Switch Volmer-Heyrovsky to Volmer-Tafel Pathway for Efficient Acidic Electrocatalytic Hydrogen Evolution by Correlating Pt Single Atoms with Clusters.

As cost-effective catalysts, platinum (Pt) single-atom catalysts (SACs) have attracted substantial attention. However, most studies indicate that Pt SACs in acidic hydrogen evolution reaction (HER) follow the slow Volmer-Heyrovsky (VH) mechanism instead of the fast kinetic Volmer-Tafel (VT) pathway. Here, this work propose that the VH mechanism in Pt SACs can be switched to the faster VT pathway for efficient HER by correlating Pt single atoms (SAs) with Pt clusters (Cs). Our calculations reveal that the correlation between Pt SAs and Cs significantly impacts the electronic structure of exposed Pt atoms, lowering the adsorption barrier for atomic hydrogen and enabling a faster VT mechanism. To validate these findings, this work purposely synthesize three catalysts: l-Pt@MoS2, m-Pt@MoS2 and h-Pt@MoS2 with low, moderate, and high Pt-loading, having different distributions of Pt SAs and Cs. The m-Pt@MoS2 catalyst with properly correlating Pt SAs and Cs exhibits outstanding performance with an overpotential of 47 mV and Tafel slope of 32 mV dec-1. Further analysis of the Tafel values confirms that the m-Pt@MoS2 sample indeed follows the VT reaction mechanism, aligning with the theoretical findings. This study offers a deep understanding of the synergistic mechanism, paving a way for designing novel-advanced catalysts.

Metal Organic Framework Glasses: A New Platform for Electrocatalysis?

Metal organic framework (MOF) glasses are a coordination network of metal nodes and organic ligands as an undercooled frozen-in liquid, and have therefore broadened the potential of MOF materials in the fundamental research and application scenarios. On the road to deploying MOF glasses as electrocatalysts, it remains several basic scientific hurdles although MOF glasses not only inherit the structural merits of MOFs but also endow with active catalytic features including concentrated defects, metal centers and disorder structure etc. The research on the ionic conductivity, catalytic stability and reactivity of MOF glasses has yielded scientific insights towards its electrocatalytic applications. Here, we first comb the history, definition and basic properties of MOF glasses. Then, we identify the main synthetic methods and characterization techniques. Finally, we advance the potentials and challenges of MOF glasses as electrocatalysts in furthering the understanding of these themes.

Recent Progress in Metal-Catalyzed Selective Oxidation of 5-Hydroxymethylfurfural into Furan-Based Value-Added Chemicals.

5-hydroxymethylfurfural (HMF), one of the most significant biomass-derived renewable resources, has been widely utilized to create furan-based value-added chemicals such as 2,5-diformylfuran (DFF), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furan dicarboxylic acid (FDCA). Indeed, DFF, HMFCA and FFCA are key intermediate products during the oxidation of HMF to FDCA. Herein, this review aims to demonstrate the recent advances in metal-catalyzed oxidation of HMF into FDCA via two different reaction routes (HMF-DFF-FFCA-FDCA and HMF-HMFCA-FFCA-FDCA). All the four furan-based compounds are comprehensively discussed by the selective oxidation of HMF. Additionally, various metal catalysts, reaction conditions, and reaction mechanisms used to obtain the four different products are systematically reviewed. It is anticipated that this review will provide related researchers with new perspectives and speed up the development of this field.

Recent Advances in the Synthesis of MXene Quantum Dots.

Quantum dots (QDs) with ultrahigh surface-to-volume ratio, abundant edge active sites, forceful quantum confinement and other remarkable physio-chemical properties, have garnered considerable research interest. MXene QDs, as an emerging member of them, have also attracted wide attention in the last six years, and shown great achievements in many fields. This critical review systematically summarizes the various methods for synthesizing MXene QDs. The characteristics and corresponding applications of various MXene QDs are also presented. The advantages and disadvantages of various synthetic methods, and the limitations of corresponding MXene QDs are compared and highlighted. Finally, the challenges and perspectives of synthesizing MXene QDs are proposed. We hope this review will enlighten researchers to the fabrication of more advancing and promising MXene-based QDs with proprietary properties in diverse applications.

