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Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells
Journal Pre-proofs Original article Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells Humaira Seema, Zaiba Zafar, Ayesha Samreen PII: DOI: Reference:
S1878-5352(20)30024-1 https://doi.org/10.1016/j.arabjc.2020.01.020 ARABJC 2558
To appear in:
Arabian Journal of Chemistry
Received Date: Revised Date: Accepted Date:
19 August 2019 22 January 2020 23 January 2020
Please cite this article as: H. Seema, Z. Zafar, A. Samreen, Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells, Arabian Journal of Chemistry (2020), doi: https://doi.org/10.1016/j.arabjc.2020.01.020
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Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells Humaira Seemaa,ǂ,*, Zaiba Zafara, ǂ, Ayesha Samreenb
aInstitute
of Chemical Sciences, University of Peshawar, Peshawar 25120, Pakistan.
bDepartment
of Physics University of Peshawar, Peshawar 25120, Pakistan
*Corresponding Author: Email address: [email protected]. Tel. +92 91 9216652; ǂ both the authors contributed equally.
Abstract This paper reports the synthesis of reduced oxide (RGO) coated polyaniline (PANi) nanocomposites via in-situ emulsion polymerization and its application as counter electrode for dye sensitized solar cells (DSSCs).
The synthesized nanocomposites were systematically
characterized through Scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform resonance infra-red (FTIR) and Raman spectroscopy indicating the uniform intercalation of polyaniline with reduced graphene oxide. The synergy between RGO and PANi chains owing to the co-doped SDS and H2SO4 leads to the enhanced solubility and improved electrocatalytic activity, that was further confirmed through electrochemical measurements to satisfy the criteria for application as cost effective counter electrode material for scalable DSSC. The fabricated CE was highly transparent and reached the conversion efficiency which is comparable to that of Platinum with a current density of (12.58 mA. cm-2) and (13.11 mA. cm-2) respectively under an illumination of AM 1.5 G (100 mW. cm-2) simulated solar light with an overall photo conversion efficiency of 3.9 %. Thus PANi/RGO based nanocomposites could therefore serve as efficient alternative material to Pt. free CE in DSSC. 1
Key words
In-situ emulsion polymerization, reduced graphene oxide, PANi, DSSC, counter
electrode. 1. Introduction In relation to the concerns about natural resource depletion and relevant environmental problems regarding pollution, photovoltaic technology seems to be the most suitable option (Parida, Iniyan et al. 2011). Dye sensitized solar cells (DSSC), the third generation solar cell technology, has been widely investigated in view of its ease of fabrication, low cost production and eco-friendly nature providing promising photovoltaic efficiency (Thomas, Deepak et al. 2014). A DSSC is typically composed of a dye coated crystalline semiconductor coated on a transparent conductive oxide, an electrolyte and a counter electrode (CE) Fig. 1. Recently researchers have adopted several approaches to enhance the performance of DSSC via introducing novel techniques for highly efficient photoanodes, counter electrodes and electrolytes respectively (Thomas, Deepak et al. 2014). However, among these components counter electrode (CE) plays a key role in the overall power conversion efficiency of the device as it serves as the main electron collecting repository from the external circuit as well as to reduce back the electrolyte that leads to the regeneration of the oxidized dye (Acharya, Camillone III et al. 2011). Therefore, to produce higher current densities CE plays a vital role to lower the internal energy loss in DSSC. So far, the most commonly employed material on CE in DSSC is platinum (Pt.) due to its excellent catalytic, thermal and electrical properties with considerably low charge transfer resistance (Calogero, Calandra et al. 2011). However, the high cost associated with Pt. is a hurdle towards commercialization for large scale production. Currently, researchers are therefore in constant search of alternative electrode materials with sufficient efficiency to replace Pt based DSSC. In 2
the past decade, carbon nanomaterials crossed linked conductive polymers (Li, Ye et al. 2009, Han, Kim et al. 2010, Joshi, Zhang et al. 2010, Sun, Luo et al. 2010, Dong, Pint et al. 2011, Veerappan, Bojan et al. 2011, Balamurugan, Pandurangan et al. 2012, Kang, Lee et al. 2012), metal organic frameworks (MOFs)(Xiao, Li et al. 2018, Geng, Cao et al. 2019, Li, Xu et al. 2019) and transition metal oxides (Wu, Hao et al. 2008, Gong, Wang et al. 2012, Wu, Lin et al. 2012) and nobel metal incorporated polymers (Lee, Chi et al. 2013) are widely researched as electrocatalytic materials with significant conversion efficiency. However, in comparison the carbonaceous materials such as activated carbon (Park, Kim et al. 2013) , graphite (Li, Li et al. 2015), carbon nanotubes (CNTs) (Mei, Cho et al. 2010) and graphene (Seema. 2019) have attained considerable attention owing to their remarkable electronic and thermal conductivity with sufficient corrosion resistance towards I2 reduction. Graphene, a two dimensional one atom thick sheet of sp2-hybridized carbon atoms has been expected to be the most promising alternative to Pt, due to its excellent electrical, catalytic and mechanical properties (Novoselov, Geim et al. 2004, Novoselov, Morozov et al. 2007, Thomas, Deepak et al. 2014, Teng, Lv et al. 2017), however owing to the limited number of active sites; bare graphene has been demonstrated with very low electrocatalytic efficiency (Veerappan, Bojan et al. 2011). In addition graphene metal oxide hybrids, such as RGO/NiO have also been widely investigated, however the efficiency still lacks behind 3.69 % (Singh, Kumar et al. 2018) Though, On the other hand; conducting polymers with incorporated graphene have been highly considered in view of their excellent conductivity and catalytic performance such as graphene oxide sheets incorporated polyaniline(GOS-PANi) have been employed as CE synthesized via direct immersion of FTO in solution containing aniline and graphite oxide following polymerization (Hsu, Tseng et al. 2014), however the technique could not overcome the large scale manufacturing (Singh, Kaur et al. 2016). For this reason a lot of efforts has been 3
undertaken regarding graphene integrated polymers, such as polyaniline (PANi) (Tai, Chen et al. 2011, Tang, Cai et al. 2013), polypyrrole (PPy) (Jeon, Kim et al. 2011, Bu, Tai et al. 2013) and poly(3,4-ethylenedioxythiphene: polystyrene sulphonate) (PEDOT:PSS) (Wei, Wang et al. 2014) based CE and electrolytes (Hsu, Chen et al. 2014) with significant improvement in photovoltaic performance. Among conducting polymers, polyaniline has come out to be the most suitable CE material due to its ease of synthesis, low cost manufacturing and excellent redox behavior but the large scale processability still remains an issue, as PANi shows lower solubility over a limited range of solvents (Saranya, Rameez et al. 2015). This paper reports a simple and cost-effective synthesis of solution processable in-situ reduced graphene oxide-polyaniline (PANi/RGO) nanocomposite and their application in Pt. free DSSC. The present work also demonstrates the room temperature deposited RGO/PANi transparent films and CE fabrication with solar conversion efficiency of 3.98 % under AM 1.5 G solar simulated irradiation of 100 mW cm-2. The nanocomposite exhibits significant acceleration in overall power conversion efficiency which therefore paves the way to large scale manufacturing of carbon-based CE as efficient alternative to Pt. based DSSC. 2. Experimental 2.1.
