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sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells
Author’s Accepted Manuscript Metal-organic framework/sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells Tai-Ying Chen, Yi-June Huang, Chun-Ting Li, Chung-Wei Kung, R. Vittal, Kuo-Chuan Ho www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(16)30583-3 http://dx.doi.org/10.1016/j.nanoen.2016.12.019 NANOEN1670
To appear in: Nano Energy Received date: 5 November 2016 Revised date: 10 December 2016 Accepted date: 11 December 2016 Cite this article as: Tai-Ying Chen, Yi-June Huang, Chun-Ting Li, Chung-Wei Kung, R. Vittal and Kuo-Chuan Ho, Metal-organic framework/sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.12.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Metal-organic framework/sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells Tai-Ying Chena, Yi-June Huanga, Chun-Ting Lia, Chung-Wei Kunga, R. Vittala and Kuo-Chuan Hoa,b,* 1
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan;
2
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
Abstract Metal-organic framework (MOF-525) is firstly introduced as the electro-catalyst for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). When MOF-525 was mixed with the conductive binder of sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]2,5-diyl) (s-PT), a composite film of MOF-525/s-PT was successfully deposited on a flexible substrate, carbon cloth (CC). The one-dimensional carbon fibers in CC were intended to provide oriented electron transfer pathways as a conductive core, and the composite film of MOF-525/s-PT covered on each carbon fiber in CC was designed to trigger the reduction of I3− as an electro-catalytic shell. Thus, a hierarchical electron transfer network was established. In the MOF-525 nanoparticle, its nodes (zirconium oxide) and linkers (meso-tetra(4carboxyphenyl)porphyrin) were both verified to function as the electro-catalytic active sites for I3− reduction. The best MOF-525/s-PT composite counter electrode rendered 8.91±0.02% to its DSSC, showing the promising potential to replace traditional platinum (8.21±0.02%). At dim light condition (10 mW cm–2), the best cell with MOF-525/s-PT composite CE shows a great cell efficiency (η) of 9.75%, which is higher than that of the cell measured at 100 mW cm–2.
1
Keywords: counter electrode; dye-sensitized solar cell; flexible substrate; metal-organic framework; sulfonated polythiophene
*
Corresponding author: Tel: +886–2–2366–0739; Fax: +886–2–2362–3040;
E–mail: [email protected]
1. Introduction Metal-organic frameworks (MOFs) have recently received increasing attention, and they are widely proposed for different applications due to their attractive material properties [1]. Metal-organic frameworks are constructed by the coordination complexes as the repeating units, extending in one, two or in three dimensions; such a coordination complex consists of metallic clusters as the nodes and organic ligands as the linkers [2]. Thus, MOF materials often exhibit well-ordered and size-controllable pores, which provide extremely large surface area. By controlling the coordination geometry of the metallic cluster and the topology of the linker, an MOF can be designed for various applications, e.g., for gas storage [3], gas separation [4], supercapacitors [5], electrochemical sensing [6], water oxidation [7], CO2 reduction [8], drug delivery [9], and catalysis [10]. Since dye-sensitized solar cells (DSSCs) have been considered as the promising renewable and clean energy resources for the next generation, enhancement of their power conversion efficiency (η) with low-cost materials became the key issue in the last two decades [11]. Several kinds of MOF have been used in DSSC to assist the fabrications of photoanode, electrolyte, and counter electrode [12]. In the case of photoanode, copper-MOF had been used as the co-sensitizers (0.27%), ZIF-8 had been used for blocking layer (5.21%), and MOF-5 had been used to synthesize the ZnO scattering layer with hierarchical structure (3.67%) [12]. In the case of electrolyte, magnesium-MOF was added into a polymer-based electrolyte to increase the cell efficiency of DSSC due to the enhancement in the interfacial connection between the TiO2 and electrolyte (4.80%) [13]. In the case of counter electrode, cobalt-MOF 2
was introduced as a precursor to synthesize efficient cobalt sulfide; the Co-MOF derived CoS offered large surface area and thus gave 8.1% efficien to its DSSC [14]. To summary the above-mentioned literatures, the light harvesting ability, blocking character, and large surface area of MOF are beneficial to the DSSCs. However, the electrochemical properties of the MOF itself have never been investigated for DSSCs. In a DSSC, the counter electrode (CE) is responsible for regenerating the redox mediator (I–/I3–), and thus keeps the DSSC working [15]. Besides, the cell efficiency of DSSCs is directly influenced by the electro-catalytic ability of the CE. Platinum (Pt) is generally chosen as the electro-catalyst; however, Pt is very expensive and easily gets poisoned by iodide [15]. Many Pt-free electro-catalysts have been developed according to the requirements for a CE of a DSSC, e.g., excellent large surface area, electro-catalytic ability, and good conductivity [15]. From the standpoint of these requirements, we reasoned that MOF-525 may be one of the potential candidates to replace Pt, due to its extremely high Brunauer−Emmett−Teller (BET) surface area of about 2620 m2 g-1, high stability, and redox activity derived from linkers [6, 16, 17]. A MOF-525 consists of cuboctahedral nodes of zirconium metal oxide (Zr6O4(OH)4) and the linkers of meso-tetra(4-carboxyphenyl)porphyrin (H4TCPP). The former is a zirconium (Zr)-based compound and the latter consists of four pyrrole-based entities interconnected at their α carbon atoms via methine bridges (=CH−). Among the Zrbased compounds, ZrO2, ZrN, and ZrC have been reported to provide electro-catalytic active sites for triiodide reduction; these compounds have rendered for their DSSCs cell efficiencies (η) of 2.60% [18], 1.20% [18], and 3.85% [18], respectively. In the case of the pyrrole-based materials, pristine polypyrrole (PPy) and its composite materials both exhibited good electrocatalytic abilities toward triiodide reduction, and their cells showed power conversion efficiencies of up to 7% [19, 20]. From the above literatures, it is understood that both the node (Zr6O4(OH)4) and the linker (H4TCPP) of MOF-525 have great potential to offer catalytic abilities for the reduction of triiodide ions to iodide ions. 3
In our study, we successfully synthesized MOF-525 nanoparticles and mixed them with the conducting polymer, sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]-2,5-diyl) (s-PT). The obtained slurry was coated on a carbon cloth (CC)-substrate via a simple dropcoating process. The carbon cloth with its composite film (hereafter MOF-525/s-PT) was used as the counter electrode for the DSSC. Scheme 1(a) (bottom) shows SEM image of the CC and its actual image in hand (middle). The CC is a flexible and low-weight conductive substrate which has been regularly used in DSSCs [21-23]. The CC is interlaced by bunches of conductive carbon fibers [24]; each carbon fiber is supposed to act as a one-dimensional (1D) pathway for electron transfer. The CC substrate is so thin (thickness = 0.36 mm) and has an extremely low weight that a very soft flower with needle-like petals could withstand its weight without enduring the slightest strain, as shown in the upper side portion of Scheme 1(a). Thus, a conductive network can be provided by this CC substrate. In a CC substrate, each carbon fiber was wrapped around by the MOF-525/s-PT composite film. Accordingly, there is a core–shell structure where the carbon fiber is a 1D conductive core and the MOF525/s-PT film is the electro-catalytic shell, as represented in Scheme 1(b). The cross sectional image of the core-shell structure is shown in Figure S1 of the Supporting Information. Therefore, a hierarchical electron transfer network could be established, as shown in Scheme 1(b). First of all, a main stream of electrons could occur through the 1D carbon fiber (core of the core-shell structure). Second, a minor stream of electrons can pass through the layer of sPT to reach the electro-catalytic active sites of MOF-525, where the reduction of triiodide ions takes place (Scheme 1(b)). The carbon cloth was intended to provide oriented electron transfer pathways, the MOF-525 nanoparticles could bestow the composite film with large surface area and good electro-catalytic ability, and the s-PT polymer was expected to prevent the MOF-525 nanoparticles from being aggregated, facilitate the linkage among MOF-525 nanoparticles, and create electron transfer pathways from the substrate to the MOF-525. This MOF-525/s-PT composite film aims to trigger the I3– reduction in a DSSC, as shown in 4
Scheme 1(c); its attractive electro-catalytic ability and the outstanding photovoltaic performance are systematically investigated, and compared with the traditional Pt material. It is noted that this is the first study on the electro-catalytic ability of MOF-525 and its influence on the photovoltaic performance of the DSSCs.
2. Experimental Section 2.1. Synthesis of MOF-525 MOF-525 nanoparticles were synthesized via a published four-step route [25] as follows: (1) two precursors of 1.35 g of benzoic acid and 105 mg of zirconyl chloride octahydrate were dissolved together and sonicated in 8 mL of N,N-dimethylformamide (DMF, ≥99.8%); the dissolved and sonicated solution was transferred into a vial of 20 mL volume; (2) the solution was heated and maintained at 80 °C for 2 h in a gravity convection oven; (3) after cooling the solution to room temperature, 47 mg of meso-tetra(4-carboxyphenyl)porphyrin (H4TCPP, >97%) was added to it, and the thus obtained solution was sonicated at room temperature for 20 min; (4) the last solution was then heated on the bottom of a gravity convection oven at 80 °C for 24 h. After washing with DMF and acetone several times, soaking into acetone for one day, and drying under vacuum, a product of MOF-525 with dark red color was obtained with good yield.
2.2. Fabrication of the counter electrodes and the DSSCs Substrates of fluorine-doped tin oxide (FTO) conducting glasses and carbon cloths (CC) were used to prepare the photoanodes and counter electrodes (CE) of the DSSCs, respectively; the substrates were sequentially cleaned with a neutral cleaner, de-ionized water, acetone, and isopropanol. In order to compare the performances of electro-catalytic films, four types of films were prepared as follows: (1) a Pt film (30 nm-thick) was obtained on an FTO by a DC sputtering technique; (2) bare s-PT film was prepared on a CC by a drop-coating technique using 50 μL of a s-PT solution; (3) by using the same drop-coating technique, five 5
types of MOF-525/s-PT composite films, namely MOF-525/s-PT-1, MOF-525/s-PT-2, MOF525/s-PT-3, MOF-525/s-PT-4, and MOF-525/s-PT-5 were obtained on CCs using 50 μL of different MOF-525/s-PT slurries, containing 1, 2, 3, 4, and 5 wt% of MOF-525 nanoparticles, respectively, with respect to the weight of s-PT; (4) bare MOF-525 film was prepared on a CC by the same drop-coating technique using 50 μL of DMF containing 3wt% of MOF-525, with respect to the weight of DMF. In brief, one Pt/FTO electrode and seven kinds of CC-based electrodes were prepared for the CEs of the DSSCs. To characterize the films, scanning electron microscopy (SEM), cyclic voltammetry (CV), rotating disk electrode (RDE), Tafel polarization curves, and electrochemical impedance spectra (EIS) were used. In order to assemble the DSSC, a 15 μm-thick TiO2 film adsorbed with an N719 dye (including a 10 μm transparent layer and a 5 μm scattering layer, with an active area of 0.20 cm2) was used as the photoanode; that was fabricated in accordance with a published procedure [24]. The photoanode was assembled with the CE using a 60 μm-thick Surlyn® as the spacer. Finally, an electrolyte, containing 1.2 M 1,2–dimethyl–3–propylimidazolium iodide (DMPII), 0.035 M iodine (I2), 0.1 M guanidinium thiocyanate (GuSCN), and 0.5 M 4tert-butylpyridine (tBP) in 3-methoxypropionitrile/acetonitrile (MPN/ACN, at the volume ratio of 2:8), was injected into the gap between these two electrodes by capillarity. Photovoltaic performance of DSSCs was measured via the photocurrent density–voltage (J– V) curves and incident photo-to-electron efficiency (IPCE) spectra. The standard deviation data for each DSSC were obtained using three cells. The Supporting Information gives details of analytic techniques and materials used in this study.
