Thermally Stable Boron-Doped Multiwalled Carbon Nanotubes as a Pt

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Thermally Stable Boron-doped Multi-walled Carbon Nanotubes as a Pt-free Counter Electrode for Dye-sensitized Solar Cells Yow-An Leu, Min-Hsin Yeh, LuYin Lin, Ta-Jen Li, Ling-Yu Chang, Sheng-Yen Shen, Yan-Sheng Li, Guan-Lin Chen, Wei-Hung Chiang, Jiang-Jen Lin, and Kuo-Chuan Ho ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01895 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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ACS Sustainable Chemistry & Engineering

Thermally Stable Boron-doped Multi-walled Carbon Nanotubes as a Pt-free Counter Electrode for Dyesensitized Solar Cells Yow-An Leua,b,§, Min-Hsin Yehb,c,§, Lu-Yin Lind*, Ta-Jen Lib, Ling-Yu Changa, Sheng-Yen Shena, Yan-Sheng Lic, Guan-Lin Chenc, Wei-Hung Chiangc*, Jiang-Jen Lina*, and Kuo-Chuan Hoa,b* a

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617,

Taiwan b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan

c

Department of Chemical Engineering, National Taiwan University of Science and Technology,

Taipei, 10607, Taiwan d

Department of Chemical Engineering and Biotechnology, National Taipei University of

Technology, Taipei 10608, Taiwan §

These authors contributed equally to this work.

KEYWORDS Boron-doped; Carbon nanotube; Counter electrode; Dye-sensitized solar cell; Polymeric dispersant; Pt-free catalyst.

ACS Paragon Plus Environment

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ABSTRACT

A polymeric dispersant is usually used for preparing the uniform multi-walled carbon nanotube (CNT) film for replacing Pt as the catalyst on the counter electrode (CE) of dye-sensitized solar cells (DSSCs). However, it is necessary to remove the polymeric dispersant completely due to its nonconductive property. Herein, boron-doped CNT (BCNT) with enhanced thermal stability as the benefit to the dispersant removal was prepared as the catalyst on the CE of a DSSC, and the cells with pristine CNT and Pt as the catalyst on the CEs were compared. The BCNT based DSSC shows the best light-to-electricity conversion efficiency (η) of 7.91 ± 0.21%, which is higher than that of the cell with a CNT CE (η = 6.02 ± 0.19%) and is comparable to that of the cell with a Pt CE (η = 8.03 ± 0.11%).


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INTRODUCTION The counter electrode (CE) is an essential component in the dye–sensitized solar cell (DSSC) to convert I3– to I– in the electrolyte and thereby completes the cycle of operation of the cell.1 Platinum is usually employed as the catalyst of CEs due to its high conductivity, stability, and high electrocatalytic activity for the I3– reduction.2-5 However, applying the expensive platinum leads to the high cost of DSSCs. Recently, carbon materials are widely studied to replace Pt due to their advantages of low cost, high conductivity, and good electrocatalytic ability for the reduction of I3- ions.6-11 Various allotropes and structure including carbon black (CB),12 carbon nanotube (CNT),13,

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and graphene,15-17 carbon nanotube/graphene composite18 have been

proposed as the substitute to Pt. To further improve the chemical and physical properties of carbon materials, heteroatom doping such as nitrogen and boron doping are applied. The heteroatom doping creates more edge sites and oxygen-rich function groups, resulting in better electrocatalytic properties.19 Semimetallic characteristics, reduced resistivity, improved thermal stability, and reduced thermal conductivity were observed for the doped carbon materials.20-25 N-doped carbon materials, e.g., N-doped graphene26-30, N-doped CNT31, 32 and N-doped graphene/CNT composite33 have been proposed for DSSCs to improve the light-to-electricity conversion efficiency due to their better conductivity and electrocatalytic abilities. However, only few literature reported the effect of boron-doped carbon materials as the CE of DSSCs. Recently, Haiqiu et al. used boron-doped graphene as the catalyst on the CE of DSSCs based on an I−/I3− redox couple to achieve an enhanced light-to-electricity conversion efficiency (η) of 6.73% due to the better electrocatalytic ability and conductivity of the boron-doped carbon materials.34 Jung et al. applied boron-doped


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graphene as the CE of DSSCs based on cobalt redox couple to obtain an improved η of 9.21% due to its low charge transfer resistance and better electrochemical stability.35 On the other hands, to fabricate homogeneous carbon films through drop coating process, the solution with carbon materials used should be well dispersed. Smith et al. reported the surface composition of CNT which plays a crucial role in determining the colloidal stability in aqueous solution.36 To further increase the dispersibility of CNT, the dispersant is necessary to add in the solution with carbon materials. Without the dispersant, the aggregated carbon materials in the coated film would retard the electron transfer through the film and, therefore, lower the device efficiency.37 Several published literature have evoked the effect of adding the dispersant on the homogeneity of the carbon film and the resultant device efficiency.