Materials and Reagents Graphene oxide was synthesized using modified Hummer’s method (Zaaba, Foo et al.
2017). Sulfuric acid (H2SO4, 95-98 %), Ammonium persulfate (NH4)2S2O8), Sodium dodecyl sulfate (CH₃(CH₂)₁₁SO₄Na) Hydrochloric acid (HCl, 36-37 %), Potassium permanganate (KMnO4, 99.9 %), aniline monomer (C6H5NH2 ≥ 99.5 %), and Acetone (CH3COCH3 ≥ 99.9 %) all were purchased from Sigma Aldrich and were used as such. Mesoporous Titania coated photoanodes (with a thick transparent TiO2 layer 11.5 μm and a 3 μm scattering layer), Iodolyte Z-100, N719 4
(Ruthenizer 535-bisTBA), Fluorine doped tin oxide glass (FTO) and Meltonix 1170-60 Gaskets (60 μm) were purchased from Solara-nix Switzerland. 2.2 . Synthesis of H2SO4 and SDS co-doped Polyaniline via emulsion polymerization PANi-SDS- H2SO4 (PANi) was synthesized via in-situ emulsion polymerization (Bilal, Gul et al. 2015) In a typical experiment 7.7 mmol SDS was slowly added to 50 mL of constantly stirring chloroform at room temperature, which was followed by the addition 16.42 mmol aniline monomer. Then to the above mixture 12.50 mmol aqueous solution of H2SO4 was added drop wise, followed by the addition of pre-cooled 25 mL of 1.25 mmol APS solution, that result in the formation of a milky white emulsion. For complete polymerization the mixture was kept stirring at room temperature for 24 h, a dark green colored product was obtained. The contents in the flask were then transferred in to a separating funnel in order to separate the aqueous and organic phases. Afterwards, the organic phase was washed repeatedly with acetone and deionized water. The resultant green color product was then collected by filtration and washed with excess of acetone, and dried in vacuum oven at temperature at 60 C˚ for 24 h. 2.3. One step synthesis of PANi-SDS-H2SO4/RGO nanocomposite The PANi-SDS-H2SO4/RGO (PANi/RGO) nanocomposite was synthesized using one step emulsion polymerization process using graphene oxide precursor, in presence of aniline monomer. Graphene oxide was synthesized using modified Hummers method, as reported in the literature (Seema, Shirinfar et al. 2017). Amount of graphene oxide was varied from 3, 5, 10, 20 and 40 wt.% with aniline net weight respectively. A certain amount of graphene oxide with an equal molar ratio of aniline (16.42 mmol) and SDS (7.7 mmol) were added to a 50 mL of chloroform under constant stirring at room temperature. A 25 mL solution of (12.50 mmol) H2SO4 was added drop wise to the above mixture with immediate addition of a pre cooled (1.25 5
mmol) APS oxidant solution. To complete polymerization the mixture was allowed to stir at room temperature for 24 h. The contents of the flask were then poured in to acetone in order to precipitate out the polymer composite, repeatedly washed with deionized water, filtered and dried at 60 C˚ in vacuum. 2.4.
Cell Fabrication
2.4.1.
Working Electrode
Mesoporous TiO2 coated FTO glass was used as the working electrode that mainly comprises of two Titania layers; a thick transparent TiO2 layer 11.5 μm with a 3 μm scattering layer, purchased from Solara nix Switzerland.
2.4.2. Fabrication of PANi/RGO Counter Electrode 0.05 wt. % dispersion of PANi and PANi/RGO nanocomposites were prepared in ethanol using ultrasonic bath to form a stable dispersion. The CEs were prepared by drop casting the PANi and RGO/PANi dispersions on a pre-cleaned FTO substrates respectively and were allowed to dry at room temperature respectively Fig. 2. 2.4.3. DSSC Assembly The mesoporous Titania photoanode was sintered around 450 oC for about 30 minutes. After the temperature is been lowered to 80 oC, the photoanodes were immersed in N-719 dye solution 0.5mM) for 24 h in dark at room temperature. After staining, the photoanodes were rinsed with ethanol and dried in nitrogen gas. Then, a portion of Meltonix 1170-60 gasket having thickness of 60 μm was well positioned on the active side of photoanode, the CE with the active side facing towards the photoanode was kept over the gasket and gently pressed and heated at 100 oC 6
to ensure perfect sealing. After sealing, an iodide-triodide electrolyte was injected through the drilled hole in CE and sealed immediately by spot sealing tape. In order to avoid the photo bleaching of dye molecules, the assembled devices were characterized for photovoltaic performance by Keithley 4200-SCS integrated with a digital capacitance meter (model 4210CVU under dark and AM 1.5 simulated illuminations (OAI TriSOL, AM1.5G Class AAA, USA). The power was set to 100 mWcm-2 using Newport Oriel PV reference cell system (Model 91150V). The active area of the cell was 0.36 cm2. 3.
Characterizations and Measurements
The FTIR spectra of the samples were taken by 100 Perkin Elmer FTIR spectrometer (PerkinElmer, Inc.). The X-Ray diffraction analysis were carried out using JDX-3532 (JEOL JAPAN) and 2θ angle using Cu Kɑ (1.5418 Å) as radiation source. The morphology of the samples was investigated through Scanning electron microscopy using Carl Zeiss EVO 50 scanning electron microscope (SEM). Raman spectra were recorded using LabSpec 6 (HORIBA Scientific). The electrochemical investigation was performed using a CHI model 900B SECM (CH Instruments, Austin, TX). The DSSC fabricated with PANi/RGO as CE were characterized for photovoltaic performance by Keithley 4200-SCS integrated with a digital capacitance meter (model 4210-CVU) under dark and AM 1.5 simulated illuminations (OAI TriSOL, AM1.5G Class AAA, USA). The power was set to 100 mWcm-2 using Newport Oriel PV reference cell system (Model 91150V). 4. 4.1.
Results and discussion FTIR Analysis
The typical peaks observed in FTIR spectrum of PANI, are 1542, 1366, 1246, 1149 and 820 cm-1 respectively that are in close agreement with that reported in the literature (Misoon,Seok 2012) Fig. 3.The peaks at 1494 and 1542 cm-1 can be attributed to the C=C and C=N stretching 7
vibrations of the quinoid and benzenoid rings, respectively indicating the oxidation state of PANi, that corresponds to the emeraldine salt (ES). The peaks at 1246 and 1366 cm-1 are assigned to the C-N+ stretching vibration and C-N stretching of the benzenoid ring respectively. The characteristic peak of C=N quinoid unit at 1108 cm-1 of doped conducting polymers due to the delocalization of electrical charges, caused by deprotonation, can be taken as measure of electron conductivity of PANi in accordance to the results reported by MacDiarmid et al. (Quillard, Louarn et al. 1994) In addition, the peak at 820 cm-1 could be ascribed to C-H out of plane bending vibration. Moreover, the presence of SDS in PANi chain is confirmed by the symmetric and asymmetric stretching vibration of the SO3-1 at 1004 and 1052 cm-1 respectively. The degree of doping, and presence of SDS are further confirmed by the presence of peak at 2930 cm-1, that is due to the stretching vibration of methylene in SDS. The FTIR spectrum of the nanocomposites however shows a little difference from that of the doped PANi spectrum, exhibiting the characteristic peaks of the doped PANi indicating the presence of –SO3-1 along with that of the C-H stretching vibration of the long alkyl chain of SDS and reduced graphene oxide in PANi/RGO nanocomposite. The persistence of the peak at 1532 cm-1 shows the restoration of the sp2 network in RGO. With increased loading of graphene, it is observed that the intensity of these S=O vibrations decreases. The band at 3436 cm-1 related to the –OH stretching vibration also get less intense which shows the reduction of GO. Similarly, in case of nanocomposite, the vibrational modes of quinoid and benzenoid rings are shifted towards low wavenumbers, at 1538 and 1469 cm
-1
respectively, with considerable decrease in their
intensities, as compared to that in doped PANi. This is due the growth of PANi chains on the uncovered graphene surface (Yu, Che et al. 2005) that restrict vibrations in the neighborhood graphene, resulting in decreased intensities of quinoid and benzene rings. 4.2.
