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Lifetime and performance assessment of commercial electric double-layer capacitors based on cover-layer formation Moritz Teuber, Maya Strautmann, Julia Drillkens, and Dirk Uwe Sauer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

Lifetime and Performance Assessment of Commercial Electric Double-Layer Capacitors Based on Cover-Layer Formation Teuber, M.1,2,*, Strautmann, M.1,2, Drillkens, J. 1,2, and Sauer, DU,2,3 1

Chair for Electrochemical Energy Conversion and Storage Systems, Institute for Power

Electronics and Electrical Drives (ISEA), RWTH Aachen University, Jägerstrasse 17-19, 52066 Aachen, Germany 2

Jülich Aachen Research Alliance, JARA-Energy, Templergraben 55, 52056 Aachen, Germany

3

Institute for Power Generation and Storage Systems (PGS) @ E.ON ERC, RWTH Aachen

University, Mathieustrasse 19, 52074 Aachen, Germany * Corresponding author: [email protected]

Keywords: Aging of Electric Double-Layer Capacitors, Accelerated Aging Test, Lifetime Prediction, Post-Mortem Analysis, Cover Layer Formation

ACS Paragon Plus Environment

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Abstract

The lifetime and performance of energy storage systems are essential characteristics for a major success of clean energy innovations, especially regarding the automotive sector. In this area, electric double layer capacitors (EDLC) represent the cutting edge of non-faradaic, high power and long lifetime energy storages. Usually, the degradation is neglected while operating or it is assessed by referring to simple rules of thumb. To better address the aging effects, commercial specimens have been investigated in-depth. The works can be split into two parts: firstly, extensive accelerated aging for more than 3 years to statistically analyze the degradation trend and significantly improve the current rules. Secondly, cell opening and surface characterization of the electrodes to gain a profound understanding of the ongoing processes and to correlate aging mechanisms to the statistics of the first part.

It is found that a prominent cover layer forms during degradation on the positive electrode scaling with lost capacitance. The used methods for in depth characterization include microscopy, x-ray diffraction and thermogravimetric analysis with subsequent mass spectrometry. The newly formed layer consists of polytetrafluoroethylene (also known as Teflon) which is still passable by charge carriers, albeit with a longer time constant. Additionally, the negative electrode shows corrosion and loss of contact. The composition of the cover layer has not been known yet. Thus, materials and especially electrolyte development can benefit from the results.


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1

Introduction: The Electric Double-Layer Capacitor (EDLC) is an energy storage technology that started to

gain traction in the process of decarbonizing our energy systems. These devices store energy by electrostatically accumulating charge carriers at a solid-liquid interface. Like in regular capacitors, charge is stored by the separation of charges. This process is non-faradaic and thus charge transfer reactions are normally not involved. The lack of electrochemical processes is the reason for the main drawback of EDLC: the energy density is low compared to chemical energy storage devices like batteries. At the same time, this gives rise to the unique characteristics which open up new fields of usage: supercapacitors are characterized by very high-power density, long lifetime (including cyclability) and a low voltage decay over time1. These features suit modern applications with high-power requirements that traditional batteries cannot meet. New electric drive concepts like start-stop technology, hybrid usage with different energy storage technologies and regenerative breaking foster the transition to an ecoconscious lifestyle. Electrically driven public transport becomes more and more relevant also including ferries and off-grid tram usage2, 3, 4, 5, 6, 7. The state-of-the-art electric double layer capacitor utilizes graphitic active material coated on aluminum current collectors for both electrodes. Highly porous activated carbon is used in combination with a polymeric binder and carbon black as conductive agent, coated on aluminum current collectors. The large surface area in combination with the small ion-carbon distance is the cause for the high capacitance (see Figure 1). The separator is either cellulose- (e.g. paper) or polymer-based (e.g. polypropylene). A standard electrolyte is composed of the solvent acetonitrile (C2H3N) and the salt tetraethylammonium tetrafluoroborate (TEA+BF4-).


