www.advmat.de www.MaterialsViews.com
COMMUNICATION
Holey Tungsten Oxynitride Nanowires: Novel Anodes Efficiently Integrate Microbial Chemical Energy Conversion and Electrochemical Energy Storage Minghao Yu, Yi Han, Xinyu Cheng, Le Hu, Yinxiang Zeng, Meiqiong Chen, Faliang Cheng, Xihong Lu,* and Yexiang Tong The serious global energy crisis and environmental pollution are rising strong demands in the development of novel energyconversion devices for the direct production of electricity from renewable resources and energy-storage devices for electrical storage.[1–5] In this regard, microbial fuel cells (MFCs)[6–8] and asymmetric supercapacitors (ASCs)[9,10] are two emerging systems required for promising electrochemical energy conversion and storage; however, one issue limiting the practical application of MFCs as power generators is their relatively low output power density compared to other energy-conversion devices.[9,10] Meanwhile, the progress on ASC anodes is relatively slow, which makes it hard for them to satisfy the demand of high energy, matching with that of cathode.[11–13] Further breakthroughs in electrode materials hold the key to fundamental advances in both MFCs and ASCs. Numerous studies have been conducted to develop new materials and optimize structures for high performances in either MFCs or ASCs. However, reports available on the realization of both energy conversion and storage in one electrode with the same material and structure are rare, even though it would open up many opportunities for the integration of renewable energy harvesting and storage with high power output. As a typical refractory transition metal oxynitride, cubic tungsten oxynitride is typically constructed with the nonmetal atoms (N, O) occupying interstitial sites in the metal lattice, typically octahedral in face-centered cubic (fcc), which results in the maximization of valence electrons in the nonmetal atoms (Figure 1a). Such unique crystal structure enables tungsten oxynitride (WON) to have a number of attractive properties such as high hardness, excellent conductivity, good thermal stability, and high melting point.[14–16] More importantly, the M. Yu, Y. Han, X. Cheng, L. Hu, Y. Zeng, M. Chen, Prof. X. Lu, Prof. Y. Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry KLGHEI of Environment and Energy Chemistry School of Chemistry and Chemical Engineering Sun Yat-Sen University 135 Xingang West Road, Chemical North Building 325 Guangzhou 510275, China E-mail: [email protected] M. Chen, Prof. F. Cheng Biosensor Research Centre Dongguan University of Technology 251 Xueyuan Road, Dongguan 523808, China
DOI: 10.1002/adma.201500493
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
good biocompatibility could strengthen the enrichment of bacteria, which enables WON to have promising application as the anode in MFCs.[17–19] In addition, comparing with other metal nitrides and metal oxynitrides, the extraordinary chemical inertness further gives WON great promise as a durable anode.[20–22] However, no investigations of WON as anode material for either MFCs or ASCs have been reported thus far. In this work, we constituted the first demonstration of WON as a high-performance anode material for MFCs and ASCs. Holey WON nanowires with quantities of micro-/mesopores were successfully synthesized on carbon cloth. A novelly structured WON with superior conductive and hydrophilic properties enables it to exhibit superior and efficient performance as anodes in both ASCs and MFCs. More importantly, an integrated MFC–ASC system has been developed, which simultaneously possesses energy conversion and storage functions. The MFC–ASC device could be self-charged to 0.9 V, and supply electrical energy for 163 s at a discharge current density of 50 mA cm−3. Holey WON nanowires were prepared on flexible and conductive carbon cloth via a two-step process. WO3 nanowires were firstly synthesized on a carbon-cloth substrate through a seed-assisted hydrothermal method. As observed from scanning electron microscopy (SEM) images (Figure S1, Supporting Information), the entire surface of the carbon fiber was covered by uniform nanowires with diameters ranging from 50 to 80 nm after hydrothermal reaction. Then, these nanowires were annealed under an ammonia atmosphere at a temperature of 700 °C for 1 h. The film color turned from light blue to black, implying the conversion from WO3 to WON occurred. Meanwhile, the wire-like morphology was almost preserved with a slight curve (Figure 1b). Figure 1c displays the typical X-ray diffraction (XRD) patterns of WO3 and WON samples. It is revealed that hexagonal WO3 (JCPDS = #33-1387) completely converted into cubic W0.62(N0.62O0.38) (JCPDS = #25-1254), and both samples are well-crystalline. Additionally, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were applied to analyze the detailed variation of microstructure. Figure S2a, Supporting Information, demonstrates that the WO3 nanowire with a smooth surface is uniform in diameter, and its single-crystalline nature can be concluded from its SAED pattern. Regular lattice fringes with d-spacings of 0.37 and 0.39 nm, corresponding to the (001) and (110) planes of WO3, further confirmed its high-quality crystallization (Figure S2b, Supporting Information). Interestingly, when WO3 turned into WON, a smooth nanowire was replaced by a rough one with a polycrystalline nature (Figure 1d). The obvious
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. a) Crystal structure of cubic WON. b) SEM images of as-prepared WON nanowires. c) XRD spectra collected for as-prepared WO3 and WON nanowires. d) TEM image and e) HRTEM images of a WON nanowire. The inset in (d) is the SAED pattern. f) HAADF-STEM image of WON nanowire and the corresponding EELS mapping image.
graphic-contrast variation in its high-resolution TEM image clearly shows that numerous pores with sizes of 1–5 nm are scattered on the nanowire (Figure 1e). Pore size distributions of the two electrodes calculated from the nitrogen adsorption branches are presented in Figure S3, Supporting Information. Apparently, an obvious enrichment of micro- and mesopores is found for WON electrodes when it is compared with those of WO3 electrodes. It is hoped that these mesopores will provide efficient channels, allowing fast and easy access of electrolyte ions to the surface of electrode; meanwhile, a large surface area is guaranteed by both micro- and mesopores, which enables a large amount of active sites for ion adsorption. The electron energy loss spectroscopy (EELS) mapping reveals that nitride is uniformly distributed in the whole nanowires, suggesting the thorough conversion of WO3 (Figure 1f). X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition of our products. Besides elemental N, no other impurities were detected to be introduced into WON as shown in XPS survey spectra (Figure S4, Supporting Information). The O 1s peak still appeared without any shift of the peak position after nitridation, while a broad peak of N 1s located at 397.5 eV obtained for WON confirmed the introduction of N element (Figure 2a,b). Figure 2c demonstrates that the oxidation state of W was in-depth. Notably, two additional peaks at the lower binding energies of 33.0 eV (W 4f7/2) and 34.7 eV (W 4f5/2) are observed in W 4f spectra after nitridation, which are associated with lower oxidation states of W (W–N) as might be expected in WON.[14] Moreover, the contact angle is decreased from 71° to 32° after the nitridation process, which indicates the surface wettability is significantly enhanced (Figure 2d). Such better architecture for accommodation of the electrolyte may speed up the electrolyte penetration and spreading in the channels of inner WON nanowires. Hence, it
2
wileyonlinelibrary.com
is anticipated that our as-fabricated WON nanowires with their unique holey structure would be an ideal electrode, decreasing the diffusion length of ions and increasing the contact area with electrolyte, as well as improving active material utilization. To optimize the capacitive behavior of WON nanowires, the effect of the nitridation temperature was firstly studied (Figure S5 and S6, Supporting Information). 700 °C was selected as the optimal temperature. Subsequently, the favorable double-layer energy-storage behavior and high-rate handling capability of the WON electrode fabricated under 700 °C were confirmed by the nearly rectangular shapes of the cyclic voltammetry (CV) curves (Figure S7a, Supporting Information) and the fairly linear slopes of the galvanostatic charge/discharge curves (Figure S7b, Supporting Information). No obvious ohmic drop was observed, which indicates the low inner-pore ion-transport resistance and short diffusion distance during the charge/discharge process. Figure 2e depicts a comparison of its calculated volumetric capacitances based on the discharge curves with the values of recently reported SC anodes. Significantly, the as-prepared WON electrode yielded a remarkable volumetric capacitance of 4.95 F cm−3 at 12.5 mA cm−3 (4.76 F cm−3 at 10 mV s−1), which is considerably higher than that of most reported anodes.[23–28] When the current density reached 500 mA cm−3, a high volumetric capacitance of 3.35 mF cm−3 was still achieved, which means more than 67% of its initial capacitance could be retained. This is an outstanding rate capability among SC anodes, even better than some carbon based anodes with high conductivity.[23–25,27–33] Additionally, more than 93% of original capacitance was retained after an ultra-long term of 100 000 cycles. To the best of our knowledge, this is the best cycling performance reported for SC electrodes based on metal nitride and oxynitride.[25,34–40] CV curves and morphologies of WON after 100, 50 000, and 100 000 cycles were further compared in the
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. a) Core-level O 1s, b) N 1s, and c) W 4f XPS spectra collected for WO3 and WON. d) Contact angles of a water droplet on WO3 and WON. e) Volumetric capacitance of WON electrode calculated based on galvanostatic charge–discharge curves as a function of current density. The volumetric capacitances of other previously reported anodes are added for comparison.[25,27] f) Cycling performance of WON electrode measured at 100 mV s−1. Insets are CV curves of WON electrode after 100, 50 000, 100 000 cycles.
