An Efficient and Earth‐Abundant Oxygen‐Evolving Electrocatalyst Based on Amorphous Metal Borides

Cost‐effective and efficient oxygen‐evolving electrocatalysts are urgently required for energy storage and conversion technologies. In this work, an amorphous trimetallic boride nanocatalyst (Fe–Co–2.3Ni–B) prepared by a simple approach is reported as a highly efficient oxygen evolution reaction electrocatalyst. It exhibits an overpotential (η) of 274 mV to deliver a geometric current density (j_geo) of 10 mA cm^?2, a small Tafel slope of 38 mV dec^?1, and excellent long‐term durability at a mass loading of 0.3 mg cm^?2. The impressive electrocatalytic performance originates from the unique amorphous multimetal–metalloid complex nanostructure. From application's point of view, this work holds great promise as this process is simple and allows for large‐scale production of cheap, yet efficient, material.     

Crystalline Ni(OH)2/Amorphous NiMoOx Mixed‐Catalyst with Pt‐Like Performance for Hydrogen Production

The achievement of effective alkaline hydrogen production from water electrolysis is an active field of research. Herein, an integrated electrode composed of crystalline Ni(OH)₂ and amorphous NiMoO_x is fabricated onto nickel foam (denoted as Ni(OH)₂–NiMoO_x/NF). The hydrogen evolution reaction (HER) kinetics are optimized along with phase transformation process during soaking operation. An overpotential of 36 mV to drive 10 mA cm^?2 along with the low Tafel slope of 38 mV dec^?1 reveals the catalyst's excellent HER performance and a Heyrovsky‐step‐controlled HER mechanism. When assembled into a urea‐assisted water electrolyzer, a voltage of 1.42 V can reach 10 mA cm^?2. Further experiments and Fourier transform infrared spectroscopy (FTIR) results illustrate the synergy effect between crystalline and amorphous areas and the optimized water dissociation step. Crystalline Ni(OH)₂ serves as the scissor for water dissociation in an alkali environment to produce H*, while the amorphous NiMoO_x layer serves as the location for H* adsorption and H₂ desorption.     

Dual‐Functional N Dopants in Edges and Basal Plane of MoS2 Nanosheets Toward Efficient and Durable Hydrogen Evolution

Herein, the authors explicitly reveal the dual‐functions of N dopants in molybdenum disulfide (MoS₂) catalyst through a combined experimental and first‐principles approach. The authors achieve an economical, ecofriendly, and most efficient MoS₂‐based hydrogen evolution reaction (HER) catalyst of N‐doped MoS₂ nanosheets, exhibiting an onset overpotential of 35 mV, an overpotential of 121 mV at 100 mA cm^?2 and a Tafel slope of 41 mV dec^?1. The dual‐functions of N dopants are (1) activating the HER catalytic activity of MoS₂ S‐edge and (2) enhancing the conductivity of MoS₂ basal plane to promote rapid charge transfer. Comprehensive electrochemical measurements prove that both the amount of active HER sites and the conductivity of N‐doped MoS₂ increase as a result of doping N. Systematic first‐principles calculations identify the active HER sites in N‐doped MoS₂ edges and also illustrate the conducting charges spreading over N‐doped basal plane induced by strong Mo 3d –S 2p –N 2p hybridizations at Fermi level. The experimental and theoretical research on the efficient HER catalysis of N‐doped MoS₂ nanosheets possesses great potential for future sustainable hydrogen production via water electrolysis and will stimulate further development on nonmetal‐doped MoS₂ systems to bring about novel high‐performance HER catalysts.     

Doped‐MoSe2 Nanoflakes/3d Metal Oxide–Hydr(Oxy)Oxides Hybrid Catalysts for pH‐Universal Electrochemical Hydrogen Evolution Reaction

