Enhancing ZnO-based supercapacitors through carbon-induced defect centers

Abstract: This study explores the effects of eco-friendly reducing and capping agents on synthesizing zinc oxide (ZnO) nanoparticles for use as electrode materials in supercapacitors. The researchers successfully produced ZnO nanoparticles with different sizes and shapes using a sol–gel method and four different capping agents: tartaric acid, chitosan, ascorbic acid, and hydroxybenzoic acid. The properties of the ZnO nanoparticles were thoroughly examined through morphological, structural, and electrochemical studies. The defect structure of the materials was analyzed using photoluminescence spectroscopy, while electron paramagnetic resonance spectroscopy revealed the presence of carbon-based signals related to doping the host material with carbon during synthesis. Specific capacitance measurements indicated that supercapacitors using the C-doped ZnO nanomaterial as electrode materials demonstrated potential for energy-storage applications. Specifically, when tartaric acid was used as a capping agent, the maximal specific capacitance, energy density, and power density values reached 103.1 F/g, 14.3 Wh/kg, and 167 kW/kg, respectively. These results show promise for the development of next-generation supercapacitor devices based on ZnO. Impact statement: This article aims to elucidate the impact of eco-friendly reducing and capping agents used in the synthesis procedure of zinc oxide nanoparticles employed as electrode materials in supercapacitor applications. ZnO nanoparticles were successfully synthesized by a sol–gel method with four different capping agents: tartaric acid, chitosan, ascorbic acid, and hydroxybenzoic acid. Thorough morphological, structural, and electrochemical studies were conducted to elucidate their properties. Photoluminescence spectroscopy distinguished dominant defect structures inside the nanomaterials. At the same time, electron paramagnetic resonance spectroscopy analyzed the intrinsic and extrinsic paramagnetic defect structures, revealing the presence of carbon-based signals related to doping the host material with C during the synthesis procedures. Specific capacitance measurements were performed, which showed that symmetrical supercapacitors using the C-doped ZnO nanomaterial as electrode materials have great potential in energy-storage applications. The maximal specific capacitance, energy density, and power density values obtained reached 103.1 F/g, 14.3 Wh/kg, and 167 kW/kg, respectively, when tartaric acid was employed as a capping agent. The results are promising compared to the literature and could be a starting point in developing new-generation supercapacitor devices based on carbon-doped ZnO.