Two-electron Transfer Process Research Articles

Effect of structure of metal-free nitrogen-doped coal-based carbon on catalytic oxygen reduction

In carbon-based oxygen reduction reaction (ORR) catalysts, metal-free nitrogen-doped carbon-based materials generally have higher catalytic stability compared to metal catalysts with reduced catalytic activity due to the occurrence of metal agglomeration during the catalytic reaction. In this paper, a metal-free nitrogen-doped carbon-based ORR catalyst NCOH5-900 with a half-wave potential of 0.846 V and onset potential of 0.94 V was prepared using coal as the carbon source, dicyandiamide as the nitrogen source and KOH as the activator. The material has good ORR catalytic performance with half-wave potential and onset potential comparable to Pt/C catalyst with the same mass loading, with better resistance to methanol toxicity and catalytic stability. The physical and chemical properties of the prepared catalyst were controlled by adjusting the amount of nitrogen doping and activation temperature to establish the relationship between the material structure and the ORR performance. The results suggest that for the metal-free nitrogen-doped carbon-based material, the good ORR catalytic performance is attributed to the abundant edge defects, high graphitization, good mixed structure of micro/mesopores, large nitrogen content, and high ratio of pyridine to graphitic nitrogen. Finally, the catalytic mechanism of NCOH5-900 for ORR is investigated using in-situ attenuated total reflection Fourier infrared (ATR-FTIR) and in-situ Raman, confirming the four-electron reduction process of O2 via a continuous dual step of two-electron transfer process.

Unveiling the Unique Reactivity of Anionic Mn(I) Complexes via Metal-Ligand Cooperation: Nucleophilic Attack on C(sp3)-X Bonds.

Metal-ligand cooperation (MLC) has emerged as a pivotal strategy for the catalytic activation of small molecules within both synthetic and biological arenas. Leveraging this approach, a suite of potent catalytic reactions─encompassing hydrogenation, hydroelementation, and dehydrogenative processes─have been realized, with notable advances in manganese catalysis in recent years. However, the activation of alkyl halides by Mn complexes, which typically requires strong reductants to form Mn(-I) complexes that are incompatible with standard cross-coupling conditions, remains a significant challenge. This limitation underscores the urgent need to investigate alternative methods for activating C(sp3)-X bonds using higher valence state Mn complexes. In response to this challenge, we present the synthesis, characterization, and reactivity of a new anionic Mn(I) complex featuring a redox-active dianionic ligand that induces multiple MLC functionalities. We have discovered an innovative mechanism of MLC, characterized by a single ligand transferring two electrons to the metal center. This novel process facilitates an orbital-symmetry-allowed nucleophilic attack on C(sp3)-X bonds, preserving manganese's oxidative state at +1. To the best of our knowledge, this is the first instance where the MLC strategy via a two-electron transfer process has been utilized to execute an SN2 nucleophilic attack at a C(sp3)-X bond by a relatively electron-deficient metal center like Mn(I). Additionally, the dianionic ligand of the anionic Mn(I) complex exhibits ambident nucleophilicity by reacting with different electrophiles, further highlighting its versatile reactivity.

Secondary metal ion-induced electrochemical reduction of U(VI) to U(IV) solids

Recent studies have shown that aqueous U(VI) ions can be transformed into U(VI) precipitates through electrocatalytic redox reactions for uranium recovery. However, there have been no reports of U(IV) solids, such as UO2, using electrochemical methods under ambient conditions since low-valence states of uranium are typically oxidized to U(VI) by O2 or H2O2. Here we developed a secondary metal ion-induced strategy for electrocatalytic production of U(IV) solids from U(VI) solutions using a catalyst consisting of atomically dispersed gallium on hollow nitrogen-doped carbon capsules (Ga-Nx-C). This method relies on the presence of secondary metal ions, e.g., alkaline earth metals, transition metals, lanthanide metals, and actinide metals, which promote the generation of UO2 or bimetallic U(IV)-containing oxides through a two-electron transfer process. No U(IV) solid products were generated in the presence of alkali metal ions. Mechanistic studies revealed that the strong binding affinity between U(IV) and alkaline earth metals (Ca2+/Mg2+/Sr2+/Ba2+), transition metals (Ni2+/Zn2+/Pb2+/Fe3+, etc.) and lanthanide/actinide metals (Ce4+/Eu3+/Th4+/La3+) suppressed re-oxidation of U(IV) to U(VI), leading to the generation of U(IV)O2 and Mx(M = Ce, Eu, Th, La)U(IV)yO2. This work provides fundamental insights into the electrochemical behavior of uranium in aqueous media, whilst guiding uranyl capture from nuclear waste and contaminated water.

