Self-Activated SiO2/C Nanocomposite from Silica-Rich Marine Mucilage for Supercapacitor Applications

Bioresources are continually being explored as potential precursors for sustainable supercapacitor electrode materials. In this work, diatom-rich marine mucilage (MM) collected from the Marmara Sea, Turkey was used as a biomass source and converted to SiO2/Carbon nanocomposite (SiO 2 /C) through pyrolysis and acid etching. Diatom frustules acted as a highly porous silica source while algal organic matter delivered the carbonaceous part of the composite. The natural salts found in marine mucilage served as a self-activating agent and avoided the use of corrosive chemicals for the development of pores. The SiO 2 /C exhibited a competitive capacitance of 210 F g–1 at a current rate of 0.5 A g–1 in 1 M sodium sulfate (Na2SO4) aqueous electrolyte solution. The porous and durable silica skeleton improved capacitance by expanding the electrode/electrolyte interface, and the interconnected hierarchical pores ensured high electrochemical stability during long-term cycling. The mucilage-derived nanocomposite retained 80% and 70% of its capacitance after 4000 and 10,000 charge–discharge cycles, respectively. This work presents a potential solution for the management of marine mucilage by converting it into a high-value electrode material.

Climate change and energy security concerns expedite the development of renewable energy conversion and storage technologies. High-power supercapacitors, which can withstand frequent charge-discharge cycles at high current rates, hold great potential for mitigating power fluctuations of grid-integrated wind or solar-based energy systems and promoting widespread application of electric vehicles. 1,2 Activated carbons have been the most commonly used electrode materials in commercial devices due to their high specific surface area, tunable porosity, and abundant surface functionality as they promote fast ion/electron transport. 3 Renewable biomass resources have attracted particular attention as diverse, cost-effective, and greener precursors for producing electrode materials for next-generation high-performance supercapacitors.
Marine biomass, such as micro-and macroalgae, are considered viable contenders for future research as they grow remarkably fast without the need for agricultural land, fresh water or human activity. 4,5 In recent years, climate-change driven temperature anomalies have triggered harmful but nontoxic marine events that produce high amounts of biowaste, such as algal blooms and mucilage outbreaks. [6][7][8] Unicellular photosynthetic microalgae (diatoms) are the main suspects of the mucilage phenomenon, which is characterized by massive gelatinous aggregates covering large areas of coastal regions. 6 Although its composition varies depending on the type of species, growth stage, harvest season, and water quality, mucilage is a valuable nanocomposite material composed of proteins, polysaccharides, lipids, live and dead microorganisms, and seawater salts. 9,10 The conversion of this marine biowaste into a value-added product presents an important challenge towards a biobased circular economy.
Diatoms, which are responsible for up to 40% of marine productivity, constitute a massive resource of biosilica (SiO 2 ) due to their intricately structured silica cell walls called frustules. [11][12][13][14] The lack of synthetic analogs has increased their use as shapepreserving precursors to obtain functional nanocomposites for various applications. 14, 15 Certain metal oxides lacking redox activity (e.g., SiO 2 ) were proven to display high differential capacitance (dQ/ dE) in aqueous media. Improved electric double layer (EDL) capacitance is a result of surface complexation which enables close association of counter ions to the charged oxide surface. [16][17][18][19] Consequently, in recent years, efforts have been devoted to employment of environmentally friendly and low-cost SiO 2 in energy storage applications. [20][21][22][23] If SiO 2 is utilized effectively, its environmental advantages would be superior to many metal oxides. 24 Moreover, coupling with carbonaceous materials or metal oxides can grant SiO 2 with better supercapacitive performance because of the improved conductivity and chemical stability. [25][26][27] Biosilicabased nanocomposites have been limited to metal oxides until the recent work of our research group, which demonstrated the remarkable energy storage performance (300 F g −1 ) of the diatom-derived SiO 2 -carbon nanocomposite. 16 In the summer of 2021, significant portions of the surface of the Marmara Sea were covered with marine mucilage, 28,29 where ∼11,000 metric tons were mechanically collected to clean the sea surface. 30 However, much more mucilage was present in the water column with devastating effects on aquatic organisms. Thus, there has been a big social demand to explore an efficient and costeffective process for the conversion of this untapped bio-resource into value-added products. 31 In this work, marine mucilage samples, which were collected from the southern Marmara coast, were transformed into a nanostructured SiO 2 /C composite through scalable one-step pyrolysis. The considerable amount of inorganic species (including alkali and alkaline Earth metals in the form of cations or salts) contained in the sample served as porogens/selfactivating agents during pyrolysis and eliminated the need for external chemicals or physical activating agents (KOH, H 3 PO 4 , ZnCl 2 , or CO 2 gas and steam). 5,[32][33][34] To the best of our knowledge, marine mucilage-derived SiO 2 /C nanocomposite and its use as an electrode material have never been reported before. The SiO 2 /C nanocomposite electrode demonstrated comparable or even superior electrochemical performance when compared to published work containing synthetic silica, which often requires complex manufacturing and/or activation techniques.

