Theoretical Insights into the Selectivity of Nitrite Reduction to NH2OH on Single-Atom Catalysts.

Electroreduction of nitrate/nitrite to high-value-added products, including NH2OH, is an important way to achieve sustainable production of green energy. However, this electrosynthesis of NH2OH still suffers from poor selectivity due to the various competing reactions. Here, we screen out Ni-N4 and Cu-N4 catalysts for highly efficient nitrite electroreduction to NH2OH by adopting density functional theory (DFT) calculations. DFT calculations reveal that the high selectivity of Ni-N4 and Cu-N4 is ascribed to their weak adsorption of *NH2OH and *NH intermediates, thereby preventing the further reduction of NH2OH. Moreover, using *NO as a model intermediate, we studied the relationship between the 3d orbital occupancy and adsorption strength of the intermediate. It is found that Ni-N4 and Cu-N4 with fully occupied dxz, dyz, and dz2 orbitals have poor adsorption of *NO intermediate. This work provides a new route for NH2OH synthesis and offers perspectives on the crucial factors in determining the catalytic selectivity.

Local Charge Transfer Unveils Antideactivation of Ru at High Potentials for the Alkaline Hydrogen Oxidation Reaction.

The activity of Ru-based alkaline hydrogen oxidation reaction (HOR) electrocatalysts usually decreases rapidly at potentials higher than 0.1 V (vs a reversible hydrogen electrode (RHE)), which significantly limits the lifetime of fuel cells. It is found that this phenomenon is caused by the overadsorption of the O species due to the overcharging of Ru nanoparticles at high potentials. Here, Mn1Ox(OH)y clusters-modified Ru nanoparticles (Mn1Ox(OH)y@Ru/C) were prepared to promote charge transfer from overcharged Ru nanoparticles to Mn1Ox(OH)y clusters. Mn1Ox(OH)y@Ru/C exhibits high HOR activity and stability over a wide potential range of 0-1.0 V. Moreover, a hydroxide exchange membrane fuel cell with a Mn1Ox(OH)y@Ru/C anode delivers a high peak power density of 1.731 W cm-2, much superior to that of a Pt/C anode. In situ X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculations reveal that Mn in Mn1Ox(OH)y clusters could receive more electrons from overcharged Ru at higher potentials and significantly decrease the overadsorption of the O species on Ru, thus permitting the HOR on Ru to proceed at high potentials. This study provides guidance for the design of alkaline HOR catalysts without activity decay at high potentials.

Optimizing Hydrogen and Hydroxyl Adsorption over Ru/WO2.9 Metal/Metalloid Heterostructure Electrocatalysts for Highly Efficient and Stable Hydrogen Oxidation Reactions in Alkaline Media.

The development of high-performance, stable and platinum-free electrocatalysts for the hydrogen oxidation reaction (HOR) in alkaline media is crucial for the commercial application of anion exchange membrane fuel cells (AEMFCs). Ruthenium, as an emerging HOR electrocatalyst with a price advantage over platinum, still needs to solve the problems of low intrinsic activity and easy oxidation. Herein, Ru nanoparticles are anchored on the oxygen-vacancy-rich metalloid WO2.9 by interfacial engineering to create abundant and efficient Ru and WO2.9 interfacial active sites for accelerated HOR in alkaline media. Ru/WO2.9/C displays excellent catalytic activity with mass activity (8.29 A mgNM -1) and specific activity (1.32 mA cmNM -2), which are 2.5/3.3 and 21.8/8.3 times that of PtRu/C and Pt/C, respectively. Moreover, Ru/WO2.9/C exhibits excellent CO tolerance and operational stability. Experimental and theoretical studies reveal that the improved charge transfer from Ru to WO2.9 in the metal/metalloid heterostructure significantly tune the electronic structure of Ru sites and optimize the hydrogen binding energy (HBE) of Ru. While, WO2.9 provides abundant hydroxyl adsorption sites. Therefore, the equilibrium adsorption of hydrogen and hydroxyl at the interface of Ru/WO2.9 will be realized, and the oxidation of metal Ru would be avoided, thereby achieving excellent HOR activity and durability.

