Proton exchange membrane electrocatalysts

Abstract The durability of fuel cell components constitutes a major barrier towards their financial viabihty. Lifetime requirements include 5000 and 40 000 h for automotive and stationary applications respectively, and thus it is impractical to evaluate fuel cell components lifetime using prolonged time periods. Accelerated durability testing protocols for fuel cell components (proton exchange membrane, electrocatalyst and supports) have been developed to obtain cell component lifetimes in shorter e q)erimental time and are discussed in detail, along with fiiel/air impurities testing protocols. 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

Significant (and even spectacular) results were contributed by the group of Norskov to the field of electrocatalysis [102-105]. Theoretical calculations led to the design of novel nanoparticulate anode catalysts for proton exchange membrane fuel cells (PEMFC) which are composed of trimetallic systems where which PtRu is alloyed with a third, non-noble metal such as Co, Ni, or W. Remarkably, the activity trends observed experimentally when using Pt-, PtRu-, PtRuNi-, and PtRuCo electrocatalysts corresponded exactly with the theoretical predictions (cf. Figure 5(a) and (b)) [102]. 

Fernandez JL, Raghuveer V, Manthiram A, Bard AJ. 2005a. Pd-Ti and Pd-Co-Au electrocatalysts as a replacement for platinum for oxygen reduction in proton exchange membrane fuel cells. J Am Chem Soc 127 13100-13101. 

Ferreira PJ, La O GJ, Shao-Hom Y, Morgan D, Makharia R, Kocha S, Gasteiger HA. 2005. Instability of Pt/C electrocatalysts in proton exchange membrane fuel cells—A mechanistic investigation. J Electrochem Soc 152 A2256-A2271. 

Fang, B., et al., High Pt loading on functionalized multiwall carbon nanotubes as a highly efficient cathode electrocatalyst for proton exchange membrane fuel cells. Journal of Materials Chemistry, 2011. 21(22) p. 8066-8073. 

Wee, J. H., Lee, K. Y, and Kim, S. H. Fabrication methods for low-Pt-loading electrocatalysts in proton exchange membrane fuel cell systems. Journal of Power Sources 2007 165 667-677. 

In addition, in a DAFC, the proton exchange membrane is not completely alcohol tight, so that some alcohol leakage to the cathodic compartment will lead to a mixed potential with the oxygen electrode. This mixed potential will decrease further the cell voltage by about 0.1-0.2 V. It turns out that new electrocatalysts insensitive to the presence of alcohols are needed for the DAFC. 

Hydrogen as the most efficient and cleanest energy source for fuel-cell power is produced by partial oxidation followed by the water gas-shift reaction and reforming of hydrocarbons or methanol [58]. A small amount of CO (0.3-1%) in the so-produced H2 must be selectively removed because CO greatly poisons Pt/C and Pt-M/C electrocatalysts in proton-exchange-membrane (PEM) fuel cells [59, 60]. PROX of CO in excess H2 is a key reaction in the practical use of H2 in PEM fuel-cell systems. 

Other technical hurdles must be overcome to make fuel cells more appealing to automakers and consumers. Durability is a key issue and performance degradation is usually traceable to the proton exchange membrane component of the device. Depending on the application, 5,000 40,000 h of fuel cell lifetime is needed. Chemical attack of the membrane and electrocatalyst deactivation (due to gradual poisoning by impurities such as CO in the feed gases) are critical roadblocks that must be over come. 

Raghuveer, V., Manthiram, A., and Bard, A.J., Pd-Co-Mo electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells, J. Phys. Chem. B, 109, 22909, 2005. 

Pt-based electrocatalysts are usually employed in proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMSC). In direct-methanol fuel cells (DMFCs), aqueous methanol is electro-oxidized to produce COj and electrical current. To achieve enhanced DMFC performance, it is important to develop electrocatalysts with higher activity for methanol oxidation. Pt-based catalysts are currently favored for methanol electro-oxidation. In particular, Pt-Ru catalysts, which gave the best results, seem to be very promising catalysts for this application. Indeed, since Pt activates the C-H bounds of methanol (producing a Pt-CO and other surface species which induces platinum poisoning), an oxophilic metal, such as Ru, associated to platinum activates water to accelerate oxidation of surface-adsorbed CO to... 

Carbon supported Pt and Pt-alloy electrocatalysts form the cornerstone of the current state-of-the-art electrocatalysts for medium and low temperature fuel cells such as phosphoric and proton exchange membrane fuel cells (PEMECs). Electrocatalysis on these nanophase clusters are very different from bulk materials due to unique short-range atomic order and the electronic environment of these cluster interfaces. Studies of these fundamental properties, especially in the context of alloy formation and particle size are, therefore, of great interest. This chapter provides an overview of the structure and electronic nature of these supported... 

Fig. 6 Proton exchange membrane fuel cell the membrane-electrode assembly (MEA) consists of the Nation membrane with the electrocatalyst on the surface in contact with porous carbon...

The PE MFC has a solid ionomer membrane as the electrolyte, and a platinum, carbon-supported Pt or Pt-based alloy as the electrocatalyst. Within the cell, the fuel is oxidized at the anode and the oxidant reduced at the cathode. As the solid proton-exchange membrane (PEM) functions as both the cell electrolyte and separator, and the cell operates at a relatively low temperature, issues such as sealing, assembly, and handling are less complex than with other fuel cells. The P EM FC has also a number of other advantages, such as a high power density, a rapid low-temperature start-up, and zero emission. With highly promising prospects in both civil and military applications, PEMFCs represent an ideal future altemative power source for electric vehicles and submarines [6]. 

Carbon aerogels and xerogels have been used as supports for Pt and Pt-based electrocatalysts for proton-exchange membrane fuel cells (PEMFCs), also known as polymer-electrolyte fuel cells [56,58,83-90], These fuel cells are convenient and environmentally acceptable power sources for portable and stationary devices and electric vehicle applications [91], These PEMFC systems can use H2 or methanol as fuel. This last type of fuel cell is sometimes called a DMFC (direct methanol fuel cell). 

Polymer electrolyte fuel cells, also sometimes called SPEFC (solid polymer electrolyte fuel cells) or PEMFC (polymer electrolyte membrane fuel cell) use a proton exchange membrane as the electrolyte. PEEC are low-temperature fuel cells, generally operating between 40 and 90 °C and therefore need noble metal electrocatalysts (platinum or platinum alloys on anode and cathode). Characteristics of PEEC are the high power density and fast dynamics. A prominent application area is therefore the power train of automobiles, where quick start-up is required. 

---