Article pubs.acs.org/accounts
Bacterial Cellulose: A Robust Platform for Design of Three Dimensional Carbon-Based Functional Nanomaterials Zhen-Yu Wu,† Hai-Wei Liang,† Li-Feng Chen, Bi-Cheng Hu, and Shu-Hong Yu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, Hefei Science Center, CAS, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *
CONSPECTUS: Three dimensional (3D) carbon nanomaterials exhibit great application potential in environmental protection, electrochemical energy storage and conversion, catalysis, polymer science, and advanced sensors fields. Current methods for preparing 3D carbon nanomaterials, for example, carbonization of organogels, chemical vapor deposition, and self-assembly of nanocarbon building blocks, inevitably involve some drawbacks, such as expensive and toxic precursors, complex equipment and technological requirements, and low production ability. From the viewpoint of practical application, it is highly desirable to develop a simple, cheap, and environmentally friendly way for fabricating 3D carbon nanomaterials in large scale. On the other hand, in order to extend the application scope and improve the performance of 3D carbon nanomaterials, we should explore efficient strategies to prepare diverse functional nanomaterials based on their 3D carbon structure. Recently, many researchers tend to fabricate high-performance 3D carbon-based nanomaterials from biomass, which is low cost, easy to obtain, and nontoxic to humans. Bacterial cellulose (BC), a typical biomass material, has long been used as the raw material of nata-de-coco (an indigenous dessert food of the Philippines). It consists of a polysaccharide with a β-1,4-glycosidic linkage and has a interconnected 3D porous network structure. Interestingly, the network is made up of a random assembly of cellulose nanofibers, which have a high aspect ratio with a diameter of 20−100 nm. As a result, BC has a high specific surface area. Additionally, BC hydrogels can be produced on an industrial scale via a microbial fermentation process at a very low price. Thus, it can be an ideal platform for design of 3D carbon-based functional nanomaterials. Before our work, no systematic work and summary on this topic had been reported. This Account presents the concepts and strategies of our studies on BC in the past few years, that is, converting cheap biomass into high value-added 3D carbon nanomaterials and designing diverse functional materials on 3D carbon structure. We first briefly introduce the history, constituent, and microstructure features of BC and discuss its advantages as a raw material for preparing the CNF aerogels. Then, we summarize the methods and strategies for preparing various 3D carbon-based nanomaterials from BC. In addition, the potential applications of the developed CNF aerogel based functional materials are also highlighted in this Account, including stretchable conductors, oxygen reduction reaction catalysts, supercapacitors, lithium-ion battery, and oil cleanup. Finally, we give some prospects on the future challenges in this emerging research area of designing CNF aerogel based functional nanomaterials from BC.
1. INTRODUCTION
For realizing practical applications, it is prerequisite to achieve high quality 3D carbon nanomaterials in a large scale via low-cost and sustainable methods. Traditionally, carbon aerogels were obtained from carbonization of organic aerogels, which was first invented by Pekala in 1989.6 Although this method can realize the mass production of carbon aerogels, it usually needs to use toxic and expensive chemical reagents, such as resorcinol and formaldehyde. In addition, the obtained carbon aerogels always have a low graphitization degree and tend to break under compression, which hampers their
Three dimensional (3D) carbon nanomaterials, such as carbon aerogel, carbon nanotube (CNT) sponge, and graphene foam, composed of interconnected network structure are attracting much interest due to their fascinating physical properties, including low apparent density, large specific surface area (SSA), abundant pore structure, high electrical conductivity, good chemical stability, and environmental compatibility.1−3 Based on their unique 3D network structures and physical properties, a kaleidoscope of 3D functional carbon nanomaterials were fabricated and used widely in adsorption, energy storage and conversion, catalysis, tissue engineering, advanced sensors, etc.3−5 © XXXX American Chemical Society
Received: August 17, 2015
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formula is (C6H10O5)n, but BC presents several distinct advantages compared with plant cellulose.10 From the viewpoint of constituents, BC is pure cellulose free from other plant components such as hemicelluloses, lignin, and pectin.7 As a result, BC does not require extra processing to remove these unwanted impurities. In terms of microstructure, BC possesses a unique and sophisticated 3D porous network structure, which consists of cellulose nanofibers with a high aspect ratio and a diameter of 20−100 nm, thus providing high SSA and porosity and unique mechanical properties.8,11 Additionally, BC also demonstrates some other unique physical properties, including high crystallinity (70−80%), high degree of polymerization (up to 8000), high water content to 99%, and moldability.8 Meanwhile, as a type of cellulose, BC also exhibits good biocompatibility, hydrophilicity, and nontoxicity.11 Due to the aforementioned characteristics of BC, it has been employed as raw material to fabricate diverse BC-based materials, such as BC/nanoparticles, BC/inorganic solid, BC/ polymers, and BC/nanostructured carbon composites.8,12 They are widely used in biological and medical fields, environmental sciences, optoelectronics, etc.7−12 However, there is no systematic work on converting BC into CNF aerogels and designing BC-derived CNF aerogel functional nanomaterials to achieve multiple functions. Our group pioneered the field of design and application of 3D carbon-based functional nanomaterials from BC since 2012;13−20 now many groups have joined this field and further broadened it.21−30
