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Novel Method for Fast Characterization of High-Surface-Area Electrocatalytic Materials Using a Carbon Fiber Microelectrode D. Strmcnik,*,† N. Hodnik,‡ S. B. Hocevar,‡ D. van der Vliet,† M. Zorko,‡ V.R. Stamenkovic,† B. Pihlar,§ and N.M. Markovic† Materials Science DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439, National Institute of Chemistry, HajdrihoVa 19, 1000 Ljubljana, SloVenia, and Faculty of Chemistry and Chemical Technology, UniVersity of Ljubljana, AskerceVa 5, 1000 Ljubljana, SloVenia ReceiVed: September 15, 2009; ReVised Manuscript ReceiVed: December 21, 2009
A carbon fiber microelectrode (CFME) was used for characterization of the nanoparticle catalysts as an alternative to the well-established rotating disk electrode (RDE) method. We found that the novel CFME method yielded comparable results to the RDE method when investigating the adsorption/desorption processes as well the specific activity for reactions such as the oxygen reduction reaction. Its major advantage over the RDE method is a fast sample preparation and rapid measurement, reducing significantly the time of a single sample characterization from 2-3 h to a favorable 5-10 min. Introduction Platinum and platinum bimetallic nanoparticle catalysts have been employed in commercial prototype energy-conversion and chemical production devices since the mid 70s.1,2 As many industrial catalysts, however, the catalysts are put into use long before their structure and properties are clearly understood, and that was certainly the case, for example, for the Pt-based fuel cell catalysts. By design, nanoparticle electrocatalysts possess an extremely high metallic surface area, for example, >105 cm2/g of metal, providing a high number of active sites per unit volume of the electrode. The main focus of the research related to the nanoscale catalysts has been placed on the development of more active, more stable, and less expensive materials that can be employed in catalyst synthesis (see, for instance, refs 3–8). These goals come along with a special set of challenges in obtaining true kinetic measurements for the electrochemical reactions rates. Stonehart and co-workers9,10 were among the first to pursue rigorous measurements of electrochemical kinetics with nanoparticle catalysts. Currently, there are two methods commonly used for routine testing of the electrocatalysts, membrane electrode assembly (MEA) and rotating disk electrode (RDE). In the MEA method, the catalyst is tested under real conditions and is sandwiched between the proton-conducting membrane and diffusion media, which is usually a high-surface-area carbon matrix. The major obstacles of this method are a relatively large amount of catalyst needed, the cost associated with a single experiment (consumption of catalyst and membrane), data analysis (the measured signal is a result of the processes occurring at both cathodic and anodic sides), and the time necessary to assemble the MEA and perform the experiment. In principle, this method is generally used to test the most promising catalysts. These limitations were overcome with a breakthrough in testing methodology that occurred with the introduction of a solublized Nafion. The final refinement which enabled a very thin (e.g., 1-10 µm) layer of the supported catalyst to be “glued” * To whom correspondence should be addressed. † Argonne National Laboratory. ‡ National Institute of Chemistry. § University of Ljubljana.
onto a glassy carbon rotating disk electrode (RDE) was provided by Schmidt et al.11 The RDE method is nowadays the most commonly used method for catalyst testing. Still, this method encounters a variety of problems, which are related mainly to the thickness, adhesion, and homogeneity of the catalyst layer. More importantly, the major drawback of this method is a significant amount of time required for the sample preparation and for measurement. Considering that electrochemical characterization of a single sample takes approximately 2-3 h, it is therefore extremely tedious to test a large number of samples. Nevertheless, the RDE is currently the most reliable method for kinetic measurements of nanoparticle catalysts, and it is the first choice for the laboratory-scale experiments. In addition to MEA and RDE methods, a single-particle submicroelectrode12,13 and microelectrode arrangements with Pt/C catalysts have been reported in the literature as attempts to study the kinetics of fuel cell reactions.14 Although potentially well-designed, these methods do not meet all requirements for catalytic measurements. These include a reliable way of mounting the catalyst into a microelectrode configuration and issues with the transport of reactants and products through the thick catalyst-Nafion layer and contamination problems, particularly in a nonadsorbing electrolyte, for example, 0.1 M HClO4. In general, microelectrodes are most commonly employed as sensors for an almost infinite variety of species from inorganic ions and gases to enzymes and DNA.15–17 However, despite a couple of attempts, no reliable microelectrode method for testing of high-surface-area electrocatalytic materials has been developed. Herein, by using the properties of carbon fiber microelectrodes (d ) 7 µm), we demonstrated an alternative method for determination of specific activities of nanoscale catalysts which enables considerably faster sample preparation and rapid measurement together with negligible consumption of the catalyst material. Furthermore, this method provides results that are within the experimental error of those obtained by the wellestablished RDE method. Presuming that the data acquired by the RDE method are in quantitative agreement with the kinetic factors obtained with a membrane electrode assembly (MEA),18 the utilization of the new carbon fiber microelectrode method
10.1021/jp908939e 2010 American Chemical Society Published on Web 01/22/2010
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Figure 1. SEM micrographs with and without the catalyst.
