Recent Advances in Carbon-Based Metal-Free Electrocatalysts

Ternary Heteroatom-Doped Carbon As a High-Performance Metal-Free Catalyst for Electrochemical Ammonia Synthesis

The electrochemical nitrogen reduction reaction (NRR) has garnered much attention, but the major challenge remains with efficient electrocatalysts. Metal-free carbonaceous materials, doped with heteroatoms and structural defects, present a promising alternative to metal-based catalysts. This study introduces a novel strategic stepwise synthesis strategy of defective nitrogen-doped carbon material, further doped with secondary heteroatoms boron and fluorine (FBDG). These secondary atoms in combination create additional active sites for nitrogen adsorption and activation and suppress the hydrogen evolution reaction (HER). The synergistic effect of three heteroatoms and induced defects in the catalyst enhances electron-donor behavior, improving π bonding within the carbon framework and facilitating the electron transfer processes during NRR, resulting in a significantly high Faradaic efficiency of 38.1% in the case of metal-free electrocatalysts. The theoretical calculation reveals that FBDG possesses a sufficient charge density to reduce nitrogen at a low overpotential following an alternating free energy pathway. The reaction intermediates are thereby identified by in situ ATR-FTIR studies. For the rapid screening of ammonia, we used a rotating ring disk system (RRDE) and did a kinetic study. The high efficiency, stability, and cost-effectiveness of FBDG position it as a strong contender for sustainable ammonia production and pave the way for future advancements in NRR.

A Review on Electrochemical Water Splitting Electrocatalysts for Green H2 Production: Unveiling the Fundamentals and Recent Advances

Green H2 production via electrochemical water splitting has emerged as a pivotal solution for achieving a sustainable energy future. This Review delves into the fundamentals of water splitting, focusing on the O2 evolution reaction (OER) and H2 evolution reaction (HER), and focuses on the critical role of electrocatalysts in these processes. Precious metals such as paltinum and iridium remain the benchmarks for catalytic performance; however, their scarcity and high cost necessitate the development of alternative materials. Recent advances in Earth-abundant catalysts, including transition-metal oxides, carbides, nitrides, and sulfides, have shown promise in balancing activity, durability, and affordability. The integration of nanostructuring techniques and computational modeling has enabled the design of catalysts with enhanced active site exposure and electronic properties. Furthermore, the Review highlights challenges such as material degradation, high overpotentials, and gas crossover, along with potential solutions like protective coatings, bifunctional catalysts, and advanced electrolyzer designs. Future prospects emphasize the role of artificial intelligence, hybrid systems, and sustainable manufacturing in accelerating progress. This comprehensive review underscores the significance of bridging fundamental research with technological innovations to scale up green hydrogen production, addressing energy demands while mitigating environmental impacts.

Constructing Pentagonal Topological Defects in Carbon Aerogels for Flexible Zinc-Air Batteries

In the context of energy conversion, the design and synthesis of high-performance metal-free carbon electrocatalysts for the oxygen reduction reaction (ORR) is crucial. Herein, a one-step nitrogen doping/extraction strategy is proposed to fabricate 3D nitrogen-doped carbon aerogels (NCA-Cl) with rich pentagonal carbon topological defects. The NCA-Cl electrocatalyst exhibits superb ORR activity, displaying a half-wave potential of 0.89 V vs RHE and 0.74 V vs RHE under alkaline (0.1 m KOH) and acidic (0.1 m HClO4) media, respectively, thanks to the balanced *OOH intermediate adsorption and desorption induced by the pentagonal carbon topological defects and nitrogen dopants. The aqueous zinc-air battery (ZAB) equipped with the NCA-Cl cathode delivers a peak power density of 206.6 mW cm-2, a specific capacity of 810.6 mAh g-1, and a durability of 400 h, and the flexible ZAB also performed convincingly. This work provides an effective strategy for the formation of topological carbon defects for the enhancement of the electrocatalytic activity of carbon-based catalysts.

A Deep Dive Into the Study of Nitrogen-Doped Carbons as Electrocatalysts for the Oxygen Reduction Reaction via Design of Experiments

A design of experiments (DoE) approach is applied to the study of nitrogen (N)-doped carbons prepared via a molten salt templating method using the eutectic salt lithium chloride/potassium chloride (LiCl/KCl) and the precursors sucrose and melamine (N precursor). This approach is used to deconvolute effects from surface composition and porosity on the electrocatalytic performance of N-doped carbons as oxygen reduction reaction (ORR) electrocatalysts. Additionally, DoE is implemented to reveal the synthesis-structure-function relationship for the prepared materials over an entire design space. From this work, it is evident that the N precursor content has the greatest impact on the tunability of material properties (e.g., N-content, pyridinic N content, surface area, pore size distribution, etc.) followed by pyrolysis temperature and salt mass. Additionally, without adequate porosity (surface area ≥ 500 m2 g-1, micropore volume > 0.15 cc g-1, etc.) and electrochemically active surface area, activity and selectivity for the ORR via N-functionalization is significantly reduced. Optimization of the studied design space indicates that an N precursor content of 35 wt.%-38 wt.%, pyrolysis temperature ≤ 900 °C, and a salt mass < 15 g would garner the necessary N-content (∼7-8 at%) and porosity to achieve the most active and selective N-doped carbon ORR electrocatalysts.

