Electrocapacitive and electrocatalytic performances of hydrochar prepared by one-step hydrothermal carbonization without further activation

Hydrochar was produced from extracted avocado seed using one-step hydrothermal carbonization (HTC) at a temperature of 200 °C for 12 h. The effects of various feedstock solutions on the specific surface area, morphology, pore characteristics, crystallinity, and chemical bonding were investigated to confirm the changes in the electrochemical performances of the produced hydrochar. The presence of potassium permanganate (KMnO4) and ammonia (NH4OH) solution in the HTC process successfully produced a porous graphite-like structure of hydrochar with the highest surface area and specific capacitance. Moreover, it also exhibits excellent electrocatalytic performance toward the Oxygen Reduction Reaction (ORR), with a current density of 2.15 mA cm−2 via the 2-electron pathway. These results imply that the HTC process can produce hydrochar with high electrocapacitive and electrocatalytic performances even without further activation at high temperatures.


Introduction
Lignocellulosic waste can be utilized to produce several materials. One of them is char for activated carbon [1]. Activated carbon can be produced from biomass with different thermal conversion technology for some applications such as adsorbent [2], catalyst [3], or energy storage and conversion device [4]. Pyrolysis and hydrothermal carbonization (HTC) are two methods of thermochemical conversion to produce biomass-based activated carbon.
Pyrolysis has been developed to convert lignocellulosic waste into value-added materials such as activated carbon [4], aerogel carbon [5], nanofiber carbon [6], and others. However, pyrolysis requires inert atmospheres and temperatures up to 950°C [7]. Compared to the pyrolysis method that has been developed hundreds of years previously, HTC is a more promising technology because it can be carried out without a pre-drying process, has a high yield, low ash content, and requires relatively low temperature (180°C-250°C) [8][9][10]. The carbon produced from the HTC process is known as hydrochar.
The properties of hydrochar are determined by temperature, reaction time, and the catalyst used. Therefore, many studies have explored producing hydrochar from biomass such as corn straw [11], wheat straw [10], soybean hulls [8], canola [12], and bamboo [13]. However, current hydrochar is unsuitable for more extensive applications of electrocapacitive and electrocatalytic materials because of its low electrical conductivity and cycling stability. As a result, most hydrochar applications are still limited to adsorbers, such as dye removal or heavy metals adsorbent, which only focus on increasing surface area and selectivity [14][15][16].
Several studies have been conducted to improve the performance of hydrochar, including the use of an acid/ base solution as a catalyst [8,10], activation with activator agents at elevated temperatures [17], and further pyrolysis with temperatures up to 900°C [18,19]. This process has been shown to improve the properties of hydrochar, but it requires a lengthy and high-temperature process. Furthermore, to our knowledge, synthesizing high electrochemical performance hydrochar through a low-temperature one-step HTC process without further activation or pyrolysis has not been reported.
On the other hand, many researchers observed the role of several non-noble metal oxides, such as manganese dioxides (MnO 2 ), as an alternative to improve the lack of carbon materials in terms of electrocapacitive and electrocatalytic performances [20,21]. Furthermore, MnO 2 can be produced by hydrothermal process from the reduction of KMnO 4 by lignin [21]. So, under suitable operating conditions, the KMnO 4 reduction process and biomass carbonization are expected to occur in a one-step HTC process.
In this study, we were inspired by the high content of antioxidants in avocado seed waste which led to many studies of avocado seed extraction for some applications in health and pharmaceutical. The extraction process produces essential oil as the main product and extracted solid residue as a by-product. The solid residue is a lignocellulosic waste that needs to be explored for its potential as new high-value material. To the best our knowledge, the one-step synthesis of avocado seed-based hydrochar for electrochemical energy application has never been investigated.
This work uses a facile method to synthesize high electrochemical performance hydrochar through a onestep HTC process without further activation or pyrolysis at high temperatures. We believe that our study will be a new milestone in the development of hydrochar materials research in electrochemical energy applications. Furthermore, the facile method will reduce both time and energy consumption when applied at a further application on a large industrial scale. In addition, the effect of various feedstock solutions was studied for their impact on hydrochar properties and their electrochemical performances.

