Cobalt-Exchanged Poly(Heptazine Imides) as Transition Metal-Nx Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic Generation of Singlet Oxygen via ROS-Mediated Redox Chain Reaction for Efficient Disinfection

The risk of harmful microorganisms to ecosystems and human health has stimulated exploration of singlet oxygen (1O2)-based disinfection. It can be potentially generated via an electrocatalytic process, but is limited by the low production yield and unclear intermediate-mediated mechanism. Herein, we designed a two-site catalyst (Fe/Mo-N/C) for the selective 1O2 generation. The Mo sites enhance the generation of 1O2 precursors (H2O2), accompanied by the generation of intermediate •HO2/•O2-. The Fe site facilitates activation of H2O2 into •OH, which accelerates the •HO2/•O2- into 1O2. A possible mechanism for promoting 1O2 production through the ROS-mediated chain reaction is reported. The as-developed electrochemical disinfection system can kill 1 × 107 CFU mL-1 of E. coli within 8 min, leading to cell membrane damage and DNA degradation. It can be effectively applied for the disinfection of medical wastewater. This work provides a general strategy for promoting the production of 1O2 through electrocatalysis and for efficient electrochemical disinfection.

Triggering efficient reconstructions of Co/Fe dual-metal incorporated Ni hydroxide by phosphate additives for electrochemical hydrogen and oxygen evolutions

Alkaline electrochemical water splitting has been considered as an efficient way for the green hydrogen production in industry, where the electrocatalysts play the critical role for the electricity-to-fuel conversion efficiency. Phosphate salts are widely used as additives in the fabrication of electrocatalysts with improved activity, but their roles on the electrocatalytic performance have not been fully understood. Herein, we fabricate Co, Fe dual-metal incorporated Ni hydroxide on Ni foam using NaH2PO4 ((Co, Fe)NiOxHy-pi) and NaH2PO2 ((Co, Fe)NiOxHy-hp) as additive, respectively. We find that (Co, Fe)NiOxHy-hp with NaH2PO2 in the fabrication shows high activity and stability for both HER and OER (a overpotential of -0.629 V and 0.65 V at 400 mA cm-2 for HER and OER, respectively). Further experiment reveals that the reconstructed structures of electrocatalyst by using NaH2PO2 (hp) endow high electrocatalytic performances: (1) in-situ generated active metal improves the accumulation, transportation and activity of hydrogen species in the HER process; and (2) in-situ generated poor-crystalline hydroxide endows superior charge/mass transportation and kinetics improvements in the OER process. Our study may provide an insightful understanding on the catalytic performance of non-precious metal electrocatalysts by controlling additives and guidance for the design and synthesis of novel electrocatalysts.

Spatial Position Regulation of Cu Single Atom Site Realizes Efficient Nanozyme Photocatalytic Bactericidal Activity

Recently, single-atom nanozymes have made significant progress in the fields of sterilization and treatment, but their catalytic performance as substitutes for natural enzymes and drugs is far from satisfactory. Here, a method is reported to improve enzyme activity by adjusting the spatial position of a single-atom site on the nanoplatforms. Two types of Cu single-atom site nanozymes are synthesized in the interlayer (CuL /PHI) and in-plane (CuP /PHI) of poly (heptazine imide) (PHI) through different synthesis pathways. Experimental and theoretical analysis indicates that the interlayer position of PHI can effectively adjust the coordination number, coordination bond length, and electronic structure of Cu single atoms compared to the in-plane position, thereby promoting photoinduced electron migration and O2 activation, enabling effective generate reactive oxygen species (ROS). Under visible light irradiation, the photocatalytic bactericidal activity of CuL /PHI against aureus is ≈100%, achieving the same antibacterial effect as antibiotics, after 10 min of low-dose light exposure and 2 h of incubation.

Sulfidation of CoCuOx Supported on Nickel Foam to Form a Heterostructure and Oxygen Vacancies for a High-Performance Anion-Exchange Membrane Water Electrolyzer

Anion-exchange membrane water electrolyzer (AEMWE) is attracting attention for hydrogen production owing to its ability to employ nonprecious metal catalysts and high energy conversion efficiency. Spinel-structured transition metal oxides exhibit excellent potential in oxygen evolution reaction (OERs). Nevertheless, the research on highly active and durable spinel-structured electrodes for the anodic OER of AEMWE is deficient. Herein, a self-supported S-CoCu oxide/nickel foam (S-CoCuOx/NF) anode was synthesized through a two-step method (electrodeposition and sulfidation). The formation of abundant oxygen vacancies and heterostructure collaboratively enhances the electron and mass transfer, resulting in an overpotential of 313 mV at 100 mA cm-2 for OER. For the lab-scale AEMWE system with the S-CoCuOx/NF anode, a current density of 1 A cm-2 was obtained at 1.87 V (cell voltage) with high durability for 110 h (1 A cm-2) at 60 °C. The results will provide insights into developing the spinel structure-derived anode for high-performance AEMWE.

Synergistically Regulating the Electronic Structure of CoS by Cation and Anion Dual-Doping for Efficient Overall Water Splitting

Precisely regulating the electronic construction of the reactive center is an essential method to improve the electrocatalysis, but achieving efficient multifunctional characteristics remains a challenge. Herein, CoS sample dual-doped by Cu and F atoms, as bifunctional electrocatalyst, is designed and synthesized for water electrolysis. According to the experimental results, Cu atom doping can perform primary electronic adjustment and obtain bifunctional properties, and then the electronic structure is adjusted for the second time to achieve an optimal state by introducing F atom. Meanwhile, this dual-doping strategy will result in lattice distortion and expose more active sites. As expected, dual-doped Cu-F-CoS show the brilliant electrocatalytic activity, revealing ultralow overpotentials (59 mV for HER, 213 mV for OER) at 10 mA cm-2 in alkaline electrolyte. Besides, it also exhibits distinguished water electrolysis activity with cell voltage as low as 1.52 V at 10 mA cm-2 . Our work can provide an atomic-level perception for adjusting the electronic construction of reactive sites by means of dual-doping engineering and put forward a contributing path for the electrocatalysts with multifunctional designing.

New Strategy for the Persistent Photocatalytic Reduction of U(VI): Utilization and Storage of Solar Energy in K+ and Cyano Co-Decorated Poly(Heptazine Imide)

The photocatalytic conversion of soluble U(VI) into insoluble U(IV) is a robust strategy to harvest aqueous uranium, but remains challenging owing to the intermittent availability of solar influx and reoxidation of U(IV) without illumination. Herein, a dual platform based on K⁺ and cyano group co-decorated poly(heptazine imide) (K-CN-PHI) is reported that can drive persistent U(VI) extraction upon/beyond light. K-CN-PHI achieves the photocatalytic reduction of U(VI) with a reaction rate of 0.89 min^-1 , being 47 times greater than that over pristine carbon nitride (PCN). This system can further be triggered by light to form long-living radicals, driving the reduction of U(VI) in the dark for over 3 d. The flexible structural K⁺ as counterions stabilize the electrons trapped by cyanamide groups, enabling the long lifetime of the generated radicals. The results collectively prove K-CN-PHI to be a novel and efficient photocatalyst enabling persistent U(VI) extraction around the clock, and broadening the practical applications of the photocatalytic extraction of U(VI).

Fine-tune d-band center of cobalt vanadium oxide nanosheets by N-doping as a robust overall water splitting electrocatalyst

