Self-Doped Interwoven Carbon Network Derived from Ulva fasciata

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Self-doped interwoven carbon network derived from Ulva fasciata for all solid supercapacitor devices: Solvent-less approach to a scalable synthetic route Jai Prakash Chaudhary, Rajeev Gupta, Ashesh Mahto, Nilesh Vadodariya, Kalpana Dharmalingm, Nataraj Sanna Kotrappanavar, and Ramavatar Meena ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02831 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 15, 2018

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ACS Sustainable Chemistry & Engineering

Self-doped interwoven carbon network derived from Ulva fasciata for all solid supercapacitor devices: Solvent-less approach to a scalable synthetic route Jai Prakash Chaudhary,a,b,c Rajeev Gupta,a Ashesh Mahto,a,d Nilesh Vadodariya,a,b Kalpana Dharmalingm,e Nataraj Sanna Kotrappanavar,d*Ramavatar Meenaa,b* aAcademy

of Scientific and Innovative Research (AcSIR)-Central Salt & Marine Chemicals Research Institute, G. B Marg, Bhavnagar, 364002, India bNatural Products & Green Chemistry Discipline, CSIR- Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar, 364002, India cDepartment of Chemical Engineering, Indian Institute of Technology, Gandhinagar, Gujarat, 382355, India dCentre for Nano and Material Sciences, Jain University, Jain Global Campus, Bengaluru, 562112, Karnataka, India eCSIR-Central Electrochemical Research Institute -Madras Unit, Taramani, Chennai 600 113, Tamilnadu, India *Corresponding Authors: [email protected]; [email protected] Mailing address: Dr. J. P. Chaudhary, Lab No. 209, Block-5, Chemical Engineering Department, IIT Gandhinagar, Gujart-382355, India; Dr. Rajeev Gupta, Organic chemistry division, Shree P. M. Patel Institute of P.G. Studies & Research in Science, Near Sardar Baug, Anand, Gujarat 388001, India; Ashesh Mahto, Lab no. 307, NPGC Division, CSIR-CSMCRI, G. B. MARG, Bhavnagar, Gujarat-364002, India; Nilesh Vadodaria, Lab no. 307, NPGC Division, CSIR-CSMCRI, G. B. MARG, Bhavnagar, Gujarat-364002, India; Dr. Kalpana D., CSIR- Central Electrochemical Research Institute, Madras Unit, Taramani, Chennai-600113, Tamilnadu, India; Dr. S. K. Nataraj, Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagaram, Bangalore - 562112, India; Dr. R. Meena, NPGC Division, CSIRCSMCRI, G. B. MARG, Bhavnagar, Gujarat-364002, India. Abstract: The surging growth of portable and wearable electronics along with increasing demand for electric vehicles has led to an exponential increase in the demand for light weight, robust power sources with high energy and power density. Here, we demonstrate a single-step conversion process of seaweed Ulva fasciata to interconnected nanoporous carbon. Pyrolysis of marine origin green biomass resulted in inherently heteroatom-doped electrochemically active 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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graphene nanocomposite. Present study outlines a simple and easily scalable electrode material production which resulted in a relatively high and stable specific capacitance through a double layer charge storage mechanism. Temperature dependent morphology and texture variation was observed for the pyrolyzed samples, with best synergy between physical properties and application achieved at 800˚ C (UF-800). The physical characteristics of electrode material highlights high electrical conductivities of ~9100mS/m and BET surface area of >376 m2/g. An all-solid supercapacitor device using H3PO4-PVA film as separator-cum-electrolyte and UF-800 as electroactive material exhibited high gravimetric capacitance of >330 F/g with a power density of 10 kW/kg. Further, the symmetrical two electrode supercapacitor demonstrate ideal electrical double layer capacitive behavior stable up to 5000 cycles (97.5% capacitance retention). A high capacitance of 200 F/g at 5 A/g with a nominal loss of 2.5% in gravimetric capacitance reflects the collusiveness of high electrical conductivity of the composite walls and the micro and nano-pores created during pyrolysis. Keywords: All Solid Supercapacitor, Ulva Fasciata, Graphene Composite, Solvent-Less Synthesis Introduction: Carbon based nanomaterials (graphitic and activated carbon, graphene and its composites) have been drawing increasing attention owing to its extraordinary high electrical conductivity, mechanical strength, thermal stability and desirable surface texture features1,2. These unique properties make such carbon-based materials to be applied in various fields including biomedical applications3, charge storage devices4,5, photovoltaic devices6,7 and transistors8,9 to name a few. Carbon, in its different form attracts enormous significance, especially in preparing user-friendly electronic gadgets at affordable prices. Energy storage devices like battery and supercapacitor are one such family hugely contributing to the success of miniature electronic gadgets. Evidently, energy storage device market is projected with compound annual growth rate (CAGR) >35%, especially supercapacitor market is projected to reach around $4.2 billion by 2020.10 As up-to-date technologies have enabled in achieving high efficiency energy storage devices, electronic industry, automobile industry and consumer product manufactures have recognized the importance of green energy sources. Growing demand for electric vehicles in European continent, North America, China and increasing interest in India is driving the global supercapacitor market. Innovations in consumer 2 ACS Paragon Plus Environment

