Article pubs.acs.org/JPCA

Effect of Temperature and Pressure on the Kinetics of the Oxygen Reduction Reaction Edmund C. M. Tse† and Andrew A. Gewirth*,†,‡ †

Department of Chemistry, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States ‡ International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 812-8581, Japan S Supporting Information *

ABSTRACT: Fundamental understanding of the oxygen reduction reaction in aqueous medium at temperatures above 100 °C is lacking due to the practical limitations related to the harsh experimental conditions. In this work, the challenge to suppress water from boiling was overcome by conducting the electrochemical investigation under pressurized conditions. A striking improvement in the kinetics of the electrocatalytic reduction of O2 by about 150 fold relative to room temperature and pressure was recorded under an O2 pressure of 3.4 MPa at 200 °C in basic aqueous environment. To deconvolute the combined effect of temperature and pressure, the underlying variables that dictate the observed O2 reduction kinetics of Pt and carbon electrodes were examined individually. O2 availability at the electrode−solution interface was controlled by the interplay between the diffusion coefficient and concentration of O2. Accurate knowledge of the temperature and pressure dependence of O2 availability at the electrode surface, the Tafel slope, the transfer coefficient, and the electrochemical active surface area was required to correctly account for the enhanced O2 reduction kinetics.

1. INTRODUCTION Polymer electrolyte membrane (PEM) fuel cells are attractive portable power conversation devices for clean and sustainable transportation in the near future.1−3 Unlike combustion engines, fuel cells are not limited by the thermodynamic aspects of the Carnot cycle on the conversion of heat to mechanical work.4,5 Despite the intrinsic advantage offered by fuel cells, the widespread deployment of fuel cells has been hindered by several technical problems and design issues.6−10 One of the major impediments is the sluggish oxygen reduction reaction (ORR) to water: O2 + 4 e− + 4 H+ → 2 H2O (in acid) and O2 + 4 e− + 2 H2O → 4 OH− (in base). While both forms of the ORR are kinetically slow, the acidic version is particularly so.11 At present, Pt or one of its alloys is the catalyst of choice to efficiently mediate the kinetically slow ORR in the fuel-cell industry.12−15 In the field of academic research, Pt is the standard benchmark for nonprecious metal and metal-free ORR catalysis. However, the performance of Pt is still far from satisfactory for practical fuel cell applications, with an overpotential of ∼300 mV. The origin of the observed overpotential lies in the strong binding affinity of OH− for Pt surfaces at potentials above 0.9 V versus RHE,16 a claim reinforced by DFT calculations.17−19 Much prior work is dedicated to destabilizing the Pt−OH bond by incorporating alloy elements. Pt3Ni,20 currently the best Pt-based ORR catalyst among other Pt-alloys,21 suffers from Ni dissolution under operating conditions.22 © XXXX American Chemical Society

Pt also serves as a standard catalyst in fuel cell anodes. The material suffers from poisoning issues by adventitious CO in the H2 stream remaining from the reforming process.23,24 10 ppm of CO is detrimental to H2 oxidation kinetics.25 By increasing the cell temperature to 130 °C, CO tolerance is enhanced up to 1000 ppm. We wondered if we could apply a similar strategy to promote Pt−OH bond dissociation at intermediate temperatures (100−200 °C). Temperature-programmed desorption (TPD) results demonstrate that CO desorbs from Pt surface at ∼230 °C,26 with an activation energy of ∼130 kJ/mol for CO desorption from Pt.27 OH− desorbs from Pt at >600 °C, with an activation barrier of ∼210 kJ/mol.28−36 The drastic difference between CO and OH− desorption is due to the formation of intricate hydrogenbonded hexagonal networks or multilayer islands of H2O with surface OH− groups to form complex structures that increase the desorption activation energy of OH−.34,37,38 Laser-induced fluorescence (LIF) studies provide further evidence of the exceptionally strong Pt−OH bond. OH− dissociation either involves a charged species or leaves behind a charged surface that is energetically highly unfavorable.36,39 This analysis suggests that direct elimination of OH− from a Pt surface at intermediate temperatures is unlikely. A close examination of the Pt-CO poisoning studies reveals that the Pt−CO bond is significantly destabilized at 130 °C, Received: November 14, 2014

