Convective heat transfer enhancement with graphene nanoplatelet/platinum hybrid nanofluid

The work here looked into heat transfer performance in addition to friction loss of graphene nanoplatelet (GNP) - Platinum (Pt) hybrid nanofluids. The experiments were performed with non-changing limit parameters of heat-flux. Nanofluid movement was turbulent at a weight percentage ranging between 0.02 and 0.1%, with the Reynold number from 5000 to 17,500. The experimental findings revealed that compared with the base liquid, all nanofluid samples had higher heat transfer abilities. Nusselt number elevation and the increment of the heat transfer coefficient were found to be dependent on Reynold number, and the weight concentration of the nanocomposite. The greatest value recorded for Nusselt number was 28.48%, accompanied by a 1.109-fold penalty. There was a rise in friction factor with regards to the highest load of nanocomposite (0.1 wt%), with the Reynolds number of 17,500.     

PtFe nanotubes/graphene hybrid: Facile synthesis and its electrochemical properties

PtFe nanotubes are synthesized by galvanic exchange reactions using Co nanowires as template and reducing agent, followed by mixed with graphene to prepare PtFe nanotubes/graphene hybrid catalyst. The morphology and crystal structure of as-made hybrid are characterized by transmission electron microscope and X-ray diffraction. Its electro-catalytic properties toward methanol oxidation are investigated by cyclic voltammetry and chronoamperometry. The average diameter and wall thickness of the PtFe nanotubes supported on graphene are ca. 50 nm and 10 nm, respectively. As an electro-catalyst for methanol electro-oxidation, PtFe nanotubes/graphene catalyst displays higher specific activity and stability than commercial PtRu/C catalyst and PtFe nanoparticles/graphene catalyst.     

Thermal properties and characterization of palmitic acid/nano silicon dioxide/graphene nanoplatelet for thermal energy storage

Palmitic acid (PA), nano silicon dioxide (nano SiO₂), and graphene nanoplatelets (GNPs) were fabricated to composite phase change materials (PCMs) for thermal energy storage. PA acted as PCM, nano SiO₂ was used as supporting material. GNP as thermal conductivity promoter was added to modify composite PCM. Nano SiO₂ has good adsorption property and can adsorb liquid PCM to prevent leakage. Leakage measurement indicated that PA maximum content in composite PCM is 70 wt%. Chemical and crystal structures, and microstructure of composite PCM were tested by Fourier transformation infrared spectroscope, X-ray diffractometer and scanning electronic microscope, which showed that the raw materials are well mixed by physical action. Differential scanning calorimeter result presented that composite PCM possess phase change temperature at about 60°C and latent heat of 128.42 kJ/kg. Thermogravimetric analyzer and thermal cycle experiment showed that composite PCM have outstanding thermal stability and durability. Thermal conductivity apparatus measurement results indicated that thermal conductivity of composite PCM with 5 wt% GNP is 1.65 times that of composite PCM without GNP. Therefore, this composite PCM are potential materials for thermal energy storage.     

A review of nanofluid preparation,stability, and thermo‐physical properties

This paper represents a comprehensive review on the preparation and stability of nanofluid, the convective heat transfer coefficient and different thermo‐physical properties such as thermal conductivity, specific heat capacity, viscosity, and so on. Here, for each thermo‐physical property, measurement methods, enhancement mechanisms, and criticisms of different studies are also presented. However, based on the available literature, it is concluded that a nanofluid has, in general, better thermo‐physical properties even at a very low particle concentration (typically 1% or less) than conventional heat transfer fluids. The only drawback is high viscosity which leads to a higher pressure drop. At a very low particle concentration, this drawback can be minimized. Three tables are provided for three thermo‐physical properties namely thermal conductivity, specific heat capacity, and viscosity, which can be used as a ready reference for calculating the nanofluid properties.     

