Water Dissociation Mechanism Research Articles

A systematic analysis and optimization of bicarbonate electrolysis based on a bipolar membrane through multiscale simulation

Bicarbonate electrolysis facilitated by bipolar membrane has emerged as a compelling alternative to gaseous CO2 electrolysis, primarily because of its enhanced product concentration and streamlined processes. However, despite increasing attention, a comprehensive understanding of bicarbonate electrolysis has been eclipsed by the complexity of the intertwined electrode reactions and water dissociation promotion mechanisms of bipolar membranes. A physics-based modeling approach has substantial potential to untangle the impact of intertwined physicochemical processes and expedite the optimization of bicarbonate electrolysis. In this study, a multiscale model coupled with a 1D bipolar membrane and a 2D cathode model is presented to explore the effects of bipolar membranes on cathode reactions and performance optimization using H+ flux as a bridge. For a bipolar membrane, the attenuation mechanism of water dissociation involving ohmic and concentration losses is presented and described using mathematical eqs. A quantification metric, the water dissociation efficiency of a bipolar membrane, is proposed to denote the degree of promoted water dissociation. Two noteworthy parameters related to the fixed functional group concentration are determined and detailed: the onset current density required to stimulate water dissociation and the limited water dissociation efficiency caused by attenuation. For the cathode, a synergistic enhancement mechanism among the operating parameters, membrane properties, and cathode properties is identified. The combined effect of these parameters on the electrolytic performance surpasses the sum of their individual enhancements. Following parameter optimization, there is a notable increase in the current density and Faraday efficiency of bicarbonate electrolysis by 68.6 mA/cm2 and 18.3%, respectively. This study provides a holistic understanding of bicarbonate electrolysis and substantial guidance for its future design.

Bipolar Membrane Seawater Splitting for Hydrogen Production: A Review.

The growing demand for clean energy has spurred the quest for sustainable alternatives to fossil fuels. Hydrogen has emerged as a promising candidate with its exceptional heating value and zero emissions upon combustion. However, conventional hydrogen production methods contribute to CO2 emissions, necessitating environmentally friendly alternatives. With its vast potential, seawater has garnered attention as a valuable resource for hydrogen production, especially in arid coastal regions with surplus renewable energy. Direct seawater electrolysis presents a viable option, although it faces challenges such as corrosion, competing reactions, and the presence of various impurities. To enhance the seawater electrolysis efficiency and overcome these challenges, researchers have turned to bipolar membranes (BPMs). These membranes create two distinct pH environments and selectively facilitate water dissociation by allowing the passage of protons and hydroxide ions, while acting as a barrier to cations and anions. Moreover, the presence of catalysts at the BPM junction or interface can further accelerate water dissociation. Alongside the thermodynamic potential, the efficiency of the system is significantly influenced by the water dissociation potential of BPMs. By exploiting these unique properties, BPMs offer a promising solution to improve the overall efficiency of seawater electrolysis processes. This paper reviews BPM electrolysis, including the water dissociation mechanism, recent advancements in BPM synthesis, and the challenges encountered in seawater electrolysis. Furthermore, it explores promising strategies to optimize the water dissociation reaction in BPMs, paving the way for sustainable hydrogen production from seawater.

Comparison of H2O Adsorption and Dissociation Behaviors on Rutile (110) and Anatase (101) Surfaces Based on ReaxFF Molecular Dynamics Simulation.

The relationship between structure and reactivity plays a dominant role in water dissociation on the various TiO2 crystallines. To observe the adsorption and dissociation behavior of H2O, the reaction force field (ReaxFF) is used to investigate the dynamic behavior of H2O on rutile (110) and anatase (101) surfaces in an aqueous environment. Simulation results show that there is a direct proton transfer between the adsorbed H2O (H2Oad) and the bridging oxygen (Obr) on the rutile (110) surface. Compared with that on the rutile (110) surface, an indirect proton transfer occurs on the anatase (101) surface along the H-bond network from the second layer of water. This different mechanism of water dissociation is determined by the distance between the 5-fold coordinated Ti (Ti5c) and Obr of the rutile and anatase TiO2 surfaces, resulting in the direct or indirect proton transfer. Additionally, the hydrogen bond (H-bond) network plays a crucial role in the adsorption and dissociation of H2O on the TiO2 surface. To describe interfacial water structures between TiO2 and bulk water, the double-layer model is proposed. The first layer is the dissociated H2O on the rutile (110) and anatase (101) surfaces. The second layer forms an ordered water structure adsorbed to the surface Obr or terminal OH group through strong hydrogen bonding (H-bonding). Affected by the H-bond network, the H2O dissociation on the rutile (110) surface is inhibited but that on the anatase (101) surface is promoted.

