RESUMO
Highly effective yet affordable non-noble metal catalysts are a key component for advances in hydrogen generation via electrolysis. The synthesis of catalytic heterostructures containing established Ni in combination with surface NiO, Ni(OH)2, and NiOOH domains gives rise to a synergistic effect between the surface components and is highly beneficial for water splitting and the hydrogen evolution reaction (HER). Herein, the intrinsic catalytic activity of pure Ni and the effect of partial electrochemical oxidation of ultra-smooth magnetron sputter-deposited Ni surfaces are analyzed by combining electrochemical measurements with transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The experimental investigations are supplemented by Density Functional Theory and Kinetic Monte Carlo simulations. Kinetic parameters for the HER are evaluated while surface roughening is carefully monitored during different Ni film treatment and operation stages. Surface oxidation results in the dominant formation of Ni(OH)2, practically negligible surface roughening, and 3-5 times increased HER exchange current densities. Higher levels of surface roughening are observed during prolonged cycling to deep negative potentials, while surface oxidation slows down the HER activity losses compared to as-deposited films. Thus, surface oxidation increases the intrinsic HER activity of nickel and is also a viable strategy to improve catalyst durability.
RESUMO
The conductivity and the state of the surface of supports are of vital importance for metallization via electrodeposition. In this study, we show that the metallization of a carbon fiber-reinforced polymer (CFRP) can be carried out directly if the intermediate graphene oxide (GO) layer is chemically reduced on the CFRP surface. Notably, this approach utilizing only the chemically reduced GO as a conductive support allows us to obtain insights into the interaction of rGO and the electrodeposited metal. Our study reveals that under the same contact current experimental conditions, the electrodeposition of Cu and Ni on rGO follows significantly different deposition modes, resulting in the formation of three-dimensional (3D) and free-standing metallic foils, respectively. Considering that Ni adsorption energy is larger than Ni cohesive energy, it is expected that the adhesion of Ni on rGO@CFRP is enhanced compared to Cu. In contrast, the adhesion of deposited Ni is reduced, suggesting diffusion of H+ between rGO and CFRP, which promotes the hydrogen evolution reaction (HER) and results in the formation of free-standing Ni foils. We ascribe this phenomenon to the unique properties of rGO and the nature of Cu and Ni deposition from electrolytic baths. In the latter, the high adsorption energy of Ni on defective rGO along with HER is the key factor for the formation of the porous layer and free-standing foils.
RESUMO
We demonstrate electrodeposition as a synthesis method for fabrication of Al coatings, up to 10 µm thick, containing a high density of genuine growth twins. This has not been expected since the twin boundary energy of pure Al is very high. TEM methods were used to analyze deposited Al and its nanoscaled twins. DFT methods confirmed that the influence of the substrate is limited to the layers close to the interface. Our findings are different from those achieved by sputtering of Al coatings restricted to a thickness less than 100 nm with twins dominated by epitaxial effects. We propose that in the case of electrodeposition, a high density of twins arises because of fast nucleation and is additionally promoted by a monolayer of adsorbed hydrogen originating from water impurities. Therefore, electrodeposition is a viable approach for tailoring the structure and properties of thicker, deposited Al coatings reinforced by twins.
