*2.4. Electrochemical Test of the EOP Biochar in Direct Carbon Fuel Cell*

The direct carbon fuel cell (DCFC) used in this study was similar to the one employed in our previous works [19,29]. Samarium-doped ceria (SDC) combined with molten carbonate (MC) consisting of a Li2CO3/Na2CO<sup>3</sup> (Chemical reagent, Tianjin, China) eutectic mixture in a mole ratio of 2:1 was used as the electrolyte material. The SDC sample (Ce0.8Sm0.2O1.9) was synthesized via an oxalate co-precipitation process. All the chemicals were used as received without further purification. The stock solution was made by mixing and dissolving 60.3913 g of cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99%, Chemical reagent, Tianjin, China) and samarium nitrate hexahydrate (Sm(NO3)3·6H2O) in 100 mL of distilled water. A total of 6.0624 g of samarium oxide (Sm2O3, 99.5%, Chemical reagent, Tianjin, China) was dissolved in 10 mL of nitric acid (Chemical reagent, Tianjin, China) and 15 mL of distilled water to obtain samarium nitrate. Aqueous oxalate acid solutions (H2C2O3·2H2O, Chemical reagent, Tianjin, China) with a concentration of 0.1 mol·L −1 were used as the precipitant. In a typical synthetic procedure, 550 mL of distilled water mixed with the oxalate acid solution (Chemical reagent, Tianjin, China) was dripped at a speed of 3 mL min−<sup>1</sup> into the precipitant solution under 250 rpm vigorous stirring at room temperature to form a white precipitate. The resultant suspension, after homogenizing for 1 h, was filtered via suction filtration. The precipitate cake was washed repeatedly with distilled water and ethanol, followed by drying at 100 ◦C for 24 h to obtain the SDC precursor. The obtained SDC precursor was sintered at 700 ◦C for 2 h to form a pale yellow

SDC powder (Ce0.8Sm0.2O1.9). The binary carbonate powder, i.e., Li2CO3/Na2CO<sup>3</sup> in a mole ratio of 2:1, was prepared. The composite electrolyte material powder was obtained through mixing the two powders (carbonate powder and SDC powder) in a weight ratio of 3:7 by ball milling for 2 h. The used composite cathode powder consisted of 30 wt% composite electrolyte and 70 wt% LixNi1-xO powders. The composite cathode powder was also prepared through 2 h ball mill mixing, sintering at 700 °C for 2 h, and grinding. The lithiated nickel oxide (LixNi1-xO) was prepared through 2 h ball mill mixing of NiO with lithium hydroxide mon-

*C* **2023**, *9*, x FOR PEER REVIEW 8 of 27

der) in a weight ratio of 3:7 by ball milling for 2 h.

*2.4. Electrochemical Test of the EOP Biochar in Direct Carbon Fuel Cell* 

The direct carbon fuel cell (DCFC) used in this study was similar to the one employed

in our previous works [19,29]. Samarium-doped ceria (SDC) combined with molten carbonate (MC) consisting of a Li2CO3/Na2CO3 (Chemical reagent, Tianjin, China) eutectic mixture in a mole ratio of 2:1 was used as the electrolyte material. The SDC sample (Ce0.8Sm0.2O1.9) was synthesized via an oxalate co-precipitation process. All the chemicals were used as received without further purification. The stock solution was made by mixing and dissolving 60.3913 g of cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99%, Chemical reagent, Tianjin, China) and samarium nitrate hexahydrate (Sm(NO3)3·6H2O) in 100 mL of distilled water. A total of 6.0624 g of samarium oxide (Sm2O3, 99.5%, Chemical reagent, Tianjin, China) was dissolved in 10 mL of nitric acid (Chemical reagent, Tianjin, China) and 15 mL of distilled water to obtain samarium nitrate. Aqueous oxalate acid solutions (H2C2O3·2H2O, Chemical reagent, Tianjin, China) with a concentration of 0.1 mol·L−1 were used as the precipitant. In a typical synthetic procedure, 550 mL of distilled water mixed with the oxalate acid solution (Chemical reagent, Tianjin, China) was dripped at a speed of 3 mL min−1 into the precipitant solution under 250 rpm vigorous stirring at room temperature to form a white precipitate. The resultant suspension, after homogenizing for 1 h, was filtered via suction filtration. The precipitate cake was washed repeatedly with distilled water and ethanol, followed by drying at 100 °C for 24 h to obtain the SDC precursor. The obtained SDC precursor was sintered at 700 °C for 2 h to form a pale yellow SDC powder (Ce0.8Sm0.2O1.9). The binary carbonate powder, i.e., Li2CO3/Na2CO3 in a mole ratio of 2:1, was prepared. The composite electrolyte material

