Integration of 3D interconnected sodium hydroxide modified biochar with the tile-like architecture of cobalt oxides for asymmetric supercapacitors

Two state-of-the-art electrodes were successfully synthesized and used to assemble both symmetric and asymmetric type supercapacitors. 3DFAB was fabricated by direct pyrolysis of green macroalgae in the presence of NaOH. Possible mechanisms of NaOH activation is proposed, which explains the formation of oxygen functional groups through quick penetration of OH- and NaOH into the vacancies. To obtain CoTLM, the tile-like architecture of cobalt oxides was introduced to the 3D interconnected functional algal biochar (3DFAB) by a simple one-pot hydrothermal method under mild conditions. For the symmetric supercapacitors, the maximum specific capacitance of RAB, 3DFAB, and CoTLM were 177 F g -1 , 322 F g -1 , and 529 F g -1 at a scan rate of 5mVs -1 . Regarding cobalt-based asymmetric systems, the maximum capacitance for the RAB//CoTLM and 3DFAB//CoTLM were ∼ 300 F g −1 and 400 F g −1 , respectively. The 3DFAB//CoTLM showed a higher energy density compared with RAB//CoTLM, while the RAB//CoTLM exhibited better cycling performance (99.1% of its initial capacitance after 4k cycles at the current density of 4 A g 1 ). this study, a cost-effective ACSs was fabricated using a new interconnected tile-like microstructure containing cobalt oxide particles, referred to as CoTLM, and functional algal biochar composed of a 3D interconnected mesopores network (3DFAB). The CoTLM composites were synthesized by impregnation of Co(NO 3 ).6H 2 O on the surface of algal biochar under hydrothermal carbonization. FAB was prepared by direct pyrolysis using green macroalgae as the carbon precursor and NaOH as the activator.


Introduction
Industrial and agricultural pollutants have significantly changed the level of nutrients, primarily nitrogen and phosphorus, in oceans, seas, and rivers 1 .These changes, directly or indirectly, cause damage to the environment on a global scale. The eutrophication, the excessive blooming of macroalgae, is a visible, alarming, and devastating phenomenon occurring as a result of such human activities. To mitigate the issue of eutrophication, many researches have been conducted to evaluate the viability of using these harmful microalgae as a source of biofuel 3,4 . However, macroalgae biofuel production has not been fully commercialized, due to the major economic and technical challenges. To minimize waste and improve the efficiency of the circular economy, biochar obtained as a byproduct of the thermochemical conversion of macroalgae could be further processed for versatile applications in energy conversion and storage sectors [5][6][7][8][9] .
The best-known example is the application of biochar as a promising alternative to its commercial competitors in supercapacitors due to their obvious advantages such as low cost, accessibility, reduced environmental impact, and good stability 10,11 . Algal biochar has a unique advantage over agricultural wastes in the way that macroalgae undergoes selfactivation and nitrogen self-doping during the thermal process due to the abundance of potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and nitrogen (N) that exist in the algae structure 12 . However, there is still much work to be done in reaching the desired electrochemical properties in supercapacitors by rationally integrating the physical and chemical modification methods. Algal biochar needs to be further be processed to achieve a highly improved EDLC and pesocapacitance performance. Most state-of-the-art modified biochar composites with excellent capacitive performance have been reviewed comprehensively by Norouzi et al 13 . The simplest and most popular chemical surface modification is the use of NaOH or/and KOH activators before or after the thermal process. For example, Hu et al. successfully synthesized porous particulate activated carbons from low sulfonate content alkaline lignin by hydrothermal carbonization in the presence of NaOH and NaOH modifiers. The synthesized material showed a hierarchical structure with improved SBET, functional groups, and EDLC performance 14 . In addition to chemical modification, it is beneficial to simultaneously enhance the pesocapacitance of the sample by introducing pseudocapacitive materials into the biochar structure. To this end, transition metal oxides or hydroxides are usually embedded in the porous structure of the surface-modified biochar [15][16][17][18] . Among the available pseudocapacitive materials, cobalt hydroxides or oxides are favorable candidates for application in electrochemical capacitors due to their low cost, great reversibility, high conductivity, multiple oxidation states, and high specific capacitance. Cobalt oxide (Co3O4) has an extremely high theoretical specific capacitance of up 3560 F g -1 , which has been recently receiving more attention within the electrochemistry research groups 19,20 .
