Highly Crystalline Iridium-Nickel Nanocages with Subnanopores for Acidic Bifunctional Water Splitting Electrolysis.

Developing efficient bifunctional materials is highly desirable for overall proton membrane water splitting. However, the design of iridium materials with high overall acidic water splitting activity and durability, as well as an in-depth understanding of the catalytic mechanism, is challenging. Herein, we successfully developed subnanoporous Ir3Ni ultrathin nanocages with high crystallinity as bifunctional materials for acidic water splitting. The subnanoporous shell enables Ir3Ni NCs optimized exposure of active sites. Importantly, the nickel incorporation contributes to the favorable thermodynamics of the electrocatalysis of the OER after surface reconstruction and optimized hydrogen adsorption free energy in HER electrocatalysis, which induce enhanced intrinsic activity of the acidic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Together, the Ir3Ni nanocages achieve 3.72 A/mgIr(η=350 mV) and 4.47 A/mgIr(η=40 mV) OER and HER mass activity, which are 18.8 times and 3.3 times higher than that of commercial IrO2 and Pt, respectively. In addition, their highly crystalline identity ensures a robust nanostructure, enabling good catalytic durability during the oxygen evolution reaction after surface oxidation. This work provides a new revenue toward the structural design and insightful understanding of metal alloy catalytic mechanisms for the bifunctional acidic water splitting electrocatalysis.

Subsize Pt-based intermetallic compound enables long-term cyclic mass activity for fuel-cell oxygen reduction.

Pt-based alloy catalysts may promise considerable mass activity (MA) for oxygen reduction but are generally unsustainable over long-term cycles, particularly in practical proton exchange membrane fuel cells (PEMFCs). Herein, we report a series of Pt-based intermetallic compounds (Pt3Co, PtCo, and Pt3Ti) enclosed by ultrathin Pt skin with an average particle size down to about 2.3 nm, which deliver outstanding cyclic MA and durability for oxygen reduction. By breaking size limitation during ordered atomic transformation in Pt alloy systems, the MA and durability of subsize Pt-based intermetallic compounds can be simultaneously optimized. The subsize scale was also found to enhance the stability of the membrane electrode through preventing the poisoning of catalysts by ionomers in humid fuel-cell conditions. We anticipate that subsize Pt-based intermetallic compounds set a good example for the rational design of high-performance oxygen reduction electrocatalysts for PEMFCs. Furthermore, the prevention of ionomer poisoning was identified as the critical parameter for assembling robust commercial membrane electrodes in PEMFCs.

Crystalline Order Propagates Across Thick Layers of Water in Solutions of Supramolecular Nanotubes.

3D crystalline order with 1 nm resolution is observed in aqueous solutions of supramolecular nanotubes containing 94 % water, at concentrations as low as 6 wt%. 50 of star-like organic ions arrange into supramolecular rings which, in turn, stack on top of each other to form long hollow tubes with 15 nm outer diameter. Cryo-TEM and X-ray diffraction show that the parallel nanotubes arrange on a perfect hexagonal lattice. Unexpectedly, fiber diffraction on sheared solutions revealed numerous hkl Bragg reflections on several layer lines indicating longitudinal interlock between the tubes and 3D crystalline order with molecular-scale details transferred across 10 nm thick layers of water. The observed high 3D order is attributed to long-range attraction between like-charged tubes and amplified charge modulation by the extremely high intra-tube correlation length.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction.

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Directional Manipulation of Electron Transfer by Energy Level Engineering for Efficient Cathodic Oxygen Reduction.

Electron transfer plays an important role in determining the energy conversion efficiency of energy devices. Nitrogen-coordinated single metal sites (M-N4) materials as electrocatalysts have exhibited great potential in devices. However, there are still great difficulties in how to directionally manipulate electron transfer in M-N4 catalysts for higher efficiency. Herein, we demonstrated the mechanism of electron transfer being affected by energy level structure based on classical iron phthalocyanine (FePc) molecule/carbon models and proposed an energy level engineering strategy to manipulate electron transfer, preparing high-performance ORR catalysts. Engineering molecular energy level via modulating FePc molecular structure with nitro induces a strong interfacial electronic coupling and efficient charge transfer from carbon to FePc-ß-NO2 molecule. Consequently, the assembled zinc-air battery exhibits ultrahigh performance which is superior to most of M-N4 catalysts. Energy level engineering provides a universal approach for directionally manipulating electron transfer, bringing a new concept to design efficient and stable M-N4 electrocatalyst.

