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C-supported Pt nanoparticles
Electrochimica Acta 56 (2010) 418–426
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Electrocatalysis of the hydrogen oxidation in the presence of CO on RhO2 /C-supported Pt nanoparticles K.S. Freitas, P.P. Lopes, E.A. Ticianelli ∗ Instituto de Química de São Carlos, USP, C.P. 780, São Carlos, SP 13560-970, Brazil
a r t i c l e
i n f o
Article history: Received 13 May 2010 Received in revised form 26 August 2010 Accepted 27 August 2010 Available online 17 September 2010 Keywords: Hydrogen oxidation CO oxidation Oxide support RhO2 MS on line
a b s t r a c t This work presents a study on the kinetics of the hydrogen oxidation reaction (HOR) in the absence and in the presence of CO in ultra thin porous layer and in PEM fuel cell electrodes formed with Pt supported on RhO2 /C substrates. Together with the electrochemical measurements, the structural and electronic properties of these catalysts were characterized, enabling to correlate their structural and electronic properties with the HOR kinetics. The results show that the presence of Rh oxides leads to an emptying of the Pt 5d band and a consequent reduction of the back-donation of electrons from Pt to CO, weakening the Pt–CO bond and diminishing the CO degree of coverage on Pt, leaving more sites available to HOR. These changes in the electronic spectra do not lead to any perceptible change in the kinetics or the reaction of pure hydrogen. Also, the formation of CO2 monitored by the MS experiments in the fuel cell anode outlet indicates that the bifunctional mechanism is also operative, but the major CO tolerance is achieved by the electronic effect induced by the RhO2 support. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction HOR involves the transfer of 2 electrons per hydrogen molecule on the polycrystalline Pt electrode, which can occur in several stages through different mechanisms [1–3]. The kinetic processes of hydrogen adsorption and oxidation are believed to follow the steps of the H2 dissociative adsorption (Tafel reaction) followed by the oxidation of adsorbed hydrogen (Volmer reaction): k1
H2 + 2Pt←→2Pt–H k2
Tafel
Pt–H←→Pt + H+ + e−
Volmer
(1) (2)
According to this mechanism, the current of the hydrogen oxidation is produced exclusively by the Volmer step and a large current density leads to a significant consumption of adsorbed hydrogen. When the fuel contains small amounts of CO, the active sites for hydrogen adsorption are limited only to the vacancies left by the CO adsorbed layer, making the Tafel reaction the rate determining step (rds). Thus, the effect of lowering the CO steady-state coverage will increase the magnitude of the hydrogen electro-oxidation current. Besides lowering the CO content in the hydrogen flow, there are basically two other mechanisms to decrease the Pt–CO surface cov-
∗ Corresponding author. Tel.: +55 0 16 3373 9945; fax: +55 0 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.059
erage, both related to the addition of another metal in contact with Pt. The first is related to using a metal that affects the electronic properties of the Pt atoms in order to lower the occupancy of the 5d band. The electronic influence on the CO adsorption at Pt sites has also been assigned to a down-shift of the Pt 5d band center, mainly caused by the lattice mismatch and the electronic interaction between Pt and other metals. This would weak the Pt–CO interaction, which lower the CO coverage. The second is the bifunctional mechanism which consists of the oxidation of adsorbed CO by oxy/hydroxy species formed on the second metal sites at lower potentials than observed on pure Pt. The second element in the alloy must enable the formation of the oxidized surface at low potentials, which on the other hand can corrode the alloy faster, due to its lower oxidation potential than Pt. PtRh supported alloy has been studied concerning the molecular hydrogen oxidation and CO tolerance [4], and the results showed found that the catalyst with 24% of Rh shows the best performance for the hydrogen contaminated with CO. This increased activity is associated to the excess of unpaired electrons per atoms in the alloy when compared to pure platinum. Several works in the literature have observed that Pt combined with metal oxides have greater performance as a CO-tolerant anode when compared to pure Pt. This increase in activity has been attributed to the greater availability of active sites for hydrogen adsorption in Pt-transition metal catalysts. Within this context, various transition metal oxides have been proposed as CO tolerant, such as RuOx , SnOx , VOx , WOx and MoOx [5–8]. For Pt supported in oxide substrates, for instance RhO2 /C, the increased activity
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for CO oxidation may be due to the fact that these materials are more able to form oxygenated species (metal-OH), increasing the rate of CO oxidation by the Langmuir–Hinshelwood mechanism (bi-functional mechanism), and leading to an intrinsically lower surface coverage, thus increasing the surface vacancies necessary for HOR. This study evaluates the performance of platinum supported RhO2 /C materials, for the hydrogen oxidation reaction in the absence and in the presence of CO, in acidic medium. The electrocatalysts were studied by EDX, XRD and in situ XAS to characterize the structural and electronic properties and the composition. The electrochemical investigations were carried out in two steps, the first is an evaluation of the kinetics of HOR using the ultra thin porous coating layer in a rotating disk electrode (RDE), and the second is a direct use of the materials in PEMFC anodes fed with H2 and H2 /CO. The cell measurements consisted of determining the steady-state polarization response, having an on-line mass spectrometer (on-line MS) coupled to the anode gas outlet, in order to investigate and understand the CO oxidation mechanism.
2. Experimental 2.1. Preparation of RhO2 /C support The RhO2 support was prepared using a sol–gel method, similar to that previously described [9,10]. The Rh(III) acetylacetonate (Alfa Aesar) was dissolved in ethanol and then the adequate amount of acetic acid was added. The carbon black (Vulcan XC-72R) was added and then the sol–gel solution was subjected to an ultrasonic treatment (FALC Ultrasonic model 06) for 5 min (homogenization step). After that, a short heat treatment was performed to remove the residual solvents and the polymeric chains, during which the xerogel is degraded to form Rh oxide after the elimination of organic residues and water. A final 2-h densification treatment was conducted at the same temperature used in the previous step (400 ◦ C), under argon atmosphere. Three distinct RhO2 contents were prepared for several contents of the carbon support, to reach 5%, 10% and 15% (w/w) of RhO2 /C. 2.2. Incorporation of the Pt nanoparticles in the RhO2 /C supports The Pt/RhO2 /C catalysts were prepared by the chemical reduction of Pt ions using the formic acid method [11]. For this purpose, a solution was prepared by mixing a formic acid solution, water and the previously prepared support. The solution was treated in an ultrasonic bath to form a uniformly dispersed ink. The solution was then kept at 80 ◦ C and the platinum salt (hexachloroplatinic acid, H2 PtCl6 Alfa Aesar) was slowly added under stirring. Next, the solution was cooled to room temperature and a washing–filtering procedure was performed to eliminate any contamination from the dissolved salts in the final product. The filtered powder was dried at 100 ◦ C for 2 h. The Pt content with respect to carbon was kept at 20% (w/w) Pt/C for all RhO2 /C supports. For the sake of simplicity, the distinct Pt/RhO2 /C electrocatalysts will be differentiated by the rhodium content. 2.3. Characterization The X-ray diffraction (XRD) experiments were conducted using a Rigaku-Rotaflex Mod. Ru-200B and a Rigaku Ultima IV equipment, with CuK␣ radiation at a scan rate of 2◦ min−1 in the range of 20 and 100◦ to characterize the Pt/RhO2 /C catalysts. The Scherrer’s equation was used to estimate the crystallite size [12,13]; the lattice parameter for the fcc structure was calculated using the following
419
equation:
a=
h2 + k2 + l2 . 2 sin
(3)
The atomic ratios of the Pt/RhO2 /C electrocatalyst were obtained by energy dispersive X ray (EDX) analysis using a Zeiss Digital Scanning Electron Microscope DSM 960 with a Link Analytical microanalysis (QX 2000), working with the electron beam acceleration potential difference of 63 kV. The measurements of in situ X-ray absorption spectroscopy (XAS) were performed at the Pt L3 absorption edge (11,564 eV), using an appropriate spectroelectrochemical cell [14]. The Pt L3 absorption edge is related to electronic transitions of the 2p3/2 orbital to the 5d band (5d5/2 states), and the magnitude of the white-line located at 5 eV above edge is closely related to the occupation of the 5d band. For these measurements the working electrodes were formed with the catalyst material dispersed with Nafion® (ca. 35 wt.%) and isopropyl alcohol, containing about 7 mg cm−2 of Pt loading. The measurements were made at several electrode potentials, referenced to the RHE (reversible hydrogen electrode). The counter electrode was formed by a platinum screen. Both the cell and the counter electrode have a window in the center to allow free passage of the X-ray beam, which only crosses the working electrode. All experiments were conducted at the D04B XAFS beam line in the Brazilian Synchrotron Light Laboratory (LNLS), Brazil. The data acquisition system for XAS measurements comprised three ionization detectors (incident Io , transmitted It and reference Ir ). The reference channel was used primarily for the internal calibration of the edge positions by using a pure foil of the metal. Nitrogen was used in the Io , It and If chambers. Due to the low critical energy of the LNLS storage ring (2.08 keV), third-order harmonic contamination of the Si(1 1 1) monochromatic beam is expected to be negligible above 5 keV [15].