Quench-Induced Surface Engineering Boosts Alkaline Freshwater and Seawater Oxygen Evolution Reaction of Porous NiCo2 O4 Nanowires.

The electrochemical oxygen evolution reaction (OER) by efficient catalysts is a crucial step for the conversion of renewable energy into hydrogen fuel, in which surface/near-surface engineering has been recognized as an effective strategy for enhancing the intrinsic activities of the OER electrocatalysts. Herein, a facile quenching approach is demonstrated that can simultaneously enable the required surface metal doping and vacancy generation in reconfiguring the desired surface of the NiCo2 O4 catalyst, giving rise to greatly enhanced OER activities in both alkaline freshwater and seawater electrolytes. As a result, the quenched-engineered NiCo2 O4 nanowire electrode achieves a current density of 10 mA cm-2 at a low overpotential of 258 mV in 1 m KOH electrolyte, showing the remarkable catalytic performance towards OER. More impressively, the same electrode also displays extraordinary activity in an alkaline seawater environment and only needs 293 mV to reach 10 mA cm-2 . Density functional theory (DFT) calculations reveal the strong electronic synergies among the metal cations in the quench-derived catalyst, where the metal doping regulates the electronic structure, thereby yielding near-optimal adsorption energies for OER intermediates and giving rise to superior activity. This study provides a new quenching method to obtain high-performance transition metal oxide catalysts for freshwater/seawater electrocatalysis.

Zn2+ Pre-Intercalation Stabilizes the Tunnel Structure of MnO2 Nanowires and Enables Zinc-Ion Hybrid Supercapacitor of Battery-Level Energy Density.

Although there has been tremendous progress in exploring new configurations of zinc-ion hybrid supercapacitors (Zn-HSCs) recently, the much lower energy density, especially the much lower areal energy density compared with that of the rechargeable battery, is still the bottleneck, which is impeding their wide applications in wearable devices. Herein, the pre-intercalation of Zn2+ which gives rise to a highly stable tunnel structure of Znx MnO2 in nanowire form that are grown on flexible carbon cloth with a disruptively large mass loading of 12 mg cm-2 is reported. More interestingly, the Znx MnO2 nanowires of tunnel structure enable an ultrahigh areal energy density and power density, when they are employed as the cathode in Zn-HSCs. The achieved areal capacitance of up to 1745.8 mF cm-2 at 2 mA cm-2 , and the remarkable areal energy density of 969.9 µWh cm-2 are comparable favorably with those of Zn-ion batteries. When integrated into a quasi-solid-state device, they also endow outstanding mechanical flexibility. The truly battery-level Zn-HSCs are timely in filling up of the battery-supercapacitor gap, and promise applications in the new generation flexible and wearable devices.

Negative Pressure Pyrolysis Induced Highly Accessible Single Sites Dispersed on 3D Graphene Frameworks for Enhanced Oxygen Reduction.

Herein, we report a negative pressure pyrolysis to access dense single metal sites (Co, Fe, Ni etc.) with high accessibility dispersed on three-dimensional (3D) graphene frameworks (GFs), during which the differential pressure between inside and outside of metal-organic frameworks (MOFs) promotes the cleavage of the derived carbon layers and gradual expansion of mesopores. In situ transmission electron microscopy and Brunauer-Emmett-Teller tests reveal that the formed 3D GFs possess an enhanced mesoporosity and external surface area, which greatly favor the mass transport and utilization of metal sites. This contributes to an excellent oxygen reduction reaction (ORR) activity (half-wave potential of 0.901 V vs. RHE). Theoretical calculations verify that selective carbon cleavage near Co centers can efficiently lower the overall ORR theoretical overpotential in comparison with intact atomic configuration.