Apart from designing a hybrid biochar electrode, selecting two dissimilar electrode materials with wellseparated potential windows, called Asymmetric supercapacitors (ASCs), plays a vital role in reaching higher energy density, power density, and cycle life 21,22 . In this study, a cost-effective ACSs was fabricated using a new interconnected tile-like microstructure containing cobalt oxide particles, referred to as CoTLM, and functional algal biochar composed of a 3D interconnected mesopores network (3DFAB). The CoTLM composites were synthesized by impregnation of Co(NO3).6H2O on the surface of algal biochar under hydrothermal carbonization. FAB was prepared by direct pyrolysis using green macroalgae as the carbon precursor and NaOH as the activator.

Results and dissuasions
FTIR analysis was carried out to indicate the rate and degree of decomposition during the synthesis based on functional groups. All three samples have similar FTIR spectra, but at different intensities ( Fig. 2a). The spectra of glycosyl-units of cellulose are detected in the range of 1150-1070 cm -1 due to the stretching vibrations of CH-, OH-, and CH2-groups 23 . The strongest peaks at these ranges are observed in the CoTLM spectrum, showing the more intense glycosidic bond-breaking reactions. The FT-IR spectrum of CoTLM exhibits a broader peak at 2000-3400 cm -1 which can be attributed to its higher hydrophilic and conductive nature. Peaks at 1457, 1592, and 1710 cm -1 are assigned to the ester and carboxylic acid functional groups in cellulose-based polysaccharides 24,25 . These peaks have been pronounced in 3DFAB, as compared to the RAB, as the result of a series of reactions which have been shown in Eq 1-6 and Fig. 1.
During the pyrolysis process, NaOH vigorously reacts with carbonyl, hydroxyl, carboxyl, ether, and ester functional groups to produce free radicals as well as a number of vacancies. At the same time, many vacancies are created due to the NaOH reaction with the C-C and C-H groups. Then, many oxygen functional groups are formed by quick penetration of OH-and NaOH into the vacancies. Finally, NaOH reacts with oxygen functional groups over in carbon fragments at 400-700 °C, which can produce K2CO3 (Eq 1-6).
According to the literature, most of the acidic functional groups attached to the surface of 3DFAB should react with cobalt ions (CO +2 and CO +3 ) during HTC to produce water and cobalt nanoparticles (eq 7 and 8).
Thus, a lower intensity of -O containing groups was excepted, but was not found. This could be due to the improved hydrolysis reactions in HTC by which polysaccharides as macro-intermediates are produced from the unreacted cellulose remaining after the pyrolysis.
These elements belong to the ash portion of the algae. The RAB contains 15% of sulfur and silica, which are considered essential micronutrients for the normal growth of algae 29 . Oxygen content in 3DFAB has more than doubled due to the elimination of AAEMs by NaOH treatment and acid reflux. However, improved oxygen contents in CoTLM are mostly related to the attachment of oxygen functional groups produced under the hydrothermal treatment. Based on surface oxygen contents, we conclude that part of the CoTLM surface were developed by oxygen functional groups which make them more polar and hydrophilic than the original biochar derived from green algae. During the HTC in the presence of Co(NO3).6H2O, cobalt particles were efficiently dispersed over the surface and caused improved oxidation-reduction reactions (pseudocapacitance) and higher electrical conductivity of the sample.   Fig. 4 displays the N2 adsorption-desorption isotherms, and BJH pore diameters of RAB, 3DFAB, CoTLM samples. All the N2 adsorption-desorption isotherms exhibited the same kinetic of the reaction with a typical IV hysteresis loop at a relative pressure between 0.45 and 0.95 which confirms the hierarchical porous structure of samples [30][31][32] . The majority of RAB pore volume is in the range of 0.10 cm 2 g -1 nm -1 . However, the pore volume in 3DFAB and CoTLM samples fluctuates between 0.10 and 0.14 cm 2 g -1 nm -1 , owing to the 3D architecture that occurred after the treatment. BET surface area of RAB, 3DFAB, and CoTLM were 243, 1020, and 605 m 2 g -1 , respectively. The characterization results reveal that both 3DFAB and CoTLM have a higher surface area, larger pore volume, and smaller pore diameter compared to the original biochar.
The higher surface area and interconnected 3D pore network in the 3DFAB could be due to reaction 1 (Eq1) in which carbon reacts vigorously with sodium hydroxide to form Na2O(s) along with hydrogen and carbon dioxide gas. Considering the superior textural properties of 3DFAB, it might be a highly promising start material for accommodating conductive materials in nanoscale. Herein, CoTLM was obtained through the dispersion of cobalt oxide particles within the interior of the interconnected 3D pore network of 3DFAB.
