Bulk and Surface Chemistry Modification of Highly Porous Carbon for Supercapacitors
Candelaria, Stephanie Lynn
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Highly porous carbon prepared through sol-gel processing is an excellent electrode material for supercapacitors, also known as electrochemical capacitors, because of its high surface area and pore volume, good conductivity, and low cost. Research has mostly focused on increasing the surface area of the carbon to improve charge storage, but there are limits to how large this value can be while still maintaining good electrochemical performance of the material. Alternatively, carbon can be modified with foreign elements, either in the bulk or on the pore surfaces, to induce pseudocapacitive reactions that can increase capacitance. Modified samples can also be tested at higher working voltages, which significantly increase the energy and power densities. In this work, highly porous carbon is modified with nitrogen and tested as electrodes in supercapacitors. Modification significantly increases the wettability of the carbon, and the capacitance increases with increasing nitrogen added. However, too much nitrogen can cause pore blockage and decrease accessible surface area, limiting the capacitance. Highly porous carbon, both with and without nitrogen on the pore surfaces, is then tested at higher working voltages. Increasing the voltage from 2V to 3V significantly improves both the energy density and power density. Finally, porous carbon was also synthesized from lignin, a complex polymer derived from natural resources and a waste product in the paper industry. Purification removed over half of the impurities and resulted in porous carbon with four times the internal surface area compared to unpurified lignin. When tested in devices, lignin-derived carbon shows promise as low-cost, renewable material for high performance supercapacitors.