Carbon Catalyst Layer Microstructural Effects on Measured and

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Pt/Carbon Catalyst Layer Microstructural Effects on Measured and Predicted Tafel Slopes for the Oxygen Reduction Reaction Dustin W. Banham, Jeff N. Soderberg, and Viola I. Birss* Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4 ReceiVed: NoVember 12, 2008; ReVised Manuscript ReceiVed: March 26, 2009

The important effect of the physical and morphological properties of porous cathodes on the oxygen reduction reaction (ORR) Tafel slope, which is normally viewed as a mechanistic parameter, was investigated using a variety of Pt/carbon (Nafion) catalyst layers in room temperature O2-saturated sulfuric acid solutions. Consistent with previous theoretical predictions, it was found experimentally that increasing the pore length, the catalyst layer resistance, and the exchange current density, and decreasing the pore diameter, all serve to cause the Tafel slope to be larger than its mechanistically predicted value, thus leading to performance loss. Theoretical Tafel slopes, calculated using a model for the migration-induced distribution of potentials in an electroactive porous layer and employing the catalyst layer properties examined experimentally, showed very good agreement with the measured Tafel slopes. These results reveal the importance of minimizing the ohmic resistance of porous electrocatalyst layers, of relevance in a wide range of applications, e.g., in fuel cells, electrolysis processes, batteries, etc. Introduction A fuel cell is an energy conversion device that converts chemical energy directly into electrical energy via electrochemical reactions. The benefits of fuel cells include their very high efficiency and the production of electricity with little to no emission of particulates, NOx, or SOx pollutants.1 Proton exchange membrane (PEM) fuel cells, operating on pure H2 and air at approximately 80 °C, have a rapid start up time and are therefore well-suited for transportation and portable device applications.1 In typical PEM fuel cells, Pt nanoparticles distributed on carbon powder particles serve as the anode and cathode electrocatalysts, while Nafion serves as both the separator and as the ion-conducting component of the catalyst layers. It is well-known that one of the most significant performance losses in PEM fuel cells arises from the sluggish oxygen reduction reaction (ORR) kinetics at the cathode.2 Therefore, efforts are underway to improve the stability and utilization of Pt nanoparticles, including increasing the surface area/volume ratio in the catalyst layer,3 leading to higher ORR rates. In another direction, lower cost Pt alloy4 and non-noble metal catalysts5 have been shown to give better activity than pure Pt, while other work is focused on optimizing catalyst morphology, ultimately to better utilize the costly electrocatalytic material and also enhance durability. The kinetics of an activation-controlled electrochemical reaction, such as the cathodic (subscript ‘c’) ORR and its reverse process, the anodic (subscript ‘a’) oxygen evolution reaction (OER), can be described by the Butler-Volmer equation:6

i ) io(eRaηF/RT - e-RcηF/RT)

(1)

where io is the exchange current density (the inherent rate of the reaction), η is the overpotential vs the equilibrium potential for the process, R is the transfer coefficient for the ORR and OER, T is the temperature, R is the gas constant, and F is * To whom correspondence should be addressed.

Faraday’s constant. Using the high field (high η) approximation for the ORR process, eq 1 can be simplified, as follows:

i ) io(e-RcηF/RT)

(2)

A plot of log(i) vs η (Tafel plot) allows io to be determined from the intercept, while the Tafel slope (dη/dlog(i), eq 3) gives the R value.

TS )

-2.303RT RcF

(3)

In theory, the measured R value can then be used to obtain mechanistic information:

R)

γ + rβ υ

(4)

where γ is the number of electrons preceding the ratedetermining step (rds), υ is number of times the rds occurs for one full occurrence of the reaction, r is the number of electrons passed in the rds, and β describes the symmetry of the activation barrier for the reaction.6 Generally, for simple single electron exchange reactions, the activation barrier is symmetrical and thus β is very close to 0.5.6 At 25 °C, assuming a β value of 0.5, the smallest nonzero allowable value of R is 0.5, which, from eq 3, corresponds to a Tafel slope (TS) of ∼120 mV/ decade of current. It has been noted in the literature7,8 that there are two distinct Tafel regions for the ORR at Pt electrocatalysts in acidic media, ∼60 mV/decade (R ) 1) at low overpotentials, and 120 mV/ decade (R ) 0.5) at higher overpotentials.7,9 This has sometimes been interpreted as indicating that the rds has changed from a slow chemical step after a rapid first electron transfer (ET) step at low overpotentials to a slow first ET step at higher η.6,10,11 It has also been suggested10,11 that this change in Tafel slope reflects a change in how the oxygen-containing species involved in the ORR adsorb on Pt. Assuming that the first electron transfer step is slow, a 60 mV Tafel slope (low overpotentials) is consistent with adsorption obeying a Temkin isotherm, while

10.1021/jp809987g CCC: $40.75  2009 American Chemical Society Published on Web 05/14/2009

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a Langmuir isotherm is assumed at higher overpotentials (giving the predicted 120 mV Tafel Slope). Irrespective of the explanations used, a small Tafel slope and a proportionally large R value (eq 3) is desired for all electrocatalytic applications. Another possible reason for the evolution to higher TS values as the overpotential is increased, even sometimes giving a slope twice the normal 120 mV value at room temperature, is related to the presence of distributed potentials within a porous, electrocatalytically active layer,12-14 arising from mass transport limitations in the pores.12,13,15 Thus, the measurement of Tafel slopes at porous electrodes can lead to an incorrect inference about the reaction mechanism or the operative adsorption isotherm. Transport limitations giving rise to distributed potentials within porous electroactive layers have generally been analyzed as one of two limiting types. The situation when concentration overpotentials (diffusion) within pores are present was studied in depth by Perry et al.,16 showing that two distinct Tafel regions can then be obtained. An optimal effective diffusion coefficient for oxygen in the catalyst layer was recently determined, based on this model.17 In contrast, de Levie12 examined the limiting case of ohmic (migration controlled) overpotentials in the pores and predicted that the Tafel slope can become twice its expected value because of the physical and morphological properties of porous electrodes.12 A summary of these two limiting cases was provided by Bockris et al.18 In the present work, we focus only on the case of migration (ohmic) limitations in porous catalyst layers and thus have experimentally modified only those catalyst layer properties that are predicted to affect its resistance. Using the single pore model,12 it has been demonstrated that the current generated in each individual pore of a porous layer on an electrode surface is given by the following equation:

