Activated carbon from agave wastes (agave tequilana) for supercapacitors via potentiostatic oating test

Isi Keyla Rangel-Heredia Universidad Autonoma de Nuevo Leon Facultad de Ciencias Quimicas Luis Carlos Torres-González (  luis.torresgn@uanl.edu.mx ) Universidad Autonoma de Nuevo Leon Facultad de Ciencias Quimicas https://orcid.org/0000-00019212-4331 Eduardo Maximiano Sánchez-Cervantes Universidad Autónoma de Nuevo León: Universidad Autonoma de Nuevo Leon Lorena Leticia Garza-Tovar Universidad Autónoma de Nuevo León: Universidad Autonoma de Nuevo Leon


Introduction
In the last decade, much attention has been focused on the applications of different novel carbonaceous materials (agriculture waste, biomass, etc.) as electrode materials because of their low cost, high conductivity and unusual features. These materials are stable in diverse electrolytic solutions and are able to perform stable in a wide temperature range. [1] A supercapacitor (SC) is an electrochemical energy storage device able to store charges in the electrode/electrolyte interface, SC demonstrate high power density and extended cycling life for energy applications, such as hybrid and electric vehicles, brake energy recovery systems, as well as to enhance the energy e ciency of instruments that require a peak power source, such as elevators, cranes, and locomotives [2][3]. The exploration for advances in SC technology is crucial to satisfy needs for the development of the future energy storage systems to help the use of sustainable energy sources, such as solar and wind electricity in agricultural areas, as well as an improved application of electric vehicles. In order, to perform highly and constant electrical energy storage capacity and high speci c power in SC [4][5][6][7], the electrodes fabricated from carbon materials [8] should have a microporous structure with elevated surface, high conductivity, propitious pore size distribution, and long cyclability [8][9].
Lately, the most stimulated research to increase the energy and power density of SC has been centred on novel carbon materials. However, advanced carbon materials, such as carbon nanotubes, graphene, [10] and resultant carbon from metal carbide have proved high capacitance and high-power density in electrochemical [11][12][13] and hybrid SC; those advanced carbonaceous materials would be considerably reduced by their high cost.
Electrode materials for SC have been substantially developed because of the increasing demand for a new type of electrical energy storage with a high speci c power, good electrical conductivity and a long durability [14].
The most important advantage of these storage devices is the ability of a high dynamic charge propagation, which can be useful for the hybrid power sources, electrical vehicles, digital telecommunication systems, UPS (uninterruptible power supply) for computers, and pulse laser techniques. Another bene t of the electrochemical capacitor systems is the possibility of full discharge and that a short-circuit between the two electrodes is also not harmful [15]. The typical electrochemical accumulators cannot ful ll such demands due to the physicochemical processes and electrode polarization that go along with the chemical to electrical energy conversion.
Carbons derived from biomass (biomass carbon or biochar) have been applied as electrode materials in electrochemical energy systems. The major sources of biomass are residues coming from forest crops, agricultural crops, industry, domestic sources and marine wastes [16][17][18].
In the work reported by Xiao Li et al., they prepared a series of nanoporous carbons from sun ower seed shells by two different strategies and used them as electrode materials for electrochemical double layer capacitors (EDLC). The pore structure of the carbons depends on the activation temperature and dosage of KOH. The capacitive performances of these carbons were much better than ordered mesoporous carbons and commercial wood-based active carbon, which has been used due to its superior speci c surface area electrode material for EDLC [19][20][21]. In this work, we prepared an activated carbon (AC) from agave wastes and tested as electrode for SC. This material in conjunction with the activating agent and the electrolyte seems to be very attractive precursors (cheap, abundant, etc.) to be used in SC, representing the novelty of the present work. The textural, structure and electrochemical performance of AC were investigated in detail.

Preparation of activated carbon
The agave wastes for the study comes from the Agave Tequilana plants used in a tequila facility in Arandas, in central Mexico. This agave is the only variety legally allowed for the tequila production. The bagasse bers were cut, weighted, and oven dried at 95 °C for 24 h. The rst pyrolysis process was made under nitrogen at a temperature of 500 °C until charring (~ 6 to 12 h). The transformation into AC was achieved by a chemical activation process with KOH in a 1:4 (mass ratio), with a second pyrolysis step at 800 °C for 1 h under nitrogen ow. After activation, the carbon was rst washed with 0.01 M HCl to remove ashes (inorganic materials) followed by washing with distilled water until reaching a neutral pH 7. After that, carbon was dried at 105 °C for 2 h and stored in desiccators for further characterization.

Characterization
A Bruker D2 Phaser diffractometer was used for characterizing the carbonize bers by XRD with Cu Kα radiation (l = 1.5418 Å). The results of the Raman spectra were analyzed from AC samples employing a Raman Microscope, Thermo Scienti c, with a 780 nm excitation laser. The speci c surface area was obtained by the Brunauer-Emmett-Teller (BET) technique from the liquid nitrogen adsorption isotherm in a relative pressure (P P 0 -1 ) range of 0.01-1.0 with Quantachrome Company equipment, the pore volume and the pore size distribution were calculated by the Quenched Solid State Functional Theory (QSDFT) for Pore Size Analysis.