The used composite cathode powder consisted of 30 wt% composite electrolyte and 70 wt% LixNi1−xO powders. The composite cathode powder was also prepared through 2 h ball mill mixing, sintering at 700 ◦C for 2 h, and grinding. The lithiated nickel oxide (LixNi1−xO) was prepared through 2 h ball mill mixing of NiO with lithium hydroxide monohydrate (LiOH·H2O, 90%, Chemical reagent, Tianjin, China) powders in a 1/1 mol% ratio, sintering at 700 ◦C for 2 h, and grinding. The fine NiO powder was produced through heating a proper amount of Ni(NO3)2·6H2O (99%, Chemical reagent, Tianjin, China) powder at 700 ◦C for 2 h until it combusted. ohydrate (LiOH.H2O, 90%, Chemical reagent, Tianjin, China) powders in a 1/1 mol% ratio, sintering at 700 °C for 2 h, and grinding. The fine NiO powder was produced through heating a proper amount of Ni(NO3)2·6H2O (99%, Chemical reagent, Tianjin, China) powder at 700 °C for 2 h until it combusted. The SDC/MC powder (0.25 g) was first uniaxially pressed in a die at 1 MPa for 60 s to form yellow electrolyte discs. A total of 0.15 g of the composite cathode powder (LixNi1-

The SDC/MC powder (0.25 g) was first uniaxially pressed in a die at 1 MPa for 60 s to form yellow electrolyte discs. A total of 0.15 g of the composite cathode powder (LixNi1−xO-SDC/MC) was added using a 60-mesh sieve to the electrolyte discs. Then, a single isostatic pressing at 500 MPa (30 kpsi) was performed for 30 s to form the DCFC cell pellet. The cell pellet was then sintered at 700 ◦C for 2 h in air. The sintered DCFC pellet had a diameter of 13 mm and was 1 mm thick. Silver paste was brush painted on both sides of the pellet for the current collection. The experimental protocol of the DCFC pellet preparation is described in Figure 2. xO-SDC/MC) was added using a 60-mesh sieve to the electrolyte discs. Then, a single isostatic pressing at 500 MPa (30 kpsi) was performed for 30 s to form the DCFC cell pellet. The cell pellet was then sintered at 700 °C for 2 h in air. The sintered DCFC pellet had a diameter of 13 mm and was 1 mm thick. Silver paste was brush painted on both sides of the pellet for the current collection. The experimental protocol of the DCFC pellet preparation is described in Figure 2.

**Figure 2.** Experimental protocol of **Figure 2.** Experimental protocol of DCFC pellet preparation. DCFC pellet preparation.

The obtained pellet was mounted in a stainlesssteel cell holder (Figure 3) serving for the current collection and for gas distribution. The DCFC pellet was placed with the cathode downwards and then sealed into a tubular furnace controlled by a K-type thermocouple.

Typical carbon material loading was 300 mg for the EOP biochar fuel. The obtained EOP biochar from carbonization was ground and sieved between 40 and 60 meshes to fit the application condition as a fuel in a DCFC system. A sealant was cured in situ during the heating of the cell to isolate the cathode and anode chambers. The fuel chamber was continuously purged under nitrogen at a 100 mL·min−<sup>1</sup> flow rate. The cell temperature was raised to 100 ◦C in an ambient air environment in order to ensure seal formation. Then, the inert gas purge was started, and the temperature was increased up to 700 ◦C. The cell was held at this temperature to assess the electrochemical performance of the DCFC fed by the EOP biochar. The cathode chamber was fed by a mixture of O<sup>2</sup> (60 mL·min−<sup>1</sup> ) and CO<sup>2</sup> (120 mL·min−<sup>1</sup> ). A Versastat 3 Potentiostat-Galvanostat equipped with the Versastudio software for automatic data collection was used to generate the DCFC cell polarization curves.

**Figure 3.** Operation mode of the DCFC. **Figure 3.** Operation mode of the DCFC.

### Typical carbon material loading was 300 mg for the EOP biochar fuel. The obtained **3. Results and Discussions**

ple.