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Figure 1. Schematic representation of the working principle of electric double layer capacitors. To access even the smallest pores, the solvation shell can be partially or fully stripped of the anions and cations. Energy storage systems constitute an important share of the costs in many modern applications. EDLC in particular must make up for their high cost by durability, hence the interest in understanding the aging processes to ultimately reduce life cycle costs, improve safety and reliability. The cell performance recedes due to faradaic, meaning charge carrier consuming, side-reactions which occur at the solid-liquid interfaces and which are promoted by operating conditions like elevated temperature, voltage and current8, 9. Despite ongoing research, aging processes are not fully understood on the microscopic scale10, 11, 12, 13, 14, 15. The exceeding of the operating conditions above nominal values causes an increase of these processes and is usually assessed by empiric rules. These estimations cannot predict the lifetime behavior of the devices


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precisely enough which is why we show an updated approach to evaluate the lifetime of supercapacitors here. By analyzing the degradation in detail, we can then tailor novel operating scenarios which make full use of the capacitor’s energy and power densities. Furthermore, the probable mechanisms causing the receding performance are experimentally investigated by opening the cells after end-of-life. The results allow drawing new conclusions about the impact of specific operating conditions on the reliability and lifetime.

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Device under test: The devices under investigation are commercially produced round cells with the above-

mentioned chemistry. The casing is 50 mm wide and 75 mm high and is made out of aluminum 99.5. The capacitor weighs 170 g. As will be detailed later, 39 capacitors were subject to the testing and accelerated aging. Regular measurements called check-ups were performed to determine capacitance and resistance. The specimens were cooled down to room temperature prior to the check-ups and immediately reheated to testing condition afterwards. The following procedure was repeated three times using the last repetition for the calculations. 1.

Constant current (CC) charge to rated working voltage RWV = 2.5 V

2. Pause with t = 1 min 3.

Constant voltage (CV) charge at RWV for t = 15 min

4.

Pause with t = 1 min

5.

CC discharge to V = 0.0 V

6. Pause with t = 1 min Usually, the charging and discharging currents are scaled to the active mass used for coating the current collectors. However, this value is not easily accessible for commercial cells (e.g.


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separating active material from additives such as carbon black). Therefore, the current was set according to the available capacitance of the device to 30 mA/F (equal to a C-rate of 42). This value is rather low compared to the high-power density of the device. While discharging, the voltage drop after t = 0.1 s was used for calculating the internal resistance of the cell. This “timebased” value of determining the resistance value was chosen to account for the logging accuracy of the measurement device. The discharged coulombs were used to calculate the capacitance. Figure 2 illustrates a CC-CV charge and subsequent discharge. The measured and calculated characteristics of the fresh specimen are summarized in Table 1. The values are based on the measurements on 39 cells. Additionally, one standard deviation is given. The specific energy and specific power are calculated using equations (1) and (2) below: C ∗ V2 ∆C ∗ V2 Em = ± 2 ∗ m ∗ 3600 s/h 2 ∗ m ∗ 3600 s/h

(1)

Figure 2. Charge-discharge curve of the supercapacitors. A constant current charge is followed by a pause (1 min) and a constant voltage charge for 15 min. Afterwards, a constant current discharge is used to determine capacitance and resistance.


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𝑉2 𝑃𝑚 = ± 4∗𝑅∗𝑚

(

―

𝑉2 4 ∗ 𝑅2 ∗ 𝑚

)

∗ ∆𝑅

(2)

Capacitance C and resistance R are here taken from the measured data. Their standard deviation was incorporated as error propagation to calculate specific energy Em and specific power Pm. Table 1. Electrical parameters of investigated cell based on the measurements on 39 specimens. One standard deviation is given. Parameter

Value

C – capacitance

(734.34 ± 35.56) F

R – resistance

(0.69 ± 0.24) mΩ

Em – specific energy

(3.75 ± 0.18) Wh/kg

Pm – specific power

(13.32 ± 4.63) W/kg

Capacitance C and resistance R are here taken from the measured data. Their standard deviation was incorporated as error propagation to calculate specific energy Em and specific power Pm.

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Accelerated lifetime testing: To assess the degradation rate and to determine the failure modes, electric double-layer

capacitors are aged artificially. This means, the operating conditions are elevated compared to standard conditions to artificially shorten the lifetime of the devices. Two modes can be distinguished: calendric aging and cyclic aging. Calendric aging aims at investigating basic aging effects by neglecting the influence of current flow. Thus, only the voltage and temperature are changed. Cyclic aging investigates the deterioration caused by applying power/current profiles.


