A Transition State Theory Perspective on the Relation of Reversible Metal Hydride First-Order Kinetics to Equilibrium Thermodynamics

19 May 2021, Version 2
This content is a preprint and has not undergone peer review at the time of posting.


In the event of hydrogen desorption from reversible metal hydrides, equilibrium thermodynamics exert a rate-limiting effect: if system pressure reaches equilibrium pressure, the reaction rate becomes zero. This is usually dealt with by an empiric term of either polynomial or logarithmic nature to first-order kinetics. This paper approaches the matter from a transition state theory perspective, combining the classic Eyring-Polanyi equation with insights on reversible metal hydride chemical overpotential for scrutinizing the relation of Arrhenius first-order kinetics to van’t Hoff equilibrium pressure. The outcome, tested for the example of 4 mol % Ti-doped NaAlH4, suggests theoretical coherency and provides a method for identifying the factor by which an experiment deviates from ideal first-order kinetics. Adopting Arrhenius-Eyring-Polanyi first-order kinetics as baseline for modelling kinetic behaviour of metal hydride sorption reactions not only covers a blind spot in the Arrhenius approach but creates a standard for result comparability.


Equilibrium thermodynamics
Kinetic Modeling
Eyring equation
Reversible metal hydrides
Kinetics analysis
chemical hydrogen storage systems
sodium alanate
Chemical overpotential

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