Boosting the electrocatalytic performance of NiFe layered double hydroxides for the oxygen evolution reaction by exposing the highly active edge plane (012)

Correlating Thickness and Phase of Single Co(OH)2 Micro-Platelets to the Intrinsic Activity of Oxygen Evolution Electrocatalysis

Morphology, crystal phase, and its transformation are important structures that frequently determine electrocatalytic activity, but the correlations of intrinsic activity with them are not completely understood. Herein, using Co(OH)2 micro-platelets with well-defined structures (phase, thickness, area, and volume) as model electrocatalysts of oxygen evolution reaction, multiple in situ microscopy is combined to correlate the electrocatalytic activity with morphology, phase, and its transformation. Single-entity morphology and electrochemistry characterized by atomic force microscopy and scanning electrochemical cell microscopy reveal a thickness-dependent turnover frequency (TOF) of α-Co(OH)2. The TOF (≈9.5 s-1) of α-Co(OH)2 with ≈14 nm thickness is ≈95-fold higher than that (≈0.1 s-1) with ≈80 nm. Moreover, this thickness-dependent activity has a critical thickness of ≈30 nm, above which no thickness-dependence is observed. Contrarily, β-Co(OH)2 reveals a lower TOF (≈0.1 s-1) having no significant correlation with thickness. Combining single-entity electrochemistry with in situ Raman microspectroscopy, this thickness-dependent activity is explained by more reversible Co3+/Co2+ kinetics and larger ratio of active Co sites of thinner α-Co(OH)2, accompanied with faster phase transformation and more extensive surface restructuration. The findings highlight the interactions among thickness, ratio of active sites, kinetics of active sites, and phase transformation, and offer new insights into structure-activity relationships at single-entity level.

Construction of Ferric-Oxide-Doped Nickel-Iron Hydroxide Electrocatalysts by Magnetic-Field-Assisted Chemical Corrosion toward Boosted Oxygen Evolution Reaction

Transition-metal-based oxygen evolution reaction (OER) catalysts have attracted widespread attention due to their inexpensive prices, unique layered structures, and rich active sites. Currently, designing low-cost, sustainable, and simple synthesis methods is essential for the application of transition-metal-based catalysts. Here, magnetic field (MF)-assisted chemical corrosion, as a novel technology, is adopted to construct superior OER electrocatalysts. The produced Ni(Fe)(OH)2-Fe2O3 electrode exhibits an overpotential of 272 mV at a current density of 100 mA cm-2, presenting a 64 mV reduction compared to the electrode without an MF. The experimental results indicate that an MF can induce the directional growth of Fe2O3 rods and reduce their accumulation. In addition, an external MF is beneficial for the lattice dislocation of the obtained catalysts, which can increase the surface free energy, thus reducing the activation energy and accelerating the electrochemical reaction kinetics. This work effectively combines a magnetic field with chemical corrosion and electrochemical energy, which offers a novel strategy for the large-scale development of environmentally friendly and superior electrocatalysts.

Generic heterostructure interfaces bound to Co9S8 for efficient overall water splitting supported by photothermal

The increase of reaction temperature of electrocatalysts and the construction of heterogeneous structures is regarded as an efficient method to improve the electrocatalytic water splitting activity. Here, we report an approach to enhance the local heat and active sites of the catalyst by building a heterostructure with Co9S8 to significantly improve its electrocatalytic performance. The as-fabricated Co9S8@Ce-NiCo LDH/NF electrode possesses a notable photothermal ability, as it effectively converts near-infrared (NIR) light into the local heat, owing to its significant optical absorption. Leveraging these favorable qualities, the prepared Co9S8@Ce-NiCo LDH/NF electrode showed impressive performance in both hydrogen evolution reaction (HER) (η100 = 144 mV) and oxygen evolution reaction (OER) (η100 = 229 mV) under NIR light. Compared to the absence of the NIR light, the presence of NIR irradiation leads to a 24.6 % increase in catalytic efficiency for HER and a 15.8 % increase for OER. Additionally, other dual-functional electrocatalysts like NiCo-P, NiFeMo, and NiFe(OH)x also demonstrated significantly enhanced photothermal effects and improved catalytic performance owing to the augmented photothermal conversion when combined with Co9S8. This work offers novel pathways for the development of photothermal-electrocatalytic systems that facilitate economically efficient and energy-conserving overall water splitting processes.

