Enhanced Electrochemical Catalytic Efficiencies of Electrochemically Deposited Platinum Nanocubes as a Counter Electrode for Dye-Sensitized Solar Cells
© Wei et al. 2015
Received: 27 August 2015
Accepted: 26 November 2015
Published: 2 December 2015
Platinum nanocubes (PtNCs) were deposited onto a fluorine-doped tin oxide glass by electrochemical deposition (ECD) method and utilized as a counter electrode (CE) for dye-sensitized solar cells (DSSCs). In this study, we controlled the growth of the crystalline plane to synthesize the single-crystal PtNCs at room temperature. The morphologies and crystalline nanostructure of the ECD PtNCs were examined by field emission scanning electron microscopy and high-resolution transmission electron microscopy. The surface roughness of the ECD PtNCs was examined by atomic force microscopy. The electrochemical properties of the ECD PtNCs were analyzed by cyclic voltammetry, Tafel polarization, and electrochemical impedance spectra. The Pt loading was examined by inductively coupled plasma mass spectrometry. The DSSCs were assembled via an N719 dye-sensitized titanium dioxide working electrode, an iodine-based electrolyte, and a CE. The photoelectric conversion efficiency (PCE) of the DSSCs with the ECD PtNC CE was examined under the illumination of AM 1.5 (100 mWcm−2). The PtNCs in this study presented a single-crystal nanostructure that can raise the electron mobility to let up the charge-transfer impedance and promote the charge-transfer rate. In this work, the electrocatalytic mass activity (MA) of the Pt film and PtNCs was 1.508 and 4.088 mAmg−1, respectively, and the MA of PtNCs was 2.71 times than that of the Pt film. The DSSCs with the pulse-ECD PtNC CE showed a PCE of 6.48 %, which is higher than the cell using the conventional Pt film CE (a PCE of 6.18 %). In contrast to the conventional Pt film CE which is fabricated by electron beam evaporation method, our pulse-ECD PtNCs maximized the Pt catalytic properties as a CE in DSSCs. The results demonstrated that the PtNCs played a good catalyst for iodide/triiodide redox couple reactions in the DSSCs and provided a potential strategy for electrochemical catalytic applications.
KeywordsDye-sensitized solar cells Platinum nanocubes Electrochemical deposition Counter electrode
In the recent years, Grätzel and many scientists have been attracted to study potential candidates of dye-sensitized solar cells (DSSCs) for next-generation solar cells [1–3]. DSSCs have many outstanding features such as the highlight of good photoelectric conversion efficiency (PCE) and the potential for its low cost and simple fabrication. The DSSCs have three major parts to assemble the sandwich structure, including the working electrode (WE), the electrolyte, and the counter electrode (CE). First of all, the WE is composed of porous nanocrystal titanium dioxide (TiO2) nanoparticles adsorbed with dye molecules on a conductive glass substrate. Secondly, electrolyte contains iodide/triiodide redox couples (I−/I3 −) in an organic solvent between the WE electrode and the CE electrode. Finally, CE is an electrocatalytic activation layer deposited on a conductive glass, which is typically a platinum (Pt) film deposited with a fluorine-doped tin oxide (FTO) glass. Until recently, many scientists focused on the performance improvement of the WE for the efficient sunlight harvesting [4–6]. However, CE also plays an important role in DSSCs; there are three major functions of the CE in DSSCs: (1) transfer electrodes to make the cell as a complete circuit, (2) regenerate the I3 − in the interface of the electrolyte/CE, and (3) reduce the I−/I3 − redox couples in order to keep low potential to minimize energy losses. The most common nanomaterials applied to the CE for DSSC investigations were Pt [7, 8], carbon nanotubes [9, 10], graphene [11, 12], graphene oxide , molybdenum sulfide [14–16], etc. Among those electrocatalytic materials, Pt exhibits the best electrocatalytic performance, which was widely used in fuel cells, biosensors, and chemical sensor development of the advanced technologies for kinds of the environmental, renewable energy, and industrial process [17–20]. In the DSSCs, Pt was one of the best materials to enhance the performance for CE due to its functional electrocatalytic activity for the I−/I3 − couple redox reactions, and it also performs the high conductivity property [21, 22]. The investigations of preparations for kinds of Pt morphologies and nanostructures as a CE have attracted much attention for the improvement of DSSC’s efficiency [23–26]. It has been found that Pt has distinct electrocatalytic activity on specific crystalline planes for the electrochemical applications , which suggests that the maximum electrocatalytic activity may be achieved by controlling the single crystallinity in fabricating the Pt nanomaterials. However, the challenge to synthesize the unique morphology of single-crystal Pt nanomaterial still remains [27–29].
