Chapter 6 Novel New Approach to the Synthesis of Nanoporous Graphitic Carbon as a
6.3 Physico-chemical characterization
Scanning electron microscopy (SEM) was used to characterize the surface morphology of the synthesized 3D nanoporous g-C. Figure 6-2 shows a typical SEM image of 3D nanoporous g-C where the Pt and Ru content is zero. As is evident from the image, the Si substrate is well covered by 3D nanoporous carbon with pore sizes of 100–900 nm.
Figure 6-2 shows the SEM image of the resulting 3D nanoporous g-C.
The graphitization of the synthesized 3D nanoporous carbon was characterized by Raman spectroscopy. The Raman spectra of the 3D nanoporous carbon in the frequency range between 1200 and 1800 cm-1 are shown in Fig. 6-3. The two main peaks at 1594 and 1359 cm
-1 illustrated in the spectra corresponds to G and D bands, respectively. The G band represents the E2g vibration of g-C with a sp2 electronic configuration and D band represents the A1g
mode of diamond-like carbon with a sp3 electronic configuration. To determine the degree of graphitization, we estimated the intensities of D and G peaks from their peak heights and hence their ratio (ID/IG). As shown in Fig. 6-3, the ID/IG ratio less than 1, indicate the better graphitization of 3D nanoporous carbon [141]. Raman spectra suggest formation of g-C in 3D nanoporous carbon structure.
Figure 6-3 Raman spectrum of 3D nanoporous g-C deposited on Si substrate.
SEM was further used to characterize the surface morphologies of the synthesized Pt/3D nanoporous g-C, and Pt50-Ru50/3D nanoporous g-C. Fig. 6-4 (A) shows a typical SEM image
of Pt/nanoporous g-C where 3D nanoporous g-C surface is deposited by nanoparticles Pt. The pore size distribution of the synthesized 3D nanoporous g-C is 100-800 nm.
Figure 6-4 shows the plane-view SEM images (A) Pt/3D nanoporous g-C, and (B) typical Pt50-Ru50/3D nanoporous g-C.
(A)
(B)
Figure 6-5 shows the TEM images (A) Pt/3D nanoporous g-C, and (B) typical Pt50-Ru50/3D nanoporous g-C.
It is evident that the some pores are filled due to the large size of the Pt nanoparticles.
Figure 6-4 (B) shows a typical SEM image of Pt50-Ru50/nanoporous g-C, where 3D nanoporous g-C surface deposited by nanoparticles Pt and Ru. From the SEM image, we can see that the Pt50-Ru50/3D nanoporous g-C has more porous surface than the Pt/3D nanoporous g-C, showing has higher surface area than Pt/3D nanoporous g-C. Because the Pt and Pt50 -Ru50 alloy catalysts on the g-C support were very small in size, Pt and Pt-Ru alloy catalyst was hardly observed from the SEM images of Fig. 6-4 (A and B).
The transmission electron microscopy (TEM) measurements were performed to reinvestigate the presence of Pt and Pt-Ru nanoparticles. A TEM image of the Pt/3D nanoporous g-C is shown in Fig. 6-5 (A). As seen from TEM images, Pt nanoparticles showing the agglomeration. And the Pt nanoparticles were ~ 20 nm in size. Figure 6-5 (B) shows the TEM image of the g-C with the deposited Pt-Ru alloy catalysts. It can be clearly seen that Pt-Ru alloy catalyst was dispersed on g-C. In addition, the composition of individual particles was probed using energy-dispersive X-ray analysis (EDX). An EDX spectrum of Pt
(B)
(A)
and Pt50-Ru50 alloy catalysts is shown in Fig. 6-6 and 6-7. The EDX indicated that Pt and Pt-Ru alloy catalysts was deposited on the g-C.
Figure 6-6 shows the EDX spectrum of the Pt catalyst on the g-C.
Figure 6-7 shows the EDX spectrum of the Pt50-Ru50 alloy catalyst on the g-C, showing the presence of Pt and Ru nanoparticles on the 3D nanoporous g-C.
