A Facile Route for Constructing Effective Cu-Nx Active Sites for Oxygen Reduction Reaction

Copper-cobalt diatomic bifunctional oxygen electrocatalysts based on three-dimensional porous nitrogen-doped carbon frameworks for high-performance zinc-air batteries

Transition-metal-loaded carbon-based electrocatalysts are promising alternatives to conventional precious metal electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in high-performance zinc-air batteries. However, efficiently doping transition-metal single atoms onto carbon-based frameworks is a significant challenge. Herein, an improved template-sacrificing method combining a two-step carbonization process is proposed to fabricate Cu/Co diatomic sites coanchored on a three-dimensional nitrogen-doped carbon-based framework. By optimizing the addition amounts of silica (template) and zinc chloride (foaming agent), as well as adjusting the high-temperature treatment conditions, the porous microstructure of the nitrogen-carbon framework is fine-tuned to achieve the optimal diatomic Cu/Co loading. This catalyst exhibits excellent bifunctional oxygen electrocatalytic performance, facilitating both ORR and OER, and outperforming commercial precious metal electrocatalysts. The synergistic catalytic effect of the isolated dual-metal sites for high-performance electrocatalysis of ORR and OER is thoroughly investigated through comparative studies with nitrogen-doped carbon frameworks without Cu or Co. An aqueous zinc-air battery is assembled to demonstrate its exceptional performance, including a high open-circuit voltage of 1.48 V, a high peak power density of 311 mW cm-2, and remarkable durability, exceeding 600 h. Additionally, a zinc-air battery containing a gel-polymer electrolyte is assembled to showcase its potential application in wearable electronic devices.

Atomically Dispersed Metal-Nitrogen-Carbon Catalysts for Acidic Oxygen Reduction Reaction

Designing efficient and cost-effective electrocatalysts toward oxygen reduction reaction (ORR) under demanding acidic environments plays a critical role in advancing proton exchange membrane fuel cells (PEMFCs). Metal-nitrogen-carbon (M-N-C) catalysts with atomically dispersed metals have gained attention for their affordability, excellent catalytic performance, and distinctive features including consistent active sites and high atomic utilization. Over the past decade, significant achievements have been made in this field. This review offers a comprehensive summary of the latest developments in atomically dispersed M-N-C catalysts for ORR in acidic environments along with their applications in PEMFCs. The ORR mechanisms, PEMFC configuration, and operational principles are presented first, followed by an in-depth discussion of strategies to improve the activity and stability of the PEMFC using atomically dispersed M-N-C catalysts at the cathode. Lastly, this review highlights the unresolved challenges and proposes future research pathways for advancing high-performance atomically dispersed M-N-C catalysts and PEMFCs.

Metal Atom-Support Interaction in Single Atom Catalysts toward Hydrogen Peroxide Electrosynthesis

Single metal atom catalysts (SACs) have garnered considerable attention as promising agents for catalyzing important industrial reactions, particularly the electrochemical synthesis of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (ORR). Within this field, the metal atom-support interaction (MASI) assumes a decisive role, profoundly influencing the catalytic activity and selectivity exhibited by SACs, and triggers a decade-long surge dedicated to unraveling the modulation of MASI as a means to enhance the catalytic performance of SACs. In this comprehensive review, we present a systematic summary and categorization of recent advancements pertaining to MASI modulation for achieving efficient electrochemical H2O2 synthesis. We start by introducing the fundamental concept of the MASI, followed by a detailed and comprehensive analysis of the correlation between the MASI and catalytic performance. We describe how this knowledge can be harnessed to design SACs with optimized MASI to increase the efficiency of H2O2 electrosynthesis. Finally, we distill the challenges that lay ahead in this field and provide a forward-looking perspective on the future research directions that can be pursued.