3. Results and Discussion 3.1. Surface Morphology Field–emission scanning electron microscopy (FE-SEM) images of various electrodes are shown in Figure 1; the insets in the images show the corresponding images at high
6
magnifications. Figure 1(a) shows an ultra-thin, flat, and uniform film of platinum (Pt) on an FTO substrate. Since the FTO substrate generally shows a rough surface composed of many triangular pyramid-like structure, the covered Pt thin film follows the same morphology as FTO. Figure 1(b) shows the image of bare s-PT film on the carbon cloth. The s-PT is well covered on the carbon fibers of the carbon cloth. The surface of the s-PT is very flat and compact, which implies its limited active sites for any electrochemical catalysis. Compared to the surface of this bare film of s-PT, all surfaces of MOF-525/s-PT films look rougher, apparently due to the presence of MOF-525 in the s-PT (Figure 1(c) to (g)). With an increase in the amount of MOF-525 in the s-PT film (from 1 to 5 wt% w.r.t. s-PT), an increase in the amount of cubic-like nanoparticles of MOF-525 can be seen on the surface of the s-PT film (Figure 1(c) to (g)); the thus obtained nanoparticles were estimated to have an average particle size of 200 nm. Since MOF-525 has an extremely large BET surface area (2620 m2 g-1) and a large pore size (1.8 nm) [25], which renders the diffusion of iodide and triiodide ions, its increase in the composite film is expected to increase the active surface area of the composite film for electrocatalysis tremendously. In the cases of MOF-525/s-PT-4 (Figure 1(f)) and MOF-525/s-PT-5 (Figure 1(g)), the MOF-525 nanoparticles appear stacked on each other and are found to form an aggregated layer, which may reduce the electron transport rate in the composite films due to poor conductivity of MOF-525. Figure 1(h) shows the image of a bare MOF-525 film; the film consists of innumerable number of MOF-525 nano-cubes. This bare MOF-525 film appears partially detached from the CC substrate, indicating a very weak adhesion between the MOF-525 nanoparticles and the CC substrate. With reference to this bare MOF-525 film, all MOF-525/s-PT films adhere well to their CC substrates; this phenomenon is apparently due to the presence of the binder s-PT in the composite film. In brief, it can be said that the MOF-525/s-PT composite films have larger active areas than that of the film of s-PT and much better adhesion to their CC substrates than that of the MOF-525 to its CC substrate. 7
3.2. X–ray photoelectron spectra and X–ray diffraction analyses X–ray photoelectron spectra (XPS, Figure S2) and X–ray diffraction (XRD, Figure S3) patterns were used to investigate the interactions between MOF-525 and s-PT in a MOF525/s-PT composite film. Figure S2(a) shows the Zr 3d orbital of MOF-525, which is embedded in s-PT. All of the three peaks in Figure S2(a) belong to the Zr–O chemical bonds, indicating that there is no chemical bond between the Zr6O4(OH)4 nodes of MOF-525 and sPT. Figure S2(b) shows the S 2p orbital of s-PT, which is in a composite MOF-525/s-PT film. There are four peaks in Figure S2(b); three peaks at higher binding energy level refer to the S–C chemical bonds, and the other one peak refers to the S–O chemical bonds. It is clear that there is no bonds between the sulfur atoms in s-PT and MOF-525. Figure S3 shows the XRD of the composite MOF-525/s-PT film; the peaks at 4.5o, 6.4o, 7.8o, and 9.1o are highly consistent with the reported XRD pattern of pure MOF-525 [26]. It can be said that the crystalline MOF-525 is not influenced by the s-PT. In all, the interactions between MOF-525 and s-PT in a composite MOF-525/s-PT film may belong to the Van Der Waals forces.
3.3. Photovoltaic performance. Various MOF-525/s-PT composite electrodes containing 1~5 wt% of MOF-525 nanoparticles (w.r.t. the weight of s-PT) are denoted as MOF-525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, MOF-525/s-PT-4, and MOF-525/s-PT-5, where the last number indicates the wt% of MOF-525 with respect to the weight of s-PT. Photocurrent density–voltage (J–V) curves of the DSSCs with these electrodes as the CEs are shown in Figure 2(a). The corresponding photovoltaic parameters are summarized in Table 1. With the increase of content of MOF-525 in the composite film, the cell efficiencies of these DSSCs show an increasing tendency from 6.49±0.03% (MOF-525/s-PT-1) to 8.91±0.02% (MOF-525/s-PT-3), and then a decreasing tendency from 8.91±0.02% (MOF-525/s-PT-3) to 5.97±0.02% (MOF8
525/s-PT-5). Table 1 shows that the values of open-circuit voltage (Voc) and fill factor (FF) of a DSSC change hardly due to the increase in the amount of MOF-525 in the composite film, but the value of short–circuit current density (Jsc) of the DSSC varies greatly with the increase in the amount of MOF-525 in the composite film. Thus, the variations in the cell efficiency of the DSSCs are certainly attributed to the variations in the Jsc of the cells. Two factors are attributed to the variations in the Jsc: (1) electro-catalytic active area of the counter electrode and thereby electro-catalytic activity of it and (2) electrical conductivity of the counter electrode. The FE-SEM images in Figure 1 indicate clearly that the electro-catalytic active area of a composite film increases in proportion to the increase in the content of MOF-525 in the composite film. A steady increase in the Jsc is therefore expected from the viewpoint of increased electro-catalytic active sites; Table 1 shows that this however is not the case. The Jsc increases initially and then it decreases. With the increases in the contents of MOF-525 from 1 to 3 wt%, the Jsc increases as expected due to the increases in the electro-catalytic active surface areas or electro-catalytic activities of the corresponding CEs. With further increases of MOF-525 from 3 to 5 wt%, the Jsc decreases; this can be attributed to the excesses in the amounts of MOF-525 in the composite films of MOF-525/s-PT-4 and MOF-525/s-PT-5, which can hinder electron transportation in the composite films and electron transfer at the interfaces of CC substrate/composite film and composite film/electrolyte. Among all the DSSCs with the MOF-525/s-PT composite films, the cell with MOF-525/s-PT-3 exhibits the highest η of 8.91±0.02%, with a Voc of 0.80±0.00 V, Jsc of 16.14±0.13 mA cm–2, and FF of 0.70±0.00. On the other hand, J–V curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare sPT, bare MOF-525, and Pt are shown in Figure 2(b); the pertinent data are summarized in Table 1. The cell with bare s-PT reaches an η of 6.94±0.02%, with a Voc of 0.79 V, Jsc of 12.32 mA cm–2, and FF of 0.71. It can be seen in Table 1 that the DSSCs with the CEs of MOF525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, and MOF-525/s-PT-4 show higher η’s than 9
that of the cell with bare s-PT CE; this is clearly attributed to the increased electro-catalytic activities of these composite films bestowed by the added MOF-525 nanoparticles in these films. The cell with bare MOF-525 exhibits an η of 4.78±0.02% with a Voc of 0.78±0.00 V, Jsc of 12.68±0.16 mA cm–2, and FF of 0.48±0.00. For comparison, we directly employed a bare CC substrate (without loading any kind of electro-catalyst) as the CE in a DSSC, and this cell showed a very poor η of 1.65% with a Voc of 0.79 V, Jsc of 10.38 mA cm–2 and FF of 0.20 (Figure S4 in Supporting Information). When the performance of the cell using bare CC substrate (η=1.65%) is compared with that of the cell using only MOF-525 (η=4.78±0.02%), it can be clearly seen that the increase in the η for the latter case is due to the catalytic film of MOF-525. Thus, it is well established that MOF-525 exhibits electro-catalytic activity toward I3- reduction. To further explore the inner electro-catalytic sites in MOF-525, another type of MOF (UiO-67) was used to deliver the performance of the Zr6O4(OH)4 node of MOF-525; UiO-67 consisted the same Zr6O4(OH)4 node and the non-catalytic linker of biphenyl-4,4′dicarboxylic acid [17] and is synthesized in according to the published procedures [27]. H4TCPP was directly used to give the information of the linker of MOF-525. These two kinds of materials are separately coated on the CC substrates and denoted as UiO-67 and H4TCPP. The J–V curves of the DSSCs with the CEs of UiO-67 and H4TCPP are depicted in Figure S5(a) and Figure S5(b) of the Supporting Information. The cell with UiO-67 shows the η of 4.08%, and the cell with H4TCPP exhibits the η of 4.48%. Both cells with UiO-67 and H4TCPP give higher η than that of the cell with bare CC (1.65%, Figure S4) and comparable η to that of the cell with bare MOF-525 (4.78%, Table 1). Thus, it can be deduced that both Zr6O4(OH)4 node and pyrrole-based linker, H4TCPP, function as the inner electro-catalytic sites in MOF-525 and thus bring good electro-catalytic activity toward triiodide to MOF-525. In brief, in comparison to the cell with bare s-PT CE, the cell with the composite film of MOF-525/s-PT-3 shows much higher Jsc and η, owing to the presence of MOF-525 in the 10
composite film at an optimum level. In comparison to the cell with bare MOF-525, the cell with the composite film of MOF-525/s-PT-3 exhibits much higher Jsc, FF, and η, because the s-PT in the composite film enhances the adhesion between the substrate and the composite film, prevents MOF-525 nanoparticles from being aggregated, facilitates the linkage among MOF-525 nanoparticles, and creates electron transfer pathways from the substrate to the MOF-525. In conclusion, within the composite film of MOF-525/s-PT, the s-PT plays the roles of a conducting binder and matrix for the composite film, the MOF-525 enhances surface area of the composite film with respect to that of the s-PT film, and both Zr6O4(OH)4 node and H4TCPP linker in the MOF-525 play the role of electro-catalytic active sites. The DSSC with MOF-525/s-PT-3 achieves the cell efficiency of 8.91±0.02%, which is better than that of the cell with Pt (8.21±0.02%). When the DSSC with MOF-525/s-PT-3 was illuminated at dim light condition of 10 mW cm–2, the cell shows even higher η of 9.75% with a Voc of 0.71 V, Jsc of 1.91 mA cm–2, and FF of 0.72 (Figure 2(c)). It is notable that the cell efficiency under dim illumination is 10% higher than that of the cell at normal condition (100 mW cm– 2
). Furthermore, the DSSCs with MOF-525/s-PT-3 and Pt both show good performance in the
long-term stability tests; they maintain 83% and 87% of cell efficiency to their initial values after 7 days, respectively (Figure S6(a)). In conclusion, the MOF-525/s-PT composite film has great potential to replace Pt in a DSSC and can be further used for indoor or wearable DSSCs. Incident photon–to–current conversion efficiency (IPCE) curves of the DSSCs with the electrodes of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt were obtained at the shortcircuit condition, in the wavelength range of 400-800 nm, as shown in Figure 2(d). The IPCE values were calculated via Equation (1), where λ is the wavelength, Jsc(λ) is the specific shortcircuit photocurrent density obtained at a specific wavelength, and φ is the incident radiation flux [28]. IPCE (λ)=
1240 ×Jsc (λ) λ×φ
×100%
(1) 11
Both the DSSCs with MOF-525/s-PT-3 and Pt give good IPCE values of about 80~90%, in the wavelength region of 400 to 600 nm, showing that these counter electrodes enable sufficient regeneration of iodide ions. In contrast, the cells with bare s-PT and bare MOF-525 show significantly lower IPCE values, owing to limited surface area of s-PT and poor conductivity of bare MOF-525, respectively. Besides, a short-circuit current density value of a DSSC can also be obtained from its IPCE spectra (denoted as Jsc-IPCE), by integrating the area under the IPCE curve; the values were calculated to be 12.97, 9.52, 10.15, and 12.16 mA cm-2 for the cells with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt, respectively. These JscIPCE
values is perfectly consistent with the tendency of Jsc values attained from the J–V curves.
