Thisnew edition describes the state of the art regarding molecular catalysts suchas MN4 metal complexes, like porphyrins and phthalocyanines. This volumefocuses on the particular case of the electrocatalysis of the reduction of O₂for practical applications in fuel cells and air batteries. Indeed, active andstable materials have been developed in the last 5 years where MN4 catalyticsystems can be obtained by the pyrolysis of starting materials that do notnecessarily involve MN4 complexes. These latter systems constitute a new classof stable and highly active non-precious metal catalysts for ORR that canreplace expensive Platinum containing electrodes. The book also offersfuture projections and points out new fields of research and developmentof these non-precious metal catalysts.