Paula Ratajczak DESIGN OF HIGH VOLTAGE AC/AC ELECTROCHEMICAL CAPACITORS IN AQUEOUS ELECTROLYTE

Poznan University of Technology Faculty of Chemical Technology

Institute of Chemistry and Technical Electrochemistry Field of study: Chemical Technology

Paula Ratajczak

DESIGN OF HIGH VOLT A GE

AC/AC ELE CT ROCHEM I CA L CAPACIT ORS IN AQUE OUS ELE CTROLY TE

P r o je k t o w a n ie w yso k o na p ię c io w yc h k o nd e n sa t o r ó w e l e k t r o c h e m i c z n y c h ,

p r a c u ją c yc h w e le k t r o lit a c h w o d n yc h

DOCTORAL DISSERTATION

P ro mo t e r : prof. François Béguin

Poznań 2015

Paula Ratajczak P a g e2 Badania do niniejszej pracy prowadzone były przy wsparciu przez projekt ECOLCAP realizowany w ramach Programu Welcome, finansowanego przez Fundację Nauki Polskiej (FNP)zgodnie z Działaniem 1.2. „Wzmocnienie potencjału kadrowego nauki”, Programu Operacyjnego Innowacyjna Gospodarka wspieranego przez Unię Europejską

Kierownik projektu: Profesor François Béguin

This thesis’ research was supported by ECOLCAP project funded in the frame of the Welcome Programme implemented by the Foundation for Polish Science (FNP) within the Measure 1.2. ‘Strengthening the human resources potential of science’, of the Innovative Economy Operational Programme supported by European Union.

Project leader: Professor François Béguin

Paula Ratajczak P a g e3 Część praca badawczej została wsparta przez projekt LIDER finansowany przez Narodowe Centrum Badań i Rozwoju LIDER/018/513/L-4/12/NCBR/201„Kondensator elektrochemiczny o wysokiej gęstości energii i mocy operujący w roztworach sprzężonych par redoks:

Kierownik projektu: dr inż. Krzysztof Fic

A port of the research work was supported by the LIDER project funded by the National Centre for Research and Development (NCBiR) LIDER/018/513/L-4/12/NCBR/201

"Electrochemical capacitor with high energy density and power operating in coupled redox couples solutions”.

Project leader: Dr Eng. Krzysztof Fic

Paula Ratajczak P a g e4

I am sincerely grateful to my supervisor, Prof. François Béguin, for his guidance and all the efforts he put in my PhD work

I am also greatly thankful to Dr hab Eng Krzysztof Jurewicz, for our collaborative work on carbon materials and supercapacitors

It is also a great pleasure to thank Prof. Dr hab Elżbieta Frąckowiak,

and Dr Eng Krzysztof Fic for helping me to develop the skills and knowledge in electrochemistry and carbon materials

My sincere gratitude is also dedicated

to all the ECOLCAP group members, especially,

Dr Qamar Abbas,

M.Sc Eng Piotr Skowron

and M.Sc Eng. Paweł Jeżowski

for their experimental support

Paula Ratajczak P a g e5

TABLE OF CONTENTS

INTRODUCTION _____________________________________________________ 9 CHAPTER I

LITERATURE REVIEW ON ELECTROCHEMICAL CAPACITORS __________ 16 I.1. General properties of electrochemical capacitors______________________ 17

1.1. The electrical double-layer models ______________________________________________ 17 1.2. Operation principle of an EDLC _______________________________________________ 19 1.3. Energy and power of electrochemical capacitors ___________________________________ 21 1.4. Pseudo-capacitive contribution _________________________________________________ 23

I.2. Electrode materials for electrochemical capacitors ____________________ 25

2.1. Commonly used carbon materials_______________________________________________ 25 2.2. Redox-active electrode materials _______________________________________________ 31

I.3. Structural and textural properties of activated carbons__________________ 31

3.1. Manufacturing of porous carbons _______________________________________________ 31 3.2. Surface functional groups on carbons ____________________________________________ 33 3.3. Effect of porous texture of activated carbons on the capacitive performance______________35

I.4. Electrolytes for electrochemical capacitors___________________________ 39

4.1. Aqueous electrolytes_________________________________________________________ 40 4.2. Organic electrolytes _________________________________________________________ 48 4.3. Ionic liquids _______________________________________________________________ 49

I.5. Conclusion ___________________________________________________ 51 CHAPTER II

ELECTROCHEMICAL TECHNIQUES

FOR ELECTROCHEMICAL CAPACITORS INVESTIGATION _______________ 53 II.1. Cyclic voltammetry ____________________________________________ 54

Paula Ratajczak P a g e6 II.2. Constant current charging/discharging ______________________________ 56 II.3. Impedance spectroscopy _________________________________________ 58 II.4. Accelerated ageing test __________________________________________ 60

CHAPTER III

STATE OF HEALTH OF AQUEOUS ELECTROCHEMICAL CAPACITORS WITH STAINLESS STEEL CURRENT COLLECTORS

UNDER ACCELERATED AGEING _____________________________________ 63 III.1. High voltage ageing assessment of AC/AC electrochemical capacitors

in lithium sulfate electrolyte ______________________________________ 65

1.1. Exploring the high operating voltage of AC/AC electrochemical capacitors

in lithium sulfate electrolyte ___________________________________________________ 65 1.2. Degradation of ECs electrochemical performance under accelerated ageing ______________ 67

III.2. Factors contributing to ageing in aqueous electrolyte __________________ 74

2. 1. Oxidation of carbon electrodes and corrosion of stainless steel current collectors __________ 74 2.1.1. Post-floating analysis of ECs by electrochemical techniques __________________________ 74 2.1.2. Post-floating analyses on carbon electrodes _______________________________________ 78 2.1.3. Effect of temperature on ageing ________________________________________________ 82 2.2. Gas evolution during floating __________________________________________________ 83

III.3. Conclusion ___________________________________________________ 87

CHAPTER IV

STRATEGIES FOR IMPROVING THE LONG TIME PERFORMANCE

OF HIGH VOLTAGE CAPACITORS IN AQUEOUS ELECTROLYTES ________ 89 IV.1. Corrosion reduction of positive current collector ______________________ 90

1.1. Alternative nickel current collectors _____________________________________________ 91 1.2. Improvement of the current collector/electrode interface _____________________________ 95 1.2.1. Carbon electrodes glued to stainless steel current collectors __________________________ 95 1.2.2. Nickel foil substrate _________________________________________________________ 97 1.2.3. Carbon conductive sub-layer _________________________________________________ 100 1.3. Addition of corrosion inhibitor ________________________________________________ 103

