Sustainable Materials from Fish Industry Waste for Electrochemical Energy Systems

Fish industry waste is attracting growing interest for the production of environmentally friendly materials for several different applications, due to the potential for reduced environmental impact and increased socioeconomic benefits. Recently, the application of fish industry waste for the synthesis of value-added materials and energy storage systems represents a feasible route to strengthen the overall sustainability of energy storage product lines. This review focused on an in-depth outlook on the advances in fish byproduct-derived materials for energy storage devices, including lithium-ion batteries (LIBs), sodium-ion (NIBs) batteries, lithium-sulfur batteries (LSBs), supercapacitors and protein batteries. For each of these, the latest applications were presented together with approaches to improve the electrochemical performance of the obtained materials. By analyzing the recent literature on this topic, this review aimed to contribute to further advances in the sustainability of energy storage devices.


Introduction
The energy crisis, climate change, increased energy consumption and growing awareness of environmental protection needs have imposed the challenge of sustainable development, pushing industrial and academic research toward efficient, clean, ecological and high-performance materials and equipment for energy storage and conversion [1]. The energy produced by renewable resources needs to be stored by electrochemical energy storage devices from which it can be extracted at a later time to perform necessary tasks [2]. These devices are required to have increasingly improved energy and power density. Moreover, electrochemical energy storage technology is crucial for the sustainable development of wearable electronics [3]. Therefore, it is essential to find high-performing, low-cost and environmentally friendly materials. Additionally, the developed materials should be able to be produced at a large-scale for usage in various industries [4][5][6][7][8].
One of the most invaluable, renewable and sustainable resources for the synthesis of high-performance materials for energy storage is biomass [9]. The term biomass indicates all renewable organic materials deriving from plants, algae, trees, crops, wood wastes, agricultural and forestry wastes, animal and poultry wastes, fishery and aquaculture waste and food processing waste [10,11]. In 2016, the total biomass waste in the world was approximately 550 gigatons of carbon, and is increasing every year [12]. This waste is either burnt or left in the ocean which leads to environmental pollution and the emission of greenhouse gas [13,14]. Biomass is exploited for energy production through thermochemical processes, including combustion, gasification and pyrolysis, and biochemical processes, including fermentation and anaerobic digestion [10,15,16].
Thanks to biomass' recyclability, abundance and low cost, the application of biomass as a precursor to produce green carbon materials for energy storage is economically and higher than the material produced for LIBs (about ten thousand tons for both anodes and cathodes) [72].
The increasing interest toward the valorization of fish waste into sustainable materials for electrochemical storage systems is highlighted by the growing number of scientific publications on this topic during the last decade and, in particular, in the last three years, as reported in Figure 1. Although the production and application of carbon materials from overall biomass were extensively reviewed [3,73,74], very little attention has been paid to value-added carbons derived from fish industry waste, which are only briefly described. In the authors' opinion, the very significant and recent advances in fish waste-derived materials deserve to be reviewed, owing to their strategic importance for the sustainability of energy storage devices.
This review focused on the application of biomass derived from fish industry waste as a sustainable material for energy storage devices including lithium-ion batteries (LIBs), sodium-ion (NIBs) batteries, lithium-sulfur batteries (LSBs), supercapacitors and protein batteries. For each, recent applications over the last decade were presented, together with strategies to improve the performance of the obtained materials.

Applications in Lithium-Ion Batteries (LIBs)
To date, the most developed electrochemical energy storage devices are lithium-ion batteries (LIBs), which are currently applied in various fields including smartphones, laptops and electric vehicles, owing to their relatively high energy density and long cycle life [75][76][77]. Nevertheless, commercial graphite anodes cannot satisfy the increasing demand of the high energy density in LIBs. Predictions have claimed that the demand for lithium will be tripled by the year 2025 [78]. Another limitation is represented by the massive anode volume changes during Li + insertion and extraction, which leads to the pulverization of the lithium−alloy particles and fast capacity drop during chargedischarge cycles [79]. To overcome these limitations, research on alternatives for graphite anodes has focused on nanoporous carbons (NPCs). NPCs have drawn interest because of their potentially higher specific capacity and stability and their well-organized porous structure. These can prepare rapid ion diffusion channels, which is advantageous when attempting to obtain high Li + storage capacity [35]. A variety of NPCs have been investigated, such as carbon nanofibers [80,81] carbon nanocages [82,83], nitrogen- Although the production and application of carbon materials from overall biomass were extensively reviewed [3,73,74], very little attention has been paid to value-added carbons derived from fish industry waste, which are only briefly described. In the authors' opinion, the very significant and recent advances in fish waste-derived materials deserve to be reviewed, owing to their strategic importance for the sustainability of energy storage devices.
