Evaluating the catalytic activity of transition metal dimers for the oxygen reduction reaction.

Various experimental investigation had proved that metal dimers possess excellent oxygen reduction reaction (ORR) activity compared to single metal atom catalysts, due to the synergistic effect exerted by two metal atoms. However, it is still unclear how the electrocatalytic activity is enhanced in a fundamental aspect. In this study, we systematically investigated five 3d transition metals (Fe, Co, Ni, Cu and Zn) by density functional theory (DFT) to explore the ability of metal dimers to catalyze the ORR. It is found that different combinations of different metal atoms have different adsorption strengths to oxygenated intermediates, which helps to screen suitable catalyst materials. The scaling relationship of the free energy of adsorption of oxygen-containing species was calculated for various metal-dimer systems. The classical volcanic diagram is derived, and it is found that the CoZnOH embedded nitrogen-doped graphene (the overpotential is 0.61 V) shows the best catalytic properties, and it is predicted that when the adsorption free energy of OH is equal to 0.95 eV, the optimal overpotential is 0.29 V. Electronic structure calculations show that the pairing of different metal atoms alters the d-band center which in turn change the adsorption properties and hence ORR catalytic performance.

Cumulative effect of transition metals on nitrogen and fluorine co-doped graphite nanofibers: an efficient and highly durable non-precious metal catalyst for the oxygen reduction reaction.

Nitrogen and fluorine co-doped graphite nanofibers (N/F-GNF) and their cumulative effect with Fe and Co have been developed as an alternative non-precious metal catalyst for efficient oxygen reduction reaction (ORR) in acidic media. The synergistic effect between the doped hetero atoms and the co-ordinated Fe and Co towards ORR activity and durability of the catalyst is deeply investigated. A high ORR onset potential comparable with commercial Pt/C catalyst is observed with the Fe-Co/NF-GNF catalyst, which indicates that this catalyst is a potential alternative to Pt/C. A fivefold increase in mass activity is achieved by the Fe-Co/NF-GNF catalyst compared to the simple N/F-GNF catalyst, which endorses the significant role of transition metal atoms in enhancing ORR activity. The advanced Fe-Co/NF-GNF catalyst also exhibits complete tolerance to CH3OH and CO. The Fe-Co/NF-GNF catalyst also exhibits excellent durability towards the ORR with only a 10 mV negative shift in its half wave potential after a 10 000 repeated potential cycling test, whereas in the case of a commercial Pt/C catalyst there was an ∼110 mV negative shift under similar environmental conditions. More stringent corrosive test cycles were also performed by maintaining the cell as high as 1.4 V with a later decrease to 0.6 V vs. RHE for 300 cycles, which showed the excellent durability of the Fe-Co/NF-GNF catalyst in comparison with the Pt/C catalyst. XPS analysis of the Fe-Co/NF-GNF catalyst presents the ORR active chemical states of N (pyridinic-N and graphitic-N) and F (semi-ionic-F) and the co-ordinated sites of Fe and Co species with the dopants. The excellent performance and durability of the Fe-Co/NF-GNF catalyst is due to the synergistic effect between the hetero atoms dopants (N and F) and strong co-ordinating bonds of M-N-C, which protect the graphene layers around the metallic species and greatly mitigates the leaching of Co and Fe during the long term cycling test. The high activity and long term durability of the Fe-Co/NF-GNF catalyst make it a promising ORR electrocatalyst for the fuel cell cathode reaction.