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Effects of nanoparticle additives on the properties of agarose polymer electrolytes
Journal of Power Sources 248 (2014) 988e993
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effects of nanoparticle additives on the properties of agarose polymer electrolytes Ying Yang a, b, Jiarui Cui a, Pengfei Yi a, Xiaolu Zheng a, Xueyi Guo a, *, Wenyong Wang b, ** a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
h i g h l i g h t s TiO2, Co3O4, and NiO nanoparticle additive modified agarose electrolytes. Pore-filling of the agarose electrolyte improved by Co3O4 nanoparticle additive. Improved DSSC efficiency/stability with agaroseeCo3O4/NiO nanoparticle electrolytes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2013 Received in revised form 3 October 2013 Accepted 6 October 2013 Available online 17 October 2013
In this work we study the effects of TiO2, Co3O4, and NiO nanoparticle additives on the properties of agarose polymer electrolytes. We characterize the crystallinity, surface morphology, ion concentration, ionic conductivity, and pore filling capability of the modified electrolytes using a variety of experimental methods. Solid-state dye-sensitized solar cells are also fabricated using the modified agarose electrolytes, and their photovoltaic and electrochemical performances are examined. The DSSC based on Co3O4modified agarose electrolyte shows the best device performance including a power conversion efficiency of 2.11% and good device stability after 400 h of testing. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Polymer electrolyte Solid-state dye-sensitized solar cells Stability
1. Introduction Because of their low fabrication cost and potentially flexible structures, dye-sensitized solar cells (DSSCs) have attracted significant research interest since the ground-breaking work of Grätzel and coworkers [1]. However, it is well known that typical liquid electrolytes utilized in DSSCs have issues such as being volatile and corrosive, which cause difficulties for device packaging and limit long-term performance of the cells. A promising approach to bypassing these issues is to replace the liquid electrolytes with solid-state or quasi-solid-state electrolytes including p-type inorganic semiconductors, organic hole transport materials, or polymer electrolytes [2e6]. Among them, the polymer electrolytes have certain advantages such as high ionic conductivity and good
* Corresponding author. Tel.: þ86 731 88877863; fax: þ86 731 88836207. ** Corresponding author. Tel.: þ1 307 766 6523; fax: þ1 307 766 2652. E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Guo), [email protected] (W. Wang). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.10.016
penetration into porous photoanode thin films, which could benefit the performance of solid-state DSSCs based on these materials. We have recently carried out a series of studies of agarose-based electrolytes for DSSC applications [7,8]. Agarose is a linear agarobiose (d-galactose and 3, 6-anhydro-l-galactopyranose) disaccharide obtained from seaweed, which is considered to be an effective polymer matrix for the formation of cross-linking networks with other electrolyte components, which can benefit the ionic transport in the material [7]. In our previous work we have prepared agarose polymers modified by magnetic nanoparticle additives, such as Fe3O4 or NiO additives, and have studied the effects of magnetic field treatment on the properties of the composite electrolytes [9,10]. In this work we have continued this investigation and have utilized non-magnetic TiO2 nanoparticles and magnetic Co3O4 and NiO nanoparticles to modify the agarose electrolytes. We have performed a comparison study of the effects of these nanoparticle additives on the properties of the agarose electrolytes, such as electrolyte crystallinity, ion concentration, ionic conductivity, and pore filling capability. We have also fabricated solid-state DSSCs based on the modified agarose electrolytes
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and have characterized their photovoltaic and electrochemical performance and device stability. 2. Experimental 2.1. Sample preparation 0.4 g agarose (Acros Organics, Thermo Fisher, Waltham, MA, USA) was dissolved in 20 g N-methyl-2-pyrrolidinone (NMP) (Sinopharm Chemical Reagent Co., Ltd (SCRC), Shanghai, China) under continuously stirring at 75 C in a water bath. After 4 h, 0.0124 g TiO2 (P25, Degussa-Evonic, AG, Essen, Germany), 0.0124 g Co3O4, or 0.0124 g NiO nanoparticles (20 nm in diameter, Aladdin, Shanghai, China) were added into the agarose solution, and the solution was stirred at 75 C for another 2 h. 0.08143 g solid LiI (Acros) and 0.01542 g I2 (SCRC, Shanghai, China) were then added to the above polymer solution under ambient conditions, and the solution was stirred continuously until a homogeneous viscous liquid was formed. To prepare the photoanode film, TiO2 nanoparticle paste was deposited from TiO2 suspensions prepared in advance by the doctoreblade technique on a conducting glass substrate (Fedoped SnO2, 15 U sq1, Yaohua FRP Co. Ltd., Qinghuangdao, Hebei Province, China), followed by sintering at 450 C for 30 min. The TiO2 photoanode film was preheated at 120 C for 30 min before being immersed into 0.5 mM N719 dye solution (dietetrabutylammonium cis-bis (isothiocyanato)bis (2,20 -bipyridyl-4, 40 -dicarboxylato) ruthenium (II) (Chemsolarism, Suzhou, Jiangsu Province, China). The assembly of dye-sensitized solar cells was performed following the procedure published in a previous report [7]. 2.2. Sample characterization Differential Scanning Calorimetry (DSC) measurement was carried out using SDT Q600 (TA Instruments, New Castle, DE, USA) to measure the melting temperature (Tm) of the agarose electrolytes with different nanoparticle additives at a heating rate of 5 C min1 from 20 to 400 C under N2 environment. The surface morphologies of the nanoparticle additive modified agarose electrolytes and the cross-sections of the corresponding DSSCs were examined by a Scanning Electron Microscope (SEM) (JSM-6360LV, JEOL Company, Tokyo, Japan). The UV-Vis spectra of the polymer electrolytes were measured using a U-4100 spectrophotometer (Hitachi, Tokyo, Japan). The ionic conductivities of the agarose electrolytes modified with different nanoparticle additives were characterized using an electrochemical analyzer (CHI604D, Chenhua, Shanghai, China) in the frequency range of 10 Hze100 MHz under a 10 mV perturbation. The performance of the DSSCs based on the modified agarose electrolytes was characterized using the electrochemical analyzer as a current densityevoltage (JeV) curve recorder under 100 mW cm2 illumination that was provided by a Xe lamp (CHFe XM500, Trust-tech, Beijing, China). The intensity of the incident light was calibrated by a Si-1787 photodiode (Hamamatsu Photonics K K, Hamamatsu, Japan) and the tested area of DSSCs is 0.25 cm2. The recombination resistances of the DSSCs were measured using the electrochemical analyzer in the frequency range of 105e103 Hz under a perturbation voltage of 10 mV.
