Hydrogen Bond Network Of Water Research Articles

Diethylene glycol reduced the water activity of aqueous electrolyte via regulating interphase chemistry for highly durable Zinc-ion batteries

The uncontrollable dendrite growth and parasitic side reactions of Zn lead to poor cycling stability, hindering the practical application of aqueous zinc-ion batteries. Here, we develop a new hybrid electrolyte with zinc trifluoromethanesulfonate dissolved in water and co-solvent diethylene glycol (DEG). DEG can not only alter the Zn2+ solvation structure with the co-participation of DEG and anion to reduce active water molecules, but also interrupt the hydrogen-bond network of the original water. DEG is more easily absorbed on Zn surface than H2O to regulate electric double layer. Based on these unique effects, DEG can regulate interface chemistry, forming a robust solid electrolyte interphase and adjusting Zn2+ stripping/plating behaviors, which suppress side reactions and dendrite growth. Their synergetic effect results in the great improved cycling stability of Zn anode with 5000 h in symmetric cell at 1 mA cm−2 and 1 mAh cm−2. The work shed light on the design of high-performance aqueous electrolyte to advance zinc-ion batteries.

Why does urea not alter the vibrational spectroscopic signatures of water?

Usually, external solutes alter the local structural arrangement of the water hydrogen bond (H-bond) network. However, urea has a negligible effect on the local structure of water, irrespective of its concentration. In this work, we aim to find the microscopic origin of the inertness of urea toward water’s local structure by using theoretical isotope dilute IR spectroscopy. To get a microscopic resolution of different local environments, we investigate frequency distributions, population of structurally different ensembles, H-bond configurations, and H-bond strength. We find that urea has a negligible effect on the vibrational spectroscopic signatures of water. The solvation shell-wise decomposition of frequency distribution further reveals that the urea has almost zero perturbation to the local structure of any of its solvation shells. However, the frequency distribution of waters near the C=O group and NH2 group reveals a non-negligible impact by urea on water. The frequency distribution of the CO-region is red-shifted and narrower than the bulk water response, indicating a stronger H-bond and enhanced structural ordering of the waters in the CO-region; however, an opposite response is observed for the NH-region. The H-bond strength outcome further supplements this finding. We infer that the O-H oscillators in the CO-region and NH-region have non-negligible counteracting perturbation from the urea molecule. Yet, the overall response is free from any such effects. The insignificant populations and counteracting responses of these two ensembles play a major role.

Alkali hydroxide (LiOH, NaOH, KOH) in water: Structural and vibrational properties, including neutron scattering results.

Structural and vibrational properties of aqueous solutions of alkali hydroxides (LiOH, NaOH, and KOH) are computed using quantum molecular dynamics simulations for solute concentrations ranging between 1 and 10M. Element-resolved partial radial distribution functions, neutron and x-ray structure factors, and angular distribution functions are computed for the three hydroxide solutions as a function of concentration. The vibrational spectra and frequency-dependent conductivity are computed from the Fourier transforms of velocity autocorrelation and current autocorrelation functions. Our results for the structure are validated with the available neutron data for 17M concentration of NaOH in water [Semrouni et al., Phys. Chem. Chem. Phys. 21, 6828 (2019)]. We found that the larger ionic radius [rLi+<rNa+<rK+] and higher concentration disturb the hydrogen-bond network of water, resulting in more disordered cationic hydration shell. Our abinitio simulation data for solute concentrations ranging between 1 and 10M can be used to guide future elastic and inelastic neutron-scattering experiments.

How Solvation Energetics Dampen the Hydrogen Evolution Reaction to Maximize Zinc Anode Stability

AbstractAqueous zinc metal batteries (AZMB) are emerging as a promising alternative to the prevailing existing Lithium‐ion battery technology. However, the development of AZMBs is hindered due to challenges including dendrite formation, hydrogen evolution reaction (HER), and ZnO passivation on the anode. Here, a tetraalkylsulfonamide (TAS) additive for suppressing HER, dendrite formation, and enhancing cyclability is rationally designed. Only 1 mm TAS is found that can effectively displace water molecules from the Zn2+ solvation shell, thereby altering the solvation matrix of Zn2+ and disrupting the hydrogen bond network of free water, as demonstrated through 67 Zn and 1H nuclear magnetic resonance spectroscopy, high‐resolution mass spectrometry (HRMS), and density functional theory (DFT) studies. Voltammetry synchronized with in situ monitoring of the electrode surface reveals suppressed dendritic growth and HER in the presence of TAS. Electrochemical mass spectrometry (ECMS) captures real‐time HER suppression during Zn electrodeposition, revealing the ability of TAS to suppress the HER by an order of magnitude. A ≈25‐fold cycle life improvement from ≈100 h to over 2500 h in coin cells cycled in the presence of TAS. Furthermore, by suppressing passivation product formation, it is demonstrated that strategy robustly maximizes the stability of Zn metal anodes.