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electronics industry including mobile phones, electronic gadgets, cameras, defibrillators, and other household electronics have created a demand for smarter energy storage devices. Wearable electronics from healthcare sector will demand non-toxic, biocompatible and flexible energy storage devices, propelling the growth and demand for green energy material-based supercapacitors/batteries.11-15 As predictable from the literature and the product strategy, it is expected that large percentage of energy market would represent manufacturing and services of energy storage technologies like supercapacitors and batteries. However, this forecast depends on non-fossil fuel-based energy generation and storage methods. It is increasingly apparent that green, sustainable materials sources are gaining remarkable attention from scientific community and technologists.16-20 Therefore, all-solid energy storage devices like flexible and portable supercapacitors, batteries, and/or roll-up devices which implicit light weight and environment friendly nature will be very useful in the future. These solid-state energy storage devices overcome the problems of conventional liquid solution electrolytes-based devices, ease of fabrication and handling being the most important ones. Carbon based composites are becoming a promising candidate for electrode materials to be used in supercapacitor devices due to their extraordinary cyclic stability, low cost, easy fabrication and non-toxicity.21,

22

Unfortunately, low energy density, owing to their limited capacitance

limits their widespread application. Therefore, enhanced specific capacitance of the electrode materials with long and stable lifetime is the future requisites to achieve wider applicability of such electrode materials. Doping of heteroatoms on to the carbon sheets is a versatile approach to enhance the specific capacitance by imparting pseudo-capacitance behavior and increasing the electrical conductivity of carbon materials.23-26 Nowadays, alternative methods to produce graphitic and graphenic carbon from bio-based natural resources are gaining popularity& needs to be explored further. Synthesis of these materials in bulk by simple, eco-friendly and costeffective procedures makes it viable for supercapacitor applications.27-30 In this direction, we demonstrate the use of heteroatoms-enriched seaweed biomass source for producing with ease of scalability, graphitic-graphenic carbon composites. Interestingly, seaweeds grow inevitably assimilating nutrients like sulfur and nitrogenous compounds from the ecosystem. Apparently, it would be an ideal candidate for electrochemical applications, to make use of self-heteroatoms doped seaweed as a precursor to obtain highly conductive electrode materials.31-35 Further, India 3 ACS Paragon Plus Environment


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has been gifted with one of the longest seacoast and houses abundant seaweeds resource, in such situation seaweed cultivation; sourcing and sustainability would be maintained without harming the eco-system.36-39 In this study, we make use of seaweed Ulva fasciata commonly known as a ‘green seaweed’ for preparing heteroatoms-doped graphene with high electrical conductivity and desirable porosity useful for charge storage applications. The seaweed Ulva fasciata was sourced and directly converted to heteroatom doped, interwoven carbon composite without using any pre or post treatment processes. Figure 1 depicts a schematic description of the current study. Further, doped seaweed carbons were extensively characterized using FTIR, XRD, Raman spectra, SEM, TEM, and other physical methods before testing it as a supercapacitor electrode material. Electrochemical studies on seaweed derived carbon showed excellent charge-discharge cyclic stability with high specific capacity and high energy density responses. Moreover, a high retention of gravimetric capacitance was achieved even at high current densities, making it an excellent candidate for all-solid supercapacitor devices. Materials and Methods: Fresh Ulva fasciata seaweed biomass was collected from southwest coast of Gujarat (20°42'34.9"N 70°55'08.9"E), India over a period of one year, sun dried and stored in the institute’s storage. This was done so as to avoid discrepancies in sample preparation. Before its use, the dried seaweed was washed under tap water to remove impurities and other unwanted things attached on its surface. The seaweed was then air-dried and transferred into a hot air oven at 80 oC for 24 h. The dried seaweed was then grinded in a household mixer and stored in an air-tight container for further use. Polyvinyl alcohol (PVA, molecular weight 1,25,000 Da), N-Methylpyrrolidone (NMP), Acetylene black were purchased from Sigma Aldrich, Polyvinylidene fluoride (PVDF) was obtained from Solvay and Phosphoric Acid from Merck. Milli-Q water was used throughout the process until specified otherwise. All the chemicals were used as received without any further purification. 2g dried and powdered seaweed (Ulva fasciata) biomass was directly pyrolyzed at 700oC, 800oC and 900oC for 3h achieved at a ramp rate of 5oC/min under nitrogen atmosphere. As obtained carbon material was successively washed with Milli-Q water 4-5 times to remove water-soluble salts and other impurities and finally dried in a vacuum desiccator. On the other hand, PVAH3PO4 polyelectrolyte separator film was prepared by mixing 0.8 g of H3PO4 in 10 ml of 10 wt. 4 ACS Paragon Plus Environment

