Hydrogen Evolution Reaction Activity Research Articles

Guanine-Assisted Contrived Low Pt-Integrated Mo2C/C for Hydrogen Evolution Reaction.

Due to the high cost of the available Pt electrocatalysts, the large-scale water electrolysis production of hydrogen has been hindered. Hydrogen generation via electrochemical water splitting is a renewable energy essential to a sustainable society, creating a distinct material interface that shows Pt-like properties with long-term stability crucial to hydrogen evolution reactions (HERs). Here, we synthesized the guanine-assisted facile synthesis of 1 wt % Pt/Mo2C/C having a layered type morphology via solid state calcined process followed by chemical reduction. The well-developed 1 wt % Pt/Mo2C/C heterostructure is analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) to understand the percentage of Pt doped on Mo2C/C. The as-synthesized 1 wt % Pt/Mo2C/C heterostructure exhibits a better HER activity than a commercial Pt/C with a small overpotential of 19 mV to reach a current density at 10 mA cm-2 with a Tafel slope of 28 mV/dec. The catalyst 1 wt % Pt/Mo2C/C shows a long-term stability of 42 h in 0.5 M H2SO4. The layered sheet structure with the N-doped carbon (C) nanosheet, encapsulating well-dispersed Pt within the layers, significantly enhances the reaction kinetics of the 1 wt % Pt/Mo2C/C. This design creates a synergistic effect among Mo2C, Pt, and the carbon matrix, improving catalytic performance. Theoretical calculations using the density functional theory (DFT) indicate the active sites for hydrogen evolution on Pt-integrated Mo2C/C. The 1 wt % Pt/Mo2C/C possessed a significantly reduced ΔGH* value (-0.06 eV) as compared to the pristine Mo2C/C material (ΔGH* = 0.34 eV), suggesting a higher catalytic activity. This simple method offers a fresh means to make clearly defined carbides and sheds light on creating low-Pt catalysts for a scalable HER.

Design of RuOx Electrocatalysts Containing Metallic Ru on the Surface to Accelerate the Alkaline Hydrogen Evolution Reaction.

The development of water splitting technology in alkaline medium requires the exploration of electrocatalysts superior to Pt/C to boost the alkaline hydrogen evolution reaction (HER). Ruthenium oxides with strong water dissociation ability are promising candidates; however, the lack of hydrogen combination sites immensely limits their performance. Herein, we reported a unique RuOx catalyst with metallic Ru on its surface through a simple cation exchange method. We demonstrated that the formation of metallic Ru on RuOx greatly enhances the interaction between the catalyst and adsorbed hydrogen (*H), resulting in extremely high HER activity in alkaline media. Moreover, we proposed the potential of zero charge (Epzc) as a descriptor of ruthenium-base catalysts for alkaline HER for the first time and revealed that the existence of metallic Ru optimizes the Epzc of RuOx toward the hydrogen region. As a result, the designed RuOx catalyst achieves an overpotential of only 18 mV at the current density of 10 mA cm-2. Furthermore, RuOx requires 1.80 V to reach 800 mA cm-2 in the anion exchange membrane water electrolyzer, outperforming the benchmark Pt/C.

{110} Surface-Exposed Bi3.15Nd0.85Ti3O12 Ferroelectric Nanosheet Arrays on Porous Ceramics as Efficient and Recyclable Piezo-Photocatalysts.

Bismuth-layered ferroelectric nanomaterials exhibit great potential for piezo-photocatalysis. However, a major challenge lies in the difficulty of recovering the catalytic powders, raising concerns regarding secondary pollution of water. In this work, a novel hierarchical porous ferroelectric ceramic containing {110} surface-exposed Bi3.15Nd0.85Ti3O12 (BIT-Nd) nanosheet arrays is grown on a porous ceramic matrix for efficient and recyclable piezo-photocatalysis. By controlling the BIT-Nd loading level of the nanosheets, the piezo-photocatalytic degradation efficiency of a Rhodamine B(RhB)(C0 = 10 mgL-1) solution reached an optimum value of 97.1% in 100 min with a first-order kinetic rate constant, k, of up to 0.0321 min-1 in Bi3.15Nd0.85Ti3O12-20 (BITNd-20) with a mass ratio of hydrothermal products to ceramics of 20%. In the presence of BITNd-20, a surprising H2 yield rate of 130 µmol·h-1 is achieved without using anycocatalyst or scavenger. Specially, the beneficial role of snowflake structures on piezoelectric potential amplification and introducing nanosheets with exposed {110} surfaces on hydrogen evolution reaction (HER) activity, piezoelectric potential output, and catalytic performance of porous ceramics has been revealed. This unique design strategy provides a new approach to enhance the piezo-photocatalytic activity by addressing environmental issues and enhancing catalytic performance to yield cleaner energy.

