Modulating oxygen electronic orbital occupancy of Cr-based MXenes via transition metal adsorbing for optimal HER activity

Modulating the surface electronic properties of the 2D MXenes is of significant importance to boost their hydrogen evolution reaction (HER) activity. Herein, a series of transition metal adatoms are employed to tune the surface electronic properties of Cr-based MXenes with oxygen function group for realizing impressive HER performance. The Results show that the charge of surface oxygen atoms, which is affected by both the host metal atoms and the adsorbed transition metal atoms, play critical role in the adsorption strength of hydrogen. The optimal performance is achieved by depositing Cr atom on the Cr₂TiC₂O₂ MXene, which results in the adsorption free energy of hydrogen very close to zero (0.03 eV). Systematic electronic structure analyses confirm that the charge transfer from the adsorbed transition metals to the neighboring surface oxygen atoms could tune the orbital occupancy of oxygen and their adsorption strength to hydrogen atom and therefore the HER activity. These findings and concepts may be useful for the design of advanced MXene-based HER catalysts.     

Photocatalytic properties of two-dimensional graphene and layered transition-metal dichalcogenides based photocatalyst for photoelectrochemical hydrogen generation: An overview

Hydrogen production through photoelectrochemical (PEC) water-splitting process has drawn significant research attention because it is a promising clean source of energy for improving earth climate in the future. Two-dimensional (2D) graphene and transition metal dichalcogenides (TMDCs), as the core of the system, have become versatile materials for the development of photocatalyst due to their distinct optical, electrical, thermal and mechanical properties. TMDCs have received significant consideration because of low-cost and earth-abundant catalysts that can replace noble metals, such as Pt. Therefore, comprehensive discussions on the structure and properties of 2D graphene and layered TMDCs materials are presented. We also gather and review various fabrication methods for TMDCs-based and graphene-TMDCs-based photocatalysts that can affect the PEC performance and hydrogen evolution. The inherent limitations and several future trends on 2D graphene and layered TMDCs-based photocatalyst for PEC water-splitting application are also discussed.     

Recent advances during CH4 dry reforming for syngas production: A mini review

Given the continuing issues of environment and energy, methane dry reforming for syngas production have sparked interest among researchers, but struggled with the process immaturity owing to catalyst deactivation. This review summarizes the recent advances in the development of efficient and stable catalysts with strong resistance to coking and metal sintering, including the application of novel materials, the assessment of advanced characterizations and the compatibility to improved reaction system. One feasible option is the crystalline oxide catalysts (perovskite, pyrochlore, spinel and LDHs), which feature a fine metal dispersion and surface confinement effect via a metal exsolution strategy and exhibit superior reactivity and stability. Some new materials (h-BN, clays and MOFs) also extend the option because of their unique morphology and microstructure. It also is elaborated that progresses were achieved in advanced characterizations application, leading to success in the establishment of reaction mechanisms and attributions to the formed robust catalysts. In addition, the perspective described the upgrade of reaction system to a higher reaction efficiency and milder reaction conditions. The combination of efficient reaction systems and robust catalysts paves a way for a scaling-up application of the process.     

Facile synthesis of NiS2 nanowires and its efficient electrocatalytic performance for hydrogen evolution reaction

Recently, the first-row transition metal dichalcogenides MX₂ (M = Fe, Co, Ni; X = S, Se) have been widely reported as promising catalysts for hydrogen evolution reaction (HER) because of its excellent catalytic activity and earth-abundance. The rational nanostructure designs have been proved as an effective way to improve their catalytic performance. However, the reported one dimension (1D) NiS₂ nanowires for HER suffer from a large Tafel slope. Here, we report a facile synthesis of 1D NiS₂ nanowires and its high efficient catalytic activity in HER. This nanowire structure with large surface area and active sites enables highly efficient electrocatalytic performance in HER with a much smaller Tafel slope (83.5 mV/dec) compared to that of bulk NiS₂ (136 mV/dec) as well as long-term stability. Our work builds up a structure–performance relationship and enriches the synthetic strategy to other efficient catalysts such as first-row transition metal dichalcogenides or transition metal phosphide.     

