Electrocatalysis in Fuel Cells

Methanol and ethanol, having high energy density, likely production from renewable sources, and ease of storage and distribution, are ideal combustibles for fuel cells wherein their chemical energy can be converted directly into electrical energy. However, the slow, incomplete oxidation of methanol and ethanol on platinum-based anodes as well as the high price and limited reserves of platinum has hampered the practical application of direct alcohol fuel cells. Extensive research efforts have been dedicated to developing high-activity electrocatalysts. This chapter presents an overview of the recent progress in methanol and ethanol electrocatalysis on platinum-based materials, with special attention focused on the research effort to reduce platinum content.

Fuel cells are being extensively studied to help ease the potential energy crisis. There have been many recent advances in direct methanol fuel cells (DMFC). These devices have shown promise for portable power applications because of the high gravimetric energy density of methanol. One major limitation of the technology is its extensive use of platinum in its anode catalyst. This chapter will provide a review of the work done to eliminate or significantly decrease the amount of platinum in DMFC catalysts by replacing platinum with tungsten monocarbide (WC) and monolayer coverages of platinum or palladium on WC.

Direct liquid fuel cells for portable electronic devices are plagued by poor efficiency due to high overpotentials and accumulation of intermediates on the electrocatalyst surface. Direct formic acid fuel cells have a potential to maintain low overpotentials if the electrocatalyst is tailored to promote the direct electrooxidation pathway. Through the understanding of the structural and environmental impacts on preferential selection of the more active formic acid electrooxidation pathway, a higher performing and more stable electrocatalyst is sought. This chapter overviews the formic acid electrooxidation pathways, enhancement mechanisms, and fundamental electrochemical mechanistic studies.

Direct formic acid fuel cells offer an alternative power source for portable power devices. They are currently limited by unsustainable anode catalyst activity, due to accumulation of reaction intermediate surface poisons. Advanced electrocatalysts are sought to exclusively promote the direct dehydrogenation pathway. Combination and structure of bimetallic catalysts have been found to enhance the direct pathway by either an electronic or steric mechanism that promotes formic acid adsorption to the catalyst surface in the CH-down orientation. Catalyst supports have been shown to favorably impact activity through either enhanced dispersion, electronic, or atomic structure effects.

The faster kinetics of the alcohol oxidation reaction in alkaline direct alcohol fuel cells (ADAFCs), opening up the possibility of using less expensive metal catalysts, as silver, nickel, and palladium, makes the alkaline direct alcohol fuel cell a potentially low-cost technology compared to acid direct alcohol fuel cell technology, which employs platinum catalysts. In this work an overview of catalysts for ADAFCs, and of testing of ADAFCs, fuelled with methanol, ethanol, and ethylene glycol, formed by these materials, is presented.

Direct alcohol alkaline fuel cells (DAAFCs) are potential power sources for a variety of portable applications as they provide unique advantages over hydrogen-based fuel cell devices. Alcohols (such as methanol, ethanol, ethylene glycol, and glycerol) have high volumetric energy density and are easier to store and transport than hydrogen. Palladium-based nanocatalysts have continued to receive much research attention because of their cost advantages, relative abundance, and unique properties in the electrocatalytic oxidation of alcohols in alkaline media compared to platinum catalysts. Recent efforts have focused on the discovery of palladium-based electrocatalysts with little or no platinum for oxygen reduction reaction (ORR). This chapter is an overview of the recent developments in the employment of palladium-based nanocatalysts, containing little or no platinum, for the electrooxidation of alcohols in alkaline media.