Photo-Excited High-Spin State Ni (III) Species in Mo-Doped Ni3S2 for Efficient Urea Oxidation Reaction.

Designing robust catalysts for increasing the sluggish kinetics of the urea oxidation reaction (UOR) is challenging. Herein, the regulation of spin states for metal active sites by photoexcitation to facilitate the adsorption of urea and intermediates is demonstrated. Mo-doped nickel sulfide nanoribbon arrays (Mo-Ni3S2@NMF) with excellent light-trapping capacity are successfully prepared. Under AM 1.5G illumination, the activity of the Mo-Ni3S2@NMF exhibits a 50% improvement in the UOR current. Compared with those under dark conditions, Mo-Ni3S2@NMF achieve 10 mA cm-2 at 1.315 VRHE for UOR and 1.32 Vcell for urea electrolysis, which are decreases of 15 and 80 mV, respectively. The electron spin resonance, in situ Fourier transform infrared spectroscopy analysis and density functional theory calculations reveal that illumination led to the formation of Ni3+ active sites in a high-spin state, which strengthens the d-p orbital hybridization of Ni-N, hence facilitating the adsorption of urea. C─N cleavage of the *CONN intermediate is further inhibited, which promotes the oxidation of urea molecules via the active N2 pathway, thereby accelerating the UOR rate.

Ultralow-Loading Ruthenium-Iridium Fuel Cell Catalysts Dispersed on Zn-N Species-Doped Carbon.

Developing low-loading platinum-group-metal (PGM) catalysts is one of the key challenges in commercializing anion-exchange-membrane-fuel-cells (AEMFCs), especially for hydrogen oxidation reaction (HOR). Here, ruthenium-iridium nanoparticles being deposited on a Zn-N species-doped carbon carrier (Ru6Ir/Zn-N-C) are synthesized and used as an anodic catalyst for AEMFCs. Ru6Ir/Zn-N-C shows extremely high mass activity (5.87 A mgPGM -1) and exchange current density (0.92 mA cm-2), which is 15.1 and 3.9 times that of commercial Pt/C, respectively. Based on the Ru6Ir/Zn-N-C AEMFCs achieve a peak power density of 1.50 W cm-2, surpassing the state-of-the-art commercial PtRu catalysts and the power ratio of the normalized loading is 14.01 W mgPGM anode -1 or 5.89 W mgPGM -1 after decreasing the anode loading (87.49 µg cm-2) or the total PGM loading (0.111 mg cm-2), satisfying the US Department of Energy's PGM loading target. Moreover, the solvent and solute isotope separation method is used for the first time to reveal the kinetic process of HOR, which shows the reaction is influenced by the adsorption of H2O and OH-. The improvement of the hydrogen bond network connectivity of the electric double layer by adjusting the interfacial H2O structure together with the optimized HBE and OHBE is proposed to be responsible for the high HOR activity of Ru6Ir/Zn-N-C.

4f-2p-3d Orbital Coupling in Ce-Ni3S2 Enhancing the Urea Oxidation Reaction.

The electrocatalytic urea oxidation reaction (UOR) provides a promising alternative to the oxygen evolution reaction (OER) for various renewable energy-related systems owing to its lower thermodynamic barriers. However, its optimization and commercial utilization were hampered due to a lack of mechanistic understanding. Here, we demonstrate a Ce-doped Ni3S2 catalyst supported on Ni foam (Ce-Ni3S2/NF) with superior activity toward UOR. The resultant Ce-Ni3S2/NF catalyst exhibits a lower Tafel slope of 20.3 mV dec-1, a higher current density of 100 mA cm-2 at 1.39 V versus RHE, and better durability than those for Ni3S2/NF. Based on in situ synchrotron radiation X-ray absorption spectroscopy, in situ Fourier transform infrared (FTIR), and in situ Raman spectroscopy, we observe the structural reconstruction of sulfide and identify the adsorbed intermediates during UOR. Density functional theory (DFT) calculations reveal that Ce can regulate the electronic structure of Ni through Ce(4f)-O(2p)-Ni(3d) orbital electronic coupling. The modulated Ni sites have weaker adsorption of carbonaceous intermediates, thus accelerating the UOR. This work provides a promising route for the design of high-activity UOR catalysts.

