Hollow CoFe-based hybrid composites derived from unique S-modulated coordinated transition bimetal complexes for efficient oxygen evolution from water splitting under alkaline conditions

The oxygen evolution reaction (OER) is an important reaction in water splitting. However, the high cost and slow-rate catalysts hinder commercial applications. Although an important process for manufacturing of hollow structures, it is difficult to construct complicated hollow structures with an excellent and regulable shape for multi-component materials. In this study, we demonstrate that sulfur-Co,Fe bimetallic nitrogen carbon hollow composite hybrids (x-S-CoFe@NC) can be synthesized by regulating the amount of sulfur and using the hydrothermal method. For OER, 32-S-CoFe@NC exhibits excellent electrocatalytic activity with a low overpotential of 232 mV, which is higher than those of 0-S-CoFe@NC (270 mV), 23-S-CoFe@NC (247 mV), and RuO₂ (243 mV) catalysts at 10 mA cm^-2. In addition, with air as the cathode, a rechargeable Zn-air battery with outstanding long-life cycling stability for 80 hours based on 32-CoFe@NC + Pt/C is proposed. The advanced technique described here supplies a new route for preparing hollow transition bimetal carbon hybrids with an adjustable composite arrangement for electrocatalysis and water splitting.

Defects engineering on CrOOH by Ni doping for boosting electrochemical oxygen evolution reaction

The design and construction of active centres are key to exploring advanced electrocatalysts for oxygen evolution reaction (OER). In this work, we demonstrate thein situconstruction of point defects on CrOOH by Ni doping (Ni-CrOOH/NF). Compared with pure CrOOH/NF, Ni-CrOOH/NF showed enhanced OER activity. The effect of the amount of Ni introduced on the OER performance was investigated. Ni_0.2-CrOOH/NF, the best introduction of Ni, uses a low overpotential of 253 mV to achieve a current density of 10 mA cm^-2with a high turnover frequency of 0.27 s^-1in 1.0 M NaOH. In addition, the electrocatalytic performance of Ni_0.2-CrOOH/NF showed little deterioration after 1000-cycle cyclic voltammetry scanning. In the potentiostatic test, activity was stable for at least 20 h.

Self-supported and defect-rich CoP nanowire arrays with abundant catalytic sites as a highly efficient bifunctional electrocatalyst for water splitting

In 1.0 M KOH, p-CoP/NF shows outstanding HER and OER activity. Furthermore when p-CoP/NF is assembled into a two-electrode cell, a voltage of only 1.55 V is needed to achieve 10 mA cm−2, and it can maintain long-term stability.

Chromium doping: A new approach to regulate electronic structure of cobalt carbonate hydroxide for oxygen evolution improvement

Highly efficient catalysts are required to solve the intrinsically sluggish kinetics of oxygen evolution reaction (OER). Herein, chromium doped cobalt carbonate hydroxide nanowire array on Ni foam (Cr-CoCH/NF) has been synthesized for the enhancement of OER activity and stability. Compared with pure CoCH/NF, Cr_0.2-CoCH/NF, the optimal doping of Cr, shows a low overpotential of 203 mV at the current density of 10 mA cm^-2 and a small Tafel slope of 84 mV dec^-1 in 1.0 M NaOH. In addition, there is little deterioration in electrocatalytic performance after 1000-cycle cyclic voltammetry and the high activity can be maintained over 25 h. Density functional theory calculations reveal the Cr doping can regulate the electronic structure of nearby Co active center to achieve great enhancement of OER activity.

High-Capacity Anode Material for Lithium-Ion Batteries with a Core-Shell NiFe2O4/Reduced Graphene Oxide Heterostructure

A novel composite consisting of transition-metal oxide and reduced graphene oxide (rGO) has been designed as a highly promising anode material for lithium-ion batteries (LIBs). The anode material for LIBs exhibits high-rate capability, outstanding stability, and nontoxicity. The structural characterization techniques, such as X-ray diffraction, Raman spectra, and transmission electron microscopy, indicate that the material adopts a unique core-shell structure with NiFe₂O₄ nanoparticles situated in the center and an rGO layer coated on the surface of NiFe₂O₄ particles (denoted as NiFe₂O₄/rGO). The NiFe₂O₄/rGO material with a core-shell structure exhibits an excellent electrochemical performance, which shows a capacity of 1183 mA h g^-1 in the first cycle and maintains an average capacity of ∼1150 mA h g^-1 after 900 cycles at a current density of 500 mA g^-1. This work provides a broad field of vision for the application of transition-metal-oxide materials in electrodes of lithium-ion batteries, which is of great significance for further development of lithium-ion batteries with excellent performance.