CoTLM can not only facilitate charge transfer and ion diffusion but also take advantage of the pseudocapacitive nature of Co3O4 nanoparticles 33 . Raman spectroscopy was performed to further analyze the carbon structure of RAB, 3DFAB, and CoTLM.
According to the Raman spectra of samples (Fig. 4b), two main peaks were recorded at around 1345 cm -1 (D band) and 1570 cm -1 (G band) which are ascribed to the turbostratic and ideal graphitic carbon structure.
ID/IG can reflect the disordered degree in the modified samples. The ID/IG ratio for RAB, 3DFAB and CoTLM was 0.91, 1.05, and 1.65, respectively. Raman spectrum of CoTLM shows some characteristic peaks at 486, 529, 608, and 678 cm -1 , which are related to the vibrational modes of Eg, F 2g , F 2g , and A1g, respectively 34 .
Electrochemical performance CV measurements. Fig. 5a gives  CCV measurements. Fig. 6 shows the cyclic performance of the RAB, 3DFAB, CoTLM under 4000 cycles at the scan rate of 50 mV s −1 . For the CoTLM electrode, 5% of the capacitance is dropped due to the volume alternation in the electrode material caused by electrolyte intercalation and deintercalation reactions during long potential cycling 38 . However, two other non-metal-modified electrodes, RAB and 3DFAB, exhibited superior cycling stability with 100.9% and 101.5% retention of their initial capacitance after 4000 cycles, respectively. This phenomenon has already been reported in the literature as the gradual activation of the surface with increasing the number of cycles. The results indicate that the electrochemical stability of the 3DFAB is higher than that of others, which makes it a suitable electrical double layer capacitor and a great candidate for accommodating pseudocapacitors. Capacitance retention as a function of cycle number, at the current density of 1 A g -1 , is plotted for RAB, 3DFAB, and CoTLM in Fig. 7d-f, respectively. As shown, three electrodes have a nearly identical capacitance drop after 4000 cycles. In all samples, the capacitance is promoted around 1%, indicating their superior cyclic performance. This slight increase in capacitance after 4000 cycles is due to the improved access of electrolyte ions to the new activated sites at higher cycles 12 . Ω) than that of RAB (5.1 Ω) and 3DFAB (3.9 Ω), confirming its remarkable electrical conductivity.
Moreover, a higher slope is recorded for CoTLM electrode, revealing lower Rs (0.71 Ω) in this electrode as compared to RAB (0.84 Ω) and 3DFAB (81 Ω). The Warburg resistance, symbol Zw, is the straight line at low-frequency regions. These lines show the variations in ion diffusion path lengths. As seen in Table 2, RAB, 3DFAB, and CoTLM have a Zw of 0.11, 0.13, 0.21 Ω, respectively. We can conclude that modification of RAB resulted in shorter ion diffusion paths and fewer barriers to ion movement 41 .

Electrochemical performances of the and 3DFAB//CoTLM ASC devices
For the ASC, 3DFAB has been considered as a positive faradic electrode due to its superior surface area and suitable potential window. On the other hand, CoTLM showed a stable and suitable potential window in the region of negative chosen electrodes. The working potential range of 3DFAB was -0.9−0 V, while that of CoTLM was -0.1−0.9 V. Thus, the cell voltage of this two-electrode combination has extended up to 1.7 V, which is significantly higher than that obtained in symmetric type supercapacitors. CV diagrams and charge/discharge curves of 3DFAB//CoTLM devices are shown in Fig. 9a and b. All the CV curves of the ASC devices remained unchanged, indicating that electrons and ions can easily move within the pore structure of the electrodes even at 100 mV s -1 . At the same time, charge-discharge curves have almost retained their symmetrical shape at different current densities, suggesting their high coulombic efficiency and good electrochemical reversibility of these two asymmetric systems.  Fig. 10a shows 3D-CCV curves of 3DFAB//CoTLM electrode measured at a scan rate of 50 mV s -1 for 4000 cycles. As shown in Fig. 10b, the asymmetric systems showed superior cyclic stability and during the cycling, the additional peaks did not appear. Up to 98.5% of the capacitance was retained for 3DFAB//CoTLM, The results were further processed into a Capacitance-Cycle number diagram to clarify the above statements. To further support the above results, the cycle performance of 3DFAB//CoTLM electrode at the current density of 4 A g -1 is shown in Fig. 11a. Notably, 3DFAB//CoTLM electrode retained 99.1% of its initial capacitance after 4000 cycles at the current density of 4 A g -1 . Although the capacity decay after long-term cycling for asymmetric cobalt-based electrodes has been observed previously, the reasons have remained somewhat uncertain. There are three main hypotheses for such phenomenon: i) slight changes in ternary oxides and/or hydroxide are caused by intensive reaction with KCl, and ii) damage in the morphology of tile-like microstructure with a 3D architecture was due to the long-term cycling test.
