Platinum Catalysts on Niobium Diboride Microparticles for Oxygen Reduction Reaction

Platinum thin films on non-precious stable support materials are a promising form as oxygen reduction reaction (ORR) catalyst in the face of the tradeoff issue between the activity and durability for conventional Pt-based nanoparticles. However, support materials having high wettability against platinum are scarce and this type of catalyst has not been realized. In this study, we pursued to form a Pt thin film catalyst using diboride as a catalyst support. Platinum was deposited on NbB2 microparticles with the atomic layer deposition (ALD) method without exposing the NbB2 particles to air after the synthesis. The Pt/NbB2 catalysts with different Pt loadings were synthesized by changing the number of the ALD cycles, and the resulting catalyst surfaces were found to consist of Pt nanoparticles with the sizes of 2–3 nm uniformly dispersed on the NbB2 particles. The obtained highest values of the mass activity (MA) and area-specific activity (SA) were 165 A·g Pt−1 and 295 μA·cm Pt−2, respectively, which are lower than the values for conventional Pt/C catalysts. The effects of heat treatment of the Pt/NbB2 were also examined, and the surfaces of the Pt/NbB2 heat-treated at 800 °C were found to consist of flat Pt dense layers and bare NbB2 with the domain sizes larger than 10 nm. This observation suggests that a Pt thin film catalyst was successfully fabricated. The so-called CO annealing was applied to smooth the Pt surfaces and improve the activity, but the resulting catalyst was found to exhibit a lower ORR activity than the Pt/NbB2 catalyst before the heat treatment. Thus, further efforts are needed for practically utilizing the Pt/NbB2 catalyst with the unique Pt morphology.