*3.2. Supercapacitors & Batteries Application*

The research on possible use of biomass-derived material as source for electrode fabrication has recently increased the attention on the use of HTC pre-treatment. The present review will consider only the methods involving the use of waste biomass as starting material. This consideration is due to the fact that some studies had focused the attention mostly on the activation step, using synthetized glucose or lignin as starting material, while in the present work only the biomass-derived precursors were considered. Agricultural wastes, as well as plant wastes or high lignocellulosic materials were reported as suitable for energy storage applications [85–88]. Application for supercapacitors has been more and more studied in recent years. Supercapacitors can deliver great power output, ensuring at the same time long life and great efficiency after hundreds of cycles [86]. Supercapacitors store charge reversibly through ions immobilization in the electric double layer, on which non-faradaic and pseudocapacitive reaction occurs [87]. Hence, carbon-based material for supercapacitors applications must ensure good electric conductivity, great surface area with possible hierarchical pore distribution, together with a reasonable cost. For such reason, biomass material represents a greatly attractive option in this field [87].

Soybean root was tested by Guo et al. [89], resulting in a great electrode capacitance and cycling stability. Authors performed an acid HTC treatment at 180 ◦C for 18 h by using a 5% *w*/*w* H2SO4 solution. The obtained hydrochars were further active with KOH at 800 ◦C for 3 h, under a N2 atmosphere. The Brunauer–Emmet–Teller (BET) and Scanning Electron Microscope (SEM) analysis showed the formation of a hierarchical interconnected porous structure composed of large macro and meso-pores on the external surface, which further develop into a capillary net of micropore sites. These latter spots are the ones that mainly contribute to ions storage [90]. Cyclic voltammetry (CV) showed great electrical response even at a high scanning rate, reporting an almost rectangular trend of the curve even at 800 mV s<sup>−</sup>1, as well as a significant capacitance of 328 F g−<sup>1</sup> for a current density of 1Ag<sup>−</sup>1.

Tests on bamboo shells were reported by [65], resulting in a lower capacitance with respect to the ones reported by Gou et al. [89], and equal to 209 F g−<sup>1</sup> at 0.5 A g−1. Similarly in this case, authors performed an acid HTC treatment with 1% *w*/*w* H2SO4 solution and the obtained hydrochars, before being activated in KOH at 600–800 ◦C, were carbonized with melamine under N2 at a temperature of 600 ◦C, with a char-to-melamine ratio of 1:4 *w*/*w*. To better understand the effect of melamine

treatment on the final product, a blank assay was also conducted. The results showed a remarkable increase in surface area and pore development, as well as in the capacitance properties of the hydrochar. Melamine for hydrochar activation was also studied by Sevilla et al. [91], mixing it with K2C2O4 and the hydrochar obtained from eucalyptus sawdust produced at 250 ◦C for 4 h. The use of K2C2O4, instead of the more frequently used potassium hydroxide, according to the authors, is due to three main reasons: its lower corrosive potential, the higher obtainable product yield and its less disruptive action on the morphology of chars. The use of K2C2O4 instead of KOH can almost double the char production, for the same amount of reactant adopted, while increasing at the same time the electrical properties. Hydrochar from pine cones, hemp waste, tobacco rods, peony pollen and argy worm-wood were successfully tested as precursors for supercapacitors application through KOH activation [63,92–95]. Excellent capacitance properties were found by Zhao et al. [63] when working with hydrothermally carbonized tobacco rods. In this work, authors firstly treated the biomass through HTC at 200 ◦C for 12 h and then activated it at 800 ◦C for 1 h with a char-to-KOH ratio of 1:3 in mass. Results showed that the activation stage increased the specific surface area up to 2115 m<sup>2</sup> g−1, raising significantly also the specific capacitance. Indeed, the produced electrodes showed superb electrical properties with a capacitance as high as 286 and 212 F g−1, when the current density was, respectively, 0.5 and 30 A g<sup>−</sup>1. Higher capacitance was recorded when using malva nut as biomass precursor by Ye et al. [96]. HTC was performed at 200 ◦C for 18 h and hydrochars were further activated at 700 ◦C in a ratio of 3:1 with KOH. Capacitance up to 279 and 219 F g−<sup>1</sup> were achieved for current densities of 1 and 20 A g<sup>−</sup>1, respectively. Acid HTC with KOH activation resulted in lower performance when pine cones were used as starting biomass [93]. Manyala et al. tested the addition of 0.5 ml of sulfuric acid into 80 ml of de ionized (DI) water, as medium for the HTC reaction. The further activation was performed for 1 h at the temperatures of 600, 700, 800 and 900 ◦C, with a KOH-to-char ratio of 1:1 (*w*/*w*). When temperature reached 900 ◦C a significant reduction in porosity and electrical properties of the char was observed. This condition is probably due to pore walls collapsing at higher temperature, which enlarge their average dimensions, reducing at the same time the amount of micropores which act as a storage site during charging. A wider inspection of the possible activating agent was conducted by Jain et al. [97] on coconut shells. Biomass was firstly treated in autoclave with H2O2 or ZnCl2, testing different dosages and temperatures, and further activated under CO2 at 800 ◦C for 2 h. Three different HTC conditions were tested in this work:


