**1. Introduction**

The use of biomass and waste as fuel represents not only a feasible but also promising alternative to conventional fossil fuels. Electricity from biomass and waste has received attention in recent years. The main advantage of biomass energy-based electricity is that renewable fuel is often a byproduct, a residue, or a waste produced from different sources, such as wood residues, agricultural residues, or agro-industrial wastes and byproducts. In this context, the national Waste Management Agency estimates that Tunisia produces around six million tons of organic wastes per year, where 73% comes from household wastes, farms, and the agro-food industry, while 17% is due to olive oil pressing residues. Tunisia, as one of the world's top five producers of olive oil, collects a huge amount (around 300.000 tons/year) of exhausted olive pomace (EOP) residues yearly. EOP is one of the main byproducts obtained from olive oil extraction, which represents an important economic sector in Mediterranean countries and specifically in Tunisia. This agro-industrial waste has a variable composition, depending on the olive variety and the olive oil processing methods. EOP consists of a lignocellulosic matrix with polyphenolic compounds, uronic acids, oily residues, water-soluble fats, proteins, water-soluble carbohydrates, and watersoluble phenolic substances [1,2], and it is rich in potassium and poor in phosphorus and micronutrients [2]. Nevertheless, the disposal of EOP is one of the major environmental problems due to its phytotoxicity and antimicrobial properties [3]. It also increases the hydrophobicity and the infiltration rate of soil and decreases the water retention rate [4,5].

**Citation:** Grioui, N.; Elleuch, A.; Halouani, K.; Li, Y. Valorization of Exhausted Olive Pomace for the Production of a Fuel for Direct Carbon Fuel Cell. *C* **2023**, *9*, 22. https://doi.org/10.3390/c9010022

Academic Editors: Indra Neel Pulidindi, Pankaj Sharma, Aharon Gedanken and Shuguang Deng

Received: 1 December 2022 Revised: 26 January 2023 Accepted: 2 February 2023 Published: 14 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

However, thanks to its lignocellulosic structure, EOP can be directly combusted in boilers and furnaces [6–8], generating pollutant emissions and bad smells. Other applications for this material have been developed in recent years, such as the recovery of high-addedvalue compounds, including oligosaccharides, sugars (d-glucose and d-xylose), phenols compounds (antioxidants), bioethanol, xylitol, and furfural [9,10]. To date, thermochemical processes, such as pyrolysis, gasification, and hydrothermal carbonization (HTC), are in progress to eliminate the problem of uncontrolled emissions and to recover energy from EOP [11–15]. At a moderately high temperature (400–500 ◦C) under an inert atmosphere, carbonization (slow pyrolysis) appears as a promising way to convert EOP, mainly to biochar. Furthermore, recent experimentation of biochar in a DCFC system has proven to be a very attractive and eco-friendly technique for power generation, which has gained attention from scientists and engineers. Indeed, several carbon materials emitted from biomass have been examined as fuels in DCFC, which basically converts, at a high temperature (600–900 ◦C), the chemical energy stored in these biomass-based carbon materials to electricity through electrochemical oxidation reactions without a gasification or combustion process [16,17]. Compared to other fuel cell types, a DCFC system has a higher attainable efficiency (80%) for power generation and a lower emission of carbon dioxide per unit of the generated electricity [18]. In addition, such a system has an entropy term (∆*S*) close to zero, explaining the 100% theoretical efficiency of the system [19]. DCFC can be categorized into three types according to the types of electrolyte materials employed: molten carbonates, molten hydroxides, and solid oxides. All of them require a high temperature (>800 ◦C) to achieve a promising DCFC performance. A DCFC based on solid oxide electrolyte is similar to a SOFC, but it is fed by solid carbon instead of hydrogen. Several studies have been conducted to analyze SOFCs' short-term and even long-term durability, while few studies have been conducted to analyze DCFCs' durability, with some studies being in an early stage. However, SOFCs suffer from poor robustness and durability in the presence of impurities, such as sulfur and chlorine [20,21]. DCFCs need to be durable in the presence of such impurities to effectively utilize solid carbon resources (such as coal, biomass, biochar, and wastes) as fuel. A test to achieve the maximum long-term durability for a DCFC was developed by Jiang et al. [22]. The cell was tested for 100 h in their study. It is noticeable that durability analyses of fuel cells have an important impact on the future of fuel cell technologies in terms of performance optimization and further development, commercialization, and deployment [23].

