Cell voltage versus electrode potential range in aqueous supercapacitors

Supercapacitors with aqueous electrolytes and nanostructured composite electrodes are attractive because of their high charging-discharging speed, long cycle life, low environmental impact and wide commercial affordability. However, the energy capacity of aqueous supercapacitors is limited by the electrochemical window of water. In this paper, a recently reported engineering strategy is further developed and demonstrated to correlate the maximum charging voltage of a supercapacitor with the capacitive potential ranges and the capacitance ratio of the two electrodes. Beyond the maximum charging voltage, a supercapacitor may still operate, but at the expense of a reduced cycle life. In addition, it is shown that the supercapacitor performance is strongly affected by the initial and zero charge potentials of the electrodes. Further, the differences are highlighted and elaborated between freshly prepared, aged under open circuit conditions, and cycled electrodes of composites of conducting polymers and carbon nanotubes. The first voltammetric charging-discharging cycle has an electrode conditioning effect to change the electrodes from their initial potentials to the potential of zero voltage, and reduce the irreversibility

Optimizing the performance of supercapacitors based on carbon electrodes and protic ionic liquids as electrolytes

International audienceProtic ionic liquids (PILs) were implemented as electrolytes for supercapacitors using activated carbons with various porous textures as electrode material. The carbon with the largest specific surface area and highest amount of narrow mesopores (pore diameter: 2–7 nm) was found to give the highest specific capacitance in pyrrolidinium nitrate (PyNO3) ionic liquid. However, it should be noted that when the pH value of this ionic liquid was adjusted around 11, higher specific capacitance was achieved, revealing a better electrochemical performance of carbon electrodes in basic media (i.e., capacitance values of 121 and 208 F g−1 for an electrolyte based on PyNO3 with a pH value of 7 and 11, respectively). This ionic liquid contained a small amount of water, which restricted the maximum voltage of symmetric capacitors to a value of 1.2 V, even after PyNO3 had been partially dried (H2O content around 1110 ppm). Therefore, the triethylammonium bis(trifluoromethylsufonyl)imide – NEt3H TFSI – PIL was prepared in order to expand the potential window; after drying this PIL contained 200 ppm water. The results obtained with NEt3H TFSI suggest that maximum voltages as high as 2.5 V can be achieved. This clearly shows that the presence of water in PILs has a negative effect on the performance of supercapacitors

Oxygen reduction on Pt/[TaOPO 4 /VC], Pt/[NbOPO 4 /VC], Pt/[Ta 2 O 5 /VC], and Pt/[Nb 2 O 5 /VC] electrocatalysts in alkaline electrolyte

New opportunities exist for the electrocatalysts in oxygen reduction reaction (ORR) in alkaline fuel cells (AFCs) in terms of electrocatalytic activity and material stability. In alkaline electrolyte, the electrocatalytic process is more facile than in acidic electrolyte due to the weakening of the competitive adsorption by the unreactive anion (1, 2). Compared to acid electrolytes, a much wider range of electrode materials are stable in alkaline electrolyte, including a number of less expensive materials; therefore, less platinum or even some non-noble metals (3-7) can be considered.. Recent studies have shown that platinum supported on metal oxide MO x (M=Ce, Ti, Mo, W, Nb, Ta) gave enhanced electrocatalytic activity for ORR ACKNOWLEDGEMENTS We are grateful to the Office of Naval Research for continued support of our research