Current progress on the preparation of Al-air battery cathode
Carbon nanofibers (CNFs) free-standing film are being developed as cathode of the Al-air battery. These CNFs are obtained by electrospinning. Electrospinning is the procedure of adopting electrostatic forces to pull a polymer solution jet to produce nanofibers. Under the action of electrostatic field, the droplet of polymer solution at the tip of the tube deforms into a cone (Taylor cone). When the electrostatic force is higher than the surface tension, the charged jet is ejected and it moves to a ground plate regarded as a counter electrode, where the non-woven nanofibers are collected forming a mat. Different collectors can be used, but in our case, we normally use a rotatory collector. Schematic procedure and our electrospinnings are showed in Figure 1.
Figure 1. Schematic procedure of electrospinning process (left) and electrospinning equipment at LEITAT (right).
There are some parameters that can be modulated to change the properties of the final nanofibers, as the operating voltage, viscosity, the flowrate or the rotation speed.
The CNFs are obtained after the application of a specific thermal treatment to the PNFs, consisting in two steps:
- First a stabilization step where the thermoplastic chain molecule is transformed into a non-meltable ladder polymer by cyclization, dehydrogenation, and oxidation.
- During subsequent carbonization in a non-oxidizing atmosphere, oxygen and the heterocyclic nitrogen are split off and planar polyaromatics are formed, giving place to a more graphitised material with an increased electrical conductivity.
Figure 2 shows the final CNFs used as cathodes for this project our tubular furnace in which we obtain the final material.
Figure 2. CNF mat (left) and tubular furnace used at LEITAT for the synthesis of the CNF mats.