In situ STM investigation of metal hydride electrodes in alkaline electrolyte during electrochemical cycles

Abstract

For more than a decade metal hydride electrodes have been investigated by ex situ methods. In situ STM (Scanning Tunnelling Microscopy) allows investigation of single metal hydride grains during the charge and discharge process in the electrolyte. The advantage of STM measurements is its manifold analysing feasibility, e.g. surface morphology, surface conductivity and atomic forces with spatial resolution, and even sometimes atomic resolution. Samples of alloy grains pressed into a gold pellet and the polished surfaces of bulk alloys were investigated in concentrated alkaline electrolyte (6 M KOH) by means of STM. The volume expansion between the metal and the hydride was measured on a single grain in the electrolyte. This value of ΔV/V=14.9% is very close to the value observed from X-ray diffraction measurements (15.4%). Multiphase samples show a very inhomogeneous surface conductivity. The identification of the phases with different surface conductivities is a key in understanding the hydrogen absorption mechanism in the electrolyte.