Ultrathin NiFe Research Articles

Tuning built-in potential of NiP/NiFe LDH p-n junction towards efficient electrocatalytic water and urea oxidation

Oxygen evolution reaction (OER) and urea oxidation reaction (UOR) are critical half-reactions to realize sustainable hydrogen production. However, the sluggish dynamics derived from the multi-electron pathways necessitate the exploration for efficient catalysts. Herein, a p-n junction nanorod array composed of Mo doped NiP nanorod and ultrathin NiFe LDH nanosheets (NiMoP/NiFe LDH) is proposed to regulate charge distribution of interface for triggering decomposition of water and urea. The p-n junction with a powerful built-in potential (EBI) of 1.2 V at the interface facilitates the adsorption and fracture of chemical group in water and urea molecules. Consequently, the heterostructured electrode demonstrates exceptional electrochemical performance with ultralow potentials of 1.43 and 1.33 V (vs. RHE) at 10 mA cm−2 for OER and UOR, respectively. The overall urea electrolysis driven by NiMoP/NiFe LDH needs only 1.51 V to deliver 100 mA cm−2. This work demonstrates a promising strategy to regulate the EBI of semiconductor heterostructure for energy conversion and sewage treatment.

Ultrathin Ni-Fe MOF Nanosheets: Efficient and Durable Water Oxidation at High Current Densities.

Efficient, durable, and economical electrocatalysts are crucial for advancing energy technology by facilitating the oxygen evolution reaction (OER). Here, ultrathin Ni-Fe metal-organic skeleton (MOF) nanosheets were created in situ on nickel foam (NiFe-UMNs/NF). The catalyst exhibited excellent OER catalytical abilities, with only 269 mV overpotentials at 250 mA cm-2. Besides, when integrated with Pt/C/NF, NiFe-UMNs/NF held the potential for application in industrial alkaline water electrolysis with an initial voltage retention of approximately 86% following a continuous operation of 100 h at a current density of 250 mA cm-2. The super performance of the NiFe-UMNs/NF catalyst was attributed to ultrathin morphology, super hydrophilicity, and synergistic effects between Ni and Fe within the MOF. In situ Raman showed that NiFe-UMNs were converted to NiFeOOH as the active species in the OER process. Density functional theory revealed that iron doping accelerated the rate-determining step and reduced the OER reaction energy barrier. This work elucidated a promising electrocatalyst for OER and enriched the practical implementation of MOF materials.

Short-time potentiostatic assisted borate to induce the generation of ultrathin NiFe LDH active phase for industrial-level water oxidation

Self-supported NiFe-based oxygen evolution reaction (OER) catalysts with high activity and durability are essential for the industrialization of green hydrogen. Herein, Boron-interfered NiFe LDH nanosheets (B2-NiFe-(a-10)) with abundant wrinkle structure are prepared by introducing nonmetallic sources into the surface reconstruction of layered double hydroxide (NiFe LDH) through a novel short-time potential constant activation strategy. It enhances interactions with the electrolyte, decreases Arrhenius activation energy (Ea), accelerates efficient charge transfer and O2 escape rate. The as-reconstructed B2-NiFe-(a-10) exhibits outstanding alkaline OER activity with an ultralow overpotential of 290 mV at 100 mA cm−2 and excellent durability (100 h). Additionally, density functional theory calculations further reveal that the interaction optimizes the adsorption of oxygenated intermediates (*OH→*O), induces the center of the d-band toward the Fermi energy level, and lowers the dissociation barrier of H2O. Moreover, the anion-exchange membrane water electrolyzer (AEMWE) achieves a current density of 1 A cm−2 at 1.97 V.

