X-Ray Absorption Spectroscopy Study of the Pd-H System during HOR in an Operating PEMFC
Authors: Siebel Armin , Gorlin Yelena , Durst Julien , Tromp Moniek , Gasteiger Hubert A. 
Published: February 2020

Volcano plots are frequently used in electrocatalysis to categorize and compare different catalysts and ultimately allow to predict the activity of novel materials on the basis of their binding energy for adsorbed intermediates.1 An accurate computation of the binding energy requires a good knowledge of the catalyst bulk and surface structure under reaction conditions. Applied to the hydrogen oxidation/evolution reaction (HOR/HER), calculations suggested that Pd should have an activity comparable to that of Pt due to their similar M-H bond strength.2 This suggestion, however, was recently disproven by unravelling the true HOR activity of the platinum-group metals (PGM): in fact, Pd is ≈100 times less active than Pt.3 This might be related to the fact that Pd is known to form a bulk hydride phase when exposed to H2 which is accompanied by an expansion of its crystal lattice. The stoichiometry x of this Pd-Hx phase depends on the gas phase activity of H2: an increasing pressure leads to a larger x.4 Moreover, the H2 partial pressure is related to the electrode potential via a simple Nernst relationship, and thus, it is possible that the hydride phase disappears after a certain overpotential is reached and the Pd lattice reverts to its original state leading to a change of the active surface.5 While there are examples of HOR polarization curves measured using the rotating-disk electrode (RDE) method, their results remain inconclusive, which might be a consequence of the limited mass-transport in liquid electrolytes compared to the gas phase.3,6 Thus, it is necessary to study the Pd-H interaction in a fuel cell where diffusion of H2 is much faster, in a so-called H2-pump configuration.7 In inert atmosphere, x can be evaluated from the anodic charge in a CV after electrolytic charging. However, in H2 atmosphere this is not possible due to large HOR currents masking a potential hydride desorption feature. Consequently, alternative characterization techniques are necessary to determine whether palladium catalysts under HOR/HER conditions are present as metallic hydrogen or as Pd-Hxphase.
The absorption of H2leads to a change of the palladium lattice parameter which can be obtained from the Extended X-ray Absorption Fine Structure (EXAFS), measured at the K-edge of Pd.
In this work, we show the correlation between gas-phase activity of H2 and electrode potential by recording absorption isotherms in N2 atmosphere at temperatures between 20-100 °C (cf. fig. 1). Subsequently, we apply this methodology to study the Pd-H phase composition during the HOR in a H2-pump experiment at 80 °C. In our operando measurements in H2 atmosphere, we clearly show that at overpotentials up to 70 mVRHE (RHE = reversible hydrogen electrode), a bulk hydride phase with a composition of ≈Pd-H0.6is present with an associated lattice parameter which is ≈3.2 % larger than in metallic Pd. Thus, the HOR on palladium catalysts takes places on the hydride phase of palladium rather than on a palladium metal phase.
References:
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Figure 1