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The Journal of Physical Chemistry A which is ca. 100 °C lower than the desorption temperature (vide supra), suggesting that the poisoning effect of CO is sufficiently mitigated at temperatures well below the CO desorption temperature. To our surprise, H2O desorbs from an oxide-covered Pt surface at ∼130 °C,40−42 which is even lower than that of CO from Pt, with an activation energy of ∼80 kJ/ mol.43 In essence, Pt−OH2 is the protonated form of Pt−OH. In conjunction with the TPD results, another study concludes that Pt−OH bonds are weaker at a liquid−solid junction than at a gas−solid interface.44 Hence, facilitating the release of H2O from Pt surface at intermediate temperatures appears to be a relevant and compelling strategy to enhance the activity of Pt. Apart from the opportunity to enable efficient cleavage of the Pt−OH bonds, raising the operating temperature can potentially enhance the ORR kinetics of Pt, as predicted by the Arrhenius equation.45 Both full- and half-cell studies suggest that the power density of Pt increases at elevated temperatures ( Na+ > K+,76 so we conducted our experiments in KOH.

reduction, which is possible at high temperatures under pressurized conditions.77 Caution! Experiments at high temperature and pressure can lead to personal injury and property damage (SI note 1). Studies that potentially generate H2 or H2O2 in situ under a highly oxidizing atmosphere should be carried out with extreme caution inside a chemical f ume hood with a blast shield properly installed. A modified version of a near- and supercritical system with a stationary three-electrode cell configuration previously described was used.77 High-temperature and -pressure electrochemical studies were carried out using a #4590 micro reactor (Parr Instruments) with a removable PTFE insert (50 mL, Parr Instruments). Temperature was varied (±1 °C) with an integrated #4848 control panel (Parr Instruments). Linear sweep voltammograms (LSVs) were collected using a 760D electrochemical workstation (CH Instruments), and electrochemical impedance spectra (EIS) were collected using a SP150 potentiostat (Bio-Logic). The electrodes were connected to the Cu wire feedthroughs by mechanical contact. This method allowed for easy disassembly and cleaning of the cell and electrodes. Electrical junctions were covered with Teflon tape, leaving only the electrodes exposed. The Teflon tape retains its integrity at temperatures below 250 °C.77 A solution volume of 15 mL was used for each experiment. After heating to the desired temperature, the system was equilibrated for 1 h to minimize thermal gradients and convection.77 For Pt studies, Pt wires (99.95%, Alfa Aesar) were used as both working and counter electrodes. Pt wires were soaked in freshly prepared aqua regia (3:1 HCl/HNO3, AR ACS grade, Macron Fine Chemicals) overnight and then HClO4 (70%, Ultrex II UltraPure reagent grade, J. T. Baker). The Pt wires were subsequently rinsed with Milli-Q water and flamed using a H2 torch (99.9999%, research grade, S. J. Smith Welding Supply) and used immediately. Electrode areas were determined by using the characteristic H2 adsorption and desorption waves at 20 °C under an Ar pressure of 0.1 MPa and were typically ca. 0.37 cm2. For carbon studies, a glassy carbon (GC) disk with an embedded metal pin (modified MF-2012, Bioanalytical Systems) and a carbon rod (spectroscopically pure, grade 1, Ted Pella) were used as the working and counter electrodes, respectively. The GC disk working electrode was polished successively with 9, 3, 1, 0.25, and 0.05 μm diameter diamond polish (Buehler) and sonicated in water after each stage. The carbon rod counter electrode was polished with 400-grit sandpaper (Buehler). Electrochemical potentials were measured and reported with respect to a tungsten wire reference electrode.78,79 A tungsten wire (99.95%, Alfa Aesar) was freshly cut and immersed in KOH (0.1 M) for 60 min prior to use.80 Because the focus of this study was the effect of temperature and pressure on the current response, no effort was made to report the electrochemical potentials with respect to the reversible hydrogen electrode (SI note 2).77 Attempts to use other bare or oxidecovered metal wires (Ag, Pt, Pd, Ir, Ni, Au, Cu, and Ta) were unsuccessful. All O2 reduction LSVs were collected at a scan rate of 50 mV/s and corrected for uncompensated Ohmic resistance (Ru), which was obtained from EIS measurements (SI note 3). Scanning electron microscopy (SEM) of the Pt wire was performed using a Hitachi S-4700 Cold FE-SEM (Hitachi High Technologies) with an acceleration voltage of 20 kV. Energy-

