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Electrochemical oxygen reduction could proceed via either 4e−-pathway toward maximum chemical-to-electric energy conversion or 2e−-pathway toward onsite H2O2 production. Bulk Pt catalysts are known as the best monometallic materials catalyzing O2-to-H2O conversion, however, controversies on the reduction product selectivity are noted for atomic dispersed Pt catalysts. Here, we prepare a series of carbon supported Pt single atom catalyst with varied neighboring dopants and Pt site densities to investigate the local coordination environment effect on branching oxygen reduction pathway. Manipulation of 2e− or 4e− reduction pathways is demonstrated through modification of the Pt coordination environment from Pt-C to Pt-N-C and Pt-S-C, giving rise to a controlled H2O2 selectivity from 23.3% to 81.4% and a turnover frequency ratio of H2O2/H2O from 0.30 to 2.67 at 0.4 V versus reversible hydrogen electrode. Energetic analysis suggests both 2e− and 4e− pathways share a common intermediate of *OOH, Pt-C motif favors its dissociative reduction while Pt-S and Pt-N motifs prefer its direct protonation into H2O2. By taking the Pt-N-C catalyst as a stereotype, we further demonstrate that the maximum H2O2 selectivity can be manipulated from 70 to 20% with increasing Pt site density, providing hints for regulating the stepwise oxygen reduction in different application scenarios. Controlling O2 reduction pathways can help optimize catalytic activity and product selectivity. Here the authors report facile manipulation of 2e‒ /4e‒ pathways on Pt-coordinated motifs by varying the Pt site density or the coordination environment. [ABSTRACT FROM AUTHOR]

Electrochemical nitrogen reduction reaction (NRR) is intensively investigated by researchers for its potential to be the next‐generation technology to produce ammonia. Many attempts have been made to explore the possibility of electrochemical ammonia production catalyzed by noble metals. However, the produced ammonia in most reported cases is in ppm level or even lower, which is susceptible to potential contaminants in experiments, leading to fluctuating or even contradictory results. Herein, a rigorous procedure was adopted to systematically evaluated the performance of commercial noble metal nanocatalysts toward NRR. No discernible amount of ammonia was detected in either acidic or alkaline solutions. Further, nitrogen‐containing contaminants in catalysts that might cause false positive results were detected and characterized. An effective way to remove pre‐existing pollutants by consecutive cyclic voltammetry scan was proposed, helping to obtain reliable and reproducible results. [ABSTRACT FROM AUTHOR]

The simultaneous construction of two different chiralities via a simple operation poses considerable challenge. Herein a cationic gold-catalyzed asymmetric hydroarylation of ortho-alkynylaryl ferrocenes derivatives is developed, which enable the simultaneous construction of axial and planar chirality. The here identified TY-Phos derived gold complex is responsible for the high yield, good diastereoselectivity (>20:1 dr), high enantioselectivities (up to 99% ee) and mild conditions. The catalyst system also shows potential application in the synthesis of chiral biaryl compounds. The cause of high enantioselectivity of this hydroarylation is investigated with density functional theory caculation. The simultaneous construction of two different types of chiralities is challenging. Here, the authors report a cationic gold-catalyzed asymmetric hydroarylation of ortho-alkynylaryl ferrocene derivatives, which enabled the simultaneous construction of axial and planar chirality. [ABSTRACT FROM AUTHOR]

Ordered mesoporous SBA‐15 supported chiral phosphine gold (I) complexes were successfully developed. The gold(I) complexes were covalently immobilized on mercaptopropyl modified SiO2 surface through radical reaction. In this way, the chiral carbon and chiral sulfur of the homogenous catalyst were perfectly remained in situ. And thus, the prepared catalysts demonstrated highly effective asymmetric cycloaddition activity, which was likely because the chiral induction effect of the Au complex was hardly influenced by this immobilization method. Moreover, they could be easily recovered by simple filtration and directly used for the subsequent catalytic runs. The prepared catalyst was highly stable and efficient without obvious activity loss after eight cycles. [ABSTRACT FROM AUTHOR]