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% PVA solutions.40 The solution was placed at 80oC for 2h under stirring condition to obtain a homogeneous solution which was finally casted onto a glass plate and allowed to dry at room temperature. After ~72 h, polyelectrolyte film was cut into small pieces of required size to be used in the symmetrical supercapacitor device. Instrumentation: Calcination was carried out in MTI (USA) tube furnace (Model- OTF-1200XS). FT-IR spectra were recorded on a Perkin-Elmer Spectrum GX (FT-IR System, USA) instrument. Elemental analysis was done using CHNS (Vario EL III Analyser) and X-Ray Photoelectron Spectrophotometer (Thermo Scientific MULTILAB 2000 Base system). Powder X-ray diffraction (XRD) measurements of the prepared materials were recorded on an EMPYREAN, PANalytical powder X-ray Diffractometer using Cu-Kα (λ = 0.15406 nm) radiation operating at 40 kV and 30 mA in the range of 5 to 80° two theta angle at a scan rate of 0.1° s-1. Raman spectroscopy measurements were done using a micro-Raman system (Horiba Jobin-Yvon Lab RAM HR800 UV-Vis m-Raman) with argon sourced laser excitation at 514.5 nm employing power of 10 mW in the range 100–4000 cm-1. The morphology of the carbon material was probed by field emission scanning electron microscopy FE-SEM (model JEOL JSM 7100F) at acceleration voltage of 15 kV& Transmission electron microscope (TEM) (on a JEOL HR-TEM -JEOL JEM 2100, Japan) instrument operated at an accelerating voltage of 200 kV. Texture analysis was carried out on static volumetric gas adsorption analyzer (Micrometrics Inc. USA, Model ASAP) at 77.4 K using N2 gas. Electrochemical measurements: All the electrochemical measurements were performed at room temperature. The conductivity of each sample was measured using Source Meter Unit (Keithley 2635A) with a two-probe method by sandwiching the material pellet (prepared by 50 mg material using hydraulic press by applying 5-ton pressure) in between two platinum foil current collectors in a spring-loaded sample holder. The current-voltage (I-V) plots were recorded in ±1.0 V bias voltage window. Solid-state symmetrical supercapacitor devices are advantageous over liquid systems due to the ease of handling, thus to see the practicability of the seaweed derived electrode material, all solid supercapacitor device was prepared. For this purpose,a slurry of 85 wt % nanoporous carbon (UF-800), 10 wt % polyvinylidene fluoride (PVDF), and 5 wt % acetylene black in required amount of N-Methylpyrrolidone (NMP) was loaded (1.8 mg) on one side of two nickel foils (current collector) with an area of 1 cm2 followed 5 ACS Paragon Plus Environment


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by oven drying at 70oC for 10 h. Nickel foils coated with electrode materials were sandwiched using a PVA-H3PO4 solid polyelectrolyte film to assemble the symmetrical device. This supercapacitor device was connected to a potentiostat/galvanostat (Autolab PGSTAT 204) for electrochemical measurements. Parameters such as specific capacitance, energy density and power density were calculated through galvanostatic charge-discharge curves using equations 1, 2 and 3, respectively.41,

42

Cyclic voltammograms (CV), galvanostatic charge-discharge cycles

(CD) were recorded on potentiostat/galvanostat and electrochemical impedance spectroscopy (EIS) was done on HIOKI IM 3570 IMPEDANCE ANALYZER with a built-in frequency response analyzer, JAPAN). Electrochemical impedance spectroscopic measurement was performed at ambient conditions over the frequency range 4 Hz to 5 MHz with sinusoidal voltage amplitude of 50 mV superimposed on 0 V Direct circuit, DC (vs open circuit). 𝐶=

4𝐼𝛥𝑡 …………………………(𝐸𝑞 1) 𝑚𝛥𝑉

𝐸=

𝐶∆𝑉2 …………………………(𝐸𝑞 2) 8

𝑃=

𝐸

…………………..………..(𝐸𝑞 3) ∆𝑡

Where C is specific capacitance (F/g) calculated from galvanotatic charge-discharge data, I is discharge current (A), ΔV is potential window (V), Δt is discharge time (sec), m is weight of the loaded active material on current collectors (g), E is specific energy density (W.h.kg-1), P is specific power density (W.kg-1).