2. EXPERIMENTAL METHODS Chemicals were obtained from commercial sources and used without further purification unless otherwise specified. Potassium hydroxide solution (0.1 M, analytical titration grade, Fisher Scientific) was prepared using Milli-Q water (>18 MΩ cm−1). Solutions were sparged with Ar (99.999%, ultra high purity, S. J. Smith Welding Supply) or O2 (99.999%, ultra high purity, S. J. Smith Welding Supply) for 60 min prior to each experiment. Ar is used instead of N2 to avoid N2 B

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Figure 1. Linear sweep voltammograms (LSVs) of Pt in 0.1 M KOH with a scan rate of 50 mV/s at 110 (black), 140 (red), 170 (blue), and 200 °C (green) with an initial Ar (dashed line) and O2 (solid lines) pressure of (a) 3.4 and (b) 1.4 MPa at 20 °C.

dispersive X-ray spectroscopy (EDX) was performed with an Oxford Instruments ISIS EDX Microanalysis System.

Scheme 1. Constants and Variables Affecting the Measured Currenta

3. RESULTS AND DISCUSSION Figure 1a shows a representative set of LSVs obtained from a Pt wire immersed in a solution of 0.1 M KOH at temperatures between 110 and 200 °C with an initial O2 pressure of 3.4 MPa at 20 °C. Figure 1a also displays an LSV obtained at 200 °C with an initial Ar pressure of 3.4 MPa at 20 °C (dashed line). The lack of substantial response from the Ar-saturated electrolyte (seen at all temperatures) means that the cathodic current seen in the presence of O2 is associated with the ORR. The Figure demonstrates that as the temperature increases the current increases as well. Figure 1b shows LSVs obtained from the same electrolyte but with an initial O2 pressure at 20 °C of 1.4 MPa. The lower O2 pressure used in Figure 1b corresponds to lower currents relative to Figure 1a. The oscillatory response seen at higher temperatures is associated with mass transport due to minor thermal convection.77 Figure S1a−c in the SI reports the temperature-dependent O2 reduction LSVs of Pt with an initial O2 pressure of 0.7, 2.1, and 2.8 MPa at 20 °C. The experimental correlation between temperature, pressure, and current is presented in Figure 2. This correlation indicates qualitatively that ORR current density is a function of O2 pressure and temperature. Scheme 1 shows the essential components affecting the measured current in LSVs. F and R are constants, and υ is held at 50 and 100 mV/s for voltammetries under O2 and Ar, respectively (SI note 4). n is

i = current (Å), F = Faraday’s constant (C mol−1), R = ideal gas constant (J mol−1 K−1), υ = scan rate (V s−1), n = number of electrons, A = electrode area (cm2), α = transfer coefficient, D0 = diffusion coefficient (cm2 s−1), C0* = bulk concentration (mol cm−3), T = temperature (K), P = pressure (MPa). a

