Efficient bifunctional catalysts synthesized from three-dimensional Ni/Fe bimetallic organic frameworks for overall urea electrolysis

Crown ether-like octanuclear molybdenum(V) clusters for cation binding and gas adsorption

Octanuclear polyoxomolybdenum-based porous materials, Na8[Mo8O8(μ2-O)8(μ2-OH)8(3-apz)4]2·26H2O (1, 3-Hapz = 3-aminopyrazole), K8[Mo8O8(μ2-O)8(μ2-OH)8(3-apz)4]2·7H2O (2) and (NH4)4[Mo8O8(μ2-O)8(μ2-OH)4(3-apz)8]·20.5H2O (3), have been successfully synthesized by a hydrothermal method and fully characterized. X-ray structural analyses show that microporous materials 1-3 contain round pores formed by octanuclear molybdenum-oxygen groups connected sequentially with pore sizes of 4.0, 4.0, and 4.8 Å, respectively. Both 1 and 2 are composed of two {Mo8} rings, which are connected by strong intramolecular hydrogen bonds between bridging hydroxy groups and oxygen atoms to form dimeric structures. The central pores in 1 and 2 are occupied by Na+ and K+, respectively, while they are empty in 3. This reflects the structural expansion and contraction effects induced by different cations. Through intermolecular stacking, 1-3 also exhibit channels with sizes of 14.0 × 6.4, 4.6 × 2.6, and 5.4 × 5.4 Å, respectively, which were used for the studies of gas adsorption. The results show that 1-3 can selectively adsorb CO2 and O2, including the empty hole in 3, while they show little or no affinity for gases H2, N2, and CH4. Moreover, an additional polyoxomolybdenum-based species (Mo8O26)n·4n(3-H2apz) (4) has been obtained with protonated 3-aminopyrazole in the absence of a reducing agent, which can serve as an intermediate for the polyoxomolybdenum-based porous products.

Controlled synthesis of ACo2O4 (A = Fe, Cu, Zn, Ni) as an environmentally friendly electrocatalyst for urea electrolysis

Water electrolysis is relatively an environmentally friendly hydrogen production technology, but due to the slow transfer of four electrons in the anodic oxidation reaction, it needs a theoretical voltage of up to 1.23 V. Therefore, in this experiment, a series of transition metal oxides, ACo₂O₄ (A = Fe, Cu, Zn, Ni), was synthesized on Ni foam current collectors by a hydrothermal and calcination method, and the material was applied in urea electrolysis to produce hydrogen. What is noteworthy is that the CuCo₂O₄ electrode has a unique flower-like nanoneedle structure, and has a larger electrochemical active area, more reactive active sites, and a faster charge transfer rate. In 1.0 M KOH and 0.5 M urea solution, CuCo₂O₄ provides a potential of only 1.268 V at a current density of 10 mA cm^-2 during the urea oxidation reaction (UOR), while in 1.0 M KOH solution, with the same current density, the oxygen evolution reaction (OER) is required to provide a potential of 1.53 V, indicating that the UOR can effectively replace the OER. Density functional theory calculations show that the CuCo₂O₄ material exhibits Gibbs free energy of the hydrogen closest to zero, thus promoting the electrochemistry performance of the electrode. In a cell composed of CuCo₂O₄//CuCo₂O₄, the current density of 10 mA cm^-2 can be achieved by providing a potential of only 1.509 V. This work offers a novel scheme for reducing energy consumption of the OER and improving catalytic performance of the UOR.

Controlled synthesis of Crx-FeCo2P nanoarrays on nickel foam for overall urea splitting

Urea splitting is a highly promising technology for hydrogen production to cope with the fossil energy crisis, which requires the development of catalysts with high electrocatalytic activity. In this article, Cr_x-FeCo₂P/NF catalysts were synthesized by hydrothermal and low-temperature phosphorylation and used in the overall urea splitting process. Cr_0.15-FeCo₂P/NF and Cr_0.1-FeCo₂P/NF exhibited excellent urea oxidation reaction (UOR) activity (potential of 1.355 V at 100 mA cm^-2) and hydrogen evolution reaction (HER) activity (overpotential of 173 mV at 10 mA cm^-2) in 0.5 M urea solution containing 1 M KOH. In the assembled Cr_0.15-FeCo₂P/NF//Cr_0.1-FeCo₂P/NF electrolytic cell, only a small voltage of 1.50 V is needed to reach 10 mA cm^-2. Density functional theory (DFT) calculation results demonstrate that an appropriate amount of Cr doping accelerates the kinetic performance of hydrogen production as well as improving the metallic properties of the electrode.

