Impact of Support Physicochemical Properties on the CO Oxidation

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Impact of Support Physicochemical Properties on the CO Oxidation and the Oxygen Reduction Reaction Activity of Pt/SnO2 Electrocatalysts Annett Rabis, Tobias Binninger, Emiliana Fabbri, and Thomas J. Schmidt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09976 • Publication Date (Web): 18 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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The Journal of Physical Chemistry

Impact of Support Physicochemical Properties on the CO Oxidation and the Oxygen Reduction Reaction Activity of Pt/SnO2 Electrocatalysts Annett Rabis1, Tobias Binninger1, Emiliana Fabbri 1*, Thomas J. Schmidt1,2 1

Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

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Laboratory of Physical Chemistry, ETH Zürich, 8093 Zürich, Switzerland

*Corresponding author: [email protected] +41 56 310 27 95

ACS Paragon Plus Environment

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Abstract The interaction between Pt catalysts for the electrochemical oxygen reduction reaction (ORR) and corrosion resistant SnO2 supports has been studied by varying the Pt morphology and the SnO2 physicochemical properties in a model electrode study. Different Pt morphologies ranging from isolated particles to thin films have been deposited by magnetron sputtering on oxidized and reduced SnO2 model film electrodes as well as on glassy carbon (GC). Furthermore, three different surface probe reactions, namely the hydrogen underpotential deposition (Hupd), the CO oxidation, and the oxygen reduction reaction (ORR) have been studied to investigate the support influence on the Pt electrocatalytic properties. A marked effect of the type of support, i.e., tin oxide vs. carbon, on the Pt electrochemically active surface area calculated from the Hupd charge was observed. Furthermore, a pronounced CO oxidation activity for Pt deposited on SnO2 supports was observed compared to that of Pt supported on GC. The intrinsic ORR activities of Pt/SnO2 and Pt/GC catalysts varied with both the Pt morphology and the SnO2 stoichiometry. Whereas very similar ORR activities of all catalysts were found at high Pt loadings where an extended surface Pt morphology is expected, a strong support-dependence was observed for isolated Pt particles at low Pt loadings. Pt nanoparticles supported on reduced SnO2 and on GC showed very comparable ORR activities, about five times higher than that of Pt nanoparticles on oxidized SnO2. Post mortem XPS investigations revealed that the reduced ORR activity of the latter catalysts can be explained with a stronger oxidation of Pt nanoparticles when supported on oxidized SnO2. This finding highlights the fundamental importance of tailoring the oxide support properties in order to maximize the Pt electrocatalyst performance.


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1. Introduction State-of-the-art cathode material for polymer electrolyte fuel cells (PEFCs) consists of Pt nanoparticles supported on high surface area carbon (Pt/C).1 It is widely established that standard carbon supports are prone to oxididation/corrosion under typical PEFC cathode operation conditions.1 Within the search for suitable substitutes, SnO2-x turned out being potentially stable supports for Pt catalysts.1-8 However, whether the oxygen reduction reaction (ORR) activity of Pt/SnO2-x catalysts can compete with that of standard Pt/C catalysts is still controversial. Saha et al.

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showed that Pt nanoparticles supported on SnO2 nanowires display higher ORR activity

than Pt/C catalysts. Zhang et al. 5 developed a mesoporous, high surface area SnO2 support for Pt and found a similar ORR activity as for the Pt/C reference catalyst. Differently, Pt/SnO2 electrocatalysts prepared by Takasaki et al.

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obtained lower ORR activity compared to that of

Pt/C reference electrodes. Depending on the fabrication method and conditions, SnO2-x-based oxides can possess different physicochemical properties e.g., oxygen stoichiometry, electrical conductivity, crystal/particle sizes or surface terminations. Currently, it is not well understood if and how the different physicochemical properties of the SnO2-x supports influence the ORR activity of Pt catalysts. Indeed, most of the studies on this topic focus on comparing the ORR activity of Pt/SnO2-x catalysts to that of standard Pt/C catalysts,4-6 but a systematic study aiming at the understanding of the impact of the support physical and chemical properties on the Pt ORR activity is currently missing. For that purpose, a model catalyst system is required, allowing the direct modification of a single property while keeping others as constant as possible. The utilization of model Pt/SnO2 catalysts prepared by magnetron sputtering represents a suitable approach to specifically


