Polymer-chelation approach to high-performance Fe-Nx-C catalyst towards oxygen reduction reaction

Pyrolyzed Fe-N_x-C with atomically dispersed Fe-N_x sites are hailed as the most promising alternative to the noble metal Pt-based catalysts towards oxygen reduction reaction (ORR). However, the conventional micropore-confinement synthetic approach usually causes the insufficient utilization of active sites and mass transport resistance as the sites are located inside the micropore. We herein report a polymer-chelation strategy to directly disperse the Fe-N_x active sites onto the carbon surface. The N-rich monomer was in-situ polymerized on the carbon support and then chelated with Fe. The strong Fe-N chelating interaction is crucial to suppress Fe aggregation when undergoing the high-temperature pyrolysis. Due to the enriched surface sites, hierarchically porous structure and excellent conductivity of carbon support, the optimal catalyst (denoted as Fe-N_x-C@C-900) exhibits impressive ORR activity of onset and half-wave potential of 1.02 and 0.87 V, respectively, superior to the Pt/C benchmark.     

Revealing the true origin of size-dependent Pd/C catalytic behavior towards formic acid decomposition

Formic acid decomposition (FAD) is considered a promising hydrogen production route to facilitate the ambient storage and on demand release of hydrogen energy. To optimize the catalysts for FAD, efforts have been paid to explore the underlying reason for the varied catalytic activity among catalysts with similar composition but differed structure. However, such endeavors are highly challenging due to the deeply intermingled effects of electronic structure, particle size, and facets, etc. Herein, to separately evaluate the respective effects of these factors, a series of catalysts with the same surface electronic structure and different particle size was prepared by cation dipole adjustment method. The performance and characterization results showed that the catalysts with different sizes and facets exhibited similar intrinsic activity with deviation of less than 5%. However, they showed 252% deviation of site stability, indicating that only the optimized electronic structure could enhance the intrinsic activity and a smaller particle size could extend the catalyst's life.     

Transition metal decorated bismuthene for ammonia synthesis: A density functional theory study

The electrochemical nitrogen reduction reaction (NRR) for the ammonia production under ambient conditions is regarded as a sustainable alternative to the industrial Haber–Bosch process. However, the electrocatalytic systems that efficiently catalyze nitrogen reduction remain elusive. In the work, the nitrogen reduction activity of the transition metal decorated bismuthene TM@Bis is fully investigated by means of density functional theory calculations. Our results demonstrate that W@Bis delivers the best efficiency, wherein the potential-determining step is located at the last protonation step of *NH₂ + H⁺ + e^– → *NH₃ via the distal mechanism with the limiting potential U_L of 0.26 V. Furthermore, the dopants of Re and Os are also promising candidates for experimental synthesis due to its good selectivity, in despite of the slightly higher U_L of NRR with the value of 0.55 V. However, the candidates of Ti, V, Nb and Mo delivered the relative lower U_L of 0.35, 0.37, 0.41 and 0.43 V might be suffered from the side hydrogen evolution reaction. More interestingly, a volcano curve is established between U_L and valence electrons of metal elements wherein W with 4 electrons in d band located at the summit. Such phenomenon originates from the underlying acceptance-back donation mechanism. Therefore, our work provides a fundament understanding for the material design for nitrogen reduction electrocatalysis.     

Bidirectionally polarizing surface chemistry of heteroatom-doped carbon matrix towards fast and longevous lithium-sulfur batteries

Herein, a bidirectional polarization strategy is proposed for hosting efficient and durable lithium-sulfur battery (Li-S) electrochemistry. By co-doping electronegative N and electropositive B in graphene matrix (BNrGO), the bidirectional electron redistribution enables a higher polysulfide affinity over its mono-doped counterparts, contributing to strong sulfur immobilization and fast conversion kinetics. As a result, BNrGO as the cathode host matrix realizes excellent cycling stability over 1000 cycles with a minimum capacity fading of 0.027% per cycle, and superb rate capability up to 10 C. Meanwhile, decent areal capacity (6.46 mAh/cm²) and cyclability (300 cycles) are also achievable under high sulfur loading and limited electrolyte. This work provides instructive insights into the interaction between doping engineering and sulfur electrochemistry for pursuing superior Li-S batteries.     

A new semiconductor-based SERS substrate with enhanced charge collection and improved carrier separation: CuO/TiO2 p-n heterojunction

In this paper, CuO/TiO₂ p-n heterojunction was developed as a new surface enhanced Raman scattering (SERS) substrate to magnify Raman signal of 4-mercaptobenzoic acid (4-MBA) molecule. In the heterojunction-molecule system, CuO as an “electron capsule” can not only offer more electrons to inject into the surface state energy level of TiO₂ and consequently bring additional charge transfer, but also improve photogenerated carrier separation efficiency itself due to strong interfacial coupling in the interface of heterojunction, which together boost SERS performance of the heterojunction substrate. As expected, owing to the enhanced charge collection capacity and the improvement of photogenerated carrier separation efficiency derived from internal electric field and strong interface coupling provided in the interface of heterojunction, this substrate exhibits excellent SERS detection sensitivity towards 4-MBA, with a detection limit as low as 1 × 10⁻¹⁰ mol/L and an enhancement factor of 8.87 × 10⁶.     

