Iron-doped metal-organic framework with enhanced oxygen evolution reaction activity for overall water splitting

Water electrolysis is an energy conversion technology to provide green and clean hydrogen energy. Developing a high-efficient and durable electrocatalyst is a critical material for water electrolysis. Therefore, we synthesize a series of iron-doped metal-organic frameworks (MOFs) by a facile one-pot hydrothermal method. In the conventional three-electrode-cell, the Co/Fe (1:1)-MOF catalyst exhibits an overpotential of 317 mV at a current density of 10 mA cm⁻² in the oxygen evolution reaction (OER). Furthermore, the electrolysis performance of Co/Fe (1:1)-MOF catalyst is further evaluated in a home-made anion-exchange-membrane water electrolysis cell. With the Co/Fe (1:1)-MOF as the OER catalyst and commercial Pt/C as the hydrogen-evolution-reaction catalyst, the cell presents an overpotential of 490 mV at a large current density of 500 mA cm⁻², which is superior to the benchmark cell with commercial IrO₂ as the OER catalyst in the alkaline media. Theoretical calculation demonstrates that the introduction of Fe dopant into MOFs significantly reduces the binding energy of 1O and 1OOH intermedium during the OER progress. Consequently, the electrocatalytic activity is increased, which is perfectly consistent with the experimental results. This work suggests that the iron-doped MOFs structure significantly improves the electrocatalytic activity and provides a facile strategy to produce hydrogen at a large current density for industrial water electrolysis.     

Enhanced water splitting through different substituted cobalt-salophen electrocatalysts

Synthesis of stable catalysts for water splitting is important for the renewable and clean energy production. Here, water oxidation activities of cobalt (II) complexes CoL¹-CoL³ (1–3) with salophen type ligands (N,N′-bis(salicylidene)-4-chloro-1,2-phenylendiamine (H₂L¹), N,N′-bis(salicylidene)-4-bromo-1,2-phenylendiamine (H₂L²) and N,N′-bis(salicylidene)-4-nitro-1,2-phenylendiamine (H₂L³)) are studied by electrochemical techniques, FE-SEM images and XRD patterns. Linear sweep voltammetry studies indicate that 2 and 3 have superior activities and only require the overpotential of 316 and 247 mV vs. RHE at current density of 10 mA/cm² with Tafel slopes of 75 and 50 mVdec^?1 at pH = 11. Experiments show relationships between the stability of the complexes and their catalytic activity. It is revealed that substituents on ligands affect the catalytic behaviors. Experiments show that in the presence of 2 and 3, the complexed cobalt ions are likely candidates as molecular catalysts for water oxidation. It is speculated that the O–O bond formation occurs by oxidizing the active center of cobalt complexes.     

Hollow ZnxCo1-xSe2 microcubes derived from Metal–Organic framework as efficient bifunctional electrocatalysts for hydrogen evolution and oxygen evolution reactions

Herein, we designed a simple and universal method to prepare cobalt-based bimetallic Zn_xCo_1-x-MOFs precursors, which were used as templates to synthesize effective bifunctional electrocatalyst hollow porous Zn_xCo_1-xSe₂ microcubes by one-step hydrothermal method. The cubic morphology of the Zn_xCo_1-x-MOFs precursors was well inherited. Particularly, the Zn_0.1Co_0.9Se₂ exhibited superior HER and OER performance in acidic solution and alkaline solution, respectively. Benefiting from the hollow porous structure, the synergistic effect of Zn–Co–Se and the incorporation of a small number of zinc atoms.

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CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media

The exploration of catalysts with high activity and low cost for water splitting is still necessary. Herein, a nanowire-like morphology CoO/NF electrode is synthesized using facile hydrothermal reaction and calcination treatment. The urea can regulate its morphology during the synthetic process of CoO/NF. Electrochemical studies reveal that the as-obtained CoO/NF exhibits excellent electrocatalytic performance with overpotential of 307 mV at current density of 10 mA cm⁻² and Tafel slope of 72 mV dec⁻¹ for oxygen evolution reaction, and CoO/NF delivers current density of 10 mA cm⁻² at overpotential of 224 mV for hydrogen evolution reaction. The results of the oxygen evolution reaction stability show that the overpotential of CoO/NF electrode is only increased by 4 mV at current density of 10 mA cm⁻². The two-electrode water splitting with CoO/NF electrodes as both anode and cathode needs a cell potential of 1.76 V to reach 10 mA cm⁻². Therefore, this simple method to prepare CoO/NF electrode can enhance the properties of electrocatalysts, which makes CoO/NF a promising material to replace noble metal-based catalysts.     

