Carbon-Supported Single-Atom Catalysts for Formic Acid Oxidation and Oxygen Reduction Reactions

Waste Plastic-Supported Pd Single-Atom Catalyst for Hydrogenation

As worldwide plastic pollution continues to rise, innovative ideas for effective reuse and recycling of waste plastic are needed. Single-atom catalysts (SACs), which are known for their high activity and selectivity, present unique advantages in facilitating plastic degradation and conversion. Waste plastic can be used as a support or raw material to create SACs, which reduces waste generation while simultaneously utilizing waste as a resource. This work successfully utilized waste plastic polyurethane (PU) as a support, through a unique Rapid Thermal Processing Reactor (RTPR) to synthesize an efficient Pd1/PU SACs. At 25 °C and 0.5 MPa H2, Pd1/PU displayed outstanding activity and selectivity in the hydrogenation of styrene, as well as remarkable stability. Pd1/PU performed well in hydrogenating a variety of common substrates. These findings highlight the great potential of SACs in plastic waste reuse and recycling, offering intriguing solutions to the global plastic pollution problem.

Advances of Transition Metal-Based Electrochemical Non-enzymatic Glucose Sensors for Glucose Analysis: A Review

Glucose concentration is a crucial parameter for assessing human health. Over recent years, non-enzymatic electrochemical glucose sensors have drawn considerable attention due to their substantial progress. This review explores the common mechanism behind the transition metal-based electrocatalytic oxidation of glucose molecules through classical electrocatalytic frameworks like the Pletcher model and the Hydrous Oxide-Adatom Mediator model (IHOAM), as well as the redox reactions at the transition metal centers. It further compiles the electrochemical characterization techniques, associated formulas, and their ensuing conclusions pertinent to transition metal-based non-enzymatic electrochemical glucose sensors. Subsequently, the review covers the latest advancements in the field of transition metal-based active materials and support materials used in non-enzymatic electrochemical glucose sensors in the last decade (2014-2023). Additionally, it presents a comprehensive classification of representative studies according to the active metal catalysts components involved.

Precise Design and Modification Engineering of Single-Atom Catalytic Materials for Oxygen Reduction

Due to their unique electronic and structural properties, single-atom catalytic materials (SACMs) hold great promise for the oxygen reduction reaction (ORR). Coordinating environmental and engineering strategies is the key to improving the ORR performance of SACMs. This review summarizes the latest research progress and breakthroughs of SACMs in the field of ORR catalysis. First, the research progress on the catalytic mechanism of SACMs acting on ORR is reviewed, including the latest research results on the origin of SACMs activity and the analysis of pre-adsorption mechanism. The study of the pre-adsorption mechanism is an important breakthrough direction to explore the origin of the high activity of SACMs and the practical and theoretical understanding of the catalytic process. Precise coordination environment modification, including in-plane, axial, and adjacent site modifications, can enhance the intrinsic catalytic activity of SACMs and promote the ORR process. Additionally, several engineering strategies are discussed, including multiple SACMs, high loading, and atomic site confinement. Multiple SACMs synergistically enhance catalytic activity and selectivity, while high loading can provide more active sites for catalytic reactions. Overall, this review provides important insights into the design of advanced catalysts for ORR.

Regulating Polysulfide Transformation and Deposition Kinetics in Lithium-Sulfur Batteries Based on 3D Conductive Framework

The polysulfide shuttle effect and slow liquid-solid conversion are supposed to be the main bottlenecks limiting lithium-sulfur battery practicality. Although a great deal of research has been devoted to the nucleation and transformation kinetics of polysulfides, many implicit details cannot be captured. In this work, we design a conducting network, FeN_x-NPC, derived from hemin, and induce a 3D nucleation mode. Different from the control group with the 2D nucleation mode, a higher Li₂S deposition and earlier nucleation are observed. Here, in situ impedance is applied to further understand the potential relationship between nucleation mode and liquid-solid transformation, and DRT results from impedance data are systematically compared from two aspects: (1) single battery under different voltages and (2) different batteries under the same voltage. It reveals that the 3D nucleation mode ensures more growth sites, on which a covered thin Li₂S layer exhibits no charge transfer limitation. What is more, the porous structure with in situ-derived nanotubes favors Li⁺ faster diffusion. Hence, these advantages allow Li-S cells to deliver high capacity (about 1423 mA h g^-1 at 0.1 C), low capacity attenuation (0.029% per cycle at 2 C), and excellent rate performance (620 mA h g^-1 at 5 C).

