A Novel Activated-Charcoal-Doped Multiwalled ... - ACS Publications

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A Novel Activated-Charcoal-Doped Multiwalled Carbon Nanotube Hybrid for Quasi-Solid-State Dye-Sensitized Solar Cell Outperforming Pt Electrode Alvira Ayoub Arbab,† Kyung Chul Sun,‡ Iftikhar Ali Sahito,† Muhammad Bilal Qadir,† Yun Seon Choi,† and Sung Hoon Jeong*,† †

Department of Organic and Nano Engineering, Hanyang University, Seoul 133-791, South Korea Department of Fuel Cells and Hydrogen Technology, Hanyang University, Seoul 133-791, South Korea

‡

S Supporting Information *

ABSTRACT: Highly conductive mesoporous carbon structures based on multiwalled carbon nanotubes (MWCNTs) and activated charcoal (AC) were synthesized by an enzymatic dispersion method. The synthesized carbon configuration consists of synchronized structures of highly conductive MWCNT and porous activated charcoal morphology. The proposed carbon structure was used as counter electrode (CE) for quasi-solid-state dye-sensitized solar cells (DSSCs). The AC-doped MWCNT hybrid showed much enhanced electrocatalytic activity (ECA) toward polymer gel electrolyte and revealed a charge transfer resistance (RCT) of 0.60 Ω, demonstrating a fast electron transport mechanism. The exceptional electrocatalytic activity and high conductivity of the AC-doped MWCNT hybrid CE are associated with its synchronized features of high surface area and electronic conductivity, which produces higher interfacial reaction with the quasi-solid electrolyte. Morphological studies confirm the forms of amorphous and conductive 3D carbon structure with high density of CNT colloid. The excessive oxygen surface groups and defect-rich structure can entrap an excessive volume of quasi-solid electrolyte and locate multiple sites for iodide/triiodide catalytic reaction. The resultant D719 DSSC composed of this novel hybrid CE fabricated with polymer gel electrolyte demonstrated an efficiency of 10.05% with a high fill factor (83%), outperforming the Pt electrode. Such facile synthesis of CE together with low cost and sustainability supports the proposed DSSCs’ structure to stand out as an efficient next-generation photovoltaic device. KEYWORDS: carbon nanotube, activated carbon, gel electrolyte, electrocatalytic activity, dye-sensitized solar cell

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INTRODUCTION

MWCNTs are distinctive nanostructures with unique electronic characteristics of high chemical stability and conductivity. MWCNTs are extensively used as conductive matrix for charge transport networks due to their coexistence tubular morphology and diffusive electron transport performance. In this manner, their unique mechanical properties provide support for the formation of a conductive matrix structure for a fast electron transport mechanism. On the other hand, MWCNTs possess insufficient electrocatalytic activity for iodide/triiodide ions due to their perfect tubular planes and defect-free surface configuration. Further, one of the main problems one comes across in an MWCNT is related to its dispersion for advance uses. Most commonly, the oxidative functionalization or ball-milling grinding are commonly used for stable CNT dispersions. Unfortunately, the stated approaches aggregate nanotubes irreversibly due to their strong van der Waals forces, while a strong dispersant destroys their sp2 bond configuration.14,15 To overcome this problem, we

Quasi-solid-state dye-sensitized solar cells (DSSCs) ensure extensive consideration due to their facile fabrication, sustainability, and low cost.1−4 Power conversion efficiencies of DSSCs have reached 13%, rendering their credibility to other silicon photovoltaic devices.5 A standard DSSC consists of ntype TiO2 nanocrystalline semiconductor oxide particulates as anode material and a Pt (platinum)-coated FTO glass as cathode fabricated with an I−/I3− redox couple liquid electrolyte.6,7 Conventional DSSCs undergo stability problems that result from their use of liquid electrolytes along with Pt CE, which causes poor sustainability such as electrolyte leakage and electrode corrosion. Still, the extensive fabrication of DSSCs using a Pt catalyst is hindered by its insufficient scarcity and high cost. Moreover, Pt catalyst can be disintegrated to PtI4 in (I−/I3−) liquid electrolyte, which will reduce its long-term stability.8 Defect-rich nanocarbon structures composed of activated carbon, carbon nanotubes, graphite, and graphene have been recognized as efficient CE materials for DSSCs.9−13 Table S1 (Supporting Information) provides a list of previously reported Pt-free, carbon based CE for DSSCs. © 2016 American Chemical Society

Received: October 2, 2015 Accepted: February 25, 2016 Published: February 25, 2016 7471

DOI: 10.1021/acsami.5b09319 ACS Appl. Mater. Interfaces 2016, 8, 7471−7482

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Diagram of (a) Enzymatic Dispersion of MWCNT and Doping of Activated Charcoal, (b) Filtration of AC-Doped MWCNT, followed by Binder Mixing, (c) Fabrication of AC-Doped MWCNT Hybrid Counter Electrode

