Local Coordination Environment Research Articles

Heteroatom-Doped Asymmetric Metal-Nx -C Single Atom Catalysts for Electrochemical CO2 Reduction Reaction.

CO2 utilization is one of the hottest research topics worldwide. As a class of newly emerging and promising catalysts for electrochemical CO2 reduction (ECR) reaction, heteroatom-doped metal-Nx -C single atom catalysts (M-Nx -C SACs) attract extensive attentions. Nowadays, great progress, including structure modulation, identification of local coordination environment and ECR mechanism via advanced synthetic strategies, characterization techniques and theoretical calculations, have been achieved over heteroatom doped asymmetric M-Nx -C SACs, which boost the ECR performances and deepen the understanding of ECR mechanism. In this context, we summarize recent progresses in heteroatom-doped asymmetric M-Nx -C SACs, with emphasis on synthetic strategies and their applications in ECR reaction, along with current understanding on the ECR mechanisms both experimentally and theoretically. Finally, the challenges and perspectives for the heteroatom-doped asymmetric M-Nx -C SACs towards ECR are proposed.

Fine-tuned local coordination environment of Pt single atoms on ceria controls catalytic reactivity

Constructing single atom catalysts with fine-tuned coordination environments can be a promising strategy to achieve satisfactory catalytic performance. Herein, via a simple calcination temperature-control strategy, CeO2 supported Pt single atom catalysts with precisely controlled coordination environments are successfully fabricated. The joint experimental and theoretical analysis reveals that the Pt single atoms on Pt1/CeO2 prepared at 550 °C (Pt/CeO2-550) are mainly located at the edge sites of CeO2 with a Pt–O coordination number of ca. 5, while those prepared at 800 °C (Pt/CeO2-800) are predominantly located at distorted Ce substitution sites on CeO2 terrace with a Pt–O coordination number of ca. 4. Pt/CeO2-550 and Pt/CeO2-800 with different Pt1-CeO2 coordination environments exhibit a reversal of activity trend in CO oxidation and NH3 oxidation due to their different privileges in reactants activation and H2O desorption, suggesting that the catalytic performance of Pt single atom catalysts in different target reactions can be maximized by optimizing their local coordination structures.

Atomically Interlocked Chemistry Activated by Interstitial Sites in LiMn2O4 Cathode

AbstractThe extensive applications of spinel LiMn2O4 (LMO) are severely plagued by grievous capacity degradation and structural collapse, mainly ascribed to deleterious Jahn−Teller distortion and subsequent dissolution of Mn2+. Herein, highly stable LMO with atomic interlocking effect is rationally designed via engineering Al into the unoccupied 16c sites. The local coordination environment of the surficial MnO6 octahedron is reconstructed by robust Al−O band coherency, giving strengthened lattice oxygen skeleton and constraining heterophase evolution with the suppression of Jahn−Teller distortion, validated by theoretical calculations coupled with synchrotron X‐ray absorption spectrum. Concomitantly, with the occupation of Al in interstitial site, the migration of Mn is effectively restrained, directly observed by scanning transmission electron microscopy, leading to the inhibition of inactivation as well as dissolution loss of Mn. Resultantly, splendid long cycling stability of Al‐LMO after 1000 loops with only 0.019% capacity fading per cycle is presented. Given this, this elaborate study can provide an ingenious avenue for regulating the structure/interface chemistry architecture in electrode materials.

Facile vacancies engineering of CoFe-PBA nanocubes for enhanced oxygen evolution

Oxygen evolution reaction (OER) is known as the key for the large-scale production of hydrogen through electrochemical processes. Developing highly efficient OER electrocatalysts with well-defined structure remains a considerable challenge. Herein, a unique and facile Ar plasma treatment was introduced to modify cobalt iron Prussian blue analogs (CoFe-PBA) immobilized nickel foam (NF). The in situ generated carbon-nitrogen vacancies can effectively tune the local coordination environment and electronic structure of metal active sites in CoFe-PBA, thus improving the OER performance. The combination of CoFe-PBA and NF presents a 3D interworking network structure with highly exposed active sites and excellent electrical conductivity, guaranteeing stable and efficient OER process. The as-prepared electrocatalyst exhibited excellent OER performance with an overpotential of 256 mV at the current density of 50 mA cm−2 and a small Tafel slope of 39 mV dec−1. This work presents a facile strategy for the design and fabrication of highly efficient PBA-based OER electrocatalyst, and enriches our understanding of the structure-property relationship of the plasma-engineered electrocatalyst.

