Ni Active Sites Research Articles

Metallic and Dimensional Optimization of Metal-Organic Frameworks for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) have been considered as one of the most promising energy storage systems owing to their high theoretical energy density and abundant sulfuric resources. However, their commercial application is limited by rapid capacity decline and low Coulombic efficiency. Metal-organic frameworks (MOFs) made of metallic nodes and organic ligands can suppress polysulfide shuttling and promote redox kinetics. In this paper, the effects of crystallographic dimensions and metallic categories on chemical performance of LSBs have been meticulously explored electrochemical performance. As a result, exposed Ni active sites in a lamellar Ni-MOF was found to deliver a superior electrochemical performance. The as-assembled LSBs with 2D-Ni-MOF/CNTs cathode deliver a much superior initial discharge capacity, (820 mAh g-1 at 0.5 C), and exhibit excellent cycling stability over 550 cycles than those analogues of 3D stereoscopic Ni-MOF and 2D lamellar Co-MOF. This work proposed a perspective in elevating LSBs performance through synergistic optimization of the MOFs dimensions and the metallic nuclei in the cathodes.

In-situ surface reconstruction of single-crystal (NiFe)3Se4 nano-pyramid arrays for efficient oxygen evolution

Transition metal-based selenides (TMSe) are considered as efficient pre-electrocatalysts towards oxygen evolution reaction (OER). However, the key factor in determining the surface reconstruction of TMSe under OER condition is not yet clear. Herein, we uncover that the crystallinity of TMSe will obviously impact the conversion degree from TMSe to transition metal oxyhydroxides (TMOOH) during OER. A novel single-crystal (NiFe)3Se4 nano-pyramid array grown on NiFe foam is fabricated by a facile one-step polyol process, which exhibits an excellent OER activity and stability, only requiring 170 mV to reach a current density of 10 mA cm−2 and can sustain for more than 300 h. In situ Raman spectrum studies reveals that the single-crystal (NiFe)3Se4 is partially oxidized on its surface during OER, generating a dense heterostructure of (NiFe)OOH/(NiFe)3Se4. Benefiting from the in situ formed heterointerface, the adsorption of OER intermediates on Ni active sites calculated by density functional theory (DFT) analysis is optimized, leading to the reduced energy barrier, which accounts for the enhanced intrinsic activity. This work not only reports a novel single-crystal (NiFe)3Se4 nano-pyramid array electrocatalyst with high-efficient OER performance, but also gains a deep insight into the role of the crystallinity of TMSe on the surface reconstruction during OER.

Atomically Dispersed NiNx Site with High Oxygen Electrocatalysis Performance Facilely Produced via a Surface Immobilization Strategy

Nonprecious-metal heterogeneous catalysts with atomically dispersed active sites demonstrated high activity and selectivity in different reactions, and the rational design and large-scale preparation of such catalysts are of great interest but remain a huge challenge. Current approaches usually involve extremely high-temperature and tedious procedures. Here, we demonstrated a straightforward and scalable preparation strategy. In two simple steps, the atomically dispersed Ni electrocatalyst can be synthesized in a tens grams scale with quantitative yield under mild conditions, and the active Ni sites were produced by immobilizing preorganized NiNx complex on the substrate surface via organic thermal reactions. This catalyst exhibits excellent catalysis performances in both oxygen evolution and reduction reactions. It also exhibited tunable catalysis activity, high catalysis reproducibility, and high stability. The atomically dispersed NiNx sites are tolerant at high Ni concentration, as the random reactions and metal nanoparticle formation that generally occurred at high temperatures were avoided. This strategy illustrated a practical and green method for the industrial manufacture of nonprecious-metal single-site catalysts with a predictable structure.