Zn-doped tubular graphene nitride for visible light and sacrificial-agent-free H2O2 photosynthesis in water

Graphitic carbon nitride (g-C3N4) has become a favored universal photocatalyst. Despite its numerous advantages, the photocatalytic efficiency of g-C3N4 is hindered by the substantial recombination of photoexcited charge carriers and holes. In this work, we demonstrate that Zn-doped tubular carbon nitride photocatalyst (Zn-tCN) can serve as highly efficient catalysts for H2O2 photosynthesis. Mechanism studies confirm that the presence of Zn in g-C3N4 prolongs the lifetimes of photogenerated carriers and inhibits their recombination, which triggers the reduction of O2 to reaction intermediates (O2−), as supported by in situ electron paramagnetic resonance (EPR) spectroscopy. More importantly, sacrificial agent experiments coupled with in situ EPR results confirmed that the reaction mechanism involves a concerted two-electron transfer process. The optimal catalyst displays a H2O2 productivity of 162.4μmol g–1h−1 under visible-light irradiation without a sacrificial agent, which is 11.7 times higher than that of pristine g-C3N4 (13.8μmol g–1h−1). This work proposes a synthetic strategy for the preparation of high-performance Zn-doped g-C3N4, which offers insights and perspectives for developing highly active photocatalysts and deepening the understanding of photocatalytic mechanisms.

Regulated Interfacial Proton and Water Activity Enhances Mn2+/MnO2 Platform Voltage and Energy Efficiency

Aqueous zinc battery (AZB) is one of the most promising energy storage systems for large-scale applications due to its high safety, low cost, and environmental friendliness. To match the high capacity of zinc metal (~820 mAh g-1), various cathode materials, such as oxides, open structure analogues, chalcogens, and halogens, have been explored for AZBs. Among them, manganese dioxide (MnO2) has been considered to be one of the most attractive candidates due to its high capacity (~308 mAh g-1 for one-electron transfer) and adequate redox potential (~0.70 V vs. SHE). Recently, the MnO2 dissolution/deposition charge storage mechanism has been demonstrated to further increase the voltage and capacity of Mn-based AZBs. The dissolution/deposition (Mn2+/MnO2) is a two-electron transfer process, which doubles the theoretical capacity of MnO2 cathode to ~616 mAh g-1 and considerably increases the redox potential (~1.229 V vs. SHE). The charge storage via the dissolution/deposition mechanism pushes the aqueous-based energy density even higher, to over 1000 Wh kg-1.However, this charge storage mechanism involves liquid-solid transition, as well as the thermodynamics and kinetics of this reaction are highly dependent on the acidity and interfacial environment. Yet, the opposite effect of interfacial H+ on the dissolution/deposition processes and the role of interfacial H2O are rarely discussed. Here we introduce tetrafluoroborate anion (BF4 -) into the sulfate-based electrolyte to regulate the interfacial H+ and H2O activity. First, BF4 - hydrolysis increases the electrolyte’s acidity, promoting MnO2 dissolution. Second, BF4 - forms an H-bond network with interfacial H2O that assists H+ diffusion while retaining sufficient H2O supply to facilitate MnO2 deposition. As a result, the cathode-free Zn//MnO2 electrolytic cell achieves a high plateau of ~1.92 V and energy efficiency of ~84.23 % in the BF4 - containing electrolyte. Significantly, the cell delivers 1000 cycles at 1 C with ~100 % Coulombic efficiency and a high energy efficiency retention of 93.65 %. Our findings disclose a new strategy to promote Mn2+/MnO2 platform voltage and energy efficiency for future large-scale energy storage systems.

