Recent developments in electrocatalysts and future prospects for oxygen reduction reaction in polymer electrolyte membrane fuel cells

The main difficulty in the extensive commercial use of polymer electrolyte membrane fuel cells（PEMFCs）is the use of noble metals such as Pt-based electrocatalyst at the cathode, which is essential to ease the oxygen reduction reaction（ORR） in fuel cells（FCs）. To eliminate the high loading of Pt-based electrocatalysts to minimize the cost, extensive study has been carried out over the previous decades on the non-noble metal catalysts. Development in enhancing the ORR performance of FCs is mainly due to the doped carbon materials, Fe and Co-based electrocatalysts, these materials could be considered as probable substitutes for Pt-based catalysts. But the stability of these non-noble metal electrocatalysts is low and the durability of these metals remains unclear. The three basic reasons of instability are:（i） oxidative occurrence by H2O2,（ii） leakage of the metal site and（iii） protonation by probable anion adsorption of the active site. Whereas leakage of the metal site has been almost solved, more work is required to understand and avoid losses from oxidative attack and protonation. The ORR performance such as stability tests are usually run at low current densities and the lifetime is much shorter than desired need. Therefore,improvement in the ORR activity and stability are the key issues of the non-noble metal electrocatalyst.Based on the consequences obtained in this area, numerous future research directions are projected and discussed in this paper. Hence, this review is focused on improvement of stability and durability of the non-noble metal electrocatalyst.