Causes and Mechanisms of Supercapacitor Ag(e)ing and Degradation

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This entry is adapted from the peer-reviewed paper 10.3390/molecules28135028

The most prominent and highly visible advantage attributed to supercapacitors of any type and application, beyond their most notable feature of high current and power capability, is their high stability in terms of lifetime, number of possible charge/discharge cycles or other stability-related properties. Actual devices show more or less pronounced deterioration of performance parameters during time and use, they show ageing.

supercapacitor ultracapacitor electrochemical capacitor electrochemical double layer capacitor aging ageing degradation

1. Introduction

Among the various properties and parameters describing the performance of a supercapacitor, the stored energy, the power (current) possibly delivered and received, and the internal resistance ESR (electrical series resistance) are most relevant for the use and the proper operation of such a device. In device-related terms, the capacitance and the ESR are commonly considered when evaluating a device. Although there does not appear to be a standard, it seems that a capacitance loss of 20% compared with the initial value and an increase of the ESR by 100% are the failure criteria that describe a device as worn out. The statement in ^[1] suggesting that the 80% criterion was set in the IEC 62391-1 standard attributed to ^[2] is misleading; the authors in the latter communication refer to this standard only because it describes the procedure of measuring the capacitance without proving any further criterion. Despite the claims praising the exceptional stability, i.e., constancy of said parameters, they tend to change over time and use, i.e., calendar ageing in terms of time passed and cyclic ageing in terms of run charge/discharge cycles can be noticed ^[3]. Both forms of ageing (the spelling "aging", also found in the literature, is not used in the following text) can be studied with different methods, but results may be different according to the various methods and different parameters used.

Presumably the most popular approach is running galvanostatic charge/discharge cycles (GCD) within a defined electrode potential window for an electrode (an electrode material) or a cell voltage window for a complete device. Comparison of the recovered charge (or, less frequently, energy, as discussed below in more detail) or the capacitance calculated from the recorded data for the first and the last cycle yields a percentage change, in most cases reported as capacity (the term capacitance is used as a synonym in many reports) retention. In earlier publications, this was termed degradation, with degradation reported in percentages appearing somewhat awkward. The term degradation is currently applied to material and device changes resulting in poorer performance. Recently, the effects of various factors on the degradation (not ageing) of supercapacitors have been analysed in a report that is somewhat difficult to understand ^[4]. At least for electrochemical double-layer capacitors (EDLC), ageing under cycling conditions was found to be faster than plain calendar ageing under similar conditions regarding applied voltage ^[5]. Basically the same information on ageing can be obtained for both electrodes and devices from cyclic voltammograms.

2. Causes and Mechanisms

2.1. Causes and Mechanisms on the Material and Component Level

2.1.1. EDLC-type Materials and Electrodes

Chemically and electrochemically aged carbon, i.e., exposed to acetonitrile-based electrolyte solutions, without and with electrode potential control at various temperatures, has been studied ^[6]. Evidence of both chemical and electrochemical reactions at “non inert” carbon surfaces (different types of carbon were studied) and the covalent attachment of most elements found in the electrolyte solution on the carbon surface (more on the positive electrode), yielding the formation of surface functional groups, was noticed. In a subsequent study, the authors identified further degradation contributions, including structural modification of the porous structure, clogging of pore openings and decomposition of cell constituents ^[7]. Infrared spectroscopy confirmed that performance losses were mostly due to positive electrode degradation ^[8]. Electrodes with N-doped graphene showed performance degradation attributed to surface oxidation as evidenced with X-ray photoelectron spectroscopy (XPS), causing increased Ohmic resistance ^[9]. The concluded inhibition of electron transfer remains unclear; presumably, electron transport (conduction) in the absence of an electron transfer Faradaic reaction was the intended meaning. The detrimental effects of surface functional groups have been highlighted based on a detailed study ^[10]. Ageing of an AC derived from carbonized polypyrrole PPy has been studied using a combination of electrode potential/cell voltage float and the cycling protocol of both single electrodes and complete cells with an aqueous KNO₃ electrolyte solution ^[11]. Essentially, self-discharge and capacitance retention were studied, but neither degradation nor ageing were actually addressed. Activated carbon spheres used in the positive and negative electrodes of EDLC devices showed decreasing mechanical strength with growing current density ^[12]. Structural degradation as a further effect was tentatively assumed but not demonstrated. Degradation of freestanding electrodes prepared from monolithic carbon spherogels results in loss of the central cavity and sphere degradation of thin-walled samples; therefore, thicker walls are preferable ^[13]. The particularly detrimental effect of even small concentrations of chloride ions on carbon stability has been highlighted ^[14]. Ageing and degradation of eight different carbon materials used in the negative electrode of EDLC devices with electrolyte solutions of propylene carbonate or acetonitrile and Et₄NBF₄ have been studied ^[15]. Different from degradation in the range of higher electrode potentials (i.e., of positive electrodes), degradation initially happens on the basal plane, not at edge sites. Degradation of N-doped graphene nanoflakes in contact with an ionic liquid electrolyte has been attributed to the intercalation of tetramethylammonium ions between graphene layers resulting in exfoliation ^[16].