This review focused on the application of biomass derived from fish industry waste as a sustainable material for energy storage devices including lithium-ion batteries (LIBs), sodium-ion (NIBs) batteries, lithium-sulfur batteries (LSBs), supercapacitors and protein batteries. For each, recent applications over the last decade were presented, together with strategies to improve the performance of the obtained materials.

Applications in Lithium-Ion Batteries (LIBs)
To date, the most developed electrochemical energy storage devices are lithium-ion batteries (LIBs), which are currently applied in various fields including smartphones, laptops and electric vehicles, owing to their relatively high energy density and long cycle life [75][76][77]. Nevertheless, commercial graphite anodes cannot satisfy the increasing demand of the high energy density in LIBs. Predictions have claimed that the demand for lithium will be tripled by the year 2025 [78]. Another limitation is represented by the massive anode volume changes during Li + insertion and extraction, which leads to the pulverization of the lithium-alloy particles and fast capacity drop during charge-discharge cycles [79]. To overcome these limitations, research on alternatives for graphite anodes has focused on nanoporous carbons (NPCs). NPCs have drawn interest because of their potentially higher specific capacity and stability and their well-organized porous structure. These can prepare rapid ion diffusion channels, which is advantageous when attempting to obtain high Li + storage capacity [35]. A variety of NPCs have been investigated, such as carbon nanofibers [80,81] carbon nanocages [82,83], nitrogen-enriched nanocarbons [84,85], etc. Their porous structures can reduce the diffusion length of Li-ions, while their high specific surface area offers abundant active sites for Li + storage reactions [86].
Recent literature demonstrates that fish waste can successfully be used as a sustainable source for nanoporous carbon materials; it is enriched with elements such as nitrogen, oxygen, hydrogen and sulfur, and characterized by cost-effectiveness and thermal stability. Depending on the type of fish industry waste, different routes have been reported in recent studies for obtaining porous carbon electrodes for Li-ion batteries, as schematically reported in Figure 2. reactions [86].
Recent literature demonstrates that fish waste can successfully be used as a sustainable source for nanoporous carbon materials; it is enriched with elements such as nitrogen, oxygen, hydrogen and sulfur, and characterized by cost-effectiveness and thermal stability. Depending on the type of fish industry waste, different routes have been reported in recent studies for obtaining porous carbon electrodes for Li-ion batteries, as schematically reported in Figure 2. Crustacean shells are effective biotemplates for preparing nanostructured anodes for rechargeable Li-ion batteries, as demonstrated by Yao et al. [72]. They obtained hollow carbon nanofibers from crab shells encapsulating sulfur and silicon. The processing route involved several steps, as schematized in Figure 2. After air calcination of the crab shells, the organic components were removed and CaCO3 templates containing twisted hollow nanometric channels were obtained, with diameters close to those of commonly anodized aluminum oxide templates. The CaCO3 framework was coated with a thin layer of carbon via heat treatment in nitrogen. Then, the obtained active electrodes were inserted into the nanochannels, where they were treated by sulfur and silicon through thermal infusion and chemical vapor deposition, respectively. After dissolving the CaCO3 framework by acid treatment, the researchers obtained hollow carbon nanofiber arrays encapsulating sulfur or silicon. The hollow nanostructures provided sufficient space for the volume expansion of sulfur/silicon during the discharge/charge processes and the thin walls of the hollow carbon nanofibers allowed rapid lithium-ion transport from the electrolyte to sulfur/silicon. As reported in Table 1, the Li-ion battery prepared with this crab shell- Crustacean shells are effective biotemplates for preparing nanostructured anodes for rechargeable Li-ion batteries, as demonstrated by Yao et al. [72]. They obtained hollow carbon nanofibers from crab shells encapsulating sulfur and silicon. The processing route involved several steps, as schematized in Figure 2. After air calcination of the crab shells, the organic components were removed and CaCO 3 templates containing twisted hollow nanometric channels were obtained, with diameters close to those of commonly anodized aluminum oxide templates. The CaCO 3 framework was coated with a thin layer of carbon via heat treatment in nitrogen. Then, the obtained active electrodes were inserted into the nanochannels, where they were treated by sulfur and silicon through thermal infusion and chemical vapor deposition, respectively. After dissolving the CaCO 3 framework by acid treatment, the researchers obtained hollow carbon nanofiber arrays encapsulating sulfur or silicon. The hollow nanostructures provided sufficient space for the volume expansion of sulfur/silicon during the discharge/charge processes and the thin walls of the hollow carbon nanofibers allowed rapid lithium-ion transport from the electrolyte to sulfur/silicon. As reported in Table 1, the Li-ion battery prepared with this crab shelltemplated carbon/silicon anode showed high specific capacity (1580 mAh/g at 1C) and high cycling performance [72].