A Planar-Structured Dinuclear Cobalt(II) Complex with Indirect Synergy for Photocatalytic CO2-to-CO Conversion.

Dinuclear metal synergistic catalysis (DMSC) has been proved an effective approach to enhance catalytic efficiency in photocatalytic CO2 reduction reaction, while it remains challenge to design dinuclear metal complexes that can show DMSC effect. The main reason is that the influence of the microenvironment around dinuclear metal centres on catalytic activity has not been well recognized and revealed. Herein, we report a dinuclear cobalt complex featuring a planar structure, which displays outstanding catalytic efficiency for photochemical CO2-to-CO conversion. The turnover number (TON) and turnover frequency (TOF) values reach as high as 14457 and 0.40 s-1 respectively, 8.6 times higher than those of the corresponding mononuclear cobalt complex. Control experiments and theoretical calculations revealed that the enhanced catalytic efficiency of the dinuclear cobalt complex is due to the indirect DMSC effect between two CoII ions, energetically feasible one step two-electron transfer process by Co2 I,I intermediate to afford Co2 II,II(CO2 2-) intermediate and fast mass transfer closely related with the planar structure.

Unveiling Phenoxazine's Unique Reversible Two-Electron Transfer Process and Stable Redox Intermediates for High-Performance Aqueous Zinc-ion Batteries.

The low specific capacity determined by the limited electron transfer of p-type cathode materials is the main obstruction to their application towards high-performance aqueous zinc-ion batteries (ZIBs). To overcome this challenge, boosting multi-electron transfer is essential for improving the charge storage capacity. Here, as a typical heteroaromatic p-type material, we unveil the unique reversible two-electron redox properties of phenoxazine in the aqueous electrolytes for the first time. The second oxidation process is stabilized in the aqueous electrolytes, a notable contrast to its less reversibility in the non-aqueous electrolytes. A comprehensive investigation of the redox chemistry mechanism demonstrates remarkably stable redox intermediates, including a stable cation radical PNO⋅+ characterized by effective electron delocalization and a closed-shell state dication PNO2+. Meanwhile, the heightened aromaticity contributes to superior structural stability during the redox process, distinguishing it from phenazine, which features a non-equivalent hybridized sp2-N motif. Leveraging these synergistic advantages, the PNO electrodes deliver a high capacity of 215 mAh g-1 compared to other p-type materials, and impressive long cycling stability with 100 % capacity retention over 3500 cycles. This work marks a crucial step forward in advanced organic electrodes based on multi-electron transfer phenoxazine moieties for high-performance aqueous ZIBs.

Designing a self-breathing electrode modulated by spatial hydrophobic microenvironments with stabilized H2O2 generation for wastewater treatment

A novel self-breathing air-diffusion rod graphite electrode (ADE-RG) engineered with spatial hydrophobic microenvironments was developed to enhance H2O2 production without external air aeration. The microenvironments, created through thermal shaking-induced turbulence and centrifugal effect, significantly improved the air-diffusion capability. This innovation led to the increase in H2O2 generation (3.36 times higher than that in the unmodified system) due to a locally enhanced electric field, increased oxygen transfer rate, and improved selectivity for two-electron transfer process. Furthermore, incorporating polyepoxysuccinic acid into the electrolyte not only prevents electrode scaling but also minimizes H2O2 decomposition, culminating in the H2O2 concentration of 50.64 ± 2.92 mg L−1 cm−2. Notably, the system performed prominently in treating industrial rifampicin wastewater, achieving a degradation efficiency of 95.29 ± 0.47%, even under high-loading conditions (initial COD was 22,898.50 ± 405.17 mg L−1). These results indicate the significant potential of the self-breathing ADE-RG system in broad-scale wastewater treatment applications.

