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TiO2-C electrocatalyst
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Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst B. Ruiz-Camacho a,*, J.H. Martı´nez-Gonza´lez a, R.G. Gonza´lez-Huerta b, M. Tufin˜o-Vela´zquez c a
Ingenierı´a en Energı´a, Universidad Polite´cnica de Guanajuato, Av. Universidad Norte s/n, Juan Alonso, Corta´zar, Guanajuato C.P. 38483, Mexico b ESIQIE-IPN, Laboratorio de Electrocata´lisis, UPALM, C.P. 07738 Me´xico, D.F., Mexico c ESFM-IPN, Laboratorio de Fı´sica Avanzada, UPALM, C.P. 07738 Me´xico, D.F., Mexico
article info
abstract
Article history:
The electrochemical activity and thermal stability of the Pt/TiO2-C were evaluated in the
Received 28 November 2013
oxygen reduction reaction (ORR) in acid medium at different temperatures. The platinum
Received in revised form
was selectively deposited onto the TiO2 (Ebg ¼ 2.3 eV) by the photo-irradiation of platinum
5 February 2014
precursor (Pt4þ/Pt0). The Pt/TiO2-C electrocatalyst prepared was characterized by XRD,
Accepted 15 February 2014
TEM/EDS, cyclic and lineal voltammetry techniques. TEM images indicated that platinum
Available online xxx
nanoparticles (<5 nm) were deposited in agglomerates form around the oxide sites. EDS and XRD results confirm the composition and crystalline structure of Pt/TiO2-C. The
Keywords:
thermal stability and electrochemical activity of the Pt/TiO2-C for ORR at different tem-
Pt/TiO2-C
peratures (298e343 K) is higher than Pt/C commercial sample (Pt-Etek). A more favorable
Fuel cell performance
apparent enthalpy of activation for Pt/TiO2-C was greatly influenced by addition of oxide in
Oxygen reduction
the catalyst compare to Pt-Etek. Single H2/O2 fuel cell performance results of Pt/TiO2-C
Enthalpy of activation
show an improvement of the power density with the increase of the temperature.
Thermal stability
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The demand for energy is increasing in our modern society. The dependence on oil-based fuels for transportation is the major cause of air pollution in the growing urban areas of the world. It has generated a great concern to find alternative sources of efficiently generated clean energy [1,2]. Nowadays, Proton Exchange Membrane Fuel Cell (PEMFC) has received a lot of attention because of their applications (transportation and portable devices), high efficiencies and low pollution
emissions [1]. Currently, from different types of fuel cells that are being researching, PEMFC is characterized by its fast start up and low operation temperature [2,3]. Platinum supported on high surface area of carbon (e.g., black carbon, Vulcan X-72) is one of the most active catalysts used in the cathode electrode for low temperature fuel cells, PEMFC and Direct Methanol Fuel Cell (DMFC) [4,5]. However, the degradation of carbon support, at the cathode working conditions in PEMFC, is a major cause to the loss of the cell performance during long term operation. A decrease in Pt electrochemical surface area over time caused by the
* Corresponding author. Tel.: þ52 461 4414300x4317. E-mail addresses: [email protected], [email protected] (B. Ruiz-Camacho). http://dx.doi.org/10.1016/j.ijhydene.2014.02.109 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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carbon corrosion is considered a major contributor of this loss in performance of PEMFC. As a consequence, the cathode electrodes present short life time [6e8]. For these reason, a supporting materials stable is necessary, it should have a relationship between electric conductivity and corrosion resistance. Therefore, one of the major efforts in cathode catalysts research is to increase the stability as well as to reduce cost and to improve the electrochemical activity for the cathodic reaction (oxygen reduction reaction, RRO) [9e11]. In order to improve the support stability, several investigations suggest that the addition of semiconductor oxides such as MOx (M ¼ W, Ti, Sn, Nb, Mn, Ce, etc.) promote the fuel cell performance [7e23]. Previously, it has been reported that Pt supported on carbon-modified TiO2 synthesized by photo-deposition method exhibit an enhancing in the electrochemical activity and stability for ORR carried out at RT [11,22e25]. This is due to the synthesis methodology that produces a synergetic effect between the PteTiO2 interactions, also is well knowledge that the TiO2 are intrinsically more resistant to electrochemical corrosion on acidic and oxidative conditions of PEMFC [6]. In the present work, a kinetic study of the ORR over the Pt/ TiO2-C catalyst at different temperatures (298e343 K) as well as their evaluation on a PEM fuel cell was investigated.
Experimental procedure Preparation of Pt/TiO2-C catalysts The synthesis of composites TiO2-C was prepared via route as was described previously [11]. The carbon Vulcan XC-72 (200 mg) was ultrasonically stirred with 2-propanol at room temperature for 2 h. The desired amount of metal oxide chemical isopropoxide precursor was added to the carbon containing suspension, and stirred for 1 h. Water was finally added in excess to hydrolyze the corresponding metal isopropoxide to TiO2 (anatase). The 5 wt.% TiO2-C obtained powder was filtered, washed with water. Finally, it was dried in vacuum at room temperature for 24 h. After that, TiO2-C (52.6 mg) was dispersed on water in a reactor with an optical quartz window. 6 mL of H2PtCl6 solution in 2-propanol was added into the cell and stirred for 3 h under UV-illumination (159 W). 10 wt.% Pt supported on 5 wt.% TiO2-C (Pt/TiO2-C) was obtained by photo-reduction of Pt4þ to Pt0 on the oxide sites of the composite.
Physical characterization The crystalline structure of the prepared catalysts was obtained by recording their X-ray diffraction (XRD) patterns on a A) Bruker D8 AXS equipment using a Cu anode (Ka, l ¼ 1.5406 and a Bragg-Brentano configuration. The angle 2q was varied between 30 and 90 with a step width of 0.2 min1 and 35 kV. Particle size distribution and the elemental composition of sample were obtained with a high resolution transmission electron microscopy (TEM) using a JEOL-JEM-2200 field emission operated at 200 kV.
