• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    恒電位氧化改性石墨氈及其氧還原電極的電化學性能

    2017-09-06 11:30:05何夢嬌閆康平王貴欣孫羽涵鐘宜霏羅春暉
    無機化學學報 2017年2期
    關鍵詞:伏安電位電化學

    何夢嬌 閆康平 王貴欣 孫羽涵 鐘宜霏 羅春暉

    (四川大學化學工程學院,成都610065)

    恒電位氧化改性石墨氈及其氧還原電極的電化學性能

    何夢嬌 閆康平*王貴欣 孫羽涵 鐘宜霏 羅春暉*

    (四川大學化學工程學院,成都610065)

    分別采用循環(huán)伏安改性法和恒電位氧化法對石墨氈進行改性處理,并采用循環(huán)伏安法對其電化學性能進行研究,實驗結果表明,恒電位氧化改性較循環(huán)伏安改性的石墨氈有較好的氧還原活性。通過XRD、FTIR、接觸角和CV針對恒電位氧化處理石墨氈進行了進一步的測試。測試結果顯示,隨恒電位氧化時間的增加,石墨氈表面親水性含氧官能團增加,潤濕性增強。恒電位氧化改性處理25 min的石墨氈氧還原峰電位及電流密度分別為~-0.43 V和~0.003 4 mA·cm-2,顯示出很好的電化學催化性能?;谝陨辖Y果,恒電位氧化法改性處理能夠極大提高石墨氈的氧陰極活性。

    氧陰極;石墨氈;改性;恒電位氧化;循環(huán)伏安

    0 Introduction

    Lithium-oxygen(Li-O2)batteries occupy a leading position in the electric vehicle power competition,in which the substance O2involved in the positive reaction comes from the air outside instead of storing an oxidizer internally with the notable features of high energy density and no pollution[1-4].The battery shows a theoretical specific energy of 1.1×104Wh·kg-1, which is 5~10 times[1-6]more than that of the state-of-the-art Li-ion batteries.However,Li-O2batteries are plaguedbymanyproblems,especiallythelittle catalyticeffectsofthecathodematerialused currently,which limit its rapid development to a great extent[7-9].

    Graphite felt has the advantages of high corrosion resistance,largespecificsurfacearea,excellent conductivity and good thermal stability,which is suitable for applying as the electrode material of Li-O2batteries.However,the electrode prepared directly with graphite felt exhibits a poor electrochemical performance due to the low surface energy of graphite fiber and the existence of large amount of hydrophobic groups[10].

    Itwasreportedthatthesurfaceenergyof graphitefeltcouldbeenhancedbydifferent modifications.Guan et al.[11]investigated the effects of the heat treatment on different carbon materials,and concluded that the electrochemical properties of the treated carbon were improved significantly.Zhong[12], Sun et al.[13-14]foundthattheacidand heat treatment on graphite felt could greatly improve the surface activity of carbonLiu et al.[15]used the modifiedgraphitefeltsastheelectrodeofthe vanadiumbatteries,andfoundthattheelectrochemical performances of the graphite felts modified by electrochemical oxidation were better than that by acid and heat treatments.

    Presently,most of the researches are focused on the effects of the current density on the generation of surfacefunctionalgroupsandoftheelectrolyte concentration on the anodic oxidation degree in the method of galvonostatic or potentiostatic oxidation[16-18]. In this work,the graphite felts were respectively modified by cyclic voltammetry and potentiostatic oxidation,and the electrochemical performances of the modified materials in aqueous solution were studied. The influences of the potentiostatic oxidation duration on wettability,functional groups and the electrochemical properties were investigated.It was shown that the electrocatalytic performance of the graphite felt treated by potentiostatic oxidation was improved obviously,andtheoptimaloxidationtimewas determined.

    1 Experimental

    The Graphite felts obtained by polyacrylonitrile graphitization with the carbon purity of≥99.5%were cut into 5 cm×6 cm wafers with the thickness of 2 mm,further ultrasonic cleaned with anhydrous ethanol and distillated water,and finally dried at 70℃for 24 h in an electric thermostatic drying oven.

    Then the graphite felt wafers were respectively modified at room temperature by cyclic voltammetry (CV)in the potential range of 2.0~-0.01 V with the scan rate of 10 mV·s-1and potentiostatic oxidation (PO)at 2.0 V on a PAR273A Potentiostat/Galvanostat and a 5210 lock-in amplifier controlled by Powersuite software(Princeton Applied Research,USA),in which the as-prepared graphite felt wafer,the graphite electrode,the saturated calomel electrode(SCE)and 1 mol·L-1H2SO4solution were used as the working electrode(WE),thecounter electrode(CE),the reference electrode(RE),and the electrolyte,respectively.

    The crystalline structures of the graphite felts were examined by X-ray diffraction(XRD on a Philips X′Pertpro MPD)with Cu Kα radiation(λ=0.154 06 nm,U=40 kV,I=40 mA)ranging from 10°to 70°at a scan rate of 0.04°·s-1.The functional groups on the surface of the graphite felts were evaluated by Fourier transform infrared spectroscopy(FTIR)on a thermo Nicolet Magna IR 560 spectrometer in the range of 900~3 800 cm-1.The hydrophilicity of the graphite felts was analyzed by sessile drop contact angle measurementusingacontactanglemeasuring apparatus(JC2000C1,Shanghai Zhongchen digital technic apparatus Co.,Ltd).

