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Porous molybdenum tungsten oxynitrides enable long-life supercapacitors with high capacitance
Journal of Power Sources 442 (2019) 227247
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Porous molybdenum tungsten oxynitrides enable long-life supercapacitors with high capacitance Zifan Wang a, 1, Xiaoqing Liu a, 1, Jie Liu b, Yinxiang Zeng a, Jianying Shi a, **, Yexiang Tong a, Xihong Lu a, c, * a
MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China College of Chemistry and Chemical Engineering, Yantai University, Yantai, 264005, People’s Republic of China c Institute of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an, 710048, People’s Republic of China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The porous oxynitride sample shows complete hydrophilicity and high conductivity. � The oxynitride sample displays a remarkable areal capacitance of 2.43 F cm 2. � A high-performance aqueous asym metric supercapacitor is achieved.
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxynitride Transition metal oxide Supercapacitor Conductivity High capacitance
Binary transition metal oxides with multiple active sites are considered as one of the most potential anode candidates for high-performance supercapacitors (SCs) but their inferior conductivity is utterly disappointing. Herein, a molybdenum tungsten oxynitride with a porous architecture is designed and synthesized by nitriding its bimetallic oxide counterpart under ammonia atmosphere. The ingenious introduction of nitrogen atoms into the binary oxide not only leads to an enlarged specific surface area with complete hydrophilicity, but also results in an optimized electrical conductivity, which synergistically strengthen its supercapacitive properties. As a proof of concept, the oxynitride electrode displays a remarkable areal capacitance of 2.43 F cm 2 at 4 mA cm 2 and exhibits high cycling stability (no noticeable decay after 20,000 cycles), considerably superior to that of the pristine sample. A high-performance aqueous asymmetric SC with long lifespan is also achieved by coupling the oxynitride anode with a MnO2 cathode. Specifically, the device delivers an energy density of 1.67 mW h cm 3 and holds a power density of 0.23 W cm 3, outstripping plenty of recently reported asymmetric SCs. This work provides a new train of thought for boosting the performance of energy storage systems based on binary tran sition metal oxides.
* Corresponding author. MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China. ** Corresponding author. E-mail addresses: [email protected] (J. Shi), [email protected] (X. Lu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227247 Received 15 August 2019; Received in revised form 12 September 2019; Accepted 1 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 442 (2019) 227247
specific, the novel oxide-derived functional electrode reached a remarkable areal capacitance of 2.43 F cm 2 at a current density of 4 mA cm 2, which was approximately two times higher than that of the pristine sample. Such a high capacitance considerably surpassed those electrodes based on molybdenum and tungsten oxides. The 56.9% capacitance retention at 20 mA cm 2 further verifies its outstanding rate performance. Moreover, the oxynitride anode presents an impressive cycling stability with no obvious capacitance decay after 20,000 cycles under the scan rate of 100 mV s 1. A high-performance ASC device was achieved by coupling this anode with a MnO2 cathode in an aqueous electrolyte. The device was quite stable (89.8% capacity retention after 20,000 cycles), delivered an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2, and held a power density of 0.23 W cm 3 at 20 mA cm 2, overmatching many of reported ASC systems on the basis of monometallic or bimetallic oxide anodes.
1. Introduction The thriving market of the hybrid electric vehicles and portable electronic devices (mobile phones, touch pads and wearable gadgets) stimulates the rapid development of the exploration of highperformance energy storage systems in recent years [1–4]. Super capacitors (SCs), also known as electrochemical capacitors, have drawn increasing research and industrial interest owing to their higher power density and greater life expectancy compared with their competitive counterparts such as batteries, fuel cells, and conventional capacitors [5–8]. At present, there are three main categories of active materials employed to advance the electrochemical performance of SCs: (i) carbonaceous matrix, (ii) conducting polymers and (iii) transition metal oxides (TMOs) [9–11]. With respect to the former two types, TMOs usually own a much higher specific capacitance taking advantages of their multiple oxidation states that are in favor of fast charge storage. Such distinct merits could be further scaled up by the strategic combi nation of two different kinds of TMOs because the resultant binary metal oxides, such as NiCo2O4 [12–14], ZnCo2O4 [15,16], and NiMnO3 [17], are demonstrated to show better electrical conductivity, higher stability and embrace more valence-tunable sites in contrast to their mono metallic counterparts [18–20].These advantageous properties are anticipated to ameliorate the relatively deficient energy density of SCs to superior levels. Yet, the charge storage capability of the state-of-the-art SCs based on TMOs, either monometallic or binary supercapacitive electrodes, is still far below expectations because