insets of Figure 2f and Figure S8 in the Supporting Information. As expected, both the CV shapes and the microstructure of WON were well preserved. The core-level W 4f XPS spectrum of the WON electrode after 100 000 cycles was further recorded to investigate the variation of its composition (Figure S9, Supporting Information). Almost no change was found, implying its superior chemical and mechanical stability. A simple ASC device based on the as-prepared WON nanowires as anode, a MnO2 electrode as cathode and LiCl/polyvinyl alcohol (PVA) polymer gel as electrolyte was further assembled. MnO2 film was directly grown on carbon fabric cloth by a facile electrochemical method (see Experimental Section), as shown in Figure S10 in the Supporting Information. The optimized volumetric ratio of the MnO2 to WON was calculated to be 1.2:1 (Figure S11, Supporting Information). A stable electrochemical voltage of 1.8 V was reached for our optimized MnO2//WONASCs (Figure S12a,b, Supporting Information). CV curves at various scan rates exhibited rectangular-like shapes without obvious redox peaks, which indicates their ideal capacitive behaviors and fast charge/discharge properties (Figure S12c, Supporting Information). Moreover, the symmetrical triangle shapes of charge/discharge curves also confirmed the excellent electrochemical performance of as-fabricated MnO2//WONASCs as shown in Figure S12d in the Supporting Information. Figure 3a presents the calculated volumetric capacitance and Coulombic efficiency based on these charge/discharge curves. The MnO2//WON-ASCs achieved the highest volumetric capacitance of 2.73 F cm−3 at 34.13 mA cm−3. More importantly, when the current density was increased 20-fold from 34.18 to 125 mA cm−3, about 1.90 F cm−3 was expressed with 69.6% capacitance retention, suggesting the superior rate capability
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
of our as-fabricated ASCs. It is also notable that the volumetric capacitance immediately recovered to its initial value after the current density returned to 34.13 mA cm−3 and the Coulombic efficiency always kept above 95%, which reflected the high reversibility during the charge and discharge process again. Furthermore, given the good flexibility of the carbon-cloth substrate, it is believed that our as-fabricated MnO2//WONASCs will inherit this attractive property. As demonstrated in Figure 3b, the CV curves collected for the ASCs under different bending conditions are almost the same, and the variation of volumetric capacitance is even less than 2%. Figure 3c compares the Ragone plots of MnO2//WON-ASCs with other previously reported ASC devices. The maximum energy density of 1.27 mW h cm−3 was obtained at a power density of 0.62 W cm−3 for our MnO2//WON-ASCs, which is much higher than other ASCs in some of the latest papers.[23,27–29,41–47] Besides, a high power density of 1.35 W cm−3 was achieved while the energy density was still 0.40 mW h cm−3. Two such ASC devices were connected in series, which could power light-emitting diode (LED) indicators (3 V) well, after being charged at 125 mA cm−3 for 30 s. Finally, the ASCs exhibited a remarkable stability with 95.2% retention of initial capacitance after 10 000 cycles at 100 mV s−1 (Figure S13, Supporting Information). Subsequently, the WON electrode was utilized as the anode in an Escherichia coli cell catalyzed MFC with single-chamber configuration for performance evaluation (denoted as WONMFC, Experimental Section). Since the anode is directly related to electron transfer between anode and bacteria, the high conductivity and good biocompatibility of our WON electrode will serve it well. The configuration of the WON-MFC is illustrated in Figure 4a. Typically, a 40 wt% Pt/C loaded carbon-paper
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) Rate performance and Coulombic efficiency of the as-fabricated MnO2//WON–ASCs at various current densities from 6.25 to 125 mA cm−3. b) CV curves collected at a scan rate of 100 mV s−1 for the ASC devices under different bending conditions. Insets are the device pictures under test conditions. c) Ragone plots of the ASC devices. The values reported for other ASC devices are added for comparison.[23,27–29,41–47] The inset is an LED indicator powered by the tandem ASCs.
cathode was directly exposed in the air, and a WON anode inoculated with an electrogenic bacterial strain, an E. coli. cell, was used to generate electrons from the organic substrate. The single-chamber MFC devices were jointed by a cation exchange membrane (CEM). Note that the performance of an MFC can be influenced by many factors, such as the cathodic reaction, buffer system, inoculated bacterial strain, cell configuration, organic substrate, etc.; thus, it is hard to compare MFC performance directly with other reports as different parameters were adopted. To prove the advanced performance of our WON anode, another two MFCs were assembled using conventional carbon cloth (most commercial MFC anodes) and 40 wt% Pt/C (common anodic electrocatalyst of MFC) loaded carbon cloth as anodes, respectively, while keeping identical configuration and operation conditions (denoted as carbon cloth-MFC and Pt/C-MFC). Figure 4b,c compare their polarization curves and power outputs determined by varying the external load resistance. It can be seen that the open-circuit potential is 0.55 V for the WON-MFC, close to that for the carbon cloth-MFC (0.59 V) and Pt/C-MFC (0.60 V). With decreasing load resistance, the cell voltage decreased more slowly, which is mainly attributed to the superior charge transfer rate. Moreover, the maximum power density obtained for WON-MFC achieved 7.1 mW cm−3 at a current density of 33.7 mA cm−3, while only 3.7 and
4
wileyonlinelibrary.com
5.0 mW cm−3 were reached for carbon cloth-MFC and Pt/C-MFC respectively. Furthermore, current and potential measurements on the MFC with WON anode were carried out in constant-load mode with an external resistance of 50 Ω, as shown in Figure S14 in the Supporting Information. It is noted that, after bacteria enrichment for about 4 h, the power density rises to a plateau, which means the bacteria could keep active for a long period. Figure 4d shows the SEM images of the WON anode after 10 h of operation. The uniformly anchored rod-shaped E. coli cells between WON nanowires further revealed the high biocompatibility of the WON electrode and the superior charge-transfer pathway. The remarkable behavior of WON electrode as both ASC and MFC anode enables it to be a promising bridge for the integration of energy conversion and storage. To this end, an attempt was made to assemble an MFC–ASC device using both WON anodes (see Experimental Section). Figure 5a schematically presents the structure and operation principle of the integrated MFC–ASC device integrated by one MFC device with one ASC device. MFC-charging is enabled by turning on switch no. 1, The electrons generated through the decomposition of glucose were transferred to the WON electrode. Meanwhile, positive charges were transported toward and stored in the MnO2 electrode. After enough charges were stored, the MFC–ASC device
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a) Schematic diagram illustrating the configuration of a single-chamber MFC device with WON anode and 40 wt% Pt/C loaded carbon paper cathode. Comparisons of b) polarization curves and c) power outputs for carbon cloth MFCs, Pt/C MFCs and WON MFCs. d) SEM image of a WON anode after constant-load discharging for 10 h.
