Low-Temperature Supercapacitors Enabled by Date-Seed-Derived Activated Carbon and NaClO4-Based Aqueous Ternary Electrolyte

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Data may be preliminary. 23 June 2025 V1 Latest version Share on Low-Temperature Supercapacitors Enabled by Date-Seed-Derived Activated Carbon and NaClO4-Based Aqueous Ternary Electrolyte Authors : Nazym Makanova , Ayaulym Belgibayeva , Gulnur Kalimuldina 0000-0001-9185-3217 , Vladimir Pavlenko , Aliya Mukanova , Zhumabay Bakenov , and Arailym Nurpeissova 0000-0002-9657-2964 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.175070195.51562548/v1 Published Results in Engineering Version of record Peer review timeline 189 views 92 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Supercapacitors are crucial at both room and low temperatures (LTs) due to the demand for high-power energy and rapid charge-discharge capabilities. Herein, a new ternary electrolyte system (TES) consisting of NaClO4, water, and a non-toxic co-solvent, propylene glycol, was designed. Additionally, date-seed-derived activated carbon (AC) was utilized for supercapacitor assembly to demonstrate that bio-derived AC can perform as effectively as commercial alternatives at LTs. High-surface-area (2423 m2 g-1), free-standing AC electrodes were incorporated into a symmetric supercapacitor employing the NaClO4-based TES. The interconnected micro-mesoporous structure of the AC enhances charge storage and transport, ensuring reliable operation across a range of temperatures; while the 6 m TES remains liquid down to -80 °C and maintains favorable physicochemical properties, with a viscosity of 104.9 mPa·s and a conductivity of 7.18 mS cm-1 at -20 °C. The combined system exhibits excellent electrochemical performance, retaining nearly 100% Coulombic efficiency and capacitance over 10,000 cycles at 1 A g-1 at -40 °C. At this temperature, stable charge storage is sustained with minimal IR drop, and the electrochemical stability window (ESW) extends up to 2.0 V. This widened ESW allowed us to increase the energy density of supercapacitors up to 73 W kg-1, which is essential for their performance. The combination of a bio-waste-derived electrode and a non-toxic, water-based electrolyte presents a sustainable and scalable approach to energy storage. Article category: Full Paper Subcategory: Supercapacitor Title: Low-Temperature Supercapacitors Enabled by Date-Seed-Derived Activated Carbon and NaClO 4 -Based Aqueous Ternary Electrolyte Nazym Makanova, Ayaulym Belgibayeva, Gulnur Kalimuldina, Vladimir Pavlenko, Aliya Mukanova, Zhumabay Bakenov * , Arailym Nurpeissova * Nazym Makanova, Ayaulym Belgibayeva,Vladimir Pavlenko, Aliya Mukanova, Zhumabay Bakenov, Arailym Nurpeissova National Laboratory Astana, Kabanbay Batyr Ave. 53, Astana 010000, Kazakhstan E-mail: [email protected] Nazym Makanova, Zhumabay Bakenov Department of Chemical and Materials Engineering, School of Engineering and Digital Sciences, Nazarbayev University, Kabanbay Batyr Ave. 53, Astana 010000, Kazakhstan E-mail: [email protected] Ayaulym Belgibayeva, Aliya Mukanova, Zhumabay Bakenov, Arailym Nurpeissova Institute of Batteries, Kabanbay Batyr Ave. 53, Astana 010000, Kazakhstan Gulnur Kalimuldina Department of Mechanical and Aerospace Engineering, School of Engineering and Digital Sciences, Nazarbayev University, Kabanbay Batyr Ave. 53, Astana 010000, Kazakhstan Keywords: supercapacitors, aqueous electrolytes, ternary electrolytes, bio-waste derived, low-temperature applications Supercapacitors are crucial at both room and low temperatures (LTs) due to the demand for high-power energy and rapid charge-discharge capabilities. Herein, a new ternary electrolyte system (TES) consisting of NaClO 4 , water, and a non-toxic co-solvent, propylene glycol, was designed. Additionally, date-seed-derived activated carbon (AC) was utilized for supercapacitor assembly to demonstrate that bio-derived AC can perform as effectively as commercial alternatives at LTs. High-surface-area (2423 m 2 g -1 ), free-standing AC electrodes were incorporated into a symmetric supercapacitor employing the NaClO 4 -based TES. The interconnected micro-mesoporous structure of the AC enhances charge storage and transport, ensuring reliable operation across a range of temperatures; while the 6 m TES remains liquid down to -80 °C and maintains favorable physicochemical properties, with a viscosity of 104.9 mPa·s and a conductivity of 7.18 mS cm -1 at -20 °C. The combined system exhibits excellent electrochemical performance, retaining nearly 100% Coulombic efficiency and capacitance over 10,000 cycles at 1 A g -1 at -40 °C. At this temperature, stable charge storage is sustained with minimal IR drop, and the electrochemical stability window (ESW) extends up to 2.0 V. This widened ESW allowed us to increase the energy density of supercapacitors up to 73 W kg -1 , which is essential for their performance. The combination of a bio-waste-derived electrode and a non-toxic, water-based electrolyte presents a sustainable and scalable approach to energy storage. 1. Introduction Supercapacitors store energy through the reversible electrostatic accumulation of ions at the electrode/electrolyte interface, enabling rapid charge and discharge with high power density. [1–3] They excel over batteries with diffusion-controlled mechanisms at LTs due to their surface-controlled energy storage mechanism [1,2] ; however, their energy density is limited, and performance declines below -30 °C because of increased electrolyte resistance and reduced ion mobility [4] . To mitigate these limitations, improvements in electrode materials and electrolyte compositions are essential for enhancing performance under such conditions. The electrolyte composition is a critical factor that requires thorough research, as its ESW directly enhances the energy density of the electrical double-layer capacitors (EDLCs) by enabling higher operating voltages. [5] Low-viscosity electrolytes improve ion mobility and ensure optimal performance at LTs, while high ionic conductivity supports efficient charge transport, especially under these conditions. Among the electrolyte components, salt is the most important, as it provides the ions that carry charge. [6,7] Therefore, sodium perchlorate (NaClO₄) has been used as a promising salt for LT applications due to its high oxygen evolution reaction onset potential (TFSI⁻ > ClO₄ ⁻ > NO₃ ⁻ > SO₄ ²⁻ ) [6–8], high solubililty (209 g per 100 mL of water), high dissociation, meaning high ionic conductivity, moderate toxicity, and compatibility with various co-solvents. [8,9] In addition to its favorable electrochemical and physical properties, NaClO₄ is tenfold cheaper compared to other commonly-used salts, such as LiTFSI. [6] Its ability to support broad ESWs and maintain ionic conductivity