Electrochemical investigations on cathodic reduction processes at graphite electrode in LiCl-KCl eutectic melt

Sequence of redox processes in LiCl-KCl eutectic at graphite electrode was studied in temperature range 698-798 K using cyclic voltammetry and electrochemical impedance spectroscopy. There were two reduction steps observed when potential was scanned from −0.25 to −1.50 V (vs.▪); a single-step two-electron transfer process due to reduction of carbon to ▪ followed by reduction of ▪. Electrolysis at −1.20 V at 773 K led to formation of ▪ on graphite surface. Complex impedance data were modelled using equivalent circuits and it was established that ▪ film formed around graphite electrode restricting both diffusion of ▪ in melt and metallic ▪ into graphite framework. These results are relevant in terms of their consequences in molten salt electrorefining of metal fuels.

Heteroatom-Coordinated Palladium Molecular Catalysts for Sustainable Electrochemical Production of Hydrogen Peroxide.

Currently, hydrogen peroxide (H2O2) manufacturing involves an energy-intensive anthraquinone technique that demands expensive solvent extraction and a multistep process with substantial energy consumption. In this work, we synthesized Pd-N4-CO, Pd-S4-NCO, and Pd-N2O2-C single-atom catalysts via an in situ synthesis approach involving heteroatom-rich ligands and activated carbon under mild reaction conditions. It reveals that palladium atoms interact strongly with heteroatom-rich ligands, which provide well-defined and uniform active sites for oxygen (O2) electrochemically reduced to hydrogen peroxide. Interestingly, the Pd-N4-CO electrocatalyst shows excellent performance for the electrocatalytic reduction of O2 to H2O2 via a two-electron transfer process in a base electrolyte, exhibiting a negligible amount of onset overpotential and >95% selectivity within a wide range of applied potentials. The electrocatalysts based on the activity and selectivity toward 2e- ORR follow the order Pd-N4-CO > Pd-N2O2-C > Pd-S4-NCO in agreement with the pull-push mechanism, which is the Pd center strongly coordinated with high electronegativity donor atoms (N and O atoms) and weakly coordinated with the intermediate *OOH to excellent selectivity and sustainable production of H2O2. According to density functional theory, Pd-N4 is the active site for selectivity toward H2O2 generation. This work provides an emerging technique for designing high-performance H2O2 electrosynthesis catalysts and the rational integration of several active sites for green and sustainable chemical synthesis via electrochemical processes.

Nanostructured Fe-Doped Nickel Sulfide on Ni Foam As Electrocatalyst for Oxygen Evolution Reaction

In recent years, with the increasing demand for clean and renewable energy, growing research efforts have been dedicated to the generation of hydrogen through water electrolysis [1,2]. To industrialize hydrogen production, it is important to find low-cost and high-activity electrocatalysts for hydrogen evolution reaction (HER) and, especially, oxygen evolution reaction (OER) [3]. OER is more challenging because it involves a four-electron transfer process, while HER is a two-electron transfer process. In this study, Ni3S2 nanorods were synthesized on Ni foam (NiS/Ni foam, Fig. 1a) by a new one-step solvothermal reaction, with S powder as the sulfur source and Ni foam as the nickel source. Experimental results indicated that the NiS/Ni foam electrocatalyst had an overpotential of 276 mV at 10 mA/cm2 for OER in 1 M KOH electrolyte. To boost the electrocatalytic activity of Ni3S2, we developed an effective method of doping with Fe via hydrothermal reaction. The obtained nanostructured Fe-doped Ni3S2 on Ni foam (Fe-NiS/Ni foam, see Fig. 1b) achieved OER at an extremely low overpotential of 218 mV at 10 mA/cm2. Remarkably, Fe-NiS/Ni foam exhibited high stability at an industrially-relevant high current density (500 mA/cm2 for 24h). Currently, we are studying its phase transfer process with in situ XPS. REFERENCES Jiao, S., et al., Energy & Environmental Science, 2021. 14(4): p. 1722-1770.Boppella, R., et al., Coordination Chemistry Reviews, 2021. 427.Du, J., F. Li, and L. Sun, Chem Soc Rev, 2021. 50(4): p. 2663-2695. Figure 1

Enhancing Photoredox Catalysis in Aqueous Environments: Ruthenium Aqua Complex Derivatization of Graphene Oxide and Graphite Rods for Efficient Visible-Light-Driven Hybrid Catalysts.