### EOP biochar from carbonization was ground and sieved between 40 and 60 meshes to fit *3.1. Feedstock Characterization*

the application condition as a fuel in a DCFC system. A sealant was cured in situ during the heating of the cell to isolate the cathode and anode chambers. The fuel chamber was continuously purged under nitrogen at a 100 mL·min−1 flow rate. The cell temperature was raised to 100 °C in an ambient air environment in order to ensure seal formation. The physico-chemical properties of the EOP raw materials are significantly important for the assessment of their byproduct potential and quality. Proximate and ultimate analyses are among the easiest ways to study the fuel characteristic of solid materials. The results of these analyses are presented in Table 1.

The obtained pellet was mounted in a stainlesssteel cell holder (Figure 3) serving for the current collection and for gas distribution. The DCFC pellet was placed with the cathode downwards and then sealed into a tubular furnace controlled by a K-type thermocou-

Then, the inert gas purge was started, and the temperature was increased up to 700 °C. The cell was held at this temperature to assess the electrochemical performance of the DCFC fed by the EOP biochar. The cathode chamber was fed by a mixture of O2 (60 The EOP biomass feedstock is mainly composed of carbon (39.45%) and oxygen (41.2%), with a low percentage of nitrogen (2.68%) and nearly no sulfur content (<0.8%). The EOP biomass composition aligns with other solid wastes reported in the literature [53].

mL·min−1) and CO2 (120 mL·min−1). A Versastat 3 Potentiostat-Galvanostat equipped with the Versastudio software for automatic data collection was used to generate the DCFC cell Based on the elemental composition results, the approximate molar chemical formula of the EOP biomass is CH1.69O0.78N0.058.

polarization curves. **3. Results and Discussions**  *3.1. Feedstock Characterization*  The proximate analysis results show that the EOP contains 7.31%, 56.5%, 10.91%, and 25.28% of moisture, volatile matter, ash, and fixed carbon, respectively (Table 1). The values obtained from the proximate analysis are in the range reported for most agricultural and forest wastes [54,55].

The physico-chemical properties of the EOP raw materials are significantly important for the assessment of their byproduct potential and quality. Proximate and ultimate analyses are among the easiest ways to study the fuel characteristic of solid materials. The results of these analyses are presented in Table 1. The chemical analysis (Table 1) also shows that the EOP has higher lignin content (49%) than cellulose and hemicellulose contents (holocellulose) (40%). This finding is in line with that reported by Jauhiainen et al. in [5,9], which means that the EOP has a different nature then common biomasses and wastes.

The EOP biomass feedstock is mainly composed of carbon (39.45%) and oxygen (41.2%), with a low percentage of nitrogen (2.68%) and nearly no sulfur content (<0.8%). The EOP biomass composition aligns with other solid wastes reported in the literature The high heating value (HHV) of the EOP (14.43 MJ·kg−<sup>1</sup> ) is also in the range of reported HHV for other biomass feedstocks (11–40 MJ·kg−<sup>1</sup> ) cited in the literature and used as fuels [56].

### [53]. *3.2. EOP Biochar Characterization*

Based on the elemental composition results, the approximate molar chemical formula of the EOP biomass is CH1.69O0.78N0.058. The proximate analysis results show that the EOP contains 7.31%, 56.5%, 10.91%, and 25.28% of moisture, volatile matter, ash, and fixed carbon, respectively (Table 1). The values obtained from the proximate analysis are in the range reported for most agricultural In this section, the deep characterization of the physico-chemical and structural properties of the EOP biochar are presented, aiming to assess its possible application as a fuel in a DCFC system. The performance and the durability of DCFCs are dependent on biochar properties. Some properties appear as inhibitors, whereas others are favorable for the electrochemical conversion of biochar in DCFCs.

