Can Aqueous Na2SO4-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor?

Ye, Long; Feng, Tianjie; Kirk, Donald W.; Jia, Charles Q.

doi:10.3390/en17153710

Open AccessFeature PaperArticle

Can Aqueous Na₂SO₄-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor?

The Green Technologies Lab, Department of Chemical Engineering & Applied Chemistry, University of Toronto, Toronto, ON M5S 3E5, Canada

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Author to whom correspondence should be addressed.

Energies 2024, 17(15), 3710; https://doi.org/10.3390/en17153710

Submission received: 6 July 2024 / Revised: 22 July 2024 / Accepted: 25 July 2024 / Published: 27 July 2024

(This article belongs to the Section D2: Electrochem: Batteries, Fuel Cells, Capacitors)

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Abstract

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The performance of supercapacitors is significantly influenced by both the nature and the concentration of the electrolyte employed. This study investigates the impact of a neutral electrolyte on the electrochemical properties of the maple-derived monolithic wood biochar (MWB)–sodium sulfate (Na₂SO₄) supercapacitor. The goal is to determine if a neutral electrolyte, in this case Na₂SO₄, can increase the supercapacitor energy density compared to a previous study employing a KOH electrolyte. Starting from examining the ion sizes and conductivities of salt species in KOH and Na₂SO₄ electrolytes, the difference in voltage window, measured specific capacitance, and resistance are discussed. By switching the electrolyte from 4 M KOH to 0.5/1 M Na₂SO₄, the voltage stability window was extended from 0.8 V to 1.4 V. For 1 M Na₂SO₄, the supercapacitor attains a specific capacitance of 46 F/g at 5 mA/g, accompanied by an energy density of 12.5 Wh/kg and a maximum power density of 300 W/kg. The MWB electrode, derived from naturally abundant wood, when combined with the non-toxic Na₂SO₄ electrolyte, offers an environmentally friendly and cost-effective energy storage solution. With a prolonged lifetime and minimal maintenance requirements, MWB-Na₂SO₄ supercapacitors emerge as a promising choice for diverse applications.

Keywords:

electrical energy storage; monolithic wood biochar; neutral electrolyte

1. Introduction

The increasing global energy demand, driven by population growth and technological advancements, has expedited the depletion of non-renewable energy reservoirs, notably fossil fuels. Compounding this issue is the intermittent nature of many renewable energy sources like solar and wind power, which often fail to synchronize with peak demand periods [1]. Consequently, extensive research efforts have been made to address this issue by developing and enhancing energy storage devices that optimize the storage of limited resources.

Supercapacitors stand out as promising energy storage devices with applications ranging from commercial electronics to renewable energy harvesting. Their versatility enables them to power various devices, significantly contributing to sustainable energy solutions. However, their performance is highly dependent on the properties of the electrode materials and electrolytes employed [2,3,4].

Carbon-based materials have garnered attention for their widespread use in supercapacitor electrodes due to their availability, low cost, robustness, and environmentally friendly nature. Graphene is often suggested due to its high relative surface area of 2600 m²/g, but its large-scale production poses challenges [5,6,7]. Current research has extensively focused on using carbon powders such as graphite oxide (GO) and reduced graphite oxide (RGO) as electrode materials. Nevertheless, these powdered carbons need to be coated using a binder to form a thin film, then rolled into a cylinder with the current collector and separator and packed into a module [8]. The active mass portion for energy storage is low, leading to a low energy density with a large mass and volume.

On the other hand, prismatic cells made from wood-derived monoliths can achieve a higher active mass portion. Monolithic wood biochar (MWB) is produced by heating wood at high temperatures with no oxygen, which results in a high carbon content. MWBs belong to nanoporous carbons (NPCs), which have morphological features that resemble the anatomical elements in a tree. It comprises carbon nanotube arrays and integrated graphene sheets for electrical conductivity and charge storage [9]. Despite these promising characteristics, research in this area remains limited.

Besides electrode materials, the electrolytes employed also play a crucial role in the performance of supercapacitors. Organic electrolytes are often used in commercial supercapacitors due to their large voltage window, offering high energy densities, but they are not environment friendly, with high flammability and often toxicity. Aqueous electrolytes often have the advantages of being less expensive, less hazardous, and more environmentally friendly, thus considerably simplifying the fabrication process of supercapacitors [10].

However, aqueous electrolytes suffer from water’s narrow electrochemical stability window, thermodynamically between 0 and 1.23 V vs. the standard hydrogen electrode at 101.32 kPa and 25 °C [11]. At low ½ cell potentials, water can be reduced to H₂ gas in the hydrogen evolution reaction (HER); at high ½ cell potentials, water can be oxidized to O₂ gas in the oxygen evolution reaction (OER). The ongoing release of gas from water during battery cycling or idling reduces the Coulombic efficiency (CE) and raises significant safety concerns regarding the risk of explosion [11]. To avoid gas evolution, the operating voltage must be kept low, limiting the supercapacitor’s energy density.

Water electrolysis is typically facilitated by hydronium and hydroxyl ions. One unique feature of neutral pH electrolytes is the lack of free H⁺ and OH⁻ in the solution, resulting in a high overpotential on both HER and OER [10]. Many neutral electrolytes have reported wider electrochemical stability windows [10]. Nonetheless, neutral pH electrolytes generally have low ionic conductivity and capacitance. The question is, can a neutral electrolyte increase supercapacitor energy density?

