Microstructural Development and Rheological Study of a Nanocomposite Gel Polymer Electrolyte Based on Functionalized Graphene for Dye-Sensitized Solar Cells

Microstructural Development and Rheological Study of a Nanocomposite Gel Polymer Electrolyte Based on Functionalized Graphene for Dye-Sensitized Solar Cells

Table of Contents





Abstract

For a liquid electrolyte-based dye-sensitized solar cell (DSSC), long-term device instability is known to negatively affect the ionic conductivity and cell performance. These issues can be resolved by using the so-called quasi-solid-state electrolytes. Despite the enhanced ionic conductivity of graphene nanoplatelets (GNPs), their inherent tendency toward aggregation has limited their application in quasi-solid-state electrolytes. In the present study, the GNPs were chemically modified by polyethylene glycol (PEG) through an amidation reaction to obtain a dispersible nanostructure in a poly(vinylidene fluoride-co-hexafluoro propylene) copolymer and polyethylene oxide (PVDF–HFP/PEO) polymer-blended gel electrolyte. Maximum ionic conductivity (4.11 × 10−3 S cm−1 ) was obtained with the optimal nanocomposite gel polymer electrolyte (GPE) containing 0.75 wt% functionalized graphene nanoplatelets (FGNPs), corresponding to a power conversion efficiency of 5.45%, which was 1.42% and 0.67% higher than those of the nano particle-free and optimized-GPE (containing 1 wt% GNP) DSSCs, respectively. Incorporating an optimum dosage of FGNP, a homogenous particle network was fabricated that could effectively mobilize the redox-active species in the amorphous region of the matrix. Surface morphology assessments were further performed through scanning electron microscopy (SEM). The results of rheological measurements revealed the plasticizing effect of the ionic liquid (IL), offering a proper insight into the polymer–particle interactions within the polymeric nanocomposite. Based on differential scanning calorimetry (DSC) investigations, the decrease in the glass transition temperature (and the resultant increase in flexibility) highlighted the influence of IL and polymer–nanoparticle interactions. The obtained results shed light on the effectiveness of the FGNPs for the DSSCs.

Keywords

dye-sensitized solar cells, quasi-solid-state electrolyte, functionalized graphene, the microstructure of polymer electrolyte, photovoltaic performance

Introduction

Since the introduction of dye-sensitized solar cells (DSSCs) by O’Regan and Grätzel [1] in the early 1990s, the efficiency of the energy conversion has been continuously improved to reach an all-time high of 14% for liquid electrolyte (LE)-based cells [2]. Despite their lower efficiency compared to their counterparts (e.g., 25% for the silicon-based solar cells [3]), DSSCs are still considered as a potential substituent for Si-based photovoltaic cells, thanks to their environmental friendliness and cost-effectiveness. The electrolyte is the most essential part of a DSSC, playing a pivotal role in its function by providing the required medium for charge transport. A majority of high-efficiency DSSCs are based on liquid electrolytes. The extensive use of DSSCs is, however, restricted by several phenomena such as degradation, leakage, dye desorption, thermal instability, and electrode corrosion, mostly due to the application of the conventional LEs [4]. In this regard, numerous studies have been devoted to substituting the conventional LEs with proper alternatives, like ionic liquids [5,6], solid-state perovskites [7], and solid/quasi-solid-state polymer gels and their composites [8,9].

Owing to their excellent ionic conductivity, gel polymer electrolytes (GPEs) have been interesting materials in a number of different fields. Generally speaking, a GPE is composed of a polymer base, salt, and a solvent/ionic liquid (IL), where the polymer entraps the IL, the salt provides free ions to enhance the conductivity, and the solvent/IL dissolves the salt and serves as a conducting medium. The polymer not only provides the required mechanical stability but also serves as a gelation agent [10]. The brilliant performance of the PVDF–HFP/PEO as a blend membrane for electrochemical devices has branded it as an applicable material for DSSCs. Owing to the considerable electronegativity and small ionic radius of the fluorine (F), the PVDF–HFP exhibits a slow recombination rate but high ionic conductivity along with the interface between the semiconductor photoanode and the polymer electrolyte in a DSSC [11]. As of present, the PEO-based electrolytes have been used to boost the ion exchange through two mechanisms: (a) cation complexation and (2) high-rate diffusion of the ion carriers into the amorphous region of the membrane [12,13]. Therefore, the blending of these components will lead to higher ionic conductivity by combining the advantages of the polymers, especially in terms of the higher mechanical stability of PVDF–HFP, with the higher diffusion rate of the ion carriers in the amorphous domain of the PEO. The improvement in ionic conductivity of the blend is mainly caused by the reduced crystallinity and generation of the ion pathways [14].

