macrocyclic cobalt complex nanocomposites as counter electrodes for Pt-free dye-sensitized solar cells

Accepted Manuscript Title: Fabrication of reduced graphene oxide/macrocyclic cobalt complex nanocomposites as counter electrodes for Pt-free dye-sensitized solar cells Authors: Chih-Hung Tsai, Chun-Jyun Shih, Wun-Shiuan Wang, Wen-Feng Chi, Wei-Chih Huang, Yu-Chung Hu, Yuan-Hsiang Yu PII: DOI: Reference:

S0169-4332(17)33167-7 https://doi.org/10.1016/j.apsusc.2017.10.208 APSUSC 37551

To appear in:

APSUSC

Received date: Revised date: Accepted date:

7-8-2017 20-10-2017 29-10-2017

Please cite this article as: Chih-Hung Tsai, Chun-Jyun Shih, Wun-Shiuan Wang, Wen-Feng Chi, Wei-Chih Huang, Yu-Chung Hu, Yuan-Hsiang Yu, Fabrication of reduced graphene oxide/macrocyclic cobalt complex nanocomposites as counter electrodes for Pt-free dye-sensitized solar cells, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.208 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 proof before it is published in its final 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.

Fabrication of reduced graphene oxide/macrocyclic cobalt complex nanocomposites as counter electrodes for Pt-free dye-sensitized solar cells Chih-Hung Tsaia,*, Chun-Jyun Shiha, Wun-Shiuan Wangb, Wen-Feng Chib, Wei-Chih Huanga, YuChung Hub, Yuan-Hsiang Yub a

Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401,

Taiwan b

Department of Chemistry, Fu Jen Catholic University, New Taipei City 24205, Taiwan

*Corresponding author. E-mail address:[email protected] (Chih-Hung Tsai) Graphical Abstract

Highlights 

Graphene oxide/macrocyclic Co complex composite materials were synthesized.



The GO/Co composites were successfully used as counter electrodes in DSSCs.

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GO/Co CEs exhibit high electrical conductivity and excellent catalytic properties.

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The PCE of the DSSCs with GO/Co (1:10) CE was higher than that of the Pt CE. 1

ABSTRACT In this study, macrocyclic Co complexes were successfully grafted onto graphene oxide (GO) to produce GO/Co nanocomposites with a large surface area, high electrical conductivity, and excellent catalytic properties. The novel GO/Co nanocomposites were applied as counter electrodes for Pt-free dye-sensitized solar cells (DSSCs). Various ratios of macrocyclic Co complexes were used as the reductant to react with the GO, with which the surface functional groups of the GO were reduced and the macrocyclic ligand of the Co complexes underwent oxidative dehydrogenation, after which the conjugated macrocyclic Co systems were grafted onto the surface of the reduced GO to form GO/Co nanocomposites. The surface morphology, material structure, and composition of the GO/Co composites and their influences on the power-conversion efficiency of DSSC devices were comprehensively investigated. The results showed that the GO/Co (1:10) counter electrode (CE) exhibited an optimal power conversion efficiency of 7.48%, which was higher than that of the Pt CE. The GO/Co (1:10) CE exhibited superior electric conductivity, catalytic capacity, and redox capacity. Because GO/Co (1:10) CEs are more efficient and cheaper than Pt CEs, they could potentially be used as a replacement for Pt electrodes. Keywords: dye-sensitized solar cells; graphene oxide; nanocomposites; counter electrode; macrocyclic cobalt complex. 1. Introduction The advantages of solar energy as a source of renewable energy are that it is inexhaustible, is unaffected by geographical location, and causes no environmental pollution [1]. In 1991, Grätzel et al. used TiO2 nanoparticles with a large surface area to develop dye-sensitized solar cells (DSSCs) [2]. Many research teams have studied DSSCs in recent years because they possess advantages such as a simple fabrication process, high efficiency, low cost, and flexibility [3-11]. The structure of a DSSC mainly comprises a TiO2 nanoparticle working electrode, a dye, an electrolyte, and a Pt counter electrode (CE) [12]. Although Pt is commonly used for the CE, its high cost increases the price of DSSCs, which inhibits their commercialization and mass production, prompting researchers to actively search for cheaper electrode materials that can replace Pt and improve the efficiency of 2

