CrN-Encapsulated Hollow Cr-N-C Capsules Boosting Oxygen Reduction Catalysis in PEMFC

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021CrN-Encapsulated Hollow Cr-N-C Capsules Boosting Oxygen Reduction Catalysis in PEMFC Hui Yang, Xu Wang, Tao Zheng, Nelly Cantillo Cuello, Gabriel Goenaga, Thomas A. Zawodzinski, He Tian, Joshua T. Wright, Robert W. Meulenberg, Xiangke Wang, Zhenhai Xia and Shengqian Ma Hui Yang College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206 , Xu Wang State Key Laboratory of Silicon Materials, Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 , Tao Zheng Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203 , Nelly Cantillo Cuello Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996 , Gabriel Goenaga Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996 , Thomas A. Zawodzinski Chemical and Biomolecular Engineering Department, University of Tennessee, Knoxville, TN 37996 , He Tian *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Silicon Materials, Center of Electron Microscopy, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 , Joshua T. Wright Department of Physics, Illinois Institute of Technology, Chicago, IL 60616 , Robert W. Meulenberg Department of Physics and Astronomy, Frontier Institute for Research in Sensor Technologies, University of Maine, Orono, ME 04469 , Xiangke Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206 , Zhenhai Xia Department of Materials Science and Engineering, University of North Texas, Denton, TX 76203 and Shengqian Ma *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, University of North Texas, Denton, TX 76201 https://doi.org/10.31635/ccschem.020.202000645 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Understanding the origin of the catalytic activity for the development of efficient catalysts is critical yet challenging. Herein, we report a simple strategy for the synthesis of chromium nitride nanoparticles (CrNNPs) encapsulated into hollow chromium–nitrogen–carbon capsules ([email protected]–Cr–Nx–C). The [email protected]–Cr–Nx–C demonstrated excellent electrocatalytic activity for the oxygen reduction reaction (ORR) in acidic solutions. When applied as a cathode material in a proton-exchange membrane fuel cell (PEMFC), the [email protected]–Cr–Nx–C achieved outstanding initial performance, which is better than that of the PEMFC with H–Cr–Nx–C cathode material. The [email protected]–Cr–Nx–C cathode also showed good stability over 110 h of operation. These results demonstrated that the coexistence of atomically dispersed CrNx sites and sufficient CrNNPs is essential for excellent PEMFC performances. Density functional theory (DFT) studies further corroborated that CrNNPs can boost the ORR activity of H–Cr–Nx–C. This finding opens a new avenue for the fabrication of low-cost, highly active, and durable ORR catalysts for PEMFC and other applications. Download figure Download PowerPoint Introduction The emergence of the proton-exchange membrane fuel cell (PEMFC) has provided enormous opportunities to develop the next generation of power sources for electric vehicles due to advantages such as high energy conversion efficiency and environmental protection.1,2 However, the widespread use of PEMFC is currently limited by the scarcity and high cost of platinum-group metal (PGM) electrocatalysts, particularly on the cathode.3–7 Since the oxygen reduction reaction (ORR) plays a crucial role in the PEMFC, a significant amount of effort have been devoted to the search for a PGM-free ORR catalyst in acidic solutions, and great progress has been made. To address the above issues, porous carbons decorated with atomically dispersed transition metal–nitrogen (M–Nx–C; M = Fe, Co, and Mn) moieties have emerged as high-performance ORR electrocatalytic catalysts for PEMFC in acidic solutions.8–22 It is generally believed that the catalysis occurs on the exposed MNx active sites of the M–Nx–C catalysts, and increasing the density of MNx moieties while minimizing other metal-based phases is an effective strategy for improving the catalytic activity. Thus, many efforts have been paid to identifying more active MNx coordination configurations.23–33 However, the interactive catalytic contribution of the metal-based phases (nanoparticles and/or clusters) from aggregation has received less focus and needs further exploration.34–41 Besides the activity, the performance of a PEMFC with M–Nx–C catalysts as cathode typically degrades rapidly (∼40–80%) in the first 100 h of testing in PEMFC.42–44 Among studied M–Nx–C catalysts, Fe–Nx–C has been the most widely studied because of their high activity for ORR in PEMFC. Nevertheless, the atomically dispersed FeNx center suffers a serious stability issue from oxidative corrosion via Fenton reaction.42,43,45–48 In these cases, the design and preparation of highly active, durable, PGM-free catalysts, as well as understanding their origin of catalytic activity, are still one of the long-standing challenges in PEMFC engineering. In this work, we report that the chromium nitride nanoparticles (CrNNPs) encapsulated into hollow chromium–nitrogen–carbon capsules (denoted as [email protected]–Cr–Nx–C) were successfully prepared by pyrolysis of a [email protected]–tannic acid ([email protected]–TA) core–shell nanocrystals. On account of the enriched CrNx sites, sparse CrNNPs, and abundant micro/mesopores, [email protected]–Cr–Nx–C exhibited outstanding electrocatalytic activity for ORR in an acidic solution. Subsequently, the [email protected]–Cr–Nx–C demonstrated an excellent open-circuit voltage (OCV) and high current and power densities when used as a cathode catalyst in PEMFC. Durability tests indicated that the [email protected]–Cr–Nx–C catalyst has very promising stability over 110 h of operation. The H–Cr–Nx–C showed lower activities compared with the [email protected]–Cr–Nx–C counterpart. The benefits of encapsulated CrNNPs to boost ORR performance of [email protected]–Cr–Nx–C were confirmed by density functional theory (DFT) calculations. Our work opens a new avenue for the rational development of more economical catalysts for PEMFC. Experimental Methods General procedure for preparation of [email protected]–TA composite [email protected]–TA was synthesized by following our reported procedure.24 Typically, 200 mg of ZIF-8 nanocrystals was dispersed in 10 mL of deionized water. Separately, 3 mL of TA aqueous solution (24 mM, pH 8 adjusted by 6 M KOH) was added to the above ZIF-8 suspension. The mixture was then stirred for 5 min. The precipitates were centrifuged, washed with methanol three times, and dried in an oven at 40 °C. Preparation of [email protected]–TA composite In a typical procedure, 30 mL of a methanolic solution containing obtained [email protected]–TA and 200 mg of Cr(NO3)3·9H2O was stirred for 3 h. The solid product was centrifuged, washed with methanol three times, and dried in an oven at 40 °C. [email protected]–TA(100) composite was synthesized via a similar synthetic procedure, only reducing the amount of Cr(NO3)3·9H2O to 100 mg. Preparation of [email protected]–Cr–Nx–C and H–Cr–Nx–C catalysts In a typical procedure, the [email protected]–TA(200) core–shell nanoparticles were heated to 900 °C with a ramp rate of 2 °C·min−1 under a nitrogen flow. Atter continuing pyrolyzed at 900 °C for 3 h, the black powder was obtained. Subsequently, the product was then immersed in 3 M H2SO4 at 90 °C for 12 h, then separated by filtration and washed thoroughly until the filtrate became neutral, and dried in an oven at 40 °C. The black powder was pyrolyzed at 900 °C for 3 h under a nitrogen flow. Waiting for the furnace cooling to room temperature, the final product ([email protected]–Cr–Nx–C) was obtained. H–Cr–Nx–C was prepared from the [email protected]–TA(100) using the same protocol. Other instrumentation, characterization methods, and experimental details, including material synthesis, rotating ring-disk electrode (RRDE) tests, membrane electrode assembly (MEA) preparation and single fuel cell tests, X-ray absorption fine structure (XAFS) measurements, and DFT calculations, are available in the Supporting Information. Results and Discussion The preparation of [email protected]–Cr–Nx–C essentially involves a three-step process (Scheme 1): (step I) uniform coating of the ZIF-8 template with K–TA polymer yield [email protected]–TA core–shell structure49; (step II) replacement of the potassium cations in the K–TA shell by Cr(III) resulted in [email protected]–TA; powder X-ray diffraction (PXRD) patterns, scanning electron microscopy (SEM) images, and energy-dispersive X-ray spectroscopy (EDXS) spectra showed the crystallinity of the ZIF-8 core, and dodecahedral morphology was retained, as expected ( Supporting Information Figures S1–S4). The amorphous Cr–TA shell layer with ∼10 nm thickness remained with the ZIF-8 core as confirmed by transmission electron microscopy (TEM; Supporting Information Figure S4b). (step III) Upon pyrolysis and acid etching, the [email protected]–TA carbonized into hollow N-doped carbon capsules, and the coordinated Cr3+ ions were transformed into atomically dispersed CrN(C)x moieties anchored on carbon shell, as well as CrNNPs support on the internal surface simultaneously. The broad peak detected in the PXRD pattern of [email protected]–Cr–Nx–C at 24.7 can be attributed to the graphitic (002) reflection from the carbon (Figure 1a). The sharp diffraction peaks at 37.47, 43.54, 63.27, and 75.91 can be assigned to the (111), (200), (220), and (311) reflections for cubic CrN, suggesting that phase nitride was formed during pyrolysis. SEM and TEM images of [email protected]–Cr–Nx–C showed hollow capsule-like morphology with embedded large CrNNPs (Figures 1b and Supporting Information Figure S6). This suggested the chromium ions in the TA layer randomly aggregated into surface-supported nanoparticles in the capsule-forming step during heat treatment. Aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) was further used to investigate the chromium-containing hollow capsule. The fringes with a lattice spacing of 0.21 and 0.24 nm can be indexed to the (200) and (111) planes of cubic CrN, respectively (Figure 1c). Low magnification HAADF-STEM images and EDXS mappings of [email protected]–Cr–Nx–C illustrate the distribution of the C, N, and Cr species in both nanocrystals and hollow capsules (Figures 1d–1h). In addition to CrNNPs, a large number of prominent and isolated bright spots are evident against a dark background, suggesting the atomically dispersed chromium moieties anchor on the capsules (Figure 1g). Figure 1 | (a) PXRD patterns of prepared materials. (b) TEM image of [email protected]–Cr–Nx–C. (c) Aberration-corrected HAADF-STEM image of an individual CrN nanoparticle marked with a red-dashed ring in 1 d. (d–h) HAADF-STEM image and EDXS mappings of [email protected]–Cr–Nx–C. Download figure Download PowerPoint Scheme 1 | Synthetic scheme for the preparation of [email protected]–Cr–Nx–C. Download figure Download PowerPoint To further probe the atomically dispersed chromium moieties in [email protected]–Cr–Nx–C, we prepared the H–Cr–Nx–C under similar synthetic procedures, only reducing the Cr3+ content in [email protected]–TA polymer (see the details in the Supporting Information). The CrN diffraction peaks disappeared in the PXRD pattern, indicated no crystalline CrN formed in H–Cr–Nx–C (Figure 1a). No obvious aggregation of chromium atoms could be found in the TEM image (Figure 2a). Aberration-corrected HAADF-STEM (Figures 2b–2d) and corresponding EDXS mapping images (Figures 2e–2i) further showed the high density of atomically dispersed (highlighted by red and yellow circles) chromium sites well anchored on the hollow N-doped carbon capsules. Taking into account these results together, we concluded that chromium ions tend to be more likely involved in the formation of atomically dispersed chromium sites anchored on hollow N-doped carbon capsules before they aggregated. However, if the chromium ions concentration is high enough, in addition to the formation of isolated CrN(C)x species, the rest aggregates to produce CrNNPs during pyrolysis. Figure 2 | (a) TEM image of H–Cr–Nx–C. (b–d) Aberration-corrected HAADF-STEM images of H–Cr–Nx–C. Single Cr atoms are highlighted by red and yellow circles. (e–i) HAADF-STEM image and EDXS mappings of H–Cr–Nx–C. Download figure Download PowerPoint The X-ray photoelectron spectroscopy (XPS) spectra of [email protected]–Cr–Nx–C are shown in Figures 3a and 3b and Supporting Information