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Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction
Journal of Energy Chemistry 23(2014)701–707
Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction Anqi Zhaoa , Justus Masab ,
Wei Xiaa∗
a. Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum, Germany; b. Analytical Chemistry and Center for Electrochemical Sciences-CES, Ruhr-University Bochum, D-44780 Bochum, Germany [ Manuscript received June 9, 2014; revised August 11, 2014 ]
Abstract Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO2−x and TiO2−x Ny were obtained by thermal treatment of anatase TiO2 under flowing H2 and NH3 , respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N2 -physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO4 . Pt/TiO2−x and Pt/TiO2−x Ny showed higher ORR activities than Pt/TiO2 as indicated by higher onset potentials. Oxygen deficiency in TiO2−x and TiO2−x Ny contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO2−x exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO2−x support. Key words TiO2 ; oxygen-deficiency; Pt catalyst; oxygen reduction reaction
1. Introduction Fuel cells as electrochemical energy conversion devices have attracted considerable attention over the past few decades due to their high efficiency and low emission [1,2]. The oxygen reduction reaction (ORR) at the cathode is one of the key factors in the performance of a fuel cell [3]. In recent years, there has been extensive effort towards the development of ORR electrocatalysts with high efficiencies. Platinum-based catalysts are among the most widely investigated materials for the ORR under acidic conditions [4,5]. Carbon black is the most commonly used support for electrocatalysts because of several advantages including high electronic conductivity, low cost and high surface area. However, the oxidation of carbon black occurs during long-term operation which leads to loss of ORR performance. It is known that metal species on carbon surfaces promote the decomposition of surface oxygen groups leading to the degradation of carbon [6]. In electrolyte under electrochemical conditions Pt particles supported on carbon catalyze the oxidation of the carbon in a similar way [7,8]. The degradation of carbon ∗
is a major reason for the loss of the active surface area of carbon supported Pt catalysts, which occurs through migration/agglomeration or dissolution (and possible re-deposition) of Pt [9−11]. Hence, high Pt loadings are often needed to compensate for the loss of Pt active surface area in long-term operation. Owing to the intrinsic instability of carbon, it is necessary to look for alternative supports that are resistant against electrochemical corrosion and stable in the long term. It is highly desirable to find support materials that promote strong metal-support interactions, which can improve the stability of Pt and modify its electronic structure for electrocatalysis. Additionally, the electronic conductivity and surface area of the support should be sufficient for electrocatalytic ORR. TiO2 is a promising alternative to carbon black due to its high corrosion resistance. In contrast to the inert surface of carbon materials, there is strong interaction between Pt and TiO2 , i.e., the strong metal-support interaction, which stabilizes the Pt particles. It was reported that highly dispersed Pt nanoparticles on porous TiO2 exhibited a higher active surface area than those on carbon in the ORR [12]. The high
Corresponding author. Tel: +49-234-3223566; Fax: +49-234-3214115; E-mail: [email protected]
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60202-3
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dispersion was attributed to strong metal-support interaction between Pt and TiO2 . The combination of Pt/TiO2 and conductive carbon particles provided a higher electrochemical active surface area than Pt/C, which enhanced the ORR performance under acidic conditions [13]. In other studies, Pt was deposited on carbon-coated anatase TiO2 , and the Pt catalysts showed superior activities for methanol oxidation and ORR [14,15]. In addition, Pt nanoparticles supported on mesoporous TiO2 prepared by an ultrasound-assisted reduction method showed a higher electrocatalytic activity and stability than commercial Pt/C [16]. Very recently, the composite of N-doped carbon nanotubes and mesoporous TiO2 was used as support for Pt, and accelerated stress tests disclosed that the catalyst was more stable than Pt/C under electrochemical conditions [17]. TiO2 is a wide band gap semiconductor and its conductivity is insufficient for electrocatalysis without modification. Recent studies indicate that oxygen-deficiency or nitrogen doping can improve charge transfer in TiO2 [18]. Oxygen vacancies can be introduced into TiO2 by thermal treatment in a reducing atmosphere at elevated temperatures, which leads to oxygen-deficient TiO2 (TiO2−x ) or sub-stoichiometric phases like Ti4 O7 [19,20]. Electrochemical studies showed that Ti4 O7 had a much higher onset corrosion potential than Vulcan XC-72, indicating a much higher oxidation resistance under real PEM fuel cell operating conditions [21]. However, a larger Pt cluster size was formed because of the decrease of the interaction between support and Pt particles. In this work, modified TiO2 was used as support for Pt catalysts for the ORR under acidic conditions. TiO2−x and TiO2−x Ny used as support were obtained by thermal treatment of anatase TiO2 nanoparticles in H2 and NH3 , respectively. The Pt catalysts were synthesized by a modified ethylene glycol method. The modified TiO2 supports and supported Pt catalysts were characterized by various techniques. The electrocatalytic activity and long-term stability of the catalysts were studied.
glycol (1.0 mol/L) was dropped into the H2 PtCl6 solution until the pH reached 12. The obtained mixture was stirred at room temperature for 1 day, and then refluxed at 160 ◦ C for 3 h in inert atmosphere. The obtained dark brown Pt colloid was stored at room temperature in air for further applications. For deposition of Pt on TiO2 , 300 mg of the support (TiO2 , TiO2−x and TiO2−x Ny ) were mixed with 50 mL of Pt colloid, and the pH of the mixture was adjusted to 2 by adding 1.0 mol/L HCl (in ethylene glycol). The mixture was refluxed at 160 ◦ C for 1.5 h before cooling to room temperature. After filtration, washing with ethanol, and drying at 60 ◦ C in air, the obtained catalysts were reduced at 300 ◦ C for 1 h under flowing hydrogen. The samples were ground in an agate mortar before further tests. 2.2. Characterization
2. Experimental
N2 -physisorption measurements were carried out at 77 K using an Autosorb-1 MP Quantachrome system. Prior to the measurements, all samples were degassed at 300 ◦ C for 2 h. The surface areas were calculated from the linear part of the Brumauer-Emmett-Teller (BET) plots. The pore volume and the pore size distribution were derived from the desorption profiles of the isotherms using the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) was performed using a PANalytical theta-theta powder diffractometer with a Cu Kα source. Scans were run from 10o to 80o with a step width of 0.03o and a collection time of 20 s per step. Elemental analysis was carried out with an ICP-OES instrument (PU701) from Philips-Unicam. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultrahigh vacuum (UHV) set-up equipped with a monochromatic Al Kα X-ray source (1486.6 eV; anode operating at 14.5 kV and 45 mA) and a high resolution Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was maintained at about 7×10−10 mbar. The XP spectra were recorded in the fixed transmission mode with a pass energy of 200. Charging effects were compensated by applying a flood gun.
