Effect of Synthesis Route on Oxygen Reduction Reaction Activity of

ARTICLE pubs.acs.org/JPCC

Effect of Synthesis Route on Oxygen Reduction Reaction Activity of Carbon-Supported Hafnium Oxynitride in Acid Media Mitsuharu Chisaka,* Yuta Suzuki, Tomohiro Iijima, and Yoji Sakurai Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi, Aichi 441-8580, Japan

bS Supporting Information ABSTRACT: The effect of the synthesis route on the oxygen reduction reaction (ORR) activity of carbon-supported hafnium oxynitride (HfOxNy-C) catalysts in acid media was investigated. Four types of HfOxNy-C catalysts were synthesized by heating carbonsupported hafnium oxide (HfOx-C) prepared via three different impregnation methods and a polymerized complex method at 1023
1223 K for 24
50 h under NH3 gas. Field emission transmission electron microscope images, X-ray diffraction patterns, X-ray photoelectron spectra, cyclic voltammograms, and rotation disk electrode voltammograms were analyzed. Nanosized HfOxNy particles with a cubic Hf2ON2 phase were dispersed onto C powders by any of the four synthesis routes. However, the level and uniformity of the HfOxNy particle size and the crystallinity depended on the synthesis routes rather than the synthesis conditions. The ORR activity of the HfOxNy-C catalysts increased with the increasing crystallinity of the Hf2ON2 phase in the HfOxNy particles. Catalysts prepared via an impregnation method using hydrolysis of hafnium tetrachloride in the dispersion, consisting of nitric acid-refluxed C powders and water, showed (a) the most uniform and smallest HfOxNy particle sizes, in the order of a few nanometers, (b) the highest crystallinity, and (c) the highest ORR current and activity levels.

1. INTRODUCTION Platinum supported on carbon black (Pt-C) has long been used as an oxygen reduction reaction (ORR) catalyst in fuel cell cathodes operating at low temperatures in acid environments, such as polymer electrolyte membrane fuel cells (PEMFCs, typically below 363 K) and phosphoric acid fuel cells (PAFCs, ∼473 K). The development of alternative catalysts has been motivated by the high costs of the current catalysts and scarce platinum resources. Since Jasinski’s discovery of cobalt phthalocyanine,1 half a century ago, numerous non-platinum-group metal catalysts, such as macrocycles or polypyrrole polymer matrixes containing trapped cobalt or iron,2
7 carbon and nitrogen compounds prepared in the presence8
19/absence19
25 of cobalt or iron, group 4 and 5 metal oxide compounds,26
33 and metal nitrides,34
36 have been developed. Some of them exhibit high ORR activity, comparable to, or higher than, that of Pt-C in alkaline media.17,18,25 However, platinum-based catalysts still exhibit the best ORR activity in acid media. It is not clear why it is difficult to achieve high ORR activity in acid media; however, the PEMFC cathode operating conditions (i.e., the low pH of ∼1 or less15 and the high potential of typically ∼0.6
0.9 V vs standard hydrogen electrode [SHE]) limit the catalyst choices because many elements dissolve in water under such conditions.37 Some group 4 and 5 metal oxide compounds developed by Ishihara and Ota et al. are attractive because of their high stability in 0.1 mol dm
3 sulfuric acid (H2SO4), evaluated by measuring the solubility using inductively coupled plasma atomic emission spectroscopy (ICP-AES).26
29 An important issue with these catalyst types is increasing their r 2011 American Chemical Society