XPS Depth Profiling of Surface Restructuring Responsible for Hydrogen Evolution Reaction Activity of Nickel Sulfides in Alkaline Electrolyte

Electrochemical water splitting provides a sustainable method for hydrogen production. However, the primary challenge for electrochemical hydrogen generation is the high cost and limited availability of platinum-based noble-metal catalysts. Transition-metal chalcogenides have been identified as low-cost and efficient electrocatalysts to promote the hydrogen evolution reaction (HER) in alkaline electrolytes. Nonetheless, the identification of active sites and the underlying catalytic mechanism remain elusive. In this study, phosphorus-doped nickel sulfide has been successfully synthesized, demonstrating enhanced activity for alkaline HER. Investigating surface chemistry through X-ray photoelectron spectroscopy (XPS), depth profiling revealed that surface restructuring occurs during the HER process. The presence of phosphorus significantly influences this transformation, promoting the formation of a novel active Ni-O layer. This Ni-O layer is responsible for enhanced catalytic activity by upshifting the d-band center and increasing the density of states near the Fermi level, along with expanding the electrochemical surface area. This study reveals that the surface restructuring of transition-metal sulfides is highly tied to the electronic structure of the parent catalysts. Gaining a comprehensive understanding of this surface restructuring is essential for predicting and exploring more efficient non-precious transition-metal sulfide electrocatalysts.

Fabricating Lattice-Confined Pt Single Atoms With High Electron-Deficient State for Alkali Hydrogen Evolution Under Industrial-Current Density.

The confining effect is essential to regulate the activity and stability of single-atom catalysts (SACs), but the universal fabrication of confined SACs is still a great challenge. Here, various lattice-confined Pt SACs supported by different carriers are constructed by a universal co-reduction approach. Notably, Pt single atoms confined in the lattice of Ni(OH)2 (Pt1/Ni(OH)2) with a high electron-deficient state exhibit excellent activity for basic hydrogen evolution reaction (HER). Specifically, Pt1/Ni(OH)2 just requires 15mV to get 10mA cm-2 and the mass activity of Pt1/Ni(OH)2 is 15 times of commercial Pt/C. Moreover, Pt1/Ni(OH)2 assembled in an alkaline water electrolyzer shows 1030h durability under the industrial current density of 800mA cm-2. In situ spectroscopy techniques reveal Pt─H and "free" OH radical can be directly observed for Pt1/Ni(OH)2, confirming the lattice-confined Pt single atoms play a key role during HER. Further density functional theory uncovers the Pt 3d orbital strongly hybridizes with O 2p and Ni 3d orbitals in Ni(OH)2, which quickly optimizes the electronic state of the Pt site, thus largely reducing the energy barrier of the rate-determining step to 0.16eV for HER. Finally, this synthesis method is extended to construct other 9 lattice-confined SACs.

Molybdenum and Sulfur Doping To Create Active Sites and Regulate d-Band Centers of Ni-Mo-S Electrocatalysts for the Hydrogen Evolution Reaction.

Defining the active sites and further optimizing their activity are of great significance for enhancing the hydrogen evolution reaction (HER) performances, especially for inexpensive Ni-based catalysts doped with metals and nonmetal elements. This work reports the role of the incorporated molybdenum and sulfur in enhancing the HER activity of nickel. The prepared molybdenum and sulfur coincorporated Ni (NMS) electrocatalysts exhibit excellent HER performance, with an overpotential and Tafel slope of 77.0 mV (10 mA·cm-2) and 64.4 mV·dec-1, respectively, because of the large electrochemically active surface area, quick reaction kinetics, and charge-transfer capability. The theoretical calculation results reveal that the incorporated Mo atoms play the role of reaction sites, and the introduced S atoms can further enhance the activity of Mo atom. The high HER activity of the Mo atom can be attributed to the regulated d-band center by optimizing the composition of NMS, which tunes the interaction between Mo and intermediate species. The elucidated mechanism can make it possible to design Ni-based electrocatalysts doped with metals and nonmetals for efficient hydrogen evolution.

