- Email: [email protected]
Amorphous cobalt hydrogen phosphate nanosheets with remarkable electrochemical performances as advanced electrode for supercapacitors
Journal of Power Sources xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Amorphous cobalt hydrogen phosphate nanosheets with remarkable electrochemical performances as advanced electrode for supercapacitors Ya Wang a, Wenpo Li a, *, Lulu Zhang a, Xin Zhang a, Bochuan Tan a, Jiangyu Hao a, Jian Zhang a, Xu Wang a, Qin Hu a, Xihong Lu b, ** a
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, China MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, China
b
H I G H L I G H T S
� ACHP nanosheets are prepared by a one-step low temperature hydrothermal method. � NaHMP is used as a novel phosphorus source. � ACHP electrode displays 97.6% capacitance maintained after 10000 GCD cycles. A R T I C L E I N F O
A B S T R A C T
Keywords: Supercapacitors Hydrothermal method Nanosheet-like architecture Electrochemical performance
The development of simple and efficient approaches to achieve high-performance electrode materials for supercapacitors is urgently needed at present. In this work, a facile and efficient hydrothermal approach (50 � C) is reported to prepare flower-like amorphous cobalt hydrogen phosphate (ACHP) architectures with significantly enhanced electrochemical activity as robust supercapacitor electrodes. These ACHP nanoflowers with numerous thin nanosheets are readily grown on nickel foam by using eco-friendly sodium hexametaphosphate (NaHMP) as phosphorus source and their formation mechanism is also revealed. The ACHP displays a high specific capaci tance (411.2 F g 1 at 1 A g 1) as well as excellent rate capability (remained capacitance retention of 82.0% at 10 A g 1) due to the unique nanosheet-like architecture and the influence of structural water. In addition, this ACHP also exhibits an ultra-long lifespan that can maintain more than 97.6% capacitance retention after 10000 charge/ discharge cycles. Therefore, this easy and effective one-step low temperature hydrothermal method offers a sustainable development strategy for energy storage devices in practical application.
1. Introduction To adapt the rapid development of the economy, energy storage devices such as fuel cells, lithium-ion batteries, sodium-ion batteries and supercapacitors need to be continuously updated [1–3]. Among them, supercapacitors have become a research hotspot due to their high power density and long cycle life [4–10], and have been widely used in con sumer electronics, portable electronic devices, voltage regulators in power lines, and so on [11,12]. Based on the different work mechanism, supercapacitors can be splitted into pseudocapacitors (reversible faradic reactions), electrical double-layer capacitors (charge accumulation) and hybrid supercapacitors (composed of supercapacitors and lithium-ion
batteries) [13–16]. Pseudocapacitors, especially transition metal compounds-based materials, such as RuO2 [12], Co3O4 [17], Ni(OH)2 [18], Co3(PO4)2 [19], are widely reported because they have higher energy density than that of electrical double-layer capacitors (carbon- based materials) [20]. Transition metal phosphates, as a kind of pseudocapacitive electrode materials, have been widely reported due to natural abundance, non toxicity and broad applications [21,22]. Huan Pang et al. successfully prepared CoHPO4⋅3H2O ultrathin nanosheets by hydrothermal method (at 200 � C for 6 days) with a specific capacitance of 413 F g 1 at 1.5 A g 1 [23]. Mirghni et al. has synthesized Co3(PO4)2⋅4H2O by hydro thermal at 200 � C, which exhibited a specific capacitance of 29 mAh g 1
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (W. Li). https://doi.org/10.1016/j.jpowsour.2019.227487 Received 27 October 2019; Received in revised form 15 November 2019; Accepted 18 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ya Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227487
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
stability of products with a heating rate of 10 � C min 1 from 25 to 800 � C under nitrogen atmosphere. Electrochemical measurements: All electrochemical measure ments were evaluated in a 2 M KOH solution on a CHI760E (Shanghai Chenhua, China) electrochemical workstation at room temperature, including galvanostatic charge/discharge (GCD) test, electrochemical resistance spectroscopy (EIS) process with the frequency range of 100 kHz to 0.01 Hz and cyclic voltammetry (CV) test. In a three-electrode system, the saturated calomel electrode (SCE), the processed nickel foam and platinum electrode (20 mm � 20 mm) represent the reference electrode, the working electrode and the counter electrode, respectively.
[24]. Co11(HPO3)8(OH)6 nanorods were synthesized by hydrothermal at 220 � C for 4 days with 783 F g 1 at a current density of 0.5 A g 1 [25]. Co3(OH)2(HPO4)2 nano-needles were prepared by adding (NH4)3PO4, which produced NH3 at 200 � C [26]. Sodium hypophosphite can be reacted with sodium hydroxide to produce toxic phosphine gas at high temperatures [27]. Although cobalt phosphate with improved electro chemical properties has been successfully synthesized, most hydro thermal synthesis requires high reaction temperature and long reaction time. Furthermore, in most cases, harmful gases are produced under high temperature environments. Thereby, this kind of synthesis method is adverse to the practical application of cobalt phosphate electrode materials. Besides, almost all reports about hydrothermal methods indicate that the hydrothermal temperature is not less than 90 � C. Therefore, sample, green and low-cost synthetic methods have become a new goal pursued by researchers to obtain high electrochemical per formances for supercapacitors. In addition to the synthesis method, the selection of phosphorus sources is also critical. In this work, we reported a one-step low temperature hydrothermal method (50 � C) to prepare a novel flower-like ACHP nanosheet grown on nickel foam using a NaHMP as phosphorus source, which is harmless and environmentally benign. The synthesis mechanism of NaHMP and Co2þ was investigated, involving the hydrolysis and ionization equilibria of Co3(PO4)2 and Co(H2PO4)2. The ACHP electrode exhibits good electro chemical properties, high specific capacitance of 411.2 F g 1, good rate capability of 82.0% (411.2 F g 1 at 1 A g 1 to 337.2 F g 1 at 10 A g 1) and long-term stability with 97.6% capacitance maintained after 10000 GCD cycles at 2 A g 1 current density, owing to the nanosheets structure and the influence of structural water and hydroxyl. More importantly, the synthetic environment is low-energy-consumption, sample, effective and pollution-free, showing promising potential material for practical application.
