1、文章编号:摇 1007鄄8827(2015)05鄄0391鄄06用于染料敏化太阳能电池的多壁碳纳米管基对电极的制备与表征郑摇 威,摇 齐摇 涛,摇 张永超,摇 石海英,摇 田均庆(哈尔滨理工大学 材料科学与工程学院, 黑龙江 哈尔滨 150040)摘摇 要:摇 经酸化处理的多壁碳纳米管(MWCNTs)与纳米石墨复合后沉积在 FTO 导电玻璃基底上制备出染料敏华太阳能电池薄膜对电极。 利用 SEM、TEM、EDS 与 IR 光谱对其进行表征。 以 MgO 掺杂的 TiO2薄膜为光阳极对电池通过循环伏安法(CV 曲线)、电化学阻抗谱(EIS)和伏安特性曲线(J鄄V)进行光电性能分析。 结果表明:酸
2、化处理的 MWCNTs 与纳米石墨复合对电极展现出优异的光催化性能,有利于电池光电性能的提高。 电池开路电压及短路电流密度分别可达 0. 53 V、4. 67 mA/ cm2,其光电转换效率达到 4. 10%,与铂对电极的性能相当。关键词:摇 染料敏化太阳能电池;对电极;多壁碳纳米管;石墨;光催化活性中图分类号: 摇 TM914. 4+2文献标识码: 摇 A收稿日期: 2015鄄06鄄28;摇 摇 修回日期: 2015鄄09鄄29基金项目:哈尔滨市科技创新人才项目(2013RFXXJ004).通讯作者:郑摇 威,博士,教授. E鄄mail: zhengwei1972 sina. com.Fab
3、rication and characterization of a multi鄄walledcarbon nanotube鄄based counter electrode for dye鄄sensitized solar cellsZHENG Wei,摇 QI Tao,摇 ZHANG Yong鄄chao,摇 SHI Hai鄄ying,摇 TIAN Jun鄄qing(College of Material Science and Engineering, Harbin University of Science and Technology, Harbin150040, China)Abstr
4、act: 摇 A counter鄄electrode (CE) for dye鄄sensitized solar cells (DSSCs) was prepared by coating a slurry containing acid鄄oxi鄄dized multi鄄wall carbon nanotubes and nano鄄graphite powder onto a fluorine鄄doped tin oxide conducting glass substrate. The sampleswere characterized by SEM, TEM, EDS and IR spe
5、ctroscopy. The CE performance in the DSSCs with MgO鄄doped TiO2as the pho鄄toanode was investigated by measurements of current鄄voltage curves, cyclic voltammetry and electrochemical impedance spectrosco鄄py. Results show that the cell with the CE exhibits the best photoelectric properties of all the ca
6、rbon鄄based CEs investigated. Theshort鄄circuit current density (Jsc) is 4. 67 mA/ cm2, the open鄄circuit voltage (Voc) is 0. 53 V and photoelectric conversion efficiencyis up to 4. 10%, which are comparable with those of the Pt鄄based CE in DSSCs.Keywords:摇 DSSC; Counter electrodes; MWCNTs; Graphite; P
7、hotocatalytic activityReceiced date: 2015鄄06鄄11;摇 Renised date: 2015鄄10鄄08Foundation item: Project of Harbin Science and Technology Innovation Talents (2013RFXXJ004).Corresponding author: ZHENG Wei, Ph. D, Professor. E鄄mail: zhengwei1972 sina. comEnglish edition available online ScienceDirect ( http
8、:蛐蛐www. sciencedirect. com蛐science蛐journal蛐18725805 ).DOI: 10. 1016/ S1872鄄5805(15)60198鄄61摇 IntroductionDye鄄sensitized solar cells (DSSCs) are relativelypromising thin film solar cells with high conversionefficiency and low cost, which are regarded as a cred鄄ible alternative to other photovoltaic c
9、ells1,2. As apart of DSSCs, the counter electrode (CE) is madeof Pt element, an expensive material, which offers anexcellent electrical conductivity and high catalytic ac鄄tivity3. But the drawbacks such as dissolution of theplatinum thin film in the organic corrosive electrolyteand the expensive pri
10、ce need developing further morestable and cost鄄effective CE materials4.Carbonnano鄄materials have been the candidates because oftheir excellent conductive and mechanical property5.Wang6prepared a few鄄layer graphene film on indi鄄um鄄tin鄄oxide ( ITO) glass that has a photoelectricconversion efficiency o
11、f 0. 26%, much less than thatusing the Pt film as the CE in DSSCs. This can be at鄄tributed to a drop of electrical conductivity owing tothe presence of defects in graphene. From then on,graphene鄄based CEs were modified in many ways to摇第 30 卷摇 第 5 期2015 年 10 月新摇 型摇 炭摇 材摇 料NEW CARBON MATERIALSVol. 30摇
12、 No. 5Oct. 2015摇improve their performance in DSSCs. Xue7prepareda N鄄doped graphene foam and Li8manufactured car鄄bon nanotubes on a piece of graphene paper as theCEs to improve their catalytic activity. Both of theseCE materials are three dimensional (3D). The carbonnanotubes were doped with metals o
13、r metal oxides toincrease their electrode conductivity9,10.The gra鄄phene/ organic composites have also been made agreat progress as CE materials11鄄13.We fabricated a CE, using MWCNTs as 3D ma鄄trix and nanographite powder as filler, to form the CEfilm on fluorine鄄tin鄄oxide (FTO) glass. MWCNTswere tre
