Incorporating iron in nickel cobalt layered double hydroxide nanosheet arrays as efficient oxygen evolution electrocatalyst

Precise modulation of nickel-molybdenum alloy (MoNi4)/molybdenum dioxide nanowires via a ternary nickel-cobalt-iron complex for enhanced electrochemical overall water splitting

Developing renewable and clean energy technologies necessitates the design of efficient bifunctional catalysts that can facilitate electrochemical water splitting without relying on inert metals. This study presents a novel three-step strategy for fabricating nickel cobalt iron (NiCoFe)-modified nickel-molybdenum alloy/molybdenum dioxide (MoNi4/MoO2) nanowires on nickel foam (NF) substrates, denoted as NiCoFe-MoNi4/MoO2/NF. The synthesized catalyst demonstrates exceptional performance, achieving an impressively low overpotential (13 mV) at 10 mA·cm-2 current density for the hydrogen evolution reaction (HER) and 230 mV at 50 mA·cm-2 for the oxygen evolution reaction (OER). Its performance surpasses many noble-metal catalysts, achieving overall water splitting at just 1.51 V under 50 mA·cm-2. The distinctive one-dimensional (1D) nanostructure and synergistic interplay between the NiCoFe complex and the MoNi4/MoO2 framework enhance mass transfer, expose additional active sites, and enhance intrinsic activity, contributing to outstanding efficiency. Incorporating cobalt (Co) and iron (Fe) into the ternary complex greatly improved the efficiencies of both HER and OER, providing a promising approach for developing high-performance, cost-effective bifunctional electrocatalysts and promoting advancements in sustainable energy conversion technologies.

Regulating the thickness of the carbon coating layer in iron/carbon heterostructures to enhance the catalytic performance for oxygen evolution reaction

The exploration of high-performance electrocatalysts for the oxygen evolution reaction (OER) is crucial and urgent for the fast development of green and renewable hydrogen energy. Herein, an ultra-fast and energy-efficient preparation strategy (microwave-assisted rapid in-situ pyrolysis of organometallic compounds induced by carbon nanotube (CNT)) is developed to obtain iron/carbon (Fe/C) heterogeneous materials (Fe/Fe₃C particles wrapped by carbon coating layer). The thickness of the carbon coating layer can be adjusted by changing the content and form of carbon in the metal sources during the fast preparation process. Fe/Fe₃C-A@CNT using iron acetylacetonate as metal sources possesses unique Fe/C heterogeneous, small Fe/Fe₃C particles encapsulated by the thin carbon coating layer (1.77 nm), and obtains the optimal electron penetration effect. The electron penetration effect derives from the redistribution of charge between the surface carbon coating layer and inner Fe/Fe₃C nanoparticles efficiently improving both catalytic activity and stability. Therefore, Fe/Fe₃C-A@CNT shows efficient OER catalytic activity, just needing a low overpotential of 292 mV to reach a current density of 10 mA cm^-2, and long-lasting stability. More importantly, the unique control strategy for carbon thickness in this work provides more opportunity and perspective to prepare robust metal/carbon-based catalytic materials at the nanoscale.

Synthesis of Hollow Leaf-Shaped Iron-Doped Nickel–Cobalt Layered Double Hydroxides Using Two-Dimensional (2D) Zeolitic Imidazolate Framework Catalyzing Oxygen Evolution Reaction

Layered double hydroxides (LDHs) have been reported as one of the most effective materials for oxygen evolution reaction (OER) catalysts, which are prone to hydrolysis and oxidation under OER conditions. Metal–organic frameworks (MOFs) are porous materials with high crystallinity and internal surface area. The design of LDHs based on MOFs has attracted increasing attention owing to their high surface area, exposed catalysis sites, and fast charge/mass transport kinetics. Herein, we report a novel approach to fabricate a leaf-shaped iron-doped nickel–cobalt LDH (L-Fe-NiCoLDH) derived from a two-dimensional (2D) zeolitic imidazolate framework with a leaf-like morphology (ZIFL). Iron doping played a significant role in enhancing the specific surface area, affecting the OER performance. L-Fe-NiCoLDH showed high OER performance with an overpotential of 243 mV at 10 mA cm−2 and high durability after 20 h. The design of LDHs based on the leaf morphology of MOFs offers tremendous potential for improving OER efficiency.