Experimental
Synthesis of the SiO 2 /C nanocomposite.-The large floating marine mucilage (MM) aggregates were collected from the surface of the Marmara Sea, Turkey (5 May 2021, from the coast of the Gemlik district in Bursa). The collected samples were centrifuged and dried at 60°C to remove excess water. The dried mucilage was ground to a powder using a pestle and mortar and pyrolyzed under Argon at 650°C for 2 h. The biochar (BC) obtained was etched overnight with a 0.1 M HCl solution. The as-prepared sample was collected by centrifugation and washed repeatedly with DI water and dried under vacuum at 80°C. Scheme 1 provides a summary of the z E-mail: ece.unur@btu.edu.tr synthetic process. In addition, the marine mucilage was washed with deionized water to remove surface salts, dried completely, and named as MM-w.
Material characterizations.-Elemental analyses were performed using a Thermo Scientific FlashSmart Elemental Analyzer. The thermogravimetric analyzer (TGA, TA Instruments SDT 650) was used to monitor the thermal stabilities under air flow (10°C min −1 ). The inorganic residue/ash contents of the samples were quantified from the TGA curves at 550°C. For the marine mucilage (MM), an additional TGA analysis was performed by heating the material to 550°C (at a rate of 10°C min −1 ) and dwelling at that temperature for 2 h to ensure stabilization of the residual weight. Surface functional groups were studied by Fourier transform infrared (FT-IR) spectroscopy (Perkin Elmer Spectrum Two ® using a diamond crystal). X-ray diffraction (XRD) patterns were collected with a Bruker D8-Discover diffractometer (CuKα radiation, step   size: 0.02°, dwelling time: 0.5 s). Surface imaging and EDS (energy dispersive X-ray spectroscopy) analyses were conducted via Zeiss GeminiSEM 300 scanning electron microscope (SEM) on Au-Pd sputter coated samples. Dispersive Raman spectra were obtained by the Renishaw InVia spectrometer with 532 nm laser excitation. The N 2 sorption isotherms were recorded at 77 K on a Quantachrome Autosorb-6B instrument after degassing the samples for 10 h at 200°C . The Brunauer, Emmett & Teller (BET) and the nonlocal density functional theory (NLDFT) were used to calculate specific surface areas and pore size distributions, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher Scientific K-Alpha instrument using a monochromated Al Kα radiation (1486.68 eV) and a multichannel detector. The survey and high-resolution spectra were obtained with pass energies of 50 eV and 200 eV, respectively, by taking 10 scans to improve the resolution.