14、ated in a strong acid to reduce their aggrega鄄tion and then filled with nanographite powder to in鄄crease their electrical conductivity. This modificationprocedure to prepare the composite structure is benefi鄄cial for providing fast electron transport channels forcharge injection between MWCNTs and F
15、TO14.DSSCs were fabricated using several 3D compos鄄ite samples as the CEs and a dye鄄sensitized MgO鄄doped TiO2as the photoanode material for evaluationof their property. The principle of DSSCs assembledin laboratory is illustrated in Fig. 1.Fig.1摇 Schematic representation of a DSSC withMWCNTs as the
16、counter鄄electrode.2摇 Experimental摇 摇 Butyl titanate(CP), Titanium isopropoxide (逸99. 7%), magnesium acetate and ethylene celluloseare analytical reagents, and butyl titanate is chemicalreagent. Emulsifier OP鄄10 (逸99. 0%) was providedby Xingtai Lanxing additive plant (Hebei, China)and MWCNTs (the out
17、side diameter50 nm, length10鄄20 滋m, purity 98%, specific surface area 40 m2/ g) were bought from Beijing Dk nano S & TLtd (Beijing, China). The nano鄄graphite with an av鄄erage grain size of about 200 nm was supplied byHarbin TR S & T Ltd (Harbin, China).2. 1摇 Preparation of photoanodeTiO2colloids wer
18、e prepared according to the pro鄄cedure reported earlier15, which were deposited onthe FTO glasses (200伊150 mm, thickness 2. 2 mm,resistance 14 赘, transmittance 90%) and sintered at450 益 for 1 h. Then the coated glasses were first im鄄mersed into an magnesium acetate solution in ethanol,then a 0. 5 mm
19、ol/ L N3dye solution in ethanol for24 h. Finnaly, the coated and immersed glasses weredried at room temperature to obtain the dye鄄sensitizedMgO/ TiO2photoanode.2. 2摇 Preparation of counter electrodesThe MWCNTs were first oxidized by an acidmixture containing H2SO4(98%) and HNO3(67%)with a volume rat
20、io of 3 颐1 under sonication, then fil鄄tered and washed by deionized water to be neutral. Apaste for the CEs was made, which was consisted ofanhydrous alcohol, OP鄄10 as the dispersant, ethylenecellulose as the adhesive, the acid鄄oxidized MWCNTsand nanographite.The paste was deposited on thecleaned FT
21、O glass substrates and heat鄄treated at400 益 for 2 h under the nitrogen to obtain the CEs.DSSCswereassembledusingthedye鄄sensitizedMgO/ TiO2as the anode and CEs as the cathodes,which were impregnated with an organic liquid elec鄄trolyte containing LiI, I2, ethylene glycol and aceto鄄nitrile. Four CEs we
22、re prepared according to the for鄄mulation listed in Table 1. For a comparison, the Ptelectrode was produced by magnetron sputtering onFTO glass.Table 1摇 The raw materials of 5 counter鄄electrodes.SampleMaterials of counter鄄electrodes1Graphite + Acid鄄treated MWCNTs2Acid鄄treated MWCNTs3Graphite + MWCNT
23、s4MWCNTs5Pt2. 3摇 CharacterizationThe morphology and microstructure of the pho鄄toanode and CEs were observed by SEM (Sirion200,Philip). The infrared spectra of samples were recor鄄ded byanIRspectrophotometer( T60SXBFTI,Nicole). The surface sheet resistance of the CEs wasmeasured by a 4鄄point probe tes
24、ter ( SX1934: SZ鄄82).The photoelectric performance of the DSSCswas analyzed by EIS, CV and I鄄V curves with anelectrochemical workstation (LK98B, Tianjin) underXenon lamp illumination (80 mW/ cm2).3摇 Results and discussion摇 摇 Fig. 2(a) and (b) are the SEM images of MgO/ TiO2thin film on the FTO subst
25、rate, which show auniform TiO2nano鄄particle size distribution and athickness of about 100 滋m. Meanwhile, an energy293摇新摇 型摇 炭摇 材摇 料第 30 卷dispersive spectrometer (EDS) is utilized to prove thepresence of Mg element as demonstrated in Fig. 3.There are C, Ti, Mg and O elements.Fig. 2摇 SEM images of MgO
