A Mild Substitution–Polymerization Strategy Enables Non-Invasive Hydrogel Stabilization of Electrocatalysts for Nitrate-to-Ammonium Conversion

The insufficient durability of electrocatalysts constitutes a critical bottleneck for electrocatalytic nitrate-to-ammonium reduction (NRA), and most existing stabilization strategies are implemented under harsh modification conditions. Herein, a sequential polymerization substitution polymerization strategy is proposed to fabricate a porous NDI-PPy conductive hydrogel catalytic electrode on a self-supported Ag-FeOOH substrate. Initially, an ND hydrogel network is constructed through the copolymerization of N-acryloxysuccinimide (NAS) and N,N-dimethylacrylamide (DMA). Amine-rich aromatic units were subsequently introduced through mild substitution with 4,4′-iminodianiline (IDA) to form the NDI hydrogel, and conductive polypyrrole (PPy) is further grown via Fe3+-triggered in situ polymerization of pyrrole. This non-invasive strategy is designed to preserve the catalytic functionality of the Ag-FeOOH substrate, while the comparative electrochemical results suggest improved cycling durability and enhanced apparent electrode-level performance after hydrogel modification. The optimized NDI-PPy/Ag-FeOOH electrode exhibits excellent initial NRA performance, including 89.34% NO3−-N removal efficiency, 94.27% NH4+-N selectivity, 89.53% Faradaic efficiency, and an NH4+-N yield rate of 2.95 mg h−1 cm−2. Systematic comparative tests are conducted on identical substrates modified with conventional PAM, SA, PVA, and PAA hydrogels, as well as NDI- and PPy-containing hydrogel systems. The comparative results suggest that amine functionalization may contribute to improved electrode stability, while the incorporation of PPy is associated with enhanced ammonium production under the tested conditions. This study provides a novel and non-invasive hydrogel-modified paradigm for the durability optimization of electrocatalytic NRA catalysts.