Band Engineering and Morphology Control of Oxygen‑Incorporated Graphitic Carbon Nitride Porous Nanosheets for Highly Efficient Photocatalytic Hydrogen Evolution

Graphitic carbon nitride(g-C3N4)-based photocatalysts have shown great potential in the splitting of water.However,the intrinsic drawbacks of g-C3N4,such as low surface area,poor diffusion,and charge separation efficiency,remain as the bottleneck to achieve highly efficient hydrogen evolution.Here,a hollow oxygen-incorporated g-C3N4 nanosheet(OCN)with an improved surface area of 148.5 m2 g^−1 is fabricated by the multiple thermal treatments under the N2/O2 atmosphere,wherein the C–O bonds are formed through two ways of physical adsorption and doping.The physical characterization and theoretical calculation indicate that the O-adsorption can promote the generation of defects,leading to the formation of hollow morphology,while the O-doping results in reduced band gap of g-C3N4.The optimized OCN shows an excellent photocatalytic hydrogen evolution activity of 3519.6μmol g^−1 h^−1 for~20 h,which is over four times higher than that of g-C3N4(850.1μmol g^−1 h^−1)and outperforms most of the reported g-C3N4 catalysts.     

Pyrazine-Functionalized Donor–Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor–accepter (D–A) COF material, named PyPz-COF, constructed from electron donor 4,4′,4″,4′″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4′-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g⁻¹ h⁻¹ with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g⁻¹ h⁻¹). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H₃PO₄ proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10⁻² S cm⁻¹ at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.     

In Situ Constructed Perovskite–Chalcogenide Heterojunction for Photocatalytic CO2 Reduction

Perovskite nanocrystals (PNCs) are promising candidates for solar-to-fuel conversions yet exhibit low photocatalytic activities mainly due to serious recombination of photogenerated charge carriers. Constructing heterojunction is regarded as an effective method to promote the separation of charge carriers in PNCs. However, the low interfacial quality and non-directional charge transfer in heterojunction lead to low charge transfer efficiency. Herein, a CsPbBr₃–CdZnS heterojunction is designed and prepared via an in situ hot-injection method for photocatalytic CO₂ reduction. It is found that the high-quality interface in heterojunction and anisotropic charge transfer of CdZnS nanorods (NRs) enable efficient spatial separation of charge carriers in CsPbBr₃–CdZnS heterojunction. The CsPbBr₃–CdZnS heterojunction achieves a higher CO yield (55.8 µmol g⁻¹ h⁻¹) than that of the pristine CsPbBr₃ NCs (13.9 µmol g⁻¹ h⁻¹). Furthermore, spectroscopic experiments and density functional theory (DFT) simulations further confirm that the suppressed recombination of charge carriers and lowered energy barrier for CO₂ reduction contribute to the improved photocatalytic activity of the CsPbBr₃–CdZnS heterojunction. This work demonstrates a valid method to construct high-quality heterojunction with directional charge transfer for photocatalytic CO₂ reduction. This study is expected to pave a new avenue to design perovskite–chalcogenide heterojunction.     

Single-Site Metal–Organic Framework and Copper Foil Tandem Catalyst for Highly Selective CO2 Electroreduction to C2H4

Tandem catalysis is a promising way to break the limitation of linear scaling relationship for enhancing efficiency, and the desired tandem catalysts for electrochemical CO₂ reduction reaction (CO₂RR) are urgent to be developed. Here, a tandem electrocatalyst created by combining Cu foil (CF) with a single-site Cu(II) metal–organic framework (MOF), named as Cu–MOF–CF, to realize improved electrochemical CO₂RR performance, is reported. The Cu–MOF–CF shows suppression of CH₄, great increase in C₂H₄ selectivity (48.6%), and partial current density of C₂H₄ at −1.11 V versus reversible hydrogen electrode. The outstanding performance of Cu–MOF–CF for CO₂RR results from the improved microenvironment of the Cu active sites that inhibits CH₄ production, more CO intermediate produced by single-site Cu–MOF in situ for CF, and the enlarged active surface area by porous Cu–MOF. This work provides a strategy to combine MOFs with copper-based electrocatalysts to establish high-efficiency electrocatalytic CO₂RR.     

