A Proton Exchange Membrane (PEM) water electrolysis cell is a zero-gap cell that uses a thin (100-200 micrometers thick) film of proton-conducting polymer (usually a perfluoro-sulfonic material) as solid electrolyte . The strongly acidic pH of these membranes requires the use of platinum group metal (PGM) electrocatalysts. Platinum black (unsupported Pt nanoparticles) or carbon-supported Pt nanoparticles are commonly used at the cathode to catalyze the two-step hydrogen evolution reaction (HER). Unsupported iridium dioxide particles are commonly used at the anode to catalyze the multi-step oxygen evolution reaction (OER). In current state-of-art, PGM loadings can be significantly low, less than 0.5 mg.cm-2
[2,3], but low loadings usually tend to reduce electrochemical performances on the long-term. More recently, molecular electrocatalysis has been used to implement alternative cheap materials based on transition metals such as cobalt, nickel or even iron, in very small quantities. Complexes of these elements have demonstrated interesting properties and characteristics that make same suitable candidates for the replacement of expensive PGMs . Besides the selection of electrocatalysts of appropriate activity, charge transfer overvoltages can also be reduced by increasing as much as possible the roughness factor (i.e. the dimensionless ratio of the expended surface area of the catalyst-electrolyte interface to the geometrical surface area of the cell) of the two electrode-electrolyte interfaces. Hence, the structure of catalytic layers also plays a critical role in the water splitting efficiency. Specific tools such as cyclic voltammetry and electrochemical impedance spectroscopy can be used to measure in situ
the electrochemical activity of these catalytic layers, in view of further optimization or analysis of ageing processes . The purpose of this communication is to provide a brief overview of existing electrocatalysts that can be used in PEM water electrolysis cells, both conventional and non-conventional, and to discuss some specific issues related to the in situ
and in operando
characterization of such materials, in order to relate electrochemical activity to microstructural aspects.
 W.T. Grubb, L.W. Niedrach, Batteries with Solid Ion-Exchange Membrane Electrolytes. II. Low Temperature H2-O2 Fuel Cells, J. Electrochem. Soc., 107(2) (1960) 131 – 134.
 C. Rozain, E. Mayousse, N. Guillet, P. Millet, Influence of iridium oxide loadings on the performance of PEM water electrolysis cells: Part I – Pure IrO2-based anodes, J. Appl. Catalysis B: Environmental, 182 (2016) 153 – 160.
 C. Rozain, N. Guillet, E. Mayousse, P. Millet, Influence of iridium oxide loadings on the performance of PEM water electrolysis cells: Part II – Advanced anodic electrodes, J. Appl. Catalysis B: Environmental, 182 (2016) 123 – 131.
 M-T. Dinh Nguyen, A. Ranjbari, L. Catala, F. Brisset, P. Millet and A. Aukauloo, Implementing Molecular Catalysts for Hydrogen Production in Proton Exchange Membrane Water Electrolysers, Coord. Chem. Review, 256 (2012) 2435 – 2444.
 P. Millet, ‘PEM water Electrolysis for hydrogen production: Principles and Applications’, chapter 10, PEM electrolyzer characterization tools, D. Bessarabov, H. Wand, H. Li, N. Zhao, CRC Press (2015).