Twinned Tungsten Carbonitride Nanocrystals Boost Hydrogen Evolution Activity and Stability.

Synergistic integration of two active metal-based compounds can lead to much higher electrocatalytic activity than either of the two individually, due to the interfacial effects. Herein, a proof-of-concept strategy is creatively developed for the successful fabrication of twinned tungsten carbonitride (WCN) nanocrystals, where W2 C and WN are chemically bonded at the molecule level. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure (XAFS) spectroscopy analyses demonstrate that the intergrowth of W2 C and WN in the WCN nanocrystals produces abundant N-W-C interfaces, leading to a significant enhancement in catalytic activity and stability for hydrogen evolution reaction (HER). Indeed, it shows 14.2 times higher and 140 mV lower in the respective turn-over frequency (TOF) and overpotential at 10 mA cm-2 compared to W2 C alone. To complement the experimental observation, the theoretical calculations demonstrate that the WCN endows more favorable hydrogen evolution reaction than the single W2 C or WN crystals due to abundant interfaces, beneficial electronic states, lower work function, and more active W sites at the N-W-C interfaces.

In situ electrochemical oxidation of electrodeposited Ni-based nanostructure promotes alkaline hydrogen production.

Highly active and stable electrocatalysts based on non-precious metals for hydrogen evolution reaction (HER) in alkaline solution are urgently required for enabling mass production of clean hydrogen in industry. Herein, core-shell NiOOH/Ni nanoarchitectures supported on the conductive carbon cloth have been successfully prepared by a facile electrodeposition process of Ni, and a subsequent in situ electrochemical oxidation. When explored as an alkaline HER electrocatalyst, the as-synthesized NiOOH/Ni nanoarchitecture requires only a low overpotential of ∼111 mV to attain a current density of -10 mA cm-2, demonstrating its strong catalytic capability of hydrogeneration. The excellent HER activity could well be attributed to the decreasing charge transfer resistance and competitive electrochemical active area of the amorphous NiOOH, compared with inactive Ni substrate. The feasible methodology established in this study can be easily expanded to obtain a series of nano-sized metal oxyhydroxide materials for various energy conversion and storage applications, where Ni-based nanomaterials are among the highly active ones.

Electronic Structure Control of Tungsten Oxide Activated by Ni for Ultrahigh-Performance Supercapacitors.

Tuning the electron structure is of vital importance for designing high active electrode materials. Here, for boosting the capacitive performance of tungsten oxide, an atomic scale engineering approach to optimize the electronic structure of tungsten oxide by Ni doping is reported. Density functional theory calculations disclose that through Ni doping, the density of state at Fermi level for tungsten oxide can be enhanced, thus promoting its electron transfer. When used as electrode of supercapacitors, the obtained Ni-doped tungsten oxide with 4.21 at% Ni exhibits an ultrahigh mass-specific capacitance of 557 F g-1 at the current density of 1 A g-1 and preferable durability in a long-term cycle test. To the best of knowledge, this is the highest supercapacitor performance reported so far in tungsten oxide and its composites. The present strategy demonstrates the validity of the electronic structure control in tungsten oxide via introducing Ni atoms for pseudocapacitors, which can be extended to other related fields as well.

RuP2 -Based Catalysts with Platinum-like Activity and Higher Durability for the Hydrogen Evolution Reaction at All pH†Values.

Highly active, stable, and cheap Pt-free catalysts for the hydrogen evolution reaction (HER) are under increasing demand for future energy conversion systems. However, developing HER electrocatalysts with Pt-like activity that can function at all pH values still remains as a great challenge. Herein, based on our theoretical predictions, we design and synthesize a novel N,P dual-doped carbon-encapsulated ruthenium diphosphide (RuP2 @NPC) nanoparticle electrocatalyst for HER. Electrochemical tests reveal that, compared with the Pt/C catalyst, RuP2 @NPC not only has Pt-like HER activity with small overpotentials at 10 mA cm-2 (38 mV in 0.5 m H2 SO4 , 57 mV in 1.0 m PBS and 52 mV in 1.0 m KOH), but demonstrates superior stability at all pH values, as well as 100 % Faradaic yields. Therefore, this work adds to the growing family of transition-metal phosphides/heteroatom-doped carbon heterostructures with advanced performance in HER.

