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Dec 16, 2006 - Activity, Selectivity, and Methanol Tolerance of Se-Modified Ru/C Cathode ... 0-1) in methanol free and methanol containing electrolyte...
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J. Phys. Chem. C 2007, 111, 1273-1283

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Activity, Selectivity, and Methanol Tolerance of Se-Modified Ru/C Cathode Catalysts L. Colmenares, Z. Jusys,* and R. J. Behm* Institute of Surface Chemistry and Catalysis, Ulm UniVersity, 89069 Ulm, Germany ReceiVed: July 20, 2006; In Final Form: October 20, 2006

The reaction characteristics of Se-modified Ru/C catalysts (RuSey/C; y ) 0-1) for the direct methanol fuel cell cathode application were determined in model studies, combining quantitative differential electrochemical mass spectrometry, rotating ring-disk electrode, and wall-jet disk electrode measurements. The experiments were performed under fuel cell relevant, but nevertheless, well-defined reaction conditions (continuous mass transport, elevated temperatures, and negligible internal diffusion resistance). Se modification affects both the activity for the O2 reduction reaction (ORR) and the selectivity for H2O and H2O2 formation, while methanol oxidation is in all cases negligible. Specifically, Se modification improves the O2 reduction activity and reduces the tendency for H2O2 formation in the technically relevant potential region of 0.6-0.8 V. The presence of methanol has little effect on the ORR characteristics on the RuSey/C catalysts, while on Pt/C, it leads to a rapid increase of the H2O2 yield at potentials of 0.4 V. Temperature-dependent measurements of the ORR activity result in activation energies of 18-24 kJ mol-1 for the RuSey/catalysts (at 0.7 V), comparable to the value obtained on Pt/C (18 kJ mol-1). On Ru/C, the ORR activity increases up to 50 °C (Eact ) 24 kJ mol-1) and then drops again at higher temperatures, due to surface blocking via thermally activated oxide/hydroxide formation. The additional overpotential for O2 reduction of 0.2 V as compared to the Pt/C catalyst and, in particular, the significant H2O2 yield of at least 1% at typical cathode operation potentials, which will adversely affect the long-term stability of membrane and electrode, require significant improvements before application as cathode catalyst becomes feasible.

1. Introduction In direct methanol fuel cells (DMFCs), the crossover of methanol from anode to cathode through the polymer membrane results in parasitic methanol oxidation on the cathode, which can lead to severe performance losses due to the so-called mixed potential formation (i.e., a decrease of the cathode potential and thus of the cell voltage during the simultaneously occurring methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR)).1-5 Because of the high sensitivity of the commonly used Pt/C cathode catalysts toward the presence of methanol,6-11 which results from its high activity for the MOR under typical cathode operation potentials, the development of highly ORR active and methanol tolerant cathode catalysts is one of the major challenges in low temperature fuel cell electrocatalysis. Searching for methanol tolerant cathode electrocatalysts, several groups of Pt free catalysts have been investigated, including Co or Fe containing organic macrocycles such as porphyrins12,13 or phenanthrolines,14,15 or Ru chalcogenide materials (for a review, see ref 16). Studies on the latter materials, in particular, on RuSey compound catalysts,17,18 revealed a high methanol tolerance in both model studies17,19-29 and also in fuel cell tests.18,30 Despite numerous model studies, however, the physical origin and mechanism for the catalytic properties of these compound materials are far from being understood and are still under debate. The performance of these catalysts, including their ORR activity and their selectivity for H2O formation in the absence and presence of methanol, has rarely been investigated under fuel cell relevant, but nevertheless, well-defined reaction conditions, including continuous * Authors to whom correspondence should be addressed. E-mail: (Z.J.) [email protected], (R.J.B.) [email protected].

reactions, controlled mass transport and internal diffusion properties within the catalyst layer, elevated temperatures, and the presence of both O2 and methanol in the electrolyte (simulated methanol crossover). Because of the disastrous consequences of even very small amounts of H2O2 formation, due to continuing long-term corrosion of membrane and electrode materials,31-34 a technical relevant evaluation of the ORR performance requires the highly sensitive detection of H2O2 formation (two-electron pathway) under reaction conditions, far below the 1% level. As part of a collaborative project, we have investigated the ORR performance of a series of Se surface-modified, carbon supported RuSey/C catalysts with different Se contents (y ) 0-1) in methanol free and methanol containing electrolyte. Details on the synthesis, the physical characterization of the resulting catalyst, by ex situ spectroscopic and microscopic methods, and on their stability in the potential range of 0.050.95 VRHE have been described in refs 28 and 29. In a preceding first paper,35 we reported results of detailed studies on the electrochemical surface properties of these catalysts, which have been evaluated by different electrochemical procedures including base voltammetry, COad stripping with on-line mass spectrometric CO2 product detection, Ru surface oxidation transients, and Cu underpotential deposition (Cu-upd) and stripping. That paper also included room temperature measurements on the ORR performance of these catalysts (i.e., on their ORR activity and selectivity for the H2O formation (four-electron pathway) under controlled mass transport conditions) at constant electrode potentials and in the absence of internal diffusion resistances. In the present paper, we have extended these measurements to elevated temperatures, up to 80 °C, as a further step toward more realistic reaction conditions, probing the ORR performance