was connected to the external circuit through turning off switch no. 1 then turning on switch no. 2, driving other electronic devices with a power output that ASC could achieve. As a demonstration, three MFC devices in series were used to integrate with one ASC device (Figure S15, Supporting Information). Figure 5b shows a typical MFC-charging and galvanostatic discharging curve. First, the open circle potential of our ASC was about 0.081 V. As soon as the switch no. 1 was turned on, the voltage underwent a rapid increase and slowly reached 0.9 V after 500 s. Then the MFC–ASC device was able to supply the
electrical energy for 163 s at a discharge current density of 50 mA cm−3. The successful fabrication of MFC–ASC devices that could realize energy conversion and storage simultaneously is believed to tremendously promote their practical applications. In conclusion, we have designed a novel kind of holey WON nanowires on a carbon fabric cloth substrate via a simple twostep method. The unique nanostructure consisting of numerous micro-/mesopores, excellent conductivity, outstanding hydrophilia, high chemical inertness, strong mechanical intensity and good biocompatibility enabled our WON nanowires to be
Figure 5. a) Schematic illustration of the configuration and general working mechanism for an MFC–ASC device integrated by one MFC device with one ASC device. b) A typical MFC-charging and galvanostatic discharging curve. Three MFC devices in series were used to charge an ASC device. The galvanostatic discharging process was performed at a current density of 50 mA cm−3.
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
superior ASC and MFC anodes. Solid-state flexible ASCs based on WON anodes and MnO2 cathodes were successfully assembled. The as-fabricated devices achieved a maximum energy density of 1.27 mW h cm−3 and a maximum power density of 1.35 W cm−3, which are considerably higher than most of the reported ASCs in recent papers. On the other hand, the MFC device equipped with a WON anode exhibited a remarkable output power density of 7.1 mW cm−3. To date, this is the first report concerning the application of WON materials in both ASC and MFC. Additionally, an integrated MFC–ASC device has been developed, which simultaneously possesses energy conversion and storage functions. This fascinating design and the encouraging results provide a promising research direction for the next-generation self-powered systems.
Experimental Section Preparation of Tungsten Oxynitride (WON) Nanowires: All the reagents used were of analytical grade and were used directly without any purification. Free-standing WON nanowires were grown on carbon fabric cloth by a two-step process. WO3 nanowires were firstly synthesized on carbon fabric cloth via a seed-assisted hydrothermal method. Carbon fabric cloth (2 cm × 3 cm) was cleaned with ethanol and distilled water, followed by being immersed in a solution of containing 0.695 g of Na2WO4·2H2O, 10 mL of 3 M HCl, and 2 mL of H2O2 (30 vol% aqueous solution) for 5 min and blow-dried with compressed air. The dried carbon fabric cloth was further heated on a hotplate in air at 300 °C for 5 min, forming WO3 nanoparticles on the carbon fabric cloth. 1.33 g of H2WO4 was dissolved in a mixed solution of 22 mL of distilled water and 6 mL of H2O2 (30vol% aqueous solution), then 16 mL of ethanol and 0.04 g of NH4Cl were added into 4 mL of the previous solution and stirred into a pellucid solution. 20 mL of this clear solution mixture together with the carbon fabric cloth coated with WO3 nanoparticles were transferred to a Teflon-lined stainless-steel autoclave (25 mL volume). The sealed autoclave was heated in an electric oven at 180 °C for 12 h, and then allowed to cool down slowly at room temperature. A light-blue WO3 film was uniformly coated on the carbon fabric cloth surface. The sample was thoroughly washed with deionized (DI) water and dried. Finally, the WO3 nanowires were converted to WON nanowires by annealing in NH3 at various temperatures for 1 h. The mass loading of WON nanowires on carbon fabric cloth is about 167.5 mg cm−3. Preparation of MnO2 on Carbon Fabric Cloth: MnO2 on carbon fabric cloth was prepared through an anodic electrodeposition method, which uses a CHI 760D workstation. The electrodeposition was conducted with a solution (17 mL) containing 0.1 M manganese acetate and 0.2 M sodium sulphate at 1.0 V for 90 s at room temperature. The mass loading of MnO2 is 35 mg cm−3. Fabrication of Solid-State ASCs: The solid-state MnO2//WON–ASCs were assembled by separating MnO2 and WON electrodes with a separator (NKK separator, Nippon Kodoshi Corporation) and polyvinyl alcohol (PVA)/LiCl gel as the electrolyte. In order to balance the charge between the electrodes, the volumetric ratio of MnO2 electrode to WON electrode was calculated to be 1.2:1. The PVA/LiCl gel was prepared via a solution-casting method. 4.24 g of LiCl and 2.00 g of PVA were dissolved in 20 mL of distilled water, then the solution was heated at 85 °C under vigorous stirring until they completely dissolved in water and formed a jelly-like solution. Two electrodes and separator were soaked in the PVA/LiCl solution, and then the gel was allowed solidify at room temperature for 6 h. Then, they were assembled together and kept at 40 °C for 6 h to remove excess water in the electrolyte. The area and thickness of the fabricated MnO2//WON–ASCs were about 0.5 cm2 and 0.08 cm, respectively. Fabrication of MFCs Equipped with WON Anodes: E. coli K-12 (ATCC 25922) was firstly cultivated anaerobically at 37 °C in a culture medium
6
wileyonlinelibrary.com
(a mixture of 10.0 peptone, 5.0 g of NaCl, and 3.0 g of beef powder per liter), which was sterilized in an autoclave at 121 °C for 20 min. After growing for 18 h, the E. coli culture (10 mL) was inoculated to anolyte (saturated with nitrogen for 20 min before inoculation). A cubic MFC with a single chamber (4 cm × 5 cm × 5 cm) was used in this work, which consisted of a poly(methyl methacrylate) chamber and a membrane cathode assembly on one side (4 cm × 4 cm). Our WON electrode (4 cm × 4 cm) was directly used as anode. The cathode was prepared by hot-pressing carbon paper on one side of a cation exchange membrane (CEM). The carbon paper cathode was pasted with 12.5 mg cm−3 commercial Pt-catalyst (40 wt% Pt/C) in a mixture of Nafion (5%). The anolyte was phosphate-buffered basal medium (PBBM) with 5 × 10−3 M 2-hydroxy-1,4-naphthoquinone (HNQ), 10.0 g L–1 glucose, and 5.0 g L–1 yeast extract. The PBBM consisted of 8 g NaCl, 0.2 g of KCl, 3.63 g of Na2HPO4·12H2O, 0.24 g of KH2PO4 per litre. Before inoculation, the cell was washed with 1 mol L−1 HCl and 1 mol L−1 NaOH to remove possible metal and biomass contamination, and rinsed by sterile water. To initiate the MFC experiment, the anode chamber was inoculated with 10 mL of the above-mentioned cell suspension. Before each test, the suspension was purged over nitrogen for 20 min to remove oxygen from the solution. Fabrication of Integrated MFC-ASC Device with Both WON Anodes: First, an MFC device equipped with WON anodes was fabricated. Then, a MnO2 electrode (4 cm × 4 cm) was also inserted into the anode chamber of MFC device. When the MnO2 electrode was connected with Pt/C MFC cathode, MFC-charging was enabled. After enough charges were stored, the MFC–ASC device was connected to the external circuit through disconnecting MnO2 electrode with Pt/C MFC cathode, then connecting the MnO2 electrode and WON electrode with external circuit. To obtain high output voltage and power, multiple MFC devices in series were utilized to integrate with one ASC devices. For example, Figure S15 in the Supporting Information illustrates the circuit diagram of three MFC devices integrated with one ASC device. Characterization: The microstructures and compositions of electrode materials were analyzed by field-emission SEM (FE-SEM) (JSM-6330F), TEM (FEI Tecnai G2 F30), XPS (XPS, ESCALab250, Thermo VG), and X-ray diffractometry (XRD) (D8 ADVANCE). The contact angles of water drops deposited on the surface of film electrodes were measured at 25 °C using a contact angle meter (SL150, Kino industrial co., LTD, USA). Cyclic voltammetry (CV), galvanostatic charge/discharge measurements were conducted employing an electrochemical workstation (CHI 760D). The electrochemical studies of the individual electrode were performed in a conventional three-electrode cell, with a Pt counter electrode, a saturated calomel reference electrode (SCE) and 5 M LiCl electrolyte. In the performance measurement of the MFC, the current and voltage outputs of MFC were monitored by a Arbin battery tester (BT2000, Shenzhen, China).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge the financial support of this work received by the Natural Science Foundation of China (Grant No. 21403306, 21273290, and J1103305), National Training Programs of Innovation and Entrepreneurship for Undergraduates, and the Opening Fund of Laboratory Sun Yat-Sen University.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: January 30, 2015 Revised: March 12, 2015 Published online:
