Silk Fibroin-Regulated Biomimetic Mineralization of Ni(OH)2 for Energy Storage Applications

Silk Fibroin-Regulated Biomimetic Mineralization of Ni(OH)2 for Energy Storage Applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Silk Fibroin-Regulated Biomimetic Mineralization of Ni(OH)2 for Energy Storage Applications Siva Kumar Ramesh, Jinkwon Kim, Seog Woo Rhee This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8385555/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 11 Apr, 2026 Read the published version in Korean Journal of Chemical Engineering → Version 1 posted 4 You are reading this latest preprint version Abstract Understanding and controlling biomimetic hybrid materials is essential to obtain novel hierarchical structures and expand the range of potential applications. Here, the biomineralization approach was applied as a rational integration method of silk fibroin and nickel hydroxide. Under protein-directed self-assembly conditions, hollow structures of hierarchical hybrid nickel hydroxide microflowers were systematically formed due to structural disorders such as stacking faults. Notably, the uniform, well-dispersed, and tunable morphology of biomineralized hybrid nickel hydroxides hollow microflowers is achieved by controlling the concentration of silk fibroin and nickel (II) chloride. The effects of organic solvents and nickel precursors on the morphology of nickel hydroxides were also elucidated. The fibroin/Ni(OH) 2 hybrid material exhibits good charge storage performance, rate capability, proton diffusion coefficient, and outstanding cyclic stability owing to its high ionic conductivity and the presence of β-sheet crystallites. Biomineralization Biomimetic materials Silk fibroin Ni(OH)2 Energy storage Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. INTRODUCTION The integration of inorganic minerals with biomacromolecules is considered an indispensable process in human daily life and drives us to create supramolecular nanostructures with intricate forms and diverse functionalities [1,2]. This self-assembly process of biomaterials enables construction of well-defined hierarchical structures through environmentally friendly synthesis [3,4]. This approach will not only accelerate progress in the field of biomineralization, but also contribute to advances in materials science as well as biomedicines [5,6]. In this regard, the biomineralization process may provide a viable strategies to induce advantageous topological and material properties while maintaining high biocompatibility [7]. The nucleation and crystal growth steps of the crystallization process control the tunable properties such as shape, size, aggregation, orientation, and texture [8]. Among various biomaterials, the silk fibroin, which is extracted from Bombyx mori and has a 3d fibrous network, has been the most extensively characterized in numerous studies presumably due to its robust mechanical strength, biocompatibility, and biodegradability [9–11]. Silk fibroin comprises heavy chain (≈ 390 kDa) and light chain (≈ 26 kDa) proteins, linked by disulfide bonds, and the amino acids sequence constitutes a highly repetitive GAGAGS motif [12]. The unique properties of silk fibroin can be utilized to create a variety of material forms such as porous sponges [13], fibers [14,15], hydrogels [16], non-woven mats[17], tubes [18], and films[19]. This versatility allows silk fibroin to be applied in diverse applications such as tissue engineering [20], drug delivery systems [21], smart wearables [22], optical devices [23], and others [24]. Nguyen et al . and Chen et al . outlined the mechanisms of the gelation process of silk fibroin into the hierarchical network [25,26]. During this process, single domain networks assemble into a multidomain network upon the introduction of external agents. This interaction induces the growth of β-sheet crystallites, thereby promoting the formation of hierarchical structure. Particularly, hydrophilic groups and polar groups in the silk fibroin can strongly bind to metal ions, triggering biomineralization and crystallization [27]. On the other hand, the exploration of advanced materials for energy storage devices emphasizes the growing demand for clean, sustainable energy solutions worldwide. Supercapacitors have become prominent energy storage devices and exhibit remarkable power performance, excellent reversibility, and an extended life cycle [28,29]. Moreover, Supercapacitors produce less thermochemical heat due to their simple charge storage mechanisms and are therefore widely used in industrial power and energy systems, consumer electronics, and memory backup systems [30]. Among various alternatives, transition metal hydroxides are considered industrially important electrochemical active materials due to its notable theoretical capacity [31,32]. In particular, β-Ni(OH) 2 is thermodynamically stable, easy to be crystallized, earth-abundant, and excellent in redox activity [33]. However, it suffers from poor conductivity, few electrode active sites on the surface, and structural instability. The integration of carbon materials and β-Ni(OH) 2 not only improves structural integrity but also provides performance by improving electrical conductivity and reducing the energy barrier for ion diffusion [34,35]. Herein, we demonstrated biomimetic mineralization to enable the preparation of a hierarchical hybrid nickel hydroxide hollow microflower by regulating the silk fibroin biomacromolecules as a template in a facile hydrothermal condition. The morphology of hybrid Ni(OH) 2 was optimized by controlling silk fibroin and Ni 2+ concentration and changing organic solvents and nickel precursors. Silk fibroin acts as a structural guide to form hierarchical microspheres while providing sites for nuclei formation and crystal growth during the nickel hydroxide precipitation process. Furthermore, biomineralized Ni(OH) 2 offers practical applications in energy storage. This hybrid material exhibits significantly better rate performance, proton diffusion coefficient, and cycling stability compared to the control sample due to its high ionic conductivity, presence of β-sheet crystallites, and structural disorder. 2. EXPERIMENTAL AND RESULTS 2.1 Synthesis of biomineralized nickel hydroxides Silk fibroin was extracted from Bombyx mori silkworm cocoons, as previously known method [36]. The final concentrations of regenerated silk fibroin concentrations were approximately 4% – 6% (w/v) and stored at − 60°C until further use. 60 mmol L − 1 of NiCl 2 ⋅6H 2 O was added to 50 mL of various fibroin concentrations (0 to 0.5% (w/v)) solution and NH 4 OH was added to the solution to adjust to pH 10. The solution was stirred for 30 minutes and transferred to an autoclave, which was heated at 180°C for 12 h. Then, after cooling the product was centrifuged, washed with DI water three times, and freeze-dried to obtain the final product. For control experiments, Ni(OH) 2 was prepared without fibroin solution using the above-mentioned method. To elucidate the effect of Ni 2+ concentration on the morphology of biomineralized Ni(OH) 2 , the silk fibroin concentration was kept constant at 0.3% (w/v) and the concentration of Ni 2+ was changed from 20 to 80 mmol L − 1 . Furthermore, to study the effect of the anion of nickel precursor on the morphology of biomineralized Ni(OH) 2 , the various nickel salts such as 60 mmol L − 1 of Ni(NO 3 )⋅6H 2 O, NiSO 4 ⋅7H 2 O, Ni(OCl 4 ) 2 ⋅6H 2 O, and Ni(CH 3 CO 2 ) 2 ⋅4H 2 O were used instead of NiCl 2 .6H 2 O with 0.3% (w/v) silk fibroin. Moreover, to study the effect of solvents on the morphology of biomineralized Ni(OH) 2 , 20 mL of ethanol, methanol and DMF and 30 mL of DI water were added into the 0.3% (w/v) silk fibroin and 60 mmol L − 1 of NiCl 2 ⋅6H 2 O, instead of 50 mL of DI water. In addition, the time-dependent experiments were carried out with 0.3% (w/v) silk fibroin and 40 mmol L − 1 of NiCl 2 .6H 2 O. The sample codes of biomineralized Ni(OH) 2 with 0, 0.1, 0.3, and 0.5% (w/v) are denoted as Ni(OH) 2 , Ni(OH) 2 /SF-0.1, Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5, respectively. 2.2 Materials Characterization The powder X-ray diffraction (XRD) analysis was conducted on a Rigaku MiniFlex 600 diffractometer (Cu Kα radiation λ = 1.5406 Å) to characterize the crystal structures of the samples. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded on an IR affinity-1S, Shimadzu Co., spectrophotometer. Thermogravimetric analysis (TGA) was performed by the NETZSCH TGA instrument (TG_209_F1_Libra) at a heating rate of 10 K min − 1 in an air atmosphere. Field emission scanning electron microscopy (FESEM, Hitachi HF-4800), transmission electron microscopy (TEM) with scanning transmission electron microscopy (STEM) (FETEM, Tecnai G 2 F30 S-Twin, acceleration voltage: 300 kV) were conducted to investigate the morphology and microstructure of the samples. The ASAP-2420 (Micromeritics, USA) instrument was used to determine the Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution. X-ray photoelectron spectroscopy was conducted using a MultiLab ESCA 2000 XPS system to gain information about surface valence states. 