Atomically dispersed Pt-N4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction

Engineering antibonding orbital occupancy of NiMoO4-supported Ru nanoparticles for enhanced chlorine evolution reaction

Chlorine evolution reaction (CER) is crucial for industrial-scale production of high-purity Cl2. Despite the development of classical dimensionally stable anodes to enhance CER efficiency, the competitive oxygen evolution reaction (OER) remains a barrier to achieving high Cl2 selectivity. Herein, a binder-free electrode, Ru nanoparticles (NPs)-decorated NiMoO4 nanorod arrays (NRAs) supported on Ti foam (Ru-NiMoO4/Ti), was designed for active CER in saturated NaCl solution (pH = 2). The Ru-NiMoO4/Ti electrode exhibits a low overpotential of 20 mV at 10 mA cm-2 current density, a high Cl2 selectivity exceeding 90%, and robust durability for 90h operation. The marked difference in Tafel slopes between CER and OER indicates the high Cl2 selectivity and superior reaction kinetics of Ru-NiMoO4/Ti electrode. Further studies reveal a strong metal-support interaction (SMSI) between Ru and NiMoO4, facilitating electron transfer through the Ru-O bridge bond and increasing the Ru 3d-Cl 2p antibonding orbital occupancy, which eventually results in weakened Ru-Cl bonding, promoted Cl desorption, and enhanced Cl2 evolution. Our findings provide new insights into developing electrodes with enhanced CER performance through antibonding orbital occupancy engineering.

Progress in Anode Stability Improvement for Seawater Electrolysis to Produce Hydrogen

Seawater electrolysis for hydrogen production is a sustainable and economical approach that can mitigate the energy crisis and global warming issues. Although various catalysts/electrodes with excellent activities have been developed for high-efficiency seawater electrolysis, their unsatisfactory durability, especially for anodes, severely impedes their industrial applications. In this review, attention is paid to the factors that affect the stability of anodes and the corresponding strategies for designing catalytic materials to prolong the anode's lifetime. In addition, two important aspects-electrolyte optimization and electrolyzer design-with respect to anode stability improvement are summarized. Furthermore, several methods for rapid stability assessment are proposed for the fast screening of both highly active and stable catalysts/electrodes. Finally, perspectives on future investigations aimed at improving the stability of seawater electrolysis systems are outlined.

Nb Doping Induced the Formation of Protective Layer to Improve the Stability of Fe-Ni3S2 for Seawater Electrolysis

The seawater electrolysis to produce hydrogen is a significant topic on alleviating the energy crisis. Here, the Fe, Nb-Ni3S2 catalyst is prepared by metal-doping strategy, and it shows high oxygen evolution reaction (OER) activity in alkaline medium, and only needs 1.491 V to deliver a current density of 100 mA cm-2 in simulated seawater. Using Fe, Nb-Ni3S2 as a bifunctional catalyst, the two-electrode electrolyzer only requires a voltage of 1.751 V (without impedance compensation) to drive the current density of 50 mA cm-2, and can run over 150 h stably in the simulated seawater. Importantly, In situ Raman test demonstrates that the outstanding performance of Fe, Nb-Ni3S2 in simulated seawater is ascribed to the in situ formed sulfate protective layer induced by Nb doping, which can effectively inhibit the corrosion of chloride ion, while the protective layer is absent for Fe-Ni3S2. The stable operation of simulated seawater electrolysis under industrial current density further confirms the stability improvement mechanism of forming protective layer. In short, this study provides a new strategy of using Nb dopants inducing the formation of protective layer to enhance the stability of seawater electrolysis.

A Simple Organo-Electrocatalysis System for the Chlor-Related Industry

Consuming a substantial quantum of energy (~165 TW h), the chlor-alkali industry garners considerable scholarly and industrial interest, with the anode reaction involving the oxidation of chloride ions being a paramount determinant of reaction rates. While the dimensionally stable anode (DSA) displays commendable catalytic activity and longevity, they rely on precious metals and exhibit a non-negligible side reaction in sodium hypochlorite (NaClO) production, underscoring the appeal of metal-free alternatives. However, the molecules and systems currently available are characterized by intricate complexity and are not amenable to large-scale production. Herein, we have successfully developed an economical and highly efficient molecular catalyst, demonstrating superior performance compared with the former organic molecules in the chloride ion oxidation process (COP) for the production of both chlorine gas (Cl2) and NaClO. The molecule of 2N only needs 92 mV to reach a current density of 1000 mA cm-2, with a small cost of only 0.002 $ g-1. Furthermore, we propose a novel mechanism underpinned by non-covalent interactions, serving as the foundation for an innovative approach to the design of efficient anodes for the COP.

Carbon-metal versus metal-metal synergistic mechanism of ethylene electro-oxidation via electrolysis of water on TM2N6 sites in graphene

Acetaldehyde (AA) and ethylene oxide (EO) are important fine chemicals, and are also substrates with wide applications for high-value chemical products. Direct electrocatalytic oxidation of ethylene to AA and EO can avoid the untoward effects from harmful byproducts and high energy emissions. The most central intermediate state is the co-adsorption and coupling of ethylene and active oxygen intermediates (*O) at the active site(s), which is restricted by two factors: the stability of the *O intermediate generated during the electrolysis of water on the active site at a certain applied potential and pH range; and the lower kinetic energy barriers of the oxidation process based on the thermo-migration barrier from the *O intermediate to produce AA/EO. The benefit of two adjacent active atoms is more promising, since diverse adsorption and flexible catalytic sites may be provided for elementary reaction steps. Motivated by this strategy, we explored the feasibility of various homonuclear TM2N6@graphenes with dual-atomic-site catalysts (DASCs) for ethylene electro-oxidation through first-principles calculations via thermodynamic evaluation, analysis of the surface Pourbaix diagram, and kinetic evaluation. Two reaction mechanisms through C-TM versus TM-TM synergism were determined. Between them, a TM-TM mechanism on 4 TM2N6@graphenes and a C-TM mechanism on 5 TM2N6@graphenes are built. All 5 TM2N6@graphenes through the C-TM mechanism exhibit lower kinetic energy barriers for AA and EO generation than the 4 TM2N6@graphenes through the TM-TM mechanism. In particular, Pd2N6@graphene exhibits the most excellent catalytic activity, with energy barriers for generating AA and EO of only 0.02 and 0.65 eV at an applied potential of 1.77 V vs. RHE for the generation of an active oxygen intermediate. Electronic structure analysis indicates that the intrinsic C-TM mechanism is more advantageous than the TM-TM mechanism for ethylene electro-oxidation, and this study also provides valuable clues for further experimental exploration.

