Film-based electrocatalyst

Another film-based electrocatalyst which we have investigated involves oxidatively electropolymerized Ni(TAP) (where TAP is... 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

Constantinos G. Tsiafoulis et al. report the electrochemical behaviour of a composite film based on ferrocene intercalated V205.nH20 xerogel (FeCp2-VXG) with photocrosslinkable polyvinyl alcohol with styrylpyridinium residues (PVA-SbQ), in order to be used as an electrocatalyst and host protein platform to develop an amperometric biosensor. 

Finally, the oxidation of D-glucose at Pt-based electrocatalysts incorporated in polypyrrole [55,56] or in polyaniline [57] was also considered. The first work [55] was carried out in Pt-doped polypyrrole films in a neutral medium (phosphate buffer) in view of biosensor applications. Then the use of Pt-Pd catalysts dispersed in PPy led to higher current densities of glucose oxidation than on pure metal dispersed in PPy. This may be related to the decrease of catalytic poisoning (by adsorbed CO as shown by infrared reflectance spectroscopy [58]), due to the presence of Pd. 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

Prussian blue-based nano-electrode arrays were formed by deposition of the electrocatalyst through lyotropic liquid crystalline [144] or sol templates onto inert electrode supports. Alternatively, nucleation and growth of Prussian blue at early stages results in nano-structured film [145], Whereas Prussian blue is known to be a superior electrocatalyst in hydrogen peroxide reduction, carbon materials used as an electrode support demonstrate only a minor activity. Since the electrochemical reaction on the blank electrode is negligible, the nano-structured electrocatalyst can be considered as a nano-electrode array. 

Zhao et al. prepared magnetite (FesO nanoparticles modified with electroactive Prussian Blue [44]. These modified NPs were drop-cast onto glassy-carbon electrodes. They observed the redox processes commonly observed for PB (similar to that seen in Figure 4.8), and also demonstrated that the Prussian White material produced by PB reduction at 0.2 V served as an electrocatalyst for Fi202 reduction. They also prepared LbL films in which PB NPs and glucose oxidase were alternated between PD DA layers [99]. These were demonstrated to act as electrocatalysts for Fi202 reduction. Based on the ability to sense the product of the enzymatic reaction, these structures were shown to act as glucose sensors. 

Three techniques have been described in the literature to prepare combinatorial libraries of fuel cell electrocatalysts solution-based methods [8, 10-14], electrodeposition methods [15-17] and thin film, vacuum deposition methods [18-21]. Vacuum deposition methods were chosen herein for electrocatalyst libraries in order to focus on the intrinsic activity of the materials, e.g., for ordered or disordered single-phase, metal alloys. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

Other complications that arise are (a) that the surface compositions of glassy metals to be used as electrocatalysts are rarely identical with the corresponding bulk compositions, as was shown in recent Auger surface analysis experiments by Vracar and Conway (134), and (b) that when such alloys are used as anodes for O2 evolution in water electrolysis an oxide film of appreciable thickness is formed, and the distribution of elements of the alloy in the film is not usually the same as in the parent metal owing to some preferential anodic leaching of any base-metal components that are present in the alloy. 

Figure 11 Tafel plots for the electro-oxidation of 0.1 M ethanol in 0.1 M HCIO4 on different Pt-based electrodes dispersed in a 0.5-pm PAni film containing 600 pgcm of electrocatalysts. 

A typical example includes the yttria-stabilized-zirconia-based high-temperature potentiometric oxygen sensor which is widely used in automotive applications. Platinum thick films are applied, forming both the cathode and anode of the sensor. The thick electrode has a porous structure which provides a larger electrode surface area compared to non-porous structures. For current measurement, a porous electrode is desirable since it leads to a larger current output. If the metallic film serves as the electrocatalyst, a porous structure is also desirable, for it provides more catalytic active sites. On the other hand, electrodes formed by the thick-film technique do not have an exact, identical... 

A general approach for incorporating electrocatalysts into p-chlorosulfonated polystyrene films, devised by Ellis and Meyer, is based on the reactivity of the chlorosulfonate group attached to the polymer film deposited on the electrode surface toward compounds, which are selected to be redox-active, containing amino, hydroxy, and carboxylate groups. Various metal redox complexes have been incorporated into polystyrene films in this way. ... 

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