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Advances in Physical Chemistry
Volume 2011 (2011), Article ID 530397, 16 pages
http://dx.doi.org/10.1155/2011/530397
Review Article

Platinum Monolayer Electrocatalysts for the Oxygen Reduction Reaction: Improvements Induced by Surface and Subsurface Modifications of Cores

Department of Chemistry, Brookhaven National Laboratory, Upton, NY 11973, USA

Received 18 May 2011; Accepted 22 August 2011

Academic Editor: Wolfgang Schmickler

Copyright © 2011 Yun Cai and Radoslav R. Adzic. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper demonstrates that the ORR activity of electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low-coordination sites, generation (via addition or segregation) of an interlayer between and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the , involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation electrocatalysts.

1. Introduction

Fuel cells are considered to be one of the most promising clean energy technologies that can help resolve the energy crisis and problems of environmental pollution. However, the promise of their widespread application is seriously hindered by the necessarily high content of Pt in the cathode catalysts and the slow kinetics of oxygen reduction reaction on the best available Pt-based catalysts. New catalysts are needed that reduce considerably the Pt content while affording the possibility of enhanced catalytic activity. In recent years, we developed a new approach for designing and synthesizing electrocatalysts that significantly reduce the Pt content and surpass the oxygen reduction reaction (ORR) activity of the state-of-the-art carbon-supported Pt electrocatalysts [1]. These electrocatalysts consist of a monolayer of Pt on carbon-supported metal or metal-alloy nanoparticles and has the highest utilization of Pt since almost every Pt atom are present on the surface and participate in the electrocatalytic reactions. These electrocatalysts are referred as platinum monolayer (PtML) electrocatalysts. The Pt monolayer deposition process involves the galvanic displacement of an underpotentially deposited Cu monolayer on a suitable substrate by Pt [2].

Since the first development, PtML electrocatalysts with various core materials were designed and synthesized for assorted reactions in fuel-cell applications [1, 310]. Many of them exhibit excellent activities and stabilities. The interaction between the Pt monolayer and the substrate material has been demonstrated to induce a synergistic effect for ORR kinetics [6, 1113]. When Pt atoms deposit on a foreign metal, due to the difference in atomic radius, the Pt surface will undergo compressive or tensile strain. As illustrated in Figure 1 [1], Pt atoms deposited on a Ru substrate would have a large compressive strain, but they would have only a small compressive strain on Pd and a tensile strain when deposited on Au. Santos et al. also observed such strain effect and demonstrated a strong correlation between the strain and electrocatalytic activities [14]. D-band center shift induced by the surface strain has been demonstrated to be a major factor determining the catalysts’ activity [15]. Moreover, electronic (ligand) effect from the electronic coupling between PtML and its supporting substrate play an additional role in determining the catalytic activity [16]. Density functional theory (DFT) calculations have shown that the binding energies and reactivity of small adsorbates have a great correlation with the position of d-band center on strained surfaces and metal overlayers [17, 18]. The interaction with the proper substrate can also decrease the oxidation of Pt monolayer and thus enhance the stability of the catalyst.

530397.fig.001
Figure 1: Models of pseudomorphic monolayers of Pt on three different substrates inducing compressive strain (Ru(0001) and Pd(111)) and expansive strain (Au(111)) [1].

In our earlier study (Figure 2) [6], we demonstrated that the electrocatalytic activity for ORR of Pt monolayer on six different single crystal surfaces has a volcano-type dependence on the position of their d-band center from DFT calculations. On the top of the “volcano” plot is the Pt monolayer on a Pd(111) surface, that is, the most active electrocatalyst among six Pt monolayer-substrate couples, plus the single Pt(111) surface. Our experiments on various Pt monolayer nanoparticles also demonstrated that the variation of the core materials significantly affect the performance of the catalyst [1, 310]. Among the PtML electrocatalysts we studied for the ORR, the palladium and palladium alloy cores outperform the rest in terms of both activity and stability [6, 19].

fig2
Figure 2: (a) Polarization curves for O2 reduction on platinum monolayers (PtML) on Ru(0001), Ir(111), Rh(111), Au(111), and Pd(111) in a 0.1 M HClO4 solution on a disk electrode. The curve for Pt(111) is taken from [19, 20] and included for comparison. The rotation rate is 1600 rpm, and the sweep rate is 20 mVs−1 (50 mVs−1 for Pt(111)); = current density, RHE = reverible hydrogen electrode [6]. (b) Kinetic currents ( , square symbols) at 0.8 V for O2 reduction on the platinum monolayers supported on different single-crystal surfaces in a 0.1 M HClO4 solution and calculated binding energies of atomic oxygen (BEo, filled circles) as functions of calculated d-band center ( ); relative to the Fermi level) of the respective clean platinum monolayers. The current data for Pt(111) is taken from [19, 20] and included for comparison. Labels: (1) PtML/Ru(0001), (2) PtML/Ir(111), (3) PtML/Rh(111), (4) PtML/Au(111), (5) Pt(111), and (6) PtML/Pd(111) [6].