κπr )A tan A ( 4RT LF

(5)

( ) ( )

(6)

I)

2

where A is given by:

A)

ioL2F 2κRTr

1/ 2

exp

ηoF 4RT

Here, ηo is the overpotential at the outer surface of the pore, κ is the ionic conductivity of the pore (assumed here to be limited by the conductivity of the electrolyte inside the pore, instead of the conductivity of the electrocatalytic material), r is the pore radius, L the pore length, and the other parameters have the same meaning as indicated above.19,7 Ohm’s law for the pore is given by:

(

V ) IR R )

L κπr2

)

(7)

From eq 7, it is apparent that large values of I (proportional to io, as shown in eq 2) or L, and small values of r or κ, will result in a large potential drop down the pore, and thus the total current produced by the pore will be lowered. It was predicted by others12,13,15 and confirmed in our previous work7 that, under these high current conditions, a doubled TS should be obtained. However, these predictions have never been verified by the controlled experimental variation of these four porous, electroactive layer parameters. Therefore, the main goal of the present work is to experimentally determine the effect on the Tafel slope for the ORR at porous Pt/C catalysts of the controlled alteration of the four porous layer parameters (L, κ, r, io). It is shown that the data and the ohmic pore model predictions agree very well in terms

of the values of the Tafel slopes, being higher than predicted mechanistically. While diffusional limitations are also likely to be present in most porous catalyst layers, we have selected experimental conditions that should minimize their impact, and thus diffusional effects have not been included in the present treatment. The very good correlation obtained between the theoretical (ohmic model) and experimental Tafel slopes suggests that, in the low current range examined here, resistive losses are more prominent than diffusional gradients through the pores of the catalyst layer. The results reported here lead to a significantly better understanding of the causes of high Tafel slopes at PEM fuel cell cathodes, as well as at porous electrodes used for many other electrochemical reactions. Also, this work provides guidance in terms of the properties of porous Pt/C catalyst layers that will generate high ORR activities without causing the Tafel slope to increase as a result of layer morphology or thickness issues. Experimental Methods Preparation of Catalyst/Nafion Ink. One gram of 11% (w/ w) Nafion ethanolic solution (EW 1100) was diluted with 10 mL of absolute ethanol to give a 1% (w/w) Nafion mixture. An amount of 0.02 g of 5-40% (w/w) Pt on activated carbon (Johnson Matthey) was then added to 0.85 g of the 1% (w/w) Nafion solution. To vary particle size, the 20% (w/w) Pt/C was ground for 15 min using a mortar and pestle. The catalyst ink was then sonicated for 15 min before application onto the surface of a glassy carbon (GC) electrode. Preparation of Glycerol-Containing Inks. An amount of 2.8 g of 11% (w/w) Nafion was diluted with 3.4 g of ethanol to make a 5% (w/w) Nafion/ethanol solution. An amount of 0.13 g of this solution was added to 0.02 g of 20% (w/w) Pt/C powder and 0.3 g water was then added. Finally, 0.4 g of glycerol was added and the ink was sonicated for 25 min. It was then treated at 60 °C in air for 18.5 h20 to form the ink. Electrochemical Evaluation of Catalysts and Determination of ORR Tafel Characteristics. A 7 mm diameter GC rotating disk electrode (RDE) was used to measure the ORR activity of the catalysts. A three-electrode electrochemical cell was used, containing a platinized Pt mesh counter electrode, a reversible hydrogen reference electrode (RHE), and the catalystcoated GC working electrode (WE). The cell solution was typically 0.5 M H2SO4, or mixtures of H2SO4 and K2SO4 of constant ionic strength. These solutions were purged with vigorously bubbling N2 (Praxair 99%) or O2 (Praxair medical grade) and then maintained under the appropriate atmosphere. To apply the sonicated Nafion/Pt/C ink to the GC disk, a micropipet was used for controlled volume application. A 14 µL aliquot resulted in a loading of 0.11 mg/cm2 of Pt when using the 20% (w/w) Pt/C catalyst. The electrode was then dried at room temperature for approximately 15 min before drying at 100 °C for 10 min, all in air. When investigating the effect of pore length, additional layers of the same ink were applied and each layer was allowed to dry in air for 15 min before the addition of another aliquot of ink. The Pt-containing catalyst layers were all subjected to electrochemical cleaning, involving cycling of the potential from -0.05 to 1.7 V at 100 mV/s for 20 cycles. At the end of the final anodic scan, the potential was held at 1.2 V for 10 min. After this, the upper and lower potential limits were set to 1.1 and 0.05 V, and potential cycling was performed until the signal stabilized. Cyclic voltammograms (CVs) were then collected

