Membrane electrocatalytic

The electrocatalytic layers are contacted by gas diffusion layers (GDLs), allowing the gases, H2 and O2, as reactants to be passed to the interface and liquid H2O as product removed fi om the interface. These GDLs consist of sheets of carbon cloth or paper with typically 100 pm thickness and 50 % porosity (Fig. 2). This arrangement of membrane, electrocatalytic layers, and gas diffusion layers is colloquially called membrane-electrode-assembly (MEA). 

The concept of a promoter can also be extended to the case of substances which enhance the performance of an electrocatalyst by accelerating the rate of an electrocatalytic reaction. This can be quite important for the performance, e.g., of low temperature (polymer electrolyte membrane, PEM) fuel cells where poisoning of the anodic Pt electrocatalyst (reaction 1.7) by trace amounts of strongly adsorbed CO poses a serious problem. Such a promoter which when added to the Pt electrocatalyst would accelerate the desired reaction (1.5 or 1.7) could be termed an electrocatalytic promoter, or electropromoter, but this concept will not be dealt with in the present book, where the term promoter will always be used for substances which enhance the performance of a catalyst. 

D. Eng, and M. Stoukides, Catalytic and Electrocatalytic Methane Oxidation with Solid Oxide Membranes, Catalysis Reviews - Science and Engineering 33, 375-412 (1991). 

The catalytic-electrocatalytic reactor consists of a membrane electrode assembly, such as Pt-black/Nafion/Pd/C sandwiched between sheets of porous carbon cloth, housed in a fuel cell assembly. 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

Apart from the problems of low electrocatalytic activity of the methanol electrode and poisoning of the electrocatalyst by adsorbed intermediates, an overwhelming problem is the migration of the methanol from the anode to the cathode via the proton-conducting membrane. The perfluoro-sulfonic acid membrane contains about 30% of water by weight, which is essential for achieving the desired conductivity. The proton conduction occurs by a mechanism (proton hopping process) similar to what occurs... 

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

Figure 17.16 Electrocatalytic H2 oxidation by Ralstonia metallidurans CH34 membrane-bound hydrogenase on a PGE RDE in the presence of O2. The eiectrode is rotated at 2000 revmin and poiarized at +0.142 V vs, SHE in buffered aqueous solution at pH 5.6 and 30 °C, close to 1 bar H2. Reprinted with permission from Vincent et al. [2007]. Copyright (2007) American Chemical Society.

G. D Arrigo, C. Spinella, G. Arena, and S. Lorenti. Fabrication of miniaturized Si-based electrocatalytic membranes. Materials Science and Engineering C 23 (2003) 13-18. 

Concentrating on the operation of the so-called membrane electrode assembly (MEA), E includes irreversible voltage losses due to proton conduction in the PEM and voltage losses due to transport and activation of electrocatalytic processes involved in the oxygen reduction reaction (ORR) in the cathode catalyst layer (CCL) ... 

This section addresses the role of chemical surface bonding in the electrochemical oxidation of carbon monoxide, CO, formic acid, and methanol as examples of the electrocatalytic oxidation of small organics into C02 and water. The (electro)oxidation of these small Cl organic molecules, in particular CO, is one of the most thoroughly researched reactions to date. Especially formic acid and methanol [130,131] have attracted much interest due to their usefulness as fuels in Polymer Electrolyte Membrane direct liquid fuel cells [132] where liquid carbonaceous fuels are fed directly to the anode catalyst and are electrocatalytically oxidized in the anodic half-cell reaction to C02 and water according to... 

It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated on an electrode catalyzes electrocatalytic proton reduction,215 but the activity was not very high. We have found that metal porphyrins and metal phtahlocyanines when incorporated into a polymer membrane coated on an electrode show high activity in electrocatalytic proton reduction to produce H2.22,235 Some data are summarized in Table 19.2. It was shown that this catalyst is more active than a conventional platinum base electrode. 

It was found that when metal phthalocyanine (MPc) is incorporated into a poly(vinylpyridine) membrane, it works as a very active catalyst for electrocatalytic C02 reduction to produce carbon monoxide.255 By investigating visible spectral change of CoPc in a poly(vinylpyridine) membrane coated on a graphite electrode by an in situ potential step chronospectrometry (PSCS) during... 

