One-pot sonochemical synthesis of hierarchical MnWO4 microflowers as effective electrodes in neutral electrolyte for high performance asymmetric supercapacitors

In this article, manganese tungstate (MnWO₄) microflowers as electrode materials for high performance supercapacitor applications are prepared by a one-pot sonochemical synthesis. The crystalline structure and morphology of MnWO₄ microflowers are characterized through X-ray diffraction, field emission scanning electron microscopy. The electrochemical properties of the MnWO₄ microflowers are investigated using cyclic voltammograms, galvanostatic charge/discharge and electrochemical impedance spectroscopy. The MnWO₄ microflowers as electrode materials possess a maximum specific capacitance of 324 F g⁻¹ at 1 mA cm⁻² in the potential window from 0 to +1 V and an excellent cycling stability of 93% after 8000 cycles at a current density of 3 mA cm⁻². An asymmetric supercapacitor device is fabricated using the MnWO₄ and iron oxide (Fe₃O₄)/multi-wall carbon nanotube as the positive and negative electrode materials, it can be cycled reversibly at a potential window at 1.8 V. The fabricated ASC device can deliver a high energy density of 34 Wh kg⁻¹ at a power density of 500 W kg⁻¹ with cycling stability of 84% capacitance retained after 3000 cycles. The above results demonstrate that MnWO₄ microflowers can be used as promising high capacity electrode materials in neutral electrolyte for high performance supercapacitors.     

Open and porous NiS2 nanowrinkles grown on non-stoichiometric MoOx nanorods for high-performance alkaline water electrolysis and supercapacitor

Hierarchical hybrid heterostructures are regarded to be promising materials for highly efficient bifunctional electrocatalysts and high-performance supercapacitors due to their intriguing morphological features and remarkable electrochemical properties. Herein, we demonstrate the rational construct of cost-effective MoO_x@NiS₂ hybrid nanostructures as bifunctional electrocatalysts and the electrode material of supercapacitor. Microstructural analysis shows that the hybrid is a kind of hierarchical heterostructure composed of open and porous NiS₂ nanowrinkles in situ grown on non-stoichiometric MoO_x nanorods, which greatly improves the conductivity, and effectively maximized the electrochemical surface area. As expected, the MoO_x@NiS₂ hybrid show remarkable electrocatalytic performance in alkaline media, such as overpotentials of 101 mV at 10 mA cm^?2 for hydrogen evolution reaction (HER) and 278 mV at 20 mA cm^?2 for oxygen evolution reaction (OER), and a low cell voltage of 1.62 V to deliver a current density of 10 mA cm^?2. Moreover, the hybrid nanostructures present a high specific capacitance 1050 A/g at 1 A/g with ultra-long stability in 6 M KOH. The strategy proposed here introduces a new perspective about the development of efficient earth-abundant bifunctional elecrocatalysts and electrode materials for superior energy conversion and storage devices.     

Physical insights into alkaline overall water splitting with NiO microflowers electrodes with ultra-low amount of Pt catalyst

Electrochemical water splitting represents a promising alternative to conventional carbon-based energy sources. The hydrogen evolution reaction (HER) is a key process, still if conducted in alkaline media, its kinetics is slow thus requiring high amount of Pt based catalysts. Extensive research has been focused on reducing Pt utilization by pursuing careful electrode investigation. Here, a low-cost chemical methodology is reported to obtain large amount of microflowers made of interconnected NiO nanowalls (20 nm thick) wisely decorated with ultralow amounts of Pt nanoparticles. These decorated microflowers, dispersed onto graphene paper by drop casting, build a high performance HER electrode exhibiting an overpotential of only 66 mV at current density of 10 mA cm^?2 under alkaline conditions. Intrinsic activity of catalyst was evaluated by measuring the Tafel plot (as low as 82 mV/dec) and turnover frequencies (2.07 s^?1 for a Pt loading of 11.2 μg cm^?2). The effect of Pt decoration has been modelled through energy band bending supported by electrochemical analyses. A full cell for alkaline electrochemical water splitting has been built, composed of Pt decorated NiO microflowers as cathode and bare NiO microflowers as anode, showing a low potential of 1.57 V to afford a current density of 10 mA cm^?2 and a good long-term stability. The reported results pave the way towards an extensive utilization of Ni based nanostructures with ultralow Pt content for efficient electrochemical water splitting.     

