Improvement of Cathode Performance on Pt-CeOx by Optimization of

Oct 30, 2012 - NIMS Beamline Station at SPring-8, National Institute for Materials ... Shipra Chauhan , Toshiyuki Mori , Takuya Masuda , Shigenori Ued...
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Improvement of Cathode Performance on Pt-CeOx by Optimization of Electrochemical Pretreatment Condition for PEFC Application Keisuke Fugane,†,‡ Toshiyuki Mori,*,†,‡ Ding Rong Ou,§ Pengfei Yan,‡ Fei Ye,∥ Hideki Yoshikawa,⊥ and John Drennan# †

Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita13, Nishi8, Kita-ku, Sapporo, Hokkaido 060-8628 Japan ‡ Hetero interface design group, Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044 Japan § Laboratory of Fuel Cells, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning 116023, China ∥ School of Materials Science and Engineering, Dalian University of Technology, 2 Linggong Road, Dalian, Liaoning 116024, China ⊥ NIMS Beamline Station at SPring-8, National Institute for Materials Science, Sayo, Hyogo 679-5148 Japan # Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia, Brisbane, Qld 4072, Australia S Supporting Information *

ABSTRACT: Pt-CeOx/C electrocatalysts for the improvement of oxygen reduction reaction (ORR) activity on cathode were prepared by a combined process of precipitation and co-impregnation methods. The Pt-CeOx/C electrocatalysts pretreated by the optimized electrochemical conditioning process showed high ORR activity as compared with homemade Pt/C electrocatalyst. Also, it showed high stability in the cyclic voltammetry (CV) test up to 1000 cycles into 0.5 M H2SO4 aqueous solution. On the basis of the data of cyclic voltammogram of 30 cyclic sweeps, X-ray photoelectron spectroscopy, electron energy loss spectroscopy, high resolution transmission electron microscope image, and selected area electron diffraction analysis, it is concluded that the Pt-CeOx heterointerface involving the defect cluster formed by using optimized electrochemical pretreatment conditions on Pt in Pt-CeOx/C electro-catalyst contributes to the promotion of ORR activity and retention of its stability in long CV tests up to 1000 cycles.

I. INTRODUCTION

One effective way to overcome this problem is an improvement of ORR on Pt cathode using the appropriate oxide promoters. The ORR activity on Pt-oxide promoters such as Pt-WO3/conductive carbon (C),7 Pt-TiO2/C,7 Pt-Ta2O5/C,8 Pt-NbO2/C,9 and Pt-cerium oxide (CeOx)/C10−14 has been examined. The improvement of ORR on Pt-oxide supports was observed. In the previously reported Pt-oxide promoter electrocatalyst series, Pt-CeOx appears to be a unique system. It is because the unique role of CeOx in the reduction of overpotential for ORR at the interface of Pt and CeOx was observed by using in situ electrochemical X-ray absorption fine structure (XAFS) analysis.15 The in situ XAFS analysis indicates that the electrochemical redox reaction of cerium (Ce3+ ↔ Ce4+) at the interface of Pt and cerium(3+,4+) oxide inhibited Pt oxide formation on the Pt surface and contributed to the improvement of ORR activity on the Pt surface in the cathodic condition. During 1000 cyclic sweeps in cyclic voltammetry (CV) experiment observed for the Pt-CeOx/C, however, the

Polymer electrolyte fuel cells (PEFCs) have attracted considerable attention as a clean and efficient power source for generation of electricity using air and hydrogen. Also, PEFCs are among the promising candidates for practical use of fuel cell vehicles and residential scale cogeneration system.1 A design of highly active and cost-effective cathodes is required for achieving key milestones in this challenge. Recently, the non-platinum cathodes such as carbon-based materials (i.e., graphene nanosheets,2 carbon alloy particles3,4) have been proposed for PEFC application. However, the most suitable cathode in PEFCs has been platinum (Pt) nanoparticles loaded on conductive carbons because of the reliable cathode performance and the stable performance of the PEFC device. Alternatively, Pt cathode still has a problem which must be resolved. The large overpotential loss of the oxygen reduction reaction (ORR) appears at the surface of the Pt cathode because of the slow kinetics of ORR on Pt in the potential ranging from 0.75 to 1.0 V (vs RHE).5,6 As a consequence of this, a reduction of the overpotential loss on Pt surfaces is a key issue for the future development of PEFCs. © 2012 American Chemical Society

Received: July 27, 2012 Revised: October 21, 2012 Published: October 30, 2012 16692

