Non-noble electrocatalysts for the PEM fuel cell oxygen reduction reaction

Abstract

Proton exchange membrane (PEM) fuel cells, including direct liquid (methanol, ethanol, and formic acid) fuel cells (DLFCs), have drawn a great deal of attention in recent years as energy conversion devices, due to their high efficiency and low/zero emissions. It is generally recognized that one of the key advantages of the PEM fuel cell stack is its fitness for the automobile industry as a zero-emission power supply. But while great progress has been made in the last several decades in the research and development of PEM fuel cells [1-3], the major challenges hindering fuel cell commercialization, i.e., high cost and low durability, are still unsolved. One of the major contributors to cost is fuel cell catalysis (platinum (Pt)-based catalysts). Therefore, new alternative catalysts to reduce or replace expensive Pt are necessary. In this effort to reduce the cost of fuel cell catalysts, two major approaches are currently very active: Pt loading reduction and non-noble electrocatalyst exploration. In the case of Pt loading reduction [4], two principle avenues have been explored: 1) increase Pt catalytic activity and reduce Pt content through alloying with other transition metals such as Cr, Ni, Fe, Co, etc., and 2) improve Pt utilization by increasing the surface area and dispersion of Pt nanoparticles using high-surface carbon supports. The first approach can effectively create active alloy catalysts with strong activity towards the oxygen reduction reaction (ORR). However, there are many concerns about the long-term stability of these Pt-alloy catalysts due to the leaching of the non-noble metals in the fuel cell operating environment. This leaching causes the following problems: (a) catalytic activity degradation or loss, (b) degradation of membrane proton conductivity by metal ionic contamination, and (c) increase of catalyst layer resistance at the cathode. The second approach, to improve Pt utilization by evenly dispersing Pt nanoparticles on a high-surface carbon support, effectively reduces Pt loading. However, Pt particle size reduction is limited (> 2-3 nm), and further increasing the active surface area seems to be impossible [5]. For such supported Pt catalysts, the agglomeration of Pt nanoparticles is the major problem during fuel cell lifetime testing. This agglomeration reduces the active Pt area, resulting in performance degradation in long-term operation. Most importantly, due to the limited global supply of Pt, which is in high demand for jewelry and other industries in addition to fuel cells, the price of Pt has dramatically increased in the last several decades. This suggests that all gains in reducing Pt loading for fuel cells will be offset by this price increase. It is thus debatable whether or not Pt load reduction is a solution for reducing fuel cell cost. Other precious metals such as Pd, Ir, and Ru, which also show catalytic activity towards the ORR and can be used as fuel cell catalysts, have also seen dramatic price rises in the last several decades. Clearly they are not suitable solutions either. In order to reduce fuel cell catalyst cost, alternative electrocatalysts that are both cost-effective and highly active must be developed. Non-noble catalyst development has become more and more intensive in recent years [6]. For example, perovskite-type and spinel-type oxides and tungsten carbides have been explored as alternative electrocatalysts to platinum. They show promising catalytic activities towards the oxygen reduction and hydrogen oxidation reactions. However, most of these catalysts demonstrate activity and stability in alkaline solutions. In a PEM fuel cell, which uses strong acidic electrolytes, these catalysts are not favorable. Currently, heat-treated transition metal macrocycles seem to be one of most promising non-noble electrocatalysts in strong acid electrolytes [7]. Although they show activity levels close to those of Pt-based catalysts for the ORR in acidic media, their level of stability is the major drawback when they are employed as PEM fuel cell catalysts. Non-noble catalysts should be the future catalysts for sustainable fuel cell commercialization. Despite several challenges, such as low catalytic activity, poor stability, and our present lack of fundamental understanding of the catalytic mechanism, non-noble catalysts are still very attractive because of their globe abundance and low cost. It is believed that in the face of urgent pressures to reduce environmental pollution and yet maintain or increase energy sources-in particular, the strong drive for fuel cell commercialization-non-noble catalyst exploration and development will intensify. Breakthroughs in the development of non-noble catalysts that exhibit strong performance and stability in fuel cell operation will come in the near future. In this chapter, we present the history of non-noble catalyst development, current progress and challenges, and future R&D directions. © 2008 Springer-Verlag.