This study builds upon the previous studies exploring the maple-derived MWB–potassium hydroxide supercapacitor. In this study, 0.5/1 M sodium sulfate (Na₂SO₄) is used as the electrolyte instead of 4 M potassium hydroxide (KOH). The energy density and maximum power are calculated and compared based on the following equations:

E = 1/2 CV²

(1)

Pmax = V²/4R

(2)

where E is the energy density (J/g), C is the specific capacitance (F/g), V is the operating voltage window (V), Pmax is the maximum power (W), and R is the resistance (Ω).

To attain high energy and maximum power density, the electrolyte selection should prioritize high ionic conductivity (minimizing R), a wide electrochemical stability window (maximizing V), and the ability to form an excellent capacitive interface with the electrode (maximizing C) [10].

Na₂SO₄, as a neutral pH electrolyte, is cost-effective and non-corrosive and possesses a broad potential window. However, when dissolved, the hydrated ions of Na₂SO₄ are considerably larger than those of KOH, as indicated in Table 1.

In maple-derived MWB electrodes characterized by micropores with a predominant pore width between 0.35 nm and 0.55 nm, accessibility to the hydrated ions in aqueous Na₂SO₄ may be limited. Consequently, the specific capacitance with Na₂SO₄ may be lower than that with KOH. Moreover, the molar conductivities of Na⁺ and (SO₄)²⁻ are significantly lower than that of K⁺ and OH⁻, leading to higher electrolyte resistance.

The main objective of the project is to explore whether maple-derived MWB, using a neutral pH electrolyte Na₂SO₄, can demonstrate energy density or maximum power density comparable to or higher than those employing a hydroxyl-conducting electrolyte such as KOH. It is also known that the concentration of electrolyte is a controlling parameter for a given active material where the optimal performance is typically achieved at a particular concentration [10]. Given that a concentration of 0.5 M or 1 M Na₂SO₄ was frequently selected in many previous studies on different biochar materials, this investigation also adopts these two concentrations for analysis.

2. Materials and Methods

2.1. Selection of Wood

In this study, the local Canadian wood species, Acer saccharum, was chosen. Commonly known as sugar maple, Acer saccharum is a hardwood with an approximate density of 0.71 g/cm³, possessing both tracheids and vessels. These anatomical features can be preserved during the pyrolysis process. The retained tracheids and vessels can act as conduits for electrolyte transport. In previous studies, sugar maple exhibited impressive performance when paired with a 4 M KOH electrolyte, showcasing a specific capacitance of up to 150 F/g.

2.2. Pyrolysis of Wood

In brief, the maple wood block was positioned within a stainless steel mesh enclosure, suspended at the center of a vertical quartz tube furnace. To establish an inert environment, Grade 5.0 nitrogen gas was continuously passed through at a consistent flow rate of 400 mL/min.

Pyrolysis was conducted using the batch procedure, comprising six sequential steps with holding temperatures set at 90, 200, 400, 600, 800, and 1000 °C. The associated ramp rates for each stage were 8.3, 2.5, 1.3, 3.3, 6.7, and 6.7 °C/min (Table 2). Following the 1000 °C step, the sample underwent natural cooling, reaching room temperature over approximately 15 h.

2.3. Electrode Fabrication

Following the retrieval of MWB from the cylindrical furnace, MWB pieces were cut based on the desired electrode thickness, and the cutting process was performed perpendicular to the axial direction. Electrode dimensions were measured using a Vernier caliper. The cut MWB piece underwent sanding with coarse-to-fine sandpaper up to 1500 grit to achieve the desired final thickness. Any residual powder was eliminated through air blowing.

When two pieces of MWB were being made for the symmetrical two-electrode supercapacitor cell assembly, the difference in mass was kept within 0.0002 g and the difference in length, width, and thickness were all kept within 0.05 mm. The processed electrodes’ dimensions were around 10 mm × 10 mm × 2 mm, and the mass was 0.1275 g, yielding a density of approximately 0.65 g/cm³. The electrodes were then sonicated in ethanol for five minutes to remove residual carbon particles and subsequently air-dried. The electrodes were then boiled in 4 M KOH for 2 h as a pretreatment, ensuring hydrophilicity and improved electrolyte penetration into the micropores. The KOH-treated electrodes were used to measure the baseline performance in 4 M KOH.

2.4. Supercapacitor Cell Assembly with Different Electrolytes

The supercapacitor cell assembly involved sandwiching two MWB electrodes between acrylic compression plates, nickel mesh current collectors (nickel gauze, 40 mesh woven from 0.13 mm wire—Wire Cloth), and a polysulfone separator. This entire configuration was encapsulated within a 100 mL glass beaker, and different electrolytes were introduced into the jar until the cell was completely submerged. The entire cell was then covered with a polytetrafluoroethylene lid, as illustrated in Figure 1.

Following cell assembly, 50 mL of 4 M KOH (Sigma-Aldrich, St. Louis, MO, USA) solution was introduced into the beaker until the cell was completely submerged. This Maple_4MKOH cell was used as a baseline for comparing energy density and power density.

After all performance testing for Maple_4MKOH (listed in Section 2.5) was completed, the 4 M KOH electrolyte was removed, and the cell was washed with DI water until the pH became neutral. Then, 50 mL of 0.5 M Na₂SO₄ (Sigma-Aldrich) solution was introduced into the beaker until the cell was completely submerged. This condition was named Maple_0.5MNa₂SO₄.

