DOE OSTI2011
The concern about energy sources, their availability, and related environmental effects, is at an all time high. Proton Exchange Membrane Fuel Cells (PEMFCs) - with an efficiency higher than that of internal combustion engines, light weight, low operating temperature, and fast-start-up capability - are strong candidates for automotive applications. Transportation applications could be especially important in shaping up the new energy economy since they may entail a substantial decrease in the adverse environmental effects linked to the use of fossil fuels and prolong their availability. The largest portion of the cost of PEMFCs reflects the large amount of Pt needed in the cathode's catalytic layer due to the low catalytic activity of Pt for the oxygen reduction reaction (ORR). Recently, considerable advances have been made in fuel cell electrocatalysis yielding improved electrocatalysts, and increasing our understanding of the kinetics of the ORR in combination with significant advances in theoretical treatments. Some of these studies involved: (1) alloying Pt to synthesize bi-metallic catalysts, (2) core-shell nanoparticles catalysts, (3) the role of size, structure, and shape of nanoparticles, and (4) de-alloying of bimetallic alloys. However, a complete understanding of the ORR kinetics on Pt, the best single element catalyst, and of its low efficiency, is yet to be achieved. These problems, compounded with the high Pt content in current cathode catalysts, and with their gradual loss of performance under operating conditions, still hamper commercialization of fuel cells. In order to minimize the amount of noble metal electrocatalysts and maximize their utilization, while achieving high catalytic activity, numerous synthetic approaches have been attempted. The electrocatalysts were prepared using vacuum deposition methods, wet chemistry methods, or electrodeposition techniques. Electrodeposition in particular has several attractive features with respect to the application in catalyst synthesis such as simplicity of operation and ease of control of the deposition conditions. In this article we describe the application of metal electrodeposition in the synthesis of a new type of electrocatalysts comprising a Pt monolayer (Pt{sub ML}) on metal or alloy nanoparticles. The concept of Pt{sub ML} electrocatalysts offers a possible solution to the impasse caused by a slow ORR kinetics and the consequent large Pt content of conventional electrocatalysts. A Pt{sub ML} shell on a nanoparticle substrate core ensures that every Pt atom is available for catalytic activity; in other words, a Pt{sub ML} achieves the ultimate reduction in Pt loading and complete Pt utilization. Also, through geometric and electronic interaction with the substrate a Pt{sub ML} can change its electronic properties and be more active and durable than pure-Pt electrocatalysts.
30 DIRECT ENERGY CONVERSION↗