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Investigating the Role of Strain towards the Oxygen Reduction Activity on Model Thin Film Pt Catalysts Sandra Elisabeth Temmel, Emiliana Fabbri, Daniele Pergolesi, Thomas K. Lippert, and Thomas J. Schmidt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01836 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016
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Investigating the Role of Strain towards the Oxygen Reduction Activity on Model Thin Film Pt Catalysts Sandra E. Temmel (1), Emiliana Fabbri (1), Daniele Pergolesi (1), Thomas Lippert (1,2), Thomas J. Schmidt (1,3,*) (1) Energy&Environment Division, Paul Scherrer Institut, Villigen PSI, 5232, Switzerland (2) Laboratory of Inorganic Chemistry, ETH Zürich, 8093, Switzerland (3) Laboratory of Physical Chemistry, ETH Zürich, 8093, Switzerland
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ABSTRACT:
Environmentally friendly energy conversion devices such as fuel cells are becoming more and more attractive. However, major impediments towards a large-scale application still arise on the material side, related to the cost and the poor performance of the cathode catalyst. State-of-the art electrocatalysts are all Pt-based materials, suffering from poor electrochemical oxygen reduction kinetics. Tuning the inter-atomic distance of Pt atoms represents a promising strategy to reduce the adsorption strength of oxygenated species to the Pt surface and thus improve the kinetics. In this context, model Pt electrocatalysts of strain-induced varied interatomic spacing were fabricated and tested. Strained Pt films with high crystalline quality can be obtained through an epitaxial growth on appropriate single crystal substrates like (111) SrTiO3 which have a different lattice parameter than Pt using pulsed laser deposition. Through a proper selection of deposition parameters, the extent of strain in the Pt films can be controlled. This study shows that strain modifies significantly the electrochemical surface properties. In particular, cyclic voltammetry and CO oxidation experiments provide valuable insights into the effect of strain on the adsorption properties of spectator species (e.g. OHad, bisulfates) relevant for ORR kinetics. Furthermore, the strained Pt films exhibit a remarkably higher oxidation reduction reaction activity as compared to the fully relaxed bulk structure as obtained from ORR polarization curves. This research highlights the importance of proper model systems with defined physical properties to establish design principles for better performing catalysts.
KEYWORDS: oxygen reduction reaction, platinum, electrocatalysis, thin films, pulsed laser deposition, strain, model catalyst
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Over the past decades, increasing attention has been turned to polymer electrolyte fuel cells (PEFCs) as environmental friendly energy conversion devices for automotive and stationary applications due to their reduced CO2 emissions and high energy conversion efficiencies 1. Stateof-the-art catalysts for the oxygen reduction reaction (ORR) occurring at the PEFC cathode side are Pt-based materials. However, besides the use of a noble metal catalyst, which severely affects the cost of the device, the ORR suffers from poor kinetics, leading to overpotentials of about 0.3 to 0.4 V and consequent performance losses 2. For a successful large-scale commercialization of this technology, a more than 4-fold increase in mass-specific Pt catalytic activity is required 3. To minimize the drawbacks related to the PEFC cathode, considerable efforts have been devoted worldwide to either i) decreasing the precious metal loading through high surface to volume ratio catalyst by screening the optimum particle size 4-5 or shape 6, ii) decreasing the amount of “buried” precious metal atoms through the design of porous 7, hollow 8 or nano frame 9-10 electrocatalysts, iii) substituting Pt for a more cost-effective and active material 11-12 or iv) improving the intrinsic electrocatalytic activity of Pt 13-23. Focusing on the latter strategy, different routes have been investigated. Extensive studies suggest that the slow ORR kinetics on Pt originates from the too strong binding of oxygenated species to the surface 24. In this context, several approaches have been investigated to alter the electronic properties of Pt to reduce the binding strength 15. Among them, alloying Pt with 3d transition metals such as Ni, Co and Fe has attracted great interest. Such Pt alloys exhibit up to 4 times higher catalytic activities compared to pure Pt. Their performance increase was indeed attributed to a weakening of the bond between Pt and the adsorbed oxygen resulting from their interactions with the surrounding O-bonded transition metal atoms 16. Furthermore, the d-band model developed by Norksov and co-authors 24 suggests that the adsorption strength of the reaction intermediates during ORR can be weakened by lowering the 3 ACS Paragon Plus Environment
