Polymeric electrocatalysts

Finally, it has been reported that carbon electrodes modified with thin polymeric films of polypyridyl metal complexes containing a dispersion of metal particles (Rh° or Pd°) can be used as electrocatalyst for reduction of C02 to hydrocarbons in MeCN. Apparently CH4 is the dominant reduction product (up to 18% of faradaic efficiency).123,124 It should be noted that the product distribution is reminiscent of a Fischer-Tropsch process since C2, C3, and C4 hydrocarbons are also formed. 

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

The porous electrodes used in PAFCs are described extensively in the patent literature (6) see also the review by Kordesch (5). These electrodes contain a mixture of the electrocatalyst supported on carbon black and a polymeric binder, usually PTFE (about 30 to 50 wt%). The PTFE binds the carbon black particles together to form an integral (but porous) structure, which is supported on a porous carbon paper substrate. The carbon paper serves as a structural support for the electrocatalyst layer, as well as the current collector. A typical carbon paper used in PAFCs has an... 

Advances in fuel cell technology over the last four decades have come primarily from improved electrocatalysts, membrane electrode assembly fabrication strategies, and cell/stack/system engineering. Apart from Nafion, new ion conducting polymeric materials have played only a minor role in significantly increasing cell performance. However, new materials... 

Electrocatalysis of proton reduction by metal complexes in solution has been widely studied [106-111] and confinement of molecular electrocatalysts for this process in polymeric films has also received some attention [111, 112]. This area has received much impetus from biochemical and structural studies of the iron-only... 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Phosphoric-acid fuel cell (PAFC) — In PAFCs the -> electrolyte consists of concentrated phosphoric acid (85-100%) retained in a silicon carbide matrix while the -> porous electrodes contain a mixture of Pt electrocatalyst (or its alloys) (-> electrocatalysis) supported on -> carbon black and a polymeric binder forming an integral structure. A porous carbon paper substrate serves as a structural support for the electrocatalyst layer and as the current collector. The operating temperature is maintained between 150 to 220 °C. At lower temperatures, phosphoric acid tends to be a poor ionic conductor and poisoning of the electrocatalyst at the anode by CO becomes severe. 

Fig. 23. Potential-current curves for some metal ligand electrocatalysts (284) solid lines, porphyrins dashed line, polymeric phthalocyanine.

Polymeric phthalocyanine films prepared from 51a have been investigated as electrocatalysts, in biological applications and as amperometric biosensors [157]. The electropolymerized films exhibited better activities than analogous low molecular weight phthalocyanine films. Some examples are given ... 

Preformed polymers coated on a variety of electrode surfaces have found application in electroanalysis. Polymeric films containing coordinated redox groups were among the first PMEs developed because of their obvious potential as electrocatalysts. Preformed polymers are normally adsorbed on the surface of the electrodes in a mechanism assumed to involve a combination of adsorption processes and the insolubility of the polymer in the electrolyte being used. 

De Souza REB, Elausino AEA, Rascio DC, Oliveira RTS, Teixeira Neto E, Calegaro ML, Santos MC (2009) Ethanol oxidation reaction on PtCe02/C electrocatalysts prepared by the polymeric precursor method. Appl Catal B Environ 91 516-523... 

Interest in organometallic maaomolecules has grown exponentially ever since Arimoto and Haven first polymerized vinylferrocene in 1955 [1]. Organometallic polymers are known to possess unique optical, magnetic, and thermal properties which allow for potential applications as chemical sensors, electrocatalysts, modified electrodes, and photo-active molecular devices [2-7]. Organoiron polymers are one of the most prevalent classes of organometallic polymers, with many reports on their synthesis and properties published over the past 50 years [8-11]. Of the many varieties of organoiron species, ferrocene and cationic cyclopentadienyliron complexes are most commonly incorporated into polymers. 

Reports on the use of monomeric or polymeric porphyrins as electrocatalysts for the detection of phenols are limited. Chen and Chen reported on the use of FeTMPyP in solution, in the presence of DNA or electrodeposited on a GCE previously modified with DNA, for the catalysis of the reduction of p-nitrophenol. ... 

Zagal, Bediuoi, and coworkers have continued to study monomeric MPc complexes as electrocatalysts for the oxidation of thiols in recent years . From their study on the catalytic behavior of adsorbed or polymeric CoTAPc towards the oxidation of 2-mercaptoethanol (2-ME), Griveau et al derived the mechanism given by Equations (7.4)-(7.7) for the electrocatalytic process. 

Araki, Toma, and coworkers, reported that thin films of the polymeric tetraruthenated porphyrin material were one of the most active electrocatalysts for the oxidation of nitrite to nitrate , with activities 30 times higher than that of electrostatically assembled porphyrin films . Electrostatically assembled porphyrin films of tetramthenated cobalt porphyrin/(/ne50-tetra(4-sulfonatephenyl) porphyrinate zinc(II) could be used to detect nitrite with a detection limit of 0.1 The bilayered film allowed for the determination of nitrite at less... 

From a perusal of the relevant literature one may readily realize that these catalysts ontperform any known anode electrocatalyst for the ethanol oxidation in half cells, especially in terms of specific cnrrent densities which are higher than 3600 A gPd. Moreover, as shown in a forthcoming Section of this Chapter, anodes catalyzed with Pd-(Ni-Zn)/C and Pd-(Ni-Zn-P)/C exhibit nmivalled performance in both passive and active DAFCs eqnipped with anion-exchange polymeric membranes. 

---