Electrochemical surface area

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

If only nonprecious atoms (e.g., Co or Fe) were selectively dissolved from the alloy surface, the roughness of the resulting Pt surface layer would be increased with the number of sweeps. However, as seen in Fig. 10.2a, the value of AQ (a measure of the electrochemical surface area) rather decreases gradually and reaches a steady value. 

Cyclic voltammograms of PtSn microelectrodes in 0.5 M sulfuric acid solution are shown in Fig. 15.6. The potential range was -200 to 800 mV (vs. SCE) and the scan rate was 100 mV/s. It can be seen clearly that hydrogen desorption from the PtSn-2 electrode is seriously inhibited compared with that from the PtSn-1 electrode. From the hydrogen desorption peak areas in the CV curves and the Pt single crystallite hydrogen desorption constant of 210 /xC/cm Pt, the electrochemical surface areas (ESA) for PtSn-1 and PtSn-2 were calculated to be 391 and 49 cm /mg, respectively. However, it is evident from XRD and TEM results that the two catalysts have similar particle size and so they should possess the similar physical surface area. The difference... 

Also corrosion problems of the carbon support have been considered as a cause of electrocatalyst durabihty loss [32], in particular carbon oxidation can occur through electrochemical oxidation at the cathode, with formation of CO2 (C -I- 2H2O = CO2 -I- 4H -F 4e ), or through water gas shift reaction, with the production of CO (C H2O = CO H2). Both these routes are catalyzed by Pt [56, 57] and subtract caibon useful for platinum loading, with consequent metal sintering and decrease of the electrochemical surface area [58]. 

UTCFC has modified the carbothermal synthesis process (U.S. Patent 4,677,092, US 4,806,515, US 5,013,618, US 4,880,711, US 4,373,014, etc.) to prepare 40 wt% ternary Pt alloy catalysts. Various high-concentration Pt catalyst systems were synthesized and the electrochemical surface area (EGA) and electrochemical activity values compared to commercially available catalysts (see Table 3). The UTCFC catalysts showed EGA and activity values comparable to the commercial catalysts. A rotating disk electrode technique for catalyst activity measurements has been developed and is currently being debugged at UTCFC. 

ECA Electrochemical Surface Area GDATP General Dynamic Armaments and... 

The cell performances were estimated in correlation to the electrochemical surface area (ESA) of the catalyst and mass-transport limitations in the cathode catalyst layer. The experimental data are discussed in terms of temperature, methanol concentration, cathode gas humidity and flow rate. 

The total active area of the catalyst is typically much higher than the electrochemical surface area determined from cyclic voltammetry measurements. That means that not all catalyst particles are in contact with the ionic conductor Nafion and only a certain amount of it participates in electrochemical reaction. 

Fig. 1 represents the in-situ electrochemical characterization of the cathode catalyst layers based on supported and unsupported catalysts. At two times lower loading of carbon supported catalyst (MEA 3), its limiting current density and thus, electrochemical surface area (ESA) is 3 times higher than the corresponding surface area of unsupported catalyst (MEA 1). This is due to the smaller size of the Pt partieles in carbon supported eatalyst and thus, higher surfaee area in contaet with Nafion (Table 1). 

Table 1. Comparison of the total Pt area and Electrochemical Surface Area (ESA) estimated from cyclic voltammetry.

Figure 1.4. Loss in electrochemical surface area with cycling to potentials above 1V relative to a standard hydrogen electrode. From Ref [43].

Preparation for more available electrochemical surface area by surface reconditioning methods to achieve optimum charge transfer. 

The main soluble intermediates could be readsorbed and oxidized to form CO2 or extracted from the surface under configuration of continuous flow rate. The last situation represents a loss of energy due to an incomplete methanol oxidation. This is well elucidated in the experiments where the extraction of solution in front of the electrode results in lower current than in experiments without sample collection [9]. For supported platinum, Jusys et al. [10] observed that an increasing conversion to CO2 would be attained with increasing Pt load by the cost of faster consumption of formaldehyde facts that are attributed to an increased readsoption rate on electrodes with enlarged electrochemical surface area. 

Table 7.4. Summary of the Electrocatalytic Data for PtCo/C and PtNi/C Catalysts. EGA Electrochemical Surface Area, MA ORR Mass Activity, and SA ORR...

In addition to the proton conductivity of the electrolyte, the performance of a fuel cell is largely dependent on the electrocatalytic activity of the anodic and cathodic interface. This depends both on the structure of the gas-electrocatalyst-electrolyte three phase boimdaries and on the electrocatalytic activity of the charge transfer reaction that takes place along the electrochemical interface. The former case determines the extent of the electrochemical surface area (ESA), while the latter is directly related to the physicochemical properties of the Pt based catalyst and the extent to which its catalytic properties are affected by its contact /interaction with the polymer electrolyte. 

Kwon et al., by the use of cyclic voltammetry following the H2 oxidation, noticed that the electrochemical surface area (ESA) decreases significantly when it was measured immediately after the cell operation and after being at open circirit and imder dry N2 flow for a longer time. The latter was attributed to the dehydration of the electrochemical interface, thus affecting the wetting of the electrocatalyst by the phosphoric acid. 

Much of the knowledge in Pt/C durability derives from the experience with phosphoric acid fuel cells (PAFCs) at operating temperatures of about 200°C. Catalyst degradation is witnessed as an apparent loss of platinum electrochemical surface area over time, " associated with platinum crystal growth. These changes are ascribed to different processes which include... 

---