A Non-Precious-Metal Catalyst Derived from a Cp2-Co+-PBI Composite for Cathodic Oxygen Reduction under Both Acidic and Alkaline Conditions

One major limitation for polymer electrolyte membrane fuel cells is the sluggish cathode kinetics. Development of efficient noble-free catalysts is the key resolution to the problem of the oxygen reduction reaction (ORR) in both acid and alkaline solutions. Herein, we report a new type of efficient non-precious-metal catalyst for the ORR through the direct pyrolysis of poly[2,2'-(1,1'-cobaltocenium)-5,5'-dibenzimidazole]. The cobalt oxides were produced after pyrolysis at 900 degrees C (Cp-2-Co+ -PBI-900, where PBI is polybenzimidazole). The obtained catalysts exhibit higher electrocatalytic activity and stability for the ORR under both alkaline and acidic conditions. Structural characterization manifested that Cp-2-Co+-PBI-800 had the highest graphitic N content and Cp-2-Co+-PBI-900 was also produced. In alkaline media, Cp-2-Co+-PBI-900 showed the highest ORR activity with onset potential of 998 mV (vs. RHE), which was only 22 mV higher than that of Pt/C under identical conditions. Besides, in acidic media, Cp-2-Co+-PBI-800 exhibited excellent ORR activity with an onset potential of 847 mV (vs. RHE) after leaching in 6 M HCl solution for 12 h. Both optimal catalysts displayed high durability, especially in acidic media. The halfwave potential was also improved by 11 mV after 5000 CV scanning cycles in N-2. The catalysts possessed diverse active sites in different working conditions. In acid conditions, cobalt acted as the promotor, whereas, in alkaline conditions, CoO was the activity site. Moreover, graphite N and pyridine N were the main activity sites in acid and alkaline conditions, respectively. PBI has a long-chain and pi-conjugated system, indicating that the PBI precursor can be used as a non-precious-metal catalyst.