Driving the Oxygen Evolution Reaction by Nonlinear Cooperativity in Bimetallic Coordination Catalysts

Electrosynthesis of Transition Metal Coordinated Polymers for Active and Stable Oxygen Evolution

Transition metal coordination polymers (TM-CP) are promising inexpensive and flexible electrocatalysts for oxygen evolution reaction in water electrolysis, while their facile synthesis and controllable regulation remain challenging. Here we report an anodic oxidation-electrodeposition strategy for the growth of TM-CP (TM=Fe, Co, Ni, Cr, Mn; CP=polyaniline, polypyrrole) films on a variety of metal substrates that act as both catalyst supports and metal ion sources. An exemplified bimetallic NiFe-polypyrrole (NiFe-PPy) features superior mechanical stability in friction and exhibits high activity with long-term durability in alkaline seawater (over 2000 h) and anion exchange membrane electrolyzer devices at current density of 500 mA cm-2. Spectroscopic and microscopic analysis unravels the configurations with atomically distributed metal sites induced by d-π conjugation, which transforms into a mosaic structure with NiFe (oxy)hydroxides embedded in PPy matrix during oxygen evolution. The superior catalytic performance is ascribed to the anchoring effect of PPy that inhibits metal dissolution, the strong substrate-to-catalyst interaction that ensures good adhesion, and the Fe/Ni-N coordination that modulates the electronic structures to facilitate the deprotonation of *OOH intermediate. This work provides a general strategy and mechanistic insight into building robust inorganic/polymer composite electrodes for oxygen electrocatalysis.

Nitrogen-Rich Covalent Organic Polymer for Metal-Free Tandem Catalysis and Postmetalation-Actuated High-Performance Water Oxidation

Development of high-performing catalytic materials for selective and mild chemical transformations through adhering to the principles of sustainability remains a central focus in modern chemistry. Herein, we report the template-free assembly of a thermochemically robust covalent organic polymer (COP: 1) from 2,2'-bipyridine-5,5'-dicarbonyl dichloride and 2,4,6-tris(4-aminophenyl)triazine as [2 + 3] structural motifs. The two-dimensional (2D) layered architecture contains carboxamide functionality, delocalized π-cloud, and free pyridyl-N site-decked pores. Such trifunctionalization benefits this polymeric network exhibiting tandem alcohol oxidation-Knoevenagel condensation. In contrast to common metal-based catalysts, 1 represents a one of a kind metal-free alcohol oxidation reaction via extended π-cloud delocalization-mediated free radical pathway, as comprehensively supported from diverse control experiments. In addition to reasonable recyclability and broad substrate scope, the mild reaction condition underscores its applicability in benign synthesis of valuable product benzylidene malononitrile. Integration of 2,2'-bipyridyl units in this 2D COP favors anchoring non-noble metal ions to devise 1-M (M: Ni2+/ Co2+) that demonstrate outstanding electrochemical oxygen evolution reaction in alkaline media with high chronoamperometric stability. Electrochemical parameters of both 1-Co and 1-Ni outperform some benchmark, commercial, as well as a majority of contemporary OER catalysts. Specifically, the overpotential and Tafel slope (280 mV, 58 mV/dec) for 1-Ni is better than 1-Co (360 mV, 78 mV/dec) because of increased charge accumulation as well as a higher number of active sites compared to the former. In addition, the turnover frequency of 1-Ni is found to be 6 times higher than that of 1-Co and ranks among top-tier water oxidation catalysts. The results provide valuable insights in the field of metal-free tandem catalysis as well as promising electrochemical water splitting at the interface of task-specific functionality fuelling in polymeric organic networks.

High-entropy-perovskite subnanowires for photoelectrocatalytic coupling of methane to acetic acid

The incorporation of multiple immiscible metals in high-entropy oxides can create the unconventional coordination environment of catalytic active sites, while the high formation temperature of high-entropy oxides results in bulk materials with low specific surface areas. Here we develop the high-entropy LaMnO3-type perovskite-polyoxometalate subnanowire heterostructures with periodically aligned high-entropy LaMnO3 oxides and polyoxometalate under a significantly reduced temperature of 100 oC, which is much lower than the temperature required by state-of-the-art calcination methods for synthesizing high-entropy oxides. The high-entropy LaMnO3-polyoxometalate subnanowires exhibit excellent catalytic activity for the photoelectrochemical coupling of methane into acetic acid under mild conditions (1 bar, 25 oC), with a high productivity (up to 4.45 mmol g‒1cat h‒1) and selectivity ( > 99%). Due to the electron delocalization at the subnanometer scale, the contiguous active sites of high-entropy LaMnO3 and polyoxometalate in the heterostructure can efficiently activate C - H bonds and stabilize the resulted *COOH intermediates, which benefits the in situ coupling of *CH3 and *COOH into acetic acid.

