Reversed I1Cu4 single-atom sites for superior neutral ammonia electrosynthesis with nitrate

Electrochemical ammonia (NH3) synthesis from nitrate reduction (NITRR) offers an appealing solution for addressing environmental concerns and the energy crisis. However, most of the developed electrocatalysts reduce NO3- to NH3 via a hydrogen (H*)-mediated reduction mechanism, which suffers from undesired H*-H* dimerization to H2, resulting in unsatisfactory NH3 yields. Herein, we demonstrate that reversed I1Cu4 single-atom sites, prepared by anchoring iodine single atoms on the Cu surface, realized superior NITRR with a superior ammonia yield rate of 4.36 mg h-1 cm-2 and a Faradaic efficiency of 98.5% under neutral conditions via a proton-coupled electron transfer (PCET) mechanism, far beyond those of traditional Cu sites (NH3 yield rate of 0.082 mg h-1 cm-2 and Faradaic efficiency of 36.5%) and most of H*-mediated NITRR electrocatalysts. Theoretical calculations revealed that I single atoms can regulate the local electronic structures of adjacent Cu sites in favor of stronger O-end-bidentate NO3- adsorption with dual electron transfer channels and suppress the H* formation from the H2O dissociation, thus switching the NITRR mechanism from H*-mediated reduction to PCET. By integrating the monolithic I1Cu4 single-atom electrode into a flow-through device for continuous NITRR and in situ ammonia recovery, an industrial-level current density of 1 A cm-2 was achieved along with a NH3 yield rate of 69.4 mg h-1 cm-2. This study offers reversed single-atom sites for electrochemical ammonia synthesis with nitrate wastewater and sheds light on the importance of switching catalytic mechanisms in improving the performance of electrochemical reactions.

Integrative catalytic pairs for efficient multi-intermediate catalysis

Single-atom catalysts (SACs) have attracted considerable research interest owing to their combined merits of homogeneous and heterogeneous catalysts. However, the uniform and isolated active sites of SACs fall short in catalysing complex chemical processes that simultaneously involve multiple intermediates. In this Review, we highlight an emerging class of catalysts with adjacent binary active centres, which is called integrative catalytic pairs (ICPs), showing not only atomic-scale site-to-site electronic interactions but also synergistic catalytic effects. Compared with SACs or their derivative dual-atom catalysts (DACs), multi-interactive intermediates on ICPs can overcome kinetic barriers, adjust reaction pathways and break the universal linear scaling relations as the smallest active units. Starting from this active-site design principle, each single active atom can be considered as a brick to further build integrative catalytic clusters (ICCs) with desirable configurations, towards trimer or even larger multi-atom units depending on the requirement of a given reaction.

Emerging Xene-Based Single-Atom Catalysts: Theory, Synthesis, and Catalytic Applications

In recent years, the emergence of novel 2D monoelemental materials (Xenes), e.g., graphdiyne, borophene, phosphorene, antimonene, bismuthene, and stanene, has exhibited unprecedented potentials for their versatile applications as well as addressing new discoveries in fundamental science. Owing to their unique physicochemical, optical, and electronic properties, emerging Xenes have been regarded as promising candidates in the community of single-atom catalysts (SACs) as single-atom active sites or support matrixes for significant improvement in intrinsic activity and selectivity. In order to comprehensively understand the relationships between the structure and property of Xene-based SACs, this review represents a comprehensive summary from theoretical predictions to experimental investigations. Firstly, theoretical calculations regarding both the anchoring of Xene-based single-atom active sites on versatile support matrixes and doping/substituting heteroatoms at Xene-based support matrixes are briefly summarized. Secondly, controlled synthesis and precise characterization are presented for Xene-based SACs. Finally, current challenges and future opportunities for the development of Xene-based SACs are highlighted.

Aluminum-incorporation activates vanadium carbide with electron-rich carbon sites for efficient pH-universal hydrogen evolution reaction

Vanadium carbide (VC) is the greatest potential hydrogen evolution reaction (HER) catalyst because of its platinum-like property and abundant earth reserves. However, it exhibits insufficient catalytic performance due to the unfavorable interaction of reaction intermediates with catalysts. In this work, using NH4VO3 as the main raw material, the flow ratio of CH4 to Ar was accurately controlled, and a non-transition metal Al-doped into VC (100) nano-flowers with carbon hybrids on nickel foams (Al-VC@C/NF) was prepared for the first time as a high-efficiency HER catalyst by chemical vapor carbonization. The overpotential of Al-VC@C/NF catalysts in 0.5 M H2SO4 and 1 M KOH at a current density of 10 mA cm-2 are only 58 mV and 97 mV, respectively, which are the best HER performance among non-noble metal vanadium carbide based catalysts. Simultaneously, Al-VC@C/NF exhibits small Tafel slope (45 mV dec-1 and 73 mV dec-1) and excellent stability in acidic and alkaline media. Theoretical calculations demonstrate that doped Al atoms can induce electron redistribution on the vanadium carbide surface to form electron-rich carbon sites, which significantly reduces the energy barrier during the HER process. This work provides a new tactic to modulate vanadium-based carbons as efficient HER catalysts through non-transition metal doping.