application and degrades their performance. Currently, CNT sponge, graphene foam, and CNT/graphene hybrid aerogel are considered to be the most promising 3D carbon nanomaterials.3 The chemical vapor deposition (CVD) technique is a popular strategy to synthesize these 3D carbon nanomaterials.3,5 However, the expensive precursors, complex equipment, and technological requirements involved in the CVD process dramatically hamper large-scale production for practical applications. Self-assembly of CNT and graphene building blocks into macroscopic structures was also extensively used for constructing 3D carbon nanomaterials.3,5 Unfortunately, expensive building blocks cannot afford large-scale production. Therefore, it is highly desirable but remains challenging to develop a low-cost, ecofriendly, and easily scalable method to produce high quality 3D carbon-based nanomaterials. In this Account, we present our developed strategy to tackle the above problem by employing industrially produced cheap bacterial cellulose (BC) with intrinsic nanofibrous network structure as raw material to fabricate 3D carbon nanomaterials, that is, so-called carbon nanofiber (CNF) aerogels. Furthermore, a series of CNF aerogel-based functional nanomaterials were obtained via rational design and synthesis. Diverse emerging applications of these CNF aerogel-based materials are also highlighted in our Account (Scheme 1). Scheme 1. Schematic Illustration of Design, Synthesis and Application of CNF Aerogel-Based Functional Nanomaterials from BC
2.2. Why We Chose BC as Raw Material
The reason should be attributed to the following three aspects. First, dried BC aerogel has desired constituents with relatively high carbon content of ca. 44.4% and a 3D interconnected nanofibrous network microstructure.7,8 Therefore, it is a suitable precursor for fabricating 3D carbon-based nanomaterials. Second, BC can be easily obtained in large amounts via an industrial-scale microbial fermentation process.13 In fact, BC has long been used as the raw material of nata-de-coco, a popular dessert in the Philippines.9 Now, many food companies in China can produce several tons per day of BC (Figure 1). The industrial-scale production ability for BC promises that BC-derived CNF aerogel based functional nanomaterials can be fabricated in a large scale at very low cost level. Last but not least, BC possesses abundant surface hydroxyl groups.8,12 This facilitates chemical modification of BC for further functionalization, resulting in diverse 3D functional carbon-based nanomaterials. On the basis of these advantages, we will demonstrate in following text that BC can be employed as a versatile raw material for fabricating a series of 3D carbon-based nanomaterials, which have a wide range of application in many fields (Scheme 1). 2.3. BC-Derived CNF Aerogels and Their Polymer Composites
2. DESIGN AND FABRICATION OF DIVERSE CNF AEROGEL BASED FUNCTIONAL MATERIALS
High temperature pyrolysis under inert atmosphere can easily convert carbon-rich materials into conductive carbon materials, accompanying some liquid and gaseous products. Yoshino et al. first reported that BC could be converted into highly conductive graphitic carbon films by heat treatment at temperatures above 2400 °C.31 Prior to our work, laboratorymade BC had also been successfully converted into 3D CNF materials by other groups, but their productivity was very low.32,33 To obtain BC-derived CNF aerogels in large-scale, we chose commercially available BC hydrogel pellicles (Figure 1b) as precursors and used a freeze-drying technique to remove the
2.1. The Brief Introduction of BC
BC produced by several species of bacteria is an emerging nanomaterial and has gained particular interest.7,8 Adrian Brown first reported BC when he worked with Bacterium aceti in 1886.9 A solid mass was accidentally formed at the surface of fermentation medium in his routine experiment, which was later identified as cellulose and named as BC for its bacterial synthesis process. Like plant cellulose, BC is a polysaccharide with a β-1,4-glycosidic linkage whose molecular B
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Figure 2. Scanning electron microscopy (SEM) images of (a) the BC and (b) the CNF aerogel, respectively. The insets in panels a and b are corresponding digital photographs. (c) The photographs show the sequential compression process of the BC and the CNF aerogel. (d) The photograph of CNF aerogel in a hot flame. (e, f) SEM images of the fracture surface of the CNF/PDMS composite. The inset in panel e shows the photograph of a bent composite. Reproduced with permission from refs 13 and 14. Copyright 2012 Nature Publishing Group and 2013 Wiley-VCH.
Figure 1. (a) Photograph of the production line for BC in a food factory. (b) Photograph of large quantity of BC pellicles produced by an industry-scaled microbial fermentation process. These photographs were provided by Ms. C. Y. Zhong (Hainan Yeguo Foods Co., Ltd., China).
water and prevent their 3D nanofibrous network from collapsing. Subsequently, the obtained BC aerogels were pyrolyzed under N2 or Ar atmosphere to get black CNF aerogels.14 It was found that the CNF aerogel shrank to half of original size of BC in three dimensions after the pyrolysis process. However, the intrinsic 3D nanofibrous structure of BC was well inherited by the obtained CNF aerogel (Figure 2a,b). Remarkably, the CNF aerogels possessed an ultralow apparent density of 4−6 mg cm−3, extraordinary compressibility, and excellent fire-resistant properties (Figure 2b−d). Additionally, our detailed study demonstrated that the electrical conductivity and surface hydrophobic properties of CNF aerogels could be easily tuned by pyrolysis temperature. Furthermore, in our group, a similar method was also adopted to achieve other 3D carbon-based nanomaterials, such as hydrothermal carbon derived carbon aerogels and elastomeric silver−carbon hybrid nanocable/nanotube sponges.34,35 The nanocarbon/polymer (especially 3D nanocarbon/ polymer) composites as a class of rising star materials have received widespread attention. Owing to the combined properties of the individual components, the nanocarbon/ polymer composites potentially have integrated and improved properties. For example, Cao’s group reported a CNT sponge nanocomposite made by directly infiltrating epoxy fluid into the CNT framework.36 The obtained composite showed simultaneous improvement in mechanical and electrical properties, leading to its application for electromechanical sensors. Similar to their method, we employed our developed cheap CNF aerogel as precursor and prepared a flexible CNF/polydimethylsiloxane (PDMS) composite by infiltrating CNF aerogel with the polymer resin under vacuum (inset of Figure 2e).13 Obviously, CNFs were homogeneously buried in the whole polymer matrix (Figure 2e,f). The obtained CNF/PDMS
