Exhibit F 

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Electrochemistry Communications 142 (2022) 107373

Available online 27 September 2022

1388-2481/© 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).

Recent advances and challenges of current collectors for supercapacitors 

Alisher Abdisattar a,b,e, Mukhtar Yeleuov a,b,e, Chingis Daulbayev d,e,*, Kydyr Askaruly a,b,e, 

Aidos Tolynbekov a,e,f

, Azamat Taurbekov a,c,e, Nikolay Prikhodko a,g 

a Institute of Combustion Problems, Almaty, Kazakhstan b Satbayev University, Almaty, Kazakhstan c Al-Farabi Kazakh National University, Almaty, Kazakhstan d National Laboratory Astana, Nazarbayev University, Nur-Sultan, Kazakhstan e Bes Saiman Group, Almaty, Kazakhstan f Institute of Nuclear Physics, Almaty, Kazakhstan g Almaty University of Power Engineering and Telecommunications, Almaty, Kazakhstan 

ARTICLE INFO 

Keywords: 

Energy storage 

Supercapacitor 

Current collector 

Electric double layer capacitor 

Pseudocapacitor 

ABSTRACT 

Global energy and environmental issues are driving the development of modern advances in efficient and 

environmentally friendly energy storage systems. Such systems must meet a range of requirements, which 

include high energy and power density, long service life, flexibility, industrial scalability, security and reliability. 

Progressive achievements in the field of energy storage are associated with the development of various kinds of 

batteries and supercapacitors. Supercapacitors are state-of-the-art energy storage devices with high power 

density, long lifespan, and the ability to bridge the power/energy gap between conventional capacitors and 

batteries/fuel cells. However, supercapacitors have limitations associated with low energy density, which can be 

solved by using various types of current collectors, since current collectors are one of the main massive com

ponents of supercapacitors. This review gives a complete understanding of the effect of current collectors on the 

actual performance and properties of supercapacitors. We reviewed current collectors based on carbon and 

metal-containing materials, and supercapacitor configurations to identify possible improvements in electro

chemical performance in terms of specific capacitance, energy density, power density, service life and variability 

in their application. 

1. Introduction 

At present, the rapid pace of development of the world economy and 

global environmental problems require the development of new revo

lutionary strategies for solving energy problems [1–3]. Along with the 

limited supply of fossil fuels, there has been an increase in the number of 

electronic devices, which leads to an increase in energy demand. One of 

the solutions to these problems is the use of alternative energy sources, 

such as solar and wind, which are environmentally friendly sources of 

energy and are increasingly being used [4]. Meanwhile, their significant 

drawback is the dependence of energy production on external factors 

(presence or absence of sunny weather and wind). All this leads to the 

fact that alternative energy sources should be used in conjunction with 

modern technologies for efficient energy storage [5]. Modern advances 

in the commercial production of batteries and supercapacitors (also 

called ultracapacitors) are used not only to solve the problem of storing 

energy obtained from alternative sources but also in everyday human 

life. Currently, lithium-ion batteries are the main driving force in the 

energy storage sector. However, various types of batteries, such as 

lithium-ion batteries, have a low power density, which limits their use in 

various integrated power modules. Unlike batteries, which have a high 

energy density, supercapacitors are of particular interest in terms of 

their high power density and long life (Fig. 1a, b) [6,7]. At the same 

time, the use of various types of supercapacitors, such as electric double 

layer capacitors, makes it possible to obtain high power, while another 

type of supercapacitor, the pseudocapacitor, is best suited for obtaining 

high energy density. 

Modern achievements in the field of supercapacitors, as well as for 

batteries, are mainly associated with active electrode materials such as 

activated carbon [8], graphene [9], carbon nanotubes [10,11], nanorods 

[12–14], MXene [15–17], and silicon oxide [18–20]. Therefore, many 

research and review articles on active electrode materials have been 

* Corresponding author at: National Laboratory Astana, Nazarbayev University, Nur-Sultan, Kazakhstan. 

E-mail address: chingis.daulbayev@nu.edu.kz (C. Daulbayev). 

Contents lists available at ScienceDirect 

Electrochemistry Communications 

journal homepage: www.elsevier.com/locate/elecom 

https://doi.org/10.1016/j.elecom.2022.107373 

Received 5 August 2022; Received in revised form 20 September 2022; Accepted 21 September 2022 

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2

published in recent decades. Much effort has been put into reviewing the 

active electrode materials and electrolytes of supercapacitors. However, 

to the best of our knowledge, few reviews on current collectors have 

been published so far, and only a limited number of materials or designs 

are considered [21]. Current collectors, along with active electrode 

materials, are one of the main massive components (~15–20 % of the 

total weight of a supercapacitor cell), which significantly influence the 

gravimetric/volumetric specific energy density, power density, and 

long-term cycle stability. This brings us to the need for a more 

comprehensive overview of current collectors. 

The key players in the supercapacitor market are the AVX Corpora

tion, SPEL Technologies Private Limited, Tesla, Nippon Chemi-Con, 

Skeleton Technologies, CAP-XX, LS Mtron, Ioxus Inc., Evans Capacitor 

Company, KORCHIP Corporation and Panasonic Corporation. Most of 

these companies produce supercapacitors based on carbon electrodes 

with organic electrolytes and aluminum current collectors. Previous 

research studies on the supercapacitors with such configuration have 

shown that after a certain number of charge–discharge cycles the carbon 

layer often peels off from the smooth metallic current collector resulting 

in poor electrochemical performance. In order to avoid the peeling effect 

of the carbon layer from aluminum current collectors, recently manu

facturers are starting to use modified current collectors such as etched 

aluminum foil and carbon-coated Al foil instead of smooth foils. Such 

modifications of the current collectors increases the strength of the bond 

between the active electrode material and the current collector and 

maintains a stable operation over hundreds of thousands of char

ge–discharge cycles [23,24]. The current collector is an indispensable 

component bridging supercapacitor and external circuit (power sources 

or electrical appliances), as well as the supporting substrate for loading 

the active electrode material, greatly influencing the capacity, rate 

capability, heat transfer property, and long-term stability of 

supercapacitors. 

