Abstract
Given the harsh operating conditions in hydrogen/oxygen fuel cells, there has been considerable efforts to make Pt(M) alloy nanoparticles as a stable oxygen reduction reaction (ORR) catalyst that have had a marginal success. Core-shell catalysts have unique properties of reducing the precious metal for the reaction and at the same time increasing the ORR activity by combined geometrical and electronic effects. Multiple mechanism has been proposed to explain the enhanced ORR activity of Pt(M) alloys. Watanabe et al. had explained the enhanced ORR activity based on the lattice parameter changes by alloying non-precious metals to Pt. Their results showed that the ORR activity increased in the order of PtFe/C > PtCo/C > PtNi/C, whereas the calculated lattice parameter decreased in the order of PtFe/C < PtCo/C < PtNi/C. Mukerjee et al. investigated the enhancement mechanism on some PtM alloys for the ORR by using XANES and EXAFS. Based on the d-band calculations that decreased in the order of PtFe/C < PtCo/C < PtNi/C, the ORR activity increased in the order of PtFe/C > PtCo/C > PtNi/C. So overall the trend in ORR activity for Pt(M) alloy nanoparticles have been in the order of PtFe/C > PtCo/C > PtNi/C. Here we report on a new class of catalyst consisting of nitride metal cores enclosed by thin Pt shells that can be easily synthesized. Our theoretical analysis and the experimental data indicate that metal nitride nanoparticle cores could significantly enhance the ORR activity as well as the durability of the core-shell catalysts as a consequence of combined geometrical, electronic and segregation effects on the Pt shells. Adding nitride metal in the core increases the ORR activity in the order of PtNiN/C > PtFeN/C > PtCoN/C. DFT calculations have shown a volcano type behavior with PtNiN/C at the top of the curve revealing the fact that, among the catalysts investigated, it has the best combination of both the surface strain and d-band center shifts. Figure 1
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