Wet-chemistry synthesis of two-dimensional Pt- and Pd-based intermetallic electrocatalysts for fuel cells

Two-dimensional (2D) noble-metal-based nanomaterials have attracted tremendous attention and have widespread promising applications as a result of their unique physical, chemical, and electronic properties. Especially, 2D Pt- and Pd-based intermetallic nanoplates (IMNPs) and nanosheets (IMNSs) are widely studied for fuel cell (FC)-related reactions, including the cathodic oxygen reduction reaction (ORR) and anodic formic acid, methanol and ethanol oxidation reactions (FAOR, MOR and EOR). Wet-chemistry synthesis is a powerful strategy to prepare metallic nanocrystals with well-controlled dispersity, size, and composition. In this review, a fundamental understanding of the FC-related reactions is firstly elaborated. Subsequently, the current wet-chemistry synthesis pathways for 2D Pt- and Pd-based IMNPs and IMNSs are briefly summarized, as well as their electrocatalytic applications including in the ORR, FAOR, MOR, and EOR. Finally, we provide an overview of the opportunities and current challenges and give our perspectives on the development of high-performance 2D Pt- and Pd-based intermetallic electrocatalysts towards FCs. We hope this review offers timely information on the synthesis of 2D Pt- and Pd-based IMNPs and IMNSs and provides guidance for the efficient synthesis and application of them.

Boosting Electrochemical Catalysis and Nonenzymatic Sensing Toward Glucose by Single-Atom Pt Supported on Cu@CuO Core-Shell Nanowires

It is critical to develop high-performance electrocatalyst for electrochemical nonenzymatic glucose sensing. In this work, a single-atom Pt supported on Cu@CuO core-shell nanowires (Pt₁ /Cu@CuO NWs) for electrochemical nonenzymatic glucose sensor is designed. Pt₁ /Cu@CuO NWs exhibit excellent electrocatalytic oxidation toward glucose with 70 mV lower onset potential (0.131 V) and 2.4 times higher response current than Cu NWs. Sensors fabricated using Pt₁ /Cu@CuO NWs also show high sensitivity (852.163 µA mM^-1 cm^-2 ), low detection limit (3.6 µM), wide linear range (0.01-5.18 µM), excellent selectivity, and great long-term stability. The outstanding sensing performance of Pt₁ /Cu@CuO NWs, investigated by experiments and density functional theory (DFT) calculations, is attributed to the synergistic effect between Pt single atoms and Cu@CuO core-shell nanowires that generates strong binding energy of glucose on the nanowires. The work provides a new pathway for exploring highly active SACs for electrochemical nonenzymatic glucose sensor.

A Zn-based catalyst with high oxygen reduction activity and anti-poisoning property for stable seawater batteries

Seawater batteries (SWBs) are a key part of the future underwater energy network for maritime safety and resource development due to their high safety, long lifespan, and eco-friendly nature. However, the complicated seawater composition and pollution, such as the S^2-, usually poison the catalyst and lead to the degradation of the battery performance. Here, Zn single-atom catalysts (SACs) were demonstrated as effective oxygen reduction reaction catalysts with high anti-poisoning properties by density functional theory calculation and the Zn SACs anchoring on an N, P-doped carbon substrate (Zn-SAC@PNC) was synthesized by a one-pot strategy. Zinc active sites ensure the anti-poisoning property toward S^2-, and N, P-doped carbon helps improve the activity. Therefore, Zn-SAC@PNC exhibits superior activity (E_1/2: 0.87 V, Tafel slope: 69.5 mV dec^-1) compared with Pt/C and shows a lower decay rate of the voltage after discharge in lean-oxygen natural seawater. In the presence of S^2-, Zn-SAC@PNC can still maintain its original catalytic activity, which ensures the stable operation of SWBs in the marine environment with sulfur-based pollutants. This study provides a new strategy to design and develop efficient cathode materials for SWBs.

