Ni-N4 Site Research Articles

Boosting aqueous non-flow zinc–bromine batteries with a two-dimensional metal–organic framework host: an adsorption-catalysis approach

We develop a high-performance aqueous non-flow zinc–bromine battery with nickel polyphthalocyanine as a cathode host, where the atomically dispersed Ni–N4 sites enable strong bromine adsorption and efficient bromine redox reactions.

Local compressive strain regulation of atomically dispersed NiN4 sites for enhancing CO2 electroreduction to CO.

Electroreduction of CO2 to valuable chemicals powered by renewable electricity provides a sustainable approach to reduce the environmental issues originating from CO2 emission. However, insufficient current density and production selectivity hinder its further application. In this case, precisely regulating the CO2 reduction reaction (CO2RR) active sites is an excellent strategy to simultaneously reduce the reaction barrier and suppress the hydrogen evolution reaction (HER) pathway. Herein, the strain regulation of atomically dispersed NiN4 active sites is investigated in helical carbon. Ni-N coordination in the curved carbon lattice displays a reduced distance compared to that in a straight lattice, inflicting local compressive strain on NiN4. The resultant catalyst shows the highest CO selectivity of up to 99.4% at -1.4 V (vs. RHE), the FECO is maintained at over 85% over a wide potential range from -0.8 to -1.8 V (vs. RHE), and the maximum partial current density for CO reaches a high of 458 mA cm-2 at -1.8 V (vs. RHE). Theoretical investigations show the superior CO2 electroreduction performance of curved NiN4 stems from its remarkable ability to generate the *COOH intermediate and to suppress the hydrogen combination simultaneously. Our findings offer a novel strategy to rationally regulate the local three-dimensional structure of single-atom sites for efficient electrocatalysis.

Simple and straightforward method to prepare highly dispersed Ni sites for selective nitrobenzene coupling to Azo/Azoxy compounds

Azo and azoxy compounds are essential substances for the chemical industry. The development of catalysts to produce these molecules directly from nitroarenes is highly desirable. Nevertheless, this reaction is extremely challenging since the catalyst should not only activate nitro-compounds but also avoids the complete reduction to amines. Beyond noble-metals, such as Au, nickel-containing catalysts have shown attention for nitroarenes coupling to produce azo/azoxy compounds. Recently, outstanding results were reported for the nitrobenzene coupling using Ni-N4 sites obtained via pyrolysis of Ni(phen). However, pyrolysis methods lack on the control of metal loading and morphology of the metal sites, clearly, a mix of nickel nanoparticles and atomic dispersed metal sites were obtained. Here, we present a simple and straightforward method to synthesize highly dispersed nickel sites stabilized on highly crystalline carbon nitrides with poly(heptazine imide) structure (PHI). This method allows fine control of the metal loading and leads to mostly highly dispersed nickel sites, enabling to investigate in depth the coordination sphere of the Ni-sites during the reaction. The synthesized catalysts (Ni-PHI) were applied to nitrobenzene coupling reactions, showing high yields towards azo/azoxybenzene (98 %) and the highest TOF (435 h−1) for this reaction in the literature under mild conditions. Raman spectroscopy analyses show that nickel sites are crucial to adsorb nitrobenzene molecules through N atoms, favoring the production of nitrosobenzene. Furthermore, Ni-PHI exhibit high stability under different cycles and can yield azo/azoxy molecules with a broad scope, revealing the wide versatility of this catalyst.

