Among current technologies for hydrogen production as an environmentally friendly fuel, water splitting has attracted increasing attention. However, the efficiency of water electrolysis is severely limited by the large anodic overpotential and sluggish reaction rate of the oxygen evolution reaction (OER). To overcome this issue, the development of efficient electrocatalyst materials for the OER has drawn much attention. Here, we show that organometallic Ni­(II) complexes immobilized on the sidewalls of multiwalled carbon nanotubes (MWNTs) serve as highly active and stable OER electrocatalysts. This class of electrocatalyst materials is synthesized by covalent functionalization of the MWNTs with organometallic Ni bipyridine (bipy) complexes. The Ni-bipy-MWNT catalyst generates a current density of 10 mA cm–2 at overpotentials of 310 and 290 mV in 0.1 and 1 M NaOH, respectively, with a low Tafel slope of ∼35 mV dec–1, placing the material among the most active OER electrocatalysts reported so far. Different simple analysis techniques have been developed in this study to characterize such a class of electrocatalyst materials. Furthermore, density functional theory calculations have been performed to predict the stable coordination complexes of Ni before and after OER measurements.

Exploiting the concept of diffusion permeability of multilayer gel membrane and porous multilayer we have derived a simple analytical equation for the limiting diffusion current at rotating disk electrode (RDE) covered by a thin layer with variable tortuosity and porosity, under the assumption of negligible convection in the porous film. The variation of limiting diffusion current with the porosity and tortuosity of the film can be described in terms of the equivalent thickness of stagnant solution layer, i.e., the average ratio of squared tortuosity to porosity. In case of monolayer of monodisperse spherical particles, the equivalent layer thickness is an algebraic function of the surface coverage. Thus, by means of cyclic voltammetry of RDE with a deposited particle monolayer we can determine the monolayer surface coverage. The effect of particle layer adsorbed on the surface of RDE increases non-linearly with surface coverage. We have tested our theoretical results experimentally by means of cyclic voltammetry measurements of limiting diffusion current at the glassy carbon RDE covered with a monolayer of 3 μm silica particles. The theoretical and experimental results are in a good agreement at the surface coverage higher than 0.7. This result suggests that convection in a monolayer of 3 μm monodisperse spherical particles is negligibly small, in the context of the coverage determination, in the range of very dense particle layers.

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