and forest wastes [54,55]. The chemical analysis (Table 1) also shows that the EOP has higher lignin content (49%) than cellulose and hemicellulose contents (holocellulose) (40%). This finding is in The ultimate analysis of the EOP biochar is also presented in Table 1 in comparison with the raw material. Carbon, as the main element in the obtained biochar (65.7%), is present in significantly high amount compared to the EOP biomass feedstock (39.45%). However, the oxygen (29.97%) content significantly decreases in comparison to the EOP raw material (41.2%). The low oxygen content in the EOP biochar sample is due to the dehydration and decarbonylation/decarboxylation reactions occurring during carbonization. The nitrogen and sulfur contents in the EOP biochar are low (0.89% and 0.08%, respectively) but still in an acceptable level for fuel application. Nitrogen and sulfur in the EOP biochar

can be considered as impurities that may affect the DCFC performance negatively [57,58]. The atomic ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) are considered to be important indicators of a fuel's quality. It is observed that the H/C and O/C ratios of the EOP biochar are lower than those of the raw EOP biomass. These lower values are due to the increase in carbon content and the decrease in oxygen and hydrogen contents during carbonization due to dehydrogenative polymerization and dehydrative polycondensation reactions [58]. In fact, the low H/C ratio (0.62) of the EOP biochar is indicated on its structural modification, which shifts to a higher content of aromatic compounds that are more resistant to thermal degradation and, thus, remain recalcitrant [59].

When neglecting the biochar's sulfur content, the approximate molar chemical formula of the biochar can be expressed as CH0.62O0.34N0.01.

On the basis of the dry biochar elemental composition, the HHV is calculated using Equation (11) and is found to be equal to 22.21 MJ·kg−<sup>1</sup> . The HHV of the EOP biochar is higher than the EOP biomass and most coals, confirming its potential use as an energy source.

The proximate analysis of the EOP biochar (Table 1) shows a high ash content (21%) and a low moisture content (1.38%). However, the volatile matter and fixed carbon contents of the EOP biochar are 46% and 31.62%, respectively. The EOP biochar exhibits a high content in ash (21.55%), compared to only 6.4% in the case of AS biochar [19] and 7.49% in the case of OW biochar [29]. The ash content increases after the carbonization of the EOP biomass (Table 1), which follows the same tendency of all carbonized biomasses. Considering the ash content, the literature has the unanimous idea that low ash content is desirable for biochar utilization as a fuel in DCFC systems. The mineral matter forming the ash reduces the lifetime of a DCFC by blocking the active reaction sites of the anode, but it is important to note that some mineral impurities, which derive from the biochar fuel source, are considered as inhibitors, while other minerals act as catalysts for the DCFC electrochemical and chemical reaction processes [19].

An analysis of ash composition is critically recommended in order to check if an EOP biochar falls under the IBI standards, which states that, "because of the known presence of heavy metals and organic pollutants in bio-solids, care must be taken during thermochemical conversion to avoid harmful air emissions as well as the accumulation of toxicants in the final carbonaceous skeleton of EOP biochar [60]". In addition, the composition of minerals is crucial to analyze before using a biochar as a fuel in a DCFC system, as previously revealed. Rady et al. [61] mentioned that Al2O<sup>3</sup> and SiO<sup>2</sup> act as the main inhibitor for the DCFC electrochemistry. However, systematic studies on the effects of these species at various concentrations are required before they can be accurately categorized as either inhibitors or catalysts and to know the extent of their inhibitive or catalytic effects.

An investigation of the ash composition was carried out using XRF analysis; the results show that the EOP biochar contains a high amount of silica compared to the alkali and alkaline earth metals (Mg, Ca, K, . . . ) (Table 2), together representing more than 30% of the ash composition. Si is the dominant compound in the EOP biochar (55.21%), followed by Ca (16.3%) and K (13.72%), but Al is present with a low percentage fraction of about 1.79%.

The high content of SiO<sup>2</sup> may cause passivation in the DCFC and an instability in the electrochemical performance of the cell regarding its inhibitive character. This finding has been confirmed by Vutetakis et al. [62], who studied the effects of various mineral impurities on a fluidized bed DCFC and observed a sharp drop in current at high overpotentials, which was explained by the passivation of electrodes since a film was formed on their surfaces due to dissolved Al2O3, SiO2, and TiO<sup>2</sup> from the coal ashes.

At a low gasification temperature (about 700 ◦C), it has been confirmed that a silica structure keeps its physical criteria but undergoes chemical changes through the presence of alkali metals [63]. In a biochar containing very high silica contents, such as rice husks and bagasse, alkali silicates may be formed through the reaction of silica with alkali metals. These resulting silicates are mesoporous with a limited surface area and can be induced in

some other way to have a limited pore volume, as present in the residual biochar. It was also demonstrated that progressive heating of these silicates revealed the ability of trapping coke deposited within the pore media. As a result, the ash residuals showed significant organic contents, even after extensive additional oxidation in air [64].