of their frustrating semi-conductive nature and limited active sites. To tackle aforementioned issues, transition metal oxynitrides (TMONs), an emerging specie of supercapacitive electrodes, are recently developed on top of TMOs by strategically introducing N atoms into the metal oxide lattice [21–23]. Typically, they possess a unique set of benign physicochemical characteristics including chemical inertness, good wettability, satisfactory thermal stability and strong mechanical intensity [24–26]. More importantly, TMONs generally embed intrinsic high conductivity, ensuring more efficient mass transport and charge transfer. These fascinating properties enable them to function as promising anode candidates for high-performance SC constructions. For instance, a fiber electrode containing 3D porous micropillars of molyb denum oxynitride was demonstrated to be capable of yielding remark able areal specific capacitance of 736.6 mF cm 2 at a scan rate of 10 mV s 1 [27]. Gong’s group achieved an all-solid-state SC by employing an innovative anode comprising the hybrid of molybdenum oxynitride and cellulose nanofibril-reduced graphene oxide composite. Its extraordinary high energy density up to 114 W h kg 1 was even comparable to that of Li ion batteries [28]. We also previously built a high-performance asymmetric supercapacitors (ASCs) by using a self-designed holey tungsten oxynitride anode [29]. Inspired by the superior electrochemical behaviors of binary metal oxides, we wish that the construction of bimetallic transition oxynitrides would be an effec tive strategy to further elevate the charge storage capability of their monometallic counterparts. However, to the best of our knowledge, the most cutting-edge studies have been exclusively focusing on the monometallic oxynitride compounds so far and there is hardly report concerning the application of bimetallic transition metal nitrides for SC assembly. To testify our hypothesis, herein, we firstly synthesized a molybde num tungsten oxide (denoted as MoWOx) on the carbon cloth surface to work as the precursor of bimetallic transition oxynitrides via a facile seed-assisted hydrothermal method. In order to obtain a molybdenum tungsten oxynitride, the bimetallic oxide sample was then calcinated under the ammonia atmosphere. The as-prepared binary oxynitride (denoted as N-MoWOx) with a porous architecture subtly inherited the complementary advantages of bimetallic oxides (numerous active sites) and metal oxynitrides (satisfactory conductivity and wettability), allowing efficient charge storage. As expected, the nitriding step effec tively strengthened the capacitive performance of the anode. To be
2. Experimental section 2.1. Sample preparation Firstly, MoWOx was tightly grown onto carbon cloth surface through a seed-assisted hydrothermal method. 0.6 g Na2MoO4⋅2H2O (A.R. Shanghai Macklin Biochemical Co., Ltd.) and 0.8 g Na2WO4⋅2H2O (A.R. Tianjin Damao Chemical Reagent Factory.) were dissolved in 50 mL distilled water. A piece of carbon cloth (1.5 cm � 2 cm � 0.04 cm) was then immerged into the above solution for 10s, and each side of the sample was heated on a hotplate in ambient at 300 � C for 30s respec tively to form MoWOx seeds on carbon fiber skeletons. 2.4 g Na2MoO4⋅2H2O and 3.3 g Na2WO4⋅2H2O were dissolved in the mixture of 95 mL distilled water and 5 mL concentrated hydrochloric acid (A.R. Guangzhou Chemical Reagent Factory.), then 10 mL of this solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave together with 10 mL ethanol absolute (Guangdong Guanghua Sci-Tech Co., Ltd.) and the as-prepared carbon fiber paper. The autoclave was heated in an electric oven with a heating speed of 5 � C min 1 to 180 � C, kept for 6 h and then naturally cooled down to room temperature. Afterward, the paper was thoroughly washed with distilled water and dried. The mass loading of MoWOx nanospheres on carbon fiber paper is about 33.97 mg cm 2. The N-MoWOx was obtained through thermal treatment of the MoWOx sample in a pine furnace under ammonia atmosphere at 600 � C for 1 h. 2.2. Assembly of the aqueous supercapacitor device The aqueous ASC device was assembled using an electrodeposited MnO2 (see details in the electronic supporting information (ESI)) as the cathode and N-MoWOx as the anode in a 5 M LiCl (A.R. Guangzhou Chemical Reagent Factory.) aqueous electrolyte (Fig. S1). The area ratio of MnO2 electrode to N-MoWOx electrode was set to be 1:1 (0.5 cm � 1 cm for both). 2.3. Material characterization and electrochemical measurements The morphologies, microstructures, and compositions of the prod ucts were characterized by field emission scanning electron microscopy (FE-SEM, JSM-6330F), transmission electron microscopy (TEM, Tec naiG2 F30), X-ray diffractometry (XRD, DMAX2200 VPC) and X-ray photoelectron spectroscopy (XPS, ESCA Lab250). All the electro chemical measurements were carried out using an electrochemical workstation (CHI 760) with a standard three-electrode electrolytic cell in a 5 M LiCl aqueous solution at room temperature. A graphite electrode and a saturated calomel electrode (SCE) were used as the counter elec trode and the reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range between 0.01 Hz and 100 kHz with an amplitude of 5 mV at the opencircuit potential. 2
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Fig. 1. (a) Schematic illustration of the synthesis process for N-MoWOx on carbon skeleton. SEM images of the (b) carbon fiber, (c) MoWOx and (d) N-MoWOx.