After all performance testing for Maple_0.5MNa₂SO₄ (listed in Section 2.5) was completed, the 0.5 M Na₂SO₄ electrolyte was removed, and 50 mL of 1 M Na₂SO₄ solution was introduced into the beaker to make Maple_1MNa₂SO₄.

In the literature, 4 M KOH is the most widely used concentration due to its established outstanding performance as an aqueous electrolyte in supercapacitors. In contrast, much lower concentrations of 0.5 M and 1 M Na₂SO₄ were selected for this study due to the significantly lower solubility of Na₂SO₄.

2.5. Supercapacitor Cell Performance Characterization

Two current collectors were connected to a potentiostat in a two-symmetrical-electrode setup. The performance was assessed through various techniques, including cyclic voltammetry (CV), galvanostatic cycling (GC), electrochemical impedance spectroscopy (EIS), and constant voltage charging followed by constant current discharge. All these techniques were conducted using the Solartron Sl 1280B Electrochemical Measurement Unit.

Prior to any performance testing, the Maple_4MKOH cell underwent pre-conditioning through GC cycling with 4 M KOH at 200 mA/g for 100 cycles across the voltage range of 0–0.8 V. Similarly, the Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ cells underwent pre-conditioning through GC cycling with 0.5 M or 1 M Na₂SO₄ at 200 mA/g for 100 cycles across the voltage range of 0–0.8 V.

For structural studies (SEM and TEM) on the MWB, please refer to the two included papers in Refs. [13,14].

2.5.1. Cyclic Voltammetry

The Maple_4MKOH cell underwent cycling from 0 V to 0.8 V to confirm the known operating window. For the Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ cells, scanning began from −0.8 V to + 0.8 V, incrementing by 0.2 V on both sides until an HER/OER peak appeared at 5 mV/s.

CV scans were also conducted at various scan rates, ranging from 1 mV/s to 50 mV/s, to complement the electrochemical performance test for Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄.

2.5.2. Constant Voltage Charging Followed by a Constant Current Discharge

Constant voltage charging was used to double-check the safe operating voltage window for the Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ cells. The two cells were maintained at a constant voltage close to the voltage corresponding to the appearance of the OER peak in CV for 10 min to observe the potential formation of bubbles from water electrolysis.

In addition, Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ underwent prolonged constant voltage charging at 1.2 V, followed by a constant current discharge at two distinct current densities. This technique determined the capacitance limitation and Coulombic efficiency (CE).

2.5.3. Galvanostatic Cycling

Galvanostatic cycling (GC) involves charging and discharging a cell at a constant current normalized to the electrode’s mass, resulting in a current density expressed as mA/g. In this study, voltage vs. time data were systematically collected to calculate specific capacitances across various charge and discharge rates, along with cell resistance, energy, and power density. The capacitance of the cell was also normalized to mass, presenting a specific capacitance in F/g. This report will comprehensively present capacitance data, including specific capacitance and current density.

To capture a wide range of behaviors, a current density range of 5–100 mA/g was selected, as beyond this range, a substantial IR drop was observed for Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄. GC data include voltage responses over time under set current for two or more cycles at six different current densities (5 mA/g, 10 mA/g, 20 mA/g, 50 mA/g, and 100 mA/g). Data points were collected at regular intervals of one second, ensuring a detailed and thorough examination of the electrochemical performance.

In the case of Maple_4MKOH, GC experiments were carried out within the voltage range of 0 V to +0.8 V. For Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄, the GC experiments were extended to +1.4 V based on the voltage window suggested by CV and constant voltage charging. To ensure reproducibility of the results, each experiment was meticulously repeated five times, and the specific capacitance and resistance were calculated based on the best trial results in each case.

2.5.4. Electrochemical Impedance Spectroscopy

The resistance obtained from GC data is the equivalent series resistance (ESR); it provides limited information on the specific elements contributing to this resistance. To overcome this limitation, electrochemical impedance spectroscopy (EIS) was employed to measure resistance from different components. A Nyquist plot was generated by plotting real and imaginary Z values on the x and y axes, respectively, as a function of frequency between the range of 0.05 and 20,000 Hz.

Nyquist plots offer both qualitative and quantitative insights into resistive components in a supercapacitor cell. In this study, the Randles circuit model (depicted in Figure 2) was adopted for modeling an electrode immersed in an electrolyte. This model is simply the series combination of the bulk electrolyte resistance or ionic resistance (R_s) with the double layer capacitance (C_dl). The charge transfer resistance (R_ct) associated with the faradaic reaction is in parallel with C_dl. The rate of faradaic reaction is assumed to be controlled by diffusion of the reactants to the electrode surface, and therefore, the diffusional resistance element (Z_w) is in series with R_ct [15].

3. Results

3.1. Voltage Stability Window

The operating voltage window for Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ was determined based on the CV data, as illustrated in Figure 3. The appearance of peaks beyond ±1.4 V indicated the occurrence of HER or OER in the electrochemical processes. To avoid unwanted side reactions and ensure stable performance, a potential window of 1.4 V was chosen.

This suggested potential window was further confirmed through potentiostatic tests. Operating Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ at 1.4 V for 10 min resulted in no observable bubble formation, indicating a relatively stable electrochemical condition. Additionally, at a slightly higher potential of 1.6 V, there was minimal bubble formation—the bubbles being very tiny, almost negligible. This experiment underscores the practical feasibility and safety of operating Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ within the specified potential window of 1.4 V.