Controlled synthesis of P-Co3O4@NiCo-LDH/NF nanoarrays as binder-free electrodes for water splitting

The design and development of robust and environmentally friendly electrocatalytic materials are of great significance to the hydrogen production industry for the electrolysis of water. A series of P-Co₃O₄@NiCo-LDH/NF materials was firstly successfully synthesized by a hydrothermal method, high temperature calcination and an electrochemical deposition approach when sodium hypophosphite was used as the source of P and Ni(NO₃)₂·6H₂O as the source of nickel and introduced cobalt at the same time. The structure, composition, morphology and electrochemical performance of the P-Co₃O₄@NiCo-LDH/NF electrocatalytic material were determined by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and electrochemical performance testing. It is worth noting that the P-Co₃O₄@NiCo-LDH-2/NF material presents excellent hydrogen evolution reaction performance in 1 M KOH alkaline solution. It only needs an overpotential of 181 mV to drive a current density of 100 mA cm^-2, which is one of the best catalytic activities reported so far. The experimental results and theoretical calculations demonstrate that the electrocatalytic activity of the P-Co₃O₄@NiCo-LDH-2/NF material is attributed to the faster electron transfer rate, exposure of more active sites, optimal water adsorption energy and better electrical conductivity.

In situsynthesis of Fe-doped CrOOH nanosheets for efficient electrocatalytic water oxidation

The oxygen evolution reaction (OER) is a process in electrochemical water splitting with sluggish kinetics that needs efficient non-noble-metal electrocatalysts. There have been few studies of CrOOH electrocatalysts for water oxidation due to their low performance. Herein,in situsynthesized Fe-doped CrOOH nanosheets on Ni foam (Fe-CrOOH/NF) were designed as electrocatalysts and performance in the OER was obviously improved. The effect of the amount of Fe doping was also investigated. Experiments revealed that the best performance of Fe-CrOOH/NF requires low overpotentials of 259 mV to reach 20 mA cm^-2together with a turnover frequency of 0.245 s^-1in 1.0 M KOH, which may suggest a new direction for the development of Fe-doped OER electrocatalysts.

Micro–nanoarchitectures of electrodeposited Ni–ITO nanocomposites on copper foil as electrocatalysts for the oxygen evolution reaction

A simple conventional three electrodes system was used for the preparation of Ni and Ni–ITO nanocomposites as an electrocatalyst for oxygen evolution reaction.

Amorphous Ni-P nanoparticles anchoring on nickel foam as an efficient integrated anode for glucose sensing and oxygen evolution

Ever-growing efforts have been devoted to developing cost-effective and earth-abundant electrocatalysts with high-performance in biosensing and energy energy conversion. In this work, amorphous nickel-phosphorus (Ni-P) nanoparticles anchoring on Ni foam (Ni-P/NF) were prepared through a facile electroless deposition approach. The morphology and composition were characterized by scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. As an integrated anode, Ni-P/NF exhibits high performance towards glucose electrochemical sensing, with a high sensitivity of 13.89 mA mM^-1 cm^-2, a low detection limit of 1 µM, a wide detection ranges from 2 µM to 0.54 mM, and a quick response (<10 s), as well as good selectivity and reliability for real sample analysis in human serum. In addition to electrocatalytic glucose oxidation, Ni-P/NF shows remarkable catalytic activity towards oxygen evolution reaction (OER) in alkaline solution and it only needs an overpotential of 360 mV to afford 50 mA cm^-2. Moreover, Ni-P/NF shows excellent durability under alkaline OER condition. All these results demonstrate Ni-P/NF as highly efficient integrated anode in both biosensing and energy conversion.

