Bulk Thermodynamics Determines Surface Hydrogen Concentration in Membranes

Abstract

nonconverted reactants. In this step, hydrogen may be separated from the mixture, and reused in the reaction. In a future scenario with hydrogen as a main energy carrier, the separation and/or purification of the energetically costly hydrogen will become even more important. [1][2][3] A promising way is the use of hydrogen selective membranes made of hydrogen absorbing metals, such as Pd and its alloys. [4,5] The permeability of such membranes is determined by the surface properties of both sides (dissociation/recombination) and by the bulk permeability (diffusion and solubility). [4] There has been substantial research effort in finding cheaper materials with a higher permeability than that of Pd (e.g., V, Nb, Ta, and their alloys [6][7][8][9][10] ), however, expensive Pd and Pd based alloys remain the superior membrane materials owing to their favorable surface properties. [5,11] Cheap materials such as V-based alloys will revolutionize the technology, if their surface properties can be modified to match those of Pd. Despite this rather straightforward goal, there is a knowledge gap of what these desirable surface properties are. Most works refer to concepts from surface science which describe the physisorption, dissociation (barrier), and chemisorption of hydrogen. [12] However, additional steps-the hopping to subsurface sites and adjacent bulk sites-are needed to model the permeation process adequately. Nevertheless, due to the complex interaction of steps, the predictive power of modeling is limited [4,6,13] and-more importantly-experimental verification only possible by comparison with very basic experiments (permeation kinetics, e.g. ref.[14]) due to the lack of operando hydrogen analysis.Baldi et al. have demonstrated electron energy loss spectroscopy as an analysis method for bulk hydrogen in nano-particles. [15] In this paper, we further developed the method to be able to probe the surface hydrogen content of hydride thin films in situ by reflecting electron energy loss spectroscopy (REELS). The method is applied in an experimental approach, in which the surface properties of membranes can be intentionally modified and their hydrogen content determined under operating conditions. We demonstrate the existence of a rate-limiting step by direct observation of the dependence of the permeation on the hydrogen content in a Pd/V composite membrane. The modeling yields the relevance of the individual layers, making it possible to link the results to the ones obtained from hydrogen uptakeThe functioning of hydrogen selective metal membranes relies on the finetuned interaction of their surfaces with the inner bulk. The link between hydrogen permeation and surface concentration is unveiled using a novel combination of thin film preparation and surface science analysis to probe and intentionally modify the surface properties of palladium vanadium composite membranes. The in situ preparation allows for observation of ultraclean hydrogenation and the quantification of the permeation as a function of temperature and p...

Why Hydrogen Dissociation Catalysts do not Work for Hydrogenation of Magnesium

“…The growth of a MgH 2 layer is still possible under dynamic conditions as applied in Figure 6, because the local chemical potential in the sample is higher than required to form the hydride. [ 66 ] The local chemical potential is defined by chemical potential at the membrane feed side (which is well above the plateau pressure), and the flux of hydrogen through the entire thin film system, which depends on the kinetics. If the kinetics was desorption limited, the complete Mg film would be hydrogenated; in particular the top layer would be MgH 2 , too.…”

Hydrogen Interactions with Defects in Materials

“…Therefore, the surface conditions have to be considered as they influence the hydrogen permeation. [68] Low hydrogen permeation coatings can be used as hydrogen barriers, to protect the material from external hydrogen. Ro ¨nnebroe et al recently reviewed the actual hydrogen barrier coatings like oxides, nitrides, carbon, carbides, MAX-phases (layered, hexagonal carbides and nitrides, with M being an early transition metal, A being an A-group element and X being either carbon and/or nitrogen) and metals.…”

“…Therefore, the surface conditions have to be considered as they influence the hydrogen permeation. 68…”