Compared to batteries, supercapacitors do not have a wide range of 

applications due to the two limiting factors of low energy density and 

high cost [25,26]. One possible solution to increase the energy density 

and reduce the cost of a supercapacitor is to develop new types or 

improve the existing types of current collectors along with active elec

trode materials used for supercapacitors. The growing interest in various 

materials used for supercapacitor current collectors is indicated by a 

number of research articles that have made it possible to consider the 

prospects and problems of choosing the type of material for super

capacitor current collectors [27–32]. All this indicates that the choice of 

current collector has a significant impact on the efficiency of both 

supercapacitors and various types of batteries. To the best of our 

knowledge, there are currently no review articles on this issue. From this 

point of view, we believe that today, the issue of a comprehensive and 

detailed analysis of materials used as current collectors for 

supercapacitors, as well as how and by what means their efficiency can 

be improved, is an acute issue. 

2. The principle of operation of supercapacitors 

Supercapacitors are electrochemical capacitors with a high energy 

density, which is hundreds of times greater than that of conventional 

electrolytic capacitors. Unlike conventional capacitors where charges 

are accumulated electrostatically, for supercapacitors, the accumulation 

of charges occurs as a result of electrostatic double-layer capacitance 

(EDLC) and electrochemical pseudocapacitance between the electrodes 

of the capacitor and the electrolyte [33]. These reactions are surface 

interactions, indicating that an increase in specific surface area leads to 

an increase in charge accumulation. Supercapacitors are systems con

sisting of two electrodes (anode and cathode), an electrolyte and a 

separator. Charge storage in the case of EDLC occurs predominantly at 

the electrode, and electrolyte ions are adsorbed at the electrolyte/elec

trode interface, while the separator acts as a semipermeable membrane 

that allows ion transport and prevents short circuiting of the device. The 

supercapacitor electrode includes a current collector and an active 

electrode layer coated on the surface of the current collector. The cur

rent collector serves to efficiently transfer electrons from the active 

material to external power sources or electrical appliances. 

To date, there are three main types of supercapacitors that have 

different mechanisms of charge accumulation (Fig. 2a, b, c): electro

static double-layer supercapacitors (EDLCs) [34], hybrid capacitors 

[35,36], and redox supercapacitors or pseudocapacitors [37,38]. For 

EDLCs, the accumulation of charges occurs as a result of the formation of 

an electric double layer due to electrostatic attractive forces that attract 

electrolyte ions to the surface of the electrodes. The electric voltage on 

the electrodes leads to the formation of one layer of charge, while to 

compensate for this layer, ions of opposite polarity from the electrolyte 

accumulate on the surface of the electrodes, forming a second ionic layer 

[39]. Between these two electrical layers are the solvent molecules (the 

Helmholtz plane). 

Due to the significant difference in the geometric dimensions of 

electrons and ions, the density of the electron layer on the electrode 

surface is higher than that for the second ion layer. At the same time, the 

electrode potential changes linearly and becomes exponentially depen

dent when approaching the diffusion layer. This charge accumulation 

mechanism makes it possible to achieve a high speed for the charge/ 

discharge cycle, while the service life is higher compared to other types 

of supercapacitors, and because no chemical reactions occur during the 

charge/discharge process, the structure of the electrodes remains un

changed. Thus, EDLCs are characterized by a long service life, and the 

specific surface area of the active material affects the capacitive char

acteristics of the supercapacitor. 

Fig. 1. (a) Energy and power density for different types of energy-storage devices (supercapacitors, fuel cells, conventional capacitors, and a typical battery); (b) 

Comparison of the electrochemical behaviour of a typical supercapacitor and a typical battery. Reprinted with permission from [22]. 

A. Abdisattar et al. 

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In contrast to EDLCs, the mechanism of charge accumulation for 

pseudocapacitors is the so-called surface Faradaic electron chargetransfer reaction [41]. These reversible reduction and oxidation re

actions take place on the electrodes of the capacitor and largely deter

mine its service life, since long-term chemical reactions during fast 

charge and discharge cycles can cause structural changes in the elec

trode [42]. The ongoing reactions are not accompanied by chemical 

transformations in the electrode material itself, but due to redox re

actions, adsorption and intercalation, the process of charge transfer 

occurs. Therefore, materials with a mesoporous structure are preferable 

for the electrode material, which has a positive effect on the diffusion of 

electrolyte ions during charge transfer reactions [43]. Various types of 

materials are currently used as electrodes, including polymers with high 

electronic conductivity, metal oxides, and their composites [44–47]. 

Pseudocapacitors are characterized by high energy density and high 

capacitance values. Unlike EDLCs, pseudocapacitors have a fairly low 

power density. 

Unlike EDLCs and pseudocapacitors, the hybrid supercapacitor has 

electrodes made from two different types of materials. In addition, the 

mechanism of charge accumulation includes both the formation of a 

double electric layer and Faradaic reactions [48]. This allows hybrid 

supercapacitors to have high energy density, low self-charging rate and 

high specific capacitance compared to symmetrical capacitors. The use 

of different materials as electrodes allows the use of the advantages of 

both EDLCs and pseudocapacitors, and the combination of these two 

charge storage mechanisms makes it possible to obtain capacitors with 

high energy density and power. 

As a rule, in the manufacture of supercapacitors, special attention is 

given to the choice of material for the electrode, which mainly affects 

the parameters such as high specific capacitance, speed of charge and 

discharge cycles, and their stability. For example, carbon materials 

(activated carbon, graphene, carbon nanotubes, etc.) are widely used as 

electrodes for EDLCs, with transition metal oxides (RuO2, MnO2, CoOx, 

NiO, Fe2O3) and reports on the use of MXene and various composites 

being based on them. In addition, manufacturers pay attention not only 

to the active electrode material but also to the current collector, which is 

one of the main key components of a supercapacitor that affects the 

overall specific parameters. 

