Carbon-based Porous Materials for Electrochemical Reactions

Abstract

The exploration of highly active and durable cathodic oxygen reduction reaction (ORR) catalysts with economical production cost is still the bottleneck to realize the large-scale commercialization of some emerging technologies, such as fuel cells and metal-air batteries. At present, the composite that contains expensive platinum (Pt) particles dispersed on a porous carbon support (e.g., activated carbon (AC)) is the most efficient ORR catalyst. In a common sense, the AC itself normally shows very low activity for the ORR, so the Pt particles are vital. Imagine that if we remove all of the Pt particles, can the remaining AC still play the similar role? The current work aims to make the inert AC active for the electrochemical reactions by creating unique defects in the AC. First of all, different porous carbon materials with variable specific surface areas were synthesized by an easy and scalable chemical activation method. It is shown that all the activated samples demonstrate obviously improved ORR activity. Afterwards, the unique defects were introduced into the activated ACs via a facile nitrogen doping and removal approach to further enhance their catalytic performance, based on the defective mechanism that was proposed in our group, in which the nitrogen was incorporated into the ACs under an ammonia atmosphere at 500 °C and subsequently removed at 1050 °C under a nitrogen environment. The results showed that the doped nitrogen did not enhance the ORR performance of the synthesized samples directly, but the produced defects possibly served as the active sites for the ORR, which finally contributed to the catalytic performance improvement. Particularly, the resulting defective carbon (D-AC) derived from the highest surface area AC (3508 m2/g) also exhibits the best ORR performance in alkaline medium with low overpotential. For example, the ORR activity of the D-AC is comparable to the commercial Pt/C (20 wt% Pt) in terms of 4-electron pathway, half-wave potential and limiting current density, namely, 3.6, 0.771 V and 4.4 mA·cm-2 vs 3.9, 0.785 V and 5.0 mA·cm-2, respectively. Meanwhile, the D-AC also exhibits excellent HER activity, which is better than most of the reported metal-free HER catalysts, but with much lower production cost.