Polarization‐Sensitive and Self‐Driven Pyro‐Phototronic Photodetectors Based on MoS2‐Water Heterojunctions

AbstractPolarization‐sensitive and self‐driven pyroelectric‐based photodetectors have recently gained interest due to their potential application in artificial electronic eyes, biomedical imaging, and optical switches. Here, a photodetector based on light modulation‐induced polarization and depolarization of water molecules on the surface of a 2D MoS2 crystal is reported. The MoS2‐water heterostructure photodetector serves as a self‐driven pyro‐phototronic device that converts light‐induced thermal energy to electrical signals, leading to a transient photoresponsivity as high as 24.6 mA W−1 and a specific detectivity of 2.85 × 108 Jones under 470 nm wavelength at zero bias. Due to the formation of a built‐in electric field at the MoS2‐water interface, this structure also has a high steady–state responsivity of 3.62 A W−1 and detectivity of 9.18 × 108 Jones at 3 V bias, along with a fast response time of ≈0.74 ms. Moreover, due to the rearrangement of the hydrogen bond network in the liquid water upon visible light illumination, the MoS2‐water photodetector is light polarization‐sensitive. The simple fabrication process, low cost, polarization sensitivity, and high performance of the MoS2‐water structure make it an excellent candidate for liquid‐compatible photodetectors.

Atomic-resolution structure analysis inside an adaptable porous framework

We introduce a versatile metal-organic framework (MOF) for encapsulation and immobilization of various guests using highly ordered internal water network. The unique water-mediated entrapment mechanism is applied for structural elucidation of 14 bioactive compounds, including 3 natural product intermediates whose 3D structures are clarified. The single-crystal X-ray diffraction analysis reveals that incorporated guests are surrounded by hydrogen-bonded water networks inside the pores, which uniquely adapt to each molecule, providing clearly defined crystallographic sites. The calculations of host-solvent-guest structures show that the guests are primarily interacting with the MOF through weak dispersion forces. In contrast, the coordination and hydrogen bonds contribute less to the total stabilization energy, however, they provide highly directional point interactions, which help align the guests inside the pore.

The effect of surface hydrophobicity and hydrophilicity on ion-ion interactions at water-solid interfaces.

Condensation and arrangement of ions at water-solid interfaces are of great importance in the formation of electrical double layers (EDL) and the transport of ions under a confined geometry. So far, the microscopic understanding of interfacial ion configurations is still far from complete, especially when the local ion concentration is high and ion-ion interactions become prominent. In this study, we directly visualized alkali metal cations within the hydrogen-bonding network of water on graphite and Cu(111)-supported graphene surfaces, using qPlus-based noncontact atomic force microscopy (NC-AFM). We found that the codeposition of the alkali cations and water molecules on the hydrophobic graphite surface leads to the formation of an ion-doped bilayer hexagonal ice (BHI) structure, where the ions are repelled from each other and scattered in a disordered distribution. In contrast, the hydrated alkali cations aggregate in one dimension on the more hydrophilic graphene/Cu(111) surface, forming a nematic state with a long-range order. Such a nematic state arises from the delicate interplay between water-ion and water-water interactions under surface confinement. These results reveal the high sensitivity of ion-ion interactions and ionic ordering to the surface hydrophobicity and hydrophilicity.

Hydration effects on thermal transitions and molecular mobility in Xanthan gum polysaccharides.