Several unique features of PtML electrocatalysts open various possibilities for designing electrocatalysts with specific catalytic properties by choosing appropriate substrates. The strong dependence of the catalytic activity on the interaction between Pt monolayer and its substrate suggests that the electrocatalytic activity of Pt monolayer catalysts may be further improved via the modification of substrate materials. Since palladium and palladium alloy are the best candidates as a substrate to support PtML, they will be our major focus in the efforts of achieving further enhanced activity and stability. In the ORR kinetics, the reaction rate is either limited by the dissociation of O2 or the protonation of O2 for metal surface that binds oxygen too weakly, or by the removal of adsorbed O and OH species if the metal binds oxygen too strong [21]. In the case of Pd, the oxygen binding energy is still slightly higher than the optimum value for the ORR kinetics [16], therefore in the design of the new generation PtML electrocatalysts, we are targeting at fine-tuning the core materials to further weaker the oxygen binding on Pd substrate. In this review, we will describe several examples demonstrating the improvements of the Pt monolayer electrocatalysts for the ORR via the modifications of surface and subsurface of the core materials.

2. Direct Modification of the Surface of the Core [22]

Although the Pd(111) substrate for Pt monolayer has been demonstrated possessing an oxygen binding energy close to the optimum value, the nanoparticle Pd, however, exhibits much more difficulty in the removing of OH species due to the existence of considerable amount of low-coordination sites, edges, defects, adatoms, and so forth, which bind oxygen much stronger than that on terrace sites [23]. To obtain a smoother Pd nanoparticle surface, we developed a strategy, referred to as bromide treatment, to successfully remove significant amount of low-coordination sites on Pd/C and Pd3Co/C and produce more (111) facets while maintaining the particle size.

The schematic in Figure 3 illustrates how the bromide solution removes the atoms on low-coordination sites during the potential cycles. In brief, the process starts from the chemisorption of a bromine layer in an alkaline solution, followed by the reductive desorption of bromine. After immersing the electrode in the bromide-containing solution, immediately is adsorbed on the surface, preferentially on the low-coordination sites, since the atoms on those sites bind with Br more strongly than those on terrace sites. In the cathodic scan, the reductive desorption of the bromine triggers the migration of low-coordinated Pd to terrace sites to minimize the total surface-free energy. Moreover, the reduction of Br to introduces a positive charge in the adjacent Pd, and the low-coordinated atoms have a looser structure than the high-coordinated ones. Hence, it is likely that the low-coordinated Pd forms a Pd-Br2 pair in solution, that is, redeposited onto the surface in the following anodic scan, accompanied by the oxidative adsorption of bromide. Meanwhile, during the cycling, the adsorbed bromine layer undergoes rearrangement to attain the stable adlayer structure on a given surface, for example, ( ) R30°-Br on Pd(111), (2 × 2)-Br on Pd(100) [24]. Therefore, during this Br-rearrangement on the surface, the adsorbed-Br may draw the dangling atoms to fill the defect sites to form a large terrace patches containing an ordered Br-adlayer if the original terrace size is large enough to stabilize it.

530397.fig.003
Figure 3: Illustration of the removal of surface low-coordination sites via the oxidative adsorption and reductive desorption of a bromine layer [22].

Figure 4 shows the TEM images of palladium nanoparticles before and after bromide treatment. Various shapes of particles with obtruding edges are present in the commercial E-TEK sample. In contrast, the Br-treated Pd/C nanoparticles are rounded, and their size distribution is narrow.

fig4
Figure 4: High-resolution TEM images of Pd/C before (a) and after (b) Br-treatment [22].

The formation of a well-ordered 2D metal overlayer via UPD is known to depend on the substrate’s crystallographic orientation and density of imperfection [25]. The less the amount of imperfections at the surface the better the formation of more ordered commensurate 2D phases is, provided that the atomic sizes are not too dissimilar. In this case, more Pt(111) 2D patches instead of 3D clusters would form on the bromide-treated surface, which in turn, should benefit the oxygen-reduction reaction.

Figure 5 shows the polarization curves for the ORR. The sample with PtML on Br-treated-Pd/C nanoparticles shows significant enhancement of kinetics of the ORR, compared with the untreated nanoparticles, especially at potentials more negative than the half-wave potential (marked by a circle), the so-called combined diffusion-kinetic control region. In this region, the adsorption and dissociation of molecular oxygen competes with strongly adsorbed hydroxyl species (OHads) for the same sites [20, 23, 26]. Furthermore, OHads does not only block the active sites on Pt, but also changes the adsorption energy of intermediates adjacent to it formed during the reaction [20]. On a smooth surface, OH binding is less strong with respect to that on edge sites, and thus, the inhibition of the ORR kinetics decreases. We summarize the corresponding mass activity and specific activity for the ORR in the inset of Figure 5; 0.25-fold and 0.5-fold enhancements were found, respectively.