Pt/Carbon Catalyst Layer Microstructural Effects

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in a N2-purged aqueous solution at 25 °C with no electrode rotation to obtain a baseline for comparison to the ORR data. The cell solution was then saturated with O2, and the CV analysis of the ORR was carried out. A range of rotation rates (500-2000 rpm) was employed using a Pine analytical rotor (model ASR-2), with the electrochemistry controlled by an EG&G PARC 175 function generator in conjunction with a Hokuto Denko HA-301 potentiostat. All CV data reported here were collected at 1000 rpm and were recorded by computer software (Chart 4 by PowerLab). The baseline CV in N2-saturated conditions was then subtracted from each CV collected in aerated conditions to reveal only the ORR electrochemistry. Koutecky-Levich plots21 (log(I × IL)/(IL - I) vs E, where IL is the limiting current) were used to correct for mass transport limitations and more accurately obtain the Tafel slope. All CV data collected in low conductivity solutions were IR-corrected (resistance of the solution was measured using a Yellow Springs Instrument Co. 35 Series conductivity cell), and all samples were subjected to duplicate or triplicate electrochemical evaluations. Surface Characterization. Scanning electron microscopy (SEM) analysis was performed using a Philips/FEI ESEM (Microscopy and Imaging Facility, University of Calgary) with Energy Dispersive X-ray analysis (EDX) capabilities. The samples were prepared on a glass slide and sputter-coated with gold before SEM analysis. Surface area (BET) and pore size measurements were performed using a Micromeritics Tristar 3000 instrument. A seven-point analysis was used at a N2 partial pressure of 0.03 to 0.3 (p/p°) to determine the real surface area of the samples. Results and Discussion Effect of Porous Layer Characteristics on Experimentally Measured Tafel Slopes. The focus of the experiments and theoretical treatment in the present work is on resistive losses within porous electrocatalytic layers. While concentration gradients are also likely to be present, we believe that their impact is minimal, partly because of the low currents that were used. This assumption is supported by the fact that the theoretical Tafel slopes that are obtained (Section 2) from eqs 5 and 6, which completely neglect diffusional gradients within the pores, track the experimentally determined Tafel plots so closely. It should also be noted that no attempt was made to determine io for the ORR at our Pt/C electrocatalysts in the present work, as it is difficult to obtain a reliable value that is not overestimated.22 Impact of Pore Length (Catalyst Layer Thickness). In this part of the work, a key assumption made was that the average pore length increases proportionally with catalyst layer thickness, which was controllably increased by the deposition of successive aliquots of the same Pt/C/Nafion ink. The problem of thick catalyst layers on fuel cell performance has been discussed previously, suggesting that mass transport under forced convection conditions in a rotating ring disk electrode configuration can be distorted by internal diffusion within the catalyst layer. It was argued,23-25 based on film thicknesses estimated from droplet sizes, that thin films allow full utilization of the catalyst, permitting kinetic information about the ORR to be obtained in model studies.22,26 The present work is based on the presumption that the pore diameter and tortuosity do not change significantly with film thickness. Scanning electron microscopy (SEM) shows (Figure 1) that the films are generally somewhat thicker in the center of the deposit, thinning out toward the edges. Even so, the one-, two-, and four-layer films were found to be 17.5 ( 2.5, 35 (

Figure 1. Cross-sectional SEM images of (a) one and (b) four layer 20% (w/w) Pt/C + 1% (w/w) Nafion/ethanol film. In b) the catalyst layer has peeled away from the glass substrate.

5, and 70 ( 10 µm thick (Figure 1), indicating a close to linear relationship between the total layer thickness and the number of applied aliquots of Pt/C/Nafion ink. Figure 1 also demonstrates that both the top-down and crosssectional morphologies of the four-layer film are very similar to the one layer film. The coatings do not show any layering, as might have been expected from the application of a succession of aliquots. The homogeneity of the microstructure as a function of depth into the films therefore supports the assumed constant tortuosity and size of the pores with increasing catalyst layer thickness. The CV response for the 1, 2, and 4 layers of Pt/C on GC is shown in room temperature (RT) N2-saturated 0.5 M H2SO4 in Figure 2a, revealing the characteristic hydrogen underpotential deposition (Hupd) peaks of Pt at 0.05-0.30 V and Pt oxide formation at anodic potentials above 0.8 V. These Pt features are seen to be superimposed on the carbon double layer and pseudocapacitance response.27 In all of the CV evaluations, a 10 mV/s sweep rate was used, found to be sufficiently slow so that all sites have time to react (seen by the linear relationship between sweep rate and CV charge). Also, the CVs collected in N2-saturated solutions (Figure 2a) were found to be independent of electrode rotation rate (ω), as expected. The Pt CV currents increase close to linearly with Pt loading, as shown in the inset of Figure 2a specifically for the hydrogen desorption peak at 0.2 V. This is important, as it supports the supposition that, even for the thickest catalyst layer, the H2SO4

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Figure 2. (a) CVs (10 mV/s) of 17.5, 35, and 70 µm thick 20% (w/w) Pt/C + 1% (w/w) Nafion/ethanol film in RT N2-saturated 0.5 M H2SO4. Inset of Fig. 2a shows the anodic Hupd current (0.43, 1.12, and 1.8 mA) at 0.2 V for one-, two-, and four-layer films, respectively, plotted against layer thickness. (b) ORR CV response (10 mV/s) at different rotation rates in 0.5 M H2SO4 at 25 °C for 17.5 µm thick layer. Inset of Fig. 2b shows limiting current at 0.05 V plotted against ω1/2.

