Atomic Design of Alkyne Semihydrogenation Catalysts via Active Learning.

Alkyne hydrogenation on palladium-based catalysts modified with silver is currently used in industry to eliminate trace amounts of alkynes in alkenes produced from steam cracking and alkane dehydrogenation processes. Intensive efforts have been devoted to designing an alternative catalyst for improvement, especially in terms of selectivity and catalyst cost, which is still far away from that as expected. Here, we describe an atomic design of a high-performance Ni-based intermetallic catalyst aided by active machine learning combined with density functional theory calculations. The engineered NiIn catalyst exhibits >97% selectivity to ethylene and propylene at the full conversion of acetylene and propyne at mild temperature, outperforming the reported Ni-based catalysts and even noble Pd-based ones. Detailed mechanistic studies using theoretical calculations and advanced characterizations elucidate that the atomic-level defined coordination environment of Ni sites and well-designed hybridization of Ni 3d with In 5p orbital determine the semihydrogenation pathway.

Stable Operation Induced by Plastic Crystal Electrolyte Used in Ni-Rich NMC811 Cathodes for Li-Ion Batteries.

The LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material has been of significant consideration owing to its high energy density for Li-ion batteries. However, the poor cycling stability in a carbonate electrolyte limits its further development. In this work, we report the excellent electrochemical performance of the NMC811 cathode using a rational electrolyte based on organic ionic plastic crystal N-ethyl-N-methyl pyrrolidinium bis(fluorosulfonyl)imide C2mpyr[FSI], with the addition of (1:1 mol) LiFSI salt. This plastic crystal electrolyte (PC) is a thick viscous liquid with an ionic conductivity of 2.3 × 10-3 S cm-1 and a high Li+ transference number of 0.4 at ambient temperature. The NMC811@PC cathode delivers a discharge capacity of 188 mA h g-1 at a rate of 0.2 C with a capacity retention of 94.5% after 200 cycles, much higher than that of using a carbonate electrolyte (54.3%). Moreover, the NMC811@PC cathode also exhibits a superior high-rate capability with a discharge capacity of 111.0 mA h g-1 at the 10 C rate. The significantly improved cycle performance of the NMC811@PC cathode can be attributed to the high Li+ conductivity of the PC electrolyte, the stable Li+ conductive CEI film, and the maintaining of particle integrity during long-term cycling. The admirable electrochemical performance of the NMC811|C2mpyr[FSI]:[LiFSI] system exhibits a promising application of the plastic crystal electrolyte for high voltage layered oxide cathode materials in advanced lithium-ion batteries.

Surface-Stabilized High-Entropy Layered Oxyfluoride Cathode for Lithium-Ion Batteries.

High-entropy materials have been demonstrated to improve the structural stability and electrochemical performance of layered cathode materials for lithium-ion batteries (LIBs). However, structural stability at the surface and electrochemical performance of these materials are less than ideal. In this study, we show that fluorine substitution can improve both issues. Here, we report a new high-entropy layered cathode material Li1.2Ni0.15Co0.15Al0.1Fe0.15Mn0.25O1.7F0.3 (HEOF1) based on the partial substitution of oxygen with fluorine in previously reported high-entropy layered oxide LiNi0.2Co0.2Al0.2Fe0.2Mn0.2O2. This new compound delivers a discharge capacity of 85.4 mAh g-1 and a capacity retention of 71.5% after 100 cycles, showing significant improvement from LiNi0.2Co0.2Al0.2Fe0.2Mn0.2O2 (first 57 mAh g-1 and 9.8% after 50 cycles). This improved electrochemical performance is due to suppression of the surface M3O4 phase formation. Although still an early stage study, our results show an approach to stabilize the surface structure and improve the electrochemical performance of high-entropy layered cathode materials.

Single-Atom Anchored Curved Carbon Surface for Efficient CO2 Electro-Reduction with Nearly 100% CO Selectivity and Industrially-Relevant Current Density.