Flexographic printing of graphene nanoplatelet ink to replace platinum as counter electrode catalyst in flexible dye sensitised solar cell

Abstract

A semitransparent catalytically active graphene nanoplatelet (GNP) ink was developed suitable for roll to roll printing onto a flexible indium tin oxide substrate at a speed of 0·4 m s^?1. Dye sensitised solar cells using this ink as a catalyst demonstrated efficiencies of 2·0%, compared with 2·6% for sputtered platinum. Given further optimisation, GNP inks have the potential to replace chemically reduced or sputtered platinum. This would have the benefit of replacing the chemical reduction or sputtering operations as well as providing potential material cost benefits.     

A review of nanofluid stability properties and characterization in stationary conditions

A new engineering medium, called nanofluid attracted a wide range of researches on many cooling processes in engineering applications, which are prepared by dispersing nanoparticles or nanotubes in a host fluid. In this paper, the stability of nanofluids is discussed as it has a major role in heat transfer enhancement for further possible applications. It also represents general stabilization methods as well as various types of instruments for stability inspection. Characterization, analytical models and measurement techniques of nanofluids after preparation by a single step or two-step method are studied.     

One-step synthesis of graphene supported platinum nanoparticles as electrocatalyst for PEM fuel cells

Here in, we describe an ultrafast, single-step microwave irradiation route (MW) to prepare graphene supported Pt nanoparticles, during which the small Pt nanoparticles are distributed uniformly on a reduced graphene oxide surface. This route provides evident advantages namely low cost, easiness, low time consuming and high yield in comparison to actual chemical methods to develop efficient Pt/rGO catalyst with Pt content close to state-of-the-art commercial composition. The structure and composition of prepared samples have been studied by specific techniques, while the electrocatalytic stability has been studied using ex-situ and in-situ measurements. High performance and electrochemically stable catalyst for PEM fuel cells was developed using the sample with highest loading and good dispersion. The fabricated Pt-rGO-based MEA was investigated for durability under fuel starvation in comparison with commercial Pt/C-based MEA. The electrocatalytic activity was investigated and the electrochemical response revealed the higher stability during accelerated degradation test under fuel starvation in comparison with commercial Pt/C. This study promotes the applicability of described preparation method to noble or transition metal nanoparticles embedded on graphene-based materials.     

High stability and activity of Pt electrocatalyst on atomic layer deposited metal oxide/nitrogen-doped graphene hybrid support

A novel nanostructured support of ZrO₂/nitrogen-doped graphene nanosheets (ZrO₂/NGNs) hybrid was synthesized successfully by atomic layer deposition (ALD) technology to significantly improve the activity and stability of Pt electrocatalyst. Electrochemical test shows that Pt–ZrO₂/NGNs catalyst has 2.1 times higher activity towards methanol oxidation reaction (MOR) than Pt/NGNs catalyst, due to the promotion by ZrO₂ to the MOR on Pt surface. Pt–ZrO₂/NGNs catalyst has higher electrochemical surface area (ECSA) and better oxygen reduction reaction (ORR) activity than Pt/NGNs catalyst. Pt–ZrO₂/NGNs catalyst has also demonstrated 2.2 times higher durability than that of Pt/NGNs. The enhanced activity and durability were attributed to the unique triple-interaction of ZrO₂–Pt–NGNs. These findings indicate that metal oxide-metal-support is a promising catalyst structure for low temperature fuel cells.     

A hybrid artificial neural network-genetic algorithm modeling approach for viscosity estimation of graphene nanoplatelets nanofluid using experimental data

Predicting the viscosity of graphene nanoplatelets nanofluid with the help of multi-layered perceptron artificial neural network and genetic algorithm was the main aim of this study. In order to achieve the experimental results nanofluid which contains graphene nanoplatelets and deionized water at 20 to 60 °C and 0.025, 0.05, 0.075, and 0.1 wt% is used. Furthermore, genetic algorithm in artificial neural network is used to improve the learning process. In other words, different weights have been chosen for neurons' relations. Also, the bias preoccupation is based on improvements by genetic algorithm. On the other hand, for analyzing the accuracy of the presented model which gives us the nanofluid viscosity predictions MAPE, RMSE, R², and MBE indexes were used. The values of the presented indexes are 0.777, 0.086, 0.985, − 0.0009 respectively. In case of comparison the results show that the presented model which is the combination of genetic algorithm and artificial neural network is compatible with experimental work.     