26、 / TiO2photoanode: (a) the surface of the film and (b) the cut section of the film.Fig. 3摇 Energy dispersive spectrum of the compositefilm of MgO/ TiO2photoanode.摇 摇 Fig. 4 (a) and (b) are the high magnificationimages of the untreated and acid鄄oxidized MWCNTson substrates. MWCNTs form a 3D mesoporou
27、s net鄄work. The acid鄄oxidized MWCNTs have rough sur鄄face, which could be caused by the strong acid oxida鄄tion. In addition, the acid鄄oxidized MWCNTs have ahigher specific surface area than the untreated ones,which is helpful for the catalysis of CEs. The contac鄄ting area between electrolyte and coun
28、ter electrodes iscorrespondingly increased after the acid oxidation.Some misalignment or defect of MWCNTs could beformed after ultrasonic dispersing, which are activesites for catalytic reduction of I-3/ I2.Fig. 4摇 SEM images of the counter鄄electrodes (a) before and (b) after the acid oxidition.摇摇Fi
29、g. 5 shows IR spectra of the untreated andacid鄄oxidized MWCNTs.The acid鄄oxidized MWC鄄NTs have several distinctive peaks.The peak at3 350 cm-1is widened and extended to the low wave鄄number direction, which is attributed to the stretchingvibrationpeakofOHinCOOHfunctionalgroup. The peak at 1 728 cm-1詤詤
30、representsCOstretching vibration at the ends of the acid鄄oxidizedMWCNTs. The peak at 1 140 cm-1, shifted to thelow wavenumber direction, corresponds to the stretc鄄hing vibration of CO and COOH. The peak at1 610 cm-1is attributed to the bending vibration ofOH in water molecule. These polar groups pla
31、y akey role in enhancing the solubility and dispersing a鄄bility of the acid鄄oxidized MWCNTs in the solvent.Due to the weak thermal stability, these oxygen鄄con鄄taining groups are easily decomposed to release CO2or CO after the high temperature treatment. Corre鄄spondingly, surface defects and porosity
32、 increase,which contributes to a high specific surface area andcatalytic activity.Therefore, the acid treatment isbeneficial to the performance of the CEs.The surface resistance of different CEs was meas鄄ured with a four鄄point probe method and is listed in393第 5 期ZHENG Wei et al: Fabrication and cha
33、racterization of a multi鄄walled carbon . . . . .摇Table 2. The resistance of the CE with the acid鄄oxi鄄dized MWCNTs and nanographite powder is less thanthat of the CE with the untreated MWCNTs. It can beexplained that nanographite with an excellent conduc鄄tivity could be distributed into porous networ
34、k of theacid鄄oxidized MWCNTs, which increases electricalconductivity by enhancing the connection of the net鄄work.Fig. 5摇 Infrared spectra of the acid鄄oxidized MWCNTsand the untreated ones.摇 摇 Fig. 6(a) and 6(b) show the SEM and TEMimages of the CE with the acid鄄oxidized MWCNTsand nano鄄graphite powde
35、r, respectively.A well dispersing of nanographite into the net鄄work formed by the acid鄄oxidized MWCNTs can beobserved.The acid鄄oxidized MWCNTs and nano鄄graphite form a 3D structure.The acid鄄oxidizedMWCNTs have diameters of 10鄄20 nm and lengths of100鄄500 nm, and the nanographite powder has sizesof 20
36、0鄄300 nm.The cyclic voltammetry(CV) curves are shownin Fig. 7. The two peaks represent I3-reduction andI-oxidation reaction.According to Randles鄄Sevcikformula15, the larger is current density maximum(Jp) of I3-/ I-, the larger the migration rate of elec鄄trons in the electrode and the higher the acti
37、vity ofelectrochemical reduction. Within the five curves, Jpis the highest, up to 2. 1 mA/ cm2for the CE contai鄄ning the acid鄄oxidized MWCNTs and nanographite,indicating that the addition of nanographite into theacid鄄oxidized MWCNTs can improve the catalytic ac鄄tivity. Furthermore, its ratio of maxi
