Catalyst layer modeling: Structure, properties and performance

Abstract

Polymer Electrolyte Fuel Cells (PEFCs) are promising electrochemical devices for the direct conversion of chemical energy of a fuel into electrical work [1-5]. Enormous research programs worldwide explore PEFCs as power sources that could replace internal combustion engines in vehicles and provide power to portable and stationary applications. Typically PEFCs operate below ∼80 °C. Anodic oxidation of H2 produces protons that migrate through the polymer electrolyte membrane (PEM) to the cathode, where reduction of O 2 produces water. Meanwhile, electrons, produced at the anode, perform work in external electrical appliances or engines. Unrivalled thermodynamic efficiencies, high energy densities, and ideal compatibility with hydrogen distinguish PEFCs as a primary solution to the global energy challenge. In spite of the abundance of promising demonstrations of vehicles and devices powered by PEFCs, success at the commercial stage is far from being guaranteed. New generations of fuel cells have to surpass established energy conversion technologies in power density, operational flexibility, stability, and cost. To give an idea of the magnitude of this challenge: currently used PEFC stacks in prototype cars are about 100 times more expensive than conventional combustion engines (∼$30/kW), albeit distinctly inferior in durability [5]. Ten years ago it was a common view among stakeholders in industry and academia that requisite progress could be achieved with engineering-type optimization, relying on existing materials. Nowadays, it is widely recognized, that progress in PEFC technology hinges on breakthroughs-not incremental changes!-in design, fabrication and implementation of innovative materials. Specific targets for improvement involve (i) increasing kinetic rates of electrocatalytic processes at electrodes, (ii) minimizing parasitic voltage losses due to transport of protons and electrons in conduction media and of reactants and water in porous diffusion media, (iii) providing uniform distributions of reaction rates, (iv) balancing water and heat fluxes, and (v) improving the long-term stability of materials. In operational PEFCs all components have to cooperate well in order to optimize the complex interplay of transport and reaction. This optimization involves more than 50 parameters [6]. The toughest competitions between random composite morphology and complex coupled processes unfold in the catalyst layers (CLs), the cathode catalyst layer (CCL) in particular [7]. All species and all processes that occur in the cell also occur in the catalyst layers: electrochemical reaction, diffusion of hydrogen or hydrocarbon-based fuels (anode) and oxygen (cathode), migration and diffusion of protons, migration of electrons, water transport by diffusion, permeation, electroosmotic drag, as well as vaporization/condensation of water. Electrical current is generated/consumed at nanoparticles of Pt, which are randomly dispersed on a high-surface carbon matrix [8]. During fabrication, the colloidal solution of carbon/Pt and ionomer self-organizes into a phase-segregated composite with interpenetrating percolating phases for the transport of electrons, protons, and gases [7]. As explored in [7], the process of microstructure formation depends on the type of the supported catalyst (carbon, Pt), the type and amount of ionomer added, the type of dispersion medium used during ink preparation, and the fabrication conditions (temperature, pressure). Electrochemical reactions occur only at those Pt particles where the three phases meet. Major constraints of this design are: (1) statistical limitations of the Pt utilization due to the random three-phase morphology and (2) highly non-uniform reaction rate distributions that arise when the thickness of the layer ( LCL ∼ 10 μm ) is large compared to the so-called "reaction penetration depth" δCL that is determined by the interplay of transport and reaction. These conditions lead to underutilization of Pt and irreversible voltage losses, with those due to oxygen reduction in the CCL (∼400 mV) diminishing cell efficiency by 30-40%. An increase by a factor 10 in the surface area of Pt reduces these losses by 60-120 mV. Pt is, however, an expensive and limited resource. For a 60 kW fuel cell vehicle, the current cost of Pt is over $2400 [5]. A simple estimate shows that replacing combustion engines in all existing vehicles by fuel cell drive systems at no penalty in power would by far exhaust all known reserves of Pt. It is thus evident that drastic improvements in power density, durability, and cost would be impossible without a breakthrough in the concept of CLs. Outline. In this chapter we will provide a detailed overview of recent efforts in theory, molecular modeling, and performance modeling of CLs in modern PEM fuel cells. Our major focus will be on state-of-the-art CLs with porous carbonaceous substrates and random composite morphology. We will evaluate the pertinent design and compare it with recent advanced design strategies. In Section 8.2, we will provide a general discussion of structure and processes in catalyst layers and how they transpire in the evaluation of performance. Section 8.3 reviews the state of the art in theory and modeling. In Section 8.4, we will discuss aspects related to self-organization phenomena in catalyst layer inks during fabrication, controlled by fundamental interactions between the various components. These phenomena determine the evolution of effective properties for transport and electrocatalytic activity. Thereafter, we will present catalyst layer models in Section 8.5 that involve parameters related to structure, processes, and operating conditions and describe the concepts that have been developed to relate these ingredients to performance. From the insights presented we will draw conclusions about potential improvements of catalyst effectiveness, voltage efficiency, water handling capabilities, and stability through optimized operating conditions and through radically new structural design. In this chapter we will not include the modeling of the electrochemical impedance response of porous electrodes or catalyst layers. Recent comprehensive coverage on this important topic can be found in the literature [3, 9]. © 2008 Springer-Verlag.