2.3 Electrochemical Characterization All electrochemical behaviors were investigated using a PGSTAT 302N Autolab (Metrohm) in a three-electrode system where nickel foam substrate (1 cm×1 cm), a platinum rod, and Ag/AgCl (saturated KCl solution) served as the working electrode, counter electrode, and reference electrode, respectively and 2 M KOH used as the electrolyte solution. The slurry was prepared with active material, carbon black and polyvinylidene fluoride in N -methyl pyrrolidone in a mass ratio of 8:1:1. Then the slurry was coated on a nickel foam substrate and dried in a vacuum oven at 60°C overnight. Cyclic voltammetry was carried out by applying various scan rates in the potential range of 0-0.6 V. Galvanostatic charge-discharge (GCD) experiments were performed with varying current densities (0.5, 1, 2, 3, 5, 7, and 10 A g − 1 ) in the potential range of 0–0.5 V. The specific capacity was calculated from the GCD equation C s = I × Δt/m × ΔV where Cs is the specific capacitance, I is current, Δt is the discharge time, m is mass, and ΔV is the potential window. The results expressed in C g − 1 were then converted to mA h g − 1 by dividing it by 3.6. The proton diffusion coefficient was determined using the Randles-Sevcik equation, Ip = (2.687 × 10 5 ) n 3/2 ACν 1/2 D 1/2 where Ip is the peak current, n is the number of electrons transferred during the redox reaction, A is the electrode surface area, C is the concentration of the electroactive species, D is the diffusion coefficient, and ν is the scan rate. 3. RESULT AND DISCUSSION The biomineralization of Ni(OH) 2 was performed by hydrothermal method using silk fibroin, as illustrated in Fig. 1 . Proteins act as biotemplates in the biomineralization process and play a major role in controlling the size, shape, composition, and crystal structure of the inorganic materials. A series of experiments were conducted by changing the concentrations of silk fibroin from 0 to 0.5% (w/v) with a constant Ni 2+ concentration of 60 mmol L − 1 in order to examine this biomineralization process. X-ray diffraction analysis was carried out to determine the crystal phase of biomineralized Ni(OH) 2 . XRD patterns of the contour heat map are shown in Fig. 2 a and b. The biomineralized Ni(OH) 2 and Ni(OH) 2 were assigned to β-NiOH (JCPDS-14-0117) with a hexagonal structure (a = 3.126 Å, c = 4.605 Å, space group P-3m1). The ideal Ni(OH) 2 consists of edge-sharing NiO 6 octahedra, oxygen packing is ABAB and hydrogen atoms are located in tetrahedral environments exactly above or below the oxygen atoms and in this packing HO 4 tetrahedra share only edges and it obeys Pauling’s third rule. When the fibroin concentration increases in Fig. 2 , we can see the line broadening of the peaks such as (001) and (h0l) peaks presumably owing to the stacking faults and interstratification, respectively [37,38]. In detail, the stacking faults lead to two types: growth faults and deformation faults, which causes oxygen atoms to migrate to C position and form fcc blocks. The oxygen packings are randomly distributed in the growth faults (ABABCBCBCBABAB) and deformation faults (ABABCACABCBCABAB) [38,39]. A typical stacking fault pattern is induced as the fibroin concentration increases and the width of the (10l) peaks broadens as the amount of stacking faults increases. The electrostatic interaction between hydrogen atoms and nickel ions is very low in the ideal structure (ABAB), whereas the samples with stacking faults (ABCB or ABCA) show higher electrostatic interaction between two polyhedra sharing one face, which violates Pauling's third rule. Therefore, in the samples with stacking faults, hydrogen atoms are expected to be more destabilized and have higher electrochemical activity [38]. Moreover, interstratification appears to broaden all the non-hk0 reflections. The (001) peak also broadens when more fibroin is present in the sample, which may be related to the presence of a few percentages of α-motifs [39]. Particularly, (100) and (110) reflections remain essentially unchanged in all samples. Figure 2 c shows the IR spectra of Ni(OH) 2 , Ni(OH) 2 /SF-0.1, Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5. The sharp absorption peaks around 3600 to 3650 cm − 1 are assigned to the stretching vibrational modes of hydroxyl groups, and the absorption peaks around 1640 cm − 1 correspond to the bending vibrational modes of absorbed water molecules. The peaks observed at about 450 to 700 cm − 1 are ascribed to Ni-O and Ni-OH [40]. The peaks observed in the region of 1400 to 1700 cm − 1 and 1200 to 1300 cm − 1 are ascribed to the silk fibroin and the peak intensity is increased as the concentration of silk fibroin increases. Notably, the absorption bands of silk fibroin around 1600–1700 are characteristic of amide I (C = O stretching vibrations), 1450–1500 cm − 1 are assigned to amide II (C-N stretching and the N-H in-plane bending vibrations) and 1200–1300 cm − 1 are associated with amide III (NH bending vibrations) [41]. Thermogravimetric analysis was carried out to gain information about the thermal behavior of the silk fibroin in biomineralized Ni(OH) 2 , Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5 (Fig. 2 d). The first weight loss below 160°C occurs due to the removal of surface and intercalated water molecules. For, Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5, the weight loss from 280 to 900°C corresponds to the loss of the polypeptides in silk fibroin. Of the total weight loss, 22% was observed for Ni(OH) 2 , whereas and 28% and 40% was observed for Ni(OH) 2 /SF-0.3 and Ni(OH) 2 /SF-0.5, respectively, indicating existence of silk fibroin in biomineralized Ni(OH) 2 . The main weight loss in all samples is attributed to the elimination of water molecules from Ni(OH) 2 to form NiO. Representative SEM images of the biomineralized Ni(OH) 2 displayed in Fig. 3 demonstrate the remarkable correlation between the concentration of silk fibroin and crystal morphology. As a comparison, a hydrothermal reaction in the absence of silk fibroin (Fig. 3 a and b) results in hexagonal platelets (approximately 100–200 nm in size) with a relatively smooth surface. Interestingly, at low fibroin concentration the biomineralized Ni(OH) 2 (Fig. 3 c-f) petal-like flakes self-assembled into microflower morphology with a size of 2.5 to 3 µm and individual flakes thickness is about 20–50 nm. The individual flakes exhibit curvature and interconnect to form microspheres. However, as the fibroin concentration increased (Fig. 3 g and h), nanoparticles were aggregated into plate-like structures instead of flower morphology. This kind of morphological change is probably owing to strong interaction between fibroin and primary nucleated nanoparticles on the surface that restrained the intrinsic crystal growth. The microflower structures appear similar at Ni(OH) 2 /SF-0.1 and Ni(OH) 2 /SF-0.3, but, in both cases, the morphology lacks uniformity and nanoflakes still remain without stacking. These results indicate that silk fibroin plays important roles in the Ni(OH) 2 precipitation process, i.e. precise control over hierarchical nanostructures and subsequent architecture tuning. TEM analysis was employed to the Ni(OH) 2 /SF-0.3 sample to obtain insights into the morphology and microstructure. The TEM image shown in Fig. 4 a reveals that petal-shaped nanoflakes self-assembled to a unique hollow microflower, which is consistent with the SEM image. An enlarged TEM image in Fig. 4 b-d indicates that petal-shaped nanoflakes of the shell part are relatively thick due to dense aggregation, with a thickness ranging from about 20 to 30 nm. The thin and soft flakes reflect the ultrathin feature. A closer observation in Fig. 4 c exhibits self-assembled Ni(OH) 2 flakes consist of smaller nanocrystallites. In addition, the high-angle annular dark-field scanning TEM (HAADF-STEM) image exhibits the presence of porous and hollow interior of Ni(OH) 2 /SF-0.3 particle. Elemental mapping by energy-dispersive X-ray spectroscopy (EDS) (Fig. 4 e) images of Ni(OH) 2 /SF-0.3 displays the homogenous distribution of Ni, O and C elements throughout the Ni(OH) 2 particle, indicating the existence of fibroin. XPS was examined to determine the elemental composition and detailed electronic states of Ni(OH) 2 /SF-0.3. The deconvoluted XPS spectra have been summarized in Fig. S1 . In the core level of Ni 2p spectra (Fig. S1 a), the two deconvoluted peaks at 857.6 and 875.0 eV are attributed to Ni 2+ 2p 3/2 and Ni 2+ 2p 1/2 , respectively. The spin-orbit splitting difference of the peak at around 17.4 eV confirms the presence of Ni 2+ . In addition, two peaks at higher binding energies of 860.0 and 878.5 eV and two accompanying peaks can be ascribed to the oxidized Ni species [34,42]. Fig. S1 b exhibits the O 1s spectrum, three peaks at 528.9, 531.8, and 533.7 eV are the characteristic peaks of M-O bond, hydroxide, and physisorbed/chemisorbed H 2 O, respectively [42]. 42 Moreover, C 1s spectra of Ni(OH) 2 /SF-0.3 (Fig. S1 c) can be deconvoluted to multiple peaks. The peaks in the range of 284.8, 286.1, 287.9 and 289.1 eV (C1, C2, C3 and C4,) were ascribed to silk fibroin carbons in –C–H– or –C–C–, –C–O–, C = O or –COOH and O–C = O groups, respectively [43]. In addition, the presence of silk fibroin carbon in Ni(OH) 2 /SF-0.3 can be confirmed by the Raman spectroscopy. As shown in Fig. S1 d, the two peaks located at 1350 and 1550 cm − 1 correspond to the sp 2 -type D band (distorted carbon) and G band (graphitic carbon), respectively. The specific surface area and pore diameter of Ni(OH) 2 /SF-0.3 were determined to be 13.3 m 2 g − 1 and 8.5 nm, respectively, by N 2 adsorption-desorption analysis. To determine the effect of Ni 2+ concentration on the uniformity of microflower morphology of biomineralized Ni(OH) 2 , the morphology of biomineralized Ni(OH) 2 crystals was monitored under various Ni 2+ concentration conditions. Fibroin concentration was fixed with 0.3% (w/v). At low Ni 2+ concentrations (20 or 30 mmol L − 1 ), thin and brittle flakes are formed because the nucleation and growth of nanoflakes as well as their self-assembly were hindered by silk fibroin. These flakes are easily aggregated and do not form a completely dense morphology as shown in Fig. 5 a to d. Particularly, only when Ni 2+ concentration reaches a certain value (40 mmol L − 1 ), biomineralized Ni(OH) 2 microflowers were produced. As shown in Fig. 5 e and f, the microflowers obtained at this condition are uniform in size and shape and well-dispersed. As the concentration of Ni 2+ increased (60 and 80 mmol L − 1 ), the morphology (Fig. 5 g-j) of microflowers collapses and its overall appearance becomes very irregular. That is, uniform flower-shaped spherical crystals are formed at specific Ni 2+ (40 mmol L − 1 ) and fibroin (0.3% w/v) concentrations. Furthermore, as shown in Fig. S2, the line broadening of the XRD peaks occurred at low concentrations of Ni 2+ due to stacking faults and interstratification defects. These types of defects are reduced at higher Ni 2+ concentrations. In general, upon addition of methanol or ethanol, silk fibroin quickly self-assembles into hydrogels because of strong polar effects, whereas DMF, an aprotic solvent, requires