Atomically Dispersed Ru-doped Ti4O7 Electrocatalysts for Chlorine Evolution Reaction with a Universal Activity

Chlorine has been supplied by the chlor-alkali process that deploys dimensionally stable anodes (DSAs) for the electrochemical chlorine evolution reaction (ClER). The paramount bottlenecks have been ascribed to an intensive usage of precious elements and inevitable competition with the oxygen evolution reaction. Herein, a unique case of Ru2+-O4 active motifs anchored on Magnéli Ti4O7 (Ru-Ti4O7) via a straightforward wet impregnation and mild annealing is reported. The Ru-Ti4O7 performs radically active ClER with minimal deployment of Ru (0.13 wt%), both in 5 m NaCl (pH 2.3) and 0.1 m NaCl (pH 6.5) electrolytes. Scanning electrochemical microscopy demonstrates superior ClER selectivity on Ru-Ti4O7 compared to the DSA. Operando X-ray absorption spectroscopy and density functional theory calculations reveal a universally active ClER (over a wide range of pH and [Cl-]), through a direct adsorption of Cl- on Ru2+-O4 sites as the most plausible pathway, together with stabilized ClO* at low [Cl-] and high pH.

Asymmetric Ru-In atomic pairs promote highly active and stable acetylene hydrochlorination

Ru single-atom catalysts have great potential to replace toxic mercuric chloride in acetylene hydrochlorination. However, long-term catalytic stability remains a grand challenge due to the aggregation of Ru atoms caused by over-chlorination. Herein, we synthesize an asymmetric Ru-In atomic pair with vinyl chloride monomer yield (>99.5%) and stability (>600 h) at a gas hourly space velocity of 180 h-1, far surpassing those of the Ru single-atom counterparts. A combination of experimental and theoretical techniques reveals that there is a strong d-p orbital interaction between Ru and In atoms, which not only enables the selective adsorption of acetylene and hydrogen chloride at different atomic sites but also optimizes the electron configuration of Ru. As a result, the intrinsic energy barrier for vinyl chloride generation is lowered, and the thermodynamics of the chlorination process at the Ru site is switched from exothermal to endothermal due to the change of orbital couplings. This work provides a strategy to prevent the deactivation and depletion of active Ru centers during acetylene hydrochlorination.

Atomically dispersed dinuclear iridium active sites for efficient and stable electrocatalytic chlorine evolution reaction

The electrochemical chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have long been used as CER catalysts, they suffer from high cost and poor selectivity due to the competing oxygen evolution reaction (OER). Single-atom catalysts (SACs), featuring high atom utilization efficiency, have captured widespread interest in diverse applications. However, the single-atom sites in SACs are generally recognized as independent motifs and the interplay of adjacent sites is largely overlooked. Herein, we report a "precursor-preselected" cage-encapsulated strategy to synthesize atomically dispersed dinuclear iridium active sites bridged by oxygen that are supported on nitrogen-doped carbon (Ir2-ONC). The dinuclear Ir2-ONC catalyst exhibits a CER onset potential of 1.375 V vs. normal hydrogen electrode, a high faradaic efficiency of >95%, and a high mass activity of 14321.6 A gIr -1, much better than the Ir SACs, which demonstrates the significance of coordination and electronic structure regulation for atomically dispersed catalysts. Density functional theory calculations and ab initio molecular dynamics simulations confirm that the unique dinuclear structure facilitates Cl- adsorption, resulting in improved catalytic CER performance.

Pt Single-Atoms on Structurally-Integrated 3D N-Doped Carbon Tubes Grid for Ampere-Level Current Density Hydrogen Evolution

To date, the excellent mass-catalytic activities of Pt single-atoms catalysts (Pt-SACs) toward hydrogen evolution reaction (HER) are categorically confirmed; however, their high current density performance remains a challenge for practical applications. Here, a binder-free approach is exemplified to fabricate self-standing superhydrophilic-superaerphobic Pt-SACs cathodes by directly anchoring Pt-SAs via Pt-NxC4-x coordination bonds to the structurally-integrated 3D nitrogen-doped carbon tubes (N-CTs) array grid (denoted as Pt@N-CTs). The 3D Pt@N-CTs cathode with optimal Pt-SACs loading is capable of operating at a high current density of 1000 mA cm-2 with an ultralow overpotential of 157.9 mV with remarkable long-term stability over 11 days at 500 mA cm-2. The 3D super-wettable free-standing Pt@N-CTs possess interconnected vertical and lateral N-CTs with hierarchical-sized open channels, which facilitates the mass transfer. The binder-free immobilization adding to the large surface area and 3D-interconnected open channels endow Pt@N-CTs cathodes with high accessible active sites, electrical conductivity, and structural stability that maximize the utilization efficiency of Pt-SAs to achieve ampere-level current density HER at low overpotentials.

Energy-Efficient Electrosynthesis of High Value-Added Active Chlorine Coupled with H2 Generation from Direct Seawater Electrolysis through Decoupling Electrolytes

Direct saline (seawater) electrolysis is a well-recognized system to generate active chlorine species for the chloride-mediated electrosynthesis, environmental remediation and sterilization over the past few decades. However, the large energy consumption originated from the high cell voltage of traditional direct saline electrolysis system, greatly restricts its practical application. Here, we report an acid-saline hybrid electrolysis system for energy-saving co-electrosynthesis of active chlorine and H2. We demonstrate that this system just requires a low cell voltage of 1.59 V to attain 10 mA cm-2 with a large energy consumption decrease of 27.7 % compared to direct saline electrolysis system (2.20 V). We further demonstrate that such acid-saline hybrid electrolysis system could be extended to realize energy-saving and sustainable seawater electrolysis. The acidified seawater in this system can absolutely avoid the formation of Ca/Mg-based sediments that always form in the seawater electrolysis system. We also prove that this system in the half-flow mode can realize real-time preparation of active chlorine used for sterilization and pea sprout production.