Dealloying-Induced Zeolite-like Metal Framework of AB2 Laves Phase Intermetallic Electrocatalysts

Exploring an efficient and robust electrocatalyst for hydrogen evolution reaction (HER) at high pH and temperature holds the key to the industrial application of alkaline water electrolysis (AWE). Herein, we design an open tunnel structure by dealloying a series of Laves phase intermetallics, i.e., MCo2 and MRu0.25Co1.75 (M = Sc and Zr). The dealloying process can induce a zeolite-like metal framework for ScCo2 and ScRu0.25Co1.75 by stripping Sc metal from the center of a tunnel structure. This structural engineering significantly lowers their overpotentials at a current density of 500 mA/cm2 (η500) ca. 80 mV in 1.0 M KOH. Through a simple process, ScRu0.25Co1.75 can be easily decorated on a carbon cloth substrate and only requires 132 mV to reach 500 mA/cm2. More importantly it can maintain activity over 1000 h in industrial conditions (6.0 M KOH at 333 K), showing its potential for practical industrial applications.

Renaissance of elemental phosphorus materials: properties, synthesis, and applications in sustainable energy and environment

The polymorphism of phosphorus-based materials has garnered much research interest, and the variable chemical bonding structures give rise to a variety of micro and nanostructures. Among the different types of materials containing phosphorus, elemental phosphorus materials (EPMs) constitute the foundation for the synthesis of related compounds. EPMs are experiencing a renaissance in the post-graphene era, thanks to recent advancements in the scaling-down of black phosphorus, amorphous red phosphorus, violet phosphorus, and fibrous phosphorus and consequently, diverse classes of low-dimensional sheets, ribbons, and dots of EPMs with intriguing properties have been produced. The nanostructured EPMs featuring tunable bandgaps, moderate carrier mobility, and excellent optical absorption have shown great potential in energy conversion, energy storage, and environmental remediation. It is thus important to have a good understanding of the differences and interrelationships among diverse EPMs, their intrinsic physical and chemical properties, the synthesis of specific structures, and the selection of suitable nanostructures of EPMs for particular applications. In this comprehensive review, we aim to provide an in-depth analysis and discussion of the fundamental physicochemical properties, synthesis, and applications of EPMs in the areas of energy conversion, energy storage, and environmental remediation. Our evaluations are based on recent literature on well-established phosphorus allotropes and theoretical predictions of new EPMs. The objective of this review is to enhance our comprehension of the characteristics of EPMs, keep abreast of recent advances, and provide guidance for future research of EPMs in the fields of chemistry and materials science.

Interfacial Engineering of Copper-Nickel Selenide Nanodendrites for Enhanced Overall Water Splitting in Alkali Condition

Fabricating heterogeneous interfaces is an effective approach to improve the intrinsic activity of noble-metal-free catalysts for water splitting. Herein, 3D copper-nickel selenide (CuNi@NiSe) nanodendrites with abundant heterointerfaces are constructed by a precise multi-step wet chemistry method. Notably, CuNi@NiSe only needs 293 and 41 mV at 10 mA cm-2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Moreover, the assembled CuNi@NiSe system just requires 2.2 V at 1000 mA cm-2 in anion exchange membrane (AEM) electrolyzer, which is 2.0 times better than Pt/C//IrO2 . Mechanism studies reveal Cu defects on the Cu2-x Se surface boost the electron transfer between Cu atoms and Se atoms of Ni3 Se4 via Cu2-x Se/Ni3 Se4 interface, largely lowering the reaction barrier of rate-determining step for HER. Besides, the intrinsic activity of Ni atoms for in situ generated NiOOH is largely enhanced during OER because of the electron-modulating effect of Se atoms at Ni3 Se4 /NiOOH interface. The unique 3D structure also promotes the mass transfer during catalysis process. This work emphasizes the essential role of interfacial engineering for practical water splitting.

