Quantification of activated carbon functional groups and active surface area by TPD-MS and their impact on supercapacitor performance

Abstract

Carbon oxygenated functional groups and active sites play an important role in the interactions with the electrolytes in aqueous supercapacitors. For the first time, correlations between each type of O-surface groups and electrochemical performance are established by means of thermodesorption coupled with mass spectrometry (TPD-MS). A set of five activated carbons and one soft-salt templated carbon, were studied in three different pH electrolytes, 1M H₂SO₄, 1M KOH and 1M Na₂SO₄. Linear correlations between surface groups and capacitance were found: acidic groups such as carboxylic acid and phenol-ether groups improve capacitance, whereas carbonyl-quinone groups are detrimental. Moreover, active surface area (ASA) is for the first time measured for activated carbons thanks to a new protocol, which minimises material burn-off during oxygen chemisorption. In addition, a new approach consisting in the quantification of the ASA is proposed. It has been highlighted that certain active sites are linearly correlated to an improvement of capacitance. Although the oxygen surface groups and ASA improve the capacitance via pseudo-capacitance phenomena, the capacitive mechanisms, governed by the porosity of the activated carbons, are shown to be predominant. Among all materials, the soft-salt templated carbon gives the best electrochemical performance. Indeed, it combines a large quantity of carboxylic acid and phenol-ether surface groups as well as appropriate ASA. Moreover, it has a high specific surface area (2556 m²·g^-1) and optimal pore size (0.88 nm). All these characteristics, provide a high capacitance, a high rate capability and a high capacitance retention after 10,000 cycles.