3. Results and discussions The ACHP nanosheets were prepared via a facile and eco-friendly low-temperature hydrothermal method by using (NaPO3)6 as a phos phorus source. SEM and TEM are employed to explore the morphology and microstructure of the as-obtained samples. The SEM images of asprepared ACHP (Fig. 1a and b) present a flower-like morphology, which is assembled by uniform nanosheets with a thickness of around 40 nm. From Fig. 1c, the results of TEM were consistent with those of SEM. In addition, the average size of the nanoflower can be measured by TEM pattern, ranging from 600 nm to 1 μm. The ACHP displayed no obvious lattice fringes and weak diffraction rings from the HRTEM image (Fig. 1d) and the SAED image (inset of Fig. 1d), indicating it owns a very low crystallinity. No significant diffraction peaks were seen in the XRD patterns (Fig. S2a), which may be existed the ultrathin nano structures resulting in lower crystallinity. Therefore, the results of XRD are consistent with that of TEM. For the element-mapping image, it can be observed that Co (green), P (red) and O (yellow) elements are ho mogeneously distributed, as illustrated in Fig. 1e, f and g. Besides, the atomic ratio of Co∶P∶O determined by EDX (Fig. S1, Supporting Infor mation) is about 1∶1∶5 in ACHP. The FTIR spectra (4000 500 cm 1) of the as-synthesized samples are displayed in Fig. 2. For ACHP, the absorption bands at 3205 and 1644 cm 1 are derived from O–H stretching and bending mode of structural water [28]. Two obvious absorption bands at 1085 and 893 cm 1 can be associated with the υ3 (P–O) in HPO4 group [29,30]. The possible reactions of NaHMP are proposed as follows [31]:
2. Experimental section Materials: Cobalt chloride hexahydrate (AR), sodium hexameta phosphate (AR), potassium hydroxide (AR) and ethanol (AR) were purchased from Chuandong Chemical Co., Ltd, polyvinylpyrrolidone (PVP, MW:10000) was purchased from Adamas-beta Co., Ltd. Synthesis of ACHP: The nickel foam (1 cm � 1 cm) was ultrasoni cally washed in acetone and hydrochloric acid for 30 and 60 min respectively. Subsequently, it was washed several times with deionized water and ethanol. In this experiment, 0.238 g CoCl2⋅6H2O, 0.238 g (NaPO3)6 and 0.1 g PVP were sequentially added into 40 mL of distilled water, then and stirred to completely dissolve. After that, the pink mixture was transferred into an autoclave (50 mL) and the clean nickel foam (1 cm � 1 cm) was then set into the solution, and sealed at 50 � C for 20 h. Finally, the nickel foam was taken out and washed with deionized water and ethanol and dried overnight under vacuum to obtain the ACHP. Setting different hydrothermal temperatures (50, 70, 90 and 110 � C), ACHP50, ACHP70, ACHP90 and ACHP110 were obtained respectively. As comparison, the as-prepared products were annealed at 200 � C (ACHP-200), 400 � C (ACHP-400) and 500 � C (ACHP-500) for 30 min at rate of 10 � C min 1 in nitrogen atmosphere. Materials characterization: X-ray diffraction (XRD, PANalytical X’Pert powder) was used to characterize the crystallized phase of the samples, in which the 2-Theta ranged from 10 to 80. Fourier transform infrared (FTIR) spectra were carried out on a Nicolet iS50 spectrometer using the KBr disks in the range of 400–4000 cm 1. X-ray photoelectron spectroscopy (XPS) with Thermo Scientific K-Alpha was used to measure the surface chemical composition of samples. Scanning electron micro scopy (SEM) with TESCAN MIRA 3 FE at 10 kV, transmission electron microscopy (TEM) with FEI Talos F200S G2, selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the surface microscopic morphology and micro structure. Thermal gravimetric and differential thermal analysis (TGA, Mettler Toledo 1600LF, Switzerland) was demonstrated the thermal
ðNaPO3 Þ6 þ 6H2 O→6Naþ þ 6H2 PO4
(1)
Because the ionization degree of H2PO4 is greater than the hydro lysis, so the next reaction is as follows: H2 PO4 → HPO24 þ Hþ
(2)
HPO24 → PO34 þ Hþ
(3)
During the hydrothermal reaction process, Co3(PO4)2 and Co (H2PO4)2 are continuously dissolved and reprecipitated or PO34 and H2PO4 are continuously hydrolyzed and ionized, eventually reaching an equilibrium. Therefore, the final product exists in the form of CoHPO4. In addition, the characteristic bands of 735 and 549 cm 1 were due to the Co–O stretching vibration [32]. The ACHP exhibits three charac teristic peaks. All the ACHP-based samples display similar three char acteristic bands. The difference is that the structural water gradually decomposed to weaken the absorption peaks at 3205 and 1644 cm 1, respectively. The XRD diagram of ACHP-500 also displayed weak diffraction peaks (Fig. S2b) and the morphology of ACHP-500 was similar to that of ACHP (Fig. S3), suggesting that the ACHP was only occurred dehydration reaction at 200, 400 and 500 � C, which corre sponded to the results of FTIR. X-ray photoelectron spectroscopy (XPS) is employed to determine the surface chemical bonding state and chemical composition of ACHP. Fig. 3a demonstrates the survey XPS spectra, which observed the pres ence Co, P and O elements. In the Co 2p spectrum (Fig. 3b), the two 2
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
Fig. 1. SEM and TEM images of ACHP electrode: (a and b) SEM images, (c and d) TEM images and the inset of (d) is SAED images, (e, f and g) elemental mapping of Co, O and P.
dominating peaks at 781.5 eV in Co 2p3/2 and 797.5 eV in Co 2p1/2 and two shake-up satellite peaks at 785.7 and 802.9 eV are attributed to Co2þ [33,34]. Fig. 3c displays the P 2p spectrum, including two main peaks at 133.5 eV of P 2p3/2 and 134.3 eV of P 2p1/2, which can be due to the HPO4 [26,35]. This result is consistent with that of FTIR. For the O 1s spectrum (Fig. 3d), there are three main peaks at 530.7, 531.4 and 532.5 eV in accordance with Co–O, O–H and adsorbed water molecules, respectively [36,37]. Thus, according to the above results (EDX, FTIR and XPS), the molecular formula of ACHP can be written as CoHPO4⋅H2O. Fig. S4 exhibits the TGA analysis curve with a rising rate of 10 � C min 1 from 25 to 800 � C at N2 atmosphere. For ACHP, a weight loss was about 12% from 25 to 120 � C, owing to the decomposition of physically absorbed water and some residual alcohol on the surface of the sample. A weight loss was about 12% from 120 to 500 � C, which attributed to the decomposition of structural water. After 500 � C, the mass remained
basically unchanged. Therefore, TGA data, coincident with the results mentioned above (EDX, FTIR and so on), further indicate that the mo lecular formula of amorphous substance can be calculated as CoHPO4⋅H2O. The electrochemical characteristics of as-obtained products were examined in 2 M KOH electrolyte and all electrochemical measurements were carried out in a beaker type three-electrode configuration. Fig. 4a illustrates the CV plots (potential window of 0–0.45 V) of ACHP elec trode at different scan rates (6, 10, 20, 30 and 50 mV s 1). The CV plots were similar at all scan rates, suggesting a good rate capability [38]. In addition, the obvious redox peaks in these CV curves demonstrate the presence of a pseudocapacitive behavior, owing to the reversible Faradic redox reaction of Co2þ/Co3þ [23]: CoHPO4 ⋅H2 O þ OH – e →CoPO4 þ 2H2 O The GCD plots (potential window of 0–0.43 V) of ACHP electrode at 3
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
capability. The good electrochemical performances (high specific capacitance and excellent rate capability) of ACHP are attributed to the following reason: the nanosheet structure can provide more active sites. The EIS analysis of ACHP electrode at a frequency of 100 kHz to 0.01 Hz is shown in Fig. 4d. The ACHP electrode exhibits low charge transfer resistance and electrolyte resistance, suggesting a good electric conductivity. Cycle stability is an important indicator for testing the application of supercapacitor materials in energy storage equipment. The cyclability of ACHP electrode was performed by repeating the GCD tests at a current density of 2 A g 1 (Fig. 4e). Obviously, a rapid capacitance increase during 1–1000 cycles was due to the electrochemical activation process, which may be related to the essential characteristics of electrode ma terials and electrolyte ion permeability [35]. The final capacitance retention of ACHP electrode was about 97.6% after 10000 cycles. In addition, the Coulombic efficiency of ACHP electrode was close to 100%. Table S1 is a comparison of the electrochemical performance of ACHP electrode with those previously reported works of literature. From Table S1, the cycle performance of the ACPH material is much higher than those of other cobalt phosphate based materials. This superior performance may be related to ultra-thin nanosheet structures, which can provide more effective active sites. It shows that the electrode ma terials with excellent cycle performance and good rate capability can be synthesized by the low temperature hydrothermal method. Therefore, these excellent electrochemical properties are sufficient to show that ACHP is a promising electrode material. Fig. 4f shows the capacitances of ACHP, ACHP-200, ACHP-400 and ACHP-500 electrodes at current densities. It can be observed from the Fig. 4f that the capacitance values of the ACHP-200 are the same as that of the ACPH, and its rate capability is also similar to that of the ACPH. However, with the decrease of
Fig. 2. FTIR spectrum of the as-prepared samples.