Therefore, it is estimated that the AC-doped MWCNT hybrid can entrap excessive gel electrolyte efficiently and possess higher catalytic reduction of I− ions present in the quasi-solid electrolyte, thus improving the photovoltaic performance of the DSSCs. Herein, we prepared different types of AC-doped MWCNT hybrid structures fabricated with coal (CL), coconut shell (CC), and pine tree (PN) carbon, named as Type A, Type B, and Type C hybrids, respectively. Furthermore, for optimization, different wt % of pine tree type AC were also used to formulate the optimized hybrid assembly. The initial photovoltaic results show that the AC-doped MWCNT hybrid structure fabricated with 0.8% pine tree AC demonstrated a high efficiency of 10.05%, outperforming the Pt electrode with 9.30% PCE. Owing to high electrocatalytic activity and exceptionally low charge transfer resistance (RCT) of the ACdoped MWCNT hybrid, it outperforms the Pt CE. In carbon CEs, it is mainly the FF among photovoltaic parameters that affects the efficiency of DSSCs. Concisely, higher FF and high efficiency of the AC-doped MWCNT hybrid CE are attributed to the synchronized nature of the high surface area with the conductive carbon matrix structure, which is responsible for high charge storage and electron transport mechanism at CEs. This facile and effective fabrication of carbon CEs will provide an essential background toward the progress of sustainable, low-cost solar cells.

have recently established an enzymatic dispersion technique to produce a highly stable CNT dispersion without compromising their electronic features.15 Furthermore, a highly defective or defect-free CNT has been hardly suitable for the CE in DSSCs. In this regard, coordination of micropatterned charcoal with CNT could produce electrocatalytic active sites with minimum change of conjugation aspect ratio of nanotubes and maintain their conductivity. For this purpose, activated charcoal would be the best candidate to induce into the MWCNT matrix. Activated charcoal has a high specific surface area and maintains a multi-edge porous morphology, which act as active catalytic sites for the charge storage and electrochemical reactions.16 However, activated charcoal still has some shortcomings with respect to low substrate adhesion and comparatively low conductivity, which give rise to large internal impedance. Amorphous carbon CEs possess lower fill factor (FF) due to their low conductivity and electron transfer efficiency across the porous morphology. Moreover, the addition of conducting paths of MWCNTs into mesoporous activated charcoal can increase electrical conductivity and capacitance of CEs. In order to synchronize high electrocatalytic activity and conductivity, we carefully subjected mesoporous carbon with enzyme dispersed MWCNT, followed by ultrasonication to convert the MWCNT into an AC-doped MWCNT hybrid. The synthesized AC-doped MWCNT structure possessed highly conductive amorphous carbon formed by pillaring the multiple tubular network of the MWCNT. The photovoltaic performance of quasi-solid-state DSSCs is mostly lower, because of poor mobility of charge carriers in the gel electrolyte and lower interfacial kinetics between the high density electrolyte and CE.17−19 On the other hand, the AC-doped MWCNT can improve electrolyte contacts and can simply be fabricated.

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EXPERIMENTAL DETAILS

Materials. All chemicals used were of analytical grade. Activated carbon (Dy-carbon, Korea) of three different grades (pine tree, coconut shell, and coal type) was used. MWCNT powder (carbon content > 95%) of 5 μm length, 6−9 nm diameter, was purchased from Sigma Aldrich Co. Enzyme lipase from candida rugose (Type 7472


