Catalyst layer degradation, diagnosis and failure mitigation

Abstract

Faced with rapidly rising air pollution-related health risks, sky-rocketing oil prices, and diminishing natural resources, scientists and engineers are now seeking clean and efficient alternatives to petroleum as energy sources. The hydrogen fuel cell, using hydrogen and oxygen from air as fuel, could achieve efficiencies of electric power generation in the 50-65% range. As a clean electric power source, fuel cells can be used to power vehicles, back-up the power supply for electric devices, and store electricity in power stations by converting water into hydrogen and oxygen during off-peak hours. The only by-products are water and heat. The proton exchange membrane fuel cell (also called polymer electrolyte membrane fuel cell, PEMFC) is a highly promising power source candidate for zero emission vehicles, stationary applications, backup power units, materials handling, and small electronics. Fuel cells are currently the only technology that can effectively provide pollution-free energy for both transportation and electric utilities. The use of fuel cell vehicles (FCVs) will partially reduce the global dependency on petroleum as a fuel. In the past several decades, a variety of fuel cells have been developed. These can be classified by the type of electrolyte used in the cells, and include: 1) polymer electrolyte membrane fuel cell, 2) alkaline fuel cell (AFC), 3) phosphoric acid fuel cell (PAFC), 4) molten carbonate fuel cell (MCFC), and 5) solid oxide fuel cell (SOFC). Hydrogen has many merits as a fuel for fuel cells in most applications. It is highly reactivity when suitable catalysts are used. It can be produced from hydrocarbons, obtained as a by-product of chemical plans, or generated by water electrolysis using solar or nuclear power. It also has high energy density. Therefore, hydrogen has been chosen as a PEMFC fuel for many applications. The most common and economical oxidant is gaseous oxygen, which is readily available from air. A typical single PEMFC consists of an anode, a cathode, a proton conductive membrane, and two bipolar plates (Figure 23.1). In a PEMFC, hydrogen is fed continuously to the anode (negative electrode) and oxygen or air is fed continuously to the cathode (positive electrode) through a flow field on the bipolar plates. At the anode, hydrogen is oxidized and a proton is transferred to the cathode through the proton conductive membrane. Meanwhile, electrons are produced and flow through an external circuit to the cathode. Oxygen (often obtained from air) is electro-catalyzed on the cathode catalyst surface and reduced to oxygen anions after receive four electrons per molecule, called the oxygen reduction reaction (ORR). The electrochemical reactions take place at the cathode to produce water. The functions of the proton conductive membrane are to provide a proton pathway, to separate the hydrogen and oxygen gases, and to electrically isolate the anode and cathode. Both anode and cathode in a PEMFC have gas distribution layers (GDLs) that are usually made of carbon fiber and/or sub-micron carbon particles with a bonding agent. The GDL functions as a gas distribution medium to allow gas flow from the gas channels in a bipolar plate to the catalyst layer. The GDL also conducts electrons from the anode catalyst layer to the cathode catalyst layer for electrochemical reaction. The functions of porous electrodes in fuel cells are: 1) to provide a surface site for gas ionization or de-ionization reactions, 2) to provide a pathway for gases and ions to reach the catalyst surface, 3) to conduct water away from the interface once these are formed, and 4) to allow current flow. A membrane electrode assembly (MEA) forms the core of a fuel cell and the key electrochemical reactions take place in the MEA. MEA performance is severely affected by electrode composition, structure, and geometry, and especially by cathode structure and composition, due to poor oxygen reduction kinetics and transport limitations of the reactants in the cathode catalyst layer. Catalytic layers in the anode and cathode are also critical components in a PEMFC. Platinum (Pt) or a Pt-Ru alloy supported by nano-carbon particles is often used as the catalyst in the anode [1-6]. Favorable electronic properties have been suggested by density-functional theory studies, which have shown that the CO adsorption energy is the lowest on the Pt monolayer located above Ru compared with the adsorption energies on pure Pt, pure Ru, and a Pt-Ru mixed surface layer over Pt [7, 8]. The bifunctional mechanism [4] suggests that Ru provides an active surface for oxidative removal of adsorbed CO at the neighboring Pt sites. Thus, a Pt-Ru alloy catalyst with high CO tolerance is used in the anode, especially when a reformate H 2-rich gas (in which CO content is high) is used for fuel. An ideal cathode should have: 1) a large interface between gas phases and/or polymer electrode and catalyst, 2) highly efficient proton transport, 3) high transportation capacity for oxygen and easy by-product removal (i.e., condensed or gaseous water), 4) high electronic conductivity, 5) excellent chemical resistance and good mechanical properties to maintain an effective porous structure during fuel cell operation, and 6) high tolerance to contamination. Pt nano-particles supported on carbon particle surfaces are commonly used in cathode catalyst layers [9-11]. Using platinum supported on high-surface-area carbon (e.g., Vulcan XC72R) as the electrocatalyst rather than pure Pt black yields a ten-fold reduction in platinum loading. To conduct protons to reactive spots, a proton conducting polymer (e.g., Nafion) is impregnated into the catalyst layer [12]. This breakthrough technology creates a Pt catalyst with a particle size as low as ∼3 nm and a very high active surface area, as well as proton transport networks . The structured cathode creates a three-dimensional network that makes mass transport and electrochemical reactions at the heterogeneous interface more efficient [13]. Recently, platinum binary or ternary alloys such as Pt-Co, Pt-Cr, or Pt-Co-Cu are being tested in PEMFCs to decrease the precious metal content in the catalyst layer and increase the ORR efficiency. Reliability and durability are the most important considerations in PEMFCs. The durability of fuel cell systems operating under automotive conditions has not been clearly established. Fuel cell power systems need to be as durable and reliable as current automotive engines, i.e., 5,000-hour lifespan (150,000 miles equivalent) and be able to function over the full range of external environmental conditions (-40 °C to +40 °C). Analysis of various standardized US vehicle drive cycles reveals that a vehicle experiences roughly 300,000 load (voltage) cycles between peak power (0.7 V) and idle (0.9 V) through its life of 5,500 h. The long-term vehicle performance degradation target of ≤ 3 μV/h over 5,500 h requires an absolute voltage loss of <17 mV over the projected 300,000 large voltage cycles (0.7↔0.9 V) [14]. The durability of catalysts has become a major concern in fuel cell development. Under fuel cell operating conditions, the cathode catalyst layer can degrade through platinum sintering and dissolution, especially in conditions of load-cycling and high electrode potentials. Carbon support corrosion is another challenge at high electrode potentials and can worsen under load cycling and hightemperature operation. Stationary power requires that PEMFC systems operate on natural gas or LPG that achieves 40% electrical efficiency and 40,000 hours of durability at low cost. CO tolerance, long-term stability of the anode catalyst layer, and low loading of noble metals for cost reduction are also some of the challenges that need to be overcome for stationary power applications. Although significant attention has been focused upon fundamentally understanding catalyst degradation and the development of novel catalysts and structures, we are not yet close to making durable electrodes at low cost for the commercializing of fuel cells. In this chapter, we will focus on degradation of anode and cathode catalysts, and diagnostic methods for electrode failure after fuel cell operations. Failure mechanisms will also be discussed. © 2008 Springer-Verlag.