Fig. 2. (a) TEM and (b) HRTEM images of the N-MoWOx. The red rectangular indicates the HRTEM location. (c) TEM selected area elemental mapping images of Mo, W, O and N of the N-MoWOx. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3. Results and discussions
element does exist in the pristine sample and the loss of the corre sponding XRD signal should be attributed to its amorphous structure. In addition, the MoWOx is kind of hydrophilic with a water contact angle of around 59.2� whereas the nitriding treatment enables the liquid drop to completely spread out (corresponding to a contact angle of 0� ) (Fig. 3b). The absolutely optimized surface wettability is capable of reducing the electrode/electrolyte interface resistance, and further facilitating the accessibility of various ions to the electrode surface, similar as those reported heteroatom-doped materials [30–33]. The XPS spectra of the metallic elements (Mo and W) with and without nitrogen atoms were then analyzed in detail to illustrate the superiority of the oxynitride microstructure. For the MoWOx, the two major peaks at 233.2 and 236.3 eV sample (Fig. 3c) are assigned to Mo 3d5/2 and Mo 3d3/2 while the characteristic two peaks located at 35.9 and 38.1 eV (Fig. 3d) are ascribed to W 4f7/2 and W 4f5/2 [29,34]. Obviously, the nitriding step leads to an apparently negative shift for both the Mo 3d and W 4f peaks, which signifies the partial decrease of the valence states of Mo and W initiated by the introduction of N dopants on the sample surface [14]. Therefore, there exists enriched valence states of the two metallic ele ments in the N-MoWOx sample, which is very likely to offer more active sites for charge storage. To evaluate the supercapacitive performance of MoWOx and NMoWOx samples, a series of electrochemical measurements were carried out in 5 M LiCl aqueous solution using a typical three-electrode system. Fig. 4a presents the cyclic voltammetry (CV) curves of the two samples collected at a scan rate of 100 mV s 1. The CV loop of the N-MoWOx shows a quasi-rectangular shape and a symmetric feature corresponding to its highly reversible double-layer capacitive behavior and fast charging/discharging rate. Apparently, in contrast to the MoWOx sam ple, the N-MoWOx surface enforces a significantly enlarged rectangular area, suggesting a remarkable capacitive enhancement have been
The preparation process of the sample is schematically illustrated in Fig. 1a. In brief, the MoWOx was first grown onto the flexible carbon cloth skeleton as the binary metal oxide precursor through a seedassisted hydrothermal method. In order to realize the introduction of N atoms to the metal oxide lattice, the sample was then subjected to thermal treatment under ammonia atmosphere. The microstructure comparison of carbon skeleton, MoWOx and N-MoWOx by the SEM imaging (Fig. 1b–d) reveals that the MoWOx coverage on the smooth carbon fiber appears as tightly interconnected convex balls with rela tively rough surface, and the subsequent ammonia treatment severely engraves the MoWOx surface, resulting in a rough, irregular and loose architecture. Moreover, the sample after ammonia corrosion consists of numerous hierarchical nanopores whose diameter varies from tens of nanometers to hundreds of nanometers (Fig. 1d inset). Such porous structure is supposed to provide a large specific area for energy storage. The high-resolution TEM (HRTEM) images in Fig. 2a–b elucidate the wall-thickness of these pores is at nanoscale, and the N-MoWOx presents an interplanar spacing of about 0.24 nm. Furthermore, the high-angle annular dark-field scanning TEM (HAADF-STEM) and selected area elemental mapping show the Mo, W, O, and N distribute homogeneously at the N-MoWOx surface, demonstrating N is uniformly doped into the MoWOx sample (Fig. S2 & Fig. 2c). This is in good agreement with the results we obtained by XPS surveys (Fig. S3). According to the XRD patterns in Fig. 3a, the precise components of MoWOx and N-MoWOx are H2W1.5O5.5H2O (JCPDF No. 48-0719) and MoWO2.4N2.1 (JCPDF No.50-0134) respectively. The interlayer distance of 0.24 nm in Fig. 2b is also found to be well indexed to the (111) plane of MoWO2.4N2.1. These results conjointly unravel that ammonia corro sion induces the crystalline variation of the oxide sample. Note that Mo 3
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Fig. 3. Physicochemical characterization of the MoWOx and N-MoWOx. (a) XRD patterns. (b) Contact angles of a water droplet. (c) Mo 3d XPS spectra. (d) W 4f XPS spectra.
Fig. 4. Electrochemical behavior comparison of the MoWOx and N-MoWOx. (a) The CV curves obtained at 100 mV s 1. (b) The GCD curves at different current densities of the N-MoWOx. (c) The GCD curves at 4 mA cm 2 and 20 mA cm 2. (d) The Nyquist plots. (e) Areal capacitances. (f) The cycling stabilities.
successfully realized through the ammonia treatment. The galvanostatic charge/discharge (GCD) profiles exhibit that the N-MoWOx sample al ways keeps a symmetrical triangular shape at a wide current density range varying from 4 mA cm 2 to 20 mA cm 2 (Fig. 4b) and its
discharging time invariably exceeds the non-doped counterpart (Fig. S4), confirming again oxynitride structure plays an auxo-action for boosting capacitive performance [15]. Moreover, the iR drop of the N-MoWOx is 29 mV at a current density of 4 mA cm 2, substantially 4
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Fig. 5. Energy storage performance test of the N-MoWOx//MnO2. (a) The CV curves of the N-MoWOx anode and MnO2 cathode collected at 100 mV s 1. (b) The CV curves collected within various operation voltage windows. (c) The GCD curves at various current densities. (d) Plots of the current density against the areal capacitance and capacitance retention ratio.
