Construction and Reactivity Mapping of a Platinum Catalyst Gradient Using the Scanning Electrochemical Microscope

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Abstract

We describe a method for the characterization of electro-oxidation catalysts that involves the fabrication and reactivity mapping of samples possessing a catalyst gradient. The objective of this work is to demonstrate a method for catalyst preparation and screening that directly measures the activity of spatially localized catalyst samples toward electro-oxidation reactions relevant to the fuel cell anode in an effort to discover and characterize new catalyst formulations. In this report, a well-defined gradient in the surface coverage of platinum is created on an electronically conductive but catalytically inactive indium−tin-oxide (ITO) substrate by the application of a nonuniform electric field during platinum electro-deposition. A linear variation in applied potential is imposed on an ITO substrate to induce a nonuniform platinum deposition rate, which results in the formation of a coverage gradient. The reactivity of this catalyst gradient is measured directly as a function of spatial position using a scanning electrochemical microscope in the feedback mode. Surface imaging using a noncatalytic redox couple (Ru(NH3)63+/2+) depicts a uniform and highly reactive electrode surface over both ITO and platinum domains. In contrast, imaging with a catalytic probe (H+/H2), which senses variations in the substrate activity toward the hydrogen oxidation reaction, clearly illustrates a variation in surface reactivity that is a function of the local substrate composition. The presence of a nonuniform platinum coverage generates a variation in the hydrogen oxidation rate constant. The local reaction rate, as deduced by scanning electrochemical measurements, is proportional to the local platinum surface coverage as determined with electron microscopy. This work demonstrates a unique method for the preparation of catalyst gradient samples coupled with a characterization method that can measure catalytic activity for electro-oxidation reactions on a local scale.

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