Synthesis of nickel hydroxide/reduced graphene oxide composite thin films for water splitting application

Facile synthesis of highly efficient and low-cost electrocatalyst for oxygen evolution reaction (OER) is important for large-scale hydrogen production. Herein, nickel hydroxide/reduced graphene oxide (Ni(OH)₂/rGO) composite thin film was fabricated using dip-coating followed by electrodeposition method on Ni foam substrate at room temperature. The deposited composite film shows amorphous nature with ultra-thin Ni(OH)₂ nanosheets vertically coated on rGO surface, which provides large electrochemical surface area and abundant catalytically active sites. It exhibits a low overpotential of 260 mV @10 mA cm⁻² as compared to the pristine electrodes and excellent long-term stability up to 20 hours in 1 M KOH solution. The electrochemical active surface area and Tafel slope of the composite electrode are 20.2 mF cm⁻² and 35 mV dec⁻¹, respectively. The superior water oxidation performance is a result of high catalytically active sites and improved conductivity of the composite electrode.     

Construction of NiCo2S4/Ni3S2 nanoarrays on Ni foam substrate as an enhanced electrode for hydrogen evolution reaction and supercapacitors

Developing a multifunctional and sustainable electrode material for hydrogen evolution reaction and supercapacitors is a highly feasible avenue for producing the high energy density and renewable energies. In our study, nanostructured NiCo₂S₄/Ni₃S₂/NF nanoarrays are rational developed in experiments via a simple hydrothermal reaction. Ascribed to the 3D nanostructured NiCo₂S₄/Ni₃S₂ with numerous exposure active sites and large contact areas for the electrolyte, the binder-free feature of NiCo₂S₄/Ni₃S₂/NF facilitates a low charge transfer resistance, as well as the synergetic effect of NiCo₂S₄ and Ni₃S₂. The obtained electrocatalyst showed ultrahigh electrocatalytic activity with an overpotential of 111 mV at 10 mA cm⁻² and a Tafel slope of 57 mV dec⁻¹. In addition, the electrode showed an area specific capacity of 6.13 F cm⁻² at 10 mA cm⁻² and superior rate capability (2.72 F cm⁻² at 80 mA cm⁻²), accompanied by excellent cycling stability. This results presented in our work can provide an effective strategy for rational design of other hybrid materials with excellent electrochemical performance in the application of electrocatalysis and supercapacitors.     

Cobalt oxide nanosheets anchored onto nitrogen‐doped carbon nanotubes as dual purpose electrodes for lithium‐ion batteries and oxygen evolution reaction

Transition metal oxides (TMOs) have been extensively explored as promising electrode materials for electrochemical energy storage and catalysis. However, TMOs intrinsically have low electronic conductivity and suffer severe volume change during electrochemical cycling. In this study, we develop an effective strategy to enhance conductivity and buffer volume changes of TMOs, in which networked nitrogen‐doped carbon nanotubes (N‐CNTs) are incorporated into Co₃O₄ nanosheets system. Based on the whole mass of Co₃O₄ and N‐CNT, the composites can maintain a stable discharge capacity of ~590 mAh g^?1 after 80 cycles at a current density of 0.5 A g^?1. Moreover, the composites also exhibit greatly enhanced catalysis ability towards oxygen evolution reaction (OER), ie, small Tafel slope of 84 mV dec^?1, low overpotential of 310 mV at a current density of 10 mA cm^?2, and almost no activity decay throughout 30‐hour continuous operation. This study lays a new route for smartly designing advanced electrode materials for energy storage and electrochemical catalysis.     

Honeycomb micro/nano-architecture of stable β-NiMoO4 electrode/catalyst for sustainable energy storage and conversion devices