A ruthenium aqua photoredox catalyst has been successfully heterogeneneized on graphene oxide (GO@trans-fac-3) and graphite rods (GR@trans-fac-3) for the first time and have proven to be sustainable and easily reusable systems for the photooxidation of alcohols in water, in mild and green conditions. We report here the synthesis and total characterization of two Ru(II)-polypyridyl complexes, the chlorido trans-fac-[RuCl(bpea-pyrene)(bpy)](PF6) (trans-fac-2) and the aqua trans-fac-[Ru(bpea-pyrene)(bpy)OH2](PF6)2 (trans-fac-3), both containing the N-tridentate, 1-[bis(pyridine-2-ylmethyl)amino]methylpyrene (bpea-pyrene), and 2,2'-bipyridine (bpy) ligands. In both complexes, only a single isomer, the trans-fac, has been detected in solution and in the solid state. The aqua complex trans-fac-3 displays bielectronic redox processes in water, assigned to the Ru(IV/II) couple. The trans-fac-3 complex has been heterogenized on different types of supports, (i) on graphene oxide (GO) through π-stacking interactions between the pyrene group of the bpea-pyrene ligand and the GO and (ii) both on glassy carbon electrodes (GC) and on graphite rods (GR) through oxidative electropolymerization of the pyrene group, which yield stable heterogeneous photoredox catalysts. GO@trans-fac-3- and GR/poly trans-fac-3-modified electrodes were fully characterized by spectroscopic and electrochemical methods. Trans-fac-3 and GO@trans-fac-3 photocatalysts (without a photosensitizer) showed good catalytic efficiency in the photooxidation of alcohols in water under mild conditions and using visible light. Both photocatalysts display high selectivity values (>99%) even for primary alcohols in accordance with the presence of two-electron transfer processes (2e-/2H+). GO@trans-fac-3 keeps intact its homogeneous catalytic properties but shows an enhancement in yields. GO@trans-fac-3 can be easily recycled by filtration and reused for up to five runs without any significant loss of catalytic activity. Graphite rods (GR@trans-fac-3) were also evaluated as heterogeneous photoredox catalysts showing high turnover numbers (TON) and selectivity values.

Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene.

The structure of and bonding in two base-free terminal actinide imido metallocenes, [η5-1,2,4-(Me3C)3C5H2]2An═N(p-tolyl) (An = U (1), Th (1')) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An═N(p-tolyl) moieties is significantly larger for 1 than for 1', which makes the bonds between the [η5-1,2,4-(Me3C)3C5H2]2U2+ and [(p-tolyl)N]2- fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(L) (L = OPMe3 (6), dmap (9), PhCN (14), and 2,6-Me2PhNC (17)) with Me3PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me2PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS2, isothiocyanates, thio-ketones, ketones, lactides, and acyl nitriles, forming the four- or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p-tolylCHO, the tetranuclear complex [η5-1,2,4-(Me3C)3C5H2]4[OCH(p-tolyl)CH(p-tolyl)O]2U4O4 (10) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N,N'-diisopropylcarbodiimide (DIC), 1, and the four-membered metallaheterocycle [η5-1,2,4-(Me3C)3C5H2]2U[N(p-tolyl)C(═NiPr)N(iPr)] (12). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph3CN3, CuI, Ph2S2, and Ph2Se2, yielding the uranium(V) imido complexes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(X) (X = N3 (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN3) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η5-1,2,4-(Me3C)3C5H2]2U=N(p-tolyl)[=NN=(9-C13H8)] (21), respectively.

