Comparative Investigation of Hydrogen Embrittlement of Palladium Deposits from Ionic Liquid and Aqueous Electrolyte

Abstract

Protective coatings are necessary for most electrical contacts, since they are exposed to different environmental conditions such as high humidity or temperature. These coatings have to fulfill certain requirements, e.g., sufficient electrical conductivity, good wear and chemical resistance, good adhesion as well as appropriate hardness. For that purpose, noble-metal coatings and alloys are often used, deposited by electrochemical techniques. The chemical resistance of gold (Au) layers is excellent. At the same time, the electrical conductivity of Au is high (44.6 m V À1 mm À2 ) and therefore, it is, in principle, suitable as material for electrical contacts. [1] However, Au is very expensive and its hardness is quite low. A lower-priced alternative material is palladium (Pd), which is already widely used in several applications (e.g., telephone-type relays) because of its very low electrical noise levels and high hardness despite of having both a significantly lower electrical conductivity (9.3 m V À1 mm À2 ) and a lower chemical resistance than Au. [1,2] Nevertheless, one of the main issues of wet electrochemical methods is hydrogen embrittlement (HE) of the layers and, in consequence, a possible tearing and spalling of the protective layer during operation. Current efficiencies that are typically below 80% are achieved for Pd depositions from aqueous electrolytes (AE). [3] Hence, hydrogen formation and codeposition cannot be avoided during the process. The related occurrence of high tensile residual stresses up to 700 MPa limits the Pd layer thickness to <2.5 mm. [3,4] At the same time, the layer hardness increases and a brittle fracture behavior due to mechanical stress is expected. [5,6] To prevent these effects and increase the life time of the contacts, alternative deposition processes have to be considered instead of AE. Anhydrous ionic liquids (IL) seem to be the most promising candidates. IL have further advantages like non-volatility (no emission of volatile organic compounds into the atmosphere) and non-flammability. [7] Even though harmful direct contamination of the environment with IL has to be avoided, they are referred to as "green solvents" by several authors. [7][8][9] At the same time, there are more than 10 6 possible binary IL, which allows broad tuning of their properties (e.g., environmental, physical, and chemical behavior). [7] A future spread of the use of IL will also reduce their currently high costs, one of the main disadvantages of IL.Up to now, only few studies of Pd depositions using IL can be found in the literature. [10][11][12][13][14][15][16][17] Their main focus is the deposition process itself. In a recent publication, first wear investigations of Pd IL layers were performed. [18] According to the knowledge of the authors, no comparative investigations on HE of Pd layers (AE and IL) are available in the literature. This study presents the effects of HE on hardness, residual stresses, and on wear of Pd contact materials allowing for conclusions on the operational behavior.

Electrosorption of Hydrogen in Pd-Based Metallic Glass Nanofilms

“…52 Moreover, hydrogen embrittlement can occur when unbonded hydrogen remains for a long time in the samples. 17,53 For a study of the structure of the MGTF−Si assembly, detailed TEM investigations were carried out on a cross-section specimen. The bright-field TEM image in Figure 4a shows that the entire MGTF layer has a uniform thickness of 55 ± 1 nm.…”

“…This is an undesired way of hydrogen storage because, in this case, the hydrogen is not absorbed electrochemically but rather through physisorption, and cannot be easily desorbed . Moreover, hydrogen embrittlement can occur when unbonded hydrogen remains for a long time in the samples. , For a study of the structure of the MGTF–Si assembly, detailed TEM investigations were carried out on a cross-section specimen. The bright-field TEM image in Figure a shows that the entire MGTF layer has a uniform thickness of 55 ± 1 nm.…”

Electrochemical Reduction of Carbon Dioxide to Ethane Using Nanostructured Cu2O-Derived Copper Catalyst and Palladium(II) Chloride

“…Hydrogen embrittlement is a possible explanation for the desorption of the Pd 0 adsorbates from the surface of the Cu 2 O-derived Cu catalyst. , Hydrogen that is formed through the HER, could diffuse into the Pd metal to form α- and β- phase PdH x , which have lattice constants of 3.893 and 4.04 Å respectively. β-PdH x is thermodynamically unstable and spontaneously decomposed to give H 2 and α-PdH x .…”