A reliable method for the determination of regions of electrochemical stability of electrolytes for electrochemical double layer capacitors (EDLC) by means of potential window opening measurements is ...presented. For this purpose, a new evaluation method for the anodic and cathodic potential limits of electrochemical systems utilizing high surface area carbon electrodes is suggested. A change of 5% in the faradaic current contribution within a potential step of 1V to the overall current is proposed as new stability criterion. The results of seven ionic liquids (IL) and three standard electrolytes dissolved in acetonitrile (AN) or propylene carbonate (PC) at RT are presented. For two electrolytes a temperature dependent study is conducted. The feasibility of the new criterion is verified by full cell constant voltage aging experiments. The IL EMIMBF4 displays a maximum cell voltage of 3.5V at RT, which is, compared to standard electrolytes, an increase of about 0.5V.
During the lifetime of a polymer electrolyte fuel cell, the pore structure of the Pt/C catalyst layer may change as a result of carbon corrosion. Three-dimensional visualization of porosity changes ...is important to understand the origin of fuel cell performance deterioration. A focused ion beam/scanning electron microscopy (FIB/SEM) approach was adopted together with electron tomographic studies to visualize the three-dimensional pore structure of a Pt/C catalyst. In the case of pristine catalyst layers, the pores form an interconnected network. After 1000 start-up/shut-down cycles, severe carbon corrosion leads to a collapse of the support structure. The porosity of the degraded catalyst layer shrinks drastically, resulting in a structure of predominantly isolated pores. These porosity changes hinder the mass transport in the catalyst layer, consequently leading to a substantial loss of fuel cell performance. FIB/SEM serial sectioning and electron tomography allows three-dimensional imaging of the catalyst pore structure, which is a prerequisite for modeling and optimizing mass transport in catalyst layers.
The application of high surface area activated carbon (AC) as a convenient quasi-reference electrode (QRE) in non-aqueous electrolyte solutions is evaluated. By calibration against the ...ferrocene/ferrocenium ion redox system, the potential of the AC QRE investigated in this work was found to lie close to
+3
V vs. Li/Li
+ for several non-aqueous electrolyte solutions and to exhibit drifts of less than
+1
mV/day at room temperature for certain electrolyte solutions. AC QREs are thus proposed as suitable alternatives to other QREs with the distinct advantages that no additional ionic impurities are introduced into the electrochemical system under investigation, no potential-determining redox system is required and the electrode potential is less influenced by changes in the concentration of redox-active species in the electrolyte solution.
Sub-micron thick yttria-stabilized zirconia (YSZ) layers (
t
=
400–700
nm) containing 3 (3YSZ) or 8
mol.% (8YSZ) Y
2O
3 with microstructures ranging from isotropic amorphous to columnar ...polycrystalline and a variable porosity can be grown by pulsed laser deposition (PLD) varying the substrate temperature and oxygen background pressure. The dependence of the mechanical and optical properties on the film microstructure and chemical composition was investigated by nanoindentation experiments and transmission spectrophotometry. Dense amorphous YSZ layers exhibit a higher optical transmissivity, 0.2
eV lower band gap energy (5.5 vs. 5.7
eV), and up to 25% lower hardness (11.9 vs. 16.0
GPa) and reduced elastic moduli (231 vs. 278
GPa) compared with crystalline films, irrespective of the dopant level. High refractive indices,
n
600
nm, in the range 2.18–2.23, i.e. close to single crystal reference data, are obtained for the low pressure PLD regime. Within these boundaries the index decreases with increasing Y
2O
3 content and is consistently slightly smaller for amorphous layers compared with crystalline films of the same composition, due to a lower atomic packing density. The gradual decrease in the refractive index for YSZ layers grown at background pressures above 1.0
Pa marks the development of pores in the form of inter-columnar voids and can be used for sensitive quantification of the film porosity. The lattice order affects the fracture behaviour, as amorphous coatings show plastic deformation mediated by shear bands, while the crystalline layers yield hoop and surface cracks upon indentation. The crystalline 3YSZ films exhibit an enhanced fracture toughness compared with 8YSZ, which is related to the stress-induced transformation to the monoclinic phase in partially stabilized zirconia.
