For a long time, cruising range is the bottleneck that restricts the development of electric vehicles. We call it process anxiety. Improving cruising range on the one hand has to increase the capacity of the battery pack, but it is more important to increase the specific energy of the battery. Currently, ternary material lithium The weight-to-energy ratio of an ion-powered battery is generally 200Wh/kg. With the continuous advancement of technology, it is expected to launch a mass-produced high specific power battery with a specific energy of 300Wh/kg by 2020, but this still cannot meet the future development of electric vehicles. The demand. With regard to the development of the next generation of high specific power batteries, there are currently several routes to choose from. One is the all-solid-state lithium metal battery, which is currently widely accepted and recognized as the technological route to the United States’ Battery 500. The plan is to achieve the target of achieving a specific energy of 500Wh/kg by developing lithium metal secondary battery technology. Japan's solid electrolyte technology is at the leading level in the world, and its developed ionic conductivity of sulfide electrolyte can even be compared with that of liquid electrolyte. The other line is the metal-air battery, such as the current mainstream Li-air and Na-air batteries. On the specific energy exceeds 2000Wh / kg, much higher than the lithium-ion battery.
Recently, Qichen Wang of Central South University and National University of Defense Technology prepared a Zn-air battery using an N-doped graphene material NDGs-800 as an air electrode catalyst. The use of a large number of graphene oxide GO defects has increased the 2The catalytic efficiency of the air electrode greatly improves the performance of the Zn-air battery, and the specific energy is as high as 872.3 Wh/kg. It has a very broad application prospect in the field of energy storage in the future.
The most critical point for metal-air batteries is the design of the air electrode. The air electrode must have both catalytic O 2Reduction and oxygen evolution reaction. Common oxygen electrodes are mostly noble metals (Pt) and rare earth oxides, but they are difficult to balance. 2The reduction and oxygen evolution of the two reactions. So people pay attention to the carbon electrode, the study shows that the defects and porous structure in the carbon material can be O 2The reduction and oxygen evolution in the electrode provide numerous active sites to enhance the metal-air battery performance. The redox graphene is just such a very good option. The redox graphene itself has many defects. Qichen Wang introduced more defects in graphene through N doping, and the large specific surface area and porous structure of graphene was also 2The reduction and oxygen evolution provide a large number of active sites, thereby significantly improving the performance of Zn-air batteries.
The method for synthesizing N-doped graphene is shown in Figure a above. First, a certain number of g-Cs are used. 3N4The tablets were added to an aqueous solution of graphene oxide GO, sonicated for 1 h, and the mixed solution was then hydrothermally treated at 180° C. for 12 h to generate a black mixed gel, which was then freeze-dried for 48 h to remove H 2O. The dried material in a tube furnace, in N 2Under the protection of heating to 600-900 ° C, heat treatment 3h, N-doped graphene material NDGs-x (x represents the processing temperature).
The structure of N-doped graphene is shown in the figure above. From figure b and c, it can be seen that it has an open-cell structure and typical graphene characteristics. Atomic force microscope (above figure e) shows that the thickness of graphene is 3 nm, about 9 Layers of carbon atoms, while the material has a very large specific surface area (443.2m 2/g) Volume ratio of micropores (3.43cm 3/g), can be O 2The reduction and oxygen evolution reactions provide a large number of active sites.
XPS studies have shown that N elements mainly exist in graphene oxide in three forms: pyridine N, pyrrole N, graphite N, and pyridine N+-O-. NDGs sintered at 800°C can be noticed from the following figure d. The pyridine N content of the 800 material is the highest, reaching 47.9%. The high content of pyridine N and the redox graphene GO in the presence of a wide range of defects, significantly promoted the catalytic O 2The efficiency of the reduction and oxygen evolution reactions.
More reactive sites help N-doped graphene NDGs to obtain better reactivity. From the linear voltage scan of Figure a below, it can be seen that the NDGs-800 material (red curve) shows a very high catalytic O 2Reductive activity, the initial reaction voltage is 0.95V, the half-wave voltage also reaches 0.85V, and the reaction current density at 0V reaches 5.6mA/cm 2From the figure b below we can notice the response current density of NDGs-800 at 0.8V (13.91mA/cm 2) Even higher than the reaction current density of the Pt/C composite catalyst (13.32 mA/cm 2) is much higher than NDGs-900 (6.03mA/cm 2), NDGs-600 (55.55mA/cm 2) and NDSs-700 (2.80mA/cm 2) This makes the NDGs-800 material the best non-metallic reaction catalyst.
Although NDGs-800 catalyzes O 2The activity of the reduction reaction is very high, but we still need to investigate the activity of the NDGs-800 catalytic oxygen evolution reaction. From the figure below we can see that the NDGs-800 material is at 10mA/cm 2At the current density, the overpotential of the oxygen evolution reaction is lower than that of RuO. 2/C catalyst is 375mV high, indicating that the oxygen evolution catalytic efficiency of NDGs-800 materials is inferior to that of RuO. 2/C catalyst, this is where NDGs-800 material needs improvement in subsequent studies.
Qichen Wang used a combination of NDGs-800 material for a Zn-air battery (structure is shown in the figure below). The battery has an open-circuit voltage of 1.45V and a power density of 115.2mW/cm. 2, better than Pt/C catalyst (1.43V, 110.3mW/cm 2), The specific capacity of the Zn anode by using NDGs-800 material reaches 750.8mAh/g (current density 10mA/cm 2), The specific energy of the battery reached 872.3Wh/kg. The battery also showed very excellent cycle performance at 10mA/cm. 2At a current density of 234 cycles (20 min per cycle), the cell has almost no fall off, which is much better than that of a Pt/C+Ir/C catalyst.
The N-doped graphene material NGDs-800 material developed by Qichen Wang makes full use of a large number of defects in graphene oxide GO and introduces more defects through N doping. 2The reduction and oxygen evolution reactions provide a large number of active sites, greatly improving the catalytic efficiency, especially in the catalytic O 2In terms of reduction, the catalytic efficiency is even higher than that of Pt/C electrode, and it also shows excellent stability in charge-discharge cycles, and has broad application prospects. However, the activity of catalytic oxygen evolution reaction of NDGs-800 is still not as good as that of RuO. 2/C catalyst, this is where the follow-up needs improvement.