DSSCs. Several previous studies have endeavored to replace Pt CEs with other more cost-effective CE materials possessing favorable electrochemical properties such as carbon materials [13-15], conducting polymers [16-23], and inorganic compounds [24]. Recently, metal selenides have also been used as CE materials for DSSCs, which show good catalytic activity [25, 26]. Novel carbon materials and conductive polymers including fullerenes, carbon nanotubes, graphene, and PEDOT:PSS have become popular materials for DSSC CEs, among which graphene delivers superior DSSC performance because of its excellent properties. Graphene is composed of a single layer of tightly arranged carbon atoms, which bond through sp2 orbital hybridization to form a two-dimensional planar structure with hexagonal honeycomb carbon rings. Graphene has superior thermal conductivity and mechanical strength, high carrier mobility, low resistance, and excellent optical transmittance, with a mere 2.3% light absorption under white light irradiation [27-29]. Research teams have recently begun to apply graphene to preparing DSSC CEs. Zhang et al. dispersed graphene nanosheets (GNs) into a mixed solution of terpineol and ethyl cellulose, and used a screenprinting technique to coat the GNs onto fluorine-doped tin oxide (FTO) glass substrates to investigate the effects of annealing temperature on DSSC device performance. They found that DSSC devices with GN CEs annealed at 450 °C achieved a maximum conversion efficiency of 2.94% [13]. Kavan et al. prepared graphene flakes on an FTO glass substrate as CEs that demonstrated satisfactory catalytic activity [14]. Wan et al. used a low-cost method to prepare graphene films on various substrates at room temperature and found that graphene was suitable for use in DSSCs and other devices such as super capacitors, fuel cells, and chemical sensors [15]. Early transition metals have the potential to substitute for Pt because they are abundant, low-cost resources. Among the early transition metals, the VIII metal elements in the fourth period, including iron (Fe), cobalt (Co), and nickel (Ni), have been used for energy and environmental catalysis, as alternative catalysts of noble metals (e.g., Pt, Rh, and Pd) for application in a variety of important industrial catalytic reactions [30]. The early transition metals have relatively lower electron transport efficiency between particles than Pt does and are more labile than Pt, which restricts the electrocatalytic properties of DSSC CEs. Therefore, it is still challenging to obtain high-performance electrocatalytic CEs for DSSCs by using the group VIII transition metals. Several design items were 3

considered to develop cost-effective electrocatalytic CEs based on the group VIII transition metals. First, the metals should be in the form of nanoparticles to improve their catalytic activity; second, the metals should have highly conductive networks for electron transfer; third, a high density of catalytic sites should exist on the surface of the CEs; fourth, the CEs should have high surface area or roughness for effective contact with the electrolytes; and fifth, the stability of the nanoparticles should be improved with encapsulating or supporting materials, such as graphene. DSSC CEs with high electrical conductivity and superior catalytic properties were recently developed by grafting group VIII nanometals Ni or Fe onto the surface of graphene [31, 32]. Co compound possesses exceptional electrical conductivity and catalytic properties; for example, LiCoO2 is widely used as the positive electrode of Li-ion batteries [33]. Recently, Co compound has been applied in research on DSSCs. Grätzel et al. demonstrated that CoS is very effective in catalyzing the reduction of triiodide to iodide in a DSSC and has the potential to replace Pt [34]. Freitag et al. applied [Co(bpy)3]3+/2+ complexes to a DSSC electrolyte and found this system’s performance was superior to that of an I−/I3− electrolyte [35]. Researchers constructed nanocomposites consisting of cobalt compound and conductive carbon-based materials (e.g., graphene, and conductive polymers) to combine the advantages of both individual compositions for DSSC CEs, such as CoS/graphene [36], Co3O4/graphene [37], and CoS/(PEDOT:PSS) [38]. In the present study, graphene oxide (GO) was reacted with macrocyclic Co complexes to produce novel nanocomposites with a large surface area, high electrical conductivity, and excellent catalytic properties to serve as the DSSC CE. Macrocyclic Co complexes were used to react with the GO, whereby the molecular grafting of Co complexes would lead to a uniform distribution of Co catalytic sites on the surface of reduced GO (RGO). The redox reaction between the functional groups of the GO and macrocyclic Co complexes were included in the reaction, in which the GO was transformed to RGO and the ligand of macrocyclic Co complexes was transformed to a conjugated macrocyclic system through oxidative dehydrogenation [39], after which the macrocyclic Co system adsorbed to the RGO surface by the - interaction between the conjugated macrocyclic Co complexes and sp2 aromatic rings of RGO, and formed GO/Co nanocomposites. The surface morphology, material structure, and composition of the

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GO/Co composites and their influences on the power conversion efficiency of DSSC devices were investigated. 2. Experiments 2.1 Preparation of Graphene Oxide (GO) Scheme 1(a) shows the synthetic procedure of GO, which follows an improved Hummers method [40]. The typical procedure is described as follows. First, 400 mL of H2SO4 and H3PO4 at a 9:1 ratio were mixed uniformly in a beaker to form a solution, after which 18 g of KMnO4 was slowly added. The solution turned dark green after stirring for 1 hour; then, 3 g of graphite was slowly added to the solution for exothermic reaction. The solution was controlled at a temperature of 50 °C and stirred continuously for 24 hours. A H2O2 diluent was prepared by adding 3 mL of H2O2 to 400 mL of distilled ice water. After cooling it to room temperature, the graphite-oxidized solution was placed in an ice bath, and the H2O2 diluent was added and stirred continuously for 2 hours. The acidic component of the solution was then separated through centrifugation at 9,000 rpm, and the resulting precipitate was cleaned several times using distilled water and ethanol until the solution turned neutral, after which the precipitate was collected through centrifugation. Finally, the precipitate was dried in a vacuum at 50 °C for 12 hours to obtain GO. 2.2 Synthesis of Macrocyclic Co Complexes Step 1: Preparation of 2,3-butanedihydrazone Scheme 1(b) shows the synthetic procedure of 2,3-butanedihydrazone. Referring to the synthesis method of Busch and Bailar [41], hydrazine hydrate (10 g, 0.2 mol) and 2,3-butanedione (8.6 g, 0.1 mol) were dissolved individually in 20 mL anhydrous ethanol and reflux-heated for 2 hours. White needle-like crystals formed when the solution cooled to room temperature, and the crystals were collected through filtration and washed with alcohol to remove yellow impurities. Finally, the crystals were vacuum-dried in an oven at 50 °C for 12 hours to obtain 2,3-butanedihydrazone, with a yield of 87.6%. Step 2: Synthesis of Macrocyclic Co Complex: [Co(C10H20N8)(H2O)2](BF4)2 5

