Effects of the physisorption properties of human hair-derived activated carbon as a potential electrode for symmetric supercapacitor

Materials for Renewable and Sustainable Energy https://doi.org/10.1007/s40243-024-00294-3 (2025) 14:22 ORIGINAL PAPER Effects of the physisorption properties of human hair-derived activated carbon as a potential electrode for symmetric supercapacitor Rashed A. M. Adam1 · Delvina J. Tarimo1 · Vusani M. Maphiri1 · Abdulmajid A. Mirghni1 · Oladepo Fasakin1 · Ncholu Manyala1 Received: 29 September 2024 / Accepted: 29 December 2024 © The Author(s) 2025 Abstract Herein, human hair-derived activated carbon (HH-AC) with remarkable physisorption properties such as high surface area and well-balanced micro- and mesopores, is synthesized by chemical activation method using potassium hydroxide (KOH). The activated carbon is synthesized at different ratio of charred human hair and activator as 1:1, 1:2 and 1:3 for HH AC(11), HH-AC(12) and HH-AC(13), respectively. These activated materials are characterized by a powder X-ray diffraction (XRD), Laser Raman spectroscopy, Scanning electron microscope (SEM), and N2 adsorption/desorption isotherms. To examine the influence of the micro-mesopore ratio with high surface area on supercapacitor behavior, all samples are tested in a three-electrode using 2.5 moles of potassium nitrate (2.5 M KNO3) as electrolyte solution. The results show that HH-AC(12) sample which has micro to mesopore-balanced (50 : 50) exhibited superior electrochemical performance with specific capacitance of 215 F g−1 and 125.8 F g−1 in the negative and positive potential, respectively at 1 A g−1 . The sample HH-AC(11), which is dominated by micropores, showed lower rate capability and specific capacitance despite the huge surface area.Whereas the HH-AC(13) sample with mostly mesopores achieved higher rate capability compared to the others. The HH-AC(12) is further examined in a 2-electrode setup to form a symmetric device. The results show a specific energy of 16 Wh kg−1 and a specific power of 375 W kg−1 at 0.5 A g−1 . The device demonstrates outstanding capacitance retention of 97% after 10,000 cycles. Thus, ACs with micro to mesopores-balanced are potential candidates for supercapacitor applications. Keywords Supercapacitor · Activated carbon · Energy storage · Microporous · Mesoporous Introduction Electrochemical energy storage (EES) devices include rechargeable batteries, fuel cells, and supercapacitors (SCs) have gained prominence in our modern society due to their use in many applications such as mobile electronic devices, and renewable energy (solar and wind systems) [1, 2]. SCs are popular in EES devices due to their high specific power and long-life span. However, in comparison with batteries and fuel cells, SCs have a relatively low specific energy [3]. The properties of the active electrode include specific surface area (SSA), pore structure, and electrical conductivity Ncholu Manyala 1 Department of Physics, University of Pretoria, Pretoria 0002, South Africa are crucial in electrochemical performance of SCs. The type of electrode materials determines the energy storage mechanism, whereby carbonaceous materials, such as activated carbon (AC), single and multi-well carbon nanotubes, graphene (G), and graphene oxide GO), exhibited electric double-layer capacitor (EDLC) behavior [4, 5, 6]. While oxides materials (e.g., manganese dioxide ( MnO2 ), vanadium oxide ( V2 O5 )), and conductive polymers e.g., polyaniline (PANI) show pseudocapacitive behavior [7, 8]. Amongst carbonaceous materials, ACs are widely used as SCs electrode due to their higher SSA, which is the key to the good electrochemical performance of EDLCs. ACs can be produced by physical activation using carbon dioxide ( CO2 ), steam, or air and chemical activation method with an activating agent such as NaOH, KOH, H3 PO4 , and ZnCl2 [9]. The chemical activation creates more pores and higher mass yield than physical activation [10]. Comparing to other activators, KOH is most effective due to its 13 22 Page 2 of 17 strong basic characteristics and ability to develop micropores and mesopores structure with the high SSA [11, 12, 13, 14]. Therefore, the chemical activation with KOH was selected for the present study. Common biomass sources for producing activated carbon include coconut shells, wood, and bamboo [15]. In contrast, new biomass sources such as human hair, garlic peels, banana peels, chicken bone, and kapok peels have recently been used to produce ACs for energy storage applications, due to their high SSA, tunable pore structure, and good electrical conductivity [16, 17, 18, 19]. The porous materials are classified by the International Union of Pure and Applied Chemistry (IUPAC) into three families based on pore size diameter: micropores ( < 2 nm ), mesopores ( 2 − 50 nm), and macropores ( > 50 nm) [20]. The porosity of ACs can be controlled by adjusting the carbonization temperature or the impregnation ratios of the carbon precursor to the activation agent [18, 21]. Previous reports have shown that the type of porosity influences the performance of supercapacitors. For instance, Zhang et al. [22] studied the influence of commercial AC pore structure on the performance of SC and found that AC with an ultramicropore volume ( 0.41 m3 g−1 ) and a reasonable mesopore volume ( 0.15 m3 g−1 ) exhibit better electrochemical performance compared to other samples. Also, Liang et al. [18] investigated activated carbon obtained from kapok peels containing both micropores and a mesopores structure. The results showed that ACs has a sheet structure and a high SSA of SBET = 1258 m2 g−1 consisting of micropore SSA of SSA of Smic=652 m2 g-1 and meso-pore SSA of Smes=606 m2 g-1, recorded a maximum capacitance of 332.3 F g−1 at 1 A g−1 in a 3-electrode system. However, there are no explicit studies that have investigated the effects of different micro to mesopores density ratios on activated carbon and correlated them with SC performance. The micropores carbon can provide abundant adsorption sites to absorb ions, but the large number of micropore sites has negative effects on the performance of the supercapacitor because the small pore size partially restricts the diffusion of the electrolyte ions in the electrode, besides, micropore carbon exhibits low-rate capability (low specific capacitance at high specific current) [23]. Consequently, activated carbons with mesopore structures are often used as supercapacitor electrodes because the channels of the mesopores facilitate ion diffusion in the material. However, the high-density ratio of mesopores in the material reduces the SSA [24], which limits the performance of the supercapacitor. Therefore, a synthesized electrode material with balanced micro to mesopores was proposed to utilize both micropores and mesopores for SCs application. Herein, human hair (HH) was used to synthesize activated carbon, and different concentration of activating agent 13 Materials for Renewable and Sustainable Energy (2025) 14:22 leading to different micro to mes (...truncated)