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ACS Applied Materials & Interfaces VII) was obtained from Sigma Aldrich. Binding agent carboxymethyl cellulose (sodium salt of MW 250 000g) was purchased from Sigma Aldrich. Transparent fluorine-doped tin oxide glass (FTO 8Ω cm−2, Pilkington Co.) substrate was used. D719, cis-diisothiocyanatobis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) bis(tetrabutyl ammonium) from Everlight Co. was obtained. For photoanode layer formation, a 20 nm size of TiO2 (P25 by Degussa Co.) was used. For DSSC fabrication, ionomer sealant (Solaronix) of 60 μm in thickness was used. Fabrication of AC-Doped MWCNT Hybrid Counter Electrode. The AC-doped MWCNT nanostructure was synthesized in the following steps. First of all, an aqueous solution of lipase enzyme (1 mg/mL) was prepared in ethanol. MWCNT (0.4 g) was dispersed in 100 mL of lipase solution and stirred for 8 h at room temperature. Solubilized lipase enzymes were substantially distributed and wrapped onto the external surface of MWCNT. Lipase enzymes considerably release the apparent aggregation and reduce tube−tube distance of the CNT (Scheme 1a). Next, activated charcoal of different kinds (i.e., pine tree, coconut shell, and coal) were agitated into the individual MWCNT suspension and kept at stirring for 8 h at room temperature. In order to assist the induction of activated charcoal into the MWCNT tubular configuration, an additional 50 mL of ethanol was added into the concentrated carbon solution and kept for 3 h of ultrasonication. The micro-sized charcoal particles fragmented, compelling induction into the nanotubular cavity of the CNT. Concentrated carbon solution was vacuum filtered by using a 0.5 μm pore size PTFE filter and washing twice with deionized water to eliminate enzymatic contaminations. After that, 15 mL of carboxymethyl cellulose polymeric binder solution was mixed with carbon cake and ground in an agate mortar to get a consistent carbon suspension (Scheme 1b). The resultant carbon paste was stored overnight at room temperature for aging and consistency. The fluorinated tin oxide glass (FTO, sheet resistance of 8 Ω/sq) was thoroughly cleaned with acetone, distilled water, and ethanol, respectively. To avoid the layer cracking, ACdoped MWCNT paste was deposited onto the cleaned, air-dried FTO glass via facile tape casting under air drying (at 50 °C) technique (Scheme 1c). Carbon layer thickness was maintained by using 3M tape. Carbon CEs were dried at 150 °C for 15 min and annealed at 300 °C for 30 min in a high temperature air furnace. Pt- and dopedfree MWCNT CEs were also prepared. For Pt CE, 10 mM chloroplatinic acid hexahydrate (H2PtCl6) solution was prepared in isopropanol and drop-cast on FTO glass and annealed at 400 °C for 20 min in a high temperature furnace. MWCNT CEs were fabricated without AC particles and deposited on the FTO substrate as explained earlier. Preparation of Polymer Gel Electrolyte. For the preparation of polymeric gel electrolyte, 0.6 M 1-butyl-3-methyl-imidazolium iodide (BMII), 0.05 M iodine (I2), 0.1 M lithium perchlorate (LiI), 0.1 M guanidine thiocyanate (GuNCS), 0.5 M 4-tert-butyl pyridine (TBP), and 3 wt % of polyethylene oxide polymer were dissolved in acetonitrile solution. Cell Fabrication. Photoanodes of DMT design were fabricated as mentioned in our previous work.20,21 A two-fold coating of 20 nm sized TiO2 nanoparticles (DM) with a 12 μm layer thickness was coated on FTO glass. For the overlayer (scattering layer), a single layer of titania nanotubes (TNT) was used. Therefore, a double main layer with a titania nanotube scattering layer termed as (DMT) pattern, was used for all cell fabrication. TiCl4-treated FTO glass (15 × 15 mm2) coated with the DMT pattern was heat-treated in following steps of 70, 325, 375, 450, and 500 °C for 30, 5, 5, 15, and 15 min, respectively, in a high temperature furnace. The annealed photoanodes were further treated with TiCl4 and immersed in 0.3 mM D719 dye solution. The photoanodes were kept in dye for at least 12 h. As shown in Figure 1, DSSCs were fabricated with a photoanode and AC-doped MWCNT CE, filled with polymer gel electrolyte by using two 60 μm ionomer sealants. For measuring the charge transfer resistance (RCT) at CEs, identical electrodes were assembled in the cell and separated by gel electrolyte, similar to the one used in DSSCs. Characterization. The structural and morphological characteristics of different samples of AC-doped MWCNT hybrids were examined by

Figure 1. Illustrations of AC-doped MWCNT hybrid based DSSCs assembly.

a field emission scanning electron microscope (FE-SEM, JEOL JSM6700F, 15 kV accelerating voltage). For better SEM images, a Pt coating was used on all carbon samples. Charcoal doping in CNTs was investigated with a transmission electron microscope (TEM, JEOL JEM-2100F, 200 kV accelerating voltage). PerkinElmer and Brunauer−Emmett−Teller data using the Quantachrome Autosorb-6 Sorption System was used to investigate the surface area and pore size volume of the as-synthesized carbon hybrids. In order to study the crystal structure of the carbon hybrids, a wide-angle X-ray diffraction (WAXD) test was performed with a Rigaku Denki X-ray generator (Rigaku, D/MAX-2500) using Cu Kα radiation (λ = 1.54181 A) at 40 kV and 60 mA. Surface chemical composition of the carbon hybrids was confirmed with XPS (X-ray photospectroscopy) using a MultiLab ESCA 2000 system VG from Thermo Scientific, USA. For XPS, monochromatized Al Kα radiation with an energy step size of 0.05 eV was used. Raman spectrometry (Jasco NRS-3100) was used to investigate the formation of defects and crystalline structure. For the Raman test, excitation wavelengths of a 532 nm green laser were used. Electrical resistance of all carbon samples was tested by using a standard four-point probe head system method (RM3000 resistivity test unit by Jandel Engineering, Switzerland). In order to check the optimized thickness of the carbon hybrid over FTO glass, an electronic outside micrometer (0−25 mm, 0.01 mm) by Schut Geometrical Metrology was used. To examine the electrocatalytic activity of the carbon hybrid, cyclic voltammetry (CV) test was performed by a three-electrode system. For this test, an argon-purged electrolyte composed of 0.01 M LiClO4, 10 mM LiI, and 1 mM I2 prepared in acetonitrile solution was used, while the CV of all samples was examined at a scan rate of 20 mVs−1 with an ultimate electrochemical workstation (Bio-Logic Co.). Charge transfer resistance (RCT) at CEs was examined by the electrochemical impedance spectroscopy (EIS) technique. The power conversion efficiency of DSSCs fabricated with different CEs was evaluated using a K101-Lab 20 source measuring unit (Mac Science Co.). A solar simulator with a 160 W xenon arc lamp was used as light source. The light intensity was calibrated with a KIER-calibrated Si solar cell (McScience Co.). Power conversion efficiency (PCE) of the cells was measured under a masked frame.22 The DSSCs with an active area of 4 × 5 mm2, measured by an electronic microscope (Camscope, ICS-305B, sometech Co.), were used for photovoltaic tests. 7473


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Figure 2. (a−c) SEM images of AC-doped MWCNT hybrid at different magnifications. (d) EDS of AC-doped MWCNT. (e) Cross-sectional image of AC-doped MWCNT hybrid.