A High-Efficiency CoSe Electrocatalyst with Hierarchical Porous Polyhedron Nanoarchitecture for Accelerating Polysulfides Conversion in Li–S Batteries

Lithium–sulfur (Li–S) batteries are recognized as promising candidates for next-generation electrochemical energy storage systems owing to their high energy density and cost-effective raw materials. However, the sluggish multielectron sulfur redox reactions are the root cause of most of the issues for Li–S batteries. Herein, a high-efficiency CoSe electrocatalyst with hierarchical porous nanopolyhedron architecture (CS@HPP) derived from a metal–organic framework is presented as the sulfur host for Li–S batteries. The CS@HPP with high crystal quality and abundant reaction active sites can catalytically accelerate capture/diffusion of polysulfides and precipitation/decomposition of Li₂S. Thus, the CS@HPP sulfur cathode exhibits an excellent capacity of 1634.9 mAh g⁻¹, high rate performance, and a long cycle life with a low capacity decay of 0.04% per cycle over 1200 cycles. CoSe nanopolyhedrons are further fabricated on a carbon cloth framework (CC@CS@HPP) to unfold the electrocatalytic activity by its high electrical conductivity and large surface area. A freestanding CC@CS@HPP sulfur cathode with sulfur loading of 8.1 mg cm⁻² delivers a high areal capacity of 8.1 mAh cm⁻² under a lean electrolyte. This work will enlighten the rational design of structure–catalysis engineering of transition-metal-based nanomaterials for diverse applications.     

Selective and Stable CO2 Electroreduction to CH4 via Electronic Metal–Support Interaction upon Decomposition/Redeposition of MOF

The CO₂ electroreduction to fuels is a feasible approach to provide renewable energy sources. Therefore, it is necessary to conduct experimental and theoretical investigations on various catalyst design strategies, such as electronic metal–support interaction, to improve the catalytic selectivity. Here a solvent-free synthesis method is reported to prepare a copper (Cu)-based metal–organic framework (MOF) as the precursor. Upon electrochemical CO₂ reduction in aqueous electrolyte, it undergoes in situ decomposition/redeposition processes to form abundant interfaces between Cu nanoparticles and amorphous carbon supports. This Cu/C catalyst favors the selective and stable production of CH₄ with a Faradaic efficiency of ≈55% at −1.4 V versus reversible hydrogen electrode (RHE) for 12.5 h. The density functional theory calculation reveals the crucial role of interfacial sites between Cu and amorphous carbon support in stabilizing the key intermediates for CO₂ reduction to CH₄. The adsorption of COOH* and CHO* at the Cu/C interface is up to 0.86 eV stronger than that on Cu(111), thus promoting the formation of CH₄. Therefore, it is envisioned that the strategy of regulating electronic metal–support interaction can improve the selectivity and stability of catalyst toward a specific product upon electrochemical CO₂ reduction.     

Fundamental Understanding of Nonaqueous and Hybrid Na–CO2 Batteries: Challenges and Perspectives

Alkali metal–CO₂ batteries, which combine CO₂ recycling with energy conversion and storage, are a promising way to address the energy crisis and global warming. Unfortunately, the limited cycle life, poor reversibility, and low energy efficiency of these batteries have hindered their commercialization. Li–CO₂ battery systems have been intensively researched in these aspects over the past few years, however, the exploration of Na–CO₂ batteries is still in its infancy. To improve the development of Na–CO₂ batteries, one must have a full picture of the chemistry and electrochemistry controlling the operation of Na–CO₂ batteries and a full understanding of the correlation between cell configurations and functionality therein. Here, recent advances in CO₂ chemical and electrochemical mechanisms on nonaqueous Na–CO₂ batteries and hybrid Na–CO₂ batteries (including O₂-involved Na–O₂/CO₂ batteries) are reviewed in-depth and comprehensively. Following this, the primary issues and challenges in various battery components are identified, and the design strategies for the interfacial structure of Na anodes, electrolyte properties, and cathode materials are explored, along with the correlations between cell configurations, functional materials, and comprehensive performances are established. Finally, the prospects and directions for rationally constructing Na–CO₂ battery materials are foreseen.     

Interfacial Electronic Modulation of Dual-Monodispersed Pt–Ni3S2 as Efficacious Bi-Functional Electrocatalysts for Concurrent H2 Evolution and Methanol Selective Oxidation

Constructing the efficacious and applicable bifunctional electrocatalysts and establishing out the mechanisms of organic electro-oxidation by replacing anodic oxygen evolution reaction(OER) are critical to the development of electrochemicallydriven technologies for efficient hydrogen production and avoid CO₂ emission. Herein, the hetero-nanocrystals between monodispersed Pt(～ 2 nm) and Ni₃S₂(～ 9.6 nm) are constructed as active electrocatalysts through interfacial...     

