氢化二氧化钛 超电容 Hydrogenated TiO2 Nanotube Arrays for Supercapacitors

Hydrogenated TiO2Nanotube Arrays for Supercapacitors

Xihong Lu,?,?Gongming Wang,?Teng Zhai,?Minghao Yu,?Jiayong Gan,?Yexiang Tong,*,?and Yat Li*,??KLGHEI of Environment and Energy Chemistry,MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry,School of Chemistry and Chemical Engineering,Sun Yat-Sen University,Guangzhou510275,People’s Republic of China

?Department of Chemistry and Biochemistry,University of California,Santa Cruz,California95064United States

*Supporting Information

upercapacitors,as an energy storage device,have attracted growing interest in recent years.1?6According to the mechanism of charge storage,supercapacitors can be classified as(1)electrical double layer capacitors(EDLCs)that are based on electrostatic charge diffusion and accumulation at the electrode/electrolyte interface;and(2)pseudocapacitors that are dominated by Faradaic reactions on electrode materials.7?9 For EDLCs,carbon-based materials are commonly used as electrodes due to their outstanding long-term electrochemical stability as a result of high electrical conductivity and extraordinary chemical stability.8,10?12However,the limited charge accumulation in electrical double layer restricts the specific capacitances of EDLCs in a range of relatively small values between90and250F g?1.On the contrary, pseudocapacitors made of metal oxides(e.g.,RuO2,13,14 NiO x,15CoO x,16MnO217,18)or conducting polymers(e.g., polyaniline19)have achieved substantially higher specific capacitances of300?1200F g?1through surface redox reactions.13?19Nevertheless,pseudocapacitors are usually suffered from the poor electrical conductivity of electrode materials and the irreversibility of Faradaic reactions on electrode surface,which cause gradual loss of capacitance. There are two major approaches to enhance electrochemical capacitance and stability of an electrode.The first approach is to develop nanostructured electrodes with extremely large effective area.Nanotubes,8,11?14nanowires,17?19nano-sheets,and mesoporous nanostructures have been studied,and these nanostructures exhibit higher specific capacitances than their bulk counterparts.The second approach is to increase the electrical conductivity of electrodes by mixing them with highly conductive materials.For instance,carbon materials as excellent electrical conductors have been widely used to form composites with metal oxide electrodes.22?25 Despite an increase of specific capacitances in carbon-based composites such as NiO/graphene23or MnO2/CNTs24 compared to their metal oxide counterparts,the incorporation of carbon materials does not change the intrinsic properties of metal oxides.It is highly desirable to fundamentally improve the electrical conductivity and pseudocapacitive behavior of metal oxide electrodes.

Highly ordered TiO2NTAs hold great promise as super-capacitor electrodes.TiO2is known to be chemically stable,and importantly the open-end nanotube structure offers an extremely large,solvated ion accessible surface area.However, the previous reported specific capacitances of TiO2NTAs (100?911μF cm?2)26?29are significantly smaller than that obtained from other metal oxides such as MnO2and RuO2.

Received:January14,2012

Revised:February9,2012

Published:February24,2012

The relatively small specific capacitance was attributed to the poor electrochemical activity and poor electrical conductivity of TiO2.30,31We hypothesize that hydrogenation of TiO2NTAs could address these limitations and thus improve specific capacitance and stability of TiO2materials.It was motivated by our recent demonstration that the donor densities of TiO2 nanostructures are significantly improved by controlled introduction of oxygen vacancy(Ti3+sites)states via thermal treatment in hydrogen atmosphere.32Furthermore,we anticipate that hydroxyl groups will be introduced on TiO2 surface during hydrogenation,which could modify the electrochemical activity of TiO2and therefore increase its pseudocapacitance,as observed in other electrode materi-als.33?35

Highly ordered TiO2NTAs were fabricated on a Ti fiber (99.7%,1mm in diameter)by anodic oxidation in a glycerol aqueous solution containing0.75%NH4F,as illustrated in Figure1a.The as-prepared TiO2NTAs were vertically aligned

on the Ti fiber(Figure1b and Figure S1Supporting Information)with a uniform diameter of～100nm and a length of～1.2μm.X-ray diffraction studies reveal that these TiO2NTAs are amorphous(Figure1c).To enhance the mechanical stability and electrical conductivity,the as-prepared TiO2NTAs were annealed at400°C for60min in air(denoted

as air?TiO2)and hydrogen atmosphere(denoted as H?TiO2), respectively.Scanning electron microscopy(SEM)studies confirm that there are no obvious morphological changes for TiO2tubes upon thermal treatment while the amorphous structure transforms to anatase phase(Figure1c).Furthermore, transmission electron microscopy(TEM)analysis shows that the TiO2NTs are polycrystalline structures with inner diameter and wall thickness of about75and20nm,respectively(Figure S2,Supporting Information).The well-resolved lattice fringes of0.35nm are corresponding to the(101)plane of anatase TiO2(JCPDF#21-1272),again confirming the anatase structure.