different current densities are depicted in Fig. 4b. Approximate sym metrical GCD curves are observed at all current densities, indicating a good Coulombic efficiency and low polarization [39]. According to formula S1, the specific capacitance of ACHP electrode is calculated to be 411.2, 395.8, 385.1, 367.4 and 337.2 F g-1 at 1, 2, 3, 5 and10 A g-1, respectively (Fig. 4c). The capacitance retention of ACHP is found to remain 82.0% from 1 A g 1 to 10 A g-1, showing its outstanding rate
Fig. 3. XPS spectra of ACHP electrode: (a) survey spectrum, (b) Co 2p spectrum, (c) P 2p spectrum, (d) O 1s spectrum. 4
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
Fig. 4. (a) CV plots of ACHP electrode at various scan rates, (b) GCD plots of ACHP electrode at various current densities, (c) specific capacitance of ACHP electrode under various current densities, (d) EIS pattern of ACHP electrode, (e) cycling behavior of ACHP electrode at 2 A g 1, (f) the plots of specific capacitance for ACHP, ACHP-200, ACHP-400 and ACHP-500 electrodes under various current densities.
Fig. 5. (a) CV plots of ACHP at different small scan rates, (b) curves of log(i) vs. log(υ) for ACHP, (c) the percentages of diffusion-controlled and capacitive-controlled for ACHP electrode, (d) capacitance contribution (violet range) of ACHP in CV curves at 10 mV s 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
structural water content, the specific capacitance of ACHP-400 and ACHP-500 electrodes decreases significantly, and the rate capability becomes worse. The reason for this phenomenon is that the presence of structural water provides a pathway with less resistance for ion inser tion/exclusion due to the lubrication [40]. In order to explore the effect of hydrothermal temperature on ma terials, SEM characterization, CV and CP tests were carried out. Figs. S5a–c shows the SEM images of ACHP50, ACHP70 and ACHP90 samples at same magnifications, respectively. It can be clearly observed from the comparison chart that with the increase of hydrothermal temperature, the flowers become larger, the nanosheets become thicker, and the nanosheets collapse. Fig. S6a shows the CV plots of ACHP50, ACHP70 and ACHP90 electrodes with a potential window 0.1 to 0.5 V (vs Hg/HgO) at a scan rate of 50 mV s 1. Obviously, all CV curves have a pair of redox peaks of Co2þ/Co3þ, which indicates their pseudocapaci tive properties. Compared with the other two samples, ACHP50 shows the largest current area. Fig. S6b shows the GCD plots in a potential window 0–0.43 V at a current density of 2 A g 1. The Specific capaci tances of ACHP50, ACHP70 and ACHP90 electrodes were calculated to be 395.8, 345.1 and 294.0 F g 1, respectively. ACHP electrode exhibits the highest capacitance values. With the increase of hydrothermal temper ature, the nanosheets become thicker and collapse, which prevents electrolyte ions from fully contacting the nanosheets, resulting in the reduction of electrochemical performance. This analysis is also consis tent with the results of SEM. To explore the charge storage mechanism of ACHP electrode, Fig. 5a displays the CV plots at various scan rates of 2–10 mV s 1, according to the following equation [41]:
which attributed to the existence of structural water, hydroxyl and nanosheets structure. This low-temperature hydrothermal synthesis method provides a new idea for the preparation of other energy storage materials, and provides more possibilities for industrial production. Declaration of competing interest I solemnly promise that this work submitted to the Journal of Power Sources has not been previously published, in whole or in part, and is not under consideration for publication elsewhere and the work described is approved for publication by all co-authors and the responsible author ities where the work was carried out. Acknowledgements This work was supported by the Municipal Natural Science Foun dation of Chongqing (No. cstc2019jcyj-msxmX0347). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227487. References [1] Z. Dai, P.-G. Ren, Y.-L. Jin, H. Zhang, F. Ren, Q. Zhang, J. Power Sources 437 (2019) 226937. [2] S. Huang, L. Yang, M. Gao, Q. Zhang, G. Xu, X. Liu, J. Cao, X. Wei, J. Power Sources 437 (2019) 226934. [3] Z. Zhu, R. Zhang, J. Lin, K. Zhang, N. Li, C. Zhao, G. Chen, C. Zhao, J. Power Sources 437 (2019) 226941. [4] D. Guo, X. Song, L. Tan, H. Ma, H. Pang, X. Wang, L. Zhang, ACS Sustain. Chem. Eng. 7 (2018) 2803–2810. [5] J. Yang, X. Xiao, P. Chen, K. Zhu, K. Cheng, K. Ye, G. Wang, D. Cao, J. Yan, Nano Energy 58 (2019) 455–465. [6] 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. 2 (2012) 1328–1332. [7] X. Zhang, S. Deng, Y. Zeng, M. Yu, Y. Zhong, X. Xia, Y. Tong, X. Lu, Adv. Funct. Mater. 28 (2018), 1805618. [8] Y. Liu, X. Cao, L. Cui, Y. Zhong, R. Zheng, D. Wei, C. Barrow, J.M. Razal, W. Yang, J. Liu, J. Power Sources 437 (2019) 226897. [9] Z. Wang, J. Du, M. Zhang, J. Yu, H. Liu, X. Chai, B. Yang, C. Zhu, J. Xu, J. Power Sources 437 (2019) 226827. [10] J. Yi, Y. Qing, C. Wu, Y. Zeng, Y. Wu, X. Lu, Y. Tong, J. Power Sources 351 (2017) 130–137. [11] C. Young, R.R. Salunkhe, J. Tang, C.C. Hu, M. Shahabuddin, E. Yanmaz, M. S. Hossain, J.H. Kim, Y. Yamauchi, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315. [12] H.S. Huang, K.H. Chang, N. Suzuki, Y. Yamauchi, C.C. Hu, K.C. Wu, Small 9 (2013) 2520–2526. [13] H. Liu, Q. Xue, J. Zhao, Q. Zhang, Electrochim. Acta 260 (2018) 330–337. [14] S.H. Lee, J.H. Kim, J.R. Yoon, Sci. Rep. 8 (2018) 8179. [15] B.