Three-Dimensionally Costabilized Metal Catalysts toward an Oxygen Reduction Reaction.

Improving the long-term stability of metal catalysts is crucial to developing polymer electrolyte fuel cells (PEFCs). In this work, we first report an inorganic (TiO2)-organic (perfluorosulfonic acid, PFSA) costabilized Pt catalyst supported on graphene nanosheets (GNS) (Pt-PFSA-TiO2/GNS). Herein, TiO2, as a robust wall, impedes the collision between the metal nanoparticles (NPs) in plane along the horizontal x and y axes, while PFSA mainly anchors the metal NPs to constrain detachment along the vertical z axis. The resulting catalyst displays higher oxygen reduction reaction (ORR) activity in comparison to that of commercial Pt/C. Significantly, the stability is particularly better than that of only PFSA- or TiO2-decorated catalysts (Pt-PFSA/GNS or Pt-TiO2/GNS) and far better than that of Pt/C. After 6000 potential cycles, the half-wave potential (E1/2) of Pt-PFSA-TiO2/GNS decreases by only 16 mV, far less than that of Pt/C (56 mV). The excellent electrochemical property of Pt-PFSA-TiO2/GNS is predominantly attributed to the synergistic effect of PFSA and TiO2 in costabilizing the Pt NP by anchoring and blocking Pt NPs in all three spatial directions. The structural dynamics and mechanism of enhanced properties are also discussed.

Correlating Single-Atomic Ruthenium Interdistance with Long-Range Interaction Boosts Hydrogen Evolution Reaction Kinetics.

Correlated single-atom catalysts (c-SACs) with tailored intersite metal-metal interactions are superior to conventional catalysts with isolated metal sites. However, precise quantification of the single-atomic interdistance (SAD) in c-SACs is not yet achieved, which is essential for a crucial understanding and remarkable improvement of the correlated metal-site-governed catalytic reaction kinetics. Here, three Ru c-SACs are fabricated with precise SAD using a planar organometallic molecular design and π-π molecule-carbon nanotube confinement. This strategy results in graded SAD from 2.4 to 9.3 Å in the Ru c-SACs, wherein tailoring the Ru SAD into 7.0 Å generates an exceptionally high turnover frequency of 17.92 H2 s-1 and a remarkable mass activity of 100.4 A mg-1 under 50 and 100 mV overpotentials, respectively, which is superior to all the Ru-based catalysts reported previously. Furthermore, density functional theory calculations confirm that Ru SAD has a negative correlation with its d-band center owing to the long-range interactions induced by distinct local atomic geometries, resulting in an appropriate electrostatic potential and the highest catalytic activity on c-SACs with 7.0 Å Ru SAD. The present study promises an attractive methodology for experimentally quantifying the metal SAD to provide valuable insights into the catalytic mechanism of c-SACs.

Tuning Active Metal Atomic Spacing by Filling of Light Atoms and Resulting Reversed Hydrogen Adsorption-Distance Relationship for Efficient Catalysis.