We performed X-ray photoelectron spectroscopy(XPS) studies to examine the effect of hydrogenation on the chemical composition and oxidation state of TiO2nanotubes.Figure2a shows the normalized Ti2p core level XPS spectra of air?TiO2and H?TiO2samples.Two broad peaks centered at～465.1 and～458.9eV that correspond to the characteristic Ti2p1/2 and Ti2p3/2peaks of Ti4+are observed for both samples.36,37In comparison to air?TiO2,the peaks of the H?TiO2sample show a negative shift in binding energy,suggesting that they have different bonding environments.By subtracting the normalized Ti2p spectra of H?TiO2with air?TiO2sample,38 there are two extra peaks centered at ca.463.5and457.9eV (Figure2a).These two peaks are consistent with the characteristic Ti2p1/2and Ti2p3/2peaks of Ti3+,39,40 confirming the presence of Ti3+ions in the H?TiO2sample. The result suggests that oxygen vacancies(Ti3+sites)are created in H?TiO2nanotubes during hydrogenation.Same conclusion was obtained from Raman analysis(Figure S3, Supporting Information).The characteristic Raman peaks of

Figure1.(a)A schematic diagram showing the fabrication of H?TiO2 NTAs.(b)SEM image of H?TiO2NTAs.(c)XRD spectra collected

from untreated TiO2,air?TiO2,and H?TiO2NTAs,respectively.Ti signals are originated from the Ti fiber.Figure2.(a)Overlay of normalized Ti2p core level XPS spectra of air?TiO2(black solid curve)and H?TiO2NTAs(red dashed curve), together with their difference spectrum(“H?TiO2”minus“air?TiO2”).(b)Normalized O1s core level XPS spectra of air?TiO2and H?TiO2NTAs.Black circles are the experimental data,which are deconvoluted into two peaks centered at529.9eV(green dashed curve)and531.4?532.0eV(pink dashed curve).The red curve is the summation of the two deconvoluted peaks.(c)Mott?Schottky plots of the untreated TiO2,air?TiO2,and H?TiO2NTAs.

anatase are negatively shifted and broaden for H ?TiO 2sample compared to air ?TiO 2sample,suggesting the increased amount of oxygen vacancies.41,42Figure 2b compares the O 1s core level XPS spectra of air ?TiO 2and H ?TiO 2NTAs.Both samples exhibit the peak of 529.9eV that corresponds to the characteristic peak of Ti ?O ?Ti.36Additional peaks centered at 531.4and 532.0eV are attributed to Ti ?OH,which has been reported to be located at the binding energy of ～1.5?1.8eV higher than the peak of Ti ?O ?Ti.17,40The Ti ?OH peak intensity of the H ?TiO 2sample is substantially higher than that of air ?TiO 2sample,indicating the TiO 2surface are functionalized by hydroxyl groups after hydro-genation.To investigate the effect of hydrogenation on the electrical properties of TiO 2,electrochemical impedance measurements were conducted on the untreated TiO 2,air ?TiO 2,and H ?TiO 2samples.Mott ?Schottky plots were generated based on capacitances that were derived from the electrochemical impedance obtained at each potential with 10kHz frequency in the dark.As shown in Figure 2c,all TiO 2samples exhibit a positive slope in the Mott ?Schottky plots,indicating n-type semiconductor character.Carrier densities of TiO 2samples were calculated using Mott ?Schottky equation =εε???

????????????????????

??N e V 2d d C d 0011

2where N d is the donor density,e 0is the electron charge,εis the dielectric constant of TiO 2(31for anatase),43ε0is the permittivity of vacuum,and V is the potential applied at the electrode.The carrier densities of the untreated TiO 2,air ?TiO 2

and H ?TiO 2NTAs are calculated to be 3.4×1020cm ?3,3.6×1020cm ?3,and 1.4×1023cm ?3,respectively.Noteworthy is that the Mott ?Schottky equation was derived based on a planar electrode model,and we used the projected area (instead of effective surface area)of the nanotube structures for the calculation that could cause errors in determining the carrier densities.Nevertheless,a qualitative comparison of carrier densities between these samples is valid,as they have similar morphology and surface area.As expected,the untreated TiO 2

sample exhibits the lowest carrier density due to their amorphous structure.Upon annealing in air,the carrier density

of TiO 2increases as a result of improved crystallinity by the

formation of anatase.Significantly,hydrogenation leads to a 3orders of magnitude enhancement in carrier density of TiO 2