Single Nickel Atom Catalysts Enable Fast Polysulfide Redox for Safe and Long-Cycle Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have attracted great interest due to their low cost, high theoretical energy density, and environmental friendliness. However, the sluggish conversion of lithium polysulfides (LiPS) to S and Li2 S during the charge/discharge process leads to unsatisfactory rate performance of lower to 0.1 C (1 C = 1675 mA g-1 ) especially for Li-S pouch batteries, thus hindering their practical applications in high power batteries. Here, well-defined and monodispersed Ni single-atom catalysts (SACs) embedded in highly porous nitrogen-doped graphitic carbons (NiSA-N-PGC) are designed and synthesized to form Ni-N4 catalytic sites at the atomic level. When serving as a bifunctional electrocatalyst, the Ni-N4 catalytic sites cannot only promote the interfacial conversion redox of LiPS by accelerating the transformation kinetics, but also suppress the undesirable shuttle effect by immobilizing LiPS. These findings are verified by both experimental results and DFT theoretical calculations. Furthermore, Li ions show low diffusion barrier on the surface of Ni-N4 sites, resulting in enhanced areal capacity of batteries. As a result, the Li-S battery delivers stable cycling life of more than 600 cycles with 0.069% capacity decay per cycle at a rate of 0.5 C. More importantly, the Li-S pouch cells with NiSA-N-PGC show an initial capacity of 1299 mAh g-1 at a rate of 0.2 C even with high sulfur loading of 6 mg cm-2 . This work opens up an avenue for developing single-atom catalysts to accelerate the kinetic conversion of LiPS for highly stable Li-S batteries.

Surface-Exposed Single-Ni Atoms with Potential-Driven Dynamic Behaviors for Highly Efficient Electrocatalytic Oxygen Evolution.

Trapping the active sites on the exterior surface of hollow supports can reduce mass transfer resistance and enhance atomic utilization. Herein, we report a facile chemical vapor deposition strategy to synthesize single-Ni atoms decorated hollow S/N-doped football-like carbon spheres (Ni SAs@S/N-FCS). Specifically, the CdS@3-aminophenol/formaldehyde is carbonized into S/N-FCS. The gas-migrated Ni species are anchored on the surface of S/N-FCS simultaneously, yielding Ni SAs@S/N-FCS. The obtained catalyst exhibits outstanding performance for alkaline oxygen evolution reaction (OER) with an overpotential of 249 mV at 10 mA cm-2 , a small Tafel slope of 56.5 mV dec-1 , and ultra-long stability up to 166 hours without obvious fading. Moreover, the potential-driven dynamic behaviors of Ni-N4 sites and the contribution of the S dopant at different locations in the matrix to the OER activity are revealed by the operando X-ray absorption spectroscopy and theoretical calculations, respectively.

Hydrogen release mechanisms of MgH2 over NiN4-embedded graphene nanosheet: First-principles calculations

In this study, first-principles calculations were performed to investigate the catalytic effect of NiN4-G on the dehydrogenation of MgH2. Side-on MgH2 can be adsorbed stably on the NiN4-G monolayer and is preferentially adsorbed on the NiN4 site compared with the graphene site. The hydrogen desorption process, in which MgH2 dissociated to the Mg atom on the NiN4 site or graphene site and an H2 molecule in the vacuum, should overcome lower barriers than pure MgH2. This is because the corresponding Mg–H bond is weakened owing to the electron transfer between the Mg atom and the substrate. The hydrogen desorption enthalpies of the (MgH2)5 cluster on the NiN4 active and graphene sites are significantly smaller (0.11 eV and 1.50 eV, respectively) when H2+H2 is released from the cluster compared with those of the undoped MgH2 cluster (2.48 eV). Therefore, the NiN4-G monolayer can provide the double effect of the NiN4 active and graphene sites on improving the dehydrogenation performance of MgH2.

Single-Product Faradaic Efficiency for Electrocatalytic of CO2 to CO at Current Density Larger than 1.2 A cm-2 in Neutral Aqueous Solution by a Single-Atom Nanozyme.

Electroreduction of CO2 to CO is a promising approach for the cycling use of CO2 , while it still suffers from impractical current density and durability. Here we report a single-atom nanozyme (Ni-N5 -C) that achieves industrial-scale performance for CO2 -to-CO conversion with a Faradaic efficiency (FE) exceeded 97 % over -0.8--2.4 V vs. RHE. The current density at -2.4 V vs. RHE reached a maximum of 1.23 A cm-2 (turnover frequency of 69.7 s-1 ) with an FE of 99.6 %. No obvious degradation was observed over 100 hours of continuous operation. Compared with the planar Ni-N4 site, the square-pyramidal Ni-N5 site has an increase and a decrease in the and dxz/yz orbital energy levels, respectively, as revealed by density functional theory calculations. Thus, the Ni-N5 catalytic site is more superior to activate CO2 molecule and reduce the energy barriers as well as promote the CO desorption, thus boosting the kinetic activation process and catalytic activity.