Several research works [24–26] tried to decrease the operating temperatures of DCFCs and SOFCs by using thinner electrolyte layers or employing low-temperature, solid ionconducting materials. In this sense, a solid composite electrolyte composed of a molten salt phase, such as mixed carbonates, and an oxygen ion-conducting porous solid phase, such as samarium doped ceria (SDC), has been employed firstly for SOFCs and then adopted for DCFCs. The conductivity of this kind of composite electrolytes in an intermediate temperature (IT) range of 400 to 700 ◦C is around 10−<sup>2</sup> to 1 S m−<sup>1</sup> much higher than a conventional solid electrolyte in a solid oxide fuel cell and a molten electrolyte in a molten carbonate fuel cell (MCFC) [27]. The high conductivity of such a composite electrolyte has been explained by the enhancing effect of the co-ionic conduction in the two phases. SOFCs based on a composite electrolyte have shown remarkably good cell performance. Similarly, the operation of a DCFC at an intermediate temperature range (600–750 ◦C) has been investigated in order to overcome the problem of high temperatures [28].

One potential alternative electrolyte consisting of a mixture of samarium-doped ceria (SDC) and molten carbonate (Li2CO3/Na2CO<sup>3</sup> in a mole ratio of 2:1) has received more attention, thanks to its double ionic conduction ability toward carbonate (CO2<sup>−</sup> 3 ) and oxide (O2−) ions [17,19,28,29].

Anode reactions:

$$\text{C} + 2\text{O}^{2-} \rightarrow \text{CO}\_2 + 4\text{e}^- \tag{1}$$

$$\text{C} + 2\text{CO}\_3^{2-} \rightarrow 3\text{ CO}\_2 + 4\text{e}^- \tag{2}$$

Cathode reactions:

$$\text{O}\_2 + 4\text{ e}^- \rightarrow 2\text{O}^{2-} \tag{3}$$

$$\rm O\_2 + 2\rm CO\_2 + 4e^- \rightarrow 2\rm CO\_3^{2-} \tag{4}$$

When a DCFC is fueled by a biochar composed mainly of carbon, hydrogen, and oxygen (CxHyOz), other side reactions can take place within the anode. Indeed, it appears that the use of biochar as a direct fuel for DCFC systems demonstrates a peculiar behavior, notably within the anodic active electrochemical zone, regarding its heterogeneous elemental composition (CHO contents), and the presence of surface functional groups moves the anodic reaction mechanisms from a theoretically complete carbon oxidation reaction to a series of inter-related light gaseous and carbon oxidation reactions, apart from several side chemical reactions.

Effectively, through the biochar skeleton, various volatiles and light gases (CO, CO2, H2, and CH4) may be generated at a high temperature. The formed CO<sup>2</sup> and H2O at the anodic active electrochemical reaction sites (AERS) can be directly used as a gasifying agent within the anode compartment and can further chemically react with solid carbon toward CO and H<sup>2</sup> formation [29]:

