Tailoring the Metallic and Oxide Sites of Pd/C and Ni/C Catalysts for Optimum Hydrogen Oxidation Reaction (HOR) and Durability in Alkaline Media

Abstract

In recent years, Pd- and Ni-based, or Pd-Ni alloys catalysts for HOR in alkaline electrolytes obtained increasing attention due to their higher durability than Pt-based catalysts. [1-3] However, to prevent passivation in high pH media and maintain active metal/oxide sites, it is necessary to protect the surface of Pd and Ni. It is well documented that different types of carbon cover layers surrounding Ni particles, such as graphite or carbon nanotube, can be beneficial for several applications, e.g. hydrogenation, dehydrogenation, methanol oxidation [4-6], and HOR. Gao et al. reported different degree of graphite coverage-Ni particles of controlled metal/oxide ratio that can enhance the HOR activity of Ni in alkaline electrolytes [7]. In this work, we explore different carbon layer properties, morphology and thickness that cover Pd and Ni particles. Thicker cover layers lead to low HOR activity initially because of the un-balanced metal/oxide sites (the amount of metal sites is higher originally), but can be improved (activated) by voltage pulsing (0.1 - 1.23 V vs. RHE, 3 s each potential, 300 cycles) in 0.1 M KOH, 25°C, Ar gas, without agglomerating the particles (as shown in Figure 1). After full activation, protected Pd and protected Ni were compared with bare Pd and bare Ni (no protecting layers) before and after accelerated stress test (AST) in the potential range of [0.1, 1.23] V vs. RHE. IL-TEM, XRD and XPS techniques show that for protected Pd and Ni, the passivation and degradation happen much slower than for bare Pd and bare Ni (which are immediately deactivated upon AST); also, the proportion of metal or oxide sites can be tailored by different activation procedure, which renders these protected catalysts suitable for long-term alkaline HOR. Figure 1 IL-TEM of Pd C1 (Pd-capped 1) at its initial state vs. post-activation (300 cycles-Ar). Top: 200 k magnified micrographs. Bottom: 500 k magnified micrographs.[1] Lafforgue, C.; Zadick, A.; Dubau, L.; Maillard, F.; Chatenet, M. Selected Review of the Degradation of Pt and Pd-Based Carbon-Supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation. Fuel Cells 2018, No. 0, 1–10.[2] Sankar, S.; Anilkumar, G. M.; Tamaki, T.; Yamaguchi, T. Binary Pd−Ni Nanoalloy Particles over Carbon Support with Superior Alkaline Formate Fuel Electrooxidation Performance. ChemCatChem 2019, 11 (19), 4731–4737.[3] Zhao, Y.; Wang, G.; Xiao, L.; Lu, J.; Zhuang, L. Hydrogen Oxidation Reaction on Pd-Ni(OH)2 Composite Electrocatalysts in an Alkaline Electrolyte. ChemistrySelect 2020, 5 (26), 7803–7807. )[4] Ortega-Domínguez, R. A.; Vargas-Villagrán, H.; Peñaloza-Orta, C.; Saavedra-Rubio, K.; Bokhimi, X.; Klimova, T. E. A Facile Method to Increase Metal Dispersion and Hydrogenation Activity of Ni/SBA-15 Catalysts. Fuel 2017, 198, 110–122.[5] Jiao, C.; Sun, L.; Xu, F.; Liu, S. S.; Zhang, J.; Jiang, X.; Yang, L. NiCo Nanoalloy Encapsulated in Graphene Layers for Improving Hydrogen Storage Properties of LiAlH4. Sci. Rep. 2016, 6 (January), 4–11.[6] Tong, X.; Qin, Y.; Guo, X.; Moutanabbir, O.; Ao, X.; Pippel, E.; Zhang, L.; Knez, M. Enhanced Catalytic Activity for Methanol Electro-Oxidation of Uniformly Dispersed Nickel Oxide Nanoparticles-Carbon Nanotube Hybrid Materials. Small 2012, 8 (22), 3390–3395.[7] Yang, Y.; Sun, X.; Han, G.; Liu, X.; Zhang, X.; Sun, Y.; Zhang, M.; Cao, Z.; Sun, Y. Enhanced Electrocatalytic Hydrogen Oxidation on Ni/NiO/C Derived from a Nickel-Based Metal–Organic Framework. Angew. Chemie - Int. Ed. 2019, 58 (31), 10644–10649.[8] Gao, Y.; Peng, H.; Wang, Y.; Wang, G.; Xiao, L.; Lu, J.; Zhuang, L. Improving the Antioxidation Capability of the Ni Catalyst by Carbon Shell Coating for Alkaline Hydrogen Oxidation Reaction. ACS Appl. Mater. Interfaces 2020, 12 (28), 31575–31581. Figure 1