Cellulose nanocrystal-derived carbon electrodes for sustainable potassium-ion charge storage systems

We have here produced carbon electrode materials derived from Crystalline NanoCellulose (CNC) for low-cost potassium-ion based energy storage systems through conventional annealing as well as through a fast and energy efficient microwave assisted carbonization process. A two-step 4-minute synthesis with ZnCl2 activation in a domestic microwave leads to a micro/mesoporous carbon with high surface area (SBET~1800 m2 g−1). These CNC-derived carbons if assessed in symmetric supercapacitor C/C cells cycled with 0.5 M K2SO4 aqueous electrolyte showing reversible capacitance values up to 66 F g−1 at current densities of 5 A g−1, retaining 83% of its initial capacitance after 10.000 cycles without any conducting additive. Due to its large electrochemical window of 1.7 V, a competitive energy density for an aqueous system of 20.9 W h kg−1 is achieved. A hybrid aqueous capacitor built with this carbon as negative electrode and coupled with a Prussian White as positive results in cell capacitance values up to 135 F g−1 under a voltage operation window of 1.8 V in 0.5 M K2SO4. On the other hand, non-activated carbons produced through a 2.25 hours thermal annealing at 900 °C, present much lower surface area (SBET~450 m2 g−1), most of it due to its high micropore volume. This low external and mesoporous surface area carbon is a competitive anode material for potassium-ion batteries with a reversible capacity of ~200 mA h g−1 cycled at 28 mA g−1 using 3.9 M KFSI in DME electrolyte (favourably most of it below 1 V vs K+/K) in a potassium half-cell with >80% retention in 100 cycles. The present research shows that sustainable CNC derived carbons produced through energy efficient methods are competitive electrode materials in low-cost K based energy storge systems.