Fig. 1. Thermograms of pure and nanoparticle-modified agarose polymer electrolytes.
nanoparticle additives in the temperature range of 20e400 C, which were obtained using DSC measurements. The thermal data including Tm and melting enthalpy (DHm) are shown in Table 1. It can be seen from Fig. 1 that the pure agarose polymer exhibits an intense melting endotherm at w280 C, which indicates a high degree of crystallinity of the polymer. After adding TiO2, Co3O4, or NiO nanoparticle additives, all of the modified agarose polymers exhibit melting endotherms at lower temperatures in the range of 227e240 C. The modified polymers also show lower values of melting enthalpy compared to that of the pure agarose as shown in Table 1, suggesting a reduction in the crystallinity of the modified polymers. This reduction in crystallinity may be attributed to the cross-linking effect of the nanoparticle additives with the agarose polymer matrix [11]. It is known that most of the crystalline phase of a polymer is formed by polymer chain rearrangement [12]; however, after adding the nanoparticle additives, the cross-linking interactions between the polymer matrix and the additives occur and the rearrangement of the polymer chains is inhibited, which can prevent the crystallization of the polymer and results in a reduction in crystallinity. From Table 1 it is also noticed that the melting enthalpy of TiO2-modified agarose polymer is substantially larger than that of a Co3O4 or NiO-modified material. This difference might be associated with the crystallinity of TiO2-modified agarose polymer which is higher than that of Co3O4 or NiOmodified one. This difference could be associated with nanoparticle aggregation that is related to the surface properties of the nanoparticle additives. According to the study by MartineGonzález et al., TiO2 nanoparticles show a more acidic surface than that of Co3O4 nanoparticles with similar size [13]. On the other hand, it is known that the acidity and basicity of transition metal oxides are determined by the electronegativity and radius of the metal ions [14]. Because the Co3O4 and NiO nanoparticles used in this experiment have similar particle size and also have similar metal ion radius and electronegativity [15], their surface acidity is considered to be similar. Therefore, the TiO2 nanoparticles have a more acidic surface than those of Co3O4 or NiO nanoparticles. Since a more acidic oxide surface corresponds to more surface hydroxyl groups
3. Results and discussion 3.1. Thermal properties of the agarose polymer electrolytes with different nanoparticle additives
Table 1 Thermal properties of pure and nanoparticle additive modified agarose polymers.
Fig. 1 shows the thermograms of pure agarose and agarose polymer electrolytes modified with TiO2, Co3O4, and NiO
Tm ( C) DHm (J g1)
Pure agarose
TiO2-modified
Co3O4-modified
NiO-modified
280.6 175.5
226.4 159.8
240.3 111.5
227.5 104.5
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Y. Yang et al. / Journal of Power Sources 248 (2014) 988e993
Fig. 2. Surface morphologies of agarose electrolytes with different nanoparticle additives: (a) TiO2, (b) Co3O4, and (c) NiO.