    The electrochemical performances of the graphite felt materials were examined by cyclic voltammetric (CV)measurements using the same apparatus for the modification introduced above,except that the counter electrode was replaced by a platinum net.The CV measurements of the pristine graphite felts and the CV modified samples were carried out in the potential range of-0.8 and+0.6 V,and the graphite felts modified by PO were carried out in the potentialrange of-0.9 and+0.4 V,which were performed at room temperature with a scan rate of 10 mV·s-1.

    The graphite felts involved in CV measurements were partly sealed with wax to remain an effective area of 1 cm×0.5 cm exposed.All electrodes were mounted in an airtight container[11],and dipped in the electrolyte of 0.1 mol·L-1LiOH solution,which was saturated by oxygen flushing for 30 min before CV measurements.

    2 Results and discussion

    2.1 XRD characterization of the raw graphite felt

    The XRD pattern of the pristine graphite felt is shown in Fig.1,which exhibits a sharp diffraction peak at~26°,and two weak peaks at~43°and~53°, corresponding respectively to(002),(100)and(004) faces of the hexagonal structure[19-20],consisting well with the diffraction pattern of graphite(JPCDS:No.65-6212).

    Fig.1X-ray diffraction pattern of the pristine graphite felt

    2.2 Modification of the graphite felts

    2.2.1 Determination of the modification methods

    The cyclic voltammetric curves of the graphite felts with and without modification are given in Fig.2.

    The CV curves of both modified samples exhibit obvious reduction peaks compared to that of the untreated graphite felt,implying that the electrochemical reaction activity is improved by modification,which was also confirmed by Georgioua et al.[21]and Shao et al.[17].

    Fig.2Cyclic Voltammetric curves of graphite felts with different treatment(a)without modification; (b)mmodified by CV;(c)modified by PO for 25 min

    The reduction peak potentials and the reduction peak current densities of the CV modified sample along with the PO modified sample are~-0.39 V and~0.000 6 mA·cm-2along with~-0.43 V and~0.003 4 mA·cm-2,respectively.Apparently,the reduction peak potentials of the two modified samples are of the same order of the magnitude,whereas the reduction peak current density of the PO modified sample is~4.7 times higher than that of the CV modified.Theoretically,the reduction peak potential refers to the oxygen reduction reaction(ORR)activity,while the reduction peakcurrentdensitytotheamountofreduced oxygen[10-11].Higher current density implies more oxygen consumption,namely the PO modification is more effective.Therefore,potentiostaticoxidationwas determined as the modification method to perform the further investigations in this work.

    2.2.2 PO modification of the graphite felts

    The change of the current intensity with the potentiostatic oxidation time is shown in Fig.3.

    The current intensity descends sharply due to polarization within the beginning 61 s followed by a slight increase,and maintains nearly stable after 200 s.The area under the curve represents the electric consumption during oxidation process,which could be determined by integrating the equation of dQ=dt×dI. Apparently,with the increase of the modification time, the electric consumption increases correspondently,implying that the oxidation degree of graphite felt materials increases simultaneously.The mechanism is discussed in detail below in this article.

    Fig.3Current-time relationship curve during PO modification

    2.3 FTIR characterization of the graphite felt

    The dispersibility,conductivity and wettability of the graphite-felt electrode have remarkable effects on theelectrochemicalperformances,whichresults eventually from the surface functional groups on graphite felts[22-24].The effects of potentiostatic oxidation modifications on the surface functional groups werecharacterizedbyFouriertransforminfrared spectroscope in this work.The typical FTIR spectra of the graphite felts with and without potentiostatic oxidation(PO)treatment are shown in Fig.4.

    The FTIR spectrum of the pristine graphite felt exhibits two broad peaks at around 701 and 1 142 cm-1,which correspond to the out-of-plane C-H bending vibration and the C-O stretching vibration in COOH[25],respectively.All as-oxidized samples present extra five spectral peaks compared with the pristine graphite felt.The peaks at around 2 900 and 1 650 cm-1correspond to the existence of CH&CH2and stretching of C=C,respectively.The peak at around 1 480 cm-1is attributed to the stretching vibrations of asymmetric OH or COOH.The peaks ranging from 1 180 to 1 049 cm-1are the absorbing of C-O[24-28]. Apparently,some hydrophilic functional groups such as OH and COOH[27,29]are generated by modification, which was also confirmed by Yue et al.[30].A schematic description of the formation mechanism of the functional groups is given in Fig.5[31].It is reasonable to suggest that potentiostatic oxidation is an effective method to improve the wettability of the graphite felts.

    Fig.4FTIR spectra of the graphite felts modified by PO for(a)0 min;(b)15 min;(c)20 min;(d)25 min; (e)30 min

    The FTIR spectra of all treated samples show the similar absorption peaks wavenumbers as mentioned above,indicatingthatthetypesofthesurface functional groups of the modified graphite felts are also similar.However,the absorption peak intensities differ from each other of different oxidation times.The functional groups such as OH,CH and CH2exhibit small intensity variation,while the peak intensities of the C-O stretching and the stretching vibrations of asymmetricCOOHincreasedistinctlywiththe oxidation time increasing,implying that the amount of the corresponding hydrophilic functional groups suchas COOH increases.That means,the hydrophilicity of the graphite felts should also increase with the oxidation time increasing,which is further discussed below in this article.

    2.4 Contact angle characterization of the graphite felt surface

    The effects of the potentiostatic oxidation on the wettability of graphite felts were investigated by observing the contact angle,which is illustrated in Fig.6.It is shown that the contact angles of the untreatedandthemodifiedsamplesarequite different.The absorption capacity of the untreated graphite felt is relative low,whereas the graphite felt modified for 30 min absorbs the water completely. The effect of the oxidation time on the contact angle is summarized in Fig.7.