The mechanism responsible for the excellent electrochemical performance of fish waste-derived porous carbon materials in LIBs occurs due to their uniform interconnected porous structure, which is beneficial for the rapid penetration of electrolytes, fast Li + diffusion and the provision of active sites for the storage of Li + ions [87]. The electrochemical properties of electrode materials can be improved by heteroatom doping, which induces defects and increases available active sites [88]. Nitrogen-doped porous carbons derived from crawfish shells were prepared by Wang et al. [89] by modifying the initial calcination treatment in a nitrogen atmosphere, followed by acid treatment to eliminate CaCO 3 . Then, N-doped porous carbon underwent a thermal treatment with cobalt acetate tetrahydrate to become nano-filled with nanometric cobalt oxide (Co 3 O 4 ) nanoparticles. The N-doped porous carbon and Co 3 O 4 -N-doped porous carbon were used to prepare a working electrode via a slurry coating procedure with high electrochemical lithium storage performance. N-doped porous carbon had a capacity of about 400 mAh/g after 100 cycles, which was greater than that of commercial graphite (372 mAh/g). This demonstrated that N-doped porous carbon could potentially replace graphite in industrial production. More interestingly, as reported in Table 1, the N-doped PC-Co 3 O 4 nanocomposite with 10 nm Co-based nanofiller presented a high reversible capacity of 1060 mAh/g after 100 cycles, acceptable rate capability, superior cyclic performance and excellent primary Coulombic efficiency (86.7%) [39,67].
In addition to the use of prawn shells (PSC), prawn meat (PMC) was used by Lian et al. [87] to prepare porous carbon materials to be applied as anodes in lithium-ion batteries. After calcination under an inert atmosphere, the obtained carbon structure was washed, centrifuged, dried and then combined with polyvinylidene fluoride (PVDF), acetylene black (AC) and N-methyl-2-pyrrolidone solvent. The initial discharge/charge capacities of PSC and PMC materials for the first 3 cycles at the current density of 30 mA/g were 1803/910 and 1200/694 mAh/g with the coulombic efficiency of 50.4% and 57.8%, respectively (see Table 1). Solid electrolyte interface formation was cited as the reason for the low initial coulombic efficiencies. For PSC and PMC, coulombic efficiency reached 91% and 93% after the first cycle and 94% and 95% after the third cycle, respectively [87]. The best performance was obtained by PSC due to the presence of a more uniform nanoporous structure compared to PMC and a higher level of N-doping.
The steps for the preparation of porous carbons from fish scales are schematized in Figure 2. As reported by Selvamani et al. [22], after air calcination, activation in alkaline solution and heat treatment, the obtained carbon was characterized by a high specific surface area and excellent electrochemical behavior, even under high charge/discharge situations. The galvanostatic charge/discharge curves at the current density of 75 mA/g demonstrated an initial discharge capacity around 541 mAh/g in ionic liquid electrolyte. After 75 cycles, the coulombic efficiency was 94% with a reversible capacity of 509 mAh/g. At the current densities of 400 and 4000 mAh/g, the reversible capacities were 390 mAh/g and 179 mAh/g, respectively. In addition to the aforementioned properties, the electrode was stable before and after cycling [22].
Very recently, it was demonstrated that collagen extracted by fish waste could be used to obtain porous materials for LIB electrodes, as schematized in Figure 2. Odoom-Wubah et al. [60] extracted collagen from Tilapia fish with an alkaline treatment followed by an acid treatment. The marine collagen was impregnated by Palladium nitrate followed by a heat treatment in nitrogen and then used in combination with polyvinylidene fluoride (PVDF), N-methyl-2pyrrolidone and carbon black as an anode material for Li, Na and Mg half-cells. Results of electrochemical measurements revealed that the reversible capacities for Li, Na and Mg-based cells were 270 (Table 1), 120 ( Table 2) and 100 mAh/g, respectively [60]. The proof of concept of extracted porous carbon from marine collagen was demonstrated, but further studies are still required to optimize the preparation and performance.
In Table 1 the performance of porous carbon-based materials obtained from fish industry waste for LIBs is compared with those of commercial graphite-based anodes for LIB in terms of specific capacity and cycle life. This latter indicates the number of charge/discharge cycles of the battery until the end of its lifetime. For LIBs, the cycle life is significantly dependent on the depth of discharge, which is an indication of the amount of storage capacity of the battery. It is typically in a range between 300 and 500 cycles for commercial LIBs, even if some manufactures have claimed 1000 cycles [90].