surface area in order to ensure that sufficient current is obtained per unit mass and/or volume as these catalysts are thin films prepared using a sputtering method26,29/electrophoretic deposition method,27 plates,28 or particles in the order of micrometers.30,31 The surface area is quite an important factor common to all types of non-platinum-group metal catalysts used in PEMFC cathode catalyst layers with controlled thicknesses. Ohmic losses as well as oxygen diffusion losses could lower the cell voltage, even when using Pt-C as a catalyst in the cathode catalyst layers with a thickness of ∼10 μm or less.38
40 The maximum cathode catalyst layer thickness using non-platinum-group metal catalysts should be less than 100 μm.41 This can be ensured by decreasing the catalyst size, preferably in the order of nanometers. We recently developed a new cathode catalyst in PEMFCs by using one of the group 4 metals, hafnium oxynitride supported on carbon black (HfOxNy-C), with an approximate size of 5
20 nm.33 The catalysts were synthesized by heating carbonsupported hafnium oxide (HfOx-C), prepared using an impregnation method under ammonia (NH3) gas in various conditions. They exhibited higher ORR activity than NH3-treated C under identical conditions, indicating that HfOxNy is active toward ORR. The leaching test with HfOxNy-C in 0.1 mol dm
3 H2SO4 at 303 K with ICP-AES showed the saturation of the dissolved hafnium mass with increasing immersion time up to 24 h, similar Received: July 17, 2011 Revised: September 5, 2011 Published: September 09, 2011 20610

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to the pioneering work of Ishihara et al.26
29 It was proven that the ORR activity was mostly affected by the heating temperature under NH3 gas; however, HfOxNy active sites should be identified in order to increase the ORR activity and current. Several synthesis routes and/or precursors could be used to prepare these carbon-supported metal-oxide-based catalysts; however, the effect of the synthesis route on the ORR activity is not clear because of the unknown active sites. The purpose of this study was to clarify (a) the effect of the synthesis route on ORR activity and (b) the physical and chemical properties of HfOxNy-C. Four types of HfOxNy-C catalysts were prepared via three different impregnation methods and a polymerized complex method, and then they were doped with nitrogen by heat treatment under flowing NH3 gas. Their surface morphology, crystal structure, and surface composition and chemical states were evaluated by field-emission transmission electron microscopy (FE-TEM), X-ray diffraction (XRD) analysis, and X-ray photoelectron spectroscopy (XPS), respectively. The ORR activity was evaluated by obtaining cyclic voltammograms (CVs) and rotating disk electrode (RDE) voltammograms.

tube furnace that was slowly evacuated and purged with N2 gas. The following heating cycle was used to generate HfOx-C: the temperature was increased from room temperature to 773
1273 at 500 K h
1, maintained at this temperature for 2
12 h, and cooled to room temperature at an uncontrolled rate. N2 gas with a flow rate of 300 standard cubic centimeters per minute (sccm; 1 sccm = 1.67  10
8 m3 s
1) was used throughout the above heating cycle. HfOxNy-C was synthesized by heating HfOx-C according to the following cycle: the temperature was increased again from room temperature to 1073
1223 at 500 K h
1, kept at this temperature for 24 or 50 h, and cooled to 773 at 25 K h
1. The cooling rate was not controlled below 773 K. The flowing gas was N2 with 1000 sccm and NH3 with 200 sccm when the temperature was below and above 773 K, respectively. For simplicity, HfOxNy-C catalysts synthesized by heating HfOx-C (synthesis route) under NH3 gas are hereafter denoted by HfOxNy-C (synthesis route). The catalyst sample masses were measured before and after NH3 treatment, and the mass loss, Δm, was calculated using the following equation