Electrochemical characterization All electrochemical measurements were carried out in a single three-electrode test electrochemical cell in a 0.5 M H2SO4 aqueous solution. A platinum mesh was used as the counter electrode, and a standard saturated calomel electrode (SCE ¼ 0.244 V/NHE) as the reference electrode. The potentials in this paper were related to normal hydrogen electrode (NHE). The electrochemical measurements were performed in a potentiostat/galvanostat Gamry Instruments reference 3000 and a RDE710 rotation speed controller. Glassy carbon disk with a cross-sectional area of 0.19 cm2 was used as a support for the thin films and used as an ink-type working electrode. RDE experiments were carried out in a water thermostated three-compartment cell for temperature control Luggin capillary. The temperature of the cell was controlled by a thermo-circulator (LabTech). The catalytic ink was prepared with 1 mg of Pt/TiO2-C catalyst, 25 mL of 5 wt.% solution Nafion (Du Pont, 1100 EW) and 125 mL of ultra-pure water. For RDE experiments, 10 mL of this sonicated mixture were deposited on glassy carbon electrode (0.06 mgcat cm2). The 10 wt.% Pt/C commercial catalyst (Pt-Etek) was tested at the same conditions for comparison purposes. Before ORR measurements, cyclic voltammetry (CV) in nitrogen-saturated electrolyte was performed to clean the electrode surface from 0.05 to 1.2 V/NHE at 100 mV s1. Hydrodynamic experiments were recorded at oxygen atmosphere in the rotation range of 200, 400, 900, 1600 and 2500 rpm at 5 mV s1 from 1.0 to 0.2 V/NHE. The thermal study was realized at 298, 313, 323, 333 and 343 K using the RDE technique in acid medium.
H2/O2 fuel cell measurements A catalyst coating membrane method was used for MEA preparation. Nafion115 (Dupont Fluoro Products, thickness 125 mm) membranes was chemical treatment, first in a 3% (w/ w) hydrogen peroxide (H2O2) solution for 1 h at 60 C, then in deionized water for 1 h at 60 C, then in 2 M sulfuric acid (H2SO4) for 1 h at 60 C, and finally in deionized water for 1 h at 60 C, according to the procedure previously described in literature [2]. Before spraying, membranes were dried and flattened. The cathodic slurry was obtained by mixing synthesized catalyst with ethyl alcohol and 5% w Nafion solutions, followed by sonication for 20 min. The platinum (Pt/ TiO2-C) loading on cathodic electrode was 0.1 mg cm2. The commercial electrode (0.4 mg cm2 40 wt.% Pt/C, Fuel Cells Etc-gas diffusion layer GDL-CT) was used in the anode side of the assembly. The MEA with Nafion membrane was prepared by direct spraying of the cathodic slurry on the membrane and then hot pressing the latter between gas-diffusion layers (GDLs) at 120 C and 11 kg cm2 for 2 min. The effective electrode area for the anode and cathode sides was 5 cm2. The commercial catalyst E-Tek was tested using the same procedure and conditions. The MEAs were tested with commercial fuel cell system (Compucell GT, Electrochem 890B) in single cell, which active geometrical area was 5 cm2. The gas pressures (gp) at the anode and cathode sides were kept at 30 psi for H2 and 34 psi for O2 respectively. The fuel cell test station was operated with
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 1 e TEM images of Pt/TiO2-C with different scale bar: (a) 20 nm and (b) 5 nm, respectively. Inset: X ray diffraction patterns of Pt/TiO2-C and TiO2-C. (c) Energy dispersive X-ray spectroscopy (EDS) of Pt/TiO2-C catalyst.
high purity H2 and O2 at 100 cm3 min1. The temperature of the humidified reactant gases was kept 5 C above the temperature of the cell. The performance was measured under steady-state conditions from 25 to 80 C.
Results and discussion Physical characterization results Fig. 1(a) and (b) shows the HRTEM images of Pt/TiO2-C prepared with different scale bar: 10 nm and 2 nm, respectively. The catalysts show a poor dispersion of the Pt nanoparticles onto the TiO2-C according to Fig. 1(a). Fig. 1(b) displays Pt nanoparticles with a particle size less than 5 nm are in form of spheres around the atomic planes of TiO2. The synthesis method used (photo-deposition) in this work favors the deposition of platinum onto oxide sites due to the electronehole pairs generated on the oxides that reduce the noble metal anions [11,23,26e28]. The inset in Fig. 1(a) shows the XRD diffraction patterns of Pt/TiO2-C sample. The powder electrocatalyst showed five diffraction peaks at 2q values of 39.8 , 46.2 , 67.4 , 81.2 and 85.7 correspond to (111), (200), (220), (311) and (222) planes of face-centered cubic platinum structure. Inset of Fig. 1(b) shows the X-ray diffraction patterns of TiO2-C support synthesized by solegel a RT. A broad reflection found at 2q w25 corresponds to (110) planes of the carbon support. On the
sample no visible diffraction peaks of the metal oxide were observed, indicating that TiO2 is highly disordered or amorphous due to the absence of heat treatment during the preparation process. According to literature, a thermal treatment is necessary to favors the crystallography growth of the TiO2 anatase phase, where the peaks become visible only after annealing at 400 C in air atmosphere for 2 h [14]. The metal load is another important factor that suggest that 5 wt.% TiO2 corresponding to anatase are embedded into the carbon matrix [6,7]. Fig. 1(c) presents an elemental composition of Pt/ TiO2-C obtained for EDS technique. The patterns of diffraction peaks from both TiO2 and Pt were identified. Also, Cu signals corresponding to the supported grids were observed.
Electrochemical characterization Cyclic voltammetry (CV) The cyclic voltammograms curves of Pt/TiO2-C and Pt-Etek electrocatalysts in N2-saturated 0.5 M H2SO4 at room temperature are shown in Fig. 2. Briefly both samples exhibit a small H-adsorption/desorption peaks at 0.05e0.3 V and Ptoxide-formation/reduction peaks at 0.9/0.75 V/NHE. In the double layer region of Pt/TiO2-C between 0.3 and 0.8 V/NHE a large capacitive current was observed due to the oxide nature and oxide species segregation [11]. The anodic peak at 0.7 V may be attributed to the oxidation of Ti-species from the dissociation of water due to the presence of TiO2, which was not seen in the Pt/C sample [16]. The Pt oxide reduction is
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 2 e Cyclic voltammograms curves of (e) Pt/TiO2-C and (- -) Pt-Etek catalysts under nitrogen-saturated H2SO4 0.5 M at room temperature. Scan rate: 100 mV sL1.
catalytic activity of Pt/TiO2-C according to the intensities and shift in the oxidation and reduction peaks. Previous reports indicated that a shift to the higher potential values of Pt oxide reduction peak is related with an improved in the Pt catalytic activity for reduction reactions (ORR) [6,7,11e17,26e28]. The electrochemical surface area (ESA) was estimated by integrating the voltammograms corresponding to hydrogen adsorptionedesorption area (0.05e0.3 V) from the electrode surface [29]. For calculation of ESA, 210 mC cm2 was assumed as the monolayer charge [30]. The ESA values calculated demonstrate that the Pt/C-Etek catalyst (3.2 cm2 mg Pt) has a larger surface area than the Pt/TiO2-C (1.1 cm2). These results indicate that more active sites are available on the Pt/C catalyst. The low quantity of ESA obtained on the Pt/TiO2-C sample is possibly caused by poor distribution of the Pt particles as was indicated in the HRTEM results. In the literature, several investigations suggested that ESA for Pt/TiO2-C increase with the Ti-oxide presence due to decreasing Pt particles size [15e17]. However, in some reports the presence of metal oxides inhibits the hydrogen adsorption as in this study [31,32].
