Improving nitrate-to-ammonia conversion efficiency on electrodeposited nickel phosphide via surface δ-FeOOH modification

Abstract

Electrocatalytic nitrate reduction to ammonia is an appealing and promising approach for recovering industry and agricultural wastewater. However, there is a lack of efficient electrocatalysts for NO₃⁻ conversion at low overpotentials and a range of nitrate concentrations, which could enable nitrogen recovery from various wastewater streams. Here, we report an affordable and efficient electrodeposited amorphous-Ni₃P electrode modified with δ-FeOOH, forming a heterojunction catalyst that efficiently converts nitrate into ammonia. The optimum heterojunction performance achieved a high faradaic efficiency of 98% ± 0.72 at −0.3 V vs. RHE with an NH₃ production yield of 8.49 mg h⁻¹ cm⁻², ranking Ni₃P/δ-FeOOH among the most active electrocatalysts so far reported for this process. Ni₃P/δ-FeOOH showed good stability using ethyl cellulose as a green and fluoride-free binder without losing the superficial modification. Mechanistic studies by in situ Raman spectroscopy show that the role of δ-FeOOH is the stabilisation of the NO_x intermediates. At the same time, Ni₃P provides *H species available for NO_x reduction, thus resulting in highly selective NO₃ hydrogenation. In contrast, in the absence of δ-FeOOH, Ni₃P exhibits significantly lower NH₃ faradaic efficiency due to simultaneous H₂ evolution. Therefore, the present results show the synergistic effect between δ-FeOOH and Ni₃P in the electrocatalytic nitrate reduction, with δ-FeOOH playing a crucial role in stabilising the reaction intermediates and enhancing the ammonia selectivity.