*3.3. Feasibility Testing of the EOP Biochar as Fuel in DCFC*

The direct carbon fuel cell voltage versus current density curve and the power density versus current density curve of the EOP biochar are shown in Figure 9, in comparison to the performances recorded for the DCFCs fed by biochars from almond shell (AS) [19] and olive wood (OW) [29] that were prepared under the same carbonization process.

The DCFC pellets used for testing the electrochemical performance of the EOP biochar sample are also the same as those used for the AS and OW biochar tests in terms of electrolyte and cathode materials, cell geometry, gas flow rates, and operating temperature (700 ◦C) [19,29]. On the basis of the experimental observation after the cell tests, in the case of the EOP biochar, the fuel utilization must be reduced in comparison to the case of the AS and OW biochars tested in previous works. This could be related to the high content of ash. This result is in agreement with the literature [73,74]. In the case of corn straw carbon, Li et al. [73] noted a better fuel utilization rate and a lower weight of remains after discharging in the DCFC anode, which contains only 1.6 wt% of ash.

The obtained results prove the feasibility of DCFC operation by using the EOP biochar as a fuel, showing an open circuit voltage of 0.8 V, a peak power density Pmax of about 10 mW·cm−<sup>2</sup> , and a limiting current density of 142 mA·cm−<sup>2</sup> at 700 ◦C. The main useful criterion of the evaluation of DCFC electrochemical performance and quality is the achieved maximum power density. The recorded Pmax of about 10 mW·cm−<sup>2</sup> in this study can be considered as promising if compared to the reported power densities of several biochars from pistachio shells (3.7 mW·cm−<sup>2</sup> ), pecan shells (3.2 mW·cm−<sup>2</sup> ), and sawdust (1.4 mW·cm−<sup>2</sup> ),which were all tested in DCFCs at 700 ◦C [31]. Nevertheless, this performance appears so modest compared to those obtained by the AS and OW biochar-fed DCFCs reported in Figure 9, which exhibited a performance 10 times higher than that of the EOP biochar.

*3.3. Feasibility Testing of the EOP Biochar as Fuel in DCFC* 

**Figure 9.** (**a**) I–V and (**b**) I–P curves of the DCFC powered by the EOP biochar at 700 °C, compared to those of the DCFCs powered by AS biochar [19] and OW biochar [29]. **Figure 9.** (**a**) I–V and (**b**) I–P curves of the DCFC powered by the EOP biochar at 700 ◦C, compared to those of the DCFCs powered by AS biochar [19] and OW biochar [29].

The broad band at 3400–3200 cm−1 indicates the presence of O-H stretching vibration. The bands at the region between 3000 and 2800 cm−1 are related to the presence of aliphatic C-H stretching vibrations. The bands at 1600–1500 cm−1 denote the presence of aromatic C=C ring stretching. The bands at 1405 cm−1 and 1318 cm−1 are assigned to the O-H bending vibrations in acid, alcohols, and phenol groups. The band at 1016 cm−1 indicates the C-O stretching vibrations in carbonyl compounds, such as alcohols, phenols, ester, ether, and acid. The bands at 900–600 cm−1 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.

The direct carbon fuel cell voltage versus current density curve and the power density versus current density curve of the EOP biochar are shown in Figure 9, in comparison to the performances recorded for the DCFCs fed by biochars from almond shell (AS) [19] and olive wood (OW) [29]that were prepared under the same carbonization process.

The DCFC pellets used for testing the electrochemical performance of the EOP biochar sample are also the same as those used for the AS and OW biochar tests in terms of electrolyte and cathode materials, cell geometry, gas flow rates, and operating temperature (700 °C) [19,29]. On the basis of the experimental observation after the cell tests, in the case of the EOP biochar, the fuel utilization must be reduced in comparison to the case It is worth highlighting that four peculiar phenomena are observed in the recorded performance curve, including the low OCV value, the high activation loss at low current densities, the limited peak power density (Pmax), and the reduced limiting current density (ilim). We have to address the physico-chemical properties of the EOP-derived biochar sample that are certainly the cause of these phenomena, as previous characterization have demonstrated some peculiar properties when compared to other biochars prepared under the same carbonization conditions, such as AS and OW biochars [19,29]. This difference is certainly related to the raw biomass's material properties and the kinetics of the complex chemical degradation reactions occurring during the carbonization process.