3.4. Cyclic voltammetry Cyclic voltammetry (CV) was applied to find out the redox kinetics of iodide/triiodide (I– /I3–) at the surface of the electro-catalytic electrode. As a CE in a DSSC, the main purpose of the electro-catalytic film is to facilitate the reduction of I3– at the CE/electrolyte interface. In the CV, the cathodic peak current density (Jpc) of the electrode can be extracted from its CV curve and can be used to evaluate the electro-catalytic activity of the electrode toward the reduction of I3–; the pertinent reaction at the electrode is as Equation (2) [15]. -
I3 +2e- →3I-
(2)
In this type of cyclic voltammetry, a higher cathodic peak current density (Jpc) of the electrode indicates a higher overall electro-catalytic activity of the electrode [29]. Figure 3(a) shows the cyclic voltammograms of the electrodes with the films of bare s-PT, bare MOF525, MOF-525/s-PT-3, and Pt; Table 2 gives their Jpc values. The Jpc values for MOF-525/sPT-3, bare s-PT, bare MOF-525 and Pt electrodes are 2.03, 0.85, 0.79 and 1.07 mA cm-2, respectively (Table 2). Obviously, the MOF-525/s-PT-3 composite film shows a higher Jpc value than that of bare s-PT film, indicating the fact that the presence of MOF-525 has indeed enhanced the electro-catalytic activity of the composite film with respect to that of bare s-PT 12
film. The intrinsic electro-catalytic activity of MOF-525 and its large surface area are apparently the reasons for its favorable role in enhancing the Jpc of the composite film. Moreover, the Jpc value of the composite film (2.03 mA cm-2) is much higher than that of the Pt film (1.07 mA cm-2), showing the better electro-catalytic activity of MOF-525/s-PT-3 than that of Pt. In addition to the electro-catalytic activity, MOF-525/s-PT-3 composite film also shows promising ability to resist the iodide corrosion. The CV curve of MOF-525/s-PT-3 counter electrode shows unfailing performance after 100 cycles, as shown in Figure S6(b). On the other hand, MOF-525 contains the H4TCPP linker, which may have light harvesting property, in accordance with the absorption spectra of pure MOF-525 (Figure S7). Therefore, at an applied potential of -0.5 V (a sufficient potential to execute I3– reduction), the plot of current density vs. time (Figure S8) was recorded under illuminated and dark conditions using the bare MOF-525 film. In Figure S8, the current-density measured under the light illumination is the same as that measured under dark condition. Thus, the catalytic ability of MOF-525 is not influenced by its light soaking.
3.5. Rotating disk electrode. In addition to the CV analysis, rotating disk electrode (RDE) technique was applied to determine two important parameters, the intrinsic heterogeneous rate constant (k0) and the effective catalytic surface area (Ae), which largely influence the overall electro-catalytic activity of an electrode. At the formal potential (E0’) of I–/I3–, various values of reciprocal current (i-1) were obtained from linear sweep voltammetry (LSV) curves for an electrode under various rotating speeds (50, 100, 200, 400, 600, 800, and 1000 rpm). Thus, plots of reciprocal current (i-1) vs. reciprocal of square root of rotating speed (ω-0.5) were obtained for MOF-525/s-PT-3, bare s-PT, and Pt electrodes, as shown in Figure 3(b). In the case of bare MOF-525, we could not get electrochemical signals for bare MOF-525 film, because the bare MOF-525 film directly peeled off from the substrate. The values of k0 and Ae were obtained 13
respectively for the mentioned electrodes from the intercept and slope of fitting lines in these plots[[30]] by using a simplified Koutecký–Levich equation (Equation (3)), where i is the specific current obtained from the LSV curve at the formal potential (E0’) of I–/I3–, n is the number of electrons transferred for I3– reduction, C is the concentration of I3– ions (1.0 mM), D is the apparent diffusion coefficient of I– (3.62×10-6 cm2 s-1), F is Faraday constant, ν is the kinematic viscosity of electrolyte, and ω is the angular velocity converted from the rotating speed. 1 i
=
1 nFAe
k0 C
+
1 0.62nFAe
D2 /3 υ-1 /6
(3)
ω1/2 C
As summarized in Table 2, the Pt film shows the highest k0 value of 3.56×10-3 cm s-1 and a low Ae value of 0.32 cm2; this reveals that the flat Pt film observed in its FE-SEM image possesses great intrinsic electro-catalytic activity but limited effective surface area. Compared with bare s-PT film, the MOF-525/s-PT-3 film exhibits higher k0 and Ae values, higher by about 1.2 times and 2.2 times, respectively. Thus, it is proved that addition of MOF-525 to sPT not only enhances the heterogeneous rate constant of s-PT, but also its electrochemical active surface area; these two factors have apparently enabled a higher catalytic activity to the composite film, compared to that of the s-PT film. The enhanced surface area of the composite film is due to the extremely large BET surface area of MOF-525. Thus, it is established that the MOF-525 nanoparticles function as a major electro-catalyst for I3– reduction in the MOF-525/s-PT composite electrode. Compared with the Pt film, the MOF525/s-PT-3 film shows a lower k0 (about 0.73 times) but a much larger Ae (about 2.2 times). Therefore, the overall electro-catalytic activity of the counter electrode with the composite film is higher than that of Pt counter electrode, as can be seen by their CVs in Figure 3(a) and from the values of Jpc and J0 in Table 2.
3.6. Tafel polarization curve In addition to the CV and RDE techniques, Tafel polarization was used to explore the 14
electro-catalytic activities of the electrodes; the data were obtained using a symmetric cell (see details in Supporting Information). The symmetric cell consisted of the same electrode as the anode and the cathode. Figure 4(a) shows the Tafel polarization curves for the electrodes of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. The Tafel curves can simply be placed into three zones: (1) the polarization zone (|V| < 120 mV), (2) the Tafel zone (120 mV< |V| < 400 mV), and (3) the diffusion zone (|V| > 400 mV) [30]. The exchange current density (J0) of an electro-catalytic composite film can be attained by extrapolating the cathodic and anodic curves and reading the cross point at 0 V in the Tafel zone. A higher value of J0 expresses a better electro-catalytic activity for the electrode [31]. As summarized in Table 2, the J0 values of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt are 4.07, 1.71, 2.09, and 3.31 mA cm-2, respectively. The tendency of these J0 values is MOF-525/s-PT-3 > Pt > bare MOF-525> bare s-PT, which well agrees with the tendency of the Jsc values of their DSSCs obtained from J–V curves. Thus, we validated the electro-catalytic activities of these films by CV and RDE using a lower concentration of I–/I3– but just by Tafel polarization curves using a higher concentration of I–/I3– (see Supporting Information). It is substantiated the fact that the outstanding electro-catalytic activity of MOF-525/s-PT composite electrode is majorly given by the MOF-525 nanoparticles.