**Table 2.** Mineral composition of the EOP biochar ashes obtained using XRF analysis.

Furthermore, alkali and alkaline earth metals are identified as effective catalysts for some chemical reaction mechanisms in several applications, such as gasification [64], insitu catalytic fast pyrolysis [65], and even in a DCFC application.

Indeed, the presence of alkali and alkaline earth metals promotes the water–gas shift reaction under the gasification process and enhances the yield of H<sup>2</sup> and CO2. Additionally, they not only boost the breakage and decarboxylation/decarbonylation reaction of the thermally labile hetero atoms of tar, but they also enhance the thermal decomposition of heavier aromatics. These impurities could also significantly enhance the decomposition of levoglucosan. It has been proven that alkaline earth metals show greater effect than alkali metals for these series of decomposition reactions, as reported previously. *C* **2023**, *9*, x FOR PEER REVIEW 12 of 27

Figure 4 presents the X-ray diffraction (XRD) patterns of the EOP biochar, which was performed to investigate its crystallographic structure and disorder. Figure 4 presents the X-ray diffraction (XRD) patterns of the EOP biochar, which was performed to investigate its crystallographic structure and disorder.

The pattern of the EOP is different from other biochars prepared under similar car-

The XRD arrangement for AS biochar [19], OW biochar [29], and corn cob biochar [49], in comparison to graphite, also demonstrate one distinct peak at 2θ = 29°, 26°, and 24°, respectively. This peak corresponds to the (002) reflection, and its presence has been revealed at comparatively lower values when compared to graphite, proving that the latter three biochar samples exhibit a crystallographically disordered structure similar to the

In the case of the EOP biochar, this peak intensity is weak due to the overlap with the intensive peak of silica seen at the same diffraction angle range. This is expected due to the high content of silica present in the EOP biochar, as reported in Table 2. Meanwhile, this behavior was previously reported by Ahmad et al. [69] in their XRD analysis of date palm waste biochar and its derived composite made of the same biochar mixed with silica. The peak of (002) graphitic basal plane depicted in the case of the biochar pattern is totally

The quantitative crystallite parameters of the EOP biochar, including the interplanar

including mainly silica (SiO2), sylvite (KCl), calcite (CaCO3), and dolomite (CaMg(CO3)2). The obtained XRD pattern of the EOP biochar sample has a similar trend to the XRD patterns reported by other researchers, who used a different feedstock, such as wood, grass, corn straw, peanut straw, and claimed that the high numbers of observed peaks is ascribed to various crystal components [66–68]. For example, peaks at 2θ = 20.88 and 25° were designated as silica, while the peak at 2θ = 50.18° was designated as feldspar in a raw silica sample [69]. Thus, a comparison of the EOP biochar pattern with the Powder Diffraction and Standards (PDF) was conducted to identify the corresponding peak of each mineral. The XRD pattern of the EOP biochar indicates similarity in the broad peak corresponding to the (002) graphitic basal plane reflection of graphite. This peak exists at around 2θ = 25°, slightly shifting from the peak location of graphite commonly found at 26.5°,thus indicating the amorphous and turbostatic structure of the EOP biochar. The shift was also reported by Konsolakis et al. [31] when investigating the XRD patterns of

**Figure 4.** XRD patterns of the EOP biochar. distance (d002) and the stacking height (Lc), are also shown in Table 3. The obtained value **Figure 4.** XRD patterns of the EOP biochar.

biochars from pistachio shells, pecan shells, and sawdust.

removed in the case of the silica-composited biochar pattern.

EOP biochar investigated here.

The pattern of the EOP is different from other biochars prepared under similar carbonization conditions [19,29] as it presents a series of sharp peaks at different diffraction angles, confirming the existence of high inorganic material content in the EOP biochar, including mainly silica (SiO2), sylvite (KCl), calcite (CaCO3), and dolomite (CaMg(CO3)2). The obtained XRD pattern of the EOP biochar sample has a similar trend to the XRD patterns reported by other researchers, who used a different feedstock, such as wood, grass, corn straw, peanut straw, and claimed that the high numbers of observed peaks is ascribed to various crystal components [66–68]. For example, peaks at 2θ = 20.88 and 25◦ were designated as silica, while the peak at 2θ = 50.18◦ was designated as feldspar in a raw silica sample [69]. Thus, a comparison of the EOP biochar pattern with the Powder Diffraction and Standards (PDF) was conducted to identify the corresponding peak of each mineral.