V-O Species-Doped Carbon Frameworks Loaded with Ru Nanoparticles as Highly Efficient and CO-Tolerant Catalysts for Alkaline Hydrogen Oxidation.

Efficient and CO-tolerant catalysts for alkaline hydrogen oxidation (HOR) are vital to the commercial application of anion exchange membrane fuel cells (AEMFCs). Herein, a robust Ru-based catalyst (Ru/VOC) with ultrasmall Ru nanoparticles supported on carbon frameworks with atomically dispersed V-O species is prepared elaborately. The catalyst exhibits a remarkable mass activity of 3.44 mA µgPGM, which is 31.3 times that of Ru/C and even 4.7 times higher than that of Pt/C. Moreover, the Ru/VOC anode can achieve a peak power density (PPD) of 1.194 W cm-2, much superior to that of Ru/C anode and even better than that of Pt/C anode. In addition, the catalyst also exhibits superior stability and exceptional CO tolerance. Experimental results and density functional theory (DFT) calculations demonstrate that V-O species are ideal OH- adsorption sites, which allow Ru to release more sites for hydrogen adsorption. Furthermore, the electron transfer from Ru nanoparticles to the carbon substrate regulates the electronic structure of Ru, reducing the hydrogen binding energy (HBE) and the CO adsorption energy on Ru, thus boosting the alkaline HOR performance and CO tolerance of the catalyst. This is the first report that oxophilic single atoms distributed on carbon frameworks serve as OH- adsorption sites for efficient hydrogen oxidation, opening up new guidance for the elaborate design of high-activity catalysts for the alkaline HOR.

Highly Flexible K-Intercalated MnO2 /Carbon Membrane for High-Performance Aqueous Zinc-Ion Battery Cathode.

The layered MnO2 is intensively investigated as one of the most promising cathode materials for aqueous zinc-ion batteries (AZIBs), but its commercialization is severely impeded by the challenging issues of the inferior intrinsic electronic conductivity and undesirable structural stability during the charge-discharge cycles. Herein, the lab-prepared flexible carbon membrane with highly electrical conductivity is first used as the matrix to generate ultrathin δ-MnO2 with an enlarged interlayer spacing induced by the K+ -intercalation to potentially alleviate the structural damage caused by H+ /Zn2+ co-intercalation, resulting in a high reversible capacity of 190 mAh g-1 at 3 A g-1 over 1000 cycles. The in situ/ex-situ characterizations and electrochemical analysis confirm that the enlarged interlayer spacing can provide free space for the reversible deintercalation/intercalation of H+ /Zn2+ in the structure of δ-MnO2 , and H+ /Zn2+ co-intercalation mechanism contributes to the enhanced charge storage in the layered K+ -intercalated δ-MnO2 . This work provides a plausible way to construct a flexible carbon membrane-based cathode for high-performance AZIBs.

Octahedral/Tetrahedral Vacancies in Fe3 O4 as K-Storage Sites: A Case of Anti-Spinel Structure Material Serving as High-Performance Anodes for PIBs.

Potassium-ion batteries (PIBs) have attracted more and more attention as viable alternatives to lithium-ion batteries (LIBs) due to the deficiency and uneven distribution of lithium resources. However, it is shown that potassium storage in some compounds through reaction or intercalation mechanisms cannot effectively improve the capacity and stability of anodes for PIBs. The unique anti-spinel structure of magnetite (Fe3 O4 ) is densely packed with thirty-two O atoms to form a face-centered cubic (fcc) unit cell with tetrahedral/octahedral vacancies in the O-closed packing structure, which can serve as K+ storage sites according to the density functional theory (DFT) calculation results. In this work, carbon-coated Fe3 O4 @C nanoparticles are prepared as high-performance anodes for PIBs, which exhibit high reversible capacity (638 mAh g-1 at 0.05 A g-1 ) and hyper stable cycling performance at ultrahigh current density (150 mAh g-1 after 9000 cycles at 10 A g-1 ). In situ XRD, ex-situ Fe K-edge XAFS, and DFT calculations confirm the storage of K+ in tetrahedral/octahedral vacancies.

Missing-Linker-Confined Single-Atomic Pt Nanozymes for Enzymatic Theranostics of Tumor.

Conventional nanozymes often possess low active site density. Pursuing effective strategies for constructing highly active single-atomic nanosystems with maximum atom utilization efficiency is exceptionally attractive. Herein, we develop a facile "missing-linker-confined coordination" strategy to fabricate two self-assembled nanozymes, i.e., conventional nanozyme (NE) and single-atomic nanozyme (SAE), which respectively consist of Pt nanoparticles and single Pt atoms as active catalytic sites anchored in metal-organic frameworks (MOFs) with encapsulated photosensitizers for catalase-mimicking enhanced photodynamic therapy. Compared to a Pt nanoparticle-based conventional nanozyme, a Pt single-atomic nanozyme shows enhanced catalase-mimicking activity in generating oxygen for overcoming tumor hypoxia, thus exhibiting a more efficient reactive oxygen species generation and high tumor inhibition rate.

Bidirectional Tandem Electrocatalysis Manipulated Sulfur Speciation Pathway for High-Capacity and Stable Na-S Battery.

The sluggish polysulfide redox kinetics and the uncontrollable sulfur speciation pathway, leading to serious shuttling effect and high activation barrier associated with sulfur cathode. We describe here the use of core-shell structured composite matrixes containing abundant catalytic sites for nearly fully reversible cycling of sulfur cathodes for Na-S batteries. The bidirectional tandem electrocatalysis provide successive reversible conversion of both long- and short-chain polysulfides, whereas Fe2 O3 accelerates Na2 S8 /Na2 S6 to Na2 S4 conversion and the redox-active Fe(CN)6 4- -doped polypyrrole shell catalyzes Na2 S4 reduction to Na2 S. The electrochemically reactive Na2 S can be readily charged back to sulfur with minimal overpotential. Simultaneously, stable cycling of Na-S pouch cell with a high reversible capacity of 696 mAh g-1 is also demonstrated. The bidirectional confined tandem catalysis renders the manipulation of sulfur redox electrochemistry for practical Na-S cells.

Hollow Cuprous Oxide@Nitrogen-Doped Carbon Nanocapsules for Cascade Chemodynamic Therapy.

Cuprous-based nanozymes have demonstrated great potential for cascade chemodynamic therapy (CDT) due to their higher catalytic efficiency and simple reaction conditions. Here, hollow cuprous oxide@nitrogen-doped carbon (HCONC) dual-shell structures are designed as nanozymes for CDT oncotherapy. This HCONC with a size distribution of 130 nm is synthesized by a one-step hydrothermal method using cupric nitrate and dimethyl formamide as precursors. The thin-layer carbon (1.88 nm) of HCONC enhances the water-stability and reduces the systemic toxicity of cuprous oxide nanocrystals. The dissolved Cu+ of HCONC in acid solution induces a Fenton-like reaction and exhibits a fast reaction rate for catalyzing H2 O2 into highly toxic hydroxyl radicals (·OH). Meanwhile, the formed Cu+ consumes oversaturated glutathione (GSH) to avoid its destruction of ROS at the intracellular level. In general, both cellular and animal experiments show that HCONC demonstrates excellent antitumor ability without causing significant systemic toxicity, which may present tremendous potential for clinical cancer therapy.

Lewis-Basic EDTA as a Highly Active Molecular Electrocatalyst for CO2 Reduction to CH4.

The most active catalysts so far successful in hydrogenation reduction of CO2 are mainly heterogeneous Cu-based catalysts. The complex coordination environments and multiple active sites in heterogeneous catalysts result in low selectivity of target product, while molecular catalysts with well-defined active sites and tailorable structures allow mechanism-based performance optimization. Herein, we firstly report a single ethylenediaminetetraacetic acid (EDTA) molecular-level immobilized on the surface of carbon nanotube as a catalyst for transferring CO2 to CH4 with an excellent performance. This catalyst exhibits a high Faradaic efficiency of 61.6 % toward CH4 , a partial current density of -16.5 mA cm-2 at a potential of -1.3 V versus reversible hydrogen electrode. Density functional theory calculations reveal that the Lewis basic COO- groups in EDTA molecule are the active sites for CO2 reduction reaction (CO2 RR). The energy barrier for the generation of CO from *CO intermediate is as high as 0.52 eV, while the further protonation of *CO to *CHO follows an energetic downhill path (-1.57 eV), resulting in the high selectivity of CH4 . This work makes it possible to control the product selectivity for CO2 RR according to the relationship between the energy barrier of *CO intermediate and molecular structures in the future.

Self-Assembled Single-Site Nanozyme for Tumor-Specific Amplified Cascade Enzymatic Therapy.

Nanomaterials with enzyme-mimicking activity (nanozymes) show potential for therapeutic interventions. However, it remains a formidable challenge to selectively kill tumor cells through enzymatic reactions, while leaving normal cells unharmed. Herein, we present a new strategy based on a single-site cascade enzymatic reaction for tumor-specific therapy that avoids off-target toxicity to normal tissues. A copper hexacyanoferrate (Cu-HCF) nanozyme with active single-site copper exhibited cascade enzymatic activity within the tumor microenvironment: Tumor-specific glutathione oxidase activity by the Cu-HCF single-site nanozymes (SSNEs) led to the depletion of intracellular glutathione and the conversion of single-site CuII species into CuI for subsequent amplified peroxidase activity through a Fenton-type Harber-Weiss reaction. In this way, abundant highly toxic hydroxyl radicals were generated for tumor cell apoptosis. The results show that SSNEs could amplify the tumor-killing efficacy of reactive oxygen species and suppress tumor growth in vivo.

Energetic Metal-Organic Frameworks Derived Highly Nitrogen-Doped Porous Carbon for Superior Potassium Storage.

The carbonaceous materials with low cost and high safety have been considered as promising anodes for potassium-ion batteries (PIBs). However, it is still a challenge to design a carbonaceous material with long cycle life and high rate performance due to the poor K+ reaction kinetics. Herein, this article reports a N-doped porous carbon framework (NPCF) with a high nitrogen content of 13.57 at% within high doping level of the pyrrolic N and pyridinic N, which exhibits a high reversible capacity of 327 mA h g-1 over 100 cycles at a current density of 100 mA g-1 , excellent rate capability (144 and 105 mA h g-1 at 10 and 20 A g-1 , respectively) and great cyclability of 258.9 mA h g-1 after 2000 cycles at 1 A g-1 . Such a high rate performance and excellent cycling stability anode material is seldom reported in PIBs. Density functional theory (DFT) calculations reveal that the pyrrolic and pyridinic N-doping are helpful to enhance the K adsorption ability, thereby increasing the specific capacity.

The Enhancement of the Catalytic Oxidation of CO on Ir/CeO2 Nanojunctions.

Heterogeneous catalysts facilitate various chemical reactions through the changing of their surface charge density. Herein, we demonstrate an interface engineering strategy to enhance the Ir catalytic performance for oxidation of CO by constructing Ir/CeO2 nanojunctions using a wet chemical reduction process. The as-prepared Ir/CeO2 nanojunctions show a complete CO conversion temperature (T100) of 110 °C under 1 vol % CO and retains long-term durability even after 24 h. The Ir atoms and CeO2 supports were connected by O atoms, which changes the surface electronic structure of Ir atoms. DFT calculations demonstrate that the significant increase in electron density at the interface between Ir and CeO2 plays a pivotal role in the enhancement of CO oxidation.

Carbon-based hybrid nanogels: a synergistic nanoplatform for combined biosensing, bioimaging, and responsive drug delivery.