Precisely tuning the spacing of the active centers on the atomic scale is of great significance to improve the catalytic activity and deepen the understanding of the catalytic mechanism, but still remains a challenge. Here, we develop a strategy to dilute catalytically active metal interatomic spacing (dM-M) with light atoms and discover the unusual adsorption patterns. For example, by elevating the content of boron as interstitial atoms, the atomic spacing of osmium (dOs-Os) gradually increases from 2.73 to 2.96 Å. More importantly, we find that, with the increase in dOs-Os, the hydrogen adsorption-distance relationship is reversed via downshifting d-band states, which breaks the traditional cognition, thereby optimizing the H adsorption and H2O dissociation on the electrode surface during the catalytic process; this finally leads to a nearly linear increase in hydrogen evolution reaction activity. Namely, the maximum dOs-Os of 2.96 Å presents the optimal HER activity (8 mV @ 10 mA cm-2) in alkaline media as well as suppressed O adsorption and thus promoted stability. It is believed that this novel atomic-level distance modulation strategy of catalytic sites and the reversed hydrogen adsorption-distance relationship can shew new insights for optimal design of highly efficient catalysts.

Dispersed FeOx nanoparticles decorated with Co2SiO4 hollow spheres for enhanced oxygen evolution reaction.

Oxygen evolution reaction (OER) has drawn ever-increasing attention because of its essential role in various renewable-energy technologies. In spite of tremendous research efforts, developing high-performance OER catalysts at low cost remains a great challenge. Inspired by two earth-abundant elements Fe and Si, herein, we report a Fe-Co2SiO4 composite consisting of well dispersed iron oxide (FeOx) decorated Co2SiO4 hollow nanospheres as an economical and promising OER catalyst. Although Co2SiO4 or FeOx alone has little OER activity, their composite exhibits satisfied performance, that is highly related to geometric effect and bimetal component electronic interactions. The Fe-Co2SiO4 composite exhibits comparable catalytic activity to most of transition mental oxide/hydroxide relevant composites at 10 mA cm-2. It is even 1.6 times higher than commercial RuO2 electrocatalyst at high current density 100 mA cm-2 in alkaline solution. In this work, surface decoration of transition metal silicate provides a new horizon to design high-performance and economical OER catalysts.

Scalable Assembly of High-Quality Graphene Films via Electrostatic-Repulsion Aligning.

Assembling pristine graphene into freestanding films featuring high electrical conductivity, superior flexibility, and robust mechanical strength aims at meeting the all-around high criteria of new-generation electronics. However, voids and defects produced in the macroscopic assembly process of graphene nanosheets severely degrade the performance of graphene films, and mechanical brittleness often limits their applications in wide scenarios. To address such challenges, an electrostatic-repulsion aligning strategy is demonstrated to produce highly conductive, ultraflexible, and multifunctional graphene films. Typically, the high electronegativity of titania nanosheets (TiNS) induces the aligning of negatively charged graphene nanosheets via electrostatic repulsion in the film assembly. The resultant graphene films show fine microstructure, enhanced mechanical properties, and improved electrical conductivity up to 1.285 × 105 S m-1 . Moreover, the graphene films can withstand 5000 repeated folding without structural damage and electrical resistance fluctuation. These comprehensive improved properties, combined with the facile synthesis method and scalable production, make these graphene films a promising platform for electromagnetic interference (EMI) shielding and thermal-management applications in smart and wearable electronics.

A chainmail effect of ultrathin N-doped carbon shell on Ni2P nanorod arrays for efficient hydrogen evolution reaction catalysis.

Exploring innovation strategies has huge potential to significantly improving both activity and stability of current catalysts. Here, a chainmail design is proposed to enable the electronic interaction of ultrathin nitrogen-doped carbon shell with Ni2P nanorod core arrayed on nickel foam (Ni2P@NC/NF) for simultaneously promoting the activity and stability in both alkaline and neutral hydrogen evolution reaction (HER). The easy penetration of valence electrons from active Ni2P core to NC shell enables the obvious improvement of HER performance compared to pure Ni2P. In 1 M KOH and 1 M PBS solution, the resultant Ni2P@NC/NF requires the ultralow overpotentials of only 93 and 96 mV to drive the current density of 10 mA cm-2 with the Faradaic efficiency of 96% and 94%, respectively. Remarkably, such a chainmail design also reveals an obviously improved stability with almost negligible performance degradation under the current density of 20 mA cm-2 for 30 h. Theoretical calculations confirm that the nitrogen-doped carbon shell improves the durability of transition metal phosphides by increasing the dissolution resistance of Ni atoms. The proposed concept may create a new pathway for synchronizing high activity and robust stability in manipulating heterogeneous catalytic properties.