Clean hydrogen production is highly promising to meet future global energy demands. The design of earth‐abundant materials with both high activity for hydrogen evolution reaction (HER) and electrochemical stability in both acidic and alkaline environments is needed, in order to enable practical applications. Here, the authors report a non‐noble 3d metal Cl‐chemical doping of liquid phase exfoliated single‐/few‐layer flakes of MoSe₂ for creating MoSe₂/3d metal oxide–hydr(oxy)oxide hybrid HER‐catalysts. It is proposed that the electron‐transfer from MoSe₂ nanoflakes to metal cations and the chlorine complexation‐induced neutralization, as well as the in situ formation of metal oxide–hydr(oxy)oxides on the MoSe₂ nanoflakes' surface, tailor the proton affinity of the catalysts, increasing the number and HER‐kinetics of their active sites in both acidic and alkaline electrolytes. The electrochemical coupling between doped‐MoSe₂/metal oxide–hydr(oxy)oxide hybrids and single‐walled carbon nanotubes heterostructures further accelerates the HER process. Lastly, monolithic stacking of multiple heterostructures is reported as a facile electrode assembly strategy to achieve overpotential for a cathodic current density of 10 mA cm^?2 of 0.081 and 0.064 V in 0.5 m H₂SO₄ and 1 m KOH, respectively. This opens up new opportunities to address the current density versus overpotential requirements targeted in pH‐universal hydrogen production.     

H2‐Directing Strategy on In Situ Synthesis of Co‐MoS2 with Highly Expanded Interlayer for Elegant HER Activity and its Mechanism

MoS₂ has drawn great attention as a promising Pt‐substituting catalyst for the hydrogen evolution reaction (HER). This work utilizes H₂ as the structure directing agent (SDA) to in situ synthesize a range of Co‐MoS₂‐n (n = 0, 0.5, 1.0, 1.4, 2.0) with expanded interlayer spacings (d = 9.2 – 11.1 Å), which significantly boost their HER activities. The Co‐MoS₂‐1.4 with an interlayer spacing of 10.3 Å presents an extremely low overpotential of 56 mV (at 10 mA cm^?2) and a Tafel slope of 32 mV dec^?1, which is superior than most reported MoS₂‐based catalysts. Density function theory calculations are used to gain insights that i) the H₂ can be dissociatively adsorbed on MoS₂ and greatly affect the related surface free energy by regulating the interlayer spacing; ii) the expanded interlayer spacing can significantly decrease the absolute value of ΔG_H, thereby leading to greatly promoted HER activity. Additionally, the large amounts of 1T phase (73.9–79.2%) and Co‐Mo‐S active sites (40.9–91.3%) also contribute to the enhanced HER activity of the synthesized samples. Overall, a simple new strategy for in situ synthesis of Co‐MoS₂ with an expanded interlayer spacing is proposed, which sheds light on other 2D energy material designs.     

Active Edge Sites Engineering in Nickel Cobalt Selenide Solid Solutions for Highly Efficient Hydrogen Evolution

An effective multifaceted strategy is demonstrated to increase active edge site concentration in Ni_0.33Co_0.67Se₂ solid solutions prepared by in situ selenization process of nickel cobalt precursor. The simultaneous control of surface, phase, and morphology result in as‐prepared ternary solid solution with extremely high electrochemically active surface area (C _dl = 197 mF cm^?2), suggesting significant exposure of active sites in this ternary compound. Coupled with metallic‐like electrical conductivity and lower free energy for atomic hydrogen adsorption in Ni_0.33Co_0.67Se₂, identified by temperature‐dependent conductivities and density functional theory calculations, the authors have achieved unprecedented fast hydrogen evolution kinetics, approaching that of Pt. Specifically, the Ni_0.33Co_0.67Se₂ solid solutions show a low overpotential of 65 mV at ?10 mV cm^?2, with onset potential of mere 18 mV, an impressive small Tafel slope of 35 mV dec^?1, and a large exchange current density of 184 µA cm^?2 in acidic electrolyte. Further, it is shown that the as‐prepared Ni_0.33Co_0.67Se₂ solid solution not only works very well in acidic electrolyte but also delivers exceptional hydrogen evolution reaction (HER) performance in alkaline media. The outstanding HER performance makes this solid solution a promising candidate for mass hydrogen production.     

Molybdenum Disulfide/Nitrogen‐Doped Reduced Graphene Oxide Nanocomposite with Enlarged Interlayer Spacing for Electrocatalytic Hydrogen Evolution

Facile design of low‐cost and highly active catalysts from earth‐abundant elements is favorable for the industrial application of water splitting. Here, a simple strategy to synthesize an ultrathin molybdenum disulfide/nitrogen‐doped reduced graphene oxide (MoS₂/N‐RGO‐180) nanocomposite with the enlarged interlayer spacing of 9.5 Å by a one‐step hydrothermal method is reported. The synergistic effects between the layered MoS₂ nanosheets and N‐doped RGO films contribute to the high activity for hydrogen evolution reaction (HER). MoS₂/N‐RGO‐180 exhibits the excellent catalytic activity with a low onset potential of ?5 mV versus reversible hydrogen elelctrode (RHE), a small Tafel slope of 41.3 mV dec^?1, a high exchange current density of 7.4 × 10^?4 A cm^?2, and good stability over 5 000 cycles under acidic conditions. The HER performance of MoS₂/N‐RGO‐180 nanocomposite is superior to the most reported MoS₂‐based catalysts, especially its onset potential and exchange current density. In this work, a novel and simple method to the preparation of low‐cost MoS₂‐based electrocatalysts with the extraordinary HER performance is presented.     

