Platinum-based alloy catalysts for PEM fuel cells

Abstract

Proton exchange membrane (or polymer electrolyte membrane) (PEM) fuel cells are one type of clean energy converting device that can contribute to sustainable world development. PEM fuel cells use hydrogen or hydrocarbons for fuel and air as the oxidant, reacting through a silent electrochemical process at low temperatures (60-90 °C) to convert chemical energy to electricity with zero or low emissions. This technology is attractive mainly due to its environmentally friendly nature and highly efficient energy conversion as compared to traditional energy technologies such as internal combustion engines (ICEs). At our current stage of development, two kinds of PEM fuel cells are the most promising for commercialization: the hydrogen (H2)-fuelled PEM fuel cell, a major candidate for automobile applications to replace oil-dependant ICE technology; and the methanol-fuelled PEM fuel cell or direct methanol fuel cell (DMFC), which shows great potential for applications in portable electronic devices. For the H2-fuelled PEM fuel cell, the dominant challenges to commercialization are high cost and low reliability/durability. Both cost and reliability strongly depend on the electrocatalysts that drive the electrochemical reactions at the cathode and anode sides of a fuel cell. At this time, the most practical electrocatalysts in PEM fuel cells are platinum (Pt)-based catalysts. Due to the high cost and limited supply of Pt, reducing Pt loading in catalyst layers from the current level of 0.6-0.8 mgPt cm-2 to ∼0.15 mgPt cm-2 is one of the key targets for automotive applications [1]. The most common strategy in Pt loading reduction to achieve this target is alloying other metal elements with Pt to form Pt alloying catalysts. This alloying practice not only reduces the noble metal content but also increases the catalytic activity. As is commonly recognized, the kinetics of the oxygen reduction reaction (ORR) at the cathode is the rate determining step in the whole fuel cell reaction, and requires much more catalyst than is needed for the anodic hydrogen oxidation reaction (HOR). Therefore, developing Pt-based alloys to increase the mass activity of the ORR by a factor of four is the activity benchmark and requirement for commercially feasible PEM fuel cells. Meanwhile, Pt-based alloys with carbon monoxide (CO) tolerance are also necessary for the HOR at the anode, in order to overcome the problem of catalyst poisoning caused by CO impurity in the hydrogen gas. In the case of the DMFC, which uses methanol as fuel, cell performance is normally lower than in H2-fuelled cells, and Pt consumption in the catalysts/catalyst layers is also significantly higher due to the sluggish methanol oxidation reaction (MOR) at the anode. For early DMFC commercialization the target total catalyst loading for the entire membrane electrolyte assembly (MEA) (composed of both cathode and anode catalyst layers as well as membrane) must be reduced from the current 2.0-8.0 mgPt cm-2 to a level ≤ 1.0 mgPt cm -2 without performance compromise [2]. Beside efforts to reduce cathode catalyst loading, developing Pt-based alloys with high activity towards the methanol oxidation reaction at the DMFC anode is equally important. In addition, in a DMFC, methanol crossover through the membrane from anode to cathode can also cause performance loss at the cathode. Therefore, methanol-tolerant Pt-based alloys at the cathode are also required for DMFCs. It is generally recognized that the development of Pt-based alloys is one feasible strategy for Pt load reduction and activity enhancement in fuel cells, including both hydrogen-fuelled PEMFCs and DMFCs. In this chapter, we introduce the research and development of Pt-based alloy catalysts for both the ORR and the MOR. The alloying effect and corresponding mechanism as well as the stability of Pt-based alloy catalysts towards the ORR and MOR will be described in detail. The topics of CO-and methanol-tolerant Pt-based alloy catalysts will not be discussed in this chapter (please see instead Chapter 16, "COTolerant Catalysts"). © 2008 Springer-Verlag.