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In this work, the effects causing the receding performance of commercial supercapacitors is to be investigated. Therefore, no current was applied (calendric aging). The calendric aging of commercial EDLCs has been investigated in literature but few of those tests lasted longer than some months16, 17, 18, 19, 20. Depending on the test conditions, some cells were tested for more than 3 years in this work. The lifetime of supercapacitors is often predicted using a heuristic lifetime expectancy. It is defined for nominal conditions (here V0 = 2.5 V and T0 = 25 °C) according to the following equation (3):

𝑡𝑙𝑖𝑓𝑒𝑡𝑖𝑚𝑒 = 𝑡0

( ∗ 0.5

𝑇 ― 𝑇0 𝛼

𝑉 ― 𝑉0

) ( +

𝛽

)

(3)

The constants α = 10 K and β = 100 mV are empirically deducted and are used as a rule of thumb21. This indicates lifetime halves if one of the operating conditions voltage or temperature exceed the nominal conditions by α or β (e.g. if the device is operated at 45 °C instead of 35 °C). Under nominal conditions, EDLC are usable for up to t0 = 10 – 20 years. It is not possible to test new specimens for this time. Therefore, the equation (3) above is used to accelerate the tests by increasing the operating conditions (temperature and voltage) and then correlating back to nominal conditions. An acceleration factor AF is introduced according to the following equation (4) which describes how much faster the test should be: ∆𝑇 ∆𝑉 + ( 𝛼) (𝛽) 𝐴𝐹 = 2

(4)

This implies a variety of voltage and temperature combinations to achieve a desired degradation time. For example, testing a supercapacitor at 35 °C (instead of 25 °C) and 2.6 V


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(instead of 2.5 V) yields AF = 4 and thus a 4 times shortened test. However, the mechanisms causing the degradation are not fully understood to date. As mentioned before, this is why EDLC's have been aged calendrically in this work by applying different constant temperatures (oven Memmert UFE500) and constant voltages with a constant power supply (Rohde&Schwarz HM8143) (see Table 2). Three specimens have been tested for each condition. The devices receding electrical characteristics were measured regularly using the current and voltage profile in Figure 2. In contrast to previous works, not all cells were stored at nominal voltage or above but also at lower voltages. Thus, it was investigated if lowering the voltage has a beneficial influence on the lifetime of EDLCs (as suggested by equation (4)). Table 2. Aging conditions for the calendaric aging tests. Three cells have been tested at each condition totaling to 39 cells. The acceleration factor AF gives the factor by which the test should be shortened compared to testing at nominal conditions. 2.3

2.4

2.5

2.6

2.7

2.8

45

X (AF =4)

X (AF = 16)

X (AF = 32)

55

X (AF = 8)

X (AF = 32)

X (AF = 64)

65 75

X (AF = 4)

X (AF = 8)

X (AF = 16)

X (AF = X (AF = 64) 32)

X (AF = 32)

X (AF = 64)

The end-of-life of the capacitors is here defined to be 80 % of the initial capacitance. The degradation curves of our tests are shown in Figure 3. The curves illustrated with a strong color are the mean of the single cell values for each condition depicted in a lighter color. The normalized capacitance was calculated using the capacitance of the fresh cell. The previously


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Figure 3. Capacitance degradation curves for the specimens of test matrix. The cells were aged calendrically (without applying a current) at the conditions given in the legend. The parameters were determined using the abovementioned checkup routine. Increased voltage and temperature negatively influence the lifetime of the devices.

made assumptions do not hold up. The degradation trend does not follow an exponential function but instead, different aging phases can be observed which can roughly be divided in a stronger capacitance loss at the beginning, a following linear section and a rapid decrease. Measurements performed at the same conditions line up nicely at the beginning resulting in a low standard deviation. But as the degradation gets stronger, especially after transitioning in the aforementioned rapid decrease in capacitance, the spread gets wider which can be clearly seen in Figure 3 for higher aging factors AF = 16 or AF = 32. This might hint at quality variations during production. The standard deviation was calculated but is deliberately not shown here to better highlight the discrepancies found. The lifetime depending on the operating conditions can


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now be newly analyzed. At the beginning of the test, the capacitance decreases strongly for most conditions. In lithium-ion batteries, this can be caused by the passive electrode parts, also called overhang22. However, our electrodes have almost no overhang which is why we attribute this observation to a so called “Burning-Phase”. This describes fast, irreversible redox reactions of parasitic groups on the surface of the electrodes and electrolyte molecules at the beginning of use23. Afterwards, a linear capacitance decrease can be observed which is attributed to electrochemical degradation reactions meaning an irreversible loss of capacitance. This degradation trend is representative of the chemical processes detailed below in section 3.1 and 422. Furthermore, the deduction of the device’s lifetime is based on this linear curve trend. Thus, the chemical analyses carried out within section 4 describe this linear phase. A third stage of the curve can be distinguished showing a transition into an accelerating stage. This is indicated by a steep slope of the mostly linear aging curve at this stage. This effect implies a self-amplifying reaction: -

an increased resistance causes a stronger self-heating which accelerates the aging reactions, which in return cause the resistance to increase faster.