Optical properties and stability of water-based nanofluids mixed with reduced graphene oxide decorated with silver and energy performance investigation in hybrid photovoltaic/thermal solar systems

The present work investigates the effect of the nanoparticles concentration on the optical and stability performance of a water-based nanofluid in solar photovoltaic/thermal (PV/T) systems experimentally and numerically. A novel nanofluid is formulated with the inclusion of the reduced graphene oxide decorated with silver (rGO-Ag) nanoparticles in water. Five different concentrations of nanoparticles in the range from 0.0005 to 0.05 wt% is suspended in water to prepare the samples. Optical properties are measured using UV-Vis. The UV-Vis absorption analysis reveals that all samples show consistent optical absorption coefficient (α) at higher value (more than 3 cm⁻¹) in the range of 1.5 to 4 eV. The application of optical filtration (OF) using water/rGO-Ag nanofluid in hybrid PV/T system presented more solar energy absorption through the OF. The hybrid system shows better performance at concentrations less than 0.0235 wt% compared to the PV system without integration with optical filtration. The hybrid solar PV/T system with OF using water/rGO-Ag nanofluid is able to produce thermal energy with efficiencies between 24% and 30%.     

Experimental investigation of the thermal transport properties of graphene oxide/Co3O4 hybrid nanofluids

The in-situ growth and chemical co-precipitation method was used for the synthesis of uniform dispersion of Co₃O₄ nanoparticles on the graphene oxide (GO) nanosheet. The reductions of aqueous cobalt chloride in the presence of GO with sodium borohydrate result in the formation of hybrid GO/Co₃O₄ nanoparticles. The synthesized GO/Co₃O₄ nanoparticles were characterized using X-ray power diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The hybrid nanofluids were prepared by dispersing synthesized GO/Co₃O₄ nanoparticles in water, ethylene glycol, and ethylene glycol/water mixtures. The properties such as thermal conductivity and viscosity were estimated experimentally at different volume concentrations and temperatures. The thermal conductivity enhancement of water-based nanofluid is 19.14% and ethylene glycol-based nanofluid is 11.85% at 0.2% volume concentration and at a temperature of 60 °C respectively compared to their respective base fluids. Similarly, the viscosity enhancement of water-based nanofluid is 1.70-times and ethylene glycol-based nanofluid is 1.42-times at 0.2% volume concentration and at a temperature of 60 °C respectively. The obtained thermal conductivity and viscosity data is compared with the literature values.     

An effective electrocatalyst based on platinum nanoparticles supported with graphene nanoplatelets and carbon black hybrid for PEM fuel cells

A facile synthesis at room temperature and at solid-state directly on the support yielded small, homogeneous and well-dispersed Pt nanoparticles (NPs) on CB-carbon black, GNP-graphene nanoplatelets, and CB-GNP-50:50 hybrid support. Synthesized Pt/CB, Pt/GNP and Pt/CB:GNP NPs were used as electrocatalysts for polymer electrolyte membrane fuel cell (PEMFC) reactions. HRTEM results displayed very small, homogeneous and well-dispersed NPs with 1.7, 2.0 and 4.2 nm mean-diameters for the Pt/CB-GNP, Pt/GNP and Pt/CB electrocatalysts, respectively. Electrocatalysts were also characterized by RAMAN, XRD, BET and CV techniques. ECSA values indicated better activity for graphene-based supports with 19 m² g⁻¹_Pt for Pt/GNP and 55 m² g⁻¹_Pt for Pt/CB-GNP compared to 10 m² g⁻¹_Pt for Pt/CB. Oxygen reduction reaction (ORR) studies and fuel cell tests were in parallel with these results where highest maximum power density of 377 mW cm⁻² was achieved with Pt/CB-GNP hybrid electrocatalyst. Both fuel cell and ORR studies for Pt/CB-GNP indicated better dispersion of NPs on the support and efficient fuel cell performance that is believed to be due to the prevention of restacking of GNP by CB. To the best of our knowledge, Pt/GNP and Pt/CB-GNP electrocatalysts are the first in literature to be synthesized with the organometallic mild synthesis method using Pt(dba)₃ precursor for the PEMFC applications.     