Stabilization versus competing de-metalation, trans-metalation and (cyclo)-dehydrogenation of Pd porphyrins at a copper surface

Metal-porphyrins are studied intensively due their potential applications, deriving from the variety of electronic and chemical properties, tunable by selecting metal centers and functional groups. Metalation, de- and trans-metalation processes are fundamental in this sense to investigate both the synthesis and the stability of these molecular building blocks. More specifically, Pd coordination in tetrapyrroles revealed to be potentially interesting in the fields of cancer therapy, drug delivery and light harvesting. Thus, we focused on the stability of palladium tetraphenyl porphyrins (PdTPPs) on a copper surface by means of combined spectroscopy and microscopy approaches. We find that PdTPPs undergo coverage-dependent trans-metalation accompanied by steric rearrangements already at room temperature, and fully trans-metalate to CuTPPs upon mild annealing. Side reactions such as (cyclo)-dehydrogenation and structural reorganization affect the molecular layer, with Pd-Cu alloying and segregation occurring at higher temperature. Instead, oxygen passivation of the Cu support prevents the metal-involving reactions, thus preserving the layer and increasing the chemical and temperature stability of the Pd porphyrins.

Nickel-Iron-Layered Double Hydroxide Electrocatalyst with Nanosheets Array for High Performance of Water Splitting

Developing high-performance and cost-competitive electrocatalysts have great significance for the massive commercial production of water-splitting hydrogen. Ni-based electrocatalysts display tremendous potential for electrocatalytic water splitting. Herein, we synthesize a novel NiFe-layered double hydroxide (LDH) electrocatalyst in nanosheets array on high-purity Ni foam. By adjusting the Ni/Fe ratio, the microstructure, and even the behavior of the electrocatalyst in the oxygen evolution reaction (OER), changes significantly. The as-obtained material shows a small overpotential of 223 mV at 10 mAcm-2 as well as a low Tafel slope of 48.9 mV·dec-1 in the 1 M KOH electrolyte. In addition, it can deliver good stability for at least 24 h of continuous working at 10 mAcm-2. This work proposes a strategy for engineering catalysts and provides a method for the development of other Ni-based catalysts with excellent performance.

Structural engineering of atomic catalysts for electrocatalysis

As a burgeoning category of heterogeneous catalysts, atomic catalysts have been extensively researched in the field of electrocatalysis. To satisfy different electrocatalytic reactions, single-atom catalysts (SACs), diatomic catalysts (DACs) and triatomic catalysts (TACs) have been successfully designed and synthesized, in which microenvironment structure regulation is the core to achieve high-efficiency catalytic activity and selectivity. In this review, the effect of the geometric and electronic structure of metal active centers on catalytic performance is systematically introduced, including substrates, central metal atoms, and the coordination environment. Then theoretical understanding of atomic catalysts for electrocatalysis is innovatively discussed, including synergistic effects, defect coupled spin state change and crystal field distortion spin state change. In addition, we propose the challenges to optimize atomic catalysts for electrocatalysis applications, including controlled synthesis, increasing the density of active sites, enhancing intrinsic activity, and improving the stability. Moreover, the structure-function relationships of atomic catalysts in the CO2 reduction reaction, nitrogen reduction reaction, oxygen reduction reaction, hydrogen evolution reaction, and oxygen evolution reaction are highlighted. To facilitate the development of high-performance atomic catalysts, several technical challenges and research orientations are put forward.

Design Principles of Single-Atom Catalysts for Oxygen Evolution Reaction: From Targeted Structures to Active Sites

Hydrogen production from electrolytic water electrolysis is considered a viable method for hydrogen production with significant social value due to its clean and pollution-free nature, high hydrogen production efficiency, and purity, but the anode oxygen evolution reaction (OER) process is complex and kinetically slow. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites often exhibit high catalytic activity and are expected to be extensively applied. The catalytic performance of OER can be further improved by precise regulation of the structure through electronic effects, coordination environment, heteroatomic doping, and so on. In this review, the mechanisms of OER under different conditions are introduced, the latest research progress of SACs in the field of OER is systematically summarized, and then the effects of various structural regulation strategies on catalytic performance are discussed, and principles and ideas for the design of SACs for OER are proposed. In the end, the outstanding issues and current challenges in this field are summarized.

Fabrication of Two-Dimensional Homo-Bimetallic Porphyrin Framework Thin Films for Optimizing Nonlinear Optical Limiting

Developing efficient metal-organic framework (MOF) optical devices with tunable third-order nonlinear optical (NLO) properties is an important challenge for scientific research and practical application. Herein, 2D monometallic and hetero/homo-bimetallic porphyrin MOF thin films (ZnTCPP(M) M = H2, Fe, Zn) were fabricated using the liquid-phase epitaxial (LPE) layer-by-layer (LBL) method to investigate the metal substitution dependent third-order NLO behavior. The prepared homo-bimetallic ZnTCPP(Zn) thin film exhibited enhanced third-order NLO performance with a higher third-order nonlinear susceptibility of ∼4.21 × 10-7 esu compared to monometallic and hetero-bimetallic counterparts. Additionally, theoretical calculations were performed to complement the experimental findings and revealed that the enhanced NLO effect of the ZnTCPP(Zn) thin film is mainly attributed to the enhanced local excitation. These findings not only provide a comprehensive understanding of the relationship between metal types and the NLO behavior of porphyrin MOF thin films but also offer valuable insights into the design and optimization of NLO devices.