530397.fig.005
Figure 5: Polarization curves of oxygen reduction at PtML on Br-treated and -untreated Pd/C. Inset: comparison of mass activity and specific activity on PtML/Pd/C [22].
2.1. Br-Treatment of Pd Co/C

The oxygen-reduction reaction on PtML/Pd3Co/C attained a Pt mass activity 2~3-fold that of commercial Pt/C [7]. However, its poor stability due to the dissolution of Co hampered its industrial applications. Because the surface Co leaches during the reaction, more low-coordination sites are generated, thereby accelerating further dissolution of the Co, and degrading the catalyst. Our bromide-treatments of Pd/C nanoparticles demonstrated an excellent method of removing the low-coordination sites by having a bromine-adlayer and the resulting advantages for the ORR.

Figure 6 shows the morphologies of the original Pd3Co/C nanoparticles, and of PtML on Pd3Co/C after the Br-treatment. The particles in the original Pd3Co/C have average diameter of 4.25 nm, but a wide size distribution, from 1.1 to 10 nm. Many small particles with a diameter of 3 nm were present and considerable number of defects and edges on the particles are observed. The Pt monolayer catalysts supported on Br-treated Pd3Co/C nanoparticles show a much more uniform particle size, averaging 5.8 nm. Considering this size includes the thickness of a Pt monolayer, the average Br-treated Pd3Co/C nanoparticles should be around 5.5 nm at most. The Br-treated nanoparticles exhibit a large area of well-defined (111)-crystalline structure, even after the deposition of the platinum monolayer while the untreated-Pd3Co/C particles have much smaller patches with poorly defined crystalline structures. Seemingly, after bromide treatment, most particles with diameters less than 3 nm dissolved and were redeposited on larger particles, suggesting the occurrence of Ostwald ripening [27] during the Br-treatment.

fig6
Figure 6: High-resolution TEM images of the original Pd3Co/C and of PtML on Br-treated Pd3Co/C [22].

Figure 7 shows the CV and polarization curves for ORR on PtML catalysts supported on Br-treated Pd3Co/C after various potential cycles. The activity and surface area remains unchanged after 25,000 potential cycles between 0.6 and 1.0 V. It is a significant improvement of the stability since the same test with Johnson Matthey’s PtML/Pd3Co/C resulted in a negative shift of the half-wave potential of more than 30 mV in the first 5000 cycles.

fig7
Figure 7: Voltammetry curves for the PtML/Br-treated-Pd3Co/C (a), and polarization curves for the ORR (b) in a stability test involving 25,000 potential cycles [22].

The above examples of surface modifications of Pd and Pd3Co core demonstrated that the removal of surface low-coordination sites via bromide-treatment enables to obtain nanoparticles with smooth surfaces having a high density of (111)-oriented facets and a slightly contracted structure. The after-treated cores become excellent substrate for Pt monolayer and significant enhancements were realized of the mass activity and specific activity for the ORR.

3. Addition of an Interlayer between the Pt Monolayer and the Core

3.1. Pt /Pd/IrCo/C [28]

Iridium is one of the most stable transition metals; its dissolution potential is comparable with platinum [29]. Its particular activity for the ORR is enhanced by alloying it with cobalt or platinum [30, 31]. However, Ir is not a good support for a Pt monolayer because it causes the Pt lattice to contract too strongly, and consequently, significantly decreases the d-band center; that is, it lowers the reactivity of Pt, entailing a very weak adsorption of O2 on Pt and a slow ORR kinetics [1, 6, 32]. Moreover, the relatively low ORR activity on Ir alone and its alloys, confirmed experimentally and theoretically, results from the disproportionately strong binding energy of oxygen at the surface [6]. Thus, it is a challenge to design an electrocatalytic system containing a highly stable and less expensive core, such as iridium or its alloys, to support a Pt monolayer yielding an efficient catalyst. Oxygen binding of Pd, contract to Ir, located in the other side of the volcano plot, is much stronger. Therefore, to reduce the effect of Ir on the d-band center of a Pt monolayer, and to assure it can make a moderately strong bond with diatomic oxygen at the surface platinum, we placed a Pd monolayer (interlayer) between the Pt monolayer and the IrCo core. Thus, while Ir causes the Pt lattice to contract too strongly, and consequently entails low activity, a Pd support entails only a small contraction that suffices to increase the ORR kinetics on platinum.

Figure 8 depicts the voltammetry curves in the absence of O2 for the PtML/IrCo with and without a palladium interlayer. The effects of the Pd interlayer on the properties of a Pt monolayer are reflected in the shift of Pt oxidation in comparison with the curve without it. These effects are more pronounced on the ORR kinetics as shown in Figure 9. For both the as-prepared IrCo and the Ir3Co cores, the palladium interlayer engenders a positive shift of about 50 mV and 150 mV, respectively, in the half-wave potential compared to the corresponding electrocatalysts lacking a palladium interlayer, clearly demonstrating that this interlayer significantly increases the electrocatalytic activity for the ORR.