2. EXPERIMENTAL METHODS

where mi and mf denote the sample mass before and after NH3 treatment, respectively. 2.2. Characterization. Using a field emission transmission electron microscope (JEM-2100F, JEOL), the morphology of the selected catalysts was investigated. The catalysts’ crystal structure was analyzed using an X-ray diffractometer (RINT-2500, Rigaku Co.) with a Cu
Kα radiation generated at 40 kV and 200 mA in the scan range of 10
90° at a scan rate of 2° min
1. The surface compositions and chemical states of the catalysts were analyzed using an X-ray photoelectron spectrometer (Quantera SXM, ULVAC-PHI Inc.) with an Al
Kα X-ray source (1486.6 eV). The N 1 s spectra were analyzed by fitting with two symmetric peaks33,42 assigned to Hf
N bonding (N1, 395
396 eV) and C
N bonding (N2, 398
400 eV), which consisted of four types of nitrogen functional groups, namely, pyridinic-N (398.6 ( 0.3 eV), pyrrolic-N/pyridine-N (400.5 ( 0.3 eV), quaternary-N/pyridinic-N-H (401.3 ( 0.3 eV), and pyridinic-N+-O
(402
405 eV).43 The atomic ratio of N1 to Hf, that is, y in HfOxNy-C catalysts, was calculated using the relative sensitivity factors provided by the manufacturer. 2.3. ORR Activity Measurements. To evaluate the ORR activity of the catalysts, the CVs and RDE voltammograms were measured. The catalysts were dispersed in isopropyl alcohol by sonication for 1200 s to obtain a slurry. The mass fraction of the catalyst samples in the slurry was 2.5%. A 10 mm3 aliquot of the slurry was dropped onto a glassy carbon (GC) disk electrode with a geometrical surface area, S, of 0.1963 cm2 (j = 5 mm; HR2-D1-GC5, Hokuto Denko Co.) for catalyst loading, that is, the catalyst mass divided by S of 1 mg cm
2 and air-dried at 318 K for 600 s. The HfOxNy loading, mHS
1, for each HfOxNy-C catalyst was calculated under the assumption that the mass loss during the NH3 treatment was due to the gasification of C11

2.1. Synthesis of Catalysts. HfOxNy-C catalysts were synthesized by heating four types of HfOx-C, namely, HfOx-C (IMCl1), HfOx-C (IM-Cl2), HfOx-C (IM-OR), and HfOx-C (PCMCl) under NH3 gas. In the x = 2 case, the mass ratio of HfOx:C was set to 1:4. The synthesis routes were as follows. HfOx-C (IM-Cl1). The preparation details of this synthesis route are described in our previous paper.33 Briefly, using a stirrer, asreceived C (Vulcan XC-72, Cabot Co.) was dispersed in a mixture containing ethanol and water. Using continuous stirring, hafnium tetrachloride (HfCl4, Mitsuwa Chemical Co.) was then dissolved in the dispersion, and the mixture was sonicated for 1200 s. HfOx-C (IM-Cl2). First, C powders were refluxed with nitric acid for 6 h, washed three times with water, and dried overnight at 363 K. Second, the refluxed C powders were dispersed in water with a stirrer. Third, HfCl4 was dissolved in the dispersion with continuous stirring, and then 0.5 mol dm
3 NH3 solution was added dropwise to adjust the pH of the dispersion to 7.5
8.0 for the hydrolysis of HfCl4. Fourth, the solvent was evaporated with a rotary evaporator and dried overnight in an oven at 363 K. Finally, the resulting powders were washed three times with water to remove ammonium chloride. HfOx-C (IM-OR). The precursor powders were prepared in an argon-filled glovebox. As-received C powders were dispersed in dehydrated ethanol using a stirrer. Hafnium isopropoxide (HfO4C12H28, Sigma-Aldrich Co.) was then dissolved in the dispersion by continuous stirring at 353 K. Stirring was continued until the excess ethanol evaporated. HfOx-C (PCM-Cl). First, citric acid (Kishida Chemical Co.) was dissolved in water with a stirrer. Second, HfCl4 and ethylene glycol (Kishida Chemical Co.) were added to the dispersion. The molar ratio of these materials was 1:2:8 (Hf/citric acid/ethylene glycol). Third, to accelerate polyesterification reactions between citric acid and ethylene glycol, the temperature was increased to 423 K. After the gelation of the mixture, water and as-received C powders were added. Stirring was continued throughout the preparation until the excess water evaporated. Finally, these mixtures were dried overnight in an oven at 363 K to obtain precursor powders. The precursor powders were then placed in an alumina boat and heated in a horizontal quartz

Δm ¼ 100 3 ðmi
mf Þ=mi

mH S
1 ¼ fH =ð100
ΔmÞ

ð1Þ

ð2Þ

where fH denotes the mass fraction of HfOx in HfOx-C powders (20% (w/w), except for HfOx-C [PCM-Cl] powders as they contain carbonized polymer mass originated from citric acid and ethylene glycol). The HfOx-C (PCM-Cl) powders were oxidized in air at 1273 K for 2 h to burn off any carbon material. By measuring the masses before and after the oxidation, the fH of HfOx-C (PCM-Cl) was then determined to be 15% (w/w). 20611

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Figure 1. FE-TEM images of (a) HfOxNy-C (IM-Cl1), (b) HfOxNy-C (IM-Cl2), (c) HfOxNy-C (IM-OR), and (d) HfOxNy-C (PCM-Cl) catalysts synthesized by heating HfOx-C powders at 1223 K for 24 h under NH3 gas. The HfOx-C powders were obtained by heating precursor powders at 1073 K for 2 h under N2 gas.

