Urea electro-oxidation efficiently catalyzed by nickel-molybdenum oxide nanorods

Multi-win situation of wastewater purification, carbon emission reduction and resource utilization: Conversion of refractory organics and nitrate to urea and ammonia in a flow-through electrochemical integrated system

The advanced oxidation process is an efficient technology for the degradation and detoxification of refractory organics to ensure water safety. However, most researches focus on improving pollutant degradation but overlook carbon emission and resource utilization. In this study, a flow-through electrochemical integrated system was constructed to simultaneously realize bisphenol A (BPA) oxidation into small non-toxic organics and CO2, and generated CO2 coupled with nitrate-containing wastewater conversion to urea and ammonia on a porous cathode (Zr-Fe/CN). The synergistic effect between anodic BPA oxidation with cathodic CO2 and NO3-reduction improves the electron utilization efficiency and thus increasing the BPA degradation, urea yield rate (UYR) and NH3 yield rate (NYR) by 13.4 % 18.4 % and 8.3 %, respectively. Furthermore, the flow-through operation mode significantly increased the mass transfer efficiency and quickly carried generated CO2 from the anode into the cathode to improve CO2 utilization efficiency. Compared to the parallel plate electrode reactor, the BPA degradation efficiency, UYR and NYR in the flow-through reactor increased from 59.46 % to 84.49 % (the initial concentration of BPA was 40 mg/L), 9.94 mmol h-1g-1 to 19.55 mmol h-1g-1, and 80.31 mmol h-1g-1 to 106.06 mmol h-1g-1 within 60 min, respectively. Moreover, the total carbon conversion efficiency (from BPA to urea) increased from 20.2 % to 42.4 % and the total Faraday efficiency (FE) increased from 78.6 % to 96.3 %. This work provides a multi-win strategy of harmless, resource-based and carbon emission reduction for wastewater treatment.

Catalytic urea electrooxidation on nickel‐metal hydroxide foams for use in a simplified dialysis device

Electrocatalytic urea removal is a promising technology for artificial kidney dialysis and wastewater treatment. Urea electrooxidation was studied on nickel electrocatalysts modified with Cr, Mo, Mn, and Fe. Mass transfer limits were observed for urea oxidation at physiological concentrations (10 mmol L). Urea oxidation kinetics were explored at higher concentrations (200 mmol L), showing improved performance, but with lower currents per active site. A simplified dialysis model was developed to examine the relationship of mass transfer coefficients and extent of reaction on flowrate, composition, and pH of the reacting stream. For a nickel hydroxide catalyst operating at 1.45 V, 37 , and pH 7.1, the model shows a minimum geometric electrode area of 1314 cm2 is needed to remove 3.75 g urea h with a flow rate of 200 mL min for continuous operation.

Synthesis of Ni decorated MoOx nanorod catalysts for efficient overall urea-water splitting

Substituting slow oxygen evolution reaction (OER) with thermodynamically favorable urea oxidation reaction (UOR) is considered as one of the feasible strategies for achieving energy-saving hydrogen production. Herein, a uniform layer of NiMoO4 nanorods was grown on nickel foam by a hydrothermal method. Then, a series of Ni-MoOx/NF-X nanorod catalysts comprising Ni/NiO and MoOx (MoO2/MoO3) were prepared through regulating annealing atmosphere and reduction temperature. The optimized Ni-MoOx/NF-3 with a large accessible specific area can act as a bifunctional catalyst for electrocatalytic anodic UOR and cathodic hydrogen evolution reaction (HER). At a current density of 100 mA cm-2, the introduction of urea can significantly reduce the overpotential of Ni-MoOx/NF-3 by 210 mV compared to OER. In addition, Ni-MoOx/NF-3 has a higher intrinsic activity than other catalysts. It only requires -0.21 and 1.38 V to reach 100 mA cm-2 in HER and UOR, respectively. Such an excellent performance can be attributed to the synergistic function between Ni and MoOx. The presence of metallic Ni and reduced MoOx in pairs is beneficial for improving the electrical conductivity and modulating the electronic structure, resulting in enhancing the electrocatalytic performance. When assembling Ni-MoOx/NF-3 into an overall urea-water splitting system, it can achieve energy-saving hydrogen production and effective removal of urea-rich wastewater.