Tailoring Electrocatalytic Properties of sp2-Bonded Carbon Nanoforms Through Doping

The symmetry of the valence and conduction bands in graphene and carbon nanotubes allows for easy modification of the electronic structure, which is correlated with their electrocatalytic activity. Modifying the electronic structure of the sp2-bonded nanocarbons by substituting carbon atoms with electron donors/acceptors and through covalent functionalization can facilitate heterogeneous electron transfer (HET), which is beneficial for designing carbon-based, high-performance electrocatalysts. Based on the Gerischer-Marcus model, we discuss how we can match the density of π-electron states (DOS) of a nanocarbon electrode to the redox potential of redox species using electron and hole doping. Along with the results, this article provides guidance on how to match the properties of nanocarbons to specific electroactive analytes, oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER).

Emerging Nitrogen and Sulfur Co-doped Carbon Materials for Electrochemical Energy Storage and Conversion

The growing global energy demands, coupled with the imperative for sustainable environmental challenges, have sparked significant interest in electrochemical energy storage and conversion (EESC) technologies. Metal-free heteroatom-doped carbon materials, especially those codoped with nitrogen (N) and sulfur (S), have gained prominence due to their exceptional conductivity, large specific surface area, remarkable chemical stability, and enhanced electrochemical performance. The strategic incorporation of N and S atoms into the carbon framework plays a pivotal role in modulating electron distribution and creating catalytically active sites, thereby significantly enhancing the EESC performance. This review examines the key synthetic strategies for fabricating N, S codoped carbon materials (NSDCMs) and provides a comprehensive overview of recent advancements in NSDCMs for EESC applications. These encompass various electrochemical energy storage systems such as supercapacitors, alkali-ion batteries, and lithium-sulfur batteries. Energy conversion processes, including hydrogen evolution, oxygen reduction/evolution, and carbon dioxide reduction are also covered. Finally, future research directions for NSDCMs are discussed in the EESC field, aiming to highlight their promising potential and multifunctional capabilities in driving further advancements in electrochemical energy systems.

Multiscale Understanding of Anion Exchange Membrane Fuel Cells: Mechanisms, Electrocatalysts, Polymers, and Cell Management

Anion exchange membrane fuel cells (AEMFCs) are among the most promising sustainable electrochemical technologies to help solve energy challenges. Compared to proton exchange membrane fuel cells (PEMFCs), AEMFCs offer a broader choice of catalyst materials and a less corrosive operating environment for the bipolar plates and the membrane. This can lead to potentially lower costs and longer operational life than PEMFCs. These significant advantages have made AEMFCs highly competitive in the future fuel cell market, particularly after advancements in developing non-platinum-group-metal anode electrocatalysts, anion exchange membranes and ionomers, and in understanding the relationships between cell operating conditions and mass transport in AEMFCs. This review aims to compile recent literature to provide a comprehensive understanding of AEMFCs in three key areas: i) the mechanisms of the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in alkaline media; ii) recent advancements in the synthesis routes and structure-property relationships of cutting-edge HOR and ORR electrocatalysts, as well as anion exchange membranes and ionomers; and iii) fuel cell operating conditions, including water management and impact of CO2. Finally, based on these aspects, the future development and perspectives of AEMFCs are proposed.

Modulating Oxygen Reduction Activity in Chalcogenophene-Incorporated Organic Electrocatalysts through Main-Group Element Engineering

Organic small molecules (OSMs) with well-defined structures are crucial integral components of cathode catalysts for fuel cells. Despite the acknowledged potential of heteroatom doping to enhance the catalytic performance of metal-free carbon-based catalysts, there exists a notable gap in conducting molecular structure and catalytic activity, particularly under the premise of maintaining a constant molecular skeleton and with a clear molecular structure. Herein, the charge distribution is modulated by introducing different chalcogens into the same molecular skeleton through main-group engineering. Among these OSMs, the Se-containing small molecule OSM-Se combined with carbonized calcium alginate exhibits a notable quasi-four-electron-transfer oxygen reduction reaction pathway, displaying a superior half-wave potential (E1/2) of 0.73 V, accompanied by outstanding electrochemical stability. Density functional theory calculations demonstrate that Se-containing small molecules can enhance the capabilities of catalysts in adsorbing and dissociating oxygen molecules, and contribute to reducing the reaction barrier of the oxygen reduction reaction. This study presents a straightforward yet highly effective approach for metal-free carbon-based OSM electrocatalysts.

Synthesis, characterization, biological activity and computation-based efficacy of cobalt(II) complexes of biphenyl-2-ol against SARS-CoV-2 virus

Cobalt(II) complexes of biphenyl-2-ol of composition, CoCl2-n(OC6H4C6H5-2)n(H2O)4 (where n = 1 or 2), were prepared by reacting cobaltous(II) chloride with equi- and bimolar ratios of sodium salt of biphenyl-2-ol. The structural characterization of the synthesized complexes was accomplished by NMR, FTIR, thermogravimetry (TGA), high resolution mass spectroscopy (HRMS), electronic spectroscopic techniques coupled with density functional theory (DFT). The stability of the complexes in different pH media of solvent was studied. Chemical reactivity parameters of the newly synthesized complexes, computed using DFT, indicated greater reactivity of complex 2 over complex 1 and free ligand as indicated by its low HOMO-LUMO energy gap corresponding to 1.71 eV. Molecular docking (MD) studies were carried out in order to study the binding affinities between amino acid residues of DNA duplex (PDB ID: 1BNA) and SARS-CoV-2 (PDB ID: 7T9K) with newly synthesized complexes. Complex 2 has shown promising antivirus behaviour with an inhibition constant value of 0.0423 µmol-1 with amino acid residues of SARS-CoV-2 virus. Toxicity of the complexes was predicted using ProTox-II online server. Antibacterial studies have indicated the complexes to exhibit greater efficacy than the free ligand, while the antioxidant activities have suggested them to display enhanced antioxidant behaviour as compared to reference compounds.Communicated by Ramaswamy H. Sarma.

