Metal–Organic Polymer-Derived Interconnected Fe–Ni Alloy by Carbon Nanotubes as an Advanced Design of Urea Oxidation Catalysts
Arindam Modak, Roopathy Mohan, Kalaiyarasi Rajavelu, Rivka Cahan, Tatyana Bendikov, Alex Schechter
Abstract
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.