38, 39

For example, Ma et al.

introduced graphene oxide as a surfactant to obtain the stable colloidal dispersion of carbon nanotubes for fabricating CNT-based CE for DSSC.40 According to our previous works, the family of imide type polymeric dispersant, poly(oxyethylene)-segmented imide (POEM), for the dispersion of carbon materials was proposed. It contains the organic functionalities of aromatic/imide ring, which provides π-π stacking and the ethylene oxide (EO) segment for forming interaction with the medium. This POEM was widely used in dispersing nanomaterial such as carbon nanotube,41-43 graphene,44 carbon black,39 platinum nanoparticle,45 and further applied to the counter electrode of DSSC for controlling the film morphology and ultimately achieving the high power conversion efficiency. However, from the point view of the thin film fabrication, the polymeric dispersant as a template should be further removed after coating the uniform thin film. Generally, the heat treatment or the solvent washing method is often utilized to remove the residual dispersant.46 In our previous work, POEM was applied to disperse the platinum nanoparticle and CNT materials, and 390 oC was chosen as the optimized annealing


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temperature to remove the nonconductive POEM in these composite films for obtaining an efficient thin film for the CE of the DSSC with an η of 8.47%.42 Higher temperature could remove more residual POEM, however, the low degradation temperature of CNT limits the process for high-temperature annealing. The remaining POEM lowers the conductivity of the film, thus limiting the eventual device efficiency. Herein, boron-doped CNT (BCNT) with enhanced thermal stability as the benefit to the polymeric dispersant, POEM removal was achieved and further applied as the catalyst on the CE of a DSSC. During the film preparing process, the BCNT with higher thermal stability to sustain higher temperature (500 oC) than CNT (400 oC) were observed. These films were further applied as CE in DSSC. BCNT based DSSC shows the best light-to-electricity conversion efficiency (η) of 7.91 ± 0.21%, which is higher than that of the cell with a CNT CE (η = 6.02 ± 0.19%) and is comparable to that of the cell with a Pt CE (η = 8.03 ± 0.11%). The better performance for the BCNT based DSSC is attributed to the higher conductivity and better catalytic ability resulting from the fewer POEM residues in the catalytic film and the more defects produced by boron doping, respectively. The effect of the extent of POEM removal and the boron atom doping on the film conductivity and device efficiency were further discussed. EXPERIMENTAL SECTION Materials Poly(oxyethylene)–diamine or poly(oxypropylene–oxyethylene–oxypropylene) segmented polyether of bis(2–aminopropyl ether) (POE2000, M.W.=2,000) is a crystalline and water– soluble compound (waxy solid, melting point 37–40 oC, amine content 0.95 meq g–1) with a formula of 39 oxyethylene and 6 oxypropylene units. Tetrahydrofuran (THF, 95%) was


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purchased from Teida Chemicals. Lithium iodide (LiI, synthetical grade), iodine (I2, synthetical grade), and poly(ethylene glycol) (PEG, M.W.=20,000) were obtained from Merck. Monomer, 4,4’–oxydiphthalic anhydride (ODPA, 97% purified by sublimation), lithium perchlorate (LiClO4), acetonitrile (ACN, 99.99%) isopropyl alcohol (IPA, 99.5%), 2–methoxyethanol (≥99.5%), and ethanol absolute (≧ 99.8%) were obtained from Sigma-Aldrich. Titanium(IV) isopropoxide (TTIP, >98%), ethanol (EtOH, absolute), 4-tert-butylpyridine (TBP, 96%), and tert-butanol (tBuOH, 99.5%) were acquired from Acros. 3-Methoxypropionitrile (MPN, 99%) was obtained from Fluka. Boron trioxide powder (B2O3, ACS reagent, ≧99%) and sodium dodecyl sulfate (SDS) were purchased from Alfa Aesar (Lancashire, United Kingdom). Cis– diisothiocyanato–bis(2,2’–bipyridyl–4,4’–dicarboxylato)ruthenium

(II)

bis(tetra–

butylammonium) (N719 dye), 1,2-dimethyl-3-propylimidazolium iodide (DMPII) and TiNanoxide HT/SP (particle size 13 nm) were received from Solaronix S. A., Aubonne, Switzerland. Deionized water (DI–water) with a resistivity≧18.2 MΩ–cm was produced by Purelab Maxima (ELGA, UK). All the chemicals were used as received without further treatments. Synthesis of polymeric dispersant (POEM) The requisite dispersant poly(oxyethylene)-segmented imide (POEM) in this work was synthesized from an aromatic dianhydride and a poly(oxyethylene)-diamine, with the optimized molar ratio of 5 : 6. The method is similar to that described previously.44 To a 100 mL, threenecked, round-bottomed flask (RB flask), equipped with a mechanical stirrer, nitrogen inletoutlet lines, and a thermometer, 6.0 mmol of POE 2000 (12.0 g in 15 mL of THF) was added. Then 5.0 mmol of ODPA (1.6 g in 10 mL of THF) was added to the RB flask through a funnel in a drop-wise manner. The reactants were gradually heated to 180 oC for 3 h to completely form