Electrospun Pt-TiO2 nanofibers Doped with HPA for Catalytic Hydrodeoxygenation

Electrospinning is utilized to fabricate catalytic nanofiber scaffold for biocrude upgrading in hydrodeoxygenation (HDO) following computational studies suggesting the need for nano-catalysts for efficient HDO conversion and selectivity. Here, Pt-TiO2 nanofibers are fabricated through electrospinning, followed by wet impregnation with a heteropoly acid (HPA), tungstosilicic acid. Intensive heat treatments were incorporated during and after processes to obtain a HPA doped Pt-TiO2 nano-catalyst. Catalytic HDO was performed in a batch reactor with phenol as the raw biocrude dissolved in hexadecane. The HPA doped Pt-TiO2 catalyst demonstrated promising HDO performance of 37.2% conversion and a 78.9% selectivity to oxygen free benzene and the remainder 21.1% as diphenyl ester as a result of esterification by acidic components of the catalyst. Additionally, BET surface area characterization show a low surface area 16.9 m2 g-1 significantly lower than existing commercial catalysts and a mesoporous nature suitable for selectivity. The presence of HPA on the anatase nanofiber compensated for low platinum nanoparticles crystallinity on the nanofibers. This work might create needed alternatives for preparing HDO catalysts for efficient aromatics production.

Metal-Free Carbon Co-Catalysts for Up-Conversion Photo-Induced Catalytic Cancer Therapy

Near-infrared (NIR)-responsive metal-free carbon co-catalysts that convert glucose into H2O2 to generate reactive oxygen species (ROS) are developed from phosphorus-doped carbon nitride (P-C3N4) and graphene quantum dots (GQD) composites, for enhanced photocatalytic cancer therapy by light exposure in the targeted tumor microenvironment. Upon irradiation, the NIR light is converted by GQD with up-conversion function into visible light to excite P-C3N4 for photocatalytic conversion of glucose into H2O2, which subsequently decomposes into ROS. ROS thus generated exhibits an excellent anticancer efficacy for efficient cancer therapy with minimal side effects, as evidenced by both in vitro and in vivo studies. This study demonstrates, for the first time, a cancer therapeutic of GQD/P-C3N4 composite that utilizes a two-step cascade effect using initially NIR-triggered GQD nanoparticles to activate P-C3N4 to photocatalytically generate ROS for effective and targeted cancer therapy.

Optimizing Surface State Electrons of Topological Semi-Metal by Atomic Doping for Enhanced Hydrogen Evolution Reaction

Topological materials carrying topological surface states (TSSs) have extraordinary carrier mobility and robustness, which provide a new platform for searching for efficient hydrogen evolution reaction (HER) electrocatalysts. However, the majority of these TSSs originate from the sp band of topological quantum catalysts rather than the d band. Here, based on the density functional theory calculation, it is reported a topological semimetal Pd3Sn carrying TSSs mainly derived from d orbital and proposed that optimizing surface state electrons of Pd3Sn by introduction heteroatoms (Ni) can promote hybridization between hydrogen atoms and electrons, thereby reducing the Gibbs free energy (ΔGH) of adsorbed hydrogen and improving its HER performance. Moreover, this is well verified by electrocatalytic experiment results, the Ni-doped Pd3Sn (Ni0.1Pd2.9Sn) show much lower overpotential (-29 mV vs RHE) and Tafel slope (17 mV dec-1) than Pd3Sn (-39 mV vs RHE, 25 mV dec-1) at a current density of 10 mA cm-2. Significantly, the Ni0.1Pd2.9Sn nanoparticles exhibit excellent stability for HER. The electrocatalytic activity of Ni0.1Pd2.9Sn nanoparticles is superior to that of commercial Pt. This work provides an accurate guide for manipulating surface state electrons to improve the HER performance of catalysts.

Unlocking Efficiency: Minimizing Energy Loss in Electrocatalysts for Water Splitting

Catalysts play a crucial role in water electrolysis by reducing the energy barriers for hydrogen and oxygen evolution reactions (HER and OER). Research aims to enhance the intrinsic activities of potential catalysts through material selection, microstructure design, and various engineering techniques. However, the energy consumption of catalysts has often been overlooked due to the intricate interplay among catalyst microstructure, dimensionality, catalyst-electrolyte-gas dynamics, surface chemistry, electron transport within electrodes, and electron transfer among electrode components. Efficient catalyst development for high-current-density applications is essential to meet the increasing demand for green hydrogen. This involves transforming catalysts with high intrinsic activities into electrodes capable of sustaining high current densities. This review focuses on current improvement strategies of mass exchange, charge transfer, and reducing electrode resistance to decrease energy consumption. It aims to bridge the gap between laboratory-developed, highly efficient catalysts and industrial applications regarding catalyst structural design, surface chemistry, and catalyst-electrode interplay, outlining the development roadmap of hierarchically structured electrode-based water electrolysis for minimizing energy loss in electrocatalysts for water splitting.

Highly Curved Defect Sites: How Curvature Effect Influences Metal-Free Defective Carbon Electrocatalysts

Topological defects are widely recognized as effective active sites toward a variety of electrochemical reactions. However, the role of defect curvature is still not fully understood. Herein, carbon nanomaterials with rich topological defect sites of tunable curvature is reported. The curved defective surface is realized by controlling the high-temperature pyrolytic shrinkage process of precursors. Theoretical calculations demonstrate bending the defect sites can change the local electronic structure, promote the charge transfer to key intermediates, and lower the energy barrier for oxygen reduction reaction (ORR). Experimental results convince structural superiority of highly-curved defective sites, with a high kinetic current density of 22.5 mA cm-2 at 0.8 V versus RHE for high-curvature defective carbon (HCDC), ≈18 times that of low-curvature defective carbon (LCDC). Further raising the defect densities in HCDC leads to the dual-regulated products (HCHDC), which exhibit exceptionally outstanding ORR activity in both alkaline and acidic media (half-wave potentials: 0.88 and 0.74 V), outperforming most of the reported metal-free carbon catalysts. This work uncovers the curvature-activity relationship in carbon defect for ORR and provides new guidance to design advanced catalysts via curvature-engineering.