Pyrrolic N‐Stabilized Monovalent Ni Single‐Atom Electrocatalyst for Efficient CO2 Reduction: Identifying the Role of Pyrrolic–N and Synergistic Electrocatalysis

AbstractEngineering the electronic structure of metal, N‐doped carbon catalysts is a potential strategy for increasing the activity and selectivity of CO2 electroreduction reaction (CO2RR). However, establishing a definitive link between structure and performance is extremely difficult due to constrained synthesis approaches that lack the ability to precisely control the specific local environment of MNC catalysts. Herein, a soft‐template aided technique is developed for the first time to synthesize pyrrolic N4Ni sites coupled with varying N‐type defects to synergistically enhance the CO2RR performance. The optimal catalyst helps attain a CO Faradaic efficiency of 94% at a low potential of −0.6 V and CO partial current density of 59.6 mA cm−2 at −1 V. Results of controlled experimental investigations indicate that the synergy between NiN4 and metal free defect sites can effectively promote the CO2RR activity. Theoretical calculations revealed that the pyrrolic N coordinated NiN4 sites and C atoms next to pyrrolic N (pyrrolic NC) have a lower energy barrier for the formation of COOH* intermediate and optimum CO* binding energy. The pyrrolic N regulate the electronic structure of the catalyst, resulting in lower CO2 adsorption energy and higher intrinsic catalytic activity.

Mechanism of Catalytic Transfer Hydrogenation for Furfural Using Single Ni Atom Catalysts Anchored to Nitrogen-Doped Graphene Sheets.

Catalytic transfer hydrogenation (CTH) of α,β-unsaturated aldehydes using single metal atom catalysts supported on nitrogen-incorporated graphene sheet (M-Nx-Gr) materials has attracted increasing attention recently, yet the reaction mechanism remains to be explored. Compared to the Ni-N4-Gr model in which the dissociation of isopropanol is highly unfavorable as a result of steric hindrance and inertness of the Ni-N4 site embedded in graphene, the Ni-N3 site in Ni-N3-Gr is more active and facilitates the formation of *H with isopropanol as the H donor, where the dissociation of H from isopropanol with an energy barrier of 0.83 eV is the rate-determining step. An alternative reaction path starts from the coadsorption of isopropanol and furfural molecules at the Ni-N3 site, followed by a direct hydrogen transfer between the two molecules; however, the rate-determining step has a much higher energy barrier of 1.32 eV. Our calculations suggest that the hydrogenation of the aldehyde group is kinetically more favorable than the C═C hydrogenation, revealing the high chemoselectivity of furfural to furfuryl alcohol. Our investigations reveal that the CTH mechanism using the Ni-N3-Gr catalyst is different from that on traditional metal oxides, where the former has only one single active site, while two active sites are required for the latter. The proposed reaction mechanism of CTH for furfural in this study should be helpful to guide the design of single metal atom catalysts with appropriate N coordination for application in chemoselective hydrogenation reactions.

Selective transfer hydrogenation coupling of nitroaromatics to azoxy/azo compounds by electron-enriched single Ni-N4 sites on mesoporous N-doped carbon

Developing an active and selective non-precious metal catalyst to boost transfer hydrogenation coupling (THC) of nitroaromatics to their corresponding aromatic azoxy/azo compounds is desirable, but remains a great challenge. Herein, for the first time, we report a facile microwave-assisted pyrolysis strategy to successfully prepare the unique Ni-N4 sites on open-mouth quasi-stellate mesoporous N-doped carbon (NC) with enlarged electronic density led by the shortening Ni-N coordination distance (h-Ni1N4 /NC) by microwave heating the 1,10-phenanthroline-Ni complex coated stellate mesoporous SiO2 followed by alkali-leaching, distinct from the single-atom Ni-N5 sites on NC (l-Ni1N5/NC) prepared by a conventional pyrolysis method, unambiguously confirmed by diverse characterization technologies and DFT calculations. More importantly, h-Ni1N4/NC efficiently boosts the selective THC of nitrobenzene to azoxy under mild conditions using isopropyl alcohol (IPA) as hydrogen donor, showing much higher turnover frequency (33.4 h−1) and selectivity (95.6%) than traditional l-Ni1N5/NC catalyst (13.5 h -1°/85.3%) and supported Ni nanoparticles on NC (10.1 h -1, no detectable azoxy). Kinetic experiments and theoretical calculations reveal that the enriched electron density of Ni atoms in single Ni-N 4 sites efficiently lowers activation barrier for IPA/nitrobenzene activation and azoxy formation over h-Ni1N4/NC, which contributes to the outstanding catalytic performance for azoxybenzene production. Moreover, the approach was extended to the transformation of diverse nitroaromatics to corresponding azoxy compounds with high yield. In addition, by varying reaction parameters, h-Ni1N4/NC catalyst enables highly active and selective production of azo compounds via THC reaction of nitroaromatics.