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electronic d-band center due to the consequential increased filling of the antibonding states. In the case of Pt, one possibility to manipulate the electronic d-band center is to reduce the inter-atomic distance through strain 24. This can be achieved through a special structural design by preparing (sub) monolayer(s)(MLs) of Pt on suitable substrates 25 or at the surface of nanoparticles (NPs) 14 of smaller lattice constant compared to Pt, thus forcing the Pt lattice to adapt to the underlying lattice (straining the Pt lattice). In this regard, core-shell NPs have attracted large interest due to the advantage of combining high electrochemical surface area (ECSA) with strained Pt surface. Following these guidelines, Pt monolayers have been prepared at the surface of metal NPs 26 or Pt alloys NPs 27 through electrochemical displacement 28, (electro) chemical dealloying 27 or segregation through thermal annealing 14. For instance, Strasser et al. reported 14 that the Pt rich shell of dealloyed Pt-Co NPs exhibited lattice parameters smaller than bulk Pt. They showed that the compressive state of the Pt shell led to more favorable oxygen-platinum bonding at the surface, which in turn greatly facilitated the ORR. PtxGd NPs with a compressive strain compared to pure Pt and a core/Pt-rich shell structure after partial Gd leaching show a strong correlation between the ORR activity and the compressive strain. 23 Therefore, the authors concluded that the ORR enhancement was driven by the compressive strain within the Pt shell induced by the alloy core. A more recent publication by Escudero-Escribano et al. 22 showed that other Pt-rare earth alloys, such as PtTb, PtGd and PtSm, display a Pt overlayer with smaller lattice parameter than bulk Pt and higher ORR activity than polycrystalline Pt or Pt-Co and Pt-Ni alloys prepared in the same way. As mentioned before, an alternative elegant approach to strained electrocatalysts consists of preparing ultra-thin (ML range) Pt films on substrates instead of on NPs. Among others, ORR activity of Pt MLs electrodeposited on different metal substrates show a volcano-type relationship between substrate-induced strain and ORR activity, with the best performance being 4 ACS Paragon Plus Environment
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predicted by DFT calculations to be the 0.4 % strained Pt/Pd system 29. Quite recently, the effect of different numbers of (sub)monolayers of Pt electrodeposited onto an 100 nm gold film was investigated 30. It was argued that in the sub monolayer range, effects other than strain also played a decisive role in determining the catalyst activity. Besides, it was shown that a several MLs (> 2 nominal MLs) tensely strained Pt film indeed exhibited a poorer ORR activity compared to bulk Pt, corroborating the d-band model theory. However, within these core-shell and ML(s) approaches several side-effects contributing to the electrocatalytic activity that enhance or hinder the ORR performance cannot be completed excluded: i) the presence of transition metal atoms incorporated in the Pt surface layer, ii) the inhomogeneity of the Pt shell, iii) (electronic) influence of the underlying alloy or the (conductive) support, iv) the size, size distribution and shape effect of the nanoparticles and, v) the potential influence of the crystalline quality and/or crystallographic properties of the Pt surface. This may hinder the unambiguous identification of a relationship between Pt interatomic distance and ORR activity. For this reason, it is of fundamental as well as practical interest to better understand the role of strain in electrocatalysis. Model systems like thin films with defined physical properties offer this possibility and are, therefore, used in this study to investigate the influence of the strain-induced variations of Pt interatomic distances on its electrocatalytic activity towards ORR. For highly ordered thin films (semi) coherently grown on substrates, strain refers to the change in lattice constant due to the forced matching of the lattice parameters of the thin film material to the substrate’s lattice and is given by =
−
(1)
where and are the fully relaxed and strained lattice constants, respectively. 5 ACS Paragon Plus Environment
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In our previous study 31 we developed a strategy to prepare strained, epitaxial, 12 nm thick Pt films grown on single crystalline (111) strontium titanate (SrTiO3, STO) substrates by pulsed laser deposition (PLD). These (111)-oriented strained Pt films demonstrated a different catalytic behavior towards CO electrooxidation compared to a single crystal (111) Pt. In particular, a strain-induced shift toward more positive potentials of the main CO oxidation peak was measured indicating a weaker OHad affinity at the strained surface. Since it is well established that the slow kinetics of the ORR originates from a too strong binding of adsorbed oxygenated species to the Pt surface 24, this shift represents a promising result for future ORR kinetic studies. Results and Discussions In this work, epitaxial (111) Pt surfaces with different stress states were successfully prepared on single crystalline (111) STO using PLD. The growth mechanism, morphology, surface roughness and epitaxial orientation were carefully investigated using various characterization techniques. These Pt films were then used to isolate the effect of strain on the electrochemical properties in cyclic voltammetry, CO oxidation, and ORR experiments.