Novel sulfate solid supported binary Ru-Ir oxides for superior electrocatalytic activity towards OER and CER

Electrolysis for producing hydrogen powered by renewable electricity can be dramatically expanded by adapting different electrolytes (brine, seawater or pure water), which means the anode materials must stand up to complex electrolyte conditions. Here, a novel catalyst/support hybrid of binary Ru3.5Ir1Ox supported by barium strontium sulfate (BaSrSO4) was synthesized (RuIrOx/BSS) by exchanging the anion ligands of support. The as-synthesized RuIrOx/BSS exhibits compelling oxygen evolution (OER) and chlorine evolution (CER) performances, which affords to 10 mA cm-2 with only overpotential of 244 mV and 38 mV, respectively. The performed X-ray adsorption spectra clearly indicate the presence of an interface charge transfer effect, which results in the assignment of more electrons to the d orbitals of the Ru and Ir sites. The theoretical calculations demonstrated that the electronic structures of the catalytic active sites were modulated to give a lower overpotential, confirming the intrinsically high OER and CER catalytic activity.

Engineering the Coordination Environment of Ir Single Atoms with Surface Titanium Oxide Amorphization for Superior Chlorine Evolution Reaction

The chlorine evolution reaction (CER) is essential for industrial Cl2 production but strongly relies on the use of dimensionally stable anode (DSA) with high-amount precious Ru/Ir oxide on a Ti substrate. For the purpose of sustainable development, precious metal decrement and performance improvement are highly desirable for the development of CER anodes. Herein, we demonstrate that surface titanium oxide amorphization is crucial to regulate the coordination environment of stabilized Ir single atoms for efficient and durable chlorine evolution of Ti monolithic anodes. Experimental and theoretical results revealed the formation of four-coordinated Ir1O4 and six-coordinated Ir1O6 sites on amorphous and crystalline titanium oxides, respectively. Interestingly, the Ir1O4 sites exhibited a superior CER performance, with a mass activity about 10 and 500 times those of the Ir1O6 counterpart and DSA, respectively. Moreover, the Ir1O4 anode displayed excellent durability for 200 h, far longer than that of its Ir1O6 counterpart (2 h). Mechanism studies showed that the unsaturated Ir in Ir1O4 was the active center for chlorine evolution, which was changed to the top-coordinated O in Ir1O6. This change of active sites greatly affected the adsorption energy of Cl species, thus accounting for their different CER activity. More importantly, the amorphous structure and restrained water dissociation of Ir1O4 synergistically prevent oxygen permeation across the Ti substrate, contributing to its long-term CER stability. This study sheds light on the importance of single-atom coordination structures in the reactivity of catalysts and offers a facile strategy to prepare highly active single-atom CER anodes via surface titanium oxide amorphization.

Enhancing Ru-Cl interaction via orbital hybridization effect in Ru0.4Sn0.3Ti0.3 electrode for efficient chlorine evolution

Chlorine evolution reaction (CER) is a commercially valuable electrochemical reaction used at an industrial scale. However, oxygen evolution reaction (OER) during the electrolysis process inevitably leads to the decreased efficiency of CER. It is necessary to improve the selectivity of CER by minimizing or even eliminating the occurrence of OER. Herein, a ternary metal oxide (Ru0.4Sn0.3Ti0.3) electrode was fabricated and employed as an active and robust anode for CER. The Ru0.4Sn0.3Ti0.3 electrode exhibits an excellent CER performance in 6.0 M NaCl solution, with a low potential of 1.17 V (vs. saturated calomel electrode, SCE) at 200 mA cm-2 current density, a high Cl2 selectivity of over 90 %, and robust durability after consecutive operation for 160 h under 100 mA cm-2. The maximum O2-Cl2 potential difference between OER and CER further demonstrates the high Cl2 selectivity of Ru0.4Sn0.3Ti0.3 electrode. Theoretical studies show that the strong Ru 3d-Ti 3d orbitals hybridization effect makes the d-band center (εd) of Ru 3d and Ti 3d orbitals positively and negatively shifted, respectively, endowing Ru site with enhanced Cl adsorption ability (i.e. enhanced Ru-Cl interaction) and Ru0.4Sn0.3Ti0.3 electrode with superior CER activity. This work offers valuable insights into the development of advanced electrodes for CER in practical application.

Protective effects of Pt-N-C single-atom nanozymes against myocardial ischemia-reperfusion injury

Effective therapeutic strategies for myocardial ischemia/reperfusion (I/R) injury remain elusive. Targeting reactive oxygen species (ROS) provides a practical approach to mitigate myocardial damage following reperfusion. In this study, we synthesize an antioxidant nanozyme, equipped with a single-Platinum (Pt)-atom (PtsaN-C), for protecting against I/R injury. PtsaN-C exhibits multiple enzyme-mimicking activities for ROS scavenging with high efficiency and stability. Mechanistic studies demonstrate that the excellent ROS-elimination performance of the single Pt atom center precedes that of the Pt cluster center, owing to its better synergistic effect and metallic electronic property. Systematic in vitro and in vivo studies confirm that PtsaN-C efficiently counteracts ROS, restores cellular homeostasis and prevents apoptotic progression after I/R injury. PtsaN-C also demonstrates good biocompatibility, making it a promising candidate for clinical applications. Our study expands the scope of single-atom nanozyme in combating ROS-induced damage and offers a promising therapeutic avenue for the treatment of I/R injury.

Organocatalyst Supported by a Single-Atom Support Accelerates both Electrodes used in the Chlor-Alkali Industry via Modification of Non-Covalent Interactions

Consuming one of the largest amount of electricity, the chlor-alkali industry supplies basic chemicals for society, which mainly consists of two reactions, hydrogen evolution (HER) and chlorine evolution reaction (CER). Till now, the state-of-the-art catalyst applied in this field is still the dimensional stable anode (DSA), which consumes a large amount of noble metal of Ru and Ir. It is thus necessary to develop new types of catalysts. In this study, an organocatalyst anchored on the single-atom support (SAS) is put forward. It exhibits high catalytic efficiency towards both HER and CER with an overpotential of 21 mV and 20 mV at 10 mA cm-2 . With this catalyst on both electrodes, the energy consumption is cut down by 1.2 % compared with the commercial system under industrial conditions. Based on this novel catalyst and the high activity, the mechanism of modifying non-covalent interaction is demonstrated to be reliable for the catalyst's design. This work not only provides efficient catalysts for the chlor-alkali industry but also points out that the SACs can also act as support, providing new twists for the development of SACs and organic molecules in the next step.

