nanoparticles on a karst-like Ni Thin film toward methanol oxidation in alkaline solutions
6.1 Introduction
Transition metals are generally very corrosion resistant in alkaline solutions, and therefore they are good electrode materials for DMAFC applications. Because nickel hydroxides formed on Ni surfaces may act as chemical oxidizing agents for methanol oxidation in alkaline solutions [121, 122], the addition of Ni in Pt/Ru alloys can enhance the electrocatalytic activity of the electrode toward MOR [32]. Nickel has also been used as the Pt catalyst support for methanol electro-oxidation in alkaline solutions to enhance CO tolerance [29, 122]. In this study, we used HNO3 wet-etch to prepare rugged Ni thin films, on which Pt nanoparticles were pulse-electrodeposited, for the study of electrocatalytic methanol oxidation in alkaline solutions. The rugged Ni thin film has a karst-like morphology, and is referred to as karst-Ni thin film thereafter. Pt nanoparticles on the karst-Ni thin film demonstrate a great improvement in the electrocatalytic performance for methanol oxidation in the alkaline electrolyte as compared with a blanket Pt thin film and Pt particles on a blanket Ni thin film.
6.2 Material characterizations
The e-beam deposited Ni thin film becomes very rugged after the HNO3 wet etch, and has a surface morphology like a karst landform; we thus refer to the rugged Ni thin film as the karst-Ni thin film. The surface of the karst-Ni thin film is full of protruding structures of irregular shapes and cavities as shown in the cross-sectional SEM image of Fig. 6-1(b).
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To illustrate the dramatic change in the surface morphology of the Ni thin film after the HNO3 wet etch, the as-deposited Ni thin film is shown in Fig. 6-1(a). The open and rugged surface of the karst-Ni thin film provides a large loading area for electrodeposited Pt nanoparticles. Fig. 6-1(c) shows the side-view SEM image of the karst-Ni thin film after the Pt pulse-electrodeposition (thereafter referred to as Pt/karst-Ni). The karst-Ni thin film retains its rugged morphology after the Pt electrodeposition, but the edge of the protruding structures becomes less sharp because of the accumulation of Pt nanoparticles.
The size of the Pt nanoparticles is too small to be clearly observed in the SEM image.
According to TEM analysis discussed later, the size of the Pt nanoparticles is about 5 nm in diameter. We also prepared a blanket Ni thin film, on which Pt particles were subsequently electrodeposited (referred to as Pt/Ni), and a blanket Pt thin film for comparison with the Pt/karst-Ni thin film on the electrocatalytic activity toward MOR.
Figure 6-1 SEM images of (a) the as-deposited metallic nickel thin film, (b) the as-prepared karst-Ni thin film, (c) the karst-Ni thin film after the pulse-electrodeposition of Pt nanoparticles, and (d) the blanket-Ni thin film with pulse-electrodeposited Pt particles.
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Fig. 6-1(d) shows an SEM image of the Pt/Ni thin film. Pt particles on the Pt/Ni thin film have a broad size distribution ranging from a few nm to hundreds of nanometers.
Figure 6-2(a) shows a cross-sectional TEM image of the Pt/karst-Ni thin film. The irregularly shaped Ni nanostructures are decorated by dark spots of nanometer scale, which randomly distribute over the karst-Ni thin film as shown in the enlarged TEM image of Figs.
6-2(b) and (c). The Ni nanostructures exhibit a slightly porous feature as indicated by some areas of bright contrast. Figure 6-2(d) shows a high resolution TEM image of two nanoparticles selected from the area marked by the square in the TEM image of Fig. 6-2(c).
The lattice fringes clearly indicate that the nanoparticles are crystalline Pt grains with a diameter of ~5 nm. XRD analysis gives an average particle size slightly larger than 5 nm.
Based on the peak width of the Pt(111) diffraction peak (not shown), the average particle
Figure 6-2 TEM images of the karst-Ni thin film with electrodeposited Pt nanoparticles in different magnifications (a), (b) and (c); HRTEM image of Pt nanoparticles on the karst -Ni support (d).
