Covalent Organic Frameworks with Intrinsic Pendant Aldehydes for Efficient Nitrate Electroreduction

ABSTRACT Nitrate (NO ₃ ⁻ ) pollution poses a critical environmental threat by contaminating water resources and disrupting the global nitrogen cycle. The electrochemical nitrate reduction reaction (NO ₃ RR) in alkaline media offers a dual solution: mitigating nitrate contamination while enabling sustainable ammonia (NH ₃ ) production. However, the scarcity of free protons (H ⁺ ) at high pH hampers efficient NO ₃ ⁻ ‐to‐NH ₃ conversion. Here, we report a sub‐stoichiometric covalent organic framework PEPy‐2CHO‐TTA, synthesized by microwave‐assisted [4 + 3 + 2] polycondensation strategy, which retains pendant unreacted aldehyde groups oriented toward the pore channels. This framework‐intrinsic integration of polar aldehyde functionalities enhances water uptake and promotes the formation of a structured hydration network within the pores, enabling localized proton transfer that overcomes proton deficiency under alkaline conditions. As a result, PEPy‐2CHO‐TTA COF achieves a Faradaic efficiency (FE) exceeding 95% and an NH ₃ yield rate of 5.87 mg h ⁻¹ cm ⁻² which is among the highest reported for metal‐free or metal‐based porous electrocatalysts. Isotope labelling using K ¹⁵ NO ₃ confirms that the produced ammonia originates exclusively from nitrate reduction. DFT calculations reveal a multi‐step eight‐electron reduction pathway with the NO‐to‐NHO transformation as the potential‐determining step. This work introduces a new design paradigm for COF electrocatalysts, where pendant aldehydes within the framework serve as molecular handles for water‐mediated proton transport, enabling efficient nitrate reduction under alkaline conditions, without external acidification or metal catalysts.