Phosphonate-derived nitrogen-doped cobalt phosphate/carbon nanotube hybrids as highly active oxygen reduction reaction electrocatalysts

Breaking the symmetry and d-orbital optimization at Co site in CoNC as bifunctional air catalysts for rechargeable liquid and flexible solid-state Zn-air batteries

By utilizing abundant earth metals and incorporating them into N-doped carbon electrocatalysts, the electrochemical kinetics and stability of oxygen reactions in zinc-air batteries (ZABs) are enhanced. However, several challenges remain. We introduce a method that focuses on microenvironmental modulation to precisely adjust the Cr-doped Co NC (Cr-Co NC) catalyst, thereby enhancing its inherent electrochemical activity and durability, and improving the oxygen reaction process. The unique Cr-N-Co configuration in the Cr-CoNC-1.00 catalyst weakens the adsorption strength of *OH intermediates by engineering the Co d-band center, thus lowering the energy barrier for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). The precisely engineered Cr-CoNC-1.00 catalyst demonstrates robust ORR and OER performance, achieving an ORR half-wave potential (E1/2) of 0.865 V and an OER overpotential (EJ=10) of 1.64 V (vs RHE), rivaling that of noble-metal catalysts (Pt/C for ORR and RuO2 for OER). In practical applications, the rechargeable liquid ZABs equipped with Cr-CoNC-1.00 delivered exceptional results (peak power density: 110 mW·cm-2, specific capacity: 816 mA·h·g-1 Zn at 10 mA·cm-2, with over 208 h of charge-discharge cycle stability). Additionally, the flexible solid-state ZABs achieved an open-circuit voltage of 1.4 V, demonstrated remarkable charge-discharge stability for over 12 h, and maintained performance under various bending conditions. This approach highlights the significant potential for developing high-efficiency bifunctional catalysts suitable for flexible zinc-air batteries.

Hierarchically Ordered Macro-/Mesoporous N,P-Codoped Carbon with Fe-Co Dual Sites for Efficient Electrocatalytic Oxygen Reduction

The development of high-performance, low-cost non-precious-metal electrocatalysts as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for promoting the commercial application of fuel cells and metal-air batteries. In this work, we report a novel type of ORR electrocatalyst with Fe and Co sites anchored on N,P-codoped hierarchically ordered macro-/mesoporous carbon (FeCo/NP-HOMMC) through a facile one-pot, controllable synthesis method with the aid of dual-templating technique. The FeCo/NP-HOMMC catalyst shows robust ORR performance under alkaline conditions with a half-wave potential (E1/2) of 0.90 V vs. reversible hydrogen electrode (RHE), significantly surpassing the commercial 20 wt% Pt/C catalyst, while also exhibiting remarkable long-term stability and great methanol tolerance. Control experiments reveal that the superior performance of the FeCo/NP-HOMMC catalyst for ORR benefits from the synergistic catalysis of highly dispersed Fe and Co dual active sites, and the advantages of the unique hierarchically ordered macro-/mesoporous structure, which can provide improved active site accessibility, excellent conductivity, and maximized mass/charge transport.

Construction of nanosensor based on cobalt phosphate-doped biochar for derivative voltammetric analysis of hydroquinone in environmental samples

A nanosensing platform was designed based on environmentally friendly cobalt phosphate-doped biochar (CoCPBC) derived from bamboo leaves for derivative voltammetric determination of hydroquinone  (HQ) in water and soil samples. Highly ambient-stable CoCPBC is prepared by hydrothermal synthesis containing cobalt nitrate, phosphoric acid (H3PO4), cetyltrimethylammonium ammonium bromide (CTAB), and bamboo leaves. Benefiting from the high effective area, the CoCPBC displays more active sites for HQ recognition and simultaneously improving the conductivity of biochar. The CoCPBC electrode shows excellent stability with retaining 99.34% of its initial current after 100   cyclic voltammetric scans. The CoCPBC nanosensor, enhanced by signal amplification, enables the determination of HQ in the concentration range 0.1-500 μM, with a detection limit of 0.052 μM. It exhibits high selectivity and excellent practicability, utilizing differential pulse voltammetry and first derivative voltammetry for detection. This work will provide an innovative method for the preparation of functional biochars from agroforestry wastes and their applications in the development of electrochemical nanosensing platforms. HIGHLIGHTS: 1. A green and sustainable method to convert forestry waste into biochar using bamboo leaves as carbon source; 2. The acid activation and surfactant pore-expanding increase specific surface area and electrochemically-active sites; 3. Co-doping enhanced the conductivity and electrocatalytic activity of biochar. 4. Realization of derivative voltammetric determination of hydroquinone.