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imide functionalities from the intermediates of cyclic amido acids. During the reaction, samples were taken periodically and monitored by FT-IR analyses. Finally, the reaction was terminated by cooling to room temperature. The product mixture was then subjected to a rotary evaporation to remove the solvent under reduced pressure. The product, namely POEM, was recovered as a yellowish waxy solid. The synthesis of POEM is represented in Figure S1a. The Fourier transform infrared (FTIR) results during the process are shown in Figure S1b, characteristic absorptions were identified at 1556, 1647 and 1720 cm-1 for amide and 1713 and 1770 cm-1 for imide functionalities, during the progression of the reaction. CNT synthesis The CNTs used in the present study were synthesized using a catalytic chemical vapor deposition (CVD). Details of the CNT growth process were similar to that described previously.47 In brief, Fe films (3.0 nm thickness) and an alumina (Al2O3) support layer (40 nm thickness) sputtered onto 2 cm × 2 cm polished silicon (Si) substrates with a silicon dioxide (SiO2) layer of 600 nm were used as the catalyst films for CNT growth. The CNTs were synthesized at one atmospheric pressure in a 3-inch quartz tube furnace with two process steps, including catalyst particle formation and CNT growth. For a typical catalyst particle formation experiment, we first flowed 200 sccm (sccm denotes standard cubic centimeter per minute at 1 atm) helium (He) and 1800 sccm hydrogen (H2) for 15 mins while ramping the temperature from room temperature to 810 oC, then keep the same gas flow rates for 15 minutes to anneal the catalyst particles. Then CNT growth began for 10 mins using a water-assisted CVD process at 810 °C with the gas mixture of 100 sccm ethylene (C2H4), 900 sccm H2, and 100 ppm water vapor as the carbon precursor, catalyst preserver, and enhancer, respectively. Water vapor of 100 ppm was supplied by passing 1000 sccm He carrier gas through a water bubbler with deionized


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(DI) water at STP (STP denotes standard condition for temperature and pressure, NIST version) condition. Water vapor concentration was monitored by a single-channel moisture meter (General Electric, MMS 35-211-1-100) coupled with a moisture probe (General Electric, M2LR) installed before the CVD reactor. All gas flows were controlled by mass flow controllers that were carefully calibrated before experiments to precisely control the gas concentrations in the CVD reactor. BCNT synthesis We develop a solution-assisted nanotube-substitution reaction approach for the highly efficient synthesis of BCNTs in bulk quantities. Details of the BCNT preparation were similar to that described previously.24 An initial and key step of this approach was dispersion of the starting CNTs in a 1 wt% SDS aqueous solution at 80 oC. We then dissolved B2O3 powder in the CNT/SDS dispersion at 80 oC and stirred using a magnetic-stirred bar under 500 rpm. After heating and degassing at 90 oC, the CNTs and the boron precursors were filtered by PTFE membrane with a pore size of 1 µm. The material on top of the membrane was collected and dried into a mixed powder. The mixed CNT containing boron precursor was placed in an alumina boat and introduced into an alumina tube reactor filled with Ar gas at atmospheric pressure. The system is first heated for 2 h at 150 oC to remove all the moisture with the Ar gas flowing at a rate of 300 sccm. Subsequently the temperature is raised to 1,000 oC for 4 h. In our experiment, after the furnace is cooled down the material is retrieved from the boat and the possible excess of physisorbed boron/boron oxide is removed by washing with hot water several times. The washed materials were filtered by the polytetrafluoroethylene (PTFE) membrane with a pore size of 1 µm. The material on top of the membrane was collected and dried into a mixed powder. In this study, the boron-doped CNTs will be denoted as BCNTs.


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Characterization of the as-prepared CNT and BCNT Ex-situ micro-characterizations of starting CNT and as-synthesized BCNT were performed by SEM, HRTEM, XRD, XPS, micro-Raman spectroscopy, and microfigure measuring instrument. SEM characterization was performed with a field-emission SEM (JEOL JSE-6500F, Japan). Cold-field emission Cs-corrected TEM (JEOL ARM-200F, Japan) with 200 kV accelerating voltage was used to observe the morphologies of both CNT and BCNT. Carboncoated copper grids (400 mesh) were used in the TEM sample preparation. XPS (VG ESCALAB 250, Thermo Fisher Scientific, UK) was performed using a monochromatic Al Kα X-ray radiation (10kV, 10 mA) to quantify the amount of boron doped into the CNTs. The source power was set at 72 W, and pass energies of 200 eV for survey scans and 50 eV for highresolution scans were used. Additional assessments of surficial characteristics of CNT and BCNT were attained by micro-Raman spectroscopy. Raman scattering studies were performed at room temperature with a Horiba Jobin Yvon LabRam HR800 spectrometer (λ = 633 nm). Spectra were normalized by the G-band intensity and averaged from 10 random positions on each sample. X-ray diffraction (XRD, Rigaku, Tokyo, Japan) analysis was performed using a Cu Kα radiation source, operated at 40 kV and 100 mA. A scan rate of 0.05 deg s-1 was employed for values of 2θ from 20 to 40°. Microfigure measuring instrument (Surfcorder ET3000) was used for measuring the thickness of the film. Preparation and characterization of counter electrodes Sputtered-Pt (s-Pt) CE was prepared by sputtering Pt for 30 nm on the tin-doped In2O3 (ITO, UR-ITO007-0.7 mm, 10 Ω sq.-1, Uni-onward Corp., Taipei, Taiwan) conducting substrate. The 10 mg of synthesized CNT or BCNT was suspended ultrasonically in a solution mixture of 5.5 mL ethanol/DI water (v/v = 1:1) and 150 mg POEM for 24 h. The slurries were drop coated on