[13], it is anticipated that the TiO2 nanoparticles possess more surface hydroxyl groups, which results in stronger hydrogen bonding interactions between the nanoparticles that can cause easy formation of nanoparticle aggregates. The formation of nanoparticle aggregates hinders the interactions between the polymer segments and nanoparticles, which causes a less efficient reduction in crystallinity of the polymer matrix. This explains why the TiO2-modified agarose polymer exhibits a higher crystallinity than that of the modified Co3O4 or NiO material. 3.2. Surface morphology study of the agarose electrolytes with different nanoparticle additives Fig. 2 shows SEM images of agarose polymer electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. The TiO2-modified electrolyte (Fig. 2a) displays a rough surface with cross-linked bundles that are due to the crystallization of the polymer matrix. However, as shown in Fig. 2b and c, the Co3O4 and NiO-modified polymer electrolytes show a smooth surface without bundles, indicating an improved reduction in crystallinity compared to that of the TiO2-modified material. This is consistent with the thermal analysis discussion presented above (Fig. 1). A low crystallinity of polymer electrolyte is considered to be advantageous for pore filling and forming good interfacial contact between the electrolyte and the nanocrystalline TiO2 photoanode. 3.3. UVeVis absorption spectra of additiveemodified agarose polymer electrolytes Fig. 3 shows the UVeVis absorption spectra of TiO2, Co3O4, and NiO nanoparticle additive modified agarose electrolytes. The peaks around 297 and 368 nm are attributed to the presence of Ie 3 in the
polymer electrolytes [16]. It can be seen from Fig. 3 that the absorption peak intensities follow the order of Co3O4 > NiO > TiO2. A higher absorption peak intensity indicates a higher Ie 3 concentration in the electrolyte, which leads to a better ionic conductivity [17]. It is known that the nanoparticle additives in a polymer electrolyte play the role of Lewis acidebase interaction centres with the ionic species in the electrolyte. The surface hydroxyl groups of the nanoparticles are expected to interact with the salt anions through the formation of hydrogen bonds [18]. As mentioned above, TiO2 nanoparticles have the highest surface acidity among the three additives and thus have the most surface hydroxyl groups. These surface hydroxyl groups not only promote nanoparticle aggregation but also interact with the anions (I and I 3 ) in the electrolyte, which subsequently lowers the free mobile anion concentration [18]. Since TiO2 nanoparticles have the most surface hydroxyl groups, the TiO2-modified agarose electrolyte exhibits the lowest free anion concentration as shown in Fig. 3. 3.4. Ionic conductivity of additive-modified agarose polymer electrolytes Table 2 shows the ionic conductivities of the agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. The TiO2-modified agarose electrolyte exhibits the lowest ionic conductivity of 2.66 103 S cm1, while the highest ionic conductivity of 4.37 103 S cm1 is observed for the Co3O4-modified electrolyte. It is known that ions transport rapidly in the amorphous phase of a polymer electrolyte [19,20]. From Figs. 1 and 2 it is concluded that the Co3O4 and NiO-modified agarose electrolytes have more amorphous phase in the electrolyte films than that of the TiO2modified material. In addition, the ion concentration in an electrolyte plays an important role in determining its ionic conductivity, and from Fig. 3 it can be seen that the Co3O4-modified electrolyte shows the highest ion concentration. Due to these two factors the highest ionic conductivity is obtained for the Co3O4modified electrolyte, and the TiO2-modified one shows the lowest ionic conductivity among the three electrolytes. 3.5. Pore filling properties of agarose polymer electrolytes with different nanoparticle additives Fig. 4 shows the cross-sectional SEM images of the TiO2 photoanodes before and after adding the agarose electrolytes modified with TiO2, Co3O4, or NiO nanoparticle additives. Fig. 4a shows the Table 2 Ionic conductivities of agarose electrolytes modified with different nanoparticle additives.
Fig. 3. UVeVis spectra of agarose electrolytes modified with different nanoparticle additives.
Ionic conductivity (S cm1)
TiO2-modified
Co3O4-modified
NiO-modified
2.66 103
4.37 103
3.33 103
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Fig. 4. Cross-sectional SEM images of TiO2 photoanodes (a) before and after introducing agarose electrolytes modified with (b) TiO2, (c) Co3O4, and (d) NiO nanoparticle additives.
bare TiO2 porous film before introducing the agarose electrolyte, where distinguishable TiO2 nanoparticles with many pores in the film can be seen. The degree of penetration of the agarose electrolytes with different nanoparticle additives into the porous TiO2 films can be seen in Fig. 4bed. The TiO2-modified electrolyte shows the worst penetration into the porous photoanode film, and the electrolyte coverage of the photoanode is very poor (Fig. 4b). On the contrary, as can be seen in Fig. 4c, the Co3O4-modified agarose
electrolyte exhibits a very good filling into the pores of the TiO2 photoanode film, and most of the TiO2 nanoparticle surface is covered by the electrolyte such that the nanoparticles become indistinguishable. The NiO-modified electrolyte also shows improved pore filling and penetration capability compared to the TiO2-modified electrolyte, but not as good as that of Co3O4-modified material. The low pore filling capability of the TiO2emodified agarose electrolyte could be attributed to its high crystallinity as discussed previously and its rough surface with cross-linked bundles as seen in Fig. 2a, which could significantly lower its flowability compared to the Co3O4 and NiO-modified electrolytes. 3.6. Effects of the nanoparticle additives on photovoltaic and electrochemical performance of the DSSCs based on agarose electrolytes
Fig. 5. JeV characteristics of the DSSCs fabricated using agarose electrolytes with TiO2, Co3O4, and NiO nanoparticle additives.
