Platinum nanoparticles, degradation

In a typical Pt/C catalyst, there are several modes of degradation that can occur through the oxidation of the carbon support. Figure 3.1 illustrates many of the mechanisms of performance loss in a PEMFC which are mainly due to the loss of TPB active sites. The first method (Figure 3.1a) in which TPB sites become inactive is from the loss of contact between the catalyst particles and the polymer electrolyte membrane. Detachment of the membrane, also known as membrane delamination, can be caused by the corrosion of the carbon support and can result in protons being unable to reach the platinum nanoparticles. This inevitably leads to lessening of the Pt utilization and catalytic activity of the MEA. 

For a TPB site to exist, the catalytic metal must be bound onto the carbon support. The metal is usually deposited onto the support as dispersed platinum nanoparticles. To maximize the surface area and to increase the utilization of the active metal, these platinum particles usually are uniformly dispersed with particle diameters ranging from 3 to 8 nm for typical Pt/C catalysts. Poor adhesion to the carbon support or degradation of the support can lead to platinum particles being unanchored from the carbon. Unanchored platinum usually leaves the cell or migrates within the MEA by the assistance of water... 

The ionomer binder used inside the catalyst layer plays a crucial role in PEMFC performance (Makharia et al. 2005). It forms a pathway for protons to reach the platinum nanoparticles supported on carbon black. lonomer degradation in the catalyst layer will cause high ionic resistance and poor performance. 

One critical issue facing the commercialization of low-temperature fuel cells is the gradual decline in performance during operation, mainly caused by the loss of the electrochemical surface area (EGA) of carbon-supported platinum nanoparticles at the cathode. The major reasons for the degradation of the cathodic catalyst layer are the dissolution of platinum and the corrosion of carbon under certain operating conditions, especially those of potential cycling. Cycling places various loads on... 

In this review, we briefly discuss the dissolution and solubility of platinum (Sect. 2), the degradation of platinum nanoparticles in fuel cells (Sect. 3), and carbon corrosion (Sect. 4). We then describe new cathode electrocatalysts wherein the platinum content can be dramatically reduced, while offering possibiUties for enhancing catalytic activity and stability (Sect. 5). 

Iliev, V., D. Tomova, L. Bilyarska, A. Eliyas and L. Petrov (2006). Photocatalytic properties of Ti02 modified with platinum and silver nanoparticles in the degradation of oxalic acid in aqueous solution. Applied Catalysis B-Environmental, 63(3 4), 266-271. 

The thermal degradation of carbon and platinum-loaded carbon in air is not expected to take place below 100°C [147], although at higher temperatures, it was shown to be accelerated by the presence of Pt nanoparticles. However, the humidification of air substantially enhances the thermal corrosion rate of carbon, by providing an additional pathway for chemical carbon oxidation through a direct reaction with water [148, 149]. 

The membrane electrode assembly (MEA) is the heart of a fuel cell stack and most likely to ultimately dictate stack life. Recent studies have shown that a considerable part of the cell performance loss is due to the degradation of the catalyst layer, in addition to membrane degradation. The catalyst layer in PEMFCs typically contains platinum/platinum alloy nanoparticles distributed on a catalyst support to enhance the reaction rate, to reach a maximum utilization ratio and to decrease the cost of fuel cells. The carbon-supported Pt nanoparticle (Pt/C) catalysts are the most popular for PEMFCs. Catalyst support corrosion and Pt dissolution/aggregation are considered as the major contributions to the degradation... 

Fig. 3 Three mechanisms for the degradation of carbon-supported platinttm nanoparticles in low-temperahrre fuel cells, (a) Particle migration and coalescence, (b) Dissolution of platinum from smaller particles and its redeposition on larger particles (electrochemical Ostwald ripening), (c) Dissolution of platinttm and its precipitation in a membrane by hydrogen molecules from the emode...

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