Fuel cell electrocatalysis electrode process

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

The use of various electrode materials to explore chemical reactions in an electric field dates back to the beginning of the nineteenth century. Although the catalytic nature of some electrodes was only appreciated much later. Grove (7) recognized their chemical or catalytic action and the need for a notable surface action in early fuel cells, just a few years after the dawn of the notion of catalysis (2). When the term electrocatalysis was deliberately introduced by Grubb in 1963 (5), it did not reflect an unnecessary complication in nomenclature, but a real need to identify and comprehend the unique and characteristic features of catalytic electrode processes. How has this need been fulfilled to date Where does the field of electrocatalysis stand compared to the development of conventional catalytic and electrochemical processes What are the new directions and goals of this discipline ... 

The term catalysis was coined by Berzelius in 1835 and is derived from the Greek kata (go down) and lysis or lyein (letting). The first authors to introduce the term catalytic electrode reactions were Bowden and Rideal in 1928 [1], who observed the different currents that appear for a certain reaction on distinct electrode surfaces but under the same electrode potentials. There is still some controversy over the first use of the term electrocatalysis. It seems from the literature that the Soviets were the pioneers in the field of electrocatalysis since 1934 [2]. The first reported work in electrocatalysis was on fuel cell processes by Grubbs in the 1950s [3]. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

Electrocatalysis is a heterogeneous process that involves the adsorption and the chemisorption of reactants or intermediates at the interface. These phenomena are encountered, for example, in fuel cells (FCs) or in organic electrosynthesis. The electrocatalytic activity of a given electrode for a certain reaction may be characterized by the current density at a chosen potential, which is proportional to the specific activity, when referred to the effective active surface. As shown in Figure 21.2, the role of a heterogeneous catalyst is to adsorb the electro-reactive species (reactant and intermediate) and transform it to another compound that can more readily undergo the desired... 

Many electrochemical reactions, e g., those involved in energy conversion processes, such as fuel cells, or those leading to a selective transformation of organic materials, are catalysed by the electrode surface [1], This constitutes the important field of electrocatalysis, which can be defined, in a first approach, as the heterogeneous catalysis of electrochemical reactions by the electrode material [2], The determining role played by electrode materials was very soon recognized, and... 

The development of membranes for fuel cells is a highly complex task. The primary functionalities, (i) transport of protons and (ii) separation of reactants and electrons, have to be provided and sustained for the required operating time. Optimization of the composition and structure of the material to maximize conductivity and mechanical robustness involves careful balancing of synthesis and process parameters. The ultimate membrane qualification test is the fuel cell experiment. It is evident that the membrane is not a stand-alone component, but is combined with the electrodes in the membrane electrode assembly (MEA). Interfacial properties, influence on anode and cathode electrocatalysis, and water management are the key aspects to be considered and optimized in this ensemble. 

Although in situ infrared spectroscopy has been applied widely in terms of the systems studied, the reflective electrodes employed have been predominantly polished metal or graphite, and so an important advance has been the study of electrochemical processes at more representative electrodes such as Pt/Ru on carbon [107,122,157], a carbon black/polyethylene composite employed in cathodic protection systems [158] and sol-gel Ti02 electrodes [159]. Recently, Fan and coworkers [160] took this concept one step further, and reported preliminary in situ FTIR data on the electro-oxidation of humidified methanol vapor at a Pt/Ru particulate electrode deposited directly onto the Nafion membrane of a solid polymer electrolyte fuel cell that was mounted within the sample holder of a diffuse reflectance attachment. As well as features attributable to methanol, a number of bands between 2200 and 1700 cm were observed in the spectra, taken under shortoperating conditions, the importance of which has already been clearly demonstrated [107]. 

A semantic point should be mentioned the term electrocatalysis cannot strictly be applied to electron transfer steps or simple electron transfer reactions since there cannot be any noncatalyzed equivalent pathway in the absence of an electrode surface. However, adsorption effects specific to the electrode surface can influence the kinetics of an electron transfer step involving adsorbed products and/or reactants and in this limited sense electrocatalysis could arise in an electron transfer reaction. The term electrocatalysis, however, applies more correctly to the influence of electrode material and the state of electrode surfaces on the behavior of those types of chemical-catalyzed steps, e.g., dissociative chemisorption in some fuel cell oxidations, that are coupled with electron transfer processes. 

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