Electrochemical properties of LaFeO3 rGO composite H O S T E D B Y Contents lists available at ScienceDirect Progress in Natural Science Materials International journal homepage www elsevier com/locat[.]
Progress in Natural Science: Materials International (xxxx) xxxx–xxxx Contents lists available at ScienceDirect HOSTED BY Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi Original Research Electrochemical properties of LaFeO3-rGO composite☆ Yongjie Yuana,b, Zhentao Donga,b, Yuan Lia,b, Lu Zhanga,b, Yumeng Zhaoa,b, Bo Wanga,b, Shumin Hana,b a b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China A R T I C L E I N F O A BS T RAC T Keywords: MH/Ni batteries LaFeO3 RGO Composite Electrochemical property LaFeO3-xwt% rGO composite (x = 8, 10, 12) was synthesized by ultraphonic stirring and lyophilization method SEM, TEM and XRD results show that the perovskite-type LaFeO3 was dispersed by rGO to form special porous structure due to the gauze-shaped wrinkles and folds structure of rGO It was found that the special porous structure can effectively increase the specific surface area and suppress particle aggregation of LaFeO3, thus improving the electrical conductivity and appreciably enhancing the electrochemical properties of LaFeO3 As compared with LaFeO3, the maximum discharge capacity of the composite (x=10) increased from 209.5 mAh g–1 to 334.6 mAh g–1 The High rate dischargeability at a discharge current density of 1500 mA g–1 (HRD1500) and the capacity retention rate after 100 charge/discharge cycles (S100) of the composite increased by 9% and 17%, respectively Introduction The perovskite-type oxides LaFeO3 have been studied as negative electrode materials of the nickel/metal hydride (MH/Ni) secondary battery at high-temperature [1], since they are facile to be synthesized, low price, environmental friendly, and excellent in electrochemical performances at high temperature [2] So they can make up for the deficiency of traditional hydrogen storage alloys (such as AB5 [3,4], AB2 [5], AB [6], and Mg-based alloys [7,8]) and meet some specialized power tools for hybrid electrical vehicles and military devices, which have to work at relatively high temperatures (40–80 °C) [9–11] Shen et al [12] prepared perovskite-type oxide LaFeO3 using stearic acid combustion method and found that it showed great discharge capacities at high temperatures They reported that the LaFeO3 had similar hydrogen storage mechanism with the traditional hydrogen storage alloys, and deduced it as the following formula: LaFeO3− δ + x H2 O + xe−⟷LaFeO3− δ Hx + x OH − Although LaFeO3 has many good characteristics as negative materials of MH/Ni secondary battery working at high temperatures, unfortunately, some inherent limiting factors weaken their practical application Firstly, the poor intrinsic conductivity of LaFeO3 leads to a relatively high resistance during the charge transferring process, and whittles down their electrochemical properties Secondly, as nanostructure perovskite-type oxides, the LaFeO3 particles are easy to aggregate during the charge and discharge process, which decreases the contact area between the electrodes and the electrolyte and sequentially restrains the electrochemical properties to a great extent [10] In order to find a way improving the performance of the perovskitetype oxides LaFeO3, Deng et al [13] successfully prepared La1−xSrxFeO3 (x = 0.2, 0.4) and by means of doping Sr in the A site to change the bulk compositions by altering elements match, and the electrochemical properties were apparently improved In order to find another way to modify the perovskite-type oxides LaFeO3 more easily, Pei et al [14,15] used to ameliorate the electrochemical properties of perovskite-type oxides LaFeO3 through carbon and carbon–polyaniline hybrid coating methods to get a thin layer of carbon or polyaniline shell on the surface of the particles LaFeO3 This obviously suppressed particle aggregation and improved the electrical conductivity during the charge and discharge process, and the carbon and polyaniline shell can also avoid the electrode corrosion from