Organocatalytic Lithium Chloride Oxidation by Covalent Organic Frameworks for Rechargeable Lithium-Chlorine Batteries

Rechargeable Li-Cl2 battery is a promising high energy density battery system. However, reasonable cycle life could only be achieved under low specific capacities due to the sluggish oxidation of LiCl to Cl2 . Herein, we propose an amine-functionalized covalent organic framework (COF) with catalytic activity, namely COF-NH2 , that significantly decreases the oxidation barrier of LiCl and accelerates the oxidation kinetics of LiCl in Li-Cl2 cell. The resulting Li-Cl2 cell using COF-NH2 (Li-Cl2 @COF-NH2 ) simultaneously exhibits low overpotential, ultrahigh discharge capacity up to 3500 mAh/g and a promoted utilization ratio of deposited LiCl at the first cycle (UR-LiCl) of 81.4 %, which is one of the highest reported values to date. Furthermore, the Li-Cl2 @COF-NH2 cell could be stably cycled for over 200 cycles when operating at a capacity of 2000 mAh/g at -20 °C with a Coulombic efficiency (CE) of ≈100 % and a discharge plateau of 3.5 V. Our superior Li-Cl2 batteries enabled by organocatalyst enlighten an arena towards high-energy storage applications.

Identifying Stable Electrocatalysts Initialized by Data Mining: Sb2 WO6 for Oxygen Reduction

Data mining from computational materials database has become a popular strategy to identify unexplored catalysts. Herein, the opportunities and challenges of this strategy are analyzed by investigating a discrepancy between data mining and experiments in identifying low-cost metal oxide (MO) electrocatalysts. Based on a search engine capable of identifying stable MOs at the pH and potentials of interest, a series of MO electrocatalysts is identified as potential candidates for various reactions. Sb2 WO6 attracted the attention among the identified stable MOs in acid. Based on the aqueous stability diagram, Sb2 WO6 is stable under oxygen reduction reaction (ORR) in acidic media but rather unstable under high-pH ORR conditions. However, this contradicts to the subsequent experimental observation in alkaline ORR conditions. Based on the post-catalysis characterizations, surface state analysis, and an advanced pH-field coupled microkinetic modeling, it is found that the Sb2 WO6 surface will undergo electrochemical passivation under ORR potentials and form a stable and 4e-ORR active surface. The results presented here suggest that though data mining is promising for exploring electrocatalysts, a refined strategy needs to be further developed by considering the electrochemistry-induced surface stability and activity.

Ag Nanoparticle-Induced Surface Chloride Immobilization Strategy Enables Stable Seawater Electrolysis

Although hydrogen gas (H2 ) storage might enable offshore renewable energy to be stored at scale, the commercialization of technology for H2 generation by seawater electrolysis depends upon the development of methods that avoid the severe corrosion of anodes by chloride (Cl- ) ions. Here, it is revealed that the stability of an anode used for seawater splitting can be increased by more than an order of magnitude by loading Ag nanoparticles on the catalyst surface. In experiments, an optimized NiFe-layered double hydroxide (LDH)@Ag electrode displays stable operation at 400 mA cm-2 in alkaline saline electrolyte and seawater for over 5000 and 2500 h, respectively. The impressive long-term durability is more than 20 times that of an unmodified NiFe-LDH anode. Meticulous characterization and simulation reveals that in the presence of an applied electric field, free Cl- ions react with oxidized Ag nanoparticles to form stable AgCl species, giving rise to the formation of a Cl- -free layer near the anode surface. Because of its simplicity and effectiveness, it is anticipated that the proposed strategy to immobilize chloride ions on the surface of an anode has the potential to become a crucial technology to control corrosion during large-scale electrolysis of seawater to produce hydrogen.

Boosting Ammonium Oxidation in Wastewater by the BiOCl-Functionalized Anode

Mixed metal oxide (MMO) anodes are commonly used for electrochlorination of ammonium (NH4+) in wastewater treatment, but they suffer from low efficiency due to inadequate chlorine generation at low Cl- concentrations and sluggish reaction kinetics between free chlorine and NH4+ under acidic pH conditions. To address this challenge, we develop a straightforward wet chemistry approach to synthesize BiOCl-functionalized MMO electrodes using the MMO as an efficient Ohmic contact for electron transfer. Our study demonstrates that the BiOCl@MMO anode outperforms the pristine MMO anode, exhibiting higher free chlorine generation (24.6-60.0 mg Cl2 L-1), increased Faradaic efficiency (75.5 vs 31.0%), and improved rate constant of NH4+ oxidation (2.41 vs 0.76 mg L-1 min-1) at 50 mM Cl- concentration. Characterization techniques including electron paramagnetic resonance and in situ transient absorption spectra confirm the production of chlorine radicals (Cl• and Cl2•-) by the BiOCl/MMO anode. Laser flash photolysis reveals significantly higher apparent second-order rate constants ((4.3-4.9) × 106 M-1 s-1 at pH 2.0-4.0) for the reaction between NH4+ and Cl•, compared to the undetectable reaction between NH4+ and Cl2•-, as well as the slower reaction between NH4+ and free chlorine (102 M-1 s-1 at pH < 4.0) within the same pH range, emphasizing the significance of Cl• in enhancing NH4+ oxidation. Mechanistic studies provide compelling evidence of the capacity of BiOCl for Cl- adsorption, facilitating chlorine evolution and Cl• generation. Importantly, the BiOCl@MMO anode exhibits excellent long-term stability and high catalytic activity for NH4+-N removal in a real landfill leachate. These findings offer valuable insights into the rational design of electrodes to improve electrocatalytic NH4+ abatement, which holds great promise for wastewater treatment applications.

Single Pd-doped arsenene coordinated with nitrogen atoms as an electrocatalyst for effective chlorine evolution reaction: DFT and machine learning studies

We designed a series of single transition metal-anchored arsenene coordinated with nitrogen atoms (TMNx@As) as electrocatalysts for chlorine evolution reaction (CER). Density functional theory (DFT) and machine learning were employed to investigate the catalytic activity of TMNx@As. It is found that the performance of TMNx@As is the best when the transition metal is Pd and the nitrogen coordination content is 66.67%. The catalytic activity of TMNx@As for chlorine evolution reaction is mainly determined by the covalent radius (Rc) and atomic non-bonded radius (Ra) of the transition metal and the fraction of N atoms in metal's coordinating atoms (fN).