Interfacial Engineering of Leaf-like Bimetallic MOF-Based Co@NC Nanoarrays Coupled with Ultrathin CoFe-LDH Nanosheets for Rechargeable and Flexible Zn-Air Batteries

Exploring high-efficiency, low-cost, and long-life bifunctional self-supporting electrocatalysts is of great significance for the practical application of advanced rechargeable Zn-air batteries (ZABs), especially flexible solid-state ZABs. Herein, ultrathin CoFe-layered double hydroxide (CoFe-LDH) nanosheets are strongly coupled on the surface of leaf-like bimetallic metal-organic frameworks (MOFs)-derived hybrid carbon (Co@NC) nanoflake nanoarrays supported by carbon cloth (CC) via a facile and scalable method for rechargeable and flexible ZABs. This interfacial engineering for CoFe-LDHs on Co@NC improves the electronic conductivity of CoFe-LDH nanosheets as well as achieves the balance of oxygen evolution reduction (OER) and oxygen reduction reaction (ORR) activity. The unique three-dimensional (3D) open interconnected hierarchical structure facilitates the transport of substances during the electrochemical process while ensuring adequate exposure of OER/ORR active centers. When applied as an additive-free air cathode in rechargeable liquid ZABs, CC/Co@NC/CoFe-LDH-700 demonstrates high open-circuit potential of 1.47 V, maximum power density of 129.3 mW cm^-2, and satisfactory specific capacity of 710.7 mAh g^-1_Zn. Further, the flexible all-solid-state ZAB assembled by CC/Co@NC/CoFe-LDH-700 displays gratifying mechanical flexibility and stable cycling performance over 40 h. More significantly, the series-connected flexible ZAB is further verified as a chain power supply for LED strips and performs well throughout the bending process, showing great application prospects in portable and wearable electronics. This work sheds new light on the design of high-performance self-supporting non-precious metal bifunctional electrocatalysts for OER/ORR and air cathodes for rechargeable ZABs.

Construction of Self-Supporting NiCoFe Nanotube Arrays Enabling High-Efficiency Alkaline Oxygen Evolution

Enhancing the intrinsic activity and modulating the electrode-electrolyte interface microenvironment of nickel-based candidates are essential for breaking through the sluggish kinetics limitation of the oxygen evolution reaction (OER). Herein, a ternary nickel-cobalt-iron solid solution with delicate hollow nanoarrays architecture (labeled as NiCoFe-NTs) was designed and fabricated via a ZnO-templated electrodeposition strategy. Owing to the synergistic nanostructure and composition feature, NiCoFe-NT presents desirable alkaline OER performance, with a η₁₀ and η₅₀₀ of 187 and 310 mV, respectively, along with favorable long-term durability. In-depth analyses identify the heterogeneous nickel-based (oxy)hydroxide species derived from the oxidative reconstruction acting as an active contributor for oxygen evolution. Impressively, the regulatory mechanism of the catalytic performance by a rationally designed nanostructure was elucidated by compressive analyses; that is, the faster gas release processes induced by nanotube arrays can modulate the heterogeneous interface states during OER, which effectively facilitates the electrochemical charge-mass transfer to promote the reaction kinetics. To assess the practical feasibility, an alkaline water electrolyzer and a CO₂ electrochemical reduction flow cell were constructed by coupling the anodic NiCoFe-NTs and cathodic nickel phosphides (Ni₂P-NF) and metallic Cu electrocatalysts, respectively, both of which achieved high-efficiency operation.

Competitive Coordination-Oriented Monodispersed Ruthenium Sites in Conductive MOF/LDH Hetero-Nanotree Catalysts for Efficient Overall Water Splitting in Alkaline Media

Rational exploration of efficient, inexpensive, and robust electrocatalysts is critical for the efficient water splitting. Conjugated conductive metal-organic frameworks (cMOFs) with multicomponent layered double hydroxides (LDHs) to construct bifunctional heterostructure catalysts are considered as an efficient but complicated strategy. Here, the fabrication of a cMOF/LDH hetero-nanotree array catalyst (CoNiRu-NT) coupled with monodispersed ruthenium (Ru) sites via a controllable grafted-growth strategy is reported. Rich-amino hexaiminotriphenylene linkers coordinate with the LDH nanotrunk to form cMOF nanobranches, providing numerous anchoring sites to precisely confine and stabilize RuN₄ sites. Moreover, monodispersed and reduced Ru moieties facilitate H₂ O adsorption and dissociation, and the heterointerface between the cMOF and the LDH further modifies the chemical and electronic structures. Optimized CoNiRu-NT displays a significant increase in electrochemical water-splitting properties in alkaline media, affording low overpotentials of 22 mV at 10 mA cm^-2 and 255 mV at 20 mA cm^-2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. In an actual electrochemical system, CoNiRu-NT drives an overall water splitting at a low cell voltage of 1.47 V to reach 10 mA cm^-2 . This performance is comparable to that of pure noble-metal-based materials and superior to most reported MOF-based catalysts.

