Flower-like Co3O4 microstrips embedded in Co foam as a binder-free electrocatalyst for oxygen evolution reaction

Designing appropriate oxygen evolution reaction (OER) electrocatalysts to meet the requirements of high efficiency, long-term durability, and low cost remains the challenge for scientific community. Cobalt oxide (Co₃O₄) has been proven as a promising candidate for OER with attractive activity and stability in alkaline media. In this study, flower-like Co₃O₄ microstrips have been successfully prepared and directly embedded in Co foam (denoted as Co₃O₄@Co foam) by a green and facile two-step strategy including hydrothermal treatment and subsequent annealing process under relatively low temperatures. It demonstrates that the OER performance of the Co₃O₄@Co foam electrode can rival to the commercial RuO₂ on glassy carbon electrode. The Co₃O₄@Co foam electrode displays high OER activity with a low overpotential of 273 mV at a current density of 10 mA cm⁻², and a low Tafel slope of 61.8 mV dec⁻¹. The flower-like Co₃O₄ microstrips greatly increase the active surface area to expose more active sites, and the directly growth of Co₃O₄ microstrips on Co foam with intimate contact improves the electron transport and ensures the stability of the Co₃O₄@Co foam electrode.     

Facile fabrication of MOF-decorated nickel iron foam for highly efficient oxygen evolution

Metal-organic frameworks (MOFs) have emerged as efficient electrocatalysts due to the features of high specific surface area, rich pore structure and diversified composition. It is still challenging to synthesize self-supporting MOF-based catalysts using simple and low-cost fabrication methods. Herein, we successfully fabricated Ni-doped MIL-53(Fe) supported on nickel-iron foam (Ni-MIL-53(Fe)/NFF) as efficient electrocatalyst. A facile two-step solvothermal method without adding any metal salts was used, which can simplify the fabrication process and reduce the experimental cost. In the fabrication process, the bimetallic Ni-MIL-53(Fe)/NFF was in situ converted from an intermediate NiFe₂O₄/NFF. The obtained material exhibits outstanding electrocatalytic oxygen evolution performance with a low overpotential of 248 mV at 50 mA cm^?2, and a small Tafel slope of 46.4 mV dec^?1. This work sheds light on the simple and efficient preparation of bimetallic MOF-based material, which is promising in electrocatalysts.     

Needle-like CoP/rGO growth on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

In this work, CoP/NF is synthesized at different temperature (250 °C, 300 °C, 350 °C) (denoted as CoP/NF-T, T = 250, 300, 350). Then, CoP/NF-300 with the best performance towards hydrogen evolution reaction (HER), is used to synthesize compounds with different ratio of reduced graphene oxide (rGO) (CoP/rGO/NF-X, X (quality ratio of rGO/CoP) = 1,3,5). In terms of morphology, under the synergistic effect of rGO, uniform and dense CoP provides the possibility to increase the electrochemical area. While CoP/rGO/NF-3 shows the minimum overpotential of 136 mV to drive 50 mA/cm, and the smallest Tafel slope 135 mV/dec among as-synthesized materials. Furthermore, CoP/rGO/NF-3 has good stability during at least 25 h. These result can be construed as the large electrochemical active area, high conductivity and long-time stability.     

Straightforward preparation of nickel selenide nanosheets supported on nickel foam as a highly efficient electrocatalyst for oxygen evolution reaction

The development of economical, durable, and efficient oxygen evolution reaction (OER) electrocatalysts is essential for large-scale industrial water electrolysis. Here, a straightforward strategy is proposed to synthesize a series of nickel selenide nanosheets supported on nickel foam (NiSe₂/NF) materials by directly selenizing nickel foam substrates at different temperatures under an inert atmosphere. When evaluated as electrocatalysts in OER, the optimal self-supported NiSe₂/NF-350 shows an excellent performance in 1.0 M KOH medium with an overpotential of 458 mV at 100 mA cm^?2, a small Tafel slope of 45.8 mV dec^?1, and a long-term stability for 36 h. Furthermore, the structural and compositional preservation for NiSe₂/NF-350 after stability test was also verified by various characterizations.     