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Figure 1. Schematic representation of Ulva fasciata extraction and conversion in to heteroatom doped electrode material. Photograph shows light up of LED using as developed electrodes in supercapacitor device. Results and Discussions: The inherent presence of nitrogen and sulfur along with high carbon content can be elucidated from the CHNS data for dried seaweed biomass (Table 1). As expected, the yield of the obtained carbon material decreased as the pyrolysis temperature was increased.43 From the CHNS data it can also be inferred that a low temperature pyrolysis (700˚C) resulted into a more carbon and heteroatom rich product. Increasing the temperature resulted into a decreased weight percentage of carbon, nitrogen and sulfur in the final material (Table 1). It can be postulated that a higher N/C ratio will enhance pseudo-Faradic reactions44, leading to a higher gravimetric capacitance for UF-700 sample followed by UF-800 and UF-900, respectively.45 Further physical and electrochemical characterizations were carried out to determine the appropriate material for energy storage application. Table 1. Elemental composition of dried seaweed and pyrolyzed samples at different temperature. 7 ACS Paragon Plus Environment


Sample Code

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Calcination Temp. (oC) NA

% Yield NA

%C

%H

%N

37.73

5.65

3.54 3.17

UF-700

700

27.01

39.80 2.039 2.10 3.84

UF-800

800

22.45

34.95

1.53

1.02 1.44

UF-900

900

20.12

30.66

1.29

0.70 1.21

Dried

%S

Seaweed

Vibrational spectroscopic analysis: FTIR analysis reveals that an increase in pyrolysis temperature leads to a concomitant increase in the functionality of the resultant sample (Fig. 2a). Presence of unsaturation is confirmed by the appearance of a peak around 1600 cm-1 in UF-800 and UF-900 samples. Incorporation of heteroatoms was also observed for UF-900 samples, which was confirmed by the appearance of peaks around 900-1000 cm-1. Presence of a sharp peak at 1525 cm-1 in all the samples might be due the formation of pyrroles and pyrimidine derivatives from nitrogen containing functionalities of the seaweed during pyrolysis. Interestingly for UF-900, two additional peaks around 500 cm-1 and 3625 cm-1 were observed, which can be attributed to calcium oxide.46 The formation of CaO might be due to the high pyrolysis temperature, leading to bond formation between calcium and degrading oxygen functionalities, both inherently present in the seaweed.47 Further, Raman analysis was done to corroborate the findings from FTIR and to get a better insight into the carbon structure composing the pyrolyzed products. The peak at ca. 1335 cm-1 is due to the disorder caused by graphitic edges (D mode) and the one around 1590 cm-1 results due to vibration of carbon atoms against each other in their respective layers (G mode)48 (Fig. 2b). The D band is forbidden in a perfect graphitic lattice and is a common feature in all graphene and graphitic materials. Another broad peak at 2850-2900 cm-1 can be observed for all the samples which is the first overtone of the D band. This band also termed as 2D is intense than both D and G bands for UF-700 sample. Also, the D and G bands are not well formed with lot of noise accompanying them, these points towards an “agglomerated carbon structure” rather than a layered morphology.48,

49

As we move to the UF-800 sample, the D and G bands are well

dispersed with 2D:G band intensity ratio close to unity, implying presence of multilayered graphene. For UF-900 sample, the 2D:G band intensity ratio decreases to ca. 0.5, pointing 8 ACS Paragon Plus Environment

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towards a less exfoliated carbon structure compared to UF-800. An interesting observation is the appearance of a sharp peak at 1015 cm-1 for UF-900 sample which confirms the presence of CaO bond formation during the pyrolysis.50,

51

A slight downshift of this peak from its original

value of around 1050-1090 might be due to charge transfer and lattice contraction effect induced by the entrapment of calcium oxide particles between successive carbon layers. Powder XRD spectra confirms the formation of CaO in UF-900 sample as evident from the appearance of several peaks corresponding to calcium oxide52 (Fig. 2 c). A common broad peak at ca. 25 2 theta value can be observed for all the samples which is a characteristic of graphene (002) and its composites.53,

54

Another less intense peak can be seen around 45˚ which

corresponds to the 100 planes of the graphene lattice (JCPDS No. 75-1621). The lack of any visible peak around 10˚ precludes the possibility of graphene oxide formation which might be due to the reducing effect of H2S gas formed during the pyrolysis process.55, 56 The appearance of a sharp peak alongside the broad peak at ca. 25˚ for UF-700 sample can be attributed to graphitic carbon57, which might have been formed due to the incomplete exfoliation of carbon matrix at low pyrolysis temperature. Pore texture and N2 adsorption isotherm analysis: Figure 2 d depicts the N2 adsorptiondesorption isotherm for all carbon samples. A cursory glance at the isotherm reveals that the UF800 sample has the highest adsorption capacity with low hysteresis. For both UF-700 and UF900 sample, the hysteresis loop extends till low p/p0 value (ca. 0.2), while for UF-800, it ends at ca. 0.4. The hysteresis loop arises due to a phenomenon termed as ‘ink bottle pore effect’ and is characteristic of mesoporous materials.58