Although single metal atoms on porous carbons (PCs) are widely used in electrochemical CO2 reduction reaction, these systems have long relied on flat graphene-based models, which are far beyond reality because of abundant curved structures in PCs; the effect of curved surfaces has long been ignored. In addition, the selectivity generally decreases under high current density, which severely limits practical application. Herein, theoretical calculations reveal that a single-Ni-atom on a curved surface can simultaneously enhance the total density of states around Fermi level and decrease the energy barrier for *COOH formation, thereby enhancing catalytic activity. This work reports a rational molten salt approach for preparing PCs with ultra-high specific surface area of up to 2635 m2 g-1 . As determined by cutting-edge techniques, a single Ni atom on a curved carbon surface is obtained and used as a catalyst for electrochemical CO2 reduction. The CO selectivity reaches up to 99.8% under industrial-level current density of 400 mA cm-2 , outperforming state-of-the-art PC-based catalysts. This work not only offers a new method for the rational synthesis of single atom catalysts with strained geometry to host rich active sites, but also provides in-depth insights for the origin of catalytic activity of curved structure-enriched PC-based catalysts.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation.

Selective catalytic reduction (SCR) of NOx with NH3 is the most efficient technology for NOx emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O3 into the simulated stack gas, we find that O3 can stoichiometrically oxidize NO to generate NO2, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V1-W1 active sites supported on TiO2(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O3 allows SCR to proceed along a NH4NO3-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O3 exclusively oxidizes NO even in the co-presence of SO2 and H2O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

Redox-Active Two-Dimensional Tetrathiafulvalene-Copper Metal-Organic Framework with Boosted Electrochemical Performances for Supercapatteries.

Metal-organic frameworks (MOFs) have attracted noticeable attention as promising candidates for electrochemical energy storage. However, the lack of electrical conductivity and the weak stability of most MOFs result in poor electrochemical performances. Here, a tetrathiafulvalene (TTF)-based complex, formulated as [(CuCN)2(TTF(py)4)] (1) (TTF-(py)4 = tetra(4-pyridyl)-TTF), is assembled by in situ generation of coordinated CN- from a nontoxic source. Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a two-dimensional layered planar structure, which is further stacked in parallel to form a three-dimensional supramolecular framework. The planar coordination environment of 1 is the first example of a TTF-based MOF. Attributed to the unique structure and redox TTF ligand, the electrical conductivity of 1 is significantly increased by 5 orders of magnitude upon iodine treatment. The iodine-treated 1 (1-ox) electrode displays typical battery-type behavior through electrochemical characterizations. The supercapattery based on the 1-ox positrode and AC negatrode presents a high specific capacity of 266.5 C g-1 at a specific current of 1 A g-1 with a remarkable specific energy of 62.9 Wh kg-1 at a specific power of 1.1 kW kg-1. The excellent electrochemical performance of 1-ox is one of the best among those reported supercapatteries, demonstrating a new strategy for developing MOF-based electrode materials.

Surface Phase Conversion in a High-Entropy Layered Oxide Cathode Material.

High-entropy transition-metal oxides are potentially interesting cathode materials for lithium-ion batteries, among which high-entropy layered oxides are considered highly promising because there exist two-dimensional ion transport channels that may, in principle, enable fast ion transport. However, high-entropy layered oxides reported to date exhibit fast capacity fading in initial cycles and thus are hardly of any practical value. Here, we investigate the structural and property changes of a five-element layered oxide, LiNi0.2Co0.2Mn0.2Fe0.2Al0.2O2, using electrochemical and physical characterization techniques. It is revealed that the M3O4 phase formed at the surface of LiNi0.2Co0.2Mn0.2Fe0.2Al0.2O2 due to the migration of metal ions from octahedral sites of the transition-metal layer to tetrahedral 8a and octahedral sites of the lithium layer hinders the intercalation of lithium ion, which leads to the low initial Coulombic efficiency and fast decay of reversible capacity. This mechanism could be generally applicable to other high-entropy layered oxides with different elemental compositions.

Oxidatively Doped Tetrathiafulvalene-Based Metal-Organic Frameworks for High Specific Energy of Supercapatteries.

Poor electrical conductivity and instability of metal-organic frameworks (MOFs) have limited their energy storage and conversion efficiency. In this work, we report the application of oxidatively doped tetrathiafulvalene (TTF)-based MOFs for high-performance electrodes in supercapatteries. Two isostructural MOFs, formulated as [M(py-TTF-py)(BPDC)]·2H2O (M = NiII (1), ZnII (2); py-TTF-py = 2,6-bis(4'-pyridyl)TTF; H2BPDC = biphenyl-4,4'-dicarboxylic acid), are crystallographically characterized. The structural analyses show that the two MOFs possess a three-dimensional 8-fold interpenetrating diamond-like topology, which is the first example for TTF-based dual-ligand MOFs. Upon iodine treatment, MOFs 1 and 2 are converted into oxidatively doped 1-ox and 2-ox with high crystallinity. The electrical conductivity of 1-ox and 2-ox is significantly increased by six∼seven orders of magnitude. Benefiting from the unique structure and the pronounced development of electrical conductivity, the specific capacities reach 833.2 and 828.3 C g-1 at a specific current of 1 A g-1 for 1-ox and 2-ox, respectively. When used as a battery-type positrode to assemble a supercapattery, the AC∥1-ox and AC∥2-ox (AC = activated carbon) present an energy density of 90.3 and 83.0 Wh kg-1 at a power density of 1.18 kW kg-1 and great cycling stability with 82% of original capacity and 92% columbic efficiency retention after 10,000 cycles. Ex situ characterization illustrates the ligand-dominated mechanism in the charge/discharge processes. The excellent electrochemical performances of 1-ox and 2-ox are rarely reported for supercapatteries, illustrating that the construction of unique highly dense and robust structures of MOFs followed by postsynthetic oxidative doping is an effective approach to fabricate MOF-based electrode materials.

An Atom-Pair Design Strategy for Optimizing the Synergistic Electron Effects of Catalytic Sites in NO Selective Reduction.

Effective adsorption and speedy surface reactions are vital requirements for efficient active sites in catalysis, but it remains challenging to maximize these two functions simultaneously. We present a solution to this issue by designing a series of atom-pair catalytic sites with tunable electronic interactions. As a case study, NO selective reduction occurring on V1 -W1 /TiO2 is chosen. Experimental and theoretical results reveal that the synergistic electron effect present between the paired atoms enriches high-energy spin charge around the Fermi level, simultaneously rendering reactant (NH3 or O2 ) adsorption more effective and subsequent surface reactions speedier as compared with single V or W atom alone, and hence higher reaction rates. This strategy enables us to rationally design a high-performance V1 -Mo1 /TiO2 catalyst with optimized vanadium(IV)-molybdenum(V) electronic interactions, which has exceptional activity significantly higher than the commercial or reported catalysts.

Efficient Catalytic Conversion of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid over Ruthenium Cluster-Embedded Ni(OH)2 Catalyst.

5-Hydroxymethylfurfural (HMF) can be oxidized to 2,5-furandicarboxylic acid (FDCA) for the production of biorenewable plastics to replace fossil resourced polyethylene terephthalate (PET). Development of a highly efficient electrocatalyst using renewable electricity as energy input is highly desired. In this work, Ru cluster-embedded Ni(OH)2 nanosheets [Ru/Ni(OH)2 ] were synthesized and exploited as electrochemical catalysts for the conversion of HMF to FDCA. Ru/Ni(OH)2 exhibited significantly improved current density (40 mA cm-2 at 1.41 V vs. reversible hydrogen electrode) of over 7.7 times in comparison with Ni(OH)2 , and nearly 100 % conversion degree for HMF and 98.5 % selectivity towards FDCA were obtained. Operando Raman experiments revealed the catalysis was facilitated by the interconversion between Ni3+ and Ni2+ . Density functional theory calculations further revealed the effect of Ru clusters of Ni(OH)2 , thereby promoting HMF adsorption capacity on Ni sites to boost HMF oxidation activity. This work provides a novel strategy using Ru clusters to modify earth abundant Ni based catalyst for HMF oxidation to obtain high-value biomass-derived products.

Breaking scaling relationships in alkynol semi-hydrogenation by manipulating interstitial atoms in Pd with d-electron gain.