Scheme 1(c) shows the synthetic procedure of [Co(C10H20N8)(H2O)2](BF4)2. First, Co(BF4)2·xH2O (3.606 g, 10 mmol) was added to an Erlenmeyer flask, after which 20 mL of water and 10 mL of CH3CN mixed solution were added and stirred evenly, producing a red solution. Next, 36.5% (1.64 g, 20 mmol) formaldehyde was added to the red solution using a dropper, after which 2,3-butanedihydrazone (2.28 g, 20 mmol) was added. The solution was then stirred for 24 hours, turning reddish brown. Next, ether (200 mL) was added to the solution, which was then placed in a freezer at 0 °C. The ether was poured out after the product precipitated, and the product was dried in a vacuum oven at 50 °C for 12 hours to obtain [Co(C10H20N8)(H2O)2](BF4)2, with a yield of 90.2%. Element analysis for identification of [Co(C10H20N8)(H2O)2](BF4)2: Calc. N%, C%, H% = 21.512%, 23.058%, 4.644%; Measured data N%, C%, H% = 21.157%, 23.066%, 4.163%. 2.3 Preparation of GO/Macrocyclic Co Complex Composites Scheme 1(d) shows the preparation process for the GO/Co nanocomposites. GO (0.2 g) was dispersed in 20 mL of distilled water, forming a brown solution. Next, 0.2 g of macrocyclic Co complex was dissolved in 30 mL of distilled water. The solution containing the Co complex was slowly added to the dispersed GO solution under N2 and then stirred at room temperature for 12 hours. The red-brown solution turned into a purple-black suspension during the reaction period. Next, the solution was centrifuged, and the unreacted macrocyclic Co complex was washed with distilled water until the solution became clear. After 12 hours of drying in a vacuum oven at 50 °C, the GO/Co composite was obtained at a weight ratio of 1:1. GO and macrocyclic Co composites with different weight ratios were prepared following the same steps to fabricate GO/Co (1:1), GO/Co (1:3), and GO/Co (1:10). The real GO/Co (weight:weight) ratios of compound codes GO/Co (1:1), GO/Co (1:3), and GO/Co (1:10) were evaluated by comparing the calculated concentrations before and after reacting with GO based on UV-vis absorption spectra, which were (1:0.82), (1:1.57), and (1:5.76), respectively. 2.4 Preparation of DSSC CEs First, two holes were drilled into the FTO transparent conductive substrates to enable the electrolyte to be injected. Subsequently, the FTO substrate was ultrasonically cleaned with deionized 6

water for 10 min, with acetone for 10 min, and with alcohol for another 10 min, and was then blowdried with nitrogen gas. Heat-resistant tape was used to form a 4.5-cm2 coating area on the substrate surface, and 10 μL of the GO/Co composite solutions was dripped onto the coating area using a micropipette. The substrates were then placed on a hot plate and annealed at 100 °C for 5 min to remove the solvent. The substrate was placed in a tube furnace and sintered at 500 °C for 30 min in a pressurized nitrogen atmosphere. The GO/Co (1:1), GO/Co (1:3), and GO/Co (1:10) CEs were completed after the substrate cooled naturally. 2.5 Characterization of the CEs A comprehensive analysis of the various GO/Co CEs was conducted. The surface morphology of the CEs was analyzed through scanning electron microscopy (SEM) to investigate the effects of differing proportions of macrocyclic Co complexes on the film surface, and the nanostructure and distribution of the CEs were studied through transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR) was employed to investigate the chemical structure of the electrode material, and energy-dispersive spectrometry (EDS) was used to analyze the element type and content of the CEs. The material properties of the CEs were analyzed through Raman spectroscopy, and the elemental and chemical composition of the electrodes was analyzed through Xray photoelectron spectrometry (XPS). The catalytic activity was measured through cyclic voltammetry (CV). CV measurements were conducted using an electrochemical analyzer, whereby the GO/Co nanocomposite electrodes were used as the working electrodes and a Pt foil was used as the CE, with Ag/Ag+ serving as the reference electrode. The scanning speed used was 50 mV/s, and the electrolyte formula used was an acetonitrile solution comprising 10 mM LiI, 1 mM I2, and 100 mM LiClO4. 2.6 Fabrication of the DSSC devices Scheme 1(e) displays the device structure of a DSSC with a CE composed of the GO/Co nanocomposites. The preparation method for the DSSC CEs is described in Section 2.4. The DSSC working electrodes were prepared as follows. First, the FTO transparent conductive glass substrate was cleaned, and 3M tape with 4-mm diameter holes was attached to the substrate. The holes, which 7