XRD Analysis 8
The crystalline phase structures of PANi and PANi/RGO composites were investigated through XRD, Fig. 4 The PANi exhibits the characteristic peak centered at 23.2o that are accompanied by the two other high intensity shoulder peaks of emeraldine salt at 20o and 18.5o respectively corresponding to the semi-crystalline planes of polyaniline of (200), (020) and (011) respectively. The characteristic peak at 20o is due to the alternating distance and layers of the polymer chain, while the peak at 23.2o is correlated to the periodicity and parallel polymer chains (Gui, Liu et al. 2014). In Fig. 4 the peak at 9.25o could be attributed to the presence of SDS, which indicates the successful doping of without affecting the phase structure of PANi and therefore increases the crystallinity and ordering of PANi (Baniasadi, SA et al. 2014, Zhang, Zhi et al. 2015). A similar diffraction pattern was observed for the PANi/RGO composite (40 wt. %) Fig. 4, with the respective broadness of the peaks from 20o – 26o indicating the presence of RGO through consequent reduction of GO, without affecting the PANi structure. 4.3.
SEM Analysis
The SEM analysis was carried out to investigate the morphology of the as synthesized PANi/RGO nanocomposites. The scanning electron micrographs of PANi showed granular morphology Fig. 5, depicting the influence of solvent system and synthesis route (Sinha, Bhadra et al. 2009). For example, Shreephati et al. investigated the effect of DBSA loading on the morphology of polyaniline synthesized through inverse emulsion polymerization which revealed febrile and porous morphology (Shreepathi,Holze 2005). Similarly, Han et al. demonstrated the spherical morphology of PANi-DBSA via reverse micelle polymerization (Han, Kusunose et al. 2009). However, the granular morphology reported for PANi in the current study can be attributed to the polymerization of aniline in presence of H2SO4 (Konyushenko, Stejskal et al. 2006, Stejskal, Sapurina et al. 2008) with low polymerization time period, which is not enough 9
for phenazine to nucleate (Sapurina,Stejskal 2008). Moreover the introduction of surfactant SDS has also some influence on the morphology of PANi that results in the assembled molecules, thus promoting the granular morphology, which accord to the previous studies involving SDBSdoped PANi synthesis (Ahn, Lee et al. 2015). In case of PANi/RGO nanocomposites, the morphology is however slightly different from that of graphene or polyaniline alone, which showed a typical sheet like morphology, corresponding to a sheet size of 1 μm, which could therefore increase the active specific surface area owing to the long alkyl chain of SDS doped PANi Fig. S1. Besides, the PANi-SDS particles also polymerize to coat the graphene oxide surface, since graphene oxide could act as a template to form micelle in the presence of a surfactant, thus further enhances the efficient intercalation and promotes the consequent coating by PANi-SDS (Ansari, Khan et al. 2014). In addition, no individual agglomerates of either PANi or graphene can be found on the surface of graphene which indicates that the nucleation and growth processes occurred only over the graphene surface. 4.4.
Raman Analysis
Raman spectroscopy serves as a common tool to characterize carbon-based materials for molecular identification. Conventionally, Raman Spectra features a typical G and D band respectively, that corresponds to the crystallinity of the material. For example, as shown in Fig. 6, the Raman spectra of polyaniline displaying the characteristic peaks at 1589, 1470 and 1188 cm-1 which corresponds to the C=C quinoid stretching , C=N stretching, C – N+ stretching and CH bending vibration of the benzenoid ring respectively, (Zhang, Liu et al. 2017). However, different from polyaniline, the PANi/RGO composites showed a prominent wide G band at 1351 cm-1 that correlates to the structural defects and the associated D band at 1565 cm-1 representing the first order scattering of the E2g mode of the sp2 carbon domains (Kim, Lee et al. 2014). The G 10
band however is wide and slightly shifted to 1565 cm-1 that can be attributed to the surface defects caused by the disruption to the conjugated structure with oxygen functional groups while reduction. In addition, the intensity ratio (ID/IG) increases (1.06) with the reduction process showing the high level of induced disorders in the nanocomposites. Furthermore, the increased intensity of the C=N stretching with a corresponding shift towards low wavenumbers also confirms the increased grafting of graphene oxide in polyaniline (40 wt. %) that results in stronger π-π interaction of the RGO with Polyaniline. 4.5.
Photovoltaic performance of PANi/RGO based counter electrodes
4.5.1. Current- Voltage (IV) Measurements Photovoltaic measurements of PANi and PANi/RGO based DSSC devices were performed under AM 1.5 G illumination (100 mW.cm−2), with corresponding photovoltaic parameters presented in the Table. 1. It can be visualized that the open circuit voltage (Voc) (0.63 mV), the short circuit current density (Jsc)(12.58 mA.cm-2 ), FF (0.55 ) and the efficiency (3.98 %) of DSSCs based on the composite catalytic films are higher than pristine PANi (Jsc = 11.03 mA. cm-2, FF = 0.47, and ɳ = 1.83) and RGO (Jsc = 9.41 mA. cm-2, FF = 0.36, and ɳ = 1.07) based CE respectively. Such enhancement is could be attributed to the improved catalytic activity due to larger surface area of the PANi/RGO composite films as revealed in SEM investigation Fig. 7 for I3- reduction and thereby leads to the higher Jsc of the cell with improved ƞ. Moreover, the surface defects present in reduced graphene oxide have significant effects on electrical conductivity as well as on electrocatalytic properties. Thus, the integration conductive network of PANi and RGO provides low charge transfer resistance with enhanced catalytic ability and higher fill factor (Wang, Xing et al. 2012). Although the Jsc (12.58 mA. cm-2) of the DSSC with PANi/RGO composite film is much higher Fig. 7 than the cell with pristine PANi-CE (Jsc = 11
11.03 mA. cm-2) but is still below the Pt. CE (13.11 mA. cm-2) which is due to the higher value of fill factor and Voc going in favor of Pt. based CE device because of the higher electronic conductivity which exhibits low charge transfer resistance and higher fill factor. 4.6. Electrocatalytic and Impedance Measurements In order to estimate the electrochemical performance of PANi/RGO based CE, CV (cyclic voltammetry) have been carried out for PANi, RGO, PANi/RGO and Pt. CE electrodes systematically in a three electrode cell using Ag/AgCl and Pt. as a reference and CE electrode respectively, with 0.01 M LiI, 0.1 M LiClO4 and 1.0 mM I2 based electrolyte system at a scan rate of 50 mV-1, Fig. 8. Each recorded CV curve can be clearly distinguished in to a couple of redox peaks, i.e. the characteristic cathodic peak current (Ipc) and anodic peak current density (Ipc), that corresponds to the oxidation and reduction of I- and I3- respectively. The magnitude of peak separation (Ep) serves as the main entity for estimation of the electrocatalytic performance. It can be clearly observed that the Ep for the respective PANi/RGO is considerably lower (Ep = 159 mV) than that of pristine PANi (203 mV) and RGO (297 mV) with respect to reference Pt. based CE (188 mV). This lower value of Ep for PANi/RGO indicates an increase in the charge transfer kinetics of I-/I3- redox reaction that occur at a much faster rate at PANi/RGO surface than the pristine PANi. However, on the other hand is lower than that of Pt. based CE due to the higher conductivity and low charge transfer resistance of the later. The fabricated CE has also been investigated through EIS, Fig. 9 represents the Nyquist curve for the corresponding PANi/RGO, PANi, and Pt. respectively using the same electrolyte system, with an applied frequency range of 40 kHz – 100 Hz to investigate the charge transfer resistance (RCT) of the fabricated cells. The equivalent circuit model is utilized for further curve fitting of the EIS of the fabricated CE, whilst the RCT could be taken as half of the semicircles occurring 12
and high frequency side. The RCT values are in extreme accordance with that of the electrocatalytic ability, where the PANi/RGO based CE possessed the least RCT of (0.60 Ω cm2) following Pt. based CE (0.28 Ω cm2) in contrast to the pristine PANi (0.69 Ω cm2) and RGO (0.98 Ω cm2) respectively. Moreover, the CE fabricated with PANi/RGO has higher electrocatalytic abilities than that of the pristine PANi, which is in consistency with both the RCT and η of the cells Table 1. The aforementioned results therefore highly favor the application pf PANi/RGO based CE as cost effective alternative for Pt. based CE in DSSC. 5.
