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A Perspective: Could Carbon Current Collectors Improve the Energy Density of Aqueous Alkaline Symmetric Supercapacitors?

  • Hemesh Avireddy ORCID logo EMAIL logo , Joan Ramon Morante and Cristina Flox
Published/Copyright: November 19, 2016
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Energy Harvesting and Systems
From the journal Energy Harvesting and Systems Volume 3 Issue 4

Abstract

The present discussion shows a perspective about using graphite as a current collector in order to achieve high energy density in a symmetric supercapacitor system. Several electrochemical modes (such as rest potential analysis, CV, PEIS, GCPL) were carried out to evaluate the electrochemical characteristics of graphite in aqueous 6 mol/L KOH. And, the resulting performance was compared to an another conventional current collector system based on nickel-stainless steel. Interestingly, widening of cell voltage was observed for graphite when compared to nickel-stainless steel. The discussion reveals the reasonable influences and validations of widening in cell voltage towards the values in energy densities. This perspective also highlights some issues related to carbon (graphite) current collectors and encloses with some promising strategies in overcoming these issues, not limiting the domain of application (either micro or macro supercapacitor devices).

Keywords: current collectors; cell voltage; energy density; Fe2O3 porous carbon nanofibers; single spinneret electrospinning

1 Introduction

One of the major goals set by the European Union was to withdraw at least 20 % of its energy consumption from the renewable resources such as wind or solar energy by the year 2020. The important criterion of these renewable resources is the equipped energy storage device. Electrochemical storage devices such as supercapacitors (SCs) could deliver requisite energy in a short interval of time. In this regard, many groups around the world focus their research in developing high performance SCs. However, SCs suffers from low energy density when compared to batteries (Béguin et al. 2014; Simon et al. 2014). In SCs, energy density is expressed as 0.5*CE2 where, C represents the cell capacitance (F) and E as the cell voltage (V). To enhance the energy density of a SC, two strategies could be implemented: increasing cell capacitance and voltage (Béguin et al. 2014; Dai et al. 2015).

In the first aspect, the capacitance could be enriched by tuning the electrochemical active materials independance with their type of charge storage phenomena, which could be classified into: (a) electrostatic sorption of ions in the material´s pores (ultra-micropores <~0.7 nm or micropores <2 nm; electric double layer capacitance, EDLC) and (b) highly reversible multiple redox reactions occurring over the electrode potential window (pseudocapacitance) or reversible redox reactions favorable at high rates (Brousse et al. 2015; Salanne et al. 2016). Extensive research has been focused on developing hybrid systems besides improving existing promising carbon materials and transition metal oxides (examples: Carbon-CNTs, Graphene, Carbide derived Carbons and porous Carbons nanofibers; Transition metal oxides- RuO2, MnO2, Co3O4 and Fe2O3) (Béguin et al. 2014; Dai et al. 2015; Gao et al. 2015; Lukatskaya et al. 2016; Zhang et al. 2015).

In regards to the second aspect, cell voltage could be enhanced by using electrolytes with wider thermodynamic potential windows such as organic (~2.5 V) and ionic liquids (~>3.5 V). However, these electrolytes could be disadvantageous because of their low ionic conductivities, high costs and nature of less environmental friendliness. Aqueous electrolytes could be an another substitute due to their nature of high ionic conductivities, cost effectiveness and environmental friendliness (Béguin et al. 2014; Demarconnay et al. 2010). However, aqueous electrolytes provides low cell voltages (~1.0 V) due to the low thermodynamic potential window of water (1.23 V), eventually leading to low energy densities (Abbas et al. 2015; Béguin et al. 2014; Demarconnay et al. 2010).

