Dendritic Nickel Structures in Air Electrodes

Summary:

Climate change and the limited availability of fossil fuels do not just increase the demand for renewable energy sources, but also for new battery technologies.

Metal-air batteries are a promising system for stationary applications. Compared to other battery technologies, they provide a very high theoretical energy density that is just limited by the size of the anode since oxygen is provided as a reactant from the ambient atmosphere.

Primary metal-air batteries with zinc anodes are commercially available for decades. In this case, both electrodes must support reactions that are considered with discharging of the battery. In secondary systems, however, reactions at the cell electrodes need to be reversible. This leads to several problems concerning the air-electrode. Oxygen is reduced during the discharge of the battery and evolved during charging. State-of-the-art air electrodes usually consists of three constituents: A catalyst (or several catalysts), a conductive support material, and a hydrophobic binder. In primary systems, carbon-particles are used as a conductive support. Carbon materials offer a high conductivity and a high specific surface area. Surface areas are crucial in electrochemical systems. However, carbon is not stable at potentials where the oxygen evolution reaction takes place at an industrial level. A corrosion-stable alternative to carbon are metals, such as silver or nickel. Those metal-based air electrodes mainly offer very low specific surface areas due to large metallic particles.

Our work is based on porous nickel supports, made by galvanic deposition. Past works show that these structures offer high specific BET surface areas compared to conventional metal-based air-electrodes. We herein specifically focus on a surface-increase and on the implementation as catalyst support in bifunctional air electrodes.

The structures were deposited on a nickel-mesh from a nickel chloride and ammonium chloride-based electrolyte. SEM analysis indicate that a highly porous structure was obtained. Those structures show small pores between dendrites and larger pores caused by hydrogen development during deposition, which is also known as dynamic hydrogen templating. Analysis via X-ray diffraction and energy-dispersive X-ray spectroscopy indicate that the deposits consists of a pure nickel-phase.

Electrochemical analysis via cyclic voltammetry indicates that the double layer capacitance drastically increases compared to a plain mesh reference samples. Herein, the double layer capacitance is an indicator for the electrochemical active surface area.