The utilization of seawater and wastewater in solid oxide electrolysis cells

RWTH Publications (RWTH Aachen)

Abstract

The utilization of seawater and wastewater is one way of reducing the pressure on freshwater sources for the production of hydrogen. High temperature electrolysis is an interesting approach due to the increased efficiencies at operating temperatures of 700 – 900 °C. In this work, commercially available solid oxide cells were analyzed for operation with saline containing water. The electrochemical performance and long term stability were evaluated, and a novel system level concept for seawater utilization in high temperature electrodes was assessed. For pure water operation, acting as reference, degradation behavior strongly depended on cell production conditions, with silicon contamination from starting materials identified as a critical factor. Thus, long term stability testing with saline water utilization should be only compared against benchmark pure water operation from the same cell production batch. The use of seawater showed no influence on the initial performance and activation energies of the cells. However, an increased degradation rate over 1000 hours of operation was observed due to polarization losses in the Ni-YSZ fuel electrode. This was correlated to silica poisoning detected by post test characterization. Additionally, the transport of salt was confirmed by deposits found on the surface of the Ni-YSZ fuel electrode. Interestingly, conventional degradation mechanisms were suppressed by seawater usage, which was attributed to a stabilization of the microstructure in the Ni-YSZ fuel gas electrode. Electrochemical measurements with saturated sodium chloride solution revealed no negative effect on the initial performance and activation energies. However, an increased degradation rate was observed, which can be attributed to polarization losses in the Ni-YSZ fuel electrode. A sodium carbonate species was detected by post test characterization on the fuel electrode surface. The findings suggested a mechanism in which gaseous hydrochloric acid is formed, contributing to accelerated Ni agglomeration in the electrode and leading to increased degradation. In the final step, the use of a falling film evaporator coupled with a high temperature electrolysis system for seawater utilization was evaluated through technical and economic analysis. This was compared with a system based on a reverse seawater osmosis unit and a conventional evaporator. It was shown that the simultaneous evaporation and purification of seawater is likely to provide an economic advantage. However, there is an increased technical risk. It was shown that seawater and wastewater utilization in high temperature electrolysis offer future potential, but the transport of salt or other impurities can be problematic for the system. In this work, it was specifically emphasized that silicon, e.g. in seawater or sodium chloride, is problematic for cell stability and must be filtered out beforehand.

Authors 1

1. Jan Uecker corresponding Aachen

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen