High-throughput electrochemical characterization of fuel cell catalysts

Abstract

Much effort has been invested into research on developing catalysts on low temp. polymer electrolyte fuel cell over the last two decades. Platinum metal is a widely used material for these catalysts, but due to high cost and insufficient availability, new catalysts are being developed. In an effort to reduce the platinum group metal (PGM) loading for the cathode in hydrogen-air fuel cells, JPL has examd. a wide range of alloy compns. where Pt metal is alloyed with valve metals (Ti, V, and Zr) and the late transition metals (Co, Ni). The results of the electrochem. measurements, voltage swept cyclic voltammograms and potentiodynamic polarization curves, were investigated using the rotating disk electrode (RDE), and the novel multi-electrode electrochem. test system developed at JPL. The samples that were studied were PtTF, and PtCoZr. In each system, measurements of the active area, revealed by measuring the area assocd. with the hydrogen-oxidn.-reaction (HER) at anodic potentials were conducted; along with polarization curves conducted to det. the oxygen-redn.- reaction (ORR) c.d. at 0.9 V, vs. NHE). The sample studied was an array of PtCoZr thin films, with up to 30 at. % Zr, and as little as 56 at. % Pt, present in the alloy. This five year old sample was measured again for its electrochem. activity. This sample exhibited outstanding electrochem. activity in its original condition (110μA /cm%2 ORR, > 10X larger than (111) Pt thin films), and large surface areas (250μC/cm%2, ∼ 2X > (111) Pt.). Previous measurements showed reproducible results with up to one year or more between measurements. Xray photo-electron-spectroscopy (XPS) measurements showed that the electrochem. treated surface was dominated by Pt-Zr surface layers, with Co, being leached from the surface, and/or reconstructing below the upper surface. The ozone ashing treatment used to prep. the surface for XPS measurements resulted in a 3X loss of electrochem. ORR activity, due to the formation of complex oxides at the specimen surface. During five years of measurements, this sample underwent aggressive durability testing; e.g., thousands of cycles up to 1.6V NHE. Regardless of the extreme conditions, this sample still shows decent ORR activity (40μA/cm%2, still > 4X larger than (111) Pt), and active areas (230μC/cm%2), comparable to polycryst. From this we can conclude that this sample is still active and very durable to harsh conditions.

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