Ammonia (NH₃) has been served as the precursor of fertilizers, ingredients for producing various chemical compounds, and the fuel for delivering hydrogen. However, the current industrial NH₃ synthesis relies on the Haber-Bosch process which is energy intensive. Electrochemical nitrate reduction reaction (NO₃^-RR), which can produce NH₃ under the ambient condition, thus becomes the attractive alternative to Haber-Bosch process. However, hydrogen evolution and the formation of nitrite are the main side reactions competing with the nitrate-to-ammonia conversion. Cu-based materials are commonly used as the electrocatalyst for NO₃^-RR to achieve a high selectivity toward NH₃. Nevertheless, the influence of the underlying support on the resulting NO₃^-RR activity of the Cu catalyst has not been fully explored in the literature.
In this study, electrically conductive and highly porous electrocatalysts composed of nanosized Cu supported by carbon, ZrO₂ loaded on carbon, and CeO₂ loaded on carbon, were synthesized by carbonizing a Cu-based metal–organic framework (MOF), Cu-installed Zr-based MOF, and Cu-installed Ce-based MOF, respectively (Figure 1). With the same loading of Cu on the electrode, the catalyst with the ceria/carbon support (CuCeO_x/C) can achieve the highest faradaic efficiencies toward NH₃ among all electrocatalysts at every overpotential. Turnover frequencies (TOF) were then calculated based on the electrochemically addressable amount of Cu sites. The results reveal that CuCeO_x/C exhibits the highest TOF among all catalysts at relatively low overpotentials owing to the support effect that facilitates the catalytic kinetics, while the Cu supported by carbon (CuO_x/C) can achieve the highest TOF than other materials at high overpotentials owing to its higher porosity and electrical conductivity.

Figure 1. Schematic representation for the electrocatalytic NO<sub>3</sub><sup>-</sup>RR process with MOF-derived Cu-based electrocatalysts containing different supports.

Figure 1. Schematic representation for the electrocatalytic NO₃^-RR process with MOF-derived Cu-based electrocatalysts containing different supports.

Meng-Dian Tsai

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