530397.fig.008
Figure 8: Cyclic voltammograms of IrCo/C (black), PtML/IrCo/C (red), and PtML/PdML/IrCo/C (blue) in argon-blanketed 0.1 M HClO4 at the scan rate of 50 mV s−1 [28].
fig9
Figure 9: Polarization curves of PtML/IrCo/C (a) and PtML/Ir3Co/C (b) with (red curves) and without (black curves) a palladium interlayer in oxygen-saturated 0.1 M HClO4. Scan rate: 10 mV s−1; rotation speed: 1600 rpm [28].

For comparison, the mass activities for oxygen reduction at 0.9 V (RHE) of the PtML/IrCo/C and PtML/PdML/IrCo/C were calculated and summarized in Figure 10; the activity on commercial Pt/C is included as a reference. The Pt loadings were calculated from the coulombic charge for the underpotential deposition of copper monolayer. It is exciting to find that the mass activity of the PtML/PdML/IrCo/C in terms of platinum mass and the platinum-group-metals’ mass is, respectively, seven-times and four-times higher than that obtained with PtML/IrCo/C (without palladium interlayer). These substantial enhancements in the specific activities again highlight the essential role of the palladium monolayer in improving the overall kinetics for the ORR.

fig10
Figure 10: Comparison of the mass activity in terms of Pt mass, (Pt + Pd) mass and the platinum-group-metal’s mass (a) and the surface-area activity (b) at 0.9 V (RHE) of PtML/IrCo/C (blue) and PtML/PdML/IrCo/C for oxygen reduction. The activity of Pt/C, the commercial catalyst, is shown in black [28].

To further explore these observations, we investigated the ORR activity on these surfaces using density functional theory, within the plane-wave approach, using the program VASP [33, 34], with the Perdew-Burke-Ernzerhof exchange-correlation functional [35] within the generalized gradient approximation. For further analysis and comparison of the relative reactivity of all surfaces at 0.9 V, we constructed an activity measure based on a microkinetic model that is detailed elsewhere [36]. In brief, the maximal activity can be expressed as , where is the rate constant of each elementary step [36]. The DFT-based activities at 1 bar, 300 K, and 0.9 V shown in Figure 11 indicate that the ORR activity on all surfaces studied is limited by the OH removal step. These values closely follow the trend in OH adsorption; therefore, OH adsorption energy is a descriptor of ORR activity of these surfaces. Both catalysts, IrCo and Ir3Co, with a Pd interlayer have a weaker OH binding than those without the interlayer, which is in agreement with the experimental results, that is, higher activity was observed on the catalysts with a Pd interlayer. Moreover, we note that the effect of the reduced lattice constant of the Ir-Co cores by the Pd interlayer clearly is reflected in the adsorption energies of the adsorbates. Ramírez-Caballero et al. [37] recently reported adsorption energies of O and OH, respectively, on Pt/Pd (bulk) as −4.21 and −2.33 eV; the values of O and OH, correspondingly, for Pt/PdIr3Co are −3.62 and −2.47 eV, and for Pt/Pd/IrCo are −3.48 and −2.10 eV.

fig11
Figure 11: Oxygen reduction activity at 0.9 V plotted in the order of strongest to weakest OH binding energy, and the binding energy of O is also given; both are shown in parenthesis in eV. (a) Ir3Co, (b) Ir-Co cores. The Pt-skin surfaces with an interlayer of Pd show the highest activities in both cases [28].

To summarize, this section describes a simple method of improving Pt monolayer core-shell catalysts that have an inadequate Pt-core interaction causing their relatively low ORR activity. Using a Pd monolayer as an interlayer between Pt and the core modifies their interaction and improves their ORR activity significantly. Employing the IrCo and Ir3Co alloy cores as supports, we demonstrated the essential role of a palladium interlayer in developing excellent platinum monolayer electrocatalysts for the ORR. Incorporating a palladium interlayer in the PtML/IrCo/C catalyst realized a sevenfold- and a fourfold-higher specific activity, respectively, in terms of platinum mass and platinum-group-metals’ mass. Our results reveal that the right choice of an interlayer, such as Pd, can facilitate the use of stable and relatively inexpensive core, such as IrCo, and tune the electrocatalytic properties of the surface platinum monolayer to achieve an efficient ORR electrocatalyst. Our DFT calculations corroborated these findings. Therefore, improved, low-cost ORR electrocatalysts with high activity, and increased potential for commercialization, can be obtained based on this approach.

3.2. Pt /(PdAu) /Pd/C [38]

The above example evidently demonstrated the meditation effect of the interlayer for the interaction between Pt monolayer and the metal core. The state-of-the-art PtML/Pd/C has been shown to be one of the electrocatalysts that is close to the optimum design for the ORR in terms of the value of oxygen binding energy and the position of d-band center. To further perfect this electrocatalyst, we modified the Pd core by the addition of a PdAu layer with only a small content of Au included, forming a new substrate for Pt monolayer.