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.     

Bimetallic metal-organic framework derived electrocatalyst for efficient overall water splitting

The development of bifunctional catalysts that can be applied to both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely regarded as a key factor in the production of sustainable hydrogen fuel by electrochemical water splitting. In this work, we present a high-performance electrocatalyst based on nickel-cobalt metal-organic frameworks for overall water splitting. The as-obtained catalyst shows low overpotential to reaches the current density of 10 mA cm⁻² with 249 mV for OER and 143 mV for HER in alkaline media, respectively. More importantly, when the electrolyzer was assembled with the as-prepared catalyst as anode and cathode simultaneously, it demonstrates excellent activity just applies a potential of 1.68 V to achieve 10 mA cm⁻² current density for overall water splitting.     

Vacancies and phosphorus atoms assembled in amorphous urchin-like Co3O4 for highly efficient overall water splitting

Designing highly efficient and low-cost electrocatalysts is essential for water splitting. Herein, urchin-like Co₃O₄ microspheres are firstly grown on nickel foam by a hydrothermal method, then Oxygen vacancies, phosphorus doping are effectively assembled in Co₃O₄ electrocatalysts. The introduction of oxygen vacancies and phosphorus doping will adjust the electronic structure of Co which increase the intrinsic catalytic activity and improve the adsorption energy of intermediates, simultaneously, progressively transform the crystal into randomly arranged atoms structure with short range order resulting in more active sites participate in the catalytic reaction. Moreover, the catalyst of vacancies Co₃O₄-Ov and phosphorus doping Co₃O₄–P demonstrate excellent performance in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media, Co₃O₄-Ov sample served as anode while Co₃O₄–P as cathode to form an electrolytic cell needs only 1.58 V to reach 20 mA cm^?2 for overall water splitting.     

Template-free synthesis of 1D hollow Fe doped CoP nanoneedles as highly activity electrocatalysts for overall water splitting

Development of low cost and high efficiency electrocatalysts for water splitting systems to produce renewable hydrogen energy is still a significant requirement. The engineering of nanostructure and element doping are effective methods to further improve the performance of catalysts. Nonmetal (such as N, P, S) doping has been extensively investigated, while the report of metal doping is relatively few. Herein, Fe doped CoP 1D hollow nanoneedles on carbon cloth (CC) are designed and fabricated by a hydrothermal method and subsequent phosphorization procedure. The conversion of Fe doped Co-hydroxide@CC to Fe–CoP can produce large number of nanopores, which are closely connected to each other, and form hollow structures within the nanoneedles. Benefiting from the effective Fe doping and the particular hollow nanoneedle structure, the obtained Fe–CoP@CC demonstrates good electrocatalytic activity for hydrogen evolution reaction (HER) both in alkaline and acidic solution, affording a current density of 10 mA cm⁻² at overpotential of 49 mV and 80 mV, respectively. Moreover, the two-electrode electrolyzer with Fe–CoP@CC as both the cathode and anode catalyst achieve a current density of 10 mA cm⁻² at a cell voltage of 1.58 V in 1.0 M KOH solution. The results illustrate that the obtained hollow Fe–CoP@CC nanoneedles can serve as an efficient catalyst for overall water splitting.     

Electrodeposited nickel–iron–carbon–molybdenum film as efficient bifunctional electrocatalyst for overall water splitting in alkaline solution

Designing and synthesizing of efficient and inexpensive bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is one of the current research topics. In this study, NiFeCMo film in nickel mesh substrate is prepared by one-step direct-current electrodeposition method. The obtained NiFeCMo film shows the excellent electrocatalytic activity, which only requires overpotentials of 254 mV for HER and 256 mV for OER to drive current density of 10 mA cm⁻², with corresponding Tafel slopes of 163.9 and 60.3 mV·dec⁻¹ in 30% KOH medium, respectively. Moreover, NiFeCMo film only needs a low cell voltage of 1.61 V to drive current density of 10 mA cm⁻² in an alkaline electrolyzer. Such remarkably HER and OER properties of NiFeCMo alloy is attributed to the increased effective electrochemically active surface area and the synergy effect among Ni, Fe, C and Mo.     

