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).
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