PEM Fuel Cell Electrocatalysts and Catalyst Layers

A fuel cell is an electrochemical device that continuously and directly converts the chemical energy of externally supplied fuel and oxidant to electrical energy. Fuel cells are customarily classified according to the electrolyte employed. The five most common technologies are polymer electrolyte membrane fuel cells (PEM fuel cells or PEMFCs), alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs). However, the popularity of PEMFCs, a relatively new type of fuel cell, is rapidly outpacing that of the others.

Unlike most other types of fuel cells, PEMFCs use a quasi-solid electrolyte, which is based on a polymer backbone with side-chains possessing acid-based groups. The numerous advantages of this family of electrolytes make the PEM fuel cell particularly attractive for smaller-scale terrestrial applications such as transportation, home-based distributed power, and portable power applications. The distinguishing features of PEMFCs include relatively low-temperature (under 90 °C) operation, high power density, a compact system, and ease in handling liquid fuel.

Oxygen (O₂) is the most abundant element in the Earth’s crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes such as biological respiration, and in energy converting systems such as fuel cells. ORR in aqueous solutions occurs mainly by two pathways: the direct 4-electron reduction pathway from O₂ to H₂O, and the 2-electron reduction pathway from O₂ to hydrogen peroxide (H₂O₂). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O₂ to superoxide (O₂ ^-) can also occur.

In proton exchange membrane (PEM) fuel cells, including direct methanol fuel cells (DMFCs), ORR is the reaction occurring at the cathode. Normally, the ORR kinetics is very slow. In order to speed up the ORR kinetics to reach a practical usable level in a fuel cell, a cathode ORR catalyst is needed. At the current stage in technology, platinum (Pt)-based materials are the most practical catalysts. Because these Pt-based catalysts are too expensive for making commercially viable fuel cells, extensive research over the past several decades has focused on developing alternative catalysts, including non-noble metal catalysts [1]. These electrocatalysts include noble metals and alloys, carbon materials, quinone and derivatives, transition metal macrocyclic compounds, transition metal chalcogenides, and transition metal carbides. In this chapter, we focus on the O₂ reduction reaction, including the reaction kinetics and mechanisms catalyzed by these various catalysts.

To assist readers, we first provide an overview of the following background information: the major electrochemical O₂ reduction reaction processes, simple ORR kinetics, and conventional techniques for electrochemical measurements.

Hydrogen (H₂), an important material and product in chemical industries, has been investigated as a new clean energy source for many decades [1–3]. With the rapid development of proton exchange membrane (PEM) fuel cell technology, in which H₂ is used as a fuel, the chemical energy stored in this H₂ can be electrochemically converted to electric energy with zero emissions and high efficiency. The dream of a hydrogen economy era therefore seems closer to reality. Beginning in the 1990s, the advantages of PEM fuel cells, including zero/low emissions, high energy efficiency, and high power density, have attracted world-wide research and development in several important application areas, including automotive engines, stationary power generation stations, and portable power devices [4]. With successful demonstrations of fuel cell technology, in particular in automotive applications, commercialization of this technology has become a strong driving force for further development in the critical areas of cost reduction and durability. The major cost of a PEM fuel cell is the platinum (Pt)-based catalysts. At our current technological stage, these Pt-based catalysts for both the cathodic O₂ reduction reaction (ORR) and the anodic H₂ oxidation reaction (HOR) are the most practical catalysts in terms of catalytic activity and lifetime stability. Therefore, research and development to improve catalytic activity and stability has shot to the fore in recent years. Although both theoretical and experimental approaches have resulted in great progress in fuel cell catalysis [2, 3, 5, 6], continuous effort is necessary to develop breakthrough fuel cell catalysts that are cost-effective and highly durable for commercial use.

Detailed investigations of CH₃OH, C₂H₅OH, and HCOOH electrooxidation started about five decades ago, with the advent of modern potentiodynamic techniques, driven initially by the goal of elucidating some of the most interesting and challenging electrode kinetic problems. Many pioneering studies that withstood the test of time and the continuous sophistication of surface electrochemistry techniques were published in the 1960s by Petry, Bagotzky, Daniel-Bek, Breiter, and others [1–14]. Important results were obtained, such as: (i) phenomenological interpretation of anodic polarization curves (e.g., determination of Tafel parameters for CH₃OH oxidation on Pt [3], C₂H₅OH oxidation in alkaline media on Ni and Pd/C [4], HCOOH oxidation on Pd [6] and Pt/C [7]); (ii) discovery of the cocatalytic effect of Ru with respect to Pt for CH₃OH oxidation [1, 8]; (iii) investigation of the potential dependent adsorption of CH₃OH and HCOOH on Pt and the associated parallel (or dual) pathway mechanism [9–12]; and (iv) mathematical modeling of flooded porous electrode behavior [13, 14].

