Assembly of ϵ-Keggin Polyoxometalate from Molecular Crystal to Zeolitic Octahedral Metal Oxide.

Zeolitic octahedral metal oxides are inorganic crystalline microporous materials with adsorption and redox properties. New ϵ-Keggin nickel molybdate-based zeolitic octahedral metal oxides have been synthesized. 31 P NMR spectroscopy shows that reduction of MoVI -based molybdates forms an ϵ-Keggin polyoxometalate that immediately transfers to the solid phase. Investigation of the formation process indicates that a low Ni concentration, insoluble reducing agent, and long synthesis time are the critical factors for obtaining the zeolite octahedral metal oxides rather than the ϵ-Keggin polyoxometalate molecule. The synthesized zeolitic nickel molybdate with Na+ is used as the adsorbent, which effectively separates C2 hydrocarbon mixtures.

Part I: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Synthesis Parameters on the Materials' Physicochemical, Structural, and Morphological Properties.

The impact of process conditions on the synthesis of NiMoO4 nanostructures using a solution combustion synthesis (SCS) method, in which agar powder and Ni(NO3)2 were utilized as fuel and as the oxidant, respectively, was thoroughly studied. The results show that the calcination temperature had a significant implication on the specific surface area, phase composition, particle size, band gap, and crystallite size. The influence of calcination time on the resulting physicochemical/structural/morphological properties of NiMoO4 nanostructures was found to be a major effect during the first 20 min, beyond which these properties varied to a lesser extent. The increase in the Ni/Mo atomic ratio in the oxide impacted the combustion dynamics of the system, which led to the formation of higher surface area materials, with the prevalence of the ß-phase in Ni-rich samples. Likewise, the change in the pH of the precursor solution showed that the combustion reaction is more intense in the high-pH region, entailing major implications on the physicochemical properties and phase composition of the samples. The change in the fuel content showed that the presence of agar is important, as it endows the sample with a fluffy, porous texture and is also vital for the preponderance of the ß-phase.

Part II: NiMoO4 Nanostructures Synthesized by the Solution Combustion Method: A Parametric Study on the Influence of Material Synthesis and Electrode-Fabrication Parameters on the Electrocatalytic Activity in the Hydrogen Evolution Reaction.

Earth-abundant NiMo-oxide nanostructures were investigated as efficient electrocatalytic materials for the hydrogen evolution reaction (HER) in acidic media. Synthesis and non-synthesis parameters were thoroughly studied. For the non-synthesis parameters, the variation in Nafion loading resulted in a volcano-like trend, while the change in the electrocatalyst loading showed that the marginal benefit of high loadings attenuates due to mass-transfer limitations. The addition of carbon black to the electrocatalyst layer improved the HER performance at low loadings. Different carbon black grades showed a varying influence on the HER performance. Regarding the synthesis parameters, a calcination temperature of 500 °C, a calcination time between 20 and 720 min, a stoichiometric composition (Ni/Mo = 1), an acidic precursor solution, and a fuel-lean system were conditions that yielded the highest HER activity. The in-house NiMoO4/CB/Nafion electrocatalyst layer was found to offer a better long-term performance than the commercial Pt/C.

A critical review on modulation of NiMoO4-based materials for photocatalytic applications.

Semiconductor photocatalysis has been widely utilized to solve the problems of energy shortage and environmental pollution. Among the explored photocatalysts, nickel molybdate (NiMoO4) has revealed many advantages for photocatalytic applications, which include visible light absorption, low cost, environment-friendly, large surface area, good electrical conductivities, and tailorable band structure. However, the recombination of photogenerated carriers, which diminishes photocatalytic efficiency, has been held as a major hurdle to the widespread application of this material. To overcome this limitation, various surface modulations such as morphology control, doping of heteroatom, deposition of noble metal nanoparticles, and fabrication of composite structures have been explored in many published studies. This article comprehensively reviews the recent progress in the modulations of NiMoO4-based materials to improve the photocatalytic efficiency. The enhanced photocatalytic capabilities of NiMoO4-based materials are reviewed in terms of such applications as pollutant removal, disinfection of bacteria, and water splitting. The current challenges and possible future direction of research in this field are also highlighted. This comprehensive review is expected to advance the design of highly efficient NiMoO4-based materials for photocatalytic applications.

Preparation, Characterization and Catalytic Activity of Nickel Molybdate (NiMoO4) Nanoparticles.