the electrolyte and ameliorate the electrochemical properties of LaFeO3 Hu et al [16] composited the perovskite-type oxides LaMnO3 with graphene for zincair battery using sol-gel process assisted with chelating effect of citric acid, in regards to the excellent physical and chemical properties of the graphene, such as high conductivity, high surface area, and stable chemical property [17,18] Results show that those composites possess excellent electrocatalytic activity for oxygen reduction reaction (ORR) and good electrochemical stability in alkaline medium ☆ Peer review under responsibility of Chinese Materials Research Society E-mail address: hanshm@ysu.edu.cn (S Han) http://dx.doi.org/10.1016/j.pnsc.2017.01.004 Received 10 October 2016; Accepted 30 November 2016 1002-0071/ © 2017 Chinese Materials Research Society Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/) Please cite this article as: Han, S., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.01.004 Progress in Natural Science: Materials International (xxxx) xxxx–xxxx Y Yuan et al In this work, we prepare LaFeO3-rGO composites with LaFeO3 and rGO using ultraphonic stirring and lyophilization method The microstructure, electrochemical properties and kinetic performance of the composites were systematically investigated Experimental 2.1 Sample preparation and characterization LaFeO3 was prepared by stearic acid combustion method in the muffle furnace [15–18], and the compound was calcined in the tube furnace at 1200 °C for h at Ar atmosphere Graphene oxide was put in a beaker with deionized water to form graphene oxides water suspension The suspension was subjected to ultrasonic vibration for h, and then the reducing agent ascorbic acid was put in the suspension and reacted at room temperature for 24 h with continuous stirring The LaFeO3 was put in the suspension with continuous stirring for another 24 h and was dried in the lyophilizer till the water evaporated The composite was heat-treated in the tube furnace for h at 600 °C to form the LaFeO3- xwt% rGO (x = 8, 10 and 12) composite ultimately The phase structure characterization was performed using X-ray powder diffraction (XRD) with a Rigaku D/Max 2500PC X-ray diffractometer (Cu Kα radiation) The patterns were recorded over the 2θ range from 10° to 90° The surface morphology of the composites was investigated using transmission electron microscopy (TEM, JEOL-2010) Fig XRD patterns of the LaFeO3-xwt% rGO (x = 0, 8, 10 and 12) composites other hand, the LaFeO3 that was not composited with rGO just agglomerated together during the preparation process and the specific surface area was limited, restricting the electrochemical properties of LaFeO3 From the TEM images of LaFeO3 and the LaFeO3−10 wt % rGO composite, it can be obviously seen that the LaFeO3 composited with rGO showed pellucid in the fringe, which may be due to the thin rGO crosslinking both in and out the LaFeO3 This indicates that LaFeO3 was composited with rGO together and rGO was enwinding around the LaFeO3 2.2 Electrochemical and kinetic measurements 3.2 Electrochemical and kinetic characteristics The electrochemical properties were tested in a DC-5 battery testing instrument 0.15 g electric-active compound and 0.75 g carbonyl nickel powders of 200–400 meshes were mixed in a agate mortar and fully grinded Then the mixture was cold-pressed into a pellet of 10 mm in diameter and mm in thickness under the pressure of 15 MPa to get an electrode tablet The electrochemical measurements were performed at 60 °C The composite worked as a working electrode and a Ni(OH)2/ NiOOH electrode was a counter electrode, and mol/L KOH solution was used as the electrolyte During the activating process, the batteries were fully charged at a current density of 60 mA g–1 for h and the same current density for the discharge process to a cut-off voltage of 0.3 V The CR was tested by that the batteries were fully charged at a current density of 300 mA g−1 and then was laid until for 24 h at 60 °C to test the residual discharge capacity The electrochemical kinetic measurements were performed in a