Influence of counter ions of ammonium for nitrogen doping and carbon properties in hydrothermal carbonization: characterization and supercapacitor performance

Abstract

Ammonium has been successfully utilized to nitrogen dope carbon structures via hydrothermal carbonization, although the influence of different attached counter ions (anions) on the resultant carbon physicochemical properties and electrochemical performance has not been examined before. Four different counter ions (SO₄²⁻, PO₄³⁻, Cl⁻, and Fe(SO₄)₂) attached to ammonium were seen to influence the hydrothermal reaction, nitrogen incorporation levels, physicochemical properties, activation ability and supercapacitor performance. For instance, nitrogen K-edge NEXAFS found differences in the levels of pyridinic and pyrrolic groups with PO₄³⁻ incorporating predominately pyridinic nitrogen groups. PO₄³⁻ also achieved the highest surface area (2132.6 m² g⁻¹), however this material was unstable as a supercapacitor, losing almost 50% of its performance over 500 cycles. SO₄²⁻ resulted in the highest level of nitrogen incorporation (5.53 at%) and hydrothermal yield (45.5%), while Fe(SO₄)₂²⁻ resulted in the lowest (2.92 at%). However, Fe(SO₄)₂²⁻ produced unique flower like structures not seen in any of the other anions. Cl⁻ produced the highest performing material, achieving 190 F g⁻¹ at 10 mV s⁻¹ in 1 M KOH and had moderate nitrogen incorporation (3.42 at%). Overall, this study indicates that the anion has substantial influence on the physicochemical properties of the material, allowing an additional level of tailoring.