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

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.

Automated summary

The conversion of low-cost and abundant precursor substances into value-added chemicals through electrochemical techniques is essential for establishing a renewable energy-chemistry cycle. This study presents the synthesis of free-standing Cu-MOF-based materials as NITRR catalysts, which exhibit remarkable NITRR performance and satisfactory half-cell energy efficiency. It can be driven by a solar cell under natural sunlight for continuous NH3 production.