Nanoporous Carbon Fabricated By CDC Method From Polysiloxane

In recent years, increasing more attentions have been paid on the fabrication and investigation of performance of nanoporous carbon(NPC), owing to its widespread application as electrode for supercapacitors and effective adsorbent for separation processes or gas storage. In comparison with conventional methods for synthesizing NPC, carbide derived carbon(CDC) technique has received many interests for some unique advantages. For example, carbon materials with high specific surface area(SSA) can be easily obtained via CDC method. Moreover, the porosity and microstructure can be finely adjusted. In this paper, NPC materials were fabricated by CDC met hod by adopting three pyrolysates(P-Si C, P-Si OC, P-Si OCH) as the carbide precursor respectively. These pyrolysates were all derived from polysiloxane(PSO). The compositional and structural evolution and etching mechanism of pyrolysates as a function of etching temperature or time were studied. Besides, the effect of post-treatment by N H3 and CO2 activation on the composition and structure was also investigated respectively. In addition, we discussed the electrochemical performance and CO2 capture performance of NPC respectively.We firstly studied the etching behavior of P-Si C. P-Si C was composed by dominant Si C crystals and a few free carbon(4 wt.%). The etching reaction for Si C can happen thermodynamically in the temperature range of 298~2000 K. However, it’s still weak for the etching reaction at 600 °C, with a very low etching rate(kl=0.02×10-6 mol·(kg·s·h)-1). A very thin porous carbon film could be observed after 3 hour etching. When the etching temperature increased from 600 °C to 750 °C, the Si proportion rapidly decreased while the C proportion uplifted obviously at the same time, which indicates that mainly Si atoms were etched by Cl2, and removed in form of gaseous Si Cl4. The etching rate also increased obviously(kl=0.4×10-6 mol·(kg·s·h)-1). A typical core-shell structure was formed for uncompleted etching. The shell was made up of nanoporous carbon while the core was mainly composed by β-Si C crystals. When the etching temperature increased from 750 °C to 900 °C, P-Si C could be totally transformed to carbon in high porosity, which came from the etching of Si C crystals by Cl2. These pores were considered as the diffuse channels for Cl2 and Si Cl4, which can help making the etching reaction conducting from outside to inside.By adopting P-Si C as the carbide precursor,microporous carbon(P-Si C-NPC)materials were obtained by CDC method(chlorination and post-treatment via NH3).P-Si C-NPC owned relatively high SSA and sigle-modal pore size distributions(PSDs)in range of 0.5~1.2 nm.Their SSA,total pore volume(Vt)and average pore size(Dmean)all increased at elevated etching temperature(900~1100°C).The SSA,Vt and Dmean for P-Si C-NPC etched at 1100°C was 1364.5 m2/g,0.716 cm3/g,2.1 nm,respectively.The proportion for micropore was very high,up to 72%.The final obtained carbon materials were homogeneously constituted by two parts:predominantly new produced amorphous carbon by chlorine and a few graphite ribbons inherited from the as-received sample.Pyrolysis temperature has a significant effect on the composition and structure of P-Si OC. At 1000 °C, P-Si OC mainly includes [Si O2C2], [Si O3C] and [Si O4] structure units, as well as a few free carbons. When the pyrolysis temperature increased, the content for [Si O2C2] and [Si O3C] decreased while the content for dominant [Si O4] and [Si C4] uplifted at the same time. The phase separation and carbothermal reduction reaction for the pyrolysates began when the pyrolysis temperature exceeded 1500 °C, which led to an obvious increase of Si C nanocrystals. The etching reaction of P-Si OC can happen at 450°C. However, small changes can be obsevered for all the element’s relative contents. Without changing etching time, when the etching temperature increased from 450°C to 525°C, the Si proportion rapidly decreased while the C proportion uplifted obviously at the same time, and the O proportion nearly kept the same. These phenomena indicated that mainly Si atoms were etched by Cl2, and removed in form of gaseous Si Cl4, leading to the generation of micropores at around 1 nm and sigle- modal PSDs. When the etching temperature increased to 600°C, the Si and O proportion both rapidly decreased while the C proportion uplifted obviously simultaneously, which indicates that a lot of Si atoms were etched by Cl2, and removed in form of gaseous Si Cl4 while O atoms escaped in form of gaseous CO or CO2, leading to the generation of mesopores(2~50 nm) and bi- modal PSDs. When the etching temperature increased to 750 °C, the Si proportion and C proportion from EDS was 0.16 at.%, 90.66 at.