Electrochemical measurements.-The working electrode was made by mixing active material, poly(vinyl difluoride) (PVDF, Kynar ® HSV 900) binder, and Super P ® carbon black (Timcal Graphite & Carbon) in a mass ratio of 80:10:10. An appropriate amount of NMP solution (N-methyl pyrrolidone, MTI) was used to form a viscous slurry. The electrode slurry was coated onto a stainless steel (SS) foil using a doctor blade. The electrode coatings were first dried overnight at room temperature and then under vacuum at 60°C for 24 h. The dried electrodes were cut into a 10mm-diameter disc shape. The mass loading of the active material on the SS foil was ∼0.4 mg cm −2 . Electrochemical measurements were conducted on a Gamry Reference 3000 electrochemical workstation using a home-made cell with Pt wire and silver/silver chloride (Ag/ AgCl (3 M KCl)) serving as counter and reference electrodes, respectively. Cyclic voltammetry (CV) and galvanostatic charge/ discharge (GCD) curves were recorded at different rates in the potential window of −0.8 to 0.2 V (vs Ag/AgCl) in neutral aqueous medium (1 M Na 2 SO 4 ). Electrochemical impedance spectra (EIS) were measured between 100 kHz and 0.01 Hz at open-circuit potential by applying an alternating potential amplitude of 5 mV. The capacitance (C, F) was calculated from the discharge curves as is the discharge current, t (s) is the discharge time, and ΔV (V) is the potential range, excluding the IR drop. The specific capacitance (C s , F g −1 ) was then calculated by dividing the capacitance by the mass of active material, m (g). The Coulombic efficiency was calculated as: η = (t d /t c ) × 100, where t d and t c are discharge and charge times, respectively. A symmetric supercapacitor was assembled and tested using 1 M Na 2 SO 4 . The corresponding specific energy (E, Wh kg −1 ) and specific power (P, W kg −1 ) of the cell were calculated according to the following equations: E = (0.5 × C s,cell × ΔV 2 )/3.6 and P = E/t d × 3600 where C s,cell is the specific capacitance of the cell.

Results and Discussion
In the summer of 2021, the Sea of Marmara in Turkey experienced one of the largest outbreaks of marine mucilage worldwide. The marine biomass that covered the shores of the Sea of Marmara (Fig. 1) was collected and transformed into an electrode material for supercapacitors. The marine mucilage (MM) was pyrolyzed into biochar (BC) in an inert atmosphere and then etched with acid to obtain a SiO 2 /C nanocomposite. The thermal stabilities of the MM and SiO 2 /C were monitored by thermogravimetric analysis (TGA) under air flow. The thermogravimetry (TG) and derivative thermogravimetry (DTG) profiles in Fig. 2 indicate a more complex degradation route for MM, demonstrating the complexity of the material. The initial weight loss observed below 200°C for both samples was associated with physical moisture release. For MM, the weight loss between 200°C and 550°C was attributed to the degradation of biomolecules (such as polysaccharides and protein amino acid residues). 35,36 The significant weight loss above 750°C can be attributed to the volatile metal loss and inorganic decomposition. 36 For the SiO 2 /C composite, steep weight loss was observed at 480°C (Fig. 2B), indicating the combustion of organic and elemental carbon content. 36 The inorganic residue/ash contents of the samples were quantified at 550°C. 6,37 The stabilization of MM weight during dwelling at 550°C (for 2 h) is depicted in the inset of Fig. 2A. The results showed that the major components of MM and SiO 2 /C are inorganic residues (73.6 and 74.2 wt%, respectively), resulting mainly from the silica frustules of diatoms. The C atom and O heteroatom contents of the MM were recorded as 6 wt% and 18.5 wt%, respectively (Table I). The C content increased to 15.6 wt% for SiO 2 /C, while a significant amount of O heteroatoms (8.4 wt% O) were retained. Heteroatoms promote EDL capacitance by improving electrical conductivity and surface wettability of the material while contributing to the pseudocapacitance through surface redox reactions. 38 SEM images of marine mucilage displayed amorphous aggregates of algal organic matter (Fig. 3A) and flat-faced cubic crystals embedded within (Fig. 3B, the area pointed by the yellow arrow).