Paula Ratajczak P a g e7 IV.2. Shifting of electrodes operating potentials __________________________ 109

2.1. Asymmetric configuration ___________________________________________________ 109 2.2. Current collectors coupling___________________________________________________ 117

IV.3. Conclusion __________________________________________________ 122

CHAPTER V

TOWARDS A NEW CONCEPT

OF HIGH VOLTAGE AC/AC CAPACITOR IN AQUEOUS ELECTROLYTES__ 124 III.1. The new concept of high voltage cell in aqueous electrolytes ___________ 125 III.2. Extension of voltage range by electrodes asymmetry _________________ 134

2.1 Adjustment of electrodes potential window by increasing m+/m- ______________________ 134 2.2. Voltage extension by use of different carbon electrodes ____________________________ 136

III.3. Conclusion __________________________________________________ 138

GENERAL CONCLUSION ____________________________________________ 138

EXPERIMENTAL ANNEX____________________________________________ 142 A.1. Cell construction _________________________________________________ 143

1.1. Materials and chemicals _____________________________________________________ 143 1.2. Preparation of electrodes ____________________________________________________ 145 1.3. Cells configurations ________________________________________________________ 146

A.2. Electrochemical characterization ____________________________________ 147 A.3. Physico-chemical and surface characterization _________________________ 147

Paula Ratajczak P a g e8 REFERENCES ______________________________________________________ 149 SCIENTIFIC ACHIEVEMENTS________________________________________ 165 ABSTRACT ________________________________________________________ 172 STRESZCZENIE ____________________________________________________ 175

Paula Ratajczak P a g e9

INTRODUCTION

Paula Ratajczak P a g e10 Energy management has a deep influence in the humans’ everyday life, considering social, economic, ecological and political aspects. During the last 50 years, the world energy consumption, mainly based on petroleum-based fuels (oil, coal and natural gas), has considerably increased (Figure 1), due to industrial development of the western countries after the 2^nd World War, accompanied by improving wealth in emerging markets and growth of the human population, especially in China and India.

Although renewable energy and nuclear power are the world fastest-growing energy sources in the recent years (each increasing around by 2.5% per year), fossil fuels still share more than 80% of the global energy consumption [1].

Figure 1 World energy consumption (based on [2]).

Over the past decade, a general awareness appeared that fossil fuel consumption presents severe drawbacks, such as an important depletion of reserves and the emission of noxious gases leading in particular to the greenhouse effect and to associated temperature increase of the planet. The industry is partly able to handle with some of these problems, by introducing modern solutions, such as reducing emissions by placing catalysts in the exhaust systems of vehicles and in the chimneys of power plants.

Notwithstanding, if fossil fuels would remain the only power source for the future, the forthcoming crunch of their availability would lead to economic dislocations and serious political problems. Therefore, the incoming environmental and economic crisis predictions have suggested to develop strategies for improving energy efficiency (e.g.,

Paula Ratajczak P a g e11 by improving buildings thermal insulation, by introducing hybridization in transportation systems, etc.) and for introducing renewables (sun and wind) in the energy mix. Due to the intermittent character of the ‘clean’ resources, to ensure a real- time balance of electricity supply to the demand over various time scales, the renewable technologies require energy-storage devices in order to adapt the energy delivery to the demand.

Figure 2 shows that energy can be stored via physical and chemical processes and further delivered in the form of electricity. The main systems are based on gravity (pumped hydro power storage), Compressed Air Energy Storage (CAES), kinetic energy (fly wheel), magnetic (Superconducting Magnetic Energy Storage (SMES), electric field (Electrical Double-Layer Capacitors (EDLCs)) and electrochemical reactions (batteries). “Pumped-hydro” is the most traditional way of storing energy on a large scale, by utilizing the excess of electric power to pump water from a lower to a higher-level reservoir. During the periods of high electricity demand, water is released to the lower elevation inducing the rotation of turbines and electricity generation.

Notwithstanding, this technology is geographically constrained and requires specific locations with a sufficient elevation difference between the two reservoirs, which makes the pumped-hydro plants non-transferable. A second interesting technology for large- scale storage uses underground air compression (CAES) and requires specific geologic characteristics. However, the required equipment to store and extract the energy, including compressors and turbine-generators, generates high cost of the CAES plants.

Moreover, CAES generates heat in excess during compression, which reduces the yield of the process.

A technology which tends to be well-suited to ensure a real-time balance of electricity supply to the demand over various time scales is based on flywheels, which feature in a rapid response time. However, due to the high rotation speed of the rotor, for long-term performance, they require maintenance, and for this reason, are still considered to be not completely safe.

Since capital cost and environmental impact are a major barrier to deployment of energy storage, magnetic energy storage (SMES) seems to be a more economic technology than, e.g., pumped hydro and CAES. However, a typical SMES system includes a coil of superconducting material, a power conditioning system and

Paula Ratajczak P a g e12 cryogenically cooled refrigerators, determining the final price of the equipment.

Moreover, SMES is not yet available on a large scale, but only for power application on a micro scale.

Figure 2 Energy storage systems which rely on physical and chemical processes.

At present, electrochemical systems (secondary batteries, electrochemical capacitors) appear as the most suited and flexible devices to adapt the electricity delivery to the demand, provided that the amount of energy involved is not extremely high. The storage batteries can convert the electrical work generated by, e.g., solar cells, into chemical free energy needed to force the reaction in a non-spontaneous direction.

Since rechargeable batteries (lead–acid, Ni-Cd, Ni-MH, Li-ion) appear in many different shapes and sizes, besides the grid energy storage applications, they are also

Paula Ratajczak P a g e13 designed for individual customers to be used in automobile starters, portable devices, light vehicles and power supplies. Due to the chemical character of the operation, the discharge rate of batteries is limited and energy is lost due to the internal resistance of the cell components. Moreover, the concentration of a relatively large amount of chemical energy into a small package may result in hazardous events, such as numerous cases of fire and explosion in case of Li-ion batteries.

Electrochemical capacitors, due to their simple construction and the electrostatic character of energy storage (Figure 2), are characterized by a fast response time, as compared to the other available devices. As they apply high surface area porous carbon electrodes immersed in an electrolytic solution, they store several orders of magnitude higher energy than conventional dielectric capacitors. The most commonly developed systems at the industrial scale are electrical-double layer capacitors (EDLCs), which store the electrical charge in the Helmholtz double-layer. Due to the specific principle of operation, where a nanoscale layer of ions from the electrolyte is attracted to the surface of a polarized electrode material, ECs display high power density of 15 kW kg^-1 when compared to 2 kW kg^-1 offered by, e.g., Li-ion batteries which store the charge through electrochemical redox reactions. Therefore, ECs are adapted for high power applications in automotive industry, opening emergency doors of aircrafts, regenerative braking and stop-start technology in vehicles or power buffer in electric drive train. Moreover, they have a high cycle life of more than 1,000,000 charge/discharge cycles. However, due to the electrostatic charge storage mechanism, ECs store lower amounts of energy (5–8 Wh kg^-1) than, e.g., Li-ion batteries (up to 180 Wh kg^-1). Therefore, an important research attention is focused on enhancing their energy density, while realizing safe, environmentally friendly and cheap systems.

