Promoting Bifunctional Water Splitting by Modification of the Electronic Structure at the Interface of NiFe Layered Double Hydroxide and Ag.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Constructing microstructures in nickel-iron layered double hydroxide electrocatalysts by cobalt doping for efficient overall water splitting

Benchmarking of oxygen evolution catalysts on porous nickel supports

For effective hydrogen production by water splitting, it is essential to develop earth-abundant, highly efficient, and durable electrocatalysts. Herein, the authors report a bifunctional electrocatalyst composed of hollow CoSx and Ni-Fe based layered double hydroxide (NiFe LDH) nanosheets for efficient overall water splitting (OWS). The optimized heterostructure is obtained by the electrodeposition of NiFe LDH nanosheets on metal-organic framework-derived hollow CoSx nanoarrays, which are supported on nickel foam (H-CoSx @NiFe LDH/NF). The unique structure of the hybrid material not only provides ample active sites, but also facilitates electrolyte penetration and gas release during the reactions. Additionally, the strong coupling and synergy between the hydrogen evolution reaction (HER) active CoSx and the oxygen evolution reaction (OER) active NiFe LDH gives rise to the excellent bifunctional properties. Consequently, H-CoSx @NiFe LDH/NF exhibits remarkable HER and OER activities with overpotentials of 95 and 250 mV, respectively at 10 mA cm-2 in 1.0 M KOH. Even at 1.0 A cm-2 , the electrode requires small overpotentials of 375 mV (for HER) and 418 mV (for OER), respectively. An electrolyzer based on H-CoSx @NiFe LDH/NF demonstrates a low cell voltage of 1.98 V at a current density of 300 mA cm-2 and good durability for 100 h in OWS application.

Constructing catalysts with new and optimizational chemical components and structures, which can operate well for both the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER) at large current densities, is of primary importance in practical water splitting technology. Herein, the NiFe2O4 nanoparticles/NiFe layered double hydroxide (LDH) nanosheet heterostructure array on Ni foam was prepared via a simple one-step solvothermal approach. The as-prepared heterostructure array displays high catalytic activity toward the OER with a small overpotential of 213 mV at 100 mA cm-2 and can afford a current density of 500 mA cm-2 at an overpotential of 242 mV and 1000 mA cm-2 at 265 mV. Moreover, it also presents outstanding HER activity, only needing a small overpotential of 101 mV at 10 mA cm-2, and can drive large current densities of 500 and 750 mA cm-2 at individual overpotentials of 297 and 314 mV. A two-electrode electrolyzer using NiFe2O4 nanoparticles/NiFe LDH nanosheets as both the anode and the cathode implements active overall water splitting, demanding a low voltage of 1.535 V to drive 10 mA cm-2, and can deliver 500 mA cm-2 at 1.932 V. The NiFe2O4 nanoparticles/NiFe LDH nanosheet array electrodes also show excellent stability against OER, HER, and overall water splitting at large current densities. Significantly, the overall water splitting with NiFe2O4 nanoparticles/NiFe LDH nanosheets as both the anode and the cathode can be continuously driven by a battery of only 1.5 V. The intrinsic advantages and strong coupling effects of NiFe2O4 nanoparticles and NiFe LDH nanosheets make NiFe2O4 nanoparticles/NiFe LDH nanosheet heterostructure array abundant catalytically active sites, high electronic conductivity, and high catalytic reactivity, which remarkably contributed to the catalytic activities for OER, HER, and overall water splitting. Our work can inspire the optimal design of the NiFe bimetallic heterostructure electrocatalyst for application in practical water electrolysis.

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

Evolution of layered double hydroxides (LDH) as high performance water oxidation electrocatalysts: A review with insights on structure, activity and mechanism