The deposition of the PdAu alloy layer on the Pd/C surface was carried out via Cu UPD method. The detail of the structure for this modified PtML electrocatalyst is illustrated in Figure 12. Three compositions of the PdAu alloy layer were studied, namely, Pd95Au5, Pd90Au10, and Pd80Au20. Their polarization curves for ORR are displayed in Figure 13. All of the catalysts show improved ORR activities over those without an alloy sublayer. The activities can be ranked in a decreasing order as Pd90Au10 > Pd95Au5 > Pd80Au20 > Pd100, where Pd100 represents the catalyst without an alloy layer. The PtML catalyst with 10 at% Au sublayer exhibits the highest activity.

530397.fig.0012
Figure 12: Illustration of the core-shell structure of PdAu alloy sublayer nanoparticles [38].
530397.fig.0013
Figure 13: ORR activities of PtML catalysts with different contents of Pd-Au interlayer on a Pd/C core obtained with electrode rotating speed at 1600 rpm in an oxygen-saturated 0.1 M HClO4 [38].

The enhanced ORR activities can be attributed to the surface strain and the d-band center shift of the PtML induced by the alloy layer. The lattice constants of Pt, Pd, and Au are 3.920, 3.890, and 4.080 Å, respectively. The mismatch between Pt and Pd produces a lateral compressive strain when a PtML is deposited on Pd (extended surfaces): , where is the lateral strain and is the lattice constant. This is for flat surfaces. For nanoparticles, surface curvature also contributes to producing a strain [39, 40]. For a 7.8 nm particle, this contribution is estimated to be about ~0.4%. Although the total surface strain of ~1.2% is still small, contraction in surface atoms in the radial direction can make a significant contribution. The enhancement in ORR activity due to the lattice contraction has been demonstrated in experiments and density functional theory calculations [18]. Huang et al. [41] have recently reported that up to 8% reduction in bond length was found in 4 nm Au nanoparticles.

When a PtML is deposited on Au, a lateral tensile strain can be generated in PtML at 3.92% that will expand Pt in the surface layer. Contraction due to the decreasing coordination numbers of surface atoms will act in the opposite direction. Therefore, when Pd-Au alloys are used as the sublayer for mediation, the interplay of lateral strain (compressive and tensile) and radial contraction (compressive strain) can lead to activity improvement of the ORR catalysts, as observed in the experiments. There should be an optimal alloy composition, that is, both too little and too much Au would decrease the ORR activity. For the results reported here, the 10 at% Au, which is estimated using the atomic radii to have about 10.5% surface coverage by Au atoms in the alloy sublayer, seems to produce the highest ORR activity.

While lattice contraction may be important, the interaction of the substrate with PtML could result in an electronic effect on the ORR activity through charge redistribution [18, 42]. As demonstrated previously, [42] the formation of PtOH is significantly prohibited for the PtML on Pd nanoparticles when compared with Pt nanoparticles. Hydroxyl formation is considered as a major factor affecting ORR activity because of site blockage by OH [26]. Au may play an important role in protecting low-coordination sites on Pt from being oxidized. Wang et al. [43] recently reported that Au coated with FePt3 stabilizes the oxidation of the surface atoms in certain facets of the nanoparticles. Likewise, the Au in the sublayer of our catalysts also has contributed to the reduction of surface oxidation, leading to the improved ORR activity of the catalysts.

The polarization curves of PtML/Pd/C with a Pd90Au10 interlayer before and after 5000 potential cycles are shown in Figure 14. Less than 1% loss in ORR activity was observed, demonstrating the excellent stability of the catalyst. The stabilization of the catalyst can be attributed to the reduction of surface oxidation from Au in the sublayer, therefore inhibiting the dissolution of metal. The stabilizing effects of Au on Pt nanoparticles have been reported by Zhang et al. [44] and other studies [43, 45, 46] have also demonstrated the same effect of Au. The addition of a Pd-Au alloy between PtML and Pd/C core enhance both the activity and stability for ORR from the induced lattice contraction (both lateral and radial). The fact that the sublayer alloy can be of different metals and compositions may open up various opportunities in tailoring catalysts for best ORR activity.

530397.fig.0014
Figure 14: Polarization curves for the ORR of the catalyst with Pd90Au10 alloy sublayer before and after 5000 potential cycles [38].

4. A Core for Pt with a Segregated Pd Layer on Pd Fe(111) [47]

Pd-M alloys (M) (M = Co, Fe, or Ti) [19, 48] have shown comparable or even slightly better ORR activity than commercial Pt catalysts. The high ORR kinetics and significantly decreased cost of materials along with their satisfactory stability make some Pd-M alloys very promising candidates to replace conventional Pt catalysts for the ORR. The enhanced ORR activity observed with bulk and high-surface-area Pd-M alloys was ascribed either to the modification of the electronic property of Pd on the segregated surface layer [16, 48, 49] or to the synergistic effect between Pd and the second metal [49, 50]. This segregated Pd surface layer that has different electronic properties from pure Pd [16] was considered to be the key factor responsible for the enhanced ORR activity on Pd-M alloys [51]. Our work on Pd3Fe(111) demonstrated that the segregated Pd layer from high-temperature annealing exhibits the same surface structure but different electronic properties. As shown in Figure 15 upon annealing to 1200 K, surface Pd signal increases and a sharp (1 × 1) LEED pattern was observed, suggesting that Pd segregates to the surface and forms a well-ordered layer. This segregated Pd layer modified the interaction of PtML and its substrate, and an enhancement of the ORR activity was observed.