保温材料热物性测试的实验及数值研究

针对保温材料导热系数低,采用常规的导热系数测试方法难以获得准确结果的问题,根据瞬态法导热系数测试原理,对常功率平面热源法进行了研究。建立传热的二维瞬态数学模型,借助FLUENT有限体积软件对常功率平面热源法中试样的温度分布和热量传递规律进行数值模拟,开发了一套保温材料导热系数测试装置。测试结果与文献数据能较好的吻合,最大误差不超过4％。测试结果可靠,测试精度较高。     

Polyol synthesis of reduced graphene oxide supported platinum electrocatalysts for fuel cells: Effect of Pt precursor,support oxidation level and pH

In this work, a comprehensive study on the polyol synthesis of platinum supported on reduced graphene oxide (Pt/rGO) catalysts, including both ex-situ and in-situ characterizations of the prepared Pt/rGO catalysts, was performed. The polyol synthesis was studied considering the influence of the platinum precursor, oxidation level of graphite oxide and pH of reaction medium. The as-prepared catalysts were analyzed using thermo-gravimetric (TG) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and cyclic voltammetry (CV). The best results in terms of platinum particle size and distribution were obtained when the synthesis was performed in acidic medium, using chloroplatinic acid as precursor and using graphene oxide with high oxidation level. The most promising graphene-supported catalyst was used to prepare a polymer electrolyte membrane fuel cell electrode. The membrane electrode assembly (MEA) prepared with graphene-based electrode was compared with a MEA prepared with catalyst based on commercial platinum supported in carbon black (Pt/C). Single cell characterization included polarization curves and in-situ electrochemical impedance spectroscopy (EIS). The graphene-based electrode presented promising albeit unstable electrochemical performance due to water management issues. Additionally, EIS measurements revealed that the MEA made with Pt/rGO catalyst presented a lower mass transport resistance than the commercial Pt/C.     

Preparation and electroactivity of polymer-functionalized graphene oxide-supported platinum nanoparticles catalysts

Polymer-functionalized graphene oxide or pristine graphene oxide supported platinum nanoparticles (Pt NPs) was prepared to study the surface modification effects. The catalysts were characterized by transmission electron microscopy, energy dispersive spectrometry, X-ray diffraction and thermogravimetric analysis. The electrochemical activities of Pt NPs were measured by cyclic voltammograms. The poly(diallyldimethylammonium chloride) (PDDA) was used as a modifier agent which formed a functionalized layer on graphene oxide (GO) sheets. As a result, the electrochemical active surface area (ESA) of PDDA functionalized GO supported Pt (Pt/PDDA–graphene) was shown to 66 m²/g that indicated higher hydrogen adsorption amount than 55 m²/g of the pristine Pt/graphene. In addition, an average particle size of Pt/PDDA–graphene NPs was measured to 1.8 nm slightly smaller than 2.0 nm of pristine Pt/graphene NPs.     

Effect of CNT structures on thermal conductivity and stability of nanofluid

Thermal conductivity and stability of carbon nanotube (CNT) structures in water-based nanofluid, as well as their dependence to temperature and time variation are of a great concern. In order to investigate such dependence, five different structures, namely SWNT (single wall CNT), DWNT (double wall CNT), FWNT (few wall CNT) and two different multiwalls were applied in this study. The experiments reveal that the maximum UV–VIS absorbance of the solution corresponds to the dispersion of SWNT in the base fluid. The results from zeta size distribution and thermal conductivity demonstrate that as the number of nanotube wall increase, both stability and thermal conductivity decrease.     