Applications in Sodium-Ion Batteries (NIBs)
Li-ion batteries cannot meet the growing needs of the energy storage market because Li is an expensive, limited and unequally distributed resource [75,95]. Sodium-ion rechargeable batteries are attracting great attention due to their similarity to LIBs and the use of sodium ions (Na + ) as the charge carriers [96]. Though sodium cannot compete with lithium's energy density, that shortcoming is compensated due to its availability and price [97]. Therefore, compared to more widespread LIB, Na-ion batteries (NIB) have lower cost and do not use scarce resources [98]. However, sodium has two disadvantages. First, its weight is three times higher than lithium; even if only 5% of the overall battery weight is related to lithium, NIBs are heavier. In addition, the Na + ion has a larger ionic radius than the Li + ion, leading to more sluggish diffusion kinetics and more significant volumetric changes during repeated charging/discharging cycles. Therefore, Na-ion batteries are less powerful, primarily due to the low ability of the graphite anodes to absorb sodium. A possible solution for achieving higher storage capacities could be the replacement of the graphite anodes commonly used today with electrodes of graphene or hard carbon [99]. The latter is a disordered, mainly sp 2 , non-graphitic carbonaceous material consisting of single layers of carbon atoms that are arranged in a planar hexagonal network, but irregular and disordered along the c-axis [100,101]. These carbon-based materials can be obtained from biomass [47,102,103]. Recent studies have reported the suitability of porous carbon-based materials obtained from fish industry waste as electrodes for NIB batteries. Previous investigations demonstrated that by a low cost, simple and environmentally friendly approach, nitrogen-doped hierarchically porous carbon material obtained from prawn shells could be obtained. The mechanisms underlying their great suitability for replacing common carbon electrodes is due to their porous structure, high inherent nitrogen content and the presence of macro, meso-and micropores that facilitate the storage and transport channels for Li and Na ions. Detailed investigation on this subject was conducted by Elizabeth et al. [91], who obtained a porous N-doped structure from prawn shells using a protocol similar to that reported in Figure 2. Experimental results revealed that, due to the high Nitrogen content in porous carbon material, electrical conductivity and active sites for Li/Na storage were increased, which led to an improved electrochemical performance. Galvanostatic charge/discharge tests showed that the initial capacity at the current density of 0.1 A/g was 1013 mAh/g, which was three times more than the capacity of conventional graphite carbon material. The formation of solid-electrolyte interphase and the irreversible trapping of Li in the pores were responsible for the heavy capacity fade at the beginning. Cyclic charge/discharge tests were conducted on the porous carbon as an anode material for sodium-ion batteries. In the first cycle, the charge specific capacity was around 660 mAh/g and the discharge capacity was 370 mAh/g with coulombic efficiency of 56%. This low coulombic efficiency was due to the irreversible capacity loss because of the solid-electrolyte interphase formation and Na trapping in the porous structure. At the current density of 0.4 A/g, the electrode reversible capacity was 234 mAh/g after 150 cycles, which was superior in comparison to other biomass-derived carbon materials used in the literature. This was ascribed to the hierarchical porous structure and N-doping of carbon. Moreover, TEM results also confirmed that, after 200 cycles, the porous structure showed little damage, which was an indication of the high structural stability of the electrode material [91].
In addition to the porous carbon derived from crustacean shells, suitable electrode materials can be obtained from fish waste, as demonstrated by Odoom-Wubah et al. [60], who extracted collagen from Tilapia waste (according to a procedure described in the previous paragraph) to obtain an anode material for NIB, achieving a specific capacity of 120 mAh/g. Other investigations have been performed in order to apply sustainable anodic materials obtained from fish collagen to Magnesium-ion batteries (MIBs) [60], or from seafood-derived chitin for Potassium-ion batteries (KIBs) [106], respectively. Reversible capacities of 105 mAh/g [60] and 154 mAh/g [106] were obtained for MIBs and KIBs, respectively.

Applications in Lithium-Sulfur Batteries (LSBs)
Lithium-sulfur cells are rechargeable batteries with higher energy density and lower cost than lithium-ion cells due to the use of sulfur cathodes (S8) for energy storage. During the discharge cycle, lithium sulphide (Li 2 S) is formed at the cathode through the migration of Li + ions in the electrolyte. At the mean redox potential of about 2.2 V vs. Li/Li + , the theoretical capacity of LSBs batteries is 1672 mAh/g, at which time sulfur goes through an overall redox reaction of S 8 + 16Li + +16e − ↔ 8Li 2 S [107,108]. The sulfur is reoxidized to S8 during the refilling phase. Nonetheless, the reduction of sulfur to lithium sulphide is a more complicated phenomenon and includes the formation of lithium polysulphides at decreasing chain length, followed by reducing at the surface of the cathode material during the discharge of the cell [109].
