Larger content of SnO2 may block the active Pt sites for ORR and will be discussed later. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) reveals a narrow particle size distribution (sub-2 nm) of SnOx-Pt/C (Pt:Sn atomic ratio to 1:10) (Figure ). As a control sample, Figure For insights into the atomic surface structure of SnOx-Pt/C particles, HRSTEM images were conducted. For SnOx-Pt/C (1:10) catalysts, SnOx islands are clearly observed on Pt particle surface (Figure ), where the enlarged images show the two or three atom layers (Figure ). By comparing Z-contrast and measuring the lattice distance of labeled area in HRSTEM images (Figure ,f), we confirmed that the formed SnOx is located on the surface of Pt particles as heterojunction structures in which the bright atom is Sn and the light element O cannot be observed. By measuring the distance between the Sn atomic plane and the Pt atomic plane, it was found that there are two interface structures (Figure ). One is that Sn and Pt are directly adjacent (Sn-Pt atomic plane spacing is 0.24 nm); the other is The Sn-Pt atomic distance is 0.28 nm, and there is a layer of O between the Sn and Pt atomic planes. When the tin content increases (Pt:Sn atomic ratio is 1:5), more independent SnOx nanoparticles are observed on the surface of SnOx-Pt/C (1:5) (Figure ). The lattice spacing was measured to be 0.34 nm (Figure ), which is consistent with the SnO2 (110) interplane spacing (Figure ). However, the high-resolution results of the pure Pt nanocluster comparison sample showed that no SnOx oxide-like features were found on its surface (Figure ). Then elemental mapping by energy- To investigate the surface chemical states and electronic structure of SnOx-Pt/C catalyst, we further conducted X-ray photoelectron spectroscopy (XPS) analysis. The Pt 4f (Figure ) and Sn 3d (Figure ) spectra of the SnOx-Pt/C catalyst show that the Pt is mainly in the metallic state, and majority of the surface Sn is in the oxidized states which are consistent with reported SnOx-Pt catalysts. The binding energies of Pt 4f of SnOx-Pt/C catalyst reveal the metallic state at 71.9 and 75.2 eV, which are shifted to low-energy compared with the values for the Pt/C catalyst. Meanwhile, the highest peak of the SnOx-Pt/C catalysts centered at 487.8 eV can be assigned to the Sn(4+). The two sub-peaks peaks of Sn 3d3/2 and 3d5/2 with binding energies at 486.6 and 485.3 eV can be assigned to the Sn(2+) and Sn(0), respectively. The XPS peak for SnO2/C catalyst is assigned to a tetravalent state (Figure ). The above analysis confirms that Sn in SnOx-Pt/C is mainly in the tetravalent state, and Sn exists in the mixed form of SnO or SnO2. We assume that electrons are transferred from Sn to Pt, and Pt tends to gain electrons. To reveal the local coordination environment of the SnOx-Pt/C catalysts, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were performed. In Figure , the Pt LIII-edge XANES spectra reveal the filling of d-orbitals of SnOx-Pt/C catalyst is different from that of Pt foil, PtO2 foil, and Pt/C (Figure ). For SnOx-Pt/C catalyst, the electron filling degree of 3d orbital of Pt atoms is higher than that of the Pt/C catalyst. Meanwhile, the K-edge spectra of Sn reflect the change in the oxidation level of SnOx-Pt/C catalyst, along with the spectra for the Sn foil, SnO2 foil, SnO foil, and SnO2/C reference (Figure ). The valence state of tin in SnOx-Pt/C existed in both tetravalent state and bivalent state, in agreement with the XPS results. As the Sn atoms lose electrons, the electron filling degree of Pt increases. We therefore infer the electron transfers from SnOx to Pt in SnOx-Pt/C catalyst. The extended EXAFS spectra of SnOx-Pt/C catalyst show no indication of Pt-Sn (Figure ), and the Sn-O coordination is in good agreement to that of SnO2 reported previously (Figure ).