Atomically Precise Integration of Multiple Functional Motifs in Catalytic Metal-Organic Frameworks for Highly Efficient Nitrate Electroreduction

Surface Engineering of PtSe2 Crystal for Highly Efficient Electrocatalytic Ethanol Oxidation

The development of efficient electrocatalysts for ethanol oxidation reaction (EOR) is crucial for the potential commercialization of direct ethanol fuel cells, yet it faces significant challenges between catalytic performance and cost-effectiveness. 2D materials have recently emerged as a promising group of electrocatalysts due to their large surface area, efficient charge transport, tunable band structures, and excellent catalytic activity. In this study, the novel 2D layered noble-metal dichalcogenide, PtSe2, is explored for efficient ethanol oxidation electrocatalysis from a microscopic perspective based on an on-chip microelectrochemical platform. While pristine PtSe2 demonstrates similar EOR activities to Pt, argon plasma treatment significantly enhances the performance on EOR activity, If/Ib ratio, onset and peak potentials, and durability. Detail investigations reveal that plasma treatment results in the exposure of PtSe2 surface, which is responsible for significantly enhanced EOR activity and poison-resistance as also confirmed by theoretical calculations. In situ electrical transport measurements for monitoring the catalyst surface intermediates, elucidate that both optimized OHads coverage and appropriate ethanol molecular adsorption on PtSe2 are the key for the high performance. This work demonstrates noble-metal dichalcogenides as promising EOR electrocatalysts, and establishes on-chip electrocatalytic microdevice as a promising probing platform for diverse electrocatalytic measurements.

Rational Ligand Design of Conjugated Coordination Polymers for Efficient and Selective Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NRA) offers an attractive route for converting nitrate pollutants to ammonia under mild conditions. Among other catalysts, single-atom catalysts (SACs) with high metal-atom-utilization efficiency and low-coordinated metal sites hold immense potential to be extensively applied, which unfortunately encounter a formidable challenge to obtain simultaneous improvement of NRA activity and selectivity. Here, a novel and general strategy is reported to achieve efficient and selective NRA catalysis on conjugated coordination polymers featuring with high-density and well-defined nitrogen (N)-coordinated single-atom metal sites via precise regulation of N‑heterocyclic ligands toward accelerating the hydrogenation kinetics necessitated in the NRA pathway. Taking cobalt (Co) as an example, two CoN4-centered conjugated coordination polymer electrocatalysts (CoN4-pyrr and CoN4-pyri) are synthesized with pyrrole and pyridine ligands are investigated as a proof-of-concept study. As revealed, the CoN4-pyrr can markedly outperform the CoN4-pyri toward NRA electrocatalysis. Experimental and theoretical results suggest that, relative to the N atoms of pyridine ligand in CoN4-pyri, the N atoms of pyrrole ligand in CoN4-pyrr can enable a faster transfer of hydrogen radicals to the Co active sites for accelerating the hydrogenation kinetics of *NO intermediate at the rate-determining step of NRA pathway.

Efficient Ammonia Electrosynthesis from Pure Nitrate Reduction via Tuning Bimetallic Sites in Redox-Active Covalent Organic Frameworks

Electrocatalytic nitrate reduction reaction (NITRR) represents a promising approach for ammonia synthesis, but existing application has been constrained by the complex proton-coupled electron transfer and the sluggish kinetics induced by various intermediates. Herein, we synthesized a series of metalized covalent organic frameworks: NiTP-MTAPP MCOFs (M = 2H, Co, Cu, and Fe), based on dual redox-active centers: thiophene-substituted Ni-bis(dithiolene) ligand-Ni[C2S2(C4H2SCHO)2]2 and metallic porphyrin. Through regulating the adsorption and desorption of species at the catalytic sites, we have identified the optimal NITRR electrocatalyst: NiTP-CoTAPP MCOF, which achieved the highest faradaic efficiency (FE) of approximately 85.6% at -0.8 V (vs. RHE) in pure nitrate solution, with an impressive yield rate of 160.2 mmol h-1 g-1 cat. The generation of active hydrogen at [NiS4] sites achieved dynamic equilibrium with the timely hydrogenation reaction at CoN4 sites, effectively suppressing the hydrogen evolution reaction. Moreover, the incorporation of thiophene (TP) groups and metal ions facilitates charge transfer. Density functional theory (DFT) calculations demonstrated the reduction in energy barriers at different catalytic sites. The CoN4-NiS4 system exhibited the optimal adsorption-to-desorption capability and the lowest energy barrier (0.58 eV) for the rate-determining step (*NO → *HNO), which is supported by the moderate d-band center and Bader charge value.