530397.fig.0015
Figure 15: LEIS spectra (1 keV He+) for a Pd3Fe(111) surface annealed to various temperatures. Inset shows a (1 × 1) LEED pattern for a Pd3Fe(111) surface annealed to 1200 K. The electron beam energy was 55 eV [47].

When the annealed Pd3Fe(111) electrode was treated with 0.1 M HClO4, only a small amount, if any, of Fe in the topmost layer immediately dissolved since no voltammetric features related to Fe dissolution were observed, leaving a pure Pd skin-like layer on the surface (referred to hereafter as Pd/annealed-Pd3Fe(111)) and verifying the migration of Pd to the surface during annealing.

Figure 16(a) shows the cyclic voltammetry scans for a Pd/annealed-Pd3Fe(111) surface immersed in 0.1 M HClO4 at 0.05 V and subsequently cycled between 0.05 V and various positive potential limits at a scan rate of 20 mV s−1. A reversible process is observed between +0.05 and +0.40 V, a finding that has not been observed on a Pd(111) electrode (Figure 16(b)) [52, 53]. Baldauf and Kolb [54] reported that absorption ( ) occurs only for Pd films thicker than 2 ML. This reversible process that is very similar to the occurred on a PdML/Pt(111) electrode [55] and therefore is tentatively assigned to on a segregated Pd layer on a Pd3Fe(111) surface. The surface oxidation of a Pd/annealed-Pd3Fe(111) electrode does not occur until the positive potential limit extends beyond 0.73 V (red and blue line in Figure 16(a)); this indicates the positively shifted onset of oxidation of ~0.1 V relative to that of Pd(111). The charge density under the cathodic peak at ~0.73 V, reflecting the surface coverage of oxygen-containing species, is considerably smaller for the Pd/annealed-Pd3Fe(111) electrode than for the Pd(111) one (Figure 16(b)). These important new properties of the alloy’s surface play a role in determining the kinetics of the ORR. A similar electrochemical phenomenon occurred on a Pt skin surface on Pt3Ni alloys [56]. The voltammetry curves stayed unchanged, even after driving the positive potential limit very high (i.e., 0.95 V).

fig16
Figure 16: (a) Voltammetry curves for an annealed Pd3Fe(111) in 0.1 M HClO4 solution with different positive potential windows. (b) Voltammetry curves for an annealed (red line) and a polished Pd3Fe(111) (blue line), and Pd(111) (black line) in 0.1 M HClO4 solution. Scan rate: 20 mV s−1 [47].

To assess the role of Fe in determining the electrochemical and catalytic properties of the Pd layer on the Pd3Fe(111) surface, the Fe atoms in several topmost layers were removed almost completely by polishing and exposing such surface to acid. The remaining Pd layers on the Pd3Fe(111) electrode have some similarity to the Pt3Ni skeleton surface described by Stamenkovic et al. [57] except this pure Pd surface layer likely was thicker. This electrode showed very similar electrochemical behavior to that of a Pd(111) electrode, which verifies the role of Fe in this catalyst.

Figure 17(a) provides a set of polarization curves for the ORR on the PtML-covered surfaces of Pd/annealed-Pd3Fe(111) and Pd(111), and Pt(111) obtained in 0.1 M HClO4 at room temperature. Figure 17(b) shows their corresponding specific activities at 0.9 V. A Pt monolayer supported on Pd/annealed-Pd3Fe(111) shows the highest ORR kinetics among these three surfaces, by a factor of at least 2 compared with that of the Pt(111) surface, and also demonstrates significantly increased ORR activity in comparison with that of the PtML/Pd(111) surface.

fig17
Figure 17: (a) Polarization curves for the ORR on a Pt monolayer covered annealed-Pd3Fe(111) (red), annealed-Pd(111) (green), and Pt(111) surfaces (black) at 1600 rpm in oxygen-saturated 0.1 M HClO4 at room temperature; scan rate: 20 mV s−1. (b) The bar plot for the comparison of ORR-specific activities of the corresponding three surfaces at 0.9 V (RHE) [47].

To elucidate the fundamental aspects of the ORR activity of composite core-shell structures, DFT calculations were conducted for model systems composed of a single PtML on a Pd monolayer supported on the close-packed surfaces of several different substrates. On the basis of this analysis, the binding energy of OH on PtML/Pd(111) is weaker than that of OH on Pt(111) and OH removal is easier on PtML/Pd(111) than it is on Pt(111) (Table 1). As a result, the ORR activity is increased on PtML/Pd(111) compared to that on Pt(111).

tab1
Table 1: Binding energies of OH on the surfaces of Pt(111), PtML/Pd(111) and PtML/Pd/Pd3Fe(111).