NTAs.The increased carrier density can be attributed to the increased oxygen vacancy states,which are known to be

electron donors for TiO 2.32,44The formation of oxygen

vacancies (Ti 3+sites)is supported by the XPS results.To evaluate the electrochemical properties of TiO 2and H ?TiO 2NTAs,electrochemical measurements were conducted in

a three-electrode electrochemical cell with a graphite rod

counter electrode and an Ag/AgCl reference electrode in 0.5M Na 2SO 4solution.Figure 3a shows the cyclic voltammetric (CV)curves of the untreated TiO 2,air ?TiO 2and H ?TiO

2nanotube-arrayed electrodes collected at the scan rate of 100mV s ?1.In comparison to the untreated TiO 2and air ?TiO

2

samples,H ?TiO 2sample delivers an obvious pseudocapacitive characteristic,which can be attributed to the oxidation/

reduction of surface hydroxyl groups.33?35Furthermore,the CV curves of H ?TiO

2NTAs obtained at various scan rates

exhibit quasi-rectangular shapes (Figure S4,Supporting Information).The shapes of these CV curves remain unchanged as the scan rate increase from 10to 1000mV s ?1,indicating good capacitive behavior and high-rate capability of H ?TiO 2NTAs.Figure 3b shows the calculated areal

Figure 3.(a)CV curves of the untreated TiO 2,air ?TiO 2,and H ?TiO 2NTAs obtained at a scan rate of 100mV s ?1.(b)Areal capacitance of TiO 2

samples measured as a function of scan rate.(c)Galvanostatic charge/discharge curves of TiO 2samples collected at a current density of 100μA

cm ?2.(d)Cycle performance of TiO 2samples measured at a scan rate of 100mV s ?1for 10000cycles.

capacitance of these electrodes as a function of scan rate (detailed calculation see Supporting Information).The areal capacitances of the H ?TiO 2sample is significantly higher than the untreated TiO 2and air ?TiO 2samples.For instance,the

H ?TiO 2electrode achieves an areal capacitance of 3.24mF

cm ?2at a scan rate of 100mV s ?1,which is a 124-and 40-fold enhancement compared to the untreated TiO 2(0.026mF

cm ?2)and air ?TiO 2(0.08mF cm ?2)samples.This areal capacitance is also substantially higher than the values recently reported for TiO 2NTAs (0.538mF cm ?2)26,28and TiO 2nanoparticles (0.12mF cm ?2).30The enhanced electrochemical performance of TiO 2NTAs can be attributed to two major improvements upon hydrogenation.First,H ?TiO 2samples

exhibit improved electrical conductivity (increased carrier density)that facilitates the transport of charge carriers.Second,hydrogenation increases the density of hydroxyl groups on TiO 2NT surface,and thereby enhances the pseudocapacitance.Moreover,the H ?TiO 2sample shows good rate capacitance.The areal capacitance of the H ?TiO 2sample drops from 3.8to

2.6mF cm ?2with an good retention of 68.4%of the initial capacitance,when the scan rate increases from 10to 1000mV s ?1.In contrast,the untreated TiO 2and air ?TiO

2samples

retain only 21.2and 9.5%of the initial capacitance,respectively.The rate capability is related to the rate of ion diffusion (mass transport)in the electrode and the electrode conductivity.Given that the morphologies of these TiO 2samples are similar,they should have similar ion diffusion rate.Therefore,the improved rate capacitance in H ?TiO 2sample should be due to the enhanced electrical conductivity of electrode.The electrochemical performance of TiO 2samples was further studied by galvanostatic charge/discharge measure-ments.Figure 3c shows the charge/discharge curves of different TiO 2electrodes collected at a current density of 100μA cm ?2.The charge/discharge curve of the H ?TiO

2electrode is

symmetric and substantially prolonged over the untreated

TiO

2and air ?TiO 2electrodes,revealing a good capacitive

behavior.Additionally,it shows a small IR drop (0.02V),again

confirming the superior electrical conductivity of the H ?TiO

2electrode.Galvanostatic charge/discharge curves were also collected for the H ?TiO 2sample at various current densities

(Figure S6Supporting Information).A slightly nonlinear

sloping potential profile was observed in these charge/discharge curves,indicating Faradaic reactions occur on the H ?TiO 2NT

surface,29,30,45which is consistent with the CV results.The areal

capacitances of the H ?TiO

2sample derived from the

discharging curves measured at different current densities are

plotted in Figure S7(Supporting Information).The areal capacitance of the H ?TiO

2sample measured at the current

densities of 100μA cm ?2is calculated to be 4.64mF cm ?2,

which is substantially larger than the values obtained from the untreated TiO

2and air ?TiO

2samples as well as the previously reported for TiO 2electrodes 27,29at the same current density.