Bromine-Enhanced Generation and Epoxidation of Ethylene in Tandem CO2 Electrolysis Towards Ethylene Oxide

The indirect electro-epoxidation of ethylene (C2 H4 ), produced from CO2 electroreduction (CO2 R), holds immense promise for CO2 upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu-based electrocatalysts during the conversion of C2 H4 to EO and CO2 to C2 H4 , respectively. Herein, we report a bromine-induced dual-enhancement strategy designed to concurrently promote both C2 H4 -to-EO and CO2 -to-C2 H4 conversions, thereby improving EO generation, using single-atom Pt on N-doped CNTs (Pt1 /NCNT) and Br- -bearing porous Cu2 O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X-ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br2 /HBrO generation over Pt1 /NCNT with optimal intermediate binding facilitates C2 H4 -to-EO conversion with a high FE of 92.2 %, and concomitantly, the Br- with strong nucleophilicity protects active Cu+ species in Cu2 O effectively for improved CO2 -to-C2 H4 conversion with a FE of 66.9 % at 800 mA cm-2 , superior to those in the traditional chloride-mediated case. Consequently, a single-pass FE as high as 41.1 % for CO2 -to-EO conversion can be achieved in a tandem system.

Emerging materials and technologies for electrocatalytic seawater splitting

The limited availability of freshwater in renewable energy-rich areas has led to the exploration of seawater electrolysis for green hydrogen production. However, the complex composition of seawater presents substantial challenges such as electrode corrosion and electrolyzer failure, calling into question the technological and economic feasibility of direct seawater splitting. Despite many efforts, a comprehensive overview and analysis of seawater electrolysis, including electrochemical fundamentals, materials, and technologies of recent breakthroughs, is still lacking. In this review, we systematically examine recent advances in electrocatalytic seawater splitting and critically evaluate the obstacles to optimizing water supply, materials, and devices for stable hydrogen production from seawater. We demonstrate that robust materials and innovative technologies, especially selective catalysts and high-performance devices, are critical for efficient seawater electrolysis. We then outline and discuss future directions that could advance the techno-economic feasibility of this emerging field, providing a roadmap toward the design and commercialization of materials that can enable efficient, cost-effective, and sustainable seawater electrolysis.

A Reflection on Sustainable Anode Materials for Electrochemical Chloride Oxidation

Chloride oxidation is a key industrial electrochemical process in chlorine-based chemical production and water treatment. Over the past few decades, dimensionally stable anodes (DSAs) consisting of RuO2 - and IrO2 -based mixed-metal oxides have been successfully commercialized in the electrochemical chloride oxidation industry. For a sustainable supply of anode materials, considerable efforts both from the scientific and industrial aspects for developing earth-abundant-metal-based electrocatalysts have been made. This review first describes the history of commercial DSA fabrication and strategies to improve their efficiency and stability. Important features related to the electrocatalytic performance for chloride oxidation and reaction mechanism are then summarized. From the perspective of sustainability, recent progress in the design and fabrication of noble-metal-free anode materials, as well as methods for evaluating the industrialization of novel electrocatalysts, are highlighted. Finally, future directions for developing highly efficient and stable electrocatalysts for industrial chloride oxidation are proposed.

Ru-Doped Co3O4 Nanoparticles as Efficient and Stable Electrocatalysts for the Chlorine Evolution Reaction

The electrochemical chlorine evolution reaction (CER) is one of the most important electrochemical reactions. Typically, iridium (Ir)- or ruthenium (Ru)-based mixed metal oxides have been used as electrocatalysts for the CER due to their high activities and durabilities. However, the scarcity of Ir and Ru has indicated the need to develop alternative earth-abundant transition-metal-based CER catalysts. In this study, we report a Co3O4 nanoparticle (NP) catalyst synthesized by a hydrothermal method. Furthermore, Ru was successfully incorporated into the Co3O4 NPs (RuxCo3-xO4 NPs) for further improvement of catalytic performance in chlorine generation. Electrokinetic analyses combined with in situ X-ray absorption near-edge structure (XANES) results suggested an identical CER mechanism for the Co3O4 NPs and RuxCo3-xO4 NPs. Various characterization techniques demonstrated that the homogeneous substitution of Ru4+ ions into the Co3+ octahedral sites enhanced the structural disorder and changed the electronic state of Co3O4, resulting in additional exposed active sites. Remarkably, the Ru0.09Co2.91O4 NP electrode exhibited outstanding stability for more than 150 h even at a high current density of 500 mA/cm2, which shows its commercial viability for active chlorine generation.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Self-adaptive amorphous CoOxCly electrocatalyst for sustainable chlorine evolution in acidic brine

Electrochemical chlorine evolution reaction is of central importance in the chlor-alkali industry, but the chlorine evolution anode is largely limited by water oxidation side reaction and corrosion-induced performance decay in strong acids. Here we present an amorphous CoOxCly catalyst that has been deposited in situ in an acidic saline electrolyte containing Co2+ and Cl- ions to adapt to the given electrochemical condition and exhibits ~100% chlorine evolution selectivity with an overpotential of ~0.1 V at 10 mA cm-2 and high stability over 500 h. In situ spectroscopic studies and theoretical calculations reveal that the electrochemical introduction of Cl- prevents the Co sites from charging to a higher oxidation state thus suppressing the O-O bond formation for oxygen evolution. Consequently, the chlorine evolution selectivity has been enhanced on the Cl-constrained Co-O* sites via the Volmer-Heyrovsky pathway. This study provides fundamental insights into how the reactant Cl- itself can work as a promoter toward enhancing chlorine evolution in acidic brine.

Unusual Ligand Assistance in Molecular Electrocatalysis via Interfacial Proton Charge Assembly

We show that the ability of the ligand to reorganize the electric double layer (EDL) often dominates the electrocatalysis contrary to their inductive effect in the spectrochemical series, leading to counterintuitive electrocatalysis. With water oxidation and chlorine evolution as the probe reactions, the same catalytic entity with carboxy functionalized ligand exhibited surprisingly higher electrochemical activity in comparison to the aggressively electron-withdrawing nitro functionalized ligands, which is contrary to their actual location in the spectrochemical series. Spectroscopic and electrochemical analyses suggest the enrichment of catalytically active species in the carboxy substituted ligand via proton charge assembly in the EDL that in turn enhances the kinetics of the overall electrochemical process. This demonstration of less obvious ligands becoming indispensable in electrocatalysis suggests a blind designing of ligands solely based on their inductive effect should be reconsidered as it will prevent the utilization of the maximum potential of the molecule in electrocatalysis.