-

access to certain electrode areas is blocked by e.g. electrical insulation (binder/aluminum dissolution) or formed cover layers (see below). This increases the current density for the remaining electrode and thus also increases heat generation and the rate of unwanted side reactions.

The last phase seems to start having reached around 80 % remaining capacitance. This unpredictability of the degradation speed after roughly 20 % capacitance loss is the main reason why the lifetime of a device is typically set to this value. Comparing our tests to the previous rule of thumb (lifetime halving factors α and β), an adjustment of these parameters is necessary. The


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formula significantly underestimates the influence of elevated temperatures and overestimates the influence of elevated voltages. Maximum temperature considerations The temperature of 75 °C was chosen as maximum temperature. The saturation vapor pressure p of acetonitrile depending on temperature T can be approximated with the well-known Magnus-formula (equation (5)):

( )

𝑝 = 𝑎 ∗ 𝑒𝑥𝑝

𝑏∗𝑇 𝑇+𝑐

Fitting this equation to experimental data from literature24,

(5)

25, 26, 27, 28,

the constants can be

determined to be a = 33.89 hPa, b = 13.69, c = 247.76 °C. Solving this equation for T, the temperature for the liquid-gaseous equilibrium at ambient pressure (p0 = 1013.25 hPa) has been determined to Tp0 = (81.76 ± 0.29) °C. Based on these considerations, acetonitrile does not evaporate when heated to 75 °C. If gaseous products occur inside of the cells housing, they stem from electrochemical side reactions producing gas. Additionally, the inner pressure will rise and thus push the evaporation process to higher temperatures.

3.1. Analysis of lifetime dependency on voltage and temperature: From the results of the accelerated aging tests the dependency of the aging behavior on ambient temperature and cell voltage has been analyzed. As has been shown in the previous section, the impact of voltage and temperature must be analyzed separately for capacitance decrease and resistance increase respectively. Assuming the general Arrhenius dependencies, they were used to deduct new factors which halve the lifetime. The logarithmic plot is shown in Figure 4. The Arrhenius-like behavior is still applicable since linear regression is possible in this


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Figure 4. Lifetime of the capacitors exemplarily shown depending on temperature for capacitive aging. Different curve slopes can be distinguished dependent on the applied voltage. Adapted with permission from9, Copyright 2017 RWTH Aachen University. depiction. The following analysis is based on the end-of-life criteria of 80 % of the initial capacitance respectively doubling of the internal resistance. All cells have reached the end-of-life criterion except a couple of specimens stored at 45 °C, their lifetime was linearly extrapolated to reach the 80 %. The lifetime curves in Figure 4 show no significant temperature dependence, meaning the slopes of the linear regression lines differ from each other9. That's why every aging condition has to be regarded separately, meaning not only a single but multiple factors α and β have to be deducted from the tests. This means the impact of temperature and voltage changes for every condition and cannot be generally approximated for all cells. It is remarkable that the lifetime of those cells stored at RWV and 55 °C showed shorter lifetime than expected compared to the other aging conditions. The temperature and voltage


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increase ΔT and ΔV which half the lifetime can be deducted from the slopes and are summarized in table Table 3.

Table 3. Lifetime halving factors taken from results presented in Figure 4. The values for capacitive and resistive aging are the slopes of the fitted curves. Reproduced with permission from9, Copyright 2017 RWTH Aachen University.

ΔV

ΔT

Capacitive

Resistive

45 °C

119 mV

76 mV

55 °C

138 mV

156 mV

65 °C

133 mV

118 mV

2.5 V

10.2 K

10.5 K

2.6 V

7.1 K

6.7 K

2.7 V

5.6 K

6.3 K

2.8 V

5.8 K

7.1 K

These factors given in the table above can be used to generally evaluate the dependency of the degradation behavior on voltage and temperature. Particularly the influence of the temperature has been underestimated by the simple approach discussed. Depending on the voltages above RWV, a lifetime halving temperature of about 5-7 K above nominal conditions can be assumed. The previously assumed 10 K only fit our measurement data at RWV29. This is why temperature as operating condition should be observed the most carefully. Users of supercaps should be aware of the fact that even a seemingly small increase to 35 °C decreases the lifetime significantly and indicates deratings in the usability. The dependency on voltages above RWV is


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lower than previously assumed. Only resistive aging at T = 45 °C is significantly dependent on overvoltage. These measurements also show that there are complex aging mechanisms taking place in the usually only electrostatic double layer capacitor. Some of those ongoing processes have been addressed in the literature (see above). This examination here using industrially produced cells in larger quantities gives now more insight into application relevant aging characteristics.