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vary certain SnO2-x and/or Pt properties.7-9 Furthermore, by magnetron sputtering deposition, supports with similar surface areas can be obtained and no binder or other additives are necessary for the catalyst electrode preparation. In recent publications,7-8 we have shown that reactive magnetron sputtering is a reliable method to produce well-defined SnO2 films having different physicochemical properties. Two SnO2 films which differ in their chemical bulk and surface oxygen stoichiometry as well as in the preferential crystal orientation have been prepared and used as substrates for Pt catalysts. A twofold increase in the kinetic ORR currents has been previously observed by depositing a fixed Pt loading on reduced SnO2 compared to Pt supported on oxidized SnO2 films.7 However, from our previous study it was not possible to deduce the origin of the differences in ORR activities. Building on the studies described in ref. 7, in the present work, different Pt loadings were deposited on reduced and oxidized SnO2 thin films resulting in two sets of Pt catalysts with varying morphologies, i.e. from isolated Pt particles via network structures to Pt films almost fully covering the support. As reference, Pt nanoparticles and films were also deposited on glassy carbon support. This approach allows investigating the sensitivity of the ECSA, CO oxidation activity and specific ORR activity with respect to both the Pt morphology and the support stoichiometry of SnO2-supported catalysts.

2. Experimental Methods Pt/SnO2 catalysts were prepared by magnetron sputtering as previously described in references 78

. In short, SnO2 thin films were deposited on glassy carbon (GC) disk substrates by reactive DC

magnetron sputtering using a metallic tin target and O2 as a reactive gas. Varying the oxygen


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partial pressure allowed the fabrication of both sub-stoichiometric and fully oxidized SnO2 films. The sub-stoichiometric SnO2-x obtained cassiterite crystal structure after annealing. This SnO2-x is termed ‘reduced SnO2’ in the following with the surface as well as the bulk composition being about SnO1.7.7-8 The fully oxidized, stoichiometric SnO2 film kept its preferential (110) orientation throughout the annealing step,7-8 and it is termed ‘oxidized SnO2’ in the following. Different amounts of Pt were sputtered on both types of SnO2 thin films and additionally directly on a glassy carbon (GC) disk surface. The calibration of the Pt sputtering parameters has been obtained by preparing Pt films with different thicknesses on Silicon wafers and measuring the Pt film thickness by Rutherford Backscattering Spectroscopy (RBS). The Pt loading [µg cm-2] was then calculated considering the Pt density and the film thickness. The Pt sputter deposition was carried out in Argon with a gas pressure of 4.3x10-3 mbar and a cathode power of 50 W. Pt loadings of 0.5, 1.0, 2.0, 4.0, and 6.0 µg cm-2 have been obtained by varying the deposition time. Transmission electron microscopy (TEM) images (FEI Tecnai F30 FEG) were taken to investigate the morphology of the sputtered Pt catalysts. For this purpose, Pt was deposited in the same way on amorphous carbon TEM grids since the SnO2/GC substrates were not suitable for TEM measurements. However, the possibility of a slight support influence on Pt morphology must be taken into account. The oxidation states of the different Pt catalysts have been analyzed by X-ray photoemission spectroscopy (XPS) using an ESCALAB 220iXL spectrometer (Thermo Fischer Scientific) equipped with an Al Kα monochromatic X-ray source. XPS analysis has been performed on as-prepared samples and after electrochemical characterization. Electrochemical measurements were conducted at room temperature in a glass cell with threeelectrode configuration together with a programmable potentiostat (Biologic VMP3). An Hg/Hg2SO4 reference electrode was used, calibrated vs. the reversible hydrogen electrode


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(RHE). The GC disks (0.196cm2) used as substrates for the preparation of all catalysts in this study could be inserted directly as working electrodes in an interchangeable rotating disk electrode (RDE) setup (PINE Instruments). After preparation, the electrodes were immersed in N2-purged 0.1 M HClO4 (SupraPure, Merck AG) under potential control. All electrodes were pretreated by cyclic voltammetry (CV) with 20 potential cycles at 50 mV s-1 between 0.05 and 1.1 VRHE. Continuous potentiodynamic carbon monoxide (CO) oxidation curves have been recorded (sweep rate of 20 mV s-1) in CO-saturated electrolyte while rotating the electrode at a velocity of 1600 rpm. For ORR experiments, the electrolyte was saturated with O2 and polarization curves were recorded in the cathodic potential scan direction at 5 mV s-1 with an electrode rotation velocity of 1600 rpm. The electrochemically active surface area (ECSA) of the Pt

catalysts

was

determined

from

the

hydrogen

underpotential

deposition

(Hupd)

adsorption/desorption charges following a procedure described previously.10 In short, instead of the standard Hupd background correction method, a CV recorded in CO-saturated electrolyte has been used for the subtraction of background charges resulting from the SnO2 support electrochemical redox behavior.10 In order to avoid an overlapping of the CO oxidation onset with the Hupd region, an electrode rotation of 1600 rpm was necessary. The ECSA-derived quantities used in this study are defined as follows:

•

‘ECSA’ is the Pt electrochemically active surface area in cm2Pt.