Figure 3. TEM images of AC-free and AC-doped MWCNTs.

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RESULTS AND DISCUSSION Choice of Activated Carbon for CNT Doping. MWCNTs are unique nanoscale materials with the unique features of high surface area, with electrical conductivity, and chemical stability. MWCNTs composed of a nearly perfect atomically smooth basal plane configuration may not be appropriate for promising electrochemical applications. In the present research, we report a novel method for doing of defect-

rich activated charcoal with MWCNTs to synchronize the mutual features of electrocatalytic electivity and conductivity. In order to make an efficient AC-doped MWCNT hybrid structure, a suitable activated carbon needs to be recognized for use in the doing process. On the other hand, little investigation has been reported on the optimization of the different charcoal doping. Therefore, photovoltaic performance of several kinds of carbon based CEs has not met with the 7474


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ACS Applied Materials & Interfaces revolution required in spite of their distinctive electrochemical applications. Initially, three different kinds of activated charcoal were used, composed of coal, coconut shell, and pine tree derived. The characteristics of three different kinds of activated charcoal are listed in Table S2. Further, Figure S1 (Supporting Information) shows the surface morphology of three different activated charcoals (ACs). All activated carbon types consist of an amorphous configuration and showed a mesoporous structure. It was observed that pine tree derived carbon has a sharp panel assembly and maximum pore volume structure. Coconut shell derived carbon has a multilateral configuration with low pore volume, whereas coal type AC has a round shape with a less porous surface. Mesoporous and defect-rich conductive carbon CE has the advantage of fast electrocatalytic reaction with high diffusion of concentrated iodide species. Morphology of AC-Doped MWCNT Hybrid. A scanning electron microscope (SEM) image illustrates a three-dimensional complex of fragmented mesoporous carbon colloid interconnected with a highly conductive MWCNT network (Figure 2).The micropatterned carbon colloid structure was revealed (Figure 2a) at low magnification, which shows many tilted and vertical charcoal collapse, whereas a clear vision of the interconnected MWCNT nanostructure can be clearly visible (Figure 2b,c). Energy-dispersive spectroscopy (EDS) confirms the magnificent existence of carbon with a large extent of oxygen in the hybrid structure (Figure 2d). The small sodium (Na) peak shows the presence of sodium salt of the carboxymethyl cellulose binder, used for carbon sheet formation. Figure 2e shows a cross-sectional view of the ACdoped MWCNT hybrid layered on FTO glass. The average layer thickness is about 3 μm, which is consistent with our previous research work.23 This carbon sheet possesses excellent electrical property, and the corresponding sheet resistance is about 6 Ω sq−1, which is even less than that of Pt CE. The calculated resistance of different carbon CEs is listed in Tables S3−S5. The AC-free and AC-doped MWCNT solutions were transferred onto the TEM grid for further observation. As shown in Figure 3, a flat tubular MWCNT structure was observed in the AC-free MWCNT substrate. It is visibly confirmed that MWCNTs were suitably dispersed with lipase enzyme without obvious agglomeration. Further, it was observed that flat fragmented flakes of AC are decorated with the CNT network. TEM discloses the bamboo-like structure of MWCNTs with a large number of transverse walls with a number of very small activated charcoal particles distributed systematically on the inner walls of the CNTs. The induction of charcoal collapse inside MWCNTs can increase the charge transfer mechanism from the porous carbon to the CNT network, and improve the electrocatalytic function of the CNT matrix, thereby improving the reduction ability of I3− present in the gel electrolyte. In order to examine the surface area and porosity of ACdoped MWCNTs, the N2 adsorption−desorption isotherm and the pore size volume were investigated. The AC-doped MWCNT layer showed typical N2 hysteresis of adsorption− desorption curves (Figure 4), indicating the specific properties of amorphous carbon. AC-doped MWCNTs showed a higher amount of N2 absorption at high relative pressure, signifying the presence of high mesoporous carbon in the hybrid structure. AC-doped MWCNTs have a high surface area of 500.0965 m2/g, with 1.102288 cm3/g pore volume, whereas AC-free MWCNTs have a low surface area of 337.1920 m2/g

Figure 4. N2 absorption desorption isotherms of AC-free and ACdoped MWCNTs.

