A self-supported copper/copper oxide heterostructure derived from a copper-MOF for improved electrochemical nitrate reduction

Abstract

The conversion of low-cost and abundant precursor substances into value-added chemicals through electrochemical techniques is a key element to build a renewable energy-chemistry cycle. Among the various available possibilities, the electrochemical nitrate reduction reaction (NITRR) has attracted more and more interest, as it can provide a novel route for ammonia (NH₃) synthesis driven by electricity from renewable sources. However, the faradaic efficiency (FE) and production rate of NH₃ are still limited by the multiple involved electron–proton-transfer steps and strong competition from the hydrogen evolution reaction. The rational design of efficient catalysts can contribute to overcoming these challenges. Herein, we report the synthesis of free-standing Cu-MOF-based materials as NITRR catalysts. Under electrochemical reduction conditions, the Cu-MOF electrode encounters both chemical reconstruction and structural change. The Cu species are partially reduced and form a unique Cu/Cu₂O/CuO heterostructure, resulting in remarkable NITRR performance. At a potential of −0.3 V vs. RHE, the FE of NH₃ is as high as 99.5% with a production rate of 5.9 mg h⁻¹ cm⁻¹. A satisfactory half-cell energy efficiency of 35.1% is achieved as well. Desirable stability is confirmed by a cycling test. Such an electrolyzer can be driven by a solar cell under the irradiation of natural sunlight for continuous NH₃ production.