Pt Atomic Single-Layer Catalyst Embedded in Defect-Enriched Ceria for Efficient CO Oxidation.

The local coordination structure of metal sites essentially determines the performance of supported metal catalysts. Using a surface defect enrichment strategy, we successfully fabricated Pt atomic single-layer (PtASL) structures with 100% metal dispersion and precisely controlled local coordination environment (embedded vs adsorbed) derived from Pt single-atoms (Pt1) on ceria-alumina supports. The local coordination environment of Pt1 not only governs its catalytic activity but also determines the Pt1 structure evolution upon reduction activation. For CO oxidation, the highest turnover frequency can be achieved on the embedded PtASL in the CeO2 lattice, which is 3.5 times of that on the adsorbed PtASL on the CeO2 surface and 10-70 times of that on Pt1. The favorable CO adsorption on embedded PtASL and improved activation/reactivity of lattice oxygen within CeO2 effectively facilitate the CO oxidation. This work provides new insights for the precise control of the local coordination structure of active metal sites for achieving 100% atomic utilization efficiency and optimal intrinsic catalytic activity for targeted reactions simultaneously.

Local coordination atom and metal types of single-atom catalysts to regulate catalytic performance of C2H2 selective hydrogenation

The local coordination environment of anchoring single-atom metal is the design core of single-atom catalysts (SACs). In this study, several kinds of single-atom M (M = Co, Ni, Cu, Pt, Pd and Ru) anchored over the dual-coordination N and other heteroatom (B/C/N/O/S/P)-doped graphene catalysts (M-N3-B/C/N/O/S/P SACs) were theoretically predicted and indirectly supported by the reported experiment to tune catalytic performance of C2H2 selective hydrogenation. Co-N3C catalyst was screened out to exhibit high C2H4 selectivity and activity, and better stability in C2H2 selective hydrogenation. More electrons loss of single-atom Co corresponds to weaker C2H4 adsorption energy to improve C2H4 selectivity. Further, C2H4 activity has an inverted volcano relationship with d-band center in Co SACs. This work provides a theoretical basis to screen and design SACs with better structural stability and excellent catalytic performance by adjusting the local coordination atom and metal types of metal SACs.

Storage of Lithium-Ion by Phase Engineered MoO3 Homojunctions.

With high theoretical specific capacity, the low-cost MoO3 is known to be a promising anode for lithium-ion batteries. However, low electronic conductivity and sluggish reaction kinetics have limited its ability for lithium ion storage. To improve this, the phase engineering approach is used to fabricate orthorhombic/monoclinic MoO3 (α/h-MoO3) homojunctions. The α/h-MoO3 is found to have excessive hetero-phase interface. This not only creates more active sites in the MoO3 for Li+ storage, it regulates local coordination environment and electronic structure, thus inducing a built-in electric field for boosting electron/ion transport. In using α/h-MoO3, higher capacity (1094 mAh g-1 at 0.1 A g-1) and rate performance (406 mAh g-1 at 5.0 A g-1) are obtained than when using only the single phase h-MoO3 or α-MoO3. This work provides an option to use α/h-MoO3 hetero-phase homojunction in LIBs.

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N 5 Site for Enhanced Electrocatalytic Reaction

Decrypting the Influence of Axial Coordination on the Electronic Microenvironment of Co-N ₅ Site for Enhanced Electrocatalytic Reaction

Spatial Well‐defined Bimetallic Two‐Dimensional Polymers with Single‐Layer Thickness for Electrocatalytic Oxygen Evolution Reaction

Innovative bimetallic materials provide more possibilities for further improving the performance of oxygen evolution reaction (OER) electrocatalysts. However, it is still a great challenge to rationally design bimetallic catalysts because there is not a practical way to decouple the factors influencing the intrinsic activity of active sites from others, thus hindering in-depth understanding of the mechanism. Herein, we provide a rational design of bimetallic Ni, Co two-dimensional polymer model OER catalyst. The well-defined architecture, identical density of active sites and monolayer characteristic allow us to decouple the intrinsic activity of active sites from other factors. The results confirmed that the relative position and local coordination environment has significant effect on the synergistic effect of the bimetallic centres. The highest electrocatalytic activity with the turnover frequency value up to 26.19 s-1 was achieved at the overpotential of 500 mV.

