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 威, 齐 涛, 张永超, 石海英, 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)

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 威, 齐 涛, 张永超, 石海英, 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)

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-V)进行光电性能分析。结果表明:酸化处理的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。

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

1 Introduction

Dye-sensitized solar cells(DSSCs)are relatively promising thin film solar cells with high conversion efficiency and low cost,which are regarded as a credible alternative to other photovoltaic cells［1,2］.As a part of DSSCs,the counter electrode(CE)is made of Pt element,an expensive material,which offers an excellent electrical conductivity and high catalytic activity［3］.But the drawbacks such as dissolution of the platinum thin film in the organic corrosive electrolyte and the expensive price need developing further more stable and cost-effective CE materials［4］. Carbon nano-materials have been the candidates because of their excellent conductive and mechanical property［5］. Wang［6］prepared a few-layer graphene film on indium-tin-oxide(ITO)glass that has a photoelectric conversion efficiency of 0.26%,much less than that using the Pt film as the CE in DSSCs.This can be attributed to a drop of electrical conductivity owing to the presence of defects in graphene.From then on, graphene-based CEs were modified in many ways toimprove their performance in DSSCs.Xue［7］prepared a N-doped graphene foam and Li［8］manufactured carbon nanotubes on a piece of graphene paper as the CEs to improve their catalytic activity.Both of these CE materials are three dimensional(3D).The carbon nanotubes were doped with metals or metal oxides to increase their electrode conductivity［9,10］.The graphene/organic composites have also been made a great progress as CE materials［11-13］.

We fabricated a CE,using MWCNTs as 3D matrix and nanographite powder as filler,to form the CE film on fluorine-tin-oxide(FTO)glass.MWCNTs were treated in a strong acid to reduce their aggregation and then filled with nanographite powder to increase their electrical conductivity.This modification procedure to prepare the composite structure is beneficial for providing fast electron transport channels for charge injection between MWCNTs and FTO［14］.

DSSCs were fabricated using several 3D composite samples as the CEs and a dye-sensitized MgO-doped TiO2as the photoanode material for evaluation of their property.The principle of DSSCs assembled in laboratory is illustrated in Fig.1.

Fig.1 Schematic representation of a DSSC with MWCNTs as the counter-electrode.

2 Experimental

Butyl titanate(CP),Titanium isopropoxide(≥99.7%),magnesium acetate and ethylene cellulose are analytical reagents,and butyl titanate is chemical reagent.Emulsifier OP-10(≥99.0%)was provided by Xingtai Lanxing additive plant(Hebei,China) and MWCNTs(the outside diameter>50 nm,length 10-20 μm,purity>98%,specific surface area> 40 m2/g)were bought from Beijing Dk nano S&T Ltd(Beijing,China).The nano-graphite with an average grain size of about 200 nm was supplied by Harbin TR S&T Ltd(Harbin,China).

2.1 Preparation of photoanode

TiO2colloids were prepared according to the procedure reported earlier［15］,which were deposited on the FTO glasses(200×150 mm,thickness 2.2 mm, resistance 14 Ω,transmittance 90%)and sintered at 450℃ for 1 h.Then the coated glasses were first immersed into an magnesium acetate solution in ethanol, then a 0.5 mmol/L N3dye solution in ethanol for 24 h.Finnaly,the coated and immersed glasses were dried at room temperature to obtain the dye-sensitized MgO/TiO2photoanode.

2.2 Preparation of counter electrodes

The MWCNTs were first oxidized by an acid mixture containing H2SO4(98%)and HNO3(67%) with a volume ratio of 3∶1 under sonication,then filtered and washed by deionized water to be neutral.A paste for the CEs was made,which was consisted of anhydrous alcohol,OP-10 as the dispersant,ethylene cellulose as the adhesive,the acid-oxidized MWCNTs and nanographite.The paste was deposited on the cleaned FTO glass substrates and heat-treated at 400℃ for 2 h under the nitrogen to obtain the CEs. DSSCswereassembled using thedye-sensitized MgO/TiO2as the anode and CEs as the cathodes, which were impregnated with an organic liquid electrolyte containing LiI,I2,ethylene glycol and acetonitrile.Four CEs were prepared according to the formulation listed in Table 1.For a comparison,the Pt electrode was produced by magnetron sputtering on FTO glass.

Table 1 The raw materials of 5 counter-electrodes.

2.3 Characterization

The morphology and microstructure of the photoanode and CEs were observed by SEM(Sirion200, Philip).The infrared spectra of samples were recorded by an IR spectrophotometer(T60SXBFTI, Nicole).The surface sheet resistance of the CEs was measured by a 4-point probe tester(SX1934:SZ-82).The photoelectric performance of the DSSCs was analyzed by EIS,CV and I-V curves with an electrochemical workstation(LK98B,Tianjin)under Xenon lamp illumination(80 mW/cm2).

