Carbon Electrocatalyst Supports

There are many considerations that must be taken into account when choosing a particular carbon, or carbon structure, as an electrocatalyst support. In hot phosphoric acid at cathodic potentials, the carbon surface is capable of being oxidized to carbon dioxide. The degree of oxidation will depend on the pretreatment of the carbon (for instance, the degree of graphitization), on the carbon precursor, and the provenance. There are two important parameters that will govern the primary oxidation rate for any given carbon material in an electrochemical environment. These are electrode potential (the carbon corrosion is an electrochemical process and therefore will increase rapidly as the electrode potential is raised) and temperature. 

An early detailed work in hot phosphoric acid is that of Kinoshita and Bett49,50 who reported corrosion rates for four different carbons in phosphoric acid at temperatures up to 160 °C. All of the carbons tested showed similar behavior as a function of time. At constant potential the corrosion currents were relatively large but declined rapidly with time. These authors concluded that the rate of oxidation was dependent on the surface micro-structure for each specific carbon sample. Graphitized carbons showed lower specific oxidation rates than ungraphitized carbons. 

Following from the previous work of Kinoshita and Bett,49 50 a detailed analysis of carbon corrosion in hot phosphoric acid was carried out by Stonehart and MacDonald.68 

The reactions contributing to this oxidation current are the formation of surface oxides on the carbon (quinones, lactones, carboxylic acids, etc.) and the evolution of carbon dioxide. Binder et al.51 had previously examined the oxidation of numerous carbons in KOH, H2S04, and HjP04 but not at temperatures as high as those in the work of Kinoshita and Bett. 

Binder el al.51 had observed that the reaction products were carbon dioxide and carbon surface oxides and that the reaction rate was proportional to the BET surface area. These early results however, were not substantiated by Kinoshita and Bett. 

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Specific Activity (SA) and Mass Activity (MA) of Pt Electrocatalysts Supported on Different Carbon Powders Characterized by Specific Surface Area (S) and Particle Size (d)... 

Attwood PA, McNicol BD, Short RT. 1980. Electrocatalytic oxidation of methanol in acid electrolyte—Preparation and characterization of noble-metal electrocatalysts supported on pretreated carbon-fiber papers. J Appl Electrochem 10 213-222. 

Pt (5 wt%) supported on platelet and ribbon graphite nanofibers exhibited similar activities to those observed by Pt (25 wt°/o) on carbon black [138], This phenomenon was attributed to the crystallographic orientations adopted by the catalyst particles dispersed on graphitic nanofiber structures [139]. Also, the electrocatalysts supported on CNFs were less susceptible to CO poisoning than Pt supported on carbon black. 

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... 

In fuel cells, carbon (or graphite) is an acceptable material of construction for electrode substrates, electrocatalyst support, bipolar electrode separators, current collectors, and cooling plates. 

It is necessary to discuss four scientific topics for phosphoric acid fuel cells. Those interconnected topics are the design of the precious metal electrocatalyst properties of the phosphoric acid properties of the matrix, and those of the carbon catalyst support. 

Finally, the understanding of the chemistry of carbon and its stability in hot phosphoric acid in relation to its use as electrocatalyst supports, has led to the use of highly graphitic carbons, where the fundamental electrochemistry now is well defined. 

The ceramic sheets were cut into the correct shape to fit in the electromembrane reactor and then impregnated with the electrocatalyst. A slurry or ink of the carbon black-supported Sb-doped Sn02 was prepared in an appropriate solvent followed by ultrasonic treatment for 30-60 min. The resulting ink was sprayed onto the ceramic membrane surface by using commercially available spray guns. The resulting membranes were then dried at room temperature overnight. 

The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid three-phase boundary. COj reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. 

In the present work, CO2 electrochemical reduction was examined on higji area metal electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths on the order of nanometers. Such electrocatalysts were used in the form of gas difiusion electrodes (GDE), which are used in the fuel-cell field. The structure of this type of electrode is shown in Figure 1. The reaction takes places at the gas phase / electrolyte (liquid phase) / electrode interface, the so-called three-phase boimdary. 