lower than that of the MoWOx (~47 mV). Similar descending trend of iR drop from 240 mV to 150 mV is also observed on the N-MoWOx when the current density attains as high as 20 mA cm 2, as illustrated in Fig. 4c. It is thus highly probable that the introduction of nitrogen atoms to the electrode surface is capable of improving electrical conductivity of TMOs [35]. To validate this viewpoint, EIS measurements were then conducted. The Nyquist plots in Fig. 4d show that both samples possess a semicircular loop in the high-frequency region and a straight line in the low-frequency region. It is worth mentioning that the semicircle diam eter is positively correlated to the charge transfer resistance (Rct) while the slope is inversely proportional to ion diffusion resistance in the electrolyte. Obviously, the Rct of N-MoWOx sample is much lower than that of the pristine MoWOx sample, which is fully consistent with its smaller iR drop observed in the GCD tests. In addition, its steeper slope ensures the quick charge transfer and/or fast ion diffusion rate at the electrode/electrolyte interface. The as-observed electrical conductivity upsurge should be ascribed to the formation of metal-ligand band which enlarges the expansion of the electron cloud, reduces the electronic repulsion and d orbital energy, and thus effectively decreases the bandgap of the N-MoWOx. The corresponding areal capacitances origi nated from the discharge curves are shown in Fig. 4e. As expected, the specific areal capacitance of N-MoWOx achieves a substantially high value of 2.43 F cm 2 at 4 mA cm 2, more than two times larger than that of the pristine MoWOx sample (about 1.15 F cm 2). The N-MoWOx sample still maintains a high capacitance of 1.52 F cm 2 when the cur rent density increases to a relatively high value of 20 mA cm 2, which demonstrates its good rate capability. The extraordinary capacitance of the N-MoWOx surpasses most of recently reported SCs based on mo lybdenum and/or tungsten oxides (Fig. S5), predicting a tempting foreground for energy storage application [16,17]. The long-term durability of the electrode represents a significative index for its overall performance evaluation. To highlight the advanta geous property of N-MoWOx sample, we compare its cycling stability with the pristine MoWOx sample. As depicted in Fig. 4f, the undoped
MoWOx electrode displays a poor electrochemical stability, experi encing a dramatic capacitance decrease of about 30% at the first 1000 cycles and the continuous capacity fading leads to only 20% retention after 20,000 cycles. Such frustrating durability is caused by the obvious exfoliation of active materials from the carbon skeleton during the persistent cycling test, as visualized by the SEM imaging (Fig. S6). In contrast, after 20,000 cycles test, the N-MoWOx electrode almost sees no capacitive decay and its surface microstructures is well preserved (Fig. S7), which dually testified its superior stability. In brief, the comprehensive upgrading of the capacitive performance of the NMoWOx sample should be conjointly attributed to its hierarchically porous structure, adorable hydrophilicity, satisfactory physiochemical stability and rewarding conductivity. To further assess the feasibility of the N-MoWOx electrode as an advanced anode for building ASCs, a typical manganese oxide was electrochemically deposited to function as the cathode (Fig. S8 a-b, experimental details in ESI). Its crystalline phase was confirmed to be MnO2 according to the high-resolution Mn 2p and Mn 3s spectra in XPS analysis (Fig. S8 c-d). Specifically, the Mn 2p signal consists of two peaks at 654.3 eV and 642.5 eV corresponding to the Mn 2p1/2 and Mn 2p2/3, respectively. The difference in the binding energy of the Mn 3s signal with two peaks at 89.2 and 84.3 eV is nearly 4.90 eV, confirming the existence of the Mn4þ oxidation state. We choose MnO2 because it represents one of the most stable manganese oxides under ambient conditions and has been extensively used as a competitive cathode in various SCs. Similarly, in this present work, the electrodeposited MnO2 cathode manifested satisfactory capacitive performance (Fig. S9). An aqueous ASC device (denoted as N-MoWOx//MnO2) was then assembled by equipping these two electrodes in a 5 M LiCl aqueous solution, and its electrochemical behaviors were systematically tested. CV curves in Fig. 5a–b reveal that there is an evident potential dif ference between the two electrodes and the as-assembled device is able to reach a wide voltage window from 0 to 1.8 V. Fig. 5c displays the GCD curves collected at a wide current density ranging from 4 to 20 mA cm 2. 5
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basis of metal oxide electrodes. Declaration of competing interest There are no conflicts to declare. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (21822509, U1810110, 21802173, and 31530009), Science and Technology Planning Project of Guangdong Province (2018A050506028) and Natural Science Foundation of Guangdong Province (2018A030310301). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227247. Fig. 6. The Ragone plot of the N-MoWOx//MnO2 with the comparison of other recently developed energy storage devices.
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The highly symmetric shape of triangular profiles is indicative of the good reversibility and fast charging/discharging ability of our ASC. At a discharge current density of 4 mA cm 2, the N-MoWOx//MnO2 device delivers a maximum areal capacitance of 296.4 mF cm 2. When the discharge current density is raised to 20 mA cm 2, the resultant areal capacitance remains 173.4 mF cm 2, giving a prominent rate capability of 58.5% (Fig. 5d). Besides, this device also presents good cycling per formance, attaining more than 89.8% capacity retention after 20,000 cycles (Fig. S10). For a full device, the energy density and power density are two important parameters for the entire performance evaluation. Our SC device delivers an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2 and holds a power density of 0.23 W cm 3 at 20 mA cm 2. To highlight the visible merits of our device, we further compared its optimal energy densities to those of recently reported SCs. As displayed in Fig. 6, the energy density of 1.67 mW h cm 3 outweighs those re ported ASCs such as H–CoMoO4//H–Fe2O3 (1.13 mWh cm 3) [36], MnO2/graphene//VOS@C (0.87 mW h cm 3) [37], MnO2/ZnO//rGO (0.234 mW h cm 3) [38], GaN//GP (0.30 mW h cm 3) [39], and those symmetric devices made of MnO2@MWCNT fibers (1.5 mW h cm 3) [40], N-rGO (0.30 mW h cm 3) [41], GaN (0.026 mW h cm 3) [42], and amorphous TiO2 nanotubes (0.85 mW h cm 3) [43]. 4. Conclusions To conclude, an advanced molybdenum tungsten oxynitride material is designed and synthesized by etching its TMO precursor with ammonia treatment at high temperature. The subtle introduction of N atoms not only generates an enlarged specific surface area consisting of abundant hierarchical nanopores, but also results in an optimized electrical con ductivity and wettability. By virtue of these beneficial merits, the NMoWOx electrode exhibits better electrochemical performance in contrast to its pristine counterpart. To be specific, it possesses a high areal capacitance of 2.43 F cm 2 at 4 mA cm 2, about two times larger than that of the MoWOx. In addition, a long-term cycling durability is observed on the oxynitride sample, as testified by its negligible capaci tance decay after 20,000 cycles. Furthermore, a high-performance aqueous ASC system is achieved by coupling the N-MoWOx anode with a homemade MnO2 cathode. This device is quite stable (89.8% capacity retention after 20,000 cycles), delivers an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2, and holds a power density of 0.23 W cm 3 at 20 mA cm 2, overmatching most of recently reported ASC devices. This work offers an enlightening idea for con structing high-performance energy storage systems, especially those on 6
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Porous molybdenum tungsten oxynitrides enable long-life supercapacitors with high capacitance Zifan Wang a, 1, Xiaoqing Liu a, 1, Jie Liu b, Yinxiang Zeng a, Jianying Shi a, **, Yexiang Tong a, Xihong Lu a, c, * a
MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China College of Chemistry and Chemical Engineering, Yantai University, Yantai, 264005, People’s Republic of China c Institute of Advanced Electrochemical Energy, Xi’an University of Technology, Xi’an, 710048, People’s Republic of China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� The porous oxynitride sample shows complete hydrophilicity and high conductivity. � The oxynitride sample displays a remarkable areal capacitance of 2.43 F cm 2. � A high-performance aqueous asym metric supercapacitor is achieved.