Materials
The raw material was collected from the solid residue of the avocado seed extraction using hexane as a solvent, as explained in our previous study [22]. Lignocellulosic component information of raw material (table 1) was analyzed using the Chesson-Datta method [23]. Chemicals used for feedstock solutions in the HTC process are potassium permanganate (KMnO 4 97.5%; UNI-Chem Indonesia), sulfuric acid (H 2 SO 4 96%; Merck), ammonia (NH 4 OH 25%; Merck), and demineralized water (UD Sumber Ilmiah Persada, Indonesia). All chemical reagents were of analytical grade and used without further purification.

Hydrochar synthesis
Hydrochar was prepared by a hydrothermal method from extracted avocado seed powder (EASP). Then, 5 g of avocado seed powder was mixed with demineralized water at a 1:10 solid/water ratio (w/v). HTC process was carried out in a 100 ml Teflon-lined stainless-steel autoclave hydrothermal reactor. The reactor was sealed and heated at 200°C for 12 h, then naturally cooled down to room temperature. The solid product was collected by vacuum filtration, washed with demineralized water until pH around 7, dried at 80°C for 12 h, and weighed. The sample was coded as HC-H. Different feed solutions were used to investigate the effect of KMnO 4 presence with 1 mmol KMnO 4 addition (HC-K). The various feedstock solution is carried out by adding H 2 SO 4 for acidic conditions (pH ∼3) and NH 4 OH for alkaline conditions (pH ∼9) to obtain the expected pH. Then, the hydrochar sample was denoted as HC-KS and HC-KN for acid and alkaline conditions, respectively.

Characterization
Nitrogen adsorption-desorption isotherms analysis using the Brunauer-Emmett-Teller (BET) method (NOVA 1200e, Quantachrome) was used to determine the specific surface area and pore characteristics of the hydrochar. Before measurements, the samples were degassed for 3 h at 300°C with flowing nitrogen gas. The multiple-point Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area (SSA). The pore diameter and volume were calculated using the Barret-Joyner-Halenda (BJH) method. The micropore volume and surface area were calculated using the t-plot method. The hydrochar's crystallographic structures were then investigated using a Philips Expert Pro x-ray diffractometer equipped with Cu K (0.1541841 nm) radiation over the 2-range of 10°-70°. Additionally, Raman spectra were obtained using a Raman imaging microscope to analyze the carbon structure and graphitic type of carbon using a HORIBA micro confocal Raman Spectrometer with a laser at a wavelength of 532 nm. Furthermore, the Gaussian function was used to fit the peaks to determine peak positions and maximum heights in the measured Raman spectra. The chemical functional groups of the hydrochar samples were then identified using Fourier-transform infrared spectroscopy (FTIR; Shimadzu IRTracer-100). Moreover, the hydrochar morphology and pore structure were examined using a scanning electron microscope (SEM, HITACHI FLEXSEM-100).

Electrochemical test
The electrocapacitive performance of hydrochar was measured using cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) in 0.1 M Na 2 S 2 O 3 aqueous solution with a three-electrode system and a potentiostat/galvanostat instrument (Autolab PGSTAT 302N, Metrohm). A platinum spiral act as a counter electrode in the three-electrode system, Ag/AgCl act as a reference electrode, and the hydrochar samples act as a working electrode. The working electrode was made by dispersing the hydrochar samples in a paste with poly(vinylidene difluoride) (PVDF; Sigma-Aldrich Pte. Ltd, Singapore) as the binder and 1-methyl-2pyrrolidinone (NMP; Sigma-Aldrich Pte. Ltd, Singapore) as the solvent. The paste with mass loading of ±0.01 g was then applied to a nickel foam surface and dried at 50°C to form a layer. The CV was determined by scanning the potential between 1.0 and 0 V (versus Ag/AgCl) at a rate of 10 mV s −1 .
The hydrochar was mixed with PVDF and NMP to make carbon ink as a working electrode for measuring the Oxygen Reduction Reaction (ORR). The carbon ink with mass loading of ±0.002 g was dripped onto a polished glassy carbon electrode with a 3 mm diameter and dried at 50°C to form a thin carbon layer. Using potassium hydroxide (KOH) as an electrolyte solution, the CV was measured by scanning the potential between 1.0 and 1.0 V (versus Ag/AgCl) in 0.1 M KOH solution at a scan rate of 10 mV s −1 . The experiment was conducted in oxygen-and nitrogen-saturated 0.1 M KOH electrolyte solutions for comparison. The hydrochar performance to ORR activity was then investigated using Linear Sweep Voltammetry (LSV) on a Rotating Disk Electrode (RDE). The measurement was carried out with a rotating speed of 400-2800 rpm, 400 rpm intervals, and a scan rate of 10 mV s −1 . It should be noted that the mass of the active material must be weighed presicely because it will be used in further calculations.

