Synthesis and thermo-physical properties of deep eutectic solvent-based graphene nanofluids

Design Strategy and Application of Deep Eutectic Solvents for Green Synthesis of Nanomaterials

The first report of deep eutectic solvents (DESs) was released in 2003 and was identified as a new member of ionic liquid (IL), involving innovative chemical and physical characteristics. Using green solvent technology concerning economical, practical, and environmental aspects, DESs open the window for sustainable development of nanomaterial fabrication. The DESs assist in different fabrication processes and design nanostructures with specific morphology and properties by tunable reaction conditions. Using DESs in synthesis reactions can reduce the required high temperature and pressure conditions for decreasing energy consumption and the risk of environmental contamination. This review paper provides the recent applications and advances in the design strategy of DESs for the green synthesis of nanomaterials. The strategy and application of DESs in wet-chemical processes, nanosize reticular material fabrication, electrodeposition/electrochemical synthesis of nanostructures, electroless deposition, DESs based nano-catalytic and nanofluidic systems are discussed and highlighted in this review.

Electrochemical Behavior of Graphene in a Deep Eutectic Solvent

Graphene electrodes and deep eutectic solvents (DESs) are two emerging material systems that have individually shown highly promising properties in electrochemical applications. To date, however, it has not been tested whether the combination of graphene and DESs can yield synergistic effects in electrochemistry. We therefore study the electrochemical behavior of a defined graphene monolayer of centimeter-scale, which was produced by chemical vapor deposition and transferred onto insulating SiO₂/Si supports, in the common DES choline chloride/ethylene glycol (12CE) under typical electrochemical conditions. We measure the graphene potential window in 12CE and estimate the apparent electron transfer kinetics of an outer-sphere redox couple. We further explore the applicability of the 12CE electrolyte to fabricate nanostructured metal (Zn) and metalloid (Ge) hybrids with graphene by electrodeposition. By comparing our graphene electrodes with common bulk glassy carbon electrodes, a key finding we make is that the two-dimensional nature of the graphene electrodes has a clear impact on DES-based electrochemistry. Thereby, we provide a first framework toward rational optimization of graphene-DES systems for electrochemical applications.

Thermal Conductivities of Choline Chloride-Based Deep Eutectic Solvents and Their Mixtures with Water: Measurement and Estimation

The thermal conductivities of selected deep eutectic solvents (DESs) were determined using the modified transient plane source (MTPS) method over the temperature range from 295 K to 363 K at atmospheric pressure. The results were found to range from 0.198 W·m^-1·K^-1 to 0.250 W·m^-1·K^-1. Various empirical and thermodynamic correlations present in literature, including the group contribution method and mixing correlations, were used to model the thermal conductivities of these DES at different temperatures. The predictions of these correlations were compared and consolidated with the reported experimental values. In addition, the thermal conductivities of DES mixtures with water over a wide range of compositions at 298 K and atmospheric pressure were measured. The standard uncertainty in thermal conductivity was estimated to be less than ± 0.001 W·m^-1·K^-1 and ± 0.05 K in temperature. The results indicated that DES have significant potential for use as heat transfer fluids.

Influence of solvents on the enhancement of thermophysical properties and stability of multi-walled carbon nanotubes nanofluid

Multi-walled carbon nanotubes (MWCNTs) are a contemporary class of nanoparticles that have a prominent thermal, electrical and mechanical properties. There have been numerous studies on the enhancement of thermophysical properties of nanofluids. However, there is only limited research on thermal and stability analysis of MWCNT nanofluids with various kinds of solvents or base fluids, namely propylene glycol, ethanol, ethylene glycol, polyethylene glycol, methanol and water. This paper reports the enhancement of thermophysical properties and stability of MWCNTs with six different base fluids in the presence of sodium dodecyl benzene sulfonate surfactant with a mass concentration of 0.5 wt%. Thermal and dispersion stabilities were determined using a thermogravimetric analyzer (TGA) and Zeta potential, along with a visual inspection method to evaluate the agglomeration or sedimentation of MWCNT nanoparticles over a period of one month. Ultraviolet-visible spectroscopy and Fourier transform infrared spectroscopy were utilized to identify the molecular components and light absorption of the formulated nanofluids at their maximum wavenumber (4500 cm^-1) and wavelength (800 nm). In addition, thermophysical properties such as thermal conductivity, specific heat capacity, viscosity and density with a peak temperature of 200 °C were also experimentally evaluated. The TGA results illustrated that MWCNT/ethylene glycol nanofluid achieved maximum thermal stability at 140 °C and it revealed a maximum zeta potential value of -61.8 mV. Thus, ethylene glycol solution was found to be the best base liquid to homogenize with MWCNTs for acquiring an enhanced thermophysical property and a long-term stability.