3. Main requirements for current collectors in supercapacitors 

The efficiency of supercapacitors is determined by the choice of 

active electrode material, electrolyte, separator, and current collector. 

Each component plays a role and largely determines the characteristics 

of the supercapacitor. For example, the internal resistance of a super

capacitor is affected by the conductivity of the electrode material and 

the current collector, as well as the resistivity of the electrolyte and the 

thickness of the separator. 

The ideal current collector should have high electrical conductivity, 

low contact resistance with the active electrode material, stable and 

strong bonding strength with the active electrode material, high specific 

surface area, high mechanical strength/modulus, low weight, high 

thermal stability, high electrochemical stability, and environmental and 

cost aspects must be considered. To date, metal current collectors (foil, 

porous metal) and current collectors based on carbon materials have 

been widely used in supercapacitors. However, there are general criteria 

for selecting current collectors, regardless of the properties of the 

supercapacitor, to ensure its high efficiency and stable operation: 

Fig. 2. Schematic illustration of (a) an electrical double-layer capacitor (EDLC); (b) pseudocapacitor (PC); (c) hybrid supercapacitor; (d) exploded view of a wound 

supercapacitor. Reprinted with permission from [40]. 

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1. High electrical conductivity. Current collectors provide efficient 

transfer of electrons to external circuits, which depends on their 

conductivity. In addition, the charge/discharge process may be 

accompanied by heat generation at the current collector with low 

conductivities. For the operation of a supercapacitor, not only the 

conductivity of the current collectors is important, but also the 

electronic resistance at the boundary between the current collector 

and the active electrode layer. The electronic resistance of current 

collect is typically negligible especially for metallic current collec

tors. Physico-chemical modification of the current collector surface 

and deposition of an additional intermediate layer between the 

active electrode layer and the current collector are the possible so

lutions for reducing the electronic resistance at the interface between 

the current collector and the active electrode layer. 

2. Electrochemical stability. Insufficient resistance of current collectors 

to reactions occurring during the processes of charge and discharge 

can lead to an undesirable decrease in capacitance characteristics 

and a reduction in the service life of supercapacitors. The choice of 

current collectors and their modifications is carried out mainly 

depending on the type of electrolyte and the electrode material (and 

the method of its integration with the current collector). 

3. Mechanical strength. Another important parameter is the mechani

cal strength of the current collector, as it maintains a stable 

connection between the electrodes and their integrity during 

charging and discharging. Current collectors are not only a charge 

transfer channel between the active electrode layer and the external 

circuit, but also a supporting carrier substrate, the mechanical 

properties of which are very important in the production of super

capacitor cells and the cell operation in harsh conditions. 

4. Optimal density. An important factor is the optimal mass ratio of the 

active electrode material loaded on the current collector to the mass 

of the current collector, since the current collectors are an inactive 

component. However, current collectors account for about 15-20 % 

of the total weight of the supercapacitor. An increase in the ratio of 

the active electrode material mass to the current collector mass 

contributes to a decrease in the total mass of the supercapacitor and, 

consequently, to an increase in the specific energy density of the 

supercapacitor. 

5. Sustainability and cost. The choice of material for the current col

lector also depends on its cost, availability, and possible recycling. 

These factors are of decisive importance not only in terms of com

mercial interest but also as an environmentally friendly recycling of 

used current collectors and the supercapacitors themselves. 

4. Materials for current collectors 

4.1. Carbon-based current collectors 

Metal current collectors are widely used for the manufacture of 

supercapacitors; however, they have a number of significant disadvan

tages associated with corrosion, high interfacial resistance between the 

metal current collector and the active electrode material, the charac

teristic low gravimetric energy density of the entire cell and the 

impossibility of creating flexible supercapacitors, which are of great 

interest from the point of view of their potential applications [49–51]. 

On the other hand, unlike metal and metal-containing current collectors, 

the use of carbon materials as current collectors makes it possible not 

only to obtain flexible supercapacitors but also to reduce the interfacial 

resistance between the active electrode material and the current col

lector [52–54]. For example, the use of conductive carbon fibres as a 

current collector for EDLCs produced from polymerized ionic liquids 

(PILs) by electrospinning demonstrates excellent charge storage pa

rameters (commercial Ketjen black carbon as the active material), su

perior to EDLCs based on carbon-coated aluminium foil [55]. At the 

same time, an EDLC with a current collector based on carbon fibres can 

operate at a current density of up to 40 A/g without a significant loss in 

capacitance, has high stability at high cell voltages and long cycles of 

work, and has good conductivity, comparable to a platinum current 

collector. At the same time, the disadvantage of such carbon fibres is the 

high cost and scalability of the method, which limits its possible com

mercial application. 

Another promising carbon material for current collectors is 

conductive carbon paper. Yes, Peihui Luo and Lily Huang [56] demon

strated a simple method for making electrodes from graphene hydrogel 

and carbon paper, which was used as a current collector. Due to its high 

porosity and high electrical conductivity, graphene hydrogel electrodes 

using carbon paper as a current collector showed a high gravimetric 

capacitance of 294F/g at a current density of 1.18 A/g, which remained 

at 66 % (193F/g) even with increasing current density up to 28.24 A/g. 

Along with carbon paper, for the manufacture of disposable, nontoxic 

and flexible supercapacitors, graphite foil is also used as an effective 

current collector material (activated carbon as the active material). 

Maede Arvania and others [52] showed that graphite foil can be used as 

a current collector. However, studies indicate the effect of the porosity of 

the current collector on the chemical stability of the supercapacitor. For 

example, graphite foil (150 μm thick) showed high capacitance and low 

equivalent series resistance (ESR) but poor chemical stability. Presum

ably, this behaviour is explained by the influence of oxygen, which leads 

to oxidation and evaporation of the electrolyte, leading to a decrease in 

mass and an increase in leakage current. 