Since the energy density of ECs strongly depends on the applied maximum voltage, most of the industrial devices are based on organic electrolytes, although environment unfriendly and unsafe, which allow reaching 2.7 – 2.8 V. Aqueous electrolytes such as H2SO4 and KOH have been also investigated for high power systems, but unfortunately voltage must be limited to less than 1 V in order to avoid electrolyte decomposition. Lately, it has been demonstrated by our research team that, by employing aqueous alkali sulfate and gold current collectors, voltage up to 2 V can be reached, due to a high over-potential of hydrogen evolution at the negative electrode.

Paula Ratajczak P a g e14 Taking into account the numerous advantages of water-based media over the organic ones, such as high conductivity, low cost, safety in operation and environmental friendliness, the ultimate aim of this doctoral dissertation is to develop a carbon-based, environmentally friendly and low-cost electrochemical capacitor (EC) operating in an aqueous electrolyte with cheap current collectors. To pursue this objective, the undertaken research requires considering and facing some obstacles which cause the cell performance to fade and reliability of the EC to decline. The perturbation phenomena occurring during long time operation of the capacitor are essentially related to aqueous electrolyte decomposition under high voltage operation, which can lead to oxidation of AC electrodes and/or internal pressure evolution and corrosion of metallic current collectors. Overall, the dissertation is divided into five chapters.

Chapter I is a literature review presenting the state-of-art on AC-based electrochemical capacitors. The operation principle and general properties of electrical double-layer capacitors (EDLCs) are described, and the common electrode materials employed for these devices are briefly introduced. The influence of structural and textural properties of carbons on the performance of electrochemical capacitors is summarized, with a special attention to the effect of porous texture on the capacitive.

ECs based on organic electrolytes, ionic liquids and aqueous media are critically compared, with a special emphasis placed on neutral aqueous solutions. Finally, in order to outline the pathway for the performed investigations, the drawn conclusions contain issues which still require to be resolved for improving high-voltage operation of carbon based electrochemical capacitors, while utilizing cheap stainless steel or nickel collectors and aqueous electrolytes.

To attain information about the performance of electrochemical capacitors, chapter II presents a survey of the electrochemical techniques used in our investigations.

In order to accelerate ageing of the analyzed devices, a test (so-called ‘floating’), initially developed by industry for systems with organic electrolyte, has been implemented and validated during our research on ECs in aqueous media.

The further parts of the dissertation are dedicated to attempts for extending the operating voltage of carbon-based ECs. The properties and performance of environmentally friendly AC/AC electrochemical capacitors using neutral salt aqueous electrolytes, e.g., essentially lithium sulfate, with cheap current collectors are presented

Paula Ratajczak P a g e15 in chapter III. Since all the previous works with promising neutral sulfate electrolytes were conducted with expensive gold collectors, chapter III identifies the possible perturbation phenomena which occur during long-term operation in aqueous solution.

The actual effect of operating voltage on the state-of-health (SOH) of the device, evaluated by measuring cell capacitance and resistance evolution together with internal pressure evolution, is presented. The changes of physicochemical and surface properties of the cells’ constituents after long time operation, such as modifications of surface functionality and porosity of the carbon-based electrodes and corrosion of stainless steel current collectors are disclosed.

The strategies proposed in chapter IV to improve the long time performance of AC/AC electrochemical capacitors in the neutral salt aqueous electrolyte are particularly intended to reduce the corrosion of stainless steel collectors and decrease its destructive effect on ECs operation. The undertaken tactics involve the replacement of the corrodible steel current collectors, the protection of the active material/collector interface and the addition of sodium molybdate corrosion inhibitor to lithium sulfate electrolyte. Cells with asymmetric configuration of electrodes and coupled kinds of current collectors are presented in the second part of chapter IV to avoid the decomposition of aqueous electrolyte by shifting the operating electrodes potentials to lower values.

Chapter V introduces a new concept of AC / AC cell using potassium hydroxide and sodium sulfate as catholyte and anolyte, respectively, and a cationic exchange membrane. Due to the pH difference between the two electrolytes, the cell can operate at higher voltage than the thermodynamic stability limit of water, e.g., 1.23 V. The effect of cell asymmetry, either by electrodes mass balancing or by use of different ACs, is critically discussed with regard to fit the electrodes potential extrema within the thermodynamic limits of water oxidation and hydrogen evolution. Besides, the proof-of- concept allows a better understanding of the over-potential origin at the negative electrode of AC/AC capacitors in neutral aqueous electrolytes.

Finally, the manuscript ends with a general conclusion and perspectives for future research in the directions investigated and presented in this dissertation.

Paula Ratajczak P a g e16

CHAPTER I

LITERATURE REVIEW

ON ELECTROCHEMICAL CAPACITORS

Paula Ratajczak P a g e17 This chapter presents an overview on electrochemical capacitors literature appeared during the last decades. After a short introduction about the operation principle and general properties of electrical double-layer capacitors (EDLCs), the review will be focused on the fundamental role played by porous carbons and electrolytes on the electrochemical performance of EDLCs. The effect of pore size on the electrical double-layer capacitance (C_dl) and the strategies to adjust the pore size to the size of electrolyte ions will be emphasized. Particular attention will be paid to the ways by which the researchers exploit the potentialities of electrolytic solutions and carbons to increase the energy density by capacitance and voltage enhancement.

Electrolytes with extended stability window which are designed and customized for ECs will be presented, with a special emphasis on aqueous media. The sources of capacitance enhancement through faradaic contributions arising from oxygenated functional groups on the surface of carbons, redox-active electrode materials, electrochemical hydrogen storage and finally redox-active electrolytes will be also discussed.

On the basis of this literature review, the chapter finishes with a conclusion introducing the consecutive parts of the thesis, and emphasizing issues required to be improved for designing a high voltage ecologically friendly capacitor in salt aqueous electrolyte.

I.1. General properties of electrochemical capacitors

Electrochemical capacitors store energy in an electrical double-layer by electrostatic interaction at the interface created between the conductive solid material and the electrolyte [3, 4]. Contrary to conventional capacitors (such as aluminum electrolytic capacitors) which contain a dielectric material sandwiched between two electrodes facing each other, EDLCs use the electrical double-layer in their function.