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size of Pt nanoparticles on the karst-Ni support is about 5.6 nm according to the Scherrer equation.
Many studies have shown that transition metal oxide supports can greatly enhance the electrocatalytic activity of Pt toward methanol oxidation [15, 20]. The enhancement is generally ascribed to the bi-functional mechanism and/or the electronic effect, which are closely related to surface properties of the catalyst support [5, 106, 123]. To understand the surface property of the karst-Ni thin film, we used XRD and XPS to characterize chemical phases present on the HNO3-etched Ni nanostructures. As shown in Fig. 6-3, the karst-Ni thin film has the same x-ray diffraction feature as the as-deposited blanket Ni thin film; the three diffraction peaks situated at 45o, 52o and 76o are due to the (111), (200) and (220) lattice planes, respectively, of the face centered cubic (FCC) lattice structure of metallic nickel. Although the XRD result suggests that the karst-Ni thin film is composed of metallic Ni, the XPS analysis shows that little metallic Ni is present on the surface of the karst-Ni thin film. The XPS spectra of the as-deposited Ni thin film and the karst-Ni thin film are shown in Fig. 6-4. The as-deposited blanket Ni thin film has a broad Ni 2p3/2
peak with the shoulder feature marked by the dashed line at 852.7 eV, which corresponds to
Figure 6-3 X-ray diffraction spectra of the as-deposited Ni thin film and the karst-Ni thin film
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the 2p3/2 electron binding energy of metallic Ni. The rest of the broad Ni 2p3/2 peak at the higher binding energy side indicates the presence of an oxidized surface layer, which was formed during the shelf period waiting for the XPS analysis. For the as -prepared karst-Ni thin film and a karst-Ni thin film after 10 cycles of the CV scan (-0.9 to 0.2 V vs. SCE) in the aqueous solution of 1 M KOH + 1 M CH3OH, the Ni 2p3/2 signal is primarily emitted from oxidized Ni species with little contribution from the metallic phase. As discussed later in more detail, the oxidized surface layer comprise various oxidized Ni species, such as NiO, Ni(OH)2 and NiOOH. Combined with the XRD result, the XPS analysis suggests that the karst-Ni nanostructures has a metallic core, which is overlaid by a Ni oxide layer with a thickness larger than the escape depth of the Ni 2p3/2 photoelectron (~5 nm). The metallic Ni core is desirable for fast electrochemical kinetics for MOR in the alkaline solution because it has a good electrical conductivity.
Figure 6-4 Ni(2p) XPS spectra of the as-deposited Ni thin film, the as-prepared karst-Ni thin film and a karst-Ni thin film after a 10-cycle CV scan in the aqueous solution of 1 M CH3OH + 1 M KOH. The dashed line marks the Ni 2p3/2 binding energy of metallic Ni.
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6.3 Electrochemical measurements
Figure 6-5 shows cyclic voltammograms (CV) of methanol electro-oxidation in the aqueous solution of 1 M CH3OH and 1 M KOH for the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes. The CVs were taken over the range between -0.9 and +0.2 V at a scan rate of 20 mVs-1. Because methanol oxidation electrocatalyzed by Ni in alkaline solutions occurs in a higher potential range (0.36-0.45 V vs. SCE) [121], the anodic peak in the CV curve must represent the contribution entirely from the MOR electrocatalyzed by Pt.
The Pt/karst-Ni electrode exhibits a very good electrocatalytic activity toward MOR in the alkaline solution as shown by the much higher anodic peak maximum in the forward scan compared with the other two electrodes. The large forward anodic current of the Pt/karst-Ni electrode can be ascribed to its large electrochemical surface active area (ESA).