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ORR activity was decreased.14 To overcome this problem and maximize the ORR activity, the design paradigm for fabrication of high-quality Pt-CeOx/C, which has the functional interface accompanied by the interaction between Pt and cerium oxide in the electrodes to be developed. In the present work, the influence of electrochemical pretreatment conditions on cathode performance and its stability in the long CV test up to 1000 cycles observed for the Pt-CeOx/C is examined. In addition, the role of the interface fabricated by using optimized pretreatment conditions for enhancement of ORR activity is concluded.

surface area calculated by using Levich’s equation (sweep rate: 10 mV s−1) was used for the calculation of the current density. The electrochemically active surface area observed by CO stripping method16 (sweep rate: 20 mV s−1) was used for the estimation of the particle sizes of homemade Pt/C and Pt-CeOx/C. The measured potentials were converted to the reversible hydrogen electrode (RHE) scale. The electrochemical pretreatment of the Pt-CeOx/C surface was necessary to obtain active and stable electrochemical performance. The standard electrochemical pretreatment in the present work was performed by means of 30 cyclic sweeps in 0.5 M H2SO4 aqueous solution using the potential raging from 0 to 1.5 V (vs RHE). The electrochemical pretreatment process was performed in N2-saturated 0.5 M H2SO4 aqueous solution. The homemade Pt/C was examined for comparison of the ORR activity. In order to optimize the electrochemical pretreatment condition, the influence of sweep range in the electrochemical pretreatment process on ORR activity and its stability in a long CV test of up to 1000 cycles were examined. For the discussion about the defect cluster structure in the interface of Pt and CeOx, the simulation work was performed. The lattice energy is the sum of long-range Coulombic interactions and shortrange interactions described by parametrized pair potentials, Sij. The short-range interactions are modeled using a Buckingham potential form Sij = A exp(−rij/ρ) − Crij−6, where A, ρ, and C are adjustable parameters. The values of these parameters are listed in Table 1.17,18

II. EXPERIMENTAL SECTION Pt-CeOx/C cathodes were prepared in a stepwise process. The pure CeO2 powder was synthesized by a hot ammonium carbonate precipitation method. The preparation detail of pure CeO2 was described in the previously published paper.14 To impregnate fine Pt particles on the prepared CeO2 particles, H2PtCl6·6H2O powder was dissolved in ethanol at the glovebox where the moisture level was kept below 10 ppm. The previously prepared pure CeO2 powders were dispersed in the H2PtCl6 ethanol solution and dried in the glovebox. The dried mixture was calcined at 400 °C for 1 h in a 10% H2/He mixed gas flow. Then, the prepared Pt-CeOx and carbon black (C; Vulcan XC−72R, Cabot Co.) were dispersed in C2H5OH and dried in a N2 gas flow. The surface chemical states of Pt-CeOx/C were analyzed by using soft X-ray photoelectron spectroscopy (SXPS). The XPS spectra were observed using a standard laboratory Al Kα X-ray source (hν = 1486.6 eV). The binding energy of Pt 4f and Ce 3d core level peaks were corrected using the binding energy of C 1s core level peak as 284.5 eV. The peak separation analysis was performed using software UNIFIT2009 (Leipzig University, Germany). The microstructural features of Pt-CeOx/C were investigated by transmission electron microscopy (TEM), using a JEM-2000EX electron microscope operating at 200 kV. Selected-area electron diffraction (SAED) analysis and TEM observation were applied to identify the crystal structure and to investigate the microstructure of the Pt-CeOx interface in Pt-CeOx/C, respectively. To identify oxidation states of platinum and CeOx in Pt-CeOx/C, an electron energy loss spectroscopy (EELS) study was performed by using a FEI TecnaiF30 electron microscope (operating at 300 kV) equipped with a Gatan Imaging Filtering system. The cathode property of Pt-CeOx/C was examined by using voltammetry in 0.5 M H2SO4 aqueous solution in the potential raging from 0.8 to 0 V (vs Ag/AgCl). The water was purified using RO (reverse osmosis)−EDI (electro-deionization) system (MILLPORE, Elix Advantage, Japan), and the ultrapure reagent grade H2SO4 (Kishida Chemicals Company, Japan) was used for preparation of electrolyte solution. The electrochemical measurements were carried out using a standard three-electrode glass cell with a rotating disk electrode after pretreatment at 28 °C. A Pt foil and Ag/AgCl were used as counter and reference electrodes, respectively. The samples for the electrochemical measurement were prepared from a suspension of the cathode materials (2 mg mL−1) in a C2H5OH solution. The suspension was spread onto the surface of a Au electrode (area: 0.20 cm2) using a micropipet. The amount of cathode material on the Au electrode was 5 μL. The quantity of Pt loaded in the homemade Pt/C and Pt-CeOx/C catalysts was 2 μg. To evaluate the ORR activity, the hydrodynamic voltammograms were observed in O2-saturated 0.5 M H2SO4 aqueous solution. The final hydrodynamic voltamometric curves were determined by the subtraction of observed hydrodynamic voltammograms in N2-saturated 0.5 M H2SO4 aqueous solution from the aforementioned observed hydrodynamic voltammograms in O2saturated 0.5 M H2SO4 aqueous solution at the same rotation rate (2000 rpm). In the electrochemical measurement, the potential was swept from the positive side to the negative side. The sweep rate was 10 mV s−1. For a fair comparison, the same amount of each catalyst by mass was loaded on a Au rotation disk electrode, and the geometric