10.1021/jp0645925 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

1274 J. Phys. Chem. C, Vol. 111, No. 3, 2007 and the MOR activity in both methanol free and methanol containing electrolytes under close to realistic but nevertheless well-defined reaction conditions as described previously. In our previous paper, we reported that increasing amounts of modifying Se lead to a gradual decrease of the sites active for CO and/or Cu-upd adsorption,35 although even at the highest amount of Se deposited on the Ru nanoparticle surface (RuSe/ C), ca. 10% of the Ru sites were still available for CO (or Cuupd) adsorption. This result agrees with the proposal of Zaikovskii et al., who based on their spectroscopic results, proposed that the process of Ru surface modification by Se applied in this study results in an amorphous (porous) RuSey shell covering a Ru core.29 Furthermore, we found a steady increase in the inherent, active surface area normalized ORR activity with increasing Se content, which is compatible with both the formation of a new active phase and/or the activation of Ru sites by hindering (hydr)oxide adsorption on Ru sites via (co-)adsorbed Se species.35 We reported also that for the Se modified RuSey/C catalysts, the hydrogen peroxide yield exhibited a clear minimum of ca. 1-2% at moderate Se loadings (y ) 0.3-0.6) under cathode relevant reaction conditions, which is equivalent to an optimum selectivity for O 2 reduction to water (four-electron pathway) of ca. 98-99% at this composition. The observed H2O2 formation on the RuSey/C catalysts contrasts the negligible H2O2 production on Pt/C catalysts under similar conditions. In the present paper, we focus on the quantitative evaluation of (i) the mass-specific, geometric, and active surface area normalized ORR kinetic current densities as a function of catalyst composition at elevated temperatures, (ii) the temperature dependence and the activation energy of the ORR, (iii) the mutual influence of MOR and ORR at elevated temperatures, and (iv) the selectivity in the MOR (complete oxidation of methanol to CO2) and in the ORR (H2O2 formation) in the presence of the respective other reactant. The activity and selectivity of the ORR were investigated by rotating ring-disk electrode (RRDE) measurements (activity/selectivity of the ORR at 60 °C, Section 3.1.1.) and by variable temperature measurements in a novel wall-jet disk electrode (WJDE) setup (ORR activity, 30-80 °C, Section 3.1.2.); the methanol tolerance was studied by differential electrochemistry mass spectrometry (DEMS) measurements of the MOR in O2 free electrolyte (room temperature measurements) and by potentiodynamic RRDE measurements of the simultaneously occurring MOR and ORR at 60 °C in methanol containing the O2-saturated electrolyte (mixed-reactant feed). Finally, the impact of these findings for practical application of RuSey/C catalysts as possible DMFC cathode material will be discussed. 2. Experimental Procedures The synthesis and physical characterization of the carbonsupported RuSey/catalysts (y ) 0-1), which were prepared at the Forschungszentrum Karlsruhe (Prof. Bo¨nnemann’s group), is described in detail in ref 29. In short, colloidal Ru precursor particles were synthesized following the borate route36,37 supported on Vulcan XC 72 (Cabot Corp.) carbon and activated by an oxidation-reduction treatment at 250 °C. The Ru/C catalyst was subsequently modified by controlled amounts of selenium via reductive annealing of the H2SeO3 impregnated catalysts at 200 °C in a H2 gas flow. This results in catalysts with a Ru metal loading of 20 wt %, a mean diameter of the initial Ru particles of 3.9 ( 1 nm, and, after surface modification by increasing amounts of Se, particles with a steadily decreasing Ru core and a growing binary RuSey shell.29 A commercial Pt/C

Colmenares et al. catalyst (E-Tek, Inc.) supported on Vulcan XC-72 carbon was used as reference (Pt 20 wt %, Pt particle mean diameter 3.7 ( 1 nm after conditioning38). Thin-film electrodes for the electrochemical measurements in the different setups, DEMS, RRDE, and WJDE, were prepared according to the experimental protocol described in refs 38 and 39 by sequential deposition and drying of the catalyst suspension and aqueous Nafion solution on mirror-polished glassy carbon disks (6 mm diameter for RRDE and WJDE and 9 mm for DEMS). The resulting Nafion film (thickness ca. 0.1 µm) is sufficiently stable to attach the catalyst particles permanently to the glassy carbon but thin enough to avoid additional diffusion limitations.38,40 The DEMS setup, which was described in detail in previous publications,41,42 consisted of a differentially pumped doublechamber design, a quadrupole Balzers 112 mass spectrometer (MS), and a Pine Instruments potentiostat with computerized data acquisition. A constant electrolyte flow rate of about 10 µL s-1, driven by the hydrostatic pressure in the electrolyte supply bottle, ensured a fast transport of the species formed at the electrode into the mass spectrometric compartment (time delay 1-2 s), where the gaseous products were evaporated into the MS through a porous membrane (Scimat, 60 µm thick, 50% porosity, 0.2 µm pore diameter). Two Pt wire electrodes at the inlet and outlet of the flow cell and a saturated calomel electrode (SCE) connected to the outlet through a Teflon capillary served as counter and reference electrodes, respectively (all potentials are referenced to that of the reversible hydrogen electrode, RHE). The RRDE measurements were conducted in a thermostated three-compartment electrochemical cell in 0.5 M sulfuric acid (Merck, Suprapure), using a Pine Instruments RRDE setup at a rotation speed of 1600 rpm. A Pt wire counter electrode and a SCE reference electrode, connected through a salt bridge, were used. All RRDE measurements were carried out at a constant temperature of 60 ( 1 °C. Hydrogen peroxide production in the O2-saturated 0.5 M H2SO4 electrolyte was monitored in a RRDE configuration, using a polycrystalline Pt ring biased at 1.2 VRHE (collection efficiency of the ring electrode N ) Iring/ Idisk ) 0.20 ( 0.0140). The hydrogen peroxide fractional yields in the ORR were calculated from the RRDE experiments as XH2O2 ) (2Iring/N)/(Idisk + Iring/N).40 The mass transport normalized kinetic ORR currents at the disk electrode were calculated as Ik ) IlimI/(Ilim - I), with Ilim taken as the ORR current at 0.4 VRHE, and are presented as geometric area and active surface area specific kinetic ORR current densities in Tafel plots. The wall-jet (impinging-jet) flow cell was machined from Kel-F and consisted of two parts, namely, (i) the cell body (working volume ca. 2 mL) with the inlet capillary (1 mm inner diameter) positioned in the center of the cell and (ii) the electrode holder for a 6 mm diameter glassy carbon disk, which was installed using a standard Teflon U-cup from Pine Instruments. Both parts were aligned and tightly interconnected by pressing them against a Teflon o-ring, using three screws. The inlet capillary nozzle was positioned at ca. 3 mm distance from the working electrode and served as a Luggin capillary for connecting the saturated calomel reference electrode (SCE), which was maintained at room temperature in a separate vessel and connected to the supply bottle via a wetted valve. The Pt wire counter electrode surrounded the external wall of the inlet capillary. For calibration of the reference electrode, the hydrogen oxidation/evolution reaction was measured on a Pt/C catalyst electrode at the different temperatures determining the zerocurrent potential. The electrolyte flow in the wall-jet setup was