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
COMMUNICATION
[1] Z. Yang, J. Deng, H. Sun, J. Ren, S. Pan, H. Peng, Adv. Mater. 2014, 26, 7038. [2] Z. Yu, J. Thomas, Adv. Mater. 2014, 26, 4279. [3] J. Xiao, L. Wan, S. Yang, F. Xiao, S. Wang, Nano Lett. 2014, 14, 831. [4] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. W. Lou, Adv. Mater. 2012, 24, 5166. [5] X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang, H. J. Fan, Nano Lett. 2014, 14, 1651. [6] B. E. Logan, K. Rabaey, Science 2012, 337, 686. [7] H. Wang, F. Qian, G. Wang, Y. Jiao, Z. He, Y. Li, ACS Nano 2013, 7, 8728. [8] F. Qian, H. Wang, Y. Ling, G. Wang, M. P. Thelen, Y. Li, Nano Lett. 2014, 14, 3688. [9] X. H. Lu, M. H. Yu, G. M. Wang, Y. X. Tong, Y. Li, Energy Environ. Sci. 2014, 7, 2160. [10] C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 2013, 13, 2078. [11] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 2013, 25, 3302. [12] X. Xia, C. Zhu, J. Luo, Z. Zeng, C. Guan, C. F. Ng, H. Zhang, H. J. Fan, Small 2014, 10, 766. [13] Y. Qiu, Y. Zhao, X. Yang, W. Li, Z. Wei, J. Xiao, S. F. Leung, Q. Lin, H. Wu, Y. Zhang, Z. Fan, S. Yang, Nanoscale 2014, 6, 3626. [14] Y. M. Zhao, W. B. Hu, Y. D. Xia, E. F. Smith, Y. Q. Zhu, C. W. Dunnill, D. H. Gregory, J. Mater. Chem. 2007, 17, 4436. [15] F. Xu, A. Fahmi, Y. Zhao, Y. Xia, Y. Zhu, Nanoscale 2012, 4, 7031. [16] S. H. Mohamed, A. Anders, Surf. Coat. Technol. 2006, 201, 2977. [17] M. Rosenbaum, F. Zhao, U. Schroder, F. Scholz, Angew. Chem. Int. Ed. 2006, 45, 6658. [18] Y. Wang, B. Li, L. Zeng, D. Cui, X. Xiang, W. Li, Biosens. Bioelectron. 2013, 41, 582. [19] M. Peuster, V. Kaese, G. Wuensch, C. von Schnakenburg, M. Niemeyer, C. Fink, H. Haferkamp, G. Hausdorf, J. Biomed. Mater. Res. B 2003, 65, 211. [20] O. Kartachova, Y. Chen, R. Jones, Y. Chen, H. Zhang, A. M. Glushenkov, J. Mater. Chem. A 2014, 2, 12940. [21] O. Kartachova, A. M. Glushenkov, Y. Chen, H. Zhang, Y. Chen, J. Mater. Chem. A 2013, 1, 7889. [22] O. Kartachova, A. M. Glushenkov, Y. Chen, H. Zhang, X. J. Dai, Y. Chen, J. Power Sources 2012, 220, 298. [23] X. Lu, M. Yu, G. Wang, T. Zhai, S. Xie, Y. Ling, Y. Tong, Y. Li, Adv. Mater. 2013, 25, 267. [24] G. Xu, B. Ding, P. Nie, L. Shen, J. Wang, X. Zhang, Chem. Eur. J. 2013, 19, 12306.
[25] X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong, Y. Li, Nano Lett. 2012, 12, 5376. [26] W. Yu, W. Lin, X. Shao, Z. Hu, R. Li, D. Yuan, J. Power Sources 2014, 272, 137. [27] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M. S. Balogun, Y. Tong, Adv. Mater. 2014, 26, 3148. [28] W. Zilong, Z. Zhu, J. Qiu, S. Yang, J. Mater. Chem. C 2014, 2, 1331. [29] X. Xiao, T. Ding, L. Yuan, Y. Shen, Q. Zhong, X. Zhang, Y. Cao, B. Hu, T. Zhai, L. Gong, J. Chen, Y. Tong, J. Zhou, Z. L. Wang, Adv. Energy Mater. 2012, 2, 1328. [30] Q. Qu, S. Yang, X. Feng, Adv. Mater. 2011, 23, 5574. [31] Q. Qu, Y. Zhu, X. Gao, Y. Wu, Adv. Energy Mater. 2012, 2, 950. [32] J. Chang, M. Jin, F. Yao, T. H. Kim, V. T. Le, H. Yue, F. Gunes, B. Li, A. Ghosh, S. Xie, Y. H. Lee, Adv. Funct. Mater. 2013, 23, 5074. [33] H. Wu, K. Lian, ECS Trans. 2014, 58, 67. [34] X. Zhou, C. Shang, L. Gu, S. Dong, X. Chen, P. Han, L. Li, J. Yao, Z. Liu, H. Xu, Y. Zhu, G. Cui, ACS Appl. Mater. Interfaces 2011, 3, 3058. [35] D. Choi, G. E. Blomgren, P. N. Kumta, Adv. Mater. 2006, 18, 1178. [36] D. Shu, H. Cheng, C. Lv, M. A. Asi, L. Long, C. He, X. Zou, Z. Kang, Int. J. Hydrogen Energy 2014, 39, 16139. [37] C. M. Ghimbeu, E. Raymundo-Piñero, P. Fioux, F. Béguin, C. Vix-Guterl, J. Mater. Chem. 2011, 21, 13268. [38] C. Chen, D. Zhao, X. Wang, Mater. Chem. Phys. 2006, 97, 156. [39] S. Dong, X. Chen, L. Gu, X. Zhou, H. Wang, Z. Liu, P. Han, J. Yao, L. Wang, G. Cui, L. Chen, Mater. Res. Bull. 2011, 46, 835. [40] T. T. Chen, H. P. Liu, Y. J. Wei, I. C. Chang, M. H. Yang, Y. S. Lin, K. L. Chan, H. T. Chiu, C. Y. Lee, Nanoscale 2014, 6, 5106. [41] X. Yu, B. Lu, Z. Xu, Adv. Mater. 2014, 26, 1044. [42] T. Zhai, S. Xie, M. Yu, P. Fang, C. Liang, X. Lu, Y. Tong, Nano Energy 2014, 8, 255. [43] Y. C. Chen, Y. G. Lin, Y. K. Hsu, S. C. Yen, K. H. Chen, L. C. Chen, Small 2014, 10, 3803. [44] C. Long, D. Qi, T. Wei, J. Yan, L. Jiang, Z. Fan, Adv. Funct. Mater. 2014, 24, 3953. [45] M. H. Yu, W. Wang, C. Li, T. Zhai, X. H. Lu, Y. X. Tong, NPG Asia Mater. 2014, 6, e129. [46] Z. Su, C. Yang, C. Xu, H. Wu, Z. Zhang, T. Liu, C. Zhang, Q. Yang, B. Li, F. Kang, J. Mater. Chem. A 2013, 1, 12432. [47] T. Zhai, X. Lu, Y. Ling, M. Yu, G. Wang, T. Liu, C. Liang, Y. Tong, Y. Li, Adv. Mater. 2014, 26, 5869.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
7
COMMUNICATION
Holey Tungsten Oxynitride Nanowires: Novel Anodes Efficiently Integrate Microbial Chemical Energy Conversion and Electrochemical Energy Storage Minghao Yu, Yi Han, Xinyu Cheng, Le Hu, Yinxiang Zeng, Meiqiong Chen, Faliang Cheng, Xihong Lu,* and Yexiang Tong The serious global energy crisis and environmental pollution are rising strong demands in the development of novel energyconversion devices for the direct production of electricity from renewable resources and energy-storage devices for electrical storage.[1–5] In this regard, microbial fuel cells (MFCs)[6–8] and asymmetric supercapacitors (ASCs)[9,10] are two emerging systems required for promising electrochemical energy conversion and storage; however, one issue limiting the practical application of MFCs as power generators is their relatively low output power density compared to other energy-conversion devices.[9,10] Meanwhile, the progress on ASC anodes is relatively slow, which makes it hard for them to satisfy the demand of high energy, matching with that of cathode.[11–13] Further breakthroughs in electrode materials hold the key to fundamental advances in both MFCs and ASCs. Numerous studies have been conducted to develop new materials and optimize structures for high performances in either MFCs or ASCs. However, reports available on the realization of both energy conversion and storage in one electrode with the same material and structure are rare, even though it would open up many opportunities for the integration of renewable energy harvesting and storage with high power output. As a typical refractory transition metal oxynitride, cubic tungsten oxynitride is typically constructed with the nonmetal atoms (N, O) occupying interstitial sites in the metal lattice, typically octahedral in face-centered cubic (fcc), which results in the maximization of valence electrons in the nonmetal atoms (Figure 1a). Such unique crystal structure enables tungsten oxynitride (WON) to have a number of attractive properties such as high hardness, excellent conductivity, good thermal stability, and high melting point.[14–16] More importantly, the M. Yu, Y. Han, X. Cheng, L. Hu, Y. Zeng, M. Chen, Prof. X. Lu, Prof. Y. Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry KLGHEI of Environment and Energy Chemistry School of Chemistry and Chemical Engineering Sun Yat-Sen University 135 Xingang West Road, Chemical North Building 325 Guangzhou 510275, China E-mail: [email protected] M. Chen, Prof. F. Cheng Biosensor Research Centre Dongguan University of Technology 251 Xueyuan Road, Dongguan 523808, China
