Electrical double layer capacitors with sucrose derived carbon electrodes in ionic liquid electrolytes

Activated carbons were prepared via a pyrolysis of sucrose followed by activation in the stream of CO₂ gas for 2-6 h at 900 °C to tune the pore size distribution (PSD) and increase the specific surface area (SSA). The porosity of the activated sucrose derived carbons (ASCs) has been characterized using N₂ sorption measurements. Increasing activation time led to the significant increase in SSA and pore volume of ASCs, among which sucrose derived carbon with 6 h activation time (ASC-6 h) exhibited the highest SSA of 1941 m² g⁻¹ and the highest micropore volume of 0.87 cm³ g⁻¹. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge cycle tests have been applied to investigate the capacitive performance of the ASC electrodes in ionic liquids (ILs) at room and elevated temperatures. The ASC-6 h electrodes in ethyl-dimethyl-propyl-ammonium bis (trifluoromethylsulfonyl) imide (EdMPNTf₂N) showed specific capacitance in excess of 170 F g⁻¹ at 60 °C, whereas the same electrodes in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) showed slightly lower capacitance but significantly better rate performance.     

Wide-temperature range operation supercapacitors from nanostructured activated carbon fabric

Electrochemical power sources that offer high energy and power densities and, can also withstand a harsh temperature range have become extremely desirable in applications ranging from civilian portable electronic devices to military weapons. In this report, we demonstrated a wide temperature withstanding supercapacitor which can be operated from 100 °C to −40 °C within a voltage window from −2 V to 2 V. The performance of the supercapacitor coin cells, assembled with nanostructured activated carbon fabric (ACF) as the electrode material and 1 M tetraethylammonium tetrafluoroborate (TEABF₄) in polypropylene carbonate (PC) solution as the electrolyte, was systematically studied within the set temperature window. The ACF supercapacitor yielded ideal rectangular shapes in cyclic voltammograms within 0-100 °C with an average mass capacitance of 90 F g⁻¹ and, 60 F g⁻¹ at −25 °C. The capacitance was still over 20 F g⁻¹ at the extremely low temperature of −40 °C. Another exciting feature of the ACF supercapacitors was that they resumed their room temperature capacitance when cooled from 100 °C and defrosted from −40 °C, demonstrating an excellent repeatability and stability. The charge-discharge behavior of the ACF supercapacitors showed long-cycle stability at extreme temperatures. These high electrochemical performances make this type of supercapacitors very promising in many practical applications.     

Electrochemical characterization of single cell and short stack PEM electrolyzers based on a nanosized IrO2 anode electrocatalyst

A nanosized IrO₂ anode electrocatalyst was prepared by a sulfite-complex route for application in a proton exchange membrane (PEM) water electrolyzer. The physico-chemical properties of the IrO₂ catalyst were studied by termogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrochemical activity of this catalyst for oxygen evolution was investigated in a single cell PEM electrolyzer consisting of a Pt/C cathode and a Nafion^® membrane. A current density of 1.26 A cm⁻² was obtained at 1.8 V and a stable behavior during steady-state operation at 80 °C was recorded. The Tafel plots for the overall electrochemical process indicated a slope of about 80 mV dec⁻¹ in a temperature range from 25 °C to 80 °C. The kinetic and ohmic activation energies for the electrochemical process were 70.46 kJ mol⁻¹ and 13.45 kJ mol⁻¹, respectively. A short stack (3 cells of 100 cm² geometrical area) PEM electrolyzer was investigated by linear voltammetry, impedance spectroscopy and chrono-amperometric measurements. The amount of H₂ produced was 80 l h⁻¹ at 60 A under 330 W of applied electrical power. The stack electrical efficiency at 60 A and 75 °C was 70% and 81% with respect to the low and high heating value of hydrogen, respectively.     

Fabrication of MnO2-pillared layered manganese oxide through an exfoliation/reassembling and oxidation process

MnO₂-pillared layered manganese oxide has been first fabricated by a delamination/reassembling process followed by oxidation reaction and then by heat treatment. The structural evolution of MnO₂-pillared layered manganese oxide has been characterized by XRD, SEM, DSC-GTA, IR and N₂ adsorption-desorption. MnO₂-pillared layered manganese oxide shows a relative high thermal stability and mesoporous characteristic. The layered structure with a basal spacing of 0.66 nm could be maintained up to 400 °C. The electrochemical properties of the synthesized MnO₂-pillared layered manganese oxide have been studied using cyclic voltammetry in a mild aqueous electrolyte. Sample MnO₂–BirMO (300 °C) shows good capacitive behavior and cycling stability, and the specific capacitance value is 206 F g⁻¹.     

