Electrocatalytic H2 oxidation reaction

Abstract

Hydrogen (H2), an important material and product in chemical industries, has been investigated as a new clean energy source for many decades [1-3]. With the rapid development of proton exchange membrane (PEM) fuel cell technology, in which H2 is used as a fuel, the chemical energy stored in this H2 can be electrochemically converted to electric energy with zero emissions and high efficiency. The dream of a hydrogen economy era therefore seems closer to reality. Beginning in the 1990s, the advantages of PEM fuel cells, including zero/low emissions, high energy efficiency, and high power density, have attracted world-wide research and development in several important application areas, including automotive engines, stationary power generation stations, and portable power devices [4]. With successful demonstrations of fuel cell technology, in particular in automotive applications, commercialization of this technology has become a strong driving force for further development in the critical areas of cost reduction and durability. The major cost of a PEM fuel cell is the platinum (Pt)-based catalysts. At our current technological stage, these Pt-based catalysts for both the cathodic O2 reduction reaction (ORR) and the anodic H 2 oxidation reaction (HOR) are the most practical catalysts in terms of catalytic activity and lifetime stability. Therefore, research and development to improve catalytic activity and stability has shot to the fore in recent years. Although both theoretical and experimental approaches have resulted in great progress in fuel cell catalysis [2, 3, 5, 6], continuous effort is necessary to develop breakthrough fuel cell catalysts that are cost-effective and highly durable for commercial use. With respect to fuel cell catalysis, most research has been focused on cathode ORR catalysts development, because the ORR kinetics are much slower than the anodic HOR kinetics; in other words, the fuel cell voltage drop polarized by load is due mainly to the cathode ORR overpotential [7, 8]. However, in some cases the overpotential of the anodic HOR can also contribute a non-negligible portion of the overall fuel cell voltage drop [8]. Therefore, the catalytic HOR on the fuel cell anode catalyst is also worth examining. On the other hand, aside from its importance in fuel cell applications, hydrogen electrooxidation catalysis is also a model system for the fundamental understanding of electrochemical kinetics, electrocatalysis, and electrochemical surface science, which have been studied for over a century [3, 5]. Indeed, the hydrogen evolution/oxidation reaction (HER/HOR) is the simplest and most widely studied electrochemical process. Almost all the basic laws of electrode kinetics and the concepts of electrocatalysis were developed and verified by the examination of these two reactions. This chapter summarizes the kinetics and mechanisms of the electrocatalyzed HOR on different electrode materials, including platinum group metals, carbides, and transition metals. Advances in CO-tolerant electrocatalysts for the HOR in fuel cells are also briefly introduced. Despite its wide range of topics, the main purpose of this chapter is to provide a fundamental understanding of the electrocatalysis of the HOR, the most important reaction other than the ORR in the PEM H2 fuel cell. © 2008 Springer-Verlag.