4

Cell tear down analysis: Afterwards, the previously described set of electrical tests was complemented by a post-

mortem study: this includes opening the casing and performing optical, and physicochemical experiments on the single cell components. The cells were opened, and samples taken to further examine the impact of the aging conditions on the inner parameters of the capacitor. The aim of this analysis is to achieve an indepth understanding of the aging processes at each electrode and to detect possible causes for the capacitance decrease respectively the resistance increase. One fresh specimen and four differently aged capacitors were chosen according to their lost capacitance. This was done assuming the same deterioration processes occur and are accelerated by increased temperature and voltage: the remaining storage capabilities were (relative to their initial capacitance): C1 = 1.00, C2 = 0.77, C3 = 0.57, C4 = 0.15, C5 = 0.07. Every respective EDLC was discharged to V =0 V (with a CC and CV phase) prior to opening. Having been transferred to an argon-filled glovebox, the aluminum casing was cut open with a can opener close to the positive terminal. Remaining electrolyte was pipetted into an air-tight flask and the jelly roll was removed from the shell. As the cells dry out with progressing deterioration, it was impossible to take electrolyte


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samples from some cells. An outer foil, probably acting as vapor barrier, had to be removed before the electrodes and separator could be unrolled and separated. Some observations made while performing the tear down analysis are noted in Table 4. A cover layer forms on the positive electrode rendering it stiff and heavy. In contrast, the active material of the negative electrode loses the contact to the current collector with progressing aging. This different deterioration of the two electrodes has already been mentioned in the literature30, 14, 15 and our results here accord to it. Samples with varying diameters were punched from the middle of each electrode for the following analyses.

Table 4. Overall impression of aged electrodes during cell opening. These observations also correlate positively with the progress of deterioration. A “0” indicates no changes, a single “+” or “-“ a slight change and a double “++” or “--” a major change in the electrodes characteristics. Observation

Electrode

Contact loss to current collector

0

++

Brittleness

0

++

Stiffness

++

0

Residues

++

+

4.1. Scanning electron microscopy: Scanning electron microscopy gives a first impression of the aging processes. Argon milling was used to prepare a clean cross section for each sample. Figure 5 shows images of a pristine electrode and the positive and negative electrode of the most aged cell (C5 = 0.07). Positive and negative electrode of a pristine cell do not differ. Two parts can be distinguished on each


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subfigure: on the left back-scattered electrons (BSE) were detected, secondary electrons (SE) on the right. Both modes are shown to get a good contrast both for the active material graphite as well as for the aluminum current collector.

(a)

(b)

(c)

Figure 5. Cross sections of a new (a) and the most aged positive (b) and negative (c) electrodes. Back-scattered and secondary electron imaging is shown for each subfigure. Distinct aging mechanisms for each potential (pos. vs. neg.) can be observed. The new electrodes show a homogeneous distribution of the active material and a sharp graphite-aluminum interface. Some surficial features can be observed on the outer parts of the current collector which probably stem from the normal surface roughness and from sample


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preparation. In contrast to the pristine electrode, the aged positive electrode (Figure 5 (b)) is distorted and has cracks throughout the graphite. We suggest that the mechanical damages can be traced back to cell opening and uncoiling the jelly roll: the electrode material felt stiff and some force needed to be applied to roll out the electrode. Furthermore, the outer layer is dense and less porous as compared to the fresh electrode (well observable in the top left part of Figure 5 (b)). Additionally, some fibers can be observed, mainly in the formed cracks in the active material. Huang et al. show that they stem from recrystallized PTFE during aging31. Additionally, a weight increase of m/m0 = 1.32 (aged electrode compared to new) supports the formation of a cover layer which is a major driver for the deterioration of electrical parameters. Additionally, prominent white spots in the secondary electron image of Figure 5 (b) suggest a deposit of additional material inside the electrode. A subsequent EDX analysis identified this to be aluminum (not shown in picture). The amount of aluminum in the electrode samples was tracked using optical emission spectroscopy (ICP-OES). The results are shown in Figure 6. The aluminum content of the negative electrode drops whereas the amount on positive electrode rises. This is in good accordance with the SEM imaging: the aluminum corrodes on the negative electrode and deposits on the positive electrode and is detectable via EDX. Current collector corrosion has already been investigated in literature: Kurzweil et al.32 and Huang et al.31 report etched pits on aged charge collectors. HF dissolves the oxide layer, producing water 6 HF + Al2O3  2 AlF3 + H2O. The process might entail an increased contact resistance and could lead to electrically isolated areas of the active material which then also causes a drop in capacitance.


