Highly durable electrocatalyst with low-loading platinum-cobalt 1 nanoparticles dispersed over single-atom Co-N-Graphene nanofiber for 2 efficient fuel cell 3

Development of highly-active, durable and cost-effective oxygen reduction electrocatalyst for 11 proton exchange membrane fuel cell is crucial and greatly desired to enable fuel cell powered 12 vehicles that are competitive with internal combustion engine automobiles. The support’s 13 structure is known to strongly influence the performance of Pt particles. Here, we present a new 14 catalyst containing PtCo core-shell nanoparticle supported over hierarchical tailored porous 15 carbon nanofibers with densely populated single-atomic Co-Nx sites embedded in N-doped 16 graphene. In a fuel cell with a total Pt loading (anode + cathode) of 0.091 mg cm -2 , the new 17 catalyst delivered unprecedented mass activity of 2.28 A mg Pt-1 at 0.9 V iR-free , Pt utilization of 18 11.1 kW g Pt-1 at 150 kPa abs , and high durability with 80% retention of initial mass activity after 19 30,000 accelerated-stress-test cycles, significantly higher than that of the state-of-the-art 20 Pt 3 Co/C. In-situ X-ray absorption spectroscopy revealed structure reversibility of the catalyst 21 The experiments reveal that Co-N-GF

where high surface area Co-N-C PGM-free catalyst was used as support for low loading PtCo NPs with protecting layers are effective approaches for Pt to achieve sustained stability 10 . In a 55 catalyst, micropores host the active site 11 (e.g. Co-N 4 ), and mesopores govern the mass transfer 56 and ionomer distribution. Therefore, to maximize the Pt performance in term of activity and 57 stability by introducing PGM-free catalyst as support, a rational design of the PGM-free catalyst 58 with high surface area, balanced micro-and meso-(pore size 2 nm -50 nm) porosity, excellent 59 corrosion resistance, electron conductivity, and strong affinity to Pt NPs, is essential 4,9 . 60 Additionally, in an ideal PGM-free catalyst, within each micropore, a dense population of active 61 sites should be freely accessible to the reactant and product. And the meso-/macro-pores should 62 offer minimal transport resistance and be interconnected through a robust, continuous conductive 63 matrix. As a consequence, the accessibility of reactant to the active sites is enhanced, giving rise 64 to the excellent electrocatalytic activity for PGM-free catalyst toward ORR 4,12-14 . 65 In this work, we introduce a new design of a low loading PtCo catalyst (LPCNGF), where a Co-MOF) family, was selected for this study as the source for Co-Nx active site formation 15 . 72 Also, it served as the metallic Co source which was the nucleation center for further PtCo alloy here not only promise a high-quality graphene with hierarchical pore structure, but also a high Raman spectrum (Fig. 1c) confirms the graphene nature of the carbon fiber as evidenced by the 110 peaks at 2653 cm -1 and 2915 cm -1 corresponding to 2D-band and D+G-band of graphene 22 .

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The catalyst has an interconnected network structure, with "beads" lining up on the "string" ( 112 Fig. 2a, Supplementary Fig. 1b-1c). The "beads" were converted from Co-MOF, maintaining 113 6 MOF-like morphology with hollowed structure. High-angle annular dark-field scanning 114 transmission electron microscopy (HAADF-STEM) images, combined with energy-dispersive 115 X-ray spectroscopy (EDS) element mapping, revealed the formation of PtCo alloy and 116 homogenous distribution of N in the carbon matrix ( Fig. 2b-2c, 2e). The average particle size 117 was ~ 3 nm ( Supplementary Fig. 1d, 1e). The formation of PtCo alloy was further confirmed by 118 X-ray Diffraction (XRD) (Supplementary Fig. 6). The atomic ratio of Co:N:Pt:C was  corresponding to a low H 2 O 2 yield of < 3% (Fig. 3b). The Pt/C and the state-of-the-art Pt 3 Co/C 140 catalyst from Umicore (Pt 3 Co/C(Um)) were tested under the same condition as benchmarks.

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Even with higher Pt loading in Pt/C and Pt 3 Co/C(Um) than that in LPCNGF, the half-wave  iR-corrected polarization shown in Supplementary Fig. 13). The peak power density of LPCNGF 190 reached 1.11 W cm -2 at BOL (150 kPa abs ), corresponding to an effective Pt utilization of 0.044 191 g Pt kW -1 at cathode, which is 1.7 times that of the recently reported Pt/nitrogen modified C 192 (0.075 g Pt kW -1 , 230 kPa abs ) 18 , and rank among the highest reported in the literatures (Fig. 4f).

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The LPCNGF delivered a MA of 2.28 A g Pt -1 at 0.9 V iR-free at BOL (Fig. 4d,    PtCo alloy is further confirmed by EXAFS (Supplementary Fig. 16b). It is worth noting that R Pt-238 Co (2.19 Å) is similar to the R Co-Co (2.18 Å) ( Supplementary Fig. 16a), suggesting that the 239 majority of the hetero-atomic interactions in LPCNGF are located in the Co-rich core, consistent 240 with the Pt thin-shell /single-PtCo core structure. The surface structure of LPCNGF catalyst is 241 schematically outlined in Fig. 5f.

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To examine evolution of the local atomic structure or metal oxidation states in LPCNGF 243 during ORR, we collected in-situ XANES and EXAFS spectra at the Co K-edge and Pt L-edge in 244 O 2 saturated 0.1 M HClO 4 at various applied potentials (Fig. 6). When 0.8 V potential was 245 applied, XANES spectrum at Co K-edge showed that the absorption energy was slightly blue-246 shifted compared to the dry sample ( Supplementary Fig. 17a), suggesting the absorption of 247 oxygenated species (O* or OH*) on the surface Co. As the applied potential increased to 1.3 V,

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the absorption edge at 7728 eV shifted to 7729 eV with slightly enhanced WL intensity (Fig. 6a,   249 inset), indicating charge transfer from Co to the oxygenate intermediate. EXAFS spectrum at the 250 Co K-edge at 0.8 V (Fig. 6c, Supplementary Fig. 17b)  decreased by 0.09 Å relative to that at 0.8 V (Fig. 6c, inset), implying oxidation of Co. decreased Pt-Co peak intensity with increasing potential (Fig. 6d, inset), suggesting that the 264 surface becomes disordered during ORR. As the potential reached 1.3 V, a new weak peak at 265 1.72 Å appeared. This peak is assigned to a Pt-OH bond, which is 0.12 Å larger than the Pt-O 266 bond (1.6 Å) 28, 29 . When the potential decreased to 0.6 V, both XANES and EXAFS spectra at

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Catalyst stability was studied by CV between 0.4 V and 1.0 V with 10,000 cycles at a scan rate of 50 mV s -1 .

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Electrochemical surface area was determined by integrating hydrogen adsorption charge on CV curve after double-  The data that support the findings of this study are available within the paper and its Supplementary