Thermal stability of oxygen reduction reaction favorably modified by the presence of TiO2, the onset of the oxide species reduction is shifted to more positive potentials on the Pt/TiO2-C samples as compared to Pt-Etek. The cyclic voltammetry results allow making an estimation of the
Fig. 3 shows the typical currentepotential curves at 298 and 343 K of Pt/TiO2-C and Pt-Etek for the ORR in acid medium. The curves obtained at different rotating speed (200, 400, 900, 1600 and 2500 rpm) for both temperatures show the three distinct
Fig. 3 e ORR currentepotential curves for Pt/TiO2-C (left) and Pt-Etek (right) electrocatalysts measured in O2 saturated H2S04 0.5 M at 5 mV sL1 and different rotation rate (100, 200, 400, 900, 1600 and 2500 rpm) at 298 and 343 K. Inset: Tafel plots calculated from the mass transfer corrected data for the respective temperature. Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Table 1 e Effect of the temperature on the electro-kinetic parameters of Pt/TiO2-C and Pt-Etek catalysts. Catalyst
Temperature, K
Pt/TiO2-C
298 313 323 333 343
Pt-Etek
298 313 323 333 343
jk at 0.9 V, (mA cm2)
Transfer coefficient a
Slop Tafel, b (mV dec1)
0.73 0.77 0.81 0.83 0.86
74 73 72 71 70
3.12 3.62 3.76 3.86 3.99
106 106 106 106 106
0.81 0.84 0.88 0.89 0.92
72 69 66 64 63
7.00 8.43 8.82 9.50 1.08
107 107 107 107 106
96 116 127 147 192 35.3 42.7 44.7 55.6 70.0
region characteristics of ORR: (I) the kinetic region, where the kinetic current, ik; is independent of the velocity rotating; (II) the mixed region where the current is determined by kinetic as well as diffusion process; and (III) the mass-transfer region, where the diffusion current, id, is a function of the velocity of rotating. The inset of Fig. 3 corresponds to the mass corrected Tafel plots obtained for the respective sample at each temperature. Table 1 summarizes the electrochemical parameters obtained from 298 K to 343 K for both catalysts. Fig. 4 shows the electrochemical activity of Pt/TiO2-C and Pt-Etek for ORR at rotating speed of 900 rpm obtained at different temperatures from 298 to 343 K. As expect in both materials is observed a shift towards positive potentials in the kinetic region. Instead of the curves at 900 rpm for different temperatures shows a overlaps of currentepotential curves, the changes in the kinetic region with temperature are presented in the inset of Fig. 4. In short a comparison of the ORR electrochemical activity of both catalysts with temperature is presented in Fig. 5. Pt/ TiO2-C sample exhibit an ORR half-wave potential of 90 mV more positive compared to Pt-Etek at RT (Fig. 5-a). The same observation was found at 343 K (Fig. 5-b). As expected both catalysts exhibit a shift of the ORR currentepotential curve towards positive potentials values with the increase of the temperature however the oxide presence has a significant influence in the electrochemical results.
Exchange current density, jo (mA cm2)
The mass transport corrected Tafel plots for the ORR are shown in Fig. 6. In agreement with the previous electrochemical results, it is evident that the current density for oxide-carbon support catalyst is higher than Pt-Etek in all range of temperature. At 0.9 V/NHE is observed an rise of the kinetic current density (jk, mA cm2) from 96 mA cm2 to 192.3 mA cm2 for Pt/TiO2-C when temperature increase from RT to 343 K. Pt-Etek displays a jk of 35.3 mA cm2 and 70 mA cm2, respectively. These jk results are agreed with several reports [33,34], for example of K. Tiido et al. (2013), how reports that at RT nano-sized Pt (69 mA cm2) catalyst supported onto titanium dioxide functionalized with graphene exhibited a relatively higher electrocatalytic activity to the four-electron reduction of oxygen to water compare to commercial Pt/C (40 mA cm2). For a better understand, the inset of Fig. 6 correspond to Tafel plot in linear scale. These results confirms that Pt/TiO2-C sample have more electrochemical activity than Pt-Etek, that is Pt-oxide interaction modifies the electronic properties of Pt and the bond PtePt. The temperature dependence of the exchange current density was analyzed via conventional Arrhenius analysis [35,36]. The inset of Fig. 7 shows the Arrhenius plot (log jo versus 1/T) of a) Pt/TiO2-C and b) Pt-Etek electrocatalyst in 0.5 M H2SO4. The apparent enthalpy of activation, DH#, was calculated from the linear regression of the slope of the Arrhenius equation represented by (1).
Fig. 4 e ORR polarization curves of Pt/TiO2-C and Pt-Etek measured in O2 saturated H2SO4 0.5 M at scan rate: 5 mV sL1 and electrode rotation rate of 900 rpm at different temperature 298, 313, 323, 333 and 343 K. Inset: comparison of the IeV polarization curve obtained at RT and 343 K for each sample. Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 5 e Oxygen reduction current densities of Pt/TiO2-C compared to Pt-Etek at (a) 298 K and (b) 343 K. Electrolyte: H2S04 0.5 M, Scan rate: 5 mV sL1 and electrode rotation rate of 900 rpm.