for a fast catalyst screening is of both fundamental and, in particular, practical importance. Experimental Section Microelectrode Preparation, Catalyst Attachment, and Characterization. The preparation of a single carbon fiber substrate microelectrode is described in detail elsewhere19 and will not be described herein. The attachment of the catalyst onto the 7 µm carbon fiber, as seen in SEM micrographs of a bare and modified carbon fiber microelectrode in Figure 1, was achieved through extinguishing of the diffuse double layer of the suspended particles, which enables the stability of the suspension (the catalyst suspension is prepared by sonicating 1 mg of catalyst per 1 mL of water for 3-5 min). By adding a strong electrolyte (1 M HClO4, ratio 1:1 vs suspension), the thickness of the diffuse part of the double layer is drastically reduced, initiating coagulation. The attenuation of the particles’ charge was monitored with a Muteck particle charge detector using the principle of measuring the streaming potential caused by forcing a suspension of charged particles over two platinum electrodes. The streaming potential is proportional to the ζ (zeta) potential, which is the potential between the bulk of the solution and the plane of shear within the particle’s diffuse double layer. An increase in the electrolyte concentration reduces the diffuse double layer thickness and consequently the zeta and streaming potential, which are measured. For more details, the reader is referred to ref 20; a more detailed discussion will be avoided here since we only used the particle charge detector to demonstrate the qualitative effect of the addition of a strong electrolyte to the suspended particles. In addition, Figure 2 depicts a decrease in the streaming potential with the addition of 1 M HClO4 to the point where coagulation is observed. During this process, the thickness of the catalysts attached to the carbon fiber was rather thin but large enough (i.e., a few ng) for a proper electrochemical measurements. As we pointed out above, a major advantage of this method is a short time required to modify the carbon fiber and to “dry” the catalysts (ca. 1-2 min vs 1-2 h in the case of RDE measurements), a very small amount of catalysts required for electrochemical testing, no mass-transfer resistances through the thin catalyst layer, and no requirement for using the RDE setup, that is, the rotator. Electrochemical Cell. A hanging drop three-electrode electrochemical cell was designed for the electrochemical experiments, as shown in Figure 3. A platinum wire was used as a counter electrode, the reference electrode was a Ag/AgCl low leak flexible electrode (WPI, inc.) embedded in the Teflon syringe, and the working electrode was a carbon fiber micro-
Figure 2. Change of streaming potential with the addition of 1 M HClO4 to 40 mL of catalyst suspension (1 mg/10 mL). Ranges of stability and coagulation are marked with dashed lines.
electrode modified with catalyst. Assuming that the impurities might be the most pronounced concern when performing the electrocatalysis on microelectrodes, special care was taken to minimize the contamination. In particular, all parts in contact with the electrolyte were made of Teflon, and the experiments were carried out in a droplet of ∼50 µL of electrolyte of 0.1 M HClO4 (EMD). The Teflon capillary with the hanging droplet was encased in a glass tube purged by a desired gas, for example, argon, hydrogen, or oxygen. Electrochemical Measurement. A carbon fiber microelectrode modified with the catalyst was inserted into the hanging droplet. Two carbon-supported catalysts were used in this study, a 5 nm Pt catalyst and a Pt3Co alloy catalyst, both provided by TKK (Tokyo, Japan). Voltammetric measurements were performed under an argon atmosphere using a scan rate of 100 mV/s. The active area of the catalyst was measured based on the HUPD charge. The gas was then switched to oxygen or hydrogen. It took ∼30 s to reach saturation, again significantly faster compared to the RDE method, where it usually takes 20 min to reach the saturation. The polarization curve was recorded using the same sweep rate, the capacitive currents were subtracted, and a correction was done for mass transport using a well-known relation (eq 1), thus enabling extraction of the so-called kinetic currents.