Electrodeposited crystalline-amorphous Ni-Ni(OH)2 electrocatalysts for efficient hydrogen evolution via alkaline water splitting

Transition metal electrocatalysts with crystalline-amorphous heterostructures have garnered particular attention owing to the pronounced synergistic effect between a crystalline phase with high electrical conductivity and an amorphous phase with abundant catalytic active sites. Herein, we construct crystalline-amorphous Ni-Ni(OH)2 hybrids via a facile electrodeposition approach for efficient alkaline hydrogen evolution reaction (HER). The Ni-Ni(OH)2 heterostructures are directly electrodeposited on the surface of Cu nanowires (NWs) in-situ grown on Cu foam with a well-maintained nanowire morphology. The Cu NWs on Cu foam substrate provide a three-dimensional structure, and enable efficient electron transportation to the Ni-Ni(OH)2 sites. Moreover, in the crystalline-amorphous Ni-Ni(OH)2 heterostructures, amorphous Ni(OH)2 nanosheets accelerate the water dissociation, while crystalline Ni active sites promote the formation and combination of adsorbed hydrogen atoms. Such a promising synergistic effect leads to the enhanced HER performance with low overpotentials of 73 and 336 mV at the current densities of 10 and 100 mA cm−2, respectively, and a small Tafel slope of 33 mV dec−1. In addition, the integrated structure enables the Ni-Ni(OH)2 electrode with a durable stability of 35 mA cm−2 for 100 h. This work offers an effective approach to construct crystalline-amorphous materials for efficient alkaline HER.

Stimulate the hidden catalysis potential and exposure of nickel site in NiSe@CNTs result in ultra-high HER/OER activity and stability

We present an electrodeposition strategy to construct heterostructures nickel selenide (NiSe) and carbon nanotubes (CNTs), namely NiSe@CNTs, to tune the electronic structure of the Ni sites, thus stimulating the hidden catalytic potential of the Ni sites. As expected, the optimized NiSe@CNTs exhibits unprecedentedly ultra-high activity for HER (27 mV at 10 mA cm−2) and OER (145 mV at 100 mA cm−2) and outstanding long-term durability (730 h). Based on the systematic characterization and DFT results, the Ni sites with increased electron density induced by π-electron off-domain of CNTs not only greatly optimize the H* adsorption kinetics but also reduce the energy barrier of the *O→*OOH step, demonstrating the enhanced intrinsic catalytic activity. Furthermore, the introduced CNTs not only avoid aggregation of nanoparticles but also reduce particle size, thus increasing the number of exposed Ni active sites. This study provides guidance for the exploration of catalysts with ultra-high activity and durability.

Face mask-derived Ni, N-doped graphene sheets for electrocatalytic CO2-to-CO reduction.

The online version contains supplementary material available at 10.1007/s10008-023-05444-7.

Steam reforming of biomass gasification tar over Ni-based catalyst supported by TiO2-SiO2 composite

Gasification is an important option for converting biomass into bio-syngas, but it also produces biomass gasification tar, which lowers the gasification efficiency and threatens the safety of the gasifiers. In this work, the TiO2-SiO2 composite-supported Ni-based catalyst was prepared for tar catalytic steam reforming, and its performance was systematically studied, with benzene being the model compound of tar. The results revealed that adding SiO2 to TiO2 improved the conversion of benzene and increased the yield of H2 in the syngas, which respectively reached 61.48% and 49.89 mmol/gbenzene over the 10Ni/1TiO2-0.30SiO2 catalyst, with promising thermal stability and anti-carbon deposition capacity. Furthermore, the presence of SiO2 increased the catalyst’s specific surface area and lowered the sizes of Ni crystals, thereby promoting the dispersion of active Ni sites. Meanwhile, the modification of SiO2 enhanced the catalyst’s redox ability and decreased its acidity, which suppressed the sintering of the catalyst and restricted the accumulation of carbon deposition, while the physical morphology and chemical graphitization degree of the deposits were slightly influenced. Therefore, the catalytic activity and anti-carbon deposition performance of the TiO2-SiO2 composite-supported catalyst were improved during tar catalytic steam reforming.

Evaluating Fe-Site and Vacancy Dependent Intrinsic Activity of NiFe Layered Double Hydroxides through Cavity Microelectrodes

We report evaluating the intrinsic activity of Ni(OH)2, NiFe layered double hydroxides (LDHs), and NiFe-LDH having vacancies for oxygen evolution reaction (OER) by the use of cavity microelectrodes (CMEs) with controllable mass loading. The number of active Ni sites (NNi-sites) ranging from 1 × 1012 to 6 × 1012 is quantitatively correlated with OER current, which reveals that the introduction of Fe-sites and vacancies increases the turnover frequency (TOF) from 0.027 to 0.118 and 0.165 s-1, respectively. Electrochemical surface area (ECSA) is further quantitatively correlated with NNi-sites, which indicates that NNi-sites per unit ECSA (NNi-per-ECSA) is decreased by the introduction of Fe-sites and vacancies. Therefore, the difference of OER current per unit ECSA (JECSA) is reduced compared with that of TOF. The results demonstrate that CMEs provide a good platform to evaluate intrinsic activity with TOF, NNi-per-ECSA, and JECSA more reasonably.