Spatial Well‐defined Bimetallic Two‐Dimensional Polymers with Single‐Layer Thickness for Electrocatalytic Oxygen Evolution Reaction

AbstractInnovative bimetallic materials provide more possibilities for further improving the performance of oxygen evolution reaction (OER) electrocatalysts. However, it is still a great challenge to rationally design bimetallic catalysts because there is not a practical way to decouple the factors influencing the intrinsic activity of active sites from others, thus hindering in‐depth understanding of the mechanism. Herein, we provide a rational design of bimetallic Ni, Co two‐dimensional polymer model OER catalyst. The well‐defined architecture, identical density of active sites and monolayer characteristic allow us to decouple the intrinsic activity of active sites from other factors. The results confirmed that the relative position and local coordination environment has significant effect on the synergistic effect of the bimetallic centres. The highest electrocatalytic activity with the turnover frequency value up to 26.19 s−1 was achieved at the overpotential of 500 mV.

Realizing the bifunctional electrocatalysis via local charge rearrangement of α-CrOOH-modulated Co@CoMoOx for overall water splitting

Rational design of highly active, durable and cost-efficient bifunctional catalysts for electrochemical water splitting is critical for aggressive reform of energy technologies. Herein, earth-abundant transition-metal (TM) oxide-(oxy)hydroxide (Co@CoMoOx-α-CrOOH) is synthesized as overall water splitting catalyst via a facile electrodeposition approach. Detailed X-ray absorption spectra (XAS) reveal that incorporation of α-CrOOH can implicitly modulate the local coordination environment and the electronic structure of Co/Mo/Cr cations, as well as their synergistic interaction that contributes to more rapid charge-transfer kinetics and enhanced catalytic activity. The precisely designed Co@CoMoOx-α-CrOOH/NF electrode displays prominent overall water-splitting efficiency that require a cell voltage only 1.57 V to achieve 10 mA cm−2 along with remarkable stability over 24 h in alkaline solution, which is comparable to a Pt/C/ NF || IrO2/NF water-splitting device at the state of the art. This work provides a sustainable strategy to design efficient and cost-effective TM oxide-based catalysts for water-splitting application.

Me-N-C Electrocatalysts for Electrochemical CO2 Reduction to High-Value Products

The development of sustainable carbon-neutral energy technologies to mitigate greenhouse gas emissions has become imperative and urgent. Of special interest and importance in this context is the use of electricity from intermittent renewable energy sources (wind, solar) for electrochemical conversion of carbon dioxide (CO2) to easily storable and transportable value-added products: fuels and feedstock chemicals. Nanoparticles of non-precious metals, e.g., Cu, Sn, Co, show high activity for electrochemical CO2 reduction reaction (CO2RR) but suffer from poor selectivity, resulting in a mixture of products that require tedious and costly separation. Recently, transition metal- and nitrogen-doped carbon (Me-N-C) materials have been emerged as promising CO2RR catalysts thanks to their well-defined structures and good activity. However, their selectivity, while respectable for the generation of carbon monoxide (CO), is low for high energy-content products. A limited understanding of reaction pathways and degradation mechanism of Me-N-C catalysts for CO2RR has additionally stemmed a rational design of these materials.In this presentation, we summarize our study of the activity, selectivity, and stability of Me-N-C (Me = Fe, Co, Ni, or Cu, etc.) catalysts for the CO2RR, focusing on the role of the local coordination environment at metal centers and metal-carbon substrate interactions. We also report the obtained bimetallic M1M2-N-C catalysts, designed to enable the formation of multi-carbon products through CO2RR and utilizing high surface-area three-dimensional carbon matrix as support for metal sites with improved CO2RR activity. The main objective of this work is to use Me-N-C catalysts to produce high energy-density chemicals, enhance mechanistic understanding, and bring CO2RR closer to practical applications.

