Highly Efficient Bifunctional NiFe-MOF Array Electrode for Nitrate Reduction to Ammonia and Oxygen Evolution Reactions

Abstract

Electrocatalytically converting nitrates in sewage to ammonia, which can not only achieve the purpose of eliminating sewage but also obtaining valuable ammonia, is an effective supplement to the traditional Haber–Bosch process. Although significant progress has been made in cathodic catalyst design, the overall ammonia electrolysis from nitrate reduction is still restricted by the anodic oxygen evolution heavily relying on noble-based catalysts. Herein, a bimetallic NiFe-MOF nanosheet array electrode is fabricated and serves as an efficient bifunctional catalyst for nitrate reduction and oxygen evolution reactions. The introduction of Fe to Ni-MOF facilitates the formation of a nanosheet structure with higher electrochemical active surface area, as well as provides synergetic NiFe sites. The NiFe-MOF electrode reaches a greatly enhanced ammonia yield rate of 0.94 mmol cm^–2 h^–1 and a Faradaic efficiency of 90.8% at the cathode and −0.6 V versus reversible hydrogen electrode, as well as an enhanced oxygen evolution reaction with a declined overpotential of 424 mV at 50 mA cm^–2. As a bifunctional catalyst in the overall electrocatalysis, the performance of NiFe-MOF in the nitrate reduction reaction is comparable with that using Pt mesh as a counter electrode.