Nanostructured NiMoO4 as active electrocatalyst for oxygen evolution

Enhanced Oxygen Evolution Reaction Performance of NiMoO4/Carbon Paper Electrocatalysts in Anion Exchange Membrane Water Electrolysis by Atmospheric-Pressure Plasma Jet Treatment

NiMoO4 was grown on carbon paper (CP) by a hydrothermal method. A rapid and high-temperature atmospheric-pressure plasma jet (APPJ) process was used to generate more oxygen-deficient NiMoO4 on the CP surface to serve as an electrode material for the oxygen evolution reaction (OER). After 60 s of APPJ treatment, the overpotential of the electrode at 100 mA/cm2 decreased to 790 mV and that at 10 mA/cm2 decreased to 368 mV. Additionally, the charge transfer resistance decreased from 2.8 to 1.2 Ω, indicating that APPJ treatment effectively reduced the electrode overpotential and impedance. The effect of NiMoO4/CP/APPJ-60 s on the anion exchange membrane water electrolysis (AEMWE) system was also tested. At a system temperature of 70 °C and current density of 100 mA/cm2, the energy efficiency reached 95.1%, and the specific energy consumption decreased from 4.02 to 3.83 kWh/m3. These results demonstrate that the APPJ-treated NiMoO4/CP electrode can effectively enhance the OER performance in water electrolysis and improve the energy efficiency of the AEMWE system. This approach shows promise in replacing precious metal electrodes, thereby potentially reducing the cost and providing an environmentally friendly alternative.

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.

FeCoNi molybdenum-based oxides for efficient electrocatalytic oxygen evolution reaction

The search for highly efficient and inexpensive electrocatalysts is crucial to the advancement of environmentally friendly and sustainable energy sources. Here, adopting a one-step hydrothermal method, we have effectively fabricated a self-supported multi-metal molybdenum-based oxide (FeCoNi-MoO4) on nickel foam (NF). In addition to changing the catalyst's microstructure, the introducing of Fe and Co, enhanced its active center count, improved its electronic structure, and in turn reduced the difficulty for high-valence Ni and Fe species to form, which accelerates the oxygen evolution reaction (OER) kinetics by promoting the development of the actual active materials, NiOOH and FeOOH. FeCoNi-MoO4 has outstanding OER performance, requiring just 204 mV overpotentials at 10 mA cm-2 and 271 mV at 100 mA cm-2. Its exceptional OER kinetics at both low and high currents are indicated by a Tafel slope of 50.6 mV dec-1, which is attributed to the combined effect of its multi-metal composition and a higher number of active sites. Moreover, the FeCoNi-MoO4 electrode was operated continuously for over 48 h. Furthermore, the density functional theory (DFT) results demonstrated that the introducing of Fe and Co, which quickens the rate of electron transfer during the electrocatalytic process, improves the ability of oxygen intermediate species to adsorb, and ultimately lowers the overpotential, is responsible for the increased electrocatalytic activity of FeCoNi-MoO4. This work offers hope for further developments in the sector by proposing an efficient approach for creating multi-active electrocatalysts that are stable, economical, and efficient.

A stable N-doped NiMoO4/NiO2 electrocatalyst for efficient oxygen evolution reaction

Recently, there has been a significant interest in the study of highly active and stable transition metal-based electrocatalysts for the oxygen evolution reaction (OER). Non-noble metal nanocatalysts with excellent inherent activity, many exposed active centers, rapid electron transfer, and excellent structural stability are especially promising for the displacement of precious-metal catalysts for the production of sustainable and "clean" hydrogen gas through water-splitting. Herein, efficient electrocatalyst N-doped nickel molybdate nanorods were synthesized on Ni foam by a hydrothermal process and effortless chemical vapor deposition. The heterostructure interface of N-NiMoO4/NiO2 led to strong electronic interactions, which were beneficial for enhancing the OER activity of the catalyst. Excellent OER catalytic activity in 1.0 M KOH was shown, which offered a small overpotential of 185.6 mV to acquire a current density of 10 mA cm-2 (superior to the commercial benchmark material RuO2 under the same condition). This excellent electrocatalyst was stable for 90 h at a constant current density of 10 mA cm-2. We created an extremely reliable and effective OER electrocatalyst without the use of noble metals by doping a nonmetal element with nanostructured heterojunctions of various active components.

Ru Cluster Incorporated NiMoO(P)4 Nanosheet Arrays as High-Efficient Bifunctional Catalyst for Wind/Solar-To-Hydrogen Generation Systems

Developing cost-efficient bifunctional water splitting catalysts is crucial for sustainable hydrogen energy applications. Herein, ruthenium (Ru)-incorporated and phosphorus (P)-doped nickel molybdate (Ru-NiMoO(P)4 ) nanosheet array catalysts are synthesized. Due to the synergy of Ru clusters and NiMoO(P)4 by the modulated electronic structure and the rich active sites, impressively, Ru-NiMoO(P)4 exhibits superior OER (194 mV @ 50 mA cm-2 ) and HER (24 mV @ 10 mA cm-2 ) activity in alkaline media, far exceeding that of commercial Pt/C and RuO2 catalysts. Meanwhile, as bifunctional catalyst, to drive the overall water splitting at the current density of 10 mA cm-2 , Ru-NiMoO(P)4 requires only 1.45 V and maintaining stable output for 100 h. Furthermore, Ru-NiMoO(P)4 also possesses excellent capability for seawater electrolysis hydrogen production. Moreover, the successful demonstration of wind and solar hydrogen production systems provide the feasibility of the ultra-low Ru loading catalyst for large-scale hydrogen production in the future.

