Chapter 7 Synthesis of 2D Continuous Pt Island Networks for Methanol Electrooxidation
7.3 Structural characterization
Figure 7-2 (A) and (B) illustrates a representative scanning electron microscopy (SEM) image of the blanket Pt on silicon substrate and Ru decorated on blanket Pt, respectively.
Figure 7-2 shows the SEM images: (A) blanket Pt on flat Si substrate; and (B) Ru on blanket Pt/Si.
(A)
(B)
From the SEM images, the blanket Pt and the Ru decorated on the blanket Pt were completely covered on the silicon substrate after the electrochemical deposition. Figure 7-3 show the surface morphology of the pulse electrodeposited Pt on the Si substarte. The Pt islands are mutually connected over the Si substrate. The Pt islands forming the continuous Pt island film have size distribution from ~200 nm to ~800 nm.
Figure 7-3 shows the SEM image of the continuous Pt island network on the flat silicon substrate.
Figure 7-4 shows the x-ray photoelectron spectrum (XPS) of (a) blanket Pt/Si; (b) Ru decorated blanket Pt; and (c) continuous Pt island network.
800 700 600 500 400 300 200 100 0
O(1s)
Ru(4d)
Pt(4d)
C(1s) Si(2s) Si(2p)
Pt(4f)
(b) (c) (a)
Rela tiv e X P S In ten sity
Binding Energy (eV)
X-ray photoelectron spectroscopy (XPS) shown in Fig. 7-4 indicated that Pt and Ru were successfully deposited on the silicon substrate by the pulse electrodeposition.
7.4 Electrochemical measurements
The electroactive surface area (ESA) of the electrodes was determined by the CO-stripping cyclic voltammetry, which was performed by flowing a10% CO/N2 gas mixture in the 1 M H2SO4 aqueous solution at +100 mV for 35 min, using a Pt wire as the counter electrode and a saturated calomel reference electrode (SCE). Before scanning, the solution was purged with N2 gas for 30 min to removed CO remained in the solution. Representative CO-stripping voltammograms for the continuous Pt island network/Si, the Ru decorated Pt film and the blanket Pt/Si electrodes are illustrated in Fig. 7-5.
Figure 7-5 shows the CO stripping cyclic voltammetry curves recorded at room temperature in a CO saturated 1 M H2SO4 solution at a scan rate of 20 mV s-1
A high ESA of 67 m2 g-1 was obtained for the continuous Pt island network/Si electrode by integrating the CO-electrooxidation peak of first CO stripping cycle, assuming an oxidation charge value of 420 μC cm−2 for a monolayer of CO adsorbed on a smooth platinum surface
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
[90]. The ESA of the continuous Pt island network/Si electrode is much higher than that of the Ru decorated Pt film electrode (21 m2 g-1) and that of the blanket Pt electrode (16 m2 g-1).
This shows that the continuous Pt island network/Si electrode has a relatively high ESA, most likely due to the interconnect structure of the Pt islands. Such well-defined continuous Pt island network structure provides abundant active sites for the electrooxidation reaction of methanol.
From the CO-stripping curve, we noticed a lower onset potential and smaller peak potential for CO oxidation on the continuous Pt island network electrode in comparison to the Ru decorated Pt film and the blanket Pt film/Si. Examination of the CO oxidation curves reveals that the onset potential of the continuous Pt island network/Si electrode (~0.43 V) is lower than that of the Ru decorated/Pt (~0.48) and the blanket Pt/Si (~0.60 V). The CO oxidation peak potential for the continuous Pt island network/Si (~0.57 V) is also lower than that for the Ru decorated Pt (~0.60 V) and the blanket Pt/Si (~0.64 V), probably due to an enhanced CO oxidation rate on the Pt islands surrounded by the chemical SiO2 layer, which was formed on the Si substarte in the electrolyte. The presence of the oxide layer on the silicon substrate can promote the oxidation of CO adsorbed on the active Pt sites via the bifunctional mechanism [105]. The oxygen-containing species on SiO2 (such as hydroxyl surface group) can transform CO-like poisoning species adsorbed on Pt to CO2, releasing the active sites on Pt for further electrochemical reaction, and hence the continuous Pt island network on the flat Si substrate possess higher activity towards CO oxidation compared to the blanket Pt on silicon and the Ru decorated Pt film.