Toward Green and Sustainable Zinc-Ion Batteries: The Potential of Natural Solvent-Based Electrolytes.

Zinc-ion batteries (ZIBs) are emerged as a promising alternative for sustainable energy storage, offering advantages such as safety, low cost, and environmental friendliness. However, conventional aqueous electrolytes in ZIBs face significant challenges, including hydrogen evolution reaction (HER) and zinc dendrite formation, compromising their cycling stability and safety. These limitations necessitate innovative electrolyte solutions to enhance ZIB performance while maintaining sustainability. This review explores the potential of natural solvent-based electrolytes derived from renewable and biodegradable resources. Natural deep eutectic solvents (DES), bio-ionic liquids, and biomass-derived organic compounds present unique advantages, including a wider electrochemical stability window, reduced HER activity, and controlled zinc deposition. Examples include DESs based on choline chloride (ChCl), glycerol-based systems, and biomass-derived solvents such as γ-valerolactone (GVL) and aloe vera, demonstrating improved cycling stability and dendrite suppression. Despite their promise, challenges such as high viscosity, cost, and scalability remain critical barriers to commercialization. This review underscores the need for further research to optimize natural solvent formulations, enhance Zn anode compatibility, and integrate these systems into practical applications. By addressing these challenges, natural solvent-based electrolytes can pave the way for safer, high-performance, and environmentally sustainable ZIBs, particularly large-scale energy storage systems.

Cucurbit[n]uril-Derived Electrocatalysts for Oxygen Evolution, Oxygen Reduction, and Hydrogen Evolution Reactions.

Various sustainable energy conversion techniques like water electrolyzers, fuel cells, and metal-air battery devices are promising to alleviate the issues in fossil fuel consumption. However, their broad employment has been mainly inhibited by the lack of advanced electrocatalysts to accelerate the sluggish kinetics of the three involved half-reactions including oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). Recent advances have witnessed the cucurbit[n]uril (CB[n])-directed strategy as a prominent tool to develop high performance electrocatalysts with either OER, ORR, or HER activities. In this review, the recent progress on CB[n]-derived electrocatalysts ranging from molecular complexes to heterogeneous nanostructures and single-atoms for the three half-reactions are reviewed, and future opportunities are discussed. A concise introduction to the fundamentals of CB[n]s regarding their synthesis, structure, and chemistry is given first. Subsequently, the systematic summary of CB[n]-derived electrocatalysts and their performance for the OER/ORR/HER are discussed in detail, with a specific emphasis on correlating their structure and activities by combining diverse physiochemical characterizations, electrochemical experiments, and theory simulations. Finally, a brief conclusion and perspective for future opportunities regarding CB[n]-derived electrocatalysts for many other electrocatalytic applications are proposed.

RuCu Nanorod Arrays Synergistically Promote Efficient Water-Splitting

In the realm of green hydrogen energy, utilizing ruthenium (Ru) as a precious metal electrocatalyst for the hydrogen evolution reaction (HER) instead of platinum (Pt/C) is an excellent choice. Unfortunately, there are not enough active sites or electronic structures on a single Ru-based catalyst to significantly improve the oxygen evolution reaction (OER). Therefore, creating bifunctional water electrolysis catalysts that are stable and highly active in a variety of media continues to be a major challenge. The study describes a new method for creating an electrocatalyst (RuCuCl/NF-2) by using Ru to regulate an inert CuCl precursor. The enhanced mass transfer performance of the distinctive coral structure and the synergistic effect of RuCu emphasize its excellent water electrolysis activity, which is based on the self-assembly of Cu nanoparticles into a conical membrane structure. Overtaking the commercial benchmark Pt/C (~38 mV to reach 10 mA cm−2), the RuCuCl/NF-2 displays HER activity (~25 mV to reach 10 mA cm−2) in 1M KOH. This sheds light on how to create more sophisticated bifunctional electrocatalysts.

Ligand-induced changes in the electrocatalytic activity of atomically precise Au25 nanoclusters.

