Interfacial Water Structure Research Articles

In-situ Raman Observation on Gas Diffusion Electrode/Polyelectrolyte Interface

Polyelectrolytes (PEs) serve as the critical medium for electrochemical reactions in membrane-based electrochemical devices, such as fuel cells and membrane electrolyzers. To optimize membrane-based electrochemical device performance and elucidate reaction mechanisms, there is a pressing need for detailed microscopic molecular information at gas diffusion electrode/PE interfaces. In this work, we designed a novel membrane-based-electrochemical-device-like gas diffusion electrode/polyelectrolyte electrochemical in-situ Raman cell. The cell's configuration and gas transfer characteristics closely mimic those of MBEDs under working conditions. We created a Pt/Nafion(s) interface by hot-pressing satellite Au@SiO2-Pt loaded carbon cloth with a Nafion membrane, and used this interface for electrochemical in-situ surface enhanced Raman spectroscopy (SERS) observation, including oxygen adsorption/desorption processes, structure of interfacial water and functional groups of Nafion under Ar. The cell enables negatively polarize the potential down to -1.6 V vs. RHE without stripping of the solid/solid interface, despite vigorous H2 generation. The stability of the interface under extreme conditions demonstrates rapid gas transfer at the interface. This observation underscores the potential of our in-situ Raman cell for studying various gas-involved reactions under conditions that closely resemble those in operational MBEDs.

Hydroxyl-Binding Induced Hydrogen Bond Network Connectivity on Ru-based Catalysts for Efficient Alkaline Hydrogen Oxidation Electrocatalysis.

Understanding the role of adsorbed intermediates at the polarized catalyst-electrolyte interface on the structure of electrical double layer (EDL) is essential for developing highly efficient electrocatalysts. Here, we prepared a series of unconventional face-centered-cubic (fcc) phase Ru-based catalysts (i.e. fcc-Ru, fcc-RuCr, and fcc-RuCrW) by rational tuning the binding energetics of hydroxyl intermediate to engineer the electrochemical interface and boost the performance of alkaline hydrogen oxidation reaction (HOR). The introduction of oxyphilic metals Cr and W can regulate the orbital occupation of Ru, promote the adsorption of hydroxyl species, resulting in an anomalous behavior that HOR performance under alkaline media exceeds acidic media. Experimental results and theoretical calculations unravel that the modulated adsorption of hydroxyl species on the electrode surface are responsible for the reconstruction of interfacial water structure and dynamic evolution of free water molecules from nearest to the electrode surface to above the gap region in the EDL, thereby leading to significantly increased water connectivity and hydrogen bond network. Our work reveals a new understanding of the surface intermediates in controlling the dynamic process of interfacial water and hydrogen bonding network in HOR electrocatalysis, and will guide rational design of advanced electrocatalysts through electrochemical interfacial engineering.

Hydroxyl‐Binding Induced Hydrogen Bond Network Connectivity on Ru‐based Catalysts for Efficient Alkaline Hydrogen Oxidation Electrocatalysis

Understanding the role of adsorbed intermediates at the polarized catalyst‐electrolyte interface on the structure of electrical double layer (EDL) is essential for developing highly efficient electrocatalysts. Here, we prepared a series of unconventional face‐centered‐cubic (fcc) phase Ru‐based catalysts (i.e. fcc‐Ru, fcc‐RuCr, and fcc‐RuCrW) by rational tuning the binding energetics of hydroxyl intermediate to engineer the electrochemical interface and boost the performance of alkaline hydrogen oxidation reaction (HOR). The introduction of oxyphilic metals Cr and W can regulate the orbital occupation of Ru, promote the adsorption of hydroxyl species, resulting in an anomalous behavior that HOR performance under alkaline media exceeds acidic media. Experimental results and theoretical calculations unravel that the modulated adsorption of hydroxyl species on the electrode surface are responsible for the reconstruction of interfacial water structure and dynamic evolution of free water molecules from nearest to the electrode surface to above the gap region in the EDL, thereby leading to significantly increased water connectivity and hydrogen bond network. Our work reveals a new understanding of the surface intermediates in controlling the dynamic process of interfacial water and hydrogen bonding network in HOR electrocatalysis, and will guide rational design of advanced electrocatalysts through electrochemical interfacial engineering.

Unveiling the Structure and Dissociation of Interfacial Water on RuO2 for Efficient Acidic Oxygen Evolution Reaction.

