Biomass-derived carbon sponges for use as sodium-ion capacitor electrodes

Abstract

S-doped carbon sponges have been synthesized via an eco-friendly approach based on a salt-templating strategy followed by an easy S-doping process. Gluconic acid was chosen as a sustainable carbon precursor, sodium carbonate as a low toxicity, water-removable template, and sulfur as an environmentally benign, earth-abundant S-dopant. These carbon sponges are characterized by a 3D structure of thin interconnected carbon walls with a highly disordered structure, dilated mean interlayer spacing (d > 0.36 nm) and high content of electrochemically active covalent sulfur (up to 13% by weight, mainly as thiophene groups). As a result of these properties, they provide high sodium storage capacity at both low and high discharge rates: 524 mA h g⁻¹ at 0.1 A g⁻¹ and 161 mA h g⁻¹ at 10 A g⁻¹. A Na-ion capacitor was assembled with the S-doped carbon sponges with optimized sodium storage performance as the negative electrode and a gluten-derived highly porous 3D carbon (S_BET ∼ 2600 m² g⁻¹) as the positive electrode. The sodium-ion capacitor with an optimized positive-to-negative electrode mass ratio of 1 can be steady cycled with low capacity fade of 0.0022% per cycle, and is able to deliver a high specific energy of 72 W h kg⁻¹ under a high-power regime (24.4 kW kg⁻¹).