Ni Single Atoms Research Articles

Ruthenium Core–Shell Engineering with Nickel Single Atoms for Selective Oxygen Evolution via Nondestructive Mechanism

AbstractTo develop effective electrocatalytic splitting of acidic water, which is a key reaction for renewable energy conversion, the fundamental understanding of sluggish/destructive mechanism of the oxygen evolution reaction (OER) is essential. Through investigating atom/proton/electron transfers in the OER, the distinctive acid–base (AB) and direct‐coupling (DC) lattice oxygen mechanisms (LOMs) and adsorbates evolution mechanism (AEM) are elucidated, depending on the surface‐defect engineering condition. The designed catalysts are composed of a compressed metallic Ru‐core and oxidized Ru‐shell with Ni single atoms (SAs). The catalyst synthesized with hot acid treatment selectively follows AB‐LOM, exhibiting simultaneously enhanced activity and stability. It produces a current density of 10/100 mA cm−2 at a low overpotential of 184/229 mV and sustains water oxidation at a high current density of up to 20 mA cm−2 over ≈200 h in strongly acidic media.

Hydroxyl improving the activity, selectivity and stability of supported Ni single atoms for selective semi-hydrogenation.

Atomically dispersed metal catalysts with high atomic utilization and selectivity have been widely studied for acetylene semi-hydrogenation in excess ethylene among others. Further improvements of activity and selectivity, in addition to stability and loading, remain elusive due to competitive adsorption and desorption between reactants and products, hydrogen activation, partial hydrogenation etc. on limited site available. Herein, comprehensive density functional theory calculations have been used to explore the new strategy by introducing an appropriate ligand to stabilize the active single atom, improving the activity and selectivity on oxide supports. We find that the hydroxyl group can stabilize Ni single atoms significantly by forming Ni1(OH)2 complexes on anatase TiO2(101), whose unique electronic and geometric properties enable high performance in acetylene semi-hydrogenation. Specifically, Ni1(OH)2/TiO2(101) shows favorable acetylene adsorption and promotes the heterolytic dissociation of H2 achieving high catalytic activity, and it simultaneously weakens the ethylene bonding to facilitate subsequent desorption showing high ethylene selectivity. Hydroxyl stabilization of single metal atoms on oxide supports and promotion of the catalytic activity are sensitive to transition metal and the oxide supports. Compared to Co, Rh, Ir, Pd, Pt, Cu, Ag and Au, and anatase ZrO2, IrO2 and NbO2 surfaces, the optimum interactions between Ni, O and Ti and resulted high activity, selectivity and stability make Ni1(OH)2/TiO2(101) a promising catalyst in acetylene hydrogenation. Our work provides valuable guidelines for utilization of ligands in the rational design of stable and efficient atomically dispersed catalysts.

A Ni or Co single atom anchored conjugated microporous polymer for high-performance photocatalytic hydrogen evolution

A single nickel or cobalt atom anchored pyridyl-functionalized conjugated microporous polymer promoted the photogenerated electron transfer, offering efficient hydrogen evolution performance under visible-light irradiation.

Incorporation of nickel single atoms into carbon paper as self-standing electrocatalyst for CO2reduction

The incorporation of Ni single atoms into carbon paper produces self-standing electrode for efficient CO2reduction to CO in water.

Coordination environment engineering on nickel single-atom catalysts for CO2 electroreduction.

Coordination engineering has recently emerged as a promising strategy to boost the activity of single atom catalysts (SACs) in electrocatalytic CO2 reduction reactions (CO2RR). Understanding the correlation between activity/selectivity and the coordination environment would enable the rational design of more advanced SACs for CO2 reduction. Herein, via density functional theory (DFT) computations, we systematically studied the effects of coordination environment regulation on the CO2RR activity of Ni SACs on C, N, or B co-doped graphene. The results reveal that the coordination environments can strongly affect the adsorption and reaction characteristics. In the C and/or N coordinated Ni-BXCYNZ (B-free, X = 0), only Ni acts as the active site. While in the B, C and/or N coordinated Ni-BXCYNZ (X ≠ 0), the B has transition-metal-like properties, where B and Ni function as dual-site active centers and concertedly tune the adsorption of CO2RR intermediates. The tunability in the adsorption modes and strengths also results in a weakened linear scaling relationship between *COOH and *CO and causes a significant activity difference. The CO2RR activity and the adsorption energy of *COOH/*CO are correlated to construct a volcano-type activity plot. Most of the B, C, and/or N-coordinated Ni-BXCYNZ (X ≠ 0) are located in the left region where *CO desorption is the most difficult step, while the C and/or N coordinated Ni-BXCYNZ (X = 0) are located in the right region where *COOH formation is the potential-determining step. Among all the possible Ni-BXCYNZ candidates, Ni-B0C3N1 and Ni-B1C1N2-N-oppo are predicted to be the most active and selective catalysts for the CO2RR. Our findings provide insightful guidance for developing highly effective CO2RR catalysts based on a codoped coordination environment.