Pd catalysts are widely used in alkynol semi-hydrogenation. However, due to the existence of scaling relationships of adsorption energies between the key adsorbed species, the increase in conversion is frequently accompanied by side reactions, thereby reducing the selectivity to alkenols. We report that the simultaneous increase in alkenol selectivity and alkynol conversion is achieved by manipulating interstitial atoms including B, P, C, S and N in Pd catalysts. A negative linear relationship is observed between the activation entropies of 2-methyl-3-butyn-2-ol and 2-methyl-3-buten-2-ol which is highly related to the filling of d-orbital of Pd catalysts by the modification of p-block elements. A catalyst co-modified by B and C atoms has the maximum d charge of Pd that achieves a 17-fold increase in the turn-over frequency values compared to the Lindlar catalysts in the semi-hydrogenation of 2-methyl-3-butyn-2-ol. When the conversion is close to 100%, the selectivity can be as high as 95%.

Tetrathiafulvalene-Cobalt Metal-Organic Frameworks for Lithium-Ion Batteries with Superb Rate Capability.

Although pristine metal-organic framework (MOF) anodes for lithium-ion batteries (LIBs) show moderate activities and relatively stable cycling, the poor rate capability of the MOF anodes limited their applications in the development of a new generation of energy storage. Herein, the electric active CoII ion is selected to coordinate with redox-active S-rich tetrathiafulvalene (TTF) derivatives to create two TTF-Co-MOFs, formulated as [Co2(py-TTF-py)2(BDC)2]·2DMF·H2O (TTF-Co-MOF 1) and [Co2(py-TTF-py)2(BPDC)2]·3DMF·3H2O (TTF-Co-MOF 2), where py-TTF-py = 2,6-bis(4'-pyridyl)tetrathiafulvalene, H2BDC = terephthalic acid, H2BPDC = biphenyl-4,4'-dicarboxylic acid, and DMF = N,N-dimethylformamide. Crystallographic characterization indicated that the two MOFs possess similar 2-fold-interpenetrating 3D frameworks but with two different pore sizes. The pore-size-dependent performances of the TTF-Co-MOFs were explored to optimize the MOFs as the anode materials for LIBs. TTF-Co-MOF 1 presents a high reversible specific capacity of 1186.6 mAh g-1 at 200 mA g-1 after 287 cycles. The rate capability is greatly enhanced by the introduction of CoII into TTF-based MOFs with specific capacities of 1028.6 mAh g-1 at 5 A g-1 and 966.5 mAh g-1 at 10 A g-1. On the basis of the series analysis of theoretical calculations, electrochemical impedance spectroscopy, and crystal structures, it is found that the CoII metal centers play a bridging role in charge transport within the MOF framework, which is beneficial for the transportation of Li ions. The competitive performances of TTF-Co-MOF 1 are attributed to the synergistic effect of the CoII metal centers and S-rich TTF ligand as well as suitable porosity. The study shed some light for the fabrication of advanced energy storage devices through the rational design of MOF-based anode materials.

2D Lead Iodide Perovskite with Mercaptan-Containing Amine and Its Exceptional Water Stability.

Two dimensional (2D) hybrid perovskites have attracted a great deal of interest because of their appropriate photovoltaic efficiency and environmental stability. Although some 2D hybrid perovskites with sulfur-containing amines have been reported, the cation having the mercaptan group has not been well explored yet. In this work, cysteamine (Cya, HS(CH2)2NH2), a mercaptan-containing amine, was introduced into 2D hybrid perovskite. Two 2D lead iodides with different structures, (HCya)2PbI4 (1) and (HCya)7Pb4I15 (2), were isolated as a red low-temperature phase and a yellow high-temperature phase, respectively. X-ray single-crystal structural analysis showed that the red phase 1 is a single layered corner-shared perovskite and that the yellow phase 2 is a corner/edge-shared quasi-2D perovskite. A thermo-induced reversible 1 to 2 phase transition was found in this synthetic system. The configuration of HCya cation greatly influences the crystallization equilibrium, generating different structures of the lead halides. The single-crystal structure of 1 is discussed in comparison with that of (HAE)2PbI4 (AE = HO(CH2)2NH2), an analogue of 1. The different effects of OH and SH groups on the 2D frameworks are studied based on their hydrogen bonding properties. More remarkably, although the two perovskites have similar structures, the (HCya)2PbI4 (1) has an intrinsic water stability that is much more stable than (HAE)2PbI4, which should be attributed to the affinity of the SH group with lead on the surface of the lead halide.

Tetrathiafulvalene-based double metal lead iodides: structures and electrical properties.

