# Carbon Dioxide Adsorption on Carbon Nanofibers with Different Porous Structures
## 2 Materials and Methods  
### Preparation of Electrospun Nanofibers  
Polyacrylonitrile (PAN)-based carbon nanofibers with different porous structures were prepared using electrospinningning processes, including solid, hollow, and porous nanofibers. The hollow nanofibers were synthesized using a coaxial electrospinning process in which PAN (Mw = 15 kDa, Sigma-Aldrich, St. Louis, MO, USA) was the shell polymer and poly(methyl methacrylate) (PMMA) (Mw = 120 kDa, Sigma-Aldrich) was selected as the sacrifice core. Initially, PAN and PMMA were dissolved in N, N-dimethylacetamide (DMAc, Sigma-Aldrich) individually to form the polymer solutions with 10 wt.% PAN or 30 wt.% PMMA. The resulting solutions were subjected to magnetic stirring at 60 \({}^{\circ}\)C for 24 h in order to obtain homogeneous polymer solutions. The core-shell fibers were prepared using a coaxial spinneret system in an electrospinning unit (FES-COE, Falco Tech Enterprise Co., Ltd., New Taipei City, Taiwan). The inner diameter of the outer needle (18 gauge) was 0.96 mm, and the inner and outer diameters of the inner needle (21 gauge) were 0.52 and 0.82 mm, respectively. The electrospinning was carried out under an applied electrical voltage of 15 kV onto a metal drum collector (O 10 cm) covered with aluminum foil and rotated at 300 rpm, and a tip-to-collector distance of 20 cm was used. The shell and core solutions were pumped at a flow rate of 1.0 and 0.5 mL/h [22], respectively, and pushed by two syringe pumps (NE-1000, New Era Pump Systems, Inc., Farmingdale, NY, USA).  
The core-shell fiber was stabilized in air from room temperature to 280 \({}^{\circ}\)C at a heating rate of 1 \({}^{\circ}\)C/min, then the temperature was held at 280 \({}^{\circ}\)C for 12 h to complete the cyclization and dehydrogenation reactions before the carbonization process to convert PAN from thermoplastic to a non-plastic compound [28; 31; 32]. The stabilized nanofibers were cooled down to room temperature and then carbonized in a nitrogen atmosphere with a flow of 100 sccm in a tubular furnace at 900 \({}^{\circ}\)C with a heating rate of 5\({}^{\circ}\) C/min, and maintained for 1 h [28] before the sample was cooled down to room temperature. After carbonization, activation of the samples was carried out by raising the temperature to 850 \({}^{\circ}\)C at a rate of 10\({}^{\circ}\) C/min under flowing nitrogen. CO\({}_{2}\) was used as the activation agent with a flow rate of 100 sccm, switched in and held for 1 h. This sample was labeled as PANh15. For comparison, the PMMA (core polymer) was replaced with PAN, and the other conditions remained the same. This solid sample was the control sample and was denoted as PANf. Another type of core-shell fiber was synthesized under an applied electrical voltage of 12 kV, and the shell and core solutions were pumped at a flow rate of 0.7 and 0.3 mL/h, respectively. The other treatments remained the same, and this sample was denoted as PANh12.  
In addition, the porous fibers were obtained from a copolymer that was electrospun using a traditional single spinneret system, where PMMA was selected as the pore generator. First, a mixture of PAN and PMMA was dissolved in DMAc by stirring for 24 h at 60 \({}^{\circ}\)C to obtain a blend solution for electrospinning. The blend ratio of PAN and PMMA was 2:1 (_w/w_), and the total polymer concentration was 15 wt.%. The blend solution was loaded into a syringe with a 21-gauge spinneret (0.52 mm inner diameter). The infusion rate of the solution in the syringe was 1.0 mL/h. The applied voltage was 15 kV, and a tip-to-collector distance of 15 cm was chosen. The follow-up processes remained the same, and this sample was labeled as PANp.