Letter pubs.acs.org/acscatalysis
Platinum-Based Electrocatalysts for the Oxygen-Reduction Reaction: Determining the Role of Pure Electronic Charge Transfer in Electrocatalysis Xiaoming Wang,* Yuki Orikasa, and Yoshiharu Uchimoto Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan S Supporting Information *
ABSTRACT: In the oxygen-reduction reaction (ORR), electronic charge transfer (ECT) derived from alloy components and support materials generates noticeable impact on the electrocatalytic activity of Pt. However, generally, ECT will not individually occur; thus, its role remains controversial. Here, using different amount of Au to decorate Pt nanoparticles, ECT from Au to Pt is isolated to correlate with the ORR activity of Pt. The linear correlation, where pure ECT (assessed by Pt 5d orbital vacancy) depresses the adsorption of oxygenated species to enhance the ORR activity, predicts that the maximum activity enhancement should be smaller than 200%. These findings highlight that the ECT effect in the ORR is weaker than the previously reported size, facet, or strain effects, which establishes a basis for understanding exceptional ORR electrocatalysis and developing efficient Pt-based electrocatalysts. KEYWORDS: Fuel cells, oxygen-reduction reaction, pure electronic charge transfer, Pt-based electrocatalysts, Au-decorated Pt nanoparticles
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Scheme 1. Traditional Pt-Based ORR Electrocatalysts: (a) Carbon-Supported Pt NPs, (b) Carbon-Supported Pt Alloy NPs, (c) Metal Oxide/Carbide-Supported Pt NPs; and Selected Pt-Based ORR Electrocatalyst with Pure ECT; (d) Carbon-Supported Au-Decorated Pt NPs
ne of the great challenges in the commercialization of fuel-cell technology is to reduce Pt usage in the cathodic oxygen-reduction reaction (ORR).1−3 To achieve this target, many methods, such as alloying with secondary transition metals4,5 and supporting onto transition-metal oxides/ carbides,6−8 have been successfully employed in previous studies. The common feature in these methods is that alloy components or support materials can induce electronic charge transfer (ECT) to Pt. Thus, for understanding exceptional oxygen-reduction electrocatalysis and developing efficient Ptbased electrocatalysts, it is quite necessary to clarify ECT effect on the ORR activity of Pt. Fundamental understanding on this effect, in principle, requires an explicit relationship between ECT and the ORR activity. The difficulties in this quest are the isolation and assessment of ECT. For theoretical studies, although ideal Ptbased models can be used to isolate ECT, the ECT is difficult to be observed.9 This disadvantage is largely induced by the unobvious ECT between two metallic atoms with different element electronegativity, especially for a structural model of single crystal. In contrast, for experimental studies, the ECT can be quantified,10 while traditional Pt-based electrocatalysts cannot be employed to isolate ECT. As shown in Schemes 1a−c, for traditional Pt-based electrocatalysts, ECT derived from alloy components and support materials is usually accompanied by the unavoidable changes of Pt structure including particle size, crystal facet, or lattice strain,4,8 all of which can affect the electronic structure or catalytic activity of Pt.11−14 These complexities decide that the role of ECT in the ORR activity is still controversial,15,16 which largely disturbs us to understand enhanced ORR electrocatalysis and develop desired Pt-based electrocatalysts.6,17 © XXXX American Chemical Society
In this work, by searching all nanostructures,18 Au-decorated Pt nanoparticles (NPs, Scheme 1d) have been selected to experimentally determine the role of ECT in the ORR activity of Pt. Compared to traditional Pt-based electrocatalysts (Schemes 1a−c), we deem that Au-decorated Pt NPs have several advantages. First, the Pt particle size can be the same. Second, the location of Au on Pt should be random; thus, the exposure of the crystal facets of all Pt clusters remains unchanged. Third, decorated Au should generate a negligible impact on the lattice strain of Pt. As a result, the influence of these main structural factors including particle size, crystal facet, and lattice strain on the electronic structure or catalytic activity of Pt can be excluded. Thus, the only electronic interaction Received: February 18, 2016 Revised: May 25, 2016
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DOI: 10.1021/acscatal.6b00497 ACS Catal. 2016, 6, 4195−4198
Letter
ACS Catalysis