In situ monitoring of the Pt film growth and morphological characterization The growth mode was in situ monitored via reflection high energy electron diffraction (RHEED). The incident electron beam is focused parallel to the (110) direction of the substrate. A representative RHEED image of the STO substrate prior to deposition is shown in Figure 1A, which also reports the corresponding in-plane reflexes (11) and (22). For simplicity, the mirrored in-plane reflexes (1-1) and (2-2) are not specifically indicated. The well recognizable spots lying on the Laue semi-circle arise from the intersection of the Ewald sphere and the narrow rods of the reciprocal lattice of the substrate surface and indicate a smooth surface with good crystalline quality 32. A scheme of the substrate and the crystallographic orientations used for the RHEED 6 ACS Paragon Plus Environment
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alignment are depicted in Figure 1E and 1I, respectively. When growing a 6 nm thick Pt film, a spotty RHEED pattern (Figure 1B) evolves showing a change from a reflection to a transmission operation mode of the electron beam. This characteristic RHEED pattern originates from a three dimensional (3D) Volmer-Weber growth mechanism of the film that was observed at the beginning of the film nucleation as illustrated in Figure 1F. A 3D island growth indeed represents the typical growth mode for metals
33
and is confirmed by scanning electron
microscopy (SEM) images (Figure 1J) in which the separated islands are clearly recognizable. Due to the transmission mode, the spots can no more be ascribed to a particular in-plane reflex. For the 12 nm thick Pt films, the spots in the RHEED pattern become more elongated/streaky (Figure 1C), indicating a less rough surface and the formation of terraces. As a result, a percolated network, decreasing the surface roughness, is formed as illustrated in Figure 1G. This observation is well in agreement with SEM images in Figure 1K demonstrating that the Pt films now consist of a percolated network instead of separated island. The RHEED spots can be assigned to the in-plane (22) and (2-2) reflexes. For the face center cubic structure of Pt, the (110) and (1-10) reflexes are forbidden. Their absence in the RHEED pattern together with the two visible (22) and (2-2) reflexes confirms the cube-on-cube in-plane epitaxial matching of the film to the substrate. For the largest film thickness, 24 nm, the RHEED pattern (Figure 1D) only consists of streaked spots lying on the Laue circle, with the previously observed (22) and (2-2) reflexes being more pronounced. The pattern suggests a relatively flat Pt film surface with the inplane epitaxy still preserved. Yet, the streaked spots, as compared to the clear spots for the pattern of the substrate (Figure 1A), imply that the surface is smooth but consists of crystallographic domains 32. Still, it is safe to conclude that the Pt film is homogenously grown as depicted in Figure 1H. SEM images (Figure 1L) support again these findings. The observed growth mechanism is characteristic for films of noble metals 33. 7 ACS Paragon Plus Environment
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Figure 1. A thickness dependent morphology evolution was observed for the (111) Pt films. The growth was monitored in situ via reflection high energy electron diffraction. Images of the substrate prior to deposition (A), and of 6 nm (B), 12 nm (C) and 24 nm (D) thick Pt films are shown. A scheme illustrating the growth from the pure substrate (E), to the separated islands (F), over a percolated network (G) and a homogenous film (H) is also depicted. Those schemes were confirmed by SEM images for the 6 nm (J), the 12 nm (K) and the 24 nm (L) Pt films. A scheme of the crystallographic orientations used for the RHEED alignment is depicted in (I). 8 ACS Paragon Plus Environment
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Correlation between thickness determination via X-ray Reflectometry and roughness analysis via Atomic Force Microscopy The thickness of the Pt films was determined by X-ray reflectometry (XRR). The respective XRR patterns are displayed in Figure 2. For the largest film thickness, well-defined oscillation fringes can be observed, indicating a well-ordered film-substrate and film-air interface. With decreasing film thickness, the XRR pattern becomes less defined at larger 2θ angles, suggesting a roughening of the Pt film surface. Atomic force microscopy (AFM) analysis revealed that with smaller film thickness the root mean square (rms) of the distribution of the surface height averaged over the measured area decreased. In literature, an rms value of 0.53 nm has been reported for extremely smooth Pt films 34. Compared to the rms value of 0.33 nm measured for the 24 nm film, this represents a further strong evidence for the smoothness of the thickest Pt film. Furthermore, the AFM images are well in agreement with the SEM images, confirming the thickness-dependent morphology, evolving from separated islands, over a percolated network to a homogenous surface.