Polysulfide functionalized reduced graphene oxide for electrocatalytic hydrogen evolution reaction and supercapacitor applications

Functionalized carbon based 2D materials are promising candidates for low cost and environment friendly electrocatalyst for hydrogen evolution reaction (HER) and supercapacitor applications. To overcome the limitations posed by the noble metals and transition metal based composites, we have successfully synthesized metal free polysulfide functionalized reduced graphene oxide (GPS) in a simple chemical route. Structure and morphology of the material are characterized via XRD, FTIR, Raman, TEM, XPS measurements. The material behaves as an efficient HER electrocatalyst in acidic medium as well as energy storage device. It shows an onset potential of 97 mV and overpotential of 254 mV to reach a high current density of 10 mA/cm². DFT calculations are carried out to understand the structural stability and identification of active sites of the material. Boosting catalytic activity via increasing the number of active sites is an elegant approach. In this material we have used the S atoms of polysulfide polymer to facilitate hydrogen adsorption and desorption, thus improving the hydrogen evolution ability. The supecapacitor attains the high specific capacitance 347 F/g at the current density of 1 A/g. The origin of such performances is due to synergistic effect of both the graphene network and the polysulfide functionalizations.     

Tungsten-molybdenum oxide nanowires/reduced graphene oxide nanocomposite with enhanced and durable performance for electrocatalytic hydrogen evolution reaction

Hydrogen has attracted huge interest globally as a durable, environmentally safe and renewable fuel. Electrocatalytic hydrogen evolution reaction (HER) is one of the most promising methods for large scale hydrogen production, but the high cost of Pt-based materials which exhibit the highest activity for HER forced researchers to find alternative electro-catalyst. In this study, we report noble metal free a 3D hybrid composite of tungsten-molybdenum oxide and reduced graphene oxide (GO) prepared by a simple one step hydrothermal method for HER. Benefitting from the synergistic effect between tungsten-molybdenum oxide nanowires and reduced graphene oxide, the obtained W-Mo-O/rGO nanocomposite showed excellent electro-catalytic activity for HER with onset potential 50 mV, a Tafel slope of 46 mV decade^?1 and a large cathodic current, while the tungsten-molybdenum oxide nanowires itself is not as efficient HER catalyst. Additionally, W-Mo-O/rGO composite also demonstrated good durability up to 2000 cycles in acidic medium. The enhanced and durable hydrogen evolution reaction activity stemmed from the synergistic effect broadens noble metal free catalysts for HER and provides an insight into the design and synthesis of low-cost and environment friendly catalysts in electrochemical hydrogen production.     

Activating basal plane of Janus VSSe for efficient hydrogen evolution reaction by non-noble metal element doping: A first-principal study

Searching electrocatalysts with excellent hydrogen evolution reaction (HER) performance is very important for developing clean hydrogen energy. Two-dimensional (2D) materials have been widely studied as HER electrocatalysts, however, the basal planes of 2D materials, which dominate the surface area, are usually with poor activity. In this work, we theoretically studied the HER activity of Janus 2H–VSSe with or without non-noble metal element doping. Density functional theory (DFT) calculations suggest that doping As and Si atoms in the S or Se sites of VSSe and the C and Ge atoms in the Se site of VSSe greatly promote the HER performance of the basal plane of VSSe, resulting in hydrogen adsorption free energy close to zero (i.e. ?0.022, ?0.040, 0.066, 0.065, ?0.030, 0.058 eV, respectively), which are better than the Pt catalyst (?0.09 eV). The doped atoms strengthen the interaction between their pz-orbital and the hydrogen s-orbital, resulting in a lower bonding state in energy and higher bind strength for the hydrogen atom. This work opens up a new way to design highly efficient and low-cost catalysts for HER.     

