Layered Double Hydroxide Catalysts Research Articles

Iron (III)-Facilitated reconstruction in NiMn layered double hydroxides for initiating rapid oxygen evolution reaction

Layered double hydroxides (LDHs) are emerging as efficient oxygen evolution reaction (OER) electrocatalysts due to their structural advantages and compositional flexibility. Despite their promise, Mn-based LDHs are hampered by poor conductivity, limiting their OER efficacy. This research introduces a hydrothermal and chemical etching technique to fabricate NF/NiMn LDH@NiMnFe(OH)x structures, significantly improving OER performance. The study reveals that integrating Fe3+ into the NiMn LDH fosters a synergistic Ni–Fe interaction, markedly boosting electrocatalytic efficiency. The NF/NiMn LDH@NiMnFe(OH)x-90 shows a low overpotential of 250 mV at 10 mA cm−2 and a Tafel slope of 49.9 mV dec−1 in 1 M KOH, surpassing both its NM precursor and other Ni-based LDH catalysts, including commercial RuO2. In situ, Raman spectroscopy indicates a pivotal phase transition in NF/NiMn LDH@NiMnFe(OH)x to NiOOH at elevated potentials, essential for OER activity. Raman peaks at 463 and 585 cm−1 confirm this structural evolution. The OER activity boost is attributed to increased oxygen vacancies and enhanced conductivity. Moreover, NF/NiMn LDH@NiMnFe(OH)x-90 maintains stability with negligible degradation after 24 h, underscoring its durability. These findings offer a strategic approach to designing high-performance OER electrocatalysts, leveraging nickel-based layered double-metal oxides for potential water-splitting applications, and emphasize the significance of surface engineering and compositional tuning.

Dual Cocatalytic Sites Synergize NiFe Layered Double Hydroxide to Boost Oxygen Evolution Reaction in Anion Exchange Membrane Water Electrolyzer

AbstractNickel‐iron layered double hydroxide (LDH) is a promising cost‐efficient catalyst to replace noble metals for alkaline oxygen evolution reaction (OER), yet its intrinsic activity under high current density conditions is not satisfactory, which greatly constrains the industrial application of NiFe LDH catalysts. Herein, a new class of integrated Co and W co‐doped NiFe LDH catalysts is reported with dual cocatalytic sites for alkaline OER catalysis. The optimized Co2.8, W3.8‐NiFe LDH integrated catalyst has superior alkaline OER activity (255 mV@1000 mA cm−2) and excellent catalytic stability (200 h@500 mA cm−2). The turnover frequency value of Co2.8, W3.8‐NiFe LDH can reach 4.02 s−1 at 1.49 V versus RHE, which is 9.6 times higher than that of NiFe LDH and superior to the noble metal catalysts. Moreover, it can achieve 1.0 A cm−2 at 1.86 V and maintain 300‐h stable operation in anion exchange membrane water electrolyzer. Theoretical and experimental studies indicate that W sites promote the *OH adsorption and Co sites favor protons desorption of OH*. These dual cocatalytic sites jointly promote *O coverage, effectively accelerating alkaline OER kinetic.

Graph Neural Network Model Accelerates Biomass Adsorption Energy Prediction on Iron-group Hydrotalcite Electrocatalysts.

Iron-group layered double hydroxides (LDH) have demonstrated excellent biomass electrooxidation performance. However, the development of these materials relies on extensive experiments and high computational costs. Therefore, we developed a graph neural network (GNN) (named GALE-Net 2.0) for predicting the adsorption energies in the electrocatalytic reaction of 5-hydroxymethylfurfural (HMF). A data set of the adsorption energies of organic molecules on the LDH was constructed. The GNN model predicted that the 1:2 CoNi-doped LDH catalyst would demonstrate excellent HMF electrooxidation performance. The calculation time was reduced from 24 h with the density functional theory (DFT) calculations to 1 h with the GALE-Net 2.0. The mean absolute error of the GNN model was 0.17 eV, which is consistent with the accuracy of the DFT calculations. Moreover, the model showed some generality as it successfully predicted the adsorption energy of furan derivatives. Our results suggest that GALE-Net 2.0 can accelerate the design of electrocatalysts.