Table 1. Short-Range Potential Parameters species 4+

2‑

Ce -O O2‑-O2‑ Pt2+-O2‑ Ce3+-O2‑

A (eV)

ρ (Å)

C (eV Å6)

ref

1809.68 9547.96 2561.61 2010.18

0.3547 0.2192 0.3200 0.3449

20.40 32.00 0.00 23.11

17 18 − −

The short-range interactions are set to zero beyond a cutoff of 20 Å. The effect of ionic polarizability of O2− and Ce4+ ions is included via the shell model.19 In this model, the ions comprise a massive core with charge X|e| and a mass less shell with charge Y|e|. The overall charge state of each ion is equal to (X + Y)|e|. The core and shell are connected by an isotropic harmonic spring of force constant k. These shell parameters are listed in Table 2.17,18 To predict how a lattice

Table 2. Shell Parameters species 4+

Ce O2‑

Y|e|

K (eV Å−2)

ref

−0.20 −2.04

177.84 6.3

17 18

accommodates defects, the Mott−Littleton two-region approach20 was used for the simulation of the defect formation energy, which is coded in the GULP program.21 In this approach, the lattice for energy minimization is partitioned into two regions: a spherical inner region I, at the center of which the defects are introduced, and an outer region II, which extends to infinity. To ensure a smooth transition between regions I and II, an interfacial region IIa is introduced. In the present work, we used an inner region I with a radius of 3.0a0 (a0 is 5.411 Å, which is the lattice constant of CeO2) and an interfacial region IIa with a radius of 6.0a0. To calculate the intrinsic defect energy, the Mott-Littleton tworegion method was applied. Also, the binding energy Eb is calculated to investigate the preference and stability of defect clusters. It is described as Eb = ΣEisolated − Ecluster, where ΣEisolated is the sum of the defect energy for all individual components and Ecluster is the entire defect energy of this cluster.

III. RESULTS Hydrodynamic voltammograms of ORR observed for Pt-CeOx/ C cathodes which were electrochemically pretreated by using 16693

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Figure 1. Voltammograms observed for Pt-CeOx/CB electrodes which were electrochemically pretreated by using various electrochemical pretreatment conditions such as the potential range of 0 to 1.0 V (vs RHE) (A) and the potential range of 0.9 to 1.0 V (vs RHE) (B).

potential observed for homemade Pt/C was almost constant within the error range in the present observation. According to the observation of Shao-Horn et al.,22 the effect of dissolution/reprecipitation in acidic solution is negligible if the particle size of Pt was greater than 5 nm. The particle size of Pt in homemade Pt/C, which was estimated by using the observed electrochemical active surface areas of Pt, varied from 24 nm to 27 nm. Therefore, the agglomeration of nanosized Pt and lowering of activity on Pt/C could not be observed in Figure 1. The small change of activity observed for the homemade Pt/C was just drift in the error range. Alternatively, the electrode potential observed for Pt-CeOx/ C at −0.03 mA cm−2 in ORR after electrochemical pretreatment by using the potential ranging from 0 to 1.0 V (vs RHE) reached the highest level as compared with other Pt-CeOx/C cathodes, which were electrochemically pretreated by using potential ranging from 0 to Z V (vs RHE; Z = 1.1, 1.2, 1.3, 1.4, and 1.5). Since the particle size of Pt in Pt-CeOx/C, which was estimated by using the CO stripping method, varied from 17 nm to 19 nm, it is concluded that the Pt particle size difference observed for both Pt/C and Pt-CeOx/C catalysts is negligible in the comparison of activity between them. Also, this improvement in activity observed for Pt-CeOx/C was beyond the range of observation error in the present work. To conclude the effect of aforementioned electrochemical pretreatment observed for Pt-CeOx/C, the influence of electrochemical pretreatment conditions on the change of Tafel slopes at Temkin-type adsorption region in ORR observed for Pt-CeOx/C (Figure 2B, |Ik| ≤ 2.0 mA cm−2) and Pt/C (Figure 2C, |Ik| ≤ 0.3 mA cm−2) was examined. From the hydrodynamic voltammograms observed for Pt-CeOx/C by using a rotation disk electrode (as shown in Figure 1A), the kinetic current densities (Ik = I × Id/(Id − I); Ik = kinetic current density, Id = diffusion-limited current density) were calculated. Tafel plots were drawn by using the relationship between applied potential and |Ik| in the ORR, as shown in Figure 2B,C. The Tafel slope values obtained for Pt/C cathodes which were electrochemically pretreated by using different electrochemical pretreatment conditions were almost same. The Tafel slope obtained from Pt/C cathodes was approximately −58 mV decade−1 in the Temkin-type adsorption