with 1.030778 cm3/g pore volume. Highly porous and conductive hybrid carbon will offer more accessibility to the I3− ions and improve the electrochemical reduction reaction.24 X-ray diffraction (XRD) was used to examine the phase structure of the AC-doped MWCNT hybrid. In a typical XRD spectrum (Figure 5a), a diffraction peak centered at 26.4° can be indexed to the (002) reflection of the graphitic carbon. Furthermore, the XRD pattern exhibited small peaks at 42.54° and 54.44° indexed to (100) and (004) diffraction intensities. From these measurements, it can be established that the ACdoped MWCNT hybrid consists of a graphite-like ordered structure with amorphous carbon particles. Further, metallic peaks of FTO glass centered at 33.5, 37.6, 61.5, 51.5, and 65.5° were shown in the spectra. To probe amorphous carbon in the MWCNT structure, we observed X-ray photoelectron spectroscopic (XPS) measurements. As seen in Figure 5b, the XPS survey spectrum of the AC-doped MWCNT confirms a main C1 carbon peak at 284.4 eV, together with a high amount of O1 oxygen peak was observed at 534.8 eV, owing to absorption and wrapping of lipase enzymes over the MWCNT. Further, the C 1s spectrum (Figure 5c) could be integrated into four divergent peaks through binding energies of 284.4, 284.9, 287.0, and 294.0 eV, corresponding to sp2-hybridized carbon, sp3hybridized carbon, C−O, and π−π interactions, respectively. Likewise, the O 1s spectrum (Figure 5d) could be integrated into two divergent peaks with binding energies of 531.8 and 533.9 eV, corresponding to the contribution of C−O and C O groups, respectively. Peak intensities of these oxygenated carbons in the hybrid structure are higher than those of MWCNTs, which shows the successful doping of mesoporous carbon within MWCNTs. Figure 6 shows the Raman spectrum of the as-synthesized AC-doped MWCNT hybrid. A comparatively higher D band for the AC-doped MWCNT hybrid was observed than that of MWCNT. This can be attributed to structural distortion affected by doping of mesoporous carbon. The intensity ratio of the crystalline G and defect-rich D bands (ID/IG) is related to the sp2 carbon cluster sizes in the structure and is practically proportional to the density of defects produced in the hybrid structure. Alternatively, a very high ID/IG ratio indicates that the AC-doped MWCNT (ID/IG = 1.22) consists of a partial 7475


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Figure 5. (a) XRD of AC-doped MWCNT hybrid. (b) XPS survey spectra of MWCNT, AC, and AC-doped MWCNT hybrid. (c) Deconvoluted C 1s spectra for AC-doped MWCNT structure. (d) Deconvoluted O 1s spectra for AC-doped MWCNT structure.

activity (ECA) toward the reduction of I3− species to I− at the CE. A cyclic voltammetry (CV) test was carried out to examine the electrocatalytic properties of AC-doped MWCNT prepared at different parameters. The CV curves of different carbon CEs investigated by means of a three-electrode system are presented in Figure 7. Two characteristic sets of oxidation and reduction peaks were observed for different types of AC-doped MWCNT structures, respectively. The negative peaks were assigned to the reduction reaction, and the positive peaks were assigned to the oxidation reaction.25 Electrocatalytic activity (ECA) of the CE can be assessed by three parameters such as cathodic peak potential (ECP), cathodic current density (IPC), and the peak-to-peak separation (Epp) of potential difference between the reduction and oxidation peaks of the (I3−/I−) couple. In this way, ECA is inversely interrelated with the magnitude of Epp and directly with the current density IPC.26 The calculated values are given in Table S3 (Supporting Information). Variation of different kinds of carbon content around MWCNT affected the ECA of the hybrid structure. It can be seen that (Figure 7a) the AC-doped MWCNT prepared with Type A, Type B, and Type C have cathodic peak potentials of −0.08, −0.04, −0.08 V, respectively. The Type C electrode possesses a larger reduction current density, as compared to its counterparts. These results are ascribed to the high conductivity and surface area of pine tree type carbon, which supports catalytic properties of the hybrid

Figure 6. Raman spectra of MWCNT and AC-doped MWCNT hybrid.

mesoporous carbon structure with a tubular graphite nanotube network. Electrocatalytic Activity. The synergic nature of the ACdoped MWCNT hybrid with high conductivity and defect-rich structure is expected to demonstrate improved electrocatalytic 7476


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Figure 8. Cyclic voltammetry of Pt and AC-doped MWCNT hybrid (Type C, 0.8%) electrodes.

Epp value of Pt CE (0.32 V). Additionally, the high current density of AC-doped MWCNT (−2.889 mA cm−2) compared to that of Pt (−1.16 mA cm−2) clearly indicates the superior electrocatalytic nature of our suggested AC-doped MWCNT hybrid. The results are tabulated in Table S5 (Supporting Information).These results are compatible with the photovoltaic performance of DSSCs. In order to observe long-term stability of Pt and AC-doped MWCNT hybrid CEs toward iodide/triiodide reduction reaction, a comparison of cyclic voltammetry (redox peak current density vs no. of cycles) for the Pt and AC-doped MWCNT hybrid was performed. The peak current densities of the AC-doped MWCNT show no sign of degradation after 30 consecutive cycles (Figure 9). The curves’ shape and redox peak current density are not changed, suggesting that the CE possesses advantages including stable redox activity, excellent electrochemical stability, and strong adhesion on FTO glass. However, the Pt catalyst showed slight depletion in redox peak after 20 cycles, which indicates its low stability electrolyte reduction reaction. On the other hand, it is a well-known fact that VOC values in DSSCs lie between the potential of the photoanode and the iodide/triiodide redox equilibrium potential of the electrolyte. Therefore, on the basis of the same photoanode and gel electrolyte, it is expected that VOC values depend on the redox equilibrium potential of the CE.29 Therefore, the effect of ACdoped MWCNT catalytic materials on the photovoltaic performance will be intensively discussed in the photovoltaic section. Electrochemical Properties. The reduction of internal impedance through nanostructural design has become a significant concern in the realm of DSSCs applications. Therefore, to evaluate the impedance effect of the AC-doped MWCNT CE, electrochemical reaction rates of the (I−/I3−) redox couple were measured with electrochemical impedance spectroscopy (EIS) using symmetrical cells as defined in earlier literature.30 The symmetrical cell comprising two identical electrodes was used to evaluate the charge transfer resistance (RCT) at the CE, so as to avoid impedance losses of the photoanode.31 Figure 10 presents the Nyquist plots of different CEs. The typical Nyquist plot shows a well-defined semicircle with one small curve. The high frequency region (around 100