Figure S11. In the Cr 2p region, the XPS spectrum provides clear signals at 577.3 and 586.8 eV due to Cr 2p3/2 and Cr 2p1/2, attributed to chromium(III) (Figure 3a).50,51 No metallic chromium was detected by XPS. The N 1s XPS spectrum can be deconvoluted into four components, pyridinic-N (398.5 eV), pyrrolic-N (399.2 eV), graphitic-N (400.9 eV), and quaternary-N (404.5 eV) (Figure 3b).24 Pyridinic and pyrrolic nitrogen atoms doped into carbon shells can serve as the anchor points for trapping atomically dispersed chromium(III) ions via coordination bonds. These XPS results suggest that a portion of chromium(III) bonded with N-formed CrN nanoparticles. The rest of chromium(III) occupy sites in the N-doped carbon shell, similar to molecular species chromium porphyrins. The XPS spectra of H–Cr–Nx–C were very similar to those of [email protected]–Cr–Nx–C evaluated under the same test environments (Figures 3c and 3d and Supporting Information Figure S12). Figure 3 | (a and b) Cr 2p and N 1s XPS spectra of [email protected]–Cr–Nx–C. (c and d) Cr 2p and N 1s XPS spectra of H–Cr–Nx–C. (e) XANES spectra of [email protected]–Cr–Nx–C and H–Cr–Nx–C together with Cr3C2, Cr2O3, and Cr foil. (f) Cr K-edge Fourier transform (FT) EXAFS spectra of [email protected]–Cr–Nx–C and H–Cr–Nx–C together with Cr3C2, Cr2O3, and Cr foil. (g and h) Corresponding EXAFS R space fitting curves for [email protected]–Cr–Nx–C and H–Cr–Nx–C, respectively. (i) N2 adsorption and desorption isotherms of catalysts. Download figure Download PowerPoint X-ray absorption near-edge structure (XANES) and extended XAFS (EXAFS) measurements were further employed to probe the possible atomic structures of chromium sites in the catalysts. Cr foil, Cr2O3, and Cr3C2 were also analyzed as references. Normalized XANES spectra revealed that the Cr K-edge absorption edges of [email protected]–Cr–Nx–C and H–Cr–Nx–C were situated close to that of Cr2O3, which corroborated the conclusion obtained from XPS spectra that the chromium sites formed have an oxidation state of approximately 3+ (Figure 3e).52,53 Moreover, fingerprint peaks at 5991.8 eV were observed in both catalysts, implying that the chromium atoms have been stabilized by strict square-planar configurations with D4h symmetry.24,45 The EXAFS spectra reveal that the oscillations of [email protected]–Cr–Nx–C and H–Cr–Nx–C are significantly different compared with Cr foil, Cr2O3, and Cr3C2 references (Figure 3f). [email protected]–Cr–Nx–C and H–Cr–Nx–C display a single well-resolved peak at 1.53 Å in R space, which implies a Cr–N or Cr–O scattering path.45,54 The absence of Cr–C and Cr–O second shell scattering paths suggest that no chromium carbide or chromium oxide formed, which is consistent with the presence of Cr–N bonds in these catalysts. Moreover, the EXAFS spectrum of [email protected]–Cr–Nx–C shows evidence for Cr–Cr scattering, suggestive of long-range order structure, and the presence of CrN. Therefore, while the XANES spectra point to [email protected]–Cr–Nx–C and H–Cr–Nx–C being very similar from an electronic structure standpoint, the EXAFS supports the formation of CrNNPs in [email protected]–Cr–Nx–C. Fitting the EXAFS data of H–Cr–Nx–C to the CrN4 species was consistent with the isolated chromium centers being coordinated by four nitrogen donors with a Cr–N bond length of 1.94(6) Å (Figure 3h and Supporting Information Table S2). These CrN4 sites are integrated into porphyrin-like basal planes, as depicted in the computational models (vide infra) ( Supporting Information Figure S15a). Based on an arsenal of complementary technologies, we conclude that [email protected]–Cr–Nx–C comprises CrNNPs and chromium cations in the 3+ oxidation states coordinated by porphyrin-like four nitrogen atoms without axial-bonded oxygen species (Figures 3g and Supporting Information S15d). [email protected]–Cr–Nx–C is comprised of 7.51 wt % nitrogen, as revealed by elemental analysis ( Supporting Information Table S1). H–Cr–Nx–C contains less chromium and more nitrogen (relative proportion) compared with [email protected]–Cr–Nx–C. The Raman spectra of [email protected]–Cr–Nx–C and H–Cr–Nx–C exhibited similar D (1350 cm−1) and G (1590 