Atomically Dispersed Semimetallic Selenium on Porous Carbon Membrane as an Electrode for Hydrazine Fuel Cells

Poly(ionic liquid)/Wood Composite-Derived B/N-Codoped Porous Carbons Possessing Peroxidase-like Catalytic Activity

The pursuit of efficient and cost-effective metal-free heterogeneous catalytic systems remains a challenging task in materials research. Heteroatom-doped carbonaceous materials are increasingly recognized as powerful metal-free catalysts, often demonstrating catalytic performance comparable to or even surpassing metal-based alternatives. This is attributed to their tunable physicochemical properties, tailorable structural features, and environmentally friendly profile. In a straightforward single-step synthetic approach, we utilized wood as an eco-friendly and renewable carbon source, in conjunction with a poly(ionic liquid) as a heteroatom source and pore-making agent. The combination of both biobased and synthetic polymers in this method yielded sustainable, high-performance catalysts characterized by enhanced stability and reusability. The inclusion of sacrificial pore-inducing templates resulted in the formation of abundant defects serving as catalytically active sites, while codoping with boron and nitrogen further enhanced these sites, significantly impacting catalytic activities, as established by peroxidase-like activity in this study. The optimized codoped porous carbon membrane exhibited excellent peroxidase-type activity and catalyzed the oxidation reaction of 3,3',5,5'-tetramethylbenzidine by hydrogen peroxide. This high activity was largely due to the dual heteroatom codoping effect and the mixed micro/macroporous structure of the membrane. Our work presents a versatile and eco-friendly method for fabricating hierarchically porous B/N codoped carbon membranes, offering a manageable, convenient, and recyclable biomimetic artificial enzyme with superior catalytic capabilities. This work introduces a practical and robust colorimetric method that can be used in healthcare and environmental rehabilitation.

Understanding Synergistic Catalysis on Cu-Se Dual Atom Sites via Operando X-ray Absorption Spectroscopy in Oxygen Reduction Reaction

The construction and understanding of synergy in well-defined dual-atom active sites is an available avenue to promote multistep tandem catalytic reactions. Herein, we construct a dual-hetero-atom catalyst that comprises adjacent Cu-N₄ and Se-C₃ active sites for efficient oxygen reduction reaction (ORR) activity. Operando X-ray absorption spectroscopy coupled with theoretical calculations provide in-depth insights into this dual-atom synergy mechanism for ORR under realistic device operation conditions. The heteroatom Se modulator can efficiently polarize the charge distribution around symmetrical Cu-N₄ moieties, and serve as synergistic site to facilitate the second oxygen reduction step simultaneously, in which the key OOH*-(Cu₁ -N₄ ) transforms to O*-(Se₁ -C₂ ) intermediate on the dual-atom sites. Therefore, this designed catalyst achieves satisfied alkaline ORR activity with a half-wave potential of 0.905 V vs. RHE and a maximum power density of 206.5 mW cm^-2 in Zn-air battery.

Progress in Hydrogen Production Coupled with Electrochemical Oxidation of Small Molecules

The electrochemical oxidation of small molecules to generate value-added products has gained enormous interest in recent years because of the advantages of benign operation conditions, high conversion efficiency and selectivity, the absence of external oxidizing agents, and eco-friendliness. Coupling the electrochemical oxidation of small molecules to replace oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode in an electrolyzer would simultaneously realize the generation of high-value chemicals or pollutant degradation and the highly efficient production of hydrogen. This Minireview presents an introduction on small-molecule choice and design strategies of electrocatalysts as well as recent breakthroughs achieved in the highly efficient production of hydrogen. Finally, challenges and future orientations are highlighted.

Green Orange Peel-Mediated Bioinspired Synthesis of Nanoselenium and Its Antibacterial Activity against Methicillin-Resistant Staphylococcus aureus