Chiral dihydrobenzofuran units are frequently present in molecules with significant biological and pharmaceutical activities. Herein, we present the first enantioselective formal [4+1] annulation of Morita‐Baylis‐Hillman carbonates with o‐quinone methides (o‐QMs) catalyzed by a newly designed chiral phosphine catalyst. Under the mild and eco‐friendly conditions, a wide range of polysubstituted dihydrobenzofurans were obtained in good yields with excellent enantioselectivities. [ABSTRACT FROM AUTHOR]

Abstract: The first example of highly enantioselective intramolecular hydroarylation of allyl aryl ethers was realized by palladium‐catalyzed reductive heck reactions utilizing a new chiral sulfinamide phosphine ligand (N‐Me‐XuPhos). N‐Me‐XuPhos can be easily prepared on gram scale from readily available starting materials in a one‐pot synthesis approach. A series of optically active 2,3‐dihydrobenzofurans bearing a quaternary stereocenter were obtained in good yields and with excellent enantioselectivities. The practicality of this reaction was validated in the straightforward synthesis of CB2 receptor agonists. Moreover, deuterium was efficiently incorporated into the products. [ABSTRACT FROM AUTHOR]

Abstract: A highly efficient phosphine‐catalyzed [4+2] annulation of electron‐deficient diene and alkyl vinyl ketone was developed for the first time, which provides an easy access to functionalized cyclohexenes. This method has the advantages of mild reaction conditions, widely functional group tolerance, high yields and transition metal free. [ABSTRACT FROM AUTHOR]

An efficient ferrocene-derived bifunctional phosphine-catalyzed enantioselective oxa-[4+2] cycloaddition of α-substituted allenones with a broad range of enones is investigated for the preparation of stereodefined dihydropyrans in good to excellent yields (up to 99 %) and excellent enantioselectivity (up to 99 % ee). Furthermore, a series of valuable chiral polyheterocyclic frameworks can be efficiently achieved in good yields with excellent enantioselectivities. [ABSTRACT FROM AUTHOR]

The development of an efficient method for the site-selective substitution of unprotected phenols has long been considered as an attractive but challenging task. Herein, we describe a highly chemo- and ortho-selective substitution reaction of phenols with α-aryl α-diazoacetates with commercially available (C6F5)3B as the catalyst. This reaction proceeds under simple and mild conditions with high efficiency, it features a wide substrate scope and can be easily scaled up. [ABSTRACT FROM AUTHOR]

A novel phosphine-catalyzed, highly enantioselective umpolung addition of trifluoromethyl ketimines to Morita-Baylis-Hillman carbonates was developed and it provides facile access to optically active trifluoromethyl amines with a chiral tertiary stereocenter under mild reaction conditions. The salient features of this reaction include general substrate scope, mild reaction conditions, good yields, high enantioselectivity, ease of scale-up to gram scale, and further transformations of the products. [ABSTRACT FROM AUTHOR]

A novel type of highly efficient chiral sulfinamide bisphosphine catalysts (Wei-Phos) were developed. These could be easily prepared from commercially available starting materials. Wei-Phos has shown good performance in the very challenging intermolecular cross-Rauhut-Currier reactions of vinyl ketones and 3-acyl acrylates or 2-ene-1,4-diones, leading to the R-C products in high yields with up to 99%ee under 2.5- 5 mol% catalyst loading. The highly regio- and enantioselective cross-Rauhut-Currier reactions of 2-ene-1,4-diones and vinyl ketone have yet reported so far. [ABSTRACT FROM AUTHOR]

A novel class of chiral sulfinamide phosphine catalysts (Xiao-Phos) are reported, which can be easily prepared from inexpensive commercially available starting materials. The Xiao-Phos catalysts showed good performance in enantioselective intramolecular Rauhut-Currier reactions, generating α-methylene-γ-butyrolactones in high yields with up to 99 % ee under mild conditions. Moreover, kinetic resolution and parallel kinetic resolution were also observed with the use of two different substituted racemic precursors. [ABSTRACT FROM AUTHOR]

Iron ORR: An ordered, mesoporous, Fe‐porphyrin‐like material was created through the nanocasting and pyrolysis of traditional Fe–N4 porphyrins. The resulting nonprecious metal electrocatalyst was used for the oxygen reduction reaction in both alkaline and acidic media. [ABSTRACT FROM AUTHOR]