XRD patterns in Fig. 4A proved these crystals to be halite (NaCl), which precipitated upon natural evaporation of seawater. During evaporation, the gelatinous mucilage layer functions as a semipermeable membrane and confines salt crystals. 39 The SEM images of the SiO 2 /C nanocomposite (Figs. 3C-3G) revealed various species of diatoms -mixed, stacked, crushed, and fragmented-with a low number of intact frustules. The frustules were composed of intricately patterned structures, some of which had rows of tiny holes. The intense peak at 2θ ∼ 27 o in the XRD spectrum of SiO 2 /C was attributed to reflections from (101) planes of the quartz phase (qtz) of the diatom silica (Fig. 4A), which was observed in the XRD pattern of the marine mucilage only after washing the NaCl out (MM-w). The EDS results in Fig. 3H confirmed that the SiO 2 /C  composite is mainly composed of C, O, and Si (with weight percentages of 17.7%, 38.8% and 29.5%, respectively). Along with silica, the SiO 2 /C composite contains trace amounts of certain inorganic elements (in the form of cations or salts), such as Ca, Mg, Al, K, and Fe, whose capacity to form complexes with polysaccharides allows them to participate in mucilage aggregation. 10 The FTIR spectra of marine mucilage (MM), biochar (BC), and acid-washed biochar (SiO 2 /C) are shown in Fig. 4B. The broad band at ∼3350 cm −1 and its shoulder at ∼3250 cm −1 , were attributed to the O-H group of carbohydrates and proteins and the NH 2 group of amino acids, respectively. These peaks disappeared after pyrolysis due to dehydration and degradation reactions. The weak stretching bands were recognized between 2950 and 2850 cm −1 , originating from the CH and CH 2 aliphatic groups of proteins and carbohydrates. 40,41 Proteins were further identified by the C=O amide I (1640 cm −1 ), C-N amide II (1550 cm −1 ), and C-N amide III (1230 cm −1 ) bands. 40 Degradation of protein structures with pyrolysis generated a broader band at 1620 cm −1 , resulting from C=C aromatic ring stretching of amorphous carbon. Inorganic carbonate was detected by two typical bands: the C=O stretching band at around 1420 cm −1 and the C=O bending band at ∼880 cm −1 37 . The absence of these bands in the SiO 2 /C spectrum suggested that the pyrolysis followed by acid wash effectively excluded the carbonate salts (MgCO 3 and CaCO 3 ) from the structure. 37 The broad bands at ∼1040-990 cm −1 and the small peaks at ∼870-780 cm −1 were associated with Si-O-Si asymmetric stretching and symmetric bending vibrations, respectively. After pyrolysis, the asymmetric stretching vibration was shifted to lower wavenumbers, presumably due to the densification of the SiO 2 , creating stronger bonds. 42 The Si-O-Si rocking modes were also observed at ∼450 cm −1 . 42 The presence of more intense silica-related peaks in the SiO 2 /C spectrum confirmed the effective removal of the halite matrix and the preservation of biosilica in the nanocomposite structure. 16 The structural features and graphitization degree of SiO 2 /C nanocomposite were further analyzed with Raman spectroscopy (Fig. 5). The bands observed at ∼1353 cm −1 (D-band) and 1594 cm −1 (G-band) indicated structural disorder and the interplane sp 2 C-C stretching vibrations of ordered graphite, respectively. 25 The intensity ratio of the G to D band was 1.1, implying the presence of partially graphitized carbon in the SiO 2 /C structure. 