Several researchers proved that the overall DCFC efficiency is ascribed mainly to the gas–active electrochemical zone interactions, rather than to the extremely pure carbon–active electrochemical zone contact. The reverse Boudouard reaction (Equation (5)), which represents the main non-electrochemical reaction possible to occur at high DCFC temperatures (>700 ◦C), has a determining role in the DCFC performance as its gaseous product (CO) can readily diffuse within the active electrochemical reaction site at a considerably faster rate than solid carbon, contributing highly to the enhancement in the power output. In addition to CO gas, the in situ generation of hydrogen (H2) is another key point when the DCFC is fed by hydrogenated carbonaceous materials and is also a cause of the high values of DCFC power densities.

In this context, one of the carbonaceous material's properties that can affect the Pmax recorded by DCFCs is the carbon content. A biochar containing low carbon generates low

amounts of obstructive matters (CO and CO2), which potentially affects the pathways of chemical and electrochemical reactions occurring in the cell anode. The carbon content in the EOP biochar sample (65.7%) is lower than that of the AS biochar (71.8%) [19] and that of the OW biochar (84.07%) [29], which could be one of the causes of the limited performance recorded by the DCFC fed by the EOP biochar. Indeed, the EOP low carbon content may hinder and reduce the amount of CO<sup>2</sup> and CO gases produced in situ at the anode compartment, subsequently limiting the extent of the CO electrochemical reactions within the anode of the DCFC (Equations (9) and (10)).

Additionally, another biochar property, which may explain the limited power density recorded in this case study, is related to the proximate composition, which has a direct effect on the evolved gases emitting off from the carbonaceous skeleton via thermal cracking reactions. Nevertheless, the literature shows that there is no consensus on the influence of biochar volatile matter content on DCFC performance. Chien et al. [75] reported that a high content of volatile matter causes a limitation in DCFC performance, while a medium volatile matter content is more appropriate for the use of a biochar as fuel in the DCFC. In contrast, Kaklidis et al. [30] affirmed that the volatile matter content greatly improves the electrochemical output of DCFCs.

Fuel ratio is a ratio of the amount of fixed carbon to volatile matter and is a distinctive feature of solid fuels, which can be used to categorize them as a coal rank according to the ASTM 388 [33]. Compared to the OW and AS biochars [19,29], which have a respective fuel ratio of about 0.563 and 0.245, the EOP biochar has a higher fuel ratio (0.897) as its fixed carbon is relatively high (about 36.45%), whereas the volatile matter is limited (40.62%). The improved fuel ratios of biochars are known to be significant, indicating greater combustion efficiencies and reduced pollutant emissions during the burning process of biochars, when compared to the burning of their raw forms. Contrarily, it appears that in the case of DCFCs, lower fuel ratio in the biochars is a preferred property for better cell output.

Additionally, the original lignocellulosic composition of the biomass can provide insight into the mass yield of the produced biochar through pyrolysis and, subsequently, its performance as a fuel in a DCFC system.

According to the DCFC performance recorded for the EOP biochar, the biomass composition is dominated by lignin, rather than by holocellulosic content, which is not preferred despite its potential in increasing the yields of the biochar. Correspondingly, the high ash content in the biomass favors charring reactions rather than devolatilization reactions [76], leading to an enhancement in the yield of the biochar.

Seeing that the objective is not focused on the biochar quantity produced from a biomass but rather its quality and potential to be electrochemically active, leading to a promising cell output, we notice that a biomass with a high lignin and ash content is not preferred in the production of a biochar fuel for DCFC systems.