3.7. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) was used to measure the interfacial resistances in the symmetric cell. As shown in Figure 4(b), the EIS spectrum displays two semicircles in the frequency range of 10 mHz to 65 kHz [15, 32]. As the equivalent circuit [15, 32] shown in the inset of Figure 4(b), the onset point of first semicircle in the high frequency region refers to the ohmic series resistance (Rs) of the substrate and its catalytic layer, the radius of the first semicircle in the middle frequency region refers to the charge transfer resistance (Rct-EIS) at the electro-catalytic film/electrolyte interface, and the diameter 15
of the second semicircle in the low frequency region refers to the Warburg diffusion resistance of the electrolyte. A lower Rs value indicates a better ohmic contact between the electrocatalytic film and the substrate [33]. In Table 2, the comparable Rs values of the cell with the electrodes of bare s-PT and composite MOF-525/s-PT-3 are 16.25 Ω cm2 and 16.42 Ω cm2, respectively, which both are lower than the Rs values of the cells with Pt (19.42 Ω cm2) and bare MOF-525 (20.26 Ω cm2). From the Rs values of bare MOF-525 and MOF-525/s-PT-3, it can be verified that the conductive binder of s-PT certainly provides intensive adhesion for the composite film to the substrate. Among all, the bare MOF-525 electrode has the largest Rs value; this indicates its weakest adhesion to its CC substrate. It is well known that a poor adhesion between a counter electrode film and its substrate causes a direct energy loss for the pertinent DSSC and a decrease in its FF value [11]. Thus, the Rs values for these electrodes follow an order of bare s-PT < MOF-525-3 < Pt < bare MOF-525, which is in complete agreement with the tendency of the FF values of their DSSCs. On the other hand, a low Rct-EIS value implies rapid passage of a large amount of charge at the interface of the pertinent electrode with the electrolyte. The Rct-EIS values of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt are 1.42, 4.82, 5.04, and 3.01 Ω cm2, respectively. Although the composite film of MOF-525/s-PT-3 shows a slightly higher Rs value than that of bare s-PT, it exhibits the lowest Rct-EIS value among all the films; this is one of the factors for the best electro-catalytic activity of MOF-525/s-PT-3 among all the films. Owing to lack of surface area, the bare s-PT shows a high Rct-EIS value (4.82 Ω cm2); owing to lack of proper adhesion to the substrate (as can be seen from the pertinent FF value in Table 1), the bare MOF-525 shows the highest Rct-EIS value (5.04 Ω cm2); when these two materials (s-PT and MOF-525) are combined, their composite film nullifies these two disadvantages and exhibits the least Rct-EIS value of 1.42 Ω cm2. 4. Conclusion
16
A novel composite film of a zirconium-based porphyrinic metal-organic framework (MOF-525) and sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]-2,5-diyl) (s-PT) was successfully deposited on a flexible substrate of carbon cloth (CC) to form a hierarchical electron transfer route. The interlaced bunch of carbon fiber in CC provided the electron transfer network, each carbon fiber in CC plays an important role of the one-dimensional conductive core, and the MOF-525/s-PT composite film covered on each carbon fiber in CC worked as the electro-catalytic shell. Among all the films, the MOF-525/s-PT composite film containing 3 wt% of MOF-525 nanoparticles (MOF-525/s-PT-3) shows the best electrocatalytic activity, which is substantiated via its highest Jpc (cyclic voltammetry), largest J0 (Tafel polarization curve), and lowest Rct-EIS (electrochemical impedance spectroscopy) values. The best electro-catalytic activity of MOF-525/s-PT-3 composite film is attributed to three key facts as follows. (1) MOF-525/s-PT-3 composite film has extremely large active areas given by the porous MOF-525 nanoparticle (the largest Ae from RDE). (2) MOF-525/s-PT-3 composite film possesses good adhesion to its CC substrate, which supplied by the conductive binder of s-PT (low Rs value from EIS). (3) MOF-525/s-PT-3 provides rapid reduction rate of I3– due to that both of the node (Zr6O4(OH)4) and the linker (H4TCPP) of MOF-525 works as electro-catalytic active sites (good k0 value from RDE and the results from J-V curve). Thus, the cell with MOF-525/s-PT-3 exhibited the highest η of 8.91±0.02%, which is better than that of the cell with traditional Pt (8.21±0.02%). There is no doubt that the MOF-525/s-PT-3 film is a promising electro-catalytic material to replace the expensive Pt in DSSCs. Besides, the cell with MOF-525/s-PT-3 is expected to have better cell efficiency, after adjusting the particle size and pore size of MOF-525 in the future. The low-cost, easy to scale-up, and cheap fabrication process of MOF-525/s-PT-3 film profits the industrialization of DSSCs. When the DSSC with MOF-525/s-PT-3 was illuminated at dim light intensities of 10 mW cm– 2
, the cell efficiency is up to 9.75%, which is higher than that of the cell under normal
17
condition (100 mW cm–2). The high efficiency of the cell with MOF-525/s-PT-3 at low light intensities broaden the horizon of the DSSC applications in indoor or wearable DSSCs.
Acknowledgements This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 105-2815-C-002-108 and 105-2221-E-002-229-MY3. We acknowledge the support from the Academia Sinica (AC) and National Taiwan University (NTU).
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Vitae Mr. Tai-Ying Chen, studying under the guidance of Prof. Kuo–Chuan Ho, is currently an undergraduate student in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan. His research interests surround on metal-organic frameworks applied as the new catalysts for the counter electrode of dye-sensitized solar cells. Mr. Yi-June Huang received his BS and MS degrees in Department of Chemical Engineering at Chung Yuan Christian University, Taiwan, in 2012 and in 2014, respectively. He, studying under the guidance of Prof. Kuo–Chuan Ho, is currently a PhD student in Department of Chemical Engineering, National Taiwan University, Taiwan. His research interests mainly focus on applied microemulsions technique to synthesize metal, metal selenide, and macroporous carbon precursor materials with various nano-structures for electro-catalyst materials in electrochemical devices, e.g, dye-sensitized solar cells and energy storage materials. Dr. Chun–Ting Li received her BS and MS degrees in Department of Chemical and Material Engineering at Chang Gung University, Taiwan, in 2010 and in 2011, respectively. She received her PhD degree in Department of Chemical Engineering at National Taiwan University, Taiwan, in 2015. Her research interests principally focus on synthesizing and developing electro-catalyst materials in electrochemical devices, including dye-sensitized solar cells as well as energy conversion & storage materials/systems with particular attention to transition metal compounds. Besides, she specializes in material and electrochemical analyses techniques.
21
Dr. Chung-Wei Kung received his BS and PhD degrees in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, in 2011 and 2015, respectively. Currently he is a postdoctoral researcher in Department of Chemistry, Northwestern University. His research focuses on the synthesis of nanostructural thin films of various materials including metal oxides, metal sulfides, conducting polymers and metal-organic frameworks, and their electrochemical applications, such as electrochemical sensors, electrocatalysis, supercapacitors, and electrochromic devices. Dr. R. Vittal is a Senior Researcher at National Taiwan University. He was a Research Assistant Professor at Korea University, Seoul during 2002 to 2007. He worked as a Scientist at Central Electrochemical Research Institute, Karaikudi, India during 1982 to 2002. Previous to this he was a Chemist at Nuclear Fuel Complex, Hyderabad, India. He has published about 90 papers with an average impact factor of about 5. Additionally he has 13 conference papers and 41 presentations in national and international seminars/symposia. His research interests include photovoltaic devices (dye-sensitized solar cells, quantum dot-sensitized solar cells, organic/plastic/polymer solar cells), electrochemical sensors, and electrochromic devices. Prof. Kuo-Chuan Ho received his BS and MS degrees in Department of Chemical Engineering from National Cheng Kung University, Taiwan in 1978 and 1980, respectively. He received his PhD in Chemical Engineering at the University of Rochester, USA in 1986. Currently he is a Distinguished Professor jointly appointed by the Department of Chemical Engineering and Institute of Polymer Science and Engineering at National Taiwan University. His research interests mainly surround applications of chemically modified electrodes to sensing and electro-optical devices, including dyesensitized solar cells and electrochromic devices.
22
Tables captions: Table 1
Photovoltaic parameters of the DSSCs with various CEs, measured at 100 mW cm–2 light intensity. The standard deviation data of each DSSC were obtained using three cells.
Table 2
Electro-catalytic and electrochemical impedance parameters of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
Scheme captions: Scheme 1 (a) The sketch of carbon cloth counter electrode, the networked carbon fiber in the carbon cloth. (b) The core-shell structure of MOF-525/s-PT composite film coated on the carbon cloth. Figure Captions: Figure 1
FE-SEM images of the films of (a) Pt, (b) bare s-PT, (c) MOF-525/s-PT-1, (d) MOF-525/s-PT-2, (e) MOF-525/s-PT-3, (f) MOF-525/s-PT-4, (g) MOF-525/s-PT5, and (h) bare MOF-525; the insets in the images show the corresponding images at high magnifications.
Figure 2
(a) Photocurrent density–voltage curves of the DSSCs with the CEs of MOF525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, MOF-525/s-PT-4, and MOF525/s-PT-5. The standard deviation data for each DSSC were obtained using three cells. (b) Photocurrent density–voltage curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. (c) Photocurrent density– voltage curves of the DSSC with MOF-525/s-PT-3, when illuminated at light intensities of 10 and 100 mW cm–2. (d) Incident photon–to–current conversion efficiency curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
Figure 3
(a) CV curves of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. (b) Plots of i–1 vs. ω–0.5 for these electro-catalytic electrodes.
Figure 4
(a) Tafel polarization plots and (b) electrochemical impedance spectra for the films of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
23
Table 1 Counter electrode
Voc (V)
Jsc (mA cm-2)
FF
η (%)
MOF-525/s-PT-1
0.80±0.00
12.67±0.14
0.70±0.01
6.96±0.03
MOF-525/s-PT-2
0.79±0.01
12.86±0.08
0.71±0.00
7.21±0.03
MOF-525/s-PT-3
0.80±0.00
16.14±0.13
0.70±0.00
8.91±0.02
MOF-525/s-PT-4
0.80±0.00
12.66±0.11
0.70±0.00
6.97±0.02
MOF-525/s-PT-5
0.80±0.00
10.95±0.20
0.70±0.00
5.97±0.02
Bare s-PT
0.79±0.00
12.32±0.05
0.71±0.00
6.94±0.01
Bare MOF-525
0.78±0.00
12.68±0.16
0.48±0.00
4.78±0.02
Pt
0.77±0.00
15.82±0.04
0.66±0.00
8.21±0.02
Table 2 Counter electrode
Jpc (mA cm-2)
k0 (cm s-1)
Ae (cm2)
J0 (mA cm-2)
Rs (Ω cm2)
Rct-EIS (Ω cm2)
MOF-525/s-PT-3
2.03
2.60 × 10-3
0.62
4.07
16.42
1.42
Bare s-PT
0.85
2.25 × 10-3
0.28
1.71
16.25
4.82
Bare MOF-525
0.79
N.A.
N.A.
2.09
20.26
5.04
Pt
1.07
3.56 × 10-3
0.32
3.31
19.42
3.01
24
Scheme 1
Figure 1
25
Figure 2
26
Figure 3
27
Figure 4 Highlights 1. The MOF is firstly used as the electro-catalyst for the CE of a DSSC. 2.
MOF-525/s-PT composite film overcomes the insulating problem of MOF.
3.
MOF-525/s-PT composite film shows large surface area and good catalytic activity.
4.
DSSC with MOF-525/s-PT CE shows a high efficiency of 8.91%. 28
5.
DSSC with MOF-525/s-PT CE shows a high efficiency of 9.75% under dim illumination.