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.     

Metal-organic frameworks derived N,S,O-doped carbon sheets coated CoP/Co3S4 hybrids for enhanced electrocatalytic hydrogen evolution reaction

Evidence shows that embedding metal-based hybrid into carbon matrix is an up-and-coming method to improve the efficiency and decrease the cost of electrocatalysts. Herein, by using a metal-organic framework (MOF) with 4,4-bipyridine and 2,5-thiophenedicarboxylic acid as a precursor, a CoP/Co₃S₄ hybrid embedded into N, S, O-doped carbon sheets (CoP/Co₃S₄@NSOC) was constructed through pyrolysis and phosphorization processes. The lamellar morphology, hetero-atom doping, and graphite carbon were favorable for fast electron and mass transfer. Moreover, the strong intrinsic activities of CoP and Co₃S₄ promoted electrocatalytic performance. In the electrochemical experiments, CoP/Co₃S₄@NSOC showed the lowest overpotential of 132 mV@10 mA cm^?2 for hydrogen evolution reaction (HER) among all the precursors. In addition, the electrocatalytic activity and structure of CoP/Co₃S₄@NSOC exhibited long-term stability over 60 h. The present work provides a feasible strategy for the construction of robust MOF-derived electrocatalysts.     

N-doped carbon coated FeNiP nanoparticles based hollow microboxes for overall water splitting in alkaline medium

Stable, earth-abundant and efficient electrocatalysts for overall water splitting are urgently needed. In this work, we have reported the synthesis of FeNiP/NC hollow microboxes (FeNiP/NC) based bifunctional electrocatalyst via the phosphorization process using rationally designed cube-type metal-organic framework (FeNi-MOF) as both the template and carbon source. The FeNiP/NC, which were obtained by assembling the uniform FeNiP nanoparticals together through N-doped carbon, manifests outstanding catalytic performances for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in KOH solution. Notably, FeNiP/NC displays exceptional activity when it was utilized as both anode and cathode toward overall water splitting with potential of 1.54 V at a current density of 10 mA cm^?2 in alkaline electrolyte, which is much better than FeP/NC and Ni₂P/NC electrocatalyst. More importantly, the improvement of the catalytic activities of FeNiP/NC mainly benefits from the well dispersion of FeNiP nanoparticles on the surface of carbon support, the large active surface area and the doping of N and C derived from organic ligands. In addition, the enhanced electrocatalytic performance of FeNiP/NC for OER is closely related with the in-situ formed surficial MOOH (M = Fe, Ni) active sites, which has been confirmed by X-ray photoelectron spectroscopy (XPS) analysis.     

Vertically aligned NiP2 nanosheets with interlaced mesh network for highly efficient water splitting under alkaline and acid solutions

Chemically polished Ti foil has been shown to be an excellent substrate for the growth of low-interface-resistance electrocatalysts. Herein, we grow vertically aligned interlaced-mesh-network NiP₂ nanosheets on deeply polished Ti foil (NiP₂ NS/Ti) by the gas-phase phosphidation of Ni(OH)₂. The produced interconnected nanosheets provide abundant electron transfer pathways and facilitate electron transfer, as confirmed by electrochemical impedance spectroscopy measurements and the presence of numerous voids that allow for fast gas release and ion migration. In view of the above advantages, the NiP₂ NS/Ti shows high electrocatalytic activity for the hydrogen evolution reaction in both alkaline and acidic solutions, wherein overpotentials of only 134 and 140 mV are required to attain a current density of 10 mA cm⁻², respectively. In addition, the above electrode also features a small Tafel slope (60.5 mV dec⁻¹ in alkaline and 49.5 mV dec⁻¹ in acidic solutions) and long-term stability, and can therefore be employed as an efficient and cheap three-dimensional cathode for electrochemical water splitting.     