Ru/MoO2 decorated on CNT networks as an efficient electrocatalyst for boosting hydrogen evolution reaction

Generally, electrochemical hydrogen evolution reaction (HER) is hampered by slow kinetics and low round-trip efficiency. Electrocatalysts with a hierarchical structure and large surface area are expected to overcome these problems. Herein, we prepared a Ru/MoO₂/carbon nanotubes (RMC) hybrid with a hierarchical structure by a convenient solid-phase reaction (SPR) method, and studied the electrochemical activity for HER. After annealing as-prepared RuO₂/MoO₂/carbon nanotubes (ROMC) precursor in a tubular furnace under Ar atmosphere, RuO₂ and MoS₂ were in-situ transformed into Ru metal and MoO₂ phase by the redox SPR. Through various tests, we have confirmed that the new formed Ru metal and MoO₂ phase are combined and uniformly coated on the outer surface of CNTs. Interestingly, the RMC-500 exhibits the best HER performance with a low overpotential of 16 mV at l0 mA cm^?2, small Tafel slope of 45 mV dec^?1, higher electrochemical active surface area, and long-time durability in alkaline electrolyte.     

Solvothermal synthesis of hierarchical LiFePO4 microflowers as cathode materials for lithium ion batteries

Hierarchical LiFePO₄ microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as a precursor, which is obtained by a simple water evaporation method beforehand. The hierarchical LiFePO₄ microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of 6-8 μm, width of 1-2 μm and thickness of around 50 nm. When ethanol is replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO₄ micro-octahedrons instead of hierarchical microflowers can be prepared. Then both of them are respectively modified with carbon coating through a post-heat treatment and their morphologies are retained. As a cathode material for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO₄ microflowers deliver high initial discharge capacity (162 mAh g⁻¹ at 0.1 C), excellent high-rate discharge capability (101 mAh g⁻¹ at 10 C), and cycling stability, which exhibits better electrochemical performances than carbon-coated LiFePO₄ micro-octahedrons. These enhanced electrochemical properties can be attributed to the hierarchical micro/nanostructures, which can take advantage of structure stability of micromaterials for long-term cycling. Furthermore the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.     

Ethylene glycol assisted solvo-hydrothermal synthesis of NGr-Co3O4 nanostructures for ethanol electrooxidation

Different morphologies of Co₃O₄ nanostructures grown on nanographitic flakes have been synthesized based on the decomposition of cobalt acetate precursor in water, ethylene glycol and water (50%)-ethylene (50%) solution at 180 °C for 18 h via a solvo-hydrothermal process. A subsequent electrophoretic deposition process is used to form composite layers on stainless steel plates. The composite layer fabricated in the water (50%)-ethylene (50%) solution is found to have the best electrocatalytic performance for ethanol electrooxidation due to the hierarchical nanoporous microstructures of Co₃O₄ exhibiting the lowest onset voltage for ethanol electrooxidation (around −0.3 V vs. Ag/AgCl) among the fabricated electrocatalyst layers. Moreover, the ethanol electrooxidation current for the electrode fabricated in the water (50%)-ethylene glycol (50%) reaches roughly 6.6 mA/cm², while it is about 3.3 mA/cm² at 0.5 V (vs. Ag/AgCl) for the electrode fabricated in pure ethylene glycol (values obtained after 1-h stability tests). The electrode fabricated in pure water is confirmed to not exhibit significant ethanol electrooxidation.     

