High Voltage Electrodes for Li-Ion Batteries and Efficient Water Electrolysis: An Oxymoron?

We demonstrate that key parameters for efficient electrocatalytic oxidation of water are the energetics of the redox complexes associated with their ionization and electrochemical potentials coupled to the change of metal-oxygen band hybridization. We investigate the catalytic activity of the LiCoPO4-LiCo2P3O10 tailored compound, which is a 5 V cathode material for Li-ion batteries. The reason for the weak catalytic activity of the lithiated compound toward the oxygen evolution reaction is a large energy difference between the electronic states involved in the electrochemical reaction. A highly active catalyst is obtained by tuning the relative energetic position of the electronic levels involved in the charge transfer reaction, which in turn are governed by the lithium content. A significant lowering of the overpotential from >550 mV to ∼370 mV at 10 mA cm-2 is achieved via a decrease of the ionization potential and shifting the electrochemical potential near the electronic states of the molecule, thereby facilitating water oxidation.

Magnetic and Electrocatalytic Properties of Nanoscale Cobalt Boride, Co3B.

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 to 22 nm. The particles are ferromagnetic with a saturation magnetization of 91 A m2 kg-1 (corresponding to 1.02 µB/Co) and a coercive field of 0.14 T at 5 K, retaining the semihard 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 during heating in the magnetometer. 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.

Synthesis of a Highly Efficient Oxygen-Evolution Electrocatalyst by Incorporation of Iron into Nanoscale Cobalt Borides.

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. (Co0.7 Fe0.3 )2 B shows an overpotential of η=0.33 V (1.56 V vs. RHE) at 10 mA cm-2 in 1 m KOH with a very low onset potential of ≈1.5 V vs. RHE, comparable to the performance of IrO2 and RuO2 . XPS shows that the original catalyst is modified under the reaction conditions and indicates that CoOOH and Co(OH)2 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 (Co1-x Fex )2 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.

A Molecular Approach to Manganese Nitride Acting as a High Performance Electrocatalyst in the Oxygen Evolution Reaction.

The scalable synthesis of phase-pure crystalline manganese nitride (Mn3 N2 ) 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 Mn3 N2 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 Mn3 N2 into an amorphous active MnOx overlayer under operation conditions intimately connected with metallic Mn3 N2 , 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.

Electrodeposition of Nickel Nanoparticles for the Alkaline Hydrogen Evolution Reaction: Correlating Electrocatalytic Behavior and Chemical Composition.

Ni nanoparticles (NPs) consisting of Ni, NiO, and Ni(OH)2 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-2 as well as a small Tafel slope of 88 mV dec-1 in 1 m KOH, values that are comparable to those of Pt foil. These NPs consist of approximately 25 % Ni and Ni(OH)2 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 Ni0 , NiO, and Ni(OH)2 into the less active Ni(OH)2 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.

CoOx thin film deposited by CVD as efficient water oxidation catalyst: change of oxidation state in XPS and its correlation to electrochemical activity.

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)).