Partially amorphous NiFe layered double hydroxides enabling highly-efficiency oxygen evolution reaction at high current density

Layered double hydroxide (LDH) serves as an innovative catalyst for water electrolysis, showcasing outstanding performance in the oxygen evolution reaction (OER) under alkaline conditions. However, it faces challenges due to its low electrical conductivity and limited accessibility to active sites. In this work, the flexibility advantages of disordered amorphous and ordered crystals in NiFe LDH were combined to improve OER performance and maintain long-term stability. This combination induces a variety of effects, including improving the intrinsic activity, changing the OER mechanism from adsorb evolution mechanism (AEM) to lattice oxygen mechanism (LOM), and promoting the reaction kinetics of the catalyst. Moreover, the porous structure of NiFe LDH can efficiently alleviate the issue of local acidic environment induced by prolonged OER reaction, satisfying the criteria for long-term stability. Therefore, the NiFe-2.0 LDH catalyst only requires an ultralow overpotential of 189 mV at a current density of 10 mA cm−2 with Tafel slope of 43 mV dec−1. More importantly, the catalyst not only displays excellent electrocatalytic activity with an overpotential of 289 mV but also represents an outstanding stability over 80 h at an ultra-high current density of 1 A cm−2. This study provides a promising strategy for optimizing the catalytic activity and stability of catalyst at ampere current density, which is expected to achieve commercial applications.

Tailored Engineering of Layered Double Hydroxide Catalysts for Biomass Valorization: A Way Towards Waste to Wealth.

Layered double hydroxides (LDH) have significant attention in recent times due to their unique characteristic properties, including layered structure, variable compositions, tunable acidity and basicity, memory effect, and their ability to transform into various kinds of catalysts, which make them desirable for various types of catalytic applications, such as electrocatalysis, photocatalysis, and thermocatalysis. In addition, the upcycling of lignocellulose biomass and its derived compounds has emerged as a promising strategy for the synthesis of valuable products and fine chemicals. The current review focuses on recent advancements in LDH-based catalysts for biomass conversion reactions. Specifically, this review highlights the structural features and advantages of LDH and LDH-derived catalysts for biomass conversion reactions, followed by a detailed summary of the different synthesis methods and different strategies used to tailor their properties. Subsequently, LDH-based catalysts for hydrogenation, oxidation, coupling, and isomerization reactions of biomass-derived molecules are critically summarized in a very detailed manner. The review concludes with a discussion on future research directions in this field which anticipates that further exploration of LDH-based catalysts and integration of cutting-edge technologies into biomass conversion reactions hold promise for addressing future energy challenges, potentially leading to a carbon-neutral or carbon-positive future.

Development of an Effective Au : Pd Bimetallic Heterogeneous Catalyst for Oxidative Lignin Depolymerization to Low Molecular Weight Aromatics

AbstractLignin, a phenolic‐rich biomass component, holds promise for producing value‐added products. However, its complex structure and recalcitrance present challenges for its effective utilization. To overcome this, a AuPd/Li−Al layered double hydroxide (LDH) catalyst was developed to facilitate lignin depolymerization. Various AuPd bimetallic nanoparticle compositions were supported on a basic Li−Al LDH support via a sol‐immobilization method and characterized using N2‐physisorption, X‐ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The catalysts were then evaluated in the oxidation of several benzylic alcohols using O2 as the oxidant, from which Au7Pd3/Li−Al LDH and Au1Pd1/Li−Al LDH were identified as possessing promising catalytic activity. Further investigations focused on the aerobic oxidation of β‐O‐4 linked lignin model dimers under atmospheric pressure. The tested catalysts demonstrated sequential oxidation of the model compounds, leading to cleavage of the β‐O‐4 linkage. Finally, the catalysts were applied to the oxidative deconstruction of γ‐valerolactone (GVL) extracted maple lignin at 120 °C. Au1Pd1/Li−Al LDH emerged as the most effective catalyst, yielding a range of aromatic monomers with a total monomer yield of 27 %. These results highlight the potential of the Au1Pd1/Li−Al LDH catalyst system as an eco‐friendly approach for lignin depolymerization under mild conditions.

Bimetallic synergistic effect of Co and Fe for the highly efficient catalytic decomposition of ozone

AbstractGround‐level ozone pollution is a major concern for human health, ecosystems, and material degradation. This article presents a Co‐ and Fe‐based bimetallic layered double hydroxide (CoFe‐LDH) catalyst for ozone decomposition. The CoFe‐LDH catalyst exhibited 100% ozone decomposition efficiency over 24 h at a reaction rate of 35.3 μmol g−1 min−1. The synergetic effects between Co and Fe regulated the valence state, increased the number of Co vacancies, and increased the specific surface area and density of the hydroxyl groups on the surface. Moreover, the two metals facilitated different dissociation steps and the desorption step became exothermic after ozone enrichment on the surface, rendering every step concurrently spontaneous, which has not been observed in previously reported ozone decomposition catalysts. This study demonstrates the potential of LDHs for ozone pollution control and offers new insights into the design of synergistic bimetallic catalysts.

