"The green-sustainable renewable energy source has always been an alternative to fossil fuels. Higher energy density with fast refuelling technology and sustainable reaction triggers the interest towards the Hydrogen fuel cells/Polymer electrolyte membrane (PEM) based electromobility vehicles. In that system, Oxygen reduction reaction (ORR) plays a crucial role, concerning the slow kinetic reaction associated rate-limiting cathodic system, resulting in the limited performance of the energy generation systems. Hence our book addresses the various fundamental issues of the ORR reaction kinetics theories, measurement tools and techniques.It elaborates in detail the different nano electrocatalysts of the Nobel, Non-Nobel metals and nanocarbon materials on ORR. The book also covers different applications of PEM fuel cells and metal-air batteries"--