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the FTO glass. Thermally stable FTO was selected as the conductive substrate for the CNT based electrodes. The loading amount of CNT catalyst on the FTO glass was optimized. The CNT film with 0.3 mg cm-2 loading, with a film thickness of 6.50 µm annealed at 400 oC, shows the best cell efficiency of 6.16% as shown in Figure S2 and Table S1. Cyclic voltammetry (CV) was performed to investigate the electrocatalytic abilities of the CEs. The CV was carried out with a three-electrode electrochemical system, by using an electrode with the film of CNT or BCNT as the working electrode, Pt foil as the counter electrode, and an Ag/Ag+ electrode as the reference electrode in an ACN solution, containing 10.0 mM LiI, 1.0 mM I2, and 0.1 M LiClO4.18, 48, 49 All the current densities were subtracted by the background current density in the CV measurements. A symmetric cell was used to investigate the electrocatalytic abilities of the CEs by electrochemical impedance spectroscopy (EIS)50 51 and Tafel-polarization curves52, 53. Preparation of TiO2 photoanode A fluorine-doped SnO2 (FTO, TEC-7, ~7 Ω sq.-1, NSG America, Inc., New Jersey, USA) conducting glasses were first cleaned with a neutral cleaner and then washed with deionized water, acetone, and isopropanol sequentially. TiO2 paste and the photoanode were prepared, as reported previously.54, 55 For this, 72 mL of TTIP was added to 430 mL of 0.1 M HNO3 with constant stirring, and the contents were heated to 88 oC for 8 h; the mixture was cooled down to the room temperature, and the resultant colloid was filtered and heated in an autoclave at 240 oC for 12 h. The TiO2 colloid was concentrated to 8 wt%, and PEG (25 wt% with respect to TiO2) was added to the colloid to prevent the film from cracking during drying. The conducting surface of the FTO was treated with a solution of TTIP in 2-methoxyethanol (weight ratio of 1:3) for obtaining a good mechanical contact between the conducting glass and the TiO2 film. The TiO2 film was made of three different layers, and a heat treatment of 450 °C for 30 min was applied


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after each coating of the TiO2 layer by using a doctor blade technique. The first layer of TiO2 was directly coated on the obtained FTO conducting glass by using a commercial TiO2 paste (TiNanoxide HT/SP), the second layer was applied on the first layer of TiO2 by using the above obtained colloidal TiO2 solution, and the third layer functioning as a scattering layer was coated on the second layer by using a paste made up of 100 nm and 20 nm TiO2 particles (PT-501A, Ya Chung industrial Co., Ltd., Taipei, Taiwan) with the weight ratio of 1:1. A portion of 0.4 × 0.4 cm2 was selected from the composite film as the active area by removing the side portions by scrapping. The TiO2 film was gradually heated to 450 oC (rate = 10 oC/min) in an oxygen atmosphere, and subsequently sintered at that temperature for 30 min. After sintering at 450 oC and cooling to 80 oC, the TiO2 electrode was immersed for 24 h in a 3×10-4 M N719 dye solution in a mixed solvent of ACN/tBA (volume ratio: 1/1). Cell assembly and measurements The above-prepared TiO2 electrode was coupled with the CE; these two electrodes were separated by a 60 µm-thick Surlyn® (SX1170-60, Solaronix S.A., Aubonne, Switzerland) and sealed by heating. A mixture of 0.1 M LiI, 0.6 M DMPII, 0.05 M I2, and 0.5 M tBP in MPN/ACN (volume ratio = 1:1) was used as the electrolyte. The electrolyte was injected into the gap between the two electrodes by capillarity, and the hole was sealed with hot-melt glue after the electrolyte injection. The assembled device was illuminated by a class A quality solar simulator (XES-301S, AM1.5G, San-Ei Electric Co., Ltd., Osaka, Japan) and the incident light intensity (100 mW cm-2) was calibrated with a standard Si Cell (PECSI01, Peccell Technologies, Inc., Kanagawa, Japan). Photoelectrochemical characteristics of the DSSCs were recorded with a potentiostat/galvanostat (PGSTAT 30, Autolab, Eco-Chemie, Utrecht, the Netherlands). RESULTS AND DISCUSSION


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Characterization of CNT and BCNT The morphology and atomic structure of the as-produced CNTs and BCNTs are investigated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). The SEM characterization indicates that the as-produced CNTs and BCNTs remained as the tubular structure after high-temperature treatment (Figure 1a and Figure 1c). Moreover, the representative high-resolution TEM (HRTEM) image reveals the long hollow tube structure of the as-produced CNTs and BCNTs with diameter around 10 nm and the absence of amorphous carbon coatings on its sidewalls (Figure 1b and Figure 1d).

Figure 1. TEM and FE-SEM images of the as-prepared CNTs (a, b) and BCNTs (c, d).


