Advanced electrocatalysts for hydrogen and oxygen evolution in proton exchange membrane electrolyzers

In this chapter, water electrolysis in polymer electrolytes water electrolyzers (PEWEs) is discussed in terms of its kinetics aspects, proposed reaction mechanisms towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic and alkaline environments, highlighting the activity trends for platinum group metal (PGM)-based and the most promising families of PGM-free electrocatalysts. A comprehensive literature review of several synthesis strategies adopted to boost HER and OER activity of PGM-free electrocatalysts, including intrinsic and extrinsic modifications, is discussed. Despite the improvements made, the activity and durability of PGM-free materials at the anode and cathode of PEWEs, particularly in PEM water electrolyzers, still need to be improved for their commercialization. To meet the activity-durability trade-off, there is an urgent call for developing and optimizing advanced synthesis strategies with deep physicochemical and electrochemical characterization, elucidating the nature of the active sites and their behavior under simulated and actual operating conditions.