2.1. Synthesis of catalysts
2.3. Electrochemical tests
Commercial anatase TiO2 (Sachtleben Chemie, Duisburg, Germany) with a specific surface area of 100 m2 ·g−1 was used as starting material. Thermal treatment was carried out in a horizontal quartz-tube reactor with an inner diameter of 30 mm. The treatment was performed at 450 ◦ C for 10 h under flowing oxygen (10 vol% O2 in N2 ), hydrogen (50 vol% H2 in He) and ammonia (5 vol% NH3 in He). A total flow of 100 mL·min−1 was applied with 1 g TiO2 in each experiment. The obtained samples were denoted as TiO2 , TiO2−x and TiO2−x Ny , respectively. Supported Pt catalysts were synthesized by a modified ethylene glycol method [22]. In a typical experiment, 0.45 g of H2 PtCl6 ·6H2 O (Aldrich) was dissolved in 250 mL of ethylene glycol (99.8%, Aldrich). A solution of NaOH in ethylene
Electrochemical measurements were performed in a conventional three-electrode cell using glassy carbon (F 4 mm; HTW, Germany) modified with the catalysts as the working electrode, Ag/AgCl/3 mol/L KCl as the reference electrode and Pt foil as the counter electrode. The reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE). Prior to the experiment, the glassy carbon electrode was polished on polishing cloth using different alumina pastes (3.0–0.05 µm) to obtain a mirror-like surface, followed by ultrasonic cleaning in water. For the preparation of the working electrode, 5.0 mg of the catalyst and carbon black in a weight ratio of 1 : 0.2 were dispersed ultrasonically for 60 min in a mixture of water (490 µL), ethanol (490 µL) and Nafionr (5%, 20 µL). 5.3 µL of the resulting catalyst suspen-
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sion was dropped onto a polished glassy carbon electrode to obtain a catalyst loading of 210 µg·cm−2. The electrode was dried in air at room temperature before measurement. Linear sweep voltammetry (LSV) measurements were carried out in a 0.1 mol/L HClO4 electrolyte saturated with argon or oxygen at room temperature using an Autolab potentiostat/galvanostat (PGSTAT12, Eco Chemie, Utrecht, The Netherlands). All experiments for oxygen reduction were carried out at room temperature at a scan rate of 10 mV·s−1 after purging with argon or oxygen for 20 min. Before each measurement, the catalyst was scanned in Ar-saturated electrolyte and the obtained background voltammogram was subtracted from that measured in the O2 -saturated electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit potential in a solution of potassium hexacyanoferrate (II)/potassium hexacyanoferrate (III) (0.05 mol/L) in KOH (0.1 mol/L) using ac perturbation of 10 mV in the frequency range from 0.1 to 100000 Hz.
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essentially identical diffraction patterns like that of the TiO2 sample, indicating that there was no phase transition to rutile or other phases during the treatment under flowing hydrogen or ammonia at 450 ◦ C for 10 h. The crystallite sizes of the TiO2 , TiO2−x and TiO2−x Ny samples were 30.8, 31.3 and 30.8 nm, respectively, as calculated using the Scherrer equation from the strongest anatase diffraction peak (101). Even though the difference cannot be observed from XRD patterns, the color changed clearly from white to dark yellow after thermal treatment in the applied reducing atmosphere.
3. Results and discussion Figure 1 shows the N2 adsorption-desorption isotherms of the TiO2 , TiO2−x and TiO2−x Ny samples. All adsorptiondesorption isotherms show type IV behavior with H3 hysteresis loop according to the IUPAC classification [23,24]. The hysteresis loops had adsorption and desorption branches in the p/p0 range from 0.65 to 0.95, which is usually associated with capillary condensation in mesoporous structures. These mesoporous structures are thought to be produced by inter-aggregated TiO2 nanoparticles. There was no significant change in the isotherms after treatment under different atmospheres, which indicates that the microstructure of TiO2 was not significantly influenced during the treatment despite of the removal of oxygen and/or the incorporation of nitrogen. The texture parameters of the three supports are summarized in Table 1. The BET surface area, average pore size and pore volume of the TiO2 sample were 103 m2 ·g−1 , 12.9 nm and 0.33 cm3 ·g−1 , respectively. After treatment under H2 and NH3 the BET surface area decreased slightly to 99 and 97 m2 ·g−1 , respectively. Small changes in the average pore size and pore volume were also observed. As a whole, the samples after treatment under different atmospheres show similar textural properties. Table 1. Textural properties of TiO2 , TiO2–x and TiO2–x Ny determined by nitrogen physisorption studies Samples TiO2 TiO2−x TiO2−x Ny
SBET (m2 ·g−1 ) 103 99 97
Average pore size (nm) 12.9 13.8 13.6
Pore volume (cm3 ·g−1 ) 0.33 0.34 0.33
X-ray diffraction patterns of the TiO2 , TiO2−x and TiO2−x Ny samples are shown in Figure 2. All the diffraction peaks of the TiO2 sample can be assigned to the typical peaks of the anatase phase with a space group of I41/amd (JCPDS No. 21-1272) [19]. Both TiO2−x and TiO2−x Ny exhibited
Figure 1. N2 adsorption-desorption isotherms of TiO2 , TiO2−x and TiO2−x Ny samples
Figure 2. XRD patterns of TiO2 , TiO2−x and TiO2−x Ny samples
Electrochemical impedance spectroscopy (EIS) measurement was carried out covering the frequency range of 0.1 to 100000 Hz using an ac perturbation amplitude of 10 mV at the open circuit potential of an Fe(II)/Fe(III) redox couple. The corresponding Nyquist plots of the TiO2 , TiO2−x and TiO2−x Ny samples are shown in Figure 3. The charge transfer resistance was about 224 W , 160 W and 169 W for the electrodes modified with TiO2 , TiO2−x and TiO2−x Ny , respectively. Since charge transfer resistance across the electrode/electrolyte interface is directly related to the conductivity of the electrode, we can conclude that the treatment of
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TiO2 under H2 and NH3 at 450 ◦ C to form oxygen deficient TiO2−x and TiO2−x Ny (additionally doped with nitrogen) respectively, resulted in substantial improvement of its electrical conductivity. Fast charge transfer is one of the most important requirements of a good electrocatalytic material. In this case, the formation of oxygen deficiency, or oxygen deficiency and simultaneous nitrogen doping of TiO2 is expected to be able to enhance the electron transfer process during the ORR.