-G. Lee, S.-H. Lee, J. Power Sources 343 (2017) 545–549. [16] S.-H. Lee, J.-M. Kim, Energy 150 (2018) 816–821. [17] X.-h. Xia, J.-p. Tu, Y.-j. Mai, X.-l. Wang, C.-d. Gu, X.-b. Zhao, J. Mater. Chem. 21 (2011) 9319. [18] B.P. Bastakoti, H.S. Huang, L.C. Chen, K.C. Wu, Y. Yamauchi, Chem. Commun. 48 (2012) 9150–9152. [19] P. Arunachalam, M. Shaddad, A. Alamoudi, M. Ghanem, A. Al-Mayouf, Catalysts 7 (2017) 119. [20] D.S. Patil, S.A. Pawar, J.C. Shin, H.J. Kim, J. Power Sources 435 (2019) 226812. [21] M. Pramanik, R.R. Salunkhe, M. Imura, Y. Yamauchi, ACS Appl. Mater. Interfaces 8 (2016) 9790–9797. [22] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017), 1600539. [23] H. Pang, S. Wang, W. Shao, S. Zhao, B. Yan, X. Li, S. Li, J. Chen, W. Du, Nanoscale 5 (2013) 5752–5757. [24] A.A. Mirghni, D. Momodu, K.O. Oyedotun, J.K. Dangbegnon, N. Manyala, Electrochim. Acta 283 (2018) 374–384. [25] Y. Zhang, M. Zheng, M. Qu, M. Sun, H. Pang, J. Alloy. Comp. 651 (2015) 214–221. [26] Y. Zhang, J. Shi, C. Cheng, S. Zong, J. Geng, X. Guan, L. Guo, Appl. Catal. B Environ. 232 (2018) 268–274. [27] D. Yin, Z. Jin, M. Liu, T. Gao, H. Yuan, D. Xiao, Electrochim. Acta 260 (2018) 420–429. [28] L. Zhang, H. Yao, Z. Li, P. Sun, F. Liu, C. Dong, J. Wang, Z. Li, M. Wu, C. Zhang, B. Zhao, J. Alloy. Comp. 711 (2017) 31–41. [29] W.-X. Lu, B. Wang, W.-J. Chen, J.-L. Xie, Z.-Q. Huang, W. Jin, J.-L. Song, ACS Sustain. Chem. Eng. 7 (2019) 3083–3091.
(6)
i ¼ aυb
where υ and i are the sweep rate and the peak current, a and b represent empirical parameters. The determination of b-value can be obtained by the slope of log(i) vs. log(υ) (Fig. 5b). For b ¼ 0.5, the charge storage mechanism is dominated by the diffusion-controlled process. In another case (b ¼ 1), the charge storage mechanism is determined by the capacitive-controlled process [42]. As shown in Fig. 5b, the b-value of ACHP electrode is around 0.73, which suggests that ACHP is predomi nantly by both capacitance-controlled and diffusion-controlled pro cesses. The specific ratio of capacitive-controlled can be calculated by the following equation [43,44]: (7)
i (V) ¼ k1υ þ k2υ1/2 1/2
i is total current (V), k1υ and k2υ stand for capacitance process and diffusion process, respectively. Thereby, the different scan rates of 2, 4, 6, 8 and 10 mV s 1 corresponds to the capacitance contributions were 76.7, 81.3, 84.3, 86.8 and 88.7% for ACHP electrode, respectively, suggesting that the capacitive-controlled dominated the whole reaction process, especially at high scan rate (Fig. 5c). As presented in Fig. 5d, the yellow area is the total contribution and the violet area represents a capacitance contribution of 88.7% at 10 mV s 1. It can be concluded that the ACHP electrode has good storage efficiency. 4. Conclusions In conclusions, we successfully prepared ACHP nanosheets with high electrochemical performance via a one-step simple low-temperature hydrothermal. This synthetic method achieves two remarkable effects: saving time and, more importantly, reducing the hydrothermal tem perature to reduce energy consumption. A new phosphorus source, NaHMP, is firstly used in supercapacitors because it is environmentally friendly and not produced toxic phosphine gas in low temperature hy drothermal reaction. The as-obtained ACHP possesses 411.2 F g 1 spe cific capacitance at a current density of 1 A g 1, 82% rate capability ranging from 1 A g 1 to 10 A g 1, outstanding cycle stability with 97.6% capacitance retention after 10000 cycles and good Coulombic efficiency, 6
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
[30] N. Iturraran, K. Huraux, Y. Bao, D.T. Gawne, J. Guilment, J. Non-Cryst. Solids 489 (2018) 64–70. [31] D.-d. Wang, X.-t. Liu, Y.-k. Wu, H.-p. Han, Z. Yang, Y. Su, X.-z. Zhang, G.-r. Wu, D.j. Shen, J. Alloy. Comp. 798 (2019) 129–143. [32] M. Ishaq, M. Jabeen, W. Song, L. Xu, W. Li, Q. Deng, Electrochim. Acta 282 (2018) 913–922. [33] H. Dan, K. Tao, Q. Zhou, Y. Gong, J. Lin, ACS Appl. Mater. Interfaces 10 (2018) 31340–31354. [34] J. Li, X. Kong, M. Jiang, X. Lei, J. Mater. Sci. 53 (2018) 16263–16275. [35] S. Wang, Z. Zhu, P. Li, C. Zhao, C. Zhao, H. Xia, J. Mater. Chem. 6 (2018) 20015–20024. [36] K. Li, D. Guo, J. Kang, B. Wei, X. Zhang, Y. Chen, ACS Sustain. Chem. Eng. 6 (2018) 14641–14651. [37] C. Lamiel, V.H. Nguyen, D.R. Kumar, J.-J. Shim, Chem. Eng. J. 316 (2017) 1091–1102.
[38] X. Zou, Y. Zhou, Z. Wang, S. Chen, B. Xiang, Y. Qiang, S. Zhu, J. Alloy. Comp. 744 (2018) 412–420. [39] Q. Zong, H. Yang, Q. Wang, Q. Zhang, Y. Zhu, H. Wang, Q. Shen, Chem. Eng. J. 361 (2019) 1–11. [40] J.B. Mitchell, W.C. Lo, A. Genc, J. LeBeau, V. Augustyn, Chem. Mater. 29 (2017) 3928–3937. [41] J. Wang, J. Polleux, J. Lim, B. Dunn, J. Phys. Chem. C 111 (2007) 14925–14931. [42] H. Zhang, H. He, J. Luan, X. Huang, Y. Tang, H. Wang, J. Mater. Chem. 6 (2018) 23318–23325. [43] C. Yang, Y. Zhang, J. Zhou, C. Lin, F. Lv, K. Wang, J. Feng, Z. Xu, J. Li, S. Guo, J. Mater. Chem. 6 (2018) 8039–8046. [44] H. Zhou, C. Liu, J.-C. Wu, M. Liu, D. Zhang, H. Song, X. Zhang, H. Gao, J. Yang, D. Chen, J. Mater. Chem. 7 (2019) 9708–9715.
