Catalyst electrochemical surface area

Loss of catalyst electrochemical surface area (ECSA), as discussed above, could be caused by Pt dissolution and migration into the membrane. In addition, increase in Pt particle size during fuel cell operation is another cause of ECSA loss in the catalyst layer. Loss of Pt surface area vs. time during fuel cell operation has been observed in both the phosphoric acid fuel cell [87-89] and PEMFC operations [9, 33, 90]. An increase in Pt particle size from 2-3 mn up to more than 10 mn during durability testing in the catalyst layer has been reported, determined by X-ray diffraction [46] or TEM image analysis [9, 33-38, 90]. 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

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 ... 

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... 

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. 

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 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. 

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... 

Fig. 22.15 The impact of Ft loading on the OER polarization, (a) Upper line Pt-NSTF substrate with 0.05, 0.10 and 0.15 mg/cm PL Lower line same Pt-NSTF substrates with 10 pg/cm of Ir-Ru voltage recorded 15 s after constant current pulse from —1 mA/cm to 12, 44, and 200 mA/cm. (b) Relative electrochemical surface area changes of Pt with (solid bars) and without (patterned bars) the OER catalyst before any current pulses, after 20 x 44 mA/cm and after 200 x 200 mA/cm (Ir-Ru on Pt-NSTF only)...

Fig. 10.13 (a) Loss of electrochemical surface area (ECSA) of Pt/C (E-TEK), platinum-black (PtB E-TEK), and PtNT catalysts with number of potential cycles in Ar-purged 0.5 M H2SO4 solution at 60°C (0-1.3 V versus RHE, sweep rate 50 mV/s). (b) ORR curves (shown as current-voltage relations) of Pt/C, platinum black (PtB), PtNTs, and PdPtNTs in 02-saturated 0.5 M H2SO4 solution at room temperature (1,600 rpm, sweep rate 5 mV/s). Inset Mass activity (top) and specific activity (bottom) for the four catalysts at 0.85 V (Reproduced from [130]. With permission)... 

Fig. 17.14 Hours of lifetime at 120°C (before catastrophic failure of the PEM) versus fluoride ion release rates (by IC) for NSTF and Pt/C catalyst-based membrane electrode assemblies (MEAs) having the same type PEM and GDL. Cells of 100 cm2 were operated at 0.4 A/cm2, 120°C, 300 kPa, 61/84% inlet relative humidity (RH). Electrochemical surface area and crossover were measured daily at 75°C. Total lifetimes were -- CSOO h for the NSTF MEAs due to diagnostic testing at 75°C. End-of-life criteria were severe falloff of cell voltage and corresponding ramp-up of F-ion release indicative of membrane pinhole formation. From reference [5]...

Zhao et al. [158] described a method to functionalize the CNTs surface by using ionic liquids. The Pt nanoparticles catalysts supported on CNTs were prepared from a microwave heated, ethylene glycol solution with ionic liquids. They reported that the electrochemical surface area of Pt the nanoparticles supported on ionic liquids treated CNTs is 21% higher than the commercial Pt/C catalyst. 

Cyclic voltammetry (CV) is one of the most widely used electrochemical techniques for acquiring qualitative information about electrochemical reactions. Measurement using cyclic voltammetry can rapidly provide considerable information about the thermodynamics of redox processes and the kinetics of heterogeneous electron-transfer reactions, as well as coupled chemical adsorption and reactions. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it can rapidly reveal the locations of the redox potentials of the electroactive species. CV is also used to measure the electrochemical surface area (ECSA, m /g catalyst) of electrocatalysts (e.g., Pt/C catalyst) in a three-electrode system with a catalyst coated glass carbon disk electrode as a working electrode [52]. Figure 21.9 shows a typical CV curve on Pt/C. Peaks 1 and 2 correspond to hydrogen electroadsorption on Pt(lOO) and Pt(l 11) crystal surfaces, respectively. The H2 electroadsorption can be expressed as Equation 21.35 ... 

During the electrode fabrication process, it is not guaranteed that all of the ECS A will be available for electrochemical reaction, due to either insufficient contact with the solid electrolyte or electrical isolation of the catalyst particles. Therefore, Pt utilization is one of the most important parameters for evaluating a catalyst layer and an electrode. Using the CV technique, Pt utilization can be determined by measuring the electrochemical surface area of the Pt catalyst and the active Pt surface area (SA, m ) in a catalyst layer. Pt utilization can de defined as the ratio of the active Pt surface area in a catalyst layer to the electrochemical surface area of the Pt catalyst. 

---