Distinguishing homogeneous advanced oxidation processes in bulk water from heterogeneous surface reactions in organic oxidation

Clarifying the reaction pathways at the solid-water interface and in bulk water solution is of great significance for the design of heterogeneous catalysts for selective oxidation of organic pollutants. However, achieving this goal is daunting because of the intricate interfacial reactions at the catalyst surface. Herein, we unravel the origin of the organic oxidation reactions with metal oxide catalysts, revealing that the radical-based advanced oxidation processes (AOPs) prevail in bulk water but not on the solid catalyst surfaces. We show that such differing reaction pathways widely exist in various chemical oxidation (e.g., high-valent Mn3+ and MnOX) and Fenton and Fenton-like catalytic oxidation (e.g., Fe2+ and FeOCl catalyzing H2O2, Co2+ and Co3O4 catalyzing persulfate) systems. Compared with the radical-based degradation and polymerization pathways of one-electron indirect AOP in homogeneous reactions, the heterogeneous catalysts provide unique surface properties to trigger surface-dependent coupling and polymerization pathways of a two-electron direct oxidative transfer process. These findings provide a fundamental understanding of catalytic organic oxidation processes at the solid-water interface, which could guide the design of heterogeneous nanocatalysts.

Controlling π-π Interactions of Highly Soluble Naphthalene Diimide Derivatives for Neutral pH Aqueous Redox Flow Batteries.

Organic redox-active molecules are a promising platform for designing sustainable, cheap, and safe charge carriers for redox flow batteries. However, radical formation during the electron-transfer process causes severe side reactions and reduces cyclability. This problem is mitigated by using naphthalene diimide (NDI) molecules and regulating their π-π interactions. The long-range π-stacking of NDI molecules, which leads to precipitation, is disrupted by tethering four ammonium functionalities, and the solubility approaches 1.5m in water. The gentle π-π interactions induce clustering and disassembling of the NDI molecules during the two-electron transfer processes. When the radical anion forms, the antiferromagnetic coupling develops tetramer and dimer and nullifies the radical character. In addition, short-range-order NDI clusters at 1m concentration are not precipitated but inhibit crossover. They are disassembled in the subsequent electron-transfer process, and the negatively charged NDI core strongly interacts with ammonium groups. These behaviors afford excellent RFB performance, demonstrating 98% capacity retention for 500 cycles at 25mA cm-2 and 99.5% Coulombic efficiency with 2m electron storage capacity.

Defect-stabilized and oxygen-coordinated iron single-atom sites facilitate hydrogen peroxide electrosynthesis.

The selective two-electron electrochemical oxygen reduction reaction (ORR) for hydrogen peroxide (H2O2) production is a promising and green alternative method to the current energy-intensive anthraquinone process used in industry. In this study, we develop a single-atom catalyst (CNT-D-O-Fe) by anchoring defect-stabilized and oxygen-coordinated iron atomic sites (Fe-O4) onto porous carbon nanotubes using a local etching strategy. Compared to O-doped CNTs with vacancy defects (CNT-D-O) and oxygen-coordinated Fe single-atom site modifying CNTs without a porous structure (CNT-O-Fe), CNT-D-O-Fe exhibits the highest H2O2 selectivity of 94.4% with a kinetic current density of 13.4 mA cm-2. Fe-O4 single-atom sites in the catalyst probably contribute to the intrinsic reactivity for the two-electron transfer process while vacancy defects greatly enhance the electrocatalytic stability. Theoretical calculations further support that the coordinated environment and defective moiety in CNT-D-O-Fe could efficiently optimize the adsorption strength of the *OOH intermediate over the Fe single atomic active sites. This contribution sheds light on the potential of defect-stabilized and oxygen-coordinated single-atom metal sites as a promising avenue for the rational design of highly efficient and selective catalysts towards various electrocatalytic reactions.