the potential ranging from 0 to 1.5 V (vs RHE) were demonstrated in Figure 1A,B. In addition, the hydrodynamic voltammogram observed for Pt-CeOx/C after electrochemical pretreatment from 0 to 1.0 V (vs RHE) in the aforementioned figures was shifted to the higher potential side as compared with the electrochemically pretreated Pt-CeOx/C by using the potential ranging from 0 to Z V (vs RHE; Z = 0.9, 0.95, 1.1, 1.2, 1.3, 1.4, and 1.5). The observation results in Figure 1A,B clearly present that Pt-CeOx/C after electrochemical pretreatment from 0 to 1.0 V (vs RHE) reveals the highest ORR activity in all examined Pt-CeOx/C cathodes. The upper limit potential (Z) in the potential range of electrochemical pretreatment (0 to Z V (vs RHE), Z = 1.0, 1.1, 1.2, 1.3, 1.4, and 1.5) dependence of the electrode potentials, which was observed for Pt-CeOx/C and homemade Pt/C cathodes at −0.03 mA cm−1 in ORR, was shown in Figure 2A. The electrode potential observed for Pt-CeOx/C decreased with increasing upper limit potential. On the contrary, the

Figure 2. Relationship between the upper limit positive potential of electrochemical pretreatment and the observed electrode potential at −0.03 mA cm−2 (A): Pt/C (●) and Pt-CeOx/C (□). Tafel plots were derived from the kinetic current density Ik of Pt-CeOx/C(B) and Pt/C (C) after various electrochemical pretreatment conditions. 16694

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region (Figure 2C) and the Tafel line was shifted within the error range of the present observation. In contrast, the Tafel slope values obtained for Pt-CeOx/C varied from −56 mV decade−1 to −73 mV decade−1 using the different electrochemical pretreatment conditions (Figure 2B). The Tafel slope value obtained for Pt-CeOx/C which was electrochemically pretreated by using the potential ranging from 0 to 1.0 V (vs RHE) was −73 mV decade−1, although the Tafel slope value which was obtained for pretreated Pt-CeOx/C in the potential ranging from 0 to 1.5 V (vs RHE) was −56 mV decade−1. The aforementioned difference of Tafel slope values obtained for Pt-CeOx/C cathodes was beyond the range of observation error in the present work. Omura et al. suggested that Tafel slope observed for the Ptbased catalyst in 0.1 M H2SO4 aqueous solution shifted to the positive side as compared with the Tafel slope observed for same electrocatalyst in 0.1 M HClO4 aqueous solution.23 This indicates that adsorbed sulfate ion on Pt in electrocatalyst makes low level kinetics of ORR on Pt. Contrary to their observation, the Tafel slope observed for Pt-CeOx/C shifted to the negative side, which indicates improvement of the kinetics of ORR on Pt in Pt-CeOx/C in the present work. This result suggests that the electrochemical pretreatment condition in the potential ranging from 0 to 1.0 V (vs RHE) could effectively promote the ORR on Pt-CeOx/C. To conclude the influence of modification of surface and interface structure on the promotion effect observed for PtCeOx/C by the optimization of electrochemical pretreatment condition, the surface and interface structural features of PtCeOx/C and Pt/C were characterized by using EELS and SXPS. Especially, the modification of the electronic structure of Pt particles accompanied by the interaction of CeOx in PtCeOx/C, which was electrochemically pretreated in H2SO4 aqueous solution, can be examined by using EELS analysis. Figure 3 demonstrates the EELS spectra of the Pt N6,7 edge and Pt O2,3 edge observed for Pt/C and Pt-CeOx/C. After fabrication of Pt-CeOx/C (i.e., before the electrochemical pretreatment), the energies corresponding to the peak intensity of the Pt O2,3 edge and Pt N6,7 edge observed for Pt-CeOx/C conspicuously shifted to higher energy loss side (i.e., observed shift of Pt O2,3 edge: from 74 to 81 eV; observed shift of Pt N6,7 edge: from 87 to 105 eV; as shown in Figure 3B,C) as compared with the energy loss peak observed for Pt/C (i.e., Pt O2,3 edge: approximately 74 eV; Pt N6,7 edge: approximately 87 eV; Figure 3A). Since it was reported that the shift of Pt O2,3 edge and Pt N6,7 edge is influenced by the appearance of Pt cations accompanied by the interaction of CeOx in Pt-loaded ceria systems,24 the authors suspect that the conspicuous shifts of energy loss peaks of Pt O2,3 edge and Pt N6,7 edge observed for Pt-CeOx/C reflect the change of electronic structure of Pt accompanied by the interaction of CeOx in Pt-CeOx/C. The excess ceria increases the resistivity on the electrodes and decreases the charge transfer between Pt and CeOx. For a conditioning process on the surface of electrodes, the electrochemical pretreatment is required. After the electrochemical pretreatment, however, the aforementioned shifts became small because of dissolution of CeOx from the surface of Pt-CeOx/C into H2SO4 aqueous solution. On the basis of the EELS analysis data, it is confirmed that the electronic structure of Pt in Pt-CeOx/C has a strong influence from the electrochemical pretreatment condition. In addition, the peak shift of the Pt O2,3 edge and Pt N6,7 edge observed for pretreated Pt-CeOx/C in the potential ranging from 0 to 1.0 V