Another carbon material used as a current collector is carbon 

nanotubes. Michel dos Santos Clem et al. [57] used poly(3,4- 

ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) ink with 

the addition of multiwalled carbon nanotubes (MWCNTs). In this work, 

the composite ink was screen printed on filter paper and used as both an 

active electrode material and a current collector (Fig. 2a, b). Such a 

supercapacitor exhibits a capacity retention of up to 72 % after 1000 

cycles with a specific capacitance of 20.3F⋅g− 1 at a current density of 1 

mA⋅g− 1 at an energy density and power of 3.1 W⋅h/kg and 420 W/kg, 

respectively. 

Printing technology is also being used for other types of carbon 

materials, such as graphene filaments as current collectors (graphite 

oxide is the active material). S.A. Baskakov et al. [58] demonstrated a 

supercapacitor based on graphene oxide, where a graphene filament was 

used as a current collector (Black Magic 3D) made in the form of a 

composite of polylactic acid (PLA) with the addition of carbon nano

materials with high conductivity. On the other hand, the laser writing 

method is also used to make high-capacitance supercapacitors by con

verting graphene oxide into porous graphene films, which is current 

collector free active material [59]. Yes, Cho E - C et al. demonstrated a 

simple and scalable method for obtaining graphene-based super

capacitors with a large working area (4 × 4 cm2

) using an infrared CO2 

laser. Such a supercapacitor has reversible capacitances of 115.2, 97.0 

and 78.4F/g at current densities of 0.5, 2 and 6 A/g, respectively, and 

only slight capacity loss after 4000 charge/discharge cycles and 2000 

bending cycles, indicating high cycle life and mechanical stability 

(Fig. 3a-d). Although the use of graphene films makes it possible to 

achieve high capacitance values and good mechanical stability in com

parison with metal current collectors, their use for the manufacture of 

supercapacitors with large areas is extremely difficult due to the use of 

lithography technology. 

Graphene is also used as a current collector for supercapacitors. 

Pourjavadi et al. [60] used porous graphene as a current collector to 

accommodate carbon fibre-functionalized polyaniline to make a standalone supercapacitor electrode. The specific capacitance for this type 

of supercapacitor was 1.42F/cm2 (710F/g) at a current density of 4 mA/ 

cm2 (2 A/g), while the capacitance of the electrodes decreased to 44.4 

and 96, respectively, 8 % of the initial values after 1000 cycles, and lost 

~ 30 % of its initial capacity at bending angles of ~ 120◦, which once 

again indicates the prospects for the use of carbon materials for the 

production of flexible supercapacitors. 

We should also note the difficulties associated with the multilayer 

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structure of supercapacitors, made in the form of a sandwich structure. 

The use of two substrates leads to a noticeable increase in volume and 

mass, which directly affects the gravimetric and volumetric capacities. 

This often results in the need for additional metal connections in the case 

of series and/or parallel connections of several supercapacitors to in

crease the voltage or capacitance. All this has led to the need for 

developing microsupercapacitors, which consist of interdigitated elec

trodes on a single substrate, obtained by screen printing. This structure 

provides a narrow space between the cathode and anode, which in turn 

leads to short circuit prevention and improved mechanical stability and 

flexibility. As evidenced by a study on the fabrication of graphene-based 

(actuating both as current collector and active material) tandem 

microsupercapacitors, which have a high capacitance (3.16 mF/cm2

) at 

a scan rate of 500 mV/s1

, and demonstrating outstanding cyclic stability 

with a capacitance retention of 91.1 % after 10,000 cycles [63]. At the 

same time, this screen printing method allows one to simultaneously 

print a large number of microsupercapacitors (from 100 to 1000) in a 

few seconds using graphene ink (actuating both as current collector and 

active material) [61]. At the same time, a tandem battery pack, con

sisting of 130 of these microsupercapacitors, allows one to obtain an 

output voltage of more than 100 V, which indicates excellent modularity 

(Fig. 3e-h). Screen printing allows the fabrication of stable super

capacitors, as evidenced by a systematic study of a large set of super

capacitors [64]. In this case, the current collectors and active layers in 

the supercapacitors under study were printed with paints from carbon 

materials such as graphite and activated carbon. It has been found that 

the best choices for using such devices are applications where low cur

rents (<1 mA) are sufficient, such as in distributed sensors. 

In addition to screen printing, 3D printing is also being used, 

allowing the fabrication of complex and adaptable 3D carbon structures 

in combination with bondless self-assembling nanostructures. At the 

same time, such supercapacitors have a good charge rate and demon

strate high gravimetric and surface capacitances (186F/g and 968 mF/ 

cm2

, respectively) (Fig. 3i, j) [62]. On the other hand, as noted above, 

one of the limiting factors of traditional supercapacitors is the negative 

effect of the substrate mass, and the use of electrophoretic deposition on 

graphene airgel as a current collector (manganese and iron oxides were 

used as the active materials) makes it possible to create a bulk structure 

[65]. Such electrodes have a high gravimetric capacitance normalized to 

the total mass of the active material and current collector (mt) and 

amount to 143F/g. With such a structure, it is easy to avoid the excess 

mass of the current collector. When layer-by-layer deposition of the 

active material occurs on the current collector, some layer of the current 

collector will not interact with the active material but only the surface 

layer. This case is clearly demonstrated with metal current collectors, as 

evidenced by the electrochemical performance of nickel foam, which is 

superior to that of nickel foil. Therefore, metal-free supercapacitors with 

a carbon-based current collector have good flexibility, making them 

suitable for microelectronics applications. However, attempts to obtain 

an electrode with large areas and capacitance lead to capacitance losses 

and an increase in resistance, which makes them unsuitable when large 

capacitances and power are needed. 