Electrochemical measurements
VMP300 Bio-Logic instrument was used to test the working potential window and electrochemical properties, all measurements were performed in air at room temperature. Electrodes were made by mixing carbon prepared in this work with carbon black acetylene, from Alfa Aesar and a 60 wt% dispersion of Polytetra uoroethylene, from Aldrich, to make a paste with a weight ratio of 85:10:5 respectively. Paste was then dried at 65 °C in a vacuum oven. Analysis was tested in a two-electrode symmetric coin cell arrangement where each electrode had ~10 mg of mass and ~150 μm of thickness. Finally, cellulose lter paper was used to separate the electrodes and immersed in 1 M Li 2 SO 4 solution as a neutral electrolyte.
The electrochemical behavior of the device was characterized before and after the oating test sequence by cyclic voltammetry (CV) and by electrochemical impedance spectroscopy (EIS), these measurements were made from 100 kHz to 0.01 Hz as the frequency range with 0 V of bias voltage and a potential amplitude of 10 mV.
For the accelerated ageing oating tests, experiments were carried out as follows: each sequence consisted of 10 h at maximum potentiostatic voltages (1.5, 1.6 and 1.8 V) and after every potentiostatic step, ve galvanostatic charge discharge (GCD) cycles were applied under a 1 A g -1 of constant current. All periods of charge discharge cycles were reiterated twenty times with a total voltage holding time of 200 h. Speci c capacitances were calculated for every 10 h stage at the fth discharge curve, respectively ( Fig. 1).

Structure and textural characterization
The XRD diffractogram (Fig. 2) shows a strong peak at 23° in 2θ and two weak peaks at 10° and 44° which reveals a typical pattern for an amorphous and disordered structure. To obtain more insight from the disorder structure a Raman spectroscopy analysis was carried out. Fig. 3 shows the Raman spectra and as can be seen it shows two intense bands, at 1340 cm -1 (D band) and at 1560 cm -1 (G band) the presence of the characteristic G peak arises from the stretching of the C-C bond in graphitic materials and is usual to all sp 2 carbon materials [22]. The depicted D peak is due to lattice vibrations of the sp 2 -sp 3 bonds of the amorphous carbon present in interstitial sites and the distorted graphitic lattice.
It can be seen that the D peak shows a higher intensity compared to the G peak, with an intensity ratio of R = (I D I G -1 ) = 1.24, that con rms the presence of a high degree of a disordered carbon material, as observe from the characteristic behavior of these peaks [23].
The surface texture of the AC was investigated by N 2 gas adsorption -desorption analysis, speci c surface areas were determined via the BET method (S BET ). The N 2 adsorption and desorption isotherms for the AC are shown in Fig. 4. The analysis reveals a type I (b) isotherm according to IUPAC [24] for a microporous material. The isotherm is concave to the P P 0 -1 axis and the amount of adsorbed gas approaches without any limiting adsorption at high relative pressure (P P 0 -1 ). It is suggested that this type of isotherm could be found in microporous solids having a pore size distribution with a mixture of wide micropores and narrow mesopores instead of type I(a) isotherms that are found in microporous materials having mainly narrow micropores (< 1 nm) [24]. The textural properties of the AC are summarized in Table   1. The apparent BET reveals a high speci c surface area value of 1460 m 2 g −1 and the total pore volume estimated was 0.69 cm 3 g -1 , the mean pore width was found about 1.8 nm as shown in Fig. 5. For the QSDFT estimation of textural analysis a slit model pore was applied. It is worth to notice that the presence of micropores and mesopores are important for charge storage since micropores are responsible for ion traps during the energy storage and the mesopores act as channels for ion transport from the electrolyte to the interface between the electrode and electrolyte [25].

