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Quantitative Resolution of Complex Stoichiometric Changes during Electrochemical Cycling by Density Functional Theory-Assisted Electrochemical Quartz Crystal Microbalance
journal contribution
posted on 2020-03-18, 12:08 authored by Tzu-Ho Wu, Ivan Scivetti, Jia-Cing Chen, Jeng-An Wang, Gilberto Teobaldi, Chi-Chang Hu, Laurence J. HardwickThe capability to
simultaneously measure changes of mass and charge
of electro-active materials during a redox process makes Electrochemical
Quartz Crystal Microbalance (EQCM) a powerful technique to monitor
stoichiometric changes during reversible electrochemical processes.
In principle, quantitative resolution of the stoichiometry of the
electro-active sample during electrochemical cycling can be obtained
by solving the system of equations for the EQCM mass and charge balance.
Such a system of equations couples the measured changes in mass and
charge through the stoichiometry of the redox process. Unfortunately,
whenever more than two chemically inequivalent species are involved
in the redox process, the system of equations is mathematically undetermined,
having more variables (stoichiometric coefficients) than equations.
In these cases, current best practice is the arbitrary reduction of
the number of variables in the mass and charge balance equation, using
chemical intuition to set some of the stoichiometric coefficients
to fixed values. For layered ion-intercalation host materials, widespread
practical approximations are the use of arbitrarily defined solvation
numbers for the intercalating ions or the neglect of ion-induced displacement
of structural solvent inside the host. Here, we propose an alternative
approach based on the use of Density Functional Theory (DFT) to sample
and screen, on an energy basis, the whole unreduced spectrum of stoichiometric coefficients compatible with EQCM measurements,
leading to DFT energy-assisted resolution of stoichiometric changes
during cycling. We illustrate the approach by taking nickel hydroxide
Ni(OH)2 as a case system and studying its ion intercalation-driven
phase transformations in the presence of different LiOH, NaOH, and
KOH electrolytes. Quantitative resolution of the Ni(OH)2 stoichiometry during electrochemical cycling unambiguously reveals
ion intercalation to displace structural water from the layered host,
promoting electrochemical degradation and aging of the material. The
process is found to be strongly dependent on the size of the electrolyte
cation, with larger cations displacing larger amounts of structural
water and resulting in faster degradation rates.
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Complex Stoichiometric Changeselectrolyteelectrochemical cyclingDensity Functional Theorydegradationcharge balance equationDensity Functional Theory-Assisted Electrochemical Quartz Crystal Microbalanceredox processElectrochemical Quartz Crystal Microbalancestoichiometric coefficientsapproachDFT energy-assisted resolutionNiEQCMQuantitativecationion intercalation-driven phase transformationsKOHsamplestoichiometryelectro-activestoichiometric changesvariableion-intercalation host materials
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