TABLE 1: Layer Thicknessa and ORR Tafel Slopeb Obtained for 20% (w/w) Pt/C + 1% (w/w) Nafion/Ethanol Ink layer thickness (µm)

average Tafel slope (mV/dec)

17.5 ( 2.5 35 ( 5 70 ( 10

61 ( 3 65 76 ( 5

a Acquired from SEM images. b Tafel slope obtained in 25 °C 0.5 M H2SO4 at 10 mV/s and 1000 rpm. The 25 µm thick film was evaluated only once, while the one- and four-layer catalysts were each tested three times.

solution (and thus also dissolved O2) is able to fully penetrate the layers and access and react with all of the ORR active Pt sites. Figure 2b shows the ORR CVs (after subtraction of the CV currents in N2-saturated solution) at a 17.5 µm thick 20% (w/ w) Pt/C + 1% (w/w) Nafion/ethanol film. The inset of Figure 2b shows the expected linear relationship6 between the limiting current and ω1/2 for the four-layer catalyst. Importantly, the ORR currents in the kinetic (Tafel) region, which is the primary focus of this paper, are independent of ω, as expected. Table 1 summarizes the ORR Tafel data in the range of 0.85 to 1.0 V, over which a 60 mV Tafel slope is normally reported at smooth Pt electrodes.18 The results show an increase in the Tafel slope from 61 to 76 mV/decade of current density as the layer thickness was increased from 17.5 ( 2.5 to 70 ( 10 µm. The effect of increasing pore resistance with increasing catalyst layer thickness is also seen in the CVs in Figure 2a by the increasing anodic and cathodic Hupd peak separation, consistent with the Tafel data (Figure 2b and Table 1). While a full doubling of the Tafel slope is not seen, it was shown in our previous work7,19 that Tafel slopes should not be expected to abruptly change from their real (e.g., 60 mV/dec) to doubled (e.g., 120 mV/dec) values. Efforts to increase the layer thickness further to reach the doubled Tafel slope conditions were not successful because of poor adhesion of these very thick films on the GC surface. However, it will be shown below that the rather small change in Tafel slope when going from 17.5 to 70 µm thick catalyst layers is exactly what is expected. Effect of Ionic Conductivity of Electrode Layer on ORR Tafel Slopes. To change the catalyst layer conductivity, only the pore solution properties were altered, rather than modifying the solid catalytic phase. This avoided changing other film parameters, such as the layer porosity and intrinsic activity. Thus, in a first approach, the concentration of the H2SO4 solution (and thus the cathode layer pore medium) was altered from 0.005 to 0.5 M (Table 2), using the 20% (w/w) Pt/C + 1% (w/w)

TABLE 2: Conductivity and Corresponding Tafel Slope in Solutions of Varying Conductivity

solution 0.5 M H2SO4 0.01 M H2SO4 + 0.137 M K2SO4a 0.05 M H2SO4 0.01 M H2SO4 + 0.0457 M K2SO4a 0.01 M H2SO4 + 0.0217 M K2SO4a

ionic strength (M)

conductivity (S/m)

Tafel slope (mV/decade of current)

0.64 0.64

9.1 2.56

62 ( 3 87 ( 3

0.088 0.16

2.41 1.24

92 ( 3 103 ( 3

0.088

0.86

118 ( 3

a

Solutions were prepared by the addition of K2SO4 in sufficient quantities to raise the ionic strength of 0.01 M H2SO4 to that of 0.05, 0.1, and 0.5 M H2SO4.

Nafion/ethanol ink. Notably, the ORR rate will decrease with increasing pH, but the mechanism (and hence the theoretical Tafel slope) of the reaction should not change at pH < 2.2.28 The results showed that the Tafel slope increased from 60 mV in 0.5 M acid, to 116 mV in 0.005 M acid. While the Tafel slope now demonstrates a notable dependence on the pore solution resistance, the Tafel slope in the most dilute solutions may not be valid because of double layer effects, which will be encountered in solutions of low ionic strength and can result in distorted Tafel plots.29 Furthermore, significant lowering of the H+ concentration could introduce diffusion limitations, thus complicating data interpretation. Therefore, in a second approach, solutions were made up to have a constant H2SO4 concentration (0.01 M) and the same, known, ionic strength as the 0.05, 0.1, and 0.5 M H2SO4 solutions alone, thus giving different solution conductivities. This was achieved by the addition of appropriate amounts of K2SO4, also taking into consideration the partial dissociation of H2SO4 and of HSO4- as a function of pH. The results of the Koutecky-Levich analysis of the ORR in the activationcontrolled region of the CVs collected in these variable conductivity solutions, as well as in the pure H2SO4 solutions, are summarized in Table 2 (listed in order of decreasing conductivity), while Figure 3 shows a plot of the measured Tafel slope as a function of the solution resistance (inversely proportional to the conductivity). It is seen that solutions of the same ionic strength do not necessarily have the same conductivity, as expected, as H+ has a significantly higher rate of transport vs K+, for example. The ORR Tafel slopes were again measured over the potential range (1.07-0.95 V vs RHE) over which Temkin conditions



Figure 3. ORR Tafel slope vs inverse of solution conductivity. All Tafel slopes were obtained at 25 °C at 1000 rpm and 10 mV/s.