Regulated coordination environment of Ni single atom catalyst toward high-efficiency oxygen electrocatalysis for rechargeable Zinc-air batteries

Single atom catalysts (SACs) are emerging as a highly promising catalyst category in oxygen electrocatalysis; however, their real-world implementation in rechargeable zinc-air batteries (ZABs) is restricted by the insufficient bifunctional activity. Here, a secondary single-atom regulation strategy is developed to tailor the coordination environment and thus modify the catalytic behaviors of single atom Ni catalysts. Through introducing atomically dispersed Fe moieties, the coordination environment of single Ni atom is well regulated with a higher Ni-N/O coordination number due to the long-range electronic penetration interaction, which triggers more optimized adsorption strength between active sites and reaction intermediates. As a result, the obtained Fe, Ni dual atom catalyst (FeNi-SAs@NC) requires only 298 mV overpotential to deliver 10 mA cm−2 in oxygen evolution reaction (OER) catalysis outperforming the benchmarking IrO2 (313 mV), Fe-SAs@NC (335 mV) and Ni-SAs@NC (356 mV) electrocatalysts. Moreover, FeNi-SAs@NC performs superior oxygen reduction reaction (ORR) activity to the single-atom counterparts and Pt/C benchmark. The practical application of the FeNi-SAs@NC is further validated by a higher ZAB performance (260 mW cm−2 vs 71 mW cm−2 for Pt/C-IrO2), improved long-term durability (100 h at 20 mA cm−2) and encouraging performance in all-solid-sate ZABs (70 mW cm−2).

Synergistic effect between Ni single atoms and acid–base sites: Mechanism investigation into catalytic transfer hydrogenation reaction

Catalytic Transfer Hydrogenation (CTH) of unsaturated functional groups to obtain high value-added products has attracted considerable attention in heterogeneous catalysis. Herein, we report a Ni single atoms (SAs) catalyst anchoring to MgAl mixed metal oxide (MMO) based on structural topotactic transformation from hydrotalcite precursor, which displays an excellent catalytic behavior toward CTH of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) (conversion: 100%; selectivity: 100%; 2 h). A combination research including AC-STEM, XAFS and in situ CO-IR demonstrates that Ni species anchored by NiO coordination with different dispersion could be precisely obtained by tuning the reduction temperature: the Ni SAs (only NiO coordination) were obtained below 450 °C, and the NiNi coordination appears and increases gradually with the rise of the reduction temperature. The selective poisoning experiment, kinetic isotope effect (KIE), in situ FT-IR measurement and DFT calculation reveal the synergistic effect between metal Ni SAs and acid–base sites toward CTH of CAL. Specifically, the Ni SAs play a crucial role in dehydrogenation reaction of 2-propanol (2-POL) to supply activated H, the base sites induce α-CH cleavage in 2-POL (the rate-determining step), and the Lewis acid sites serve as the active center for CO adsorption in CAL molecule. This work demonstrates a synergetic effect between Ni SAs and acid–base sites, which would pave the way in the rational design and synthesis of high-performance SAs materials used in CTH reactions.

Anchoring Ni single atoms on sulfur-vacancy-enriched ZnIn2S4 nanosheets for boosting photocatalytic hydrogen evolution