Figure 4. XRD patterns of Pt supported on TiO2 , TiO2−x and TiO2−x Ny . (1) Pt/TiO2 , (2) Pt/TiO2−x , (3) Pt/TiO2−x Ny , (4) Pt/TiO2−x Ny -NH3 Table 2. Pt content determined by elemental analysis and crystallite size of titania and Pt derived from XRD studies
Figure 3. EIS Nyquist plots of TiO2 , TiO2−x and TiO2−x Ny samples at open circuit potentials
After the deposition of Pt nanoparticles on the three supports, the obtained catalysts were reduced at 300 ◦ C for 1 h in a H2 atmosphere. For comparison, Pt nanoparticles supported on TiO2−x Ny was also treated in NH3 at 300 ◦ C for 1 h, and the obtained sample is denoted as Pt/TiO2−x Ny -NH3 . This treatment is supposed to be able to reduce Pt and simultaneously introduce additional nitrogen into the catalyst support. The Pt content of the samples determined by ICP-OES is shown in Table 2. It can be seen that all the samples have similar Pt loadings pointing to the high reproducibility of Pt deposition. X-ray diffraction measurements were performed after Pt nanoparticle deposition. As expected, all the diffraction peaks of anatase TiO2 were maintained after Pt deposition (Figure 4). The crystallite size of the titania support calculated using Scherrer equation was kept largely unchanged after Pt deposition (Table 2). Two diffraction peaks originating from Pt nanoparticles can be observed at 2θ of 39.8o and 46.1o (Figure 4), which are the characteristic (111) and (200) peaks of Pt (JCPDS No. 04-0802), respectively [25]. The average crystallite size of Pt nanoparticles calculated from Pt diffraction peaks (111) and (200) was in the range from 5.8 to 6.9 nm for the samples. The Pt/TiO2−x Ny -NH3 sample exhibited smaller Pt particle sizes than the Pt/TiO2−x Ny sample, indicating that comparatively smaller particles are formed under ammonia than under a hydrogen atmosphere. This is in good agreement with our previous results, where the sintering of nanoparticles was less severe under ammonia than under hydrogen [26].
Samples Pt/TiO2 Pt/TiO2−x Pt/TiO2−x Ny Pt/TiO2−x Ny -NH3
Pt loading (wt%) 8.77 8.65 8.65 8.65
dsupport (nm) 31.6 31.6 32.0 31.4
dPt (nm) 5.8 5.8 6.9 6.1
High-resolution XPS measurements were carried out on the Pt catalysts supported on modified TiO2 supports. Figure 5(a) shows the XP Ti 2p spectra. The two main peaks of Pt/TiO2 at the binding energies of 459.2 and 464.9 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 , respectively [27,28]. The peak separation between the Ti 2p1/2 and Ti 2p3/2 lines was 5.7 eV, which is consistent with the +4 oxidation state of Ti [29]. Although oxygen deficiency exists in the TiO2−x and TiO2−x Ny samples, their Ti 2p spectra did not show clear shift or shoulder, indicating that the amount of oxygen vacancies is very small and irresolvable by XPS. Moreover, the interaction between Ti and N atoms cannot be observed in the Pt/TiO2−x Ny and Pt/TiO2−x Ny -NH3 samples. These results demonstrate that the Ti remains largely in the +4 oxidation state, and the structure of the samples was not significantly affected by the reductive treatment, which is in good agreement with earlier results. Hence, Ti of lower oxidation states like Ti3+ must be in a very limited amount in the samples, which, however, could change the electronic structure of the samples [18]. Nitrogen was hardly detectable in the Pt/TiO2−x Ny sample (Figure 5b). The Pt deposition and subsequent reduction treatment may have partially removed incorporated nitrogen in the TiO2−x Ny used as support. Post-treatment of Pt/TiO2−x Ny in NH3 led to a clear N 1s peak in Pt/TiO2−x Ny NH3 (Figure 5b). The nitrogen contribution in similar samples was often assigned to Ti-O-N or Ti-N-O species [30]. Although Ti-O-N and Ti-N-O species may be present in the samples, they might not be detectable due to very low concentrations. Hence, it is more likely that the N 1s contribution
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in Pt/TiO2−x Ny -NH3 is related to N–H or adsorbed NH3 , as reported in literature [31]. The surface atomic concentration of nitrogen was determined to be 2.9 at% in the Pt/TiO2−x Ny NH3 sample. The XP O 1s spectra of supported Pt catalysts appear to be asymmetric (Figure 5c). All the spectra can be deconvoluted
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into two peaks at binding energies of 530.4 eV and 532.1 eV, respectively. The positions of the two peaks agree well with the values reported for oxygen in titanium dioxide (O2− ) and hydroxyl (OH− ) group on the surface, respectively [28,29]. The calculated Ti : O atomic ratio is 0.49 for all the samples, which fits well with the atomic composition of TiO2 .