7
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Amorphous cobalt hydrogen phosphate nanosheets with remarkable electrochemical performances as advanced electrode for supercapacitors Ya Wang a, Wenpo Li a, *, Lulu Zhang a, Xin Zhang a, Bochuan Tan a, Jiangyu Hao a, Jian Zhang a, Xu Wang a, Qin Hu a, Xihong Lu b, ** a
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 401331, China MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou, 510275, China
b
H I G H L I G H T S
� ACHP nanosheets are prepared by a one-step low temperature hydrothermal method. � NaHMP is used as a novel phosphorus source. � ACHP electrode displays 97.6% capacitance maintained after 10000 GCD cycles. A R T I C L E I N F O
A B S T R A C T
Keywords: Supercapacitors Hydrothermal method Nanosheet-like architecture Electrochemical performance
The development of simple and efficient approaches to achieve high-performance electrode materials for supercapacitors is urgently needed at present. In this work, a facile and efficient hydrothermal approach (50 � C) is reported to prepare flower-like amorphous cobalt hydrogen phosphate (ACHP) architectures with significantly enhanced electrochemical activity as robust supercapacitor electrodes. These ACHP nanoflowers with numerous thin nanosheets are readily grown on nickel foam by using eco-friendly sodium hexametaphosphate (NaHMP) as phosphorus source and their formation mechanism is also revealed. The ACHP displays a high specific capaci tance (411.2 F g 1 at 1 A g 1) as well as excellent rate capability (remained capacitance retention of 82.0% at 10 A g 1) due to the unique nanosheet-like architecture and the influence of structural water. In addition, this ACHP also exhibits an ultra-long lifespan that can maintain more than 97.6% capacitance retention after 10000 charge/ discharge cycles. Therefore, this easy and effective one-step low temperature hydrothermal method offers a sustainable development strategy for energy storage devices in practical application.
1. Introduction To adapt the rapid development of the economy, energy storage devices such as fuel cells, lithium-ion batteries, sodium-ion batteries and supercapacitors need to be continuously updated [1–3]. Among them, supercapacitors have become a research hotspot due to their high power density and long cycle life [4–10], and have been widely used in con sumer electronics, portable electronic devices, voltage regulators in power lines, and so on [11,12]. Based on the different work mechanism, supercapacitors can be splitted into pseudocapacitors (reversible faradic reactions), electrical double-layer capacitors (charge accumulation) and hybrid supercapacitors (composed of supercapacitors and lithium-ion
batteries) [13–16]. Pseudocapacitors, especially transition metal compounds-based materials, such as RuO2 [12], Co3O4 [17], Ni(OH)2 [18], Co3(PO4)2 [19], are widely reported because they have higher energy density than that of electrical double-layer capacitors (carbon- based materials) [20]. Transition metal phosphates, as a kind of pseudocapacitive electrode materials, have been widely reported due to natural abundance, non toxicity and broad applications [21,22]. Huan Pang et al. successfully prepared CoHPO4⋅3H2O ultrathin nanosheets by hydrothermal method (at 200 � C for 6 days) with a specific capacitance of 413 F g 1 at 1.5 A g 1 [23]. Mirghni et al. has synthesized Co3(PO4)2⋅4H2O by hydro thermal at 200 � C, which exhibited a specific capacitance of 29 mAh g 1
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (W. Li). https://doi.org/10.1016/j.jpowsour.2019.227487 Received 27 October 2019; Received in revised form 15 November 2019; Accepted 18 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Ya Wang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227487
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
stability of products with a heating rate of 10 � C min 1 from 25 to 800 � C under nitrogen atmosphere. Electrochemical measurements: All electrochemical measure ments were evaluated in a 2 M KOH solution on a CHI760E (Shanghai Chenhua, China) electrochemical workstation at room temperature, including galvanostatic charge/discharge (GCD) test, electrochemical resistance spectroscopy (EIS) process with the frequency range of 100 kHz to 0.01 Hz and cyclic voltammetry (CV) test. In a three-electrode system, the saturated calomel electrode (SCE), the processed nickel foam and platinum electrode (20 mm � 20 mm) represent the reference electrode, the working electrode and the counter electrode, respectively.
[24]. Co11(HPO3)8(OH)6 nanorods were synthesized by hydrothermal at 220 � C for 4 days with 783 F g 1 at a current density of 0.5 A g 1 [25]. Co3(OH)2(HPO4)2 nano-needles were prepared by adding (NH4)3PO4, which produced NH3 at 200 � C [26]. Sodium hypophosphite can be reacted with sodium hydroxide to produce toxic phosphine gas at high temperatures [27]. Although cobalt phosphate with improved electro chemical properties has been successfully synthesized, most hydro thermal synthesis requires high reaction temperature and long reaction time. Furthermore, in most cases, harmful gases are produced under high temperature environments. Thereby, this kind of synthesis method is adverse to the practical application of cobalt phosphate electrode materials. Besides, almost all reports about hydrothermal methods indicate that the hydrothermal temperature is not less than 90 � C. Therefore, sample, green and low-cost synthetic methods have become a new goal pursued by researchers to obtain high electrochemical per formances for supercapacitors. In addition to the synthesis method, the selection of phosphorus sources is also critical. In this work, we reported a one-step low temperature hydrothermal method (50 � C) to prepare a novel flower-like ACHP nanosheet grown on nickel foam using a NaHMP as phosphorus source, which is harmless and environmentally benign. The synthesis mechanism of NaHMP and Co2þ was investigated, involving the hydrolysis and ionization equilibria of Co3(PO4)2 and Co(H2PO4)2. The ACHP electrode exhibits good electro chemical properties, high specific capacitance of 411.2 F g 1, good rate capability of 82.0% (411.2 F g 1 at 1 A g 1 to 337.2 F g 1 at 10 A g 1) and long-term stability with 97.6% capacitance maintained after 10000 GCD cycles at 2 A g 1 current density, owing to the nanosheets structure and the influence of structural water and hydroxyl. More importantly, the synthetic environment is low-energy-consumption, sample, effective and pollution-free, showing promising potential material for practical application.
