Inductive Effect on Single-Atom Sites.

Single-atom catalysts exhibit promising electrocatalytic activity, a trait that can be further enhanced through the introduction of heteroatom doping within the carbon skeleton. Nonetheless, the intricate relationship between the doping positions and activity remains incompletely elucidated. This contribution sheds light on an inductive effect of single-atom sites, showcasing that the activity of the oxygen reduction reaction (ORR) can be augmented by reducing the spatial gap between the doped heteroatom and the single-atom sites. Drawing inspiration from this inductive effect, we propose a synthesis strategy involving ligand modification aimed at precisely adjusting the distance between dopants and single-atom sites. This precise synthesis leads to optimized electrocatalytic activity for the ORR. The resultant electrocatalyst, characterized by Fe-N3P1 single-atom sites, demonstrates remarkable ORR activity, thus exhibiting great potential in zinc-air batteries and fuel cells.

Construction of Co4 Atomic Clusters to Enable Fe-N4 Motifs with Highly Active and Durable Oxygen Reduction Performance.

Fe-N-C catalysts with single-atom Fe-N4 configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe-N4 catalysts. The integration of Fe-N4 configurations with highly uniform Co4 ACs on the N-doped carbon substrate (Co4 @/Fe1 @NC) is realized through a "pre-constrained" strategy using Co4 molecular clusters and Fe(acac)3 implanted carbon precursors. The as-developed Co4 @/Fe1 @NC catalyst exhibits excellent ORR activity with a half-wave potential (E1/2 ) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm-2 in a H2 -O2 fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe-N4 that modified with Co4 ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.

Molecular Recognition Regulates Coordination Structure of Single-Atom Sites.

Coordination engineering for single-atom sites has drawn increasing attention, yet its chemical synthesis remains a tough issue, especially for tailorable coordination structures. Herein, a molecular recognition strategy is proposed to fabricate single-atom sites with regulable local coordination structures. Specifically, a heteroatom-containing ligand serves as the guest molecule to induce coordination interaction with the metal-containing host, precisely settling the heteroatoms into the local structure of single-atom sites. As a proof of concept, thiophene is selected as the guest molecule, and sulfur atoms are successfully introduced into the local coordination structure of iron single-atom sites. Ultrahigh oxygen reduction electrocatalytic activity is achieved with a half-wave potential of 0.93 V versus reversible hydrogen electrode. Furthermore, the strategy possesses excellent universality towards diversified types of single-atom sites. This work makes breakthroughs in the fabrication of single-atom sites and affords new opportunities in structural regulation at the atomic level.

Ultrathin Nanotube Structure for Mass-Efficient and Durable Oxygen Reduction Reaction Catalysts in PEM Fuel Cells.

It remains a challenge for platinum-based oxygen reduction reaction catalysts to simultaneously possess high mass activity and high durability in proton-exchange-membrane fuel cells. Herein, we report ultrathin holey nanotube (UHT)-structured Pt-M (M = Ni, Co) alloy catalysts that achieve unprecedented comprehensive performance. The nanotubes have ultrathin walls of 2-3 nm and construct self-supporting network-like catalyst layers with thicknesses of less than 1 µm, which have efficient mass transfer and 100% surface exposure, thus enabling high utilization of Pt atoms. Combined with the high intrinsic activity produced by the alloying effect, the catalysts achieve high mass activity. Moreover, the nanotube structure not only avoids the agglomeration problem of nanoparticles, but the low curvature of the tube wall also gives UHT a low surface energy (less than 1/3 of that of the same size nanoparticle), so UHT is more resistant to the Ostwald ripening and is stable. For the first time, the U.S. DOE mass activity target and dual durability targets for load and start-stop cycles are achieved on one catalyst. This study provides an effective structural strategy for the preparation of electrocatalysts with high atomic efficiency and excellent durability.

Activating the Electrocatalysis of MoS2 Basal Plane for Hydrogen Evolution via Atomic Defect Configurations.