Among water splitting techniques, electrochemical water splitting is enhanced using efficient catalysts to complete hydrogen evolution (HER) and oxygen evolution (OER) reactions. However, when it comes to commercial level processing to create water electrolyzers including AEM and PEM electrolyzers, these catalysts mostly in powder state require to be immobilized onto a current collector using a suitable polymeric binder. This coating process is very important to maintain the catalyst strength, reducing the interfacial resistance between catalyst and current collector etc. However, peeling off of the catalysts and thereby catalyst aggregation is often confronted during long term operation causing large decrease in electrolyzer performance. In this scenario, self-supported catalysts which are directly grown or developed on conductive substrates or forming free-standing films are identified as a solution to overcome this problem while progressing to realize efficient water electrolyzers. Some major advantages of the self-supported catalysts include; direct use of catalysts as anode/cathode electrodes, excellent synergistic effect between the catalyst and substrate, reduced peeling off catalysts and more importantly greater charge transfer between catalyst layer and current collector. In the present work, we developed self-supporting multi metal catalysts over nickel foam which can be used as electrode materials for integrating water electrolyzers capable of high performance and durability in alkaline conditions. Oxygen evolution reaction (OER) studies under half cell conditions in 1 M KOH using the developed self-supported catalysts involving Fe and Ru over nickel foam displayed an over potential of 185 mV at 10 mA cm-2, while 111 mV was for observed during hydrogen evolution reaction (HER). The presentation will include water splitting performance data using the processed catalysts under alkaline conditions and also the detailed electrochemical and spectroscopic results during and post OER/HER. Further, the synergistic interactions among the metal species, creation of active species/sites and changes in electron charge transfer leading to the excellent activity and stability will also be discussed. Fig 1. HER and OER CV profiles in half cell conditions using the self-supported catalysts References Zhang et al, Homogeneously dispersed multimetal oxygen-evolving catalysts, Science 2016, 352,333-337 Kwon, H. Han, S. Choi, K. Park, S. Jo, U. Paik, T. Song, Current Status of Self-Supported Catalysts for Robust and Efficient Water Splitting for Commercial Electrolyzer, ChemCatChem 2019, 11, 5898–59 Miyanishi, T. Yamaguchi, Highly conductive mechanically robust high M wpolyfluorene anion exchange membrane for alkaline fuel cell and water electrolysis application, Polym. Chem. 2020, DOI: 10.1039/D0PY00334DA. Miller, K. Bouzek, J. Hnat, S. Loos, C. I. Bernacker, T. Weißgarber, L. Rontzsch, J. Meier-Haack, Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions, Sustainable Energy Fuels, 2020,4, 2114-2133 Acknowledgements: This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan Figure 1

Tetrafunctional electrocatalyst for oxygen reduction, oxygen evolution, hydrogen evolution, and carbon dioxide reduction reactions

Efficient bifunctional electrocatalyst for water splitting is essential for replacing fossil-fuel energy sources with clean energy-dense hydrogen fuel (142 MJ/kg). Efficient electrocatalyst can be obtained by either increasing active site density or specific activity on individual active sites. The active site densities can be increased through roughening the potential energy surface or exposing the facets which has higher active site densities. The specific activity can be increased through modulation of strain or charge densities on active sites which can be achieved through introduction of dopants, defects or stabilization of “non-native phases” that are all the other crystalline and amorphous states that differ in terms of discrete translational symmetry in the sub-surface region from the “native” phase (or bulk ground-state). While for a given composition, there is a unique native state for a given set of thermodynamic condition while, there can be many non-native structures having different bond-angles, bond-distances and surface atom densities from the native phase, leading to different electrocatalytic properties. In this context, polymorphic engineering via stabilizing ‘non-native phase’ offers a potential approach for improving both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts and its activity. The beneficial effect of polymorphic engineering with regards to bifunctional electrochemical OER and HER is demonstrated by first principle calculation by taking CoSe2 as a model electrocatalyst which has marcasite (Space Group-58) and pyrite (Space Group-205) as the native (N) and non-native (NN) structures, respectively. The first principle computations predict pyrite (NN) structure of CoSe2 would have better electrochemical activity towards OER and HER than its marcasite (N) counterpart which is confirmed through experimental results in literature too. Though the co-ordination number of Co remains same in both the structures, the co-ordination symmetry surrounding Co atom varies. This results in differential charge distribution in constituting Co- and Se-atoms consequently resulting in variable density of state (DOS) near Fermi level (Figure 1) thereby affecting the binding energies (BE) of reaction intermediates of OER and HER. Pyrite (NN) phase of CoSe2 has a greater electron density near Fermi Level in comparison to its marcasite (N) counterpart due to differential co-ordination symmetry. A greater electron density near Fermi-level is indicative of lower work function and consequently lower polarization resistance during water splitting. A greater electron density near Fermi level is contributed by Co-3d orbitals which is the common active site for both OER and HER. The greater electron density and lower work function in Pyrite (NN) results in stronger metal-hydrogen BE (0.03 eV) resulting in lower overpotential of HER. Hydrogen adsorption on Se sites occurs only at higher HER overpotential due to weak Se-hydrogen BE (0.59 eV). This results in observation of twin Tafel slopes during HER on CoSe2 electrocatalyst as the potential determination step (PDS) switches from Volmer to Heyrovsky step with participation of Se during HER. The lower work function and higher electron density near Fermi level in Pyrite (NN) structure results in weaker metal-oxygen bond thereby promoting multi-electron OER activity. The OER intermediates (-OH, -O, -OOH) has a higher BE over Co- than Se-sites. The transformation of Oads à HOOads on Co-sites of CoSe2 (001) structure is the potential determination step with an onset potential of 1.66 V (vs RHE).The desorption of O2 from Se site is found to be the potential-determination-step (PDS) for OER (η=0.79 V). Furthermore, pristine CoSe2 acts as a precursor for OER which undergoes dissolution to form a surface Co-O structure which has a greater activity than pristine pyrite CoSe2 surfaces (η=0.31 V). This energetics is more favourable for pyrite (NN) structure than marcasite (N) structure for dissolution process to form surface Co-O structure due to stronger Co-Se bonds present in the latter case. Furthermore, point-defects which can aid both OER and HER, can be more easily formed in pyrite (NN) structure than marcasite (N) structure due to the aforementioned reason. The present study underlines the importance of stabilization of non-native structures which has a great potential to produce higher electrocatalytic activity thus providing greater options in search of better water splitting electrocatalyst. Figure 1