cm−1)-bands, suggesting a similar disorientated degree of graphene ( Supporting Information Figure S10). Nitrogen adsorption–desorption isotherms were recorded to probe the accessible porosity of the catalysts at 77 K. The isotherm showed rapid N2 uptake at a low relative pressure (P/P0 < 0.1), followed by a more gradual increase at pressures between 0.1 < P/P0 < 0.95, suggesting that micropores and mesopores are predominant in existence (Figure 3i). Large hysteresis loops were observed at pressures between P/P0 0.95 and 0.4 during the desorption process, indicating the existence of cavities in these catalysts. The calculated Brunauer–Emmett–Teller (BET) surface areas are 913 and 1054 m2 g−1 for [email protected]–Cr–Nx–C and H–Cr–Nx–C, respectively ( Supporting Information Table S1). The pore structures of the catalysts were confirmed by their micropore and mesopore size distributions with void diameters clustered around 1.5 and 2.8 nm, as determined using a DFT model ( Supporting Information Figure S8). On this basis, their porosities were expected to offer a higher collision probability of the active sites and reactants without mass transfer limitations, as anticipated for efficient electrocatalysis. Metal–nitrogen–carbon materials have attracted widespread interest as potential substitutes for PGM catalysts in PEMFC.7,19,43,48 In this light, [email protected]–Cr–Nx–C and H–Cr–Nx–C were evaluated as electrocatalysts for the PEMFC. First we tested their ORR performance in O2-saturated 0.1 M HClO4 with RRDE operated at 1600 rpm (Figure 4a). The onset potentials, half-wave potentials, and ultimate current densities calculated from these measurements are shown in Supporting Information Table S3. [email protected]–Cr–Nx–C exhibited a good ORR activity in terms of the onset potential of 0.85 V and a half-wave potential of 0.72 V, which were higher than those of the H–Cr–Nx–C catalyst. The limiting current density of [email protected]–Cr–Nx–C is very close to that of Pt/C (30 wt % of Pt) catalyst. The Tafel slopes of [email protected]–Cr–Nx–C and H–Cr–Nx–C are 57 and 55 mV dec−1, respectively, lower than those of commercial Pt/C (68 mV dec−1), indicating its excellent ORR kinetic qualities ( Supporting Information Figure S13). Meanwhile, Tafel slopes of both catalysts suggest the same reaction pathway toward the ORR. [email protected]–Cr–Nx–C and H–Cr–Nx–C further showed a low peroxide yield of around 5–20%, and the electron-transfer numbers were found to be 3.9 (Figure 4b). These results suggest the high-efficiency four-electron complete reduction of oxygen to water is preferable47,55 Figure 4 | (a) Linear sweep voltammetry curves of catalysts. (b) H2O2 yield and electron-transfer numbers of catalysts in 0.1 M HClO4 by RRDE tests. (c) I–V polarization and power density curves for H2–O2 PEMFC with [email protected]–Cr–Nx–C and H–Cr–Nx–C, respectively, as cathode catalysts at 80 °C. (d) Fuel cell durability tests with [email protected]–Cr–Nx–C as cathode over 110 h operation. Download figure Download PowerPoint RRDE tests preliminarily revealed that the obtained catalysts showed good oxygen reduction activity in acidic solutions. However, the high ORR activity measured from the half-cell does not necessarily imply a high PEMFC performance due to the significantly different working conditions. We further carried out PEMFC performance tests on [email protected]–Cr–Nx–C and H–Cr–Nx–C catalysts. These materials were utilized as cathode materials for fabricating MEAs studied in PEMFCs. The polarization curves were measured by a fuel cell test station (5 cm2 membrane electrode) at 80 °C and back pressures of 29.4 psi. Commercial Pt/C (30 wt % of Pt on Vulcan XC72 was used with a of as the for the oxidation a catalyst of 2 mg The measured were 0.85 and V for [email protected]–Cr–Nx–C and H–Cr–Nx–C, respectively, in good with RRDE tests in HClO4 (Figure [email protected]–Cr–Nx–C exhibited a current density of and a peak power density of at V, which is higher than those of the with H–Cr–Nx–C and The power density of the cell with [email protected]–Cr–Nx–C was at V, to the of at V for H–Cr–Nx–C. These excellent of [email protected]–Cr–Nx–C are with