Nanosized crosslinked polymer networks, named as nanogels, are playing an increasingly important role in a diverse range of applications by virtue of their porous structures, large surface area, good biocompatibility and responsiveness to internal and/or external chemico-physical stimuli. Recently, a variety of carbon nanomaterials, such as carbon quantum dots, graphene/graphene oxide nanosheets, fullerenes, carbon nanotubes, and nanodiamonds, have been embedded into responsive polymer nanogels, in order to integrate the unique electro-optical properties of carbon nanomaterials with the merits of nanogels into a single hybrid nanogel system for improvement of their applications in nanomedicine. A vast number of studies have been pursued to explore the applications of carbon-based hybrid nanogels in biomedical areas for biosensing, bioimaging, and smart drug carriers with combinatorial therapies and/or theranostic ability. New synthetic methods and structures have been developed to prepare carbon-based hybrid nanogels with versatile properties and functions. In this review, we summarize the latest developments and applications and address the future perspectives of these carbon-based hybrid nanogels in the biomedical field.

Dual Graphitic-N Doping in a Six-Membered C-Ring of Graphene-Analogous Particles Enables an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

Graphene-based materials still exhibit poor electrocatalytic activities for the hydrogen evolution reaction (HER) although they are considered to be the most promising electrocatalysts. We fabricated a graphene-analogous material displaying exceptional activity towards the HER under acidic conditions with an overpotential (57 mV at 10 mA cm-2 ) and Tafel slope (44.6 mV dec-1 ) superior to previously reported graphene-based materials, and even comparable to the state-of-the art Pt/C catalyst. X-ray absorption near-edge structure (XANES) and solid-state NMR studies reveal that the distinct feature of its structure is dual graphitic-N doping in a six-membered carbon ring. Density functional theory (DFT) calculations show that the unique doped structure is beneficial for the activation of C-H bonds and to make the carbon atom bonded to two graphitic N atoms an active site for the HER.

Cobalt Nanocrystals Encapsulated in Heteroatom-Rich Porous Carbons Derived from Conjugated Microporous Polymers for Efficient Electrocatalytic Hydrogen Evolution.

Cobalt nanocrystals encapsulated in N,O-dual-doped porous carbons as efficient and stable electrocatalysts for hydrogen evolution reaction (HER) are reported. A heteroatom-rich-conjugated microporous polymer is first chemically deposited on a carbon fiber cloth, and after addition of a cobalt salt, pyrolyzed to produce a heteroatom-doped C/Co nanocrystal composite. With this process, the use of additional binders for preparation of electrodes can be avoided. With a trace cobalt loading (0.46 wt%), the electrodes achieve a low Tafel slope of 46 mV dec-1 and overpotential of only 69 mV at a current density of 10 mA cm-2 in 0.5 m H2 SO4 . Experimental and computational studies reveal that the superior HER behavior is due to a decreased free energy of hydrogen adsorption, induced by i) electrons transferred from the cobalt nanocrystals to graphite layers and ii) N,O-dual doping reduced the Fermi level of neighboring C atoms.

A Flexible Sulfur-Enriched Nitrogen Doped Multichannel Hollow Carbon Nanofibers Film for High Performance Sodium Storage.

Heteroatom doping is regarded as a promising method to enhance the sodium storage performance of carbon materials. In this work, a sulfur-enriched N-doped multichannel hollow carbon nanofiber (denoted as S-NCNF) film is prepared through electrospinning technology and heat treatment with sublimed sulfur as the flexible anode for sodium ion batteries (NIBs). The S-NCNF film displays outstanding electrochemical performance, particularly with a high rate capacity (132 mA h g-1 at the current density of 10 A g-1 ) and remarkable long cycling stability (reversible specific capacity of 187 mA h g-1 at 2 A g-1 over 2000 cycles). The improved sodium storage performance results from the unique 3D structure, abundant defects, and increased interlayer spacing of S-NCNFs. The density functional theory calculations demonstrate that nitrogenous carbon nanofibers doping with sulfur could not only promote the adsorption of sodium but also favor electrons' transfer. This strategy has been demonstrated as a general process to design free-standing carbon-based thin film with other heteroatom doping.