Amorphous-crystalline interface coupling induced highly active ultrathin NiFe oxy-hydroxide design towards accelerated alkaline oxygen evolution

Amorphous NiFe-based oxyhydroxides can provide abundant active sites for oxygen evolution reaction (OER) in alkaline water splitting, but their disordered atomic arrangement leads to severe charge accumulation and electrochemical polarization at high current density. To address this issue, a crystalline Fe2O3 supported ultrathin amorphous NiFe oxy-hydroxide amorphous-crystalline interface coupling design is proposed. In this design, the interfacial Fe3+ in Fe2O3 as a strong Lewis acid is conducive for trapping the electrons in amorphous NiFe(OH)x, reducing the surface charge accumulation, electrochemical polarization, and improves its OER performance. Meanwhile, in-situ Raman, in-situ IR and XPS analysis reveal that, the electrons transfer is accompanied with Ni2+ oxidation into the highly OER active Ni3+ species. Benefited from this design, the OER overpotentials of the resulting NiFe(OH)x/Fe2O3NAs catalyst are measured to be 111 and 233 mV at 10 and 100 mA cm−2, respectively, superior than many reported OER electrocatalysts. The anion exchange membrane water electrolyzer using NiFe(OH)x/Fe2O3NAs delivers a current density of 600 mA cm−2 at a small cell voltage of 1.75 V, and exhibits excellent stability performance in high-current–density condition. This work provides a promising alternative strategy for the development of high-performance amorphous OER catalysts.

Ultrathin 2D–2D NiFe LDH/MOF heterojunction nanosheets: an efficient oxygen evolution reaction catalyst for water oxidation

An ultrathin NiFe LDH/MOF catalyst is grown on carbon paper by an in situ self-dissociation–assembly strategy. Studies reveal that the strong electronic interaction between Fe and Ni species with rich active sites results in good OER and OWS performance.

Phosphorylated Co(OH)2@NiFe-LDH core–shell heterostructure for highly efficient alkaline water splitting

The development of transition metal bifunctional electrocatalysts with high activity, stability and cost-effectiveness is necessary for the overall water decomposition. In this work, a phosphorylated core–shell heterostructure bifunctional catalytic material with a nickel foam carrier, a Co(OH)2 nanowire array core and an ultrathin NiFe layered double hydroxide (NiFe-LDH) shell is synthesized. The solid contact between the NiFe-LDH shell and the highly conductive Co(OH)2 core brings about excellent electrical connectivity and an integrated hierarchical nanostructure that greatly improves the active specific surface area. Meanwhile, the phosphorylated hierarchical core–shell nanostructure enables the presence of a large amount of amorphous material at the edges, which leads to excellent activity and stability for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER).

Ultrathin Fe2O3 shell-encapsulated NiFe alloy nanoparticles embedded in tubular carbon matrix for enhanced oxygen evolution reaction

The exploitation of efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is the key to the industrialization of the hydrogen production technology from water splitting. Herein, Ni1Fe2@Fe2O3 composites with core-shell structure embedded in Metaplexis japonica fluff-derived carbon microtube matrix (Ni1Fe2@Fe2O3@C) are prepared by nitrate soaking and subsequent high-temperature calcination, which are used to catalyze the OER. The existence of carbon matrix greatly prevents the agglomeration of metal particles and creates an efficient electron transfer path for OER. The synergy among the carbon matrix, Ni1Fe2 core and Fe2O3 shell together enhances the OER performance, thus endowing Ni1Fe2@Fe2O3@C with the considerable catalytic activity with the overpotential of 271 mV at 10 mA cm−2 and a Tafel slope of 78 mV dec−1. Ni1Fe2@Fe2O3@C also shows good stability due to the double protection of metal particles by biomass carbon matrix and ultrathin Fe2O3 shell. The water splitting device composed of Pt/C as the cathode and Ni1Fe2@Fe2O3@C as the anode exhibits water splitting activity close to that of Pt/C || RuO2 system in alkaline medium, and the stability retention rate of the system reaches to 93 % after 30 h electrolysis.