1.1. The electrical double-layer models

Over the last two centuries, scientists have developed various modelsof the EDL defining how ions from the electrolyte aggregate at the surface of polarized electrodes and in their vicinity. Helmholtz was the first to describe the phenomena which occur at the solid conductor-electrolyte boundary, and suggested that the interface consists of

Paula Ratajczak P a g e18 two electrical layers which are: (i) electrons at the surface of the electrode, (ii) and a monolayer of ions in the electrolytic solution [5].

One of the shortcomings of the Helmholtz model was the assumption of stationary conditions where ions accumulate on the electrode surface. It did not take into account that, due to their motion, ions are not only compacted at the surface of the electrode, but form a diffuse space charge. Therefore, in the 1900’s Gouy and Chapman formulated a model according to which the capacitance depends also on the applied potential and ions concentration n [6], and is expressed by the equation (1):

𝑪

=

𝒄𝒐𝒔𝒉

where 𝜅 is the Debye-Hückel length [m] described in equation (2):

𝜿 = √

z - the valency of ions, n - the number of ions per cm³, T- the absolute temperature [K], and k – the Boltzmann constant (1.3806488 10^-23 J K^-1).

More than twenty years later, Stern included in his model both a compact and a diffuse layer [7], while Grahame divided this combined Stern layer into two regions [8]:

(i) a layer of adsorbed ions at the surface of the electrode, referred to as the inner Helmholtz plane (IHP) (ii) and an outer Helmholtz plane (OHP) formed by the diffuse ions in the vicinity of the electrode surface. From the Grahame model, the capacitance C of the double-layer is described by equation (3):

𝟏

𝑪

= _𝑪 ^𝟏

𝑯

+ _𝑪 ^𝟏

𝑮𝑪 (3)

with 𝐶_𝐻, which corresponds to the specific capacitance of the Helmholtz’ compact double-layer, and 𝐶_𝐺𝐶 which results from the diffuse layer described by Gouy and Chapman.

The currently used model (BMD model) of the electrical double-layer was described by Bockris, Devanathan and Muller [9], who proposed that a water layer is

Paula Ratajczak P a g e19 present at the surface of the electrode and some other water molecules are displaced by specifically adsorbed ions (e.g., redox ions) which contribute to the pseudocapacitance.

The BMD model may be extended to charge-transfer reactions occurring in organic electrolytes with polar solvents, e.g., acetonitrile (AN), contributing to the potential drop across the electrode/electrolyte plane. As presented on the example of a negatively polarized electrode (Figure 3), the inner Helmholtz plane (IHP) passes through the centers of the specifically adsorbed ions and solvent molecules, which are oriented parallel to the electric field. Then, the outer Helmholtz plane (OHP) passes through the solvated ions centers, which are outside the IHP. Behind the outer Helmholtz plane, there is a diffuse layer region.

Figure 3 Schematic representation of the BMD double-layer model on a negatively polarized electrode (based on [9]).

1.2. Operation principle of an EDLC

In general, EDLCs are made from two identical electrodes made from a porous material (the most commonly carbon) coated on a current collector and separated by a porous membrane soaked with the electrolyte. When a device is connected to a power

Paula Ratajczak P a g e20 supply the ions from the electrolyte aggregate on the surface of positively and negatively polarized electrodes (Figure 4). As energy accumulation proceeds during charging, the device is equivalent to two capacitors in series of capacitance C₊ and C_- and resistance R_f+ and R_f-. The electrical double-layer capacitance of each electrode C_dl is given by formula (4) [3]:

𝑪

=

where S is the surface area of the electrode/electrolyte interface, εr - the relative permittivity of the electrolyte, ε0 - vacuum permittivity (ε0= 8.854·10⁻¹² F m^-1), d - the EDL thickness.

.

Figure 4 Schematic representation of the charged state of a symmetric electrical double- layer capacitor using porous carbon electrodes and its simplified equivalent circuit [10].

Even in a symmetric capacitor, due to the different size of cations and anions in the electrolyte, the two electrodes display different capacitance values. Due to the series equivalent circuit, the capacitance C of the total system is given by equation (5):

Paula Ratajczak P a g e21

𝟏

𝑪 = _𝑪 ^𝟏

+

+ _𝑪 ^𝟏

−

According to this relationship, the electrode with the smallest capacitance determines the capacitance of the system.

1.3. Energy and power of electrochemical capacitors

The stored energy is directly related to ECs’ capacitance C and operating voltage window U, according to equation (6):

𝑬 =

𝑪𝑼

Likewise, the maximum power density also depends on the applied voltage and is given by formula (7):

𝑷 =

𝒔 (7)

with Rs which states for the equivalent series resistance (ESR) of the device. During the charging and discharging processes, as the charges pass, the EDL flows to and from the electrolyte/electrode interface, and electrical losses take place. The main contributions to ESR come from [11]:

• electrolyte resistance;

• electrode material resistance;

• electrode/current-collector interfacial resistance;

• ionic (diffusion) resistance of: (i) ions reaching small pores; (ii) ions moving through the separator.

In order to customize energy storage devices for a wide range of applications, energy and power are plotted versus each other in a so-called Ragone plot. Figure 5 shows the significantly large area covered by the ECs, which can deliver more power (up to 15 kW kg^-1) than redox systems such as Li-ion batteries (up to 2 kW kg^-1) [12].

However, the specific energy reached by ECs is much lower than for Li-ion batteries, (5–8 Wh kg^-1 compared to up to 180 Wh kg^-1, respectively) [13].

Paula Ratajczak P a g e22 Figure 5 Ragone plot of various electrochemical energy storage systems (adapted from [14]).

The diagonal dashed lines in Figure 5 are obtained by dividing the energy density by power, and inform how fast the energy can be distributed. This time constant of the device τ reveals the electrical losses during the charge storage, and is related to the equivalent series resistance R_s and capacitance of the system C according to formula (8):

𝝉 = 𝑹 _𝒔 𝑪

As seen in Figure 5, the charging/discharging process of EDLCs is very fast; this is due to the purely physical character of the storage mechanism in the electrical double-layer.

Paula Ratajczak P a g e23 Since EDLCs are able to deliver all the stored energy within few seconds, they are particularly adapted for applications which require energy pulses during short periods of time, e.g., electric and hybrid vehicles, cranking of diesel engines and renewable energy harvesting, tramways, buses, cranes, forklifts, wind turbines, electricity load leveling in stationary and transportation systems, in opening emergency doors of aircrafts, etc. [12, 15].

