Engineering the Electronic Structure of 2D WS2 Nanosheets Using Co Incorporation as CoxW(1‐x)S2 for Conspicuously Enhanced Hydrogen Generation

Transition metal dichalcogenides (TMDs), as one of potential electrocatalysts for hydrogen evolution reaction (HER), have been extensively studied. Such TMD‐based ternary materials are believed to engender optimization of hydrogen adsorption free energy to thermoneutral value. Theoretically, cobalt is predicted to actively promote the catalytic activity of WS₂. However, experimentally it requires systematic approach to form Co_xW_(1?x)S₂ without any concomitant side phases that are detrimental for the intended purpose. This study reports a rational method to synthesize pure ternary Co_xW_(1?x)S₂ nanosheets for efficiently catalyzing HER. Benefiting from the modification in the electronic structure, the resultant material requires overpotential of 121 mV versus reversible hydrogen electrode (RHE) to achieve current density of 10 mA cm^?2 and shows Tafel slope of 67 mV dec^?1. Furthermore, negligible loss of activity is observed over continues electrolysis of up to 2 h demonstrating its fair stability. The finding provides noticeable experimental support for other computational reports and paves the way for further works in the area of HER catalysis based on ternary materials.     

Graphene Decorated with Uniform Ultrathin (CoP)x–(FeP)1–x Nanorods: A Robust Non‐Noble‐Metal Catalyst for Hydrogen Evolution

Developing high‐performance but low‐cost hydrogen evolution reaction (HER) electrocatalysts with superior activity and stability for future sustainable energy conversion technologies is highly desired. Tuning of microstructure, configuration, and chemical composition are paramount to developing effective non‐noble electrocatalysts for HER. Herein, a universal “nanocasting” method is reported to construct graphene decorated with uniform ternary (CoP)_x –(FeP)_1?x (0 ≤ x ≤ 1) nanorods hybrids with different chemical compositions [(CoP)_x –(FeP)_1?x –NRs/G] as a highly active and durable nonprecious‐metal electrocatalyst for the HER. The optimized (CoP)_0.54–(FeP)_0.46–NRs/G electrocatalyst exhibits overpotentials of as low as 57 and 97 mV at 10 mA cm^?2, Tafel slopes of 52 and 62 mV dec^?1, exchange current densities of 0.489 and 0.454 mA cm^?2, and Faradaic efficiency of nearly 100% in acidic and alkaline media, respectively. More importantly, this electrocatalyst also exhibits high tolerance and durability in a wide pH range and keeps catalytic activity for at least 3000 cycles and 24 h of sustained hydrogen production. The excellent catalytic performance of the (CoP)_x –(FeP)_1?x –NRs/G electrocatalyst may be ascribed to its unique mesoporous structure and strong synergistic effect between CoP and FeP. Thus, the work provides a feasible way to fabricate cheap and highly efficient electrocatalyst as alternatives for Pt‐based electrocatalysts for HER in electrochemical water splitting.     

Composition and Interface Engineering of Alloyed MoS2xSe2(1–x) Nanotubes for Enhanced Hydrogen Evolution Reaction Activity

Hierarchical MoS_2xSe_2(1?x) nanotubes assembled from several‐layered nanosheets featuring tunable chalcogen compositions, expanded interlayer spacing and carbon modification, are synthesized for enhanced electrocatalytic hydrogen evolution reaction (HER). The chalcogen compositions of the MoS_2xSe_2(1?x) nanotubes are controllable by adjusting the selenization temperature and duration while the expanded (002) interlayer spacing varies from 0.98 to 0.68 nm. It is found that the MoS_2xSe_2(1?x) (x = 0.54) nanotubes with expanded interlayer spacing of 0.98 nm exhibit the highest electrocatalytic HER activity with a low onset potential of 101 mV and a Tafel slope of 55 mV dec^?1. The improved electrocatalytic performance is attributed to the chalcogen composition tuning and the interlayer distance expansion to achieve benefitting hydrogen adsorption energy. The present work suggests a potential way to design advanced HER electrocatalysts through modulating their compositions and interlayer distances.     

