Electrocatalysts requirements

Understanding the activity and selectivity properties of electrocatalysts requires the characterization of catalyst surfaces, determination of adsorption characteristics, identification of surface intermediates and of all reaction products and paths, and mechanistic deliberation for complex as well as model reactions. Electrochemical and classical methods for adsorption studies are well documented in the literature (5, 7-9, 25, 24, 373. Here, we shall outline briefly some prominent electrochemical methods and some nonelectrochemical techniques that can provide new insight into electrocatalysis. Electrode kinetic parameters can be determined by potentionstatic methods using the methodology of Section II1,D,3. 

Based on Eq. (5) one can compare two electrocatalytic materials. For example, if particular electrode reaction takes place on two different electrode materials with same current density, more effective electrocatalyst requires lower overvoltage. If overvoltage is the same for both materials, higher current density will be measured at more efifeetive electrocatalyst. Now, let us consider the power (/ ) necessary for electrode reaction to take place in electrolytic cell, or power gained by some power source. In both cases general expression is ... 

The alcohol tolerance of O2 reduction by bilirubin oxidase means that membraneless designs should be possible provided that the enzymes and mediators (if required) are immoblized at the electrodes. Minteer and co-workers have made use of NAD -dependent alcohol dehydrogenase enzymes trapped within a tetraaUcylammonium ion-exchanged Nafion film incorporating NAD+/NADH for oxidation of methanol or ethanol [Akers et al., 2005 Topcagic and Minteer, 2006]. The polymer is coated onto an electrode modified with polymethylene green, which acts as an electrocatalyst... 

Nonenzymatic regeneration of NAD(P)H requires the regioselective transfer of two electrons and a proton to NAD(P)+. Various rhodium(III) complexes are effective electrocatalysts capable of mimicking hydrogenase enzymes.48-54... 

The reduction of C02 can be driven electrochemically at metallic cathodes, however, it requires large overpotentials (<—1.5 V) and electrode poisoning often occurs.65 Those problems can be addressed by adding catalysts. Metal complexes are a priori good candidates as electrocatalysts. It is expected that their reduction will be accompanied by the appearance of a vacant coordination site able to bind C02 and thus activate its reduction in the metal coordination sphere.1... 

Synchotron based techniques, such as surface X-ray scattering (SXS) and X-ray absorption spectroscopy (XAS), have found increased use in characterization of electrocatalysts during electrochemical reactions.37 These techniques, which can be used for characterization of surface structures, require intricate cell designs that can provide realistic electrochemical conditions while acquiring spectra. Several examples of the use of XAS and EXAFS in non-precious metal cathode catalysts can be found in the literature.38 2... 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Some Ni(II) complexes show catalytic activity for the electrocatalytic reduction of C02 in water, where an intermediate formation of Ni(I) species has been proposed. To obtain a useful electrocatalyst in the electroreduction of C02, the selectivity of the process is highly important. As many electrochemical systems available for reducing C02 require the presence of water, the reduction of molecular hydrogen is always a competing reaction that needs to be avoided. 

Despite extensive studies, the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. The main reason is that it is very difficult to meet all requirements for high efficiency. For example, high catalytic activity and sufficient passivation at the electrode surface are incompatible. It was found, however, that a semiconductor electrode modified with small metal particles can meet all the requirements and thus becomes an ideal type semiconductor electrode. Cu, Ag, and Au were chosen because they were reported to work as efficient electrocatalysts for the C02 reduction. p-Si electrodes modified with these metals in C02-staurated aqueous electrolyte under illumination produce mainly methane and ethylene.178 This is similar to the metal electrodes but the metal-particle-coated electrodes work at approximately 0.5 V more positive potentials, contrary to continuous metal-coated p-Si electrodes. 

Table 13.2 summarises the different approaches used to construct enzyme electrochemical biosensors for application to food analysis based on the different types of enzymes available. Generally, the main problems of many of the proposed amperometric devices have been poor selectivity due to high potential values required to monitor the enzyme reaction, and poor sensitivity. Typical interferences in food samples are reducing compounds, such as ascorbic acid, uric acid, bilirubin and acetaminophen. Electrocatalysts, redox mediators or a second enzyme coupled reaction have been used to overcome these problems (see Table 13.2), in order to achieve the required specifications in terms of selectivity and sensitivity. 

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