Figure 3. EELS spectra of the Pt N6,7, Pt O2,3, and CeN4,5 edge recorded from the Pt/C and the Pt-CeOx/C cathodes.

(vs RHE) was bigger than that of the Pt O2,3 edge and Pt N6,7 edge observed for pretreated Pt-CeOx/C in the potential ranging from 0 to 1.5 V (vs RHE). Also, the ORR activity observed for pretreated Pt-CeOx/C in Figure 3B was higher than that of pretreated Pt-CeOx/C in Figure 3C. Those results suggest that there is a close relationship between the electronic structure modification of Pt accompanied by the interaction of CeOx in Pt-CeOx/C and ORR activity on Pt in Pt-CeOx/C. The Pt-remained CeOx interaction observed for pretreated PtCeOx/C in the potential ranging from 0 to 1.0 V (vs RHE) would be larger than that of pretreated Pt-CeOx/C in the potential ranging from 0 to 1.5 V (vs RHE). Since no Ce N4,5 edge was observed for the pretreated PtCeOx/C in Figure 3B and C and additional supporting data about the interaction between Pt and remaining ceria on Pt was required, the surface chemical states of pretreated Pt-CeOx/C were analyzed by using SXPS as shown in Figure 4. The Pt 4f spectra in Figure 4 show two spin−orbital splitting doublets Pt 4f7/2 − 4f5/2. The peaks at 71.1 and 72.8 eV are attributed to metallic Pt (Pt0)25 and divalent Pt (Pt2+, PtO),26 respectively, while the peak at 74.7 eV is due to the tetravalent Pt (Pt4+, PtO2).27 The other extra peak around the binding energy of 72.1 eV, labeled Pt-O-α (α: Vo•• or Ce), would correspond to the slightly ionized Pt. Since few Pt cation spectra were observed for pretreated Pt/C (Figure 4A) and both Pt2+ and Pt-O-α (α: Vo•• or Ce) spectra were observed for pretreated Pt-CeOx/C (Figure 4B,C), it is concluded that both Pt2+ and Pt-O-α (α: Vo•• or Ce) spectra observed for pretreated PtCeOx/C were observed from the interaction between Pt and CeOx on Pt. The formation of Pt-O-α (α: Vo•• or Ce) bonds at the interface is supported by the first principle calculation of Yang et al.28 Also, the pretreated Pt-CeOx/C sample which shows larger integrated peak intensity of Pt2+ and Pt-O-α (α: Vo•• or Ce) spectra in Figure 4 revealed a larger EELS peak 16695

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To develop the Pt-CeOx/C for fuel cell application, the long CV test up to 1000 cycles was performed in H2SO4 aqueous solution as shown in Figure 5A. As the results of long CV test

Figure 4. Pt 4f SXPS spectra of Pt/C prior to and after electrochemical pretreatment (0−1.0 and 0−1.5 V (vs RHE)) (A), Pt 4f (B), and Ce 3d (C) SXPS spectra of Pt-CeOx/C prior to and after electrochemical pretreatment Pt-CeOx/C (0−1.0 and 0−1.5 V (vs RHE)).