•

‘ms-ECSA’ is the Pt mass-specific ECSA in m2Pt g-1Pt, i.e. the ECSA normalized w.r.t. the Pt loading.

•

‘rf’ is the Pt roughness factor in cm2Pt cm-2geo, i.e. the ECSA normalized w.r.t. the geometrical area of the disk electrode (0.196cm2).


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3. Results 3.1 Morphology and ECSA of Pt/SnO2 and Pt/GC catalysts In order to obtain an understanding of the morphology of the sputtered Pt catalysts as a function of the Pt loading (i.e. deposition time), Pt was also sputtered on TEM grids with the same deposition conditions used for the oxides and the GC supports. Figure 1 shows TEM images of sputtered Pt catalysts with different Pt loadings (0.5, 1.0, 2.0, 4.0, and 6.0 µg cm-2). Isolated and homogeneously distributed Pt particles with diameters of approximately 2 nm were obtained for the lowest investigated Pt loading. Pt agglomerates and network structures were observed for Pt loadings between 1.0 to 4.0 µg cm-2, while higher loadings resulted in the formation of dense Pt films fully covering the substrate. However, due to a possible influence of the substrate, Pt morphologies on the oxide and GC supports could follow slightly different morpholgy trends compared to Figure 1. Nevertheless, TEM samples are expected to provide at least an approximate picture of the Pt morphologies on the latter.


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Figure 1. TEM images of different Pt loadings sputtered on amorphous carbon TEM grid together with a sketch of the sample cross section. Figure 2a-c shows CVs of catalysts with Pt loadings of 1.0 and 6.0 µg cm-2 supported on reduced and oxidized SnO2 and on GC recorded in N2-saturated 0.1 M HClO4 electrolyte. For most of the catalysts, characteristic Pt features can be clearly observed: Hydrogen adsorption/desorption (Hupd) region at potentials below 0.4 VRHE, the ‘double-layer region’ (0.4–0.6 VRHE), and the Pt surface oxidation/reduction region at E > 0.6 VRHE. For the 1.0 µg cm-2 Pt supported on oxidized SnO2, the Hupd region is dominated by the support background currents and the characteristic hydrogen adsorption/desorption features are not clearly visible. The position of the Pt-OH reduction peak maximum (Ered,max) in the potential region between 0.6–0.8 VRHE is related to the adsorption strength of oxygenated species on the Pt surface, with a lower value of Ered,max corresponding to stronger adsorption of oxygenated species. Ered,max values estimated from the


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CVs in Figure 2a-c are summarized in Figure 2d. On all supports, lower values of Ered,max are found for lower Pt loadings, which can be explained with well-known Pt particle size effects leading to an increased oxidation tendency of smaller Pt particles. Turning to the effect of the support, in comparison to Pt/GC, lower values are found especially for the Pt/oxidized SnO2 catalyst that also reveals the strongest sensitivity of the Pt reduction peak position towards the Pt loading. These results point to an SnO2-induced strengthening of Pt-OH surface species. This effect appears strongly dependent on the stoichiometry of the SnO2 support and is enhanced with decreasing Pt loadings.

Figure 2. CVs of catalysts with Pt loadings of 1.0 and 6.0 µg cm-2 supported on reduced SnO2 (a), oxidized SnO2 (b), and GC (c) recorded in N2-saturated 0.1 M HClO4 electrolyte at a sweep


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rate of 50 mV s-1. The table (d) summarizes the Pt-OH reduction peak maximum positions (Ered,max) of the corresponding CVs.

The ECSA and the roughness factors (rf) of the Pt/SnO2 and Pt/GC catalysts was determined by an advanced Hupd method described in detail in Ref.10. Unfortunately, the extremely low amount of Pt for the 0.5 µg cm-2 loading led to inaccurate ECSA determination, and therefore the electrochemical results related to those samples are not presented in the following. In Figure 3, rfs are shown as function of the Pt loading ranging from 1.0 to 6.0 µg cm-2. As expected, rf values increase with increasing Pt loading. For Pt supported on reduced and oxidized SnO2 the rf values are very similar, while significantly higher rf values were obtained for Pt/GC electrodes: Pt/GC catalysts with a Pt loading 1.0 µg cm-2 had rf values of 0.92 ± 0.05 cm2Pt cm-2geo, while Pt/SnO2 catalysts with an identical Pt loading revealed values of 0.55 ± 0.1 cm2Pt cm-2geo for Pt/oxidized SnO2 and 0.51 ± 0.08 cm2Pt cm-2geo for Pt/reduced SnO2, respectively. This corresponds to ms-ECSA values of 92 m2 g-1, 55 m2 g-1, and 51 m2 g-1 for the three systems, respectively. Different reasons can account for the lower ECSAs of Pt/SnO2 catalysts compared to those of Pt/GC: (i) a support-dependent Pt catalyst morphology or (ii) a modification of the hydrogen adsorption on the Pt surface as a function of the support physicochemical properties. The observation that for the same Pt loading almost identical ECSAs have been obtained from Hupd analysis for the Pt catalysts supported on the oxidized and reduced SnO2, in spite of the fact that both supports have very distinctive physicochemical properties, does not indicate a pronounced support-effect on the Pt hydrogen adsorption properties. Therefore, it is most likely that the difference in ECSAs between Pt/GC and Pt/SnO2 samples is due to the former reason, i.e. a support-dependent catalyst morphology when moving from carbon to an oxide support.


