Electrosorption of Hydrogen in Pd-Based Metallic Glass Nanofilms

Abstract

As an efficient potential hydrogen storage and conversion system, hydrogen electrosorption and evolution mechanisms in Pd-based metallic glass thin films (MGTFs) are investigated. In this study, thin films of 55 nm thickness were deposited by dc magnetron sputtering. The amorphous structure of MGTFs and the atomically smooth interface between the MGTF and substrate were confirmed by transmission electron microscopy, whereas the compositiondependent surface roughness was obtained via atomic force microscopy. The shifts in the broad diffraction maxima for the Si and Cu additions were evaluated by X-ray diffraction. The Pd thin film (PdTF) and MGTF working electrodes were chronoamperometrically saturated in 0.5 M H 2 SO 4 solution. The formation of palladium hydride (PdH x ) in the MGTFs was investigated by X-ray photoelectron spectroscopy. Cyclic voltammograms were subsequently recorded (between −0.2 and 1.4 V) at sweep rates of 0.02 V s −1 . Electrochemical impedance spectroscopy of MGTFs and PdTF was performed in full spectrum including sorption, desorption, and evolution of hydrogen in a conventional three-electrode configuration. Electrochemical circuit modeling provided the relationship between the composition-dependent hydrogen evolution and H absorption/adsorption processes. The adsorption capacitance parameter Y ad corresponding to αand β-hydride formation in the case of Pd 0.79 Si 0.16 Cu 0.05 MGTF is ∼5 times higher than that of the crystalline Pd thin film which is in line with the decrease in the charge-transfer resistance R ct . Addition of Cu disturbs the symmetry of the glass formers, leading to remarkable changes in interfacial hydrogen bonding and diffusion of hydrogen into sublayers. Compared to other Pd-based micron-sized materials, our findings show excellent volumetric hydrogen storage capacity 4 times higher than that of the traditional counterparts of several microns, and 50% higher than the Pd thin films of the same thickness, together with high tunable capacitance, charge-transfer resistance, and diffusivity depending on the glass-forming characteristics of the nanosized MGTF.

Ultrahigh hydrogen-sorbing palladium metallic-glass nanostructures

“…On the other hand, our previous findings obtained from the electrochemical circuit modeling (ECM) show that hydrogen absorption in Pd-based glassy thin films is mainly independent of this roughness factor. 17 Compared to the inhomogenously scattered bumps obtained from the 3D profile of PdTF as shown in Fig. S1b (ESI †), the MG shows a more homogenous dispersion of surface asperities.…”

“…This results in broadening of the first sharp peak of the Pd sample, where this peak is strongly influenced by the contributions of Pd-Si and Pd-Cu bonds. 17 The first diffuse peak maximum of Pd 84 Si 16 (2y = 40.281) shifts to 2y = 40.541 for the 5 at% Cu addition (Pd 79 Si 16 Cu 5 ). The more Pd is replaced with Cu, the bigger the shift in 2y towards larger values.…”

“…Si 2p 3 , Cu 2p 1/2 , and Cu 2p 3/2 peaks do not shift after hydrogenation (as confirmed in our previous work). 17 Thus, no chemical interaction between Si and H, and Cu and H has been detected after hydrogenation. For this reason, Pd is the main metal interacting with the sorbed hydrogen from the active sites of the MG sample.…”

“…16 In our previous study, the electrosorption (adsorption, absorption, desorption, and evolution) behaviour of hydrogen and kinetic parameters such as charge transfer resistance, as well as the double layer and adsorption capacitance were investigated using a combination of cyclic voltammetry and electroimpedance spectroscopy. 17 Depending on the Cu content, PdSi-based metallic glass nanofilms can remarkably enhance metal-hydrogen interaction and palladium hydride (PdH x ) formation. Although this phenomenon can be linked to the non-translational structural motifs on the nm scale and the free volume in Pd-based metallic glasses, which probably generates additional interstitial sites for a higher amount of hydrogen sorption, [18][19][20] the mechanism and extent of palladium hydride formation in these multicomponent amorphous nanofilms are still unclear.…”

“…In our previous study, the replacement of Pd with Cu by only 5% was proven to have a tremendous increase in the double layer and adsorption capacitance of the PdSi-based MG thin films. 17 The adsorption capacitance parameter, Y ad , calculated from the impedance values of the constant phase element (details given in the ESI † -Materials and methods, Electrochemical measurements) is shown in Fig. 2b.…”