Anion–Cation Double Substitution in Transition Metal Dichalcogenide to Accelerate Water Dissociation Kinetic for Electrocatalysis

Until now, many works have shown that the hydrogen evolution reaction (HER) performance can be improved by anion or cation substitution into the crystal lattice of pyrite‐structure materials. However, the synergistic effects of anion–cation double substitution for overall enhancement of the catalytic activity remains questionable. Here, the simultaneous incorporation of vanadium and phosphorus into the CoS₂ moiety for preparing 3D mesoporous cubic pyrite‐metal Co_1‐xV_xSP is presented. It is demonstrated that the higher catalytic activity of CoS₂ after V incorporation can be primarily attributed to abundance active sites, whereas P substitution is responsible for improving HER kinetics and intrinsic catalyst. Interestingly, due to the synergistic effect of P–V double substitution, the 3D Co_1‐xV_xSP shows superior electrocatalysis toward the HER with a very small overpotential of 55 mV at 10 mA cm^?2, a small Tafel slope of 50 mV dec^?1, and a high turnover frequency of 0.45 H₂ s^?1 at 10 mA cm^?2, which is very close to commercial 20% Pt/C. Density functional theory calculation reveals that the superior catalytic activity of the 3D Co_1‐xV_xSP is contributed by the reduced kinetic energy barrier of rate‐determining HER step as well as the promotion of the desorption H₂ gas process.     

Highly Efficient Oxygen Evolution by a Thermocatalytic Process Cascaded Electrocatalysis Over Sulfur‐Treated Fe‐Based Metal–Organic‐Frameworks

The oxygen evolution reaction (OER) is a bottleneck process for water splitting and finding highly efficient, durable, low‐cost, and earth‐abundant electrocatalysts is still a major challenge. Here a sulfur‐treated Fe‐based metal–organic‐framework is reported as a promising electrocatalyst for the OER, which shows a low overpotential of 218 mV at the current density of 10 mA cm^?2 and exhibits a very low Tafel slope of 36.2 mV dec^?1 at room temperature. It can work on high current densities of 500 and 1000 mA cm^?2 at low overpotentials of 298 and 330 mV, respectively, by keeping 97% of its initial activity after 100 h. Notably, it can achieve 1000 mA cm^?2 at 296 mV with a good stability at 50 °C, fully fitting the requirements for large‐scale industrial water electrolysis. The high catalytic performance can be attributed to the thermocatalytic processes of H⁺ capture by –SO₃ groups from *OH or *OOH species, which cascades to the electrocatalytic pathway and then significantly reduces the OER overpotentials.     

Tailoring Active Sites in Mesoporous Defect‐Rich NC/Vo‐WON Heterostructure Array for Superior Electrocatalytic Hydrogen Evolution

Tailoring active sites in earth‐abundant non‐noble metal electrocatalysts are required toward widespread applications in sustainable energy fields. Herein, an integrated mesoporous heterostructure array is reported by a hydrogenation/nitridation‐induced in situ growth strategy. Highly conductive oxygen‐vacancies‐rich tungsten oxynitride (V_o‐WON) nanorod array acts as the backbone encapsulated by ultrathin nitrogen‐doped carbon (NC) nanolayers, forming high‐quality shell/core NC/V_o‐WON heterostructures. Density functional theory calculations reveal that defect‐rich heterostructure arrays not only enhance the conductivity and modulate electronic structure but also promote the adsorption and dissociation of reactants and offer substantial potential sites. As expected, porous NC/V_o‐WON array exhibits a small overpotential of 16 mV at the current density of 10 mA cm^?2 and a low Tafel slope of 33 mV per decade in alkaline media, accompanied by negligible loss upon a large current density over 100 h. Benefiting from outstanding electrocatalytic hydrogen evolution reaction performance and stability, this defective heterostructure could serve as a prominent alternative electrocatalyst for renewable energy applications.     