Mechanism of Water Dissociation on Metal Oxide Surfaces in Bipolar Membrane Electrolyzers: A First-Principles Calculation

Bipolar membranes for water electrolysis, consisting of an acidic cation exchange layer and an alkaline anion exchange layer are a promising means for hydrogen production. The electrolyzer operates the oxygen evolution reaction (OER) under locally basic conditions and the hydrogen evolution reaction (HER) under locally acidic conditions. Water dissociation (WD) occurs within the junction, which is spatially separated from the HER and the OER. It has been shown that the addition of a layer of metal oxide nanoparticles at the junction significantly lowers the overpotential for the process of WD [Science, 2020, 369,1099]. However, the atomistic mechanism by which the metal oxide surface catalyzes the WD is still unclear. We will discuss the theoretical understanding of the WD mechanism at the metal-oxide/water interfaces and investigate the descriptors such as surface pKa, dielectric constants, and adsorption energies, etc. We will combine ab intio molecular dynamics simulations with the electronic structure calculations at the hybrid functional level. Further details of the reaction mechanisms during the WD process will also be explored with the nudged-elastic-band calculations.

Tuning the Interfacial Electrical Field of Bipolar Membranes with Temperature and Electrolyte Concentration for Enhanced Water Dissociation

A coupled experimental and numerical study was performed for a fundamental understanding of the impact of operating conditions, i.e., temperature and electrolyte concentration, as well as interfacial abruptness, on the bipolar membrane (BPM) performance. A comprehensive multiphysics-based model was developed to optimize the operation condition and interfacial properties of BPM, and the model was used to guide the design and engineering of high-performing BPMs. The origin of the enhanced BPM performance at a high temperature was identified, which was attributed to the intrinsic reaction rate enhancement as well as the increase in electrolyte ionic conductivity. The experimentally demonstrated current density–voltage characteristics of BPMs clearly exhibited three distinctive regions of operation: ion-crossover region, water dissociation region, and water-limiting region, which agreed well with the multiphysics simulation results. In addition, the model revealed that a sharper interfacial abruptness led to improved BPM performance due to the enhanced interfacial electric field at the water dissociation region. The decrease of the electrolyte concentration, which increased the dielectric constant of the electrolyte, enhanced the interfacial electric field, leading to improved electrochemical performances. The present study offers an in-depth perspective to understand the species transport as well as water dissociation mechanism under various operation conditions and membrane designs, providing the optimal operation conditions and membrane designs for maximizing the BPM performance at high current densities.

Mechanism of Water Dissociation with an Electric Field and a Graphene Oxide Catalyst in a Bipolar Membrane

Bipolar membranes (BPMs) have recently been incorporated into energy storage devices to increase the overall battery potential and maintain a constant pH gradient by catalyzing internal H2O dissociation. In this Article, we performed a mechanistic and kinetic study of the H2O dissociation reaction on graphene oxide (GO) embedded in BPM using Car–Parrinello molecular dynamics (CPMD) and CPMD-based metadynamics methods. The synergistic effect of active sites (*OH, *C–O–C, and *C═C) on the GO catalyst and the electric field (E) strength across the GO catalyst were investigated for H2O splitting in a BPM. The results indicate the dominant activity of surface *OH sites, providing the lowest activation barrier (0.57 eV) among different sites on GO. Moreover, a higher E (>108 V/m) will significantly facilitate the dissociation reaction by polarizing the H–O bond in H2O. Our findings provide avenues for improving the BPM efficiency to achieve higher energy and power densities for next-generation energy storages.

Review on electrochemical carbon dioxide capture and transformation with bipolar membranes

Anthropogenic carbon dioxide (CO2) emission from the combustion of fossil fuels aggravates the global greenhouse effect. The implementation of CO2 capture and transformation technologies have recently received great attention for providing a pathway in dealing with global climate change. Among these technologies, electrochemical CO2 capture technology has attracted wide attention because of its environmental friendliness and flexible operating processes. Bipolar membranes (BPMs) are considered as one of the key components in electrochemical devices, especially for electrochemical CO2 reduction and electrodialysis devices. BPMs create an alkaline environment for CO2 capture and a stable pH environment for electrocatalysis on a single electrode. The key to CO2 capture in these devices is to understand the water dissociation mechanism occurring in BPMs, which could be used for optimizing the operating conditions for CO2 capture and transformation. In this paper, the references and technologies of electrochemical CO2 capture based on BPMs are reviewed in detail, thus the challenges and opportunities are also discussed for the development of more efficient, sustainable and practical CO2 capture and transformation based on BPMs.