3D MnCo2O4@CoS nanoarrays with different morphologies as an electrocatalyst for oxygen evolution reaction

The efficiency and stability of electrocatalysts are the key factors for measuring oxygen evolution reaction. In this work, the MnCo₂O₄ structure assembled from well-arranged nanowires or nanosheet arrays has been grown vertically on nickel foam by in-situ hydrothermal method. Interestingly, different morphology of MnCo₂O₄ can be easily regulated by adding NH₄F to a mixed solvent to achieve conversion from nanowires to nanosheets. In addition, further synthesis of unique three-dimensional hierarchical core/shell MnCo₂O₄@CoS nanowires or nanosheets arrays was performed primarily by electrochemical deposition. Both MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets exhibit high efficiency and long-lasting stability for the oxygen oxidation reaction. The lower overpotential of only 280 mV and 270 mV at 20 mA cm⁻² for the MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets were obtained with lower Tafel slopes of 139. 19 mV dec⁻¹ and 131.81 mV dec⁻¹ in 1.0 M potassium hydroxide respectively comparing with our other MnCo₂O₄@CoS catalysts. The results demonstrate that the crystal morphology of MnCo₂O₄@CoS does not significantly influence their electrocatalytic activity in water oxidation reactions by comparing nanostructured MnCo₂O₄@CoS nanowires and MnCo₂O₄@CoS nanosheets. The high catalytic activity of the MnCo₂O₄@CoS nanoarrays is attributed to the possession of more active sites, larger specific surface area, abundant oxygen vacancy, and fast electron transport rate. Not only that, the durability of the MnCo₂O₄@CoS nanoarrays is also excellent after continuous oxygen evolution test of 1000 cycles. The results of XRD, SEM and XPS show that MnCo₂O₄@CoS-7 cycles nanowires and MnCo₂O₄@CoS-7 cycles nanosheets materials can be used as a highly efficient and stable catalyst for oxygen evolution reaction.     

Facile synthesis of tubular CoP as a high efficient electrocatalyst for pH-universal hydrogen evolution

A facile three-step approach for tubular CoP preparation and its catalytic activity for HER and OER are reported. The CoP microtubes show superior HER performance in a wide pH range with low overpotentials of 91, 101 and 113 mV at 10 mA cm^?2 in 0.5 M H₂SO₄, 1 M KOH and 1 M PBS, respectively. Additionally, it also depicts superior OER performance with an overpotential of 300 mV at 10 mA cm^?2, which is lower than reported precious metal oxides. The improved electrocatalytic performance of tubular CoP is likely attributed to the porous tube-like structural features, which not only afford rich exposed active sites, but also accelerate the charge or mass transfer efficiency, and thus efficiently promote the HER performance. The synthesis of tubular CoP confirms the importance of morphology features and provides a new insight to rationally design and synthesize highly effective non-noble metal phosphide-based pH-universal electrocatalysts for HER.     

Low-pressure-plasma-processed NiFe-MOFs/nickel foam as an efficient electrocatalyst for oxygen evolution reaction

A NiFe bimetallic metal organic framework (MOF) deposited on nickel foam and processed by low-pressure plasmas with 95%Ar+5%H₂, pure Ar, and 95%Ar+5%O₂ gases is used as an electrocatalyst for the oxygen evolution reaction. An alkaline solution (1 M KOH) with 95%Ar+5%H₂ plasma processed NiFe-MOFs/NF exhibits the best electrocatalytic performance with the lowest overpotential of 149 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1. Furthermore, electrical impedance spectroscopy and cyclic voltammetry show that after 95%Ar+5%H₂ plasma treatment, the interfacial impedance greatly reduces, and the electrical double-layer capacitance slightly increased.     