A R T I C L E I N F O
A B S T R A C T
Keywords: Oxynitride Transition metal oxide Supercapacitor Conductivity High capacitance
Binary transition metal oxides with multiple active sites are considered as one of the most potential anode candidates for high-performance supercapacitors (SCs) but their inferior conductivity is utterly disappointing. Herein, a molybdenum tungsten oxynitride with a porous architecture is designed and synthesized by nitriding its bimetallic oxide counterpart under ammonia atmosphere. The ingenious introduction of nitrogen atoms into the binary oxide not only leads to an enlarged specific surface area with complete hydrophilicity, but also results in an optimized electrical conductivity, which synergistically strengthen its supercapacitive properties. As a proof of concept, the oxynitride electrode displays a remarkable areal capacitance of 2.43 F cm 2 at 4 mA cm 2 and exhibits high cycling stability (no noticeable decay after 20,000 cycles), considerably superior to that of the pristine sample. A high-performance aqueous asymmetric SC with long lifespan is also achieved by coupling the oxynitride anode with a MnO2 cathode. Specifically, the device delivers an energy density of 1.67 mW h cm 3 and holds a power density of 0.23 W cm 3, outstripping plenty of recently reported asymmetric SCs. This work provides a new train of thought for boosting the performance of energy storage systems based on binary tran sition metal oxides.
* Corresponding author. MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chem & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, People’s Republic of China. ** Corresponding author. E-mail addresses: [email protected] (J. Shi), [email protected] (X. Lu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jpowsour.2019.227247 Received 15 August 2019; Received in revised form 12 September 2019; Accepted 1 October 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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specific, the novel oxide-derived functional electrode reached a remarkable areal capacitance of 2.43 F cm 2 at a current density of 4 mA cm 2, which was approximately two times higher than that of the pristine sample. Such a high capacitance considerably surpassed those electrodes based on molybdenum and tungsten oxides. The 56.9% capacitance retention at 20 mA cm 2 further verifies its outstanding rate performance. Moreover, the oxynitride anode presents an impressive cycling stability with no obvious capacitance decay after 20,000 cycles under the scan rate of 100 mV s 1. A high-performance ASC device was achieved by coupling this anode with a MnO2 cathode in an aqueous electrolyte. The device was quite stable (89.8% capacity retention after 20,000 cycles), delivered an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2, and held a power density of 0.23 W cm 3 at 20 mA cm 2, overmatching many of reported ASC systems on the basis of monometallic or bimetallic oxide anodes.
1. Introduction The thriving market of the hybrid electric vehicles and portable electronic devices (mobile phones, touch pads and wearable gadgets) stimulates the rapid development of the exploration of highperformance energy storage systems in recent years [1–4]. Super capacitors (SCs), also known as electrochemical capacitors, have drawn increasing research and industrial interest owing to their higher power density and greater life expectancy compared with their competitive counterparts such as batteries, fuel cells, and conventional capacitors [5–8]. At present, there are three main categories of active materials employed to advance the electrochemical performance of SCs: (i) carbonaceous matrix, (ii) conducting polymers and (iii) transition metal oxides (TMOs) [9–11]. With respect to the former two types, TMOs usually own a much higher specific capacitance taking advantages of their multiple oxidation states that are in favor of fast charge storage. Such distinct merits could be further scaled up by the strategic combi nation of two different kinds of TMOs because the resultant binary metal oxides, such as NiCo2O4 [12–14], ZnCo2O4 [15,16], and NiMnO3 [17], are demonstrated to show better electrical conductivity, higher stability and embrace more valence-tunable sites in contrast to their mono metallic counterparts [18–20].These advantageous properties are anticipated to ameliorate the relatively deficient energy density of SCs to superior levels. Yet, the charge storage capability of the state-of-the-art SCs based on TMOs, either monometallic or binary supercapacitive electrodes, is still far below expectations because of their frustrating semi-conductive nature and limited active sites. To tackle aforementioned issues, transition metal oxynitrides (TMONs), an emerging specie of supercapacitive electrodes, are recently developed on top of TMOs by strategically introducing N atoms into the metal oxide lattice [21–23]. Typically, they possess a unique set of benign physicochemical characteristics including chemical inertness, good wettability, satisfactory thermal stability