(Invited) Electrostatic Site Potential in Electrolytes As an Emerging Descriptor for Reversible Metal Electrodes

In order to maximize the electrochemical stability of a battery with metal anode, suppressing the side reaction between the metal and the electrolyte is essential. When the deposition potential positions outside of the stability window, optimization of SEI is a general approach to realize kinetic hinderance of side reactions. While another effective approach should be upshifting the deposition potential itself because it makes the side reactions much slower and milder, or even thermodynamic stability could be gained if the potential is upshifted into the stability window of the electrolyte. It is well known that electrode potentials vary significantly depending on the electrolyte. Typical example can be found in salt concentrated electrolyte, where the metal deposition potential significantly upshifts over 0.5 V upon increasing the concentration. However, the mechanism behind the potential shift has remained unclear. By careful analyses of the local coordination environment by molecular dynamics simulations and the accurate experimental measurement of the deposition potential, we revealed that the potential shift is primarily determined by electrostatic destabilization of metal cations rather than the simple Nernst-type expression depending on cation activity. The shallow site potential of metal cations is attributable to the dominant coordination to more charge-dispersive, larger size anions rather than to relatively electro-positive solvents, thus decreasing the overall Coulombic energy gain of metal cations. Such simple yet hitherto-overlooked mechanism can be a useful guideline in the development of better electrolytes for reversible metal electrode.

Quantifying the Anomalous Local and Nanostructure Evolutions Induced by Lattice Oxygen Redox in Lithium-Rich Cathodes.

Due to their accessible lattice oxygen redox (l-OR) at high voltages, Li-rich layered transition metal (TM) oxides have shown promising potential as candidate cathodes for high-energy-density Li-ion batteries. However, this l-OR process is also associated with unusual electrochemical issues such as voltage hysteresis and long-term voltage decay. The structure response mechanism to the l-OR behavior also remains unclear, hindering rational structure optimizations that would enable practical Li-rich cathodes. Here, this study reveals a strong coupling between l-OR and structure dynamic evolutions, as well as their effects on the electrochemical properties. Using the technique of neutron total scattering with pair distribution function analysis and small-angle neutron scattering, this study quantifies the local TM migration and formation of nanopores that accompany the l-OR. These experiments demonstrate the causal relationships among l-OR, the local/nanostructure evolutions, and the unusual electrochemistry. The TM migration triggered by the l-OR can change local oxygen coordination environments, which results in voltage hysteresis. Coupled with formed oxygen vacancies, it will accelerate the formation of nanopores, inducing a phase transition, and leading to irreversible capacity and long-cycling voltage fade.

Iron-induced electron modulation of nickel hydroxide/carbon nanotubes composite to effectively boost the oxygen evolution activity

The practical potential of nickel hydroxide for oxygen evolution reaction (OER) is hindered by its low electron transport efficiency and weak coupling between hydroxyl groups and active sites. Here, we introduce iron into the nickel hydroxide to effectively adjust the local coordination environment and electron structure. This conception is realized by a facile surface-redox-etching method to grow Fe-decorated nickel hydroxide nanoparticles on the three-dimensional (3D) carbon nanotube networks (Fe-Ni(OH)2/CNT). It is surprising that the Fe-Ni(OH)2/CNT catalyst displays extraordinary OER performance with a low overpotential of 63 mV and 228 mV at 10 mA cm−2 and 100 mA cm−2, respectively, outstripping most OER electrocatalysts. The superior OER performance is ascribed to the 3D CNT networks and synergistic interaction between Ni and Fe in the lattice, which not only accelerate electron transfer but also induce strong electron coupling with OH*, boosting the reaction kinetics of OER.

Single Atom Catalysts with Out-of-Plane Coordination Structure on Conjugated Covalent Organic Frameworks.

Adjusting the local coordination environment of single-atom electrocatalysts is a viable way to improve catalytic performance. The diversity of coordination geometric structures is limited to the traditional in-plane configuration, with only a little consideration paid to out-of-plane configurations due to the lack of suitable carriers and fabrication methods. This study reports out-of-plane coordination of Co-based single-atom catalysts mediated by the conjugated bipyridine-rich covalent organic framework (COF). The bipyridine nitrogen on the COF layer backbone of these catalysts serves as the linker center for cobalt sites anchoring, while the complementary moieties are coordinated at the other side of the Co metal and reside beyond the COF backbone plane, thus yielding out-of-plane coordination. The electrochemical experiments and density functional theory calculations reveal that catalysts with multiple out-of-plane coordinations exhibit different electrocatalytic oxygen evolution activities and catalytic pathways. The out-of-plane coordination enabled by COFs provides a strategy for designing single-atom electrocatalysts, expanding the application of COFs in the field of electrocatalysis.