more time for gelation of fibroin [44]. Regenerated silk fibroin has a random coil structure, which converts to a to a β-sheet structure upon exposure to polar solvents. This transition is facilitated by the increased ability of polar solvents to remove the water molecules bound around the silk fibroin chains through intermolecular hydrogen bonding. The morphological changes of biomineralized Ni(OH) 2 /SF-0.3 in various mixed solvents are shown in Fig. 6 . Ni(OH) 2 /SF-0.3 synthesized in methanol/water mixed solvent exhibits a doughnut-shaped morphology, as shown in Fig. 6 a and 6 b. However, in ethanol/water solvent (Fig. 6 c and 6 d), nanoflakes aggregate to form a micro-pad structure. In DMF/water solvent (Fig. 6 e and 6 f), marigold flower-shaped Ni(OH) 2 /SF-0.3 crystals are produced. These results suggest that the hierarchical structures of biomineralized inorganic crystals using silk fibroin can be controlled by the addition of an appropriate organic solvent. Different solubility products of nickel salts may affect biomineralization process of Ni(OH) 2 and fibroin. Among different nickel sources, only nickel chloride and nickel acetate produce the 3d flower-like morphology, as shown in Fig. S3. Using nickel nitrate, non-uniform nanosheets of less than 500 nm were obtained. When nickel sulfate and perchlorate were used, thin plates of irregular shape were formed. To elucidate the detailed biomineralization process of Ni(OH) 2 and fibroin, time-dependent experiments were carried out and the morphological changes were monitored by SEM. The SEM images shown in Fig. 7 provide insight into how Ni(OH) 2 /fibroin nanoflakes form microflower structures through a self-assembly process. In the initial stages of the reaction, small crystals are formed as the Ni 2+ cations react with silk fibroin. As the reaction temperature was increased, silk fibroin begins to aggregate (or formation of micelles) due to polypeptide chain folding and hydrophobic interactions [45]. There may be a strong interaction between the polypeptide chains of silk fibroin and nickel ions, owing to amphiphilic nature of the fibroin. The nanocrystals of Ni(OH) 2 subsequently come together to form loosely attached aggregates, which induces the growth of flakes. As the reaction time increases, these aggregated flakes continue to grow and self-organize into spherical microflower structures. Occasionally, nanoflakes are assembled to much larger hollow structures instead of microflowers, as shown in Fig. 7 d. The hollow interior is believed to have been formed through a mechanism similar to Ostwald ripening [46]. In general, the individual nanosheets have high surface energy due to the presence of two main exposed planes [47], and nanosheets tend to aggregate each other into spherical clusters to minimize the surface energies. The initial solid phase may not be well crystallized and Ostwald ripening governs growth/recrystallization process. The inner crystallites dissolve and migrate outward, creating channels connecting inner space and outer space in the hydroxide shells [48]. During the hydrothermal process at 180°C, the silk fibroin macromolecular chain may be prone to degrade into several polypeptide chains. It is important to note that complete degradation into individual amino acids is not possible because decomposition temperature should be higher than 200°C. Concurrently, the essential GAGAGS motifs are likely to endure in these segments, as evidenced by earlier findings on thermal and enzyme degradation [49,50]. It is known that degradation begins in the amorphous region of the silk fibroin, which leaves the fundamental self-assembling capability unaffected, especially the significant GAGAGS motifs. This hierarchical self-assembly and morphological evolution mechanism is schematically illustrated in Fig. 8 . To understand the influence of structure and morphology on the electrochemical properties, CV and GCD of Ni(OH) 2 , Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5 were measured in 2 M KOH solution. Figure 9 a-c reveals the CV curves of the Ni(OH) 2 , Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5 samples at different scan rates ranging from 10 to 100 mV/s in the potential range 0-0.6 V, and each curve shows a distinct single pair of redox peaks. These observations demonstrate that the capacity response is driven by the oxidation of Ni 2+ to Ni 3+ , and the current response of the sample electrode trends to be further enhanced with increasing scan rate. Moreover, the peak current values of the Ni(OH) 2 /SF-0.3 (Fig. 9 b) are significantly larger than the Ni(OH) 2 /SF-0.5 (Fig. 9 c) and Ni(OH) 2 (Fig. 9 a). Lower peak separation was observed for Ni(OH) 2 /SF-0.5, which may be due to the increased concentration of silk fibroin. The peak current values of Ni(OH) 2 /fibroin decrease with increasing fibroin concentration, which is due to the reduction of the electrochemical surface area by aggregation. The redox reaction of Ni(OH) 2 consists of three key processes in an alkaline solution: the diffusion of the electrolyte to the electrode surface, the interaction of protons on the hydroxide structure and the electron transfer between the Ni(OH) 2 material and the electrode [51]. The electrochemical reaction of biomineralized Ni(OH) 2 electrode is limited by proton diffusion. Proton diffusion coefficient can be calculated from the Randles-Sevcik equation using CV experiments with varying scan rates. Fig. S4a shows that the connection between peak current, Ip and v 1/2 plot gives a reasonably linear relationship, thereby confirming that the electrode reaction is controlled by proton diffusion. The Ni(OH) 2 /SF-0.3 (5.54 × 10 − 9 cm 2 s − 1 ) shows a higher proton diffusion coefficient than the Ni(OH) 2 /SF-0.5 (1.27× 10 − 9 cm 2 s − 1 ) and Ni(OH) 2 (4.32 × 10 − 9 cm 2 s − 1 ). To further understand the ion transport mechanism of all the electrodes, the b values were obtained using the power law equation which is the constraint parameter and provided in Fig. S4b. The b values of all the electrodes are around 0.5, suggesting that energy storage is a diffusion-controlled process. The Nyquist plot in Fig. 9 g reveals that Ni(OH) 2 /SF-0.3 exhibits lower charge transfer resistance than Ni(OH) 2 /SF-0.5 and Ni(OH) 2 , indicating faster electron transfer at the electrode-electrolyte interface and enhanced ion diffusion within the electrode structure [52]. Figure (9d-f and h) and Table S2 show the discharge curves and respective calculated specific capacities for Ni(OH) 2 , Ni(OH) 2 /SF-0.3, and Ni(OH) 2 /SF-0.5. Each discharge plot reveals three identifiable regions corresponding to two sloping regions and a potential plateau region characteristic of battery-type behaviour. Briefly, the sloping regions are attributed to the pseudocapacitive behaviour arising from surface or near-surface charge storage, and the battery-type plateau region is closely related to redox interactions between Ni 2+ and Ni 3+ . Particularly, the discharge curve of Ni(OH) 2 /SF-0.3 exhibits higher discharge voltage and a longer plateau region than the other two samples, suggesting the excellent capacity of charge storage of Ni(OH) 2 /SF-0.3 electrode. Notably, the hybrid Ni(OH) 2 /SF-0.3 exhibits the highest specific capacity of 170 and 168.5 mAh g − 1 at the current density of 0.5 and 1 A g − 1 and even at the highest current density of 10 A g − 1 it delivers a specific capacity of 95 mA h g − 1 . The specific capacity of Ni(OH) 2 /SF-0.3 is also superior to the Ni(OH) 2 , Ni(OH) 2 /SF-0.5 and other Ni-based reported electrodes (Table S3). Incorporating a certain amount of silk fibroin into NiOH 2 results in a better charge-discharge profile due to shorter diffusion path lengths of ions, high ionic conductivity, and better contact on the electrode/electrolyte interface, which leads to an increase in the specific capacity of the material. The specific capacity of all samples (Fig. 9 h) implies a decreasing trend with increasing current density, which is due to rapid changes of polarization in the electrode, limiting access to active sites, limited duration for ions to sufficiently interact with the active substance and causing incomplete redox reactions. The assessment of electrode stability is imperative for practical applications of supercapacitors. Figure 9 i depicts the stability of GCD 2000 cycles of all electrodes, which were examined at the current density of 2 A g − 1 . The Ni(OH) 2 /SF-0.3 illustrates better cycling stability compared to other samples and capacity retention of 80% after continuous charge-discharge 2000 cycles. The capacity retention of Ni(OH) 2 /SF-0.3 after 500th and 1000th discharge cycles was calculated to be 87%. The Ni(OH) 2 /SF-0.3 and Ni(OH) 2 exhibits capacity retention of about 70% and 50% after 2000 cycles. The decrease in the specific capacity is ascribed to the alteration in mechanical stress of the electrode active material resulting from the insertion or de-insertion of electrolyte ions [53]. These results confirm the excellent cycling stability and rate performance of Ni(OH) 2 /SF-0.3. Overall, Ni(OH) 2 /SF-0.3 exhibits superior energy storage performance in comparison to Ni(OH) 2 /SF-0.5 and Ni(OH) 2 . The enhancement is attributed to its hierarchical hybrid nickel hydroxides hollow microflower, which provides a beneficial inner cavity and large surface area, increase the number of electrochemical active sites while mitigating structural strain during charge-discharge cycling [34,54]. The presence of β-sheet crystallites in the Ni(OH) 2 /SF-0.3 enhances the ionic conductivity of composites and facilitates efficient electron transport, contributing to improved electrochemical performance [55–57]. In contrast, the lower performance of Ni(OH) 2 /SF-0.5 is due to excessive fibroin content, which leads to particles aggregation, reduced active sites, and non-hollow structure morphology, thereby hindering charge storage capability. 