Energy density and power density are two main factors that should be considered in scaling up the ACS devices. Fig. 11b shows the Ragone plots of the 3DFAB//CoTLM electrode. The energy density acquired from the 3DFAB//CoTLM ASC device is 54.44 Wh kg −1 with a power density of 800 W kg −1 , which is significantly larger than those reported for cobalt and iron-based composite ASCs 28 .

Experimental section
Green macroalgae wastes (Cladophora glomerata) were collected by hand from different locations of the Speed River, Guelph, Ontario, Canada during the months of June and July 2019 (See Figure 12b). The exact sampling locations are shown in Fig. 12c. The algae were carefully washed with deionized water to remove sand, salt, and other contaminants attached to their surface. Afterwards, any surplus of water was drained, and the samples were then dried at 105 °C in a furnace overnight. The dried algae were then ground and mixed to ensure uniform consistency and composition of the batch (Fig. 12d). RAB was prepared using a macro TGA at Bio-Renewable Innovation Lab, University of Guelph, Canada (Fig. 12a)., Five grams of green macroalgae (sieved into the particle size <150 µm in diameter) were placed into a reactor consisting of a stainless-steel tube of 175 mm height and 15 mm and then the system was purged by the nitrogen gas before starting the reaction. Afterwards, the purged reactor was inserted inside a Muffle Furnace (Model F48055-60, USA) to heat the pyrolysis reactor. The experiments were performed at a heating rate of 15 ˚C/min, reaching a temperature of 700 ˚C. The k-type thermocouple was connected to a data-logger to continuously visualize and record the temperature profile on the computer. RAB was utilized as a start material to synthesize3DFAB and CoTLM.
Synthesis of 3DFAB. Four grams of green macroalgae were mixed with 6 M NaOH and refluxed for 5 hours at 100 ˚C. The resulting suspension was centrifuged and dried before being placed in the pyrolysis reactor.
Afterwards, 3DFAB derived from green macroalgae was synthesized by pyrolysis at 700 ˚C for 2 h and subsequent reflux with H2SO4 and HNO3 (1:3 by volume) at 80 ˚C for 6 h.
Synthesis of CoTLM. A suspension composed of 1 g 3DFAB, 100 mL of distilled water, Ammonium Hydroxide as pH adjusters (to set pH at ≈11), and 0.25 g of Co(NO3).6H2O was transferred into a Teflonlined stainless-steel autoclave and heated at 150 °C for 15 h. The resulting products were centrifuged at 5000 rpm. CoTLM was obtained after being washed with distilled water and ethanol several times. working electrode, an Ag/AgCl reference electrode, and a graphite rod counter electrode. We implemented the same instruction for determining CV, CCV, and GCD, but in a two-electrode system. All electrochemical data were collected using an Autolab 302N, at 25 °C in 3 M KCl electrolyte.

Conclusions
In summary, we have fabricated a cost-effective ASC using RAB and 3DFAB as negative electrodes and CoTLM as a positive electrode. CoTLM were synthesized by integrating pyrolysis and hydrothermal carbonization methods. XRD, FTIR, and FESEM-EDS analyses confirmed the unique hierarchical architecture and superior surface area of CoTLM, resulting high specific capacitance, and excellent cycling stability. The working potential range of 3DFAB was -0.9−0 V, while that of CoTLM was -0.1−0.9 V. Thus, the cell voltage of this two-electrode combination has extended up to 1.7 V, which is significantly higher than that obtained in symmetric type supercapacitors. The energy density acquired from the 3DFAB//CoTLM ASC device is 54.44 Wh kg −1 with a power density of 800 W kg −1 , which is significantly larger than that of the RAB//CoTLM (23.22 Wh kg -1 at a power density of 850 W kg -1 ). These values are significantly higher than those reported for cobalt and iron-based composite ASCs.
Author Contributions: ON and SP equally contributed in designing and performing the experiments, performing the experimental work, and interpreting the results. HN helped in measuring the electrochemical properties. FD and AD have helped in scientific discussion to revise manuscript. ON wrote the manuscript.
Animesh Dutta supervised the group during the entire process of project.