Ranitidine degradation in layered double hydroxide activated peroxymonosulfate system: impact of transition metal composition and reaction mechanisms.

Ranitidine, a competitive inhibitor of histamine H2 receptors, has been identified as an emerging micropollutant in water and wastewater, raising concerns about its potential impact on the environment and human health. This study aims to address this issue by developing an effective removal strategy using two types of layered double hydroxide (LDH) catalysts (i.e., CoFeLDH and CoCuLDH). Characterization results show that CoFeLDH catalyst has superior catalytic properties due to its stronger chemical bond compared to CoCuLDH. The degradation experiment shows that 100% degradation of ranitidine could be achieved within 20min using 25mg/L of CoFeLDH and 20mg/L of peroxymonosulfate (PMS). On the other hand, CoCuLDH was less effective, achieving only 70% degradation after 60min at a similar dosage. The degradation rate constant of CoFeLDH was 10 times higher than the rate constant of CoCuLDH at different pH range. Positive zeta potential of CoFeLDH made it superior over CoCuLDH regarding catalytic oxidation of PMS. The catalytic degradation mechanism shows that sulfate radicals played a more dominant role than hydroxyl radicals in the case of LDH catalysts. Also, CoFeLDH demonstrated a stronger radical pathway than CoCuLDH. XPS analysis of CoFeLDH revealed the cation percentages at different phases and proved the claim of being reusable even after 8 cycles. Overall, the findings suggest that CoFeLDH/PMS system proves to be a suitable choice for attaining high degradation efficiency and good stability in the remediation of ranitidine in wastewater.

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation.

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

Photoelectrochemical performance of a ternary WO3/BiVO4/NiCo LDH photoanode for high-efficiency water oxidation

Recently, tungsten oxide (WO3) and bismuth vanadate (BiVO4) have been considered as an intriguing combination for constructing a heterojunction for efficient photoelectrochemical (PEC) applications. Herein, we report the preparation of ternary WO3/BiVO4/NiCo layered double hydroxide (LDH) photoanodes for boosting photogenerated carrier transport and promotion of catalytic activity. WO3/BiVO4 heterostructures were synthesized by a hydrothermal process of WO3 nanoplates on a fluorine-doped tin oxide (FTO) glass substrate, followed by spin-coating for BiVO4 materials. We investigated the effect of a NiCo LDH catalyst on PEC performance by controlling the growth temperatures of the NiCo LDH via hydrothermal synthesis. Moreover, the X-ray diffraction (XRD), Fourier‒transform infrared (FTIR) and X‒ray photoelectron spectroscopy (XPS) analyses revealed that the WO3/BiVO4 heterojunction was retained after the NiCo LDH growth process regardless of synthesis temperatures. The PEC results verified that the WO3/BiVO4 heterostructure with the optimized NiCo LDH catalyst (WBN60) possessed the best photocurrent density of 3.2 mA/cm2 at 1.23 V vs. reversible hydrogen electrode (RHE), which created improved charge separation and surface oxidation kinetics of photogenerated carriers. Consequentially, the smallest charge transfer resistance and recombination rate were achieved in the WBN60 electrode compared to others, indicating a much lower photoelectrode‒electrolyte interfacial resistance and the enhancement of photogenerated carrier transport by suppressing recombination.

Efficient and Durable Oxidation Removal of Formaldehyde over Layered Double Hydroxide Catalysts at Room Temperature.

Room temperature catalytic oxidation (RTCO) using non-noble metals has emerged as a highly promising technique for removal of formaldehyde (HCHO) under ambient conditions; however, non-noble catalysts still face the challenges related to poor water resistance and low stability under harsh conditions. In this study, we synthesized a series of layered double hydroxides (LDHs) incorporating various dual metals (MgAl, ZnAl, NiAl, NiFe, and NiTi) for formaldehyde oxidation at ambient temperature. Among the synthesized catalysts, the NiTi-LDH catalyst showed an HCHO removal efficiency and CO2 yield close to 100.0%, and exceptional water resistance and chemical stability on running 1300 min. The abundant hydroxyl groups in LDHs directly bonded with HCHO, leading to the production of CO2 and H2O, thus inhibiting the formation of CO, even in the absence of O2 and H2O. The coexistence of O2 effectively reduced the reaction barrier for H2O molecule dissociation, facilitating the formation of hydroxyl groups and their subsequent backfill on the catalyst surface. The mechanisms underlying the involvement and regeneration of hydroxyl groups in room temperature oxidation of formaldehyde were elucidated with the combined in situ DRIFTS, HCHO-TPD-MS, and DFT calculations. This work not only demonstrates the potential of LDH catalysts in environmental applications but also advances the understanding of the fundamental processes involved in room temperature oxidation of formaldehyde.

Lattice-matched spinel/layered double hydroxide 2D/2D heterojunction towards large-current-density overall water splitting

It is fascinating and challenging to construct earth-abundant first-row transition metal-based bifunctional electrocatalysts for large-scale hydrogen production via water electrolysis. Here we report a progressive conversion from spinel nanosheets to layered double hydroxide (LDH) nanosheets through the preferential dissolution of alternated layers in spinel. This process enables the formation of a lattice-matched and chemically bonded edge-to-edge spinel/LDH 2D/2D heterojunction. Significant charge redistribution at the 2D/2D heterointerface accelerates water dissociation kinetics and optimize adsorption energy of hydrogen- and oxygen-containing intermediates. The synthesized CoFe2O4/CoFe-LDH 2D/2D heterojunction demonstrates exceptional performance in the hydrogen evolution reaction, achieving an overpotential of 337 mV at a high current density of 1000 mA cm−2, surpassing previously reported non-noble metal-based spinels and LDHs. Furthermore, its oxygen evolution reaction performance is comparable to that of typical spinel and LDH catalysts. The in-depth insights can guide rational interfacial engineering of heterogeneous catalysts for energy and environmental applications.

Effective visible light-driven binary ZnO catalyst decorated on layered double hydroxides for simultaneous photocatalytic degradation of Ciprofloxacin and Ampicillin

BackgroundThe direct use of clean solar energy to convert organic pollutants in industrial wastewater into harmful products is a viable tactic and a hot topic to save water bodies and avoid water pollution. Pharmaceutical wastewater is complicated and contains a variety of contaminants, making its treatment difficult. MethodThis study focuses on the simultaneous degradation of an aqueous solution containing a combination of contaminants, Ampicillin (AMP) and Ciprofloxacin (CIP). Using hydrothermal process, we successfully fabricated a ZnO-NiCoMn layered double hydroxide (LDH) sheet. The prepared catalyst was examined using a range of surface analytical optical techniques. Significant findingsThe ZnO-NiCoMn LDH demonstrates exceptional photocatalytic performance, as demonstrated by the maximal CIP and AMP degradation rates of 96 % and 94 % in 100 min under visible light, respectively, according to the data. ZnO-adorned NiCoMn LDH sheets exhibited a rod-like structure, as demonstrated by FE-SEM and HR-TEM. Because of the highest charge separation and superior electron (e−) transport between ZnO and NiCoMn LDH material, the binary catalyst degraded CIP and AMP by 96 % and 94 % in 100 min, respectively, more than the bare catalyst. The binary ZnO-NiCoMn LDH catalyst could withstand up to five CIP and AMP degradation cycles.

Effect of divalent cations on the basicity and catalytic performance of layered double hydroxide heterogeneous catalyst in microwave-assisted aldol-condensation of benzaldehyde with 1–heptanal

In this study, we investigated the catalytic activity of bimetallic layered double hydroxide catalysts (M–Al–LDH) in the aldol condensation reaction between benzaldehyde (B) and 1–heptanal (H). Specifically, we used M–Al–LDH, where M represents magnesium (Mg), nickel (Ni), and zinc (Zn) with the same molar ratio of M/Al = 3, as heterogeneous basic catalysts for the aldol–condensation to produce jasminaldehyde (J). To prepare the M–Al–LDH catalysts, we employed the co-precipitation method using the nitrate precursors under a nitrogen atmosphere. Sodium hydroxide was used as the precipitating agent. The resulting solids were characterized using various techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA/DTA), and BET textural analysis, to study their structural properties. The performance of the catalysts was evaluated and compared in the aldol condensation reaction of benzaldehyde with 1–heptanal under solvent-free conditions, which were carried out in a microwave oven. We also investigated the influence of several reaction parameters on the conversion of 1–heptanal and the selectivity ratio (J/P) in detail to determine the optimal reaction conditions. The results obtained showed that the optimized Mg–Al–NO3 catalyst exhibited a higher conversion of 1–heptanal (99.7%) with a good selectivity ratio (J/P) of 4.00 compared to the other materials studied. Furthermore, the reaction proceeded rapidly in all cases, indicating the practicality of our protocol for the synthesis of jasminaldehyde.