Nanoporous nickel with rich adsorbed oxygen for efficient alkaline hydrogen evolution electrocatalysis

Sluggish water dissociation kinetics severely limits the rate of alkaline electrocatalytic hydrogen evolution reaction (HER). Therefore, finding highly active electrocatalysts and clarifying the mechanism of water dissociation are challenging but important. In this study, we report an integrated nanoporous nickel (np-Ni) catalyst with high alkaline HER performance and the origin of the corresponding enhanced catalytic activity. In 1 mol L−1 KOH solution, this np-Ni electrode shows an HER overpotential of 20 mV at 10 mA cm−2, along with fast water dissociation kinetics. The excellent performance is not only attributed to the large surface area provided by the three-dimensional interconnected conductive network but also from the enhanced intrinsic activity induced by the unique surface properties. Further studies reveal that the types of oxygen species that naturally form on the Ni surface play a key role in water dissociation. Remarkably, when the lattice oxygen almost disappears, the Ni surface terminates with adsorbed oxygen (Oads), exhibiting the fastest water dissociation kinetics. Density functional theory calculation suggests that when Oads acts as the surface termination of Ni metal, the orientation and configuration of polar water molecules are strongly affected by Oads. Finally, the H—OH bond of interfacial water molecules is effectively activated in a manner similar to hydrogen bonding. This work not only identifies a high-performance and low-cost electrocatalyst but also provides new insights into the chemical processes underlying water dissociation, thus benefiting the rational design of electrocatalysts.

Development and Application of a ReaxFF Reactive Force Field for Cerium Oxide/Water Interfaces.

Ceria (CeO2) is a well-known catalytic oxide with many environmental, energy production, and industrial applications, most of them involving water as a reactant, byproduct, solvent, or simple spectator. In this work, we parameterized a Ce/O/H ReaxFF for the study of ceria and ceria/water interfaces. The parameters were fitted to an ab initio training set obtained at the DFT/PBE0 level, including the structures, cohesive energies, and elastic properties of the crystalline phases Ce, CeO2, and Ce2O3; the O-defective structures and energies of vacancy formation on CeO2 bulk and CeO2 (111) surface, as well as the absorption and reaction energies of H2 and H2O molecules on CeO2 (111). The new potential reproduced reasonably well all the fitted properties as well as the relative stabilities of the different ceria surfaces, the oxygen vacancies formation, and the energies and structures of associative and dissociative water molecules on them. Molecular dynamics simulations of the liquid water on the CeO2 (111) and CeO2 (100) surfaces were carried out to study the coverage and the mechanism of water dissociation. After equilibration, on average, 35% of surface sites of CeO2 (111) are hydroxylated whereas 15% of them are saturated with molecular water associatively adsorbed. As for the CeO2 (100) surface, we observed that water preferentially dissociates covering 90% of the available surface sites in excellent agreement with recent experimental findings.

Modeling Water Electrolysis in Bipolar Membranes

Bipolar membranes (BPMs) have proven useful in numerous electrochemical energy conversion and storage applications, including fuel cells and electrolyzers. However, water dissociation in bipolar membrane electrolysis cells (BPMECs) is a complicated phenomenon that occurs via several different pathways. In this work, we develop a model based on the Poisson-Nernst-Planck system that includes a multistep water-dissociation mechanism to observe the fundamental processes that contribute to BPMEC performance. The model, which is validated to in-house experimental data, demonstrates that the junction potential is the most significant contributor to the total electrolysis voltage. We investigated the effects of water-dissociation catalysts and found that the optimal catalyst depends on how the catalyst is integrated into the BPM (although values near 7 are typically best, in accordance with conventional wisdom). We also simulated the water content across the BPM and found that dry-out is not a significant issue when the membrane is in contact with liquid water on both sides. The species conservation approach taken here leads to a physical understanding of the system without using any fitting parameters.

Water Dissociation and Hydroxyl Ordering on Anatase TiO_{2}(001)-(1×4).

We studied the interaction of water with the anatase TiO_{2}(001) surface by means of scanning tunneling microscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. Water adsorbs dissociatively on the ridges of a (1×4) reconstructed surface, resulting in a (3×4) periodic structure of hydroxyl pairs. We observed this process at 120K, and the created hydroxyls desorb from the surface by recombination to water, which occurs below 300K. Our calculations reveal the water dissociation mechanism and uncover a very pronounced dependence on the coverage. This strong coverage dependence is explained through water-induced reconstruction on anatase TiO_{2}(001)-(1×4). The high intrinsic reactivity of the anatase TiO_{2}(001) surface towards water observed here is fundamentally different from that seen on other surfaces of titania and may explain its high catalytic activity in heterogeneous catalysis and photocatalysis.