4. CONCLUSION In summary, we found that silk fibroin can be act as a template to control the crystallization process of hierarchical hybrid nickel hydroxides hollow microflowers. The morphology of biomineralized Ni(OH) 2 is primarily determined by the silk fibroin and Ni 2+ concentration, but is also significantly influenced by the polarity of organic solvents governing the nucleation and growth processes. In addition, the anion of nickel salt appears to influence the formation of hierarchical microsphere morphology by anion-fibroin interactions. The hierarchical hollow structure of nickel hydroxides is thought to be formed by Ostwald ripening during the hydrothermal process. The resulting hybrid materials exhibit an excellent charge storage capability, enhanced proton diffusion coefficient, low charge-transfer resistance, and superior cyclic stability. These properties are attributed to its hollow structure, high ionic conductivity, the presence of β-sheet crystallites and structural disorders, which collectively contribute to improved electrochemical performance. This work highlights the biomimetic mineralization strategy for achieving controllable growth and nucleation of inorganic and biomaterial-based hybrid materials, offering a promising pathway for large-scale synthesis of functional materials with widespread applications. Declarations AUTHOR INFORMATION Corresponding Author *Jinkwon Kim - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea; Phone.: +82-41-850-8496; Fax: +82-41-850-8613. E-mail: [email protected] *Seog Woo Rhee - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea; [email protected] Author Siva Kumar Ramesh - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea Author Contributions Siva Kumar Ramesh - conceptualization, methodology, software, data curation, investigation and writing-original draft preparation, Seog Woo Rhee - project administration, supervision, and writing-reviewing and editing. Jinkwon Kim - conceptualization, project administration, supervision, and writing-reviewing and editing. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF-2021R1I1A2060355). The authors thank Oh Sung Hyun for the schematic Fig.s. 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16:19:44","extension":"html","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":97698,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/7adfa7dde76c0b01ba3c79d0.html"},{"id":98992102,"identity":"b86dee8f-6695-43f0-a73f-f1c771f3ac89","added_by":"auto","created_at":"2025-12-25 10:45:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":285433,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the synthesis of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/5105edf82df06e2d6c50590f.png"},{"id":99311880,"identity":"dddde219-86cb-4fd5-bb3a-f4b5852145eb","added_by":"auto","created_at":"2025-12-31 16:17:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":806446,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns, (b) Projection of XRD patterns, (c) IR, and (d) TGA of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/b6521b2a551f607dbbb47b98.png"},{"id":98992108,"identity":"a77a676c-91eb-4960-9924-e49f897776b5","added_by":"auto","created_at":"2025-12-25 10:45:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1504191,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of biomineralized Ni(OH)\u003csub\u003e2 \u003c/sub\u003eusing various fibroin concentrations with a constant Ni (II) concentration (60 mmol L\u003csup\u003e-1\u003c/sup\u003e): (a,b) Ni(OH)\u003csub\u003e2\u003c/sub\u003e, (c,d) Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.1, (e,f) Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and (g,h) Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5.\u0026nbsp;\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/66a1d5e8cd65ef1c86ee1663.png"},{"id":98992129,"identity":"9435a2dc-666b-460e-8201-284a1156471d","added_by":"auto","created_at":"2025-12-25 10:45:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":494278,"visible":true,"origin":"","legend":"\u003cp\u003eTEM and STEM images of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/da4e3585c828418b855d33c9.png"},{"id":98992105,"identity":"2452babc-c9bf-478b-b1cd-14bf1b6dd929","added_by":"auto","created_at":"2025-12-25 10:45:28","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1545901,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of biomineralized Ni(OH)\u003csub\u003e2 \u003c/sub\u003eusing various Ni\u003csup\u003e2+\u003c/sup\u003e concentrations with constant silk fibroin concentration (0.3% w/v): (a,b) 20 mmol L\u003csup\u003e-1\u003c/sup\u003e\u003csub\u003e,\u003c/sub\u003e (c,d) 30 mmol L\u003csup\u003e-1\u003c/sup\u003e, (e,f) 40 mmol L\u003csup\u003e-1\u003c/sup\u003e, (g,h) 60 mmol L\u003csup\u003e-1\u003c/sup\u003e,\u003csup\u003e \u003c/sup\u003eand (i,j) 80 mmol L\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/9842962727f4a45568385ce9.png"},{"id":98992103,"identity":"89b95dc4-a9a9-4bb1-8b8d-5446bb11119e","added_by":"auto","created_at":"2025-12-25 10:45:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1375795,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of biomineralized Ni(OH)\u003csub\u003e2 \u003c/sub\u003eusing mixed solvents: (a,b) methanol/water, (c,d) ethanol/water, and (e,f) DMF/water-treated biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/26a308fb248a8a59a1d4a8b2.png"},{"id":98992122,"identity":"d122f4c2-5b29-4b2e-bfd2-0e976e328914","added_by":"auto","created_at":"2025-12-25 10:45:29","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":717088,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the time-dependent experiment of biomineralized Ni(OH)\u003csub\u003e2 \u003c/sub\u003e(a) 30 minutes, (b) 1 h, and (c,d) 2 h.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/522af882454c073504fdcc9b.png"},{"id":99312224,"identity":"c03dbff7-c21f-4df7-82f6-55453e4b91be","added_by":"auto","created_at":"2025-12-31 16:18:21","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":504230,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of the growth process of the biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/d8f4e278c7cf87b0b0c147c4.png"},{"id":99312389,"identity":"27364b28-0617-46cb-81bd-dcd2e4f6ba70","added_by":"auto","created_at":"2025-12-31 16:18:55","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":300990,"visible":true,"origin":"","legend":"\u003cp\u003eCV curves and GCD curves of (a,d) Ni(OH)\u003csub\u003e2\u003c/sub\u003e, (b,e) Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, (c,f) Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5, (g) EIS, (h) Specific capacity values vs different current density, and (i) Cyclic performance of Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/14ccba5d2ffdae73c1aceaae.png"},{"id":106808827,"identity":"a05b87c8-bc08-40c4-ade2-ca2d61a13a13","added_by":"auto","created_at":"2026-04-13 16:02:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8132560,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/8c3b5dc7-4580-4c3e-a3bf-cb9437fff2ef.pdf"},{"id":99312215,"identity":"a505add0-1175-4a43-b14f-bf48ac5bccd1","added_by":"auto","created_at":"2025-12-31 16:18:20","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":302399,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSYNOPSIS TOC\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"Screenshot20251225160642.png","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/618a85ed90ed9363e99445a9.png"},{"id":99312986,"identity":"ea6be6e8-026d-40da-b9d4-c402413a0fed","added_by":"auto","created_at":"2025-12-31 16:19:41","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":17012143,"visible":true,"origin":"","legend":"","description":"","filename":"Supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-8385555/v1/073c7bfb8c8f5f7c279d6ed6.docx"}],"financialInterests":"","formattedTitle":"Silk Fibroin-Regulated Biomimetic Mineralization of Ni(OH)2 for Energy Storage Applications","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eThe integration of inorganic minerals with biomacromolecules is considered an indispensable process in human daily life and drives us to create supramolecular nanostructures with intricate forms and diverse functionalities [1,2]. This self-assembly process of biomaterials enables construction of well-defined hierarchical structures through environmentally friendly synthesis [3,4]. This approach will not only accelerate progress in the field of biomineralization, but also contribute to advances in materials science as well as biomedicines [5,6]. In this regard, the biomineralization process may provide a viable strategies to induce advantageous topological and material properties while maintaining high biocompatibility [7]. The nucleation and crystal growth steps of the crystallization process control the tunable properties such as shape, size, aggregation, orientation, and texture [8].\u003c/p\u003e \u003cp\u003eAmong various biomaterials, the silk fibroin, which is extracted from Bombyx mori and has a 3d fibrous network, has been the most extensively characterized in numerous studies presumably due to its robust mechanical strength, biocompatibility, and biodegradability [9\u0026ndash;11]. Silk fibroin comprises heavy chain (\u0026asymp;\u0026thinsp;390 kDa) and light chain (\u0026asymp;\u0026thinsp;26 kDa) proteins, linked by disulfide bonds, and the amino acids sequence constitutes a highly repetitive GAGAGS motif [12]. The unique properties of silk fibroin can be utilized to create a variety of material forms such as porous sponges [13], fibers [14,15], hydrogels [16], non-woven mats[17], tubes [18], and films[19]. This versatility allows silk fibroin to be applied in diverse applications such as tissue engineering [20], drug delivery systems [21], smart wearables [22], optical devices [23], and others [24]. Nguyen \u003cem\u003eet al\u003c/em\u003e. and Chen \u003cem\u003eet al\u003c/em\u003e. outlined the mechanisms of the gelation process of silk fibroin into the hierarchical network [25,26]. During this process, single domain networks assemble into a multidomain network upon the introduction of external agents. This interaction induces the growth of β-sheet crystallites, thereby promoting the formation of hierarchical structure. Particularly, hydrophilic groups and polar groups in the silk fibroin can strongly bind to metal ions, triggering biomineralization and crystallization [27].