Interestingly, François Béguin´s group was able to show an excellent cyclable 1.9 V carbon/carbon symmetric SC using 1 mol/L Li2SO4 (Abbas et al. 2015; Fic et al. 2012; Q. Gao et al. 2012). The high cell voltage was achieved because of the optimization in high over potential of di-hydrogen evolution, contributed from water reduction and OH- ions produced from negative carbon pores (Abbas et al. 2015; Demarconnay et al. 2010; Gao et al. 2012). However, this system was designed with a cell composed of gold current collectors which could reduce the feasibility of commercialization. In order to reduce the high cost involved due to the usage of gold, stainless steel was implemented and a cell voltage of 1.5 V was achieved (Abbas et al. 2015). Beyond 1.5 V, for example at 1.6 V, reduction of capacitance was observed due to increase in cell resistance which was likely due to the accumulation and generations of corrosion products. These products could be contributed due to the irreversible oxygen evolving reactions at the positive end (Abbas et al. 2015; Ratajczak et al. 2013; Ratajczak et al. 2014). Furthermore, since thanks to the nickel´s ability to avoid corrosion at the positive end as compared with stainless steel, stainless steel was replaced by nickel. Although mixed nickel compounds are formed due to oxidation of Ni(OH)2 to NiOOH, apparently not much influence has been observed on the electrochemical performances. This effect might be due to the good conductivity of mixed nickel compounds, which could provide high mobility to electronic and ionic species (Abbas et al. 2015). While, on close observations on these discussions, the limiting factor of cell voltage could be from current collectors. In this regard, as carbon/carbon system with gold current collectors could show 1.9 V, we percept about using carbon itself as a current collector which is currently the motivation of this work.

Therefore, in the present discussion, we show a perspective about using carbon as current collector in a symmetric prototype cell configuration. Graphite was chosen primitively to validate this perspective in a traditional aqueous alkaline medium (6 mol/L KOH). The agenda of varying the electrolyte from 1 mol/L Li2SO4 to 6 mol/L KOH was due to the high solubility of KOH in water when compared with that of Li2SO4 (KOH=121 g/100 ml; Li2SO4, monohydrate=34.9 g/100 ml at 25 °C), which could provide high ionic conductivity and mobility to electrolyte ions (Béguin et al. 2014). In addition, we also compare this perspective with nickel-stainless steel current collectors. In addition to this, flexibile self standing Fe2O3 porous carbon nanofibers (Fe2O3-PCNFs) were prepared via single spinneret electrospinning technique under a high feed rate. These electrode materials are promising active materials for both micro and macro-supercapacitors because of the presence in two-fold charge storage effects (EDLC (Zhang et al. 2015) and redox reactions (Li et al. 2015)).

2 Methods and Materials

2.1 Chemicals

Polyacrylonitrile (99 %, PAN; Mw: 150 000) and Ferric chloride, (99 %, FeCl3) were purchased from Sigma Aldrich. Whereas N, N-dimethylformamide (99 %, DMF) as a solvent from Panreac and Potassium hydroxide (KOH, Mw: 56.11; pellets) from Emsure. All chemicals were used without further purification.

2.2 Fabrication of Electrode Materials

The mixture of 3 wt. % of FeCl3 and 12 wt. % of PAN was dissolved in 20 g of DMF solution by magnetic stirring for an hour at 80 °C to obtain a homogenous electrospinning blend solution. The weight percent (wt. %) in case of PAN would represent the weight with respect to the weight content of DMF solvent, whereas, in case of FeCl3, weight content of Fe in FeCl3 with respect to PAN. The blend solutions were electrospun on an electrospinning rotating type current collector system obtained from Yflow® Nanotechology Solutions (Flox et al. 2013) (as shown in Figure 1), with optimized electrospinning parameters to fabricate a mat. The applied voltage, feed rate and distance between the spinneret and current collector was around 14 kV (1.75 kV/cm2), 1.50 mL/h and 8 cm, respectively. The mats were stabilized in the air at 270 °C (7 h; 1 °C/min) and carbonized in Ar/H2 (5 %) at 500 °C and 800 °C (1 h at each step; 5 °C/min) to obtain self-supporting flexible Fe2O3-PCNFs films (see Figure 1). These Fe2O3-PCNFs films were subjected to different material and electrochemical characterization techniques.

Figure 1: Right: An ongoing single spinneret electrospining process, Feed rate: 1.5 mL/h (Stage: production of mats). Left: Flexible and self-standing Fe2O3-PCNFs electrodes prepared via single spinneret electrospining technique. (Equipment: Yflow ® Nanotechnology solutions) (Flox et al. 2013).
Figure 1:

Right: An ongoing single spinneret electrospining process, Feed rate: 1.5 mL/h (Stage: production of mats). Left: Flexible and self-standing Fe2O3-PCNFs electrodes prepared via single spinneret electrospining technique. (Equipment: Yflow ® Nanotechnology solutions) (Flox et al. 2013).