Hollow carbon spheres with isolated Ni atoms on both external and internal surfaces for efficient CO2 electroreduction

Electrochemical CO2 reduction is a promising method to mitigate excessive CO2 emission and realize CO2 valorization. However, the lacking of efficient catalysts hinders the practical applications of this important process. Here, we report a new type of hollow carbon spheres containing isolated Ni atoms on both external and internal surfaces by an integrated bottom-up and top-down strategy. For such unique structure, the material exhibits good activity for electrochemical CO2 reduction with much higher current density and Faradaic efficiency for CO production than the materials with common structures (i.e., the solid spheres with Ni sites only on external surface). The enhanced activity of the material is attributed to the favored mass transport and charge transfer guaranteed by the presence of hollow structure as well as active Ni sites on both external and internal surfaces. Furthermore, we demonstrate that a secondary pyrolysis of the material can improve its activity and stability for CO2 reduction. Such treatment can increase the degree of graphitization and reaction kinetics of the material during electrocatalysis. The final material exhibits a large current density (150 mA cm−2) and a high Faradaic efficiency for CO production (94%) in a flow cell, showing good potential for practical applications. This work extends the concept of structural regulation of catalysts and provides the guidance for the design of high-performance catalysts for electrochemical CO2 reduction.

Monometallic interphasic synergy via nano-hetero-interfacing for hydrogen evolution in alkaline electrolytes

Electrocatalytic synergy is a functional yet underrated concept in electrocatalysis. Often, it materializes as intermetallic interaction between different metals. We demonstrate interphasic synergy in monometallic structures is as much effective. An interphasic synergy between Ni(OH)2 and Ni-N/Ni-C phases is reported for alkaline hydrogen evolution reaction that lowers the energy barriers for hydrogen adsorption-desorption and facilitates that of hydroxyl intermediates. This makes ready-to-serve Ni active sites and allocates a large amount of Ni d-states at Fermi level to promote charge redistribution from Ni(OH)2 to Ni-N/Ni-C and the co-adsorption of Hads and OHads intermediates on Ni-N/Ni-C moieties. As a result, a Ni(OH)2@Ni-N/Ni-C hetero-hierarchical nanostructure is developed, lowering the overpotentials to deliver −10 and −100 mA cm−2 in alkaline media by 102 and 113 mV, respectively, compared to monophasic Ni(OH)2 catalyst. This study unveils the interphasic synergy as an effective strategy to design monometallic electrocatalysts for water splitting and other energy applications.

Modulating Electronic Structure of Atomically Dispersed Nickel Sites through Boron and Nitrogen Dual Coordination Boosts Oxygen Reduction

AbstractAtomically dispersed 3D transitional metal active sites with nitrogen coordination anchored on carbon support have emerged as a kind of promising electrocatalyst toward oxygen reduction reaction (ORR) in the field of fuel cells and metal–air cells. However, it is still a challenge to accurately modulate the coordination structure of single‐atom metal sites, especially first‐shell coordination, as well as identify the relationship between the geometric/electronic structure and ORR performance. Herein, a carbon‐supported single‐atom nickel catalyst is fabricated with boron and nitrogen dual coordination (denoted as Ni‐B/N‐C). The hard X‐ray absorption spectrum result reveals that atomically dispersed Ni active sites are coordinated with one B atom and three N atoms in the first shell (denoted as Ni‐B1N3). The Ni‐B/N‐C catalyst exhibits a half‐wave potential (E1/2) of 0.87 V versus RHE, along with a distinguished long‐term durability in alkaline media, which is superior to commercial Pt/C. Density functional theory calculations indicate that the Ni‐B1N3 active sites are more favorable for the adsorption of ORR intermediates relative to Ni‐N4, leading to the reduction of thermodynamic barrier and the acceleration of reaction kinetics, which accounts for the increased intrinsic activity.