Nickel molybdate (NiMoO4) nanoparticles were synthesized via calcination of an oxalate complex in static air at 500 °C. The oxalate complex was analyzed by thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). The as-synthesized nickel molybdate was characterized by Brunauer-Emmett-Teller technique (BET), X-ray diffraction (XRD), and transmission electron microscopy (TEM) and its catalytic efficiency was tested in the reduction reaction of the three-nitrophenol isomers. The nickel molybdate displays a very high activity in the catalytic reduction of the nitro functional group to an amino. The reduction progress was controlled using Ultraviolet-Visible (UV-Vis) absorption.

Vanadium-doped Ni microspheres loaded with phosphatization of NiMoO4 contributing to enhanced electron transfer for stable overall water splitting.

At present, there are few reports on the micron-sized catalysts for overall water splitting. In this study, phosphating method were used to construct the self-supporting catalyst (V doped Ni microspheres coated by NiMoO4/Ni12P5) with microspherical structure, providing a short path and a stable structure to guarantee quick electron transfer and excellent catalytic performance. Hence, oxygen evolution reaction (OER) only needs 254 mV to reach a current density of 50 mA cm-2 in 1.0 mol/L KOH, after 114 h without attenuation. The catalyst can achieve a current densitiy of 10 mA cm-2 with a voltage of only 158 mV for hydrogen evolution reaction (HER). When micron scale V-Ni@NiMoO4/Ni12P5 is used as both anode and cathode for overall water splitting, the device can operate at a current density of 10 mA cm-2 for more than 200 h of good stability. Its superior catalytic performance can be attributed to the construction of micron size and phosphating. DFT calculations indicate that the introduction of P better activates the adsorbed *OH and H2O*, reduces reaction the energy barrier, and improves the catalytic activity.

Engineering electronic structures and oxygen vacancies of manganese-doped nickel molybdate porous nanosheets for efficient oxygen evolution reaction.

The design strategy of designing effective local electronic structures of active sites to improve the oxygen evolution reaction (OER) performance is the key to the success of sustainable alkaline water electrolysis processes. Herein, a series of manganese-doped nickel molybdate porous nanosheets with rich oxygen vacancies on the nickel foam (Mn-NiMoO4/NF PNSs) synthesized by the facile hydrothermal and following annealing routes are used as high-efficiency and robust catalysts towards OER. By virtue of unique nanosheets architectures, more exposed active site, rich oxygen vacancies, tailored electronic structures, and improved electrical conductivity induced by Mn incorporation, as predicted, the optimized Mn0.10-NiMoO4/NF PNSs catalyst exhibits superior the OER performance with a low overpotential of 211 mV at 10 mA‧cm-2, a small Tafel slope of 41.7 mV‧dec-1, and an excellent stability for 100 h operated at 100 mA‧cm-2 in 1.0 M KOH electrolyte. The in-situ Raman measurements reveal the surface dynamic reconstruction. Besides, the results of density functional theory (DFT) calculations unveil the reaction mechanism. This study provides an effective design strategy via Mn incorporation to synergistically engineer electronic structures and oxygen vacancies of metal oxides for efficiently boosting the OER performance.

2D Hierarchical NiMoO4 Nanosheets/Activated Carbon Nanocomposites for High Performance Supercapacitors: The Effect of Nickel to Molybdenum Ratios.

Supercapacitors have the potential to be used in a variety of fields, including electric vehicles, and a lot of research is focused on unique electrode materials to enhance capacitance and stability. Herein, we prepared nickel molybdate/activated carbon (AC) nanocomposites using a facile impregnation method that preserved the carbon surface area. In order to study how the nickel-to-molybdenum ratio affects the efficiency of the electrode, different ratios between Ni-Mo were prepared and tested as supercapacitor electrodes, namely in the following ratios: 1:1, 1:2, 1:3, 1:4, and 1:5. X-ray diffraction, X-ray photoelectron spectroscopy, FESEM, HRTEM, and BET devices were extensively used to analyze the structure of the nanocomposites. The structure of the prepared nickel molybdates was discovered to be 2D hierarchical nanosheets, which functionalized the carbon surface. Among all of the electrodes, the best molar ratio between Ni-Mo was found to be 1:3 NiMo3/AC reaching (541 F·g-1) of specific capacitance at a current density of 1 A·g-1, and 67 W·h·Kg-1 of energy density at a power density of 487 W·Kg-1. Furthermore, after 4000 repetitive cycles at a large current density of 4 A·g-1, an amazing capacitance stability of 97.7% was maintained. This remarkable electrochemical activity for NiMo3/AC could be credited towards its 2D hierarchical structure, which has a huge surface area of 1703 m2·g-1, high pore volume of 0.925 cm3·g-1, and large particle size distribution.

