Electrode Material and Structure

Electrochemical reactions at the electrode-electrolyte interface are surface phenomena and require large exposed solid surface area as reaction sites. In order to achieve large active surface area and for efficient transport of reactant gases to the reaction sites, the electrodes are made in the form of a highly porous structure. The pore structure typically used in PEMFC is in the form of a macro- or microporous carbon cloth or paper through which reactant gas diffuses toward the interface. The electrodes are characterized by the thickness and pore structure. 

Another important aspect of the electrodes is the use of catalyst to accelerate hydrogen oxidation and oxygen reduction reaction at the anode and cathode electrodes. The catalyst loading is characterized by the mass of catalyst (mg) per unit surface area (cm ), that is, mg/cm of the electrode. In the early design of an electrode in PEMFC, the catalyst layers are applied to the gas 

Impregnation of electrode within a thin layer of electrolyte membrane forms a three-phase active reaction zone or triple-phase boundaries (TPBs). Kim et al. (1995) developed a Nafion-impregnated electrode with a platinum loading of 0.4 mg/cm and a Nafion content of 0.6 mg/cm.  

Platinum catalyst particles are supported on larger and finely divided carbon particles. A carbon-based power XC-72 is commonly used. The reaction regions are characterized by the active surface area where electrode, electrolyte, and catalyst are present. 

This carbon-supported catalyst is then fixed in a fhin layer on the electrode surface to enhance the electrochemical reaction and to reduce activation 

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Complex investigations on electrode materials and electrode kinetics were carried out with the aim to elaborate electrodes with high electrochemical activity. The route and mechanism of the electrode reactions, dependence of polarization on electrode materials and structure, gas atmosphere, and other factors were studied. As a result of this research the electrodes based on Ni, Co, Cu, manganite, and cobaltite having high working characteristics were elaborated. 

For the first time, a totally solid-state electric double layer capacitor (EDLC) was fabricated using PEO-KOH-H2O as the SPE and the polymer electrolyte could replace large amount of liquid KOH electrolyte [17,18]. The ideal rectangular shape of cyclic voltammety result for this solid-state EDLC was obtained, and the real value of specific capacitance was 90 F g". It was only slightly lower than that of liquid electrolyte supercapacitor, and it might be related to the electrode material and structure. 

Since gas aossover is the fundamental mechanism for membrane chemical degradation, lowering the gas crossover rate can potentially reduce membrane chemical degradations. As discussed earher, hydrocarbon membranes can effectively reduces gas crossover rate (Aoki et al. 2006a). Further development of membrane/electrode materials and structures can potentially lower the amount of gas aossover and overall degradation rate. 

Use of materials and structures that maximize utilization of reactants are preferred, as this allows the electrodes to be thin while making the most economical use of the consumable electrode. Factors that limit utilization include a tendency for passivation of the active component (by formation of a uniform, insoluble, non-conductive product over the active surface), and formation of islands (electrical isolation of one or more active portions by non-uniform current distribution). 

The previous sections dealt with a generalized theory of heterogeneous electron-transfer kinetics based on macroscopic concepts, in which the rate of the reaction was expressed in terms of the phenomenological parameters, and a. While useful in helping to organize the results of experimental studies and in providing information about reaction mechanisms, such an approach cannot be employed to predict how the kinetics are affected by such factors as the nature and structure of the reacting species, the solvent, the electrode material, and adsorbed layers on the electrode. To obtain such information, one needs a microscopic theory that describes how molecular structure and environment affect the electron-transfer process. 

Within the last two decades Electron Spin Resonance-(ESR) spectroscopy has become a standard experimental technique in electrochemical research. The main interest was in the field of electrochemical generation of radicals to characterize their structure by ESR spectroscopy or to prove their presence in electrode reactions. The studies have been extended to the kinetics of radical reactions and the set up of reaction mechanism, to the solvation phenomena in radical electron densities and to radical conformation and ion complex structure. The latest development is the study of the electrode materials and their surface layers in electrochemical systems by simultaneous ESR spectroscopic and electrochemical measurements, e.g., of polymer modified electrodes. 

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