Cathodes oxygen reduction paths

Fig. 5. (a) Working principle of a solid oxide fuel cell, (b) Sketch of possible reaction paths of the oxygen reduction reaction, taking place on a particle of a solid oxide fuel cell cathode. 

Fig. 43. Double-logarithmic plot of the electrode polarization resistance versus the microelectrode diameter measured with impedance spectroscopy (ca. 800 °C) at (a) a cathodic dc bias of -300 mV, and (b) at an anodic dc bias of +300 mV. In (b) the first data point of the 20-pm microelectrode is not included in the fit. (c) Sketch illustrating the path of the oxygen reduction reaction for cathodic bias, (d) Path of the electrochemical reaction under anodic bias the rate-determining step occurs close to the three-phase boundary.

The processes that take place in the pit and in its vicinity are shown in Fig. 20M(a). At the concentration of chloride ions found in seawater, the passive layer on aluminum breaks down, and anodic dissolution of the metal can occur. This happens mostly inside the pit, where the supply of oxygen is slow. On the other hand, oxygen reduction can readily take place on the surface of the metal outside the pits, where its diffusion path is short. Thus, the cathodic area is typically... 

Horita, T., Yamaji, K., Sakai, N., Yokokawa, H., Kawada, T., Kato, T. Oxygen reduction sites and diffusion paths at Lao <>Sro iMnO j x/yttria-stabilizcd zirconia interface for different cathodic overvoltages by secondary-ion mass spectrometry. Solid State Ionics 2000,127, 55-65. 

Because of the irreversible and not well-understood change of the electrocatalyst surface above 1.0 V, early mechanistic studies were conducted under ill-defined conditions. Thus, while anodic evolution of Oj takes place always in the presence of oxygen-covered electrodes, the cathodic reaction proceeds on either oxygen-covered or oxygen free surfaces with different mechanisms (77,158). The electrochemical oxide path, proposed for oxide-covered platinum metals in alcaline electrolytes (759,160), has been criticized by Breiter (7), in view of the inhibition of oxygen reduction by the oxygen layers. Present evidence points to the peroxide-radical mechanism (77,... 

In the case of direct current (DC) interference, a cathodic reaction (e. g. oxygen reduction or hydrogen evolution) takes place where the current enters the buried structure, while an anodic reaction (e. g. metal dissolution) occurs where the current returns to the original path, through the soil (Figure 9.1). Metal loss results in the anodic points, where the current leaves the structure usually, the attack is localised and can have serious consequences especially on pipelines. Effects of AC stray current are more complex however, alternate currents are known to be much less dangerous than direct ones. 

There are four environmental requirements that must be met for an electrochemical corrosion process to proceed an electron path, an ion path, a cathode, and an anode [2]. The electron path can be as obvious as an external circuit cormection between the anode and the cathode in an experimental setup or more subtly the transport of free electrons from an anodic to a cathodic site on a corroding metal surface. The ionic path refers to the medium that transports cations away from the anodic site and anionic or other reactive species transport to cathodic reaction sites. The cathode can be a separate material or a temporary reaction site on the corroding metal where a reduction reaction—such as the oxygen reduction reaction or hydrogen evolution reaction—occurs. 

Figure 25A shows a cyclic voltammetric i-E curve for 25 pM horse heart cytochrome c in 1 mM Tris HCl buffer (pH 7) containing 20 mM NaCl [124]. The measurement was made in a specially constructed, thin-layer electrochemical cell with a path length of approximately 100 pm and a cell volume of approximately 6 pL. The diamond OTE was a free-standing diamond disk (380 pm thick and 8 mm in diameter). The scan was initiated at 0.15 V and recorded at 2 mV/s. The oxidation and reduction peaks present at-0.075 and 0.010 V(A p = 85 mv) are not as well defined as they are for other redox systems at this same electrode (e.g., ferrocene). The cathodic current at -0.15 V is believed to be due to some residual oxygen reduction, as the thin layer cell could not be efficiently deoxygenated with nitrogen. 

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]. 

As the oxygen ions are transported across the electrolyte between adjacent anode and cathode, the ionic conduction path is mainly determined by the size of the inter-electrode gap. Several experimental studies performed under different operating conditions (gas composition, temperature) and for cells constructed from different cell component materials showed that a decrease in the inter-electrode gap leads to a decrease in the ohmic cell resistance and an increase in power output [9, 20, 62, 68]. These observations were also confirmed by a computational analysis where maximum cell performance was obtained for the smallest gap size studied [64]. The OCV, however, was found to be independent of changes of the inter-electrode gap [20]. When the anode and cathode are located in grooves in the electrolyte, ions can also be transferred through the electrolyte material filling the inter-electrode gap, and a reduction in ohmic cell resistance can be achieved [65]. 

The key point of the MEA development for the PEFC is how to promote the reactions on the surface of the platinum catalyst in the CLs, particularly at the cathode. To increase the reaction area that can contribute to the catalyst reduction, the supply channels of oxygen and hydrogen to the reactive sites, the conductive channels for protons, and the exit paths for the generated water all become important. In the present essay, mainly the development history and the current design for the reduction of Pt in the CL will be described. 

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