Inducing the SnO2-based electron transport layer into NiFe LDH/NF as efficient catalyst for OER and methanol oxidation reaction

• Introduce the electron transport layer SnO 2 between NiFe LDH and conductive substrate NF was first reported. • SnO 2 nanosheets are interwoven to form a sponge-like 3D structure, which promotes the formation of relatively smaller and thinner NiFe LDH nanosheets, thereby generating more active sites and reducing electron transmission resistance. • It was verified that the electronic structure of Ni and Fe was optimizing via the introduction of SnO 2 . • NiFe LDH@SnO 2 /NF possesses a low overpotential of 234 mV at 10 mA cm −2 for OER and 1.396 V at 10 mA cm −2 for methanol oxidation reaction. In an electrocatalyst with a heterointerface structure, the different interfaces can efficiently adjust the catalyst's conductivity and electron arrangement, thereby enhancing the activity of the electrocatalyst. Ultrathin and smaller NiFe LDH was successfully constructed on the surface of SnO 2 nanosheet supported NF by layer by layer assembly, and exhibits lower overpotential of 234 mV at a current density of 10 mA cm −2 , which only increases by 6.4% even at a high current density of 100 mA cm −2 . The excellent OER activity of catalyst is attributed to the contribution of the semiconductor SnO 2 electron transport layer. Through experiments and characterization, 3d structure SnO 2 nanosheets control the growth of ultra-thin nickel-iron, the hierarchical interface between SnO 2 and NiFe LDH can change the electron arrangement around the iron and nickel active centers at the interface, resulting the valence states of iron slightly increased and Ni 3+ content increased. The result will promote the oxidation of water. Meanwhile, the SnO 2 semiconductor as electron transport layer is conducive to trapping electrons generated in oxidation reaction, promoting electrons transferring from the NiFe LDH active center to the Ni substrate more quickly, and enhance the activity of NiFe LDH. It also shows excellent activity in an electrolyte solution containing 0.5 M methanol and 1 M KOH, and only 1.396 V (vs. RHE) is required to drive a current density of 10 mA cm −2 .

In situ/operando structural tracking of Ni-Fe LDH

Deciphering <i>in situ</i> structure of layer double hydroxides (LDHs) and their corresponding mechanism of oxygen evolution reaction (OER) is essential to further development of LDH-based OER catalysts. In this issue of <i>Chem Catalysis</i>, Ko et al. demonstrated a scalable synthesis of ultrathin Ni-Fe LDH nanostructure, its chemical oxidation process, and its catalytic mechanism by <i>in situ/operando</i> spectroscopies, paving a solid pathway to potential practical application of LDH-based catalysts.

Ultrathin binary MOF nanozyme with boosted activity via introduction of active iron sites for detecting sulfide ion

Convenient and reliable sulfide ion (S2-) detection is necessary due to its harmfulness. Herein, a new colorimetric sensing platform for S2- was established based on the binary NiFe metal-organic framework (MOF) nanozyme. The tremella-like NiFe MOF with ultrathin nanosheets was synthesized via introduction of active Fe sites in the Ni MOF by a rapid microwave-assisted method just in 11 min. The ultrathin NiFe MOF exhibited 2.8-fold higher peroxidase-like activity and 42.5-fold higher affinity to H2O2 than the Ni MOF, which could efficiently catalyze the decomposition of H2O2 into hydroxyl radicals (•OH) and superoxide anion radicals (O2•−) for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB). The DFT study reveals that the dissociation energies of H2O2 and adsorption energies of reactive oxygen intermediates on Fe sites are more negative than those on Ni sites, proving the crucial role of Fe sites in the elevated peroxidase-like activity of NiFe MOF. By taking advantage of the unique redox reaction between H2O2 and S2-, a simple and sensitive colorimetric method for S2- was proposed based on TMB/H2O2/NiFe MOF system with a wide linear range of 0.5 – 60 μM and a low detection limit of 28 nM (3σ). The proposed NiFe MOF-based colorimetric method exhibited good selectivity to S2- among some common cations and anions, and it was successfully applied in the real water samples.

Suppressing Magnetic Damping Related to Two-Magnon Scattering in Ultrathin NiFe Films by Interface Engineering

Designing the ferromagnetic/nonmagnetic (FM/NM) heterostructure with desirable low magnetic damping is important for spintronic devices to reduce the critical current density of magnetization switching. For ultrathin FM/NM films, magnetic damping from the two-magnon scattering (TMS) effect is a critical component, and suppressing TMS damping becomes one of the crucial ways to achieve low damping. In this paper, we have demonstrated the depressed linewidth in Ta (1 nm)/NiFe (3 nm)/NM (4 nm)/Ta (1.5 nm) heterostructure with Cu instead of Pt as the NM layer, where the maximal linewidth of ferromagnetic resonance (FMR) decreases from 440 to 190 Oe. And the decrease of TMS damping from 55 to 0 Oe is one of the main reasons for the lower FMR linewidth. The nonzero surface magnetic anisotropy implies that the suppression of interfacial defect scattering causes reduced TMS damping. Meanwhile, a noticeable enhancement of TMS damping from the lack of seed layer may be due to the lattice structure change. Thereby, our findings provide a way toward the design of low-damping spintronic devices.

