Electrocatalysis operating conditions

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

Although in situ infrared spectroscopy has been applied widely in terms of the systems studied, the reflective electrodes employed have been predominantly polished metal or graphite, and so an important advance has been the study of electrochemical processes at more representative electrodes such as Pt/Ru on carbon [107,122,157], a carbon black/polyethylene composite employed in cathodic protection systems [158] and sol-gel Ti02 electrodes [159]. Recently, Fan and coworkers [160] took this concept one step further, and reported preliminary in situ FTIR data on the electro-oxidation of humidified methanol vapor at a Pt/Ru particulate electrode deposited directly onto the Nafion membrane of a solid polymer electrolyte fuel cell that was mounted within the sample holder of a diffuse reflectance attachment. As well as features attributable to methanol, a number of bands between 2200 and 1700 cm were observed in the spectra, taken under shortoperating conditions, the importance of which has already been clearly demonstrated [107]. 

Enzymes are efficient catalysts for cathodic and anodic reactions relevant to fuel cell electrocatalysis in terms of overpotential, active site activity, and substrate/reaction specificity. This means that design constraints (e.g., fuel containment and anode-cathode separation) are relaxed, and very simple devices that may take up ambient fuel or oxidant from their environment are possible. While operation is generally confined to conditions close to ambient temperature, pressure, and pH, and power densities over about 10 mW cm are rarely achieved, enzyme fuel cells may be particularly useM in niche environments, for example scavenging trace H2 released into air, or sugar and O2 from blood. Thus, trace or unusual fuels become viable for energy production. 

The bond strength measured or estimated at a constant temperature or at a specified facet of single-crystal planes are, of cause, important factors in the discussion of electrocatalysis. But, if they are obtained under different conditions from the practical ones, the conclusion might not be correct. The author believes that the general understanding on the size effect still needs further experimental and theoretical studies. Particularly careful attention should be paid to the operation temperature, the coagulation of nanoparticles during the experiment, or the territory. Of course, evaluation on the stability of such small particles is essential in the practical application. 

Porous metallic structures have been used for electrocatalysis (Chen and Lasia, 1991 Kallenberg et al., 2007). Porous electrodes are made with conductive materials that can degrade under high temperatures at high anodic potential conditions. This last problem is of less importance for fuel cell anodes, which operate at relatively low potentials, but it can be of importance for electrochemical reactors. Porous column electrodes prepared by packing a conductive material (carbon fiber, metal shot) forming a bar are frequently used. Continuous-flow column electrolytic procedures can provide high efficiencies for electrosynthesis or removal of pollutants in industrial situations. Theoretical analysis for the electrodeposition of metals on porous solids has been provided by Masliy et al. (2008). 

A study of electrocatalysis is also important to understand the mechanism of the reaction that operates the process. Since the reaction mechanisms are very difficult to discern, the possibility of the variation of different parameters, such as the electrode potential, the nature of the electrode material, the electrolyte composition, etc., brings into focus a wide possibility of pathways. The fixing of the experimental conditions allows us to choose the appropriate route for substance preparation, complete energy conversion, inhibition of the corrosion process, and eliminating the side reaction pathways. 

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... 

It is evident therefore that it is now possible to characterize electrode-solution interfaces at the molecular level fundamental measurements can be made on systems of practical importance. The significance of the development of these methods (as well as of other new methods in the pipeline ) lies at least in part in the scope this gives for the design and selection of appropriate conditions for electrocatalysis, synthesis, corrosion inhibition, metal plating, operation of battery systems etc. 

Chemical reactions are temperature sensitive, and indeed, chemical rate constants and reactions mechanism are expected to vary considerably with temperature. Most investigations on the electrocatalysis of the ORR are usually performed at ambient conditions, which do not necessarily represent the behavior of the materials and the reaction at the conditions of practical interest. For example, in proton exchange membrane fuel cells, the temperature of operation is between 80 and 100 °C. Significant discrepancy in behavior may arise if reactions and materials are tested at ambient conditions and their behavior at high temperatures is merely deduced firom extrapolation. Schafer et al. introduced variable temperature SECM, with an operational range of 0-100 °C, by integrating a temperature control unit (Peltier element) into an SECM setup, as shown in the schematic of Fig. 23 [66]. At the heart of the temperature control unit is the Peltier element, which is housed in a stainless steel block. 

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