Notwithstanding, the charge/discharge mechanism in EDLCs is fully reversible, with efficiency close to 100%. Therefore, the commercially available devices display a high cycle life of more than 1,000,000 charge/discharge cycles [16].

1.4. Pseudo-capacitive contributions

Whilst the main mode of energy storage in EDLCs originates from electrostatic charging, there are also pseudo-capacitive contributions associated with fast faradic reactions at the electrode-electrolyte interface (Figure 6). In this case, the relation between the charge exchanged dq and the change of potential dE is given by the formula (9) [3, 17] as in a capacitors:

𝑪 = _𝒅𝑬 ^𝒅𝒒

Figure 6 Schemes of EDL and faradaic energy storage in electrochemical capacitors [18].

Paula Ratajczak P a g e24 The pseudo-capacitive contributions are mainly associated with, e.g., redox reactions of electroactive species and electrosorption of nascent hydrogen or metal atoms (underpotential deposition). The contribution to capacitance from redox reactions comes from faradaic electron transfer involving an electrochemically active material and/or electrolyte species at the surface of an electrode. In the equilibrium state, the value of potential E is described by the Nernst equation (10) [19]:

𝑬 = 𝑬 ^𝟎 − ^𝑹𝑻 _𝒛𝑭 𝒍𝒏 _𝒂 ^𝒂

𝒓𝒆𝒅 ⁽¹⁰⁾

where E⁰ is the standard electrode potential, R- gas constant (8.314472 J K^-1 mol^-1); T - absolute temperature, z – number of moles of electrons transferred in the half-reaction, F- Faraday constant (9.648 533 ^. 10⁴ C mol^-1), a - chemical activity of reducer (ared) and oxidant (aox). When an electric current is applied, the equilibrium is disrupted and the electrode potential is changed to a value which depends on the amount of charge transferred q, where q is the product of the moles number z and Faraday constant F. The change of potential value is influenced by several factors: (i) the ionic conductivity of the electrolyte, (ii) the transport of species which participate in the reaction; (iii) and phase transition phenomena.

Another source of pseudocapacitance includes the reversible adsorption of atomic species at the surface of an electrode, accompanied by a partial transfer of charge, depending on the charge of the adsorbed atomic species A and the charge density at the electrode surface area S, as described by equation (11) [20]:

𝑨 ^± _𝒄 + 𝑺 _𝟏−𝜽

± 𝒆 ⁻ _𝑬 ↔ 𝑺 _𝜽

𝑨 _𝒂𝒅𝒔

where, c - concentration of adsorbable ions, 1-θA is the fractional free surface area available for adsorption, θA - coverage, E - potential. This specific process occurs when the adsorption of, e.g., anions is not only electrostatic in origin but also depends on electronic interactions between the valence electrons of the adsorbed anions and the surface orbitals of the electrode.

Paula Ratajczak P a g e25 Since the dissertation is focused on aqueous electrolytes, the pseudocapacitive effects which are likely to appear in these electrolytic solutions are presented in paragraph 2.2.

I.2. Electrode materials for electrochemical capacitors

Since the electrodes are the key part of electrochemical capacitors (ECs), the kind of selected electrode materials is very essential to determine the properties of ECs.

In this section, the storage principles and characteristics of electrode materials, including carbonaceous materials for EDLCs and redox-active electrodes for ECs are briefly depicted. Since the objective of this dissertation is related to the design of a low cost and environment friendly capacitor operating in aqueous electrolyte, special attention will be paid in the next section (I.3.) to the influence of surface properties of activated carbons (AC) for achieving high power and energy density.

2.1. Commonly used carbon materials

In order to obtain a system characterized by high energy and power and excellent cycle life, materials with good physical properties and chemical inertness should be applied. Therefore, porous carbons are the most widely used electrode materials for EDLCs, due to their [11]:

• high electrical conductivity,

• high specific surface-area (from around 1 to around 2600 m² g⁻¹),

• good corrosion resistance,

• relatively easily controlled porous texture,

• processability and compatibility in composite materials,

• low cost of production

• various forms (powders, fibers, nanotubes, graphene, foams, fabrics, composites, etc.).

Figure 7 presents the most commonly used carbons as electrodes for EDLCs, which include: activated carbons (ACs) [4, 21], carbon nanotubes (CNTs) [22], onion- like carbons (OLCs) [23], graphene [24] and carbide-derived carbons (CDCs) [25].

Nonetheless, low cost and high specific capacitance are the essential criteria which determine the choice of activated carbon as material for EDLCs.

Paula Ratajczak P a g e26 Figure 7 Electron microscopy images of high surface area carbon materials: (a) scanning electron microscopy (SEM) of AC particles [26]; (b) SEM of AC fabrics [27];

(c) SEM of AC fibers [27]; (d) SEM of vertically aligned CNT forest [28]; (e) SEM of CNT fabric [28]; (f) SEM of randomly oriented CNTs within CNT paper mats [29]; (g) transmission electron microscopy (TEM) of carbon onions [30]; (h) SEM of multilayer graphene flakes [31]; (i) SEM of carbide derived carbons (CDC) [32].

Activated carbon

Activated carbon (AC) is a very complex and highly disordered material made of nano-scale units. In the early model of non-graphitizable carbon proposed by Franklin (Figure 8a) [33], the units constituted of few graphene layers [34] are oriented randomly and connected with each other. The cross-links are sufficiently strong to impede the movement of the layers to a more parallel arrangement. However, after the model proposed by Stoeckli [35], it is believed that ACs sometimes involve single fragments of graphene curved layers connected with each other, as presented in Figure 8b. It was found by high-resolution electron microscopy that high temperature treatment of non-graphitizable carbon entails the production of faceted particles made of misoriented stacks of parallel graphene layers [36].

Paula Ratajczak P a g e27 Compared to some other forms of carbons (e.g., CNTs, OLCs), ACs are characterized by a lower conductivity, which for supercapacitor electrodes is usually compensated by using a percolator (carbon black or CNTs addition) and by appropriate electrodes manufacturing process [37, 38, 39].

Figure 8 (a) 2D model of a non-graphitizable carbonaceous material [33]; (b) 3D model of carbonaceous material [40].

Carbon nanotubes

Carbon nanotubes (CNTs) form a cylindrical 1D structure which contains either one rolled-up graphene layer (single-wall CNT - SWCNT) or several ones (multiwalled CNT - MWCNT) (Figure 9). Generally, they are produced either by catalyst assisted chemical vapor deposition (CCVD) using a hydrocarbon feedstock, such as methane, acetylene and propylene [41] or by CVD deposition in the nano-channels of an anodic alumina template [42].

In contrast to ACs and CDCs, CNTs have relatively low SSA and low density, which limit the volumetric capacitance and energy density of CNT-based EDLCs.