tri-electrode system and a reference electrode (Hg/HgO) was added The anode polarization curves, potential static step discharge, and electrochemical impendence spectroscopy were performed as we reported previously [7] High temperature proton perovskite oxides are known as to have high electric capacity at high temperature (40–80 °C) [2] Fig shows the discharge curves of LaFeO3 and the LaFeO3-xwt% rGO (x=8, 10 and 12) composites at 60 °C It indicated that when LaFeO3 was composited with rGO, it has high electrochemical capacity, the discharge curves show a long and horizontal potential plateau compared to pristine LaFeO3, and its maximum discharge capacity increased from 209.5 mAh g–1 to 334.6 mAh g–1 (rGO content was 10%) This is probably for the reasons that rGO improved the surface conductivity of LaFeO3 due to the special outstanding physicochemical properties of rGO and the special porous structures that improved the discharge capacity of the LaFeO3 as anode of MH/Ni secondary battery Charge retention (CR) is also a very important property for secondary batteries It represents the ability for the battery to keep charge during the use process The CR of LaFeO3 and the composites were displayed in Table According to our measurements reflected in Table 1, the CR of LaFeO3 composite with rGO was considerably high, approximately reaching to 97.71% (rGO content was 10%), compared to 90.22% of LaFeO3 without rGO This is probably due to that when the rGO crosslinking both in and out the LaFeO3, it effectively restrained the charge overbrim from the electrode to the electrolyte and improved its charge retention ability Fig shows the high rate dischargeability (HRD) HRD reflects the discharge capability at large discharge current densities and can be dominated by the charge transfer rate at the electrode surface and the diffusion rate of hydrogen atoms in the bulk of the electrode [19] HRD can be calculated by the following equation: Results and discussions 3.1 LaFeO3 and composite characterization Fig shows the XRD patterns of the LaFeO3 and the LaFeO3- xwt % rGO (x = 0, 8, 10 and 12) composites We can see from the patterns that when the perovskite-type oxides LaFeO3 were composited with rGO, they still had the same perovskite-type structure compare to the standard LaFeO3 patterns in the JCPDS card (JCPDS No 37–1493) The results imply that the composition process didn’t alter its crystal structure Fig shows the SEM and TEM images of LaFeO3 and the LaFeO3−10 wt% rGO composite We can see from the SEM images that LaFeO3 was wrapped by the rGO and the special porous structure was formed due to the gauze-shaped wrinkles and folds structure of rGO It has been found that the specific surface area of the composite was increased and then the proton exchange property between the surface of the composite and the electrolyte was improved On the HRD = Cd / C max (1) × 100% −1 Cd is the discharge capacity at the current density of 1500 mA g , Cmax is the discharge capacity at the current density of 60 mA g−1 According to Fig 4, LaFeO3 composited with rGO have high performance in comparison with those without rGO This was could be ascribed that when LaFeO3 was composited with rGO, its specific surface area was greatly enhanced and that could maximize the surface conductive electroactive to improve the reaction kinetics of electrode materials Progress in Natural Science: Materials International (xxxx) xxxx–xxxx Y Yuan et al (a) (b) 4μm μm (c) (d) 200 nm 100 nm Fig SEM and TEM images of of the LaFeO3-xwt% rGO (x = 0, 8, 10 and 12) composites: (a) SEM of LaFeO3; (b) SEM of LaFeO3−10 wt% rGO; (c) TEM of LaFeO3; (d) TEM of LaFeO3−10 wt% rGO 1.4 Table Electrochemical properties of the LaFeO3-xwt% rGO (x = 0, 8, 10 and 12) composites 1.2 Voltage (V) 1.0 x = 12 x = 10 x= x= 0.8 0.6 0.2 50 100 150 200 250 300 Cmax (mAh g−1) HRD1500 (%) CR (%) S100 (%) x=0 x=8 x=10 x=12 209.5 304.5 334.6 311.1 33.82 34.42 42.32 38.95 90.22 94.31 97.71 96.34 72.19 84.86 88.84 80.94 maximum discharge capacity within 100 charge-discharge cycles We can see from Table that the S100 of the composites were 