and strong mechanical intensity [24–26]. More importantly, TMONs generally embed intrinsic high conductivity, ensuring more efficient mass transport and charge transfer. These fascinating properties enable them to function as promising anode candidates for high-performance SC constructions. For instance, a fiber electrode containing 3D porous micropillars of molyb denum oxynitride was demonstrated to be capable of yielding remark able areal specific capacitance of 736.6 mF cm 2 at a scan rate of 10 mV s 1 [27]. Gong’s group achieved an all-solid-state SC by employing an innovative anode comprising the hybrid of molybdenum oxynitride and cellulose nanofibril-reduced graphene oxide composite. Its extraordinary high energy density up to 114 W h kg 1 was even comparable to that of Li ion batteries [28]. We also previously built a high-performance asymmetric supercapacitors (ASCs) by using a self-designed holey tungsten oxynitride anode [29]. Inspired by the superior electrochemical behaviors of binary metal oxides, we wish that the construction of bimetallic transition oxynitrides would be an effec tive strategy to further elevate the charge storage capability of their monometallic counterparts. However, to the best of our knowledge, the most cutting-edge studies have been exclusively focusing on the monometallic oxynitride compounds so far and there is hardly report concerning the application of bimetallic transition metal nitrides for SC assembly. To testify our hypothesis, herein, we firstly synthesized a molybde num tungsten oxide (denoted as MoWOx) on the carbon cloth surface to work as the precursor of bimetallic transition oxynitrides via a facile seed-assisted hydrothermal method. In order to obtain a molybdenum tungsten oxynitride, the bimetallic oxide sample was then calcinated under the ammonia atmosphere. The as-prepared binary oxynitride (denoted as N-MoWOx) with a porous architecture subtly inherited the complementary advantages of bimetallic oxides (numerous active sites) and metal oxynitrides (satisfactory conductivity and wettability), allowing efficient charge storage. As expected, the nitriding step effec tively strengthened the capacitive performance of the anode. To be
2. Experimental section 2.1. Sample preparation Firstly, MoWOx was tightly grown onto carbon cloth surface through a seed-assisted hydrothermal method. 0.6 g Na2MoO4⋅2H2O (A.R. Shanghai Macklin Biochemical Co., Ltd.) and 0.8 g Na2WO4⋅2H2O (A.R. Tianjin Damao Chemical Reagent Factory.) were dissolved in 50 mL distilled water. A piece of carbon cloth (1.5 cm � 2 cm � 0.04 cm) was then immerged into the above solution for 10s, and each side of the sample was heated on a hotplate in ambient at 300 � C for 30s respec tively to form MoWOx seeds on carbon fiber skeletons. 2.4 g Na2MoO4⋅2H2O and 3.3 g Na2WO4⋅2H2O were dissolved in the mixture of 95 mL distilled water and 5 mL concentrated hydrochloric acid (A.R. Guangzhou Chemical Reagent Factory.), then 10 mL of this solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave together with 10 mL ethanol absolute (Guangdong Guanghua Sci-Tech Co., Ltd.) and the as-prepared carbon fiber paper. The autoclave was heated in an electric oven with a heating speed of 5 � C min 1 to 180 � C, kept for 6 h and then naturally cooled down to room temperature. Afterward, the paper was thoroughly washed with distilled water and dried. The mass loading of MoWOx nanospheres on carbon fiber paper is about 33.97 mg cm 2. The N-MoWOx was obtained through thermal treatment of the MoWOx sample in a pine furnace under ammonia atmosphere at 600 � C for 1 h. 2.2. Assembly of the aqueous supercapacitor device The aqueous ASC device was assembled using an electrodeposited MnO2 (see details in the electronic supporting information (ESI)) as the cathode and N-MoWOx as the anode in a 5 M LiCl (A.R. Guangzhou Chemical Reagent Factory.) aqueous electrolyte (Fig. S1). The area ratio of MnO2 electrode to N-MoWOx electrode was set to be 1:1 (0.5 cm � 1 cm for both). 2.3. Material characterization and electrochemical measurements The morphologies, microstructures, and compositions of the prod ucts were characterized by field emission scanning electron microscopy (FE-SEM, JSM-6330F), transmission electron microscopy (TEM, Tec naiG2 F30), X-ray diffractometry (XRD, DMAX2200 VPC) and X-ray photoelectron spectroscopy (XPS, ESCA Lab250). All the electro chemical measurements were carried out using an electrochemical workstation (CHI 760) with a standard three-electrode electrolytic cell in a 5 M LiCl aqueous solution at room temperature. A graphite electrode and a saturated calomel electrode (SCE) were used as the counter elec trode and the reference electrode, respectively. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency range between 0.01 Hz and 100 kHz with an amplitude of 5 mV at the opencircuit potential. 2
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Fig. 1. (a) Schematic illustration of the synthesis process for N-MoWOx on carbon skeleton. SEM images of the (b) carbon fiber, (c) MoWOx and (d) N-MoWOx.
