Controlled synthesis of single cobalt atom catalysts via a facile one-pot pyrolysis for efficient oxygen reduction and hydrogen evolution reactions

An asymmetrically coordinated Zn-Co diatomic site catalyst for efficient hydrogen evolution reactions

We propose an innovative preparation method, namely, a two-step pyrolysis process, to synthesize Zn-Co bimetallic catalysts with excellent hydrogen evolution performance. In the synthesized Zn1Co1-SNC catalyst, there exists a strong interaction between Zn and Co, along with synergistic effects with S/N atoms, collectively promoting the stability of the catalyst structure. Experimental results demonstrate that the overpotential of this catalyst at 10 mA cm-2 current density is only 49 mV, and it maintains excellent hydrogen evolution performance even after 5000 cycles.

Single-atom and cluster catalysts for thermocatalytic ammonia synthesis at mild conditions

Ammonia (NH3) is closely related to the fields of food and energy that humans depend on. The exploitation of advanced catalysts for NH3 synthesis has been a research hotspot for more than one hundred years. Previous studies have shown that the Ru B5 sites (step sites on the Ru (0001) surface uniquely arranged with five Ru atoms) and Fe C7 sites (iron atoms with seven nearest neighbors) over nanoparticle catalysts are highly reactive for N2-to-NH3 conversion. In recent years, single-atom and cluster catalysts, where the B5 sites and C7 sites are absent, have emerged as promising catalysts for efficient NH3 synthesis. In this review, we focus on the recent advances in single-atom and cluster catalysts, including single-atom catalysts (SACs), single-cluster catalysts (SCCs), and bimetallic-cluster catalysts (BCCs), for thermocatalytic NH3 synthesis at mild conditions. In addition, we discussed and summarized the unique structural properties and reaction performance as well as reaction mechanisms over single-atom and cluster catalysts in comparison with traditional nanoparticle catalysts. Finally, the challenges and prospects in the rational design of efficient single-atom and cluster catalysts for NH3 synthesis were provided.

Environmental energy enhanced solar-driven evaporator with spontaneous internal convection for highly efficient water purification

Solar-driven interfacial evaporation for water purification is limited by the structural design of the solar evaporator and, more importantly, by the inability to separate the water from volatile organic compounds (VOCs) present in the water source. Here, we report a three-dimensional (3D) bifunctional evaporator based on N-doped carbon (CoNC/CF), which enables the separation of fresh water from VOCs by activating PMS during the evaporation process with a VOC removal rate of 99%. There is abundant van der Waals interaction between peroxymonosulfate (PMS) and CoNC/CF, and pyrrolic N is confirmed as the active site for binding phenol, thus contributing to the separation of phenol from water. With the advantageous features of sufficient light absorption, adequate water storage capacity, and spontaneous internal convection flow on its top surface, the 3D evaporator achieves a high evaporation rate under one sun (1 kW/m2) at 3.16 kg/m2/h. More notably, through careful structural design, additional energy from the environment and water can be utilized. With such a high evaporation rate and satisfactory purification performance, this work is expected to provide a promising platform for wastewater treatment.

Relationship between Structure and Performance of Atomic-Scale Electrocatalysts for Water Splitting

Atomic-scale electrocatalysts greatly improve the performance and efficiency of water splitting but require special adjustments of the supporting structures for anchoring and dispersing metal single atoms. Here, the structural evolution of atomic-scale electrocatalysts for water splitting is reviewed based on different synthetic methods and structural properties that create different environments for electrocatalytic activity. The rate-determining step or intermediate state for hydrogen or oxygen evolution reactions is energetically stabilized by the coordination environment to the single-atom active site from the supporting material. In large-scale practical use, maximizing the loading amount of metal single atoms increases the efficiency of the electrocatalyst and reduces the economic cost. Dual-atom electrocatalysts with two different single-atom active sites react with an increased number of water molecules and reduce the adsorption energy of water derived from the difference in electronegativity between the two metal atoms. In particular, single-atom dimers induce asymmetric active sites that promote the degradation of H2 O to H2 or O2 evolution. Consequently, the structural properties of atomic-scale electrocatalysts clarify the atomic interrelation between the catalytic active sites and the supporting material to achieve maximum efficiency.

