A and B site Co-doping of CaMnO3: a route to enhanced heat storage properties

The successful commercialization of concentrating solar power (CSP) plants requires effective energy storage for the supply of power on demand during solar transients. High-temperature thermal energy storage (>= 700 degrees C up to 1200 degrees C) has the potential to address storage needs and power capacity owing to the efficiency gains from a high-temperature operation. Recently, doped CaMnO<INF>3</INF> has been identified as a promising candidate for thermochemical heat storage in CSP plants. Herein, we aim at tuning the CaMnO<INF>3</INF> heat storage temperature window and enhancing the heat storage properties beyond that of singly doped compositions by co-doping with equal amounts of La and Fe on the A and B sites, respectively, ((La<INF>x</INF>Ca<INF>1-x</INF>)(Fe<INF>x</INF>Mn<INF>1-x</INF>)O<INF>3</INF>). Two doping levels are investigated (x = 0.05 and 0.10). X-ray absorption spectroscopy and diffraction studies revealed that in both materials, Fe and Mn adopt, respectively, the 3+ and 4+ oxidation states under ambient conditions and the dopants are incorporated into the intended sites. Interestingly, the heat storage capacity did not vary monotonically with dopant content. The highest heat storage capacity was attained from La<INF>0.05</INF>Ca<INF>0.95</INF>Fe<INF>0.05</INF>M<INF>0.95</INF>O<INF>3-delta</INF>. This surprising result is a consequence of the substantial large extent of reduction enabled by the slightly lower enthalpy than that of La<INF>0.1</INF>Ca<INF>0.9</INF>Fe<INF>0.1</INF>Mn<INF>0.9</INF>O<INF>3-delta</INF>. Under technologically relevant conditions, operating over a temperature window value ranging from 700 to 1200 degrees C and under an oxygen partial pressure of about 10-3 atm, the thermochemical heat storage capacities of La<INF>0.05</INF>Ca<INF>0.95</INF>Fe<INF>0.05</INF>Mn<INF>0.95</INF>O<INF>3 delta</INF> and La<INF>0.1</INF>Ca<INF>0.9</INF>Fe<INF>0.1</INF>Mn<INF>0.9</INF>O<INF>3-delta</INF> are 378.5 +/- 1.0 kJ kg<INF>ABO<INF>3</INF></INF>-1 and 282.3 +/- 1.5 kJ kg<INF>ABO<INF>3</INF></INF>-1, respectively, and exceed the values not only of the undoped material but also of other singly doped analogs for the first material. Furthermore, with respect to the singly Fe-doped CaMnO<INF>3</INF>, we narrowed the operating temperature range from 400-1200 degrees C to 700-1200 degrees C, which is the target temperature for the CSP plants. Hence, we demonstrated that by co-doping, it is possible to tailor reduction enthalpy and extent together with the operating temperature range.