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Figure 6. Aluminum content of the positive and negative electrode samples measured via ICP-OES. A loss of weight on the negative electrode is opposed by a weight gain on the positive electrode. However, no strict correlation in between “lost” and “gained” aluminum can be seen indicating a loss of aluminum to the rest of cell, e.g. the casing.

Different aging processes take place at the negative electrode (Figure 5 (c)). Cracks and a rough graphite/aluminum interface agree with the weak adhesion of the active material as already observed when uncoiling the jelly roll. Partial current collector corrosion and/or dissolution provokes these symptoms as well as the deposition of metal on the opposite electrode.

4.2. X-ray diffraction analysis: The next step in analysing the aging processes of the capacitors was a structural investigation by x-ray diffraction (XRD). A Panalytical Empyrean was used for the following investigations. The electrode samples were analyzed on an aluminum holder in Bragg-Brentano theta-2 theta configuration using a copper anode (Cu Kα with a wavelength of λ = 0.154184 nm). Before


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analyzing the data, two corrections were done: 1. the intensity changes introduced by using a programmable divergence slit were reverted; 2. a standard height error of Δh = 0.75 mm stemming from sample preparation was mathematically corrected using following equation (6): ∆𝜃 = 2𝜃 ―

(180𝜋 ∗ 2 ∗ ∆ℎ ∗𝑅𝑐𝑜𝑠(𝜃))

(6)

R = 240 mm here describes the goniometer radius. We analyzed samples from both positive and negative electrode and from different positions along the length of the electrode. XRD patterns do not differ significantly between the positions so we show averages of the measurements. The findings described below were not observed on the negative electrode which is why positive electrode results are shown. The resulting diffraction patterns are shown in Figure 7 and the observed peaks are given more detailed in Table 5. Measurements stemming from capacitors at different aging stages are shown. New material (blue) and aged samples, meaning more lost capacitance, are given. Almost all peaks for

(a)

(b)

Figure 7. X-ray diffractograms of the positive electrode at different deterioration states of capacitance (a). The 2θ = 18.04° intensity increases with progressive aging and is characteristic for the PTFE(100) peak (b).


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new and aged samples correlate to the used materials for manufacturing the electrode: carbon and aluminum. It should be noted that activated carbon was used for preparing the electrode. Table 5. Listing of the observed peaks with XRD. The active material graphite and the current collector aluminum can be distinguished indicating a penetration of both materials by the x-rays. The emergence of the peak at 2θ = 18.04 is characteristic for PTFE and indicates progressive cover layer formation. 2θ / °

hkl

Association

Ireal / %

Iideal / %

18.04

(100)

(C2F4)n

6.8

100.0

26.61

(002)

C

100.0

100.0

38.48

(111)

Al

3.1

100.0

(011)

C

(002)

Al

54.72

(004)

C

6.3

6.4

65.19

(022)

Al

77.4

28.7

78.24

(113)

Al

9.6

32.0

82.45

(222)

Al

-

9.1

87.08

(006)

C

0.8

1.3

44.69

52.9

16.8 47.7

The measured intensities do not match the ones proposed by the database, this is most obvious looking at the Al peaks which deviate strongly. This effect is caused by a preferred orientation of the aluminum stemming from production of the current collector foil. An interesting observation is the presence of the first peak at 2θ = 18.04° which cannot be assigned to carbon or aluminum. After a more in-depth investigation this peak was assigned to the hexagonal crystal structure of poly(tetrafluoroethylene) (PTFE), also known as Teflon. It is not uncommon to find PTFE in double layer capacitor electrodes because it is often used as


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binder material. However, its peak is almost not distinguishable in new electrodes. Figure 7 (b) shows the PTFE peak intensity plotted against the remaining capacity. The intensity correlates positively with the capacitance decrease. This indicates the formation of additional and/or the recrystallization of existing PTFE. This observation is in line with the positive electrode’s stiffness. Additionally, a small amorphous spectrum bulge just above 2θ = 20° can be observed which is caused by the inactive, non-crystalline parts of the electrode, mostly binder.