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated PtII species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous PtII catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of PtII moieties on the single-atom catalysts beyond ideally four-coordinated PtII-N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtII species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d8 metal ions.

High-Performing Atomic Electrocatalyst for Chlorine Evolution Reaction

Electrocatalysts facilitating chlorine evolution reaction (ClER) play a vital role in chlor-alkali industries. Owing to a huge amount of chlorine consumed worldwide, inexpensive high-performing catalysts for Cl₂ production are highly demanded. Here, a superb ClER catalyst fabricated through uniform dispersion of Pt single atoms (SAs) in C₂ N₂ moieties of N-doped graphene (denoted as Pt-1) is presented, which demonstrates near 100% exclusive ClER selectivity, long-term durability, extraordinary Cl₂ production rate (3500 mmol h^-1 g_Pt ^-1 ), and >140 000-fold increased mass activity over industrial electrodes in acidic medium. Excitingly, at the typical chlor-alkali industries' operating temperature (80 °C), Pt-1 supported on carbon paper electrode requires a near thermoneutral ultralow overpotential of 5 mV at 1 mA cm^-2 current density to initiate the ClER, consistent with the predicted density functional theory (DFT) calculations. Altogether these results show the promising electrocatalyst of Pt-1 toward ClER.

CO2-mediated organocatalytic chlorine evolution under industrial conditions

During the chlor-alkali process, in operation since the nineteenth century, electrolysis of sodium chloride solutions generates chlorine and sodium hydroxide that are both important for chemical manufacturing1-4. As the process is very energy intensive, with 4% of globally produced electricity (about 150 TWh) going to the chlor-alkali industry5-8, even modest efficiency improvements can deliver substantial cost and energy savings. A particular focus in this regard is the demanding chlorine evolution reaction, for which the state-of-the-art electrocatalyst is still the dimensionally stable anode developed decades ago9-11. New catalysts for the chlorine evolution reaction have been reported12,13, but they still mainly consist of noble metal14-18. Here we show that an organocatalyst with an amide functional group enables the chlorine evolution reaction; and that in the presence of CO2, it achieves a current density of 10 kA m-2 and a selectivity of 99.6% at an overpotential of only 89 mV and thus rivals the dimensionally stable anode. We find that reversible binding of CO2 to the amide nitrogen facilitates formation of a radical species that plays a critical role in Cl2 generation, and that might also prove useful in the context of Cl- batteries and organic synthesis19-21. Although organocatalysts are typically not considered promising for demanding electrochemical applications, this work demonstrates their broader potential and the opportunities they offer for developing industrially relevant new processes and exploring new electrochemical mechanisms.

Electrosynthesis of chlorine from seawater-like solution through single-atom catalysts

The chlor-alkali process plays an essential and irreplaceable role in the modern chemical industry due to the wide-ranging applications of chlorine gas. However, the large overpotential and low selectivity of current chlorine evolution reaction (CER) electrocatalysts result in significant energy consumption during chlorine production. Herein, we report a highly active oxygen-coordinated ruthenium single-atom catalyst for the electrosynthesis of chlorine in seawater-like solutions. As a result, the as-prepared single-atom catalyst with Ru-O₄ moiety (Ru-O₄ SAM) exhibits an overpotential of only ~30 mV to achieve a current density of 10 mA cm^-2 in an acidic medium (pH = 1) containing 1 M NaCl. Impressively, the flow cell equipped with Ru-O₄ SAM electrode displays excellent stability and Cl₂ selectivity over 1000 h continuous electrocatalysis at a high current density of 1000 mA cm^-2. Operando characterizations and computational analysis reveal that compared with the benchmark RuO₂ electrode, chloride ions preferentially adsorb directly onto the surface of Ru atoms on Ru-O₄ SAM, thereby leading to a reduction in Gibbs free-energy barrier and an improvement in Cl₂ selectivity during CER. This finding not only offers fundamental insights into the mechanisms of electrocatalysis but also provides a promising avenue for the electrochemical synthesis of chlorine from seawater electrocatalysis.

Ni-Fe Oxides/TiO2 Heterojunction Anodes for Reactive Chlorine Generation and Mediated Water Treatment

Reactive chlorine-mediated electrochemical water treatment necessitates selective chlorine evolution reaction (ClER) versus parallel oxygen evolution reaction (OER) in mild pH (7-10), with minimal deployments of precious electrocatalysts. This study reports Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode prepared by a straightforward sol-gel coating with thermal decomposition at 425 °C. The ClER current efficiency (CE, 70%) and energy efficiency (2.3 mmol W h^-1) were comparable to benchmarking Ir₇Ta₃O_y/TiO₂ at 30 mA cm^-2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable Ni_xFe_1-xO_y/TiO₂ (x: 0.33, 0.8-1) with the flat-band potential and p-band center, as experimental descriptors for surface charge density, nominated the outer TiO₂ to be the active ClER center. The underlying Ni_0.33Fe_0.67O_y, characterized as biphasic NiFe₂O₄ and NiO, effectively lowered the O binding energy of TiO₂ by electronic interaction across the junction. The OER activity of Ni_0.33Fe_0.67O_y superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni_0.33Fe_0.67O_y could also facilitate the ClER as an ohmic contact. Stability tests and NH₄⁺ degradation in synthetic and real wastewater confirmed the feasibility of Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode for mediated water treatments in mild pH.

