Study of CO2-hydrate formation in contact with bulk nanobubbles: An investigation from experiment and molecular-dynamics simulations

HYPOTHESIS

Nanobubbles (NBs) have been extensively investigated as a sustainable promoter for gas hydrate nucleation, which also contribute to the hydrate memory effect. However, less attention afforded to their effects on the hydrate-growth process, thus lacking a complete perspective of the overall effects from NBs on hydrate formation. We hypothesize that their effect on CO2 hydrate growth may vary depending on the properties of NBs.

EXPERIMENTS AND SIMULATIONS

This study investigates CO2-hydrate nucleation and growth with a dual methodology. Laboratory experiments were conducted using bulk NBs generated either from CO2 hydrate dissociation or electric-field-based electrostriction in virgin water. Simultaneously, molecular dynamics simulations examined hydrate growth in contact with a single NB containing CO2 molecules.

FINDINGS

Experimental results indicate that NBs promote hydrate nucleation, with finer ones from electric fields leading to a slight promotion at the onset of hydrate growth, followed by inhibition. MD simulations reveal that while NBs can serve as a gas source for growth, denser NBs hinder the process due to stronger gas-water interactions and CO2 clustering. These results suggest that optimizing NB size and concentration is critical for maximizing gas-hydrate formation efficiency in industrial applications such as gas storage and carbon capture.

Large Temperature Dependence of Redox Potential Driven by Semiclathrate Hydrate Formation for Thermo-Electrochemical Conversion

The temperature dependence of the redox potential (temperature coefficient) is a critical parameter for redox couples employed in thermoelectrochemical conversion devices, such as thermogalvanic cells and thermally regenerative electrochemical cycles (TRECs). We developed a novel strategy for boosting the temperature coefficient of ferro/ferricyanide through the formation/dissociation of a semiclathrate hydrate (SCH). The aqueous solution with ferro/ferricyanide and tetrabutylammonium fluoride (TBAF) showed SCH formation/dissociation by small temperature variations, which contributed to a huge temperature coefficient (-13.8 mV K-1) near ambient temperature. The large-temperature coefficient was attributed to a significant change in the TBAF concentration in the liquid phase caused by SCH formation/dissociation, resulting in the rearrangement of the ion pair of ferricyanide and cations. We introduced the electrolyte to a charging-free TREC device driven by a small temperature swing (9 K) and achieved the highest normalized power density (4.8 mW m-2 K-2). This electrolyte design strategy will pave the way for electrochemical energy harvesting from small temperature changes such as diurnal temperature variations. In addition, this study creates a new research field for semiclathrate hydrate chemistry.

Pressure-induced evolution in the durability of nickel-metal hydride batteries under high-current charge

We found that an AAA-type battery (min. 750 mAh) pressurized with Ar or N₂ at pressures of up to 5 MPa exhibited a significant durability enhancement even under high-current conditions. As an example of a charge-discharge cycle test at 3 amperes, the residual ratio of capacity at atmospheric pressure decreased to approximately 90% of the standard capacity before 50 cycles. However, at a pressure of 3 MPa of N₂, the capacity remained at more than 90% until 180 cycles. With an increase in the pressure, the residual ratio of capacity was further improved. It has been considered that, at the positive electrode of the Ni-MH battery, the chemical reaction from nickel(II) hydroxide (Ni(OH)₂) crystals to nickel oxide hydroxide (NiOOH) crystals occurs during the charging process. However, X-ray diffraction (XRD) results in the present study do not support this solid-solid reaction between these two types of crystal. To address this contradiction, we propose a different reaction mechanism by introducing the concept of non-crystalline fine particles of compounds, which are undetected by XRD. This mechanism clearly explains how the pressure affects the durability improvement. Pressurized batteries, which are capable of fast charge-discharge operation under high-current conditions, can provide a new route for application fields of unconventional energy storage.

Supercooling suppression in the tetrahydrofuran clathrate hydrate formation

The addition of silver(ii) oxide effectively diminishes the degree of supercooling in the tetrahydrofuran hydrate formation.

Cyclic Formation Stability of 1,1,1,2-Tetrafluoroethane Hydrate in Different SDS Solution Systems and Dissociation Characteristics Using Thermal Stimulation Combined with Depressurization

Cold storage using hydrates for cooling is a high-efficiency technology. However, this technology suffers from problems such as the stochastic nature of hydrate nucleation, cyclic hydrate formation instability, and a low cold discharge rate. To solve these problems, it is necessary to further clarify the characteristics of hydrate formation and dissociation in different systems. First, a comparative experimental study in pure water and sodium dodecyl sulfate (SDS) solution systems was conducted to explore the influence of SDS on the morphology of the hydrate and the time needed for its formation under visualization conditions. Subsequently, the cyclic hydrate formation stability was investigated at different test temperatures with two types of SDS solution systems-with or without a porous medium. The induction time, full time, and energy consumption time ratio of the first hydrate formation process and the cyclic hydrate reformation process were analyzed. Finally, thermal stimulation combined with depressurization was used to intensify hydrate dissociation compared with single thermal stimulation. The results showed that the growth morphology of hydrate and the time required for its formation in the SDS solution system were obviously different than those in pure water. In addition, the calculation and comparison results revealed that the induction time and full time of cyclic hydrate reformation were shorter and the energy consumption time ratio was smaller in the porous medium. The results indicated that a porous medium could improve the cyclic hydrate formation process by making it more stable and by decreasing time and energy costs. Thermal stimulation combined with depressurization at different backpressures (0.1, 0.2, 0.3, and 0.4 MPa) effectively promoted the decomposition of hydrates, and with the decrease in backpressure, the dissociation time decreased gradually. At a backpressure of 0.1 MPa, the dissociation time was reduced by 150 min. The experimental results presented the formation and dissociation characteristics of 1,1,1,2-tetrafluoroethane hydrates in different systems, which could accelerate the application of gas hydrates in cold storage.

Thermodynamic Properties and Crystallographic Characterization of Semiclathrate Hydrates Formed with Tetra-n-butylammonium Glycolate

Semiclathrate hydrates are a crystalline host-guest material, which forms with water and ionic substances such as tetra-n-butylammonium (TBA) salts. Various anions can be used as a counter anion to the TBA cation, and they can modify thermodynamic properties of the semiclathrate hydrates, which are critical for applications, for example, cold energy storage and gas separation. In this study, the semiclathrate hydrates of the TBA glycolate were newly synthesized. Measurements for melting temperatures and a heat of fusion and a crystal structure analysis were performed. In comparison with the other similar materials, such as acetates, propionates, lactates, and hydroxybutyrates, the glycolate greatly changed the melting temperature and the heat of fusion. The preliminarily determined crystal structure showed that the glycolate anion builds a relatively porous structure compared to the previously reported hydrates formed with hydroxycarboxylates. The present study showed that substitution of a hydrophobic group by a hydrophilic group is an effective method to control the thermodynamic properties as well as to improve environmental, biological, and chemical properties.