To stably coat the GC surface with catalyst samples, 10 mm3 of a 0.5% Nafion solution (prepared by diluting a 5% w/w Nafion solution [510211, Sigma-Aldrich Co.] with ethanol) was dropped onto the surface and air-dried at 318 K for 600 s. Before the slurry was dropped, the surface was polished with 1.0 and 0.05 μm alumina slurries and washed with water and acetone, followed by drying at 318 K in air. Using a conventional three-electrode cell, electrochemical measurements were performed in 0.1 mol dm
3 H2SO4 at room temperature. The catalyst-coated GC disk electrode, a carbon paper (TGP-H-120, Toray), and a Ag/AgCl electrode (HX-R4, Hokuto Denko Co.) were used as the working, counter, and reference electrodes, respectively. The working electrode was set in a rotator (HR-201, Hokuto Denko Co.) equipped with a rotating-speed controller (HR-202, Hokuto Denko Co.). The reference electrode was set in a separate compartment with a Luggin capillary. All working electrode potentials were referenced to an SHE. Using a potentiostat (HABF-501A, Hokuto Denko Co.) equipped with a function generator (HB-111A, Hokuto Denko Co.), CVs were recorded in the potential range of 0.05
1.2 V vs SHE, at a scan rate of 5 mV s
1, without rotations, after bubbling with N2 or O2 for more than 1800 s. Following the CV measurements, RDE voltammograms were recorded for the identical potential range, at the identical scan rate, with a rotation speed of 1500 rpm under an O2 atmosphere.

3. RESULTS 3.1. Morphology and Crystal Structure. Figure 1 shows the FE-TEM images of four HfOxNy-C types synthesized by heating HfOx-C powders at 1223 K for 24 h under NH3 gas. The HfOx-C powders were prepared by heating the precursor powders at 1073 K for 2 h under flowing N2 gas. The amorphous carbon appeared as gray areas, whereas HfOxNy particles appeared as black spots. The size of the HfOxNy particles in HfOxNy-C (IMCl2) was more uniform and much smaller than that of the others, approximately a few nanometers in size. Because the sizes of HfOx in HfOx-C (IM-Cl1) and HfOxNy in HfOxNy-C (IM-Cl1)

Figure 2. XRD patterns of (A) HfOx-C (IM-Cl1), (B) HfOx-C (IM-Cl2), (C) HfOx-C (IM-OR), and (D) HfOx-C (PCM-Cl) prepared by heating precursor powders at 1073 K for 2 h under N2 gas.

were similar,33 it can be assumed that the observed values follow the trend and their sizes are similar to the sizes of HfOx particles in HfOx-C. Figure 2 shows the XRD patterns of four HfOx-C types prepared by heating the precursor powders at 1073 K for 2 h under flowing N2 gas. The broad peaks at the 2θ values of 20
30° and 40
45° were assigned to graphite (002) and (101), respectively. The mixture of monoclinic and cubic HfO2 phases was observed only in the case of HfOx-C (PCM-Cl), whereas the other three HfOx-C types exhibited only a monoclinic HfO2 phase. Romas-Gonzalez et al.44 synthesized carbon-free HfO2 nanoparticles by heating precursor powders prepared using the polymerized complex method at 773 or 1073 K in air for 2 h. They reported that the crystal structure of HfO2 could be controlled by varying the molar ratio of HfCl4 to citric acid regardless of the heat-treatment temperature as follows: 0.1, single monoclinic phase; 0.5 or 1, mixture of monoclinic and cubic phases. In this 20612

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Figure 3. XRD patterns of (i) HfOx-C (IM-Cl1), (ii) HfOx-C (IM-OR), and (iii) HfOx-C (PCM-Cl) prepared by heating precursor powders at 1273 K for 2 h under N2 gas.