\u003c/p\u003e \u003cp\u003eOn the other hand, the exploration of advanced materials for energy storage devices emphasizes the growing demand for clean, sustainable energy solutions worldwide. Supercapacitors have become prominent energy storage devices and exhibit remarkable power performance, excellent reversibility, and an extended life cycle [28,29]. Moreover, Supercapacitors produce less thermochemical heat due to their simple charge storage mechanisms and are therefore widely used in industrial power and energy systems, consumer electronics, and memory backup systems [30]. Among various alternatives, transition metal hydroxides are considered industrially important electrochemical active materials due to its notable theoretical capacity [31,32]. In particular, β-Ni(OH)\u003csub\u003e2\u003c/sub\u003e is thermodynamically stable, easy to be crystallized, earth-abundant, and excellent in redox activity [33]. However, it suffers from poor conductivity, few electrode active sites on the surface, and structural instability. The integration of carbon materials and β-Ni(OH)\u003csub\u003e2\u003c/sub\u003e not only improves structural integrity but also provides performance by improving electrical conductivity and reducing the energy barrier for ion diffusion [34,35].\u003c/p\u003e \u003cp\u003eHerein, we demonstrated biomimetic mineralization to enable the preparation of a hierarchical hybrid nickel hydroxide hollow microflower by regulating the silk fibroin biomacromolecules as a template in a facile hydrothermal condition. The morphology of hybrid Ni(OH)\u003csub\u003e2\u003c/sub\u003e was optimized by controlling silk fibroin and Ni\u003csup\u003e2+\u003c/sup\u003e concentration and changing organic solvents and nickel precursors. Silk fibroin acts as a structural guide to form hierarchical microspheres while providing sites for nuclei formation and crystal growth during the nickel hydroxide precipitation process. Furthermore, biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e offers practical applications in energy storage. This hybrid material exhibits significantly better rate performance, proton diffusion coefficient, and cycling stability compared to the control sample due to its high ionic conductivity, presence of β-sheet crystallites, and structural disorder.\u003c/p\u003e"},{"header":"2. EXPERIMENTAL AND RESULTS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Synthesis of biomineralized nickel hydroxides\u003c/h2\u003e \u003cp\u003eSilk fibroin was extracted from Bombyx mori silkworm cocoons, as previously known method [36]. The final concentrations of regenerated silk fibroin concentrations were approximately 4% \u0026ndash; 6% (w/v) and stored at \u0026minus;\u0026thinsp;60\u0026deg;C until further use. 60 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of NiCl\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO was added to 50 mL of various fibroin concentrations (0 to 0.5% (w/v)) solution and NH\u003csub\u003e4\u003c/sub\u003eOH was added to the solution to adjust to pH 10. The solution was stirred for 30 minutes and transferred to an autoclave, which was heated at 180\u0026deg;C for 12 h. Then, after cooling the product was centrifuged, washed with DI water three times, and freeze-dried to obtain the final product. For control experiments, Ni(OH)\u003csub\u003e2\u003c/sub\u003e was prepared without fibroin solution using the above-mentioned method. To elucidate the effect of Ni\u003csup\u003e2+\u003c/sup\u003e concentration on the morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e, the silk fibroin concentration was kept constant at 0.3% (w/v) and the concentration of Ni\u003csup\u003e2+\u003c/sup\u003e was changed from 20 to 80 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Furthermore, to study the effect of the anion of nickel precursor on the morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e, the various nickel salts such as 60 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO, NiSO\u003csub\u003e4\u003c/sub\u003e\u0026sdot;7H\u003csub\u003e2\u003c/sub\u003eO, Ni(OCl\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO, and Ni(CH\u003csub\u003e3\u003c/sub\u003eCO\u003csub\u003e2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026sdot;4H\u003csub\u003e2\u003c/sub\u003eO were used instead of NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO with 0.3% (w/v) silk fibroin. Moreover, to study the effect of solvents on the morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e, 20 mL of ethanol, methanol and DMF and 30 mL of DI water were added into the 0.3% (w/v) silk fibroin and 60 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of NiCl\u003csub\u003e2\u003c/sub\u003e\u0026sdot;6H\u003csub\u003e2\u003c/sub\u003eO, instead of 50 mL of DI water. In addition, the time-dependent experiments were carried out with 0.3% (w/v) silk fibroin and 40 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of NiCl\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO. The sample codes of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e with 0, 0.1, 0.3, and 0.5% (w/v) are denoted as Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.1, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Materials Characterization\u003c/h2\u003e \u003cp\u003eThe powder X-ray diffraction (XRD) analysis was conducted on a Rigaku MiniFlex 600 diffractometer (Cu Kα radiation λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;) to characterize the crystal structures of the samples. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded on an IR affinity-1S, Shimadzu Co., spectrophotometer. Thermogravimetric analysis (TGA) was performed by the NETZSCH TGA instrument (TG_209_F1_Libra) at a heating rate of 10 K min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in an air atmosphere. Field emission scanning electron microscopy (FESEM, Hitachi HF-4800), transmission electron microscopy (TEM) with scanning transmission electron microscopy (STEM) (FETEM, Tecnai G\u003csup\u003e2\u003c/sup\u003e F30 S-Twin, acceleration voltage: 300 kV) were conducted to investigate the morphology and microstructure of the samples. The ASAP-2420 (Micromeritics, USA) instrument was used to determine the Brunauer\u0026ndash;Emmett\u0026ndash;Teller (BET) specific surface area and pore size distribution. X-ray photoelectron spectroscopy was conducted using a MultiLab ESCA 2000 XPS system to gain information about surface valence states.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Electrochemical Characterization\u003c/h2\u003e \u003cp\u003eAll electrochemical behaviors were investigated using a PGSTAT 302N Autolab (Metrohm) in a three-electrode system where nickel foam substrate (1 cm\u0026times;1 cm), a platinum rod, and Ag/AgCl (saturated KCl solution) served as the working electrode, counter electrode, and reference electrode, respectively and 2 M KOH used as the electrolyte solution. The slurry was prepared with active material, carbon black and polyvinylidene fluoride in \u003cem\u003eN\u003c/em\u003e-methyl pyrrolidone in a mass ratio of 8:1:1. Then the slurry was coated on a nickel foam substrate and dried in a vacuum oven at 60\u0026deg;C overnight. Cyclic voltammetry was carried out by applying various scan rates in the potential range of 0-0.6 V. Galvanostatic charge-discharge (GCD) experiments were performed with varying current densities (0.5, 1, 2, 3, 5, 7, and 10 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in the potential range of 0\u0026ndash;0.5 V. The specific capacity was calculated from the GCD equation C\u003csub\u003es\u003c/sub\u003e = I\u0026thinsp;\u0026times;\u0026thinsp;Δt/m\u0026thinsp;\u0026times;\u0026thinsp;ΔV where Cs is the specific capacitance, I is current, Δt is the discharge time, m is mass, and ΔV is the potential window. The results expressed in C g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were then converted to mA h g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e by dividing it by 3.6. The proton diffusion coefficient was determined using the Randles-Sevcik equation, Ip = (2.687 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e) n\u003csup\u003e3/2\u003c/sup\u003eACν\u003csup\u003e1/2\u003c/sup\u003eD\u003csup\u003e1/2\u003c/sup\u003e where Ip is the peak current, n is the number of electrons transferred during the redox reaction, A is the electrode surface area, C is the concentration of the electroactive species, D is the diffusion coefficient, and ν is the scan rate.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULT AND DISCUSSION","content":"\u003cp\u003eThe biomineralization of Ni(OH)\u003csub\u003e2\u003c/sub\u003e was performed by hydrothermal method using silk fibroin, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Proteins act as biotemplates in the biomineralization process and play a major role in controlling the size, shape, composition, and crystal structure of the inorganic materials. A series of experiments were conducted by changing the concentrations of silk fibroin from 0 to 0.5% (w/v) with a constant Ni\u003csup\u003e2+\u003c/sup\u003e concentration of 60 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in order to examine this biomineralization process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eX-ray diffraction analysis was carried out to determine the crystal phase of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e. XRD patterns of the contour heat map are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and b. The biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e and Ni(OH)\u003csub\u003e2\u003c/sub\u003e were assigned to β-NiOH (JCPDS-14-0117) with a hexagonal structure (a\u0026thinsp;=\u0026thinsp;3.126 \u0026Aring;, c\u0026thinsp;=\u0026thinsp;4.605 \u0026Aring;, space group P-3m1). The ideal Ni(OH)\u003csub\u003e2\u003c/sub\u003e consists of edge-sharing NiO\u003csub\u003e6\u003c/sub\u003e octahedra, oxygen packing is ABAB and hydrogen atoms are located in tetrahedral environments exactly above or below the oxygen atoms and in this packing HO\u003csub\u003e4\u003c/sub\u003e tetrahedra share only edges and it obeys Pauling\u0026rsquo;s third rule. When the fibroin concentration increases in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, we can see the line broadening of the peaks such as (001) and (h0l) peaks presumably owing to the stacking faults and interstratification, respectively [37,38]. In detail, the stacking faults lead to two types: growth faults and deformation faults, which causes oxygen atoms to migrate to C position and form fcc blocks. The oxygen packings are randomly distributed in the growth faults (ABABCBCBCBABAB) and deformation faults (ABABCACABCBCABAB) [38,39]. A typical stacking fault pattern is induced as the fibroin concentration increases and the width of the (10l) peaks broadens as the amount of stacking faults increases. The electrostatic interaction between hydrogen atoms and nickel ions is very low in the ideal structure (ABAB), whereas the samples with stacking faults (ABCB or ABCA) show higher electrostatic interaction between two polyhedra sharing one face, which violates Pauling's third rule. Therefore, in the samples with stacking faults, hydrogen atoms are expected to be more destabilized and have higher electrochemical activity [38]. Moreover, interstratification appears to broaden all the non-hk0 reflections. The (001) peak also broadens when more fibroin is present in the sample, which may be related to the presence of a few percentages of α-motifs [39]. Particularly, (100) and (110) reflections remain essentially unchanged in all samples.