had an area of 0.126 cm2, served as the coating region of the device, to which 25 nm of TiO2 paste was applied using the doctor blade coating method, and the substrates were heated at 150 °C for 10 min. After they cooled to room temperature, the TiO2 paste was recoated to obtain an approximately 12-μm-thick TiO2 working electrode. Another layer of TiO2 paste (200 nm particle size) was applied using the doctor blade coating method to serve as the scattering layer, after which the substrate was placed in a furnace and sintered at 500 °C for 30 min. After the TiO2 electrode cooled to 80 °C, it was immersed in a prepared dye for 24 hours. For this experiment, 0.5 mM N719 dye was used together with 0.5 mM chenodeoxycholic acid (CDCA) as the coabsorbent, the solvent of which was a mixture of acetonitrile and tert-butyl alcohol with a volume ratio of 1:1. A 60-μm-thick sealing foil was then cut to a size of 2.5 × 2.5 cm with a 0.8 × 0.8-cm area cut out of the center. The sealing foil was used together with the working electrode to assemble the CE, which was continuously pressurized at 3 kg/cm2 for 3 min at a temperature of 130 °C to seal the upper and lower electrodes. After the assembled device cooled, approximately 5 μL of electrolyte (0.6 M BMII, 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine, 0.1 M guanidine thiocyanate, and a mixture of acetonitrile and valeronitrile with a volume ratio of 5:1) was injected using a micropipette. The 0.8 × 0.8-cm sealing foil was then used together with thin glass to seal the holes of the CE at a temperature of 130 °C and pressure of 3 kg/cm2 to prevent the electrolyte from leaking and volatilizing. Finally, the device surface was wiped clean with alcohol to complete the fabrication of the DSSCs. The characteristics of the DSSC device were analyzed in terms of its current density–voltage (J–V), incident photon-to-electron conversion efficiency (IPCE), and electrochemical impedance spectroscopy (EIS) measurements. 3. Results and Discussion 3.1 Characterization of the CEs Fig. 1 shows the SEM results for the various GO/Co CEs. The surface morphologies of the GO/Co nanocomposites were analyzed at 10k and 30k magnification. The figure clearly shows that the Co metal nanoparticles adhered to the surface of the reduced GO (RGO). In the GO/Co (1:1) CE, the Co metal nanoparticles were relatively small and evenly distributed. The GO/Co (1:3) CE exhibited the fewest nanoparticles, and they were also the largest and most unevenly distributed. The

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GO/Co (1:10) CE had the most Co nanoparticles, and they were also the smallest and most evenly distributed. Fig. S1 (in Supplementary Materials) shows the SEM image of the GO/Co (1:10) CE at 150k magnification, confirming the even distribution of the Co nanoparticles on the GO surface. The SEM results show that GO provided a high specific surface area and oxygen-containing functional groups, and the successful grafting of macrocyclic CO complexes on the RGO promoted the even distribution of the Co nanoparticles on the RGO surface. Among the various GO/Co CEs, the GO/Co (1:10) CE exhibited the optimal Co nanoparticle distribution, which may enhance the electron transport rate and electrical conductivity of the CE. Fig. 2 shows the TEM results for each of the GO/Co nanocomposite materials. The figure shows that the nanocrystalline particles of the Co complex were successfully deposited onto the graphene surface. The size and distribution of the macrocyclic Co nanoparticles were analyzed at 100k and 400k magnification. The TEM images at 100k magnification show that the GO/Co (1:1) nanocomposite exhibited scattered, large Co nanoparticles with a clearly even distribution. The GO/Co (1:3) nanocomposite exhibited a dense formation of crystal particles that were more numerous compared with the GO/Co (1:1) nanocomposite. The GO/Co (1:10) nanocomposite clearly demonstrated a dense and even distribution of Co nanoparticles. The distributions of Co nanoparticles in the GO/Co (1:3) and GO/Co (1:10) nanocomposites were more even than that in the GO/Co (1:1) nanocomposite. The TEM image at 400k magnification shows that the Co nanoparticles in the GO/Co (1:3) nanocomposite were uniformly dispersed and smaller than those in the GO/Co (1:1) nanocomposite. The dispersion of Co nanoparticles in the GO/Co (1:10) nanocomposite was satisfactory; in addition to its even distribution of nanoparticles, the difference in the size of the nanoparticles was smaller, and particle agglomeration was less frequent. Fig. 3 shows the FTIR results for the GO, each GO/Co CE, and the macrocyclic Co complex [Co(C10H20N8)(H2O2](BF4)2. The results revealed that GO absorption peaks at 3010–3680 cm−1 (–OH stretching), 1732 cm−1 (–C=O stretching), 1627 cm−1 (–C=C stretching), 1220 cm−1 (–C(O)–OH bending), and 1045 cm−1 (–C–O–C stretching), verifying the presence of hydroxyl, carboxylic, and epoxy groups. The characteristic peaks of macrocyclic Co complexes were observed at 3000–3600 cm−1 (N–H and O-H stretching), 2883 cm−1 (–CH3), 1627 cm−1 (–C=C), 1590 cm−1 (–C=N stretching), 9