dlog jo DH# ¼ dð1=TÞ 2:303R
(1)
An apparent enthalpy of activation DH# ¼ 32 kJ mol1 was calculated for Pt/TiO2-C from the slope of this plots. In the case of Pt-Etek sample shows a value of 45.37 kJ mol1 of enthalpy activation. Both values are in agreement with apparent activation energies reported for platinum-base electrocatalysts for the ORR in acid medium, w20e50 kJ mol1 [31,35e38]. For Pt/TiO2-C and Pt-Etek electrocatalysts, the electrochemical parameters results obtained with the temperature indicates that Tafel slope, b, is dependent on temperature according to the relation given by equation (2) [36]: b¼
dE 2:303RT ¼ dlog i anF
(2)
where: n and a are the number of electrons transferred and the transfer coefficient, respectively. Theoretically, b is temperature dependent if a is assumed to be invariant with
temperature. Temperature dependence of the Tafel slope and transfer coefficient are shown in Fig. 7(a) Pt/TiO2-C and Fig. 7(b) Pt-Etek, respectively. According to Table 1 it can be observed that an experimental Tafel slope is practically invariant with temperature for both samples, leading to a dependence of the transfer coefficient with temperature. An increased linear variation of the charge transfer coefficient with respect to the absolute temperature of da/dT ¼ 2.9 103 K1 for Pt/TiO2-C compared to Pt-Etek, da/dT ¼ 2.5 103 K1. This behavior represents a significant feature and has been considered as an exception in the ORR rather than a rule in this electrochemical process [36e39]. In general the transfer coefficient a varies with the absolute temperature by the linear relationship, equation (3): a ¼ aH þ TaS
(3)
where aH is the enthalpic and aS the entropic component to a. The term aH is related to the change of electrochemical enthalpy of activation with electrode potential; mean while aS is related to the change of electrochemical entropy of activation with electrode potential. aH and aS can be evaluated from the slope and intercept of a nominated Conway plot (plot not included) according to the equation (4) [38]: 1 aH F aS F ¼ þ b 2:303RT 2:303R
(4)
Pt/TiO2-C shows a aH ¼ 2.9 104 and aS ¼ 3.66 103 K1. Therefore, the transfer coefficient for the molecular oxygen reduction in 0.5 M H2SO4 can be written as: a ¼ (2.9 104 K1) *T þ 3.66 103. These means that the both transfercoefficient are important factors that determining the catalytic activity of this reaction. However, aS shows a value of a larger order of magnitude than aH. Similar results were found for Pt-Etek, the transfer coefficient obtained was a ¼ (9.39 104 K1)*T þ 5.9 103.
Fuel cell measurements Fig. 6 e Tafel plots of Pt/TiO2-C and Pt-Etek electrocatalyst obtained from the mass transfer corrected data at 298 K and 343 K. Inset: Linear scale of the same Tafel plots.
To PEMFC, the oxygen gas and hydrogen gas take reactions at the interface of the catalyst and membrane into the MEA, in this gas/solid/liquid interface water molecules are needed to
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 7 e Variation of Tafel slope and transfer coefficient with temperature for (a) Pt/TiO2-C and (b) Pt-Etek. Inset : Arrhenius plot of the exchange current density for the ORR.
form chemical channels into the membrane. Each MEA was prepared by spraying catalyst ink on cathodic side of the pretreated Nafion 115 membrane. Then, the catalyzed membrane was put between porous carbon cloth for the cathode and commercial cloth electrode for the anode. The MEAs were inserted into the fuel cell for the testing processes. Studying the temperature effect on the MEA performance, curves of the cell voltage and power density against current density were recorded at 25, 40, 60 and 80 C, fed with H2/O2 under 30/34 psi pressure. Fig. 8(a) shows the fuel cell performance at different temperatures for the MEA, which was fabricated with Pt/TiO2-C cathode catalysts and Fig. 6(b) shows the performance fuel cell with Pt-Etek cathode catalysts. Open circuit voltages, Eoc, was around 0.94 V on Pt/TiO2-C cathode at the operating temperature. An improvement of the MEA performance with an increase in the operating temperature was observed in both systems. The measured maximum power density, Wmax, to Pt/TiO2-C cathode was 80 mW cm2 at 40 C, increasing to 110 mW cm2 at 80 C, 30 mW cm2 higher than power at 40 C. This behavior confirms that the oxygen reduction reaction on Pt/TiO2-C catalyst is activated by temperature. While the result of power density with Pt/TiO2-C
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Fig. 8 e H2/O2 PEMFC performance of a) Pt/TiO2-C and b) PtEtek cathode catalysts. 40 wt.% Pt/C was used as anode catalysts with a loading of 0.4 mg cmL2. For cathode catalysts, the loading was 0.1 mg cmL2. (B) 80 C, (e) 60 C, (C) 40 C and (-)25 C.
catalyst is lower than that obtained with commercially available Pt-Etek the relatively low output performance observed with the Pt/TiO2-C catalyst may be attributed to poor Pt particles distribution deposited on oxides sites of TiO2carbon agglomeration as was indicated in TEM results. Additional effort in catalyst preparation and assembly characterizations should be carried out to optimize the fuel cell operation.
Conclusions The irradiation effect generates electronehole pairs on the oxide that produced the photo-reduction of Pt4þ to Pt0 under UV-irradiation. Pt nanoparticles in agglomerates form are deposited onto oxide sites as TEM results confirm. The photoreduction of platinum under TiO2 increase the Pt-support interaction that produces a synergetic effect increasing the
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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electrochemical activity for the ORR compared to the Pt-Etek. However is necessary to increase the distribution of TiO2 onto the carbon obtained during preparation by solegel and reduce the time irradiation that produces the growth of the metaloxide particles. The effect of temperature on electrochemical parameters exhibited that the Tafel slop is an independent factor leading to a dependence of the transfer coefficient. A more favorable apparent enthalpy of activation DH# ¼ 32 kJ mol1 for Pt/TiO2-C was obtained compare to the Pt-Etek (DH# ¼ 45.3 kJ mol1). Single H2/O2 fuel cell results of Pt/ TiO2-C shows an improvement of the power density with the increase of the temperature. Pt/TiO2-C shows relative lower power density compared in relation to Pt-Etek electrocatalyst due to the poor distribution of oxide sites of TiO2-carbon caused by the synthesis method. Instead of Pt/TiO2-C catalysts has a poor distribution compared with the required for use in PEMFC, this material synthesized shows a good electrocatalytic activity for the purposes for which it was prepared.
Acknowledgments The authors are grateful to the Universidad Polite´cnica de Guanajuato (UPG) under the project PROMEP No. 103.5/12/3400. This work has been partially supported by the IPN under multidisciplinary project SIP-1540 and Secretarı´a de Ciencia Tecnologı´a e Innovacio´n del DF, SECITI DF, agreement ICYTDF/ 193/2012.
