Transition Metal Carbides and Nitrides in Energy Storage and Conversion.

Since the initial MXenes were discovered in 2011, several MXene compositions constructed using combinations of various transition metals have been developed. MXenes are ideal candidates for different applications in energy conversion and storage, because of their unique and interesting characteristics, which included good electrical conductivity, hydrophilicity, and simplicity of large-scale synthesis. Herein, we study the current developments in two-dimensional (2D) MXene nanosheets for energy storage and conversion technologies. First, we discuss the introduction to energy storage and conversion devices. Later, we emphasized on 2D MXenes and some specific properties of MXenes. Subsequently, research advances in MXene-based electrode materials for energy storage such as supercapacitors and rechargeable batteries is summarized. We provide the relevant energy storage processes, common challenges, and potential approaches to an acceptable solution for 2D MXene-based energy storage. In addition, recent advances for MXenes used in energy conversion devices like solar cells, fuel cells and catalysis is also summarized. Finally, the future prospective of growing MXene-based energy conversion and storage are highlighted.

Recent advances and prospects of MXene-based materials for electrocatalysis and energy storage

Updates on the development of nanostructured transition metal nitrides for electrochemical energy storage and water splitting

Owing to the first-rate residences of graphene, the research had been achieved on novel two-dimensional (2D) materials. The MXene is a 2D inorganic compound that consists of transition metal carbides, nitrides, and carbonitrides. Ti₃C₂, the primary 2D-layered MXene, was reported in 2011. This material that is a layered bulk material analogous to graphite, changed into derived from its 3D phase, is Ti₃AlC₂ MAX. The start of this review through reviewing the structures and morphologies and their fabrication routes. After this, the discussion converts to the mechanical, electrical, and electronic properties of MXenes. The awareness then turns to their thrilling capacity in energy storage, conversion, and utilization. Energy storage programs encompass electrodes in rechargeable lithium batteries and supercapacitors. In phrases of energy conversion and utilization, photocatalytic hydrogen evolution, solar steam manufacturing, photothermal therapies, photovoltaic devices are presented. Highlights The study shows that transition metal carbides and nitrides (MXenes), own circle of two-dimensional (2D) inorganic compounds. The substances composed of some atomic layers of transition metal carbides, nitrides, or carbonitrides. Ti₃C₂, the primary 2D-layered MXene, were remote in 2011 This material that is a layered material analogous to graphite, changed into derived from its 3D phase, is Ti₃AlC₂ MAX Conversion of mechanical, electrical, and electronic properties of MXenes which turns to their thrilling capacity in energy storage, conversion, and utilization