2.3 Material Characterization

Field Emission Scanning Electron Microscopy (FESEM) images and Energy-Dispersive X-ray Spectroscopy (EDS) were obtained by Zeiss Serie Auriga. Powered X-ray diffraction (P-XRD) analysis was observed using Bruker D8 advance equipment with CuKα1 radiation (λ=1.5405 Å). Transmission Electron Microscopy (TEM) images were obtained by Zeiss LIBRA 120 at different operating voltages (mainly from 50 kV to 75 kV). Thermogravimetric analysis (TGA) was obtained by using PerkinElmer (TGA 4000) analyzer in the air with a ramp rate of 10 °C/min. Textural behavior was analyzed from N2 sorption isotherms with Micromeritics ASAP 2020 porosimeter at –196 °C. Brunauer–Emmett–Teller (BET) Surface Area Analysis was used to estimate surface area, whereas Barrett-Joyner-Halenda (BJH) method for Pore Size & Volume analysis. Samples were degassed at 200 °C in N2 for 12 h before the analysis.

2.4 Electrochemical Characterization

In order to know the individual electrochemical performance of the fabricated Fe2O3-CNFs films, a three electrode test cell configuration was used which was obtained from Bio-Logic (France). Saturated Calomel Electrode (SCE) and platinum mesh was used as a reference and the counter electrode. The working electrode in this case, was fabricated by laminated Fe2O3-PCNFs on nickel foam and stainless mesh to ensure good contacts. The average mass loading of active materials (including CNFs) was about 1.65 mg/cm2.

Two different cells were used: (i) PTFE Swagelok cell named C-I and (ii) home-made zero-gap (filter press) prototype cell, named C-II. In the Swagelok cell, nanoporous membranes were sandwiched by two stainless steel pistons (grade 304) with cross-sectional geometrical area of 1.13 cm2. Fe2O3-PCNFs films were laminated on nickel foam (purity>99 %) using press-die equipment (5 MPa, 10 s) to form a working electrode. The home-made prototype cell, as shown in Figure 2(a)), was constructed by squeezing a nanoporous membrane with two graphite bipolar plates etched with serpentine flow channels. Each end of graphite plate was connected via cupper to make an electrical contact with cables of Potentiostat. Viton gaskets were used to avoid leakage of the electrolyte and metallic end-plates made up of aluminum were used to enclose the cell to ensure a zero-gap cell. (as shown in Figure 2(b)). The cell was injected with electrolyte through the inlets connected via tygon tubing. The electrochemical measurement was done in static mode. The geometrical area of the graphite to Fe2O3-PCNFs films was about 4 cm2. In both cell configurations, equal mass loading of electrodes was used at both ends. The average mass loading of single electrode was about 2.6 mg/cm2 and 2.9 mg/cm2 for C-I and C-II respectively.

Figure 2: (a) Schematic representation of the prototype cell configuration with graphite as current collectors.
Figure 2:

(a) Schematic representation of the prototype cell configuration with graphite as current collectors.

The electrochemical characterization techniques such potentiostatic impedance spectroscopy (PEIS), cyclic Voltammetry (CV) and galvanostatic charge discharge cycles were performed by using a multichannel VMP3 potentiostat/galvanostat (Bio-Logic, France). PEIS was measured at a rest potential with amplitude of 5 mV/s from 200 kHz to 10 mHz. Long term stability experiments were measured in galvanostatic charge & discharge cycle mode using a cell current density of 1 A/g (>5000 cycles). The gravimetric capacitances (F/g) (for single electrode and cell) and energy density (Wh/kg) were calculated by considering the system as a non-linear SCs (see reference for methods (Laheäär et al. 2015).