As determined from the CO stripping CV measurement in the 1 M KOH solution (discussed later in Fig. 6-6), the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes have an ESA of 511.2, 258.9 and 75.8 m2/g, respectively, assuming an oxidation charge of 0.484 mC for a
Figure 6-5 Cyclic voltammogarms of methanol electro-oxidation for the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes in the aqueous solution of 1 M CH3OH + 1 M KOH. The scan rate was 20 mVs-1. The current density is normalized to the sample surface area.
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monolayer of CO molecules adsorbed on a smooth Pt surface [20]. The peak potential of MOR for the Pt/karst-Ni electrode is -0.31 V vs. SCE with the onset potential at -0.54 V, which is herein defined as the MOR potential at which the current density reaches 10% of the peak maximum. For the Pt/Ni electrode, the MOR peak potential and the onset potential are situated at -0.29 V and -0.49 V vs. SCE, respectively. The blanket-Pt electrode has the MOR peak potential (-0.25 V) and the onset potential (-0.45 V) less negative than the two Ni-supported electrodes, suggesting that the Ni supports can improve the electrocatalytic activity.
In addition to a large ESA, efficient removal of carbonaceous intermediates or residuals from the Pt catalyst surface during MOR is crucial to the electrocatalytic performance of an electrode. In CV measurements for methanol electro-oxidation in alkaline electrolytes, the anodic peaks in the forward scan is associated with chemisorption of methanol molecules and oxidation of intermediate organic species, and the anodic peak in the reverse scan is due to oxidation of weakly bonded CHO species, which are incompletely oxidized intermediates in the forward scan [34]. The ratio of the forward anodic current (If) to the reverse anodic current (Ib), If/Ib, is generally used as a simple index to signify the ability of the Pt catalyst to resist CO poisoning during MOR [20, 39, 124]. A small If/Ib value indicates that the methanol electro-oxidation reaction has a poor kinetics, leaving excessive carbonaceous adspecies on the Pt catalyst surface. From Fig. 6-5, the If/Ib ratios of the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes are 6.25, 5.67 and 4.92, respectively, indicating that electrodes using Ni as the Pt catalyst support have better CO tolerance in the KOH electrolyte than the blanket-Pt electrode. The CO stripping CV measurement discussed below provides direct evidences of better CO tolerance of the Ni supported Pt catalyst in the alkaline solution.
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The CO-stripping cyclic voltammograms of the three electrodes are shown in Fig. 6-6.
For both the Ni supported Pt electrodes, a single broad peak is observed within the range between -0.8 and -0.1 V vs. SCE, while the blanket-Pt electrode has a doublet peak feature.
In an acidic solution, CO adspecies on Pt usually exhibit two electro-oxidation peaks in CO stripping CVs [46, 60, 125], and the observation of the two potential peaks is ascribed to CO oxidation occurring on different lattice planes of the Pt electrocatalyst [125]. The doublet peak feature observed in the KOH solution may result from CO electro -oxidation at different lattice sites on the polycrystalline Pt thin film as well. The absence of the doublet peak feature for the two Ni-supported electrodes are likely due to that the electrodeposited Pt particles do not have a preferential distribution in the orientation planes that yield the characteristic doublet feature for CO electro-oxidation. The onset potential and the anodic peak potential are listed in pair for the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes, respectively, as follows: (-0.75 V, -0.62 V), (-0.72V, -0.52 V) and (-0.69, -0.42), where in the parenthesis the first value is the onset potential. The lower CO stripping onset potential indicates that the Pt/karst-Ni electrode has a smaller overpotential for the CO electro-oxidation. Combined with the high ESA, the lower onset and peak potentials of
Figure 6-6 CO stripping cyclic voltammograms of the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes in the CO saturated 1 M KOH solution. The scan rate was 20 mVs-1. The current density is normalized to the sample surface area.
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CO electro-oxidation make the Pt/karst-Ni electrode much more electrocatalytically active toward MOR in the alkaline solution than the other two electrodes.