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Figure 2. X-ray reflectometry patterns recorded for the Pt films of different thickness. Atomic force microscopy images (∆z represents the height of the image) revealed a thickness-dependent roughness of the Pt films and further confirmed the specific morphology for all Pt films of different thickness
Strain investigations by X-ray diffraction The crystallographic orientation and strain of the Pt films was studied by X-ray diffraction (XRD) (Figure 3A). The sharp peak at 39.99 º corresponds to the (111) out-of-plane reflex of the single crystal STO substrate. The XRD plot shows that the Pt films are (111) epitaxially oriented with the STO substrate. The (111) is the first allowed reflex for the Pt structure and no other reflexes were observed for larger 2θ angles. The dashed line indicates the angular position of the (111) diffraction peak of the fully relaxed crystalline structure of Pt, (nearly) coinciding with the Pt film reflex of the thickest Pt film. This indicates that at 24 nm, the Pt film is fully relaxed. In contrast, the reflexes for the 6 nm and 12 nm Pt films are shifted to smaller 2θ angles, indicating 10 ACS Paragon Plus Environment
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an enlarged out-of-plane lattice. An out-of-plane tensile strain of the unit cell of Pt implies the presence of an in-plane compressive strain, which is the expected result in the case of an epitaxial cube-on-cube growth where the Pt lattice parameter (3.923 Å) adapts to the smaller lattice parameter of the STO substrate (3.905 Å). The out-of-plane strain can be directly determined from the Pt film peak position in the θ2θ scan, while the in-plane lattice strain , the crucial parameter expected to influence the electrochemical reactions, can then be calculated as =
2 1 −
(2)
where ν denotes the Poisson ratio 35. Their relation and dependency on the film thickness is depicted in Figure 3B. The inverse proportionality between strain and thickness is evident: The larger the film thickness, the smaller the strain. This is expected since the probability for propagating misfit dislocation to release the accumulated elastic energy increases with larger film thickness 33.
Figure 3. A. X-ray diffraction pattern of 6 nm, 12 nm and 24 nm thick Pt films. The shift of the (111)Pt peak towards smaller 2θ angles becomes more pronounced with decreasing film thickness. B. Both in-plane and out-of-plane strain are plotted over the respective Pt film thickness. The strain is clearly larger for smaller film thicknesses. 11 ACS Paragon Plus Environment
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Figure 3B shows that the estimated in-plane strain of the 6 nm Pt film is around 0.4% which is very close to the theoretical lattice misfit of the two materials (0.46%). This suggests that the 6 nm thick Pt film is almost coherently strained, i.e. the theoretical lattice mismatch is almost fully converted into interfacial lattice distortions. The 12 nm thick films shows a residual strain of about 0.3 % implying that part of the maximum theoretical strain is indeed released, resulting in a less strained surface as compared to the 6 nm thick films. Above a certain thickness, it is not energetically favorable anymore to preserve the strained lattice. As a result, the crystalline structure of the film relaxes and the topmost layers will have the same lattice parameter as the fully relaxed structure. This can be observed for the 24 nm thick Pt films for which the XRD analysis showed no detectable strain. It is important to take into account that XRD probes the average strain of a sample over the whole film thickness. As a result, the calculated in-plane strain values cannot be directly converted into absolute surface strain values, but rather into values indicating a trend in the surface strain. Concerning the comparison with available literature data, it is worth mentioning that commonly, when strain-related electrochemical activities of Pt-based catalysts are reported, no direct proof of the strained Pt surface is given. The extent of strain is either estimated assuming the Pt lattice parameter to be the same as the lattice parameter of the underlying substrate or NP, or theoretically calculated using density functional theory. It is worth highlighting here that, to the best of the authors’ knowledge, this is the first time that a direct strain assessment of a Pt surface has been performed using XRD and will then be correlated to the ORR activity. In experimental 29 and theoretical 36 studies that compared a range of differently strained ML Pt catalysts, it was found that an in-plane compressive strain of 0.4% (estimated as discussed above) resulted in the best ORR performance among the studied systems. The Pt films fabricated for this 12 ACS Paragon Plus Environment
