Electrocatalysis electrode potential

This last equation contains the two essential activation terms met in electrocatalysis an exponential function of the electrode potential E and an exponential function of the chemical activation energy AGj (defined as the activation energy at the standard equilibrium potential). By modifying the nature and structure of the electrode material (the catalyst), one may decrease AGq, thus increasing jo, as a result of the catalytic properties of the electrode. This leads to an increase in the reaction rate j. 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution into the present and future of the industrial world. It illustrates the transition of electrochemical sciences from a solid chapter of physical electrochemistry (covering mainly electron transfer reactions, concepts of electrode potentials and stmcture of the electrical double layer) to the field in which electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is supported by high-level theory, connects to other areas of science, and includes focus on electrode surface structure, reaction environment, and interfacial spectroscopy. 

The aim of this review is to first provide an introduction of electrocatalysis with the hope that it may introduce the subject to non-electrochemists. The emphasis is therefore on the surface chemistry of electrode reactions and the physics of the electrode electrolyte interface. A brief background of the interface and the thermodynamic basis of electrode potential is presented first in Section 2, followed by an introduction to electrode kinetics in Section 3. This introductory material is by no means comprehensive, but will hopefully provide sufficient background for the rest of the review. For more comprehensive accounts, please see texts listed in the references.1-3... 

Electrocatalysis and the Rate of Electrochemical Reactions For a given electrochemical reaction A + ne B, which involves the transfer of n electrons at the electrode/ electrolyte interface, the equilibrium potential, called the electrode potential, is given by the Nernst law ... 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

This reaction is about the simplest that involves intermediate radicals (adsorbed H atoms waiting to combine to form H2). A study of potential energy diagrams such as that described below can be used to comprehend why a change in the electrode potential changes the reaction rate, and thus to understand the basis of electrocatalysis. 

The approach to P given above is a simplification, although it does show why the effect of the change in the electrode potential on the charge-transfer rate is less titan that expected if the full potential were applied, an important realization. Another virtue of the early theory is the basis it gives to a theory of electrocatalysis. 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Figure 3.4. Fuel cell negative electrode potential 0, and positive electrode potential 0., as a function of current. The main cause of the diminishing potential difference A0., for increasing current is at first incomplete electrocatalysis at the electrodes, for larger currents also ohmic losses in the electrolyte solution, and finally a lack of ion transport (cf. Bockris and Shrinivasan, 1969). From B. Sorensen, Renewable Energy, 2004, used by permission from Elsevier.

Figure 8-13. Left Electrocatalysis of A. xylosoxidans Cu-nitrite reductase on cysteamine-modified Au(lll)-electrode 5 mM sodium acetate buffer, pH 6.0. Scan rate 10 mV s . Dashed line no KNO2 present. Solid lines Potassium nitrite concentrations (pM) a 70, b 110, c 250, d 800. Right In situ STM of A xylosoxidans Cu-nitrite reductase. Same conditions. Potassium nitrite concentration 200 pM. Working electrode potential +0.38 V. Bias voltage -1.10 V. Tunneling current 0.1 uA. From ref. 127 with permission.

It is remarkable that the erudite Rideal (a specialist in catalysis) did not introduce the term electrocatalysis in 1928. The reason, I think, was that electrochemistry in 1928 was generally focused on the electrode potential at a constant current and not the current (= rate) at a constant... 

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]. 

A study of electrocatalysis is also important to understand the mechanism of the reaction that operates the process. Since the reaction mechanisms are very difficult to discern, the possibility of the variation of different parameters, such as the electrode potential, the nature of the electrode material, the electrolyte composition, etc., brings into focus a wide possibility of pathways. The fixing of the experimental conditions allows us to choose the appropriate route for substance preparation, complete energy conversion, inhibition of the corrosion process, and eliminating the side reaction pathways. 

Not only is the value of jQ important in electrocatalysis but also the experimental Tafel slope at the operating electrode potential. As expected in an electrocatalytic process, this complex heterogeneous reaction exhibits at least one intermediate (reactant or product) adsorbed species. Therefore, a single or simple Tafel slope for the entire process is not expected, but rather surface coverage and electrolyte composition potential dependent Tafel slopes within the whole potential domain are expected. Instead of calculating the most proper academic Tafel slope, the experimental current vs. potential curve is required for the selected electrocatalysts [4,6]. 

Electrocatalysis in metallic corrosion may be classified into two groups Adsorption-induced catalyses and solid precipitate catalyses on the metal surface. In general, the bare surface of metals is soft acid in the Lewis acid-base concept and tends to adsorb ions and molecules of soft base forming the covalent binding between the metal surface and the adsorbates. The Lewis acidity of the metal surface however may turn gradually to be hard as the electrode potential is made positive, and the bare metal surface will then adsorb species of hard base such as water molecules and hydroxide ions in aqueous solution. Ions and molecules thus adsorbed on the metal surface catalyze or inhibit the corrosion processes. Solid precipitates, on the other hand, are produced by the combination of hydrated cations of hard acid and anions of hard base forming the ionic bonding between the cations and the anions on the metal surface. 

DMFC research at LANE in FY 2002 has focused primarily on fundamental issues relevant to potential portable and transportation applications of direct methanol fuel cells, such as cathode and anode electrocatalysis, electrode composition and structure, membrane properties and MEA design. Substantial progress has been achieved in cathode research. 

In contrast to the situation during depositions from nitrate-based solutions, the addition of EY accelerated the reduction of O2 and the subsequent film growth [261]. Addition of EY even at a concentration as low as 1 mM largely enhanced the cathodic current With increasing additions of EY, the current systematically increased. The increase of current was caused by electrocatalysis of EY in the O2 reduction to different extent, however, when different electrode potentials were discussed [27] and led to an increased rate of ZnO deposition with a Earadaic efficiency of 100% [265]. Quite different mechanisms of electrocatalysis for the reduction of O2 were found relevant for EY in its neutral or reduced state [27]. EY/... 

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