Molecularly imprinted polypyrrole counter electrode for gel-state dye-sensitized solar cells
Graphical abstract
Introduction
Dye-Sensitized Solar Cells (DSSCs) have attracted intense interest into the scientific and technological community as light harvesting technology due to the ability to work in diffuse light conditions, low costs processes preparation, and environmental low impact compared with traditional photovoltaic devices [[1], [2], [3], [4]]. The main challenges for the DSSCs commercialization are the increasing of stability and the materials costs reduction of these devices, particularly related to the quantity of Platinum loaded [1,3]. In fact, Pt is very expensive and scarce in nature; it also suffers of slow dissolution when I3−/3I− electrolyte is used, due to the corrosive properties of the redox couple that decreases the long term stability of the final device [5,6]. For these reasons, the research activity is focused on development of highly efficient Platinum-free counter electrode (CE) and replacement of the liquid electrolyte with the solid or gel state one in order to increase the DSSCs stability [1,7]. Among the different materials, conducting polymers (CPs) are potential candidates due to their high conductivity, low cost, good stability and high catalytic activity towards triiodide reduction. The most studied and efficient CPs are polyaniline (PANI), polypyrrole (PPy), poly(3,4-propylenedioxythiophene) (PProDOT) and poly(3,4-ethylenedioxythiophene) (PEDOT) [[8], [9], [10]]. PPy exhibits the best performances as CE: its electronic and catalytic properties can be easily tuned by doping process and the film morphology can be controlled optimizing the synthesis conditions [[11], [12], [13], [14]]. However, for the development of efficient CE materials, the whole chemical composition of the electrolyte was not taken into account up to now. The DSSCs electrolyte is commonly a complex matrix containing, other than triiodide, other compounds used as additives and stabilizers typically tetrabutylammonium molecule and compounds based on nitrogen-containing heterocyclic molecules such as analogues and derivatives of pyridine, alkylaminopyridine, alkylpyridine, benzimidazole, pyrazole [15,16]. These molecules can interfere and compete with the triiodide reduction due to their redox properties. In fact, literature reports that pyridine can be reduced to pyridinium ion at −1.0 V vs SCE, benzimidazole and derivate compounds can be reduced in the range of −1.68 and −2.72 V vs Ag/AgCl (KCl sat.) and finally pyrazole can be reduced at −1,6 and 0,8 V vs Ag/AgCl (KCl sat.) [[17], [18], [19], [20]]. Moreover, in efficient electrolytes high iodide concentrations are present leading to the formation of polyiodide species like I5−, I7− and I9− (in general I2n+1-) [16,21]. A new strategy that takes into account the high chemical and electrochemical complexity of the electrolyte for enhancing the selectivity of triiodide and consequently the final efficiency of polymeric CE is here presented. Materials well known for their high selectivity are the so-called Molecular Imprinted Materials widely used into the sensors field [[22], [23], [24]]. In this study, the first one in the solar cells field, a novel approach based on Molecularly Imprinted Polymer of PPy (PPy-MIP) will be proposed to enhance the efficiency of the DSSCs CE. Traditionally, to produce a MIP, the polymerization of the monomer occurs in the presence of a template molecule which is incorporated in the polymeric matrix and that corresponds to the target one. Functional monomers are chosen to interact with the template molecule since the formation of a stable template-monomer complex is fundamental for the success of the molecular recognition (i.e. self-assembling). The polymer obtained is a porous matrix possessing microcavities with structure complementary to that of the template. Thus, the removal of the template molecules from the polymer, by washing with solvent leaves binding sites that are complementary in shape to the template. Consequently, the resultant polymer recognizes and binds selectively only the template molecules [22,23]. MIPs can be prepared following different processes such as electropolymerization, drop-coating and in-situ chemical polymerization. The MIP films prepared by electrochemical method have superior properties in terms of adherence on the substrate surface, easy preparation method and control on the film properties [25,26]. About that, polypyrrole already possess the functional properties for MIP application i.e. hydrogen atom in order to strongly interact with the template molecule [25]. Inside the DSSCs electrolyte, the target molecule at the CE is the triiodide one that must be reduced in order to complete the working mechanism of the device. The same binding mechanism is necessary for the development of MIP based CE for DSSC applications, where triiodide must be chemically bound with the PPy structure. Lu et al. [27] showed in fact that triiodide molecules can be linked with the porous PPy structure by electrostatic interaction and van der Waals forces. Unfortunately, for the realization of the CE using the MIP approach triiodide cannot be used as template during the electropolymerization process because it can trigger the polymerization of pyrrole monomer instead of e.g. the anodic current applied during the electrochemical deposition process. Thus a different template with molecular dimension comparable with the target one must be used during the MIP production process. In order to demonstrate the potentiality of this approach, two molecules with suitable chemical properties for MIP application but different surface area were investigate as template: the first one with similar surface to the triiodide one and the second one with higher surface. The theoretical surface area values were calculated using the method reported by Ferrera et al. [28] based on the usage of the van der Waals radii and using a dedicated software [29]. In particular, considering the triiodide (target) surface area equal to 121.15 Å2, the first molecule analyzed was the 2-aminoacetic acid (called Glycine, pKa1 = 2.34 and pKa2 = 9.60) that shows at pH below 2 shows a surface area of 118.60 Å2 while the second molecule was L-2-aminopropionic acid (called l-Alanine, pKa1 = 2.34 and pKa2 = 9.69) which exhibits a surface area of 147.24 Å2 in the same conditions. The chemical structures of these molecules are shown in Fig. 1.
Section snippets
Thin films preparation
The electropolymerization of Molecular Imprinting Polymers (MIP) and Non-Imprinting ones (NIP) based on PPy was realized onto Fluorine-doped Tin Oxide glasses (FTO, Sigma – Aldrich, Germany) applying a constant voltage of 1.0 V vs Saturated Calomel Electrode (SCE). The synthesis was carried out using a conventional three-electrodes system with Pt foil as a counter-electrode and SCE as reference electrode. The polymerizations of NIP-PPy were performed in an aqueous solution containing 50 × 10−3
Results and discussion
MIP and NIP PPy were electropolymerized on FTO surface at 1.0 V vs SCE with and without Glycine and l-Alanine as template molecules. The first template concentration studied was fixed at 25 × 10−3 M (PPy MIP25 mM) and these electrodes were prepared with the same amount of charge that flows onto the FTO surface. In this way, the obtained films have comparable thickness. The electrical charge was equal to 4 × 10−2 C cm −2 and the films thickness was confirmed by profilometry, giving an average
Conclusions
In summary, the Molecular Imprinting approach generally used for sensor applications was successfully applied also in the DSSCs field. This technique was able to strongly enhanced the catalytic properties of PPy CEs towards the triiodide selectivity without adding other active materials or changing the polymerization process parameters. The triiodide target molecules however cannot be used as template due to the oxidant properties of iodine, thus Glycine was identified as suitable molecule due
Declarations of interest
None.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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