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Since rf values of the SnO2 supported catalysts are significantly lower than for GC supported ones even for 6.0 µg cm-2 Pt loading, incomplete coverage of the oxide supports can be deduced even at high Pt loading in contrast to the morphologies observed for Pt sputtered on amorphous carbon TEM grid. This conclusion is further supported by the non-zero slope of the rf vs. loading curve between 4.0 and 6.0 µg cm-2 Pt loading. In contrast, for Pt/GC, the negligible variation of rf between 4.0 and 6.0 µg cm-2 Pt loading indicates dense Pt films on GC support at these loadings in agreement with observations on TEM samples.

Figure 3. Roughness factor (rf) as a function of the Pt loading for Pt supported on GC and on oxidized and reduced SnO2.

3.2 Electrochemical CO oxidation In order to gain more insight into the surface-electrochemical Pt properties of the different catalysts, continuous CO oxidation experiments have been performed. In Figure 4a-c, potentiodynamic CO oxidation polarization curves for Pt supported on reduced and oxidized SnO2 and on GC with loadings of 1.0 and 6.0 µg cm-2 are shown. In Figure 4d, the CO oxidation


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curves for the three different catalysts with same Pt loading of 6.0 µg cm-2 are directly compared. For the highest Pt loading 6.0 µg cm-2, diffusion limiting currents are observed for all the supports but significant CO oxidation currents are already obtained at potentials below 0.6 VRHE for the Pt/reduced SnO2 catalyst. In contrast, no significant CO oxidation current is observed for the Pt/GC and the Pt/oxidized SnO2 catalyst below 0.8 VRHE. In case of the lowest Pt loading (1.0 µg cm-2) no diffusion limiting currents are reached for the Pt/oxidized SnO2 catalysts showing low currents in the entire potential range and indicating a non-active surface for the CO oxidation reaction. In contrast, 1.0 µg cm-2 Pt deposited on the reduced SnO2 shows remarkable CO oxidation currents even at relatively low potentials compared to Pt/GC. These results indicate a strong support influence on the Pt CO oxidation activity, with an outstanding activity obtained for the Pt/reduced SnO2 sample especially at low Pt loading.


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Figure 4. CVs of Pt/reduced SnO2 (a), Pt/oxidized SnO2 (b) and Pt/GC (c) catalysts with loadings of 1.0 and 6.0 µg cm-2 in CO-saturated 0.1 M HClO4 electrolyte at a sweep rate of 20 mV s-1 and at an RDE rotation velocity of 1600 rpm. (d) Direct comparison of the anodic scans for the three catalysts with 6.0 µg cm-2 Pt loading.

3.3 Oxygen reduction reaction activity Polarization curves recorded in O2-saturated electrolyte are shown in Figure 5 for the Pt catalysts supported on reduced and oxidized SnO2 and on GC, with Pt loadings of 1.0 and 6.0 µg cm-2. Similar polarization curves with identical limiting currents in the oxygen mass transportcontrolled region between 0.2–0.5 VRHE were obtained for all catalysts with Pt loadings of 6.0 µg cm-2 (Figure 5b). Differently, decreased limiting currents and a strong negative shift of half-wave potentials were observed for catalysts with a Pt loading of 1.0 µg cm-2 supported on reduced and oxidized SnO2 compared to the Pt/GC catalyst (Figure 5a).

Figure 5. ORR polarization curves of Pt catalysts deposited on reduced and oxidized SnO2 and on GC, with loadings of 1.0 (a) and 6.0 µg cm-2 (b).


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Mass transport-corrected kinetic currents were derived from the ORR polarization curves using the Koutecky-Levich equation and were normalized w.r.t. the corresponding ECSA to obtain the specific kinetic ORR current (jspec). In Figure 6, the resulting jspec values at 0.9 VRHE are plotted vs. the Pt loading for all investigated catalysts.