at LTs makes it a compelling choice for advanced electrolyte systems. Water-in-salt electrolytes (WISE) have been proposed as a promising solution in preventing water decomposition due to their minimal number of free water molecules. [8, 10–13] This reduction in free water broadens the ESW, enabling higher operating voltages and thus increasing the energy density of supercapacitors. [1, 14] At LTs issues such as high viscosity, low wettability, and salt precipitation at low temperatures limit their practicality. [11, 12] To tackle these problems, the integration of co-solvents like methanol [15] , ethanol [16] , ethylene glycol (EG) [17, 18] , acetonitrile (ACN) [11, 19] , dimethyl sulfoxide (DMSO) [20, 21] has been explored, but safety and toxicity concerns associated with these substances prompt the search for safer alternatives. In this regard, propylene glycol (PG), specifically 1,2-PG, on the contrary, is non-toxic and is used in the food industry. [22] Moreover, its low freezing point (-60 o C), high solubility in water, and relatively high flash point (104 o C) make 1,2-PG a good candidate as a co-solvent for safer and greener electrolytes. [23, 24] Also, its ability to form hydrogen bonds with water molecules and ’bind’ them in a solvation shell decreases the number of free water molecules [25] , which again plays a favorable role in the overall electrolyte composition. The next promising approach to designing supercapacitors for LT operation involves optimizing electrode material properties, such as particle size [26, 27] , porosity [28] , and pore size distribution [28–30] , which directly affect the diffusivity of guest ions. Using bio-waste-derived ACs is gaining significant attention [31–34] due to their high specific surface area (SSA). Among them, date-seed-derived AC offers certain advantages. It is abundant, renewable, and exhibits a high SSA, excellent electrical conductivity, and robust chemical stability. [35,36] These properties make it a cost-effective and environmentally friendly alternative to conventional commercial ACs, positioning it as a strong candidate for advanced supercapacitor applications. While various commercial ACs such as YP-50 F and YP-80 F have demonstrated promising LT performance in supercapacitor applications [6,11–13, 37] , date-seed-derived ACs, to the best of our knowledge, have not yet been explored. Thus, in this study, a TES was developed, consisting of 1,2-PG, NaClO₄, and water. Date-seed-derived AC was utilized as the electrode material to evaluate the performance of this TES. By leveraging the complementary properties of sustainable electrode materials and an optimized electrolyte, the system was specifically designed to broaden the ESW, enhance energy density, and overcome the challenges of LT performance in supercapacitors. 2. Results and Discussion 2.1. Physical properties of the activated carbon from date seeds Purification and carbonization of date seeds were performed as reported elsewhere with the only difference in the introduction of LT pre-carbonization at 300 °C and addition of activating agent KOH during the high-temperature carbonization at 800 °C. [38] The changes in the morphology of the pre-carbonized seeds after the activation and carbonization were observed by SEM analysis and presented in Figure 1 . As shown in Figure 1a and b, pre-carbonized seeds exhibit a unique morphology with irregular particle shapes and sizes, characterized by natural cavities. These cavities, inherent to the date seed structure, serve as advantageous features by acting as nucleation sites for pore development. As a result of further activation and carbonization, the morphology of the obtained AC in Figure 1c and d changes from dense to more sponge-like structure with an abundant network of interconnected macropores, enhancing the surface area and enabling efficient charge storage in supercapacitor applications. [28, 32, 39] This structure ensures efficient electrolyte penetration and ion diffusion, which is especially important when operating at LT. [28] Figure 1. SEM images of date-seed-derived pre-carbonized bio-char (a, b, c) and AC (d, e, f). The amorphous nature of the obtained AC was studied using the XRD and the result is presented in Figure 2a . The shape of the XRD pattern is characteristic for typical amorphous carbon structure. [32,38,40,41] The sample exhibits two broad peaks at nearly 23° and 43°, corresponding to the (002) plane of graphitic carbon and the (100) plane of disordered carbon structure, respectively. The d 002 value of 0.387 nm, significantly higher than that of crystalline graphite (0.335 nm), indicates a low level of graphitization in the sample, with increased structural defects and reduced crystallinity, typical for hard carbon from biomaterials [44, 45]. Similarly, the d 100 value of 0.210 nm, which provides information about the in-plane carbon-carbon bond distance within the graphene sheets, indicates the disordered and amorphous nature of the AC structure. The obtained results are in good agreement with the results reported for the AC in other studies. [32, 44] The degree of graphitization and structural defects in the prepared AC were further characterized by Raman analysis. As shown in Figure 2b , two characteristic peaks are observed: the D band, associated with structural defects and disorder, appears at 1331 cm⁻¹, and the G band, which corresponds to sp² hybridized carbon domains, appears at 1559 cm⁻¹. The D band is broad, indicating significant structural defects, while the G band, though more intense, is also somewhat broad, suggesting the presence of graphitic domains but with an overall amorphous structure. The I D /I G ratio of 0.854 reconfirms a low degree of graphitization and the presence of defects in the sample. [38, 43] The nitrogen adsorption/desorption isotherms of the AC and their pore size distribution determined by application of the 2D-NLDFT theory are presented in Figure 2c . In accordance with the IUPAC recommendations [45, 46] , the obtained isotherm is classified as IV-type with H4 hysteresis loop, which suggests the presence of a micro-mesoporous structure with a slit-shaped pores. The SSA, calculated using the BET equation, for the AC is 2423 m² g -1 , and the pore volume, determined by the QSDFT method, is 1.19 cm³ g -1 . The pore size distribution in Figure 2d also confirms these observations. As it can be seen from the graph, the carbon material contains a high amount of micropores (< 2 nm). [29,47] Additionally, there are some mesopores, indicated by a broad peak centered at 3.75 nm, with an average pore width ranging from 3 to 4 nm. The micro- and mesoporosity of the material is important since micropores are responsible for charge storage through ion adsorption, while mesopores facilitate the transportation of the electrolyte