Amorphous NiFe-layered double hydroxides nanosheets for oxygen evolution reaction

Cobalt phosphide nanoarrays on a borate-modified nickel foam substrate as an efficient dual-electrocatalyst for overall water splitting

Developing efficient non-noble metal dual-functional electrocatalysts for overall water splitting is essential for the production of green hydrogen. Given the significant advantages of self-supporting electrodes, regulating the growth of self-supporting nanoarrays on a conductive substrate is conducive to improving the electrocatalytic activity. In this work, aligned cobalt phosphide (CoP) nanowire arrays grown on borate-modified Ni foam substrate (CoP/R-NF) were utilized as a bifunctional electrocatalyst for both hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) in alkaline solution. The borate interfacial layer regulated the growth behavior of CoP nanowires, promoting a tip-enhanced electric field effect, facilitating an enhanced bimetallic synergistic effect. The CoP/R-NF electrode showed substantial catalytic activity for HER (η10 = 35 mV, 70 mV dec-1) and OER (241 mV, 32 mV dec-1). Moreover, a low cell voltage of 1.50 V to drive 10 mA cm-2 current density for overall water-splitting was achieved in an alkaline water electrolyzer, with long-term durability of 200 h at 100 mA cm-2, indicating the potential application of CoP/R-NF as a bifunctional catalyst for clean and renewable energy utilization. Such a synthetic strategy could pave the way for the development of non-noble bifunctional electrocatalysts for comprehensive water splitting.

Structural Changes of NiFe Layered Double Hydroxides During the Oxygen Evolution Reaction: A Diffraction and Total Scattering Operando Study

NiFe-layered double hydroxides (LDHs) are promising electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. Here, operando X-ray diffraction (XRD) and X-ray total scattering are used with Pair Distribution Function (PDF) analysis to investigate the atomic structure of the catalytically active material and follow structural changes under operating conditions. XRD shows an interlayer contraction under applied oxidative potential, which relates to a transition from the α-LDH to the γ-LDH phase. The phase transition is reversible, and the α-LDH structure is recovered at 1.3 VRHE. However, PDF analysis shows an irreversible increase in the stacking disorder under operating conditions, along with a decrease in the LDH sheet size. The analysis thus shows that the operating conditions induce a breakdown of the particles leading to a decrease in crystallite size.

Long-term Durability of Seawater Electrolysis for Hydrogen: From Catalysts to Systems

Direct electrochemical seawater splitting is a renewable, scalable, and potentially economic approach for green hydrogen production in environments where ultra-pure water is not readily available. However, issues related to low durability caused by complex ions in seawater pose great challenges for its industrialization. In this review, a mechanistic analysis of durability issues of electrolytic seawater splitting is discussed. We critically analyze the development of seawater electrolysis and identify the durability challenges at both the anode and cathode. Particular emphasis is given to elucidating rational strategies for designing electrocatalysts/electrodes/interfaces with long lifetimes in realistic seawater including inducing passivating anion layers, preferential OH-adsorption, employing anti-corrosion materials, fabricating protective layers, immobilizing Cl- on the surface of electrocatalysts, tailoring Cl- adsorption sites, inhibition of OH- binding to Mg2+ and Ca2+, inhibition of Mg and Ca hydroxide precipitation adherence, and co-electrosynthesis of nano-sized Mg hydroxides. Synthesis methods of electrocatalysts/electrodes and innovations in electrolyzer are also discussed. Furthermore, the prospects for developing seawater splitting technologies for clean hydrogen generation are summarized. We found that researchers have rethought the role of Cl- ions, as well as more attention to cathodic reaction and electrolyzers, which is conducive to accelerate the commercialization of seawater electrolysis.

