S. Jayaraman et Ac. Hillier, Construction and reactivity mapping of a platinum catalyst gradient using the scanning electrochemical microscope, LANGMUIR, 17(25), 2001, pp. 7857-7864
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.