Facile Synthesis of Carbon-Supported IrxSey Chalcogenide

Jan 18, 2008 - The Journal of Physical Chemistry C .... In the absence of methanol, IrxSey/C catalysts showed a comparable oxygen reduction reaction (...
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J. Phys. Chem. C 2008, 112, 2058-2065

Facile Synthesis of Carbon-Supported IrxSey Chalcogenide Nanoparticles and Their Electrocatalytic Activity for the Oxygen Reduction Reaction Gang Liu†,‡ and Huamin Zhang*,† Lab of PEMFC Key Materials and Technologies, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China ReceiVed: September 1, 2007; In Final Form: NoVember 13, 2007

Vulcan XC-72R carbon-supported IrxSey chalcogenide catalysts with different Se/Ir atomic composition were synthesized via a microwave-assisted polyol process using H2IrCl6 and Na2SeO3 as the Ir and Se precursors. IrxSey chalcogenide catalysts were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The data were discussed with respect to the Se-free carbon-supported Ir nanoparticles prepared by the same microwave-assisted polyol process. The XRD results revealed that IrxSey chalcogenide catalysts showed characteristic reflections of cubic metallic iridium, and the average particle size increased with the increase of Se content in the chalcogenide catalysts. TEM images indicated that IrxSey chalcogenide catalysts were well dispersed on the surface of the carbon support with a narrow particle size distribution when y/x e 1/3. When y ) x, because of the particle agglomeration, we were not able to build particle size distribution and calculate the average particle size for Ir50Se50/C catalyst. Cyclic voltammogram (CV), rotating disk electrode (RDE) and single-cell measurements were conducted to evaluate the electrocatalytic activity of IrxSey chalcogenide catalysts. The CV results suggested that Se covered some surface area of Ir particles, and also, the surface properties of Ir particles were changed after Se modification. The RDE measurements were carried out with the absence/presence of methanol. In the absence of methanol, IrxSey/C catalysts showed a comparable oxygen reduction reaction (ORR) activity with Pt/C catalyst. However, in the presence of methanol, IrxSey/C catalysts showed a better ORR activity than Pt/C catalyst. The performance of the membrane electrode assembly (MEA) prepared with the most active Ir85Se15/C as the cathode catalyst in a single proton exchange membrane fuel cell (PEMFC) was also tested and achieved a maximum power density of 500 mW cm-2 at the current density of 1500 mA cm-2.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are regarded as a possible alternative power source for stationary and mobile applications, due to their high power density and near-zero pollutant emission.1 In PEMFC, carbon-supported platinum (Pt) is usually used as the catalyst for the electroreduction of O2.2 Since the oxygen reduction reaction (ORR), which involves four-electron transfer, is kinetically sluggish, the significant overpotential for the ORR, even on pure Pt, is in excess of 300 mV, which highly limits the efficiency of PEMFCs.3 In addition, Pt is an expensive metal with limited abundance, and it is thus of great interest to develop non-platinum alternatives used for PEMFCs. Non-platinum catalysts such as transition metal oxides,3-4 transition metal (e.g., Fe, Co) macrocyclic compounds5-7 and Ru-based chalcogenides8-10 have been proposed as potential catalysts for the ORR. Focusing on the latter group of catalysts, due to the relatively high ORR activity compared to other nonplatinum catalysts and to its methanol-tolerant ability, a serious of chalcogenide catalysts (RuxXy, X ) S, Se and Te) have been synthesized and tested for the electroreduction of O2. RuxSey resulted in being the most active one.11-16 The presence of Se * To whom correspondence should be addressed. Tel: +86-411-8437 9072. Fax: +86-411-84665057. E-mail: [email protected]. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