The principal challenges of Li-S batteries are the low conductivity of sulfur and its massive volume change during discharging, called the shuttle effect, which is responsible for the progressive loss of active material from the electrode. This determines the short service life of the battery [110]. This shortcoming is mainly related to the weaknesses of Li 2 S as a cathode material due to its high melting point, high activation potential, poor solubility, poor capability rate and capacity fading [111]. The search for a suitable cathode is a necessary step for the commercialization of Li-S batteries. The most common and established approach to mitigate the shuttle effect consists in using a carbon-sulfur composite material, wherein sulfur is infiltrated into the pores of porous carbons such as activated carbons, carbon nanotubes, graphene and other nano/microstructured materials [112,113]. Carbon acts both as a trap for lithium polysulphides released at the cathode, and as a conductive matrix. By confining the sulfur within a conductive matrix, it is possible to increase the contribution of the active material to the battery capacity [114].
Recent literature demonstrated that the dissolution of polysulfides in LSBs can be successfully prevented by introducing porous carbons derived from crustacean shells and fish scales, obtaining a porous sulfur/carbon nanocomposite. This nanocomposite adsorbs the polysulfide generated during chemical reactions and controls the volume expansion of the sulfur particles, thus increasing the life cycle of the battery and providing a high specific capacity for charge and discharge cycles. Yao et al. [72] prepared nanostructured carbon cathodes from crab shells, based on carbon hollow nanofibers encapsulating sulfur according to a procedure already described in paragraph 2 ( Figure 2). As reported in Table 3, the initial capacity at a C/5 rate was 1230 mAh/g. After 100 cycles, the capacity of the cell reached 880 mAh/g, or 71% retention, which was superior compared to other conventional sulfur electrodes.
A successful strategy for tuning the porous structures and nitrogen doping of carbons obtained from shrimp shells is based on selective removal of CaCO 3 , followed by a KOH activation process at various temperatures. This was demonstrated by Qu et al. [115], who prepared electrodes by mixing the micro-mesoporous carbons with acetylene black, polyvinylidene difluoride and N-methyl pyrrolidone. The obtained structure effectively encapsulated sulfur and suppressed the diffusion of dissolved polysulfides. Among the different tested porous carbons, those with a micro-mesoporous structure led to the highest capacity of around 920 mAh/g at the current density of 100 mA/g and cycle stability of 82% within 100 cycles (Table 3) [115]. Figure 3 shows the electrochemical performance of the carbon-sulfur composites which were used as cathode material in LSBs. The CV curves in Figure 3a show a reasonable reactive reversibility and cycling stability, while the discharge/charge profiles in Figure 3b indicate the intense capability to suppress of the lithium polysulfides dissolution in the organic electrolyte during the discharge/charge process. The optimum cycle performance is reported in Figure 3c,d. A hierarchical porous carbon structure can be derived from fish scales after carbonization, alkaline activation and calcination in the nitrogen atmosphere, as demonstrated by Zhao et al. [116]. Through the use of a proper sulfur impregnation treatment, nanocomposite sulfur/carbon cathodes with different S content for LSBs were  A hierarchical porous carbon structure can be derived from fish scales after carbonization, alkaline activation and calcination in the nitrogen atmosphere, as demonstrated by Zhao et al. [116]. Through the use of a proper sulfur impregnation treatment, nanocomposite sulfur/carbon cathodes with different S content for LSBs were prepared. The best performance was achieved using LSB with the nanocomposite cathode with 58.8% sulfur, which achieved an initial capacity of 1039 mAh/g and an excellent reversible discharge capacity of 1023 mAh/g after 70 cycles. These results were ascribed to the porous structure of the nanocomposites. The pores, on one hand, acted as micro-reactors for sulfur, which is beneficial for the cyclability of the cathode. On the other hand, they also provided channels for the mass transport of Li ions during the electrochemical reaction. Moreover, pores led to even penetration of the electrolyte to the cathode materials [116].
Further improvement was achieved by the studies of Gao et al. [117], who obtained porous carbon from fish scales for the preparation of sulfur-carbon nanocomposite cathodes which were coated with an additional layer of porous carbon. As reported in Table 3, the nanocomposite electrode with the porous coating showed the best performance. By covering the cathode material containing porous carbon with an additional porous layer, the cycle performance increased from 878 mAh/g to 1094 mAh/g after 50 cycles. In addition, capacity retention was around 80% and the coulombic efficiency was about 97% after 100 cycles. This is due to the ability to adsorb the polysulfides on the cathode surface, during the discharge process, which is related to the large porous carbon surface area. Consequently, porous carbon derived from fish scales improved the capacity of the sulfur cathode by suppressing the polysulfide dissolution in the electrolyte and preventing the accumulation of sulfur material [117]. Further studies from the same authors led to increased performance of the electrode when the fish scale porous carbon was mixed with sulfur, carbon black and polymer binder [71].