Atomically precise gold nanoclusters have shown great promise as model electrocatalysts in pivotal electrocatalytic processes such as the hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO2RR). Although the influence of ligands on the electronic properties of these nanoclusters is well acknowledged, the ligand effects on their electrocatalytic performances have been rarely explored. Herein, using [Au25(SR)18]- nanoclusters as a prototype model, we demonstrated the importance of ligand hydrophilicity versus hydrophobicity in modulating the interface dynamics and electrocatalytic performance. Our first-principles calculations revealed that Au25 protected by hydrophilic -SCH2COOH ligands exhibits faster kinetics in stripping the thiolate ligand and better HER activity due to enhanced proton transfer facilitated by boosted interface hydrogen bonding. Conversely, Au25 protected by hydrophobic -SCH2CH3 ligands demonstrates enhanced CO2RR performance by minimizing water interference to stabilize the key *COOH intermediate and lower the barrier for CO formation. Experimental validation using synthesized hydrophilic and hydrophobic ligand-protected Au25 nanoclusters (NCs), such as [Au25(MPA)18]- (MPA = mercaptopropionic acid), [Au25(MHA)18]- (MHA = 6-mercaptohexanoic acid), and [Au25(SC6H13)18]-, confirms these findings, where the hydrophilic ligand-protected Au25 NCs exhibit better activity and stability in the HER, while the hydrophobic ligand-protected Au25 NCs achieve higher faradaic efficiency and current density in the CO2RR. The mechanistic insights in this study provide valuable guidance for the rational design of surface microenvironments in efficient nanocatalysts for sustainable energy applications.

Employing Octahedral Net Charge Descriptors for Designing High-Performance Aqueous Proton Batteries.

Recently, aqueous proton batteries have shown promise for electrochemical energy storage using MXene electrodes. However, designing high-performance MXene proton batteries remains challenging due to the inevitable hydrogen evolution reaction (HER), the vast chemical composition space of MXene, and the unclear proton transport mechanism. To tackle these challenges, we established a general descriptor based on structural units of MXenes, termed the octahedral net charge descriptor (Qoct). This descriptor correlates well with HER activity, capacity, and proton transport performance. Based on the descriptor, prediction reveals a dual-proton storage mechanism per site in N-functionalized MXene. Meanwhile, the accuracy of the descriptor is verified across a broader MXene chemical space. Additionally, the kinetic process shows the topological transformation energy barrier of the interfacial solution is profoundly influenced by Qoct, thereby impacting the proton transfer performance. This universal descriptor originates from the different electron filling states on the molecular orbitals of the octahedron. Overall, this work provides an efficient strategy for designing MXene proton batteries and can be extended to other battery and catalysis fields.

Employing Octahedral Net Charge Descriptors for Designing High‐Performance Aqueous Proton Batteries

Recently, aqueous proton batteries have shown promise for electrochemical energy storage using MXene electrodes. However, designing high‐performance MXene proton batteries remains challenging due to the inevitable hydrogen evolution reaction (HER), the vast chemical composition space of MXene, and the unclear proton transport mechanism. To tackle these challenges, we established a general descriptor based on structural units of MXenes, termed the octahedral net charge descriptor (Qoct). This descriptor correlates well with HER activity, capacity, and proton transport performance. Based on the descriptor, prediction reveals a dual‐proton storage mechanism per site in N‐functionalized MXene. Meanwhile, the accuracy of the descriptor is verified across a broader MXene chemical space. Additionally, the kinetic process shows the topological transformation energy barrier of the interfacial solution is profoundly influenced by Qoct, thereby impacting the proton transfer performance. This universal descriptor originates from the different electron filling states on the molecular orbitals of the octahedron. Overall, this work provides an efficient strategy for designing MXene proton batteries and can be extended to other battery and catalysis fields.

Unlocking the Key to Photocatalytic Hydrogen Production Using Electronic Mediators for Z-Scheme Water Splitting.

A prevalent challenge in particulate photocatalytic water splitting lies in the fact that while numerous photocatalysts exhibit outstanding hydrogen evolution reaction (HER) activity in organic sacrificial reagents, their performance diminishes markedly in a Z-scheme water splitting system using electronic mediators. This underlying reason remains undefined, posing a long-standing issue in photocatalytic water splitting. Herein, we unveiled that the primary reason for the decreased HER activity in electronic mediators is due to the strong adsorption of shuttle ions on cocatalyst surfaces, which inhibits the initial proton reduction and results in a severe backward reaction of the oxidized shuttle ions. To address this, taking typical visible-light-responsive photocatalysts, BaTaO2N and SrTiO3:Rh, as examples, we have developed a strategy via selective surface modification of metal cocatalysts (such as Pt, Ru) with chromium oxide species (CrOx) to prevent the adsorption of shuttle ions. It is demonstrated that the photocatalytic HER activities of BaTaO2N and SrTiO3:Rh can be improved by one to two orders of magnitude in diverse shuttle ions. The introduced CrOx substantially weakens the interaction between the metal cocatalysts and shuttle ions, promotes proton adsorption for the HER reaction, and also suppresses the backward reaction between shuttle ions. Owing to the improved HER activity, the photocatalytic performance of Z-scheme water splitting is significantly enhanced, providing a feasible strategy for constructing efficient Z-scheme systems in heterogeneous photocatalysis.