strongly enhances the electrocatalytic activity of Ru toward ORR.12-14 Schmidt et al. suggested that the electrocatalytic activity of RuxSey catalysts in the ORR is dominated by the catalytic behavior of the Ru surface sites.15 This interpretation is supported by the maximum in activity at moderate Se content, where further addition of Se leads to decrease in the activity.14-15 The active sites of Se-modified Ru catalysts consist, seemingly, of the Se-free Ru sites. The role of Se atoms in RuxSey is to inhibit the Ru particles against oxidation and to provide the electronic effect for electrocatalysis.12,14-16 Ru-based chalcogenides (MozRuxSey) were first synthesized from a solid-state reaction of pure Ru, Se, and Mo elements at high temperatures between 1200 and 1700 °C.8,17 As they require an environment of extremely high reaction temperature and longduration heat treatment in argon, this process for catalyst synthesis is therefore complicated and costly. Furthermore, the synthesized chalcogenides are strict with regard to the purity of the reactants. Recently, low-temperature methodologies have been developed to prepare amorphous chalcogenides by the thermolysis of Ru-carbonyls in organic solvents (xylene or 1,2dichlorobenzene) in the presence of selenium (Se).9,14 However, because this method involves some complex chemical reactions, several polynuclear compounds with amorphous structures could be produced. It is very difficult to separate the RuxSey chalcogenide from the byproducts, and the yield of the final product is always below 100% (normally ∼ 40-60%).18 The Ru-

10.1021/jp077032u CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

IrxSey Chalcogenides

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TABLE 1: Sample Description no.

description

1 2 3 4 5

20 wt.% Ir 19.7 wt.% Ir + 0.3 wt.% Se 18.7 wt.% Ir + 1.3 wt.% Se 17.6 wt.% Ir + 2.4 wt.% Se 14.2 wt.% Ir + 5.8 wt.% Se

Se:Ir (atomic ratio) abbreviation 0 5:95 15:85 25:75 50:50

Ir/C Ir95Se5/C Ir85Se15/C Ir75Se25/C Ir50Se50/C

carbonyl precursors are relatively expensive, and the solvents used in this method are toxic and environmentally damaging. New synthesis routes for Ru-based chalcogenides that are simple, fast, cost-reductive, and environmentally friendly are definitely needed. Furthermore, the catalytic activity of Ru-based chalcogenides is still not high enough and should be further improved for the practical application in PEMFCs. Iridium (Ir) is one of the most stable metals among Pt-group metals in acidic media.19 Although Ir shows much lower activity toward ORR than Pt does, it is interesting to investigate the ORR activity of Ir catalysts that after the modification of Se.20-22 In this work, carbon-supported IrxSey chalcogenide nanoparticles were prepared by a microwave-assisted polyol process using H2IrCl6 and Na2SeO3 as the Ir and Se precursors. The structures and the electrocatalytic activities of the synthesized carbon-supported IrxSey chalcogenide catalysts were fully characterized. Modification of Ir nanoparticles with Se enhanced their electrocatalytic activity for the oxygen reduction. IrxSey chalcogenide catalysts also showed better methanol tolerance than Pt/C catalyst. 2. Experimental Section 2.1. Catalyst Synthesis. The synthesis was carried out with the aid of an LG WD700 microwave oven (700 W, 2450 MHz). Vulcan XC-72R carbon black (Carbot Corp., SBET) 250 m2 g-1) was used as the support. The typical synthesis procedure for the Ir85Se15/C catalyst (atomic ratio of Se:Ir ) 15:85, the Ir and Se total loading is 20%) is as follows: First, 3.89 mL of 0.03121 M chloroiridic acid (H2IrCl6) in ethylene glycol (EG) and 0.55 mL of 0.03866 M sodium selenite (Na2SeO3) in water were mixed with 50 mL EG. The mixed solution was homogenized for 20 min in an ultrasonic bath, followed by the addition of a 2 M EG solution of sodium hydroxide to increase the pH to >10. The mixed solution was performed in the abovementioned microwave oven with total power for 90 s. The final temperature reached in the performed solution was about 190 °C. Then, 100 mg of Vulcan XC-72R carbon black was added to adsorb the as-prepared IrxSey clusters. The resulting solid was filtered, washed with copious distilled water, and dried in a vacuum oven at 60 °C for 8 h. The catalyst Ir85Se15/C was finally obtained after heat-treated in hydrogen at 400 °C for 1 h. Similarly, Se/C, Ir/C, and IrxSey/C with different Se/Ir atomic ratios were also prepared by the same procedure. The sample descriptions are given in Table 1. 2.2. Catalyst Characterizations and Evaluation. XRD measurements were conducted on a PAN-alytical powder diffratometer (Philips X’Pert PRO) using Ni-filtered Cu-KR radiation (λ ) 1.54056 Å) as radiation source. The XRD patterns were recorded between 20° and 90° at a step size of 0.020°. The average size of Ir particles in the as-prepared catalyst was calculated using Scherrer’s equation from a full width at halfmaximum (fwhm) of the Ir (220) diffraction line.23 TEM images were recorded on a JEOL JEM-2011 electron microscope operated at 120 KV. The Ir/C and IrxSey/C samples were placed in a vial containing ethanol and, then, were ultrasonically agitated to form homogeneous slurry. A drop of

the slurry was dispersed on a holey, amorphous carbon film on a Cu grid for analysis. Two hundred metal particles were calculated to obtain the particle distribution diagram of every catalyst. The mean particle diameter dm was calculated by the following formula:24

dm )