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Figure 2. (a) FTIR spectral overlap showing the change in functionality with pyrolysis temperature, (b) Raman spectra for all the UF-x samples showing an anomalous peak at 1015 in UF-900 sample, (c) Powder XRD spectra for all the pyrolyzed samples showing emergence of peaks corresponding to Calcium Oxide in UF-900, (d) Nitrogen adsorption-desorption isotherms with hysteresis loop for all UF-x samples. Table 2 summarizes the textural analysis data and as expected, UF-800 shows the highest BET surface area of 374 m2g-1. The pore sizes and pore volumes are tabulated alongside which were calculated according to BJH (Barrett-Joyner-Halenda) model and MP (micropore analysis). Comparing these values with pore size distribution plots (Fig. 3 a & b) gives a better insight into the porosity of these carbon materials. UF-900 and UF-700 are more mesoporous compared to UF-800 as evident for the pore size distribution according to BJH model (Fig 3a), which is in corroboration with higher hysteresis loop area (Fig. 2d). The MP plot shows a comparatively 10 ACS Paragon Plus Environment

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intense distribution of micropores at 0.6 nm for UF-800 sample (Fig 3b). A highly interconnected microporous material is desirable for energy storage applications as it provides unhindered ion transport with negligible passage resistance.59 Macroporous passage with mesoporous cavities connecting the available microporous sites results into efficient ion storage and their movement thereof.60 Table 2. Surface area and pore characteristics of pyrolyzed seaweed samples at different temperature Sample Code

BET Langmuir Surface Surface 2 -1 area (m g ) area (m2g-1)

Pore Volume (BJH) (cm3g-1)

Pore size (BJH) (nm)

Pore Volume (MP) (cm3g-1)

Pore size (MP) (nm)

UF-700

139.66

142.14

0.063

2.52

0.064

0.6

UF-800

376.82

374.55

0.097

2.52

0.217

0.6

UF-900

115.3

115.87

0.095

6.44

0.050

0.6

Figure 3. (a) Pore size distribution for all UF-x samples analyzed using BJH model, (b) MP plot for all the samples showing intense distribution at ca. 0.6 nm. X-Ray Photoelectron Spectroscopy analysis: Figure 4 shows the XPS survey spectra overlap for all the pyrolyzed samples. Apart from the peaks corresponding to carbon and heteroatoms, metal impurities were also detected with prominent peaks corresponding to Mg 1s and Ca 2p1/2 orbital electrons visible on a cursory 11 ACS Paragon Plus Environment


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glance. There have been numerous reports on metal accumulation in marine macroalgae61, 62. Relevant to the current research work, few studies had been conducted to quantify the presence of trace metals in Ulva fasciata seaweed collected from Gujarat coastline in India. Parekh et al. and Kumar et al. in two independent studies have confirmed the presence of transition metals viz. Fe, Zn, Cu, Mn and alkaline earth metals viz. Ca and Mg in Ulva fasciata seaweed63, 64. Under pyrolytic conditions, these metals might form different compounds depending on the availability of counter anion and reaction environment65. Presence of amorphous carbon matrix with only a trace amounts of these metallic compounds makes their detection by powder XRD analysis difficult. This is mainly due to the absence of long-range order resulting in diffused XRD peaks. Peaks corresponding to Fe 3s and 3p, Ca 2p1/2 and 2s, Zn 2p1/2 and Mg 1s orbitals have been designated in the XPS spectra (Fig. 4), Among these, Ca 2s orbital peak of comparatively low intensity was observed at ca. 440 eV only for the UF-900 sample. Detailed spectra analysis for individual elements is discussed in the supporting information section (ESI, Fig. S1 and S2, Table S1).

Figure 4. XPS survey spectra overlap for all the samples showing presence of metal impurities in conjunction with heteroatoms doped carbon. Electrical Conductivity measurements: The electrical conductivity (κ) of self-doped inter woven carbon networks (UF-x) were evaluated using current-voltage (I–V) plots recorded in the range of -1.0V to 1.0V (Fig. 5 a-c). The curves of each sample exhibited a linear response of current to the applied voltage which suggests ohmic nature of the materials. As calculated electrical conductivities are 58.57mScm-1, 88.90mScm-1 and 91.00 mScm-1for UF-700, UF-800 12 ACS Paragon Plus Environment

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and UF-900 samples, respectively. These values are of the same order of magnitude as reported for highly conductive graphene-based materials.66-68 A simple yet cost effective route for synthesis of porous, heteroatoms doped graphene from renewable biomass resource makes this technique green and easily scalable. It is clear that with increasing calcination temperature, conductivity also increases, although not linearly. Here, the difference in the conductivity values of UF-800 and UF-900 is not significant, thus temperature beyond 800˚C is not really required to enhance the electrical conductivity of the resultant material. Also, looking back at the BET surface areas of both the samples, UF-800 surface area is much higher than that of UF-900. These properties of UF-800 may work synergistically towards providing highly accessible sites for the semi-solid electrolyte ions with low hindrance towards their transport. This type of interwoven mesoporous and good conductivity enables UF-800 to be considered for further electrochemical characterizations and a potential charge storage material in solid state device.