The electrochemical properties of a gel polymer electrolyte can be improved by adding a filler. Primarily, fillers are selected for the following reasons: (a) reducing the crystallinity and self-aggregation along polymer chains, leading to a more amorphous phase for the gel polymer electrolyte; (b) enhancing salt dissociation while increasing the number of charge carriers; (c) developing a more conductive pathway to improve ionic conductivity; (d) enhancing the stability and mechanical strength of the polymer electrolyte [15,16]. Therefore, the size [17] and content [18] of the filler nanoparticles (NPs) as well as their interaction (chemical/physical or interface/interphase) with the various constituents could dramatically affect the final properties of the nanocomposite. Graphene nanoplatelets (GNPs) have exhibited large potentials, thanks to their excellent mechanical, thermal and electrical features as well as their capability of forming interconnected networks at relatively low contents. Such improvements in characteristics at such low contents have been assigned to the high aspect ratio of the GNPs [19]. Yet, fine dispersion and distribution of carbonaceous NPs in the polymer matrix have remained unsolved issues [20,21]. Homogeneous dispersion of carbonaceous NPs in the polymer can be enhanced by chemical or physical functionalization [22,23]. Many studies have been published about polymeric electrolytes containing GNP and FGNP [24–27]. Marchezi et al. [28] prepared a gel polymer electrolyte composed of PEO, γ-butyrolactone (GBL), LiI, I2, and different concentrations of reduced graphene oxide (RGO). Upon using only 0.5 wt% of RGO, they achieved an efficiency of 5.07% with the highest short-circuit photocurrent (JSC) and open-circuit potential (Voc). They suggested that NPs acted as a multifunctional component in the GPE. In a very recent study, Gomari et al. [29] pioneeringly grafted polyethylene glycol (PEG) onto graphene and employed it in a PEO electrolyte to enhance ionic conductivity by lowering the nanocomposite crystallinity. According to their results, PEG-grafted graphene was achieved through hydrogen bonding with oxygen atoms of PEO chains, possibly deteriorating the crystallinity of the PEO-based electrolyte. PEG-grafted graphene could contribute in either of two ways: enhancing the segmental motion of PEO chains for higher flexibility and promoting better distribution through the polymer matrix. Recently, Prabakaran et al. [30] prepared a polymer electrolyte based on PVDF–HFP/PEO (60/40 wt/wt) and 0.8 wt% RGO, ending up with maximal efficiency of 4.6%. Rehman et al. [31] reported a DSSC using a polyvinyl acetate (PVAc)/graphene nanocomposite-based gel electrolyte, which could realize high efficiency of 4.57%.

The microstructure of GPE plays a vital role in the final performance of the device. Concerning the blend, the miscibility of two components in the amorphous domain can result in homogenous pathways for ion carriers [32]. In the case of nanocomposites, the viscoelasticity and rheological features are highly dependent on the distribution of nanofillers in the polymer matrix. These properties could also help better understand the molecular interactions occurring between the filler and the polymer matrix, and hence, provide guidelines for improving the performance of electrochemical devices [33,34]. Composed of organic cations and counter-ions, ILs have drawn a considerable deal of attention as alternatives to organic solvents, making them capable of playing an important role in a GPE system. Muhammad et al. [35] prepared a DSSC with 8 wt% IL (1-methyl-3-propylimidazolium iodide, i.e., MPImI) in polymer electrolytes based on hexanoyl chitosan/poly(vinyl chloride) (PVC), and the device performed at an efficiency (η) of 4.55%. The noncovalent interactions of CNTs with a polymer matrix (e.g., polyurethane and PVDF) in the presence of an IL leads to the fine dispersion and distribution of the NPs, bringing about some synergistically improved properties. Among the crystalline and amorphous phases of GPEs, the amorphous phase is significantly related to ionic conductivity [36]. According to Vyas and Chandra, EMIMBF4 is an ionic liquid capable of providing free ions and increasing the amorphous phase of the polymer in the presence of single-walled carbon nanotubes (SWCNT), hence, serving as a plasticizer and giving rise to enhanced ionic conductivity [37].