Moreover, the crystallographic structure of carbonaceous materials is a key property over others that directly affects the electrical conductivity and the reactivity of biochars. A high degree of order in the structure provides enhanced electrical conductivity. We should keep in mind that when carbon is applied as fuel in a DCFC, the reactivity, which is known to be higher in non-graphitic structures, is more important than high electrical conductivity values [77]. It is often stated in the literature that the electrical conductivity of solid carbonaceous materials is not correlated with the DCFC power output, regardless of the diversity of anode configurations. Logically, higher electrical conductivity values are generally recorded for graphitic or crystalline structures of carbon materials, which should induce a reduced global DCFC ohmic resistance and, consequently, a better DCFC performance. Unfortunately, that has never been observed when these high-ordered carbon materials were employed as fuels in DCFC systems. The need for crystalline carbon materials was revealed when using carbon materials simultaneously as the anode, the current collector, and the fuel in a few DCFC configurations. In contrast, when using an anode current collector (silver for example) to collect electrons, the dependence of the cell performance on the electrical conductivity of carbon materials was more curtailed.

Meanwhile, the crystalline structure of solid carbon materials has often been measured using XRD analysis. Chien et al. [41] reported that a DCFC's electric power generation and long-term stability have a strong correlation with the crystalline structure of solid carbons. A high power density and good stability are obtained when using less crystalline carbon, which is characterized by a broad (002) diffraction peak et 2θ = 20–30◦ on the XRD pattern.

In this study, the obtained XRD pattern of the EOP biochar demonstrates a high number of peaks, which is different to the AS [19] and OW [29] biochar patterns that exhibit only two peaks. As previously mentioned, the EOP biochar has a high ash content, which explains the diversity of peaks recorded as belonging to silica, dolomite, sylvite, etc. This represents one of the features of a low reactive carbon material and explains the low recorded peak power density value. Based on the limited performance recorded by the DCFC fed by the EOP biochar (Figure 9), it is shown that the biochar's disordered structure is not sufficient to judge its potential as fuel in a DCFC system, but it is necessary to assess more of its content in terms of mineral crystalline materials.

More specifically, the presence of silica is a cause of the low EOP biochar reactivity and conductivity to electrons because the silica non-crystalline particles act as insulators to electron transfer and as inhibitors to electrochemical oxidation reactions.

As previously mentioned, the reactivity of carbon materials appears to be a key property affecting the maximum power value (Pmax) generated by the DCFC. This is evidenced through the results obtained by TPO analysis. The reactivity of the EOP biochar is also lower than that of the AS and OW biochars and can be considered as another reason for the limited DCFC performance shown in Figure 9; stage III of the EOP biochar's TPO profile (Figure 6) starts at almost the same temperature as the AS and OW biochars but ends later at about 650 ◦C in comparison to 580 ◦C and 590 ◦C in the case of the AS and OW biochars, respectively.

Eom et al. [78] affirmed that the ratio of total oxygen to carbon, i.e., surface functional groups containing oxygen, has the dominant effect on the electrochemical reaction relative to the surface area in a DCFC. However, in our case, the EOP exhibits a higher O/C ratio, which means higher surface oxygen groups, compared to the AS and OW biochars [19,29], but the DCFC performance is comparatively limited. It is also known that when the amount of oxygen-containing surface groups increases remarkably, it favors a decrease in the electrical conductivity due to the preponderance of insulating effects caused by the functional groups on the surface [79].

Regarding the hybrid character of the DCFC anode, which allows the simultaneous electrochemical conversion of both solid carbon and reformed gaseous fuels (CO and H2), an analysis of the nature of the surface oxygenated functional groups, apart from their extent, enables an estimation of their production potential of the gaseous species mentioned above and, subsequently, allows an assessment of their contribution to the complex reaction scheme of anode chemistry and electrochemistry. The FTIR spectrum of the EOP biochar (Figure 8) indicates the absence of the peak of C=O at about 1700–1710 cm−<sup>1</sup> , which reflects the absence of carbonyl, carboxyl, lactone, and anhydride groups within the surface chemical structure of the EOP biochar. These oxygen functional groups are known to contribute to the generation of CO and CO<sup>2</sup> gases within the anode and, thus, cause the occurrence of several chemical reactions, such as the water–gas shift reaction, the Boudouard reaction, the water shift reaction, and the methanation reaction. Elleuch et al. [29] proposed a chemical mechanism of OW biochar devolatilization within a DCFC anode and showed that CO<sup>2</sup> evolves from the decomposition of carboxylic acid functionality at low temperatures and/or lactones at high temperatures, while CO arises from carbonyls at high temperatures.