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Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst B. Ruiz-Camacho a,*, J.H. Martı´nez-Gonza´lez a, R.G. Gonza´lez-Huerta b, M. Tufin˜o-Vela´zquez c a
Ingenierı´a en Energı´a, Universidad Polite´cnica de Guanajuato, Av. Universidad Norte s/n, Juan Alonso, Corta´zar, Guanajuato C.P. 38483, Mexico b ESIQIE-IPN, Laboratorio de Electrocata´lisis, UPALM, C.P. 07738 Me´xico, D.F., Mexico c ESFM-IPN, Laboratorio de Fı´sica Avanzada, UPALM, C.P. 07738 Me´xico, D.F., Mexico
article info
abstract
Article history:
The electrochemical activity and thermal stability of the Pt/TiO2-C were evaluated in the
Received 28 November 2013
oxygen reduction reaction (ORR) in acid medium at different temperatures. The platinum
Received in revised form
was selectively deposited onto the TiO2 (Ebg ¼ 2.3 eV) by the photo-irradiation of platinum
5 February 2014
precursor (Pt4þ/Pt0). The Pt/TiO2-C electrocatalyst prepared was characterized by XRD,
Accepted 15 February 2014
TEM/EDS, cyclic and lineal voltammetry techniques. TEM images indicated that platinum
Available online xxx
nanoparticles (<5 nm) were deposited in agglomerates form around the oxide sites. EDS and XRD results confirm the composition and crystalline structure of Pt/TiO2-C. The
Keywords:
thermal stability and electrochemical activity of the Pt/TiO2-C for ORR at different tem-
Pt/TiO2-C
peratures (298e343 K) is higher than Pt/C commercial sample (Pt-Etek). A more favorable
Fuel cell performance
apparent enthalpy of activation for Pt/TiO2-C was greatly influenced by addition of oxide in
Oxygen reduction
the catalyst compare to Pt-Etek. Single H2/O2 fuel cell performance results of Pt/TiO2-C
Enthalpy of activation
show an improvement of the power density with the increase of the temperature.
Thermal stability
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The demand for energy is increasing in our modern society. The dependence on oil-based fuels for transportation is the major cause of air pollution in the growing urban areas of the world. It has generated a great concern to find alternative sources of efficiently generated clean energy [1,2]. Nowadays, Proton Exchange Membrane Fuel Cell (PEMFC) has received a lot of attention because of their applications (transportation and portable devices), high efficiencies and low pollution
emissions [1]. Currently, from different types of fuel cells that are being researching, PEMFC is characterized by its fast start up and low operation temperature [2,3]. Platinum supported on high surface area of carbon (e.g., black carbon, Vulcan X-72) is one of the most active catalysts used in the cathode electrode for low temperature fuel cells, PEMFC and Direct Methanol Fuel Cell (DMFC) [4,5]. However, the degradation of carbon support, at the cathode working conditions in PEMFC, is a major cause to the loss of the cell performance during long term operation. A decrease in Pt electrochemical surface area over time caused by the
* Corresponding author. Tel.: þ52 461 4414300x4317. E-mail addresses: [email protected], [email protected] (B. Ruiz-Camacho). http://dx.doi.org/10.1016/j.ijhydene.2014.02.109 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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carbon corrosion is considered a major contributor of this loss in performance of PEMFC. As a consequence, the cathode electrodes present short life time [6e8]. For these reason, a supporting materials stable is necessary, it should have a relationship between electric conductivity and corrosion resistance. Therefore, one of the major efforts in cathode catalysts research is to increase the stability as well as to reduce cost and to improve the electrochemical activity for the cathodic reaction (oxygen reduction reaction, RRO) [9e11]. In order to improve the support stability, several investigations suggest that the addition of semiconductor oxides such as MOx (M ¼ W, Ti, Sn, Nb, Mn, Ce, etc.) promote the fuel cell performance [7e23]. Previously, it has been reported that Pt supported on carbon-modified TiO2 synthesized by photo-deposition method exhibit an enhancing in the electrochemical activity and stability for ORR carried out at RT [11,22e25]. This is due to the synthesis methodology that produces a synergetic effect between the PteTiO2 interactions, also is well knowledge that the TiO2 are intrinsically more resistant to electrochemical corrosion on acidic and oxidative conditions of PEMFC [6]. In the present work, a kinetic study of the ORR over the Pt/ TiO2-C catalyst at different temperatures (298e343 K) as well as their evaluation on a PEM fuel cell was investigated.
Experimental procedure Preparation of Pt/TiO2-C catalysts The synthesis of composites TiO2-C was prepared via route as was described previously [11]. The carbon Vulcan XC-72 (200 mg) was ultrasonically stirred with 2-propanol at room temperature for 2 h. The desired amount of metal oxide chemical isopropoxide precursor was added to the carbon containing suspension, and stirred for 1 h. Water was finally added in excess to hydrolyze the corresponding metal isopropoxide to TiO2 (anatase). The 5 wt.% TiO2-C obtained powder was filtered, washed with water. Finally, it was dried in vacuum at room temperature for 24 h. After that, TiO2-C (52.6 mg) was dispersed on water in a reactor with an optical quartz window. 6 mL of H2PtCl6 solution in 2-propanol was added into the cell and stirred for 3 h under UV-illumination (159 W). 10 wt.% Pt supported on 5 wt.% TiO2-C (Pt/TiO2-C) was obtained by photo-reduction of Pt4þ to Pt0 on the oxide sites of the composite.
Physical characterization The crystalline structure of the prepared catalysts was obtained by recording their X-ray diffraction (XRD) patterns on a A) Bruker D8 AXS equipment using a Cu anode (Ka, l ¼ 1.5406 and a Bragg-Brentano configuration. The angle 2q was varied between 30 and 90 with a step width of 0.2 min1 and 35 kV. Particle size distribution and the elemental composition of sample were obtained with a high resolution transmission electron microscopy (TEM) using a JEOL-JEM-2200 field emission operated at 200 kV.