Lubricant Effects on White Etching Cracking Failures in Thrust Bearing Rig Tests

White etching cracking (WEC) is a subsurface bearing failure mechanism influenced by a number of factors, including lubricant composition. Certain metal-containing lubricants have been reported to promote WEC-induced failure; however, the exact mechanisms linking lubricant effects on WEC propensity are still not fully understood. An interesting field that has not been elucidated is the influence of additive concentration and tribofilm growth on WEC initiation, propagation, and failure. The investigations conducted in this work involved two series of oil formulations: one with additives that give rise to WEC (WEC oils) in different combinations and concentrations and another with additives that do not cause WEC (non-WEC oils). A mini traction machine (MTM) in combination with a spacer layer imaging machine (SLIM) was employed to study the growth of tribofilms and their influence on friction response. Insights from the MTM-SLIM study allowed for better interpretation of FE8 bearing tests. When using oils that contribute to WEC formation, the tribofilm-induced WEC mechanism was confirmed, with cracks initiating as early as after 20 h of FE8 testing. Metal-containing additives were found to favor the formation of WECs by generating a high-friction tribofilm and increasing the water content in the lubricant. Furthermore, the source of subsurface H associated with WEC failure is investigated using heavy water (D2O)-saturated oil. A mechanism of water dissociation induced in tribofilm growth (incubation period) is proposed in this article.

High Reactivity of the ZnO(0001) Polar Surface: The Role of Oxygen Adatoms

Understanding the mechanism of water dissociation on metal oxide surfaces is of particular interest in catalytic reactions. In this work, the interaction of water with the ZnO(0001) polar surface is investigated, and the role of oxygen adatoms in water splitting is uncovered. The individual surface energies and electronic properties of ZnO polar surfaces are investigated on the basis of density functional theory calculations. The oxygen adatoms on the ZnO(0001) surface introduce in-gap surface states, resulting in a direct-to-indirect band gap transition. Water strongly interacts with oxygen adatoms to spontaneously form hydroxyl groups, recovering the direct band-gap characteristics of ZnO polar surfaces. Furthermore, water prefers to adsorb at the step edges of cavities after all of the oxygen adatoms are consumed, and the hydrogen-bonded network among water molecules triggers the dissociation of water at the edge sites, which is also confirmed by molecular dynamics calculations. Our results provide ato...

Surface Fe vacancy defects on haematite and their role in light-induced water splitting in artificial photosynthesis.

Haematite (α-Fe2O3) is a potential candidate for photo-electrochemical water splitting. It is abundant and its electronic properties fit those needed for this kind of device. However, very little is known about the intermediate steps of this photon-induced water splitting process. We propose here that surface iron vacancies can be the main defects responsible for the activity of haematite in the photoelectrochemical reaction. We perform DFT+U calculations and explicitly add holes to show that these defects are common in iron-terminated (0001) surfaces. As holes tend to be localized at these centers, they should be available for the dissociation of water under sunlight. Our calculations also reveal that the water adsorption energy close to the vacancy is 1 eV stronger than far from it, and when the formation of multi-holes is considered, a thermodynamically stable water dissociation mechanism can be developed. We determined that both Fe[double bond, length as m-dash]O and Fe-OOH intermediate steps are stable, although Fe-OOH quickly leads to the formation of O2, having therefore a very short lifetime. Phonon calculations on these structures reveal the appearance of peaks in the 800-900 cm-1 frequency range only for the intermediate steps, connected to Fe[double bond, length as m-dash]O vibrations, in agreement with recent measurements.

Cluster assisted water dissociation mechanism in MOF-74 and controlling it using helium

We show that the water dissociation reaction H2O → OH + H in the confined environment of MOF-74 channels can be precisely controlled by the addition of noble gas He.

Alumina-supported Rh, Rh2, and RhI(CO) as catalysts for hydrogen evolution from water

The hydrogen evolution activity of hydroxylated supported rhodium on alumina surface is investigated by ab initio molecular dynamics techniques. We investigate here the details of the mechanism of water dissociation and hydrogen evolution reaction of Rhn (n=1,2) and RhI(CO) supported on the (0001) α- and (110) γ-Al2O3 surfaces. We find that H2 evolution reaction is feasible using these catalysts except supported RhI(CO). Insights into the effects of oxidation state, surface hydroxylation, surface structure, and cluster size on the hydrogen evolution reaction are reported.