Achieving Tunable Selectivity and Activity of CO2 Electroreduction to CO via Bimetallic Silver–Copper Electronic Engineering

Limited comprehension of the reaction mechanism has hindered the development of catalysts for CO₂ reduction reactions (CO₂RR). Here, the bimetallic AgCu nanocatalyst platform is employed to understand the effect of the electronic structure of catalysts on the selectivity and activity for CO₂ electroreduction to CO. The atomic arrangement and electronic state structure vary with the atomic ratio of Ag and Cu, enabling tunable d-band centers to optimize the binding strength of key intermediates. Density functional theory calculations confirm that the variation of Cu content greatly affects the free energy of *COOH, *CO (intermediate of CO), and *H (intermediates of H₂), which leads to the change of the rate-determining step. Specifically, Ag₉₆Cu₄ reduces the free energy of the formation of *COOH while maintaining a relatively high theoretical overpotential for hydrogen evolution reaction(HER), thus achieving the best CO selectivity. While Ag₇₀Cu₃₀ shows relatively low formation energy of both *COOH and *H, the compromised thermodynamic barrier and product selectivity allows Ag₇₀Cu₃₀ the best CO partial current density. This study realizes the regulation of the selectivity and activity of electrocatalytic CO₂ to CO, which provides a promising way to improve the intrinsic performance of CO₂RR on bimetallic AgCu.     

Regulating the Electronic Structure of Bismuth Nanosheets by Titanium Doping to Boost CO2 Electroreduction and Zn–CO2 Batteries

The electrochemical carbon dioxide reduction reaction (E-CO₂RR) to formate is a promising strategy for mitigating greenhouse gas emissions and addressing the global energy crisis. Developing low-cost and environmentally friendly electrocatalysts with high selectivity and industrial current densities for formate production is an ideal but challenging goal in the field of electrocatalysis. Herein, novel titanium-doped bismuth nanosheets (Ti Bi NSs) with enhanced E-CO₂RR performance are synthesized through one-step electrochemical reduction of bismuth titanate (Bi₄Ti₃O₁₂). We comprehensively evaluated Ti Bi NSs using in situ Raman spectra, finite element method, and density functional theory. The results indicate that the ultrathin nanosheet structure of Ti Bi NSs can accelerate mass transfer, while the electron-rich properties can accelerate the production of *CO₂⁻ and enhance the adsorption strength of *OCHO intermediate. The Ti Bi NSs deliver a high formate Faradaic efficiency (FE_formate) of 96.3% and a formate production rate of 4032 µmol h⁻¹ cm⁻² at −1.01 V versus RHE. An ultra-high current density of −338.3 mA cm⁻² is achieved at −1.25 versus RHE, and simultaneously FE_formate still reaches more than 90%. Furthermore, the rechargeable Zn–CO₂ battery using Ti Bi NSs as a cathode catalyst achieves a maximum power density of 1.05 mW cm⁻² and excellent charging/discharging stability of 27 h.     

Band gap tuning and nonlinear optical characterization of TiO2–SiO2 nanocomposites with respect to composition and phase

We report on the linear and nonlinear optical studies on TiO₂–SiO₂ nanocomposites with varying percentage ratio. It is found that optical band gap of the material varies with respect to the amount of the SiO₂ in the composite. Nonlinear optical characterization of these samples was studied by using open as well as closed aperture Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz). The nanocomposites showed enhanced nonlinear optical properties than pure TiO₂ and this can be attributed to the surface states and weak dielectric confinement of TiO₂ nanoparticles by SiO₂ matrix. The nanocomposites were thermally treated and similar studies were performed. The anatase form of TiO₂ in the nanocomposites showed superior properties relative to the amorphous and rutile phase of the composite. The involved mechanism is explained by taking into account the dominant role played by the excitons in the TiO₂ nanoparticles.     

Achieving Ultralong-Cycle Zinc-Ion Battery via Synergistically Electronic and Structural Regulation of a MnO2 Nanocrystal–Carbon Hybrid Framework

Aqueous rechargeable zinc-ion batteries (ZIBs) have attracted burgeoning interests owing to the prospect in large-scale and safe energy storage application. Although manganese oxides are one of the typical cathodes of ZIBs, their practical usage is still hindered by poor service life and rate performance. Here, a MnO₂–carbon hybrid framework is reported, which is obtained in a reaction between the dimethylimidazole ligand from a rational designed MOF array and potassium permanganate, achieving ultralong-cycle-life ZIBs. The unique structural feature of uniform MnO₂ nanocrystals which are well-distributed in the carbon matrix leads to a 90.4% capacity retention after 50 000 cycles. In situ characterization and theoretical calculations verify the co-ions intercalation with boosted reaction kinetics. The hybridization between MnO₂ and carbon endows the hybrid with enhanced electrons/ions transport kinetics and robust structural stability. This work provides a facile strategy to enhance the battery performance of manganese oxide-based ZIBs.     

