Iodine Dopants Facilitate Tandem Catalysis via Transformation Adsorption Behavior for Efficient Electrochemical Nitrate Reduction Reaction

Electrochemical nitrate reduction reaction (NO₃RR) has great potential for simultaneously achieving nitrate-rich wastewater treatment and ammonia (NH₃) synthesis. Given that the NO₃RR process encompasses distinct steps of deoxygenation and hydrogenation, the active sites of most catalysts frequently demonstrate similar adsorption behavior. In this work, the second-shell iodine-doped modified cobalt single atoms (I-CoN₄) and cobalt atomic clusters (Co AC) anchored on nitrogen-doped carbon (Co SAAC/INC) are synthesized through a mild etching synchronization doping strategy. In the neutral electrolyte, Co SAAC/INC exhibits a high NH₃ yield rate of 18.64 mg h^-1 mg_cat^-1 at -0.7 V versus reversible hydrogen electrode (vs RHE) and a satisfactory FE of 97.2% at -0.6 V vs RHE. Combining in situ electrochemical Fourier infrared spectroscopy, online differential electrochemical mass spectrometry, confirmatory experiments, and density functional theory calculations demonstrates that second-shell iodine heteroatom doping effectively regulates the electronic structure of CoN₄ site and induced the favorable adsorption from H₂O/*H to NO₃^-, while the adjacent Co AC site accelerates the dissociation of H₂O and provides abundant active hydrogen (H_ads) for the subsequent hydrogenation step, thus constructing the tandem catalytic sites with I-CoN₄ site to promote NO₃RR. Furthermore, the Zn-NO₃^- battery with Co SAAC/INC as the cathode shows high power density (15.41 mW cm^-2) and excellent NH₃ synthesis efficiency, while simultaneously achieving NO₃^- pollutant removal, NH₃ synthesis, and energy supply. This work not only clarifies the transformation of the NO₃RR mechanism induced by heteroatom doping but also provides insights into the construction of the tandem catalytic sites.