Water Activation Triggered by Cu-Co Double-Atom Catalyst for Silane Oxidation

High-performance catalysts sufficient to significantly reduce the energy barrier of water activation are crucial in facilitating reactions that are restricted by water dissociation. Herein we present a Cu-Co double-atom catalyst (CuCo-DAC), which possesses a uniform and well-defined CuCoN6 (OH) structure, and works together to promote water activation in silane oxidation. The catalyst achieves superior catalytic performance far exceeding that of single-atom catalysts (SACs). Various functional silanes are converted into silanols with up to 98 % yield and 99 % selectivity. Kinetic studies show that the activation energy of silane oxidation by CuCo-DAC is significantly lower than that of Cu single-atom catalyst (Cu-SAC) and Co single-atom catalyst (Co-SAC). Theoretical calculations demonstrate two different reaction pathways where water splitting is the rate-determining step and it is accelerated by CuCo-DAC, whereas H2 formation is key for its single-atom counterpart.

Recent Advances in Engineering of 2D Materials-Based Heterostructures for Electrochemical Energy Conversion

2D materials, such as graphene, transition metal dichalcogenides, black phosphorus, layered double hydroxides, and MXene, have exhibited broad application prospects in electrochemical energy conversion due to their unique structures and electronic properties. Recently, the engineering of heterostructures based on 2D materials, including 2D/0D, 2D/1D, 2D/2D, and 2D/3D, has shown the potential to produce synergistic and heterointerface effects, overcoming the inherent restrictions of 2D materials and thus elevating the electrocatalytic performance to the next level. In this review, recent studies are systematically summarized on heterostructures based on 2D materials for advanced electrochemical energy conversion, including water splitting, CO2 reduction reaction, N2 reduction reaction, etc. Additionally, preparation methods are introduced and novel properties of various types of heterostructures based on 2D materials are discussed. Furthermore, the reaction principles and intrinsic mechanisms behind the excellent performance of these heterostructures are evaluated. Finally, insights are provided into the challenges and perspectives regarding the future engineering of heterostructures based on 2D materials for further advancements in electrochemical energy conversion.

Effects of tuning the structural symmetry of cobalt porphyrin on electrocatalytic oxygen reduction reactions

Metalloporphyrins have attracted significant attention as highly promising alternatives to Pt-based electrocatalysts in the realm of oxygen reduction reactions (ORRs). While the structure of porphyrin is widely recognized as a pivotal factor influencing the ORR performance, the impact of molecular symmetry, which is one of the key properties of the molecular structure, has rarely been understood and its effects remain largely unexplored. Herein, we designed and synthesized two triphenylamine (TPA)-substituted cobalt porphyrins, the asymmetric aBz-TCoP and the symmetric Bz-2TCoP, which are doped onto carbon black to construct composite catalysts for ORRs. The electronic structures of both porphyrins are determined through density functional theory (DFT) calculations, and the morphology and electronic states of the composites are examined by spectroscopic techniques. A series of electrochemical measurements demonstrate the superior activity, selectivity and durability of Bz-2TCoP/C to aBz-TCoP/C in ORRs conducted in both acidic and alkaline electrolytes. The improved ORR properties of the symmetric porphyrin may stem from the steric properties rather than the electronic properties of the chemical structure. This work represents a preliminary study on the effects of porphyrin structural symmetry on electrocatalysis and provides a potential strategy for further structural modifications of metalloporphyrins, as non-noble metal electrocatalysts, to enhance the ORR performance.

Boron-Doped Activated Carbon Supports for Cobalt-Catalyzed Oxygen Evolution in Alkaline Electrolyte

Activated carbons (ACs) are the most widely used and attractive support materials for electrocatalytic applications because of their significant surface areas, high electrical conductivities, and moderate affinities toward supported metal catalysts. However, the corrosive behavior of ACs at oxidative potentials causes an inevitable reduction in the active surface area of supported catalysts, resulting in the continuous deterioration of their electrocatalytic performance. Therefore, the introduction of corrosion-resistant durable catalyst supports is essential for sustainable and efficient electrocatalysis. Here, we modified ACs to obtain different boron (B)-doped structures via doping-temperature controls to investigate the corrosion resistance of B-doped ACs. With increasing doping temperature, the B-doped ACs exhibited a decreased defect density and enhanced crystallinity owing to the accelerating dopant-induced graphitization. We found that the substitution of B atoms into the carbon lattice improved the structural integrity of the carbon structure, and cyclic voltammetry (CV) tests suggested that the highly B-substituted structures caused electrochemical surface passivation against carbon corrosion. Moreover, B-doped ACs significantly contributed to the increase in loading mass of cobalt (Co)-based catalyst on them and the electrochemical durability toward the oxygen evolution reaction as catalyst-support hybrid. The B22 (B-doped AC obtained at a 2200 °C B-doping temperature)-supported Co catalyst with the lowest oxidation current exhibited a voltage change of 32 mV at a current density of 10 mA/cm² (ΔE_j=10) after 10,000 cycles, which was a factor of ∼7 higher cycle durability and stability than that of the conventional IrO₂ catalyst (ΔE_j=10 = 205 mV). Here, we propose that surface engineering by B-doping to improve the structural integrity of ACs is an attractive method for designing durable electrocatalytic support materials.