    Fig.6Contact angle characterization of graphite felts modified by PO for(a)0 min;(b)25 min;(c)30 min

    Fig.7Contact angle of graphite felts at different PO time

    The contact angle of the pristine sample as shown in Fig.7 is 146°,which exhibits the strong hydrophobic property,that agrees with the results reported by Sun et al.[13].The contact angles of the modifiedsamplesaresmallerthanthatofthe untreated sample.With the extension of oxidation time from 15 to 30 min,the contact angles reduced gradually.The graphite felt modified for 30 min shows the strongest hydrophilic property with the contact angle 0°.The change tendency of the contact angle withtheoxidationtimeindicatesthatthe hydrophilicity is improved by PO modification,and the hydrophilic degree ascends with the modification timeincreasing.ReferringtotheFTIRresults obtained above,it could be concluded thatthe extended modification time intensifies the oxidation degree,which results in the increase of the wettability and the number of the hydrophilic functional groups on the surface of graphite felt.

    2.5 Electrochemical behavior

    Tofurtherevaluatetheelectrochemical performances of the modified graphite felts,the cyclic voltammetrymeasurementswereperformed.The cyclic voltammetric curves of the samples with and without potentiostatic oxidation are shown in Fig.8. The oxygen reduction potentials and the corresponding current densities of the reduction peaks for the graphite felts modified by potentiostatically oxidizingfor 15,20,25,30 min respectively are presented in Fig.9.

    Fig.8Cyclic Voltammetric curves of graphite felts modified by PO for(a)0 min,(b)15 min, (c)20 min,(d)25 min and(e)30 min

    Fig.9Change of the reduction peak potentials and the current densities with PO time

    No oxygen reduction peaks appear on the cyclic voltammetriccurveofthepristinegraphitefelt, implying that the ORR activity of the untreated sample is relative poor due to its strong hydrophobicity as discussed above.All cyclic voltammetric curves of the modified graphite felts exhibit the reduction peaks,but the corresponding reduction peak potentials and current densities are relative different. The reduction peak potential and the current density of the graphite felt modified for 15 min are the lowest. With the modification time increasing from 15 to 25 min,the oxygen reduction peak potentials change from~-0.49 to~-0.43 V representing the reducing of polarization,and then the current densities increase from~0.002 4 to~0.003 4 mA·cm-2gradually, indicating that the electrochemical performances of the modified graphite felts are improved with the modification time increasing.However,both oxygen reduction peak potential and current density of the material modified for 30 min exhibit the contrary change tendency,suggesting that the ORR activity of thegraphitefeltisdegradedforthelongtime oxidation.

    Not only the amount but also the type of the oxygen-containing functional groups on the surface of thegraphitefeltsincreaseafterpotentiostatic oxidation.Furthermore,with the modification time increasing,the amount of the hydrophilic functional groups increases correspondingly and the hydrophilicity of the graphite felt is accordingly improved[16].The hydrophilic oxygen-containing groups are redox-active, which are able to directly participate in the electrochemical reaction.Therefore,the modified graphite felts exhibit the oxygen reduction reaction activity, which was confirmed by the existence of the reduction peaks on CV curves[32].However,with the oxidation time increasing over 25 min,the ORR activity decreases,indicating that excessive hydrophilic functional groups have negative effects on the electrochemical performances.An interpretation of the phenomenon is schematically described in Fig.10.The surface of the unmodified graphite felts is mostly covered by oxygen due to its low wettability as shown in Fig.10(a),which ORR activity is very poor.The hydrophilic functional groups on the surface obtained by PO modification make the graphite felts to possess the electrochemical reaction activity.At the same time,oxygen as the reactant takes also part in the reaction,where the oxygen channels must be provided to ensure the contact between oxygen and the graphite felts as illustrated in Fig.10(b).Over high wettability resulted from the longtime oxidation causes that most area of the surface is surrounded by water,which blocks the contactofoxygenwiththegraphitefelts,andaccordingly hinders the electrochemical reactions[33]as shown in Fig.10(c).Consequently,the optimal potentiostatic oxidation is supposed to not only increase the amount of the hydrophilic functional groups on the surface but also remain the enough space for oxygen to pass through the water layer and contact with the electrode materials.The graphite felts modified by potentiostatically oxidizing at 2.0 V for 25 min exhibits the preferable electrochemical performance in this work,which is determined as the optimal PO modification conditions.

    Fig.10Schematic description of the solid-liquid interfaces(a)hydrophobic surface; (b)partially hydrophilic surface;(c)hydrophilic surface

    3 Conclusions

    Theoptimalmodificationconditionsforthe graphite felts as the electrode of Li-O2batteries were determined in this work.Both potentiostatic oxidation modification and cyclic voltammetric treatment are able to improve the electrochemical performances of graphite felts.The graphite felts modified by potentiostatic oxidation are more electrochemical active than that treated by cyclic voltammetry.The improvement of the oxygen reduction reaction activity for the PO modified graphite felts is attributed to the increase of the hydrophilicity,owing to the formation of the hydrophilic oxygen-containing functional groups on the surface.Furthermore,the amount of the hydrophilicfunctionalgroupsincreaseswiththePO modification time increasing.However,over high wettability from long time oxidation results in the contact difficulty between oxygen and the graphite felt electrode,and accordingly hinders the electrochemical reactions.As a result,the graphite felts modified by potentiostatically oxidizing at 2.0 V for 25 min exhibits the optimal electrochemical performances.