3. Results and discussions The ACHP nanosheets were prepared via a facile and eco-friendly low-temperature hydrothermal method by using (NaPO3)6 as a phos phorus source. SEM and TEM are employed to explore the morphology and microstructure of the as-obtained samples. The SEM images of asprepared ACHP (Fig. 1a and b) present a flower-like morphology, which is assembled by uniform nanosheets with a thickness of around 40 nm. From Fig. 1c, the results of TEM were consistent with those of SEM. In addition, the average size of the nanoflower can be measured by TEM pattern, ranging from 600 nm to 1 μm. The ACHP displayed no obvious lattice fringes and weak diffraction rings from the HRTEM image (Fig. 1d) and the SAED image (inset of Fig. 1d), indicating it owns a very low crystallinity. No significant diffraction peaks were seen in the XRD patterns (Fig. S2a), which may be existed the ultrathin nano structures resulting in lower crystallinity. Therefore, the results of XRD are consistent with that of TEM. For the element-mapping image, it can be observed that Co (green), P (red) and O (yellow) elements are ho mogeneously distributed, as illustrated in Fig. 1e, f and g. Besides, the atomic ratio of Co∶P∶O determined by EDX (Fig. S1, Supporting Infor mation) is about 1∶1∶5 in ACHP. The FTIR spectra (4000 500 cm 1) of the as-synthesized samples are displayed in Fig. 2. For ACHP, the absorption bands at 3205 and 1644 cm 1 are derived from O–H stretching and bending mode of structural water [28]. Two obvious absorption bands at 1085 and 893 cm 1 can be associated with the υ3 (P–O) in HPO4 group [29,30]. The possible reactions of NaHMP are proposed as follows [31]:
2. Experimental section Materials: Cobalt chloride hexahydrate (AR), sodium hexameta phosphate (AR), potassium hydroxide (AR) and ethanol (AR) were purchased from Chuandong Chemical Co., Ltd, polyvinylpyrrolidone (PVP, MW:10000) was purchased from Adamas-beta Co., Ltd. Synthesis of ACHP: The nickel foam (1 cm � 1 cm) was ultrasoni cally washed in acetone and hydrochloric acid for 30 and 60 min respectively. Subsequently, it was washed several times with deionized water and ethanol. In this experiment, 0.238 g CoCl2⋅6H2O, 0.238 g (NaPO3)6 and 0.1 g PVP were sequentially added into 40 mL of distilled water, then and stirred to completely dissolve. After that, the pink mixture was transferred into an autoclave (50 mL) and the clean nickel foam (1 cm � 1 cm) was then set into the solution, and sealed at 50 � C for 20 h. Finally, the nickel foam was taken out and washed with deionized water and ethanol and dried overnight under vacuum to obtain the ACHP. Setting different hydrothermal temperatures (50, 70, 90 and 110 � C), ACHP50, ACHP70, ACHP90 and ACHP110 were obtained respectively. As comparison, the as-prepared products were annealed at 200 � C (ACHP-200), 400 � C (ACHP-400) and 500 � C (ACHP-500) for 30 min at rate of 10 � C min 1 in nitrogen atmosphere. Materials characterization: X-ray diffraction (XRD, PANalytical X’Pert powder) was used to characterize the crystallized phase of the samples, in which the 2-Theta ranged from 10 to 80. Fourier transform infrared (FTIR) spectra were carried out on a Nicolet iS50 spectrometer using the KBr disks in the range of 400–4000 cm 1. X-ray photoelectron spectroscopy (XPS) with Thermo Scientific K-Alpha was used to measure the surface chemical composition of samples. Scanning electron micro scopy (SEM) with TESCAN MIRA 3 FE at 10 kV, transmission electron microscopy (TEM) with FEI Talos F200S G2, selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy (EDX) were used to analyze the surface microscopic morphology and micro structure. Thermal gravimetric and differential thermal analysis (TGA, Mettler Toledo 1600LF, Switzerland) was demonstrated the thermal
ðNaPO3 Þ6 þ 6H2 O→6Naþ þ 6H2 PO4
(1)
Because the ionization degree of H2PO4 is greater than the hydro lysis, so the next reaction is as follows: H2 PO4 → HPO24 þ Hþ
(2)
HPO24 → PO34 þ Hþ
(3)
During the hydrothermal reaction process, Co3(PO4)2 and Co (H2PO4)2 are continuously dissolved and reprecipitated or PO34 and H2PO4 are continuously hydrolyzed and ionized, eventually reaching an equilibrium. Therefore, the final product exists in the form of CoHPO4. In addition, the characteristic bands of 735 and 549 cm 1 were due to the Co–O stretching vibration [32]. The ACHP exhibits three charac teristic peaks. All the ACHP-based samples display similar three char acteristic bands. The difference is that the structural water gradually decomposed to weaken the absorption peaks at 3205 and 1644 cm 1, respectively. The XRD diagram of ACHP-500 also displayed weak diffraction peaks (Fig. S2b) and the morphology of ACHP-500 was similar to that of ACHP (Fig. S3), suggesting that the ACHP was only occurred dehydration reaction at 200, 400 and 500 � C, which corre sponded to the results of FTIR. X-ray photoelectron spectroscopy (XPS) is employed to determine the surface chemical bonding state and chemical composition of ACHP. Fig. 3a demonstrates the survey XPS spectra, which observed the pres ence Co, P and O elements. In the Co 2p spectrum (Fig. 3b), the two 2
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
Fig. 1. SEM and TEM images of ACHP electrode: (a and b) SEM images, (c and d) TEM images and the inset of (d) is SAED images, (e, f and g) elemental mapping of Co, O and P.
dominating peaks at 781.5 eV in Co 2p3/2 and 797.5 eV in Co 2p1/2 and two shake-up satellite peaks at 785.7 and 802.9 eV are attributed to Co2þ [33,34]. Fig. 3c displays the P 2p spectrum, including two main peaks at 133.5 eV of P 2p3/2 and 134.3 eV of P 2p1/2, which can be due to the HPO4 [26,35]. This result is consistent with that of FTIR. For the O 1s spectrum (Fig. 3d), there are three main peaks at 530.7, 531.4 and 532.5 eV in accordance with Co–O, O–H and adsorbed water molecules, respectively [36,37]. Thus, according to the above results (EDX, FTIR and XPS), the molecular formula of ACHP can be written as CoHPO4⋅H2O. Fig. S4 exhibits the TGA analysis curve with a rising rate of 10 � C min 1 from 25 to 800 � C at N2 atmosphere. For ACHP, a weight loss was about 12% from 25 to 120 � C, owing to the decomposition of physically absorbed water and some residual alcohol on the surface of the sample. A weight loss was about 12% from 120 to 500 � C, which attributed to the decomposition of structural water. After 500 � C, the mass remained
basically unchanged. Therefore, TGA data, coincident with the results mentioned above (EDX, FTIR and so on), further indicate that the mo lecular formula of amorphous substance can be calculated as CoHPO4⋅H2O. The electrochemical characteristics of as-obtained products were examined in 2 M KOH electrolyte and all electrochemical measurements were carried out in a beaker type three-electrode configuration. Fig. 4a illustrates the CV plots (potential window of 0–0.45 V) of ACHP elec trode at different scan rates (6, 10, 20, 30 and 50 mV s 1). The CV plots were similar at all scan rates, suggesting a good rate capability [38]. In addition, the obvious redox peaks in these CV curves demonstrate the presence of a pseudocapacitive behavior, owing to the reversible Faradic redox reaction of Co2þ/Co3þ [23]: CoHPO4 ⋅H2 O þ OH – e →CoPO4 þ 2H2 O The GCD plots (potential window of 0–0.43 V) of ACHP electrode at 3
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
capability. The good electrochemical performances (high specific capacitance and excellent rate capability) of ACHP are attributed to the following reason: the nanosheet structure can provide more active sites. The EIS analysis of ACHP electrode at a frequency of 100 kHz to 0.01 Hz is shown in Fig. 4d. The ACHP electrode exhibits low charge transfer resistance and electrolyte resistance, suggesting a good electric conductivity. Cycle stability is an important indicator for testing the application of supercapacitor materials in energy storage equipment. The cyclability of ACHP electrode was performed by repeating the GCD tests at a current density of 2 A g 1 (Fig. 4e). Obviously, a rapid capacitance increase during 1–1000 cycles was due to the electrochemical activation process, which may be related to the essential characteristics of electrode ma terials and electrolyte ion permeability [35]. The final capacitance retention of ACHP electrode was about 97.6% after 10000 cycles. In addition, the Coulombic efficiency of ACHP electrode was close to 100%. Table S1 is a comparison of the electrochemical performance of ACHP electrode with those previously reported works of literature. From Table S1, the cycle performance of the ACPH material is much higher than those of other cobalt phosphate based materials. This superior performance may be related to ultra-thin nanosheet structures, which can provide more effective active sites. It shows that the electrode ma terials with excellent cycle performance and good rate capability can be synthesized by the low temperature hydrothermal method. Therefore, these excellent electrochemical properties are sufficient to show that ACHP is a promising electrode material. Fig. 4f shows the capacitances of ACHP, ACHP-200, ACHP-400 and ACHP-500 electrodes at current densities. It can be observed from the Fig. 4f that the capacitance values of the ACHP-200 are the same as that of the ACPH, and its rate capability is also similar to that of the ACPH. However, with the decrease of
Fig. 2. FTIR spectrum of the as-prepared samples.