Carbon nanorod supported metal alloy nanocubes using polydopamine as location reagent for water splitting

Transition metal catalysts were supposed to be the most likely substitute for commercial noble metal catalysts, and the development of highly active and long-term catalyst for water splitting are the future trend. Herein, Ni rectangular nitrogen doped carbon nanorods@Fe–Co nanocubes (Ni-CNRs@Fe–Co cubes) were fabricated via a facile template-free method. This simple strategy not only realizes the structure tailoring, but also achieves high-quality nitrogen-doping. Specifically, nickel dimethylglyoxime [Ni(dmg)₂] with rectangular rodlike structure was firstly synthesized by solution method, then metal-organic frameworks Fe–Co nanocube with different contents were loaded on rectangular carbon nanorods with polydopamine as the locating and the connecting agent, and finally Ni-CNRs@xFe-Co cubes were obtained by a one-step calcination. A series of electrochemical tests were researched on materials with different metal contents in the 1 M KOH solution. The Ni-CNRs@Fe–Co cubes show excellent electrocatalytic activity in the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). For HER and OER, the Tafel slopes were 83.3 mV dec⁻¹ and 71 mV dec⁻¹, the onset potential were −167 mV and 1.62 V, and reached the current densities of 10 mA cm⁻², the overpotential just needed 196 mV and 433 mV, respectively. This novel synthetic strategy will provide a template-free way for cheap electrocatalysts of non-precious metal for OER and HER.     

The P/NiFe doped NiMoO4 micro-pillars arrays for highly active and durable hydrogen/oxygen evolution reaction towards overall water splitting

Water splitting is an efficient strategy to produce purity hydrogen and convert intermittent electricity from renewable wind and solar sources. In this work, dense NiMoO₄ micro-pillars arrays (MPAs) were in-situ grown on nickel foam (NF) through facile hydrothermal method, then the NiMoO₄/NF were converted into NiMoO₄–P/NF and NiFe/NiMoO₄/NF via phosphating and electrodeposition method, respectively. The NiMoO₄–P/NF electrode required small overpotentials of 34 mV@10 mA cm⁻² and 130 mV@100 mA cm⁻² for hydrogen evolution reaction (HER). The NiFe/NiMoO₄/NF electrode exhibited excellent oxygen evolution reaction (OER) activity with overpotentials of 210 mV@10 mA cm⁻² and 300 mV@100 mA cm⁻². The overall water splitting using the anode-cathode couple of NiFe/NiMoO₄/NF||NiMoO₄–P/NF only consumes low voltages of 1.47 V@10 mA cm⁻² for 100 h and 1.66 V@100 mA cm⁻² for 50 h in 1 M KOH. The electronic modification and the well-designed hierarchical structure contribute the high energy-efficient and stabile overall water splitting.     

Cobalt/cerium-based metal-organic framework composites for enhanced oxygen evolution electrocatalysis

Rational design of multivariate metal-organic framework (MOF)-based electrocatalysts with favorably heterostructures is highly desirable for the oxygen evolution reaction (OER). In this work, we report the rational construction of a series of CoCe-based MOFs (Co_1-xCe_x-BTC, x = 0, 0.1, 0.15 and 0.2) for OER electrocatalysis, and demonstrate the catalytically inactive Ce-BTC as a new OER promoter greatly boosts the electroctalaytic performance of Co-BTC. The resulting Co_0·9Ce_0.1-BTC exhibits the outstanding OER performances with a low overpotential of 308 mV at a current density of 10 mA cm^?2 and a long-term stability within 25 h, which is favorably comparable to those of the commercial IrO₂ and other previously reported MOF-based OER electrocatalysts.     