An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production

At present, the energy shortage and environmental pollution are the burning global issues. For centuries, fossil fuels have been used to meet worldwide energy demand. However, thousands of tons of greenhouse gases are released into the atmosphere when fossil fuels are burned, contributing to global warming. Therefore, green energy must replace fossil fuels, and hydrogen is a prime choice. Photocatalytic water splitting (PWS) under solar irradiation could address energy and environmental problems. In the past decade, solar photocatalysts have been used to manufacture sustainable fuels. Scientists are working to synthesize a reliable, affordable, and light-efficient photocatalyst. Developing efficient photocatalysts for water redox reactions in suspension is a key to solar energy conversion. Semiconductor nanoparticles can be used as photocatalysts to accelerate redox reactions to generate chemical fuel or electricity. Carbon materials are substantial photocatalysts for total WS under solar irradiation due to their high activity, high stability, low cost, easy production, and structural diversity. Carbon-based materials such as graphene, graphene oxide, graphitic carbon nitride, fullerenes, carbon nanotubes, and carbon quantum dots can be used as semiconductors, photosensitizers, cocatalysts, and support materials. This review comprehensively explains how carbon-based composite materials function as photocatalytic semiconductors for hydrogen production, the water-splitting mechanism, and the chemistry of redox reactions. Also, how heteroatom doping, defects and surface functionalities, etc., can influence the efficiency of carbon photocatalysts in H2 production. The challenges faced in the PWS process and future prospects are briefly discussed.

Secondary-Heteroatom-Doping-Derived Synthesis of N, S Co-Doped Graphene Nanoribbons for Enhanced Oxygen Reduction Activity

The rareness and weak durability of Pt-based electrocatalysts for oxygen reduction reactions (ORRs) have hindered the large-scale application of fuel cells. Here, we developed an efficient metal-free catalyst consisting of N, S co-doped graphene nanoribbons (N, S-GNR-2s) for ORRs. GNRs were firstly synthesized via the chemical unzipping of carbon nanotubes, and then N, S co-doping was conducted using urea as the primary and sulfourea as the secondary heteroatom sources. The successful incorporation of nitrogen and sulfur was confirmed by elemental mapping analysis as well as X-ray photoelectron spectroscopy. Electrochemical testing revealed that N, S-GNR-2s exhibited an E_onset of 0.89 V, E_1/2 of 0.79 V and an average electron transfer number of 3.72, as well as good stability and methanol tolerance. As a result, N, S-GNR-2s displayed better ORR property than either N-GNRs or N, S-GNRs, the control samples prepared with only a primary heteroatom source, strongly clarifying the significance of secondary-heteroatom-doping on enhancing the catalytic activity of carbon-based nanomaterials.

N, P-doped carbon supported ruthenium doped Rhenium phosphide with porous nanostructure for hydrogen evolution reaction using sustainable energies

Developing efficient and cost-effective catalysts for hydrogen evolution reaction (HER) is vital to hydrogen energy's commercial applications. In this study, N,P-doped carbon supported ruthenium (Ru) doped triruthenium tetraphosphide (Re₃P₄) (Ru-Re₃P₄/NPC) with porous nanostructure is prepared using the low-toxic melamine phosphate as the carbon and phosphorous source. The in-situ generated N,P-doped carbon layers play a pivotal role in regulating the electrocatalytic activity by avoiding the aggregation of the nanoparticles and increasing the specific surface area. Moreover, Ru doping contributes to the remarkable electrocatalytic performance of the prepared nanomaterials. Impressively, the as-synthesized Ru-Re₃P₄/NPC presents remarkable electrocatalytic performances toward HER with small overpotentials of 39 mV, 115 mV, and 88 mV to deliver 10 mA cm^-2 in alkaline, neutral, and acidic media. Moreover, the prepared electrocatalyst can drive water-splitting with a small potential of 1.45 V@10 mA cm^-2 and use sustainable energies, including solar, wind, and thermal, as electric resources. This work paves a novel and valuable way to enhance the electrocatalytic performances of metal phosphides.

The synergistic effect of carbon edges and dopants towards efficient oxygen reduction reaction

Decoration with alien atoms and increasing the edge content are two valid ways to activate the oxygen reduction reaction (ORR) property of nanocarbons. To further enhance their intrinsic activity and explore the underlying ORR mechanism, graphene nanoribbons (GNRs) were selected as an ideal catalyst model. Theoretical simulations have predicted that with the synergistic effect between heteroatom-doping and edge sites, the ORR activity can be significantly improved. Inspired by this, N-GNRs were synthesized via the oxidative unzipping of CNTs followed by nitrogen incorporation with urea. Ample edges and nitrogen doping sites were detected by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. As a result, N-GNRs exhibited remarkably higher ORR properties in terms of onset and half-wave potentials, Tafel slopes, electron transfer number and methanol tolerance than either GNRs, the control sample without doping, or N-CNTs, the control sample without abundant edges, simply clarifying the significance of synergy between dopants and edges. Thus, this work provides a simple but efficient strategy to fabricate high-performance oxygen reduction catalysts.