Figure 6. Specific ORR activities at 0.9 VRHE of Pt catalysts with different loadings supported on GC, oxidized and reduced SnO2. For all the supports, the highest specific ORR activity was reached for the highest Pt loading, i.e. for the sample with the Pt morphology closest to an extended surface, with values at 0.9 VRHE of 0.40, 0.34, and 0.36 mA cmPt-2 for Pt supported on GC, oxidized and reduced SnO2, respectively. Specific activities of Pt/GC and Pt/reduced SnO2 are very similar with overlapping error margins for all Pt loadings and show a decreasing trend as the Pt loading decreases, i.e. moving from Pt extended surfaces to isolated nanoparticles, as expected due to the well-known Pt morphology influence on the ORR activity.11 The same is true also for the Pt/oxidized SnO2 at the higher Pt loadings of 4.0 and 6.0 µg cm-2. In contrast, significant differences in jspec were observed for Pt loadings ≤ 2.0 µg cm-2 on the oxidized SnO2 support with a jspec value of only 0.05 mA cmPt-2 at


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1.0 µg cm-2 Pt loading. The corresponding values for Pt/GC and Pt/reduced SnO2 with 1.0 µg cm-2 Pt loading are 0.24 and 0.21 mA cmPt-2, respectively. Thus, whereas the Pt/GC and Pt/reduced SnO2 catalysts largely maintain high ORR activity also at low Pt loadings, the Pt/oxidized SnO2 catalysts reveal a significant decrease in activity with decreasing Pt loading. 3.4 X-ray photoelectron spectroscopy (XPS) investigations XPS analysis has been performed to obtain more insights into the effect of the SnO2 supports on the oxidation state of Pt catalysts. Measurements carried out on the as-prepared electrodes showed that the Pt catalysts were mainly present in the metallic state (4f7/2 peak of Pt located at approx. 71 eV) for all Pt loadings independent of the support. A slight shift towards higher binding energy of the Pt 4f spectra was observed for loadings < 2.0 µg cm-2, suggesting that Pt nanoparticles were slightly more oxidized than extended surfaces.

Figure 7. Pt 4f XPS spectra of Pt supported on reduced SnO2 (a), oxidized SnO2 (b), and GC (c) for the different Pt loadings. XPS Pt 4f spectra of samples after the electrochemical measurements were also recorded for Pt loadings of 1.0 and 6.0 µg cm-2 shown in Figure . The Pt 4f spectra of the 6.0 µg cm-2 Pt catalysts supported on GC, reduced and oxidized SnO2 are nearly identical (Figure a). However, significant differences in the oxidation of the Pt catalysts after electrochemical measurements


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depending on the support were observed for a Pt loading of 1.0 µg cm-2 (Figure b-d). The Pt 4f7/2 peak maxima for Pt/GC (Figure b) and Pt/reduced SnO2 (Figure c) were located at approx. 71.0 and 71.5 eV, respectively, indicating that the Pt particles were mainly present in the metallic state even after the electrochemical measurements. Differently, the Pt 4f7/2 spectrum of Pt/oxidized SnO2 revealed only a minor contribution at 71.0 eV (metallic Pt) and a large contribution at 73.3 eV, corresponding to PtO2. Additionally, a strong general decrease of the Pt 4f peak intensity compared to the Sn 3d5/2 peak intensity was observed for Pt/oxidized SnO2 after electrochemical experiments suggesting a significant loss of Pt mass during electrochemical measurements for this catalyst.


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Figure 8. Pt 4f XPS spectra of Pt supported on GC, reduced and oxidized SnO2 after the electrochemical characterization: Comparison of catalysts with a loading of 6.0 µg cm-2 (a), and Pt/GC (b), Pt/reduced SnO2 (c) and Pt/oxidized SnO2 (d) with a loading of 1.0 µg cm-2.

4. Discussion Before further interpreting the results, it must be discussed whether the specific differences between the reduced and oxidized SnO2 remained preserved during electrochemical testing. For this purpose, Supporting Information Figure S1 presents CVs of the bare SnO2 thin films in the potential range between 0 and 1.0 V vs. RHE relevant for the present work. The physicochemical differences between reduced and oxidized SnO2 lead to distinct CV shapes of both types of SnO2 with an increased electrochemical redox activity of the reduced SnO2. Although the pronounced oxidation peak around 0.4 V vs. RHE of the reduced SnO2 becomes weaker during cycling, the general distinctiveness of the CV of the latter in comparison to the oxidized SnO2 is preserved. Therefore, it can be concluded that the sub-stoichiometric nature of the reduced SnO2 was largely conserved during electrocatalytic tests conducted in this study. The physicochemical nature of the support material was found to have direct influence on the morphological and electrochemical properties of the sputtered Pt catalysts. For all Pt loadings, Pt morphologies on the oxidized and the reduced SnO2 supports were very similar judging from the ECSA as determined by electrochemical hydrogen adsorption. In contrast, significantly higher ECSA was obtained for the Pt catalysts supported on the GC. Whereas these results show that the sputtered Pt morphology was dependent on the fundamental chemical nature of the substrate,


