As an alternative approach for carbonaceous fuel conversion and CO2 capture, the so-called chemical looping strategy utilizes redox properties of first-row transition metal oxides to simplify the conventional energy conversion processes. In a typical chemical looping process, carbonaceous feedstock is oxidized into products such as CO2 by active lattice oxygen (O2-) in the transition metal oxide particles, a.k.a. oxygen carrier. In a subsequent step, the O2--deprived oxygen carrier particles are replenished by a gaseous oxidant, generating hydrogen or heat. The cyclic redox operation, often carried out in circulating fluidized bed reactors, has the potential to significantly reduce the exergy loss for carbonaceous energy conversion and carbon dioxide capture.
While a number of supported metal oxides have demonstrated promising redox performances, further improvements of the activity and redox stability of these oxygen carriers are of critical importance for successful deployment of this novel technology. To date, oxygen carrier development largely relies on a trial-and-error type of approach. Using iron-containing oxides as an example, we present a rationalized strategy for oxygen carrier optimization: to arrive at oxygen carriers with superior activity, the rate limiting step for the redox reactions is first identified. Mixed ionic-electronic conductive support that de-bottlenecks such a rate limiting step is then used to improve the metal oxide activity by two orders of magnitude. Investigation of oxygen carrier deactivation mechanisms further sheds light for designing oxygen carriers with both high activity and extended lifetime. Besides their applications in chemical looping combustion, transition metal oxides with tailored nano-structures for methane partial oxidation, solar-based water-splitting, and oxidative dehydrogenation are also exemplified.