Defect-Enriched Hollow Porous Carbon Nanocages Enable Highly Efficient Chlorine Evolution Reaction

Metal-free carbon-based catalysts are crucial for the electrocatalytic chlorine evolution reaction (CER) to reduce the usage of noble metals and industrial cost. However, the corresponding catalytic activity of high overpotential and low durability hinders their wide application. Here, a hollow porous carbon (HPC) nanocage with a controlled oxygen electronic state around designed carbon defects for CER activity is reported. Alkali etching can bring defects in zeolite with a hollow structure. In a hard template strategy, the type of carbon defects is directly related to etching degree of the zeolite template. More importantly, the oxygen atoms can be "borrowed" from the zeolite framework by the defective carbon. The electron density around unsaturated O atoms can be decreased on the minor defects in carbon compared with that on large defects which is favorable for the adsorption of Cl^- . Consequently, the as-synthesized HPC nanocages with minor defects show excellent electrocatalytic performance for CER with a low overpotential of 94 mV at current density of 10 mA cm^-2 with good stability, which is superior to the commercial precious metal catalyst of dimensionally stable anode (DSA), and the best in the reported carbon materials. The designed carbon materials provide an option for metal-free industrial catalysts with significant CER activities.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Preferential Adsorption of Ethylene Oxide on Fe and Chlorine on Ni Enabled Scalable Electrosynthesis of Ethylene Chlorohydrin

Industrial manufacturing of ethylene chlorohydrin (ECH) critically requires excess corrosive hydrochloric acid or hypochlorous acid with dealing with massive by-products and wastes. Here we report a green and efficient electrosynthesis of ECH from ethylene oxide (EO) with NaCl over a NiFe₂ O₄ nanosheet anode. Theoretical results suggest that EO and Cl preferentially adsorb on Fe and Ni sites, respectively, collaboratively promoting the ECH synthesis. A Cl radical-mediated ring-opening process is proposed and confirmed, and the key Cl and carbon radical species are identified by high-resolution mass spectrometry. This strategy can enable scalable electrosynthesis of 185.1 mmol of ECH in 1 h with 92.5 % yield at a 55 mA cm^-2 current density. Furthermore, a series of other chloro- and bromoethanols with good to high yields and paired synthesis of ECH and 4-amino-3,6-dichloropyridine-2-carboxylicacid via respectively loading and unloading Cl are achieved, showing the promising potential of this strategy.

Nano-sphere RuO2 embedded in MOF-derived carbon arrays as a dual-matrix anode for cost-effective electrochemical wastewater treatment

The electrodes' activity, surface area and cost hinder the deployment of electrochemical wastewater treatment. Using an economical microfiber-based carbon felt (CF) substrate, we design RuO₂ nanospheres confined by Co_xO cooperated carbon nanoarrays (RuO₂-Co_xO@TCF) to augment noble metal utilization and thus reduce the catalyst cost. RuO₂-Co_xO@TCF anode with vertical diffusion channels exhibits rapid generation ability of oxidizing species particularity in the presence of Cl^- ions, which play a crucial role in azo bond cleavage and benzene ring chlorination of methyl orange. As a result, the catalyst shows 99.5 % color removal and ∼ 70 % mineralization efficiency at a concentration of 60 ppm. In synthetic dyeing wastewater, RuO₂-Co_xO@TCF delivers a stable total organic carbon (TOC) removal throughout ten cycling tests. Moreover, the electricity consumption of RuO₂-Co_xO@TCF is far below the reference anode, showing great promise for dye degradation and remediation of industrial wastewater.

Electrochemical removal of ammonium nitrogen in high efficiency and N2 selectivity using non-noble single-atomic iron catalyst

Ammonia nitrogen (NH₄⁺-N) is a ubiquitous environmental pollutant, especially in offshore aquaculture systems. Electrochemical oxidation is very promising to remove NH₄⁺-N, but suffers from the use of precious metals anodes. In this work, a robust and cheap electrocatalyst, iron single-atoms distributed in nitrogen-doped carbon (Fe-SAs/N-C), was developed for electrochemical removal of NH₄⁺-N from in wastewater containing chloride. The Fe-SAs/N-C catalyst exhibited superior activity than that of iron nanoparticles loaded carbon (Fe-NPs/N-C), unmodified carbon and conventional Ti/IrO₂-TiO₂-RuO₂ electrodes. And high removal efficiency (> 99%) could be achieved as well as high N₂ selectivity (99.5%) at low current density. Further experiments and density functional theory (DFT) calculations demonstrated the indispensable role of single-atom iron in the promoted generation of chloride derived species for efficient removal of NH₄⁺-N. This study provides promising inexpensive catalysts for NH₄⁺-N removal in aquaculture wastewater.

Selectively Enhanced Electrocatalytic Oxygen Evolution within Nanoscopic Channels Fitting a Specific Reaction Intermediate for Seawater Splitting

Abundant availability of seawater grants economic and resource-rich benefits to water electrolysis technology requiring high-purity water if undesired reactions such as chlorine evolution reaction (CER) competitive to oxygen evolution reaction (OER) are suppressed. Inspired by a conceptual computational work suggesting that OER is kinetically improved via a double activation within 7 Å-gap nanochannels, RuO₂ catalysts are realized to have nanoscopic channels at 7, 11, and 14 Å gap in average (d_gap ), and preferential activity improvement of OER over CER in seawater by using nanochanneled RuO₂ is demonstrated. When the channels are developed to have 7 Å gap, the OER current is maximized with the overpotential required for triggering OER minimized. The gap value guaranteeing the highest OER activity is identical to the value expected from the computational work. The improved OER activity significantly increases the selectivity of OER over CER in seawater since the double activation by the 7 Å-nanoconfined environments to allow an OER intermediate (*OOH) to be doubly anchored to Ru and O active sites does not work on the CER intermediate (*Cl). Successful operation of direct seawater electrolysis with improved hydrogen production is demonstrated by employing the 7 Å-nanochanneled RuO₂ as the OER electrocatalyst.

Efficient electrochemical generation of active chlorine to mediate urea and ammonia oxidation in a hierarchically porous-Ru/RuO2-based flow reactor

The electrochemical chlorination of urea to CO₂ and N₂ end-products, via active-chlorine-mediated oxidation under nearly neutral conditions, is an effective treatment for medium-concentrated urea-containing wastewater. Herein, we design a novel flow reactor integrated with three-dimensional hierarchically porous Ru/RuO₂ architectures anchored on a Ti mesh. The hierarchically macroporous electrode can create sufficient exposure of catalytically active sites and facilitate the microscopic mass transport and diffusion inside the active layer, thereby contributing to the increased removal efficiency of urea-N and ammonia-N. The combined results of electrochemical measurements, UV-visible spectrometry and in situ Raman spectrometry, show that the OCl^- species produced by chlorine evolution reaction (CER) are the main active constituents for removing urea-N. Theoretical calculations reveal thLTWAat the Ru/RuO₂ possesses a moderate Cl binding strength, lower theoretical overpotentials of CER and a higher conductivity, compared with pure RuO₂. On this basis, we assemble a circular flow reactor with the hierarchically porous electrodes in a two-electrode system to obtain an enhanced microfluidic process, which during 9 days of uninterrupted operation, at a high electrolysis current of 500 mA, achieve a total nitrogen removal of 92.6% and an energy consumption of 7.94 kWh kg^-1 N, demonstrating the promising application of the novel process.