    [1]Abraham K M,Jiang Z.J.Electrochem.Soc.,1996,143(1):1-5

    [2]Girishkumar G,Mccloskey B,Luntz A C,et al.J.Phys. Chem.Lett.,2010,1(14):2193-2203

    [3]Kraytsberg A,Ein-Eli Y.J.Power Sources,2011,196(3):886-893

    [4]Liu T,Leskes M,Yu W,et al.Science,2015,350(6260):530-533

    [5]Kuboki T,Okuyama T,Ohsaki T,et al.J.Power Sources, 2005,146(1/2):766-769

    [6]Cheng F,Chen J.Chem.Soc.Rev.,2012,41(6):2172-92

    [7]Yuasa M,Matsuyoshi T,Kida T,et al.J.Power Sources, 2013,242(35):216-221

    [8]Débart A,Bao J,Armstrong G,et al.J.Power Sources,2007, 174(2):1177-1182

    [9]Rychcik M,Skyllas-Kazacos M.J.Power Sources,1988,22 (1):59-67

    [10]Lu Y,Li W,Sun F,et al.Carbon,2010,48(11):3079-3090

    [11]Guan P,Wang G,Luo C,et al.Electrochim.Acta,2014,129 (16):318-326

    [12]Zhong S,Padeste C,Kazacos M,et al.J.Power Sources, 1993,45(1):29-41

    [13]Sun B,Skyllas-kazacos M.Electrochim.Acta,1992,37(7): 1253-1260

    [14]Sun B,Skyllas-kazacos M.Cheminform,1992,23(49):18

    [15]LIU Di(劉迪),TAN Ning(譚寧),HUANG Ke-Long(黃可龍), et al.Chinese J.Power Sources(電源技術),2006,30(3):224-223

    [16]Ishifune M,Suzuki R,Mima Y,et al.Electrochim.Acta, 2005,51(1):14-22

    [17]Shao Y,Yin G,Zhang J,et al.Electrochim.Acta,2006,51 (26):5853-5857

    [18]Noel M,Santhanam R.J.Power Sources,1998,72(1):53-65

    [19]Lee G W,Kim J,Yoon J,et al.Thin Solid Films,2008,516 (17):5781-5784

    [20]CHEN Teng-Yuan(陳騰遠),ZHANG Chen-Jun(陳晨軍),LI Zai-Jun(李在均),et al.Chinese J.Inorg.Chem.(無機化學學報),2014,30(12):2691-2698

    [21]Georgiou P,Walton J,Simitzis J.Electrochim.Acta,2010, 55(3):1207-1216

    [22]Seredych M,Hulicova-Jurcakova D,Gao Q L,et al.Carbon, 2008,46(11):1475-1488

    [23]Qiao W,Korai Y,Mochida I,et al.Carbon,2002,40(3):351-358

    [24]Nian Y R,Teng H.J.Electroanal.Chem.,2003,540(2):119-127

    [25]Mawhinney D B,Naumenko V,Kuznetsova A,et al.J.Am. Chem.Soc.,2000,122(10):2383-2384

    [26]El-Hendawy A N A.J.Anal.Appl.Pyrolysis,2006,75(2): 159-166

    [27]Szabó T,Berkesi O,Forgó P,et al.Chem.Mater.,2006,18 (11):2740-2749

    [28]Szabó T,Tombácz E,Illés E,et al.Carbon,2004,44(3):537-545

    [29]Li L,Quinlivan P A,Knappe D R U.Carbon,2002,40(12): 2085-2100

    [30]Yue Z R,Jiang W,Wang L,et al.Carbon,1999,37(11):1785-1796

    [31]HUANG Qiao(黃橋),SUN Hong-Juan(孫紅娟),YANG Yong -Hui(楊勇輝).Chinese J.Inorg.Chem.(無機化學學報), 2011,27(9):1721-1726

    [32]Frackowiak E,Béguin F.Carbon,2001,39(6):937-950

    [33]Moreira J,Ocampo A L,Sebastian P J,et al.Int.J.Hydrogen Energy,2003,28(6):625-627

    Electrochemical Performance of Graphite Felts Modified by Potentiostatic Oxidization for Oxygen Reduction Cathode

    HE Meng-JiaoYAN Kang-Ping*WANG Gui-XinSUN Yu-HanZHONG Yi-YeiLUO Chun-Hui*
    (College of Chemical Engineering,Sichuan University,Chengdu 610065,China)

    The graphite felts were respectively modified by cyclic voltammetry(CV)and potentiostatic oxidation (PO),which electrochemical performances were evaluated by cyclic voltammetric experiments.As a result,PO modification is more effective on improving the oxygen reduction reaction(ORR)activity of the graphite felts than CV treatment.The PO modified graphite felts were further investigated by XRD,FTIR,Contact angle and CV.It is found that the wettability of the graphite felts increases with the increase of potentiostatic oxidation time,due to the increase of the hydrophilic oxygen-containing functional groups on surface.The graphite felt modified by PO for 25 min in this work exhibits the preferable electrochemical performances with the reduction potential~-0.43 V and the current density~0.003 4 mA·cm-2of the reduction peak on CV curve.Consequently,potentiostatic oxidation is an effective and feasible treatment for improving the electrochemical properties of the graphite felts as the electrode material of Li-O2batteries.

    oxygen cathode;graphite felts;modification;potentiostatic oxidation;cyclic voltammetry