88.84%, 84.86%, 80.94% (rGO content was 10%, 8%, 12%), respecitvely, compared to 72.19% of the LaFeO3 without rGO The value of S100 almost increased by 17% The recession of the discharge capacity was ascribed to the corrosion by the electrolyte and aggregation of the oxide itself during the charge/discharge process This phenomenon can be retarded and restrained by compositing with rGO according to the experimental date above This is probably due to that rGO has high mechanical strength and is stable, when it composited with LaFeO3 the oxide corroded by the electrolyte during the charge and discharge process could be prevented and the agglomeration of the oxide also could be restrained Fig shows the electrochemical impedance spectrums of LaFeO3 and the LaFeO3−10 wt% rGO composite, and they contain three parts: a small semi circle at the high frequency (Rct: the contact resistance 0.4 0.0 -50 Samples 350 Discharge capacity (mAhg -1) Fig Discharge curves of the LaFeO3-xwt% rGO (x = 0, 8, 10 and 12) composites The rGO itself owns high conductivity, high surface area and stable chemical property, and therefore promotes the stability of the electrode at high rate dischargeability The capacity retentions of the samples at the 100th cycle (S100) were listed in Table and it can be calculated by C100/Cmax100%, here C100 is the discharge capacity after 100 cycle number, Cmax is the Progress in Natural Science: Materials International (xxxx) xxxx–xxxx Y Yuan et al -3.0 100 LaFeO 90 0.12 0.08 0.1 -2.5 70 Log i(A) HRD (%) 80 60 -2.0 LaFeO3-rGO LaFeO3 -1.5 50 40 -1.0 30 200 400 600 800 1000 1200 1400 1600 -1.6 -1 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 Potential (V,vs.Hg/HgO) Discharge current density (mA g ) Fig Tafel polarization curves of the LaFeO3-xwt% rGO (x = 0,10) composites Fig HRD of the LaFeO3-xwt% rGO (x = 0, 8, 10 and 12) composites 0.2 2.5 Current density (A) LaFeO rGO-LaFeO Z''/ohm 2.0 1.5 1.0 0.1 0.0 -0.1 LaFeO -rGO -0.2 LaFeO 0.5 -0.3 0.0 0.0 0.5 1.0 1.5 2.0 2.5 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 3.0 0.0 0.2 Overpotential (V) Z'/ohm Fig Cyclic voltammograms of the LaFeO3-xwt% rGO (x = 0,10) composites Fig EIS of the LaFeO3-xwt% rGO (x = 0, 10) composites Fig shows the cyclic voltammetry curves of LaFeO3 and the LaFeO3−10 wt% rGO The reduction and oxidation peak represent the kinetic properties of the electrode, and the peak area represents the hydrogen storage capacity for the electrode The oxidation peak current of hydrogen of the anode and the peak area in the cyclic voltammetry curves for the composite were improved a lot compared to LaFeO3 We can see from Fig that the hydrogen adsorption peak improved a lot obviously at −0.9 ˜ −0.7 V (vs Hg/HgO) for the composite compared to LaFeO3 This is probably due to the reason that when rGO composited on the LaFeO3 its surface structure can be changed to form honeycomb texture and accelerate the absorption of hydrogen during the charge and discharge process Fig shows the linear polarization curves of LaFeO3 and the LaFeO3−10 wt% rGO composites We can see from Fig that when the overpotential range from −5 to mV the polarizing current has linear relationship with overpotential, and the slope of the linear polarization curve of LaFeO3 with rGO composite increases a lot compared with original LaFeO3 The exchange current density I0 can be calculated according to the following equation [19]: between the granule of LaFeO3), a large semi circle in the intermediate frequency area (Rcp: the charge transfer resistance between the electrode surface of LaFeO3), and an oblique line of the low-frequency region (Rel: the diffusion impedance of hydrogen in the LaFeO3) [20] The electrochemical impedance spectrum is fitted by Z-View program software along with equivalent circuit The results show that the decreased Rct of LaFeO3-rGO composite reflected from the decrease in radius of the small semi circle in the intermediate frequency It means that when LaFeO3 was composited with rGO, the Rct decreased benefiting from the excellent electrical conductivity The conductivity of the composite was