Understanding the structure and dynamic process of interfacial water molecules at the catalyst-electrolyte interface on acidic oxygen evolution reaction (OER) kinetics is highly desirable for the development of proton exchange membrane water electrolyzers. Herein, we construct a series of p-block metallic elements (Ga, In, Sn) doped RuO2 catalysts with manipulated electronic structure and Ru-O covalency to investigate the effect of electrochemical interfacial engineering on the improvement of acidic OER activity. Associated with operando attenuated total reflectance surface-enhanced infrared absorption spectroscopy measurements and theoretical analysis, we uncover the free-H2O enriched local environment and dynamic evolution from 4-coordinated hydrogen-bonded water and 2-coordinated hydrogen-bonded water to free-H2O on the surface of Ga-RuO2, are responsible for the optimized connectivity of hydrogen bonding network in the electrical double layer by promoting solvent reorganization. In addition, the structurally ordered interfacial water molecules facilitate high-efficiency proton-coupled electron transfer across the interface, leading to reduced energy barrier of the follow-up dissociation process and enhanced acidic OER performance. This work highlights the key role of structure and dynamic process of interfacial water for acidic OER, and demonstrates the electrochemical interfacial engineering as an efficient strategy to design high-performance electrocatalysts.

Comprehensive review of stabilising factors, demulsification methods, and chemical demulsifiers of oil-water emulsions

With most crude oil extracted in the form of an emulsion, a substantial quantity of interfacially active substances such as asphaltenes and resins adsorbed at the oil–water interface can form a stable interfacial film through intermolecular interactions (hydrogen bonds or π-π stacking), which hinder the coalescence of water droplets. This process improves interface stability, making oil–water separation difficult, thus considerably increasing the petroleum industry’s cost. Hence, comprehending the stability mechanism of oil–water emulsions and destroying the stable oil–water interfacial structure is vital for effective demulsification. The paper systematically reviews types, stability factors, demulsification technologies and principles of oil–water emulsions, summarises common types of demulsifiers, explains the action mechanism of demulsifiers from structure-activity relationships, and proposes future development prospects in this field, thus providing support for formulating demulsification theoretical strategies.

From Graphene Oxide to Graphene: Changes in Interfacial Water Structure and Reactivity Using Deep Neural Network Force Fields

From Graphene Oxide to Graphene: Changes in Interfacial Water Structure and Reactivity Using Deep Neural Network Force Fields

Revealing the molecular structures of α-Al2O3(0001)-water interface by machine learning based computational vibrational spectroscopy.

Solid-water interfaces are crucial to many physical and chemical processes and are extensively studied using surface-specific sum-frequency generation (SFG) spectroscopy. To establish clear correlations between specific spectral signatures and distinct interfacial water structures, theoretical calculations using molecular dynamics (MD) simulations are required. These MD simulations typically need relatively long trajectories (a few nanoseconds) to achieve reliable SFG response function calculations via the dipole moment-polarizability time correlation function. However, the requirement for long trajectories limits the use of computationally expensive techniques, such as abinitio MD (AIMD) simulations, particularly for complex solid-water interfaces. In this work, we present a pathway for calculating vibrational spectra (IR, Raman, and SFG) of solid-water interfaces using machine learning (ML)-accelerated methods. We employ both the dipole moment-polarizability correlation function and the surface-specific velocity-velocity correlation function approaches to calculate SFG spectra. Our results demonstrate the successful acceleration of AIMD simulations and the calculation of SFG spectra using ML methods. This advancement provides an opportunity to calculate SFG spectra for complicated solid-water systems more rapidly and at a lower computational cost with the aid of ML.

Osmolyte-Induced Modulation of Hofmeister Series.

Natural selection has driven the convergence toward a selected set of osmolytes, endowing them with the necessary efficiency to manage stress arising from salt diversity. This study combines atomistic simulations and experiments to investigate how two osmolytes, glycine and betaine, individually modulate the Hofmeister ion ordering of alkali metal salts (LiCl, KCl, and CsCl) near a charged silica interface. Both osmolytes are found to prevent salt-induced aggregation of the charged entities, yet their mode and degree of relative modulation depend on their intricate interplay with specific salt cations. Betaine's ion-mediated surface interaction maintains Hofmeister ion ordering, whereas glycine alters the relative Hofmeister order of the cation by salt-specific ion desorption from the surface. Experimental validation through surface-enhanced Raman spectroscopy supports these findings, elucidating osmolyte-mediated alterations in interfacial water structures. These observations based on an inorganic interface are reciprocated in amyloid beta 40 dimerization dynamics, highlighting osmolytes' efficacy in mitigating salt-induced aggregation. A molecular analysis suggests that the differential modes of interaction, as found here for glycine and betaine, are prevalent across classes of zwitterionic osmolytes.