Visual appearance
The resulting hydrochar visual appearance of HC-H, HC-K, HC-KS, and HC-KN is significantly different. As shown in figure 1, the hydrochar of HC-KS and HC-KN have a deeper black color than HC-H and HC-K, which are blackish brown. This condition is an early indicator that adding acid (H 2 SO 4 ) and alkaline (NH 4 OH) medium can increase the intensity of carbonization during the HTC process, as evidenced by HC-KS and HC-KN.

Yield analysis
As shown in table 2, the highest yield exists in HC-H, followed by HC-K, HC-KN, and HC-KS. Lignocellulosic degradation and decomposition are the main reason for yield reduction. The yield indicates that HTC using H 2 O at 200°C for 12 h cannot convert biomass into hydrochar because the decomposition of lignocellulose components has not occurred completely. It needs a longer reaction time than 12 h for lignin, cellulose, and hemicellulose degradation to form carbon by HTC at 200°C [24].
HC-K yield has decreased by 3.62% compared to HC-H due to the presence of KMnO 4 as a strong oxidizer. It can oxidize the aromatic ring of lignin and play a role in the degradation of lignin and its derivatives in the hydrochar formation [25]. Furthermore, adding acid and alkaline medium to KMnO 4 solution during HC-KS and HC-KN synthesis accelerates the decomposition and degradation of lignocellulosic components during the HTC process [8]. Some studies reported that adding H 2 SO 4 as a strong acid is possible to spontaneously hydrolyze lignocellulose even under room temperature and atmospheric conditions [26,27]. Consequently, the HC-KN yield is higher than HC-KS.

Specific surface area and pore characteristics of hydrochar
The BET-specific surface area (SSA) and pore structure parameters of hydrochar (table 3) show that the use of different feedstock solutions in the HTC process can affect the characteristics of hydrochar. The BET-SSA of HC-H is only 25.26 m 2 g −1 , the lowest SSA in this study. As a comparison, the BET SSA of hydrochar from other biomass via the HTC process with water as a feedstock medium is presented in table 4. Hydrochar from avocado seeds has a higher SSA than others. This result could be attributed to the higher total cellulose and lower lignin content of the raw materials used than other biomass, as obtained from some references. These conditions would be advantageous in the HTC process of avocado seed because lignin has been shown to interfere with the cellulose and hemicellulose hydrolysis process and slow down the component decomposition and hydrochar formation [13]. Furthermore, a larger SSA of 310.11 m 2 g −1 was obtained on HC-K, implying that KMnO 4 decomposed lignin components into smaller fragments, which water hydrolysis cannot occur on HC-H samples. On the other hand, the SSA of HC-KS increased to 486.71 m 2 g −1 , but smaller than the SSA of HC-KN of 573.99 m 2 g −1 . It is caused by the tendency of H 2 SO 4 to degrade biomass components spontaneously and vigorously into smaller fragments with large pore formations.
The linear plot of the adsorption-desorption isotherm curve (figure 2) shows that the hydrochar has a type-IV classification isotherm curve with a hysteresis loop on the curve, indicating the presence of mesopore structure. The pore size of HC-K, HC-KS, and HC-KN hydrochar samples was in the range of 2.596 to 3.238 nm, which is larger than the HC-H pore size of 2.379 nm.   Table 3. Specific surface area and pore characteristics of hydrochar.