In situ assembly of Ni3S2 nanosheets encapsulated with NiFe(oxy)hydroxides for efficient water oxidation

Morphology control plays a pivotal role in achieving an exceptionally efficient electrocatalyst with abundant active sites and outstanding electrical conductivity. In this study, we employed a sophisticated chemical nanoengineering technique to fabricate an exquisitely thin NiFe(OH)x electrocatalyst on Ni3S2 nanosheets. Firstly, the Ni3S2 nanosheets were synthesized through an innovative in situ one-step sulfurization reaction of the Ni(OH)2 nanosheets grown on Ni foam. Subsequently, a remarkable ultrathin layer of NiFe(OH)x was precisely deposited onto the surface of the Ni3S2 to form a captivating core-shell structure using a chemical dipping method. The resulting electrode, denoted as NiFe(OH)x/Ni3S2/NF, exhibited exceptional electrocatalytic activity and durability towards the oxygen evolution reaction (OER), owing to its expansive specific surface area, rapid electron transport, and robust interlayer bonding. Notably, this electrode achieved an impressive current density of 100 mA cm-2 at an astonishingly low overpotential of 218 mV while maintaining a low Tafel slope of 37.9 mV dec-1 and remarkable stability for up to 12 days in 1 M KOH aqueous solution. This work presents an alluring novel approach for constructing highly efficient ultrathin catalysts for water splitting.

Nb2 AlC MAX Nanosheets Supported Ru Nanocrystals as Efficient Catalysts for Boosting pH-Universal Hydrogen Production

MAX phase combines both ceramic and metallic properties, which exhibits widespread application prospects. 2D MAX nanosheets have more abundant surface-active sites, being anticipated to improve the performance of surface-related applications. Herein, for the first time, 2D Nb2 AlC nanosheets (NSs) as novel supports anchored with Ru catalysts for overall water splitting are developed. The optimized catalyst of Ru@Nb2 AlC NSs exhibit Pt-comparable kinetics and superior catalytic activity toward hydrogen evolution reaction (HER) (low overpotentials of 61 and 169 mV at 10 and 100 mA cm-2 , respectively) with excellent durability (5000 cycles or 80 h) in alkaline media. In particular, Ru@Nb2 AlC NSs achieve a mass activity of ≈4.8 times larger than the commercial Pt/C (20 wt.%) catalyst. The post-oxidation resultant catalyst of RuO2 @Nb2 AlC NSs also exhibit boosting HER and oxygen evolution reaction activities and ≈100% Faraday efficiency for overall water splitting with a cell voltage of 1.61 V to achieve 10 mA cm-2 . Therefore, the novel category of 2D MAX supports anchored with Ru nanocrystals offers a novel strategy for designing a wide range of MAX-supported metal catalysts for the renewable energy field.

Self-Supported Spray-Coated NiFe-LDH Catalyst on a Stainless Steel Substrate for Efficient Oxygen Evolution Reaction

In the quest for more efficient and cost-effective electrocatalytic systems, careful selection of catalysts and substrates plays a pivotal role. This study introduces an approach by synthesizing and depositing NiFe-layered double hydroxide (NiFe-LDH) catalysts on commercial AISI 304 substrates by using a low-temperature spray-coating technique. Through systematic investigations, the influence of processing conditions, such as the synthesis, ultrasonic power for having a stable nanoparticle's dispersion, and spray cycle optimization on the electrochemical and morphological properties of the coatings, is thoroughly explored. The results showcase exceptional catalytic performance, achieving an overpotential of 230 mV at 10 mA/cm2, with enhanced stability even at high current densities of 500 mA/cm2. The study highlights the significance of meticulous processing optimization and presents a scalable methodology that holds great potential for developing catalysts for oxygen evolution reactions (OER) and facilitates their integration into industrial processes.

