Electrocatalysts morphology

When considering the morphology of prepared electro-catalysts are different to each other especially to the commercial one, one can think that the structure of electrode which was optimized to the commercial catalyst may not be optimum. So, the for the better electrode structures was conducted by investigating the effect of NFP. Fig. 2 is a schematic of electrode which depicts the effect of Nafion content[9]. For the conventional electrocatalysts, the range of 30 35 % NFP is reported as optimum value[10]. 

Recently, rhodium and ruthenium-based carbon-supported sulfide electrocatalysts were synthesized by different established methods and evaluated as ODP cathodic catalysts in a chlorine-saturated hydrochloric acid environment with respect to both economic and industrial considerations [46]. In particular, patented E-TEK methods as well as a non-aqueous method were used to produce binary RhjcSy and Ru Sy in addition, some of the more popular Mo, Co, Rh, and Redoped RuxSy catalysts for acid electrolyte fuel cell ORR applications were also prepared. The roles of both crystallinity and morphology of the electrocatalysts were investigated. Their activity for ORR was compared to state-of-the-art Pt/C and Rh/C systems. The Rh Sy/C, CojcRuyS /C, and Ru Sy/C materials synthesized by the E-TEK methods exhibited appreciable stability and activity for ORR under these conditions. The Ru-based materials showed good depolarizing behavior. Considering that ruthenium is about seven times less expensive than rhodium, these Ru-based electrocatalysts may prove to be a viable low-cost alternative to Rh Sy systems for the ODC HCl electrolysis industry. 

Hwang JT, Chung JS. 1993. The morphological and surface properties and their relationship with oxygen reduction activity for platinum-iron electrocatalysts. Electrochim Acta 38 2715-2723. 

Fig. 14.12 Effect of pore morphology of the carbon support (CMK-3 and WMC) on the activity of Pt electrocatalyst (Reprinted from [148] with permission from Elsevier).

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

Since Ru02 and Ir02 are usually prepared by thermal decomposition of suitable precursors on an inert support, the morphology of the active layer is very like that of a compressed powder [486]. The surface area plays an important role since the roughness factor can be between 102 and 103. However, the low Tafel slope observed is a clear indication of electrocatalytic effects and the high surface area is the factor which extends the low Tafel slope to much higher current densities. Thus, the combination of these two factors renders these oxides very efficient electrocatalysts for H2 evolution. 

Fig- 13.29. Distribution of particle sizes in 10 wt. % Pt-C electrocatalyst. Particle sizes are average diameters. (Reprinted from E. A. Ti-cianelli, M. N. Beery, and S. Srinivasan, Dependence of Performance of Solid Polymer Electrolyte Fuel Cells with Low Platinum Loading on Morphologic Characteristics of the Electrodes, J. Appl. Electrochem. 21 601 copyright 1991, Fig. 9. 

Physical properties of the carbon substrate, such as electronic conductivity, surface area, and surface morphology are important, since the former can contribute to resistive losses in the electrocatalyst structure and the latter may determine the sites on which the platinum electrocatalyst crystallites may be located. Both the initial deposition of the platinum crystallites on the carbon substrate and the subsequent surface area loss of the platinum crystallites under operating conditions by a surface migration mechanism can be influenced by the surface carbon structure. 

Pt-based electrocatalysts have proven to be ideally suited to the Ap analysis primarily because of the extensive morphological characterizations (X-ray diffraction, single crystal electrochemical evaluations, UHV spectroscopies, etc.) performed over the past decades. In contrast, chalcogenide electrocatalysts are comprised of nanoscale amorphous clusters making a detailed analysis of the strac-ture/property relationships inherently difficult. In light of these considerations, we have recently applied the Ap technique to a novel mixed-phase chalcogenide electrocatalyst (RhxSy, commercially available from A-TEX, Inc). Rh Sy shows remarkable per-... 

Raney nickel electrocatalysts have also found useful applications as active electrodes for the HER (179, 180). The activity of Raney Ni catalysts is established after leaching out the base metal, Al or Zn. Choquette et al. (181) have examined the changes in morphology and composition of Raney-Ni composite catalytic electrodes accompanying dissolution of the base metal in concentrated NaOH. The depletion of Al from the Raney particles is, of course, accompanied by a major increase in real area with time of leaching and also, interestingly, with possible phase transformations (181). The electro-catalytic activity is, however, surprisingly, practically independent of time. 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

The noble metals or their oxides are the most convenient substrates for most electrochemical reactions taking place in fuel cells or in industrial electrolysis, for example. Because of this, the activated electrodes are introduced, consisting of a conducting, inert support coated with a thin layer of electrocatalyst. In this way, not only the chemical nature of the electrode can be modified but also its morphology and structure in dependence on the procedure of preparation.1... 

In Chapter 4 by Popov et al., the aspects of the newest developments of the effect of surface morphology of activated electrodes on their electrochemical properties are discussed. These electrodes, consisting of conducting, inert support which is coated with a thin layer of electrocatalyst, have applications in numerous electrochemical processes such as fuel cells, industrial electrolysis, etc. The inert electrodes are activated with electrodeposited metals of different surface morphologies, for example, dendritic, spongy-like, honeycomblike, pyramid-like, cauliflower-like, etc. Importantly, the authors correlate further the quantity of a catalyst and its electrochemical behavior with the size and density of hemispherical active grain. 

The optimization of an electrochemical reactor calls for a full description of the process to accomplish the specific objective. The problem of the optimization of the electrocatalyst is of real importance in most of the recently developed technical electrodes that were prepared without detailed studies. It must be borne in mind that the strong experimental conditions in which the large electrical currents and large ionic forces of the electrolytes prevail change the morphology and the composition of the catalyst. 

In the previous example the supported metal oxide onto which the metal precursor was adsorbed did not reduce which will be the case for many promoted systems. In many systems, however, the supported metal oxide will reduce, especially through hydrogen spillover, and a bimetallic catalyst can be synthesized. The idea is illustrated in Figure 3.11a for the Pd/Co/C electrocatalyst system. The idea will be to adsorb Pd complexes onto a well-dispersed, carbon-supported C03O4 phase, and reduce to get bimetallic Pd/Co particles that are perhaps core-shell in morphology. 

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