Fundamentals of Electrocatalysis

The science of electrocatalysis provides the connection between the rates of electrochemical reactions and the bulk and surface properties of the electrodes on which these reactions proceed. The degree to which an electrode will influence the reaction rates differs for different electrochemical reactions. In complex electrochemical reactions having parallel pathways, such as reactions involving organic substances, the electrode material might selectively influence the rates of certain individual steps and thus influence the selectivity of the reaction that is, the overall direction of the reaction and the relative yields of primary and secondary reaction products. 

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be realized for many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by many orders of magnitude between metals. 

Research in electrocatalysis was strongly stimulated in the early 1960s by efforts toward the development of various types of fuel cells. Studies were initiated on the various factors influencing the rates not only of hydrogen evolution but also of other reactions, particularly cathodic oxygen reduction and the complete oxidation of simple organic substances ( fuels ) to carbon dioxide. The 

Fuel Cells Problems and Solutions, Second Edition. Vladimir S. Bagotsky. 

In 1965, synergistic (nonadditive) catalytic effects were discovered in electrochemical reactions. It was shown in particular that the electrochemical oxidation of methanol on a combined platinum-ruthenium catalyst will occur with rates two to three orders of magnitude higher than at pure platinum even though pure ruthenium is catalytically altogether inactive. 

---

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

Santos E, Schmickler W. 2007c. Fundamental aspects of electrocatalysis. Chem Phys 332 39-48. 

These questions are of both applied and fundamental importance, since answering them will further our understanding of electrocatalysis. 

In this paper, we will discuss the thermodynamic principles involved in fuel cells as well as the kinetic aspects of their half cell reactions. In the kinetic considerations, we will also touch, briefly, on the fundamental problem of electrocatalysis. We will then proceed to describe different types of fuel cells and finally present the status of this new electrical generation device. 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

Two years ago, Advances in Catalysis featured a chapter on chemisorbed intermediates in electrocatalysis. In this issue we follow up with a chapter by Wendt, Rausch, and Borucinski, Advances in Applied Electrocatalysis. The successful commercial application of electrocatalysis requires a detailed, fundamental knowledge of the many catalytic phenomena such as adsorption, diffusion, and superimposition of catalyst micro- and nanostructure on the special requirements of electrode behavior. Considerable understanding of the status and limitations of electrolysis, fuel cells, and electro-organic syntheses has been obtained and provides a sound basis for future developments. 

The search for new electrode materials is expected to be guided by the fundamental understanding of the factors governing the activity. In electrochemistry, this branch of the discipline is known by the name of electrocatalysis . Strictly speaking, electrocatalysis is the science devoted to the relationship between the properties of materials and the electrode reaction rate. The scope of electrocatalysis as a science is to establish a predictive basis for the design and the optimization of electrocatalysts. 

Fundamentals of ab initio calculations, including density functional theory (DFT) methods, help to understand several key aspects of fuel cell electrocatalysis at the molecular level. 

In the field of electrocatalysis the situation seems to be somewhat better as far as the problems of clean surfaces and the existence of chemical effects are concerned. Unfortunately, so far only a few reactions have been studied. Thus, much more systematic fundamental research has to be done. From the results already available one can extract the hope that ion-bombardment will be of future importance for fields involving electrocatalytical reactions such as, for example, hydrogen technology, energy conversion, fuel cells and electrochemical redox reactions. 

Development of supported Pt electrocatalysts came as a result of intensive research on fundamental and applied aspects of electrocatalysis [especially for kinetically difficult oxygen reduction reaction (ORR)] fueled by attempts at commercialization of medium-temperature phosphoric acid fuel cells (PAFCs) in the late 1960s and early 1970s. Dispersion of metal crystallites in a conductive carbon support resulted in significant improvements in all three polarization zones (activation, ohmic, and... 

SERS may lead to detection techniques and assays with good sensitivity and selectivity. The area of electrocatalysis is continuously expanding. It has great potential in the area of electrochemical energy systems, especially fuel cells and batteries. The fundamental questions on nucleation, electrocrystallization, faceting, energy dissipation in hot metals and redox potential of nanoparticles are some of the issues that will remain the focus of immediate research. 

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

---