DOI: 10.1002/adma.201500493
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
good biocompatibility could strengthen the enrichment of bacteria, which enables WON to have promising application as the anode in MFCs.[17–19] In addition, comparing with other metal nitrides and metal oxynitrides, the extraordinary chemical inertness further gives WON great promise as a durable anode.[20–22] However, no investigations of WON as anode material for either MFCs or ASCs have been reported thus far. In this work, we constituted the first demonstration of WON as a high-performance anode material for MFCs and ASCs. Holey WON nanowires with quantities of micro-/mesopores were successfully synthesized on carbon cloth. A novelly structured WON with superior conductive and hydrophilic properties enables it to exhibit superior and efficient performance as anodes in both ASCs and MFCs. More importantly, an integrated MFC–ASC system has been developed, which simultaneously possesses energy conversion and storage functions. The MFC–ASC device could be self-charged to 0.9 V, and supply electrical energy for 163 s at a discharge current density of 50 mA cm−3. Holey WON nanowires were prepared on flexible and conductive carbon cloth via a two-step process. WO3 nanowires were firstly synthesized on a carbon-cloth substrate through a seed-assisted hydrothermal method. As observed from scanning electron microscopy (SEM) images (Figure S1, Supporting Information), the entire surface of the carbon fiber was covered by uniform nanowires with diameters ranging from 50 to 80 nm after hydrothermal reaction. Then, these nanowires were annealed under an ammonia atmosphere at a temperature of 700 °C for 1 h. The film color turned from light blue to black, implying the conversion from WO3 to WON occurred. Meanwhile, the wire-like morphology was almost preserved with a slight curve (Figure 1b). Figure 1c displays the typical X-ray diffraction (XRD) patterns of WO3 and WON samples. It is revealed that hexagonal WO3 (JCPDS = #33-1387) completely converted into cubic W0.62(N0.62O0.38) (JCPDS = #25-1254), and both samples are well-crystalline. Additionally, transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were applied to analyze the detailed variation of microstructure. Figure S2a, Supporting Information, demonstrates that the WO3 nanowire with a smooth surface is uniform in diameter, and its single-crystalline nature can be concluded from its SAED pattern. Regular lattice fringes with d-spacings of 0.37 and 0.39 nm, corresponding to the (001) and (110) planes of WO3, further confirmed its high-quality crystallization (Figure S2b, Supporting Information). Interestingly, when WO3 turned into WON, a smooth nanowire was replaced by a rough one with a polycrystalline nature (Figure 1d). The obvious
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. a) Crystal structure of cubic WON. b) SEM images of as-prepared WON nanowires. c) XRD spectra collected for as-prepared WO3 and WON nanowires. d) TEM image and e) HRTEM images of a WON nanowire. The inset in (d) is the SAED pattern. f) HAADF-STEM image of WON nanowire and the corresponding EELS mapping image.
graphic-contrast variation in its high-resolution TEM image clearly shows that numerous pores with sizes of 1–5 nm are scattered on the nanowire (Figure 1e). Pore size distributions of the two electrodes calculated from the nitrogen adsorption branches are presented in Figure S3, Supporting Information. Apparently, an obvious enrichment of micro- and mesopores is found for WON electrodes when it is compared with those of WO3 electrodes. It is hoped that these mesopores will provide efficient channels, allowing fast and easy access of electrolyte ions to the surface of electrode; meanwhile, a large surface area is guaranteed by both micro- and mesopores, which enables a large amount of active sites for ion adsorption. The electron energy loss spectroscopy (EELS) mapping reveals that nitride is uniformly distributed in the whole nanowires, suggesting the thorough conversion of WO3 (Figure 1f). X-ray photoelectron spectroscopy (XPS) was utilized to investigate the composition of our products. Besides elemental N, no other impurities were detected to be introduced into WON as shown in XPS survey spectra (Figure S4, Supporting Information). The O 1s peak still appeared without any shift of the peak position after nitridation, while a broad peak of N 1s located at 397.5 eV obtained for WON confirmed the introduction of N element (Figure 2a,b). Figure 2c demonstrates that the oxidation state of W was in-depth. Notably, two additional peaks at the lower binding energies of 33.0 eV (W 4f7/2) and 34.7 eV (W 4f5/2) are observed in W 4f spectra after nitridation, which are associated with lower oxidation states of W (W–N) as might be expected in WON.[14] Moreover, the contact angle is decreased from 71° to 32° after the nitridation process, which indicates the surface wettability is significantly enhanced (Figure 2d). Such better architecture for accommodation of the electrolyte may speed up the electrolyte penetration and spreading in the channels of inner WON nanowires. Hence, it
2
wileyonlinelibrary.com
is anticipated that our as-fabricated WON nanowires with their unique holey structure would be an ideal electrode, decreasing the diffusion length of ions and increasing the contact area with electrolyte, as well as improving active material utilization. To optimize the capacitive behavior of WON nanowires, the effect of the nitridation temperature was firstly studied (Figure S5 and S6, Supporting Information). 700 °C was selected as the optimal temperature. Subsequently, the favorable double-layer energy-storage behavior and high-rate handling capability of the WON electrode fabricated under 700 °C were confirmed by the nearly rectangular shapes of the cyclic voltammetry (CV) curves (Figure S7a, Supporting Information) and the fairly linear slopes of the galvanostatic charge/discharge curves (Figure S7b, Supporting Information). No obvious ohmic drop was observed, which indicates the low inner-pore ion-transport resistance and short diffusion distance during the charge/discharge process. Figure 2e depicts a comparison of its calculated volumetric capacitances based on the discharge curves with the values of recently reported SC anodes. Significantly, the as-prepared WON electrode yielded a remarkable volumetric capacitance of 4.95 F cm−3 at 12.5 mA cm−3 (4.76 F cm−3 at 10 mV s−1), which is considerably higher than that of most reported anodes.[23–28] When the current density reached 500 mA cm−3, a high volumetric capacitance of 3.35 mF cm−3 was still achieved, which means more than 67% of its initial capacitance could be retained. This is an outstanding rate capability among SC anodes, even better than some carbon based anodes with high conductivity.[23–25,27–33] Additionally, more than 93% of original capacitance was retained after an ultra-long term of 100 000 cycles. To the best of our knowledge, this is the best cycling performance reported for SC electrodes based on metal nitride and oxynitride.[25,34–40] CV curves and morphologies of WON after 100, 50 000, and 100 000 cycles were further compared in the
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. a) Core-level O 1s, b) N 1s, and c) W 4f XPS spectra collected for WO3 and WON. d) Contact angles of a water droplet on WO3 and WON. e) Volumetric capacitance of WON electrode calculated based on galvanostatic charge–discharge curves as a function of current density. The volumetric capacitances of other previously reported anodes are added for comparison.[25,27] f) Cycling performance of WON electrode measured at 100 mV s−1. Insets are CV curves of WON electrode after 100, 50 000, 100 000 cycles.
