Adsorption Of Hydroxyl Ions Research Articles

Enhanced oxygen evolution reaction in flexoelectric thin-film heterostructures

Recently, the flexoelectric effect has triggered considerable interest in energy-related applications, such as flexo-actuation, flexo-photovoltaic, and flexo-catalysis, because of its ubiquitous feature allowing the creation of electric polarity, i.e., the flexoelectric polarization (Pflexo), in non-polar materials by strain gradient. Here, we show a flexoelectric strategy in electrocatalytic water splitting. Remarkably enhanced oxygen evolution reaction (OER) properties are achieved in strain-gradient LaFeO3 (LFO) thin-film heterostructures owing to the promotion of kinetic processes by Pflexo. The improved OER is demonstrated by increased current density of ∼300% in linear sweep voltammetry and lowered charge transfer resistance by two orders of magnitude in electrochemical impedance spectroscopy. These are ascribed to the flexoelectric-induced downward bending of the LFO band, as revealed by density functional theory calculations and band structure measurements. With Pflexo in the thin-film heterostructure catalysts, the adsorption of hydroxyl ions is strengthened on the polar LFO surface, and the transfer of electrons is accelerated from the reactants/key intermediates to the catalyst across the band-tilted LFO layer. These findings indicate the significance of flexoelectric effect in OER kinetics and open a new perspective for exploiting catalytic mechanisms and performances in water splitting.

Electron-interactive 1D porous vertical NiCo2O4 nanowire and 3D N-doped graphene aerogel enable high hydroxyl-ion adsorption capacity for superior energy storage

Supercapacitors are a kind of highly efficient energy-storage device with long cycle life and high power density, however, their specific capacitance is insufficient for further application and development. In this work, a novel composite with one-dimensional (1D) NiCo2O4 nanowire arrays (NiCo2O4-NWA) vertically supported on three-dimensional (3D) nitrogen-doped graphene aerogel (NGA) is designed and facilely realized by hydrothermal reaction and thermal treatment. The NiCo2O4 nanowires, with length of ∼1 μm and diameter of ∼25 nm, display a porous structure and are regularly arranged on both sides of the sheets in nitrogen-doped graphene aerogel. The aerogel-supporting NiCo2O4 nanowires demonstrate a remarkably high specific capacitance of 2742 F g−1 at 1 A g−1, and an outstanding cyclic stability with capacitance retention of 90% even after 10,000 cycles at 80 A g−1. The density functional theory calculation shows the narrow band gap (0.32 eV) and strong electronic interaction and hydroxyl-ion adsorption effect of NiCo2O4 supported on NGA, resulting in superior specific capacitance and energy storage performance. The 3D nitrogen-doped graphene aerogel supporting 1D nanowire arrays represent a promising high-performance electrode material for application in the supercapacitors and other electrochemical energy-conversion and storage equipment.

Effects of carbonate on ferrihydrite transformation in alkaline media.

Alkaline media widely exist in natural and engineered systems such as semiarid/arid areas, radioactive waste sites, and mine tailings. In these settings, the commonly occurring iron (oxyhydr)oxides differed in their ability to influence the fate of nutrients and contaminants. Due to the substantially increased atmospheric carbon dioxide (CO2) concentration, carbonate stands to increase in these media. However, how increasing carbonate affects the transformation of poorly crystalline iron (oxyhydr)oxides (e.g., two-line ferrihydrite) under alkaline conditions still remains unclear. Here, kinetics of ferrihydrite transformation were evaluated at pH ∼10 as a function of [carbonate] = 0-286 mM using synchrotron-based X-ray and vibrational spectroscopic techniques. The results showed that carbonate slowed down ferrihydrite transformation slightly and suppressed goethite formation, but promoted hematite formation regardless of its concentration. At low carbonate concentration (11.42 mM), the effect of carbonate on product formation was obvious due to the weak inner-sphere complex; however, at high carbonate concentration (80-286 mM), the effect was retarded because of the adsorption equilibrium of carbonate as well as the initial carbonate adsorption followed by desorption. Moreover, carbonate modified the morphology of hematite from rhombic to ellipsoidal to honeycomb and goethite from rod-like to needle-like to spindle-like due to the inner-sphere adsorption-desorption of carbonate and adsorption of hydroxyl ions on reactive sites of iron (oxyhydr)oxides in alkaline media. The results suggest that the concurrently increasing carbonate with enhanced atmospheric CO2 could control the transformation and occurrence of iron (oxyhydr)oxides in natural and engineered environments and have important implications for the biogeochemical cycles of iron and carbon.

Green desorption combined with peptization technology for disposing of Cr(VI)-V(V)-containing hazardous aluminum waste to prepare high-valued pseudo-boehmite

Cr(VI)-containing aluminum residues are inevitable industrial hazardous waste in the chromate industry. To reduce and utilize these residues, a combined desorption and peptization technology for disposing of Cr(VI)-V(V)-containing boehmite hazardous waste (γ-AlOOH-Cr-V) with NaOH solutions was proposed, which realized the removal of V(V) and Cr(VI) effectively and the improvement of peptization index (PI) of obtained pseudo-boehmite (PB) synergistically. The results showed that PB with high PI (98.1%) and low content of V(V) (0.0656%) and Cr(VI) (0.0018%) could be achieved by desorbing the γ-AlOOH-Cr-V, while the surface area and pore volume would be decreased during desorption with the increase of NaOH concentration. Besides the leaching toxicity of Cr(VI) of obtained PB of 0.09 mg/L was extremely lower than that of the standard of hazardous waste. It was noted that the evolution of structural hydroxyl groups governed the improvement of peptization performance. DFT results showed that proper interlayer water content was beneficial to the adsorption of hydroxyl ions on the structural hydroxyl groups of PB. This work not only reduced the emission of Cr(VI)-V(V)containing hazardous waste, but also paved the way for the high-valued utilization of co-associated aluminum and vanadium resources in chromite.

A streaming-potential-based microfluidic measurement of surface charge at immiscible liquid-liquid interface

Surface charging at immiscible liquid-liquid interface is essential to the emulsion stability, surfactant adsorption, and various engineering applications such as drug delivery and mineral flotation. However, droplet electrophoresis, as a widely-used electrokinetic method to measure the surface charge density, has various limitations in physical modeling and sample preparation. In this work, an alternative experimental method based on streaming potential setup is proposed. A Y-Y shaped microchannel was used to make a flat and stable liquid-liquid interface. The inner wall was coated with polymer to suppress the interference of the solid-liquid interfacial electrokinetics in the liquid-liquid one. The experimental setup was first verified by revisiting the aqueous solution-silicon surface charging, after which the surfactant-free decane-KCl solution interface charging was investigated. The negative surface charge at the decane-KCl solution interface is confirmed and found to increase when increasing the pH. This result is compatible with the probable charging mechanism that the acquired negative surface charge results from hydroxyl ion adsorption onto the interface. The proposed method enables the simplicity and flexibility for further side-by-side studies on the liquid-liquid interface charging mechanism and will inspire the quantitative macroscopic interfacial modeling for numerous scenarios, such as the droplet electrophoresis and interfacial electro-hydrodynamics.

Novel 2D/2D NiCo2O4/ZnCo2O4@rGO/CNTs self-supporting composite electrode with high hydroxyl ion adsorption capacity for asymmetric supercapacitor

The energy storage device has been urgently studied and developed to meet the increasing demand for energy and sustainable development. Due to the excellent conductivity of graphene and high performance of ZnCo2O4 and NiCo2O4, we design a self-supporting electrode based on vertically grown two-dimensional/two-dimensional (2D/2D) NiCo2O4/ZnCo2O4 hierarchical flakes on the carbon-based conductive substrate (NiCo2O4/ZnCo2O4@graphene/carbon nanotubes, [email protected]). The density functional theory calculations indicate that the high OH− adsorption capacity of the NiCo2O4/ZnCo2O4 nanosheets can significantly enhance the electrochemical reaction activity. [email protected] shows a high capacitance of 1128.6 F g−1 at 1 A g−1. The capacitance retains 84.0% after 6000 cycles even at 10 A g−1. A hybrid supercapacitor is fabricated using [email protected] and activated carbon, exhibiting a large energy density of 50.8 W h kg−1 at the power density of 800 W kg−1. After 9000 charge/discharge cycles, the supercapacitor still has 86.1% capacitance retention. The [email protected] film has showed the potential as promising electrodes in high efficiency electrochemical energy storage devices.