Identifying Active Sites of Pt‐NiFe LDH Catalysts for CO Oxidation Using In‐Situ DRIFTS with Multiple Microscopy and Spectroscopy Techniques

AbstractLayered double hydroxide (LDH) nanostructures are commonly used to study the interfacial effects of Pt‐based catalysts in CO oxidation. However, the specific role and detailed intermediates involved in CO oxidation remain unclear. Density functional theory (DFT) predicted that NiFe LDH loaded with single Pt atoms would exhibit satisfactory CO oxidation activity. On this basis, we prepared Pt1NiFe LDH catalysts with various Fe : Ni ratios by using the one‐pot method and achieved favorable dispersion of single Pt atoms. Pt1/FN‐1, which had the optimal Fe : Ni ratio (1 : 1), exhibited satisfactory catalytic performance for CO oxidation and a high turnover frequency, as determined through DFT calculations and experimental characterization. In‐situ diffuse reflectance infrared Fourier transform spectroscopy confirmed that CO directly reacted with Pt1/FN‐1 to produce CO2 and that introduced O2 formed bicarbonate species to produce CO2 together with the adsorbed CO on Pt1 sites. Overall, our work provides new insights into the catalytic mechanism of the well‐known Pt1NiFe LDH catalysts in CO oxidation.

Performance and mechanism of efficient activation of peroxymonosulfate by Co-Mn-ZIF derived layered double hydroxide for the degradation of enrofloxacin

Due to the uncontrolled synthesis of layered double hydroxides (LDHs), a simple and efficient strategy was needed to construct high-performance LDHs. Herein, Co-Mn-ZIF was used as the sacrificial template to form cobalt-manganese-nickel LDH (M−Co−Mn−Ni LDH) through the process of deconstruction and reconstruction. Due to the sufficient in-situ etching of the Co-Mn-ZIF template, M−Co−Mn−Ni LDH exhibited a larger specific surface area (482.4 m2/g) and pore volume (3.3 × 10−2 cm3/g) as well as easier diffusion and interaction with reaction sites of organic molecules. The prepared M−Co−Mn−Ni LDH showed high catalytic performance for the degradation of enrofloxacin (ENR) by activating peroxymonosulfate (PMS), with the removal rate of 88.5 % within 30 min. The quenching reaction and electron paramagnetic resonance (EPR) analysis results indicated that SO4•− and 1O2 were main active substances in the oxidation process of ENR. In addition, the Co-Mn-Ni LDH/PMS system was stable and almost unaffected by pH values, organic and inorganic substances, and it could achieve high removal efficiency in various pollutant samples. This work provided a convenient and efficient approach for the construction of high-performance MOFs derived LDHs catalysts.

Removal of Food Dyes by Peroxymonosulfate and Hydrogen Peroxide Activation Using Layered Double Hydroxide Catalysts: Optimization of Reaction Conditions by Box-Behnken Design

Bu çalışmada, çeşitli tabakalı çift hidroksit (TÇH) katalizörlerinin peroksimonosülfat ve hidrojen peroksit aktivasyonu performansları, model gıda boyası olarak seçilen tartrazinin sulu çözeltilerden uzaklaştırılmasında test edilmiştir. Cu-Fe-TÇH, Co-Fe-TÇH ve Ni-Fe-TÇH’nin peroksimonosülfat ve hidrojen peroksit varlığında katalitik aktiviteleri karşılaştırılmıştır. Farklı oksidanlar kullanılarak gerçekleştirilen katalizör tarama deneylerinde Co-Fe-TÇH ve peroksimonosülfat en uygun katalizör ve oksidan olarak belirlenmiştir. Katalizör yüklemesi, pH ve oksidan/boya molar oranının etkileşimli etkileri araştırılmış ve Box-Behnken Design ve tepki yüzeyi metodolojisi kullanılarak reaksiyon koşulları optimize edilmiştir. 2 g/L katalizör yüklemesi, pH 3 ve 11,36 oksidan/boya molar oranı olarak belirlenen optimum reaksiyon koşullarında %87.35 organik madde giderimi ve %97.47 renk giderimi elde edilmiştir.

