Hydrogen Absorption into Metals

Besides hydrogen adsorption and evolution, hydrogen absorption into metals might occur. It is observed in Pd and certain alloys of the type AB5 (e.g., LaNis) or AB2 and is used in metal hydride batteries. The theory developed here is also applicable to other reactions, e.g., Li intercalation in Li-ion batteries. Let us consider first the simplest adsorption-absorption reaction [272]. 

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Lasia A, Gregoire D. General model of electrochemical hydrogen absorption into metals. J Eleetrochem Soc 1995 142(10) 3393-3399. 

Hydrogen evolution can be used where reduction of hydrogen ions is the cathodic reaction, e.g. in acidic solutions. The method can be cumbersome, because the solubility of hydrogen in the solution and hydrogen absorption into metals must be considered. The method is most practical at high rates of corrosion in acids, but is not too commonly used. In one case, the technique has been used for rapid screening of acid inhibitors. 

An alternative to storing hydrogen as a gas or liquid is to store it by absorption into metal hydrides. These have the advantage of low volume over most other storage methods, but tend to be heavier. Additionally, the pressures of operation are typically below 10 bar, much lower than compressed gas cylinders. This allows the tank to be constructed in geometries not used for pressure vessels, which facilitates better space utilisation. 

Using a constant corrosion rate multiplied by the adsorption efficiency measured as described above, the rate of hydrogen absorption into the metal was calculated, and susceptibility to HIC was assumed to be established once a critical hydrogen concentration (Hc) was reached. A more detailed discussion of this simple conservative model, including a description of the determination of Hc from mechanical experiments, is described elsewhere (33). The conservatism in the model arises from the assumption that all the hydrogen absorbed is retained by the metal rather than released by oxidation as the corrosion process proceeds through the metal. As was emphasized in the introduction, such a conservatism is acceptable in a model where safety is the primary requirement. The approach described would be too conservative for an industrial service model. 

These results were obtained, using hydrogen absorption into pure aluminum between 300 and 1050 °C and were calculated by temporal process degassing. Eichenauer et al. also reported that the square root Sievert s law is satisfied within the limit of error in the solid and in the liquid state. Utilizing the fact that diffusion in the metal is responsible for the hydrogen degassing rate of the solid aluminum they deduced the diffusion coefficient to be D = 0.11 exp (—9780/ RT) cm s . The heat of solution was calculated from these measurements by Birnbaum et al. [7] and Ichimura et al. [8] and found to be in the range of + 0.6 to + 0.7 eV. 

Amorphous metals do not have the high free volumes found in oxide and organic glasses. Their structures are so dense that the inert gases cannot pass through the doorways between interstices. Since hydrogen molecules dissociate on absorption into metals, they can diffuse as very small protons through these materials. The permeability of amorphous metals, just as for crystalline metals, appears to be primarily controlled by the nature of oxide films on their surface. 

These equations can be solved for semi-infinite external diffusion, where both Red and Ox forms are in the solution outside the sphere (diffusion to a spherical or hemispherical hanging mercury electrode, metallic solid spherical electrode), or they may diffuse inside the sphere (amalgam formation at mercury electrode, intercalation of Li into particles, hydrogen absorption into spherical hydrogenabsorbing particles). 

A. Lasia, Applications of electrochemical impedance spectroscopy to hydrogen adsorption, evolution and absorption into metals, in Modern Aspects of Electrochemistry, vol. 35, ed. by B.E. Conway, R.E. White (Kluwer/Plenum, New York, 2002), p. 1... 

Chemisorption (i.e., absorption of hydrogen), which involves dissociation of hydrogen molecules into hydrogen atoms and chemical bonding of the atoms to a host matrix. Thus, the hydrogen is integrated in the lattice of a metal, an alloy or a chemical compound. 

Palladium hydride is not a stoichiometric chemical compound but simply a metal in which hydrogen is dissolved and stored in solid state, in space between Pd atoms of crystal lattice of the host metal. Relatively high solubility and mobility of H in the FCC (face-centered-cubic) Pd lattice made the Pd H system one of the most transparent, and hence most studied from microstructural, thermodynamic, and kinetic points of view. Over the century that followed many metal-hydrogen systems were investigated while those studies were driven mostly by scientific curiosity. Researchers were interested in the interaction of hydrogen molecule with metal surfaces adsorption and diffusion into metals. Many reports on absorption of in Ni, Fe, Ni, Co, Cu, Pd, Pt, Rh, Pd-Pt, Pd-Rh, Mo-Fe, Ag-Cu, Au-Cu, Cu-Ni, Cu-Pt, Cu-Sn, and lack of absorption in Ag, Au, Cd, Pb, Sn, Zn came from Sieverts et al. [30-33]. 

Concurrent stream of the development of nanomaterials for solid-state hydrogen storage comes from century-old studies of porous materials for absorption of gasses, among them porous carbon phases, better known as activated carbon. Absorption of gases in those materials follows different principles from just discussed absorption in metals. Instead of chemisorption of gas into the crystalline structure of metals, it undergoes physisorption on crystalline surfaces and in the porous structure formed by crystals. The gases have also been known to be phy-sisorbed on fine carbon fibers. 

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