Along with the rapid consumption of fossil fuels and serious environmental pollution, the development of renewable and clean energy has been receiving a lot of attention recently. Among the various renewable energy sources, hydrogen (H2) is considered as most promising substance in future energy society owing to its high gravimetric energy density and environmental friendliness. Electrochemical water electrolysis system is powerful strategy for high-purity hydrogen production without emission of any pollutants. It consists of two electrocatalytic reactions: oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, the practical efficiency of water electrolysis is limited due to the intrinsically sluggish reaction kinetics of OER and HER. To date, the precious metal-based electrocatalysts such as RuO2 and Pt/C for OER and HER, respectively, were employed as state-of-the-art electrocatalyst to enhance the performance of water electrolysis. However, due to the high expense, mono-functionality, and unsatisfactory stability of the precious metal-based materials, research on bifunctional electrocatalysts with low cost and high activity has become an urgent topic. To date, numerous scientific endeavors have been dedicated to the advancement of highly efficient non-noble metal-based bifunctional electrocatalysts such as transition metal oxides, hydroxides, sulfides, carbides, nitrides, and phosphides. Among these, Ni3S2 has emerged as a particularly promising electrocatalyst for water splitting, garnering substantial attention owing to its high electric conductivity, rich redox property, abundance in the Earth's crust, and environmentally friendly characteristics. Nevertheless, the electrocatalytic activity of Ni3S2 lags behind that of noble metal counterparts. Therefore, elaborate modification of Ni3S2 catalyst should be conducted to further improve its intrinsic activity toward OER and HER. Among the various strategies, heteroatom doping into Ni3S2 structure could be adopted as powerful technique for enhancement of electrocatalytic performance. The homogeneously incorporated dopants not only generate lattice disorders but elaborately modulate the surface electronic property. Specifically, high-valent cation dopants such as vanadium and molybdenum can effectively mediate the change of chemical states of Ni active sites during the step-wise electrocatalytic procedure. Meanwhile, additionally introduced anion dopants, which have lower electronegativity compared with sulfur, can reduce the electron trapping phenomenon induced by excessive electron transfer from nickel to sulfur. Inspired by above features, we synthesized V and P co-doped Ni3S2 nanoneedles directly grown on nickel foam (V-Ni3S2-P/NF) through facile two-step preparation method. First, V doped Ni3S2 nanoneedles were homogeneously grown on NF (V-Ni3S2/NF) by hydrothermal process without use of nickel precursor. The vanadium ions promote the nucleation of Ni3S2 species and lead the preferential growth toward 1D direction during the hydrothermal reaction. After that, the V-Ni3S2/NF was annealed in the tube furnace with the NaH2PO2 as a phosphorus precursor under the Ar flow. Under the mild annealing condition, the PH3 gases generated from thermal decomposition of NaH2PO2 reacted with the V-Ni3S2 species. Consequently, the V-Ni3S2 was partially phosphidated into V-Ni3S2-P throughout anion exchange process. As a result, the V-Ni3S2-P/NF electrocatalyst exhibited excellent bifunctional activity toward both OER and HER compared with mono-doped and un-doped counterparts and precious metal-based electrocatalysts. Furthermore, the outstanding electrocatalytic performance of V-Ni3S2-P/NF was well-maintained over the 100 h of continuous operation under alkaline condition. This study will contribute to the development of efficient and cost-effective electrocatalysts for future energy conversion and storage technologies.

Phosphorus-doped cobaltous oxide core@shell microspheres with enhanced performances in energy conversion and storage

Balancing Scales

Recent advances of two-dimensional transition metal nitrides for energy storage and conversion applications

Abstract2D transition metal carbides and/or nitrides (MXenes), by virtue of high electrical conductivity, abundant surface functional groups and excellent dispersion in various solvents, are attracting increasing attention and showing competitive performance in energy storage and conversion applications. However, like other 2D materials, MXene nanosheets incline to stack together via van der Waals interactions, which lead to limited number of active sites, sluggish ionic kinetics, and finally ordinary performance of MXene materials/devices. Constructing 2D MXene nanosheets into 3D architectures has been proven to be an effective strategy to reduce restacking, thus providing larger specific surface area, higher porosity, and shorter ion and mass transport distance over normal 1D and 2D structures. In this review, the commonly used strategies for manufacturing 3D MXene architectures (3D MXenes and 3D MXene‐based composites) are summarized, such as template, assembly, 3D printing, and other methods. Special attention is also given to the structure–property relationships of 3D MXene architectures and their applications in electrochemical energy storage and conversion, including supercapacitors, rechargeable batteries, and electrocatalysis. Finally, the authors propose a brief perspective on future opportunities and challenges for 3D MXene architectures/devices.

The family of two-dimensional (2D) materials-solids with high aspect ratios and thicknesses consisting of a few atomic layers-has grown far beyond graphene. 2D transition metal carbides, nitrides and carbonitrides, known as MXenes, are one of the latest additions to this family. This rapidly growing class of 2D materials finds applications in fields ranging from energy storage to electromagnetic interference shielding and transparent conductive coatings. However, while over twenty carbide MXenes have been synthesized, very few transition metal nitrides (TMNs), and no nitride MXenes, had previously been reported. Two-dimensional TMNs, including nitride MXenes, have several potential advantages over their carbide analogs. They theoretically have higher values of electrical conductivity than carbide MXenes, which has implications on outperforming carbides in electrochemical and other applications. Compared to carbides, they are superior candidates for promising plasmonic devices and spintronic devices that incorporate magnetic 2D materials. Although there are theoretically as many nitride MXenes as carbide MXenes predicted, synthesizing nitride MXenes and 2D TMNs in general faces several challenges. Synthesis methods that have produced over two dozen 2D carbides MXenes have failed to yield 2D TMNs. The major focus of this dissertation is investigating routes of synthesizing 2D TMNs including, but not limited to, selective etching of layered bulk metal nitride precursors. Three promising routes of synthesis are explored, and their electronic and magnetic properties of the synthesized materials are also characterized. Discovering how to synthesize 2D TMNs will remove the barrier between merely studying their theoretically predicted properties and finally applying these outstanding properties in devices for energy storage, spintronics and beyond.

