We report here the plasma-enhanced chemical vapor deposition and electrocatalytic characterization of pure NiO x and NiO x (OH) y . Whereas NiO x is deposited if oxygen is used as a reactive gas, the use of air as a reactive gas leads to the deposition of the NiO x (OH) y , which is electrochemically more active than the NiO x . By recording X-ray photoelectron spectra from the as-deposited catalysts and after their electrochemical investigations, we determined that the electrochemical activity correlates with the amount of hydroxide sites on the surface. Such a behavior was already observed for CoO x and CoO x (OH) y . As a consequence, CoNiO x (OH) y was deposited using air as a reactive gas to study the influence of nickel on the electronic structure of CoO x (OH) y and its effect on the electrochemical activity.

In the last decades several different catalysts for the electrochemical water splitting reaction have been designed and tested. In so-called benchmark papers they are compared with respect to their efficiency and activity. In order to relate the different catalyst to each other the definition of well-defined procedures is required. Two different methods are mainly used: Either the normalization with respect to the geometric surface area or to the catalyst loading. Most often only one of these values is available for a sample and the other one cannot be estimated easily. One approach in electrocatalysis is to determine the Helmholtz double layer capacitance (DLC) and deduce the electrochemical active surface area (ECSA). The DLC can be obtained from two different methods, either using differential capacitance measurement (DCM) or impedance spectroscopy (EIS). The second value needed for the calculation of the ECSA is the specific capacitance, which is the capacitance for a perfectly flat surface of given catalyst material. Here, we present the determination of the different capacitance values using manganese oxide as the exemplary model for the oxygen evolution reaction (OER). We determine the capacitance by DCM and EIS to calculate the ECSA using literature values for the specific capacitance. The obtained values are comparable from the two methods, but are much larger than the surface areas obtained by atomic force microscopy. Therefore, we consider the possibility of using the measured AFM area together with the Helmholtz capacitance to determine the specific capacitances for this material class. The comparison of these results with literature values illustrates the actual limits of the ECSA method, which will be discussed in this paper.

The scalable synthesis of phase-pure crystalline manganese nitride (Mn N ) from a molecular precursor is reported. It acts as a superiorly active and durable electrocatalyst in the oxygen evolution reaction (OER) from water under alkaline conditions. While electrophoretically deposited Mn N on fluorine tin oxide (FTO) requires an overpotential of 390 mV, the latter is substantially decreased to merely 270 mV on nickel foam (NF) at a current density of 10 mA cm with a durability of weeks. The high performance of this material is due to the rapid transformation of manganese sites at the surface of Mn N into an amorphous active MnO overlayer under operation conditions intimately connected with metallic Mn N , which increases the charge transfer from the active catalyst surface to the electrode substrates and thus outperforms the electrocatalytic activity in comparison to solely MnO -based OER catalysts.

To reduce energy losses in water electrolysers a fundamental understanding of the water oxidation reaction steps is necessary to design efficient oxygen evolution catalysts. Here we present CoOx/Ti electrocatalytic films deposited by thermal and plasma enhanced chemical vapor deposition (CVD) onto titanium substrates. We report electrochemical (EC), photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements. The electrochemical behavior of the samples was correlated with the chemical and electronic structure by recording XPS spectra before and after each electrochemical treatment (conditioning and cyclovoltammetry). The results show that the electrochemical behavior of CoOx/Ti strongly depends on the resulting electronic structure and composition. The thermal deposition leads to the formation of a pure Co(II)Ox which transforms to a mixed Co(II)Co(III)Ox during the OER. This change in oxidation state is coupled with a decrease in overpotential from η = 0.57 V to η = 0.43 V at 5 mA cm(-2). Plasma deposition in oxygen leads to a Co(III)-dominated mixed CoOx, that has a lower onset potential as deposited due to a higher Co(III) content in the initial deposited material. After the OER XPS results of the CoOx/Ti indicate a partial formation of hydroxides and oxyhydroxides on the oxide surface. Finally the plasma deposition in air, results in a CoOxOH2 surface, that is able to completely oxidizes during OER to an oxyhydroxide Co(III)OOH. With the in situ formed CoOOH we present a highly active catalyst for the OER (η = 0.34 at 5 mA cm(-2); η = 0.37 V at 10 mA cm(-2)).