A series of high-nuclear planar equilateral triangle-shaped {Ln6(μ3-OH)6} cluster encapsulated polyoxoniobates with frequency dependent magnetic property

The integration of lanthanide (Ln) ions and polyoxoniobates (PONbs) is challenging, and the known Ln-substituted PONbs are still scarce. This work introduces high-nuclear iso-Ln-oxo clusters into the PONb system. The first series of high-nuclear Ln-oxo clusters encapsulated heterometallic polyoxoniobates H₉[Na(H₂O)₄][Cu(en)₂]₁₀{Ln₆(μ₃-OH)₆(SiNb₁₈O₅₄)₃}·18H₂O (1-Ln, en = ethylenediamine, Ln = Dy, Gd, Tb, Ho, Er, Tm, Yb, Lu) based on flower-like {Ln₆(μ₃-OH)₆(SiNb₁₈O₅₄)₃} ({Ln₆Si₃Nb₅₄}) clusters have been successfully synthesized via one-pot hydrothermal synthesis strategy. The flower-like polyoxoanion {Ln₆Si₃Nb₅₄} is consisted of three heteropolyoxoniobate {SiNb₁₈O₅₄} clusters and one unique planar equilateral triangle-shaped {Ln₆(μ₃-OH)₆} cluster, which presents the highest nuclear iso-Ln-oxo cluster in PONb chemistry. In {Ln₆(μ₃-OH)₆} cluster, each pair of μ₃-OH groups link three Dy³⁺ ions to form a small approximate equilateral triangle-shaped {Dy₃(OH)₂} cluster. Furthermore, the three {Dy₃(OH)₂} clusters comprise a bigger approximate equilateral triangle-shaped {Dy₆(μ₃-OH)₆} cluster. The reported hexanuclear {Ln₆} cluster skeletons are mostly octahedral, however, such equilateral triangle-shaped skeleton of the hexanuclear Ln-oxo cluster is first observed. The 1-Dy exhibits good water vapor adsorption capacity and ferromagnetic properties.     

Nitrogen-doped mesoporous carbon nanospheres loaded with cobalt nanoparticles for oxygen reduction and Zn–air batteries

Mesoporous carbon supported with transition metals nanoparticles performs desired activities for oxygen reduction reaction (ORR) and clean energy conversion devices such as Zn–air batteries. In this work, we synthesized N-doped mesoporous carbon loaded with cobalt nanoparticles (CoMCN) through self-assembly method. There are sufficient mesopores on the carbon substrate which stem from the pore-forming agent. These mesopores can provide enough accessible active sites and profitable charge/mass transport for ORR. The high content of pyridinic and graphitic N is beneficial for promoting O₂ adsorption and reduction. The smaller value of I_D/I_G indicates the higher degree of graphitization of CoMCN, providing better electronic conductivity. The half-wave potential of CoMCN is 0.865 V in basic solution, which is 24 mV more positive than that of the commercial Pt/C (0.841 V). In addition, CoMCN performs excellent methanol tolerance and stability under both basic and acidic conditions. The Zn–air battery assembled with CoMCN performs the larger power density and open-circuit voltage than the commercial Pt/C-based battery, indicating the potential application in energy conversion systems. This work provides thoughtful ideas for fabricating transition metal nanoparticles based porous carbon for electrocatalysis and metal–air batteries.     

TBAI/H2O-cooperative electrocatalytic decarboxylation coupling-annulation of quinoxalin-2(1H)-ones with N-arylglycines

The first example of TBAI/H₂O cooperative electrocatalytic coupling-annulation of quinoxalin-2(1H)-ones with N-arylglycines was developed. A broad range of tetrahydroimidazo[1,5-a]quinoxalin-4(5H)-ones were obtained in good to excellent yields with exclusive chemoselectivities and excellent regioselectivities. The H-hydrogen bond served as a key factor for the electrocatalytic production of aminomethyl radical at lower oxidative potential.     

Organic Functional Free Radicals

This special column presents a snapshot of recent progress of various organic free radicals including the design, synthesis, characterization, properties and applications. We hope that it can help readers stimulate interest to explore novel persistent/stable organic free radicals and promote the research of organic radical based multi-functional materials.     

Highly Selective Transmembrane Transport of Exogenous Lithium Ions through Rationally Designed Supramolecular Channels

Lithium ions have been applied in the clinic in the treatment of psychiatric disorders. In this work, we report artificial supramolecular lithium channels composed of pore-containing small aromatic molecules. By adjusting the lumen size and coordination numbers, we found that one of the supramolecular channels developed shows unprecedented transmembrane transport of exogenous lithium ions with a Li⁺/Na⁺ selectivity ratio of 23.0, which is in the same level of that of natural Na⁺ channels. Furthermore, four coordination sites inside channels are found to be the basic requirement for ion transport function. Importantly, this artificial lithium channel displays very low transport of physiological Na⁺, K⁺, Mg²⁺, and Ca²⁺ ions. This highly selective Li⁺ channel may become an important tool for studying the physiological role of intracellular lithium ions, especially in the treatment of psychiatric disorders.