A novel, mild, robust catalyst Y(OTf)3for C–C bond heterolysis of N-tosyl aziridines was developed and the resulting metallo-azomethine ylides may readily undergo [3+2] dipolar cycloaddition with an electron-rich olefin viaa stepwise reaction pathway with high regio- and diastereoselectivity leading to substituted pyrrolidines. [ABSTRACT FROM AUTHOR]

Selective synthesis of ring-fused tetrahydroquinolines and tetrahydroazepines from the same starting materials was achieved by subtle use of the oxophilic Sc(OTf)3or carbophilic IPrAuOTf as the catalyst. [ABSTRACT FROM AUTHOR]

This article focuses on a study related to mixed-metal Pt monolayer electrocatalysts for enhanced oxygen reduction kinetics. Electrocatalytic oxygen reduction reaction (ORR) has been the focus of considerable attention because of its slow kinetics and the need for better electrocatalysts with decreased Pt content for fuel cell cathodes. New electrocatalysts have a very high activity compared to that of pure Pt ORR catalysts and a considerably lower Pt content. The kinetic current density is more than times larger than that for the pure Pt monolayer, whose activity is already considerably higher than that of pure Pt.

• Nanobubble transport and bubble accumulation behaviors are present in CL. • Bubble accumulation behavior is influenced by thickness, porosity and wettability of CL. • Water transport at some of bubble covered active sites may be carried by the ionomer network after bubble accumulation reaches equilibrium. The presence of nano-scale oxygen bubble transport and accumulation within the anodic catalyst layer (CL) in proton exchange membrane water electrolyzer (PEMWE) remains controversial. In this study, we obtain several crucial evidence to resolve this debate using a Pt-coated mesh porous transport layer (PTL) as an oxygen sensor and with the help of multiple visualization methods. The evidence includes the oxygen reduction current detected from the anodic CL after electrolysis was stopped, continued bubble generation and growth on the CL surface when no oxygen evolution reaction happening in CL, and the gas outlet holes observed from the CL surface. The evidence supports that nano-scale oxygen bubble transport and accumulation indeed happen within anodic CL. The influence of characteristics of the CL on the reversible performance change induced by bubble accumulation behavior further reveals the possible mechanism of bubble transport in CL. The bubbles are generated at the entire CL, and its transport behavior is shaped by the porous structure of CL. Increasing the bubble transport distance, tortuosity, and pore hydrophobicity within the CL all lead to a significant increase in the rate and degree of bubble accumulation within the CL. [Display omitted] [ABSTRACT FROM AUTHOR]

The equivalent weight (EW) and side-chain length of perfluorosulfonic acid (PFSA) ionomers have been found to significantly influence the performance of different proton exchange membrane (PEM)-based electrochemical devices. However, few reports have investigated their influence on PEM water electrolyzers (PEMWE). In this study, we investigate the influence of EW and side-chain length on PEMWE's membrane electrode assemblies (MEAs) which are fabricated by four different PFSA ionomers. It is found that the change in PFSA ionomer EWs barely influences the microstructure of the anode catalyst layer (ACL) but affects its surface wettability and MEA performance. Different from that in PEM fuel cells, MEA in PEMWEs prepared by PFSA ionomers with higher EWs shows better cell performance. The voltage difference comes from the change in the mass transport overpotential which is up to 113 mV with the comparison between PFSA ionomers with the highest EW and the lowest EW, which attributes to the changed wettability and water-oxygen transport of the ACLs. The influence of PFSA ionomer side-chain lengths on cell performance is not observed. This work provides insight into the effect of PFSA ionomers and offers information on how to design high-performance PFSA ionomers for PEMWE applications. [Display omitted] • PFSA ionomers with higher EWs present better MEA performance in PEMWE. • Effect of PFSA ionomer EW on PEMWE is opposite to that on PEMFC. • Change in side-chain length of PFSA ionomers has no influence on PEMWE performance. • Future development of higher-performance PFSA ionomers is indicated. [ABSTRACT FROM AUTHOR]