JeV characteristics of the DSSCs fabricated with agarose electrolytes modified by TiO2, Co3O4, and NiO nanoparticle additives are obtained under 1 Sun (100 mW cm2), and the results are presented in Fig. 5. Table 3 summarizes the cell performance parameters obtained from Fig. 5. Compared to the DSSC fabricated with TiO2-modified electrolyte, improvement in power conversion efficiency (h) is observed for the cells fabricated with Co3O4 or NiOmodified electrolyte, and the cell fabricated with Co3O4-modified electrolyte shows the highest efficiency at 2.11%. The short-circuit current (Jsc) also exhibits the same trend. It is known that in order to have an enhanced photovoltaic performance the reactions are required to be efficient in DSSCs [21]:
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Table 3 Performance parameters of the DSSCs fabricated using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. Agarose electrolyte modified with
Jsc(mA cm2)
Voc(V)
FF
h (%)
TiO2 Co3O4 NiO
5.28 7.24 6.20
0.605 0.635 0.625
0.55 0.46 0.52
1.71 2.11 2.02
I þ I 2 4 I3 þ I2 4 I5
(1)
3I þ 2dyeþ / I 3 þ 2dye (TiO2 photoanode)
(2)
I 3 þ 2e 4 3I (counter electrode)
(3)
From the UVeVis measurement results presented in Fig. 3, the TiO2-modified electrolyte shows the lowest I 3 concentration. This low Ie 3 concentration can cause an inefficient I3 diffusion process, which consequently leads to a decreased Jsc. Hence, the DSSC fabricated with TiO2emodified electrolyte exhibits the lowest Jsc. The openecircuit voltage (Voc) of this DSSC is also low compared to those of the cells containing Co3O4 or NiO-modified electrolyte (Table 3). As discussed above, TiO2 nanoparticles have more surface hydroxyl groups than Co3O4 or NiO nanoparticles, which can react with the anions of the LiI salt [18]. This can lead to more mobile Liþ in the TiO2-modified electrolyte compared with the Co3O4 or NiO-modified material. These lithium ions can be adsorbed on the surface of the porous TiO2 photoanode film and subsequently decrease the electron concentration in the TiO2 conduction band, which causes a low Voc of the DSSC with TiO2-modified electrolyte [22,23]. Electrochemical impedance spectroscopy (EIS) measurement was also carried out to study the charge transport and recombination processes in the DSSCs. The EIS Nyquist plots are shown in Fig. 6, and three semicircles are observed in the plots, which correspond to the charge transfer process at the Pt/electrolyte interface (frequency range of 105e103 Hz), charge recombination process at the TiO2/dye/electrolyte interface (frequency range of 103e1 Hz), and ion diffusion process occurring in the electrolyte (frequency range of 1e103 Hz), respectively [24]. The charge recombination resistances (R2) of the DSSCs obtained from the Nyquist plots follow the order Co3O4 (23.1 U) > NiO (19.1 U) > TiO2
Fig. 6. EIS Nyquist plots of the DSSCs fabricated using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives.
(17 U). The cell with Co3O4-modified electrolyte shows the highest recombination resistance, while the cell with TiO2-modified electrolyte shows the lowest. According to a previous study, an increased R2 indicates a suppressed charge recombination reaction between the photoanode conduction band electrons and the triiodide ions [I 3 þ 2ecb (TiO2) ¼ 3I ] at the TiO2 photoanode/ electrolyte interface, which can lead to a higher Voc [25]. Hence, the R2 measurement results are consistent with the Voc data presented in Table 3. 3.7. Stability of the DSSCs based on agarose electrolytes with different nanoparticle additives Fig. 7 shows the normalized power conversion efficiency as a function of time for the DSSCs using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. It can be seen that the normalized efficiency of the DSSC with TiO2-modified electrolyte exhibits a w50% decrease after 164 h testing (without sealing), while the cells containing Co3O4 or NiO-modified electrolyte can maintain the initial efficiencies after 400 h testing under the same condition. This improvement in DSSC stability may be attributed to the better pore filling capability of Co3O4 and NiO-modified electrolytes as discussed. It is known that the organic solvent in electrolytes can cause device sealing issues; however, the organic solvent has also been considered as gelator in quasi-solid-state polymer electrolytes, which plays the role of a cross-linker that can benefit the ionic transport process [26]. During DSSC fabrication, after drying at an elevated temperature to form quasi-solidstate electrolyte film in the cell, a small amount of organic solvent still remains in the polymer electrolyte, which can maintain the effective ionic transport pathways [26]. As time goes by, the organic solvent will evaporate slowly and the efficiency of the DSSC will subsequently decrease. To improve the stability of solid-state DSSCs, it is important to have sufficient polymer electrolyte in the porous photoanode film to slow down this solvent evaporation process. As discussed previously, the TiO2-modified electrolyte studied in this work shows a high crystallinity and a rough surface morphology (Figs. 1 and 2), which affects its flowability. Compared to Co3O4 and NiOemodified electrolytes, the TiO2-modified agarose electrolyte has the lowest penetration into the porous photoanode film (Fig. 4), and a large portion of the electrolyte stays on top of the photoanode, which subsequently promotes the evaporation of the organic solvent. Hence, the DSSC using the TiO2-modified electrolyte shows the most rapid degradation (Fig. 7).
Fig. 7. Power conversion efficiency as a function of time for the DSSCs based on agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives.
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4. Conclusions
References
In this work we have studied the effects of non-magnetic (TiO2) and magnetic (Co3O4 and NiO) nanoparticle additives on the properties of agarose polymer electrolytes. Using DSC, SEM, UVeVis, and conductivity measurement methods we have characterized the crystallinity, surface morphology, ionic conductivity, and pore filling capability of the nanoparticle additive modified electrolytes. The photovoltaic and electrochemical performance and the stability of the DSSCs based on the modified polymer electrolytes have also been investigated. It has been observed that among the three types of modified agarose electrolytes that with the Co3O4 nanoparticle additive shows the best pore-filling capability, and the DSSC based on this electrolyte also shows the best power conversion efficiency of 2.11%. This improved performance has been associated with the lower surface acidity, hence fewer surface hydroxyl groups of Co3O4 nanoparticles compared to that of the TiO2 nanoparticles, which decreases the crystallinity of the agarose electrolyte and increases the ion concentration. This observation might be used to direct the design of solid-state DSSCs based on polymer electrolytes with further enhanced performance. The focus of our study has been to synthesize and understand the properties of nanoparticle additivemodified agarose electrolytes. It would be interesting to examine the use of this material to replace spiro-MeOTAD [(2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9, 90 -spirobifluorene] used in DSSCs using organic primary amine halide perovskites to examine whether if this electrolyte would further improve device performance. See, for example, the very recent results of Grätzel and coworkers for a DSSC with 15% efficiency [27].