Excellent rate capability supercapacitor electrodes with highly hydroxyl ion adsorption capacity enabled by P-doped MnCo2O4 nanotube arrays

Doping or substitution of non-metallic ions is considered to be an effective way to improve the rate capability and cyclic stability of metal oxide electrodes. Nevertheless, current research lacks in-depth studies on the effect of doping on the rate capability. Herein, P-doped MnCo2O4 nanotubes with vertical array structure are rationally synthesized on carbon fibers via mild solution method and chemical heat treatment strategy. The density functional theory calculation and impedance test indicate that the introduced P atoms in MnCo2O4 are conducive to boost the conductivity and surface reactivity, as well as reaction kinetics. The electrochemicalmeasurementshowthat the novel P-doped MnCo2O4 nanotube electrodes deliver a highest specific capacitance of 996.7 F g−1. It also exhibits excellent rate performance of 81.7% original capacitance retention when the current density is increased by 30 times. An asymmetric supercapacitor P-doped MnCo2O4//activated carbon can realize a high energy density of 42.7 Wh kg−1at 750 W kg−1. Moreover, the device also exhibits a comparable cyclic retention of 81.9% at the end of the 8000th cycle. This work not only offers a potential electrode for energy storage but also provides a strategy for the preparation of hollow nanotubes structure with high electrochemical activities.

Excellent Rate Capability Supercapacitor Electrodes with Highly Hydroxyl Ion Adsorption Capacity Enabled by P-Doped Mnco2o4 Nanotube Arrays

Excellent Rate Capability Supercapacitor Electrodes with Highly Hydroxyl Ion Adsorption Capacity Enabled by P-Doped Mnco2o4 Nanotube Arrays

Novel CoZnNi oxyphosphide-based electrode with high hydroxyl ion adsorption capacity for ultra-high volumetric energy density asymmetric supercapacitor

Achieving a high volumetric energy density supercapacitor is of great significance for portable energy storage devices while still a major challenge. Herein, we design and fabricate self-supporting electrodes using CoZnNi oxyphosphide nanoarrays sandwiched graphene/carbon nanotube (CZNP/GC) film with highly exposed active sites. Benefitting from the modified electronic structures, high accessible surface areas, and the integrated structure, the well-designed CZNP/GC electrode exhibits an ultra-high volumetric capacitance of 2096.4 F cm−3 at a current density of 1 A g−1. Moreover, a high-performance negative electrode of carbon/rGO/CNTs (C/GC) is also fabricated using the same CoZn-metal-organic frameworks precursor. The assembled asymmetric supercapacitor CZNP/GC//C/GC displays an ultra-high volumetric energy density of 71.8 W h L−1 at 960 W L−1. After 6000 charge-discharge cycles, the device still maintains 85.6% of the original capacitance. The density functional theory calculation is studied and the negative adsorption energy proves that the OH– adsoption process onto the surface of as-prepared electrode is thermodynamically favorable, facilitating the electrochemical reaction. This work provides a new option in constructing tailorable electrodes with a well-defined hierarchical structure for supercapacitor and beyond.