Electrochemical characterization All electrochemical measurements were carried out in a single three-electrode test electrochemical cell in a 0.5 M H2SO4 aqueous solution. A platinum mesh was used as the counter electrode, and a standard saturated calomel electrode (SCE ¼ 0.244 V/NHE) as the reference electrode. The potentials in this paper were related to normal hydrogen electrode (NHE). The electrochemical measurements were performed in a potentiostat/galvanostat Gamry Instruments reference 3000 and a RDE710 rotation speed controller. Glassy carbon disk with a cross-sectional area of 0.19 cm2 was used as a support for the thin films and used as an ink-type working electrode. RDE experiments were carried out in a water thermostated three-compartment cell for temperature control Luggin capillary. The temperature of the cell was controlled by a thermo-circulator (LabTech). The catalytic ink was prepared with 1 mg of Pt/TiO2-C catalyst, 25 mL of 5 wt.% solution Nafion (Du Pont, 1100 EW) and 125 mL of ultra-pure water. For RDE experiments, 10 mL of this sonicated mixture were deposited on glassy carbon electrode (0.06 mgcat cm2). The 10 wt.% Pt/C commercial catalyst (Pt-Etek) was tested at the same conditions for comparison purposes. Before ORR measurements, cyclic voltammetry (CV) in nitrogen-saturated electrolyte was performed to clean the electrode surface from 0.05 to 1.2 V/NHE at 100 mV s1. Hydrodynamic experiments were recorded at oxygen atmosphere in the rotation range of 200, 400, 900, 1600 and 2500 rpm at 5 mV s1 from 1.0 to 0.2 V/NHE. The thermal study was realized at 298, 313, 323, 333 and 343 K using the RDE technique in acid medium.
H2/O2 fuel cell measurements A catalyst coating membrane method was used for MEA preparation. Nafion115 (Dupont Fluoro Products, thickness 125 mm) membranes was chemical treatment, first in a 3% (w/ w) hydrogen peroxide (H2O2) solution for 1 h at 60 C, then in deionized water for 1 h at 60 C, then in 2 M sulfuric acid (H2SO4) for 1 h at 60 C, and finally in deionized water for 1 h at 60 C, according to the procedure previously described in literature [2]. Before spraying, membranes were dried and flattened. The cathodic slurry was obtained by mixing synthesized catalyst with ethyl alcohol and 5% w Nafion solutions, followed by sonication for 20 min. The platinum (Pt/ TiO2-C) loading on cathodic electrode was 0.1 mg cm2. The commercial electrode (0.4 mg cm2 40 wt.% Pt/C, Fuel Cells Etc-gas diffusion layer GDL-CT) was used in the anode side of the assembly. The MEA with Nafion membrane was prepared by direct spraying of the cathodic slurry on the membrane and then hot pressing the latter between gas-diffusion layers (GDLs) at 120 C and 11 kg cm2 for 2 min. The effective electrode area for the anode and cathode sides was 5 cm2. The commercial catalyst E-Tek was tested using the same procedure and conditions. The MEAs were tested with commercial fuel cell system (Compucell GT, Electrochem 890B) in single cell, which active geometrical area was 5 cm2. The gas pressures (gp) at the anode and cathode sides were kept at 30 psi for H2 and 34 psi for O2 respectively. The fuel cell test station was operated with
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 1 e TEM images of Pt/TiO2-C with different scale bar: (a) 20 nm and (b) 5 nm, respectively. Inset: X ray diffraction patterns of Pt/TiO2-C and TiO2-C. (c) Energy dispersive X-ray spectroscopy (EDS) of Pt/TiO2-C catalyst.
high purity H2 and O2 at 100 cm3 min1. The temperature of the humidified reactant gases was kept 5 C above the temperature of the cell. The performance was measured under steady-state conditions from 25 to 80 C.
Results and discussion Physical characterization results Fig. 1(a) and (b) shows the HRTEM images of Pt/TiO2-C prepared with different scale bar: 10 nm and 2 nm, respectively. The catalysts show a poor dispersion of the Pt nanoparticles onto the TiO2-C according to Fig. 1(a). Fig. 1(b) displays Pt nanoparticles with a particle size less than 5 nm are in form of spheres around the atomic planes of TiO2. The synthesis method used (photo-deposition) in this work favors the deposition of platinum onto oxide sites due to the electronehole pairs generated on the oxides that reduce the noble metal anions [11,23,26e28]. The inset in Fig. 1(a) shows the XRD diffraction patterns of Pt/TiO2-C sample. The powder electrocatalyst showed five diffraction peaks at 2q values of 39.8 , 46.2 , 67.4 , 81.2 and 85.7 correspond to (111), (200), (220), (311) and (222) planes of face-centered cubic platinum structure. Inset of Fig. 1(b) shows the X-ray diffraction patterns of TiO2-C support synthesized by solegel a RT. A broad reflection found at 2q w25 corresponds to (110) planes of the carbon support. On the
sample no visible diffraction peaks of the metal oxide were observed, indicating that TiO2 is highly disordered or amorphous due to the absence of heat treatment during the preparation process. According to literature, a thermal treatment is necessary to favors the crystallography growth of the TiO2 anatase phase, where the peaks become visible only after annealing at 400 C in air atmosphere for 2 h [14]. The metal load is another important factor that suggest that 5 wt.% TiO2 corresponding to anatase are embedded into the carbon matrix [6,7]. Fig. 1(c) presents an elemental composition of Pt/ TiO2-C obtained for EDS technique. The patterns of diffraction peaks from both TiO2 and Pt were identified. Also, Cu signals corresponding to the supported grids were observed.
Electrochemical characterization Cyclic voltammetry (CV) The cyclic voltammograms curves of Pt/TiO2-C and Pt-Etek electrocatalysts in N2-saturated 0.5 M H2SO4 at room temperature are shown in Fig. 2. Briefly both samples exhibit a small H-adsorption/desorption peaks at 0.05e0.3 V and Ptoxide-formation/reduction peaks at 0.9/0.75 V/NHE. In the double layer region of Pt/TiO2-C between 0.3 and 0.8 V/NHE a large capacitive current was observed due to the oxide nature and oxide species segregation [11]. The anodic peak at 0.7 V may be attributed to the oxidation of Ti-species from the dissociation of water due to the presence of TiO2, which was not seen in the Pt/C sample [16]. The Pt oxide reduction is
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 2 e Cyclic voltammograms curves of (e) Pt/TiO2-C and (- -) Pt-Etek catalysts under nitrogen-saturated H2SO4 0.5 M at room temperature. Scan rate: 100 mV sL1.
catalytic activity of Pt/TiO2-C according to the intensities and shift in the oxidation and reduction peaks. Previous reports indicated that a shift to the higher potential values of Pt oxide reduction peak is related with an improved in the Pt catalytic activity for reduction reactions (ORR) [6,7,11e17,26e28]. The electrochemical surface area (ESA) was estimated by integrating the voltammograms corresponding to hydrogen adsorptionedesorption area (0.05e0.3 V) from the electrode surface [29]. For calculation of ESA, 210 mC cm2 was assumed as the monolayer charge [30]. The ESA values calculated demonstrate that the Pt/C-Etek catalyst (3.2 cm2 mg Pt) has a larger surface area than the Pt/TiO2-C (1.1 cm2). These results indicate that more active sites are available on the Pt/C catalyst. The low quantity of ESA obtained on the Pt/TiO2-C sample is possibly caused by poor distribution of the Pt particles as was indicated in the HRTEM results. In the literature, several investigations suggested that ESA for Pt/TiO2-C increase with the Ti-oxide presence due to decreasing Pt particles size [15e17]. However, in some reports the presence of metal oxides inhibits the hydrogen adsorption as in this study [31,32].
