Rational coordination regulation in carbon-based single-metal-atom catalysts for electrocatalytic oxygen reduction reaction

Preparation and biomedical applications of single-metal atom catalysts

Nanocatalysts, including nanozymes, photocatalysts and sonocatalysts, have been investigated to trigger catalytic reactions in vivo to regulate biological microenvironments and stimulate therapeutic effects. Compared with lower metal atom utilization rate and catalytic activity of conventional nanocatalysts, single-metal atom catalysts (SACs) usually possess higher catalytic activity and selectivity owing to their well-defined structures and maximized atom utilization. Their properties are, however, strongly dependent on their composition and the preparation procedure. Here we describe the design, preparation and functionalization of SACs with single-metal atoms positioned within nitrogen-doped carbon supports. The SACs are prepared by pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) or polydopamine-derived materials. Their properties depend on, for example, the metal chosen and atoms available for coordination; four example procedures are described: Cu-N4 from Cu-ZIF-8, Ir-N5 from Ir@ZIF-8 plus melamine, Co-PN3 from triphenylphosphine@Co-ZIF-8 and Cu-SN3 from ZnS@Cu-polydopamine. These SACs need to be functionalized to, for example, reduce aggregation and in vivo corona formation before they can be used in biological applications. In this Protocol, functionalization with the proteins (that is, cholesterol oxidase and pyruvate oxidase) is used as an example. The Protocol provides advice regarding physicochemical and functional characterization, as well as for performing experiments in tumor-bearing mice. The functional experiments were designed with the aim of identifying nanocatalysts with peroxidase-like activity that generate reactive oxygen species within areas of the tumor microenvironment that have increased levels of hydrogen peroxide. SAC synthesis takes 3-4 days, functional modification requires one extra day and the most basic and essential in vitro and in vivo assays require 2-3 months.

Tunable B-Doped Cobalt Phosphide Nanosheets Engineered via Phosphorus Activation of Co-MOFs for High Efficiency Alkaline Water-Splitting

Introducing secondary heteroatoms and simultaneous in situ surface modification can enhance electrocatalysts by affecting their porosity for adjusting electrochemically active surface area (ECSA), number of active sites, and electronic properties, thus mitigating the sluggish kinetics of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline media. Here, mesoporous 3D heterostructures of boron-doped cobalt phosphide@nitrogen-doped carbon nanosheet network arrays are successfully grown on Ni foam as free-standing bifunctional electrocatalysts with controlled phosphorous levels (B-CoPx@NC/NF, x = 0.25, 0.5, and 1). Boron doping induces the Co active sites to bind O* and OOH* intermediates. Meanwhile, an optimal phosphorous content also leads to ideal adsorption strength at each reaction step, satisfying the Sabatier principle well. The optimal B-CoP0.5@NC/NF requires low overpotentials of 248 mV for OER and 95 mV for HER with long-term stability. The B-CoP0.5@NC/NF (+/-) electrolyzer exhibits a low cell potential of 1.59 V at 10 mA cm-2 for overall water-splitting, with superior activity compared to the RuO2/NF(+)//20%Pt/NF(-) electrolyzer at high current densities above 50 mA cm-2. Such exceptional bifunctional activities are attributed to the modulated electronic structure, lower charge-transfer resistance, higher ECSA, and inductive effect of B-doping, thus boosting both OER and HER activities in alkaline media.

Current status of developed electrocatalysts for water splitting technologies: from experimental to industrial perspective

The conversion of electricity into hydrogen (H2) gas through electrochemical water splitting using efficient electrocatalysts has been one of the most important future technologies to create vast amounts of clean and renewable energy. Low-temperature electrolyzer systems, such as proton exchange membrane water electrolyzers, alkaline water electrolyzers, and anion exchange membrane water electrolyzers are at the forefront of current technologies. Their performance, however, generally depends on electricity costs and system efficiency, which can be significantly improved by developing high-performance electrocatalysts to enhance the kinetics of both the cathodic hydrogen evolution reaction and the anodic oxygen evolution reaction. Despite numerous active research efforts in catalyst development, the performance of water electrolysis remains insufficient for commercialization. Ongoing research into innovative electrocatalysts and an understanding of the catalytic mechanisms are critical to enhancing their activity and stability for electrolyzers. This is still a focus at academic institutes/universities and industrial R&D centers. Herein, we provide an overview of the current state and future directions of electrocatalysts and water electrolyzers for electrochemical H2 production. Additionally, we describe in detail the technological framework of electrocatalysts and water electrolyzers for H2 production as utilized by relevant global companies.