Significant influence of controllable surface oxygen vacancies of CuO for enhancing sensitivity of glucose detection

The construction of oxygen vacancies, as one of defects engineering, is a promising strategy to boost the electrocatalytic activity. Herein, CuO nanoflakes enriched with oxygen vacancies were successfully synthesised via a simple one-pot method with stirring. High-angle annular dark-field scanning, X-ray photoelectron spectroscopy and electron spin resonance spectroscopy were performed to confirm the existence of enriched oxygen vacancies. The as-synthesised CuO nanoflakes enriched with oxygen vacancies and exhibited an excellent performance for glucose detection with a lower onset-potential of +0.22 V, ultrahigh sensitivity of 4870 μA mM−1 cm−2 and low detection limit of 0.5 μM. The superior performance of the CuO nanoflakes may be attributed to surface oxygen vacancies, which accelerate the adsorbed hydroxyl ions to promote the formation of Cu3+ species, thereby, reducing the one-potential and facilitating the glucose oxidation. This work would serve the guide for the rational design of other efficient catalysts for glucose detection via constructing oxygen vacancies.

Trifle Pt coupled with NiFe hydroxide synthesized via corrosion engineering to boost the cleavage of water molecule for alkaline water-splitting

Developing cost-effective and highly efficient hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts with small overpotentials and superior long-term stabilities is imperative for catalyzing water electrolysis. Herein, corrosion engineering is applied to synthesize NiFe-LDH incorporating noble metals on a 3D porous Ni foam at room temperature to afford the NF-Na-Fe-Pt electrocatalyst. The optimized balance adsorption of hydroxyl ions and atomic hydrogen that are manifested by the synergism between NiFe-LDH and trifle noble metals endow the nanomaterial with an excellent electrocatalytic performance and long-term durability for mediating the water-splitting process in 1 M KOH. Only 31 and 261 mV are required to drive 10 mA cm−2 of the prepared NF-Na-Fe-Pt electrocatalyst for HER and OER, respectively. DFT calculations demonstrate that Pt and NiFe-LDH synergistically accelerate the Volmer reaction and adsorption/desorption of atomic H to enhance alkaline HER. Moreover, the NF-Na-Fe-Pt electrocatalyst also exhibit remarkable catalytic activity for water splitting.

Nanocoral CuCo2S4 thiospinels: Oxygen evolution reaction via redox interaction of metal ions

Understanding the electrocatalytic activity and redox mechanism in mixed metal sulfides has steered the present work. Mixed metal sulfide, CuCo2S4, synthesised by hydrothermal method, reveals the presence of highly crystalline nanocoral sheets of CuCo2S4 with mixed oxidation states of Co and Cu ions. Electrochemical measurements show superior oxygen evolution reaction (OER) activity and high exchange current density over CuCo2S4 in comparison with pristine (Co3S4 and Cu2S) and sintered CuCo2S4. In 0.5 M KOH, the nanocoral CuCo2S4 catalyst and its sintered form show a current density of 10 mA cm−2 at overpotential of 368 mV and 511 mV, respectively. A close inspection of the electrocatalytic performance in correlation with XPS spectra reveals the synergistic effect of Co and Cu ions through a redox interaction. Density Functional Theory calculations also indicate strong dependence of redox activity on redox couples as accompanied with changes in the partial charges of different constituent ions in the catalyst upon adsorption of hydroxyl ions which is a key step in OER. The study not only details sulfide-based catalytic compound for OER but also provides conclusive insights into the surface redox processes thereby providing an alternative direction to the search on novel OER catalysts.

Patterns in the Organic-Acid Leaching of Fluorine from Rocks

Experiments were carried out on fluorine leaching from rocks (alkaline agpaitic granite, dacite, and feldspar–quartz aleurite with 0.005 M solutions of acetic, tartaric, citric, and oxalic acids. The previously revealed decrease of fluorine leaching with organic acids compared to distilled water under an acidity increase within the 6.3–3.5 range of pH values was confirmed. In terms of leaching ability related to distilled water, the organic acids constitute a series: oxalic > citric > acetic ~ tartaric. It has been proposed that a decrease of pH values causes the sorption of fluorides resulting from the substitution of adsorbed hydroxyl ions. In view of this, the less intense fluorine leaching in acids compared to distilled water might be ascribed to sorption immobilization of fluoride ions on a solid phase. A positive correlation between the levels of leached fluorine and magnesium points to the existence of some general but undetermined mechanism of the transition of these elements from minerals into aquatic solutions.