The high prospects of exploiting the oxygen reduction reaction (ORR) for lucrative technologies, for example, in the fuel cells industry, chlor-alkali electrolysis, and metal-air batteries, to name but a few, have prompted enormous research interest in the search for cost-effective and abundant catalysts for the electrocatalytic reduction of oxygen. This chapter describes and discusses the electrocatalysis of oxygen reduction by metallomacrocyclic complexes and the prospect of their potential to be used in fuel cells. Since the main interest of most researchers in this field is to design catalysts which can achieve facile reduction of O₂ at a high thermodynamic efficiency, this chapter aims to bring to light the research frontiers uncovering important milestones towards the synthesis and design of promising metallomacrocyclic catalysts which can accomplish the four-electron reduction of O₂ at low overpotential and to draw attention to the fundamental requirements for synthesis of improved catalysts. Particular attention has been paid to discussion of the common properties which cut across these complexes and how they may be aptly manipulated for tailored catalyst synthesis. Therefore, besides discussion of the progress attained with regard to synthesis and design of catalysts with high selectivity towards the four-electron reduction of O₂, a major part of this chapter highlights quantitative structure–activity relationships (QSAR) which govern the activity and stability of these complexes, which when well understood, refined, and carefully implemented should lead to rational design of better catalysts. A brief discussion about nonmacrocyclic copper (I) complexes, particularly Cu(I) phenanthrolines, and those with a laccase-like structure which exhibit promising activity for ORR has been included in a separate section at the end.

Non-precious metal catalysts have shown good activity towards oxygen reduction reaction, both in basic and acidic media. The use of NPMCs in fuel cells and metal–air batteries has been hampered by two main issues: the synthesis complexity, translating into a high fabrication cost, and by relatively low stability when compared to platinum-based catalysts, especially in acidic media. In order to overcome these issues, a new class of non-precious metal oxygen reduction catalysts was developed that involves heat treatment as a key step in the NPMC synthesis. This chapter provides a review of the progress in research on heat-treated non-precious metal catalysts of oxygen reduction since the early 1970s until today. The focus of this chapter is on the activity and morphology of the state-of-the-art heat-treated ORR catalysts and trends in the development of more active and durable materials.

The development of high-performance non-precious metal catalysts (NPMC) for use at the cathode of polymer electrolyte membrane fuel cells will provide immense economic advantages over the current platinum-based catalyst technologies, perpetuating the sustainable widespread commercialization of these devices. It is imperative to develop NPMC that can effectively combine excellent oxygen reduction activities, high catalyst utilization, and long-term operational durability. This chapter focuses on recent advances made in the past 3–4 years and research trends in this field, with a particular focus on pyrolyzed carbon-supported nitrogen-coordinated transition metal (Fe and/or Co) complexes which have high potential of replacing conventional platinum-based catalysts.

Despite decades of research on Fe (or Co)-based electrocatalysts for the oxygen reduction reaction (ORR) in acidic medium, such as that in PEM fuel cells, the role of the metal is still one that raises a great deal of controversy. Consequently, the nature of the catalytic site in these non-noble metal ORR catalysts is still a topic of debate. One camp within the scientific community believes that the metal is an integral and electrochemically active part of the catalytic site, while the other believes that the metal is merely a chemical catalyst for the formation of special oxygen-reducing N-doped carbon structures. After presenting the case for the importance of non-noble catalysts at the cathode of PEM fuel cells, we introduce the three models of active sites that were advocated during the 1980s by van Veen, Yeager, and Wiesener and discuss how they have evolved, especially that of Yeager. Wiesener’s model is analyzed in detail through the work of several research groups that have been staunch supporters. The oxygen reduction mechanism on Fe-based and N-doped carbon catalytic sites is also reviewed. It is concluded that all the active sites proposed by van Veen, Yeager, and Wiesener in the 1980s, while different, are in fact simultaneously present in Fe (or Co)-based catalysts active for ORR in acidic medium, except that their activity and relative population in these catalysts are different, depending on the choice of the metal precursor, nitrogen precursor, structural properties of the carbon support, and the synthesis procedure.