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study show in-plane strains of 0.3% and 0.4% for the 12 nm and 6 nm thick Pt films, respectively. However, the latter could not be considered for further electrochemical measurements since no sufficient electrical pathway was provided through the separated islands morphology. Nevertheless, the 0.3% in-plane compressive strain of the 12 nm thick films represents a promising strain value for the purpose of this investigation and can serve as model systems to investigate the role of strain on the ORR activity. In the following, the 24 nm thick films will be named as “relaxed”, whereas the 12 nm thick films will be referred to as “strained”. Electrochemical studies by cyclic voltammetry, CO oxidation and oxygen reduction reaction experiments It is widely accepted that the ORR on Pt-based materials occurs via a 4e- series pathway, involving intermediate adsorbed oxygen containing species (e.g. O2,ad, OOHad etc.) 13. Extensive studies suggest that the slow ORR kinetics on Pt commonly originate from too strong binding energies of these oxygenated species to the catalyst surface, which consequently hinder OHad removal 24, 37. Using DFT calculations 24, it was indeed demonstrated that on (111) Pt surfaces, the critical step for the ORR to proceed is the OHad removal. It has been shown 13 that, in a simple picture, the overall ORR reaction rate “” can be expressed as a function of the total surface coverage of all adsorbed species “ “ on Pt by and can be expressed as ∝ 1 − exp
!
(3)
with α representing the transfer coefficient, F [C mol-1] the Faraday constant, R [J mol-1 K-1] the universal gas constant, T [K] the temperature and η [V] the applied overpotential with respect to the ORR equilibrium potential. A detailed explanation of Equation 3 is given in ref. 13. Literature reports that supposing a small coverage of ORR intermediates under reaction 13 ACS Paragon Plus Environment
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conditions relevant for ORR kinetics, only two kind of adsorbates have to be taken into account in 0.5 M sulfuric acid: The (bi)sulfate anions and the OHad 13. Therefore, the pre-exponential term determines the overall reaction rate at given overpotential and the relation given in Equation 3 can thus be simplified to ∝ "1 − #$ − % &' (
(4)
where θOH and θsulfates stand for the coverage of Pt sites by OHad and (bi)sulfate anions, respectively. In the following, cyclic voltammograms (CVs) and CO oxidation experiments will, among others, be utilized to separate the influence of strain on these two spectator species. Furthermore, the coverage-dependent term "1 − #$ − % &' ( will help to identify the role of strain in ORR activity measurements. CVs were recorded on “strained” and “relaxed” Pt films and compared to a Pt(111) single crystal. The different features in a CV measurement are well-known and arise from specific electrochemical reactions occurring at the electrode. In particular, the packing and coordination of the surface atoms varies with different Pt(hkl) crystallographic orientation resulting in individual fingerprints for any surface orientation 38. The shape of the Pt CV, therefore, serves as a valuable tool to investigate the adsorption properties of protons and of spectator species on Pt films and to corroborate the crystallographic surface orientation beforehand determined by XRD. A representative CV of a Pt(111) single crystal is depicted in Figure 4.A. The three typical features of Pt(111) can be observed 39. These are the box-shaped feature at low potentials (the under potential deposition hydrogen (Hupd) region), the so-called butterfly around 0.4 V arising from the specific adsorption of (bi)sulfate anions and the peak pair around 0.7 V corresponding to the reversible OH adsorption/desorption. The sharp peak of the butterfly originates from a phase transition of the adsorbed (bi)sulfate ions on long-range ordered terraces and can only be 14 ACS Paragon Plus Environment
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observed for a (nearly) defect-free single crystal surface prepared by a careful defect annealing procedure 40.