1360 cm−1 (–C–N stretching), and approximately 1100 cm−1 (BF4). For the GO/Co composites, the disappearance of the characteristic peaks at 1732 cm−1 (C=O of carboxylic acid) and 1045 cm−1 (epoxide group) in the GO indicated that the macrocyclic Co complexes had grafted onto the RGO and that the GO was reduced. The disappearance of the characteristic peak of BF4 in GO/Co composites indicated that a conjugated macrocyclic system with a di-anion ligand of macrocyclic Co complex was formed on the surface of RGO, which supports the oxidative dehydrogenation reaction of the octaaza-bis-α-diimine Co macrocyclic complex transformed into a fully conjugated macrocyclic system by the deprotonation and oxidized by GO. A driving force for well-distributed, fully conjugated molecules onto the sp2 aromatic hexagonal lattice of graphene through - interaction was expected. Increasing the proportion of macrocyclic Co complexes in the GO/Co composites also increased the peak intensities at 1694 cm−1 (–C=C stretching), 1590 cm−1 (–C=N stretching), and 1360 cm−1 (–C–N stretching), respectively. This increase in macrocyclic Co complex proportions augmented the macrocyclic Co complex peak while attenuating the GO peak. These results verified that the redox reaction of the GO and macrocyclic Co complexes successfully produced GO/Co composites. Fig. 4 shows the EDS results for each GO/Co CE. The GO/Co (1:1) CE exhibited the lowest Co weight percentage (1.17%). The GO/Co (1:3) CE (3.25%) had the next highest weight percentage, followed by the GO/Co (1:10) CE (3.3%). The EDS results demonstrated that the Co content in the GO/Co CE increased with the weight ratio of the macrocyclic Co complexes. Fig. 5 presents the Raman spectra of the GO and various GO/Co CEs. In the figure, the D band near 1350 cm−1 indicated a defective structure due to the sp3 hybridization of carbon atoms, whereas the G band at 1580 cm−1 indicated a defect-free structure due to the sp2 hybridization of carbon atoms. Graphite is a hexagonal honeycomb lattice formed by sp2 carbon bonds, so the vibration of the carbon atoms produces a characteristic peak (G band) at 1566 cm−1. When graphite is oxidized, the graphene surface is grafted with numerous –OH, –COOH, and C–O–C to form GO, and the original sp2 hybridization of the carbon atom bonding converts to sp3 hybridization bonding, damaging the structure and resulting in defects. The change in the electron structure causes the 2D band near 2700 cm−1 to disappear and a D band to appear at 1359 cm−1, with the peak G band being at 1570.3 cm−1. Fig. 5 shows that the 10

composite still retained D and G band signals when GO was mixed with various proportions of macrocyclic Co complexes, indicating that after the macrocyclic Co complex-grafted GO was made into GO/Co CE, it still retained the features of the RGO structure. Fig. 6 displays the C1s XPS results for GO and the various GO/Co CEs. Fig. 6(a) shows the C1s spectrum of the GO, which revealed three absorption peaks with binding energies (BEs) of 284.5 eV for C=C/C–C, 286.2 eV for C–O, and 288.2 eV for –COOH. The observation of C–O and –COOH in the XPS image of the GO indicates that graphite was effectively chemically transformed into GO. Figs. 6(b) and 6(c) are the C1s spectra for the GO/Co (1:1) and GO/Co (1:3) CEs, respectively, and show characteristic peaks consistent with that of GO after the analysis of the C1s spectra, with C–O still appearing at 286.2 eV. However, the integral area of –COOH at 288.2 eV was obviously smaller, indicating an incomplete reaction between the macrocyclic Co complex and GO in the GO/Co (1:1) and GO/Co (1:3) CEs, which caused most of the GO to retain its functional group. Fig. 6(d) shows the C1s spectra of the GO/Co (1:10) CE, with BEs of 284.5 eV for C=C/C–C, 285.7 eV for C–N, and 288.2 eV for –COOH. Compared with Fig. 6(a), the GO underwent a strong reduction reaction that led to the loss of its C–O and –COOH bonds. Therefore, the C–O at 286.2 eV disappeared, whereas the integral value of the –COOH peak area at 288.2 eV decreased significantly. In addition, the appearance of C–C and C–N bonds indicated the presence of macrocyclic Co complexes, which also verified that the complex grafted onto the GO through an oxidative dehydrogenation reaction. Fig. 7 shows the full spectrum XPS results for GO and various GO/Co CEs. The peak binding energy (BE) values of various GO/Co CEs were 285, 399, 532, and 780 eV; these BEs indicated the presence of C1s, N1s, O1s, and Co2p3, respectively. The elemental content and C/O ratio for various CEs obtained from the XPS peak intensities are shown in Table 1. Regarding GO, graphite was converted into GO with oxygen-containing functional groups; thus, the C/O ratio of GO was 2.02. For GO/Co CEs, the appearance of the Co2p3 characteristic peak indicated that the macrocyclic Co complex had grafted onto the GO. In the GO/Co composite, the redox reaction between the GO and macrocyclic Co complex resulted in bonding between the oxygen-containing functional groups in the GO and the secondary amine in the complex, leading to a decline in the oxygen-containing functional groups of