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i.e. tin oxide vs. carbon, the detailed stoichiometry of the SnO2 appeared to have negligible influence on the ECSA. However, a very different picture was found regarding the electrocatalytic properties of the Pt. Interestingly, all catalysts behaved differently regarding CO oxidation. The Pt CO oxidation activity varied strongly with the supports and for the SnO2-supported Pt a significant difference was observed also as a function of the Pt loading. While 6.0 µg cm-2 of Pt supported on oxidized SnO2 showed relatively good CO oxidation activity, very low CO oxidation currents were observed for the lowest Pt loading on this support, probably due to the highly oxidized nature of Pt nanoparticulate catalysts supported on the oxidized SnO2, as revealed by XPS investigations. In contrast, the highest CO oxidation activity was observed for Pt/reduced SnO2, particularly for the lowest Pt loading, indicating a positive support influence on the Pt CO oxidation electrochemistry. CO oxidation is a suitable probe for the presence of adsorbed oxygenated species on the Pt surface (predominantly OHad), due to the involvement of such co-adsorbed species in the CO oxidation mechanism12-13. The early onset of CO oxidation on the Pt/reduced SnO2 catalysts indicates that a certain amount of oxygenated species are already adsorbed on the Pt surface at relatively low potentials in the double-layer region. This is supported by the lower potential for the Pt-OH reduction peak maximum (Ered,max) observed in the CV of Pt/reduced SnO2 at low Pt loading shown in Figure 2 in comparison to Pt/GC. Summarizing, the CO oxidation results can be interpreted as follows: Both SnO2 supports lead to an increased tendency of Pt to adsorb OH species, which lead to an earlier onset of CO oxidation in comparison to Pt/GC, cf. Figure 4d. This effect becomes stronger at lower Pt loadings, leading to highest activity of the 1.0 µg cm-2 Pt/reduced SnO2. However, on the oxidized SnO2, this enhancement of OH adsorption on the Pt becomes so strong that it turns from a beneficial reversible OHad to


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the formation of a detrimental irreversible Pt oxide layer, thus explaining the breakdown of CO oxidation activity for the 1.0 µg cm-2 Pt/oxidized SnO2 catalyst. The specific ORR activity of Pt supported on reduced SnO2 was found to be almost identical to the one of Pt/GC for all Pt loadings despite the different Pt ECSA values. It has been demonstrated that the specific ORR activity of Pt catalysts strongly depends on the Pt ECSA, i.e. on the Pt morphology, following a master curve obtained by normalizing the values from different studies to an arbitrary Pt specific surface area (35 m2Pt/gPt), being the lower the ECSA the higher the specific ORR activity.11 Since for all loadings, Pt/reduced SnO2 samples revealed lower ECSAs than those of corresponding Pt/GC samples, the former could be expected to display higher specific ORR activity (by about a factor of 1.65, according to the master curve reported in ref. 11) than that of the latter catalysts. Therefore, the observation of the specific ORR activity of Pt/reduced SnO2 samples being almost identical to that of Pt/GC indicates that the reduced SnO2 support had an additional negative effect on the Pt ORR activity which compensated the expected beneficial effect of the Pt morphology. This negative support effect could be ascribed to a higher tendency of the Pt catalysts supported on reduced SnO2 to reversibly adsorb OH species compared to Pt supported on GC, in agreement with the interpretation of increased CO oxidation activity of the Pt/reduced SnO2 discussed above. However, whereas increased OHad has a beneficial effect on the CO oxidation activity, it has a negative effect on the ORR activity due to blocking of active Pt sites (basically directly influencing the preexponetial (1-Θ)-term in the rate equation described previously14). Since OH adsorption can be regarded as a reversible Pt “oxidation” process, differences between Pt supported on reduced SnO2 and on GC cannot be seen by XPS in ultra-high vacuum.


