Experimental and theoretical investigation of gas purity in alkaline water electrolysis

Nowadays hydrogen, which is required in huge quantities for many important industrial processes such as ammonia synthesis, is still being produced through inexpensive, but greenhouse gas emitting processes like steam reforming and coal gasification. In the course of the energy turnaround hydrogen is often seen as the fuel of the future. Within the framework of the power-to-gas concept (PtG), particularly water electrolysis is often discussed as the key technology for future synthesis of hydrogen. Alkaline water electrolysis has been applied in the industry for decades, but no further research activities have been undertaken for quite some time. For realization and improvement of the PtG concept precise knowledge, especially about the dynamic behavior of the electrolysis process, is indispensable. Usually the acceptable part-load operation of an alkaline water electrolyzer is limited to about 10 % - 40 % of the nominal load. Below this working range the hydrogen quality is significantly reduced through contamination with oxygen, which is also being produced in the process. The increasing hydrogen impurity is mainly based on two aspects. Firstly, the product gases diffuse through the separator into the opposite half-cell to a certain extent. Secondly, the mixing of the hydrogen and oxygen saturated electrolyte leads to a decrease of the product gas quality in the part-load regime as the saturation of the electrolyte is approximately independent of the electrolyzer load. The mixing of the catholyte and anolyte cycle is necessary to compensate an electrolyte concentration gradient which is caused by the occurring half-cell reactions. Particularly through the use of renewable energy sources an intermitting operation of the process may lead to a safety plant shutdown at around 2 vol% H2 in O2 in the lower working range. In addition, the current development of alkaline water electrolysis focuses on the increase of the electrolysis pressure to avoid the need of additional mechanical hydrogen compression, which further intensifies the problem of product gas contamination. In this study, classical mixing of catholyte and anolyte as well as several other electrolyte management concepts are examined with respect to the resulting gas purity. Next to the classical strategy, the complete electrolyte separation or the application of periodic separation-mixing-sequences are conceivable, which promise a reduction of the product gas contamination. In order to investigate these concepts, experiments are carried out in a custom-built laboratory electrolyzer under industrially relevant conditions, which allow an evaluation of the influence of various process parameters and the quantification of the prevailing crossover mechanisms. In addition, a model is being developed that can be used for the support of the experiments and for the optimization of the process. The results show that a reduction of the electrolyte flow rate and system pressure, an increase of the electrolyte temperature, and an increase of the electrolyte concentration lead to a reduced contamination of the products when the electrolyzer is operated with mixed electrolyte cycles. The analysis of the results further reveals that the main source of contamination is not the permeation of the gases through the separator, but the dissolution in the electrolyte and transport to the other half-cell by electrolyte recycling. Consequently, a significant reduction of gas crossover can be achieved by the separation of the cycles or a dynamic process strategy, which involves a continuous alternation between merged and separated electrolyte cycles. This process management provides an almost constant electrolyte concentration while improving the product gas quality simultaneously.