Using Operando XRD to Characterize High-Temperature Oxidation and Reduction of Fe-X Energy Storage Materials

High temperature reduction/oxidation cycling of Fe materials is an attractive energy storage paradigm for the solid-oxide metal-air rechargeable battery (SOMARB). Using laboratory XRD sources, <i>operando</i> characterization of different Fe-X alloys during cycling enables detailed examination of the effects of the alloying element on the redox cycling process. Using these techniques, the extreme degradation resistance of hierarchically porous Fe-Mo and Fe-W metallic foams during redox cycling is shown. The foams are able to maintain their multi-level porosity through sintering inhibition, chemical vapor transport during reduction, and phase evolution that encourages mixing rather than segregation. These factors result in faster reaction kinetics with increased cycling, particularly over the first five cycles. These results are contrasted with Fe-Ni, Fe-Co, and Fe-Cu foams which show enhanced reduction kinetics but ultimately degrade due to poor sintering inhibition and lack of microscale porosity. These results guide further development of the energy storage material for SOMARBs and can be translated across domains into carbon utilization and chemical looping combustion.