Electrodeposited polyoxometalate–Cu2+1O hybrid on copper foam: synergistic electron transfer for efficient nitrate electroreduction to ammonia

Abstract

Electrocatalytic nitrate reduction (NO₃RR) is a sustainable strategy to address nitrate pollution and replace the energy-intensive Haber–Bosch process for ammonia synthesis, but it is hindered by complex multi-electron transfer pathways and severe hydrogen evolution competition. Herein, a hybrid electrocatalyst (Mo₇/Cu₂₊₁O/Cu@CF) was successfully fabricated via electrodeposition, integrating [Mo₇O₂₄]⁶⁻ (denoted as Mo₇) with Cu₂₊₁O on copper foam (CF). The optimized catalyst exhibits exceptional NO₃RR performance in neutral media: at −0.7 V vs. the reversible hydrogen electrode (RHE), it achieves a high ammonia yield rate of 7.16 mg h⁻¹ cm⁻² and a faradaic efficiency (FE) of 95.7%, along with outstanding stability over 10 hours of continuous electrolysis. Structural characterization (XRD, XPS, and TEM) confirms the formation of a hybrid structure with strong electronic coupling at the Mo₇/Cu₂₊₁O interface, facilitating efficient interfacial electron transfer. In situ Fourier-transform infrared (FTIR) spectroscopy reveals the reaction pathway as NO₃⁻ → *NO₃ → *NO₂ → *NO → *NH₂OH → *NH₃, while ¹⁵N isotope-labeling experiments verify that NH₃ originates exclusively from nitrate. The superior performance stems from the synergistic effect between Mo₇ (electron reservoir) and Cu₂₊₁O (active site matrix), which optimizes the electronic structure of active sites and suppresses the hydrogen evolution reaction. This work provides a promising catalyst for sustainable ammonia production and a general design principle for high-performance electrocatalysts via polyoxometalate–metal oxide hybridization.