[7, The prepared SnOx-Pt/C catalyst has Sn on the surface in the form of oxides. Due to partial oxidation and the existence of defects, part of Sn loses electrons. Pt serves as an electron acceptor, and its d orbital filling degree increases. Since no obvious alloy was observed, we will mainly explore the role of this SnOx-Pt interface structure in catalysis. Next, the ORR performance of SnOx-Pt/C catalysts was firstly investigated on rotating disk electrode in 0.1 M HClO4 solution (Figure ). The electrochemical properties of the SnO2-Pt/C catalysts with different Pt:Sn ratios were evaluated after aging. Owing to relatively low electrical conductivity and the physical block of SnO2/C with small surface area (Figure ), as shown in Figure , the electrochemical surface areas (ECSAs) estimated from H adsorption/desorption for SnOx-Pt/C catalysts with different Pt:Sn ratios are quite different from that of Pt/C catalyst. Specifically, the ECSAs of SnOx-Pt/C (1:10) (69.3 m 2 g -1 ) is similar to that of SnOx-Pt/C (1:20) (72.5 m 2 g -1 ) and that of the Pt/C catalyst (71.4 m 2 g -1 ), and the SnOx-Pt/C (1:5) has a smallest ECSA (20.6 m 2 g -1 ). The cyclic voltammetry (CV) curves of SnOx-Pt/C (1:5) displays a typical redox peaks between 0.85 and 0.35 V (vs RHE in the following discussion unless otherwise specified), which can be ascribed to either the OH adsorption at the SnO2 sites or the reversible redox couple of Sn4+/Sn2+, possibly overlap with the peak region of Pt dissolution and redeposition on the Pt surface. The negative shift of overpotential of OH adsorption on SnOx might facilitate the hydrogenation of reactive intermediates on the Pt surface of SnOx-Pt/C, which is beneficial to activity enhancement. The varying Pt:Sn ratios brings noticeable change in the mass activity of SnOx-Pt/C, specifically, the SnOx-Pt/C (1:10) catalyst shows a similar mass activity to that of SnOx-Pt/C (1:20) catalyst and both of them are much better to that of SnOx-Pt/C (1:5) catalysts (Figure ). Furthermore, the electrochemical stability of SnOx-Pt/C (Pt:Sn is 1:10) catalyst was investigate by the accelerated degradation test (ADT) in the voltage range from 0.6 to 1.0 V. As the CV curves depicted, the ECSAs of SnOx-Pt/C catalyst (Figure ) show a volcano trend with ADT progresses, while the ECSA of Pt/C catalyst (Figure ) slightly increases after 10000 cycles and then declines dramatically. It is noted that the ECSA of SnOx-Pt/C catalyst remains as high as 62.5 m 2 g -1 even after 30,000 ADT cycles. By contract, the ECSA of Pt/C catalyst decrease to 58 m 2 g -1 just after 10,000 ADT cycles. This demonstrates that the SnOx-Pt/C catalyst is more stable than Pt/C catalyst under the same ADT conditions. Meanwhile, the mass activity of both SnOx-Pt/C catalyst and Pt/C catalyst was calculated by the Koutecky-Levich equation based on the LSV curves (Figure and). The mass activities of SnOx-Pt/C catalyst increases to 212 A mg -1 after 20,000 cycles test, then deceases to 191 A mg -1 as 30,000 cycles finished. These performances are much higher than Pt/C catalyst, of which the mass activity never exceeds 150 A mg -1 throughout the whole ADT test and finally drops to 125 A mg -1 after 20,000 cycles. Pt/C catalysts (e-f) before, after 10000, after 20000, after 30000 and after 40000 potential cycles between 0.6 and 1.2 V versus RHE with a sweep rate of 50 mV/s. The CV curves were recorded at room temperature in N2-purged 0.1 M HClO4 solution with a sweep rate of 100 mV/s. The LSV curves were recorded at room temperature in O2-purged 0.1 M HClO4 solution with a sweep rate of 5 mV/s.