Exploration of metal-free 2D electrocatalysts toward the oxygen electroreduction

The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal-air batteries. Recently, 2D non-metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface-to-volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal-free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal-free 2D materials. Initially, the merits of metal-free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal-free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal-free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal-free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.

Lattice Strain with Stabilized Oxygen Vacancies Boosts Ceria for Robust Alkaline Hydrogen Evolution Outperforming Benchmark Pt

Earth-abundant metal oxides are usually considered as stable but catalytically inert toward hydrogen evolution reaction (HER) due to their unfavorable hydrogen intermediate adsorption performance. Herein, a heavy rare earth (Y) and transition metal (Co) dual-doping induced lattice strain and oxygen vacancy stabilization strategy is proposed to boost CeO2 toward robust alkaline HER. The induced lattice compression and increased oxygen vacancy (Ov) concentration in CeO2 synergistically improve the water dissociation on Ov sites and sequential hydrogen adsorption at activated Ov-neighboring sites, leading to significantly enhanced HER kinetics. Meanwhile, Y doping offers stabilization effect on Ov by its stronger Y─O bonding over Ce─O, which endows the catalyst with excellent stability. The Y,Co-CeO2 electrocatalyst exhibits an ultra-low HER overpotential (27 mV at 10 mA cm-2) and Tafel slope (48 mV dec-1), outperforming the benchmark Pt electrocatalyst. Moreover, the anion exchange membrane water electrolyzer incorporated with Y,Co-CeO2 achieves excellent stability of 500 h under 600 mA cm-2. This synergistic lattice strain and oxygen vacancy stabilization strategy sheds new light on the rational development of efficient and stable oxide-based HER electrocatalysts.

Promoted Overall Water Splitting Catalytic Activity and Durability of Ni3Fe Alloy by Designing N-Doped Carbon Encapsulation

Combining an electrochemically stable material onto the surface of a catalyst can improve the durability of a transition metal catalyst, and enable the catalyst to operate stably at high current density. Herein, the contribution of the N-doped carbon shell (NCS) to the electrochemical properties is evaluated by comparing the characteristics of the Ni3Fe@NCS catalyst with the N-doped carbon shell, and the Ni3Fe catalyst. The synthesized Ni3Fe@NCS catalyst has a distinct overpotential difference from the Ni3Fe catalyst (ηOER = 468.8 mV, ηHER = 462.2 mV) at (200 and -200) mA cm-2 in 1 m KOH. In stability test at (10 and -10) mA cm-2, the Ni3Fe@NCS catalyst showed a stability of (95.47 and 99.6)%, while the Ni3Fe catalyst showed a stability of (72.4 and 95.9)%, respectively. In addition, the in situ X-ray Absorption Near Edge Spectroscopy (XANES) results show that redox reaction appeared in the Ni3Fe catalyst by applying voltages of (1.7 and -0.48) V. The decomposition of nickel and iron due to the redox reaction is detected as a high ppm concentration in the Ni3Fe catalyst through Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis. This work presents the strategy and design of a next-generation electrochemical catalyst to improve the electrocatalytic properties and stability.

Functionality Modulation Toward Thianthrene-based Metal-Free Electrocatalysts for Water Splitting

The development of metal-free bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) is significant but rarely demonstrated. Porous organic polymers (POPs) with well-defined electroactive functionalities show superior performance in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Precise control of the active sites' local environment requires careful modulation of linkers through the judicious selection of building units. Here, a systematic strategy is introduced for modulating functionality to design and synthesize a series of thianthrene-based bifunctional sp2 C═C bonded POPs with hollow spherical morphologies exhibiting superior electrocatalytic activity. This precise structural tuning allowed to gain insight into the effects of heteroatom incorporation, hydrophilicity, and variations in linker length on electrocatalytic activity. The most efficient bifunctional electrocatalyst THT-PyDAN achieves a current density of 10 mA cm─2 at an overpotential (η10) of ≈65 mV (in 0.5 m H2SO4) and ≈283 mV (in 1 m KOH) for HER and OER, respectively. THT-PyDAN exhibits superior activity to all previously reported metal-free bifunctional electrocatalysts in the literature. Furthermore, these investigations demonstrate that THT-PyDAN maintains its performance even after 36 h of chronoamperometry and 1000 CV cycling. Post-catalytic characterization using FT-IR, XPS, and microscopic imaging techniques underscores the long-term durability of THT-PyDAN.

Hydrogen Spillover Accelerates Electrocatalytic Semi-hydrogenation of Acetylene in Membrane Electrode Assembly Reactor

Electrocatalytic acetylene semi-hydrogenation (EASH) offers a promising and environmentally friendly pathway for the production of C2H4, a widely used petrochemical feedstock. While the economic feasibility of this route has been demonstrated in three-electrode systems, its viability in practical device remains unverified. In this study, we designed a highly efficient electrocatalyst based on a PdCu alloy system utilizing the hydrogen spillover mechanism. The catalyst achieved an operational current density of 600 mA cm-2 in a zero-gap membrane electrode assembly (MEA) reactor, with the C2H4 selectivity exceeding 85%. This data confirms the economic feasibility of EASH in real-world applications. Furthermore, through in situ Raman spectroscopy and theoretical calculations, we elucidated the catalytic mechanism involving interfacial hydrogen spillover. Our findings underscore the economic viability and potential of EASH as a greener and scalable approach for C2H4 production, thus advancing the field of electrocatalysis in sustainable chemical synthesis.