Multi-functionality is a highly desirable feature in designing new electrode material for both energy storage and conversion devices. Here, we report a well-integrated and stable β-NiMoO₄ that was fabricated on three dimensional (3D) nickel foam (NF) by a simple hydrothermal approach. The obtained β-NiMoO₄ with interesting honeycomb like interconnected nanosheet microstructure leads to excellent electrochemical activity. As an electrode for Supercapatteries, β-NiMoO₄–NF showed a high specific capacity of 178.2 mA h g⁻¹ (916.4 F g⁻¹) at 5 mA cm⁻² current density. Most importantly, the fabricated symmetric device exhibits a maximum specific energy of 35.8 W h kg⁻¹ with the power output of 981.56 W kg^-1 and excellent cyclic stability. In methanol electro-oxidation, the β-NiMoO₄ –NF catalyst deliver the high current density of 41.8 mA cm⁻² and much lower onset potential of 0.29 V with admirable long term stability. Apart from the above electrochemical activity, the β-NiMoO₄ –NF honeycomb microstructure demonstrates a promising non-noble electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and showed a considerable overpotential of 351 mV (OER) and 238 mV (HER). The attractive multifunctional electrochemical activity of β-NiMoO₄–NF could be originates from their unique honeycomb micro/nano structure which can acts as an “ion reservoir” and thus leads to superior energy storage and conversion processes.     

Facile one pot sonochemical synthesis of CoFe2O4/MWCNTs hybrids with well-dispersed MWCNTs for asymmetric hybrid supercapacitor applications

In this study, a facile sonochemical strategy is used for the fabrication of CoFe₂O₄/MWCNTs hybrids as an electrode material for supercapacitor applications. FE-SEM image demonstrates the uniformly well-distributed MWCNTs as well as porous structures in the prepared CoFe₂O₄/MWCNTs hybrids, suggesting 3D network formation of conductive pathway, which can enhance the charge and mass transport properties between the electrodes and electrolytes during the faradic redox reactions. The as-fabricated CoFe₂O₄/MWCNTs hybrids with the MWCNTs concentration of 15 mg (CFC15) delivers maximum specific capacitance of 390 F g⁻¹ at a current density of 1 mA cm⁻², excellent rate capability (275 F g⁻¹ at 10 mA cm⁻²), and outstanding cycling stability (86.9% capacitance retention after 2000 cycles at 3 mA cm⁻²). Furthermore, the electrochemical performance of the CFC15 is superior to those of pure CoFe₂O₄ and other CoFe₂O₄/MWCNTs hybrids (CFC5, CFC10 and CFC20), indicating well-dispersion MWCNTs and uniform porous structures. Also, as-fabricated asymmetric supercapacitor device using the CoFe₂O₄/MWCNTs hybrids as the positive electrode and activated carbon as the negative electrode materials shows the outstanding supercapacitive performance (high specific capacitance, superior cycling stability and good rate capability) for energy storage devices. It delivers a capacitance value of 81 F g⁻¹ at 3 mA cm⁻², ca. 92% retention of its initial capacitance value after 2000 charge-discharge cycles and excellent energy density (26.67 W h kg⁻¹) at high power density (~319 W kg⁻¹).     

Enhanced electrocatalytic property of Pt/C electrode with double catalyst layers for PEMFC

The objective of this study was to fabricate an efficient structural catalyst electrode of Pt/C consisting of double catalyst layers (DCL) with catalyst-ink spray and electrophoresis deposition (EPD) methods. The prepared Pt/C DCL electrode with Pt-dispersed and Pt-concentrated catalyst layers demonstrated better electrochemical properties than individual Pt/C single catalyst layer (SCL) electrodes. An S₁E₁ DCL electrode with Pt loading weight ratio of 1:1 between the Pt-dispersed and Pt-concentrated layers exhibited a higher electrochemical surface area (ECSA, 57.2 m²/g_Pt) and lower internal resistance (20 Ω) than an individual Pt-dispersed SCL electrode prepared with only the spray method (S₁E₀, 31.9 m²/g_Pt and 132 Ω) and an individual Pt-concentrated SCL electrode prepared with only the EPD method (S₀E₁, 34.1 m²/g_Pt and 120 Ω). The S₁E₁ DCL electrode exhibited 2.1 and 1.7 times higher mass activity for methanol oxidation reaction (MOR) than S₁E₀ and S₀E₁ SCL electrodes, respectively (1,230 mA/mg_Pt for S₁E₁ vs. 595 mA/mg_Pt for S₁E₀ and 715 mA/mg_Pt for S₀E₁). In addition, the S₁E₁ DCL electrode demonstrated high MOR durability after 1,000 sequential cycles while losing 30% activity. Meanwhile, S₀E₁ and S₁E₀ SCL electrodes rapidly lost 52% and 55% activity, respectively. These improved electrochemical performances of DCL electrode were owing to the advantages of separating Pt catalysts into two layers, which provides more Pt catalytic active sites to the electrolyte than those in SCL electrodes. Our observation may aid in minimizing the usage amount of Pt catalysts (~0.16 mg_Pt/cm²) compared to those in present commercial Pt/C composites (~0.3 mg_Pt/cm²) as well as maximize efficient Pt utilization. More importantly, with regard to proton exchange membrane fuel cell (PEMFC) activity as a crucial in-situ characterization of a catalyst, a membrane electrode assembly (MEA) containing S₁E₁ as the anode electrode could generate mass maximum power density of 3.84 W/mg_Pt, 3.6 times higher than the present commercial one (1.07 W/mg_Pt).     

