Hydrogen Storage in Activated Carbon with High Surface Area
Hydrogen is a renewable and eco-friendly fuel regarded as an ideal alternative to fossil fuels. However, the storage of hydrogen is still a tricky problem which is only 0.59 wt% even under the conditions of 273 K and 6.0 MPa. In order to improve the hydrogen storage capacity, the performance of activated carbon with high surface area, two kinds of pore size distribution is studied, since activated carbon shows good hydrogen adsorption properties at low temperature due to its complex porous structure.
Preparation of activated carbon samples with an ultra-high specific surface area
The activated carbon samples use petroleum coke as the precursor, KOH as the activating agent, are activated under an inert atmosphere at different temperatures and times. The specifications of the two samples are as follows.
| Activated Carbon | Surface Area | Micropores | Mesopores |
|---|---|---|---|
| Sample A | 2610 m2/g | 20.94% | 79.06% |
| Sample B | 2644 m2/g | 47.88% | 52.12% |
Effect of pressure on hydrogen desorption
A test is carried out to compare the effects of pressure on hydrogen desorption amount of the two activated carbon samples at a temperature of 273 K. The results show that the higher the mesopore content of activated carbon, the greater the effect of pressure on hydrogen desorption: At 273 K temperature and 9.0 MPa pressure, the hydrogen desorption amount of sample A is 26.67 mmol/g, while that of sample B is only 22.52 mmol/g.
The two prepared activated carbon samples both have a specific surface area above 2600 m2/g, a well-developed, complex pore structure where hydrogen molecules are stored. The unbalanced forces acting on the atoms on the pore walls of activated carbon form an adsorption potential field between the walls, the strength of which varies with the pore size: The smaller the pore size, the greater the strength of the field, and the stronger the adsorption capacity to hydrogen molecules.
For pores with small sizes (micropores), due to the strong adsorption potential field, they can firmly adsorb hydrogen molecules on the surface of pore wall. And because of the limitation of pore size, hydrogen molecules are mostly adsorbed in a monolayer and form an adsorption phase, which may cause some of the adsorbed hydrogen molecules difficult to desorb. Moreover, the hydrogen molecules are difficult to enter small pores due to large air resistance, so the large surface area of high micropore content activated carbon cannot be fully utilized, and its storage capacity of hydrogen is relatively weaker compared to that with high mesopore content.
For pores with relatively large sizes, the larger distance between two pore walls leads to a weaker strength of the potential field, so the hydrogen molecules are easily desorbed during the desorption process. Apart from the adsorption phase, there are also many gas phase hydrogen molecules due to the effect of pressure in the larger pores, whose intermolecular force is way larger than adsorption phase molecules’. The adsorption phase hydrogen molecules have a larger density than gas phase, and their contribution to the storage capacity is also much greater. The increase of adsorption pressure can increase the density of both gas phase and adsorption phase hydrogen, so the hydrogen desorption amount of activated carbon increases accordingly.
Effect of temperature on hydrogen desorption
The hydrogen desorption performance of the two samples at 3.0 MPa and different temperatures is tested. The results show that with the increase of temperature, the hydrogen desorption amount of activated carbon samples decreases. (1) When the temperature increases from 273 K to 298 K, the desorption amount is relatively less affected by the temperature: The reduction rate of sample A is 3.64%, and that of sample B is 12.5%. (2) When the temperature increases from 298 K to 323 K, the desorption amount decreases sharply: The amount of samples A and B decreases by 6.88 mmol/g, 5.43 mmol/g, respectively. (3) When the temperature is higher than 323 K, the decrease rate is not significant. (4) The temperature ranges of the steep drop of the two samples are different, those of samples A and B are 308-323 K and 298-308 K, respectively.
A low temperature is conducive to the adsorption of hydrogen molecules on the surface of activated carbon and increases the density of both adsorption phase and gas phase hydrogen, so the activated carbon can have better performance in the storage of hydrogen. And hydrogen molecules are harder to desorb at low temperatures.
Because sample B has higher micropore content than sample A, its stronger adsorption potential field magnifies the effect of temperature on the hydrogen storage capacity, and its temperature range of the steep drop is lower than sample A. When the temperature reaches a certain level, the amount of hydrogen molecules that can be adsorbed on the surface of activated carbon becomes very small, and those in the activated carbon adsorption system mainly rely on compression. Therefore, the decrease rate of desorption amount becomes stable when temperatures above 320 K.