X-ray photoelectron spectroscopic (XPS) system is a convenient tool to investigate the presence of Pt and Pt-Ru alloy catalysts on the g-C. The wide-scan XPS spectrum of the Pt/3D nanoporous g-C and Pt50-Ru50/3D nanoporous g-C are shown in Fig. 6-8. From the spectrum it is clear that the Pt and Pt-Ru alloy catalysts was deposited on the 3D nanoporous
g-C. The relatively strong O(1s) XPS signal suggests that the oxygen surface group was present on the 3D nanoporous g-C. This is greatly affects the nanoparticles deposition and electrocatalytic characteristics of the Pt and Pt-Ru nanoparticles. On the other hand, the weak signals of Si(2s) and Si(2p) were detected from XPS, indicates nanoporous g-C completely covered the Si surface. This is consistent with our SEM observation as shown in Fig. 6-4 (A and B).
Figure 6-8 shows the XPS survey spectrum of the 3D nanoporous g-C supported Pt and Pt-Ru alloy catalysts.
6.4 Electrocatalytic activity
The hydrogen adsorption and desorption was used to investigate the electrochemical active surface area (ECSA) of these electrodes. Figure 6-9 shows the CV curves of the Pt/3D nanoporous g-C and Pt50-Ru50/3D nanoporous g-C electrodes in a 1 M H2SO4 aqueous solution. The potential was scanned between -0.4 and 1.2 V (vs. SCE) at the potential scan rate of 25 mV s-1.
Figure 6-9 shows the cyclic voltammograms of the Pt50-Ru50/3D nanoporous g-C and Pt/3D nanoporous g-C electrodes in aqueous solution in N2 saturated aqueous solution of 1 M H2SO4. The scan rate was 25 mV s-1.
The peaks for the adsorption/desorption of hydrogen appearing between -0.35 and 0.15 V vs. SCE are clearly shown. The wide peak potentials shift slightly from Pt/3D nanoporous g-C (~ 0.14 V, –0.27 V) to that of Pt50-Ru50/3D nanoporous g-C (~ 0.05 V, –0.30 V). The double layer capacitance remarkably enhanced by addition of ruthenium into the platinum.
The higher the ruthenium content, the larger the double layer capacitance [142]. The hydrogen adsorption charge (QH) has been calculated by taking the assumption that the double layer capacitance is constant across the entire investigated potential range. The QH of the Pt/3D nanoporous g-C and Pt50-Ru50/3D nanoporous g-C electrodes are 16.12 and 28.23 mC cm-2, respectively. QH shows the number of sites of Pt available for hydrogen adsorption and desorption. These results show that, even their geometric surface area is the same, the active surface area of Pt50-Ru50/3D nanoporous g-C electrode is much larger than that of the Pt/3D nanoporous g-C electrode, and the high ECSA were favorable to electrochemical oxidation
toward methanol.
The electro-active surface area (ESA) of the Pt/3D nanoporous g-C, and the Pt50-Ru50/3D nanoporous g-C electrodes was measured by CO-stripping CV in 1 M H2SO4 solution at a scan rate of 25 mVs-1. For CO-stripping CV measurements, CO adsorption on the Pt catalyst was conducted by flowing a 10% CO/N2 gas mixture into 1 M H2SO4 electrolyte for 35 min at 0.1 V (vs. SCE), followed by purging with nitrogen gas for 30 min to eliminate any residual CO from the solution. The electrochemical active area of the electrodes was determined assuming the formation of a monolayer of linearly adsorbed CO molecules and the columbic charge required for oxidation of COads to be 420 μC cm−2 [90]. The CO-stripping CV curves for the Pt/3D nanoporous g-C and the Pt50-Ru50/3D nanoporous g-C electrodes are shown in Fig. 6-10 (A and B). The ESAs for Pt/3D nanoporous g-C and Pt50-Ru50/3D nanoporous g-C electrodes are 14.2 and 26.92 mC cm-2, respectively. The above results indicate that the Pt50 -Ru50/3D nanoporous g-C electrode possessed a much higher ESA compared to the Pt/3D nanoporous g-C electrode. This is consistent with the results presented in Fig. 6-9, further showing the large ESA of the synthesized 3D nanoporous Pt50Ru50 electrodes. However, one of the most important factors in direct methanol fuel cells is efficient removal of adsorbed CO-like species on the surface of the catalysts. The CO-stripping CV curves were also used to evaluate the CO tolerance over both Pt/3D nanoporous g-C, and Pt50-Ru50/3D nanoporous g-C electrodes. As shown in Fig. 6-10 (A and B) during the first cycle, when the potential was scanned from −0.3 to 0.0 V, the CV curve is flat indicating that hydrogen adsorption is completely suppressed due to that available active Pt sites were completely covered by the CO. The CO electrooxidation current peaks of the Pt50Ru50/3D nanoporous g-C and Pt/3D nanoporous g-C electrodes were centered at ~0.44 V and ~0.48 V, respectively. The earlier onset of the CO electrooxidation shown by the shoulder at 0.11 V for the Pt50Ru50/3D nanoporous g-C electrode vs. 0.36 V for the Pt/3D nanoporous g-C electrode and the higher current density in this region demonstrates that the Pt50Ru50/3D nanoporous g-C electrode can
oxidize the adsorbed CO-like adspecies more efficiently than Pt/3D nanoporous g-C at the lower electrode potentials.