Sol-gel derived mixed phase (Mn,Ti)-oxides/graphene nanoplatelets for hybrid supercapacitors

In this investigation, (Mn, Ti)- oxides were prepared using pluronic 123 templating assisted sol-gel method. Sol-A with Ti precursor in ethanol was acidified with HCl whereas sol-B containing Mn precursor was prepared in ethanol containing 5 wt% pluronic 123 surfactant. Gelation was accomplished by the addition of de-ionized water. As-prepared gels were aged for 24 hours, dried at 80°C for 12 hours and calcined at 500°C to 1200°C for 5 hours to obtain powdered electrode materials, which were analyzed by x-ray diffraction, scanning electron microscopy/energy-dispersive x-ray, and Brunauer-Emmett-Teller (BET) surface area analysis. Hybrid supercapacitors were fabricated using the sol-gel derived (Mn, Ti)- oxides and Gr-nanoplatelets electrodes with aqueous KOH (potassium hydroxide) as electrolyte. Fabricated supercapacitors were charged with 2.0 V and 0.01 A for 10 minutes. Charged supercapacitors were tested via cyclic voltammetry to determine specific capacitance. For the powdered materials prepared with Mn:Ti at 65:35 wt% and calcined at 500°C, x-ray diffraction analysis revealed the presence of TiO₂-rutile, Mn₂O₃ and Mn₃O₄ as 20%, 60%, and 18%, respectively. At higher calcination temperature, TiO₂-rutile and Mn₂O₃ phases were found to be absent with the presence of higher perovskite (TiMnO₃) phase. Both pore volume and BET specific surface area was found to decrease with increase in calcination temperature. The specific capacitance was found be dependent on Mn:Ti wt% used to prepare the powdered materials as well as the calcination conditions. The gel prepared with Mn:Ti of 30:70 wt% followed by 2-step calcination yielded a maximum specific capacitance.     

Effect of morphology of carbon nanomaterials on thermo-physical characteristics,optical properties and photo-thermal conversion performance of nanofluids

Graphite nanoparticles (GNPs), single-wall carbon nanotubes (SWCNTs) and graphene (GE) were dispersed into an ionic liquid (IL) to prepare nanofluids at different mass fractions, respectively. The thermo-physical characteristics, radiative properties, and photo-thermal conversion performance of the IL-based nanofluids containing the carbon nanomaterials with different morphologies were investigated in details. It is shown that all the nanofluids exhibit an increase in thermal conductivity and a decrease in viscosity as composed with the base liquid, and the enhancement and reduction ratios varied with their morphologies. The GE-dispersed nanofluids exhibit the highest thermal conductivity enhancement ratios as compared to the GNPs- and SWCNTs-dispersed ones at the same mass fractions. Among the nanofluids containing different carbon nanomaterials, the GE-dispersed nanofluids show the lowest transmittance and possess the highest extinction coefficients. It is revealed that the photo-thermal conversion performance of the IL has been enhanced by the addition of the carbon nanomterials, and the GE-dispersed nanofluids exhibit the highest photo-thermal conversion efficiency among the nanofluids containing different carbon nanomaterials. The superiorities in thermal conductivity, optical property and photo-thermal conversion efficiency make the GE-dispersed nanofluids show great potential for use as high-performance HTFs in solar thermal systems such as working fluids for DASCs.     

Performance of a hybrid photovoltaic/thermal system utilizing water‐Al2O3 nanofluid and fins

Innovative hybrid solar panels combining photovoltaic cells along with an efficient heat exchanger with attached fins to the parallel plates and water‐Al₂O₃ nanofluid as a working fluid is presented in this work. Twenty‐seven fins at the upper wall and 27 fins at the lower wall in labyrinth arrangement are used in simulations with fin lengths of 0, ¼, ½, and ¾ of the flow path height. Moreover, nanosolid particles dispersed in the base fluid range as 0 ≤ ? ≤ 0.2 . In addition, Reynolds number Re at the inlet was varied such that 10 < Re < 80. Numerical finite element analysis using COMSOL software is utilized to investigate flow and thermal characteristics as well the overall efficiency of the hybrid system. Results show that as the Reynolds number, the length of the fin, and the volume fraction of the nanosolid particles increase, the overall efficiency increases. Moreover, increasing nanoparticle volume fraction and the fin length was found to increase the friction coefficient.