In Situ Formation of FeNi Nanoparticles on Polypyrrole Hydrogel for Efficient Electrocatalytic Nitrate Reduction to Ammonia.

The electrocatalytic reduction of nitrate to ammonia (NH3) under mild environmental conditions is attracting increasing attention, in which efficient and inexpensive transition metal catalysts, with the advantages of abundancy and low cost, play a key role. However, synergistic activity and selectivity promotion are still highly challenging. Herein, we developed a hydrogel-assisted strategy to prepare FeNi nanoparticles via the in situ adsorption and reduction of Fe/Ni precursors on a polypyrrole hydrogel. After optimization, the maximum NH3 yield reached 0.166 mmol h-1 cm-2, with a Faradaic efficiency of 88.9% and a selectivity of 86.6%. This excellent electrochemical performance was attributed to the mesoporous hydrophilic structure of the polypyrrole hydrogel, which facilitates the homogeneous loading of FeNi nanoparticles and provides a channel for both charge and mass transfer during nitrate reduction, which is important for the conversion of NO3- to NH3. Electrochemical active surface area determination and impedance spectroscopy showed that the introduction of hydrogel increased the active sites and improved the charge transfer. This study provides an effective strategy for improving the selectivity and yield of NH3 in transition metal electrocatalysts by utilizing the three-dimensional hydrogel network and electrical conductivity.