3 Results and Discussions

3.1 Characterization of Fe2O3-PCNFs

The P-XRD pattern (as shown by Figure 3(a)) shows a broad peak in the 2 theta ranges of 20° to 30° and a narrow peak at 50, inferring the presence of amorphous carbon with graphitic impurities assigned to carbon (JCPDS card number of 01-075-1621). The remaining P-XRD peaks match the tetragonal structure of the maghemite phase of iron oxide (γ-Fe2O3; JCPDS card number: 25–1402). Moreover, the P-XRD pattern also shows a presence of low intensity peaks corresponding to the rhombohedral structure of hematite (α-Fe2O3; JCPDS card number: 33–0664) which might be due to the presence of a reducing atmosphere (Ar/H2) or reducing effect of CNFs (Wu et al. 2014). The FESEM and TEM image, as shown in Figure 3(b) and 3(c), represents the morphological characteristics of continuously oriented CNFs incorporated into Fe2O3 nanostructures (nano-ball like morphology). In addition, an EDS element mapping was also evaluated, as shown in figure S4 (a)-(d), providing evidence of C, Fe and O distribution in the nanocomposite, which also indicates the Fe2O3 incorporation onto the surface of CNFs matrix. The thickness of the Fe2O3-CNFs film was around 210 μm (see supplementary, figure S1). Moreover, visible pores were also observed, providing a configuration of Fe2O3-PCNFs. These textural properties were further analyzed by N2 adsorption measurements and their corresponding isotherms. BJH Pore Size & Volume distribution was represented by Figure 4(a) and 4(b) respectively. The isotherm shows a type I(b) and type IV(a) behavior with H2(a) or/and H2(b) type hysteresis, inferring broad range microporous (<~ 2.5 nm) and cylindrical mesoporous, which might be influenced by pore percolation caused by cavitation-induced evaporation (Thommes et al. 2015). In our case, the mesoporous was created by evaporation or etching of Fe2O3 during carbonization in Ar/H2 (Ryu et al. 2013)). These pores exhibited microporosity between 1.79 nm to 2.0 nm, whereas mesoporosity occurred between 2.0 nm to 10.58 nm and 10.58 nm to 53.1 nm. The presence of a wide range of porosity could provide better adsorption of electrolyte ions (EDLC) (Zhang et al. 2015). TGA measurement was conducted to calculate the wt. % of Fe2O3 incorporation on the CNFs matrix and is shown in Figure 4(c). Based on the results, the wt. % of Fe2O3 in CNF matrix was found to be around 17.9 wt. %.

Figure 3: (a) P-XRD pattern of Fe2O3-PCNFs, (b) FESEM images of Fe2O3-PCNF (Inset image: Fe2O3 nanoballs like structures incorporated on CNF matrix, scale-200 nm) (c) TEM image of Fe2O3-PCNF (visible pores). A (Red): pores due to detachment of Fe2O3; B (white) and C (Purple): Fe2O3 nanoballs.
Figure 3:

(a) P-XRD pattern of Fe2O3-PCNFs, (b) FESEM images of Fe2O3-PCNF (Inset image: Fe2O3 nanoballs like structures incorporated on CNF matrix, scale-200 nm) (c) TEM image of Fe2O3-PCNF (visible pores). A (Red): pores due to detachment of Fe2O3; B (white) and C (Purple): Fe2O3 nanoballs.

Figure 4: Textural analysis of Fe2O3-PCNFs (a) Nitrogen sorption curves, (b) corresponding adsorption pore size & volume distribution. (c) TGA curves, temperature for Initial measurement of weight %: 100 °C (X-axis).
Figure 4:

Textural analysis of Fe2O3-PCNFs (a) Nitrogen sorption curves, (b) corresponding adsorption pore size & volume distribution. (c) TGA curves, temperature for Initial measurement of weight %: 100 °C (X-axis).