The effects of impurities on fuel cells, often referred to as fuel cell contamination, is one of the most important issues in fuel cell operation and applications. Contamination is closely associated with proton exchange membrane fuel cell (PEMFC) durability and stability, both of which are important factors in the development and commercialization of PEMFC technology. Studies have identified that the membrane electrode assembly (MEA), the heart of the PEMFC, is the fuel cell component most affected by contamination. Impurities in the air and fuel streams damage the MEA by affecting both the anode and cathode catalyst layers (CLs), the gas diffusion layers (GDLs), as well as the proton exchange membrane (PEM), causing MEA performance degradation or even fuel cell failure. In general, PEMFC contamination effects can be categorized into three major types: (1) kinetic losses caused by the poisoning of both anode and cathode catalyst sites or a decrease in the catalyst activity; (2) ohmic losses due to an increase in the resistance of membrane and ionomer, caused by alteration of the proton transportation path; and (3) mass transfer losses due to changes in structure and in the ratio between the hydrophobicity and hydrophilicity of CLs, GDLs, and the PEM. Among those effects, the most significant is the kinetic effect of the anode and cathode electrocatalysts. This chapter presents PEMFC contamination with a focus on the anode and cathode catalyst layers. Catalyst contamination mechanisms, experimental results, modeling, as well as mitigation strategies are also covered in detail. For further information, such as contamination effects on other parts of PEMFCs, the reader is referred to a recent review paper [1].

Proton exchange membrane fuel cells (PEMFCs) have attracted intensive attention as a result of their applicability to transportation systems and portable electronic products [1]. The important challenges in PEMFC research arise in the catalyst layers (CLs) because these are complex and heterogeneous. The catalyst layers need to be designed so as to generate high rates of the desired reactions and minimize the amount of catalyst necessary for reaching the required levels of power output. To meet the goal, the following requirements need to be considered: (1) large three-phase interface in the CL, (2) efficient transport of protons, (3) easy transport of reactant and product gases and removal of condensed water, and (4) continuous electronic current passage between the reaction sites and the current collector. A CL with a thickness around several micrometers is a critical component of a PEMFC and requires more elaborate treatment [2]. The CL is in direct contact with the membrane and the gas diffusion layer (GDL), as shown in Figure 7.1. It is also referred to as the active layer [3]. Gottesfeld and Zawodzinski provided a good overview of the CL structure and functions [4]. The overall CL performance depends on all these critical factors and is therefore essential to identify the electrode structures and operation conditions. In this section, the functions and the technical impacts of the CLs will be described.

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 O₂ 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.

This chapter deals with aspects of the synthesis of fuel cell catalysts. Practical catalysts for low-temperature fuel cells are typically in the nano-size range and are frequently formed or deposited on high-surface-area supports. Pt is the most commonly used catalyst for both cathode and anode in proton exchange membrane fuel cells (PEMFCs). In the case of the cathode, combined catalyst systems such as Pt nanoparticles supported on Au or Pt alloy catalysts, as well as Pt-skin catalysts formed in combination with the iron group metals have also attracted attention. Much work has been carried out on the development of “non-noble” metal (Pt-free) catalysts, the synthesis of which will be discussed in Section 9.5. In the case of the anode, bi-metallic catalysts are typically employed unless the fuel is neat H₂. Pt-Ru is the state-of-the-art catalyst for both methanol and reformate fuel cells. For the latter, other anode catalysts such as Pt/MoO_x and Pt/Sn are also considered promising.

In recent years, fuel cells have attracted considerable attention due to their high energy efficiency with zero emissions [1]. Electrocatalysts are some of the key materials used in low-temperature fuel cells such as the polymer electrolyte membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC). Creating high-performance catalysts is widely recognized as a key step for the further development and commercialization of low-temperature fuel cells.

The physical characterization of electrocatalysts is very important for several areas of research: (1) preparing new types of electrocatalysts with high activity and high selectivity, (2) recognizing electrocatalyst structures, and (3) investigating the mechanisms of catalysts and certain additives.

