Non-Precious Electrocatalysts for Oxygen Reduction Reaction in Alkaline Media: Latest Achievements on Novel Carbon Materials

Low temperature fuel cells (LTFCs) are considered as clean energy conversion systems and expected to help address our society energy and environmental problems. Up-to-date, oxygen reduction reaction (ORR) is one of the main hindering factors for the commercialization of LTFCs, because of its slow kinetics and high overpotential, causing major voltage loss and short-term stability. To provide enhanced activity and minimize loss, precious metal catalysts (containing expensive and scarcely available platinum) are used in abundance as cathode materials. Moreover, research is devoted to reduce the cost associated with Pt based cathode catalysts, by identifying and developing Pt-free alternatives. However, so far none of them has provided acceptable performance and durability with respect to Pt electrocatalysts. By adopting new preparation strategies and by enhancing and exploiting synergetic and multifunctional effects, some elements such as transition metals supported on highly porous carbons have exhibited reasonable electrocatalytic activity. This review mainly focuses on the very recent progress of novel carbon based materials for ORR, including: (i) development of three-dimensional structures; (ii) synthesis of novel hybrid (metal oxide-nitrogen-carbon) electrocatalysts; (iii) use of alternative raw precursors characterized from three-dimensional structure; and (iv) the co-doping methods adoption for novel metal-nitrogen-doped-carbon electrocatalysts. Among the examined materials, reduced graphene oxide-based hybrid electrocatalysts exhibit both excellent activity and long term stability.