Synergistic Modulation of Electronic Interaction to Enhance Intrinsic Activity and Conductivity of Fe–Co–Ni Hydroxide Nanotube for Highly Efficient Oxygen Evolution Electrocatalyst

The large-scale hydrogen production and application through electrocatalytic water splitting depends crucially on the development of highly efficient, cost-effective electrocatalysts for oxygen evolution reaction (OER), which, however, remains challenging. Here, a new electrocatalyst of trimetallic Fe–Co–Ni hydroxide (denoted as FeCoNiO_xH_y) with a nanotubular structure is developed through an enhanced Kirkendall process under applied potential. The FeCoNiO_xH_y features synergistic electronic interaction between Fe, Co, and Ni, which not only notably increases the intrinsic OER activity of FeCoNiO_xH_y by facilitating the formation of *OOH intermediate, but also substantially improves the intrinsic conductivity of FeCoNiO_xH_y to facilitate charge transfer and activate catalytic sites through electrocatalyst by promoting the formation of abundant Co³⁺. Therefore, FeCoNiO_xH_y delivers remarkably accelerated OER kinetics and superior apparent activity, indicated by an ultra-low overpotential potential of 257 mV at a high current density of 200 mA cm⁻². This work is of fundamental and practical significance for synergistic catalysis related to advanced energy conversion materials and technologies.     

Phase Relations in the Na2CO3–ZrO2–SiO2–H2O System at 0.1 and 0.05 GPa and 450°C

The phase relations in the Na₂CO₃–ZrO₂–SiO₂–H₂O system were studied at 0.1 and 0.05 GPa and 450°C using large-particle-size and nanocrystalline zirconias. Four silicates were obtained when use was made of readily soluble ZrO₂(nanocr): ZrSiO₄, Na₂ZrSi₆O₁₅ · 3H₂O, Na₂ZrSi₃O₉ · 2H₂O, and Na₄Zr₂Si₅O₁₆ · H₂O. In the system containing poorly soluble ZrO₂(cr), only Na₂ZrSi₆O₁₅ · 3H₂O was found to crystallize. It is shown that the structures of all the Na–Zr silicates contain invariant six-polyhedron structural precursors, each made up of two ZrO₆ octahedra and four SiO₄ tetrahedra, and belong to a homologous series of structures based on the silicate Na₂ZrSi₂O₇, which forms via direct packing of cyclic subpolyhedral precursors.     

Sequential Electrodeposition of Bifunctional Catalytically Active Structures in MoO3/Ni–NiO Composite Electrocatalysts for Selective Hydrogen and Oxygen Evolution

Exploring earth-abundant and highly efficient electrocatalysts is critical for further development of water electrolyzer systems. Integrating bifunctional catalytically active sites into one multi-component might greatly improve the overall water-splitting performance. In this work, amorphous NiO nanosheets coupled with ultrafine Ni and MoO₃ nanoparticles (MoO₃/Ni–NiO), which contains two heterostructures (i.e., Ni–NiO and MoO₃–NiO), is fabricated via a novel sequential electrodeposition strategy. The as-synthesized MoO₃/Ni–NiO composite exhibits superior electrocatalytic properties, affording low overpotentials of 62 mV at 10 mA cm⁻² and 347 mV at 100 mA cm⁻² for catalyzing the hydrogen and the oxygen evolution reaction (HER/OER), respectively. Moreover, the MoO₃/Ni–NiO hybrid enables the overall alkaline water-splitting at a low cell voltage of 1.55 V to achieve 10 mA cm⁻² with outstanding catalytic durability, significantly outperforming the noble-metal catalysts and many materials previously reported. Experimental and theoretical investigations collectively demonstrate the generated Ni–NiO and MoO₃–NiO heterostructures significantly reduce the energetic barrier and act as catalytically active centers for selective HER and OER, synergistically accelerating the overall water-splitting process. This work helps to fundamentally understand the heterostructure-dependent mechanism, providing guidance for the rational design and oriented construction of hybrid nanomaterials for diverse catalytic processes.     