The improvement in the electro-oxidation activity of CO adspecies is generally ascribed to the bi-functional mechanism involved in the CO oxidation reaction and the electronic effect (ligand effect) due to charge transfer between the catalyst and the support [47, 126]. In the bi-functional mechanism model, oxygen containing adspecies, such as hydroxyl surface groups, can oxidize CO adspecies on the Pt catalyst, thereby avoiding CO poisoning. The CO electro-oxidation reaction via the bi-functional mechanism can be expressed by the following equation.
COPt + 2OH－M → CO2(g) + H2O +2e－ (6-1) where the subscript M represents Pt atoms or hydroxylated sites (e.g. Ni-OH) immediately adjacent to a CO-bonded Pt site. Chemical reaction processes on a surface are generally described by either the Langmuir-Hinshelwood (L-H) mechanism or the Eley-Rideal (E-R) mechanism. For methanol electro-oxidation in acidic electrolytes, the bifunctional mechanism can be best understood by the L-H mechanism, in which adsorbed reactants migrate on the surface and reactions take place by collision between adspecies. The CO electro-oxidation reaction via the L-H mechanism requires that OH adspecies be present on the Pt catalyst surface. Because of the deficiency in OH- ions in an acidic solution, OH adspecies on a Pt catalyst particle are primarily produced by dissociative adsorption of water molecules or OH spillover from neighboring hydroxylated sites. The ease of the CO oxidation reaction via the L-H mechanism greatly depends on adsorption properties of CO and OH adspecies, such as the (CO)-Pt bond strength and the surface diffusivity of CO and OH adspecies. Opposite to acidic solutions, alkaline electrolytes contain abundant OH -ions, and thus the E-R mechanism should also be considered an important reaction pathway leading to the CO electro-oxidation and removal. In an E-R surface reaction, the reaction product is formed by direct collision of a reactant species from the solution phase with a
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reactant adspecies. According to the E-R mechanism model, CO adspecies on the Pt catalyst can be readily oxidized by OH- ions diffusing from the bulk alkaline electrolyte.
Because all the three electrodes of this study are immersed in the KOH solution of the same chemical ingredients, the collision rate per unit area of OH- ions with CO adspecies on the three electrode must be the same. Under such condition, the E-R mechanism should be more prevailing on the electrode with CO adsorption structures allowing more effective CO oxidation and easier (CO)-Pt bond breaking. From the above discussion, for both the E-R and the L-H mechanisms, the electrochemical activity of the CO oxidation reaction is closely related to adsorption properties of the CO adspecies. Because CO adsorption properties are governed by surface properties of the Pt catalyst, which can be modified by the Ni support, the electronic interaction between the Pt catalyst and the Ni support can greatly influence the electrochemical activity of CO oxidation on the Ni-supported electrodes of this study. According to XPS analysis, the Pt/karst-Ni and the Pt/Ni electrodes have a large negative shift in the Pt 4f7/2 binding energy, indicating that charge transfer occurs between the Pt catalyst and the Ni support.