Novel preparation of high activity 1T-phase MoS2 ultra-thin flakes by layered double hydroxide for enhanced hydrogen evolution performance

In this study, 1T phase molybdenum disulfide Ultra-thin Flakes (1T-MoS₂ UFs) with high HER activity was successfully prepared by spatial confinement of layered double hydroxides (LDHs). The growth of MoS₂ tends to be thin layer and high 1T phase due to the binding force of LDHs, this novel strategy is different from other phase transition methods, like alkali metal ion intercalation, electron irradiation, and stress induction, etc. The 1T-MoS₂ UFs were used as hydrogen evolution reaction (HER) catalysts, when current density is 10 mA/cm², the potential (vs. RHE) is 254 mV and the Tafel slope is only 64 mV/dec under the optimal condition. The superior HER activity is due to highly active 1T phase and rich defects.     

In-situ grown of Ni2P nanoparticles on 2D black phosphorus as a novel hybrid catalyst for hydrogen evolution

Nickel phosphide-based nanomaterials have been acted as efficient catalysts for the hydrogen evolution reaction (HER), however, the design of novel and high performance HER catalyst is still a challenge. Herein, we report a novel 2D material black phosphorus (BP) as support for constructing Ni₂P-based hybrid catalyst by a one-pot thermal decomposition approach. TEM results indicated that the monodispersed Ni₂P nanoparticles with small size and good dispersion supported on the surface of layered BP, which implied that more catalytic active sites may be exposed for HER. The as-synthesized Ni₂P/BP hybrid exhibits high HER electrocatalytic performance with low onset overpotential (70 mV), small Tafel slope (81 mV dec^?1), large double-layer capacitance (1.24 mF cm^?2), high conductivity and good stability, which can be assigned to the strong synergistic effect between Ni₂P and BP. Therefore, BP may be a suitable support for constructing excellent catalysts in electrocatalysis.     

Recent advancement in the performance of solar cells by incorporating transition metal dichalcogenides as counter electrode and photoabsorber

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) architectures have revealed fascinating characteristics such as direct band gap, strong light absorption, and novel electrochemical properties, which make them promising materials for photovoltaic applications. The review focuses on (1) the study of electrochemical and photovoltaic properties of TMDCs thereby using them as counter electrodes (CEs) in dye‐sensitized solar cells (DSSCs) and (2) analyzing the light absorption and charge transport performance of TMDCs heterostructures with different 3D materials. We have further investigated different materials in combination with TMDCs such as reduced graphene oxide nanocomposite, graphene flakes, and molybdenum as CEs in DSSCs. Conventionally, platinum (Pt) is used as a CE material for DSSCs that displays excellent catalytic activity and high electrical conductivity but due to the high cost and scarcity of Pt limits the large‐scale production. Therefore, the excellent electrochemical properties and cost‐effectiveness of TMDCs make them promising contender to replace Pt as CEs in DSSCs. Additionally, the photovoltaic properties of TMDCs and their heterostructures with various materials such as silicon, gallium arsenide, indium phosphate, tungsten disulfide, boron nitride, and organic polymers are reviewed. TMDCs are also investigated as hole transport layer (HTL) and electron transport layer (ETL) with various organic polymers such as P3HT, PCBM, PEDOT:PSS, PTB7, and spiro‐OmeTAD for organic and perovskite‐based solar cells (SCs). The utilization of TMDCs as CEs and photoabsorbers enhances the power conversion efficiency (PCE) to generate cost‐effective and high performance SC devices that can be exploit for future technological applications.     