composite fully retained the high electrical conductivity of the CNF aerogel and well achieved excellent flexibility from elastic PDMS. Impressively, when the pyrolysis temperature of BC was 1450 °C, the electrical conductivity of the composite could reach 0.41 S cm−1, much higher than traditional CNT- and graphene-based composites prepared by solution mixing with a similar filler content.13 We believe that such infiltrating method can be extended easily to prepare other CNF/polymer composites. 2.4. Various BC-Derived Doped CNF Aerogels
Doping carbon materials with heteroatoms (N, B, S, and P) can effectively change their electronic characteristics, spin structures, and surface chemical properties.37 Consequently, the heteroatom doped carbon (HDC) materials manifest significantly enhanced performance in supercapacitors, oxygen reduction reaction (ORR), and lithium-ion batteries (LIBs) when compared with undoped carbon materials. In recent years, multifarious HDC nanomaterials have been successfully developed to meet ever increasing demands for energy and catalysis fields. Since 2012, our group has been making great efforts to develop diverse high performance HDC materials based on our continual works on different carbon-based precursors, especially on BC.15−19,38−42 According to the methodology, our strategy to prepare HDC nanomaterials from BC can be categorized into three types. (1) Annealing BC with heteroatom-rich compounds. For example, we fabricated various doped CNF aerogels by pyrolysis of BC pellicles that had adsorbed different toxic organic dyes (Figure 3a).17 The elemental mapping of the typical sample obtained from methylene blue (C16H18ClN3S) C
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Figure 3. (a) The fabrication process of various doped CNF aerogels via a dyeing method. (b) Elemental mapping images of an individual N-S-CNF. Reproduced with permission from ref 17. Copyright 2014 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
Figure 4. (a) SEM, (b) transmission electron microscopy (TEM), and (c) elemental mapping images of CNF/MnO2. (d) SEM, (e) TEM, and (f) elemental mapping images of N-CNF/Mo2C. Insets of panels a and d are the photographs of CNF/MnO2 and N-CNF/Mo2C. (g, i, k, m) SEM and (h, j, l, n) TEM images of CNF/SnO2, CNF/Ge, CNF/Ni3S2, and CNF/FeOOH, respectively. Insets of panels e, h, j, l, and n are corresponding high-resolution TEM images. Reproduced with permission from refs 15, 20, 21, 22 and 28. Copyright 2013 Wiley-VCH, 2014 and 2015 Elsevier Ltd.
thermal reaction of CNF aeroegls with urea and aqueous ammonia solution.15,16 Due to the mild reaction conditions, low energy consumption, and easy scale-up properties, this hydrothermal method could be extended to synthesize other HDC nanomaterials. (3) Annealing BC-derived CNF aerogel in a heteroatom-donor atmosphere. The common drawback with the above methods is that the obtained doped CNF aerogels had low SSA and pore volume. To settle this problem, we developed a new N-CNF aerogel with high SSA (916 m2 g−1) and large pore volume (0.71 cm3 g−1) by annealing CNF aerogel in NH3 atmosphere.19 The main reason for improved SSA and pore volume is that large amounts of micropores were
showed N and S atoms were successfully doped in the carbon framework and homogeneously distributed in the CNF aerogel (Figure 3b). The synthetic method has some significant advantages, such as being green, general, and easily scalable. In another example, P-doped, N,P-codoped, and B,P-codoped CNF aerogels could be successfully prepared by the pyrolysis of BC immersed in H3PO4, NH4H2PO4, and H3BO3/H3PO4 solution, respectively.18 The N,P-codoped CNF aerogels showed great potential as supercapacitor electrode materials. (2) Hydrothermal treatment of BC-derived CNF aerogels with heteroatom-containing molecules in solution. Recently, two types of N-doped CNF aerogels were fabricated by hydroD
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Figure 5. (a) Variation of the ΔR/R0 of the composite as a function of tensile strain of 80% in the first two stretch−release cycles. (b) ΔR/R0 as a function of stretching cycles at a strain of 80%. The inset shows the 5th, 100th, and 1000th stretching cycle. (c) ΔR/R0 of the composite at a bend radius up to 1.0 mm in the first bending cycle. (d) ΔR/R0 as a function of bend cycles at a maximum bend radius of 1.0 mm. The inset shows the 20th, 1000th, and 5000th bending cycle. (e) Photographs of lighting a commercial LED using the CNF/PDMS composites under different conditions. Reproduced with permission from ref 13. Copyright 2012 Nature Publishing Group.
CNF/Mo2C aerogel. Figure 4d−f clearly indicate that highly crystalline Mo2C nanoparticles were homogeneously decorated on the N-CNF aerogel. Owing to some oxygen-containing moieties existing in the CNF, they can effectively direct the growth of inorganic nanoparticles on the CNF aerogel. Consequently, many CNF aerogel−metal or −metal compound nanoparticle composites, including CNF/SnO2 (Figure 4g,h),21 CNF/Ge (Figure 4i,j),21 CNF/Ni3S2 (Figure 4k,l),22 CNF/FeOOH (Figure 4m,n),28 and CNF/Ru,30 have been successfully synthesized via the in situ growth method. Take CNF/SnO2 for an example: A certain amount of SnCl2 in glycol was first mixed with NaOH in water. Then the CNF aerogel was added into the solution under stirring, and the solution with the CNF was heated to obtain a uniform CNF/SnO2 composite. In contrast, large aggregates of SnO2 nanoparticles were obtained when the CNF aerogel scaffold was absent, which strongly suggested that the CNF played a crucial role in the formation of homogeneously distributed SnO2 nanoparticles.