    O613.71

    A

    1001-4861(2017)02-0315-08

    10.11862/CJIC.2017.018

    2016-05-18。收修改稿日期:2016-10-27。

    *通信聯(lián)系人。E-mail:cyankp@scu.edu.cn,luochunhui@scu.edu.cn

    猜你喜歡
    伏安電位電化學
    用伏安法測電阻
    電位滴定法在食品安全檢測中的應用
    電化學中的防護墻——離子交換膜
    關于量子電化學
    電化學在廢水處理中的應用
    Na摻雜Li3V2(PO4)3/C的合成及電化學性能
    電鍍廢水處理中的氧化還原電位控制
    淺談等電位聯(lián)結
    基于LABVIEW的光電池伏安特性研究
    電子制作(2016年23期)2016-05-17 03:53:41
    通過伏安特性理解半導體器件的開關特性
    在线天堂中文资源库| 美女主播在线视频| 久久久久久伊人网av| 色视频在线一区二区三区| 国产精品99久久99久久久不卡 | 亚洲,欧美精品.| 亚洲欧美精品自产自拍| 日韩制服骚丝袜av| 国产免费视频播放在线视频| 日韩一区二区三区影片| 日韩在线高清观看一区二区三区| 一级片免费观看大全| 日韩大片免费观看网站| 丰满迷人的少妇在线观看| 精品少妇久久久久久888优播| 热re99久久国产66热| 午夜福利影视在线免费观看| 精品少妇内射三级| 日本猛色少妇xxxxx猛交久久| 校园人妻丝袜中文字幕| 久久久久人妻精品一区果冻| 日韩av免费高清视频| 久久热在线av| 国产精品熟女久久久久浪| 毛片一级片免费看久久久久| 26uuu在线亚洲综合色| 七月丁香在线播放| 久久久久久久久免费视频了| 夫妻午夜视频| 久久午夜福利片| 97精品久久久久久久久久精品| 丝袜喷水一区| 看免费成人av毛片| 亚洲一码二码三码区别大吗| 街头女战士在线观看网站| 成人国语在线视频| 国产精品女同一区二区软件| 免费日韩欧美在线观看| 飞空精品影院首页| 丁香六月天网| 80岁老熟妇乱子伦牲交| 五月开心婷婷网| 欧美日本中文国产一区发布| 女性生殖器流出的白浆| 久久久久久久久久久免费av| 女性生殖器流出的白浆| 男女下面插进去视频免费观看| 波多野结衣一区麻豆| 免费久久久久久久精品成人欧美视频| 国产免费视频播放在线视频| 亚洲精品成人av观看孕妇| 老汉色∧v一级毛片| 国产免费一区二区三区四区乱码| 麻豆精品久久久久久蜜桃| 亚洲精品第二区| 亚洲国产毛片av蜜桃av| 欧美老熟妇乱子伦牲交| 久久精品人人爽人人爽视色| 精品福利永久在线观看| 69精品国产乱码久久久| 欧美日韩亚洲国产一区二区在线观看 | 亚洲内射少妇av| 国产97色在线日韩免费| 男女国产视频网站| 有码 亚洲区| 一级毛片 在线播放| 两性夫妻黄色片| 高清黄色对白视频在线免费看| 天堂中文最新版在线下载| 亚洲欧美精品自产自拍| 日韩一本色道免费dvd| 香蕉国产在线看| 久久99一区二区三区| 女性生殖器流出的白浆| 九九爱精品视频在线观看| 高清欧美精品videossex| 一边亲一边摸免费视频| 亚洲成国产人片在线观看| 亚洲综合色网址| 制服人妻中文乱码| 校园人妻丝袜中文字幕| 最近的中文字幕免费完整| 久久久久久人妻| 黄色 视频免费看| 丁香六月天网| 亚洲,欧美,日韩| 一级,二级,三级黄色视频| 亚洲伊人久久精品综合| 男女国产视频网站| 国产精品熟女久久久久浪| 另类精品久久| 丁香六月天网| 亚洲国产日韩一区二区| 男女边摸边吃奶| av免费观看日本| 欧美日韩精品成人综合77777| 精品一区二区三区四区五区乱码 | 韩国精品一区二区三区| 80岁老熟妇乱子伦牲交| 免费高清在线观看日韩| 国产精品国产三级专区第一集| 日韩伦理黄色片| 亚洲精品久久久久久婷婷小说| 91国产中文字幕| 国产精品人妻久久久影院| 少妇人妻 视频| 午夜激情久久久久久久| 丝袜美足系列| 亚洲国产色片| 一级a爱视频在线免费观看| 免费看av在线观看网站| 国产成人精品一,二区| 国产日韩欧美在线精品| 亚洲美女搞黄在线观看| 欧美 日韩 精品 国产| 女的被弄到高潮叫床怎么办| 蜜桃国产av成人99| 韩国高清视频一区二区三区| 91精品伊人久久大香线蕉| 一二三四中文在线观看免费高清| 国语对白做爰xxxⅹ性视频网站| 97在线视频观看| 亚洲欧美成人综合另类久久久| 黑丝袜美女国产一区| 一本一本久久a久久精品综合妖精 国产伦在线观看视频一区 | videos熟女内射| 青春草视频在线免费观看| 