Van Marum Colloquium: Shedding Synchrotron Light on Catalyst Atomic Rearrangement and Strain Dynamics in Electrochemical Environment
- Thursday 9 March 2023
2333 CC Leiden
In the case of heterogeneous catalysis (where the reactions occur onto the catalyst surface), the most promising approach in material design follows the Sabatier principle: the ability of the surface to bind adsorbates and the strength of the bonds define the reaction thermodynamics and kinetics. In this respect, the catalyst surface chemistry and structure (crystallographic orientation of the facets and/or strain) are used as levers to optimize its performance. However, it is largely accepted that the structure of PEMWE anode and PEMFC cathode catalysts does not remain unaltered over the long term operation due to harsh (electro)chemical conditions and it is of a high interest to understand the structural change, and dissolution, during the electrocatalysis. In this contribution we use X-ray diffraction techniques to qualitatively and quantitatively describe those changes on both model and applied Pt based electrochemical systems.
To elucidate the role of the Pt surface structure on the surface restructuring and dissolution, we performed in-depth comparative studies of Pt(111) and Pt(100) in perchloric acid solution by in situ SXRD, online inductively coupled plasma mass spectrometry (ICP-MS) and DFT . Clear differences were found, which mirror the differences in the dissolution behavior of the two electrode surfaces. Pt(100) not only start to dissolve at lower potential but its oxidation also immediately changes its surface structure in an irreversible manner. These observations demonstrate clearly that Pt dissolution is linked to the way Pt atoms are initially extracted from the surfaces during oxidation.
In the case of Pt nanocatalyst, monitoring the structural features of the catalyst during cyclic voltammetry permits studying adsorption and oxidation processes. Deconvoluting both effects (adsorption and oxidation) is possible by following different parameters of the analyzed XRD patterns with high temporal resolution . Carefully determining the charge passed through the circuit during the CV also allows to establish a near-linear correlation between the expansion of the bulk lattice parameter of Pt nanoparticles and their oxide surface coverage . Since the latter is known to be a descriptor of catalysts activity toward numerous reactions (oxygen reduction and fuel oxidation), the measurement of the lattice parameter according to this simple diffraction approach allows experimental access to this descriptor during operando measurements in device-relevant sample environments. This constitutes a main advantage compared to spectroscopic techniques and is foreseen to find a wide range of applications.
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