Int. J. Hydrogen Energy, Vol. 20, No. 9, pp. 697-700, 1995 Copyright @ International Association for Hydrogen Energy Elsevier Science Ltd 0360-3199(94)00119--7 Printed in Great Britain. All rights reserved 036&3199/95 $9.50 + 0.00 NEW MATERIALS FOR WATER ELECTROLYSIS DIAPHRAGMS V. M. ROSA, M. B. F. SANTOS and E. P. DA SILVA Laboratbrio de Hidrogznio da UNICAMP, Universidade Estadual de Campinas, Cx. p. 6039-13083-970, Campinas, SP, Brazil (Received for publication 16 November 1994) Abstract-The perspective of hydrogen utilization as a clean fuel, mainly produced from renewable energy sources, compels the refinement of the water electrolysis process,thus reducing hydrogen generation costs.This paper presents the research now in progress at the Laboratbrio de Hidrog&nio of the Univesidade Estadual de Campinas, consisting of tests of some materials for asbestosdiaphragm substitution in water electrolyzers, their corrosion resistance being analyzed in highly alkaline media (28-30wt% KOH) and at a temperature of about 100°C. The results obtained with polysulfone-coated asbestos tissues are also presented. The ionic conductivity of samples in KOH solution as well as their dimensional and weight variations for a period up to 1000h were measured. The effect of bubbles in the cell conductivity was not investigated. Asbestos tissues were submitted to the same tests for comparison. NOMENCLATURE C Conductivity of solution k Cell constant l/a Length/area R Resistance of solution Z Impedance Greek letters P Resistivity of solution w Frequency INTRODUCTION Hydrogen is an important raw material, with a large field of application Cl]: chemical and petrochemical, metal- lurgic, foods, etc. It is also receiving much attention as an alternative and clean energy source because of petro- leum scarcity and because of the environmental problems that it causes. Besides, most of the renewable energy sources, such as solar and aeolian sources, produce electrical energy which must be stored. Among the possible processes of electric storage, hydrogen gener- ation by water electrolysis is notable because of its high efficiency (8G90 %). In this process hydrogen and oxygen are produced from water molecules. These gases are separated inside the electrolyzer by a diaphragm, whose presence must not disturb ionic transport in the solution between the electrodes. The required characteristics of the materials to be used as diaphragms or membranes are: high corrosion resistance in a strongly alkaline medium; the possibility of manufacture of porous tissues or porous thin films; low ionic resistivity in an alkaline medium in a way which does not decrease the process efficiency; resistance to the atomic oxygen produced. It is common to use asbestos tissues as diaphragms in water electrolysis, but there are limitations such as low corrosion resistance, cost and toxicity. Asbestos is a fibrous silicate which has a lot of varieties [2]. The main one is crisotila or white asbestos, whose structural formula is Mg,Si,O,(OH),. Because of its alkaline struc- ture, crisotila cannot be used in an acidic medium; however, in a basic medium, corrosion can occur if the temperature is high enough [3]: Mg,Si,O,(OH), + 4 KOH 03 Mg(OH), + 2 K,SiO + H,O. The corrosion rate increases with temperature, so it is difficult to increase electrolyzer efficiency by increasing the solution temperature [4] when crisotila is used as the diaphragm. As well as the corrosion rate increasing, mechanical deformation increases and the mechanical resistance decreases. Another great problem of asbestos is its toxicity [2]. It is well known that asbestos fibers can produce asbestosis, mesothelioma, pleural thickening and cancer. When dust-laden air is inhaled, fibers of small size may enter the bronchial tree and even the alveoli. In the case of asbestosis, the inhalation of asbestos causes lung fibrosis. Furthermore, the asbestos used in electro- 697