Catalyst degradation acid loss

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... 

Other than the membrane degradation which leads to an MEA failure, the main causes of the voltage decay of HT-PEMFC MEAs in the literature are catalyst degradation and acid loss. Eigure 16.2 shows a schematic diagram that... 

The high operation temperature and the presence of phosphoric acid in HT-PEMFC MEAs are reported to accelerate the catalyst degradation [48, 55, 58, 60]. The reported values of the fraction of remaining active surface area (SA/SAo) after cell operation are summarized in Table 16.3. The table indicates that the most influential factor in loss of the surface area is the operation temperature. The 3D KMC simulation results indicated that the fraction of the remaining Pt surface area after 15,500 h at 150 °C was 0.79, and the value decreased to 0.60 when the operation temperature increased to 190 °C despite the considerably shorter cell operation time of... 

A nttmber of degradation mechanisms have been proposed for PBI-based MEAs, such as phosphoric acid loss from the membrane, faster catalyst dissolution in the hot acid meditrm, Pt catalyst sintering, thermal stress on fuel cell parts, thermal degradation of the catalyst support and carbon support corrosion. In particttlar, phosphoric acid loss has been specrrlated as a major degradation... 

Extensive studies on catalyst degradation in PEMFCs and phosphoric acid fuel cells (PAFCs) demonstrated that its cause can be attributed mainly to a loss of ECA in the cathode. PEMFCs and PAFCs use similar catalysts, although the degradation... 

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 ... 

One of the most critical parameters in the SBA production is the process temperature. Since the catalyst performance is slowly degrading because of SO3H loss, to maintain constant productivity, the temperature of the catalyst bed needs to be gradually raised to approximately 170 °C. This leads to further loss of acid functionality and an increase in the level of di-,sec-butyl ether. 

Degradation of poisoning phosphite [27] may lead to the formation of an aldehyde acid, as shown in Equation 2.8. The concentration of aldehyde acid and phosphorus or phosphoric acids should be monitored and controlled to minimize losses of the desired catalyst modifying ligand. 

In carrying out the alkylation of benzene the propylene tetramer is reacted with an excess of benzene in the presence of a Friedel-Crafts catalyst such as aluminum chloride, boron trifluoride, or hydrofluoric acid. With careful control of this reaction, yields of alkylate boiling from 500° to 650° F. are of the order of 80% of theory with the losses due to slight olefin degradation and dialkylation. Inspection of commercial aromatic products, believed to be typical of this process, indicates the composition to be that shown in... 

Bronsted acids such as sulfuric and hydrochloric acids and their ammonium salts, while serving as excellent catalysts, also lead to undesirable fibre degradation and unstable finish baths. Often citric acid is combined with a Lewis acid to provide an additional boost to the reaction, especially for the short shock condensation. A particularly powerful co-catalyst for ether modified DMDHEU products is sodium borotetrafluoride Na(BF ). But this flash catalyst may cause cellulose depolymerisation leading to high strength loss. 

It can be observed that the non-oxidative stability of all systems increases as a function of age time under both environments. Each system exhibits an initial rapid increase in thermal stability over the first 24 hours of aging followed by a more gradual increase over the remainder of the study. It is reasonable to assume that these initial increases are in part due to the loss of pro-degradants such as 2-ethylhexanoic acid and other reaction by-products such as propanol from the elastomer system (2-ethylhexanoic acid is a hydrolysis product of the active organotin catalyst, tin(II) 2-ethylhexanoate and propanol is formed during the TPOS/silanol condensation reaction). 

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