Catalyst-Support Interactions Promoted Acidic Electrochemical Oxygen Evolution Catalysis: A Mini Review

In the context of the growing human demand for green secondary energy sources, proton-exchange membrane water electrolysis (PEMWE) is necessary to meet the high-efficiency production of high-purity hydrogen required for proton-exchange membrane fuel cells (PEMFCs). The development of stable, efficient, and low-cost oxygen evolution reaction (OER) catalysts is key to promoting the large-scale application of hydrogen production by PEMWE. At present, precious metals remain irreplaceable in acidic OER catalysis, and loading the support body with precious metal components is undoubtedly an effective strategy to reduce costs. In this review, we will discuss the unique role of common catalyst-support interactions such as Metal-Support Interactions (MSIs), Strong Metal-Support Interactions (SMSIs), Strong Oxide-Support Interactions (SOSIs), and Electron-Metal-Support Interactions (EMSIs) in modulating catalyst structure and performance, thereby promoting the development of high-performance, high-stability, low-cost noble metal-based acidic OER catalysts.

Towards the Future of Polymeric Hybrids of Two-Dimensional Black Phosphorus or Phosphorene: From Energy to Biological Applications

With the advent of a new 2D nanomaterial, namely, black phosphorus (BP) or phosphorene, the scientific community is now dedicated to focusing on and exploring this 2D material offering elusive properties such as a higher carrier mobility, biocompatibility, thickness-dependent band gap, and optoelectronic characteristics that can be harnessed for multiple applications, e.g., nanofillers, energy storage devices, field effect transistors, in water disinfection, and in biomedical sciences. The hexagonal ring of phosphorus atoms in phosphorene is twisted slightly, unlike how it is in graphene. Its unique characteristics, such as a high carrier mobility, anisotropic nature, and biocompatibility, have attracted much attention and generated further scientific curiosity. However, despite these interesting features, the phosphorene or BP poses challenges and causes frustrations when it comes to its stability under ambient conditions and processability, and thus in order to overcome these hurdles, it must be conjugated or linked with the suitable and functional organic counter macromolecule in such a way that its properties are not compromised while providing a protection from air/water that can otherwise degrade it to oxides and acid. The resulting composites/hybrid system of phosphorene and a macromolecule, e.g., a polymer, can outperform and be exploited for the aforementioned applications. These assemblies of a polymer and phosphorene have the potential for shifting the paradigm from exhaustively used graphene to new commercialized products offering multiple applications.

Exceptional catalytic activity of oxygen evolution reaction via two-dimensional graphene multilayer confined metal-organic frameworks

Oxygen evolution reaction (OER) plays a key role in many renewable energy technologies such as water splitting and metal-air batteries. Metal-organic frameworks (MOFs) are appealing to design efficient OER electrocatalysts, however, their intrinsic poor conductivity strongly hinders the activity. Here, we show a strategy to boost the OER activity of poor-conductive MOFs by confining them between graphene multilayers. The resultant NiFe-MOF//G gives a record-low overpotential of 106 mV to reach 10 mA cm⁻² and retains the activity over 150 h, which is in significant contrast to 399 mV of the pristine NiFe-MOF. We use X-ray absorption spectroscopy (XAS) and computations to demonstrate that the nanoconfinement from graphene multilayers not only forms highly reactive NiO₆-FeO₅ distorted octahedral species in MOF structure but also lowers limiting potential for water oxidation reaction. We also demonstrate that the strategy is applicable to other MOFs of different structures to largely enhance their electrocatalytic activities.

While metal-organic frameworks offer a diverse array of structural motifs for electrocatalysis, poor conductivity and mass permeability limit performances. Here, authors confine low-conductivity metal-organic frameworks between graphene multilayers to enhance oxygen evolution performances.

A Paradigmatic Approach to Find the Valency-Based K-Banhatti and Redefined Zagreb Entropy for Niobium Oxide and a Metal-Organic Framework

Entropy is a thermodynamic function in chemistry that reflects the randomness and disorder of molecules in a particular system or process based on the number of alternative configurations accessible to them. Distance-based entropy is used to solve a variety of difficulties in biology, chemical graph theory, organic and inorganic chemistry, and other fields. In this article, the characterization of the crystal structure of niobium oxide and a metal-organic framework is investigated. We also use the information function to compute entropies by building these structures with degree-based indices including the K-Banhatti indices, the first redefined Zagreb index, the second redefined Zagreb index, the third redefined Zagreb index, and the atom-bond sum connectivity index.

Photo-induced reversible nitric oxide capture by Fe-M(CO2H)4 (M = Co, Ni, Cu) as a building block of mixed-metal BTC-based MOFs

Metal-organic frameworks incorporating mixed-metal sites (MM-MOFs) have emerged as promising candidates in the development of sensing platforms for the detection of paramagnetic species. In this context, the present study explores the photo-induced switching behavior of mixed-metal Fe-M (M = Co, Ni, Cu) formate (Fe-M(CO₂H)₄), as an experimentally feasible strategy for the reversible capture of nitric oxide (NO). Using Fe-M(CO₂H)₄ as a building block of synthesized MOFs based on BTC (benzene-1,3,5-tricarboxylic acid), molecular simulations of NO adsorption on Fe-M(CO₂H)₄ were conducted to provide a template for evaluating the behavior of BTC-based MOFs towards NO. Accordingly, the relationship between the magnetic properties and adsorption behaviors of Fe-M(CO₂H)₄ towards NO gas molecules was evaluated before and after photoexcitation. We show that the photo-induced effect on the magnetic properties of Fe-M(CO₂H)₄ changes the interaction strength between NO and the Fe-M(CO₂H)₄ systems. NO chemisorption over Fe-Ni(CO₂H)₄ indicates that nickel-doped Fe-BTC MOFs can be efficiently applied for capturing purposes. Moreover, our calculations show a switching behavior between physisorption and chemisorption of the NO molecules over Fe-Co(CO₂H)₄, occurring through magnetic modulation under UV-Vis irradiation. As far as we know, this is the first study that proposes light-controlled reversible NO capture using MOFs. The present study provides a promising platform for reversible NO capture using MM-MOF-incorporated BTC building blocks.

