A hydrogen radical pathway for efficacious electrochemical nitrate reduction to ammonia over an Fe-polyoxometalate/Cu electrocatalyst

Abstract

Electrochemical nitrate (NO₃⁻) reduction to ammonia (NH₃), which is a high value-added chemical or high-energy density carrier in many applications, could become a key process overcoming the disadvantages of the Haber–Bosch process; however, current electrocatalysts have severe drawbacks in terms of activity, selectivity, and stability. Here, we report the hydrogen radical (H*) pathway as a solution to overcome this challenge, as demonstrated by efficacious electrochemical NO₃⁻ reduction to NH₃ over the Fe-polyoxometalate (Fe-POM)/Cu hybrid electrocatalyst. Fe-POM, composed of Preyssler anions ([NaP₅W₃₀O₁₁₀]¹⁴⁻) and Fe cations, facilitates efficient H* generation via H₂O + e⁻ → H* + OH⁻, and H* transfer to the Cu sites of the Fe-POM/Cu catalyst enables selective NO₃⁻ reduction to NH₃. Operando spectroelectrochemical spectra substantiate the occurrence of the H* pathway through direct observation of Fe redox related to H* generation and Cu redox related to NO₃⁻ binding. With the H* pathway, the Fe-POM/Cu electrodes exhibit high activity for NO₃⁻ reduction to NH₃ with 1.44 mg cm⁻² h⁻¹ in a 500 ppm NO₃⁻/1 M KOH solution at −0.2 V vs. RHE, which is about 36-fold higher than that of the pristine Cu electrocatalyst. Additionally, it attains high selectivity with a faradaic efficiency of up to 97.09% at −0.2 V vs. RHE while exhibiting high catalytic stability over cycles.