How long will it last?

Abstract

Alkaline water electrolysis will become increasingly important for supplying the world with sufficient renewable energy, and with raw material for the chemical and pharmaceutical industry. Zero Emission Fuels B.V. (ZEF), a technology start-up based in the Netherlands, is developing a small scale alkaline electrolysis cell (AEC), which is integrated in a methanol producing micro-plant. The challenge of this project is to look into the use of polymers for the encapsulation of the ZEF AEC. The conditions of the ZEF AEC are not to be neglected. The system will run in a 30wt% KOH solution at a temperature of 90 °C and a pressure of 50 bar. Not many polymers will be able to withstand these conditions for a desired lifetime of 20 years. Using CES EduPack, a selection of 30 potentially suitable polymers has been made, from which high density polyethylene (HDPE), 40% glass-reinforced polyphenylene sulfide (PPS-40%gf) and polysulfone (PSU) are further investigated. Based on a literature review and a simple KOH ageing test, HDPE is found unsuitable for application in the ZEF AEC. PPS-40%gf and PSU have been subjected to durability testing for a range of different conditions involving a variety of KOH concentrations (15, 30 and 45wt%), and two oxygen partial pressures (O2 at 5 bar and air with pO2=20%) at different temperatures (90, 120 and 170 °C). The purpose of the ageing experiments is to give a better understanding of the effect of these parameters on the structure and integrity of the polymer; and to eventually be able to acquire a lifetime prediction. Extensive characterisation of the exposed samples has been carried out using different techniques, including weight measurement, tensile testing, DMA, creep-recovery testing, DSC, FTIR, XRD and SEM. After 12 weeks of ageing, it is found that glass-filled polymers are unsuitable for application in a strong alkaline solution at elevated temperatures, due to the dissolution of the glass fibres, which leads to a reduction in mechanical and barrier properties. However, the PPS matrix and PSU are found to be resistant to thermo-oxidative and chemical degradation in the tested ageing conditions. Only subtle changes in mechanical, visco-elastic and thermal behaviour are observed, which can be assigned to the effects of physical ageing. Due to the undesirable brittle nature of PPS, it can be concluded that PSU is the most promising candidate for the long-term application in alkaline electrolysis.