The XRD pattern of the EOP biochar indicates similarity in the broad peak corresponding to the (002) graphitic basal plane reflection of graphite. This peak exists at around 2θ = 25◦ , slightly shifting from the peak location of graphite commonly found at 26.5◦ , thus indicating the amorphous and turbostatic structure of the EOP biochar. The shift was also reported by Konsolakis et al. [31] when investigating the XRD patterns of biochars from pistachio shells, pecan shells, and sawdust.

The XRD arrangement for AS biochar [19], OW biochar [29], and corn cob biochar [49], in comparison to graphite, also demonstrate one distinct peak at 2θ = 29◦ , 26◦ , and 24◦ , respectively. This peak corresponds to the (002) reflection, and its presence has been revealed at comparatively lower values when compared to graphite, proving that the latter three biochar samples exhibit a crystallographically disordered structure similar to the EOP biochar investigated here.

In the case of the EOP biochar, this peak intensity is weak due to the overlap with the intensive peak of silica seen at the same diffraction angle range. This is expected due to the high content of silica present in the EOP biochar, as reported in Table 2. Meanwhile, this behavior was previously reported by Ahmad et al. [69] in their XRD analysis of date palm waste biochar and its derived composite made of the same biochar mixed with silica. The peak of (002) graphitic basal plane depicted in the case of the biochar pattern is totally removed in the case of the silica-composited biochar pattern.

The quantitative crystallite parameters of the EOP biochar, including the interplanar distance (d002) and the stacking height (Lc), are also shown in Table 3. The obtained value proves the turbostatic structure of the EOP biochar as the d<sup>002</sup> value (0.3622 nm) is slightly higher than the graphite d<sup>002</sup> value (0.36 nm), and the L<sup>c</sup> (24.02 nm) is much higher than the L<sup>c</sup> (13.8 nm) value assigned to graphite.


**Table 3.** Summarized properties of the EOP biochar sample.

The electrochemical reactions taking place in the DCFC anode occur predominantly on the carbon surface, which is generally represented by the pore walls. A higher porosity of carbon materials implies an extended specific surface area and, subsequently, more available reaction sites for electrochemical reactions and better DCFC efficiency. This provides major advantages for the biochar over other fuels with a smooth surface area, such as graphite and raw biomass. In this context, the N<sup>2</sup> BET adsorption technique was performed on the EOP biochar; unfortunately, the obtained result does not permit the prediction of its micro-structural properties. This could be due to the mesoporous and macroporous structure of the EOP biochar. Micropores will be filled in a single step over a narrow range of relative pressure before the formation of a monolayer coverage on the biochar surface. This will disturb the adsorption of N<sup>2</sup> and, subsequently, does not give useful data. This EOP biochar porous structure may be related to the operating conditions of the carbonization experiment being carried out at 400 ◦C for four hours. It is evident that, as carbonization temperature increases, pore blocking substances are driven off or are thermally cracked, thus increasing the externally accessible surface area [70]. However, the extended carbonization holding time (4 h) used in this study can have the opposite effect since the reactions continue at the pore surface area, causing a decrease in micropores and a shift toward meso- and macropores [70].

The scanning electron microscopy (SEM) images of the EOP biochar sample are shown in Figure 5a,b. The SEM images of the biochar produced from EOP pyrolysis at 400 ◦C show a hardly visible porosity (Figure 5a). The presence of crystalline phases with cubic, tubular, and elongated shapes on the particle's surfaces show that the particles are rough and grainy. The pore sizes are not uniform and are in the range of tens of nanometers to microns (Figure 5b). *C* **2023**, *9*, x FOR PEER REVIEW 14 of 27

**Figure 5.** Scanning electron micrographs (SEM) of the EOP biochar: (**a**) at 100 μm resolution and (**b**) at 5 μm resolution **Figure 5.** Scanning electron micrographs (SEM) of the EOP biochar: (**a**) at 100 µm resolution and (**b**) at 5 µm resolution.