Figure 4. XRD patterns of (a) HfOxNy-C (IM-Cl1), (b) HfOxNy-C (IM-Cl2), (c) HfOxNy-C (IM-OR), and (d) HfOxNy-C (PCM-Cl) catalysts shown in Figure 1.

study, the ratio was set at 0.5. Although there are differences between their work and this study (i.e., the heating atmosphere [air and N2] and the presence of C), a cubic phase appeared in both. At ambient pressures, pure HfO2 in the bulk can adopt three different crystal structures. It is stable in the monoclinic phase at room temperature and transforms to a tetragonal and cubic phase at 1993 and 2873 K, respectively.45 However, the stabilization of the tetragonal and cubic phases at room temperature can be achieved by the following two ways: (1) incorporation of cation dopants and/or ionized oxygen defects compensating trivalent dopants46 and (2) decreasing the size of HfO2.47 The size of HfOx in HfOx-C (PCM-Cl) can be assumed to be not much less than that in the other three types of HfOx-C. However, the HfOx-C (PCM-Cl) synthesis route was different from the others. In particular, the HfOx precursor particles were surrounded by the polymer matrix originating from citric acid and ethylene glycol before the heat treatment under N2 gas; then, during the heat-treatment, the carbonization of the polymer, as well as the crystallization of HfOx, proceeded. This difference and/or the incorporation of some oxygen defects could be the reason for the formation of the cubic phase in HfOx-C (PCM-Cl). The full width at half-maximum (fwhm) values of the strongest

diffraction peak for HfOx-C (IM-Cl1), HfOx-C (IM-Cl2), HfOx-C (IM-OR), and HfOx-C (PCM-Cl) were 0.65, 0.57, 1.63, and 5.46°, respectively. The fwhm can be decreased by increasing the HfOx size or crystallinity. The results obtained from the FE-TEM images (Figure 1) and the XRD patterns (Figure 2) indicate that the crystallinity of HfOx in HfOx-C (IM-Cl1) and HfOx-C (IMCl2) was higher than that in HfOx-C (IM-OR) and HfOx-C (PCM-Cl). Even though four oxygen atoms were present for every hafnium atom in the starting material of HfOx-C (IM-OR), HfO4C12H28, its crystallinity was much lower than that of HfOx-C (IM-Cl1) and HfOx-C (IM-Cl2) prepared from HfCl4, indicating that sufficient oxygen was provided during these two preparation processes. As shown in Figure S1 (Supporting Information), the fwhm was also affected by the heating temperature and time; however, it depended mostly on the synthesis route. The XRD patterns of the three HfOx-C types prepared by heating precursor powders at 1273 K for 2 h are shown in Figure 3. Cubic Hf2ON2 phases appeared in all HfOx-C types, suggesting the doping of nitrogen into HfOx-C by the heat treatment at 1273 K in a N2 atmosphere, which was clearly confirmed in the N 1 s spectra of HfOx-C (PCM-Cl) and HfOx-C (IM-OR) (Figure S2, Supporting Information). Pialoux48 reported that HfO2 could be 20613

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Figure 5. Hf 4f and N 1 s spectra (solid lines) of (a) HfOxNy-C (IM-Cl1), (b) HfOxNy-C (IM-Cl2), (c) HfOxNy-C (IM-OR), and (d) HfOxNy-C (PCM-Cl) catalysts shown in Figure 1. Hf 4f spectra (dashed lines) of precursor powders before the heat treatment under N2 gas and (dotted-dashed lines) HfOx-C are also shown.

