Nitrogen release and pore formation through KOH activation of nitrogen-doped carbon materials: an evaluation of the literature

The simultaneous use of KOH and nitrogen to manufacture carbon materials provides these materials with properties that the presence of only one of these additives would not give them, such as high porosity and reactivity. However, it is difficult to obtain nitrogen-doped carbon materials with both high porosity and high nitrogen content, as the KOH significantly reduces the nitrogen content. In this review the complex relationships between nitrogen content and nitrogen precursor amount, KOH amount and the activation temperature are discussed, with a focus on the different N-functional groups and the porosity of the fabricated carbons. Generally, increasing activation temperature and increasing KOH amount decrease the nitrogen content due to reactions with the N-containing substructures of carbon, resulting in the release of nitrogen as N2, HCN and other N gases. Increasing these parameters can also result in the reduction of pyridine-N while the amount of quaternary-N increases simultaneously. Besides this, an increase in the amount of nitrogen precursor leads to an increase in the porosity of N-doped materials. However, too high amounts of the nitrogen precursor generate an excess of nitrogen which blocks the pore system and consequently reduces the porosity of the doped carbons.