Platinum electrocatalyst

Despite their high cost, they are used in industrial electrolyses, fuel cells, and many electrochemical devices. The large investments associated with platinum electrocatalysts usually are paid back by appreciably higher efficiencies. 

Kinoshita K, Lundquist JT, Stonehart P. 1973. Potential cychng effects on platinum electrocatalyst surfaces. J Electroanal Chem 48 157-166. 

Stonehart P, Kohlmayr G. 1972. Effect of poisons on the kinetic parameters for platinum electrocatalyst sites. Electrochim Acta 17 369-382. 

Kobayashi T, Babu PK, Gancs L, Chung JH, Oldfield E, Wieckowski A. 2005. An NMR determination of CO diffusion on platinum electrocatalysts. J Am Chem Soc 127 14164-14165. 

Dameron, A.A., et ah, Aligned carbon nanotube array functionalization for enhanced atomic layer deposition of platinum electrocatalysts. Applied Surface Science, 2012. 258(13) ... 

M. Van Brussel, G. Kokkinidis, I. Vandendael, and C. Buess-Herman, High performance gold-supported platinum electrocatalyst for oxygeu reductiou, Electrochem. Commun. 4, 808—813 (2002). 

M. Morita, Y. Iwanaga, and Y. Matsuda, Anodic-oxidation of methanol at a gold-modified platinum electrocatalyst prepared by RE-sputtering on a glassy-carbon support, Electrichim. Acta 36, 947-951 (1991). 

Figure 13.19 shows two performance curves. The platinum electrocatalyst is supported on C in one. The marked advantage of the more conducting TiC as support (less IR drop through the support) is shown. 

The application of gold as an electrocatalytic component within the fuel cell itself has to date been limited primarily to the historical use of a gold-platinum electrocatalyst for oxygen reduction in the Space Shuttle/Orbiter alkaline fuel cells (AFC)88 and the recent use of gold for borohydride oxidation in the direct borohydride alkaline fuel cell (DBAFC).89,90 Electrocatalysts with lower cost, improved carbon monoxide tolerance and higher... 

The most important operational features are the relationships between the crystallite sizes of the platinum electrocatalysts to the specific (A.real m"2 Pt) and the mass (Ag 1 Pt) activities. These features are most directly applicable to the efficiency and utilization of the catalyst in operating fuel-cells. 

For practical purposes, there is a trade-off between increasing the inter-crystallite distance on the carbon surface (to maximize the platinum crystallite specific activity by lowering the platinum loading in the electrode) and the electrode thickness, since as the loading becomes smaller, then the electrode thickness must increase to obtain a sufficient mass of platinum electrocatalyst in order to achieve the required current-density. Unfortunately, thicker electrodes then lead to progressive ohmic losses within the electrode structures. 

Physical properties of the carbon substrate, such as electronic conductivity, surface area, and surface morphology are important, since the former can contribute to resistive losses in the electrocatalyst structure and the latter may determine the sites on which the platinum electrocatalyst crystallites may be located. Both the initial deposition of the platinum crystallites on the carbon substrate and the subsequent surface area loss of the platinum crystallites under operating conditions by a surface migration mechanism can be influenced by the surface carbon structure. 

Whereas the rate-determining step for hydrogen molecule oxidation now is recognized69,70 to be the dissociative chemisorption of the hydrogen molecule on dual sites at the platinum surface, the rate of this step is so high that in most electrochemical environments platinum electrocatalysts are almost always operating under diffusion control. 

The poisoning influence of carbon monoxide diminishes as the temperature increases and at a 1 % and 2 % CO concentration, the hydrogen molecule oxidation rate approaches that of an unpoisoned platinum electrocatalyst at 200 °C. 

Kinoshita reported a correlation between the fraction of Pt surface atoms on the (10 0) and (111) crystal faces of the platinum particles of cubo-octahedral structure with varying particle sizes and the specific catalytic activity of platinum electrocatalysts 54). For cubo-octahedral particles, which have both (1 1 1) and (10 0) faces, an optimum in mass activity at a 3.5-nm platinum particle diameter was reported. Under these conditions, the surface fraction of platinum on (1 00) and (111) faces shows a maximum according to calculations of the coordination number with changing average particle size (55). 

Stevens, D.A. and Dahn, J.R., Thermal degradation of the support in carbon-supported platinum electrocatalysts for PEM fuel cells. Carbon, 43, 179, 2005. 

L.D. Burke, J.K. Casey, The role of hydrous oxide species on platinum electrocatalysts in the methanol/air fuel cell. Electrochim. Acta 1992, 37(10), 1817-1829. 

Typically, carbon electrodes with a platinum electrocatalyst are used for both the anode and cathode. The PEM used in the fuel cell serves to keep the fuel separate from the oxidant, and also as an electrolyte. 

Phosphoric acid concentrated to 100% is used as an electrolyte in this cell that operates between 150°C and 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor and carbon monoxide poisoning of the platinum electrocatalyst in the anode becomes severe. The relative stability of concentrated phosphoric acid is high compared to other common acids. Consequently, the PAFC is capable of operating at the high end of the acid temperature range (100°C-220°C). In addition, the use of concentrated acid (100%) minimizes the water vapor pressure, so water management in the cell is not difficult. The matrix universally used to retain the acid is silicon carbide and the electrocatalyst in both the anode and the cathode is platinum. 

K. Yasuda and T. Ioroi, Carbon back supported platinum electrocatalysts for polymer electrolyte fuel cell, Hyomen (Surface), 2000, 38, 55-67. 

Even compounds with a complex structure such as cellulose can be oxidized quantitatively to CO2 using platinized-platinum electrocatalysts (100). This is of interest because of the difficulty of chemical or biochemical oxidation of cellulose. A cellulose-air fuel cell could yield about 1 mw cm of power. The disposal of body wastes could be accelerated electrochemically. 

Platinum was the initial choice for both the anodic and cathodic electrocatalysts for the following reasons (1) platinum shows minimum degradation or corrosion in acid or when used as an anodic or cathodic electrocatalyst (2) recent technical advances have been made in forming efficient high surface area porous platinum electrocatalyst structures at the surface of PEM membranes and (3) preliminary work... 

Also, a group of researchers at University of Texas at Austin used inkjet for rapid screening of non-platinum electrocatalysts such as Pd-Ti and Pd-Co-Au which show electrochemical performance similar to that found with commercial platinum catalysts [35]. 

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