to the electrode/electrolyte interface. [47] not-yet-known not-yet-known not-yet-known unknown Figure 2. a) XRD pattern, b) Raman spectrum, c) N2 adsorption-desorption isotherms, and d) pore size distribution of the date-seed-derived AC. 3.2. Physicochemical properties of the developed NaClO 4 -based TES Electrolyte performance in energy storage systems is strongly influenced by the interactions between its components. [5,9] The interplay between salt ions, solvents, and co-solvents governs key physicochemical properties such as ion transport, thermal stability, and phase behavior. In aqueous and mixed-solvent electrolytes, hydrogen bonding and ion-solvent coordination dictate freezing point depression, viscosity, and conductivity. [9] Understanding these interactions is critical for optimizing electrolyte formulations for LT applications. FTIR analysis was conducted to investigate solvent-salt interactions at varying concentrations. The spectra in Figure 3a exhibited a shift in the 3600–3000 cm -1 region, corresponding to O-H stretching, indicating interactions between 1,2-PG, water, and the ClO 4 - anion through hydrogen bonding. Previous studies [7,48,49] attributed this shift to water molecules weakly hydrogen-bonded to ClO 4 - anions due to the ”structure-breaking” effect of ClO 4 , which disrupts strong hydrogen bonds in water. A reduction in peak intensity suggested significant reorganization of the hydrogen-bonding network with increasing salt concentration (6 m and 8 m). The shift to lower wavenumbers was attributed to solvent-separated ion pairs (M∙∙∙(H 2 O) n ∙∙∙ClO 4 - ) and stronger localized hydrogen bonds. The peak at 1650 cm -1 , corresponding to water bending vibrations, remained unchanged, indicating an equal amount of water in all solutions. [48] Peaks in the 1200–1000 cm -1 region, assigned to C–C and C–O stretching, increased in intensity with salt concentration, reflecting stronger ionic interactions between the 1,2-PG solvent and NaClO 4 . The 620 cm -1 peak, characteristic of the ionic form of NaClO 4 , was associated with its lattice vibrations. [7, 48] The increasing intensity of this peak with higher salt concentrations indicated a greater number of dissociated salt ions in solution. Another crucial factor affecting both the ESW and LT performance of electrolytes is water activity. A high water activity leads to a narrow stability window due to water decomposition at lower voltages, while a controlled reduction in water activity helps extend this window by suppressing side reactions and enhancing electrochemical stability. [8,60] Additionally, lower water activity minimizes ice crystallization, enabling improved charge transport and ionic conductivity under subzero conditions. By carefully tuning the solvent composition and salt concentration, it is possible to optimize water activity and achieve a balance between stability and performance. The presence of PG played a crucial role in modifying the water activity of the electrolyte. By forming hydrogen bonds with water molecules, PG disrupted the natural water network, thereby reducing water activity. The measured water activity values, shown in Figure 3b , exhibited a systematic decrease with increasing salt concentration. A significant drop was observed when increasing the concentration from 2 m to 4 m, indicating a major shift in water structuring and ion solvation. Beyond 4 m, water activity continued to decrease, suggesting progressive binding of water molecules to salt ions and further restructuring of the hydrogen-bonding network. This trend is consistent with findings in previous studies, where the competition between ion solvation and solvent structuring leads to a gradual limitation of free water molecules at higher concentrations [60]. The continued reduction in water activity with increasing salt concentration suggests that ionic interactions dominate over bulk solvent effects, reinforcing the role of solvation dynamics in defining electrolyte behavior. not-yet-known not-yet-known not-yet-known unknown Figure 3. ( a) FTIR spectra, (b) water activity, (c) DCS thermograms, (d) rheological properties of developed TES at different temperatures. A DSC test was conducted to determine the crystallization temperature of the electrolytes, and the results are presented in Figure 3c . No exothermic peaks were observed from RT down to -80 °C, indicating the absence of crystallization within this range. This suggests that the crystallization temperature of the samples is below -80 °C. The stable liquid state at LTs can be attributed to the consistent water-to-1,2-PG ratio, which limits phase separation and freezing. Given the low freezing point of 1,2-PG (-60 °C) and high concentrations of NaClO4, these findings confirm that the electrolyte remains in a liquid state throughout the studied temperature range. Low viscosity and high conductivity are crucial for liquid-based energy storage systems, as they enable efficient ion transport and enhance electrochemical performance.[6,11–13] The viscosity and ionic conductivity of the NaClO4-based TES were measured between -50 °C and +30 °C and are presented in Figure 3d . As expected, viscosity increased with decreasing temperature. The 6 m TES exhibited moderate viscosity at subzero temperatures while maintaining the highest ionic conductivity, reaching 1.1 mS cm⁻¹ at -40 °C. This can be attributed to the balance between ion concentration and solvent structure, where a sufficient number of charge carriers are available despite increasing viscosity. The 4 m solution, with lower viscosity, maintained relatively high conductivity. In contrast, the viscosity of the 8 m solution increased exponentially at lower temperatures due to extensive ion pairing and clustering, severely restricting ion mobility and leading to a sharp decline in conductivity. These trends indicate that optimizing the salt concentration is essential to maintaining both fluidity and efficient charge transport in LT conditions. Density measurements in Figure S1 revealed that electrolyte density increases with both salt concentration and decreasing temperature. This trend reflects the stronger ion-solvent interactions at higher concentrations, leading to a more compact liquid structure. At LTs, reduced molecular mobility enhances these interactions, further increasing density.