Figure 7-6 shows the cyclic voltammograms of the three electrodes recorded in 1 M CH3OH/1 M H2SO4 aqueous solution at a potential scan rate of 20 mV s-1. The CV curve of the continuous Pt island network/Si shows that the methanol oxidation peak had the maximum around 0.63V vs. SCE and a very low onset potential of ~0.38 V. Also shown in Fig.
7-6 is the CV curves of the blanket Pt film and the Ru decorated Pt film, which show a much
smaller current density with a higher onset potential. The negative onset potential shift indicated that the continuous Pt island network/Si can effectively reduce overpotentials in the methanol electrooxidation reaction [91-94].
Figure 7-6 shows the cyclic voltammograms in 1 M CH3OH+1 M H2SO4 at a scan rate of 25 mV s-1.
Noted that the methanol oxidation peak in the forward scan for the continuous Pt island network/Si electrode was much larger than the peak in the region of 0.3 - 0.5 V in the reverse scan. In the cyclic voltammetric scan, the anodic peaks in the forward scan and in the reverse scan are associated with electrooxidation of methanol and removal of incompletely oxidized carbonaceous species (CO-like poisoning species) on the electrode, respectively. The catalyst tolerance against CO adsorption may be evaluated by the ratio of the current density of the forward anodic peak (If) to that of the reverse anodic peak (Ib), (If/Ib) [143]. For the continuous Pt island/Si electrode, the (If/Ib) ratio was calculated to be ~19. This ratio was more than 9 times and 20 times larger than that of the Ru decorated Pt film and the blanked Pt
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
film, respectively.
Figure 7-7 shows the chronoamperometry curves in 1 M CH3OH+1 M H2SO4 at the potential of 0.4 V.
Chroamperometry technique was employed to further test the activity of theses three electrodes. Figure 7-7 shows the chronoamperogram of electroactivity of the three electrodes at the oxidation potential of ~0.4 V in the 1 M CH3OH/1 M H2SO4 aqueous solutions at 25○C.
Steady-state currents for methanol electrooxidation were measured for more than 800 s. At the oxidation potential of 0.4 V, the steady-state currents at 800 s for the continuous Pt island network/Si, Ru decorated Pt film and blanket Pt/Si are ~10, ~4, and ~0.05 mA cm-2, respectively. The observation implied that most CO-like poisoning species could be oxidized and removed from the Pt catalyst so that the catalytic oxidation of methanol could be kept proceeding efficiently on the continuous Pt island network/Si electrode. Because oxygen containing species on the SiO2 surface layer can promote the CO removal as described above, the improvement of the electrooxidation activity can be ascribed to the synergistic effect of the Pt island catalyst and the SiO2 surface layer. These results are very consistent with the CV studies shown in Fig. 7-5 and 7-6.
Figure 7-8 shows the Tafel plots for the electrochemical oxidation of 1 M CH3OH/1 M H2SO4 aqueous solution at a scan rate of 1 mV/s.
Tafel plot for electrochemical oxidation of 1 M CH3OH/1 M H2SO4 aqueous solution at a scan rate of 1 mV s-1 is shown in Fig. 7-8. The blanket Pt/Si and Ru decorated Pt film have a Tafel slop of ~115 and ~137 mV dec-1, respectively. On the other hand, the continuous Pt island network/Si exhibits a much larger Tafel slope (~245 mV dec-1), suggesting a great difference in the electrooxidation mechanism for the continuous Pt island network/Si electrode from the other two electrodes. This might be ascribed to the Pt island network structure and the presence of active oxygenated on the SiO2 surface layer [153, 154]. The mechanistic difference could result in the better catalytic activity and CO tolerance of the 2-D continuous Pt island network/Si electrode compared to the blanket Pt/Si and the Ru decorated/Pt film electrode.
Figure 7-9 shows the electrochemical impedance spectra in 1 M CH3OH + 1 M H2SO4 at the potential 0.3 V. The inset in figure shows the equivalent circuit model.