It is noteworthy to compare this voltage window with that of Maple_4MKOH. In the case of Maple_4MKOH, the operating voltage window was restricted to 0.8 V. This implies that, despite the different electrolyte compositions, the specified potential window for Na₂SO₄ (1.4 V) is notably higher than that of KOH (0.8 V). This difference has significant implications for the energy density of the cell, which will be discussed in Section 3.5.

3.2. Capacitance

3.2.1. Pseudo-Capacitance from CV

In the realm of capacitive energy storage, Faradaic reactions improve the capacitance by combining surface-based double layer capacitance (electrostatic energy storage) with volume-based pseudo-capacitance from the products of Faradaic reactions (chemical energy storage) [17].

The CVs in Figure 4 for Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ have a pair of peaks: one for the reduction during the cathodic sweep and another for the oxidation during the anodic sweep. This suggests the presence of quasi-reversible Faradaic reactions within the operating voltage window of ±1.4 V.

As illustrated in Figure 5, free-diffusing species and adsorbed species have different peak shapes. Based on this information, it can be inferred that Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ with Na₂SO₄ as the electrolyte exhibit peak shapes characteristic of free-diffusing species.

The peak potential in CV enables the rapid determination of the formal potential of the redox species, albeit sometimes only approximatively. Assuming that the kinetic inhibition is absent, and the reaction is controlled solely by diffusion and potential, the redox reactions occur as soon as the potential for oxidation or reduction is reached. If the electrolyte’s reduced and oxidized species have the same diffusion coefficient, the formal potential should be precisely in the middle of the two peak potentials of reduction and oxidation [18].

According to the CV data obtained from Maple_1MNa₂SO₄ at a scan rate of 1 mV/s, the calculated formal potential of the redox species is −0.0139 V. The most probable redox reaction associated with this potential is S₂O₆²⁻ + 2e⁻ → 2S₂O₃²⁻, featuring a formal potential of 0.080 V. Another potential redox reaction may be SO₄²⁻ + 4H⁺ + 2e⁻ → H₂SO₃²⁻ + H₂O, given the standard electrode potential is 0.172 V [19].

Another parameter that delivers information about the system is the peak current. In CV, the observed currents result from capacitive and Faraday currents. Capacitive current exhibits a linear growth with the scan rate, while Faraday current increases proportionally to the square root of the scan rate. Consequently, the capacitive current escalates faster than the Faraday peak current with an increasing scan rate [18]. Beyond a specific scan rate, the capacitive current dominates in the CV, overshadowing the distinctive shape and peaks caused by the Faraday current, making them less identifiable, as shown in Figure 6.

3.2.2. Measured Specific Capacitance from GC

Capacitance was obtained from the GC data as shown in Figure 7. The measured specific capacitances of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ are plotted versus current density in Figure 8, and the numerical data are shown in Table 3.

The specific capacitance values display notable variations among the three cells, primarily influenced by the electrolyte type rather than its concentration. Specifically, Maple_4MKOH consistently exhibits significantly higher specific capacitance than Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ across all tested current densities.

The observed discrepancy in specific capacitance can be attributed to the differences in the sizes of hydrated ions and ion diffusion coefficients between the two electrolytes. In particular, the solvated ions in KOH, namely K⁺ and OH⁻, possess smaller hydrated diameters (0.662 nm and 0.600 nm, respectively) compared to the solvated ions in Na₂SO₄ (Na⁺ with a diameter of 0.716 nm and (SO₄)²⁻ with a diameter of 0.758 nm).

Ion diffusion coefficients affect how quickly ions move within the electrolyte and access the electrode surface during electrochemical processes. The higher ion diffusion coefficients in KOH (OH⁻: 5.27 × 10⁻⁹ m²/s; K⁺: 1.96 × 10⁻⁹ m²/s) compared to Na₂SO₄ (Na⁺: 1.33 × 10⁻⁹ m²/s; SO₄²⁻: 1.07 × 10⁻⁹ m²/s) indicate a faster movement of ions in the former [20].

Biochar samples are rich in mesopores (2–50 nm), particularly between 4 and 5 nm, regardless of wood species [21]. As the diameters of the hydrated Na⁺ and SO₄²⁻ ions are less than 1 nm, they will be able to access mesopores and form the electrical double layer, although their migration speed will be lower than that of K⁺ and OH⁻. Consequently, the measured capacitance with Na₂SO₄ will be smaller than that of KOH. As shown in Figure 8 and Table 3, the difference in measured capacitance between Na₂SO₄ and KOH became smaller when the current density was lower, which was expected as ion migration speeds were influenced by ion size and pore size. Smaller ions will be able to access smaller pores, resulting in higher capacitance and energy density. However, deeper access to smaller pores hinders ion migration and the capacitance measured with high current densities.

The measured specific capacitance decreases with increasing current density. At lower current densities, it took a longer time for the supercapacitor cell to be charged to the set voltage, allowing ions to penetrate deeper pores and form a greater electrical double layer where charges are stored. At 5 mA/g, the measured specific capacitance for 4 M KOH is 83.1 F/g; at 100 mA/g, the value becomes 76.1 F/g. For 0.5 M Na₂SO₄ and 1 M Na₂SO₄, the measured specific capacitance decreases from 43.5 F/g and 46 F/g to 9.6 F/g. Compared with 4 M KOH, where 90% capacitance was retained at 100 mA/g, the magnitude of the decrease in capacitance in Na₂SO₄ is much larger, with only 25% capacitance retained. The substantial capacitance difference at higher current densities is attributed to the challenge faced by ions with larger sizes in penetrating deeply into the electrodes. Faster charging/discharging at elevated currents becomes a limiting factor for larger ions, hindering their effective utilization and resulting in a notable drop in specific capacitance compared to smaller ion counterparts.