iK )
iiL iL - i
(1)
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Figure 3. The apparatus for measuring the catalyst properties. The catalyst is applied on the surface of a single carbon fiber microelectrode. The Pt counter electrode (CTR) is located at the bottom of the electrolyte droplet. The reference electrode (REF) is located in the syringe.
(ik stands for kinetic current, i is the measured current, and iL is the diffusion limited current. Note that due to differences in catalyst loading and distribution, the diffusion limiting currents did vary significantly. However, the so-called kinetic current density did not depend on the loading or the distribution of the catalyst on the surface of the carbon fiber. After the correction for mass transfer and calculation of the specific activity (kinetic current per active surface area obtained by integration of the HUPD charge), the values revealed reasonable accuracy, as shown in the Results and Discussion section. Care must be taken, though, that the current values are not taken too close to the diffusion-controlled potential region, where the errors are bigger and give less accurate values. RDE Measurements. The experimental procedure for the RDE measurements have been described in detail elsewhere;21,22 thus, we describe it only briefly at this point. A volume of 20-40 µL of the catalyst suspension was pipetted onto a polished glassy carbon substrate (6 mm diameter, 0.283 cm2 geometrical surface area), leading to a Pt loading of ∼15 µgPt/ cm2. After the evaporation of the water in an argon stream, the electrode was transferred (with protection of the surface via a drop of ultrapure water) into the electrochemical cell and immersed, under potential control at 0.05 V, in an argonsaturated 0.1 M HClO4 solution. The potential was then continuously cycled between 0.05 and 1.0 V until a stable cyclic voltammogram was recorded. For the oxygen reduction reaction (ORR), the electrolyte was purged with oxygen (Airgas), the measurements were performed at 293 and 333 K, and the specific current density was calculated at 0.9 V.
To demonstrate that the CFME method can be as reliable as the RDE method for probing adsorption and catalytic properties of the high-surface-area catalysts, we summarized in Figure 4 cyclic voltammograms and polarization curves for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) on Pt and Pt3Co catalysts in 0.1 M HClO4. As shown in Figure 4a and b, the voltammetric features obtained on catalysts attached to the CFME and RDE are almost identical. For example, no significant difference was obtained in the adsorption/desorption of HUPD (0.05-0.4 V) as well as the adsorption of oxygenated species above 0.7 V and their reduction (0.6-0.9 V). Table 1 summarizes the positions of the characteristic voltammetric features obtained by both methods. Almost exactly the same values (mostly within 10 mV) for both methods indicated that the catalyst behaved very similarly in both cases. This was an important observation assuming that the integration of the HUPD charge is usually used to estimate the active surface area of the catalyst.18,21,22 In turn, this value was used for evaluating the specific activity (per real surface area) of Pt and Pt3Co catalysts. Although specific examples for the ORR and the HOR are summarized in Figure 4, for our purposes here, we will focus on quantitative evaluation of kinetic parameters for the ORR; the results for the HOR are used solely to demonstrate a qualitative similarity in the polarization curves obtained by CFME and RDE methods. From Figure 4c and d, polarization curves obtained on catalysts attached to the CFME and RDE revealed substantial differences. However, these differences could be attributed mostly to the geometry differences of the two systems, RDE being planar and the carbon fiber microelectrode being cylindrical. Consequently, the mass transport was much faster in the case of the CFME compared to that for the RDE, which extended the kinetically controlled potential region significantly.23 Furthermore, the gray dashed lines, which represent the ORR polarization curves on the 5 nm Pt catalyst, showed that the diffusion-limited currents varied in the case of the CFME (as explained in the Experimental Section) as opposed to those for RDE. Note, however, that this did not present a lack of reliability of the CFME method. To corroborate this presumption, we summarized the mass-transport-corrected current densities at 0.9 V for both methods