43 The broad band observed between 2650-2850 cm −1 was due to the secondorder scatterings. The absence of silica-related Raman signals in the spectrum can be attributed to the presence of a carbon coating. 16 The specific surface area (SSA) and porosity of the samples were examined by N 2 -physisorption at 77 K. The marine mucilage exhibited low porosity and SSA. The type IV (IUPAC) isotherm ( Fig. 6A) with increasing adsorption volume at low relative pressures (P/P 0 < 0.1) and H3 type hysteresis indicated enhanced micro-and mesoporosity for the SiO 2 /C nanocomposite, respectively. 44 The SSA of the mucilage increased ∼20 times from 7 m 2 g −1 to 149 m 2 g −1 and the pore volume ∼12 times from 0.01 cm 3 g −1 to 0.12 cm 3 g −1 upon pyrolysis and acid treatment. The SiO 2 /C nanocomposite possesses additional macropores as indicated by high N 2 uptake at high relative pressures (0.95 < P/P 0 < 1). The pore size distribution (PSD) curves (Fig. 6B) also confirmed the formation of a hierarchically porous interconnected network. The observed pore tuning might be explained by the templating effect of in situ salts. 45,46 During pyrolysis, the salts on the surface of marine mucilage melted into clusters of various sizes and shapes (e.g., cubes and spheres). These clusters filled the voids of the raw precursor and precipitated below the eutectic temperature. The macropores, mesopores, and micropores are created by releasing the occupied gaps after cooling and washing with hydrochloric acid and water. 46 Furthermore, silica skeleton provided a confined space for the polycondensation of organic gases into carbonaceous material. 27 The rigid skeleton also acted as a hard template and suppressed pore shrinkage during pyrolysis. 16,47 The hierarchically porous structure can increase the penetration depth of electrolyte ions and facilitate ion diffusion, which are beneficial for achieving higher specific capacitance and rate capability.
XPS analysis provided further information on the surface elemental composition of MM and SiO 2 /C. As illustrated in (Fig. 7A), the XPS survey spectra confirmed the presence of elemental Ca, Mg and Cl on the surface of the raw marine mucilage. After pyrolysis and acid etching, four main peaks around 533 eV, 285 eV, 155 eV and 104 eV are observed in XPS survey spectrum of the SiO 2 /C composite, which are ascribed to the O 1s, C 1s, Si 2s and Si 2p peaks, respectively, indicating that almost no salts remained on the surface of the SiO 2 /C composite. This also proves the templating effect of the in situ salts. High-resolution XPS spectra of C 1s, O 1s, and Si 2p were displayed in Figs. 7B-7D. The background has been subtracted using a Shirley spline. The C 1s spectrum can be deconvoluted into three peaks at 284.8, 285.9, and 289.2 eV, which are associated with the binding energies of C=C, C-O-C, and O-C=O chemical states in SiO 2 /C, respectively. 27 The O 1s spectrum could be divided into 3 peaks with binding energies of 531.0 eV, 532.9 eV and 533.8 eV, which are related to O=C/ silicates, SiO 2 qtz and O-C bonds, respectively. The Si 2p region was resolved into two Gaussian peaks to account for the spin-orbit lines Si 2p 3/2 (103.6 eV) and Si 2p 1/2 (104.2 eV) of Si 4+ in SiO 2 qtz that are separated by Δ = 0.6 eV. There is no contribution from silicon suboxides (+1 through +3) or elemental Si, indicating that the majority of Si atoms exist as SiO 2 . 48 The atomic ratio of O 1s to Si 2p is 2.7, which exceeds the theoretical value for SiO 2 , indicate the existence of oxygen-containing functional groups within SiO 2 /C composite.