Pt Single Atoms on CrN Nanoparticles Deliver Outstanding Activity and CO Tolerance in the Hydrogen Oxidation Reaction

The large-scale application of proton exchange membrane fuel cells is currently hampered by high cost of commercial Pt catalysts and their susceptibility to poisoning by CO impurities in H₂ feed. In this context, the development of CO-tolerant electrocatalysts with high Pt atom utilization efficiency for hydrogen oxidation reaction (HOR) is of critical importance. Herein, Pt single atoms are successfully immobilized on chromium nitride nanoparticles by atomic layer deposition method, denoted as Pt SACs/CrN. Electrochemical tests establish Pt SACs/CrN to be a very efficient HOR catalyst, with a mass activity that is 5.7 times higher than commercial PtRu/C. Strikingly, the excellent performance of Pt SACs/CrN is maintained after introducing 1000 ppm of CO in H₂ feed. The excellent CO-tolerance of Pt SACs/CrN is related to weaker CO adsorption on Pt single atoms. This work provides guidelines for the design and construction of active and CO-tolerant catalysts for HOR.

Dual-Use of Seawater Batteries for Energy Storage and Water Desalination

Seawater batteries are unique energy storage systems for sustainable renewable energy storage by directly utilizing seawater as a source for converting electrical energy and chemical energy. This technology is a sustainable and cost-effective alternative to lithium-ion batteries, benefitting from seawater-abundant sodium as the charge-transfer ions. Research has significantly improved and revised the performance of this type of battery over the last few years. However, fundamental limitations of the technology remain to be overcome in future studies to make this method even more viable. Disadvantages include degradation of the anode materials or limited membrane stability in aqueous saltwater resulting in low electrochemical performance and low Coulombic efficiency. The use of seawater batteries exceeds the application for energy storage. The electrochemical immobilization of ions intrinsic to the operation of seawater batteries is also an effective mechanism for direct seawater desalination. The high charge/discharge efficiency and energy recovery make seawater batteries an attractive water remediation technology. Here, the seawater battery components and the parameters used to evaluate their energy storage and water desalination performances are reviewed. Approaches to overcoming stability issues and low voltage efficiency are also introduced. Finally, an overview of potential applications, particularly in desalination technology, is provided.

Steering Catalytic Selectivity with Atomically Dispersed Metal Electrocatalysts for Renewable Energy Conversion and Commodity Chemical Production

Electrocatalysis is a key driver in promoting the paradigm shift from the current fossil-fuel-based hydrocarbon economy to a renewable-energy-driven hydrogen economy. The success of electrocatalysis hinges primarily on achieving high catalytic selectivity along with maximum activity and sustained longevity. Many electrochemical reactions proceed through multiple pathways, requiring highly selective catalysts.Atomically dispersed metal catalysts have emerged as a new frontier in heterogeneous catalysis. In addition to the widely perceived advantages of maximized active site utilization and substantially reduced metal content, they have shown different catalytic selectivities in some electrocatalytic reactions compared to the traditional nanoparticle (NP)-based catalysts. Although there have been significant advances in their synthesis, the highly energetic nature of a single atomic site has made the preparation of atomically dispersed metal catalysts rely on empiricism rather than rational design. Consequently, the structural comprehension of a single atomic site and the understanding of its unusual electrocatalytic selectivity remain largely elusive.In this Account, we describe our endeavors toward developing general synthetic approaches for atomically dispersed metal catalysts for the discovery of new selective and active electrocatalysts and to understand their catalytic nature. We introduce synthetic approaches to produce a wide range of nonprecious- and precious-metal-based atomically dispersed catalysts and control their coordination environments. Metallomacrocyclic-compound-driven top-down and metal salt/heteroatom layer-based bottom-up strategies, coupled with a SiO₂-protective-layer-assisted method, have been developed that can effectively generate single atomic sites while mitigating the formation of metallic NPs. The low-temperature gas-phase ligand exchange method can reversibly tune the coordination structure of the atomically dispersed metal sites. We have used the prepared atomically dispersed metal catalysts as model systems to investigate their electrocatalytic reactivity for renewable energy conversion and commodity chemical production reactions, in which high selectivity is important. The reactions of our interest include the following: (i) the oxygen reduction reaction, where O₂ is reduced to either H₂O or H₂O₂ via the four-electron or two electron pathway, respectively; (ii) the CO₂ reduction reaction, which should suppress the hydrogen evolution reaction; and (iii) the chlorine evolution reaction, which competes with the oxygen evolution reaction. The type of metal center to which the reactant is directly bound is found to be the most important in determining the selectivity, which originates from the dramatic changes in the binding energy of each metal center with the reactants. The coordination structure surrounding the metal center also has a significant effect on the selectivity; its control can modulate the oxidation state of the metal center, thereby altering the binding strength with the reactants.We envisage that future advances in the synthesis of atomically dispersed metal catalysts, combined with the growing power of computational, spectroscopic, and microscopic methods, will bring their synthesis to the level of rational design. Elaborately designed catalysts can overcome the current limits of catalytic selectivity, which will help establish the field of atomically dispersed metal catalysts as an important branch of catalysis.

Boosting Thermal Stability of Volatile Os Catalysts by Downsizing to Atomically Dispersed Species

Os-based catalysts present remarkable catalytic activity; however, their use has been limited by the undesirable side reactions that generate highly toxic and volatile OsO₄ even at room temperature. Herein, we demonstrate that the thermal stability of Os-based catalysts can be dramatically improved by downsizing Os nanoparticles (NPs) into atomically dispersed species. We observed that Os NPs were converted into OsO₄ after calcination at 250 °C followed by sublimation, whereas single Os sites retained their structure after calcination. Temperature-programmed oxidation analysis confirmed that Os NPs started to undergo oxidation at 130 °C, whereas atomically dispersed Os preserved its state up to 300 °C. The CO oxidation activity of the atomically dispersed Os catalyst at 400 °C (100% conversion) was stably preserved over 30 h. By contrast, the activity of Os NP catalyst declined drastically. This study highlights the unique catalytic behavior of atomically dispersed catalysts, which is distinct from that of NP-based catalysts.