Hydrogen storage in nano-structured carbon materials

Full text of publication follows: Energy and environment are two major concerns in our modern society due to the coming shortage in fossil energy sources and the growing of greenhouse gas emissions. The challenge for the coming years is to discover new energy resources and to develop devices that are compatible with a sustainable development and generate few (or zero) emission. One of these devices is the fuel cell feed by hydrogen, whose application fields are very large. In particular, the proton exchange membrane fuel cell (PEMFC) is the most realistic device for automotive application. However, hydrogen storage remains one of the most important challenges regarding its development. Although different techniques are available for storing hydrogen, no ideal solution has been found yet. Compression needs elaborated tanks in shape for supporting high pressures, liquefaction requires an expensive hydrogen cooling and adapted tanks. Chemical storage by hydrides imposes heavy devices. A last technique consists in adsorption of hydrogen on solid surface of nano-structured materials, specially carbon-based ones. This technique enables to store the hydrogen with a reduction the total gas pressure in the tank as compared to pure compression. Literature review shows that measuring the hydrogen storage capacity is a complex task. Controversies related to the material itself as well as to the experimental technique used appear regularly in the publications. For this reason, an experimental set-up was built and proper calibrated in our laboratory at CEP-ENSMP in Sophia-Antipolis. It was designed for testing a large amount of material and it enable to measure the hydrogen uptake by an adsorbent in comparison to compression. CEP-ENSMP is capable to synthesize several families of nano-porous carbon materials in large quantities. These families comprise carbon aerogels and those obtained by a plasma process. Carbon aerogels are nano-structured materials characterised by a high surface area. Their porosity and their pore size distribution depend on the operating conditions. Materials synthesised by the plasma process are essentially composed of nano-structured carbon soot of different qualities such as carbon black, carbon nano-fibres, nano-tubes or new exotic structures. This paper is focused on the determination of hydrogen uptake on carbon materials synthesised at CEP. Firstly, the volumetric test bench will be presented. A major importance will be devoted to the calibration protocol, the estimation of experimental errors and the study of sensitivity of the experimental parameters, including human and technical errors. Secondly, carbon nano-materials produced by CEP will be described. Special attention will be paid to the characterisation of structural properties (SEM, BET,...). Thirdly, the measurements carried out for determining the ability of these materials to store hydrogen in comparison to compression will be shown. Finally, the relationship between the experimental results and the structural properties of the materials will be discussed. This research was carried out in the framework of the 'Reseau Pile a Combustible' project funded by ADEME