Table 1 summarizes data on carbon-based current collectors, 

including their fabrication methods; mechanical characteristics such as 

flexibility and stretchability; and electrochemical characteristics of 

these supercapacitors in terms of capacitance, impedance properties, 

rate capability and cyclic stability. 

4.2. Current collectors based on metal with various modifications 

Metal current collectors, in contrast to carbon and polymer mate

rials, attract special attention due to their high conductivity and low 

cost. At the same time, they provide mechanical integrity and flexibility. 

However, as noted above, one of the main factors limiting their use is the 

corrosion of metal current collectors in a water-based electrolyte. At the 

same time, recent advances in the field of supercapacitors are mainly 

associated with aqueous electrolytes, which, in comparison with other 

types (organic electrolytes, ionic liquids and water-in-salt electrolytes), 

allow reaching high values of power density up to 10 kW/kg. An analysis 

of the available literature has shown that to solve this problem, methods 

Fig. 3. (a) and (b) Digital photograph of polyimide-derived graphene (PIDG) patterned on a polyimide substrate; (c) and (d) SEM image and corresponding EDS 

mapping of the PIDG pattern; (e) and (f) digital images of screen-printed graphene-microsupercapacitors (SPG-IMSC); (g) Demonstration of a letter-shaped SPG-IMSC 

pack for powering 3 LCDs; (h) Demonstration of powering a sports watch with a SPG-IMSC pack connecting 8 cells in series; (i) Synthesis and self-assembly of the 

MnO2 nanostructures onto the surface of the 3D-printed polymer network; (j) SEM images of 3D-printed polymer/MnOx hybrid materials. Reprinted with permission 

from [59,61,62]. 

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are used based on the use of expensive corrosion-resistant metals 

(platinum, nickel, stainless steel), conductive composites based on 

metals and carbon materials, and modification of metal current collec

tors by physical and chemical treatment (laser irradiation, chemical and 

electrochemical deposition) (Fig. 4) [69]. In addition, research in this 

field shows that after a certain number of charge–discharge cycles, the 

carbon layer often peels off from the untreated smooth metallic current 

collector in activated carbon-based supercapacitors with organic elec

trolytes resulting in declined electrochemical performances. All these 

publications indicate the prospects for improving metal current collec

tors by modifying both physicochemical methods and carbon materials, 

which help suppress corrosion, improve interfacial contact and reduce 

internal resistance. 

Ordinary graphite can be used as a carbon material, the hydrophobic 

layer of which is deposited on aluminium foil to improve the interfacial 

contact between the active material and the current collector [70]. The 

deposited graphite layer has high adhesion and highly ordered packing 

on aluminium foil, which makes it possible to create a strong interfacial 

contact between the active material layer and the modified current 

collector without voids. In addition, graphite helps to suppress the for

mation of a resistive oxide film on the surface of the aluminium foil and 

suppresses corrosion, which makes it possible to achieve high capaci

tance values and improved cycle life. The surface of aluminium current 

collector was treated by laser which reduced oxygen content and lead to 

a better interaction with active material. 95 % capacitance retention was 

demonstrated by the aluminium current collector protected with 

Table 1 

Carbon-based current collectors. 

Year Current 

collector (CC) 

Active material Capacitance Impedance 

(Rct and Rs, 

Ohm) 

Electrolyte Method Rate 

capability 

Cyclic 

stability 

Flexibility or 

Stretchability 

Ref. 

2019 carbon fibers Ketjen black 

carbon 

Impedance 

< 1.5 

1 M Na2SO4 electrospinning 

and cross-linking 

– – – [55] 

2020 carbon paper graphene 

hydrogel 

294F/g at 1.18 

A/g, 193F/g at 

28.24 A/g 

Rct < 2 1 M KOH hydrothermal 

treatment (of 

active material) 

66 % 

retention at 

28.24 A/g 

– – [56] 

2020 graphite foil 

T68A 

activated 

carbon 

0.326 ESR = 1.3 NaCl:DI 

water = 1:5 

printing 

technology 

– – – [52] 

2020 graphite foil 

Sigraflex 

activated 

carbon 

0.361 ESR = 1.3 NaCl:DI 

water = 1:5 

printing 

technology 

– – – [52] 

2018 PEDOT:PSS/ 

multiwall 

carbon 

nanotube ink 

PEDOT:PSS/ 

multiwall 

carbon 

nanotube ink 

20.3F/g at 10-3 

A/g 

Rs = 58,3 Rct 

= 2.762 

gel-like 

electrolyte 

(PVA/ 

H3PO4) 

screen printed – 72 % 

after 

1000 

cycles 

– [57] 

2017 polymer-nanocarbon PANI 250F/g at 20 

mV/s 

– 1 M H2SO4 printing 

technology 

– – – [58] 

2020 porous 

graphenePEDOT 

porous 

graphenePEDOT 

115.2 at 0.5 A/ 

g, 78.4 at 6 A/ 

g 

Rct = 66.8 1 M H2SO4 laser technology – slight 

losses 

after 

4000 

cycles 

Slight losses 

after 2000 

cycles 

[59] 

2018 porous 

graphene 

carbon 

fibers@PANI 

710F/g at 2 A/ 

g, 1.42F/cm2 

at 4⋅10 -3 A/ 

cm2 

Rct = 3 1 M H2SO4 chemical vapor 

deposition 

– 96.8 % 

after 

1000 

cycles 

– [60] 

2017 exfoliated (EG) 

planar 

graphene 

exfoliated (EG) 

planar 

graphene 

4.9 mF/cm2 at 

2 mV/s 

ESR = 180 H2SO4/PVA printing 

technology 

64.6 % at 500 

mV/s 

91.1 % 

after 

10,000 

– [63] 