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Figure 4. Cyclic voltammograms of the Pt(111) single crystal (A), the “relaxed” Pt (B) and the “strained” Pt(C) performed at room temperature in 0.5 M H2SO4 at 50 mV s-1 are illustrated. The surface properties of the “relaxed” Pt are clearly comparable to the single crystalline surface, whereas the “strained” Pt exhibits markedly different features. In (D), the specific charge evaluation Qspec derived from the CVs of (A), (B) and (C) is plotted over the electrode potential to evaluate the specific coverage θad on the respective Pt surfaces. Two different regions can be clearly distinguished: The Hupd deposition region and the adsorption region for bisulfates and OHad.
Yet, this procedure is not applicable to the “relaxed” or “strained” Pt films, which could suffer severe damage at high temperature. Most likely, droplet coalescence (dewetting) or surface strain release would be the consequence. The absence of this peak can thus generally be correlated to a higher level of crystalline surface defects. With this in mind, since the steps and edges that are commonly present on non-ideal surfaces prevent a long-range order, it is not surprising that the “relaxed” Pt (Figure 4B) exhibits the typical box-shaped and suppressed butterfly features, while the sharp peak is missing. In contrast, the CV of the “strained” Pt (Figure 4C) differs greatly from the two previous ones and clearly lacks the distinct features discussed above: Neither the box-shape nor the suppressed butterfly features can be observed. The shape suggests that the surface is indeed strained up to the surface. Furthermore, if other effects were dominant (e.g. edges of the percolated network), the characteristic features of polycrystalline Pt 41 should be present in the CV. We, therefore, conclude that the smeared Hupd region observed for the “strained” Pt is the result of an in-plane strained surface as suggested by the XRD measurements To evaluate the adsorbate coverage θad (see Equation 6 in the Experimental Section) of the respective Pt surfaces, the specific charge Qspec (see Equation 7 in the Experimental Section) is derived from the CVs in Figure 4A, B and C and plotted over the electrode potential (Figure 4.D). It can be distinguished between the Hupd desorption region (~0.07-0.33 V) and the adsorption region for (bi)sulfate anions and oxygenated (e.g. OHad) species (~0.33-1 V). 16 ACS Paragon Plus Environment
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Overall, considering a full monolayer of Hupd and assuming that exactly one hydrogen atom is adsorbed on one single Pt surface atom, the theoretical maximum Qspec on a Pt(111) surface is 42 240 μC cm01 . However, it has been reported 42 that due to the densely packed Pt(111) ./
structure, only a Hupd-coverage of 0.6 ML is experimentally reached, giving rise to a Qspec of 160 μC cm01 ./ . This is in good agreement with the results in our study. The Pt(111) single crystal (dashed black line, Figure 4.D) illustrates the Qspec progress of an exemplary (nearly) defect free surface. Starting from Qspec equal to 160 μC cm01 ./ , a steep constant decline due to the uniform desorption of protons with increasing potential can be observed. Above 0.33 V, Qspec rises up to 80 μC cm01 ./ at about 0.44 V. This corresponds to the adsorption of 0.3 ML of fully discharged HSO4--anions typically observed on Pt(111) 42. The subsequent plateau originates from the equal charge distribution in the double layer region and is followed by a small increase of Qspec (~ 15 42 μC cm01 ./ ) which can be ascribed to the reversible OHad adsorption on bisulfate-free Pt sites .