∑i nidi/∑i ni

where ni is the number of particles with diameter di. Electrochemical characterization was performed on CHI 660 electrochemical station (CH Corporation, USA) with an RDE system (EG&G model 636). A standard three-electrode electrochemical cell was used. A large-area Pt foil (3 cm2) and a saturated calomel electrode (SCE) served as the counter and the reference electrode, respectively. The catalyst layer on the glassy carbon electrode (GCE) (4 mm in diameter) was prepared as follows. A mixture containing 5.0 mg Ir/C or IrxSey/C or Pt/C (20%, Johnson Matthey), 1.0 mL ethanol, and 50 µL Nafion solution (5 wt.%, Dupont corp.) was ultrasonically blended in a weighing bottle for 30 min to obtain a homogeneous ink. A precise amount of the paint ink was spread on the surface of the GCE (area: 0.1256 cm2) and the electrode was dried in the air to obtain a thin active catalyst layer. The Ir or Pt loading on the GCE was maintained at 0.177 mg cm-2. The electrochemical measurements were carried out in 0.5 M H2SO4 solution with and without 0.5 M CH3OH at room temperature. All electrode potentials in this paper were quoted to reversible hydrogen electrode (RHE). The CV data were recorded in the potential range of 0 to 0.8 V versus RHE with a scan rate of 50 mV s-1 after bubbling high-purity nitrogen through the electrolyte for 30 min. The RDE curves were obtained in the potential range of 0.95 to 0.2 V versus RHE with the applied scan rate of 5 mV s-1. Prior to the RDE test, the electrolyte was saturated with O2 by bubbling O2 for 30 min. The Ir85Se15/C catalyst was evaluated as the cathode catalyst in a single PEMFC and compared with the 20% Pt/C catalyst (Johney Matthey Corp.). The electrode was prepared as follows: mixing Ir85Se15/C catalyst, 5% Nafion, and ethanol to form a homogeneous mixture, spraying the mixture onto the wet-proofed carbon paper (SGL, 39% PTFE). The Ir loading in the electrode was about 0.35 mg cm-2, and the dry Nafion loading was about 1.12 mg cm-2. This electrode was used for the cathode. The Pt/C (20%, Johnson Matthey) cathode was prepared by the same procedure. The Pt loading in the Pt/C cathode was about 0.30 mg cm-2 and the dry Nafion loading was about 1.00 mg cm-2. The anode adopted the commercial 28.4% Pt/C electrocatalyst (TKK Corp.) with Pt loading of 0.30 mg cm-2. The dry Nafion loading in the anode was about 0.4 mg cm-2. The MEA was fabricated by hot-pressing the anode and the cathode to the Nafion 212 membrane (50 µm, DuPont) at 140 °C and 1 MPa for 1 min. The MEA active area was 5 cm2. The single-cell performances were tested at 80, 60, and 40 °C with saturated humidification, respectively. The single cell was fed with pure hydrogen and oxygen and operated at 0.2 MPa. The electrochemical impedance spectra (EIS) of the cells were recorded using a KIKUSUI KFM 2023 frequency-response analyzer. The anode of the single cell was used as the reference electrode and counter electrode, respectively. The cathode was used as the working electrode. The impedance spectra were measured in the constant current density mode by sweeping frequencies over the 0.05 Hz-1 kHz range and recorded 10 points per decade. The voltage modulation was 10 mV. The impedance spectra of single cells in the high-frequency range show

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Liu and Zhang TABLE 2: Comparison of the Electrochemical Area (ECA) and Mean Particle Size among Ir/C and IrxSey/C Catalysts catalyst Ir/C Ir95Se5/ C Ir85Se15/ C Ir75Se25/ C Ir50Se50/ C