Theoretical screening of di-metal atom (M = Fe,Co, Ni,Cu, Zn) electrocatalysts for ammonia synthesis

Electrochemical N₂ reduction reaction (NRR) has received much attention in recent times. Aiming for, discovering potential electrocatalysts with superior activity, stability and selectivity, a series of 3 d transition metal dimers were studied by density functional theory (DFT) calculations. The investigation reveals that most of the metal dimers have admirable stability, and partial density of states (PDOS) confirms that the unoccupied and occupied d orbitals of metal atoms are the key for effective activation of N₂. Especially, two metal dimers bonded to nitrogen-doped graphene, FeFe and CoCo, can selectively adsorb and activate N₂ for efficient conversion. Their limiting potentials are −0.44 and −0.45 V, which are superior than to most of the catalysts and they can well suppress the hydrogen evolution reaction (HER). Moreover, the desorption free energy of NH₃ is 0.54 and 0.57 eV respectively for FeFe and CoCo, guarantees the good durability of the catalysts.     

Transition metal atoms anchored on nitrogen-doped α-arsenene as efficient electrocatalysts for nitrogen electroreduction reaction

Electrocatalytic reduction of N₂ to NH₃ under ambient conditions, inspired by biological nitrogen fixation, is a new approach to address the current energy shortage crisis. As a result, developing efficient and low-cost catalysts is critical. The catalytic activity, catalytic mechanism, and selectivity of α-arsenene (α-Ars) catalysts anchored with various transition metal atoms and doped with different numbers of N atom were investigated for N₂ reduction reaction (NRR) in this paper. Results reveal that compared with WN₃-α-Ars which is coordinated with three N atoms, asym-WN₂As-α-Ars that coordinated with two N atoms not only exhibits high catalytic activity (U_L = ?0.36 V), but can also successfully suppress the hydrogen evolution reaction (HER). It is manifested that reducing the number of coordination atoms can promote the selectivity of the transition metal (TM) loaded N-doped arsenene catalysts. Furthermore, activity origin analyses show both the charge on 1N–NH and φ form volcano-type relationship with the limiting potential. The active center of the catalyst, which acts as the charge transporter and has the moderate ability to retrieve charges, is the most efficient in NRR. Overall, this research creates high performance NRR catalysts by varying the number of coordinating N atoms, which provides a novel idea for the development of new NRR catalysts.     

Role of graphene and transition metal dichalcogenides as hole transport layer and counter electrode in solar cells

Photovoltaic (PV) technology got much attention in the past few decades in developing advanced and environment friendly solar cells (SCs). However, high cost, unstable nature, and low efficiency are major limitations towards commercialization of SCs. To overcome the issues, two-dimensional materials (2DMs) have been exploited due to low cost, high catalytic activity, fast charge separation, and better electrochemical performance. The review emphasis on (a) the electrochemical performance of graphene and transition metal dichalcogenides (TMDCs) as a hole transport layer (HTL) in SCs and (b) to explore low-cost and effective counter electrode (CE) based on graphene and TMDCs for dye-sensitized solar cell (DSSC). The review presents a comparative analysis of 2DMs as HTL and CE to attain highly efficient and low-cost PV devices. Multiple combinations of the material with graphene, graphene oxide (GO), reduced graphene oxide (rGO), tungsten disulfide (WS₂), molybdenum disulfide (MoS₂) as HTL, and CE material in PV cells are discussed and comparatively analyzed. Numerous strategies are briefly discussed to enhance the efficiency of SCs by utilizing graphene and TMDCs based HTL and CEs. The review focuses on the recent progress in developing low-cost and highly efficient PV devices by using 2DMs. Our study reveals that GO/PEDOT:PSS demonstrate a maximum power conversion efficiency (PCE) of 13.1% when fabricated at different revolutions. Moreover, our statistical analysis unveils that efficiency of the cell can be enhanced by optimizing the layer thickness, which provide a route to develop highly efficient and better performance SCs that can be exploited for future commercial applications.     