Carbon Catalysts Empowering Sustainable Chemical Synthesis via Electrochemical CO2 Conversion and Two-Electron Oxygen Reduction Reaction

Carbon materials hold significant promise in electrocatalysis, particularly in electrochemical CO2 reduction reaction (eCO2 RR) and two-electron oxygen reduction reaction (2e- ORR). The pivotal factor in achieving exceptional overall catalytic performance in carbon catalysts is the strategic design of specific active sites and nanostructures. This work presents a comprehensive overview of recent developments in carbon electrocatalysts for eCO2 RR and 2e- ORR. The creation of active sites through single/dual heteroatom doping, functional group decoration, topological defect, and micro-nano structuring, along with their synergistic effects, is thoroughly examined. Elaboration on the catalytic mechanisms and structure-activity relationships of these active sites is provided. In addition to directly serving as electrocatalysts, this review explores the role of carbon matrix as a support in finely adjusting the reactivity of single-atom molecular catalysts. Finally, the work addresses the challenges and prospects associated with designing and fabricating carbon electrocatalysts, providing valuable insights into the future trajectory of this dynamic field.

A first-principles study of electro-catalytic reduction of CO2 on transition metal-doped stanene

Employing first-principles calculations based on density functional theory, this work examines the activity of 3d transition metal-doped stanene for electro-catalytic CO2 reduction through the first two electron transfer steps to CO. Our results related to CO2 activation, the first and a crucial step of the reduction process revealed that, among the entire 3d transition metal row studied, only Ti- and Fe-doped stanene can bind and significantly activate the CO2 molecule, while the rest of the TM single atoms are inert in activating the molecule. The activation of the CO2 molecule on Ti- and Fe-doped stanene has been observed in the presence of water as well. In addition, the formation of OCHO has been observed to be energetically preferred over COOH formation as a reaction intermediate, indicating the preference for the formate path of the reduction reaction. Furthermore, despite the strong adsorption of H2O on the catalyst surface, the presence of water seems to enhance CO2 adsorption on the catalysts, contrary to what has been observed recently in graphene-based catalysts. Finally, our difference charge density and the Bader charge calculations reveal that the ability of Ti- and Fe-doped stanene in activating the CO2 molecule and their potential catalytic activity for CO2 reduction is to be attributed to the charge transfer between the catalyst and the molecule, providing new insights into the rational design of 2D catalysts beyond graphene.

Vanadium nitride decorated carbon cloth anode promotes aniline degradation and electricity generation of MFCs by efficiently enriching electroactive bacteria and promoting extracellular electron transfer

To increase the colonization of electroactive bacteria and accelerate the rate of extracellular electron transfer, a simple coated anode of microbial fuel cell was designed. Here, we took advantage of vanadium nitride (VN) particles to modify the carbon cloth (VN@CC). Compared with bare carbon cloth, the designed VN@CC bioanodes exhibited a larger electrochemically active area, better biocompatibility, and smaller charge transfer impedance. The MFC with VN@CC bioanodes achieved the maximum power density of 3.89 W m-2 and chemical oxygen demand removal rate of 84% when 1000 mg L-1 aniline was degraded, which were about 1.88 and 2.8 times that of CC. The morphology of biofilm and 16s rRNA gene sequence analysis proved that the VN@CC bioanodes facilitated the enrichment of electroactive bacteria (99.02%) and increased the ratio of fast electron transfer in the extracellular electron transfer, thus enhancing the MFC performance of aniline degradation and power output. This work disclosed that it was feasible to increase the overall performance of MFC by enhancing the EET efficiency and presented valuable insights for future work.

Boosting Carbon Dioxide Reduction in a Photocatalytic Fuel Cell with a Bubbling Fluidized Cathode: Dual Function of Titanium Carbide

Photoelectrochemical reduction of carbon dioxide (CO2) is a promising avenue to realize resourceful utilization of carbon dioxide and mitigate the energy shortage. Herein, a photocatalytic fuel cell with a bubbling fluidized cathode (PFC-BFC) is proposed to increase the performance of the photocatalytic CO2 reduction reaction (CO2RR). Titanium carbide (Ti3C2) is first used as a fluidized cathode catalyst with the dual features of superior capacitance and high CO2RR catalytic activity. Compared with the conventional PFC system, the as-proposed PFC-BFC system exhibits a higher gas production performance. Particularly, the generation rate and Faraday efficiency for CH4 production reach to 37.2 μmol g-1 h-1 and 72%, which are 10.9 and 6.5 times higher than that of the conventional PFC system, respectively. The bubbling fluidized cathode allows a rapid electron transfer between catalysts and the current collector and an efficient diffusion of catalysts in the whole solution, thus remarkably increasing the effective reaction area of the CO2RR. In addition, the fluidized reaction mechanism of charging/discharging-coupled CO2RR is investigated. Significantly, a magnified PFC-BFC system is designed and exhibits a similar gas generation rate compared to that of the small-scale system, indicating a good potential of scaling up in the industry applications. These results demonstrated that the proposed PFC-BFC system can maximize the utilization of catalyst active sites and enhance the reaction kinetics, providing an alternative design for the application of CO2RR.