Fig. 2. (a) TEM and (b) HRTEM images of the N-MoWOx. The red rectangular indicates the HRTEM location. (c) TEM selected area elemental mapping images of Mo, W, O and N of the N-MoWOx. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
3. Results and discussions
element does exist in the pristine sample and the loss of the corre sponding XRD signal should be attributed to its amorphous structure. In addition, the MoWOx is kind of hydrophilic with a water contact angle of around 59.2� whereas the nitriding treatment enables the liquid drop to completely spread out (corresponding to a contact angle of 0� ) (Fig. 3b). The absolutely optimized surface wettability is capable of reducing the electrode/electrolyte interface resistance, and further facilitating the accessibility of various ions to the electrode surface, similar as those reported heteroatom-doped materials [30–33]. The XPS spectra of the metallic elements (Mo and W) with and without nitrogen atoms were then analyzed in detail to illustrate the superiority of the oxynitride microstructure. For the MoWOx, the two major peaks at 233.2 and 236.3 eV sample (Fig. 3c) are assigned to Mo 3d5/2 and Mo 3d3/2 while the characteristic two peaks located at 35.9 and 38.1 eV (Fig. 3d) are ascribed to W 4f7/2 and W 4f5/2 [29,34]. Obviously, the nitriding step leads to an apparently negative shift for both the Mo 3d and W 4f peaks, which signifies the partial decrease of the valence states of Mo and W initiated by the introduction of N dopants on the sample surface [14]. Therefore, there exists enriched valence states of the two metallic ele ments in the N-MoWOx sample, which is very likely to offer more active sites for charge storage. To evaluate the supercapacitive performance of MoWOx and NMoWOx samples, a series of electrochemical measurements were carried out in 5 M LiCl aqueous solution using a typical three-electrode system. Fig. 4a presents the cyclic voltammetry (CV) curves of the two samples collected at a scan rate of 100 mV s 1. The CV loop of the N-MoWOx shows a quasi-rectangular shape and a symmetric feature corresponding to its highly reversible double-layer capacitive behavior and fast charging/discharging rate. Apparently, in contrast to the MoWOx sam ple, the N-MoWOx surface enforces a significantly enlarged rectangular area, suggesting a remarkable capacitive enhancement have been
The preparation process of the sample is schematically illustrated in Fig. 1a. In brief, the MoWOx was first grown onto the flexible carbon cloth skeleton as the binary metal oxide precursor through a seedassisted hydrothermal method. In order to realize the introduction of N atoms to the metal oxide lattice, the sample was then subjected to thermal treatment under ammonia atmosphere. The microstructure comparison of carbon skeleton, MoWOx and N-MoWOx by the SEM imaging (Fig. 1b–d) reveals that the MoWOx coverage on the smooth carbon fiber appears as tightly interconnected convex balls with rela tively rough surface, and the subsequent ammonia treatment severely engraves the MoWOx surface, resulting in a rough, irregular and loose architecture. Moreover, the sample after ammonia corrosion consists of numerous hierarchical nanopores whose diameter varies from tens of nanometers to hundreds of nanometers (Fig. 1d inset). Such porous structure is supposed to provide a large specific area for energy storage. The high-resolution TEM (HRTEM) images in Fig. 2a–b elucidate the wall-thickness of these pores is at nanoscale, and the N-MoWOx presents an interplanar spacing of about 0.24 nm. Furthermore, the high-angle annular dark-field scanning TEM (HAADF-STEM) and selected area elemental mapping show the Mo, W, O, and N distribute homogeneously at the N-MoWOx surface, demonstrating N is uniformly doped into the MoWOx sample (Fig. S2 & Fig. 2c). This is in good agreement with the results we obtained by XPS surveys (Fig. S3). According to the XRD patterns in Fig. 3a, the precise components of MoWOx and N-MoWOx are H2W1.5O5.5H2O (JCPDF No. 48-0719) and MoWO2.4N2.1 (JCPDF No.50-0134) respectively. The interlayer distance of 0.24 nm in Fig. 2b is also found to be well indexed to the (111) plane of MoWO2.4N2.1. These results conjointly unravel that ammonia corro sion induces the crystalline variation of the oxide sample. Note that Mo 3
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Fig. 3. Physicochemical characterization of the MoWOx and N-MoWOx. (a) XRD patterns. (b) Contact angles of a water droplet. (c) Mo 3d XPS spectra. (d) W 4f XPS spectra.
Fig. 4. Electrochemical behavior comparison of the MoWOx and N-MoWOx. (a) The CV curves obtained at 100 mV s 1. (b) The GCD curves at different current densities of the N-MoWOx. (c) The GCD curves at 4 mA cm 2 and 20 mA cm 2. (d) The Nyquist plots. (e) Areal capacitances. (f) The cycling stabilities.
successfully realized through the ammonia treatment. The galvanostatic charge/discharge (GCD) profiles exhibit that the N-MoWOx sample al ways keeps a symmetrical triangular shape at a wide current density range varying from 4 mA cm 2 to 20 mA cm 2 (Fig. 4b) and its
discharging time invariably exceeds the non-doped counterpart (Fig. S4), confirming again oxynitride structure plays an auxo-action for boosting capacitive performance [15]. Moreover, the iR drop of the N-MoWOx is 29 mV at a current density of 4 mA cm 2, substantially 4
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Fig. 5. Energy storage performance test of the N-MoWOx//MnO2. (a) The CV curves of the N-MoWOx anode and MnO2 cathode collected at 100 mV s 1. (b) The CV curves collected within various operation voltage windows. (c) The GCD curves at various current densities. (d) Plots of the current density against the areal capacitance and capacitance retention ratio.
lower than that of the MoWOx (~47 mV). Similar descending trend of iR drop from 240 mV to 150 mV is also observed on the N-MoWOx when the current density attains as high as 20 mA cm 2, as illustrated in Fig. 4c. It is thus highly probable that the introduction of nitrogen atoms to the electrode surface is capable of improving electrical conductivity of TMOs [35]. To validate this viewpoint, EIS measurements were then conducted. The Nyquist plots in Fig. 4d show that both samples possess a semicircular loop in the high-frequency region and a straight line in the low-frequency region. It is worth mentioning that the semicircle diam eter is positively correlated to the charge transfer resistance (Rct) while the slope is inversely proportional to ion diffusion resistance in the electrolyte. Obviously, the Rct of N-MoWOx sample is much lower than that of the pristine MoWOx sample, which is fully consistent with its smaller iR drop observed in the GCD tests. In addition, its steeper slope ensures the quick charge transfer and/or fast ion diffusion rate at the electrode/electrolyte interface. The as-observed electrical conductivity upsurge should be ascribed to the formation of metal-ligand band which