Carbon supported nickel phosphide as efficient electrocatalyst for hydrogen and oxygen evolution reactions

Hydrogen production through water splitting is an efficient and green technology for fulfilling future energy demands. Carbon nanotubes (CNT) supported Ni₂P has been synthesized through a simpler hydrothermal method. Ni₂P/CNT has been employed as efficient electrocatalysts for hydrogen and oxygen evolution reactions in acidic and alkaline media respectively. The electrocatalyst has exhibited low overpotential of 137 and 360 mV for hydrogen and oxygen evolution reactions respectively at 10 mA cm^?2. Lower Tafel slopes, improved electrochemical active surface area, enhanced stability have also been observed. Advantages of carbon support in terms of activity and stability have been described by comparing with unsupported electrocatalyst.     

Controlled synthesis of NiCo2O4@Ni-MOF on Ni foam as efficient electrocatalyst for urea oxidation reaction and oxygen evolution reaction

Heterostructured materials with special interfaces and features give a unique character for much electrocatalytic process. In this work, the introduction of exogenous modifier Ni-MOF improved the reaction kinetics and morphology of the NiCo₂O₄@Ni-MOF/NF catalyst. As-obtained NiCo₂O₄@Ni-MOF/NF has excellent oxygen evolution reaction (OER) performance and urea oxidation reaction (UOR) performance. The catalyst need overpotential of 340 mV at a current density of 100 mA cm^?2 for OER and a potential of 1.31 V at the same current density for UOR. The Tafel slopes of NiCo₂O₄@Ni-MOF/NF is 38.34 and 15.33 mV dec^?1 for OER and UOR respectively, which is more superior than 78.58 and 66.73 mV dec^?1 of NiCo₂O₄/NF. The nanosheets microstructure is beneficial to the adsorption and transport of electrolyte and the presence of a large number of mesoporous channels can also accelerate gas release, and then improves activity of the catalyst. Density functional theory calculation demonstrate that NiCo₂O₄ plays a role in absorbing water, while the existence of in situ generated NiOOH can promote the electron transfer efficiency. It is synergies of NiCo₂O₄ and in situ generated NiOOH that enhance the decomposition of water on the surface of the NiCo₂O₄@Ni-MOF/NF. This investigation provides a new strategy for the application of spinel oxide and MOF materials.     

Modulated amorphous Fe–Ni–P nanosphere chains anchored on graphene aerogel grafted nickel foam: High-performance electrocatalyst for oxygen evolution reaction

Substrate materials with large special surface area and high conductibility play a crucial role in preparing promising oxygen evolution reaction (OER) catalysts. Herein, binder-free Fe–Ni–P nanospheres anchored on graphene aerogel grafted nickel foam (FNP/GA@NF) are assembled with different molar ratios of Fe/Ni. GA@NF with prominent conductivity supplies ample accessible sites to amorphous Fe–Ni–P nanospheres to attach and offers a strong skeleton guaranteeing long-term stability during OER process. As revealed via spectroscopic measurement, tremella-like nanospheres arranged in a nanochain structure have been observed on the surface of GA@NF. Such a nanochain provides abundant paths for freely electronic transformation leading to faster kinetics. Besides, FNP/GA@NF possesses distinguished electrocatalytic activities in 1.0 M KOH solution. Especially, when the molar ratio is 3:2, FNP/GA@NF catalyst requires overpotentials of only 320 and 413 mV to arrive current density of 50 and 100 mA cm^?2. Furthermore, by the reason of robust framework, it shows superior durability even up to 24 h chronoamperometry test. This work opens an evolutive direction to construct binderless substrates to improve conductivity and catalytic activity of non-noble catalysts for OER.     

