Nanostructured\nSulfur-Doped Carbon from Biomass and\nIts Layer-by-Layer Self-Assembly for High-Performance Supercapacitor\nElectrodes

Here, we show that biomass derived from waste wood from\nforest\n(silver birch trees, <i>Betula pendula</i>) is an excellent\nstarting material for fabricating activated carbon for supercapacitors.\nThe carbon was prepared via hydrothermal carbonization with H<sub>3</sub>PO<sub>4</sub> followed by pyrolysis. The effect of sulfur\ndoping on its physicochemical and electrochemical properties was evaluated.\nThe samples were named BCM (biomass carbon material) for non-doped\nand S-BCM (sulfur–biomass carbon material) for the doped samples.\nWe further show that sulfur doping (with around 7% sulfur content)\nradically increases these nanoparticles’ performance, leading\nto higher capacitance and stability. The sulfur doping increased the\nspecific surface area to 2124 m<sup>2</sup> g<sup>–1</sup> compared\nto non-doped (1972 m<sup>2</sup> g<sup>–1</sup>), as reflected\nin the enhancement of the number of micropores. In addition, according\nto Raman spectroscopy analysis, the sulfur doping increased the structural\ndefects based on the <i>I</i><sub>D</sub>/<i>I</i><sub>G</sub> values (S-BCM = 2.23 and BCM = 1.98). Furthermore, the\nsulfur doping increased the hydrophilicity of the carbon particles,\nallowing us to disperse them in water and use layer-by-layer self-assembly\nto fabricate supercapacitor electrodes with nanometer-layer precision.\nThe assembled S-BCM electrodes exhibited a higher capacitance than\nthose of pristine carbon with the highest values measured at 79.1\nF/g at 1 A/g. They also had higher stability with a capacitance retention\nof 85.3% after 10 000 charge–discharge cycles. Our work\nshows a promising route for making advanced high-performance electrode\nmaterials for supercapacitors using waste byproducts, which is especially\nrelevant for the Nordic hemisphere, to minimize carbon footprint while\nenabling advanced energy storage devices as we aim at reach a smooth\ntransition toward a fossil-free future.