those of other reported M–Nx–C catalysts ( Supporting Information and S6). Durability is for M–Nx–C catalysts as generally exhibited with of during the first 100 h of testing in PEMFC.42–44 However, the [email protected]–Cr–Nx–C exhibited good stability with a current rate of over 110 h of (Figure The above measurements indicated that [email protected]–Cr–Nx–C exhibited performance to H–Cr–Nx–C, suggesting that the presence of CrNNPs the activity of the CrNx active Subsequently, DFT were further to an understanding of the high electrocatalytic performance of [email protected]–Cr–Nx–C. It is well that an active catalyst can to the ORR and to complete ORR. Based on the above and XAFS we both the CrN4 and CrNNPs moieties in [email protected]–Cr–Nx–C as possible active sites for ORR Therefore, models of CrN4 and CrNNPs were calculated for the ( Supporting Information Figures Table We that both catalysts could complete the reduction of to in a four-electron this an is first on a Cr then to further into and and transformed into The of ORR on these structures are shown in Figure The ORR on the structures is for due to its adsorption to the The adsorption of to the is compared with is still than H–Cr–Nx–C. The ORR of H–Cr–Nx–C, and [email protected]–Cr–Nx–C were calculated to be 0.95, and V, respectively. with H–Cr–Nx–C and CrN [email protected]–Cr–Nx–C has lower indicating that the presence of CrNNPs can significantly the of and the desorption of on the CrN4 active sites, the ORR activity of the CrN4 (Figure The and density of states are further analyzed in Supporting Information Figures and respectively. The presence of CrN to the H–Cr–Nx–C the from to of CrN4 catalytic active center in H–Cr–Nx–C, which plays an role in the desorption of The high electronic of the atomically dispersed Cr anchored on hollow N-doped carbon shell in the [email protected]–Cr–Nx–C is close to the which the electronic transfer from metal to during the and to a better catalytic This is consistent with the experimental results and to the excellent experimental ORR performance of [email protected]–Cr–Nx–C in PEMFC. Figure 5 | (a) The of ORR four-electron pathway in four structures in this (b) The ORR reaction of [email protected]–Cr–Nx–C. Download figure Download PowerPoint The above results that the [email protected]–Cr–Nx–C is an excellent catalyst for ORR in acidic PEMFC. The excellent electrocatalytic activity of this catalyst can be attributed The hollow and porosity of the carbon shell, which have excellent and the of to the catalytic active The atomically dispersed porphyrin-like CrNx sites are anchored on the of N-doped porous carbon capsules, which for the high and of active The anchored CrNx moieties an catalytic activity to the Fenton to good of the embedded CrNNPs to the H–Cr–Nx–C capsules, which provides a large of exposed between CrNNPs and CrNx sites to the adsorption and the desorption during electrocatalytic ORR. This that the activity of CrNx sites by CrN nanoparticles. The [email protected]–Cr–Nx–C has been successfully from the between CrNNPs and atomically dispersed CrNx sites, [email protected]–Cr–Nx–C showed very activity for ORR in an acidic Subsequently, the [email protected]–Cr–Nx–C catalyst as a cathode demonstrated excellent initial activity and durability in a PEMFC. DFT further confirmed that the presence of CrN nanoparticles the active activity of CrNx sites for ORR in PEMFC. This that a promising this of materials as ORR electrocatalysts in PEMFC. The strategy reported for the synthesis of these catalysts is and This a new for the design of catalysts for applications. Supporting Information Supporting Information is available and instrumentation, experimental procedures, pore size ORR curves and calculations, and DFT calculations. of The no of The the support from the Robert A. from North China Electric Power University and the Science of China are also and and support from the Science Materials Research are by the Department of and the This used of the a of Science operated for the of Science by Laboratory under and for Fuel and for the Oxygen Reduction at Fuel Wang K. of in

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