Nickel cobalt oxide nanowires with iron incorporation realizing a promising electrocatalytic oxygen evolution reaction

Developing cost-effective electrocatalysts for water electrolysis is a promising strategy to enhance conversion and storage efficiency of sustainable energy. Transition metal oxides have been considered as alternative oxygen evolution reaction (OER) catalysts to replace noble metal-based catalysts. Here, we report a series of Fe-doped NiCo₂O₄ (NCO) nanowires with different Fe-doped concentrations, synthesized by a facile solvothermal and calcinations process, as high-efficiency electrocatalysts for OER. Due to abundant catalytically active sites, high-charge transport capability and specific surface area, these as-obtained NCO nanowires exhibit low overpotential and small Tafel slope. Specifically, NCO-0.1 shows the outstanding OER performance with a low overpotential of 297 mV at a current density of 10 mA cm^-2 and a small Tafel slope of about 68 mV dec^-1 in 1.0 M KOH. This study offers a promising electrocatalyst for the OER in water splitting.

Manganese-Modulated Cobalt-Based Layered Double Hydroxide Grown on Nickel Foam with 1D-2D-3D Heterostructure for Highly Efficient Oxygen Evolution Reaction and Urea Oxidation Reaction

Hydrogen production by energy-efficient water electrolysis is a green avenue for the development of contemporary society. However, the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR) occurring at the anode are impeded by the sluggish reaction kinetics during the water-splitting process. Consequently, it is promising to develop bifunctional anodic electrocatalysts consisting of nonprecious metals. Herein, a bifunctional CoMn layered double hydroxide (LDH) was grown on nickel foam (NF) with a 1D-2D-3D hierarchical structure for efficient OER and UOR performance in alkaline solution. Owing to the significant synergistic effect of Mn doping and heterostructure engineering, the obtained Co₁ Mn₁ LDH/NF exhibits satisfactory OER activity with a low potential of 1.515 V to attain 10 mA cm^-2 . Besides, the potential of the Co₁ Mn₁ LDH/NF catalyst for UOR at the same current density is only 1.326 V, which is much lower than those of its counterparts and most reported electrocatalysts. An urea electrolytic cell with a Co₁ Mn₁ LDH/NF anode and a Pt-C/NF cathode was established, and a low cell voltage of 1.354 V at 10 mA cm^-2 was acquired. The optimized strategy may result in promising candidates for developing a new generation of bifunctional electrocatalysts for clean energy production.

Construction of FeCo2O4@N-Doped Carbon Dots Nanoflowers as Binder Free Electrode for Reduction and Oxidation of Water

Electrochemical water splitting is known as a potential approach for sustainable energy conversion; it produces H₂ fuel by utilizing transition metal-based catalysts. We report a facile synthesis of FeCo₂O₄@carbon dots (CDs) nanoflowers supported on nickel foam through a hydrothermal technique in the absence of organic solvents and an inert environment. The synthesized material with a judicious choice of CDs shows superior performance in hydrogen and oxygen evolution reactions (HER and OER) compared to the FeCo₂O₄ electrode alone in alkaline media. For HER, the overpotential of 205 mV was able to produce current densities of up to 10 mA cm⁻², whereas an overpotential of 393 mV was needed to obtain a current density of up to 50 mA cm⁻² for OER. The synergistic effect between CDs and FeCo₂O₄ accounts for the excellent electrocatalytic activity, since CDs offer exposed active sites and subsequently promote the electrochemical reaction by enhancing the electron transfer processes. Hence, this procedure offers an effective approach for constructing metal oxide-integrated CDs as a catalytic support system to improve the performance of electrochemical water splitting.