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driven by a piston pump (Ismatek), which was connected via a Teflon tube to the outlet of the wall-jet cell in the upper part of the cell. The electrolyte leaving the pump was continuously recycled through a Teflon tube back to the thermostated electrolyte supply reservoir, which was extensively purged with the respective gases (>1000 standard cubic centimeters per minute), passing the gas through a fritted glass bubbler. The measurements were performed at an electrolyte volumetric flow rate of 1 mL s-1, which corresponds to a linear velocity of ca. 2 m s-1 for the electrolyte impinging via the inlet capillary (0.8 mm diameter). Gases were supplied from MTI Gase (Ar N6.0 and O2 N5.7). Suprapure grade sulfuric acid (Merck), analytical grade methanol (Merck), and Millipore Milli-Q water were used for preparing the solutions. 3. Results and Discussion 3.1. Activity and Selectivity for O2 Reduction. The ORR activity and selectivity of the RuSey/C catalysts at elevated temperatures were determined from RRDE measurements at 60 °C and compared with results obtained on a commercial Pt/C catalyst. Because of the temperature limitations of the RRDE measurements (insufficient thermal stability of the commercial RRDE head and significant losses of O2 from the solution at temperatures above 60 °C40), we performed additional measurements in a wall-jet setup, which allows model studies closer to realistic fuel cell cathode operation conditions (higher flux of O2 over the electrode surface, temperatures up to 80 °C), as an alternative to a pressurized thin-layer flow cell.43,44 3.1.1. Rotating Ring-Disk Electrode Measurements. Figure 1 shows the Faradaic currents (disk currents, Figure 1a) and the H2O2 oxidation currents (ring currents, Figure 1b) of the ORR at 60 °C obtained during the positive-going scan on the Pt/C and RuSey/C catalyst thin-film disk electrodes (a) and on the polycrystalline Pt ring electrode (b) (bias 1.2 V). The mass transport limited ORR current in the O2-saturated solution on the Pt/C catalyst was reached in the potential range from 0.06 to 0.6 V, followed by a mixed mass transport/kinetically limited ORR region at potentials of a positive 0.7 V up to ca. 0.95 V. These observations fully agree with previously reported data.40,45 For the Ru/C catalyst (Figure 1a, solid black line), these two regions extend over the potential ranges from 0.06 to 0.4 and 0.4 to 0.85 V, respectively, in agreement with results presented in refs 21 and 35. Hence, on the Ru/C catalyst, the kinetically controlled region of the ORR is shifted negatively by ca. 0.25 V, as compared to the Pt/C catalyst. This shift of the ORR is attributed to the onset of OHad formation and Ru surface oxidation at lower potentials than on the Pt electrode (see refs 46-52 and references cited therein). Furthermore, even in the mass transport limited potential region, the transport limited ORR current determined on Pt/C is not reached, indicating that, also in this region, the reaction is not purely mass transport limited but is also kinetically hindered. (We will nevertheless denote it as mass transport limited potential range henceforth.) The nonzero slope of the ORR current in this region is attributed to a continuous change in surface activity, which most likely arises from a continuous change in surface composition with increasing OHad formation/surface oxidation, diminishing the ORR activity. Modification of the Ru surface by Se further decreases the ORR current in the mass transport limited regime, due to site-blocking effects, and shifts the kinetically controlled ORR region back to more positive values up to an 80 mV positive shift is achieved for the RuSe0.3/C and RuSe0.6/C catalysts as compared to the Ru/C catalyst (Figure 1a). This

Figure 1. Oxygen reduction current (a), H2O2 formation current (b), and H2O2 yields. (Inset of panel b) Pt/C and Se-modified RuSey/C (y ) 0-1) catalysts measured by R(R)DE: positive-going potential scans in O2-saturated 0.5 M H2SO4 electrolyte. Catalyst active phase (Pt, RuSey) loading 28 µg cm-2 (disk), polycrystalline Pt ring (E ) 1.2 V, collection efficiency 20%), rotation rate 1600 rpm, potential scan rate 10 mV s-1, and 60 °C temperature. For catalyst assignments, see figure.

positive shift indicates a reaction promoting effect of Se in the kinetically controlled region, which can be correlated to Se induced Ru surface stabilization toward adsorbed oxide/ hydroxide formation. The latter adlayer is expected to inhibit O2 reduction. However, the onset of O2 reduction and the kinetically controlled ORR region still remains ca. 0.2 V negatively shifted as compared to the Pt/C catalyst. Hence, Se modification of the Ru/C catalyst decreases the inhibiting effect of the oxy-species adsorption on Ru35 but does not at all lead to the activity reached by the Pt/C catalyst. For the most Serich RuSe/C catalyst, the kinetically limited ORR region is shifted back again to more cathodic values, due to enhanced site-blocking, but is still better than that of the unmodified Ru/C catalyst. We also evaluated the selectivity of the ORR, determined by the amount of H2O2 formation under present reaction conditions by RRDE measurements. Hydrogen peroxide, which may be produced in the ORR on the disk electrode in addition to the majority product H2O (Figure 1a), is detected via its oxidation on the Pt ring electrode (Figure 1b). The H2O2 production during the ORR on the Pt/C catalyst in the positivegoing potential scan reproduces previously reported data.40,45 (To avoid contributions from H2 evolution at lower potentials on the disk electrode and the interference of hydrogen oxidation with H2O2 oxidation at the ring electrode, the low potential limit in the present study was set to slightly less negative values, by ca. 20 mV, than in the previous studies.) H2O2 production on the Pt/C catalyst occurs at potentials of negative 0.6 V (Figure 1b, gray line), which was attributed to the reaction of O2 on a surface blocked by (bi)sulfate and hydrogen adsorption.9,53 At the low potential limit, the H2O2 yield reaches about 1% on the

1276 J. Phys. Chem. C, Vol. 111, No. 3, 2007 Pt/C catalyst (inset of Figure 1b, gray line). H2O2 production on the Pt/C catalyst is very low at potentials typical for cathode operation, which is important for operation as a DMFC cathode. In contrast to the Pt/C catalyst, the H2O2 production on the unmodified Ru/C catalyst (Figure 1b, solid black line) is very low at the low potential limit (i.e., O2 is reduced mainly to water on the largely metallic but still partly OHad covered Ru surface,47-49 which may be related to the absence of a strongly bound H adlayer under these conditions as it is present on Pt).9,49,53 In the positive-going scan, the H2O2 production increases, in agreement with previously reported room temperature data,21,35,52 and passes two broad, overlapping peaks in the potential region between 0.1 and 0.8 V. These peaks can be related to the adsorption of the bisulfate and hydroxyl species on the Ru surface (first peak) and the subsequent conversion into surface oxides (second peak), which hinders the dissociative adsorption of O2 molecules and thus leads to incomplete O2 reduction to hydrogen peroxide. Correspondingly, the H2O2 yield in the ORR over the Ru/C catalyst is nearly constant in the potential region from 0.25 to 0.5 V, then starts to significantly increase at higher potentials (inset of Figure 1b, solid black line). Note that this increase in the relative H2O2 yield mainly results from the stronger decay of the disk current in the kinetically controlled ORR region as compared to the slower decay of the ring current (Figure 1a). As a result, at potentials relevant for DMFC cathode operation, the Ru/C catalyst produces significant amounts of H2O2, which makes its technical application questionable. In contrast, H2O2 formation on Pt/C under these conditions is negligible (gray line in the inset in Figure 1b). Modification of the Ru/C catalyst by increasing amounts of Se results in a gradual increase of the (absolute) rate of H2O2 production and of the (relative) H2O2 yield in the low potential region, at potentials of negative 0.5 V, and at the low potential limit as compared to the Ru/C catalyst (Figure 1b). Hence, the RuSey surface resulting after modification29 must be more active for H2O2 formation than Ru itself, and also, the OHad/oxyspecies covered the Ru surface in the potential range of 0.20.6 V. Interestingly, in the kinetically controlled potential region, H2O2 formation is lower on the modified RuSey/C catalysts, at least for the RuSe0.15/C and RuSe0.6/C catalysts, than on the unmodified Ru/C catalyst. In this potential range, at potentials relevant for cathode operation, Se modification improves both the ORR activity and the selectivity for H2O formation (fourelectron pathway) as compared to the unmodified Ru/C catalyst (Figure 1b). On a microscopic scale, we explain the reduced tendency for H2O2 formation after Se modification by a combination of two effects. First of all, on unmodified Ru, the selectivity becomes increasingly worse when going to higher potentials, which we relate to the increasing oxide coverage on the surface.35,47-50 (It should be noted that covering active metal surfaces by a dense adsorbate layer tends to increase the H2O2 yield, as had been shown, e.g., for COad, Se or H-upd covered Pt surfaces.9,54-59) Second, the Se modification increases the total ORR rate, which is reflected by the shift of the kinetically controlled part of the I-U curve to higher potentials. These two effects reduce the H2O2 yields to 1-2% in this potential region, which is significantly better than on Ru/C but still much higher than on the Pt/C catalyst (inset of Figure 1b). At even higher potentials, at the onset of the ORR, the H2O2 yield increases steeply, mainly caused by the faster decay of the H2O formation rate. Because of the low absolute currents, this potential region appears to be of little interest for fuel cell operation, and this is, in fact, the situation that is met during low load operation. These effects were investigated recently in