Substitution of fluorine-containing binders by materials from renewable sources is of growing interest. Unfortunately, stability in terms of capacitance retention for electrodes prepared with such binders varies widely, with results not yet being practically viable ^[17]. Collapse (i.e., restacking or agglomeration in more general terms) of graphene- and MXene-based materials ^[18] results in capacitance and performance degradation ^[19]^[20]; in addition, oxidation may have a negative effect on performance ^[21]. A similar effect resulting in poorer performance is restacking of carbon nanotubes (CNTs) during fabrication, subsequently hindering ion movement ^[22]. This type of degradation does not appear to happen during the operation time of a device and is not a form of ageing.

In a study limited to electrodes made from multiwalled carbon nanotubes (MWCNTs) supported on stainless steel, evidence of ageing and degradation similar to that obtained with other carbon materials in contact with aqueous as well as non-aqueous electrolyte solution was obtained ^[23].

Corrosion of stainless steel current collectors in EDLC-type supercapacitors in particular at the positive electrode has been studied ^[24], and corrosion protection by a siloxane coating has been proposed to slow down or even inhibit ageing.

2.1.2. Battery-Type Materials and Electrodes

As discussed elsewhere ^[25]^[26], battery-type electrodes are mostly composed of an active material, a binder and added electronically conducting carbon to make up for the insufficient conductivity of the active material. As single active materials, metal chalcogenides of simple- (e.g., MnO₂) or complex-composition MeMe1_xMe2_yO_z (e.g., CoFe₂O₄), intrinsically conducting polymers (ICPs), and a few further inorganic materials showing electrochemical redox activity (e.g., transition metal oxynitrides) have been examined. Both organic and inorganic materials have been reviewed extensively. Because of their inherent flaws (especially the insufficient electronic conductivity and stability of many chalcogenides and lack of stability of many ICPs), composites have been prepared and studied, reviews are available ^[25]. The causes and mechanisms of material degradation of single materials and electrodes, as well as of composites and their electrodes, differ substantially because of the fundamentally different chemistries and properties of the materials. In the case of MnO₂-based electrodes, the dissolution of manganese ions is a frequently encountered mode of degradation and electrode ageing ^[27]^[28]^[29]. As pointed out, in addition to the formation of actually solvated manganese ions, detachment of particles becoming possibly inactive, redeposition in structurally different forms and further phenomena may occur. Detachment of active material particles (somewhat misleadingly called delamination in ^[30]) may also occur with composite materials. MnO₂ combined with cotton fabric into a positive electrode showed some particle coarsening during redox cycling ^[31]. In the case of MoO_x as an active mass, certain redox states of the molybdenum may be prone to instability; thus, the selection of the suitable operating electrode potential window is highly important ^[32]. Volume change during redox cycling has also been identified as the reason for the electrode degradation of manganese molybdate (MnMoO₄·nH₂O); using nanosheets of this material increased stability ^[33]. An unspecified “phase separation” in composites with carbonaceous materials has been claimed to be a cause of electrode performance degradation ^[34]. α-Co(OH)₂ has been claimed to have a higher storage capability than β-Co(OH)₂ but is converted into the latter phase upon redox cycling, resulting in capacitance losses ^[35]. Formation of α-Co(OH)₂ via hydrolysis of ZIF-67 results in a stable version ^[35]. The ageing of TiO₂ nanotubes in aqueous electrolyte solutions has been studied ^[36]. The results suggest a small dependency of performance degradation on synthesis procedure. Similar observations regarding the effects of synthesis details have been reported for a composite of such nanotubes coated with MnO₂ ^[37]. A very specific type of ageing was encountered with catechol-based polymeric redox materials ^[38]. A few days after preparation, a change of colour of the material already indicated an autoxidation of the catechol groups and subsequent crosslinking associated with a higher redox potential. This “ageing” increased the operating cell voltage and thus its energy density. Studies of ageing and degradation of a composite electrode of reduced graphene oxide rGO/Fe₃O₄ used in a sodium sulphate electrolyte solution revealed the formation of FeS, which showed poor stability during further charge/discharge ^[39]. Performance degradation of an electrode of Ni_0.34Co_0.66Se₂ nanorods was attributed to selenium losses ^[40]. Degradation of vanadium nitride, suggested as an electrode material for microsupercapacitors, has been attributed to the formation of oxides on the material surface ^[41].

