Microporous Solids En Route to Heterogeneous Electrocatalysis: The Oxygen Reduction Reaction

The recent progress and challenges in developing highly active microporous materials as heterogeneous electrocatalysts for oxygen reduction reaction (ORR) are discussed. Due to the slow kinetics of ORR, the search for novel highly stable catalysts that can outperform Pt-based ones is a growing demand. Functional microporous materials and exceptional chemical stability are of great interest due to their unique catalytic properties. The crucial rule of porosity and catalyst design strategies are thoroughly discussed with emphasis on the influence of the degree of microporosity on the material's intrinsic catalytic activity and the current best practice for catalyst testing and activity reporting. This Review focuses on microporous materials, which are classified into crystalline as metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), or amorphous porous-organic polymers (POPs) as to their recent utilization in constructing efficient ORR electrocatalysts. The fundamental challenges in obtaining stable catalysts with appreciable kinetics from non-noble metal ions, and ways to form them utilizing microporous solids as catalyst matrices, are reviewed. The fundamental criteria for future generation ORR catalysts based on microporous solids, including stability in acidic medium, high electrical conductivity, and accessibility of the active sites within the matrix are highlighted for guidance to future works.