Construction and reactivity mapping of a platinum catalyst gradient using the scanning electrochemical microscope

We describe a method for the characterization of electro-oxidation catalyst s that involves the fabrication and reactivity mapping of samples possessin g a catalyst gradient. The objective of this work is to demonstrate a metho d for catalyst preparation and screening that directly measures the activit y of spatially localized catalyst samples toward electro-oxidation reaction s 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 linea r variation in applied potential is imposed on an ITO substrate to induce a nonuniform platinum deposition rate, which results in the formation of a c overage gradient. The reactivity of this catalyst gradient is measured dire ctly as a function of spatial position using a scanning electrochemical mic roscope in the feedback mode. Surface imaging using a noncatalytic redox co uple (Ru(NH3)(6)(3+/2+)) depicts a uniform and highly reactive electrode su rface over both ITO and platinum domains. In contrast, imaging with a catal ytic probe (H+/H-2), which senses variations in the substrate activity towa rd the hydrogen oxidation reaction, clearly illustrates a variation in surf ace reactivity that is a function of the local substrate composition. The p resence of a nonuniform platinum coverage generates a variation in the hydr ogen oxidation rate constant. The local reaction rate, as deduced by scanni ng electrochemical measurements, is proportional to the local platinum surf ace coverage as determined with electron microscopy. This work demonstrates a unique method for the preparation of catalyst gradient samples coupled w ith a characterization method that can measure catalytic activity for elect ro-oxidation reactions on a local scale.