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Further structural information about the as-produced BCNTs can be obtained from X-ray diffraction (XRD) and Raman spectra measurements. In XRD patterns (Figure 2a), the asproduced BCNTs have a diffraction peak at 2θ = 25.87°, which is 0.02° higher than that of pristine CNTs. On the basis of Bragg's law, this result suggests that the d value of the asproduced BCNTs was smaller than that of pristine CNTs and can be attributed to the substitution of boron in the carbon network with the presence of BC3 domains slightly influencing the periodic atomic arrangement of the hexagonal carbon network.56 Raman spectra of pristine CNTs and as-produced BCNTs are shown in Figure 2b. Generally, the value of the intensity ratio of D


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Figure 2. (a) XRD pattern and (b) Raman spectra of pristine CNTs and as-prepared BCNTs; (c) XPS survey scan for pristine CNT and BCNT containing an enlarged plot for B1S peaks.


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band to G band (ID /IG) is used to estimate the degree of disorder and defects of carbon materials. The as-produced BCNTs shows higher ID/IG ratio of 1.54 than that of the pristine CNTs (ID/IG = 1.51), which is consistent with the previous study,57 confirming that boron atoms were doped into the carbon network and more defects were created. The element components and B-doping configuration within the as-produced BCNTs were further characterized by XPS. The survey scan of XPS for CNT and BCNT are illustrated in the Figure 2c. The peaks at around 190, 285, and 534 eV respectively were assigned to B1s, C1s, and O1s for the as-produced BCNTs while only peaks for C1s and O1s were obtained for pristine CNTs. This result supports that B atoms existed in the structure of the as-produced BCNTs. We carefully estimate the B dopant concentration in the as-produced sample using XPS elemental quantification and found that the B dopant concentration is 0.4 atomic percentage (at%) in the asproduced BCNTs. Figure 3a and Figure 3b show the representative HR-XPS results of C1s peak and B1s peak of the as-produced BCNTs. The high-resolution BCNT C1s scan indicated the presence of four types of carbon bonding, namely, C-C (284.6 eV), C-O (285.6 eV), C-O-B (289.4 eV) and C-B (283.9 eV). The C-B peak in HR-XPS result confirms the existence of bonding between boron and carbon in the BCNTs, indicating the successful doping of boron. For BCNT B1s scan, three types of boron bonding including BC3 (190 eV), BC2O (190.9 eV) and BCO2 (191.8 eV) are obtained. The fractions of each bonding type are summarized in Table 1. Significantly, the percentage of BCO2 bond is the highest (90.20%) in the as-produced BCNTs. Previous reports suggest that the distribution of various B–C bonds is critical to the electrochemical properties of boron doped carbons.58 The oxygen-containing BC2O and BCO2 dopant species can induce redox reactions and further enhance the electrochemical activity while


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the graphite-like BC3 dopant species plays a major role in improving the electronic conductivity.59

Figure 3. HR-XPS of (a) C1s peaks, and (b) B1s of the as-prepared BCNTs. Table 1 The percentages of XPS spectra peak deconvolution for CNTs and BCNTs.

Sample

C (at%)

C1s peaks O (at%)

CNT

99.55

0.45

0

BC3 (%) 0

BCNT

98.01

1.59

0.40

6.01

B (at%)

B1s peaks BC2O (%) 0 3.79

BCO2 (%) 0 90.20

Thermal properties of BCNT The thermal properties of POEM, CNT and BCNT were conducted by using the thermogravimetric analysis (TGA) as shown in Figure 4a, and the structure of POEM is shown in Figure 4b. The weight of POEM starts to lose at 200 oC, while the weight losses of CNT and BCNT occur at 500 oC. The weights of CNT and BCNT fail entirely at the temperature lower than 650 and 700 oC, respectively, indicating the better thermal stability of BCNT due to the relatively strong B-C bonds for this case.60 The pictures of the CNT and BCNT films annealed at


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100, 300, 400, 450 and 500 oC are shown in Figure 4c. With the annealing temperature below 400 oC, smooth films without any cracks were observed for both of the CNT and BCNT films. However, the CNT film ruptured gradually due to the degradation for the cases with the annealing temperature at 450 oC, and the film was almost removed from the substrate at 500 oC. On the contrary, the BCNT films remained unchanged even for the case annealed at 500 oC, again indicating its high thermal stability. The BCNT film annealed at 500 oC is expected to have better performance for the pertinent DSSC due to the less residual of POEM in the BCNT film since it is annealed at high temperature.

Figure 4 (a) TGA spectra of CNT, BCNT, and POEM, (b) the chemical structure of POEM, and (c) the photograph of CNT and BCNT films after annealing at different temperatures. Photovoltaic performance of the DSSCs with the CE containing CNT and BCNT with various annealing temperatures


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The CNT and BCNT electrodes are applied as the CE of the DSSCs. The performance of the DSSCs with CNT and BCNT electrodes annealed at 70, 300, 400, 450, and 500 oC were compared by considering the extent of the POEM dispersant removal and the effect of the boron atom doping. The results are shown in Figure 5 and Table 2. The fill factor (FF) and the shortcircuit current density (JSC) of the DSSC with the CNT or BCNT film annealed at 70 oC on its CE are relatively low since the insufficient heat treatment cannot completely remove the nonconducting polymeric dispersant, POEM, from the slurry. The unfavorable electron-transfer occurring in the film having the nonconductive POEM would lead to a low conductivity and a poor catalytic ability. Therefore, the relatively lower light-to-electricity conversion efficiencies (η) of 2.48 ± 0.21% and 2.57 ± 0.08% for the DSSCs with CNT and BCNT films annealed at 70 o