insets of Figure 2f and Figure S8 in the Supporting Information. As expected, both the CV shapes and the microstructure of WON were well preserved. The core-level W 4f XPS spectrum of the WON electrode after 100 000 cycles was further recorded to investigate the variation of its composition (Figure S9, Supporting Information). Almost no change was found, implying its superior chemical and mechanical stability. A simple ASC device based on the as-prepared WON nanowires as anode, a MnO2 electrode as cathode and LiCl/polyvinyl alcohol (PVA) polymer gel as electrolyte was further assembled. MnO2 film was directly grown on carbon fabric cloth by a facile electrochemical method (see Experimental Section), as shown in Figure S10 in the Supporting Information. The optimized volumetric ratio of the MnO2 to WON was calculated to be 1.2:1 (Figure S11, Supporting Information). A stable electrochemical voltage of 1.8 V was reached for our optimized MnO2//WONASCs (Figure S12a,b, Supporting Information). CV curves at various scan rates exhibited rectangular-like shapes without obvious redox peaks, which indicates their ideal capacitive behaviors and fast charge/discharge properties (Figure S12c, Supporting Information). Moreover, the symmetrical triangle shapes of charge/discharge curves also confirmed the excellent electrochemical performance of as-fabricated MnO2//WONASCs as shown in Figure S12d in the Supporting Information. Figure 3a presents the calculated volumetric capacitance and Coulombic efficiency based on these charge/discharge curves. The MnO2//WON-ASCs achieved the highest volumetric capacitance of 2.73 F cm−3 at 34.13 mA cm−3. More importantly, when the current density was increased 20-fold from 34.18 to 125 mA cm−3, about 1.90 F cm−3 was expressed with 69.6% capacitance retention, suggesting the superior rate capability
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
of our as-fabricated ASCs. It is also notable that the volumetric capacitance immediately recovered to its initial value after the current density returned to 34.13 mA cm−3 and the Coulombic efficiency always kept above 95%, which reflected the high reversibility during the charge and discharge process again. Furthermore, given the good flexibility of the carbon-cloth substrate, it is believed that our as-fabricated MnO2//WONASCs will inherit this attractive property. As demonstrated in Figure 3b, the CV curves collected for the ASCs under different bending conditions are almost the same, and the variation of volumetric capacitance is even less than 2%. Figure 3c compares the Ragone plots of MnO2//WON-ASCs with other previously reported ASC devices. The maximum energy density of 1.27 mW h cm−3 was obtained at a power density of 0.62 W cm−3 for our MnO2//WON-ASCs, which is much higher than other ASCs in some of the latest papers.[23,27–29,41–47] Besides, a high power density of 1.35 W cm−3 was achieved while the energy density was still 0.40 mW h cm−3. Two such ASC devices were connected in series, which could power light-emitting diode (LED) indicators (3 V) well, after being charged at 125 mA cm−3 for 30 s. Finally, the ASCs exhibited a remarkable stability with 95.2% retention of initial capacitance after 10 000 cycles at 100 mV s−1 (Figure S13, Supporting Information). Subsequently, the WON electrode was utilized as the anode in an Escherichia coli cell catalyzed MFC with single-chamber configuration for performance evaluation (denoted as WONMFC, Experimental Section). Since the anode is directly related to electron transfer between anode and bacteria, the high conductivity and good biocompatibility of our WON electrode will serve it well. The configuration of the WON-MFC is illustrated in Figure 4a. Typically, a 40 wt% Pt/C loaded carbon-paper
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) Rate performance and Coulombic efficiency of the as-fabricated MnO2//WON–ASCs at various current densities from 6.25 to 125 mA cm−3. b) CV curves collected at a scan rate of 100 mV s−1 for the ASC devices under different bending conditions. Insets are the device pictures under test conditions. c) Ragone plots of the ASC devices. The values reported for other ASC devices are added for comparison.[23,27–29,41–47] The inset is an LED indicator powered by the tandem ASCs.
cathode was directly exposed in the air, and a WON anode inoculated with an electrogenic bacterial strain, an E. coli. cell, was used to generate electrons from the organic substrate. The single-chamber MFC devices were jointed by a cation exchange membrane (CEM). Note that the performance of an MFC can be influenced by many factors, such as the cathodic reaction, buffer system, inoculated bacterial strain, cell configuration, organic substrate, etc.; thus, it is hard to compare MFC performance directly with other reports as different parameters were adopted. To prove the advanced performance of our WON anode, another two MFCs were assembled using conventional carbon cloth (most commercial MFC anodes) and 40 wt% Pt/C (common anodic electrocatalyst of MFC) loaded carbon cloth as anodes, respectively, while keeping identical configuration and operation conditions (denoted as carbon cloth-MFC and Pt/C-MFC). Figure 4b,c compare their polarization curves and power outputs determined by varying the external load resistance. It can be seen that the open-circuit potential is 0.55 V for the WON-MFC, close to that for the carbon cloth-MFC (0.59 V) and Pt/C-MFC (0.60 V). With decreasing load resistance, the cell voltage decreased more slowly, which is mainly attributed to the superior charge transfer rate. Moreover, the maximum power density obtained for WON-MFC achieved 7.1 mW cm−3 at a current density of 33.7 mA cm−3, while only 3.7 and
4
wileyonlinelibrary.com
5.0 mW cm−3 were reached for carbon cloth-MFC and Pt/C-MFC respectively. Furthermore, current and potential measurements on the MFC with WON anode were carried out in constant-load mode with an external resistance of 50 Ω, as shown in Figure S14 in the Supporting Information. It is noted that, after bacteria enrichment for about 4 h, the power density rises to a plateau, which means the bacteria could keep active for a long period. Figure 4d shows the SEM images of the WON anode after 10 h of operation. The uniformly anchored rod-shaped E. coli cells between WON nanowires further revealed the high biocompatibility of the WON electrode and the superior charge-transfer pathway. The remarkable behavior of WON electrode as both ASC and MFC anode enables it to be a promising bridge for the integration of energy conversion and storage. To this end, an attempt was made to assemble an MFC–ASC device using both WON anodes (see Experimental Section). Figure 5a schematically presents the structure and operation principle of the integrated MFC–ASC device integrated by one MFC device with one ASC device. MFC-charging is enabled by turning on switch no. 1, The electrons generated through the decomposition of glucose were transferred to the WON electrode. Meanwhile, positive charges were transported toward and stored in the MnO2 electrode. After enough charges were stored, the MFC–ASC device
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a) Schematic diagram illustrating the configuration of a single-chamber MFC device with WON anode and 40 wt% Pt/C loaded carbon paper cathode. Comparisons of b) polarization curves and c) power outputs for carbon cloth MFCs, Pt/C MFCs and WON MFCs. d) SEM image of a WON anode after constant-load discharging for 10 h.
was connected to the external circuit through turning off switch no. 1 then turning on switch no. 2, driving other electronic devices with a power output that ASC could achieve. As a demonstration, three MFC devices in series were used to integrate with one ASC device (Figure S15, Supporting Information). Figure 5b shows a typical MFC-charging and galvanostatic discharging curve. First, the open circle potential of our ASC was about 0.081 V. As soon as the switch no. 1 was turned on, the voltage underwent a rapid increase and slowly reached 0.9 V after 500 s. Then the MFC–ASC device was able to supply the
electrical energy for 163 s at a discharge current density of 50 mA cm−3. The successful fabrication of MFC–ASC devices that could realize energy conversion and storage simultaneously is believed to tremendously promote their practical applications. In conclusion, we have designed a novel kind of holey WON nanowires on a carbon fabric cloth substrate via a simple twostep method. The unique nanostructure consisting of numerous micro-/mesopores, excellent conductivity, outstanding hydrophilia, high chemical inertness, strong mechanical intensity and good biocompatibility enabled our WON nanowires to be
Figure 5. a) Schematic illustration of the configuration and general working mechanism for an MFC–ASC device integrated by one MFC device with one ASC device. b) A typical MFC-charging and galvanostatic discharging curve. Three MFC devices in series were used to charge an ASC device. The galvanostatic discharging process was performed at a current density of 50 mA cm−3.