Stability of Ni-Fe-Layered Double Hydroxide Under Long-Term Operation in AEM Water Electrolysis

Anion exchange membrane water electrolysis (AEMWE) is an attractive method for green hydrogen production. It allows the use of non-platinum group metal catalysts and can achieve performance comparable to proton exchange membrane water electrolyzers due to recent technological advances. While current systems already show high performances with available materials, research gaps remain in understanding electrode durability and degradation behavior. In this study, the performance and degradation tracking of a Ni3Fe-LDH-based single-cell is implemented and investigated through the correlation of electrochemical data using chemical and physical characterization methods. A performance stability of 1000 h, with a degradation rate of 84 µV h-1 at 1 A cm-2 is achieved, presenting the Ni3Fe-LDH-based cell as a stable and cost-attractive AEMWE system. The results show that the conductivity of the formed Ni-Fe-phase is one key to obtaining high electrolyzer performance and that, despite Fe leaching, change in anion-conducting binder compound, and morphological changes inside the catalyst bulk, the Ni3Fe-LDH-based single-cells demonstrate high performance and durability. The work reveals the importance of longer stability tests and presents a holistic approach of electrochemical tracking and post-mortem analysis that offers a guideline for investigating electrode degradation behavior over extended measurement periods.

Two-Dimensional SnS Mediates NiFe-LDH-Layered Electrocatalyst toward Boosting OER Activity for Water Splitting

NiFe-layered double hydroxides (NiFe-LDHs), as promising electrocatalysts, have received significant research attention for hydrogen and oxygen generation through water splitting. However, the slow oxidation kinetics of NiFe-LDH, due to the limited number of active sites and the low conductivity, hinders the improvement of the water-splitting efficiency. Therefore, to overcome the obstacles, two-dimensional (2D) SnS was first explored to tailor the prepared NiFe-LDH via the hydrothermal method. A NiFe-LDH/SnS heterojunction is built, which is observed from the microstructural investigations. SnS incorporation could greatly improve the conductivity of the NiFe-LDH sheets, which was reflected by the reduced charge transfer resistance. Moreover, SnS layers modulated the electronic environment around the active sites, favoring the adsorption of intermediates during the oxygen evolution reaction (OER) process, which was verified by density functional theory calculations. A synergistic effect induced by the NiFe-LDH/SnS heterostructure promoted the OER activities in electrical, electronic, and energetic aspects. Consequently, the as-prepared NiFe-LDH/SnS electrocatalyst greatly improved the electrocatalytic performance, exhibiting 20% and 27% reductions in the overpotential and Tafel slope compared with those of pristine NiFe-LDH, respectively. The results provide a strategy for regulating NiFe-based electrocatalysts by using emerging 2D materials to enhance water-splitting efficiency.

Synthesis of NiFe-layered double hydroxides using triethanolamine-complexed precursors as oxygen evolution reaction catalysts: effects of Fe valence

The synthesis of highly efficient NiFe-layered double hydroxides (NiFe-LDHs) to catalyze the oxygen evolution reaction (OER) is urgent and challenging. Herein, NiFe-FeCl3-x and NiFe-FeCl2-x samples (where FeCl3 and FeCl2 represent the Fe sources and x represents the imposed reaction time: 6, 12, and 24 h) were prepared via one-pot hydrothermal synthesis using Fe sources characterized by Fe(III) or Fe(II) valence states. In the presence of triethanolamine, when FeCl3 was used as the Fe source, pure NiFe-LDH was obtained, whose crystallinity increased with increasing hydrothermal treatment time. In contrast, when FeCl2 was used as the Fe source, a mixture of NiFe-LDH, Fe2O3, and trace amounts of Fe3O4 was obtained. The content of NiFe-LDH in the mixture increased under longer hydrothermal treatment and NiFe-FeCl3-x catalysts exhibited better OER performance than NiFe-FeCl2-x catalysts. Specifically, NiFe-FeCl3-6 afforded the highest OER performance with an overpotential of 246.8 mV at 10 mA cm-2 and a Tafel slope of 46.1 mV dec-1. Herein, we investigated the effects of the valence state of Fe precursors on the structures and OER activities of the prepared catalysts; the mechanism of NiFe-LDH formation via hydrothermal synthesis in the presence of triethanolamine was also proposed.

