Iron as Recyclable Metal Fuel: Unraveling Oxidation Behavior and Cyclization Effects Through Thermogravimetric Analysis, Wide‐Angle X‐ray Scattering and Mössbauer Spectroscopy

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Abstract
The carbon‐free chemical storage and release of renewable energy is an important task to drastically reduce CO2 emissions. The high specific energy density of iron and its recyclability makes it a promising storage material. Energy release by oxidation with air can be realized by the combustion of micron‐sized iron powders in retro‐fitted coal fired power plants and in fixed‐bed reactors under milder conditions. An experimental parameter study of iron powder oxidation with air was conducted based on thermogravimetric analysis in combination with wide‐angle X‐ray scattering and Mössbauer spectroscopy. In agreement with literature the oxidation was found to consist of a very fast initial oxidation of the outer particle layer followed by much slower oxidation due to diffusion of iron ions through the Fe2O3/Fe3O4 layer being the rate‐limiting step. Scanning electron microscopy analysis of the iron particle before and after oxidation reveal a strong particle morphology transformation. This impact on the reaction was studied by cyclization experiments. Up to 10 oxidation‐reduction cycles show that both, oxidation and reduction rates, increase strongly with cycling due to increased porosity.

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