Metal–Organic Polymer-Derived Interconnected Fe–Ni Alloy by Carbon Nanotubes as an Advanced Design of Urea Oxidation Catalysts

The electrochemical urea oxidation reaction (UOR) is considered as a promising renewable source for harvesting energy from waste. We report a new synthetic design approach to produce an iron–nickel alloy nanocatalyst from a metal–organic polymer (MOP) by a single-step carbonization process at 500 °C, thus forming a core–shell of iron–nickel-coated carbon (C@FeNi) nanostructures wired by embedded carbon nanotubes (CNTs) (CNT/C@FeNi). Powder X-ray diffraction confirmed the formation of metallic FeNi3 alloy nanoparticles (∼20 to 28 nm). Our experimental results showed that MOP containing CNTs acquired an interconnected hierarchical topology, which prevented the collapse of its microstructure during pyrolysis. Hence, CNT/C@FeNi shows higher porosity (10 times) than C@FeNi. The electrochemical UOR in alkaline electrolytes on these catalysts was studied using cyclic voltammetry (CV). The result showed a higher anodic current (3.5 mA cm–2) for CNT/C@FeNi than for C@FeNi (1.1 mA cm–2) at 1.5 V/RHE. CNT/C@FeNi displayed good stability in chronoamperometry experiments and a lower Tafel slope (33 mV dec–1) than C@FeNi (41.1 mV dec–1). In this study, CNT/C@FeNi exhibits higher exchange current density (3.2 μA cm–2) than does C@FeNi (2 μA cm–2). The reaction rate orders of CNT/C@FeNi and C@FeNi at a kinetically controlled potential of 1.4 V/RHE were 0.5 and 0.9, respectively, higher than the 0.26 of β-Ni(OH)2, Ni/Ni(OH)2 electrodes. The electrochemical impedance result showed a lower charge-transfer resistance for CNT/C@FeNi (61 Ω·cm–2) than for C@FeNi (162 Ω·cm–2), due to faster oxidation kinetics associated with the CNT linkage. Moreover, CNT/C@FeNi exhibited a lower Tafel slope and resistance and higher heterogeneity (25.2 × 10–5 cm s–1), as well as relatively high faradic efficiency (68.4%) compared to C@FeNi (56%). Thus, the carbon-coated FeNi3 core connected by CNT facilitates lower charge-transfer resistance and reduces the UOR overpotential.