A Combined SAXS and XAS Setup for Operando Electrocatalyst Degradation Studies
Authors: Povia Mauro , Herranz Juan , Binninger Tobias , Abbott Daniel F. , Kim Baejung , Nachtegaal Maarten , Schmidt Thomas J. 
Published: February 2020

The noble metal nanoparticles (NPs, including oxides and/or alloys) customarily used to catalyze the electrochemical reactions at play in low temperature fuel cells and electrolyzers are prone to potential-induced dissolution1 which leads to changes in their composition and morphology. Minimizing the impact of these processes on the devices’ operation requires a better understanding of their mechanism under application-relevant operando conditions. In this regard, small angle X-ray scattering (SAXS) provides information about a sample’s NP size distribution and agglomeration, while X-ray absorption spectroscopy (XAS) is sensitive to the electronic (oxidation state) and geometric (identity and number of coordinating atoms, bond distances) structure of the element of interest. The combination of these two techniques in an operando setup would result in novel insight on the degradation mechanisms affecting noble metal nanoparticles.

With this motivation, we have developed a novel setup in which the gas ionization chambers and flight tube plus 2D-detector required for XAS and SAXS acquisition respectively, are mounted in parallel on a movable table that allows to transition among techniques within a couple of minutes. This configuration allows to use both techniques in their optimal configuration, without losing signal quality while compromising on one technique while combining it with the other (as done in the past).2 To validate this approach it was used to study the operando degradation of two carbon-supported Pt-NP (Pt/C) catalysts customarily used in polymer electrolyte fuel cells, which constitute a relatively well understood system owing to the large volume of work that has been devoted to its study.3 Specifically, commercial Pt on Vulcan and Pt on graphitized Black Pearls catalysts (Pt/V and Pt/BP-g, respectively – both with 30 wt. % Pt) were processed into electrodes and electrochemically tested in an operando flow cell developed in-house.4 The degradation protocol consisted of 250 potential cycles between 0.5 and 1.5 V vs. the reversible hydrogen electrode (RHE) at 50 mV∙s-1, using 0.1 M HClO4 at 60 °C as the electrolyte. Complementing results derived from both techniques with respect to changes in particle diameter as well as oxidation state of the catalyst particles are then used to quantifiy NP-growth rates, correlation of particle size with Pt-Pt bond distance as well as NP oxide coverages. In summary, this contribution will introduce a novel, combined SAXS and XAS setup devoted to (electro)catalyst research, and exemplify its potential application based on the results derived for two Pt/C catalysts, which are fully consistent with previous literature on such materials.

Cherevko, A. R. Zeradjanin, A. A. Topalov, N. Kulyk, I. Katsounaros, and K. J. J. Mayrhofer, ChemCatChem
6, 2219 (2014).

Nikitenko, A. M. Beale, A. M. J. van der Eerden, S. D. M. Jacques, O. Leynaud, and M. G. O‘Brien, J. Synchrotron Rad.
15, 632 (2008).

Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, F. Holby, and D. Morgan, Top. Catal.
46, 285 (2007).

Binninger, E. Fabbri, A. Patru, M. Garganourakis, J. Han, D. Abbott, O. Sereda, R. Kötz, A. Menzel, M. Nachtegaal, and T. J. Schmidt, J. Electrochem.
Soc.
163, H906 (2016).

A. Gilbert, N. N. Kariuki, R. Subbaraman, A. J. Kropf, M. C. Smith, E. F. Holby, D. Morgan, and D. J. Myers, J. Am. Chem. Soc.
134, 14823 (2012).

Lei, J. Jelic, L. C. Nitsche, R. Meyer, and J. Miller, Top. Catal.
54, 334 (2011).