Two-Lever Design Rule for High-Entropy Oxide Oxygen Carriers: Minimize Spin Polarization, Maximize Fe–O–Ni Covalency

What is missing in high-entropy oxide (HEO) oxygen carrier design is an actionable rule that links composition to redox performance. We advance a two-lever principle─decrease spin polarization and increase Fe–O–Ni covalency─that lowers the oxygen-vacancy formation energy at the targeted Fe–O–Ni-linked sites and improves CO space–time yield (STY) by 4.7 times. Guided by this rule, we prepare compositionally diverse HEOs and benchmark them in chemical looping reverse water–gas shift (CL-RWGS). Spectroscopy and temperature-programmed reduction indicate that strengthening the Fe–O–Ni covalency increases the fraction of labile lattice oxygen, while electronic-structure calculations connect suppressed spin polarization to lower oxygen vacancy formation energy across representative local environments. A FeMgAlNiZn HEO following the rule achieves a STY of 8.6 mmol CO ·kg cat –1 ·s –1 under CL-RWGS at 650 °C, substantially outperforming FeMgCoZnMn HEO that violates the rule, and maintains performance over extended cycling. The combined experimental–computational evidence establishes a mechanism-anchored, composition-level guideline for HEO oxygen carriers: avoid cations with a high spin state shift to maintain phase stability, promote Fe–O–Ni linkages to enhance covalency, and tune the electronic structure to minimize spin polarization. This design framework enables rapid, rational navigation of the vast HEO space for carbon-efficient CO 2 to CO conversion.