Wet-chemistry synthesis of Li4Ti5O12 as anode materials rendering high-rate Li-ion storage

Compared with traditional anode materials, spinel-structured Li₄Ti₅O₁₂ (LTO) with “zero-strain” characteristic offers better cycling stability. In this work, by a wet-chemistry synthesis method, LTO anode materials have been successfully synthesized by using CH₃COOLi·2H₂O and C₁₆H₃₆O₄Ti as raw materials. The results show that sintering conditions significantly affect purity, uniformity of particle sizes, and electrochemical properties of as-prepared LTO materials. The optimized LTO product calcined at 650°C for 20 hours demonstrates small particle sizes and excellent electrochemical performances. It delivers an initial discharge capacity of 242.3 mAh g⁻¹ and remains at 117.4 mAh g⁻¹ over 500 cycles at the current density of 60 mA g⁻¹ in the voltage range of 1.0 to 3.0 V. When current density is increased to 1200 mA g⁻¹, its discharge capacity reaches 115.6 mAh g⁻¹ at the first cycle and remains at 64.6 mAh g⁻¹ after 2500 cycles. The excellent electrochemical performances of LTO can be attributed to the introduction of rutile TiO₂ phase and small particle sizes, which increases electrical conductivity and electrode kinetics of LTO. Therefore, as-synthesized LTO in this study can be regarded as a promising anode candidate material for lithium-ion batteries.     

The effect of Fe on chemical stability and oxygen evolution performance of high surface area SrTix-1FexO3-δ mixed ionic-electronic conductors in alkaline media

Development of environmentally friendly, high performing oxygen evolution reaction (OER) catalysts is an important research challenge. In this work, iron doped strontium titanates with a general formula SrTi_1-xFe_xO_3-δ (x = 0.35, 0.50, 0.70, 0.90, and 1.00) denoted as STFx, were synthesized via a solid state reaction technique and characterized in terms of oxygen evolution reaction electrocatalysis in an alkaline electrolyte (0.1 M KOH). The produced powders were characterized by a high specific surface area (>20 m² g⁻¹), beneficial for OER. The evaluation of specific activity indicated the following trend of increasing performance: STF35 < STF50 < STF70 < SFO < STF90. The lowest overpotential at 10 mAcm⁻² _GEO of 410 mV (350 mV at 25 μA cm⁻²_OX) was achieved by STF90 with the corresponding Tafel slope of 60 mV dec⁻¹. The two materials with the highest Fe content (i.e. STF90 and SFO) showed, however, poor chemical stability in alkaline solution demonstrated by the dissolution of Sr. Based on the good electrochemical performance (~460 mV at 10 mA cm⁻²_GEO, ~405 mV at 25 μA cm⁻²_OX) and chemical stability for at least 30 days (no Sr dissolution) of STF50, it can be considered an interesting, working at room temperature OER catalyst based on non-toxic and abundant elements.     

Homogeneous core–shell NiCo2S4 nanorods as flexible electrode for overall water splitting

Exploring low-cost and highly efficient Water splitting electrocatalyst has been recognized as one of the most challenging and promising ways. NiCo₂S₄ core-shell nanorods supported on nickel foam (NF) have been fabricated by a facile hydrothermal method. The electrochemical performance of NiCo₂S₄@NiCo₂S₄ for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is studied. NiCo₂S₄@NiCo₂S₄/NF exhibits a significantly improved OER and HER performance with an overpotential of 200 mV at 40 mA/cm² and an overpotential of 190 mV at 10 mA/cm². The combination of low charge-transfer resistance, enhanced interaction and charge transport as well as large electrochemical double-layer capacitance enables superior OER and HER. The NiCo₂S₄@NiCo₂S₄/NF nanorod electrode shows excellent electrocatalytic activity with a low voltage 1.57 V and stability with long hour electrolysis, which is highly satisfactory for a prospective bifunctional electrocatalyst.     