Conclusions
In current study, reduced graphene oxide-based polymer composites; PANi/RGO has been synthesized
through in situ emulsion polymerization. At suitable loadings of graphene oxide,
hybrid polymeric material with uniformly coated graphene oxide can be obtained.
The
PANi/RGO CE has been prepared through drop casting on FTO substrate using the less hazardous ethanol as a dispersion solvent. The resultant devices showed a better performance than the CE with individual components, and achieved an efficiency of 3.9 % which is very close to that of Pt. thus suggesting that composite material is a potential alternative towards cost effective and solution processable DSSCs. Acknowledgment We acknowledge the financial support extended by the HEC Pakistan project number 7737/KPK/ NRPU/R&D/ HEC/2017.
13
Fig. 1. Working principle of DSSC
Fig. 2. Room temperature fabrication of PANi/RGO counter electrode
14
350
(a) (b) (c)
Transmittance %
300 250 200 150 100 50 0 500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers cm-1
Fig. 3. FTIR spectra of (a) PANi (b) PANi/RGO-20 wt. % (c) PANi/RGO-40 wt. %
15
PANi RGO
Intensity (a.u)
PANi/RGO
PANi 10
20
30
40
50
60
70
80
2 θ (degree) Fig. 4. XRD pattern of PANi and PANi/RGO-40 wt.%
16
Fig. 5. SEM images of (a-b) PANi (c-d) PANi/RGO-40 wt. %
17
D
(a) (b) (c)
G
Instensity (a.u)
ID/IG= 1.06
ID/IG= 0.84
1000
3000
2000
Raman Shift cm-1
Fig. 6. Raman Spectra of (a) PANi (b) PANi/RGO-20 wt. % (c) PANI/RGO-40 wt. %
15
Current Density (Am cm-2)
RGO PANi PANi/RGO (40 wt. %) Pt.
10
5
0 0
100
200
300
400
500
600
700
Voltage /mV Fig. 7. IV-curve of PANi, RGO and PANI/RGO-40wt. % composite 18
Device
(%)
Jsc
Voc
FF (%)
Rct
(mA.cm-2)
(mV)
Pt. PANi RGO
13.11 11.03 9.41
0.64 0.62 0.56
0.63 0.47 0.36
4.75 1.83 1.07
0.28 0.69 0.98
PANi/RGO 40 wt.%
12.58
0.63
0.55
3.98
0.60
(Ω cm2)
Table 1. Comparison of the parameters of DSSC devices with respect to CE films
3
Current Density (Am cm-2)
Pt. PANi/RGO
2
PANi RGO
1 3I-
0
2e- + I3-
-1
2e- + I3-
3I-
-2 -3 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Potential (V vs Ag/AgCl) Fig. 8. CV curves of CE with Pt., RGO, PANi and PANi/RGO-40 wt.%
19
Pt.
0.8
PANi/RGO PANi
-Z''(ohm)
RGO
0.6
0.4
0.2
0.0 13.5
14.0
14.5
15.0
15.5
16.0
16.5
Z' (ohm) Fig. 9. EIS of CE with Pt., RGO,PANi and PANi/RGO (40 wt. %).