improved a lot in comparison with that of LaFeO3, and therefore, the HRD of the composite was remarkably improved We can also see from the spectrum that the Rcp also reduced a lot, which is probably due to that when the rGO composited with LaFeO3, they have triaxial network frame porous structure according to the SEM images Therefore they have more specific surface area and improve the charge transition from the electrode surface to the electrolyte Fig shows the tafel polarization curves of LaFeO3 and the LaFeO3−10 wt% rGO composites It represents the anti-corrosion properties of the electrode and the tafel curve was got from the potentiodynamic polarization test by electrochemical workstation and analyzed using CHIE604E electrochemical analyzer The corrosion currents for LaFeO3 and the LaFeO3−10 wt% rGO composites were 133.75 mA cm–1 and 19.31 mA cm−1, respectively This is probably attributed to the fact that when LaFeO3 was composited with rGO, it was crosslinked intertwined both in and out of the LaFeO3 and then their anti-corrosion properties was improved a lot by the protection of rGO I0 = IRT Fη (2) where I is polarized current density (mA g–1), R is gas constant (R = 8.314 J (mol K)–1), T is the absolute temperature (K), F is faraday constant (F = 96485 C mol–1), η is over potential (mV), and the result shows that the I0 of LaFeO3 with rGO composite reaches to 233.23 mA g–1 compared to the original LaFeO3, 171.13 mA g–1 These signify that the ability of charge transfer improved a lot with rGO composite for LaFeO3, and this result was identical with the EIS tested above Progress in Natural Science: Materials International (xxxx) xxxx–xxxx Y Yuan et al 100 Acknowledgement LaFeO3-rGO This work was financially supported by the National Natural Science Foundation of China (NOs 51571173 and 21303157), and the Natural Science Foundation of Hebei Province (NO B2014203114) LaFeO3 -1 Current density (mA g ) 80 60 References 40 [1] [2] [3] [4] 20 -6 [5] -4 -2 [6] Overpotential (mV) [7] Fig Linear polarization curves of the LaFeO3-xwt% rGO (x = 0,10) composites [8] [9] [10] [11] [12] Conclusions LaFeO3-xwt% rGO composites (x = 8, 10 and 12) with a spatial porous structure were prepared using LaFeO3 and rGO by ultrasonics, stirring, and lyophilization method The LaFeO3 was crosslinking intertwined around by rGO and formed a special porous structure The electrochemical properties of the LaFeO3-rGO composite were significantly improved In compared with LaFeO3, the maximum discharge capacity of the LaFeO3−10 wt% rGO composite was elevated from 210 mAh g–1 to 335 mAh g–1, HRD1500 was improved from 33.8% to 42.3%, and the I0 increased from 171.1 mA g–1 to 233.2 mA g–1 And the capacity retention (CR) of the composite increased by 8% The improvement in electrochemical properties of LaFeO3 can be attributed to the increasing conductivity and inhibiting particle aggregation in composites by rGO [13] [14] [15] [16] [17] [18] [19] [20] T Esaka, H Sakaguchi, S Kobayashi, J Solid State Ion 166 (3) (2004) 351–357 G Deng, Y Chen, M Tao, et al., J Electrochim Acta 55 (3) (2010) 884–886 Y Wang, M Zhao, J Int J Hydrog Energy 37 (4) (2012) 3276–3282 C Wan, H Yan, X Ju, et al., J Int J Hydrog Energy 37 (17) (2012) 13234–13242 M.Y Song, D Ahn, I.H Kwon, et al., J Electrochem Soc 148 (9) (2001) A1041–A1044 H Yukawa, Y 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Electrochem Soc 161 (12) (2014) A1844–A1850 J Liu, S Han, Y Li, et al., J Electrochim Acta 111 (2013) 18–24 ... images of of the LaFeO3- xwt% rGO (x = 0, 8, 10 and 12) composites: (a) SEM of LaFeO3; (b) SEM of LaFeO3? ??10 wt% rGO; (c) TEM of LaFeO3; (d) TEM of LaFeO3? ??10 wt% rGO 1.4 Table Electrochemical properties. .. EIS of the LaFeO3- xwt% rGO (x = 0, 10) composites Fig shows the cyclic voltammetry curves of LaFeO3 and the LaFeO3? ??10 wt% rGO The reduction and oxidation peak represent the kinetic properties of. .. shows the discharge curves of LaFeO3 and the LaFeO3- xwt% rGO (x=8, 10 and 12) composites at 60 °C It indicated that when LaFeO3 was composited with rGO, it has high electrochemical capacity, the