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
superior ASC and MFC anodes. Solid-state flexible ASCs based on WON anodes and MnO2 cathodes were successfully assembled. The as-fabricated devices achieved a maximum energy density of 1.27 mW h cm−3 and a maximum power density of 1.35 W cm−3, which are considerably higher than most of the reported ASCs in recent papers. On the other hand, the MFC device equipped with a WON anode exhibited a remarkable output power density of 7.1 mW cm−3. To date, this is the first report concerning the application of WON materials in both ASC and MFC. Additionally, an integrated MFC–ASC device has been developed, which simultaneously possesses energy conversion and storage functions. This fascinating design and the encouraging results provide a promising research direction for the next-generation self-powered systems.
Experimental Section Preparation of Tungsten Oxynitride (WON) Nanowires: All the reagents used were of analytical grade and were used directly without any purification. Free-standing WON nanowires were grown on carbon fabric cloth by a two-step process. WO3 nanowires were firstly synthesized on carbon fabric cloth via a seed-assisted hydrothermal method. Carbon fabric cloth (2 cm × 3 cm) was cleaned with ethanol and distilled water, followed by being immersed in a solution of containing 0.695 g of Na2WO4·2H2O, 10 mL of 3 M HCl, and 2 mL of H2O2 (30 vol% aqueous solution) for 5 min and blow-dried with compressed air. The dried carbon fabric cloth was further heated on a hotplate in air at 300 °C for 5 min, forming WO3 nanoparticles on the carbon fabric cloth. 1.33 g of H2WO4 was dissolved in a mixed solution of 22 mL of distilled water and 6 mL of H2O2 (30vol% aqueous solution), then 16 mL of ethanol and 0.04 g of NH4Cl were added into 4 mL of the previous solution and stirred into a pellucid solution. 20 mL of this clear solution mixture together with the carbon fabric cloth coated with WO3 nanoparticles were transferred to a Teflon-lined stainless-steel autoclave (25 mL volume). The sealed autoclave was heated in an electric oven at 180 °C for 12 h, and then allowed to cool down slowly at room temperature. A light-blue WO3 film was uniformly coated on the carbon fabric cloth surface. The sample was thoroughly washed with deionized (DI) water and dried. Finally, the WO3 nanowires were converted to WON nanowires by annealing in NH3 at various temperatures for 1 h. The mass loading of WON nanowires on carbon fabric cloth is about 167.5 mg cm−3. Preparation of MnO2 on Carbon Fabric Cloth: MnO2 on carbon fabric cloth was prepared through an anodic electrodeposition method, which uses a CHI 760D workstation. The electrodeposition was conducted with a solution (17 mL) containing 0.1 M manganese acetate and 0.2 M sodium sulphate at 1.0 V for 90 s at room temperature. The mass loading of MnO2 is 35 mg cm−3. Fabrication of Solid-State ASCs: The solid-state MnO2//WON–ASCs were assembled by separating MnO2 and WON electrodes with a separator (NKK separator, Nippon Kodoshi Corporation) and polyvinyl alcohol (PVA)/LiCl gel as the electrolyte. In order to balance the charge between the electrodes, the volumetric ratio of MnO2 electrode to WON electrode was calculated to be 1.2:1. The PVA/LiCl gel was prepared via a solution-casting method. 4.24 g of LiCl and 2.00 g of PVA were dissolved in 20 mL of distilled water, then the solution was heated at 85 °C under vigorous stirring until they completely dissolved in water and formed a jelly-like solution. Two electrodes and separator were soaked in the PVA/LiCl solution, and then the gel was allowed solidify at room temperature for 6 h. Then, they were assembled together and kept at 40 °C for 6 h to remove excess water in the electrolyte. The area and thickness of the fabricated MnO2//WON–ASCs were about 0.5 cm2 and 0.08 cm, respectively. Fabrication of MFCs Equipped with WON Anodes: E. coli K-12 (ATCC 25922) was firstly cultivated anaerobically at 37 °C in a culture medium
6
wileyonlinelibrary.com
(a mixture of 10.0 peptone, 5.0 g of NaCl, and 3.0 g of beef powder per liter), which was sterilized in an autoclave at 121 °C for 20 min. After growing for 18 h, the E. coli culture (10 mL) was inoculated to anolyte (saturated with nitrogen for 20 min before inoculation). A cubic MFC with a single chamber (4 cm × 5 cm × 5 cm) was used in this work, which consisted of a poly(methyl methacrylate) chamber and a membrane cathode assembly on one side (4 cm × 4 cm). Our WON electrode (4 cm × 4 cm) was directly used as anode. The cathode was prepared by hot-pressing carbon paper on one side of a cation exchange membrane (CEM). The carbon paper cathode was pasted with 12.5 mg cm−3 commercial Pt-catalyst (40 wt% Pt/C) in a mixture of Nafion (5%). The anolyte was phosphate-buffered basal medium (PBBM) with 5 × 10−3 M 2-hydroxy-1,4-naphthoquinone (HNQ), 10.0 g L–1 glucose, and 5.0 g L–1 yeast extract. The PBBM consisted of 8 g NaCl, 0.2 g of KCl, 3.63 g of Na2HPO4·12H2O, 0.24 g of KH2PO4 per litre. Before inoculation, the cell was washed with 1 mol L−1 HCl and 1 mol L−1 NaOH to remove possible metal and biomass contamination, and rinsed by sterile water. To initiate the MFC experiment, the anode chamber was inoculated with 10 mL of the above-mentioned cell suspension. Before each test, the suspension was purged over nitrogen for 20 min to remove oxygen from the solution. Fabrication of Integrated MFC-ASC Device with Both WON Anodes: First, an MFC device equipped with WON anodes was fabricated. Then, a MnO2 electrode (4 cm × 4 cm) was also inserted into the anode chamber of MFC device. When the MnO2 electrode was connected with Pt/C MFC cathode, MFC-charging was enabled. After enough charges were stored, the MFC–ASC device was connected to the external circuit through disconnecting MnO2 electrode with Pt/C MFC cathode, then connecting the MnO2 electrode and WON electrode with external circuit. To obtain high output voltage and power, multiple MFC devices in series were utilized to integrate with one ASC devices. For example, Figure S15 in the Supporting Information illustrates the circuit diagram of three MFC devices integrated with one ASC device. Characterization: The microstructures and compositions of electrode materials were analyzed by field-emission SEM (FE-SEM) (JSM-6330F), TEM (FEI Tecnai G2 F30), XPS (XPS, ESCALab250, Thermo VG), and X-ray diffractometry (XRD) (D8 ADVANCE). The contact angles of water drops deposited on the surface of film electrodes were measured at 25 °C using a contact angle meter (SL150, Kino industrial co., LTD, USA). Cyclic voltammetry (CV), galvanostatic charge/discharge measurements were conducted employing an electrochemical workstation (CHI 760D). The electrochemical studies of the individual electrode were performed in a conventional three-electrode cell, with a Pt counter electrode, a saturated calomel reference electrode (SCE) and 5 M LiCl electrolyte. In the performance measurement of the MFC, the current and voltage outputs of MFC were monitored by a Arbin battery tester (BT2000, Shenzhen, China).
