Electrochemical active surface area values

The research is therefore focused at the cathode. The state-of-the-art catalyst Pt/ C shows only a low specific activity in the order of 0.2 mA cm Pt at 900 mV (IR-free, at 1 bar 80°C) This is compensated by the large Electrochemically Active Surface Area values (ECSA) obtained with these catalysts, which can be in the order of 60-90 m g. The corresponding mass activities, which are the product of the ECSA and the specific activity, are between 0.12 and 0.18 A mg about a factor 3 lower than the target, resulting in a required Pt loading in the order of 0.4-0.6 mg cm Pt [46]. 

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]. 

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. 

Equations (31) and (32) can be used to analyze impedance spectra without knowledge of structural electrode parameters (thickness, density, etc). However, we need this information in order to transform the ohmic parameters obtained by a fit into specific electrochemical parameters. In particular, this information can be used to calculate the effective surface area of the particles. Particles used in practical batteries can usually be treated either as thin plates (Levi and Aurbach [1997]) or as pseudospherical in shape (Barsoukov [2003]), and have a narrow size distribution due to sieving. Particle size values are provided by material manufacturers. The number of particles in a given volume can be estimated from the ratio of their crystallographic density of particles, Op, to the density of the composite-electrode film, a. This allows one to calculate the electrochemically active surface area for a composite electrode for thin-plate particles as 5 = xAdalUOp] and for spherical particles as 5 = 3xAdal[rCp. Here x is the fraction of active material in the composite A is the geometric area of the electrode d is the thickness of the composite electrode <7 is the density of the composite electrode Op is the true density of particles and I and r are the thickness of the plate and radius of spherical particles, respectively. 

A small amount of Pt was loaded on WC, and the Pt/WC electrocatalyst was tested for hydrogen oxidation. Electrocataljdic activities can be evaluated by measuring electrochemical active surface area (EAS, m /gpt) from cyclovoltammetry. 10 wt% Pt/WC prepared via TPRe of various tungsten oxide based precursors showed 5.9-11.4 m /gpt in 0.5 Af H2SO4 electrolyte at room temperature (28). 7.5 wt% Pt/W2C and Pt/WC fabricated by polymer-induced carburization led to high EAS value of 327 m /gpt (33) and 316 m /gpt (34) in 1 Af H2SO4 electrolyte. 

A maximum power density of about 850 mW cm was reported for Aquivion -based MEAs at 130 °C. This value is significantly better compared to the power density of 600 mW cm measured with Nafion membrane under similar conditions in pressurized mode (Fig. 2.23). The voltage efficiency at the maximum power density is also larger for Aquivion (0.57 V) than Nafion (0.4 V). A larger electrochemical active surface area and a lower cross-over for Aquivion - vs. Nafion -based MEAs may provide a suitable explanation for the better performance in the first case. Moreover, the higher conductivity of Aquivion vs. Nation can also account for this the latter aspect can be mainly interpreted in terms of lower equivalent weight. 

FIGURE 10.23 Results comparison of all parts in every cell, (a) Electrochemically active surface area coefficient, (b) Average particle diameter values, (c) Contact angle values, (d) Impedance values. (Reprinted from Int. /. Hydrogen Energy, 35, P. Pei, X. Yiran, P. Chao. Analysis on the PEM fuel cells after accelerated life experiment, 3147-3151, Copyright (2010), with permission from Elsevier.)... 

The peculiar ionomer morphology influences the formation of a thin film of liquid water between Pl/C surface and ionomer. Resulting distributions of ionomer and water at the agglomerate surface and in agglomerate pores are essential structural properties. These distributions determine the real value of the electrochemically active surface area of the catalyst, Secsa the proton concentration (or pH) at the catalyst surface, and the proton conductivity of the layer. 

The mean value between Qh des and Qu-ads is denoted as Qh upd and can be used to evaluate the electrochemical active surface area (ESCA) [179] ... 

If one knows the amount of charge necessary to deposit 1 ML of hydrogen per imit area of the electrode (Qh,o) electrochemically active surface area is determined as Qh/ Qh,o- Qh,o is well defined for single crystal surfaces, and taken as an average for polycrystalline surface. However, there is a rule that a value for polycrystalline surface is very close to corresponding value for (100) single crystal surface [14], For Pt the accepted value of Qh,o is 210 p,C cm", and this value is commonly used for studies in acidic solutions. However, the value of 150 p,C cm was recently proposed for the studies of Pt catalysts in alkaline solutions [15]. 

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... 

The mass activity MA (in A g ) of the Pt catalyst is, of course, the product of the specific activity js (in A m ) and the specitic surface area 5mass (in ni g ) MA = js mass- Because S ass is inversely proportional to the particle diameter dpt, the use of supported Pt nanoparticles is effective for increasing MA, if js is a constant independent of dpt- However, even at pure Pt, conflicting results on the values of js and P(H202) have been reported, suggesting the presence of differences in electrochemical properties between bulk and supported nanoparticles. For example, Bregoli [1978]... 

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