• A new PEMWE performance improvement method based on bubble evolution enhancement. • Detailed investigation of the surface morphology and wettability change of catalyst coated membrane fabricated by imprinting method before and after assembly. • Significant mass transport overpotential reduction up to 59% at 5 A cm-2 in PEMWE cell. The demand for cost reduction in proton exchange membrane water electrolyzer (PEMWE) is driving it to increase the operating current density and higher current densities require better bubble management. In this work, we demonstrate that introducing patterned grooves to the catalyst layer surface (CL) is able to enhance the bubble evolution and improve electrolyzer performance. The patterned grooves microstructure on CL provides additional bubble nucleation sites and faster bubble evolution. Smaller average bubble size and higher bubble detachment frequency during water electrolysis are observed through a bubble evolution visualization system and further confirmed by a frequency analysis method. In addition, patterned grooves also improve the hydrophilicity and aerophobicity of CL surfaces. In the PEM water electrolyzer cell, the patterned grooves on CL boost the cell performance and reach high current densities up to 5.0 A cm-2. With better bubble management, patterned grooves significantly reduce the mass transport overpotential at each current density. At 5.0 A cm-2, the mass transport overpotential is reduced by 59%. This work provides a new idea of bubble management in PEMWE and is highly informative for designing CLs for PEMWE and other gas-evolving electrochemical energy conversion applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Although the properties of perfluorosulfonic acid (PFSA) membranes (10–100 s μm thick) in proton exchange membrane fuel cells (PEMFCs) have been well studied for decades, the limited understanding of nano-sized PFSA ionomer within catalyst layer hampers the improvement of ultra-low platinum loading PEMFC. Recently, the vital role of PFSA ionomer in the local transfer of proton, oxygen, and water in fuel cell catalyst layer has been recognized, thus its properties have attracted more attention. In this review, a comprehensive comparison between PFSA bulk membrane and ultrathin PFSA ionomer film is summarized as morphology, physical properties (e.g., water uptake, surface wettability, diffusivity), and mechanical properties (e.g., crystallization, antiplasticization, glass transition point). Afterwards, the characteristics of ionomer in different dispersion media and the microstructure of PFSA ionomer in the formed catalyst layers are illuminated. The proton conduction and mass transport (i.e., oxygen and water) behaviors in the confined PFSA ionomer are focused and highlighted to reveal the ionomer's influence on fuel cell performance. Extrapolating from the insight into structural and transport properties of PFSA ionomer, we are optimistic that the performance of ultralow Pt loading PEMFCs could be greatly improved via modifying ionomer chemical structure and engineering ionomer/catalyst interaction. [Display omitted] • Different properties between PFSA bulk membrane and PFSA ionomer thin films. • Microstructure and mass transport behaviors within nanoscale PFSA ionomer films. • Future strategies for improving the performance of the ultralow Pt loading PEMFCs. [ABSTRACT FROM AUTHOR]

A transition-metal free borylation of aryl iodides is readily achieved with diboranes (II) and (V) in the presence of cesium carbonate to give arylboronic ethers in moderate yields. [ABSTRACT FROM AUTHOR]

Non-precious metal catalysts (NPMCs) are regarded as the ultimate alternative to high-cost Pt-based catalysts for oxygen reduction reaction in proton exchange membrane fuel cells (PEMFCs). Indeed, great progresses have been made in the rotating disk electrode (RDE) performance of NPMCs, while their corresponding fuel cell performance remains far from satisfying real demands due to the fact that the properties of NPMC-based electrode need to be clarified and optimized. In this work, a series of properties including the oxygen reduction activities, catalyst layer proton conduction resistance and oxygen transport resistance are investigated on membrane electrode assemblies (MEAs) fabricated from home-made Fe–N–C catalysts. It is found that both the oxygen reduction activities and catalyst layer proton conduction increase with the catalyst loading. Unexpectedly, the total oxygen transport resistance is quite large for the MEA with a lower catalyst loading, and the resistance first decreases and then enlarges with the increase in catalyst loading, resulting from a comprehensive effect between local transport and bulk transport. This provides a novel meaningful guide that compared to using Pt-based MEA technique directly, special and deliberate designs are needed for MEAs based on NPMCs to balance the cathode catalyst layer (CCL) activity, proton resistance and oxygen transport resistance simultaneously. • ORR activity in Fe–N–C MEA increase with the catalyst loading. • Proton conductivity in Fe–N–C MEA increase with the catalyst loading. • Oxygen transport in CCL is comprehensive in terms of local and bulk transport. • Balance is needed between ORR activity, proton conductivity and O 2 transport. [ABSTRACT FROM AUTHOR]