Successive Deprotonation Steering the Structural Evolution of Supramolecular Assemblies on Ag(111)

In this study, we demonstrate the structural evolution of a two-dimensional (2D) supramolecular assembly system, which is steered by the thermally activated deprotonation of the primary organic building blocks on a Ag(111) surface. Scanning tunneling microscopy revealed that a variety of structures, featuring distinct structural, chiral, and intermolecular bonding characters, emerged with the gradual thermal treatments. According to our structural analysis, in combination with density function theory calculations, the structural evolution can be attributed to the successive deprotonation of the organic building blocks due to the inductive effect. Our finding offers a facile strategy towards controlling the supramolecular assembly pathways and provides a comprehensive understanding of the 2D crystal engineering on surfaces.

Metal-Corrole-Based Porous Organic Polymers for Electrocatalytic Oxygen Reduction and Evolution Reactions

Integrating molecular catalysts into designed frameworks often enables improved catalysis. Compared with porphyrin-based frameworks, metal-corrole-based frameworks have been rarely developed, although monomeric metal corroles are usually more efficient than porphyrin counterparts for the electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). We herein report on metal-corrole-based porous organic polymers (POPs) as ORR and OER electrocatalysts. M-POPs (M=Mn, Fe, Co, Cu) were synthesized by coupling metal 10-phenyl-5,15-(4-iodophenyl)corrole with tetrakis(4-ethynylphenyl)methane. Compared with metal corrole monomers, M-POPs displayed significantly enhanced catalytic activity and stability. Co-POP outperformed other M-POPs by achieving four-electron ORR with a half-wave potential of 0.87 V vs. RHE and reaching 10 mA cm^-2 OER current density at 340 mV overpotential. This work is unparalleled to develop and explore metal-corrole-based POPs as electrocatalysts.

Recent Advances in Porphyrin-Based Systems for Electrochemical Oxygen Evolution Reaction

Oxygen evolution reaction (OER) plays a pivotal role in the development of renewable energy methods, such as water-splitting devices and the use of Zn–air batteries. First-row transition metal complexes are promising catalyst candidates due to their excellent electrocatalytic performance, rich abundance, and cheap price. Metalloporphyrins are a class of representative high-efficiency complex catalysts owing to their structural and functional characteristics. However, OER based on porphyrin systems previously have been paid little attention in comparison to the well-described oxygen reduction reaction (ORR), hydrogen evolution reaction, and CO2 reduction reaction. Recently, porphyrin-based systems, including both small molecules and porous polymers for electrochemical OER, are emerging. Accordingly, this review summarizes the recent advances of porphyrin-based systems for electrochemical OER. Firstly, the electrochemical OER for water oxidation is discussed, which shows various methodologies to achieve catalysis from homogeneous to heterogeneous processes. Subsequently, the porphyrin-based catalytic systems for bifunctional oxygen electrocatalysis including both OER and ORR are demonstrated. Finally, the future development of porphyrin-based catalytic systems for electrochemical OER is briefly prospected.

Salt-Templated Nanoarchitectonics of CoSe2-NC Nanosheets as an Efficient Bifunctional Oxygen Electrocatalyst for Water Splitting

Recently, the extensive research of efficient bifunctional electrocatalysts (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) on water splitting has drawn increasing attention. Herein, a salt-template strategy is prepared to synthesize nitrogen-doped carbon nanosheets encapsulated with dispersed CoSe₂ nanoparticles (CoSe₂-NC NSs), while the thickness of CoSe₂-NC NSs is only about 3.6 nm. Profiting from the ultrathin morphology, large surface area, and promising electrical conductivity, the CoSe₂-NC NSs exhibited excellent electrocatalytic of 10 mA·cm⁻² current density at small overpotentials of 247 mV for OER and 75 mV for HER. Not only does the nitrogen-doped carbon matrix effectively avoid self-aggregation of CoSe₂ nanoparticles, but it also prevents the corrosion of CoSe₂ from electrolytes and shows favorable durability after long-term stability tests. Furthermore, an overall water-splitting system delivers a current density of 10 mA·cm⁻² at a voltage of 1.54 V with resultants being both the cathode and anode catalyst in alkaline solutions. This work provides a new way to synthesize efficient and nonprecious bifunctional electrocatalysts for water splitting.