On the other hand, the EOP biochar's FTIR spectrum (Figure 8) shows the presence of C-O single-bond groups ascribed to the high content of phenols and ethers. These groups are more difficult to decompose but are able to produce CO and CO<sup>2</sup> at very high temperatures in the presence of catalysts. In conclusion, it seems that the existence of C=O bonds is preferred over C-O bonds for the easiest way to generate gases within the anode of a DCFC and for the further electrochemical contribution of these gases to the DCFC electrochemistry; these bonds are absent in the case of the EOP biochar. Moreover, the high O/C content indicates the extended availability of surface oxygen groups, although this information remains not informative. We need to identify the chemical functional groups that are known to be easily decomposed, such as carbonyl groups.

The DCFC fed by the EOP biochar delivers a limited current density, which is also correlated with its distinct physico-chemical properties. HHV is a fuel property that indicates the chemical energy density that will be converted into electricity through DCFC systems. The HHV of the EOP biochar is higher than that of the EOP raw biomass due to the conversion of oxygenated compounds into carbonaceous hydrocarbons, which reduces the oxygen content in the biochar and increases the carbon content. As shown in Figure 9, the limiting current density delivered by the DCFC fueled by the EOP biochar (142 mA·cm−<sup>2</sup> ) is so low compared to that of the AS biochar (450 mA·cm−<sup>2</sup> ) and the OW biochar (550 mA·cm−<sup>2</sup> ). The HHV also follows the same trend: OW biochar (27.31 MJ·kg−<sup>1</sup> ) [29] > AS biochar (24.7 MJ·kg−<sup>1</sup> ) [19] > EOP biochar (22.21 MJ·kg−<sup>1</sup> ). It is, thus, evident that the higher the biochar energy density is, the higher the current density delivered by the cell is; however, there is a high disproportion between the ordered HHV values and the decrease in the limiting current density.

In order to investigate this behavior between the HHV and the limiting current of the cell, we selected another biochar (corn cob, noted as CC) tested in a similar DCFC system by Yu et al. [49]. The results illustrated in Figure 10a show that the limiting current density recorded by the DCFC follows a polynomial trend as a function of the biochar's HHVs, according to the following equation:

<sup>i</sup>lim <sup>=</sup> <sup>−</sup>14.955 HHV<sup>2</sup> + 821.23 HHV <sup>−</sup> 10,723 with R<sup>2</sup> = 0.9962. This correlation could predict the DCFC's output when applying different types of carbon materials as fuel in similar DCFC operating conditions (700 ◦C) and materials.

As the biochar's HHVs range from 22 to 32.5 MJ·kg−<sup>1</sup> , we used the previously mentioned polynomial law to assess the dependency between the biochar's property HHV and its impact on the cell's current output. The obtained curve (Figure 10b) shows that the biochar has a HHV ranging between 24 and 30 MJ·kg−<sup>1</sup> , resulting in a promising DCFC current density (higher than 400 mA·cm−<sup>2</sup> ).

The electrochemical reactions in a DCFC system occur predominantly on the carbon surface represented by the pores walls. Consequently, a higher porosity results in a larger surface area. It is known that a high interfacial surface enlarges the availability of sites for electrochemical reactions and, consequently, leads to an increase in the electron flux generated via the electrochemical reactions of the DCFC.

Figure 6 shows the distribution of the pore diameter within the EOP biochar, which is similar to that of the AS and OW biochars but different from derived biochars from agricultural wastes, which are often considered to be entirely microporous (D < 2 nm). The presence of the meso–macroporosity in the EOP biochar helps ameliorate the mass transport and the kinetic processes, which results in the promising current density output recorded when testing the EOP biochar (142 mA·cm−<sup>2</sup> ).