In this study, green orange peel (GOP) was feasibly evidenced in preparing selenium nanoparticles (SeNPs). Acting as reducing agents, polyphenolic compounds were extracted from GOP at the optimal extraction conditions (at 70 °C for 1.5 h, mass ratio of dried orange peel/distilled water of 5/100). The formation of SeNPs was observed at the wavelength range of 250-300 nm by ultraviolet-visible spectroscopy (UV-vis), and their highest yield could be reached at the following conditions: volume ratio of extract/selenious acid solution (V _Ext/V _Se) of 40/10, synthesis duration of 4 h, selenious acid concentration (C _Se) of 80 mM, and reaction temperature of 120 °C. The highly crystalline structure of SeNPs in the hexagonal phase was characterized by powder X-ray diffraction (XRD) with a lattice parameter of 4.3 Å; meanwhile, their spheres with an average crystal size of 18.3 nm were estimated by high-resolution transmission electron microscope (HR-TEM). The rationale of bioreducing agents extracted from green orange peel for the formation of SeNPs was also recognized by Fourier-transform infrared spectroscopy (FT-IR). The antibacterial investigation of the SeNP sample was assessed against antibiotic-resistant bacteria, typically methicillin-resistant Staphylococcus aureus (MRSA), by executing the zone of inhibition and the minimum inhibitory concentration (MIC) tests. The SeNP sample demonstrated excellent antibacterial activity with an average diameter of inhibition zones of 20.0 ± 0.7 mm and an MIC of 4.94 μg/L. A comparison of the physicochemical properties of SeNPs synthesized from GOP extract by the hydrothermal method with SeNP products from other green reducing agents and other methods as well as its antibacterial activity compared with other nanoparticles and some antibiotics was conducted to highlight the superiority of GOP-mediated green-synthesized SeNPs.

Understanding hydrazine oxidation electrocatalysis on undoped carbon

Carbons are ubiquitous electrocatalytic supports for various energy-related transformations, especially in fuel cells. Doped carbons such as Fe-N-C materials are particularly active towards the oxidation of hydrazine, an alternative fuel and hydrogen carrier. However, there is little discussion of the electrocatalytic role of the most abundant component - the carbon matrix - towards the hydrazine oxidation reaction (HzOR). We present a systematic investigation of undoped graphitic carbons towards the HzOR in alkaline electrolyte. Using highly oriented pyrolytic graphite electrodes, as well as graphite powders enriched in either basal planes or edge defects, we demonstrate that edge defects are the most active catalytic sites during hydrazine oxidation electrocatalysis. Theoretical DFT calculations support and explain the mechanism of HzOR on carbon edges, identifying unsaturated graphene armchair defects as the most likely active sites. Finally, these findings explain the 'double peak' voltammetric feature observed on many doped carbons during the HzOR.

Engineering the Local Atomic Environments of Indium Single-Atom Catalysts for Efficient Electrochemical Production of Hydrogen Peroxide

The in-depth understanding of local atomic environment-property relationships of p-block metal single-atom catalysts toward the 2 e^- oxygen reduction reaction (ORR) has rarely been reported. Here, guided by first-principles calculations, we develop a heteroatom-modified In-based metal-organic framework-assisted approach to accurately synthesize an optimal catalyst, in which single In atoms are anchored by combined N,S-dual first coordination and B second coordination supported by the hollow carbon rods (In SAs/NSBC). The In SAs/NSBC catalyst exhibits a high H₂ O₂ selectivity of above 95 % in a wide range of pH. Furthermore, the In SAs/NSBC-modified natural air diffusion electrode exhibits an unprecedented production rate of 6.49 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M KOH electrolyte and 6.71 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M PBS electrolyte. This strategy enables the design of next-generation high-performance single-atom materials, and provides practical guidance for H₂ O₂ electrosynthesis.

Atomically Dispersed Selenium Sites on Nitrogen-Doped Carbon for Efficient Electrocatalytic Oxygen Reduction

Owing to their unique electronic structure and maximum atom utilization efficiency, single-atom catalysts have received widespread attention and exhibited efficient activity. Herein, we report the preparation of non-metal Se single atoms embedded in nitrogen-doped carbon (NC) via a high-temperature reduction strategy for electrocatalytic oxygen reduction reaction (ORR). Selenium dioxide is reduced to selenium by NC at high temperature and partially anchored to form C-Se-C bond. Impressively, the obtained single-atom catalyst exhibits outstanding ORR activity and stability that even surpasses state-of-the-art noble metal catalysts and many previously reported nanocatalysts. Experimental and theoretical calculations reveal that the Se single atoms can serve as the ORR active sites and contribute to lowering the reaction barrier. Our discoveries demonstrate the promising prospects for utilizing metal-free single-atom-based materials for efficient electrocatalysis.