Catalytic Activity Enhancement on Alcohol Dehydrogenation via Directing Reaction Pathways from Single- to Double-Atom Catalysis

To further improve the intrinsic reactivity of single-atom catalysts (SACs), the controllable modification of a single site by coordinating with a second neighboring metal atom, developing double-atom catalysts (DACs), affords new opportunities. Here we report a catalyst that features two bonded Fe-Co double atoms, which is well represented by an FeCoN₆(OH) ensemble with 100% metal dispersion, that work together to switch the reaction mechanism in alcohol dehydrogenation under oxidant-free conditions. Compared with Fe-SAC and Co-SAC, FeCo-DAC displays higher activity performance, yielding the desired products in up to 98% yields. Moreover, a broad diversity of benzyl alcohols and aliphatic alcohols convert into the corresponding dehydrogenated products with excellent yields and high selectivity. The kinetic reaction results show that lower activation energy is obtained by FeCo-DAC than that by Fe-SAC and Co-SAC. Moreover, computational studies demonstrate that the reaction path by DACs is different from that by SACs, providing a rationale for the observed enhancements.

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

Water oxidation is still one of the most important challenges to develop efficient artificial photosynthetic devices. In recent decades, the development and study of molecular complexes for water oxidation have allowed insight into the principles governing catalytic activity and the mechanism as well as establish ligand design guidelines to improve performance. However, their durability and long-term stability compromise the performance of molecular-based artificial photosynthetic devices. In this context, heterogenization of molecular water oxidation catalysts on electrode surfaces has emerged as a promising approach for efficient long-lasting water oxidation for artificial photosynthetic devices. This review covers the state of the art of strategies for the heterogenization of molecular water oxidation catalysts onto electrodes for (photo)electrochemical water oxidation. An overview and description of the main binding strategies are provided explaining the advantages of each strategy and their scope. Moreover, selected examples are discussed together with the the differences in activity and stability between the homogeneous and the heterogenized system when reported. Finally, the common design principles for efficient (photo)electrocatalytic performance summarized.

Free-base porphyrin polymer for bifunctional electrochemical water splitting

Water splitting is considered a promising approach for renewable and sustainable energy conversion. The development of water splitting electrocatalysts that have low-cost, long-lifetime, and high-performance is an important area of research for the sustainable generation of hydrogen and oxygen gas. Here, we report a metal-free porphyrin-based two-dimensional crystalline covalent organic polymer obtained from the condensation of terephthaloyl chloride and 5,10,15,20-tetrakis(4-aminophenyl) porphyrin which is stabilized by an extensive hydrogen bonding network. This material exhibits bifunctional electrocatalytic performance towards water splitting with onset overpotentials, η, of 36 mV and 110 mV for HER (in 0.5 M H₂SO₄) and OER (in 1.0 M KOH), respectively. The as-synthesized material is also able to perform water splitting in neutral phosphate buffer saline solution, with 294 mV for HER and 520 mV for OER, respectively. Characterized by electrochemical impedance spectroscopy (EIS) and chronoamperometry, the as-synthesized material also shows enhanced conductivity and stability compared to its molecular counterpart. Inserting a non-redox active zinc metal center in the porphyrin unit leads to a decrease in electrochemical activity towards both HER and OER, suggesting the four-nitrogen porphyrin core is the active site. The high performance of this metal-free material towards water splitting provides a sustainable alternative to the use of scarce and expensive metal electrocatalysts in energy conversion for industrial applications.

Water splitting is considered a promising approach for renewable and sustainable energy conversion.

Oriented Assembly of 2D Metal-Pyridylporphyrinic Framework Films for Giant Nonlinear Optical Limiting

The development of metal-organic frameworks (MOFs) with nonlinear optical (NLO) properties is of pronounced significance for optical devices. Herein, a series of 2D MOFs ZnTPyP(M) (TPyP = 5,10,15,20-tetrakis(4-pyridyl)porphyrin, M = Cu, Ni, Mn, H₂) films with [010]-orientation growth composed of ultrathin nanosheets from a pyridylporphyrinic ligand are first obtained by using a liquid-phase epitaxial (LPE) layer-by-layer (lbl) growth approach. ZnTPyP(M) films show a giant nonlinear optical limiting (OL) response and can be modulated by tuning the type of metalloporphyrinic ligands. As a result, ZnTPyP(Cu) film exhibits the highest nonlinear absorption coefficient of 5.7 × 10^-6 m/W compared to other reported NLO materials. Density functional theory calculations were consistent with the experimental results, revealing that the tunable π-π* local excitation and the increased delocalization of the metalloporphyrinic group regulate the NLO performance of ZnTPyP(M) films. These findings provide new insight into the effect of 2D porphyrinic MOFs toward the NLO response and offer new film candidates for nonlinear OL application.

Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal-Organic Coordination Networks

In recent studies, porphyrin derivatives have been frequently used as building blocks for the fabrication of metal-organic coordination networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions (UHV). The porphyrin core can host a variety of 3d transition metals, which are usually incorporated in solution. However, the replacement of a pre-existing metal atom in the porphyrin core by a different metallic species has been rarely reported under UHV. Herein, we studied the influence of cyanophenyl and pyridyl functional endgroups in the self-assembly of structurally different porphyrin-based MOCNs by the deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized (Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy (STM). A comparative analysis of the influence of the cyano and pyridyl endgroups on the formation of different in-plane coordination motifs is performed. Each porphyrin derivative formed two structurally different Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination nodes, respectively. Interestingly, the codeposited Fe atoms did not only bind to the functional endgroups but also reacted with the porphyrin core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange reaction took place in the porphyrin core where the codeposited Fe atoms replaced the Zn atoms. This was evidenced by the appearance of molecules with an enhanced (centered) STM contrast compared with the appearance of Zn-TPyP, which suggested the formation of a new molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered STM contrast after the deposition of Fe atoms, which was attributed to the binding of the Fe atoms on the top site of the Co-substituted porphyrin core. In summary, the deposition of metal atoms onto organic layers can steer the formation of structurally different MOCNs and may replace pre-existing metal atoms contained in the porphyrin core.