Anion-Directed Assembly of a Bimetallic Pd/Ag Nanocluster: Synthesis, Characterization, and HER Activity.

Palladium-doped silver nanoclusters (NCs) have been highlighted for their unique physicochemical properties and potential applications in catalysis, optics, and electronics. Anion-directed synthesis offers a powerful route to control the morphology and properties of these NCs. Herein, we report a novel Pd-doped Ag NC, [Pd(H)Ag13(S){S2P(OiPr)2}10] (PdHAg13S), synthesized through the inclusion of sulfide and hydride anions. This NC features a unique linear S-Pd-H axis enclosed in a 4-5-4 stacked arrangement of silver atoms. The distinctive hydride environment was characterized by NMR spectroscopy, and the total structure was determined by single-crystal X-ray diffraction (SCXRD) and supported by computational studies. Mass spectrometry and X-ray photoelectron spectroscopy (XPS) further confirmed the assigned composition. This unique construct exhibits promising hydrogen evolution reaction (HER) activity. Our findings highlight the potential of anion-directed synthesis for creating novel bimetallic NCs with tailored structures and catalytic properties.

Testing mixed metal bimetallic, and monometallic, cryptates for electrocatalytic hydrogen evolution.

Appropriately designed catalysts help to minimise the energy required to convert the energy-poor feedstock H2O into energy-rich molecular H2. Herein, two families of pyridazine-based cryptates, mononuclear [MIILi](BF4)2 and mixed metal dinuclear [MIICuILi](BF4)3 (M = Fe, Co, Cu or Zn; Li is the Schiff base cryptand made by 2 : 3 condensation of tris(2-aminoethyl)amine and 3,6-diformylpyridazine), are investigated as potential electrocatalysts for the hydrogen evolution reaction (HER) in MeCN with acetic acid as the proton source. The synthesis and structures of a new mixed metal cryptate, [ZnIICuILi](BF4)3, and the tetrafluoroborate analogue of the previously reported perchlorate salt of the mono-zinc cryptate, [ZnIILi](BF4)2·0.5H2O, are reported. Electrocatalytic HER testing showed that a deposit forms on the glassy carbon working electrode during electrolysis and it is the active species responsible for the very modest activity observed. The deposits formed by the heterobinuclear cryptates had higher activities (2.0 < TON2h < 3.5) than the deposits formed by the mononuclear cryptates (TON2h < 0.75). But unfortunately the control, using CuI(MeCN)4BF4, had a similar TON2h (2.3) to those seen for the heterobinculear cryptates, which indicates that it is the deposit formed by the CuI cation present in the heterobinuclear cryptates that is likely responsible for the observed, very modest, HER activity.

Decoding the Hume-Rothery Rule in a Bifunctional Tetra-metallic Alloy for Alkaline Water Electrolysis.

The 90-year-old Hume-Rothery rule was adapted to design an outstanding bifunctional tetra-metallic alloy electrocatalyst for water electrolysis. Following the radius mismatch principles, Fe (131 pm) and Ni (124 pm) are selectively incorporated at the Pd (139 pm) site of Mo0.30Pd0.70 nanosheets. Analogously, Cu (132 pm) alloys with only Pd, while Ag (145 pm) alloys with both Pd and Mo (154 pm). The face-centered cubic Mo0.30Pd0.35Ni0.23Fe0.12 nanosheets with 10-12 atomic layers, featuring in-plane compressive strain along the {111} basal plane, show 1/3 (422) reflection from local hexagonal symmetry. The more electronegative Pd attracts electron density from Ni/Fe in Mo0.30Pd0.35Ni0.23Fe0.12, synergistically boosting the mass activities for hydrogen and oxygen evolution reactions to 89 ± 5 and 38.6 ± 3.1 A g-1 at ±400 mV versus RHE, respectively. Full water electrolysis continues for ≥550 h, requiring cell voltages of 1.51 and 1.63 V at 10 and 100 mA cm-2, delivering 45 mL h-1 green H2.

Metallic PtC monolayer as a promising hydrogen evolution electrocatalyst.