Wrinkled Rh2P Nanosheets as Superior pH‐Universal Electrocatalysts for Hydrogen Evolution Catalysis

Searching for highly efficient and durable electrocatalysts for the hydrogen evolution reaction (HER) that function effectively at all pHs is of great interest to the scientific community, however it is still a grand challenge, because the HER kinetics of Pt in alkaline solutions are approximately two to three orders of magnitude lower than that in acidic solution. Herein, a new class of wrinkled, ultrathin Rh₂P nanosheets for enhancing HER catalysis at all pHs is reported. They exhibit a small overpotential of 18.3 mV at 10 mA cm^?2, low Tafel slope of 61.5 mV dec^?1, and good durability in alkaline media, much better than the commercial Pt/C catalyst. Density functional theory calculations reveal that the active open‐shell effect from the P‐3p band not only promotes Rh‐4d for increased proton–electron charge exchange but also provides excellent p–p overlapping to locate the O‐related species as distributary center, which can benefit the HER process in alkaline media. It is also demonstrated that the present wrinkled, ultrathin Rh₂P nanosheets are highly efficient and durable electrocatalysts toward HER in both acid and neutral electrolytes. The present work opens a new material design for ultrathin 2D metal phosphide nanostructures for the purpose of boosting HER performance at all pHs.     

A Cu‐Based Nanoparticulate Film as Super‐Active and Robust Catalyst Surpasses Pt for Electrochemical H2 Production from Neutral and Weak Acidic Aqueous Solutions

Electrocatalysts that are stable and highly active at low overpotential (η) under mild conditions as well as cost‐effective and scalable are eagerly desired for potential use in photo‐ and electro‐driven hydrogen evolution devices. Here the fabrication and characterization of a super‐active and robust Cu‐Cu_xO‐Pt nanoparticulate electrocatalyst is reported, which displays a small Tafel slope (44 mV dec^?1) and a large exchange current density (1.601 mA cm^?2) in neutral buffer solution. The catalytic current density of this catalyst film reaches 500 mA cm^?2 at η = ?390 ± 12 mV and 20 mA cm^?2 at η = ?45 ± 3 mV, which are significantly higher than the values displayed by Pt foil and Pt/C electrodes in neutral buffer solution and even comparable with the activity of Pt electrode in 0.5 m H₂SO₄ solution.     

Carbon‐Tailored Semimetal MoP as an Efficient Hydrogen Evolution Electrocatalyst in Both Alkaline and Acid Media

The electrolysis processes such as hydrogen evolution reaction (HER) require high efficient catalysts with robust surface stability. A high conductivity is also necessary to speed up the charge transport between the catalyst and the electrolyte. Recently, the observation of exceedingly high conductivity in the topological semimetal MoP, has provided a model catalyst to investigate the correlation between the electrical transport and the electrocatalytic activity for the HER. Thus, MoP is encapsulated in a Mo, P codoped carbon layer (MoP@C). This composite material exhibits outstanding HER performance, with an extremely low overpotential of 49 mV at a current density of 10 mA cm^?2 and a Tafel slope of 54 mV dec^?1 in an alkaline medium. In addition, electron transport analysis indicates that MoP exhibits high conductivity and mobility due to the existence of triple‐point fermions and a complex Fermi surface. Furthermore, the presence of P? C and Mo? C bonds at the interface between the carbon layer and the MoP particles modulates the band structure of MoP@C and facilitates fast electron transfer, accumulation, and subsequent delocalization, which are in turn responsible for the excellent HER activity.     

Hydrothermal Synthesis of Monolithic Co3Se4 Nanowire Electrodes for Oxygen Evolution and Overall Water Splitting with High Efficiency and Extraordinary Catalytic Stability

Cobalt selenide has been proposed to be an effective low‐cost electrocatalyst toward the oxygen evolution reaction (OER) due to its well‐suited electronic configuration. However, pure cobalt selenide has by far still exhibited catalytic activity far below what is expected. Herein, this paper for the first time reports the synthesis of new monoclinic Co₃Se₄ thin nanowires on cobalt foam (CF) via a facile one‐pot hydrothermal process using selenourea. When used to catalyze the OER in basic solution, the conditioned monolithic self‐supported Co₃Se₄/CF electrode shows an exceptionally high catalytic current of 397 mA cm^?2 at a low overpotential (η) of 320 mV, a small Tafel slope of 44 mV dec^?1, a turnover frequency of 6.44 × 10^?2 s^?1 at η = 320 mV, and excellent electrocatalytic stability at various current densities. Furthermore, an electrolyzer is assembled using two symmetrical Co₃Se₄/CF electrodes as anode and cathode, which can deliver 10 and 20 mA cm^?2 at low cell voltages of 1.59 and 1.63 V, respectively. More significantly, the electrolyzer can operate at 10 mA cm^?2 over 3500 h and at 100 mA cm^?2 for at least 2000 h without noticeable degradation, showing extraordinary operational stability.     