3 Results and discussion

Fig.2(a)and(b)are the SEM images of MgO /TiO2thin film on the FTO substrate,which show a uniform TiO2nano-particle size distribution and a thickness of about 100 μm.Meanwhile,an energydispersive spectrometer(EDS)is utilized to prove the presence of Mg element as demonstrated in Fig.3. There are C,Ti,Mg and O elements.

Fig.2 SEM images of MgO/TiO2photoanode:(a)the surface of the film and(b)the cut section of the film.

Fig.3 Energy dispersive spectrum of the composite film of MgO/TiO2photoanode.

Fig.4(a)and(b)are the high magnification images of the untreated and acid-oxidized MWCNTs on substrates.MWCNTs form a 3D mesoporous network.The acid-oxidized MWCNTs have rough surface,which could be caused by the strong acid oxidation.In addition,the acid-oxidized MWCNTs have a higher specific surface area than the untreated ones, which is helpful for the catalysis of CEs.The contacting area between electrolyte and counter electrodes is correspondingly increased after the acid oxidation. Some misalignment or defect of MWCNTs could be formed after ultrasonic dispersing,which are active sites for catalytic reduction of/I2.

Fig.4 SEM images of the counter-electrodes(a)before and(b)after the acid oxidition.

Fig.5 shows IR spectra of the untreated and acid-oxidized MWCNTs.The acid-oxidized MWCNTs have several distinctive peaks.The peak at 3 350 cm-1is widened and extended to the low wavenumber direction,which is attributed to the stretching vibrationpeakof－OH in －COOH functional group.The peak at 1 728 cm-1＝represents C O stretching vibration at the ends of the acid-oxidized MWCNTs.The peak at 1 140 cm-1,shifted to the low wavenumber direction,corresponds to the stretching vibration of C－O and－COOH.The peak at 1 610 cm-1is attributed to the bending vibration of O－H in water molecule.These polar groups play a key role in enhancing the solubility and dispersing ability of the acid-oxidized MWCNTs in the solvent. Due to the weak thermal stability,these oxygen-containing groups are easily decomposed to release CO2or CO after the high temperature treatment.Correspondingly,surface defects and porosity increase, which contributes to a high specific surface area and catalytic activity.Therefore,the acid treatment is beneficial to the performance of the CEs.

The surface resistance of different CEs was measured with a four-point probe method and is listed inTable 2.The resistance of the CE with the acid-oxidized MWCNTs and nanographite powder is less than that of the CE with the untreated MWCNTs.It can be explained that nanographite with an excellent conductivity could be distributed into porous network of the acid-oxidized MWCNTs,which increases electrical conductivity by enhancing the connection of the network.

Fig.5 Infrared spectra of the acid-oxidized MWCNTs and the untreated ones.

Fig.6(a)and 6(b)show the SEM and TEM images of the CE with the acid-oxidized MWCNTs and nano-graphite powder,respectively.

A well dispersing of nanographite into the network formed by the acid-oxidized MWCNTs can be observed.The acid-oxidized MWCNTs and nanographite form a 3D structure.The acid-oxidized MWCNTs have diameters of 10-20 nm and lengths of 100-500 nm,and the nanographite powder has sizes of 200-300 nm.

The cyclic voltammetry(CV)curves are shown in Fig.7.The two peaks represent I3-reduction and I-oxidation reaction.According to Randles-Sevcik formula［15］,the larger is current density maximum (Jp)of I3-/I-,the larger the migration rate of electrons in the electrode and the higher the activity of electrochemical reduction.Within the five curves,Jp is the highest,up to 2.1 mA/cm2for the CE containing the acid-oxidized MWCNTs and nanographite, indicating that the addition of nanographite into the acid-oxidized MWCNTs can improve the catalytic activity.Furthermore,its ratio of maximum values oxidation and reduction is about 0.89,close to 1,implying an excellent reversibility and stability of the CE materials.

Fig.6 SEM and TEM images of the counter-electrode with the acid-oxidized MWCNTs and nano-graphite powder.

Fig.7 CV curves for different CEs at a scanning speed of 50 mV/s.

To further understand the catalytic performance of the CEs in DSSCs,the electrochemical impedance spectra(EIS)are shown in Fig.8.Typically,a Nyquist curve contains two or three semi-circles.The first semi-circle at a high-frequency range is attributed to the charge-transfer resistance at the interface between the CE and electrolyte.The second semi-circle at a low-frequency is related to the electron transport and electron capture of TiO2photoanode/electrolyte interface,as well as the Warburg diffusion of the redox species in the electrolyte［16,17］.