Ni doped Bi2WO6 for electrochemical OER activity

Due to over depletion of fossil fuels, researchers started to find hydrogen energy to compete with the energy demands. Bi₂WO₆ and Ni (5% and 10%) doped Bi₂WO₆ were prepared via hydrothermal route. Structural confirmation of undoped and doped Bi₂WO₆ nanostructures was estimated by using standard characterization studies. The nanoflake and nanoneedle like morphology of undoped and Ni doped Bi₂WO₆ was confirmed in nanoscale range. The highest OER activity was achieved for 10% Ni doped Bi₂WO₆ nanostructure electrode with the excellent current density of 272 mA/g with overpotential of 242 mV in the fabricated three electrode half cell set up. The higher electron transport offered by Ni ions to Bi₂WO₆ host has been reported with the electrochemical mechanism. Hence, the unusual robust electrodes for electrochemical potential applications by tuning its property via suitable foreign ion dopant could be the great beginning of this recent year research. In such a way, this work would be the better way of swapping of nobel metal catalysts for electrochemical OER activity.     

A pH-differential dual-electrolyte microfluidic electrochemical cells for CO2 utilization

CO₂ can be converted to useful fuels by electrochemical processes. As an effective strategy to address greenhouse effect and energy storage shortage, electrochemical reduction of CO₂ still needs major improvements on its efficiency and reactivity. Microfluidics provides the possibility to enhance the electrochemical performance, but few studies have focused on the virtual interface. This work demonstrates a dual electrolyte microfluidic reactor (DEMR) that improves the thermodynamic property and raises the electrochemical performance based on a laminar flow membrane-less architecture. Freed from hindrances of a membrane structure and thermodynamic limitations, DEMR could bring in 6 times higher reactivity and draws electrode potentials closer to the equilibrium status (corresponded to less electrode overpotentials). The cathode potential was reduced from −2.1 V to −0.82 V and the anode potential dropped from 1.7 V to 1 V. During the conversion of CO₂, the peak Faradaic and energetic efficiencies were recorded as high as 95.6% at 143 mA/cm² and 48.5% at 62 mA/cm², respectively, and hence, facilitating future potential for larger-scale applications.     

Template assisted hydrothermal synthesis of CoSnO3 hollow microspheres for electrocatalytic oxygen evolution reaction

The practical complications suffered by the most recognized electrochemical energy systems, such as, water-electrolyzers and metal-air batteries reside in the half-cell oxygen evolution reaction. To resolve this problem, continuous colossal efforts are required to develop the active, affordable and sustainable electrocatalysts. Shape-tailoring of the catalysts, constructed from non-noble metals is one of the emerging strategies to augment the activity of the material toward electrochemical reactions. In the present work, we demonstrate the template-assisted hydrothermal synthesis of hierarchical CoSnO₃ hollow microspheres, constructible from the wafer-thin sheets of CoSnO₃. The hierarchical CoSnO₃ hollow microspheres possess a high specific surface area of 153.59 m²/g, and mesoporous configuration, which are the essential pre-requisites of an electrochemical system. In addition to this, the proposed CoSnO₃ hollow microspheres possess adequate electroactive surface area (793.5 cm²) and happens to be a suitable candidate for driving the oxygen evolution reaction with a low overpotential of 282 mV and Tafel slope of 96.5 mV/dec in alkaline medium. The higher turnover frequency (0.0045 s⁻¹), high specific and mass activities (2.195 mA/cm²_EASA and 28.752 mA/mg, respectively) were observed for CoSnO₃ hollow spheres. Furthermore, the chronoamperometric measurement reveals a good stability of CoSnO₃ hollow microspheres in alkaline condition, satisfying the fundamental demand of an energy system.     