The surface area of a wooden biochar is about 131.4 m<sup>2</sup> ·g −1 [17], which is limited but higher than that of the analyzed EOP biochar (52.495 m<sup>2</sup> ·g −1 ). These surface area values are in good agreement with those reported in the literature, especially with the fact that these samples have resulted from a carbonization process with no further activation step [71]. We should also notice that the AS and OW biochars exhibit very limited specific surface areas in the range between 30 and 40 m<sup>2</sup> g −1 , which is in a comparable range with that of the EOP biochar sample.

There is a strong correlation between the pore volume and surface area measurements based on N<sup>2</sup> adsorption, but the use of pore volume may be preferable due to the low surface area values in the case of biochar obtained from carbonization [41].

**Figure 10.** Evolution of (**a**) experimentally recorded current densities and (**b**) modeled polynomial current densities delivered by the DCFC system as a function of the biochar's HHVs.

The total pore volume of the EOP biochar is three and two times lower than the OW and AS biochars, respectively. Similarly, its porosity is very limited (about 36%). This may be caused by the high content of silica within the EOP biochar sample, which hinders its porousness and leads to the limited current density (ilim) delivered by the DCFC (Figure 9). The ilim also depends on the diffusion ability of species, which appears to be restricted in this case. Porousness could, thus, be enhanced by removing silica from the biochar, which can be transformed into activated carbon when exposed to higher temperature.

The open circuit voltage (OCV) of the DCFC fed by the EOP biochar is lower than the theoretical OCV value (ca. 1.02 V) of the DCFC. This low OCV is not related to the low densification of the composite electrolyte, which is proven through an SEM investigation of the electrolyte after the test, as shown in Figure 11. In Figure 11a, the porous structure of the DCFC pellet at room temperature is clearly observed. The composite electrolyte and the cathode layers indicate different morphologies. A large particle size distribution ranging from 90 to 500 nm and from 45 to 312 nm for the LixNi1−xO cathode particles and the composite electrolyte particles, respectively, are obtained. This is not the case in the second micrograph of the DCFC pellet obtained after the electrochemical test (Figure 11b). After the DCFC test, it becomes difficult to distinguish between the pellet layers (cathode/electrolyte). Additionally, a distinction of the SDC particles from the carbonate phase is not possible because the SDC particles are coated completely by the solidified carbonate, which has once melted, proving the fact that the electrolyte is well densified and prevents the cathode gas from leaking through during the DCFC electrochemical test. The composite electrolyte provides the pathways for the transfer of both oxide and carbonate ions. Thus, the nature of the fuel may be the cause of this OCV tendency. This behavior is different from that recorded in our previous studies using the AS [19] and OW [29] biochars. This deviation can

be ascribed to the distinct and complex schemes of chemical and electrochemical reactions occurring within the anodic side, yielding a lower concentration of reactive gaseous species (CO and H2) (Figure 12). *C* **2023**, *9*, x FOR PEER REVIEW 22 of 27 *C* **2023**, *9*, x FOR PEER REVIEW 22 of 27

**Figure 11.** Scanning electron micrograph of the electrolyte structure: (**a**) before cell testing at room temperature and (**b**) after cell testing at T=700 °C. **Figure 11.** Scanning electron micrograph of the electrolyte structure: (**a**) before cell testing at room temperature and (**b**) after cell testing at T = 700 ◦C. **Figure 11.** Scanning electron micrograph of the electrolyte structure: (**a**) before cell testing at room temperature and (**b**) after cell testing at T=700 °C.

**Figure 12.** Presentation of the anodic reaction mechanisms of the EOPbiochar as a fuel in the **Figure 12.** Presentation of the anodic reaction mechanisms of the EOPbiochar as a fuel in the **Figure 12.** Presentation of the anodic reaction mechanisms of the EOPbiochar as a fuel in the DCFC.