%, indicating a near completion of etching reaction. In comparison with P-Si C, it is easier to be etched by Cl2 for P-Si OC.By adopting P-Si OC as the carbide precursor, microporous and mesoporous carbon(P-Si OC-NPC) materials were obtained by CDC method. P-Si OC-NPC owned high SSA and bi- modal PSDs. Their pore structures were strongly associated with the pyrolysis temperature. The decreasing of SSA for P-Si OC-NPC was observed as the pyrolysis temperature increased. Nevertheless, the increasing of Dmean and widening of PSDs accured at the same time; The Vt decreased in the range of 1000~1400 °C, and greatly increased when the pyrolysis tempe rature got up to1600°C. At 1000 °C, the SSA, Vt and Dmean was 2223.6 m2/g, 1.873 cm3/g, 3.4 nm respectively. The proportion for micropore, mesopore and macropore was 58.7%, 29.0%, 12.3% respectively. At 1600 °C, the SSA, Vt and Dmean was 863.1 m2/g, 2.553 cm3/g, 11.8 nm respectively. The proportion for micropore, mesopore and macropore was 6.8 %, 86.5 %, 6.7 % respectively. The final obtained carbon materials were also homogeneously constituted by two parts: predominantly new produced amorphous carbon by chlorine and a few carbons in graphene structure or graphite ribbons inherited from the as-received sample.The amorphous P-Si OCH is composed by[Si O2C2Hx]and dominant[Si O3CHx] structure units.The H content decreased at elevated pyrolysis temperature.The etching behavior for P-Si OCH is similar with that for P-Si OC.However,some differences were observed.On the one hand,except Si atoms,H atoms in P-Si OCH could be also etched by Cl2;On the other hand,in comparison with P-Si OC,it is easier to etched by Cl2 for P-Si OCH,whose etching rate(kl=0.99×10-6 mol·(kg·s·h)-1)is higher than that for P-Si OC(kl=0.72×10-6 mol·(kg·s·h)-1).P-Si OCH could be transformed to carbon after 3hours etching at 750°C.Most porosity came from the etching of P-Si OCH while the inferior part came from the porosity existed in P-Si OCH before etching.By adopting P-Si OCH as the carbide precursor, microporous and mesoporous carbon(P-Si OCH-NPC) materials were obtained by CDC method. P-Si OCH-NPC owned high SSA and bi- modal PSDs. Their SSA and Vt increased at elevated etching temperature. When the pyrolysis temperature and etching temperature was 800, 900 °C respectively, the SSA, Vt and Dmean got up to 2142.6 m2/g, 2.195 cm3/g, 4.1 nm respectively. The proportion for micropore, mesopore and macropore was 46.8%, 33.3 %, 19.9 % respectively. In comparison with P-Si OC-NPC, the proportion of macropores for P-Si OCH-NPC was higher. Besides, their structures were homogeneously amorphous.The geometries and macrostructures of NPC are finely inherited from the pyrolysates obtained at different pyrolysis temperatures from PSO. Their microstructures are basically amorphous, with a relatively low crystal degree. After the post-treatment by NH3, the residue Cl atoms in NPC were almost removed. Luck ily, their crystallinity had no decrease. Increasing the pyrolysis temperature and etching temperature will help improving the crystallinity.In order to further adjusting the pore structure or microstructure of NPC, we studied the effect of CO2 activation(temperature or time) and thermal-treatment in Ar(temperature or time) on the structure of NPC respectively. The results showed that CO2 activation can increase the pore volume, without affecting their crystallinity. A relatively good activation conditio n is: 800~950°C for the activation temperature, 1~3 h for the activation time. The pore volume and pore size increased when the activation temperatrure or time was uplifted. For P-Si C-NPC, the SSA and Vt can be improved up to 46.5 %, 86.4 % respectively after 2 hours activation at 950°C; Under the same condition, the SSA and Vt is improved up to 22.2 %, 8.6 % respectively for P-Si OC-NPC. By thermal- treatment in Ar atmostphere at 1800~2100 °C for 2 h, the crystallinity of P-Si OC-NPC was obviously improved, the SSA decreased owing to the enlargement of pore size or decreasing of micropore volume. At 1500°C and below, the structure is stable.The NPC materials obtained by CDC method from PSO showed good capacitance as the electrode for supercapacitors. The capacitance reached to 148.7 F/g at the current density of 1 A/g for P-Si OC-NPC electrode with KOH aqueous solutions as electrolyte, and this value could retain 74.6 % when the current density increased to 10 A/g. The P-Si OC-NPC electrode also showed good capacitance retention ratio up to 94.3 % after 2000 charge-discharge cycles at a current density of 1 A g-1. Increasing pyrolysis temperature would lead to a continual decline of specific capacitance. The P-Si C-NPC electrode showed high capacitance of 454.4 F/g at the current density of 0.5 A/g. However, this value rapidly decreased when the current density increased. The P-Si C-NPC electrode showed a better capacitance retention ratio up to 100 % after 2000 charge-discharge cycles at a current density of 1 A g-1.The NPC materials obtained by CDC method from PSO showed excellent CO 2 capture performance as the adsorbent. The capture performance of P-Si C-NPC materials is better compared with other two materials(P-Si OC-NPC, P-Si OCH-NPC) owing to their relatively larger super- micropore volume(d<0.6 nm). The maximum value for CO2 uptake reached up to 5.80 mmol/g at 0 °C under the ambient pressure(or 1 atm). The results from TGA test showed that the P-Si OC-NPC materials exhibited excellent desorption performance and cycling performance. The CO2 uptake can be improved to 6.01 mmol/g at 0 °C under the ambient pressure when the P-Si C-NPC experienced the CO2 activation, which was found to be able to increase the super-micropore volume.