C on their CE were obtained respectively (Table 2). With the annealing temperature increasing

to 300 oC, both FF and JSC of the DSSCs with CNT and BCNT films are improved a lot, due to the gradual removal of the nonconductive POEM, resulting in enhanced η's of 4.95 ± 0.13% and 5.91 ± 0.05% for the DSSCs with CNT and BCNT films on their CE, respectively. The higher η of the DSSC with BCNT as compared to that of the cell with CNT is due to the better catalytic ability and more conductive boron doped into CNT for the former case. The doped boron not only creates more defects to act as active sites for catalyzing the reduction of I3- but also induce the semi-metallic characteristic which leads to a higher conductivity. Same results were found for the film annealing at 400 oC. The device with BCNT film as CE shows an efficiency of 6.53 ± 0.06%, superior to the device with CE of CNT film (6.02 ± 0.19%). However, for the cases with the annealing temperature of 450 oC, a large decrease in the performance of the DSSC with a CNT film on its CE was observed, while the enhanced performance of the cell with a BCNT film on its CE was found. The decrease in the η of the DSSC with a CNT film on its CE annealed


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Figure 5 Photocurrent density-voltage curves of the DSSCs with CEs containing the films of (a) CNTs, (b) BCNTs under various annealed temperatures, i.e., 70, 300, 400, 450 and 500 oC, obtained at 100 mW cm-2 (AM 1.5G).


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Page 20 of 35

Table 2 Photovoltaic parameters of the of the DSSCs with CEs containing the films of CNTs and BCNTs under various annealed temperatures, i.e., 70, 300, 400, 450 and 500 oC, obtained at 100 mW cm-2 (AM 1.5G). The sheet resistance (Rsh) of the corresponding films are also listed in the table.

VOC

JSC

CEs*

η

Rsh

(%)

(Ω/□)

FF

(V)

(mA cm-2)

CNT (70 oC)

0.64 ± 0.00

10.0 ± 0.6

0.39 ± 0.05

2.48 ± 0.21

2.79 x 10

CNT (300 oC)

0.68 ± 0.01

13.2 ± 0.21

0.54 ± 0.00

4.95 ± 0.13

1.11 x 10

CNT (400 oC)

0.70 ± 0.03

14.0 ± 0.3

6.02 ± 0.19

8.83 x 10

CNT (450 oC)

0.69 ± 0.12

14.0 ± 0.53

0.32 ± 0.05

3.07 ± 0.32

7.98 x 10

CNT (500 oC)

0.70 ± 0.02

7.20 ± 0.15

0.25 ± 0.01

1.24 ± 0.07

1.61 x 10

BCNT (70 oC)

0.64 ± 0.00

9.76 ± 0.52

0.42 ± 0.04

2.57 ± 0.08

1.99 x 10

BCNT (300 oC)

0.66 ± 0.00

14.6 ± 0.13

0.62 ± 0.01

5.91 ± 0.05

9.75 x 10

BCNT (400 oC)

0.70 ± 0.09

15.1 ± 0.24

0.70 ± 0.01

6.53 ± 0.06

5.56 x 10

BCNT (450 oC)

0.70 ± 0.00

16.3 ± 0.02

0.63 ± 0.00

7.21 ± 0.01

4.94 x 10

BCNT (500 oC)

0.72 ± 0.03

17.3 ± 0.5

0.63 ± 0.00

7.91 ± 0.21

4.59 x 10

Pt

0.70 ± 0.02

17.1 ± 1.1

0.67 ± 0.02

8.03 ± 0.11

4.38 x 10

0.61 ± 0.00

2

2 1 4 6 2 1 1 1 1

1

*All performance data are based on three DSSC samples.

at 450 oC is due to the decrease in its FF. Since the degradation of CNT starts to occur at 450 oC, the CNT film cannot sustain at this temperature, as observed in the images shown in Figure 4c that half of the CNT film was gone. On the contrary, for the DSSC with BCNT film on its CE, the better thermal stability of BCNT rendered a homogenous film on the CE, as shown in Figure 4c, and the high temperature would remove more POEM, resulting in the enhanced η of the


20

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pertinent DSSC. Furthermore, when the annealing temperature was raised to 500 oC, the CNT was all gone from the film, but the highly thermal stable BCNT still remained on the substrate, as shown in Figure 4c. The poor performance of the DSSC with the CNT film on its CE was obtained since there was barely CNT on the CE. However, the DSSC with the BCNT film on the CE annealed at 500 oC shows much better performance with the FF of 0.63 ± 0.00, JSC of 17.3 ± 0.5 mA cm-2, and η of 7.91 ± 0.21%. These values are higher than those of other cases with BCNT, because higher extent of the removal of the nonconductive POEM and more thermally stable catalyst of BCNT are obtained at this high temperature. Impressively, the η of the DSSC with the BCNT film annealed at 500 oC on its CE is comparable to that of the cell with sputtered Pt on its CE (η = 8.03 ± 0.11%). It can be concluded that the lower the annealing temperature, the more the residual of POEM left in the catalytic films. However, if the annealing temperature is too high, the CNT film would crack and damage severely. According to the performance of DSSC with CEs containing the films of CNTs and BCNTs under various annealed temperatures, the optimized annealing temperatures for fabricating CNT and BCNT electrodes were 400 and 500 oC, respectively. Even at the annealing temperature of 400 oC, the η of the DSSC with BCNT electrode (6.53 ± 0.06%) is still higher than the cell with CNT electrode (6.02 ± 0.19%), indicating the superior performance of the BCNT annealed below its high-temperature endurance limit mentioned above. For the comparison of the DSSC cells with the above two best CNT and BCNT films and Pt, they all had similar values of the open-circuit voltage (VOC), while the short-circuit current density (JSC) of cell was enhanced significantly from 14.0 ± 0.3 to 17.3 ± 0.5 mA cm-2 for the CNT and BCNT CEs, respectively. The JSC value of the DSSC with BCNT CE even exceeds that of Pt (17.1 ± 1.1 mA cm-2). It is expected that with boron doping in the CNT, more defect sites or