2.4. RDE measurements These experiments used a conventional one-compartment glass cell with a Luggin capillary, which were conducted at room temperature (25 ± 1 ◦ C). The working electrode was composed of the catalysts deposited as a thin layer over the pyrolitic graphite (polished to a mirror-finish prior to each experiment, 5 mm diameter disk, 0.196 cm2 ) of a rotating disk electrode. An aqueous suspension of 1.0 mg mL−1 of the metal/C was prepared by ultrasonically dispersing the catalyst into pure water for about 15 min. An aliquot of the dispersed suspension was pipetted onto the pyrolitic carbon substrate surface and dried under ambient conditions. The catalyst load resulted close to 14 g cm2 . After the water evaporation, 20 L of a diluted Nafion® solution (0.05 wt.%, DuPont) were pipetted onto the electrode surface in order to attach the catalyst particles to the pyrolitic carbon [16]. A large area platinum foil served as the counter electrode and a reversible hydrogen electrode (RHE) system was used as the reference electrode. All the experiments were carried out in 0.5 mol L−1 H2 SO4 , prepared from high purity reagents (Mallinckrodt) and water purified in a Milli-Q (Millipore) system. The electrolyte was saturated with pure N2 and H2 gases. The electrochemical behavior was evaluated in a potential range limited by the hydrogen and oxygen evolutions, between 0.05 and 1.1 V at 50 mV s−1 . HOR steady-state polarization curves were obtained with the RDE at several rotation speeds (900, 1600 2500 and 3600 rpm) using a rotation system from PINE Instruments (AFCPR), where the experiments were recorded in the potential range between 0 and 0.25 V vs. RHE at 2 mV s−1 using an AUTOLAB bipotentiostat (PGSTAT30).
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2.5. Single-cell measurements The polarization curves were obtained in a PEMFC single cell that exhibited an electrode exposed area of about 5 cm2 with an on-line Mass Spectrometer (on-line MS) coupled to the anode gas outlet, monitoring at real-time the HOR products in the presence of CO. The electrodes consisted of the Pt/RhO2 /C materials deposited over a gas diffusion layer with Pt load of 0.4 mg cm−2 previously covered with a Nafion® film (35.5% w/w). The methodology for preparing the gas diffusion layer and the catalyst deposition method is described elsewhere [17,18]. The cell was operated with pure H2 or H2 /CO (100 ppm CO) and pure O2 at the anode and cathode, respectively. The reagents were pre-humidified after passing through the H2 and O2 humidifier chambers at temperatures of 100 ◦ C and 90 ◦ C, respectively. The temperature of the single cell was maintained at 85 ◦ C and the anode and cathode compartments were kept at 2.0 and 1.7 atm, respectively. These experimental conditions were employed since they represent the standard conditions for a PEMFC operation [17,18]. The curves were obtained galvanostatically and before each data acquisition the system was kept at 0.7 V with pure H2 for 2 h and then at 0.8 V with H2 /CO (100 ppm CO) for 2 h more. The cell current density decreased monotonically in the same way for all catalysts during the CO adsorption period, after which a steady state response was reached. During the MS experiments, the cell potential was changed stepwise and the time of each step was of approximately 5 min to stabilize the signal of the spectrometer. The measured mass to charge ratios were the CO (m/z 28), CO2 (m/z 44) and CH4 (m/z 15, CH3 + form), in which any change in the ionic current is related to the consumption or production of the observed species. The measurements were performed in an OmniStar (GSD 301Pfeiffer Vacuum, Prisma QMS 200) mass spectrometer with the ionization energy of 70 eV and emission current of 100 mA. The detection of ions was performed by an electron multiplier (common voltage = 1000 V) built-into the equipment. In order to compare the response obtained by the on-line MS experiments for the different materials under study, the ionic current signal of the m/z 44 in the experiment was normalized by subtracting the ion current signal for the different mass when CO is in the fuel stream (I44 ) from 0 ) and the ion current when the cell was fed only with pure H2 (I44 0 dividing this difference by the (I44 ) value. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD profiles of the support and the catalysts with different compositions (RhO2 /C and Pt/RhO2 /C). The Pt/C profile was added for comparison. The broad peak at 2 ≈ 25◦ observed for all catalysts is a result of the (0 0 2) reflection of the hexagonal structure of the carbon support (Vulcan XC–72R). The broad reflection of the Pt catalysts indicates that they are formed by small-sized nanocrystals and the absence of the characteristic diffraction peaks of the other metal, in addition to Pt, may be due to the proximity of the lattice parameter values for both Pt fcc and RhO2 fcc structures, and the smaller magnitude of the RhO2 signal. Table 1 lists the lattice parameter values and the mean crystallite sizes obtained from the XRD analysis, calculated using the 2 2 0 peak for which any contribution of RhO2 /C may be neglected. It is seen that for the Pt/RhO2 /C there is no significant change in the lattice parameter when compared to the pure Pt, indicating that the reduction of Pt over the RhO2 /C support does not induce any alloying effect. This was to be expected since the occurrence of any alloying would be linked to the reduction of the rhodium oxide layer and to a high atomic mobility of both Rh and Pt atoms, which is not
Fig. 1. X-ray diffraction patterns for Pt supported in RhO2 /C in three compositions, Pt/C and the support 15% RhO2 /C.
the case. The average size of the Pt nanocrystals was found to be 3.8, 2.9 and 2.7 nm for the Pt/RhO2 /C 5%, 10% and 15% RhO2 /C, respectively. These values indicate that higher RhO2 content induces a stabilization of the Pt nanoparticles in smaller aggregates. The electronic properties of platinum in the materials were investigated by X-ray absorption near edge structure (XANES), with the specific purpose of evaluating the effect of the presence of RhO2 /C. Fig. 2a and b shows XANES spectra obtained at the Pt L3 edge for the Pt/RhO2 /C electrocatalysts in an acid medium at 50 mV and for the 10% Pt/RhO2 /C at four distinct electrode potentials. The absorption at the Pt L3 edge (11564 eV) corresponds to 2p3/2 –5d electronic transitions and the magnitude of the absorption hump (white line) located at ca. 5 eV above the edge is directly related to the occupancy of the 5d electronic states. The higher the hump, the lower the occupancy and vice-versa. Fig. 2a shows an increase in the white line magnitude when compared to the Pt/C for higher RhO2 contents. The presence of this oxide may cause an electron withdrawing effect from the Pt 5d-states, increasing the vacancy/atom fraction, and causing a reduction in the density of states near the Fermi level, which leads to a decrease in the 5d band energy center. These electronic modifications may cause the CO-tolerance effects, as will be discussed later. In summary, although the XRD analysis indicates that there is no Pt–Rh alloying in the Pt/RhO2 /C materials, the XAS results clearly demonstrate an electronic effect on the Pt atoms. Fig. 2b denotes only negligible changes of the white line showing that the Pt 5d band occupancy is not affected very much by the electrode potential. This observation is not the same as that presented for Pt/C, in which there is emptying of the Pt 5d band, in agreement with the presence of an electron withdrawing effect of the oxygen present in a well-known surface oxide layer formed above 0.8 V [14]. These results indicate that the presence of Rh inhibit Table 1 Structural parameters obtained by XRD for the Pt/RhO2 /C, RhO2 /C and Pt/C. Catalyst
Pt/RhO2 /C RhO2 /C Pt/Ca (Etek) Pt metal a
Proportion by weight of the second metal 5% RhO2 /C 10%RhO2 /C 15%RhO2 /C 15% 100% –
Mean crystallite size (nm)
Lattice parameter (nm)
Pt 3.8 2.9 2.7 – 2.9 –
Pt 0.3922 0.3921 0.3920 – 0.3920 0.3920
The Pt content with respect to carbon was kept at 20% (w/w) Pt/C in all materials.