Finally, porous carbons derived from fish industry waste can be used as separators in LSBs. This was demonstrated by Shao et al. [118], who used a Ca(OH) 2 -carbon framework derived from crab shells with the advantages of facilitating electron and ion transfer during redox reactions and efficiently trapping the dissolved polysulfides. Ai et al. [119] prepared hierarchical porous carbons by carbonizing tuna bones followed by an activation process in alkaline solution and N 2 atmosphere. The obtained powder was used to fabricate a separator by mixing with acetylene black powder, a binder, deionized water and isopropyl alcohol. The discharge capacity of the LSB with this novel separator was 1237 mAh/g, significantly higher than that of commercial separators based on Norit carbon and PP, which were 1080 and 777 mAh/g, respectively. As reported in Table 3, the discharge capacities after 100 cycles and coulombic efficiencies of the LSB with this novel separator were 1044 mAh/g and 99%, respectively. Moreover, FBPC separators demonstrated higher reversible capacities even at higher current densities, in comparison to commercial separators. This excellent performance of FBPC material may be due to its micropores/mesopores heteroatoms-doped hierarchical porous structure, which helps the dissolution of polysulfides, facilitates the electrolyte infiltration and increases the number of active sites [119].

Applications in Supercapacitors
Supercapacitors, also called ultracapacitors, enable the storage and delivery of energy at considerable rates, much higher than batteries, thanks to the same charge-separation mechanism of the capacitors (i.e., by ion electrosorption or fast redox-processes), but with much higher capacitance and lower voltage limits [3]. The supercapacitor performance bridges the gap between electrolytic capacitors and rechargeable batteries [121][122][123]. Thanks to their high power density, rapid charge/discharge rates and enduring cycle life, they are employed to balance grid-scale power spikes, in regenerative braking systems and for the starters of cars. [3]. Supercapacitors are mainly classified into three types: electric double-layer capacitors (EDLCs), pseudo capacitors and hybrid super capacitors. Among them, EDLCs store charges electrostatically with the advantages of double-layer charge accumulation at the electrode-electrolyte interface, excellent cycle stability, and long life cycle [121,124]. The huge capacitance is due to an extremely small distance between the opposite charges, as defined by the electric double-layer, and to the very high surface-area of highly porous electrodes [121].
The power and energy-storage capabilities of carbon-based EDLCs are attributed to the physical and chemical properties of the carbon electrodes and there has been great attention paid to active material development with increased surface area [46]. Preferred materials include graphene, carbon aerogel, carbon nanotubes and their composites; however, these are scarce, expensive and tend to agglomerate during synthesis [125]. Moreover, due to the complicated preparation, large-scale production is difficult to implement. Therefore, it is essential to find materials for lower cost and efficient carbon electrodes with comparable electrochemical performances [126]. Recently, the production of active carbons for supercapacitors with graphite-like and microporous structures from agricultural biomass has attracted significant attention as a sustainable, green, abundant and economical route [3,124,[127][128][129][130]. Moreover, very recent literature has demonstrated the feasibility of obtaining similar outcomes from fish industry waste, including the pioneering works of Liu et al. [86] and Chen et al. [69]. These teams produced hierarchically ordered mesoporous carbon structures from crab shells and fish scales, respectively, according to a procedure whose steps can be schematized in Figure 2 depending on the waste typology. The process parameters of each step were tailored in order to enhance the surface area and optimize the electrochemical performance, as reported in Table 4.
The excellent electrochemical performance of fish waste-derived porous carbon materials in supercapacitors derives from an optimal combination of porosity, surface functionality and conductivity [131]. Generally, pure carbon electrodes are hydrophobic, with ineffective surface areas for charge storage in aqueous electrolytes. Heteroatom doping, for example with nitrogen, phosphorus and sulfur, is an effective route to improve specific capacitance by developing wettability, inducing additional pseudocapacitance and reaching a fast charge transfer under high current loads [1,132]. Nonetheless, many heteroatom doping procedures require post-treatments for the carbon materials, making the synthesis more complicated [133].
Gao et al. [134] performed several carbonizations and activation processes at high temperatures to obtain nitrogen-doped activated carbon materials from prawn shells for high-performance supercapacitors. The results of CV and galvanostatic charge/discharge experiments revealed that the specific capacitance of their AC-700 sample was around 280 F/g, a much higher value compared to the other activated carbon materials in the literature, which was due to the perfect combination of electric double layer capacitance and pseudocapacitance derived from high specific surface area and the moderate N-doping level. More recently, Wang et al. [135] and Raj et al. [136] obtained supercapacitors with electrodes made of heteroatom-doped porous carbons from fish scales and squid gladii, respectively, with very good performance reported in Table 4.