Intrinsic Lability of NiMoO4 to Excel the Oxygen Evolution Reaction

Nickel-based bimetallic oxides such as NiMoO₄ and NiWO₄, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO₄ nanorods and NiWO₄ nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO₄/NF and NiWO₄/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH)_ED (ED: electrochemically derived) from NiMoO₄ precatalyst, while NiWO₄ remains stable. A controlled study, stirring of NiMoO₄/NF in 1 M KOH without applied potential, confirms that NiMoO₄ hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH)_ED. Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO₄ compared to the Ni-O-W bond in NiWO₄ causes the transformation of NiMoO₄ into NiO(OH)_ED. A comparison of the OER performance of electrochemically derived NiO(OH)_ED, NiWO₄, ex-situ-prepared Ni(OH)₂, and NiO(OH) confirmed that in-situ-prepared NiO(OH)_ED remained superior with a substantial potential of 238 (±6) mV at 20 mA cm^-2. The notable electrochemical performance of NiO(OH)_ED can be attributed to its low Tafel slope value (26 mV dec^-1), high double-layer capacitance (C_dl, 1.21 mF cm^-2), and a low charge-transfer resistance (R_ct, 1.76 Ω). The NiO(OH)_ED/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm^-2. Post-characterization of the anode proves the structural integrity of NiO(OH)_ED even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)_ED/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm^-2. Similar to that on NF, NiMoO₄ deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO₄ to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH)_ED succeeding the NiO phase is intrinsic to its superior activity.

Solution combustion synthesis of Ni-based hybrid metal oxides for oxygen evolution reaction in alkaline medium

Oxygen evolution reaction (OER) has arisen as an outstanding technology for energy generation, conversion, and storage. Herein, we investigated the synthesis of nickel-based hybrid metal oxides (Ni x M1-x O y ) and their catalytic performance towards OER. Ni x M1-x O y catalysts were synthesized by solution combustion synthesis (SCS) using the metal nitrates as oxidizer and glycine as fuel. Scanning electron microscope (SEM) micrographs display a porous morphology for the hybrid binary Ni x M1-x O y , the common feature of combusted materials. X-ray diffraction (XRD) of Ni x M1-x O y depicted well-defined diffraction peaks, which confirms the crystalline nature of synthesized catalysts. The particle size of as-synthesized materials ranges between 20 and 30 nm with a mesoporous nature as revealed by N2-physisorption. The electrocatalytic performance of the as-prepared materials was evaluated towards OER in alkaline medium. Among them, Ni x Co1-x O y showed the best catalytic performance. For instance, it exhibited the lowest overpotential at a current density of 10 mA cm-2 (404 mV), onset potential (1.605 V), and Tafel slope (52.7 mV dec-1). The enhanced electrocatalytic performance of Ni x Co1-x O y was attributed to the synergism between cobalt and nickel and the alteration of the electronic structure of nickel. Also, Ni x Co1-x O y afforded the highest Ni3+/Ni2+ when compared to other electrocatalysts. This leads to higher oxidation states of Ni species, which promote and improve the electrocatalytic activity.

Crystal-amorphous NiO/MoO2 with high-density interface for hydrogen evolution

Distinctive interfacial catalysis play a vital role for achieving ultrahigh activity for hydrogen evolution reaction (HER) under alkaline media. In fact, the optimized regulation of electronic structure of heterostructures interface...

Triple captured iron by defect abundant NiO for efficient water oxidation

Fe-doped NiO host in well-defined nanorod assembly (Fe-NiMoO4@NiO-30) with large surface area is designed to achieve the triple capture of Fe via adjustable surface reconstruction and impregnation to optimize OER...

Aerosol-Assisted Chemical Vapor Deposition Growth of NiMoO4 Nanoflowers on Nickel Foam as Effective Electrocatalysts toward Water Oxidation

The fabrication of active and durable catalysts derived from transition metals is highly desired for the realization of efficient water oxidation reactions. This is particularly important to address the slow oxygen evolution reaction (OER) kinetics and hence can contribute to the conversion and storage of sustainable energy. In this study, the deposition of crystalline flowerlike 2D nanosheets of nickel molybdate (NiMoO4) directly on nickel foam (NF) through an aerosol-assisted chemical vapor deposition process is reported. The NiMoO4 nanosheets were developed on NF by altering the deposition time for 60 and 120 min at a fixed temperature of 480 °C. The structural determination by XRD and XPS analyses revealed a highly crystalline single phase NiMoO4. The micrographs of NiMoO4 show that the surface consisted of vertically aligned 2D nanosheets assembled into flowerlike structures. The nanosheets produced after 60 min deposition time on a network of NF is found to perform better for OER as compared to the one developed for 120 min. A reference current density of 10 mA cm-2 was achieved at an overpotential (η) of 320 mV, which was better as compared to that reported for the benchmark OER catalyst in 1.0 M KOH. Moreover, a small Tafel value (75 mV dec-1) and good OER stability for >15 h were also observed.

