Chemical Diffusivity for Hydrogen Storage: Pneumatochemical Intermittent Titration Technique

Abstract

Point relaxations for the pressure–composition isotherms were analyzed as a solid-state diffusion problem of hydrogen solutes from the gas phase provided in a limited amount by a pulse-like supply. For the large differential hydrogen storage, typical in the PCI plateau region, represented by small effective volume ratio λ parameter in the diffusion equation, the relaxation occurs much faster than in the galvanostatic or potentiostatic relaxations for the same chemical diffusivity values. The analysis was successfully applied to Mg/MgH2 system and the apparent chemical diffusivity and the hydrogen self-diffusivity were evaluated as a function of the state of hydrogen storage, in an analogous manner to the well-known electrochemical titration techniques. The increase in the apparent chemical diffusivity and hydrogen self-diffusivity in the plateau region with the increasing state of storage can be consistently explained by the lower hydrogen self-diffusivity in MgH2 than in Mg solid solution. The hydrogen self-diffusivity variation in the single phase MgH2 region suggests the vacancy mechanism. The method is named as pneumatochemical intermittent titration technique (PnITT).

Moving boundary diffusion problem for hydration kinetics evidenced in non-monotonic conductivity relaxations of proton conducting perovskites

“…(10) may be interpreted as (4/π 2 )/(k(II)d) for a surface-reaction controlled transport. Apparent exponential decay behavior is not necessarily related to the diffusion-controlled behavior upon a stepwise change in the surface activity [28,32]. In-situ optical spectroscopy indicates less prominent development of the oxygenation profiles, which may be also ascribed to the substantial contribution by the Stotz-Wagner mechanism without redox effects in the water-incorporation [6].…”

Pneumatochemical Immittance Spectroscopy for Hydrogen Storage Kinetics

“…9 However, for small λ as shown in Figure 2a the √t dependence is practically unobservable. 9 The initial drop, e.g., to 10% of the (theoretical) full Δp j occurs immediately. 9 Most of the relaxation is described by the short-time error function solution or √t −1 dependence, rather than √t dependence, distinctly different from the constant surface activity condition.…”

“…The fitted C S , which corresponds to C d of the RC transmission line model for diffusion, is shown to be consistent with λ −1 , which is predetermined from PCI and used in the evaluation of pneumatochemical impedance according to eq 18. The variation of C S is compared with τ Pn obtained by the time-domain analysis 9 to indicate the thermodynamic contribution by C d since τ Pn = a 2 / D̃and D̃= a 2 /(3R d C d ). The variation of τ Pn and D̃with respect to C P should be explained by R d .…”

“…The diffusion formalism based on the λ values is nonetheless valid and the kinetic parameters derived were shown to be physically significant. 9 With given λ values, τ Pn can be evaluated by fitting Δp(t) to the error function solution, eq 3, or other approximate solutions depending on λ values for the diffusion problem of a solute in a limited amount under constant volume condition (bottom left). It is named as PnITT, pneumatochemical intermittent titration technique, since it allows the determination of the chemical diffusivity from the intermittent hydrogen injection similarly as intermittent titration techniques in the electrochemical storage kinetics.…”

“…5−8 We recently solved the diffusion problem for the pressure relaxations in a typical Sievert's setup. 9 The boundary condition for the diffusion problem is the supply of a limited amount of hydrogen by the pneumatic control for each point measurement. The hydrogen solute then diffuses into the solid-state storage material with the corresponding decrease in the pressure or concentration in the surrounding medium of a constant volume.…”