Expanding the frontiers of electrocatalysis: advanced theoretical methods for water splitting

Electrochemical water splitting, which encompasses the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), offers a promising route for sustainable hydrogen production. The development of efficient and cost-effective electrocatalysts is crucial for advancing this technology, especially given the reliance on expensive transition metals, such as Pt and Ir, in traditional catalysts. This review highlights recent advances in the design and optimization of electrocatalysts, focusing on density functional theory (DFT) as a key tool for understanding and improving catalytic performance in the HER and OER. We begin by exploring DFT-based approaches for evaluating catalytic activity under both acidic and alkaline conditions. The review then shifts to a material-oriented perspective, showcasing key catalyst materials and the theoretical strategies employed to enhance their performance. In addition, we discuss scaling relationships that exist between binding energies and electronic structures through the use of charge-density analysis and d-band theory. Advanced concepts, such as the effects of adsorbate coverage, solvation, and applied potential on catalytic behavior, are also discussed. We finally focus on integrating machine learning (ML) with DFT to enable high-throughput screening and accelerate the discovery of novel water-splitting catalysts. This comprehensive review underscores the pivotal role that DFT plays in advancing electrocatalyst design and highlights its potential for shaping the future of sustainable hydrogen production.

Electrochemical Synthesis of Nickel Hexacyanoferrate and Nickel Sulfide on Nickel Foam as Sustainable Electrocatalysts for Hydrogen Generation via Urea Electrolysis

A promising approach to energy-efficient hydrogen production is coupling the hydrogen evolution reaction (HER) with the urea oxidation reaction (UOR), significantly reducing the energy requirements. However, achieving a low-cost yet high-performance electrocatalyst for both HER and UOR remains challenging. Here, we present a facile method for synthesizing nanoporous nickel sulfide (NiS) and nickel hexacyanoferrate (NiHCF) nanocubes directly on nickel foam (NF) without any added nickel source using a cyclic voltammetry technique. In this approach, NF serves simultaneously as the substrate and nickel source, streamlining the synthesis process. The unique nanoarchitecture of NiHCF and NiS promotes highly efficient catalytic activity for both UOR and HER. NiHCF catalyzes urea oxidation by dual active sites of Ni and Fe with its synergistic interaction, without the formation of NiOOH or FeOOH. For hydrogen production, the self-supporting NiHCF/NF||NiS/NF-coupled system achieves a notably low cell voltage of 1.8 V at 100 mA cm-2, which is approximately 487 mV lower than traditional IrO2/NF||Pt/C/NF water electrolysis. This innovative electrochemical method enables the controlled synthesis of Ni-based nanoelectrocatalysts, offering a sustainable, energy-efficient pathway for H2 production from urea-rich wastewater. This environmentally friendly strategy holds significant potential to reduce the global carbon footprint, paving the way for a greener future.

Challenges and Strategies for Synthesizing High Performance Micro and Nanoscale High Entropy Oxide Materials

High-entropy oxide micro/nano materials (HEO MNMs) have shown broad application prospects and have become hot materials in recent years. This review comprehensively provides an overview of the latest developments and covers key aspects of HEO MNMs, by discussing design principles, computer-aided structural design, synthesis challenges and strategies, as well as application areas. The analysis of the synthesis process includes the role of high-throughput process in large-scale synthesis of HEOs MNMs, along with the effects of temperature elevation and undercooling on the formation of HEO MNMs. Additionally, the article summarizes the application of high-precision and in situ characterization devices in the field of HEO MNMs, offering robust support for related research. Finally, a brief introduction to the main applications of HEO MNMs is provided, emphasizing their key performances. This review offers valuable guidance for future research on HEO MNMs, outlining critical issues and challenges in the current field.