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421
Fig. 2. XANES spectra in 0.5 mol L−1 H2 SO4 , for the composition of Pt/RhO2 /C and Pt/C at 50 mV vs. RHE (a) and Pt/RhO2 /C 15% RhO2 under four electrode potentials (b).
the Pt surface oxidation, at least for potentials up to 0.9 V. On the other hand, the oxygen present in the RhO2 support may cause the observed electronic changes, without requiring any alloy formation. Therefore, this type of materials may have distinct effects over the HOR electrocatalysis and on the CO-tolerance. 3.2. Electrochemical measurements Fig. 3a and b shows the cyclic voltammetric profiles for the 15% Pt/RhO2 /C, Pt/C and 15% RhO2 /C catalysts. The features of the voltammograms for 5% and 10% Pt/RhO2 /C are essentially the same as for 15% Pt/RhO2 /C, although the RhO2 content does change the magnitude of the current density. In Fig. 3a there is the appearance of a reversible peak close to 0.55 V (vs. RHE) in the voltammogram for Pt/C, which is not present in the case of a smooth Pt electrode [21]. However, such a peak is usually observed for Pt/C in acid media and has been assigned to redox features of carbon surface groups, i.e. quinone/hydroquinone [11,17]. We must also observe that the magnitude of the currents in the cyclic voltammograms is consistent with the crystallite surface area of the catalysts, as determined from the XRD results. However, no quantitative correlations can be established since the Pt/RhO2 /C catalysts has a somewhat different behavior when compared to materials containing only Pt. The voltammetric features of the Pt/RhO2 /C catalysts are similar to those of metallic Rh. The adsorption/desorption process of hydrogen on this material is characterized by the presence of a large single peak. As no alloying was observed on the XRD results, the cyclic voltammetric response may be considered as a sum of the
individual Pt and RhO2 /C sites. However, this may not be the case since no combined features of both voltammograms are seen in the Pt/RhO2 /C response. So, it seems that the electronic modifications observed in Fig. 2a may have some effects on the hydrogen adsorption/desorption process. It is also noted that the peak formation of oxides are shifted to considerably more negative potentials when compared to Pt/C, indicating a high stability of the RhO2 oxides. This behavior is consistent with the observations related to the absence of oxide formation at 0.9 V, as evidenced by the XANES spectrum in Fig. 2b. In the presented work, no changes in the shape or size of the voltammograms were observed during the whole period of measurements for all catalysts, indicating the good stability of the materials. 3.2.1. RDE HOR results Fig. 4(a)–(c) presents the hydrogen oxidation polarization curves for the Pt/RhO2 /C (15% RhO2 /C), 15% RhO2 /C support and Pt/C for several rotation speeds and obtained in 0.5 mol L−1 solutions at a 2 mV s−1 sweep rate. For all materials it is observed that at low potentials, the currents are controlled by activation, but with the increase of the potential, mass transport starts to influence the current until a limiting value is reached above ca. 0.03 V, as would be expected because of the low solubility of hydrogen (7.14 × 10−3 mol L−1 ) [19]. It is noted that, as expected, the HOR limiting currents for the Pt/RhO2 /C catalyst assume values close to those for Pt/C. Also, the activation and kinetic-diffusion controlled regions of the curves indicate that a higher activity is observed for the Pt/RhO2 /C material compared to the Pt/C. Due to the low activity
Fig. 3. Cyclic voltammograms obtained for Pt/RhO2 /C and Pt/C (a) and for 15% RhO2 /C (b) in 0.5 mol L−1 H2 SO4 at 50 mV s−1 . The current are normalized by the geometric area of the thin porous coating layer.
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Fig. 4. Steady-state polarization curves for the HOR on Pt/RhO2 /C (a), 15% RhO2 /C (b) and Pt/C (c) at several rotation rates in 0.5 mol L−1 H2 SO4 .
for HOR in the RhO2 /C and the apparent smaller specific surface area of Pt in Pt/RhO2 /C compared with Pt/C (Fig. 3a), the enhancement of the HOR kinetics in the Pt/RhO2 /C may be attributed to changes on the platinum electronic states caused by the presence of RhO2 . Finally it should be noted that, although smaller than for the Ptbased catalysts, a considerable activity for the hydrogen oxidation reaction was observed for RhO2 /C support. Due to the ultra-thin layer electrode configuration, the film thickness of approximately 0.5 m does not affect the total current density in the hydrogen electrooxidation, which is dependent only on the reaction current kinetics and the diffusion of reactants through the hydrodynamic layer (Nernst layer) [20]. Assuming a laminar flow, the mass transport rate and consequently the limiting diffusion current (id ), is described by the Levich equation [3]: id = 0.62nFAD2/3 −1/6 C + ω1/2
active sites in the RhO2 /C catalyst. Under this circumstance a significant fraction of the H2 molecules may pass along the catalyst layer surface without reacting with any site. In order to obtain information about the HOR kinetics on the catalysts, the polarization data were analyzed in terms of mass transport corrected Tafel diagrams. The kinetic equations used for such analyses were those derived considering a reversible or an irreversible nature for the kinetics of the electrochemical reaction. For the reversible case, the kinetic equation that describes the polarization of the electrode is written as [1–3,21]: E = E 0 − 2.303
RT log(Irev ) nF
and
Irev =
ILa − I ILa
(5)
(4)
where A is the geometric area of the electrode, D the diffusion coefficient and C* the solubility of the hydrogen in the electrolyte, the kinematics viscosity of the electrolyte, and ω is the rotation speed. Fig. 5 shows the Levich plots for the systems under investigation. It is seen that the responses for all catalysts follow the predictions of Eq. (3), where for the zero rotation speed value, the lines intersect the origin, and also present a linear behavior indicating that the difusional current density limit depends exclusively on the effects of convective mass transport, showing that the Tafel step is fast enough to not determine the rate of the process, even at high rotation speeds. Small differences in the slope may be due to small differences in the geometric area of the active layer. This is expected for a situation where the thickness of the hydrodynamic layer is larger than the rugosity of the surface. In the case of the RhO2 /C catalyst the same linear profile is observed but with a lower slope when compared to PtRhO2 /C materials or Pt/C. Based on the Levich equation (Eq. (3)) and since the values of the D, v, C* are independent of the catalyst and n cannot be smaller than 2 electrons, the cause for the smaller slope may be related to a much reduced amount of
Fig. 5. Levich diagrams for the hydrogen oxidation on Pt/C, Pt/RhO2 /C and RhO2 /C for the distinct compositions in 0.5 mol L−1 H2 SO4 .
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Fig. 6. Mass transfer corrected Tafel plots assuming reversible (a) and irreversible (b) kinetics for the hydrogen oxidation on Pt/RhO2 /C on the three compositions and on the 15% RhO2 /C support.
where E is the electrode potential, E0 is the equilibrium potential of reaction and ILa is the anodic current limit and 2.303RT/nF is the slope of the Tafel curves. Note that if this mechanism occurs, the plots of (E vs . log Irev ) derived from the experimental data should be independent of the system’s linear and angular speeds. Assum-
ing the occurrence of an irreversible mechanism, the equation is [1–3,21]:
E=E
0
+b log(Iirrev )−b log CR∞
− const. and
Iirrev =
ILa xI
ILa − I
(6)
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Table 2 Theoretical Tafel slopes for the several mechanisms and kinetic conditions for the HOR, for T = 25 ◦ C and assuming the transfer coefficient ˛ = 0.5. Mechanism
Rate determining step
b (mV dec−1 )
Direct discharge (reversible) Direct discharge (irreversible) Tafel/Volmer Tafel/Volmer Heyrovsky/Volmer Heyrovsky/Volmer Heyrovsky/Volmer
– – Tafel Volmer Heyrovsky Volmer Volmer
30 60 70 120 70 40 120
where b is the Tafel slope of the process, and of the mechanism in Table 2; CR∞ is the solubility of the reactant in the liquid electrolyte. Here, also the linearity and independence of the graphics (E vs . log Iirrev ) on the angular velocity should be accomplished if the data are compatible with respect to Eq. (5). Each of the above mechanistic possibilities leads to different Tafel coefficients and the values are presented in Table 2 [1,2,22,23]. Fig. 6 shows the Tafel plots for the H2 oxidation at different rotation speeds for the Pt/RhO2 /C and RhO2 /C catalysts, respectively, constructed on the basis of the equations for the direct reversible and irreversible mechanisms. For the construction of the mass transport corrected Tafel diagrams, potential values in the range of 0–0.08 V vs. RHE were used, which correspond to the activation and mixed kinetic control of HOR. The results for Pt/C presented essentially the same behaviors as those shown for the Pt/RhO2 /C catalysts. Comparing the Tafel diagrams in Fig. 6a and b, it can be observed that for all catalysts the results follow Eq. (4) (reversible reaction), as seen by the essentially linear relationship and the independence of the angular velocity. Markovic, Grgur and Ross [24] and de Melo and Ticianelli [21] studied the hydrogen oxidation reaction on Pt in H2 SO4 solutions up to 303 K and found a Tafel coefficient of 28 mV dec−1 . In this study the value of the Tafel coefficient obtained for the HOR for Pt/RhO2 /C was in the range of 30–40 mV dec−1 , which corresponds approximately to the slope predicted by the equation developed for the (reversible) direct discharge mechanism (although in some cases the Tafel coefficient slightly exceeds the predicted value of 30 mV dec−1 for T = 25 ◦ C and n = 2). This observation is in agreement with what was previously observed for a smooth Pt electrode [21]. The possibility that the reaction occurs by the irreversible Heyrovsky/Volmer mechanism, having the Volmer step as the rds, is not supported due to the nonlinearity and dispersion of the data generated during the analyses of this possibility. Fig. 7a shows the comparison of the responses of the several Pt/RhO2 /C materials with that of the pure RhO2 /C support. It can be observed that the increase in the RhO2 content improves the catalytic activity of the Pt/RhO2 /C materials. This series follows the same trend observed for the Pt 5d band vacancy, indicating a correlation between the Pt electronic structure and the HOR activity. In Fig. 7b it is observed that the RhO2 /C support with 15% RhO2 shows the highest activity, although it is much lower when compared to the Pt-containing materials. Hence, the pure RhO2 /C may constitute a good candidate for future developments, if non-Pt catalysts for fuel cells are considered. 3.2.2. Steady-state polarization and on line MS measurements Fig. 8 shows the results of the H2 /O2 and H2 + CO/O2 single cell steady-state polarization data for the several Pt/RhO2 /C anodes. As seen, when fed with pure hydrogen the curve profiles are similar to those in the literature for Pt/C or Pt-M/C (M = Ru, Mo, etc.) alloys [25–31], indicating that the support has little effect on the anodic response of the fuel cells. This is in agreement with the RDE steady-state data, since the HOR kinetics on the studied materials showed to be similar to that of Pt/C. The only noticeable feature is
Fig. 7. Steady state polarization lines for the HOR for the (a) PtRhO2 /C materials and RhO2 /C 15% and (b) for the three RhO2 /C materials at 2500 rpm..