CoSe2 nanoparticles anchored on CoNC carbon nanoplates as bifunctional electrocatalyst for flexible rechargeable Zn-air batteries

A flexible solid rechargeable Zn-air battery for advanced energy conversion and storage has extensive applications in portable electric sources, wildlife rescue and flexible wearable systems. Herein, the CoSe₂ nanoparticles anchored on cobalt-embedded N-doping carbon nanoplates (CoSe₂/CoNC) is developed as a highly active bifunctional catalyst via pyrolysis and selenization of bimetallic zeolitic imidazolate frameworks containing Zn and Co. The introduction of inactive Zn generates strong electrochemically active surface areas due to the synergistic effect between CoSe2 nanoparticles and CoNC matrix. Further, CoSe2/CoNC exhibits prominent Zn-air battery performance and even outperforms the commercially available noble-metal catalysts. Notably, a high-rate flexible Zn-air battery enabled by an alkaline composite polyacrylic acid-carboxymethyl cellulose electrolyte delivers the open-circuit potential of 1.51 V. The battery offers high wearability and performs very well under various conditions, such as soaking, drilling and sewing.

Construction of Non-Precious Metal Self-Supported Electrocatalysts for Oxygen Evolution from a Low-Temperature Immersion Perspective

Water splitting is considered as a promising technology to solve energy shortage and environmental pollution. Since oxygen evolution reaction (OER) directly affects the efficiency of hydrogen evolution, the preparation of efficient and inexpensive OER catalysts is an urgent problem. "Low-temperature immersion" (LTI) is expected to be a prospective strategy for electrocatalyst preparation due to its simplicity and energy-saving advantages. However, there is almost no comprehensive overview on the progress of LTI engineering in the construction of non-precious metal self-supported electrocatalysts for OER. Herein, this review firstly introduces the principles and applications of LTI engineering-assisted preparation of non-precious metal self-supported electrocatalysts in terms of etching and deposition. Then the mechanism of OER is analyzed from an amorphous viewpoint, and finally some perspective insights and future challenges of this method are discussed.

Axial Phosphate Coordination in Co Single Atoms Boosts Electrochemical Oxygen Evolution

Cobalt single atoms coordinated with planar four nitrogen atoms (Co₁ N₄ ) represent an efficient electrocatalyst for oxygen evolution reaction (OER), whereas the large energy barrier of CoOH dehydrogenation limits the OER activity. Herein, axial phosphate (PO₄ ) coordination is incorporated in Co₁ N₄ single atoms of cobalt phthalocyanine@carbon nanotubes (P-CoPc@CNT), so as to boost the intrinsic OER performance through manipulating the reaction pathway. With a relative low mass loading of Co (2.7%), the P-CoPc@CNT shows remarkable alkaline OER activity with the overpotential of 300 mV and Tafel slope of 41.7 mV dec^-1 , which dramatically outperforms the CoPc@CNT without axial PO₄ coordination. Based on mechanistic analysis, the axial PO₄ coordination directly participates in the OER cycle by the transformation of axial ligand. Specially, the CoOH dehydrogenation process is replaced by the dehydrogenation of HPO₄ -Co₁ N₄ intermediate, which largely decreases the energy barrier and thus benefits the whole OER process.

Hierarchal Porous Graphene-Structured Electrocatalysts with Fe-N5 Active Sites Modified with Fe Clusters for Enhanced Performance Toward Oxygen Reduction Reaction

The local coordination environment around the active centers has a major impact on tuning the intrinsic activity of M-N-C catalysts. Herein, a porous graphene with Fe-N₅ active sites modified with Fe clusters is successfully fabricated by using Fe³⁺-SCN^- and NaHCO₃ as the metal precursor and pore-forming agent, respectively. The unique Fe-N₅ configuration accompanying Fe clusters and the improved ORR activity are confirmed by various characterization techniques and theoretical calculations. Benefiting from the pores, mass and electron transfer channels are successfully constructed, making more active sites accessible and facilitating the ORR process. As a consequence, the as-prepared catalyst has an excellent ORR activity with a half-wave potential of 0.89 V, comparable selectivity, and superior stability. In addition, a homemade primary zinc-air battery using this material as the cathode catalyst has a maximum power density of 0.205 W/cm², revealing the potential of the as-constructed CSA-Fe-N-C catalyst to replace precious Pt catalysts.