3.3. Resistance

3.3.1. Resistance from GC

Cell resistance was calculated from the IR drops (∆V_t), observed as sudden voltage decreases at the peaks marking the onset of the discharge process in the GC data, as shown in Figure 9.

R = ∆V_t/∆I = ∆V_t/2I

(3)

where R is the resistance (Ω), ∆V_t is the instantaneous voltage drop (V), and I is the charge/discharge current (A).

The resistance calculated encompasses various elements across the cell, including resistance in the current collector wires, contact resistance at the current collector and biochar electrode interface, resistance in the electrode, and resistance in the electrolyte solution. The accuracy of this calculation is constrained by the data collection frequency during GC. While ∆V_t ideally represents the instantaneous voltage drop, the data collection frequency was one point per second.

The examination of resistance values derived from GC data, detailed in Table 4, offers valuable insights into the electrochemical behavior of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ across diverse current densities.

Maple_4MKOH consistently demonstrates lower resistance values (2 Ω) across the entire range of tested current densities compared to Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ (10–12 Ω). This difference in resistance can be attributed to the variances in the type of electrolytes utilized.

Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ demonstrate relatively similar resistance values across various current densities, showcasing uniformity in their performance concerning resistance characteristics. This consistent behavior is notable, especially considering the difference in electrolyte concentrations between the two cells.

Additionally, as resistance increases, the efficient flow of electric current within the cell will be impeded, affecting the charge and discharge processes. This can contribute to a more pronounced reduction in the effective capacitance.

3.3.2. Resistance from EIS

As mentioned in the Materials and Methods section, the resistance obtained from GC data is the equivalent series resistance (ESR), with limited information on the specific elements contributing to this resistance. Although the calculation remained consistent during data collection, it is essential to recognize that this value is intended as a quick reference for assessing cell connections rather than providing high accuracy and precision. To address the limitations, EIS was employed to measure the resistance of the supercapacitor cell. This method provides a more in-depth examination of the cell’s resistance, offering valuable insights beyond the brief information obtained from GC. Figure 10 displays Nyquist plots obtained from various cells.

The bulk electrolyte resistance (R_s), charge transfer resistance (R_ct), the diffusional resistance element (Z_w), and equivalent series resistance (ESR) were obtained from Python data fitting using the Randles circuit model [22], summarized in Table 5. As depicted in Figure 11, the fitting results demonstrated adequacy at high frequencies for both the bulk electrolyte resistance (R_s) and the charge transfer resistance (R_ct). However, the fitting in the diffusionally controlled region was less satisfactory. Incorporating low-frequency data would likely lead to improved fitting accuracy, albeit at the cost of significantly extending the duration of the analysis.

The bulk electrolyte resistance, which primarily reflects ion movement in the electrolyte solution, exhibits a notable trend in alignment with the expected behavior based on the molar conductivity of the electrolytes. Maple_4MKOH, utilizing 4 M KOH, demonstrates the lowest bulk electrolyte resistance, consistent with the higher molar conductivity of K⁺ and OH⁻ ions than Na⁺ and SO₄²⁻. This result is further influenced by the higher concentration of KOH compared to the concentration of Na₂SO₄. This concentration-dependent behavior is also evident as Maple_0.5MNa₂SO₄ shows approximately twice the bulk electrolyte resistance compared to Maple_1MNa₂SO₄. Conductivity is known to be proportional to electrolyte concentration, and the decrease in concentration results in reduced ion conductance.

The charge transfer resistance is influenced by electrode–electrolyte interphases, suggesting the relevance of the electrolyte type in this context [23]. Maple_4MKOH with KOH has the lowest charge transfer resistance among the three cells. Again, for Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄, as the concentration of Na₂SO₄ increases, the charge transfer resistance decreases, correlating with the observed decreased semi-circle diameter in the Nyquist plots in Figure 11.

The diffusional resistance is the most dominant in all three cases. Again, Maple_4MKOH has the lowest diffusional resistance. The similarity in diffusional resistance between Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ implies a potential dependence on the electrolyte type, particularly the size of ions involved, as opposed to the concentration. This observation suggests that diffusional resistance is influenced by the size of hydrated ions, with larger ions facing challenges in diffusion into the electrode.

GC and EIS measure different parameters under different conditions. When GC measures overall resistance, EIS quantifies values of various resistive components in a cell. The most meaningful comparison would be between the overall resistance measured with GC and the ESR measured with EIS. As shown in Table 4 and Table 5, their differences are within the uncertainty of measurements, confirming that aqueous Na₂SO₄ was about five times more resistive than aqueous KOH. It is well known that even using the same method, for example GC, the measured resistance value will change with conditions such as the current density (Table 4).

3.4. Performance Stability

Throughout the electrochemical measurements, a notable challenge emerged with the use of Na₂SO₄, as it exhibited severe issues related to forming precipitates and solid deposits on various components, including current collector wires, electrodes, and the container. Additionally, the electrolyte tended to dry out rapidly within days when left open, complicating further testing efforts as the supercapacitor cell became obstructed by the evaporated salt block. The drying was due to water evaporation, as the test cell was open. However, a commercial cell would be sealed, which prevents water evaporation. We envisage a Na₂SO₄-MWB supercapacitor cell structurally similar to a sealed lead-acid battery.