reduced by graphite at high temperatures, under a N2 atmosphere, by substituting some oxygen atoms with nitrogen atoms. Similar carbothermal reduction was suggested by this study, especially in the case of HfOx-C (PCM-Cl), during which the HfOx particles were in close contact with carbonized polymer originating from the citric acid and ethylene glycol. These results indicate that HfOx-C powders with a high crystalline HfO2 phase could be prepared by impregnation methods using HfCl4 as the starting material. All of these powders were heat-treated under NH3 gas in various conditions. Figure 4 presents the XRD patterns of the four HfOxNy-C catalyst types synthesized by heating the HfOx-C powders shown in Figure 2, at 1223 K for 24 h under NH3 gas. Although the crystal structure of the four HfOx-C types was not the same, as shown in Figure 2, all HfOxNy-C catalyst types displayed a similar cubic Hf2ON2 phase. The fwhm values of the strongest diffraction peak for HfOxNy-C (IM-Cl1), HfOxNy-C (IM-Cl2), HfOxNy-C (IM-OR), and HfOxNy-C (PCM-Cl) were 0.49, 0.43, 0.80, and 0.90°, respectively; this order is similar to that of the four HfOx-C types. Any HfOxNy-C synthesized by heating at 1223 K under NH3 gas showed a cubic Hf2ON2

phase, regardless of the heat-treatment conditions of HfOx-C under N2 gas. 3.2. Surface Property. Figure 5 shows the Hf 4f and N 1s spectra for the four HfOxNy-C types synthesized by heating HfOx-C powders at 1223 K for 24 h under NH3 gas. Also shown are the Hf 4f spectra of the four precursor powders before the heat treatment under N2 gas and HfOx-C powders. It is well known that the Hf 4f level is spin
orbit split into the Hf 4f7/2 and Hf 4f5/2 sublevels, which can be seen in the Hf 4f spectra as a doublet. The doublet at ∼17.7 and ∼19.0 eV observed from the Hf 4f spectra of all four types of precursor powders is typical for fully oxidized Hf4+,49,50 indicating the presence of HfO2, Hf(OH)4, and/or HfOCl2, etc. Only the precursor powders of HfOx-C (IMCl2) were obtained with water-washing, and thus the contaminants from starting materials, etc., can be assumed to be negligible. Their survey scan spectra showed no Cl 2p peak, suggesting the absence of HfOCl2 and the presence of Hf(OH)4. Because the peak binding energy of Hf(OH)4 is ∼1.6 eV higher than that of HfO2, the presence of Hf(OH)4 can be observed by measuring their O 1s spectra when the sample contains only hafnium 20614

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Figure 6. RDE voltammograms of (a) HfOxNy-C (IM-Cl1), (b) HfOxNy-C (IM-Cl2), (c) HfOxNy-C (IM-OR), and (d) HfOxNy-C (PCM-Cl) catalysts shown in Figure 1. Scans were performed under N2 without rotations and under O2 with a rotation speed of 1500 rpm at a scan rate of 5 mV s
1. The loading of all catalysts was set constant at 1 mg cm
2. As the mass ratio of HfOxNy to C was not constant, the loading of HfOxNy, mH S
1, was 0.55, 0.38, 0.60, and 0.26 mg cm
2 for (a)
(d), respectively.

