Dynamics of Particle Growth and Electrochemical Surface Area Loss due to Platinum Dissolution

Abstract

A model for coalescence/sintering of Pt nanoparticles is developed to analyze particle growth and electrochemical surface area (ECSA) loss measured in aqueous tests and in catalyst-coated membrane and gas diffusion electrode-containing single fuel cells. The model combines a non-ideal solid solution theory for Pt dissolution with the dynamics of particle size evolution considering particle growth by Ostwald ripening and coalescence/sintering. Results from the model indicate that the observed growth in particle size and loss in ECSA in accelerated tests are primarily due to coalescence/sintering resulting from Pt dissolution and redeposition between particles. An enthalpy of dissolution of 49.3 kJ.mol −1 and an effective heat of fusion of 28.2 kJ.mol −1 for particle coalescence/sintering have been empirically determined from the measured temperature dependence of particle growth. The model suggests that higher Pt solubility in the presence of oxygen is responsible for enhanced particle growth and ECSA loss in accelerated tests in H 2 /air as compared to H 2 /N 2 . The model also indicates that the dissolution rate constant must be reduced by two orders of magnitude to explain the measured decrease in ECSA loss of the Pt cathode catalyst after 10,000 square potential cycles at 80 • C when the relative humidity is decreased from 100% to 30%.The inability to reach oxygen reduction reaction (ORR) activities in operating fuel cells that have been achieved in ex-situ, aqueous electrolyte measurements for ideal Pt-based crystal structures 1 and further loss of ORR activity under potential cycling 2 directly contribute to the remaining cost and durability barriers to mass commercialization of polymer electrolyte fuel cells (PEFC) for automotive applications. Ex-situ (aqueous) and in-cell studies have been conducted to investigate different aspects of the stability of Pt nanoparticles in PEFC cathode catalyst layers. Ex-situ studies in aqueous electrolyte offer the advantage that analytical techniques such as inductively-coupled plasma mass spectroscopy (ICP-MS) 3-6 and electrochemical quartz crystal microbalance (ECQM) 7 can be used to monitor Pt dissolution and Pt oxide growth under controlled potentials. The disadvantage is that the data may not be fully representative of the behavior of Pt catalyst in actual fuel cells.More common are in-cell studies that monitor loss in electrochemical surface area (ECSA) and degradation in cell performance. 8-10 The disadvantage is that the extent of Pt dissolution can only be estimated at the end of the test by using transmission electron microscopy (TEM) or X-ray diffraction (XRD) for particle size distribution. Only a small section of the electrode can usually be examined by TEM, making it difficult to get an accurate closure on Pt mass balance. The results are also difficult to interpret since more than one mechanism (Pt dissolution, transport, and precipitation) is involved.In a previous study, 5 we conducted ex-situ dissolution tests in an aqueous medium to investigate the ther...

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…An enthalpy of dissolution of 49.3 kJ.mol −1 and an effective heat of fusion of 28.2 kJ.mol −1 for particle coalescence/sintering have been empirically determined from the measured temperature dependence of particle growth. 446 A systematic study is still needed to investigate the effect of O 2 on the thermodynamics and kinetics of Pt dissolution in aqueous media. Additionally, the model suggests that a reduction in ionomer hydration limits the diffusion of Pt ions in the ionomer; data is needed to estimate the platinum dissolution rate dependence on ionomer hydration.…”

Potentiostatic and Potential Cycling Dissolution of Polycrystalline Platinum and Platinum Nano-Particle Fuel Cell Catalysts

“…40,41,48,49 It has been speculated that this accelerated ECA loss is predominantly due to Pt dissolution and that the potentialdependent loss processes are linked to the relationship between dissolution and formation and reduction of platinum oxides. 46,50,51 However, there is disagreement as to the role of Pt oxides in Pt loss. Some authors speculate that Pt dissolution primarily occurs during the positive-going potential scan/step and is assisted by the formation of oxide, either directly or as a result of surface roughening incurred during oxide reduction 20,[52][53][54] or to chemical dissolution of Pt oxides.…”

Screening Bifunctional Pt Based NSTF Catalysts for Durability with the Rotating Disk Electrode: The Effect of Ir and Ru

“…Cherevko et al 3 and Ahluwalia et al 4 demonstrated that Pt is also unstable during normal fuel cell operational potentials (≤1.0 V), i.e. when the formation of Pt oxide (and it's reduction) is somewhat negligible.…”