Figure 7. Cyclic voltammetry of (a) three types of AC-doped MWCNT electrodes. (b) Different wt % of pine tree AC (Type C).

structure.27 The larger cathodic current density (−2.638 mA cm−2) of the Type C electrode indicates the lower overpotential, and superior electrocatalytic activity for I3− reduction. Figure 7b illustrates the CVs of carbon electrodes prepared with different AC content. It was interesting to find out that, by increasing the charcoal content from 0 to 3.2 wt %, two pairs of redox peaks were clearly shown, which indicates high electrocatalytic activity. However, by increasing the AC content, low redox peaks were observed, which showed low charge transfer efficiency of the hybrid structure. It was also observed that, by increasing the AC content, the absolute value of the cathodic current density increases; i.e., the AC-doped MWCNT hybrid of 0.8 wt % AC attains the lowest EPP value of 0.43 V with a high IPC value of 2.889 mA cm−2, which revealed the high electrocatalytic activity and conductivity of CE. Further, by increasing the AC content from 1.6 to 3.2 wt %, large values of EPP with low current densities were observed, which might be due to the large defects produced by an incompatible carbon composition ratio, destroying the conductive network of the CNT matrix.28 The results are tabulated in Table S4 (Supporting Information). It should also be noted that our suggested AC-doped MWCNT hybrid shows higher ECA than the conventional Pt CE (Figure 8). AC-doped MWCNT CE exhibits a typical curve similar to the Pt electrode. AC-doped MWCNT shows an Epp of 0.43 V, which is compatible with the 7477


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Figure 9. Continuous cycle scans (30 cycles) for (a) Pt and (b) ACdoped MWCNT hybrid electrodes (0.8 wt) at 20 mV/s scan rate.

Figure 10. Nyquist plots of the dummy cells based on (a) three types of AC-doped MWCNT hybrid electrodes. (b) Different wt % of pine tree AC (Type C).

kHz) intercept on the real axis characterizes the series resistance (RS) of the CE. The first semicircle at the medium frequency region is associated with the charge transfer resistance (RCT) and the corresponding constant phase element (CPE) of electrolyte and CE, while the other small semicircle at the low frequency region determines the Nernst diffusion impedance (ZW) of electrolyte. In this respect, we mainly put emphasis on the RS and RCT values of counter electrodes, as the Nernst diffusion impedance value is insignificant for our focus. The RS and RCT values were attained by fitting Nyquist plots by using a Randles-type equivalent circuit (inset of Figure 10a) from EC-lab software. According to the values obtained from the plots given in Table 1, the Type C CE has the RCT of 0.93 Ω, signifying low charge-transfer resistance (RCT) as compared to its counterparts. This result is in accordance with the high catalytic activity of I3− ions reduction in the CV results. The RCT value of Type A and Type B have a considerably high RCT with 2.70 and 2.18 Ω, respectively. The RS of the three electrodes varied negligibly, which reveals a good bonding strength of the sheet with the FTO glass substrate. The result demonstrates that pine tree charcoal possesses high electrical conductivity and catalytic activity with a high charge transfer mechanism than its counterparts.

To investigate RCT at optimum charcoal dosage, dummy symmetrical cells were fabricated with CEs containing different amounts of (PN) charcoal particles, filled with gel electrolyte. The calculated results (Figure 10b and Table 2) showed the influence of charcoal content on the electrochemical characteristics of symmetrical cells. The RCT of CE decreases gradually with increasing carbon content from 0 to 0.8 wt %. In brief, with the CE of 0.8 wt % AC, the Nyquist plot highlights a striking reduction of RCT with 0.60 Ω. This result proves that the synchronized combination of conductive MWCNT and mesoporous carbon can reduce a large amount of iodide/ triiodide of PEO based electrolyte, which provides a fast charge transfer mechanism. However, when the AC content is beyond 0.8 wt %, RCT values were gradually increased due to less conductivity and high defects formation reduces the reduction reaction of the electrolyte, previously proved in CV results. Thus, there is a major reduction in impedance when using the AC-doped MWCNT hybrid instead of Pt as a catalyst with gel electrolyte, generating a significant difference for the performance of quasi-solid-state DSSCs. As seen in Figure 11 and Table 3, the RCT of Pt is 3.37 Ω, which is much higher as compared to that of our suggested CE. Figure S4 (Supporting 7478