Into the scaling relations of TM-N 3 (N 4)/graphene toward ENRR, candidates are proposed and limitations are clarified. [Display omitted] • ENRR potential evaluated for 58 types of SACs on nitrogen-doped graphene. • Scaling relations exist in protonation steps related to N 2 H and NH 2. • 2D-volcanic plot established between *N 2 to *N 2 H and *NH 2 to *NH 3. • 6 candidates identified where V-N 4 /graphene shows best ENRR activity and stability. In this study, we performed the first-principles density functional theory calculations to systematically predict the activity of fifty-eight types of different single atom electrocatalysts on nitrogen doped graphene for electrochemical ammonia synthesis. Two strong linear relations were revealed among the reaction energies on these single atom structures, including positive correlation between the adsorption energy of N 2 H and the free energy change for *N 2 transition to *N 2 H, as well as negative correlation between the adsorption energy of NH 2 and the free energy change for *NH 2 transition to *NH 3. Using the developed scaling relations and some additional factors including nitrogen adsorption, hydrogen evolution reaction, water adsorption, ammonia desorption and structure stability, we have computationally identified six candidate structures as promising active sites for ammonia synthesis. Especially, V-N 4 /graphene was predicted to exhibit the best stability, the highest activity with the limiting potential of −0.71 V, and suppression of hydrogen evolution reaction. [ABSTRACT FROM AUTHOR]

The durability of PEMFCs is greatly influenced by the non-uniform degradation of catalyst particles across the thickness of cathode catalyst layer (CCL). Gradient CCL is one of the effective structures to improve both cell performance and durability. Here we employ mathematical modeling to evaluate several gradient CCL structures with Pt/C catalysts in terms of the evolution of electrochemical surface area (ECSA) and Pt mass during cycling. Results based on the two-layer models considering particle size gradient only show that larger Pt particles near the CCL/membrane interface can remarkably mitigate the loss of ECSA and Pt mass, but accelerate the Pt dissolution near the gas diffusion layer (GDL). An increase in the number of layers, corresponding to a more smooth decrease of particle size from the CCL/membrane interface to GDL, can effectively retain high ECSA after cycling. The CCL with an optimum performance is achieved by manipulating the particle size gradient and Pt loading gradient simultaneously, which gains more uniform distribution of Pt active surface for improving the end-of-test performance and durability. This work provides a strategy to achieve highly durable PEMFCs by combining the gradient of particle size and Pt loading in the CCL structure. Image 1 • The structure of cathode catalyst layer able to be optimized by mathematical method. • Larger Pt particles near the membrane greatly mitigate the degradation of cathode. • Three-layers CCL performs better than two-layers one for retaining high ECSA. • Gradient design of particle size and Pt loading provides high durability for PEMFCs. [ABSTRACT FROM AUTHOR]

Understanding the degradation process of Pt-based catalysts is a crucial step to improve the durability of PEMFCs. A mathematical model is established to study the electrochemical surface area (ECA) loss and structural/compositional evolutions of core-shell Pt 3 Co catalysts along the cathode under a certain circumstance. Three major processes are included in this model: Pt dissolution and re-precipitation on the Pt shell, Co leaching from the bulk and deposition of Pt ions in the membrane due to crossover hydrogen. The results show that the significant ECA loss next to the membrane is mainly attributed to the severe particle corrosion rather than variation of the particle size distribution. The Pt shell thickness and Pt/Co atomic ratio decrease from gas diffusion layer (GDL)/CCL interface to the cathode catalysts layer (CCL)/membrane interface, while the Co mass loss shows an inverse trend. This work demonstrates that mathematical modeling is effective to predict the structural and compositional evolutions of the catalyst particles across the CCL, and therefore is useful to evaluate the complete performance degradation of Pt 3 M catalysts for PEMFCs in the future. Image 10784 • The present model able to predict the degradation of core-shell Pt 3 Co in PEMFCs. • More severe particles corrosion plays a major role in ECA loss near the membrane. • Structural/compositional changes of Pt 3 Co catalysts are firstly captured by modeling. • The model can be revised for evaluating the degradation of other Pt-based catalysts. [ABSTRACT FROM AUTHOR]

A novel class of chiral phosphine catalysts is introduced and successfully employed in the enantioselective intramolecular Rauhut-Currier reaction. [ABSTRACT FROM AUTHOR]