Zn-Leaching Induced Rapid Self-Reconstruction of NiFe-Layered Double Hydroxides for Boosted Oxygen Evolution Reaction.

Optimizing the active centers through reconstruction is recognized as the key to construct high-performance oxygen evolution reaction (OER) catalysts. Herein, a simple and rapid in situ leaching strategy to promote the self-reconstruction of NiFe-layered double hydroxides (LDHs) catalysts is reported. The trace Zn dopants are introduced in advance by a facile and one-step hydrothermal method, followed by leaching over the electrochemical activation process, which can remarkably reduce the formation potential of NiFeOOH active centers to enable the deeper self-reconstruction for the formation of abundant highly active centers. Moreover, the self-restructured NiFeOOH-VZn cannot only significantly lower the dehydrogenation energy barrier for the transformation from Ni(OH)2 to NiOOH, but also decrease the free energy barrier of rate determining step for the *OH converted to *O through a deprotonation process, thus significantly boosting the OER behaviors. As a proof of concept, the obtained NiFeOOH-VZn catalyst just requires a low overpotential of 240 mV at 10 mA cm-2, and delivers robust stability at 50 mA cm-2 over 120 h, which outperforms the benchmark of noble metal RuO2 and those of most non-noble metal catalysts ever reported.

Freestanding 2D NiFe Metal-Organic Framework Nanosheets: Facilitating Proton Transfer via Organic Ligands for Efficient Oxygen Evolution Reaction.

The oxygen evolution reaction (OER) is crucial to electrochemical hydrogen production. However, designing and fabricating efficient electrocatalysts still remains challenging. By confinedly coordinating organic ligands with metal species in layered double hydroxides (LDHs), an innovative LDHs-assisted approach is developed to facilely synthesize freestanding bimetallic 2D metal-organic framework nanosheets (2D MOF NSs), preserving the metallic components and activities in OER. Furthermore, the research has demonstrated that the incorporation of carboxyl organic ligands coordinated with metal atoms as proton transfer mediators endow 2D MOF NSs with efficient proton transfer during the electrochemical OHads  â†’ Oads transition. These freestanding NiFe-2D MOF NSs require a small overpotential of 260 mV for a current density of 10 mA cm-2 . When this strategy is applied to LDH nanosheets grown on nickel foam, the overpotential can be reduced to 221 mV. This outstanding OER activity supports the capability of multimetallic organic frameworks for the rational design of water oxidation electrocatalysts. This strategy provides a universal path to the synthesis of 2D MOF NSs that can be used as electrocatalysts directly.

Structural Regulation of NiFe LDH under Spontaneous Corrosion to Enhance the Oxygen Evolution Properties.

Electrochemical water splitting holds promise for sustainable hydrogen production but restricted by the sluggish reaction kinetics at the anodic oxygen evolution. Herein, we present a room-temperature spontaneous corrosion strategy to convert inexpensive iron (Fe) on iron foam substrates into highly active and stable self-supporting nickel iron layered hydroxide (NiFe LDH) catalysts. The corrosion evolution mechanisms are elucidated combining ex-situ scanning electron microscopy (SEM) and X-ray photo electron spectroscopy (XPS) techniques, demonstrating precise control over the concentration of Ni2+ and reaction time to achieve controllable micro-structures of NiFe LDH. Taking advantage of the self-supporting morphology and hierarchical micro-/nano- structure, the NiFe LDH with optimized Ni2+ concentration and reaction time exhibits significant small overpotentials of 160 mV and 200 mV for the OER at current densities of 10 mA cm-2 and 100 mA cm-2 respectively, showcasing excellent OER activities. Furthermore, this catalyst demonstrates superior reaction kinetics, high electrochemical stability, and excellent integral water splitting performance when coupled with a commercial Pt/C cathode. The energy-efficient, cost-effective, and scalable spontaneous corrosion strategy opens new avenues for the development of high-electrochemical-interface catalysts.

Enhancing Electrocatalytic Water Oxidation of NiFe-LDH Nanosheets via Bismuth-Induced Electronic Structure Engineering.

The design and synthesis of high-efficiency electrocatalysts are of great practical significance in electrocatalytic water splitting, specifically in accelerating the slow oxygen evolution reaction (OER). Herein, a self-supported bismuth-doped NiFe layered double hydroxide (LDH) nanosheet array for water splitting was successfully constructed on nickel foam by a one-step hydrothermal strategy. Benefiting from the abundant active sites of two-dimensional nanosheets and electronic effect of Bi-doped NiFe LDH, the optimal Bi0.2NiFe LDH electrocatalyst exhibits excellent OER performance in basic media. It only requires an overpotential of 255 mV to drive 50 mA cm-2 and a low Tafel slope of 57.49 mV dec-1. The calculation of density functional theory (DFT) further shows that the incorporation of Bi into NiFe LDH could obviously overcome the step of H2O adsorption during OER progress. This work provides a simple and effective strategy for improving the electrocatalytic performance of NiFe LDHs, which is of great practical significance.