The water distribution across the membrane electrolyte assembly (MEA) of a working polymer electrolyte fuel cell (PEFC) was observed
in situ using neutron radiography. In order to resolve the ...distribution between the different layers of the MEA,
in plane imaging (cell membrane parallel to the beam) was used. Unprecedented spatial resolution for neutron radiography was obtained using a new detector system available at PSI combined with specific anisotropic resolution enhancement methods. A detrimental effect on performance of excessive water content in the cathode GDL was observed. Depending on the operating condition, a strong separation of the water content between ribs and channel was observed, particularly in the cathode GDL.
In situ XPS analysis of the electrochemical system EMIMBF
4/Pt was performed in a three electrode electrochemical cell as a function of electrode potential. The so-called electrochemical shift of the ...binding energy of electrolyte species with the applied electrode potential is confirmed. The reductive degradation of EMIMBF
4 at a potential of −
1.9
V vs. a carbon quasi reference electrode is demonstrated.
►
In situ XPS studies in a three-electrode electrochemical cell. ► The electrochemical shift was confirmed to be 1.0
eV/V. ► The N 1s line indicates a degradation product of EMIM. ► The degradation product is attributed to species in the electrolyte.
Partially reduced graphite oxide (GOpr) prepared from natural as well as synthetic graphite was used as electrode material for supercapacitors in 1
M Et
4NBF
4 in acetonitrile electrolyte. As a ...function of the degree of reduction of graphite oxide (GO) the graphite layer distance was varied between 0.46 and 0.33
nm. The initial specific capacitance of all samples was negligibly small around the open circuit potential, which was in agreement with the small BET surface area of the reduced GO powder of around 5
m
2/g. During the first potential cycle, however, electrochemical activation resulted in a specific capacitance of up to 220
F/g for samples with a graphene layer distance of 0.44
nm. The potential for anodic and cathodic electrochemical activation was found to be a function of the GO layer distance. Dilatometrie investigations showed a significant swelling and shrinking of the samples.
Laboratory-scale electrochemical capacitor cells with bound activated carbon electrodes and acetonitrile-based electrolyte were aged at various elevated constant cell voltages between 2.75
V and 4.0
...V. During the constant voltage tests, the cell capacitance as well as the capacitance and resistance of each electrode was determined. Following each aging experiment, the cells were analyzed by means of electrochemical impedance spectroscopy, and the individual electrodes were characterized by gas adsorption and X-ray photoelectron spectroscopy. At cell voltages above 3.0
V, the positive electrode ages much faster than the negative. Both the capacitance loss and resistance increase of the cell could be totally attributed to the positive electrode. At cell voltages above 3.5
V also the negative electrode aged significantly. X-ray photoelectron spectroscopy indicated the presence of degradation products on the electrode surface with a much thicker layer on the positive electrode. Simultaneously, a significant decrease in electrode porosity could be detected by gas adsorption.
▶ Aging of supercapacitors at high voltage (3.5V) depends on electrode mass ratio. ▶ Strategy for reduced aging is similar for acetonitrile and propylene carbonate. ▶ Increase of the mass of positive ...electrode leads to reduced aging. ▶ For a mass balanced cell the potential of the positive electrode is reduced.
The accelerated degradation of carbon based supercapacitors utilizing 1M Et4NBF4 in acetonitrile and in propylene carbonate as electrolyte is investigated for a constant cell voltage of 3.5V as a function of the positive over total electrode mass ratio. The degradation rate of the supercapacitor using acetonitrile as a solvent can be decreased by increasing the mass of the positive electrode. With a mass ratio (positive electrode mass/total electrode mass) of 0.65 the degradation rate is minimum. For the capacitor utilizing propylene carbonate as a solvent a similar effect was observed. The degradation rate was smallest for a mass ratio above 0.5.
The aging behavior of electrochemical double layer capacitors (EDLCs) based on activated carbon electrodes bound with poly(tetrafluoroethylene) (PTFE) was tested in electrolyte solutions based on ...acetonitrile (AN) and propylene carbonate (PC) at a constant elevated cell voltage of 3.5
V. The aging was quantified in terms of capacitance loss and resistance increase for the full cell and the individual electrodes. It is shown that the enhanced aging rate of symmetric EDLCs in either solvent at elevated voltages is dominated by the aging of a single electrode, and that the polarity of this limiting electrode depends directly on the solvent. In AN, the positive electrode ages much more rapidly than the negative, while in PC the negative electrode exhibits faster aging than the positive. After aging, the electrodes were investigated by nitrogen adsorption and X-ray photoelectron spectroscopy, revealing significant modifications of the electrode surface and providing clear evidence for the deposition of electrolyte degradation products on the electrodes.
