(326f) Uncovering the Dynamics of Oxygen Reduction Reaction Catalysts during Synthesis and Application Using in-Situ/Operando Techniques to Improve Catalyst Designs

Hydrogen fuel cells are key technologies for the storage and conversion of renewable energy.¹ Especially the improvement of oxygen reduction reaction (ORR) catalysts is crucial for a widespread application of hydrogen fuel cells due to the kinetically hindered ORR. Current state-of-the-art catalysts consists out of expensive platinum (Pt). One way to reduce the fuel cell device costs is therefore the increase of Pt utility and durability, another is to replace Pt by non-Pt-group metals. For both strategies the understanding of the catalyst synthesis mechanism and the dynamic nature of the catalysts during electrochemical application play a key role during the development of improved catalysts.

To understand utility and stability limitations of Pt catalysts, a catalyst synthesis approach allowing to control different material properties independent of each other is favorable. The polyol process is such a toolbox approach to prepare tailored “surfactant-free” Pt nanoparticle catalysts. Stable Pt nanoparticles are formed without the presence of “surfactants” that would otherwise require removal. A Pt nanoparticle size control is reached by the amount of hydroxide present during the nanoparticle synthesis and the narrow size distributions allow to investigate a particle size-dependent catalyst performance.² In order to understand the particle formation mechanism, the nanoparticle synthesis was slowed down by a visible-light-induced Pt precursor reduction instead of the common thermal induction. This enabled the in-situ identification of reaction intermediates and the proposition of a synthesis mechanism, an important step forward to understand the nanoparticle size control during the polyol process.³

When designing and optimizing catalysts, a decent electrochemical stability under realistic fuel cell device conditions is desirable and therefore the identification of their degradation mechanism is of high interest. Four nanoparticle degradation mechanisms of the oxygen reduction catalyst are expected: particle dissolution (material loss), followed by redeposition via Ostwald ripening (particle growth), i.e., larger particles grow at the expense of smaller ones via the formation of ionic species, particle agglomeration/coalescence (also particle growth), i.e., merging of particles, or particle detachment (materials loss without size change).⁴ To reduce the stability testing time and increase catalyst degradation, accelerated stress test (AST) protocols are applied. For oxygen reduction catalysts fuel cell application conditions are simulated. The gas diffusion electrode (GDE) setup is a straightforward testing device that enables the performance of such ASTs and allows realistic testing conditions with respect to catalyst loading and temperature close to fuel cell application conditions. Combining ex-situ small angle X-ray scattering (SAXS) to determine the particle size changes and electrochemically active surface area (ECSA) measurements after the performance of ASTs revealed despite differing initial nanoparticle sizes of Pt on carbon deposited (Pt/C) catalysts similar end-of-treatment sizes. As degradation mechanisms particle coalescence or Ostwald ripening and particle detachment were identified.⁵

Trying to differentiate between both growing mechanisms (particle coalescence and Ostwald ripening) and to promote the electrochemical Ostwald ripening, a bimodal Pt/C catalyst consisting of two distinguishable Pt/C particle size populations was probed. To uncover the catalyst dynamics on a fundamental level during the electrochemical operation, in-situ SAXS was performed while performing ASTs. Due to the comparison of the particle size changes of the bimodal Pt/C catalyst to the single size distribution Pt/C catalysts, electrochemical Ostwald ripening could be identified.⁶ A grazing incidence configuration during the in-situ SAXS allowed the detection of a degradation depending on the depth profile within the Pt/C catalyst layer suggesting a future depth-dependent catalyst design.⁷ Therefore, in-situ SAXS was shown as a strong technique to track degradation mechanisms of nanoparticle catalyst systems to develop improved catalysts.

To reduce catalyst material costs non-Pt group are of high interest but due to stability limitations more accessible in alkaline environment. In a recent work the dynamic nature of MnO_x deposited on a thin Ag film (MnO_x@Ag) was investigated in-situ/operando by X-ray absorption near-edge spectroscopy (XANES). The observed MnO_x valence dependency on the microenvironment and catalysis offers new opportunities for catalyst optimization by pre-conditioning or in-situ stabilization.⁸

Overall, in-situ/operando tracking of the dynamic nature of catalysts during preparation and application allows to understand parameter impacts and catalyst limitations. The identifications of those challenges offer great opportunities to develop improved catalysts.

References:

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(8) Schröder, J. A. Zamora Zeledón, G. A. Kamat, M. E. Kreider, L. Wei, A. Mule, A. Torres, K. Yap, D. Sokaras, A. Gallo, M. Burkes Stevens, T. F. Jaramillo, in preparation.