assumed to be 4 because polycrystalline Pt catalyzes the ORR via a 4e− pathway.81 Electrochemical active surface area measured prior to each experiment is taken as A and is assumed to be constant throughout the experiment. α, D0, and C*0 are temperature-dependent.82 P is also influenced by T and indirectly affects the current measured by affecting the bulk concentration. We aim to elucidate the effect of T, the independent variable, on i, the observable quantity. To quantify the relationship between temperature, pressure and current, we search for a mathematical expression that includes variables listed in Scheme 1 to describe the trend observed in Figure 2. The LSVs in Figure 1 and Figure S1 in the SI exhibit microelectrode-like steady-state current behavior, not macroelectrode-like peak current behavior. One straightforward method to discern the differences between the two behaviors is to vary the scan rate. Steady-state current is independent of scan rate, but peak current is scan-ratedependent. Figure S2 in the SI shows the O2 reduction LSVs of Pt at various scan rates. At high scan rates, we observed LSVs with typical peaking behavior for an irreversible process. We attribute the limiting current behavior observed at low scan rates to the excess availability of O2 at the surface. At slow scan rates, O2 depletion at the surface is relatively slow compared with O2 diffusion from bulk solution to surface. The fast replenishment of O2 is due to the combined effect of high O2

Figure 2. Tridimensional plot of O2 reduction current density of Pt in 0.1 M KOH versus O2 pressure versuss cell temperature. The color scheme quantifies the O2 reduction current density. C

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the experimental and calculated ORR current densities of Pt. The calculated values are found using eq 1 with the calculated D0(T) and C0*(T) of O2, assuming α = 0.5. The general trend is correctly predicted, yet the calculated values deviate from the observed values, suggesting that α is likely not 0.5. To obtain values of α, we measure the Tafel slopes of the O2 reduction LSVs of Pt at intermediate temperatures.

concentration and increased O2 mobility at high temperature and high pressure conditions. The inset of Figure S2 in the SI describes the linear relationship between the square roots of scan rates and the observed peak current densities. This linear relationship suggests that our system is in the macroelectrode regime, allowing us to calculate the theoretical limiting current using the peak current equation for irreversible electrochemical processes (eq 1).83 i = 0.4958F 3/2R−1/2υ1/2nAC0*D01/2α1/2T −1/2

η= (1)

2 773.8 ⎛⎜ 506.4 ⎞⎟ − ⎝ T ⎠ T

(2)

Next, eq 3 establishes the connection between the temperature dependence of C*0 and PO2,86 which is also temperaturedependent by our experimental design. We determine the change in pressure by applying the van der Waals equation (eq 4), allowing us to compute C*0 using eq 3, where a (m6 Pa mol−2) and b (m3 mol−1) are van der Waals constants of O2,87 ng is the number of moles of O2, and V (m3) is the volume of the head space in the reactor. ⎡ ⎛ T ⎞ ⎟ − (299.378 + 0.092T ) C0* = PO2 exp⎢0.046T 2 + 203.35T ln⎜ ⎝ 298 ⎠ ⎣ ⎤ (T − 298) − 20591⎥/[8.3144T ] ⎦ (3)

⎡ a(ng )2 ⎤ ⎢P + ⎥(V − ng b) = ng RT ⎢⎣ V 2 ⎥⎦

(5)

