7 - Reliably measuring hydrogen uptake in storage materials

Abstract

This chapter addresses the technical aspects of measuring hydrogen uptake, including methodology and apparatus. It discusses the most direct and authoritative approaches to measuring hydrogen uptake and focuses on the most common techniques, which are based on measurement of the hydrogen pressure in a closed system (manometry) or of the mass of the sample and its loading of absorbed or adsorbed hydrogen (gravimetry). The chapter gives prominence to a problem affecting both classes of technique that of the influence of inaccurate knowledge of the volume occupied by the sample itself on the reliability of the data, and proposes a new approach to measuring hydrogen uptake, the variable-volume hydrogenator, as the best solution to this all-pervading problem. It considers the possibilities and problems of measuring the uptake of hydrogen using manometric and gravimetric apparatus. The universal dependence of the measurement on the density of the gas around the sample means that some measure of gas density beyond the ideal-gas approximation should almost always be used. The chapter describes the possible operating regimes (isochoric, isobaric, isoplethic, and constant flow), presents detailed models of the Sieverts manometric hydrogenator and a generic gravimetric system based on a microbalance, and discusses the characterization of the system of hydrogenator plus sample. It also reviews the problems caused by the invariable need to accurately know the effective volume of the sample.