The electrochemical impedance spectroscopy (EIS) measurements were used to evaluate the charge-transfer resistance and the capacitance of these three electrodes during methanol electrooxidation. Figure 7-9 shows three Nyquist plots recorded in 1 M CH3OH + 1 M H2SO4
at the oxidation potential of ~0.3 V, where Zr and Zi represent the real and imaginary components of the impedance, respectively. The equivalent circuit model shown in the inset of Fig. 7-9 was used to fit the experimental data. The Rs resistor represents the resistance of the electrolyte solution, Rct the charge transfer resistance and CPE represents the constant phase element [151]. As shown in Fig. 7-9, the EIS well fits with the proposed model. The Ru decorated Pt film and the blanket Pt/Si electrodes have charge-transfer resistances about ~16 and ~68 Ω-cm-2, which is over 3 times and 11 times larger than that for the continuous Pt island network/Si (~6 Ω-cm-2).
7.5 Summary
A two-dimensional continuous island Pt network electrode have been successfully
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Ru decorated on Blanket Pt/flat Si Pt Network/flat Si
fabricated by potentiostatic pulse plating on the flat silicon substrate, and electrochemical measurements confirm that this catalyst structure on the silicon substrate has a better electroactivity toward methanol oxidation than the blanket Pt/Si and the Ru decorated Pt film/Si electrodes. The good electrooxidation performance can be ascribed to the synergistic effect of the Pt island catalyst and the surrounding SiO2 surface layer, which significantly enhanced the CO tolerance and thus improved the electrooxidation activity of the Pt catalyst.
Chapter 8
Synthesis of 3D Platinum Nanoflowers and Their Electrochemical Characteristics
8.1 Introduction
Recently, synthesis of nanostructured materials with a high specific surface area has attracted great interest in the development of fuel cell catalysts [105, 155, 156], applications for photocatalytic activity [157], biosensors [158], chemical sensors [159, 160], and in the reduction of pollutant emission from automobiles [161]. Among all precious catalyst metals, platinum has unique chemical and physical characteristics and has a wide range of industrial and environmental applications. But it is extremely susceptible to poisons such as CO-like species, which result in a dramatic decrease in efficiency. However, a critical problem with Pt catalyst is the high cost due to limited supply. Thus, one of the major challenges in fuel cell development is to reduce the usage of platinum catalyst. One effective approach to accomplish this goal is to synthesize Pt nanostructures well dispersed on the electrode so that a high surface/volume ratio can be obtained. The use of both larger surface area supports, such as porous substrates [153, 162-163] and porous platinum [16, 155, 164-170] has been explored. Herein, we report a new and facile method of synthesizing 3D platinum nanoflowers on the silicon substrate by potentiostatic pulse plating. The 3D nanostructured platinum catalyst shows excellent electrocatalytic activity toward oxidation of methanol and CO adspecies.
8.2 Synthesis of the 3D Pt nanoflowers
A p-type 4-inch Si wafer of low resistivity (0.002 Ω-cm) was used as the substrate for the synthesis of the 3D Pt nanofloweres. 1 M H2PtCl6 was first mixed with 1 M H2SO4 in an
aqueous solution at room temperature, and the mixture was stirred at 25○C for 5 h to make the mixture solution homogeneous. The Pt catalyst was then electrodeposited on the flat silicon substrate in the mixture solution by potentiostatic pulse plating in a three electrode cell system with a saturated calomel reference electrode (SCE). The time periods for the positive potential pulse (+0.05 V) and the negative potential pulse (-0.02 V) were 5 ms and 1 ms, respectively.
Under the bipolar pulse electrodeposition conditions, 3D Pt nanoflowers could be synthesized on the Si substrate. After Pt electrodeposition, the sample was washed by DI water to removed contamination from the sample surface and dried in the ambient. For comparison, a blanket Pt thin film was also prepared on the Si substrate by electrodeposition, which will be referred to Pt thin film catalyst thereafter.
8.3 Characterization of the 3D Pt nanoflowers
Surface morphology of the 3D Pt nanoflowers was investigated by scanning electron microscopy (SEM). Figure 8-1 shows the SEM micrograph of the bipolar-pulse electrodeposited Pt catalyst thin film, which was composed of rose-like particles with a size between ~400 and ~800nm.
Figure 8-1 shows the scanning electron micrographs of the 3D Pt nanoflower on the Si substrate.
Figure 8-2 shows the XPS survey spectrum of the 3D Pt nanoflower on the silicon substrate.
Figure 8-3 shows the energy window of Pt (4f) electrons.