The detrimental impacts of climate change coupled with increasing global energy demand has resulted in a significant research effort to develop clean technologies for energy generation, conversion, storage, distribution as well as the removal of CO2 from various industrial sectors. Undoubtedly, electrocatalysis will play a major role in each of these aspirations, which is reflected in the topics covered in this Special Issue. The contributions included here range from the more mature areas of fuel-cell-relevant reactions and electrochemical water splitting to rapidly emerging reactions such as CO2 reduction and nitrogen conversion to ammonia with further mechanistic insights provided by new experimental techniques and computational studies. Electrocatalytic reactions are at the heart of fuel-cell technology and therefore understanding and improving the efficiency of these reactions remains a highly active area of research. This is reflected in this Special Issue by the work that encompasses many aspects of fuel cells including reactions at the anode and cathode, dissolution of the catalyst, the role of the catalyst support, and understanding the dynamics between the electrodes in a fuel cell. Özaslan and co-workers investigate the role of the capping agent on Pt nanocubes and how it influences both the structural stability of the catalyst and ORR performance. Sandbeck, Cherevko et al. also investigate Pt and determine that dissolution occurs to a different extent on different Pt single-crystal basal planes and polycrystalline Pt. However, not only is corrosion of the catalyst an issue but Maillard and co-workers demonstrate that corrosion of the carbon catalyst support used in proton exchange membrane fuel cells (PEMFCs) is also problematic and involves a Pt-catalyzed decarboxylation mechanism which leads to CO and CO2 evolution. Kunze-Liebhäuser and co-workers demonstrate that the high stability of zirconium oxycarbide lends it well to anodic reactions such as alcohol or CO oxidation. Tremiliosi-Filho et al. show that the electrocatalytic oxidation of ethanol on disordered Pt(111) surfaces is highly influenced by the presence of defects on the surface and provides insights into the operation of real catalysts. Varela and co-workers gain new insights into fuel cell operation by inserting an external reference electrode in a direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) under stationary and oscillatory conditions. Electrolysis of water coupled to renewable energy sources is a promising method to produce green hydrogen with zero emissions. The past decade has witnessed remarkable progress in the understanding of both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) for water electrolyzers. This Special Issue reflects the ongoing efforts in: 1) elucidation of structure–activity–stability relations, 2) investigations of the role of the interface structure and the support, 3) combining electrochemical methods with in situ advanced characterization, and 4) studying the electrocatalytic HER and OER using both model extended surfaces and nanoparticles. Model studies on single-crystalline surfaces are essential to gain detailed knowledge on the relations between the electrode structure and the electrocatalytic properties. Feliu and co-workers combine cyclic voltammetry, in situ spectroscopy, and laser-induced temperature jump technique to investigate the interfacial properties of Ni-modified Pt(111) surfaces in phosphate electrolyte for the HER. Arenz, Aschauer and co-workers also use Pt single-crystalline surfaces to establish the structure-sensitivity of the OER in acidic electrolyte by combining experimental work and theoretical calculations. The slow kinetics of the OER causes significant overpotentials in water electrolyzers. To improve the efficiency of water electrolysis, it is necessary to design and develop more active and stable OER electrocatalysts. Zhao and co-workers review the field of OER electrocatalysis with a special focus on multimetallic-based catalysts to improve the OER performance in both acidic and alkaline electrolytes. In acidic media, where polymer electrolyte membrane electrolyzers operate, catalysts based on Ir are required. Escudero-Escribano and co-workers show that both the composition and concentration of the acidic electrolyte play an important role in the performance of Ir nanoparticles for the OER. In alkaline electrolyzers, earth-abundant materials are typically used as OER catalysts. Cao, Zheng, and co-workers present highly active and stable hollow nanocubes based on Co–Fe hydroxides for OER in alkaline media. Abu Sayeed and O'Mullane present an electrodeposition method to fabricate bimetallic Co–Fe oxides for both the HER and OER. Boettcher and co-workers report an electrochemical study on the structure–activity relations for Fe (oxy)hydroxides on different metallic supports, showing how the activity can be significantly tuned by the substrate–electrocatalyst interactions. Schuhmann, Andronescu, and co-workers report a method to improve the OER activity of Ni–Fe (oxy)hydroxides by enhancing their electrical conductivity. Arenz, Delgado, and co-workers report an accelerated durability test for high-surface-area Ni-based (oxy)hydroxides for OER in alkaline media. Katayama and co-workers present a method to fabricate Cu-deposited catalysts for the OER in alkaline media. Finally, the OER is also a very relevant reaction in batteries. Risch and co-workers combine electrochemical methods with X-ray absorption to investigate the electrocatalytic OER on LiMn2O4 in LiOH electrolytes. Electrocatalysis encompasses a wide variety of chemical transformations that are not solely limited to fuel-cell-relevant or water-splitting reactions, which are currently of intense interest. Kortlever and co-workers have reviewed another reaction gaining significant attention, namely electrochemical CO2 conversion into fuels and valuable chemicals. In particular, they focus on the effect of the electrolyte employed in the electrocatalytic reaction and its influence on efficiency and selectivity. Herranz et al. also study the electrochemical CO2 reduction reaction using a thin-film Cu2O electrode; they investigate the oxidation state of the catalyst via post mortem analysis with XPS, where complete reduction of the surface to Cu was found; their work has implications for previous work on rough Cu2O electrodes. Scherson and co-workers investigate hydroxylamine oxidation on polycrystalline electrodes and determine that N2 is formed which is dependent on the pH and applied potential. Symes et al. investigate the effect of ultrasound on the electro-oxidation of sulfate solutions to generate useful and powerful oxidants like persulfate and find that at low sulfate concentration and low current density, the use of ultrasound results in a lower concentration of this oxidant. Stimming and co-workers investigate the V(II)/V(III) and V(IV)/V(V) redox reactions employed in redox flow batteries; by developing a method to accurately assess the electrochemically active surface area of the working electrodes they conclude that in disagreement to the received wisdom that porous carbon does not catalyze vanadium redox chemistry. The development of in situ or operando techniques is a key research area that is continuously being developed to gain a better understanding of the mechanisms of electrocatalytic reactions. This is particularly important when considering the validation of theoretical predictions, which is described below, and covered in this Special Issue. Kibler et al. have used in situ scanning tunneling microscopy (STM) to study the adsorption of unreactive acetate on Au(111) surfaces where a phase transition within an adsorbed adlayer is observed, providing key information on the role of reactive adsorbates such as formate on electrocatalytic reactions. Cuesta and co-workers review the area of