Suppression of hysteresis in absorption of hydrogen by a Pd-Au alloy

Abstract

Hydrogen absorption by Pd exhibits hysteresis loops (provided the temperature is not too high) and represents one of the classical examples of first-order phase transitions in metals. Experiments indicate that addition of even a small amount of Au is able to suppress hysteresis. From this perspective, we analyze the energetics of hydrogen in a Pd-Au alloy by using extensive density-functional-theory (DFT) calculations. The dependence of the hydrogen binding energy on the number (n) of Au atoms forming an adsorption site is found to be appreciably nonlinear. With the DFT input for statistical calculations, we reproduce special features of the hydrogen absorption isotherms and explain the rapid decrease of the corresponding critical temperature with increasing Au fraction. The key factor here is that the phase transition is related primarily to absorption in sites formed only by Pd. With increasing Au amount, the fraction of such sites rapidly decreases, the distance between H atoms located there becomes on average larger, the interaction between them becomes weaker, and accordingly the driving force for the phase transition decreases. It is of interest that all these effects can be illustrated by taking only the configurations with n 2 into account. This means that in the context under consideration the fine details of the dependence of the hydrogen binding energy on n are in fact not too important.

A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd-Au-H

“…5b-c), the SRO parameter is negative, indicating that H is more likely to occupy sites next to Pd than next to Au. This confirms what has been observed both with Mössbauer spectroscopy [60] and firstprinciples calculations [24], namely that it is energetically unfavorable for hydrogen to occupy the sites closest to a Au atom. For next-nearest Pd/Au-H/vacancy neighbors (Fig.…”

“…3b). As has been previously reported [24] and can be expected from the ECIs (Fig. 2), occupation of hydrogen at such sites is energetically particularly favorable, and renders this long-range ordered phase particularly stable.…”

“…In Pd-Au hydride, hydrogen primarily occupies octahedral interstitial sites in the face-centered cubic (FCC) lattice formed by Pd-Au (although in some configurations, tetrahedral site may be preferred [24,35]). The full system can thus be viewed as a rock salt structure, i.e., two intertwined FCC sublattices, one occupied by Pd and Au, and the other populated with H and vacancies.…”

“…A few previous studies have successfully used combinations of density-functional theory (DFT) calculations and statistical methods to predict properties of hydrogen in Pd-Au. For example, Mamatkulov and Zhdanov [24] recently developed a model based on DFT calculations that successfully predicted absorption isotherms, highlighting that once octahedral sites without surrounding Au atoms become few, the phase transition and its associated hysteresis disappears. These calculations assumed a Pd-Au lattice with random configuration, which remained unaffected by insertion of hydrogen.…”

High-Performance Nanostructured Palladium-Based Hydrogen Sensors—Current Limitations and Strategies for Their Mitigation

“…Mechanistically, this can be understood as the Au atoms, which occupy Pd lattice sites, slightly expanding the Pd host. Thereby, they reduce the strain-induced energy barrier created upon hydrogen sorption, and thus hysteresis. , To this end, Wadell et al have shown that alloying Pd with 25 at % Au reduces sensor uncertainty to below 5% across the 1–1000 mbar H 2 pressure range, which is on par with the corresponding performance target by the US DoE. Similarly, alloying with metals with smaller atomic radius than Pd, e.g., Cu and Ni, also suppresses hysteresis at ca.…”