Ruthenium and Nickel Molybdate-Decorated 2D Porous Graphitic Carbon Nitrides for Highly Sensitive Cardiac Troponin Biosensor.

Two-dimensional (2D) layered materials functionalized with monometallic or bimetallic dopants are excellent materials to fabricate clinically useful biosensors. Herein, we report the synthesis of ruthenium nanoparticles (RuNPs) and nickel molybdate nanorods (NiMoO4 NRs) functionalized porous graphitic carbon nitrides (PCN) for the fabrication of sensitive and selective biosensors for cardiac troponin I (cTn-I). A wet chemical synthesis route was designed to synthesize PCN-RuNPs and PCN-NiMoO4 NRs. Morphological, elemental, spectroscopic, and electrochemical investigations confirmed the successful formation of these materials. PCN-RuNPs and PCN-NiMoO4 NRs interfaces showed significantly enhanced electrochemically active surface areas, abundant sites for immobilizing bioreceptors, porosity, and excellent aptamer capturing capacity. Both PCN-RuNPs and PCN-NiMoO4 NRs materials were used to develop cTn-I sensitive biosensors, which showed a working range of 0.1-10,000 ng/mL and LODs of 70.0 pg/mL and 50.0 pg/mL, respectively. In addition, the biosensors were highly selective and practically applicable. The functionalized 2D PCN materials are thus potential candidates to develop biosensors for detecting acute myocardial infractions.

Phosphorus doping and phosphates coating for nickel molybdate/nickel molybdate hydrate enabling efficient overall water splitting.

Earth-abundant transition metal-based bifunctional electrocatalysts are promising alternatives to noble metals for overall water electrolysis, but restricted by low activity and durability. Herein, a three-dimensional phosphorus-doped nickel molybdate/nickel molybdate hydrate @phosphates core-shell nanorod clusters on nickel foam self-supported electrode was fabricated by a combined hydrothermal and phosphating process. The phosphorus doping and phosphate coating induced by phosphating process bring in a synergistic effect to improve the electrical conductivity, provide abundant active surface sites and accelerate the surface reaction for nickel molybdate/nickel molybdate hydrate (NiMoO4/NiMoO4·nH2O) heterostructures. These advantages enable the self-supported electrode to exhibit high hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity in 1.0 M KOH with low overpotentials of 148 and 260 mV at 10 mA cm-2, respectively. When it was employed both as anode and cathode, a cell voltage of 1.62 V is only required to reach the current density of 10 mA cm-2 in alkaline solution. Especially, the self-supported electrode reveals outstanding durability, which could maintain over 25 h at 10 mA cm-2 for HER, OER or overall water splitting. This work provides a novel avenue to enhance the electrocatalytic performance of the catalysts by synergistically modulating the intrinsic electrical conductivity, active surface sites and surface reaction.

Influence of Nickel molybdate nanocatalyst for enhancing biohydrogen production in microbial electrolysis cell utilizing sugar industrial effluent.

Biohydrogen production in Microbial Electrolysis Cell (MEC) had inspired the researchers to overcome the challenges associated towards sustainability. Despite microbial community and various substrates, economical cathode catalyst development is most significant factor for enhancing hydrogen production in the MEC. Hence, in this study, the performance of MEC was investigated with a sugar industry effluent (COD 4200 ± 20 mg/L) with graphite anode and modified Nickel foam (NF) cathode. Nickel molybdate (NiMoO4) coated NF achieved a higher hydrogen production rate 0.12 ± 0.01 L.L-1D-1 as compared to control under favorable conditions. Electrochemical characterizations demonstrated that the improved catalytic activity of novel nanocatalyst with lower impedance favoring faster hydrogen evolution kinetics. The MEC with the novel catalyst performed with 58.2% coloumbic efficiency, 20.36% cathodic hydrogen recovery, 11.96% overall hydrogen recovery and 54.38% COD removal efficiency for a 250 mL substrate during 5 days' batch cycle. Hence, the potentiality of modified cathode was established with the real time industrial effluent highlighting the waste to wealth bio-electrochemical technology.

From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy.

Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Zn Metal Atom Doping on the Surface Plane of One-Dimesional NiMoO4 Nanorods with Improved Redox Chemistry.