However, high electrical conductivity and open porosity of CNTs allow fast transport of ions, and thus the system to reach high power.

Paula Ratajczak P a g e28 Figure 9 (a) Structure of a single-wall carbon nanotube (SWCNT) and (b) multi-walled carbon nanotube (MWCNT) [43].

Carbon onions

Carbon onions, also called carbon nano-onions (CNOs) or onion-like carbons (OLCs) owe their name to the layered structure reminiscent to an onion, which contains spherical closed carbon shells of fullerene or polyhedral nanostructure (Figure 10). They offer a specific surface area up to 500-600 m² g^-1 which is fully accessible to ions [30].

They are produced via several techniques, such as electron beam irradiation, condensation of carbon vapor and vacuum precursor. Due to their 0D structure, small diameter (<10 nm), high electrical conductivity, relatively easy dispersion as compared to 1D nanotubes and 2D graphene, OLCs appear as a promising electrode material [44].

However, due to their high cost and low capacitance (about 30 F g^-1), they are more preferably used as conductive agent to carbon based electrodes for high-power EDLCs.

Figure 10 3D structure of onion-like carbon (OLC) [45].

Paula Ratajczak P a g e29

Carbide-derived carbons (CDCs)

Carbide-derived carbons (CDCs), also known as tunable nanoporous carbons, are a class of highly porous carbon materials derived from binary (e.g. SiC, TiC) or ternary carbides (e.g., Ti2AlC, Ti3SiC2), polymer-derived ceramics (e.g., Si-O-C or Ti- C) or carbonitrides (Si-N-C) by selective etching of the metal atoms [46]. The most commonly used preparation method of CDCs is a reactive extraction of the metal from carbides with chlorine, where carbon grows from the outside to the core of particles (Figure 11). To avoid sintering and aggregation of the material, generally, the synthesis temperature does not exceed 1200 °C. In the last few years, CDCs attracted a lot of attention as electrode materials for ECs and hydrogen storage applications, due to their high specific surface area (up to 3100 m² g⁻¹ for CDCs synthesized by electrospinning of polycarbosilane with subsequent pyrolysis and chlorination) and broad range of pore sizes (0.3 – 30 nm) [47]. Owing to the highly tunable porosity, SiC-CDC enables to reach gravimetric capacitance of 75 F g^-1 in 1.5 mol L^-1 TEABF₄/AN [48]. For the further developments of this manuscript, structural/textural properties of CDCs and activated carbons (ACs) will be considered as comparable.

Figure 11 Scheme of the carbide conversion to carbide-derived carbon (CDC) depending on the reaction time [49].

Graphene

Graphene is a 2D structured carbon material with fully accessible surface area (reaching in theory 2670 m² g^-1) and high conductivity. However, due to the strong π-π interactions, the graphene sheets tend to restack (Figure 12), which is a critical issue entailing a decrease of accessible surface area and reduction of ions diffusion rates.

Therefore, techniques such as exfoliation and reduction of graphene oxide (GO), e.g.,

Paula Ratajczak P a g e30 via microwave irradiation or heating of GO in propylene carbonate (PC), are applied to increase the gravimetric capacitance of graphene-based electrodes (190 F g^-1 in aqueous and 120 F g^-1 in organic electrolytes) [50]. Recently free-standing holey graphene frameworks (HGF) with efficient ion transport pathways were reported [51]. The HGF were prepared through hydrothermal reduction graphite oxide (GO) with simultaneous low temperature etching of graphene, owing to the presence of H₂O₂ molecules. Due to the formation of nanopores in the graphene sheets, this 3D self-assembled structure enables to reach high and stable capacitance values (298 F g^-1) in 1-ethyl-3- methylimidazolium tetra-fluoroborate/acetonitrile (EMIMBF4/AN) during 25,000 galvanostatic cycles with current density of 25 A g^-1.

Figure 12 Model of a layered microscopic segment of graphene sheets. [52]

2.2. Redox-active electrode materials

In the past decades, many redox-active materials have been studied to gain additional charge from electrochemical reactions, such as conducting polymers [53] or transition metal oxides (RuO2, MnO2, Fe3O4) [54, 55, 56].However, due to the faradaic charge storage mechanism, ECs with redox active electrodes do not exhibit long time operation with a high efficiency.

Over the years, one of the most studied materials with pseudocapacitive behavior has been conductive ruthenium oxide (RuO₂) in acidic electrolytes. During the transitions from the Ru^+II oxidation state to Ru^+IV, a fast and reversible electron transfer with simultaneous electrosorption of protons on the surface of RuO₂ particles takes place, according to reaction (12) [14]:







( )

2

H e RuO OH

RuO 





Paula Ratajczak P a g e31 where 0 ≤  ≤ 2. The three distinct oxidation states of ruthenium during insertion or de- insertion of protons (Ru^+II, Ru^+III and Ru^+II) occur within 1.2 V, and allow ECs with amorphous RuO₂ reaching specific capacitance values of more than 600 F g^-1 [57].

Although, capacitance enhancement in Ru-based aqueous electrochemical capacitors is very attractive, their applications are limited due to the very high price and voltage window of only 1 V.

Therefore, less expensive oxides have been studied, such as iron, vanadium, and cobalt oxides, with particular emphasis on manganese oxide. In capacitors with MnO2 electrodes, the charge storage mechanism is based on the adsorption of cations from the electrolyte (C⁺ = K⁺, Na⁺…) and incorporation of protons. Therefore, these reversible surface redox reactions are fast and close to those in pure EDLC, according to the reaction (13):

H

MnOOC e

z zH

C

MnO

 



 (   )



In neutral aqueous electrolytes, MnO₂ micro-powders or micrometer-thick films exhibit specific capacitance of ~150 F g^–1 within a voltage window of less than 1 V. Therefore, MnO₂ electrodes are frequently used in asymmetric configuration with an AC negative electrode, as an attractive alternative to conventional pseudocapacitors or EDLCs.

I.3. Structural and textural properties of activated carbons

To improve the performance of electrodes, researchers try to optimize the properties of carbons, focusing essentially on conductivity and specific surface area.

However, to better understand the role of carbon materials in ECs, it is also important to consider their structural/nanotextural diversity and surface functionality in more details.

