Nanophase nickel- zirconium alloys have been investigated for use as electrically conductive coatings and catalyst supports in fuel cells. Heretofore, noble metals have been used because they resist corrosion in the harsh, acidic fuel-cell interior environments. However, the high cost of noble metals has prompted a search for less-costly substitutes.
Nickel-zirconium alloys belong to a class of base metal alloys formed from transition elements of widely different d-electron configurations. These alloys generally exhibit unique physical, chemical, and metallurgical properties that can include corrosion resistance. Inasmuch as corrosion is accelerated by free-energy differences between bulk material and grain boundaries, it was conjectured that amorphous (glassy) and nanophase forms of these alloys could offer the desired corrosion resistance.
For experiments to test the conjecture, thin alloy films containing various proportions of nickel and zirconium were deposited by magnetron and radiofrequency co-sputtering of nickel and zirconium. The results of x-ray diffraction studies of the deposited films suggested that the films had a nanophase and nearly amorphous character.
For tests of corrosion resistance, films of these alloys were deposited on graphite foils to form working electrodes. In each test, the working electrode was immersed in a 2 N sulfuric acid solution and polarized at a succession of potentials in range of 0.05 to 0.75 V versus a normal hydrogen electrode. The steady-state current sustained by the working electrode was monitored at each applied potential. For the alloys containing less than 70 atomic percent nickel, the steady-state current densities were less than 1 nA/cm2. Inasmuch as current densities less than 100 nA/cm2 are generally considered indicative of good corrosion resistance, these measurements can be interpreted as indicating excellent corrosion resistance. There was also visual evidence of excellent corrosion resistance (see figure).
One alloy, comprising 55 atomic percent nickel and 45 atomic percent zirconium, was selected for further tests. In one test, part of a nickel foil was coated with this alloy, then the foil was immersed in sulfuric acid for 48 hours. At the end of the test, the alloy coat remained shiny, while the uncoated part of the foil had become corroded. For another test, a thin film of the alloy was incorporated as a catalyst-support layer in an anode in a fuel cell. The fuel cell was then operated at a temperature of 90 °C for several tens of hours. The fuel cell exhibited stable current densities, indicating that the alloy is stable under fuel-cell operating conditions.
This work was done by Sekharipuram Narayanan, Jay Whitacre, and Thomas Valdez of for Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-40415, volume and number of this NASA Tech Briefs issue, and the page number.
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Nanophase Nickel-Zirconium Alloys for Fuel Cells
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Overview
The document discusses the development and evaluation of nanophase nickel-zirconium alloys for use in fuel cells, highlighting their corrosion resistance and potential applications in energy technologies. The research, conducted by S. R. Narayanan, Jay Whitacre, and Thomas Valdez at NASA's Jet Propulsion Laboratory, focuses on the unique properties of these alloys, which are prepared through sputter-deposition techniques.
Key findings indicate that these nickel-zirconium alloys exhibit excellent corrosion resistance when tested in sulfuric acid environments. Specifically, a composition of nickel-zirconium with a 55/45 ratio was identified as particularly effective, as evidenced by its ability to maintain a shiny, intact surface while uncoated areas of nickel underwent significant corrosion. This protective characteristic was further demonstrated through experiments where nickel foil was partially coated with the alloy and immersed in sulfuric acid, confirming the alloy's ability to shield underlying materials from corrosive damage.
The document also details the fabrication of a fuel cell membrane-electrode assembly using the nickel-zirconium alloy as the anode, combined with a Nafion membrane as the electrolyte and a conventional platinum black electrode as the cathode. The assembly was tested under standard fuel cell conditions, with methanol supplied to the anode and oxygen to the cathode, operating at 90°C. The results showed stable current densities for methanol oxidation, indicating that the nickel-zirconium underlayer is both conductive and stable in fuel cell environments.
Overall, the research underscores the potential of nanophase nickel-zirconium alloys to enhance the performance and durability of fuel cells, making them a promising alternative to traditional materials. The document concludes with a note on ongoing investigations into the catalytic properties of these thin-film materials, suggesting that further advancements could lead to significant improvements in fuel cell technology.
This work is part of NASA's broader efforts to explore innovative materials for aerospace and energy applications, aiming to leverage these developments for commercial and scientific advancements. The findings contribute to the understanding of nanophase materials and their practical applications in energy systems.