Tailoring Semiconductor Lateral Multijunctions for Giant Photoconductivity Enhancement

Semiconductor heterostructures have played a critical role as the enabler for new science and technology. The emergence of transition‐metal dichalcogenides (TMDs) as atomically thin semiconductors has opened new frontiers in semiconductor heterostructures either by stacking different TMDs to form vertical heterojunctions or by stitching them laterally to form lateral heterojunctions via direct growth. In conventional semiconductor heterostructures, the design of multijunctions is critical to achieve carrier confinement. Analogously, successful synthesis of a monolayer WS₂/WS_{2(1?x )}Se_2x /WS₂ multijunction lateral heterostructure via direct growth by chemical vapor deposition is reported. The grown structures are characterized by Raman, photoluminescence, and annular dark‐field scanning transmission electron microscopy to determine their lateral compositional profile. More importantly, using microwave impedance microscopy, it is demonstrated that the local photoconductivity in the alloy region can be tailored and enhanced by two orders of magnitude over pure WS₂. Finite element analysis confirms that this effect is due to the carrier diffusion and confinement into the alloy region. This work exemplifies the technological potential of atomically thin lateral heterostructures in optoelectronic applications.     

Super-Hydrophilic Microporous Ni(OH)x/Ni3S2 Heterostructure Electrocatalyst for Large-Current-Density Hydrogen Evolution

Exploiting active and stable non-precious metal electrocatalysts for alkaline hydrogen evolution reaction (HER) at large current density plays a key role in realizing large-scale industrial hydrogen generation. Herein, a self-supported microporous Ni(OH)x/Ni₃S₂ heterostructure electrocatalyst on nickel foam (Ni(OH)x/Ni₃S₂/NF) that possesses super-hydrophilic property through an electrochemical process is rationally designed and fabricated. Benefiting from the super-hydrophilic property, microporous feature, and self-supported structure, the electrocatalyst exhibits an exceptional HER performance at large current density in 1.0 M KOH, only requiring low overpotential of 126, 193, and 238 mV to reach a current density of 100, 500, and 1000 mA cm⁻², respectively, and displaying a long-term durability up to 1000 h, which is among the state-of-the-art non-precious metal electrocatalysts. Combining hard X-rays absorption spectroscopy and first-principles calculation, it also reveals that the strong electronic coupling at the interface of the heterostructure facilitates the dissociation of H₂O molecular, accelerating the HER kinetics in alkaline electrolyte. This work sheds a light on developing advanced non-precious metal electrocatalysts for industrial hydrogen production by means of constructing a super-hydrophilic microporous heterostructure.     

Controlling Photoluminescence Enhancement and Energy Transfer in WS2:hBN:WS2 Vertical Stacks by Precise Interlayer Distances

2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS₂:hBN:WS₂ are fabricated utilizing chemical vapor deposition (CVD)‐grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS₂ drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS₂ constituents. Such doping effects originate from the interfaces that WS₂ monolayers reside on and interact with. The electron density in the WS₂ is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back‐gated voltages. Other influential factors include the strain in WS₂ and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.     

Wafer‐Scale and Low‐Temperature Growth of 1T‐WS2 Film for Efficient and Stable Hydrogen Evolution Reaction

The metallic 1T phase of WS₂ (1T‐WS₂), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS₂ (2H‐WS₂) is inherently stable, methods for synthesizing 1T‐WS₂ are limited and complicated. Herein, a uniform wafer‐scale 1T‐WS₂ film is prepared using a plasma‐enhanced chemical vapor deposition (PE‐CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T‐WS₂ films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as‐grown 1T‐WS₂ are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a₀ × a₀ superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T‐WS₂ film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T‐WS₂ films synthesized with CVD and hydrothermal methods. The 1T‐WS₂ does not transform to stable 2H‐WS₂, even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T‐phase 2D materials for electrocatalysis applications.     

