Ni_xFe_100-x for urea and oxygen evolution: a matter of compromise

Abstract

The combination of aqueous electrolysis, either for hydrogen generation or CO2 conversion, with wastewater treatment offers an elegant way to tackle issues associated with our energy transition and the need for clean drinking water. However, it requires an anode capable of doing both the oxidation of the targeted pollutant and the oxygen evolution reaction (OER), as most pollutants are present in too low concentration to be practical for industrial electrolysis. In this study, we focussed on the oxidation of urea on NixFe100-x catalysts. These catalysts were prepared by pulsed laser ablation in liquid, a versatile and green technique to prepare electrocatalysts. Transmission electron microscopy of the nanoparticles indicates the production of monodisperse nanoparticles, with an average diameter increasing from 7.8 ± 2.8 to 19.7 ± 3.9 nm with a higher iron fraction. The composition could be controlled between pure Ni and NiFe bimetallic nanoparticles with up to 56 ± 3% of iron, by controlling the composition of the target. A brief optimisation of the electrode preparation (loading, catalyst-to-carbon ratio) yielded an optimum at about 30 µg/cm2 of catalyst with a catalyst-to-carbon ratio of 20:80. During the electrocatalytic tests, Ni was the best catalyst for urea oxidation, with a maximum peak current of 619 mA/mg. However, Ni75Fe25 was the best OER catalyst, showing a peak current of 1150 mA/mg. The difference increased further during CA at 0.5 V, during which Ni75Fe25 outperformed pure Ni by almost a factor of 3 after 30 min.