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A different behavior has been observed for Pt supported on oxidized SnO2 which showed significantly lower specific ORR activities than those of Pt/GC and Pt/reduced SnO2 for lower Pt loadings. Since the ECSA of Pt were almost identical on both types of SnO2, morphological effects can be excluded as origin of this significant decrease of jspec for Pt/oxidized SnO2 compared to Pt/reduced SnO2. Furthermore, it can be excluded that these differences in ORR activities arise from variations of the respective support conductivities, because for Pt loadings ≥ 4.0 µg cm-2, very similar ORR activities were observed for all catalysts. Therefore, the decrease of the intrinsic ORR activity of the Pt/oxidized SnO2 catalysts at low Pt loadings can be explained in the same way as the suppression of CO oxidation discussed above: The oxidized SnO2 support leads to an even stronger enhancement of OH adsorption on the Pt to a level that turns the reversible OH adsorption into an irreversible Pt oxide layer which leads to a strong decrease of ORR activity in comparison to Pt/reduced SnO2 and Pt/GC at low Pt loadings. This irreversible Pt oxidation on the oxidized SnO2 support is in agreement with the strong negative shift of the Pt reduction potential Ered,max obtained from CV analysis. This hypothesis is furthermore supported by XPS investigations performed after electrochemical characterization, which revealed that Pt nanoparticles on oxidized SnO2 support were prone to irreversible oxidation and mass loss. In contrast, Pt nanoparticles supported on reduced SnO2 and on GC did not experience such strong oxidation and no decrease in the Pt 4f signal was observed by XPS. It is well established that the initial formation on Pt-OH catalysts is reversible up to a certain potential before irreversible oxide formation occurs.15-19 The negative shift of the Pt-OH reduction peak observed for Pt deposited on oxidized SnO2 could justify the assumption that also the potential for the irreversible formation of PtOx is shifted to lower values. Consequently, strongly oxidized Pt nanoparticles could be present during the cathodic scan of ORR polarization


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experiments with a detrimental effect on the ORR activity. Furthermore, PtOx is known to be prone to dissolution under ORR conditions,20 which, in turn, could explain the Pt mass loss observed by XPS after electrochemical characterization. Therefore, Pt oxidation and Pt dissolution could be identified as main reasons for the low ORR activity of Pt nanoparticles supported on oxidized SnO2. However, it must be said that both types of SnO2 support led to an enhancement of Pt-OH adsorption/oxidation with a stronger effect of the oxidized SnO2 support. It is interesting to contemplate recently published ‘capacitive electronic metal–support interactions (EMSI)21 between the tin oxide support and the Pt nanoparticles as origin of this enhanced Pt oxidation tendency. It has been shown21 that such capacitive EMSI lead to an additional surface charge on the electrochemically active Pt nanoparticle surface, which is strongest for lowest Pt loadings in agreement with the experimental observations in the present study. This additional surface charge could shift the potential of zero charge of the tin oxide supported Pt nanoparticles, thus explaining an earlier onset of reversible OH adsorption and of irreversible Pt oxidation.

5. Conclusions Investigation of the electrochemical properties of sputtered Pt catalysts supported on reduced and oxidized SnO2 thin films and on glassy carbon demonstrated the impact of the support on the Pt electrocatalytic properties. Whereas the ECSA-morphology of sputtered Pt was largely determined by the fundamental chemical nature of the substrate and independent of the detailed SnO2 stoichiometry, the Pt electrocatalytic activity towards both ORR and CO oxidation was extremely sensitive to the latter. The strong decrease in specific ORR activity for Pt/oxidized


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SnO2 at low Pt loadings was explained by a strong increase of the adsorption strength of OH on the Pt surface leading to the formation of an irreversible Pt oxide layer, which was confirmed by strong negative shifts of the Pt reduction peak maxima observed in CV experiments and by endof-test XPS analysis of the Pt oxidation state after ORR. Also the reduced SnO2 support led to an enhancement of the OH adsorption on the supported Pt, although in this case the effect was reversible and less than for the oxidized SnO2 support. In contrast to the ORR activity, this enhanced reversible OH adsorption greatly facilitated the electrochemical CO oxidation on Pt/reduced SnO2. These findings clearly reveal a strong impact of the detailed physicochemical properties of the SnO2 support on the CO oxidation and ORR activity of supported Pt catalysts, in particular at low Pt loading, and highlight the importance of creating a favorable catalyst metal–support interaction that avoids an increased oxidation of the Pt catalyst. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ASSOCIATED CONTENT Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website. CVs of reduced and oxidized SnO2 thin films between 0 and 1 V vs. RHE, sweep rate at 100 mV/s, electrolyte is Ar saturated 0.1 M HClO4.

ACKNOWLEDGMENT The authors thank Umicore GmbH & Co KG and the Competence Center for Energy and Mobility Switzerland (CCEM) for financial support within the project DuraCat.