Electrocatalysts for application in low-temperature fuel cells (including PEMFCs and DMFCs) constitute a special type of heterogeneous catalyst. The most important difference between an electrocatalyst and a normal heterogeneous catalyst is that the former should have good conductivity, whereas most typical heterogeneous catalysts are insulators; therefore, most characterization techniques for electrocatalysts are the same as for regular heterogeneous catalysts, but some special techniques are required for electrocatalysts because of their conductivity.

Since a fuel cell is an electrochemical device, electrochemical methods are deemed to play important roles in characterizing and evaluating the cell and its components such as the electrode, the membrane, and the catalyst. The most popular electrochemical characterization methods include potential step, potential sweep, potential cycling, rotating disk electrode, rotating ring-disk electrode, and impedance spectroscopy. Some techniques derived from these methods are also used for fuel cell characterization.

An electrochemical reaction involves at least the following steps: transport of the reactants to the surface of the electrode, adsorption of the reactants onto the surface of the electrode, charge transfer through either oxidation or reduction on the surface of the electrode, and transport of the product(s) from the surface of the electrode. The purpose of the electrochemical characterizations is to determine the details of these steps.

Traditionally, chemists and material scientists rely on the slow and serendipitous trial-and-error process for discovering and developing new chemicals or materials. However, the conventional one-at-a-time, or one-by-one, methods are not capable of matching the pace of present-day material development. For example, in the area of drug discovery, it has been estimated theoretically that the number of possible drugs with molecular structures and weights attractive for pharmaceutical activity screening is 10¹⁸. This number is ~10³ times larger than the number of chemical substances available from commercial sources or in-house collections. This number is also ~5×10¹⁰ times larger than that listed in the Chemical Abstracts Service (CAS) database. It is impossible to screen the pharmaceutical activities of such a large number of potential drugs by using traditional one-by-one methods. For organic substances, if each substance contains more than 30 atoms of H, C, O, N, and S elements, the number of potential stable molecular structures is expected to be ~10⁶³. For inorganic substances, possibilities include ~3×10³ binaries, ~7×10⁴ ternaries, ~1×10⁶ quaternaries, and ~6×10¹² decanaries, which can be made from 75 useful and stable elements in the periodic table. These numbers exclude those with stoichiometric and structural diversity and different orders. Traditional one-by-one approaches will never be able to deal with the screening and optimization of these substances.

Proton exchange membrane (or polymer electrolyte membrane) (PEM) fuel cells are one type of clean energy converting device that can contribute to sustainable world development. PEM fuel cells use hydrogen or hydrocarbons for fuel and air as the oxidant, reacting through a silent electrochemical process at low temperatures (60–90 °C) to convert chemical energy to electricity with zero or low emissions. This technology is attractive mainly due to its environmentally friendly nature and highly efficient energy conversion as compared to traditional energy technologies such as internal combustion engines (ICEs). At our current stage of development, two kinds of PEM fuel cells are the most promising for commercialization: the hydrogen (H₂)-fuelled PEM fuel cell, a major candidate for automobile applications to replace oil-dependant ICE technology; and the methanol-fuelled PEM fuel cell or direct methanol fuel cell (DMFC), which shows great potential for applications in portable electronic devices.

The use of nanotechnology towards improving clean energy solutions is very important. There is a growing awareness that nanotechnology will have a profound impact on energy generation, storage, and utilization by understanding the significant differences of energy states and transport in nanostructures compared to macrostructures. Nanotechnology-based solutions are being developed for a wide range of energy solutions such as solar cells, hydrogen generation and storage, batteries, and fuel cells.

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.

Fuel cell systems offer the promise of economically delivering power with environmental and other benefits. Recently, polymer electrolyte membrane fuel cells (PEMFCs) have passed the demonstration phase and have partly reached the commercialization stage due to impressive research efforts. Nevertheless, there are still some technological challenges to be solved. Among those challenges, (i) choice of fuel (gasoline, methanol, or hydrogen), (ii) efficient fuel processing, with reduction of weight, volume, and carbon monoxide (CO) residuals, and (iii) development of anode electrocatalysts tolerant to CO at levels of 50 ppm (with a noble metal loading of 0.1 mg cm^-2 or less) are deemed to be the most significant barriers that PEMFCs must overcome to achieve complete commercialization. The first and second challenges are closely related to the source and purity of hydrogen as the fuel.