Linker Mediated Electronic-State Manipulation of Conjugated Organic Polymers Enabling Highly Efficient Oxygen Reduction

Conjugated polymers with tailorable composition and microarchitecture are propitious for modulating catalytic properties and deciphering inherent structure-performance relationships. Herein, we report a facile linker engineering strategy to manipulate the electronic states of metallophthalocyanine conjugated polymers and uncover the vital role of organic linkers in facilitating electrocatalytic oxygen reduction reaction (ORR). Specifically, a set of cobalt phthalocyanine conjugated polymers (CoPc-CPs) wrapped onto carbon nanotubes (denoted CNTs@CoPc-CPs) are judiciously crafted via in situ assembling square-planar cobalt tetraaminophthalocyanine (CoPc(NH2)4) with different linear aromatic dialdehyde-based organic linkers in the presence of CNTs. Intriguingly, upon varying the electronic characteristic of organic linkers from terephthalaldehyde (TA) to 2,5-thiophenedicarboxaldehyde (TDA) and then to thieno/thiophene-2,5-dicarboxaldehyde (bTDA), their corresponding CNTs@CoPc-CPs exhibit gradually improved electrocatalytic ORR performance. More importantly, theoretical calculations reveal that the charge transfer from CoPc units to electron-withdrawing linkers (i.e., TDA and bTDA) drives the delocalization of Co d-orbital electrons, thereby downshifting the Co d-band energy level. Accordingly, the active Co centers with more positive valence state exhibit optimized binding energy toward ORR-relevant intermediates and thus a balanced adsorption/desorption pathway that endows significant enhancement in electrocatalytic ORR. This work demonstrates a molecular-level engineering route for rationally designing efficient polymer catalysts and gaining insightful understanding of electrocatalytic mechanisms.

General Design Concept of High-Performance Single-Atom-Site Catalysts for H2O2 Electrosynthesis

Hydrogen peroxide (H2O2) as a green oxidizing agent is widely used in various fields. Electrosynthesis of H2O2 has gradually become a hotspot due to its convenient and environment-friendly features. Single-atom-site catalysts (SASCs) with uniform active sites are the ideal catalysts for the in-depth study of the reaction mechanism and structure-performance relationship. In this review, the outstanding achievements of SASCs in the electrosynthesis of H2O2 through 2e- oxygen reduction reaction (ORR) and 2e- water oxygen reaction (WOR) in recent years, are summarized. First, the elementary steps of the two pathways and the roles of key intermediates (*OOH and *OH) in the reactions are systematically discussed. Next, the influence of the size effect, electronic structure regulation, the support/interfacial effect, the optimization of coordination microenvironments, and the SASCs-derived catalysts applied in 2e- ORR are systematically analyzed. Besides, the developments of SASCs in 2e- WOR are also overviewed. Finally, the research progress of H2O2 electrosynthesis on SASCs is concluded, and an outlook on the rational design of SASCs is presented in conjunction with the design strategies and characterization techniques.

Engineering organic polymers as emerging sustainable materials for powerful electrocatalysts

Cost-effective and high-efficiency catalysts play a central role in various sustainable electrochemical energy conversion technologies that are being developed to generate clean energy while reducing carbon emissions, such as fuel cells, metal-air batteries, water electrolyzers, and carbon dioxide conversion. In this context, a recent climax in the exploitation of advanced earth-abundant catalysts has been witnessed for diverse electrochemical reactions involved in the above mentioned sustainable pathways. In particular, polymer catalysts have garnered considerable interest and achieved substantial progress very recently, mainly owing to their pyrolysis-free synthesis, highly tunable molecular composition and microarchitecture, readily adjustable electrical conductivity, and high stability. In this review, we present a timely and comprehensive overview of the latest advances in organic polymers as emerging materials for powerful electrocatalysts. First, we present the general principles for the design of polymer catalysts in terms of catalytic activity, electrical conductivity, mass transfer, and stability. Then, the state-of-the-art engineering strategies to tailor the polymer catalysts at both molecular (i.e., heteroatom and metal atom engineering) and macromolecular (i.e., chain, topology, and composition engineering) levels are introduced. Particular attention is paid to the insightful understanding of structure-performance correlations and electrocatalytic mechanisms. The fundamentals behind these critical electrochemical reactions, including the oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, oxygen evolution reaction, and hydrogen oxidation reaction, as well as breakthroughs in polymer catalysts, are outlined as well. Finally, we further discuss the current challenges and suggest new opportunities for the rational design of advanced polymer catalysts. By presenting the progress, engineering strategies, insightful understandings, challenges, and perspectives, we hope this review can provide valuable guidelines for the future development of polymer catalysts.