Equation 5 shows the Tafel slope equation with η = overpotential, na = number of electron transferred in the ratedetermining step (RDS), and i0 = the exchange current density (SI note 5). The electrochemical community has not yet reached a consensus on whether the Tafel slope or the transfer coefficient of the ORR is temperature-sensitive.88 A study proposed that a Tafel slope of ∼60 mV/dec in the low current density region, where Temkin adsorption kinetics dominate on an oxide-covered Pt surface, is sensitive toward temperature.25 In the high current density region where Langmuirian conditions are favored on an oxide-free Pt surface, a temperature-insensitive Tafel slope of ∼120 mV/dec is observed.25 However, other studies suggested otherwise.89−91 First, we compare the observed Tafel slopes in both the Langmuir and Temkin regions to the theoretical Tafel slopes predicted from eq 5. Figure 4a,b shows the observed (colored symbols) and predicted (na = 1, 3, 4, black lines) Tafel slopes, assuming α is 0.5. The observed Tafel slopes are temperaturedependent and match with the predicted Tafel slopes in the Langmuir region but are temperature-insensitive and deviate from the predicted Tafel slopes in the Temkin region. These observations contradict a published report,92 likely due to differences in experimental conditions. The ORR kinetics in ref 92 are likely altered by a contamination effect from the polymer electrolyte, as pointed out by authors in another study.93 Because the observed Tafel slopes in the Temkin region are temperature-independent, we suspect that the transfer coefficient, α, instead is temperature-dependent. Figure 4c,d shows the α na values calculated from the observed Tafel slopes in the Langmuir and Temkin regions, respectively. We found α na to be close to 0.5, indicating that α = 0.5 and na = 1 in the Langmuir region. These values are consistent with results suggesting that the ORR catalyzed by Pt proceeds via a 1e− step in the Langmuir region.94 Experimentally, we find α is ∼0.5 ± 0.1, which is within the typical range of 0.3 ≤ α ≤ 0.7.83 The Temkin region for Pt ORR typically refers to a 2e− reduction process.82 Assuming na = 2, α increases from 0.7 to 1 as temperature increases from 110 to 200 °C. An α of unity in the Temkin region is not uncommon; previous studies suggest a fast electron-transfer step that is followed by a chemical RDS.88,89 To validate our observations on Pt and to avoid complications due to different Tafel slopes, we next use glassy carbon (GC) as a simpler test system. Figure 5a displays a representative set of LSVs obtained from a GC electrode immersed in a solution of 0.1 M KOH at temperatures between 110 and 200 °C with an initial O2 pressure of 0.7 MPa at 20 °C. Similar to the case of Pt, the ORR limiting current density on GC increases as a function of temperature. The experimental correlation between temperature, pressure, and current is described in Figure 5b. This correlation indicates that ORR current density on GC is a function of O2 pressure and temperature. Figure 5c shows the observed Tafel slopes of GC

Equation 1 shows that in addition to the direct dependence on temperature, current is further indirectly affected by temperature through the temperature dependence of D0, C0*, and α. Knowledge of D0(T), C0*(T), and α(T) is necessary to accurately predict the temperature dependence of the current. These will be discussed later. First, the diffusion coefficient of O2 (D0) does not depend on the pressure of O2 (PO2),84 so we directly compute D0 at various temperatures using eq 2.85 log10 D0 = − 4.410 +

RT RT ln i0 − ln i αnaF αnaF

(4)

The values of D0 and C*0 for O2 together determine the availability of O2 at the electrode surface. Figure S3 in the SI describes the relationship between observed O2 reduction current with the diffusion coefficient of O2 and the bulk concentration of O2. Figure 3 shows the comparison between

Figure 3. Experimental (symbols) and theoretical (lines) ORR current densities of Pt with initial O2 pressures of 0.7 (red), 1.4 (blue), 2.1 (green), 2.8 (orange), and 3.4 (purple) MPa at 20 °C in 0.1 M KOH, assuming α = 0.5. D

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Figure 4. Experimental Tafel slopes in the (a) Langmuir and (b) Temkin region and α na in the (c) Langmuir and (d) Temkin region of ORR catalyzed by Pt in 0.1 M KOH (symbols) with initial O2 pressures of 0.7 (red), 1.4 (blue), 2.1 (green), 2.8 (orange), and 3.4 (purple) MPa at 20 °C. Theoretical values are predicted with na = 1 (black solid line), 3 (black dotted line), or 4 (black dashed−dotted line).

Figure 5. (a) LSVs of GC in 0.1 M KOH with a scan rate of 50 mV/s at 110 (black), 140 (red), 170 (blue), and 200 °C (green) with an initial O2 pressure of 0.7 MPa at 20 °C. (b) Tridimensional plot of O2 reduction current density of GC in 0.1 M KOH vs O2 pressure vs cell temperature. The color scheme quantifies the O2 reduction current density. (c) Experimental (symbols) Tafel slopes and (d) α na of ORR catalyzed by GC in 0.1 M KOH. Theoretical values (lines) are predicted with na = 1 (black solid line) or 2 (black dashed line).

calculated from the Tafel slopes. The product of α and na is slightly temperature-sensitive and lies between the theoretical values calculated with α = 0.5 and na = 1 or 2. The typical na value of GC is 1,90 which equals to the observed na at 0.1 MPa, 20 °C. Hence, we conclude that the α values of GC increase