In this work, the xanthan gum (XG) polysaccharide is studied over a wide range of temperatures and water fractions 0 ≤ hw ≤ 0.70 (on a wet basis) by employing differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The investigation reveals that the critical water fraction for ice formation is about 0.35. Glass transition temperature (Tg) was determined through calorimetry experiments for all the samples studied. Water acts as a strong plasticizer, i.e., decreasing Tg, for water fractions up to about 0.35. A secondary (local) relaxation process is recorded in both dry and hydrated samples, which is sensitive to the presence of water molecules. This fact indicates that this process originates due to the orientation of small polar groups of the side chain, or/and due to the local main chain dynamics. Two types of long-range charge transport processes were resolved. The first is related to the conductive paths being formed via bulk-like ice structures (at high hydration levels), whereas the second can be attributed to proton mobility via the hydrogen bond (HB) network of non-freezing water existing in XG. Interestingly, this process is exactly the same in all the hydrated samples with hw > 0.25. With respect to the sample with hw = 0.27, a Vogel-Tammann-Fulcher (VTF)-like polarization process has also been recorded which seems to be related to long-range charge mobility via interconnected water clusters. As far as we are aware, this is the first time that XG is studied in terms of glass transition and molecular mobility over a wide range of hydration levels combining DSC and BDS techniques.

Ammonia Electrosynthesis from Nitrate Using a Ruthenium-Copper Cocatalyst System: A Full Concentration Range Study.

Electrochemical synthesis of ammonia via the nitrate reduction reaction (NO3RR) has been intensively researched as an alternative to the traditional Haber-Bosch process. Most research focuses on the low concentration range representative of the nitrate level in wastewater, leaving the high concentration range, which exists in nuclear and fertilizer wastes, unexplored. The use of a concentrated electrolyte (≥1 M) for higher rate production is hampered by poor hydrogen transfer kinetics. Herein, we demonstrate that a cocatalytic system of Ru/Cu2O catalyst enables NO3RR at 10.0 A in 1 M nitrate electrolyte in a 16 cm2 flow electrolyzer, with 100% faradaic efficiency toward ammonia. Detailed mechanistic studies by deuterium labeling and operando Fourier transform infrared (FTIR) spectroscopy allow us to probe the hydrogen transfer rate and intermediate species on Ru/Cu2O. Ab initio molecular dynamics (AIMD) simulations reveal that adsorbed hydroxide on Ru nanoparticles increases the density of the hydrogen-bonded water network near the Cu2O surface, which promotes the hydrogen transfer rate. Our work highlights the importance of engineering synergistic interactions in cocatalysts for addressing the kinetic bottleneck in electrosynthesis.

Comparative Investigation of the Microstructure of MgCl2 Aqueous Solutions Using Different X-ray Scattering Sources, Raman Spectroscopy, and Atomistic Simulations.

Aqueous solutions of magnesium chloride (MgCl2(aq)) are often used to test advances in the theory of electrolyte solutions because they are considered an ideal strong 2:1 electrolyte. However, there is evidence that some ion association occurs in these solutions, even at low concentrations. Even a small ion-pairing constant can have a significant impact on the chemical speciation of ions, so it is important to determine whether ion pairing actually occurs. In this study, MgCl2(aq) with concentrations ranging from 1 to 35% was studied using three methods: X-ray scattering (XRS) with the Shanghai Synchrotron Radiation Facility (SSRF) and silver-anode laboratory sources, Raman spectroscopy, and molecular dynamics (MD) simulations with the COMPASS-II and Madrid force fields. XRS results were analyzed in the framework of PDF theory to obtain the reduced structure function F(Q) and the reduced pair distribution function G(r). The F(Q) values from synchrotron radiation and laboratory sources both showed that the tetrahedral hydrogen bonds in bulk water were destroyed with the increased MgCl2 concentration. The results of G(r) indicated that the main peaks centered at 2.05 and 2.80 Å can be ascribed to the interactions of Mg-O and O-O, respectively. The peak at 3.10 Å is attributed to the combined effect of O-O and Cl-O. By comparing the structural information on MgCl2 solution obtained from the two light sources, it was found that both SSRF and silver-anode laboratory sources can reflect the above-mentioned structural information on MgCl2 solution. The radial distribution function (RDF) obtained from MD simulations of MgCl2 solutions assigned the peaks at 2.0, 2.8, and 3.2 Å to the Mg-O, O-O, and Cl-O interatomic pairs, respectively. The decrease in the O-O coordination number confirms that the hydrogen-bonding network of water is disrupted by increasing MgCl2 observed by X-ray scattering. The proportion of Mg-Cl contact ion pairs gradually increases with MgCl2 concentration as does the coordination number. Raman spectroscopy results show that the bond type changes from double donor double acceptor (DDAA) to single donor-single acceptor (DA) with increasing concentration, providing explicit details of the hydrogen-bond evolution in the aqueous solution.