29
PII: DOI: Reference:
S2211-2855(16)30583-3 http://dx.doi.org/10.1016/j.nanoen.2016.12.019 NANOEN1670
To appear in: Nano Energy Received date: 5 November 2016 Revised date: 10 December 2016 Accepted date: 11 December 2016 Cite this article as: Tai-Ying Chen, Yi-June Huang, Chun-Ting Li, Chung-Wei Kung, R. Vittal and Kuo-Chuan Ho, Metal-organic framework/sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.12.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Metal-organic framework/sulfonated polythiophene on carbon cloth as a flexible counter electrode for dye-sensitized solar cells Tai-Ying Chena, Yi-June Huanga, Chun-Ting Lia, Chung-Wei Kunga, R. Vittala and Kuo-Chuan Hoa,b,* 1
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan;
2
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
Abstract Metal-organic framework (MOF-525) is firstly introduced as the electro-catalyst for the counter electrode (CE) of a dye-sensitized solar cell (DSSC). When MOF-525 was mixed with the conductive binder of sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]2,5-diyl) (s-PT), a composite film of MOF-525/s-PT was successfully deposited on a flexible substrate, carbon cloth (CC). The one-dimensional carbon fibers in CC were intended to provide oriented electron transfer pathways as a conductive core, and the composite film of MOF-525/s-PT covered on each carbon fiber in CC was designed to trigger the reduction of I3− as an electro-catalytic shell. Thus, a hierarchical electron transfer network was established. In the MOF-525 nanoparticle, its nodes (zirconium oxide) and linkers (meso-tetra(4carboxyphenyl)porphyrin) were both verified to function as the electro-catalytic active sites for I3− reduction. The best MOF-525/s-PT composite counter electrode rendered 8.91±0.02% to its DSSC, showing the promising potential to replace traditional platinum (8.21±0.02%). At dim light condition (10 mW cm–2), the best cell with MOF-525/s-PT composite CE shows a great cell efficiency (η) of 9.75%, which is higher than that of the cell measured at 100 mW cm–2.
1
Keywords: counter electrode; dye-sensitized solar cell; flexible substrate; metal-organic framework; sulfonated polythiophene
*
Corresponding author: Tel: +886–2–2366–0739; Fax: +886–2–2362–3040;
E–mail: [email protected]
1. Introduction Metal-organic frameworks (MOFs) have recently received increasing attention, and they are widely proposed for different applications due to their attractive material properties [1]. Metal-organic frameworks are constructed by the coordination complexes as the repeating units, extending in one, two or in three dimensions; such a coordination complex consists of metallic clusters as the nodes and organic ligands as the linkers [2]. Thus, MOF materials often exhibit well-ordered and size-controllable pores, which provide extremely large surface area. By controlling the coordination geometry of the metallic cluster and the topology of the linker, an MOF can be designed for various applications, e.g., for gas storage [3], gas separation [4], supercapacitors [5], electrochemical sensing [6], water oxidation [7], CO2 reduction [8], drug delivery [9], and catalysis [10]. Since dye-sensitized solar cells (DSSCs) have been considered as the promising renewable and clean energy resources for the next generation, enhancement of their power conversion efficiency (η) with low-cost materials became the key issue in the last two decades [11]. Several kinds of MOF have been used in DSSC to assist the fabrications of photoanode, electrolyte, and counter electrode [12]. In the case of photoanode, copper-MOF had been used as the co-sensitizers (0.27%), ZIF-8 had been used for blocking layer (5.21%), and MOF-5 had been used to synthesize the ZnO scattering layer with hierarchical structure (3.67%) [12]. In the case of electrolyte, magnesium-MOF was added into a polymer-based electrolyte to increase the cell efficiency of DSSC due to the enhancement in the interfacial connection between the TiO2 and electrolyte (4.80%) [13]. In the case of counter electrode, cobalt-MOF 2
was introduced as a precursor to synthesize efficient cobalt sulfide; the Co-MOF derived CoS offered large surface area and thus gave 8.1% efficien to its DSSC [14]. To summary the above-mentioned literatures, the light harvesting ability, blocking character, and large surface area of MOF are beneficial to the DSSCs. However, the electrochemical properties of the MOF itself have never been investigated for DSSCs. In a DSSC, the counter electrode (CE) is responsible for regenerating the redox mediator (I–/I3–), and thus keeps the DSSC working [15]. Besides, the cell efficiency of DSSCs is directly influenced by the electro-catalytic ability of the CE. Platinum (Pt) is generally chosen as the electro-catalyst; however, Pt is very expensive and easily gets poisoned by iodide [15]. Many Pt-free electro-catalysts have been developed according to the requirements for a CE of a DSSC, e.g., excellent large surface area, electro-catalytic ability, and good conductivity [15]. From the standpoint of these requirements, we reasoned that MOF-525 may be one of the potential candidates to replace Pt, due to its extremely high Brunauer−Emmett−Teller (BET) surface area of about 2620 m2 g-1, high stability, and redox activity derived from linkers [6, 16, 17]. A MOF-525 consists of cuboctahedral nodes of zirconium metal oxide (Zr6O4(OH)4) and the linkers of meso-tetra(4-carboxyphenyl)porphyrin (H4TCPP). The former is a zirconium (Zr)-based compound and the latter consists of four pyrrole-based entities interconnected at their α carbon atoms via methine bridges (=CH−). Among the Zrbased compounds, ZrO2, ZrN, and ZrC have been reported to provide electro-catalytic active sites for triiodide reduction; these compounds have rendered for their DSSCs cell efficiencies (η) of 2.60% [18], 1.20% [18], and 3.85% [18], respectively. In the case of the pyrrole-based materials, pristine polypyrrole (PPy) and its composite materials both exhibited good electrocatalytic abilities toward triiodide reduction, and their cells showed power conversion efficiencies of up to 7% [19, 20]. From the above literatures, it is understood that both the node (Zr6O4(OH)4) and the linker (H4TCPP) of MOF-525 have great potential to offer catalytic abilities for the reduction of triiodide ions to iodide ions. 3
In our study, we successfully synthesized MOF-525 nanoparticles and mixed them with the conducting polymer, sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]-2,5-diyl) (s-PT). The obtained slurry was coated on a carbon cloth (CC)-substrate via a simple dropcoating process. The carbon cloth with its composite film (hereafter MOF-525/s-PT) was used as the counter electrode for the DSSC. Scheme 1(a) (bottom) shows SEM image of the CC and its actual image in hand (middle). The CC is a flexible and low-weight conductive substrate which has been regularly used in DSSCs [21-23]. The CC is interlaced by bunches of conductive carbon fibers [24]; each carbon fiber is supposed to act as a one-dimensional (1D) pathway for electron transfer. The CC substrate is so thin (thickness = 0.36 mm) and has an extremely low weight that a very soft flower with needle-like petals could withstand its weight without enduring the slightest strain, as shown in the upper side portion of Scheme 1(a). Thus, a conductive network can be provided by this CC substrate. In a CC substrate, each carbon fiber was wrapped around by the MOF-525/s-PT composite film. Accordingly, there is a core–shell structure where the carbon fiber is a 1D conductive core and the MOF525/s-PT film is the electro-catalytic shell, as represented in Scheme 1(b). The cross sectional image of the core-shell structure is shown in Figure S1 of the Supporting Information. Therefore, a hierarchical electron transfer network could be established, as shown in Scheme 1(b). First of all, a main stream of electrons could occur through the 1D carbon fiber (core of the core-shell structure). Second, a minor stream of electrons can pass through the layer of sPT to reach the electro-catalytic active sites of MOF-525, where the reduction of triiodide ions takes place (Scheme 1(b)). The carbon cloth was intended to provide oriented electron transfer pathways, the MOF-525 nanoparticles could bestow the composite film with large surface area and good electro-catalytic ability, and the s-PT polymer was expected to prevent the MOF-525 nanoparticles from being aggregated, facilitate the linkage among MOF-525 nanoparticles, and create electron transfer pathways from the substrate to the MOF-525. This MOF-525/s-PT composite film aims to trigger the I3– reduction in a DSSC, as shown in 4
Scheme 1(c); its attractive electro-catalytic ability and the outstanding photovoltaic performance are systematically investigated, and compared with the traditional Pt material. It is noted that this is the first study on the electro-catalytic ability of MOF-525 and its influence on the photovoltaic performance of the DSSCs.
2. Experimental Section 2.1. Synthesis of MOF-525 MOF-525 nanoparticles were synthesized via a published four-step route [25] as follows: (1) two precursors of 1.35 g of benzoic acid and 105 mg of zirconyl chloride octahydrate were dissolved together and sonicated in 8 mL of N,N-dimethylformamide (DMF, ≥99.8%); the dissolved and sonicated solution was transferred into a vial of 20 mL volume; (2) the solution was heated and maintained at 80 °C for 2 h in a gravity convection oven; (3) after cooling the solution to room temperature, 47 mg of meso-tetra(4-carboxyphenyl)porphyrin (H4TCPP, >97%) was added to it, and the thus obtained solution was sonicated at room temperature for 20 min; (4) the last solution was then heated on the bottom of a gravity convection oven at 80 °C for 24 h. After washing with DMF and acetone several times, soaking into acetone for one day, and drying under vacuum, a product of MOF-525 with dark red color was obtained with good yield.
2.2. Fabrication of the counter electrodes and the DSSCs Substrates of fluorine-doped tin oxide (FTO) conducting glasses and carbon cloths (CC) were used to prepare the photoanodes and counter electrodes (CE) of the DSSCs, respectively; the substrates were sequentially cleaned with a neutral cleaner, de-ionized water, acetone, and isopropanol. In order to compare the performances of electro-catalytic films, four types of films were prepared as follows: (1) a Pt film (30 nm-thick) was obtained on an FTO by a DC sputtering technique; (2) bare s-PT film was prepared on a CC by a drop-coating technique using 50 μL of a s-PT solution; (3) by using the same drop-coating technique, five 5
types of MOF-525/s-PT composite films, namely MOF-525/s-PT-1, MOF-525/s-PT-2, MOF525/s-PT-3, MOF-525/s-PT-4, and MOF-525/s-PT-5 were obtained on CCs using 50 μL of different MOF-525/s-PT slurries, containing 1, 2, 3, 4, and 5 wt% of MOF-525 nanoparticles, respectively, with respect to the weight of s-PT; (4) bare MOF-525 film was prepared on a CC by the same drop-coating technique using 50 μL of DMF containing 3wt% of MOF-525, with respect to the weight of DMF. In brief, one Pt/FTO electrode and seven kinds of CC-based electrodes were prepared for the CEs of the DSSCs. To characterize the films, scanning electron microscopy (SEM), cyclic voltammetry (CV), rotating disk electrode (RDE), Tafel polarization curves, and electrochemical impedance spectra (EIS) were used. In order to assemble the DSSC, a 15 μm-thick TiO2 film adsorbed with an N719 dye (including a 10 μm transparent layer and a 5 μm scattering layer, with an active area of 0.20 cm2) was used as the photoanode; that was fabricated in accordance with a published procedure [24]. The photoanode was assembled with the CE using a 60 μm-thick Surlyn® as the spacer. Finally, an electrolyte, containing 1.2 M 1,2–dimethyl–3–propylimidazolium iodide (DMPII), 0.035 M iodine (I2), 0.1 M guanidinium thiocyanate (GuSCN), and 0.5 M 4tert-butylpyridine (tBP) in 3-methoxypropionitrile/acetonitrile (MPN/ACN, at the volume ratio of 2:8), was injected into the gap between these two electrodes by capillarity. Photovoltaic performance of DSSCs was measured via the photocurrent density–voltage (J– V) curves and incident photo-to-electron efficiency (IPCE) spectra. The standard deviation data for each DSSC were obtained using three cells. The Supporting Information gives details of analytic techniques and materials used in this study.