Re-Dispersion of Platinum From CNTs Substrate to α-MoC1 - x to Boost the Hydrogen Evolution Reaction

Developing high-performance electrocatalysts toward hydrogen evolution reaction (HER) is important for clean and sustainable hydrogen energy, yet still challenging. Herein, an α-MoC_1 - x induced redispersing strategy to construct a superior HER electrocatalyst (Pt/CNTs-N + α-MoC_1 - x) by mechanical mixing of α-MoC_1 - x with Pt/CNTs-N followed by thermal reduction is reported. It is found that thermo-activation treatment enables partial Pt atoms to redisperse on α-MoC_1 - x substrate from carbon nanotubes, which creates dual active interfaces of Pt species dispersed over carbon nanotubes and α-MoC_1 - x. Benefiting from the strong electronic interaction between the Pt atom and α-MoC_1 - x, the utilization efficiency of the Pt atom and the zero-valence state of Pt is evidently enhanced. Consequently, Pt/CNTs-N + α-MoC_1 - x catalyst exhibits excellent HER activity with low overpotentials of 17 and 34 mV to achieve a current density of 10 mA cm⁻² in acidic and alkaline electrolytes, respectively. Density functional theory calculations further reveal that the synergistic effect between Pt and α-MoC_1 - x makes it accessible for the dissociation of water molecules and subsequent desorption of hydrogen atoms. This work reveals the crucial roles of α-MoC_1 - x additives, providing practical solutions to enhance platinum dispersion, and thereby enhance the catalytic activity in HER.     

Carbon‐Encapsulated WOx Hybrids as Efficient Catalysts for Hydrogen Evolution

Developing non‐noble metal catalysts as Pt substitutes, with good activity and stability, remains a great challenge for cost‐effective electrochemical evolution of hydrogen. Herein, carbon‐encapsulated WO_x anchored on a carbon support (WO_x@C/C) that has remarkable Pt‐like catalytic behavior for the hydrogen evolution reaction (HER) is reported. Theoretical calculations reveal that carbon encapsulation improves the conductivity, acting as an electron acceptor/donor, and also modifies the Gibbs free energy of H* values for different adsorption sites (carbon atoms over the W atom, O atom, W? O bond, and hollow sites). Experimental results confirm that WO_x@C/C obtained at 900 °C with 40 wt% metal loading has excellent HER activity regarding its Tafel slope and overpotential at 10 and 60 mA cm^?2, and also has outstanding stability at ?50 mV for 18 h. Overall, the results and facile synthesis method offer an exciting avenue for the design of cost‐effective catalysts for scalable hydrogen generation.     

Discovery of Superconductivity in 2M WS2 with Possible Topological Surface States

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T_c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.     

Integrated Freestanding Two‐dimensional Transition Metal Dichalcogenides

This paper reports on the integration of freestanding transition metal dichalcogenides (TMDs). Monolayer (1‐L) MoS₂, WS₂, and WSe₂ as representative TMDs are transferred on ZnO nanorods (NRs), used here as nanostructured substrates. The photoluminescence (PL) spectra of 1‐L TMDs on NRs show a giant PL intensity enhancement, compared with those of 1‐L TMDs on SiO₂. The strong increases in Raman and PL intensities, along with the characteristic peak shifts, confirm the absence of stress in the TMDs on NRs. In depth analysis of the PL emission also reveals that the ratio between the exciton and trion peak intensity is almost not modified after transfer. The latter shows that the effect of charge transfer between the 1‐L TMDs and ZnO NRs is here negligible. Furthermore, confocal PL and Raman spectroscopy reveal a fairly consistent distribution of PL and Raman intensities. These observations are in agreement with a very limited points contact between the support and the 1‐L TMDs. The entire process reported here is scalable and may pave the way for the development of very efficient ultrathin optoelectronics.     

Interstitial Hydrogen Atom to Boost Intrinsic Catalytic Activity of Tungsten Oxide for Hydrogen Evolution Reaction

Tungsten oxide (WO₃) is an appealing electrocatalyst for the hydrogen evolution reaction (HER) owing to its cost-effectiveness and structural adjustability. However, the WO₃ electrocatalyst displays undesirable intrinsic activity for the HER, which originates from the strong hydrogen adsorption energy. Herein, for effective defect engineering, a hydrogen atom inserted into the interstitial lattice site of tungsten oxide (H_0.23WO₃) is proposed to enhance the catalytic activity by adjusting the surface electronic structure and weakening the hydrogen adsorption energy. Experimentally, the H_0.23WO₃ electrocatalyst is successfully prepared on reduced graphene oxide. It exhibits significantly improved electrocatalytic activity for HER, with a low overpotential of 33 mV to drive a current density of 10 mA cm⁻² and ultra-long catalytic stability at high-throughput hydrogen output (200 000 s, 90 mA cm⁻²) in acidic media. Theoretically, density functional theory calculations indicate that strong interactions between interstitial hydrogen and lattice oxygen lower the electron density distributions of the d-orbitals of the active tungsten (W) centers to weaken the adsorption of hydrogen intermediates on W-sites, thereby sufficiently promoting fast desorption from the catalyst surface. This work enriches defect engineering to modulate the electron structure and provides a new pathway for the rational design of efficient catalysts for HER.     