Interfacial Coupling of Graphene with Nickel Nanoparticles for Water Splitting and Urea Oxidation: A Spectroelectrochemical Investigation

Nickel nanoparticle and graphene interfaces of various stoichiometries were created through electrodeposition techniques. The catalytic behavior of the electrodeposited films was investigated through spectro-electrochemical methodologies. UV-vis absorbance spectra of the electrodeposited films are significantly different in the air and alkaline medium. Furthermore, UV-vis and Raman spectroscopy confirmed the coupling of Ni nanoparticles (Ni-NP) with the graphene framework, along with NiO and Ni(OH)2 . A combination of Raman and impedance spectroscopy revealed that the surface adsorption and charge transfer properties of the electrodeposited films are entirely dependent on the defects on graphene structure as well as distribution of Ni-NP on graphene. The electrodeposited films possess heterogeneous catalytic properties with a low overpotential of 50 mV (10 mA/cm-2 ) for hydrogen evolution reaction, as well as 601 mV and 391 mV (at 50 mA/cm-2 ) for the oxygen evolution reaction and urea oxidation reaction, respectively. In addition, eelectrodeposited samples show extraordinary overall water splitting performance by achieving a current density of 10 mA/cm2 at a very low applied potential of 1.38 V. This synergistic coupling of Ni and graphene renders the electrodeposited samples promising candidates as electrodes for overall water splitting in alkaline and urea-supplemented solutions.

Large scale uniform Ni-P plated carbon fiber for boosting urea electro-oxidation and electro-detection

Seeking an excellent electrocatalyst is the trickiest issue for the application of urea electro-oxidation and electro-detection. Phosphorus-doped nickel plating on carbon fibers (Ni-P/CF) is synthesized by simple electroless plating. SEM results exhibit that the Ni-P densely and uniformly grows onto the surface of carbon fibers (CF), forming carbon fibers-like nanoarchitectures. Benefiting from the carbon fibers-like nano architectures with abundant exposed active sites on the surface of CF, electron transfer can be synchronously facilitated, and Ni-P/CF displays superior urea electrooxidation (UOR) performance with potentials of 1.40 V to reach 100 mA cm-2. Impressively, it can maintain at 20 mA cm-2 for 48 h without evident activity attenuation, demonstrating robust durability. Cycle stability shows that the voltage has only increased by 10 mV at 300 mA cm-2 from the 10th to 20000th cycles. Most importantly, Ni-P/CF at a length of 100 cm with good reproducibility was successfully synthesized, denoting great potential for large-scale industrial production. Therefore, this work not only affords cost-effective tactics for urea-rich wastewater degradation but also can achieve practical medical applications.

Electrochemical-based processes for produced water and oily wastewater treatment: A review

The greatest volume of by-products produced in oil and gas recovery operations is referred to as produced water and increasing environmental concerns and strict legislations on discharging it into the environment cause to more attention for focusing on degradation methods for treatment of produced water especially electrochemical technologies. This article provides an overview of electrochemical technologies for treating oily wastewater and produced water, including: electro-coagulation, electro-Fenton, electrochemical oxidation and electrochemical membrane reactor as a single stage and combination of these technologies as multi-stage treatment process. Many researchers have carried out experiments to examine the impact of various factors such as material (i.e, electrode material) and operational conditions (i.e., potential, current density, pH, electrode distance, and other factors) for organic elimination to obtain the high efficiency. Results of each method are reviewed and discussed according to these studies, comprehensively. Furthermore, several challenges need to be overcome and perspectives for future study are proposed for each method.

Multistep Dissolution of Lamellar Crystals Generates Superthin Amorphous Ni(OH)2 Catalyst for UOR