different current densities are depicted in Fig. 4b. Approximate sym metrical GCD curves are observed at all current densities, indicating a good Coulombic efficiency and low polarization [39]. According to formula S1, the specific capacitance of ACHP electrode is calculated to be 411.2, 395.8, 385.1, 367.4 and 337.2 F g-1 at 1, 2, 3, 5 and10 A g-1, respectively (Fig. 4c). The capacitance retention of ACHP is found to remain 82.0% from 1 A g 1 to 10 A g-1, showing its outstanding rate
Fig. 3. XPS spectra of ACHP electrode: (a) survey spectrum, (b) Co 2p spectrum, (c) P 2p spectrum, (d) O 1s spectrum. 4
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
Fig. 4. (a) CV plots of ACHP electrode at various scan rates, (b) GCD plots of ACHP electrode at various current densities, (c) specific capacitance of ACHP electrode under various current densities, (d) EIS pattern of ACHP electrode, (e) cycling behavior of ACHP electrode at 2 A g 1, (f) the plots of specific capacitance for ACHP, ACHP-200, ACHP-400 and ACHP-500 electrodes under various current densities.
Fig. 5. (a) CV plots of ACHP at different small scan rates, (b) curves of log(i) vs. log(υ) for ACHP, (c) the percentages of diffusion-controlled and capacitive-controlled for ACHP electrode, (d) capacitance contribution (violet range) of ACHP in CV curves at 10 mV s 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 5
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
structural water content, the specific capacitance of ACHP-400 and ACHP-500 electrodes decreases significantly, and the rate capability becomes worse. The reason for this phenomenon is that the presence of structural water provides a pathway with less resistance for ion inser tion/exclusion due to the lubrication [40]. In order to explore the effect of hydrothermal temperature on ma terials, SEM characterization, CV and CP tests were carried out. Figs. S5a–c shows the SEM images of ACHP50, ACHP70 and ACHP90 samples at same magnifications, respectively. It can be clearly observed from the comparison chart that with the increase of hydrothermal temperature, the flowers become larger, the nanosheets become thicker, and the nanosheets collapse. Fig. S6a shows the CV plots of ACHP50, ACHP70 and ACHP90 electrodes with a potential window 0.1 to 0.5 V (vs Hg/HgO) at a scan rate of 50 mV s 1. Obviously, all CV curves have a pair of redox peaks of Co2þ/Co3þ, which indicates their pseudocapaci tive properties. Compared with the other two samples, ACHP50 shows the largest current area. Fig. S6b shows the GCD plots in a potential window 0–0.43 V at a current density of 2 A g 1. The Specific capaci tances of ACHP50, ACHP70 and ACHP90 electrodes were calculated to be 395.8, 345.1 and 294.0 F g 1, respectively. ACHP electrode exhibits the highest capacitance values. With the increase of hydrothermal temper ature, the nanosheets become thicker and collapse, which prevents electrolyte ions from fully contacting the nanosheets, resulting in the reduction of electrochemical performance. This analysis is also consis tent with the results of SEM. To explore the charge storage mechanism of ACHP electrode, Fig. 5a displays the CV plots at various scan rates of 2–10 mV s 1, according to the following equation [41]:
which attributed to the existence of structural water, hydroxyl and nanosheets structure. This low-temperature hydrothermal synthesis method provides a new idea for the preparation of other energy storage materials, and provides more possibilities for industrial production. Declaration of competing interest I solemnly promise that this work submitted to the Journal of Power Sources has not been previously published, in whole or in part, and is not under consideration for publication elsewhere and the work described is approved for publication by all co-authors and the responsible author ities where the work was carried out. Acknowledgements This work was supported by the Municipal Natural Science Foun dation of Chongqing (No. cstc2019jcyj-msxmX0347). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227487. References [1] Z. Dai, P.-G. Ren, Y.-L. Jin, H. Zhang, F. Ren, Q. Zhang, J. Power Sources 437 (2019) 226937. [2] S. Huang, L. Yang, M. Gao, Q. Zhang, G. Xu, X. Liu, J. Cao, X. Wei, J. Power Sources 437 (2019) 226934. [3] Z. Zhu, R. Zhang, J. Lin, K. Zhang, N. Li, C. Zhao, G. Chen, C. Zhao, J. Power Sources 437 (2019) 226941. [4] D. Guo, X. Song, L. Tan, H. Ma, H. Pang, X. Wang, L. Zhang, ACS Sustain. Chem. Eng. 7 (2018) 2803–2810. [5] J. Yang, X. Xiao, P. Chen, K. Zhu, K. Cheng, K. Ye, G. Wang, D. Cao, J. Yan, Nano Energy 58 (2019) 455–465. [6] 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. 2 (2012) 1328–1332. [7] X. Zhang, S. Deng, Y. Zeng, M. Yu, Y. Zhong, X. Xia, Y. Tong, X. Lu, Adv. Funct. Mater. 28 (2018), 1805618. [8] Y. Liu, X. Cao, L. Cui, Y. Zhong, R. Zheng, D. Wei, C. Barrow, J.M. Razal, W. Yang, J. Liu, J. Power Sources 437 (2019) 226897. [9] Z. Wang, J. Du, M. Zhang, J. Yu, H. Liu, X. Chai, B. Yang, C. Zhu, J. Xu, J. Power Sources 437 (2019) 226827. [10] J. Yi, Y. Qing, C. Wu, Y. Zeng, Y. Wu, X. Lu, Y. Tong, J. Power Sources 351 (2017) 130–137. [11] C. Young, R.R. Salunkhe, J. Tang, C.C. Hu, M. Shahabuddin, E. Yanmaz, M. S. Hossain, J.H. Kim, Y. Yamauchi, Phys. Chem. Chem. Phys. 18 (2016) 29308–29315. [12] H.S. Huang, K.H. Chang, N. Suzuki, Y. Yamauchi, C.C. Hu, K.C. Wu, Small 9 (2013) 2520–2526. [13] H. Liu, Q. Xue, J. Zhao, Q. Zhang, Electrochim. Acta 260 (2018) 330–337. [14] S.H. Lee, J.H. Kim, J.R. Yoon, Sci. Rep. 8 (2018) 8179. [15] B.