First, in the synthesis of all carbon-supported Au-decorated Pt samples, the same Pt (TKK, 4.1 nm) has been employed, which guarantees that the particle size of Pt remains unchanged. It should be noted, when the Cu mediator on Pt is displaced by Au, since the H2SO4 electrolyte cannot provide the required Cl− ions, Pt is difficult to be displaced by Au (3Pt + 2AuCl4− + 4Cl− = 3PtCl42− + 2Au). In the similar case of the displacement of Pd by Au,20 it has been demonstrated that only the apexes of Pd can be displaced by Au. Second, the information on the crystal facets of Pt in all carbon-supported Au-decorated Pt NPs can be obtained from the electrochemical property. The normalized adsorption and desorption of H+, which indicates the exposure of the different crystal facets of Pt, have been shown in Figure S4 in the Supporting Information. As observed, the adsorption and desorption behaviors of H+ are same. Since there is no adsorption and desorption of H+ on Au,21 Figure S4 indicates that decorated Au will not generate the selective exposure of the crystal facets of Pt. Third, because of the electronic perturbation of Au on Pt, the electron orbits of Pt might rehybridize and thus the lattice strain of Pt will vary. So, the lattice strain of Pt in carbonsupported Au-decorated Pt samples has been assessed by using the average Pt−Pt bond distance, which is usually determined by fitting the EXAFS from XAS.22 In this work, to avoid the overlap of the EXAFS spectra from XAS signals at Pt LIII (11 560 eV) and Au LIII (11 920 eV) edges (Figure S2), the EXAFS spectra from XAS signal at the Pt K-edge has been used to obtain the Pt−Pt bond distance. In Figure 2a, the k3-
(pure ECT) between Pt and Au can be achieved to correlate with the ORR activity of Pt. The synthesis of carbon-supported Au-decorated Pt NPs has been shown in Figure S1 in the Supporting Information. Via the displacement of an underpotentially deposited Cu mediator, Au can be decorated onto the surface of carbonsupported Pt (Tanaka Kikinzoku Kogyo (TKK), 4.1 nm). During the post-treatment of cyclic voltammetry (50 mV s−1; 0−1.4 VRHE; three cycles) in N2-saturated 0.1 M HClO4, monodispersed Au atoms have a tendency to become stable Au clusters, largely due to the excessive lateral stress (atomic radius: Au, 1.44 Å; Pt, 1.39 Å) and the limited unpaired 6s electrons (electron configuration: Au, 5d 10−x 6s 1+x ; Pt, 5d9+x6s1−x), as reported by the previous publication.19 By changing the potential and the number of times for Cu underpotential deposition, different amounts of Au can be decorated onto Pt NPs. The amount of decorated Au can be calculated according to underpotentially deposited Cu mediator. The calculated molar ratio between Pt and Au varies from 1:0.15 to 1:0.9. This result can be determined by X-ray absorption spectroscopy (XAS),19 where the absorption intensity ratio of extended X-ray absorption fine structure (EXAFS) spectra (Figure S2 in the Supporting Information) at Pt LIII and Au LIII edges varies from 1:0.22 to 1:0.8. The decoration of Au on Pt can be confirmed by cyclic voltammogram (Figure S3 in the Supporting Information), where the reduction peak of Au and Pt increases and decreases, respectively. Here, the decrease of the electrochemical surface area (ECSA) of Pt indicates the decoration of Au on Pt. In addition, according to the change of the ECSA of Pt, we know that the coverage of Au on Pt is in the range from 12.3% to 32.0%. The detailed structure of Audecorated Pt NPs can be proved by the transmission electron microscopy (TEM) images of a representative sample (molar ratio, Pt:Au = 1:0.8; Au coverage: 32%). In Figure 1, the low-
Figure 2. (a) Fourier-transformed EXAFS spectra derived from XAS at the Pt K-edge, and (b) average Pt−Pt bond distance fitted from the EXAFS spectra of carbon-supported Pt and Au-decorated Pt NPs (Au coverage, 15.6%, 21.4%, 32.0%).
weighted Fourier-transformed EXAFS spectra of carbonsupported Pt before and after the decoration of Au (coverage, 15.6%, 21.4%, 32.0%) have been shown. As observed, the same Pt−Pt (containing negligible Pt−Au) amplitudes in 1.7−3.0 Å indicate almost identical Pt−Pt bond distance in all samples. In Figure 2b, all average Pt−Pt bond distance fitted from the EXAFS spectra (Figure S5 and Table S1 in the Supporting Information) distributes in a narrow range of 2.752 ± 0.001 Å. Such a minor fluctuation is much smaller than the minimal error range (±0.003 Å) of Pt−Pt bond distance. Thus, decorated Au does not generate the change in the lattice strain of Pt. In the above characterizations, the main structural changes of Pt, such as the particle size, crystal facet, and lattice strain, have been excluded. In this sense, the electronic interaction between Pt and Au has been successfully isolated. It should be noted that decorated Au on Pt also produces some minor structural issues. On the one hand, Au cluster will generate some Au−Au pairs for the ORR. On the other hand, Au cluster will impact
Figure 1. (a and b, inset) TEM images of carbon-supported Audecorated Pt NPs (molar ratio, Pt: Au = 1:0.8; Au coverage, 32.0%).