Electrodeposited laser – nanostructured electrodes for increased hydrogen production

In the present work, a novel approach has been employed to effectively enlarge the electrocatalytic area of the electrodes in an alkaline electrolysis setup. This approach consists of a two-step electrode fabrication process: In the first step, ultrashort laser pulses have been used to nanostructure the electrode surface. In the second step, electrodeposition of nickel particles was performed in a modified Watt's bath. The resulting electrodes have been found to exhibit a significantly increased hydrogen evolution reaction (HER) activity. Compared to the laser-nanostructured electrode (LN) and an untreated (i.e., flat) electrode, the electrodeposited-laser-nanostructured (ELN) electrode provides (i) enhanced electrochemical values (ii) a significant increase of double-layer capacitance (C_DL) (values up to 1945 μF cm^?2) compared to that of an LN electrode (288 μF cm^?2) (iii) higher J_peaks at CVs sweeps and (iv) lower Tafel slopes (?121 mV dec^?1 compared to ?157 mv dec^?1). The ELN electrode provides an overpotential value of |η|₁₀₀ = 264 mV, which shows a noteworthy 34% decrease compared to a flat Ni electrode and a 15% decrease to an (LN) electrode. Scanning electron microscopy (SEM) revealed that the electrodeposition of nickel on the LN nickel electrodes results in a dendrite-like morphology of the surface. Thus, the enhancement of the HER has been attributed to the dendrite-like geometry and the concomitant enlargement of the electrocatalytic area of the electrode, which presents an electrochemical active surface area (ECSA) = 97 cm^?2 compared to 2.8 cm^?2 of the flat electrode. The electrodes have also been tested in actual hydrogen production condition, and it was found that the ELN electrode produces 4.5 times more hydrogen gas than a flat Ni electrode and 20% more hydrogen gas than an LN electrode (i.e. without the extra nickel electrodeposition).     

Manipulating the charge separation via piezoelectric field and heterojunction to enhance the photoelectrochemical water splitting ability of Bi2WO6/BiOBr photoanode

The unsatisfactory separation efficiency of photogenerated charge is one of the significant problems restricting the application of photoelectrochemical water splitting. Herein, we successfully prepared a Bi₂WO₆/BiOBr nanoplate arrays structure with the heterostructure for efficient piezoelectric photoelectric water splitting. A charge transfer path is formed by constructing a heterojunction to accelerate the spatial separation of photogenerated charges. The Bi₂WO₆/BiOBr shows the better photoelectrochemical performance with high photocurrent density of 0.068 mA/cm² at 1.23 V vs. RHE which is 1.8 times higher than simple Bi₂WO₆. In addition, after the introduction of piezoelectric polarization, the carrier separation efficiency is further enhanced under the synergistic action of the piezoelectric polarization and the heterojunction, and the photocurrent of Bi₂WO₆/BiOBr photoanode is increased to 0.088 mA/cm² at 1.23 V vs. RHE. By comparing the photoelectrocatalytic performance of the samples before and after the introduction of piezoelectric field, it further explains the role of piezoelectric built-in electric field in promoting carrier separation. This work provides a new method to improve the carrier separation efficiency by combining heterojunction and piezoelectric polarization.     

Unprecedented solar water splitting of dendritic nanostructured Bi2O3 films by combined oxygen vacancy formation and Na2MoO4 doping

We demonstrate the synergetic effect of Na₂MoO₄-doping and vacuum-annealing on dendritic nanostructured bismuth oxide (Bi₂O₃) thin films prepared by electrodeposition for visible-light-assisted photoelectrochemical (PEC) water oxidation. After evaluating various extents of Na₂MoO₄-doping as well as vacuum-annealing temperatures, it was evidenced that both Na₂MoO₄-doping and vacuum-annealing significantly improved the efficiency and PEC water oxidation performance. Compared to the undoped Bi₂O₃ photoanode, the optimized Na₂MoO₄-doped Bi₂O₃, after vacuum-annealing, resulted in more than 25-fold enhancement in the photoanodic current density to 1.06 mA/cm² at 1.23 V_RHE under AM1.5 G illumination. The PEC enhancement is credited mainly to the increased PEC surface active sites in the Na₂MoO₄-doped vacuum annealed sample. Confirmed by combined XPS and Mott-Schottky (M ? S) analysis, vacuum annealing resulted in surface oxygen vacancies that can contribute to the photocatalytic activity. Besides, Na₂MoO₄-doping resulted in reduced dimensions of the dendritic structure, revealed by FE-SEM and XRD measurements, resulting in larger surface area and, therefore, larger surface/electrolyte contact. This dual strategy (metal doping + vacuum annealing) can be generalized to assemble photoanodes of other materials used for the production of solar fuels. Our results make a valuable step towards efficient Bi₂O₃/BiVO₄ pn heterojunctions.     