Urea oxidation reaction (UOR) is an ideal replacement of the conventional anodic oxygen evolution reaction (OER) for efficient hydrogen production due to the favorable thermodynamics. However, the UOR activity is severely limited by the high oxidation potential of Ni-based catalysts to form Ni³⁺ , which is considered as the active site for UOR. Herein, by using in situ cryoTEM, cryo-electron tomography, and in situ Raman, combined with theoretical calculations, a multistep dissolution process of nickel molybdate hydrate is reported, whereby NiMoO₄ ·xH₂ O nanosheets exfoliate from the bulk NiMoO₄ ·H₂ O nanorods due to the dissolution of Mo species and crystalline water, and further dissolution results in superthin and amorphous nickel (II) hydroxide (ANH) flocculus catalyst. Owing to the superthin and amorphous structure, the ANH catalyst can be oxidized to NiOOH at a much lower potential than conventional Ni(OH)₂ and finally exhibits more than an order of magnitude higher current density (640 mA cm^-2 ), 30 times higher mass activity, 27 times higher TOF than those of Ni(OH)₂ catalyst. The multistep dissolution mechanism provides an effective methodology for the preparation of highly active amorphous catalysts.

Molybdenum carbide/Ni nanoparticles-incorporated carbon nanofibers as effective non-precious catalyst for urea electrooxidation reaction

In this study, molybdenum carbide and carbon were investigated as co-catalysts to enhance the nickel electro-activity toward urea oxidation. The proposed electrocatalyst has been formulated in the form of nanofibrous morphology to exploit the advantage of the large axial ratio. Typically, calcination of electropsun polymeric nanofibers composed of poly(vinyl alcohol), molybdenum chloride and nickel acetate under vacuum resulted in producing good morphology molybdenum carbide/Ni NPs-incorporated carbon nanofibers. Investigation on the composition and morphology of the proposed catalyst was achieved by XRD, SEM, XPS, elemental mapping and TEM analyses which concluded formation of molybdenum carbide and nickel nanoparticles embedded in a carbon nanofiber matrix. As an electrocatalyst for urea oxidation, the electrochemical measurements indicated that the proposed composite has a distinct activity when the molybdenum content is optimized. Typically, the nanofibers prepared from electrospun nanofibers containing 25 wt% molybdenum precursor with respect to nickel acetate revealed the best performance. Numerically, using 0.33 M urea in 1.0 M KOH, the obtained current densities were 15.5, 44.9, 52.6, 30.6, 87.9 and 17.6 mA/cm² for nanofibers prepared at 850 °C from electropsun mats containing 0, 5, 10, 15, 25 and 35 molybdenum chloride, respectively. Study the synthesis temperature of the proposed composite indicated that 1000 °C is the optimum calcination temperature. Kinetic studies indicated that electrooxidation reaction of urea does not follow Arrhenius's law.

Mo doping and Se vacancy engineering for boosting electrocatalytic water oxidation by regulating the electronic structure of self-supported Co9Se8@NiSe

Oxygen evolution reactions (OERs) are regarded as the rate-determining step of electrocatalytic overall water splitting, which endow OER electrocatalysts with the advantages of high activity, low cost, good conductivity, and excellent stability. Herein, a facile H₂O₂-assisted etching method is proposed for the fabrication of Mo-doped ultrathin Co₉Se₈@NiSe/NF-X heterojunctions with rich Se vacancies to boost electrocatalytic water oxidation. After step-by-step electronic structure modulation by Mo doping and Se vacancy engineering, the self-standing Mo-Co₉Se₈@NiSe/NF-60 heterojunctions deliver a current density of 50 mA cm^-2 with an overpotential of 343 mV and a cell voltage of only 1.87 V at 50 mA cm^-2 for overall water splitting in 1.0 M KOH. Our study opens up the possibility of realizing step-by-step electronic structure modulation of nonprecious OER electrocatalysts via heteroatom doping and vacancy engineering.

Coordination Polymer Framework-Derived Ni-N-Doped Carbon Nanotubes for Electro-Oxidation of Urea

Electrochemical oxidation of urea (UOR) is critical in the removal of urea from wastewater and energy conservation and storage. Nickel-based catalysts are widely used for urea-ORR, but in all cases, the nickel must be hybridized with carbon materials to improve its conductivity. In this manuscript, we demonstrate the synthesis of a nickel-decorated carbon nanotube (Ni-NCNT) by simple microwave pyrolysis of Dabco (1,4-diazabicyclo[2.2.2]octane)-based coordination polymer frameworks (CPF). The surface structure, morphology and chemical composition of Ni-NCNT were characterized by Raman spectrum, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy (EDS) analysis. SEM studies showed micrometer-long bamboo-shaped CNTs with nickel nanoparticles anchored to the walls and inside the nanotubes. A structural study by TEM and Raman spectra showed that carbon nanotubes are rich in defects due to the presence of nitrogen, and this was confirmed by energy-dispersive X-ray spectroscopy (EDS) maps. When applied as electrocatalysts in urea oxidation reactions (UOR), our newly developed Ni-NCNT shows excellent electrocatalytic activity and stability, making it a versatile catalyst in energy generation and mitigating water contamination.