21


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Page 22 of 35

active sites can be created for the reduction of I3-, thus leading to the better catalytic ability of the BCNT CE. In addition, it is reported that the CNT with boron doping can inhibit the photoconversion reaction for converting hydroiodic acid to hydrogen triiodide in aqueous solutions which obstruct the reduction of I3-.61 The fill factor (FF) of the DSSCs with BCNT CE shows higher value of 0.63 ± 0.00 than that of CNT (FF = 0.61 ± 0.00), due to the less residual of the nonconductive POEM in the film which was annealed at higher temperature (500 oC) and the smaller resistivity owing to the reduced band gap with boron doping20 for the former case. The highest value of FF for the DSSC with the Pt CE results from its best electrical conductivity among these three cases. On the whole, the DSSC with BCNT CE achieved the highest η of 7.91 ± 0.21% which is superior to that of CNT (η = 6.02 ± 0.19%). The better performance of the DSSC with the BCNT CE is due to the less residual of the nonconductive POEM in the film, leading to higher electrical conductivity and better electrocatalytic ability owing to the more edge sites for the BCNT catalyst. Conductivities of CNT, BCNT, and Pt films with various annealing temperatures Four-point probe measurements were conducted to understand how the extent of POEM removal and boron doping affect the conductivity of the film. The sheet resistances (RSh) of the CNT and BCNT films annealed at 70, 300, 400, 450, and 500 oC were obtained and summarized in Table 2. The high sheet resistance values of 2.79 x 102 and 1.99 x 102 Ω/□ were respectively obtained for the CNT and BCNT films annealed at 70 oC, due to the residual of the nonconductive POEM. As the annealing temperature increases to 400 oC, the RSh of the CNT film decreases to a value of 8.83 x 101 Ω/□, since the extent of the removal of POEM is higher. However, higher RSh values of 7.98 x 104 and 1.61 x 106 Ω/□ were obtained for the CNT films annealed at 450 oC and 500 oC, respectively, resulting from the thermal instability of CNT which


22

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decomposed and vanished from the substrate. As for the BCNT films, the RSh value gradually decreases with increasing annealing temperature, due to the higher extent of the removal of the nonconductive POEM in the films and the more thermally stable BCNT. Therefore, the BCNT film annealed at 500 oC reaches the lowest RSh value of 4.59 x 101 Ω/□, which is comparable to that of the sputtered Pt film (4.38 x 101 Ω/□). CV, Tafel polarization curves and EIS for determining the electrocatalytic abilities of the counter electrodes with CNT, BCNT, and Pt films The electrocatalytic abilities of the CNT and BCNT films with best device performance were further evaluated. The electrocatalytic abilities for catalyzing the redox reaction of I3-/Iwere examined by the cycle voltammetry (CV) curves, using a three-electrode electrochemical system as shown in Figure 6. The redox reaction of I3-/I- occurring at the CE is shown as the following,62 I3- + 2e- ↔ 3I-

(1)

The electrocatalytic ability of a CE for the I3− reduction in a DSSC can be visualized in terms of its cathodic peak current density (ipc). The peak is not obvious for the CNT electrode, but a much clear peak with the ipc value closed to that of Pt electrode was observed for the BCNT electrode. The results indicate that the catalytic ability of BCNT electrode is better than that of the CNT electrode and even comparable to that of Pt electrode. The enhanced catalytic ability for promoting the I3- reduction is attributed to the synergetic effect of the oxygen-containing active site for BC2O and BCO2 dopant species in the BCNTs.59 Moreover, Fig. S3 shows the long-term stability data of the BCNT counter electrode (500 oC) for 100 continuous CV cycles, where the values of both anodic and cathodic peak current densities remained rather steady, suggesting the promising properties of the BCNT counter electrode.