Crystal interpenetration featured NiWSe@NF acicular nanowires for performance enhanced water splitting

Synergistic composite catalysts have always been the research focus of water splitting. Whereas, amorphous phases usually occurred on grain boundaries, which greatly hinders electron transfer and reduces the catalytic performance. Herein, a NiWSe@NF acicular nanowire electrode was fabricated by a simple hydrothermal strategy, affording outstanding activity and durability, requiring overpotentials of only 103 mV (HER) and 203 mV (OER) under 10 mA cm^?2, respectively. Experimental studies and theoretical analysis demonstrate that the excellent catalytic activity is attributed to the interpenetrate structure, which eliminates the formation of amorphous phase and provides a high-speed channel for electron transmission. The crystal interpenetration on grain boundaries adjusts the electronic, promotes the intermediates adsorption and reduce reaction energy barrier. The super-hydrophilic arisen from crystal interpenetration would also reduce the adhesion of generated bubbles and avoid performance attenuation. This work provides a new perspective for the development of high-efficiency composite catalysts.     

Highly active Ni/Co-metal organic framework bifunctional electrocatalyst for water splitting reaction

Rational design of electrocatalycally active materials with excellent performance for renewable energy conversion is of great interest. We have developed a nanosheet array of Ni/Co metal-organic framework (MOF) grown on CoO modified Ni foam (CoO/NF) substrate via the solvothermal process. The high surface area and low resistance of Ni/Co-MOF@CoO/NF (NC@CoO/NF) catalyst contribute to efficient water splitting. We have prepared a series of NC-n/CoO/NF (n = 1–4) catalysts to optimize the molar ratio of the Co atom in Ni MOF-74. Among them, NC-2@CoO/NF shows an excellent electrochemical performance in alkaline medium, i.e., low overpotential of 290 and 139 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. For a two-electrode system with NC-2@CoO/NF, a low cell voltage of 1.54 V at 10 mA cm^?2 has been obtained for overall water splitting which is much smaller than that with commercial Ir/C– Pt/C pair. This excellent performance can be attributed to the synergistic effects of Ni/Co-MOF and CoO/NF. In addition, the as-prepared NC-2@CoO/NF exhibits excellent long-term stability. The computational simulation also supports experimental results.     

In situ grown Cu-Based metal-organic framework on copper foam as high-performance electrocatalysts for oxygen evolution reaction

It is of great significance to develop highly efficient and robust oxygen evolution reaction (OER) electrocatalysts derived from earth-abundant and inexpensive elements for future hydrogen economy via electrochemical water splitting. Herein, Cu-based metal-organic framework (MOF) is directly supported on conductive Cu foam (CF) by a simply chemical oxidation of Cu substrate to grow Cu(OH)₂ nanowire arrays, followed by solvothermal treatment to obtain in situ grown Cu-based MOF electrode (MOF [Cu(OH)₂]/CF). The as-prepared 3D electrode shows superior OER activity with a low potential of 330 mV to deliver a current density of 10 mA cm⁻², a Tafel slope of 108 mV dec⁻¹, and excellent durability in alkaline media (1.0 M KOH). After electrolysis, XRD confirms that the initial MOFs have been transformed into CuO species, which are essentially active components for OER performance. This demonstrates that the MOFs can serve as efficient precursors for formation of highly active Cu oxide catalysts towards OER. This work provides a new strategy to develop MOFs-derived electrocatalysts for future clean energy conversion and storage systems.     

Metal-organic framework derived ellipse-like NiS2 nanocatalyst for stable electrochemical hydrogen generation in alkaline electrolyte

The development of non-precious and high-efficient electrocatalysts to enhance the activity and stability in alkaline media is impending for massive hydrogen and oxygen production. In this study, NiS₂ nanoparticles array with ellipse-like topography was fabricated via simple hydrothermal and sulfurization treatment. The NiS₂-400 featured with the unique loose stacking topographic architecture contributes to more exposed active sites, the smaller contact resistance between electrode/electrolyte, faster ion diffusion and electron transfer. As a result, NiS₂-400 electrode requires only overpotentials of 116 and 178 mV to drive current densities of 10 and 50 mA cm^?2 in 1.0 M KOH towards the hydrogen evolution reaction (HER), coupled with a Tafel slope of 93.0 mV dec^?1. Moreover, the resultant NiS₂-400 nanoparticles exhibit excellent electrochemical stability for more than 50 h. In addition, the density functional theory (DFT) calculation further confirms that the (200) facet acts as the predominant active site, contributing to the enhanced HER performance.