Colmenares et al. more detail, using a highly sensitive flow cell setup for room temperature measurements at constant electrode potentials.35 These measurements indicate compatible H2O2 yields over the same catalyst series. From the data in Figure 1, we extracted the kinetic current densities normalized to the geometric area of the thin-film electrode (0.28 cm2) and to the active surface area, respectively, utilizing the active surface areas determined by COad stripping in ref 35. These current densities, obtained at 60 °C on the Semodified RuSey/C catalysts, are plotted as a function of the electrode potential and Se content in Figure 2, where Figure 2a,b represents the geometric and active surface area normalized kinetic current densities, respectively. Figure 2a shows that at high over-potentials, the geometric surface area normalized current density exhibits a maximum for the RuSe0.3/C catalysts, while at lower overpotentials (0.75 V reaction potential), this was found at Ru/Se ratios of around 1:0.3 to 1:0.6, in agreement with previously reported room temperature data (see ref 35 and references cited therein). The behavior is completely different when the kinetic current is normalized to the Ru active surface area (inherent ORR activity, Figure 2b). The latter shows a continuous increase in kinetic current density with increasing Se content. This observation fully agrees with our previous results determined at 25 °C in a thin-layer flow cell on the same catalysts.35 Most simply, and in agreement with previous results (see ref 35 and references cited therein), this can be explained by a stabilization of the Ru surface toward oxidation by Se modification, which reduces the formation of reaction inhibiting (hydr)oxide species on the Ru surface and thus increases the (inherent) ORR activity of the RuSey/C catalysts with higher Se content. Alternatively, an increase in the ORR activity with Se content could be explained by the formation of a new active phase.23,29 These effects, however, are largely masked in the geometric area normalized current density (Figure 2a) by the decrease in accessible active surface area with increasing Se loading. The H2O2 yields (ORR selectivity) on the RuSey/C catalysts and their dependence on the Se content and on the electrode potential are summarized in Figure 2c. For an unmodified Ru/C catalyst, the H2O2 formation yields are highest at low overpotentials (ca. 6% at 0.75 V, Figure 2c), which is associated with Ru surface blocking by (hydr)oxide species. When the Ru/C catalyst is modified with Se, the formation of H2O2 decreases significantly at the corresponding potentials, until reaching a minimum (around 1%) at Ru/Se ratios of about 1:0.3 to 1:0.6. This can be tentatively interpreted as the optimum Ru/ Se composition for O2 reduction to water rather than H2O2 formation. At 0.75 V and high Se contents (RuSe/C), the H2O2 yield increases again. We explain these trends in H2O2 formation by two competing effects: (i) an increased ORR activity on Se-modified Ru surface sites (Ru sites adjacent to a Se deposit atom) and (ii) the necessity of at least two neighboring free sites on the Ru substrates for dissociative O2 adsorption. At high Se coverages/contents, the probability for these dimers decreases sharply. It is important to note, however, that even at the highest Se content, where based on COad stripping35 10% of the Ru sites are still accessible, H2O2 formation at the typical cathode potentials is only about 1-2%. Hence, most of the accessible Ru sites are still present as dimers or larger ensembles. This conclusion is supported by a recent report from Mo et al., who observed a 100% H2O2 yield on a fully Se covered Pt surface under similar conditions.56 3.1.2. Wall-Jet Disk Electrode Measurements. We start with discussing the influence of the temperature on the O2

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Figure 3. Potentiodynamic (10 mV s-1) oxygen reduction current obtained on (a) Pt/C, (b) unmodified Ru/C, and (c) Se-doped RuSe0.15/C thin-film electrodes in WJDE measurements as a function of temperature (30-80 °C). Right plots show a magnified presentation of the kinetically controlled potential region. Catalyst active phase (Pt, RuSey) loading 28 µg cm-2, electrolyte 0.5 M H2SO4 saturated with O2, and flow rate 1 mL s-1.

Figure 2. Geometric (a) and active surface area normalized (b) kinetic current densities and H2O2 yields (c) in the ORR as a function of applied potential and Se content (XSe) on Se-modified RuSey/C (y ) 0-1) catalysts. Data extracted from Figure 1.

concentration and the diffusion properties in the WJDE setup, using measurements on the Pt/C electrode as an example. First of all, the mass transport limited currents of ca. 2.3 mA (geometric current densities of about 7 mA cm-2) attained under present reaction conditions (flow rate of 1 mL s-1) indicate a much faster mass transport to the wall-jet electrode as compared to the RRDE measurements (Figure 1a); it would be equivalent to RRDE measurements at rotation rates of 3600 rpm.40 Second,

the mass transport limited ORR current does not decrease significantly in the WJDE configuration up to 70 °C (Figure 3a), in contrast to the current decay at temperatures above 60 °C in the RRDE setup,40 although both operate as open systems (at ambient pressures). The observation can be rationalized by a more efficient O2 purging in the wall-jet via a relatively large, fritted bubbler, operating at gas flow rates of over 1000 standard cubic centimeters per minute in the former case, as compared to the lower bubbling rates in the RRDE configuration (typically, ca. 500 scm3 min-1, saturating the electrolyte under a glass bell), making the WJDE set-up compatible to a pressurized thin-layer flow cell.43 Furthermore, the time for delivery of the O2-saturated electrolyte to the catalyst surface in the WDJE setup is short (∼0.05 s) under current experimental conditions (see Section 2.). (For a quantitative evaluation of the influence of the temperature on the diffusion properties, see refs 60-62.) Figure 3 shows the WJDE data of the ORR activity in the positive-going scan on the Pt/C (Figure 4a, left), unmodified Ru/C (Figure 3b, left), and RuSe0.15/C (Figure 3c, left) catalyst electrodes at different temperatures between 30 and 80 °C. A magnified presentation of the respective current/voltage plots in the potential range of the mixed mass transport/kinetically controlled potential regime is given in the corresponding righthand column. Similar to the RRDE measurements, the mass transport limited ORR current on the Pt/C catalyst is reached at potentials of negative 0.7 V, independent of the temperature,