shift in Figure 3. The XPS result of Pt surface agreed well with the EELS result of Pt bulk. The Ce 3d spectra showed the complicated and characteristic spectral shapes. This is a result of the spin−orbit splitting of 3d5/2 and 3d3/2 states and photoemission final state effects. The 3d spectra consist of (Ce3+ + Ce4+) mixture spectrum. Assignment details of Ce 3d spectra were described in a previously published paper.14 The (Ce3+ + Ce4+) mixture spectra were clearly observed for pretreated Pt-CeOx/C samples, even though the remaining CeOx on Pt in pretreated samples was not clearly observed in EELS analysis. This suggests that the amount of remaining CeOx on Pt in pretreated samples was under the detectable level of EELS analysis in the present work. The characterization results for the interface and surface of Pt-CeOx/C, which were demonstrated in Figure 3 and Figure 4, indicate that the observed EELS peak shift in Figure 3 could be related to the appearance of Pt cations in the interface accompanied by the interaction between Pt and remaining CeOx on Pt in pretreated Pt-CeOx/C. Also, this modification of electronic structural features of Pt accompanied by the interaction with a small amount of CeOx would contribute to the increase of Tafel slope value and improvement of ORR activity on Pt in the pretreated Pt-CeOx/C.

Figure 5. (A) Stability of mass activity in long CV test up to 1000 cycle sweeps; Pt/C (●) and Pt-CeOx/C pretreated by using potential ranging from 0 to 1.0 V (vs RHE) (□). Sweep rate: 50 mV s−1. (B) TME image and selected area electron diffraction patterns taken from 200 cycled Pt-CeOx/C (TEM image: 1−1; SAED pattern: 1−2) and 1000 cycled Pt-CeOx/C (TEM image: 2−1; SAED pattern: 2−2).

up to 1000 cycles in Figure 5A, the ORR activity observed for pretreated Pt-CeOx/C by using optimized pretreatment condition was almost constant during 1000 cycles test as well as homemade Pt/C. The highlighted point in Figure 5A is high stability of interface structure of Pt and the remained CeOx on Pt in Pt-CeOx/C. Figures 5B 1−1 and 1−2 show the TEM image and SAED pattern taken from 200 cycled Pt-CeOx/C. The small amount of residual CeOx partially covered the Pt surface as shown in Figure 5B 1−1. Also, the diffraction rings can be assigned to metallic Pt and CeOx (i.e., CeO2 (d(111) ≈ 3.1 Å, d(200) ≈ 2.7 Å)29 and/or Ce2O3 (d(222) ≈ 3.2 Å, d(400) ≈ 2.8 Å)30) in the SAED pattern (Figure 5B 1−2). In the results of the highresolution TEM observation (Figure 5B 2−1) and the SAED analysis (Figure 5B 2−2) which were performed for 1000 16696

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Figure 6. Schematic diagram of proposed defect cluster structure in the Pt-CeOx interface: (A) PtCe″-Vo•• (Eb = 1.1 eV), (B) PtCe″-2Vo••-2CeCe′ (Eb = 5.5 eV), (C) {6(A) clusters + (B) cluster} (Eb = 21.9 eV), (D) {12x(A) clusters + 2x(B) clusters} (Eb = 32.9 eV), (E) {2x(D) clusters}, (F) C-type rare earth structure. Eb = binding energy of cluster.

one Ce3+ into Ce4+ sublattice site as shown in eq 1, and it is designated as PtCe′′-2Vo••-2CeCe′. However, this idea lacks detailed information about the defect clustering of oxygen vacancies in the interface region. It was not so easy to understand the discussion about the relationship between the formation of the Pt−O−Ce bond and the promotional effect of the electrocatalytic reaction. To discuss this key point based on the experimental data in the present work, the defect clustering of oxygen vacancies in the Pt-CeOx interface was discussed by using computer simulation. The stable configurations of PtCe″-Vo•• and PtCe″-2Vo••2CeCe′ in the united cell of fluorite-type structure ceria were shown in Figure 6A,B, respectively. The highest binding energy of PtCe″-Vo•• cluster (as shown in Figure 6A) was Eb = 1.1 eV. This binding energy was higher than that of other configurations of PtCe″-Vo•• cluster in the fluorite structure. Since the ceria matrix prefers the defect cluster if the binding energy (Eb) of the defect cluster was a high enough level in the present simulation work, the stability of configuration of PtCe″Vo•• cluster, which is demonstrated in Figure 6A, is higher than that of the other configurations of the PtCe″-Vo•• cluster in the ceria. On the other hand, the binding energies of PtCe″-2Vo••2CeCe′, which was shown in Figure 6B, was Eb = 5.5 eV. Also, this binding energy was higher than that of other configurations of the PtCe″-2Vo••-2CeCe′ cluster. This indicates that the configuration of the PtCe″-2Vo••-2CeCe′ cluster as displayed in Figure 6B was more stable than the other configurations of the PtCe″- 2Vo••-2CeCe′ defect cluster in the fluorite structure. To conclude the interface structure of Pt and CeOx using the (A) type cluster and (B) type cluster in Figure 6, the following three combinations were made in the CeOx structure: Figure 6C,D,E shows the defect cluster combination of six (A) type defect clusters and one (B) type cluster (i.e., 6x(A) clusters + (B) cluster), for which the binding energy of (C) type cluster is 21.9 eV; the combination of twelve (A) type defect clusters and two (C) type clusters (i.e., 12x(A) clusters + 2x(C) clusters), for which the binding energy of (D) type cluster is 32.9 eV; and the combination of two (D) type defect clusters (i.e., 2x(D)

