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

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 FeNi 3 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 FeNi 3 core connected by CNT facilitates lower charge-transfer resistance and reduces the UOR overpotential.

Recent advances in the pre-oxidation process in electrocatalytic urea oxidation reactions

“…103 In addition, a bimetallic FeNi alloyed catalyst anchored on conductive CNTs derived from a metal-organic polymer was proposed by the Schechter group, which exhibits an enhanced pre-oxidation reaction as well as UOR activity compared to the FeNi alloy loaded on carbon, further proving the beneficial role of CNTs in promoting such electrochemical processes via regulating the charge transfer ability. 104 Conductive polymers could also be engaged in optimizing the charge transfer and facilitating the pre-oxidation reaction for the UOR. The Song group indicated that the hierarchical Ni nanostructures grown on polypyrrole/graphene oxide (PPy/GO) could promote the generation of catalytically active NiOOH species owing to the fast charge transfer brought by PPy as well as the large surface area guaranteed by the GO substrate, which leads to obviously enhanced UOR activity compared with the unsupported sample.…”

Advances in Catalytic Electrooxidation of Urea: A Review

“…In another very recent work, Modak et al used a single‐step carbonization process to convert a metal–organic polymer into core–shell‐type FeNi‐coated carbon nanostructures interconnected by carbon nanotubes (CNT/C@FeNi). [ 97 ] This material was found to be an effective UO catalyst, exhibiting an oxidation current density of 3.5 mA cm −2 at 1.5 V versus RHE. In the absence of CNT, the corresponding material, C@FeNi, displayed a much lower UO current density of 1.1 mA cm −2 .…”

Recent Advances in the Valorization of Lignin: A Key Focus on Pretreatment, Characterization, and Catalytic Depolymerization Strategies for Future Biorefineries

“…Earlier studies have reported the application of bimetallic to be quite noteworthy in contrast to monometallic catalysts. [182][183][184][185] Zhang et al [182] used monometallic (Ni, Ru) and bimetallic NiRu catalyst for the depolymerization of organosolv lignin (130 °C, 1 h). Significantly higher aromatic monomer yields were reported for bimetallic (0.8 wt%) compared to the monometallic (0.16 wt%) catalyst, which further improved for bimetallic catalyst at longer reaction time (Table 10, Entry 5, 6).…”

“…Earlier studies have reported the application of bimetallic to be quite noteworthy in contrast to monometallic catalysts. [ 182–185 ] Zhang et al. [ 182 ] used monometallic (Ni, Ru) and bimetallic NiRu catalyst for the depolymerization of organosolv lignin (130 °C, 1 h).…”