ECA (m2 g-1 Ir)

mean particle size by XRD (nm)

mean particle size by TEM (nm)

37.0 3.2 1.1 0.8

1.4 1.7 2.2 2.4 3.3

1.53 1.71 2.39

chalcogenides and the different preparation method maybe resulted in the different particle size change trend. As can be seen in Figure 1b, the Ir/C sample showed poorly defined Ir peaks. The Ir (111) and (200) overlapped severely, and the other iridium characteristic peaks were almost not observed. These results suggested that the Ir particles were very small and highly dispersed on the carbon support. The mean particle size of the Ir particles in Ir/C and IrxSey/C samples were calculated using Scherrer’s equation from a full width at half-maximum (fwhm) of the Ir (220) diffraction line. The Scherrer’s equation is as follows:23

L)

Figure 1. XRD patterns of (a) Se/C (as-prepared with no further treatment) and (b) Ir/C and IrxSey/C catalysts heat-treated in hydrogen atmosphere at 400 °C for1 h; (I) Ir/C, (II) Ir95Se5/C, (III) Ir85Se15/C, (IV) Ir75Se25/C, (V) Ir50Se50/C.

inductive behavior, characteristic to the experimental setup. Such inductive characteristics have been reported by other researchers in impedance studies on fuel cells.25 To avoid complications resulting from these characteristics, we limited the highfrequency range to 1 kHz. 3. Results and Discussion XRD was conducted to obtain the structural information for the Se/C, Ir/C, and IrxSey/C catalysts. Figure 1a shows the XRD pattern of 20% Se/C. The Se/C showed characteristic reflections of hexagonal selenium (JCPDS Powder Diffraction File No. 651876). This proved that the Se was easily reduced from Na2SeO3 under the experimental conditions and gave a base for the synthesis of the IrxSey chalcogenide. Figure 1b shows the XRD patterns of 20% Ir/C and 20% IrxSey/C (with different Se/Ir atomic ratio). The IrxSey/C samples presented the characteristic peaks of face-centered-cubic (fcc) crystalline iridium (JCPDS Powder Diffraction File No. 65-1686). The characteristic peaks at about 41°, 47°, 69°, 83°, and 88° are corresponding to the Ir (111), (200), (220), (311), and (222) planes, respectively. No iridium oxide diffraction peaks were presented in the whole range, indicating the complete reduction of H2IrCl6. As the increase of Se amount, the fcc Ir peaks of IrxSey/C samples turned to be more complete and sharp, indicating the size increase of Ir nanoparticles. However, according to RuxSey chalcogenides, the Ru particle size decreased with the increase of Se amount.12 The different properties of the IrxSey

0.9λKR1 B2θcosθmax

where L is the average size of metal crystallites, λKR1 is the X-ray wavelength (Cu-KR, λKR1 ) 1.54056 Å), θmax is the angle value of Ir (220) peak, and B(2θ) is the fwhm of the Ir (220) diffraction line. The detailed results can be seen in Table 2. It is well-established that the metal particle morphology and the particle size strongly affect the properties of the catalyst.26-27 With this in mind, we carried out TEM analyses on the Se-free Ir/C catalyst and IrxSey/C catalysts to examine the effect of the modification of Se in the IrxSey system on the particle morphology and the particle size. Figure 2 shows the TEM images of Ir/C and IrxSey/C catalysts. The corresponding particle size distribution histograms are reported in Figure 3. As can been seen in Figure 2, parts a, b, and c, the Ir/C, Ir95Se5/C, and Ir85Se15/C catalysts presented a somewhat better dispersion on the carbon support. However, the Ir50Se50/C catalyst tended to aggregate. At low Se loadings (Se:Ir e 1/3), the morphologies of IrxSey/C catalysts did not change a lot from the Se-free Ir/C catalyst. The Ir nanoparticles were well dispersed on the carbon surface, and the mean particle size increased slightly from 1.53 to 2.39 nm with the increase of Se amount from 0 to 1.3%. These results are in good agreement with the XRD measurement (Table 2). At high Se loading (Se:Ir ) 1), distinct particle agglomeration can be seen in the TEM image of Ir50Se50/C catalyst (Figure 2d). A majority of area in Figure 2d is gray. No particles can be seen in the broad gray region (Figure 2e). This made us not able to build particle size distribution and calculate the average particle size for the Ir50Se50/C catalyst. However, the mean particle size of Ir50Se50/C catalyst was only 3.3 nm estimated from the XRD pattern of Figure 2(V). This indicated that the Ir particles in Ir50Se50/C catalyst might be relatively well dispersed on the carbon support, but, covered by thick amorphous shells of Se or IrxSey particles. Although the observation is insufficient to unambiguously prove the coverage of Se or IrxSey particles on Ir particles, the CV data in Figure 4 will give a further illustration. Figure 4a shows the complete cyclic voltammograms (CVs) for Ir/C and IrxSey/ C catalysts in N2-saturated 0.5 M H2SO4 solution at room temperature. In the hydrogen adsorption/ desorption (Ha/d) region, the Ir/C catalyst showed large Ha/d