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Figure 4. Arrhenius plots of the active phase (Pt, RuSey) mass-specific ORR current densities at a constant overpotential of 0.525 V (i.e., the reaction potential changes from 0.7 V (30 °C) via 0.692 V (40 °C), 0.684 V (50 °C), 0.675 (60 °C), and 0.667 V (70 °C) to 0.659 V (80 °C) obtained on Pt/C and Se-doped RuSey/C (y ) 0-1) thin-film electrodes in WJDE measurements. Catalyst active phase (Pt, RuSey) loading 28 µg cm-2. Inset: calculated activation energies in kJ mol-1. For catalyst assignments, see figure.

followed by the mixed mass transport/kinetically controlled region (0.7 V up to ca. 0.9 V). Figure 3a (right) shows that on the Pt/C catalyst, the increase in temperature, from 30 to 80 °C, results in a continuous increase of the ORR activity, equivalent to a decrease of the ORR overpotential.63 For Ru/C (Figure 3b), the trend is opposite, with higher temperatures leading to a further increase in the ORR over-potential. This is attributed to an enhanced, thermally activated OHad formation and Ru surface oxidation. Hence, the additional ORR over-potential of the Ru/C catalyst relative to Pt/C increases significantly with higher temperatures. This is partly avoided (at least up to 70 °C) if the Ru surface is modified by Se, as shown for the RuSe0.15/C catalyst (Figure 3c). On this catalyst, the ORR activity in the kinetically controlled ORR region is shifted positively with increasing temperature. The shift is even more pronounced than on Pt/C, most likely due to increased stability of Ru toward surface oxidation induced by Se modification.35 Nevertheless, also at 80 °C, the onset of the ORR on the RuSey/C catalysts occurs still at more negative potentials (about 0.2 V) than on the Pt/C catalyst; the difference in the onset potential of the two catalysts hardly changes with temperature. Arrhenius plots of the mass-specific ORR current densities at a constant over-potential of 0.525 V (see exact reaction potentials in Figure 4 caption) obtained on the Pt/C and RuSey/C catalysts are shown in Figure 4. This value of the over-potential was chosen to obtain measurable rates in the kinetically controlled regime on all catalysts. The over-potential at each temperature was determined following the procedure described in ref 43, using the reference potentials determined experimentally for each temperature (see Experimental Procedures). Both, on the

Colmenares et al. Pt/C catalyst and on the Se-modified Ru/C catalysts, the ORR activity increased continuously with temperature. For these catalysts, the increase in ORR activity is reasonably welldescribed by a logarithmic increase with 1/T. Only, at the highest temperatures, it deviates slightly due to a decreasing O2 concentration in the electrolyte. For the Ru/C catalyst, the temperature effects are more complicated. Here, the ORR activity increases exponentially with 1/T up to 50 °C and, then, for higher temperatures, decreases again. This behavior is most likely due to a thermally activated (hydr)oxide formation on the Ru surface under these conditions. The activation enthalpies calculated from a least-squares regression over the temperature range of 30-60 °C (Ru/C 30-50 °C) at a constant overpotential (0.525 V) are listed in Figure 4. For the Pt/C catalyst, the ORR activation enthalpy of ∼18 kJ mol-1 determined under our experimental conditions is comparable to the values reported by Grgur et al.,63 who reported values of ca. 20 kJ mol-1 on all three low index Pt singlecrystal surfaces. Likewise, Anderson et al.64 reported for Pt/C and Pt-alloy based catalysts activation energies in the range of 20-30 kJ mol-1 over the potential range of 0.7-0.9 V (overpotential above 0.53-0.33 V). The latter authors also determined the activation energies as a function of the electrode potential, yielding values in the range of 13.89-17.08 kJ mol-1 for the potential range of 0.76-0.81 V. Paulus et al. determined an activation enthalpy of around 26 kJ mol-1 on a Pt/C thin-film electrode in 0.5 M H2SO4 at an over-potential of 0.35 V in RDE measurements. For the unmodified Ru/C catalyst, the measured ORR activation energies have not been reported so far. Comparison of the value of about 23 kJ mol-1 determined in the temperature range between 30 and 50 °C with recent density functional calculations,51 where O2 dissociation on Ru(0001) was found to exhibit a pronounced negative activation energy, clearly indicates that the observed barrier is not related to O2 dissociation on the bare Ru substrate but must be due to other effects. Most likely, it is related to the presence of an oxide/ hydroxide adlayer on the Ru substrate. For the Se-modified RuSey/C catalysts, where Se prevents the decrease of the ORR activity due to OHad/surface oxide formation, the activation energies are in the same range as for Pt/C and Ru/C (see Figure 4). Although it would be tempting to explain the rather similar values of the ORR activation barrier on these different catalysts by an identical rate limiting step in all cases, the situation is more complex because of the simultaneous change in OH adsorption energy/H2O splitting barrier (stabilization against OHad formation), which was discussed before. The ORR activity of RuSey/C catalysts under fuel cell relevant conditions in the WJDE setup (T ) 80 °C and high O2 flux at an electrolyte flow rate 1 mL s-1) in the positive-going scan is illustrated in Figure 5 for different Se amounts. Similar to the observations in the 60 °C RRDE measurements (see Section 3.1.1. and Figure 1), the ORR currents on the unmodified Ru/C catalyst and on the modified RuSey/C catalysts in the mass transport limited region are affected also by kinetic effects, which can be attributed to changes in the surface activity due to OH adsorption.35 Other than in the RRDE experiments, we find no distinct difference between the mass transport limited currents on the RuSey/C and Pt/C catalysts. Most likely, this is due to slight contamination of the Pt surface during the cathodic scan, which is illustrated also by the weaker typical H-upd features. RuSey/C catalysts, and in particular Ru/C, are expected to be much less sensitive to contaminant adsorption due to the OH adlayer present over a wide potential regime. Furthermore, at more cathodic potentials, these effects are much less

Se-Modified Ru/C Cathode Catalysts

Figure 5. Potentiodynamic (10 mV s-1) oxygen reduction current obtained on Pt/C and Se-modified RuSey/C (y ) 0-1) thin-film electrodes in WJDE measurments. Right plot is a magnification of the kinetically controlled current region. Catalyst active phase (Pt, RuSey) loading 28 µg cm-2, electrolyte 0.5 M H2SO4 saturated with O2, flow rate 1 mL s-1, and 80 °C temperature. For catalyst assignments, see figure.