3. Results and Discussion 3.1. Surface Morphology Field–emission scanning electron microscopy (FE-SEM) images of various electrodes are shown in Figure 1; the insets in the images show the corresponding images at high
6
magnifications. Figure 1(a) shows an ultra-thin, flat, and uniform film of platinum (Pt) on an FTO substrate. Since the FTO substrate generally shows a rough surface composed of many triangular pyramid-like structure, the covered Pt thin film follows the same morphology as FTO. Figure 1(b) shows the image of bare s-PT film on the carbon cloth. The s-PT is well covered on the carbon fibers of the carbon cloth. The surface of the s-PT is very flat and compact, which implies its limited active sites for any electrochemical catalysis. Compared to the surface of this bare film of s-PT, all surfaces of MOF-525/s-PT films look rougher, apparently due to the presence of MOF-525 in the s-PT (Figure 1(c) to (g)). With an increase in the amount of MOF-525 in the s-PT film (from 1 to 5 wt% w.r.t. s-PT), an increase in the amount of cubic-like nanoparticles of MOF-525 can be seen on the surface of the s-PT film (Figure 1(c) to (g)); the thus obtained nanoparticles were estimated to have an average particle size of 200 nm. Since MOF-525 has an extremely large BET surface area (2620 m2 g-1) and a large pore size (1.8 nm) [25], which renders the diffusion of iodide and triiodide ions, its increase in the composite film is expected to increase the active surface area of the composite film for electrocatalysis tremendously. In the cases of MOF-525/s-PT-4 (Figure 1(f)) and MOF-525/s-PT-5 (Figure 1(g)), the MOF-525 nanoparticles appear stacked on each other and are found to form an aggregated layer, which may reduce the electron transport rate in the composite films due to poor conductivity of MOF-525. Figure 1(h) shows the image of a bare MOF-525 film; the film consists of innumerable number of MOF-525 nano-cubes. This bare MOF-525 film appears partially detached from the CC substrate, indicating a very weak adhesion between the MOF-525 nanoparticles and the CC substrate. With reference to this bare MOF-525 film, all MOF-525/s-PT films adhere well to their CC substrates; this phenomenon is apparently due to the presence of the binder s-PT in the composite film. In brief, it can be said that the MOF-525/s-PT composite films have larger active areas than that of the film of s-PT and much better adhesion to their CC substrates than that of the MOF-525 to its CC substrate. 7
3.2. X–ray photoelectron spectra and X–ray diffraction analyses X–ray photoelectron spectra (XPS, Figure S2) and X–ray diffraction (XRD, Figure S3) patterns were used to investigate the interactions between MOF-525 and s-PT in a MOF525/s-PT composite film. Figure S2(a) shows the Zr 3d orbital of MOF-525, which is embedded in s-PT. All of the three peaks in Figure S2(a) belong to the Zr–O chemical bonds, indicating that there is no chemical bond between the Zr6O4(OH)4 nodes of MOF-525 and sPT. Figure S2(b) shows the S 2p orbital of s-PT, which is in a composite MOF-525/s-PT film. There are four peaks in Figure S2(b); three peaks at higher binding energy level refer to the S–C chemical bonds, and the other one peak refers to the S–O chemical bonds. It is clear that there is no bonds between the sulfur atoms in s-PT and MOF-525. Figure S3 shows the XRD of the composite MOF-525/s-PT film; the peaks at 4.5o, 6.4o, 7.8o, and 9.1o are highly consistent with the reported XRD pattern of pure MOF-525 [26]. It can be said that the crystalline MOF-525 is not influenced by the s-PT. In all, the interactions between MOF-525 and s-PT in a composite MOF-525/s-PT film may belong to the Van Der Waals forces.
3.3. Photovoltaic performance. Various MOF-525/s-PT composite electrodes containing 1~5 wt% of MOF-525 nanoparticles (w.r.t. the weight of s-PT) are denoted as MOF-525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, MOF-525/s-PT-4, and MOF-525/s-PT-5, where the last number indicates the wt% of MOF-525 with respect to the weight of s-PT. Photocurrent density–voltage (J–V) curves of the DSSCs with these electrodes as the CEs are shown in Figure 2(a). The corresponding photovoltaic parameters are summarized in Table 1. With the increase of content of MOF-525 in the composite film, the cell efficiencies of these DSSCs show an increasing tendency from 6.49±0.03% (MOF-525/s-PT-1) to 8.91±0.02% (MOF-525/s-PT-3), and then a decreasing tendency from 8.91±0.02% (MOF-525/s-PT-3) to 5.97±0.02% (MOF8
525/s-PT-5). Table 1 shows that the values of open-circuit voltage (Voc) and fill factor (FF) of a DSSC change hardly due to the increase in the amount of MOF-525 in the composite film, but the value of short–circuit current density (Jsc) of the DSSC varies greatly with the increase in the amount of MOF-525 in the composite film. Thus, the variations in the cell efficiency of the DSSCs are certainly attributed to the variations in the Jsc of the cells. Two factors are attributed to the variations in the Jsc: (1) electro-catalytic active area of the counter electrode and thereby electro-catalytic activity of it and (2) electrical conductivity of the counter electrode. The FE-SEM images in Figure 1 indicate clearly that the electro-catalytic active area of a composite film increases in proportion to the increase in the content of MOF-525 in the composite film. A steady increase in the Jsc is therefore expected from the viewpoint of increased electro-catalytic active sites; Table 1 shows that this however is not the case. The Jsc increases initially and then it decreases. With the increases in the contents of MOF-525 from 1 to 3 wt%, the Jsc increases as expected due to the increases in the electro-catalytic active surface areas or electro-catalytic activities of the corresponding CEs. With further increases of MOF-525 from 3 to 5 wt%, the Jsc decreases; this can be attributed to the excesses in the amounts of MOF-525 in the composite films of MOF-525/s-PT-4 and MOF-525/s-PT-5, which can hinder electron transportation in the composite films and electron transfer at the interfaces of CC substrate/composite film and composite film/electrolyte. Among all the DSSCs with the MOF-525/s-PT composite films, the cell with MOF-525/s-PT-3 exhibits the highest η of 8.91±0.02%, with a Voc of 0.80±0.00 V, Jsc of 16.14±0.13 mA cm–2, and FF of 0.70±0.00. On the other hand, J–V curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare sPT, bare MOF-525, and Pt are shown in Figure 2(b); the pertinent data are summarized in Table 1. The cell with bare s-PT reaches an η of 6.94±0.02%, with a Voc of 0.79 V, Jsc of 12.32 mA cm–2, and FF of 0.71. It can be seen in Table 1 that the DSSCs with the CEs of MOF525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, and MOF-525/s-PT-4 show higher η’s than 9
that of the cell with bare s-PT CE; this is clearly attributed to the increased electro-catalytic activities of these composite films bestowed by the added MOF-525 nanoparticles in these films. The cell with bare MOF-525 exhibits an η of 4.78±0.02% with a Voc of 0.78±0.00 V, Jsc of 12.68±0.16 mA cm–2, and FF of 0.48±0.00. For comparison, we directly employed a bare CC substrate (without loading any kind of electro-catalyst) as the CE in a DSSC, and this cell showed a very poor η of 1.65% with a Voc of 0.79 V, Jsc of 10.38 mA cm–2 and FF of 0.20 (Figure S4 in Supporting Information). When the performance of the cell using bare CC substrate (η=1.65%) is compared with that of the cell using only MOF-525 (η=4.78±0.02%), it can be clearly seen that the increase in the η for the latter case is due to the catalytic film of MOF-525. Thus, it is well established that MOF-525 exhibits electro-catalytic activity toward I3- reduction. To further explore the inner electro-catalytic sites in MOF-525, another type of MOF (UiO-67) was used to deliver the performance of the Zr6O4(OH)4 node of MOF-525; UiO-67 consisted the same Zr6O4(OH)4 node and the non-catalytic linker of biphenyl-4,4′dicarboxylic acid [17] and is synthesized in according to the published procedures [27]. H4TCPP was directly used to give the information of the linker of MOF-525. These two kinds of materials are separately coated on the CC substrates and denoted as UiO-67 and H4TCPP. The J–V curves of the DSSCs with the CEs of UiO-67 and H4TCPP are depicted in Figure S5(a) and Figure S5(b) of the Supporting Information. The cell with UiO-67 shows the η of 4.08%, and the cell with H4TCPP exhibits the η of 4.48%. Both cells with UiO-67 and H4TCPP give higher η than that of the cell with bare CC (1.65%, Figure S4) and comparable η to that of the cell with bare MOF-525 (4.78%, Table 1). Thus, it can be deduced that both Zr6O4(OH)4 node and pyrrole-based linker, H4TCPP, function as the inner electro-catalytic sites in MOF-525 and thus bring good electro-catalytic activity toward triiodide to MOF-525. In brief, in comparison to the cell with bare s-PT CE, the cell with the composite film of MOF-525/s-PT-3 shows much higher Jsc and η, owing to the presence of MOF-525 in the 10
composite film at an optimum level. In comparison to the cell with bare MOF-525, the cell with the composite film of MOF-525/s-PT-3 exhibits much higher Jsc, FF, and η, because the s-PT in the composite film enhances the adhesion between the substrate and the composite film, prevents MOF-525 nanoparticles from being aggregated, facilitates the linkage among MOF-525 nanoparticles, and creates electron transfer pathways from the substrate to the MOF-525. In conclusion, within the composite film of MOF-525/s-PT, the s-PT plays the roles of a conducting binder and matrix for the composite film, the MOF-525 enhances surface area of the composite film with respect to that of the s-PT film, and both Zr6O4(OH)4 node and H4TCPP linker in the MOF-525 play the role of electro-catalytic active sites. The DSSC with MOF-525/s-PT-3 achieves the cell efficiency of 8.91±0.02%, which is better than that of the cell with Pt (8.21±0.02%). When the DSSC with MOF-525/s-PT-3 was illuminated at dim light condition of 10 mW cm–2, the cell shows even higher η of 9.75% with a Voc of 0.71 V, Jsc of 1.91 mA cm–2, and FF of 0.72 (Figure 2(c)). It is notable that the cell efficiency under dim illumination is 10% higher than that of the cell at normal condition (100 mW cm– 2
). Furthermore, the DSSCs with MOF-525/s-PT-3 and Pt both show good performance in the
long-term stability tests; they maintain 83% and 87% of cell efficiency to their initial values after 7 days, respectively (Figure S6(a)). In conclusion, the MOF-525/s-PT composite film has great potential to replace Pt in a DSSC and can be further used for indoor or wearable DSSCs. Incident photon–to–current conversion efficiency (IPCE) curves of the DSSCs with the electrodes of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt were obtained at the shortcircuit condition, in the wavelength range of 400-800 nm, as shown in Figure 2(d). The IPCE values were calculated via Equation (1), where λ is the wavelength, Jsc(λ) is the specific shortcircuit photocurrent density obtained at a specific wavelength, and φ is the incident radiation flux [28]. IPCE (λ)=
1240 ×Jsc (λ) λ×φ
×100%
(1) 11
Both the DSSCs with MOF-525/s-PT-3 and Pt give good IPCE values of about 80~90%, in the wavelength region of 400 to 600 nm, showing that these counter electrodes enable sufficient regeneration of iodide ions. In contrast, the cells with bare s-PT and bare MOF-525 show significantly lower IPCE values, owing to limited surface area of s-PT and poor conductivity of bare MOF-525, respectively. Besides, a short-circuit current density value of a DSSC can also be obtained from its IPCE spectra (denoted as Jsc-IPCE), by integrating the area under the IPCE curve; the values were calculated to be 12.97, 9.52, 10.15, and 12.16 mA cm-2 for the cells with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt, respectively. These JscIPCE
values is perfectly consistent with the tendency of Jsc values attained from the J–V curves.