in situ infrared spectroscopy identifying key theoretical aspects of the technique and highlight recent uses in studying the electrochemical CO2 reduction reaction for the detection of reaction intermediates. Horch et al. demonstrate the integration of ultra-high-vacuum equipment with an electrochemical cell as a way of producing complex surface structures not attainable by regular electrochemical methods. They study increased step density on Pt(111), Cu(111), and Pt/Cu(111) electrodes by STM and the effect on their electrochemical behavior in acidic and alkaline electrolytes. Computational electrocatalysis is a rapidly emerging field that is employed to gain a more fundamental mechanistic understanding of quite complex reactions such as those covered in this Special Issue. This field not only provides support for experimental observation but is being used to predict the activity of electrocatalysts not yet synthesized in the laboratory. Malek and co-workers provide a perspective where they critically assess the use of artificial-intelligence-driven modelling and computational approaches for such a task and take CO2 conversion as a test case. In addition, Tang and Jiang used first-principles density functional theory (DFT) to predict that Ti, Sc, and Fe dimer clusters supported on phosphorene constitute promising electrocatalysts for N2 reduction to NH3. Baletto et al. developed a multi-scale approach to study the catalytic properties of MgO(100) supported Pt nanoparticles for the ORR, where reconstruction of the interface layer is predicted to increase activity. A major challenge, however, in the application of theoretical models is the incorporation of the electrochemical interface and the electrolyte into the simulation while keeping computational times manageable. Rossmeisl and co-workers have used ab initio methods to construct a thermodynamically realistic interface to present simulated cyclic voltammograms of Cu basal plane electrodes that are validated by comparison to experimental data over a large pH range, which therefore provides an atomistic understanding of the interfacial structure of Cu electrodes. Calle-Vallejo et al. tackle the challenge of modelling the role of solvation and its influence on the adsorption energy of species at surfaces. They evaluate the influence of van der Waal interactions on the solvation of *OH adsorbed on alloys of Pt. Chan and co-workers present a hybrid continuum/ab initio method where they introduce a capacitor model for the relationship between the reaction energetics and the potential and charge. This results in an order of magnitude reduction in computational costs to determine electrochemical reaction energetics. To conclude, the topics outlined in this Special Issue highlight the beneficial impact that electrocatalysis can play in developing a cleaner and more sustainable society. The future of this field is indeed bright and brings together not only the expertise of electrochemists but material scientists, theoreticians, engineers, and surface scientists. The outcome is the continuous development of new materials with enhanced performance underpinned by the greater understanding of reaction mechanisms via integration of electrochemical systems with sophisticated in situ techniques and validation with increasingly realistic simulation environments. Professor Anthony O'Mullane received his PhD degree (2001) from University College Cork (Ireland) and completed postdoctoral fellowships at Technische Universitat Darmstadt (Germany), the University of Warwick (UK), and Monash University (Australia). He previously held a position (2008) at RMIT University (Australia) until moving to Queensland University of Technology (QUT) in 2013. He is a Fellow of the Royal Society of Chemistry and Fellow of the Royal Australian Chemical Institute (FRACI). He is the immediate past-Chair of the Electrochemistry Division of the RACI and served as vice chair of the Physical Electrochemistry Division of the International Society of Electrochemistry. His research interests are the electrochemical synthesis and characterization of nanostructured materials; electrocatalysis (water splitting, fuel-cell-relevant reactions); catalysis (water remediation); room-temperature liquid metals; Li-metal-based batteries; and the application of electrochemical methods to various aspects of physical, chemical, and biological science. He has published over 170 journal articles in these areas. María Escudero-Escribano is an assistant professor at the University of Copenhagen (Denmark) since 2017. She received her PhD in Chemistry from the Autonomous University of Madrid (Spain) in 2011. She completed postdoctoral fellowships at the Technical University of Denmark and Stanford University (US). At the University of Copenhagen, María leads the Nanoelectrocatalysis Group, which investigates tailored electrochemical interfaces for sustainable energy conversion and production of renewable fuels and chemicals. She is the Chair of the Danish Electrochemical Society since 2018 and holds a Villum Young Investigator Grant from the Villum Foundation. María has received numerous awards in recognition of her early-career achievements, including the European Young Chemist Award 2016 (Gold Medal, 35-year-old level), the Energy Technology Division Young Investigator Award 2018 from the Electrochemical Society, the Princess of Girona Scientific Research Award 2018, the Young Researchers Award 2019 from the Spanish Royal Society of Chemistry, and the Clara Immerwahr Award 2019. Ifan Stephens is Senior Lecturer at the Department of Materials at Imperial College London. Prior to his appointment to Imperial in 2017, he was at the Department of Physics at the Technical University of Denmark (DTU); he was first employed as a postdoctoral researcher, then as assistant professor, and finally as associate professor and leader of the Electrocatalysis Group there. In 2015, Massachusetts Institute of Technology (MIT) appointed Ifan as the Peabody Visiting Associate Professor. He taught and conducted research at the Department of Mechanical Engineering at MIT for a whole semester. Ifan′s research aims to enable the large-scale electrochemical conversion of renewable energy to fuels and valuable chemicals and vice versa. Such processes will be critical in order to allow the increased uptake of renewable energy. Ifan has published 66 papers on topics including oxygen reduction, oxygen evolution, CO2 reduction and N2 reduction. Ifan′s research on H2O2 electrosynthesis led to the establishment of the spinout HPNow, which he co-founded. Katharina Krischer is a Professor of Physics at the Technical University of Munich (TUM), Germany. She is also a member of the Catalysis Research Center of TUM and serves on editorial boards of several journals on electrochemistry or nonlinear sciences. She did her Ph.D. at the Fritz-Haber-Institut, Berlin, in the group of Prof. Ertl. After postdoctoral training at Princeton University, USA, she returned as a group leader to the Fritz-Haber-Institut, and completed her habilitation in 1998. In 2002 she moved to Munich to take on her current position. Her research interests cover two broad topics, electrochemistry and nonlinear dynamics. She works on photoelectrochemistry, solar fuels, and semiconductor electrochemistry as well as on nonlinear phenomena during electrochemical reactions. Furthermore, she has a strong interest in theory, bridging the gap between physico-chemical continuum models describing self-organization phenomena at the solid-liquid interface and normal form approaches and abstract mathematical models. She has coauthored about 130 publications in peer-reviewed journals and a text book on "Physics of Energy Conversion". She was elected a fellow of the International Society of Electrochemistry and is a member of the German Physical Society (DPG) and the Society of German Chemists (GDCh).