Furthermore, it was noted that the supercapacitor employing Na₂SO₄ as the electrolyte exhibited a noticeable decline in performance when left idle for several days without undergoing electrochemical tests. This observation underscores the sensitivity of the Na₂SO₄-based cell to periods of inactivity and highlights the importance of regular testing or activation procedures to maintain optimal performance. Additional precycles were implemented for performance recovery, as shown in Figure 12. In contrast, with 4 M KOH, a single precycle trial performed after cell assembly proved to be sufficient.

3.5. Energy Density and Power Density

Switching from 4 M KOH to 0.5 M or 1 M Na₂SO₄ increased the potential window from 0.8 V (in KOH) to 1.4 V despite the reduction in capacitance and the increase in resistance. This expanded potential range enables a wider variety of operating voltages, offering increased flexibility in energy storage applications.

The voltage window plays a crucial role in influencing energy density, given that energy density is proportional to the square of the voltage window. As a result, a wider voltage window can potentially compensate for the sacrifice in specific capacitance. This effect is particularly evident at lower current densities, where the sacrifice in capacitance is less pronounced, and the expanded voltage window becomes a dominant factor. Table 6 shows that at 5 and 10 mA/g current densities, Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ exhibit higher energy densities compared to Maple_4MKOH. However, while the broader voltage window still contributes to the increased energy density at higher current densities, the decrease in specific capacitance becomes a more substantial factor. At 20 mA/g, the measured specific capacitance for 0.5 M or 1 M Na₂SO₄ is only around 28% of that for 4 M KOH. Therefore, energy densities for KOH are higher than Na₂SO₄ when the current densities are 20 mA/g or higher. Table 7 and Figure 13 show that Maple_0.5M Na₂SO₄ and Maple_1MNa₂SO₄ consistently demonstrate higher power densities than Maple_4MKOH across all examined current densities. This trend corresponds with the observed superior energy densities for Na₂SO₄ at 5 and 10 mA/g current densities and significantly shorter discharge times for Na₂SO₄ at all tested current densities. Table 8 offers valuable information on the maximum power densities achieved by each cell. Contrary to the calculated power densities in Table 7, Maple_4MKOH consistently exhibits the highest maximum power density at all current densities. This suggests that, despite Maple_4MKOH showing lower power densities under certain operating conditions, it can sustain higher power output under peak conditions due to its significantly lower ESR.

4. Discussion

The MWB electrode, derived from naturally abundant wood, when combined with the non-toxic Na₂SO₄ electrolyte, offers an environmentally friendly and cost-effective energy storage solution.

In general, as the salt concentration increases, electrolyte conductivity increases, which is desirable. However, if the concentration is too high, solid crystals may form, which can block pores and hinder the supercapacitor’s performance, particularly the energy density. As Na₂SO₄ is less soluble in water than KOH, lower concentrations (0.5 M and 1.0 M) were used. The data in Table 6 showed that the performance difference between 0.5 M and 1.0 M was limited, suggesting that ion migration speed in the electrode was a significant factor controlling the performance of the Na₂SO₄-MWB cell. Ion migration speed depends more on the ion size and porosity and tortuosity of MWB. The superior conductivity of MWB allows super-thick electrodes (up to 10 mm) [13], compared with ~ 0.05 mm in commercial supercapacitors. Moreover, MWB is binder-free. Consequently, the areal mass loading in MWB cells was very high. For the same reason, the packing efficiency (volume fraction of active material of a cell) of MWB cell is much higher than commercial cells. Given the electrode thickness (2 mm) and MWB density (0.65 g/cm³), the areal mass loading would be 130 mg/cm², orders of magnitude greater than commercial cells.

Opting for 0.5 M Na₂SO₄ provides comparable performance and cost savings, making it a practical and economical choice. Additionally, using 0.5 M Na₂SO₄, which is less likely to reach saturation, adds another dimension of stability to the supercapacitor’s long-term performance.

The proposed sizes of MWB supercapacitors for storing 15 kWh, representing daily energy consumption, were calculated using the energy density values of the three cells. The results are summarized in Table 9. The sample calculation refers to Appendix A.

The calculations are based on assumptions of 100% Coulombic efficiency and a volume fraction of the active mass in the capacitor of 0.5, which serve as simplified representations for analysis. Coulombic efficiency is the percentage of charges released in a charge–discharge cycle. For a detailed explanation on Coulombic efficiency, please see Appendix B. It is recognized that attaining a Coulombic efficiency of 100% is only sometimes possible in practical situations. The results from Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄, which underwent constant voltage charging followed by a constant current discharge, showed a Coulombic efficiency of approximately 70% (Appendix B). This highlights the practical variability that can occur in actual experimental conditions.

Regarding the assumption concerning the volume fraction, it is considered practical given that the MWB used in the study does not require a binder as a supportive material, in contrast to powdered carbon electrodes. These assumptions provide a baseline for analysis and interpretation, and the observed variations emphasize the importance of considering real-world complexities in the application of these findings.

As shown in Table 9, the volume required for maple-derived MWB-Na₂SO₄ supercapacitors to store 15 kWh is approximately 16 m³, significantly larger than the compact Tesla Powerwall, which has a volume of 0.13 m³ [24]. The Tesla Powerwall’s compact design is tailored for efficient space utilization, making it ideal for residential and commercial applications where space is limited. However, it is essential to note that the MWB-Na₂SO₄ cell has not been fully optimized to maximize energy density. This indicates significant potential for further enhancement and improved performance in future iterations.

