Electrochemical activity catalysts

On either side of the membrane are the anode and cathode catalyst electrode layers. The electrochemically active catalyst sites require the three-phase interface (1) Ft catalyst surface, electrically connected to the external path to provide electron transport paths (2) ionomer or electrolyte contact to transport protons and (3) reactant gas phase access. 

This failure mechanism can have significant impact on the ability of the anode to tolerate adsorbed contaminants. Similar to the impact of carbon corrosion on the cathode, the reduced electrochemically active catalyst surface area becomes very sensitive to the presence of contaminants. This is very important, for example, for operation on reformate where even small amounts of carbon monoxide can result in significant performance loss. 

One additional function of catalyst support material is to enhance the platinum utilization in the electrode. This leads to an enlarged three phase reaction zone in the MEA and higher electrochemical active catalyst areas at the same catalyst loading of the electrode. For this reason, support materials with adequate surface area and porosity have to be chosen. Antolini [9] shows the benefits of using meso porous stractured carriers with pore sizes between 2 and 50 nm. Here, free pore volume is available for the electrolyte which enables an additional rise of three phase boundary zone and an increased interaction between catalyst and electrolyte [5, 6, 9-15]. 

Roughness Factor In some cases, the ratio of the actual electrochemically active catalyst surface area to the planform geometric surface area is used as a separate parameter to delineate morphology effects, called the roughness factor a ... 

Figure 9.10 The area shown under the dashed line represents hydrogen adsorption and can be used to determine electrochemically active catalyst area. CV done on a 5 cm active area fuel cell, in-situ. 50 seem hydrogen on anode, 200 seem nitrogen on cathode. RH anode/cathode = 100/100%, scan rate 20 mV/sec.

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). 

C.A. Cavalca, and G.L. Haller, Solid Electrolytes as Active Catalyst Supports Electrochemical Modification of Benzene Hydrogenation Activity on Pt/p"(Na)Al203, /. Catal. Ill, 389-395(1998). 

C.G. Vayenas, and S. Neophytides, Electrochemical Activation of Catalysis In situ controlled promotion of catalyst surfaces, in Catalysis-Special periodical Report, Royal Society of Chemistry, Cambridge (1996), pp. 199-253. 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

It also shows that electrochemical promotion is due to electrochemically controlled migration (backspillover) of ions (acting as promoters) from the solid electrolyte to the gas-exposed catalytically active catalyst-electrode surface. 

C.G. Vayenas, S. Bebelis, I.V. Yentekakis, C. Karavasilis, and J. Yi, Non-Faradaic Electrochemical Modification of Catalytic Activity Solid Electrolytes as Active Catalyst Supports, Solid State Ionics 72, 321-327 (1994). 

Most of the electrochemical promotion studies surveyed in this book have been carried out with active catalyst films deposited on solid electrolytes. These films, typically 1 to 10 pm in thickness, consist of catalyst grains (crystallites) typically 0.1 to 1 pm in diameter. Even a diameter of 0.1 pm corresponds to many (-300) atom diameters, assuming an atomic diameter of 3-10 10 m. This means that the active phase dispersion, Dc, as already discussed in Chapter 11, which expresses the fraction of the active phase atoms which are on the surface, and which for spherical particles can be approximated by ... 

Jiang R, Chu D. 2000. Remarkably active catalysts for the electroreduction of O2 to H2O for use in acidic electrolyte containing concentrated methanol. J Electrochem Soc 147 4605-4609. 

Crown A, Kin H, Lu GQ, de Moraes IR, Rice C, Wieckowski A. 2000. Research toward designing high activity catalysts for fuel cells Structure and Reactivity. J New Mater Electrochem Syst 3 275. 

Liu YC, Yu CC, Yang KH (2006) Active catalysts of electrochemically prepared gold nanoparticles for the decomposition of aldehyde in alcohol solutions. Electrochem Commun... 

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

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... 