Hybrid ternary rice paper/polypyrrole ink/pen ink nanocomposites as components of flexible supercapacitors

The rapid development of the portable and wearable devices has inspired the ever-growing pursuit of flexible energy storage equipment which can power these devices. Here, rice paper (RP) was integrated with a homemade LGS-polypyrrole (LGS-PPy) ink and commercial pen ink in order to construct a flexible electrode of a supercapacitor via a facial dip-coating method. The obtained RP/LGS-PPy ink/Pen ink composites showed a high areal specific capacitance of 1568 mF/cm² at 0.2 mA/cm², owing to the uniform deposition of a LGS-PPy layer which was covered by the pen ink coating. Furthermore, the assembled symmetric supercapacitor fabricated from the RP/LGS-PPy ink/Pen ink electrode exhibited impressive electrochemical performances in terms of specific capacitance (317.5 mF/cm² at 2 mA/cm²), power density (846 μW/cm² at the energy density of 23.5 μWh/cm²), and life times (82.1% capacitance retention after 5000 cycles). In addition, the capacitance of the as-prepared device remained essentially unchanged even after bending at 180°, thus demonstrating this device's outstanding flexibility. The low cost of raw materials, robust fabrication strategy as well as the moderate performances make the RP-based supercapacitor a promising candidate for future flexible energy storage devices.     

Cobalt decorated graphitic carbon nitride photoanode for electrochemical ethanol oxidation: A sustainable way towards clean energy

Energy conversion devices based on liquid fuels have gained considerable attention in recent times to meet the increasing global demand for energy. In this work, graphitic carbon nitride (gCN) nanosheets have been synthesized by pyrolysis of urea and Co has been decorated in different molar ratios over its surface by the solution phase method. The prepared catalysts have been utilized for photo electrooxidation of ethanol, an anodic half-cell reaction in direct ethanol fuel cells. Electrochemical studies show that the catalyst containing 3 mol % of Co shows the best activity with a peak current density of 6.91 mA/cm² obtained at a peak potential of 0.28 V with maximum current density of 40 mA/cm². The effect of light on the catalytic activity has also been studied. On illuminating the surface of the electrode with light, an increment of 85% in current density is observed which indicates higher charge transfer that enhanced the photoactivity of the catalyst. This study confirms the practical applicability of the non-expensive carbonaceous material Co–C₃N₄ utilization as a photoanode in future energy systems.     

Hierarchical mesoporous SnO2/BiVO4 photoanode decorated with Ag nanorods for efficient photoelectrochemical water splitting

Bismuth vanadate has been extensively investigated as a potential visible light photoanode for PEC water splitting. The performance of BiVO₄ is restricted by fast charge recombination and slow oxygen evolution reaction kinetic. To address these issues, hierarchical SnO₂ (HSN) mesoporous support is developed via a novel sol-electrophoretic approach, and BiVO₄ film is decorated with silver nanorods (Ag NRs). The photocurrent density of HSN/BiVO₄ photoanode is 3.98 mA/cm² at 1.23 V vs. reversible hydrogen electrode (RHE) and onset potential (V_onset) of 0.5 V vs. RHE. The PEC performance is attributed to the appropriate band alignment between SnO₂ and BiVO₄, as well as the hierarchical structure of SnO₂. Ag-HSN/BiVO₄ photoanode shows photocurrent density of 4.30 mA/cm² at 1.23 V vs. RHE and V_onset of 0.28 V vs. RHE. The enhanced photocurrent and negatively shifted V_onset can be attributed to radiative localized surface plasmon resonance decay and catalytic effect of Ag NRs, respectively.     