Defective blue titanium oxide induces high valence of NiFe-(oxy)hydroxides over heterogeneous interfaces towards high OER catalytic activity

Nickel-iron (oxy)hydroxides (NiFeOxHy) have been validated to speed up sluggish kinetics of the oxygen evolution reaction (OER) but still lack satisfactory substrates to support them. Here, non-stoichiometric blue titanium oxide (B-TiOx) was directly derived from Ti metal by alkaline anodization and used as a substrate for electrodeposition of amorphous NiFeOxHy (NiFe/B-TiOx). The performed X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations evidenced that there is a charge transfer between B-TiOx and NiFeOxHy, which gives rise to an elevated valence at the Ni sites (average oxidation state ∼ 2.37). The synthesized NiFe/B-TiOx delivers a current density of 10 mA cm-2 and 100 mA cm-2 at an overpotential of 227 mV and 268 mV, respectively, which are better than that of pure Ti and stainless steel. It also shows outstanding activity and stability under industrial conditions of 6 M KOH. The post-OER characterization studies revealed that the surface morphology and valence states have no significant change after 24 h of operation at 500 mA cm-2, and also can effectively inhibit the leaching of Fe. We illustrate that surface modification of Ti which has high corrosion resistance and mechanical strength, to generate strong interactions with NiFeOxHy is a simple and effective strategy to improve the OER activity and stability of non-precious metal electrodes.

012 facets modulated LDH composite for neurotoxicity risk assessment through direct electrochemical profiling of dopamine

Rising concerns of pesticide-induced neurotoxicity and neurodegenerative diseases like Parkinson's, Alzheimer's, and Multiple Sclerosis, are exacerbated by overexposure to contaminated waterbodies. Therefore, evaluating the risk accurately requires reliable monitoring of related biomarkers like dopamine (DA) through electrochemical detection. Layered double hydroxides (LDHs) have shown great potential in sensors. However, to meet the challenges of rapid detection of large patient cohorts in real-time biological media, they should be further tailored to display superior analytical readouts. Herein, a ternary LDH (Ni2CoMn0.5) was integrated with the sheets of thermally reduced graphene oxide (trGO), to expose more highly active edge planes of the LDH, as opposed to its generally observed inert basal planes. The improvement in detection performance through such a modulated structure-property is a prospect that hasn't been previously explored for any other LDH-based materials employed in sensing applications. The 2 folds superior electrochemical activity exhibited by the face-on oriented LDH with trGO as compared to the pristine LDH material was further employed for direct detection of DA in real blood plasma samples. Moreover, the designed sensor exhibited exceptional selectivity towards the detection of DA with a limit of detection of 34.6 nM for a wide dynamic range of 0.001-5 mM with exceptional stability retaining 88.56% of the initial current even after storage in ambient conditions for 30 days.

Grain Boundary Effects of Hierarchical Ni-Fe (Oxy)hydroxide Nanosheets in Water Oxidation

The robust and scalable oxygen evolution electrocatalysts that can deliver high current densities at low applied potential is a great challenge for the large-scale industrial application in hydrogen production. Here, the preparation of a grain-boundary-rich Ni-Fe (oxy)hydroxide catalyst on Ni foam is reported using a scalable coating approach followed by a chemical precipitating treatment. This facile method effectively assembles the hierarchical Ni-Fe (oxy)hydroxide nanosheet in the ultrasmall crystalline domains (<4 nm) with rich grain boundaries. The hierarchical nanosheet structure with the grain boundaries provides more accessible catalytic sites, facile charge, and mass transfer. Benefiting from the abundant grain boundaries in the hierarchical nanosheets, the as-prepared Ni-Fe (oxy)hydroxide electrodes deliver current densities of 500 and 1000 mA cm-2 at overpotentials of only 278 and 296 mV for the oxygen evolution reaction. The prepared electrode also exhibits long-term durability at a high current density in alkaline conditions.

