In the past few decades, the rapid consumption of non-renewable fossil fuels has caused significant environmental and social problems on a global scale. Renewable energy such as wind power, solar energy, and tides have been greatly developed and widely applied, and other efficient energy sources with attractive performance, low cost, and environmental benefits are in high demand at the same time.
Fuel cells and metal-air batteries as a clean and efficient energy source have been expected to be an encouraging solution to the energy crisis. The cathode oxygen reduction reaction (ORR) of fuel cells and metal-air batteries is at heart of key for the performance when converting chemical energy into electricity, so high-performing electrocatalysts for the ORR are essential to meet practical applications.
Up to now, in order to overcome the sluggish kinetics of ORR, Pt-based electrocatalysts are the most common cathode catalyst used for fuel cells. However, the high cost and limited availability hampered further development and commercialization pace of fuel cells and metal-air batteries. Therefore, the development of inexpensive and efficient non-noble catalysts to replace the Pt-based electrocatalysts has become the most pressing issue in current catalyst technologies.
For the study, boric acid, triphenylphosphine, cyanamide, and ferrous chloride were used as the precursor of a boron source, a phosphorus source, a nitrogen source, and a transition metal, respectively, to synthesize a series of transition metal iron-modified multi-element doped porous graphene foams catalysts via a simple silicate templated method.
The most efficient one, i.e., PGF-Fe-NBP, received the onset potential of 0.95 V and a half-wave potential of 0.84 V in alkaline medium, which is very close to that of the commercial 40% Pt/C catalyst. Even in an acidic medium, PGF-Fe-NBP received the onset potential of 0.85 V and a half-wave potential of 0.68V. In addition, it also obtained superb electro activities of low H2O2%, a high electron transfer number, and excellent stability in both alkaline and acidic media.
The study further explored the effect of single-element, dual-element, and multi-element doped graphene foams. The order of the ORR performance of iron-free modified catalysts in alkaline medium is: PGF-NB> PGF-B> PGF-NBP> PGF-P. In acidic medium, the catalyst showed the same order of activity as in alkaline medium. For the Fe-modified catalysts, the order of the ORR performance in alkaline medium was: PGF-Fe-NB> PGF-Fe-NBP> PGF-Fe-B> PGF-Fe-P. However, the order of the ORR performance of the catalysts was as follows: PGF-Fe-NBP> PGF-Fe-NB> PGF-Fe-B> PGF-Fe-P in acidic medium.
It is also found that iron modification could (i) promote the formation of a relatively large number of mesoporous and a larger specific surface area, which is beneficial for more active sites; (ii) assist with the formation of more defects in the carbon materials and increase the degree of graphitization of the catalyst; and (iii) promote doping amount of heteroatoms in graphene, which is beneficial to form more active sites and then to enhance the ORR performance.
These findings will be useful for the further rational design of high-performing catalysts for ORR. Furthermore, it is worth noting that the transition metal iron-modified multi-element doped porous graphene foams catalysts make it very promising as the ORR catalyst for fuel cells and metal-air batteries.
These findings are described in the article entitled Heteroatom (B, N and P) doped porous graphene foams for efficient oxygen reduction reaction electrocatalysis, recently published in the International Journal of Hydrogen Energy. The idea of this work was initiated by Dr. Yixiao Cai and Prof. Jinli Qiao, and conducted by masters students Fang Dong, Cong Liu and Junyu Liu from Donghua University.