Dual Zn5-NiS4 Sites in a Redox-Active Metal-Organic Framework Enables Efficient Cascade Catalysis for Nitrate-to-Ammonia Conversion

Electrocatalytic Nitrate Reduction to Ammonia (NO3RR) offers a promising solution to both environmental pollution and the sustainable energy conversion. Here we propose an efficient cascade catalytic mechanism based on a dual Zn5-NiS4 sites, orderly assembled in a redox-active metal-organic framework structure, which separately promotes the reaction kinetics of nitrate-to-nitrite and nitrite-to-ammonia conversions. Specifically, the Zn5 clusters adsorb and selectively reduce the NO3 - to NO2 -, whereas [NiS4] acts as an analogue to the ferredoxins, subsequently boosts the reduction of NO2 - to produce NH3. To this end, the bimetallic Zn5-NiS4TP MOF was synthesized based on the redox-active ligand [Ni(C2S2(TPCOOH)2)2]. A maximum ammonia production rate of 23477.59 μg ⋅ h-1 ⋅ mg-1 cat. and faradaic efficiency 92.87 % was achived by Zn5-NiS4TP MOF under neutral conditions. To validate the critical role of dual Zn5-NiS4 sites, Mn5-NiS4TP and Cd2-NiS4TP were synthesized as control samples, together with Zn-TTFTB, Zn-NiS4Ph and other Zn5-cluster-based MOFs applied for the investigation of electrocatalytic nitrate reduction. Our results indicated that substitution by -thienyl instead of -phenyl group increases the S-heteroatom content, improves the conductivity and facilitates electron transfer. Furthermore, Density Functional Theory (DFT) calculations of the energy changes for the reduction of each species could rationalize experimental results.

Single-atom catalysts for electrocatalytic nitrate reduction into ammonia

Ammonia (NH3) is a versatile and important compound with a wide range of uses, which is currently produced through the demanding Haber-Bosch process. Electrocatalytic nitrate reduction into ammonia (NRA) has recently emerged as a sustainable approach for NH3synthesis under ambient conditions. However, the NRA catalysis is a complex multistep electrochemical process with competitive hydrogen evolution reaction that usually results in poor selectivity and low yield rate for NH3synthesis. With maximum atom utilization and well-defined catalytic sites, single atom catalysts (SACs) display high activity, selectivity and stability toward various catalytic reactions. Very recently, a number of SACs have been developed as promising NRA electrocatalysts, but systematical discussion about the key factors that affect their NRA performance is not yet to be summarized to date. This review focuses on the latest breakthroughs of SACs toward NRA catalysis, including catalyst preparation, catalyst characterization and theoretical insights. Moreover, the challenges and opportunities for improving the NRA performance of SACs are discussed, with an aim to achieve further advancement in developing high-performance SACs for efficient NH3synthesis.