The effect of zinc (Zn) doping and defect formation on the surface of nickel molybdate (NiMoO4) structures with varying Zn content has been studied to produce one-dimensional electrodes and catalysts for electrochemical energy storage and ethanol oxidation, respectively. Zn-doped nickel molybdate (Ni1-xZnxMoO4, where x = 0.1, 0.2, 0.4, and 0.6) nanorods were synthesized by a simple wet chemical route. The optimal amount of Zn is found to be around 0.25 above which the NiMoO4 becomes unstable, resulting in poor electrochemical activity. This result agrees with our density functional theory calculations in which the thermodynamic stability reveals that Ni1-xZnxMoO4 crystallized in the ß-NiMoO4 phase and is found to be stable for x≤0.25. Analytical techniques show direct evidence of the presence of Zn in the NiMoO4 nanorods, which subtly alter the electrocatalytic activity. Compared with pristine NiMoO4, Zn-doped NiMoO4 with the optimized Zn content was tested as an electrode for an asymmetric supercapacitor and demonstrated an enhanced specific capacitance of 122 F g-1 with a high specific energy density of 43 W h kg-1 at a high power density of 384 W kg-1. Our calculations suggest that the good conductivity from Zn doping is attributed to the formation of excess oxygen vacancies and dopants play an important role in enhancing the charge transfer between the surface and OH- ions from the electrolyte. We report electrochemical testing, material characterization, and computational insights and demonstrate that the appropriate amount of Zn in NiMoO4 can improve the storage capacity (∼15%) due to oxygen vacancy interactions.

Carbon-Free, High-Capacity and Long Cycle Life 1D-2D NiMoO4 Nanowires/Metallic 1T MoS2 Composite Lithium-Ion Battery Anodes.

Both metallic 1T MoS2 and conductive molybdate compounds exhibit interesting electrochemical properties, however, the properties of composite electrodes based on these materials have not been investigated. Here, 1T MoS2 single crystal nanosheets and NiMoO4 single crystal nanowires are synthesized and formed into a carbon-free composite lithium-ion anode using blade- and spray-coating. The composite anodes deliver charge mass specific capacity of 940.1 mAh g-1, while the discharge mass specific capacity is up to 941.6 mAh g-1, with a capacity retention ratio of 84.2% after 750 cycles. The charge and discharge volumetric capacity (porosity of 15.6%, full electrode basis, excluding the current collector) are 1238.7 mAh cm-3 and 1240 mAh cm-3, respectively, and the active materials volume fraction is 82.5%. These capacities significantly exceed that of single 1T MoS2 or single NiMoO4 anodes we reported. We calculate if matched vs a cathode with an average discharge voltage of 4.0 V the gravimetric energy density of the composite electrodes would be 3389.8 Wh kg-1. Electrochemical measurements indicate that the composite electrode has excellent electrochemical reversibility, suggesting that the structure has played a crucial role in the cycling process.

Homogeneous Core/Shell NiMoO4@NiMoO4 and Activated Carbon for High Performance Asymmetric Supercapacitor.

Here, we report the extraordinary electrochemical energy storage capability of NiMoO4@NiMoO4 homogeneous hierarchical nanosheet-on-nanowire arrays (SOWAs), synthesized on nickel substrate by a two-stage hydrothermal process. Comparatively speaking, the SOWAs electrode displays superior electrochemical performances over the pure NiMoO4 nanowire arrays. Such improvements can be ascribed to the characteristic homogeneous hierarchical structure, which not only effectively increases the active surface areas for fast charge transfer, but also reduces the electrode resistance significantly by eliminating the potential barrier at the nanowire/nanosheet junction, an issue usually seen in other reported heterogeneous architectures. We further evaluate the performances of the SOWAs by constructing an asymmetric hybrid supercapacitor (ASC) with the SOWAs and activated carbon (AC). The optimized ASC shows excellent electrochemical performances with 47.2 Wh/kg in energy density of 1.38 kW/kg at 0-1.2 V. Moreover, the specific capacity retention can be as high as 91.4% after 4000 cycles, illustrating the remarkable cycling stability of the NiMoO4@NiMoO4//AC ASC device. Our results show that this unique NiMoO4@NiMoO4 SOWA has great prospects for future energy storage applications.

Freestanding hierarchical nickel molybdate@reduced graphene oxide@nickel aluminum layered double hydroxides nanoarrays assembled from well-aligned uniform nanosheets as binder-free electrode materials for high performance supercapacitors.