Boosting the Electrocatalytic Formic Acid Oxidation Activity via P-PdAuAg Quaternary Alloying

Direct formic acid fuel cells (DFAFCs) are considered promising sustainable power sources due to their high energy density, nonflammability, and low fuel crossover. However, serious CO poisoning and activity attenuation of the anodic formic acid oxidation reaction (FAOR) greatly restrict the output and durability of DFAFCs. Inspired by the specific relationship between the composition, type, and property of alloys, in this work, we synthesize a series of hybrid substitutional/interstitial quaternary alloys P-PdAuAg by means of a novel polyphosphide route to address these issues. Due to the simultaneous interstitial P-doping and metal (Au, Ag, Pd) co-reduction, the P-PdAuAg quaternary alloy obtained is only 3 nm in diameter with abundant defects. It not only achieves a new high mass activity of 8.08 A mg_Pd^-1 (6.78 A mg_catalyst^-1) but also maintains high stability in the high potential range and harsh reaction conditions. Both the activity and anti-poisoning ability are far exceeding those of the currently reported FAOR catalysts. Detailed density functional theory (DFT) calculations reveal that the superb electrochemical performances originate from the shift of the d-band center of Pd as a result of the synergistic electronic/ligand effects between Pd, Au, Ag, and P. The introduction of interstitial P inhibits the occurrence of an indirect reaction pathway on Pd, while Au and Ag suppress the adsorption of CO and optimize the sequential dehydrogenation steps, leading to boosted reaction kinetics and CO tolerance. This work pioneered a facile way for the synthesis of Pd-based substitutional/interstitial hybrid alloys, providing a promising means of further improving the performance of alloying catalysts.

Single-atom catalysts for electrochemical applications

The field of small molecule electro-activated conversion is becoming a new star in modern catalytic research toward the carbon-neutral future. The advent of single-atom catalysts (SACs) is expected to greatly accelerate the kinetics of electrocatalytic reactions such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), etc., owing to their maximum atomic efficiency, unique quantized energy level structure and strong interaction between well-defined active sites and supports. In this feature article, our group's proposed synthesis methodology applied in electrocatalysis is mainly summarized. Furthermore, we elaborate on how to achieve the stabilization of single metal atoms against migration and agglomeration during the preparation of SACs. Moreover, the electrochemical applications of SACs with a focus on the above heterogeneous reactions are presented. Finally, the prospects for the development and deficiencies of these SACs for electrocatalytic reactions are discussed.

Computational Study on the Catalytic Performance of Single-Atom Catalysts Anchored on g-CN for Electrochemical Oxidation of Formic Acid

The electrochemical formic acid oxidation reaction (FAOR) has attracted great attention due to its high volumetric energy density and high theoretical efficiency for future portable electronic applications, for which the development of highly efficient and low-cost electrocatalysts is of great significance. In this work, taking single-atom catalysts (SACs) supported on graphitic carbon nitrides (g-CN) as potential catalysts, their catalytic performance for the FAOR was systemically explored by means of density functional theory computations. Our results revealed that the strong hybridization with the unpaired lone electrons of N atoms in the g-CN substrate ensured the high stability of these anchored SACs and endowed them with excellent electrical conductivity. Based on the computed free energy changes of all possible elementary steps, we predicted that a highly efficient FAOR could be achieved on Ru/g-CN with a low limiting potential of −0.15 V along a direct pathway of HCOOH(aq) → HCOOH* → HCOO* → CO2* → CO2(g), in which the formation of HCOO* was identified as the potential-determining step, while the rate-determining step was located at the CO2* formation, with a moderate kinetic barrier of 0.89 eV. Remarkably, the moderate d-band center and polarized charge of the Ru active site caused the Ru/g-CN catalyst to exhibit an optimal binding strength with various reaction intermediates, explaining well its superior FAOR catalytic performance. Hence, the single Ru atom anchored on g-CN could be utilized as a promising SAC for the FAOR, which opens a new avenue to further develop novel catalysts for a sustainable FAOR in formic-acid-based fuel cells.