Boudouard reaction:

$$\text{?C} + \text{CO}\_2 \rightarrow \text{CO} \tag{5}$$

Water shift reaction:

$$\text{C} + \text{H}\_2\text{O} \rightarrow \text{CO} + \text{H}\_2\tag{6}$$

The latter reactions are strongly favored at a high temperature and present a key role in DCFC performance as their gaseous products (CO and H2) can easily diffuse and reach the reaction sites much more rapidly than solid carbon materials, contributing largely to power generation through the following reactions:

$$\rm H\_2 + \rm CO\_3^{2-} \rightarrow \rm CO\_2 + \rm H\_2O + 2e^- \tag{7}$$

$$\text{H}\_2 + \text{O}^{2-} \rightarrow \text{H}\_2\text{O} + 2\text{e}^- \tag{8}$$

$$\text{CO} + \text{O}^{2-} \rightarrow \text{CO}\_2 + 2\text{e}^- \tag{9}$$

$$\text{CO} + \text{CO}\_3^{2-} \rightarrow 2\text{CO}\_2 + 2\text{e}^- \tag{10}$$

Based on these series of electrochemical reactions, the overall DCFC efficiency can be attributed mainly to CO and H2-AERS interactions, rather than to the extremely limited solid carbon-AERS contact [19,29–31].

Practically all the experimental and numerical studies on DCFC anode kinetic mechanisms affirmed that the hypothesized4-electron carbon electrochemical oxidation reaction (Equation (2)) is not sufficient to explain the recorded DCFC performance. The occurrence of a 2-electron CO oxidation (Equation (9)) and chemical Boudouard reaction (Equation (5)) is often illustrated. This chemical reaction is temperature and CO/CO<sup>2</sup> content dependent. The reverse Boudouard reaction is known to be fast at 700 ◦C. However, in a molten carbonate medium, it exhibits a strange behavior. Its rate may be much slower. Meanwhile, the 2-electron CO oxidation (Equation (9)), which is a consequence of the occurrence of the reverse Boudouard reaction, is known to have a limited kinetic rate below 650 ◦C and a significantly accelerated rate starting from 700 ◦C. The backward sense of the latter electrochemical reaction can also possibly occur, increasing the CO concentration within the anode. In this sense, Chen et al. [32] developed a macro-homogeneous model to assess the kinetics of the three aforementioned reactions and their dependence on several properties, such as anodic bed thickness and carbon conductivity. They concluded that the electrochemical mechanism is approximately three times as fast as the chemical reverse Boudouard reaction near the anodic current collectors.

Various research studies have focused on the investigation of biomass-based carbon materials' potential as fuels in DCFCs to reveal their efficacy as energy carriers. It has been found that both the performance and lifetime of DCFCs are notably affected by the physicochemical properties of biomass-based carbon fuels [18,31,33]. Actually, the recorded low performance of biochar-fueled DCFCs hinders their further development.