Local Nanostrain Engineering of Monolayer MoS2 Using Atomic Force Microscopy-Based Thermomechanical Nanoindentation

Strain engineering in two-dimensional materials (2DMs) has important application potential for electronic and optoelectronic devices. However, achieving precise spatial control, adjustable sizing, and permanent strain with nanoscale resolution remains challenging. Herein, a thermomechanical nanoindentation method is introduced, inspired by skin edema caused by mosquito bites, which can induce localized nanostrain and bandgap modulation in monolayer molybdenum disulfide (MoS2) transferred onto a poly(methyl methacrylate) film utilizing a heated atomic force microscopy nanotip. Via adjustment of the machining parameters, the strains of MoS2 are manipulated, achieving an average strain of ≤2.6% on the ring-shaped expansion structure. The local bandgap of MoS2 is spatially modulated using three types of nanostructures. Among them, the nanopit has the largest range of bandgap regulation, with a substantial change of 56 meV. These findings demonstrate the capability of the proposed method to create controllable and reproducible nanostrains in 2DMs.

Amazing enhancement of OER performances: creating a well-designed functional Ni and N-doped carbon layer as a support material for fabricating a NiFe-LDH electrocatalyst

A well-designed support material between catalyst and substrate can always significantly enhance the performance of an electrode on water oxidation. In this work, a functional Ni and N-doped carbon layer (NNC) was designed on carbon paper (CP) via pyrolysis by using a controlled electrodeposited polyporphyrin as a precursor. Consequently, the fabricated NiFe-LDH/NNC/CP achieved a catalytic current density of 100 mA cm-2 at a small overpotential of 231 mV with a low Tafel slope of 26.0 mV dec-1, as well as high durability for more than 360 h. The insights are that N-doping reinforces the hydrophilicity and the catalyst binding capacity, while Ni-doping intensifies the conductivity.

One-step achievement of Fe-doped and interfacial Ru nanoclusters co-engineered Ni(OH)2 electrocatalyst on Ni foam for promoted oxygen evolution reaction

The creation of inexpensive, high-performance catalysts to reduce the overpotential of the oxygen evolution reaction (OER) process is critical for the electrolysis of water for hydrogen production. Therefore, we applied a one-step hydrothermal method using cation exchange reaction (CER) to prepare Fe-doped and interfacial Ru nanoclusters co-engineered Ni(OH)₂ nanosheets directly grafted on Ni foam (Ru@Fe-Ni(OH)₂/NF) for OER process. Results of electrochemical tests reveal that Ru@Fe-Ni(OH)₂/NF has excellent OER activity, and its overpotential (η) is only 266.4 mV when the current density is 50 mA cm^-2 in 1 M KOH solution, even lower than that of commercial OER catalyst RuO₂ (355 mV). The Tafel slope also decreases from 133.8 mV dec^-1 for pristine Ni(OH)₂/NF material to 24.1 mV dec^-1 for Ru@Fe-Ni(OH)₂/NF, indicating the higher charge transfer rates and fastest kinetics for water oxidation. At an overpotential of 300 mV the optimal turnover frequency (TOF) of 0.062 s^-1 for Ru@Fe-Ni(OH)₂/NF is achieved compared to that of Ni(OH)₂/NF (0.014 s^-1, NN), demonstrating the fast reaction kinetics of the as-prepared electrocatcalyst. After 24 h stability test, the catalytic activity of Ru@Fe-Ni(OH)₂/NF was only attenuated by 2 %, showing excellent OER stability and durability. Our results show that we have successfully developed an efficient OER catalyst for green and efficient electrocatalytic hydrolysis to produce H₂ and O₂, providing a promising method for clean H₂ production.