Thermal stability of oxygen reduction reaction favorably modified by the presence of TiO2, the onset of the oxide species reduction is shifted to more positive potentials on the Pt/TiO2-C samples as compared to Pt-Etek. The cyclic voltammetry results allow making an estimation of the
Fig. 3 shows the typical currentepotential curves at 298 and 343 K of Pt/TiO2-C and Pt-Etek for the ORR in acid medium. The curves obtained at different rotating speed (200, 400, 900, 1600 and 2500 rpm) for both temperatures show the three distinct
Fig. 3 e ORR currentepotential curves for Pt/TiO2-C (left) and Pt-Etek (right) electrocatalysts measured in O2 saturated H2S04 0.5 M at 5 mV sL1 and different rotation rate (100, 200, 400, 900, 1600 and 2500 rpm) at 298 and 343 K. Inset: Tafel plots calculated from the mass transfer corrected data for the respective temperature. Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Table 1 e Effect of the temperature on the electro-kinetic parameters of Pt/TiO2-C and Pt-Etek catalysts. Catalyst
Temperature, K
Pt/TiO2-C
298 313 323 333 343
Pt-Etek
298 313 323 333 343
jk at 0.9 V, (mA cm2)
Transfer coefficient a
Slop Tafel, b (mV dec1)
0.73 0.77 0.81 0.83 0.86
74 73 72 71 70
3.12 3.62 3.76 3.86 3.99
106 106 106 106 106
0.81 0.84 0.88 0.89 0.92
72 69 66 64 63
7.00 8.43 8.82 9.50 1.08
107 107 107 107 106
96 116 127 147 192 35.3 42.7 44.7 55.6 70.0
region characteristics of ORR: (I) the kinetic region, where the kinetic current, ik; is independent of the velocity rotating; (II) the mixed region where the current is determined by kinetic as well as diffusion process; and (III) the mass-transfer region, where the diffusion current, id, is a function of the velocity of rotating. The inset of Fig. 3 corresponds to the mass corrected Tafel plots obtained for the respective sample at each temperature. Table 1 summarizes the electrochemical parameters obtained from 298 K to 343 K for both catalysts. Fig. 4 shows the electrochemical activity of Pt/TiO2-C and Pt-Etek for ORR at rotating speed of 900 rpm obtained at different temperatures from 298 to 343 K. As expect in both materials is observed a shift towards positive potentials in the kinetic region. Instead of the curves at 900 rpm for different temperatures shows a overlaps of currentepotential curves, the changes in the kinetic region with temperature are presented in the inset of Fig. 4. In short a comparison of the ORR electrochemical activity of both catalysts with temperature is presented in Fig. 5. Pt/ TiO2-C sample exhibit an ORR half-wave potential of 90 mV more positive compared to Pt-Etek at RT (Fig. 5-a). The same observation was found at 343 K (Fig. 5-b). As expected both catalysts exhibit a shift of the ORR currentepotential curve towards positive potentials values with the increase of the temperature however the oxide presence has a significant influence in the electrochemical results.
Exchange current density, jo (mA cm2)
The mass transport corrected Tafel plots for the ORR are shown in Fig. 6. In agreement with the previous electrochemical results, it is evident that the current density for oxide-carbon support catalyst is higher than Pt-Etek in all range of temperature. At 0.9 V/NHE is observed an rise of the kinetic current density (jk, mA cm2) from 96 mA cm2 to 192.3 mA cm2 for Pt/TiO2-C when temperature increase from RT to 343 K. Pt-Etek displays a jk of 35.3 mA cm2 and 70 mA cm2, respectively. These jk results are agreed with several reports [33,34], for example of K. Tiido et al. (2013), how reports that at RT nano-sized Pt (69 mA cm2) catalyst supported onto titanium dioxide functionalized with graphene exhibited a relatively higher electrocatalytic activity to the four-electron reduction of oxygen to water compare to commercial Pt/C (40 mA cm2). For a better understand, the inset of Fig. 6 correspond to Tafel plot in linear scale. These results confirms that Pt/TiO2-C sample have more electrochemical activity than Pt-Etek, that is Pt-oxide interaction modifies the electronic properties of Pt and the bond PtePt. The temperature dependence of the exchange current density was analyzed via conventional Arrhenius analysis [35,36]. The inset of Fig. 7 shows the Arrhenius plot (log jo versus 1/T) of a) Pt/TiO2-C and b) Pt-Etek electrocatalyst in 0.5 M H2SO4. The apparent enthalpy of activation, DH#, was calculated from the linear regression of the slope of the Arrhenius equation represented by (1).
Fig. 4 e ORR polarization curves of Pt/TiO2-C and Pt-Etek measured in O2 saturated H2SO4 0.5 M at scan rate: 5 mV sL1 and electrode rotation rate of 900 rpm at different temperature 298, 313, 323, 333 and 343 K. Inset: comparison of the IeV polarization curve obtained at RT and 343 K for each sample. Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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Fig. 5 e Oxygen reduction current densities of Pt/TiO2-C compared to Pt-Etek at (a) 298 K and (b) 343 K. Electrolyte: H2S04 0.5 M, Scan rate: 5 mV sL1 and electrode rotation rate of 900 rpm.