A review on sensing and catalytic activity of nano-catalyst for synthesis of one-step ammonia and urea: Challenges and perspectives

One of the most significant chemical operations in the past century was the Haber-Bosch catalytic synthesis of ammonia, a fertilizer vital to human life. Many catalysts are developed for effective route of ammonia synthesis. The major challenges are to reduce temperature and pressure of process and to improve conversion of reactants produce green ammonia. The present review, briefly discusses the evolution of ammonia synthesis and current advances in nanocatalyst development. There are promising new ammonia synthesis catalysts of different morphology as well as magnetic nanoparticles and nanowires that could replace conventional Fused-Fe and Promoted-Ru catalysts in existing ammonia synthesis plants. These magnetic nanocatalyst could be basis for the production of magnetically induced one-step green ammonia and urea synthesis processes in future.

Efficient heterogeneous Ni/Ni2P catalyst for urea-assisted water electrolysis

The built-in electrophilic/nucleophilic domain promoted the favorable adsorption of urea molecules on the surface/interface of heterogeneous Ni/Ni2P for urea oxidation.

Pd Nanoparticles Coupled to NiMoO4-C Nanorods for Enhanced Electrocatalytic Ethanol Oxidation

The interfacial interaction including chemical bonding or electron transfer and even physisorption in composite electrocatalysts has a considerable effect on electrocatalytic oxidation reaction. Herein, we report a tremendously enhanced catalytic activity and excellent durability for the ethanol electro-oxidation reaction in NiMoO₄-C-supported Pd composites (Pd/NiMoO₄-C) compared to the commercial Pd/C (10%) catalyst. The X-ray powder diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements disclose that the strong electron transfer between NiMoO₄ nanorods and Pd nanoparticles likely induces the formation of more electrochemical active centers and improves the adsorption-desorption capacity of reactants and corresponding intermediates. In addition, the Pd/NiMoO₄-C composite exhibits superior specific activity for ethanol oxidation compared to the Pd/NiMoO₄ catalyst with physically incorporated carbon black, which further reveals that the stronger anchoring effect between Pd and C and higher electrical conductivity in Pd/NiMoO₄-C composites are also conducive to promote the ethanol oxidation reaction. These discoveries provide an effective and simple method for the design of advanced electrocatalysts and provide more insights into optimizing the electronic interaction between the catalyst and support in general.

Ni-Fe phosphide deposited carbon felt as free-standing bifunctional catalyst electrode for urea electrolysis

A free-standing catalyst electrode for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER) in a urea electrolysis cell was synthesized by electroplating a Ni-Fe alloy onto carbon felt, followed by phosphidation (P-NiFe@CF). The prepared P-NiFe@CF catalyst consisted of Ni5P4, NiP2, and FeP with 3D flower-like P-NiFe architecture on CF. P-NiFe@CF exhibited excellent electrocatalytic activity for the UOR (demanding only 1.39 V (vs. RHE) to achieve 200 mA cm-2), and for the HER with a low overpotential of 0.023 V (vs. RHE) at 10 mA cm-2, indicating its feasibility as a bifunctional catalyst electrode for urea electrolysis. A urea electrolysis cell with P-NiFe@CF as both the free-standing anode and cathode generated a current density of 10 mA cm-2 at a cell potential of 1.37 V (vs. RHE), which is considerably lower than that of water electrolysis, and also lower than previously reported values. The results indicate that the P-NiFe@CF catalyst electrodes can be used as free-standing bifunctional electrodes for urea electrolyzers.