[4] This information is useful in understanding electrolyte mass transport properties, as higher density solutions may exhibit reduced diffusivity but improved electrode wetting. Additionally, the observed density variations provide insights into the overall structural organization of the electrolyte, supporting the trends observed in viscosity and ionic conductivity. Overall, these findings demonstrate that the NaClO4-based TES is a promising electrolyte for energy storage applications, offering a broad liquid-phase temperature range, stable ionic transport properties, and a tunable balance between viscosity and conductivity. The ability to suppress crystallization and maintain conductivity under extreme conditions makes this system suitable for LT applications. The next section explores its impact on electrochemical performance of date-seeds derived AC across varying concentrations. math_shortcuts 3.3. Effect of electrolyte concentration on the electrochemical performance of supercapacitors The electrochemical performance of supercapacitors is significantly influenced by the electrolyte properties, particularly ion transport and charge storage efficiency. [4, 17] To assess the impact of electrolyte concentration, GCD measurements were conducted, comparing NaClO 4 -based TES with a conventional 6 M KOH electrolyte, as presented in Figure 4 . The results indicate that the 2 m TES exhibited lower charge-discharge times compared to KOH, while the 4 m TES provided nearly identical performance. Increasing the salt concentration to 6 m and 8 m led to slightly higher charge-discharge times, with both concentrations performing almost similarly. These results suggest that at sufficiently high salt concentrations, the NaClO 4 -based electrolyte can match or even surpass the performance of KOH, offering a viable alternative for supercapacitor applications. Figure 4 GCD profiles of cells with different electrolytes. The power and energy densities of EDLC supercapacitors depend on the nominal voltage at which they can operate (Equation 3 and 4). [6] Determining and increasing this nominal voltage is highly important because a higher nominal voltage allows for greater energy storage. To determine this value, the window opening method was employed with a low scanning rate of 5 mV s -1 and an incremental step of 100 mV. For accurate determination of the nominal voltage, symmetric supercapacitors with AC on both sides and equal masses of working electrodes were used to closely replicate industrial EDLCs. The CVs of cells with 6 m TES, obtained at RT and presented in Figure 5 a, maintained a nearly rectangular shape up to 1.7 V. At higher voltages, the onset of the water decomposition process was observed, as indicated by a deviation in the CV curve. To verify the reliability of the obtained results, symmetric cells were galvanostatically charged and discharged at a current density of 0.2 A g -1 (calculated based on the average mass of the electrodes). Consistent with observations from the CV curves, the typical triangular shape of the GCD profile was maintained up to 1.7 V at RT in Figure 5b . At higher voltages, the lines became nonlinear, indicating uneven energy distribution during charge/discharge, i.e. water decomposition. The stability window of the electrolytes was evaluated based on Coulombic efficiency and presented in Figure 6c . A threshold of 97% Coulombic efficiency was used to determine the practical stability limits based on previous reports. [6,8] The 2 m TES was found to be stable only up to 1.4 V, indicating a lower onset potential for side reactions. In contrast, the 4 m, 6 m, and 8 m TES solutions demonstrated a wider stability window of up to 1.7 V, aligning with the previously discussed reduction in water activity at higher salt concentrations. A further increase in the operating voltage to 2.1 V was possible for the 8 m electrolyte; however, this was accompanied by a significant drop in Coulombic efficiency. This decline can be attributed to the fact that at voltages exceeding 1.7 V, side reactions such as oxygen evolution become more pronounced, contributing to irreversible capacity loss and reduced cycling efficiency. Figure 5 Voltage window opening (5 mV s -1 ) on symmetric supercapacitors in the 6 m TES at RT a) CV curves, b) GCD (0.2 A g -1 ) profiles. c) Plots of energy efficiency vs. maximum voltage of 2, 4, 6 and 8 m TESs. The widening of the stability window at higher concentrations can be explained by the suppression of water reactivity due to strong solvation interactions with Na + and ClO 4 - ions. [5] As previously discussed, water activity measurements showed a systematic reduction with increasing salt concentration, indicating that free water molecules become increasingly coordinated with salt ions, reducing their availability for electrochemical decomposition. However, at extremely high concentrations, the increased viscosity and slower ion transport limit the practical benefits, as observed in the 8 m TES. These findings highlight the importance of optimizing electrolyte concentration for supercapacitor performance. While higher concentrations enhance charge storage due to increased ion availability and broaden the ESW by reducing free water activity, excessive salt content leads to viscosity-induced transport limitations and increased susceptibility to parasitic reactions at elevated voltages. The 6 m TES demonstrated the most effective balance between conductivity, charge transport, stability window, and capacitance retention. The next section explores the LT performance of these optimized supercapacitors. 3.4 Low-temperature performance of developed supercapacitors Supercapacitor applications in extreme environments require electrolytes that retain ionic conductivity and electrochemical stability at subzero temperatures. To evaluate the LT performance of NaClO₄-based TES, the temperature and rate capability of cells with 4 m, 6 m, and 8 m electrolytes were tested at 1.7 V down to -50 °C. This voltage was selected based on the previously determined stability window, ensuring a balance between energy storage and charge efficiency. GCD measurements in Figure 6a showed that all three electrolytes exhibited similar capacitance at RT, reaching about 160 F g -1 at 0.2 A g -1 . However, at lower temperatures and higher current densities, differences became evident. The 6 m TES maintained higher capacitance compared to 4 m and 8 m, particularly at high current densities, indicating more efficient ion transport. The 8 m TES experienced a sharp decline in performance, failing to operate at 1 A g -1 at -50 °C, whereas both 4 m and 6 m TES still retained some capacitance, demonstrating better charge transport properties under extreme cold conditions. Figure 6 (a) Rate capability performances of 4, 6 and 8 m TES at LTs, (b) GCD profiles of 6 m electrolyte at LTs, c) specific capacitance values at different current densities and temperatures, (d) Nyquist plots of symmetric AC/AC supercapacitors at different temperatures (Inset: enlarged plots), (e) CV curves with voltage window opening at -30 o C, (f) GCD profiles with voltage window opening on symmetric AC/AC cell with the 6m ternary electrolyte, g) plots of coulombic efficiency (%) vs. maximum voltage at -30 o C of TESs, and (h) cycling performance at -30 °C. At 0.2 