Morphology and elemental composition of hydrochar
The morphology of hydrochar was investigated by SEM analysis (figure 3) to support the BET-SSA and pore structure parameters data discussed above. The morphology of HC-H appears solid with no particle pores. It implies that water cannot convert biomass into porous carbon material during the HTC process. Interestingly,   there are significant differences in the morphology of hydrochar synthesized in acid and alkaline conditions. 3Dconnecting morphology with large pore formation of HC-KS is probably caused by the evaporation of the sulfur at high temperatures [33]. Besides that, HC-KN shows a sponge-like morphology with a relatively smooth surface and smaller uniform pore size that spread to the inside of particles. It is also related to other studies that found the chemical structure of precursors played an essential role in carbon material formation [10,34]. According to the summary of elemental composition data (table 5), carbon (40.87%-48.78%) and oxygen (18.14%-42.79%) are the main components of produced hydrochar. The amount of carbon component in the product is not significantly affected by the different feedstock solutions. HC-KS has the lowest oxygen content (18.14%) because more intensive dehydration and decarboxylation reactions occur due to the addition of concentrated sulfuric acid [35]. Large amounts of nitrogen or sulfur have also been noticed in HC-KN and HC-KS, respectively, corresponding to nitrogen or sulfur source in NH 4 OH or H 2 SO 4 as the feedstock solution. The Manganese (Mn) component in all samples except HC-H is predicted as MnO 2 structure, as a reduction result of KMnO 4 due to the presence of lignin from biomass that can contribute to enhance the electrochemical performance of hydrochar [21,36].
Furthermore, energy dispersive x-ray (EDX) mapping analysis ( figure 4) confirm that the carbon (C) and the other elements are uniformly distributed except for HC-KS, which could be due to strong-spontaneous degradation because of sulfuric acid presence as mentioned earlier.
3.5. FTIR spectra of hydrochar FTIR spectra (figure 5) were measured to identify the functional groups present in the hydrochar samples. The broad peak around 3000-3700 cm −1 confirms that the oxygen presence as −OH in hydrochar might be caused by the dehydration process during HTC [13]. The bands in the HC-H sample at 1021, 900, and 850 cm −1 are  related to aromatic C−H in-plane deformation of lignin, aromatic C−H out-plane vibration of lignin, and C−H stretching vibration assigned to glucose rings of cellulose, respectively [37,38]. These chemical bonds were significantly reduced in the HC-K sample and completely disappeared in HC-KS and HC-KN, showing that water did not have a better ability to decompose lignocellulose than others, as confirmed by the SEM image as discussed previously. The bands at around 1699 and 1610 cm −1 correspond to the vibration of C=C groups of aromatic structure and C=O (carbonyl, ester, or carboxyl) groups, respectively [13,24]. Another peak in the HC-KS sample at around 1192 and 1120 cm −1 suggests the presence of functional groups of C=S and C−S caused by sulfuric acid addition [39][40][41]. The typical difference peak of around 1580 cm −1 in the HC-KN sample indicates the presence of functional groups C=C, C=O, and C=N, which might overlap in this range [5]. Furthermore, HC-KN has low-intensity peaks around 1103, 1371, and 1438 cm −1 , indicating C-N bonding. Chemical bonds of C-N and C=N are formed because of ammonia in the feedstock solution.
3.6. Crystallography patterns of hydrochar X-ray diffraction (XRD) was used to investigate the crystallinity of hydrochar at different feedstock solutions. Based on the XRD pattern (figure 6), HC-H has three characteristic peaks of approximately 12.73°, 20.28°, and 21.38°, which could be indexed to cellulose II [5]. Besides that, peaks with higher intensity at around 17°and 22.95°belong to lignin and microparticle lignin, respectively [42]. Therefore, it indicates that there are still lignin and cellulose components inside. The decline intensity peak occurred significantly at HC-K, indicating that lignin and cellulose components are degraded more efficiently. The peak that disappears in both XRD patterns represents the complete degradation of lignocellulose components in HC-KS and HC-KN. A broad peak of about 24°in HC-KS indicates the presence of amorphous carbon [5]. Meanwhile, the peak diffraction in HC-KN widened and shifted slightly to the left to 26.4°. The diffraction peak shift indicated that carbon had been disrupted and defected. The results correspond with other studies showing that ammonia can cause carbon structure defects [5,43].

