Structural insights into iron-based phase transformation oxides for highly efficient thermochemical water splitting

Two-step thermochemical water splitting (TWS) is a promising green hydrogen production technology in which metal oxides are used as redox-active materials to split steam. Among the several challenges toward the commercialization of two-step TWS, the limited performance of existing redox materials has been identified as a critical issue. Here, we report Fe-poor Ni ferrites (NFOs), a class of phase transformation oxides, as highly promising redox materials for two-step TWS that overcome the limitations of existing materials. These materials achieve a superior H₂O-to-H₂ conversion of 0.528 %/g_oxide under more favorable reaction conditions, outperforming state-of-the-art materials that exhibit < 0.250 %/g_oxide. A redox-active cation in Fe-poor NFOs is hypothesized and experimentally validated, establishing the fundamental structure-property relationships. Our results show that the extent of redox swing between the two active cations strongly correlates with water splitting performance. Density functional theory calculations reveal that both the number of active sites and the surface reaction energy play critical roles in determining the redox swing extent and, consequently, the water splitting performance. This study not only introduces Fe-poor NFOs as new state-of-the-art materials for two-step TWS, but also provides fundamental insights that can be broadly applied in the design of highly efficient redox oxides.