Regulation of Structure and Magnetic Properties for Nickel(II)/Cobalt(II) Coordination Polymers with 1,4‐Bis(imidazole‐1‐ylmethyl)benzene Ligand

AbstractTwo new coordination polymers Ni(bix)3 ⋅ 2(NO3) ⋅ (H2O) 1 and Co(bix)2 ⋅ 2(NO3) 2 (bix=1,4‐bis(imidazole‐1‐ylmethyl)benzene) have successfully been synthesized via hydrothermal reactions. These two compounds have similar local coordination environments but different topological networks due to the flexibility of the bix ligand. Additionally, magnetic studies reveal that 1 and 2 also exhibit different magnetic behaviors. Although magnetic exchange pathways are same as MII‐bix‐MII (M=Ni 1, Co 2), 1 shows purely paramagnetic state almost down to 2 K with weak antiferromagnetic interaction; while stronger antiferromagnetic interaction in 2 is suggested. Meanwhile, an anomaly around 38 K in zero‐field‐cooled mode is observed but suppressed in field‐cooled mode, which may be suggesting the spin canting for 2. The field dependent magnetizations show magnetization of 2.17 μB and 2.44 μB at 7 T for 1 and 2, respectively. Also, the magnetic interaction J/kB of −3.3 K is estimated for 2.

A Framework for the Relationships between Stability and Functional Properties of Electrochemical Energy Materials

The path toward a renewable energy future relies on the development of materials for electrochemical energy technologies that are not only highly functional but also stable. In this Perspective, we will discuss a framework for the relationships between function and stability, starting by revisiting how we measure stability that should focus on changes to the nature and number of active centers (materials stability) together with monitoring how current/potential change over time during operation (reaction durability). By shifting our focus to materials properties, we then discuss how subtractive and additive processes such as dissolution, deposition, and (de)intercalation contribute to changes in the local surface composition, coordination environment, and phase restructuring that accompany the degradation of electrochemical materials. Ultimately, gaining insights into the processes responsible for changes to active centers during operation will help us better predict and control the durability of materials for clean energy.

Hierarchal Porous Graphene-Structured Electrocatalysts with Fe-N5 Active Sites Modified with Fe Clusters for Enhanced Performance Toward Oxygen Reduction Reaction.

The local coordination environment around the active centers has a major impact on tuning the intrinsic activity of M-N-C catalysts. Herein, a porous graphene with Fe-N5 active sites modified with Fe clusters is successfully fabricated by using Fe3+-SCN- and NaHCO3 as the metal precursor and pore-forming agent, respectively. The unique Fe-N5 configuration accompanying Fe clusters and the improved ORR activity are confirmed by various characterization techniques and theoretical calculations. Benefiting from the pores, mass and electron transfer channels are successfully constructed, making more active sites accessible and facilitating the ORR process. As a consequence, the as-prepared catalyst has an excellent ORR activity with a half-wave potential of 0.89 V, comparable selectivity, and superior stability. In addition, a homemade primary zinc-air battery using this material as the cathode catalyst has a maximum power density of 0.205 W/cm2, revealing the potential of the as-constructed CSA-Fe-N-C catalyst to replace precious Pt catalysts.

Systemic development of Mn4+-activated double perovskite red phosphors for plant growth applications

Developing efficient and stable red phosphor has always been considered as important academic research of LED plant-growth lamps. In this study, Mn4+-activated double perovskite red phosphors, A2BB’O6:Mn4+ (A = Sr, Ba; B = Lu, Gd; B’ = Nb, Ta), were successfully synthesized by solid-phase method. To elucidate the structure-activity relationship between the double perovskite structure and the luminescence of Mn4+, the crystal structure, electronic structure and luminescence properties of Sr2LuNbO6:Mn4+ were first studied in detail using X-ray powder diffraction data, density functional theory calculations and spectroscopy data. The coordination environment of Mn4+ was analyzed by investigating the crystal-field parameters, Racah parameters and nephelauxetic parameter. Then, the effect of structural evolution on the luminescence of Mn4+ was investigated by regulating the local coordination environment of Mn4+ through cationic modification. The results demonstrate that Mn4+ enters the octahedral lattice site with a strong crystal field and covalent environment, which can be excited by a near-ultraviolet LED chip to emit narrow-band far-red light in the double perovskite oxides. Moreover, the Mn–O bond length, octahedral distortion and crystal-system type all affect the luminescence of Mn4+. This systemic development provides some guidance for the exploration of novel Mn4+-activated double perovskite red phosphors.