**1. Introduction**

Supercapacitors, i.e., electrochemical capacitors, have a high-power density, fast charging rate, cyclability, and low cost of operation [1,2]. Thus, they have found many uses in electronics, energy management, mobile electrical systems, as well as industrial power arrays [3,4]. In an electrochemical double-layer supercapacitor, the energy is stored in the electrostatic charge separation on the surface between the electrode and electrolyte. High porosity and good mechanical properties allow porous carbon materials to be widely used as electrode materials [5].

In a supercapacitor, energy is stored either by an electrochemical double layer in the electrode–electrolyte interphase (nonfaradaic) and/or by reduction–oxidation reactions (faradaic) on electrode surfaces [6–12]. For electrochemical double-layer capacitors (EDLC), ions in the electrolyte will accumulate at the surface of the solid electrode materials. The capacitance (CDL) of the electrode depends on the layer thickness where the ions and solvent molecules reside, d; and the surface area of the electrode, A (see equation below). E and Er are the permittivity of the vacuum and the solvent, respectively.

$$\mathbf{CDL} = \mathbf{c} \times \mathbf{c} \mathbf{r} \text{ A/d}$$

For high-energy and power-density EDLC, it is imperative for the electrode to possess a high surface area for cations and anions to accumulate. Porous carbons, such as graphene-based nanocarbons, carbon nanotubes, and activated carbons, are used as electrodes in EDLC supercapacitor devices due to their high surface area, high electrical conductivity, and electrochemical stability. The low-cost and established electrode fabrication technologies in the industry create

**Citation:** Albaiz, A.; Alsaidan, M.; Alzahrani, A.; Almoalim, H.; Rinaldi, A.; Jalilov, A.S. Active Carbon-Based Electrode Materials from Petroleum Waste for Supercapacitors. *C* **2023**, *9*, 4. https://doi.org/10.3390/ c9010004

Academic Editors: Indra Pulidindi, Pankaj Sharma and Aharon Gedanken

Received: 10 November 2022 Revised: 17 December 2022 Accepted: 22 December 2022 Published: 28 December 2022

**Copyright:** © 2022 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/).

a sustainable approach for using carbons as electrodes in supercapacitor devices. Activated carbon with a BET surface area of 3150 m2/g was reported to show a capacitance of 312 F/g. This capacitance translates into a specific capacitance value of 9.9 µF/cm<sup>2</sup> [13].

Porous carbons from various synthesis routes have been explored as high-surface area electrodes for supercapacitors [14,15]. Asphalt, a petroleum waste, has been used as a precursor to high-surface area carbons that were used as electrodes in batteries and supercapacitors [16–18]. Asphalt is a low-cost material obtained from the heaviest fraction of crude oil. Asphalt contains some amount of volatile organic species that are removed during a carbonization process at about >400 ◦C. Asphalt-derived carbons with surface areas of >4000 m2/g were achieved after a sequential process of removal of volatile organic followed by high-temperature activation with KOH [19,20].

This energy-intensive activation step to create a high surface area can be compensated by using alternative and inexpensive activating agents from industrial waste. Coal-fired power plants, on the other hand, are the large-point sources of carbon dioxide (CO2) [21] and generate solid waste such as fly ash and bottom ash, which mainly contain lime (CaO) and magnesium oxide (MgO) [22,23]. However, using such alternative and inexpensive mineral feedstock from industrial waste as activating agents is challenging due to the slow chemical kinetics and high activation energy [10]. Furthermore, most of the methods of activation of carbons require high temperatures, usually above 800 ◦C. The combination of using asphalts as petroleum waste as a carbon source and solid industrial waste as activating agents can be an attractive cost-effective route towards synthesizing high-surfacearea carbons for supercapacitor application.

In this work, we aim to use vacuum residue (VR) as the carbon precursor for generating low-density porous carbon materials with KOH, MgO, and Ca(OH)<sup>2</sup> activation under mild conditions of activation at 400 ◦C. The resultant activated carbon materials will be tested as electrode materials for supercapacitors.