Water structures and anisotropic dynamics at Pt(211)/water interface revealed by machine learning molecular dynamics

Abstract Water molecules at solid–liquid interfaces play a pivotal role in governing interfacial phenomena that underpin electrochemical and catalytic processes. The organization and behavior of these interfacial water molecules can significantly influence the solvation of ions, the adsorption of reactants, and the kinetics of electrochemical reactions. The stepped structure of Pt surfaces can alter the properties of the interfacial water, thereby modulating the interfacial environment and the resulting surface reactivity. Revealing the in situ details of water structures at these stepped Pt/water interfaces is crucial for understanding the fundamental mechanisms that drive diverse applications in energy conversion and material science. In this work, we have developed a machine learning potential for the Pt(211)/water interface and performed machine learning molecular dynamics simulations. Our findings reveal distinct types of chemisorbed and physisorbed water molecules within the adsorbed layer. Importantly, we identified three unique water pairs that were not observed in the basal plane/water interfaces, which may serve as key precursors for water dissociation. These interfacial water structures contribute to the anisotropic dynamics of the adsorbed water layer. Our study provides molecular-level insights into the anisotropic nature of water behavior at stepped Pt/water interfaces, which can influence the reorientation and distribution of intermediates, molecules, and ions—crucial aspects for understanding electrochemical and catalytic processes.

De Novo Design and Characterization of Amphiphilic Peptides with Basic Side Chains for Tailored Interfacial Chemistries.

Lysine-leucine (LK) peptides have been used as model systems and platforms for 2D material design for decades. LK peptides are amphiphilic sequences designed to bind and fold at hydrophobic surfaces through hydrophobic leucine side chains and hydrophilic lysine side chains extending into the aqueous subphase. The hydrophobic periodicity of the sequence dictates the secondary structure at the interface. This robust design makes them ideal candidates for controlling interfacial chemistry. This study presents the de novo design and characterization of two novel peptides: LRα14 and LHα14, which substitute lysine with arginine and histidine, respectively, in the helical LKα14 sequence. This modification is intended to expand the LK peptide platform to a new basic interfacial chemistry. We explore the stability of the new LRα14 and LHα14 designs with respect to changes in pH and salt concentration in bulk solution and at the interface using circular dichroism (UV-CD) and vibrational sum-frequency generation spectroscopy, respectively. Notably, the structural stability of the peptides remains unaffected across a wide range of pH and ionic strength values. At the same time, the variation of side-chain chemistry leads to a wide spectrum of interfacial water structures. By extension of the LK platform to include arginine and histidine, this study broadens the toolbox for designing tailored interfacial chemistries with applications in material and biomedical sciences.

Role of Interfacial Water Structure in the Electroreduction of NO over Cu2O.

The electrochemical nitric oxide reduction reaction (NORR), which utilizes water as the sole hydrogen source, has the potential to facilitate ammonia production while concurrently mitigating pollutants. However, limited research has been dedicated to characterizing the structure of interfacial water due to the challenges associated with probing this intricate system, impeding the development of more efficient catalysts for the NORR process. Herein, the Cu2O microcrystals with distinct exposed facets, including {100}, {110}, and {111}, are employed for the model catalysts to investigate interfacial water structure and intermediate species in the NORR process. The results from shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) indicated that the NORR performance in 0.1 M Na2SO4 (with heavy water as the solvent) was positively correlated to the proportion of hydrated Na+ ion water. In addition, a sequence of intermediates from the NORR, including *NOH, *NH, *NH2, and *NH3, was detected by employing a combination of multiple in situ characterization methods. Furthermore, in conjunction with experimental results and theoretical calculations, we revealed the potential reaction pathway of NORR. This study offers novel insights into the NORR mechanism and valuable guidance for the design of high-performance catalysts for ammonia production.