4.3

Thermogravimetric analysis with mass spectrometry:

The previously found cover layer investigated via SEM and XRD is to be investigated further. Thermogravimetric analysis with subsequent mass spectrometry was used in order to further investigate the formed species. Temperatures of up to 400°C were used to evaporate any organic species present on the surface of the elctrodes. The activated carbon material was scraped off the aluminum current collector and crushed with a spatula.


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The two samples, a pristine electrode and the most aged positive electrode behaved differently

Figure 8. Mass and heat flow during TGA of a pristine and an aged anode. The first drop in mass corresponds to the vaporization of water (endothermic). The decomposition of TEABF4 sets on at around 250 °C (exothermic). during this preparation: the pristine material came off rather easily, while the aged sample stuck firmly to the current collector and agglutinated. This observation is in accordance with the microscopy findings. The samples are exposed to ambient air during this preparation. The Mettler Toledo TGA/DSC 1 apparatus heats the samples from room temperature to 400 °C at a rate of 5 °C/min, then holds temperature for 1 h and cools down at 20 °C/min. A blank measurement with an empty crucible serves as correction. Figure 8 shows the mean relative mass and mean heat flow from the six samples (three samples of pristine and aged positive electrode respectively). During the first stage of heating up to around 110 °C the material loses adsorbed water. Accordingly, the mass spectra show an increase in molecules with m/z = 18 for H20. Figure 9 illustrates the development of selected peaks in the mass spectra during the heating program. The process is related to a dip in the heat flow: the samples take up the


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enthalpy of vaporization. The pristine sample (a) material held more water than the aged sample (b), maybe just related to water absorption from ambient air during sample preparation. During the subsequent heating phase, up to about 280 °C, the mass spectrometer detects molecules with m/z = 31, 44 – functional groups and CO2. CO2 forms from various carbonaceous materials in the presence of O and heat. Surface groups on carbons produce CO2 when decomposing: carboxylic groups at 200-250 °C, lactones at 190-650 °C and anhydrides at 350-400 °C33. The reported temperatures vary. Furthermore, a molecule with m/z = 43 dispels from the pristine sample. The most likely explanation is the presence of remaining acetone C3H6O. It was used during sample preparation for this test. Upon further heating, a number of fragments appear at m/z of 20, 31, 33, 47, 48, 50, 51, 69, 101. The signal peak of these decomposition products lies at 336 °C for the pristine electrode material and at 308 °C for the aged anode. The fragments relate to the decomposition of TEABF4. The reported decomposition temperature of TEABF4 lies around 340-445 °C.


24

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(a)

(b)

Figure 9. Production of fragments during the heating programm. Fragments with the selected m/z = 18 corresponds to water, m/z = 44 to CO2. The fragments 101, 69 and 47 relate to TEABF4. Further fragments are assigned to hydrocarbons. In the aged anode material (b), decomposition reactions seem to spread over a wider range and set on at lower temperatures than in the pristine electrode material (a).

However, looking at the heat flow and weight change in Figure 8, the decompositions already starts below 300 °C and peaks around 375 °C. Thus, the observed peaks in the mass spectrometry align with the decomposition of TEABF4. Cation decomposition and exemplary reactions of the salt are shown in Figure 10. The cleavage of one ethyl chain from TEA+ via Hofmann elimination (flourine ion) is well known in tetraethylammonium salts (see formula (7) in Figure 10)34, 35. The observed ratios of 101, 20 and 28 can be explained with this reaction path. Bourgeat-Lami et al.36 examined the decomposition of TEA+ with hydroxide and/or Si-Oanions (Y-) in porous zeolites. They report C2H4 and HF as products of the Hofmann elimination. This corresponds to the observed molecule masses 101, 20 and 28. The solid TEABF4 can also decompose into triethylamine, boron trifluoride and fluoroethane in an exothermic reaction (see formula (8) in Figure 10)34. The corresponding molecule masses


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are 101, 48 and 68, while 101, 47 and 69 appear in the measured mass spectra. A swap of carbon and boron or the addition of hydrogen might cause this. Furthermore, m/z = 69 can be assigned to several fluoropolymers. PTFE breaks into its monomers C2F2 with mass m/z = 100, starting at 350 °C37. During a third heating phase, fragments with a m/z ratio of 25, 26, 39, 41, 43, 55, 56, 57 are observable. Those can be related to longer polymer chains. As a summary, the TEABF4 salt decomposes and forms gaseous products from 250 °C on. If the resulting hydrocarbon fragments polymerize in the pores of activated carbon under the influence of acidic groups, this reaction pathway might also lead to a polymer deposit of hydroor fluorocarbons during aging. Decomposition reactions spread over a wider range and set on at lower temperatures in the aged anode.