P-Block Atomically Dispersed Antimony Catalyst for Highly Efficient Oxygen Reduction Reaction

Main-group (s- and p-block) metals are generally regarded as catalytically inactive due to the delocalized s/p-band. Herein, we successfully synthesized a p-block antimony single-atom catalyst (Sb SAC) with the Sb-N₄ configuration for efficient catalysis of the oxygen reduction reaction (ORR). The obtained Sb SAC exhibits superior ORR activity with a half-wave potential of 0.86 V and excellent stability, which outperforms most transition-metal (TM, d-block) based SACs and commercial Pt/C. In addition, it presents an excellent power density of 184.6 mW cm^-2 and a high specific capacity (803.5 mAh g^-1 ) in Zn-air battery. Both experiment and theoretical calculation manifest that the active catalytic sites are positively charged Sb-N₄ single-metal sites, which have closed d shells. Density of states (DOS) results unveil the p orbital of the atomically dispersed Sb cation in Sb SAC can easily interact with O₂ -p orbital to form hybrid states, facilitating the charge transfer and generating appropriate adsorption strength for oxygen intermediates, lowering the energy barrier and modulating the rate-determining step. This work sheds light on the atomic-level preparing p-block Sb metal catalyst for highly active ORR, and further provides valuable guidelines for the rational design of other main-group-metal SACs.

The Electro-Oxidation of Hydrazine: A Self-Inhibiting Reaction

The electro-oxidation of hydrazine to form dinitrogen is reported over a wide range of both pH and unbuffered conditions at glassy carbon electrodes. It is shown that hydrazine molecules are only electro-active in their unprotonated form, N₂H₄, whereas the protonated species N₂H₅⁺ is electro-inactive. The oxidation of N₂H₄ releases four protons per molecule which are diffusing away from the electrode to rapidly (on the voltammetric time scale) protonate unreacted N₂H₄ molecules diffusing to the electrode converting them into the electro-inactive form, N₂H₅⁺; the reaction is self-inhibiting, and the currents flowing are significantly reduced compared to those expected for a simple electrolytic conversion to an extent reflecting the pH and buffer content of the solution local to the electrode. The local pH in turn is controlled partly by the quantity of protons released electrolytically. The self-inhibition is modeled by solving the relevant transport equations with coupled homogeneous chemical kinetics, utilizing Marcus-Hush electron transfer, giving predicted reduced currents reflecting the pK_a and kinetics of the N₂H₄/N₂H₅⁺ equilibrium in excellent agreement with experimental voltammetric wave shapes.

Mimicking Hydrazine Dehydrogenase for Efficient Electrocatalytic Oxidation of N2H4 by Fe-NC

Pursuing nonprecious doped carbon with Pt-like electrocatalytic N₂H₄ oxidation activity for hydrazine fuel cells (HzFCs) remains a challenge. Herein, we present a Fe/N-doped carbon (Fe-NC) catalyst with mesopore-rich channel and highly dispersed Fe-N sites incorporated in N-doped carbon, as an analogue of hydrazine dehydrogenase (HDH), showing the structure-dependent activity for electrocatalytic oxidation of N₂H₄. The maximal turnover frequency of the N₂H₄ oxidation reaction (HzOR) over the Fe-N sites (62870 h^-1) is 149-fold that over the pyridinic-N sites of N-doped carbon. The Fe mass activity of HzOR and maximal power density of HzFCs driven by Fe-NC approximately surpass those of Pt/C by 2.3 and 2.2 times, respectively. Theoretical calculation reveals that the Fe-N sites improve the dehydrogenation process of HzOR-related intermediates. One of the roles of the mesoporous structure in Fe-NC resembles that of a substrate channel in HDH for enhancing the transport of N₂H₄ besides exposing Fe-N sites and improving storage capacity of HzOR-related species.

Porous Carbon Membrane-Supported Atomically Dispersed Pyrrole-Type FeN4 as Active Sites for Electrochemical Hydrazine Oxidation Reaction

The rational design of catalytically active sites in porous materials is essential in electrocatalysis. Herein, atomically dispersed Fe-N_x sites supported by hierarchically porous carbon membranes are designed to electrocatalyze the hydrazine oxidation reaction (HzOR), one of the key techniques in electrochemical nitrogen transformation. The high intrinsic catalytic activity of the Fe-N_x single-atom catalyst together with the uniquely mixed micro-/macroporous membrane support positions such an electrode among the best-known heteroatom-based carbon anodes for hydrazine fuel cells. Combined with advanced characterization techniques, electrochemical probe experiments, and density functional theory calculation, the pyrrole-type FeN₄ structure is identified as the real catalytic site in HzOR.