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 diffractionand compositional analysis, a formation mechanism is proposed, based on in-situ nitridation by the decomposing gel matrix. The Fe3N nanoparticlesponge is shown to be a promising catalyst for ammonia decomposition, an easy process for CO2-free hydrogen supply and off-gas treatment.