Figure 6. Geometric surface area (a) and active surface area (panel b, see ref 35, Figure 2) normalized Tafel plots of the kinetically controlled ORR currents extracted from Figure 5. For catalyst assignments, see figure.

pronounced, most likely due to oxidation of the (organic) contaminants;65 therefore, the activation energies are little affected by such effects. This is supported also by the close similarity of the Eact values determined for Pt/C in the present WJDE experiments and literature data. For a better comparison, the kinetic ORR currents at 80 °C from Figure 5 are replotted in Tafel plots, normalized to the geometric surface area (surface area 0.28 cm2, Figure 6a) and to the Ru active surface area (Figure 6b) determined by COad stripping (see ref 35). Using the technically relevant geometric area normalization (Figure 6a), which, except for the highest Se content, is close to the Ru mass specific activity, the ORR activity of the RuSey/C catalysts is significantly lower than that of the Pt/C catalysts at similar potentials. For the Ru active

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1279 surface area normalized kinetic current densities (Figure 6b), the inherent activity increases significantly with increasing amounts of Se, and the activity on the RuSe/C catalyst comes close to that of the Pt/C catalyst. For low and medium Se contents, the increase in absolute ORR current is magnified by the effect of the decreasing active surface area; for the Se-rich RuSe/C catalyst, the decrease in active surface area overcompensates the decrease in absolute ORR current, resulting in a further increase of the active surface area normalized ORR current. The increase in the ORR activity with increasing Se content (in the active surface area normalized Tafel plots) could be explained by two different effects, namely, by the formation of a new active phase and/or by the hindering of (hydr)oxide adsorption on Ru sites by modifying the Se species. In both cases, the slope of the Tafel plots changes continuously, as has been noted before for supported Pt/C9,40,45,66,67 and Ru/C21 catalysts, and does not show distinct regions with slopes of 60 or 120 mV dec-1. A slope of 120 mV dec-1, which would be expected for a two-electron transfer as a rate limiting step, is reached at potentials around 0.7 V. Figure 7 summarizes the geometric (Figure 7a,c,e) and active surface area (Figure 7b,d,f) normalized kinetic ORR current densities at different temperatures (30 °C: Figure 7a,b; 60 °C: Figure 7c,d; and 80 °C: Figure 7e,f) as a function of Se content and electrode potential. Figure 7a,c,e (geometric normalized) shows that the ORR activity on the unmodified Ru/C increases when the temperature is raised from 30 to 60 °C but decreases significantly when the temperature approaches 80 °C. This decrease in the ORR activity of the Ru/C catalysts was already shown in Figure 4 for the determination of the activation energy, where a maximal activity for ORR on Ru/C was achieved around 50 °C (at an over-potential of 0.525 V). At high temperatures, blocking of the Ru surface by (hydr)oxide adsorption is significantly diminished on the Se-modified RuSey/C catalysts. At a low Se content (Ru/Se ratio; 1:0.15 to 1:0.3), the ORR activity increases with increasing temperature up to 80 °C (Figure 7c-e). On the high Se content RuSey/C catalysts (Ru/Se ratio, 1:0.6 to 1:1), the geometric area normalized ORR activity shows only slight changes with the temperature. It reaches a maximum at around 60-70 °C (see also Figure 4). On these catalysts, the high content of modifying Se plays a double role: stabilization of the Ru surface versus oxidation and simultaneous blocking of the Ru surface sites.35 Finally, when looking at the effect of varying the over-potential, the significant differences in the geometric area normalized ORR activity observed at high over-potentials are much less pronounced at low over-potentials in the present temperature range. At 80 °C, the maximum geometric area normalized current density occurs at a Ru/Se ratio of 1:0.15 for all potentials investigated. Going to the inherent ORR activity (active surface normalized current density, see Figure 7b,d,f) leads to significant changes in the dependence of the ORR activity with Se content. Similar to previous observations in thin-layer flow cell measurements at room temperature35 and in R(R)DE measurements at up to 60 °C (see Section 3.2.1.), the activity increases continuously with an increasing Se content, independent of the temperature. As has been discussed before (Section 3.1.1. and ref 35), we attribute the increase in inherent activity to the stabilization of Ru sites against oxide/hydroxide formation, providing more OH free Ru sites for the ORR and/or to the increasing formation of a novel, active RuSe phase.23,29 3.2. Influence of Methanol Oxidation on the O2 Reduction Reaction. In the following sections, the methanol tolerance of

1280 J. Phys. Chem. C, Vol. 111, No. 3, 2007

Colmenares et al.

Figure 7. Geometric (a, c, and e) and active surface area normalized (b, d, and e, see ref 35, Figure 2) kinetic current densities in the ORR as a function of applied potential and Se content (XSe) obtained on Se-modified RuSey/C (y ) 0-1) thin-film electrodes in WJDE measurements at different temperatures: 30 °C (a and b); 60 °C (c and d); and 80 °C (e and f).

the RuSey/C catalysts under realistic conditions, simulating the cathode situation during methanol crossover, will be discussed. First, the activity and selectivity of the RuSey/C and Pt/C catalysts for the MOR were investigated by DEMS at room temperature (Section 3.2.1.); subsequently, the mutual interaction of the simultaneously occurring MOR and ORR at elevated temperatures (60 °C) and under enforced mass transport conditions was evaluated in RRDE measurements (Section 3.2.2.). 3.2.1. Methanol Oxidation on RuSey/C and Pt/C Catalysts. DEMS measurements of methanol (0.01 M) oxidation on the RuSey/C catalyst electrodes and, for comparison, over Pt/C catalyst electrodes, are shown in Figure 8 panel a, Faradaic current and panel b, mass spectrometric m/z ) 44 current. Methanol oxidation on the Pt/C catalyst (solid gray lines) reproduces previously reported results.5,41,68 Methylformate formation is not detectable at the low methanol concentration used here (10 mM),5 due to both the lower amounts of formic acid produced and the low methanol bulk concentration. Nevertheless, by converting the CO2 formation charge to the corresponding partial Faradaic current for six-electron oxidation of methanol to CO2, and using the calibration constant determined for COad stripping on the Pt/C catalyst (see ref 68), we obtain a current efficiency for CO2 formation of about 60% over a complete potential cycle, which clearly indicates a significant contribution of incomplete methanol oxidation to formic acid and/or formaldehyde under these conditions.68 In contrast to the MOR on the Pt/C catalyst, we found almost no measurable Faradaic current for methanol oxidation on the RuSey/C catalysts (Figure 8a); the CVs for these catalysts in the 0.01 M methanol containing supporting electrolyte are nearly identical to the base CVs.35 These results agree well with previously reported data, indicating a poor catalytic activity of Ru/C and RuSey/C catalysts for methanol oxidation.14,69 However, in examining the CO2 mass spectrometric current more carefully (by increasing the MS signal sensitivity by a factor of 10), it is possible to monitor the CO2 formation rate for the MOR on RuSey/C catalysts (Figure 8b). This shows a very small but resolvable CO2 formation rate on the RuSey/C catalysts,