Good cycling stability is one of the most important

characteristics for high-performance supercapacitors.TiO 2electrodes were tested at a scan rate of 100mV s ?1for 10000cycles.As shown in Figure 3d,the capacitances of TiO

2and

air

?TiO 2electrodes drop 34.8and 44.3%,respectively,after 10

000cycles.Significantly,the H ?TiO

2electrode exhibits an

excellent long-term stability with only 3.1%reduction of

capacitance after 10000cycles.There is no structural modification of the H ?TiO

2electrode after 10000cycles of

measurement (Figure S8a,Supporting Information).Addition-

ally,the first and the 10000th CV curves of the H ?TiO 2sample are more or less the same,indicating the redox reactions on H ?TiO

2electrode surface (pseudocapacitance)are very

stable (Figure S8b,Supporting Information).The remarkable

Figure 4.(a)CV curves collected at a scan rate of 100mV s ?1for H ?TiO 2NTAs hydrogenated at different temperatures.(b)Areal capacitances

collected for H ?TiO 2electrodes as a function of scan rate.(c)Ragone plots of H ?TiO 2electrodes obtained at different temperatures.(d)Areal capacitances collected at a scan rate of 100mV s ?1for H ?TiO 2electrodes for 10000cycles.

cycling performance for the H?TiO2electrode is ascribed to their enhanced electrical conductivity and highly stable surface redox reaction.

The above-mentioned experimental results confirm that the electrochemical performance of TiO2is strongly correlated to their electrical conductivity(carrier density).As the carrier density of H?TiO2is expected to be depend on the hydrogenation conditions,it motivated us to study the effect of hydrogenation temperature on the structural and electro-chemical performance of H?TiO2NTAs.As shown in Figure 4a,the capacitive current density of H?TiO2electrode increases with the increase of hydrogenation temperature. XRD studies(Figure S9,Supporting Information)reveal that there is no phase change observed for TiO2NTAs when the hydrogenation temperature increases from300to600°C.On the other hand,XPS analysis indicates that the amount of surface hydroxyl groups gradually increases with the increase of hydrogenation temperature(Figure S10,Supporting Informa-tion).Mott?Schottky studies reveal a2orders of magnitudes enhancement of carrier density in H?TiO2sample when the hydrogenation temperature increased from300to400°C (Figure S11,Supporting Information)possibly due to the improved crystallinity of TiO2as well as the creation of oxygen vacancy states.These results suggest that the formation of surface hydroxyl groups and the reduction of TiO2are more favorable at high temperature.As expected,the H?TiO2 sample hydrogenated at600°C yields the highest areal capacitance of14.95mF cm?2at a scan rate of10mV s?1 compared to other H?TiO2samples prepared at lower temperatures due to the improved electrical conductivity and pseudocapacitance arising from the increased amount of surface hydroxyl groups.To our knowledge,this is the best areal capacitance ever achieved by TiO2materials,and it is even comparable to the capacitance obtained from a recently reported Ni?NiO core?shell structure(9mF cm?2).46 Moreover,Ragone plots(Figure4c)show that the H?TiO2 electrodes achieve the highest power density of17.5mW/cm2 and the maximum energy density of1.9mWh/cm2.We also performed cycling measurements to examine the long-term stability of these H?TiO2electrodes(Figure4d).While the H?TiO2electrodes obtained between300and500°C exhibit excellent cycling performance with capacitance drop less than 8%after10000cycles,the H?TiO2sample obtained at600°C shows a relatively large reduction of～26%in areal capacitance after10000cycles.The relatively low stability of the600°C sample could be due to the hydrogen embrittlement of Ti fiber that degrades the contact between TiO2NTs and the fiber substrate.The Ti fiber became fragile and the H?TiO2NTAs can be easily peeled off from the Ti fiber after hydrogenation at 600°C.A possible solution is to prepare TiO2NTAs on other substrates such as carbon fabrics that are stable for high-temperature hydrogenation.Investigation of additional sub-strates is in progress.