Surface Engineered Single-atom Systems for Energy Conversion

Single-atom catalysts (SACs) are demonstrated to show exceptional reactivity and selectivity in catalytic reactions by effectively utilizing metal species, making them a favorable choice among the different active materials for energy conversion. However, SACs are still in the early stages of energy conversion, and problems like agglomeration and low energy conversion efficiency are hampering their practical applications. Substantial research focus on support modifications, which are vital for SAC reactivity and stability due to the intimate relationship between metal atoms and support. In this review, a category of supports and a variety of surface engineering strategies employed in SA systems are summarized, including surface site engineering (heteroatom doping, vacancy introducing, surface groups grafting, and coordination tunning) and surface structure engineering (size/morphology control, cocatalyst deposition, facet engineering, and crystallinity control). Also, the merits of support surface engineering in single-atom systems are systematically introduced. Highlights are the comprehensive summary and discussions on the utilization of surface-engineered SACs in diversified energy conversion applications including photocatalysis, electrocatalysis, thermocatalysis, and energy conversion devices. At the end of this review, the potential and obstacles of using surface-engineered SACs in the field of energy conversion are discussed. This review aims to guide the rational design and manipulation of SACs for target-specific applications by capitalizing on the characteristic benefits of support surface engineering.

Atomic-Level Regulation of Cu-Based Electrocatalyst for Enhancing Oxygen Reduction Reaction: From Single Atoms to Polymetallic Active Sites

The slow kinetics of cathodic oxygen reduction reactions (ORR) in fuel cells and the high cost of commercial Pt-based catalysts limit their large-scale application. Cu-based single-atom catalysts (SACs) have received increasing attention as a promising ORR catalyst due to their high atom utilization, high thermodynamic activity, adjustable electronic structure, and low cost. Herein, the recent research progress of Cu-based catalysts is reviewed from single atom to polymetallic active sites for ORR. First, the design and synthesis method of Cu-based SACs are summarized. Then the atomic-level structure regulation strategy of Cu-based catalyst is proposed to improve the ORR performance. The different ORR catalytic mechanism based on the different Cu active sites is further revealed. Finally, the design principle of high-performance Cu-based SACs is proposed for ORR and the opportunities and challenges are further prospected.

Copper Single-Atom Catalysts-A Rising Star for Energy Conversion and Environmental Purification: Synthesis, Modification, and Advanced Applications

Future renewable energy supply and green, sustainable environmental development rely on various types of catalytic reactions. Copper single-atom catalysts (Cu SACs) are attractive due to their distinctive electronic structure (3d orbitals are not filled with valence electrons), high atomic utilization, and excellent catalytic performance and selectivity. Despite numerous optimization studies are conducted on Cu SACs in terms of energy conversion and environmental purification, the coupling among Cu atoms-support interactions, active sites, and catalytic performance remains unclear, and a systematic review of Cu SACs is lacking. To this end, this work summarizes the recent advances of Cu SACs. The synthesis strategies of Cu SACs, metal-support interactions between Cu single atoms and different supports, modification methods including modification for carriers, coordination environment regulating, site distance effect utilizing, and dual metal active center catalysts constructing, as well as their applications in energy conversion and environmental purification are emphatically introduced. Finally, the opportunities and challenges for the future Cu SACs development are discussed. This review aims to provide insight into Cu SACs and a reference for their optimal design and wide application.

Defect-Confinement Strategy for Constructing CuO Clusters on Carbon Nanotubes for Catalytic Oxidation of AsH3 at Room Temperature

The efficient removal of the highly toxic arsine gas (AsH3) from industrial tail gases under mild conditions remains a formidable challenge. In this study, we utilized the confinement effect of defective carbon nanotubes to fabricate a CuO cluster catalyst (CuO/ACNT), which exhibited a capacity much higher than that of CuO supported on pristine multiwalled carbon nanotubes (MWCNT) (CuO/PCNT) for catalytically oxidizing AsH3 under ambient conditions. The experimental and theoretical results show that nitric acid steam treatment could induce MWCNT surface structural defects, which facilitated more stable anchoring of CuO and then improved the oxygen activation ability, therefore leading to excellent catalytic performance. Density functional theory (DFT) calculations revealed that the catalytic oxidation of AsH3 proceeded through stepwise dehydrogenation and subsequent recombination with oxygen to form As2O3 as the final product.