The degradation of ICPs, particularly noticeable during electrode potential cycling in, e.g., CVs, has been observed frequently; for overviews, see, e.g., ^[42]^[43]. In-depth studies, beginning with the more or less pseudocapacitive behaviour of ICP-electrode-specific degradation processes have been reported. Using Raman spectroscopy, structural changes of PPy on the molecular level, and “the low mechanical stability of the C=C bonds in PPy”, were related to the observed capacitance degradation based on observed band shifts ^[44]. The decreasing intensity of Raman bands attributed to the bipolaronic state of the oxidized PPy may indicate a decreasing charge storage capability because of the decreasing number of available redox-active sites caused by the irreversible transformation associated with molecular chain deformations. During charge/discharge cycling, changes of PPy in a composite with rGO were observed with various analytical tools without yielding a coherent conclusion ^[45]. Electrochemical degradation of a PPy film with p-toluene sulfonic counteranions has been studied with various methods ^[46]. The impedance data suggest the growing Ohmic resistance of the ICP, limiting current flow and thus decreasing effective capacitance; further experimental evidence, such as growth of a carbonyl peak in infrared spectroscopy, is mentioned, but is hard to correlate with the somewhat diffuse experimental procedure with a mix of ex situ storage and in situ electrochemical measurements. A similar conclusion regarding the growing Ohmic resistance of the ICP has been reported elsewhere ^[47]. In a study of PPy nanowires, dissolution of PPy was observed ^[48]. Morphological changes of PPy nanotubes evidenced with impedance measurements during galvanostatic cycling negatively affected capacitance ^[49]. Mechanical degradation of PPy in a PPy/bacterial cellulose composite, as evidenced with SEM, was identified as the cause of the noticed capacitance decrease ^[50].

The Influence of electropolymerization conditions, i.e., pH of the solution, pH-value and presence/absence of dissolved dioxygen, on PPy film stability has been studied ^[51]. Films grown at lower pH-values and in the absence of dioxygen were more stable according to the results of electrochemical impedance measurements. It is possible that dioxygen oxidized the PPy film. Unspecified “degradation” of PPy, evidenced with redox electrode potential shifts and changing currents in CVs, has been claimed ^[52]. XPS was employed to find the effects of preparation conditions, but apparently not of degradation. Ion trapping in polymer films as a possible contribution towards degradation has been studied in detail ^[53].

The electrochemical capacity fading of a PANI-based supercapacitor electrode was studied with XPS ^[54]. Hydrolytic degradation supported by water molecules transported into the polymer during cycling was concluded from the disappearance of the chlorine signal initially caused by the chloride counteranions in the ICP and an increase of the sulphur (from the sulphuric acid electrolyte) and oxygen signals. This suggests the need for a closer inspection of the water-transport properties of the studied anions beyond the properties already examined ^[55]^[56]^[57]. Based on further evidence from Raman spectroscopy, the degradation products p-aminophenol, p-benzoquinone and quinone imine created by breaking bonds between repeat units, and further chemical as well as electrochemical reactions, were identified ^[58]. Degradation of PANI performance has been related to molecular weight decrease, changed state of aggregation and changed morphology ^[59]. Lower molecular weight is caused by the disintegration of the ICP through bond breaking. The kinetics of PANI degradation aiming to identify optimum operation parameters have been reported ^[60]. Degradation products of PANI in a composite with cobalt–aluminium-layered double hydroxide were identified as being redox-active, contributing to storage capacity ^[61]. The identities of these degradation products were not revealed.

