Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance

Résumé

  The protocol describes the synthesis and electrochemical testing of platinum-nickel nanowires. Nanowires were synthesized by the galvanic displacement of a nickel nanowire template. Post-synthesis processing, including hydrogen annealing, acid leaching, and oxygen annealing were used to optimize nanowire performance and durability in the oxygen reduction reaction.

Résumé

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 °C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 °C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

Introduction

Proton exchange membrane fuel cells are partially limited by the amount and cost of platinum required in the catalyst layer, which can account for half of fuel cell costs¹. In fuel cells, nanomaterials are typically developed as oxygen reduction catalysts, since the reaction is kinetically slower than hydrogen oxidation. Carbon-supported Pt nanoparticles are often used as oxygen reduction electrocatalysts due to their high surface area; however, they have specific selective activity and are prone to durability losses.

Extended thin films offer potential benefits to nanoparticles by addressing these limitations. Exten....

Protocole

1. Synthesis of Pt-Ni Nanowires

1. To begin the displacement process, suspend the nickel nanowire template in water and heat it to 90 °C.
   1. Add 40 mg of commercially available, nickel nanowires to 20 mL of deionized water in a 50 mL centrifuge tube. Sonicate it for 5 min.
      ​NOTE: The nanowires are approximately 150-250 nm in diameter and 100-200 µm in length.
   2. Transfer the suspended nanowires to a 250 mL glass round bottom flask and add 60 mL of deionized water. Heat the flask to 90 °C in a mineral oil bath. Stir the reaction mixture at 500 rpm with a polytetrafluoroethylene paddle connected to a glass shaft and....

Résultats

Spontaneous galvanic displacement of Ni nanowires with Pt, using the specified amount, produced Pt-Ni nanowires that were 7.3 wt. % Pt (Figure 1 and Figure 2A). Some modification to the amount of Pt precursor may be required to reach the optimum Pt loading. Pt displacement is sensitive to the thickness of the surface Ni oxide layer, which can vary based on template age (air exposure) and upstream variability

Discussion

These protocols have been used to produce extended surface electrocatalysts with both high surface areas and specific activities in the oxygen reduction reaction⁸. By depositing Pt onto nanostructured templates, the nanowires avoided low coordinated sites and minimize particle size effects, producing specific activities more than 12 times greater than carbon-supported Pt nanoparticles. Using galvanic displacement as the synthesis approach also produced an approximate coating on the Ni template

Matériaux

Liste des matériaux utilisés dans cet article

Nom	Entreprise	Numéro de catalogue	Commentaires
Nickel nanowires	Plasmachem GmbH
250 mL round bottom flask	Ace Glass
Hot plate	VWR International
Mineral oil	VWR International
Potassium tetrachloroplatinate	Sigma Aldrich
Syringe pump	New Era Pump Systems
Rotator	Arrow Engineering
Teflon paddle	Ace Glass
Glass shaft	Ace Glass
Split hinge tubular furnace	Lindberg		Customized in-house
Schlenk line	Ace Glass
Condensers	VWR International
Nitric acid	Fisher Scientific
2-propanol	Fisher Scientific
Nafion ionomer (5 wt. %)	Sigma Aldrich
Glassy carbon working electrode	Pine Instrument Company
RDE glassware	Precision Glassblowing		Customized in-house
Platinum wire	Alfa Aesar		Customized in-house
Platinum mesh	Alfa Aesar		Customized in-house
MSR Rotator	Pine Instrument Company
Potentiostat	Metrohm Autolab