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec shows the IR spectra of Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.1, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5. The sharp absorption peaks around 3600 to 3650 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to the stretching vibrational modes of hydroxyl groups, and the absorption peaks around 1640 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the bending vibrational modes of absorbed water molecules. The peaks observed at about 450 to 700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are ascribed to Ni-O and Ni-OH [40]. The peaks observed in the region of 1400 to 1700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1200 to 1300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are ascribed to the silk fibroin and the peak intensity is increased as the concentration of silk fibroin increases. Notably, the absorption bands of silk fibroin around 1600\u0026ndash;1700 are characteristic of amide I (C\u0026thinsp;=\u0026thinsp;O stretching vibrations), 1450\u0026ndash;1500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to amide II (C-N stretching and the N-H in-plane bending vibrations) and 1200\u0026ndash;1300 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are associated with amide III (NH bending vibrations) [41].\u003c/p\u003e \u003cp\u003eThermogravimetric analysis was carried out to gain information about the thermal behavior of the silk fibroin in biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). The first weight loss below 160\u0026deg;C occurs due to the removal of surface and intercalated water molecules. For, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5, the weight loss from 280 to 900\u0026deg;C corresponds to the loss of the polypeptides in silk fibroin. Of the total weight loss, 22% was observed for Ni(OH)\u003csub\u003e2\u003c/sub\u003e, whereas and 28% and 40% was observed for Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5, respectively, indicating existence of silk fibroin in biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e. The main weight loss in all samples is attributed to the elimination of water molecules from Ni(OH)\u003csub\u003e2\u003c/sub\u003e to form NiO.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRepresentative SEM images of the biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e demonstrate the remarkable correlation between the concentration of silk fibroin and crystal morphology. As a comparison, a hydrothermal reaction in the absence of silk fibroin (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and b) results in hexagonal platelets (approximately 100\u0026ndash;200 nm in size) with a relatively smooth surface. Interestingly, at low fibroin concentration the biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-f) petal-like flakes self-assembled into microflower morphology with a size of 2.5 to 3 \u0026micro;m and individual flakes thickness is about 20\u0026ndash;50 nm. The individual flakes exhibit curvature and interconnect to form microspheres. However, as the fibroin concentration increased (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg and h), nanoparticles were aggregated into plate-like structures instead of flower morphology. This kind of morphological change is probably owing to strong interaction between fibroin and primary nucleated nanoparticles on the surface that restrained the intrinsic crystal growth. The microflower structures appear similar at Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.1 and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, but, in both cases, the morphology lacks uniformity and nanoflakes still remain without stacking. These results indicate that silk fibroin plays important roles in the Ni(OH)\u003csub\u003e2\u003c/sub\u003e precipitation process, i.e. precise control over hierarchical nanostructures and subsequent architecture tuning. TEM analysis was employed to the Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 sample to obtain insights into the morphology and microstructure. The TEM image shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea reveals that petal-shaped nanoflakes self-assembled to a unique hollow microflower, which is consistent with the SEM image. An enlarged TEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb-d indicates that petal-shaped nanoflakes of the shell part are relatively thick due to dense aggregation, with a thickness ranging from about 20 to 30 nm. The thin and soft flakes reflect the ultrathin feature. A closer observation in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec exhibits self-assembled Ni(OH)\u003csub\u003e2\u003c/sub\u003e flakes consist of smaller nanocrystallites. In addition, the high-angle annular dark-field scanning TEM (HAADF-STEM) image exhibits the presence of porous and hollow interior of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 particle. Elemental mapping by energy-dispersive X-ray spectroscopy (EDS) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee) images of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 displays the homogenous distribution of Ni, O and C elements throughout the Ni(OH)\u003csub\u003e2\u003c/sub\u003e particle, indicating the existence of fibroin.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eXPS was examined to determine the elemental composition and detailed electronic states of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3. The deconvoluted XPS spectra have been summarized in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e. In the core level of Ni 2p spectra (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea), the two deconvoluted peaks at 857.6 and 875.0 eV are attributed to Ni\u003csup\u003e2+\u003c/sup\u003e 2p\u003csub\u003e3/2\u003c/sub\u003e and Ni\u003csup\u003e2+\u003c/sup\u003e 2p\u003csub\u003e1/2\u003c/sub\u003e, respectively. The spin-orbit splitting difference of the peak at around 17.4 eV confirms the presence of Ni\u003csup\u003e2+\u003c/sup\u003e. In addition, two peaks at higher binding energies of 860.0 and 878.5 eV and two accompanying peaks can be ascribed to the oxidized Ni species [34,42]. Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb exhibits the O 1s spectrum, three peaks at 528.9, 531.8, and 533.7 eV are the characteristic peaks of M-O bond, hydroxide, and physisorbed/chemisorbed H\u003csub\u003e2\u003c/sub\u003eO, respectively [42].\u003csup\u003e42\u003c/sup\u003e Moreover, C 1s spectra of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ec) can be deconvoluted to multiple peaks. The peaks in the range of 284.8, 286.1, 287.9 and 289.1 eV (C1, C2, C3 and C4,) were ascribed to silk fibroin carbons in \u0026ndash;C\u0026ndash;H\u0026ndash; or \u0026ndash;C\u0026ndash;C\u0026ndash;, \u0026ndash;C\u0026ndash;O\u0026ndash;, C\u0026thinsp;=\u0026thinsp;O or \u0026ndash;COOH and O\u0026ndash;C\u0026thinsp;=\u0026thinsp;O groups, respectively [43]. In addition, the presence of silk fibroin carbon in Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 can be confirmed by the Raman spectroscopy. As shown in Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ed, the two peaks located at 1350 and 1550 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e correspond to the sp\u003csup\u003e2\u003c/sup\u003e-type D band (distorted carbon) and G band (graphitic carbon), respectively. The specific surface area and pore diameter of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 were determined to be 13.3 m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 8.5 nm, respectively, by N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo determine the effect of Ni\u003csup\u003e2+\u003c/sup\u003e concentration on the uniformity of microflower morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e, the morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e crystals was monitored under various Ni\u003csup\u003e2+\u003c/sup\u003e concentration conditions. Fibroin concentration was fixed with 0.3% (w/v). At low Ni\u003csup\u003e2+\u003c/sup\u003e concentrations (20 or 30 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), thin and brittle flakes are formed because the nucleation and growth of nanoflakes as well as their self-assembly were hindered by silk fibroin. These flakes are easily aggregated and do not form a completely dense morphology as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea to d. Particularly, only when Ni\u003csup\u003e2+\u003c/sup\u003e concentration reaches a certain value (40 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e microflowers were produced. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and f, the microflowers obtained at this condition are uniform in size and shape and well-dispersed. As the concentration of Ni\u003csup\u003e2+\u003c/sup\u003e increased (60 and 80 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), the morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eg-j) of microflowers collapses and its overall appearance becomes very irregular. That is, uniform flower-shaped spherical crystals are formed at specific Ni\u003csup\u003e2+\u003c/sup\u003e (40 mmol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and fibroin (0.3% w/v) concentrations. Furthermore, as shown in Fig. S2, the line broadening of the XRD peaks occurred at low concentrations of Ni\u003csup\u003e2+\u003c/sup\u003e due to stacking faults and interstratification defects. These types of defects are reduced at higher Ni\u003csup\u003e2+\u003c/sup\u003e concentrations.\u003c/p\u003e \u003cp\u003eIn general, upon addition of methanol or ethanol, silk fibroin quickly self-assembles into hydrogels because of strong polar effects, whereas DMF, an aprotic solvent, requires more time for gelation of fibroin [44]. Regenerated silk fibroin has a random coil structure, which converts to a to a β-sheet structure upon exposure to polar solvents. This transition is facilitated by the increased ability of polar solvents to remove the water molecules bound around the silk fibroin chains through intermolecular hydrogen bonding. The morphological changes of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 in various mixed solvents are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 synthesized in methanol/water mixed solvent exhibits a doughnut-shaped morphology, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb. However, in ethanol/water solvent (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed), nanoflakes aggregate to form a micro-pad structure. In DMF/water solvent (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee and \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef), marigold flower-shaped Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 crystals are produced. These results suggest that the hierarchical structures of biomineralized inorganic crystals using silk fibroin can be controlled by the addition of an appropriate organic solvent.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDifferent solubility products of nickel salts may affect biomineralization process of Ni(OH)\u003csub\u003e2\u003c/sub\u003e and fibroin. Among different nickel sources, only nickel chloride and nickel acetate produce the 3d flower-like morphology, as shown in Fig. S3. Using nickel nitrate, non-uniform nanosheets of less than 500 nm were obtained. When nickel sulfate and perchlorate were used, thin plates of irregular shape were formed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo elucidate the detailed biomineralization process of Ni(OH)\u003csub\u003e2\u003c/sub\u003e and fibroin, time-dependent experiments were carried out and the morphological changes were monitored by SEM. The SEM images shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e provide insight into how Ni(OH)\u003csub\u003e2\u003c/sub\u003e/fibroin nanoflakes form microflower structures through a self-assembly process. In the initial stages of the reaction, small crystals are formed as the Ni\u003csup\u003e2+\u003c/sup\u003e cations react with silk fibroin. As the reaction temperature was increased, silk fibroin begins to aggregate (or formation of micelles) due to polypeptide chain folding and hydrophobic interactions [45]. There may be a strong interaction between the polypeptide chains of silk fibroin and nickel ions, owing to amphiphilic nature of the fibroin. The nanocrystals of Ni(OH)\u003csub\u003e2\u003c/sub\u003e subsequently come together to form loosely attached aggregates, which induces the growth of flakes. As the reaction time increases, these aggregated flakes continue to grow and self-organize into spherical microflower structures. Occasionally, nanoflakes are assembled to much larger hollow structures instead of microflowers, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ed. The hollow interior is believed to have been formed through a mechanism similar to Ostwald ripening [46]. In general, the individual nanosheets have high surface energy due to the presence of two main exposed planes [47], and nanosheets tend to aggregate each other into spherical clusters to minimize the surface energies. The initial solid phase may not be well crystallized and Ostwald ripening governs growth/recrystallization process. The inner crystallites dissolve and migrate outward, creating channels connecting inner space and outer space in the hydroxide shells [48]. During the hydrothermal process at 180\u0026deg;C, the silk fibroin macromolecular chain may be prone to degrade into several polypeptide chains. It is important to note that complete degradation into individual amino acids is not possible because decomposition temperature should be higher than 200\u0026deg;C. Concurrently, the essential GAGAGS motifs are likely to endure in these segments, as evidenced by earlier findings on thermal and enzyme degradation [49,50]. It is known that degradation begins in the amorphous region of the silk fibroin, which leaves the fundamental self-assembling capability unaffected, especially the significant GAGAGS motifs. This hierarchical self-assembly and morphological evolution mechanism is schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo understand the influence of structure and morphology on the electrochemical properties, CV and GCD of Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 were measured in 2 M KOH solution. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea-c reveals the CV curves of the Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 samples at different scan rates ranging from 10 to 100 mV/s in the potential range 0-0.6 V, and each curve shows a distinct single pair of redox peaks. These observations demonstrate that the capacity response is driven by the oxidation of Ni\u003csup\u003e2+\u003c/sup\u003e to Ni\u003csup\u003e3+\u003c/sup\u003e, and the current response of the sample electrode trends to be further enhanced with increasing scan rate. Moreover, the peak current values of the Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb) are significantly larger than the Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ec) and Ni(OH)\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea). Lower peak separation was observed for Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5, which may be due to the increased concentration of silk fibroin. The peak current values of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/fibroin decrease with increasing fibroin concentration, which is due to the reduction of the electrochemical surface area by aggregation. The redox reaction of Ni(OH)\u003csub\u003e2\u003c/sub\u003e consists of three key processes in an alkaline solution: the diffusion of the electrolyte to the electrode surface, the interaction of protons on the hydroxide structure and the electron transfer between the Ni(OH)\u003csub\u003e2\u003c/sub\u003e material and the electrode [51].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical reaction of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e electrode is limited by proton diffusion. Proton diffusion coefficient can be calculated from the Randles-Sevcik equation using CV experiments with varying scan rates. Fig. S4a shows that the connection between peak current, Ip and v\u003csup\u003e1/2\u003c/sup\u003e plot gives a reasonably linear relationship, thereby confirming that the electrode reaction is controlled by proton diffusion. The Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 (5.54 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) shows a higher proton diffusion coefficient than the Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 (1.27\u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and Ni(OH)\u003csub\u003e2\u003c/sub\u003e (4.32 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;9\u003c/sup\u003e cm\u003csup\u003e2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). To further understand the ion transport mechanism of all the electrodes, the b values were obtained using the power law equation which is the constraint parameter and provided in Fig. S4b. The b values of all the electrodes are around 0.5, suggesting that energy storage is a diffusion-controlled process. The Nyquist plot in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eg reveals that Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 exhibits lower charge transfer resistance than Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 and Ni(OH)\u003csub\u003e2\u003c/sub\u003e, indicating faster electron transfer at the electrode-electrolyte interface and enhanced ion diffusion within the electrode structure [52]. Figure\u0026nbsp;(9d-f and h) and Table S2 show the discharge curves and respective calculated specific capacities for Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3, and Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5. Each discharge plot reveals three identifiable regions corresponding to two sloping regions and a potential plateau region characteristic of battery-type behaviour. Briefly, the sloping regions are attributed to the pseudocapacitive behaviour arising from surface or near-surface charge storage, and the battery-type plateau region is closely related to redox interactions between Ni\u003csup\u003e2+\u003c/sup\u003e and Ni\u003csup\u003e3+\u003c/sup\u003e. Particularly, the discharge curve of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 exhibits higher discharge voltage and a longer plateau region than the other two samples, suggesting the excellent capacity of charge storage of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 electrode. Notably, the hybrid Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 exhibits the highest specific capacity of 170 and 168.5 mAh g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the current density of 0.5 and 1 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and even at the highest current density of 10 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e it delivers a specific capacity of 95 mA h g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The specific capacity of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 is also superior to the Ni(OH)\u003csub\u003e2\u003c/sub\u003e, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 and other Ni-based reported electrodes (Table S3). Incorporating a certain amount of silk fibroin into NiOH\u003csub\u003e2\u003c/sub\u003e results in a better charge-discharge profile due to shorter diffusion path lengths of ions, high ionic conductivity, and better contact on the electrode/electrolyte interface, which leads to an increase in the specific capacity of the material. The specific capacity of all samples (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eh) implies a decreasing trend with increasing current density, which is due to rapid changes of polarization in the electrode, limiting access to active sites, limited duration for ions to sufficiently interact with the active substance and causing incomplete redox reactions.