3.1. Manufacturing of porous carbons

The vast majority of carbon based electrode materials is derived from organic precursors by so-called carbonization process which involves heat treatment of a sample in inert atmosphere. Therefore, the structural and textural properties of carbons are dependent on the precursor, its state (e.g., solid material, gel) and conditions of processing [58]. The common natural organic precursors for activated carbon synthesis

Paula Ratajczak P a g e32 include: coal, peat, fruit stones, nut shells, wood, petroleum coke, pitch, lignite, starch, sucrose, corn grain, leaves, coffee grounds, straw etc. [59, 60, 61, 62, 63, 64, 65, 66]. In general, carbonized samples from natural organic precursors have a relatively low porosity with a large number of interstices which block the pore entrances. Therefore, the pre-carbonized product must be further physically or chemically activated in order to open the porosity and to create new pores. The physical activation is conducted by gasification of the pre-carbonized char at temperatures ranging from 700 to 1000 °C, in the presence of an oxidizing agent (such as CO2, steam, air or mixture of these gases), which increases the pore volume and surface area of the material by a controlled carbon burn-off, according to equations (14) to (17) [67, 68]:

2

2

O CO H

H

C   

CO CO

C 

 2

2

2

CO

O

C  

CO O

C 2

2 



The production of ACs by chemical activation is carried out at slightly lower temperatures (∼400–700 °C) and generally results in smaller pores and more uniform pore size distribution [11]. The process involves the reaction of a precursor or a char with a chemical reagent (such as KOH [69, 70], ZnCl2 [71, 72] or H3PO4 [73, 74]). As reported, by activation with potassium hydroxide, it is possible to obtain ACs with specific surface area above 2500 m² g⁻¹ [75, 76]. Nonetheless, to remove residual reactants as well as any inorganic residues (e.g., ash) which originate from the carbon precursor or are introduced during preparation, post-activation washing is always required.

Although it is generally believed that the activation process is required to open the pores of carbonized precursors, carbons with well-developed porosity and good capacitance values, as well as reproducible properties can be obtained by simple one- step carbonization of synthetic polymers, e.g., through a rapid microwave heating of polypyrrole (PPy) [77]. Recently, it has been also presented that self-activation proceeds during carbonization of appropriate biomass precursors, e.g., tobacco [78] or seaweeds

Paula Ratajczak P a g e33 [79, 80], where the second stage of chemical or physical activation is unnecessary. Due to the presence of naturally embedded group I and II elements (such as potassium, calcium, magnesium, sodium), during the thermal treatment, carbonization and self- activation of the precursor occur simultaneously. For the Burley tobacco, the optimal self-activation temperature is considered as 800 °C. At higher temperatures, annealing of the materials dominates and provokes a decrease of specific surface area and average pore size [78].

3.2. Surface functional groups on carbons

As presented in Figure 8b, carbon materials are constituted of fragments of graphene layers connected with each other, each fragment containing edges and defect like vacancies, leading to the development of surface functional groups [68]. As a result of incomplete carbonization of the porous material, a part of the chemical structure is associated with heteroatoms which are in the vast majority oxygen and hydrogen, and in a lesser degree nitrogen and sulfur (Figure 13). Therefore, in addition to electrical double-layer charging, faradic electron transfer reactions involving the surface functional groups may be involved in energy storage [81, 82, 83]. In order to enhance this contribution, the surface functionality of ACs is generally developed through: (i) electrochemical polarization [84], (ii) chemical treatment [85], (iii) and plasma treatment [86].

There are three types of surface oxides present on the carbon material, namely, acidic, basic and neutral (Figure 13) [11]. Surface oxides with acidic nature are formed when carbons are exposed to di-oxygen at 200-750 °C or by reactions with oxidizing agents at room temperature. These surface groups include carboxylic, lactonic and phenolic functionalities. The basic and neutral groups are formed after heat treatment of AC to eliminate the surface functionalities, and further exposition of AC to di-oxygen at low temperature. The basic oxygen-containing groups include ethers, carbonyls and pyrone structures. Although, the acidic or basic nature of quinone/hydroquinone functionalities is not strongly marked, their contribution to capacitance and creation of catalytic active sites for, e.g., oxidative dehydrogenation reactions cannot be neglected [87]. The contribution of quinone/hydroquinone pairs to capacitance can be observed in cyclic voltammograms by cathodic and anodic waves at ~0 V vs Hg/Hg2SO4.

Paula Ratajczak P a g e34 Nonetheless, the recent trend is to introduce the quinone/hydroquinone redox pair into the electrolyte, which is simpler than, e.g., grafting of quinone derivatives on the surface of carbon [88, 89].

Figure 13 Possible functional groups on the surface of carbons related to the presence of heteroatoms: (a) oxygen, (b) nitrogen, and (c) sulfur. Acidic and basic functionalities are indicated in red and blue, respectively (adapted from [90]).

Different techniques are available to analyze the surface functional groups on carbons, such as Temperature-Programmed Desorption (TPD), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FTIR), and chemical or electrochemical titration methods (i.e., Boëhm titration) [91]. Nowadays, the most popular method for characterization of surface oxides starts to be TPD. In this

Paula Ratajczak P a g e35 technique, the functionalities present on the carbon surface are thermally decomposed releasing primarily CO2, CO and/or H2O at different temperatures [92]. The nature of the groups is evaluated from the type of released gas and the decomposition temperature [93]. The TPD patterns of CO and CO₂ evolution are a sum of peaks, therefore, to estimate the amount of each type of oxygenated surface group, the spectra can be deconvoluted by using, e.g. a multiple Gaussian function (Figure 14) [92].

Figure 14 Deconvolution of TPD patterns for a carbon sample oxidized with 5 mol L^-1 nitric acid for 6 hours at boiling temperature: (a) CO₂ pattern; (b) CO pattern; TPD experimental data ■/■; individual peaks ---; sum of the individual peaks -) (adapted from [92]).

Apart from the capacitive contribution, the presence of functional groups on the surface of AC influence the double-layer properties of carbon, such as wettability, rest potential, ESR, leakage current and self-discharge characteristics [3, 11]. As the amount of oxygen associated with the carbon surface increases, the hydrophilicity of carbon increases. Therefore, ACs with high oxygen content can be easier wetted by water than pure carbons without oxygenated surface functionalities.

3.3. Effect of porous texture of activated carbons on the capacitive performance

The nature of the organic precursor and the conditions of AC synthesis, such as carbonization/activation temperatures and kind of used activating agent, influence the pore size distribution of carbon materials. Due to the complex interconnected network of internal pores, the BET specific surface area of AC ranges between 500-3000 m² g^-1.