Bridging sheet size controls densification of MXene films for robust electromagnetic interference shielding.

Numerous voids among the incompact layer-structure of MXene films result in their low ambient stability and poor innate conductivity for electromagnetic interference (EMI) shielding. Herein, we report a bridging-sheet-size-controlled densification process of MXene films by applying graphene oxide (GO) as a bridging agent. Specifically, the sheet size of GO is tailored to quantify a negative correlation of sheet size with densification for directing the preparation of most compact MXene-GO films. Benefiting from the shortest electron-transport-distance in the most compact structure, the conductivity of the MXene-GO film achieves 1.7 times (∼1.6 × 105 S/m) that of MXene film. The EMI shielding performance (5.2 × 106 dB/m) reaches the record-value among reported MXene films at 10 µm-scale thickness. Moreover, the compact structure boosts the ambient stability of MXene-GO films where the conductivity and EMI shielding performance remain 88.7% and 90.0% after 15 days, respectively. The findings rationale the structure-activity relationship of compact MXene films for flexible electronics.

Localizing Tungsten Single Atoms around Tungsten Nitride Nanoparticles for Efficient Oxygen Reduction Electrocatalysis in Metal-Air Batteries.

Combining isolated atomic active sites with those in nanoparticles for synergizing complex multistep catalysis is being actively pursued in the design of new electrocatalyst systems. However, these novel systems have been rarely studied due to the challenges with synthesis and analysis. Herein, a synergistically catalytic performance is demonstrated with a 0.89 V (vs reversible hydrogen electrode) onset potential in the four-step oxygen reduction reaction (ORR) by localizing tungsten single atoms around tungsten nitride nanoparticles confined into nitrogen-doped carbon (W SAs/WNNC). Through density functional theory calculations, it is shown that each of the active centers in the synergistic entity feature a specific potential-determining step in their respective reaction pathway that can be merged to optimize the intermediate steps involving scaling relations on individual active centers. Impressively, the W SAs/WNNC as the air cathode in all-solid-state Zn-air and Al-air batteries demonstrate competitive durability and reversibility, despite the acknowledged low activity of W-based catalyst toward the ORR.

Ultrafast Macroscopic Assembly of High-Strength Graphene Oxide Membranes by Implanting an Interlaminar Superhydrophilic Aisle.

A macroscopic-assembled graphene oxide (GO) membrane with sustainable high strength presents a bright future for its applications in ionic and molecular filtration for water purification or fast force response for sensors. Traditionally, the bottom-up macroscopic assembly of GO sheets is optimized by widening the interlaminar space for expediting water passage, frequently leading to a compromise in strength, assembly time, and ensemble thickness. Herein, we rationalize this strategy by implanting a superhydrophilic bridge of cobalt-based metal-organic framework nanosheets (NMOF-Co) as an additional water "aisle" into the interlaminar space of GO sheets (GO/NMOF-Co), resulting in a high-strength macroscopic membrane ensemble with tunable thickness from the nanometer scale to the centimeter scale. The GO/NMOF-Co membrane assembly time is only 18 s, 30800 times faster than that of pure GO (154 h). More importantly, the obtained membrane attains a strength of 124.4 MPa, which is more than 3 times higher than that of the GO membrane prepared through filtration. The effect of hydrophilicity on membrane assembly is also investigated by introducing different intercalants, suggesting that, except for the interlamellar spacing, the interlayered hydrophilicity plays a more decisive role in the macroscopic assembly of GO membranes. Our results give a fundamental implication for fast macroscopic assembly of high-strength 2D materials.