The Role of Mixed Conduction in W-WO3 Redox Kinetics

The tungsten - tungsten trioxide (W-WO3) materials system has recently shown excellent promise as a medium for grid level energy storage and for solar energy conversion.  In both these applications the thermal energy transfer is accompanied by a redox reaction in the W-WO3 couple.  For viability of this system as an energy storage medium, it is important to have facile redox kinetics that allows us to rapidly store and release energy.  Facile kinetics are also required in the utilization of this system in solar syngas production.  In this paper we show that the addition of an ionic conducting phase, namely yttria stabilized zirconia (YSZ) greatly enhances the redox kinetics of the W-WO3 couple.  Specifically, the reduction kinetics of the WO3-W reaction are facilitiated by the addition of YSZ.  As a control experiment, a redox experiment with a filler material that is not an ionic conductor did not show such an enhancement in the kinetics. Chemical reaction rates were determined by transient thermogravimetry, and the data were analyzed using relevant kinetic and transport theory.