\u003c/p\u003e \u003cp\u003eThe assessment of electrode stability is imperative for practical applications of supercapacitors. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ei depicts the stability of GCD 2000 cycles of all electrodes, which were examined at the current density of 2 A g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 illustrates better cycling stability compared to other samples and capacity retention of 80% after continuous charge-discharge 2000 cycles. The capacity retention of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 after 500th and 1000th discharge cycles was calculated to be 87%. The Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 and Ni(OH)\u003csub\u003e2\u003c/sub\u003e exhibits capacity retention of about 70% and 50% after 2000 cycles. The decrease in the specific capacity is ascribed to the alteration in mechanical stress of the electrode active material resulting from the insertion or de-insertion of electrolyte ions [53]. These results confirm the excellent cycling stability and rate performance of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3.\u003c/p\u003e \u003cp\u003eOverall, Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 exhibits superior energy storage performance in comparison to Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 and Ni(OH)\u003csub\u003e2\u003c/sub\u003e. The enhancement is attributed to its hierarchical hybrid nickel hydroxides hollow microflower, which provides a beneficial inner cavity and large surface area, increase the number of electrochemical active sites while mitigating structural strain during charge-discharge cycling [34,54]. The presence of β-sheet crystallites in the Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.3 enhances the ionic conductivity of composites and facilitates efficient electron transport, contributing to improved electrochemical performance [55\u0026ndash;57]. In contrast, the lower performance of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/SF-0.5 is due to excessive fibroin content, which leads to particles aggregation, reduced active sites, and non-hollow structure morphology, thereby hindering charge storage capability.\u003c/p\u003e"},{"header":"4. CONCLUSION","content":"\u003cp\u003eIn summary, we found that silk fibroin can be act as a template to control the crystallization process of hierarchical hybrid nickel hydroxides hollow microflowers. The morphology of biomineralized Ni(OH)\u003csub\u003e2\u003c/sub\u003e is primarily determined by the silk fibroin and Ni\u003csup\u003e2+\u003c/sup\u003e concentration, but is also significantly influenced by the polarity of organic solvents governing the nucleation and growth processes. In addition, the anion of nickel salt appears to influence the formation of hierarchical microsphere morphology by anion-fibroin interactions. The hierarchical hollow structure of nickel hydroxides is thought to be formed by Ostwald ripening during the hydrothermal process. The resulting hybrid materials exhibit an excellent charge storage capability, enhanced proton diffusion coefficient, low charge-transfer resistance, and superior cyclic stability. These properties are attributed to its hollow structure, high ionic conductivity, the presence of β-sheet crystallites and structural disorders, which collectively contribute to improved electrochemical performance. This work highlights the biomimetic mineralization strategy for achieving controllable growth and nucleation of inorganic and biomaterial-based hybrid materials, offering a promising pathway for large-scale synthesis of functional materials with widespread applications.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAUTHOR INFORMATION\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding Author\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e*Jinkwon Kim - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea; Phone.: +82-41-850-8496; Fax: +82-41-850-8613.\u003cstrong\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/strong\u003eE-mail: [email protected]\u003c/p\u003e\n\u003cp\u003e*Seog Woo Rhee - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea; [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSiva Kumar Ramesh - Department of Chemistry, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam-do 32588, Republic of Korea\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSiva Kumar Ramesh\u003c/strong\u003e - conceptualization, methodology, software, data curation, investigation and writing-original draft preparation,\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSeog Woo Rhee\u003c/strong\u003e - project administration, supervision, and writing-reviewing and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eJinkwon Kim\u003c/strong\u003e - conceptualization, project administration, supervision, and writing-reviewing and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing financial interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Research Foundation of Korea (NRF-2021R1I1A2060355). The authors thank Oh Sung Hyun for the schematic Fig.s.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request. The data supporting this article has been included as part of the ESI.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eP. Dubey, S. Murab, S. Karmakar, P.K. Chowdhury, S. Ghosh, Modulation of Self-Assembly Process of Fibroin: An Insight for Regulating the Conformation of Silk Biomaterials, Biomacromolecules \u003cstrong\u003e16\u003c/strong\u003e 3936\u0026ndash;3944 (2015). https://doi.org/10.1021/acs.biomac.5b01258.\u003c/li\u003e\n \u003cli\u003eJ.J. De Yoreo, P.U.P.A. Gilbert, N.A.J.M. Sommerdijk, R.L. Penn, S. Whitelam, D. Joester, H. Zhang, J.D. Rimer, A. Navrotsky, J.F. Banfield, A.F. Wallace, F.M. Michel, F.C. Meldrum, H. C\u0026ouml;lfen, P.M. 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Today Nano \u003cstrong\u003e24\u003c/strong\u003e 100399 (2023). https://doi.org/10.1016/j.mtnano.2023.100399.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Biomineralization, Biomimetic materials, Silk fibroin, Ni(OH)2, Energy storage","lastPublishedDoi":"10.21203/rs.3.rs-8385555/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8385555/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUnderstanding and controlling biomimetic hybrid materials is essential to obtain novel hierarchical structures and expand the range of potential applications. Here, the biomineralization approach was applied as a rational integration method of silk fibroin and nickel hydroxide. Under protein-directed self-assembly conditions, hollow structures of hierarchical hybrid nickel hydroxide microflowers were systematically formed due to structural disorders such as stacking faults. Notably, the uniform, well-dispersed, and tunable morphology of biomineralized hybrid nickel hydroxides hollow microflowers is achieved by controlling the concentration of silk fibroin and nickel (II) chloride. The effects of organic solvents and nickel precursors on the morphology of nickel hydroxides were also elucidated. The fibroin/Ni(OH)\u003csub\u003e2\u003c/sub\u003e hybrid material exhibits good charge storage performance, rate capability, proton diffusion coefficient, and outstanding cyclic stability owing to its high ionic conductivity and the presence of β-sheet crystallites.\u003c/p\u003e","manuscriptTitle":"Silk Fibroin-Regulated Biomimetic Mineralization of Ni(OH)2 for Energy Storage Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-25 10:45:23","doi":"10.21203/rs.3.rs-8385555/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-12-24T01:05:16+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-24T00:47:36+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-23T08:38:33+00:00","index":"","fulltext":""},{"type":"submitted","content":"Korean Journal of Chemical Engineering","date":"2025-12-17T07:14:43+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"korean-journal-of-chemical-engineering","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"kjce","sideBox":"Learn more about [Korean Journal of Chemical Engineering](http://link.springer.com/journal/11814)","snPcode":"11814","submissionUrl":"https://www.editorialmanager.com/kjce/default2.aspx","title":"Korean Journal of Chemical Engineering","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Subscription","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ce8fbfcf-6faf-47f8-98af-c433469c48e5","owner":[],"postedDate":"December 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-13T16:00:39+00:00","versionOfRecord":{"articleIdentity":"rs-8385555","link":"https://doi.org/10.1007/s11814-026-00688-1","journal":{"identity":"korean-journal-of-chemical-engineering","isVorOnly":false,"title":"Korean Journal of Chemical Engineering"},"publishedOn":"2026-04-11 15:57:20","publishedOnDateReadable":"April 11th, 2026"},"versionCreatedAt":"2025-12-25 10:45:23","video":"","vorDoi":"10.1007/s11814-026-00688-1","vorDoiUrl":"https://doi.org/10.1007/s11814-026-00688-1","workflowStages":[]},"version":"v1","identity":"rs-8385555","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8385555","identity":"rs-8385555","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}