With the ever-growing global energy demands and environmental pollution issues, developing high-performance energy storage and conversion materials has become a hot topic in the material science community. In this regard, substantial progress has been made in theoretically predicting new materials for energy-related fields, experimentally synthesizing these materials, and further improving their properties for high performance in energy storage and conversion devices. In particular, two-dimensional (2D) materials have shown great potential in the field of energy storage and conversion. However, it remains challenging to explore 2D materials that render high efficiency of energy storage and conversion while guarantee long-term stability and safety. Over the past decades, theoretical calculations based on density functional theory (DFT) have become a practical toolkit to address this issue by revealing the reaction mechanism at an atomic scale and screening high-performance energy storage and conversion materials on a large scale. In particular, DFT calculations enable us to establish the relationships between the intrinsic properties of materials and their performance for energy storage and conversion, and provide theoretical guidance for screening and experimentally synthesizing the promising materials. In this review, we summarize the DFT calculations’ applications in recent studies of developing high-performance and reliable energy-related 2D materials for Li-ion battery (LIB), water splitting, fuel cells, and electrochemical carbon dioxide reduction (CRR). First, we introduce the reaction mechanism of LIB, hydrogen evolution reaction (HER), oxygen evolution reaction/oxygen reduction reaction (OER/ORR), and CRR in detail and the application of 2D material in these fields. Then, we highlight the role of DFT calculations in unveiling the intrinsic relationships between the electronic structure and the performance of 2D materials by comprehensively discussing the descriptors in predicting the performance of 2D materials. For example, the occupancy of d orbital and energy required to fill empty states serve as descriptors to predict the electrochemical performance of the electrode in ion intercalation battery. The d orbital center, lowest unoccupied states, and oxygen vacancy formation energy serve as descriptors to predict the catalytic performance of electrode in HER. The energy difference between the lowest valance electron orbital center and Fermi level, occupancy of p z orbital, and the energy difference between p z and p x /p y orbital center serve as descriptors to predict the catalytic performance of electrode in ORR. Even though these descriptors can help to further understand the relationships between the electronic structure and the performance of the electrochemical electrode, they are only reliable to specific materials and inapplicable to the electrode with a complex structure or complex reaction path, such as the electrode in CRR. Newly developed machine learning methods may bring a breakthrough to the exploration of a universal descriptor, which is a key factor in the large-scale screening of potential electrode materials with excellent performance and the dependable guidance to experimental synthesis. Finally, we summarize the disadvantage of DFT calculation, such as the underestimation of bandgap and incorrect description of van der Waals interaction, and give a perspective of DFT calculations in the study of new energy-related materials. The method to simulate the ambient environment of the electrode (including the electrolyte, external electric field, and non-cooperative transfer of proton and electron) based on DFT calculation is needed to be developed, which is vital to reflect the actual working condition of the electrode. The universal descriptor applicable to the electrode with a complex structure is also needed to explore to overcome the poor versatility of single intrinsic property of the material in predicting the performance of the electrochemical electrode.