20
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Park, S.-H., Kim, B.-K., Lee, W.-J. 2013. Electrospun activated carbon nanofibers with hollow core/highly mesoporous shell structure as counter electrodes for dye-sensitized solar cells. J. Power Sources. 239, 122-127. Quillard, S., Louarn, G., Lefrant, S., MacDiarmid, A. 1994. Vibrational analysis of polyaniline: a comparative study of leucoemeraldine, emeraldine, and pernigraniline bases. Phys. Rev. B. 50, 12496. Sapurina, I., Stejskal, J. 2008. The mechanism of the oxidative polymerization of aniline and the formation of supramolecular polyaniline structures. Polym. Int. 57, 1295-1325. Saranya, K., Rameez, M., Subramania, A. 2015. Developments in conducting polymer based counter electrodes for dye-sensitized solar cells–An overview. Eur. Polym. 66, 207-227. Seema, H., 2019. Novel self assembled magnetic Prussian blue graphene based aerogel for highly selective removal of radioactive cesium in water. Arab. J. Chem. Seema, H., Shirinfar, B., Shi, G., Youn, I.S. and Ahmed, N., 2017. Facile synthesis of a selective biomolecule Chemosensor and fabrication of its highly fluorescent graphene complex. The J. Phys. Chem. B, 121, 5007-5016. Shreepathi, S., Holze, R. 2005. Spectroelectrochemical investigations of soluble polyaniline synthesized via new inverse emulsion pathway. Chem. mater. 17, 4078-4085. Singh, J., Kaur, A., Saini, D. 2016. A Review on Analysis and Testing of Conducting Polymer and Nanocomposites on the basis of their Conducting Properties. Int. J. Eng. Sci. 7, 100-107 Singh, R., Kumar, S., Bedi, R., Saxena, V., Aswal, D., Mahajan, A. 2018. Optimization of Ni2+/Ni3+ ratio in reduced graphene oxide/nickel oxide nanohybrids for platinum free dye sensitized solar cells. J. Phys. Chem. Sol. 123, 191-197. Sinha, S., Bhadra, S., Khastgir, D. 2009. Effect of dopant type on the properties of polyaniline. J. Appl. Polym. Sci. 112, 3135-3140. Stejskal, J., Sapurina, I., Trchová, M., Konyushenko, E. N. 2008. Oxidation of aniline: polyaniline granules, nanotubes, and oligoaniline microspheres. Macromolecules. 41, 35303536. Sun, H., Luo, Y., Zhang, Y., Li, D., Yu, Z., Li, K., Meng, Q. 2010. In situ preparation of a flexible polyaniline/carbon composite counter electrode and its application in dye-sensitized solar cells. J. Phys. Chem. C. 114, 11673-11679. Tai, Q., Chen, B., Guo, F., Xu, S., Hu, H., Sebo, B., Zhao, X.-Z. 2011. In situ prepared transparent polyaniline electrode and its application in bifacial dye-sensitized solar cells. Acs Nano. 5, 3795-3799. Tang, Q., Cai, H., Yuan, S.,Wang, X. 2013. Counter electrodes from double-layered polyaniline nanostructures for dye-sensitized solar cell applications. J. Mater. Chem. A. 1, 317-323. Teng, Z., Lv, H., Wang, C., Xue, H., Pang, H.,Wang, G. 2017. Bandgap engineering of ultrathin graphene-like carbon nitride nanosheets with controllable oxygenous functionalization. Carbon. 113, 63-75. Thomas, S., Deepak, T., Anjusree, G., Arun, T., Nair, S. V.,Nair, A. S. 2014. A review on counter electrode materials in dye-sensitized solar cells. J. Mater. Chem. A. 2, 4474-4490. 23
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Author Agreement
All authors of the manuscript titled “Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells” hereby agree and approve the submission of the said manuscript to Arabian Journal of Chemistry. Regards,
Dr. Humaira Seema Lecturer, Institute of Chemical Sciences, University of Peshawar, Pakistan. Phone: +92-91-9216652 Email:[email protected]
25
S1878-5352(20)30024-1 https://doi.org/10.1016/j.arabjc.2020.01.020 ARABJC 2558
To appear in:
Arabian Journal of Chemistry
Received Date: Revised Date: Accepted Date:
19 August 2019 22 January 2020 23 January 2020
Please cite this article as: H. Seema, Z. Zafar, A. Samreen, Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells, Arabian Journal of Chemistry (2020), doi: https://doi.org/10.1016/j.arabjc.2020.01.020
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Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells Humaira Seemaa,ǂ,*, Zaiba Zafara, ǂ, Ayesha Samreenb
aInstitute
of Chemical Sciences, University of Peshawar, Peshawar 25120, Pakistan.
bDepartment
of Physics University of Peshawar, Peshawar 25120, Pakistan
*Corresponding Author: Email address: [email protected]. Tel. +92 91 9216652; ǂ both the authors contributed equally.
Abstract This paper reports the synthesis of reduced oxide (RGO) coated polyaniline (PANi) nanocomposites via in-situ emulsion polymerization and its application as counter electrode for dye sensitized solar cells (DSSCs).
The synthesized nanocomposites were systematically
characterized through Scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform resonance infra-red (FTIR) and Raman spectroscopy indicating the uniform intercalation of polyaniline with reduced graphene oxide. The synergy between RGO and PANi chains owing to the co-doped SDS and H2SO4 leads to the enhanced solubility and improved electrocatalytic activity, that was further confirmed through electrochemical measurements to satisfy the criteria for application as cost effective counter electrode material for scalable DSSC. The fabricated CE was highly transparent and reached the conversion efficiency which is comparable to that of Platinum with a current density of (12.58 mA. cm-2) and (13.11 mA. cm-2) respectively under an illumination of AM 1.5 G (100 mW. cm-2) simulated solar light with an overall photo conversion efficiency of 3.9 %. Thus PANi/RGO based nanocomposites could therefore serve as efficient alternative material to Pt. free CE in DSSC. 1
Key words
In-situ emulsion polymerization, reduced graphene oxide, PANi, DSSC, counter
electrode. 1. Introduction In relation to the concerns about natural resource depletion and relevant environmental problems regarding pollution, photovoltaic technology seems to be the most suitable option (Parida, Iniyan et al. 2011). Dye sensitized solar cells (DSSC), the third generation solar cell technology, has been widely investigated in view of its ease of fabrication, low cost production and eco-friendly nature providing promising photovoltaic efficiency (Thomas, Deepak et al. 2014). A DSSC is typically composed of a dye coated crystalline semiconductor coated on a transparent conductive oxide, an electrolyte and a counter electrode (CE) Fig. 1. Recently researchers have adopted several approaches to enhance the performance of DSSC via introducing novel techniques for highly efficient photoanodes, counter electrodes and electrolytes respectively (Thomas, Deepak et al. 2014). However, among these components counter electrode (CE) plays a key role in the overall power conversion efficiency of the device as it serves as the main electron collecting repository from the external circuit as well as to reduce back the electrolyte that leads to the regeneration of the oxidized dye (Acharya, Camillone III et al. 2011). Therefore, to produce higher current densities CE plays a vital role to lower the internal energy loss in DSSC. So far, the most commonly employed material on CE in DSSC is platinum (Pt.) due to its excellent catalytic, thermal and electrical properties with considerably low charge transfer resistance (Calogero, Calandra et al. 2011). However, the high cost associated with Pt. is a hurdle towards commercialization for large scale production. Currently, researchers are therefore in constant search of alternative electrode materials with sufficient efficiency to replace Pt based DSSC. In 2
the past decade, carbon nanomaterials crossed linked conductive polymers (Li, Ye et al. 2009, Han, Kim et al. 2010, Joshi, Zhang et al. 2010, Sun, Luo et al. 2010, Dong, Pint et al. 2011, Veerappan, Bojan et al. 2011, Balamurugan, Pandurangan et al. 2012, Kang, Lee et al. 2012), metal organic frameworks (MOFs)(Xiao, Li et al. 2018, Geng, Cao et al. 2019, Li, Xu et al. 2019) and transition metal oxides (Wu, Hao et al. 2008, Gong, Wang et al. 2012, Wu, Lin et al. 2012) and nobel metal incorporated polymers (Lee, Chi et al. 2013) are widely researched as electrocatalytic materials with significant conversion efficiency. However, in comparison the carbonaceous materials such as activated carbon (Park, Kim et al. 2013) , graphite (Li, Li et al. 2015), carbon nanotubes (CNTs) (Mei, Cho et al. 2010) and graphene (Seema. 2019) have attained considerable attention owing to their remarkable electronic and thermal conductivity with sufficient corrosion resistance towards I2 reduction. Graphene, a two dimensional one atom thick sheet of sp2-hybridized carbon atoms has been expected to be the most promising alternative to Pt, due to its excellent electrical, catalytic and mechanical properties (Novoselov, Geim et al. 2004, Novoselov, Morozov et al. 2007, Thomas, Deepak et al. 2014, Teng, Lv et al. 2017), however owing to the limited number of active sites; bare graphene has been demonstrated with very low electrocatalytic efficiency (Veerappan, Bojan et al. 2011). In addition graphene metal oxide hybrids, such as RGO/NiO have also been widely investigated, however the efficiency still lacks behind 3.69 % (Singh, Kumar et al. 2018) Though, On the other hand; conducting polymers with incorporated graphene have been highly considered in view of their excellent conductivity and catalytic performance such as graphene oxide sheets incorporated polyaniline(GOS-PANi) have been employed as CE synthesized via direct immersion of FTO in solution containing aniline and graphite oxide following polymerization (Hsu, Tseng et al. 2014), however the technique could not overcome the large scale manufacturing (Singh, Kaur et al. 2016). For this reason a lot of efforts has been 3
undertaken regarding graphene integrated polymers, such as polyaniline (PANi) (Tai, Chen et al. 2011, Tang, Cai et al. 2013), polypyrrole (PPy) (Jeon, Kim et al. 2011, Bu, Tai et al. 2013) and poly(3,4-ethylenedioxythiphene: polystyrene sulphonate) (PEDOT:PSS) (Wei, Wang et al. 2014) based CE and electrolytes (Hsu, Chen et al. 2014) with significant improvement in photovoltaic performance. Among conducting polymers, polyaniline has come out to be the most suitable CE material due to its ease of synthesis, low cost manufacturing and excellent redox behavior but the large scale processability still remains an issue, as PANi shows lower solubility over a limited range of solvents (Saranya, Rameez et al. 2015). This paper reports a simple and cost-effective synthesis of solution processable in-situ reduced graphene oxide-polyaniline (PANi/RGO) nanocomposite and their application in Pt. free DSSC. The present work also demonstrates the room temperature deposited RGO/PANi transparent films and CE fabrication with solar conversion efficiency of 3.98 % under AM 1.5 G solar simulated irradiation of 100 mW cm-2. The nanocomposite exhibits significant acceleration in overall power conversion efficiency which therefore paves the way to large scale manufacturing of carbon-based CE as efficient alternative to Pt. based DSSC. 2. Experimental 2.1.