Molecular-level insights to structure and hydrogen bonds network of 1,4-dioxane aqueous solution

We investigate aqueous 1,4-dioxane solution structure via molecular dynamics approach for the force fields by Smith et al. and OPLS-AA/CM1A with water model TIP4P. The overestimation of enthalpy of mixing is linked with the lower numbers of water-water and higher numbers of dioxane-water hydrogen bonds. The results provide the total number of hydrogen bonds per oxygen atom, as well as specific numbers of hydrogen bonds for water-water and dioxane-water pairs in solution. We show that specific quantities are required to observe the difference in the solution structure across the employed force fields, such as the hydrogen bond network percolation and the cluster analysis of dioxane molecules in solution. It is shown that for lower numbers of water-water hydrogen bonds the average dioxane cluster size is also lower. Total number of hydrogen bonds per oxygen atom and dioxane cluster sizes are in good agreement with X-Ray experiments by Takamuku et al. The water hydrogen bond network analysis adequately reproduces the experimental percolation thresholds by Hernandez et al.

Micro-Solvation of Propofol in Propylene Glycol-Water Binary Mixtures: Molecular Dynamics Simulation Studies.

The water microstructure around propofol plays a crucial role in controlling their solubility in the binary mixture. The unusual nature of such a water microstructure can influence both translational and reorientational dynamics, as well as the water hydrogen bond network near propofol. We have carried out all-atom molecular dynamics simulations of five different compositions of the propylene glycol (PG)/water binary mixture containing propofol (PFL) molecules to investigate the differential behavior of water microsolvation shells around propofol, which is likely to control the propofol solubility. It is evident from the simulation snapshots for various compositions that the PG at high molecular ratio favors the water cluster and extended chainlike network that percolates within the PG matrix, where the propofol is in the dispersed state. We estimated that the radial distribution function indicates higher ordered water microstructure around propofol for high PG content, as compared to the lower PG content in the PG/water mixture. So, the hydrophilic PG regulates the stability of the water micronetwork around propofol and its solubility in the binary mixture. We observed that the translational and rotational mobility of water belonging to the propofol microsolvation shell is hindered for high PG content and relaxed toward the low PG molecular ratio in the PG/water mixture. It has been noticed that the structural relaxation of the hydrogen bond formed between the propofol and the water molecules present in the propofol microsolvation shell for all five compositions is found to be slower for high PG content and becomes faster on the way to low PG content in the mixture. Simultaneously, we calculated the intermittent residence time correlation function of the water molecules belonging to the microsolvation shell around the propofol for five different compositions and found a faster short time decay followed up with long time components. Again, the origin of such long time decay is primarily from the structural relaxation of the microsolvation shell around the propofol, where the high PG content shows the slower structural relaxation that turns faster as the PG content approaches to the other end of the compositions. So, our studies showed that the slower structural relaxation of the microsolvation shell around propofol for a high PG molecular ratio in the PG/water mixture correlate well with the extensive ordering of the water microstructure and restricted water mobility and facilitates the dissolution process of propofol in the binary mixture.

Atomistic Investigation of the occupancy limits and stability of hydrogen hydrates as a hydrogen storage medium

Gas hydrates are nonstoichiometric inclusion compounds which occur via a hydrogen bonded water network. This network forms cages that encapsulates gas molecules such as hydrogen which are stabilized by weak van der Waals forces. This unique cage-guest interaction offers the potential for a safe, economical, and green medium for hydrogen storage. In this study, we use Density Functional Theory (DFT) to investigate the storage capacity of hydrates which primarily depends on the hydrogen occupancy in the small and large cages of the lattice structure. Energy analysis of the system with various occupancies shows that DFT is indicating occupancy limits beyond what has been previously reported and that the maximum occupancy in small and large cages is interdependent. The maximum hydrogen occupancy is 9 in large cages with an occupancy of 1 and 2 in the small cages and a maximum of 8 with an occupancy of 3 or 4 in the small cages. The occupancy limit mechanism is the breakage of hydrogen bonds due to cage volume expansion, the diffusion of hydrogen through pentagonal and hexagonal faces in small and large cages, and computational convergence.