Figure 5. Current-Voltage (I-V) plots for (a) UF-700, (b) UF-800 and (c) UF- 900. Morphological characterization: Scanning electron microscopy (SEM) images for all the pyrolyzed samples are depicted in Figure 6. UF-700 consists of macrovoids (Fig. 6 a & b) with agglomerated carbon constituting the entire sample. It seems like the carbon structure has collapsed under the pyrolytic conditions leading to the formation of such structures. In sharp contrast, UF-800 sample consists of interconnected pores as can be seen clearly in figure 6 c & d. As we increase the pyrolysis temperature to 900˚C, the porous morphology is substituted by what looks like highly stacked graphene sheets. The temperature dependent morphological change is noteworthy and is in corroboration with textural analysis data. To further probe the interesting morphology of UF-800 sample, high magnification SEM images were recorded (Fig. 7 a-d). The seemingly rough surface is actually composed of microfibrils-like carbonaceous



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fibers (Fig. 7 d). It is safe to conclude that these fibrils serve as interconnecting networks leading to a high surface area in conjunction with good microporosity.

Figure 6. SEM images of UF-700 (a & b), UF-800 (c & d), and UF-900 (e & f).


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Figure 7. High magnification SEM analysis of UF-800 sample. To confirm the aforementioned claim, high resolution transmittance electron microscopy (HRTEM) images were recorded for UF-800 sample (Fig. 8 a-f). It can be observed that the carbonaceous material is composed of wrinkled sheets of graphene connected with what seems like rolled-up graphene sheets (Fig. 8 a-c). On further magnification it was observed that these tube-like structures were evenly distributed throughout the sample and might serve as connectors between different carbon sheets (Fig. 8 d & e). Although the sheets are not thoroughly exfoliated (Fig. 8 f), it is quite conclusive that pyrolysis at 800˚C lead to the formation of highly interconnected microporous graphene composites. The wrinkled graphene sheets interconnected with nanotube-like rolled graphene provides macro/meso channels with very low ion transport resistance. Excellent electrical conductivity coupled with morphology like this can result in exceptional ion mobility and charge storage by 15 ACS Paragon Plus Environment


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ideal electric double layer capacitance (EDLC) behavior. Thus, it can be expected that supercapacitor electrodes composed of UF-800 active material can achieve high capacitance with good cyclic stability.

Figure 8. HR-TEM images of UF-800 sample at different magnifications. Electrochemical performanceConsiderable amount of literature on carbon materials for energy storage applications points towards three major factors affecting the performance and stability of these carbonaceous electrodes, controlled pore size distribution, high surface area and excellent conductivity for better ion diffusion.22, 69, 70 In the current work, UF-800 sample has better morphology, superior conductivity and higher surface area than UF-700 and UF-900. Keeping this in mind, UF-800 was employed as the electroactive material to carry out the electrochemical experiments in an all-solid symmetric supercapacitor device using PVA-H3PO4 film as the separator cum solid electrolyte and nickel foil as current collector (Fig. 9 d: inset). The cyclic voltammograms (Fig. 9 a) were recorded in the potential range of -0.6 V to 0.0 V at different scan rates of 100 mVs-1, 50 mVs-1, 10 mVs-1 and 5 mV mVs-1. Obtained CVs exhibited near-ideal rectangular shape, 16 ACS Paragon Plus Environment