Boosting Oxygen Evolution Performance of Nickel-Iron Layered Double Hydroxides by Controlling Oxygen Vacancies and Structural Disorder via n-Butyllithium Treatment

Nickel-iron-based layered double hydroxides (NiFe-LDHs) are promising catalysts for the oxygen evolution reaction (OER) because of their high activity, availability, and low cost. Defect engineering, particularly the formation of oxygen vacancies, can improve the catalytic activity of NiFe-LDHs. However, the controllable introduction of uniform oxygen vacancies remains challenging. Herein, an n-butyllithium treatment method is developed to tune oxygen vacancy defects and change the degree of amorphization in NiFe-LDHs via deep reduction, followed by partial oxidization at low temperatures. Interestingly, the Ni in the NiFe-LDHs is selectively reduced to the alloy state by n-butyllithium, whereas Fe is not. The different structural transformations of Ni and Fe during the treatment successfully produce an oxygen-defect-rich amorphous/crystalline electrocatalyst. Under optimal conditions, the treated NiFe-LDHs exhibit high OER activity with an overpotential of 223 mV at 10 mA cm-2 (68 mV lower than that of a commercial IrO2 electrocatalyst) and long-term stability. Notably, the n-butyllithium treatment can be applied to other electrocatalysts, such as CoFe-LDHs and IrO2 (treated IrO2 with an overpotential of 197 mV at 10 mA cm-2). This n-butyllithium reduction/partial oxidization treatment constitutes a novel top-down strategy for the controllable modification of metal oxide structures, with various energy-related applications.

Layered double hydroxide nanosheets-built porous film-covered Au nanoarrays as enrichment and enhancement chips for efficient SERS detection of trace styrene

Styrene, a prevalent volatile organic compounds (VOCs), is very harmful to atmosphere and humans. Consequently, the development of efficient detection technologies for styrene is of high importance, which is still in challenge! In this work, we crafted a layered double hydroxide (LDH) porous film-coated gold nanoarray, designed to act as a surface enhanced Raman spectroscopy (SERS) chip for the efficient and portable detection of gaseous styrene. This chip features a covering layer composed of cross-linked LDH nanosheets, notable for their porous structure and high specific surface area. When the covering layer is 100-300 nm in thickness, this composite chip has significant enrichment effect and strong SERS performance to gaseous styrene with a lowest detectable concentration below 1 ppb (4.64 ×10-3 mg/m3), and can response within 10 s, showing the rapid response and high sensitivity. Additionally, the chip has strong anti-interference capabilities and maintains excellent response to styrene, even in mixed benzene-VOCs gases. The exceptional SERS performances of this chip is ascribed to its LDH covering layer-induced styrene-enrichment and structurally-enhanced SERS performances. This study provides a simple route and practical chip for the rapid and ultrasensitive SERS-based detection of gaseous styrene, which is also potentially beneficial for the detection of other gaseous VOCs.

Dual Electronic Modulations on NiFeV Hydroxide@FeOx Boost Electrochemical Overall Water Splitting

Nickel-iron based hydroxides have been proven to be excellent oxygen evolution reaction (OER) electrocatalysts, whereas they are inactive toward hydrogen evolution reaction (HER), which severely limits their large-scale applications in electrochemical water splitting. Herein, a heterostructure consisted of NiFeV hydroxide and iron oxide supported on iron foam (NiFeV@FeOx /IF) has been designed as a highly efficient bifunctional (OER and HER) electrocatalyst. The V doping and intimate contact between NiFeV hydroxide and FeOx not only improve the entire electrical conductivity of the catalyst but also afford more high-valence Ni which serves as active sites for OER. Meanwhile, the introduction of V and FeOx reduces the electron density on lattice oxygen, which greatly facilitates desorption of Hads . All of these endow the NiFeV@FeOx /IF with exceptionally low overpotentials of 218 and 105 mV to achieve a current density of 100 mA cm-2 for OER and HER, respectively. More impressively, the electrolyzer requires an ultra-low cell voltage of 1.57 V to achieve 100 mA cm-2 and displays superior electrochemical stability for 180 h, which outperforms commercial RuO2 ||Pt/C and most of the representative catalysts reported to date. This work provides a unique route for developing high-efficiency electrocatalyst for overall water splitting.