(7)

(8)

Figure 10. Possible reaction paths of the cation present in the electrolyte. Formula (7) shows the cleavage of one ethyl chain from TEA+ via Hofmann elimination (flourine ion) and is well known for ammonium salts. Furthermore, the salt TEABF4 can also decompose into triethylamine, boron trifluoride and fluoroethane in an exothermic reaction, visible via TGAMS.

4.4

Cover layer formation:


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In this section, we give a possible explanation for the formation of a PTFE covering layer on the positively charged electrode of the supercaps based on the previously described findings. The formation of additional PTFE apart from the binder is evident from the XRD results. This is further supported by TGA measurements. The cleavage of one ethyl chain from TEA+ via Hoffmann elimination is well known in tetraethylammonium salts. The solid TEABF4 decomposes into triethylamine, ethene and a hydrogenated anion in an exothermic reaction. Bourgeat-Lami et al.36 examined the decomposition of TEA+ with hydroxide and/or Si-O- anions (Y-) in porous zeolites. They also report C2H4 and further YH as products of the Hofmann eliminations (see Figure 10). We here assume the presence of OH- and/or F- which seems plausible, see e.g.14, 38. This is followed up by two ensuing reactions: 1. The fluorination of hydrogen in ethene 2. The present radicals act as chain initiators for the polymerization These reaction mechanisms explain the formation of more PTFE being present as thick cover layer on the positively charged electrode. The positive ion TEA+ reacts at the negative electrode. The produced Ethene is not charged and thus distributes freely inside of the cell. The fluorination mentioned above, and subsequent polymerization can only happen at the positive electrode since strongly electronegative anions are required for this reaction. This is in good agreement with our observations: Aluminum corrosion deteriorates the negative electrode whereas cover layer formation is impeding the electrical performance of the positive electrode.

5

Conclusions:


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Page 28 of 34

The aging of electric double-layer capacitors cannot be addressed with a simple rule of thumb. 39 commercial supercapacitors have been tested in an extended aging matrix (calendaric) to determine the dependencies of lifetime on temperature and voltage more precisely. It could be shown that temperature has a stronger impact on lifetime than voltage and furthermore that the lifetime dependency on temperature increases with rising overvoltage. These novel findings refine and add to the already existing knowledge on capacitor aging. Lifetime halving factors for temperatures and voltages have been determined assuming exponential dependencies and can be used to develop new operating strategies taking full advantage of lifetime favorable conditions. These findings are interesting as they describe significant aging of commercial specimen even within the datasheet specifications which users should be aware of. Especially the temperature should be kept at the lowest possible even though the specifications would allow for higher temperatures. Thus, the lifetime can be significantly prolonged by not increasing average operating temperature by more than 5-7 K above nominal conditions (here 25 °C). Furthermore, the post-mortem results and literature findings indicate significant differences in aging processes for the two electrodes. A distinct optimization of the materials formulation on each electrode could prove beneficial regarding lifetime. For example, additives inhibiting cover layer formation on the positive electrode could be added to the electrolyte mixture whereas a protective coating on the negative current collector would increase the stability of said electrode. Additionally, aged specimens have been investigated more thoroughly with a post-mortem analysis. The formed cover layer on the positive electrode impeding normal performance was investigated. The chemical composition was analyzed using electron imaging, x-ray diffraction and thermogravimetry. It consists of polytetrafluoroethylene also being used as binder. A formation mechanism via Hoffmann elimination and subsequent fluorination and polymerization


28

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was proposed which has not been discussed to this extend yet. These novel findings help understand the measurable performance degradation of EDLC.

6

Acknowledgment: The authors thank Deutsche Forschungsgemeinschaft and the Federal Ministry for Economic

Affairs and Energy for supporting this research within the projects SA 1392/1-1 and 0327822J. We thank Rita Graff and Holger Blanke for performing OES measurements and assisting with the electrical test setups respectively.

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