction, operating temperature, buffer system, and cell configuration, it is difficult to compare MFC performance directly between different studies,13 especially when the researchers report the performance with different parameters. To accurately evaluate the performance improvement of the current MFC, a MFC with an identical configuration and operation conditions but using a conventional carbon cloth anode, of which the performance results have been reported in our previous work,22 is used as a reference cell, and its 4740

dx.doi.org/10.1021/nl302175j | Nano Lett. 2012, 12, 4738−4741

Nano Letters

Letter

(10) Qian, L.; Zhang, H. F. J. Chem. Technol. Biotechnol. 2011, 86 (2), 172−184. (11) Vickery, J. L.; Patil, A. J.; Mann, S. Adv. Mater. 2009, 21 (21), 2180−+. (12) Katuri, K.; Ferrer, M. L.; Gutierrez, M. C.; Jimenez, R.; del Monte, F.; Leech, D. Energy Environ. Sci. 2011, 4 (10), 4201−4210. (13) Xie, X.; Hu, L. B.; Pasta, M.; Wells, G. F.; Kong, D. S.; Criddle, C. S.; Cui, Y. Nano Lett. 2011, 11 (1), 291−296. (14) Yong, Y. C.; Dong, X. C.; Chan-Park, M. B.; Song, H.; Chen, P. ACS Nano 2012, 6 (3), 2394−2400. (15) Guo, C. X.; Lu, Z. S.; Lei, Y.; Li, C. M. Electrochem. Commun. 2010, 12 (9), 1237−1240. (16) Thongprachan, N.; Nakagawa, K.; Sano, N.; Charinpanitkul, T.; Tanthapanichakoon, W. Mater. Chem. Phys. 2008, 112 (1), 262−269. (17) Li, H. Q.; Wang, Y. G.; Wang, C. X.; Xia, Y. Y. J. Power Sources 2008, 185 (2), 1557−1562. (18) Rabaey, K.; Boon, N.; Hofte, M.; Verstraete, W. Environ. Sci. Technol. 2005, 39 (9), 3401−3408. (19) He, Z.; Mansfeld, F. Energy Environ. Sci. 2009, 2 (2), 215−219. (20) Xu, F. L.; Duan, J. Z.; Hou, B. R. Bioelectrochemistry 2010, 78 (1), 92−95. (21) Qiao, Y.; Li, C. M.; Bao, S. J.; Lu, Z. S.; Hong, Y. H. Chem. Commun. 2008, 11, 1290−1292. (22) Liu, J.; Qiao, Y.; Guo, C. X.; Lim, S.; Song, H.; Li, C. M. Bioresour. Technol. 2012, 114 (0), 275−280.

varying the external load resistance. The MFC based on the optimized CHI/VSG-50 anode delivers a remarkable maximum power density of 1530 mW m−2, 78 times higher than that based on carbon cloth anode (19.5 mW m−2). The open-circuit voltage of the MFC with P. aeruginosa-CHI/VSG-50 anode is 910 mV. With decreasing load resistance, the cell voltage decreases very slowly compared to the P. aeruginosa−carbon cloth anode, indicating a low polarization, which is mainly ascribed to the enhanced charge transfer rate. In summary, novel CHI/VSG scaffolds with hierarchically porous structure have been carefully designed and successfully prepared. The macropores induced by the ISISA technique provide an open space in anode interior for bacteria colonization and enhance the affinitive contact between multilayered bacteria and biocompatible VSG, thus improving direct electron transfer process. Porous VSG stimulates the P. aeruginosa to excrete more endogenous mediators and serves as primary unit in hierarchically porous architecture for larger specific surface area, thus increasing the mediated electron transfer rate. All these factors, from the delicate anode structure design and anode material selection, contribute to the remarkable 78 times maximum powder density improvement. The study could bring MFCs technology closer to practical application.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental section, supplementary Figures S1−S7 and Scheme S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (T.T.Y.T.), [email protected] (C.M.L.); Tel +65 6790 4062; Fax +65 6794 7553. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by Singapore Ministry of Education AcRF Tier 2 ARC16/11. We thank Ms Yeow Swee Kuan and Prof. Loo Say Chye Joachim for helping with BET testing.

■

REFERENCES

(1) Lovley, D. R. Nat. Rev. Microbiol. 2006, 4 (7), 497−508. (2) Qiao, Y.; Bao, S. J.; Li, C. M. Energy Environ. Sci. 2010, 3 (5), 544−553. (3) Mink, J. E.; Rojas, J. P.; Logan, B. E.; Hussain, M. M. Nano Lett. 2012, 12 (2), 791−795. (4) Logan, B. E. Nat. Rev. Microbiol. 2009, 7 (5), 375−381. (5) Qiao, Y.; Bao, S. J.; Li, C. M.; Cui, X. Q.; Lu, Z. S.; Guo, J. ACS Nano 2008, 2 (1), 113−119. (6) Rinaldi, A.; Mecheri, B.; Garavaglia, V.; Licoccia, S.; Di Nardo, P.; Traversa, E. Energy Environ. Sci. 2008, 1 (4), 417−429. (7) Logan, B.; Cheng, S.; Watson, V.; Estadt, G. Environ. Sci. Technol. 2007, 41 (9), 3341−3346. (8) Zhang, Y. Z.; Mo, G. Q.; Li, X. W.; Zhang, W. D.; Zhang, J. Q.; Ye, J. S.; Huang, X. D.; Yu, C. Z. J. Power Sources 2011, 196 (13), 5402−5407. (9) Gutierrez, M. C.; Garcia-Carvajal, Z. Y.; Hortiguela, M. J.; Yuste, L.; Rojo, F.; Ferrer, M. L.; del Monte, F. J. Mater. Chem. 2007, 17 (29), 2992−2995. 4741

dx.doi.org/10.1021/nl302175j | Nano Lett. 2012, 12, 4738−4741