Electrochemical characterization
One way to raise the energy and power density in EDLCs is by increasing the cell voltage above 1.24 V of currently available EDLCs using AC. To analyze performance of the electrodes a symmetric SC device was prepared to carry-out electrochemical tests at three different operating voltages; 1.5, 1.6, and 1.8 V in 1M Li 2 SO 4 solution as electrolyte and selected from previous CV experiments. Instead of using the long cycling charge-discharge performance test we carry out an alternative way known as potentiostatic oating test (or constant voltage hold test) more e cient for determination of the EDLCs stability limits [18][19][20][21][22][23]. This plot is sketched in gure 1 with 5 GCD cycles and 10 h of oating where for voltage tests and current are plotted. In the oating test, the capacitor is constantly exposed to voltage values above the assumed stability limit. Fig. 6a shows at 1.5 V and 200 h, the behavior of a cell, indicating 25% decay in capacitance at 170 h, with a capacitance of 297 F g -1 calculated at the fth cycle of every period, the capacitance loss of electrodes, it is possibly ascribed to higher potential in the positive electrode. On the other hand, the cell tested at 1.6 V and 200 h, it is possible to observe a 25% decay at 150 h with a capacitance of 285 F g -1 . Finally, for analysis at 1.8 V we can observe the shortest decay capacitance period of 25% at 90 h, indicating a capacitance of 264 F g -1 with a very high voltage and low stability, considering the extensively acceptable criteria for measuring end of life for SC (from 20% to 30% capacitance decay) [23]. The device at 1.5 V showed the greater stability and voltage hold for more hours, capacitance decay of 30% represents the point where SC reach their maximum active lifetime, this test was stopped at 110 h. For comparisons purposes in oating tests and according to other studies, by Bello et al., [7] their results showed capacitance loss of 25% with a constant voltage of 1.2 V for up to 120 h of voltage holding, in other cases the cell voltage increased above 2.5-2.8 V using electrolytes such as ionic liquids, with a decrease of the capacitance observed after 200 h of test at 3.75 V. The capacitance loss of 30% was observed after 340 h [14][15][16]. increased 100% at 140 h and the cell tested at 1.8 V, showed that resistance increased rapidly at 75 h, as resistance increased a moderate degradation in the electrode occurred possibly associated to higher potential in the positive electrode [24][25] This good electrochemical behavior could be ascribed to the micropore/mesopore distribution within the carbon material which provides the relevant pore sites accessible to electrolyte ions along with short diffusion paths supporting fast ions transport during the lengthy charge-discharge process [26][27][28][29]. Floating experiments were used to demonstrate stability in capacitors, as the time elapses, the electrode suffers degradation, and the advantage of an electric double layer capacitor is to increases the lifetime which allows it to be very stable above the nominal cell voltage and can be discharged to zero without harm as discussed previously in Fig. 1

. [30]
Electrochemical impedance spectroscopy is a powerful technique that allows measuring both the equivalent resistance and the capacitance at different frequencies and helps to realize the performance of the electrode at open circuit potential or at any alternal current signal. Impedance measurements studies where performed with a voltage amplitude of 10 mV in a frequency range from100 kHz to 0.01 Hz. The Nyquist plot presented in Fig. 6c (red) exhibits a semicircle in the high to mid-frequency area which means the charge transfer resistance, it is also shown a vertical spike (line) in the low frequency area, which indicates an ideal capacitive behavior of carbon electrode material. Equivalent series resistance (ESR) originated from the total resistance of the electrolyte, the resistance of the active materials and the resistance at the interface between the active electrode material and current collector. The Nyquist plot after oating test it is also presented in Fig. 6c (black), showing a little deviation to the initial curve corroborating a minuscule deterioration of the AC electrode material. Before and after oating test in the Nyquist plots was observed comparable internal resistance values (ESR = 0.5 Ω and 1.1 Ω respectively) shown in the gure, indicating a favorable contact between AC material and separators, current collectors and electrolyte.
[31] Fig. 6d presents the CV plots (20 mV) of the cell before and after voltage holding, showing an almost rectangular shape up to 1.5 V indicative of loss in capacitance as mentioned in Fig. 6a previous. The results observed in the CV plots evidently show that oating test does not have a notable degradation effect in the AC electrodes of the symmetric device.
Furthermore, it is acceptable to conclude the performance of the electrode material investigated presents successful and acceptable values, highlighting the promise of this material for electrochemical high power applications.

Conclusions
Carbon from agave wastes was activated with KOH showing a speci c surface area of 1462 m 2 g -1 and depicted a type I (b) isotherm in the textural analysis, characteristic of microporous carbons. From the analyses of Raman spectroscopy and XRD allowed us to con rm that the AC included a small degree of graphitization and that its structure mostly consists of disordered carbon. Also, the performance of the symmetric carbon SC was carried out in a button cell at different maximum voltages of 1.5, 1.6 and 1.8 V followed by ve galvanostatic charge/discharge cycles for a total voltage holding of 200 h. Further the cell capacitance was measured from the slope of every fth discharge and the ESR from the discharge ohmic drop. The SC device that exhibits the highest capacitance and performance was observed with the cell at a oating voltage of 1.5 V, with a large initial speci c capacitance of 297 F g -1 while in EIS was observed a capacitive performance before the oating test and after the analysis diffusive and resistance in material, it is possible describe this material with an ideal capacitive response for an energy storage device.

Declarations
Funding: Consejo Nacional de Ciencia y Tecnología (CONACYT) and Programa de Apoyo a la Investigación Cientí ca y Tecnológica, Universidad Autónoma de Nuevo León (PAICYT-UANL) Con icts of interest: The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.   Figure 6 Electrochemical test of the full symmetric device: (a) capacitance decay in oating test at 1.8, 1.6 and 1.5 V (b) resistance increment in oating test at 1.8, 1.6 and 1.5 V (c) Nyquist plot of the two electrodes device before and after oating test at 1.5 V and (d) Cyclic voltammetry before and after oating test at 1.5 V