Pt-Quantum-Dot-Modified Sulfur-Doped NiFe Layered Double Hydroxide for High-Current-Density Alkaline Water Splitting at Industrial Temperature.

Suitable electrocatalysts for industrial water splitting can veritably promote practical hydrogen applications. Rational surface design is exceptionally significant for electrocatalysts to bridge the gap between fundamental science and industrial expectation in water splitting. Here, Pt-quantum-dot-modified sulfur-doped NiFe layered double hydroxides (Pt@S-NiFe LDHs) are designed with eximious catalytic activity toward hydrogen evolution reaction (HER) under industrial condition. Benefiting from enhanced binding energy, mass transfer, and hydrogen release, Pt@S-NiFe LDHs exhibit outstanding activity in HER at high current densities. Notably, it obtains an impressively low overpotential of 71 mV and long-term stability of 200 h at 100 mA cm-2 , exceeding commercial 40% Pt/C and most reported Pt-based electrocatalysts. Its mass activity is 2.7 times higher than that of 40% Pt/C with an overpotential of 100 mV. Furthermore, at industrial temperature (65 °C), the electrolyzer based on Pt@S-NiFe LDH needs just 1.62 V to reach the current density of 100 mA cm-2 , superior to that of the commercial one of 40% Pt/C//IrO2 . This work provides rational ideas to develop electrocatalysts with exceptional performance for industrial high-temperature water splitting at high current densities.

Synthesis of bifunctional NiFe layered double hydroxides (LDH)/Mo-doped g-C3N4 electrocatalyst for efficient methanol oxidation and seawater splitting.

To boost the oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) of pristine NiFe-layered double hydroxides (LDH), the NiFe-LDH/Mo-doped graphitic carbon nitride (NiFe-LDH/MoCN) heterojunction was synthesized herein through hydrothermal method. The establishment of built-in electric field in NiFe-LDH/MoCN heterojunction enhanced the electrochemical oxidation activities towards both seawater splitting and methanol oxidation, via the improving electrocatalyst surface wettability and conductivity. Almost 10-fold enhancement of turnover frequency (TOF) and electrochemical active surface area (ECSA) than pure NiFe-LDH implied more active sites to participate in catalytic reactions via Mo doping and the formation of heterostructure. Moreover, the local charge redistribution demonstrated in the NiFe-LDH/MoCN interface region may favor the adsorption of methanol and OH- in the seawater. The present work may expound the strong coupling interaction and the establishment of built-in electric field in the interface between NiFe-LDH and semiconductor to enhance both methanol oxidation and seawater oxidation for NiFe-LDH.

Ultrasonic assisted preparation of ultrafine Pd supported on NiFe-layered double hydroxides for p-nitrophenol degradation.

NiFe-layered double hydroxide (NiFe-LDH)-loaded ultrafine Pd nanocatalysts (Pd/NiFe-LDHs) were prepared by a facile ultrasonic-assisted in situ reduction technology without any stabilizing agents or reducing agents. Pd/NiFe-LDHs were characterized by FT-IR, XRD, XPS, and TEM. PdNPs are uniformly dispersed on NiFe-LDHs with a particle size distribution of 0.77-2.06 nm and an average particle size of 1.43 nm. Hydroxyl groups in Fe-OH and Ni-OH were dissociated into hydrogen radicals (·H) excited by ultrasound, and ·H reduced Pd2+ to ultrafine PdNPs. Then, Pd was coordinated with O in Ni-O and Fe-O, which improved the stability of the catalysts. Pd/NiFe-LDHs completely degraded 4-NP in 5 min, and the TOF value was 597.66 h-1, which was 16.7 times that of commercial Pd/C. The 4-NP conversion rate remained at 98.75% over Pd/NiFe-LDHs after 10 consecutive catalytic cycles. In addition, the catalyst also has high catalytic activity for the reduction of Congo red, methylene blue, and methyl orange by NaBH4.

Preparation of CoS2 supported flower-like NiFe layered double hydroxides nanospheres for high-performance supercapacitors.