Fuel cells are regarded as one of the most promising candidates for stationary and mobile power generation due to their high energy yield and low environmental impact of hydrogen oxidation. The oxygen reduction reaction (ORR) at cathode is a very complex process and plays a crucial role during operation of the PEM fuel cells. However, its mechanism and the nature of intermediates involved remain vague. This chapter focuses on the recent theoretical modeling studies of ORR catalysts for PEMFC. Recent theoretical investigations on oxygen reduction electrocatalysts, such as Pt-based catalysts, non-Pt metal catalysts (Pd, Ir, CuCl), and non-precious metal catalysts (transitional metal macrocyclic complexes, conductive polymer materials, and carbon-based materials), are reviewed. The oxygen reduction mechanisms catalyzed by these catalysts are discussed based on the results.

Polymer electrolyte membrane (PEM) fuel cells are attracting much attention as promising clean alternative power sources to conventional power sources, including internal combustion engines and secondary batteries. Electrocatalysts for the oxygen reduction reaction (ORR) are a key component of PEM fuel cells, which convert chemical energy directly into electricity by coupling the ORR with the oxidation of fuel molecules at the other electrode via the diffusion of ions through the membrane. Although Pt-based ORR catalysts are given high priority in formulating electrodes for PEM fuel cells, they still suffer from multiple competitive disadvantages, including their high cost, susceptibility to CO gas poisoning, and fuel crossover effect. Due to their unique electrical and thermal properties, wide availability, environmental acceptability, corrosion resistance, and large surface area, certain carbon nanomaterials have recently been studied as metal-free ORR electrocatalysts to circumvent those issues associated with the Pt catalyst. Much effort has been devoted to developing metal-free ORR catalysts for fuel cells, which led to great advances in both fundamental and applied research. In this chapter, we present an overview on recent progresses in the development of metal-free ORR electrocatalysts for fuel cells.

The cathode catalysts for polymer electrolyte fuel cells should have high stability as well as excellent catalytic activity for oxygen reduction reaction (ORR). Group 4 and 5 metal oxide-based compounds have been evaluated as a cathode from the viewpoint of their high catalytic activity and high stability. Although group 4 and 5 metal oxides have high stability even in acidic and oxidative atmosphere, they are almost insulator and have poor ORR activity because they have large bandgaps. It is necessary to modify the surface of the oxides to improve the ORR activity. We have tried the surface modification methods of oxides into four methods: (1) formation of complex oxide layer containing active sites, (2) substitutional doping of nitrogen, (3) creation of oxygen defects without using carbon and nitrogen, and (4) partial oxidation of compounds which include carbon and nitrogen. These modifications were effective to improve the ORR activity of the oxides. The solubility of the oxide-based catalysts in 0.1 M H₂SO₄ at 30 °C under atmospheric condition was mostly smaller than that of platinum black, indicating that the oxide-based catalysts had sufficient stability compared to the platinum. The onset potential of various oxide-based cathodes for the ORR in 0.1 M H₂SO₄ at 30 °C achieved over 0.97 V vs. a reversible hydrogen electrode. This high onset potential suggests that the quality of the active sites of the oxide-based catalysts is mostly equivalent to that of platinum.

Transition metal chalcogenide materials represent nowadays a new family of alternative materials for the cathode oxygen reduction reaction (ORR). During the last decade, the efforts have been concentrated in developing this kind of materials due to their capacity to remain selective and tolerant in the presence of small organics in acid as well as in alkaline media. This is a good advantage regarding their potential use in low power systems working in mixed reactant conditions. Recent efforts have focused on the discovery and/or modification of sensitive catalytic centers. This chapter adds new challenges for the development of such “sophisticated” materials that become popular in recent years, giving a panorama of the state of the art particularly of nanodivided materials.