High Edge Selectivity of In Situ Electrochemical Pt Deposition on Edge‐Rich Layered WS2 Nanosheets

Recent studies show that the Pt electrode can be slowly dissolved into the acidic media and regenerate on the working electrode along with the long‐time hydrogen evolution reaction (HER) test. However, to date, the relationship between the Pt deposition and the intrinsic properties of the working electrode remains elusive. Herein, for the first time, the edge selectivity of in situ electrochemical Pt deposition on layered 2H‐WS₂ nanosheets, whose edge surface with rich dangling bonds is chemically active to regulate their properties, especially the interfacial reaction occurring between the electrode surface and the adjacent thin layer of the solution, is theoretically elucidated and experimentally verified by controlling the cathode polarization test using Pt wire as the counter electrode in H₂SO₄ solution. It is revealed that the layered WS₂ nanosheets with rich exposed edges show much stronger interaction with Pt atoms because the terminated S₂^2? or S^2? ligands on the edge exhibit much lower binding energy for Pt atoms compared with the apical S^2? ligands on the terrace surface. The in situ electrochemical Pt‐deposited WS₂ nanosheets with rich exposed edges can act as a highly active hybrid electrocatalyst to accelerate HER kinetics and exhibit commercial Pt‐like HER performance, especially in the alkaline media.     

Quantifying Quasi‐Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal Dichalcogenides

One of the most fundamental parameters of any photovoltaic material is its quasi‐Fermi level splitting (?µ) under illumination. This quantity represents the maximum open‐circuit voltage (V_oc) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of V_oc that can possibly be achieved from monolayer WS₂, MoS₂, WSe₂, and MoSe₂‐based solar cells, and they are compared with state‐of‐the‐art perovskites. These results show that V_oc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS₂, MoS₂, WSe₂, and MoSe₂ monolayers at 1 Sun illumination, respectively. It is also observed that ?µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ?µ heterogeneity by electrically gating the TMD monolayers in a metal‐oxide‐semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ?µ heterogeneity. The values of ?µ determined from this work reveal the potential of atomically thin TMDs for high‐voltage, ultralight, flexible, and eye‐transparent future solar cells.     

Ultrathin Transition Metal Dichalcogenide/3d Metal Hydroxide Hybridized Nanosheets to Enhance Hydrogen Evolution Activity

The vast majority of the reported hydrogen evolution reaction (HER) electrocatalysts perform poorly under alkaline conditions due to the sluggish water dissociation kinetics. Herein, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS₂ and WS₂). A series of ultrathin 2D‐hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D‐TMD nanosheets. The resultant Ni(OH)₂ and Co(OH)₂ hybridized ultrathin MoS₂ and WS₂ nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The 2D‐MoS₂/Co(OH)₂ hybrid achieves an extremely low overpotential of ≈128 mV at 10 mA cm^?2 in 1 m KOH. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides.     

Synergistic Phase and Disorder Engineering in 1T‐MoSe2 Nanosheets for Enhanced Hydrogen‐Evolution Reaction

MoSe₂ is a promising earth‐abundant electrocatalyst for the hydrogen‐evolution reaction (HER), even though it has received much less attention among the layered dichalcogenide (MX₂) materials than MoS₂ so far. Here, a novel hydrothermal‐synthesis strategy is presented to achieve simultaneous and synergistic modulation of crystal phase and disorder in partially crystallized 1T‐MoSe₂ nanosheets to dramatically enhance their HER catalytic activity. Careful structural characterization and defect characterization using positron annihilation lifetime spectroscopy correlated with electrochemical measurements show that the formation of the 1T phase under a large excess of the NaBH₄ reductant during synthesis can effectively improve the intrinsic activity and conductivity, and the disordered structure from a lower reaction temperature can provide abundant unsaturated defects as active sites. Such synergistic effects lead to superior HER catalytic activity with an overpotential of 152 mV versus reversible hydrogen electrode (RHE) for the electrocatalytic current density of j = ?10 mA cm^?2, and a Tafel slope of 52 mV dec^?1. This work paves a new pathway for improving the catalytic activity of MoSe₂ and generally MX₂‐based electrocatalysts via a synergistic modulation strategy.     