normally apply (first ET step is slow) and thus a slope of 60 mV/dec is expected.10,11 For the 0.01 M H2SO4 solutions, Table 2 shows that, as the solution conductivity (and hence the pore conductivity) is lowered, the Tafel slope increases, as predicted by porous electrode theory (eqs 5 and 6).7 Table 2 also shows that the solution with the lowest conductivity (0.86 S/m) shows a close to doubled Tafel slope of 118 mV/dec. The conductivity of the pore solution clearly has a significant effect on the experimental Tafel slope for the ORR. As the proton and oxygen concentrations were kept constant in most of these experiments, with solution conductivity being the only variable, this rules out any changes in the diffusional conditions. These data therefore show clearly that ohmic limitations are the primary origin of the distributed potentials within the pores of the Pt/C layers under the present experimental conditions, leading to larger (up to two times) Tafel slopes than mechanistically predicted. It is believed that the observed changes in the Tafel slope are not related to a change in the rate-determining step of the ORR. In fact, it has been previously shown28 that, over the pH range studied here, the mechanism of the ORR should not change. Also, the fact that the Tafel slope acquired in the 0.01 M H2SO4 + 0.137 M K2SO4 solution matches so closely with the Tafel slope obtained in 0.05 M H2SO4 (solutions of nearly equal conductivity) argues that the rate-determining step does not change as the pH increased. Effect of Exchange Current Density (% Pt) on ORR Tafel Slope. Equation 5 predicts that, in the case of distributed potentials in porous layers, the Tafel slope will increase as the current (proportional to io) in the activation-controlled region increases. The only way to alter the intrinsic io value for the ORR, per unit real surface area, is to change the catalytic material employed. However, this would cause significant changes in the three other catalyst layer characteristics. Therefore, in this work, io was changed by varying the Pt content between 5 and 40 wt % Pt/C (io should increase by a factor of 8), keeping all other factors constant. All of the catalysts were purchased from Alfa Aesar, with the exception of the 5% (w/w) Pt/C, which was prepared by a 4-fold dilution of the 20% (w/w) Pt/C catalyst with Vulcan XC-72 carbon. It is known that catalyst particle size can have a significant effect on the kinetic parameters of the ORR,30 as different crystal faces of Pt, with different ORR activities, are dominant for different crystallite sizes.31 However, powder X-ray diffraction data (not shown) were collected on all of the catalyst materials studied here, showing that there was no significant difference in Pt crystallite size (all were around 10 -12 nm). Figure 4 shows that the CV currents and the Hupd charges for the 20% (w/w) Pt/C in N2-saturated 0.05 M H2SO4 are approximately 4 times higher than for the 5% (w/w) Pt/C (26.3 mC/cm2 vs 6.8 mC/ cm2), and thus the 5% Pt concentration can be trusted.

Figure 4. CVs (10 mV/s, 0 rpm) of 5% (w/w) Pt/C and 20% (w/w) Pt/C catalyst in N2-saturated 0.5 M H2SO4 at 25 °C. The current generated by the 20% (w/w) Pt/C (0.53 mA) at 0.2 V is ∼4 times larger than that for the 5% (w/w) Pt/C (0.13 mA). Shown in the inset is a plot of the oxygen reduction activation-controlled current at 0.9 V vs Pt content.

Figure 5. Tafel plots of the ORR at 5, 10, 20, and 40 wt % Pt/C in O2-saturated, 25 °C, 0.05 M H2SO4 at 10 mV/s and 1000 rpm.

In O2-saturated 0.05 M H2SO4, the inset of Figure 4 shows that the ORR currents at 0.9 V scale linearly with Pt content, as expected. This demonstrates that all of the active Pt sites are fully accessible. Figure 5 shows that the Tafel slopes for the ORR at the 5, 10, 20 and 40% (w/w) Pt/C catalyst layers are 62, 77, 84, and 97 mV/decade of current, respectively. In accordance with porous electrode theory, the larger ORR currents at catalyst layers with higher Pt content produce a larger potential drop through the pores and thus result in a higher Tafel slope. Similar results were obtained by Higuchi et al.,32 who suggested that, at high Pt loadings, the Pt deeper in the catalyst layer may not be as active as Pt in the outer regions of the layer. These results show that even the 10% Pt/C electrode is giving a higher Tafel slope than what is mechanistically predicted in 0.05 M H2SO4. As these are conditions that could be encountered in practice within a catalytic layer operating at sufficiently high currents, this clearly demonstrates the importance of controlling porous layer morphology in order to keep the Tafel slope at its minimum value. Effect of Pore Radius on ORR Tafel Slope. Controlled modification of the radius of the pores in the Pt/C/Nafion films proved to be challenging, especially without altering any of the other porous layer parameters. As previous work6 has demonstrated that decreasing the particle size (by mortar/pestle grinding) of non-Pt-based ORR catalysts can lead to an increase in the Tafel slope, presumably due to a concomitant decrease in pore size, the 20% (w/w) Pt was first ground using a mortar and pestle in an effort to decrease particle and pore size. However, these efforts produced only a minor effect on the Tafel

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Figure 6. Koutecky-Levich plots for the ORR at ground (mortar and pestle) and unground 20% (w/w) Pt/C + 1% (w/w) Nafion/ethanol 17.5 µm thick films in O2-saturated, 25 °C, 0.5 M H2SO4, at 10 mV/s and 1000 rpm.

TABLE 3: Average ORR Tafel Slopea and BET Surface Area of Commercial Unground and Ground 20% (w/w) Pt/C Catalyst Material catalyst

BET surface area, m2/g

average tafel slope (mV/dec)

not ground 20% (w/w) Pt/C ground 20% (w/w) Pt/C

166 ( 4 176 ( 3

61 ( 3 68 ( 9

a

CVs collected in 25 °C 0.5 M H2SO4 at 10 mV/s and 1000 rpm.