3.2 Electrochemical Characterization

3.2.1 Cyclic Voltammetry

CV was conducted in order to: (i) understand the behavior of charge over the cell voltage window and (ii) identify the maximum cell operational voltage window. Figure 5 shows the CV curves for both C-I and C-II at a scan rate of 20 mV/s using Fe2O3-PCNFs as active materials (represented as gravimetric capacitance versus cell voltage, see supplementary figure S2 for current versus cell voltage). C-I exhibits almost rectangular shape behavior, where gravimetric capacitance is almost constant over the cell voltage window (Brousse et al. 2015). This behavior is typically related to the adsorption and desorption of electrolyte ions at the porous CNFs matrix, formed due to the decomposition of Fe2O3 during carbonization under Ar/H2 atmosphere (Ryu et al. 2013). On the other hand, C-II exhibits an additional hump shape in the CV curve which might have been caused from augmented charge storage. These charge storage processes can be divided into two main regions, as shown in Figure 6: region 1 and 2. In region 1, charge contribution might have been the result of electrostatic adsorption of electrolyte ions at pores (similar to C-I). However, in region 2 (additional region), charge storage could have been caused by di-hydrogen formation and chemisorptions, as well as by Fe2O3 assisted redox reactions. Further validation of this hypothesis was carried out by organizing CV in a three electrode cell experiment, as shown in Figure 7. To understand the charge storage processes, a potential window experiment was scheduled. In this experiment, working electrodes potential could be varied versus a standard reference electrode, eventually providing a behavior of current which could assist in distinguishing charge storage contributions. In this aspect, in our case, we vary the electrode potential window from 0.0 V to –1.2 V Vs. SCE (as shown in Figure 7). By contrast, three noticeable processes could be seen which were labeled as A, B and C, where A corresponds to electrostatic adsorption of electrolyte ions (EDLC), B corresponds to di-hydrogen production and chemisorptions which could be expressed by equations from [1] to [4], respectively.

Figure 5: Cyclic voltammograms of Fe2O3-PCNFs in C-I (nickel-stainless steel) and C-II (graphite). (Two electrode cell configuration, scan rate: 20 mV/s and electrolyte: 6 mol/L KOH).
Figure 5:

Cyclic voltammograms of Fe2O3-PCNFs in C-I (nickel-stainless steel) and C-II (graphite). (Two electrode cell configuration, scan rate: 20 mV/s and electrolyte: 6 mol/L KOH).

Figure 6: Cyclic voltammogram of Fe2O3-PCNFs in C-II (graphite). Region 1: electrostatic sorption of electrolyte ions. Region 2: redox reactions from Fe0/ Fe2+/Fe3+ and di-hydrogen formation and chemisorptions. (Two electrode cell configuration, scan rate: 20 mV/s and electrolyte: 6 mol/L KOH).
Figure 6:

Cyclic voltammogram of Fe2O3-PCNFs in C-II (graphite). Region 1: electrostatic sorption of electrolyte ions. Region 2: redox reactions from Fe0/ Fe2+/Fe3+ and di-hydrogen formation and chemisorptions. (Two electrode cell configuration, scan rate: 20 mV/s and electrolyte: 6 mol/L KOH).

Figure 7: Three electrode cyclic voltammograms of Fe2O3-PCNFs with different potential windows. Label A: electrostatic sorption of electrolyte ions (EDLC). Label B: di-hydrogen formation and chemisorptions and label C: redox reactions from Fe0/ Fe2+/Fe3+. (scan rate: 10 mV/s, electrolyte: 6 mol/L KOH, current collector: nickel-stainlees steel).
Figure 7:

Three electrode cyclic voltammograms of Fe2O3-PCNFs with different potential windows. Label A: electrostatic sorption of electrolyte ions (EDLC). Label B: di-hydrogen formation and chemisorptions and label C: redox reactions from Fe0/ Fe2+/Fe3+. (scan rate: 10 mV/s, electrolyte: 6 mol/L KOH, current collector: nickel-stainlees steel).

(1) H 2 O + e − → H + O H −

(2) C + x H → C H x

(3) C + x H 2 O + x e − → C H x + O H −

(4) C H x + H 2 O → C + H 3 O + + x e −

In the above equations, {CHx} and {C} represent the hydrogen insertion into the carbon and carbon substrate, respectively (Abbas et al. 2015; Béguin et al. 2014). The charge storage from di-hydrogen production and chemisorptions has been previously observed with an increase in cell voltage for carbon/carbon SC (Abbas et al. 2015). These reactions could have been possibly originated from negative and/or the positive electrode during the negative potential scan (Demarconnay et al. 2010). On the other hand, label C corresponds to redox reactions attributed to Fe0/Fe2+/Fe3+ in alkaline medium (Li et al. 2015). As a result, in case of C-II, because of these additional charge storage processes, an increase of capacitance (a hump) could be observed (when compared to C-I), achieving a single electrode gravimetric capacitance of about 63.1 F/g for C-II (whereas only 42.6 F/g could be reached with C-I) at a sweep rate of 20 mV/s. These capacitance values were appreciable when compared with those reported for Fe2O3 based 1 D and 2-D carbon electrodes in alkaline medium (as shown in Table. S1).