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Table 1. Electrochemical and Photovoltaic Performance of Cells Fabricated with Various AC-Doped MWCNT Hybrid Counter Electrodes symmetrical cell

DSSC

type of hybrid

RS (Ω)

RCT (Ω)

JSC (mA cm−2)

VOC (V)

FF (%)

PCE (%)

Type A Type B Type C

8.35 8.29 8.22

2.70 2.18 0.93

14.485 15.383 15.681

0.695 0.724 0.752

76.45 76.40 80.84

7.68 8.51 9.52

Table 2. Electrochemical and Photovoltaic Performance of Cells Fabricated with AC-Doped MWCNT Hybrid Counter Electrodes Prepared with Different wt % of Pine Tree Type AC symmetrical cell

DSSC −2

wt % of AC (PN)

RS (Ω)

RCT (Ω)

JSC (mA cm )

VOC (V)

FF (%)

PCE (%)

0 0.4 0.8 1.6 3.2

7.90 8.04 8.11 8.22 8.37

3.65 1.52 0.60 0.93 2.57

14.184 14.937 16.074 15.681 14.581

0.701 0.732 0.753 0.752 0.695

72.06 80.67 83.00 80.84 77.01

7.16 8.82 10.05 9.52 7.81

different types of AC-doped MWCNT CEs, and subsequently compared with Pt cells. The photocurrent−voltage (I−V) curves of different CEs were measured under a simulated AM 1.5 illuminations (1 sun, 100 mW/cm2). Photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and power conversion efficiency (η) of AC-doped MWCNT hybrid CEs are listed in Table 1. From Figure 12a, it can be found that DSSCs based on the Type A CE showed a rather low VOC (0.695 V), JSC (14.485 mA/cm2), and FF (76.45%) and showed a low efficiency of 7.68%. As for the type B carbon CE, high VOC (0.724 V), JSC (15.383 mA/cm2), and FF (76.40%), demonstrated enhanced conversion efficiency of the cell from 7.68% to 8.51%. However, the efficiency of the Type C (pine tree) based DSSCs increased with an enormously high VOC (0.752 V) and fill factor (80.84%), demonstrating 9.52% efficiency. This remarkable performance of the device can be due to the enormously low charge transfer resistance (0.93 Ω) and higher electrocatalytic activity at the CE with gel electrolyte, as confirmed by the EIS and CV analysis. In brief, the pine tree based (type C) CE exhibits the best performance among the three different ACdoped MWCNT hybrid structures. It was observed with the results that carbon nanotube CEs possess low surface area and electrocatalytic activity, which affect the reduction of I3− species at the CE, and subsequently lower VOC and FF. To overcome this disadvantage, mesoporous activated charcoal (AC) particles were added to the CNT dispersion to incorporate the defects and porous surface. Different amounts of AC were added, 0.4, 0.8, 1.6, and 3.2 wt % to optimize the MWCNT recipe. Figure 12b presents I−V curves of the quasi-solid-state DSSCs based on AC-doped MWCNT CEs containing different wt % of AC. It is shown in Table 2, with the addition of AC, that the efficiency first increased and then decreased. The short-current density (Jsc)

Figure 11. Comparison of Nyquist plots of Pt and AC-doped MWCNT hybrid (Type C, 0.8%) electrodes.

Information) shows the morphology of Pt and AC-doped MWCNT hybrid. It can be proved that rapid electron transport characteristics of the AC-doped MWCNT hybrid provides maximum current density attainable in our proposed solar cell. We can suggest that this effect may be due to the threedimensional and porous morphology of the AC-doped MWCNT sheet, dropping the diffusion distance of the polymer gel electrolyte and producing diffusion fields within the radial symmetry of the hybrid CE. Photovoltaic Data. The influence of the electrocatalytic characteristics of CE on the photovoltaic performance of quasisolid-state DSSCs was investigated by fabricating cells with

Table 3. Comparison of Electrochemical and Photovoltaic Performance of Cells Fabricated with Pt and AC-Doped MWCNT Hybrid CEs symmetrical cell

DSSC −2

CE

RS (Ω)

RCT (Ω)

JSC (mA cm )

VOC (V)

FF (%)

PCE (%)

Pt AC-doped MWCNT hybrid

7.97 8.11

3.37 0.60

15.402 16.074

0.729 0.753

82.81 83.00

9.30 10.05

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Figure 13. Comparison of photovoltaic performance of quasi-solidstate DSSCs based on Pt and AC-doped MWCNT hybrid (Type C, 0.8%) CE.

