Electrochemically active platinum surface area

The formation of a platinum oxide surface layer as given in Eq. (20.6) prevents dissolution or precipitation of platinum according to Eq. (20.5), since dissolution of platinum oxide as shown by Eq. (20.7) is rather slow [14, 15]. Precipitation of Pt + ions by reaction with hydrogen may occur within the membrane, forming an electrically insulated Pt band [16]. Platinum losing electrical contact with the electrode is referred to as catalyst islanding [17]. The electrochemically active platinum surface area decreases because of these mechanisms, resulting in performance... 

The results for constant potential holds over 400 h show that the loss of electrochemically active platinum surface area is neghgible at 0.87 and 1.2 V, respectively, whereas it is significant at 1.05 V [72]. The reason is that at 0.87 V the reaction kinetics are slow whereas at 1.2 V a PtO monolayer is formed, blocking any dissolution or precipitation. When the electrode is held at intermediate potentials, significant catalyst degradation occurs. 

The characterization of pure platinum catalysts and of Pt catalysts modified by lead was achieved in situ by linear potential sweep cyclic voltammetry. This technique allowed to measure the active platinum surface area in the absence and in the presence of deposited lead and to determine the surface fraction covered by lead adatoms (9-12). The adsorption stoichiometry of lead on platinum was also evaluated by electrochemical techniques and found to be equal to two (one lead atom covers two platinum atoms on the surface) (II). 

Nickel and Platinum—These two metals (in the form of Raney nickel and Pt-black) are used for electrocatalytic organic hydrogenation reactions (i.e., the electrochemical generation of hydrogen on the catalytically active, high surface area cathode followed by the chemical reaction of adsorbed hydrogen with the organic substrate). 

Especially at elevated temperature small particles tend to agglomerate, therefore a spatial separation is important. It was found [31] that the platinum surface area correlates with the BET surface of the carbon carrier material. This is easily understandable as a better dispersion of the noble metal particles leading to higher electrochemical activity. But as soon as the platinum particles reside in very small pores, smaller than 40 nm, they do not contribute to the electrochemical reaction anymore... 

Ferreira et al. (2005) used a similar accelerated catalyst degradation condition to investigate platinum surface area loss during fuel cell operation. MEAs with 0.4 mg Pt cm" loading in the anode and cathode were subjected to 10 000 cycles (0.6-1 V vs RHE 20 mV s sweep rate) at 80 °C cell temperature and fully humidified (100% RH) reactants (H2 and N2 in the anode and cathode respectively). Areduction of approximately 64% in electrochemically active surface area was observed at the end of 10 000 cycles. X-ray difiraction (XRD) and transmission electron microscopy studies (TEM) revealed a significant increase of platinum particle size (-2 tun initial... 

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

Electrochemical nuclear magnetic resonance (NMR) is a relatively new technique that has recently been reviewed (Babu et al., 2003). NMR has low sensitivity, and a typical high-held NMR instrument needs 10 to 10 NMR active atoms (e.g., spins), to collect good data in a reasonable time period. Since 1 cm of a single-crystal metal contains about 10 atoms, at least 1 m of surface area is needed to meet the NMR sensitivity requirement. This can be met by working with carbon-supported platinum... 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

In the case of the direct electrochemical approach, while the electrolysis conditions are less severe, the selection of the appropriate electrode material is still very important, and further reading on the use of stainless steel [93], platinum [94], graphite [95], doped Sn02 [92], doped Pb02 [86, 87, 96], and so on, is suggested. The economic viability of the electrochemical treatment approach is influenced in no small way by the cost and lifetime of the anode material this can easily make or break the field implementation of the process. Some authors have used high-surface area, porous anodes for cyanide treatment in order to combat the problems of mass-transport limitations so evident at cyanide concentrations below 100 ppm [88]. That system consists of a reticulated vitreous carbon porous anode that was activated for cyanide oxidation by the deposition of some copper oxide. The process looks very promising at the laboratory scale,... 

Figure 18-6 shows how a hydrogen electrode is constructed. The metal conductor is a piece of platinum that has been coated, or platinized, with finely divided platinum (platinum black) to increase its specific surface area. This electrode is immersed in an aqueous acid solution of known, constant hydrogen ion activity. The solution is kept saturated with hydrogen by bubbling the gas at constant pressure over the surface of the electrode. The platinum does not take part in the electrochemical reaction and serves only as the site where electrons are transferred. The half-reaction responsible for the potential that develops at this electrode is... 

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