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors acknowledge the financial support of this work received by the Natural Science Foundation of China (Grant No. 21403306, 21273290, and J1103305), National Training Programs of Innovation and Entrepreneurship for Undergraduates, and the Opening Fund of Laboratory Sun Yat-Sen University.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: January 30, 2015 Revised: March 12, 2015 Published online:
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500493
COMMUNICATION
[1] Z. Yang, J. Deng, H. Sun, J. Ren, S. Pan, H. Peng, Adv. Mater. 2014, 26, 7038. [2] Z. Yu, J. Thomas, Adv. Mater. 2014, 26, 4279. [3] J. Xiao, L. Wan, S. Yang, F. Xiao, S. Wang, Nano Lett. 2014, 14, 831. [4] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X. W. Lou, Adv. Mater. 2012, 24, 5166. [5] X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang, H. J. Fan, Nano Lett. 2014, 14, 1651. [6] B. E. Logan, K. Rabaey, Science 2012, 337, 686. [7] H. Wang, F. Qian, G. Wang, Y. Jiao, Z. He, Y. Li, ACS Nano 2013, 7, 8728. [8] F. Qian, H. Wang, Y. Ling, G. Wang, M. P. Thelen, Y. Li, Nano Lett. 2014, 14, 3688. [9] X. H. Lu, M. H. Yu, G. M. Wang, Y. X. Tong, Y. Li, Energy Environ. Sci. 2014, 7, 2160. [10] C. Zhou, Y. Zhang, Y. Li, J. Liu, Nano Lett. 2013, 13, 2078. [11] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 2013, 25, 3302. [12] X. Xia, C. Zhu, J. Luo, Z. Zeng, C. Guan, C. F. Ng, H. Zhang, H. J. Fan, Small 2014, 10, 766. [13] Y. Qiu, Y. Zhao, X. Yang, W. Li, Z. Wei, J. Xiao, S. F. Leung, Q. Lin, H. Wu, Y. Zhang, Z. Fan, S. Yang, Nanoscale 2014, 6, 3626. [14] Y. M. Zhao, W. B. Hu, Y. D. Xia, E. F. Smith, Y. Q. Zhu, C. W. Dunnill, D. H. Gregory, J. Mater. Chem. 2007, 17, 4436. [15] F. Xu, A. Fahmi, Y. Zhao, Y. Xia, Y. Zhu, Nanoscale 2012, 4, 7031. [16] S. H. Mohamed, A. Anders, Surf. Coat. Technol. 2006, 201, 2977. [17] M. Rosenbaum, F. Zhao, U. Schroder, F. Scholz, Angew. Chem. Int. Ed. 2006, 45, 6658. [18] Y. Wang, B. Li, L. Zeng, D. Cui, X. Xiang, W. Li, Biosens. Bioelectron. 2013, 41, 582. [19] M. Peuster, V. Kaese, G. Wuensch, C. von Schnakenburg, M. Niemeyer, C. Fink, H. Haferkamp, G. Hausdorf, J. Biomed. Mater. Res. B 2003, 65, 211. [20] O. Kartachova, Y. Chen, R. Jones, Y. Chen, H. Zhang, A. M. Glushenkov, J. Mater. Chem. A 2014, 2, 12940. [21] O. Kartachova, A. M. Glushenkov, Y. Chen, H. Zhang, Y. Chen, J. Mater. Chem. A 2013, 1, 7889. [22] O. Kartachova, A. M. Glushenkov, Y. Chen, H. Zhang, X. J. Dai, Y. Chen, J. Power Sources 2012, 220, 298. [23] X. Lu, M. Yu, G. Wang, T. Zhai, S. Xie, Y. Ling, Y. Tong, Y. Li, Adv. Mater. 2013, 25, 267. [24] G. Xu, B. Ding, P. Nie, L. Shen, J. Wang, X. Zhang, Chem. Eur. J. 2013, 19, 12306.
[25] X. Lu, G. Wang, T. Zhai, M. Yu, S. Xie, Y. Ling, C. Liang, Y. Tong, Y. Li, Nano Lett. 2012, 12, 5376. [26] W. Yu, W. Lin, X. Shao, Z. Hu, R. Li, D. Yuan, J. Power Sources 2014, 272, 137. [27] X. Lu, Y. Zeng, M. Yu, T. Zhai, C. Liang, S. Xie, M. S. Balogun, Y. Tong, Adv. Mater. 2014, 26, 3148. [28] W. Zilong, Z. Zhu, J. Qiu, S. Yang, J. Mater. Chem. C 2014, 2, 1331. [29] X. Xiao, T. Ding, L. Yuan, Y. Shen, Q. Zhong, X. Zhang, Y. Cao, B. Hu, T. Zhai, L. Gong, J. Chen, Y. Tong, J. Zhou, Z. L. Wang, Adv. Energy Mater. 2012, 2, 1328. [30] Q. Qu, S. Yang, X. Feng, Adv. Mater. 2011, 23, 5574. [31] Q. Qu, Y. Zhu, X. Gao, Y. Wu, Adv. Energy Mater. 2012, 2, 950. [32] J. Chang, M. Jin, F. Yao, T. H. Kim, V. T. Le, H. Yue, F. Gunes, B. Li, A. Ghosh, S. Xie, Y. H. Lee, Adv. Funct. Mater. 2013, 23, 5074. [33] H. Wu, K. Lian, ECS Trans. 2014, 58, 67. [34] X. Zhou, C. Shang, L. Gu, S. Dong, X. Chen, P. Han, L. Li, J. Yao, Z. Liu, H. Xu, Y. Zhu, G. Cui, ACS Appl. Mater. Interfaces 2011, 3, 3058. [35] D. Choi, G. E. Blomgren, P. N. Kumta, Adv. Mater. 2006, 18, 1178. [36] D. Shu, H. Cheng, C. Lv, M. A. Asi, L. Long, C. He, X. Zou, Z. Kang, Int. J. Hydrogen Energy 2014, 39, 16139. [37] C. M. Ghimbeu, E. Raymundo-Piñero, P. Fioux, F. Béguin, C. Vix-Guterl, J. Mater. Chem. 2011, 21, 13268. [38] C. Chen, D. Zhao, X. Wang, Mater. Chem. Phys. 2006, 97, 156. [39] S. Dong, X. Chen, L. Gu, X. Zhou, H. Wang, Z. Liu, P. Han, J. Yao, L. Wang, G. Cui, L. Chen, Mater. Res. Bull. 2011, 46, 835. [40] T. T. Chen, H. P. Liu, Y. J. Wei, I. C. Chang, M. H. Yang, Y. S. Lin, K. L. Chan, H. T. Chiu, C. Y. Lee, Nanoscale 2014, 6, 5106. [41] X. Yu, B. Lu, Z. Xu, Adv. Mater. 2014, 26, 1044. [42] T. Zhai, S. Xie, M. Yu, P. Fang, C. Liang, X. Lu, Y. Tong, Nano Energy 2014, 8, 255. [43] Y. C. Chen, Y. G. Lin, Y. K. Hsu, S. C. Yen, K. H. Chen, L. C. Chen, Small 2014, 10, 3803. [44] C. Long, D. Qi, T. Wei, J. Yan, L. Jiang, Z. Fan, Adv. Funct. Mater. 2014, 24, 3953. [45] M. H. Yu, W. Wang, C. Li, T. Zhai, X. H. Lu, Y. X. Tong, NPG Asia Mater. 2014, 6, e129. [46] Z. Su, C. Yang, C. Xu, H. Wu, Z. Zhang, T. Liu, C. Zhang, Q. Yang, B. Li, F. Kang, J. Mater. Chem. A 2013, 1, 12432. [47] T. Zhai, X. Lu, Y. Ling, M. Yu, G. Wang, T. Liu, C. Liang, Y. Tong, Y. Li, Adv. Mater. 2014, 26, 5869.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
7