enlarges the expansion of the electron cloud, reduces the electronic repulsion and d orbital energy, and thus effectively decreases the bandgap of the N-MoWOx. The corresponding areal capacitances origi nated from the discharge curves are shown in Fig. 4e. As expected, the specific areal capacitance of N-MoWOx achieves a substantially high value of 2.43 F cm 2 at 4 mA cm 2, more than two times larger than that of the pristine MoWOx sample (about 1.15 F cm 2). The N-MoWOx sample still maintains a high capacitance of 1.52 F cm 2 when the cur rent density increases to a relatively high value of 20 mA cm 2, which demonstrates its good rate capability. The extraordinary capacitance of the N-MoWOx surpasses most of recently reported SCs based on mo lybdenum and/or tungsten oxides (Fig. S5), predicting a tempting foreground for energy storage application [16,17]. The long-term durability of the electrode represents a significative index for its overall performance evaluation. To highlight the advanta geous property of N-MoWOx sample, we compare its cycling stability with the pristine MoWOx sample. As depicted in Fig. 4f, the undoped
MoWOx electrode displays a poor electrochemical stability, experi encing a dramatic capacitance decrease of about 30% at the first 1000 cycles and the continuous capacity fading leads to only 20% retention after 20,000 cycles. Such frustrating durability is caused by the obvious exfoliation of active materials from the carbon skeleton during the persistent cycling test, as visualized by the SEM imaging (Fig. S6). In contrast, after 20,000 cycles test, the N-MoWOx electrode almost sees no capacitive decay and its surface microstructures is well preserved (Fig. S7), which dually testified its superior stability. In brief, the comprehensive upgrading of the capacitive performance of the NMoWOx sample should be conjointly attributed to its hierarchically porous structure, adorable hydrophilicity, satisfactory physiochemical stability and rewarding conductivity. To further assess the feasibility of the N-MoWOx electrode as an advanced anode for building ASCs, a typical manganese oxide was electrochemically deposited to function as the cathode (Fig. S8 a-b, experimental details in ESI). Its crystalline phase was confirmed to be MnO2 according to the high-resolution Mn 2p and Mn 3s spectra in XPS analysis (Fig. S8 c-d). Specifically, the Mn 2p signal consists of two peaks at 654.3 eV and 642.5 eV corresponding to the Mn 2p1/2 and Mn 2p2/3, respectively. The difference in the binding energy of the Mn 3s signal with two peaks at 89.2 and 84.3 eV is nearly 4.90 eV, confirming the existence of the Mn4þ oxidation state. We choose MnO2 because it represents one of the most stable manganese oxides under ambient conditions and has been extensively used as a competitive cathode in various SCs. Similarly, in this present work, the electrodeposited MnO2 cathode manifested satisfactory capacitive performance (Fig. S9). An aqueous ASC device (denoted as N-MoWOx//MnO2) was then assembled by equipping these two electrodes in a 5 M LiCl aqueous solution, and its electrochemical behaviors were systematically tested. CV curves in Fig. 5a–b reveal that there is an evident potential dif ference between the two electrodes and the as-assembled device is able to reach a wide voltage window from 0 to 1.8 V. Fig. 5c displays the GCD curves collected at a wide current density ranging from 4 to 20 mA cm 2. 5
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basis of metal oxide electrodes. Declaration of competing interest There are no conflicts to declare. Acknowledgements The authors acknowledge the financial support by the National Natural Science Foundation of China (21822509, U1810110, 21802173, and 31530009), Science and Technology Planning Project of Guangdong Province (2018A050506028) and Natural Science Foundation of Guangdong Province (2018A030310301). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227247. Fig. 6. The Ragone plot of the N-MoWOx//MnO2 with the comparison of other recently developed energy storage devices.
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The highly symmetric shape of triangular profiles is indicative of the good reversibility and fast charging/discharging ability of our ASC. At a discharge current density of 4 mA cm 2, the N-MoWOx//MnO2 device delivers a maximum areal capacitance of 296.4 mF cm 2. When the discharge current density is raised to 20 mA cm 2, the resultant areal capacitance remains 173.4 mF cm 2, giving a prominent rate capability of 58.5% (Fig. 5d). Besides, this device also presents good cycling per formance, attaining more than 89.8% capacity retention after 20,000 cycles (Fig. S10). For a full device, the energy density and power density are two important parameters for the entire performance evaluation. Our SC device delivers an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2 and holds a power density of 0.23 W cm 3 at 20 mA cm 2. To highlight the visible merits of our device, we further compared its optimal energy densities to those of recently reported SCs. As displayed in Fig. 6, the energy density of 1.67 mW h cm 3 outweighs those re ported ASCs such as H–CoMoO4//H–Fe2O3 (1.13 mWh cm 3) [36], MnO2/graphene//VOS@C (0.87 mW h cm 3) [37], MnO2/ZnO//rGO (0.234 mW h cm 3) [38], GaN//GP (0.30 mW h cm 3) [39], and those symmetric devices made of MnO2@MWCNT fibers (1.5 mW h cm 3) [40], N-rGO (0.30 mW h cm 3) [41], GaN (0.026 mW h cm 3) [42], and amorphous TiO2 nanotubes (0.85 mW h cm 3) [43]. 4. Conclusions To conclude, an advanced molybdenum tungsten oxynitride material is designed and synthesized by etching its TMO precursor with ammonia treatment at high temperature. The subtle introduction of N atoms not only generates an enlarged specific surface area consisting of abundant hierarchical nanopores, but also results in an optimized electrical con ductivity and wettability. By virtue of these beneficial merits, the NMoWOx electrode exhibits better electrochemical performance in contrast to its pristine counterpart. To be specific, it possesses a high areal capacitance of 2.43 F cm 2 at 4 mA cm 2, about two times larger than that of the MoWOx. In addition, a long-term cycling durability is observed on the oxynitride sample, as testified by its negligible capaci tance decay after 20,000 cycles. Furthermore, a high-performance aqueous ASC system is achieved by coupling the N-MoWOx anode with a homemade MnO2 cathode. This device is quite stable (89.8% capacity retention after 20,000 cycles), delivers an energy density of 1.67 mW h cm 3 at a current density of 4 mA cm 2, and holds a power density of 0.23 W cm 3 at 20 mA cm 2, overmatching most of recently reported ASC devices. This work offers an enlightening idea for con structing high-performance energy storage systems, especially those on 6
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Journal of Power Sources 442 (2019) 227247
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