MgAl layered double hydroxide (LDH) for promoting ammonia synthesis in non-thermal plasma: Role of surface oxygen vacancy

Here, the MgAl layered double hydroxide (LDH) supports and relevant metal on LDH catalysts (i.e., Ni, Co, and Ru) were prepared and investigated under non thermal plasma (NTP) conditions to probe the role of surface oxygen vacancy (OV) in NTP-assisted ammonia (NH3) synthesis. The findings show that OV on the LDH carrier is highly beneficial to NH3 formation under NTP conditions, and concentration of OV on the LDH can be regulated by the post-synthesis calcination and hydrogen plasma etching. Additionally, loading of active metal species on the LDH could promote the NH3 synthesis further due to presence of multiple reaction pathways and the synergy between the surface OV and metal sites in such NTP-catalytic systems. As the result, the catalysts developed by this work showed high ammonia synthesis rates of 4.42−4.52 mmol g−1 h−1 and energy efficiencies of 1.67−1.71 gNH3 kWh−1, respectively. The findings of the work pave the way for the rational design and optimization of highly efficient catalysts with dual active sites for intensifying the NTP-catalytic ammonia synthesis.

Pomegranate-like structured FeNi-nanodots@FeNi LDH composite as a high performance bifunctional catalyst for oxygen electrocatalytic reactions in zinc-air batteries

An efficient bifunctional oxygen catalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for the widespread application of rechargeable zinc-air batteries (ZABs) with high energy density and low-cost electrically. Herein, we report an effective pomegranate-like structured FeNi–N–C@FeNi LDH catalyst using Zeolitic Imidazolate Framework (ZIF-8) as precursor via pyrolysis and hydrothermal reactions. The as-prepared FeNi–N–C@FeNi LDH composite, consisting of FeNi-nanodots embedded in carbon matrix and metal layered double hydroxides (LDHs) as outer layer, exhibits preferable performance toward ORR (E1/2 = 0.890V) and OER (Ej = 10 = 1.535V) compared to precious metal-based catalysts. Moreover, when being applied in ZABs, the FeNi–N–C@FeNi LDH composite displays a high open-circuit voltage of 1.497 V and a durability of up to 100 h, demonstrating its great possibility in practical application.

Structural Transformations in Ni (Oxy)Hydroxide Host Structures Under Operating Conditions for Oxygen Evolution Electrocatalysts

The incorporation of Fe into the brucite-like Ni(OH)2 host structure increases significantly the catalytic activity for the oxygen evolution reaction (OER).1 The resulting hydrotalcite-like crystalline structure is known as NiFe layered double hydroxide (LDH). However, even the most active NiFe LDH catalyst still shows a considerable overpotential for the OER.2 Understanding the nature of the catalytic active sites and the catalytic mechanism in NiFe LDHs are key challenges to develop better OER electrocatalysts, as well as the fundamental understanding of the role of the Ni-based host structure.In this contribution, atomic-scale details of the catalytic active phase will be presented, showing that NiFe LDHs are oxidized under applied anodic potentials from as-prepared α-phases to activated γ-phases.3 The interlayer and in-plane lattice parameters of the OER-active γ-phase were obtained by performing wide angle X-ray scattering (WAXS) measurements on NiFe LDH nanoplatelets during operating electrochemical conditions and were characterized by a contraction of about 8%. Operando WAXS was then performed for other selected catalysts belonging to the transition metal LDH family of materials. Structural similarities of the catalytically active phases will be highlighted. In addition, the structural transformations of β-Ni(OH)2 will be discussed, as potential host structure for novel multi-element catalysts. Also for Ni(OH)2, the γ-phase is found to be the catalytically active phase. However, the structural transformations are more complex, involving multiple limiting phases. Their structural stability is presented and the discussion supported by density functional theory calculations. References L. Trotochaud, S. L. Young, J. K. Ranney and S. W. Boettcher, Journal of the American Chemical Society, 2014, 136: 6744-6753.F. Dionigi and P. Strasser, Advanced Energy Materials, 2016, 6 1600621.F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. Ferreira de Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. Roldan Cuenya and P. Strasser, Nat Commun, 2020, 11 2522.