Figure 5. High-resolution XP spectra of Pt supported on TiO2 , TiO2−x and TiO2−x Ny (a) Ti 2p, (b) N 1s, (c) O 1s, (d) Pt 4f and (e) valence band
Figure 5(d) shows XP Pt 4f spectra of supported Pt catalysts. The Pt 4f7/2 and 4f5/2 peaks were observed at binding energies of 71.2 eV and 74.5 eV, respectively, with a peak separation of 3.3 eV. The result indicates that Pt is mostly in the metallic state on the supports [32]. The Pt/TiO2 sample
exhibited a higher Pt atomic concentration than the other catalysts, which is in good agreement with the elemental analysis results. The valence band spectra of the samples are shown in Figure 5(e). Slight shifts of the Fermi edge to lower binding energy side was observed in the valence band spec-
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tra of the oxygen-deficient titania samples, i.e., Pt/TiO2−x , Pt/TiO2−x Ny and especially Pt/TiO2−x Ny -NH3 (Figure 5e). The shifts provided further evidence on the improvement of conductivity after introducing oxygen deficiency and nitrogen doping. The electrochemical activities of these supported Pt catalysts for the oxygen reduction were investigated by linear sweep voltammetry in oxygen-saturated 0.1 mol/L HClO4 . The working electrode was prepared by mixing the catalyst and carbon black in a weight ratio of 1 : 0.2. The voltammograms of all the samples are shown in Figure 6(a) with pure carbon black included for comparison. Clearly, pure carbon black exhibited negligible current compared to the catalyst samples so the observed catalytic currents can be exclusively attributed to the intrinsic properties of the materials. Among the Pt-based catalysts, Pt/TiO2−x and Pt/TiO2−x Ny showed higher ORR activities than Pt/TiO2 as indicated by higher onset potentials, although the Pt/TiO2 sample has the highest Pt
loading and the smallest Pt particle size. Obviously, oxygen deficiency and doped nitrogen in the support can enhance the activity for ORR by decreasing the resistance to charge transfer. In contrast, the post-treatment of Pt/TiO2−x Ny in NH3 (the Pt/TiO2−x Ny -NH3 sample) did not positively influence the ORR performance despite of a much higher nitrogen content of 2.9 at%. It is believed that the nitrogen was mainly adsorbed nitrogen species on the surface of both support and Pt, which block active sites on Pt leading to apparently lower ORR activity of the Pt/TiO2−x Ny -NH3 sample. Figure 6(b) shows the catalytic activities of Pt/TiO2−x with different catalyst to carbon black weight ratios, namely 1 : 0.5, 1 : 1 and 1 : 2. It can be seen that the increase of carbon black as conductive additive did not enhance the catalytic performance. On the contrary, the ORR activity decreased with increasing carbon black content likely due to the decrease of Pt density when diluted with carbon black.
Figure 6. Linear sweep voltammograms of (a) Pt supported on TiO2 , TiO2−x and TiO2−x Ny and (b) samples with different Pt/TiO2−x to carbon black weight ratios. Measurements were performed in oxygen-saturated 0.1 mol/L HClO4 at a scan rate of 10 mV·s−1 without rotation
The Pt/TiO2−x sample with the best ORR activity was selected for electrochemical stability studies, which was compared with commercial Pt/C (Pt loading of 20 wt%). It is worth to note that the ORR activity of Pt/TiO2−x is clearly lower than Pt/C. The stability tests were carried out at a constant potential of 0.5 V for 60 h in air-saturated 0.1 mol/L HClO4 solution, which was exposed to air during measurement. The corresponding current-time (i-t) chronoamperometric responses are shown in Figure 7. It has to be pointed out that the initial activity of Pt/TiO2−x is much lower than Pt/C. In the first 30 min, the reduction current of Pt/C decreased dramatically due to the consumption of dissolved oxygen in the electrolyte. The current after the consumption of the initially dissolved oxygen after 30 min was set at 100% performance for both samples. For the Pt/TiO2−x sample, a very slow decrease in reduction current was observed and a relatively high current of approximately 91% was maintained after 60 h, compared to around 15% for the commercial Pt/C sample. Hence, it can be concluded that the Pt/TiO2−x has a higher electrochemical stability with a lower current decay
rate than the commercial Pt/C sample. The high stability can be mainly attributed to the stable oxide support which is resistant against oxidation and to the strong metal-support interaction between Pt and TiO2 .
Figure 7. Current-time chronoamperometric responses of Pt/TiO2−x and commercial Pt/C samples at 0.5 V in air-saturated 0.1 mol/L HClO4 solution
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4. Conclusions In summary, modified titania was tested as alternative support for Pt catalysts used in the ORR under acidic conditions. The TiO2 , TiO2−x and TiO2−x Ny samples were obtained by thermal treatment of anatase TiO2 nanoparticles in O2 , H2 and NH3 , respectively. Pt nanoparticles were deposited on modified titania by a modified ethylene glycol method. Compared to the TiO2 sample, the oxygendeficient TiO2−x and TiO2−x Ny samples exhibited smaller charge-transfer resistance. The Pt/TiO2−x and Pt/TiO2−x Ny samples showed higher ORR activity than the Pt/TiO2 sample as indicated by higher onset potentials. Obviously, oxygen deficiency in support can enhance the ORR activity by increasing the conductivity of TiO2 . Electrochemical stability studies revealed that the Pt/TiO2−x sample exhibits a higher stability with a lower current decay rate than the commercial Pt/C. This can be attributed to the stable oxide support which is resistant against oxidation, and to the strong metal-support interaction between Pt and TiO2 . Acknowledgements A. Zhao thanks the China Scholarship Council for a research grant.