Figure 8. Faradaic (a) and mass spectrometric (m/z ) 44) (b) currents during methanol oxidation on Pt/C and Se-modified RuSey/C (y ) 0-1) catalyst electrodes (CV and MSCV, respectively). Catalyst active phase (Pt, RuSey) loading 28 µg cm-2, electrolyte (0.5 M H2SO4 + 0.01 M CH3OH) flow rate 10 µL s-1, potential scan rate 10 mV s-1, and room temperature. For catalyst assignments, see figure.

which is slightly higher than that in the base electrolyte (not shown, but identical to the CO2 signal on the RuSe/C catalyst,

Se-Modified Ru/C Cathode Catalysts

Figure 9. R(R)DE measurements of methanol oxidation (0.01 M for Pt/C and 0.1 M for RuSey/C catalysts) in O2 free deaerated electrolyte (0.5 M H2SO4) and O2-saturated electrolyte on Pt/C and Se-modified RuSey/C (y ) 0-1) catalyst electrodes. (a) Disk current for the simultaneously occurring MOR and ORR on the respective catalysts and (b) ring current for H2O2 formation during ORR + MOR and H2O2 yields (inset). Catalyst active phase (Pt, RuSey) loading 28 µg cm-2 (disk), polycrystalline Pt ring (E ) 1.2 V, collection efficiency 20%), rotation rate 1600 rpm, potential scan rate 10 mV s-1, and temperature 60 °C. For catalyst assignments, see figure.

dotted line in Figure 8b). While CO2 formation on the RuSe/C catalyst in the supporting electrolyte is fully attributed to oxidation of the carbon support, the other RuSey catalysts with lower Se contents show small CO2 formation rates in methanol containing solutions. The CO2 yields in the MOR, however, are less than 1% of those obtained on the Pt/C catalyst under similar conditions (Figure 8b, gray line). Incomplete methanol oxidation to formic acid and/or formaldehyde cannot be detected because of the low MOR rates (i.e., it can neither be completely ruled out nor confirmed for these catalysts). The DEMS data demonstrate that the Ru/C and RuSey/C catalysts are almost completely methanol tolerant under these conditions, confirming the results of previous Faradaic current measurements.28 Therefore, they seem to be suitable materials for the DMFC cathode as suggested earlier.14 MS model studies shown previously were performed at a relatively low electrolyte flow and at room temperature. Therefore, more vigorous electrolyte transport and, in particular, elevated temperatures may increase the MOR rate via thermal activation of the MOR on the ruthenium-rich catalysts.70,71 3.2.2. Simultaneous Methanol Oxidation/Oxygen Reduction on RuSey/C and Pt/C Catalysts. To further clarify the methanol tolerance of RuSey/C catalysts and to elucidate the impact of methanol on the ORR selectivity (H2O and H2O2 formation), we performed RRDE experiments at elevated temperatures (60 °C), continuous enforced mass transport (1600 rpm), and higher methanol concentration (0.1 M) (Figure 9a). (Measurements on the Pt/C catalyst performed for comparison

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1281 were still carried out in the 0.01 M methanol containing solution due to its high MOR activity.) The methanol oxidation current obtained on the Pt/C catalyst in the 0.01 M methanol containing solution in the RDE configuration at 60 °C under these conditions (Figure 9a, solid gray line) was ca. 7 times higher than that found in the DEMS experiments (Figure 8a, gray line) at room temperature and moderate electrolyte flow rate (ca. 10 µL s-1), despite the similar catalyst loading and active electrode area in these experiments. This confirms the role of thermal activation of the MOR70,71 and of a vigorous mass transport of methanol to the electrode, combined with the efficient removal of incomplete methanol oxidation products (formic acid and formaldehyde) to the electrolyte bulk.72 In the O2-saturated 0.01 M methanol solution, the Faradaic current on the Pt/C catalyst shifts to cathodic currents (Figure 9a, gray dashed line) as compared to reactions in the deaerated methanol solution at potentials of negative 1.0 V, caused by the superposition of MOR and ORR currents.5,9 The overpotential of the ORR in the presence of methanol (the ORR on Pt/C in the methanol free oxygen-saturated solution was discussed in Section 3.1.1.) increases by ca. 0.3 V at currents above 0.5 mA (i < 2 mA cm-2geom) as compared to the ORR over-potential in the absence of methanol. As expected, the difference between the Faradaic currents in the two electrolytes is smaller in the range of the kinetically controlled currents than in the low potential region. This is due to the steep decrease of the ORR current in the kinetically controlled potential range, which results, for example, in a smaller difference between the MOR peak currents. It is important to realize that in addition to the superposition of the two reactions and reaction currents, the presence of both reactants, O2 and methanol, can modify the reaction characteristics and, in particular, the selectivity in both processes (MOR and ORR), which was demonstrated for Pt/C and discussed in more detail elsewhere.5,9 As expected, the MOR rates on the Ru/C and RuSey/C catalysts during the positive-going procedure in the deaerated 0.1 M methanol solution are negligible in O2 free electrolyte (Figure 2a, black lines, upper set) as compared to the Pt/C catalyst (Figure 2a, solid gray line, upper set). This holds true despite the 10-fold higher methanol concentration (0.1 M) for the RuSey/C set as compared to the 0.01 M solution for the Pt/C catalyst and even at a 10-fold magnification of the Faradaic current traces. The curves fully agree with the base CVs of the respective catalysts.35 Therefore, despite the elevated temperature (60 °C) and enhanced transport in the RDE configuration (1600 rpm), the Ru/C and RuSey/C catalysts remain essentially inactive toward the MOR as compared to the Pt/C catalyst, which suffers from significant (ca. 0.3 V) performance loss due to a formation of a mixed potential EMix (EMix corresponds to a zero net current, i.e., equal currents of the anodic and cathodic partial reactions). Most simply, this can be explained by the formation of strongly adsorbed oxygenated species on the Ru surface, which inhibit methanol dissociation and oxidation. The absence of any significant MOR activity of the RuSey/C catalysts is maintained also in O2-saturated 0.1 M methanol electrolyte (Figure 2a, lower set). Comparison of the total Faradaic current obtained under these conditions with the ORR current in the methanol free O2-saturated electrolyte on the respective catalysts (Figure 1a) shows that the Faradaic current is fully dominated by the ORR. It is important to note that the inactivity for methanol oxidation is a characteristic property already of the Ru itself, as indicated by the fully suppressed MOR on the unmodified Ru/C catalyst (see also Figure 8a).