Interestingly (from Figure 5), CV provides information of maximum operating cell voltage, which was 1.0 V and 1.5 V for C-I and C-II, respectively. A lower cell voltage of 1.0 V was observed in case of C-I, compared with 1.5 V of C-II. The reason behind the limitation of cell voltage could be attributed to the irreversible oxygen evolving reactions from nickel-stainless steel current collector of positive side (Abbas et al. 2015). Moreover, no such observations were noticed until 1.5 V in case of C-II (see supplementary, as shown by figure S3). However, validation of these voltages as the maximum operating cell voltage is quite crucial and this has been carried out by voltage efficiency test.

3.2.2 Maximum Operating Cell Voltage (Supported by Voltage-Efficiency Test)

Voltage – efficiency test provides information about the maximum operating cell voltage (See references for methods) (Abbas et al. 2015; Stoller and Ruoff n.d.). Conceptually, cell voltage is varied during this measurement, and the corresponding coulombic efficiency is usually observed. The cell voltage at which a drastic decrease in coulombic efficiency is noticed, we consider this voltage as the maximum operating cell voltage. In this point of view, we have varied cell voltage from 0.5 V to 1.1 V (for C-I) and 1.6 V (for C-II). Figure 8, shows the plot between coulombic efficiency (ɳ%, timedischarge/timecharge) versus cell voltage. In case of C-I, the drastic change in ɳ% was observed from 96.5 % to 94.4 %, when cell voltage was varied from 1.0 V to 1.1 V, whereas in case of C-II a coulombic efficiency of 97.6 % to 93.7 % was observed, when cell voltage was varied from 1.5 V to 1.6 V. As a result, the maximum operational cell voltage was concluded to be 1.0 V and 1.5 V for C-I and C-II, respectively, which correlates with our observations inferred during CV measurements (as shown in Figure 5). The wider cell voltage achieved in C-II could definitely influence the energy density, as energy density can be either improved by enhancing the cell capacitance or increasing the voltage (or both, relationship, Energy density = 0.5*CE2 (Béguin et al. 2014)).

Figure 8: Plot between voltage-efficiency (observed by conducting galvanostatic charge & discharge cycles at a cell current density of 1 A/g).
Figure 8:

Plot between voltage-efficiency (observed by conducting galvanostatic charge & discharge cycles at a cell current density of 1 A/g).

3.2.3 Energy Density

The energy density was calculated from charge & discharge curves, which is shown in Figure 9 (current density: 1 A/g). With regards to the calculation of energy density, a difference in linear and non-linear SC system was considered. In a linear SC system, the charge (i. e. capacitance) is constant over the cell voltage window, whereas it is variable in a non-linear system. Since the variation of capacitance over the cell voltage for C-II was variable (due to additional charge storage processes, as discussed before), for comparison purposes, only a non-linear system is considered for both C-I and C-II. The gravimetric cell energy density and capacitances were calculated by the methods proposed by Laheäär et al. (2015). Lack of analysis about these points, i.e, misjudging the appropriate calculation methods for linear or non-linear systems could lead to an overestimation of energy density. An example has been shown in Table 1. The calculated energy density was about 1.21 Wh/kg and 5.05 Wh/kg for both C-I and C-II, respectively. From these values of energy density, the derived gravimetric cell capacitance was 8.73 F/g and 16.16 F/g for C-I and C-II, respectively. The observed energy density of C-II was almost increased by a factor of four. This increase in energy density could be accounted for because of: (1) the increase of cell voltage from 1.0 V to 1.5 V and (2) the additional charge storage contributions of di-hydrogen production and chemisorptions and redox reactions of Fe0/Fe2+/Fe3+, which enhances the capacitance. These performance values indicate that the substitution of graphite (or carbon) instead of nickel-stainless steel current collectors could improve the energy density of SCs. However, validity at these cell voltages for long term performance is quite crucial.

