**4. Conclusions**

In this work, the electrochemical performance of a biochar produced from EOP biomass carbonization at 400 ◦C was examined as a fuel in a DCFC system supported by a 0.6 mm-thick Ce0.8Sm0.2O1.9 (SDC)-carbonate composite electrolyte layer. The cell exhibits a maximum power density of about 10 mW·cm−<sup>2</sup> and a limiting current density of about 142 mA·cm−<sup>2</sup> at 700 ◦C, which is limited if compared to the AS and OW biochar-fed DCFCs prepared under similar conditions. The OCV is about 0.87 V lower compared to the DCFC's theoretical voltage. This is ascribed to the various formed gases emitted from the porous biochar structure, which plays a key role in the DCFC's performance apart from the solid carbon skeleton. Even though the electro-oxidation of solid carbon and volatile gases has a significant effect on the performance of the DCFC, the contribution of each one has not been fully assessed. The physico-chemical and structural properties of the EOP biochar were analyzed using elemental and proximate analyses, mercury porosimetry, scanning electron microscopy, X-ray diffraction, FTIR analysis, and X-ray fluorescence analysis. The thermogravimetric analysis under a normal air atmosphere enables the conclusions about the thermal stability and reactivity of the EOP biochar. On the basis of this extensive characterization study, a direct correlation between the EOP biochar's physico-chemical characteristics and the power output is revealed.

The carbon, oxygen, ash (impurities), and volatile matter contents; the porosity and surface area; the presence of carbonyl/carboxylic groups; and the disorder of the carbon structure exert a pronounced impact on the electrochemical performance of the EOP biochar-fed DCFC.

An enhanced electrochemical reactivity of the EOP biochar in the DCFC system could be achieved by first removing silica from the biochar using a demineralization pre-treatment, followed by activation.

The present research investigated the positive and negative properties affecting the performance of a biochar as fuel in a DCFC. The major conclusions show that a promising biochar fuel for DCFCs is one that comes from a biomass feedstock with a high HHV, a high holocellulose/lignin ratio, and, especially, a low ash content, without looking at its mineral composition.

**Author Contributions:** Experimental work and writing—original draft preparation: N.G. and A.E.; review and supervision: A.E., K.H. and Y.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Higher Education and Scientific Research of Tunisia in the framework of the ERANETMED2-72-251, Medwaste project.

**Data Availability Statement:** Additional details of the experiments and data may be asked via email to the corresponding authors.

**Acknowledgments:** This research is supported by the Tunisian Ministry of Higher Education and Scientific Research in the framework of the ERANETMED2-72-251, Medwaste project. The financial support of the NSF of China under contract number21120102039, the support of the Tianjin Municipal Science and Technology commission under contract number 13JCZDJC26600, the support of the Program of Introducing Talents to the University Disciplines under file number B06006, the support of the Program for Chengjiang Scholars and Innovative Research Teams in Universities under file number IRT 0641, and the support of the Ministry of Education of China under contract number 20130032120023 are also gratefully acknowledged.

**Conflicts of Interest:** The authors declare no conflict of interest.