PANI shows two clearly separated redox transitions from the leucoemeraldine to the emeraldine and from the emeraldine to the pernigraniline state. Although both transitions are fairly reversible in terms of maintained redox activity, and thus charge storage capability ^[42]^[43], PANI in the pernigraniline state is rather susceptible to degradation by, e.g., nucleophilic attack of aqueous electrolyte solution constituents ^[62]^[63]^[64]. This is even more pronounced during overoxidation, i.e., even higher electrode potentials ^[65]. Accordingly, proper cell voltage control limiting the positive electrode redox process to the first transition, will help to avoid degradation and thus electrode and cell ageing ^[66]. The effects of defects on the molecular level in PEDOT:PSS and their influence on electrode ageing have been examined, and a detrimental effect of elevated temperature was noticed ^[67]. Ageing of a composite material of graphene oxide and PEDOT:PSS formed with glucose as a green filler by ultraviolet radiation has been studied ^[68]. The effects of the irradiation were not revealed.

The degradation of multilayer electrodes composed of different ICPs has been examined ^[69]. Multilayers of PEDOT and poly(N-methylpyrrole) (PNMPy) were more stable than multilayers of just one ICP. The former kept their porosity even after extensive cycling. Favourable interactions not further specified between the ICP layers were invoked as the reasons for the improvements ^[70]^[71]. Layer-by-layer films of poly(o-methoxyaniline) and poly(3-thiophene acetic acid) have been examined as supercapacitor electrode material ^[72]. Films casted only with the former material showed lower stability and faster degradation, attributed to counterion ingress/egress (see also ^[73]). The frequently deplored degradation of PANI was mostly attributed to swelling/shrinking during cycling ^[25], as well as to structural degradation on a molecular level ^[74], which has also been observed with composites employing this ICP, e.g., in particular at elevated temperatures with PANI/MnFe₂O₄ ^[75]. Extra-large capacitance values of PANI/graphene composites have been attributed to the redox activity of oligoaniline PANI degradation products ^[76].

In composites, slower degradation and consequently slower ageing have been attributed to inhibited dissolution of, e.g., MnO₂ when coated with PANI ^[77]. The inherent flaw of this ICP, its degradation caused by swelling/shrinking during cycling, is ameliorated by keeping the coating of PANI on the MnO₂ nanowires very thin. A long-term stability claim for a composite of PANI and MoS_x after 150 cycles (!) appears to be slightly overoptimistic ^[78].

Performance losses of metal chalcogenides and of the electrodes incorporating them can be attributed to various modes of deterioration. Structural changes during cycling, i.e., during changes of the redox states of the metal ions in a chalcogenide and the associated insert/egress of further cations, can be at least partially irreversible. In the case of hollandite α-MnO₂ DFT studies, the insertion of alkali metal cations causing distortions of the unit cell and changes of the manganese coordination are identified as possible causes of degradation ^[79]. Performance loss, and in particular capacitance loss, of Ni(OH)₂-based electrodes appears to be caused by dissolution of the nickel hydroxide, and consequently measures to inhibit dissolution by, e.g., coating with an ICP may be a remedy ^[80]. Such unwelcome structural changes and thus degradation have also been observed with iron oxides ^[81].

Negative electrodes based on Li₄Ti₅O₁₂ suggested for lithium-ion capacitors (LiC) showed degradation via lithium ion trapping at the octahedral 16c position of the titanate ^[82].