The photocatalytic H 2 evolution performance of ZnIn 2 S 4 are significantly enhanced by the synergistic effect of sulfur vacancy and Ni single atom. Structure manipulation of photocatalysts at an atomic scale is a promising way to improve its photocatalytic performance. Herein, we realize the anchoring of single Ni atoms on the ZnIn 2 S 4 nanosheets with rich sulfur vacancies. Experimental results demonstrate that single Ni atoms induce the formation of Ni-O-M (Zn/In) atomic interface, which can efficiently promote the carriers separation and prolong the carrier life time. In addition, in situ electron spin resonance spectroscopy (ESR) confirms that the single Ni atoms act as an electron trapping center for protons reduction. As a result, the single Ni atoms decorated ZnIn 2 S 4 nanosheets with rich sulfur vacancies (Ni/ZnIn 2 S 4 -RV s ) shows a hydrogen evolution rate up to 89.4 μmol h −1 , almost 5.7 and 2.3 times higher compared to that of ZnIn 2 S 4 nanosheets with poor sulfur vacancies and rich sulfur vacancies (denoted as ZnIn 2 S 4 -PV s and ZnIn 2 S 4 -RV s ). This work opens up a new perspective manipulating the single-atom cocatalyst and sulfur vacancy on sulfide supports for improving photocatalytic hydrogen evolution.

Photocatalytic CO2 Reduction to CO over Ni Single Atoms Supported on Defect‐Rich Zirconia

AbstractThe photocatalytic CO2 reduction reaction (CRR) holds great promise for curbing anthropogenic CO2 emissions, though boosting photocatalyst activity and tuning product selectivity remain key priorities. Herein, isolated Ni single atoms dispersed on defect rich zirconia (Ni‐SA‐x/ZrO2) are identified as a very promising photocatalyst for CRR under Xe lamp irradiation, showing good activity and CO selectivity in the absence of added sacrificial agents or sensitizers. Due to an abundance of accessible nickel single atomic sites, the optimized photocatalyst affords CO at a rate of 11.8 µmol g−1 h−1 (92.5% selectivity). Experimental and theoretical investigations determine that the atomically dispersed Ni sites lower the energy barrier for CO2 to CO conversion via an adsorbed COOH intermediate, while also suppressing H2 desorption in the competing water splitting reaction. The Ni single atom sites thus simultaneously promote CO2 conversion and CO selectivity, thus offering valuable new insights for the future design of improved metal single‐atom catalysts for CRR.

Sponge Assembled by Graphene Nanocages with Double Active Sites to Accelerate Alkaline HER Kinetics.

Elaborate design of novel hybrid structures for hydrogen-evolution electrocatalysts is a crucial strategy for synergistically accelerating the reaction kinetics of water splitting. Herein, we prepare a three-dimensional (3D) sponge assembled by graphene nanocages (SGNCs) in which Ni nanoparticles and Ni single atoms coexist via a facile one-pot self-templating and self-catalytic strategy. Driven by simultaneous atomization and agglomeration under higher temperature, dual active sites of single atoms and nanoparticles are formed on graphene nanocages. Benefiting from the unique 3D porous structure and dual active sites, the SGNCs exhibit excellent hydrogen evolution reaction (HER) performance, which affords the current density of 10 mA cm-2 at a low overpotential of 27 mV. Theoretical calculations reveal that the interaction between single atoms and nanoparticles promotes HER kinetics. The controlled engineering strategy of non-noble metal-based hybrid materials provides prospects for innovative electrocatalyst development.

Synthesis of a Nickel Single-Atom Catalyst Based on Ni–N4–xCx Active Sites for Highly Efficient CO2 Reduction Utilizing a Gas Diffusion Electrode

A Ni single-atom catalyst with Ni–N4–xCx active sites is prepared in a single pyrolysis step in which the Ni single atom is incorporated in the carbon framework through nitrogen and carbon coordina...

Size‐Dependent Nickel‐Based Electrocatalysts for Selective CO2 Reduction

AbstractClosing the anthropogenic carbon cycle by converting CO2 into reusable chemicals is an attractive solution to mitigate rising concentrations of CO2 in the atmosphere. Herein, we prepared Ni metal catalysts ranging in size from single atoms to over 100 nm and distributed them across N‐doped carbon substrates which were obtained from converted zeolitic imidazolate frameworks (ZIF). The results show variance in CO2 reduction performance with variance in Ni metal size. Ni single atoms demonstrate a superior Faradaic efficiency (FE) for CO selectivity (ca. 97 % at −0.8 V vs. RHE), while results for 4.1 nm Ni nanoparticles are slightly lower (ca. 93 %). Further increase the Ni particle size to 37.2 nm allows the H2 evolution reaction (HER) to compete with the CO2 reduction reaction (CO2RR). The FE towards CO production decreases to under 30 % and HER efficiency increase to over 70 %. These results show a size‐dependent CO2 reduction for various sizes of Ni metal catalysts.