Prudent electrochemical pretreatment to promote the OER by catalytically inert "Iron incorporated metallic Ni nanowires" synthesized via the "non-classical" growth mechanism

This study provides new insight towards the non-classical "amorphous to crystalline" growth mechanism for metal nanowire synthesis and reports an electrochemical strategy to activate inactive materials into efficient electrocatalysts for the OER. Despite considerable research on transition metal oxides/hydroxides, especially NiFe based hydroxides as OER electrocatalysts, poor conductivity of these materials plagues their catalytic efficiency. In contrast, lack of catalytic centers hinders the OER performance of conductive metals. Herein, we devised a suitable precondition strategy to transform only the surface of conductive metallic Ni nanowires into active catalytic centers. The resulting material with intimate contact between the electrically conductive core and electrocatalytically active surface showed promising "specific" and "geometric" electrocatalytic activity towards the alkaline OER at different pH. Upon iron incorporation, the Fe centers incorporated at the surface as well as in the bulk of the nanowires were found to further boost the OER activity of these materials. A one-pot strategy was adopted to produce iron free/incorporated Ni nanowires covered with nano-spikes. Growth analysis revealed a unique "non-classical amorphous-to-crystalline transformation" to be responsible for the formation of metallic nanowires.

Porosity-Engineering of MXene as a Support Material for a Highly Efficient Electrocatalyst toward Overall Water Splitting

The use of 2 D transition metal carbide MXenes as support materials to incorporate catalytically active compounds is of interest because of their unique properties. However, the preparation of well-dispersed catalytic phases on the inter-connected porous MXene network is challenging and has been rarely explored. This work focuses on the synthesis of basal-plane-porous titanium carbide MXene (ac-Ti₃ C₂ ) that is used subsequently as an effective host for the incorporation of a known catalytically active phase (IrCo) as an effective bifunctional electrocatalyst toward water splitting. The porous ac-Ti₃ C₂ with abundant macro/meso/micropores is prepared by a wet chemical method at room temperature and provides ideal anchor sites for intimate chemical bonding with alien compounds. The resulting IrCo@ac-Ti₃ C₂ electrocatalyst exhibits an excellent reactivity (220 mV at 10 mA cm^-2 ) towards the oxygen evolution reaction in 1.0 m KOH, which surpasses that of the benchmark RuO₂ , a low voltage cell of 1.57 V (@ 10 mA cm^-2 ) and good long-term durability. Our work demonstrates the effectiveness of porosity engineering in MXene nanosheets as a support material to shorten ion migration pathways, to increase electrolyte accessibility between inter-sheets and to overcome inherited re-stacking and aggregation issues.

Laser-Ablation-Produced Cobalt Nickel Phosphate with High-Valence Nickel Ions as an Active Catalyst for the Oxygen Evolution Reaction

Cost-effective, highly efficient and stable non-noble metal-based catalysts for the oxygen evolution reaction (OER) are very crucial for energy storage and conversion. Here, an amorphous cobalt nickel phosphate (CoNiPO₄ ), containing a considerable amount of high-valence Ni³⁺ species as an efficient electrocatalyst for OER in alkaline solution, is reported. The catalyst was converted from Co-doped Ni₂ P through pulsed laser ablation in liquid (PLAL) and exhibits a large specific surface area of 162.5 m²  g^-1 and a low overpotential of 238 mV at 10 mA cm^-2 with a Tafel slope of 46 mV dec^-1 , which is much lower than those of commercial RuO₂ and IrO₂ . This work demonstrates that PLAL is a powerful technology for generating amorphous CoNiPO₄ with high-valence Ni³⁺ , thus paving a new way towards highly effective OER catalysts.

In-situ degradation of polybrominated diphenyl ethers from thermal desorption off-gas over structured Fe-based/γ-Al2O3/Al plate-type catalyst

Thermal desorption was an efficient method for removal of decabromodiphenyl ether (BDE-209) from contaminated soil, but some less brominated diphenyl ethers (tri- to hepta-BDEs) with high toxicity were detected in the effluent gas. Herein, a novel anodic alumina supported Fe-based catalyst was developed and applied for in-situ degradation of gaseous polybrominated diphenyl ethers (PBDEs). The produced Fe/γ-Al₂O₃/Al catalyst was able to degrade PBDEs in the effluent gas, while a low activity with degradation efficiency of 70.1% was observed. As such, Cu was added into the Fe-based catalyst, and the effects of Cu loading on gaseous PBDEs degradation were systematically examined. A proper copper loading was found to increase the active Fe₃O₄ sites, thus improving the catalytic activity. Meanwhile, the degradation of gaseous PBDEs by Fe-based catalysts follows a pseudo-first-order model. A 90.2% PBDEs degradation efficiency was achieved at 375 °C on the optimized Fe/Cu/γ-Al₂O₃/Al catalyst, which demonstrated that the anodic alumina supported Fe and Cu was an excellent catalyst for gaseous PBDEs degradation system. Thus, this study provides a promising method and catalyst to achieve in-situ degradation of gaseous PBDEs.