Comparing the run of the Qspec curve of the “strained” and “relaxed” Pt films, several aspects are worth noticing. Firstly (indicated by number 1 in Figure 4.D), the Hupd charge of the 01 “relaxed” Pt is larger than the 160 23 456
conventionally observed for Pt(111) surfaces. The
physical meaning is to indicate the small presence of polycrystalline steps and edges with (100) facets that can adsorb more protons than the close packed (111) facets and thus generate a higher Qspec value. This finding is well in agreement with the lack of the sharp butterfly peak observed in the CV (Figure 4.A). Secondly (indicated by number 2 in Figure 4.D), on the “strained” Pt the Hupd charge is clearly suppressed, suggesting that the adsorption affinity towards protons has been affected by strain as also reported in literature 43-44 for Pt skin structures. Thirdly (indicated by number 3 in Figure 4.D), on both “relaxed” and “strained” Pt the bisulfate adsorption is reduced. Whereas in this region the run of the curve for the “relaxed” Pt still follows the Pt(111) single crystal (only slightly downshifted), the “strained” Pt exhibits a 17 ACS Paragon Plus Environment
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completely different behavior. In the latter case, the Qspec increases exponentially with increasing the applied potential. Fourthly (indicated by number 4 in Figure 4.D), no distinct adsorption features related to OHad are recognizable neither for the “relaxed” nor the “strained” Pt, indicating a comparably low capability of these surfaces to adsorb oxygenated species. Lastly (indicated by number 5 in Figure 4.D), in the region of interest concerning ORR (E< 1 V), a reduced Qspec and thus reduced total coverage θad of both spectator species, OHad and bisulfates, is visible on both surfaces. As has been mentioned before, θad represents a crucial factor for determining the ORR activity and thus represents a first indication of the positive influence of strain on ORR kinetics. To conclude, the CV shape of the “relaxed” Pt is comparable to the one of the Pt(111) single crystal (except for number 5 as described above), indicating that the surface of the former is truly in a relaxed state. In contrast, the Hupd features of the “strained” Pt film are suppressed, allowing to correlate the strain measured by XRD to this distinctive Hupd fingerprint. Furthermore, Qspec evaluation reveals that the total θad of spectator species is reduced on the “strained” and “relaxed” Pt as compared to a Pt(111) single crystal. Yet, this method does not offer the possibility to differentiate specifically between the effect of strain on θsulfates or on θOH. This effect can be probed in CO oxidation experiments where at low potentials COad is adsorbed on the Pt surface and subsequently oxidized to CO2 with increasing potential 45. To clarify the role of strain on OH adsorption, it is worth looking more closely into the reaction path of the COad oxidation. Since the (bi)sulfate adsorption is very fast compared to CO oxidation, it is well known that the shape and onset of the COad oxidation peak is only determined by CO oxidation kinetics 46. As a consequence, θsulfates clearly does not play a crucial role in this reaction. Furthermore, for the COad oxidation to proceed, the coadsorption of OHad on adjacent sites to COad is required. The adsorption of OH species can only take place on surface sites liberated 18 ACS Paragon Plus Environment
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ACS Catalysis
upon CO desorption or CO oxidation and is faster the larger the OH adsorption affinity of the surface. The main peak potential can thus serve as a measure to estimate the adsorption affinity of a surface towards OHad as a function of the strained surface state. Hereby, the influence of strain on θsulfates or on θOH can thus be separated. The CO oxidation voltamogramm of the Pt(111) single crystal (Figure 5A, black dashed line) is characterized by one sharp oxidation peak at 0.777 V, which (nearly) coincides with the peak potential (0.781 V) of the “relaxed” Pt (Figure 5A, blue line). This is expected since the adsorption properties of both surfaces should be similar. The shape of the CO oxidation peak of the “relaxed” Pt film is slightly broader than that of the single crystal. The broadening arises from surfaces with higher defect densities since the CO molecules can adsorb on sites of varying coordination
47
. This is in agreement with the expected larger defect density of a thin film as
compared to a single crystal and the observations from CV experiments in this study.