Operando High‐Valence Cr‐Modified NiFe Hydroxides for Water Oxidation

High-valence metal-doped multimetal (oxy)hydroxides outperform noble metal electrocatalysts for the oxygen evolution reaction (OER) owing to the modified energetics between 3d metals and high-valence dopants. However, the rational design of sufficient and subtle modulators is still challenging. With a multimetal layered double hydroxide (LDH) as the OER catalyst, this study introduces a series of operando high-valence dopants (Cr, Ru, Ce, and V), which can restrict the 3+ valence states in the LDH template to prevent phase separation and operando transfer to the >3+valence states for sufficient electronic interaction during the OER process. Through density functional theory simulations, ultrathin Cr-doped NiFe (NiFeCr) LDH is synthesized with strong electronic interaction between Cr dopants and NiFe bimetallic sites, evidenced by X-ray absorption spectroscopy. The resulting NiFeCr-LDH catalyzes the OER with ultralow overpotentials of 189 and 284mV, obtaining current densities of 10 and 1000mA cm-2 , respectively. Further, a NiFeCr-LDH anode is coupled in the anion exchange membrane electrolyzers to promote alkaline water splitting and CO2 -to-CO electrolysis, which achieves low full cell voltages at high current densities.

Spatially confined growth of ultrathin NiFe layered double hydroxide nanosheets within carbon nanofibers network for highly efficient water oxidation

The rational design of catalysts with low cost, high efficient and robust stability toward oxygen evolution reaction (OER) is greatly desired but remains a formidable challenge. In this work, a one-pot, spatially confined strategy was reported to fabricate ultrathin NiFe layered double hydroxide (NiFe-LDH) nanosheets interconnected by ultrafine, strong carbon nanofibers (CNFs) network. The as-fabricated NiFe-LDH/CNFs catalyst exhibits enhanced OER catalytic activity in terms of low overpotential of 230 mV to obtain an OER current density of 10 mA cm−2 and very small Tafel slope of 34 mV dec−1, outperforming pure NiFe-LDH nanosheets assembly, commercial RuO2, and most non-noble metal catalysts ever reported. It also delivers an excellent structural and electrocatalytic stability upon the long-term OER operation at a large current of 30 mA cm−2 for 40 h. Furthermore, the cell assembled by using NiFe-LDH/CNFs and commercial Pt/C as anode (+) and cathode (−) ((+)NiFe-LDH/CNFs||Pt/C(−)) only requires a potential of 1.50 V to deliver the water splitting current of 10 mA cm−2, 130 mV lower than that of (+)RuO2||Pt/C(−) couple, demonstrating great potential for applications in cost-efficient water splitting devices.

Thickness-dependence of magnetic damping related to two-magnon scattering in ultrathin Ni0.81Fe0.19 films

Magnetic damping is of critical importance for devices due to its influences on magnetization dynamics. When the ferromagnetic layer goes into the nanometer scale, the magnetic damping related to the two-magnon scattering (TMS) effect becomes the main damping in nonmagnetic/ferromagnetic films. Here, we experimentally demonstrated the thickness-dependence of TMS damping, one part dependent on ferromagnetic thickness (), another part independent, in both Cu/NiFe and Pt/NiFe films. The scale of TMS damping can be explained based on the ‘Arias–Mills’ mode, and its origin is the distribution of surface magnetic anisotropy due to interface defects. The rest of TMS damping is independent on is attributed to the more generic defects creating inhomogeneity, and its origin can be sorted into the ‘McMichael–Krivosik’ mechanism. The quantitative results reveal that the -dependent and -independent TMS damping are of equal importance in ultrathin NiFe films. Meanwhile, the approximately equal intrinsic damping was displayed for Cu/NiFe and Pt/NiFe thin films. Our results add a clearer understanding of TMS damping in ultrathin ferromagnetic films.