A Carbon-Free and Free-Standing Cathode From Mixed-Phase TiO2 for Photo-Assisted Li–CO2 Battery

Li–CO₂ battery provides a new strategy to simultaneously solve the problems of energy storage and greenhouse effect. However, the severe polarization of CO₂ reduction and CO₂ evolution reaction impede the practical application. Herein, anodic TiO₂ nanotube arrays are first introduced as carbon-free and free-standing cathode for photo-assisted Li–CO₂ battery, and the photo-assisted charge and discharge mechanism is first clarified from the perspective of photocatalysis. Mixed-phase TiO₂ exhibits a long cycling life of 580 h (52 cycles) at 0.025 mA cm⁻² and delivers a high discharge specific capacity of 3001 µAh cm⁻² under UV illumination. The charge voltage dramatically reduces from 4.53 to 3.03 V under UV illumination. The improvement of photo-assisted Li–CO₂ battery performance relies on the synergistic effect of the hierarchical porous structure, strong UV absorption, efficient separation, and transfer of photo-generated electrons and holes at hetero-phase junction, and the facilitation of photo-generated electrons and holes on CO₂ reduction and CO₂ evolution reaction. This work can provide useful guidance for designing efficient photocathode for photo-assisted Li–CO₂ battery and other metal–air batteries.     

Cubic NaSbS2 as an Ionic–Electronic Coupled Semiconductor for Switchable Photovoltaic and Neuromorphic Device Applications

The recent emergence of lead halide perovskites as ionic–electronic coupled semiconductors motivates the investigation of alternative solution-processable materials with similar modulatable ionic and electronic transport properties. Here, a novel semiconductor—cubic NaSbS₂—for ionic–electronic coupled transport is investigated through a combined theoretical and experimental approach. The material exhibits mixed ionic–electronic conductivity in inert atmosphere and superionic conductivity in humid air. It is shown that post deposition electronic reconfigurability in this material enabled by an electric field induces ionic segregation enabling a switchable photovoltaic effect. Utilizing post-perturbation of the ionic composition of the material via electrical biasing and persistent photoconductivity, multistate memristive synapses with higher-order weight modulations are realized for neuromorphic computing, opening up novel applications with such ionic–electronic coupled materials.     

Regulating π-Conjugation in sp2-Carbon-Linked Covalent Organic Frameworks for Efficient Metal-Free CO2 Photoreduction with H2O

The development of sp²-carbon-linked covalent organic frameworks (sp²c-COFs) as artificial photocatalysts for solar-driven conversion of CO₂ into chemical feedstock has captured growing attention, but catalytic performance has been significantly limited by their intrinsic organic linkages. Here, a simple, yet efficient approach is reported to improve the CO₂ photoreduction on metal-free sp²c-COFs by rationally regulating their intrinsic π-conjugation. The incorporation of ethynyl groups into conjugated skeletons affords a significant improvement in π-conjugation and facilitates the photogenerated charge separation and transfer, thereby boosting the CO₂ photoreduction in a solid-gas mode with only water vapor and CO₂. The resultant CO production rate reaches as high as 382.0 µmol g⁻¹ h⁻¹, ranking at the top among all additive-free CO₂ photoreduction catalysts. The simple modulation approach not only enables to achieve enhanced CO₂ reduction performance but also simultaneously gives a rise to extend the understanding of structure-property relationship and offer new possibilities for the development of new π-conjugated COF-based artificial photocatalysts.     

Thermodynamic analysis and optimization of a novel N2O–CO2 cascade system for refrigeration and heating

In this study, a natural refrigerant based cascaded system, with nitrous oxide as the low temperature fluid and carbon dioxide as the high temperature fluid, is analyzed for simultaneous cooling and heating applications. Effects of significant design and operating parameters on system performance are studied. Optimization of intermediate pressure for maximum COP for various design and operating parameters are presented as well. Results show that use of internal heat exchanger has marginal influence on system performance. Due to similar thermodynamic properties of nitrous oxide and carbon dioxide, the optimized intermediate temperature turns out to be independent of the performance of gas cooler and evaporator for a given operating condition. Due to the same reason, N₂O as low temperature fluid and CO₂ as high temperature fluid in a cascade arrangement exhibit similar behavioural trends in a system where the fluids are swapped.     

Low Temperature Synthesis of TiO2 Nanoparticles with High Photocatalytic Activity and Photoelectrochemical Properties through Sol–Gel Method

In the present work TiO₂ nanoparticles were prepared by sol–gel method and both small-sized nanoparticles and proper crystals were delivered by simultaneous use of surfactant, which was applied for the purpose of size reduction of nanoparticles, and acid ions that were used so that the formation of crystalline structures occurs appropriately. Photocatalytic activity of samples was measured by degrading RO dye, and photoelectrochemical properties were assayed using anodic photocurrent responses under Xe lamp light irradiation. The synthesized nanoparticles exhibit great photoelectrochemical and photocatalytic activity compared to that of commercial photocatalyst, Degussa P-25, which demonstrates that the method can be applied in the synthesis of other semiconductor oxides nanoparticles.