Development of a Method for Producing Nanoporous Carbon Material from Lithium Citrate

Porous carbon was prepared by direct carbonization of tri-lithium citrate tetrahydrate at 800 °C, followed by acid leaching and re-annealing. Pyrolysis produced Li2CO3 and Li2O within the carbonizate; their removal left an oxygen-functionalized disordered carbon with an openwork structure of crumpled sheets, a BET area of 342 m2 g−1 and micropores near 0.9 nm that supply about 40% of the NLDFT surface area. In a symmetric cell with 6 mol L−1 KOH, the carbon delivered a single-electrode capacitance of 91.5 F g−1 at 0.5 A g−1 and retained 80.2 F g−1 at 5 A g−1, with nearly rectangular voltammograms and coulombic efficiencies of at least 98.9%. In concentrated aqueous lithium bis(trifluoromethanesulfonyl)imide (10 mol kg−1), three symmetric coin cells extended the voltage window to 2.0 V and gave a reproducible capacitance of 57.6 ± 0.2 F g−1 at 0.2 A g−1. In batch capacitive deionization with 10 mmol L−1 NaCl at 1.2 V, the electrodes returned a stable charge of 1.4–1.5 C per cycle over 100 cycles and removed 2.54 ± 0.28 mg g−1 of salt in the first five cycles; the decline in salt adsorption on cycling identifies charge efficiency as the next property to improve.