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.
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.
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. Extended Pt surfaces typically produce specific activities an order of magnitude greater than nanoparticles, by limiting less active facets and particle size effects, and have been shown to be durable under potential cycling2,3,4. While high mass activities have been achieved in extended surface electrocatalysts, improvements have been made primarily through increases in specific activity, and the catalyst type has been limited to Pt with a low surface area (10 m2 gPt-1)3,4,5.
Spontaneous galvanic displacement combines the aspects of corrosion and electrodeposition6. The process is generally governed by the standard redox potentials of the two metals, and the deposition typically occurs when the metal cation is more reactive than the template. The displacement tends to produce nanostructures that match the template morphology. By applying this technique to extended nanostructures, Pt-based catalysts can be formed that take advantage of the high specific oxygen reduction activity of extended thin films. Through partial displacement, small amounts of Pt have been deposited, and have produced materials with high surface areas (> 90 m2 gPt-1)7,8.
These protocols involve hydrogen annealing to mix Pt and Ni zones and improve oxygen reduction activity. A number of studies have theoretically established the mechanism and experimentally confirmed an alloying effect in Pt oxygen reduction. Modeling and correlating Pt-OH and Pt-O binding to oxygen reduction activity suggest that Pt improvements can be made through lattice compression9,10. Alloying Pt with smaller transition metals has confirmed this benefit, and Pt-Ni has been investigated in a number of forms, including polycrystalline, faceted electrodes, nanoparticles, and nanostructures11,12,13,14.
Galvanic displacement has been used in Pt-oxygen reduction catalyst development with a variety of other templates, including silver, copper, and cobalt nanostructures15,16,17. The synthesis technique has also been used in the deposition of other metals and has produced electrocatalysts for fuel cells, electrolyzers, and the electrochemical oxidation of alcohols18,19,20,21. Similar protocols can also be adapted for the synthesis of nanomaterials with a wider range of electrochemical applications.
1. Synthesis of Pt-Ni Nanowires
2. Check Composition with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS).
Note: Catalyst composition should be 7.3 ± 0.3 wt. % Pt.
3. Post-synthesis Process of the Pt-Ni Nanowires by Annealing and Acid Leaching.
4. Electrochemically Characterize the Nanowires in Rotating Disk Electrode (RDE) Half-Cells8
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 variability22. The composition, however, is critical to ensuring high activity and ICP-MS was used to ensure optimum composition. RDE testing protocols have been included, since following these protocols are critical to confirming correct synthesis and processing parameters. Catalysts at this composition (7.3 wt. % Pt) produced peak oxygen reduction mass activity7. Higher amounts of Pt deposition resulted in lower electrochemical surface areas, attributed to lower Pt utilization and the formation of thicker Pt layers (Figure 2b). Lower amounts of Pt resulted in lower specific activity, potentially due to a particle size effect, although the activity drop was milder than Pt nanoparticle findings2.
Hydrogen annealing was required to integrate the Pt and Ni zones and compress the Pt lattice8. Lattice compression improved oxygen reduction activity and annealing to 250 °C produced optimal mass activity (Figure 3). Although the specific activity continued to increase at higher annealing temperatures, the electrochemical surface area decreased potentially due to Pt reordering at the surface.
Although hydrogen annealing produced high oxygen reduction activity, durability testing resulted in large performance losses and high amounts of Ni dissolution. Acid leaching was used to preferentially remove Ni, and oxygen annealing was used to improve durability and minimize Ni dissolution8,22. Acid leaching to 15.2 wt. % Pt and oxygen annealing to 175 °C produced optimal activity and durability (Table 1). If greater amounts of Ni removal occurred in the acid leaching step, high durability was achieved, but at the cost of initial performance. High Pt composition produced nanowires with lower specific activity (dealloying effect) and the materials were of less interest electrocatalytically. If lower amounts of Ni removal occurred in the acid leaching step, large amounts of Ni still remained on the surface. Oxygen annealing improved the stability of Ni near the nanowire surface, preventing access to Pt sites during electrochemical conditioning. The oxygen annealing temperature of 175 °C provided a balance between the need to stabilize subsurface Ni for durability testing, while still allowing for Pt access during conditioning. For nanowires that were 15.2 wt. % Pt, higher oxygen annealing temperatures produced lower initial activity; conversely, lower oxygen annealing temperatures resulted in higher durability losses and higher degrees of Ni dissolution.
