Morphological characterization of the Pt-CTF/CP

Pt-CTF/CP was synthesized by modifying the standard synthesis protocol of Pt-CTF9,10,22,23. Briefly, CTFs were obtained by in situ polymerization of 2,6-dicyanopyridinein-molten ZnCl 2 containing CPs (the weight ratio of 2,6-dicyanopyridine to CP was 1: 1), after which the CTF/CP was impregnated with a platinum chloride salt to obtain Pt-CTF/CP. Scanning electron microscopic (SEM) inspection reveals that the particles of Pt-CTF/CP (20–200 nm, Fig. 1a) are much smaller than that of Pt-CTF polymerized without CPs (1–5 μm, Fig. 1b), suggesting that CTF is well mixed with CPs during the in situ polymerization. Then, we conducted high-resolution transmission electron microscopy (Fig. 1c) and the corresponding high-angle annular dark-field scanning transmission electron microscopy (Fig. 1d), the latter of which is a powerful tool for discerning individual heavy atoms24,25,26. It was confirmed that the bright spots corresponding to Pt atoms (the sizes <0.5 nm) were uniformly dispersed and almost no Pt nanoparticles (the sizes >1 nm) were observed (Fig. 1d). Such bright spots could not be observed on the CPs (without Pt-CTF) as shown in Supplementary Fig. 1. Figure 1e exhibits another high-resolution transmission electron microscopy image and the corresponding elemental maps (carbon, nitrogen and platinum) obtained by energy dispersive X-ray (EDX) technique. Notably, the EDX maps revealed that Pt and N atoms are localized at the edges of CPs, strongly suggesting that CPs (or the aggregates) are covered with Pt-CTF as schematically shown in Fig. 1f.

Figure 1: Electron microscopic images of Pt-CTF/CP. Representative SEM images of (a) Pt-CTF/CP and (b) Pt-CTF. (c) A representative high-resolution transmission electron microscopy (HR-TEM) image of Pt-CTF/CP and (d) the corresponding high-angle annular dark-field scanning transmission electron microscopy image. (e) Another HR-TEM image of Pt-CTF/CP and the corresponding EDX mappings for C, N and Pt atoms. Scale bar, 1 μm (a,b) and 10 nm (c–e). (f) Schematic illustration of Pt-CTF/CP (blue: N, red: Pt and black: C, Chlorine atoms are not shown for clarity). Full size image

Next, the nitrogen adsorption–desorption isotherms were obtained to analyse the pore structure of Pt-CTF/CP (Supplementary Fig. 2). Type IV isotherm and H 2 hysteresis loop were observed, suggesting that porous structures existed in the synthesized Pt-CTF/CP27,28. The pore-size distribution calculated based on nonlocal density functional theory was shown in the inset of Supplementary Fig. 2. Although CP (Ketchenblack) is known to exhibit a peak at 3.6–3.7 nm (refs 29, 30), the synthesized Pt-CTF/CP exhibited peaks at 1.4 and 5.3 nm. The total pore volume and the BET surface area were estimated to be 0.79 cm3g−1 and 555 m2g−1, respectively.

Electrochemical characterizations of Pt-CTF/CP

Figure 2 shows current density (j) versus potential (U) curves for Pt-CTF/CP obtained in an oxygen-saturated 0.5 M H 2 SO 4 solution. Although the ORR activity of Pt-CTF (without CP) was very low (blue curve), the ORR current increased significantly upon hybridization of Pt-CTF with CPs (red curve). This enhancement in ORR activity can be explained on the basis that both the electric conductivity and the electrochemically active surface area of the material were increased by the hybridization with CPs (Fig. 1a,b and Supplementary Table 1). In contrast, when the CTFs (without Pt) were hybridized with CPs (green curve), the ORR onset potential was much negative than that of Pt-CTF/CP, indicating that the Pt atoms in Pt-CTF/CP are an active centre for ORR. To the best of our knowledge, this is the first demonstration of the application of a CTF-based material as an electrocatalyst.

Figure 2: ORR electrocatalytic activities. j versus U curves for CTF (black), CTF/CP (green), Pt-CTF (blue) and Pt-CTF/CP (red) in 0.5 M H 2 SO 4 saturated with dissolved oxygen, obtained at a scan rate of 10 mV s−1. Rotation rate 1,500 r.p.m. (Inset) Magnified curves. Full size image

Next, to investigate the methanol tolerance of Pt-CTF/CP, we intentionally added methanol to a 0.5-M H 2 SO 4 solution in the presence of oxygen. Cyclic voltammograms obtained in the presence of methanol are shown in Fig. 3. In case of a commercial 20 wt% Pt/C, the oxidation peak of methanol can be clearly observed at around +600 mV versus RHE (Fig. 3b). After the addition of 1 M methanol, the onset potential of the cathodic current shifted ~200 mV in the negative direction, reaching 580 mV versus RHE. In contrast, surprisingly, the overlap of the methanol oxidation current with that of the ORR was almost negligible for Pt-CTF/CP even in the presence of 1 M methanol. To directly compare the methanol oxidation activity of Pt-CTF/CP and 20 wt% Pt/C, we obtained cyclic voltammograms in H 2 SO 4 solution containing methanol in the absence of dissolved oxygen (Fig. 4). The peak currents for methanol oxidation with Pt-CTF/CP (Fig. 4a) were ~1/40 compared with those with Pt/C (Fig. 4b). We confirmed that methanol oxidation is inactive even in 0.5 M HClO 4 as shown in Supplementary Fig. 3, which excludes the possibility that the methanol tolerance originated from the suppression of methanol oxidation by strongly adsorbed sulfate/bisulfate31,32. Thus, the above results clearly showed that Pt-CTF/CP exhibits little activity with regard to methanol oxidation.