Raman spectra of hydrochar
From the identity of the visual appearance, morphological images, and crystallography of the XRD pattern, only HC-KS and HC-KN are completely carbonized. For further confirmation of carbon structure and graphitic type of carbon, Raman spectra were analyzed. The Gaussian function was used to fit the peaks to determine the peak positions and maximum height of the measured Raman spectra ( figure 7).
Three intense bands correspond to specific chemical structures within the carbon structure. The D band at around 1350 cm −1 is primarily caused by the vibration mode of the sp 2 carbon atom outside the plane, representing the defects in the amorphous carbon structure. The first-order scattering causes the G band at around 1590 cm −1 to the optical phonon E 2g mode in the plane, which corresponds to the vibration of the carbon atom. The 2D band at around 2750 cm −1 is caused by second-order scattering, related to the inelastic scattering of two phonons with opposite momentum. G and 2D bands are characteristic peaks of graphitic-like carbon formation that differ from amorphous carbon spectra [5,44]. Figure 7 shows that HC-KS has characteristic peaks of amorphous carbon, which is indicated by the presence of G and D peaks with no 2D peak observed. The D peak was smooth, the G peak was small, and there was no 2D, indicating that HC-KS was mostly amorphous carbon. Another result with HC-KN, a low-intensity 2D peak,  was observed, implying that the graphitic phase of carbon structure is beginning to form in the produced hydrochar. However, the results of the Raman spectra combined with the XRD pattern could conclude that HC-KN is carbon material with a combination of amorphous and graphite-like structures. Furthermore, the intensity ratios of the D band and the G band (I D /I G ) of HC-KS and HC-KN were 0.70 and 1.12, respectively. The increase of I D /I G ratio in the HC-KN further confirms that the carbon contained many defects and disorders, as discussed in the XRD pattern result of HC-KN. Other studies reported that increasing the ID/IG ratio and forming a graphite-like structure with high porosity should be conducted to achieve good electrocapacitive and electrocatalytic activity [34,[45][46][47]. Therefore, the electrochemical performances of produced hydrochar will be discussed in the following paragraph.

Electrocapacitive performance
Porous carbon-based material is commonly used as an energy storage device, and the electrocapacitive performance of carbon has been experimentally evaluated. CV curve for HC-H, HC-K, HC-KS, and HC-KN measured in 0.1 M Na 2 S 2 O 3 at a scan rate of 10 mV s −1 is depicted in figure 8.
The specific capacitance ( ) Cs was calculated from the CV potentiostat data using equation (1) below: where I is the cathodic current (ampere), V is the applied potential (volt), m is the electrode loading mass (gram), s is the scan rate (volt s −1 ), and R is the scan range of applied potential [48].
According to the results (table 6), the specific capacitance increases linearly with increasing the hydrochar surface area, as previously discussed in the BET-SSA section. It demonstrates how surface area affects material capacitance significantly. The surface area directly impacts the ion diffusion into the pores of a carbon-based electrode [49].
The CV curve of HC-KN over a potential window of −1 to +0.6 V versus Ag/AgCl at different scan rates (0.1 to 10 mV s −1 ) ( figure 9(a)) shows that the typical CV curve shape does not significantly change at any scan rate.  However, the higher applied scan rate produces a broader cyclic voltammograms curve, indicating the higher electrochemical capacitance achieved. The capacitance retention ratio demonstrates the electrocapacitive cycling performance as a function of cycle number based on repeating CV cycles at a scan rate of 1 mV s −1 ( figure 9(b)). After 1000 cycles, the specific capacitance of the HC-KN retains 84.7% of its initial value in the Na 2 S 2 O 3 electrolyte. As a comparison, table 7 shows some previous research results about carbon-based materials and their specific capacitance value from various materials and methods showing that the HC-KN hydrochar in this study has promising potential as a supercapacitor electrode.