slope, increasing it from 60 to 68 mV (Figure 6 and Table 3). This is consistent with the BET results shown in Table 3, indicating that these grinding procedures produced only a slight increase in the catalyst real surface area, and thus the particle/ pore sizes also increased by only a small amount. Therefore, another approach, involving the use of a more viscous binder to decrease the pore size, was taken. It has been reported33 that the addition of glycerol to standard Pt/C Nafion inks helps to keep the components of the ink dispersed and allows the Pt/carbon catalyst to be painted (vs aliquoted) onto the electrode surface, thus creating a more uniform film.34 It has also been recently reported that the addition of glycerol causes a decrease in pore volume by decreasing the pore radius.34 In the same work, it was shown that, although glycerol is soluble in water, it remains in the film pores even when the film is immersed in room temperature, aqueous solutions. For these reasons, the ORR Tafel behavior of the glycerol-containing Pt/C films was compared against those obtained using conventional Nafion/ethanol-based Pt/C films. It is known that the amount of Nafion used for catalyst film preparation can change the kinetic parameters of the ORR, as this greatly affects proton conductivity.25,32,35 However, this is not a factor in the present work, as the limiting currents for the ORR are essentially the same for the glycerol- and Nafion-based films, showing no change in the diffusional properties of the layers. The top-down SEM images of a standard Pt/C film formed using 1% (w/w) Nafion/ethanol (Figure 7a) vs the glycerol-based films (Figure 7b) show that the latter is significantly more uniform than the former, as predicted. The cross-sectional SEM analysis shows that a single glycerol-based 20% (w/w) Pt/C layer is around 20 µm thick (Figure 8), which is very similar to the normal one-layer film. To collect the ORR Tafel data, a glycerol-containing film was deposited on a GC substrate, giving a Pt loading of 0.52 mg/cm2, which is roughly 5 times larger than the 0.11 mg Pt/ cm2 loading resulting from the deposition of the standard Nafion/

Figure 7. Top down SEM images of (a) a Nafion/ethanol-based and (b) a glycerol-containing Nafion/ethanol-based 20% (w/w) Pt/C film (weight ratio of 1:5:20 carbon/water/glycerol20). The images show that the normal Nafion/ethanol film is quite cracked, whereas the glycerolcontaining film is very smooth.

Figure 8. SEM cross-sectional view of the glycerol-containing 20% Pt/C film on a glass slide. The film appears less porous than the standard Nafion/ethanol films (see Figure 1) and also adheres better to glass.

ethanol based Pt/C film catalyst. However, despite the higher Pt loading for the glycerol-based film, the Hupd charges (and hence the active Pt area) for the two catalysts in N2-saturated 0.5 M H2SO4 (Figure 9) are very similar. This indicates that a significant amount of the Pt in the glycerol film is blocked and is not accessible to the solution, consistent with studies that showed that glycerol can block active sites and lower electrode


Figure 9. CVs (10 mV/s) of 17.5 µm thick 20% (w/w) Pt/C for standard Nafion/ethanolic films and glycerol-containing films in N2saturated 25 °C 0.5 M H2SO4.

Figure 10. Koutecky-Levich plots for ORR at 20% (w/w) Pt/C in standard Nafion/ethanol films and in Nafion/ethanol-glycerol containing films in O2-saturated, 25 °C, 0.5 M H2SO4 at 10 mV/s and 1000 rpm.

performance.34 The use of glycerol may also have improved the hydrophilicity of the catalyst layer and thus enhanced the ORR kinetics. However, in the present work in sulfuric acid, as the CV signal is significantly smaller (per mass of Pt) for the glycerol- vs the Nafion-based films, glycerol does not seem to be having this effect and thus is likely impacting only on catalyst layer pore size. Figure 10 shows that the Tafel slope is substantially higher for the glycerol-containing film vs the standard Nafion/ethanolbased films, likely due to a decrease in the pore radius.34 Therefore, these results confirm that decreasing the average pore diameter causes a more pronounced distribution of potentials inside the porous layer, thus leading to the predicted higher Tafel slopes (Figure 10). Comparison of Theoretically Predicted and Experimental Tafel Data for the ORR at Pt/C Electrodes. Earlier porous electrode theory developed by de Levie12 provides a means of generating Tafel plots (and Tafel slope values) assuming the presence of a migration (ohmic)-induced potential distribution in a single pore and using a reasonable estimate of the four porous film properties, L, r, κ, and io. Therefore, our goal was to determine if the predicted Tafel slopes would change in the same way and to the same extent as seen in our experimental data (e.g., Figures 3, 5, 10) when the film properties were altered, while keeping all other reaction conditions fixed. However, in the present work, the exact value of only one parameter, κ, the bulk solution conductivity, is known with certainty, and it is assumed to be the same as the pore solution conductivity. For L and io, only the relative change in their values could be controlled experimentally, achieved by varying the film thickness by a factor of up to 4 and the Pt loading by a factor of up to 8, respectively. In terms of r, neither its exact value nor the extent of its change is known with precision, and only ORR


Figure 11. Theoretical and experimental Tafel plots for the ORR at 5 and 40 wt % Pt/C. The thinner lines depict the Tafel plot generated using eqs 5 and 6, while the thicker lines represent the Tafel data obtained experimentally. The parameters used to calculate the theoretical Tafel data are r ) 0.5 µm, κ ) 2 S/m, and L ) 20 µm. io was varied from 2.5 × 10-10 to 2 × 10-9 A/cm2 to reflect the change in Pt loading from 5 to 40 wt %. The experimental data were collected in O2-saturated 25 °C 0.05 M H2SO4 at 10 mV/s and 1000 rpm.