The limited OCV value is, thus, another proof of the previously outlined finding related directly to the reduced formation of gases as fuels due to the absence of carbonyl groups at the surface functionalities of the EOP biochar. This is aligned with the results reported by Kaklidis et al. [30], who clearly correlated the CO formation rate obtained under open circuit conditions with the achieved DCFC electrochemical performance and proved that the overall chemical and electrochemical processes are driven by the CO shut-The limited OCV value is, thus, another proof of the previously outlined finding related directly to the reduced formation of gases as fuels due to the absence of carbonyl groups at the surface functionalities of the EOP biochar. This is aligned with the results reported by Kaklidis et al. [30], who clearly correlated the CO formation rate obtained under open circuit conditions with the achieved DCFC electrochemical performance and proved that the overall chemical and electrochemical processes are driven by the CO shut-The limited OCV value is, thus, another proof of the previously outlined finding related directly to the reduced formation of gases as fuels due to the absence of carbonyl groups at the surface functionalities of the EOP biochar. This is aligned with the results reported by Kaklidis et al. [30], who clearly correlated the CO formation rate obtained under open circuit conditions with the achieved DCFC electrochemical performance and proved that the overall chemical and electrochemical processes are driven by the CO shuttle mechanism.

tle mechanism. Furthermore, it appears that other physico-chemical properties of the biochar, such as the mineral composition, strongly affect the gas phase/surface chemical reactions and the charge transfer phenomena, modifying the anode gas composition and, subsequently, tle mechanism. Furthermore, it appears that other physico-chemical properties of the biochar, such as the mineral composition, strongly affect the gas phase/surface chemical reactions and the charge transfer phenomena, modifying the anode gas composition and, subsequently, Furthermore, it appears that other physico-chemical properties of the biochar, such as the mineral composition, strongly affect the gas phase/surface chemical reactions and thecharge transfer phenomena, modifying the anode gas composition and, subsequently, the recorded OCV.

In the case of DCFC application, the research work by Li and co-workers [43], who studied the effect of several metallic oxides common to coal ash on the electrochemical

In the case of DCFC application, the research work by Li and co-workers [43], who studied the effect of several metallic oxides common to coal ash on the electrochemical

DCFC.

DCFC.

the recorded OCV.

the recorded OCV.

In the case of DCFC application, the research work by Li and co-workers [43], who studied the effect of several metallic oxides common to coal ash on the electrochemical performance of DCFCs through the addition of a fixed amount (8%) of the oxides of calcium, magnesium, iron, aluminum, and silica impregnated in activated carbon (AC),confirmed that MgO, CaO, and Fe2O<sup>3</sup> act as catalysts for the electrochemical process of DCFCs [43]. Elleuch et al. [29] analyzed an OW biochar's performance as a fuel in a DCFC system and affirmed that CaO and K2O act as catalysts for the anodic electrochemical mechanism of the cell. The same catalytic effect of the latter alkali and alkaline metals (K2O and CaO) was also revealed in the case of an AS biochar-fed DCFC, which is considered one of the main positive characters of the biochar, leading to the promising performance. We should also notice that these minerals are able to catalyze several devolatilization reactions occurring within the biochar skeleton, such as the Boudouard reaction and the water–gas shift reaction.

In this study, the EOP biochar's content of K2O (13.72 wt%) and CaO (16.31 wt%) minerals in the inorganic fraction of the ash is far different to both the OW and AS biochars, which were previously affirmed as promising fuels in DCFCs [19,29]. This difference is primarily related to the biomass essence.

Additionally, the catalytic contribution of these metals to the DCFC's electrochemical and the biochar's devolatilization reactions seems to be curtailed by a high content of silica in the EOP biochar. This is in agreement with other research works on gasification catalysts, which concluded that the catalytic effect of metals, such as (K), was reduced by the reaction with silica to form silicate during gasification, rather than to the accelerated devolatilization reactions [64].

Another peculiarity observed in the I–V curve of the DCFC fed by the EOP biochar (Figure 9) is related to the pronounced activation polarization. This voltage loss results from the difficulties encountered by reactive species (solid carbon, CO, and H2) when carrying out electrochemical reactions at lower current densities. The limited kinetics of reactions is also related to the physico-chemical properties of the biochar. Biochar porousness will curtail the reaction sites, and a high silica content will hinder the diffusion of species on the one hand and contribute to the insulation of some of the available reaction sites on the other hand. Altogether, these explain the high activation polarization taking place during the operation of the DCFC fed by the EOP biochar.