cycled Pt-CeOx/C, quite similar information on the crystal structure and the microstructure of the Pt-CeOx interface was observed. On the basis of the microanalysis data observed for 200 cycled and 1000 cycled Pt-CeOx/C, it is concluded that the interface of Pt and CeOx was stable in long CV experiments. Also, this result clearly indicates that the interaction between Pt and the remaining CeOx on Pt is key for a design of highquality Pt-CeOx/C with superior stability in long CV test up to 1000 cycles.

IV. DISCUSSION The experimental data in the present work indicated that the optimized electrochemical conditioning process enhanced the ORR activity on Pt-CeOx/C electrocatalysts as compared with the homemade Pt/C electrocatalyst. Also, this result suggests that the electron charge transfer in the interface between Pt and CeOx on Pt could take place and promotes the ORR reaction on Pt. To conclude the interaction between Pt and CeOx, Ou et al. discussed the influence of the formation of Pt−O−Ce bond, which consists of Ce1‑xPtxO2‑y-type clusters in the Pt-CeOx interface on the CeOx promotion effect of the electrocatalytic reaction.24 In the interface of CeOx and Pt in the Pt-CeOx/C, the Ce1‑xPtxO2‑y-type solid solution would be formed by the diffusion of Pt cations from the Pt surface into the CeOx lattice during the loading of Pt on CeOx. In this Pt loading process in H2 gas flow at 400 °C, Ce4+ cations in CeOx were reduced to Ce3+ cations as follows: CeO2

Pt + 2Ce x Ce ⎯⎯⎯⎯→ Pt″Ce + 2Vo·· + 2CeCe′

(1)

where the Krögoer-Vink notation is used for the simple explanation of solid solution formation reaction. Since PtO and PtO2 could not be observed in the observed SAED pattern and the aforementioned formation of solid solution would occur at the interface between Pt surface and a small amount of remaining CeOx on Pt, it is concluded that the Ce1‑xPtxO2‑y-type defect clusters are formed in the interface of Pt and CeOx. The Ce1‑xPtxO2‑y-type cluster would consist of two oxygen vacancies, one Pt2+ into Ce4+ sublattice site, and 16697

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Figure 7. Schematic illustration of structural change of the (A) as prepared Pt-CeOx/C, (B) pretreated Pt-CeOx/C in potential ranging from 0 to less than 1.0 V (vs RHE), (C) pretreated Pt-CeOx/C in potential ranging from 0 to 1.0 V (vs RHE), and (D) pretreated Pt-CeOx/C in potential ranging from 0 to 1.5 V (vs RHE).

fully covered the surface of Pt and the activity on Pt was depressed due to quite limited open space on the Pt surface (Figure 7B). Alternatively, when the applied potential in pretreatment condition ranged from 0 to 1.5 V (vs RHE), a quite small amount of the aforementioned functional interface between Pt and remaining CeOx on Pt would remain on the surface of Pt in Pt-CeOx/C due to the severe pretreatment condition (i.e., range of applied potential: 0 to 1.5 V (vs RHE)) and the ORR activity became low (Figure 7D). As illustrated in Figure 7C, the active interfaces which have optimum open space on the Pt surface and strong interaction between Pt and CeOx were created by using optimized electrochemical pretreatment conditions (i.e., 0 to 1.0 V (vs RHE)). EELS and SXPS analysis as shown in Figure 3 and Figure 4 supported this conclusion. On the basis of the experimental results, it is concluded that the ORR observed for Pt-CeOx/C, which is pretreated by no optimization, approaches the level of ORR activity observed for homemade Pt/C, for which the surface is cleaned up by the electrochemical pretreatment. To enhance the charge transfer in the interface and the interaction of Pt and CeOx, the electrochemical pretreatment condition was optimized. The previously reported results of in situ XAFS analysis about the function of interface between Pt and the remaining CeOx on Pt in Pt-CeOx/C15 suggest that the aforementioned sufficient interface on Pt would improve the Pt surface activity by use of the electrochemical redox reaction of cerium (Ce3+↔Ce4+) and the charge transfer in the interface between Pt surface and remaining CeOx. As consequence of this, the ORR activity observed for Pt-CeOx/C, which was pretreated by using optimized conditions, was much higher than that of Pt-CeOx/C, which was pretreated using no optimized conditions. Also, the Tafel slope at the Temkin-type adsorption region observed for Pt-CeOx/C, which is pretreated by optimized conditions, was steeper than that of Pt-CeOx/C,