Surface Anion Promotes Pt Electrocatalysts with High CO Tolerance in Fuel-Cell Performance.

Platinum reaches considerable activity and stability as an electrocatalyst but is not always capable of maintaining such performance under CO poisoning, particularly in CO residual fuels for practical proton-exchange membrane fuel cells (PEMFCs). In this work, we report that surface anions including a series of nonmetal elements on Pt nanoparticles result in outstanding CO tolerance for electrocatalysts in fuel cells. In particular, phosphorus surface-anion-modified Pt (denoted as P-Pt) possesses more than 10-fold enhancement of CO tolerance (only 8.4% decay) than commercial Pt/C, which can serve as a robust electrocatalyst both in CO poisoning half cells and full cells. Moreover, the general mechanism and principle were proposed, stating that surface anions should be selected preferentially to offer electron feedback to downshift the d-band center for the Pt surface, successfully weakening CO adsorption and leading to high-tolerance capability. We anticipate that surface anions on a Pt surface can bring robust electrocatalysts for practical PEMFCs and offer novel insights for high-performance Pt-based electrocatalysts.

Interfacial engineering of cobalt sulfide/graphene hybrids for highly efficient ammonia electrosynthesis.

Electrocatalytic N2 reduction reaction (NRR) into ammonia (NH3), especially if driven by renewable energy, represents a potentially clean and sustainable strategy for replacing traditional Haber-Bosch process and dealing with climate change effect. However, electrocatalytic NRR process under ambient conditions often suffers from low Faradaic efficiency and high overpotential. Developing newly regulative methods for highly efficient NRR electrocatalysts is of great significance for NH3 synthesis. Here, we propose an interfacial engineering strategy for designing a class of strongly coupled hybrid materials as highly active electrocatalysts for catalytic N2 fixation. X-ray absorption near-edge spectroscopy (XANES) spectra confirm the successful construction of strong bridging bonds (Co-N/S-C) at the interface between CoS x nanoparticles and NS-G (nitrogen- and sulfur-doped reduced graphene). These bridging bonds can accelerate the reaction kinetics by acting as an electron transport channel, enabling electrocatalytic NRR at a low overpotential. As expected, CoS2/NS-G hybrids show superior NRR activity with a high NH3 Faradaic efficiency of 25.9% at -0.05 V versus reversible hydrogen electrode (RHE). Moreover, this strategy is general and can be extended to a series of other strongly coupled metal sulfide hybrids. This work provides an approach to design advanced materials for ammonia production.

Sub-2 nm Ultrasmall High-Entropy Alloy Nanoparticles for Extremely Superior Electrocatalytic Hydrogen Evolution.

The development of sufficiently effective catalysts with extremely superior performance for electrocatalytic hydrogen production still remains a formidable challenge, especially in acidic media. Here, we report ultrasmall high-entropy alloy (us-HEA) nanoparticles (NPs) with the best-level performance for hydrogen evolution reaction (HER). The us-HEA (NiCoFePtRh) NPs show an average diameter of 1.68 nm, which is the smallest size in the reported HEAs. The atomic structure, coordinational structure, and electronic structure of the us-HEAs were comprehensively clarified. The us-HEA/C achieves an ultrahigh mass activity of 28.3 A mg-1noble metals at -0.05 V (vs the reversible hydrogen electrode, RHE) for HER in 0.5 M H2SO4 solution, which is 40.4 and 74.5 times higher than those of the commercial Pt/C and Rh/C catalysts, respectively. Moreover, the us-HEA/C demonstrates an ultrahigh turnover frequency of 30.1 s-1 at 50 mV overpotential (41.8 times higher than that of the Pt/C catalyst) and excellent stability with no decay after 10 000 cycles. Operando X-ray absorption spectroscopy and theoretical calculations reveal the actual active sites, tunable electronic structures, and a synergistic effect among five elements, which endow significantly enhanced HER activity. This work not only engineers a general and scalable strategy for synthesizing us-HEA NPs and elucidates the complex structural information and catalytic mechanisms of multielement HEA system in depth, but also highlights HEAs as sufficiently advanced catalysts and accelerates the research of HEAs in energy-related applications.