generated by the radicals of the decomposed NH3 etching out carbon fragments of CNF aerogels. 2.5. BC-Derived CNF Aerogel/Metal or Metal Compound Nanoparticle Composites
Decorating various kinds of metal or metal compound nanoparticles onto 3D CNF aerogels may create multiple functionalities or achieve synergistically enhanced performances that pure CNF aerogels or inorganic nanoparticles cannot provide. Currently, two methods have been used to synthesize the composites from BC, (1) direct reaction with the framework of the CNF or BC aerogel during certain conditions and (2) in situ growth of nanoparticles on the CNF aerogel. Employing BC-derived CNF aerogel as the precursor, CNF/ MnO2 composite could be easily synthesized according to the first method.15 In a typical experiment, the CNF aerogels were immersed into KMnO4/K2SO4 solution for a certain time. Since the strong oxidizing MnO4− ions could be reduced by the exterior carbon of the CNF based on the redox reaction 4MnO4− + 3C + H2O → 4MnO2 + CO32− + 2HCO3−, the CNF/MnO2 aerogel composite was obtained, as shown in Figure 4a−c. Different from the synthesis of CNF/MnO2 aerogel, N-CNF/Mo2C was fabricated by directly using BC as the precursor.20 First, the BC/(NH4)6Mo7O24 hybrid was prepared via a solution immersion method. Second, the hybrid aerogel was annealed under inert atmosphere to afford N-
3. APPLICATION OF DIVERSE CNF AEROGEL BASED FUNCTIONAL MATERIALS 3.1. Stretchable Conductors
With high electrical conductivity and excellent flexibility, CNF/ PDMS composites were desired stretchable conductors.13 E
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Figure 6. (a) CV curves of N-CNF aerogels and Pt/C catalyst in O2-saturated (solid line) and Ar-saturated 0.1 M KOH (dash line). (b) ORR polarization plots of CNF aerogel, N-CNF aerogel, and Pt/C. (c) ORR polarization plots of N-CNF aerogel and other NH3-treated carbon materials. (d) Long-term galvanostatic discharge curves of Zn−air batteries until complete consumption of Zn anode. Reproduced with permission from ref 19. Copyright 2015 Elsevier Ltd.
aerogel. Unfortunately, the ORR activity of N,S-codoped CNFs is not satisfactory because of their low SSA. To solve this issue, we developed a high SSA N-doped CNF aerogel by the NH3 activation method.19 Accordingly, it exhibited a desired ORR activity, comparable to that of Pt/C catalyst. Figure 6a shows cyclic voltammetry (CV) measurements; the N-CNF aerogel afforded a peak potential of 0.85 V, which was the same as that of Pt/C. The excellent ORR activity of N-CNF aerogel was also confirmed by ORR polarization plots, which showed that the N-CNF aerogel possessed an ORR half-wave potential of 0.80 V, only 50 mV more negative than that of Pt/C catalyst and much higher than that of pristine CNF aerogel and the most of reported values (Figure 6b). Compared with four reference carbon-based catalysts prepared under the same conditions, the N-CNF aerogel exhibited the highest ORR activity (Figure 6c) and better catalytic selectivity, which could be attributed to its unique 3D nanofibrous network structure. When used as a cathode catalyst for constructing the air electrode of Zn−air battery, the N-CNF aerogel exhibited high specific capacity of 615 mA h g−1, comparable to the battery made from Pt/C (630 mA h g−1) (Figure 6d). Recently, Chen et al. also reported a BC-derived CNF@N-doped graphene electrocatalyst made by annealing a BC/urea hybrid; it also exhibited an impressive ORR activity in alkaline media.27
Figure 5a,b shows the resistance variation (ΔR/R0) as a function of uniaxial tensile strain. During the first stretching− releasing cycle, the ΔR/R0 increased nearly linearly with increasing tensile strain, and ΔR/R0 at the maximum strain of 80% was only 14.8%. After recovering to the unstrained state, ΔR/R0 of 9.2% was observed, indicating a partial breaking of the 3D nanofibrous network (Figure 5a). Remarkably, further stretching−releasing cycles did not cause obvious increase of ΔR/R0 (Figure 5b), implying excellent electromechanical stability of our developed stretchable conductors. The changes of ΔR/R0 under bending deformations were also investigated (Figure 5c,d). The ΔR/R0 displayed a slight increase of 2.0% at a bending radius of 1.0 mm and could recover partially after unbending during the first bending cycle. Importantly, the ΔR/ R0 was only 4.1% after 5000 cycle tests, which promised long durability of CNF/PDMS composites under bending conditions. When the composite was used as a conductor to light a commercial LED, the brightness of the LED did not show obvious change under twisted or stretched conditions (Figure 5e), clearly indicating the great application potential of our developed stretchable conductor. 3.2. ORR Electrocatalysts
Developing metal-free HDC materials as alternatives to Ptbased catalysts for ORR has become an active research field in recent years.19,43 We first studied the ORR performance of dyeing BC-derived N,S-codoped CNF aerogels in alkaline media.17 An enhanced ORR activity was observed for N,Scodoped CNF aerogels when compared with the original CNF
3.3. Supercapacitor
Supercapacitors have attracted intensive research interest in recent years, due to their high power density, fast charge− discharge rates, ultralong cycle life, and great safety performF
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Figure 7. (a) Scheme of CNF/MnO2//CNF/N-ASC device. (b) Galvanostatic charge−discharge curves under different operation voltages. (c) Ragone plots of the supercapacitors. (d) A schematic diagram of the all-solid-state supercapacitor. (e) CV curves for the A-CNF/N flexible supercapacitor under different bending angles. Inset is the schematic showing the device under stress. (f) Galvanostatic charge−discharge curves of the A-CNF/N flexible supercapacitor at different current densities. Reproduced with permission from refs 15 and 16. Copyright 2013 Wiley-VCH and 2013 Royal Society of Chemistry.
