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.