Paula Ratajczak P a g e36 The parameters closely connected with the specific surface area (pore volume of carbons, size and shape of pores, tortuosity) also play an important role in charge storage. According to the IUPAC classification, there are three main kinds of pores: (i) micropores (with diameters <2 nm), mesopores (diameters from 2 to 50 nm) and macropores (diameters >50 nm) [94]. Since the macropores do not take part in the actual adsorption processes, their contribution to the total surface area is negligible. Ions are the most efficiently adsorbed in the micropores providing the high surface area, while the mesopores are intended to allow the ions to be transported to the micropores [95, 96, 97]. To enhance capacitance and to lower the ESR values, it is important to keep an appropriate volume ratio of meso/micropores, while selecting carbons for EDLCs [98, 99]. For AC/AC electrochemical capacitors in sulfuric acid, the optimum mesopore volume ratio is in the range of 20 to 50% [100]. The role of micropores is seen during slow charging (2 mVs⁻¹ scan rate), whilst the beneficial effect of mesoporous transportation channels on capacitance is pronounced at higher rates [100].

The adsorption of a gaseous medium at a fixed temperature (generally nitrogen at -192°C) is the most common method used to investigate the porosity of carbons. The characteristics of activated carbons are estimated by commercial sorption equipment, generally using in-built software based on the adsorption isotherm of a given adsorbate/adsorbent system and a model of the adsorption process [101, 102, 103].

Nevertheless, in highly porous materials, the adsorption may occur via a pore filling mechanism, rather than by surface coverage only (as it is assumed by the Langmuir and Brunauer–Emmett–Teller theory (BET) [104]). Therefore, in the narrow pores, the application of the BET equation can lead to unrealistic surface-area (SBET) estimations [105, 106]. More and more often, the regularized density functional theory (DFT) is taken into consideration as a more accurate way to correlate capacitance with SSA. In the model, slit-shaped pore geometry is assumed, and it concerns the adsorption and capillary condensation in pores of different geometry and surface chemistry [107].

Figure 15a shows that the gravimetric capacitance of ACs and carbon blacks increases almost linearly with SSA up to SBET ≈ 1500 m² g^-1, and then for carbons with higher activation degree a plateau is visible [108]. For the same carbons, the proportionality region of capacitance with SDFT is more extended than when using SBET, but still for SDFT higher than 1200 m² g^-1 a capacitance saturation phenomenon can be observed (Figure 15b). For carbons materials with SDFT around 1200 m² g^-1, due to the

Paula Ratajczak P a g e37 increase in pore volume, the carbon pore walls become too thin to accommodate additional charges, which results in capacitance saturation [108].

Figure 15 Gravimetric capacitance vs (a) BET specific surface area; (b) DFT specific surface area (adapted from [108]).

To overcome the over- or under-estimation of SSA derived from the BET equation, it is more accurate to combine gas adsorption and immersion calorimetry for porous carbons of different origins, as proposed by Stoeckli et al [109, 110]. Contrary to the anomalous increase of C/S_BET (F m^-2) for TiC-based carbons in pores of less than 1 nm when using TEABF₄ in AN electrolyte [111], the C/S_av values are constant in pores between 0.7 and 1.8 nm [112, 113]. Furthermore, in this pore size range, the volumetric capacitance (C/Wo) increases with decreasing pore width (Figure 16). Interestingly, the linearization of volumetric capacitance vs L0 led to similar trend in 1 mol L^-1 TEABF4

in AN and 6 mol L^-1 KOH electrolyte for two series of activated carbons, while assuming slit-shaped pores [114].

According to equation (4), capacitance might be also overestimated when Lo

decreases, if assuming constant electrolyte dielectric permittivity εr. In fact, since slit- shape micropores contain a constant amount of ions which are surrounded by a variable amount of solvent molecules, the relative electrolyte permittivity in micropores decreases with the solvent to ion ratio, i.e. with the decrease of L0. Therefore, the Feng model [115] which suggests a gradual decrease of relative permittivity of TEABF4/AN, explains the almost constant value of C/S in pores below 1 nm. However, the studies on microporous carbons cannot longer rely on models, which still assume that solvated

Paula Ratajczak P a g e38 ions occupy a central position in micropores, which in turn feature in well-defined shape and rigidity, and are not interconnected.

Figure 16 Volumetric capacitance of various microporous carbons in TEABF₄/AN electrolyte vs average pore width (L_o) accessible to CCl₄; W_o represents the volume of micropores deduced from the carbon tetrachloride (CCl4) isotherm, assuming that the diameters of TEA⁺ (0.68 nm) and CCl₄ (0.63 nm) are comparable[113].

From the foregoing, and considering the diameter of solvated TEA⁺ (1.3 nm) and BF4-

(1.16 nm) and desolvated TEA⁺ (0.67 nm) and BF4-

(0.48 nm) [116], it suggests that ions need to be at least partly desolvated to penetrate into the micropores [117]. Desolvation of TEA⁺ and BF4-

was confirmed by nuclear magnetic resonance (NMR) on AC electrodes extracted from capacitors charged up to different voltage values in the TEABF4/AN electrolyte. Figure 17 shows the molar proportions of TEA⁺ and BF₄^- and the relative amount of AN vs the total amount of electrolyte species after polarization at various voltages [118]. Predictably, due to charging, large TEA⁺ cations in the positive electrode are replaced by smaller BF₄^- anions, leaving the place for solvent molecules, which amount remains nearly constant up to 4.0 V. Simultaneously, in the negative electrode, small anions are replaced by larger cations, and consequently the AN concentration decreases rapidly and becomes negligible at 2.7 V (no AN molecules are left in the micropores of the AC-based electrode). The AN solvent is expelled by incoming TEA⁺ and is further stored in the mesopores.

Paula Ratajczak P a g e39 Figure 17 Molar proportions of TEA⁺ and BF₄^- in the positive and negative electrodes of an AC/AC electrochemical capacitor calculated from NMR spectra, and relative amount of AN versus the total amount of electrolyte species, after polarization at various cell potentials for 30 min (adapted from [118]).

I.4. Electrolytes for electrochemical capacitors

In order to extend the range of ECs applications, the current researches seek for strategies which improve their energy density. According to equation (6), the value of stored energy can be enhanced either by increasing the capacitance C or by extending the operating voltage U. Since the latter is closely determined by the stability window of the applied electrolyte, this paragraph is focused on pros and cons of electrolytes which are designed and customized for different ECs applications. Beside the electrochemical stability window, which is a key factor affecting the electrolyte selection, the physical properties of the electrolytic solution, such as, mobility and molar conductivity of ions, are found to be also important in terms of energy storage efficiency. It is commonly known that the charge storage capacitance and resistance of the electrode material are affected by the nature of the electrolyte, i.e. the ionic radii of unsolvated and solvated ions, the molar conductivity of ions and their mobility in the pores of electrodes [119].

Calvo et al. showed that it is possible to predict the capacitance for each electrolyte based on the information about molar conductivity of ions and surface functionality of the electrode material [120].