2017 mesoporous 

polyaniline 

graphene 

mesoporous 

polyaniline 

graphene 

7.6 mF/cm2 at 

2 mV/s 

– H2SO4/PVA printing 

technology 

– 80.9 % 

after 

5000 

cycles 

– [63] 

2022 3D pyrolytic 

carbon 

Mn3O4 186F/g or 968 

mF/cm2 at 

0.5* 10-3 A/ 

cm2 

Impedance 

> 140 

1 M H2SO4 3D printing – >92 % 

after 

5000 

cycles 

– [62] 

2018 reduced 

graphene oxide 

aerogel 

MnO2 

nanotube 

274 mF/cm2 

or 137F/g at 2 

mV/s 

Impedance 

> 20 

0.5 M 

Na2SO4 

hydrothermal 

method 

above 50 % at 

100 mV/s 

– – [66] 

2018 reduced 

graphene oxide 

aerogel 

Fe2O3 

nanotube 

286 mF/cm2 

or 143F/g at 2 

mV/s 

Impedance 

> 20 

0.5 M 

Na2SO4 

hydrothermal 

method 

above 50 % at 

100 mV/s 

– – [66] 

2017 PF407C 

graphite 

activated 

carbon 

200 – 360 mF 

at 1 mA 

ESR from 7.9 

to 12.7 

NaCl printing 

technology 

– – – [64] 

2018 paper-based 

expanded 

graphite (EG) 

polypyrrole 

(PPy) 

177.8F/g Rs≈3 1 M H2SO4 brush-painting from 510.3 to 

329.7 mF/ 

cm2 retention 

from 1 to 10 

mA/cm2 

>94.9 % 

after 

5000 

cycles 

– [67] 

2021 graphite tape MnO2 577.5 mF/cm2 

or 91.7F/g at 

0.5 mA/cm2 

Rs = 39.2 3 M LiCl peeling off a thin 

graphite layer 

with a tape 

about 33 % 

retention at 

10 mA/cm2 

78 % 

after 

5000 

cycles 

97 % after 1200 

cycles of 

bending to 180◦

[68] 

* In the Tables 1 and 2, impedance means the sum of charge transfer resistance, solution resistance and diffusion layer. Impedance was used in the cases when 

authors provided EIS spectra but did not specify its parameters. Rct – charge transfer resistance, Rs – solution resistance. The ESR was derived from IR drop of GCD 

curve. 

A. Abdisattar et al. 

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7

commercial graphite ink Acheson Electrodag 965SS after 950 days of 

test [52]. There are several research papers in which the authors have 

used commercial carbon-coated aluminum foil, also called conductive 

coated aluminum foil, as a current collector to improve the performance 

of supercapacitors [71,72]. 

Carbon-coated aluminum foil can maintain long-term electro

chemical stability on a platform with a higher voltage than pure 

aluminum, can withstand long-term immersion in electrolyte, has 

excellent surface contact ability, can provide excellent interface con

ductivity, can withstand the stress generated during the long-term cycles 

of the supercapacitor. 

Along with graphite, the use of carbon nanofibres has also been re

ported, which, due to their high surface area and simple preparation 

methods, are attractive for modifying metal current collectors. X. Chen 

et al. demonstrated a simple method for obtaining the electrode based 

on the electrospinning of MnO-polymer nanofibres followed by elec

trochemical 3D printing of a strictly controlled current collector of 

copper layer onto a composite [73]. This leads to a significant increase in 

the electronic conductivity of the electrode (by>360 %) and an increase 

in specific capacitance (by 110 %), which is presumably based on a 

decrease in interfacial resistance due to carbon fibres. Graphene oxide is 

also used to create modified composite current collectors with metals. 

One such example is the GO@AgNW (active material is activated car

bon) flexible current collector composite, which is synthesized by 

sputtering silver nanowires onto the surface of graphene oxide [74]. 

This approach makes it possible to achieve not only high corrosion 

resistance due to the presence of functional groups in graphene oxide, 

but at the same time, such a supercapacitor demonstrates a retention of 

70 % after a cyclic tensile test (~10,000 cycles, 40 % elongation) and 60 

% after an additional cyclic charge–discharge test (3000 cycles). 

Another modification method is the physical treatment of metal 

current collectors; for example, picosecond laser action on aluminium 

current collectors contributes to an increase in the capacitance of 

supercapacitors up to 110.1F/g, which is due to the removal of the oxide 

layer and an increase in contact with the electrode [32]. At the same 

time, such processing of the current collector led to an improvement in 

the cyclic stability of the supercapacitor due to a stronger contact with 

the electrode and prevention of delamination. Electrochemical etching 

modifications have also been reported for nickel current collectors, 

which, although characterized by high power density, fast charge/ 

discharge cycles and long life, have a flat surface that leads to cyclic 

instability. Improving its characteristics is possible by modifying its 

surface structure to create a larger area of contact with the electrode 

material. This is evidenced by the studies carried out in [75], which 

show that nickel foam has higher electrochemical characteristics, reac

tion reversibility, higher pseudocapacitance and weaker polarization in 

comparison with the nickel grid. A simple electrochemical etching 

method can be used to achieve these goals, while functionalization of the 

surface of the nickel current collector can significantly increase the 

capacitance characteristics of commercial electrode material (activated 

Fig. 4. a) Schematic illustration of the PPy/Au LV network electrode fabrication process and of the device structure; b) photograph of supercapacitor, demonstrating 

high transparency; (c) schematic of the graphene electrode fabrication process; (d) the Ag grid film, and the sintered Ag nanoparticles; (e) SEM and EDS mapping 

images of the Au grid and retention rate by mechanical bending of the GG sample (inset: photograph of the Au/Ag grid supercapacitor bent around a lab vial); (f) 

schematic illustration for the fabrication of Cu@Ni/porous Ni/MnCo2O4 electrodes; (g) SEM images of 3D porous Ni skeletons of Cu@Ni/porous Ni, and Cu@Ni/ 

porous Ni/MnCo2O4; (h) SEM images of nanostructured MnO2 under 80 ◦C with different reaction times on Au film and Pt@Au film; schemes of MnO2 nucleation 

model on Au substrate and on Pt@Au substrate. Reprinted with permission from [69,78,82,83]. 

