Electrochemical energy conversion devices, such as polymer electrolyte membrane fuel cells (PEMFCs) and PEM electrolyzers are considered as possible candidates for integration in future renewable energy network 1,2 . However, the oxygen reduction reaction (ORR) kinetic at the cathode of a PEMFC is slow and the overpotential at the cathode is generally high (higher than 0.200 V), whereas that of the hydrogen oxidation at the anode is ca. 0.050 V 3 . For this purpose, platinum (Pt) or its alloys are often required as electrocatalyst because of their excellent catalytic activity for the ORR 4 . Electrocatalysis involves surface reactions, therefore, a high surface/inner atoms ratio is desirable to improve the Pt nanoparticles (Pt NPs) performance. NPs with size ranging between 2 to 4 nm are generally preferred 5 .NPs can be obtained through various physical, chemical or physicochemical routes 6,7 . The chemical methods are very versatile in terms of controlling NPs shape and size by varying the reaction conditions. However, NPs chemical synthesis requires additives, which generate by-products difficult to remove and NPs with limited purity. In contrast, physical methods (such as sputtering, thermal evaporation, laser ablation, spark discharge…) on solid substrates avoid the use of additives, allowing the production of pure metallic NPs with the same material composition as that of the starting material. Moreover, these methods can be considered as green approaches using non-toxic reducing agents and avoiding the formation of byproducts 8 . Among physical techniques for preparing electrocatalytic NPs, magnetron sputtering is known to produce efficient Pt NPs deposited on a microporous carbon layer 9 or on a polymer electrolyte membrane 10 to directly fabricate the catalytic layer composite. However, using the sputtering method for the deposition of metal NPs onto such substrates often makes the control of the NPs properties (size, dispersion and morphology) complex. Moreover, this process makes also difficult to create three phase boundaries by the addition of ionomer in comparison with the conventional liquid ink preparation techniques based on classical chemical processes for carbon supported NPs. A solution consists in synthesizing NPs directly in a liquid phase using magnetron sputtering technique and to disseminate the NPs onto a porous substrate. This innovative method

Plasma increases activity: A one-step synthesis of Pt-C nanowire composites using a plasma co-deposition method is reported. Electrodes with a very low Pt loading can be obtained. Pt particles with sizes ranging from 1 to 2 nm are decorating the columnar carbon nanostructures because of strong interactions. The composite microstructure is responsible for a very high metal utilization rate as exemplified by reactions occurring in fuel cell electrodes.