the non negligible potential drop observed when using RhO2 /C as the anode electrocatalyst, indicating it is not very much active for the HOR. However, this activity is relatively high, which is important because there is no Pt in this catalyst. In the presence of CO, severe potential losses are observed for all materials, but for the
Fig. 8. Polarization curves from the PEMFC single cell containing the distinct electrode configurations as the cell anode. Closed symbols – pure hydrogen as anode feed. Open symbols – hydrogen with 100 ppm of CO as anode feed. T(cell) = 85 ◦ C, T(H2 ) = 100 ◦ C and T(O2 ) = 90 ◦ C. The flow rate was fixed at 250 mL min−1 for both anode and cathode streams. The total backpressure of the cathode and anode sides was 1.7 atm and 2.0 atm, respectively.
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Fig. 9. Anode overpotential curves (a) and CO2 production curves (b) for the diverse electrode configurations used as anodes. The CO2 production curves were obtained under the same conditions as the overpotential profiles.
Pt/RhO2 /C the CO tolerance is higher than for pure Pt/C. RhO2 /C shows the largest performance decay in the low current density region, demonstrating that CO completely poisons the surface sites that are active for HOR. To better evaluate the CO poisoning effect and the tolerance mechanism, the anode overpotential caused by the presence of CO is plotted in Fig. 9 as a function of the current density (Fig. 9a) and as a function of the rate of CO2 production, such as that obtained by the on line MS measurements (Fig. 9b). The calculated overpotential was taken as the difference of the cell potential when CO is present in the fuel stream and when pure H2 is present. Since the cathode and membrane parameters are held constant, in principle this subtraction removes the membrane and cathode overpotentials, leaving only the potential loss due to the CO poisoning effect [32,33]. In Fig. 9a it can be seen that at low current densities the corelated anodic overpotentials are small, except for the RhO2 /C that shows high potential losses due to the CO poisoning, as mentioned earlier. As discussed by Ticianelli and co-workers [31,32,34] the Sshaped overpotential profile, observed particularly for Pt/C, is a consequence of the adsorption competition between H2 and CO for the surface sites followed by the occurrence of CO oxidation. Since CO adsorbs stronger than H2 , it will leave only a small portion of free sites for the occurrence of HOR. With the increase of the cell current, this eventually leads to the appearance of a current density threshold, in which the rds becomes the H2 adsorption process (Tafel step), because there is not enough surface free sites available for the occurrence of HOR. Hence, this current threshold is directly related to the CO surface coverage. As seen in Fig. 9a the 15% and 10% Pt/RhO2 /C materials present higher and 5% RhO2 /C smaller current thresholds when compared to Pt/C. These results indicate that the CO surface coverage is distinct for the different materials. XANES results indicated that the 15% and 10% RhO2 /C present higher Pt 5d band vacancy induced by the RhO2 /C support when compared to Pt/C. For 5% RhO2 /C a small decrease in occupancy is also seen, but its effect on the CO tolerance can be neglected. As mentioned before, lower Pt 5d band occupancies induce lower CO adsorption bond strength, and vice-versa. Therefore, the lower current threshold observed for the 5% RhO2 /C and the highest values seen for the higher rhodium contents, compared to Pt/C, agree with the differences of Pt–CO bond strength produced by the distinct Pt electronic structures observed in these materials. In Fig. 9a and b, it is seen that, after the plateau, the current density starts to increase at the same time that CO2 production starts to increase, as was particularly observed for Pt/C. This is due to the occurrence of the bi-functional mechanism, which is dictated by the generation of oxygenated species. In Fig. 9b it is seen that
the CO2 generation starts at lower overpotentials when compared to Pt/C but the corresponding effect of the increase in the current density is not so clear. It should be noted that for the Pt/RhO2 /C catalysts, the voltammograms (Fig. 3a) indicate that the formation of activated water required for the occurrence of the bifunctional mechanism only occurs after 0.3 V vs. RHE, while Fig. 9b evidences CO2 formation at hydrogen electrode overpotentials as low as 0.15 V. However, it should also be noted that the RDE experiments were carried out at 25 ◦ C, while the single cell tests were conducted at 85 ◦ C. As seen in previous works [32,34], at a higher temperature there is a negative shift of the CO oxidation onset potential (or equivalently of formation of activated water) of almost 200 mV, and this explains why the CO oxidation can be operative under the fuel cell conditions. The aforementioned phenomena can be rationalized in terms of a dual active site concept. Since only Pt presents HOR activity and only RhO2 /C oxidizes the CO at low overpotentials (observed in Fig. 9b), only a close contact between these two sites will be efficient to eliminate the CO from the Pt by the bi-functional mechanism. In turn, if the number of these two neighbors is low, the overpotential curve will have a slope increase after the current threshold, indicating that not enough CO is eliminated from Pt, even though the CO2 production grows with the increase of the overpotential. In summary, the CO oxidation occurs at the RhO2 /C active sites without any reduction of the CO coverage at the Pt sites.
4. Conclusions The results of studies about Pt supported on RhO2 /C catalysts for the HOR electrocatalysis and of the CO tolerance are reported when employed at FC anodes. Structural characterization indicates that the Pt atoms are not alloyed but present modifications in the Pt 5d-band structure with crescent vacancy/atom ratio for materials with higher RhO2 /C contents. The RDE characterization of the HOR mechanism evidenced that all materials drive the same reaction pathways, which is given by the reversible direct discharge mechanism, in the potential range of 0–0.05 V vs. RHE. The RhO2 /C support showed a relatively high HOR activity despite the absence of Pt. The single cell results for all materials are in agreement with the RDE findings, with respect to a high HOR activity when the cell is fed with pure H2 . When CO is in the fuel stream, the Pt/RhO2 /C materials with higher RhO2 /C content showed lower reaction overpotential when compared to Pt/C, which was assigned to the presence of both electronic effect and the occurrence of the bi-functional mechanism.
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[16] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm, J. Electroanal. Chem. 495 (2001) 134. [17] E.A. Ticianelli, E.R. Gonzalez, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cell Technology, vol. 2, Wiley-VCH, New York, 2003, p. 490. [18] V.A. Paganin, E.A. Ticianelli, E.R. Gonzalez, J. Appl. Electrochem. 26 (1996) 297. [19] H.A. Gasteiger, N.M. Markovic, N.P. Ross Jr., J. Phys. Chem. 99 (1995) 8290. [20] T.J. Schmidt, H.A. Gasteiger, G.D. Stäb, P.M. Urban, D.M. Kolb, R.J. Behm, J. Electrochem. Soc. 145 (7) (1998) 2354. [21] R.M.Q. Mello, E.A. Ticianelli, Electrochim. Acta 42 (1997) 1031. [22] J.A. Harrinson, Z.A. Khan, J. Electroanal. Chem 30 (1971) 327. [23] J.O’m. Bockris, N. Reddy, Modern Electrochemistry, Plenum, New York, 1970. [24] N.M. Markovic, B.N. Grgur, P.N. Ross, J. Phys. Chem. B 101 (1997) 5405. [25] E.I. Santiago, G.A. Camara, E.A. Ticianelli, Electrochim. Acta 48 (2003) 3527. [26] E.I. Santiago, M.S. Batista, E.M. Assaf, E.A. Ticianelli, J. Electrochem. Soc. 151 (2004) A944. [27] E.I. Santiago, V.A. Paganin, M. Carmo, E.R. Gonzalez, E.A. Ticianelli, J. Electroanal. Chem. 575 (2005) 53. [28] L.G.S. Pereira, M.E. Pereira, E.A. Ticianelli, Quim. Nova 30 (2007) 1644. [29] L.G.S. Pereira, V.A. Paganin, E.A. Ticianelli, Electrochim. Acta 54 (2008) 1992. [30] A.C. Garcia, V.A. Paganin, E.A. Ticianelli, Electrochim. Acta 53 (2008) 4309. [31] P.P. Lopes, E.A. Ticianelli, J. Electroanal. Chem. 644 (2010) 110. [32] G.A. Camara, E.A. Ticianelli, S. Mukerjee, S.J. Lee, J. Mcbreen, J. Electrochem. Soc. 149 (2002) A748. [33] J. Zhang, T. Thampan, R. Datta, J. Electrochem. Soc. 149 (2002) A765. [34] S.J. Lee, S. Mukerjee, E.A. Ticianelli, J. McBreen, Electrochim. Acta 44 (1999) 3283.