Polynuclear Cobalt Cluster-Based Coordination Polymers for Efficient Nitrate-to-Ammonia Electroreduction

The electrocatalytic nitrate reduction reaction (NITRR) holds great promise for purifying wastewater and producing valuable ammonia (NH3). However, the lack of efficient electrocatalysts has impeded the achievement of highly selective NH3 synthesis from the NITRR. In this study, we report the design and synthesis of two polynuclear Co-cluster-based coordination polymers, {[Co2(TCPPDA)(H2O)5]·(H2O)9(DMF)} and {Co1.5(TCPPDA)[(CH3)2NH2]·(H2O)6(DMF)2} (namely, NJUZ-2 and NJUZ-3), which possess distinct coordination motifs with well-defined porosity, high-density catalytic sites, accessible mass transfer channels, and nanoconfined chemical environments. Benefitting from their intriguing multicore metal-organic coordination framework structures, NJUZ-2 and NJUZ-3 exhibit remarkable catalytic activities for the NITRR. At a potential of -0.8 V (vs. RHE) in an H-type cell, they achieve an optimal Faradaic efficiency of approximately 98.5% and high long-term durability for selective NH3 production. Furthermore, the electrocatalytic performance is well maintained even under strongly acidic conditions. When operated under an industrially relevant current density of 469.9 mA cm-2 in a flow cell, a high NH3 yield rate of up to 3370.6 mmol h-1 g-1cat. was observed at -0.5 V (vs. RHE), which is 20.1-fold higher than that obtained in H-type cells under the same conditions. Extensive experimental analyses, in combination with theoretical computations, reveal that the great enhancement of the NITRR activity is attributed to the preferential adsorption of NO3- and the reduction in energy input required for the hydrogenation of *NO3 and *NO2 intermediates.

Porous Crystalline Materials Based on Tetrathiafulvalene and Its Analogues: Assembly, Charge Transfer, and Applications

ConspectusThe directed synthesis and functionalization of porous crystalline materials pose significant challenges for chemists. The synergistic integration of different functionalities within an ordered molecular material holds great significance for expanding its applications as functional materials. The presence of coordination bonds connected by inorganic and organic components in molecular materials can not only increase the structural diversity of materials but also modulate the electronic structure and band gap, which further regulates the physical and chemical properties of molecular materials. In fact, porous crystalline materials with coordination bonds, which inherit the merits of both organic and inorganic materials, already showcase their superior advantages in optical, electrical, and magnetic applications. In addition to the inorganic components that provide structural rigidity, organic ligands of various types serve as crucial connectors in the construction of functional porous crystalline materials. In addition, redox activity can endow organic linkers with electrochemical activity, thereby making them a perfect platform for the study of charge transfer with atom-resolved single-crystal structures, and they can additionally serve as stimuli-responsive sites in sensor devices and smart materials.In this Account, we introduce the synthesis, structural characteristics, and applications of porous crystalline materials based on the famous redox-active units, tetrathiafulvalene (TTF) and its analogues, by primarily focusing on metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). TTF, a sulfur-rich conjugated molecule with two reversible and easily accessible oxidation states (i.e., radical TTF•+ cation and TTF2+ dication), and its analogues boast special electrical characteristics that enable them to display switchable redox activity and stimuli-responsive properties. These inherent properties contribute to the enhancement of the optical, electrical, and magnetic characteristics of the resultant porous crystalline materials. Moreover, delving into the charge transfer phenomena, which is key for the electrochemical process within these materials, uncovers a myriad of potential functional applications. The Account is organized into five main sections that correspond to the different properties and applications of these materials: optical, electrical, and magnetic functionalities; energy storage and conversion; and catalysis. Each section provides detailed discussions of synthetic methods, structural characteristics, the physical and chemical properties, and the functional performances of highlighted examples. The Account also discusses future directions by emphasizing the exploration of novel organic units, the transformation between radical cation TTF•+ and dication TTF2+, and the integration of multifunctionalities within these frameworks to foster the development of smart materials for enhanced performance across diverse applications. Through this Account, we aim to highlight the massive potential of TTF and its analogues-based porous crystals in chemistry and material science.

