Electrooxidation of NaBH4 in Alkaline Medium on Well-defined Pt Nanoparticles Deposited onto Flat Glassy Carbon Substrate: Evaluation of the Effects of Pt Nanoparticle Size, Inter-Particle Distance, and Loading

Well-defined Pt nanoparticles deposited at smooth glassy-carbon (GC) surfaces were elaborated and thoroughly characterized. Using such model Pt/GC surfaces enabled demonstrating that the borohydride oxidation reaction (BOR) is subjected to nanoparticle size and ensemble effects: larger particle diameter and shorter inter-particle distance yield faster BOR kinetics and larger faradaic efficiency. As previously noted for smooth Pt (and Au) surfaces, the Pt/GC nanoparticles are self-poisoned in the course of the BOR; surprisingly, such poisoning also proceeds in open-circuit conditions. The adsorbed intermediates formed in the course of the step-wise electrooxidation and heterogeneous hydrolysis processes are most likely yielding the Pt surface blocking below E = 0.6 V vs. Reversible Hydrogen Electrode (RHE). This blocking is, however, reversible, since incursions to potentials E > 0.6 V vs. RHE enable cleaning the Pt surface. Finally, comparing smooth Pt/GC surfaces to volumic active layers composed of Pt/carbon black (CB) demonstrates that the intrinsic activity/faradaic efficiency of the Pt nanoparticles may strongly be biased by mass-transport effects within the active layer. Larger (apparent) faradaic efficiency and lower BOR onset potential are observed for thick active layers, whereas the specific activity in these is artificially lowered following effectiveness factor well below unity in that case. As a result, the determination of the intrinsic activity of an electrocatalytic material should only be done with tremendous care and with a perfect control of the electrode/surface morphology, texture and structure.