Clean and sustainable energy supply is regarded as the most significant problems in the 21st century, which is ultimately related to our daily lives, global environment, economy, and human health. Although fossil fuels as the main energy sources will continue to play a crucial role in responding our energy needs in the future, they come at a tremendous price, including a rapid increase in greenhouse gas emissions and long-lasting environmental pollution. The imminent shortage of fossil fuels and growing ecological concerns is pushing scientists and engineers to exploit sustainable, clean, and highly efficient technologies to supply and store energy. With the permanently increasing demand in energy resources, massive efforts have been devoted to developing advanced energy storage and conversion systems. Novel materials hold the key to fundamental advances in energy conversion and storage, both of which are vital in order to meet the challenge of global warming and the finite nature of fossil fuels. Graphene as one of the most successful functionally nanomaterials, which have attracted great attention due to their unique properties of large surface area, superior electric and thermal conductivities, high mechanical flexibility, chemical stability, which render them great choices as alternative electrode materials for electrochemical energy storage systems. The ultrathin two-dimensional (2D) morphology of graphene with unique properties is triggering a great deal of attention toward the family of 2D structures. The types of 2D inorganic graphene analogues nanomaterials such as metal dichalcogenides have also been studied and applied in various applications including electronics, optoelectronics, energy storage devices, solar energy, electrocatalysts for hydrogen evolution reaction and so on. Layered transition metal dichalcogenides (MoS2, MoSe2, WS2, WSe2, etc.) as the typical graphene analogues, which are a chemically diverse class of compounds having band gaps from 0 to ∼2 eV and remarkable electrochemical properties. The band gaps and electrochemical properties of layered transition metal dichalcogenides can be tuned by exchanging the transition metal or chalcogenide elements. Among numerous transition metal dichalcogenides, layered metal seleniums exhibit many novel properties, especially in the electrochemical energy field, which may be beyond those existing in layered metal disulfide. The excellent electrochemical performances of the layered metal seleniums materials could be attributed to their unique intrinsic structure. Firstly, the layered metal seleniums have a higher electrical conductivity than layered metal disulfide owing to its narrower band-gap energies. In addition, the larger diameter of Se atom provides the layered metal seleniums with expanded interlayer spacing, which will afford more active reaction sites for electrolyte ion storage and reduce the energy barrier for electrolyte ion insertion. Benefiting from their remarkable electrochemical properties, these layered metal seleniums will play meaningful roles for low-cost and environmentally friendly energy storage and electrocatalysts for hydrogen evolution technologies. In this review, we summarize the physic structures, synthesis methods of 2D layered metal diseleniums, as well as its application in the field of electrochemical energy, including the Li ion battery, Na ion battery, supercapacitor, Mg ion battery and hydrogen evolution reaction. Finally, we make the prospects and the development trends on the layered metal diseleniums.