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GO and a drop in oxygen content. Therefore, the C/O ratio of the GO/Co composite was larger than that of the GO, and the C/O ratio increased with the proportion of macrocyclic Co complexes. Fig. 8(a) shows the XRD results for the various GO/Co CEs. The diffraction peaks of the GO/Co(1:1), GO/Co(1:3), and GO/Co (1:10) nanocomposites appeared at 8.8°, 7.4°, and 6.8°, respectively, indicating an increase in the spacing between GO layers was the ratio of macrocyclic Co complexes increased. This resulted in weaker XRD diffraction peaks due to the grafting of the complex on the GO, which led to a decline in the GO’s crystallinity. Fig. 8(b) displays XRD images of the GO/Co nanocomposites, which were prepared under high-temperature sintering at 500 °C for 30 min under N2. The high temperature led to the decomposition of GO (–OH, –COC–, –COOH) and the functional group of the macrocyclic Co complexes, which explains the absence of the GO peak in the figure. The decomposition of the macrocyclic structure in the sintered GO/Co resulted in a diffraction peak at 25.9°, which was similar to that of graphite, indicating that the GO had reduced to RGO. In addition, Fig. S2 (in Supplementary Materials) shows the thermal gravimetric analysis (TGA) curves of the GO/Co (1:1), GO/Co (1:3), GO/Co (1:10), GO, and Co macrocyclic complex. Table S1 shows the decomposition temperatures (Td) of various materials obtained from the TGA measurement. The TGA results confirm that the high temperature sintering process (500 °C) led to the decomposition of GO (–OH, –COC–, –COOH) and the functional group of the macrocyclic Co complexes. Fig. 9 shows the CV results for the various GO/Co CEs. The figure displays two pairs of redox peaks: the left pair represents the redox reaction of I3− + 2e− ↔ 3I−, whereas the right pair represents the redox reaction of 3I2 + 2e− ↔ 2I3−. Oxidation (reduction) reactions occurred at 0.4–1.0 V and 1.0– 1.4 V (1.0–0.6 V and 0.4–0.1 V) for the GO/Co (1:1) CE, 0.4–1.0 V and 1.1–1.4 V (1.0–0.5 V and 0.5–(−0.1 V)) for the GO/Co (1:3) CE, and 0.5–1.0 V and 1.1–1.4 V (1.0–0.6 V and 0.5–(−0.1 V)) for the GO/Co (1:10) CE. These results indicated that for each GO/Co nanocomposite, an increase in the ratios of the macrocyclic Co complexes increased the reduction current density, as shown in Fig. 9, with the GO/Co (1:10) CE exhibiting the highest reduction current density. The reduction current density of the GO/Co (1:10) was higher than that of Pt, indicating that the GO/Co (1:10) exhibited exceptional redox and electrochemical catalytic capacities. The CV results yielded the following 12

findings: in the redox reaction of I−/I3−, the GO/Co (1:3) and GO/Co (1:10) CEs demonstrated superior electrode activity and reversibility compared with the GO/Co (1:1) CE, which could be attributable to the even distribution of Co nanoparticles on the RGO surface and an increase in the number of Co particles formed by increasing the number of bonding sites between the GO and macrocyclic Co complexes, thus effectively enhancing the electrochemical catalytic capacity of the GO/Co (1:10) CE. 3.2 Analysis of the DSSC Device Characteristics The GO/Co CEs and conventional Pt CEs were subjected to photoelectric characterization. Figs. 10(a) and 10(b) show the J–V and IPCE curves, respectively, for DSSCs with various CEs. Table 2 lists the photoelectric characteristics of the DSSCs, including the short circuit current (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (Eff.). The results for the DSSC device with the Pt CE were as follows: JSC = 16.87 mA/cm2, VOC = 0.72 V, FF = 0.59, and Eff. = 7.24%. Under the same fabrication conditions, the results for the DSSC devices with the GO/Co composites were as follows. GO/Co (1:1) CE: JSC = 16.60 mA/cm2, VOC = 0.71 V, FF = 0.31, and Eff. = 3.73%; GO/Co (1:3) CE: JSC = 17.83 mA/cm2, VOC = 0.71 V, FF = 0.53, and Eff. = 6.79%; GO/Co (1:10) CE: JSC = 17.34 mA/cm2, VOC = 0.76 V, FF = 0.57, and Eff. = 7.48%. These results showed that among the GO/Co CEs, the GO/Co (1:10) CE exhibited the optimal efficiency, which was even higher than that of the Pt CE. In addition, among the three GO/Co CEs, the DSSC with the GO/Co (1:10) CE had the highest Voc and FF, mainly because the CE surface had more Co nanoparticles with a denser distribution. Therefore, the GO/Co (1:10) exhibited superior electric conductivity, catalytic capacity, and redox capacity. Because the GO/Co (1:10) CE was more efficient and cheaper than the Pt CE, this composite could potentially be used as a replacement for Pt electrodes. Previous studies have reported that using graphene-based composite materials as CEs enhanced the charge transfer reaction and catalytic activity of the CEs [42, 43]. In this study, macrocyclic Co complexes were used to react with the GO, whereby the molecular grafting of Co complexes would lead to a uniform distribution of Co catalytic sites on the surface of reduced GO. GO provided a high specific surface area and oxygen-containing functional groups, and the successful grafting of macrocyclic Co complexes on the RGO promoted the even distribution of the Co nanoparticles on the RGO surface. 13