Fuel cells present a promising technology for providing clean, efficient electric power in a variety of applications. They are the most environmentally friendly alternative to internal combustion engine technology in vehicles. They also have applications in portable electronics, as well as distributed and back-up power. The last few years have witnessed a tremendous increase in the research and development of fuel cells, including the development of new materials, new system designs, and new operating methods. While many breakthroughs have been made, technical and economic barriers for commercialization still exist. For a polymer electrolyte membrane fuel cell (PEMFC) – the most promising fuel cell technology – to be used commercially in stationary or transportation applications, cost and durability are the major challenges. In transportation applications, fuel cell technologies face more stringent cost and durability requirements: a fuel cell system needs to cost less than $50/kW with a 5,000 hour lifespan (150,000 miles equivalent) and have the ability to function over the full range of vehicle operating conditions (–40 to +90 °C). For stationary applications, a fuel cell system operating on natural gas needs to achieve 40% electrical efficiency and 40,000 hours durability at $750/kW [1]. To be commercially viable, however, fuel cell systems must also exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside the preferred operating range. As PEMFCs approach commercialization, significant progress is being made towards producing systems that achieve the optimum balance of cost, efficiency, reliability, and durability.

Proton exchange membrane fuel cells (PEMFCs), including direct methanol fuel cells (DMFCs), are considered one of the most promising types of energy converting devices due to their low/zero pollution emission, high power density, and high energy conversion efficiency. However, commercialization faces several major technical challenges, the top three being high cost, unsatisfactory durability, and operational flexibility. The last several decades have witnessed great efforts to overcome these challenges. Operating a PEMFC at temperatures greater than 90 °C is one approach [1].

The membrane electrode assembly (MEA) is the heart of proton exchange membrane fuel cells (PEMFCs), including direct methanol fuel cells (DMFCs), and determines both fuel cell performance and durability. The MEA component materials, structure, and fabrication technologies play important roles in performance improvement and optimization. For example, the catalyst layers, where the electrochemical reactions take place, are the most important of the several components in PEMFCs. An MEA contains an anode gas diffusion layer (GDL), an anode catalyst layer (CL), a proton exchange membrane (PEM), a cathode catalyst layer, and a cathode gas diffusion layer. An ideal MEA would allow all active catalyst sites in the catalyst layer to be accessible to the reactant (H₂ or O₂), protons and electrons, and would facilitate the effective removal of produced water from the CL and GDL. Over the past several decades, great efforts have been made to optimize the catalyst layer and MEA, and many catalyst layer/MEA structures and fabrication methods have been developed. As a result, MEA performance with advanced catalyst layers has been significantly improved by employing different fabrication methods [1–4], changing the catalyst layer structures [5–11], and using different components [5–8].

This chapter will address the preparation methods for catalyst inks, catalyst layers, and MEAs, with a focus on the fabrication processes.

In spite of many efforts and improvements by thousands of scientists worldwide, PEMFCs and DMFCs have not been commercially used. State-of-the-art electrocatalysts for PEMFCs rely on large quantities of platinum to achieve acceptable performance levels. This presents a significant hurdle to market acceptance of FC-powered vehicles; a commercially viable electrocatalyst will require nearly an order-of-magnitude reduction in Pt usage to meet both cost and Pt availability constraints. Pt fine particles are dispersed on carbon blacks with a large surface area to reduce the total Pt used and to enhance catalytic activity. This activity depends not only on the primary structure of the catalyst Pt/carbon composites (i.e., carbon surface area, Pt particle size, Pt surface area, etc.), but also on the secondary structure (i.e., aggregation and agglomeration of carbon grains).

The membrane electrode assembly (MEA) is a key unit of proton exchange membrane (PEM) fuel cells, including direct methanol fuel cells (DMFCs). In general, the MEA is composed of an anode gas diffusion layer (GDL), an anode catalyst layer, a membrane (the PEM), a cathode catalyst layer, and a cathode gas diffusion layer. The MEA materials, structures, components and fabrication technologies have strong effects on the corresponding fuel cell performance. In particular, the catalyst layers, where the electrochemical reactions take place, are the most important components. Theoretically, in an ideal catalyst layer all catalyst particle sites would be accessible to the reactant gas (H₂ or O₂), protons, and electrons. In order to achieve this, the distributions of the electron conductor, proton conductor, catalyst sites, and gas pores should be uniform in a catalyst layer.

As an alternative clean energy technology, the proton exchange membrane fuel cell (PEMFC) could be widely used in residential, transportation, and military applications. However, several factors currently limit fuel cell system commercialization: low stack performance, short lifetime, and high cost. Catalyst layer composition is a key component in determining both stack performance and lifetime. Therefore, catalyst layer composition optimization is highly significant for improving MEA stability and continuity, enhancing stack lifetime, and reducing the overall system cost. This chapter discusses the main factors in catalyst layer composition that determine MEA performance.

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.