Multifunctional Electrolyte toward Long‐Life Zinc‐Ion Batteries: Synchronous Regulation of Solvation, Cathode and Anode Interfaces

AbstractRechargeable aqueous zinc‐ion batteries (ZIBs) are threatened by the cathode dissolution and anode corrosion/dendrites, resulting in poor reversibility. Here, a multifunctional electrolyte is developed to regulate the cathode, anode interface, and solvation by introducing a low‐cost diethylene glycol monomethyl ether (DGME). The DGME can modulate the primary solvation sheath (PSS) of the hydrated Zn2+ and destroy the original hydrogen bond network of water, while the electric double layer (EDL) interface is reconstructed by the adsorption of DGME, which guides the uniform deposition of Zn2+ on the (002) crystal plane. Simultaneously, the DGME also effectively inhibits the dissolution of VO2 and enhances the reaction kinetics. As a proof of concept, the Zn//Cu cells deliver 2000 cycles with an ultrahigh coulombic efficiency (CE) of 99.6%. Additionally, the multifunctional electrolyte significantly improves the multiple performance of Zn//VO2 full cells. The low‐cost and multifunctional electrolyte can be readily applied to ZIBs, demonstrating its practical versatility.

Exploring the change of hydrogen bond evolution in NMP-H2O solution through 2D Raman-COS spectra analysis

The transitions of Hydrogen Bond (HB) structure with in the cluster structures of NMP-H2O solutions was conducted by combining Density Functional Theory (DFT) calculations and Two-dimensional Correlation Raman Spectroscopy (2D Raman-COS). When VNMP < 20 %, NMP molecule is incorporated into the tetrahedral structure of water through an "insertion" mechanism, by displacing some H2O molecules and establishing HB. This interaction strengthens the tetrahedral arrangement in water, causing the free OH bond length to be compressed. When VNMP > 20 %, large amounts of NMP molecules were self-assembled through dipole-dipole alignment, and the NMP·2H2O, NMP·H2O, and 2NMP·H2O clusters gradually substitute the tetrahedral structure of water, leading to the disruption of the HB network in water. Furthermore, it was observed that the pulsed laser beam accelerated the transition point of the HB structure in the NMP-H2O solution. This research provides references for the application of NMP in the fields of aqueous batteries from the view of HB with electrolyte.

Energy Dissipation into the Solvent during Proton Transfer Occurs via Acoustic Phonons.

In photochemistry, rapid energy dissipation into the solvent is mandatory to prevent radiation damages. By optical pump THz spectroscopy, we are able to follow the details of the energy dissipation mechanism upon photoexcitation of the photoacid to the hydrogen-bonded network of water: Based on the frequency maps subsequent to photoexcitation, we propose that energy transfer takes place via propagation of an acoustic phonon. The dissipation into the solvent can be rationalized by an initial first hydration shell response followed by energy dissipation via an acoustic phonon. Surprisingly, for the first 10 ps, the propagation in the water network can be described by a wave packet with a constant group velocity, indicating a long-range correlation. After 300 ps, thermalization in the liquid jet is reached and the THz spectrum reflects a Boltzmann population, corresponding a temperature increase of ΔT = 0.5 °C.

In situ study of structural changes: Exploring the mechanism of protein corona transition from soft to hard

HypothesisThe process of protein corona changes has been widely believed to follow the Vroman effect, while protein structural change during the process is rarely reported, due to the lack of analytical methods. In-situ interpretation for protein structural change is critical to processes such as the recognition and transport of nanomaterials. ExperimentsMolecular dynamics (MD) simulation was used to predict the deflection and twist of the protein tertiary structure. The structural changes of the surface protein corona during the interaction of nanoparticles (NPs) with lipid bilayer were probed in situ and real-time by sum frequency generation (SFG) spectroscopy. FindingsThe ring tertiary structure of the protein corona is altered from vertical to horizontal on particle surface, a process of the soft-to-hard structural transition, which is contributed by the hydrogen bonding force between the protein and water molecules. The negatively charged protein corona can induce the redistribution of interfacial charge, leading to a more stable hydrogen bond network of the interfacial water. Our findings suggest that the structural change from flexible to rigid is a crucial process in the soft-to-hard transition of the protein corona, which will be a beneficial supplement to the Vroman effect of protein adsorption.

A Universal Additive Design Strategy to Modulate Solvation Structure and Hydrogen Bond Network toward Highly Reversible Fe Anode for Low-Temperature All-Iron Flow Batteries.