Hollow Carbon Nanorod Confined Single Atom Rh for Direct Formic Acid Electrooxidation

Nearly theoretical 100% atomic utilization (supposing each atom could serve as independent sites to play a role in catalyz) of single-atom catalysts (SACs) makes it highly promising for various applications. However, for most SACs, single-atom sites are trapped in a solid carbon matrix, which makes the inner parts hardly available for reaction. Herein, a hollow N-doped carbon confined single-atom Rh (Rh-SACs/HNCR) is developed via a coordination-template method. Both aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy mapping confirm the uniform distribution of Rh single atoms. Owning to the unique hollow structure and effective carbon confinement, excessive conversion from pyridinic/pyrrolic N to graphic N is hindered. As a proof of concept, Rh-SACs/HNCR exhibits superior activity, stability, selectivity, and anti-poisoning capability in formic acid oxidation reaction compared with the counterpart Rh/C, Pd/C, and Pt/C catalysts. This work provides a powerful strategy for synthesizing hollow carbon confined single-atom catalysts apply in various energy-related systems.

Effect of Modifying Carbon Materials with Metal Phthalocynines and Palladium on Their Catalytic Activity in ORR

Bimetallic catalysts based on multi-walled carbon nanotubes (MWCNT), graphene oxide (GO) and ultradispersed diamonds (UDD) supports for the process of electroreduction of oxygen from alkaline electrolyte were obtained using high-temperature synthesis. The materials were characterized by low-temperature nitrogen adsorption, Raman spectroscopy, scanning electron microscopy and X-ray structure analysis. The synthesized bimetallic catalysts contain meso- and micropores. Based on the study by Raman spectroscopy, it is shown that high-temperature synthesis of MWCNT with metal phthalocyanines leads to doping of this material with nitrogen and the appearance of significant defects in the structure. Carbon nanotube-based catalysts showed enhanced activity compared to other carbon materials. Moreover, bimetallic catalysts based on cobalt phthalocyanine and palladium (MWCNT_CoPc_Pd) are characterized by higher activity on all carbon supports compared to materials contain on copper and palladium. The specific current density in the diffusion region of the MWCNT_CoPc_Pd catalyst is comparable to a commercial platinum electrode (Pt(20%)/C) and equals to 2.65 mA/cm2. The area of the electrochemically active surface of all the obtained catalysts was calculated from the CV data in a nitrogen atmosphere. The MWCNT_CoPc_Pd catalyst is characterized by high corrosivity: after 2500 revolutions, the current density in the diffusion region decreases by 7%, and, also, an increase in the values of E1/2 and Eonset is observed.

Macro/Micro-Environment Regulating Carbon-Supported Single-Atom Catalysts for Hydrogen/Oxygen Conversion Reactions

Single-atom catalysts (SACs) have attracted tremendous research interest due to their unique atomic structure, maximized atom utilization, and remarkable catalytic performance. Among the SACs, the carbon-supported SACs have been widely investigated due to their easily controlled properties of the carbon substrates, such as the tunable morphologies, ordered porosity, and abundant anchoring sites. The electrochemical performance of carbon-supported SACs is highly related to the morphological structure of carbon substrates (macro-environment) and the local coordination environments of center metals (micro-environment). This review aims to provide a comprehensive summary on the macro/micro-environment regulating carbon-supported SACs for highly efficient hydrogen/oxygen conversion reactions. The authors first summarize the macro-environment engineering strategies of carbon-supported SACs with altered specific surface areas and porous properties of the carbon substrates, facilitating the mass diffusion kinetics and structural stability. Then the micro-environment engineering strategies of carbon-supported SACs are discussed with the regulated atomic structure and electronic structure of metal centers, boosting the catalytic performance. Insights into the correlation between the co-boosted effect from the macro/micro-environments and catalytic activity for hydrogen/oxygen conversion reactions are summarized and discussed. Finally, the challenges and perspectives are addressed in building highly efficient carbon-supported SACs for practical applications.