dlog jo DH# ¼ dð1=TÞ 2:303R
(1)
An apparent enthalpy of activation DH# ¼ 32 kJ mol1 was calculated for Pt/TiO2-C from the slope of this plots. In the case of Pt-Etek sample shows a value of 45.37 kJ mol1 of enthalpy activation. Both values are in agreement with apparent activation energies reported for platinum-base electrocatalysts for the ORR in acid medium, w20e50 kJ mol1 [31,35e38]. For Pt/TiO2-C and Pt-Etek electrocatalysts, the electrochemical parameters results obtained with the temperature indicates that Tafel slope, b, is dependent on temperature according to the relation given by equation (2) [36]: b¼
dE 2:303RT ¼ dlog i anF
(2)
where: n and a are the number of electrons transferred and the transfer coefficient, respectively. Theoretically, b is temperature dependent if a is assumed to be invariant with
temperature. Temperature dependence of the Tafel slope and transfer coefficient are shown in Fig. 7(a) Pt/TiO2-C and Fig. 7(b) Pt-Etek, respectively. According to Table 1 it can be observed that an experimental Tafel slope is practically invariant with temperature for both samples, leading to a dependence of the transfer coefficient with temperature. An increased linear variation of the charge transfer coefficient with respect to the absolute temperature of da/dT ¼ 2.9 103 K1 for Pt/TiO2-C compared to Pt-Etek, da/dT ¼ 2.5 103 K1. This behavior represents a significant feature and has been considered as an exception in the ORR rather than a rule in this electrochemical process [36e39]. In general the transfer coefficient a varies with the absolute temperature by the linear relationship, equation (3): a ¼ aH þ TaS
(3)
where aH is the enthalpic and aS the entropic component to a. The term aH is related to the change of electrochemical enthalpy of activation with electrode potential; mean while aS is related to the change of electrochemical entropy of activation with electrode potential. aH and aS can be evaluated from the slope and intercept of a nominated Conway plot (plot not included) according to the equation (4) [38]: 1 aH F aS F ¼ þ b 2:303RT 2:303R
(4)
Pt/TiO2-C shows a aH ¼ 2.9 104 and aS ¼ 3.66 103 K1. Therefore, the transfer coefficient for the molecular oxygen reduction in 0.5 M H2SO4 can be written as: a ¼ (2.9 104 K1) *T þ 3.66 103. These means that the both transfercoefficient are important factors that determining the catalytic activity of this reaction. However, aS shows a value of a larger order of magnitude than aH. Similar results were found for Pt-Etek, the transfer coefficient obtained was a ¼ (9.39 104 K1)*T þ 5.9 103.
Fuel cell measurements Fig. 6 e Tafel plots of Pt/TiO2-C and Pt-Etek electrocatalyst obtained from the mass transfer corrected data at 298 K and 343 K. Inset: Linear scale of the same Tafel plots.
To PEMFC, the oxygen gas and hydrogen gas take reactions at the interface of the catalyst and membrane into the MEA, in this gas/solid/liquid interface water molecules are needed to
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e9
Fig. 7 e Variation of Tafel slope and transfer coefficient with temperature for (a) Pt/TiO2-C and (b) Pt-Etek. Inset : Arrhenius plot of the exchange current density for the ORR.
form chemical channels into the membrane. Each MEA was prepared by spraying catalyst ink on cathodic side of the pretreated Nafion 115 membrane. Then, the catalyzed membrane was put between porous carbon cloth for the cathode and commercial cloth electrode for the anode. The MEAs were inserted into the fuel cell for the testing processes. Studying the temperature effect on the MEA performance, curves of the cell voltage and power density against current density were recorded at 25, 40, 60 and 80 C, fed with H2/O2 under 30/34 psi pressure. Fig. 8(a) shows the fuel cell performance at different temperatures for the MEA, which was fabricated with Pt/TiO2-C cathode catalysts and Fig. 6(b) shows the performance fuel cell with Pt-Etek cathode catalysts. Open circuit voltages, Eoc, was around 0.94 V on Pt/TiO2-C cathode at the operating temperature. An improvement of the MEA performance with an increase in the operating temperature was observed in both systems. The measured maximum power density, Wmax, to Pt/TiO2-C cathode was 80 mW cm2 at 40 C, increasing to 110 mW cm2 at 80 C, 30 mW cm2 higher than power at 40 C. This behavior confirms that the oxygen reduction reaction on Pt/TiO2-C catalyst is activated by temperature. While the result of power density with Pt/TiO2-C
7
Fig. 8 e H2/O2 PEMFC performance of a) Pt/TiO2-C and b) PtEtek cathode catalysts. 40 wt.% Pt/C was used as anode catalysts with a loading of 0.4 mg cmL2. For cathode catalysts, the loading was 0.1 mg cmL2. (B) 80 C, (e) 60 C, (C) 40 C and (-)25 C.
catalyst is lower than that obtained with commercially available Pt-Etek the relatively low output performance observed with the Pt/TiO2-C catalyst may be attributed to poor Pt particles distribution deposited on oxides sites of TiO2carbon agglomeration as was indicated in TEM results. Additional effort in catalyst preparation and assembly characterizations should be carried out to optimize the fuel cell operation.
Conclusions The irradiation effect generates electronehole pairs on the oxide that produced the photo-reduction of Pt4þ to Pt0 under UV-irradiation. Pt nanoparticles in agglomerates form are deposited onto oxide sites as TEM results confirm. The photoreduction of platinum under TiO2 increase the Pt-support interaction that produces a synergetic effect increasing the
Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109
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electrochemical activity for the ORR compared to the Pt-Etek. However is necessary to increase the distribution of TiO2 onto the carbon obtained during preparation by solegel and reduce the time irradiation that produces the growth of the metaloxide particles. The effect of temperature on electrochemical parameters exhibited that the Tafel slop is an independent factor leading to a dependence of the transfer coefficient. A more favorable apparent enthalpy of activation DH# ¼ 32 kJ mol1 for Pt/TiO2-C was obtained compare to the Pt-Etek (DH# ¼ 45.3 kJ mol1). Single H2/O2 fuel cell results of Pt/ TiO2-C shows an improvement of the power density with the increase of the temperature. Pt/TiO2-C shows relative lower power density compared in relation to Pt-Etek electrocatalyst due to the poor distribution of oxide sites of TiO2-carbon caused by the synthesis method. Instead of Pt/TiO2-C catalysts has a poor distribution compared with the required for use in PEMFC, this material synthesized shows a good electrocatalytic activity for the purposes for which it was prepared.
Acknowledgments The authors are grateful to the Universidad Polite´cnica de Guanajuato (UPG) under the project PROMEP No. 103.5/12/3400. This work has been partially supported by the IPN under multidisciplinary project SIP-1540 and Secretarı´a de Ciencia Tecnologı´a e Innovacio´n del DF, SECITI DF, agreement ICYTDF/ 193/2012.
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Please cite this article in press as: Ruiz-Camacho B, et al., Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.02.109