3.4. Cyclic voltammetry Cyclic voltammetry (CV) was applied to find out the redox kinetics of iodide/triiodide (I– /I3–) at the surface of the electro-catalytic electrode. As a CE in a DSSC, the main purpose of the electro-catalytic film is to facilitate the reduction of I3– at the CE/electrolyte interface. In the CV, the cathodic peak current density (Jpc) of the electrode can be extracted from its CV curve and can be used to evaluate the electro-catalytic activity of the electrode toward the reduction of I3–; the pertinent reaction at the electrode is as Equation (2) [15]. -
I3 +2e- →3I-
(2)
In this type of cyclic voltammetry, a higher cathodic peak current density (Jpc) of the electrode indicates a higher overall electro-catalytic activity of the electrode [29]. Figure 3(a) shows the cyclic voltammograms of the electrodes with the films of bare s-PT, bare MOF525, MOF-525/s-PT-3, and Pt; Table 2 gives their Jpc values. The Jpc values for MOF-525/sPT-3, bare s-PT, bare MOF-525 and Pt electrodes are 2.03, 0.85, 0.79 and 1.07 mA cm-2, respectively (Table 2). Obviously, the MOF-525/s-PT-3 composite film shows a higher Jpc value than that of bare s-PT film, indicating the fact that the presence of MOF-525 has indeed enhanced the electro-catalytic activity of the composite film with respect to that of bare s-PT 12
film. The intrinsic electro-catalytic activity of MOF-525 and its large surface area are apparently the reasons for its favorable role in enhancing the Jpc of the composite film. Moreover, the Jpc value of the composite film (2.03 mA cm-2) is much higher than that of the Pt film (1.07 mA cm-2), showing the better electro-catalytic activity of MOF-525/s-PT-3 than that of Pt. In addition to the electro-catalytic activity, MOF-525/s-PT-3 composite film also shows promising ability to resist the iodide corrosion. The CV curve of MOF-525/s-PT-3 counter electrode shows unfailing performance after 100 cycles, as shown in Figure S6(b). On the other hand, MOF-525 contains the H4TCPP linker, which may have light harvesting property, in accordance with the absorption spectra of pure MOF-525 (Figure S7). Therefore, at an applied potential of -0.5 V (a sufficient potential to execute I3– reduction), the plot of current density vs. time (Figure S8) was recorded under illuminated and dark conditions using the bare MOF-525 film. In Figure S8, the current-density measured under the light illumination is the same as that measured under dark condition. Thus, the catalytic ability of MOF-525 is not influenced by its light soaking.
3.5. Rotating disk electrode. In addition to the CV analysis, rotating disk electrode (RDE) technique was applied to determine two important parameters, the intrinsic heterogeneous rate constant (k0) and the effective catalytic surface area (Ae), which largely influence the overall electro-catalytic activity of an electrode. At the formal potential (E0’) of I–/I3–, various values of reciprocal current (i-1) were obtained from linear sweep voltammetry (LSV) curves for an electrode under various rotating speeds (50, 100, 200, 400, 600, 800, and 1000 rpm). Thus, plots of reciprocal current (i-1) vs. reciprocal of square root of rotating speed (ω-0.5) were obtained for MOF-525/s-PT-3, bare s-PT, and Pt electrodes, as shown in Figure 3(b). In the case of bare MOF-525, we could not get electrochemical signals for bare MOF-525 film, because the bare MOF-525 film directly peeled off from the substrate. The values of k0 and Ae were obtained 13
respectively for the mentioned electrodes from the intercept and slope of fitting lines in these plots[[30]] by using a simplified Koutecký–Levich equation (Equation (3)), where i is the specific current obtained from the LSV curve at the formal potential (E0’) of I–/I3–, n is the number of electrons transferred for I3– reduction, C is the concentration of I3– ions (1.0 mM), D is the apparent diffusion coefficient of I– (3.62×10-6 cm2 s-1), F is Faraday constant, ν is the kinematic viscosity of electrolyte, and ω is the angular velocity converted from the rotating speed. 1 i
=
1 nFAe
k0 C
+
1 0.62nFAe
D2 /3 υ-1 /6
(3)
ω1/2 C
As summarized in Table 2, the Pt film shows the highest k0 value of 3.56×10-3 cm s-1 and a low Ae value of 0.32 cm2; this reveals that the flat Pt film observed in its FE-SEM image possesses great intrinsic electro-catalytic activity but limited effective surface area. Compared with bare s-PT film, the MOF-525/s-PT-3 film exhibits higher k0 and Ae values, higher by about 1.2 times and 2.2 times, respectively. Thus, it is proved that addition of MOF-525 to sPT not only enhances the heterogeneous rate constant of s-PT, but also its electrochemical active surface area; these two factors have apparently enabled a higher catalytic activity to the composite film, compared to that of the s-PT film. The enhanced surface area of the composite film is due to the extremely large BET surface area of MOF-525. Thus, it is established that the MOF-525 nanoparticles function as a major electro-catalyst for I3– reduction in the MOF-525/s-PT composite electrode. Compared with the Pt film, the MOF525/s-PT-3 film shows a lower k0 (about 0.73 times) but a much larger Ae (about 2.2 times). Therefore, the overall electro-catalytic activity of the counter electrode with the composite film is higher than that of Pt counter electrode, as can be seen by their CVs in Figure 3(a) and from the values of Jpc and J0 in Table 2.
3.6. Tafel polarization curve In addition to the CV and RDE techniques, Tafel polarization was used to explore the 14
electro-catalytic activities of the electrodes; the data were obtained using a symmetric cell (see details in Supporting Information). The symmetric cell consisted of the same electrode as the anode and the cathode. Figure 4(a) shows the Tafel polarization curves for the electrodes of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. The Tafel curves can simply be placed into three zones: (1) the polarization zone (|V| < 120 mV), (2) the Tafel zone (120 mV< |V| < 400 mV), and (3) the diffusion zone (|V| > 400 mV) [30]. The exchange current density (J0) of an electro-catalytic composite film can be attained by extrapolating the cathodic and anodic curves and reading the cross point at 0 V in the Tafel zone. A higher value of J0 expresses a better electro-catalytic activity for the electrode [31]. As summarized in Table 2, the J0 values of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt are 4.07, 1.71, 2.09, and 3.31 mA cm-2, respectively. The tendency of these J0 values is MOF-525/s-PT-3 > Pt > bare MOF-525> bare s-PT, which well agrees with the tendency of the Jsc values of their DSSCs obtained from J–V curves. Thus, we validated the electro-catalytic activities of these films by CV and RDE using a lower concentration of I–/I3– but just by Tafel polarization curves using a higher concentration of I–/I3– (see Supporting Information). It is substantiated the fact that the outstanding electro-catalytic activity of MOF-525/s-PT composite electrode is majorly given by the MOF-525 nanoparticles.