Wang et al. [34] assessed the potential of reed biochar as a fuel in a direct carbon fuel cell based on a SDC-carbonate composite electrolyte and achieved the best maximum power density of about 378 mW·cm−<sup>2</sup> at 750 ◦C to date. They obtained this promising performance after using a KCl-washing pre-treatment on the raw biomass before pyrolysis. The effect of KCl washing in raw reed increased the structural disorder degree of the biochar during the pyrolysis process, leading to a high oxidation activity of the reed biochar and, subsequently, a good DCFC performance. KCl is known as one of the chemical activating agents used in the preparation of activated carbons for energy storage applications, such as supercapacitors and batteries [35,36]. KOH, H2SO4, and ZnCl<sup>2</sup> are other types of agents that can contribute to the activation of biomass precursors. These chemical agents ensure an increase in total porosity and micropore development, as well as an increase in the yield of the activation process. Gómez et al. [37] proposed a reaction mechanism, which was validated by mass spectroscopy analysis and thermodynamic calculations, to carry out activation through the use of a mixture of KOH and KCl. They concluded that the role of KCl consists of arise in the solubilization of carbonates that precipitate in its absence, hence lowering the contact between the liquid KOH and the carbon particles. They affirmed that the use of KCl as an additive results in the synthesis of activated carbons with lower amounts of KOH, which are, therefore, more available to be produced at large scales. Jayakumar et al. [38] achieved 360 mW·cm−<sup>2</sup> at 700 ◦C with sugar char as a fuel when using a molten antimony anode. Cai et al. [39] used a biochar derived from orchid tree leaves as fuel. They showed that the high content of CaCO<sup>3</sup> in the leaf biochar catalyzes the reverse Boudouard reaction and enhances the performance of DCFCs. Hao et al. [40] also found that carbon from magazine waste paper contains a high amount of magnesium calcite, which improves the thermal reactivity of the carbon materials. Chien and Chuang [41] used coconut coke as a fuel for an anode-supported DCFC and recorded a maximum power density of about 80 mW·cm−<sup>2</sup> at 800 ◦C. They affirmed, through CO and CO<sup>2</sup> pulse transient studies, that the increased cell performance was attributed to an increasing extent of electrochemical oxidation of CO, a product of the Boudouard reaction. Hao et al. [42] tested a DCFC with bamboo carbon as fuel and recorded a maximum power density of 156 mW·cm−<sup>2</sup> at <sup>650</sup> ◦C. They concluded that the inherent impurities, such as calcite (CaCO3) and kaolinite (Al2Si2O5(OH)4), in the biochar might favor its thermal gasification and resulted in the enhanced performance of the intermediate-temperature DCFC. Elleuch et al. investigated almond shell (AS) [19] and olive wood (OW) [29] biochars as fuels in DCFCs supported by a 0.65 mmthick Ce0.8Sm0.2O1.9 (SDC)-carbonate composite electrolyte layer and recorded a maximum power density of 107 and 105 mW·cm−<sup>2</sup> , respectively, at 700 ◦C. They claimed that the high concentration of oxygen-containing groups is the main reason for the higher performance recorded, when compared to activated carbon. They also concluded that alkali and alkalineearth metal oxides, such as K2O, Fe2O3, and CaO, worked as the active catalysts for the anodic reaction by decreasing the electrochemical activation polarization in the case of the AS biochar. It is known that gasification reactions take place below 800 ◦C [43], and the gasification of carbon fuel is the limiting factor of DCFC performance when operating at a higher temperature range. The kinetics of these reactions can be catalyzed using several catalysts. Meanwhile, it has become a consensus that the alkali metal, alkaline earth metal, and transition metal are effective catalysts for carbon gasification [44–46], which are widely used in coal gasification research to obtain a competitive reaction rate at a lower temperature.

Li et al. [43] tested Ni, K, and Ca catalysts and claimed that all of them are suitable gasification catalysts to accelerate the carbon gasification rate, reduce the reaction temperature of the DCFC, and, thus, improve the cell performance. Cui et al. [27] tested the effect of carbonate as a catalyst and affirmed that it can also play a catalytic role in carbon gasification reactions. Tang et al. [47] used gadolinium-doped ceria (GDC) mixed with silver

as the anode to catalyze the electrochemical oxidation of CO, while a Fe-based catalyst was loaded on the carbon fuel to enhance the Boudouard reaction. They mentioned an enhancement of about 10 times higher than that of a cell without any catalyst. Recently, a strontium slag and its derived catalyst were successfully introduced with the carbon materials at the anode in order to enhance a DCFC's performance by promoting the reverse Boudouard reaction [48]. Yu et al. [49] used corncob biochar as fuel and a single cell similar to the one used by Elleuch et al. [19,29], but with adding a printed anode layer (a mixture of NiO and SDC). They showed a maximum power density of 185 mW·cm−<sup>2</sup> at 750 ◦C.

The main objective of the present research is to explore the electrochemical capability of EOP biochar as fuel in a direct carbon fuel cell (DCFC), using an experimental correlation between the DCFC's power output and the physico-chemical properties of the fuel, in order to determine the main EOP biochar limiting properties when it is used as a fuel for DCFCs. A series of physico-chemical analyses are investigated for this purpose. Furthermore, the DCFC's chemical/electrochemical mechanisms are predicted with respect to the EOP biochar's physico-chemical properties and compared to similar DCFC configurations, which have successfully operated with other biochar fuels when prepared under the same conditions [19,29].
