Efficient co-production of ammonia and formic acid from nitrate and polyester via paired electrolysis

Paired electrolysis, which integrates a productive cathodic reaction, such as the nitrate reduction reaction (NO₃^-RR) with selective oxidation at the anode, offers an intriguing way to maximize both atomic and energy efficiency. However, in a conventional design, the NO₃^-RR is often coupled with the anodic oxygen evolution reaction, leading to substantial energy consumption while yielding low-value oxygen. Here, we report a hybrid electrolysis system that combines cathodic reduction of nitrate to ammonia and anodic oxidation of polyethylene-terephthalate-derived ethylene glycol (EG) to formic acid (FA), utilizing oxygen-vacancy-rich (O_V) Co₃O₄ and Cu doped Ni(OH)₂ as the cathode and anode, respectively. Remarkably, this paired electrolysis system demonstrates a faradaic efficiency (FE) of 92% for cathodic ammonia production and a FE of 99% for anodic FA production, while reducing the cell voltage by 0.54 V compared to the conventional NO₃^-RR system at the same current density of 100 mA cm^-2. Experimental investigations combined with theoretical calculations reveal that the O_V introduction effectively addresses the insufficient NO₃^- adsorption and hydrogenation on bare Co₃O₄. Additionally, Cu incorporation increases the Ni-O covalency, resulting in an improved EG adsorption ability. This work presents a promising way for waste management via paired electrolysis.