Photocatalytic transfer of aqueous nitrogen into ammonia using nickel-titanium-layered double hydroxide

The development of solar-driven transfer of atmospheric nitrogen into ammonia is one of the green and sustainable strategies in industrial ammonia production. Nickel-titanium-layered double hydroxide (NiTi-LDH) was synthesised using the soft-chemical process for atmospheric nitrogen fixation application under photocatalysis in an aqueous system. NiTi-LDH was investigated using advanced characterisation techniques and confirmed the potential oxygen vacancies and/or surface defects owing to better photocatalytic activity under the solar spectrum. It also exhibited a bandgap of 2.8 eV that revealed its promising visible-light catalytic activities. A maximum of 33.52 µmol L-1 aqueous NH3 was obtained by continuous nitrogen (99.9% purity) supply into the photoreactor under an LED light source. Atmospheric nitrogen supply (≈78%) yielded 14.67 µmol L-1 aqueous NH3 within 60 min but gradually reduced to 3.6 µmol L-1 at 330 min. Interestingly, in weak acidic pH, 20.90 µmol L-1 NH3 was produced compared to 11.51 µmol L-1 NH3 in basic pH. The application of NiTi-LDH for visible-light harvesting capability and photoreduction of atmospheric N2 into NH3 thereby opens a new horizon of eco-friendly NH3 production using natural sunlight as alternative driving energy.

Sea urchin-like amorphous MgNiCo mixed metal hydroxide nanoarrays for efficient overall water splitting under industrial electrolytic conditions

Exploring amorphous mixed transition metal hydroxide electrocatalysts with high performance and stability for overall water splitting is a difficult challenge under industrial electrolytic conditions. Herein, a sea urchin-like amorphous MgNiCo hydroxide (Mg_xNi_1-xCo-OH, 0 < x < 1), self-assembled from nanowire arrays, is synthesized by the hydrothermal process. The synergistic effect between Mg and Ni/Co adjusts their crystal structure and morphology, which can improve the inherent activity and provide more active sites. Benefiting from the favorable structural features, Mg_0.5Ni_0.5Co-OH exhibits superior electrocatalytic oxygen and hydrogen evolution reaction (OER and HER) activity with a low overpotential of 277 and 110 mV (10 mA cm^-2) in 1 M KOH at 25 °C. Furthermore, overpotentials of 239 and 197 mV are required to achieve a current density of 50 mA cm^-2 for the OER and HER under simulated industrial electrolysis conditions (5 M KOH at 65 °C). Notably, Mg_0.5Ni_0.5Co-OH remarkably accelerates water splitting with a low voltage of 1.938 and 1.699 V for 50 mA cm^-2 in 1 M KOH at 25 °C and 5 M KOH at 65 °C, respectively. This work presents a novel amorphous strategy to design and construct sea urchin-like mixed metal hydroxide bifunctional efficient electrocatalysts for industrial applications.