2.2. Causes and Mechanisms at the Device Level

2.2.1. Devices with EDLC-Type Electrodes

Temperature, particularly elevated temperature, and too-high cell voltage have been identified repeatedly as the main sources or at least major contributors of ageing ^[83]^[84]^[85]^[86]. High-voltage supercapacitors with aqueous electrolyte solutions operating at cell voltages > 1.23 V may show gas evolution due to carbon electrode decomposition (mostly oxidation at the positive electrode) and hydrogen evolution at the negative electrode ^[87]. Using cell pressure measurements and online electrochemical mass spectrometry, it was confirmed that oxidation may begin at cell voltages as low as 0.6 V, whereas noticeable hydrogen evolution via water decomposition starts around 1.6 V. During short-term cycling, some reversible gas formation/consumption was observed. During long-term cycling, irreversible side-reactions begin which are associated with increased cell pressure and performance deterioration. Based on the reported observations, cycling was found to be more harmful for electrode integrity than keeping a fixed cell voltage; this apparent contradiction to the opposite conclusions presented above may be related to the fact that in the study discussed above ^[88]^[89], only capacitance retention was monitored, not electrode integrity or cell pressure. In a similar study with a neutral aqueous Li₂SO₄ electrolyte solution, similar observations were made ^[90]. The positive electrode caused most of the ageing of the device; it had many more surface-oxygenated functional groups than the negative electrode. These groups contributed to pore blocking and associated loss of electrochemically active surface area. With this solution, an upper safe cell voltage of 1.5 V was concluded, significantly higher than with KOH or H₂SO₄. In a similar study with a LiNO₃-based aqueous electrolyte solution, essentially the same results regarding degradation and cell ageing were obtained ^[91]. Experimental options of mass spectrometry in similar studies have been described ^[92], and a supercapacitor cell for operando GC-MS has been developed ^[93]. As an alternative, in situ Raman spectroscopy of gas evolving during cell operation has been applied successfully ^[94].

Minor differences in ageing, in particular under accelerating conditions, between electrolyte solutions using different organic solvents (e.g., acetonitrile and propylene carbonate), have been observed ^[84]. The influence of the carbon material became very obvious in a comparative study with EDLC devices and an organic solvent-based electrolyte solution with two different carbons ^[95]. One carbon showed continuous degradation of both ESR and capacitance, whereas the other one initially showed only a growth of ESR. The latter behaviour was explained by invoking the formation of a passivation layer. Unfortunately, the characterization of the two carbons did not include an examination of the carbon surface chemistry; thus, it can only be assumed that such differences may cause the different ageing behaviour. Structural details may also be relevant; the first carbon had a larger specific surface area, presumably due to a larger fraction of micropores more susceptible to clogging as observed with the capacitance decrease. In a subsequent study, again comparing two electrodes, which, in addition to data in the earlier study were characterized in terms of different numbers of surface functional groups, further details regarding an ageing mechanism were proposed ^[96]. The larger number of surface functional groups contributed to passivation layer formation. With both carbons, the presence of traces of water in the cell contributing to the proposed formation of a superacid HF·BF₃ as the first step at the positive electrode was stressed. In a review of analytical techniques for supercapacitor material characterization, the detrimental effects of surface functional groups on carbon materials for EDLC devices was stressed ^[97]. Appropriate methods for evaluating the efficiency and capacitive behaviour of supercapacitors have been critically examined; the different types of devices addressed in the title are presumably just different brands of EDLC devices ^[98]. References to redox reactions, the addition of KI and hydrogen evolution may indicate that, beyond EDLC devices, systems with Faradaic charge storage were also included. The need to distinguish between Coulomb and energy efficiency is stressed, and reporting of the latter is highly recommended. Sensitivity of devices with aqueous electrolyte solutions to cell voltage applied during cycling was confirmed ^[99], but only floating at 1.5 V yielded the highest capacitance; at 1.6 V and even more so at 1.8 V, significant degradation was observed.

Capacitance losses during operation are referred to as degradation, particularly in earlier reports. As a consequence, capacitance losses during operation (or testing) which can be reversed by keeping the device at zero voltage for some time have been referred to as reversible degradation ^[100]. Processes and mechanisms enabling this were not reported; a possible connection to incomplete discharge was indicated.

A microsupercapacitor with 10.8 V operating voltage has been described ^[101], it was constructed as a series connection of nine cells of the EDLC type with graphene electrodes prepared by laser writing on a polymer film support. After 100,000 cycles, 100% capacity retention was observed, but reasons for the stability were not provided. The strength and ductility of completely reduced GO and the good interfacial adhesion between it and the also employed MnO₂ are suggested as possible reasons of remarkable stability ^[102].

2.2.2. Devices with Battery-Type Electrodes

Supercapacitors with ICPs as active masses can be prepared in various configurations, as previously discussed in detail ^[25]^[26]. Although a symmetrical configuration with, e.g., PANI both as a positive and negative electrode is hardly preferable, it has been tested with respect to electrode degradation (and implicitly cell ageing) ^[103]. Changes in terms of charge storage capability, Ohmic resistance and charge transfer resistance of any electrode reaction at the positive electrode were much more pronounced than at the negative electrode. Consequently, an asymmetric device with PANI as the negative and AC as the positive electrode was built and successfully tested for capacity retention during cycling. Charge trapping in the negative electrode of a symmetric device with p/n-dopable conducting redox polymers has been found to be the main reason for performance degradation ^[104]. Partial recovery of charge trapping by potential cycling was found to be possible.