Facile and scalable synthesis of heterostructural NiSe2/FeSe2 nanoparticles as efficient and stable binder-free electrocatalyst for oxygen evolution reaction

Oxygen evolution reaction (OER) is a rate-limiting step in electrocatalytic water splitting due to its sluggish reaction kinetics. Therefore, it is still challengeable to develop an inexpensive and efficient OER catalyst via a facile and scalable synthesis method. To address such issues, herein, we present a facile and scalable approach to prepare ultrathin NiSe₂/FeSe₂ heterostructural nanoparticles in-situ grown on NiFe foam (NFS/NFF), which can be employed as a self-supported non-noble metal-based catalyst for OER. The NFS/NFF catalyst delivers outstanding OER performance with a small Tafel slope of 57.07 mV dec⁻¹ and a low overpotential of 274 mV at 40 mA cm⁻² and displays terrific long-term stability, surpassing the performance of commercial RuO₂ and single component NiSe₂/NF catalyst. The results of XPS manifest that there is a strong heterointerface interaction between NiSe₂ and FeSe₂. In addition, combined with density functional theory (DFT) calculations, we further confirmed that the synergistic interface effect between NiSe₂ and FeSe₂ reduces the value of the Gibbs free energy of oxygen-containing intermediates as determining step (RDS) from 3.15 eV (NiSe₂) to 2.41 eV (NiSe₂/FeSe₂ heterostructures), leading to excellent OER performance. This work provides a novel strategy to rationally design and fabricate selenide-based heterostructural nanoparticles via a facile method, which can extend to prepare other non-precious OER catalysts with high efficiency and long-term stability.     

Free-standing carbon nanotubes as non-metal electrocatalyst for oxygen evolution reaction in water splitting

Oxygen evolution reaction (OER) has significant impact on the overall electrochemical water splitting. We introduce, for the first time, a facile approach towards the fabrication of versatile electrode composed of free-standing multiwalled carbon nanotubes (MWCNTs) as electrocatalyst for the water splitting reaction. Directly extracted MWCNTs as sheets from vertically grown arrays transferred over the glass substrate, are used without any post treatment as a working electrode for OER. Onset potential of 1.60 V was achieved for MWCNTs which is significantly reduced as compared to platinum based metal electrode (1.72 V) with excellent current density. No surface modification, metal-free nature, flexibility and low cost with excellent catalytic activity proved this material as a promising candidate for the replacement of metal based electrodes in electrochemical water splitting.     

Facile synthesis of self-supported intertwined columnar NiCoP as a high efficient electrocatalyst for hydrogen evolution reaction

A new self-supported nickel-cobalt phosphide (NiCoP) on Ni foam (NiCoP/NF) is fabricated by simple immersion in Co(NO₃)₂ solution followed by subsequent phosphorization. NiCoP/NF displays intertwined and porous columnar morphology derived from topological transformation of corresponding columnar amorphous hydroxides precursor. NiCoP/NF manifests the most prominent hydrogen evolution reaction (HER) performance in both 0.5 M H₂SO₄ and 1 M KOH with the overpotentials of 49 and 57 mV to achieve 10 mA cm^?2, respectively. Also, NiCoP/NF showed excellent oxygen evolution reaction (OER) performance, requiring 256 mV to achieve 10 mA cm^?2, even superior to that of RuO₂ and IrO₂. Such impressive HER performance of NiCoP/NF is mainly attributed to the collective effects of enlarged surface area and enriched exposed active sites, affording faster charge transfer kinetic in HER process. This simple immersion method offers a new insight to design cost-effective and efficient electrocatalysts for large scale application.     

Facile synthesis of binary NiCoS nanorods supported on nickel foam as efficient electrocatalysts for oxygen evolution reaction