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Acknowledgements We acknowledge the financial support of the National Nature Science Foundation of China (61006047). We also acknowledge financial support from the University of Wyoming’s School of Energy Resources.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Effects of nanoparticle additives on the properties of agarose polymer electrolytes Ying Yang a, b, Jiarui Cui a, Pengfei Yi a, Xiaolu Zheng a, Xueyi Guo a, *, Wenyong Wang b, ** a b
School of Metallurgy and Environment, Central South University, Changsha 410083, China Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
h i g h l i g h t s TiO2, Co3O4, and NiO nanoparticle additive modified agarose electrolytes. Pore-filling of the agarose electrolyte improved by Co3O4 nanoparticle additive. Improved DSSC efficiency/stability with agaroseeCo3O4/NiO nanoparticle electrolytes.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2013 Received in revised form 3 October 2013 Accepted 6 October 2013 Available online 17 October 2013
In this work we study the effects of TiO2, Co3O4, and NiO nanoparticle additives on the properties of agarose polymer electrolytes. We characterize the crystallinity, surface morphology, ion concentration, ionic conductivity, and pore filling capability of the modified electrolytes using a variety of experimental methods. Solid-state dye-sensitized solar cells are also fabricated using the modified agarose electrolytes, and their photovoltaic and electrochemical performances are examined. The DSSC based on Co3O4modified agarose electrolyte shows the best device performance including a power conversion efficiency of 2.11% and good device stability after 400 h of testing. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Polymer electrolyte Solid-state dye-sensitized solar cells Stability
1. Introduction Because of their low fabrication cost and potentially flexible structures, dye-sensitized solar cells (DSSCs) have attracted significant research interest since the ground-breaking work of Grätzel and coworkers [1]. However, it is well known that typical liquid electrolytes utilized in DSSCs have issues such as being volatile and corrosive, which cause difficulties for device packaging and limit long-term performance of the cells. A promising approach to bypassing these issues is to replace the liquid electrolytes with solid-state or quasi-solid-state electrolytes including p-type inorganic semiconductors, organic hole transport materials, or polymer electrolytes [2e6]. Among them, the polymer electrolytes have certain advantages such as high ionic conductivity and good
* Corresponding author. Tel.: þ86 731 88877863; fax: þ86 731 88836207. ** Corresponding author. Tel.: þ1 307 766 6523; fax: þ1 307 766 2652. E-mail addresses: [email protected] (Y. Yang), [email protected] (X. Guo), [email protected] (W. Wang). 0378-7753/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2013.10.016
penetration into porous photoanode thin films, which could benefit the performance of solid-state DSSCs based on these materials. We have recently carried out a series of studies of agarose-based electrolytes for DSSC applications [7,8]. Agarose is a linear agarobiose (d-galactose and 3, 6-anhydro-l-galactopyranose) disaccharide obtained from seaweed, which is considered to be an effective polymer matrix for the formation of cross-linking networks with other electrolyte components, which can benefit the ionic transport in the material [7]. In our previous work we have prepared agarose polymers modified by magnetic nanoparticle additives, such as Fe3O4 or NiO additives, and have studied the effects of magnetic field treatment on the properties of the composite electrolytes [9,10]. In this work we have continued this investigation and have utilized non-magnetic TiO2 nanoparticles and magnetic Co3O4 and NiO nanoparticles to modify the agarose electrolytes. We have performed a comparison study of the effects of these nanoparticle additives on the properties of the agarose electrolytes, such as electrolyte crystallinity, ion concentration, ionic conductivity, and pore filling capability. We have also fabricated solid-state DSSCs based on the modified agarose electrolytes
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and have characterized their photovoltaic and electrochemical performance and device stability. 2. Experimental 2.1. Sample preparation 0.4 g agarose (Acros Organics, Thermo Fisher, Waltham, MA, USA) was dissolved in 20 g N-methyl-2-pyrrolidinone (NMP) (Sinopharm Chemical Reagent Co., Ltd (SCRC), Shanghai, China) under continuously stirring at 75 C in a water bath. After 4 h, 0.0124 g TiO2 (P25, Degussa-Evonic, AG, Essen, Germany), 0.0124 g Co3O4, or 0.0124 g NiO nanoparticles (20 nm in diameter, Aladdin, Shanghai, China) were added into the agarose solution, and the solution was stirred at 75 C for another 2 h. 0.08143 g solid LiI (Acros) and 0.01542 g I2 (SCRC, Shanghai, China) were then added to the above polymer solution under ambient conditions, and the solution was stirred continuously until a homogeneous viscous liquid was formed. To prepare the photoanode film, TiO2 nanoparticle paste was deposited from TiO2 suspensions prepared in advance by the doctoreblade technique on a conducting glass substrate (Fedoped SnO2, 15 U sq1, Yaohua FRP Co. Ltd., Qinghuangdao, Hebei Province, China), followed by sintering at 450 C for 30 min. The TiO2 photoanode film was preheated at 120 C for 30 min before being immersed into 0.5 mM N719 dye solution (dietetrabutylammonium cis-bis (isothiocyanato)bis (2,20 -bipyridyl-4, 40 -dicarboxylato) ruthenium (II) (Chemsolarism, Suzhou, Jiangsu Province, China). The assembly of dye-sensitized solar cells was performed following the procedure published in a previous report [7]. 2.2. Sample characterization Differential Scanning Calorimetry (DSC) measurement was carried out using SDT Q600 (TA Instruments, New Castle, DE, USA) to measure the melting temperature (Tm) of the agarose electrolytes with different nanoparticle additives at a heating rate of 5 C min1 from 20 to 400 C under N2 environment. The surface morphologies of the nanoparticle additive modified agarose electrolytes and the cross-sections of the corresponding DSSCs were examined by a Scanning Electron Microscope (SEM) (JSM-6360LV, JEOL Company, Tokyo, Japan). The UV-Vis spectra of the polymer electrolytes were measured using a U-4100 spectrophotometer (Hitachi, Tokyo, Japan). The ionic conductivities of the agarose electrolytes modified with different nanoparticle additives were characterized using an electrochemical analyzer (CHI604D, Chenhua, Shanghai, China) in the frequency range of 10 Hze100 MHz under a 10 mV perturbation. The performance of the DSSCs based on the modified agarose electrolytes was characterized using the electrochemical analyzer as a current densityevoltage (JeV) curve recorder under 100 mW cm2 illumination that was provided by a Xe lamp (CHFe XM500, Trust-tech, Beijing, China). The intensity of the incident light was calibrated by a Si-1787 photodiode (Hamamatsu Photonics K K, Hamamatsu, Japan) and the tested area of DSSCs is 0.25 cm2. The recombination resistances of the DSSCs were measured using the electrochemical analyzer in the frequency range of 105e103 Hz under a perturbation voltage of 10 mV.