In addition to being used as an electrode,the open-end nanotube structure of TiO2also enables them to serve as a good support for other capacitive active electrode materials to form composite structures.We used H?TiO2NTAs(hydro-genated at400°C)to support MnO2nanoparticles for supercapacitor electrodes,as MnO2is one of the most promising pseudocapacitive materials with high theoretical specific capacitance(～1400F g?1)but suffer from low electrical conductivity(10?5?10?6S cm?1).17,24,47The MnO2/H?TiO2composites were synthesized by electro-depositing MnO2onto the H?TiO2NTs(Experimental Section,Supporting Information).SEM image shows that the H?TiO2NTs are completely filled with MnO2nanoparticles after60s electrodeposition(Figure S12a,Supporting Information).Mn2p core level XPS spectrum reveals two peaks located at642.2and654.3eV,which are consistent with the characteristic Mn2p3/2and Mn2p1/2binding energies of MnO2(Figure S12b,Supporting Information).48It supports the successful preparation of MnO2.To study the role of TiO2 support on the electrochemical performance of the composite structure,we also prepared MnO2/air?TiO2NTAs for comparison.In comparison to the MnO2/air?TiO2,the CV curve of MnO2/H?TiO2composite exhibits a rectangle-like shape and enhanced capacitive current density(Figure S13, Supporting Information).The areal capacitance of the MnO2/ H?TiO2composite is two times larger than that of the MnO2/ air?TiO2composite(Figure5a).Significantly,the MnO2/H?

TiO2composite achieves a calculated specific capacitance of 912F g?1(based on the mass of MnO2,Supporting Information)at the scan rate of10mV s?1.This value is not only4times higher than that of MnO2/air?TiO2sample(217 F g?1),but also higher than the values recently reported for other MnO2based composites,such as TiN/MnO2nanotubes (～480F g?1),49SnO2@MnO2nanowires(～540F g?1),50 Zn2SnO4@MnO2nanorods(～560F g?1),51ZnO@MnO2 nanorods(～675F g?1),52and even comparable to CNTs/ MnO x composites(1250F g?1).53Furthermore,the MnO2/ H?TiO2composite shows good rate capability with a capacitance retention of69.9%when the scan rate increase from10to100mV s?1,while it is only34.6%for the MnO2/ air?TiO2composite.These results suggest that MnO2/H?TiO2samples have good electrical conductivity and the charge separation and transport in these highly ordered tubular Figure5.(a)Specific capacitances and areal capacitances of the MnO2/H?TiO2and MnO2/air?TiO2composites measured as a function of scan rate.(b)Galvanostatic charge/discharge curves of the MnO2/H?TiO2and MnO2/air?TiO2NTAs collected at a current density of200μA cm?2.

structures are efficient.Moreover,the charge/discharge curve of the MnO2/H?TiO2sample is symmetric with nearly linear slopes and a small IR drop(0.04V).The small IR drop again indicates the small equivalent series resistance of the H?TiO2 NTAs.

In summary,we have demonstrated that hydrogenation improves significantly the electrochemical performance of TiO2 NTAs as electrode materials for supercapacitors.TiO2NTAs hydrogenated at400°C yields the highest specific capacitances of3.24mF cm?2at a scan rate of100mV s?1with areal energy density of0.8mWh cm?2and power density of17.5mW cm?2. Importantly,the H?TiO2electrode exhibits excellent long-term stability with only3.1%reduction of capacitance after10000 cycles.The enhancement in capacitance can be attributed to the combined contribution from the improved donor density and the increased density of surface hydroxyl groups. Furthermore,H?TiO2NTAs were proved to be excellent supports for other capacitive active materials such as MnO2. The MnO2/H?TiO2composite achieves the highest specific capacitance of912F g?1at the scan rate of10mV s?1.These findings could open up new opportunities for TiO2materials in constructing high-performance supercapacitors as well as other

energy storage devices.

■ASSOCIATED CONTENT

*Supporting Information

Synthetic and analytical methods,capacitive equations,SEM and TEM images,XPS and XRD spectra,CV and charge/ discharge curves,and Mott?Schottky plots.This material is

available free of charge via the Internet at https://www.wendangku.net/doc/9c17935552.html,.■AUTHOR INFORMATION

Corresponding Author

*E-mail:(Y.T.)chedhx@https://www.wendangku.net/doc/9c17935552.html,;(Y.L.)yli@ https://www.wendangku.net/doc/9c17935552.html,.

Notes

The authors declare no competing financial interest.■ACKNOWLEDGMENTS

Y.L.acknowledges the financial support of this work in part by NSF(DMR-0847786),faculty startup funds granted by the University of California,Santa Cruz.Y.X.T.acknowledges the financial support of this work by the Natural Science Foundations of China(90923008and J1103305)and the Natural Science Foundations of Guangdong Province (9251027501000002).X.H.L.thanks the Academic New Artist Ministry of Education Doctoral Post Graduate(China)for

China Scholarship Council financial support.

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