A g -1 , more than 50% of the RT capacitance was retained at -50 °C for both 4 m and 6 m TES. This suggests that sufficient ion mobility and charge storage are maintained despite the temperature drop. The poorer performance of the 8 m TES can be attributed to its higher viscosity, which limits ion diffusion and hinders charge transfer. The results highlight the trade-off between increased ion availability and reduced ion mobility at higher salt concentrations, emphasizing the importance of viscosity control for LT applications. An interesting phenomenon was observed during further testing of the 6 m TES. When the cell was cycled through temperature- and rate-capability tests down to -50 °C and then returned to RT, it failed to operate ( Figure S2 ). However, at -20 °C and -30 °C, it performed consistently across different current densities, maintaining high and stable capacitance for an additional 10,000 cycles at a current density of 1 A g -1 at -40 o C (Figure S3) . This suggests that the system remains electrochemically functional within this subzero range despite being subjected to such harsh operation conditions (both high current density up to 2.0 A g -1 and LT down to -50 °C). In a separate test, shown in Figure S4 , when the cell was cycled at a constant current density of 0.2 A g -1 from RT down to -30 °C and then brought back to RT, it resumed operation with only a slight decrease in capacitance, retaining 87% of the initial capacitance. Notably, no capacitance loss was observed at subsequent testing at 0 °C and -20 o C, this can be visually observed from the CV curves, which remain almost the same when tested at -20°C for the first time, and upon returning to -20°C after testing at RT ( Figure S5) . These results indicate that the operational stability of the 6 m TES may depend on the thermal cycling conditions, with gradual cooling and heating leading to better recovery. This behavior suggests possible structural or solvation state changes in the electrolyte at extreme temperature fluctuations, which warrant further investigation. GCD curves at different temperatures in Figure 6b retained an inherently ideal triangular shape with a small IR drop down to -30 °C, indicating high electrochemical stability of the system and efficient charge transport. However, at -40 °C and -50 °C, an increase in IR drop was observed, indicating higher internal resistance. [6,15,37] The specific capacitance value for the GCD profile obtained at these temperatures was calculated by integrating the area under the curve, as recommended for non-linear GCD profiles. The resistive response of the supercapacitors was evaluated using the EIS technique. The resistivity of the assembled cell depends on several factors, including the electrode resistance, the contact resistance between the current collector and the electrode, the electrolyte resistivity, and the processes occurring within the electrolyte i.e. diffusion. [50,51] The EIS technique was employed after the continuous rate-capability tests at different temperatures. The obtained data is presented in Figure 6d , and the equivalent circuit model for the curve is shown in Figure S6 . The obtained EIS plots are typical for AC-based supercapacitors [6,11,12,37] , featuring a semi-circle at high frequencies, which is attributed to the charge transfer resistance between the current collectors and the active material. [50,51] A sloped curve corresponds to the diffusion of ions into the pores of the active material. Finally, a vertical line, almost parallel to the y-axis, characterizes the capacitive behavior of the cell. This occurs when the capacitance is no longer frequency-dependent, indicating that the entire surface of the material is covered by ions from the electrolyte .[52] In Table 1 , equivalent series resistance (ESR), charge transfer resistance (R ct ), ionic resistance (R ionic ), and equivalent distributed resistance (EDR) values are summarized for different temperatures. The ESR values are determined by the intercept with the real axis at the semi-circle’s extremity at high frequencies. The charge transfer resistance (Rct) is identified by the diameter of the semi-circle. The ionic resistance (R ionic ) is calculated by subtracting the starting and ending points of the diffusion component. The equivalent distributed resistance (EDR) is determined by linear extrapolation of the nearly vertical line at low frequencies. [52] By examining the semi-circle diameter, its volume, and along with the R ct values, it is evident that the charge transfer resistance is higher at RT than at LT. Specifically, R ct at -40 °C is almost half that at RT, with values of approximately 9 Ω and 17 Ω, respectively. However, when considering R ionic , which is determined from the diffusion component, it can be observed that R ionic is much smaller at RT (1.4 Ω) compared to LT (9 Ω at -40 o C). This indicates more efficient diffusion-related processes at RT. Conversely, the increased R ionic values at lower temperatures, along with the more pronounced slope at low frequencies, suggest significant diffusion limitations at LT, such as reduced ion mobility and slower reaction kinetics. In summary, the increased charge transfer resistance at RT is associated with greater faradaic activity and more effective ion transport compared to LT, where diffusion limitations are more pronounced. The small values of ionic resistance indicate efficient ion transport into the pores of the active material. The pronounced increase at -30 °C (24 Ω) and lower is attributed to the increased viscosity of the electrolyte, which reduces ion mobility. ESR values are highly dependent on temperature [52]. It can be observed that as the temperature decreases, ESR values increase due to the reduced ionic conductivity of the electrolyte. The electrochemical stability of the 6 m TES was evaluated at -30 °C by performing CV measurements up to different voltages, as presented in Figure 6e . The CV curves retained a rectangular shape up to 2.0 V, indicating stable capacitive behavior with minimal faradaic contributions. The lower intensity of faradaic peaks compared to room temperature suggests kinetic limitations of redox reactions at LTs, which aligns with previous observations. [6] GCD measurements were conducted under the same conditions and summarized in Figure 6f . The GCD curves retained an ideal triangular shape up to 2.0 V, with minimal deviation from linearity. Even at 2.3 V, the curves remained more triangular and the voltage profile more linear compared to those at RT, further supporting the suppression of faradaic processes at LTs. Additionally, IR drop values observed from the GCD curves were significantly lower at -30 °C than at RT, indicating reduced overall system resistance. Coulombic efficiency comparisons for 4 m, 6 m, and 8 m TES in Figure 