be explained by an energy band diagram. Pt metals do not have a band gap, while HOMO and LUMO energy levels of MWCNT and AC-doped MWCNT can be calculated from the onset oxidation potential (EOx(onset)) and the onset reduction potential (E Red (onset)) shown in Figures S2 and S3 (Supporting Information), respectively. It was observed that AC doping slightly increases the band gap of the MWCNT, from 0.01 to 0.2 eV. This result shows that induction of charcoal particles into the CNT matrix does not destroy its sp2 bond structure; in fact, it further increases its electrocatalytic activity. Therefore, a low band gap with a defect-rich porous morphology of the AC-doped MWCNT structure displays desirable characteristics as a low recombination rate of charge carriers and excellent kinetics. From the energy band diagram (Figure 14), it can be clearly seen that VOC values in DSSCs lie between the potential of the photoanode and iodide/triiodide redox equilibrium potential of the electrolyte. Therefore, fast diffusion of polymer gel electrolyte into the porous morphology of AC-doped MWCNTs enhanced equilibrium kinetics of the electrolyte mediator, and hence enhanced the VOC.32,33 The AC-doped MWCNT hybrid structure revealed a densely packed defectrich carbon structure and has a superior adhesion with FTO glass. Fortunately, this phenomenon led to an increase of the electron transport at the FTO/CE/gel electrolyte interface and a reduction of the interfacial resistance between the FTO substrate and AC-doped MWCNT layer, resulting in a high VOC and FF. In addition, the high Voc values suggest that ACdoped MWCNT CEs are more effective than MWCNT and Pt CE in catalyzing the reduction of triiodide to iodide. The internal kinetics of DSSCs with MWCNT and Pt are shown in Figures S5 and S6 (Supporting Information), respectively. It can be observed that the low porous morphology of Pt and MWCNT CE can make resistance for electron pathways, whereas a large volume of electrolyte can be entrapped by the oxygen-rich porous AC-doped MWCNT CE, which reduces the internal resistance of DSSCs. Low internal resistance, improved electrocatalytic activity, and high surface area in the AC-doped MWCNT CE strongly influence the rapid release of I− species from the electrolytic reduction at the counter electrode/electrolyte interface, and enhance its VOC.34,35 In

Figure 12. Photovoltaic performances of quasi-solid-state DSSCs based on (a) three types of AC-doped MWCNT counter electrodes. (b) Different wt % of pine tree AC (Type C).

and fill factor (FF) show the same trend. The DSSC containing 0.8% of AC hybrid structure reaches an efficiency of 10.05%, with a high FF of 83%. On the other hand, above 0.8 wt % of AC, high defects produced on the CE, which reduced the sheet adhesion on FTO, subsequently, lowering the FF and VOC of the device. Therefore, DSSCs fabricated with the CE of 3.2 wt % AC yield a low efficiency of 7.81%. Figure 13 presents the I−V curves of the quasi-solid-state DSSCs based on Pt and AC-doped MWCNT hybrid CEs. The optimized results are presented in Table 3. DSSCs assembled with conventional Pt CEs showed 9.30% efficiency, although the cell prepared with AC-doped MWCNT CE reached 10.05% efficiency, outperforming the Pt electrode. This result corresponds to one of the highest efficiency values obtained for Pt-free, D719, quasi-solid-state DSSCs. Specifically, both the open-circuit voltage (VOC) and fill factor (FF) were higher, with the AC-doped MWCNT (0.8 wt %, Type C). These results indicate the facile charge mobility at the FTO/AC-doped MWCNT/gel electrolyte interface mechanism, resulting in a high VOC and FF. In order to put emphasis explicitly on the effect of high VOC of the AC-doped MWCNT CE, internal kinetics of DSSCs can 7480


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Figure 14. Energy level diagram and mechanism of photocurrent generation in DSSCs with TiO2/gel electrolyte/AC-doped MWCNT hybrid sandwich structure.

addition, the high current density (JSC) suggests that the ACdoped MWCNT CE is more effective than the Pt CE in catalyzing the redox reaction mechanism in high density electrolyte medium, indicating improved light harvesting properties of our suggested CEs. Statistical data of the cell performance prepared with the AC-doped MWCNT CE are provided in Table S6 (Supporting Information). Consistent results with very low variation were observed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CONCLUSION Quasi-solid-state dye-sensitized solar cells were successfully fabricated using polyethylene oxide (PEO) based gel electrolyte and an AC-doped MWCNT hybrid counter electrode. The ACdoped MWCNT hybrid CE was found to have the best electrochemical features of high conductivity, excellent electron transport, and facile redox reaction. The high surface area and defect-rich AC-doped MWCNT is beneficial to locate a large volume of PEO based gel electrolyte, and reduced the charge mobility within the cell. Hence, fabricated quasi-solid-state DSSCs achieved an overall efficiency of 10.05%. The high efficiency and sustainability of quasi-solid-state DSSCs assembled with metal-free CE and gel electrolyte indicate that they are promising for large-scale industrialization.

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electrode, energy band gap of MWCNT from CV, energy band gap of AC-doped MWCNT hybrid from CV, FESEM images of Pt and AC-doped MWCNT hybrid, and statistical data of the DSSCs performance fabricated with AC-doped MWCNT hybrid counter electrode (PDF)

Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09319. Previous research work related to different carbon based counter electrodes for DSSCs, FE-SEM images of different activated charcoal, characteristics of different activated charcoal, electrical and electrocatalytic characteristics of AC-doped MWCNT hybrid counter electrodes prepared with different types of activated charcoal, electrical and electrocatalytic characteristics of AC-doped MWCNT hybrid counter electrodes prepared with different wt % of pine tree type (PN) activated charcoal, comparison of electrical and electrocatalytic characteristics of AC-doped MWCNT hybrid with Pt counter 7481


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