High-energy storage capacity of cellulose nanofiber supercapacitors using bound water
Mikio Fukuhara, Tomonori Yokotsuka, Takuya Takashina, N. Fujima, Masahiro Morita, Tatsunori Ito, Takeshi Nakatani, Toshiyuki Hashida
Abstract
Abstract The performance of electric double-layer capacitors and lithium-ion batteries deteriorates with increasing humidity. The desirable effect of bound water on the energy-storage properties of physically dry cellulose nanofiber (Na-ACF) supercapacitors with sodium (Na) carboxylate radicals was investigated using infrared and near-infrared spectroscopy, and nuclear magnetic resonance spectroscopy, alternating current impedance analyses, and first-principles calculations. The storage capacity decreased gradually upon heating to 423 K and reached zero upon exceeding 483 K, accompanied by increasing electrical resistance, forming a distorted semicircle in Nyquist diagram and drawing the phase angle to zero in Bode diagram. This is attributed to the water in the hydration gel bound to the Na + -ions that cross-link the cellulose chains, evaporating as the temperature increases, and finally becoming Na 2 O. The increased band-gap energy from the increase in bound water prevents leakage from the supercapacitor. In contrast to ordinary batteries, bound water is necessary for developing Na-ACF supercapacitors.