Red Carbon Mediated Formation of Cu2O Clusters Dispersed on the Oxocarbon Framework by Fehling's Route and their Use for the Nitrate Electroreduction in Acidic Conditions

The oligomers of carbon suboxide, known as red carbon, exhibit a highly conjugated structure and semiconducting properties. Upon mild heat treatment, it transforms into a carbonaceous framework rich in oxygen surface terminations, called oxocarbon. In this study, the abundant oxygen functionalities are harnessed as anchors to create oxocarbon-supported nanohybrid electrocatalysts. Starting with single atomic Cu (II) strongly coordinated to oxygen atoms on red carbon, the Fehling reaction leads to the formation of Cu2O clusters. Simultaneously, a covalent oxocarbon framework emerges via cross-linking, providing robust support for Cu2O clusters. Notably, the oxocarbon support effectively stabilizes Cu2O clusters of very small size, ensuring their high durability in acidic conditions and the presence of ammonia. The synthesized material exhibits a superior electrocatalytic activity for nitrate reduction under acidic electrolyte conditions, with a high yield rate of ammonium (NH4 +) at 3.31 mmol h-1 mgcat -1 and a Faradaic efficiency of 92.5% at a potential of -0.4 V (vs RHE).

Nanoengineering Metal-Organic Frameworks and Derivatives for Electrosynthesis of Ammonia

Electrocatalytic synthesis under mild conditions has become increasingly important as one of the practical alternatives for industrial applications, especially for the green ammonia (NH3) industry. A properly engineered electrocatalyst plays a vital role in the realization of superior catalytic performance. Among various types of promising nanomaterials, metal-organic frameworks (MOFs) are competitive candidates for developing efficient electrocatalytic NH3 synthesis from simple nitrogen-containing molecules or ions, such as N2 and NO3-. In this review, recent advances in the development of electrocatalysts derived from MOFs for the electrosynthesis of NH3 are collected, categorized, and discussed, including their application in the N2 reduction reaction (NRR) and the NO3- reduction reaction (NO3RR). Firstly, the fundamental principles are illustrated, such as plausible mechanisms of NH3 generation from N2 and NO3-, the apparatus of corresponding electrocatalysis, parameters for evaluation of reaction efficiency, and detection methods of yielding NH3. Then, the electrocatalysts for NRR processes are discussed in detail, including pristine MOFs, MOF-hybrids, MOF-derived N-doped porous carbons, single atomic catalysts from pyrolysis of MOFs, and other MOF-related materials. Subsequently, MOF-related NO3RR processes are also listed and discussed. Finally, the existing challenges and prospects for the rational design and fabrication of electrocatalysts from MOFs for electrochemical NH3 synthesis are presented, such as the evolution of investigation methods with artificial intelligence, innovation in synthetic methods of MOF-related catalysts, advancement of characterization techniques, and extended electrocatalytic reactions.

Fundamental Perspectives on the Electrochemical Water Applications of Metal-Organic Frameworks

HIGHLIGHTS

The recent development and implementation of metal-organic frameworks (MOFs) and MOF-based materials in electrochemical water applications are reviewed. The critical factors that affect the performances of MOFs in the electrochemical reactions, sensing, and separations are highlighted. Advanced tools, such as pair distribution function analysis, are playing critical roles in unraveling the functioning mechanisms, including local structures and nanoconfined interactions. Metal-organic frameworks (MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy-water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications (i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in detecting/removing pollutants, recovering resources, and harvesting energies from different water sources. Compared with the pristine MOFs, the efficiency and/or selectivity can be further enhanced via rational structural modulation of MOFs (e.g., partial metal substitution) or integration of MOFs with other functional materials (e.g., metal clusters and reduced graphene oxide). Several key factors/properties that affect the performances of MOF-based materials are also reviewed, including electronic structures, nanoconfined effects, stability, conductivity, and atomic structures. The advancement in the fundamental understanding of these key factors is expected to shed light on the functioning mechanisms of MOFs (e.g., charge transfer pathways and guest-host interactions), which will subsequently accelerate the integration of precisely designed MOFs into electrochemical architectures to achieve highly effective water remediation with optimized selectivity and long-term stability.