Recent Development of Nickel-Based Electrocatalysts for Urea Electrolysis in Alkaline Solution

Recently, urea electrolysis has been regarded as an up-and-coming pathway for the sustainability of hydrogen fuel production according to its far lower theoretical and thermodynamic electrolytic cell potential (0.37 V) compared to water electrolysis (1.23 V) and rectification of urea-rich wastewater pollution. The new era of the "hydrogen energy economy" involving urea electrolysis can efficiently promote the development of a low-carbon future. In recent decades, numerous inexpensive and fruitful nickel-based materials (metallic Ni, Ni-alloys, oxides/hydroxides, chalcogenides, nitrides and phosphides) have been explored as potential energy saving monofunctional and bifunctional electrocatalysts for urea electrolysis in alkaline solution. In this review, we start with a discussion about the basics and fundamentals of urea electrolysis, including the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER), and then discuss the strategies for designing electrocatalysts for the UOR, HER and both reactions (bifunctional). Next, the catalytic performance, mechanisms and factors including morphology, composition and electrode/electrolyte kinetics for the ameliorated and diminished activity of the various aforementioned nickel-based electrocatalysts for urea electrolysis, including monofunctional (UOR or HER) and bifunctional (UOR and HER) types, are summarized. Lastly, the features of persisting challenges, future prospects and expectations of unravelling the bifunctional electrocatalysts for urea-based energy conversion technologies, including urea electrolysis, urea fuel cells and photoelectrochemical urea splitting, are illuminated.

Electrocatalyst based on Ni2P nanoparticles and NiCoP nanosheets for efficient hydrogen evolution from urea wastewater

Urea electrolysis is a promising approach to produce hydrogen while simultaneously purifying urea-rich wastewater. In practice, it is highly desirable but still challenging, through the structure construction strategy, to implement a method with controllable synthesis of ultra-thin nanosheet arrays with rich interfaces, and then apply them into the catalysis operations of hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). In this work, the bifunctional electrocatalyst Ni₂P/NiCoP nanosheets anchored nickel foam (NF) were prepared with ultra-thin rich interfaces by regulating the Co- and P-doping. The results showed that the elaborated Ni₂P/NiCoP/NF electrode delivered the excellent electrocatalytic activities for both UOR and HER operations. Particularly for UOR, it required only a cell voltage of 1.41 V at 100 mA cm^-2, which was 400 mV lower than that in the traditional overall water splitting operation.

Achieving low-energy consumption water-to-hydrogen conversion via urea electrolysis over a bifunctional electrode of hierarchical cuprous sulfide@nickel selenide nanoarrays

Replacing sluggish oxygen evolution reaction with thermodynamically favorable urea oxidation reaction is a promising strategy for hydrogen-generation from water with low-energy consumption. However, the involved six-electron transfer process makes it formidable and seems critical. Hence, exploring high-efficient and low-cost bifunctional catalysts toward urea electrolysis is highly desirable. Herein, hierarchical cuprous sulfide@nickel selenide nanowire arrays were grown on copper foam (termed as Cu₂S@Ni₃Se₂) via a developed method composed of in situ chemical deposition, ion exchange and electrodeposition. The as-prepared bifunctional Cu₂S@Ni₃Se₂ not only shows remarkable hydrogen evolution reaction (HER) activity but also affords excellent urea oxidation reaction (UOR) activity. A subsequently configured Cu₂S@Ni₃Se₂//Cu₂S@Ni₃Se₂ full-cell (Cu₂S@Ni₃Se₂ working as both anode and cathode) only requires a low voltage of 1.48 V to launch a current density of 10 mA cm^-2, not only surpassing the routine water electrolysis (1.70 V), but also outperforming the state-of-the-art Pt/C//IrO₂ for urea electrolysis (1.65 V). Moreover, the performance is superior to most recently reported ones that configured with other catalysts. This work presents a solid step for hydrogen-generation from water with low-energy consumption.

Oxygen Vacancy-rich Ni/NiO@NC Nanosheets with Schottky Heterointerface for Efficient Urea Oxidation Reaction

H₂ production via electrocatalytic water splitting is greatly hindered by the sluggish oxygen evolution reaction (OER). The urea oxidation reaction (UOR) draws specific attention not only because of its lower theoretical voltage of 0.37 V compared with OER (1.23 V), but also for treating sewage water. Herein, Ni/NiO nanosheets with an ultrathin N-doped C layer containing a Schottky Ni and NiO heterointerface is constructed. Because of the self-driven charge redistribution at the heterointerface, janus charge domains are successfully created to drive the cleavage of urea molecules. Meanwhile, the synergistic effect between N-doped C and Ni/NiO restrains the deactivation of active sites in alkaline solution. The catalyst displays 1.35 V for UOR at 10 mA/cm² , 0.27 V lower than that of OER. The final potential increase is only 2 mV after long-term stability test of 12 h for UOR, much smaller than the uncoated sample (38 mV). The present work shows that C-coated transition metal nanomaterials with oxygen vacancies and a Schottky heterointerface are promising candidates for simultaneously boosting UOR with both high activity and long-term stability.

Luminescent triphenylamine-based metal-organic frameworks: recent advances in nitroaromatics detection

Luminescent metal-organic frameworks (LMOFs), as one branch of MOFs, have attracted considerable attention in recent years due to their good crystallinity, structural diversity, tunable porosity and easily induced fluorescence emission. Importantly, their photoluminescence (PL) properties can be adjusted by altering metal ions or metal clusters and organic ligands in one hybrid system. Among the various sensing applications, using LMOFs as chemical sensors to detect the explosive and environment pollution causing nitroaromatic compounds (NACs) is an important topic. In this account, we describe the recent advancements in the field of NAC detection by LMOFs based on the triphenylamine (TPA) unit as the π-conjugated fluorophore.