On-Chip Microdevice Unveils Reactant Enrichment Effect Dominated Electrocatalysis Activity in Molecular-Linked Catalysts

Molecular functionalization has been intensely studied and artificially constructed to advance various electrocatalytic processes. While there is a widely approved charge-doping effect, the underlying action for reactant distribution/transport remains long neglected. Here an on-chip microdevice unravels that the proton enrichment effect at prototypical methylene blue (MB)/MoS₂ interfaces rather than charge doping contributes to the hydrogen evolution reaction (HER) activity. Back-gated electrical/electrochemical tests detect quantitatively a strong charge injection from MB to MoS₂ realized over diploid carrier density, but these excess carriers are unqualified for the actual enhanced HER activity (from 32 to 125 mA cm^-2 at -0.29 V). On-chip electrochemical impedance further certifies that the proton enrichment in the vicinity of MoS₂, which is generated by the nucleophilic group of MB, actually dominates the HER activity. This finding uncovers the leading function of molecular-linked catalysts.

Hydrogen Production Technology Promotes the Analysis and Prospect of the Hydrogen Fuel Cell Vehicles Development under the Background of Carbon Peak and Carbon Neutrality in China

Hydrogen fuel cell vehicles have always been regarded as the main direction for developing new energy vehicles in the future due to their advantages of zero emission, high cruising range, and strong environmental adaptability. Currently, although the related technologies have gradually matured, there are still many factors hindering its development. One of the main reasons is that the price of hydrogen fuel increases the cost of using vehicles, which puts it at a competitive disadvantage compared with traditional fuel vehicles and pure electric vehicles. Herein, we summarize the recent development status of hydrogen fuel cell vehicles at home and abroad, and analyze the cost and sustainability brought by the latest scientific research progress to the hydrogen production industry, which is derived from basic research on electrocatalysts used in industrial electrocatalytic water splitting with an alkaline electrolyte. Finally, the development of hydrogen fuel cell vehicles was analyzed and prospected, which is one of the main application fields of hydrogen in the future.

In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting

The in situ growth of metal organic framework (MOF) derivatives on the surface of nickel foam is a novel type of promising self-supporting electrode catalyst. In this paper, this work reports for the first time the strategy of in situ growth of Ni-MOF, where the metal source is purely provided by a nickel foam (NF) substrate without any external metal ions. MOF-derived Ni2P/NPC structure is achieved by the subsequent phosphidation to yield Ni2P on porous N, P-doped carbon (NPC) backbone. Such strategy provides the as-synthesized Ni2P/NPC/NF electrocatalyst an extremely low interfacial steric resistance. Moreover, a unique three-dimensional hierarchical structure is achieved in Ni2P/NPC/NF, providing massive active sites, short ion diffusion path, and high electrical conductivity. Directly applied as the electrode, Ni2P/NPC/NF demonstrates excellent electrocatalytic performance towards both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with low overpotentials of only 58 mV and 208 mV to drive 10 mA cm−2, respectively, in 1 M KOH. Furthermore, Ni2P/NPC/NF acting as the overall water splitting electrodes can generate a current density of 10 mA cm−2 at an ultralow cell voltage of 1.53 V. This simple strategy paves the way for the construction of self-supporting transition metal-based electrocatalysts.

Boosting the overall electrochemical water splitting performance of pentlandites through non-metallic heteroatom incorporation

We report on synthesis of the heterotrimetallic pentlandite-type material Fe3Co3Ni3S8 (FCNS) in presence of suitable phosphorus-(FCNSP) and nitrogen-(FCNSN) donors for the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N into the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP facilitate the oxygen evolution reaction (OER) at 100 mAcm-2 in 1.0M KOH with overpotentials of 479, 440, and 427 mV, respectively, outperforming the benchmark IrO2 (564 mV) and commercial Ni metal powder (>600 mV). Likewise, FCNSN and FCNSNP reveal an improved performance toward the hydrogen evolution reaction (HER) in 0.5M H2SO4, outperforming the pristine FCNS. All materials revealed high stability and morphological robustness during OER and HER. Notably, DFT calculation suggests that N and P doping boost the OER activity of the pristine FCNS, whereas N doping enhances the HER activity.

Rational Design of Atomic Site Catalysts for Electrocatalytic Nitrogen Reduction Reaction: One Step Closer to Optimum Activity and Selectivity

The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber–Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH3 yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area: the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.

Graphical Abstract

Rational design of atomic site catalysts (ASCs) for nitrogen reduction reaction (NRR) has both scientific and industrial significance. In this review, the recent experimental and theoretical breakthroughs in the design principles of transition metal ASCs for NRR are comprehensively discussed, and the topic is also extended to double-atom catalysts and boron-based metal-free ASCs.