Amorphous–crystalline FeNi2S4@NiFe–LDH nanograsses with molten salt as an industrially promising electrocatalyst for oxygen evolution

The hydrolysis of S ion FeNi2S4 can promote the in-situ formation of ultrathin amorphous NiFe–LDH grown in situ on crystalline FeNi2S4, which can not only improve activity but also significantly enhance stability for oxygen evolution reaction.

Ultrathin MOF Coupling with Molecular Cobaloxime to Construct an Efficient Hybrid Hematite Photoanode for Photocatalytic Water Splitting

It is a great challenge to develop efficient artificial photosynthesis systems by mimicking natural photosynthesis. In this present work, an organo-inorganic hybrid photoanode was fabricated by depositing ultrathin NiFe MOF nanolayers and molecular cobaloxime cocatalyst on the surface of Ti-doped porous hematite (Ti-PH). This hybrid photoanode exhibited a high photocurrent density of 2.45 mA/cm2 at 1.23 V vs the reversible hydrogen electrode (RHE) under AM 1.5 G illumination with excellent stability. Moreover, the optimal incident photon-to-current efficiency (IPCE) of the hybrid photoanode reached 83.0% at 365 nm and the surface charge injection efficiency (ηinj) was improved to 87.5% at 1.23 V vs RHE. Detailed investigations reveal that NiFe MOFs can effectively facilitate charge separation as a hole-transport layer and molecular cobaloxime cocatalyst can passivate surface trap states to increase water oxidation kinetics.

Rich oxygen vacancies on ultrathin NiFe layered double hydroxide nanosheets raised by cerium-assisted synthesis for enhanced electrocatalytic water oxidation

Ni-Fe Layered double hydroxides (NiFe LDHs) are a promising electrocatalyst for oxygen evolution reaction (OER) under alkaline environments. However, the bulk NiFe LDHs with less exposed active sites limit the improvement of the electrocatalytic activity. Herein, an oxygen vacancy (O vac ) -enriched surface for NiFe LDHs on nickel foam (Ce asst-NiFe LDHs/NF) was obtained by cerium (III) ion as an assistant reagent via a hydrothermal approach. Note that Ce 3+ ions were verified without residue at the as-resulted Ce asst-NiFe LDHs/NF after the sediment centrifuging. Ce 3+ was here readily adsorbed on the surface of NiFe LDHs during the synthetic process, reducing their thickness (~6 nm). In addition, Ce 3+ with a lower electronegativity competes with Ni 2+ and Fe 3+ , inducing increase for O vac near metal active sites on the surface of NiFe LDHs. The Ce asst-NiFe LDHs/NF electrocatalyst with enriched O vac has exhibited extremely electrocatalytic activity for OER at the low potential with only 246 mV to achieve 200 mA cm -2 in 1 M KOH. The favorable stability of 18 h at the current density of 50 mA cm -2 increases with only ~0.2% potential. Our research paves the way to designing highly active surface of NiFe LDHs OER catalysts. • O vac near Ni 2+ on NiFe LDHs are induced by adding Ce 3+ in the synthetic process. • Ce 3+ competes O 2- with Ni 2+ /Fe 3+ due to three metals’ different electronegative. • The thickness (5 nm) of ultrathin nanosheets is reduced due to Ce 3+ adsorbing. • The electrocatalyst achieves 200 mA cm -2 at the low potential with 246 mV for OER. • The favorable stability of 18 h at 50 mA cm -2 with only ≈0.2% potential increase.