Single-Atom and Dual-Atom Electrocatalysts: Synthesis and Applications

Distinguishing themselves from nanostructured catalysts, single-atom catalysts (SACs) typically consist of positively charged single metal and coordination atoms without any metal-metal bonds. Dual-atom catalysts (DACs) have emerged as extended family members of SACs in recent years. Both SACs and DACs possess characteristics that combine both homogeneous and heterogeneous catalysis, offering advantages such as uniform active sites and adjustable interactions with ligands, while also inheriting the high stability and recyclability associated with heterogeneous catalyst systems. They offer numerous advantages and are extensively utilized in the field of electrocatalysis, so they have emerged as one of the most prominent material research platforms in the direction of electrocatalysis. This review provides a comprehensive review of SACs and DACs in the field of electrocatalysis: encompassing economic production, elucidating electrocatalytic reaction pathways and associated mechanisms, uncovering structure-performance relationships, and addressing major challenges and opportunities within this domain. Our objective is to present novel ideas for developing advanced synthesis strategies, precisely controlling the microstructure of catalytic active sites, establishing accurate structure-activity relationships, unraveling potential mechanisms underlying electrocatalytic reactions, identifying more efficient reaction paths, and enhancing overall performance in electrocatalytic reactions.

Stable, Efficient, Copper Coordination Polymer-Derived Heterostructured Catalyst for Oxygen Evolution under pH-Universal Conditions

The constructure of a heterostructured interface is an effective way to design highly durable and efficient water oxidation electrocatalysts. Herein, Cu/CuCN with heterointerfaces is the first synthesized case through a simple epitaxial-like growth method, displaying superior activity and stability under pH-universal media. Associated with high electron transport and transfer of the epitaxial interfacial area, the Cu/CuCN pre-catalyst is applied to deliver the oxygen evolution reaction (OER) with lower overpotentials of 250 mV (forward scan) and 380 mV (backward scan) at 10 mA cm^-2 and demonstrates better intrinsic activity (j_ECSA of 1.0 mA cm^-2 at 420 mV) and impressive stability (136 h) in 1.0 M KOH, which exceeds most previous catalysts. Even using a nominal voltage of 1.5 V of a AA battery can drive the overall water-splitting setup. Experiments combined with theoretical simulations further uncover the existence of CuO species at the heterointerface during basic OER, which is evidence of better OER performance with abundant active sites that accelerate the conversion kinetics.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

Cu/S-Occupation Bifunctional Oxygen Catalysts for Advanced Rechargeable Zinc-Air Batteries

The design and synthesis of low-cost and highly efficient bifunctional catalysts is an inevitable path for rechargeable zinc-air batteries (rZABs). In this work, double-carbon co-supported Co-based oxide with the Cu and S substitutions are synthesized by a one-step hydrothermal method and formed a unique honeycomb structure. As expected, the (Cu, Co)₃OS₃@CNT-C₃N₄ exhibits high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with low overpotential (0.86 V), high power density (215 mW cm^-2), and long-term discharge stability (115 h). The (Cu, Co)₃OS₃@CNT-C₃N₄-based rZAB also shows a stronger charge-discharge durability with a very low voltage gap of merely 0.5 V than that of Pt/C+RuO₂. The high catalytic performances are attributed to these following reasons: (i) the porous morphology and hierarchical structure with plentiful "catalytic buffer", which accelerates the mass transfer; (ii) a high-speed electronic transmission network established by C₃N₄ and carbon nanotube (CNT), enhancing the conductivity; (iii) the strong synergistic effect between (Cu, Co)₃OS₃@CNT and C₃N₄, which improves the kinetics of ORR/OER; and (iv) the controllable occupation of Cu ions and S ions, which effectively regulates the CoO₆ surface and increases the active site density. This work not only offers a promising ORR/OER electrode for rZAB but also provides a new pathway to understand the improvement mechanism for catalysts by the bi-ion substitutions.