Tafel data for catalyst layers which can be described as having smaller (glycerol-containing films) vs larger (no glycerol) r values were collected. Therefore, for the initial calculation, a single set of approximate porous layer properties was selected. Based on the solution conductivity measurements, the κ value for 0.5 M H2SO4 is 2 S/m. From the SEM and Tafel data for the 20% (w/w) Pt/C/Nafion ink (no glycerol) in 0.5 M H2SO4, the other parameter values were a 20 µm pore length, an io value of 1 × 10-9 A/cm2, and a pore radius estimated at 0.5 µm. Using these film property values, the current (from only a single pore) was calculated as a function of potential using eqs 5 and 6. The currents were then multiplied by a constant scaling factor to overlay them with the measured Tafel data for this particular film. This best-fit factor, found to be 5 × 106, may represent a combination of the real porosity and tortuosity of these films. This factor was then held constant for all of the films examined in the present work (changing L, io, and κ, while the effect of changing r could not be determined). Notably, the precise value of the scaling factor has no impact on the theoretical Tafel slope obtained or on the potential at which the transition from normal to higher Tafel slope occurs and is therefore not critical to the outcome of this analysis. The use of the scaling factor simply allows a best-fit to be obtained between the calculated and experimental Tafel currents. Figure 11 shows that the theoretical Tafel plot gives a 60 mV Tafel region at low currents (overpotentials) and a doubled 120 mV region at higher currents, as predicted from porous electrode theory for the situation where the first electron transfer step is rate limiting.7,12,18 Significantly, a very good match between the theoretical and experimental Tafel data is seen for the ORR at the 5 and 40% Pt/C catalysts in O2-saturated, 25 °C, 0.05 M H2SO4 (taken from Figure 5), with the only variable in this case being io. It is unfortunate that the experimental Tafel data could not be collected over a wider range of potentials to more fully test the validity of the model employed here. However, at higher overpotentials, diffusional limitations were seen to commence, while at lower overpotentials, only small (and noisy) currents were observed, primarily due to the onset of Pt oxide formation at potentials above ∼0.95 V. Calculations of the kind shown in Figure 11 were also carried out for the variable layer thickness (pore length) and κ data,

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TABLE 4: Summary of Theoretical and Experimental Tafel slopes for the ORR at Pt/C Catalyst Layers of Varying Pt Loading, Thickness, or Conductivity Effect of %Pt on Tafel slopesa % Pt (on C) experimental Tafel slope (mV/dec) theoretical Tafel slope (mV/dec)

5 62 ( 3 62

Effect of Layer Thickness on Tafel slopesb layer thickness (µm) 17 ( 2.5 experimental Tafel slope (mV/dec) 61 ( 3 theoretical Tafel slope (mV/dec) 61 Effect of Solution Conductivity on conductivity experimental Tafel slope mV/dec theoretical Tafel slope mV/dec

10 77 ( 3 76

20 84 ( 3 81

40 97 ( 3 95

2.41 89 ( 3 85

9.1 61 ( 3 66

70 ( 10 76 ( 5 72

Tafel slopesc 0.86 1.24 118 ( 3 104 ( 3 102 93

a Parameters assumed in generating theoretical Tafel plot are r ) 0.5 µm, κ ) 2 S/m, L ) 20 µm, and io from 2.5 × 10-10A/cm2 to 2 × 10-9 A/cm2 (reflecting change in Pt loading from 5 to 40 wt %). b Parameters assumed in generating theoretical Tafel plot are r ) 0.5 µm, κ ) 9.1 S/m, io ) 1 × 10-9A/cm2, and L from 20 to 70 µm (reflecting changing layer thickness). c Parameters assumed in generating theoretical Tafel plot are r ) 0.5 µm, io ) 1 × 10-9. A/cm2, L ) 20 µm and κ from 0.86 to 9.1 S/m (reflecting change in solution composition and hence conductivity).

while the pore radius data could not be simulated, as the difference in the average pore radius for the glycerol- and nonglycerol-containing films remains unknown. It is shown in Table 4 that the theoretical and experimental Tafel slopes in the regions of overlap track each other very closely, especially as the % Pt content and the film thickness is varied. Overall, the experimental and theoretical (eqs 5 and 6) Tafel slopes match very closely for a total of 10 different films (Table 4, showing three of the film parameters being controllably altered). This is very strong support for the suggested cause of the increasing Tafel slopes being the ohmically induced distributed potentials within the catalyst layer pores under the conditions of our experiments. This clearly shows the significant impact of the physical and morphological characteristics of the catalyst layer on performance, particularly on the experimentally obtained Tafel slopes and thus on the reported transfer coefficients. Conclusions The Tafel slopes associated with the oxygen reduction reaction (ORR) at Pt/C/Nafion catalyst films, deposited on glassy carbon rotating disk electrodes, were measured in room temperature O2-saturated sulfuric acid solutions. The primary goal of this work was to controllably alter catalyst layer properties (pore length, pore radius, layer conductivity, and layer activity) in order to demonstrate their impact on the measured Tafel slope and also to compare these effects with what is predicted from a model based on migration-induced distributed potentials within porous electroactive layers. While diffusional limitations are also likely to be present in porous layers, we have selected experimental conditions that should minimize their impact, and thus diffusional effects have not been included in the present treatment. The effect of pore length was determined by increasing the thickness of a 20% (w/w) Pt/C + 1% (w/w) Nafion/ethanol catalyst layer from 17.5 to 70 µm, assuming constant layer tortuosity. This caused the ORR Tafel slope to increase from the expected value of 61 ( 3 mV/dec to 76 ( 5 mV/ dec of current, as predicted by the model. The conductivity of the catalyst layer was changed by varying the properties of the solution in the pores, rather than changing the catalyst material itself. It was determined that decreasing the conductivity of the