Rational Modulation of Single Atom Coordination Microenvironments in a BCN Monolayer for Multifunctional Electrocatalysis

Single-atom (SA) catalysts (SACs) have demonstrated outstanding catalytic performances toward plenty of relevant electrochemical reactions. Nevertheless, controlling the coordination microenvironment of catalytically active SAs to further enhance their catalytic oerformences has remained elusive up to now. Herein, a systematic investigation of 20 transition metal atoms that are coordinated with 20 different microenvironments in a boroncarbon-nitride monolayer (BCN) is conducted using high-throughput density functional theory calculations. The experimentally synthesized ternary BCN monolayer contains carbon, nitrogen, and boron atoms in its 2D network, thus providing a lot of new coordination environments than those of the current Cx Ny nanoplatforms. By exploring the structural/electrochemical stability, catalytic activity, selectivity, and electronic properties of 400 (20 × 20) TM-BCN moieties, it is discovered that specific SA coordination environments can achieve superior stability and selectivity for different electrocatalytic reactions. Moreover, a universal descriptor to accelerate the experimental process toward the synthesis of BCN-SACs is reported. These findings not only provide useful guidance for the synthesis of efficient multifunctional BCN-SACs but also will immediately benefit researchers by levering up their understanding of the mechanistic effects of SA coordination microenvironments on electrocatalytic reactions.

2D Materials Beyond Post-AI Era: Smart Fibers, Soft Robotics, and Single Atom Catalysts

Recent consecutive discoveries of various 2D materials have triggered significant scientific and technological interests owing to their exceptional material properties, originally stemming from 2D confined geometry. Ever-expanding library of 2D materials can provide ideal solutions to critical challenges facing in current technological trend of the fourth industrial revolution. Moreover, chemical modification of 2D materials to customize their physical/chemical properties can satisfy the broad spectrum of different specific requirements across diverse application areas. This review focuses on three particular emerging application areas of 2D materials: smart fibers, soft robotics, and single atom catalysts (SACs), which hold immense potentials for academic and technological advancements in the post-artificial intelligence (AI) era. Smart fibers showcase unconventional functionalities including healthcare/environmental monitoring, energy storage/harvesting, and antipathogenic protection in the forms of wearable fibers and textiles. Soft robotics aligns with future trend to overcome longstanding limitations of hard-material based mechanics by introducing soft actuators and sensors. SACs are widely useful in energy storage/conversion and environmental management, principally contributing to low carbon footprint for sustainable post-AI era. Significance and unique values of 2D materials in these emerging applications are highlighted, where the research group has devoted research efforts for more than a decade.

Asymmetric Coordination Environment Engineering of Atomic Catalysts for CO2 Reduction

Single-atom catalysts (SACs) have emerged as well-known catalysts in renewable energy storage and conversion systems. Several supports have been developed for stabilizing single-atom catalytic sites, e.g., organic-, metal-, and carbonaceous matrices. Noticeably, the metal species and their local atomic coordination environments have a strong influence on the electrocatalytic capabilities of metal atom active centers. In particular, asymmetric atom electrocatalysts exhibit unique properties and an unexpected carbon dioxide reduction reaction (CO₂RR) performance different from those of traditional metal-N₄ sites. This review summarizes the recent development of asymmetric atom sites for the CO₂RR with emphasis on the coordination structure regulation strategies and their effects on CO₂RR performance. Ultimately, several scientific possibilities are proffered with the aim of further expanding and deepening the advancement of asymmetric atom electrocatalysts for the CO₂RR.