clusters), respectively. Those combinations in Figure 6 suggest that the C-type rare earth like defect structure which is quite similar to the C-type rare earth structure as show in Figure 6F can be developed in the interface by the diffusion of Pt2+ cations from Pt metal. Also, the present simulation work indicates that the interface of Pt and CeOx consists of type (B), type (C), type (D), and type (E) defect clusters; and the microanalysis results as shown in Figure 3(EELS analysis), Figure 4 (SXPS analysis), and Figure 5B (TEM and SAED analysis) show the aforementioned heterogenerous interface structural features in Pt-CeOx/C. According to eq 2, the quasi-free electrons would be introduced into the Pt-CeOx interface by the formation of aforementioned clusters. Oo x → 1/2O2 + Vo•• + 2e−

(2)

The introduced quasi-free electron in the Pt-CeOx interface would enhance the charge transfer between Pt and CeOx under the applied potential. On the basis of the aforementioned idea, it is expected that the large-scale simulation using this defect cluster model would clarify the detailed defect structure of the Pt-CeOx interface on Pt in the near future. This challenge will provide us with a big chance to design the highly functional metal−oxide interface on active metal electrodes such as Pt. During the electrochemical pretreatment process, which is required in the examination of ORR activity on electrodes, a large amount of CeOx was dissolved into the sulfuric acid solution and the heterointerface accompanied with the charge transfer between Pt and remaining CeOx on Pt would be formed (Figure 7A). When the applied potential in pretreatment condition ranged from 0 to less than 1.0 V (vs RHE), the remaining CeOx almost 16698

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which is pretreated by no optimized conditions. In addition, the high ORR activity observed for Pt-CeOx/C, which was pretreated by optimized conditions, was stable, as well as Pt/ C in long CV tests up to 1000 cycles. On the basis of all experiment data, it is found that the interface in Pt-CeOx/C, which is formed by using optimized pretreatment conditions, effectively contributes to improvement of the ORR activity on Pt, and it is stable in long CV test up to 1000 cyclic sweeps.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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V. CONCLUSIONS The electrochemical pretreatment process in the present work could modify the electronic structure of Pt particles accompanied by the interaction of CeOx in Pt-CeOx/C, and the Tafel slope value of ORR observed for Pt surface in PtCeOx/C became steep by using optimized electrochemical pretreatment condition. The stability of ORR activity observed for the electrochemically pretreated Pt-CeOx/C was examined in long CV tests up to 1000 cycles. The simulation work indicated that the defect clusters PtCe″-Vo•• and PtCe″-2Vo••2CeCe′ were formed by the diffusion of Pt from the metal side to the oxide side, and these defect clusters form C-type rare earth-like structures in the fluorite-type structure by their combinations. After the formation of a heterogeneous interface of C-type rare earth-like structure and fluorite-type structure, the quasi-free electrons would be introduced into the Pt-CeOx interface. Also, it is concluded that ORR activity on the PtCeOx/C cathode is enhanced by the combination effect of the electrochemical redox reaction of cerium (Ce3+↔Ce4+) and the charge transfer in the Pt-CeOx interface. The optimized electrochemical treatment condition contributes to the formation of this functional Pt-CeOx interface and enhancement of the aforementioned combination effect for enhancement of ORR activity. In addition, it is expected that the ORR activity on Pt-CeOx/C will be maximized by the heterointerface design between Pt and CeOx, which is based on the results in the present work.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-29-8604395. Fax: +81-29-860-4712. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Grant-in-Aid for Scientific Research (Fundamental Research B, No.22310053) by the Ministry of Education, Culture, Sports, and Technology (MEXT), Japan, is gratefully acknowledged. We also thank the Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN), National Institute for Materials Science, Japan, for partial financial support. In addition, the authors express sincere gratitude to Professor Kohei UOSAKI and Professor Takuya MASUDA about their kind advice for development of the present work. 16699

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