Electrodeposition of Defect-Rich Ternary NiCoFe Layered Double Hydroxides: Fine Modulation of Co3+ for Highly Efficient Oxygen Evolution Reaction

The low-cost, high-abundance and durable layered double hydroxides (LDHs) have been considered as promising electrocatalysts for oxygen evolution reaction (OER). However, the easy agglomeration of lamellar LDHs in the aqueous phase limits their practical applications. Herein, a series of ternary NiCoFe LDHs were successfully fabricated on nickel foam (NF) via a simple electrodeposition method. The as-prepared Ni(Co_0.5 Fe_0.5 )/NF displayed an unique nanoarray structural feature. It showed an OER overpotential of 209 mV at a current density of 10 mA cm^-2 in alkaline solution, which was superior to most systems reported so far. As evidenced by the XPS and XAFS results, such excellent performance of Ni(Co_0.5 Fe_0.5 )/NF was attributed to the higher Co³⁺ /Co²⁺ ratio and more defects exposed, comparing with Ni(Co_0.5 Fe_0.5 )-bulk and Ni(Co_0.5 Fe_0.5 )-mono LDHs prepared by conventional coprecipitation method. Furthermore, the ratio of Co to Fe could significantly tune the Co electronic structure of Ni(Co_x Fe_1-x )/NF composites (x=0.25, 0.50 and 0.75) and affect the electrocatalytic activity for OER, in which Ni(Co_0.5 Fe_0.5 )/NF showed the lowest energy barrier for OER rate-determining step (from O* to OOH*). This work proposes a facile method to develop high-efficiency OER electrocatalysts.

Design and preparation of three-dimensional hetero-electrocatalysts of NiCo-layered double hydroxide nanosheets incorporated with silver nanoclusters for enhanced oxygen evolution reactions

Layered double hydroxides (LDHs) are one of the most effective electrocatalysts. However, it is still necessary to improve the lower conductivity and limited active sites of LDHs to enhance their catalytic performance. Targeted generation of vacancies on the catalyst's surface by the incorporation of metal nanoparticles has been explored as a promising strategy to synthesize highly efficient electrocatalysts. Herein, we designed and prepared novel three-dimensional (3D) hetero-electrocatalysts of NiCo-layered double hydroxide nanosheets incorporated with silver nanoclusters on a Ni foam (labeled as Ag@NiCo-LDH/NF) by a one-pot hydrothermal method. We also conducted experimental and theoretical investigations to demonstrate the high electrocatalytic performance of the Ag@NiCo-LDH/NF hetero-electrocatalysts for OERs and the underlying mechanism. The resulting hetero-electrocatalysts show a low overpotential of 262 mV at a current density of 10 mA cm^-2, and even exhibit low overpotentials of 300 mV at a high current density of 50 mA cm^-2 and 324 mV at 100 mA cm^-2, and a small Tafel slope of 41 mV dec^-1 as well as excellent durability for 80 h for OERs in 1.0 M KOH. The excellent performance is attributed to the synergistic effects between Ag nanoclusters and LDHs. The population engineering effect of silver not only helps to modulate the intrinsic properties of active sites but also induces the generation of abundant oxygen vacancies on the surface; finally, it facilitates the rate-determining step of OERs (ΔG₃ (O* → OOH*) = 1.31 eV) to gain high performance. The one-pot silver incorporating strategy and the resulting high performance pave new ways for the further development of highly efficient electrocatalysts for OERs.