Ammonia inhibition of electricity generation in single-chambered microbial fuel cells

Batch experiments are conducted at various concentrations of initial total ammonia nitrogen (TAN) with acetate as an electron donor to examine the effects of free ammonia (NH₃) inhibition on electricity production in single-chambered microbial fuel cells (MFCs). This research demonstrates that initial TAN concentrations of over 500 mg N L⁻¹ significantly inhibit electricity generation in MFCs. The maximum power density of 4240 mW m⁻³ at 500 mg N L⁻¹ drastically decreases to 1700 mW m⁻³ as the initial TAN increases up to 4000 mg N L⁻¹. Nitrite and nitrate analysis confirms that nitrification after complete acetate removal consumes some TAN. Ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) are also inhibited by increasing the initial TAN concentrations. Another batch experiment verifies the strong inhibitory effect of TAN with only small differences between the half-maximum effective concentration (EC₅₀) for TAN (894 mg N L⁻¹ equivalent to 10 mg N L⁻¹ as NH₃) and optimum TAN conditions; it requires careful monitoring of the TAN for MFCs. In addition, abiotic control experiments reveal that granular activated carbon, which is used as an auxiliary anode material, adsorbs a significant amount of ammonia at each TAN concentration in batch MFCs.     

Investigation of cobalt-free perovskite Ba0.95La0.05FeO3−δ as a cathode for proton-conducting solid oxide fuel cells

Cobalt-free perovskite Ba_0.95La_0.05FeO_3−δ (BLF) was synthesized. The conductivity of BLF was measured with a DC four-point technique. The thermal expansion coefficient of the BLF was measured using a dilatometer. The BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte based proton conducting solid oxide fuel cells (SOFCs) were fabricated. A composite cathode with BLF + BZCY7 was used to mitigate the thermal expansion mismatch with the BZCY7 electrolyte. The polarization processes of the button cell NiO-BZCY7/BZCY7/BLF + BZCY7 were characterized using the complicated electrochemical impedance spectroscopy technique. The open circuit voltage of 0.982 V, 1.004 V, and 1.027 V was obtained at 700 °C, 650 °C, and 600 °C, respectively, while the peak power density of 325 mW cm⁻², 240 mW cm⁻², and 152 mW cm⁻², was achieved accordingly.     

Cooling strategies,summer comfort and energy performance of a rehabilitated passive standard office building

One of the first rehabilitated passive energy standard office buildings in Europe was extensively monitored over two years to analyse the cooling performance of a ground heat exchanger and mechanical night ventilation together with the summer comfort in the building. To increase the storage mass in the light weight top floor, phase change materials (PCM) were used in the ceiling and wall construction. The earth heat exchanger installed at a low depth of 1.2 m has an excellent electrical cooling coefficient of performance of 18, but with an average cooling power of about 1.5 kW does not contribute significantly to cooling load removal. Mechanical night ventilation with 2 air changes also delivered cold at a good coefficient of performance of 6 with 14 kW maximum power. However, the night air exchange was too low to completely discharge the ceilings, so that the PCM material was not effective in a warm period of several days. In the ground floor offices the heat removal through the floor to ground of 2–3 W m⁻² K⁻¹ was in the same order of magnitude than the charging heat flux of the ceilings. The number of hours above 26 °C was about 10% of all office hours. The energy performance of the building is excellent with a total primary energy consumption for heating and electricity of 107–115 kW h m⁻² a⁻¹, without computing equipment only 40–45 kW h m⁻² a⁻¹.     

Assessment of PrBaCo2O5+δ + Sm0.2Ce0.8O1.9 composites prepared by physical mixing as electrodes of solid oxide fuel cells

The performance of PrBaCo₂O_5+δ + Sm_0.2Ce_0.8O_1.9 (PrBC + SDC) composites as electrodes of intermediate-temperature solid oxide fuel cells is investigated. The effects of SDC content on the performance and properties of the electrodes, including thermal expansion, DC conductivity, oxygen desorption, area specific resistance (ASR) and cathodic overpotential are evaluated. The thermal expansion coefficient and electrical conductivity of the electrode decreases with an increase in SDC content. However, the electrical conductivity of a composite electrode containing 50 wt% SDC reaches 150 S cm⁻¹ at 600 °C. Among the various electrodes under investigation, an electrode containing 30 wt% SDC exhibits superior electrochemical performance. A peak power density of approximately 1150 and 573 mW cm⁻² is reached at 650 and 550 °C, respectively, for an anode-supported thin-film SDC electrolyte cell with the optimal composite electrode. The improved performance of a composite electrode containing 70 wt% PrBC and 30 wt% SDC is attributed to a reduction in the diffusion path of oxygen-ions within the electrode, which is a result of a three-dimensional oxygen-ion diffusion path in SDC and a one-dimensional diffusion path in PrBC.     