Movable type printing method to synthesize high-entropy single-atom catalysts

The controllable anchoring of multiple isolated metal atoms into a single support exhibits scientific and technological opportunities, while the synthesis of catalysts with multiple single metal atoms remains a challenge and has been rarely reported. Herein, we present a general route for anchoring up to eleven metals as highly dispersed single-atom centers on porous nitride-doped carbon supports with the developed movable type printing method, and label them as high-entropy single-atom catalysts. Various high-entropy single-atom catalysts with tunable multicomponent are successfully synthesized with the same method by adjusting only the printing templates and carbonization parameters. To prove utility, quinary high-entropy single-atom catalysts (FeCoNiCuMn) is investigated as oxygen reduction reaction catalyst with much more positive activity and durability than commercial Pt/C catalyst. This work broadens the family of single-atom catalysts and opens a way to investigate highly efficient single-atom catalysts with multiple compositions.

It is challenging to integrate multi-single metal atoms into one support. In this work, the authors demonstrate the production of high-entropy single-atom catalysts via a movable typing method, which enables the anchor up to eleven metals as highly dispersed single-atom active centers on the carbon support for the oxygen reduction reaction.

Considering single-atom catalysts as photocatalysts from synthesis to application

With the ever-increased greenhouse effect and energy crisis, developing novel photocatalysts to realize high-efficient solar-driven chemicals/fuel production is of great scientific and practical significance. Recently, single-atom photocatalysts (SAPs) are promising catalysts with maximized metal dispersion and tuneable coordination environments. SAPs exhibit boosted photocatalytic performance by enhancing optical response, facilitating charge carrier transfer behaviors or directly manipulating surface reaction processes. In this regard, this article systematically reviews the state-of-the-art progress in the development and application of SAPs, especially the mechanism and performance of SAPs on various reaction processes. Some future challenges and potential research directions over SAPs are outlined at the final stage.

Impacts of Metal-Support Interaction on Hydrogen Evolution Reaction of Cobalt-Nitride-Carbide Catalyst

Cobalt-nitride-carbide (Co-N-C) catalysts are promising cost-efficient transition metal catalysts for electrocatalytic hydrogen evolution, but few works investigate the metal-support interaction (MSI) effect on hydrogen evolution reaction (HER) performance. Herein, efficient Co-N-CX catalysts with controllable MSI between encapsulated Co nanoparticles and nitrogen-doped graphitic carbon nanosheets were synthesized via a facile organic-inorganic hybridization method. Results demonstrate that the Co-N-C0.025M catalyst with the coexistence of single-atom Co sites and Co nanoparticles prepared by 0.025 M cobalt nitrate shows excellent HER performance, achieving a low overpotential of 145 mV to reach 10 mA cm-2 in 0.5 M sulfuric acid, which is mainly because the optimal MSI, which leads to a moderate hydrogen adsorption energy and improved electroactive sites, not only facilitates the charge transfer to improve the HER kinetics, but also improves the durability of the catalyst by Co-N bond anchoring and encapsulation of active Co species. This work provides guidance to further reveal the influence of MSI on their catalytic activity.

Study on the Structure-Activity Relationship Between Single-Atom, Cluster and Nanoparticle Catalysts in a Hierarchical Structure for the Oxygen Reduction Reaction

Literature reports have shown that in primary structures, single-atom catalysts exhibit better performance than cluster and nanoparticles due to their maximum atom utilization and the fine-tuning of the electronic structure of the active sites. Hierarchical structures have recently been extensively studied because of increased active sites and orderliness of channels significantly improves the catalytic performance compare to primary structures especially in nanoparticles, however, the different sized effect of catalysts research has not been reported. Herein, a unique hollow double-shell structure (a distinct cavity-containing) is used as a hierarchical model to study the possible difference between single atom, cluster, and nanoparticle and to establish the corresponding structure-activity relationship. Three Co catalysts are prepared: single atoms (Co-Catalyst-1), clusters (Co-Catalyst-2, 0.5-1 nm), and nanoparticles (Co-Catalyst-3, ≈5 nm) and their oxygen-reduction capacity is evaluated. The unique electronic interactions, the strong electron-withdrawing ability of N in Co-N₄ (Co-Catalyst-1), attract electrons from the electrode to Co, specifically by expediting the generation and transformation of the rate-determining step intermediates *OOH. The variant spatial structure which is caused by Co atom aggregation, and led to surface area, pore size, and carbon disorder, is a distinct, therefore significant variation in mass and charge transport efficiency, and activities.

Expanding the interlamellar spacing of biomass-derived hybrids with intercalated nanotubes for enhanced oxygen reduction reaction

An efficient method has been designed for creating mesopores in electrocatalysts using in situ grown CNTs.