Liu et al. [133] suggested a cheap and efficient way to produce hierarchically porous carbons with N/O heteroatoms derived from fish scales to be used as anode and cathode materials for high-performance Na-ion capacitors. Electrochemical experiments revealed that PCNS-600 electrode demonstrated high gravimetric capacitances of 519 and 306 F/g at the current densities of 0.1 and 1 A/g, respectively (Figure 4). By increasing the current densities from 1 to 200 A/g, the capacitance retention was 77.8%. Even after 20,000 cycles at the current density of 5 A/g, there was no capacitance decay, indicating an excellent cyclic performance of the PCNS-600 material. Charging PCNS-600 supercapacitors for 10 s can power one LED for more than 180 s [133].  Multiheteroatom doping has been demonstrated to be a successful and reliable for improving capacitor performance, as reported in the work of Shan et al. [137], w the synergistic effects of hierarchically porous fibrous foam and multiple hetero doping enabled researchers to employ porous materials derived from fish bones as a and cathode in lithium-ion hybrid supercapacitors with very high energy density. F preparation of the half-cells, the carbon material obtained from fish bones was mixed acetylene black, poly(vinylidenedifluoride) (PVDF) and N-methyl-2-pyrrolid (NMP). They studied the positive effects of a sodium carboxymethyl cellulose (CM binder on electrochemical performance. Generally, the capacitance increased increases in the surface area of porous materials. However, as indicated in Table 4, su area and specific capacitance are not always related, mainly because both were aff Multiheteroatom doping has been demonstrated to be a successful and reliable route for improving capacitor performance, as reported in the work of Shan et al. [137], where the synergistic effects of hierarchically porous fibrous foam and multiple heteroatom doping enabled researchers to employ porous materials derived from fish bones as anode and cathode in lithium-ion hybrid supercapacitors with very high energy density. For the preparation of the half-cells, the carbon material obtained from fish bones was mixed with acetylene black, poly(vinylidenedifluoride) (PVDF) and N-methyl-2-pyrrolidinone (NMP). They studied the positive effects of a sodium carboxymethyl cellulose (CMCNa) binder on electrochemical performance. Generally, the capacitance increased with increases in the surface area of porous materials. However, as indicated in Table 4, surface area and specific capacitance are not always related, mainly because both were affected by porosity and the surface functional groups of porous carbons.

Protein Batteries Derived from Fish Industry Waste
Previous investigations revealed that waste from the fish and poultry industries are rich in protein and can successfully be used for the construction of protein batteries. The basic principle of the protein battery is based on protein, peptides and amino acid oxidation and reduction reactions. The rate of these reactions is dependent on the electrode potential differences, and can be reversible.
For the conversion of fish scales and feathers to useful material in electrochemical applications, a series of chemical, biological, thermal or thermomechanical treatments is needed. Hussain et al. [139] gathered fish scales and poultry feathers, rinsed them in several steps using tap and distilled water, dried them, and then dissolved them in the NaOH solution, followed by several heat treatments. The final fish stock solution was a rich source of protein collagen and could be used as a cathode electrode in the battery. The results of their experiments demonstrated that the derived solutions from the dissolution of fish scales and feathers contained protein hydrolysates, keratin and amino acids. The cathodic half-cell was made of fish scales and the anodic half-cell was made of feathers; they were coupled together in order to make a battery. These products participated in the oxidation and reduction reactions at the dynamic equilibrium state, the results of which were used for the generation and storage of electrical energy. The obtained energy depended on the concentration of the cell's active components, which were also related to the concentration of the original materials. Therefore, finding an optimum concentration to maximize battery capacity was critical. In order to investigate the battery charge storage capacity, Hussain et al., developed a four-cell protein battery using light-emitting diodes and connected fish scale and feather half-cells together with wires. The battery produced 0.088 Wh capacity, enough to power low-energy light sources [139].
In a successive work, Hussain et al. [140] used an alkaline solution containing collagen obtained from fish scales as the cathode electrode in the battery. Oxytocin was used as the anode material. The battery contained 5 units, including 2 cubical units, which were connected by epoxy resins. Anodic and cathodic solutions were connected by a salt bridge and the battery voltage was measured at the end. The potential difference between the battery electrodes produced electrical energy. The nature of the salt bridge, reactant concentration and the power of the charger play a crucial role in the stability and voltage of the battery.