Surface Nitrogen-Injection Engineering for High Formation Rate of CO2 Reduction to Formate.

In this study, we highlight that surface nitrogen-injection engineering brings a high formation rate for CO2 reduction to formate, which is high level among the reported electrocatalysts. Surface nitrogen-injection engineering can increase the amounts of active sites and optimize the electronic structure simultaneously. Taking an example of SnS2 precursors, the final-obtained surface N-enriched Sn(S) nanosheets (denoted as N-Sn(S) nanosheets) exhibit a 5-fold of current density and 2.45-fold of Faradaic efficiency than pristine SnS2 derived Sn(S) nanosheets (denoted as Sn(S) nanosheets). On account of high activity and selectivity, the formation rate of formate is 14 times than that of pristine samples and reaches up to 1358 µmol h-1 cm-2. Moreover, this strategy is proven to be general to other metal sulfides, such as CuS and In2S3. We anticipate that surface nitrogen-injection engineering offers new avenues to rational design of advanced electrocatalysts for CO2 reduction reaction.

Freestanding Cubic ZrN Single-Crystalline Films with Two-Dimensional Superconductivity.

The successful fabrication of freestanding two-dimensional (2D) crystals that exhibit unprecedented high crystal quality and macroscopic continuity renovates the conventional cognition that 2D long-range crystalline order cannot stably exist at finite temperatures. Current progresses are primarily limited to van der Waals (vdW) layered materials, while studies on how to obtain 2D materials from nonlayered bulk crystals remain sparse. Herein, we report the experimental realization of vdW-like cubic ZrN single crystal and emphasize the significant role of confined electrons in stabilizing the atomic structure at the 2D limit. Furthermore, the exfoliated ZrN single-crystal films with a few nanometers thick exhibit dimensional crossover effect of emerging 2D superconductivity with the unconventional upper critical field beyond Pauli paramagnetic limit, which suggests a dimensional effect in the pairing mechanism of dimensionally confined superconductors.

Ultrafine Co3 O4 Nanoparticles within Nitrogen-Doped Carbon Matrix Derived from Metal-Organic Complex for Boosting Lithium Storage and Oxygen Evolution Reaction.

Transition metal oxides have recently received great attention for application in advanced lithium-ion batteries (LIBs) and oxygen evolution reaction (OER). Herein, the ethylenediaminetetraacetic cobalt complex as a precursor to synthesize ultrafine Co3 O4 nanoparticles encapsulated into a nitrogen-doped carbon matrix (NC) composites is presented. The as-prepared Co3 O4 /NC-350 obtained by pyrolysis at 350 °C demonstrates superior rate performance (372 mAh g-1 at 5.0 A g-1 ) and high cycling stability (92% capacity retention after 300 cycles at 1.0 A g-1 ) as anode for LIBs. When evaluated as an electrocatalyst for OER, the Co3 O4 /NC-350 achieves an overpotential of 298 mV at a current density of 10 mA cm-2 . The NC-encapsualted porous hierarchical structure assures fast and continuous electron transportation, high activity sites, and strong structural integrity. This works offers novel complex precursors for synthesizing transition metal-based electrodes for boosting electrochemical energy conversion and storage.

Vibronic Superexchange in Double Perovskite Electrocatalyst for Efficient Electrocatalytic Oxygen Evolution.

Perovskites are prototype electrocatalyts benefiting from their high terrestrial availability and high stability. Electronic state regulation plays a key role in promising higher electrocatalytic efficiencies. Herein, we highlighted a vibronic superexchange in double perovskite to synergistically optimize eg electron filling state and increase the formation of active species on the surface of catalysts. Vibronic superexchange of Ni3+-O-Mn3+ in La2NiMnO6 nanoparticles brings the optimal eg electron filling state of Mn and Ni ions toward unity. Moreover, a vibronic superexchange interaction of Ni3+-O-Mn3+ induces strong Jahn-Teller distortion of MnO6 and NiO6 octahedra, elongating metal-O bonds, which helps to form the active species of Mn/Ni hydroxide/oxide on the surface of catalysts. Surprisingly, La2NiMnO6 nanoparticles exhibit superior oxygen evolution reaction (OER) catalytic performance with higher current density and lower Tafel slope than its bulk counterpart. Our finding will be a promising pathway to develop advanced precious-metal-free catalysts.

Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage.

Two-dimensional (2D) energy materials have shown the promising electrochemical characteristics for lithium ion storage. However, the decreased active surfaces and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO4) nanosheets as a model material for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol and tetrahydrofuran, the sodium ion transport in VOPO4 nanosheets has been significantly improved. In addition to advanced characterization including X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption fine structure to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO4 host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO4 nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO4 nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO4 nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg2+ and Al3+.

Dynamic Migration of Surface Fluorine Anions on Cobalt-Based Materials to Achieve Enhanced Oxygen Evolution Catalysis.

Fluorine-anion surface engineering has now been used to activate catalytic active species, representing a completely new way of reconstruction toward oxygen evolution reaction (OER) active species. The electronegativity of the fluorine anion is the strongest so that it will be much easier to form weak metal-fluorine bonds with stronger ionicity, contributing to the dynamic migration of fluorine anions and finally enriching on the surface of both cobalt-based oxide/oxyhydroxide. Surface enrichment of fluorine anions endows more hydrophilic surface character for accelerating the key process of oxygen-related intermediate adsorption. Combining with an obviously improved electron transfer capacity, the F-CoOOH/NF catalyst exhibits a greatly enhanced OER activity (270 mV at 10 mA cm-2 ) and reaction kinetics (54 mV dec-1 ) in alkaline medium. Surface anion engineering introduces a new concept for rational design advanced OER catalysts for energy conversion system.

Synthesis of Sub-2†nm Iron-Doped NiSe2 Nanowires and Their Surface-Confined Oxidation for Oxygen Evolution Catalysis.

Ultrathin nanostructures are attractive for diverse applications owing to their unique properties compared to their bulk materials. Transition-metal chalcogenides are promising electrocatalysts, yet it remains difficult to make ultrathin structures (sub-2 nm), and the realization of their chemical doping is even more challenging. Herein we describe a soft-template mediated colloidal synthesis of Fe-doped NiSe2 ultrathin nanowires (UNWs) with diameter down to 1.7 nm. The synergistic interplay between oleylamine and 1-dodecanethiol is crucial to yield these UNWs. The in situ formed amorphous hydroxide layers that is confined to the surface of the ultrathin scaffolds enable efficient oxygen evolution electrocatalysis. The UNWs exhibit a very low overpotential of 268 mV at 10 mA cm-2 in 0.1 m KOH, as well as remarkable long-term stability, representing one of the most efficient noble-metal-free catalysts.

Double-Exchange Effect in Two-Dimensional MnO2 Nanomaterials.

Electronic state transitions, especially metal-insulator transitions (MIT), offer physical properties that are useful in intriguing energy applications and smart devices. But to-date, very few simple metal oxides have been shown to undergo electronic state transitions near room temperature. Herein, we demonstrate experimentally that chemical induction of double-exchange in two-dimensional (2D) nanomaterials brings about a MIT near room temperature. In this case, valence-state regulation of a 2D MnO2 nanosheet induces a Mn(III)-O-Mn(IV) structure with the double-exchange effect, successfully triggering a near-room-temperature electronic transition with an ultrahigh negative magneto-resistance (MR). Double-exchange in 2D MnO2 nanomaterials exhibits an ultrahigh MR value of up to -11.3% (0.1 T) at 287 K, representing the highest reported negative MR values in 2D nanomaterials approaching room temperature. Also, the MnO2 nanosheet displays an infrared response of 7.1% transmittance change on going from 270 to 290 K. We anticipate that dimensional confinement of double-exchange structure promises novel magneto-transport properties and sensitive responses for smart devices.

Exclusive Ni-N4 Sites Realize Near-Unity CO Selectivity for Electrochemical CO2 Reduction.

