Electrochemical applications oxygen reduction reaction

Carbon is unique among chemical elements since it exists in different forms and microtextures transforming it into a very attractive material that is widely used in a broad range of electrochemical applications. Carbon exists in various allotropic forms due to its valency, with the most well-known being carbon black, diamond, fullerenes, graphene and carbon nanotubes. This review is divided into four sections. In the first two sections the structure, electronic and electrochemical properties of carbon are presented along with their applications. The last two sections deal with the use of carbon in polymer electrolyte fuel cells (PEFCs) as catalyst support and oxygen reduction reaction (ORR) electrocatalyst. 

Sensor evaluations or fuel cell catalyst evaluations commonly use the oxygen reduction reaction and do not rec[uire the use of any external salt. The tip can use electrochemistry to detect products as they diffuse through porous membranes. Corrosion products may be able to undergo further electrochemical reactions, or could additionally benefit from using an ion-selective electrode as the tip. Many different applications can benefit from the ability to both control and monitor electrochemical reactions, with the added dimension of being able to provide spatial resolution thanks to the SECM. 

It is clear that the oxygen reduction reaction (ORR) is one of the most important electrochemical reactions since it has multiple applications. The potential fields range from the energy conversion to corrosion science. For this reason, it has been the subject of numerous works throughout the years. In the complete reduction of oxygen to water, there are four electrons exchanged. This high number of electrons... 

Due to the importance of electrochemical reduction of oxygen in life processes such as biological respiration and industrial applications including fuel cells or batteries, that reduction process and mechanism have been investigated over the last decades. The mechanism of the oxygen reduction reaction is highly complicated and dependent on a large number of factors, such as the electrode material, catalyst, solvent electrolyte and proton activity [1]. 

Electrocatalytic Reduction of Oxygen. Oxygen reduction reaction (ORR) occurs on the cathode side of low temperature fuel cells and heavily loaded Pt/C is the most common electrocatalyst. Replacement of ORR catalysts with less expensive materials would have higher technical impact than for anode catalysts. Transition metals loaded carbides and carbide-metal codeposited carbon have been investigated for ORR application. For example, 40 wt% Pt/WC electrocatalyst prepared with RDE electrode showed a cathodic current (-5 x 10 A) similar to that of 40 wt% Pt/C with 0.5 M H2SO4, 100 mv/s and 2000 rpm (160). Also, 40 wt% Pt/WC exhibited electrochemical stability after 100 cycles of cyclic voltammetry from 0 to 1.4 V (vs RHE), whereas the cathodic current of 40 wt% Pt/C disappeared after 100 cycles. 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

Ziegelbauer JM, Gatewood D, Gulla AF, Ramaker DE, Mukerjee S (2006) X-ray absorption spectroscopy studies of water activation on RhxSy electrocatalyst for oxygen reduction reaction application. Electrochem Solid State Lett 9 A430-A434... 

The oxygen reduction reaction at the cathode (Eq. 4) can be broken down into several steps gas-phase diffusion, oxygen adsorption and dissociation at the cathode surface, surface or bulk diffusion of oxygen atoms, and incorporation into the electrolyte [4,5]. Any of these steps can limit the rate of cathodic reaction. The reaction site distributes three dimensionally aroimd the triple-phase boimdary (TPB) of electrode, electrolyte, and gas phase, as illustrated in Figme 2. In practical applications, LSM is often used as a composite with YSZ particles to increase the electrochemical reaction site. As YSZ can make a separate ionic path, the reaction site is made three dimensionally inside the electrode layer [6]. 

In recent years, much attention has been focused on electrochemical studies of metalloporphyrins, not only as mimetic compounds of the iron porphyrin unit in heme proteins but also as potential electrocatalysts . Metalloporphyrins have been found to be applicable in both homogeneous and heterogeneous catalysis - and, because oxygen can be reduced directly through a 4-electron pathway on some transition metal porphyrins, catalysis in the heterogeneous electrochemical oxygen reduction reaction has received particular attention The application of metalloporphyrins to heterogeneous electrocatalysis requires their attachment to solid electrodes which can be realized based on chemisorption, chemical reactions with previously functionalized electrodes, chemical reactions with a functionalized polymer, incorporation of the porphyrin with the polymer film and electrochemical polymerization. 

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Among the huge number of applications of functionalized ECPs, when focusing on electrochemical properties, one has to deal with electrocatalysis. Several reactions, notably of biological interest, are rather sluggish and require catalysis to operate in mild conditions oxygen reduction, nitric oxide or... 

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