Designing cost-effective electrocatalysts with fast reaction kinetics and high stability is an outstanding challenge in green hydrogen generation through overall water splitting (OWS). Layered double hydroxide (LDH) heterostructure materials are promising candidates to catalyze both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), the two OWS half-cell reactions. This work develops a facile hydrothermal route to synthesize hierarchical heterostructure MoS2@NiFeCo-LDH and MoS2@NiFeCo-Mo(doped)-LDH electrocatalysts, which exhibit extremely good OER and HER performance as witnessed by their low IR-corrected overpotentials of 156 mV and 61 mV with at a current density of 10 mA cm-2 under light assistance. The MoS2@NiFeCo-Mo(doped)-LDH‖MoS2@NiFeCo-LDH OWS cell achieves a low cell voltage of 1.46V at 10 mA cm-2 during light-assisted water electrolysis. Both materials exhibited exceptional stability under industrially relevant HER and OER conditions, maintaining a current density of 1 A cm-2 with minimal alterations in their potential and performance. The experimental and computational results demonstrate that doping the LDH matrix with high-valence Mo atoms and MoS2 quantum dots improves the electrocatalytic activity by 1) enhancing electron transfer, 2) making the electrocatalyst metallic, 3) increasing the number of active sites, 4) lowering the thermodynamic overpotential, and 5) changing the OER mechanism. Overall, this work develops a facile synthesis method to design highly active and stable MoS2@NiFeCo-Mo(doped)-LDH heterostructure electrocatalysts.

Designing cost-effective electrocatalysts with fast reaction kinetics and high stability is an outstanding challenge in green hydrogen generation through overall water splitting (OWS). Layered double hydroxide (LDH) heterostructure materials are promising candidates to catalyze both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), the two OWS half-cell reactions. This work develops a facile hydrothermal route to synthesiz hierarchical heterostructure MoS2@NiFeCo-LDH and MoS2@NiFeCo-Mo(doped)-LDH electrocatalysts, which exhibit extremely good OER and HER performance as witnessed by their low IR-corrected overpotentials of 156 and 61 mV with at a current density of 10mA cm-2 under light assistance. The MoS2@NiFeCo-Mo(doped)-LDH-MoS2@NiFeCo-LDH OWS cell achieves a low cell voltage of 1.46V at 10mA cm-2 during light-assisted water electrolysis. Both materials exhibited exceptional stability under industrially relevant HER and OER conditions, maintaining a current density of 1A cm-2 with minimal alterations in their potential and performance. The experimental and computational results demonstrate that doping the LDH matrix with high-valence Mo atoms and MoS2 quantum dots improves the electrocatalytic activity by 1) enhancing electron transfer, 2) making the electrocatalyst metallic, 3) increasing the number of active sites, 4) lowering the thermodynamic overpotential, and 5) changing the OER mechanism. Overall, this work develops a facile synthesis method to design highly active and stable MoS2@NiFeCo-Mo(doped)-LDH heterostructureelectrocatalysts.

Owing to its earth abundance, low kinetic overpotential, and superior stability, NiFe-layered double hydroxide (NiFe-LDH) has emerged as a promising electrocatalyst for catalyzing water splitting, especially oxygen evolution reaction (OER), in alkaline solutions. Unfortunately, as a result of extremely sluggish water dissociation kinetics (Volmer step), hydrogen evolution reaction (HER) activity of the NiFe-LDH is rather poor in alkaline environment. Here a novel strategy is demonstrated for substantially accelerating the hydrogen evolution kinetics of the NiFe-LDH by partially substituting Fe atoms with Ru. In a 1 m KOH solution, the as-synthesized Ru-doped NiFe-LDH nanosheets (NiFeRu-LDH) exhibit excellent HER performance with an overpotential of 29 mV at 10 mA cm-2 , which is much lower than those of noble metal Pt/C and reported electrocatalysts. Both experimental and theoretical results reveal that the introduction of Ru atoms into NiFe-LDH can efficiently reduce energy barrier of the Volmer step, eventually accelerating its HER kinetics. Benefitting from its outstanding HER activity and remained excellent OER activity, the NiFeRu-LDH steadily drives an alkaline electrolyzer with a current density of 10 mA cm-2 at a cell voltage of 1.52 V, which is much lower than the values for Pt/C-Ir/C couple and state-of-the-art overall water-splitting electrocatalysts.