ance.15,38 The advancement of supercapacitor technologies can benefit from the design and development of advanced electrode materials. 3.3.1. Symmetric Supercapacitor (SSC). As mentioned in section 2.4, P-doped, N,P-codoped, and B,P-codoped CNF aerogels had been successfully prepared by our group,18 and the N,P-codoped CNF aerogels showed the best capacitive performance among these samples. By optimization of the concentration of NH4H2PO4 solution, the highest capacitance of 204.9 F g−1 at 1.0 A g−1 was achieved, significantly higher than that of the pure CNF aerogel. The enhanced electrochemical performance was attributed to a pseudocapacitive contribution of the surface functionalities (i.e., N,P-codoping). Electrochemical impedance spectroscopy revealed that N,Pcodoped CNF aerogel based SSC had a very low chargetransfer resistance of 0.367 Ω; thus it exhibited a high maximum power density of 186.03 kW kg −1. In addition, the as-prepared SSC exhibited a very stable specific capacitance value within 4000 cycles. 3.3.2. Asymmetric Supercapacitor (ASC). High energy and power density ASCs based on BC-derived CNF aerogel materials were also successfully developed by our group in 2013.15 Optimized positive electrode materials (i.e., CNF/ MnO2) with specific capacitance of 254.64 F g−1 and negative electrode materials (i.e., CNF/N) with specific capacitance of 173.32 F g−1 were first fabricated. Then, the ASC was constructed according to the scheme in Figure 7a by using Na2SO4 as aqueous electrolyte. Charge−discharge tests indicated that the device worked very well at different voltages, even up to 2.0 V, which traditional SSCs could not reach (Figure 7b). Therefore, such an ASC had significant enhancement in both energy density and power density compared with other SSCs (Figure 7c). The maximum energy density and power density of this asymmetric device were 32.91 W h kg−1 and 284.63 kW kg−1, respectively, surpassing most of the reported MnO2-based supercapacitors. Very recently, BC-
derived N-CNF/MnO2 and CNF/Ni3S2 nanomaterials were also developed by other groups and used as positive electrode materials for ASCs.22,26 3.3.3. Flexible Supercapacitor. A flexible all-solid-state supercapacitor was also fabricated in our group by using a BCderived 3D N-doped activated CNF network (A-CNF/N).16 As shown in Figure 7d, the A-CNF/N-based flexible supercapacitor was fabricated by impregnating poly(vinyl alcohol)− H2SO4 gel electrolyte into the 3D porous network of A-CNF/ N and integrating them, where 3D A-CNF/Ns were used as both electrodes and current collectors, and the gel electrolyte served as both the electrolyte and separator. Consequently, the fabricated supercapacitor device exhibited good flexibility. The CV curves of the A-CNF/N flexible supercapacitor showed similar rectangular shapes at different curvatures (Figure 7e), indicating that the A-CNF/N flexible supercapacitor kept a constant capacitance output under various bending conditions without any decline in performance. Furthermore, the symmetric linear profile of charge−discharge curves at various current densities also revealed the good capacitive performance of the device (Figure 7f). The above results clearly indicated that BC-derived CNF materials were ideal electrode materials for designing and constructing various high performance supercapacitors. 3.4. LIB
BC-derived CNF aerogel could be directly used as the anode in LIBs.25 The initial discharge and charge capacities at a current density of 75 mA g−1 were 797 and 386 mA h g−1, respectively. After 100 cycles, the charge capacity showed a slight decrease to 359 mA h g−1, indicating excellent capacity retention ability of the CNF aerogel. Besides, CNF aerogel was also a versatile support for active LIBs anode materials, such as SnO2 nanoparticles. The obtained CNF/SnO2 aerogel exhibited much better LIB performance than bare SnO2 nanoparticles and commercial graphite.21 As shown in Figure 8a, the CNF/ G
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Figure 8. (a) Cycling performances of CNF/SnO2 and aggregated SnO2 nanoparticles over 100 cycles at 100 mA g−1. (b) Specific capacities of CNF/SnO2 at various current densities. Reproduced with permission from ref 21. Copyright 2013 Wiley-VCH.
SnO2 composites exhibited a good cycling stability and still retained a reversible capacity of ca. 600 mA h g−1 after 100 cycles at 100 mA g−1, which was ca. 2 times the theoretical specific capacity of graphite. In contrast, the bare SnO2 showed a rapid capacity decrease and delivered a specific capacity less than 100 mA h g−1 after 50 cycles. The enhanced LIB performance could be mainly explained by the fact that the CNFs in CNF/SnO2 electrode could effectively disperse the SnO 2 nanoparticles and thus alleviated the fusion of neighboring nanoparticles during the cycling process. Additionally, the SnO2/CNF composites exhibited superior rate capability (Figure 8b). Even under a very high current density of 1 A g−1, a favorable specific capacity of ca. 380 mA h g−1 could be obtained, which was still higher than the theoretical capacity of graphite. 3.5. Oil-Cleanup Sorbents
Considering its surface hydrophobicity/lipophilicity and high porosity, as well as low-cost fabrication process, BC-derived CNF aerogel is an ideal candidate for highly efficient extraction/separation of organic pollutants and oils.14,34 As shown in Figure 9a, once a small piece of the CNF aerogel was placed on the surface of oil−water mixtures, the oils were quickly absorbed by the CNF aerogel resulting in the clean water originally contaminated by the oils. Importantly, the CNF aerogel exhibited a very high sorption capacity for various kinds of organic liquids. In general, the sorption capacity ranged from 106 to 312 times its own weight (Figure 9b), much higher than most reported capacity values of other sorbents. Significantly, the CNF aerogel sorbent can be easily recycled many times by a distillation method.