Figure 6-10 shows the CO-stripping CVs in a CO saturated 1 M H2SO4 solution (A) the 3D nanoporous g-C electrode; and (B) the Pt50-Ru50/3D nanoporous g-C electrode.
The scan rate was 25 mV s-1.
(A)
(B)
We further studied the electrocatalytic activity of the Pt/3D nanoporous g-C and Pt50 -Ru50/3D nanoporous g-C electrodes in an aqueous solution of 1 M H2SO4 - 1 M methanol at a scan rate of 25 mV s-1 are shown in Fig. 6-11 (A). As seen from Fig. 6-11 (A), the peak current density of methanol electrooxidation for the Pt50-Ru50/3D nanoporous g-C catalyst at the potential of 0.59 V (vs. SCE) was 8.93 mA cm-2, which is twice of that on Pt/3D nanoporous g-C (4.1 mA cm-2 at 0.74 V).
The Pt/3D nanoporous g-C and Pt50-Ru50/3D nanoporous g-C electrodes showed onset potentials of ~0.43 and ~0.25 V, respectively. An excellent catalyst for methanol electrooxidation is one that exhibits a low onset potential. Therefore, this implies that the Pt50 -Ru50/3D nanoporous g-C for the methanol electrooxidation has superior electrocatalytic activity compared with the Pt/3D nanoporous g-C catalyst. In addition, the ratio of the forward current density (If) to the reverse anodic peak current density (Ib), (If/Ib) value could be used to describe the catalyst tolerance to carbonaceous species accumulation [143]. The If/Ib value for Pt50-Ru50/3D nanoporous g-C and Pt/3D nanoporous g-C electrodes were ~6.3 and ~1.84, respectively. A lower If/Ib value indicated poor oxidation of methanol to CO during the anodic scan and excessive accumulation of residual carbon species on the electrode surface, in other words, a greater extent of CO poisoning. It could be seen that the Pt50 -Ru50/3D nanoporous g-C electrode had the highest If/Ib ratio than that of the Pt/3D nanoporous g-C electrode, indicating that the better CO tolerance of Pt50-Ru50/3D nanoporous g-C electrode. The electrocatalytic activity of both electrodes was further examined using chronoamperometry. Fig. 6-11 (B) represents the results obtained on the Pt50-Ru50/3D nanoporous g-C and Pt/3D nanoporous g-C electrodes in an N2 saturated aqueous solution of 1 M CH3OH - 1 M H2SO4 at 25°C at a constant potential of 0.4 V (vs. SCE). As shown in Fig.
6-11 (B), the potentiostatic current decreased gradually at the initial stage. The rapidly decreased in the potentiostatic current might be due to the formation of intermediate species, for instance CO and CHO adspecies etc., during the methanol oxidation reaction [144].
Figure 6-11 (A) CVs of the Pt50-Ru50/3D nanoporous g-C and Pt/3D nanoporous g-C electrodes in aqueous solution of 1 M H2SO4 - 1 M CH3OH at a scan rate of 25 mV s-1, and (B) Chronoamperometric curves for the Pt50-Ru50/3D nanoporous g-C and Pt/3D nanoporous g-g-C electrodes in aqueous solution of 1 M H2SO4 - 1 M CH3OH at a constant potential of 0.4 V vs. SCE.
It can be clearly seen that the current density on the Pt50-Ru50/3D nanoporous g-C
0 100 200 300 400 500 600 700 0
2 4 6 8
(b) (a)
Cu rren t Den sity (m A/ cm
2)
Time (second)
Pt
50-Ru
50/g-C Pt/g-C
(A)
(B)
electrode is evidently higher than that on the Pt/3D nanoporous g-C electrode. This result further confirms the superiority of the Pt50-Ru50/3D nanoporous g-C electrode over the Pt/3D nanoporous g-C electrode with respect to catalytic activity, CO tolerance, and stability.