Moreover, Hussain et al. [141] prepared a protein battery containing proteins of fish scales as cathode and feathers as anode based on the sono-chemical oxidation-reduction of hydrolysates. Proteins were peptides or amino acids obtained via the hydrolysis of fish scales. Speed and selectivity of the products in sono-chemical reactions were determined in the storage energy. The cathodic and anodic half-cells were connected through a salt bridge and wires. Three different methods (electrical, sonic and microwave experiments) were used to charge the battery. At first, oxidized and reduced forms of the amino acids were in equilibrium; any change in concentration induced flowing of charge and energy gradient. Exposure time and a number of doses of the sonic or microwave energies seriously affected the voltage of the battery. Another determining factor was the pH of the solution of fish scales or feathers. Finding an optimum acid concentration was critical to obtain maximum power and efficiency for the electrical power storage. The energy obtained by this battery could be used for powering household devices.
Battampara et al. [142] dissolved wool and fish scale wastes in NaOH solution and used them as electrolytes to prepare a biochemical fuel cell. Each half-cell solution was connected to the other half-cell through a cotton wick, cardboard or agar-agar conductive bridge. Inert graphite electrodes were used as cathode and anode extensions from the protein solutions. In order to investigate the ability of the solution to keep the charge, a DC power supply was utilized to charge the battery. Voltage was generated based on the charge differences between the two half-cells. The amount of the generated charge was related to the half-cell potential to produce ions and the transfer efficiency. In this electrochemical cell, fish scales were able to produce charges and also behave as charge transfer mediators. The net produced voltage of the battery was determined by the fish scale constituent's oxidation-reduction potential. In general, alkaline concentration and salt bridge material can affect the biochemical fuel cell constructed by proteins in sheep wool and extracts from fish scales. The cyclic voltammetry experiment results revealed that the electrolytes could keep the charge for a long time at lower scan rates. Additionally, after 24 h, the biochemical cell could keep around 96% of its charge. Moreover, higher current densities enhanced the capacity and potential of the battery. By connecting several cells together in series, a voltage of about 1.8 V could be generated, enough to power an LED [142].
Finally, fish industry waste can potentially be applied as a catalyst [39]. So far, the most common electrocatalysts materials for the oxygen reduction reactions in fuel cells and metal-air batteries are platinum and its alloys. However, because platinum is rare, expensive and does not have long-term stability, the demand for replacing it with a cheaper and more stable electrocatalyst is high [143][144][145][146]. Researchers have found that biowastesincluding nori-marine algae [147], shrimp shell [148], sweet potato vine [149] and pomelo peel [150]-can be processed as a porous carbon catalyst for ORR activity due to their sufficient amount of amino acid and bioprotein. In comparison with the above-mentioned biowastes, fish scales are rich in collagen, protein and amino acids, and are eco-friendly, cheap and abundant. Therefore, they could successfully be applied to produce porous carbon materials [143].

Conclusions and Future Perspectives
Fish industry waste has huge potential for the development of sustainable materials for energy storage devices including lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), lithium-sulfur batteries (LSBs), supercapacitors and protein batteries. The utilization of this waste not only helps to find an inexpensive high-potential material, but also mitigates environmental pollution, providing opportunities for biomass-based industries. Numerous research studies on porous and nanostructured carbons derived from fish industry by-products have been carried out in the last decade. These materials present several advantages, including high conductivity, high tensile strength, low density, large aspect ratio and the possibility to obtain different structures by a careful selection of the starting material. Great effort has gone into synthesis and post-modification methods to optimize the porous structure and improve the electrochemical performance. In order to improve the mass transport and the performance of porous carbons, currently reported methods are based on a controlled porosity ratio between mesopores and micropores and, on the surface, heteroatom-doping. However, the intrinsic composition variability of this type of biomass affects the properties of the obtained materials, which can vary with the feedstock.
Even though different reaction mechanisms of fish waste-derived carbon materials in energy storage systems have been reported in the literature, no general consensus yet exists on a commonly accepted mechanism linking their structural and morphological properties with their electrochemical behavior. Further research is still required for a better understanding of the interactions of these sustainable carbon materials with the other components of electrochemical storage devices in order to reduce production costs and increase efficiency and durability.
The transformation of fish industry waste into value-added functional carbons still presents some limitations, namely: time-consuming synthesis, multiple steps and low yield. This inevitably increases production time, thus reducing the economic advantage, compared to other synthetic nanocarbons, which mainly derive from the low cost of the starting waste material. It is expected that, with increasing research and development, these technological limits will be overcome, making these waste-derived carbonaceous materials even more economically convenient and contributing to the increased sustainability of energy storage devices.
Despite several benefits, much work still remains to be done to move from the laboratory scale to the industrial manufacturing level. A careful design of the thermochemical process is still needed in order to tailor the porosity and electrochemical properties of the resulting materials.

Conflicts of Interest:
The authors declare no conflict of interest.