Platinum dissolution kinetics

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

Darling RM, Meyers JP. 2003. Kinetic model of platinum dissolution in PEMPCs. J Electrochem Soc 150 A1523-A1527. 

R. M. Darling, and J. P. Meyers, Kinetic model of platinum dissolution in PEM fuel cells in E. Society (Ed.), Proton Conducting Membrane Fuel Cells III, 2005, p. 44. 

R. M. Darling and J. P. Meyers, "Kinetic Model of Platinum Dissolution in PEMFCs," Journal of the Electrochemical Society, 150 (2003) A1523-A1527. 

By means of a transient ID model. Darling and Meyers [15] studied the kinetics of platinum dissolution under constant load and load cycling conditions. They followed Eqs. (20.5-20.7) assuming that the oxidation of Pt to PtO was the dominant effect and assuming slow kinetics for the chemical dissolution of platinum. The formation of platinum oxide was allowed to exceed one monolayer on the platinum particles, leading to a surface coverage of 6 > 1. This assumption is based on experimental results showing that 0 = 1 is reached at an electrode potential of — 1.15 V [69-71]. Once a PtO monolayer is formed, further dissolution or precipitation of platinum is inhibited. 

Ahluwalia RK, Arisetty S, Wang X et al (2013) Thermodynamics and kinetics of platinum dissolution from carbon-supported electrocatalysts in aqueous media under potentiostatic and potentiodynamic conditions. J Eletrcochem Soc 160 F447-F455... 

Cathode kinetics Both cells very similar No specific influence of start/ stop cycling Platinum dissolution dining start/stop cycling insignificant Increase in ORR overpotential in start/stop ceU Reduction of ECSA due to loss of structural integrity of cathode catalyst layer... 

This study was on a smooth platinum foil using gravimetric analyses and it was found that the equilibrium concentrations of platinum in the phosphoric acid were achieved rapidly, i.e., within one hour. Since it is to be expected that the kinetics of dissolution would be modified with high surface-area platinum crystallites, this equilibrium would be achieved more rapidly. Consequently, it is clear that, at the higher potentials, dissolution of the platinum is of great concern so that operating procedures must be established to prevent exposure under hot open-circuit conditions. 

Reference [147] reports the simultaneous analysis of spatiotemporal pattern formation in electrolytic oxidation of sulfide on a platinum disk using electrochemical methods and a CCD camera. A series resistor and a large electrode area facilitated measurement of kinetic instability across a wide range of the potential curve. Spatial patterns such as pulses and labyrinthine stripes, as well as temporal patterns like lionts, twinkling eyes, and alternate deposition and dissolution were observed at different experimental conditions. 

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. 

Damjanovic A, Dey A, Bockris JOM (1966) Kinetics of oxygen evolution and dissolution on platinum electrodes. Electrochim Acta 11 791-814... 

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