Figure 3: ORR activities of Pt-CTF/CP and Pt/C in the presence of methanol. j versus U curves for (a) Pt-CTF/CP and (b) 20 wt% Pt/C in 0.5 M H 2 SO 4 saturated with dissolved oxygen. Methanol concentration: (black) 0 M, (blue) 0.1 M and (red) 1.0 M. Full size image

Figure 4: Methanol oxidation activity of Pt-CTF/CP and Pt/C. j versus U curves for (a) Pt-CTF/CP and (b) 20 wt% Pt/C in 0.5 M H 2 SO 4 in the absence of oxygen. Methanol concentration: (black) 0 M, (blue) 0.1 M and (red) 1.0 M. Full size image

X-ray characterizations of Pt-CTF/CP

Characterizations of Pt-CTF/CP were conducted using various X-ray techniques to obtain information on the molecular mechanism of the methanol tolerance. The surface concentration of each element was determined by an X-ray photoelectron spectroscopy (XPS), and the results are summarized in Table 1. Peaks assignable to Pt and N were confirmed, and the Pt/N elemental ratio did not show a clear dependence on the amount of CP (the right column of Table 1), implying that the structures of Pt-CTFs are essentially identical even by hybridizing with CPs. In addition, the surface concentration of C, which was calculated with XPS, became higher with the increasing ratio of CP. Taking into account that the CP aggregates are uniformly covered by Pt-CTFs (Fig. 1), these results indicated that the thickness of Pt-CTFs is less than the escape depth of photoelectrons (ca. ~3 nm) as schematically shown in Fig. 1f, which enabled the Pt-CTF to possess electronic conduction with the CPs.

Table 1 XPS-elemental analyses. Full size table

The details of the electronic structures of Pt-CTFs were investigated by taking XPS and X-ray absorption near-edge structures (XANES; Supplementary Figs 4–7). The Pt-4f 7/2 peaks were located at 72.5 eV for Pt-CTF/CP and 72.2 eV for Pt-CTF (Supplementary Fig. 4). These binding energies were over 1 eV higher than those for metal Pt (70.9 eV), revealing that the valence state of Pt was Pt(II)33,34. In addition, XANES of Pt L 3 edge shown in Supplementary Fig. 5 exhibited that the white-line intensities at 11,562 eV for Pt-CTF (1.45) and Pt-CTF/CP (1.46) are higher than those for metal Pt (1.24), indicating that Pt atoms existed as oxidized forms in both Pt-CTF and Pt-CTF/CP35. Thus, the Pt-4f XPS and the XANES results also show that there was no formation of Pt aggregates (metal Pt) on Pt-CTFs. Next, we focused on the N-1s XPS spectrum (Supplementary Fig. 6 and Supplementary Table 2). The N-1s peak at 399.2 eV observed for the 2,6-dicyanopyridine (that is, the catalyst monomer, Supplementary Fig. 7) was not observed for the Pt-CTF/CP, indicating that the cyclical trimerization of cyano reaction groups efficiently proceeded. The N-1s peak of Pt-CTF/CP could be deconvoluted into C 2 NH (398–399 eV) and C 3 N (400–401 eV)4. The peak assigned to C 2 NH was shifted to the higher-energy side upon Pt loading on both CTF and CTF/CP, indicating that the electron density of the N atoms became lower in the presence of Pt atoms. The decrease in the electron density of the N atoms can be explained by considering the formation of Pt-N coordination bonds (this point will be argued later). All the features described above were confirmed even for Pt-CTF (that is, without CPs), indicating that the hybridization with CPs did not influence on the electronic properties of N atoms in Pt-CTFs.

Next, we conducted extended X-ray absorption fine structure (EXAFS) analyses of Pt L 3 edge to obtain information on the molecular structure of Pt-CTF/CP. Fourier transformations of k 3 -weighted EXAFS oscillations for Pt-CTF/CPs, Pt-CTFs, commercial Pt(bpy)Cl 2 (bpy: 2,2'-bipyridine), PtO 2 , K 2 PtCl 4 and Pt metal are shown in Fig. 5. The peak corresponding to a Pt–Pt bond at 2.6 Å was not observed at all for Pt-CTF/CPs. Instead, two peaks at R=1.5 and 1.9 Å assignable to Pt–N and Pt–Cl bonds, respectively, were clearly observed, indicating that Pt exists in the form of a single atom as illustrated in Fig. 1f. The ratio of the Pt–N peak to the Pt–Cl peak for Pt-CTFs corresponded to that for Pt(bpy)Cl 2 (model PtN 2 Cl 2 complex), implying that Pt atoms mainly existed as PtN 2 Cl 2 . It should be noted here that there was no clear difference in the EXAFS spectra between Pt-CTF/CP and Pt-CTF, indicating that the molecular structure of Pt-CTF is maintained upon the hybridization with CPs.