The printed structures were transferred into a tube furnace (corundum tube) and pyrolyzed at 920 °C for 2 h under an argon atmosphere (heating rate 300 K h -1 , flow rate 30 mL min -1 ). Prior to the cooling phase of the furnace, the structures were physically activated with CO2. Therefore, the argon gas supply was closed and the carbon dioxide stream opened (flow rate 30 mL min - 1 ) 45 min.