3.7. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) was used to measure the interfacial resistances in the symmetric cell. As shown in Figure 4(b), the EIS spectrum displays two semicircles in the frequency range of 10 mHz to 65 kHz [15, 32]. As the equivalent circuit [15, 32] shown in the inset of Figure 4(b), the onset point of first semicircle in the high frequency region refers to the ohmic series resistance (Rs) of the substrate and its catalytic layer, the radius of the first semicircle in the middle frequency region refers to the charge transfer resistance (Rct-EIS) at the electro-catalytic film/electrolyte interface, and the diameter 15
of the second semicircle in the low frequency region refers to the Warburg diffusion resistance of the electrolyte. A lower Rs value indicates a better ohmic contact between the electrocatalytic film and the substrate [33]. In Table 2, the comparable Rs values of the cell with the electrodes of bare s-PT and composite MOF-525/s-PT-3 are 16.25 Ω cm2 and 16.42 Ω cm2, respectively, which both are lower than the Rs values of the cells with Pt (19.42 Ω cm2) and bare MOF-525 (20.26 Ω cm2). From the Rs values of bare MOF-525 and MOF-525/s-PT-3, it can be verified that the conductive binder of s-PT certainly provides intensive adhesion for the composite film to the substrate. Among all, the bare MOF-525 electrode has the largest Rs value; this indicates its weakest adhesion to its CC substrate. It is well known that a poor adhesion between a counter electrode film and its substrate causes a direct energy loss for the pertinent DSSC and a decrease in its FF value [11]. Thus, the Rs values for these electrodes follow an order of bare s-PT < MOF-525-3 < Pt < bare MOF-525, which is in complete agreement with the tendency of the FF values of their DSSCs. On the other hand, a low Rct-EIS value implies rapid passage of a large amount of charge at the interface of the pertinent electrode with the electrolyte. The Rct-EIS values of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt are 1.42, 4.82, 5.04, and 3.01 Ω cm2, respectively. Although the composite film of MOF-525/s-PT-3 shows a slightly higher Rs value than that of bare s-PT, it exhibits the lowest Rct-EIS value among all the films; this is one of the factors for the best electro-catalytic activity of MOF-525/s-PT-3 among all the films. Owing to lack of surface area, the bare s-PT shows a high Rct-EIS value (4.82 Ω cm2); owing to lack of proper adhesion to the substrate (as can be seen from the pertinent FF value in Table 1), the bare MOF-525 shows the highest Rct-EIS value (5.04 Ω cm2); when these two materials (s-PT and MOF-525) are combined, their composite film nullifies these two disadvantages and exhibits the least Rct-EIS value of 1.42 Ω cm2. 4. Conclusion
16
A novel composite film of a zirconium-based porphyrinic metal-organic framework (MOF-525) and sulfonated-poly(thiophene-3-[2-(2-methoxyethoxy)-ethoxy]-2,5-diyl) (s-PT) was successfully deposited on a flexible substrate of carbon cloth (CC) to form a hierarchical electron transfer route. The interlaced bunch of carbon fiber in CC provided the electron transfer network, each carbon fiber in CC plays an important role of the one-dimensional conductive core, and the MOF-525/s-PT composite film covered on each carbon fiber in CC worked as the electro-catalytic shell. Among all the films, the MOF-525/s-PT composite film containing 3 wt% of MOF-525 nanoparticles (MOF-525/s-PT-3) shows the best electrocatalytic activity, which is substantiated via its highest Jpc (cyclic voltammetry), largest J0 (Tafel polarization curve), and lowest Rct-EIS (electrochemical impedance spectroscopy) values. The best electro-catalytic activity of MOF-525/s-PT-3 composite film is attributed to three key facts as follows. (1) MOF-525/s-PT-3 composite film has extremely large active areas given by the porous MOF-525 nanoparticle (the largest Ae from RDE). (2) MOF-525/s-PT-3 composite film possesses good adhesion to its CC substrate, which supplied by the conductive binder of s-PT (low Rs value from EIS). (3) MOF-525/s-PT-3 provides rapid reduction rate of I3– due to that both of the node (Zr6O4(OH)4) and the linker (H4TCPP) of MOF-525 works as electro-catalytic active sites (good k0 value from RDE and the results from J-V curve). Thus, the cell with MOF-525/s-PT-3 exhibited the highest η of 8.91±0.02%, which is better than that of the cell with traditional Pt (8.21±0.02%). There is no doubt that the MOF-525/s-PT-3 film is a promising electro-catalytic material to replace the expensive Pt in DSSCs. Besides, the cell with MOF-525/s-PT-3 is expected to have better cell efficiency, after adjusting the particle size and pore size of MOF-525 in the future. The low-cost, easy to scale-up, and cheap fabrication process of MOF-525/s-PT-3 film profits the industrialization of DSSCs. When the DSSC with MOF-525/s-PT-3 was illuminated at dim light intensities of 10 mW cm– 2
, the cell efficiency is up to 9.75%, which is higher than that of the cell under normal
17
condition (100 mW cm–2). The high efficiency of the cell with MOF-525/s-PT-3 at low light intensities broaden the horizon of the DSSC applications in indoor or wearable DSSCs.
Acknowledgements This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 105-2815-C-002-108 and 105-2221-E-002-229-MY3. We acknowledge the support from the Academia Sinica (AC) and National Taiwan University (NTU).
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Vitae Mr. Tai-Ying Chen, studying under the guidance of Prof. Kuo–Chuan Ho, is currently an undergraduate student in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan. His research interests surround on metal-organic frameworks applied as the new catalysts for the counter electrode of dye-sensitized solar cells. Mr. Yi-June Huang received his BS and MS degrees in Department of Chemical Engineering at Chung Yuan Christian University, Taiwan, in 2012 and in 2014, respectively. He, studying under the guidance of Prof. Kuo–Chuan Ho, is currently a PhD student in Department of Chemical Engineering, National Taiwan University, Taiwan. His research interests mainly focus on applied microemulsions technique to synthesize metal, metal selenide, and macroporous carbon precursor materials with various nano-structures for electro-catalyst materials in electrochemical devices, e.g, dye-sensitized solar cells and energy storage materials. Dr. Chun–Ting Li received her BS and MS degrees in Department of Chemical and Material Engineering at Chang Gung University, Taiwan, in 2010 and in 2011, respectively. She received her PhD degree in Department of Chemical Engineering at National Taiwan University, Taiwan, in 2015. Her research interests principally focus on synthesizing and developing electro-catalyst materials in electrochemical devices, including dye-sensitized solar cells as well as energy conversion & storage materials/systems with particular attention to transition metal compounds. Besides, she specializes in material and electrochemical analyses techniques.
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Dr. Chung-Wei Kung received his BS and PhD degrees in Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, in 2011 and 2015, respectively. Currently he is a postdoctoral researcher in Department of Chemistry, Northwestern University. His research focuses on the synthesis of nanostructural thin films of various materials including metal oxides, metal sulfides, conducting polymers and metal-organic frameworks, and their electrochemical applications, such as electrochemical sensors, electrocatalysis, supercapacitors, and electrochromic devices. Dr. R. Vittal is a Senior Researcher at National Taiwan University. He was a Research Assistant Professor at Korea University, Seoul during 2002 to 2007. He worked as a Scientist at Central Electrochemical Research Institute, Karaikudi, India during 1982 to 2002. Previous to this he was a Chemist at Nuclear Fuel Complex, Hyderabad, India. He has published about 90 papers with an average impact factor of about 5. Additionally he has 13 conference papers and 41 presentations in national and international seminars/symposia. His research interests include photovoltaic devices (dye-sensitized solar cells, quantum dot-sensitized solar cells, organic/plastic/polymer solar cells), electrochemical sensors, and electrochromic devices. Prof. Kuo-Chuan Ho received his BS and MS degrees in Department of Chemical Engineering from National Cheng Kung University, Taiwan in 1978 and 1980, respectively. He received his PhD in Chemical Engineering at the University of Rochester, USA in 1986. Currently he is a Distinguished Professor jointly appointed by the Department of Chemical Engineering and Institute of Polymer Science and Engineering at National Taiwan University. His research interests mainly surround applications of chemically modified electrodes to sensing and electro-optical devices, including dyesensitized solar cells and electrochromic devices.
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Tables captions: Table 1
Photovoltaic parameters of the DSSCs with various CEs, measured at 100 mW cm–2 light intensity. The standard deviation data of each DSSC were obtained using three cells.
Table 2
Electro-catalytic and electrochemical impedance parameters of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
Scheme captions: Scheme 1 (a) The sketch of carbon cloth counter electrode, the networked carbon fiber in the carbon cloth. (b) The core-shell structure of MOF-525/s-PT composite film coated on the carbon cloth. Figure Captions: Figure 1
FE-SEM images of the films of (a) Pt, (b) bare s-PT, (c) MOF-525/s-PT-1, (d) MOF-525/s-PT-2, (e) MOF-525/s-PT-3, (f) MOF-525/s-PT-4, (g) MOF-525/s-PT5, and (h) bare MOF-525; the insets in the images show the corresponding images at high magnifications.
Figure 2
(a) Photocurrent density–voltage curves of the DSSCs with the CEs of MOF525/s-PT-1, MOF-525/s-PT-2, MOF-525/s-PT-3, MOF-525/s-PT-4, and MOF525/s-PT-5. The standard deviation data for each DSSC were obtained using three cells. (b) Photocurrent density–voltage curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. (c) Photocurrent density– voltage curves of the DSSC with MOF-525/s-PT-3, when illuminated at light intensities of 10 and 100 mW cm–2. (d) Incident photon–to–current conversion efficiency curves of the DSSCs with the CEs of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
Figure 3
(a) CV curves of the electrodes with MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt. (b) Plots of i–1 vs. ω–0.5 for these electro-catalytic electrodes.
Figure 4
(a) Tafel polarization plots and (b) electrochemical impedance spectra for the films of MOF-525/s-PT-3, bare s-PT, bare MOF-525, and Pt.
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Table 1 Counter electrode
Voc (V)
Jsc (mA cm-2)
FF
η (%)
MOF-525/s-PT-1
0.80±0.00
12.67±0.14
0.70±0.01
6.96±0.03
MOF-525/s-PT-2
0.79±0.01
12.86±0.08
0.71±0.00
7.21±0.03
MOF-525/s-PT-3
0.80±0.00
16.14±0.13
0.70±0.00
8.91±0.02
MOF-525/s-PT-4
0.80±0.00
12.66±0.11
0.70±0.00
6.97±0.02
MOF-525/s-PT-5
0.80±0.00
10.95±0.20
0.70±0.00
5.97±0.02
Bare s-PT
0.79±0.00
12.32±0.05
0.71±0.00
6.94±0.01
Bare MOF-525
0.78±0.00
12.68±0.16
0.48±0.00
4.78±0.02
Pt
0.77±0.00
15.82±0.04
0.66±0.00
8.21±0.02
Table 2 Counter electrode
Jpc (mA cm-2)
k0 (cm s-1)
Ae (cm2)
J0 (mA cm-2)
Rs (Ω cm2)
Rct-EIS (Ω cm2)
MOF-525/s-PT-3
2.03
2.60 × 10-3
0.62
4.07
16.42
1.42
Bare s-PT
0.85
2.25 × 10-3
0.28
1.71
16.25
4.82
Bare MOF-525
0.79
N.A.
N.A.
2.09
20.26
5.04
Pt
1.07
3.56 × 10-3
0.32
3.31
19.42
3.01
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Scheme 1
Figure 1
25
Figure 2
26
Figure 3
27
Figure 4 Highlights 1. The MOF is firstly used as the electro-catalyst for the CE of a DSSC. 2.
MOF-525/s-PT composite film overcomes the insulating problem of MOF.
3.
MOF-525/s-PT composite film shows large surface area and good catalytic activity.
4.
DSSC with MOF-525/s-PT CE shows a high efficiency of 8.91%. 28
5.
DSSC with MOF-525/s-PT CE shows a high efficiency of 9.75% under dim illumination.
29