Enhancement of hydrogen evolution reaction by Pt nanopillar-array electrode in alkaline media and the effect of nanopillar length on the electrode efficiency

Pt nanopillar-array 3D electrodes with nanopillar length of 150, 450 and 900 nm and nanopillar density of ~10⁹ cm⁻² were fabricated. Their catalytic activity for hydrogen evolution reaction (HER) was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy. In comparison with straightly electrodeposited black Pt film and forged Pt sheet electrodes, the HER current density has been significantly improved by the nanopillar-array architecture. The overpotential of HER at current density of 10 mA cm-2 at 26 °C is as low as 78 mV, lower than the black Pt film of 107 mV and the Pt sheet of 174 mV. The improvement of HER is ascribed to the low charge transfer resistance of the 3D electrode and the high desorption capability of hydrogen bubbles at the nanotips. Interestingly, the nanopillar-array 3D electrode has an optimal nanopillar length for HER. The mechanisms for the optimal nanopillar length were investigated here.     

Ionanofluid plasticized electrolyte with improved electrical and electrochemical properties for high-performance lithium polymer battery

Herein, the electrochemical characteristics of Li/LiFePO₄ battery, comprising a new class of poly (ethylene oxide) (PEO) hosted polymer electrolytes, are reported. The electrolytes were prepared using lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) dopant salt and imidazolium ionic liquid-based nanofluid (ionanofluid) as the plasticizer. Morphological, thermophysical, electrical, and electrochemical properties of these newly developed electrolytes were studied. Using FT-IR spectroscopy, the interactions between dopant salt plasticizers and the host polymer, within the electrolytes, were evaluated. The optimized 30 wt% ionanofluid plasticized electrolyte exhibits a room temperature ionic conductivity of 6.33 × 10⁻³ S cm⁻¹, wide electrochemical voltage window (~4.94 V vs Li/Li⁺) along with a moderately high value of lithium-ion transference number (0.47). The values are substantially higher than that of similar wt% IL plasticized electrolyte (7.85 × 10⁻⁴ S cm⁻¹, ~4.44 V vs Li/Li⁺ and ~ 0.28, respectively). Finally, the Li/LiFePO₄ battery, comprising optimized 30 wt% ionanofluid plasticized electrolyte, delivers 156 mAh g⁻¹ discharge capacity at 0.1 C rate and able to retain its 92% value after 50 cycles. Such a superior battery performance as compared to the IL plasticized electrolyte cell (137 mAh g⁻¹ and 84% after 50 cycles at the same current rate) would endow this ionanofluid a very promising plasticizer to develop electrolytes for next-generation lithium polymer battery.     

Binder free and high performance of sputtered tungsten nitride thin film electrode for supercapacitor device

Here, we demonstrates the fabrication of binder free and very efficient supercapacitor electrode based on tungsten nitride (W₂N) thin film on stainless steel (SS) substrate using reactive sputtering technique. W₂N thin film as a working electrode exhibits high specific capacitance (163 F g⁻¹ at 0.5 mA cm⁻² in 1 M H₂SO₄) along with excellent cycling stability. The binder free symmetric supercapacitor (W₂N||W₂N) device delivers a high specific capacitance (80 Fg^-1) and long life span (90.46% capacitance retention after 10,000 cycles) along with high energy (12.92 Whkg⁻¹) and power (∼674 kWkg⁻¹ at 9.36 Whkg⁻¹) densities. These observed excellent electrochemical performances of the present W₂N thin film based supercapacitor device, recommend it as a potential candidate for energy storage applications.     