Systems Analysis of SO2-CO2 Co-Capture from a Post-Combustion Coal-Fired Power Plant in Deep Eutectic Solvents

In this study, CO2 and SO2 captures from post-combustion flue gas from a pulverized coal-fired power plant were evaluated using deep eutectic solvents (DES) to replace existing mono-ethanol amine (MEA) and CanSolv technologies. The system design of the DES-based CO2 and SO2 capture was based on the National Energy Technology Laboratory’s (NETL) 550 MWe pulverized coal-fired power plant model using Illinois #06 coal. Two of the most studied DES (choline chloride and urea at a 1:2 molar ratio and methyltriphenylphosphonium bromide (METPB) and ethylene glycol at a 1:3 molar ratio) for CO2 and SO2 capture were evaluated for this system analysis. Physical properties of DES were evaluated using both density functional theory (DFT)-based modeling as well as with documented properties from the literature. A technoeconomic assessment (TEA) was completed to assess DES ability to capture CO2 and SO2. Both solvents were able to fully dissolve and capture all SO2 present in the flue gas. It was also found from the system analyses that choline chloride and urea outperformed METPB and ethylene glycol (had a lower final cost) when assessed at 10–30% CO2 capture at high operating pressures (greater than 10 bar). At high system sizes (flow rate of greater than 50,000 kmoles DES per hour), choline chloride:urea was more cost effective than METPB:ethylene glycol. This study also establishes a modeling framework to evaluate future DES for physical absorption systems by both thermophysical and economic objectives. This framework can be used to greatly expedite DES candidate screening in future studies.

Efficient Volumetric Absorption Solar Thermal Platforms Employing Thermally Stable - Solar Selective Nanofluids Engineered from Used Engine Oil

We report a low cost and scalable method to synthesize solar selective nanofluids from 'used engine oil'. The as-prepared nanofluids exhibit excellent long-term stability (presently tested up to 6 months under undisturbed stagnant conditions at room temperature) and photo-thermal conversion efficiency. Moreover, these were found to retain their stability and functional characteristics even after extended periods (72 hours) of high temperature (300°C) heating, ultra violet light exposure and thermal cyclic loading. Building upon it, we have been able to successfully engineer an efficient volumetric absorption solar thermal platform that employs the as-prepared nanofluids and achieves higher steady state temperatures (approximately 5% higher) relative to the conventional surface absorption based solar thermal system under the sun. The developed volumetric absorption solar thermal platform could prove to be significant step in the evolution of efficient solar thermal systems which could potentially be deployed for host of applications ranging from solar driven heating, air-conditioning, and desalination units to solar energy electricity generation systems.

Elucidating the Properties of Graphene-Deep Eutectic Solvents Interface

The properties of five deep eutectic solvents prepared based on the selection of choline chloride ionic liquid as hydrogen bond acceptor, which are mixed with several hydrogen bond donors with selected molecular features, were studied theoretically at graphene interfaces via both density functional theory and classical molecular dynamics methods. Molecular structuring at the interfaces, angular orientation, densification, and dynamic properties were analyzed upon adsorption on the graphene surface and when the deep eutectic solvents were confined between two graphene sheets and analyzed in terms of the role of the type of hydrogen bond donor for each solvent. Likewise, the behavior of deep eutectic solvent nanodroplets on graphene was simulated leading to the calculation of contact angles and nanowetting with further studies considering the effect of an external electric field on nanodroplet properties.