In contrast the oxo-ruthenium complex c ,c -[ (bpy)2Runl(0H2) 2(//-0)]4+ and some of its derivatives are known to be active catalysts for the chemical or electrochemical oxidation of water to dioxygen.464-472 Many studies have been reported473 181 on the redox and structural chemistry of this complex for understanding the mechanism of water oxidation. Based on the results of pH-dependent electrochemical measurements, the basic structural unit is retained in the successive oxidation states from Rum-0 Ru111 to Ruv O Ruv.466... 

A recent study (1) has demonstrated that the electrochemical oxidation of hydroxide ion yields hydroxyl radical ( OH) and its anion (O"-). These species in turn are stabilized at glassy carbon electrodes by transition-metal ions via the formation of metal-oxygen covalent bonds (unpaired d electron with unpaired p electron of -OH and O- ). The coinage metals (Cu, Ag, and Au), which are used as oxygen activation catalysts for several industrial processes (e.g., Ag/02 for production of ethylene oxide) (2-10), have an unpaired electron (d10s1 or d9s2 valence-... 

The electrochemical activation of the catalyst must be possible at potentials less negative than — 0.9 V vs SCE since at more negative potentials the direct electrochemical reduction of NAD(P)+ will lead to NAD dimer formation. 

Now, by chemically, or electrochemically depositing catalyst atoms at these sites, a means is provided to reduce the activation energy. Figure 3 shows a schematic illustration of this new configuration. 

The stability during potential cycling and ORR activity of Pt (20 wt°/o) supported on MWCNTs and carbon black was also investigated [136]. Two different potential cycling conditions were used, namely lifetime (0.5 to 1.0 V vs. RHE) and start-up (0.5 to 1.5 V vs. RHE). Pt supported on MWCNTs catalyst exhibited a significantly lower drop in normalized electrochemically active surface area (ECA) values compared to Pt supported on Vulcan (Fig. 14.10), showing that MWCNTs possess superior stability to commercial carbon black under normal and severe potential cycling conditions [137]. 

The electrocatalytic activity of the nanostructured Au and AuPt catalysts for MOR reaction is also investigated. The CV curve of Au/C catalysts for methanol oxidation (0.5 M) in alkaline electrolyte (0.5 M KOH) showed an increase in the anodic current at 0.30 V which indicating the oxidation of methanol by the Au catalyst. In terms of peak potentials, the catalytic activity is comparable with those observed for Au nanoparticles directly assembled on GC electrode after electrochemical activation.We note however that measurement of the carbon-supported gold nanoparticle catalyst did not reveal any significant electrocatalytic activity for MOR in acidic electrolyte. The... 

In this chapter, two carbon-supported PtSn catalysts with core-shell nanostructure were designed and prepared to explore the effect of the nanostructure of PtSn nanoparticles on the performance of ethanol electro-oxidation. The physical (XRD, TEM, EDX, XPS) characterization was carried out to clarify the microstructure, the composition, and the chemical environment of nanoparticles. The electrochemical characterization, including cyclic voltammetry, chronoamperometry, of the two PtSn/C catalysts was conducted to characterize the electrochemical activities to ethanol oxidation. Finally, the performances of DEFCs with PtSn/C anode catalysts were tested. The microstmc-ture and composition of PtSn catalysts were correlated with their performance for ethanol electrooxidation. 

The catalyst layers (the cathode catalyst layer in particular) are the powerhouses of the cell. They are responsible for the electrocatalytic conversion of reactant fluxes into separate fluxes of electrons and protons (anode) and the recombination of these species with oxygen to form water (cathode). Catalyst layers include all species and all components that are relevant for fuel cell operation. They constitute the most competitive space in a PEFC. Fuel cell reactions are surface processes. A primary requirement is to provide a large, accessible surface area of the active catalyst, the so-called electrochemically active surface area (ECSA), with a minimal mass of the catalyst loaded into the structure. 

---