Phase‐Controlled Synthesis of Monolayer W1−xRexS2 Alloy with Improved Photoresponse Performance

Tuning bandgap and phases in the ternary 2D transition metal dichalcogenides (TMDs) alloys has opened up unexpected opportunities to engineer optoelectronic properties and explore potential applications. In this work, a salt‐assisted chemical deposition vapor (CVD) growth strategy is reported for the creation of high‐quality monolayer W_1?xRe_xS₂ alloys to fulfill a readily phase control from 1H to DT by changing the ratio of Re and W precursors. The structures and chemical compositions of doping alloys are confirmed by combining atomic resolution scanning transmission electron microscopy‐annular dark field imaging with energy dispersive X‐ray spectroscopy (EDS) and X‐ray photoelectron spectroscopy, matching well with the calculated results. The field‐effect transistors (FETs) devices fabricated based on 1H‐W_0.9Re_0.1S₂ monolayer exhibit a n‐type semiconducting behavior with the mobility of 0.4 cm² V^?1 s^?1. More importantly, the FETs show high‐performance responsivity with a value of 17 µA W^?1 in air, which is superior to that of monolayer CVD‐grown WS₂. This work paves the way toward synthesizing monolayer ternary alloys with controlled phases for potential optoelectronic applications.     

Colloidal Single‐Layer Photocatalysts for Methanol‐Storable Solar H2 Fuel

Molecular surfactants are widely used to control low‐dimensional morphologies, including 2D nanomaterials in colloidal chemical synthesis, but it is still highly challenging to accurately control single‐layer growth for 2D materials. A scalable stacking‐hinderable strategy to not only enable exclusive single‐layer growth mode for transition metal dichalcogenides (TMDs) selectively sandwiched by surfactant molecules but also retain sandwiched single‐layer TMDs' photoredox activities is developed. The single‐layer growth mechanism is well explained by theoretical calculation. Three types of single‐layer TMDs, including MoS₂, WS₂, and ReS₂, are successfully synthesized and demonstrated in solar H₂ fuel production from hydrogen‐stored liquid carrier—methanol. Such H₂ fuel production from single‐layer MoS₂ nanosheets is CO_x‐free and reliably workable under room temperature and normal pressure with the generation rate reaching ≈617 µmole g^?1 h^?1 and excellent photoredox endurability. This strategy opens up the feasible avenue to develop methanol‐storable solar H₂ fuel with facile chemical rebonding actualized by 2D single‐layer photocatalysts.     

Preparation of High‐Percentage 1T‐Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth‐abundant electrocatalysts to potentially replace precious platinum‐based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low‐density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high‐yield, large‐scale production of water‐dispersed, ultrasmall‐sized, high‐percentage 1T‐phase, single‐layer TMD nanodots with high‐density active edge sites and clean surface, including MoS₂, WS₂, MoSe₂, Mo_0.5W_0.5S₂, and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of ?140 mV at current density of 10 mA cm^?2, a Tafel slope of 40 mV dec^?1, and excellent long‐term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high‐density active edge sites, high‐percentage metallic 1T phase, alloying effect and basal‐plane Se‐vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging.     

Iron Carbides: Control Synthesis and Catalytic Applications in COx Hydrogenation and Electrochemical HER

Catalytic transformation of CO_x (x = 1, 2) with renewable H₂ into valuable fuels and chemicals provides practical processes to mitigate the worldwide energy crisis. Fe‐based catalytic materials are widely used for those reactions due to their abundance and low cost. Novel iron carbides are particularly promising catalytic materials among the reported ferrous catalysts. Recently, a series of strategies has been developed for the preparation of iron carbide nanoparticles and their nanocomposites. Control synthesis of FeC_x‐based nanomaterials and their catalytic applications in CO_x hydrogenation and electrochemical hydrogen evolution reaction (HER) are reviewed. The discussion is focused on the unique catalytic activities of iron carbides in CO_x hydrogenation and HER and the correlation between structure and catalytic performance. Future synthesis and potential catalytic applications of iron carbides are also summarized.