Fig. 1. Thermograms of pure and nanoparticle-modified agarose polymer electrolytes.
nanoparticle additives in the temperature range of 20e400 C, which were obtained using DSC measurements. The thermal data including Tm and melting enthalpy (DHm) are shown in Table 1. It can be seen from Fig. 1 that the pure agarose polymer exhibits an intense melting endotherm at w280 C, which indicates a high degree of crystallinity of the polymer. After adding TiO2, Co3O4, or NiO nanoparticle additives, all of the modified agarose polymers exhibit melting endotherms at lower temperatures in the range of 227e240 C. The modified polymers also show lower values of melting enthalpy compared to that of the pure agarose as shown in Table 1, suggesting a reduction in the crystallinity of the modified polymers. This reduction in crystallinity may be attributed to the cross-linking effect of the nanoparticle additives with the agarose polymer matrix [11]. It is known that most of the crystalline phase of a polymer is formed by polymer chain rearrangement [12]; however, after adding the nanoparticle additives, the cross-linking interactions between the polymer matrix and the additives occur and the rearrangement of the polymer chains is inhibited, which can prevent the crystallization of the polymer and results in a reduction in crystallinity. From Table 1 it is also noticed that the melting enthalpy of TiO2-modified agarose polymer is substantially larger than that of a Co3O4 or NiO-modified material. This difference might be associated with the crystallinity of TiO2-modified agarose polymer which is higher than that of Co3O4 or NiOmodified one. This difference could be associated with nanoparticle aggregation that is related to the surface properties of the nanoparticle additives. According to the study by MartineGonzález et al., TiO2 nanoparticles show a more acidic surface than that of Co3O4 nanoparticles with similar size [13]. On the other hand, it is known that the acidity and basicity of transition metal oxides are determined by the electronegativity and radius of the metal ions [14]. Because the Co3O4 and NiO nanoparticles used in this experiment have similar particle size and also have similar metal ion radius and electronegativity [15], their surface acidity is considered to be similar. Therefore, the TiO2 nanoparticles have a more acidic surface than those of Co3O4 or NiO nanoparticles. Since a more acidic oxide surface corresponds to more surface hydroxyl groups
3. Results and discussion 3.1. Thermal properties of the agarose polymer electrolytes with different nanoparticle additives
Table 1 Thermal properties of pure and nanoparticle additive modified agarose polymers.
Fig. 1 shows the thermograms of pure agarose and agarose polymer electrolytes modified with TiO2, Co3O4, and NiO
Tm ( C) DHm (J g1)
Pure agarose
TiO2-modified
Co3O4-modified
NiO-modified
280.6 175.5
226.4 159.8
240.3 111.5
227.5 104.5
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Fig. 2. Surface morphologies of agarose electrolytes with different nanoparticle additives: (a) TiO2, (b) Co3O4, and (c) NiO.
