From waste to energy and fuel: novel CuxNiy/CN catalysts from waste melamine resin for efficient nitrate reduction to ammonia†

Abstract

The conversion of nitrate (NO₃⁻) contaminants into ammonia (NH₃) through electrochemical reduction presents a viable strategy for the dual purposes of wastewater purification and ammonia production. Meanwhile, dealing with hazardous heavy metals (Cu, Ni, etc.) in the electroplating industry is a global mandate, incurring substantial cost and consuming vast amounts of energy. This work integrates the treatment of Cu and Ni in electroplating wastewater with the nitrate reduction reaction (NO₃⁻RR) for sustainable fuel and electricity generation. A facile and cost-effective Cu_xNi_y/CN catalyst, which represents a CuNi alloy on nitrogen-doped carbon foam (CN), is developed from the waste electroplating water and melamine resin. Among various Cu_xNi_y/CN catalysts, Cu_0.5Ni_0.5/CN exhibits enhanced NO₃⁻RR performance with a high ammonia yield rate (1755 μg h⁻¹ mg_cat⁻¹) and Faradaic efficiency (92.4%), outperforming most of the reported catalysts. The performance of Cu_0.5Ni_0.5/CN at low nitrate concentration verifies the effectiveness of catalysts for potential industrial application. Theoretical calculations reveal that the reduced energy barrier facilitates the hydrogenation of *NO₂ to *NO₂H over Cu_0.5Ni_0.5/CN, favoring the subsequent reduction to NH₃. A Zn–nitrate battery is designed for power generation with the Cu_0.5Ni_0.5/CN catalyst, exhibiting a voltage of 1.36 V and power density of 1.51 mW cm⁻². This study opens a new avenue to recycle both industrial (electroplating, nitrate) and domestic (melamine resin) wastes for sustainable fuel and power generation.