Crystalline ZnO and ZnO/TiO2 nanoparticles derived from tert-butyl N-(2 mercaptoethyl)carbamatozinc(II) chelate: Electrocatalytic studies for H2 generation in alkaline electrolytes

A newly synthesized zinc(II) complex, namely tert-butyl N-(2 mercaptoethyl)carbamatozinc(II) complex [Zn(Boc-S)₂] (Boc = tert-butyl N-[2-mercaptoethyl]carbamate), has been used as an organozinc precursor for the production of crystalline ZnO and ZnO/TiO₂ nanoparticles. The synthesized complex and the obtained nanomaterials were fully characterized using various spectroscopic and surface analysis techniques. Their surface morphology, chemical purity and stoichiometry have been investigated by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) as well as X-ray fluorescence. The synthesized Zn(II) molecular complex, ZnO and ZnO/TiO₂ nanomaterials have been tested in alkaline aqueous solution (1.0 MNaOH) for the hydrogen evolution reaction (HER) using various electrochemical techniques. The results revealed high HER catalytic performance of ZnO and ZnO/TiO₂ cathode materials, with the latter exhibiting higher catalytic activity recording an exchange current density (j_o) of 0.3 mA cm⁻². This current value, which approaches that of Pt wire (0.5 mA cm⁻²), cross-sectional area ~0.008 cm², is about 11 and 100 times greater than those measured for ZnO alone (0.028 mA cm⁻²) and TiO₂ alone (0.0032 mA cm⁻²), respectively. Moderate catalytic activity was recorded for the complex catalyst, namely GC-Zn(Boc-S)₂ with j_o value of (0.01 mA cm⁻²). Tafel slope values of 130 and 122 mV dec⁻¹ were calculated for ZnO and ZnO/TiO₂, respectively. Such Tafel slope values, which are close to that of the Pt wire (120 mV dec⁻¹), referred to a Volmer-controlled HER kinetics. Other important electrochemical parameters describing the kinetics of the HER, such as roughness factor (R_f) and turnover frequency (TOF) were also estimated and discussed. The high numerical values of the various HER kinetic parameters recorded for the ZnO/TiO₂ catalyst, in addition to its high stability and durability (stable for up to 10 000 continuous cathodic polarization cycles), besides maintaining its morphology and chemical composition after stability test (confirmed from SEM/EDX and XRD examinations), located it in a privileged position among the most efficient HER electrocatalysts reported in the literature.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

Efficient bifunctional hydrogen and oxygen evolution reaction electrocatalyst based on the NU-1000/CuCo2S4 heterojunction

One of the current necessities to produce clean energy is the logical design of inexpensive noble-metal free electrocatalysts with developed structure and composition for electrochemical water splitting. In this study, we introduce a new core-shell-structured bifunctional electrocatalyst of NU-1000/CuCo₂S₄ for oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and overall water splitting for the first time. Own to unique structure with rich porosity, high electrical conductivity, high stability and larger density of active sites, this nanocomposite can produce water electrolysis in a 1 M KOH solution. The electrochemical measurements show overpotentials of 335 mV for OER and 93 mV for HER at a current density of 10 mAcm⁻². Also, the NU-1000/CuCo₂S₄ nanocomposite exhibits Tafel slope values of 110 mV dec⁻¹ and 103 mV dec⁻¹ for HER and OER, respectively. Besides, NU-1000/CuCo₂S₄ presents a significant long-term stability in a 72 h run. Additionally, NU-1000/CuCo₂S₄ requires 1.55 V to deliver 10 mA cm⁻² current density in overall water splitting. According to these results, we hope to use this electrocatalyst in producing oxygen and hydrogen from water.     

Construction of hollow structure cobalt iron selenide polyhedrons for efficient hydrogen evolution reaction

The development of highly active, robust and cost-effective noble metal-free electrocatalysts for hydrogen evolution reaction (HER) in alkaline solution remains a severe challenge. In this work, a hollow structure CoSe₂-FeSe₂ heterojunction electrocatalyst (denoted by “(Co,Fe)Se₂”) was designed by a simple anion exchange reaction and selenization. Benefiting from the unique hollow structure, the (Co,Fe)Se₂ catalyst accelerates diffusion of electrolyte, besides, the CoSe₂-FeSe₂ heterojunction could provide rapid interfacial charge transportation and more active sites for the HER reaction. The (Co,Fe)Se₂ electrode material exhibits good performance for HER in 1 M KOH electrolyte. It needs an overpotential of 124 mV to obtain a current density of 10 mA cm⁻², and the Tafel slope is 65 mV dec⁻¹. Besides, (Co,Fe)Se₂ has a smaller charge transfer resistance compared with CoSe₂. At the same time, it has relatively large electrochemical active surface area due to the porosity. Most importantly, the (Co,Fe)Se₂ electrode displays good stability in alkaline conditions for 15 hours, the linear sweep voltammetry curves are almost coincident before and after 1000 cycles, the overpotential with current density of 10 mA cm⁻² increased by only 9.76% after 5000 cycles of CV. It shows great application potential in HER.     