in Table 1. This potential was chosen because it has been used in the past as a benchmark potential for evaluation of the specific activity of the ORR on different catalysts.18,22 Notice that the activity values obtained by the CFME method were slightly lower (ca. 10%) than those measured with the RDE. For example, the current densities at 0.9 V for the 5 nm Pt catalyst were 0.59 and 0.64 mA/cm2 for the CMFE and RDE, respectively. For Pt3Co catalyst, the corresponding values were 2.0 and 2.2 mA/cm2 for the CMFE and RDE methods, respectively. The standard deviation for the CFME method was 12-15%, that is, slightly higher than that obtained with the RDE (8-9%), making CFME somewhat less accurate. However, this is a negligible compromise for the time gained with the new method. Note also that the mean activity values at the RDE were within the standard deviation interval of the mean values obtained with the CFME and vice versa; therefore, we can conclude that the two methods yielded practically identical results. The fact that the CFME can be used as a reliable method for evaluating kinetic parameters of Faradic reactions on highsurface-area catalysts is of both fundamental and technical importance. While from a fundamental point of view it is
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Figure 4. Comparison of the 5 nm Pt and Pt3Co catalyst measured with the RDE (left) and CFME (right). HOR and ORR polarization curves are presented in red and blue, respectively. Cyclic voltammograms are presented in black. The dashed gray curves are the ORR polarization curves for 5 nm Pt catalyst.
TABLE 1: Positions of Characteristic Voltammetric Features, Kinetic Currents, and Standard Deviations for the ORR Obtained with the RDE and CFME Methods for the Pt and Pt3Co Catalysta catalyst/method 5 nm Pt RDE 5 nm Pt CFME Pt3Co RDE Pt3Co CFME
HUPD-A peak 145 mV 135 mV n/a n/a
HUPD-C peak
oxide-A peak
108 mV 119 mV n/a n/a
oxide-C peak
n/a n/a 882 mV 882 mV
762 mV 753 mV 734 mV 746 mV
IK @ 0.9 V 2
0.64 mA/cm 0.59 mA/cm2 2.2 mA/cm2 2.0 mA/cm2
sd
Rsd
0.05 0.07 0.2 0.3
0.08 0.12 0.09 0.15
a Capital letters A and C designate the anodic and cathodic direction of the scan, respectively; sd is the standard deviation of the kinetic current IK, and Rsd is the relative standard deviation.
important that it is indeed possible to use the properties of a microelectrode for analyzing the polarization curves for Faradic processes, the fact that this method is much faster, but equally reliable as the RDE method, opens new opportunities for fast screening of several catalysts which are of great technological importance.
Conclusion A novel carbon fiber microelectrode (CFME) method is presented as an alternative to the existing RDE method for characterization of the nanoscale catalysts. The basis of this new protocol is the use of a single carbon fiber microelectrode. The
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adhesion of the catalyst to the surface of the carbon fiber was realized by inducing coagulation of the suspended particles by means of a strong electrolyte (1 M HClO4). In this way, the microelectrode coated with the selected catalyst is ready for measurement within a few minutes. The following electrochemical characterization could be accomplished for various reactions in a rather short period of time. The whole process can take as little as 5-10 min per catalyst, which significantly simplifies characterization protocol and substantially reduces the time required for kinetic analysis. The CFME method exhibits almost exactly the same voltammetric results and activity values for the ORR as the RDE methods, with slightly higher standard deviation values. The CFME method offers great possibilities for fast screening of the nanocatalysts as well as a unique opportunity for automated characterization of the novel catalysts in the future. Acknowledgment. This work was supported under Contract No. DE-AC02-06CH11357 by the University of Chicago and Argonne, LLC, Operator of Argonne National Laboratory, and the U.S. Department of Energy. References and Notes (1) Wieckowski, A., Savinova, E. R., Vayenas, C. G., Eds.; Catalysis and Electrocatalysis at Nanoparticle Surfaces; Marcel Dekker: New York, 2003. (2) Vielstich, W.; Yokokawa, H.; Gasteiger, H. A. Handbook of Fuel Cells. Fundamentals and Technology, Vol. 5: AdVances in Electrocatalysis, Materials and Durability: Part 1; John Wiley & Sons: Chichester, U.K., 2009.
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