composites.49,51 However, the O 1s spectra were not used in this study because all samples contain contributions from functional groups on the C surface. Both HfOx-C (IM-OR) and HfOx-C (PCM-Cl) showed almost the same Hf 4f spectra as their precursor powders. However, no clear doublet was found in the HfOx-C (IM-Cl1) and HfOx-C (IM-Cl2) spectra, indicating the existence of another doublet at lower binding energies and thus suggesting the presence of hafnium suboxide49,52 at their surfaces. The identical NH3 treatment at 1223 K for 24 h of these four HfOx-C types resulted in different surface properties. Only HfOxNy-C (IM-Cl1) showed a limited peak shift to higher binding energies, whereas the other three HfOxNy-C showed the opposite trend, which is often seen in HfOxNy thin films and is attributed to the appearance of Hf
N bonding.42,50 The peak at 395
396 eV, which is assigned to Hf
N bonding, appeared in all N 1s spectra, so different amounts of nitrogen were doped into all four HfOx types; the calculated y values were 0.35, 0.39, 0.60, and 0.52 for HfOxNy-C (IM-Cl1), HfOxNy-C (IM-Cl2), HfOxNy-C (IM-OR), and HfOxNy-C (PCM-Cl), respectively. Because NH3 decomposition should produce a reducing atmosphere, the NH3 treatment can result in oxygen defects. Some researchers53,54 reported that charged oxygen defects shifted the Fermi level of HfOx upward and thus shifted the Hf 4f and O 1s levels toward higher binding energies. The peak shift observed in the Hf 4f spectra of HfOxNy-C (IM-Cl1) whose y value was the lowest among four types of HfOxNy-C could be due to the defects produced during the NH3 treatment. 3.3. ORR Activity. The RDE voltammograms of four HfOxNyC catalyst types are shown in Figure 6. The current measured under oxygen (IO), subtracted from that measured under nitrogen (IN), was assumed to be responsible for the ORR. The ORR activity depended on the synthesis route in the following order: HfOxNy-C (IM-Cl1) ≈ HfOxNy-C (IM-Cl2) > HfOxNy-C (IMOR) > HfOxNy-C (PCM-Cl). In this study, the HfOxNy-C loading on a GC electrode was set constant at 1 mg cm
2, but that of HfOxNy was not constant because mH depended on Δm (eq 2). The ORR current per unit mass of HfOxNy versus the

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Figure 7. Mass activity versus potential [(IO
IN)mH
1
E] curves of (a) HfOxNy-C (IM-Cl1), (b) HfOxNy-C (IM-Cl2), and (c) HfOxNy-C (IM-OR) catalysts, created using the data in Figure 6.

Figure 8. Onset potential versus atomic ratio of nitrogen assigned to Hf
N bonding to hafnium (EO
y) plots of HfOxNy-C catalysts.

potential [(IO
IN)mH
1
E] curves created using the data in Figure 6 are shown in Figure 7. The HfOxNy-C (IM-Cl1) and HfOxNy-C (IM-Cl2) ORR activities were similar; however, the (IO
IN)mH
1 of HfOxNy-C (IM-Cl2) was higher than that of HfOxNy-C (IM-Cl1). These results, along with the results presented in Figure 1, suggest that the active surface area of HfOxNy particles in HfOxNy-C (IM-Cl2) was higher than that of HfOxNy-C (IM-Cl1) because of their smaller particle size.

4. DISCUSSION The results shown in Figures 1
6 revealed that the synthesis route affects the morphology, crystal structure, surface property, and ORR activity of HfOxNy-C catalysts. To clarify the property critical for HfOxNy ORR activity, the potential at which |IO
IN| in CVs exceeded 2 μA (geometrical ORR current density, 10 μA cm
2) was defined as the onset potential, EO, and was plotted against two parameters: y obtained from XPS analyses and the fwhm of the strongest diffraction peak from XRD patterns in Figures 8 and 9. No clear correlation was observed between 20615

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the highest EO obtained in this study is still ∼0.1 V lower than that of other state-of-the-art catalysts. For example, Liu et al.13 reported that nitrogen-doped ordered porous carbon catalysts showed an EO of 0.88 V vs SHE in 0.5 mol dm
3 H2SO4. The U.S. Department of Energy set a practical activity target in 2015 for non-platinum-group metal catalysts at 300 A cm
3 at a cell resistance-corrected potential of 0.8 V vs a reversible hydrogen electrode in a real PEMFC cathode,57 suggesting that the EO of the present HfOxNy-C catalyst should be enhanced further. This can be ensured by increasing (i) the crystallinity of Hf2ON2 in HfOxNx-C and/or (ii) the mass ratio of HfOxNy to C without increasing particle size.

Figure 9. Onset potential versus full width at half-maximum (EO
fwhm) curves of HfOxNy-C catalysts.