Herein, the hierarchical nickel molybdate@reduced graphene oxide@nickel aluminum layered double hydroxides (NiMoO4@rGO@NiAl-LDHs) architecture assembled from well-aligned nanosheets is successfully constructed on Ni-foam through a three-step strategy. For the first time, ultrathin graphene nanosheets are introduced by a novel spraying process to avoid the stack. NiAl-LDHs is prepared via situ growth procedure in which NiAl-LDH self-assembles on the surface of graphene to prevent graphene from aggregating, resulting in an enlarged specific surface area. Electrochemical analysis manifests that NiMoO4@rGO@NiAl-LDHs yields exceptional specific capacitance (3396 Fg-1 at 1 Ag-1), favorable charge/discharge rate and approving long-term stability. Furthermore, the NiMoO4@rGO@NiAl-LDHs//AC device delivers superior specific capacitance (137.2 Fg-1 at 1 Ag-1), high energy density (48.7 Whkg-1) associated with pleasurable power output (7987 Wkg-1). Importantly, the well-aligned NiMoO4@rGO@NiAl-LDHs provides a prospective conception constructing hierarchical structural materials in the area of supercapacitor.

Urea-Assisted Room Temperature Stabilized Metastable ß-NiMoO4: Experimental and Theoretical Insights into its Unique Bifunctional Activity toward Oxygen Evolution and Supercapacitor.

Room-temperature stabilization of metastable ß-NiMoO4 is achieved through urea-assisted hydrothermal synthesis technique. Structural and morphological studies provided significant insights for the metastable phase. Furthermore, detailed electrochemical investigations showcased its activity toward energy storage and conversion, yielding intriguing results. Comparison with the stable polymorph, α-NiMoO4, has also been borne out to support the enhanced electrochemical activities of the as-obtained ß-NiMoO4. A specific capacitance of ∼4188 F g-1 (at a current density of 5 A g-1) has been observed showing its exceptional faradic capacitance. We qualitatively and extensively demonstrate through the analysis of density of states (DOS) obtained from first-principles calculations that, enhanced DOS near top of the valence band and empty 4d orbital of Mo near Fermi level make ß-NiMoO4 better energy storage and conversion material compared to α-NiMoO4. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm-2 is achieved at overpotential of 300 mV, which is much lower than that of IrO2/C. First-principles calculations also confirm a lower overpotential of 350 mV for ß-NiMoO4.

Hydrothermal synthesis and electrochemical performances of 1.7 V NiMoO4⋠xH2O||FeMoO4 aqueous hybrid supercapacitor.

One-dimensional (1D) NiMoO4⋅xH2O nanorods and ß-FeMoO4 microrods are successfully synthesized by simple hydrothermal method without using any organic solvents. X-ray diffraction (XRD) patterns reveal the single phase formation of nickel molybdate (NiMoO4⋅xH2O) and pure monoclinic phase of ß-FeMoO4. The growth of one dimensional morphology of both the molybdates are identified from scanning and transmission electron microscopes (SEM and TEM). The cyclic voltammogram envisage the pseudocapacitance behavior of NiMoO4⋅xH2O and ß-FeMoO4 through the reversible redox reactions of Ni(3+)/Ni(2+) and Fe(3+)/Fe(2+) ions. An asymmetric supercapacitor is fabricated using NiMoO4⋅xH2O nanorods and ß-FeMoO4 as a positive and negative electrode, respectively. The ß-FeMoO4||NiMoO4⋅xH2O asymmetric supercapacitor delivers a capacitance of 81 F g(-1) at a current density of 1 mA cm(-2). The cell exhibits a high energy density of 29 W h kg(-1) and good cycling stability even after 1000 cycles.

Phosphorus and sulfur co-doped nickel molybdate with rich-oxygen vacancies for efficient water splitting.

The rational design of high-performance electrocatalysts is essential for promoting the industrialization of electrocatalytic water-splitting technology. Herein, phosphorus and sulfur co-doped nickel molybdate with rich-oxygen vacancies (P, S-NiMoO4) was prepared as an efficient bifunctional self-supporting water-splitting catalyst from the perspective of enhancing the conductivity and optimizing the electronic configurations. The incorporation of P, S and oxygen vacancies greatly enhances the conductivity and charge-transfer efficiency of NiMoO4. Additionally, P and S can serve as proton carriers and electron acceptors to enhance the catalytic activity by accelerating proton activation and high-valent metal generation in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As expected, P, S-NiMoO4 demonstrates efficient bifunctional catalytic activity with an overpotential of only 31/206 mV at 10 mA cm-2 for HER/OER in 1 M KOH. Meantime, the electrolyzer assembled with P, S-NiMoO4 as electrodes requires a voltage of only 1.55 V to achieve a water-splitting current density of 50 mA cm-2 along with good stability over 110 h. This work puts forward a novel approach based on elemental doping and vacancy engineering for the design of effective and enduring catalysts for water splitting.