Point defects of heteroatoms and vacancies can activate the inert basal plane of molybdenum sulfide (MoS2 ) to improve its performance on catalyzing the hydrogen evolution reaction (HER). However, the synergy between heteroatoms and vacancies is still unclear. Here, a chemical vapor deposition-assisted in situ vanadium (V) doping method is used to synthesize monolayer MoS2 with abundant and tunable vacancies and V-dopants in the lattice. Ten delicate defect configurations are prepared to provide a complex system for the relationship investigation between microstructure and catalytic performance. The combination of on-chip electrochemical tests and theoretical calculations indicates that the HER performance greatly depends on the type and amount of defect configurations. The optimal configuration is that three V atoms are aggregated and accompanied by abundant sulfur vacancies, in which, H atoms directly interact with Mo and V atoms to form the most stable metal-bridge structure. The on-chip measurements also confirm that the sample with high concentrations of this type of defect configuration exhibits the best catalytic performance, indicating the efficient synergy in the optimal configuration. The revealed effects of defect configurations are expected to inspire the design and regulation of high-efficiency 2D catalysts.

Unveiling the high-activity origin of single-atom iron catalysts for oxygen reduction reaction.

It is still a grand challenge to develop a highly efficient nonprecious-metal electrocatalyst to replace the Pt-based catalysts for oxygen reduction reaction (ORR). Here, we propose a surfactant-assisted method to synthesize single-atom iron catalysts (SA-Fe/NG). The half-wave potential of SA-Fe/NG is only 30 mV less than 20% Pt/C in acidic medium, while it is 30 mV superior to 20% Pt/C in alkaline medium. Moreover, SA-Fe/NG shows extremely high stability with only 12 mV and 15 mV negative shifts after 5,000 cycles in acidic and alkaline media, respectively. Impressively, the SA-Fe/NG-based acidic proton exchange membrane fuel cell (PEMFC) exhibits a high power density of 823 mW cm-2 Combining experimental results and density-functional theory (DFT) calculations, we further reveal that the origin of high-ORR activity of SA-Fe/NG is from the Fe-pyrrolic-N species, because such molecular incorporation is the key, leading to the active site increase in an order of magnitude which successfully clarifies the bottleneck puzzle of why a small amount of iron in the SA-Fe catalysts can exhibit extremely superior ORR activity.

Self-Adaptive Electrode with SWCNT Bundles as Elastic Substrate for High-Rate and Long-Cycle-Life Lithium/Sodium Ion Batteries.

Massive volume change of active materials in lithium/sodium ion batteries (LIB/SIB) causes severe structural collapse of electrodes and fast capacity decay of batteries. Here, a coaxial composite of single-wall carbon nanotube bundle (SWCNTB/SnO2 ) nanoparticles (NPs)/N-doped carbon shell (SWCNTB@SnO2 @C) is constructed, where SWCNTBs with exceptional elasticity are explored as a self-adaptive substrate to supply a highly resilient conductive network. Within the confinement of hard carbon shells, SWCNTB can produce radially elastic deformation to accommodate the volume change of SnO2 during Li+ /Na+ insertion/extraction. This overcomes the problem of strain fracturing of the outer carbon shell, as well as maintains close electrical contact between SnO2 and the conductive network. The LIB/SIB with the self-adaptive SWCNTB@SnO2 @C electrode presents a series of superior battery performances, for example, a high specific capacity of 608 mAh g-1 at 10 A g-1 and 600 cycles in LIB without capacity decay.

Highly efficient nonprecious metal catalyst prepared with metal-organic framework in a continuous carbon nanofibrous network.

Fuel cell vehicles, the only all-electric technology with a demonstrated >300 miles per fill travel range, use Pt as the electrode catalyst. The high price of Pt creates a major cost barrier for large-scale implementation of polymer electrolyte membrane fuel cells. Nonprecious metal catalysts (NPMCs) represent attractive low-cost alternatives. However, a significantly lower turnover frequency at the individual catalytic site renders the traditional carbon-supported NPMCs inadequate in reaching the desired performance afforded by Pt. Unconventional catalyst design aiming at maximizing the active site density at much improved mass and charge transports is essential for the next-generation NPMC. We report here a method of preparing highly efficient, nanofibrous NPMC for cathodic oxygen reduction reaction by electrospinning a polymer solution containing ferrous organometallics and zeolitic imidazolate framework followed by thermal activation. The catalyst offers a carbon nanonetwork architecture made of microporous nanofibers decorated by uniformly distributed high-density active sites. In a single-cell test, the membrane electrode containing such a catalyst delivered unprecedented volumetric activities of 3.3 A ⋅ cm(-3) at 0.9 V or 450 A ⋅ cm(-3) extrapolated at 0.8 V, representing the highest reported value in the literature. Improved fuel cell durability was also observed.

The Solid-Phase Synthesis of an Fe-N-C Electrocatalyst for High-Power Proton-Exchange Membrane Fuel Cells.