In vast agricultural landscapes where space is abundant, MWB-Na₂SO₄ supercapacitors may emerge as a highly promising and practical solution for energy storage. These supercapacitors can be readily adjusted to accommodate the substantial energy requirements of large farms.

Farms usually have access to renewable energy sources such as solar or wind. MWB-Na₂SO₄ supercapacitors efficiently store the intermittent energy generated from these sources, providing a reliable reservoir for a consistent power supply, even during periods of low energy production. MWB-Na₂SO₄ supercapacitors offer an independent energy storage solution in remote or expansive farming areas, where connection to the main power grid might be challenging. This reduces dependency on external power sources, providing autonomy and ensuring uninterrupted operations.

We envisage a structure similar to a lead-acid battery for a commercial MWB-Na₂SO₄ supercapacitor. The environmental friendliness and cost-effectiveness were due to the long-lasting nature of a neutral electrolyte cell. Using aqueous neutral electrolytes also eliminates fire hazards and allows the use of more cost-effective materials for cell housing. A potential challenge in scaling up the MWB-Na₂SO₄ supercapacitor is the lack of information about the mechanical properties and machinability of MWB, which are the subjects of future investigation.

5. Conclusions

The energy density of commercial supercapacitors is limited to 10 Wh/kg [25]. Combining aqueous Na₂SO₄ electrolyte with thick MWB electrodes widens the charge window, eliminates the fire hazard, boosts the packing density of electrode material, and lowers the cost and environmental impacts while maintaining a high energy density. The objective of this study was to evaluate the potential of Na₂SO₄ in widening the charge window and increasing energy density. The energy density of a cell depends also on the specific capacitance and packing efficiency of the active electrode material. The superior conductivity of MWB enables super-thick electrodes and a high packing efficiency. The MWB electrodes used in this study were not optimized to maximize specific capacitance. We envisage a Na₂SO₄–MWB cell with an energy density substantially higher than 10 Wh/kg.

Author Contributions

Conceptualization, C.Q.J., T.F. and L.Y.; methodology, L.Y. and T.F.; software, L.Y.; validation, L.Y.; formal analysis, L.Y. and T.F.; investigation, L.Y.; resources, L.Y. and C.Q.J.; data curation, L.Y.; writing—original draft preparation, L.Y.; writing—review and editing, L.Y. and D.W.K.; visualization, L.Y.; supervision, C.Q.J. and D.W.K.; project administration, C.Q.J.; funding acquisition, C.Q.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by NSERC, grant number 453859.

Data Availability Statement

Dataset available on request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A. Energy Storage Calculations

Assumptions:

Volume fraction of electrode = 0.5
Density of electrode = 6.11 × 10⁵ g/m³
Coulombic efficiency = 1

For Maple_4MKOH:

Specific capacitance = 83 F/g

Voltage window = 0.8 V

Energy density = 0.5 × 83 F/g × (0.8 V)² = 26.56 J/g = 7.18 × 10⁻⁶ kWh/g

Cell energy = 0.5 × 0.1275 g × 7.18 × 10⁻⁶ kWh/g = 4.70 × 10⁻⁷ kWh/cell

# of cells required = 15 kWh ÷ 4.70 × 10⁻⁷ kWh/cell = 3.19 × 10⁷ cells

Total mass of electrodes = 3.19 × 10⁷ cells × 0.1275 × 2 = 8.13 × 10⁶ g

Volume of electrode = 8.13 × 10⁶ g ÷ 6.11 × 10⁵ g/m³ = 13.13 m³

Volume of supercapacitor = 13.13 m³ × 2 = 26.62 m³

Appendix B. Coulombic Efficiency Calculations

The Coulombic efficiency (

C E

) describes the ratio of the discharge capacity to its preceding charge capacity under specific operating conditions.

C E

can be calculated from the data obtained in Section 2.5.2, following the below equation:

C E = \frac{I_{d i s} \times Δ t}{\int I_{c h a r g e} d t}

(A1)

where

$C E$ : Coulombic efficiency
$I_{d i s}$ : applied discharge current (A)
$Δ t$ : total discharge time (s)
$\int I_{c h a r g e} d t$ : area under constant voltage charging curve for the charging duration (A·s).

Table A1. Coulombic efficiency calculated for Maple_0.5MNa₂SO₄.

0.5 M Na₂SO₄	Discharge Current Density (mA/g)
Constant Voltage Charging Time (h)	10	100
0.5	0.77	0.27
1	0.67	0.29
2	0.81	0.32
4	0.95	0.36

Table A2. Coulombic efficiency calculated for Maple_1MNa₂SO₄.

1 M Na₂SO₄	Discharge Current Density (mA/g)
Constant Voltage Charging Time (h)	10	100
0.5	0.73	0.29
1	0.64	0.32
2	0.81	0.36
4	0.99	0.44

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Figure 1. Schematic drawing for supercapacitor cell assembly.

Figure 2. EIS Nyquist plot and Randles circuit [16].

Figure 3. CV of Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ at a scan rate of 5 mV/s.

Figure 4. CV of Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ at a scan rate of 1 mV/s.

Figure 5. Schematic representation of the different peak shapes in a CV due to adsorbed or free-diffusing species, reproduced based on reference [18].

Figure 6. CV of Maple_0.5MNa₂SO₄ and Maple_1MNa₂SO₄ at a scan rate of 50 mV/s.