Regularities of fluorine leaching from the rocks by organic acids

Experiments on fluorine leaching from the rocks (alkaline agpaitic granite, dacite, and feldspathic-quartzy aleurite) by 0.005 M solutions of acetic, tartaric, citric, and oxalic acids were carried out. The previously revealed fact of decrease in the fluorine leaching by organic acids as compared with that by distilled water with an increase in acidity of solutions in the pH range from 6.3 to 3.5 is confirmed. According to the leaching ability in relation to distilled water, organic acids are formed a number: oxalic > citric > acetic ≈ tartaric. It is assumed that decrease in pH leads to sorption of fluorides as a result of the replacement of adsorbed hydroxyl ions. From this point of view, less intensive (compared with distilled water) fluorine leaching in acidic medium can be associated with sorption immobilization of fluoride ions in the solid phase. A positive correlation between amounts of leached fluorine and magnesium indicates the existence of a certain common, as well as not established mechanism for transition of these elements from minerals to aqueous solution.

Bayberry-like Pt nanoparticle decorated ZnO nanorods for the photocatalytic application

The preparation of the nanocomposites zinc oxide/platinum (ZnO/Pt) has attracted much attention with regard to improving the photocatalytic efficiency of ZnO, but most studies have focused on the use of ZnO/Pt in conventional powdered spherical nanocrystals only. In this experiment, bayberry-like Pt nanoparticles (NPs) were successfully synthesized and deposited on the surfaces of ZnO nanorods (NRs) that were hydrothermally grown on glass substrates. The bayberry-like Pt NPs, which had a diameter range of 40–70 nm and consisted of many nanodots with a diameter < 5 nm, were spread uniformly on the surface of the ZnO NRs via the drop-casting method. The presence of the Pt NPs increased the photocatalytic activity of ZnO NRs for methylene blue (MB) degradation by 54% and 32% under UV and visible light irradiation, respectively. This improvement can be attributed to the presence of Pt, which acted as an electron trap and can inhibit the rate of electron–hole recombination. It may also be credited to the morphology of the bayberry-like Pt NPs, which consisted of many nanodots that can trap light in the surroundings of the nanodots, thereby enhancing the light scattering and absorption and enlarging the surface area for oxygen and hydroxyl ion adsorption.

Bulk Nanobubbles from Acoustically Cavitated Aqueous Organic Solvent Mixtures

We investigate the existence and stability of bulk nanobubbles in various aqueous organic solvent mixtures. Bulk nanobubble suspensions generated via acoustic cavitation are characterized in terms of their bubble size distribution, bubble number density, and zeta potential. We show that bulk nanobubbles exist in pure water but do not exist in pure organic solvents, and they disappear at some organic solvent-water ratio. We monitor the nanobubble suspensions over a period of a few months and propose interpretations for the differences behind their long-term stability in pure water versus their long-term stability in aqueous organic solvent solutions. Bulk nanobubbles in pure water are stabilized by their substantial surface charge arising from the adsorption of hydroxyl ions produced by self-ionization of water. Pure organic solvents do not autoionize, and therefore, nanobubbles cannot exist in concentrated aqueous organic solvent solutions. Because of preferential adsorption of organic solvent molecules at the nanobubble interfaces, the surface charge of the nanobubbles decreases with the solvent content, but the strong hydrogen bonding near their interfaces ensures their stability. The mean bubble size increases monotonically with the solvent content, whereas the surface tension of the mixture is sharply reduced. This is in agreement with literature results on macro- and microbubbles in aqueous organic solutions, but it stands in stark contrast to the behavior of macro- and microbubbles in aqueous surfactant solutions.