A versatile electrochemical cell for hanging meniscus or flow cell measurement of planar model electrodes characterized with scanning tunneling microscopy and x-ray photoelectron spectroscopy

We demonstrate the development of a portable electrochemistry (EC) cell setup that can be applied to measure relevant electrochemical signals on planar samples in conjunction with pre- and post-characterization by surface science methods, such as scanning tunneling microscopy and x-ray photoelectron spectroscopy. The EC cell setup, including the transfer and EC cell compartments, possesses the advantage of a small size and can be integrated with standard ultra-high vacuum (UHV) systems or synchrotron end-stations by replacing the flange adaptor, sample housing, and transfer arm. It allows a direct transfer of the pre-characterized planar sample from the UHV environment to the EC cell to conduct in situ electrochemical measurements without exposing to ambient air. The EC cell setup can operate in both the hanging meniscus and flow cell mode. As a proof of concept, using a Au(111) single crystal electrode, we demonstrate the application of the EC cell setup in both modes and report on the post-EC structure and chemical surface composition as provided by scanning tunneling microscopy and x-ray photoelectron spectroscopy. To exemplify the advantage of an in situ EC cell, the EC cell performance is further compared to a corresponding experiment on a Au(111) sample measured by transfer at ambient conditions. The EC cell demonstrated here enables a wealth of future electrocatalysis measurements that combine surface science model catalyst approaches to facilitate the understanding of nano- and atomic-scale structures of electrocatalytic interfaces, the crucial role of catalyst stability, and the nature of low-concentration and atomically dispersed metal (single atom) dopants.

Tuning of Reciprocal Carbon-Electrode Properties for an Optimized Hydrogen Evolution

Closing the material cycle for harmful and rare resources is a key criterion for sustainable and green energy systems. The concept of using scalable biomass-derived carbon electrodes to produce hydrogen from water was proposed here, satisfying the need for sustainability in the field of chemical energy conversion. The carbon electrodes exhibited not only water oxidation activity but also a strong self-oxidation when being used as anode for water splitting. The carbon oxidation, which is more energy-favorable, was intentionally allowed to occur for an improvement of the total current, thus enhancing the hydrogen production on the cathode side. By introducing different earth-abundant metals, the electrode could be well adjusted to achieve an optimized water/carbon oxidation ratio and an appreciable reactivity for practical applications. This promising methodology may become a very large driver for carbon chemistry when waste organic materials or biomass can be converted using its intrinsic energy content of carbon. Such a process could open a safe path for sub-zero CO2 emission control. The concept of how and which parameter of a carbon-based electrode can be optimized was presented and discussed in this paper.

Insights into electrocatalysis by scanning tunnelling microscopy

Understanding the mechanism of electrocatalytic reaction is important for the design and development of highly efficient electrocatalysts for energy technology. Investigating the surface structures of electrocatalysts and the surface processes in electrocatalytic reactions at the atomic and molecular scale is helpful to identify the catalytic role of active sites and further promotes the development of emerging electrocatalysts. Since it was invented, scanning tunnelling microscopy (STM) has become a powerful technique to investigate surface topographies and electronic properties at the nanoscale resolution. STM can be operated in diversified environments. Electrochemical STM can be used to investigate the surface processes during electrochemical reactions. Moreover, the critical intermediates in catalysis on catalyst surfaces can be identified by STM at low temperature or ultrahigh vacuum. STM has been extensively utilized in electrocatalysis research, including the structure-activity relationship of electrocatalysts, the distribution of active sites, and surface processes in electrocatalytic reactions. In this review, progress in the application of STM in electrocatalysis is systematically discussed. The construction of model electrocatalysts and electrocatalytic systems are summarized. Then, we present the STM investigation of electrocatalyst structures and surface processes related to electrocatalysis. Challenges and future developments in the field are discussed in the outlook.

Enhancing One-Dimensional Charge Transport in Metal-organic Framework Hexagonal Nanorods for Electrocatalytic Oxygen Evolution

Metal-organic frameworks (MOFs) have exhibited huge potential in electrocatalytic fields. However, the intrinsic low conductivity and the blockage of metal active sites by organic linkers still seriously hinder their large-scale application. In this study, as a proof of principle, constructing cofacial π-π stacking in the terminal ligand (4,4'-bipyridine) of a Ni/Fe-chain-based MOF to fabricate strong π-π interaction, in combination with unique hexagonal nanorod (HXR) structure, is found to be an effective strategy to enhance one-dimensional charge carrier efficiency and thus achieve excellent activity in the oxygen evolution reaction (OER). The approach yields a high turnover frequency (4.54 s^-1 ) in well-designed bimetallic chain-based MOFs (NiFe-HXR) at an overpotential of 350 mV, which is about 8.7 and 34.9 times higher than those in Ni-HXR (0.52 s^-1 ) and IrO₂ (0.13 s^-1 ), respectively. This work effectively combines "through-bond" channel in chain-based structure of NiFe-HXR and "through-space" transport between face-to-face terminal ligands, thus resulting in outstanding OER activity. This strategy of modulating the structure chemistry and morphology of MOFs to promote the OER may open a new perspective to synthesize MOFs for energy-relevant electrochemical reactions.