Electronic structure inspired a highly robust electrocatalyst for the oxygen-evolution reaction

We demonstrated that the electronic-band structure holds the key to electrocatalytic durability towards the oxygen-evolution reaction (OER).

An Fe stabilized metallic phase of NiS2 for the highly efficient oxygen evolution reaction

This work reports a fundamental study on the relationship of the electronic structure, catalytic activity and surface reconstruction process of Fe doped NiS₂ (Fe_xNi_1-xS₂) for the oxygen evolution reaction (OER). A combined photoemission and X-ray absorption spectroscopic study reveals that Fe doping introduces more occupied Fe 3d⁶ states at the top of the valence band and thereby induces a metallic phase. Meanwhile, Fe doping also significantly increases the OER activity and results in much better stability with the optimum found for Fe_0.1Ni_0.9S₂. More importantly, we performed detailed characterization to track the evolution of the structure and composition of the catalysts after different cycles of OER testing. Our results further confirmed that the catalysts gradually transform into amorphous (oxy)hydroxides which are the actual active species for the OER. However, a fast phase transformation in NiS₂ is accompanied by a decrease of OER activity, because of the formation of a thick insulating NiOOH layer limiting electron transfer. On the other hand, Fe doping retards the process of transformation, because of a shorter Fe-S bond length (2.259 Å) than Ni-S (2.400 Å), explaining the better electrochemical stability of Fe_0.1Ni_0.9S₂. These results suggest that the formation of a thin surface layer of NiFe (oxy)hydroxide as an active OER catalyst and the remaining Fe_0.1Ni_0.9S₂ as a conductive core for fast electron transfer is the base for the high OER activity of Fe_xNi_1-xS₂. Our work provides important insight and design principle for metal chalcogenides as highly active OER catalysts.

Nanostructured Nickel Nitride with Reduced Graphene Oxide Composite Bifunctional Electrocatalysts for an Efficient Water-Urea Splitting

A three-dimensional nickel nitride with reduced graphene oxide composite on nickel foam (s-X, where s represents Ni3N/rGO@NF and the annealing temperature X can be 320, 350, or 380) electrode has been fabricated through a facile method. We demonstrate that s-350 has excellent urea oxidation reaction (UOR) activity, with a demanded potential of 1.342 V to reach 10 mA/cm2 and bears high hydrogen evolution reaction (HER) activity. It provides a low overpotential of 124 mV at 10 mA/cm2, which enables the successful construction of its two-electrode alkaline electrolyzer (s-350||s-350) for water-urea splitting. It merely requires a voltage of 1.518 V to obtain 100 mA/cm2 and is 0.145 V lower than that of pure water splitting. This noble metal-free bifunctional electrode is regarded as an inexpensive and effective water-urea electrolysis assisted hydrogen production technology, which is commercially viable.

Elemental selenium enables enhanced water oxidation electrocatalysis of NiFe layered double hydroxides

The oxygen evolution reaction (OER) is involved in various renewable energy systems, such as water-splitting, metal-air batteries and CO₂ electroreduction. Ni-Fe layered double hydroxides (LDHs) have been reported as promising OER electrocatalysts in alkaline electrolytes. Herein, we demonstrate that the introduction of elemental selenium (Se) with an optimized phase composition, i.e., monoclinic (m-) or trigonal (t-) Se, could effectively tailor the OER activity of NiFe-LDH. Compared to t-Se doped NiFe-LDH, the presence of hybrid m/t-Se could effectively tune the electronic states of Ni-O and Fe-O sites, promote the generation of OER-active γ-NiOOH, and inhibit Fe-migration during the OER process, thus enhancing the OER performance. The optimized Ni_0.8Fe_0.2-m/t-Se_0.02-LDH catalyst exhibits extraordinarily high OER activity, with an overpotential of 200 mV at 10 mA cm^-2, which is superior to those of IrO₂ and most of the reported Se-based OER catalysts. The Ni_0.8Fe_0.2-m/t-Se_0.02-LDH catalyst is further implemented as an anode for overall water splitting and demonstrates a low cell voltage of 1.50 V to achieve 10 mA cm^-2.