High-performance catalysts for the oxygen-evolution reaction in water electrolysis are usually based on expensive and rare elements. Herein, mixed-metal borides are shown to be competitive with established electrocatalysts like noble metal oxides and other transition-metal(oxide)-based catalysts. Iron incorporation into nanoscale dicobalt boride results in excellent activity and stability in alkaline solutions. (Co Fe ) B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO and RuO . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH) are formed as active surface species, whereas the Fe remains in the catalyst, contributing to an improved catalyst performance. The nanoscale borides are obtained by a one-step solution synthesis, calcined, and characterized by XRD, energy-dispersive X-ray spectroscopy, and SEM. Single crystals of (Co Fe ) B grown under chemical transport conditions were used for an unambiguous specification of the nanostructured particles by relating the cobalt/iron ratio to the lattice parameters.

The scalable synthesis of phase‐pure crystalline manganese nitride (Mn3N2) from a molecular precursor is reported. It acts as a superiorly active and durable electrocatalyst in the oxygen evolution reaction (OER) from water under alkaline conditions. While electrophoretically deposited Mn3N2 on fluorine tin oxide (FTO) requires an overpotential of 390 mV, the latter is substantially decreased to merely 270 mV on nickel foam (NF) at a current density of 10 mA cm−2 with a durability of weeks. The high performance of this material is due to the rapid transformation of manganese sites at the surface of Mn3N2 into an amorphous active MnOx overlayer under operation conditions intimately connected with metallic Mn3N2, which increases the charge transfer from the active catalyst surface to the electrode substrates and thus outperforms the electrocatalytic activity in comparison to solely MnOx‐based OER catalysts.

The use of low-temperature solution synthesis followed by a brief annealing step allows metastable single-phase Co3B nanoparticles to be obtained, with sizes ranging from 11 nm to 22 nm. The particles are ferromagnetic with a saturation magnetization of 91 A m 2 kg-1 (corresponding to 1.02 μB/Co) and a coercive field of 0.14 T at 5 K, retaining the semi-hard magnetic properties of bulk Co3B. They display a magnetic blocking temperature of 695 K and a Curie temperature near 710 K, but the measurement of these high-temperature properties was complicated by decomposition of the particles at these high temperatures. Additionally, the nanoparticles of Co3B were investigated as an electrocatalyst in the oxygen evolution reaction and showed a low onset potential of 1.55 V vs. RHE. XPS measurements were performed before and after the electrocatalytic measurements to study the surface of the catalyst, to pinpoint what appear to be the active surface species.

Ni nanoparticles (NPs) consisting of Ni, NiO, and Ni(OH) were formed on Ti substrates by electrodeposition as electrocatalysts for the hydrogen evolution reaction (HER) in alkaline solution. Additionally, the deposition parameters including the potential range and the scan rate were varied, and the resulting NPs were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy. The chemical composition of the NPs changed upon using different conditions, and it was found that the catalytic activity increased with an increase in the amount of NiO. From these data, optimized NPs were synthesized; the best sample showed an onset potential of approximately 0 V and an overpotential of 197 mV at a cathodic current density of 10 mA cm as well as a small Tafel slope of 88 mV dec in 1 m KOH, values that are comparable to those of Pt foil. These NPs consist of approximately 25 % Ni and Ni(OH) each, as well as approximately 50 % NiO. This implies that to obtain a successful HER electrocatalyst, active sites with differing compositions have to be close to each other to promote the different reaction steps. Long-time measurements (30 h) showed almost complete transformation of the highly active catalyst compound consisting of Ni , NiO, and Ni(OH) into the less active Ni(OH) phase. Nevertheless, the here-employed electrodeposition of nonprecious metal/metal-oxide combination compounds represents a promising alternative to Pt-based electrocatalysts for water reduction to hydrogen.