Bimetallic nanoalloys planted on super-hydrophilic carbon nanocages featuring tip-intensified hydrogen evolution electrocatalysis

The insufficient availability and activity of interfacial water remain a major challenge for alkaline hydrogen evolution reaction (HER). Here, we propose an “on-site disruption and near-site compensation” strategy to reform the interfacial water hydrogen bonding network via deliberate cation penetration and catalyst support engineering. This concept is validated using tip-like bimetallic RuNi nanoalloys planted on super-hydrophilic and high-curvature carbon nanocages (RuNi/NC). Theoretical simulations suggest that tip-induced localized concentration of hydrated K+ facilitates optimization of interfacial water dynamics and intermediate adsorption. In situ synchrotron X-ray spectroscopy endorses an H* spillover-bridged Volmer‒Tafel mechanism synergistically relayed between Ru and Ni. Consequently, RuNi/NC exhibits low overpotential of 12 mV and high durability of 1600 h at 10 mA cm‒2 for alkaline HER, and demonstrates high performance in both water electrolysis and chlor-alkali electrolysis. This strategy offers a microscopic perspective on catalyst design for manipulation of the local interfacial water structure toward enhanced HER kinetics.

Understanding the interfacial water structure in electrocatalysis

The structure of interfacial water molecules plays a crucial role in modulating the electrochemical surface kinetics. This article provides an in-depth understanding of the water molecule structure inside the double layer and its main influencing factors at the molecular scale.

Robust Electrocatalytic CO2 Reduction in Acid Enabled by "Molecularly Charged" Cobalt Phthalocyanine: A Profound Understanding from Electric Double Layer.

Electrocatalytic CO2 reduction (eCO2R) in acid holds promise in renewable electricity-powered CO2 utilization with high efficiency, but the hydrogen evolution reaction (HER) often prevails and results in a low eCO2R selectivity. Here, using cobalt phthalocyanine/Ketjen black (CoPc/KB) as the model catalysts, we systematically study the effect of active site density, operational current density, and hydrated cations on the acidic eCO2R selectivity and decipher it through the componential dynamics of electric double layer (EDL). The optimal CoPc-4/KB demonstrates a near-unity CO Faradaic efficiency from 50 to 400 mA cm-2 and superb operational stability (>120 h) at 100 mA cm-2. Aided by in situ Raman and infrared spectroscopies, we reveal that the proper cations establish an electrostatic shield for mitigating bulk H+ penetration and mediate the interfacial water structure for suppressing HER. This study should elicit further profound thinking on robust eCO2R system design from the perspective of multiphasic and dynamic EDL.

Interfacial Water Is Separated from a Hydrophobic Silica Surface by a Gap of 1.2 nm.

The interaction of liquid water with hydrophobic surfaces is ubiquitous in life and technology. Yet, the molecular structure of interfacial liquid water on these surfaces is not known. By using a 3D atomic force microscope, we characterize with angstrom resolution the structure of interfacial liquid water on hydrophobic and hydrophilic silica surfaces. The combination of 3D AFM images and molecular dynamics simulations reveals that next to a hydrophobic silica surface, there is a 1.2 nm region characterized by a very low density of water. In contrast, the 3D AFM images obtained of a hydrophilic silica surface reveal the presence of hydration layers next to the surface. The gap observed on hydrophobic silica surfaces is filled with two-to-three layers of straight-chain alkanes. We developed a 2D Ising model that explains the formation of a continuous hydrocarbon layer on hydrophobic silica surfaces.

Ion-specific ice provides a facile approach for reducing ice friction

HypothesisIce friction plays a crucial role in both basic study and practical use. Various strategies for controlling ice friction have been developed. However, one unsolved puzzle regarding ice friction is the effect of ion–ice interplay on its tribological properties. Experiments and SimulationsHere, we conducted ice friction experiments and summarized the specific effects of hydrated ions on ice friction. By selecting cations and anions, the coefficient of ice friction can be reduced by more than 70 percent. Experimental spectra, low-field nuclear magnetic resonance (LF-NMR), density functional theory (DFT) calculations, and Molecular dynamics (MD) simulations demonstrated that the addition of ions could break the H-bonds in water. FindingsThe link between the charge density of ions and the coefficients of ice friction was revealed. A part of the ice structure was changed from an ice-like to a liquid-like interfacial water structure with the addition of ions. Lower charge density ions led to weaker ionic forces with the water molecules in the immobilized water layer, resulting in free water molecules increasing in the lubricating layer. This study provides guidance for preparing ice-making solutions with low friction coefficients and a fuller understanding of the interfacial water structure at low temperatures.