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Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction Anqi Zhaoa , Justus Masab ,
Wei Xiaa∗
a. Laboratory of Industrial Chemistry, Ruhr-University Bochum, D-44780 Bochum, Germany; b. Analytical Chemistry and Center for Electrochemical Sciences-CES, Ruhr-University Bochum, D-44780 Bochum, Germany [ Manuscript received June 9, 2014; revised August 11, 2014 ]
Abstract Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO2−x and TiO2−x Ny were obtained by thermal treatment of anatase TiO2 under flowing H2 and NH3 , respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N2 -physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO4 . Pt/TiO2−x and Pt/TiO2−x Ny showed higher ORR activities than Pt/TiO2 as indicated by higher onset potentials. Oxygen deficiency in TiO2−x and TiO2−x Ny contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO2−x exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO2−x support. Key words TiO2 ; oxygen-deficiency; Pt catalyst; oxygen reduction reaction
1. Introduction Fuel cells as electrochemical energy conversion devices have attracted considerable attention over the past few decades due to their high efficiency and low emission [1,2]. The oxygen reduction reaction (ORR) at the cathode is one of the key factors in the performance of a fuel cell [3]. In recent years, there has been extensive effort towards the development of ORR electrocatalysts with high efficiencies. Platinum-based catalysts are among the most widely investigated materials for the ORR under acidic conditions [4,5]. Carbon black is the most commonly used support for electrocatalysts because of several advantages including high electronic conductivity, low cost and high surface area. However, the oxidation of carbon black occurs during long-term operation which leads to loss of ORR performance. It is known that metal species on carbon surfaces promote the decomposition of surface oxygen groups leading to the degradation of carbon [6]. In electrolyte under electrochemical conditions Pt particles supported on carbon catalyze the oxidation of the carbon in a similar way [7,8]. The degradation of carbon ∗
is a major reason for the loss of the active surface area of carbon supported Pt catalysts, which occurs through migration/agglomeration or dissolution (and possible re-deposition) of Pt [9−11]. Hence, high Pt loadings are often needed to compensate for the loss of Pt active surface area in long-term operation. Owing to the intrinsic instability of carbon, it is necessary to look for alternative supports that are resistant against electrochemical corrosion and stable in the long term. It is highly desirable to find support materials that promote strong metal-support interactions, which can improve the stability of Pt and modify its electronic structure for electrocatalysis. Additionally, the electronic conductivity and surface area of the support should be sufficient for electrocatalytic ORR. TiO2 is a promising alternative to carbon black due to its high corrosion resistance. In contrast to the inert surface of carbon materials, there is strong interaction between Pt and TiO2 , i.e., the strong metal-support interaction, which stabilizes the Pt particles. It was reported that highly dispersed Pt nanoparticles on porous TiO2 exhibited a higher active surface area than those on carbon in the ORR [12]. The high
Corresponding author. Tel: +49-234-3223566; Fax: +49-234-3214115; E-mail: [email protected]
Copyright©2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi: 10.1016/S2095-4956(14)60202-3
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dispersion was attributed to strong metal-support interaction between Pt and TiO2 . The combination of Pt/TiO2 and conductive carbon particles provided a higher electrochemical active surface area than Pt/C, which enhanced the ORR performance under acidic conditions [13]. In other studies, Pt was deposited on carbon-coated anatase TiO2 , and the Pt catalysts showed superior activities for methanol oxidation and ORR [14,15]. In addition, Pt nanoparticles supported on mesoporous TiO2 prepared by an ultrasound-assisted reduction method showed a higher electrocatalytic activity and stability than commercial Pt/C [16]. Very recently, the composite of N-doped carbon nanotubes and mesoporous TiO2 was used as support for Pt, and accelerated stress tests disclosed that the catalyst was more stable than Pt/C under electrochemical conditions [17]. TiO2 is a wide band gap semiconductor and its conductivity is insufficient for electrocatalysis without modification. Recent studies indicate that oxygen-deficiency or nitrogen doping can improve charge transfer in TiO2 [18]. Oxygen vacancies can be introduced into TiO2 by thermal treatment in a reducing atmosphere at elevated temperatures, which leads to oxygen-deficient TiO2 (TiO2−x ) or sub-stoichiometric phases like Ti4 O7 [19,20]. Electrochemical studies showed that Ti4 O7 had a much higher onset corrosion potential than Vulcan XC-72, indicating a much higher oxidation resistance under real PEM fuel cell operating conditions [21]. However, a larger Pt cluster size was formed because of the decrease of the interaction between support and Pt particles. In this work, modified TiO2 was used as support for Pt catalysts for the ORR under acidic conditions. TiO2−x and TiO2−x Ny used as support were obtained by thermal treatment of anatase TiO2 nanoparticles in H2 and NH3 , respectively. The Pt catalysts were synthesized by a modified ethylene glycol method. The modified TiO2 supports and supported Pt catalysts were characterized by various techniques. The electrocatalytic activity and long-term stability of the catalysts were studied.
glycol (1.0 mol/L) was dropped into the H2 PtCl6 solution until the pH reached 12. The obtained mixture was stirred at room temperature for 1 day, and then refluxed at 160 ◦ C for 3 h in inert atmosphere. The obtained dark brown Pt colloid was stored at room temperature in air for further applications. For deposition of Pt on TiO2 , 300 mg of the support (TiO2 , TiO2−x and TiO2−x Ny ) were mixed with 50 mL of Pt colloid, and the pH of the mixture was adjusted to 2 by adding 1.0 mol/L HCl (in ethylene glycol). The mixture was refluxed at 160 ◦ C for 1.5 h before cooling to room temperature. After filtration, washing with ethanol, and drying at 60 ◦ C in air, the obtained catalysts were reduced at 300 ◦ C for 1 h under flowing hydrogen. The samples were ground in an agate mortar before further tests. 2.2. Characterization
2. Experimental
N2 -physisorption measurements were carried out at 77 K using an Autosorb-1 MP Quantachrome system. Prior to the measurements, all samples were degassed at 300 ◦ C for 2 h. The surface areas were calculated from the linear part of the Brumauer-Emmett-Teller (BET) plots. The pore volume and the pore size distribution were derived from the desorption profiles of the isotherms using the Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) was performed using a PANalytical theta-theta powder diffractometer with a Cu Kα source. Scans were run from 10o to 80o with a step width of 0.03o and a collection time of 20 s per step. Elemental analysis was carried out with an ICP-OES instrument (PU701) from Philips-Unicam. X-ray photoelectron spectroscopy (XPS) measurements were carried out in an ultrahigh vacuum (UHV) set-up equipped with a monochromatic Al Kα X-ray source (1486.6 eV; anode operating at 14.5 kV and 45 mA) and a high resolution Gammadata-Scienta SES 2002 analyzer. The base pressure in the measurement chamber was maintained at about 7×10−10 mbar. The XP spectra were recorded in the fixed transmission mode with a pass energy of 200. Charging effects were compensated by applying a flood gun.