The environmentally friendly synthesis of highly active Fe-N-C electrocatalysts for proton-exchange membrane fuel cells (PEMFCs) is desirable but remains challenging. A simple and scalable method is presented to fabricate FeII -doped ZIF-8, which can be further pyrolyzed into Fe-N-C with 3 wt % of Fe exclusively in Fe-N4 active moieties. Significantly, this Fe-N-C derived acidic PEMFC exhibits an unprecedented current density of 1.65 A cm-2 at 0.6 V and the highest power density of 1.14 W cm-2 compared with previously reported NPMCs. The excellent PEMFC performance can be attributed to the densely and atomically dispersed Fe-N4 active moieties on the small and uniform catalyst nanoparticles.

Revealing Distance-Dependent Synergy between MnCo2O4 and Co-N-C in Boosting the Oxygen Reduction Reaction.

Synergistic effects have been applied to a variety of hybrid electrocatalysts to improve their activity and selectivity. Understanding the synergistic mechanism is crucial for the rational design of these types of catalysts. Here, we synthesize a MnCo2O4/Co-N-C hybrid electrocatalyst for the oxygen reduction reaction (ORR) and systematically investigate the synergy between MnCo2O4 nanoparticles and Co-N-C support. Theoretical simulations reveal that the synergy is closely related to the distance between active sites. For a pair of remote active sites, the ORR proceeds through the known 2e- + 2e- relay catalysis while the direct 4e- ORR occurs on a pair of adjacent active sites. Therefore, the formation of the undesired byproduct (H2O2) is inhibited at the interface region between MnCo2O4 and Co-N-C. This synergistic effect is further verified on an anion-exchange membrane fuel cell. The findings deepen the understanding of synergistic catalysis and will provide guidance for the rational design of hybrid electrocatalysts.

Mg-MOF-74 Derived Defective Framework for Hydrogen Storage at Above-Ambient Temperature Assisted by Pt Catalyst.

Metal-organic frameworks (MOFs) are promising candidates for room-temperature hydrogen storage materials after modification, thanks to their ability to chemisorb hydrogen. However, the hydrogen adsorption strength of these modified MOFs remains insufficient to meet the capacity and safety requirements of hydrogen storage systems. To address this challenge, a highly defective framework material known as de-MgMOF is prepared by gently annealing Mg-MOF-74. This material retains some of the crystal properties of the original Mg-MOF-74 and exhibits exceptional hydrogen storage capacity at above-ambient temperatures. The MgO5 knots around linker vacancies in de-MgMOF can adsorb a significant amount of dissociated and nondissociated hydrogen, with adsorption enthalpies ranging from -22.7 to -43.6 kJ mol-1, indicating a strong chemisorption interaction. By leveraging a spillover catalyst of Pt, the material achieves a reversible hydrogen storage capacity of 2.55 wt.% at 160 °C and 81 bar. Additionally, this material offers rapid hydrogen uptake/release, stable cycling, and convenient storage capabilities. A comprehensive techno-economic analysis demonstrates that this material outperforms many other hydrogen storage materials at the system level for on-board applications.

Honeycomb ZrCo Intermetallic for High Performance Hydrogen and Hydrogen Isotope Storage.

Hydrogen isotope storage materials are of great significance for controlled nuclear fusion, which is promising to provide unlimited clean and dense energy. Conventional storage materials of micrometer-sized polycrystalline ZrCo alloys prepared by the smelting method suffer from slow kinetics, pulverization, disproportionation, and poor cycling stability. Here, we synthesize a honeycomb-structured ZrCo composed of highly crystalline submicrometer ZrCo units using electrospray deposition and magnesiothermic reduction. Compared with conventional ones, honeycomb ZrCo does not require activation and exhibits more than 1 order of magnitude increase in kinetic property. Owing to low defects and low stress, the anti-disproportionation ability and cycling stability of honeycomb ZrCo are also obviously higher than those of conventional ZrCo. Moreover, the interfacial stress (due to hydrogenation/dehydrogenation) as a function of particle radius is established, quantitatively elucidating that small-sized ZrCo reduces stress and pulverization. This study points out a direction for the structural design of ZrCo alloy with high-performance hydrogen isotope storage.

Single-crystal ZrCo nanoparticle for advanced hydrogen and H-isotope storage.