6g showed no significant difference at -30 °C when tested at 0.2 A g -1 , with all concentrations maintaining Coulombic efficiency above 98% up to 2.0 V. This suggests that, at LTs, side reactions are equally suppressed across different concentrations, leading to similar charge-discharge efficiencies. Beyond 2.0 V, a slight decrease in efficiency was observed for all systems, but it remained above 97% even at 2.2 V, indicating minimal side reaction contributions. These results confirm that electrolyte concentration does not significantly affect Coulombic efficiency at -30 °C and that all tested formulations exhibit stable charge-discharge behavior under these conditions. The observed behavior can be attributed to several factors. At LTs, the increased electrolyte viscosity suppresses ion mobility, which slows faradaic reactions and limits non-ideal processes. This suppression enhances the EDLC response. Additionally, favorable interactions between the electrode material and electrolyte — potentially influenced by pore structure and surface area of the AC — can further improve the EDLC behavior at LTs. [17,18,29] After observing that the ESW can be extended to 2 V at -30°C, charge-discharge tests were conducted up to 2 V at lower temperatures, and the results are shown in Figure S7 . The graphs at high current densities indicate that the profile maintains an inherently triangular shape even at -40 °C. Furthermore, the supercapacitor demonstrates stable performance down to -50 °C at a current density of 0.5 A g -1. This increase in working voltage enabled us to enhance the energy density of the supercapacitor from 51 Wh kg -1 (1.7 V) to 76 Wh kg -1 (2.0 V) at -20°C (Table S1). Cycling performance at a current density of 0.2 A g -1 and a nominal voltage of 2.0 V was evaluated at -30 °C. As shown in Figure 6h , Coulombic efficiency remained close to 100%, and the specific capacitance retention was also nearly 100%, indicating excellent cycling stability (GCD profiles are shown in Figure S8 ). The absence of capacitance fading even after prolonged cycling suggests that the electrolyte maintains its structural integrity and charge storage capability under these conditions. This stable performance further highlights the reliability of the 6 m TES for long-term operation in LT environments. The ability of the 6 m TES to retain high capacitance and operate reliably at -20 °C and -30 °C, even after extensive cycling, highlights its advantage as a robust electrolyte for LT applications. The nearly 100% Coulombic efficiency and capacitance retention at -30 °C demonstrate excellent electrochemical stability and charge storage reliability. Unlike the 8 m TES, which suffers from severe viscosity-induced ion transport limitations, and the 4 m TES, which offers lower capacitance retention, the 6 m formulation provides an optimal balance between ion mobility, charge storage, and electrochemical stability. From the comparison with other electrode/electrolyte configurations ( Table 2 ), it can be seen that activated carbon derived from bio-waste can be used at LTs as effectively as (and in some cases even better than) commercial alternatives. This is due to its higher SSA (commercial AC YP-50F has an SSA of 1600 m 2 g -1 [29,53] , while bio-waste-derived AC can achieve an SSA as high as 3300 m 2 g -1 [28,47,54] ) and its tunable porous structure. Depending on the guest ion, a more microporous or mesoporous structure can be achieved. Additionally, the use of bio-waste significantly contributes to sustainability. The incorporation of non-toxic 1,2-PG, along with the stability of the electrolyte and electrode system over 10,000 cycles at 1A g -1 — following more than 800 cycles of rate capability testing ( Figure S2 and S3 ) — demonstrates the reliability of this system. Moreover, the observed stable performance at extended voltages and suppressed faradaic reactions at LTs suggest that this electrolyte system is well-suited for high-voltage applications under subzero conditions. The ability to maintain energy efficiency, stability over prolonged cycling, and minimal IR drop further supports its applicability in energy storage systems that require sustained performance in extreme environments. These findings establish the 6 m TES as a strong candidate for practical applications in supercapacitors designed for LT operations, including aerospace, electric vehicles, and grid storage solutions. math_shortcuts 3. Conclusions This study presents a high-performance supercapacitor system utilizing a NaClO₄-based TES electrolyte and a lightweight, free-standing AC electrode derived from date seeds. The unique characteristics of the AC from date seeds, with its interconnected pores and micro-mesoporous structure, contribute to stable electrochemical behavior at both LT and RTs, ensuring minimal degradation over extended cycling. The excellent physicochemical properties of the ternary electrolyte allow it to remain in a liquid state down to -80 °C, enabling the supercapacitor to operate as low as -50 °C, retaining more than 62% of the RT capacitance. The outstanding cycling stability of the developed supercapacitor was observed, maintaining nearly 100% Coulombic efficiency and capacitance retention at -30 °C, 0.2 A g -1 , and 2.0 V. Its ability to sustain high performance at extended voltages and reduced IR drop further confirms its practical viability. Unlike conventional electrolytes, which degrade under extreme cold, this formulation ensures reliable energy storage even in harsh conditions. Beyond electrochemical advantages, this work contributes to sustainable energy storage by utilizing waste-derived carbon and a nontoxic, water-based electrolyte. The combination of green chemistry principles, recyclability, and safety aligns with the demand for eco-friendly energy solutions. The developed system holds promise for applications in electric vehicles, aerospace, grid storage, and wearable electronics. not-yet-known not-yet-known not-yet-known unknown 4. Experimental part 4.1 Synthesis of AC from Date Seeds Date seeds (Bam Mazafati Dates, Iran) were thoroughly washed with deionized water (DI) using a magnetic stirrer to remove residual fruit pulp and mechanical impurities as seen in Figure 7 . Then the seeds were ground using a tube mill (IKA, Germany) at speeds varying from 5000 to 10000 rpm, with each speed maintained for 1 minute, to facilitate better washing in subsequent processes. After grinding they were soaked in an HCl (Sigma-Aldrich) solution (1:3 HCl (37%) volume ratio) for 24 hours to break down the rigid lignocellulose structure, thus preparing the material for the carbonization process, then washed with hot DI water until a neutral pH was reached. The cleaned ground seeds were pre-carbonized to remove lignocellulose from the structure and obtain biochar for the