Combinatorial methods represent an appealing experimental method for the discovery of heterogeneous catalysts. One can efficiently identify candidate materials or sample vast regions of composition space using a combination of dense catalyst libraries and high-throughput reactivity screening techniques. This is particularly appealing for the discovery of novel catalysts for low temperature fuel cells where multi-component systems have shown improved performance. For example, the poison tolerance of typical anode catalysts can be improved by the addition of oxophilic components such as ruthenium, molybdenum, tin or osmium. Consequently, a vast composition space must be sampled in order to identify catalyst compositions or regions of composition space with greater activity. Combinatorial methods represent a practical means to speed-up the catalyst discovery process. In this manuscript, we demonstrate a novel method for combinatorial catalyst discovery based upon the synthesis and reactivity mapping of catalyst composition gradients. Samples consisting of uniform variations in surface composition of metals catalysts (Pt-M| and Pt-M|-M2, where Mi, Mb = Ru, Mo, Sn or Os) are fabricated using a gel-transfer technique. A concentration gradient of source metal ions is produced in a swollen polymer gel and then transferred onto a surface by electrodeposition to create a continuous composition gradient. An in situ reactivity-mapping tool based on the scanning electrochemical microscope is used to interrogate these catalyst gradients for the hydrogen oxidation reaction in the presence of adsorbed carbon monoxide.
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