Modulating microenvironment of active moiety in Prussian blue analogues via surface coordination to enhance CO2 photoreduction

Well-arranged photosensitive units and catalytic sites in closeness are critical for enhancing charge separation effectiveness and promoting photocatalysis. Here, a heterogeneous interface is constructed, which binds a deprotonated carboxyl group on perylene diimide (PDI) photosensitizer to unsaturated metal sites generated by etching of different Prussian blue analogues (PBAs). The atomically-dispersed metal sites and anchored photosensitive units greatly accelerate charge transfer. Typically, the S-scheme heterojunction built by etched NiCo PBA and PDI (NCSP) possesses bond of O-Ni. Density functional theory (DFT) and in-situ technology reveals the electron density of the Ni active site is optimized. Consequently, activation barrier of CO2 on the Ni site was reduced, thus improving the CO generation (60.0 μmol g−1) up to about 9 times than NiCo PBA. This work reveals key insights into the precise modulation of active moiety for efficient photocatalytic CO2 conversion.

The Promoting Role of Ni on In2O3 for CO2 Hydrogenation to Methanol.

Ni-promoted indium oxide (In2O3) is a promising catalyst for the selective hydrogenation of CO2 to CH3OH, but the nature of the active Ni sites remains unknown. By employing density functional theory and microkinetic modeling, we elucidate the promoting role of Ni in In2O3-catalyzed CO2 hydrogenation. Three representative models have been investigated: (i) a single Ni atom doped in the In2O3(111) surface, (ii) a Ni atom adsorbed on In2O3(111), and (iii) a small cluster of eight Ni atoms adsorbed on In2O3(111). Genetic algorithms (GAs) are used to identify the optimum structure of the Ni8 clusters on the In2O3 surface. Compared to the pristine In2O3(111) surface, the Ni8-cluster model offers a lower overall barrier to oxygen vacancy formation, whereas, on both single-atom models, higher overall barriers are found. Microkinetic simulations reveal that only the supported Ni8 cluster can lead to high methanol selectivity, whereas single Ni atoms either doped in or adsorbed on the In2O3 surface mainly catalyze CO formation. Hydride species obtained by facile H2 dissociation on the Ni8 cluster are involved in the hydrogenation of adsorbed CO2 to formate intermediates and methanol. At higher temperatures, the decreasing hydride coverage shifts the selectivity to CO. On the Ni8-cluster model, the formation of methane is inhibited by high barriers associated with either direct or H-assisted CO activation. A comparison with a smaller Ni6 cluster also obtained with GAs exhibits similar barriers for key rate-limiting steps for the formation of CO, CH4, and CH3OH. Further microkinetic simulations show that this model also has appreciable selectivity to methanol at low temperatures. The formation of CO over single Ni atoms either doped in or adsorbed on the In2O3 surface takes place via a redox pathway involving the formation of oxygen vacancies and direct CO2 dissociation.

Properties-activity correlation of Nickel supported on fibrous Zeolite-Y for dry reforming of methane

Nickel-supported Fibrous zeolite-Y (Ni/FHY) was successfully synthesized via the microemulsion method using HY as seed, followed by catalytic evaluation in dry reforming of methane (DRM) for hydrogen production. Ni/FHY possessed good NiO distribution, improved metal-support interface, and strong basicity, accredited to the fibrous structure of FHY. This unique morphology led to the enrichment in the amount of accessible Ni active sites, thus resulting in the superior activity of Ni/FHY (XCH4=95.1%,XCO2=91.1%,H2/CO=0.89) compared to Ni/HY (XCH4=92.7%,XCO2=89.8%,H2/CO=0.78). Meanwhile, the inferior performance of Ni/HY could be caused by the poor distribution of Ni with large particles, thus contributing to high carbon deposition and Ni sintering. The unique physicochemical properties and superior catalytic activity confirmed its viability in the DRM.

Amorphous NiB2for electroreduction of NO to NH3

Amorphous NiB2is demonstrated as an efficient metal diboride catalyst for electrochemical NO reduction to NH3(NORR), which is attributed to the active Ni sites which accelerate NORR energetics and retard the hydrogen evolution.

Identifying the intrinsic active site in bimetallic Co3S4/Ni3S2 feathers on MXene nanosheets as a heterostructure for efficient oxygen evolution reaction

The NF@MXene@Co3S4/Ni3S2 Co3S4/Ni3S2 feathers fixed on MXene nanosheets with enhanced conductivity and Ni active sites of Ni3S2 shows efficient activity for OER.