Hydrogen-isotope storage materials are essential for the controlled nuclear fusion. However, the currently used smelting-ZrCo alloy suffers from rapid degradation of performance due to severe disproportionation. Here, we reveal a defect-derived disproportionation mechanism and report a nano-single-crystal strategy to solve ZrCo's problems. Single-crystal nano-ZrCo is synthesized by a wet-chemistry method and exhibits excellent comprehensive hydrogen-isotope storage performances, including ultrafast uptake/release kinetics, high anti-disproportionation ability, and stable cycling, far superior to conventional smelting-ZrCo. Especially, a further incorporation of Ti into nano-ZrCo can almost suppress the disproportionation reaction. Moreover, a mathematical relationship between dehydrogenation temperature and ZrCo particle size is established. Additionally, a microwave method capable of nondestructively detecting the hydrogen storage state of ZrCo is developed. The proposed disproportionation mechanism and anti-disproportionation strategy will be instructive for other materials with similar problems.

Fe/N/C composite in Li-O2 battery: studies of catalytic structure and activity toward oxygen evolution reaction.

Atomically dispersed Fe/N/C composite was synthesized and its role in controlling the oxygen evolution reaction during Li-O(2) battery charging was studied by use of a tetra(ethylene glycol) dimethyl ether-based electrolyte. Li-O(2) cells using Fe/N/C as the cathode catalyst showed lower overpotentials than α-MnO(2)/carbon catalyst and carbon-only material. Gases evolved during the charge step contained only oxygen for Fe/N/C cathode catalyst, whereas CO(2) was also detected in the case of α-MnO(2)/C or carbon-only material; this CO(2) was presumably generated from electrolyte decomposition. Our results reiterate the catalytic effect in reducing overpotentials, which not only enhances battery efficiency but also improves its lifespan by reducing or eliminating electrolyte decomposition. The structure of the Fe/N/C catalyst was characterized by transmission electron microscopy, scanning transmission electron microscopy, inductively coupled plasma optical emission spectroscopy, and X-ray absorption spectroscopy. Iron was found to be uniformly distributed within the carbon matrix, and on average, Fe was coordinated by 3.3 ± 0.6 and 2.2 ± 0.3 low Z elements (C/N/O) at bond distances of ~1.92 and ~2.09 Å, respectively.

Environmentally Tough and Stretchable MXene Organohydrogel with Exceptionally Enhanced Electromagnetic Interference Shielding Performances.

Conductive hydrogels have potential applications in shielding electromagnetic (EM) radiation interference in deformable and wearable electronic devices, but usually suffer from poor environmental stability and stretching-induced shielding performance degradation. Although organohydrogels can improve the environmental stability of materials, their development is at the expense of reducing electrical conductivity and thus weakening EM interference shielding ability. Here, a MXene organohydrogel is prepared which is composed of MXene network for electron conduction, binary solvent channels for ion conduction, and abundant solvent-polymer-MXene interfaces for EM wave scattering. This organohydrogel possesses excellent anti-drying ability, low-temperature tolerance, stretchability, shape adaptability, adhesion and rapid self-healing ability. Two effective strategies have been proposed to solve the problems of current organohydrogel shielding materials. By reasonably controlling the MXene content and the glycerol-water ratio in the gel, MXene organohydrogel can exhibit exceptionally enhanced EM interference shielding performances compared to MXene hydrogel due to the increased physical cross-linking density of the gel. Moreover, MXene organohydrogel shows attractive stretching-enhanced interference effectiveness, caused by the connection and parallel arrangement of MXene nanosheets. This well-designed MXene organohydrogel has potential applications in shielding EM interference in deformable and wearable electronic devices.

Iron atom-cluster interactions increase activity and improve durability in Fe-N-C fuel cells.

Simultaneously increasing the activity and stability of the single-atom active sites of M-N-C catalysts is critical but remains a great challenge. Here, we report an Fe-N-C catalyst with nitrogen-coordinated iron clusters and closely surrounding Fe-N4 active sites for oxygen reduction reaction in acidic fuel cells. A strong electronic interaction is built between iron clusters and satellite Fe-N4 due to unblocked electron transfer pathways and very short interacting distances. The iron clusters optimize the adsorption strength of oxygen reduction intermediates on Fe-N4 and also shorten the bond amplitude of Fe-N4 with incoherent vibrations. As a result, both the activity and stability of Fe-N4 sites are increased by about 60% in terms of turnover frequency and demetalation resistance. This work shows the great potential of strong electronic interactions between multiphase metal species for improvements of single-atom catalysts.

MXene-Reinforced Liquid Metal/Polymer Fibers via Interface Engineering for Wearable Multifunctional Textiles.