Fig. 19.3 Relationship between turnover number (TN) of Ru-red confined in a coaed iNafior membrane for electrocatalytic 02 evolution and the catalyst concentration ir the absence of amino acid model compound (a), in the presence of 5.0 x 1U 2Mp-Cre (b), and witl toluene (c). The solid line and dashed line are calculated curve .. The dash-dotted curve (I) is a simulated curvt in the presence of / -Cre when r0 and rd are the same as those in the absenc< of p-Cre, and curve (II) is a simulated curve in the presenci of p-Cre when r0 and kQ2 an the same as those in the absence ofp-Cre.

For laboratory-scale reactions, this electrocatalytic AD generally is performed in a glass H-type cell in which the anode and cathode compartments are separated by a semipermeable Nafion cation-exchange membrane and platinum electrodes are used. A 5% aqueous solution of phosphoric acid is used in the cathode compartment, and the reaction in the anode compartment is stirred vigorously, Under a controlled anode potential of 0.4 V (vs. Ag/AgCl) and with (DHQD)2-PHAL as chiral ligand, a-methylstyrene was converted to 7 -2-pheny 1-1,2-propanediol in 15 h with the electrical consumption of 2.1 F/mol. The product was isolated in 100% yield with 92% ee [ 37],... 

T. Ikeda, K. Miki, F. Fushimi, and M. Senda, Electrocatalytic oxidation of D-gluconate at a ubiquinone-mixed carbon paste electrode with an immobilized layer of D-gluconate dehydrogenase from bacterial membranes, Agric. Biol. Chem., 51 (1987) 747-754. 

The multilayer membrane coverage (of Fig. 6.6) improves the relative surface availability of oxygen and excludes potential interferences (common at the potentials used for detecting the peroxide product). Electrocatalytic transducers based on Prussian Blue layers (15) or metallized carbons (16), which preferentially accelerate the oxidation of hydrogen peroxide, are also useful for minimizing potential interferences. The enzymatic reaction can also be followed by monitoring the consumption of the oxygen cofactor. 

SECM is a powerful tool for studying structures and heterogeneous processes on the micrometer and nanometer scale [8], It can probe electron, ion, and molecule transfers, and other reactions at solid-liquid, liquid-liquid, and liquid-air interfaces [9]. This versatility allows for the investigation of a wide variety of processes, from metal corrosion to adsorption to membrane transport, as discussed below. Other physicochemical applications of this method include measurements of fast homogeneous kinetics in solution and electrocatalytic processes, and characterization of redox processes in biological cells. 

The SECM capacity for rapid screening of an array of catalyst spots makes it a valuable tool for studies of electrocatalysts. This technique was used to screen the arrays of bimetallic or trimetallic catalyst spots with different compositions on a GC support in search of inexpensive and efficient electrocatalytic materials for polymer electrolyte membrane fuel cells (PEMFC) [126]. Each spot contained some binary or ternary combination of Pd, Au, Ag, and Co deposited on a glassy carbon substrate. The electrocatalytic activity of these materials for the ORR in acidic media (0.5 M H2S04) was examined using SECM in a rapidimaging mode. The SECM tip was scanned in the x—y plane over the substrate surface while electrogenerating 02 from H20 at constant current. By scanning... 

There are very few studies based on this approach, but it was demonstrated that using nanostructured carbon-based electrodes, it is possible to electrocatalytically reduce C02 in the gas phase using the protons flowing through a membrane [38], Long-chain hydrocarbons and alcohols up to C9 CIO are formed, with preferential formation of isopropanol using carbon-nanotube-based electrodes [14, 39]. Productivities are still limited, but these results demonstrate the concept of a new approach to recycle C02 back to fuels. 

Baker R (2008) Substituted iron phthalocyanines electrocatalytic activity towards 02 reduction in a proton exchange membrane fuel cell cathode environment as a function of temperature. M.A.Sc. diss. The University of British Columbia, Canada... 

There is reason to pause here, and refer back to the electrocatalysis discussion in the introduction. Best electrocatalysis yields maximum power density from minimum platinum loading. For a cathode platinum particle to be electrocatalytically effective, it must be in electrical contact with the conducting porous electrode structure. It must also be in contact with the membrane, or a biconductor layer on the membrane surface. 

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