Electronic interaction and synergistic effects between catalysts and the support material have been investigated in the context of fuel-cell electrocatalysts. Electron spin resonance (ESR) has been used to demonstrate the electron donation by Pt to carbon [11] support. This has been further supported by XPS studies [12], which show that the metal acts as an electron donor to the support, their interaction depending on their respective Eermi levels. Bogotsky and Snudkin [13] have shown that the characteristics of the electrical double layer formed between the microdeposit (Pt) and the support depends to a certain extent on the difference in the work function of Pt (5.4 eV) and carbon support (pyrolytic support 4.7 eV), thereby resulting in an increase of the electron density of Pt. However, the rise in the electron density can be significant only when the particle size of the microdeposit is comparable to the thickness of the double layer. 

Electrocatalyst support The main functions of the carbon support are to (a) disperse the ultrafine electrocatalyst particles, (b) bind strongly with the... 

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

Designing alloy electrocatalysts by the so-called ad-atom method, and by alloy sputtering for oxidation of CH3OH and CO, and for CO tolerance in H2 oxidation, respectively, as well as for O2 reduction are discussed. Many years of experience are summarized and collaborations with other groups are highlighted. The particle size effect in electrocatalysis by small particle electrodes, and the effect of corrosion of carbon-black supported nanoparticles on the electrocatalytic activity are also discussed. All these factors, as well as catalyst lifetimes, are very important in fuel cell performance and in the final cost estimates for the practical fuel cell applications. 

K. Yasuda and T. Ioroi, Carbon back supported platinum electrocatalysts for polymer electrolyte fuel cell, Hyomen (Surface), 2000, 38, 55-67. 

Activated carbons possess high BET surface areas (400 to 2500 m /g) and micropore volumes (up to 1.2 cm /g), which makes them particularly attractive adsorbents. They are also used as supports for heterogeneous catalysts and sometimes, electrocatalysts [20]. In a number of patents it was claimed that addition of either activated carbons or activated carbon-supported Pt to the CLs composed of carbon black-supported catalysts improves cell performance [21],... 

Salgado IRC, Paganin VA, Gonzalez ER et al (2013) Characterization and performance evaluation of Pt-Ru electrocatalysts supported on different carbon materials for direct methanol fuel cells, hit J Hydrogen Energy 38 910-920... 

Bambagioni V, Bianchini C, Marchionni A, Filippi J, Vizza F, Teddy J, Serp P, Zhiani M (2009) Pd and Pt-Ru anode electrocatalysts supported on multi-walled carbon nanotubes and their use in passive and active direct methanol alcohol fuel cells with an anion-exchange membrane (alcohol = methanol, ethanol, glycerol). J Power Sources 190 241-251... 

Carbon constitutes the most abundant element of the different FC components. Setting aside the membrane, which is a polymer with a carbon backbone, all the other components, i.e. the CL, GDL and current collector plates (bipolar plates) are made almost entirely of graphitic carbon. The electrocatalyst support of the CL is commonly carbon black in the form of fine powder. GDLs are thin porous layers formed by carbon fibers interconnected as a web or fabric, while current collector plates are carbon monoliths with low bulk porosity. As explained above each of these components has a particular function within the fuel cell and in particular in the triple phase boundary. The structure and properties of the carbon in each of the different FC components will determine the whole performance of the cell. 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

Bing Y, Neburchilov V, Song C, Baker R, Guest A, Ghosh D, et al. Effects of synthesis condition on formation of desired crystal structures of doped-Ti02/carbon composite supports for ORR electrocatalysts. Ekctrochim Acta 2012 77(0) 225-31. 

Figure 3. SEM micrograph of the carbonized and pulverized PAA after Pd deposition. Inset SEM micrograph of PAA prepared in 0.3 M oxalic acid at 60 V. Reprinted from Zhenyou Wang, Fengping Hu and Pei Kang Shen, Carbonized porous anodic alumina as electrocatalyst support for alcohol oxidation, Electrochemistry Communications, 8 (2006) 1764-68, Copyright (2006) with permission from Elsevier.

Fang, B., Kim, J.H., Yu, J.S. Colloid-imprinted carbon with superb nanostructure as an efBcient cathode electrocatalyst support in proton exchange membrane fuel cell. Electrochem. Commun. 10(4), 659-662 (2008)... 

Carbon as Support Material in Fuel Cell Electrocatalysts 251 7.1.2.4 Other Methods and In situ Studies... 

Carbon as Support Material in Fuel Cell Electrocatalysts... 

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