The mercury intrusion porosimetry was used in this study as a complementary technique to obtain a better textural characterization of the EOP biochar. Figure 6 displays the log differential mercury intrusion volume as a function of the pore diameter of the EOP biochar. Indeed, the EOP biochar presents a porosity in the mesopore–macropore range; more specifically, it is an inter-particle porosity due to the high pore size diameter (6 nm up to 3000 nm) (Figure 5), at which the mercury intrusion occurs[71]. This finding is aligned with the SEM micrographs presented above. The pore structure and the pore size distributions are summarized in Table 3.The recorded relatively low surface area observed for the EOP biochar (52.495 m2·g−1) is probably due to the inorganic materials, The mercury intrusion porosimetry was used in this study as a complementary technique to obtain a better textural characterization of the EOP biochar. Figure 6 displays the log differential mercury intrusion volume as a function of the pore diameter of the EOP biochar. Indeed, the EOP biochar presents a porosity in the mesopore–macropore range; more specifically, it is an inter-particle porosity due to the high pore size diameter (6 nm up to 3000 nm) (Figure 5), at which the mercury intrusion occurs [71]. This finding is aligned with the SEM micrographs presented above. The pore structure and the pore size distributions are summarized in Table 3. The recorded relatively low surface area observed for the EOP biochar (52.495 m<sup>2</sup> ·g −1 ) is probably due to the inorganic materials, mainly silica particles, that partially fill or block the micropores.

**Figure 6.** Log differential intrusion as a function of the pore diameter of the EOP biochar.

Temperature programmed oxidation (TPO) is an efficient tool to evaluate the relative activity of carbon oxidation. The weight loss curve of the EOP biochar is shown in Figure

7.

mainly silica particles, that partially fill or block the micropores.

mainly silica particles, that partially fill or block the micropores.

(**b**) at 5 μm resolution

**Figure 6.** Log differential intrusion as a function of the pore diameter of the EOP biochar. **Figure 6.** Log differential intrusion as a function of the pore diameter of the EOP biochar.

**Figure 5.** Scanning electron micrographs (SEM) of the EOP biochar: (**a**) at 100 μm resolution and

The mercury intrusion porosimetry was used in this study as a complementary technique to obtain a better textural characterization of the EOP biochar. Figure 6 displays the log differential mercury intrusion volume as a function of the pore diameter of the EOP biochar. Indeed, the EOP biochar presents a porosity in the mesopore–macropore range; more specifically, it is an inter-particle porosity due to the high pore size diameter (6 nm up to 3000 nm) (Figure 5), at which the mercury intrusion occurs[71]. This finding is aligned with the SEM micrographs presented above. The pore structure and the pore size distributions are summarized in Table 3.The recorded relatively low surface area observed for the EOP biochar (52.495 m2·g−1) is probably due to the inorganic materials,

Temperature programmed oxidation (TPO) is an efficient tool to evaluate the relative activity of carbon oxidation. The weight loss curve of the EOP biochar is shown in Figure Temperature programmed oxidation (TPO) is an efficient tool to evaluate the relative activity of carbon oxidation. The weight loss curve of the EOP biochar is shown in Figure 7. *C* **2023**, *9*, x FOR PEER REVIEW 15 of 27

**Figure 7.** Temperature programmed oxidation (TPO) of the EOP biochar. **Figure 7.** Temperature programmed oxidation (TPO) of the EOP biochar.

Based on the TPO results of EOP biochar, four stages are depicted, indicating the presence of four forms of carbons with different resistance to oxidation. The initial small weight loss below 150 °C is caused by the desorption of physiosorbed water within the EOP biochar and the oxidation of volatile organic C (Stage I). A small weight loss is observed below 380 °C due to the rapid release of combustion gases from the superficial oxygen functional groups (Stage II). This could be related to the oxidation of labile organic carbons, such as aliphatic carbon. In addition, a significant weight loss in the EOP biochar starts from 380 °C and ends at around 650 °C (Stage III). During this stage, the oxidation of recalcitrant organic carbons (predominantly formed by lignin and recalcitrant carbon, such as aromatic carbon) and refractory organic carbons (poly-condensed forms of aromatic carbon) occurs between 380 and 475 °C and between 475 and 650 °C, respectively. Based on the TPO results of EOP biochar, four stages are depicted, indicating the presence of four forms of carbons with different resistance to oxidation. The initial small weight loss below 150 ◦C is caused by the desorption of physiosorbed water within the EOP biochar and the oxidation of volatile organic C (Stage I). A small weight loss is observed below 380 ◦C due to the rapid release of combustion gases from the superficial oxygen functional groups (Stage II). This could be related to the oxidation of labile organic carbons, such as aliphatic carbon. In addition, a significant weight loss in the EOP biochar starts from 380 ◦C and ends at around 650 ◦C (Stage III). During this stage, the oxidation of recalcitrant organic carbons (predominantly formed by lignin and recalcitrant carbon, such as aromatic carbon) and refractory organic carbons (poly-condensed forms of aromatic carbon) occurs between 380 and 475 ◦C and between 475 and 650 ◦C, respectively. From 650 ◦C to 1000 ◦C, inorganic carbons (carbonates) are oxidized (Stage IV).