Figure 1. Schematic of spontaneous galvanic displacement process. Schematic of the spontaneous galvanic displacement process, with a nobler metal cation (red) displacing a metal template (blue)6. Reprinted (adapted) with permission from S. M. Alia, Y. S. Yan and B. S. Pivovar, Catalysis Science & Technology, 4, 3589 (2014). Copyright 2014 Royal Society of Chemistry. Please click here to view a larger version of this figure.
Figure 2. Synthesized Pt-Ni nanowires: their composition and surface area. (A) Pt-Ni nanowire composition as a function of the amount of Pt precursor (potassium tetrachloroplatinate) added to 40 mg of Ni nanowires during galvanic displacement. (B) Electrochemical surface areas of as-synthesized Pt-Ni nanowires as a function of the level of Pt displacement7. The data points denote the average value, while the error bars denote the standard deviation of the measurement. Reprinted (adapted) with permission from S. M. Alia, B. A. Larsen, S. Pylypenko, D. A. Cullen, D. R. Diercks, K. C. Neyerlin, S. S. Kocha and B. S. Pivovar, ACS Catalysis, 4, 1114 (2014). Copyright 2014 American Chemical Society. Please click here to view a larger version of this figure.
Figure 3. Oxygen reduction mass activity of hydrogen annealed Pt-Ni nanowires as a function of the annealing temperature8. The data points denote the average value, while the error bars denote the standard deviation of the measurement. Reprinted (adapted) with permission from S. M. Alia, C. Ngo, S. Shulda, M.-A. Ha, A. A. Dameron, J. N. Weker, K. C. Neyerlin, S. S. Kocha, S. Pylypenko and B. S. Pivovar, ACS Omega, 2, 1408 (2017). Copyright 2017 American Chemical Society. Please click here to view a larger version of this figure.
Catalyst | im,i0.9V [mA mgPt‒1] |
im,f0.9V [mA mgPt‒1] |
Pt-Ni | 1653 | 1339 |
H2 | 5213 | 3962 |
Acid | 3583 | 3153 |
O2 | 5414 | 5305 |
Pt/HSC | 500 | 375 |
Table 1. Oxygen reduction mass activities prior to (im,i) and following (im,f) half-cell durability testing. Evaluated catalysts include as-synthesized (Pt-Ni), hydrogen annealed (H2), acid leached (Acid), and oxygen annealed (O2) Pt-Ni nanowires. The half-cell performance of carbon-supported Pt nanoparticles (Pt/HSC) was also provided as a reference8. Reprinted (adapted) with permission from S. M. Alia, C. Ngo, S. Shulda, M.-A. Ha, A. A. Dameron, J. N. Weker, K. C. Neyerlin, S. S. Kocha, S. Pylypenko and B. S. Pivovar, ACS Omega, 2, 1408 (2017). Copyright 2017 American Chemical Society.
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 template7. At low levels of Pt displacement, this process produced electrochemical surface areas in excess of 90 m2 gPt-1, a significant breakthrough in extended surface catalysts.
Hydrogen annealing was needed to improve performance8. Annealing to elevated temperatures improved the oxygen reduction specific activity, which was rationalized as an alloying effect caused by Pt lattice compression weakening Pt-O chemisorption9,10. Although the hydrogen annealing step improved initial activity, the high durability and Ni dissolution losses were a concern. Acid leaching and oxygen annealing were used to minimize these losses. The optimized Pt-Ni nanowires produced oxygen reduction mass activities of eleven times greater than carbon-supported Pt nanoparticles and three times greater than the as-synthesized wires. Significant improvements were also made to the nanowire durability, which lost 3% mass activity (as-synthesized lost 21%) and 0.3% of the catalyst mass to Ni dissolution (as-synthesized lost 7%).
Pt-Ni nanowires have been developed and optimized for their performance in RDE half-cells. RDE testing is often used in catalyst screening, to evaluate the fundamental properties and electrochemical capabilities of a catalyst. RDE activity, however, does not guarantee similar fuel cell performance, and membrane electrode assemblies include activity losses due to mass transport, and electronic and ionic resistance. The Pt-Ni nanowires developed in these protocols demonstrate more than an order of magnitude higher activity to Pt nanoparticles, as well as improved durability. While these results suggest that Pt-Ni nanowires could reduce fuel cell electrode loadings to meet cost-performance metrics, effectively incorporating these materials into membrane electrode assemblies remains a significant challenge.
The authors have nothing to disclose.
Financial support was provided by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under contract number DE-AC36-08GO28308 to NREL.
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 |