80 78 76 74 72 70 68
4f
7/274.7 eV 71.4 eV
4f
5/2Pt(4f)
Re lative X PS Inte nsity
Binding Energy (eV)
Figure 8-4 shows the TEM of nanopetals mechanical scratched off the 3D Pt nanoflower/Si sample. The inset shows the SAED pattern, and (B) a high resolution TEM image of a nanopetal.
The x-ray photoelectron spectrum (XPS) of the 3D Pt nanoflowers is shown in Fig. 8-2.
Because the nanoflowers did not fully cover the Si substrate, O(1s) and Si(2p) XPS signals
(A)
(B)
(111) (220)
(200) (311) (111) (220)
(200) (311)
from the native silicon oxide could still be detected. The XPS peak of the Pt(4f) shown in the Fig. 8-3. The binding energies of Pt (4f7/2) and (4f5/2) electrons were 71.4 and 74.7 eV, respectively, which are in good agreement with pure bulk platinum [171]. No XPS signal associated with oxidized Pt species (such as Pt2+ and Pt4+) and chlorine from the Pt precursor were observed, suggesting that metallic platinum was likely the only constituent of the nanoflowers synthesized by pulse electrodeposition.
Figure 8-4 (A) shows the transmission electron microscopy (TEM) image of nanopetals, which were mechanically scratched off the Pt nanoflower thin film. A selected-area electron diffraction (SAED) pattern is shown in the inset of the figure. The bright diffraction rings can be indexed as the (111), (200), (220), and (311) lattice planes of the Pt face-centered-cube (FCC) lattice structure. From the TEM image, the Pt nanopetals had a bamboo-leaf like structure, with an averaged length of ~300 nm. The high resolution lattice image further reveals that the nanopetal was made up by Pt nanocrystals of a few nanometers in size as shown in Fig. 8-4 (B).
8.4 Electrochemical characteristics
The electroactive surface area of the catalyst was evaluated from charges associated with hydrogen adsorption/desorption on the electrode surface from cyclic voltammograms (CVs) in 1 M H2SO4 aqueous solution at a scan rate of 20 mV s-1 [172]. Figure 8-5 shows CVs of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes. In the potential range characteristic of hydrogen electrosorption in the sulfuric acid solution (-0.4 – 0.0 V vs. SCE), the Pt thin film/Si electrode shows a very weak and featureless broad peak, while two peaks (at ca. -0.22 and -0.02 V vs. SCE) can be clearly observed on the CV curve of the Pt nanoflower/Si electrode. The peak feature in a cyclic voltammogram of Pt in sulfuric acid solutions strongly depends on the crystallographic orientation of the Pt surface. For the Pt(111) surface, the
cyclic voltammogram is characterized by a broad and flat hydrogen electrosorption peak in the potential range ~ -0.4 V - 0.0 V (vs. SCE) [173]. On the other hand, in the same potential range, the Pt(100) surface gives two distinct hydrogen electrosorption peaks, and a single peak can be found for the Pt(110) surface in the same potential range. Thus, from the CV curves of Fig. 8-5, we suggest that the (100) and (110) lattice planes prevailed over the (111) plane on the 3D Pt nanoflower surface.
The hydrogen adsorption charge (QH) evaluated from Fig. 7-5 is ~0.27 mC cm-2 and
~31.17 mC cm-2 for the Pt thin film/Si and the 3D Pt nanoflower/Si electrodes, respectively.
QH is usually used to quantify active sites for hydrogen adsorption/desorption on the Pt catalyst. The measured QH values show that the active surface area of the 3D Pt nanoflowers/Si electrode is much higher than that of the Pt thin film/Si electrode by a factor of >110.
Figure 8-5 shows the CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the 1 M H2SO4 solution. The scan rate was 20 mV s-1.