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characteristic of electric double layer capacitance (EDLC) behavior.71 It explicates a very rapid current response on voltage reversal and a low resistance of ion transport within the electrode material. Faradic humps arising due to doped heteroatoms resulted into small peaks at the edges of the voltammograms (at 0.0 and -0.6 V). One plausible reason might be occurrence of redox reactions at pyrrolic and pyrimidine functionalities on the carbon electrode surface.72, 73 Further, galvanostatic charge-discharge curves were recorded in the voltage range of 0.0 V to 1.0 V (Fig. 9 b) at four different applied current densities and the gravimetric capacitances were calculated using the discharge time of the solid supercapacitor device. The material showed high specific capacitance, with as calculated values of 332 Fg-1, 304 Fg-1, 270 Fg-1 and 200 Fg-1 at current densities of 0.5 Ag-1, 1.0 Ag-1, 2.5 Ag-1 and 5.0 Ag-1, respectively (Fig. 9 c). It is clear that when the current density was varied from its lowest to highest value, the supercapacitor retained 60% of its capacitance, indicating that the device had an excellent capacitance retention capability resulting from the continuous 3-D mesoporous structure and the good hydrophilicity of the electrode leading to smoother diffusion of ions. Comparatively higher value of specific capacitance at lower value of current density is due to slow and more efficient diffusion of electrolyte into the pores of UF-800, leading to a better interaction between electrolyte and electrode surface. A practical device is shown in Fig. 9 d where a LED bulb is lighted using a charged supercapacitor device. A comparison between the electrochemical performance of all the samples was carried out by recording the charge-discharge cycles at a current density of 0.5 A/g (ESI, Fig S3 a). The specific capacitance calculated from these cycles showed the superior performance of UF-800 (316 F/g) over UF-700 (141 F/g) and UF-900 (73 F/g) (ESI, S3 b). Further, the life cycle of UF-800 electrode in terms of specific capacitance retention was tested by recording a number of charge-discharge cycles (5000 cycles) in galvanostatic measurement at a constant current density of 5 A/g (Fig. 10 b). Cycle life of a supercapacitor is an important parameter which relates to the active functioning of the device over a period of time without undergoing any degradation. As the stability and long-term usability of the device is directly related to the cost factor, a long cycle life is always preferred from market perspective. It is clear from Fig. 10 b that the system is retaining almost all its performance even till 5000th cycle with no loss in its specific capacitance. The multiple charge-discharge curves of the device within potential window of 0.0 V to 1.0 V at constant current density of 5 A/g is shown in Fig. 10 a. The material shows a triangular and symmetric profile during the charge-discharge process, 17 ACS Paragon Plus Environment


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which indicates that it has good charge-discharge reversibility. A cursory glance over the initial and intermediate charge-discharge cycles shows that they are almost the same with comparable charging and discharging time periods (Fig 10 a). This is due to the robustness of the electrode material which retains its texture and surface properties even after repeated operation cycles.

Figure 9. Electrochemical measurements of UF-800 as all-solid supercapacitor device:(a) Cyclic voltammograms at 100 mVs-1, 50 mVs-1, 10 mVs-1 and 5 mVs-1 scan rates, (b) Galvanostatic charge-discharge plots at 5.0 Ag-1, 2.5 Ag-1, 1.0 Ag-1 and 0.5 Ag-1 current densities, (c) Specific capacitance v/s current density plot, (d) Photograph of a light-emitting-diode (LED) lighted by charged practical device (inset: Showing photograph of UF-800 coated Ni-foil current collectors). Gravimetric energy density-power density plot (Ragone plot) of the device is shown in Fig. 10 c. Energy and power density are two key parameters for assessing the performance of a supercapacitor device which are calculated using eq. 2 and 3 mentioned earlier (obtained gravimetric specific capacitance values are used for the calculations). It is clear from the plot that energy density and power density values are in the range of 25 Wh/kg to 50 Wh/kg and 1000 18 ACS Paragon Plus Environment

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W/kg to 10000 W/kg, respectively. UF-800 delivers an energy density of 46.1 Wh/Kg at a power density of 1000 W/Kg and it preserves 60% of its energy density as the power density increases to 10000 W/kg. These obtained values are comparable or higher to that of the other seaweed derived carbon mesoporous material-based supercapacitors reported in earlier studies. A comparative outline is given in Table 3 as well as depicted in the Ragone plot. UF-800 has a three-dimensional, open interwoven microporous structure with mesopores acting as connective cavities, this helps in the faster ion diffusion which results predominantly in enhanced charge storage capacity. UF-800 electrodes undergo faster charging-discharging with high reversibility and efficient electrical storage which results in high energy and power densities. Fig. 10 d shows electrochemical impedance spectrum, represented by Nyquist plot obtained for the all-solid supercapacitor cell. Detailed description of simulation of the Nyquist plot and equivalent circuit is given in next section.

Figure 10. Electrochemical measurements of UF-800 as all-solid supercapacitor device:(a) Charge-discharge cycle test at 5 Ag-1 showing some starting and in-between cycles,(b) Cycle test



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(till 5000 cycles) to see specific capacitance change during charge-discharge (c) Ragone plot, (d) Recorded electrochemical impedance spectrum.