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Electrocatalysis of the hydrogen oxidation in the presence of CO on RhO2 /C-supported Pt nanoparticles K.S. Freitas, P.P. Lopes, E.A. Ticianelli ∗ Instituto de Química de São Carlos, USP, C.P. 780, São Carlos, SP 13560-970, Brazil
a r t i c l e
i n f o
Article history: Received 13 May 2010 Received in revised form 26 August 2010 Accepted 27 August 2010 Available online 17 September 2010 Keywords: Hydrogen oxidation CO oxidation Oxide support RhO2 MS on line
a b s t r a c t This work presents a study on the kinetics of the hydrogen oxidation reaction (HOR) in the absence and in the presence of CO in ultra thin porous layer and in PEM fuel cell electrodes formed with Pt supported on RhO2 /C substrates. Together with the electrochemical measurements, the structural and electronic properties of these catalysts were characterized, enabling to correlate their structural and electronic properties with the HOR kinetics. The results show that the presence of Rh oxides leads to an emptying of the Pt 5d band and a consequent reduction of the back-donation of electrons from Pt to CO, weakening the Pt–CO bond and diminishing the CO degree of coverage on Pt, leaving more sites available to HOR. These changes in the electronic spectra do not lead to any perceptible change in the kinetics or the reaction of pure hydrogen. Also, the formation of CO2 monitored by the MS experiments in the fuel cell anode outlet indicates that the bifunctional mechanism is also operative, but the major CO tolerance is achieved by the electronic effect induced by the RhO2 support. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction HOR involves the transfer of 2 electrons per hydrogen molecule on the polycrystalline Pt electrode, which can occur in several stages through different mechanisms [1–3]. The kinetic processes of hydrogen adsorption and oxidation are believed to follow the steps of the H2 dissociative adsorption (Tafel reaction) followed by the oxidation of adsorbed hydrogen (Volmer reaction): k1
H2 + 2Pt←→2Pt–H k2
Tafel
Pt–H←→Pt + H+ + e−
Volmer
(1) (2)
According to this mechanism, the current of the hydrogen oxidation is produced exclusively by the Volmer step and a large current density leads to a significant consumption of adsorbed hydrogen. When the fuel contains small amounts of CO, the active sites for hydrogen adsorption are limited only to the vacancies left by the CO adsorbed layer, making the Tafel reaction the rate determining step (rds). Thus, the effect of lowering the CO steady-state coverage will increase the magnitude of the hydrogen electro-oxidation current. Besides lowering the CO content in the hydrogen flow, there are basically two other mechanisms to decrease the Pt–CO surface cov-
∗ Corresponding author. Tel.: +55 0 16 3373 9945; fax: +55 0 16 3373 9952. E-mail address: [email protected] (E.A. Ticianelli). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.059
erage, both related to the addition of another metal in contact with Pt. The first is related to using a metal that affects the electronic properties of the Pt atoms in order to lower the occupancy of the 5d band. The electronic influence on the CO adsorption at Pt sites has also been assigned to a down-shift of the Pt 5d band center, mainly caused by the lattice mismatch and the electronic interaction between Pt and other metals. This would weak the Pt–CO interaction, which lower the CO coverage. The second is the bifunctional mechanism which consists of the oxidation of adsorbed CO by oxy/hydroxy species formed on the second metal sites at lower potentials than observed on pure Pt. The second element in the alloy must enable the formation of the oxidized surface at low potentials, which on the other hand can corrode the alloy faster, due to its lower oxidation potential than Pt. PtRh supported alloy has been studied concerning the molecular hydrogen oxidation and CO tolerance [4], and the results showed found that the catalyst with 24% of Rh shows the best performance for the hydrogen contaminated with CO. This increased activity is associated to the excess of unpaired electrons per atoms in the alloy when compared to pure platinum. Several works in the literature have observed that Pt combined with metal oxides have greater performance as a CO-tolerant anode when compared to pure Pt. This increase in activity has been attributed to the greater availability of active sites for hydrogen adsorption in Pt-transition metal catalysts. Within this context, various transition metal oxides have been proposed as CO tolerant, such as RuOx , SnOx , VOx , WOx and MoOx [5–8]. For Pt supported in oxide substrates, for instance RhO2 /C, the increased activity
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for CO oxidation may be due to the fact that these materials are more able to form oxygenated species (metal-OH), increasing the rate of CO oxidation by the Langmuir–Hinshelwood mechanism (bi-functional mechanism), and leading to an intrinsically lower surface coverage, thus increasing the surface vacancies necessary for HOR. This study evaluates the performance of platinum supported RhO2 /C materials, for the hydrogen oxidation reaction in the absence and in the presence of CO, in acidic medium. The electrocatalysts were studied by EDX, XRD and in situ XAS to characterize the structural and electronic properties and the composition. The electrochemical investigations were carried out in two steps, the first is an evaluation of the kinetics of HOR using the ultra thin porous coating layer in a rotating disk electrode (RDE), and the second is a direct use of the materials in PEMFC anodes fed with H2 and H2 /CO. The cell measurements consisted of determining the steady-state polarization response, having an on-line mass spectrometer (on-line MS) coupled to the anode gas outlet, in order to investigate and understand the CO oxidation mechanism.
2. Experimental 2.1. Preparation of RhO2 /C support The RhO2 support was prepared using a sol–gel method, similar to that previously described [9,10]. The Rh(III) acetylacetonate (Alfa Aesar) was dissolved in ethanol and then the adequate amount of acetic acid was added. The carbon black (Vulcan XC-72R) was added and then the sol–gel solution was subjected to an ultrasonic treatment (FALC Ultrasonic model 06) for 5 min (homogenization step). After that, a short heat treatment was performed to remove the residual solvents and the polymeric chains, during which the xerogel is degraded to form Rh oxide after the elimination of organic residues and water. A final 2-h densification treatment was conducted at the same temperature used in the previous step (400 ◦ C), under argon atmosphere. Three distinct RhO2 contents were prepared for several contents of the carbon support, to reach 5%, 10% and 15% (w/w) of RhO2 /C. 2.2. Incorporation of the Pt nanoparticles in the RhO2 /C supports The Pt/RhO2 /C catalysts were prepared by the chemical reduction of Pt ions using the formic acid method [11]. For this purpose, a solution was prepared by mixing a formic acid solution, water and the previously prepared support. The solution was treated in an ultrasonic bath to form a uniformly dispersed ink. The solution was then kept at 80 ◦ C and the platinum salt (hexachloroplatinic acid, H2 PtCl6 Alfa Aesar) was slowly added under stirring. Next, the solution was cooled to room temperature and a washing–filtering procedure was performed to eliminate any contamination from the dissolved salts in the final product. The filtered powder was dried at 100 ◦ C for 2 h. The Pt content with respect to carbon was kept at 20% (w/w) Pt/C for all RhO2 /C supports. For the sake of simplicity, the distinct Pt/RhO2 /C electrocatalysts will be differentiated by the rhodium content. 2.3. Characterization The X-ray diffraction (XRD) experiments were conducted using a Rigaku-Rotaflex Mod. Ru-200B and a Rigaku Ultima IV equipment, with CuK␣ radiation at a scan rate of 2◦ min−1 in the range of 20 and 100◦ to characterize the Pt/RhO2 /C catalysts. The Scherrer’s equation was used to estimate the crystallite size [12,13]; the lattice parameter for the fcc structure was calculated using the following
419
equation:
a=
h2 + k2 + l2 . 2 sin
(3)
The atomic ratios of the Pt/RhO2 /C electrocatalyst were obtained by energy dispersive X ray (EDX) analysis using a Zeiss Digital Scanning Electron Microscope DSM 960 with a Link Analytical microanalysis (QX 2000), working with the electron beam acceleration potential difference of 63 kV. The measurements of in situ X-ray absorption spectroscopy (XAS) were performed at the Pt L3 absorption edge (11,564 eV), using an appropriate spectroelectrochemical cell [14]. The Pt L3 absorption edge is related to electronic transitions of the 2p3/2 orbital to the 5d band (5d5/2 states), and the magnitude of the white-line located at 5 eV above edge is closely related to the occupation of the 5d band. For these measurements the working electrodes were formed with the catalyst material dispersed with Nafion® (ca. 35 wt.%) and isopropyl alcohol, containing about 7 mg cm−2 of Pt loading. The measurements were made at several electrode potentials, referenced to the RHE (reversible hydrogen electrode). The counter electrode was formed by a platinum screen. Both the cell and the counter electrode have a window in the center to allow free passage of the X-ray beam, which only crosses the working electrode. All experiments were conducted at the D04B XAFS beam line in the Brazilian Synchrotron Light Laboratory (LNLS), Brazil. The data acquisition system for XAS measurements comprised three ionization detectors (incident Io , transmitted It and reference Ir ). The reference channel was used primarily for the internal calibration of the edge positions by using a pure foil of the metal. Nitrogen was used in the Io , It and If chambers. Due to the low critical energy of the LNLS storage ring (2.08 keV), third-order harmonic contamination of the Si(1 1 1) monochromatic beam is expected to be negligible above 5 keV [15].
