Efficient electrochemical water splitting to hydrogen and oxygen is considered a promising technology to overcome our dependency on fossil fuels. Searching for novel catalytic materials for electrochemical oxygen generation is essential for improving the total efficiency of water splitting processes. We report the synthesis, structural characterization, and electrochemical performance in the oxygen evolution reaction of Fe-doped NiO nanocrystals. The facile solvothermal synthesis in tert-butanol leads to the formation of ultrasmall crystalline and highly dispersible FexNi1-xO nanoparticles with dopant concentrations of up to 20%. The increase in Fe content is accompanied by a decrease in particle size, resulting in nonagglomerated nanocrystals of 1.5-3.8 nm in size. The Fe content and composition of the nanoparticles are determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy measurements, while Mössbauer and extended X-ray absorption fine structure analyses reveal a substitutional incorporation of Fe(III) into the NiO rock salt structure. The excellent dispersibility of the nanoparticles in ethanol allows for the preparation of homogeneous ca. 8 nm thin films with a smooth surface on various substrates. The turnover frequencies (TOF) of these films could be precisely calculated using a quartz crystal microbalance. Fe0.1Ni0.9O was found to have the highest electrocatalytic water oxidation activity in basic media with a TOF of 1.9 s(-1) at the overpotential of 300 mV. The current density of 10 mA cm(-2) is reached at an overpotential of 297 mV with a Tafel slope of 37 mV dec(-1). The extremely high catalytic activity, facile preparation, and low cost of the single crystalline FexNi1-xO nanoparticles make them very promising catalysts for the oxygen evolution reaction.

Ultrasmall, crystalline, and dispersible NiO nanoparticles are prepared for the first time, and it is shown that they are promising candidates as catalysts for electrochemical water oxidation. Using a solvothermal reaction in tert‐butanol, very small nickel oxide nanocrystals can be made with sizes tunable from 2.5 to 5 nm and a narrow particle size distribution. The crystals are perfectly dispersible in ethanol even after drying, giving stable transparent colloidal dispersions. The structure of the nanocrystals corresponds to phase‐pure stoichiometric nickel(ii) oxide with a partially oxidized surface exhibiting Ni(iii) states. The 3.3 nm nanoparticles demonstrate a remarkably high turn‐over frequency of 0.29 s–1 at an overpotential of g = 300 mV for electrochemical water oxidation, outperforming even expensive rare earth iridium oxide catalysts. The unique features of these NiO nanocrystals provide great potential for the preparation of novel composite materials with applications in the field of (photo)electrochemical water splitting. The dispersed colloidal solutions may also find other applications, such as the preparation of uniform hole‐conducting layers for organic solar cells.

The rapidly growing demand for electric vehicles and mobile electronics urgently requires the development of electrochemical energy storage systems with both high energy density and high power. [1] Supercapacitors [2] can deliver very high powers, but their attainable energy densities are far lower than those of batteries. [3] Closing the gap between the two main technologies requires the development of materials that can incorporate or liberate a large amount of charge in a very short time. Herein we report the synthesis of fully crystalline interconnected porous frameworks composed of ultrasmall lithium titanate spinel nanocrystals of a few nanometers in size. These frameworks feature a gravimetric capacity of about 175 mA h g À1 at rates of 1-50 C (0.17-8.7 A g À1 ) and can deliver up to 73 % of their maximum capacity at unprecedented high rates of up to 800 C or 140 A g À1 (corresponding to only 4.5 s of charge/discharge) without deterioration up to a thousand cycles. This titanate morphology results in the fastest ever-reported lithium insertion.A key to this performance is the design of a fully crystalline interconnected porous framework composed of ultrasmall spinel nanocrystals of a few nanometers in size. The assembly of nanoscale building blocks into interconnected porous frameworks is a promising strategy to maximize the rate performance and to enhance the power density, and is also possible for materials not accessible by electrodeposition. Nanoscaling greatly increases the interface leading to enhanced charge transfer, and drastically shortens the ion/ electron diffusion pathways by decreasing the grain size of the bulk material. [1b, 4, 5] Lithium titanate Li 4 Ti 5 O 12 (LTO) is widely used as an active material in commercial lithium ion batteries and hybrid electrochemical storage devices [6] owing to its suitable potential, relatively high capacity, and robustness. Although the lithium insertion rate in bulk LTO is intrinsically low, which is due to its low conductivity, recent reports have demonstrated that it can be substantially increased by decreasing the crystal size to the nanometer scale, [6, 7] and several approaches were developed to obtain nanoscaled material. The reported strategies include flash annealing [6] or solid-state reactions of nanosized titania, [7a] which however lead to relatively large crystals of over 30 nm. Lithium titanate with much smaller crystalline domains and enhanced insertion rates can be obtained by solvothermal [7b] and sol-gel reactions. [7c-f,i, 8] However, the power density and especially the cycling stability still need to be improved. High rate capability in combination with excellent cycling stability was achieved for hybrid materials composed of a few nanometersized crystals anchored on a conducting carbon matrix, although at the expense of significantly decreased gravimetric and volumetric capacities owing to a high content of the conducting support. [9,10] A non-supported nanosized and fully crystalline LTO with extremely high insertion/extractio...

The influence of nanoscale on the formation of metastable phases is an important aspect of nanostructuring that can lead to the discovery of unusual material compositions. Here, the synthesis, structural characterization, and electrochemical performance of Ni/Co mixed oxide nanocrystals in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is reported and the influence of nanoscaling on their composition and solubility range is investigated. Using a solvothermal synthesis in tert-butanol ultrasmall crystalline and highly dispersible Ni x Co 1−x O nanoparticles with rock salt type structure are obtained. The mixed oxides feature non-equilibrium phases with unusual miscibility in the whole composition range, which is attributed to a stabilizing effect of the nanoscale combined with kinetic control of particle formation. Substitutional incorporation of Co and Ni atoms into the rock salt lattice has a remarkable effect on the formal potentials of NiO oxidation that shift continuously to lower values with increasing Co content. This can be related to a monotonic reduction of the work function of (001) and (111)-oriented surfaces with an increase in Co content, as obtained from density functional theory (DFT+U) calculations. Furthermore, the electrocatalytic performance of the Ni x Co 1−x O nanoparticles in water splitting changes significantly. OER activity continuously increases with increasing Ni contents, while HER activity shows an opposite trend, increasing for higher Co contents. The high electrocatalytic activity and tunable performance of the nonequilibrium Ni x Co 1−x O nanoparticles in HER and OER demonstrate great potential in the design of electrocatalysts for overall water splitting.