It is well known that the catalytic activity of nanocrystals is strongly related to their surface structure because the electrochemical reactions are surface structure sensitive. Pt is an expensive and a precious metallic element owing to its scarcity, and hence, the preparations of Pt nanocatalysts with enhanced electrocatalytic activity and utilization efficiency have been an important research focus. Very recently, intense attention has been paid to the synthesis and the analysis of Pt nanocubes (PtNCs) [30, 31]. In contrast to the traditional Pt film deposition from expensive sputtering vacuum equipment, electrochemical deposition (ECD) was found to be an efficient method at room temperature and a simple and low-cost process to deposit Pt nanostructure [32–35]. In this study, we described our investigation of the PtNCs, which was directly deposited onto the fluorine-doped SnO2 conducting glass (FTO, 8 Ω/sq., 2.2 mm in thickness, TEC-7, Hartford) by a novel pulse-mode ECD (pulse-ECD) method. The surface morphology and nanostructure of the prepared pulse-ECD PtNCs were examined by field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). The surface roughness of the prepared samples was examined by atomic force microscopy (AFM). Pt loading was examined by inductively coupled plasma mass spectrometry (ICP-MS). The electrocatalytic performances were considered by cyclic voltammetry (CV), Tafel polarization, and electrochemical impedance spectrum (EIS). It was found that the influence of PtNCs with the single-crystal nanostructure was significant on the resulting electroactivities and therefore enhanced the performance of DSSCs. The PCE of the DSSCs with the pulse-ECD PtNCs on FTO as a CE was examined under the illumination of AM 1.5 (100 mWcm−2), which was revealed to exhibit an excellent PCE of 6.48 %, higher than the cells using the conventional Pt film CE (6.18 %).
Preparation of the TiO2 Working Electrodes
A FTO conducting glass was firstly cleaned with deionized (DI) water, acetone, and isopropyl alcohol (IPA, 99.5 %, Sigma-Aldrich), sequentially. The nanocrystalline TiO2 nanoparticles were used to coat as a thin film, two kinds of TiO2 nanoparticle layers were coated on the FTO glass by using the print-screen method, and the area of TiO2 nanoparticle layer was 0.049 cm2. The first layer (10 μm in thickness) served as the interlayer on which a layer of light-scattering anatase TiO2 particles (2 μm in thickness) was coated. The TiO2-coated WE was dried at 120 °C, then gradually heated to 550 °C for 30 min in ambient air, and then cooled slowly to room temperature. After calcination, the TiO2-coated WE was immersed in a N719 dye (Solaronix) solution (0.3 mM in a mixture of acetonitrile and tert-butyl alcohol) for 24 h at room temperature. The dye-adsorbed TiO2-coated WE was washed with acetonitrile (ACN) and dried at room temperature for a few seconds to remove the remaining dye.
Preparation of the Pt Counter Electrodes
There were two different methods used to prepare the Pt CEs in this study. Method I was the conventional Pt film prepared by electron beam evaporation and used as a reference electrode. The Pt film was deposited on the FTO glass to a thickness of 50 nm in order to ensure high PCE of the reference Pt film CE . Method II was the pulse-ECD PtNCs, where the FTO glass was subjected to a pulse-ECD method in a plating bath containing an Ar-saturated Pt precursor aqueous solution (0.2 mM H2PtCl6˙6H2O and 0.1 M H2SO4 in DI water) at a controlled temperature of 30 °C under ambient pressure. In each scan cycle, voltages of 0 and −0.45 VSCE were each applied for the duration of 1 s, and 500 scans were used to deposit the PtNCs on the FTO glass. The pulse-ECD PtNCs were performed using a potentiostat/galvanostat (PGSAT 302N, Autolab, EcoChemie) in a conventional three-electrode cell, and a Pt plate and a saturated calomel electrode were used as the CE and the reference electrode, respectively.
Characterization of the Pulse-ECD PtNC Electrodes
The surface morphology and the nanostructure of the pulse-ECD PtNCs were characterized by using FESEM (JEOL, JSM-6330F) and HRTEM (JEOL-2100F), respectively. The AFM (Park system, XE-70™) with a nanoscope IV controller by Digital Instruments Inc. was carried out to examine the surface roughness in an ambient environment. CV was used to examine the redox activities of the prepared CE samples in a three-electrode configuration, at a scan rate of 20 mVs−1 at room temperature. The solution used for CV measurements contained 1 mM I2, 10 mM LiI, and 0.1 M LiClO4 in ACN solution. The EIS was applied to study the charge-transfer properties of the CE. During the scan for the EIS, the frequency ranged from 105 to 10−1 Hz and used an applied electric bias potential of 10 mV. Nyquist plots were held and examined using the aforementioned potentiostat/galvanostat equivalent circuit model with Autolab FRA software. The Tafel measurements were used to calculate the exchange current density. Tafel polarization curves were performed with a scanning rate of 1 mV s−1 in the potential range of 0.1 to −0.1 V. The symmetric dummy cells were used for both EIS and Tafel measurements, and both EIS and Tafel measurements were analyzed using a potentiostat/galvanostat (PGSAT 302N, v4.9 Autolab, EcoChemie B.V.). The Pt loadings of the prepared samples were measured with the ICP-MS (SCIEX ELAN 5000, Perkin Elmer).