Each set of tests was further proposed under a wide range of biomass concentrations in the solution, ranging from 0.05 to 0.66 mg L<sup>−</sup>1. For the case of ZnCl2-to-shell ratio of 1:1, authors reported a decrease in the specific surface area as long as biomass concentration was raised, passing from 1700 m2 g <sup>−</sup><sup>1</sup> for a biomass concentration of 0.05 mg L<sup>−</sup>1, to 1350 m2 g <sup>−</sup><sup>1</sup> when the concentration was raised to 0.22 mg L<sup>−</sup>1. The tests using H2O2 presented a similar trend, with an increase in surface area from 1750 to 2450 m<sup>2</sup> g−<sup>1</sup> when the biomass concentration was raised from 0.16 to 0.5 mg L−1, but a further increase up to 0.66 mg L−<sup>1</sup> led to an almost 20% reduction in specific surface area. In terms of electrical properties, the highest energy density as well as the highest capacitance was found to be related to carbon produced with H2O2 and ZnCl2 with a biomass concentration of 0.5 mg L<sup>−</sup>1, with a capacitance of 246 and 221 F g−<sup>1</sup> for a current density of 0.25 and5Ag−1, respectively. Impressive performances were also reported in [98] when testing ZnCl2-activated hydrocar produced from Coca Cola®. Authors found one of the highest capacitances ever recorded for waste precursors, equal to 352.7 F g−<sup>1</sup> for a current density of 1 A g−1. Table 2 summarizes some of the studies reported above with also some reference values for synthetized materials used for supercapacitor applications.


**Table 2.** Performances of biomass-derived hydrochar for supercapacitors application.

\* Capacitance calculated at 0.5 A g<sup>−</sup>1. \*\* Capacitance calculated at1Ag−1.

Aside from applications for supercapacitors, hydrochar has been successfully used for the production of electrode material in lithium or sodium ion batteries as well as in the vanadium redox ones [101–104]. In [101], authors used corn stalk as substrate for the production of a bio-based lamellar molybdenum disulfide electrode to be used as anode in a lithium ion battery. To obtain the desired material, a thiourea and ammonium molybdate solution was used as reaction medium for the hydrothermal carbonization of cornstalks. Hydrochars were further treated in a furnace at 1000 ◦C in N2 atmosphere and then placed into an ammonium chloride solution at 60 ◦C, to separate the excess of calcium. The obtained material presented a discharge capacity, after 100 cycles, for a current density of 0.1 and 1 A g−<sup>1</sup> of, respectively, 1129 and 339 mAh g<sup>−</sup>1. A lithium ion battery anode was also produced from a cellulose-derived carbon nanosphere obtained from corn straw by Yu et al. [102]. A sulfuric acid bath and a following sodium hydroxide bath were used to extract cellulose from corn straw. The obtained products were hydrothermally carbonized at 200 ◦C for 24, 36, 48 and 60 h before being further carbonized at 600 ◦C in Ar, to obtain carbon nanospheres. Authors found that if reaction time is too long, it can induce particles agglomeration, reducing the available specific area and, therefore, the storage capacity. The best carbonization time was found to be 36 h, which showed a specific discharge capacity after 100 cycles of 577 mA g−<sup>1</sup> for a current density of 74 mA g<sup>−</sup>1. Aside from lithium ion, also applications for sodium ion batteries were studied. In [103], authors tested lath-shaped carbon made through HTC followed by a 800 ◦C carbonization step produced from peanut shells, to be used as anode in Sodium ion battery. In this case, the discharge capacity was found to be equal to 265 mA g−<sup>1</sup> at 30 mA g−<sup>1</sup> after 100 cycles. Moreover, tests for a possible cathode material were performed by Palomares et al. [105], using waste from vine shoots and eucalyptus wood as precursor to realize a sodium vanadium fluorophosphate electrode. Authors found that when hydrochar is further subjected to a flash thermal treatment (700 ◦C, 10 min in N2), its electrochemical properties reached a specific capacity of more than twofold that of pristine hydrochar.

Promising results were also achieved in vanadium redox flow batteries when testing activated hydrochar [104] as electrode material.