2D MXenes and their heterostructures for HER,OER and overall water splitting: A review

MXenes are a family of 2D transition metal carbides, nitrides, and carbonitrides that have surface termination groups such as –OH, –O, and –F. The presence of transition metal imparts conductivity, surface termination groups induce hydrophilicity and layered structure offers large surface area which makes MXenes a potential candidate to be utilized as an electro-catalyst with enhanced efficiency. The Water Electrolysis (WE) efficiency of an electro-catalysts is dependent on the performance of half-cell reactions i.e. Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER). The OER kinetics of most of the bi-functional electrocatalysts are considered sluggish due to which they are tested in alkaline media. However, due to the metallic nature and surface properties of MXenes, they as substrate not only improve HER performance of grown electro-catalyst but also facilitate OER kinetics which is considered sluggish for most bi-functional electrocatalysts. This review presents the significance of MXenes as HER, OER, and bi-functional electrocatalysts by discussing the electrocatalytic properties of a wide range of MXenes and how their hetero-structures affect HER, OER, and bi-functional electrocatalytic performance. In the end, the current challenges, and future perspectives of MXenes and their nanocomposites for water electrolysis have been discussed.     

Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts

A promising electrocatalyst material composed of 2D layered MoS₂-WS₂ heterostructure hierarchically assembled into a 3D highly interconnected macroporous network of graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal reaction followed by moderate thermal annealing which guarantees the uniform distribution of the MoS₂-WS₂ heterojunctions within graphene matrix. The presence of 3D conductive and porous graphene network and the combined merits of MoS₂ and WS₂ endow the resulting 3D MoS₂-WS₂/graphene nanohybrids with unique conductivity pathways and channels for electrons and with outstanding electrocatalytic performance towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as very trivial drop in the current density was noticed even after 2000 cycles. The superior electrocatalytic performance of 3D MoS₂-WS₂/graphene over other non-precious metal electrocatalysts is accredited to the robust synergism of 2D MoS₂-WS₂ with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER.     

Atomically dispersed low-cost transition metals catalyze efficient hydrogen evolution on two-dimensional SnO nanosheets

Two-dimensional (2D) electrocatalyst plays an important role in hydrogen production via water splitting. In this work, the first-principles calculation was used to investigate the hydrogen evolution reaction (HER) performance of transition metal (TM) single atom catalysts (SACs) on 2D SnO nanosheets. Among the TM considered (TM = V, Cr, Mn, Fe, Co, Ni, Cu and Pt), V, Fe, Co, Ni, Cu and Pt can effectively improve the catalytic activity of SnO. More importantly, the low-cost Co can exhibit promising HER performance with the Gibbs free energy as low as ~0.015 eV, which is competitive with the precious catalyst Pt. The theoretical exchange current densities of Co SACs can reach ~10⁻¹⁶ A/site. The exciting HER activity is mainly facilitated through the d-d hybridization between the TM and Sn atoms on the SnO surface, which introduces new electron states near the Fermi level. Our work highlights the complexity and diversity of the effect of TM SACs on SnO nanosheets and implies their potential applications as efficient HER electrocatalysts.     

The effect of heteroatoms doping for the Pt supported graphene hollow spheres on electrocatalytic properties towards oxygen reduction and hydrogen evolution reaction

Noble metal Pt is the acknowledged efficient catalyst for oxygen reduction (ORR) and hydrogen evolution reaction (HER) in commercial applications. However, due to its high price and limited reserves, its large-scale application is limited. In order to overcome this defect, the loaded Pt nanoparticles (NPs) should be small and dispersed efficiently through the design of electrode materials, so as to improve the utilization efficiency of Pt. In addition, the introduction of non-noble metal active sites can reduce the consumption of Pt efficiently. In this work, hollow graphene spheres are used as the carrier and the heteroatoms (N, Fe and Co) are introduced. The results show that the introduction of Fe and Co can form very effective heteroatom active sites (carbon encapsulated Fe/Co metals and FeCo alloy, and/or metal nitrides Fe/Co-N_x-C) in the substrate material, which improve the catalytic activity of the electrode material effectively and the utilization efficiency of Pt. In addition, the generation of Fe/Co-N_x-C active sites and the loading of Pt are also closely related to the doped N atoms. The onset potential, limiting current density (J_L), half-wave potential (E_1/2) and Tafel slope of sample FeCo-N_xHGSs/Pt (10 wt%) can exceed or comparable to those of commercial catalysts Pt/C (20 wt%) towards ORR both in acid and alkaline electrolyte. Moreover, the values of η₁₀₀ and the Tafel slope for FeCo-N_xHGSs/Pt towards HER can also exceed the commercial catalysts Pt/C (20 wt%) in acid and alkaline electrolytes. The purpose of reducing the usage amount of precious metals without reducing the catalytic performance is realized. The relationship between the ORR and HER performance of the resultant electrode catalyst and the doped heteroatoms, such as nitrogen (N), iron (Fe) and cobalt (Co) atoms, was studied and discussed in detail.     