DSSC Assembly and Photovoltaic Performance Measurement
In order to assemble the DSSCs, a 60-μm-thick hot melt spacer (SX1170-60, Solaronix) was sandwiched between the WE and CE by heating at 100 °C for a few seconds. The liquid, iodide-based electrolyte (AN-50, Solaronix) was injected into the space between the WE and CE of the DSSCs. The DSSC devices were illuminated by a grade A quality solar simulator with a light intensity of 100 mWcm−2 (AM 1.5) to measure the photocurrent–voltage curves, and the solar light was calibrated with a standard silicon cell (calibrated at NREL, PVM-81).
Results and Discussion
Characterization of the Pt Counter Electrodes
AFM roughness values of the Pt film and pulse-ECD PtNCs
Average roughness (Ra; nm)
RMS roughness (Rq; nm)
Summary of the electrochemical characteristics and Pt loadings of the reference Pt film CE and the pulse-ECD PtNC CE
I pa (mAcm−2)
I pc (mAcm−2)
R s (Ω)
R ct (Ω)
Z N (Ω)
J lim (mAcm−2)
Electrochemical Properties of the Pt Counter Electrodes
According to Fig. 3, the I pc of the PtNC electrode was −0.83 mAcm−2, and the Pt film electrode was −0.57 mAcm−2, which means a higher redox reaction rate in the interface between the PtNC electrode and electrolyte. Our prepared pulse-ECD PtNC electrode demonstrated a superior electrocatalytic activity for the I3 − ion reduction reactions.
where R, T, n, and F represented the gas constant, the temperature, the number of electrons transferred in the reduction reaction, and the Faraday constant, respectively.
According to Eq. (3), D, l, n, F, and C represented the diffusion coefficient of the triiodide, the thickness of spacer, the number of electrons involved in the reduction of triiodide at the electrode, the Faraday constant, and the concentration of triiodide, respectively.
Table 2 summarizes the values of I pa, I pc, R s, R ct, Z N, J lim, and Pt loading from the electrochemical measurement results obtained from the CV curves, Nyquist plots, Tafel polarization curves, and ICP-MS measurements. As shown in Table 1, comparing the PtNCs and Pt film, our pulse-ECD PtNC CE provided a better I pc of −0.83 mAcm−2, a lower R ct of 32.4 Ω, and a higher J lim of 34 mAcm−2. Meanwhile, our prepared pulse-ECD PtNC CE performed the superior electrocatalytic properties.
The MA values of the Pt film and pulse-ECD PtNCs
I pc (mAcm−2)
Pt Loading (mgcm−2)
The pulse-ECD PtNCs not only owned the unique single-crystal nanostructure but also decreased the Pt loading as shown in Table 2. Pulse-ECD PtNCs still kept the superior electrocatalytic properties and made a better Pt utilization efficiency as the CE for DSSCs as shown in Table 3. We herein suggested that our prepared specific crystallinity of PtNCs with a unique cubic shape exposed the low-index facets to enhance the electrocatalytic activities at the electrolyte–electrode interface; it might be that the single-crystal structure of PtNCs reduced the internal defects which led to the decrease of the electron loss pathway and enhance the catalytic efficiency. Compared with the conventional Pt film, the pulse-ECD PtNCs not only decreased the Pt loading but also held the superior electrocatalytic properties.
Characterization of the DSSCs
Summary of the photovoltaic characteristic parameters for the reference Pt film CE and the pulse-ECD PtNC CE
J sc (mAcm−2)
V oc (V)
In this work, the single-crystal nanostructure of PtNCs was successfully developed at room temperature by the pulse-mode ECD technique to be deposited onto the FTO glass as a CE for DSSCs. Our results indicated that the crystallinity of PtNCs with a unique cubic shape exposed the low-index facets, which offered the superb ability for triiodide reduction and resulted in the superior electrocatalytic activity. The electrocatalytic activity of the prepared PtNC CE, as determined by the catalyst mass activity (MA) for the triiodide reduction reaction, was 2.71 times better than that of a conventional Pt film CE. Finally, the DSSCs assembled with the pulse-ECD PtNC CE showed a superior PCE of 6.48 % to the DSSCs with a conventional Pt film CE (6.18 %). PtNCs not only reduced the Pt loading to make a better Pt utilization efficiency but also improved the PCE of DSSCs. Present work suggests that the Pt CE with the outstanding electrocatalytic activities and low Pt loading could be achieved by controlling the crystallinity of Pt.
The financial support provided by the National Science Council of Taiwan (project no NSC 102-2221-E007-080-) is greatly appreciated.
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