REFERENCES 1. Rabis, A.; Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel Cells: Recent Achievements and Future Challenges. ACS Catalysis 2012, 2, 864-890. 2. Shrestha, S.; Liu, Y.; Mustain, W. E. Electrocatalytic Activity and Stability of Pt Clusters on State-of-the-Art Supports: A Review. Catalysis Reviews: Science and Engineering 2011, 53, 256-336. 3. Wang, Y.-J.; Wilkinson, D. P.; Zhang, J. Noncarbon Support Materials for Polymer Electrolyte Membrane Fuel Cell Electrocatalysts. Chem Rev 2011, 111, 7625-7651. 4. Sudan Saha, M.; Li, R.; Cai, M.; Sun, X. High Electrocatalytic Activity of Platinum Nanoparticles on Sno2 Nanowire-Based Electrodes. Electrochemical and Solid-State Letters 2007, 10, B130-B133. 5. Zhang, P.; Huang, S.-Y.; Popov, B. N. Mesoporous Tin Oxide as an Oxidation-Resistant Catalyst Support for Proton Exchange Membrane Fuel Cells. Journal of The Electrochemical Society 2010, 157, B1163-B1172. 6. Takasaki, F.; Matsuie, S.; Takabatake, Y.; Noda, Z.; Hayashi, A.; Shiratori, Y.; Ito, K.; Sasaki, K. Carbon-Free Pt Electrocatalysts Supported on Sno2 for Polymer Electrolyte Fuel Cells: Electrocatalytic Activity and Durability. Journal of The Electrochemical Society 2011, 158, B1270-B1275. 7. Rabis, A.; Kramer, D.; Fabbri, E.; Worsdale, M.; Kotz, R.; Schmidt, T. J. Catalyzed Sno2 Thin Films: Theoretical and Experimental Insights into Fabrication and Electrocatalytic Properties. J Phys Chem C 2014, 118, 11292-11302. 8. Rabis, A.; Fabbri, E.; Foelske, A.; Horisberger, M.; Kötz, R.; Schmidt, T. J. Durable Oxide-Based Catalysts for Application as Cathode Materials in Polymer Electrolyte Fuel Cells (Pefcs). Ecs Transactions 2013, 50, 9-17.


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9. Schwanitz, B.; Rabis, A.; Horisberger, M.; Scherer, G. G.; Schmidt, T. J. Sputtered Cathodes for Polymer Electrolyte Fuel Cells: Insights into Potentials, Challenges and Limitations. Chimia 2012, 66, 110-119. 10. Binninger, T.; Fabbri, E.; Kotz, R.; Schmidt, T. J. Determination of the Electrochemically Active Surface Area of Metal-Oxide Supported Platinum Catalyst. Journal of the Electrochemical Society 2013, 161, H121-H128. 11. Fabbri, E.; Taylor, S.; Rabis, A.; Levecque, P.; Conrad, O.; Kötz, R.; Schmidt, T. J. The Effect of Platinum Nanoparticle Distribution on Oxygen Electroreduction Activity and Selectivity. ChemCatChem 2014, 6, 1410-1418. 12. Gilman, S. The Mechanism of Electrochemical Oxidation of Carbon Monoxide and Methanol on Platinum. Ii. The “Reactant-Pair” Mechanism for Electrochemical Oxidation of Carbon Monoxide and Methanol1. The Journal of Physical Chemistry 1964, 68, 70-80. 13. Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. Mechanism and Kinetics of the Electrochemical Co Adlayer Oxidation on Pt(111). Journal of Electroanalytical Chemistry 2002, 524–525, 242-251. 14. Markovic, N. M.; Schmidt, T. J.; Stamenkovic, V.; Ross, P. N. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells 2001, 1, 105-116. 15. Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. The Effect of the Particle Size on the Kinetics of Co Electrooxidation on High Surface Area Pt Catalysts. Journal of the American Chemical Society 2005, 127, 68196829. 16. Ostermaier, J. J.; Katzer, J. R.; Manogue, W. H. Crystallite Size Effects in the LowTemperature Oxidation of Ammonia over Supported Platinum. J Catal 1974, 33, 457-473. 17. Cant, N. W. Metal Crystallite Size Effects and Low-Temperature Deactivation in Carbon Monoxide Oxidation over Platinum. J Catal 1980, 62, 173-175. 18. Darling, R. M.; Meyers, J. P. Kinetic Model of Platinum Dissolution in Pemfcs. Journal of The Electrochemical Society 2003, 150, A1523-A1527. 19. Wang, X.; Kumar, R.; Myers, D. J. Effect of Voltage on Platinum Dissolution: Relevance to Polymer Electrolyte Fuel Cells. Electrochemical and Solid-State Letters 2006, 9, A225-A227. 20. Kuhn, A. T.; Randle, T. H. Effect of Oxide Thickness on the Rates of Some Redox Reactions on a Platinum Electrode. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1985, 81, 403-419. 21. Binninger, T.; Schmidt, T. J.; Kramer, D. Capacitive Electronic Metal-Support Interactions: Outer Surface Charging of Supported Catalyst Particles. Physical Review B 2017, 96, 165405.


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Impact of Support Physicochemical Properties on the CO Oxidation

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