Transfer of electrolyte ions and water dissociation in anion-exchange membranes under intense current conditions

Polarization characteristics of electromembrane systems (EMS) based on the Russian commercial heterogeneous membranes MA-40 and MA-41, the anion-exchange heterogeneous membrane AMH (Mega, Czech Republic), and the modified membrane MA-40M are studied by the method of rotating membrane disk in dilute sodium chloride solutions. The effective transport numbers of ions are found; the partial voltammetric characteristics (VAC) with respect to chloride and hydroxyl ions are measured; the limiting current densities are calculated as a function of the membrane disk rotation rate. In terms of the theory of the overlimiting state of EMS, based on experimental VAC and the dependences of the effective transport numbers on the current density, the following internal parameters of systems under study are calculated: the space charge and electric field strength distribution over the diffusion layer and the membrane. It is shown that water dissociation can be virtually completely eliminated by substituting chemically stable quaternary ammonium groups inert with respect to water dissociation in the surface layer of a heterogeneous anion-exchange membrane MA-40 for the active ternary and secondary functional amino groups. The maximum electric field strength values at the membrane/solution interface, which were found in the framework of the theory of over-limiting state, turned out to be close for all anion-exchange membranes studied, namely, (7–9) × 106 V/cm. This suggests that it is the nature of ionogenic groups in the surface layer rather than the field effect that plays the decisive role in the membrane ability to accelerate the water dissociation reaction. It is proved experimentally that in highly intense current modes of the electrodialysis process, the thermal hydrolysis of quaternary ammonium bases occurs in strongly basic MA-41 and AMH membranes by the Hofmann reaction to form ternary amino groups catalytically active in water dissociation reaction. Based on the concept on the catalytic mechanism of water dissociation, the fraction of ternary amino groups formed by thermal hydrolysis in the surface layer (the space charge region) of monopolar anion-exchange membranes MA-41 and AMH is assessed quantitatively as 0.7 and 6.5%, respectively.

Confined Water Dissociation in Microporous Defective Silicates: Mechanism, Dipole Distribution, and Impact on Substrate Properties

Interest in microporous materials has risen in recent years, as they offer a confined environment that is optimal to enhance chemical reactions. Calcium silicate hydrate (C-S-H) gel, the main component of cement, presents a layered structure with sub-nanometer-size disordered pores filled with water and cations. The size of the pores and the hydrophilicity of the environment make C-S-H gel an excellent system to study the possibility of confined water reactions. To investigate it, we have performed molecular dynamics simulations using the ReaxFF force field. The results show that water does dissociate to form hydroxyl groups. We have analyzed the water dissociation mechanism, as well as the changes in the structure and water affinity of the C-S-H matrix and water polarization, comparing the results with the behavior of water in a defective zeolite. Finally, we establish a relationship between water dissociation in C-S-H gel and the increase of hardness due to a transformation from a two- to a three-dimensional structure.

Nickel-Containing Effluent Reclaming by Bipolar Membrane-Electrodeionization Process

Electrodeionization(EDI) is an electro-membrane process with high efficiency for deep desalinating, where water dissociation mechanism occurring on the surface of ion exchange membranes and resins typically exsits and acts. But when used to treat heavy metal-containing solutions such as effluents from the electronic industry, the EDI will suffer from metal-hydroxides precipitation which would destroy the separation process irreversible. With the primary aim to solve that problem, an electrodeionization process coupled with bipolar membrane (BMEDI) was developed together with a preacidification method. The effects of separating and concentrating heavy metal ions by the BMEDI were studied with NiSO4 solution containing 24 mg×L-1 Ni2+ ions. The results suggested that, the nickel-concentration in the dilute products were both below the analysis limit at constant current densities of 40 mA·cm-2 and 10 mA·cm-2, while a concentration factor of 31 was achieved at 40 mA·cm-2. In addition, no Ni(OH)2 precipitation was found in all the compartments throughout the experiments, which provide a guarantee of good stability for long-time operation.

Research on a Kind of New Green Precision and Micro-Manufacturing Technology

In order to develop the green manufacturing and microfabrication, an innovative micro-electrochemical machining (ECM) method¬¬-the electrochemical machining in pure water was studied. On the base of experimentation principle, ECM set up was developed for engineering application. Water dissociation mechanism is found under the function of cation exchange membrane, and many factors to enhance current density were analyzed. As a result of technological test, a micro hole is machined in the thin stainless. It is confirmed that electrochemical machining technology is feasibility to make use of pure water as electrolyte.