Engineering two-dimensional metal oxides and chalcogenides for enhanced electro- and photocatalysis

Two-dimensional (2D) metal oxides and chalcogenides (MOs & MCs) have been regarded as a new class of promising electro- and photocatalysts for many important chemical reactions such as hydrogen evolution reaction, CO₂ reduction reaction and N₂ reduction reaction in virtue of their outstanding physicochemical properties. However, pristine 2D MOs & MCs generally show the relatively poor catalytic performances due to the low electrical conductivity, few active sites and fast charge recombination. Therefore, considerable efforts have been devoted to engineering 2D MOs & MCs by rational structural design and chemical modification to further improve the catalytic activities. Herein, we comprehensively review the recent advances for engineering technologies of 2D MOs & MCs, which are mainly focused on the intercalation, doping, defects creation, facet design and compositing with functional materials. Meanwhile, the relationship between morphological, physicochemical, electronic, and optical properties of 2D MOs & MCs and their electro- and photocatalytic performances is also systematically discussed. Finally, we further give the prospect and challenge of the field and possible future research directions, aiming to inspire more research for achieving high-performance 2D MOs & MCs catalysts in energy storage and conversion fields.

Single Co Atoms Implanted into N-Doped Hollow Carbon Nanoshells with Non-Planar Co-N4-1-O2 Sites for Efficient Oxygen Electrochemistry

Facile synthesis of cost-effective carbon-supported Co single atoms (Co-SAs) exhibits huge potential applications in energy storage and conversion devices. We here report the implantation of Co-SAs into hollow carbon spheres (Co-SAs-HCS) via a facile wet-chemistry strategy followed by controlled pyrolysis. Electron-rich histidine acted as a Lewis base effectively immobilizing Co²⁺ (Lewis acid) via the electrostatic effect and hydrogen bonds, thus achieving the scalable synthesis of Co-SAs-HCS. We constructed a series of histidine-Co²⁺ structure models to elucidate the formation of histidine-Co²⁺ complexes by analyzing their binding energy. X-ray absorption fine-structure results verify that central Co atoms with four N coordination atoms possess a non-planar Co-N₄ structure. Electrochemical results indicate that the as-prepared Co-SAs-HCS catalyst shows a low potential difference (0.809 V) between the oxygen evolution reaction potential at 10 mA cm^-2 and the oxygen reduction reaction half-wave potential, outperforming the commercial Pt/C catalysts (0.996 V). Moreover, an assembled Zn-air battery based on Co-SAs-HCS exhibits an unexpected long-term durability. We have demonstrated that non-planar Co-N₄-1-O₂ sites are the source for highly efficient adsorption and dissociation of O₂ molecules and then reduction of the free energy of desorption of the intermediates by density functional theory. Our findings provide a new design insight into the exploration of advanced electrocatalysts, which will be applied in the design of green energy devices in the future.

Conversion of bimetallic MOF to Ru-doped Cu electrocatalysts for efficient hydrogen evolution in alkaline media

The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction (HER) is of great importance for water splitting. Herein, we develop a facile approach for preparation of porous carbon-confined Ru-doped Cu nanoparticles (denoted as Ru-Cu@C) by direct pyrolysis of the Ru-exchanged Cu-BTC metal-organic framework. When served as the electrocatalyst for HER, strikingly, the obtained Ru-Cu@C catalyst exhibits an ultralow overpotential (only 20 mV at 10 mA cm^-2) with a small Tafel slope of 37 mV dec^-1 in alkaline electrolyte. The excellent performance is comparable or even superior to that of commercial Pt/C catalyst. Density functional theory (DFT) calculations confirm that introducing Ru atoms into Cu nanocrystals can significantly alter the desorption of H₂ to achieve a close-to-zero hydrogen adsorption energy and thereby boost the HER process. This strategy gives a fresh impetus to explore low-cost and high-performance catalysts for HER in alkaline media.

Construction of highly accessible single Co site catalyst for glucose detection

The development of high-performance glucose sensors is an urgent need, especially for diabetes mellitus diagnosis. However, the glucose monitoring is conventionally operated in an invasive finger-prick manner and their noninvasive alternatives largely suffered from the relatively poor sensitivity, selectivity, and stability, resulted from the lack of robust and efficient catalysts. In this paper, we design a concave shaped nitrogen-doped carbon framework embellished with single Co site catalyst (Co SSC) by selectively controlling the etching rate on different facet of carbon substrate, which is beneficial to the diffusion and contact of analyte. The Co SSC prompts a significant improvement in the sensitivity of the solution-gated graphene transistor (SGGT) devices, with three orders of magnitude better than those of SGGT devices without catalysts. Our findings expand the field of single site catalyst in the application of biosensors, diabetes diagnostics and personalized health-care monitoring.