2.1. Synthesis of catalysts
2.3. Electrochemical tests
Commercial anatase TiO2 (Sachtleben Chemie, Duisburg, Germany) with a specific surface area of 100 m2 ·g−1 was used as starting material. Thermal treatment was carried out in a horizontal quartz-tube reactor with an inner diameter of 30 mm. The treatment was performed at 450 ◦ C for 10 h under flowing oxygen (10 vol% O2 in N2 ), hydrogen (50 vol% H2 in He) and ammonia (5 vol% NH3 in He). A total flow of 100 mL·min−1 was applied with 1 g TiO2 in each experiment. The obtained samples were denoted as TiO2 , TiO2−x and TiO2−x Ny , respectively. Supported Pt catalysts were synthesized by a modified ethylene glycol method [22]. In a typical experiment, 0.45 g of H2 PtCl6 ·6H2 O (Aldrich) was dissolved in 250 mL of ethylene glycol (99.8%, Aldrich). A solution of NaOH in ethylene
Electrochemical measurements were performed in a conventional three-electrode cell using glassy carbon (F 4 mm; HTW, Germany) modified with the catalysts as the working electrode, Ag/AgCl/3 mol/L KCl as the reference electrode and Pt foil as the counter electrode. The reference electrode was calibrated with respect to the reversible hydrogen electrode (RHE). Prior to the experiment, the glassy carbon electrode was polished on polishing cloth using different alumina pastes (3.0–0.05 µm) to obtain a mirror-like surface, followed by ultrasonic cleaning in water. For the preparation of the working electrode, 5.0 mg of the catalyst and carbon black in a weight ratio of 1 : 0.2 were dispersed ultrasonically for 60 min in a mixture of water (490 µL), ethanol (490 µL) and Nafionr (5%, 20 µL). 5.3 µL of the resulting catalyst suspen-
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sion was dropped onto a polished glassy carbon electrode to obtain a catalyst loading of 210 µg·cm−2. The electrode was dried in air at room temperature before measurement. Linear sweep voltammetry (LSV) measurements were carried out in a 0.1 mol/L HClO4 electrolyte saturated with argon or oxygen at room temperature using an Autolab potentiostat/galvanostat (PGSTAT12, Eco Chemie, Utrecht, The Netherlands). All experiments for oxygen reduction were carried out at room temperature at a scan rate of 10 mV·s−1 after purging with argon or oxygen for 20 min. Before each measurement, the catalyst was scanned in Ar-saturated electrolyte and the obtained background voltammogram was subtracted from that measured in the O2 -saturated electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were performed at open circuit potential in a solution of potassium hexacyanoferrate (II)/potassium hexacyanoferrate (III) (0.05 mol/L) in KOH (0.1 mol/L) using ac perturbation of 10 mV in the frequency range from 0.1 to 100000 Hz.
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essentially identical diffraction patterns like that of the TiO2 sample, indicating that there was no phase transition to rutile or other phases during the treatment under flowing hydrogen or ammonia at 450 ◦ C for 10 h. The crystallite sizes of the TiO2 , TiO2−x and TiO2−x Ny samples were 30.8, 31.3 and 30.8 nm, respectively, as calculated using the Scherrer equation from the strongest anatase diffraction peak (101). Even though the difference cannot be observed from XRD patterns, the color changed clearly from white to dark yellow after thermal treatment in the applied reducing atmosphere.
3. Results and discussion Figure 1 shows the N2 adsorption-desorption isotherms of the TiO2 , TiO2−x and TiO2−x Ny samples. All adsorptiondesorption isotherms show type IV behavior with H3 hysteresis loop according to the IUPAC classification [23,24]. The hysteresis loops had adsorption and desorption branches in the p/p0 range from 0.65 to 0.95, which is usually associated with capillary condensation in mesoporous structures. These mesoporous structures are thought to be produced by inter-aggregated TiO2 nanoparticles. There was no significant change in the isotherms after treatment under different atmospheres, which indicates that the microstructure of TiO2 was not significantly influenced during the treatment despite of the removal of oxygen and/or the incorporation of nitrogen. The texture parameters of the three supports are summarized in Table 1. The BET surface area, average pore size and pore volume of the TiO2 sample were 103 m2 ·g−1 , 12.9 nm and 0.33 cm3 ·g−1 , respectively. After treatment under H2 and NH3 the BET surface area decreased slightly to 99 and 97 m2 ·g−1 , respectively. Small changes in the average pore size and pore volume were also observed. As a whole, the samples after treatment under different atmospheres show similar textural properties. Table 1. Textural properties of TiO2 , TiO2–x and TiO2–x Ny determined by nitrogen physisorption studies Samples TiO2 TiO2−x TiO2−x Ny
SBET (m2 ·g−1 ) 103 99 97
Average pore size (nm) 12.9 13.8 13.6
Pore volume (cm3 ·g−1 ) 0.33 0.34 0.33
X-ray diffraction patterns of the TiO2 , TiO2−x and TiO2−x Ny samples are shown in Figure 2. All the diffraction peaks of the TiO2 sample can be assigned to the typical peaks of the anatase phase with a space group of I41/amd (JCPDS No. 21-1272) [19]. Both TiO2−x and TiO2−x Ny exhibited
Figure 1. N2 adsorption-desorption isotherms of TiO2 , TiO2−x and TiO2−x Ny samples
Figure 2. XRD patterns of TiO2 , TiO2−x and TiO2−x Ny samples
Electrochemical impedance spectroscopy (EIS) measurement was carried out covering the frequency range of 0.1 to 100000 Hz using an ac perturbation amplitude of 10 mV at the open circuit potential of an Fe(II)/Fe(III) redox couple. The corresponding Nyquist plots of the TiO2 , TiO2−x and TiO2−x Ny samples are shown in Figure 3. The charge transfer resistance was about 224 W , 160 W and 169 W for the electrodes modified with TiO2 , TiO2−x and TiO2−x Ny , respectively. Since charge transfer resistance across the electrode/electrolyte interface is directly related to the conductivity of the electrode, we can conclude that the treatment of
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TiO2 under H2 and NH3 at 450 ◦ C to form oxygen deficient TiO2−x and TiO2−x Ny (additionally doped with nitrogen) respectively, resulted in substantial improvement of its electrical conductivity. Fast charge transfer is one of the most important requirements of a good electrocatalytic material. In this case, the formation of oxygen deficiency, or oxygen deficiency and simultaneous nitrogen doping of TiO2 is expected to be able to enhance the electron transfer process during the ORR.