Multi-Function of the Ni Interlayer in the Design of a BiVO4-Based Photoanode for Photoelectrochemical Water Splitting

BiVO₄ with an appropriate band structure is considered to be an ideal candidate for photoanodes. However, slow water oxidation kinetics and low charge separation efficiency seriously restrict its application. To address these issues, an NF/N/BVO photoanode with a hierarchical network structure was successfully constructed by direct-current magnetron sputtering of Ni followed by electrochemical deposition of nickel-iron layered double hydroxide (NiFe-LDH) on BiVO₄. A photocurrent density of 4.50 mA/cm² was obtained for NF/N/BVO, which was 2.4 times that for pristine BiVO₄. The introduction of the Ni layer contributed to the following growth of NiFe-LDH nanosheets with larger size, which acted as active sites and speeded up water oxidation kinetics. Furthermore, surface photovoltage microscopy revealed that Ni and NiFe-LDH acted as the electron collector and hole reservoir, respectively. The co-existence of the two components constituted a highly efficient surface charge separation structure, which was one of the important issues for the excellent water oxidation activity.

Doping Mo into NiFe LDH/NiSe Heterostructure to Enhance Oxygen Evolution Activity by Synergistically Facilitating Electronic Modulation and Surface Reconstruction

It is of great significance to design highly efficient electrocatalysts with abundant earth elements instead of precious metals for water splitting. Herein, Mo-doped NiFe-layered double hydroxides/NiSe heterostructure (Mo-NiFe LDH/NiSe) was fabricated by coupling Mo-doped NiFe LDH and NiSe on nickel foam (NF). The heterostructure electrocatalyst showed ultra-low overpotential (250 mV) and remarkable durability for oxygen evolution reaction (OER) at 150 mA cm^-2 . Both theoretical and experimental results confirmed that Mo doping and interfacial synergism induced the interfacial charge redistribution and the lifted d-band center to weaken the energy barrier (EB) of the formation of OOH^* . Mo doping also facilitated the surface reconstruction of NiFe LDH into Ni(Fe)OOH as the active sites under electro-oxidation process. This work provides a facile strategy for electronic modulation and surface reconstruction of OER electrocatalyst by transition metal doping and heterostructure generation.

Self-supported CoMoO4/NiFe-LDH core-shell nanorods grown on nickel foam for enhanced electrocatalysis of oxygen evolution

Developing high-performance catalysts is an effective strategy for speeding up the oxygen evolution reaction (OER) and increasing production efficiency. Here, a core-shell electrocatalyst consisting of CoMoO₄ nanorods grown in situ on nickel foam substrate covered by nickel-iron layered double hydroxide (NiFe-LDH) via electrodeposition was demonstrated (CoMoO₄/NiFe-LDH@NF). Experimental investigations revealed that self-supporting and binder-free electrodes ensured that the catalysts exposed an abundance of active sites, faster electron transfer, and excellent long-cycle stability. The NiFe-LDH shell with a crystalline-amorphous dual structure served as an accurate active material, lowering the energy barrier and contributing more catalytic sites for water oxidation. Furthermore, the core CoMoO₄ nanorods not only effectively avoided the accumulation of NiFe-LDH to increase the electrochemically active area but also acted as a highway for electrons from the active site to the substrate to promote the OER kinetics. Specifically, CoMoO₄/NiFe-LDH@NF exhibited lower overpotential (180 mV at 10 mA cm^-2) and smaller Tafel slope (34 mV dec^-1) than pure CoMoO₄@NF and NiFe-LDH@NF, revealing its excellent catalytic performance and fast intrinsic reaction kinetics. In addition, CoMoO₄/NiFe-LDH@NF exhibited long-term stability of more than 20 h at 50 mA cm^-2, further demonstrating its potential for practical applications. These findings pointed to a potential option for building innovative OER catalysts.