2.4. RDE measurements These experiments used a conventional one-compartment glass cell with a Luggin capillary, which were conducted at room temperature (25 ± 1 ◦ C). The working electrode was composed of the catalysts deposited as a thin layer over the pyrolitic graphite (polished to a mirror-finish prior to each experiment, 5 mm diameter disk, 0.196 cm2 ) of a rotating disk electrode. An aqueous suspension of 1.0 mg mL−1 of the metal/C was prepared by ultrasonically dispersing the catalyst into pure water for about 15 min. An aliquot of the dispersed suspension was pipetted onto the pyrolitic carbon substrate surface and dried under ambient conditions. The catalyst load resulted close to 14 g cm2 . After the water evaporation, 20 L of a diluted Nafion® solution (0.05 wt.%, DuPont) were pipetted onto the electrode surface in order to attach the catalyst particles to the pyrolitic carbon [16]. A large area platinum foil served as the counter electrode and a reversible hydrogen electrode (RHE) system was used as the reference electrode. All the experiments were carried out in 0.5 mol L−1 H2 SO4 , prepared from high purity reagents (Mallinckrodt) and water purified in a Milli-Q (Millipore) system. The electrolyte was saturated with pure N2 and H2 gases. The electrochemical behavior was evaluated in a potential range limited by the hydrogen and oxygen evolutions, between 0.05 and 1.1 V at 50 mV s−1 . HOR steady-state polarization curves were obtained with the RDE at several rotation speeds (900, 1600 2500 and 3600 rpm) using a rotation system from PINE Instruments (AFCPR), where the experiments were recorded in the potential range between 0 and 0.25 V vs. RHE at 2 mV s−1 using an AUTOLAB bipotentiostat (PGSTAT30).
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2.5. Single-cell measurements The polarization curves were obtained in a PEMFC single cell that exhibited an electrode exposed area of about 5 cm2 with an on-line Mass Spectrometer (on-line MS) coupled to the anode gas outlet, monitoring at real-time the HOR products in the presence of CO. The electrodes consisted of the Pt/RhO2 /C materials deposited over a gas diffusion layer with Pt load of 0.4 mg cm−2 previously covered with a Nafion® film (35.5% w/w). The methodology for preparing the gas diffusion layer and the catalyst deposition method is described elsewhere [17,18]. The cell was operated with pure H2 or H2 /CO (100 ppm CO) and pure O2 at the anode and cathode, respectively. The reagents were pre-humidified after passing through the H2 and O2 humidifier chambers at temperatures of 100 ◦ C and 90 ◦ C, respectively. The temperature of the single cell was maintained at 85 ◦ C and the anode and cathode compartments were kept at 2.0 and 1.7 atm, respectively. These experimental conditions were employed since they represent the standard conditions for a PEMFC operation [17,18]. The curves were obtained galvanostatically and before each data acquisition the system was kept at 0.7 V with pure H2 for 2 h and then at 0.8 V with H2 /CO (100 ppm CO) for 2 h more. The cell current density decreased monotonically in the same way for all catalysts during the CO adsorption period, after which a steady state response was reached. During the MS experiments, the cell potential was changed stepwise and the time of each step was of approximately 5 min to stabilize the signal of the spectrometer. The measured mass to charge ratios were the CO (m/z 28), CO2 (m/z 44) and CH4 (m/z 15, CH3 + form), in which any change in the ionic current is related to the consumption or production of the observed species. The measurements were performed in an OmniStar (GSD 301Pfeiffer Vacuum, Prisma QMS 200) mass spectrometer with the ionization energy of 70 eV and emission current of 100 mA. The detection of ions was performed by an electron multiplier (common voltage = 1000 V) built-into the equipment. In order to compare the response obtained by the on-line MS experiments for the different materials under study, the ionic current signal of the m/z 44 in the experiment was normalized by subtracting the ion current signal for the different mass when CO is in the fuel stream (I44 ) from 0 ) and the ion current when the cell was fed only with pure H2 (I44 0 dividing this difference by the (I44 ) value. 3. Results and discussion 3.1. Characterization Fig. 1 shows the XRD profiles of the support and the catalysts with different compositions (RhO2 /C and Pt/RhO2 /C). The Pt/C profile was added for comparison. The broad peak at 2 ≈ 25◦ observed for all catalysts is a result of the (0 0 2) reflection of the hexagonal structure of the carbon support (Vulcan XC–72R). The broad reflection of the Pt catalysts indicates that they are formed by small-sized nanocrystals and the absence of the characteristic diffraction peaks of the other metal, in addition to Pt, may be due to the proximity of the lattice parameter values for both Pt fcc and RhO2 fcc structures, and the smaller magnitude of the RhO2 signal. Table 1 lists the lattice parameter values and the mean crystallite sizes obtained from the XRD analysis, calculated using the 2 2 0 peak for which any contribution of RhO2 /C may be neglected. It is seen that for the Pt/RhO2 /C there is no significant change in the lattice parameter when compared to the pure Pt, indicating that the reduction of Pt over the RhO2 /C support does not induce any alloying effect. This was to be expected since the occurrence of any alloying would be linked to the reduction of the rhodium oxide layer and to a high atomic mobility of both Rh and Pt atoms, which is not
Fig. 1. X-ray diffraction patterns for Pt supported in RhO2 /C in three compositions, Pt/C and the support 15% RhO2 /C.
the case. The average size of the Pt nanocrystals was found to be 3.8, 2.9 and 2.7 nm for the Pt/RhO2 /C 5%, 10% and 15% RhO2 /C, respectively. These values indicate that higher RhO2 content induces a stabilization of the Pt nanoparticles in smaller aggregates. The electronic properties of platinum in the materials were investigated by X-ray absorption near edge structure (XANES), with the specific purpose of evaluating the effect of the presence of RhO2 /C. Fig. 2a and b shows XANES spectra obtained at the Pt L3 edge for the Pt/RhO2 /C electrocatalysts in an acid medium at 50 mV and for the 10% Pt/RhO2 /C at four distinct electrode potentials. The absorption at the Pt L3 edge (11564 eV) corresponds to 2p3/2 –5d electronic transitions and the magnitude of the absorption hump (white line) located at ca. 5 eV above the edge is directly related to the occupancy of the 5d electronic states. The higher the hump, the lower the occupancy and vice-versa. Fig. 2a shows an increase in the white line magnitude when compared to the Pt/C for higher RhO2 contents. The presence of this oxide may cause an electron withdrawing effect from the Pt 5d-states, increasing the vacancy/atom fraction, and causing a reduction in the density of states near the Fermi level, which leads to a decrease in the 5d band energy center. These electronic modifications may cause the CO-tolerance effects, as will be discussed later. In summary, although the XRD analysis indicates that there is no Pt–Rh alloying in the Pt/RhO2 /C materials, the XAS results clearly demonstrate an electronic effect on the Pt atoms. Fig. 2b denotes only negligible changes of the white line showing that the Pt 5d band occupancy is not affected very much by the electrode potential. This observation is not the same as that presented for Pt/C, in which there is emptying of the Pt 5d band, in agreement with the presence of an electron withdrawing effect of the oxygen present in a well-known surface oxide layer formed above 0.8 V [14]. These results indicate that the presence of Rh inhibit Table 1 Structural parameters obtained by XRD for the Pt/RhO2 /C, RhO2 /C and Pt/C. Catalyst
Pt/RhO2 /C RhO2 /C Pt/Ca (Etek) Pt metal a
Proportion by weight of the second metal 5% RhO2 /C 10%RhO2 /C 15%RhO2 /C 15% 100% –
Mean crystallite size (nm)
Lattice parameter (nm)
Pt 3.8 2.9 2.7 – 2.9 –
Pt 0.3922 0.3921 0.3920 – 0.3920 0.3920
The Pt content with respect to carbon was kept at 20% (w/w) Pt/C in all materials.