Stretchable conductive fibers are an important component of wearable electronic textiles, but often suffer from a decrease in conductivity upon stretching. The use of liquid metal (LM) droplets as conductive fillers in elastic fibers is a promising solution. However, there is an urgent need to develop effective strategies to achieve high adhesion of LM droplets to substrates and establish efficient electron transport paths between droplets. Here, we use large-sized MXene two-dimensional conductive materials to modify magnetic LM droplets and prepare MXene/magnetic LM/poly(styrene-butadiene-styrene) composite fibers (MLMS fibers). The MXene sheets decorated on the surface of magnetic LM droplets not only enhance the droplet adhesion to substrate but also bridge adjacent droplets to establish efficient conductive paths. MLMS fibers show several-fold improvements in tensile strength and elongation and a 30-fold increase in conductivity compared with pure LM-filled fibers. These conductive fibers can be easily woven into multifunctional textiles, which exhibit strong electromagnetic interference shielding and stable Joule heating performances even under large tensile deformation. In addition, other advantages of MLMS textiles, such as high gas/liquid permeability, strong chemical resistance (acid and alkaline conditions), high/low-temperature tolerance (-40-150 °C) and water washability, make them particularly suitable for wearable applications.

Hydrogen Passivation of M-N-C (M = Fe, Co) Catalysts for Storage Stability and ORR Activity Improvements.

M-N-C (M = Fe, Co) are highly active nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) and other applications. Although their operation stability has been extensively studied in proton-exchange-membrane fuel cells, the storage stability that determines the performance maintenance before use has not yet been understood. Here, it is found that long-term exposure of M-N-C catalysts in air would cause surface oxidation and hydroxylation, resulting in significant decrease of ORR activity and fuel-cell performances. Hydrogen passivation is demonstrated to be an effective strategy to protect the atomic M-N4 active sites and improve the storage stability of the catalysts. In addition, the hydrogen-termination can also reduce the ORR energy barrier and increase the utilization of active sites, leading to the improvements of fuel-cell activity and power density. Notably, these findings help to understand the storage-associated degradation and protection of M-N-C catalysts.

Hydrogen storage in incompletely etched multilayer Ti2CTx at room temperature.

Hydrogen storage materials are the key to hydrogen energy utilization. However, current materials can hardly meet the storage capacity and/or operability requirements of practical applications. Here we report an advancement in hydrogen storage performance and related mechanism based on a hydrofluoric acid incompletely etched MXene, namely, a multilayered Ti2CTx (T is a functional group) stack that shows an unprecedented hydrogen uptake of 8.8 wt% at room temperature and 60 bar H2. Even under completely ambient conditions (25 °C, 1 bar air), Ti2CTx is still able to retain ~4 wt% hydrogen. The hydrogen storage is stable and reversible in the material, and the hydrogen release is controllable by pressure and temperature below 95 °C. The storage mechanism is deduced to be a nanopump-effect-assisted weak chemisorption in the sub-nanoscale interlayer space of the material. Such a storage approach provides a promising strategy for designing practical hydrogen storage materials.

Carbon Fibers Embedded with Aligned Magnetic Particles for Efficient Electromagnetic Energy Absorption and Conversion.

Harvesting electromagnetic (EM) energy from the environment and converting it into useful micropower is a new and ideal way to eliminate EM radiation and while providing power for microelectronic devices. The key material of this technology is broadband, ultralight, and ultrathin EM-wave-absorbing materials, whose preparation remains challenging. Herein, a high magnetic field (HMF) strategy is proposed to prepare a biomass-derived CoFe/carbon fiber (CoFe/CF) composite, in which CoFe magnetic particles are aligned in CFs, creating magnetic coupling and fast electron transmission channels. The graphitization degree of CFs is improved via the "migration catalysis" of CoFe particles under HMF. The HMF-derived CoFe/CF shows a largely broadened EM wave absorption bandwidth under ultralight and ultrathin conditions (1.5 mm). Its absorption bandwidth increases 5-10 times compared with conventional CoFe/CF that has randomly distributed CoFe particles and surpasses the reported analogues. A device model for EM energy absorption and reuse is designed based on the HMF-derived CoFe/CF membrane, which exhibits a 300% higher capability than conventional CoFe/CF membrane in converting EM energy to thermal energy. This work offers a new strategy for the design and fabrication of broadband, ultrathin, and ultralight EM wave absorption materials and demonstrates a potential conversion approach of the waste EM energy.