The development of an efficient and cost-effective oxygen evolution reaction (OER) catalyst is important in increasing the overall efficiency of the electrochemical water splitting process to produce green hydrogen. In this work, the substitution of copper in NiFe layered double hydroxide (LDH) was utilized to improve the sluggish kinetics of the water oxidation process. The simple in situ hydrothermal method was used to introduce copper into the NiFe LDH structure. All of the prepared catalysts displayed a sheet-like morphology, with the optimized NiCuFe LDH sample exhibiting a BET specific surface area of 117.5 m2 g-1. The optimized Cu-substituted LDH exhibited a superior performance in the alkaline water splitting process by requiring a lower overpotential of 230 mV to attain a current density of 10 mA cm-2, accompanied by a low Tafel constant of 47.7 mV dec-1, by outperforming the pristine NiFe LDH. The electronic structure modification of NiFe LDH by Cu atoms favors the OER process, which is verified by the density functional theory (DFT). Further, the optimized electrode was utilized in real-world conditions of the saline-alkaline electrolyte for water splitting, necessitating a minimal overpotential of 247.5 mV to oxidize water, and the electrode demonstrated long-term stability. Thus, NiCuFe LDH is a potential OER catalyst for large-scale electrochemical water splitting applications.

The development of efficient and sustainable energy conversion technologies has driven extensive research into the hydrogen evolution reaction (HER) as a key component of water splitting. Hydrogen, as a clean and renewable energy carrier, has the potential to revolutionize energy storage and utilization. However, the efficiency of HER is largely dependent on the development of electrocatalysts capable of operating under a wide range of conditions with low overpotential and high current density. Conventional noble-metal catalysts such as platinum (Pt) exhibit excellent HER performance but suffer from high cost and limited availability, necessitating the exploration of cost-effective and earth-abundant alternatives.Graphene quantum dots (GQDs) have emerged as promising candidates due to their unique electronic properties, including a high surface-to-volume ratio, tunable bandgap, and excellent electrical conductivity. These characteristics enable GQDs to serve as effective co-catalysts or dopants in hybrid materials, enhancing charge transfer kinetics and catalytic activity. Layered double hydroxides (LDHs), particularly Co-NiFe LDHs, have demonstrated outstanding electrocatalytic performance for both HER and the oxygen evolution reaction (OER), making them attractive bifunctional catalysts for overall water splitting. The synergistic interaction of Co, Ni, and Fe within the LDH structure provides a favorable environment for enhanced electron transfer and active site availability, thereby improving catalytic efficiency.In this study, we employ machine learning-assisted density functional theory (ML-DFT) calculations to investigate the HER performance of GQDs integrated with Co-NiFe LDH. The ML-DFT approach enables efficient and accurate exploration of a vast parameter space, allowing us to predict the electronic structure, adsorption energies, and reaction pathways with high fidelity. By leveraging ML-DFT, we systematically evaluate the effects of GQD incorporation on the catalytic activity of Co-NiFe LDH. ML-DFT calculations reveal that the GQDs/Co-NiFe LDH exhibits greater potential for HER, with free adsorption energy calculations confirming enhanced catalytic activity upon GQD incorporation. Additionally, our findings confirm that the GQD/Co-NiFe LDH composite exhibits enhanced electron transfer capabilities and increased active site availability, leading to improved HER performance, effectively addressing prior concerns regarding catalytic efficiency. The incorporation of GQDs significantly modifies the electronic structure of Co-NiFe LDH, facilitating improved charge transfer and optimizing hydrogen adsorption characteristics.The integration of GQDs with Co-NiFe LDH is hypothesized to create a hybrid material with improved HER performance, attributed to the synergistic interaction between the GQD-induced electronic modulation and the catalytic active sites of the LDH. This study provides fundamental insights into the structure-property relationships governing HER activity in hybrid catalysts and highlights the potential of GQD-based materials for next-generation electrocatalysts. By bridging advanced computational techniques with materials design, our work paves the way for the rational development of cost-effective and high-performance catalysts for sustainable hydrogen production.