Fundamental understanding of electrocatalysis over layered double hydroxides from the aspects of crystal and electronic structures

Layered double hydroxides (LDHs) composed of octahedral ligand units centered with various transition metal atoms display unique electronic structures and thus attract significant attention in the field of electrocatalytic oxygen evolution reactions (OER). Intensive experimental explorations have therefore been carried out to investigate the LDHs synthesis, amorphous control, intrinsic material modifications, interfacing with other phases, strain, etc. There is still the need for a fundamental understanding of the structure-property relations, which could hinder the design of the next generation of the LDHs catalysts. In this review, we firstly provide the crystal structure information accompanied by the corresponding electronic structures. Then, we discuss the conflicts of the active sites on the NiFe LDHs and propose the synergistic cooperation among the ligand units during OER to deliver a different angle for understanding the current structure-property relations beyond the single-site-based catalysis process. In the next section of the OER process, the linear relationship-induced theoretical limit of the overpotential is further discussed based on the fundamental aspects. To break up the linear relations, we have summarized the current strategies for optimizing the OER performance. Lastly, based on the understanding gained above, the perspective of the research challenges and opportunities are proposed.

Introducing Oxygen Vacancies to NiFe LDH through Electrochemistry Reduction to Promote Oxygen Evolution Reaction

The transition metal hydroxide NiFe LDH is a promising oxygen evolution reaction (OER) catalyst. Surface engineering, such as the introduction of oxygen vacancies into NiFe LDH, has been reported to...

High catalytic performance of nano-flowered Mg-doped NiCo layered double hydroxides for the oxygen evolution reaction

Layered double hydroxides (LDHs) are one of the ideal functional materials for the oxygen evolution reaction (OER) because of their special configuration and good electrochemical activity.

Universal MOF-Mediated synthesis of 2D CoNi-based layered triple hydroxides electrocatalyst for efficient oxygen evolution reaction

Developing low-budget, stable, and high-performance electrocatalyst toward oxygen evolution reaction (OER) is of pivotal significance in the fields of energy conversion and storage. Herein, a universal metal organic framework (MOF)-mediated method for the synthesis of two-dimensional (2D) layered triple hydroxides (LTHs) nanosheets with ultrathin nature has been developed. It is interesting to disclose that the CoNi-based LTHs possess better electrochemical catalytic performance, giving superior performance to commercial RuO₂ catalysts. Remarkably, benefitting from the ultrathin nanosheet configuration, optimized electronic structure, and strong synergistic effect, the optimized CoNiFe LTHs nanosheets show excellent OER performance with an ultralow overpotential of 262 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 88.1 mV dec^-1. This work provides a promising avenue to develop low-cost and high-performance layered ternary hydroxide electrocatalysts.

Ultrathin vanadium hydroxide nanosheets assembled on the surface of Ni-Fe-layered hydroxides as hierarchical catalysts for the oxygen evolution reaction

Developing state-of-the-art non-noble metal catalysts for the oxygen evolution reaction holds a key to the production of electrolytic hydrogen. Herein, self-supported hierarchical NiFe LDH/VO(OH)2 nanoflowers/nanosheets grown on a Ni foam have been synthesized via a two-step hydrothermal method. Numerous fine VO(OH)2 nanosheets grown on NiFe LDH nanoflowers enlarge the contact area for the electrolyte penetration and facilitate ion diffusion, while the three-dimensional structure of the material also provides an extensive active surface area and plentiful accessible active sites. Moreover, the strong synergistic interaction between VO(OH)2 and NiFe LDHs subtly modulates the electronic environment, accelerating the electron/charge transfer. As a result, the catalyst exhibits excellent electrochemical performance for OER giving a voltage of 1.51 V to achieve the current density of 100 mA cm-2 and possessed a Tafel slope of 65 mV dec-1 in 1.0 M KOH. In addition, the material exhibited remarkable long-term durability and stability during the 40 h measurement. This investigation provides a promising strategy for rationally designing high-efficiency metal electrocatalysts with hierarchical multi-dimensional nanostructures for OER.

Fe-Induced electronic optimization of mesoporous Co–Ni oxide nanosheets as an efficient binder-free electrode for the oxygen evolution reaction

An efficient binder-free OER electrode CoNiFeO_x/NF with mesoporous structure was synthesized by a facile strategy of hydrothermal method and post-annealing.