Figure 4. XRD patterns of Pt supported on TiO2 , TiO2−x and TiO2−x Ny . (1) Pt/TiO2 , (2) Pt/TiO2−x , (3) Pt/TiO2−x Ny , (4) Pt/TiO2−x Ny -NH3 Table 2. Pt content determined by elemental analysis and crystallite size of titania and Pt derived from XRD studies
Figure 3. EIS Nyquist plots of TiO2 , TiO2−x and TiO2−x Ny samples at open circuit potentials
After the deposition of Pt nanoparticles on the three supports, the obtained catalysts were reduced at 300 ◦ C for 1 h in a H2 atmosphere. For comparison, Pt nanoparticles supported on TiO2−x Ny was also treated in NH3 at 300 ◦ C for 1 h, and the obtained sample is denoted as Pt/TiO2−x Ny -NH3 . This treatment is supposed to be able to reduce Pt and simultaneously introduce additional nitrogen into the catalyst support. The Pt content of the samples determined by ICP-OES is shown in Table 2. It can be seen that all the samples have similar Pt loadings pointing to the high reproducibility of Pt deposition. X-ray diffraction measurements were performed after Pt nanoparticle deposition. As expected, all the diffraction peaks of anatase TiO2 were maintained after Pt deposition (Figure 4). The crystallite size of the titania support calculated using Scherrer equation was kept largely unchanged after Pt deposition (Table 2). Two diffraction peaks originating from Pt nanoparticles can be observed at 2θ of 39.8o and 46.1o (Figure 4), which are the characteristic (111) and (200) peaks of Pt (JCPDS No. 04-0802), respectively [25]. The average crystallite size of Pt nanoparticles calculated from Pt diffraction peaks (111) and (200) was in the range from 5.8 to 6.9 nm for the samples. The Pt/TiO2−x Ny -NH3 sample exhibited smaller Pt particle sizes than the Pt/TiO2−x Ny sample, indicating that comparatively smaller particles are formed under ammonia than under a hydrogen atmosphere. This is in good agreement with our previous results, where the sintering of nanoparticles was less severe under ammonia than under hydrogen [26].
Samples Pt/TiO2 Pt/TiO2−x Pt/TiO2−x Ny Pt/TiO2−x Ny -NH3
Pt loading (wt%) 8.77 8.65 8.65 8.65
dsupport (nm) 31.6 31.6 32.0 31.4
dPt (nm) 5.8 5.8 6.9 6.1
High-resolution XPS measurements were carried out on the Pt catalysts supported on modified TiO2 supports. Figure 5(a) shows the XP Ti 2p spectra. The two main peaks of Pt/TiO2 at the binding energies of 459.2 and 464.9 eV can be assigned to Ti 2p3/2 and Ti 2p1/2 , respectively [27,28]. The peak separation between the Ti 2p1/2 and Ti 2p3/2 lines was 5.7 eV, which is consistent with the +4 oxidation state of Ti [29]. Although oxygen deficiency exists in the TiO2−x and TiO2−x Ny samples, their Ti 2p spectra did not show clear shift or shoulder, indicating that the amount of oxygen vacancies is very small and irresolvable by XPS. Moreover, the interaction between Ti and N atoms cannot be observed in the Pt/TiO2−x Ny and Pt/TiO2−x Ny -NH3 samples. These results demonstrate that the Ti remains largely in the +4 oxidation state, and the structure of the samples was not significantly affected by the reductive treatment, which is in good agreement with earlier results. Hence, Ti of lower oxidation states like Ti3+ must be in a very limited amount in the samples, which, however, could change the electronic structure of the samples [18]. Nitrogen was hardly detectable in the Pt/TiO2−x Ny sample (Figure 5b). The Pt deposition and subsequent reduction treatment may have partially removed incorporated nitrogen in the TiO2−x Ny used as support. Post-treatment of Pt/TiO2−x Ny in NH3 led to a clear N 1s peak in Pt/TiO2−x Ny NH3 (Figure 5b). The nitrogen contribution in similar samples was often assigned to Ti-O-N or Ti-N-O species [30]. Although Ti-O-N and Ti-N-O species may be present in the samples, they might not be detectable due to very low concentrations. Hence, it is more likely that the N 1s contribution
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in Pt/TiO2−x Ny -NH3 is related to N–H or adsorbed NH3 , as reported in literature [31]. The surface atomic concentration of nitrogen was determined to be 2.9 at% in the Pt/TiO2−x Ny NH3 sample. The XP O 1s spectra of supported Pt catalysts appear to be asymmetric (Figure 5c). All the spectra can be deconvoluted
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into two peaks at binding energies of 530.4 eV and 532.1 eV, respectively. The positions of the two peaks agree well with the values reported for oxygen in titanium dioxide (O2− ) and hydroxyl (OH− ) group on the surface, respectively [28,29]. The calculated Ti : O atomic ratio is 0.49 for all the samples, which fits well with the atomic composition of TiO2 .
