Breaking the Stoichiometric Limit in Oxygen-Carrying Capacity of Fe-Based Oxygen Carriers for Chemical Looping Combustion using the Mg-Fe-O Solid Solution System

The performance of oxygen carriers contributes significantly to the efficiency of chemical looping combustion (CLC), an emerging carbon capture technology. Despite their low cost, Fe₂O₃-based oxygen carriers suffer from sintering-induced deactivation and low oxygen-carrying capacity (OCC) during CLC operations. Here, we report the development of a sintering-resistant MgO-doped Fe₂O₃oxygen carrier with an optimal composition of 5MgO·MgFe₂O₄, which exhibits superior cyclic stability and an OCC of 0.45 mol O/mol Fe (2.25 mmol O/g_solid), exceeding the widely accepted OCC limit of 0.167 mol O/mol Fe (2.08 mmol O/g_solid) of unmodified commercial Fe₂O₃. This result distinguishes this report from all past studies, in which efforts to enhance the cyclic stability of Fe-based oxygen carriers would always result in dilution of the OCC. The capacity enhancement by MgO is attributed to the unique mixtures of Mg_xFe_1-xO (halite) and Mg_1-yFe_2+yO₄(spinel) solid solutions, which effectively reduce the exergonicity for the reduction from Fe³⁺to Fe²⁺, while preventing any irreversible structural transformations during the redox process. This hypothesis-driven oxygen carrier design approach provides a new avenue for tailoring the lattice oxygen activities of oxygen carriers for chemical looping applications.