This chapter provides an overview of the recent advancement in the development of non-Pt electrocatalysts for oxygen reduction reactions (ORRs) in alkaline media; catalyst materials discussed include carbon-supported transition metals (Pt/C, Pd/C, Ag/C), transition-metal macrocycles (M–N–C), and multifunctional materials (e.g., metallic alloys, metallic MnO₂, macrocycle-treated metals). The important factors affecting ORR kinetics are identified through combined theoretical simulations and experimental measurements. The inconsistencies between the ORR activities observed in fuel cell tests and those observed in rotating disk electrodes, as reported by several research groups, were analyzed in details, and plausible theoretical explanations were proposed. Several promising bifunctional catalysts and their potentials as replacements for Pt in anion-exchange-membrane fuel cell (AEMFC) applications are discussed. For the AEMFC technology to mature as a low-cost high-performance energy device, further improvement of the performance and durability of the catalysts is essential; we believe that the necessary improvements can be achieved through intelligent design of multifunctional catalysts.

This chapter reviews the recent advances on the study of the oxygen reduction reaction (ORR) on gold electrodes. The initial part is devoted to the study of the reaction on single-crystal electrodes to determine the effect of the surface structure on the reactivity of gold electrodes for this reaction. The best reactivity is found for the Au(100) electrode in alkaline medium. For the nanoparticle electrodes, the reactivity for this reaction depends on two different effects: size and surface structure effects. Regarding the size effects, the different studies found in the literature do not agree on whether the size of the nanoparticles has a significant impact on the reactivity for the ORR. This disagreement between different authors is probably due to the lack of control of the surface structure of the nanoparticles. On the other hand, significant effects are found when the surface of the nanoparticle is changed. In general, the reactivity in alkaline media increases as the fraction of {100} domains on the surface increases. In some cases, the reactivity of gold in alkaline medium is similar to that measured for platinum electrodes.

Fuel cells are clean energy devices that are expected to help address the energy and environmental problems in our society. Platinum-based nanomaterials are usually used as the electrocatalysts for both the anode (hydrogen oxidation) and cathode (oxygen reduction) reactions. The high cost and limited resources of this precious metal hinder the commercialization of fuel cells. Recent efforts have focused on the discovery of palladium-based electrocatalysts with little or no platinum for oxygen reduction reaction (ORR). This chapter overviews the recent progress of electrocatalysis of palladium-based materials including both extended surfaces and nanostructured ones for ORR.

In this chapter, we review recent works of dealloyed Pt core–shell catalysts, which are synthesized by selective removal of transition metals from a transition-metal-rich Pt alloys (e.g., PtM₃). The resulted dealloyed Pt catalysts represent very active materials for the oxygen reduction reaction (ORR) catalysis in terms of noble-metal-mass-normalized activity as well as their intrinsic area-specific activity. The mechanistic origin of the catalytic activity enhancement and the stability of dealloyed Pt catalysts are also discussed.

A wide variety of core–shell electrocatalysts have been investigated in recent years, showing benefits for the oxygen reduction reaction (ORR) in acid electrolytes. Particularly high values of activity per gram of Pt are often measured for core–shell systems in rotating disc electrode (RDE) measurements; however, fewer systems have been tested for performance and durability in membrane electrode assemblies (MEAs) under realistic proton exchange membrane fuel cell (PEMFC) conditions. This chapter discusses the various approaches, both electrochemical and chemical, used to prepare core–shell materials at both small and gram scales and highlights some of the methods used to assess the uniformity of the Pt shell and activity and durability. Available data from MEA testing is reviewed along with some of the implications on overall cost of the use of precious metals within the core. So far, a limited number of core–shell materials have been tested in MEAs, and these data tend to show a lower activity compared to testing at microgram scale under more idealized conditions, due to the combination of catalyst scale-up issues and differences in testing protocols, test conditions (temperature, pH), and catalyst instability. Thus, an increasing focus on the validation of the core–shell approach under realistic MEA test conditions is necessary, to demonstrate their true benefits as cost-effective cathode catalysts for PEMFCs.