精品亚洲成a人片在线观看| 久久精品熟女亚洲av麻豆精品| 欧美在线黄色| 少妇的逼水好多| 你懂的网址亚洲精品在线观看| 女人高潮潮喷娇喘18禁视频| 少妇的逼水好多| 黄频高清免费视频| 少妇人妻 视频| av国产精品久久久久影院| 欧美成人午夜精品| 一区二区日韩欧美中文字幕| 精品国产露脸久久av麻豆| 美女主播在线视频| 亚洲情色 制服丝袜| 国产黄色视频一区二区在线观看| 波多野结衣av一区二区av| 国产一区二区激情短视频 | 久久久精品94久久精品| 9191精品国产免费久久| av女优亚洲男人天堂| 波多野结衣av一区二区av| 人人妻人人澡人人爽人人夜夜| 国产一区亚洲一区在线观看| 蜜桃国产av成人99| 午夜福利网站1000一区二区三区| 亚洲精品久久午夜乱码| 在线观看一区二区三区激情| 国产一区二区三区av在线| 亚洲成人手机| 免费看不卡的av| 69精品国产乱码久久久| av不卡在线播放| 欧美变态另类bdsm刘玥| 免费在线观看黄色视频的| 精品久久久精品久久久| 久久精品熟女亚洲av麻豆精品| 中文欧美无线码| 毛片一级片免费看久久久久| 亚洲精华国产精华液的使用体验| 日韩大片免费观看网站| 蜜桃国产av成人99| 中文字幕另类日韩欧美亚洲嫩草| 国产精品女同一区二区软件| 日韩视频在线欧美| 看非洲黑人一级黄片| 中文字幕精品免费在线观看视频| 久久久久精品人妻al黑| www.精华液| 另类精品久久| 男女下面插进去视频免费观看| 久久人人爽av亚洲精品天堂| 国产成人精品一,二区| 大香蕉久久成人网| 热99国产精品久久久久久7| 亚洲第一青青草原| 久久久久视频综合| 午夜精品国产一区二区电影| 这个男人来自地球电影免费观看 | 久久久久久人妻| 久久99一区二区三区| 我的亚洲天堂| 精品人妻在线不人妻| 国产熟女午夜一区二区三区| 欧美日韩av久久| 99re6热这里在线精品视频| 桃花免费在线播放| 国产无遮挡羞羞视频在线观看| 亚洲人成77777在线视频| 色吧在线观看| av电影中文网址| 纵有疾风起免费观看全集完整版| 一级片免费观看大全| 亚洲av.av天堂| 久久99热这里只频精品6学生| 国产精品一二三区在线看| kizo精华| 免费高清在线观看视频在线观看| 中国国产av一级| 五月天丁香电影| 亚洲国产毛片av蜜桃av| 国产有黄有色有爽视频| 极品人妻少妇av视频| 大片免费播放器 马上看| 老熟女久久久| 黑丝袜美女国产一区| av片东京热男人的天堂| 天天躁日日躁夜夜躁夜夜| 97在线人人人人妻| 精品国产乱码久久久久久男人| 日韩一卡2卡3卡4卡2021年| 国产在视频线精品| 久久久精品免费免费高清| 亚洲欧洲国产日韩| 一级黄片播放器| 国产精品av久久久久免费| 在线观看国产h片| 日日啪夜夜爽| 最近最新中文字幕大全免费视频 | 日本免费在线观看一区| 免费观看在线日韩| 七月丁香在线播放| 高清欧美精品videossex| 国产又爽黄色视频| 精品福利永久在线观看| 男女高潮啪啪啪动态图| 日韩一区二区三区影片| 午夜福利,免费看| 两性夫妻黄色片| 久久青草综合色| 天天躁夜夜躁狠狠躁躁| 色94色欧美一区二区| 桃花免费在线播放| 不卡av一区二区三区| 亚洲精品久久成人aⅴ小说| 亚洲人成电影观看| 久久99热这里只频精品6学生| 亚洲伊人色综图| 永久免费av网站大全| 伦理电影免费视频| 国产精品久久久av美女十八| 欧美日韩一级在线毛片| 黄网站色视频无遮挡免费观看| 免费女性裸体啪啪无遮挡网站| 精品少妇久久久久久888优播| 精品视频人人做人人爽| 中文字幕色久视频| 亚洲精品一区蜜桃| 视频区图区小说| 亚洲精品成人av观看孕妇| 中文字幕另类日韩欧美亚洲嫩草| 午夜日韩欧美国产| 在线天堂中文资源库| 日韩av免费高清视频| 妹子高潮喷水视频| 在线观看人妻少妇| 国产免费一区二区三区四区乱码| 人妻系列 视频| 亚洲精品一区蜜桃| 老汉色av国产亚洲站长工具| 中文天堂在线官网| 天天躁夜夜躁狠狠久久av| 黄片播放在线免费| 久久久久久久大尺度免费视频| 亚洲成色77777| 免费不卡的大黄色大毛片视频在线观看| 丰满少妇做爰视频| 午夜影院在线不卡| 美女大奶头黄色视频| av一本久久久久| 亚洲精品中文字幕在线视频| 性高湖久久久久久久久免费观看| 成人国产麻豆网| 又黄又粗又硬又大视频| 亚洲国产看品久久| 国产精品无大码| 欧美精品亚洲一区二区| 成人毛片a级毛片在线播放| 好男人视频免费观看在线| 尾随美女入室| 亚洲av电影在线观看一区二区三区| 日韩视频在线欧美| 久久久久国产一级毛片高清牌| 九九爱精品视频在线观看| 