# Porous Doped Carbons from Anthracite for High-Performance Supercapacitors
## 1 Introduction  
During past decades, both the aggravating resource depletion and exacerbating environmental pollution have stimulated tremendous efforts regarding pursuing the renewable/eco-friendly energy storage devices to gratify the insatiable demand [1; 2]. As a result, supercapacitors as a kind of new and promising energy storage system have cut an increasing dash due to their fast charge/discharge rate, high power density, and long cycling stability [3; 4]. Supercapacitors can be categorized into the electrochemical double-layer capacitors (EDLCs) and pseudocapacitors based on the energy storage mechanisms [5]. EDLCs store energy by means of reversible ionadsorption in the double layer formed on the surface of inert electrodes, while pseudocapacitors deliver energy depending upon reversible redox reactions on active material's surface [6]. In the last few years, multifarious materials, including carbon-based materials, metal oxides/hydroxides, conducting polymers, and hybrid composites, have been extensively exccogitated as supercapacitor electrode materials [7]. Among them, carbon materials, especially the porous carbons, are acclaimed as the most promising candidates as electrode materials for EDLCs, taking into account their high merits of the large specific surface area (SSA), well-designed pore size distribution (PSD), low cost, favorable chemical/thermal stabilities, precursor availability, environmental friendliness, and relatively high conductivity [8]. Nevertheless, pragmatic applications of porous carbon electrodes are still stifled because of the fatal disadvantage of low energy density (3-5 Wh kg\({}^{-1}\)) [9]. It is well-known that the performances of supercapacitors are strongly swayed by SSA, PSD, electrical conductivity, and surface functionalities [10; 11]. The SSA and PSD can universally constitute two pivotal factors dictating the capacitive performance of EDLCs [12]. In addition, the existence of heteroatoms (such as B, N, O, P, and S) can enhance specific capacitance by improving the wettability and inducing the pseudocapacitance [13]. For example, nitrogen doping has been deemed as the most feasible protocol to step up the surface wettability, electronic conductivity, and capacitive property of EDLCs [14]. Unfortunately, a high level of nitrogen doping inevitably renders the materials susceptible to the grim capacity deterioration during the long-term cyclic test as a result of the vulnerable material stability and the functionality decomposition. So far, it still remains a grand challenge to facilely develop the optimal porous doped carbon materials from inexpensive carbon sources. Therefore, to further tweak the electrochemical signature of EDLCs, the porous carbon electrode materials should be judiciously formulated with the particular intention of maximizing SSA, controlling the PSD, and modulating the doping heteroatom. To fulfill the high-performance supercapacitors, it is thus imperative to develop an expeditious and straightforward scenario to synthesize porous carbons from the naturally rich and economically competitive carbon sources. Over years, a myriad of carbon resources, e.g., fossil-based hydrocarbons, agricultural and biomass by-products, and polymers, have been capitalized upon for the preparation of porous carbons [15]. Out of them, coals are cheap and abundant as the most commonly used materials. According to the literature, the porous carbon materials can be obtained utilizing different strategies: (i) physical activation with different oxidizing gases (e.g., H\({}_{2}\)O or CO\({}_{2}\)) [16]; (ii) chemical activation with chemical activating agents (e.g., KOH, NaOH, H\({}_{3}\)PO\({}_{4}\), or ZnCl\({}_{2}\)) [17]; (iii) the templating avenue with templates (e.g., MgO, ZnO or organic molecules) [18]. In comparison with the physical activation, chemical activation needs a lower temperature and shorter time, and, more importantly, results in a more uniform PSD and higher SSA.