further activation process in a tubular furnace at 300 °C (5°C min -1 ) for 2 hours under a nitrogen flow (50 mL min -1 ). The pre-carbonized biochar was then mixed with KOH (Sigma-Aldrich, anhydrous, ≥99.95% trace metals basis) powder for the activation process in a ratio of 1:3 by weight. Following this, 30 mL of DI water was added, and the mixture was ultrasonicated for 30 minutes to ensure thorough homogenization. Then the mixture was stirred on a magnetic stirrer at 300 rpm for an hour and dried in an oven at 100 o C overnight to remove water. The solid biochar-KOH mixture was activated in a tubular furnace at 800 °C (10 °C min -1 ) for 2 hours under a nitrogen flow (70 mL min -1 ). The resulting AC was rinsed with dilute HCl and hot DI water until a neutral pH was reached, followed by drying in the vacuum oven overnight. Figure 7. Schematic illustration of the synthesis of activated carbon from date seeds. not-yet-known not-yet-known not-yet-known unknown 2.2. Electrolyte preparation 1,2-PG (Sigma-Aldrich, ≥ 99,5%) was used as a co-solvent and stored in an Ar-filled glovebox (MBRAUN LABmaster Pro Glovebox, Germany) with < 0.1 ppm O2 and 99%) was dried at 120 °C in a vacuum drying oven for 24 hours to remove any residual water, ensuring the accuracy of the solution concentration. To prepare the TES, consisting of 2, 4, 6, and 8 mol kg -1 (m) NaClO4 in DI water:1,2-PG = 1:1 (by mass), the appropriate amount of NaClO4 was first dissolved in DI water (0.05 µS cm-1) by stirring at < 60 °C for 30 minutes to maintain homogeneity. Once the salt was fully dissolved, 1,2-PG was added to the solution. The mixture was then stirred at 300 rpm for an additional 30 minutes to ensure uniform blending. math_shortcuts 2.3. Physical characterizations To examine the surface morphology of the AC, Scanning Electron Microscopy (SEM) was performed using a ZEISS Crossbeam 540 (Carl Zeiss, Germany) operating at 5 kV. The crystallinity of AC was analyzed using X-ray diffraction (XRD) with a SmartLab diffractometer (Rigaku Co., Japan) employing Cu Kα radiation (𝜆=0.154056) . The XRD data were collected over a 2θ range from 10° to 70° at a scan rate of 3° min -1 , using an X-ray source operated at 40 kV and 30 mA. The interlayer spacings d 002 and d 100 were calculated using the 2θ values for the (002) and (100) planes using the Bragg equation (2 d sin θ = nλ). Raman spectrum was recorded to evaluate the degree of graphitization of the AC using LabRam HR Evolution Raman Spectrometer (Horiba, France). The sample was placed on a glass plate, and the Raman spectra were obtained using a 10x vertical objective in a 180° backscattering arrangement. A 532 nm laser wavelength was used to analyze the range from 1100 to 1800 cm -1 . Brunauer-Emmett-Teller (BET) surface area analysis was conducted using nitrogen physisorption experiments at −196 °C with a Quantachrome Quadrasorb SI (Anton Paar, USA). Before the measurements, approximately 100 mg of the sample was degassed in a dynamic vacuum at 300 °C for 24 hours in a FloVac Degasser (Anton Paar, USA). The SSA was calculated using the multipoint BET method in the range 𝑝/𝑝 0 =0.1–0.3, and the pore volume was calculated at 𝑝/𝑝 0 =0.9 . The pore size distribution was evaluated using the quenched solid density functional theory (QSDFT) for slit and cylindrical pores. The interaction between water, 1,2-PG, and NaClO 4 was analyzed using FTIR spectroscopy at room temperature. A Nicolet iS10 FT-IR Spectrometer (Thermo Scientific, US) was employed for the analysis. The spectrum was scanned over the range of 400–4000 cm −1 , with a scanning resolution of 4 cm −1 . The Aqualab TDL 2 (Meter Group, US) water activity meter was used to measure the water activity in the prepared solutions. Each electrolyte was placed in a sealed container before starting the measurement and was allowed to equilibrate to 25°C. After the stabilization process within the instrument’s chamber, the analysis was carried out. The operating principle of the water activity meter is based on measuring the relative humidity at the dew point of the air in equilibrium with the sample, using a chilled mirror sensor. Then, tunable diode laser absorption spectroscopy (TDL) is employed to measure the water activity, as discussed in. [38] Water molecules in the air above the sample absorb a specific wavelength of light, and the degree of this absorption is proportional to the water activity in the sample—ranging from 0 for a completely dry solution to 1 for pure water. Viscosity and density of the electrolytes were measured with an Anton Paar (USA) SVM 3001 viscometer across a wide temperature range from -20 °C to 30 °C. The ionic conductivity of the investigated solutions was measured using the method of electrochemical impedance spectroscopy (EIS) with a sinusoidal signal of 5 mV s -1 in the frequency range of 1 Hz to 100 kHz, over a wide range of temperatures from 25 °C to -50 °C. For these measurements, a 25 mL beaker cell equipped with two 1 cm × 1 cm platinum plate electrodes was used. Firstly, the cell constant (Kcell, cm -1 ) was determined using standard solutions with known conductivity values. For this purpose, 5 KCl standard solutions with different conductivities were used, and the cell constant was calculated using the formula: \(K_{\text{cell}}=\sigma\bullet R_{\text{bulk}}\) (1) where, K cell - cell constant (cm -1 ), 𝜎 - ionic conductivity of the solution (S cm -1 ), and R bulk - the bulk resistance of the solution represented by the high-frequency intercept on the real axis (Ohm). The ionic conductivity values of the investigated electrolytes were also measured using a SevenCompact S230 (Mettler Toledo, UK) conductometer, to confirm the obtained values. All the measurements were conducted in a climate chamber (Xi’an LIB Environmental Simulation Industry, China). math_shortcuts 2.4. Electrochemical characterizations Electrochemical measurements were performed using symmetric supercapacitor cells assembled in 2-electrode polytetrafluoroethylene (PTFE) Swagelok-type configurations. The free-standing electrodes were fabricated by mixing AC derived from date seeds, PTFE (60 wt% dispersion in water, Sigma-Aldrich), and acetylene black (MTI Corp., China) as a conducting additive at a mass ratio of 90:5:5 in 10 mL of isopropanol. The mixture was mixed and compacted to form a dough-like material, which was rolled into a thick sheet and dried at 100 °C under vacuum for 12 hours. The dried sheet was then rolled using a calendering machine (Hohsen corp., Japan) until reaching a thickness of 100 µm ± 10 µm. Electrodes of 8 mm diameter were then punched out using a hand-held disc cutter. The electrodes were impregnated with the electrolyte under vacuum for 5 minutes before assembling the cell. The supercapacitor cells consisted of 10 mm diameter stainless steel (316 L) rods as current collectors, with a glass microfiber separator (Whatman GF/C, uncompressed thickness = 260 µm, diameter = 10 mm) sandwiched between two AC electrodes, each weighing approximately 2.0 mg ± 0.5 mg. The separator was soaked with 250 µL of developed electrolyte to complete the cell assembly. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) at different currents ranging from 0.2 A g -1 to 2 A g -1 and EIS measurements with a sinusoidal signal of 5 mV s -1 in the frequency range of 1 mHz to 100 kHz were performed using a VMP3 multichannel potentiostat/galvanostat (Bio-Logic Instruments, France) in the aforementioned climate chamber to maintain a constant temperature (± 0.2 °C) from 25 o C (from now on RT) to –50 o C. The specific capacitance of a single electrode (F g -1 ) is: \(C_{\text{spec}}=\frac{4I\mathrm{\Delta}t}{m\mathrm{\Delta}U}\) (2) where, I – discharging current (A), ∆t – discharge time (sec), ∆U – operation voltage (V) excluding the IR drop during the discharge process, m – total mass of electrodes. The specific energy (Wh kg -1 ) (3) then can be calculated by following formula: \(E_{\text{spec}}=\frac{C_{\text{spec}}\bullet U_{\text{MAX}}^{2}}{2\bullet 3.6}\)(3) Power (W kg -1 ) density values can be calculated by (4): \(P=\frac{{3600\bullet E}_{\text{spec}}}{\mathrm{\Delta}t}\) (4) To quantify the impact of faradaic phenomena on the cells, the coulombic efficiency was calculated using the following formula: \(CE=\frac{Q_{D}}{Q_{C}}\) (5) where, Q D represents the discharge capacity, and Q C represents the charge capacity, both corresponding to their respective portions of the CV curve. These values were determined by integrating the respective sections of the curve. 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Comparison of LT electrochemical performance of supercapacitors with different electrode/electrolyte configurations Electrolyte Electrodes Lowest Operating temperature Specific capacitance Reference 7.5 m LiClO 4 -based WiS AC-YEC-8-A − 20 o C 38 F g -1 [55] 1 M Na 2 SO 4 /H 2 O/ 50%EG Commercial activated carbon (not defined) −40 °C 102.0 F g -1 [56] LiCl eutectic YP-80F AC − 70 o C 28 F g -1 [57] Ethylene glycol and 1 m Mg(ClO 4 ) 2 YP-50F AC -40 o C 22.5 F g -1 at -30 o C [58] 20% ethylene glycol in 4.5 mol L −1 LiCl aqueous solution Macklin AC -60 o C 11.4 mF cm −2 at 10 mV s −1 [17] LiClO 4 /DMSO/EG/ACN AC (the trademark is not defined) -20 o C 36.4 F g -1 at 0.1A g -1 [20] 1 m-Propylene Carbonate/H 2 O-LiTFSI 5 % hybrid electrolyte CMK-3 powders (mesoporous AC) -20 o C 34 F g -1 at 10 A g -1 [14] 8.84 m NaClO 4 YP-80F AC -35 o C 102 F g -1 [6] 13NaClO 4 -2CO(NH 2 ) 2 /H 2 O YP-50F AC -40 o C 16.62 F g -1 at -30 ◦C [59] 5 m choline nitrate + 10 vol.% methanol YP-80F AC -40 o C 78 F g -1 [15] 2 m Ca(NO 3 ) 2 in DMSO 30%/water Macklin porous carbon -50 o C 41 F g -1 at 0.25 A g -1 [21] 6 m NaClO 4 in H 2 O and 1,2-PG (1:1 m/m) Activated carbon derived from date seeds -40 o C 105 F g -1 at 1 A g -1 This work Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgments This research was funded by the research targeted programs BR21882402 “Development of new material technologies and energy storage systems for a green economy” and BR24992766 “Development of methods and technologies for environmentally friendly “green” processing of polymer waste for energy storage” from the Science Committee, Ministry of Science and Higher Education of the Republic of Kazakhstan. Authors are grateful to Dr. Malchick Fyodor (Kazakh National University) for the water activity measurements. Schematic illustration of the synthesis process was made using BioRender. Received: (will be filled in by the editorial staff) Revised: (will be filled in by the editorial staff) Published online: (will be filled in by the editorial staff) Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018. Supporting Information Title: Low-Temperature Supercapacitors Enabled by Date-Seed-Derived Activated Carbon and NaClO 4 -Based Aqueous Ternary Electrolyte not-yet-known not-yet-known not-yet-known unknown Nazym Makanova, Ayaulym Belgibayeva, Gulnur Kalimuldina, Vladimir Pavlenko, Aliya Mukanova, Zhumabay Bakenov*, Arailym Nurpeissova* Figure S1 Density of 2, 4, 6 and 8 m TES electrolytes at different temperatures. Figure S2 Rate capability performances of 4, 6 and 8 m TES at low temperatures and turning back to RT Figure S3 Cycling performance of 6m TES after rate capability at low temperatures and turning back to RT Figure S4 Performance of 6 m TES at different temperatures with repeating cycles Figure S5 CV results of the 6 m TES after repeated cycles at different temperatures not-yet-known not-yet-known not-yet-known unknown Figure S6 Equivalent circuit model for the EIS measurements Figure S7 GCD profiles of the 6m TES with widened voltage up to 2 V Table S1 : Energy and Power density values for 6 m NaClO 4 math_shortcuts Temperature ( o C) 6 m TES Voltage (V) Current density (A g -1 ) Energy Density (Wh kg -1 ) Power density (kW kg -1 ) RT 1.7 1.0 55 1.98 1.7 2.0 49 4.30 -20 1.7 1.0 51 1.93 1.7 2.0 47 3.94 2.0 1.0 76 2.36 2.0 2.0 71 4.78 -30 1.7 1.0 46 1.97 1.7 2.0 42 4.33 2.0 1.0 70 2.47 2.0 2.0 63 5.15 -40 1.7 1.0 37 2.30 1.7 2.0 27 5.40 2.0 1.0 56 2.70 2.0 2.0 42 6.05 Figure S8 First and last cycle GCD profiles of the 6m TES at -40 o C Information & Authors Information Version history V1 Version 1 23 June 2025 Peer review timeline Published Results in Engineering Version of Record 1 Mar 2026 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords aqueous electrolytes bio-waste derived low-temperature applications supercapacitors ternary electrolytes Authors Affiliations Nazym Makanova Nazarbaev Universitet National Laboratory Astana View all articles by this author Ayaulym Belgibayeva Nazarbaev Universitet National Laboratory Astana View all articles by this author Gulnur Kalimuldina 0000-0001-9185-3217 Nazarbayev University View all articles by this author Vladimir Pavlenko Nazarbaev Universitet National Laboratory Astana View all articles by this author Aliya Mukanova Nazarbaev Universitet National Laboratory Astana View all articles by this author Zhumabay Bakenov Nazarbaev Universitet National Laboratory Astana View all articles by this author Arailym Nurpeissova 0000-0002-9657-2964 [email protected] Institute of Batteries LLC View all articles by this author Metrics & Citations Metrics Article Usage 189 views 92 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Nazym Makanova, Ayaulym Belgibayeva, Gulnur Kalimuldina, et al. Low-Temperature Supercapacitors Enabled by Date-Seed-Derived Activated Carbon and NaClO4-Based Aqueous Ternary Electrolyte. Authorea . 23 June 2025. DOI: https://doi.org/10.22541/au.175070195.51562548/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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