Coupling NixSy-Ni2P heterostructure nanoarrays on Ni foam as environmentally friendly electrocatalyst for overall water splitting

The development of efficient, cheap and stable electrodes is the key to achieve the industrialization of hydrogen production from electrochemical water splitting. In this paper, Ni_xS_y-Ni₂P mixtures on Ni foam (Ni_xS_y-Ni₂P/NF) were synthesized by hydrothermal process followed by sulfurization and phosphorization approach. The combination of Ni_xS_y and Ni₂P exposes a large number of active sites, thus greatly improving the catalytic activity of the material. As expected, the Ni_xS_y-Ni₂P/NF material exhibits ultra-small overpotentials of 211 and 320 mV for water oxidation reaction at the current densities of 10 and 100 mA/cm², respectively. What is noteworthy is that the material also present superior hydrogen evolution reaction properties (122 mV@10 mA cm^?2). Moreover, when the material is acted as a bifunction electrode to drive the overall water splitting, only a cell voltage of 1.54 V is required to drive a current density of 10 mA/cm², which is one of the superior catalytic properties reported up to now. Experimental results show that the good electrochemistry performance of the Ni_xS_y-Ni₂P/NF material is attributed to the improved charge transfer rate, exposure of more active site and superior electrical conductivity. This work provides an effective way to explore environmentally friendly catalysts based on transition metal sulfide and phosphide.     

Novel 3-D urchin-like Ni– Co–W porous nanostructure as efficient bifunctional superhydrophilic electrocatalyst for both hydrogen and oxygen evolution reactions

Fabrication of an electrocatalyst with remarkable electrocatalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for the production of hydrogen energy. In this study, Ni–Co–W alloy urchin-like nanostructures were fabricated by binder-free and cost-effective electrochemical deposition method at different applied current densities and HER and OER electrocatalytic activity was studied. The results of this study showed that the microstructure and morphology are strongly influenced by the electrochemical deposition parameters and the best electrocatalytic properties are obtained at the electrode created at the 20 mA.cm⁻²applied current density. The optimum electrode requires −66 mV and 264 mV, respectively, for OER and HER reactions for delivering the 10 mA cm⁻² current density. The optimum electrode also showed negligible potential change after 10 h electrolysis at 100 mA cm⁻², which means remarkable electrocatalytic stability. In addition, when this electrode used as a for full water splitting, it required only 1.58 V to create a current density of 10 mA cm⁻². Such excellent electrocatalytic activity and stability can be related to the high electrochemical active surface area, being binder-free, high intrinsic electrocatalytic activity and hydrophilicity. This study introduces a simple and cost-effective method for fabricating of effective electrodes with high electrocatalytic activity.     

Effect of cation substitution on the water splitting performance of spinel cobaltite MCo2S4 (M = Ni,Cu and Co)

The introduction of different ions is an effective method for regulating electron distribution and increasing the electrocatalytic activity of spinel cobalt sulfide (Co₃S₄). However, the effect of doping different ions on water splitting performance has not been systematically clarified. Therefore, a detailed research is done to illuminate the doping of different ions on the water splitting performance of spinel cobalt sulfide MCo₂S₄ (M = Ni, Cu and Co) nanorods grown on Ni foam. To drive the electrocatalytic current of 50 mA/cm² and 10 mA/cm², the CuCo₂S₄/NF material only requires an overpotential of 240 mV for oxygen evolution reaction (OER) and an overpotential of 142 mV for hydrogen evolution reaction (HER). The results of density functional theory and experiment demonstrate that the strong water adsorption energy and the large electrochemical activity area make CuCo₂S₄/NF show good catalytic activity. The CuCo₂S₄/NF nanorods material presents superior electrochemistry performance with a small voltage 1.53 V. The water oxidation activity increases linearly before nonlinearly improving with the increasing of pH, indicating that the substrate changes from water to hydroxyl. It is noteworthy that CuCo₂S₄/NF will be transformed into amorphous oxide active species, which will act as a stable catalyst during the reaction.     

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.