-G. Lee, S.-H. Lee, J. Power Sources 343 (2017) 545–549. [16] S.-H. Lee, J.-M. Kim, Energy 150 (2018) 816–821. [17] X.-h. Xia, J.-p. Tu, Y.-j. Mai, X.-l. Wang, C.-d. Gu, X.-b. Zhao, J. Mater. Chem. 21 (2011) 9319. [18] B.P. Bastakoti, H.S. Huang, L.C. Chen, K.C. Wu, Y. Yamauchi, Chem. Commun. 48 (2012) 9150–9152. [19] P. Arunachalam, M. Shaddad, A. Alamoudi, M. Ghanem, A. Al-Mayouf, Catalysts 7 (2017) 119. [20] D.S. Patil, S.A. Pawar, J.C. Shin, H.J. Kim, J. Power Sources 435 (2019) 226812. [21] M. Pramanik, R.R. Salunkhe, M. Imura, Y. Yamauchi, ACS Appl. Mater. Interfaces 8 (2016) 9790–9797. [22] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Adv. Sci. 4 (2017), 1600539. [23] H. Pang, S. Wang, W. Shao, S. Zhao, B. Yan, X. Li, S. Li, J. Chen, W. Du, Nanoscale 5 (2013) 5752–5757. [24] A.A. Mirghni, D. Momodu, K.O. Oyedotun, J.K. Dangbegnon, N. Manyala, Electrochim. Acta 283 (2018) 374–384. [25] Y. Zhang, M. Zheng, M. Qu, M. Sun, H. Pang, J. Alloy. Comp. 651 (2015) 214–221. [26] Y. Zhang, J. Shi, C. Cheng, S. Zong, J. Geng, X. Guan, L. Guo, Appl. Catal. B Environ. 232 (2018) 268–274. [27] D. Yin, Z. Jin, M. Liu, T. Gao, H. Yuan, D. Xiao, Electrochim. Acta 260 (2018) 420–429. [28] L. Zhang, H. Yao, Z. Li, P. Sun, F. Liu, C. Dong, J. Wang, Z. Li, M. Wu, C. Zhang, B. Zhao, J. Alloy. Comp. 711 (2017) 31–41. [29] W.-X. Lu, B. Wang, W.-J. Chen, J.-L. Xie, Z.-Q. Huang, W. Jin, J.-L. Song, ACS Sustain. Chem. Eng. 7 (2019) 3083–3091.
(6)
i ¼ aυb
where υ and i are the sweep rate and the peak current, a and b represent empirical parameters. The determination of b-value can be obtained by the slope of log(i) vs. log(υ) (Fig. 5b). For b ¼ 0.5, the charge storage mechanism is dominated by the diffusion-controlled process. In another case (b ¼ 1), the charge storage mechanism is determined by the capacitive-controlled process [42]. As shown in Fig. 5b, the b-value of ACHP electrode is around 0.73, which suggests that ACHP is predomi nantly by both capacitance-controlled and diffusion-controlled pro cesses. The specific ratio of capacitive-controlled can be calculated by the following equation [43,44]: (7)
i (V) ¼ k1υ þ k2υ1/2 1/2
i is total current (V), k1υ and k2υ stand for capacitance process and diffusion process, respectively. Thereby, the different scan rates of 2, 4, 6, 8 and 10 mV s 1 corresponds to the capacitance contributions were 76.7, 81.3, 84.3, 86.8 and 88.7% for ACHP electrode, respectively, suggesting that the capacitive-controlled dominated the whole reaction process, especially at high scan rate (Fig. 5c). As presented in Fig. 5d, the yellow area is the total contribution and the violet area represents a capacitance contribution of 88.7% at 10 mV s 1. It can be concluded that the ACHP electrode has good storage efficiency. 4. Conclusions In conclusions, we successfully prepared ACHP nanosheets with high electrochemical performance via a one-step simple low-temperature hydrothermal. This synthetic method achieves two remarkable effects: saving time and, more importantly, reducing the hydrothermal tem perature to reduce energy consumption. A new phosphorus source, NaHMP, is firstly used in supercapacitors because it is environmentally friendly and not produced toxic phosphine gas in low temperature hy drothermal reaction. The as-obtained ACHP possesses 411.2 F g 1 spe cific capacitance at a current density of 1 A g 1, 82% rate capability ranging from 1 A g 1 to 10 A g 1, outstanding cycle stability with 97.6% capacitance retention after 10000 cycles and good Coulombic efficiency, 6
Y. Wang et al.
Journal of Power Sources xxx (xxxx) xxx
[30] N. Iturraran, K. Huraux, Y. Bao, D.T. Gawne, J. Guilment, J. Non-Cryst. Solids 489 (2018) 64–70. [31] D.-d. Wang, X.-t. Liu, Y.-k. Wu, H.-p. Han, Z. Yang, Y. Su, X.-z. Zhang, G.-r. Wu, D.j. Shen, J. Alloy. Comp. 798 (2019) 129–143. [32] M. Ishaq, M. Jabeen, W. Song, L. Xu, W. Li, Q. Deng, Electrochim. Acta 282 (2018) 913–922. [33] H. Dan, K. Tao, Q. Zhou, Y. Gong, J. Lin, ACS Appl. Mater. Interfaces 10 (2018) 31340–31354. [34] J. Li, X. Kong, M. Jiang, X. Lei, J. Mater. Sci. 53 (2018) 16263–16275. [35] S. Wang, Z. Zhu, P. Li, C. Zhao, C. Zhao, H. Xia, J. Mater. Chem. 6 (2018) 20015–20024. [36] K. Li, D. Guo, J. Kang, B. Wei, X. Zhang, Y. Chen, ACS Sustain. Chem. Eng. 6 (2018) 14641–14651. [37] C. Lamiel, V.H. Nguyen, D.R. Kumar, J.-J. Shim, Chem. Eng. J. 316 (2017) 1091–1102.
[38] X. Zou, Y. Zhou, Z. Wang, S. Chen, B. Xiang, Y. Qiang, S. Zhu, J. Alloy. Comp. 744 (2018) 412–420. [39] Q. Zong, H. Yang, Q. Wang, Q. Zhang, Y. Zhu, H. Wang, Q. Shen, Chem. Eng. J. 361 (2019) 1–11. [40] J.B. Mitchell, W.C. Lo, A. Genc, J. LeBeau, V. Augustyn, Chem. Mater. 29 (2017) 3928–3937. [41] J. Wang, J. Polleux, J. Lim, B. Dunn, J. Phys. Chem. C 111 (2007) 14925–14931. [42] H. Zhang, H. He, J. Luan, X. Huang, Y. Tang, H. Wang, J. Mater. Chem. 6 (2018) 23318–23325. [43] C. Yang, Y. Zhang, J. Zhou, C. Lin, F. Lv, K. Wang, J. Feng, Z. Xu, J. Li, S. Guo, J. Mater. Chem. 6 (2018) 8039–8046. [44] H. Zhou, C. Liu, J.-C. Wu, M. Liu, D. Zhang, H. Song, X. Zhang, H. Gao, J. Yang, D. Chen, J. Mater. Chem. 7 (2019) 9708–9715.
7