Given the mentioned concerns regarding the DSSCs, this study is aimed at increasing the efficiency of the device through developing a novel GPE. An attempt was also made to investigate the effect of PEG-grafted graphene on the microstructure and hence the ionic conductivity and conduction mechanism of the GPE. A miscible PVDF–HFP/PEO mixture was used as the basic GPE blend, while an ionic liquid served as the plasticizing agent to enhance the ionic conductivity of the GPE. Accordingly, the graphene was functionalized by PEG in the presence of a carbodiimide condensing agent to prepare the gel polymer electrolyte. Then, functionalized graphene was added to the PVDF–HFP/PEO blend at various contents to investigate the effect of the functionalized graphene dosage on ionic conductivity and DSSC performance. The microstructure and its relationship with ionic conductivity and electrochemistry properties of nanocomposite GPEs were thoroughly investigated by a combination of thermal and rheological studies and SEM and impedance analyses. To the best of our knowledge, this blend and functionalized graphene have not been addressed as an electrolyte.

Conclusions

PVDF–HFP/PEO-based polymer nanocomposites were investigated. According to the results, the ionic conductivity of the electrolyte and the DSSC performance was found to be highly dependent on the availability of free ions and incorporation of additives into the polymer electrolyte at optimal ratios. The mobility of the ions could be further affected by the free volume of the PVDF–HFP/PEO that could be enhanced by expanding the amorphous domains of the samples. As a filler, FGNPs were incorporated into the PVDF–HFP/PEO and ionic liquid systems. The FGNP-grafted PEG molecules interacted with the oxygen atoms of the PEO chains through hydrogen bonding, thereby disrupting the blend crystallinity. DSC analysis indicated a decrease in the degree of crystallization upon the incorporation of the GNPs (and FGNPs, in particular). The strong polymer–nanoparticle interactions combined with proper connectivity improved the conductivity of the electrolyte significantly, lowering the required activation energy. The developed solar cells containing an optimal dosage of FGNP (0.75 wt%) offered a Voc of 0.637 V and a Jsc of 13.81 mA/cm2 and led to a solar energy conversion efficiency of 5.45% upon exposure to 100 mW/cm2. Based on the linear steady-state voltammetry results, the FGNPs could shorten the ionic diffusion length while enhancing the diffusion coefficient to about 8.14 × 10−9 cm2 s −1. The optimum DSSC exhibited superior stability as it succeeded to retain 82.19% of its original performance after 1000 h of storage, as per the results of regular periodic tests.

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Microstructural Development and Rheological Study of a Nanocomposite Gel Polymer Electrolyte Based on Functionalized Graphene for Dye-Sensitized Solar Cells

Bibliography

author

Pedram Manafi 1, Hossein Nazockdast 2,*, Mohammad Karimi 3, Mojtaba Sadighi 4 and Luca Magagnin 5,

1 Mahshahr Campus, Amirkabir University of Technology, Mahshahr P.O. Box 63517-13178, Iran; pedram_manafi@aut.ac.ir

2 Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, Tehran P.O. Box 15875-4413, Iran

3 School of Materials and Advanced processes Engineering, Department of Textile Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran P.O. Box 15875-4413, Iran; mkarimi@aut.ac.ir

4 Department of Mechanical Engineering, Amirkabir University of Technology, Tehran P.O. Box 15875-4413, Iran; mojtaba@aut.ac.ir

5 Department of Chemistry, Materials and Chemical Engineering “Giulio Natta,” Politecnico di Milano, 20131 Milano, Italy * Correspondence: nazdast@aut.ac.ir (H.N.); luca.magagnin@polimi.it (L.M.)

Year

2020

Title

Microstructural Development and Rheological Study of a Nanocomposite Gel Polymer Electrolyte Based on Functionalized Graphene for Dye-Sensitized Solar Cells

Publish in

Polymer-Based Solar Cells

Doi

https://doi.org/10.3390/polym12071443

PDF reference and original file: Click here

 

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Nasim Gazerani was born in 1983 in Arak. She holds a Master's degree in Software Engineering from UM University of Malaysia.

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Professor Siavosh Kaviani was born in 1961 in Tehran. He had a professorship. He holds a Ph.D. in Software Engineering from the QL University of Software Development Methodology and an honorary Ph.D. from the University of Chelsea.

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Somayeh Nosrati was born in 1982 in Tehran. She holds a Master's degree in artificial intelligence from Khatam University of Tehran.