In Fig.8(a),carbon-based cells show lower impedances at the CE/electrolyte interfaces than the Pt-based one,though the first semi-circles are weakened and hided by some uncertain factors.Fig.8b shows the magnification of impedance spectra in high frequency area.Nevertheless,there is quite close charge-transfer resistance(about 5-10 Ω)between these carbon-based CEs.At the low-frequency region,the impedance value of Pt CE is still the low-est.While,why MWCNT electrodes have a high electro-catalytic property is still controversial.Due to the structural features of several coaxially arranged graphene sheets,MWCNTs present large surface area,high electrical conductivity,corrosion resistance,and excellent electrocatalytic activity for tri-iodide ion reduction［18,19］.Most support that a large inner surface area and defect-rich planes can considerably enhance the kinetics of electron transfer［20］,and also may promote the reaction of I-3reduction at the interface between MWCNTs and electrolyte,which is reflected by the high-frequency semi-circles in the Nyquist curve［21］.

Fig.8 Electrochemical impedance of DSSCs with different CEs:(a)the overall figure and(b)the magnified figure.

In Fig.9 we report I-V curves of DSSCs fabricated with different CEs when incident optical intensity is 80 mW/cm2.Compared with carbon-based CEs, Pt-based DSSC is still the most excellent,which has the short-circuit current densities(Jsc),open-circuit voltage(Voc)and photoelectric conversion efficiency of 6.07 mA/cm2,0.59 V and 4.48%,respectively. For the carbon-based DSSCs,sample 1 with the CE containing the acid-oxidized MWCNTs and nanographite achieves the best performance(Jscof 4.67 mA/cm2,Vocof 0.53 V,photoelectric conversion efficiency up to 3.10%).It is also indicated that both the acid oxidation and nanographite addition contribute to an improvement of photovoltaic properties through increasing catalytic activity.However,the weak adherence of MWCNTs to the substrate may lead to their detachment from the substrate in a corrosion electrolyte,which will be discussed in the future research.

Fig.9 Current-voltage curves of DSSCs fabricated with different CEs.

4 Conclusion

We fabricated CEs with MWCNTs as 3D network and nanographite-powder as filler to form films on FTO glass in DSSCs.MWCNTs were oxidized in a strong acid mixture to reduce their aggregation and nanographite powder was filled to increase electrical conductivity.The performance of these CEs were evaluated.Results show that the CE containing the acid-oxidized MWCNTs and nano-graphite powder exhibits the most excellent photoelectric properties among carbon-based CEs,which has a photoelectric conversion efficiency up to 3.1%,27%higher than that of the MWCNT based CE cell.Although the efficiency of the carbon-based CE DSSCs are lower than that of the Pt-based one,carbon materials are the low cost alternative CE materials for the expensive metals.

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Fabrication and characterization of a multi-walled carbon nanotube-based counter electrode for dye-sensitized solar cells

ZHENG Wei, QI Tao, ZHANG Yong-chao, SHI Hai-ying, TIAN Jun-qing
(College of Material Science and Engineering,Harbin University of Science and Technology,Harbin 150040,China)

A counter-electrode(CE)for dye-sensitized solar cells(DSSCs)was prepared by coating a slurry containing acid-oxidized multi-wall carbon nanotubes and nano-graphite powder onto a fluorine-doped tin oxide conducting glass substrate.The samples were characterized by SEM,TEM,EDS and IR spectroscopy.The CE performance in the DSSCs with MgO-doped TiO2as the photoanode was investigated by measurements of current-voltage curves,cyclic voltammetry and electrochemical impedance spectroscopy.Results show that the cell with the CE exhibits the best photoelectric properties of all the carbon-based CEs investigated.The short-circuit current density(Jsc)is 4.67 mA/cm2,the open-circuit voltage(Voc)is 0.53 V and photoelectric conversion efficiency is up to 4.10%,which are comparable with those of the Pt-based CE in DSSCs.

DSSC；Counter electrodes；MWCNTs；Graphite；Photocatalytic activity

ZHENG Wei,Ph.D,Professor.E-mail:zhengwei1972@sina.com

TM914.4+2

A

2015-06-28;

2015-09-29

哈尔滨市科技创新人才项目(2013RFXXJ004).

郑 威,博士,教授.E-mail:zhengwei1972@sina.com.

1007-8827(2015)05-0391-06

Foundation item:Project of Harbin Science and Technology Innovation Talents(2013RFXXJ004).

10.1016/S1872-5805(15)60198-6

Receiced date:2015-06-11； Renised date:2015-10-08

English edition available online ScienceDirect(http://www.sciencedirect.com/science/journal/18725805).