Materials and Reagents Graphene oxide was synthesized using modified Hummer’s method (Zaaba, Foo et al.
2017). Sulfuric acid (H2SO4, 95-98 %), Ammonium persulfate (NH4)2S2O8), Sodium dodecyl sulfate (CH₃(CH₂)₁₁SO₄Na) Hydrochloric acid (HCl, 36-37 %), Potassium permanganate (KMnO4, 99.9 %), aniline monomer (C6H5NH2 ≥ 99.5 %), and Acetone (CH3COCH3 ≥ 99.9 %) all were purchased from Sigma Aldrich and were used as such. Mesoporous Titania coated photoanodes (with a thick transparent TiO2 layer 11.5 μm and a 3 μm scattering layer), Iodolyte Z-100, N719 4
(Ruthenizer 535-bisTBA), Fluorine doped tin oxide glass (FTO) and Meltonix 1170-60 Gaskets (60 μm) were purchased from Solara-nix Switzerland. 2.2 . Synthesis of H2SO4 and SDS co-doped Polyaniline via emulsion polymerization PANi-SDS- H2SO4 (PANi) was synthesized via in-situ emulsion polymerization (Bilal, Gul et al. 2015) In a typical experiment 7.7 mmol SDS was slowly added to 50 mL of constantly stirring chloroform at room temperature, which was followed by the addition 16.42 mmol aniline monomer. Then to the above mixture 12.50 mmol aqueous solution of H2SO4 was added drop wise, followed by the addition of pre-cooled 25 mL of 1.25 mmol APS solution, that result in the formation of a milky white emulsion. For complete polymerization the mixture was kept stirring at room temperature for 24 h, a dark green colored product was obtained. The contents in the flask were then transferred in to a separating funnel in order to separate the aqueous and organic phases. Afterwards, the organic phase was washed repeatedly with acetone and deionized water. The resultant green color product was then collected by filtration and washed with excess of acetone, and dried in vacuum oven at temperature at 60 C˚ for 24 h. 2.3. One step synthesis of PANi-SDS-H2SO4/RGO nanocomposite The PANi-SDS-H2SO4/RGO (PANi/RGO) nanocomposite was synthesized using one step emulsion polymerization process using graphene oxide precursor, in presence of aniline monomer. Graphene oxide was synthesized using modified Hummers method, as reported in the literature (Seema, Shirinfar et al. 2017). Amount of graphene oxide was varied from 3, 5, 10, 20 and 40 wt.% with aniline net weight respectively. A certain amount of graphene oxide with an equal molar ratio of aniline (16.42 mmol) and SDS (7.7 mmol) were added to a 50 mL of chloroform under constant stirring at room temperature. A 25 mL solution of (12.50 mmol) H2SO4 was added drop wise to the above mixture with immediate addition of a pre cooled (1.25 5
mmol) APS oxidant solution. To complete polymerization the mixture was allowed to stir at room temperature for 24 h. The contents of the flask were then poured in to acetone in order to precipitate out the polymer composite, repeatedly washed with deionized water, filtered and dried at 60 C˚ in vacuum. 2.4.
Cell Fabrication
2.4.1.
Working Electrode
Mesoporous TiO2 coated FTO glass was used as the working electrode that mainly comprises of two Titania layers; a thick transparent TiO2 layer 11.5 μm with a 3 μm scattering layer, purchased from Solara nix Switzerland.
2.4.2. Fabrication of PANi/RGO Counter Electrode 0.05 wt. % dispersion of PANi and PANi/RGO nanocomposites were prepared in ethanol using ultrasonic bath to form a stable dispersion. The CEs were prepared by drop casting the PANi and RGO/PANi dispersions on a pre-cleaned FTO substrates respectively and were allowed to dry at room temperature respectively Fig. 2. 2.4.3. DSSC Assembly The mesoporous Titania photoanode was sintered around 450 oC for about 30 minutes. After the temperature is been lowered to 80 oC, the photoanodes were immersed in N-719 dye solution 0.5mM) for 24 h in dark at room temperature. After staining, the photoanodes were rinsed with ethanol and dried in nitrogen gas. Then, a portion of Meltonix 1170-60 gasket having thickness of 60 μm was well positioned on the active side of photoanode, the CE with the active side facing towards the photoanode was kept over the gasket and gently pressed and heated at 100 oC 6
to ensure perfect sealing. After sealing, an iodide-triodide electrolyte was injected through the drilled hole in CE and sealed immediately by spot sealing tape. In order to avoid the photo bleaching of dye molecules, the assembled devices were characterized for photovoltaic performance by Keithley 4200-SCS integrated with a digital capacitance meter (model 4210CVU under dark and AM 1.5 simulated illuminations (OAI TriSOL, AM1.5G Class AAA, USA). The power was set to 100 mWcm-2 using Newport Oriel PV reference cell system (Model 91150V). The active area of the cell was 0.36 cm2. 3.
Characterizations and Measurements
The FTIR spectra of the samples were taken by 100 Perkin Elmer FTIR spectrometer (PerkinElmer, Inc.). The X-Ray diffraction analysis were carried out using JDX-3532 (JEOL JAPAN) and 2θ angle using Cu Kɑ (1.5418 Å) as radiation source. The morphology of the samples was investigated through Scanning electron microscopy using Carl Zeiss EVO 50 scanning electron microscope (SEM). Raman spectra were recorded using LabSpec 6 (HORIBA Scientific). The electrochemical investigation was performed using a CHI model 900B SECM (CH Instruments, Austin, TX). The DSSC fabricated with PANi/RGO as CE were characterized for photovoltaic performance by Keithley 4200-SCS integrated with a digital capacitance meter (model 4210-CVU) under dark and AM 1.5 simulated illuminations (OAI TriSOL, AM1.5G Class AAA, USA). The power was set to 100 mWcm-2 using Newport Oriel PV reference cell system (Model 91150V). 4. 4.1.