A. Abdisattar et al. 

Case 2:23-cv-00334-JRG Document 1-6 Filed 07/14/23 Page 8 of 12 PageID #: 118
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8

carbon) up to 210F/g at a current density of 0.5 A/g and cycle stability 

of 89 % after 10,000 cycles [76]. 

Nickel can also be used as nanospheres up to 400 nm in diameter 

chemically deposited on the surface of filter paper, which exhibits 

excellent electrical and mechanical properties: the surface resistance is 

2.7 O/cm2 (Ro = 0.8 O/cm2) after 5000 bending cycles, and the mass 

density is only 0.35 g/cm3 [77]. The porosity achieved due to the nickel 

metal layer provides a large area of contact with the active material, 

which has a positive effect on the capacitance characteristics. In contrast 

to the thin layer of nickel nanospheres, dense double layers are advan

tageous in terms of the high corrosion resistance of the nickel coating. 

This kind of coaxial composite current collector has excellent flexibility 

and mechanical strength and allows us to obtain a high capacitance of up 

to 20.6 mF/cm and an energy density of 4.8 μWh/cm at a power density 

of 32.25 μW/cm (Fig. 4f, g) [78]. 

Unlike nickel-plated current collectors, the use of three-dimensional 

(3D) porous nickel coated with nickel hydroxide has a number of ad

vantages that make it possible to achieve the theoretical capacitance and 

high-speed performance of supercapacitors. The nickel hydroxide layer 

can reduce the pore size of commercial nickel foam from 300 μm to 

10–20 μm, which leads to a noticeable improvement in the performance 

of the supercapacitor, while it demonstrates a maximum capacitance of 

3637F/g (active material is Ni(OH)2) at a current density of 1 A/g and 

retains 97 % capacitance at a high current density of 100 A/g with a 

cyclic stability of>80 % after 10,000 cycles [79]. All these works point 

to the obvious influence of micro- and nanostructures of the current 

collector on the electrochemical properties of the supercapacitor. A 

striking example is a massive current collector consisting of nickel 

nanorods, i.e., the active material is in a thick layer of the current col

lector. This structure makes it possible to create numerous conducting 

channels, which significantly reduces the contact resistance of the thick 

electrode layer at their interfaces. At the same time, such current col

lectors have a high specific capacitance exceeding more than sixfold that 

of nickel foil, which is 795F/g for an NN@MnO2 electrode with a MnO2 

mass load of 0.054 mg/cm2 [80]. At the same time, an increase in the 

electrode load by a factor of 20 leads to a decrease in the gravimetric 

capacitance to 326F/g, but the areal capacitance increased>8 times; this 

behavior indicates the important role of interfacial contacts between the 

active material and the current collector. When the active material is a 

thick layer, the structure of the current collector must be selected 

accordingly, that is, having a large specific area of interfacial contact. 

However, massive current collectors and thick layers of active material 

have limitations associated primarily with their flexibility and me

chanical strength, as well as low energy density. 

It should be noted that a new trend in the field of current collectors is 

the use of paper as a flexible and efficient current collector due to its low 

cost, flexibility and the possibility of its modification with metals by 

various methods. Another trend is the creation of transparent super

capacitors that replicate the structure of the bioinspired quasi-fractal 

structure of leaf venation (LV), where a metal network acts as a cur

rent collector (Fig. 4a, b) [81]. Such supercapacitors have excellent 

specific capacitance, which is 5.6 mF/cm2 with a transparency of 45 % 

[82]. Flexible and transparent supercapacitors are of particular interest 

for creating next-generation electronics for various applications, such as 

projection displays, biological sensors and smart devices [83]. Largescale production technologies for such transparent intelligent super

capacitors can be based on the use of a flexible and ultralight current 

collector made of a 3D nickel microgrid and highly active Ni-Co bime

tallic hydroxide as an active material [84]. Another example of a current 

collector for a transparent supercapacitor is a lithographed silver mesh, 

which exhibits high optical transparency (~80.58 % at 550 nm), flexi

bility and stability [85]. Meanwhile, transparent supercapacitors are 

characterized by low capacitance values, and their increase leads to a 

decrease in the optical transparency of the device, which limits their 

practical application. 

Table 2 summarizes data on metal current collectors, including their 

fabrication methods; mechanical characteristics such as flexibility and 

stretchability; and electrochemical characteristics of these super

capacitors in terms of capacitance, impedance properties, rate capability 

and cyclic stability. 

5. Conclusions 

In this review, we have detailed and summarized the latest advances 

in the development and application of current collectors for nextgeneration supercapacitors with longer lifetimes, greater environ

mental friendliness and lower costs, as well as the ability to match 

properties depending on the application. To achieve these goals, it is 

necessary to pay attention to the fact that the current collector does not 

increase the total capacitance of the supercapacitor but allows the 

capacitance of the active material to be maximized. At the same time, a 

well-designed structure of the current collector and the electrode as a 

whole improves the energy and power density of the supercapacitor. 