Electrochemical Impedance Spectroscopy (EIS) analysis
Since HC-KN has the highest specific capacitance value, Electrochemical Impedance Spectroscopy (EIS) test was examined to further investigate the frequency response of an energy storage device. The Nyquist plot ( figure 10(a)) revealed a single semi-circular and linear line with a slope of ∼45°corresponding to the chargetransfer process and the semi-infinite linear mass-transport, respectively.Moreover, incomplete semicircle of HC-KN represent its superior resistance which is the reason why its specific capacitance value is still far away from commercial supercapacitor in global market [54,55]. Furthermore, Bode plots of -phase angle (°) versus log f at −1.0 Hz ( figure 10(b)) are observed, a phase angle of ∼57°was obtained for the HC-KN. Its Bode magnitude plot is characterised by two distinct regions. In the higher frequency (10 3 -10 5 Hz), a flat portion of curve with slope≈0 is observed due to response of electrolyte resistance. In the lower frequency (10 −1 −10 3 Hz), the impedance spectrum displays a linear relation with slope≈−1 between log (Z ) and log ( f ). Moreover, as seen from the Bode phase plot, the phase angle initially drop at around frequency of 10 Hz and it significantly drop again at higher frequency. The former one demonstrates that the solution resistance dominates the   impedance in this frequency range, and the latter one indicates the contribution of surface film resistance to the impedance [56].
3.10. ORR electrocatalytic performance ORR analysis was required to determine the potential of produced hydrochar in electrocatalyst applications. Therefore, CV measurements were performed on the two hydrochar samples with the highest SSA and specific capacitance, HC-KS, and HC-KN. Figure 11 shows  had different current densities at the reduction peak of 0.45 and 2.15 mA cm −2 for HC-KS and HC-KN, respectively. These results indicated that HC-KN has better performance as an electrocatalyst. It was also proved that the contribution of the defect and disorder hydrochar structure caused by the ammonia presence significantly improves ORR electrocatalytic performances of hydrochar. Further investigation regarding ORR kinetics and electrocatalytic process on hydrochar as an electrocatalyst was carried out by LSV analysis using an RDE at rotating rates from 400 to 2800 rpm. The LSV curve was measured in 0.1 M KOH O 2 -saturated solution in the potential range of-0.6-0 V with a scan rate of 10 mV s −1 .
In both HC-KS ( figure 11(c)) and HC-KN (figure 11(d)) LSV curves, the diffusion-limiting current densities gradually increased as the increasing of rotation speed caused by the enhanced oxygen flux at the electrode surface at higher speeds.
The ORR kinetics of hydrochar was further analyzed using Koutecky-Levich plots based on the potential linear range from the LSV curve. K-L plots of HC-KS (figure 11(e)) and HC-KN (figure 11(f)) have a linear relationship betweeni 1 and w -.
The excellent linearity and parallelism of the potential range of −0.4 to −0.5 V reveal the first-order reaction kinetics [57].
Next, the slope of the line is used to determine the number of electrons, n, based on the Koutecky-Levich equation [58].  Equation (5) represents 2-electron pathway for oxygen molecule reduction to hydrogen peroxide ions. Then, the hydrogen peroxide ions were further electrochemically reduced to hydroxide ions through the 2-electron pathway that follows equation (6). These showed that HC-KN hydrochar followed the two-electron ORR mechanism, which was actively catalytic toward peroxide decomposition via a two-step two-electron pathway. In contrast, the number of electrons transferred and the kinetic current density for HC-KS were reduced to 1.83 and 0.07 mA cm −2 , respectively.
Since the sample with the highest electrocatalytic activity was HC-KN, it was tested at a high cycle number to determine the stability of the electrocatalyst. Figure 12 depicts the RDE stability test of the sample in oxygensaturated 0.1 M KOH at 2000 rpm. The onset potential and limiting current density of the initial ORR curve are relatively high. Furthermore, the kinetic region decreases by only 31 mV potential after 1000 cycles. The ORR electrocatalytic performance of the hydrochar, as determined by the kinetic current density ( ) I , K the number of electrons transferred ( ) n , and cycling stability, indicated that using KMnO 4 -NH 4 OH feedstock solution in the HTC process is a better choice to improve the electrocatalytic performance of the hydrochar.
As a comparison, ORR electrocatalyst performance from various materials and synthesis methods were presented in table 9 to illustrate the advantage of this study. Among the others, the current density ( ) J of hydrochar in this study is not as high. However, considering the facile method and the energy consumed, this study showed that ammonia presence with KMnO 4 solution by one-step hydrothermal carbonization is quite a promising way to improve the electrochemical performance of hydrochar for ORR electrocatalyst.

Conclusion
Hydrochar was successfully synthesized from extracted avocado seed using a one-step HTC process with various feedstock solutions. The presence of KMnO 4 and ammonia solution in the HTC process successfully produced a porous graphite-like structure of hydrochar (HC-KN) with the highest surface area (491.17 m 2 g −1 ) and specific capacitance value (71.22 F g −1 ). Besides that, HC-KN also has excellent electrocatalytic performance toward the ORR activity, with a transferred-electron number and current density were 2.21 and 2.15 mA cm −2 , respectively. This study found that the HTC process can produce hydrochar with high electrocapacitive and electrocatalytic performances even without further activation at high temperatures.