Results and discussion FTIR Analysis
The typical peaks observed in FTIR spectrum of PANI, are 1542, 1366, 1246, 1149 and 820 cm-1 respectively that are in close agreement with that reported in the literature (Misoon,Seok 2012) Fig. 3.The peaks at 1494 and 1542 cm-1 can be attributed to the C=C and C=N stretching 7
vibrations of the quinoid and benzenoid rings, respectively indicating the oxidation state of PANi, that corresponds to the emeraldine salt (ES). The peaks at 1246 and 1366 cm-1 are assigned to the C-N+ stretching vibration and C-N stretching of the benzenoid ring respectively. The characteristic peak of C=N quinoid unit at 1108 cm-1 of doped conducting polymers due to the delocalization of electrical charges, caused by deprotonation, can be taken as measure of electron conductivity of PANi in accordance to the results reported by MacDiarmid et al. (Quillard, Louarn et al. 1994) In addition, the peak at 820 cm-1 could be ascribed to C-H out of plane bending vibration. Moreover, the presence of SDS in PANi chain is confirmed by the symmetric and asymmetric stretching vibration of the SO3-1 at 1004 and 1052 cm-1 respectively. The degree of doping, and presence of SDS are further confirmed by the presence of peak at 2930 cm-1, that is due to the stretching vibration of methylene in SDS. The FTIR spectrum of the nanocomposites however shows a little difference from that of the doped PANi spectrum, exhibiting the characteristic peaks of the doped PANi indicating the presence of –SO3-1 along with that of the C-H stretching vibration of the long alkyl chain of SDS and reduced graphene oxide in PANi/RGO nanocomposite. The persistence of the peak at 1532 cm-1 shows the restoration of the sp2 network in RGO. With increased loading of graphene, it is observed that the intensity of these S=O vibrations decreases. The band at 3436 cm-1 related to the –OH stretching vibration also get less intense which shows the reduction of GO. Similarly, in case of nanocomposite, the vibrational modes of quinoid and benzenoid rings are shifted towards low wavenumbers, at 1538 and 1469 cm
-1
respectively, with considerable decrease in their
intensities, as compared to that in doped PANi. This is due the growth of PANi chains on the uncovered graphene surface (Yu, Che et al. 2005) that restrict vibrations in the neighborhood graphene, resulting in decreased intensities of quinoid and benzene rings. 4.2.
XRD Analysis 8
The crystalline phase structures of PANi and PANi/RGO composites were investigated through XRD, Fig. 4 The PANi exhibits the characteristic peak centered at 23.2o that are accompanied by the two other high intensity shoulder peaks of emeraldine salt at 20o and 18.5o respectively corresponding to the semi-crystalline planes of polyaniline of (200), (020) and (011) respectively. The characteristic peak at 20o is due to the alternating distance and layers of the polymer chain, while the peak at 23.2o is correlated to the periodicity and parallel polymer chains (Gui, Liu et al. 2014). In Fig. 4 the peak at 9.25o could be attributed to the presence of SDS, which indicates the successful doping of without affecting the phase structure of PANi and therefore increases the crystallinity and ordering of PANi (Baniasadi, SA et al. 2014, Zhang, Zhi et al. 2015). A similar diffraction pattern was observed for the PANi/RGO composite (40 wt. %) Fig. 4, with the respective broadness of the peaks from 20o – 26o indicating the presence of RGO through consequent reduction of GO, without affecting the PANi structure. 4.3.
SEM Analysis
The SEM analysis was carried out to investigate the morphology of the as synthesized PANi/RGO nanocomposites. The scanning electron micrographs of PANi showed granular morphology Fig. 5, depicting the influence of solvent system and synthesis route (Sinha, Bhadra et al. 2009). For example, Shreephati et al. investigated the effect of DBSA loading on the morphology of polyaniline synthesized through inverse emulsion polymerization which revealed febrile and porous morphology (Shreepathi,Holze 2005). Similarly, Han et al. demonstrated the spherical morphology of PANi-DBSA via reverse micelle polymerization (Han, Kusunose et al. 2009). However, the granular morphology reported for PANi in the current study can be attributed to the polymerization of aniline in presence of H2SO4 (Konyushenko, Stejskal et al. 2006, Stejskal, Sapurina et al. 2008) with low polymerization time period, which is not enough 9
for phenazine to nucleate (Sapurina,Stejskal 2008). Moreover the introduction of surfactant SDS has also some influence on the morphology of PANi that results in the assembled molecules, thus promoting the granular morphology, which accord to the previous studies involving SDBSdoped PANi synthesis (Ahn, Lee et al. 2015). In case of PANi/RGO nanocomposites, the morphology is however slightly different from that of graphene or polyaniline alone, which showed a typical sheet like morphology, corresponding to a sheet size of 1 μm, which could therefore increase the active specific surface area owing to the long alkyl chain of SDS doped PANi Fig. S1. Besides, the PANi-SDS particles also polymerize to coat the graphene oxide surface, since graphene oxide could act as a template to form micelle in the presence of a surfactant, thus further enhances the efficient intercalation and promotes the consequent coating by PANi-SDS (Ansari, Khan et al. 2014). In addition, no individual agglomerates of either PANi or graphene can be found on the surface of graphene which indicates that the nucleation and growth processes occurred only over the graphene surface. 4.4.
Raman Analysis
Raman spectroscopy serves as a common tool to characterize carbon-based materials for molecular identification. Conventionally, Raman Spectra features a typical G and D band respectively, that corresponds to the crystallinity of the material. For example, as shown in Fig. 6, the Raman spectra of polyaniline displaying the characteristic peaks at 1589, 1470 and 1188 cm-1 which corresponds to the C=C quinoid stretching , C=N stretching, C – N+ stretching and CH bending vibration of the benzenoid ring respectively, (Zhang, Liu et al. 2017). However, different from polyaniline, the PANi/RGO composites showed a prominent wide G band at 1351 cm-1 that correlates to the structural defects and the associated D band at 1565 cm-1 representing the first order scattering of the E2g mode of the sp2 carbon domains (Kim, Lee et al. 2014). The G 10
band however is wide and slightly shifted to 1565 cm-1 that can be attributed to the surface defects caused by the disruption to the conjugated structure with oxygen functional groups while reduction. In addition, the intensity ratio (ID/IG) increases (1.06) with the reduction process showing the high level of induced disorders in the nanocomposites. Furthermore, the increased intensity of the C=N stretching with a corresponding shift towards low wavenumbers also confirms the increased grafting of graphene oxide in polyaniline (40 wt. %) that results in stronger π-π interaction of the RGO with Polyaniline. 4.5.