Reasonable design of hydrogen evolution reaction (HER) electrocatalysts with low Pt loading and excellent catalytic performance is a key challenge in finding efficient and cost attractive catalysts. Pt with its unique d-electrons provides new opportunities for the development of HER catalysts when it forms compounds with highly earth-abundant C. Herein, we focused on designing highly efficient catalysts composed of Pt and C elements using first-principles structure search simulations, identifying four stability PtCx monolayers. The novel PtC monolayer with a zigzag C chain not only possesses lower Pt loading but also shows inherent metallicity. Meanwhile, its H2O adsorption and dissociation abilities are efficient and facile. The HER activity of the PtC monolayer is comparable to that of commercial Pt, with desirable ΔGH* values and larger exchange current density, which are mainly attributed to lower charge donation of Pt, larger occupation of Pt PDOS at the Fermi level, and paired electrons of the zigzag C chain. Moreover, its excellent HER activity can be maintained even at high H coverage under strain and solvent effect. All these attractive properties render the PtC monolayer an appropriate HER catalyst.

Enhancing Alkaline Hydrogen Evolution by Regulating H and OH Binding Strength through Strong Metal-Support Interactions.

Establishing optimized metal-support interaction (MSI) between active sites and the substrate is essential for modulating the adsorption properties of key reaction intermediates during catalysis, thereby enhancing the catalytic performance. In this study, catalyst composites with varying degrees of MSI are constructed using ruthenium (Ru) and different carbon nanotubes, and their performance for alkaline hydrogen evolution reaction (HER) is systematically investigated. Detailed kinetic assessments reveal that catalysts with a strong MSI exhibit superior HER activity. For instance, Ru-O-CNT catalyst composite demonstrates an encouragingly low overpotential of 11 mV at 10 mA cm-2 and excellent stability. Electrochemical voltammetry analysis indicates that an effective MSI optimizes the binding strength of both *H and *OH, accelerating the HER process. Furthermore, we showcase that an industrial-level electrolyzer, assembled using Ru-O-CNT as the cathodic catalyst, achieves impressive performance with a low cell voltage of 1.72 V and high stability at a current density of 1 A cm-2.

Tuning Fork Scanning Electrochemical Cell Microscopy for Resolving Morphological and Redox Properties of Single Ag Nanowires.

We report a Tuning Fork Scanning Electrochemical Cell Microscopy (TF-SECCM) technique for providing morphological and electrochemical information on single redox-active entities. This new operation configuration of SECCM utilizes an electrolyte-filled nanopipette tip mounted onto a tuning fork force sensor to obtain a precise tip-sample distance control and surface morphological mapping capabilities. Redox activities of regions of interest (ROIs) can be investigated by scanning electrode potential by moving the nanopipette to any target regions while maintaining the constant force engagement of the tip with the sample. Using silver nanowires (Ag NWs) as a model system due to their extensive utilization in energy and sensing devices, TF-SECCM provides not only the topography of single Ag NWs but also their distinctive redox activities, catalytic hydrogen evolution reaction (HER) activities, and electrolyte anion adsorption/desorption features in contrast to NW bundles and a supporting substrate.

Boosting the Hydrogen Evolution Activity of a Low-Coordinated Co─N─C Catalyst via Vacancy Defect-Mediated Alteration of the Intermediate Adsorption Configuration.

The cobalt-nitrogen-carbon (Co─N─C) single-atom catalysts (SACs) are promising alternatives to precious metals for catalyzing the hydrogen evolution reaction (HER) and their activity is highly dependent on the coordination environments of the metal centers. Herein, a NaHCO3 etching strategy is developed to introduce abundant in-plane pores within the carbon substrates that further enable the construction of low-coordinated and asymmetric Co─N3 sites with nearby vacancy defects in a Co─N─C catalyst. This catalyst exhibits a high HER activity with an overpotential (η) of merely 78mV to deliver a current density of 10mA cm-2, a Tafel slope of 45.2mV dec-1, and a turnover frequency of 1.67 s-1 (at η = 100mV). Experimental investigations and theoretical calculations demonstrate that the vacancy defects neighboring the Co─N3 sites can modulate the electronic structure of the catalyst and alter the adsorption configuration of the H intermediate from the typical atop mode to the side mode, resulting in weakened H adsorption strength and thus improved HER activity. This work provides an efficient strategy to regulate the coordination environment of SACs for improved catalytic performance and sheds light on the atomic-level understanding of the structure-activity relationships.