Size-Dependent Nickel-Based Electrocatalysts for Selective CO2 Reduction.

Closing the anthropogenic carbon cycle by converting CO2 into reusable chemicals is an attractive solution to mitigate rising concentrations of CO2 in the atmosphere. Herein, we prepared Ni metal catalysts ranging in size from single atoms to over 100 nm and distributed them across N-doped carbon substrates which were obtained from converted zeolitic imidazolate frameworks (ZIF). The results show variance in CO2 reduction performance with variance in Ni metal size. Ni single atoms demonstrate a superior Faradaic efficiency (FE) for CO selectivity (ca. 97 % at -0.8 V vs. RHE), while results for 4.1 nm Ni nanoparticles are slightly lower (ca. 93 %). Further increase the Ni particle size to 37.2 nm allows the H2 evolution reaction (HER) to compete with the CO2 reduction reaction (CO2 RR). The FE towards CO production decreases to under 30 % and HER efficiency increase to over 70 %. These results show a size-dependent CO2 reduction for various sizes of Ni metal catalysts.

Dynamic evolution and reversibility of single-atom Ni(II) active site in 1T-MoS2 electrocatalysts for hydrogen evolution

1T-MoS2 and single-atom modified analogues represent a highly promising class of low-cost catalysts for hydrogen evolution reaction (HER). However, the role of single atoms, either as active species or promoters, remains vague despite its essentiality toward more efficient HER. In this work, we report the unambiguous identification of Ni single atom as key active sites in the basal plane of 1T-MoS2 (Ni@1T-MoS2) that result in efficient HER performance. The intermediate structure of this Ni active site under catalytic conditions was captured by in situ X-ray absorption spectroscopy, where a reversible metallic Ni species (Ni0) is observed in alkaline conditions whereas Ni remains in its local structure under acidic conditions. These insights provide crucial mechanistic understanding of Ni@1T-MoS2 HER electrocatalysts and suggest that the understanding gained from such in situ studies is necessary toward the development of highly efficient single-atom decorated 1T-MoS2 electrocatalysts.

Dual active sites-dependent syngas proportions from aqueous CO2 electroreduction

Electroreduction of aqueous CO2 is a sustainable approach to produce tunable syngas, which demands the rational design of dual-active-sites catalysts for individual CO2 reduction reaction (CRR) and hydrogen evolution reaction (HER). Here, the self-supporting nickel@nitrogen-doped carbon nanotubes grown on carbon nanofiber (Ni@NCNTs-CNF) is developed with Ni single atoms (SAs) and nanoparticles (NPs) functioning as CRR and HER sites respectively. Benefiting from the hierarchical nanostructures for effective electron transport and mass diffusion, as well as the high accessibility of dual-active sites, Ni@NCNTs-CNF catalysts can mediate aqueous CO2 into CO/H2 (2:1) with a 100 % Faraday efficiency and a high current density (25 mA cm−2) at low working potential (-0.8 V). By manipulating the Ni SAs/Ni NPs ratio, the CO/H2 proportion is easily modulated in a wide range from 1:4 to 5:1, independence of the working potential (-0.6 to -0.8 V). Our findings provide fresh insights into the design of dual-active-sites catalysts.

Isolated Ni single atoms in nitrogen doped ultrathin porous carbon templated from porous g-C3N4 for high-performance CO2 reduction

Abstract Electrochemical reduction of CO2 to value-added products is an effective approach to manage the global carbon balance. However, the lack of effective electrocatalyst for CO2 reduction process is a major obstacle for its development. Currently, constructing atomically dispersed non-precious metal electrocatalysts presents a promising way to build high-performance and cost-effective electrochemical CO2 reduction systems. Herein we demonstrate a novel strategy to realize the anchoring and stabilization of isolated Ni atoms in the nitrogen-doped ultrathin porous carbon nanosheets via a polydopamine-assisted g-C3N4 template method. Benefitting from the abundant atomic Ni sites and ultrahigh specific surface area of porous 2D supports (>1000 m2 g−1), the catalyst exhibits excellent activity for CO2 reduction with particularly high selectivity towards CO, achieving a faradaic efficiency of 96% at −0.86 V (vs. RHE) with a current density of 26.4 mA cm−2 in 0.1 M KHCO3 solution.