Synergistic coupling of defective Ru nanoclusters with oxygen-ligand-steered Ru single-atoms modifying interfacial water structure and key intermediates bonding toward efficient hydrogen energy conversion

Constructing highly efficient ruthenium (Ru)-based electrocatalysts and understanding their reaction mechanisms for hydrogen energy conversion are highly desirable, but extremely challenge. Herein, we integrate the well-dispersed defective Ru nanoclusters and oxygen-coordinated Ru single atoms into one electrocatalyst (RuSA-O4/RuNC) toward alkaline hydrogen oxidation and evolution reactions (HOR/HER). Synergistic catalytic mechanism of dual-sites has been revealed by in situ ATR-SEIRAS characterization, EDTA/SCN– poisoning experiments and DFT calculations. Benefiting from the electronic redistribution between RuSA-O4 and RuNC sites, the single-atom RuSA-O4 species can act as Lewis acid sites that display strong affinity toward interfacial water via an O-down (H2O↓) conformation and subsequently promote water dissociation/formation as well as hydroxide adsorption. Meanwhile, adjacent defective Ru sites have around thermoneutral value of H adsorption free energy. RuNC works with RuSA-O4 species, collaboratively reducing reaction barriers. As expected, the RuSA-O4/RuNC achieves 20.3 times mass activity of commercial Pt/C for alkaline HOR at 10 mV overpotential and excellent alkaline HER activity with a low overpotential of 22 mV to deliver 10 mA cm−2, as well as considerable HER mass activity of 17.0-times compared to commercial Pt/C at 50 mV overpotential. Besides, the synergistic dual-sites strategy can also be extended to prepare efficient PtSA-O4/PtNC and IrSA-O4/IrNC electrocatalysts.

Orientational dynamics of the water layer adjacent to Au surface accelerated by polarization effect.

The orientation and rearrangement of water on a gold electrode significantly influences its physicochemical heterogeneous performance. Despite numerous experimental and theoretical studies aimed at uncovering the structural characteristics of interfacial water, the orientational behavior resulting from electrode-induced rearrangements remains a subject of ongoing debate. Here, we employed molecular dynamics simulations to investigate the adaptive structure and dynamics properties of interfacial water on Au(111) and Au(100) surfaces by considering a polarizable model for Au atoms in comparison with the non-polarizable model. Compared to the nonpolarizable systems, the polarization effect can enhance the interaction between water molecules and the gold surface. Unexpectedly, the rotational dynamics directly associated with the orientational behavior of water adjacent to the gold surface is accelerated, thereby reducing the hydrogen bond lifetime. The underlying mechanism for this anomalous phenomenon originates from the polarization effect, which induces the attraction of the positive hydrogen atoms to the surface by the negative image charge. This leads to a change in orientation that disrupts the hydrogen bonds in the first water layer and subsequently accelerates reorientation dynamics of water molecules adjacent to the gold surface. These results shed light on the intricate interplay between polarization effects and water molecule dynamics on metal surfaces, establishing the foundation for the rational regulation of the orientation of interfacial water.

The Dependence of Hydrophobic Interactions on the Shape of Solute Surface.

According to our recent studies on hydrophobicity, this work is aimed at understanding the dependence of hydrophobic interactions on the shape of a solute's surface. It has been observed that dissolved solutes primarily affect the structure of interfacial water, which refers to the top layer of water at the interface between the solute and water. As solutes aggregate in a solution, hydrophobic interactions become closely related to the transition of water molecules from the interfacial region to the bulk water. It is inferred that hydrophobic interactions may depend on the shape of the solute surface. To enhance the strength of hydrophobic interactions, the solutes tend to aggregate, thereby minimizing their surface area-to-volume ratio. This also suggests that hydrophobic interactions may exhibit directional characteristics. Moreover, this phenomenon can be supported by calculated potential mean forces (PMFs) using molecular dynamics (MD) simulations, where different surfaces, such as convex, flat, or concave, are associated with a sphere. Furthermore, this concept can be extended to comprehend the molecular packing parameter, commonly utilized in studying the self-assembly behavior of amphiphilic molecules in aqueous solutions.

How valinomycin transports water across the bilayer: effect of the cation in the electrolyte

Surface-enhanced infrared absorption spectroscopy (SEIRAS) was employed to investigate the structure of the interfacial water in the 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC)/valinomycin floating bilayer on the gold surface in two electrolytes with different cations, KClO4 and NaClO4. The SEIRAS studies indicate that interfacial water adopts different structures in the bilayer with different cations. The structure of interfacial water in the DPhPC/valinomycin bilayer in the NaClO4 solution is similar to water in the DPhPC bilayer (without valinomycin), while the structure of water in the bilayer in the KClO4 solution has a high amount of disordered hydrogen bond network. The results of the present study confirm that valinomycin selectively transports K+ across the lipid bilayer as an ion pair and that the ion pair is bonded to residual water that is released into the submembrane region.