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Figure 2. TEM images of (a) Ir/C; (b) Ir95Se5/C; (c) Ir85Se15/C; (d) Ir50Se50/C and (e) Ir50Se50/C (at a magnification of 200 000).

peaks between 0.01 and 0.35 V versus RHE. Such hydrogen adsorption behavior on the Ir/C catalyst is in good agreement with that of the bare Ir electrode as illustrated in the literature.28,29 However, the IrxSey/C catalysts exhibited different surface properties from the Ir/C catalyst. After the modification of Se in the IrxSey chalcogenide catalysts, the Ha/d peaks rapidly decreased with the increase of Se amount. When the Se loading increased to 5.8% (atomic ratio Se:Ir ) 1), the Ha/d peaks were definitely disappeared in the same potential region (0.01 to 0.35 V versus RHE). Since the mean Ir particle sizes in Ir/C and IrxSey/C catalysts is quite close and hydrogen is hardly adsorbed on the Se surface, the suppressed hydrogen adsorption on IrxSey/C catalysts might be due to the influence of the Se, presumably the partial coverage of Se particles on Ir particles and the changed surface properties of IrxSey/C catalysts. The active surface area of Ir/C and IrxSey/C catalysts was quantitatively calculated via integrating the Ha/d peaks (Figure 4b) based on the following assumption:30-31 (i) Ha/d saturation coverage are similar for both Pt and Ir and assuming the hydrogen

monolayer adsorption/desorption charge of 220 µC cm-2, (ii) hydrogen can be adsorbed only on Ir sites rather than on Se, and (iii) Se does not cause any electronic effects on hydrogen adsorption. The calculated surface area of the catalysts was presented in Table 2. Although the loading of Se in Ir95Se5/C catalyst is small (0.3 wt.%), the active surface area of Ir95Se5/C decreased from 37.0 m2 g-1 (Ir/C) to 3.2 m2 g-1. These results showed that the added Se not only directly covered on Ir particles’ surface, but also altered Ir particles’ surface properties. As seen from Figure 4a, a reversible peak at about 0.65 V versus RHE can be seen on the CV of the Ir/C catalyst, which can be attributed to the Ir/IrOH or IrO redox process.28-29 However, in the case of IrxSey/C catalysts, no such redox peaks were seen at the same potential, indicating that the formation of Ir oxide was hindered. From these results, it was determined that the presence of Se can effectively suppress the formation of Ir oxide. These results agreed well with observations reported by Lee et al.22 Meanwhile, the same observations that the modification of Se on Ru nanoparticles led to a suppression of

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Figure 3. Histograms of particle size distributions of the catalysts (a) Ir/C; (b) Ir95Se5/C; and (c) Ir85Se15/C.

Figure 4. Complete (a) and part (b) of the cyclic voltammograms for Ir/C and IrxSey/C catalysts in N2-saturated 0.5 M H2SO4 at room temperature, recorded at 50 mV s-1.

Ru (hydr)oxide formation, were also found in the former studies.16,22,32 To evaluate the electrocatalytic activities for the ORR on IrxSey/C catalysts, the RDE tests were explored. Figure 5 shows the ORR polarization curves for Ir/C and IrxSey/C catalysts including the commercial Pt/C catalyst (20% Pt, Johnson Matthey) in O2-saturated 0.5 M H2SO4 solution at room temperature. The polarization curves were obtained at 5 mv s-1 and 1600 rpm. As shown in Figure 5, the Ir/C and IrxSey/C catalysts showed well-defined limiting currents below 0.5 V, suggesting a good electrocatalytic activity for the ORR. During the synthesis of Ir/C and Se/C, the color of H2IrCl6 solution was changed from orange to black brown and the color of Na2SeO3 solution was changed from transparent to pink after the microwave irradiation for 90 s. These phenomena suggested the reduction reactions of Ir4+ and SeO32- to Ir and Se were

Figure 5. Polarization curves for ORR on Ir/C, IrxSey/C, and Pt/C catalysts in O2-saturated 0.5 M H2SO4 at room temperature. Sweep rate: 5 mV s-1; Rotation speed: 1600 rpm.