Layered double hydroxides (LDHs) are a kind of classic pseudocapacitive materials with lamellar structure and large specific surface area, which have attracted swinging attention in the electrochemical energy storage area. The CoS2@Ni is synthesized through a hydrothermal process, followed by surface generation of the flower-like nickel-iron layered double hydroxide (NiFe-LDH) nanospheres through a hydrothermal process, which is directly used to design a binder-free electrode with a splendid capacitance capability. The as-synthesized NiFe-LDH@CoS2@Ni electrode presents an outstanding specific capacitance of 11.28 F cm-2 (3880 F g-1) at 2 mA cm-2 (1.17 A g-1) in a three electrodes system. Also, the all-solid-state asymmetric supercapacitor (ASC) is combined utilizing the NiFe-LDH@CoS2@Ni hybrid as the positive electrodes and active carbon covered Ni foam as negative electrodes, respectively. The as-fabricated ASC exhibits a high energy density of 15.84 Wh kg-1 at the power density of 375.16 W kg-1 and can be able to lighten a blue LED indicator for more than 30 min, revealing that the prepared NiFe-LDH@CoS2@Ni owns great potential in the aspect of practical applications. Therefore, the prepared NiFe-LDH@CoS2@Ni with outstanding electrochemical properties could be applied for high-performance supercapacitors.

Unassisted Water Splitting Using Standard Silicon Solar Cells Stabilized with Copper and Bifunctional NiFe Electrocatalysts.

Silicon photovoltaic cells functionalized with water-splitting electrocatalysts are promising candidates for unassisted water splitting. In these devices, the total surface of silicon solar cells is covered with electrocatalysts, causing issues with (i) stabilizing silicon solar cells in water and (ii) device efficiency due to parasitic optical absorption in electrocatalysts. We describe and validate a water-splitting device concept using a crystalline silicon solar cell where the front side is covered with an insulating Si3N5 antireflection coating. The Ag contacts, fired through the antireflection coating, are removed and subsequently substituted with NiFe layered double hydroxide (LDH) or Cu/NiFe-LDH electrocatalysts. In this device, only the site of Ag contacts, nearly 2% of the total device area, is covered by the electrocatalyst. We found that this small area of the catalyst does not limit device performance and the addition of a Cu interlayer between Si and NiFe-LDH improves device performance and stability. The unassisted water-splitting efficiency of 11.31%, measured without separating the evolved gases, is achieved using a device composed of three series-connected silicon solar cells and an NiFe-LDH/Cu/Ni-foam counter electrode in a highly alkaline electrolyte.

Effect of Mo doping and NiFe-LDH cocatalyst on PEC water oxidation efficiency.

The NiFe-layered double hydroxide (LDH) nanosheets were decorated on the surface of doped BiVO4 to structure an integrating photoanode for improving solar photoelectrochemical (PEC) water splitting efficiency, which is a dynamic research topic to solve the energy crisis and remit environmental pollution caused by fossil fuel combustion. The fabricated photoanode exhibits rapid response to visible light, enhances photocurrent density and shows significant cathodic shift compared to BiVO4. Moreover, the measured incident photon-to-current efficiency (IPCE) of the photoanode is comparable to that reported in the literature. The amount of evolution oxygen was measured and the faradaic efficiency produced oxygen was also obtained by comparing the theoretical calculation value. The enhancement is attributed to the increase of the carrier density, the effective separation of photogenerated electron-hole and consuming of the photogenerated holes accumulated at the electrode surface, which has been confirmed by electrochemical impedance spectra (EIS) and the intensity modulated photocurrent spectra (IMPS). The work may offer a promising method for designing a high efficiency and low-cost photoanode.

Layered Structure Causes Bulk NiFe Layered Double Hydroxide Unstable in Alkaline Oxygen Evolution Reaction.

NiFe-based layered double hydroxides (LDHs) are among the most efficient oxygen evolution reaction (OER) catalysts in alkaline medium, but their long-term OER stabilities are questionable. In this work, it is demonstrated that the layered structure makes bulk NiFe LDH intrinsically not stable in OER and the deactivation mechanism of NiFe LDH in OER is further revealed. Both operando electrochemical and structural characterizations show that the interlayer basal plane in bulk NiFe LDH contributes to the OER activity, and the slow diffusion of proton acceptors (e.g., OH- ) within the NiFe LDH interlayers during OER causes dissolution of NiFe LDH and therefore decrease in OER activity with time. To improve diffusion of proton acceptors, it is proposed to delaminate NiFe LDH into atomically thin nanosheets, which is able to effectively improve OER stability of NiFe LDH especially at industrial operating conditions such as elevated operating temperatures (e.g., at 80 °C) and large current densities (e.g., at 500 mA cm-2 ).