The observed lower overpotential, superior electrocatalytic activity towards methanol oxidation and better CO tolerance at the Pt50-Ru50/3D nanoporous g-C electrode could be elucidated in terms of the bifunctional mechanism described elsewhere [145], in which Ru was proposed to be able to promote the oxidation of the strongly adsorbed CO on Pt by supplying an oxygen source (Ru-OHad). Moreover, the presence of 3D nanopores of g-C supports may also contribute to the observed higher current density because of the easy transport of methanol and the oxidation products in these nanopores [146].
6.5 Summary
We have demonstrated a new, simple and efficient method to synthesis the 3D nanoporous graphitic carbon (g-C) with high surface area and utilized as a support for Pt and Pt50–Ru50
alloy catalysts. The electrochemical results show that the Pt50-Ru50/3D nanoporous g-C electrode had lower overpotential, superior electrocatalytic performance towards methanol oxidation, and better CO tolerance, showing the direct synthesis 3D nanoporous g-C may have a better future as catalysts support in electrocatalysis and fuel cells.
Chapter 7
Synthesis of 2D Continuous Pt Island Networks for Methanol Electrooxidation
7.1 Introduction
Over the past few years, synthesis of nanostructured materials has received great interest in the technology due to its wide range of applications in biosensors [147], energy system [148], catalysis [149], and in self-assembly of supramolecular structures [150].
Nanostructured Pt metals materials are very attractive because of their superior electrocatalytic performance than the blanket Pt electrode. Recently several methods have been reported for the preparation on nanostructure but it is difficult and time-consuming to prepare the nanostructures [15-18, 151]. Besides, Pt is very expensive, resource limited and irreversibly inactivated by CO-like poisoning species. Therefore, it is essential that the utilization of platinum should be kept as low as possible without sacrificing the catalytic performance. The one best way to accomplish this is to create continuous Pt island networks.
The interconnected structure could have additional advantages in enhancing catalytic activities for reactions that involve two or more reactants, because such networks supply enough absorption sites for reactant molecules over a close range [152].
Herein, we report a new and simple method for fabricating continuous Pt island networks by pulse-potentiostatic electrodeposition using Si substrates of low resistivity, which act as the current collector. And thus, the silicon support was very appropriate for use as the Pt electrocatalytic electrode in respect of electrical conductivity. As shown in Fig. 7-1, the presence of the surface oxide layer on the silicon substrate can greatly enhanced the oxidation of CO adsorbed on the active Pt sites according to the bifunctional mechanism. Moreover, the electrocatalytical study of the continuous Pt island network on the silicon substrate indicates
the potential application for electrodes in direct methanol fuel cells.
Figure 7-1 shows a schematic illustration of the continuous Pt island network on the flat silicon substrate. The right hand side exhibits the bifunctional mechanism of CO electrooxidation. The adsorbed oxygen containing species on the surface of SiO2
can facilitate the oxidation of CO-like poisoning species adsorbed on the active Pt sites.
7.2 Fabrication steps for the continuous island Pt network electrode
The fabrication steps for the continuous island Pt network electrode are described in the following: a flat Si substrate was washed with acetone followed by DI water (>18 MΩ ), then etched in 10 wt% HF at room temperature for 5 min to remove the thin native oxide layer on the silicon surface. Then, the Pt particles were electrodeposited on the etched Si in the aqueous solution of 1 M K2PtCl6/ 1 M H2SO4 (100 mL/100 mL) at room temperature by potentiostatic pulse plating in a three electrode cell system with a saturated calomel reference electrode (SCE) [105]. The time durations for the high potential pulse (+0.08 V) and the low potential pulse (-0.01 V) were 3 and 1 ms, respectively. The blanket Pt catalyst was prepared by potentiostatic pulse plating (1 M K2PtCl6/ 1 M H2SO4) on the silicon substrate. The time durations for the high potential pulse (+0.06 V) and the low potential pulse (-0.04 V) were 5
CH3OH
and 2 ms, respectively. The Ru decorated blanket Pt electrode were obtained by deposition of Ru on the blanket Pt by potentiostatic pulse plating at -0.07 and +0.02 V in a 1 M RuCl3 (200 mL) solution for 5 and 1 ms respectively.
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
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