1282 J. Phys. Chem. C, Vol. 111, No. 3, 2007 The data presented in Figure 9a demonstrate that the RuSey/C catalysts are inactive for the MOR also at elevated temperature (60 °C) and under continuous mass transport (1600 rpm), regardless of the absence or presence of O2 in the electrolyte and the simultaneously occurring ORR. In addition, the ORR activity of the RuSey/C catalysts is hardly affected by the presence of methanol. A second point to be clarified is the O2 selectivity of the ORR in the presence of methanol (i.e., the question of how much, if at all, methanol affects the formation of H2O2). To clarify this issue, we monitored the hydrogen peroxide oxidation current on the polycrystalline Pt ring detector during the ORR on Pt/C and RuSey/C catalysts in the methanol containing electrolyte, similar to the measurements in the methanol free electrolyte (Figure 1b). These measurements are meaningful because of the very low MOR activity of the Pt ring at 1.2 V (see Figure 8 and refs 9 and 21). Calibration measurements in the O2 free methanol solution resolved a slight increase of the ring current, indicative of a very low but not zero activity for the MOR at the ring potential of 1.2, but this value basically did not change with the disk potential. Therefore, this could be subtracted from the measured H2O2 oxidation current as a constant background. Hydrogen peroxide formation, which had already been detected in the absence of methanol during the ORR over RuSey/C catalysts (see Figure 1b and corresponding discussion in Section 3.1.1.), was observed also in the presence of methanol. Although the general characteristics for H2O2 formation on these catalysts are similar in the absence and presence of methanol, there is a slight increase in the H2O2 yield in the presence of methanol (Figure 9b and inset) as compared to the ORR in the absence of methanol (Figure 1b and inset) on the RuSey/C catalysts. For the Pt/C catalyst, in contrast, there is a steep increase in the H2O2 yield at potentials of negative 0.4 V, resulting in a H2O2 yield at low potentials, which is about 10-fold higher than in the absence of methanol. At potentials of positive 0.4 V, a quantitative evaluation of H2O2 yields on the Pt/C catalyst is hardly possible due to the onset of the MOR, which contributes increasingly to the overall net current measured on the disk electrode. The high H2O2 yields in the presence of methanol at cathodic potentials can be explained by the presence of a CO adlayer on the catalyst surface, which results from dissociative methanol adsorption5 and which increasingly blocks the Pt surface for dissociative adsorption of the O2 molecule. It has been demonstrated in earlier studies that strongly adsorbed adlayers such as COad lead to an increased two-electron reduction of O2 to hydrogen peroxide.54,58,59 This effect is less pronounced for RuSey/C catalysts, which are inactive for both oxidation of methanol and its dissociative adsorption on the Ru surface. Nevertheless, the RuSey/C catalysts exhibit undesirable H2O2 yields of 1-2% at typical cathode operation potentials, regardless of the presence or absence of methanol. In total, despite the very good methanol tolerance of the surface-modified RuSey/C catalysts, their tendency for H2O2 formation at potentials relevant for DMFC or PEFC cathode operation, both in the presence and in the absence of methanol and over a temperature range up to at least 60 °C, makes a technical application questionable due to the continuing longterm corrosion of membrane and electrode materials induced by H2O2.31-34 For technical applications, further improvements are necessary to (i) significantly reduce the tendency for H2O2 formation and (ii) reduce the over-potential for the ORR, which is significantly higher than on Pt/C in the absence of methanol and makes them superior to Pt/C only for higher methanol concentrations on the cathode side.

Colmenares et al. 4. Conclusion In summary, we have shown by combined DEMS, RRDE, and WJDE experiments, performed under fuel cell relevant conditions (continuous electrolyte transport and elevated temperatures up to 80 °C) and under well-defined diffusion conditions, that Se surface modification of the Ru/C catalyst prevents the decrease in ORR activity observed for the Se-free Ru/C catalyst at temperatures higher than 50 °C, most likely by inhibition of the (hydr)oxide species formation on the Ru surface. Therefore, the inherent ORR activity of the RuSey/C catalysts is increased with the Se content. At close-to-realistic reaction conditions (80 °C and high O2 flux), the highest geometric area normalized kinetic current density at low overpotentials was found at a Ru/Se ratio of about 1:0.15. The same optimum composition was determined also in RRDE measurements at 60 °C, with somewhat lower H2O2 production yields (ca. 1-2%) as compared to Ru/C and RuSe/C catalysts in the technically relevant potential region (0.6-0.8 V). Even for the optimum composition RuSey/C catalyst, however, the overpotential for the ORR exceeds that of the Pt/C catalyst by about 0.2 V, and the tendency for H2O2 formation at typical cathode operation potentials is significantly enhanced as compared to the Pt/C catalyst, from nondetectable values on Pt/C to about 1-2% on the RuSey/C catalyst. Increasing reaction temperatures lead to a continuous increase in ORR activity on the Pt and surface-modified RuSey/C catalysts, whereas on the Ru/C catalyst, the ORR rate initially increases, up to 50 °C, and then sharply decreases at higher temperatures. The latter effect is mainly attributed to thermally activated OHad/surface oxide formation on the unmodified Ru/C catalyst, which is much less effective on the Se-modified RuSey/ catalysts, indicative of a Se induced hindering of (hydr)oxide formation on the Ru sites. The activation energies determined in the temperature range of 30-60 °C (30-50 °C for Ru/C) are of the same order of magnitude, between 18 and 24 kJ mol-1. The DEMS data demonstrate that the Ru/C and RuSey/C catalysts are almost completely methanol tolerant at relatively low electrolyte flow and at room temperature, in agreement with RRDE measurements at 60 °C. Despite the elevated temperature (60 °C) and enhanced mass transport in the RRDE configuration (1600 rpm), and regardless of the absence or presence of O2 in the electrolyte, the Ru/C and RuSey/C catalysts remain to be essentially inactive toward the MOR, while the Pt/C catalyst suffers significant (ca. 0.3 V) performance loss due to mixedpotential formation. In contrast to the excellent methanol tolerance of the RuSey/C catalyst, their selectivity for H2O formation in the ORR is less satisfying, with H2O2 yields of 1-2% at typical cathode operation potentials in both the absence and the presence of methanol. The unmodified Ru/C and Se-modified RuSey/C catalysts exhibit the same tolerance toward methanol oxidation, comparable Tafel slopes and activation energies, but different selectivities for H2O2 formation (the tendency for hydrogen peroxide formation increases with the Se content). Overall, these observations are consistent with a mechanism where (i) the ORR takes place on the Se free Ru sites, (ii) Se surface modification results in a stabilization of the adjacent Ru sites against OHad formation, and (iii) O2 reduction to H2O (four-electron pathway) requires the presence of at least two adjacent Ru sites for dissociative O2 adsorption. At very high Se coverages, the sharply decreasing number of these vacancy dimers results in increasing H2O2 formation, which also can occur on a Se covered surface. At the current state of the development, practical application of these Se-modified RuSey/C catalysts in PEFC and, in

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