Over the past five years, in the research and development of current 

collectors, the attention of the authors has been directed to obtaining 

micro, flexible, stretchable, wearable and transparent supercapacitors 

where large energy capacities are not needed. To improve properties 

such as conductivity, power, rate and cycling stability, light weight and 

increased energy density are mainly dependent on the current collector 

along with the active material. Thus, studies of the influence of the 

current collector structure on the electrochemical properties of super

capacitors indicate that a nanostructured current collector has a number 

of advantages associated with a high specific surface area and electrical 

conductivity, which reduces the resistance to diffusion and ion transport 

in the electrolyte. However, the latest development trends are related to 

3D structures, where the current collector and active material together 

form a complete 3D electrode. As with other energy storage devices, 

supercapacitors are seeing a shift towards microsupercapacitors, which 

are complex circuits that combine up to 130 supercapacitors into a 

single circuit that outputs>100 V. Thus, for the development of a new 

generation of supercapacitors, it is necessary to conduct research on the 

materials and structures of current collectors in the following areas: 

1. Despite numerous studies on the use of current collectors based on 

carbon materials and various polymers, which were considered as an 

alternative to metal current collectors, the problem of ensuring full 

contact between the current collector and the active material has not 

yet been solved. From this point of view, we believe that the ten

dency to create an integral three-dimensional electrode that will act 

as both a current collector and an active material is a possible solu

tion to ensure good contact between the current collector based on 

carbon materials and the active material. 

2. On the other hand, metal current collectors, unlike carbon materials, 

attract special attention due to their high conductivity and low cost; 

at the same time, they provide mechanical integrity and flexibility. 

However, their use in water-based electrolytes limits metal current 

collectors. In the literature, there has been an increase in research 

aimed at improving metal current collectors by modifying both 

physicochemical methods and carbon material integration, which 

help to suppress corrosion, improve interfacial contact and reduce 

internal resistance. The use of methods such as laser irradiation and 

chemical and electrochemical deposition makes it possible to create 

uniform layers of a given thickness and, accordingly, masses, 

providing high porosity and improved adhesive properties. 

CRediT authorship contribution statement 

Alisher Abdisattar: Conceptualization, Methodology, Writing – 

original draft, Investigation. Mukhtar Yeleuov: Data curation, Super

vision, Writing – review & editing. Chingis Daulbayev: Conceptuali

zation, Methodology, Writing – original draft, Investigation, Writing – 

review & editing. Kydyr Askaruly: Supervision, Validation. Aidos 

A. Abdisattar et al. 

Case 2:23-cv-00334-JRG Document 1-6 Filed 07/14/23 Page 9 of 12 PageID #: 119
9

Table 2 Current collectors based on metal with various modifications. 

Year Current collector 

(CC) 

Active material Capacitance Impedance 

(Rct and Rs, 

Ohm) 

Electrolyte Method Rate capability Cycli

2021 Al graphite activated carbon 105.3F/g at 10 

mV/s, 102.4F/g at 1 A/g 

Rct ≈ 0.4 1 M Na2SO4 roll-to-roll coating 

technology 

74 % retention for 100 mV/s, 86 % retention at 20 A/g 

almoafter cycle

2019 Cu manganese@carbon 

nanofibers 

208F/g at 5 mV/s Rs + Rct < 9 0.25 M bis 

(trifluoromethane) sulfonimide lithium 

electrochemical 3D printing – – 

2020 graphene oxide 

nano flakes@silvernanowire 

graphene hybrid hydrogels 

108F/g at 3 A/g – 0.5 M H3PO4 alternating sequence of 

spray-coating 

– – 

2018 Al activated carbon 110.1F/g at 0.1 A/ 

g 

Rct = 7.3 1 M tetraethylammonium 

tetrafluoroborate in acetonitrile 

picosecond laser processing – 91.5 

after cycle

2019 Ni activated carbon 210F/g at 0.5 A/g Rct = 0.71 6 M KOH electrochemical etching 

process 

83 % retention at 20.0 A/g 

89 % 10,0

2019 Ni paper MnO2 1095 mF/cm2 at 1 

mA/cm2 

Rs ≈ 1.0 Rct =

5.5 

1 M Na2SO4 electroless deposition – 97.3 

1000 % afcycle

2018 Cu@Ni /porous Ni MnCo2O4 1280 mF/cm2 at 5 

mA/cm2 

Rs = 1.4, 

Impedance <15 

2 M KOH electroless plating and 

hydrogen bubble dynamic template electrodepostion 

– – 

2017 3D nickel Ni(OH)2 3637F/g at 1 A/g Rs = 1.32, Rct 

= 0.31 

1 M KOH electrodeposition 97 % retention at 

100 A/g 

80 % 10,0

2017 nickel nanorod 

arrays 

MnO2 795F/g or 42 mF/ 

cm2 at 5 mV/s 

Rct = 3.25, 

Impedance >30 

1 M Na2SO4 anodic aluminum oxide 

template 

– – 

2019 Au leaf venation Polypyrrole 13 mF/cm2, at the 

current I = 0.1 mA 

Impedance <5 

1 M LiCl leaf venation template 61.7 % retention 

from 0.1 to 1 mA 

93 % GCD 

2021 3D Ni micromesh Ni–Co bimetallic 

hydroxide 

75.58 μA⋅h/cm2 at 

1 mA/cm2 

ESR = 0.75 Impedance >6 

2 M KOH photolithography and 

electrochemical deposition 

60.6 % capacity retention from 1 to 50 mA/cm2 

88.7 2500 81.4 6000 

2017 Ag-grid@PET PEDOT: PS 2.79 mF/cm2 at 

0.01 V/s, 0.85 mF/ cm2 at 1.00 V/s 

Rs ≈ 2 PVA/H3PO4 lithography and scrap 

techniques 

– 92 % 

10,0

Case 2:23-cv-00334-JRG Document 1-6 Filed 07/14/23 Page 10 of 12 PageID 
Electrochemistry Communications 142 (2022) 107373

10

Tolynbekov: Supervision, Validation. Azamat Taurbekov: Supervi

sion, Validation. Nikolay Prikhodko: Project administration, Funding 

acquisition. 

Declaration of Competing Interest 

The authors declare that they have no known competing financial 

interests or personal relationships that could have appeared to influence 

the work reported in this paper. 

Data availability 

Data will be made available on request. 

Acknowledgements 

This work was supported by the Ministry of Education and Science of 

the Republic of Kazakhstan through the project AP08856683. 

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