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Arylene Diimide Derivatives as Anolyte Materials with Two-Electron Storage for Ultrastable Neutral Aqueous Organic Redox Flow Batteries

Phenazine-Based Compound as a Universal Water-Soluble Anolyte Material for the Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are emerging energy storage technologies due to their high availability, low cost of organic compounds, and the use of eco-friendly water-based supporting electrolytes. In the present work, we demonstrate a unique phenazine-based material that shows redox reversibility in neutral, basic, and acidic conditions with the redox potentials of −0.85 V (1.0 M KOH), −0.67 V (1.0 M NaCl), −0.26 V, and 0.05 V (1.0 M H2SO4) vs. the Ag/AgCl reference electrode and two-electron transfer process at all pH values. High solubility of the phenazine compound in water-based electrolytes up to 1.3 M is achieved by introducing quaternary amonium-based substituents, leading to the outstanding theoretical volumetric capacity of 70 Ah L−1. Laboratory redox flow batteries in neutral and acidic electrolytes presented >100 cycles of stable operation with a capacity loss of 0.25 mAh L−1 and 1.29 mAh L−1 per cycle, respectively. The obtained results demonstrate a material with the potential for not only fundamental understanding but also the practical application of AORFBs in the development of new-generation energy storage technologies.

Heterogeneous Fe(IV)[dbnd]O mediated fenton-like process on Fe-Zr heterojunction for selective oxidation of organic pollutants at near neutral pH

Remarkable heterogeneous Fe(IV)O (≡Fe(IV)O) mediated Fenton-like process on (1 1 0)-(1 0 0) heterojunction of Fe-Zr bimetallic oxide was proposed in this work for selective oxidation of organic pollutants under near-neutral conditions. The formation of heterojunction shortened the Fe-O bond length by 0.14 Å and increased the binding energy of Fe 2p3/2 orbital by 0.8 eV on Fe-Zr (1 1 0)-(1 0 0) interface, compared with that on Fe2O3 (1 1 0) facet. This will promote the cyclic complexes of Fe(II)-[OOH]+ formation on Fe-Zr (1 1 0)-(1 0 0) interface. Besides, energy barrier of Fe(II)-[OOH]+ on (1 1 0)-(1 0 0) heterojunction was 10.6 kJ mol−1 lower than that on Fe2O3 (1 1 0) facet. It will facilitate the inner two-electron transfer process to generate Fe(IV)O. And electrostatic repulsion and electrophilic addition enable Fe(IV)O to selectively oxidize target organic pollutants in the system containing ions and humic acids. This work provides a fundamental information toward the structure-performance relationship between heterojunction structure and Fe(IV)O formation, and it will enlighten the selective oxidation of pollutants in wastewater treatment.

Direct electroseparation of zinc from zinc sulfide in eco-friendly deep eutectic solvent: Highlighting the role of malonic acid

Zinc sulfide (ZnS), one of the main ingredients of zinc ore and emerging materials, partly processes environmental risk. Deep eutectic solvents (DESs) are green solvents to efficiently separate metals with less emission. In this study, a novel route for direct electroseparation of Zn from ZnS in eco-friendly choline chloride-urea DES with the addition of malonic acid (MA) is studied for the first time. The role of MA and the reduction behavior of Zn(II) are systematically analyzed by cyclic voltammetry. The reduction of Zn(II) is quasi-reversible and follows one-step two-electron transfer process. Pure Zn with nanostructure is obtained with the current efficiency of 86.5 % and energy consumption of 2621.8 kW·h·t−1 at the optimized parameters of 10 mA·cm−2, 353 K and 100 mM·L-1 ZnS. Mechanisms analyses indicate that in the presence of 60 mM·L-1 MA, besides [ZnCl−3]- and [ZnCl−5]3-, some ZnS are dissolved to form stable [Zn(MA)2-Cl−2]4-, [Zn(MA)2-Cl-]3- and other MA based ligands, which causes a higher Zn ion solubility of 0.13 mol·L-1 and may promote the subsequent electroseparation of ZnS. Results from this study are expected to propose a simplified route to directly separate Zn from ZnS without oxidation pretreatment to achieve hazardous waste minimization.