Electrochemical studies.-Electrochemical charge storage properties of mucilage-derived SiO 2 /C nanocomposite were evaluated for supercapacitor applications. Figure 8A shows the cyclic voltammetry (CV) profiles of the SiO 2 /C electrode at 50 mV s −1 with different mass loadings in an aqueous electrolyte (1 M Na 2 SO 4 , −0.8 to +0.2 V vs Ag/AgCl). The figure shows that the rectangular curve, which is an indicator of ideal capacitive behavior, was more defined for the electrode with a mass loading of 0.4 mg cm −2 . The mass loading we chose (0.4 mg cm −2 ) as the optimum allows comparison of our results to published work. [49][50][51][52] Figure 8B shows the CV profiles under different scan rates (2-500 mV s −1 ). The distortion of the voltammograms to an elliptical shape at high scan rates (>50 mV s −1 ) suggests the contribution of some (pseudocapacitive) surface redox reactions to the overall capacitance. To differentiate the dominant charge storage mechanism, reaction   kinetics were further analyzed by a power-law relationship: i (V) = a v b , where i and v are peak current and scan rate, respectively, while a and b are adjustable parameters. The slope derived from the log i (V) vs log v plot corresponds to the b-value, where b = 0.5 implies diffusion-limited electrochemical processes, and b = 1.0 indicates surface-controlled/capacitive kinetics. 53 The b-value of SiO 2 /C was determined to be 0.88 (Fig. 8C), representing the dominance of surface-controlled kinetics. Furthermore, the voltammetric current response can be deconvoluted into capacitive (k 1 v) and diffusioncontrolled (k 2 v 1/2 ) components by using the following equation: i (V) = k 1 v + k 2 v 1/2 (Fig. 8D), where k 1 and k 2 are constants. 54 As shown in Fig. 8E, the capacitive contribution of SiO 2 /C is 30% at 2 mV s −1 and gradually increases to 87% at 500 mV s −1 , indicating the significance of capacitive charge storage in total electrode capacitance, particularly at high scan rates. 55 Figures 9A and 9B show galvanostatic charge-discharge (GCD) curves at various current rates (0.5-10 A g −1 ), which present linear and symmetric GCD profiles. The specific capacitance values of the sample were calculated from the discharge curves and plotted against the current rate in Fig. 9C. The SiO 2 /C electrode exhibited a high specific capacitance of 210 F g −1 at 0.5 A g −1 and maintained 110 F g −1 when the current rate was raised by 40-fold to 10 A g −1 . At high current rates, charge propagation and ion diffusion through pores are hindered by time constraints, leading to lower capacitance. 56 The long-term cycling stability of the SiO 2 /C nanocomposite was evaluated at 5 A g −1 by repeated charge-discharge of the sample (Fig. 9D). Considering the rather harsh current and susceptibility of SiO 2 to volume changes, the SiO 2 /C exhibited good reversibility and structural stability. The capacitance dropped gradually by 20% and 30% after 4,000 and 10,000 cycles, respectively. The decrease in capacitance can also be deduced from the decreasing discharge times in the GCD curves of the 4,000th and 10,000th cycles ( Fig. 9Dinset). The voltage drops (IR) at the start of the discharge curves tripled from 0.06 V to 0.18 V after 10,000 cycles, probably due to increasing internal resistance contributed by the electrode and electrolyte. The relative resistance values of the material were compared before and after cycling to assess its long-term aging conditions. 57 The Nyquist plot of SiO 2 /C recorded after cycling (Fig. 10A) revealed a larger semicircle and a lower Warburg slope, indicating an increased charge transfer (R ct ) and ion diffusion resistance, respectively. The x-intercept at the high-frequency region is attributed to the series resistance (R s ), which is the sum of the intrinsic resistance of the active material and the ionic resistance of the electrolyte. An equivalent circuit (Fig. 10A-inset) was used to determine variations in resistance values. The R s value decreased from 5.49 to 4.88 ohms at the end of the cycle process. However, the charge transfer (R ct ) and the Warburg coefficient (σ), which is directly proportional to the Warburg resistance, increased nearly three (from 7.39 to 22.05 Ω) and five (from 26.47 to 128.95 Ω s −1/2 ) times. Although the shapes of the Nyquist plots and GCD curves did not indicate total capacitance loss, these results suggested that the electrode/electrolyte interface was adversely affected by the cycle tests as expected. The cycling performance and corresponding resistance parameters can be optimized for the final industrial applications of supercapacitors by changing the operating potential and applied current. 57,58 The rate-limiting charge storage kinetics can be inferred from the Bode phase plots of SiO 2 /C recorded before and after cycling (Fig. 10B). The phase angle is -90°for pure capacitive response. At a low frequency of 10 mHz, the phase angle of the electrode reaches -77.92°and -68.12°before and after the cycling test, indicating a pseudocapacitive contribution for charge storage. The cross-over frequency corresponding to -45°, marked with a horizontal solid line in Fig. 10B, indicates the transition point from capacitive to resistive behavior. The inverse of the cross-over frequency gives the relaxation time, τ = 1/f, which is a measure of how fast the device can be charged and discharged reversibly with an efficiency greater than 50%. 55 The electrode displays a relaxation time of 1.5 s and 1.8 s before and after the cycling test, respectively, indicating rapid ion diffusion at the electrode-electrolyte interface.