Facile fabrication of bifunctional SnO-NiO heteromixture for efficient electrocatalytic urea and water oxidation in urea-rich waste water

Heterostructured transition metal oxide hybrid have more attention in energy saving and environmental related field due to their higher electro-catalytic activity. In this work, we demonstrated SnO decorated with NiO nanocrystal electrocatalyst is successfully synthesized through solvothermal method and well characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy. Physical characterizations confirm that spherical shape of SnO nanoparticles are homogeneously dispersed on the surface of NiO. The kinetic study of catalytic performance towards urea oxidation reaction were measured by liner sweep voltammetry and chronoamprometry. As proposed catalyst to facilitate the rate of urea oxidation reaction can increase by SnO doped NiO catalyst. The urea oxidation on SnO-NiO nanostructured modified electrode exhibits lower onset potential of 1.12 V and enhancement of current with tafel slope of 150 mV dec^-1. The obtained results demonstrated the synthesized SnO-NiO anode material could be promising electrode for urea-rich containing wastewater remediation and hydrogen production from wastewater.

Molybdenum oxide based sensors

In this review article were considered the works of electrochemical sensors modified with molybdenum oxide. The work of sensors based on molybdenum oxide was systematized, a comparison table was developed, the sensors were classified according to the purpose of use. Methods of molybdenum oxide synthesis used to modify the working electrode in electrochemical sensors were considered. The various methods have been used to synthesize molybdenum oxide, such as a thermal, hydrothermal, electrochemical, electric spark, pulsed laser method, acid condensation, electrophoretic precipitation, pulse potential precipitation. The main parameters of the molybdenum oxide modified sensors, such as the detection limit, linear range, response time, sensitivity, and other parameters were compared. As a result of studies, it was found that molybdenum oxide is selected as a modifying material in electrochemical sensors due to the unique physicochemical properties of molybdenum oxide, in particular because of mechanical strength, electrical conductivity, electro catalytic activity, crystallinity. The features of electrochemical biosensors coated with molybdenum oxide were described for the detection of important compounds in specific samples. Sensors based on molybdenum oxide have been used for detection of glucose, dopamine, ethanol, ascorbic acid, troponin-1, norepinephrine, procalcitonin, L-lactate, bromate, chlorate, E110, tartrazine, hydrochlorothiazide, human epidermal growth factor-2, lithium,sodium,potassium. This paper provides general summarized information about current aspects of research works related to electrochemical sensors based on molybdenum oxide.

Molecular insight into optimizing the N- and P-doped fullerenes for urea removal in wearable artificial kidneys

Urea is the result of the breakdown of proteins in the liver, the excess of which circulates in the blood and is adsorbed by the kidneys. However, in the case of kidney diseases, some products, specifically urea, cannot be removed from the blood by the kidneys and causes serious health problems. The end-stage renal disease (ESRD) patients are not able to purify their blood, which endangers their life. ESRD patients require dialysis, a costly and difficult method of urea removal from the blood. Wearable artificial kidneys (WAKs) are consequently designed to remove the waste from blood. Regarding the great amount of daily urea production in the body, WAKs should contain strong and selective urea adsorbents. Fullerenes-which possess fascinating chemical properties-have been considered herein to develop novel urea removal adsorbents. Molecular dynamics (MD) has enabled researchers to study the interaction of different materials and can pave the way toward facilitating the development of wearable devices. In this study, urea adsorption by N-doped fullerenes and P-doped fullerenes were assessed through MD simulations. The urea adsorption was simulated by five samples of fullerenes, with phosphorous and different nitrogen dopant contents. For comparing the urea adsorption capacity in the performed simulations, detailed characteristics-including the energy analysis, radius of gyration, radial distribution function (RDF), root-mean-square fluctuation (RMSD), and H-bond analyses were investigated. It had been determined that the fullerene containing 8% nitrogen-with the highest reduction in the radius of gyration, the maximum RDF, a high adsorption energy, and a high number of hydrogen bonds-adsorbs urea more efficiently.

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.

Dual-functioning porous catalysts: robust electro-oxidation of small organic molecules and water electrolysis using bimetallic Ni/Cu foams

The simultaneous co-deposition of Cu within the matrix of Ni foams (utilizing DHBT) increases their intrinsic catalytic activity towards water electrolysis, urea oxidation reaction (UOR), and glycerol oxidation reaction (GOR) in alkaline media.