Shape-control of one-dimensional PtNi nanostructures as efficient electrocatalysts for alcohol electrooxidation

Bimetallic one-dimensional (1D) nanostructures such as nanowires (NWs) and nanorods (NRs), serving as high-efficiency anode electrocatalysts, have attracted extensive attention in the past decade. However, the precise design and synthesis of 1D Pt-based nanocrystals with tunable morphology and size still remain an arduous challenge. Driven by this, we report a facile yet efficient strategy for the first time to prepare PtNi ultrafine NWs (UNWs), sinuous NWs (SNWs) and ultrashort NRs (UNRs) by adjusting the amount of citric acid, ascorbic acid and glucose. Detailed analysis of their electrocatalytic properties has indicated that the as-obtained PtNi SNWs exhibit the most outstanding electrocatalytic activity toward ethylene glycol oxidation reaction (EGOR) and glycerol oxidation (GOR), 4.5 and 4.3 times higher in mass activity as well as 4.3 and 3.9 times higher in specific activity compared with the commercial Pt/C catalyst. The as-prepared PtNi SNWs are also more stable than the commercial Pt/C catalyst after successive durability tests. The proposed method provides insight into more rational designs of bimetallic nanocatalysts with 1D architectures and the as-synthesized PtNi catalysts with improved electrocatalytic performance assist in promoting the further development of direct alcohol fuel cells (DAFCs).

Aluminum-Tailored Energy Level and Morphology of Co3- x Alx O4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation

Tuning energy levels plays a crucial role in developing cost-effective, earth-abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom-occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom-engineered energy levels of spinel Co₃ O₄ for highly-effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co²⁺ ions in the tetrahedral sites and Co³⁺ ions in the octahedral sites (denoted as Co²⁺ _Td and Co³⁺ _Oh , respectively). Experimental and theoretical simulations demonstrate that Al³⁺ ions substituting Co²⁺ _Td and Co³⁺ _Oh active sites, especially Al³⁺ ions occupying the Co²⁺ _Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co_1.75 Al_1.25 O₄ nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm^-2 , among the best Co-based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al-occupied different Co sites in Co_3-x Al_x O₄ composites on OER performance, but also inspire the design of low-cost, earth-abundant, and high-active electrocatalysts toward water oxidation.

Hierarchical CoS2/Ni3S2/CoNiOx nanorods with favorable stability at 1 A cm-2 for electrocatalytic water oxidation

Herein, we have reported an easily synthesized CoS2/Ni3S2/CoNiOx water oxidation catalyst with excellent catalytic activity and superior durability. The as-prepared catalyst required overpotential (η) as low as 256 mV to exhibit a current density of 10 mA cm-2 in 1.0 M KOH. Remarkably, it sustained a current density of 1 A cm-2 for one week in 30% KOH solution with only 25 mV increment of η. Thus, it is a hopeful candidate as a highly-effective water oxidation electrode in practical applications.

Enhancement of alkaline water splitting activity by Co-P coating on a copper oxide nanowire

Hydrogen is the most attractive source of energy in the 21st century. However, high-efficiency mass production of hydrogen still faces many challenges. Although electrochemical water splitting is an ideal way to produce hydrogen, it requires low-cost and efficient electrocatalysts. In this work, a hybrid shell/core Co-P/CuO nanowire array was fabricated by Co-P film electrodeposition on a CuO nanowire array. Because of the synergy between Co-P and CuO nanowire arrays, Co-P/CuO shows remarkable activity toward the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). This bifunctional electrocatalyst achieving 20 mA cm-2 requires a cell voltage of only 1.645 V, and has superb long-term electrochemical stability and high faradaic efficiency.