Tensile-Strained Holey Pd Metallene toward Efficient and Stable Electrocatalysis

Noble metal-based metallenes are attracting intensive attention in energy catalysis, but it is still very challenging to precisely control the surface structures of metallenes for higher catalytic properties on account of their intrinsic thermodynamic instability. Herein, the synthesis of tensile-strained holey Pd metallene by oxidative etching is reported using hydrogen peroxide, which exhibits highly enhanced catalytic activity and stability in comparison with normal Pd metallene toward both oxygen reduction reaction and formic acid oxidation. The pre-prepared Pd metallene functions as a catalyst to decompose hydrogen peroxide, and the Pd atoms in amorphous regions of Pd metallene are preferentially removed by the introduced hydrogen peroxide during the etching process. The greatly enhanced ORR activity is mainly determined by the strong electrostatic repulsion between intermediate O* and the dopant O, which balances the adsorption strength of O* on Pd sites, ultimately endowing a weakened adsorption energy of O* on TH-Pd metallene. This work creates a facile and economical strategy to precisely shape metallene-based nanoarchitectures with broad applications for energy systems and sensing devices.

Recent Progress of Transition Metal Compounds as Electrocatalysts for Electrocatalytic Water Splitting

Hydrogen has enormous commercial potential as a secondary energy source because of its high calorific value, clean combustion byproducts, and multiple production methods. Electrocatalytic water splitting is a viable alternative to the conventional methane steam reforming technique, as it operates under mild conditions, produces high-quality hydrogen, and has a sustainable production process that requires less energy. Electrocatalysts composed of precious metals like Pt, Au, Ru, and Ag are commonly used in the investigation of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Nevertheless, their limited availability and expensive cost restrict practical use. In contrast, electrocatalysts that do not contain precious metals are readily available, cost-effective, environmentally friendly, and possess electrocatalytic performance equal to that of noble metals. However, considerable research effort must be devoted to create cost-effective and high-performing catalysts. This article provides a comprehensive examination of the reaction mechanism involved in electrocatalytic water splitting in both acidic and basic environments. Additionally, recent breakthroughs in catalysts for both the hydrogen evolution and oxygen evolution reactions are also discussed. The structure-activity relationship of the catalyst was deep-going discussed, together with the prospects of current obstacles and potential for electrocatalytic water splitting, aiming at provide valuable perspectives for the advancement of economical and efficient electrocatalysts on an industrial scale.

Carbon Electrode Materials for Advanced Potassium-Ion Storage

Tremendous progress has been made in the field of electrochemical energy storage devices that rely on potassium-ions as charge carriers due to their abundant resources and excellent ion transport properties. Nevertheless, future practical developments not only count on advanced electrode materials with superior electrochemical performance, but also on competitive costs of electrodes for scalable production. In the past few decades, advanced carbon materials have attracted great interest due to their low cost, high selectivity, and structural suitability and have been widely investigated as functional materials for potassium-ion storage. This article provides an up-to-date overview of this rapidly developing field, focusing on recent advanced and mechanistic understanding of carbon-based electrode materials for potassium-ion batteries. In addition, we also discuss recent achievements of dual-ion batteries and conversion-type K-X (X=O2 , CO2 , S, Se, I2 ) batteries towards potential practical applications as high-voltage and high-power devices, and summarize carbon-based materials as the host for K-metal protection and possible directions for the development of potassium energy-related devices as well. Based on this, we bridge the gaps between various carbon-based functional materials structure and the related potassium-ion storage performance, especially provide guidance on carbon material design principles for next-generation potassium-ion storage devices.

Lignin-derived carbon nanosheets boost electrochemical reductive amination of pyruvate to alanine

Efficient and sustainable amino acid synthesis is essential for industrial applications. Electrocatalytic reductive amination has emerged as a promising method, but challenges such as undesired side reactions and low efficiency persist. Herein, we demonstrated a lignin-derived catalyst for alanine synthesis. Carbon nanosheets (CNSs) were synthesized from lignin via a template-assisted method and doped with nitrogen and sulfur to boost reductive amination and suppress side reactions. The resulting N,S-co-doped carbon nanosheets (NS-CNSs) exhibited outstanding electrochemical performance. It achieved a maximum alanine Faradaic efficiency of 79.5%, and a yield exceeding 1,199 μmol h-1 cm-2 on NS-CNS, with a selectivity above 99.9%. NS-CNS showed excellent durability during long-term electrolysis. Kinetic studies including control experiments and theoretical calculations provided further insights into the reaction pathway. Moreover, NS-CNS catalysts demonstrated potential in upgrading real-world polylactic acid plastic waste, yielding value-added alanine with a selectivity over 75%.

Molybdenum Phosphide Quantum Dots Encapsulated by P/N-Doped Carbon for Hydrogen Evolution Reaction in Acid and Alkaline Electrolytes

A facile and eco-friendly strategy is presented for synthesizing novel nanocomposites, with MoP quantum dots (QDs) as cores and graphitic carbon as shells, these nanoparticles are dispersed in a nitrogen and phosphorus-doped porous carbon and carbon nanotubes (CNTs) substrates (MoP@NPC/CNT). The synthesis involves self-assembling reactions to form single-source precursors (SSPs), followed by pyrolysis at 900 °C in an inert atmosphere to obtain MoP@NPC/CNT-900. The presence of carbon layers on the MoP QDs effectively prevents particle aggregation, enhancing the utilization of active MoP species. The optimized sample, MoP@NPC/CNT-900, exhibits remarkable electrocatalytic activity and durability for the hydrogen evolution reaction (HER). It demonstrates a low overpotential of 155 mV at 10 mA cm-2 , a small Tafel slope of 76 mV dec-1 , and sustained performance over 20 hours in 0.5 M H2 SO4 . Furthermore, the catalyst shows excellent activity in 1 M KOH, with a relatively low overpotential of 131 mV and long-term durability under constant current input. The exceptional HER activity can be attributed to several factors: the superior performance of MoP QDs, the large surface area and good conductivity of the carbon substrates, and the synergistic effect between MoP and carbon species.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Nanocomposite Electrocatalysts for Hydrogen Evolution Reactions (HERs) for Sustainable and Efficient Hydrogen Energy-Future Prospects