Developing high-activity, long-durability, and noble metal-free oxygen evolution (OER) and hydrogen evolution (HER) cocatalysts are the bottlenecks for efficient overall water splitting (OWS). Here, novel cobalt vanadium oxides doped by nitrogen were synthesized by nitriding Co2V2O7@NF precursor at 300-450 °C for OER and HER reactions. N-Co2V2O7@NF (350 °C) and N-Co2VO4/VO2@NF (400 °C) show remarkable OER and HER performance with overpotentials of 310 mV and 224 mV at high current density (100 mA cm-2). Besides, they also revealed long-term solid stability even after 170 h and 700 h of continuous performance. Furthermore, the N-Co2V2O7@NF(+)||N-Co2VO4/VO2@NF(-) OWS device possesses a cell voltage of 1.93 V at 500 mA cm-2 better than RuO2@NF(+)||Pt/C@NF(-) (2.02 V) and can operate for 60 h with almost no degradation. This extraordinary performance can be attributed to the nanosheet structure, which can maximize the exposure of active sites and accelerate the mass transfer. Moreover, density functional theory (DFT) calculations suggest that N-doping can fine-tune the d-band center and band gap to facilitate intermediate adsorption and electron motion. The method presented here can be applied in other novel N-doped electrocatalysts for the energy field.

Two-Dimensional Covalent Heptazine-Based Framework Enables Highly Photocatalytic Performance for Overall Water Splitting

Screening high-efficiency 2D conjugated polymers toward visible-light-driven overall water splitting (OWS) is one of the most promising but challenging research directions to realize solar-to-hydrogen (STH) energy conversion and storage. "Mystery molecule" heptazine is an intriguing hydrogen evolution reaction (HER) building block. By covalently linking with the electron-rich alkynyl and phenyl oxygen evolution reaction (OER) active units, 10 experimentally feasible 2D covalent heptazine-based frameworks (CHFs) are constructed and screened four promising visible-light-driven OWS photocatalysts, which are linked by p-phenyl (CHF-4), p-phenylenediynyl (CHF-7), m-phenylenediynyl (CHF-8), and phenyltriynyl (CHF-9), respectively. Their HER and OER active sites achieve completely spatially separated, where HER active sites focus on heptazine units and OER active sites located on alkynyl or phenyl units. Their lower overpotentials allow them to spontaneously trigger the surface OWS reaction under their own light-induced bias without using any sacrificial agents and cocatalysts. Among them, CHF-7 shows the best photocatalytic performance with an ideal STH energy conversion efficiency estimated at 12.04%, indicating that it is a promising photocatalyst for industrial OWS. This work not only provides an innovative idea for the exploration of novel polymer photocatalysts for OWS but also supplies a direction for the development of heptazine derivatives.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Bicontinuous Nanoporous Nitrogen/Carbon-Codoped FeCoNiMg Alloy as a High-Performance Electrode for the Oxygen Evolution Reaction

The kinetics of the oxygen evolution reaction (OER) in aqueous electrolytes is relatively slow, which seriously limits the energy efficiency of electricity-to-hydrogen conversion. Herein, a bicontinuous nanoporous FeCoNiMg alloy is prepared by high heat sintering method based on the nanoscale Kirkendall effect and the surface is codoped with nitrogen and carbon elements by the nitrocarburizing method (denoted NC-FeCoNiMg). The three-dimensional (3D) nanoporous NC-FeCoNiMg alloy electrode achieves superior electrocatalytic performance for the OER in alkaline media, delivering a low Tafel slope (34.6 mV dec^-1) and small overpotentials (235 and 290 mV at 10 and 100 mA cm^-2, respectively). Under consecutive high current densities, the NC-FeCoNiMg electrode still exhibits excellent long-term stability, and the OER activity even increases after testing for 100 h at a high current density of 1000 mA cm^-2. Comprehensive studies reveal that the N/C codoping of the inner and outer surfaces dramatically improves the electrocatalytic activity of the NC-FeCoNiMg electrode. This work demonstrates an efficient nanoarchitectural construction and a surface modulation strategy to increase the electrocatalytic activity and stability of transition-metal-based electrodes for the OER, holding great promise for fulfilling the requirements for the large-scale production of clean hydrogen energy.

Water on porous, nitrogen-containing layered carbon materials: The performance of computational model chemistries

Porous, layered materials containing sp2-hybridized carbon and nitrogen atoms, offer through their tunable properties, a versatile route towards tailormade catalysts for electrochemistry and photochemistry. A key molecule interacting with these...

