Surface Entropy-Reduction Tailored Few-Atom Layer Metal on High-Entropy Alloy for Tandem Electrocatalytic Nitrate Reduction to Ammonia

To kill two birds in one stone, electrocatalytic reduction of nitrate (NO₃^-) offers a mild and reliable approach for wastewater denitrification and a green pathway for ammonia (NH₃) production. Despite the complex proton-coupled electron transfer process in NO₃^- reduction, efficient NH₃ production has been challenging. In this study, high-entropy multivalent chalcogenides (HEMCs) were synthesized, exhibiting a synergistic catalytic effect for enhanced electrochemical performance. The highest Faraday efficiency of 92.3% and NH₃ yield rate of 5.67 mg h^-1 cm^-2 were achieved at -0.6 and -0.8 V versus reversible hydrogen electrode (V vs RHE) under the catalysis of HEMC with Fe, Cu, Co, Cr, Zr, Ni, and Bi (molar ratio 1:1:1:1:1:1:1), denoted as F3CZNB. Comparatively, senary and quinary HEMCs, namely, F3CZN and F3CZ, were also synthesized. Characterization and catalytic performance analyses revealed that Fe and Cu primarily contributed to the catalytic activity, while NH₃ selectivity and electrode stability were attributed to the stoichiometric composition of the metals. Ni further enhanced conversion of NO₃^- to NH₃ by its hydrophilicity, and Bi improved conductivity and durability with d-p interaction with transition metals. The NO₃^- reduction reaction pathway was confirmed by detecting intermediates (*NO₂, *NO, *NH₂OH, *NH₂, and *NH) on the catalyst surface using in situ Fourier transform infrared spectroscopy (in situ FT-IR) and electrochemical differential mass spectrometry (DEMS). A Zn-NO₃^- battery assembled with F3CZNB demonstrated an energy density of 6.45 mW cm^-2, highlighting the potential of F3CZNB for energy conversion devices. This study sheds light on a novel protocol for synthesizing multivalent chalcogenides with potent electrocatalytic activity.