Hierarchical Co/MoO2@N-doped carbon nanosheets derived from waste lotus leaves for electrocatalytic water splitting

It is imperative for electrochemical water splitting to seek functional materials with high performance, competence, durability, economical, and eco-friendly. Herein, we develop a simplistic two-step annealing strategy to synthesize the hierarchical Co/MoO₂@nitrogen (N)-doped carbon nanosheets (CMO@NC) electrocatalysts derived from the low-cost and sustainable lotus leaves biomass for water-splitting. The optimum catalyst (CMO@NC/450) exhibits a notable low overpotential of 130 and 272 mV at a current density of 10 mA cm^?2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH, owing to their unique surface features, large surface area, abundant meso/micropores, and high pyridinic N-contents. Also, the CMO@NC/450 catalyst-equipped with a two-electrode configuration exhibits notable water splitting activity only requires a cell-potential of 1.629 V@10 mA cm^?2 in 1.0 M KOH. The results reveal that hierarchical flower-like morphology increases the contact area, prevents aggregation, and enables massive active-sites for HER and OER. Additionally, the synergistic effects between Co/MoO₂ and the N-doped-carbon heterostructure enhance charge-delocalization, ultimately improving electrocatalytic performance and stability. This work is aimed to promote the exploration and design of suitable doping structures and compositions for the development of highly effective and sustainable biomass-derived catalysts in a wide-range of electrochemical applications.     

Amorphous MoS2 nanosheets on MoO2 films/Mo foil as free-standing electrode for synergetic electrocatalytic hydrogen evolution reaction

A facile oxidation-sulfidation strategy is proposed to fabricate the vertically aligned amorphous MoS₂ nanosheets on MoO₂ films/Mo foil (MF) as free-standing electrode, which features as the integration of three merits (high conductivity, abundant exposures of active sites, and enhanced mass transfer) into one electrode for hydrogen evolution reaction (HER). Density functional theory (DFT) calculations reveal the strong interaction between MoS₂ and MoO₂, which can enhance the intrinsic conductivity with narrow bandgap, and decreases hydrogen adsorption free energy (ΔG_H1 = ~0.06 eV) to facilitate the HER process. Benefiting from the unique hierarchical structure with amorphous MoS₂ nanosheets on conductive MoO₂ films/MF to facilitate the electron/mass transfer by eliminate contact resistance, controllable number of stacking layers and size of MoS₂ slabs to expose more edge sites, the optimal MoS₂/MoO₂/MF exhibits outstanding activity with overpotential of 154 mV at the current density of 10 mA cm⁻², Tafel slope of 52.1 mV dec⁻¹, and robust stability. Furthermore, the intrinsic HER activity (vs. ECSA) on MoS₂/MoO₂/MF is significantly enhanced, which shows 4.5 and 18.6 times higher than those of MoS₂/MF and MoO₂/MF at overpotential of 200 mV, respectively.     

Synthesis and characterizations of graphene/Sm doped BiFeO3 composites photoanode for efficient photo-electrochemical water splitting