Amidoxime Group-Anchored Single Cobalt Atoms for Anti-Biofouling during Uranium Extraction from Seawater

Marine biofouling is one of the most significant challenges hindering practical uranium extraction from seawater. Single atoms have been widely used in catalytic applications because of their remarkable redox property, implying that the single atom is highly capable of catalyzing the generation of reactive oxygen species (ROS) and acts as an anti-biofouling substance for controlling biofouling. In this study, the Co single atom loaded polyacrylamidoxime (PAO) material, PAO-Co, is fabricated based on the binding ability of the amidoxime group to uranyl and cobalt ions. Nitrogen and oxygen atoms from the amidoxime group stabilize the Co single atom. The fabricated PAO-Co exhibits a broad range of antimicrobial activity against diverse marine microorganisms by producing ROS, with an inhibition rate up to 93.4%. The present study is the first to apply the single atom for controlling biofouling. The adsorbent achieves an ultrahigh uranium adsorption capacity of 9.7 mg g-1 in biofouling-containing natural seawater, which decreased only by 11% compared with that in biofouling-removed natural seawater. These findings indicate that applying single atoms would be a promising strategy for designing biofouling-resistant adsorbents for uranium extraction from seawater.

Phosphorus-Doped Graphene Electrocatalysts for Oxygen Reduction Reaction

Developing cheap and earth-abundant electrocatalysts with high activity and stability for oxygen reduction reactions (ORRs) is highly desired for the commercial implementation of fuel cells and metal-air batteries. Tremendous efforts have been made on doped-graphene catalysts. However, the progress of phosphorus-doped graphene (P-graphene) for ORRs has rarely been summarized until now. This review focuses on the recent development of P-graphene-based materials, including the various synthesis methods, ORR performance, and ORR mechanism. The applications of single phosphorus atom-doped graphene, phosphorus, nitrogen-codoped graphene (P, N-graphene), as well as phosphorus, multi-atoms codoped graphene (P, X-graphene) as catalysts, supporting materials, and coating materials for ORR are discussed thoroughly. Additionally, the current issues and perspectives for the development of P-graphene materials are proposed.

Theoretical study of p-block metal–nitrogen–carbon single-atom catalysts for the oxygen reduction reaction

A ‘double-peak’ ORR volcano plot is found for the p-block metal–nitrogen–carbon catalysts by considering both pristine and OH* self-modifying sites.

Rational Design of Graphene-Supported Single-Atom Catalysts for Electroreduction of Nitrogen

Critically, the central metal atoms along with their coordination environment play a significant role in the catalytic performance of single-atom catalysts (SACs). Herein, 12 single Fe, Mo, and Ru atoms supported on defective graphene are theoretically deigned for investigation of their structural and electronic properties and catalytic nitrogen reduction reaction (NRR) performance using first-principles calculations. Our results reveal that graphene with vacancies can be an ideal anchoring site for stabilizing isolated metal atoms owing to the strong metal-support interaction, forming stable TMC_x or TMN_x active centers (x = 3 or 4). Six SACs are screened as promising NRR catalyst candidates with excellent activity and selectivity during NRR, and RuN₃ is identified as the optimal one with an overpotential of ≥0.10 V via the distal mechanism.

An Adjacent Atomic Platinum Site Enables Single-Atom Iron with High Oxygen Reduction Reaction Performance

The modulation effect has been widely investigated to tune the electronic state of single-atomic M-N-C catalysts to enhance the activity of oxygen reduction reaction (ORR). However, the in-depth study of modulation effect is rarely reported for the isolated dual-atomic metal sites. Now, the catalytic activities of Fe-N₄ moiety can be enhanced by the adjacent Pt-N₄ moiety through the modulation effect, in which the Pt-N₄ acts as the modulator to tune the 3d electronic orbitals of Fe-N₄ active site and optimize ORR activity. Inspired by this principle, we design and synthesize the electrocatalyst that comprises isolated Fe-N₄ /Pt-N₄ moieties dispersed in the nitrogen-doped carbon matrix (Fe-N₄ /Pt-N₄ @NC) and exhibits a half-wave potential of 0.93 V vs. RHE and negligible activity degradation (ΔE_1/2 =8 mV) after 10000 cycles in 0.1 M KOH. We also demonstrate that the modulation effect is not effective for optimizing the ORR performances of Co-N₄ /Pt-N₄ and Mn-N₄ /Pt-N₄ systems.