Ammonia Treatment of Activated Carbon Powders for Supercapacitor Electrode Application

Abstract

Microporous activated carbon powder was modified by ammonia gas or ammonia/nitrogen mixture at elevated temperatures with different duration in order to obtain supercapacitor electrode materials with enhanced capacitance. XPS analysis showed higher amount of nitrogen containing functional groups (up to 4.5 at%) incorporated into carbon powders which were oxidized before ammonia treatment. The highest amount of N was introduced by treatments at 400 • C, whereas extensive activation was observed after treatment at 800 • C. The modified carbon powders demonstrated an increase in gravimetric capacitance up to 30% in 1 mol dm −3 H 2 SO 4 solution. During 10,000 constant current charge/discharge cycles, the capacitance decreased by 2.5 to 10%, depending on the modified material used.Together with the growing worldwide energy consumption and the concerns about environmental effects of using fossil fuels, there is a strong ambition to remarkably increase the role of renewable energy sources, e.g., solar and wind energy systems. Since their power is heavily fluctuating, reliable and cost-efficient storage and conversion devices have to be developed to effectively incorporate the renewables in the uninterrupted and balanced electric energy supply. There are also other attractive application areas requiring electricity management solutions, such as automotive power, portable electronic devices, etc. Electrochemical devices, such as supercapacitors, batteries and fuel cells, have been proven to be promising for energy storage and conversion, both in small and large scale. [1][2][3][4][5][6][7][8][9] Supercapacitors as primarily high-power devices are based on either aqueous or non-aqueous electrolytes, combined with various electrode materials which can be generally divided into two groups: practically double-layer capacitive high surface area carbons (activated carbons, carbon aerogels, carbon nanotubes, carbide-derived carbons, etc.) 1-7,10 and various materials that, in addition to double-layer charging, undergo pseudocapacitive redox reactions as a function of electrode potential (various d-metal oxides, N or O heteroatom functionalized carbon powders, conducting polymers, etc.). 1,2,11-21 Recently some papers have also evidenced pseudocapacitive phenomena, where the quick redox reactions on the carbon/electrolyte interface originate from the electrolyte. 22,23 Under negative polarization in protic electrolytes, nanoporous carbons can also exhibit pseudocapacitive behavior related with nearly reversible electrochemical hydrogen storage. 24 Hence, the pseudocapacitive properties of carbons can be enhanced by creating surface functional groups. 25-29 For example, treatments at elevated temperatures by mixtures of gases containing ammonia (Ar-NH 3 , N 2 -NH 3 , etc.) have been proposed to modify the surface functionality and porous texture of various carbons (activated carbon powder, activated carbon fiber, carbonized melamine-mica composite, activated rayon char, etc.). [25][26][27][28][29] The treatment conditions have to...

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