EO and y. Analyses performed on nitrogen functional groups bonded to carbon atoms also gave no clear correlation to EO. In contrast, EO increased when the fwhm decreased; that is, increasing the crystallinity of Hf2ON2 indicates that Hf2ON2 has active sites for ORR and the number increases when the crystallinity is increased. The fluorite-related crystal structure of Hf2ON2 was reported by Clarke et al.55 In the Hf2ON2 crystal, Hf4+ is located at the center of the hexahedral fluorite structure with six convex quadrilateral faces, and the two anion defect sites are located at the vertices surrounded by four Hf atoms. Zhong et al.35 proposed a reaction mechanism for their carbon-supported molybdenum nitride (Mo2N-C) catalyst during which oxygen molecules were adsorbed and dissociated at the vacancies in Mo2N linked by four atom bridges and then reduced to water through a subsequent reaction with protons and electrons. The 4-fold-type vacancies were reported to be highly active toward the molecular dissociation for O2, not only on the surface of Mo2N56 but also at the dicobalt cofacial porphyrin.3 Similar mechanisms could be proposed for the Hf2ON2 surface. The HfOxNy-C (IM-OR) or HfOxNy-C (PCM-Cl) y values could be controlled at higher levels compared with those of HfOxNy-C (IM-Cl1) and HfOxNy-C (IM-Cl2); however, the Hf2ON2 phase crystallinity in the former two catalysts was lower compared with that in the latter two catalysts, which was due to the lower HfO2 phase crystallinity in HfOx-C (IM-OR) and the HfOx-C (PCM-Cl) compared with that of HfOx-C (IM-Cl1) and HfOx-C (IM-Cl2). Therefore, the number of active sites in HfOxNy-C (IM-OR) and HfOxNy-C (PCM-Cl) could be lower, resulting in lower EO. Under the experimental conditions in this study, the crystallinity of Hf2ON2 in HfOxNx-C should be increased to enhance ORR activity. It could be increased by increasing the crystallinity of the HfO2 phase in HfOx-C, which depends on the synthesis route rather than experimental conditions, for example, heating temperature or time under N2 gas. Among the four HfOxNy-C catalyst types synthesized via different routes, HfOxNy-C (IM-Cl2) was the best in achieving simultaneously high ORR activity and high ORR current. Although the definition of EO to gauge ORR activity of different non-platinum-group metal catalysts has not yet been standardized,

5. CONCLUSION Four HfOxNy-C catalyst types were synthesized by heating HfOx-C, prepared via four different routes at 1073 or 1223 K for 24 or 50 h, under NH3 gas. The experimental results obtained by FE-TEM, XRD/XPS analyses, and electrochemical measurements revealed the following: (1) Nanosized HfOxNy particles could be supported on C by any of the four synthesis routes. The most uniform and the smallest HfOxNy particle sizes, in the order of a few nanometers, were observed for HfOxNy-C (IM-Cl2) prepared via an impregnation method using hydrolysis of HfCl4 in the dispersion consisting of the nitric acid-refluxed C powders and water. (2) The main crystal phase of HfOxNy-C was cubic Hf2ON2, regardless of the synthesis routes. (3) The ORR activity of the HfOxNy-C catalyst was increased by increasing the crystallinity of the Hf2ON2 phase. It could be increased with increasing the crystallinity of the HfO2 phase in HfOx-C, which was most dependent on the synthesis route rather than the experimental conditions, for example, heating temperature or time under N2 gas. HfOxNy-C (IM-Cl2) exhibited both high ORR activity and high current, which could be a result of higher crystallinity and smaller HfOxNy size, respectively. ’ ASSOCIATED CONTENT

bS

Supporting Information. Extended characterization results, XRD patterns and N 1s spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +81-532-44-6728. Fax: +81-532-44-6757. E-mail: chisaka@ ee.tut.ac.jp.

’ ACKNOWLEDGMENT The authors gratefully acknowledge Hirokazu Muramoto for his technical assistance in obtaining the TEM images. This work was partially supported by a Grant-in-Aid for Young Scientists (B), 23760185, from the Japanese Ministry of Education, Culture, Sports, Science and Technology; a research grant, 10-020, from TEPCO Memorial Foundation; and a research grant, R-22103, from the Research Foundation for the Electrotechnology of Chubu. ’ REFERENCES (1) Jasinski, R. Nature 1964, 201, 1212–1213. (2) Alt, H.; Binder, H.; Sandstede, G. J. Catal. 1973, 28, 8–19. (3) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027–6036. 20616

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