Iron nitride (Fe₃N) is a promising material to replace scarce and costly noble metals in many catalytic applications. Here, we report the synthesis of Fe₃N nanostructures by a simple sol–gel based route. This aqueous, one-pot method based on a self-expanding polypeptide foam represents a breakthrough in Fe₃N nanostructure synthesis, which previously has only been achieved through ammonolysis. Through extensive X-ray diffraction and compositional analysis, a formation mechanism is proposed, based on in-situ nitridation by the decomposing gel matrix. The Fe₃N nanoparticle sponge is shown to be a promising catalyst for ammonia decomposition, an easy process for CO₂-free hydrogen supply and off-gas treatment.