(colored symbols) and the predicted Tafel slopes (black lines), assuming α is 0.5. The figure demonstrates that the experimental ORR Tafel slope of GC is temperatureindependent. Given this condition, we then study the effect of temperature on α na. Figure 5d shows the α na values E

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temperature effect on α, the predicted values are in good agreement with the experimental values, with the exception of one condition at 200 °C, which will be addressed later. We justify the use of α values from the Temkin region in Figure S5 in the SI and discussion therein. We next investigate the deviation between the experimental and calculated ORR current densities observed at 200 °C. Under high pressure of O2 at 200 °C, reactor components including the tungsten wire, the thermocouple, and the electrical feedthroughs might corrode or leach.77,95 To verify if the observed enhanced ORR activity is due to the formation of Pt surface alloys from metal impurities, we examine the surface composition of the Pt wire by using EDX spectroscopy. Pt wire, which is freshly cleaned, heated to 80 °C at 0.1 MPa of O2, or heated to 200 °C at an initial O2 pressure of 3.4 MPa, exhibits no formation of W, Fe, Cu, or N alloys. No elements other than Pt, C, O, K, and Cl were detectable by EDX. Minor concentration of contaminants below the detection limit of EDX (1%) is possible, but dopant levels higher than 10% are typically required to enhance the ORR performance.96−99 We thus confirm that formation of surface alloy is not the cause to the enhanced ORR current density observed at 200 °C. Our prediction model accounts for temperature effects on α, D0, and C0*, assuming n and A to be constant. We wonder if the deviation stems from unanticipated changes in n or A at intermediate temperature under a pressurized O2 atmosphere. Studies indicate that Pt generates less H2O2 as temperature increases,100 and thus the assumption that n remains as 4 at high temperature and pressure is valid. A, measured before pressurizing and heating, is assumed to be constant. Thermal expansion of the solvent to increase the submerged surface area of the Pt wire is possible. However, a Pt disk electrode produces the same result as a Pt wire, suggesting that the change in water level does not significantly alter the submerged area of the Pt wire. Instead of a macroscopic change in the Pt surface area, we hypothesize that the deviation in observed and predicted current densities is a consequence of microscopic changes on the Pt surface. To this end, we monitor the change in electrochemical active surface area (ECSA) of Pt. To measure the ECSA of Pt in situ, we collect CVs of Pt under Ar at intermediate temperatures. We then calculate the ECSA of Pt by integrating the charge in the region where underpotential deposition of H2 on Pt (Pt−Hupd) occurs. Figure S6a in the SI displays the CVs focusing on the hydrogen adsorption− desorption region of Pt under Ar up to 50 °C. The temperature dependence of the potential range and shape of the Pt−Hupd feature are similar to those reported in literature.44,101,102 Surprisingly, at temperatures above 110 °C, we observe the disappearance of the Pt−Hupd feature (Figure S6b in the SI), which has not been reported before (SI note 6). This observation is in agreement with TPD data, which indicates that the Pt−H bond dissociates at 90 °C under vacuum.27 Because of the disappearance of the Pt−Hupd features at intermediate temperatures, our attempt to monitor changes in ECSA of Pt in situ under Ar is not fruitful. Instead of monitoring the changes in ECSA of Pt in situ, we measure ECSA of Pt after the experiment. We let the reactor cool to room temperature, and then depressurize the system. Figure 8 displays the Pt−Hupd region before (solid line) and after (dashed line) heating to 200 °C with an initial O2 pressure of 3.4 MPa at 20 °C. The ECSA of Pt increases by 28% after heating to 200 °C. By using the ECSA of Pt obtained after

with temperature, albeit the magnitude of change in α is smaller relative to that from Pt obtained in the Temkin region. Equipped with a new set of knowledge on the temperature dependence of α, we recalculate the theoretical temperature dependence of the ORR current density by using eq 1. Figure 6