Figure 5. High-resolution XP spectra of Pt supported on TiO2 , TiO2−x and TiO2−x Ny (a) Ti 2p, (b) N 1s, (c) O 1s, (d) Pt 4f and (e) valence band
Figure 5(d) shows XP Pt 4f spectra of supported Pt catalysts. The Pt 4f7/2 and 4f5/2 peaks were observed at binding energies of 71.2 eV and 74.5 eV, respectively, with a peak separation of 3.3 eV. The result indicates that Pt is mostly in the metallic state on the supports [32]. The Pt/TiO2 sample
exhibited a higher Pt atomic concentration than the other catalysts, which is in good agreement with the elemental analysis results. The valence band spectra of the samples are shown in Figure 5(e). Slight shifts of the Fermi edge to lower binding energy side was observed in the valence band spec-
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tra of the oxygen-deficient titania samples, i.e., Pt/TiO2−x , Pt/TiO2−x Ny and especially Pt/TiO2−x Ny -NH3 (Figure 5e). The shifts provided further evidence on the improvement of conductivity after introducing oxygen deficiency and nitrogen doping. The electrochemical activities of these supported Pt catalysts for the oxygen reduction were investigated by linear sweep voltammetry in oxygen-saturated 0.1 mol/L HClO4 . The working electrode was prepared by mixing the catalyst and carbon black in a weight ratio of 1 : 0.2. The voltammograms of all the samples are shown in Figure 6(a) with pure carbon black included for comparison. Clearly, pure carbon black exhibited negligible current compared to the catalyst samples so the observed catalytic currents can be exclusively attributed to the intrinsic properties of the materials. Among the Pt-based catalysts, Pt/TiO2−x and Pt/TiO2−x Ny showed higher ORR activities than Pt/TiO2 as indicated by higher onset potentials, although the Pt/TiO2 sample has the highest Pt
loading and the smallest Pt particle size. Obviously, oxygen deficiency and doped nitrogen in the support can enhance the activity for ORR by decreasing the resistance to charge transfer. In contrast, the post-treatment of Pt/TiO2−x Ny in NH3 (the Pt/TiO2−x Ny -NH3 sample) did not positively influence the ORR performance despite of a much higher nitrogen content of 2.9 at%. It is believed that the nitrogen was mainly adsorbed nitrogen species on the surface of both support and Pt, which block active sites on Pt leading to apparently lower ORR activity of the Pt/TiO2−x Ny -NH3 sample. Figure 6(b) shows the catalytic activities of Pt/TiO2−x with different catalyst to carbon black weight ratios, namely 1 : 0.5, 1 : 1 and 1 : 2. It can be seen that the increase of carbon black as conductive additive did not enhance the catalytic performance. On the contrary, the ORR activity decreased with increasing carbon black content likely due to the decrease of Pt density when diluted with carbon black.
Figure 6. Linear sweep voltammograms of (a) Pt supported on TiO2 , TiO2−x and TiO2−x Ny and (b) samples with different Pt/TiO2−x to carbon black weight ratios. Measurements were performed in oxygen-saturated 0.1 mol/L HClO4 at a scan rate of 10 mV·s−1 without rotation
The Pt/TiO2−x sample with the best ORR activity was selected for electrochemical stability studies, which was compared with commercial Pt/C (Pt loading of 20 wt%). It is worth to note that the ORR activity of Pt/TiO2−x is clearly lower than Pt/C. The stability tests were carried out at a constant potential of 0.5 V for 60 h in air-saturated 0.1 mol/L HClO4 solution, which was exposed to air during measurement. The corresponding current-time (i-t) chronoamperometric responses are shown in Figure 7. It has to be pointed out that the initial activity of Pt/TiO2−x is much lower than Pt/C. In the first 30 min, the reduction current of Pt/C decreased dramatically due to the consumption of dissolved oxygen in the electrolyte. The current after the consumption of the initially dissolved oxygen after 30 min was set at 100% performance for both samples. For the Pt/TiO2−x sample, a very slow decrease in reduction current was observed and a relatively high current of approximately 91% was maintained after 60 h, compared to around 15% for the commercial Pt/C sample. Hence, it can be concluded that the Pt/TiO2−x has a higher electrochemical stability with a lower current decay
rate than the commercial Pt/C sample. The high stability can be mainly attributed to the stable oxide support which is resistant against oxidation and to the strong metal-support interaction between Pt and TiO2 .
Figure 7. Current-time chronoamperometric responses of Pt/TiO2−x and commercial Pt/C samples at 0.5 V in air-saturated 0.1 mol/L HClO4 solution
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4. Conclusions In summary, modified titania was tested as alternative support for Pt catalysts used in the ORR under acidic conditions. The TiO2 , TiO2−x and TiO2−x Ny samples were obtained by thermal treatment of anatase TiO2 nanoparticles in O2 , H2 and NH3 , respectively. Pt nanoparticles were deposited on modified titania by a modified ethylene glycol method. Compared to the TiO2 sample, the oxygendeficient TiO2−x and TiO2−x Ny samples exhibited smaller charge-transfer resistance. The Pt/TiO2−x and Pt/TiO2−x Ny samples showed higher ORR activity than the Pt/TiO2 sample as indicated by higher onset potentials. Obviously, oxygen deficiency in support can enhance the ORR activity by increasing the conductivity of TiO2 . Electrochemical stability studies revealed that the Pt/TiO2−x sample exhibits a higher stability with a lower current decay rate than the commercial Pt/C. This can be attributed to the stable oxide support which is resistant against oxidation, and to the strong metal-support interaction between Pt and TiO2 . Acknowledgements A. Zhao thanks the China Scholarship Council for a research grant.
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