Figure 7. GC at different current densities with Maple_0.5MNa₂SO₄. The red box highlights the end-of-slope on the GC curve, used to determine the specific capacitance by analyzing the linear discharge region where the voltage decreases at a constant rate.

Figure 8. Measured specific capacitance of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ at different current densities.

Figure 9. GC at 100 mA/g with different concentrations of Na₂SO₄.

Figure 10. Nyquist Plot of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄.

Figure 11. Data fitting for R_s, R_ct, and Z_w from EIS data of different cells.

Figure 12. Three CV trials after Maple_0.5MNa₂SO₄ was left idle. Maple_0.5MNa₂SO₄ underwent constant voltage charging (0.5, 1, and 2 h) followed by constant current discharge (10 and 100 mA/g) between each trial as additional precycles.

Figure 13. Energy density and power density for Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ at different current densities.

Table 1. Solvated ion sizes for anions and cations from electrolytes used, compared with H₂SO₄ electrolyte [10,12].

Electrolyte	Ion	Hydrated Ion Diameter (nm)	Molar Conductivity (10⁻⁴ m² S mol⁻¹)
KOH	K⁺	0.662	73.48
KOH	OH⁻	0.600	198.6
Na₂SO₄	Na⁺	0.716	50.08
Na₂SO₄	SO₄²⁻	0.758	80
H₂SO₄	H⁺	0.560	350.1
H₂SO₄	SO₄²⁻	0.758	80

Table 2. Pyrolysis parameters for heating rates for the batch procedure.

Pyrolysis Stage	Heating Rate (°C/min)	Holding Temp (°C)	Dwell Time (min)	Time Required (h)
1	8.3	90	180	3.1
2	2.5	200	6	0.8
3	1.3	400	6	2.7
4	3.3	600	6	1.1
5	6.7	800	6	0.6
6	6.7	1000	6	0.6
Total				8.9 h

Table 3. Measured specific capacitance of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄.

Current Density (mA/g)	Specific Capacitance (F/g)
Current Density (mA/g)	4M KOH	0.5M Na₂SO₄	1M Na₂SO₄
5	83.1	43.5	46.0
10	82.5	32.5	32.6
20	81.8	22.8	22.5
50	78.8	14.0	13.7
100	76.1	9.6	9.6

Table 4. Measured resistance from GC data for Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄.

Current Density (mA/g)	Resistance (Ω)
Current Density (mA/g)	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
5	2.36	12.18	11.78
10	2.02	12.87	12.59
20	2.12	12.24	10.68
50	2.21	11.83	9.22
100	1.91	10.23	10.27

Table 5. EIS resistance summary from data fitting using python.

Resistance	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
R_s (Ω)	0.394	2.02	1.07
R_ct (Ω)	0.362	1.32	0.544
Z_w (Ω)	4.13	21.8	22.7
ESR (Ω) ¹	2.1	10.5	9.0

¹ ESR = real Z value when data fit line (orange line) has an infinite slope.

Table 6. Energy densities of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ across different current densities.

Current Density (mA/g)	Energy Density (Wh/kg)
Current Density (mA/g)	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
5	7.38	11.84	12.52
10	7.33	8.85	8.89
20	7.27	6.20	6.13
50	7.00	3.80	3.73
100	6.77	2.62	2.60

Table 7. Power density of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ across different current densities.

Current Density (mA/g)	Power Density (W/kg)
Current Density (mA/g)	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
5	5.05	22.08	24.57
10	10.29	42.36	44.75
20	21.34	76.51	78.66
50	59.27	169.46	169.74
100	134.34	316.17	317.46

Table 8. Maximum power density of Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄ across different current densities.

Current Density (mA/g)	Maximum Power Density (W/kg)
Current Density (mA/g)	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
5	531.74	315.53	326.24
10	621.24	298.61	305.25
20	591.93	313.98	359.84
50	567.83	324.86	416.83
100	657.02	375.67	374.21

Table 9. Summary table for Maple_4MKOH, Maple_0.5MNa₂SO₄, and Maple_1MNa₂SO₄.

Characteristics	4 M KOH	0.5 M Na₂SO₄	1 M Na₂SO₄
Specific Capacitance (F/g)	83.1	43.5	46
Voltage Window (V)	0.8	1.4	1.4
Energy Density (Wh/kg)	7.39	11.84	12.52
Max Power Density (W/kg)	531.74	315.53	326.24
Volume of Electrode (m³)	13.31	8.29	7.84
Volume of Supercapacitor (m³)	26.62	16.58	15.68

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Ye, L.; Feng, T.; Kirk, D.W.; Jia, C.Q. Can Aqueous Na₂SO₄-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor? Energies 2024, 17, 3710. https://doi.org/10.3390/en17153710

AMA Style

Ye L, Feng T, Kirk DW, Jia CQ. Can Aqueous Na₂SO₄-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor? Energies. 2024; 17(15):3710. https://doi.org/10.3390/en17153710

Chicago/Turabian Style

Ye, Long, Tianjie Feng, Donald W. Kirk, and Charles Q. Jia. 2024. "Can Aqueous Na₂SO₄-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor?" Energies 17, no. 15: 3710. https://doi.org/10.3390/en17153710

APA Style

Ye, L., Feng, T., Kirk, D. W., & Jia, C. Q. (2024). Can Aqueous Na₂SO₄-Based Neutral Electrolyte Increase Energy Density of Monolithic Wood Biochar Electrode Supercapacitor? Energies, 17(15), 3710. https://doi.org/10.3390/en17153710

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