Electrochemical reduction of carbon dioxide (CO2) to value-added carbon products is a promising approach to reduce CO2 levels and mitigate the energy crisis. However, poor product selectivity is still a major obstacle to the development of CO2 reduction. Here we demonstrate exclusive Ni-N4 sites through a topo-chemical transformation strategy, bringing unprecedentedly high activity and selectivity for CO2 reduction. Topo-chemical transformation by carbon layer coating successfully ensures preservation of the Ni-N4 structure to a maximum extent and avoids the agglomeration of Ni atoms to particles, providing abundant active sites for the catalytic reaction. The Ni-N4 structure exhibits excellent activity for electrochemical reduction of CO2 with particularly high selectivity, achieving high faradaic efficiency over 90% for CO in the potential range from -0.5 to -0.9 V and gives a maximum faradaic efficiency of 99% at -0.81 V with a current density of 28.6 mA cm-2. We anticipate exclusive catalytic sites will shed new light on the design of high-efficiency electrocatalysts for CO2 reduction.

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,N-Decorated Graphene.

The electrocatalyzed oxygen reduction and evolution reactions (ORR and OER, respectively) are the core components of many energy conversion systems, including water splitting, fuel cells, and metal-air batteries. Rational design of highly efficient non-noble materials as bifunctional ORR/OER electrocatalysts is of great importance for large-scale practical applications. A new strongly coupled hybrid material is presented, which comprises CoOx nanoparticles rich in oxygen vacancies grown on B,N-decorated graphene (CoOx NPs/BNG) and operates as an efficient bifunctional OER/ORR electrocatalyst. Advanced spectroscopic techniques were used to confirm formation of abundant oxygen vacancies and strong Co-N-C bridging bonds within the CoOx NPs/BNG hybrid. Surprisingly, the CoOx NPs/BNG hybrid electrocatalyst is highly efficient for the OER with a low overpotential and Tafel slope, and is active in the ORR with a positive half-wave potential and high limiting current density in alkaline medium.

Atomically Dispersed Iron-Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions.

Rational design of non-noble materials as highly efficient, economical, and durable bifunctional catalysts for oxygen evolution and reduction reactions (OER/ORR) is currently a critical obstacle for rechargeable metal-air batteries. A new route involving S was developed to achieve atomic dispersion of Fe-Nx species on N and S co-decorated hierarchical carbon layers, resulting in single-atom bifunctional OER/ORR catalysts for the first time. The abundant atomically dispersed Fe-Nx species are highly catalytically active, the hierarchical structure offers more opportunities for active sites, and the electrical conductivity is greatly improved. The obtained electrocatalyst exhibits higher limiting current density and a more positive half-wave potential for ORR, as well as a lower overpotential for OER under alkaline conditions. Moreover, a rechargeable Zn-air battery device comprising this hybrid catalyst shows superior performance compared to Pt/C catalyst. This work will open a new avenue to design advanced bifunctional catalysts for reversible energy storage and conversion devices.

Initial Reaction Mechanism of Platinum Nanoparticle in Methanol-Water System and the Anomalous Catalytic Effect of Water.

Understanding the detailed reaction mechanism in the early stage of noble metal nanoparticles is very critical for controlling the final crystal's size, morphology, and properties. Here, we report a systematic study on the initial reaction mechanism of Pt nanoparticles in methanol-water system and demonstrate an anomalous catalytic effect of H2O on the reduction of H2PtCl6 to Pt nanoparticles using a combination of UV-vis, X-ray absorption spectroscopy (XAS), liquid chromatography mass spectrometry (LCMS), and first-principles calculation methods. The observations reveal the transformation route [PtCl6](2-) → [PtCl5(CH3O)](2-) → [PtCl4](2-) → [PtCl3(CH3O)](2-) → [PtCl2](2-) and finally to form Pt nanoparticles in a pure CH3OH solution. With 10 vol % water adding in the CH3OH solution, a new and distinct chemical reduction pathway is found in which the precursors change from [PtCl6](2-) to [PtCl5(CH3O)(H2O)](2-) to [PtCl4](2-) to [PtCl3(CH3O)(H2O)](2-) to [PtCl2](2-) and to Pt nanoparticles. Notably, the supernumerary water molecular can significantly accelerate the rate of chemical reduction and greatly shorten the reaction time. This work not only elucidates the initial reaction mechanism of Pt nanoparticles but also highlights the pronounced influence of H2O on the reaction pathway, which will provide useful insights for understanding the formation mechanism of noble metal nanoparticles and open up a high efficient way to synthesize new functional nanomaterial.