Edge-enriched Ni-N4 atomic sites embedded enoki-mushroom-like carbon nanotubes assembling hollow fibers for CO2 conversion and flexible Zn-air battery

Explorations of multifunctional catalysts for electrochemical reactions, such as oxygen electrocatalysis and CO2 electroreduction, are key issues for energy crisis and global warming. Herein, the hollow fiber stitched by enoki-mushroom-like carbon nanotube with enriched Ni-N4 single-atomic edge-sites (Ni/N-ESC) is constructed. The nickel clusters are introduced to catalyze in-situ growth of carbon nanotubes and preferentially anchor single-atomic Ni-N sites. These hairy units stitch together similar to enoki-mushroom and constitute the highly porous matrix for hollow fibers. After self-sacrifice of nickel clusters, hollow fibers assembled by carbon nanotubes with edge-enriched single-atomic Ni-N4 sites are constructed. Both theoretical and experimental results demonstrate the edge-rich single-atomic sites manifest superior activities on both oxygen and CO2 catalysis. Meanwhile, the highly porous and conductive framework favors the fast kinetics and gives abundant active sites. Taking above advantages, the Ni/N-ESC hollow fiber manifests the outstanding activities towards both oxygen catalysis and CO2 conversions. Moreover, the flexible Zn-air battery assembled with Ni/N-ESC hollow fiber delivers the good flexibility, high power/energy density and stable property during long-term cycling at both ambient (25 °C) and freezing (-20 °C) temperatures. Therefore, this work not only provides a new platform to design high efficient catalysts for multifarious electrochemical reactions, but also promotes the development of flexible Zn-air battery working in broadened temperature range.

Nickel polyphthalocyanine with electronic localization at the nickel site for enhanced CO2 reduction reaction

Nickel phthalocyanine (NiPc) can be at first glance a compelling catalyst for CO2 reduction reaction (CO2RR) because of its Ni–N4 site. Unfortunately, the pristine NiPc possesses a low catalytic activity resulting from the poor CO2 adsorption and activation capabilities of the electron-deficiency Ni site. Herein, we develop nickel polyphthalocyanine (NiPPc) with extended conjugation to tailor the electronic density at the Ni active site. The enlarged π conjugation of NiPPc evokes the d-electrons localization, increasing the electronic density at the Ni site, which enhances its CO2 adsorption and activation. Consequently, NiPPc supported on carbon nanotubes (NiPPc/CNT) in a flow cell delivers an excellent activity of −300 mA cm−2 for CO2RR with the CO selectivity of 99.8%, which is much higher than that of NiPc dispersed on carbon nanotubes. NiPPc/CNT exhibits an outstanding stability for CO2RR of more than 30 h at a current density of −100 mA cm−2 with an ultrahigh selectivity for CO, exceeding 99.7%. This work showcases a new way of tuning the electronic density of catalytic sites.

Atomically-dispersed NiN4–Cl active sites with axial Ni–Cl coordination for accelerating electrocatalytic hydrogen evolution

The NiN4–Cl SAs/N–C electrocatalyst consisting of NiN4–Cl active sites with axial Ni–Cl coordination was synthesized. The axial Cl-coordination modulates the electronic configuration of Ni–N4 sites and enhances the interaction of Ni–H efficiently.

High-curvature carbon-supported Ni single atoms with charge polarization for highly efficient CO2 reduction.