街头女战士在线观看网站| 欧美日韩精品网址| 满18在线观看网站| 久久国内精品自在自线图片| 亚洲中文av在线| 精品第一国产精品| 久久av网站| 在线观看三级黄色| 国产色婷婷99| 性少妇av在线| 伊人久久国产一区二区| 亚洲伊人久久精品综合| 亚洲av在线观看美女高潮| 国产一区二区三区av在线| 亚洲五月色婷婷综合| 永久网站在线| 夜夜骑夜夜射夜夜干| 国产白丝娇喘喷水9色精品| 日韩成人av中文字幕在线观看| 热99久久久久精品小说推荐| av一本久久久久| 精品视频人人做人人爽| 免费久久久久久久精品成人欧美视频| 在线观看人妻少妇| 亚洲成国产人片在线观看| 乱人伦中国视频| 欧美bdsm另类| 男人舔女人的私密视频| 日韩伦理黄色片| 蜜桃国产av成人99| 观看av在线不卡| 黄网站色视频无遮挡免费观看| 精品人妻偷拍中文字幕| 九色亚洲精品在线播放| 老鸭窝网址在线观看| 亚洲综合精品二区| 国产熟女欧美一区二区| 亚洲欧美清纯卡通| 亚洲人成77777在线视频| 国产1区2区3区精品| 伦精品一区二区三区| 久久午夜综合久久蜜桃| 免费在线观看完整版高清| 国产视频首页在线观看| a级片在线免费高清观看视频| www.精华液| 青春草视频在线免费观看| 中文字幕色久视频| 亚洲精品一区蜜桃| 日韩一本色道免费dvd| 日韩av免费高清视频| 一区在线观看完整版| av国产久精品久网站免费入址| 波多野结衣av一区二区av| 永久免费av网站大全| 9色porny在线观看| 老女人水多毛片| 亚洲精品日韩在线中文字幕| 91在线精品国自产拍蜜月| 国产日韩欧美亚洲二区| 国产精品成人在线| 日韩欧美精品免费久久| 久久久精品免费免费高清| 日韩不卡一区二区三区视频在线| 97人妻天天添夜夜摸| 亚洲经典国产精华液单| 91久久精品国产一区二区三区| 老熟女久久久| 欧美日韩成人在线一区二区| 不卡av一区二区三区| 在线精品无人区一区二区三| 18禁裸乳无遮挡动漫免费视频| 婷婷色av中文字幕| 大香蕉久久成人网| 香蕉精品网在线| 男人操女人黄网站| 国产欧美日韩一区二区三区在线| 国产国语露脸激情在线看| 在线亚洲精品国产二区图片欧美| 亚洲国产av新网站| tube8黄色片| 一本—道久久a久久精品蜜桃钙片| 国产亚洲精品第一综合不卡| 日韩三级伦理在线观看| 欧美97在线视频| 可以免费在线观看a视频的电影网站 | 飞空精品影院首页| 久久久精品国产亚洲av高清涩受| 国产亚洲精品第一综合不卡| 在线观看三级黄色| 考比视频在线观看| 青青草视频在线视频观看| 亚洲国产精品999| 国产片内射在线| 亚洲第一av免费看| 天天操日日干夜夜撸| 黄色一级大片看看| 中文字幕人妻熟女乱码| 嫩草影院入口| 国产不卡av网站在线观看| 极品人妻少妇av视频| 亚洲三区欧美一区| 你懂的网址亚洲精品在线观看| 色视频在线一区二区三区| 一区二区日韩欧美中文字幕| 少妇人妻 视频| 青春草国产在线视频| 你懂的网址亚洲精品在线观看| 啦啦啦中文免费视频观看日本| 考比视频在线观看| 国产精品亚洲av一区麻豆 | 人妻系列 视频| 久久精品国产亚洲av高清一级| 国产成人免费无遮挡视频| 欧美激情 高清一区二区三区| 可以免费在线观看a视频的电影网站 | 在线观看免费视频网站a站| 97在线视频观看| 亚洲精华国产精华液的使用体验| 日本爱情动作片www.在线观看| 美女高潮到喷水免费观看| 好男人视频免费观看在线| 欧美激情高清一区二区三区 | 国产免费又黄又爽又色| 久久久精品94久久精品| 亚洲久久久国产精品| 日日摸夜夜添夜夜爱| 免费高清在线观看日韩| 亚洲精品日本国产第一区| 男的添女的下面高潮视频| 少妇人妻精品综合一区二区| 波多野结衣一区麻豆| 九九爱精品视频在线观看| 一级毛片 在线播放| 国产免费又黄又爽又色| 制服人妻中文乱码| 女人久久www免费人成看片| 亚洲国产欧美网| 色网站视频免费| 一本大道久久a久久精品| 精品福利永久在线观看| av不卡在线播放| 国产成人精品在线电影| 丰满乱子伦码专区| 熟妇人妻不卡中文字幕| 大话2 男鬼变身卡| 黄色怎么调成土黄色| 国产精品秋霞免费鲁丝片| 国产精品嫩草影院av在线观看| 久久国产精品大桥未久av| 亚洲,一卡二卡三卡| 巨乳人妻的诱惑在线观看| 亚洲伊人久久精品综合| 如何舔出高潮| 交换朋友夫妻互换小说| 久久韩国三级中文字幕| 99re6热这里在线精品视频| 精品一区在线观看国产| 精品国产一区二区三区四区第35| 国产精品美女特级片免费视频播放器 | 国产一区二区三区在线臀色熟女 | 咕卡用的链子| 亚洲av成人av| xxx96com| 