Figure 6. Experimental (symbols) and theoretical (lines) ORR current densities of GC with initial O2 pressures of 0.7 (red), 1.4 (blue), 2.1 (green), 2.8 (orange), and 3.4 (purple) MPa at 20 °C in 0.1 M KOH. α values are obtained from Tafel slopes at each temperature.

shows the experimental (symbols) and predicted (lines) ORR current densities of GC. By compensating for the temperature dependence of α, the predicted values are in close agreement with the experimental values. We conclude that eq 1 accurately describes the ORR activity of GC at intermediate temperatures in a pressurized system if we account for the temperature effect on α. We next address Pt. As previously mentioned, the Pt system is complicated due to the presence of two Tafel slopes, requiring the use of two sets of α values. In the Langmuir region, we obtain α values ∼0.5 from the temperaturedependent Tafel slopes. Using these α values centered at 0.5 yields calculated ORR current densities similar to those predicted in Figure 3 that do not match with the observed values (Figure S4 in the SI). We use α values obtained from the Temkin region to predict the ORR current density. Figure 7 shows a comparison of the calculated and experimental temperature dependence of the current density for the ORR on Pt using α values obtained from the Temkin region. Indeed, by taking into account the

Figure 7. Experimental (symbols) and theoretical (lines) ORR current densities of Pt with initial O2 pressures of 0.7 (red), 1.4 (blue), 2.1 (green), 2.8 (orange), and 3.4 (purple) MPa at 20 °C in 0.1 M KOH. α values are obtained from Tafel slopes in the Temkin region at each temperature. F

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS E.C.M.T. acknowledges a Croucher Foundation Scholarship. We thank the U.S. Department of Energy (DE-FG0295ER46260) for support of this research. This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, which are partially supported by the U.S. Department of Energy (DE-FG02-07ER46453 and DE-FG02-07ER46471). We thank R. T. Venegas for insightful suggestions on experimental design and C. J. Barile for assistance with SEM/EDX data collection and analysis.

Figure 8. CVs of Pt at 20 °C under 0.1 MPa of Ar in 0.1 M KOH with a scan rate of 100 mV/s before (solid line) and after (dashed line) heating to 200 °C with an initial O2 pressure of 3.4 MPa at 20 °C.

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heating to 200 °C, the experimental current density becomes 0.18 ± 0.03 A cm−2, which is comparable to the calculated current density. Figure S7 in the SI shows a collection of ex situ SEM images of Pt working electrodes after experiments under various conditions, further supporting the notion that the difference in observed and predicted ORR current densities of Pt is likely surface-related. With the predicted current densities being consistent with the observed values, we conclude that at intermediate temperatures the peak current equation is valid and the intrinsic catalytic ORR activity of Pt remains unchanged.

4. CONCLUSIONS We have studied the electrocatalytic reduction of O2 at temperatures above 100 °C and pressures beyond 0.7 MPa in 0.1 M KOH aqueous medium. Our results demonstrate that the O2 reduction kinetics is a function of both temperature and pressure, variables that together modulate the availability of O2 at the electrode surface. Apart from the diffusion coefficient and solubility of O2, we investigate the temperature dependence of the ORR Tafel slopes. In the Temkin region, we observe temperature-insensitive Tafel slopes, suggesting that α, the transfer coefficient, is temperature-dependent. An increase in the electrochemical active surface area of Pt electrode at 200 °C during ORR likely reflects Pt instability at elevated temperatures. Correct accounting for the many temperature- and pressure-dependent variables leads to accurate prediction of the enhanced ORR activity observed at intermediate temperatures under pressurized conditions. We subject GC to the same experimental conditions as Pt and observe similar temperaturedependent behavior of the Tafel slopes and α values. GC electrode reduces O2 via a 2 e− process even at high temperatures. Under an O2 pressure of 3.4 MPa at 200 °C, the ORR activity of both Pt and GC electrodes improves by ca. 150 fold relative to room temperature and pressure.

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ASSOCIATED CONTENT

S Supporting Information *

LSVs, scan-rate dependence studies, voltammetries under Ar, and notes as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: 217-333-8329. Fax: 217-244-3186. E-mail: agewirth@ illinois.edu. G

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