Oxygen Reduction Reaction Activity of Aluminum-Doped Hafnium Oxynitride in Acid Media

Among the non- platinum-group-metal catalysts for polymer electrolyte fuel cells (PEFCs), only oxide and oxynitride catalysts containing either group 4 or 5 metals have been found to be stable in acid media using inductively coupled plasma-atomic emission spectroscopy.^1,2 The high stability motivated us to investigate the active sites for oxygen reduction reaction (ORR). To date, ORR-active sites either in the oxide or oxynitride catalysts have been acknowledged as anion defects.^3,4 The charge neutrality of the defect-incorporated oxides is compensated by a decrease in the valence of the metal, which results in dissolution as the oxides are stable in acid media when the valence of either group 4 or 5 metals is the highest.⁵ The substitution of oxygen by an anion with larger valence (below 2–) such as nitrogen and metal by a cation with lower valence (below 4+ and 5+ for group 4 and 5 metals, respectively) are the possible routes to incorporate oxygen defects into oxides without decreasing the valence of either group 4 or 5 metal. The first route has been investigated by several groups, whereas the second one has not been reported so far for either group 4 or 5 metal oxide catalysts. In this study, we report the synthesis of aluminum-doped hafnium oxynitride catalysts supported on carbon black (Hf_1-δAl_δO_xN_y-C) and its ORR activity.

                   Hf_1-δAl_δO_x-C catalysts were synthesized using a previously reported impregnation method for obtaining HfO_x-C (denoted as IM-Cl2³) with the modification of addition of powdered AlCl₃·6H₂O precursor into a dispersion.⁶ The δ was varied over the range of 0.03–0.3. Only Hf_0.8Al_0.2O_x-C catalysts which showed the maximum activity were doped with nitrogen by heating them at 1273 K for 6 h under flowing ammonia gas to obtain Hf_0.8Al_0.2O_xN_y-C. The catalyst morphologies, crystal structures, and surface chemical states were evaluated using field-emission transmission electron microscopy (FE-TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The ORR activities were evaluated by measuring rotating disk electrode (RDE) voltammograms in 0.1 mol dm^–3 H₂SO₄ solution.

                   Figure 1 shows a FE-TEM image of Hf_0.8Al_0.2O_xN_y-C. The energy dispersive X-ray (EDX) spectra (not shown) revealed that the black particles contained both Hf and Al atoms whereas grey area contained only C atoms. The Hf_0.8Al_0.2O_xN_y particles’ size shown in Figure 1 were nanometer-order; however, it is noted that some particles aggregated into ~15 nm. All the Hf_1-δAl_δO_xN_y-C catalysts were synthesized using the impregnation method previously reported for the synthesis of Al-free HfO_xN_y-C catalysts in which the HfO_xN_y particle size was about a few nanometers,⁴ lower than that of Hf_0.8Al_0.2O_xN_y. The experimental differences between the previous⁴ and current studies are the ammonia-treatment conditions and presence of Al precursor. Because the former did not affect the particle size,² the latter could be the source of size enhancement. XRD and XPS analyses confirmed that the trivalent-Al atoms were successfully doped into tetravalent-Hf sites in HfO_x-C without lowering the Hf-valence, thereby yielding the cubic-HfO₂ phase with oxygen defects. Both the content of the cubic phase and ORR activity showed a volcano type dependence on δ, with an optimum value of 0.2. Nonetheless, the N-doping into O-sites enhanced the activity further. The durability of Hf_0.8Al_0.2O_xN_y-C catalyst was investigated using an accelerated degradation protocol recommended by the U.S. Department of Energy.⁷ It showed high durability: a decay of only 50 mV at 1 mA cm^–2 after the 20,000th potential cycling without significant changes in the chemical bonding states on their surfaces confirmed by XPS spectra obtained after the accelerated degradation tests.

References

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7. U.S. DoE, Testing Oxygen Reduction Reaction Activity with the Rotating Disc Electrode Technique, https://www1.eere.energy.gov/hydrogenandfuelcells/webinars.html

Acknowledgments

This work was partially supported by a research grant from the Kurata Memorial Hitachi Science and Technology Foundation; Grant-in-Aid for Young Scientists (B), 23760185, from the Japanese Ministry of Education, Culture, Sports, Science and Technology; Adaptable and Seamless Technology Transfer Program through target-driven R&D, AS242Z00224L, from the Japan Science and Technology Agency; a research grant from the Kao Foundation for Arts and Sciences, Japan.