The present study features Bi_1-xSm_xFeO₃ (BSFO) nanoparticles anchored on high-quality, reduced graphene oxide (RGO) sheets via a two-step ultrasonication method for photo-electrochemical (PEC) studies relating to solar hydrogen generation. Sm doping leads to the formation of pure BFO type phase without any secondary phases. The structural, morphological, optical, and local structure analyses of BSFO and BSFO@RGO have been done through X-ray diffraction, scanning electron microscope, UV–Vis spectrophotometer, and Raman spectrometer, respectively. The BSFO nanoparticles have been templated on reduced graphene oxide. The BSFO@RGO has been employed as a photoanode for PEC measurements under the simulated solar irradiation of intensity 100 mW-cm^?1. The optimum photoanode has been found with Bi_0.95S_0.05FO₃@RGO. The highest photocurrent density and solar to hydrogen (STH) conversion efficiency have been found as 2.40 mA/cm² (at 0.5 V vs. saturated calomel electrode) and 2.45%, respectively. Furthermore, the stability of the photoanode against photo corrosion has also been tested by the chronoamperometric technique. During 2 h experiment, the average photocurrent density has been maintained at 1.5 mA/cm² (at 0 V vs. SCE). The improved photocatalytic activity of BSFO@RGO has been explained based on the effect of doping, better solar spectral response, hindering the recombination loss of photo-generated charge carriers, and fast, facile charge transport. Although earlier studies have used Bi(Sm)FeO₃ photoanode, hydrogen production has been observed for the first time in the present investigation to the best of our knowledge. Also, it appears that hydrogen production at zero external bias as observed in the present study suggests a new feature for bandgap tailored BFO.     

High-performance bifunctional Fe-doped molybdenum oxide-based electrocatalysts with in situ grown epitaxial heterojunctions for overall water splitting

The design and development of low-cost, abundant reserves, high catalytic activity and durability bifunctional electrocatalysts for water splitting are of great significance. Here, simple hydrothermal and hydrogen reduction methods were used to fabricate a uniform distribution of Fe-doped MoO₂/MoO₃ sheets with abundant oxygen vacancies and heterojunctions on etched nickel foam (ENF). The Fe– MoO₂/MoO₃/ENF exhibited a small overpotential of 36 mV at 10 mA cm⁻² for hydrogen evolution reaction (HER), an excellent oxygen evolution reaction (OER) overpotential of 310 mV at 100 mA cm⁻² and outstanding stabilities of 95 h and 120 h for the HER and OER, respectively. As both cathode and anode catalysts, the heterogeneously structured Fe– MoO₂/MoO₃/ENF required a low cell voltage of 1.57 V at 10 mA cm⁻². Density functional theory (DFT) calculations show that Fe doping and MoO₂/MoO₃ heterojunctions can significantly reduce the band gap of the electrode, accelerate electron transport and reduce the potential barrier for water splitting. This work provides a new approach for designing metal ion doping and heterostructure formation that may be adapted to transition metal oxides for water splitting.     

Phosphate ions-functionalized and wettability-tuned nickel ferrite for boosted oxygen evolution performance

Nickel ferrite (NiFe₂O₄) has been explored as a promising oxygen evolution reaction (OER) electrocatalyst for water splitting owning to its earth-abundant and considerable water oxidation catalytic activity. Nevertheless, its practical electrocatalytic performance towards OER is still undesirable due to the sluggish OER kinetics and high overpotential gap on the water oxidation anode side. In this work, in order to enhance the electrochemical water oxidation performance of NiFe₂O₄, the surface of NiFe₂O₄ is functionalized with phosphate ions (Pi) by using a facile incipient impregnation and following calcination process. Results demonstrate that the OER properties of NiFe₂O₄ under alkaline conditions can be dramatically boosted by the surface Pi functionalization. In 1.0 M KOH solution, the resulting NiFe₂O₄-Pi on glassy carbon (GC) electrode demonstrates quite lower overpotential of 332 mV (10 mA/cm²) and Tafel slope of 57 mV/dec compared to that of pristine NiFe₂O₄ (443 mV@10 mA/cm² and 96 mV/dec), which is also better than that of commercial RuO₂ electrocatalysts (348 mV@10 mA/cm² and 80 mV/dec). Moreover, such electrocatalyst on nickel foam electrode also realizes superior OER durability to afford a current density of 70 mA/cm² at overpotential of only 300 mV for at least 28 h. The excellent electrocatalytic water oxidation activities of NiFe₂O₄-Pi can be attributed to the tuning electronic property and surface wettability by Pi ions functionalization. This work provides us a novel and effective approach to modify the photo-/electrocatalytic activity for transition metal oxides.