Introducing electronically active organic components into lower dimensional metal halide compounds is an effective strategy to improve the electronic properties of hybrid metal halide materials. We have previously used this strategy to explore hybrid halides with tetrathiafulvalenes (TTFs) and a series of lead iodides and bismuth halides were isolated. The electronic properties were improved notably using this modification. In this work, we expand the study of TTF based main-group metal halides to double metal halides with mixed lead and copper transition metals. Two hybrid TTF-lead-cuprous iodides, formulated as [TTF]5[Pb2Cu2I10]·H2O (1) and [TTF]2[PbCu2I6] (2), and two monometal analogues of [TTF]2[Cu4I6]·H2O (3) and [TTF]2[Ag4I6] (4) were crystallographically characterized. The anion of 1 is a 0D cluster, while that of the others is a 1D chain structure. The anion structures of 1-4 are novel and are reported for the first time. The TTF moieties are stacked to form a 2D framework in 1 and 1D columns in 2-4. We found that the semiconductor properties of the hybrids are related to electron donation from an anion to a cation. The electronic state of the TTF cations is another significant factor that affects the electronic properties of the materials. More notably, this work proved that the conductivity and photoconductivity of the mixed metal iodides are superior to those of the monometal iodides.

Unveiling the Nature of Pt Single-Atom Catalyst during Electrocatalytic Hydrogen Evolution and Oxygen Reduction Reactions.

Single-atom catalysts (SACs) have attracted significant attention due to their superior catalytic activity and selectivity. However, the nature of active sites of SACs under realistic reaction conditions is ambiguous. In this work, high loading Pt single atoms on graphitic carbon nitride (g-C3 N4 )-derived N-doped carbon nanosheets (Pt1 /NCNS) is achieved through atomic layer deposition. Operando X-ray absorption spectroscopy (XAS) is performed on Pt single atoms and nanoparticles (NPs) in both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The operando results indicate that the total unoccupied density of states of Pt 5d orbitals of Pt1 atoms is higher than that of Pt NPs under HER condition, and that a stable Pt oxide is formed during ORR on Pt1 /NCNS, which may suppress the adsorption and activation of O2 . This work unveils the nature of Pt single atoms under realistic HER and ORR conditions, providing a deeper understanding for designing advanced SACs.

Modulating Single-Atom Palladium Sites with Copper for Enhanced Ambient Ammonia Electrosynthesis.

The electrochemical reduction of N2 to NH3 is emerging as a promising alternative for sustainable and distributed production of NH3 . However, the development has been impeded by difficulties in N2 adsorption, protonation of *NN, and inhibition of competing hydrogen evolution. To address the issues, we design a catalyst with diatomic Pd-Cu sites on N-doped carbon by modulation of single-atom Pd sites with Cu. The introduction of Cu not only shifts the partial density of states of Pd toward the Fermi level but also promotes the d-2π* coupling between Pd and adsorbed N2 , leading to enhanced chemisorption and activated protonation of N2 , and suppressed hydrogen evolution. As a result, the catalyst achieves a high Faradaic efficiency of 24.8±0.8 % and a desirable NH3 yield rate of 69.2±2.5 µg h-1 mgcat. -1 , far outperforming the individual single-atom Pd catalyst. This work paves a pathway of engineering single-atom-based electrocatalysts for enhanced ammonia electrosynthesis.

Stable and Efficient Single-Atom Zn Catalyst for CO2 Reduction to CH4.

The development of highly active and durable catalysts for electrochemical reduction of CO2 (ERC) to CH4 in aqueous media is an efficient and environmentally friendly solution to address global problems in energy and sustainability. In this work, an electrocatalyst consisting of single Zn atoms supported on microporous N-doped carbon was designed to enable multielectron transfer for catalyzing ERC to CH4 in 1 M KHCO3 solution. This catalyst exhibits a high Faradaic efficiency (FE) of 85%, a partial current density of -31.8 mA cm-2 at a potential of -1.8 V versus saturated calomel electrode, and remarkable stability, with neither an obvious current drop nor large FE fluctuation observed during 35 h of ERC, indicating a far superior performance than that of dominant Cu-based catalysts for ERC to CH4. Theoretical calculations reveal that single Zn atoms largely block CO generation and instead facilitate the production of CH4.