Earth‐Abundant Iron Diboride (FeB2) Nanoparticles as Highly Active Bifunctional Electrocatalysts for Overall Water Splitting

Developing efficient, durable, and earth‐abundant electrocatalysts for both hydrogen and oxygen evolution reactions is important for realizing large‐scale water splitting. The authors report that FeB₂ nanoparticles, prepared by a facile chemical reduction of Fe²⁺ using LiBH₄ in an organic solvent, are a superb bifunctional electrocatalyst for overall water splitting. The FeB₂ electrode delivers a current density of 10 mA cm^?2 at overpotentials of 61 mV for hydrogen evolution reaction (HER) and 296 mV for oxygen evolution reaction (OER) in alkaline electrolyte with Tafel slopes of 87.5 and 52.4 mV dec^?1, respectively. The electrode can sustain the HER at an overpotential of 100 mV for 24 h and OER for 1000 cyclic voltammetry cycles with negligible degradation. Density function theory calculations demonstrate that the boron‐rich surface possesses appropriate binding energy for chemisorption and desorption of hydrogen‐containing intermediates, thus favoring the HER process. The excellent OER activity of FeB₂ is ascribed to the formation of a FeOOH/FeB₂ heterojunction during water oxidation. An alkaline electrolyzer is constructed using two identical FeB₂‐NF electrodes as both anode and cathode, which can achieve a current density of 10 mA cm^?2 at 1.57 V for overall water splitting with a faradaic efficiency of nearly 100%, rivalling the integrated state‐of‐the‐art Pt/C and RuO₂/C.     

Efficient Water Splitting Using a Simple Ni/N/C Paper Electrocatalyst

An efficient water splitting electrocatalyst is presented. Cheap and sustainable cellulose filter paper, infiltrated with nickel acetate as the nickel source, and phenanthroline as a ligand and nitrogen source are carbonized together. Nitrogen functionalities turn out to be crucial coordination sites for the supported Ni/NiO(OH) particles. This simple and scalable one step procedure leads to powders, but also to complete membranes made of ≈10 wt% Ni, supported on nitrogen functionalized carbon. The non‐noble catalyst shows a low onset potential (330 mV vs reversible hydrogen electrode), high current density (e.g., j > 25 mA cm^?­2 at η = 430 mV), excellent kinetics (Tafel slope of 44 mV dec^?1), and a very favorable stability (<5% decay after 10 h electrolysis) in the oxygen evolution. The performance is similar or even better compared to state‐of‐the‐art noble metal catalysts (e.g., IrO₂, Ir/C, Ru/C, and Pt/C). Because of the simple, cheap, and scalable preparation procedure the catalyst is highly promising for practical low price/tech applications. Interestingly, the system is also active in the hydrogen evolution reaction, leading to a promising bifunctional catalyst. The benchmark characteristics are η₁₀ = 390 mV for oxygen evolution and η₁₀ = 190 mV for hydrogen evolution, that is, an overall efficiency of 68% at 10 mA current density.     

Nitrogen and Sulfur Codoped Graphite Foam as a Self‐Supported Metal‐Free Electrocatalytic Electrode for Water Oxidation

The oxidation of water to produce oxygen gas is related to a variety of energy storage systems. Thus, the development of efficient, cheap, durable, and scalable electrocatalysts for oxygen evolution reaction (OER) is of great importance. Here, a high‐performance OER catalyst, nitrogen and sulfur codoped graphite foam (NSGF) is reported. This NSGF is prepared from commercial graphite foil and directly applied as an electrocatalytic electrode without using a current collector and a polymeric binder. It exhibits an extremely low overpotential of 0.380 V to reach a current density of 10 mA cm^?2 and shows fast kinetics with a small Tafel slope of 96 mV dec^?1 in 0.1 m KOH. This electrocatalytic performance is superior or comparable to those of previously reported metal‐free OER catalysts.     

