Microwave-hydrothermal synthesis of boron/nitrogen co-doped graphene as an efficient metal-free electrocatalyst for oxygen reduction reaction

The eloquent factor that improves the electrode performance in electrochemical industries is the oxygen reduction reaction. Various researches have also been done on enhancing oxygen reduction reaction activity using precious carbon supported metals like Pd, Pt and Ru. Thereby investigated these precious metal catalysts have some severe dis-benefit such as poor stability, expensive, limited availability. So, to develop metal-free oxygen reduction reaction electrocatalysts particularly heteroatom-doped carbon material with other atoms provides active sites for catalytic reaction.

Co-doping by two elements with higher and lower electronegativity than that of carbon produces tremendous electronic structure with a synergistic coupling effect stemming from the doped heteroatoms. Rather than singly doped carbon materials, co-doped carbon based materials possess much higher electro-catalytic activity. In electrochemical energy researches, graphene which possesses high conductivity, remarkable surface area and mechanical properties is considered as promising candidates for oxygen reduction reaction catalyst.

After a while, non doped graphene is a disappointing electrocatalyst activity whereas co-doped graphene with different heteroatoms enhancing oxygen reduction reaction catalytic activity. So far, B and N co-doped graphene has synthesized using different methods like hydrothermal, thermal annealing and chemical vapor deposition, however these methods have some drawbacks. So it is very important to develop efficient approaches beyond conventional methods.

Researchers led by Professor Moo Whan Shin from Yonsei University in South Korea proposed a simple approach to synthesize highly reduced B, N co doped graphene (HRBNG) from graphene oxide by microwave hydrothermal method. The work is published in the peer-reviewed Journal, International Journal of Hydrogen Energy.

In their microwave hydrothermal method, the first step, graphene oxides are converted into BN doped graphene oxides by the microwave hydrothermal method, thereby heteroatom doping with B and N atoms are achieved. But there remained oxygen group which is linked to graphene nano sheets. In the second step, the oxygen group undergoes microwave treatment again. Thus reduced BN co-doped graphene is achieved

Hence B and N atoms are incorporated into graphene framework and prevents formation of inactive products. Due to the assistance of microwave radiation, rapid heating is induced so the synthesis time is significantly reduced than that of conventional methods. To evaluate the qualitative and quantitative chemical analysis of functional groups on the graphene surface, X-ray photo electron spectroscopy is carried out.

This highly reduced B, N co–doped graphene method proposed by the authors enhances electrocatalytic activity towards oxygen reduction reaction in alkaline electrolytes and favors four electron transfer process for the oxygen reduction. It also better stability than that of conventional methods.

This study demonstrated the use of highly reduced BN co-doped graphene as a suitable replacement to other expensive metal catalyst and provided a new approach for developing graphene based oxygen reduction reaction catalysts.