From 650 °C to 1000 °C, inorganic carbons (carbonates) are oxidized (Stage IV). Compared to the TPO curves of graphite, the onset of stage III of the EOP biochar is much lower than that of graphite, which starts at 650 °C [72]. Moreover, the offset of this stage for EOP biochar ends earlier compared to graphite (850 °C). The enhanced thermal stability of the EOP biochar in air can be assigned to its lower graphitic degree and crys-Compared to the TPO curves of graphite, the onset of stage III of the EOP biochar is much lower than that of graphite, which starts at 650 ◦C [72]. Moreover, the offset of this stage for EOP biochar ends earlier compared to graphite (850 ◦C). The enhanced thermal stability of the EOP biochar in air can be assigned to its lower graphitic degree and crystallinity, which confirms the findings observed in its XRD analysis.

tallinity, which confirms the findings observed in its XRD analysis. FTIR spectroscopy was performed in order to analyze the surface functional groups of the EOP biochar, aiming to assess its stability. The EOP biochar FTIR spectrum is shown FTIR spectroscopy was performed in order to analyze the surface functional groups of the EOP biochar, aiming to assess its stability. The EOP biochar FTIR spectrum is shown in Figure 8.

**Figure 8.** FTIR spectrum of the EOP biochar sample.

in Figure 8.

**Figure 7.** Temperature programmed oxidation (TPO) of the EOP biochar.

Based on the TPO results of EOP biochar, four stages are depicted, indicating the presence of four forms of carbons with different resistance to oxidation. The initial small weight loss below 150 °C is caused by the desorption of physiosorbed water within the EOP biochar and the oxidation of volatile organic C (Stage I). A small weight loss is observed below 380 °C due to the rapid release of combustion gases from the superficial oxygen functional groups (Stage II). This could be related to the oxidation of labile organic carbons, such as aliphatic carbon. In addition, a significant weight loss in the EOP biochar starts from 380 °C and ends at around 650 °C (Stage III). During this stage, the oxidation of recalcitrant organic carbons (predominantly formed by lignin and recalcitrant carbon, such as aromatic carbon) and refractory organic carbons (poly-condensed forms of aromatic carbon) occurs between 380 and 475 °C and between 475 and 650 °C, respectively.

Compared to the TPO curves of graphite, the onset of stage III of the EOP biochar is much lower than that of graphite, which starts at 650 °C [72]. Moreover, the offset of this stage for EOP biochar ends earlier compared to graphite (850 °C). The enhanced thermal stability of the EOP biochar in air can be assigned to its lower graphitic degree and crys-

FTIR spectroscopy was performed in order to analyze the surface functional groups of the EOP biochar, aiming to assess its stability. The EOP biochar FTIR spectrum is shown

From 650 °C to 1000 °C, inorganic carbons (carbonates) are oxidized (Stage IV).

tallinity, which confirms the findings observed in its XRD analysis.

**Figure 8. Figure 8.** FTIR spectrum of the EOP biochar sample. FTIR spectrum of the EOP biochar sample.

in Figure 8.

The broad band at 3400–3200 cm−<sup>1</sup> indicates the presence of O-H stretching vibration. The bands at the region between 3000 and 2800 cm−<sup>1</sup> are related to the presence of aliphatic C-H stretching vibrations. The bands at 1600–1500 cm−<sup>1</sup> denote the presence of aromatic C=C ring stretching. The bands at 1405 cm−<sup>1</sup> and 1318 cm−<sup>1</sup> are assigned to the O-H bending vibrations in acid, alcohols, and phenol groups. The band at 1016 cm−<sup>1</sup> indicates the C-O stretching vibrations in carbonyl compounds, such as alcohols, phenols, ester, ether, and acid. The bands at 900–600 cm−<sup>1</sup> are related to the C-H bending vibrations in aromatic hydrocarbons. This analysis shows that the derived EOP biochar structure is mainly composed of aromatics, which explain the lower value of the H/C ratio reported in Table 1.