[13], it is anticipated that the TiO2 nanoparticles possess more surface hydroxyl groups, which results in stronger hydrogen bonding interactions between the nanoparticles that can cause easy formation of nanoparticle aggregates. The formation of nanoparticle aggregates hinders the interactions between the polymer segments and nanoparticles, which causes a less efficient reduction in crystallinity of the polymer matrix. This explains why the TiO2-modified agarose polymer exhibits a higher crystallinity than that of the modified Co3O4 or NiO material. 3.2. Surface morphology study of the agarose electrolytes with different nanoparticle additives Fig. 2 shows SEM images of agarose polymer electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. The TiO2-modified electrolyte (Fig. 2a) displays a rough surface with cross-linked bundles that are due to the crystallization of the polymer matrix. However, as shown in Fig. 2b and c, the Co3O4 and NiO-modified polymer electrolytes show a smooth surface without bundles, indicating an improved reduction in crystallinity compared to that of the TiO2-modified material. This is consistent with the thermal analysis discussion presented above (Fig. 1). A low crystallinity of polymer electrolyte is considered to be advantageous for pore filling and forming good interfacial contact between the electrolyte and the nanocrystalline TiO2 photoanode. 3.3. UVeVis absorption spectra of additiveemodified agarose polymer electrolytes Fig. 3 shows the UVeVis absorption spectra of TiO2, Co3O4, and NiO nanoparticle additive modified agarose electrolytes. The peaks around 297 and 368 nm are attributed to the presence of Ie 3 in the
polymer electrolytes [16]. It can be seen from Fig. 3 that the absorption peak intensities follow the order of Co3O4 > NiO > TiO2. A higher absorption peak intensity indicates a higher Ie 3 concentration in the electrolyte, which leads to a better ionic conductivity [17]. It is known that the nanoparticle additives in a polymer electrolyte play the role of Lewis acidebase interaction centres with the ionic species in the electrolyte. The surface hydroxyl groups of the nanoparticles are expected to interact with the salt anions through the formation of hydrogen bonds [18]. As mentioned above, TiO2 nanoparticles have the highest surface acidity among the three additives and thus have the most surface hydroxyl groups. These surface hydroxyl groups not only promote nanoparticle aggregation but also interact with the anions (I and I 3 ) in the electrolyte, which subsequently lowers the free mobile anion concentration [18]. Since TiO2 nanoparticles have the most surface hydroxyl groups, the TiO2-modified agarose electrolyte exhibits the lowest free anion concentration as shown in Fig. 3. 3.4. Ionic conductivity of additive-modified agarose polymer electrolytes Table 2 shows the ionic conductivities of the agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. The TiO2-modified agarose electrolyte exhibits the lowest ionic conductivity of 2.66 103 S cm1, while the highest ionic conductivity of 4.37 103 S cm1 is observed for the Co3O4-modified electrolyte. It is known that ions transport rapidly in the amorphous phase of a polymer electrolyte [19,20]. From Figs. 1 and 2 it is concluded that the Co3O4 and NiO-modified agarose electrolytes have more amorphous phase in the electrolyte films than that of the TiO2modified material. In addition, the ion concentration in an electrolyte plays an important role in determining its ionic conductivity, and from Fig. 3 it can be seen that the Co3O4-modified electrolyte shows the highest ion concentration. Due to these two factors the highest ionic conductivity is obtained for the Co3O4modified electrolyte, and the TiO2-modified one shows the lowest ionic conductivity among the three electrolytes. 3.5. Pore filling properties of agarose polymer electrolytes with different nanoparticle additives Fig. 4 shows the cross-sectional SEM images of the TiO2 photoanodes before and after adding the agarose electrolytes modified with TiO2, Co3O4, or NiO nanoparticle additives. Fig. 4a shows the Table 2 Ionic conductivities of agarose electrolytes modified with different nanoparticle additives.
Fig. 3. UVeVis spectra of agarose electrolytes modified with different nanoparticle additives.
Ionic conductivity (S cm1)
TiO2-modified
Co3O4-modified
NiO-modified
2.66 103
4.37 103
3.33 103
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Fig. 4. Cross-sectional SEM images of TiO2 photoanodes (a) before and after introducing agarose electrolytes modified with (b) TiO2, (c) Co3O4, and (d) NiO nanoparticle additives.
bare TiO2 porous film before introducing the agarose electrolyte, where distinguishable TiO2 nanoparticles with many pores in the film can be seen. The degree of penetration of the agarose electrolytes with different nanoparticle additives into the porous TiO2 films can be seen in Fig. 4bed. The TiO2-modified electrolyte shows the worst penetration into the porous photoanode film, and the electrolyte coverage of the photoanode is very poor (Fig. 4b). On the contrary, as can be seen in Fig. 4c, the Co3O4-modified agarose
electrolyte exhibits a very good filling into the pores of the TiO2 photoanode film, and most of the TiO2 nanoparticle surface is covered by the electrolyte such that the nanoparticles become indistinguishable. The NiO-modified electrolyte also shows improved pore filling and penetration capability compared to the TiO2-modified electrolyte, but not as good as that of Co3O4-modified material. The low pore filling capability of the TiO2emodified agarose electrolyte could be attributed to its high crystallinity as discussed previously and its rough surface with cross-linked bundles as seen in Fig. 2a, which could significantly lower its flowability compared to the Co3O4 and NiO-modified electrolytes. 3.6. Effects of the nanoparticle additives on photovoltaic and electrochemical performance of the DSSCs based on agarose electrolytes
Fig. 5. JeV characteristics of the DSSCs fabricated using agarose electrolytes with TiO2, Co3O4, and NiO nanoparticle additives.
JeV characteristics of the DSSCs fabricated with agarose electrolytes modified by TiO2, Co3O4, and NiO nanoparticle additives are obtained under 1 Sun (100 mW cm2), and the results are presented in Fig. 5. Table 3 summarizes the cell performance parameters obtained from Fig. 5. Compared to the DSSC fabricated with TiO2-modified electrolyte, improvement in power conversion efficiency (h) is observed for the cells fabricated with Co3O4 or NiOmodified electrolyte, and the cell fabricated with Co3O4-modified electrolyte shows the highest efficiency at 2.11%. The short-circuit current (Jsc) also exhibits the same trend. It is known that in order to have an enhanced photovoltaic performance the reactions are required to be efficient in DSSCs [21]:
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Table 3 Performance parameters of the DSSCs fabricated using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. Agarose electrolyte modified with
Jsc(mA cm2)
Voc(V)
FF
h (%)
TiO2 Co3O4 NiO
5.28 7.24 6.20
0.605 0.635 0.625
0.55 0.46 0.52
1.71 2.11 2.02
I þ I 2 4 I3 þ I2 4 I5
(1)
3I þ 2dyeþ / I 3 þ 2dye (TiO2 photoanode)
(2)
I 3 þ 2e 4 3I (counter electrode)
(3)
From the UVeVis measurement results presented in Fig. 3, the TiO2-modified electrolyte shows the lowest I 3 concentration. This low Ie 3 concentration can cause an inefficient I3 diffusion process, which consequently leads to a decreased Jsc. Hence, the DSSC fabricated with TiO2emodified electrolyte exhibits the lowest Jsc. The openecircuit voltage (Voc) of this DSSC is also low compared to those of the cells containing Co3O4 or NiO-modified electrolyte (Table 3). As discussed above, TiO2 nanoparticles have more surface hydroxyl groups than Co3O4 or NiO nanoparticles, which can react with the anions of the LiI salt [18]. This can lead to more mobile Liþ in the TiO2-modified electrolyte compared with the Co3O4 or NiO-modified material. These lithium ions can be adsorbed on the surface of the porous TiO2 photoanode film and subsequently decrease the electron concentration in the TiO2 conduction band, which causes a low Voc of the DSSC with TiO2-modified electrolyte [22,23]. Electrochemical impedance spectroscopy (EIS) measurement was also carried out to study the charge transport and recombination processes in the DSSCs. The EIS Nyquist plots are shown in Fig. 6, and three semicircles are observed in the plots, which correspond to the charge transfer process at the Pt/electrolyte interface (frequency range of 105e103 Hz), charge recombination process at the TiO2/dye/electrolyte interface (frequency range of 103e1 Hz), and ion diffusion process occurring in the electrolyte (frequency range of 1e103 Hz), respectively [24]. The charge recombination resistances (R2) of the DSSCs obtained from the Nyquist plots follow the order Co3O4 (23.1 U) > NiO (19.1 U) > TiO2
Fig. 6. EIS Nyquist plots of the DSSCs fabricated using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives.