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.

Boosting Electrochemical Performance of Hematite Nanorods via Quenching-Induced Alkaline Earth Metal Ion Doping

Ion doping in transition metal oxides is always considered to be one of the most effective methods to obtain high-performance electrochemical supercapacitors because of the introduction of defective surfaces as well as the enhancement of electrical conductivity. Inspired by the smelting process, an ancient method, quenching is introduced for doping metal ions into transition metal oxides with intriguing physicochemical properties. Herein, as a proof of concept, α-Fe2O3 nanorods grown on carbon cloths (α-Fe2O3@CC) heated at 400 °C are rapidly put into different aqueous solutions of alkaline earth metal salts at 4 °C to obtain electrodes doped with different alkaline earth metal ions (M-Fe2O3@CC). Among them, Sr-Fe2O3@CC shows the best electrochemical capacitance, reaching 77.81 mF cm−2 at the current of 0.5 mA cm−2, which is 2.5 times that of α-Fe2O3@CC. The results demonstrate that quenching is a feasible new idea for improving the electrochemical performances of nanostructured materials.

Towards the Hydrogen Economy—A Review of the Parameters That Influence the Efficiency of Alkaline Water Electrolyzers

Environmental issues make the quest for better and cleaner energy sources a priority. Worldwide, researchers and companies are continuously working on this matter, taking one of two approaches: either finding new energy sources or improving the efficiency of existing ones. Hydrogen is a well-known energy carrier due to its high energy content, but a somewhat elusive one for being a gas with low molecular weight. This review examines the current electrolysis processes for obtaining hydrogen, with an emphasis on alkaline water electrolysis. This process is far from being new, but research shows that there is still plenty of room for improvement. The efficiency of an electrolyzer mainly relates to the overpotential and resistances in the cell. This work shows that the path to better electrolyzer efficiency is through the optimization of the cell components and operating conditions. Following a brief introduction to the thermodynamics and kinetics of water electrolysis, the most recent developments on several parameters (e.g., electrocatalysts, electrolyte composition, separator, interelectrode distance) are highlighted.

Electrodeposition of Pt-Decorated Ni(OH)2/CeO2 Hybrid as Superior Bifunctional Electrocatalyst for Water Splitting

The facile synthesis of highly active and stable bifunctional electrocatalysts to catalyze water splitting is attractive but challenging. Herein, we report the electrodeposition of Pt-decorated Ni(OH)₂/CeO₂ (PNC) hybrid as an efficient and robust bifunctional electrocatalyst. The graphite-supported PNC catalyst delivers superior hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities over the benchmark Pt/C and RuO₂, respectively. For overall water electrolysis, the PNC hybrid only requires a cell voltage of 1.45 V at 10 mA cm⁻² and sustains over 85 h at 1000 mA cm⁻². The remarkable HER/OER performances are attributed to the superhydrophilicity and multiple effects of PNC, in which Ni(OH)₂ and CeO₂ accelerate HER on Pt due to promoted water dissociation and strong electronic interaction, while the electron-pulling Ce cations facilitate the generation of high-valence Ni OER-active species. These results suggest the promising application of PNC for H₂ production from water electrolysis.

Defect Engineering of Molybdenum-Based Materials for Electrocatalysis

Molybdenum-based electrocatalysts have been widely applied in electrochemical energy conversion reactions. The essential roles of defects, including doping, vacancies, grain boundaries, and dislocations in improving various electrocatalytic performances have been reported. This review describes the latest development of defect engineering in molybdenum-based materials for hydrogen evolution, oxygen reduction, oxygen evolution, and nitrogen reduction reactions. The types of defects, preparation methods, characterization techniques, and applications of molybdenum-based defect materials are elucidated. Finally, challenges and future research directions for these types of materials are also discussed.

Efficient bifunctional catalysts for overall water splitting: porous Fe-Mo oxide hybrid nanorods

Efficient and inexpensive bifunctional catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential for water splitting. Herein, we successfully prepare porous Fe-Mo oxide hybrid nanorods through a hydrothermal method followed by annealing at high temperature. They exhibit excellent catalytic activity for OER and HER in alkaline media, and produce a current density of 10 mA cm-2 at overpotentials of 200 and 66 mV. Besides, they work as bifunctional electrode materials for overall water splitting, achieving a current density of 10 mA cm-2 at a voltage of 1.52 V, and maintaining a current density of 60 mA cm-2 for 60 h. The unique morphology with self-supported structure can expose more active sites and facilitate charge transfer, and is not easy to peel off, thus it improves the catalytic activity and stability. This work therefore provides a valuable route for designing and fabricating inexpensive and high-performance catalytic materials for overall water splitting.