The synergistic effect between GO and the macrocyclic Co complex may enhance the electron transport rate, electrical conductivity, and catalytic capacity of the CEs. Fig. 10(b) shows the IPCE results for the DSSC devices with various CEs. Because the same N719 dye was used throughout this study, the ranges of the absorbance wavelength for the devices were identical, with the only difference being in the IPCE values. The results showed that among the three GO/Co CEs, the GO/Co (1:3) and GO/Co (1:10) CEs exhibited higher IPCE peak values (91.8% and 90.6%, respectively) than the GO/Co (1:1) CE (82.4%) and Pt CE (83.9%). The IPCE results were consistent with the trend in the JSC values in the J–V measurements. This study further analyzed the EIS characteristics of the DSSCs. Fig. 10(c) shows the EIS Nyquist plot of the DSSC devices with Pt and GO/Co CEs. EIS is useful for analyzing the charge transport characteristics of the various interfaces in DSSC devices, such as the electron transfer and charge recombination in the TiO2/dye/electrolyte interface, the electron transfer impedance in the TiO2 electrode, the charge recombination in the CE/electrolyte interface, and the transfer of I3− in the electrolyte. The frequency range studied (0.1 Hz–1 MHz) is generally split into three regions: a small semicircle in the lowest frequency range (0.1 Hz–1 Hz), a larger semicircle in the intermediate frequency range (1 Hz–1 kHz), and a smaller semicircle in the highest frequency range (>1 kHz). When light and voltage are applied to the DSSC, the small semicircle in the lowest frequency can indicate the ion diffusion impedance in the electrolyte (WD), the large semicircle in the intermediate frequency corresponds to the charge transfer impedance in the TiO2/dye/electrolyte interface (Rct), and the smaller semicircle in the highest frequency corresponds to the charge transfer impedance in the CE/electrolyte interface (RCE). To extract the quantitative impedance characteristics of the DSSCs, an equivalent circuit model (Fig. 10(c)) was used to analyze the internal impedance of the DSSCs. The extracted quantitative impedance parameters from the EIS Nyquist plots of the devices are listed in Table 3. The EIS results indicated that in the semicircle in the highest frequency region, the GO/Co (1:10) CE had the smallest impedance value; in the semicircle in the intermediate frequency region, the impedances of GO/Co (1:3) and GO/Co (1:10) CEs were lower than that of the GO/Co (1:1) CE; in the low frequency region, the GO/Co (1:10) CE exhibited the lowest impedance value. In general, the impedance values were in the descending order of GO/Co (1:1) > GO/Co (1:3) > GO/Co (1:10), 14

with the GO/Co (1:10) CE exhibiting the lowest impedance among the three GO/Co CEs, indicating that it possessed the optimal electrical conductivity and electrochemical catalytic activity. This is attributable to the GO/Co (1:10) composites having a higher proportion of macrocyclic Co complexes possessing more bonding sites with the GO’s surface. The CEs fabricated with GO/Co (1:10) composites could therefore enable rapid electron transfer between the RGO and Co nanoparticles. When the GO/Co nanocomposites were involved in the electrolyte reaction, the reduction of I3− into I− caused the electrons to diffuse to the electrolyte, resulting in dye reduction. Therefore, an increase in the Co metal nanoparticles can accelerate the diffusion of the I−/I3− redox couple, thereby enhancing the electrocatalytic performance. This study further prepared DSSCs fabricated with GO/Co (1:15) CEs. The characteristics of the DSSC based on the GO/Co (1:15) CE were analyzed by J–V, IPCE, and EIS measurements, respectively. Fig. 10(a) and Table 2 exhibit the J–V characteristics of the DSSC fabricated with GO/Co (1:15) CE, the JSC, VOC, and FF of which were 16.38 mA/cm2, 0.72 V, and 0.58, respectively, with an efficiency of 6.84%. Fig. 10(b) shows the IPCE result for the DSSC based on GO/Co (1:15) CE. The GO/Co (1:15) CEs exhibited an IPCE peak value of 80.7%. Fig. 10(c) and Table 3 show the EIS results of the DSSCs with GO/Co (1:15) CEs. The EIS results indicated that the impedance value of the GO/Co (1:15) CE was larger than that of the GO/Co (1:3) CE and GO/Co (1:10) CE, indicating a decline in charge transfer reaction and catalytic activity in the GO/Co (1:15) CE. The results showed that the power conversion efficiency of the DSSC based on GO/Co (1:15) CE was lower than that of the GO/Co (1:10) CE, indicating that adding excess macrocyclic Co complexes on the GO may raise the aggregation and uneven distribution of the Co complexes, which decreased the electrocatalytic performance of the GO/Co (1:15) CE [31, 32]. Of the experimental DSSCs, the DSSC with GO/Co (1:10) CE demonstrated the highest efficiency, indicating that this ratio enables the most favorable electrochemical catalytic capacity. Therefore, it can be used to replace conventional Pt CEs and decrease the costs of DSSCs. 4. Conclusions

15

GO/Co nanocomposites were prepared by using various proportions of macrocyclic Co complexes as reductants to react with GO. The functional groups on the GO surface were reduced to form RGO, and the macrocyclic ligands of Co complexes were oxidized to form a conjugated macrocyclic system through an oxidative dehydrogenation reaction. The driving force for inducing the grafting and deposition of macrocyclic complexes on the surface of RGO includes the interaction of carboxylic acid and epoxide functional groups with the secondary amine of the macrocyclic ligand, the redox reaction between GO and Co macrocyclic complexes, and the - interaction between the aromatic system of RGO and conjugated Co macrocyclic molecules. The surface morphology and structural composition of the nanocomposites and their influences on the efficiency of the DSSC devices were investigated. The surface morphologies of the GO/Co (1:1), GO/Co (1:3), and GO/Co (1:10) CE films were analyzed individually. SEM revealed that increasing the proportion of macrocyclic Co complexes increased the number of Co particles and resulted in a more even distribution on the graphene. Regarding the structural identification analysis, FTIR revealed that the macrocyclic Co complexes had successfully grafted onto the graphene surface. The EDS results demonstrated that increasing the proportion of macrocyclic Co complexes increased the Co weight ratio in the electrodes. The Raman results show that the macrocyclic Co complexes caused the reduction of GO, thereby reducing GO to RGO. The XPS results showed that because the GO underwent a reduced reaction with macrocyclic Co complexes, the oxygen-containing functional groups on the GO reacted with the secondary amine on the macrocyclic Co complexes, leading to a decline in the oxygen-containing functional groups on the GO and thus a decrease in the oxygen content. Therefore, the C/O ratio of the GO/Co composites was higher than that of pure GO, and the C/O ratio and percentage of Co increased with the proportion of the macrocyclic Co complexes. The CV results revealed that the GO/Co (1:10) exhibited the optimal electrochemical catalytic effect. According to the results of the DSSC device analysis, the DSSC with a GO/Co (1:10) CE exhibited the following characteristics: JSC = 17.34 mA/cm2, VOC = 0.76 V, FF = 0.57, and Eff. = 7.48%. These results are superior to those obtained using a DSSC device with a Pt CE. The superior performance, low material cost, and simple fabrication process indicate the potential of GO/Co CEs over their Pt counterparts, particularly in replacing Pt CEs. Using GO/Co CEs will reduce the fabrication costs of DSSCs and thus improve their market competitiveness.