High curvature carbon-supported isolated NiN4 sites with ideal CO2 reduction reaction performance were obtained through a top-down strategy. The charge polarization induced by the axial asymmetry of the carbon substrate can enhance the adsorption of COOH* and reduce the free energy barrier of the rate-determining step.

Metal–organic framework with atomically dispersed Ni–N4 sites for greatly-raised visible-light photocatalytic H2 production

The depletion of traditional fossil fuels and related environmental issues have increased the demand for clean and sustainable energy resources to achieve carbon neutrality. Active, robust and cheap photocatalysts are required for large-scale photocatalytic H2 evolution reaction (p-HER), which transforms solar energy into chemical energy. Here we report the decoration of Ni-imidazole framework (NiIm) with CdS nanorods for p–HER. The p–HER rate of CdS/NiIm hybrid sharply increased by about 14.23 times to 21,712 µmol h−1 g–1 when compared with that of CdS alone. Physicochemical characterizations and theoretical calculations reveal strong interactions at the CdS/NiIm interface and the presence of numerous Ni–N4 active sites on NiIm leading to the significant performance enhancement. This work demonstrates that NiIm acts as an affordable and abundant co-catalyst that remarkably enhances the p–HER, and enlightens the further development in the design and preparation of metal−organic framework-based materials for various applications in photocatalysis and related subjects.

A promising single-atom Co-N-C catalyst for efficient CO2 electroreduction and high-current solar conversion of CO2 to CO

Excavating highly efficient M-N-C electrocatalysts for electrochemical CO2 reduction (ECR) is of paramount importance. Herein, we report a single-atom Co-N-C catalyst (CoN4-CB) with a high CO Faradaic efficiency (FECO, 98.7%) in ECR comparable to that of a similarly prepared NiN4-CB catalyst. Impressively, CoN4-CB demonstrates a CO turnover frequency of 27173 h−1 and a CO current density of −33.6 mA∙cm−2 at −0.76 V, 20.2 and 6.8 times higher than that of NiN4-CB, respectively. In a solar-driven ECR system composed of a Si solar cell and a flow cell, CoN4-CB shows a remarkably large current density of 98.3 mA∙cm−2 with an average FECO of 92.1%. Theoretical calculations suggest that the energy barrier for *COOH formation largely decreases on CoN4 sites compared with NiN4 sites, leading to a low onset potential and high activity for CO production. This work will boost the development of efficient M-N-C electrocatalysts and further practical application of solar-driven ECR system.

Engineering Single-Atomic Ni-N4-O Sites on Semiconductor Photoanodes for High-Performance Photoelectrochemical Water Splitting.

Direct photoelectrochemical (PEC) water splitting is a promising solution for solar energy conversion; however, there is a pressing bottleneck to address the intrinsic charge transport for the enhancement of PEC performance. Herein, a versatile coupling strategy was developed to engineer atomically dispersed Ni-N4 sites coordinated with an axial direction oxygen atom (Ni-N4-O) incorporated between oxygen evolution cocatalyst (OEC) and semiconductor photoanode, boosting the photogenerated electron-hole separation and thus improving PEC activity. This state-of-the-art OEC/Ni-N4-O/BiVO4 photoanode exhibits a record high photocurrent density of 6.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (vs RHE), over approximately 3.97 times larger than that of BiVO4, achieving outstanding long-term photostability. From X-ray absorption fine structure analysis and density functional theory calculations, the enhanced PEC performance is attributed to the construction of single-atomic Ni-N4-O moiety in OEC/BiVO4, facilitating the holes transfer, decreasing the free energy barriers, and accelerating the reaction kinetics. This work enables us to develop an effective pathway to design and fabricate efficient and stable photoanodes for feasible PEC water splitting application.

