The ultramicroporous carbon precursor was synthesized using the same procedure reported elsewhere by our group. In short, resorcinol (10 mmol) was dissolved in water (5 mL) before adding. imidazole (2 mmol), ethylene diamine tetraacetic acid (EDTA; 0.5 mmol), and zinc chloride (ZnCl2; 2 mmol) sequentially to the solution under vigorous stirring. After stirring for 10 min formaldehyde solution (37 wt.%) containing 20 mmol of formaldehyde was quickly injected into the solution with fast stirring (pH value = 3.5). The clear precursor solution could be directly used for inkjet printing for up to 4 days at room temperature (up to 10 days with additional cooling in a fridge at 2 °C).