We review recent analyses of the various aspects related to the performance of core/shell nanocatalyst particles used as electrodes in proton exchange membrane fuel cells. These nanoparticles usually consist of a thin layer of pure Pt in the shell and a core alloy made of a combination of metal elements that are targeted to meet two main objectives: reducing the catalyst price and enhancing the activity of the surface layer with respect to an equivalent particle made of pure Pt. Even though both objectives have been shown to be met, a huge challenge remains that is related to the long-term durability of the particle. This is because the less noble components are prone to relatively easy dissolution in the harsh acid conditions in which low-temperature fuel cells operate. The catalytic behavior of the nanoparticle towards the oxygen reduction reaction (ORR) and the evolution of the catalytic particle under this complex environment require a combination of experimental modern surface science and electrochemical techniques but also the formulation of models that allow a better understanding and a rational catalyst design. In this chapter, we review the state-of-the-art modeling of core/shell catalysts for the ORR. This involves various aspects that are intrinsic to the core/shell structure: surface segregation, metal dissolution, and catalytic activity. A number of methods ranging from ab initio density functional theory to classical molecular dynamics and Kinetic Monte Carlo are included in our discussion.

The goal of catalyst development is to be able to adjust the structure and composition of catalytic materials to obtain the optimal electronic properties for desired chemical reactivity. Key features of the electronic structure that influence the reactivity of nanostructured catalysts are reviewed. Conclusions derived from the DFT electronic structure and the surface reactivity computations, with emphasis on the catalyst property intrinsically governed by the local, site-specific interactions, for nanostructured catalysts are presented.

Minute amounts of ruthenium and iridium on platinum nanostructured thin films have been evaluated in an effort to reduce carbon corrosion and Pt dissolution during transient conditions in proton exchange membrane fuel cells. Electrochemical tests showed the catalysts had a remarkable oxygen evolution reaction (OER) activity, even greater than that of bulk, metallic thin films. Stability tests within a fuel cell environment showed that rapid Ru dissolution could be managed with the addition of Ir. Membrane electrode assemblies containing a Ru to Ir atomic ratio of 1:9 were evaluated under start-up/shutdown and cell reversal conditions for OER catalyst loadings ranging from 1 to 10 μg/cm². These tests affirmed that electrode potentials can be controlled through the addition of OER catalysts without impacting the oxygen reduction reaction on the cathode or the hydrogen oxidation reaction on the anode. The morphology and chemical structure of the thin OER layers were characterized by scanning transmission electron microscopy and X-ray photoelectron spectroscopy in an effort to establish a correlation between interfacial properties and electrochemical behavior.

Moderate-temperature fuel cells are clean power generators for both stationary and mobile applications. In particular, polymer electrolyte membrane fuel cells (PEMCs) have attracted much attention due to their high gravimetric and volumetric power densities. However, due to their acidic environment, platinum-based nanocatalysts are the only feasible electrocatalyts for such systems. High cost and limited resources of this precious metal hinder the commercialization of PEMFCs. As a result, tremendous efforts are being exerted to either reduce Pt loading or substitute Pt metal with other non-noble metals. In this context, metal carbides have been extensively investigated due to their bifunctional mechanism as a catalyst as well as a catalyst support. Hence, the aim of using metal carbides is to replace carbon support since carbon suffers from corrosion problem and at the same time to reduce a substantial amount of Pt in fuel cell cathode. In this chapter, we have given an overview on metal carbides and their benefits as catalyst support for fuel cell cathode reactions.

One of the most significant roadblocks in the commercialization and widespread implementation of proton exchange membrane fuel cells is the identification of low-cost, high-stability, high-activity electrocatalysts. An overwhelming amount of the work that has been done in this area has targeted the electrochemically active material, which has been the focus of much of this book. However, a key component to any catalyst is its support. Interaction between the catalyst and support dictates some of the most critical parameters for fuel cell performance including catalyst dispersion, particle size, faceting, and stability. Some supports, like graphitic carbon, interact very weakly with Pt and have a limited influence on catalyst activity and stability. On the other hand, recent work by several groups has shown that a strongly interacting support can drastically impact both catalyst activity and stability.