Iron as a recyclable energy carrier: Redox cycling of iron oxide pellets for hydrogen storage

The reversible storage of hydrogen in metal oxides via the steam-iron process offers a promising pathway for
large-scale, carbon-free energy storage. In this study, the redox performance and mechanical stability of iron
oxide-based pellets were investigated over ten redox cycles. Pelletization produced mechanically robust pellets
(1–2 mm) suitable for fixed-bed operation. Trends previously observed for iron oxide powders (100-200 μm),
such as sintering-induced deactivation and stabilization by dopants, were confirmed in pellet form. Remarkably,
iron oxide of lower purity containing trace amounts of Al and Cr exhibited nearly constant H$_2$ production over
ten cycles, which can be attributed to the combined stabilizing effects of minor impurities, a primary particle
morphology less prone to sintering, and crack formation during redox cycling that enhances gas transport
within the pellets. Increasing the induration temperature of Al-modified iron oxide to 1300 ◦C significantly
improved the mechanical integrity of the pellets and both H2 production and porosity increased with cycle
number. These findings demonstrate the potential of additive-stabilized iron oxide pellets as durable and
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recyclable hydrogen storage material.
Novelty and significance statement: This study bridges the gap between powder-scale research and practical fixed-
bed operation by systematically evaluating pelletized iron oxides in the steam-iron process. It demonstrates
how pellet microstructure, induration temperature, and presence of stabilizing additives govern long-term
redox stability under cyclic conditions. Trace-level impurities can provide stabilization comparable to deliberate
modification, highlighting the potential of low-cost, naturally occurring iron oxides. The findings support the
development of stable, cost-effective iron-based materials for cyclic energy storage.