(17 U). The cell with Co3O4-modified electrolyte shows the highest recombination resistance, while the cell with TiO2-modified electrolyte shows the lowest. According to a previous study, an increased R2 indicates a suppressed charge recombination reaction between the photoanode conduction band electrons and the triiodide ions [I 3 þ 2ecb (TiO2) ¼ 3I ] at the TiO2 photoanode/ electrolyte interface, which can lead to a higher Voc [25]. Hence, the R2 measurement results are consistent with the Voc data presented in Table 3. 3.7. Stability of the DSSCs based on agarose electrolytes with different nanoparticle additives Fig. 7 shows the normalized power conversion efficiency as a function of time for the DSSCs using agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives. It can be seen that the normalized efficiency of the DSSC with TiO2-modified electrolyte exhibits a w50% decrease after 164 h testing (without sealing), while the cells containing Co3O4 or NiO-modified electrolyte can maintain the initial efficiencies after 400 h testing under the same condition. This improvement in DSSC stability may be attributed to the better pore filling capability of Co3O4 and NiO-modified electrolytes as discussed. It is known that the organic solvent in electrolytes can cause device sealing issues; however, the organic solvent has also been considered as gelator in quasi-solid-state polymer electrolytes, which plays the role of a cross-linker that can benefit the ionic transport process [26]. During DSSC fabrication, after drying at an elevated temperature to form quasi-solidstate electrolyte film in the cell, a small amount of organic solvent still remains in the polymer electrolyte, which can maintain the effective ionic transport pathways [26]. As time goes by, the organic solvent will evaporate slowly and the efficiency of the DSSC will subsequently decrease. To improve the stability of solid-state DSSCs, it is important to have sufficient polymer electrolyte in the porous photoanode film to slow down this solvent evaporation process. As discussed previously, the TiO2-modified electrolyte studied in this work shows a high crystallinity and a rough surface morphology (Figs. 1 and 2), which affects its flowability. Compared to Co3O4 and NiOemodified electrolytes, the TiO2-modified agarose electrolyte has the lowest penetration into the porous photoanode film (Fig. 4), and a large portion of the electrolyte stays on top of the photoanode, which subsequently promotes the evaporation of the organic solvent. Hence, the DSSC using the TiO2-modified electrolyte shows the most rapid degradation (Fig. 7).
Fig. 7. Power conversion efficiency as a function of time for the DSSCs based on agarose electrolytes modified with TiO2, Co3O4, and NiO nanoparticle additives.
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4. Conclusions
References
In this work we have studied the effects of non-magnetic (TiO2) and magnetic (Co3O4 and NiO) nanoparticle additives on the properties of agarose polymer electrolytes. Using DSC, SEM, UVeVis, and conductivity measurement methods we have characterized the crystallinity, surface morphology, ionic conductivity, and pore filling capability of the nanoparticle additive modified electrolytes. The photovoltaic and electrochemical performance and the stability of the DSSCs based on the modified polymer electrolytes have also been investigated. It has been observed that among the three types of modified agarose electrolytes that with the Co3O4 nanoparticle additive shows the best pore-filling capability, and the DSSC based on this electrolyte also shows the best power conversion efficiency of 2.11%. This improved performance has been associated with the lower surface acidity, hence fewer surface hydroxyl groups of Co3O4 nanoparticles compared to that of the TiO2 nanoparticles, which decreases the crystallinity of the agarose electrolyte and increases the ion concentration. This observation might be used to direct the design of solid-state DSSCs based on polymer electrolytes with further enhanced performance. The focus of our study has been to synthesize and understand the properties of nanoparticle additivemodified agarose electrolytes. It would be interesting to examine the use of this material to replace spiro-MeOTAD [(2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9, 90 -spirobifluorene] used in DSSCs using organic primary amine halide perovskites to examine whether if this electrolyte would further improve device performance. See, for example, the very recent results of Grätzel and coworkers for a DSSC with 15% efficiency [27].
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Acknowledgements We acknowledge the financial support of the National Nature Science Foundation of China (61006047). We also acknowledge financial support from the University of Wyoming’s School of Energy Resources.