Figure 11. Electrochemical impedance spectrum (UF-800) simulations: Simulated Nyquist plot is shown in the frequency range of 108 Hz to 10-3 Hz. The inset shows simulated Nyquist plot at the high frequency end only. Another inset shows the equivalent circuit with various circuit parameters: Rs – polyelectrolyte membrane resistance, Rct – charge transfer resistance, CPEdl double layer capacitance, W – Warburg resistance, RL – leakage resistance, Cm – mass capacitance. Electrochemical impedance spectroscopy (EIS) measurements were performed to see the resistive and capacitive behavior of all solid symmetric supercapacitor device and its simulation was done to get perfectly fitted simulated Nyquist plot and equivalent circuit which are presented in Figure 11. The resulting Nyquist plot is composed of a very small semicircle in the high frequency region and a straight inclined line at 45º angle in low frequency region. This line is termed as the Warburg line which is a result of the frequency dependent ion diffusion in the electrolyte on the electrode/electrolyte interface. An inset shows the Nyquist plot at high frequency end. An equivalent circuit model to evaluate this kind of EIS result is also shown in inset of Figure 11. The PVA-H3PO4 polyelectrolyte membrane resistance (Rs) can be calculated from the intercept on the real axis at high frequency, also known as the equivalent series resistance (ESR) and its value is around 17 Ω. Rs is followed by small semicircle which represents the interfacial charge transfer resistance (Rct=12 Ω), characterizing the rate of charge accumulation at the electrode-electrolyte interface. Low charge transfer resistance or small 20 ACS Paragon Plus Environment

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semicircle part suggests fast ion transport within the device. CPEdl is the constant phase element (CPE) representing double layer capacitance, which occurs at the electrode/electrolyte interface between the solid carbon and polyelectrolyte due to separation of electronic and ionic charges. The transition region in Nyquist plot from high frequency semicircle to middle frequency is represented by W=0.11 Ω, which is the Warburg element representing the diffusion of ions into the porous electrode. This originates due to the frequency dependence of the diffusing ions. Small value of Warburg element generally shows short diffusion path lengths of the ions in the electrolyte. At low frequency, inclined line represents the combined effect of leakage resistance (RL) and mass capacitance (Cm) where both are placed in parallel to each other. The leakage may be due to parasitic reactions in the cell which are responsible for the self-discharge of supercapacitor. At low frequency, vertical line parallel to the imaginary axis is generally shown by an ideal capacitor but the Inclination of the vertical line towards real axis indicates non-ideal behavior of the device, a behavior common to all the practical supercapacitor devices. Table 3: Comparison table of seaweed-based carbon materials (reported in literature) and UF800 (present work) Starting Material

Electrolyte

Sodium alginate Lessonia Nigrescens

1 M H2SO4 1 M H2SO4

Gravimetric Capacitance (F g-1) 198 273

Energy Density (W h kg-1) 7.4 8.8

Power Density

255 201 125 124 264 94

12.6 4.4 10.7

362

9.6

50 W kg-1

75

332

46.1

1000 W Kg-1

(Present work)

10 W kg-1 6.7 kW kg-1

Reference

32 34

seaweed+ CNTs

Lessonia Nigrescens seaweed

Laver seaweed Lessonia Nigrescens

1 M H2SO4 6 M KOH 0.5 M Na2SO4 6 M KOH 1 M H2SO4 1 M TEABF4

35

74 33

19.5 17.5

seaweed

SDG polyaniline UF-800

+ 1 M H2SO4 PVA-H3PO4



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Conclusion: Current study exploits the hidden potential of green seaweed “Ulva fasciata” as a carbon and heteroatoms source to produce electroactive material for energy storage applications. All-solid supercapacitor device fabricated using pyrolyzed seaweed showed exceptional charge storage properties with excellent cyclic stability. Ulva fasciata derived carbon material outperforms all the other supercapacitor electrode material derived from seaweeds so far. Solvent and external chemical free approach makes this process green and easily scalable with nominal waste generation. Moreover, similar approach can be adopted for other green seaweeds to develop carbon materials with diverse applications, such as energy storage, fuel cells, catalysis, waste water treatment, CO2 capture etc. Widespread availability of raw material along the coastlines of India throughout the year makes Ulva fasciata a renewable resource for the production of carbonaceous materials. Acknowledgements: CSIR-CSMCRI registration number is 116/2018. RM & JPC want to acknowledge SERB (DST, EMR/2016/004944), CSIR (OLP0088) and A&ES&CIF, CSIRCSMCRI for sample analysis. JPC and AM acknowledge AcSIR-CSMCRI. SKN and AM gratefully acknowledges the DST, Government of India for DST-INSPIRE Fellowship and Research Grant (IFA12-CH-84). Conflict of interest: The authors declare that there are no conflicts of interests. ASSOCIATED CONTENT: Supporting Information The supporting information file includes C1s deconvoluted XPS spectra for all the pyrolyzed samples (Fig. S1) followed by discussion on the same. Further, Table S1 lists the atomic percentage of different forms of carbon present in the samples. An overlap of N1s, O1s, S2p and Ca2p XPS spectra for all the samples are depicted in Fig. S2. A comparative galvanostatic charge-discharge plot for all the sample and as calculated specific capacitance are also provided (Fig. S3). Notes and References:


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Graphical Abstract Present study outlines a solvent-less, simple and easily scalable electrode material production process which resulted in a relatively high and stable specific (up to 5000 cycles) capacitance through a double layer charge storage mechanism.

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Self-Doped Interwoven Carbon Network Derived from Ulva fasciata

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