Polymer electrolyte fuel cells (PEFC) that utilize hydrogen and oxygen as the reactants have attracted a lot of attentions as an environmentally-friendly energy conversion system, as they can be operated under relatively low temperature below 100 degree Celsius and emit only water as the reaction product. Improvement of the anode catalysts is one of the essential requirements toward the popularization of the PEFC technologies. Currently, platinum (Pt) or its alloy is used as the anode catalyst in practical PEFC. As Pt is rare and expensive, there is increasing demand for reducing the loading amount of Pt on the anode. Another problem regarding to the Pt anode catalyst is that Pt is active not only for hydrogen oxidation reaction (HOR) but for oxygen reduction reaction (ORR). During the start-up conditions of PEFC, air inevitably flows into the anode chamber and cathodic ORR proceeds on the anode Pt catalysts.

Single-atom Pt catalyst is a possible candidate that solves the above two problems for the PEFC anode catalyst. Based on the single-atom nature, we can expect not only to ultimately reduce the loading amount of Pt, but also to obtain unique reaction selectivity as many reactions can proceed only on Pt ensemble sites. Recently, we demonstrated that covalent triazine frameworks (CTFs) can serve as a platform for constructing heterogeneous single-atom electrocatalysts [1-6]. Herein, we demonstrated that a Pt-modified CTF (Pt-CTF, Figure) has higher HOR and lower ORR activity compared to commercial Pt/C.

The CTF was obtained by the polymerization of 2,6-dicyanopydridine on carbon nanoparticles in molten ZnCl₂. Then, Pt atoms were grafted in the pore of CTFs by the impregnated with platinum chloride. The high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure analyses revealed that Pt atoms in Pt-CTF were atomically dispersed. We evaluated the PFEC performance with 2.8 wt % Pt-CTF (Pt amount; 0.020 mg cm^-2) as the anode catalyst, compared with the 20 wt % Pt/C catalyst (Pt amount; 0.10 mg cm^-2). The maximum power density of PEFC with Pt-CTF was determined to be 487 mW cm^-2 at 1.2 A cm^-2, a value that was nearly identical to that of PEFC with Pt/C (462 mW cm^-2 at 1.0 A cm^-2), which contained approximately 5 times more Pt anode catalyst. These results indicated that the amount of Pt required for catalytic activity is drastically lower than that of conventional Pt/C catalysts [3].

Next, we investigated the ORR activity. The electrocatalytic ORR activity of 2.8 wt% Pt-CTF was markedly lower than that of commercial 20 wt% Pt/C, although these catalysts showed the almost similar HOR activity. The selectivity likely relied on the fact that the required number of Pt sites for HOR was significantly smaller than that for ORR. Namely, Pt-CTF selectively catalyze HOR, even in the presence of dissolved oxygen, which is critical for limiting cathode degradation during the start–stop cycles of fuel cells.

[1] K. Kamiya and S. Nakanishi et al. Nature Commun. 2014, 5, 5040.

[2] K. Iwase, S. Nakanishi and ± et al. Angew. Chem. Int. Ed., 2015, 54, 11068.

[3] K. Kamai, K. Kamiya, K. Hashimoto and S. Nakanishi Angew. Chem. Int. Ed., 2016, 55, 13184.

[4] T. Yoshioka, S. Nakanishi and K. Kamiya*g J. Phys. Chem. C,, 2016, 120, 15729.

[5] R. Kamai, S. Nakanishi, K. Hashimoto^*, K. Kamiya^*, J. Electroanal. Chem., 2017, 800, 54

[6] S. Yamaguchi, K. Kamiya^*, K. Hashimoto, S. Nakanishi^*, Chem. Commun. 2017, 53, 10437