2.2.3. Hybrid Devices

LiCs combining a negative lithium intercalation electrode and a positive EDLC electrode have been subjected to post-mortem analysis after accelerated ageing tests at ambient and strongly elevated temperatures ^[105]. Pore blocking of the positive AC electrode and lithium-ion loss of the pre-lithiated negative electrode were found to be the major factors contributing to device ageing. Detrimental effects of the AC (positive electrode) surface functional groups possibly adsorbing lithium ions were indicated as an important subject of further research. Because of the different operating mechanisms of ageing of LiCs, the effects of cell voltage are different from those found in EDLC-type devices ^[106]. The particularly fast ageing at low cell voltages was considered in an adapted model based on the results of electrochemical impedance measurements. Calendar ageing of LiCs at elevated temperatures of both fully charged and discharged cells has been studied using mostly electrochemical impedance measurements ^[107]. Fully discharged cells suffered huge capacitance losses compared to the charged cells. The low operating potential of a graphite electrode in an LiC has been claimed to be a cause of lithium plating on the graphite surface and consequently performance degradation ^[108]. With granular Li₄Ti₅O₁₂ as the negative electrode in an LiC, performance degradation due to gas evolution (H₂O and HF) was found to be proportional to applied current density ^[109]. Further inspiration may be gained from observing high-power lithium-ion batteries with a performance claimed to be equivalent to many applications currently assigned to supercapacitors ^[110].

2.2.4. Devices with Organic (Solvent) Electrolyte Solutions

Causes of EDLC-type supercapacitors ageing with organic electrolytes (this abridged but over-simplified description is popular, and is used here for conciseness) and AC electrodes under voltage floating conditions (2.5 V at 4000 to 7000 h) have been studied previously ^[111]. Using several common analytical methods, various decomposition products were identified. Differences in the amount and identity of positive and negative electrodes suggested redox reactions between the electrolyte and surface functionalities on the carbon. In addition, these decomposition products plugged some pores of the carbons, as evidenced by BET measurements. Diminished accessible surface area and poorly conducting deposits were invoked as explanations for the increased ESR and decreased capacitance of the studied devices. Lower concentration of surface functionalities was suggested to be favourable for slower ageing. Gas pressure increase of cells under accelerated ageing conditions was compared for cells with acetonitrile-based electrolyte solutions containing different ammonium tetrafluoroborate salts ^[112]. Cells containing an electrolyte with an acyclic cation showed a much larger pressure increase, attributed to weaker solvent–electrolyte interactions. The influence of different organic solvents on gas pressure evolution has been examined ^[113]. With γ-butyrolactone, gas evolution started at 2.5 V cell voltage, whereas gas evolution was small even at 3.25 V. During and after ageing at elevated temperatures, gas pressure changes and the elemental composition of electrodes and their changes for an EDLC device with an electrolyte solution of acetonitrile and triethylmethylammonium tetrafluoroborate were examined ^[114]. An additional “precharge at low voltage” resulted in smaller pressure rise at high voltages, and a large pressure increase was observed at 3 V cell voltage, well above the rated operating voltage. On the electrode, deposits were found which explained the increase of ESR and decrease of capacitance.