Surfactant-free exfoliation of multilayer molybdenum disulfide nanosheets in water

In this study, we report a potentially scalable method for producing multilayer molybdenum disulfide (MoS2) nanosheets. The addition of a small amount of ammonia solution can improve the exfoliation of MoS2 nanosheets in water. The surface charge induced by spontaneous adsorption of hydroxyl ions on MoS2 surfaces favors the exfoliation process. The edge charge generated by the ionization of edge-attached groups facilities the dispersion of exfoliated nanosheets in water. It is also found that smaller MoS2 nanosheets show an improved photocatalytic performance, which stems from enhanced edge effects and a reduced flake thickness. This work opens a new vista on preparation and application of multilayer MoS2 nanosheets.

Insight into the Electro-Oxidation Mechanism of Glucose and Other Carbohydrates by CuO-Based Electrodes.

In this work, a new hypothesis for the electrocatalytic behavior of CuO electrodes is presented. Different from the established mechanism, here we discuss why CuIII species do not participate in the oxidation mechanism of carbohydrates. We show that hydroxyl ion adsorption and the semiconductive properties of the material play a more significant role in this process. The relationship between the flat band potential and the potential that begin oxidation suggests that the concentration of vacancies in the charge region acts upon the reactivity of the adsorbed hydroxyl ions through a partial charge transfer reaction. In the presence of carbohydrate molecules, the electron transfer is facilitated and involves the transfer of electrons from the adsorbed hydroxyl ions to the CuO film. This mechanism is fundamentally relevant since it helps the understanding of several experimental misleads. The results can also lead to obtaining better catalysts, since improvements in the material should focus on enhancing the semiconductive properties rather than the CuII/CuIII redox transition. The results shed light on different aspects of carbohydrate molecules oxidation that could lead to novel applications and possibly a better description of other semiconductor mechanisms in electrocatalysis.

Controlled Fabrication of Hierarchically Structured Nitrogen‐Doped Carbon Nanotubes as a Highly Active Bifunctional Oxygen Electrocatalyst

Hierarchically structured nitrogen‐doped carbon nanotube (NCNT) composites, with copper (Cu) nanoparticles embedded uniformly within the nanotube walls and cobalt oxide (CoxOy) nanoparticles decorated on the nanotube surfaces, are fabricated via a combinational process. This process involves the growth of Cu embedded CNTs by low‐ and high‐temperature chemical vapor deposition, post‐treatment with ammonia for nitrogen doping of these CNTs, precipitation‐assisted separation of NCNTs from cobalt nitrate aqueous solution, and finally thermal annealing for CoxOy decoration. Theoretical calculations show that interaction of Cu nanoparticles with CNT walls can effectively decrease the work function of CNT surfaces and improve adsorption of hydroxyl ions onto the CNT surfaces. Thus, the activities of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are significantly enhanced. Because of this benefit, further nitrogen doping, and synergistic coupling between CoxOy and NCNTs, Cu@NCNT/CoxOy composites exhibit ORR activity comparable to that of commercial Pt/C catalysts and high OER activity (outperforming that of IrO2 catalysts). More importantly, the composites display superior long‐term stability for both ORR and OER. This simple but general synthesis protocol can be extended to design and synthesis of other metal/metal oxide systems for fabrication of high‐performance carbon‐based electrocatalysts with multifunctional catalytic activities.

Spontaneous electrification of fluoropolymer-water interfaces probed by electrowetting.

Fluoropolymers are widely used as coatings for their robustness, water-repellence, and chemical inertness. In contact with water, they are known to assume a negative surface charge, which is commonly attributed to adsorbed hydroxyl ions. Here, we demonstrate that a small fraction of these ions permanently sticks to surfaces of Teflon AF and Cytop, two of the most common fluoropolymer materials, upon prolonged exposure to water. Electrowetting measurements carried out after aging in water are used to quantify the density of 'trapped' charge. Values up to -0.07 and -0.2 mC m-2 are found for Teflon AF and for Cytop, respectively, at elevated pH. A similar charge trapping process is also observed upon aging in various non-aqueous polar liquids and in humid air. A careful analysis highlights the complementary nature of electrowetting and streaming potential measurements in quantifying interfacial energy and charge density. We discuss the possible mechanism of charge trapping and highlight the relevance of molecular scale processes for the long term stability and performance of fluoropolymer materials for applications in electrowetting and elsewhere.