resolution images (Figures 1a and 1b) show that the majority of NPs are composed of neighboring clusters, and the highresolution image (the inset) shows the morphology of two isolated clusters. These results are in good agreement with a previous publication.19 Besides, for the particle size of Au, it is very difficult to be determined due to the similar properties of Pt and Au in various characterizations. As mentioned above, the main structural factors, including particle size, crystal facet, and lattice strain can impact the electronic structure or catalytic activity of Pt. Thus, for all carbon-supported Au-decorated Pt samples, it is necessary to confirm that their Pt does not have these structural changes. 4196
DOI: 10.1021/acscatal.6b00497 ACS Catal. 2016, 6, 4195−4198
Letter
ACS Catalysis
to a 9% decline in Pt 5d orbital vacancy.6 However, even so, the trend in Figure 4a predicts that the maximum enhancement in the ORR activity is still smaller than 200%. In the section of structural characterization, we have mentioned some Au-induced minor structural issues. First, exposed Au might activate O2. Generally, the two-electron reduction of O2 to H2O2 on Au depends on the pH value of electrolyte,27 the synergy of support materials,28,29 and the low working potential. In our experiment, the activation of O2 on Au is extremely difficult to occur due to the strong acidic electrolyte (0.1 M HClO4), usual support material (Carbon Black) and high working potential (0.9 VRHE). Furthermore, the adsorption energy of O2 on Au cluster has been simulated using a model of Au6 decorated Pt(111) single crystal (Figure S8 in the Supporting Information). As shown in Table S3 in the Supporting Information, the adsorption energy of O2 on Au is much smaller than that on Pt, indicating that nanosized Au cluster in this work (Figure 1) should generate the small contribution for the ORR (Figure S9 in the Supporting Information). Second, decorated Au will produce some Pt−Au pairs, and it will increase the coordination number of surrounding Pt atoms. For the former, the adsorption energy of O2 on Pt−Au (Table S3), which is still much smaller than that on Pt−Pt, confirms that Pt−Au also generates the small contribution for the ORR. For the latter, the increased coordination number of surrounding Pt atoms will induce the structural relaxation. But, the direction of relation is radial, not lateral.30 For the ORR, its activity is dependent on the lateral strain of Pt−Pt. As a result, the radial relaxation of these surrounding Pt atoms should not affect the ORR activity. Thus, if we exclude the small contribution of these minor structural factors in the enhanced ORR activity, the maximum enhancement predicted by previous ECT-activity trend should be much smaller than 200%. Furthermore, the origin of ECT to enhance ORR activity has been explored. Theoretical investigation indicates that the sluggish ORR activity of Pt should ascribe to the overlarge adsorption of O2 and its intermediates on Pt.31 Thus, it is essential to assess the adsorption ability of Pt at electrochemical conditions. The adsorption ability of Pt can be determined by the charge ratio between oxygenated species (QOH + QO, ca. 0.5−0.9 VRHE) and hydrogen (QH, ca. 0.03−0.4 VRHE) from cyclic voltammograms (Figure S3 in the Supporting Information) in N2-saturated 0.1 M HClO4. Here, the given potential is 0.9 VRHE, which will not induce the oxidation of Au, as identified by the reduction potential (higher than 0.95 VRHE) of Au in Figure S3. In Figure 4b, Pt 5d orbital vacancy has been correlated with the (QOH + QO)/QH. Compared with Figure 4a, a reversed linear trend, where the reduced Pt 5d orbital vacancy generates the depressed (QOH + QO)/QH, has been obtained. The lessened (QOH + QO)/QH means weak adsorption ability and more active sites of Pt in the ORR.32 The weak adsorption ability of Au-decorated Pt is also in good agreement with the calculation in Table S3. Thus, the eventual ORR activity can be enhanced. Compared with previously reported structural (e.g., particle size, crystal facet, and lattice strain) effects, the ECT effect in Pt-based electrocatalysts on the ORR activity is clear. For the effect of particle size (1−3 nm), the large-sized Pt with a small fraction of edge sites can generate 3-fold greater enhanced ORR activity than the small-sized Pt.33 For the effect of crystal facet, the low-index facets of Pt also possess different ORR activity. Among them, Pt(110) containing step atoms can generate
the surrounding Pt atoms. These influences will be discussed in the section of electrochemical test. The electronic interaction between Pt and Au can be assessed by Pt 5d orbital vacancy, which is calculated from the X-ray near-edge structure (XANES) (Figure S6, Table S2 in the Supporting Information) of in situ XAS (0.5 VRHE, at which no oxygenated species exist on exposed Pt surface) at Pt LIII and LII edges.10,22 As reported by various characterizations including X-ray photoelectron spectroscopy (XPS) and XAS,23,24 ECT from Au to Pt has been exhibited in Figure 3,
Figure 3. ECT from Au to Pt (Pt 5d orbital vacancy) as a function of the coverage of Au on Pt.
where the increased coverage of Au leads to the linear decrease in Pt 5d orbital vacancy. At the utmost, the decrease reaches 5%. It is widely accepted that the absolute magnitude of the ECT between two metallic atoms due to their different element electronegativity should be modest. Experimental observations also indicate that the decrease in Pt 5d orbital vacancy of many Pt-based systems is always