A facile two-step method has been applied to synthesize novel binary metal NiCoS nanorods supported on nickel foam (NF) as electrocatalysts for oxygen evolution reaction (OER). Firstly, electrodeposition process is conducted to fabricate binary Ni-Co hydroxides on NF (NiCo/NF). Then, a hydrothermal sulfuration of NiCo/NF has been adopted to prepare NiCoS nanorods arrays uniformly grown on the surface of NF (NiCoS/NF). XRD indicates that NiCoS/NF has mixed crystal phases of Ni₃S₂, CoS and Co₉S₈. SEM images display the uniform NiCoS nanorods composed of many vertical nanosheets on the surface, implying more exposed active sites. OER measurements demonstrate that NiCoS/NF has better activity with an overpotential of 370 mV to reach 100 mA cm^?2 than NiCo/NF and CoS_x/NF. Electrochemical impedance spectroscopy (EIS) tests confirm the faster charge-transfer rate of NiCoS/NF and smaller Tafel slope derived from binary NiCoS, implying the excellent electrocatalytic performances of binary metal sulfides.     

Co/Mo2C composites for efficient hydrogen and oxygen evolution reaction

Non-precious transition metal electrocatalysts with high catalytic performance and low cost enable the scalable and sustainable production of hydrogen energy through water splitting. In this work, based on the polymerization of CoMoO₄ nanorods and pyrrole monomer, a heterointerface of carbon-wrapped and Co/Mo₂C composites are obtained by thermal pyrolysis method. Co/Mo₂C composites show considerable performance for both hydrogen and oxygen evolution in alkaline media. In alkaline media, Co/Mo₂C composites show a small overpotential, low Tafel slope, and excellent stability for water splitting. Co/Mo₂C exhibits a small overpotential of 157 mV for hydrogen evolution reaction and 366 mV for oxygen evolution reaction at current density of 10 mA cm⁻², as well as a low Tafel slope of 109.2 mV dec⁻¹ and 59.1 mV dec⁻¹ for hydrogen evolution reaction and oxygen evolution reaction, respectively. Co/Mo₂C composites also exhibit an excellent stability, retaining 94% and 93% of initial current value for hydrogen evolution reaction and oxygen evolution reaction after 45,000 s, respectively. Overall water splitting via two-electrode water indicates Co/Mo₂C can hold 91% of its initial current after 40,000 s in 1 M KOH.     

Three-dimensional VOx/NiS/NF nanosheets as efficient electrocatalyst for oxygen evolution reaction

Developing effective and robust electrocatalysts that are applicable for intense conditions is promising for variable industrial oxygen evolution reaction (OER). Herein, we have developed a simple hydrothermal strategy to construct a three dimensional nanoflower-like VOx nanosheets (VOx/NiS/NF) that utilizes S-modified NF as substrate. NiS/NF can provide not only high-surface area for the growth of VOx but also better conductivity and stability derived from NiS. XRD shows the formation of amorphous VOx supported on NiS/NF. XPS confirms the existence and valence state of V, Ni and S. EDX and SEM elemental mapping reveal the composition and great distribution of V, O, Ni and S. SEM and TEM show that the thin VOx nanosheets covered on the surface of NiS/NF uniformly, which implying more exposed active sites. OER measurements display that VOx/NiS/NF has the outstanding catalytic activity with the lower overpotential (330 mV, 50 mA cm⁻²), smaller tafel slope (121 mV dec⁻¹) and lower value of semicircle of EIS than VOx/NF. The modification of NF may be the key for enhancement performances for OER due to reduced charge transfer resistance. The strong durability of VOx/NiS/NF may be attributed to the tighter integration between VOx and NiS/NF in alkaline electrolytes. The impressive results may provide a new strategy to design suitable substrate with good dispersion and conductivity to prepare effective electrocatalysts for OER.     

Direct synthesis of binder-free Ni–Fe–S on Ni foam as superior electrocatalysts for hydrogen evolution reaction

Designing the efficient, low-cost and stable electrocatalyst is of great significance for storage and conversion of the renewable energy to hydrogen. Herein, the binder-free Ni–Fe–S electrocatalysts were directly electrodeposited on Ni foam, which exhibited the excellent hydrogen evolution reaction performance with the overpotential of 51.4 mV at the current density of 10 mA cm^?2. Based on the analysis and results of as-synthesized Ni, Ni–Fe and Ni–S, the boosted electrocatalytic activity can be attributed to the composite effect between Ni and the introduced Fe and S. Additionally, the Ni–Fe–S electrocatalysts also displayed the low cell voltage (1.59 V at 10 mA cm^?2), remarkable durability and high Faraday efficiency in overall water electrocatalysis. Moreover, the water electrolysis device with Ni–Fe–S bi-electrodes can be driven by a small wind power generation and producing 4 mL H₂ in 39 min, indicating the prepared Ni–Fe–S electrocatalyst has the great potentials in producing hydrogen via renewable energy.     