Engineering the electronic and strained interface for high activity of PdMcore@Ptmonolayer electrocatalysts for oxygen reduction reaction

Alloyed nanoparticles with core-shell structures provide a favorable model to modulate interfacial interaction and surface structures at the atomic level, which is important for designing electrocatalysts with high activity and durability. Herein, core-shell structured Pd₃M@Pt/C nanoparticles with binary PdM alloy cores (M = Fe, Ni, and Co) and a monolayer Pt shell were successfully synthesized with diverse interfaces. Among these, Pd₃Fe@Pt/C exhibited the best oxygen reduction reaction catalytic performance, roughly 5.4 times more than that of the commercial Pt/C catalyst used as reference. The significantly enhanced activity is attributed to the combined effects of strain engineering, interfacial electron transfer, and improved Pt utilization. Density functional theory simulations and extended X-ray absorption fine structure analysis revealed that engineering the alloy core with moderate lattice mismatch and alloy composition (Pd₃Fe) optimizes the surface oxygen adsorption energy, thereby rendering excellent electrocatalytic activity. Future researches may use this study as a guide on the construction of highly effective core-shell electrocatalysts for various energy conversions and other applications.

Ru catalyst supported on nitrogen-doped nanotubes as high efficiency electrocatalysts for hydrogen evolution in alkaline media

Due to the potential application in the future energy conversion system, there is an increasing demand for efficient, stable and cheap platinum-free catalysts for hydrogen evolution. However, it is still a great challenge to develop electrocatalysts with high activity similar to platinum or even higher, especially those that can work under alkaline conditions. Ruthenium (Ru), as a cheap substitute for platinum, has been studied as a feasible substitute for (HER) catalyst for hydrogen evolution reaction. In this paper, we designed and developed a novel Ru catalyst (Ru@CNT) supported on nitrogen-doped carbon nanotubes. Electrochemical tests show that even under alkaline conditions (1 M KOH), Ru@CNT still shows excellent catalytic performance and good durability. It only needs 36.69 mV overpotential to reach a current density of 10 mA cm-2, and its Tafel slope is 28.82 mV dec-1. The catalytic performance of the catalyst is comparable to that of 20% Pt/C. The significant activity is mainly attributed to the chelation of highly dispersed ruthenium atoms on nitrogen-doped carbon nanotubes. Secondly, the one-dimensional pore structures supported by nitrogen heterocarbon nanotubes can provide more opportunities for active centers. Excellent HER performance makes Ru@CNT electrocatalyst have a broad application prospect in practical hydrogen production.

Facile self-template fabrication of hierarchical nickel-cobalt phosphide hollow nanoflowers with enhanced hydrogen generation performance

Developing facile methods to construct hierarchical-structured transition metal phosphides is beneficial for achieving high-efficiency hydrogen evolution catalysts. Herein, a self-template strategy of hydrothermal treatment of solid Ni-Co glycerate nanospheres followed by phosphorization is delivered to synthesize hierarchical NiCoP hollow nanoflowers with ultrathin nanosheet assembly. The microstructure of NiCoP can be availably tailored by adjusting the hydrothermal treatment temperature through affecting the hydrolysis process of Ni-Co glycerate nanospheres and the occurred Kirkendall effect. Benefitting from the promoted exposure of active sites and affluent mass diffusion routes, the HER performance of the NiCoP hollow nanoflowers has been obviously enhanced in contrast with the solid NiCoP nanospheres. The fabricated NiCoP hollow nanoflowers yield the current density of 10 mA cm^-2 at small overpotentials of 95 and 127 mV in 0.5 mol L^-1 H₂SO₄ and 1.0 mol L^-1 KOH solution, respectively. Moreover, the two-electrode alkaline cell assembled with the NiCoP and Ir/C catalysts exhibits sustainable stability for overall water splitting. The work provides a simple but efficient method to regulate the microstructure of transition metal phosphides, which is helpful for achieving high-performance hydrogen evolution catalysts based on solid-state metal alkoxides.