Electrochemically active surface area

Similarly, in the development of solid oxide fuel cells (SOFCs), it is well recognized that the microstructures of the component layers of the fuel cells have a tremendous influence on the properties of the components and on the performance of the fuel cells, beyond the influence of the component material compositions alone. For example, large electrochemically active surface areas are required to obtain a high performance from fuel cell electrodes, while a dense, defect-free electrolyte layer is needed to achieve high efficiency of fuel utilization and to prevent crossover and combustion of fuel. 

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 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 anode layer of polymer electrolyte membrane fuel cells typically includes a catalyst and a binder, often a dispersion of poly(tetraflu-oroethylene) or other hydrophobic polymers, and may also include a filler, e.g., acetylene black carbon. Anode layers may also contain a mixture of a catalyst, ionomer and binder. The presence of a ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires a ionically conductive pathway to the cathode catalyst to generate electric current (16). 

Ahn et al. have developed fibre-based composite electrode structures suitable for oxygen reduction in fuel cell cathodes (containing high electrochemically active surface areas and high void volumes) [22], The impedance data obtained at -450 mV (vs. SCE), in the linear region of the polarization curves, are shown in Figure 6.22. Ohmic, kinetic, and mass transfer resistances were determined by fitting the impedance spectra with an appropriate equivalent circuit model. 

High, electrochemically active surface area of the catalyst... 

Thus the potential in the macropores is directly affected by transport and charging phenomena in the micropores accessible to electrolyte. These properties of active carbon render it a difficult material to use as an electrode. The large electrochemically active surface area leads to considerable double-layer charging currents, which tend to ob.scure faradic current features. The network of micropores in the electrode material might be expected to result in a significant ohmic effect, which would further impair the potential resolution (IR drop on electrode material) obtainable by PACE voltammetry. CV curves recorded with different masses (and sediment layer thicknesses) of powdered samples of selected carbons in various electrolyte solutions are presented in Fig. 8 as an example [194]. Where amounts of material were greater than 20 mg, the CVs recorded were of the same shape. 

Therefore the electrochemical response with porous electrodes prepared from powdered active carbons is much increased over that obtained when solid electrodes are used. Cyclic voltammetry used with PACE is a sensitive tool for investigating surface chemistry and solid-electrolyte solution interface phenomena. The large electrochemically active surface area enhances double layer charging currents, which tend to obscure faradic current features. For small sweep rates the CV results confirmed the presence of electroactive oxygen functional groups on the active carbon surface. With peak potentials linearly dependent on the pH of aqueous electrolyte solutions and the Nernst slope close to the theoretical value, it seems that equal numbers of electrons and protons are transferred. 

Note that the farther away the electric potential of the carbon surface is from the potential of zero charge point ( pzc) l e higher the disjoining pressure is. In principle, this may result in a systematic variation of the support pore size in Me/C catalysts with potential (similar to the electrocapillary curve [96,97]) and consequently the efficiency of metal particle blocking by the pore walls. Such behavior of porous carbons obviously can influence the measurements of the electrochemically active surface area and might be one of the reasons for the observed correlation between the apparent dispersion of Pt/C catalysts, measured by cyclic voltammetry, and pHpzc of the supports [95], whereas no noticeable difference in the particle size has been observed with HRTEM. Undoubtedly, this problem needs further investigation. 

Figure 6 Comparison of the mass activity for ORR at 0.9 V as a function of electrochemically active surface area (m /gm Pt) for supported Pt and Pt alloy electrocatalysts. (From Ref. 18.)...

The highest electronic resistances were observed at low Nafion loadings, indicating that the ionomer played a significant role as a binder [211], Meanwhile, kinetic losses pass throngh a minimnm correlated with the electrochemically active snrface area of the catalyst estimated from cyclic voltammograms [209] The higher the electrochemically active surface area, the lower the kinetic losses. This volcano type of cnrve reflects the optimnm in the metal utilization factor u. Below, we try to nnderstand how carbon properties may influence these characteristics. 

In all three MEAs the rate of methanol oxidation was facilitated by the platinum-ruthenium unsupported catalyst, which in the presence of CO as a byproduct of the reaction, exhibit an electrochemical activity higher than pure Pt. However, compared to Pt supported and unsupported catalysts, the electrochemically active surface area of PtRu alloys cannot be determined by hydrogen adsorption using cyclic voltammetiy due to the overlap of hydrogen and oxygen adsorption potentials, and the tendency for hydrogen to absorb in the ruthenium lattice [xvi]. However, under the same operation conditions, cyclic voltammetry can be used for qualitative estimation of the similarity in the PtRu anode layer properties. 

To accurately calculate iVDopanu the actual surface area in contact with the electrolyte must be accurately determined. This surface area could be very different than that of the planar projected geometric area. In some cases, selective dye absorption to the surface can be used to quantify the roughness factor and obtain an electrochemically active surface area [18]. Other methods include evaluating the Cdi as a proxy for surface area using EIS measurements at low frequencies as well as cyclic voltammograms at various sweep rates [19]. 

Loss of carbon support will disconnect platinum particles from the electron conducting path thus making them electrochemically inactive. They might also combine with other particles reducing the electrochemically active surface area. 

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