Facile synthesis of bimetallic Ni–Fe phosphide as robust electrocatalyst for oxygen evolution reaction in alkaline media

Bimetallic Ni–Fe phosphide electrocatalysts were in-situ synthesized through direct phosphorization of metal salts on carbon cloth (CC). The Fe dopant remarkably enhances the OER performance of Ni₂P in alkaline medium through the electronic structure modulation of Ni. The (Fe_0.5Ni_0.5)₂P/CC electrode, composed of uniform films coated on carbon fibers, delivers a low overpotential of 260 mV with a small Tafel slope of 45 mV·dec⁻¹ at the current density of 100 mA cm⁻², outperforming most reported non-noble electrocatalysts and commercial RuO₂ electrocatalyst. The (Fe_0.5Ni_0.5)₂P/CC also displays superior electrochemical stability at high current density. An appropriate Fe dopant level facilitates the in-situ transformation of Ni–Fe phosphides into active NiFeOOH during alkaline OER. This work simplifies the synthesis procedure of metal phosphides.     

Nitrogen-doped cobalt sulfide as an efficient electrocatalyst for hydrogen evolution reaction in alkaline and acidic media

The enhancement in intrinsic catalytic activity and material conductivity of an electrocatalyst can leads to promoting HER activity. Herein, a successful nitrogenation of CoS₂ (N–CoS₂) catalyst has been investigated through the facile hydrothermal process followed by N₂ annealing treatment. An optimized N–CoS₂ catalyst reveals an outstanding hydrogen evolution reaction (HER) performance in alkaline as well as acidic electrolyte media, exhibiting an infinitesimal overpotential of ?0.137 and ?0.097 V at a current density of ?10 mA/cm² (?0.309 and ?0.275 V at ?300 mA/cm²), corresponding respectively, with a modest Tafel slope of 117 and 101 mV/dec. Moreover, a static voltage response was observed at low and high current rates (?10 to ?100 mA/cm²) along with an excellent endurance up to 50 h even at ?100 mA/cm². The excellent catalytic HER performance is ascribed to improved electronic conductivity and enhanced electrochemically active sites, which is aroused from the synergy and mutual interaction between heteroatoms that might have varied the surface chemistry of an active catalyst.     

In situ growth of NiCoFe-layered double hydroxide through etching Ni foam matrix for highly enhanced oxygen evolution reaction

It is of great significance to develop the nonprecious metal oxide electrocatalysts toward oxygen evolution reaction (OER) for water splitting. Herein we report an in-situ growth of the ternary NiCoFe-layered double hydroxide nanosheets on surface etching nickel foam (NiCoFe LDHs/NF) without adding any nickel precursor. In this method, etching Ni matrix by Fe³⁺ not only provides the slowly released nickel ions, but also intensifies the Fe–Ni interaction between the directly grown active species and Ni foam. Therefore the composition, electronic structure, and morphology of the electrocatalysts can be easily regulated only by adjusting Co²⁺:Fe³⁺ ratio in the precursor solution. The obtained NiCo₁Fe₁ LDH/NF, which is formed in 1:1 Co²⁺:Fe³⁺ solution, has highest content of Ni³⁺ and Co³⁺ active sites and the largest electrochemical active area. It exhibits an outstanding OER performance with a small overpotential of 231 mV at 10 mA cm^?2 and excellent durability at 50 mA cm^?2 in 1.0 M KOH solution.