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Fig. 2. XANES spectra in 0.5 mol L−1 H2 SO4 , for the composition of Pt/RhO2 /C and Pt/C at 50 mV vs. RHE (a) and Pt/RhO2 /C 15% RhO2 under four electrode potentials (b).
the Pt surface oxidation, at least for potentials up to 0.9 V. On the other hand, the oxygen present in the RhO2 support may cause the observed electronic changes, without requiring any alloy formation. Therefore, this type of materials may have distinct effects over the HOR electrocatalysis and on the CO-tolerance. 3.2. Electrochemical measurements Fig. 3a and b shows the cyclic voltammetric profiles for the 15% Pt/RhO2 /C, Pt/C and 15% RhO2 /C catalysts. The features of the voltammograms for 5% and 10% Pt/RhO2 /C are essentially the same as for 15% Pt/RhO2 /C, although the RhO2 content does change the magnitude of the current density. In Fig. 3a there is the appearance of a reversible peak close to 0.55 V (vs. RHE) in the voltammogram for Pt/C, which is not present in the case of a smooth Pt electrode [21]. However, such a peak is usually observed for Pt/C in acid media and has been assigned to redox features of carbon surface groups, i.e. quinone/hydroquinone [11,17]. We must also observe that the magnitude of the currents in the cyclic voltammograms is consistent with the crystallite surface area of the catalysts, as determined from the XRD results. However, no quantitative correlations can be established since the Pt/RhO2 /C catalysts has a somewhat different behavior when compared to materials containing only Pt. The voltammetric features of the Pt/RhO2 /C catalysts are similar to those of metallic Rh. The adsorption/desorption process of hydrogen on this material is characterized by the presence of a large single peak. As no alloying was observed on the XRD results, the cyclic voltammetric response may be considered as a sum of the
individual Pt and RhO2 /C sites. However, this may not be the case since no combined features of both voltammograms are seen in the Pt/RhO2 /C response. So, it seems that the electronic modifications observed in Fig. 2a may have some effects on the hydrogen adsorption/desorption process. It is also noted that the peak formation of oxides are shifted to considerably more negative potentials when compared to Pt/C, indicating a high stability of the RhO2 oxides. This behavior is consistent with the observations related to the absence of oxide formation at 0.9 V, as evidenced by the XANES spectrum in Fig. 2b. In the presented work, no changes in the shape or size of the voltammograms were observed during the whole period of measurements for all catalysts, indicating the good stability of the materials. 3.2.1. RDE HOR results Fig. 4(a)–(c) presents the hydrogen oxidation polarization curves for the Pt/RhO2 /C (15% RhO2 /C), 15% RhO2 /C support and Pt/C for several rotation speeds and obtained in 0.5 mol L−1 solutions at a 2 mV s−1 sweep rate. For all materials it is observed that at low potentials, the currents are controlled by activation, but with the increase of the potential, mass transport starts to influence the current until a limiting value is reached above ca. 0.03 V, as would be expected because of the low solubility of hydrogen (7.14 × 10−3 mol L−1 ) [19]. It is noted that, as expected, the HOR limiting currents for the Pt/RhO2 /C catalyst assume values close to those for Pt/C. Also, the activation and kinetic-diffusion controlled regions of the curves indicate that a higher activity is observed for the Pt/RhO2 /C material compared to the Pt/C. Due to the low activity
Fig. 3. Cyclic voltammograms obtained for Pt/RhO2 /C and Pt/C (a) and for 15% RhO2 /C (b) in 0.5 mol L−1 H2 SO4 at 50 mV s−1 . The current are normalized by the geometric area of the thin porous coating layer.
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Fig. 4. Steady-state polarization curves for the HOR on Pt/RhO2 /C (a), 15% RhO2 /C (b) and Pt/C (c) at several rotation rates in 0.5 mol L−1 H2 SO4 .
for HOR in the RhO2 /C and the apparent smaller specific surface area of Pt in Pt/RhO2 /C compared with Pt/C (Fig. 3a), the enhancement of the HOR kinetics in the Pt/RhO2 /C may be attributed to changes on the platinum electronic states caused by the presence of RhO2 . Finally it should be noted that, although smaller than for the Ptbased catalysts, a considerable activity for the hydrogen oxidation reaction was observed for RhO2 /C support. Due to the ultra-thin layer electrode configuration, the film thickness of approximately 0.5 m does not affect the total current density in the hydrogen electrooxidation, which is dependent only on the reaction current kinetics and the diffusion of reactants through the hydrodynamic layer (Nernst layer) [20]. Assuming a laminar flow, the mass transport rate and consequently the limiting diffusion current (id ), is described by the Levich equation [3]: id = 0.62nFAD2/3 −1/6 C + ω1/2
active sites in the RhO2 /C catalyst. Under this circumstance a significant fraction of the H2 molecules may pass along the catalyst layer surface without reacting with any site. In order to obtain information about the HOR kinetics on the catalysts, the polarization data were analyzed in terms of mass transport corrected Tafel diagrams. The kinetic equations used for such analyses were those derived considering a reversible or an irreversible nature for the kinetics of the electrochemical reaction. For the reversible case, the kinetic equation that describes the polarization of the electrode is written as [1–3,21]: E = E 0 − 2.303
RT log(Irev ) nF
and
Irev =
ILa − I ILa
(5)
(4)
where A is the geometric area of the electrode, D the diffusion coefficient and C* the solubility of the hydrogen in the electrolyte, the kinematics viscosity of the electrolyte, and ω is the rotation speed. Fig. 5 shows the Levich plots for the systems under investigation. It is seen that the responses for all catalysts follow the predictions of Eq. (3), where for the zero rotation speed value, the lines intersect the origin, and also present a linear behavior indicating that the difusional current density limit depends exclusively on the effects of convective mass transport, showing that the Tafel step is fast enough to not determine the rate of the process, even at high rotation speeds. Small differences in the slope may be due to small differences in the geometric area of the active layer. This is expected for a situation where the thickness of the hydrodynamic layer is larger than the rugosity of the surface. In the case of the RhO2 /C catalyst the same linear profile is observed but with a lower slope when compared to PtRhO2 /C materials or Pt/C. Based on the Levich equation (Eq. (3)) and since the values of the D, v, C* are independent of the catalyst and n cannot be smaller than 2 electrons, the cause for the smaller slope may be related to a much reduced amount of
Fig. 5. Levich diagrams for the hydrogen oxidation on Pt/C, Pt/RhO2 /C and RhO2 /C for the distinct compositions in 0.5 mol L−1 H2 SO4 .
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423
Fig. 6. Mass transfer corrected Tafel plots assuming reversible (a) and irreversible (b) kinetics for the hydrogen oxidation on Pt/RhO2 /C on the three compositions and on the 15% RhO2 /C support.
