Electrochemical investigation of surface area effects on PVD Al-Ni as electrocatalyst for alkaline water electrolysis
By Kjartansdóttir, Cecilía; Caspersen, Michael; Egelund, Sune & Mà¸ller, Per
Published in Electrochimica Acta
2014
Abstract
A thermo-chemical diffusion of vapour deposited aluminium onto a nickel substrate, leads to a rapid formation of an Al/Ni intermetallic layer that is particularly acceptable for dissolution of aluminium in strong alkali. The geometry and the structure of the final skeletal nickel coatings can be manipulated by altering the time interval of the diffusion. In that way the actual electrochemical surface area and, thus, the electrocatalytic activity of the coatings towards HER and OER can be influenced. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) investigations, display that the electrocatalytic surface area increases in proportion to the thickness of the developed porous nickel structure. In the case of the HER, an increase in roughness factor (Rf) from 1 (for polished Ni) to 800, results in reduction of the hydrogen overpotential of 337 mV. When further increasing Rf, up to above 2000, additional 40 mV are gained. For the OER, smaller roughness values were observed with the same activity trend as for the HER. The electrocatalyst are however found not to be stable in the anodic environment during electrolysis. The corrosion mechanism of a skeletal nickel electrocatalyst during the OER in an alkaline environment is briefly discussed. The structure and composition of the electrocatalysts are characterised by scanning electron microscopy and X-ray diffraction. The actual electrocatalytic surface area and the electrocatalytic behaviour are studied with potentiodynamic polarisation, CV and EIS.