Electrocatalytically active surface area

The foremost objective is to obtain the highest reactivity or transfer current density of desired electrochemical reactions with a minimum amount of Pt-based catalysf (DoE target for 2010 0.29 g Pf per kilowatt). This demands a huge electrocatalytically active surface area small kinetic barriers to the transport of protons, electrons, and reactant gases and proper handling of product water and waste heat. 

Major contributions to the development of the macrokinetic or macrohomogeneous theory of porous electrodes were made by Yu, A, Chizmadzhev and Yu, G, Chirkov [25-27], In these works the importance of the interplay of oxygen diffusion and interfacial kinetics had already been realized. For oxygen reduction electrodes, a large electrocatalytically active surface area per unit volume,, has to compensate for the smallness of the intrinsic activity per unit... 

Pt utilization is a statistical property of the CCL. It can be obtained from ex situ electrochemical studies. It is defined as the ratio of the electrocatalytically active surface area, accessible to electrons and solvated protons, to the total surface area ofPt ... 

The statistical surface area utilization factor Fstat has been considered under different conditions, specifically in catalyst powders and in MEAs of operational PEFCs. The electrocatalytically active surface area in the catalyst powder can be obtained from the charge under the H-adsorption or CO-stripping waves measured by... 

A third way to increase both the active surface area and the number of oxygenated species at the electrode surface is to prepare alloy particles or deposits and then to dissolve the non-noble metal component. This technique, which is similar to that used to prepare Raney-type catalysts, yields very high surface area electrodes and hence some improvements in the electrocatalytic activities compared with those of pure platinum. However, it is always difficult to be sure whether the mechanism of enhancment of the activities is due to this effect or the possible presence of remaining traces of the dissolved metal. Results with PtyCr and PtSFe were encouraging, although the effect of iron is still under discussion. From studies in a recent work on the behavior of R-Fe particles for methanol electrooxidation, it was concluded that the electrocatalytic effect is due to the Fe alloyed to platinum. ... 

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. 

The MCFC anodes are made from a porous sintered nickel with a thickness of 0.8-1.0 mm and a porosity of 55-70% with a mean pore diameter of 5pm. This porosity range provides adequate interconnected pores for mass transport of gaseous reactants and adequate surface area for the anodic electrocatalytic reactions. Because the anode kinetics is faster than that of the cathode, less active surface area is sufficient for the anodic process. Partial flooding of the comparatively thick anode is therefore acceptable at the anode interface. 

Since the rate of all electrocatalytic reactions is strictly related to the active surface area, besides the surface chemistry, the morphology of the electrocatalyst needs to be tailored. Morphology is not only related to the metal-phase area but also to the presence of micro- and macro pores in the electrocatalyst support that could facilitate or hinder the mass transport properties. All these characteristics determine the cell performance even if the relative influence of each parameter is still not known in detail. It is thus necessary to select appropriate procedures for the optimization of these characteristics, i.e. composition, structure, particle size, porosity, etc. Generally a combination of physico-chemical and electrochemical analyses carried out on different electrocatalysts indicates the system that best suits the scope of application in a DMFC. 

The primary optimization parameter of porous electrodes is the ideal electrochem-ically active surface area per unit volume sa- rough approximation, the value ECSA is proportional to the amount of the electrocatalytically active material Pt, in the case of PEFC electrodes. It is inversely proportional to the feature size d, which could represent diameters of catalyst particles, of pores in a porous catalytic medium, or of rod-like structures (nanotubes or nanorods), onto which a thin film of catalyst is deposited. On the other hand, is also roughly proportional to the energy density... 

Certain amorphous alloys with special composition and structure possess very high catalytic or electrocatalytic activity, which is superior to that of their crystalline counterparts or other conventional catalysts [131]. These materials have attracted attention due to their application as catalysts for fuel cells [132]. In general, the amorphous state is more active than the crystalline one, due to the presence of a denser active surface area, due to its homogeneity. The necessary amount of platinum in the amorphous state is reduced, so the overall cost of the catalysts is lowered. The main advantage of this kind of material is related to the possibility of changing the nature of the electrode by alloying different catalytic elements that will decorate a conducting matrix [65]. 

Clearly the majority of the electro-catalysts explored in Table 16.2 lend themselves to fuel cell applications. A key parameter to the success of the electrocatalytic sensing of ammonia involves the design of the catalyst. That is, a surface which gives rises to a large active surface area which also stabilises active intermediates and is of a composition to induce changes in the activation energy. The performance of the electro-catalyst is characterised by the mass activity (MA activity mass ) which is the current density (at a specific potential) normalised by the mass of the electro-catalyst which is related to the specific electrochemically active area (SSA area mass ) and the specific activity (SA activity area ) which is the current density normahsed by the electrochemically active surface area (ECSA) of the electro-catalyst. As such, the mass activity is the key parameter given by ... 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

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... 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

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