Figure 9. (a) A layer of gasoline (colored with Sudan III) was absorbed by a CNF aerogel completely. (b) Absorption efficiency of CNF aerogels for various organic liquids. Reproduced with permission from ref 14. Copyright 2013 Wiley-VCH.
3.6. Other Applications
4. CONCLUSIONS AND PROSPECTS The development of simple, low-cost, and sustainable methods for fabricating high-performance 3D carbon nanomaterials is the first step on the road to realize their practical applications. Furthermore, rational design and synthesis will endow 3D nanostructured carbon materials with diverse functionalities and applications. In this Account, we present our strategies to prepare 3D nanostructured carbon by employing industrial produced cheap BC with an intrinsic 3D nanofibrous network structure as a sustainable raw material. Furthermore, using different functionalization methods (including infiltrating with polymers, doping with heteroatoms, and decorating with nanoparticles), BC-derived CNF aerogels can be converted into diverse 3D functional nanomaterials. They exhibited great application potential and can compete with and even replace
BC-derived CNF aerogels and their composites have also been widely used in other fields. For example, CNF aerogels could be used as electrode materials for seawater desalination. Experimental results demonstrated that CNFs obtained at 800 °C exhibited excellent desalination performance with an electrosorption capacity of 12.81 mg g−1 in 1000 mg L−1 NaCl solution, much higher than those of CNTs (3.78 mg g−1) and electrospun CNFs (6.56 mg g−1).29 In addition, BC-derived CNF/FeOOH composites could be used as adsorbents for a harmful organic dye (methyl orange), showing a high adsorption capacity and good recyclability.28 In the catalytic field, CNF/Mn3O4 effectively catalyzed ammonium perchlorate decomposition,24 and CNF/Ru exhibited application potential as a catalyst for a Li−O2 battery.30 H
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Accounts of Chemical Research expensive CNT sponge and graphene foam in some fields. Our focused and systematic work clearly demonstrates that low-cost and sustainable BC can act as an excellent platform to design 3D carbon-based functional nanomaterials for widespread application. Despite all the above success, it should be noticed that the research of design and application of 3D carbon-based functional nanomaterials from BC is now in its early stage. A number of challenges still remain before the widespread implementation of these materials in various practical areas can be realized. For example, the yield of converting BC into CNF aerogel under the pyrolysis process is relatively low. This may be improved by metal-ion assisted carbonization or prehydrothermal treatment of BC before pyrolysis. Additionally, the performance of BC-derived CNF aerogel functional materials needs to be further improved to compete and even replace the state-of-the-art materials by controlling suitable constituents and designing unique structures. For widespread application of BC-derived CNF aerogel functional nanomaterials, more synthesis design and strategies are still needed to be developed. Finally, the concepts and methods presented in this Account, including converting cheap BC into high value-added 3D CNF aerogel and fabricating diverse functional materials on 3D carbon structure, can be applied to other biomass for preparing 3D carbon-based functional materials with broad applications in many technical fields in the future.
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carbon-based non-precious-metal electrocatalysts for energy applications. Li-Feng Chen received his Ph.D. under the supervision of Prof. ShuHong Yu at USTC in 2013. Then, he worked as a Postdoctoral researcher with Prof. Shu-Hong Yu at USTC. His research interests are carbon-based nanomaterials for energy storage devices. Bi-Cheng Hu is currently a Ph.D. candidate under the supervision of Prof. Shu-Hong Yu at USTC. His research focuses on designing carbon-based nanomaterials for energy storage and conversion. Shu-Hong Yu received his Ph.D. in 1998 from USTC. After he finished Postdoctoral research in the Tokyo Institute of Technology and the Max Planck Institute of Colloids and Interfaces, he was appointed as a full professor in 2002 at USTC and was awarded the Cheung Kong Professorship in 2006. He serves as an editorial or advisory board member of journals Accounts of Chemical Research, Chemical Science, Chemistry of Materials, Materials Horizons, Nano Research, ChemNanoMater, CrystEngComm, Particle & Particle Systems Characterization, and Crystals. His research interests include bioinspired synthesis and self-assembly of nanostructured materials and nanocomposites, and their related properties.
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(1) Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002, 102, 4243−4266. (2) Moreno-Castilla, C.; Maldonado-Hódar, F. J. Carbon aerogels for catalysis applications: An overview. Carbon 2005, 43, 455−465. (3) Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutierrez, M. C.; del Monte, F. Three dimensional macroporous architectures and aerogels built of carbon nanotubes and/or graphene: synthesis and applications. Chem. Soc. Rev. 2013, 42, 794−830. (4) Biener, J.; Stadermann, M.; Suss, M.; Worsley, M. A.; Biener, M. M.; Rose, K. A.; Baumann, T. F. Advanced carbon aerogels for energy applications. Energy Environ. Sci. 2011, 4, 656−667. (5) Chabot, V.; Higgins, D.; Yu, A.; Xiao, X.; Chen, Z.; Zhang, J. A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment. Energy Environ. Sci. 2014, 7, 1564−1596. (6) Pekala, R. W.; Hill, P. Low density, resorcinol−formaldehyde aerogels. US patent 1989, 4873218. (7) Huang, Y.; Zhu, C.; Yang, J.; Nie, Y.; Chen, C.; Sun, D. Recent advances in bacterial cellulose. Cellulose 2014, 21, 1−30. (8) Hu, W.; Chen, S.; Yang, J.; Li, Z.; Wang, H. Functionalized bacterial cellulose derivatives and nanocomposites. Carbohydr. Polym. 2014, 101, 1043−1060. (9) Lin, S.-P.; Loira Calvar, I.; Catchmark, J.; Liu, J.-R.; Demirci, A.; Cheng, K.-C. Biosynthesis, production and applications of bacterial cellulose. Cellulose 2013, 20, 2191−2219. (10) Abeer, M. M.; Mohd Amin, M. C. I.; Martin, C. A review of bacterial cellulose-based drug delivery systems: their biochemistry, current approaches and future prospects. J. Pharm. Pharmacol. 2014, 66, 1047−1061. (11) Fu, L.; Zhang, J.; Yang, G. Present status and applications of bacterial cellulose-based materials for skin tissue repair. Carbohydr. Polym. 2013, 92, 1432−1442. (12) Shah, N.; Ul-Islam, M.; Khattak, W. A.; Park, J. K. Overview of bacterial cellulose composites: A multipurpose advanced material. Carbohydr. Polym. 2013, 98, 1585−1598. (13) Liang, H.-W.; Guan, Q.-F.; Zhu, Z.; Song, L.-T.; Yao, H.-B.; Lei, X.; Yu, S.-H. Highly conductive and stretchable conductors fabricated from bacterial cellulose. NPG Asia Mater. 2012, 4, e19. (14) Wu, Z.-Y.; Li, C.; Liang, H.-W.; Chen, J.-F.; Yu, S.-H. Ultralight, Flexible, and Fire-Resistant Carbon Nanofiber Aerogels from Bacterial Cellulose. Angew. Chem., Int. Ed. 2013, 52, 2925−2929. (15) Chen, L.-F.; Huang, Z.-H.; Liang, H.-W.; Guan, Q.-F.; Yu, S.-H. Bacterial-Cellulose-Derived Carbon Nanofiber@MnO2 and Nitrogen-
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.5b00380. List of abbreviations and their meanings (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions †
Z.Y. W. and H.W. L. contributed equally to this work.