Figure 9: Galvanostatic charge & discharge cycle curves at a cell current density of 1 A/g. C-I (nickel-stainless steel) and C-II (graphite). (Electrolyte: 6 mol/L KOH. Two electrode cell configuration).
Figure 9:

Galvanostatic charge & discharge cycle curves at a cell current density of 1 A/g. C-I (nickel-stainless steel) and C-II (graphite). (Electrolyte: 6 mol/L KOH. Two electrode cell configuration).

Table 1:

Values of electrochemical performances in terms of linear and non-linear SC system. C-I (nickel-stainless steel) and C-II (graphite).

Linear System Non-linear System
Parameter C-I C-II C-I C-II
Gravimetric capacitance, Cell (F/g) 9.80 23.41 8.73 16.16
Energy density (Wh/kg) 1.36 7.31 1.21 5.05

3.2.4 Long Term Performance Studies (Charge & Discharge Cycles)

Long term performance stability in terms of capacitance retention for both C-I and C-II were analyzed by measuring 5,000 galvanostatic charge & discharge cycles at a cell current density of 1 A/g. The maximum cell voltage, as observed from CV and voltage-efficiency test, applied to C-I and C-II was 1.0 V and 1.5 V, respectively. Figure 10, shows the capacitance retention curves for both C-I and C-II, which was around 76 % and 91 %, respectively. On observation, higher capacitance retention was noticed for C-II, when compared with that of C-I. These results, indicates good stability of C-II even at 1.5 V. The lower values of capacitance retention for C-I could have been because of the oxygen evolving reactions from the positive end (Abbas et al. 2015). However, in addition to long term stability, the impedance behavior is also a crucial parameter from which the time lapse for energy deliverance could be affected (Barsoukov & Macdonald, 2005).

Figure 10: Capacitance retention curves obtained by measuring continuous galvanostatic charge & discharge cycles at a cell current density of 1 A/g (red: C-II, black: C –I). C0= refers to values of capacitance at the first cycle. C-I (nickel-stainless steel) and C-II (graphite).
Figure 10:

Capacitance retention curves obtained by measuring continuous galvanostatic charge & discharge cycles at a cell current density of 1 A/g (red: C-II, black: C –I). C0= refers to values of capacitance at the first cycle. C-I (nickel-stainless steel) and C-II (graphite).

Impedance behaviors were obtained via PEIS technique and were represented by Nyquist plot. Figure 11, shows the Nyquist plot of both C-I and C-II. For both C-I and C-II, the Nyquist plot shows a semi-circle at high frequency range which are related to electron diffusion (also known as charge transfer resistance, CTR) (Barsoukov & Macdonald, 2005). These CTRs are likely to be contributed either by electron diffusion at the pore grain boundaries of the CNF matrix (Pérez et al. 2013) or from redox reactions. The intercept at the real Z’ is generally assigned to the equivalent series resistance (ESR), provides an information about the equivalent resistance of a cell which typically includes current collectors, membranes (separators), electrolyte ionic conductivities and active material’s resistance (Barsoukov & Macdonald, 2005). The ESR for both C-I and C-II was around 0.18 ohm. cm2 and 2.9 ohm. cm2 respectively. The higher ESR for C-II might be related to the conductivity issues of graphite plate which could reduce the maximum power density. Table 2 shows the summary of the electrochemical performances. The possible strategies to reduce the ESR of the device, irrespective of application scale, i. e. either micro or grid based devices, could be via: (1) replacement of better design of current collectors in terms of their electrical contact with the active materials and/or (2) replacement with highly conductive carbon materials as current collectors.

Figure 11: Nyquist plots of C-I and C-II (normalized in terms of geometrical area). C-I (nickel-stainless steel) and C-II (graphite).
Figure 11:

Nyquist plots of C-I and C-II (normalized in terms of geometrical area). C-I (nickel-stainless steel) and C-II (graphite).

Table 2:

Summary of the electrochemical performances.