Recent Advances on Transition-Metal-Based Layered Double Hydroxides Nanosheets for Electrocatalytic Energy Conversion

Transition-metal-based layered double hydroxides (TM-LDHs) nanosheets are promising electrocatalysts in the renewable electrochemical energy conversion system, which are regarded as alternatives to noble metal-based materials. In this review, recent advances on effective and facile strategies to rationally design TM-LDHs nanosheets as electrocatalysts, such as increasing the number of active sties, improving the utilization of active sites (atomic-scale catalysts), modulating the electron configurations, and controlling the lattice facets, are summarized and compared. Then, the utilization of these fabricated TM-LDHs nanosheets for oxygen evolution reaction, hydrogen evolution reaction, urea oxidation reaction, nitrogen reduction reaction, small molecule oxidations, and biomass derivatives upgrading is articulated through systematically discussing the corresponding fundamental design principles and reaction mechanism. Finally, the existing challenges in increasing the density of catalytically active sites and future prospects of TM-LDHs nanosheets-based electrocatalysts in each application are also commented.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Regulating the Spin State of FeIII Enhances the Magnetic Effect of the Molecular Catalysis Mechanism

Aqueous-phase oxygen evolution reaction (OER) is the bottleneck of water splitting. The formation of the O-O bond involves the generation of paramagnetic oxygen molecules from the diamagnetic hydroxides. The spin configurations might play an important role in aqueous-phase molecular electrocatalysis. However, spintronic electrocatalysis is almost an uncultivated land for the exploration of the oxygen molecular catalysis process. Herein, we present a novel magnetic Fe^III site spin-splitting strategy, wherein the electronic structure and spin states of the Fe^III sites are effectively induced and optimized by the Jahn-Teller effect of Cu²⁺. The theoretical calculations and operando attenuated total reflectance-infrared Fourier transform infrared (ATR FT-IR) reveal the facilitation for the O-O bond formation, which accelerates the production of O₂ from OH^- and improves the OER activity. The Cu₁-Ni₆Fe₂-LDH catalyst exhibits a low overpotential of 210 mV at 10 mA cm^-2 and a low Tafel slope (33.7 mV dec^-1), better than those of the initial Cu₀-Ni₆Fe₂-LDHs (278 mV, 101.6 mV dec^-1). With the Cu²⁺ regulation, we have realized the transformation of NiFe-LDHs from ferrimagnets to ferromagnets and showcase that the OER performance of Cu-NiFe-LDHs significantly increases compared with that of NiFe-LDHs under the effect of a magnetic field for the first time. The magnetic-field-assisted Cu₁-Ni₆Fe₂-LDHs provide an ultralow overpotential of 180 mV at 10 mA cm^-2, which is currently one of the best OER performances. The combination of the magnetic field and spin configuration provides new principles for the development of high-performance catalysts and understandings of the catalytic mechanism from the spintronic level.

Layered Double Hydroxide Catalysts for Ozone Decomposition: The Synergic Role of M2+ and M3

In previous work, we successfully prepared a NiFe-layered double hydroxide (LDH) with superior activity and stability for catalytic ozone decomposition, which fundamentally avoids deactivation under high-humidity conditions. However, the role of the metal elements (M²⁺ and M³⁺) in LDH catalysts is not clear. Here, LDH materials containing different metals (NiFe, NiAl, NiMn, CoFe, and MgFe) were prepared by a simple co-precipitation method. It was found that the LDHs containing Ni²⁺ exhibited catalytic performance far superior to that of Co²⁺ and Mg²⁺ for ozone elimination, and NiFe-LDH had the best activity and stability among LDH materials prepared in this study. The NiFe-LDH can maintain 78% catalytic activity within 144 h at room temperature, even under a relative humidity of 65% and a space velocity of 840 L·g^-1·h^-1. Physicochemical characterizations demonstrated that chemical stability in an oxidizing atmosphere and the synergic role of M²⁺ and M³⁺ ions are crucial. The result of density functional theory calculation showed that the synergic role of Ni²⁺ and Fe³⁺ weakens the interaction between O and H in the O-H bond, which effectively lowers the reaction barrier of ozone decomposition compared with MgFe-LDH.