Hydrogen is considered a good clean and renewable energy substitute for fossil fuels. The major obstacle facing hydrogen energy is its efficacy in meeting its commercial-scale demand. One of the most promising pathways for efficient hydrogen production is through water-splitting electrolysis. This requires the development of active, stable, and low-cost catalysts or electrocatalysts to achieve optimized electrocatalytic hydrogen production from water splitting. The objective of this review is to survey the activity, stability, and efficiency of various electrocatalysts involved in water splitting. The status quo of noble-metal- and non-noble-metal-based nano-electrocatalysts has been specifically discussed. Various composites and nanocomposite electrocatalysts that have significantly impacted electrocatalytic HERs have been discussed. New strategies and insights in exploring nanocomposite-based electrocatalysts and utilizing other new age nanomaterial options that will profoundly enhance the electrocatalytic activity and stability of HERs have been highlighted. Recommendations on future directions and deliberations for extrapolating information have been projected.

Unravelling the Effect of Activators used in The Synthesis of Biomass-Derived Carbon Electrocatalysts on the Electrocatalytic Performance for CO2 Reduction

N-doped carbon materials can be efficient and cost-effective catalysts for the electrochemical CO2 reduction reaction (CO2 RR). Activators are often used in the synthesis process to increase the specific surface area and porosity of these carbon materials. However, owing to the diversity of activators and the differences in physicochemical properties that these activators induce, the influence of activators used for the synthesis of N-doped carbon catalysts on their electrochemical performance is unclear. In this study, a series of bagasse-derived N-doped carbon catalysts is prepared with the assistance of different activators to understand the correlation between activators, physicochemical properties, and electrocatalytic performance for the CO2 RR. The properties of N-doped carbon catalysts, such as N-doping content, microstructure, and degree of graphitization, are found to be highly dependent on the type of activator applied in the synthesis procedure. Moreover, the overall CO2 RR performance of the synthesized electrocatalysts is not determined only by the N-doping level and the configuration of the N-dopant, but rather by the overall surface chemistry, where the porosity and the degree of graphitization are jointly responsible for significant differences in CO2 RR performance.

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.

Flexible Solid-State Metal-Air Batteries: The Booming of Portable Energy Supplies

The rapid development of portable and wearable electronics has given rise to new challenges and provoked research in flexible, lightweight, and affordable energy storage devices. Flexible solid-state metal-air batteries (FSSMABs) are considered promising candidates, owing to their large energy density, mechanical flexibility, and durability. However, the practical applications of FSSMABs require further improvement to meet the demands of long-term stability, high power density, and large operating voltage. This Review presents a detailed discussion of innovative electrocatalysts for the air cathode, followed by a sequential overview of high-performance solid-state electrolytes and metal anodes, and a summary of the current challenges and future perspectives of FSSMABs to promote practical application and large-scale commercialization in the near future.

An inclusive review and perspective on Cu-based materials for electrochemical water splitting

In recent years, there has been a resurgence of interest in developing green and renewable alternate energy sources as a solution to the energy and environmental problems produced by conventional fossil fuel use. As a very effective energy transporter, hydrogen (H2) is a possible candidate for the future energy supply. Hydrogen production by water splitting is a promising new energy option. Strong, efficient, and abundant catalysts are required for increasing the efficiency of the water splitting process. Cu-based materials as an electrocatalyst have shown promising results for application in the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting. In this review, our aim is to cover the latest developments in the synthesis, characterisation, and electrochemical behaviour of Cu-based materials as a HER, and OER electrocatalyst, highlighting the impact that these advances have had on the field. It is intended that this review article will serve as a roadmap for developing novel, cost-effective electrocatalysts for electrochemical water splitting based on nanostructured materials with particular emphasis on Cu-based materials for electrocatalytic water splitting.

Acceleration of the semi-hydrogenation of alkynes over an N-doped porous carbon sphere-confined ultrafine PdCu bimetallic nanoparticle catalyst

Selective hydrogenation of alkynes to obtain alkenes is a key reaction in petrochemical and fine chemical industries. However, the development of stable and highly selective catalysts with uniformly dispersed active sites is still immensely challenging for the semi-hydrogenation of alkynes. In this study, N-doped porous carbon nanospheres (NPCNs) were synthesized by the nanoemulsion self-assembly and subsequently carbonization method. Ultrafine PdCu bimetallic nanoparticles (NPs) were uniformly dispersed and immobilized on NPCNs. The obtained PdCu/NPCNs catalyst exhibited an open framework and abundant active sites originating from ultrafine PdCu NPs. In the semi-hydrogenation of alkynes, the PdCu/NPCNs catalyst exhibited a remarkable performance and stability, outperforming most of the classical catalysts. The excellent performance was related to the introduction of a secondary metal Cu, which can regulate the electronic state of Pd active sites to further enhance the hydrogenation activity and selectivity. Hence, the facile approach reported herein may be useful for constructing highly dispersed bimetallic NP-based catalysts for selective hydrogenation of alkynes in the petrochemical industry.