Mesoporous MoO3–x Material as an Efficient Electrocatalyst for Hydrogen Evolution Reactions

A unique approach for the synthesis of nonstoichiometric, mesoporous molybdenum oxide (MoO_3–x) with nanosized crystalline walls by using a soft template (PEO‐b‐PS) synthesis method is introduced. The as‐synthesized mesoporous MoO_3–x is very active and stable (durability > 12 h) for the electrochemical hydrogen evolution reaction (HER) under both acidic and alkaline conditions. The intrinsic MoO₃ serves as an HER electrocatalyst without the assistance of carbon materials, noble metals, or MoS₂ materials. The results from transmission electron microscopy and N₂ sorption techniques show that the as‐synthesized mesoporous MoO_3–x has large accessible pores (20–40 nm), which are able to facilitate mass transport and charge transfer during HER. In terms of X‐ray diffraction, X‐ray photoelectron spectroscopy, temperature‐programmed oxidation, and diffusive reflectance UV–vis spectroscopy, the mesoporous MoO_3–x exhibits mixed oxidation states (Mo⁵⁺, Mo⁶⁺) and an oxygen‐deficient structure. The as‐synthesized MoO_3–x only requires a low overpotential (≈0.14 V) to achieve a 10 mA cm^?2 current density in 0.1 m KOH and the Tafel slope is as low as 56 mV dec^?1. Density functional theory calculations demonstrate a change of electronic structure and the possible reaction pathway of HER. Oxygen vacancies and mesoporosity serve as key factors for excellent performance.     

Low‐Overpotential Electrocatalytic Water Splitting with Noble‐Metal‐Free Nanoparticles Supported in a sp3 N‐Rich Flexible COF

Covalent organic frameworks (COFs) are crystalline organic polymers with tunable structures. Here, a COF is prepared using building units with highly flexible tetrahedral sp³ nitrogens. This flexibility gives rise to structural changes which generate mesopores capable of confining very small (<2 nm sized) non‐noble‐metal‐based nanoparticles (NPs). This nanocomposite shows exceptional activity toward the oxygen‐evolution reaction from alkaline water with an overpotential of 258 mV at a current density of 10 mA cm^?2. The overpotential observed in the COF‐nanoparticle system is the best in class, and is close to the current record of ≈200 mV for any noble‐metal‐free electrocatalytic water splitting system—the Fe–Co–Ni metal‐oxide‐film system. Also, it possesses outstanding kinetics (Tafel slope of 38.9 mV dec^?1) for the reaction. The COF is able to stabilize such small‐sized NP in the absence of any capping agent because of the COF–Ni(OH)₂ interactions arising from the N‐rich backbone of the COF. Density‐functional‐theory modeling of the interaction between the hexagonal Ni(OH)₂ nanosheets and the COF shows that in the most favorable configuration the Ni(OH)₂ nanosheets are sandwiched between the sp³ nitrogens of the adjacent COF layers and this can be crucial to maximizing their synergistic interactions.     

Self‐Limited on‐Site Conversion of MoO3 Nanodots into Vertically Aligned Ultrasmall Monolayer MoS2 for Efficient Hydrogen Evolution

MoS₂ has emerged as a promising alternative electrocatalyst for the hydrogen evolution reaction (HER) due to high intrinsic per‐site activity on its edge sites and S‐vacancies. However, a significant challenge is the limited density of such sites. Reducing the size and layer number of MoS₂ and vertically aligning them would be an effective way to enrich and expose such sites for HER. Herein, a facile self‐limited on‐site conversion strategy for synthesizing monolayer MoS₂ in a couple of nanometers which are highly dispersed and vertically aligned on 3D porous carbon sheets is reported. It is discovered that the preformation of well‐dispersed MoO₃ nanodots in 1–2 nm as limited source is the key for the fabrication of such an ultrasmall MoS₂ monolayer. As indicated by X‐ray photoelectron spectroscopy and electron spin resonance data, these ultrasmall MoS₂ monolayers are rich in accessible S‐edge sites and vacancies and the smaller MoS₂ monolayers the more such sites they have, leading to enhanced electrocatalytic activity with a low overpotential of 126 mV at 10 mA cm^?2 and 140 mV at 100 mA mg^?1 for HER. This state‐of‐the‐art performance for MoS₂ electrocatalysts enables the present strategy as a new avenue for exploring well‐dispersed ultrasmall nanomaterials as efficient catalysts.