Intermediate temperature fuel cell with a doped ceria-carbonate composite electrolyte

The performance of a composite electrolyte composed of a samarium doped ceria (SDC) and a binary eutectic carbonate melt phase has been examined. This material shows higher ionic conductivity than pure SDC in intermediate temperature region. SDC with different morphologies is obtained by co-precipitation, sol-gel and glycine-nitrate combustion preparation techniques. A tri-layer single cell is prepared with a cost-effective co-pressing and co-sintering technique. It is found that the surface properties of SDC and the electrolyte thickness have a great influence on the fuel cell performance. When the co-precipitated SDC is used as the electrolyte component and CO₂/O₂ gas mixture is adopted as the cathode oxidant gas, a fuel cell with an excellent performance is obtained, which has a peak power output of 1704 mW cm⁻² at a current density of 3000 mA cm⁻² at 650 °C. The influence of cathode atmosphere is examined with conductivity measurement and fuel cell performance test. The results support the concept of O²⁻/H⁺/CO₃²⁻ ternary conduction.     

Ammonia inhibition and microbial adaptation in continuous single-chamber microbial fuel cells

Here, we report that a continuous single-chamber microbial fuel cell (MFC) is applicable to wastewaters containing a high nitrogen concentration using a process of adaptation. Continuous experiments are conducted to investigate the inhibitory effect of total ammonia nitrogen (TAN) on the MFC using influents with various concentrations of TAN ranged from 84 to 10,000 mg N L⁻¹. As the TAN concentration increases up to 3500 mg N L⁻¹, the maximum power density remains at 6.1 W m⁻³. However, as the concentration further increases, TAN significantly inhibits the maximum power density, which is reduced at saturation to 1.4 W m⁻³ at 10,000 mg N L⁻¹. We confirm that the adapted electrical performance of a continuous MFC can generate approximately 44% higher power density than the conductivity control. A comparative study reveals that the power densities obtained from a continuous MFC can sustain 7-fold higher TAN concentration than that of previous batch MFCs. TAN removal efficiencies are limited to less than 10%, whereas acetate removal efficiencies remain as high as 93-99%. The increased threshold TAN of the continuous MFC suggests that microbial acclimation in a continuous MFC can allow the electrochemical functioning of the anode-attached bacteria to resist ammonia inhibition.     

Effects of the cationic structures of fluorohydrogenate ionic liquid electrolytes on the electric double layer capacitance

Electric double layer capacitance of an activated carbon electrode has been measured for fluorohydrogenate ionic liquids (FHILs) based on five different cations (1,3-dimethylimidazolium (DMIm⁺), 1-ethyl-3-methylimidazolium (EMIm⁺), 1-butyl-3-methylimidazolium (BMIm⁺), 1-ethyl-1-methylpyrrolidinium (EMPyr⁺), and 1-methoxymethyl-1-methylpyrrolidinium (MOMMPyr⁺)) at 25 °C. For all the FHILs, the capacitance increases with increase in charging voltage, and exhibits the maximum value around 2.7 V. The capacitances for FHILs are higher than those for EMImBF₄ or 1 M tetraethylammonium tetrafluoroborate in propylene carbonate (TEABF₄/PC) in the measured range (1.0 < V < 3.2). For the three imidazolium-based FHILs, the maximum capacitance decreases with increase in the size of the cation in the order, DMIm(FH)_2.3F (178 F g⁻¹) > EMIm(FH)_2.3F (162 F g⁻¹) > BMIm(FH)_2.3F (135 F g⁻¹). On the other hand, the maximum capacitance observed for MOMMPyr(FH)_2.3F (152 F g⁻¹) is larger than that for EMPyr(FH)_2.3F (134 F g⁻¹) in spite of the larger size of MOMMPyr⁺ than EMPyr⁺, which is derived from introduction of the methoxy group. Some FHILs with low melting points exhibit a sufficient capacitance even at −40 °C (64 F g⁻¹ for EMIm(FH)_2.3F).     