Dynamically activating Ni-based catalysts with self-anchored mononuclear Fe for efficient water oxidation

A surface engineering strategy to form a highly active Fe–Ni structure on the surfaces of Ni-based catalysts. The surface Fe is readily oxidized under OER conditions, compensating for the charges of Ni and increasing the activity of Ni active sites.

The adjacent Fe oxidation greatly enhancing OER activity on the Ni active site: S plays the role in optimizing local coordination and electronic structure

Oxygen evolution reaction (OER) is the bottleneck process of water splitting, and finding efficient, durable, low-cost, and earth-abundant electrocatalysts remains a major challenge. Here, FeNi2-400-S is to be a promising OER electrocatalyst which exhibits a low overpotential of 214 mV at a current density of 10 mA/cm2. X-ray analysis indicates that the introduction of S leads to a mismatch in bond distance between the metal-sulfur bond and the metal-metal bond, which can change the local electronic structure and favorably control the electronic oxidation. The active site position of FeNi2-400-S has been further confirmed by DFT, which the ∗OOH can stably adsorb on the Ni site of the oxidized Fe-Ni-S benefitting from the synergetic effect of the Ni site and the adjacent oxidized O on the Fe atom. Our findings demonstrate that the internal reconstruction of catalyst can make the optimization of local coordination and electronic structure, in which the in-situ generated vacancy can enable the outstanding OER performance.

Dual Ni active sites mediated by In to separate ethane activation and oxidation for enhanced ethene production via chemical looping scheme

Chemical looping oxidative dehydrogenation (CL-ODH) opens a unique avenue to achieve highly efficient conversion of alkanes to value-added alkenes. The challenge lies in identical metal-oxo active sites responsible for both alkane activation and oxidation causing either insufficient activity or over-oxidation thus inferior ethene yield and productivity at relatively low temperature (< 650 °C). Herein, we designed a novel redox catalyst by the incorporation of main-group In (6NiInx/HY, x indicates the atomic ratio of In/Ni) which achieved more than 92 % ethene selectivity at 40 % ethane conversion and long-term stability at 600 °C with 0.22 mmol g−1 ethene productivity at 650 °C. This was because Ni2+ Lewis acid sites conducted catalytic dehydrogenation of ethane to ethene and hydrogen, and the Ni-O-In moiety outside HY framework enabled selective oxidation of hydrogen rather than ethene due to well-regulated Ni 3d electron densities leading to the reduction of adsorption for ethene thus decreasing over-oxidation.

High‑index‑faceted and electron density-optimized Ni3S2 in hierarchical NiWO4-Ni3S2@NiO/NF nanofibers for robust alkaline electrocatalytic hydrogen evolution

Designing nonprecious electrocatalysts with outstanding performances in hydrogen evolution reaction (HER) via water splitting is attractive for renewable energy conversion. Here, a free-standing and binder-free electrode by in-situ growing a multi-level layered electrocatalyst consisted of NiWO4-coupled Ni3S2 nanofibers with secondary NiO layer on Ni foam (NiWO4-Ni3S2@NiO/NF) is devised, in which NiWO4 was recognized to boost the exposure of high-index {1 2¯ 3} facets on Ni3S2 and NiO was identified to increase the electron density of Ni3S2 via accumulating S vacancies. Such an integration of high‑index‑facet exposure and electron-density optimization endowed the resultant NiWO4-Ni3S2@NiO/NF-3 (3 represents the nominal Ni: S molar ratio in fabricating Ni3S2) electrode with a more suitable H adsorption energy on the active Ni sites and a positive shift of the d-band center of Ni3S2 toward the Fermi level, resulting in a strengthened Ni–H interaction. As a consequence, a low overpotential of 89 mV to deliver a current density of 10 mA cm−2 and a high turnover frequency of H2 generation of 1.5 × 10−3 s−1 for HER in 1 mol l−1 KOH were acquired on NiWO4-Ni3S2@NiO/NF-3, both outperforming the Ni3S2/NF-1, NiWO4-Ni3S2/NF-1, and NiWO4-Ni3S2@NiO/NF-1 electrocatalysts. Remarkably, an electrolytic cell assembled by using NiWO4-Ni3S2@NiO/NF-3 as both anode and cathode delivered an overall-water-splitting current density of 10 mA cm−2 at 1.64 V, evidencing its robust bifunctional catalytic ability in water splitting.