Dual Single‐Atomic Ni‐N4 and Fe‐N4 Sites Constructing Janus Hollow Graphene for Selective Oxygen Electrocatalysis

Nitrogen-coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as an active research frontier in a wide range of key renewable energy reactions and devices. Herein, a step-by-step self-assembly strategy is developed to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), realizing separate-sided different single-atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni-N4 or Fe-N4 planar configuration. The developed Ni-N4 /GHSs/Fe-N4 Janus material exhibits excellent bifunctional electrocatalytic performance, in which the outer Fe-N4 clusters dominantly contribute to high activity toward the oxygen reduction reaction (ORR), while the inner Ni-N4 clusters are responsible for excellent activity toward the oxygen evolution reaction (OER). Density functional theory calculations demonstrate the structures and reactivities of Fe-N4 and Ni-N4 for the ORR and OER. The Ni-N4 /GHSs/Fe-N4 endows a rechargeable Zn-air battery with excellent energy efficiency and cycling stability as an air-cathode, outperforming that of the benchmark Pt/C+RuO2 air-cathode. The current work paves a new avenue for precise control of single-atom sites on carbon surface for the high-performance and selective electrocatalysts.

Highly Active and Carbon-Resistant Nickel Single-Atom Catalysts for Methane Dry Reforming

The conversion of CH4 and CO2 to syngas using low-cost nickel catalysts has attracted considerable interest in the clean energy and environment field. Nickel nanoparticles catalysts suffer from serious deactivation due mainly to carbon deposition. Here, we report a facile synthesis of Ni single-atom and nanoparticle catalysts dispersed on hydroxyapatite (HAP) support using the strong electrostatic adsorption (SEA) method. Ni single-atom catalysts exhibit excellent resistance to carbon deposition and high atom efficiency with the highest reaction rate of 1186.2 and 816.5 mol.gNi−1.h−1 for CO2 and CH4, respectively. Although Ni single-atom catalysts aggregate quickly to large particles, the polyvinylpyrrolidone (PVP)-assisted synthesis exhibited a significant improvement of Ni single-atom stability. Characterizations of spent catalysts revealed that carbon deposition is more favorable over nickel nanoparticles. Interestingly, it was found that, separately, CH4 decomposition on nickel nanoparticle catalysts and subsequent gasification of deposit carbon with CO2 resulted in CO generation, which indicates that carbon is reacting as an intermediate species during reaction. Accordingly, the approach used in this work for the design and control of Ni single-atom and nanoparticles-based catalysts, for dry reforming of methane (DRM), paves the way towards the development of stable noble metals-free catalysts.

Reaction-driven transformation of Ni/NiO hybrid structure into Ni single atoms

N-doped carbon nanotube-supported Ni/NiO hybrid structures with Ni/NiO crystalline interfaces (NiO/Ni@NCNTs) were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution, featuring an overpotential of 87.5 mV at a current density of 10 mA cm−2. During catalyzation, reaction-driven transformation of Ni/NiO hybrid structures into Ni single atoms and clusters was observed on deformed NCNTs. These single atoms (clusters) remained anchored even after harsh washing in acid, manifesting that NiO/Ni@NCNTs may be used as a platform for stabilised site-isolated catalysis in practical electrolytic processes. Surface characterization shows that metallic Ni was shown to mainly originate from Ni atoms bonded to pyridinic N atoms of the carbon skeleton. As shown by theoretical calculations, the Gibbs free energy ΔG (H∗) of the H adsorption was minimised when a Ni atom was adjacent to an O atom on a Ni (111) surface, which indicated that metallic Ni partially covered with NiO was energetically favourable for the HER.

High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single-Atom Catalysts with Tetradentate N2 O2 Coordination in a Three-Phase Flow Cell.

Carbon-supported NiII single-atom catalysts with a tetradentate Ni-N2 O2 coordination formed by a Schiff base ligand-mediated pyrolysis strategy are presented. A NiII complex of the Schiff base ligand (R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni-N2 O2 /C catalyst showed excellent performance for the electrocatalytic reduction of O2 to H2 O2 through a two-electron transfer process in alkaline conditions, with a H2 O2 selectivity of 96 %. At a current density of 70 mA cm-2 , a H2 O2 production rate of 5.9 mol gcat. -1 h-1 was achieved using a three-phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni-N2 O2 /C catalyst could electrocatalytically reduce O2 in air to H2 O2 at a high current density, still affording a high H2 O2 selectivity (>90 %). A precise Ni-N2 O2 coordination was key to the performance.