Characterization of electrodes modified with sludge-derived biochar and its performance of electrocatalytic oxidation of azo dyes

Pyrolysis of waste sludge in sewage treatment can achieve a substantial reduction in solid waste and obtain sludge-based biochars with multiple functions. However, the electrochemical properties of sludge-derived biochar as electrode modification material and the electrocatalytic ability of biochar-modified electrodes are still unclear. In this study, sludge-based biochars were prepared at various pyrolysis temperatures (400 °C, 500 °C, 600 °C, 700 °C, and 800 °C) and then were cast on glassy carbon electrodes to fabricate composite biochar-electrodes (GC400, GC500, GC600, GC700, and GC800). The results of elemental analysis and Raman spectra showed that sludge-based biochar prepared at higher temperatures exhibited higher aromaticity and degree of defect structures. And the results of cyclic voltammetry and electrochemical impedance spectra confirmed that biochar-modified electrodes prepared at higher temperatures (>600 °C) possessed better electrocatalytic activity and electrochemical stability, and their higher oxygen evolution potential than control test could improve the electrocatalytic efficiency. In the electrocatalytic oxidation of methyl orange, the removal rate with GC800 was the highest, reaching 94.49% within 240 min, and the removal rates with other composite electrodes were 90.61% (GC700) > 86.96% (GC600) > 80.32% (GC). The free radical quenching experiment revealed that the electrocatalytic degradation of methyl orange mainly depended on the indirect oxidation of hydroxyl radicals generated by electrocatalysis, accounting for 81.3% of the removal rate. The biochar-modified electrode not only greatly improved the electrocatalytic ability of the electrode for the degradation of azo dyes, but also achieved the recycling application of products after pyrolysis of sludge waste.

Rational Designing of Bimetallic/Trimetallic Hydrogen Evolution Reaction Catalysts Using Supervised Machine Learning

Cost-efficient electrocatalysts to replace precious platinum group metals- (PGMs-) based catalysts for the hydrogen evolution reaction (HER) carry significant potential for sustainable energy solutions. Machine learning (ML) methods have provided new avenues for intelligent screening and predicting efficient heterogeneous catalysts in recent years. We coalesce density functional theory (DFT) and supervised ML methods to discover earth-abundant active heterogeneous NiCoCu-based HER catalysts. An intuitive generalized microstructure model was designed to study the adsorbate's surface coverage and generate input features for the ML process. The study utilizes optimized eXtreme Gradient Boost Regression (XGBR) models to screen NiCoCu alloy-based catalysts for HER. We show that the most active HER catalysts can be screened from an extensive set of catalysts with this approach. Therefore, our approach can provide an efficient way to discover novel heterogeneous catalysts for various electrochemical reactions.

Electrocatalytic activity for proton reduction by a covalent non-metal graphene-fullerene hybrid

A non-metal covalent hybrid of fullerene and graphene was synthesized in one step via fluorographene chemistry. Its electrocatalytic performance for the hydrogen evolution reaction and durability was ascribed to intrahybrid charge-transfer phenomena, exploiting the electron-accepting properties of C60 and the high conductivity and large surface area of graphene.

Highly efficient C(CO)-C(alkyl) bond cleavage in ketones to access esters over ultrathin N-doped carbon nanosheets

Selective oxidative cleavage of the C(CO)–C bond in ketones to access esters is a highly attractive strategy for upgrading ketones. However, it remains a great challenge to realize this important transformation over heterogeneous metal-free catalysts. Herein, we designed a series of porous and ultrathin N-doped carbon nanosheets (denoted as CN-X, where X represents the pyrolysis temperature) as heterogeneous metal-free catalysts. It was observed that the fabricated CN-800 could efficiently catalyze the oxidative cleavage of the C(CO)–C bond in various ketones to generate the corresponding methyl esters at 130 °C without using any additional base. Detailed investigations revealed that the higher content and electron density of the graphitic-N species contributed to the excellent performance of CN-800. Besides, the high surface area, affording active sites that are more easily accessed, could also enhance the catalytic activity. Notably, the catalysts have great potential for practical applications because of some obvious advantages, such as low cost, neutral reaction conditions, heterogeneous nature, high efficiency, and broad ketone scope. To the best of our knowledge, this is the first work on efficient synthesis of methyl esters via oxidative esterification of ketones over heterogeneous metal-free catalysts.

Ultrathin and metal-free N-doped carbon nanosheets showed high activity and selectivity for oxidative esterification of ketones via C(CO)–C bond cleavage to access methyl esters.

Carbon Dot Therapeutic Platforms: Administration, Distribution, Metabolism, Excretion, Toxicity, and Therapeutic Potential

Ultrasmall nanoparticles are often grouped under the broad umbrella term of "nanoparticles" when reported in the literature. However, for biomedical applications, their small sizes give them intimate interactions with biological species and endow them with unique functional physiochemical properties. Carbon quantum dots (CQDs) are an emerging class of ultrasmall nanoparticles which have demonstrated considerable biocompatibility and have been employed as potent theragnostic platforms. These particles find application for increasing drug solubility and targeting, along with facilitating the passage of drugs across impermeable membranes (i.e., blood brain barrier). Further functionality can be triggered by various environmental conditions or external stimuli (i.e., pH, temperature, near Infrared (NIR) light, ultrasound), and their intrinsic fluorescence is valuable for diagnostic applications. The focus of this review is to shed light on the therapeutic potential of CQDs and identify how they travel through the body, reach their site of action, administer therapeutic effect, and are excreted. Investigation into their toxicity and compatibility with larger nanoparticle carriers is also examined. The future of CQDs for theragnostic applications is promising due to their multifunctional attributes and documented biocompatibility. As nanomaterial platforms become more commonplace in clinical treatments, the commercialization of CQD therapeutics is anticipated.