where E is the electrode potential, E0 is the equilibrium potential of reaction and ILa is the anodic current limit and 2.303RT/nF is the slope of the Tafel curves. Note that if this mechanism occurs, the plots of (E vs . log Irev ) derived from the experimental data should be independent of the system’s linear and angular speeds. Assum-
ing the occurrence of an irreversible mechanism, the equation is [1–3,21]:
E=E
0
+b log(Iirrev )−b log CR∞
− const. and
Iirrev =
ILa xI
ILa − I
(6)
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Table 2 Theoretical Tafel slopes for the several mechanisms and kinetic conditions for the HOR, for T = 25 ◦ C and assuming the transfer coefficient ˛ = 0.5. Mechanism
Rate determining step
b (mV dec−1 )
Direct discharge (reversible) Direct discharge (irreversible) Tafel/Volmer Tafel/Volmer Heyrovsky/Volmer Heyrovsky/Volmer Heyrovsky/Volmer
– – Tafel Volmer Heyrovsky Volmer Volmer
30 60 70 120 70 40 120
where b is the Tafel slope of the process, and of the mechanism in Table 2; CR∞ is the solubility of the reactant in the liquid electrolyte. Here, also the linearity and independence of the graphics (E vs . log Iirrev ) on the angular velocity should be accomplished if the data are compatible with respect to Eq. (5). Each of the above mechanistic possibilities leads to different Tafel coefficients and the values are presented in Table 2 [1,2,22,23]. Fig. 6 shows the Tafel plots for the H2 oxidation at different rotation speeds for the Pt/RhO2 /C and RhO2 /C catalysts, respectively, constructed on the basis of the equations for the direct reversible and irreversible mechanisms. For the construction of the mass transport corrected Tafel diagrams, potential values in the range of 0–0.08 V vs. RHE were used, which correspond to the activation and mixed kinetic control of HOR. The results for Pt/C presented essentially the same behaviors as those shown for the Pt/RhO2 /C catalysts. Comparing the Tafel diagrams in Fig. 6a and b, it can be observed that for all catalysts the results follow Eq. (4) (reversible reaction), as seen by the essentially linear relationship and the independence of the angular velocity. Markovic, Grgur and Ross [24] and de Melo and Ticianelli [21] studied the hydrogen oxidation reaction on Pt in H2 SO4 solutions up to 303 K and found a Tafel coefficient of 28 mV dec−1 . In this study the value of the Tafel coefficient obtained for the HOR for Pt/RhO2 /C was in the range of 30–40 mV dec−1 , which corresponds approximately to the slope predicted by the equation developed for the (reversible) direct discharge mechanism (although in some cases the Tafel coefficient slightly exceeds the predicted value of 30 mV dec−1 for T = 25 ◦ C and n = 2). This observation is in agreement with what was previously observed for a smooth Pt electrode [21]. The possibility that the reaction occurs by the irreversible Heyrovsky/Volmer mechanism, having the Volmer step as the rds, is not supported due to the nonlinearity and dispersion of the data generated during the analyses of this possibility. Fig. 7a shows the comparison of the responses of the several Pt/RhO2 /C materials with that of the pure RhO2 /C support. It can be observed that the increase in the RhO2 content improves the catalytic activity of the Pt/RhO2 /C materials. This series follows the same trend observed for the Pt 5d band vacancy, indicating a correlation between the Pt electronic structure and the HOR activity. In Fig. 7b it is observed that the RhO2 /C support with 15% RhO2 shows the highest activity, although it is much lower when compared to the Pt-containing materials. Hence, the pure RhO2 /C may constitute a good candidate for future developments, if non-Pt catalysts for fuel cells are considered. 3.2.2. Steady-state polarization and on line MS measurements Fig. 8 shows the results of the H2 /O2 and H2 + CO/O2 single cell steady-state polarization data for the several Pt/RhO2 /C anodes. As seen, when fed with pure hydrogen the curve profiles are similar to those in the literature for Pt/C or Pt-M/C (M = Ru, Mo, etc.) alloys [25–31], indicating that the support has little effect on the anodic response of the fuel cells. This is in agreement with the RDE steady-state data, since the HOR kinetics on the studied materials showed to be similar to that of Pt/C. The only noticeable feature is
Fig. 7. Steady state polarization lines for the HOR for the (a) PtRhO2 /C materials and RhO2 /C 15% and (b) for the three RhO2 /C materials at 2500 rpm..
the non negligible potential drop observed when using RhO2 /C as the anode electrocatalyst, indicating it is not very much active for the HOR. However, this activity is relatively high, which is important because there is no Pt in this catalyst. In the presence of CO, severe potential losses are observed for all materials, but for the
Fig. 8. Polarization curves from the PEMFC single cell containing the distinct electrode configurations as the cell anode. Closed symbols – pure hydrogen as anode feed. Open symbols – hydrogen with 100 ppm of CO as anode feed. T(cell) = 85 ◦ C, T(H2 ) = 100 ◦ C and T(O2 ) = 90 ◦ C. The flow rate was fixed at 250 mL min−1 for both anode and cathode streams. The total backpressure of the cathode and anode sides was 1.7 atm and 2.0 atm, respectively.
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425
Fig. 9. Anode overpotential curves (a) and CO2 production curves (b) for the diverse electrode configurations used as anodes. The CO2 production curves were obtained under the same conditions as the overpotential profiles.
Pt/RhO2 /C the CO tolerance is higher than for pure Pt/C. RhO2 /C shows the largest performance decay in the low current density region, demonstrating that CO completely poisons the surface sites that are active for HOR. To better evaluate the CO poisoning effect and the tolerance mechanism, the anode overpotential caused by the presence of CO is plotted in Fig. 9 as a function of the current density (Fig. 9a) and as a function of the rate of CO2 production, such as that obtained by the on line MS measurements (Fig. 9b). The calculated overpotential was taken as the difference of the cell potential when CO is present in the fuel stream and when pure H2 is present. Since the cathode and membrane parameters are held constant, in principle this subtraction removes the membrane and cathode overpotentials, leaving only the potential loss due to the CO poisoning effect [32,33]. In Fig. 9a it can be seen that at low current densities the corelated anodic overpotentials are small, except for the RhO2 /C that shows high potential losses due to the CO poisoning, as mentioned earlier. As discussed by Ticianelli and co-workers [31,32,34] the Sshaped overpotential profile, observed particularly for Pt/C, is a consequence of the adsorption competition between H2 and CO for the surface sites followed by the occurrence of CO oxidation. Since CO adsorbs stronger than H2 , it will leave only a small portion of free sites for the occurrence of HOR. With the increase of the cell current, this eventually leads to the appearance of a current density threshold, in which the rds becomes the H2 adsorption process (Tafel step), because there is not enough surface free sites available for the occurrence of HOR. Hence, this current threshold is directly related to the CO surface coverage. As seen in Fig. 9a the 15% and 10% Pt/RhO2 /C materials present higher and 5% RhO2 /C smaller current thresholds when compared to Pt/C. These results indicate that the CO surface coverage is distinct for the different materials. XANES results indicated that the 15% and 10% RhO2 /C present higher Pt 5d band vacancy induced by the RhO2 /C support when compared to Pt/C. For 5% RhO2 /C a small decrease in occupancy is also seen, but its effect on the CO tolerance can be neglected. As mentioned before, lower Pt 5d band occupancies induce lower CO adsorption bond strength, and vice-versa. Therefore, the lower current threshold observed for the 5% RhO2 /C and the highest values seen for the higher rhodium contents, compared to Pt/C, agree with the differences of Pt–CO bond strength produced by the distinct Pt electronic structures observed in these materials. In Fig. 9a and b, it is seen that, after the plateau, the current density starts to increase at the same time that CO2 production starts to increase, as was particularly observed for Pt/C. This is due to the occurrence of the bi-functional mechanism, which is dictated by the generation of oxygenated species. In Fig. 9b it is seen that
the CO2 generation starts at lower overpotentials when compared to Pt/C but the corresponding effect of the increase in the current density is not so clear. It should be noted that for the Pt/RhO2 /C catalysts, the voltammograms (Fig. 3a) indicate that the formation of activated water required for the occurrence of the bifunctional mechanism only occurs after 0.3 V vs. RHE, while Fig. 9b evidences CO2 formation at hydrogen electrode overpotentials as low as 0.15 V. However, it should also be noted that the RDE experiments were carried out at 25 ◦ C, while the single cell tests were conducted at 85 ◦ C. As seen in previous works [32,34], at a higher temperature there is a negative shift of the CO oxidation onset potential (or equivalently of formation of activated water) of almost 200 mV, and this explains why the CO oxidation can be operative under the fuel cell conditions. The aforementioned phenomena can be rationalized in terms of a dual active site concept. Since only Pt presents HOR activity and only RhO2 /C oxidizes the CO at low overpotentials (observed in Fig. 9b), only a close contact between these two sites will be efficient to eliminate the CO from the Pt by the bi-functional mechanism. In turn, if the number of these two neighbors is low, the overpotential curve will have a slope increase after the current threshold, indicating that not enough CO is eliminated from Pt, even though the CO2 production grows with the increase of the overpotential. In summary, the CO oxidation occurs at the RhO2 /C active sites without any reduction of the CO coverage at the Pt sites.
4. Conclusions The results of studies about Pt supported on RhO2 /C catalysts for the HOR electrocatalysis and of the CO tolerance are reported when employed at FC anodes. Structural characterization indicates that the Pt atoms are not alloyed but present modifications in the Pt 5d-band structure with crescent vacancy/atom ratio for materials with higher RhO2 /C contents. The RDE characterization of the HOR mechanism evidenced that all materials drive the same reaction pathways, which is given by the reversible direct discharge mechanism, in the potential range of 0–0.05 V vs. RHE. The RhO2 /C support showed a relatively high HOR activity despite the absence of Pt. The single cell results for all materials are in agreement with the RDE findings, with respect to a high HOR activity when the cell is fed with pure H2 . When CO is in the fuel stream, the Pt/RhO2 /C materials with higher RhO2 /C content showed lower reaction overpotential when compared to Pt/C, which was assigned to the presence of both electronic effect and the occurrence of the bi-functional mechanism.
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