Electrodeposited of ultrathin VOx-doped NiFe layer on porous NiCo phosphide for efficient overall water splitting

An ultrathin VOx-doped NiFe alloy layer (VOx@NiFe) was electrodeposited on the surface of nickel-cobalt phosphide (NiCoP) supported by titanium mesh (TM) to achieve a highly active and low-cost bifunctional electrocatalyst for overall water splitting. VOx doping not only significantly enhances the electrochemical surface areas and the concentration of oxygen vacancies in the NiFe/NiCoP/TM composite, but also greatly improves the surface hydrophilicity. The VOx@NiFe/NiCoP/TM electrode retains a 3D porous structure of NiCoP/TM and exhibits superior electrocatalytic activity for water electrolysis. It requires the overpotentials of merely 45 and 215 mV to deliver the current density of 10 mA cm−2 in 1.0 M KOH for HER and OER, respectively. Meanwhile, the VOx@NiFe/NiCoP/TM electrode also displays robustness and outstanding durability in alkaline medium. Serving as both anode and cathode electrocatalysts in a two-electrode water electrolytic cell, the bifunctional VOx@NiFe/NiCoP/TM electrodes only require a potential of 1.52 and 1.65 V to deliver the current densities of 10 and 50 mA cm−2 with excellent durability. This work provides a design of multilevel porous structure of bifunctional electrocatalysts for energy storage and conversion.

Facet Engineering in Ultrathin Two-Dimensional NiFe Metal–Organic Frameworks by Coordination Modulation for Enhanced Electrocatalytic Water Oxidation

Crystal facet engineering has been proved as a dramatically efficient strategy to optimize the intrinsic catalytic property of catalysts. However, because of the difficulty in precise construction, facet engineering of two-dimensional metal–organic frameworks (2D MOFs) is rarely reported. Herein, we propose a new and facile coordination modulation method to realize the controllable synthesis of 2D NiFe-MOFs with a highly exposed (001) crystal facet. Specifically, acetate ions, which possess the same coordination groups with the participated terephthalic ligand, have been adopted into the MOF construction as the morphology and facet regulator. Through competitive coordination effect and ion-induced self-assembly, the fabricated NiFe-MOFs reveal a hierarchical microsphere architecture consisting of ultrathin nanosheets with thickness ∼5 nm. Further benefiting from the structural advantages, e.g., abundant active sites, highly exposed catalytic facet, and enhanced mass/charge transport, the hierarchical NiFe-MOF nanosheets exhibit an attractive overpotential of 240 mV at the current density of 10 mA cm–2, a small Tafel slope of 73.44 mV dec–1, and satisfactory stability for 16 h toward oxygen evolution reaction (OER). Besides, the increased exposure of the (001) facet will increase the proportion of high-valent nickel oxyhydroxides during OER, which will enhance the electrocatalytic performance. The universality of the modulation method is further verified by the successful synthesis of a series of 2D MOFs (Ni, Co, Cu, Zn, and corresponding bimetal MOFs). This method may serve as an important inspiration for the preparation and crystal facet engineering of 2D MOFs.

Constructing Ultrathin W-Doped NiFe Nanosheets via Facile Electrosynthesis as Bifunctional Electrocatalysts for Efficient Water Splitting.

Exploring cost-effective and efficient bifunctional electrocatalysts via simple fabrication strategies is strongly desired for practical water splitting. Herein, an easy and fast one-step electrodeposition process is developed to fabricate W-doped NiFe (NiFeW)-layered double hydroxides with ultrathin nanosheet features at room temperature and ambient pressure as bifunctional catalysts for water splitting. Notably, the NiFeW nanosheets require overpotentials of only 239 and 115 mV for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively, to reach a current density of 10 mA/cm2 in alkaline media. Their exceptional performance is further demonstrated in a full electrolyzer configuration with the NiFeW as both anode and cathode catalysts, which achieves a low cell voltage of 1.59 V at 10 mA/cm2, 110 mV lower than that of the commercial IrO2 (anode) and Pt (cathode) catalysts. Moreover, the NiFeW nanosheets are superior to various recently reported bifunctional electrocatalysts. Such remarkable performances mainly ascribe to W doping, which not only effectively modulates the electrocatalyst morphology but also engineers the electronic structure of NiFe hydroxides to boost charge-transfer kinetics for both the OER and HER. Hence, the ultrathin NiFeW nanosheets with an efficient fabrication strategy are promising as bifunctional electrodes for alkaline water electrolyzers.