As shown in Table 1, PtML/Pd/Pd3Fe(111), an appropriate model for the PtML on the annealed-Pd3Fe(111) surface, the binding of OH on which is destabilized much more than on the PtML/Pd(111) and Pt(111). This leads to PtML/Pd/Pd3Fe(111) having more OH-free sites for O2 adsorption and reactions than the PtML/Pd(111) and Pt(111) surfaces. Interestingly, we are finding that the activity of PtML/Pd(111) and Pt(111) is limited by OH removal (left side of the ORR activity volcano plotted against BEOH) whereas the activity of PtML/Pd/Pd3Fe(111) is limited by the O–O bond scission in the OOH intermediate, placing this surface on the other side of the volcano plot. Yet, a direct comparison between the BEOH and the calculated ORR activities on PtML/Pd(111) and PtML/Pd/Pd3Fe(111) at the relevant bias voltage suggests that the enhanced activity of the latter surface originates mainly from the destabilization of OH on that surface, compared to that on the former surface.

The segregated Pd layer further weakens the Pt–OH bonding, and is proven to be an excellent substrate for a Pt monolayer for the ORR, as indicated by the enhanced ORR kinetics on PtML/Pd/annealed-Pd3Fe(111) compared to that of PtML/Pd(111) and Pt(111).

5. Subsurface Modification of the Core: Introduction of Iridium to the Pd Subsurface [58]

This approach introduces a second metal to the subsurface of Pd core and thus generates a Pd-interlayer which alters both the electronic and geometric properties of the core and also affects the catalytic activity of the Pt monolayer.

The purpose of submerging Ir under the Pd surface is to influence the Pt-Pd interaction in order to improve the ORR activity of the already established highly active PtMLPd/C [42]. Also, since Ir has a much higher dissolution potential than Pd (1.16 V for Ir and 0.99 V for Pd), [29] the addition of subsurface Ir can enhance Pd stability under fuel cell operating conditions.

Figure 18 depicts the process of introducing Ir to the subsurface of Pd core for Pt monolayer. In brief, an Ir layer was first placed on Pd/C via the Cu UPD method and then was subsequently annealed at elevated temperatures to enable a thin layer of Pd to segregate to the surface. After that, a Pt monolayer was deposited via the Cu UPD. The Ir loading in IrPd/C is estimated to be 4.1 wt% and there is 11 at% of Ir in IrPd.

530397.fig.0018
Figure 18: Depiction showing the catalyst preparation (see text) [58].

In Figure 19(a), the voltammetry curve of PtMLPdIr/C (red line) is compared to PtMLPd/C (blue line) and Pt/C (black line) in de-aerated 0.1 M HClO4 while Figure 19(b) illustrates a set of polarization curves for the ORR on the same samples with the addition of the notannealed sample (PtML/IrPd/C, green line) at 1,600 rpm in oxygenated 0.1 M HClO4. From the above CVs, the segregated Pd surface layer on PdIr nanoparticles exhibits different electrochemical behavior compared to that of Pd/C electrodes. The influence of Ir is revealed in the oxidation region. The oxidation of PtML on PdIr/C is delayed compared to that of PtML/Pd/C and Pt/C. This is in qualitative agreement with DFT calculations predicting weaker Pd–O interaction on Pd/Ir(111) than on Pd(111) [19].

fig19
Figure 19: Voltammetry curves for PtMLPd/C (blue), PtMLPdIr/C (red), and Pt/C (black) in deaerated 0.1 M HClO4 solution. Scan rate, 20 mVs−1. (b) Polarization curves for the ORR on PtMLIrPd/C (green), PtMLPd/C (blue), PtMLPdIr/C (red), and Pt/C (black) at 1,600 rpm in oxygen-saturated 0.1 M HClO4 solution. Scan rate, 10 mVs−1 [58].

As shown in Figure 19(b) the PtMLPdIr/C has higher ORR activity than Pt/C and PtMLPd/C even though it has a smaller surface area. The Pt-specific activity (a) and Pt-mass activity (b) at 0.9 V of Pt/C, PtMLPd/C, and PtMLPdIr/C are summarized in Figure 20. The Pt-specific activity for PtMLPdIr/C is three-times and 25% higher than that of Pt/C and PtMLPd/C, respectively; the Pt-mass activity of PtMLPdIr/C is more than 20-times and 25% higher than that of Pt/C and PtMLPd/C, respectively.

fig20
Figure 20: (a) Pt-surface specific activity, (b) Pt-mass activities of Pt/C, PtMLPd/C, and PtMLPdIr/C at 0.9 V versus RHE [58].