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...

Ultrafast Room-Temperature Synthesis of Self-Supported NiFe-Layered Double Hydroxide as Large-Current-Density Oxygen Evolution Electrocatalyst

Water splitting is a promising sustainable technology to produce high purity hydrogen, but its commercial application remains a giant challenge due to the kinetically sluggish oxygen evolution reaction (OER). In this work, a time- and energy-saving approach to directly grow NiFe-layered double hydroxide (NiFe-LDH) nanosheets on nickel foam under ambient temperature and pressure is reported. These NiFe-LDH nanosheets are vertically rooted in nickel foam and interdigitated together to form a highly porous array, leading to numerous exposed active sites, reduced resistance of charge/mass transportation and enhanced mechanical stability. As self-supported electrocatalyst, the representative sample (NF@NiFe-LDH-1.5-4) shows an excellent large-current-density catalytic activity for OER in alkaline electrolyte, requiring low overpotentials of 190 and 220 mV to reach the current densities of 100 and 657 mA cm^-2 with a Tafel slope of 38.1 mV dec^-1 . In addition, NF@NiFe-LDH-1.5-4 as an overall water splitting electrocatalyst can stably achieve a large current density of 200 mA cm^-2 over 300 h at a low cell voltage of 1.83 V, meeting the requirement of industrial hydrogen production. This exceedingly simple and ultrafast synthesis of low-cost and highly active large-current-density OER electrocatalysts can propel the commercialization of hydrogen producing technology via water splitting.

Crystal facet and phase engineering for advanced water splitting

This review covers the principles and recent advances in facet and phase engineering of catalysts for photocatalytic, photoelectrochemical, and electrochemical water splitting. It suggests the basis of catalyst design for advanced water splitting.

NiFe Layered-Double-Hydroxide Nanosheet Arrays on Graphite Felt: A 3D Electrocatalyst for Highly Efficient Water Oxidation in Alkaline Media

It is of great importance to rationally design and develop earth-abundant nanocatalysts for high-efficiency water electrolysis. Herein, NiFe layered double hydroxide was in situ grown hydrothermally on a 3D graphite felt (NiFe LDH/GF) as a high-efficiency catalyst in facilitating the oxygen evolution reaction (OER). In 1.0 M KOH, NiFe LDH/GF requires a low overpotential of 214 mV to deliver a geometric current density of 50 mA cm^-2 (η_{50 mA cm^-2} = 214 mV), surpassing that NiFe LDH supported on a 2D graphite paper (NiFe LDH/GP; η_{50 mA cm^-2} = 301 mV). More importantly, NiFe LDH/GF shows good durability at 50 mA cm^-2 within 50 h of OER catalysis testing and delivers a faradaic efficiency of nearly 100% in the electrocatalysis of OER.

Boosting electrocatalytic oxygen evolution activity of bimetallic CoFe selenite by exposing specific crystal facets

It was of great significance to develop efficient and stable electrocatalysts for oxygen evolution reactions via simple and environmentally-friendly methods.

Hierarchical porous Ni, Fe-codoped Co-hydroxide arrays derived from metal–organic-frameworks for enhanced oxygen evolution

The multi metal organic frameworks can be successfully transformed into hierarchical porous Ni,Fe-codoped Co-hydroxide nanowire array catalysts with excellent electrocatalytic performance for the OER in alkaline media.