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Figure 5. A. CO oxidation experiments of “strained” and “relaxed” (111) Pt films in comparison to a single crystal (111) Pt performed at room temperature in 0.5 M H2SO4 at 10 mV s-1. B. Cathodic oxygen reduction reaction polarization curves measured at 10 mV s-1 in an in-house designed flow cell 48. The inset shows the remarkably higher activity of “strained” Pt films compared to “relaxed” Pt. 20 ACS Paragon Plus Environment
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In contrast, the catalytic properties of the strained interface are markedly different from that of the relaxed one (Figure 5A, orange graph). The main CO oxidation peak is shifted to more positive potentials (0.798 V), which, as described before, can be ascribed to a weakening of the OHad affinity on the strained surface. Similar findings have been reported on compressed Pt/W(111) thin films 49. This result puts in evidence that strain clearly affects the OHad adsorption energy allowing to decouple it from the effect on adsorbed (bi)sulfates. We, therefore, suggest that in the coverage-dependent term "1-#$ - % &' (, strain influences mostly θOH resulting in the fact that the ORR reaction rate “” is primarily related to this parameter. Furthermore, the charge QCO under the oxidation peak can then be used to determine the roughness factor (Rf) as given by Equation 5 in the experimental section. In electrocatalysis, the Rf is generally used as a measure of the electrochemical active surface area (ECSA). It is defined as the ratio between the area exposed to the electrolyte and the geometric surface area of the electrode. Most commonly, the Rf is determined through the desorption/adsorption charge of Hupd in a CV. However, CO monolayer oxidation experiments are typically considered to give more accurate ECSAs and Rfs for skin structured nanoparticles and thin films, respectively [54]. Roughness factors of 0.97, 1.05 and 1.25 cm2Pt cm2geo were obtained for the Pt(111) single crystal, the “relaxed” and the “strained” Pt, respectively (see Table 1). These findings are well in agreement with the results from AFM, which showed an inverse proportionality between film thickness and rms value, which is related to the surface roughness (Figure 2). The rougher the surface, the more electrochemical active sites are exposed to the electrolyte and thus the larger Rf. To conclude, the additional similarity between the single crystal and the “relaxed” Pt film in terms of roughness factor and CO oxidation peak position provides strong evidence that at a thickness of 24 nm, any strain effects have vanished at the surface. The “relaxed” Pt film can, 21 ACS Paragon Plus Environment
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therefore, serve as a valid baseline for the following electrochemical investigations concerning the ORR activity. Furthermore, significant differences concerning the main CO oxidation peak position between the relaxed and strained Pt films were observed, suggesting a different adsorption affinity of oxygenated species of the two surfaces. Through CV and CO oxidation experiments we have suggested that the ORR rate should be primarily governed by the θOH coverage, which in turn, as we have shown, strongly depends on strain. In the following, ORR polarization curves are evaluated to confirm this statement. The Ohmic drop corrected cathodic scan of the “strained” vs “relaxed” Pt film is plotted in Figure 5B. At room temperature, the reversible potential Erev of the ORR in an acidic solution is situated at 1.23 V. At low overpotentials η (0.7 < E < 0.9 V, thus η~0.3-0.5 V), a mixed diffusion/kinetically controlled region is observed. At larger overpotentials (E ~0.5< V), the current is generally purely limited by the diffusion of the reactants to the electrode surface. The same diffusion-limited current plateau for both “strained” and “relaxed” Pt films thus demonstrates that also the “strained” Pt film consists of a complete electronically connected network over the whole geometrical area. At more negative potentials than 0.5 V, the current decreases, indicating a partial change from a 4e- to a 2e- path resulting in a larger hydrogen peroxide production, commonly observed for Pt(111) single crystals 13. The shift of the polarization curve of the “strained” Pt film to more positive potentials is evidently recognizable and in good agreement with the same shift observed in CO oxidation experiments. The half wave potential E1/2 of the ORR polarization curves shifts from 0.77 V for the relaxed Pt films to 0.81 V for the strained Pt films, indicating the enhanced ORR activity for the latter 50. To evaluate the specific electrocatalytic activity of the “strained” Pt film in comparison to the “relaxed” Pt film, the current is commonly compared at 0.9 V. This potential is situated in the activation controlled regime, where mass-transport contributions are negligible. This makes it 22 ACS Paragon Plus Environment
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ACS Catalysis
possible to directly use the kinetic current from the polarization curve without significant errors due to the influence of diffusion as demonstrated in previous studies
48
. As shown in Table 1
(and inset in Figure 5B), specific activities (see Equation 8 in the Experimental Section) of 21 µA cm2Pt and 168 µA cm2Pt are obtained for the “relaxed” and “strained” Pt, respectively, corresponding to an 8-fold improvement of the “strained” Pt film towards ORR. This result is well in agreement with reports from literature where it was demonstrated that the presence of tensile strain on the catalyst surface has the opposite effect on the ORR than compressive strain, namely hindering the ORR kinetics 30.
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Table 1. Summary of all important parameters for the Pt(111) single crystal, the “relaxed” and “strained” Pt.
Pt(111) „relaxed“ Pt „strained“ Pt
Strain ɛ [%]
Qspec in Hupd region 0> [89 :; [89 :; :;?@AB ]
CO peak potential [V]
E1/2 ORR [V]
ispec at 0.9 V at RT [;? :;0> 160
85
1.05
0.781
0.77
21
0.3
Qspec
