Platinum redeposition

A final example is that of the use of controlled-potential electrolysis for the preparation of carrier-free radioactive silver. Griess and Rogers isolated tracer quantities of radioactive silver, which had been prq>ared by neutron bombardment of palladium, by selectively depositing the silver onto a platinum surface. Although a small amount of palladium was codepo ted, complete separation was achieved by anodic stripping and redeposition. 

Figure 1.2 shows the thermodynamic equilibrium for mobile platinum versus potential for an acidic solution in equilibrium with an exposed platinum surface and a lower branch of equilibrium concentration at potentials more positive than approximately 1.1 volts relative to a hydrogen electrode, where the plotted concentration denotes the concentration of mobile species in equilibrium with a platinum oxide layer covering the surface. This shows that excursions to higher potential can rapidly increase the rate of platinum dissolution prior to passivation of the surface. Once the surface is passivated, the dissolution stops and redeposition can occur, albeit incomplete redeposition, as the platinum, once rendered mobile, is free to redeposit on larger particles or diffuse away from the catalyst layer altogether [32]. 

Platinum dissolution at high potentials and redeposition on larger particles (Ostwald ripening). 

The potential dependence of ECA loss for Pt/C ORR electrocatalysts has led most researchers to point toward modified Ostwald ripening as the dominant mechanism for performance loss at the PEMFC cathode. Here, Pt ions result from the dissolution of metallic and oxidized platinum surface species, which are subsequently redeposited onto larger particles due to the highly reducing environment of the PEMFC cathode and its operating electrode potential [15]. 

Fig. 22.2 Majcn- mechanisms for platinum degradatimi in PEMFCs. (1) Coalescence via migratimi of Pt nanopaTticles, (2) particle growth via the Ostwald ripening (dissolution and redeposition), (3) detachment of Pt...

It is understood that in the cycling potential range, platinum particles experience the dissolution and redeposition that leads to decrease of surface area and therefore an activity reduction. Diagnostic measurements include the ECSA determination (under H2/N2) and polarization curve recording (by switching to mode)... 

To use the alizarin test, the electrol5d ic precipitation of mercury must take place from neutral solution, since acid or alkaline solutions attack metallic aluminum sufficiently to give a strong reaction with alizarin. When the test solution is neutral, the mercury can be precipitated directly on an aluminum cathode, and the alizarin test carried out in situ. If, however, the test solution is acid or alkaline, the mercury must be deposited on a platinum wire cathode which, after washing with water, is connected as anode against a cathode of aluminum foil. On passing the current, the mercury dissolves from the anode and is redeposited on the aluminum and forms an amalgam. 

On the basis of results from micro-Raman spectroscopy, severe side degradation is found around the platimum redeposition line (Ohma et al. 2007a). The sulfate ion release rate on the cathode side is more than that on the anode side. A similar trend is also observed for the ERR. The results strongly support the role of the platinum line in membrane chemical degradation. 

In reality, the effect of ECSA change on DMFC durability may have limited direct impact on fuel ceU performance thanks to the specific properties of many DMFC systans. These properties are (1) inaeased specific activity of larger catalyst nanoparticles, possibly compensating for the loss in the catalyst surface area (Kinoshita et al. 1973), (2) relatively high loading of DMFC catalysts making catalysts less sensitive to surface area losses, and (3) redeposition of dissolved platinum (ruthenium) species on the surface of catalyst nanoparticles, reducing the rate of catalyst loss (Yasuda et al. 2006). 

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...

Figure 10 shows TEM images of an MEA following an open-circuit endurance test in which was supplied to the anode and to the cathode. The test conditions were a cell temperature of 90 C, 30% relative humidity, anode atmosphere of H, and cathode atmosphere of O. Similar to the results of the load-cycling test, it was found that platinum from the cathode catalyst layer dissolved and was redeposited in the electrolyte membrane. Under these test conditions, redeposited platinum particles were observed near the center of the electrolyte membrane. The position of redeposited platinum particles is determined by a balance between the mixed potential of the electrolyte membrane and the partial pressures of the anode and cathode O. It was estimated that platinum particles would be redeposited near the center of the electrolyte membrane under the conditions used in this test (Fig. 11). Chemical degradation of the electrolyte membrane was observed centered on the band of redeposited platinum particles. An analysis was made of the drain water discharged from the MEA during the test and fluoride ions were detected, which suggests that the electrolyte manbrane was partially decomposed (Ohma et al. 2007).

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