Aqueous all-iron redox flow batteries (RFBs) are promising competitors for next-generation grid-scale energy storage applications. However, the high-performance operation of all-iron RFBs in a wider temperature range is greatly hindered by inferior iron plating/stripping reaction and low solid-liquid transition temperature at Fe anode. Herein, a universal electrolyte additive design strategy for all-iron RFBs is reported, which realizes a highly reversible and dendrite-free Fe anode at low temperatures. Quantum chemistry calculations first screen several organic molecules with oxygen-containing functional groups and identify N,N-Dimethylacetmide (DMAc) as a potential candidate with low cost, high solubility, and strong interactions with Fe2+ and H2 O. Combined experimental characterizations and theoretical calculations subsequently demonstrate that adding DMAc into the FeCl2 solution effectively reshapes the primary solvation shell of Fe2+ via the Fe2+ -O (DMAc) bond and breaks hydrogen-bonding network of water through intensified H-bond interaction between DMAc and H2 O, thereby affording the Fe anode with enhanced Fe/Fe2+ reversibility and lower freezing point. Consequently, the assembled all-iron RFB achieves an excellent combination of high power density (25mWcm-2 ), long charge-discharge cycling stability (95.59% capacity retention in 103h), and preeminent battery efficiency at -20°C (95% coulombic efficiency), which promise a future for wider temperature range operation of all-iron RFBs.

Resonance and relaxation effects governing the sorption of O2 and CO2 in deionized water under dynamic conditions at 21 ± 1 °C

The behavior of water samples (deionized, deaerated and washed with He) was investigated in dynamics by physicochemical and electrochemical methods. The susceptibility of water samples to sorption of O2 and CO2 from air depends on their pretreatment. Sorption of O2 and CO2 accompanied by the desorption of dissolved helium is a multivariable nonlinear process, presumably, based on resonance and relaxation phenomena within the surface layer. Sorption of O2 and CO2 in water is considered as an interaction between gas molecules and ensembles of H2O molecules, which is assumed to be based on resonance effects. Sorption of gases leads to a, decrease in the surface tension at the water/gas interface. The effect is most pronounced in case of He-washed samples. The decrease in the surface energy upon CO2 absorption and the increase in the relaxation properties of the water surface are probably factors that facilitate sorption of gases, including O2. A fraction of sorbed O2 are contained in nanobubbles formed via water surface relaxation. A change in the viscosity of water samples during the sorption of gases may serve as a relaxation factor favorably affecting the formation and stability of nanobubbles in the surface layer and in bulk water. Irradiation of water with a laser light at λ = 633 nm can generate nano-cavities in the laser beam. In case of very low dissolved O2 concentrations, nanobubbles have no noticeable effect on the oxygen reduction reaction. Water samples washed with helium demonstrate a higher physical and chemical activity compared to other water samples, the activity being manifested in sorption properties. Electrochemical properties of water samples - deionized, deaerated and washed with He - are somewhat different. This is primarily due to the different content of dissolved O2 in the samples. Nonempirical simulations helped in clarifying the experimental data. The key factor that predetermines stability and/or collapse of nanobubbles is the organization of the hydrogen-bond network of water that strongly depends on the presence of water ions and foreign particles. In the case of both deionized water and water washed with He, HCO3-anion cannot participate in stabilization of nanobubbles. On the other hand, OH– anions can stabilize nanobubbles.

Structure evolution at the gate-tunable suspended graphene-water interface.

Graphitic electrode is commonly used in electrochemical reactions owing to its excellent in-plane conductivity, structural robustness and cost efficiency1,2. It serves as prime electrocatalyst support as well as a layered intercalation matrix2,3, with wide applications in energy conversion and storage1,4. Being the two-dimensional building block of graphite, graphene shares similar chemical properties with graphite1,2, and its unique physical and chemical properties offer more varieties and tunability for developing state-of-the-art graphitic devices5-7. Hence it serves as an ideal platform to investigate the microscopic structure and reaction kinetics at the graphitic-electrode interfaces. Unfortunately, graphene is susceptible to various extrinsic factors, such as substrate effect8-10, causing much confusion and controversy7,8,10,11. Hereby we have obtained centimetre-sized substrate-free monolayer graphene suspended on aqueous electrolyte surface with gate tunability. Using sum-frequency spectroscopy, here we show the structural evolution versus the gate voltage at the graphene-water interface. The hydrogen-bond network of water in the Stern layer is barely changed within the water-electrolysis window but undergoes notable change when switching on the electrochemical reactions. The dangling O-H bond protruding at the graphene-water interface disappears at the onset of the hydrogen evolution reaction, signifying a marked structural change on the topmost layer owing to excess intermediate species next to the electrode. The large-size suspended pristine graphene offers a new platform to unravel the microscopic processes at the graphitic-electrode interfaces.