The influence of the Ir sublayer on the ORR activity could be explained by the position of the PtML d-band center ( ). Previous DFT calculations have shown that the of the metal monolayer under compressive strain tends to downshift in energy whereas tensile strain has the opposite effect [6, 15]. A surface characterized by a higher-lying tends to bind adsorbates more strongly, thereby enhancing the kinetics of dissociation reactions producing these adsorbates. On the other hand, a surface with a lower-lying tends to bind adsorbates more weakly and facilitates the formation of bonds towards larger intermediates. The PtML on Pd is compressed but the position of the for the PtML depends both on the strain (geometric effects) and on the electronic interaction between the PtML and its substrate (ligand effect) [6]. Introducing subsurface Ir causes a small contraction of the Pd surface layer due to the smaller atomic size of Ir compared to Pd. As a consequence, the PtML put on top of the Pd surface covering the PdIr subsurface will be further compressed causing additional downshift in which is manifested by weaker Pt–OH interaction leading to the reduced oxidation of this ternary system. Since the intrinsic ORR activity is largely determined by the binding energy of OH, weaker Pt–OH interaction should result in enhanced ORR activity.

To directly demonstrate the destabilization of OH on PtMLPdIr (as compared to PtMLPd or Pt), density functional theory calculations were performed on representative close packed (111) facets of the appropriate model systems. The results of these calculations are shown in Table 2. The calculations show that depositing a monolayer of Pt on a Pd substrate destabilizes OH binding (BEOH = −2.07 eV) as compared to pure Pt (BEOH = −2.17 eV) or Pd (BEOH = −2.29 eV). Since the lattice constant for Pt and Pd are quite similar, this difference is mostly a manifestation of the ligand effect. Introducing Ir into the second sublayer below a PtML and a PdML sublayer further reduces binding of OH on the Pt surface (BEOH = −2.00 eV). In the case that the catalysts were not annealed prior to depositing the Pt overlayer, OH binding is enhanced (BEOH = −2.21 eV), translating into ORR performance of the notannealed PtML/IrPd inferior to that of pure Pt, as one can see in Figure 20(b). Therefore, according to our calculations, the annealed PtML/PdIr catalyst possesses the weakest binding of OH of all systems studied here. Weaker binding of OH leads to lower OH coverage, less poisoning for ORR, and thereby higher ORR activity. Our calculations provide support to the reported distribution of Ir within the sublayers. In particular, the total energies of the two PtML/PdIr model slabs employed suggest that Pd is more stable in the first sublayer (right below the PtML) than Ir is. Therefore, upon annealing, Pd atoms are pulled from the bulk to form a Pd-rich first sublayer right below the Pt surface monolayer, with an Ir-rich second sublayer found right below the first Pd sublayer.

tab2
Table 2: Binding energy (BE) of OH (in eV) on several close-packed (111) model surfaces at 1/6 ML coverage.

This work suggests that the introduction of Ir to the subsurface of the Pd core successfully modified the electronic and geometric properties of the substrate for the PtML, which in turns benefit the ORR kinetics.

6. Other Modifications of Cores to Improve Pt Electrocatalysts

In addition to the above approaches, some of our efforts were also put on the study of unconventional core materials for PtML. Two examples will be briefly shown here.

In addition to the conventional carbon-supported metallic nanoparticles, we found that intermetallic compound can be a great alternative as the substrate for Pt monolayer [59]. Some intermetallic compound nanoparticles are stable and highly active catalysts for small organic molecular oxidation [60, 61]. Figure 21 shows the comparison of the ORR activity of the PtML electrocatalysts with intermetallic compound PtPb and PdFe cores with the standard Pt/C. The activity of the Pt/C is inferior to those PtML intermetallics. It demonstrated that the combination of highly stable, inexpensive intermetallics with Pt monolayer is promising in the design of electrocatalysts with high activity, low metal content, and high stability.

530397.fig.0021
Figure 21: Comparison of the activities for the ORR of Pt/C, core-shell PtML/Pd/C, and Pt/PtPb and Pt/PdFe in 0.1 M HClO4; 1600 rpm; 10 mV/s [59].

Another example is using multimetallic nanoparticles as the substrate for Pt monolayer. Our study on PtML/AuNi0.5Fe/C [62] demonstrated that by choosing proper combination of metals, the multimetallic core materials can be a promising substrate for PtML for the ORR from the synergetic contribution from each metal component. Figure 22 shows the ORR activity (a) and stability (b) of this catalyst. 5- and 7-fold increase in the Pt mass and specific activities, respectively, compared with Pt/C and an insignificant loss in the electrochemical surface area and almost unchanged activity were achieved.

fig22
Figure 22: (a) Polarization curves of PtML/AuNi0.5Fe/C and Pt/C in an oxygen-saturated 0.1 M HClO4; (b) polarization curves for PtML/AuNi0.5Fe/C before and after 15000 cycles of sweeping potentials between 0.6 and 1.0 V at 50 mV/s. Conditions: scan rate, 10 mV/s; rotation speed, 1600 rpm [62].

7. Summary

This review demonstrates that the ORR activity of PtML electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low-coordination sites, generation (via addition or segregation) of an interlayer between PtML and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of PtML electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the PtML, involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation PtML electrocatalysts.

Acknowledgment

This paper is supported by US Department of Energy (DOE), Divisions of Chemical and Material Sciences, under Contract no. DE-AC02-98CH10886.

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