Theoretical study of single-nonmetal-modified V2CO2 MXene as an efficient electrocatalyst for overall water splitting

The electrocatalytic water splitting consists of two half-reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which require low-cost and highly activity catalysts. Two-dimensional transition metal carbon-nitride (MXenes) are considered as the potential catalysts candidates for HER and OER due to their unique physical and chemical properties. In this work, using density functional theory (DFT), we have investigated the effect of single non-metal (NM, NM = B, N, P, and S) atoms doping, strain, and terminal types on the HER and OER activities of V₂CO₂ MXene. Results indicated that P doping V₂CO₂ (P/V₂CO₂) has best HER performance at hydrogen coverage of θ = 1/8, and N doping V₂CO₂ (N/V₂CO₂) has best OER performance among the studied systems. In addition, it can be found that there is a strong correlation between the ΔG_H and strain, ΔG_H and valence charges of the doped atoms after applying strain to the doping structures, with the correlation coefficient (R²) about equal 1. Moreover, the mixed terminal can enhance the performances of HER and OER, which obey the follow rules: mixed terminal (O1 and 1OH) > original terminal (O1) > 1OH terminal. The ab initio molecular dynamics simulations (AIMD) results revealed that the single non-metallic doped structures are stable and can be synthesized experimentally at different terminals.     

Exploration of electrocatalytic water oxidation properties of NiFe catalysts doped with nonmetallic elements (P,S, Se)

The introduction of non-metallic atoms (P, S, Se) has emerged as an effective way to improve the catalytic activity of transition metal based layered double hydroxides (LDHs) for oxygen evolution reaction. However, objective comparisons of the performance of heteroelement-doped catalysts are complicated by the lack of standardization both in the electrochemical tests and physicochemical analysis. Herein, we use a unified protocol for evaluating the catalytic activities of heteroelement-doped NiFe-LDHs and explore the reasons for the differences in their catalytic performance. Some regular results are found from comparing the properties of the heteroelement-doped catalysts: (1) the introduction of P/S/Se can optimize the redox behaviors of Ni species, which is conducive to regulating adsorption energy of intermediates and the formation of high-valent active sites; (2) the specific area of catalysts was expanded after heteroelements doping, ensuring a more favorable structure for heterogeneous catalysis; (3) All P/S/Se-doped catalysts showed better activities when compared to the original NiFe LDHs in alkaline solutions, and the catalysts doped with S showed the best performance (Se < P < S).     

Three-dimensional flower-like Co-based metal organic frameworks as efficient multifunctional catalysts towards oxygen reduction,oxygen evolution and hydrogen evolution reactions

Design and development of cost-efficient multifunctional three-dimensional (3D) metal organic frameworks (MOFs) towards oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are very significant for green energy devices. Herein, a scalable one-pot solvothermal method is developed to obtain a series of multifunctional 3D flower-like MOFs. In addition, systematic studies are also conducted on the effects of various metal cations and N-containing ligands on the structures, compositions, and multifunctional performance of the obtained MOFs. As a result, 3D flower-like Co-MOFs using Co²⁺ as a metal cation and 2,2’:6′,2″-terpyridine as a N-containing ligand exhibit the highest multifunctional performance towards ORR, OER and HER. The scalable method provides a new prospect to design and develop other MOFs-based multifunctional catalysts.