Ultrathin High‐Entropy Fe‐Based Spinel Oxide Nanosheets with Metalloid Band Structures for Efficient Nitrate Reduction toward Ammonia

Abstract Spinel oxides with tunable chemical compositions have emerged as versatile electrocatalysts, however their performance is greatly limited by small surface area and low electron conductivity. Here, ultrathin high‐entropy Fe‐based spinel oxides nanosheets are rationally designed (i.e., (Co_0.2Ni_0.2Zn_0.2Mg_0.2Cu_0.2)Fe₂O₄; denotes A⁵Fe₂O₄) in thickness of ≈4.3 nm with large surface area and highly exposed active sites via a modified sol–gel method. Theoretic and experimental results confirm that the bandgap of A⁵Fe₂O₄ nanosheets is significantly smaller than that of ordinary Fe‐based spinel oxides, realizing the transformation of binary spinel oxide from semiconductors to metalloids. As a result, such A⁵Fe₂O₄ nanosheets manifest excellent performance for the nitrate reduction reaction (NO₃⁻RR) to ammonia (NH₃), with a NH₃ yield rate of ≈2.1 mmol h⁻¹ cm⁻² at −0.5 V versus Reversible hydrogen electrode, outperforming other spinel‐based electrocatalysts. Systematic mechanism investigations reveal that the NO₃⁻RR is mainly occurred on Fe sites, and introducing high‐entropy compositions in tetrahedral sites regulates the adsorption strength of N and O‐related intermediates on Fe for boosting the NO₃⁻RR. The above findings offer a high‐entropy platform to regulate the bandgap and enhance the electrocatalytic performance of spinel oxides.