16

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[41] D.H. Busch, J.C. Bailar, The iron(I1)-methine chromophore, J. Am. Chem. Soc. 78 (1956) 11371142. [42] F. Gong, H. Wang, Z.-S. Wang, Self-assembled monolayer of graphene/Pt as counter electrode for efficient dye-sensitized solar cell, Phys. Chem. Chem. Phys. 13 (2011) 17676-17682. [43] F. Gong, Z. Li, H. Wang, Z.-S. Wang, Enhanced electrocatalytic performance of graphene via incorporation of SiO2 nanoparticles for dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 17321-17327. Table and Figure Captions Table 1 Elemental composition data of GO, GO/Co (1:1), GO/Co (1:3), and GO/Co (1:10) obtained from the XPS analysis. Table 2 Photovoltaic characteristics of DSSCs based on various counter electrodes. Table 3 The extracted quantitative impedance parameters from the EIS Nyquist plots of the DSSCs. Scheme 1 Preparation processes of (a) graphene oxide, (b) 2,3-butanedihydrazone, (c) octaaza-bis-αdiimine macrocyclic Co complex, and (d) graphene/macrocyclic Co complex-based nanocomposites. (e) DSSCs fabricated with the GO/Co nanocomposite CEs. Fig. 1 SEM images of (a) GO/Co (1:1), (b) GO/Co (1:3), and (c) GO/Co (1:10) nanocomposite CEs observed at magnification of 10k ; and (d) GO/Co (1:1), (e) GO/Co (1:3), and (f) GO/Co (1:10) nanocomposite CEs observed at magnification of 30k . Fig. 2 TEM images of (a) GO/Co (1:1), (b) GO/Co (1:3), and (c) GO/Co (1:10) nanocomposite materials observed at magnification of 100k ; and (d) GO/Co (1:1), (e) GO/Co (1:3), and (f) GO/Co (1:10) nanocomposite materials observed at magnification of 400k . Fig. 3 FTIR spectra of (a) GO, (b) GO/Co (1:1), (c) GO/Co (1:3), (d) GO/Co (1:10), and (e) [Co (C18H30N8)](BF4)2. Fig. 4 EDS results of (a) GO/Co (1:1), (b) GO/Co (1:3), and (c) GO/Co (1:10) CEs. Fig. 5 Raman spectra of GO and various GO/Co nanocomposite CEs. 21

Fig. 6 C1s XPS results of (a) GO, (b) GO/Co (1:1), (c) GO/Co (1:3), and (d) GO/Co (1:10). Fig. 7 Full spectrum XPS results of (a) GO, (b) GO/Co (1:1), (c) GO/Co (1:3), and GO/Co (1:10). Fig. 8 XRD spectra of various GO/Co nanocomposite CEs measured (a) before and (b) after annealing process. Fig. 9 Cyclic voltammograms of various CEs. Fig. 10 (a) J–V characteristics, (b) IPCE spectra, and (c) EIS Nyquist plots of DSSCs based on various CEs.

22

Table 1

Counter Electrode

Carbon

Oxygen

Nitrogen

Co

C/O Ratio

GO

66.9

33.1

-

-

2.02

GO/Co (1:1)

65.4

26.3

7.9

0.5

2.49

GO/Co (1:3)

62.8

22.7

13.4

1.2

2.77

GO/Co (1:10)

63.0

19.1

16.6

1.2

3.29

23

Table 2

24

Counter Electrode

JSC (mA/cm2)

VOC (V)

Fill Factor

Efficiency (%)

Pt

16.87

0.72

0.59

7.24

GO/Co (1:1)

16.60

0.71

0.31

3.73

GO/Co (1:3)

17.83

0.71

0.53

6.79

GO/Co (1:10)

17.34

0.76

0.57

7.48

GO/Co (1:15)

16.38

0.72

0.58

6.84

25

Table 3

Counter Electrode

RS (Ω)

RCE (Ω)

Rct (Ω)

WD (Ω)

Pt

21.15

40.81

53.29

24.12

GO/Co (1:1)

21.96

42.13

107.56

23.45

GO/Co (1:3)

21.75

35.53

75.39

22.56

GO/Co (1:10)

21.43

22.62

78.42

21.46

GO/Co (1:15)

21.27

37.29

92.14

24.73

26

Scheme 1

27

28

Figure 1

29

Figure 2

30

Figure 3

31

32

Figure 4

33

Figure 5

34

Figure 6

35

36

Figure 7

37

Figure 8

38

39

Figure 9

40

Figure 10

41

42

43