IrxSey Chalcogenides

Figure 6. Tafel plots for the ORR on Ir/C, IrxSey/C and Pt/C catalysts. Data were extracted from Figure 5.

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Figure 9. Performance curves of the single cell adopting Ir85Se15/C as cathode catalyst at different temperatures. Anode: the commercial TKK 28.4% Pt/C catalyst with Pt loading of 0.3 mg cm-2. Cathode Ir loading: 0.4 mg cm-2. H2/O2 ) 0.2 MPa/ 0.2 MPa. Pure hydrogen and oxygen are put into the cell with saturated humidification.

Figure 7. Dependence of the ORR activity of the Ir/C and IrxSey/C catalysts, given as kinetic current density at 0.7 V versus RHE, as taken from Tafel plots.

Figure 10. (a) EIS of PEM single cell at different cell temperatures with Ir85Se15/C as cathode catalyst at a constant current density of 100 mA cm-2. The single cell operating conditions are the same with the conditions in Figure 9. (b) Equivalent circuit for the single cell.

Figure 8. Polarization curves for ORR on Ir85Se15/C and Pt/C catalysts in O2-saturated 0.5 M H2SO4 with and without 0.5 M MeOH at room temperature. Sweep rate: 5 mV s-1; Rotation speed: 1600 rpm.

very quick in the microwave-assisted polyol process. Therefore, it was possible that the microwave-assisted polyol process can effectively control particles size and distribution, which may contribute to the high ORR activity of Ir/C and IrxSey/C catalysts.33 Meanwhile, the microwave irradiation promoted the further reaction of Ir and Se to IrxSeychalcogenides. Though the Ir/C and IrxSey/C catalysts showed exciting ORR activities, the ORR activities of Ir-base catalysts are still lower than that of the Pt/C catalyst, with a larger overvoltage of about 0.15 V. At low Se loadings (Se:Ir e 1/3), the IrxSey/C catalyst

possessed superior ORR activity to the Ir/C catalyst. However, when the Se loading increased to 5.8% (atomic ratio Se:Ir ) 1), the ORR activity of Ir50Se50/C catalyst decreased sharply and lowered than the Ir/C catalyst. Regarding Ir sites are the active catalytic site for the ORR,34 the role of Se is to enhance the ORR activity by modification of Ir sites. When the Se amount is in the moderate range, the Se exhibited a positive effect on the ORR activity. While the Se amount increased further, more Ir active sites were inhibited, and, hence the ORR activity decreased. This observation was further proven by the CV tests and the TEM measurement (Figure 4a and Figure 2, parts d and e). According to the rotating disk electrode theory, the current density (i) at each electrode potential (E), shown in Figure 5, should contain two contributions: the kinetic current density (ik) and the diffusion-limited current density (id). The relationship among these current densities can be expressed as the following equation:35

ik )

id‚i id - i

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TABLE 3: Comparison of the Single Cell Performance Adopting Ir85Se15/C and Pt/C as Cathode Catalysts (80 °C) output voltage (V) cathode catalyst

open circuit voltage (V)

At 50 Ma cm-

At 100 mA cm-2

At 500 mA cm-2

At 1000 mA cm-2

maximum power density (mW cm-2)

