Cathode electrode degradation

The two major mechanisms that will be referred to a number of times throughout this chapter are briefly introduced here, including cathode electrode degradation and membrane degradation. 

Visual detection of surface layers on cathodes using microscopy techniques such as SFM seems to be supportive of the existence of LiF as a particulate-type deposition.The current sensing atomic force microscope (CSAFM) technique was used by McLarnon and co-workers to observe the thin-film spinel cathode surface, and a thin, electronically insulating surface layer was detected when the electrode was exposed to either DMC or the mixture FC/DMC. The experiments were carried out at an elevated temperature (70 °C) to simulate the poor storage performance of manganese spinel-based cathodes, and degradation of the cathode in the form of disproportionation and Mn + dissolution was ob-served. °° This confirms the previous report by Taras-con and co-workers that the Mn + dissolution is acid-induced and the electrolyte solute (LiPFe) is mainly responsible. 

Another issue with platinum catalysts is that their capacity sometimes fades over time. Several factors are responsible, including a phenomenon similar to the side effects described for medications in chapter 3. Side effects occur when a medication acts on healthy tissue instead of the intended target. With platinum electrodes, the problem is that sometimes unwanted reactions occur at the electrodes. In the oxygen reactions taking place at the cathode, for example, hydroxide (OH) and other molecules sometimes form and bind to the platinum atoms. These molecules cover the platinum atoms and block access to the desired reactant, thereby reducing the catalytic activity. Sometimes the molecules even pull platinum atoms away from the surface, causing serious electrode degradation. 

As shown in Fig. 11.7, the extent of cathode thinning can be monitored by cross section measurements via SEM or optical microscopy [23]. It is accompanied by a loss of electrode void volume as is illustrated in Fig. 11.8, showing high-resolution SEM cross sections of a nondegraded cathode electrode (left-hand-side) and of a degraded cathode (right-hand-side). 

Yokokawa et al. [8] made thermodynamic analyses on the interaction between cathodes/anodes and gaseous impurities. By comparison between the thermodynamic reactivity and the electrode degradation behaviors, they have extracted... 

It is preferable for the cathode interface to have a low WE contact for efficient electron extraction. Low WF metals, such as calcium (Ca), barium (Ba) or magnesium (Mg), are usually inserted into the interfaee between Al and organic active layer to improve the device performance. " However, the low WF metal is vulnerable to oxidation under ambient eonditions, and electrode degradation is a major concern for this type of deviee. Therefore, the development of new interfacial materials to use as a eathode interlayer is still required. 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

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