Recent progress in transition metal carbides and nitrides based composites as bifunctional oxygen electrocatalyst for zinc air batteries

The continued rise in global energy consumption, along with the associated environmental hazards, pose a significant risk to our society’s infrastructure unless the main source is changed. Electrocatalysis involving hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and oxygen reduction reaction (ORR), provide a pathway for energy storage and conversion due to their enhanced environmental friendliness and efficient energy input compared to their thermocatalytic counterparts. Currently, the state-of-the-art electrocatalysts suffer from scarcity and high cost. MXenes, a novel family of two-dimensional (2D) transition metal carbide and nitride materials, show potential as cost-efficient and highly abundant electrocatalysts with limited knowledge on their electrocatalytic mechanisms. This is especially true when considering the often-overlooked nitride family of MXenes. Herein, we investigate the electrocatalytic performance of a Ti2N nitride MXene under different electrocatalytic conditions. The effect of surface phenomena is investigated through manipulation of the surface passivation layer, as evidenced by Raman and Fourier-transform infrared (FTIR) spectroscopies, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Successful decoupling of the bulk and surface phenomena is achieved through Raman laser power attenuation. Under acidic medium, the surface reactivity towards HER is poor but the bulk reactivity for NRR is favored, making it an optimal NRR catalyst. Under alkaline medium, the surface reactivity of the pristine Ti2N MXene for ORR is high, but also leads to surface passivation and thus hinders the electrocatalytic activity. Overall, these results provide fundamental insights into future optimization strategies of the Ti2N nitride MXene, along with other MXene electrocatalysts, towards electrocatalytic applications.