NP monolayer supported transition-metal single atoms for electrochemical water splitting: a theoretical study

The development of cost-effective and highly efficient electrocatalysts for water splitting is highly desirable but remains an ongoing challenge. Numerous single-atom catalysts (SACs) have achieved satisfactory performances in this area; however, non-carbon metal-free substrates have been rarely explored. Herein, we report a series of single-metal atoms supported on a novel two-dimensional NP monolayer as promising electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) by theoretical calculations. Our results disclose that Ti@NP, V@NP and Ir@NP exhibit desirable catalytic activity for the HER with extremely low of -0.004, -0.051, and 0.017 eV, respectively. More importantly, the calculated activation barriers for the Tafel reactions of these SACs are much lower than those for the benchmark Pt catalysts. In addition, Pt@NP shows the lowest η^OER of 0.495 V, followed by Rh@NP (η^OER = 0.548 V), which are even superior to that of state-of-the-art IrO₂. This work highlights the potential application of metal-free supports in SACs, which also further enriches the application of a NP monolayer in other related electrochemical processes.

Emerging Single-Atom Catalysts/Nanozymes for Catalytic Biomedical Applications

Single-atom catalysts (SACs) are a type of atomically dispersed nanozymes with the highest atom utilization, which employ low-coordinated single atoms as the catalytically active sites. SACs not only inherit the merits of traditional nanozymes, but also hold high catalytic activity and superb catalytic selectivity, which ensure their tremendous application potential in environmental remediation, energy storage and conversion, chemical industry, nanomedicine, etc. Nevertheless, undesired aggregation effect of single atoms during preactivation and reaction processes is significantly enhanced owing to the high surface free energy of single atoms. In this case, appropriate substrates are requisite to prevent the aggregation event through the powerful interactions between the single atoms and the substrates, thereby stabilizing the high catalytic activity of the catalysts. In this review, the synthetic methods and characterization approaches of SACs are first described. Then the application cases of SACs in nanomedicine are summarized. Finally, the current challenges and future opportunities of the SACs in nanomedicine are outlined. It is hoped that this review may have implications for furthering the development of new SACs with improved biophysicochemical properties and broadened biomedical applications.

A single-atom Cu-N2 catalyst eliminates oxygen interference for electrochemical sensing of hydrogen peroxide in a living animal brain

Hydrogen peroxide (H₂O₂) plays essential roles in various physiological and pathological processes. The electrochemical hydrogen peroxide reduction reaction (HPRR) has been recognized as an efficient approach to H₂O₂ sensing; however, the HPRR has always suffered from low tolerance against the oxygen reduction reaction (ORR), resulting in poor selectivity of the HPRR-based sensing platform. In this study, we find that the electrochemical HPRR occurs preferentially compared to the ORR when isolated Cu atoms anchored on carbon nitride (Cu₁/C₃N₄) are used as a single-atom electrocatalyst, which is theoretically attributed to the lower energy barrier of the HPRR than that of the ORR on a Cu₁/C₃N₄ single-atom catalyst (SAC). With the Cu₁/C₃N₄ SAC as the electrocatalyst, we fabricated microsensors that have a good response to H₂O₂, but not to O₂ or other electroactive neurochemicals. When implanted into a living rat brain, the microsensor shows excellent in vivo sensing performance, enabling its application in real-time quantitative investigation of the dynamics of H₂O₂ production induced by mercaptosuccinate and glutathione monoethyl ester in a living animal brain.

We have achieved the selective monitoring of H₂O₂ fluctuation in vivo free from O₂ interference by a single-atom Cu–N₂ electrocatalyst.

Atomically Dispersed Selenium Sites on Nitrogen-Doped Carbon for Efficient Electrocatalytic Oxygen Reduction

Owing to their unique electronic structure and maximum atom utilization efficiency, single-atom catalysts have received widespread attention and exhibited efficient activity. Herein, we report the preparation of non-metal Se single atoms embedded in nitrogen-doped carbon (NC) via a high-temperature reduction strategy for electrocatalytic oxygen reduction reaction (ORR). Selenium dioxide is reduced to selenium by NC at high temperature and partially anchored to form C-Se-C bond. Impressively, the obtained single-atom catalyst exhibits outstanding ORR activity and stability that even surpasses state-of-the-art noble metal catalysts and many previously reported nanocatalysts. Experimental and theoretical calculations reveal that the Se single atoms can serve as the ORR active sites and contribute to lowering the reaction barrier. Our discoveries demonstrate the promising prospects for utilizing metal-free single-atom-based materials for efficient electrocatalysis.

