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An overview of the electrochemical reduction of oxygen at carbon-based modified electrodes

Journal of the Iranian Chemical Society (Impact Factor: 1.41). 03/2005; 2:1-25. DOI: 10.1007/BF03245775

ABSTRACT We present an overview of the electrochemical reduction of oxygen in water, focussing on carbon-based and modified carbon electrodes. This process is of importance for gas sensing, in fuel cells and in the electrosynthesis of hydrogen peroxide.

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    ABSTRACT: Hydrogen peroxide is a valuable chemical used in various applications and there is a demand for small on-site production plants. An attractive way to produce hydrogen peroxide is by electrochemical reduction of oxygen, using electrocatalytic materials that favour formation of hydrogen peroxide instead of water. Oxygen reduction on carbon in alkaline solution is known to produce mainly hydrogen peroxide but the overpotential is large and electrocatalytic materials need to be used in combination with carbon. In the present study, the mechanism of oxygen reduction in alkaline solution was studied using NiO, Ni 0.75 Co 0.25 O and CoO powders in a matrix of carbon paste. The desired product is hydrogen peroxide and the rotating ring disc technique was used to measure the amount of hydrogen peroxide formed. Two separate processes are observed with a peak shaped wave at low overpotentials and a sigmoidal process at high overpotentials. The charge involved in the first process and the heterogeneous rate constant for the second process were determined and found to be higher in the presence of oxide compared to pure carbon paste. Maximum increase was found for the NiO containing electrode with five times higher charge in the low overpotential region and 25 times higher rate constant in the high overpotential region. The mechanism of oxygen reduction comprises redox mediated electron transfer reactions involving Ni(II)/Ni(III) states on the surface. In the low overpotential region, where oxygen reduction on carbon is mediated by native quinone groups, the increased activity is explained by an interplay between the quinone and Ni(OH) 2 /NiOOH redox couples.
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    ABSTRACT: Oxygen (O 2) 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 O 2 to H 2 O, and the 2-electron reduction pathway from O 2 to hydrogen peroxide (H 2 O 2). In non-aqueous aprotic solvents and/or in alkaline solutions, the 1-electron reduction pathway from O 2 to superoxide (O 2 -) 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 O 2 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 O 2 reduction reaction processes, simple ORR kinetics, and conventional techniques for electrochemical measurements. 2.1.1 Electrochemical O 2 Reduction Reactions [2, 3] Table 2.1 lists several typical ORR processes with their corresponding thermodynamic electrode potentials at standard conditions. The mechanism of the electrochemical O 2 reduction reaction is quite complicated and involves many 90 C. Song and J. Zhang intermediates, primarily depending on the natures of the electrode material, catalyst, and electrolyte. The mechanism catalyzed by different catalysts is discussed in detail later in this chapter. Table 2.1. Thermodynamic electrode potentials of electrochemical O 2 reductions [2, 3] a, b: The thermodynamic potentials for the 1-electron reduction reaction to form a superoxide, and its further reduction to O 2 2-, are not listed in Table 2.1 because their values are strongly dependent on the solvent used. In Table 2.1, the reduction pathways such as the 1-, 2-, and 4-electron reduction pathways have unique significance, depending on the applications. In fuel cell processes, the 4-electron direct pathway is highly preferred. The 2-electron reduction pathway is used in industry for H 2 O 2 production. The 1-electron reduction pathway is of importance in the exploration of the ORR mechanism.

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