Transition Metal-Based 2D Layered Double Hydroxide Nanosheets: Design Strategies and Applications in Oxygen Evolution Reaction

Water splitting driven by renewable energy sources is considered a sustainable way of hydrogen production, an ideal fuel to overcome the energy issue and its environmental challenges. The rational design of electrocatalysts serves as a critical point to achieve efficient water splitting. Layered double hydroxides (LDHs) with two-dimensionally (2D) layered structures hold great potential in electrocatalysis owing to their ease of preparation, structural flexibility, and tenability. However, their application in catalysis is limited due to their low activity attributed to structural stacking with irrational electronic structures, and their sluggish mass transfers. To overcome this challenge, attempts have been made toward adjusting the morphological and electronic structure using appropriate design strategies. This review highlights the current progress made on design strategies of transition metal-based LDHs (TM-LDHs) and their application as novel catalysts for oxygen evolution reactions (OERs) in alkaline conditions. We describe various strategies employed to regulate the electronic structure and composition of TM-LDHs and we discuss their influence on OER performance. Finally, significant challenges and potential research directions are put forward to promote the possible future development of these novel TM-LDHs catalysts.

Morphological and Elemental Investigations on Co–Fe–B–O Thin Films Deposited by Pulsed Laser Deposition for Alkaline Water Oxidation: Charge Exchange Efficiency as the Prevailing Factor in Comparison with the Adsorption Process

Abstract

Mixed transition-metals oxide electrocatalysts have shown huge potential for electrochemical water oxidation due to their earth abundance, low cost and excellent electrocatalytic activity. Here we present Co–Fe–B–O coatings as oxygen evolution catalyst synthesized by Pulsed Laser Deposition (PLD) which provided flexibility to investigate the effect of morphology and structural transformation on the catalytic activity. As an unusual behaviour, nanomorphology of 3D-urchin-like particles assembled with crystallized CoFe2O4 nanowires, acquiring high surface area, displayed inferior performance as compared to core–shell particles with partially crystalline shell containing boron. The best electrochemical activity towards water oxidation in alkaline medium with an overpotential of 315 mV at 10 mA/cm2 along with a Tafel slope of 31.5 mV/dec was recorded with core–shell particle morphology. Systematic comparison with control samples highlighted the role of all the elements, with Co being the active element, boron prevents the complete oxidation of Co to form Co3+ active species (CoOOH), while Fe assists in reducing Co3+ to Co2+ so that these species are regenerated in the successive cycles. Thorough observation of results also indicates that the activity of the active sites play a dominating role in determining the performance of the electrocatalyst over the number of adsorption sites. The synthesized Co–Fe–B–O coatings displayed good stability and recyclability thereby showcasing potential for industrial applications.

Graphic Abstract

A three-dimensional flower-like NiCo-layered double hydroxide grown on nickel foam with an MXene coating for enhanced oxygen evolution reaction electrocatalysis

Electrolysis of water is currently one of the cleanest and most efficient ways to produce high-purity hydrogen. The oxygen evolution reaction (OER) at the anode of electrolysis is the key factor affecting the reaction efficiency, which involves the transfer of four electrons and can slow down the overall reaction process. In this work, using nickel foam coated with MXene (Ti3C2T x ) as the carrier, a three-dimensional flower-shaped layered double hydroxide (NiCo-LDH) is grown on Ti3C2T x by a hydrothermal method to fabricate a NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst for enhanced OER performance. The results reveal that the hybrid electrocatalyst has excellent OER activity in alkaline solution, in which a low overpotential of 223 mV and a small Tafel slope of 47.2 mV dec-1 can be achieved at a current density of 100 mA cm-2. The interface interaction and charge transfer between Ti3C2T x and NiCo-LDH can accelerate the electron transfer rate during the redox process and improve the catalytic activity of the overall reaction. This NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst may have important research significance and great application potential in catalytic electrolysis of water.

2D MOF-derived porous NiCoSe nanosheet arrays on Ni foam for overall water splitting

A novel 2D porous NiCoSe nanosheet arrays were grown on Ni foam using ZIF-67 as precursors, which exhibited outstanding bifunctional electrocatalytic activity and superior durability for overall water splitting.

In situ fabrication of dynamic self-optimizing Ni3S2 nanosheets as an efficient catalyst for the oxygen evolution reaction

Ni₃S₂ nanosheets exhibit enhanced oxygen evolution reaction performance by self-optimizing their surface composition.

Recent advances in ternary layered double hydroxide electrocatalysts for the oxygen evolution reaction

The recent advances in ternary layered double hydroxide electrocatalysts, including the strategies used for the design, synthesis, and evaluation of their performance for oxygen evolution reaction are reviewed in this account.