23


-

2.0

-

3I

-

I3 + 2e

1.5

-2

Current density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 35

1.0 0.5 0.0 -0.5 -1.0

o

-1.5

-

-

I3 + 2e

CNT (400 C) o BCNT (500 C) Pt

-

3I

-2.0 -0.6

-0.4

-0.2

0.0

0.2

0.4

+ Potential (V vs. Ag/Ag ) Figure 6 Cyclic voltammograms of the electrodes with CNT, BCNT and Pt films, recorded in an electrolyte containing 10.0 mM LiI,1.0 mM I2, and 0.1 M LiClO4 in ACN. Moreover, the Tafel polarization and the electrochemical impedance spectroscopy (EIS) techniques were conducted using a symmetrical cell composed of two identical electrodes to investigate the electrocatalytic ability of the electrodes with CNT, BCNT, and Pt as the catalysts. Figure 7 shows the anodic and cathodic Tafel polarization curves represented by the logarithmic current density (Log I) versus the potential (V). The exchange current density of an electrode obtained from the slopes of cathodic or anodic curves in Tafel zone (the curve at the middle potentials with a sharp slope) reflects the electrocatalytic ability of the electrode. A smaller slope, along with a higher exchange current density, was obtained for the BCNT electrode, indicating the catalytic ability for the reduction of I3- to I- is more efficient for this case than that for the


24

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electrodes with CNT. Meanwhile, we also noticed that the slope for the BCNT electrode is close to that of Pt electrode, implying that the BCNT electrode exhibited promising electrocatalytic ability for catalyzing I3- reduction reaction.

Figure 7 Tafel curves of electrodes with CNT, BCNT and Pt films. In addition, Figure 8 shows the Nyquist plots of the symmetric cells with the first semicircle in the middle-frequency range (105~10 Hz) which represents the resistance against the heterogeneous electron transfer at the electrode/electrolyte interface (Rct), and the values of Rct were listed in the inset of Figure 8. The BCNT electrode shows lower Rct value than that of CNT, again indicating the better catalytic ability of BCNT due to the doped boron. Moreover, the Rct value of BCNT electrode is close to that of Pt electrode, confirming the comparable catalytic capabilities of these two catalysts.


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Page 26 of 35

Figure 8 EIS spectra of symmetric cells consisting of two identical electrodes with CNT, BCNT and Pt films, the starting points are normalized for Nyquist plot comparison and the inset shows the relevant equivalent circuit model and the values of Rct. CONCLUSIONS In summary, BCNTs were prepared by using a simple two-step postgrowth substitution reaction route under atmospheric pressure. It is noteworthy from a practical point of view that the proposed method is amenable to industrial-scale production since it avoids the need for a vacuum system. Due to the enhanced thermal stability of BCNT, a heat treatment with 500 oC as the annealing temperature was applied during the film preparation to remove the residual nonconductive dispersant POEM more efficiently. The resultant BCNT film shows better


26

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electrical conductivity and catalytic ability as compared to those for the CNT film. The DSSC with a BCNT CE achieved the highest η of 7.91 ± 0.21%, which is comparable to that for the cell with a Pt CE (8.03 ± 0.11%). This implies that the expensive noble metal and high cost sputtering facility can be totally eliminated. Even though our standard DSSC with a Pt CE only gave a cell efficiency of 8.03 ±0.11%, we believe that this work provides a possible solution to enhance the performance of DSSCs with carbon based counter electrodes. AUTHOR INFORMATION Corresponding Authors Tel: +886 2 2771 2171, Fax: +886 2 2731 7117, E-mail: [email protected] (Dr. L.-Y. Lin) Tel: +886 2 2737 6647, Fax: +886 2 2737 6644, E-mail: [email protected] (Dr. W.-H. Chiang) Tel: +886 2 3366 5312, Fax: +886 2 3366 5237, E-mail: [email protected] (Dr. J.-J. Lin) Tel.: +886 2 2366 0739, Fax: +886 2 2362 3040, E-mail: [email protected] (Dr. K.-C. Ho) Author Contributions Y. A. Leu and M. H. Yeh contributed equally to this work. All authors have given approval to the final version of the manuscript. Funding Sources


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Page 28 of 35

This work was supported by the Ministry of Science and Technology (MOST) of Taiwan (Grant no. MOST 103-2221-E-011-150-MY2). Notes The authors declare no competing financial interest. Supporting Information Available Synthetic process and relative characterization of POEM, performance of the DSSC containing the films of CNTs under various catalyst loadings, and long-term stability test of BCNT electrode are provided in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This work was supported in part by the Ministry of Science and Technology (MOST) of Taiwan, Industrial Technology Research Institute (ITRI), National Taiwan University (NTU), National Taiwan University of Science and Technology (NTUST), and National Taipei University of Technology (NTUT). REFERENCES 1.

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TOC graphic Thermally Stable Boron-doped Multi-walled Carbon Nanotubes as a Pt-free Counter Electrode for Dye-sensitized Solar Cells Yow-An Leua,b,§, Min-Hsin Yehb,c,§, Lu-Yin Lin*d, Ta-Jen Li b, Ling-Yu Changa, Sheng-Yen Shena, Yan-Sheng Lic, Guan-Lin Chenc, Wei-Hung Chiang*c, Jiang-Jen Lin*a, and Kuo-Chuan Ho*a,b a

Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan c Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan d Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan § These authors contributed equally to this work. b

* To whom correspondence should be addressed: [email protected] (L.-Y. Lin); [email protected] (W.-H. Chiang); [email protected] (J.-J. Lin); [email protected] (K.-C. Ho)

Thermally stable B-doped carbon nanotubes can be a promising electrocatalyst for replacing the expensive Pt in a dye-sensitized solar cell.


Thermally Stable Boron-Doped Multiwalled Carbon Nanotubes as a Pt

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