Parameters C-I C-II
Mass loading of Fe2O3-PCNFs (mg/cm2) 2.6 2.9
Cell voltage (V) 1.0 1.5
Cell capacitance (F/g) 8.73 16.16
Cell energy density (Wh/kg) 1.21 5.05
Cell power density (W/kg) 445.40 571.07
Capacitance retention (%), 5,000 cycles 76 91
Cell ESR (Ohm. cm2) 0.18 2.9
Maximum power density* (W/kg) 330 31

Source: *See reference for calculation Gromadskyi et al. (2015).

Note: C-I (nickel-stainless steel) and C-II (graphite). (ESR=equivalent series resistance).

In regards to the first aspect, good electrical contact of electrode material with current collectors, especially in case of self-standing active materials is vital. As mentioned before, the prototype cell was enclosed with metallic aluminum plates and etched graphite plates to ensure good contacts between Fe2O3-PCNFs films and graphite plate. However, these tactics could be further improved by using other advanced etching techniques (such as laser engraving). In regards to the second aspect, the graphite could be replaced by other highly conductive carbon materials such as Carbon nanotubes (CNTs) or graphene papers. Some of the authors like Marco et al. even claims that the conductivity achieved with CNTs could be as equivalent to that of gold current collectors (Notarianni et al. 2014). Graphene papers could be an another promising replacement not only because of their highly conductive property, but also due to ease in production of large geometrical area flexible current collectors which can be easily applicable to micro or grid based devices (Chee et al. 2016).

4 Summary & Conclusions

In conclusions, the perspective in using carbon (graphite) current collectors to enhance the energy density of a symmetric supercapacitior system based in an aqueous alkaline medium,was validated by using several electrochemical studies. Higher value of energy density was witnessed for graphite when compared with that of nickel-stainless steel current collector system. This improvement was accounted because of: (i) widening of cell voltage and (ii) increase of capacitance due to additional charge contributions happening due to widening of cell voltage (redox reactions: Fe0/Fe2+/Fe3+; di-hydrogen production and chemisorption). Lastly, various promising strategies was proposed to improve the resistance related issues of graphite, providing an ease in producing high energy density aqueous micro or macro-supercapacitors.

Funding statement: This work was supported by the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013).

Acknowledgements

The authors thank Doris Cadavid and Marti Biset for their support during TEM analysis and María Dolores González Candela for BET measurements. IREC also acknowledges additional support by IDIADA AUTOMOTIVE TECHNOLOGY, S.A.

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Published Online: 2016-11-19
Published in Print: 2016-10-01

©2016 by De Gruyter

Articles in the same Issue

  1. Frontmatter
  3. Editorial
  4. Coupling Supercapacitors and Aeroacoustic Energy Harvesting for Autonomous Wireless Sensing in Aeronautics Applications
  5. Variable Capacitor Energy Harvesting Based on Polymer Dielectric and Composite Electrode
  6. A Perspective: Could Carbon Current Collectors Improve the Energy Density of Aqueous Alkaline Symmetric Supercapacitors?
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https://doi.org/10.1515/ehs-2016-0015
Keywords for this article
current collectors; cell voltage; energy density; Fe2O3 porous carbon nanofibers; single spinneret electrospinning

Articles in the same Issue

  1. Frontmatter
  3. Editorial
  4. Coupling Supercapacitors and Aeroacoustic Energy Harvesting for Autonomous Wireless Sensing in Aeronautics Applications
  5. Variable Capacitor Energy Harvesting Based on Polymer Dielectric and Composite Electrode
  6. A Perspective: Could Carbon Current Collectors Improve the Energy Density of Aqueous Alkaline Symmetric Supercapacitors?
  7. High Efficiency Vibration Energy Harvesting Through Combined Isolator and Absorber Approach
  8. Modeling and Experimental Validation of Bi-Stable Beam Based Piezoelectric Energy Harvester
  9. A Monte Carlo Study on the Effect of Energy Barriers on the Thermoelectric Properties of Si
  10. Effect of the Annealing on the Low-Temperature Charge Transport Properties of Heavily Boron-Doped Nanocrystalline Silicon Films for Thermoelectric Applications
  11. Power Response of a Planar Thermoelectric Microgenerator Based on Silicon Nanowires at Different Convection Regimes
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