The Pt(4f) XPS spectrum of the Pt/karst-Ni thin film shown in Fig. 6-7 exhibits a doublet peak with the Pt 4f7/2 peak maximum at 70.7 eV, which negatively shifts from that of the blanket Pt thin film by ~0.7 eV. The negative binding energy shift of the Pt 4f doublet peak can be ascribed to either the nanometer-size effect of Pt nanoparticles or charge transfer between Pt nanoparticles and the Ni support, or a combination of the both effects. It has been widely reported that the binding energy of core level electrons of metal nanoparticles shifts from that of the bulk counterparts as a function of the particle size [116-118]. The binding energy shift generally increases with decreasing the size of metal nanoparticles; and the size effect on the energy shift becomes insignificant when the particle size is larger than 3 nm [116]. Because Pt nanoparticles on the Pt/karst-Ni electrode have a particle size around 5 nm, the large negative shift (-0.7 eV) in the Pt 4f7/2
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binding energy suggests that negative charge transfer from the karst-Ni support to Pt nanoparticles has occurred. A theoretical study showed that, upon adsorption on the NiO(100) surface, Pt atoms sit on the oxygen-top site and form strong chemical bonds with the surface as a result of a change in the electronic configuration of Pt atoms [127]. It is likely that Pt atoms adsorbed on nickel hydroxides are also subject to a strong electronic modification. Surface atoms of a smaller Pt nanoparticle should experience a larger electronic modification because of their close proximity to the interface between the nanoparticle and the Ni support, at where the charge transfer takes place. For the Pt/Ni electrode, the curve-fitted Pt 4f7/2 peak has the maximum at 71.0 eV and has a slightly larger full-width-at-half-maximum (FWHM) than the Pt/karst-Ni electrode. The less negative Pt 4f energy shift and the larger FWHM indicate that Pt particles on the Pt/Ni electrode have a smaller electronic modification with a wider distribution of the modification strength. This is because the Pt particles on the Pt/Ni electrode have a wide size distribution ranging from a few to a few hundred nanometers, and, as a result, surface
Figure 6-7 The Pt 4f XPS spectra of the Pt/karst-Ni, the Pt/Ni and the blanket-Pt electrodes. In the spectra of the two Ni-supported electrodes, the peak situated around 67.6 eV is due to the Ni 3p signal emitted from the Ni support. Curve fitting was carried out so that the Pt 4f7/2 binding energy for the two Ni-supported electrodes could be more accurately determined.
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atoms clearly experiencing the electronic modification are far fewer than atoms lacking the modification. Figure 6-8 schematically illustrates the progressive decrease in the electronic modification as a function of the distance from the interface between a Pt nanoparticle and the hydorxylated Ni support. The colored area indicates the region affected by the charge transfer between the Pt nanoparticle and the Ni support, and the gradually shaded color represents the degree of the induced electronic modification. A smaller nanoparticle is apparently has a larger portion of surface atoms that are subject to the electronic modification. Because of the greater electronic modification of Pt nanoparticles, the Pt/karst-Ni electrode must exhibit an electrocatalytic activity distinct from the Pt/Ni and the blanket-Pt electrodes. The better CO electro-oxidation performance for the Pt/karst-Ni electrode, as shown in Fig. 6-6, suggests that the electronic modification due to the charge transfer results in a (CO)-Pt adsorption structure that enhances the CO electro-oxidation activity in the alkaline solution. The Pt/Ni electrode also benefits from the electronic modification for better CO tolerance although the improvement is less significant than the Pt/karst-Ni electrode.
In a study of the mechanism of CO oxidation in NaOH solutions, Spendelow et al, found that CO electro-oxidation on the Pt(111) surface was primarily via the L-H mechanism [128]. In such case, OH adspecies must be present on the Pt catalyst surface so that the CO oxidation reaction can proceed, as shown by reaction 1 in Fig. 6-8. A previous study has shown that OH adsorption on the Pt(111) surface in alkaline solutions occurs in the potential range between 0.65 and 0.85 V vs. RHE (equivalent to -0.36 − -0.16 V vs. SCE) [129]. The potential range for OH adsorption on the Pt(111) surface is much less negative than the measured potential range (-0.8 − -0.4 V vs. SCE) for the CO electro-oxidation reaction occurring on the Pt/karst-Ni electrode. Although Pt
In a study of the mechanism of CO oxidation in NaOH solutions, Spendelow et al, found that CO electro-oxidation on the Pt(111) surface was primarily via the L-H mechanism [128]. In such case, OH adspecies must be present on the Pt catalyst surface so that the CO oxidation reaction can proceed, as shown by reaction 1 in Fig. 6-8. A previous study has shown that OH adsorption on the Pt(111) surface in alkaline solutions occurs in the potential range between 0.65 and 0.85 V vs. RHE (equivalent to -0.36 − -0.16 V vs. SCE) [129]. The potential range for OH adsorption on the Pt(111) surface is much less negative than the measured potential range (-0.8 − -0.4 V vs. SCE) for the CO electro-oxidation reaction occurring on the Pt/karst-Ni electrode. Although Pt