High-performance potassium poly(heptazine imide) films for photoelectrochemical water splitting

Potassium poly(heptazine imide) photoanode is synthesized, and owing to the improved crystallinity, it has presented a remarkable performance for solar-driven water splitting.

Mesoporous WCx Films with NiO-Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template-controlled porosity that are surface-modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft-templated mesoporous tungsten oxide (mp. WOx ) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx ), and a subsequent shape-conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+ , leading to remarkably improved utilization of OER-active sites in alkaline medium. The obtained Ni mass-specific activity is about 280 times that of mesoporous NiOx (mp. NiOx ) films. The NiO-coated WCx catalyst achieves an outstanding mass-specific activity of 1989 A gNi -1 in a rotating-disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.

State of the Art in the Preparation and Properties of Molecular Monomeric s-Heptazines: Syntheses, Characteristics, and Functional Applications

This review gives an account on the fast expanding field of monomeric (or molecular) heptazines, at the exclusion of their various polymeric forms, often referred to as carbon nitrides. While examples of monomeric heptazines were extremely limited until the beginning of this century, the field has started expanding quickly since then, as has the number of reports on polymeric materials, though previous reviews did not separate these fields. We provide here a detailed report on the synthetic procedures for molecular heptazines. We also extensively report on the different achievements realized from these new molecules, in the fields of physical chemistry, spectroscopy, materials preparation, (photo)catalysis, and devices. After a comprehensive summary and discussion on heptazines syntheses and characteristics, we show that starting from well-defined molecules allows a versatility of approaches and a wide tunability of the expected properties. It comes out that the field of monomeric heptazines is now emerging and possibly heading toward maturity, while diverging from the one of polymeric carbon nitrides. It is likely that this area of research will quickly surge to the forefront of the search for active organic molecules, with special attention to the domains of catalysis and organic-based functional materials and devices.

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Gram-Scale Synthesis of High-Loading Single-Atomic-Site Fe Catalysts for Effective Epoxidation of Styrene

Single-atomic-site (SAS) catalysts, a new frontier of catalysts, always show extremely high atom efficiency and unexpected catalytic properties. Herein, a pyrolyzing coordinated polymer (PCP) strategy is developed, which is facile and widely applicable in the synthesis of a series of SAS catalysts including SAS-Fe, SAS-Ni, SAS-Cu, SAS-Zn, SAS-Ru, SAS-Rh, SAS-Pd, SAS-Pt, and SAS-Ir. The as-obtained SAS catalysts can be easily synthesized at gram scale and the metal loading of SAS-Fe catalysts achieves a record value of 30 wt%, which meets the requirement of practical applications. Moreover, it is discovered that SAS-Fe catalysts show unprecedented catalytic performance for epoxidation of styrene using O₂ as the only oxidant (yield: 64%; selectivity: 89%), while Fe nanoparticles and ironporphyrin are inactive. This discovery is believed to pave the way for exploiting the unparalleled properties of SAS catalysts and promoting their industrial applications.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.

NiFe LDH/CuO nanosheet: a sheet-on-sheet strategy to boost the active site density towards oxygen evolution reaction

The amount of active sites of a catalyst is of great importance to enhance the oxygen evolution reaction (OER) activity. Here, the sheet-on-sheet strategy is proposed to effectively increase the density of active sites of NiFe layered double hydroxide (NiFe LDH) catalyst in terms of structural engineering. As a non-precious electrocatalyst for the OER, NiFe LDH is grown directly on CuO nanosheets. As a result, the received NiFe LDH/CuO nanosheet catalyst with sheet-on-sheet structure shows an ultralow overpotential of 270 mV at 20 mA cm⁻², much lower than that of RuO₂ as a benchmark. The CuO nanosheets, as substrate, play the vital role in downsizing the NiFe LDH, leading to the raised active site density.

The amount of active sites of a catalyst is of great importance to enhance the oxygen evolution reaction (OER) activity.