Enhancement of Electrocatalytic Oxygen Reduction Activity and Durability of Pt–Ni Rhombic Dodecahedral Nanoframes by Anchoring to Nitrogen-Doped Carbon Support

Abstract

Pt-based nanostructured electrocatalysts supported on carbon black have been widely studied for the oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells. Because sluggish ORR kinetics are known to govern the cell performance, there is a need to develop highly active and durable electrocatalysts. The ORR activity of Pt-based electrocatalysts can be improved by controlling their morphology and alloying Pt with transition metals such as Ni. Improving the catalyst durability remains challenging and there is a lack of catalyst design concepts and synthetic strategies. We report the enhancement of the ORR activity and durability of a nanostructured Pt–Ni electrocatalyst by strong metal/support interactions with a nitrogen-doped carbon (NC) support. Pt–Ni rhombic dodecahedral nanoframes (NFs) were immobilized on the NC support and showed higher ORR electrocatalytic activity and durability in acidic media than that supported on a nondoped carbon black. Durability tests demonstrated that NF/NC showed almost no activity loss even after 50 000 potential cycles under catalytic conditions, and the Ni dissolution from the NFs was suppressed at the NC support, as confirmed by energy dispersive X-ray spectroscopy analysis. Physicochemical measurements including surface-enhanced infrared absorption spectroscopy of surface-adsorbed CO revealed that the strong metal/support interactions of the NF with the NC support caused the downshift of the d-band center position of the surface Pt. Our findings demonstrate that tuning the electronic structure of nanostructured Pt alloy electrocatalysts via the strong metal/support interactions with heteroatom-doped carbon supports will allow the development of highly active and robust electrocatalysts.

Electronic Effects of Nitrogen Atoms of Supports on Pt–Ni Rhombic Dodecahedral Nanoframes for Oxygen Reduction

“…[47][48] Although a clear negative potential shift of ca. 0.1 V was previously observed for NF/NC, compared with NF/C, 8 such an obvious potential shift was not observed between NF/PBI/VACNT and NF/VACNT (Figure 4). This result is in good agreement with the results in XPS and XAS.…”

“…8,40 The electrical resistivity of supports is known to affect the catalytic activity of Pt-based catalysts immobilized. 8,16,21 NFs immobilized on more conductive NC supports could show higher activity than those on less conductive PBI-coated VACNTs. To understand electronic effects of PBI-wrapping on the surface Pt of NFs, ex situ XPS of NF/VACNT and NF/PBI/VACNT was performed in the Pt 4f region (Figure S3).…”

“…7,[13][14][15][16] Similar positive peak shifts were also observed for NF/NC. 8 Because such a binding energy shift indicates an interaction between nitrogen atoms of supports and the surface Pt of NFs, PBIcoated supports gave almost no impact on the electronic state of the surface Pt of NFs but NC supports did.…”

“…Pt-Ni rhombic dodecahedral nanoframes (NFs) were prepared based on the synthetic procedure reported. 8,[29][30] H2PtCl6•6H2O (100 mg, 0.193 mmol), Ni(NO3)2•6H2O (87.5 mg, 0.301 mmol) and oleylamine (50 mL, 0.152 mol) were mixed in a two-necked flask (100 mL). The reaction mixture was heated in an oil bath at 443 K under Ar for approximately 1 h, heated in an aluminum block at 543 K under Ar for ca.…”

“…[1][2] For example, nitrogen-doped carbon (NC) supports have been studied to improve the catalytic activity of supported metal nanoparticles for many reactions including nitrobenzene hydrogenation, 3 formic acid oxidation 4 and oxygen reduction reactions (ORR). [5][6][7][8][9][10][11][12][13][14][15][16] The ORR is a key reaction in polymer electrolyte fuel cells (PEFCs) and metal-air batteries.…”