Benchmark study of performances and durability between different stack technologies for high temperature electrolysis

Abstract

In the current landscape of high temperature electrolysis, mainly two solid oxide cell (SOC) technologies are being used: electrolyte‐supported and cathode‐supported SOCs. The geometrical differences, namely the thickness of the electrolyte, can lead to vastly different operating temperatures. Since most phenomena affecting performance and durability remain thermally activated, comparing stack technologies can be a difficult endeavor at best.While the most visible goal of the European project MultiPLHY consists of Sunfire GmbH building the first multi‐megawatt solid oxide electrolyzer, a work package is being dedicated to stack testing in a laboratory environment. A harmonized protocol was first elaborated to allow comparing different stack technologies. It includes the recording of performance maps, several galvanostatic steps in thermoneutral conditions, as well as load point and thermal cycles. Subsequently, Sunfire operated a pile‐up of two 30‐cell electrolyte‐supported stacks for over 8200 h, while a 25‐cell cathode‐supported stack was tested at CEA for 6800 h.The present article aims at presenting the findings gathered during the implementation of the protocol. This benchmark study puts forward performance maps as well as voltage and stack temperature profiles over time, and discusses some of the difficulties inherent to long‐term testing.

Electrochemical Analysis of an Electrolyte-Supported Solid Oxide Cell-Based MK35x Stack during Long-Term Electrolysis Operation

“…[6][7][8]11,12 For example, several durability tests over >1000 h of electrolyte-supported cell-based stacks have shown relative voltage increase rates as low as 0.5%-0.6%/kh. 7,8,15 While degradation rates of state-of-the-art stacks with fuel electrode supported cells seem to be higher on average, 12,[30][31][32][33] some studies report degradation rates of 0.5%-0.7%/ kh as well. 10,11,13 As a different metric, some long-term degradation studies prefer to use the resistance increase over time as a more accurate measure of degradation.…”

Recent Highlights on Solid Oxide Cells, Stacks and Modules Developments at CEA

“…Consequently, the stack temperature was regularly increased to compensate for the degradation and maintain the cells in (near) thermoneutral operation. With this operation strategy, no hydrogen production loss (the hydrogen production being directly related to the current density) has been observed over the total test duration (26)(27).…”

“…A durability test has been started, with a current density of -0.65 A cm -2 , and a steam conversion of 70%. The operation strategy was as above and as reported in (26)(27), to operate at the thermoneutral voltage and to compensate the degradation by a temperature increase. The evolution of the temperature of the hotbox (the same for the two stacks) is reported in figure 10, and compared to the initial step of the 25-100 stack tested during 6800 h, operated in the same conditions and reported in (26)(27).…”

“…The operation strategy was as above and as reported in (26)(27), to operate at the thermoneutral voltage and to compensate the degradation by a temperature increase. The evolution of the temperature of the hotbox (the same for the two stacks) is reported in figure 10, and compared to the initial step of the 25-100 stack tested during 6800 h, operated in the same conditions and reported in (26)(27). One can see a very good superposition of the two curves, demonstrating the good and reproducible operation of the two stacks in the hotbox (30).…”

“…Figure 10. Time evolutions for temperature of the hotbox in which the two stacks are operated at -0.65 A cm -2 , and a steam conversion of 70% to be close to the thermoneutral voltage; comparison with the reference 25-100 stack as published in(26)(27) …”