Ageing and failure modes of EDLC devices under constant load (different from the well-established meaning of this term in battery testing, where it suggests a constant Ohmic discharge resistor connected to the battery, the complex current–time–charge–discharge program applied here is very much different) have been studied ^[115]. Capacitance, ESR and leakage current were examined. At elevated temperatures (>70 °C) and high cell voltage, failure of devices caused by internal pressure build-up were observed. Ageing at elevated voltages (3.3 V) resulted in changes in the recorded impedance data, suggesting an increase of electrode surface in heterogeneity not observed during ageing at elevated temperatures. Leakage current decreased during constant voltage test, and therefore cannot be taken as an indicator of device ageing. Capacitance losses below the commonly accepted 80% value as an end-of-life criterion always occurred earlier than the doubling of the ESR assumed as the other criterion. EDLC supercapacitors with an acetonitrile-base electrolyte solution of alkylammonium fluoroborate studied by excessive overcharge released several decomposition products of all constituents ^[116]. The formed HF attacked the aluminium foil used as electrode support. Tests of similar systems under less abusive conditions revealed the destruction of the adhesive layer between the current collector and active material to be a cause of performance degradation and device ageing ^[117]. In further accelerated life testing of such devices, the safety and reliability of EDLC devices were tested, along with excess voltage and elevated temperature conditions ^[118]. Decomposition of acetonitrile from the electrolyte solution and destruction of the electrodes caused by the continuous stress were observed. An attempt to identify indicators of over-voltage and over-temperature stress has been reported ^[83]. As fault indicators, the equivalent series resistance, capacitance and cell voltage relaxation after disconnection from power sources were identified ^[83].

An accelerated ageing test of EDLC devices with an ionic liquid as the electrolyte combining cycling and “floating at high potential” (obviously “voltage hold”) yielded > 80% capacity retention after 100 h (!) ^[119].

The ageing of a supercapacitor with a redox-active component (KI) added to the electrolyte solutions was examined ^[120]. GCD had a more degrading effect than voltage floating tests on carbon structure, but voltage floating tests were more detrimental overall. In a redox flow capacitor with a membrane separator, membrane fouling, also seen in flow battery studies ^[121]^[122], was identified as the reason for power degradation and device ageing ^[123].

Deep eutectic solvents (DES) have been proposed for supercapacitors operating at elevated temperatures ^[1]. Because one component (in ^[1], it was acetamide) may evaporate, precipitation of the other component (LiNO₃) may occur, yielding faster ageing because of the detrimental effects of solid deposition on porous electrode performance as discussed above. A DES based on lithium bis(fluorosulfonyl)imide and formamide as the electrolyte in an EDC-device showed gas evolution at excessive cell voltages, which was in part electrochemically reversible ^[124].

To enable lower operating temperatures, EDLC cells with a mixture of water and methanol as the electrolyte solution solvent have been proposed ^[125]. The observed ageing was attributed to oxidation at the positive electrode and corrosion of the stainless steel current collector. Given the frequently stressed sensitivity of EDLC devices vs. elevated temperatures, attempts to select cell constituents suitably were reported after identifying ageing and the reasons for modified device failure at 120 °C: fusion of the separator increasing the ESR, decomposition of the separator and delamination of active masses from current collectors ^[126].

2.2.5. Devices with Solid Electrolytes

Given the numerous advantages of solid or at least semi-solid (gelled) electrolytes, further degradation processes inside the ionically conducting phase between the electrodes may contribute to device aging. In the case of an anionically conducting polymer, a decrease of ionic conductivity somehow associated with some not yet resolved change in the polymer was identified as a cause of capacity decrease ^[127].

The accelerating effect of elevated temperatures on performance degradation was also confirmed for a CNT-based all-solid-state supercapacitor ^[128].

A symmetric device with two PPy electrodes and an ionic liquid-based gel polymer electrolyte showed substantial ageing in terms of capacitance loss, attributed to the conceivable formation of a passivation layer between the electrode and electrolyte ^[129].

2.2.6. Flexible, Stretchable and Bendable Devices

In addition to supercapacitors employed in electric and electronic circuits of traditional design and construction, new applications for flexible, wearable and bendable devices sometimes demonstrate further functions, such as transparency or electrochromism. For such devices, further causes of performance degradation must be considered.

In a stretchable electrode, the detrimental effects of stress induced by stretching could be reduced by using a composite of PPy and MnO₂ nanosheets as the active material instead of MnO₂ alone ^[130]. With PPy alone as the active material, stretching-induced degradation was also low; this was attributed to the various structural advantages of the polymer. Unfortunately, PPy, as many other intrinsically conducting polymers, has other flaws as active mass in supercapacitors. For overviews, see ^[25]^[26].

Mechanical degradation by bending, folding, flexing or other forms of mechanical deformation may cause degradation of device performance identified as capacitance loss. In reports, high stability, i.e., minor degradation of a given material and device are frequently stated, but the reasons for this are not provided.

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Subjects: Electrochemistry

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Xuecheng Chen

Yuping Wu

Rudolf Holze

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Update Date: 15 Aug 2023

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