Ir85Se15/ C Pt/C

0.935 0.981

0.767 0.884

0.727 0.857

0.563 0.746

0.442 0.663

501.0 1027.2

Figure 6 compares the kinetic current densities ik for the ORR on the Ir/C, IrxSey/C, and Pt/C catalysts between 0.6 and 0.9 V versus RHE. The data were extracted from Figure 5. As shown in Figure 6, Ir85Se15/C catalyst exhibited the best activities among the other IrxSey/C catalysts and Ir/C catalyst. At the moderate Se/Ir atomic ratio (Se:Ir e 1/3), the IrxSey/C catalysts all showed better activities than Ir/C catalyst. However, still further increase of Se amount in the IrxSey/C catalysts led to a decrease in activity. When the Se/Ir atomic ratio increased to 1, the Ir50Se50/C catalyst showed a worse activity than Ir/C catalyst. The catalytic activity at 0.7 V versus RHE against the Se content in the catalysts is plotted in Figure 7. It was apparent that the catalytic activity toward oxygen reduction in terms of current densities increased with the increase of Se content in the IrxSey/C catalysts until a maximum value at ∼15 mol % Se was reached. Alonso-Vante et al.18 have reported that chalcogenide clusterlike materials of the RuxSey type are fully methanol-tolerant and have comparable catalytic activity to Pt catalyst for ORR in the presence of methanol. Recently, Papageorgopoulos et al.32 reported that RhxSy/C and RuSex/C are also much less sensitive to methanol than the conventional Pt/C. These previous studies indicated that Se and S chalcogens, when reacted with transition metals, could favor the ORR activities versus some other nonplatinum catalysts and could be tolerant to methanol poisoning. To evaluate the methanol tolerance ability of IrxSey/C chalcogenide catalysts, the ORR polarization curves for Ir85Se15/C and Pt/C catalysts in O2-saturated 0.5 M H2SO4 solution with and without 0.5 M CH3OH are shown in Figure 8. For the Pt/C catalyst, a large anodic current was presented above 0.55 V due to the CH3OH oxidation. The onset potential for ORR by Pt/C in the methanol-containing solution began at a potential no higher than 0.55 V, which was over 450 mV lower than that when CH3OH was absent. The mixed potential caused by the simultaneous reactions of oxygen reduction and CH3OH oxidation attributed to the large drop of ORR onset potential. However, as shown in Figure 8, the ORR polarization curves for Ir85Se15/C in O2-saturated methanol-containing solution was nearly identical to that obtained in O2-saturated methanol-free 0.5 M H2SO4 solution, indicating that Ir85Se15/C had a strong methanol tolerance property. The IrxSey/C catalysts with other Se/Ir atomic ratios that synthesized in this work also displayed a higher susceptibility to methanol poisoning. The catalytic activity for ORR of the most active Ir85Se15/C catalyst as cathode catalyst in the single PEMFC was tested at different temperatures and compared with that of the commercial Pt/C catalyst. The single cell performance curves of Ir85Se15/C MEA are presented in Figure 9. The comparison of the cell performances between the Ir85Se15/C MEA and the Pt/C MEA at defined current densities was shown in Table 3. The open circuit potentials of the Ir85Se15/C MEA were found to be higher than 0.9 V. At 80 °C, the polarization curve shows the limiting current density of ∼2500 mA cm-2 and the maximum power density of 500 mW cm-2 at 1500 mA cm-2, which are superior to the single cell performance with the Ru-based cathode catalysts reported in the literature.36-37 Although a maximum

current density at maximum power density (mA cm-2) 1500 2400

TABLE 4: Parameters Evaluated from Fit of EIS with the Equivalent Circuit Shown in Figure 10 T (oC)

R1 (m)

R2 (m)

Rct (m)