38、mum values oxi鄄dation and reduction is about 0. 89, close to 1, impl鄄ying an excellent reversibility and stability of the CEmaterials.Fig. 6摇 SEM and TEM images of the counter鄄electrode with the acid鄄oxidized MWCNTs and nano鄄graphite powder.Fig.7摇 CV curves for different CEs at a scanning speed of 5
39、0 mV/ s.摇摇To further understand the catalytic performanceof the CEs in DSSCs, the electrochemical impedancespectra (EIS) are shown in Fig. 8.Typically, aNyquist curve contains two or three semi鄄circles. Thefirst semi鄄circle at a high鄄frequency range is attributedto the charge鄄transfer resistance at
40、the interface be鄄tween the CE and electrolyte. The second semi鄄circleat a low鄄frequency is related to the electron transportand electron capture of TiO2photoanode/ electrolyteinterface, as well as the Warburg diffusion of the red鄄ox species in the electrolyte16,17.In Fig. 8 (a), carbon鄄based cells s
41、how lowerimpedances at the CE/ electrolyte interfaces than thePt鄄based one, though the first semi鄄circles are weak鄄ened and hided by some uncertain factors. Fig. 8bshows the magnification of impedance spectra in highfrequency area.Nevertheless, there is quite closecharge鄄transfer resistance ( about
42、5鄄10 赘) betweenthese carbon鄄based CEs.At the low鄄frequency re鄄gion, the impedance value of Pt CE is still the low鄄493摇新摇 型摇 炭摇 材摇 料第 30 卷est. While, why MWCNT electrodes have a highelectro鄄catalytic property is still controversial. Due tothe structural features of several coaxially arrangedgraphene
43、sheets, MWCNTs present large surfacearea, high electrical conductivity, corrosion resist鄄ance, and excellent electrocatalytic activity for tri鄄io鄄dide ion reduction18,19. Most support that a large in鄄ner surface area and defect鄄rich planes can considera鄄bly enhance the kinetics of electron transfer2
44、0, andalso may promote the reaction of I-3reduction at the in鄄terface between MWCNTs and electrolyte, which isreflected by the high鄄frequency semi鄄circles in theNyquist curve21.Fig. 8摇 Electrochemical impedance of DSSCs with different CEs: (a) the overall figure and (b) the magnified figure.摇 摇 In F
45、ig. 9 we report I鄄V curves of DSSCs fabrica鄄ted with different CEs when incident optical intensityis 80 mW/ cm2. Compared with carbon鄄based CEs,Pt鄄based DSSC is still the most excellent, which hasthe short鄄circuit current densities (Jsc), open鄄circuitvoltage (Voc) and photoelectric conversion effici
46、encyof 6. 07 mA/ cm2, 0. 59 V and 4. 48%, respectively.For the carbon鄄based DSSCs, sample 1 with the CEcontainingtheacid鄄oxidizedMWCNTsandnanographite achieves the best performance ( Jscof4. 67 mA/ cm2, Vocof 0. 53 V, photoelectric conver鄄sion efficiency up to 3. 10%). It is also indicated thatboth
47、the acid oxidation and nanographite addition con鄄tribute to an improvement of photovoltaic propertiesthrough increasing catalytic activity.However, theweak adherence of MWCNTs to the substrate maylead to their detachment from the substrate in a corro鄄sion electrolyte, which will be discussed in the
48、futureresearch.Fig. 9摇 Current鄄voltage curves of DSSCsfabricated with different CEs.4摇 Conclusion摇摇We fabricated CEs with MWCNTs as 3D net鄄work and nanographite 鄄powder as filler to form filmson FTO glass in DSSCs. MWCNTs were oxidized ina strong acid mixture to reduce their aggregation andnanograph
49、ite powder was filled to increase electricalconductivity.The performance of these CEs wereevaluated. Results show that the CE containing theacid鄄oxidized MWCNTs and nano鄄graphite powderexhibits the most excellent photoelectric propertiesamong carbon鄄based CEs, which has a photoelectricconversion eff
50、iciency up to 3. 1%, 27% higher thanthat of the MWCNT based CE cell. Although the effi鄄ciency of the carbon鄄based CE DSSCs are lower thanthat of the Pt鄄based one, carbon materials are the lowcost alternative CE materials for the expensive metals.References1摇 O爷Regan B, Gr覿tzel M. A Low鄄cost, high鄄ef