Single-Atom Engineering to Ignite 2D Transition Metal Dichalcogenide Based Catalysis: Fundamentals, Progress, and Beyond

Single-atom catalysis has been recognized as a pivotal milestone in the development history of heterogeneous catalysis by virtue of its superior catalytic performance, ultrahigh atomic utilization, and well-defined structure. Beyond single-atom protrusions, two more motifs of single-atom substitutions and single-atom vacancies along with synergistic single-atom motif assemblies have been progressively developed to enrich the single-atom family. On the other hand, besides traditional carbon material based substrates, a wide variety of 2D transitional metal dichalcogenides (TMDs) have been emerging as a promising platform for single-atom catalysis owing to their diverse elemental compositions, variable crystal structures, flexible electronic structures, and intrinsic activities toward many catalytic reactions. Such substantial expansion of both single-atom motifs and substrates provides an enriched toolbox to further optimize the geometric and electronic structures for pushing the performance limit. Concomitantly, higher requirements have been put forward for synthetic and characterization techniques with related technical bottlenecks being continuously conquered. Furthermore, this burgeoning single-atom catalyst (SAC) system has triggered serial scientific issues about their changeable single atom-2D substrate interaction, ambiguous synergistic effects of various atomic assemblies, as well as dynamic structure-performance correlations, all of which necessitate further clarification and comprehensive summary. In this context, this Review aims to summarize and critically discuss the single-atom engineering development in the whole field of 2D TMD based catalysis covering their evolution history, synthetic methodologies, characterization techniques, catalytic applications, and dynamic structure-performance correlations. In situ characterization techniques are highlighted regarding their critical roles in real-time detection of SAC reconstruction and reaction pathway evolution, thus shedding light on lifetime dynamic structure-performance correlations which lay a solid theoretical foundation for the whole catalytic field, especially for SACs.

Facile Synthesis of MoP-RuP2 with Abundant Interfaces to Boost Hydrogen Evolution Reactions in Alkaline Media

Exploiting efficient electrocatalysts for hydrogen evolution reactions (HERs) is important for boosting the large-scale applications of hydrogen energy. Herein, MoP-RuP₂ encapsulated in N,P-codoped carbon (MoP-RuP₂@NPC) with abundant interfaces were prepared via a facile avenue with the low-toxic melamine phosphate as the phosphorous resource. Moreover, the obtained electrocatalyst possessed a porous nanostructure, had abundant exposed active sites and improved the mass transport during the electrocatalytic process. Due to the above merits, the prepared MoP-RuP₂@NPC delivered a greater electrocatalytic performance for HERs (50 mV@10 mA cm⁻²) relative to RuP₂@NPC (120 mV) and MoP@NPC (195 mV) in 1 M KOH. Moreover, an ultralow potential of 1.6 V was required to deliver a current density of 10 mA cm⁻² in the two-electrode configuration for overall water splitting. For practical applications, intermittent solar energy, wind energy and thermal energy were utilized to drive the electrolyzer to generate hydrogen gas. This work provides a novel and facile strategy for designing highly efficient and stable nanomaterials toward hydrogen production.

Single-Atom Electrocatalysts for Multi-Electron Reduction of CO2

The multi-electron reduction of CO₂ to hydrocarbons or alcohols is highly attractive in a sustainable energy economy, and the rational design of electrocatalysts is vital to achieve these reactions efficiently. Single-atom electrocatalysts are promising candidates due to their well-defined coordination configurations and unique electronic structures, which are critical for delivering high activity and selectivity and may accelerate the explorations of the activity origin at atomic level as well. Although much effort has been devoted to multi-electron reduction of CO₂ on single-atom electrocatalysts, there are still no reviews focusing on this emerging field and constructive perspectives are also urgent to be addressed. Herein recent advances in how to design efficient single-atom electrocatalysts for multi-electron reduction of CO₂ , with emphasis on strategies in regulating the interactions between active sites and key reaction intermediates, are summarized. Such interactions are crucial in designing active sites for optimizing the multi-electron reduction steps and maximizing the catalytic performance. Different design strategies including regulation of metal centers, single-atom alloys, non-metal single-atom catalysts, and tandem catalysts, are discussed accordingly. Finally, current challenges and future opportunities for deep electroreduction of CO₂ are proposed.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

PMO12@ZIF-8/ZnO-derived hierarchical porous molybdenum carbide as efficient electrocatalysts for hydrogen evolution

MoC@NC is an N-doped hierarchical porous graphite carbon-coated MoC nanoparticles with outstanding HER activity.

P- or S-Doped graphdiyne as a superior metal-free electrocatalyst for the hydrogen evolution reaction: a computational study

By carefully controlling the type and site of the P or S dopant, graphdiyne can be utilized as a promising electrocatalyst with high efficiency for the hydrogen evolution reaction.