Nanoporous Metal Phosphonate Hybrid Materials as a Novel Platform for Emerging Applications: A Critical Review

Nanoporous metal phosphonates are propelling the rapid development of emerging energy storage, catalysis, environmental intervention, and biology, the performances of which touch many fundamental aspects of portable electronics, convenient transportation, and sustainable energy conversion systems. Recent years have witnessed tremendous research breakthroughs in these fields in terms of the fascinating pore properties, the structural periodicity, and versatile skeletons of porous metal phosphonates. This review presents recent milestones of porous metal phosphonate research, from the diversified synthesis strategies for controllable pore structures, to several important applications including adsorption and separation, energy conversion and storage, heterogeneous catalysis, membrane engineering, and biomaterials. Highlights of porous structure design for metal phosphonates are described throughout the review and the current challenges and perspectives for future research in this field are discussed at the end. The aim is to provide some guidance for the rational preparation of porous metal phosphonate materials and promote further applications to meet the urgent demands in emerging applications.

CdS Nanorods Anchored with Crystalline FeP Nanoparticles for Efficient Photocatalytic Formic Acid Dehydrogenation

Photocatalytic dehydrogenation of formic acid is a promising strategy for H₂ generation. In this work, we report the use of crystalline iron phosphide (FeP) nanoparticles as an efficient and robust cocatalyst on CdS nanorods (FeP@CdS) for highly efficient photocatalytic formic acid dehydrogenation. The optimal H₂ evolution rate can reach ∼556 μmol·h^-1 at pH 3.5, which is more than 37 times higher than that of bare CdS. Moreover, the photocatalyst demonstrates excellent stability; no significant decrease of the catalytic activity was observed during continuous testing for more than four days. The apparent quantum yield is ∼54% at 420 nm, which is among the highest values obtained using noble-metal-free photocatalysts for formic acid dehydrogenation. This work provides a novel strategy for designing highly efficient and economically viable photocatalysts for formic acid dehydrogenation.

Synthesis and electrochemical properties of metal(ii)-carboxyethylphenylphosphinates

We report herein the synthesis, structural characterization and electrocatalytic properties of three new coordination polymers, resulting from the combination of divalent metal (Ca²⁺, Cd²⁺ or Co²⁺) salts with (2-carboxyethyl)(phenyl)phosphinic acid. In addition to the usual hydrothermal procedure, the Co²⁺ derivative could also be prepared by microwave-assisted synthesis, in much shorter times. The crystal structures were solved by ab initio calculations, from powder diffraction data. Compounds M^II[O₂P(CH₂CH₂COOH)(C₆H₅)]₂ {M = Cd (1) or Ca (2)} crystallize in the monoclinic system and display a layered topology, with the phenyl groups pointing toward the interlayer space in a interdigitated fashion. Compound Co₂[(O₂P(CH₂CH₂COO)(C₆H₅)(H₂O)]₂·2H₂O (3) presents a 1D structure composed of zig-zag chains, formed by edge-sharing cobalt octahedra, with the phenyl groups pointing outside. Packing of these chains is favored by hydrogen bond interactions via lattice water molecules. In addition, H-bonds along the chains are established with the participation of the water molecules and the hydrophilic groups from the ligand. However, the solid exhibits a low proton conductivity, attributed to the isolation of the hydrophilic regions caused by the arrangement of hydrophobic phenyl groups. Preliminary studies on the electrocatalytic performance for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have been conducted for compound 3 and its pyrolytic derivatives, which were previously thoroughly characterized. By comparison, another Co²⁺ phosphinate, 4, obtained by microwave-assisted synthesis, but with distinct stoichiometry and a known structure was also tested. For the OER, the best performance was achieved with a derivative of 3, prepared by heating this compound in N₂ at 200 °C. This derivative showed overpotential (339 mV, at a current density of 10 mA cm^-2) and Tafel slope (51.7 mV dec^-1) values comparable to those of other Co²⁺ related materials.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

Design Strategies of Transition-Metal Phosphate and Phosphonate Electrocatalysts for Energy-Related Reactions

The key challenge to developing renewable energy conversion and storage devices lies in the exploration and rational engineering of cost-effective and highly efficient electrocatalysts for various energy-related electrochemical reactions. Transition-metal phosphates and phosphonates have shown remarkable performances for these reactions based on their unique physicochemical properties. Compared with transition-metal oxides, phosphate groups in transition-metal phosphates and phosphonates show flexible coordination with diverse orientations, making them an ideal platform for designing active electrocatalysts. Although numerous efforts have been spent on the development of transition-metal phosphate and phosphonate electrocatalysts, some urgent issues, such as low intrinsic catalytic efficiency and low electronic conductivity, have to be resolved in accordance with their applications. In this Review, we focus on the design strategies of highly efficient transition-metal phosphate and phosphonate electrocatalysts, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and construction of heterostructures. Guided by these strategies, recently developed transition-metal phosphate and phosphonate materials have exhibited excellent activity, selectivity, and stability for various energy-related electrocatalytic reactions, showing great potential for replacing noble-metal-based catalysts in next-generation advanced energy techniques. The existing challenges and prospects regarding these materials are also presented.