Photovoltaic performance of PANi/RGO based counter electrodes
4.5.1. Current- Voltage (IV) Measurements Photovoltaic measurements of PANi and PANi/RGO based DSSC devices were performed under AM 1.5 G illumination (100 mW.cm−2), with corresponding photovoltaic parameters presented in the Table. 1. It can be visualized that the open circuit voltage (Voc) (0.63 mV), the short circuit current density (Jsc)(12.58 mA.cm-2 ), FF (0.55 ) and the efficiency (3.98 %) of DSSCs based on the composite catalytic films are higher than pristine PANi (Jsc = 11.03 mA. cm-2, FF = 0.47, and ɳ = 1.83) and RGO (Jsc = 9.41 mA. cm-2, FF = 0.36, and ɳ = 1.07) based CE respectively. Such enhancement is could be attributed to the improved catalytic activity due to larger surface area of the PANi/RGO composite films as revealed in SEM investigation Fig. 7 for I3- reduction and thereby leads to the higher Jsc of the cell with improved ƞ. Moreover, the surface defects present in reduced graphene oxide have significant effects on electrical conductivity as well as on electrocatalytic properties. Thus, the integration conductive network of PANi and RGO provides low charge transfer resistance with enhanced catalytic ability and higher fill factor (Wang, Xing et al. 2012). Although the Jsc (12.58 mA. cm-2) of the DSSC with PANi/RGO composite film is much higher Fig. 7 than the cell with pristine PANi-CE (Jsc = 11
11.03 mA. cm-2) but is still below the Pt. CE (13.11 mA. cm-2) which is due to the higher value of fill factor and Voc going in favor of Pt. based CE device because of the higher electronic conductivity which exhibits low charge transfer resistance and higher fill factor. 4.6. Electrocatalytic and Impedance Measurements In order to estimate the electrochemical performance of PANi/RGO based CE, CV (cyclic voltammetry) have been carried out for PANi, RGO, PANi/RGO and Pt. CE electrodes systematically in a three electrode cell using Ag/AgCl and Pt. as a reference and CE electrode respectively, with 0.01 M LiI, 0.1 M LiClO4 and 1.0 mM I2 based electrolyte system at a scan rate of 50 mV-1, Fig. 8. Each recorded CV curve can be clearly distinguished in to a couple of redox peaks, i.e. the characteristic cathodic peak current (Ipc) and anodic peak current density (Ipc), that corresponds to the oxidation and reduction of I- and I3- respectively. The magnitude of peak separation (Ep) serves as the main entity for estimation of the electrocatalytic performance. It can be clearly observed that the Ep for the respective PANi/RGO is considerably lower (Ep = 159 mV) than that of pristine PANi (203 mV) and RGO (297 mV) with respect to reference Pt. based CE (188 mV). This lower value of Ep for PANi/RGO indicates an increase in the charge transfer kinetics of I-/I3- redox reaction that occur at a much faster rate at PANi/RGO surface than the pristine PANi. However, on the other hand is lower than that of Pt. based CE due to the higher conductivity and low charge transfer resistance of the later. The fabricated CE has also been investigated through EIS, Fig. 9 represents the Nyquist curve for the corresponding PANi/RGO, PANi, and Pt. respectively using the same electrolyte system, with an applied frequency range of 40 kHz – 100 Hz to investigate the charge transfer resistance (RCT) of the fabricated cells. The equivalent circuit model is utilized for further curve fitting of the EIS of the fabricated CE, whilst the RCT could be taken as half of the semicircles occurring 12
and high frequency side. The RCT values are in extreme accordance with that of the electrocatalytic ability, where the PANi/RGO based CE possessed the least RCT of (0.60 Ω cm2) following Pt. based CE (0.28 Ω cm2) in contrast to the pristine PANi (0.69 Ω cm2) and RGO (0.98 Ω cm2) respectively. Moreover, the CE fabricated with PANi/RGO has higher electrocatalytic abilities than that of the pristine PANi, which is in consistency with both the RCT and η of the cells Table 1. The aforementioned results therefore highly favor the application pf PANi/RGO based CE as cost effective alternative for Pt. based CE in DSSC. 5.
Conclusions
In current study, reduced graphene oxide-based polymer composites; PANi/RGO has been synthesized
through in situ emulsion polymerization. At suitable loadings of graphene oxide,
hybrid polymeric material with uniformly coated graphene oxide can be obtained.
The
PANi/RGO CE has been prepared through drop casting on FTO substrate using the less hazardous ethanol as a dispersion solvent. The resultant devices showed a better performance than the CE with individual components, and achieved an efficiency of 3.9 % which is very close to that of Pt. thus suggesting that composite material is a potential alternative towards cost effective and solution processable DSSCs. Acknowledgment We acknowledge the financial support extended by the HEC Pakistan project number 7737/KPK/ NRPU/R&D/ HEC/2017.
13
Fig. 1. Working principle of DSSC
Fig. 2. Room temperature fabrication of PANi/RGO counter electrode
14
350
(a) (b) (c)
Transmittance %
300 250 200 150 100 50 0 500
1000
1500
2000
2500
3000
3500
4000
Wavenumbers cm-1
Fig. 3. FTIR spectra of (a) PANi (b) PANi/RGO-20 wt. % (c) PANi/RGO-40 wt. %
15
PANi RGO
Intensity (a.u)
PANi/RGO
PANi 10
20
30
40
50
60
70
80
2 θ (degree) Fig. 4. XRD pattern of PANi and PANi/RGO-40 wt.%
16
Fig. 5. SEM images of (a-b) PANi (c-d) PANi/RGO-40 wt. %
17
D
(a) (b) (c)
G
Instensity (a.u)
ID/IG= 1.06
ID/IG= 0.84
1000
3000
2000
Raman Shift cm-1
Fig. 6. Raman Spectra of (a) PANi (b) PANi/RGO-20 wt. % (c) PANI/RGO-40 wt. %
15
Current Density (Am cm-2)
RGO PANi PANi/RGO (40 wt. %) Pt.
10
5
0 0
100
200
300
400
500
600
700
Voltage /mV Fig. 7. IV-curve of PANi, RGO and PANI/RGO-40wt. % composite 18
Device
(%)
Jsc
Voc
FF (%)
Rct
(mA.cm-2)
(mV)
Pt. PANi RGO
13.11 11.03 9.41
0.64 0.62 0.56
0.63 0.47 0.36
4.75 1.83 1.07
0.28 0.69 0.98
PANi/RGO 40 wt.%
12.58
0.63
0.55
3.98
0.60
(Ω cm2)
Table 1. Comparison of the parameters of DSSC devices with respect to CE films
3
Current Density (Am cm-2)
Pt. PANi/RGO
2
PANi RGO
1 3I-
0
2e- + I3-
-1
2e- + I3-
3I-
-2 -3 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Potential (V vs Ag/AgCl) Fig. 8. CV curves of CE with Pt., RGO, PANi and PANi/RGO-40 wt.%
19
Pt.
0.8
PANi/RGO PANi
-Z''(ohm)
RGO
0.6
0.4
0.2
0.0 13.5
14.0
14.5
15.0
15.5
16.0
16.5
Z' (ohm) Fig. 9. EIS of CE with Pt., RGO,PANi and PANi/RGO (40 wt. %).
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Author Agreement
All authors of the manuscript titled “Evaluation of solution processable polymer reduced graphene oxide transparent films as counter electrodes for dye-sensitized solar cells” hereby agree and approve the submission of the said manuscript to Arabian Journal of Chemistry. Regards,
Dr. Humaira Seema Lecturer, Institute of Chemical Sciences, University of Peshawar, Pakistan. Phone: +92-91-9216652 Email:[email protected]
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