在线观看免费视频网站a站| 91麻豆av在线| 无遮挡黄片免费观看| x7x7x7水蜜桃| 欧美一区二区精品小视频在线| 桃红色精品国产亚洲av| 国产精品免费视频内射| 大型av网站在线播放| 高清欧美精品videossex| 少妇裸体淫交视频免费看高清 | 国产av在哪里看| av网站在线播放免费| 欧美老熟妇乱子伦牲交| 亚洲中文日韩欧美视频| 满18在线观看网站| 999久久久精品免费观看国产| 午夜老司机福利片| 国产不卡一卡二| 久久国产精品人妻蜜桃| 亚洲狠狠婷婷综合久久图片| 水蜜桃什么品种好| 亚洲人成电影免费在线| 巨乳人妻的诱惑在线观看| 国产aⅴ精品一区二区三区波| 久久久水蜜桃国产精品网| 91九色精品人成在线观看| 夜夜爽天天搞| 少妇粗大呻吟视频| 久久久国产成人精品二区 | 久久久久久久久中文| 最近最新中文字幕大全免费视频| 999久久久精品免费观看国产| 亚洲人成伊人成综合网2020| 欧美黑人精品巨大| 丁香六月欧美| 69av精品久久久久久| 精品一区二区三区视频在线观看免费 | 99精品在免费线老司机午夜| 18禁美女被吸乳视频| 亚洲成人免费电影在线观看| 免费在线观看亚洲国产| 亚洲少妇的诱惑av| 两人在一起打扑克的视频| 久久人人精品亚洲av| 在线av久久热| 国产一区在线观看成人免费| 亚洲精品国产一区二区精华液| 色在线成人网| 久久久国产成人精品二区 | 女人被狂操c到高潮| 欧美日本亚洲视频在线播放| 黄片大片在线免费观看| 每晚都被弄得嗷嗷叫到高潮| 免费在线观看黄色视频的| 久久欧美精品欧美久久欧美| 欧美成狂野欧美在线观看| 黑人巨大精品欧美一区二区mp4| 色精品久久人妻99蜜桃| 美女国产高潮福利片在线看| 亚洲国产精品一区二区三区在线| 曰老女人黄片| 丝袜人妻中文字幕| 国产成人欧美| 99久久国产精品久久久| 久久欧美精品欧美久久欧美| 国产熟女午夜一区二区三区| 自线自在国产av| 少妇的丰满在线观看| 丁香欧美五月| 国产激情久久老熟女| 18禁国产床啪视频网站| 国产在线精品亚洲第一网站| 日韩欧美三级三区| 午夜免费鲁丝| 欧美黑人欧美精品刺激| av国产精品久久久久影院| 亚洲精品国产精品久久久不卡| 在线观看免费高清a一片| 嫩草影院精品99| 一本大道久久a久久精品| 黄频高清免费视频| 激情在线观看视频在线高清| 国产精品久久久久久人妻精品电影| 老司机在亚洲福利影院| www.www免费av| 天堂俺去俺来也www色官网| 国产精华一区二区三区| 免费在线观看影片大全网站| 欧美色视频一区免费| 欧美日韩亚洲高清精品| 美女 人体艺术 gogo| 国产男靠女视频免费网站| 老司机午夜福利在线观看视频| 在线观看一区二区三区| 国产高清videossex| 欧美黑人欧美精品刺激| 久久 成人 亚洲| av在线天堂中文字幕 | 日韩 欧美 亚洲 中文字幕| 国产精品 欧美亚洲| 午夜免费观看网址| 亚洲精品久久午夜乱码| 色综合站精品国产| 好看av亚洲va欧美ⅴa在| 丝袜美腿诱惑在线| 欧美乱色亚洲激情| 亚洲国产精品一区二区三区在线| 一本大道久久a久久精品| 亚洲七黄色美女视频| 757午夜福利合集在线观看| 午夜福利在线观看吧| 欧美日韩黄片免| 人人妻人人澡人人看| 村上凉子中文字幕在线| 亚洲男人的天堂狠狠| 国产蜜桃级精品一区二区三区| 亚洲情色 制服丝袜| 女生性感内裤真人,穿戴方法视频| 在线观看免费高清a一片| 中文字幕另类日韩欧美亚洲嫩草| a级毛片黄视频| 久久性视频一级片| 男人舔女人的私密视频| 成人国语在线视频| 午夜福利一区二区在线看| xxxhd国产人妻xxx| 久久中文字幕人妻熟女| 9热在线视频观看99| 国产成人影院久久av| 国产精品综合久久久久久久免费 | 午夜福利在线观看吧| 免费在线观看影片大全网站| 免费不卡黄色视频| 欧美黑人精品巨大| 看免费av毛片| 91av网站免费观看| 黑人巨大精品欧美一区二区mp4| 欧美日韩一级在线毛片| 巨乳人妻的诱惑在线观看| av天堂久久9| 五月开心婷婷网| 亚洲人成网站在线播放欧美日韩| 中文字幕色久视频| 琪琪午夜伦伦电影理论片6080| 久久香蕉精品热| 亚洲欧美精品综合久久99| 成人亚洲精品一区在线观看| 丰满饥渴人妻一区二区三| 成人亚洲精品av一区二区 | 777久久人妻少妇嫩草av网站| 97超级碰碰碰精品色视频在线观看| www.999成人在线观看| 欧美不卡视频在线免费观看 | 欧美午夜高清在线| 99国产精品一区二区三区| 日本一区二区免费在线视频| 欧美激情 高清一区二区三区| 亚洲一区中文字幕在线| 色婷婷av一区二区三区视频| 男女床上黄色一级片免费看|