Carbonization of polymers of intrinsic microporosity to microporous heterocarbon: Capacitive pH measurements

A nitrogen-containing polymer of intrinsic microporosity (PIM-EA-TB-H₂; nitrogen adsorption surface area 846 m² g⁻¹) is vacuum carbonized at 700 °C and thereby directly without post-treatment converted into a microporous heterocarbon (cPIM; N₂ adsorption surface area 425 m² g⁻¹). Nitrogen functionalities in the polymer backbone are retained in the heterocarbon and appear responsible for unusual time-, electrolyte-, and pH-dependent properties. Electrochemical characterization suggests a high specific capacitance (typically 50 F g⁻¹) but only after prolonged immersion in aqueous HClO₄. The time-dependent increase in capacitance during immersion is assigned to slow hydration and ingress of HClO₄ into hydrophobic micropores (H₂SO₄ or H₃PO₄ are more hydrophilic and much less effective). Once hydrated, the microporous heterocarbon exhibits pH-dependent capacitance “switching” over a wide pH range and analytical applications as “capacitive” pH sensor are proposed.