Electrocatalytic activity of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes toward methanol oxidation reaction (MOR) was studied by cyclic voltammetry in a nitrogen-saturated 1 M CH3OH/1 M H2SO4 solution at a scan rate of 25 mV s-1, and the CV curves are shown in Fig. 8-6. The onset potential for methanol oxidation of the 3D Pt nanoflower catalyst was ~0.38 V, which was ~0.13 V lower than that of the Pt thin film catalyst, indicating faster electrode kinetics [174]. In addition, the oxidation current peak density of the 3D Pt nanoflower catalyst in the forward scan was higher than that of the blanket Pt thin film catalyst. This implies that the 3D Pt nanoflower catalyst has a better electrocatalytic activity toward the MOR compared with the blanket Pt thin film catalyst. In the CV scan, the anodic peak in the reverse scan might be attributed to the removal of CO-like poisoning species formed on the Pt catalyst in the forward scan. The catalyst tolerance against CO adsorption may be estimated by the ratio of the forward current density (If) to the reverse anodic peak current density (Ib), (If/Ib) [143]. A high If/Ib ratio suggests efficient electrooxidation of methanol during the forward scan and less accumulation of residues on the electrodes, whereas a low ratio indicates incomplete electrooxidation of methanol and excessive accumulation of carbonaceous residues on the electrode surface. The (If/Ib) ratio of the 3D Pt nanoflower/Si electrode and the Pt thin film/Si electrode are ~2.5 and ~0.93 respectively. This ratio for the 3D Pt nanoflower/Si electrode is ~2.68 times larger than that for the Pt thin film/Si electrode, indicating that the 3D Pt nanoflowers electrode had a higher electrocatalytic activity toward MOR and thus a better CO tolerance.
To study CO tolerance of the 3D Pt nanoflower/Si electrode, CO-stripping CV measurement was carried out. Figure 8-7 shows the CO stripping CV curves of the 3D Pt nanoflower/Si and the Pt thin film/Si electrodes in 1 M H2SO4 aqueous solution. For the CO stripping analysis, CO adsorption on the Pt catalyst was conducted by flowing a 10% CO/N2
gas mixture into the (1 M H2SO4) electrolyte for 35 min at 0.1 V (vs. SCE), followed by purging with nitrogen gas for 30 min to remove any residual CO from the solution.
Figure 8-6 shows the CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the 1 M CH3OH/1 M H2SO4 solution. The scan rate was 25 mV s-1.
As shown in Fig. 8-7, during the first cycle, the CV curve was flat in the potential range between −0.3 to 0.3 V indicating that the hydrogen adsorption was suppressed due to the complete coverage of available active Pt sites by CO adspecies. In the first scan, a broad anodic peak appeared between 0.4-0.8 V, which was absent in the subsequent scan, indicating that CO adspecies were effectively oxidized during the first scan. The CO oxidation current peaks of the 3D Pt nanoflower/Si and the thin film Pt/Si electrodes were centered at ~0.58 V and ~0.64 V, respectively. The peak current density of CO electrooxidation on the 3D Pt nanoflower/Si electrode was much larger than that on the Pt thin film electrode, further indicating that the 3D Pt nanoflower/Si electrode had a much larger electroactive surface area.
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Figure 8-7 shows the CO stripping CVs of the 3D Pt nanoflower/Si and the Pt thin film /Si electrodes in the CO saturated 1 M H2SO4 solution. The scan rate was 25 mV s-1.
The above electrochemical analysis results indicate that the 3D Pt nanoflower/Si electrode possessed a much higher electrocatalytic activity towards CO oxidation and MOR compared to the Pt thin film/Si electrode. This may be ascribed to that Pt catalysts on the two electrodes had different preferential surface lattice orientations. It is known that methanol oxidation reaction catalyzed by Pt in acidic aqueous solutions is sensitive to the surface structure of the Pt catalyst [175]. According to previous reports, the electrocatalytic activity of Pt toward MOR in H2SO4 aqueous solution increases in the order Pt(100)> Pt(110)> Pt(111) [176]. As discussed above about the hydrogen eletrosorption peaks (Fig. 8-5), the Pt nanoflower/Si electrode had more surface areas with the Pt(100) and Pt(110) lattice orientations than the blanket Pt thin film electrode, of which the catalyst surface was Pt(111) preferentially oriented.
Therefore, as shown in Fig. 8-6, a lower onset potential and a much higher current density
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
were obtained during the forward scan in the methanol oxidation CV measurement for the Pt nanoflower/Si electrode. In addition, the oxidation rate of CO adspecies on Pt also varies with the Pt surface orientation, with the electrocatalytic activity increasing in the order Pt(111)
<Pt(110) <Pt(100) [177]. As a result, CO like adspecies on the Pt(100) and (110) surfaces can
<Pt(110) <Pt(100) [177]. As a result, CO like adspecies on the Pt(100) and (110) surfaces can