Funding
We acknowledge funding support from the National Natural Science Foundation of China (Grants 21521001, 21431006, 91227103, and 21503207), the Ministry of Science and Technology of China (Grants 2014CB931800 and 2013CB933900), the Chinese Academy of Sciences (Grant KJZD-EW-M01-1), and the Users with Excellence of Hefei Science Center of CAS (Grants 2015HSC-UE007 and 2015SRG-HSC038). Notes
The authors declare no competing financial interest. Biographies Zhen-Yu Wu is currently a Ph.D. candidate under the supervision of Prof. Shu-Hong Yu at University of Science and Technology of China (USTC). His research focuses on designing carbon-based nanomaterials for energy and environmental sciences. Hai-Wei Liang received his Ph.D. under the supervision of Prof. ShuHong Yu at USTC in 2011. He is currently a Postdoctoral researcher under Prof. Xinliang Feng and Prof. Klaus Müllen at the Max Planck Institute of Polymer Research (Germany). His research topics are I
DOI: 10.1021/acs.accounts.5b00380 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research Doped Carbon Nanofiber Electrode Materials: An Asymmetric Supercapacitor with High Energy and Power Density. Adv. Mater. 2013, 25, 4746−4752. (16) Chen, L.-F.; Huang, Z.-H.; Liang, H.-W.; Yao, W.-T.; Yu, Z.-Y.; Yu, S.-H. Flexible all-solid-state high-power supercapacitor fabricated with nitrogen-doped carbon nanofiber electrode material derived from bacterial cellulose. Energy Environ. Sci. 2013, 6, 3331−3338. (17) Wu, Z.-Y.; Liang, H.-W.; Li, C.; Hu, B.-C.; Xu, X.-X.; Wang, Q.; Chen, J.-F.; Yu, S.-H. Dyeing bacterial cellulose pellicles for energetic heteroatom doped carbon nanofiber aerogels. Nano Res. 2014, 7, 1861−1872. (18) Chen, L.-F.; Huang, Z.-H.; Liang, H.-W.; Gao, H.-L.; Yu, S.-H. Three-Dimensional Heteroatom-Doped Carbon Nanofiber Networks Derived from Bacterial Cellulose for Supercapacitors. Adv. Funct. Mater. 2014, 24, 5104−5111. (19) Liang, H.-W.; Wu, Z.-Y.; Chen, L.-F.; Li, C.; Yu, S.-H. Bacterial cellulose derived nitrogen-doped carbon nanofiber aerogel: An efficient metal-free oxygen reduction electrocatalyst for zinc-air battery. Nano Energy 2015, 11, 366−376. (20) Wu, Z.-Y.; Hu, B.-C.; Yu, S.-H. Mo2C Nanoparticles Embedded within 3D N-doped Carbon Nanofibers Networks for Hydrogen Evolution. Unpublished work, 2015. (21) Wang, B.; Li, X.; Luo, B.; Yang, J.; Wang, X.; Song, Q.; Chen, S.; Zhi, L. Pyrolyzed Bacterial Cellulose: A Versatile Support for Lithium Ion Battery Anode Materials. Small 2013, 9, 2399−2404. (22) Yu, W.; Lin, W.; Shao, X.; Hu, Z.; Li, R.; Yuan, D. High performance supercapacitor based on Ni3S2/carbon nanofibers and carbon nanofibers electrodes derived from bacterial cellulose. J. Power Sources 2014, 272, 137−143. (23) Meng, F.; Li, L.; Wu, Z.; Zhong, H.; Li, J.; Yan, J. Facile preparation of N-doped carbon nanofiber aerogels from bacterial cellulose as an efficient oxygen reduction reaction electrocatalyst. Chin. J. Catal. 2014, 35, 877−883. (24) Li, L.; Zhou, Y.; Li, Z.; Ma, Y.; Pei, C. One step fabrication of Mn3O4/carbonated bacterial cellulose with excellent catalytic performance upon ammonium perchlorate decomposition. Mater. Res. Bull. 2014, 60, 802−807. (25) Wang, L.; Schutz, C.; Salazar-Alvarez, G.; Titirici, M.-M. Carbon aerogels from bacterial nanocellulose as anodes for lithium ion batteries. RSC Adv. 2014, 4, 17549−17554. (26) Long, C.; Qi, D.; Wei, T.; Yan, J.; Jiang, L.; Fan, Z. NitrogenDoped Carbon Networks for High Energy Density Supercapacitors Derived from Polyaniline Coated Bacterial Cellulose. Adv. Funct. Mater. 2014, 24, 3953−3961. (27) Ye, T.-N.; Lv, L.-B.; Li, X.