Single-Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

A heterogeneous catalyst is a backbone of modern sustainable green industries; and understanding the relationship between its structure and properties is the key for its advancement. Recently, many upscaling synthesis strategies for the development of a variety of respectable control atomically precise heterogeneous catalysts are reported and explored for various important applications in catalysis for energy and environmental remediation. Precise atomic-scale control of catalysts has allowed to significantly increase activity, selectivity, and in some cases stability. This approach has proved to be relevant in various energy and environmental related technologies such as fuel cell, chemical reactors for organic synthesis, and environmental remediation. Therefore, this review aims to critically analyze the recent progress on single-atom catalysts (SACs) application in oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and chemical and/or electrochemical organic transformations. Finally, opportunities that may open up in the future are summarized, along with suggesting new applications for possible exploitation of SACs.

Stabilization and activation of molecular oxygen at biomimetic tetrapyrroles on surfaces: from UHV to near-ambient pressure

Recent advances in the development of surface science methods have allowed bridging, at least partially, the pressure gap between the ultra-high vacuum environment and some applicative conditions. This step has been particularly critical for the characterization of heterogenous catalytic systems (solid-liquid, solid-gas interfaces) and, specifically, of the electronic, structural, and chemical properties of tetrapyrroles at surfaces when arranged in 2D networks. Within a biomimetic picture, in which 2D metalorganic frameworks are expected to model and reproduce in a tailored way the activity of their biochemical proteic counterparts, the fundamental investigation of the adsorption and activation of small ligands at the single-metal atom reaction sites has progressively gained increasing attention. Concerning oxygen, biology offers a variety of tetrapyrrole-based transport and reaction pockets, as e.g. in haemoglobin, myoglobin or cytochrome proteins. Binding and activation of O2 are accomplished thanks to complex charge transfer and spin realignment processes, sometimes requiring cooperative mechanisms. Within the framework of surface science at near-ambient pressure (towards and beyond the mbar regime), recent progress has unveiled novel and interesting properties of 2D metalorganic frameworks and heterostacks based on self-assembled tetrapyrroles, thus opening possible, effective applicative routes in the fields of light harvesting, heterogenous (electro-)catalysts, chemical sensing, and spintronics.

Nanocomposites Prepared from Carbon Nanotubes and the Transition Metal Dichalcogenides WS2 and MoS2 via Surfactant-Assisted Dispersions as Electrocatalysts for Oxygen Reactions

Fuel cells are emerging devices as clean and renewable energy sources, provided their efficiency is increased. In this work, we prepared nanocomposites based on multiwalled carbon nanotubes (MWNTs) and transition metal dichalcogenides (TMDs), namely WS₂ and MoS₂, and evaluated their performance as electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), relevant to fuel cells. The one- and two-dimensional (1D and 2D) building blocks were initially exfoliated and non-covalently functionalized by surfactants of opposite charge in aqueous media (tetradecyltrimethylammonium bromide, TTAB, for the nanotubes and sodium cholate, SC, for the dichalcogenides), and thereafter, the three-dimensional (3D) MoS₂@MWNT and WS₂@MWNT composites were assembled via surfactant-mediated electrostatic interactions. The nanocomposites were characterized by scanning electron microscopy (SEM) and structural differences were found. WS₂@MWNT and MoS₂@MWNT show moderate ORR performance with potential onsets of 0.71 and 0.73 V vs. RHE respectively, and diffusion-limiting current densities of -1.87 and -2.74 mA·cm^-2, respectively. Both materials present, however, better tolerance to methanol crossover when compared to Pt/C and good stability. Regarding OER performance, MoS₂@MWNT exhibits promising results, with η₁₀ and j_max of 0.55 V and 17.96 mA·cm^-2, respectively. The fabrication method presented here is cost-effective, robust and versatile, opening the doors for the optimization of electrocatalysts' performance.

Application of Scanning Tunneling Microscopy in Electrocatalysis and Electrochemistry

Abstract

Scanning tunneling microscopy (STM) has gained increasing attention in the field of electrocatalysis due to its ability to reveal electrocatalyst surface structures down to the atomic level in either ultra-high-vacuum (UHV) or harsh electrochemical conditions. The detailed knowledge of surface structures, surface electronic structures, surface active sites as well as the interaction between surface adsorbates and electrocatalysts is highly beneficial in the study of electrocatalytic mechanisms and for the rational design of electrocatalysts. Based on this, this review will discuss the application of STM in the characterization of electrocatalyst surfaces and the investigation of electrochemical interfaces between electrocatalyst surfaces and reactants. Based on different operating conditions, UHV-STM and STM in electrochemical environments (EC-STM) are discussed separately. This review will also present emerging techniques including high-speed EC-STM, scanning noise microscopy and tip-enhanced Raman spectroscopy.

Graphic Abstract