Improved high temperature performance of La0.8Sr0.2MnO3 cathode by addition of CeO2

Ceria is proposed as an additive for La_0.8Sr_0.2MnO₃ (LSM) cathodes in order to increase both their thermal stability and electrochemical properties after co-sintering with an yttria-stabilized zirconia (YSZ) electrolyte at 1350 °C. Results show that LSM without CeO₂ addition is unstable at 1350 °C, whereas the thermal stability of LSM is drastically improved after addition of CeO₂. In addition, results show a correlation between CeO₂ addition and the maximum power density obtained in 300 μm thick electrolyte-supported single cells in which the anode and modified cathode have been co-sintered at 1350 °C. Single cells with cathodes not containing CeO₂ produce only 7 mW cm⁻² at 800 °C, whereas the power density increases to 117 mW cm⁻² for a CeO₂ addition of 12 mol%. Preliminary results suggest that CeO₂ could increase the power density by at least two mechanisms: (1) incorporation of cerium into the LSM crystal structure, and (2) by modification or reduction of La₂Zr₂O₇ formation at high temperature. This approach permits the highest LSM-YSZ co-sintering temperature so far reported, providing power densities of hundreds of mW cm⁻² without the need for a buffer layer between the LSM cathode and YSZ electrolyte. Therefore, this method simplifies the co-sintering of SOFC cells at high temperature and improves their electrochemical performance.     

Thermal insulation of muscovite/glass ceramic foam for solid oxide fuel cell

A direct foaming method of dispersed suspensions containing muscovite particulates and a glass powder (47BaO-21B₂O₃-27SiO₂-5Al₂O₃, in mol%) is used to prepare porous ceramic structures. The sintered foams exhibit extremely low thermal conductivity and slight expansion during the thermal treatment at 1000 °C. Both the foam stability and its thermal conductivity are investigated by considering foaming agents, muscovite/glass ratios, solid contents, microwave drying, wetting behaviors, and foam consolidation. One of the muscovite/glass ceramic foam, thermally treated at 950 °C for 1 h, showed the lowest thermal conductivity of 0.18 W m⁻¹ K⁻¹ at 800 °C among all of the prepared samples. Its gas permeability and compressive strength are 0.1 × 10⁻⁷ cm² and 440 kPa, respectively.     

Treatment of carbon fiber brush anodes for improving power generation in air-cathode microbial fuel cells

Carbon brush electrodes have been used to provide high surface areas for bacterial growth and high power densities in microbial fuel cells (MFCs). A high-temperature ammonia gas treatment has been used to enhance power generation, but less energy-intensive methods are needed for treating these electrodes in practice. Three different treatment methods are examined here for enhancing power generation of carbon fiber brushes: acid soaking (CF-A), heating (CF-H), and a combination of both processes (CF-AH). The combined heat and acid treatment improve power production to 1370 mW m⁻², which is 34% larger than the untreated control (CF-C, 1020 mW m⁻²). This power density is 25% higher than using only acid treatment (1100 mW m⁻²) and 7% higher than that using only heat treatment (1280 mW m⁻²). XPS analysis of the treated and untreated anode materials indicates that power increases are related to higher N1s/C1s ratios and a lower C-O composition. These findings demonstrate efficient and simple methods for improving power generation using graphite fiber brushes, and provide insight into reasons for improving performance that may help to further increase power through other graphite fiber modifications.     

Improved electrochemical performance and thermal compatibility of Fe- and Cu-doped SmBaCo2O5+δ-Ce0.9Gd0.1O1.95 composite cathode for intermediate-temperature solid oxide fuel cells

Fe- and Cu-doped SmBaCo₂O_5+δ (FC-SBCO)-Ce_0.9Gd_0.1O_1.95 (CGO) composites with various CGO contents (0-40 wt.%) are investigated as new cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on a Ce_0.9Gd_0.1O_1.95 electrolyte. The effect of CGO incorporation on the thermal expansion coefficient (TEC), electrochemical properties and thermal stability of the FC-SBCO-CGO composites is investigated. A composite cathode of 30 wt.% CGO-70 wt.% FC-SBCO (CS30-70) coated on a Ce_0.9Gd_0.1O_1.95 electrolyte shows the lowest area specific resistance (ASR), i.e., 0.049 Ω cm² at 700 °C. The TEC of the CS30-70 cathode is 14.1 × 10⁻⁶ °C⁻¹ up to 900 °C, which is a lower value than that of the FC-SBCO (16.6 × 10⁻⁶ °C⁻¹) counterpart. Long-term thermal stability and thermal cycle tests of the CS30-70 cathode are performed. Stable ARS values are observed during both type of test. An electrolyte-supported (300-μm thick) single-cell configuration of CS30-70/CGO/Ni-CGO delivers a maximum power density of 535 mW cm⁻² at 700 °C. The unique composite composition of CS30-70 demonstrates improved electrochemical performance and good thermal stability for IT-SOFCs.     

Performance improvement of direct carbon fuel cell by introducing catalytic gasification process

In this paper, the effects of catalytic gasification on the solid oxide electrolyte DCFC (direct carbon fuel cell) performance are experimentally investigated and analyzed using K, Ca, Ni as catalyst in carbon black and controlling the temperatures of cell and carbon black at 750 °C and 700-1000 °C, respectively. The average power densities are 976, 1473 and 1543 W m⁻² respectively for 900, 950 and 1000 °C pure carbon black gasification. Catalytic gasification improves the DCFC performance significantly. For the same performance of pure carbon black, the gasification temperatures decrease about 200, 130 and 150 °C with K, Ca and Ni additives, respectively. The catalytic effects for carbon black gasification with CO₂ are: K > Ni > Ca. For typical identical temperature DCFC operating at 750 °C, the power densities of 0.7 V discharging are 1477, 1034 and 1123 W m⁻² for the carbon black with K, Ca and Ni additives, respectively. It is possible to reduce the operation temperature of DCFC to the medium temperature range of solid oxide electrolyte (600-800 °C) by introducing catalytic gasification process.     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Structural,morphological and electrical properties of Gd0.1Ce0.9O1.95 prepared by a citrate complexation method

A variant of the sol–gel technique known as cation complexation is used to prepare a nanocrystalline Gd_0.1Ce_0.9O_1.95 (GDC) solid solution. A range of techniques including thermal analysis (TGA/DTA), X-ray diffraction, specific surface area determination (BET) and electron microscopy (SEM and TEM) are employed to characterise the GDC powders. GDC calcined at 500 °C is found to have an average crystallite size of 11 nm. Specific surface areas are found to be 29.7 m² g⁻¹ for the as-calcined powder and 57.5 m² g⁻¹ after ball milling at 400 rpm. Dense ceramic pellets are prepared from unmilled and ball-milled GDC powders employing different thermal treatments. Their electrical properties are studied by impedance spectroscopy. Those samples sintered at 1300 °C for 30 h (starting from ball-milled powders) exhibit the highest density (96% of theoretical density) and the highest total ionic conductivity (1.91 × 10⁻² S cm⁻¹ at 600 °C).     

Influence of the mesoporous structure on capacitance of the RuO2 electrode

The difference in capacitive performance between high and low surface area RuO₂ electrodes, synthesized with and without a mesoporous silica template, respectively, was investigated in aqueous solutions of sulfuric acid and sulfates by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). RuO₂ synthesized with the template was crystalline and the formation of the mesoporous structure with a 6.5 nm diameter was confirmed using a transmission electron microscope and the nitrogen adsorption and desorption isotherm. From the CV at the scan rate of 1 mV s⁻¹, the specific capacitance of the high surface area electrode in H₂SO₄(aq) was determined to be 200 F g⁻¹. The high surface area RuO₂ has a three times higher BET specific surface area (140 m² g⁻¹) than the low surface area sample (39 m² g⁻¹). Introducing the mesoporous structure was proved effective for increasing the capacitance per mass of the RuO₂, though not all the surface functions as a capacitor. Both the CV and EIS suggest that by increasing the charging rate or frequency, the mesoporous structure of the electrode leads to a lower capacitance decrease (higher capacitance retention) than the low surface area electrode. The EIS also indicates that the response time of the capacitor is hardly influenced by the presence of the mesoporous structure.     

Photovoltaic-thermal collectors for night radiative cooling of buildings

A new photovoltaic-thermal (PVT) system has been developed to produce electricity and cooling energy. Experimental studies of uncovered PVT collectors were carried out in Stuttgart to validate a simulation model, which calculates the night radiative heat exchange with the sky. Larger PVT frameless modules with 2.8 m² surface area were then implemented in a residential zero energy building and tested under climatic conditions of Madrid. Measured cooling power levels were between 60 and 65 W m⁻², when the PVT collector was used to cool a warm storage tank and 40-45 W m⁻², when the energy was directly used to cool a ceiling. The ratio of cooling energy to electrical energy required for pumping water through the PVT collector at night was excellent with values between 17 and 30. The simulated summer cooling energy production per square meter of PVT collector in the Madrid/Spain climatic conditions is 51 kWh m⁻² a⁻¹. In addition to the thermal cooling gain, 205 kWh m⁻² a⁻¹ of AC electricity is produced under Spanish conditions. A comparative analysis for the hot humid climate of Shanghai gave comparable results with 55 kWh m⁻² a⁻¹ total cooling energy production, mainly usable for heat rejection of a compression chiller and a lower electricity production of 142 kWh m⁻² a⁻¹.