-H.; Xu, M.; Chen, J.-S. Strongly Veined Carbon Nanoleaves as a Highly Efficient Metal-Free Electrocatalyst. Angew. Chem., Int. Ed. 2014, 53, 6905−6909. (28) Wang, Z.; Ma, Y.; He, H.; Pei, C.; He, P. A novel reusable nanocomposite: FeOOH/CBC and its adsorptive property for methyl orange. Appl. Surf. Sci. 2015, 332, 456−462. (29) Liu, Y.; Lu, T.; Sun, Z.; Chua, D. H. C.; Pan, L. Ultra-thin carbon nanofiber networks derived from bacterial cellulose for capacitive deionization. J. Mater. Chem. A 2015, 3, 8693−8700. (30) Tong, S.; Zheng, M.; Lu, Y.; Lin, Z.; Zhang, X.; He, P.; Zhou, H. Binder-free carbonized bacterial cellulose-supported ruthenium nanoparticles for Li-O2 batteries. Chem. Commun. 2015, 51, 7302−7304. (31) Yoshino, K.; Matsuoka, R.; Nogami, K.; Yamanaka, S.; Watanabe, K.; Takahashi, M.; Honma, M. Graphite film prepared by pyrolysis of bacterial cellulose. J. Appl. Phys. 1990, 68, 1720−1725. (32) Kuga, S.; Kim, D.-Y.; Nishiyama, Y.; Brown, R. M. Nanofibrillar carbon from native cellulose. Mol. Cryst. Liq. Cryst. 2002, 387, 13−19. (33) Wan, Y.; Zuo, G.; Yu, F.; Huang, Y.; Ren, K.; Luo, H. Preparation and mineralization of three-dimensional carbon nanofibers from bacterial cellulose as potential scaffolds for bone tissue engineering. Surf. Coat. Technol. 2011, 205, 2938−2946. (34) Wu, Z.-Y.; Li, C.; Liang, H.-W.; Zhang, Y.-N.; Wang, X.; Chen, J.-F.; Yu, S.-H. Carbon nanofiber aerogels for emergent cleanup of oil spillage and chemical leakage under harsh conditions. Sci. Rep. 2014, 4, 4079.
(35) Yao, H.-B.; Huang, G.; Cui, C.-H.; Wang, X.-H.; Yu, S.-H. Macroscale Elastomeric Conductors Generated from Hydrothermally Synthesized Metal-Polymer Hybrid Nanocable Sponges. Adv. Mater. 2011, 23, 3643−3647. (36) Gui, X.; Li, H.; Zhang, L.; Jia, Y.; Liu, L.; Li, Z.; Wei, J.; Wang, K.; Zhu, H.; Tang, Z.; Wu, D.; Cao, A. A Facile Route to Isotropic Conductive Nanocomposites by Direct Polymer Infiltration of Carbon Nanotube Sponges. ACS Nano 2011, 5, 4276−4283. (37) Paraknowitsch, J. P.; Thomas, A. Doping carbons beyond nitrogen: an overview of advanced heteroatom doped carbons with boron, sulphur and phosphorus for energy applications. Energy Environ. Sci. 2013, 6, 2839−2855. (38) Chen, L.-F.; Zhang, X.-D.; Liang, H.-W.; Kong, M.; Guan, Q.-F.; Chen, P.; Wu, Z.-Y.; Yu, S.-H. Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors. ACS Nano 2012, 6, 7092−7102. (39) Chen, P.; Xiao, T.-Y.; Qian, Y.-H.; Li, S.-S.; Yu, S.-H. A Nitrogen-Doped Graphene/Carbon Nanotube Nanocomposite with Synergistically Enhanced Electrochemical Activity. Adv. Mater. 2013, 25, 3192−3196. (40) Zhang, W.; Wu, Z.-Y.; Jiang, H.-L.; Yu, S.-H. Nanowire-Directed Templating Synthesis of Metal−Organic Framework Nanofibers and Their Derived Porous Doped Carbon Nanofibers for Enhanced Electrocatalysis. J. Am. Chem. Soc. 2014, 136, 14385−14388. (41) Cong, H.-P.; Wang, P.; Gong, M.; Yu, S.-H. Facile synthesis of mesoporous nitrogen-doped graphene: An efficient methanol−tolerant cathodic catalyst for oxygen reduction reaction. Nano Energy 2014, 3, 55−63. (42) Chen, P.; Wang, L.-K.; Wang, G.; Gao, M.-R.; Ge, J.; Yuan, W.J.; Shen, Y.-H.; Xie, A.-J.; Yu, S.-H. Nitrogen-doped nanoporous carbon nanosheets derived from plant biomass: an efficient catalyst for oxygen reduction reaction. Energy Environ. Sci. 2014, 7, 4095−4103. (43) Wu, Z.-Y.; Xu, X.-X.; Hu, B.-C.; Liang, H.-W.; Lin, Y.; Chen, L.F.; Yu, S.-H. Iron Carbide Nanoparticles Encapsulated in Mesoporous Fe-N-Doped Carbon Nanofibers for Efficient Electrocatalysis. Angew. Chem., Int. Ed. 2015, 54, 8179−8183.
J
DOI: 10.1021/acs.accounts.5b00380 Acc. Chem. Res. XXXX, XXX, XXX−XXX
