Thermochemical Energy Storage Using the Phase Transitions Brownmillerite -2H Perovskite - Cubic Perovskite in the CaxSr1–xCoO3−δ (x = 0 and 0.5) System

Abstract

The oxides Ca0.5Sr0.5CoO3-δ and SrCoO3-δ, which present perovskite or perovskite-related phases in different temperature domains, have been tested as materials for thermochemical energy storage. The first one, Ca0.5Sr0.5CoO3-δ, experiences a reversible phase transition upon consecutive cycles under air flow at a maximum operating temperature of 1196 K. Unfortunately, the heat stored in this process, associated to an oxygen loss/gain and a structural phase transition, is very small hindering its use for thermochemical heat storage. The as-prepared oxide SrCoO3-δ, which displays a brownmillerite structure like the Ca-containing compound, in the first heating step irreversibly segregates some Co3O4 at 823 K to yield a 2H hexagonal-perovskite. This phase reversibly transforms at 1073 K into a cubic-perovskite. These 2H⇄C transition occurs from the second to, at least, thirty cycles. The average absorbed and released heat are ~104.1±0.06 J/g and ~68.8±1.8 J/g, respectively, and therefore SrCoO3-δ presents a high exo/endo ratio. The exergy efficiency is, on average for the 30 cycles performed, as high as 63.9±1.2%. The mechanism of the phase 2H⇄C transition of SrCoO3-δ explains the good performance of this material for thermochemical energy storage.