The SiO 2 /C nanocomposite, obtained without additional activation or doping processes, showed comparable or superior electrochemical performance to previously reported SiO 2 -based nanocomposites (Table II), which are composed of synthetic silica particles prepared by relatively toxic and expensive precursors. The high specific capacitance of the SiO 2 /C composite we report here (composed of ∼74 wt% SiO 2 ) cannot be attributed solely to its interconnected hierarchical pore structure or moderate surface area (149 m 2 g −1 ). As a non-transition metal oxide, SiO 2 does not undergo traditional redox reactions and, therefore, cannot store energy via pseudocapacitive mechanism. A double-layer capacitance was also not predicted due to its electrically insulating nature. But, SiO 2 has been reported to contribute to EDL capacitance through its inherent surface potential and the surface complexation mechanism described by a "triple-layer" model, which generates a differential capacitance of 180 μF cm −2 . 18 This value significantly exceeds the typical double-layer capacitance of carbon electrodes (10-20 μF cm −2 ). 17,59 In the traditional double-layer model (i.e., the Gouy −Chapman−Stern Model); a compact layer of ions adsorbed onto the electrode surface is considered the first layer, while the ion gradient spreading into the bulk of the solution forms the second/ diffuse layer. 17,59 With respect to Davis's "triple-layer" model; the surface charge on the oxide forms the first layer, and the specifically adsorbed counter ions and diffuse layer form the second and third layers, respectively. 19 The water layer chemisorbed onto SiO 2  This work 110 @ 10 A g −1 adsorbs or releases protons to form surface charges on SiO 2 and a layer of protons/hydroxyls. The high inner layer capacitance values result from close association of counterions in the electrolyte with the highly structured protons/hydroxyls adjacent to the oxide surface. In most cases, the chemisorbed water may be a part of the counterion's hydration layer and improve the proximity. 60 A symmetric two-electrode cell was assembled to further investigate the capacitive behavior of SiO 2 /C in a practical system. The CV (Figs. 11A, B) and GCD (Fig. 11C) curves of the device were recorded in a 1 M Na 2 SO 4 aqueous electrolyte (ΔV=1.1 V). The device exhibited a maximum specific energy of 4.7 Wh kg −1 (0.1 A g −1 ) at a specific power of 76 W kg −1 , which was comparable with the previous reports 46 and the currently available devices with activated carbon electrodes (2-5 Wh kg −1 ) in the aqueous electrolyte. 68,69 Furthermore, the device remained chemically and structurally intact over 2000 charge-discharge cycles at 1.3 A g −1 and retained 95% of its initial capacitance (Fig. 11D).

Conclusions
Marine mucilage obtained from the Sea of Marmara in Turkey was converted into a hierarchically porous SiO 2 /C nanocomposite. The interconnected hierarchical pores generated by etching in situ salts allowed shorter diffusion paths for electrolyte ions, while the inherent nanopatterned structure of diatom frustules provided a larger accessible surface area for charge storage. Thus, electrolyte ions interacted more closely with SiO 2 owing to its intrinsic surface potential and improved double layer capacitance. The enhanced electrode-electrolyte interactions through a larger specific surface area and free pathways for ion transfer resulted in high specific capacitance (210 F g −1 at 0.5 A g −1 ) and rate capability. Owing to the buffering effect of carbon in the composite, the SiO 2 /C demonstrated high long-term cycling stability, with 70% capacitance retention after 10,000 cycles at 5 A g −1 . The conversion of silicarich marine mucilage into an economical nanocomposite with a complex architecture provides a promising perspective for supercapacitor applications.