Enhancement of Mass Transfer for Facilitating Industrial‐Level CO2 Electroreduction on Atomic NiN4 Sites

AbstractHerein, an active catalyst A‐Ni@CMK is developed with the atomically dispersed NiN4 moieties on the mesoporous carbon for the electrocatalytic CO2 reduction reaction (CO2RR) through a step‐by‐step pore‐filling synthetic strategy. Concretely, the as‐synthesized catalyst A‐Ni@CMK exhibits outstanding catalytic performance for CO2RR in the H‐cell with a Faradaic efficiency of CO (FECO) > 80% in a wide electrochemical potential window (−0.5 to −0.9 V vs RHE) and a large CO partial current density (jCO) of 24 and 51 mA cm−2 at −0.6 and −0.8 V versus RHE, respectively. Notably, jCO for A‐Ni@CMK can further reach the industrial‐level values of 366 mA cm−2 at −0.8 V versus RHE, respectively, with the FECO > 95% in the flow cell, surpassing those for all the thus far reported NiNC catalysts and representing one of the best values for the catalysts for CO2RR. Moreover, comparative experimental studies in combination with density functional theory calculations demonstrate that the intrinsic high activity of the atomic NiN4 sites towards CO2RR results in high selectivity. Nevertheless, the mesoporous channels favor the mass transmission for both reactant CO2 and gas‐product CO, which in turn enhances the reaction kinetics, leading to the improved overall performance of the as‐prepared A‐Ni@CMK catalyst for CO2‐to‐CO conversion.

A New Strategy for Accelerating Dynamic Proton Transfer of Electrochemical CO2 Reduction at High Current Densities

AbstractDeveloping single‐atom electrocatalysts with high activity and superior selectivity at a wide potential window for CO2 reduction reaction (CO2RR) still remains a great challenge. Herein, a porous NiNC catalyst containing atomically dispersed NiN4 sites and nanostructured zirconium oxide (ZrO2@Ni‐NC) synthesized via a post‐synthetic coordination coupling carbonization strategy is reported. The as‐prepared ZrO2@Ni‐NC exhibits an initial potential of −0.3 V, maximum CO Faradaic efficiency (F.E.) of 98.6% ± 1.3, and a low Tafel slope of 71.7 mV dec−1 in electrochemical CO2RR. In particular, a wide potential window from −0.7 to −1.4 V with CO F.E. of above 90% on ZrO2@Ni‐NC far exceeds those of recently developed state‐of‐the‐art CO2RR electrocatalysts based on NiN moieties anchored carbon. In a flow cell, ZrO2@Ni‐NC delivers a current density of 200 mA cm−2 with a superior CO selectivity of 96.8% at −1.58 V in a practical scale. A series of designed experiments and structural analyses identify that the isolated NiN4 species act as real active sites to drive the CO2RR reaction and that the nanostructured ZrO2 largely accelerates the protonation process of *CO2− to *COOH intermediate, thus significantly reducing the energy barrier of this rate‐determining step and boosting whole catalytic performance.

Two-Electron Electrochemical Reduction of CO2 on B-Doped Ni–N–C Catalysts: A First-Principles Study

We evaluate the electrocatalytic activity of graphene-supported NiN4 active center in facilitating two-electron CO2 electrochemical reduction into CO and HCOOH, as well as the competing hydrogen evolution reaction. NiN4 center is found to be more stable in zigzag-edge and armchair-edge proximity of graphene, confirming experimental evidence. In an attempt to reduce the CO2 reduction overpotential, we construct a neighboring-site environment of NiN4 center and BN substitutional defect. B-doped structures are found to be capable of reducing the CO2 reduction energy barrier through direct (HCOOH pathway) or indirect (CO pathway) participation in facilitating CO2 reduction-related key intermediates. In most Ni sites, the presence of adjacent BN site is also discovered to change the product selectivity from CO to HCOOH. Our result predicts that B-doped NiN4 sites on the interior side (NiN4BN-G) and on the armchair-edge side of tilted orientation (t-NiN4BN-AGNR) are HCOOH-selective. Although the rest of zigzag-edge and armchair-edge sites have a tendency to selectively produce CO and HCOOH, respectively, the undesirable hydrogen evolution reaction is found to be more dominant and therefore obscuring the potential. Our result demonstrates that the hydrogen evolution reaction is most likely to occur on top of a neighboring C atom instead of on the Ni center, in contrast to the commonly understood mechanism. To ultimately suppress the activity of the hydrogen evolution reaction, we predict that an effective electrocatalyst optimization cannot be realized by only selecting the best metal center and dopant pair but also by altering the neighbor’s electronic structure.