40 60 80

26.2 22.6 20.1

43.87 31.34 22.78

150.9 129 115.3

power density of about 500 mW cm-2 was achieved in our work, the performance of Ir85Se15/C MEA was still lower than that of Pt/C MEA (Table 3). However, as shown in Figure 9, the cell performance of Ir85Se15/C MEA operated at 80 °C gave a better performance than that operated at 60 °C and 40 °C. At the current density of 250 mA cm-2, the cell voltage at 80 °C is 0.65 V, which is about 25 mV and 57 mV higher than that at 60 °C and 40 °C, respectively. These initial results indicated that the Ir85Se15/C MEA performance could be improved when operated at a higher temperature. Moreover, better performance can be obtained by optimization in the catalyst preparation process and MEA manufacture. The EIS spectra at different temperatures obtained for the single cell with Ir85Se15/C as the cathode catalyst are shown in Figure 10a. The EIS measurements were conducted at a constant current density of 100 mA cm-2. It was found from the spectra that the small loop (usually not apparent) at the highest frequency (HF) was overlapped by a large loop at the middle high frequency (MHF). The small loop is due either to the ionic ohmic drop/double layer charging inside the active layer in the granular electrode structure, or to electronic contact problems between the electronic supply and the GDE gas diffusion layer.38-39 The large loop at intermediate frequencies is attributed to the charge-transfer resistance and the double layer capacity and the relaxations of the intermediate species.40 The size of the plots decreased with increasing temperature from 40 to 80 °C. The equivalent circuit for the single cell is shown in Figure 10b. R1 represents the impedance at the intersection of the HF curve with the real axis. It is attributed to the internal resistance of the cell including the total ohmic resistance of the cell, which can be expressed as a sum of the contributions from uncompensated contact resistance and ohmic resistance of the cell components such as membrane, catalyst layer, backing, end plate, and that between each of them.41 R2 and CPE1 represent the resistance and constant phase element that arose by the abovementioned potential fact at the HF small loop. Rct represents the charge-transfer resistance for oxygen reduction, and CPE2 represents the constant phase element associated by the catalyst layer capacitance properties. Table 4 shows the simulated data using the equivalent circuit by ZSimpWin. It can be seen that, R1 decreased from 26.2 mΩ to 20.1 mΩ, and R2 decreased from 43.87 to 22.78 mΩ when the temperature increased from 40 °C to 80 °C. It is supposed that the proton conductivity of the ionomer/membrane increased with the increase of the operating temperature, which contributed to the change of the properties of the electrode structure and resulted in the decrease of R1 and R2. It is well-known that the magnitude of the semicircle in the cathode impedance at the measured conditions is related to the resistance due to the

IrxSey Chalcogenides oxygen reduction kinetic. The charge-transfer resistance Rct of the single cell at 80 °C was 115.3 mΩ, which was 35.6 mΩ lower than that at 40 °C. It indicated that the ORR activity of the Ir85Se15/ C catalyst enhanced with the increase of operating temperature. This was reasonably correlated with the cell performance as shown in Figure 9. 4. Conclusions In summary, carbon-supported IrxSey chalcogenide nanoparticles were prepared by a microwave-assisted polyol process using H2IrCl6 and Na2SeO3 as the Ir and Se precursors. The IrxSey/C catalysts presented fully defined fcc Ir peaks as the increase of Se amount. Ir nanoparticles were well dispersed on the carbon surface when the Se was in a moderate amount. When the Se/Ir atomic ratio increased to 1, the Ir particles in Ir50Se50/C catalyst formed distinct agglomeration. The CV tests showed that the modification of Se on Ir particles not only covered some Ir surface, but also changed the Ir surface property. The catalytic activity toward oxygen reduction enhanced with the increase of Se amount in the IrxSey/C catalysts until a maximum value at ∼15 mol % Se was reached. Further addition of Se led to a decrease in activity. IrxSey chalcogenide catalysts also showed better methanol tolerance than Pt/C catalyst. A maximum power density of 500 mW cm-2 was achieved with the most active Ir85Se15/C as the cathode catalyst, though the performance was inferior to that of the commercial Pt/C single cell. However, the cell performance could be improved by the optimization of the catalyst preparation process, the MEA manufacture and the operating conditions. We believed that the favorable properties of these IrxSey/C catalysts, as well as the simple preparation technique, are encouraging for research and development of PEMFCs and DMFCs. Further refinement of the preparative conditions is now under way and hopefully will lead to better performance of the IrxSey/C catalysts. Acknowledgment. This work was partly supported by the project of Fundamental Research for the Application of Fuel Cell in Transportation supported by Shanghai Automotive Industry Corporation (Group). References and Notes (1) Shukla, A. K.; Christensen, P. A.; Hamnett, A.; Hogarth, M. P. J. Power Sources 1995, 55, 87. (2) AdVances in Electrochemical Science and Engineering; Gottesfeld, S., Zawodzinski, T. A., Alkire, R. C., Gerischer, H., Kolb, D. M., Tobias, C. W., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 5, Chapter 4. (3) Liu, Y.; Ishihara, A.; Mitsushima, S.; Kamiya, N.; Ota, K. I. Electrochem. Solid-State Lett. 2005, 8 (8), A400. (4) Kim, J. H.; Ishihara, A.; Misushima, S.; Kamiya, N.; Ota, K. I. Electrochim. Acta 2007, 52, 2492. (5) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63. (6) Wang, B. J. Power Sources 2005, 152, 15.

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