Achieving maximum efficiency in thermochemical green hydrogen production

Thermochemical water splitting offers a direct route for renewable hydrogen and CO production through metal oxide redox cycling under temperature and/or pressure swings, yet reported system efficiencies remain far below the theoretical limit in part because redox materials, reactors, and balance of plant are often optimized in isolation. Here we introduce a unified, physics based thermodynamic framework that co-optimizes redox equilibrium, reactor operation, and auxiliary subsystem integration to quantify the maximum achievable heat to hydrogen efficiency for a given redox material and operating envelope. The model combines mass and energy conservation with the Gibbs equilibrium criterion and explicitly accounts for the energetic penalties and recoverable work associated with O2 removal, H2 separation, heat recuperation, and power cycle utilization of intermediate temperature heat. Applying this framework to ceria, doped ceria, ferrites, and perovskites across a broad design space shows that reduction enthalpy and entropy govern oxygen chemical potential and oxidation driving force, which in turn determine subsystem loads and plant level efficiency limits. Under baseline operation, ceria reaches 19.67% heat to fuel efficiency at 1500 °C, while advanced system integration enables a peak efficiency of 46.20% at 1600 °C, revealing a clear pathway toward substantially improved performance. In contrast, perovskites, despite their higher non stoichiometry, exhibit weak oxidation driving forces that require large excess steam and cap efficiency at 19.36% at 1500 °C. These results establish thermochemical water splitting as a potent high efficiency hydrogen pathway and provide a generalizable framework for material screening, subsystem selection, and plant level design.