*3.3. Capacitors*

The ability that activated carbons obtained from glycerol may have as capacitors has been investigated very recently and a brief overview of the work done so far is presented. The first reference to this possibility was in 2019 by Gonçalves et al. and is a small part of the work concerning the adsorption capacity of activated carbon described before [53]. The authors selected three activated carbons: two with larger surface areas activated differently and one with the higher micropores/mesopores ratio. Electrodes were prepared by pressing a mixture of activated, multiwalled carbon nanotubes. The more suitable activated carbon was obtained from using ZnCl2 as the activating agent. It presented the higher micropores/mesopores ratio and not the largest surface. This characteristic was attributed to the more suitable pore distribution in this carbon and the higher micropores/mesopores ratio. More recently, Narvekar et al. [67] synthesized carbon from glycerol which was chemically activated with KOH at 800 ◦C under N2 atmosphere for 2 h. Cyclic voltammetry studies showed the activated carbon had a much higher capacitance than the commercial carbons (Vulcan XC-72 or CNT), a fact that was attributed to the carbonyl and sulphonyl

surface functionalities and large surface area with favorable pore size distribution wherein the pores are accessible to form an extended electrical double layer. More recently, Juchen et al. [68] synthesized KOH activated carbon from crude glycerol. The electrodes were prepared by mixing 90 wt% of chemically activated carbon and 10 wt% of Polyvinylidene fluoride (PVDF) in n-methyl-pyrrolidone (NMP) solvent and used for the desalination of brackish water. Figure 8 shows the results of cyclic voltammetry experiences showing the electrode capacitance, resistivity, and mass transfer effects in the desalination process. The electrodes remained stable over 50 desalination/regeneration cycles applying potentials lower than 1.2 V.

**Table 2.** Review of the textural properties and adsorption capacity of glycerol-activated carbon materials.


**Figure 8.** (**a**) Specific capacitance from cyclic voltammograms recorded at different scan rates, before desalination; (**b**) total specific capacitance, as a function of scan rate, before and after desalination applying 1.2 V; (**c**) Nyquist plots before and after desalination applying 1.2 V; (**d**) modified Randle equivalent circuit. Working and counter electrodes: PGAC. Electrolyte: 1 mol·L−<sup>1</sup> NaCl. Reproduced with permission from [68], Elsevier, 2022.

#### **4. Summary and Outlook**

The amount of glycerol produced as a by-product in the biodiesel industry has been increasing. In addition, the use of waste fats (waste and residues), for sustainability reasons, by the biodiesel industry originated glycerol, which may contain unwanted compounds (contaminants). This causes this glycerol not to be used in certain applications such as food or cosmetics, because they do not have the kosher certification as demanded by the food, pharmaceutical, and cosmetic industries. This fact reinforces the need to quickly discover other applications for this glycerol and its use for the synthesis of carbons may be a solution, as may be seen by the work developed so far and their wide applications. The carbons from glycerol have been successfully used in a wide range of applications such as catalyst for a wide range of reactions such as acetylation, etherification, synthesis of substituted imidazoles, and benzamides, among others. The activated carbons have been used as adsorbent of gases (H2S, VOCs, ethene and ethylene) and liquid (dyes and pharmaceuticals) pollutants, and capacitor materials. Nevertheless, this research is still in its initial stages in comparison with other carbons, and optimization of the synthetic procedures by changing the activated agent, temperatures, and pressure may give rise to more effective materials for a given application. Other possibilities could be surface functional group variation on the activated carbon surface, which may be achieved using different treatment parameters, or by post-synthesis modification, a possibility that has not been investigated so far. A systematic study of surface modification may help in obtaining better materials for the intended application.

Concerning practical applications, the adsorption process is the most promising for the glycerol-based active carbons. Adsorption technology is known for its simplicity, reliability, and low energy and maintenance costs, and it is already being used in many situations. The viability of this process is very dependent on the adsorbent. The use of glycerol-based activated carbons as adsorbent will depend on the possibility of producing this material using an energetic and environmentally sustainable processes.

**Author Contributions:** Conceptualization, M.B., S.C., J.P., M.L.P. and R.C.; methodology, M.B. and S.C.; validation, M.B. and S.C.; formal analysis, M.B. and S.C.; investigation, M.B. and S.C.; writing—original draft preparation, M.B. and S.C.; writing—review and editing, M.B., S.C., J.P., M.L.P. and R.C.; funding acquisition, J.P. and M.L.P.; M.B. and S.C. contributes equally for the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financed by Fundação para a Ciência e a Tecnologia (FCT) developed in the scope of the projects UIDB/00100/2020 (CQE), UIDB/04028/2020 & UIDP/04028/2020 (CERENA).

**Data Availability Statement:** No applicable.

**Acknowledgments:** This work has received funding from Fundação para a Ciência e a Tecnologia (FCT) in the scope of the projects UIDB/00100/2020 (CQE), UIDB/04028/2020 (CQE), IMS—LA/P/0056/2020 & UIDP/04028/2020 (CERENA). MB and SC acknowledge for FCT-Investigator contract-DL57 and PTDC/MEDQUI/28271/2017, respectively.

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