H2SO4 solution from 9.1 to 0.86 S/m caused the Tafel slope to approximately double in value. The impact of the activity of the catalyst layer was also examined, showing that quadrupling of the Pt content leads to four times the active Pt area and a change in Tafel slope from 62 ( 3 to 97 ( 3 mV/dec. These results correlate well with the predicted change in Tafel slope calculated using eqs 5 and 6. Finally, decreasing the pore radius through the use of glycerol as a binding agent was found to increase the Tafel slope, also as predicted from theory. It was also determined that the glycerol-containing 20% (w/w) Pt/C film is more uniform than the standard 20% (w/w) Pt/C + 1% (w/w) Nafion/ethanol film. The experimental Tafel slopes for 10 different films (produced by controllably varying the layer thickness, conductivity and Pt content) were then compared to those calculated from eqs 5 and 6. The excellent match shows that, under the conditions of our experiments, the ORR Tafel slope will be increased above its mechanistically expected 60 mV value strictly by varying the Pt/C catalyst layer properties. While these increased Tafel slopes are clearly undesirable, as they lead to lower cathode performance, these results also clearly demonstrate that caution must be exerted in interpreting observed Tafel slope values at porous electrodes using reaction mechanism considerations only. Acknowledgment. We gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support of this work, and the Alberta Ingenuity Fund for scholarship support of D.B. SEM imaging was performed at the Microscopy and Imaging Facility at the University of Calgary. The authors also thank Dr. Max Cimenti and Prof. Josephine M. Hill (University of Calgary) for assistance with the BET surface area measurements. References and Notes (1) Barbir, F. PEM Fuel Cells: Theory and Practice; Elsevier Academic Press: New York, 2005. (2) Darling, R. M.; Meyers, J. P. J. Electrochem. Soc. 2003, 150, A1523. (3) Chan, K. Y.; Ding, J.; Ren, J.; Cheng, S.; Tsang, K. Y. J. Mater. Chem. 2004, 14, 505. (4) Soderberg, J. N.; Sirk, A. H. C.; Campbell, S. A.; Birss, V. I. J. Electrochem. Soc. 2005, 152, A2017. (5) Sirk, A. H. C.; Campbell, S. A.; Birss, V. I. J. Electrochem. Soc. 2008, 155, B592. (6) Bockris, J. O. M.; Reddy, A. K. N. Modern Electrochemistry: An Introduction to an Interdisciplinary Area; Plenum Publishing Corporation: New York, 1977; Vol. 2. (7) Soderberg, J. N.; Co, A. C.; Sirk, A. H. C.; Birss, V. I. J. Phys. Chem. B 2006, 110, 10401. (8) Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electrochem. Soc. 1999, 146, 1296. (9) Meng, H.; Shen, P. K. J. Phys. Chem. B 2005, 109, 22705. (10) Damjanovic, A.; Dey, A.; Bockris, J. O. M. Electrochim. Acta 1966, 11, 791. (11) Sepa, D. B.; Vojnovic, M. V.; Damjanovic, A. Electrochim. Acta 1981, 26, 781. (12) de Levie, R. Electrochemical Respose of Porous and Rough Electrodes; John Wiley & Sons: New York, 1967; Vol. 6. (13) Tilak, B. V.; Venkatesh, S.; Rangarajan, S. K. J. Electrochem. Soc. 1989, 136, 1977. (14) Darling, R.; Newman, J. J. Electrochem. Soc. 1998, 145, 720. (15) Srinivasan, S.; Hurwitz, H. D.; Bockris, J. O. M. J. Chem. Phys. 1967, 46, 3108. (16) Perry, M. L.; Newman, J.; Cairns, E. J. J. Electrochem. Soc. 1998, 145, 5. (17) Kulikovsky, A. A. Electrochem. Solid-State Lett. 2009, 12, B53. (18) Bockris, J. O. M.; Srinivasan, S. Fuel Cells: Their Electrochemistry; McGraw-Hill: New York, 1969. (19) Bockris, J. O. M.; Reddy, A. K. N. Modern Electrochemistry: An Introduction to an Interdisciplinary Area; Plenum Publishing Corporations: New York, 1977; Vol. 2.

Pt/Carbon Catalyst Layer Microstructural Effects (20) Wilson, M. S.; Gottesfeld, S. J. Appl. Electrochem. 1992, 22, 1. (21) Tur’yan, Y. I.; Gorenbein, P.; Kohen, R. J. Electroanal. Chem. 2004, 571, 183. (22) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (23) Gloaguen, F.; Convert, P.; Gamburzev, S.; Velev, O. A.; Srinivasan, S. Electrochim. Acta 1998, 43, 3767. (24) Gloaguen, F.; Andolfatto, F.; Durand, R.; Ozil, P. J. Appl. Electrochem. 1994, 24, 863. (25) Paulus, U. A.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. Electroanal. Chem. 2001, 495, 134. (26) Neyerlin, K. C.; Gu, W.; Jorne, J.; Gasteiger, H. A. J. Electrochem. Soc. 2006, 153, A1955. (27) Frackowiak, E.; Be´guin, F. Carbon 2001, 39, 937. (28) Damjanovic, A.; Brusic, V. Electrochim. Acta 1967, 12, 615.

J. Phys. Chem. C, Vol. 113, No. 23, 2009 10111 (29) Samec, Z.; Kakiuchi, T.; Senda, M. Electrochim. Acta 1995, 40, 2971. (30) Watanabe, M.; Sei, H.; Stonehart, P. J. Electroanal. Chem. 1989, 261, 375. (31) Wikander, K.; Ekstro¨m, H.; Palmqvist, A. E. C.; Lindbergh, G. Electrochim. Acta 2007, 52, 6848. (32) Higuchi, E.; Uchida, H.; Watanabe, M. J. Electroanal. Chem. 2005, 583, 69. (33) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455. (34) Chisaka, M.; Daiguji, H. Electrochim. Acta 2006, 51, 4828. (35) Gojkovic, S. L.; Zecevic, S. K.; Savinell, R. F. J. Electrochem. Soc. 1998, 145, 3713.

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Carbon Catalyst Layer Microstructural Effects on Measured and

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