In the depth understanding of reversible ionic transfers of layered double hydroxides by in operando electrochemical and gravimetric measurements coupled with grazing incidence wide angle X-ray scattering
Résumé
A layered double hydroxide (LDH) material consists of a stacking of positively charged
brucitic layers due to the substitution of some divalent cations M(II) with trivalent cations
M(III). The charge compensation is managed by the ionic species intercalated into the
interlayer spacing i.e. water and anions A n-‐ . This leads to the general formula: [M(II) 1-‐
x M(III) x (OH) 2 ]x+ (An-‐x/n ).mH2 O. As a consequence, LDHs provide a high capacity for anion
exchange.
For wide applications in terms of energy storage device or water treatment, the reversibility
of the ion transfer between the electrolyte and the interlayer spacing of the LDH needs to be
further investigated. Tuning the oxidation state of the electroactive metallic sites with
electrochemistry reversibly modifies the positive charge excess within the layers. The low
electronical conductivity performance of the LDH is promoted by preparing thin films coated
on a working electrode surface. The related ion transfer is probed by coupling
electrochemistry with a quartz crystal microbalance sensitive to the ng (EQCM).
Here, we investigate for the first time the nature, kinetics, exchanged concentration and
mass of each ion species transferred from the electrolyte to the material related to the
oxidation and reduction of the electroactive components of the LDHs. This is made possible
with the ac-‐electrogravimetry technique, developed at LISE, that couples electrochemical
impedance and mass/potential transfer functions measurements.
The in situ grazing-‐incident-‐wide-‐angle-‐X-‐ray-‐scattering (GIWAXS, 20 keV) coupled with
EQCM at synchrotron enables to measure the change in the interlayer spacing of the LDH as
well as the mass variation of the material in solution during cyclic oxidation/reduction of the
metal sites. It confirms and demonstrates further the mechanisms of the ionic transfers
between the electrolyte and the interlayer distance.