Efficient nanointerface hybridization in a nickel/cobalt oxide nanorod bundle structure for urea electrolysis

Urea electrolysis has received great attention for the energy-relevant applications, and efficient nanostructured catalysts are required to overcome the sluggish urea oxidation kinetics. Herein, we noticed that the valence state of Ni in the hybrid Ni/Co oxide nanorods can be correlated to the catalytic capability for urea oxidation. Crystal lattice hybridization was found in the interface of Ni/Co oxide nanoparticles that assembled as a nanorod bundle structure. The more or the less of Ni2+/Ni3+ generated lower catalytic ability, and Ni/Co oxide with the optimum content of Ni2+/Ni3+ exhibited the highest catalytic ability for urea oxidation because of the efficient synergism, resulting from the formation of high valence state of Ni species and improved kinetics. A low onset potential of 1.29 V was required for the urea oxidation compared with the high onset potential of 1.52 V for water oxidation; high selectivity for urea oxidation was found in the potential below 1.50 V, and as a promising application for urea-assisted water electrolysis about 190 mV less was required to provide 10 mA cm-2 in the two-electrode system, indicating the energy-efficient nature for hydrogen evolution. The study provides some novel insights into the Ni/Co catalyst design and fabrication with efficient catalytic synergism for electrocatalysis.

Metal-organic framework-derived Ni@C and NiO@C as anode catalysts for urea fuel cells

Highly porous self-assembled nanostructured Ni@C and NiO@C were synthesized via calcination of a Ni-based metal–organic framework. The morphology, structure, and composition of as synthesized Ni@C and NiO@C were characterized by SEM, FIB-SEM, TEM, and XRD. The electro-catalytic activity of the Ni@C and NiO@C catalysts towards urea oxidation was investigated using cyclic voltammetry. It was found that the Ni@C had a higher residual carbon content and a higher specific surface area than NiO@C, thus exhibiting an enhanced electrochemical performance for urea oxidation. A direct urea fuel cell with Ni@C as an anode catalyst featured an excellent maximum power density of 13.8 mW cm⁻² with 0.33 M urea solution in 1 M KOH as fuel and humidified air as oxidant at 50 °C, additionally showing excellent stability during continuous 20-h operation. Thus, this work showed that the highly porous carbon-supported Ni catalysts derived from Ni-based metal–organic framework can be used for urea oxidation and as an efficient anode material for urea fuel cells.

Bonding state synergy of the NiF2/Ni2P hybrid with the co-existence of covalent and ionic bonds and the application of this hybrid as a robust catalyst for the energy-relevant electrooxidation of water and urea

Among the energy-relevant electrochemical reactions, the electrochemical water and urea oxidation reactions are very significant for solving the increasing energy crisis and environmental pollution. Herein, the NiF2/Ni2P hybrid catalyst, in which covalent and ionic bonds co-existed, was found to be a very active catalyst for these electrochemical reactions occuring during the electrolysis of water. The bonding states of the covalent and ionic bonds were verified by the crystal structure and surface chemical state revealed by spectral analysis. As a bifunctional catalyst for the electrooxidation of water and urea, the NiF2/Ni2P hybrid structure demonstrated higher catalytic activity, kinetics and stability in the catalytic reaction than the individual components NiF2 and Ni2P under the same conditions. Specifically, an overpotential as low as 283 mV could drive the benchmark current density of 10 mA cm-2 for the oxygen evolution reaction, significantly lower than the overpotential required for the NiF2 (393 mV) and Ni2P materials (342 mV); the maximum current density for urea electrooxidation could reach 157.35 mA cm-2 at 1.53 V, which was much higher than those of NiF2 (23.55 mA cm-2) and Ni2P (102.72 mA cm-2). The catalytic performance also outperformed those of the recently reported similar advanced catalysts, and the high performance could be attributed to the highly exposed active sites, rough surface area, excellent charge transfer ability, and especially, the synergistic effects between the covalent and ionic bonds in the catalyst system. Using a commercial Pt/C catalyst as a cathode, the cell potential for urea-assisted water electrolysis could be reduced to 1.5 V to obtain the current density of nearly 40 mA cm-2 in a two-electrode system (Pt/C||NiF2/Ni2P), about 300 mV less than that required for water electrolysis in the general alkaline electrolyte. The current study demonstrates the significance of bonding state synergy in an advanced catalyst for water electrolysis and sheds some light on catalyst development in energy chemistry.