Metal-Supported Solid Oxide Co-Electrolysis Cells
Authors: Caldogno Riccardo , Hagen Anke , Sun Xiufu 
Published: December 2024

Achieving a net-zero greenhouse gas emissions economy by 2050 is an ambitious target for the European Union to mitigate climate change. The development of carbon-neutral fuels with high energy-density is essential for the de-fossilization of the heavy transportation sector. Solid Oxide Electrolyser (SOE) technology could play a crucial role in Power-to-X applications aimed at facilitating the development of such fuels. Solid Oxide Electrolysis Cells (SOECs) boast high efficiencies in converting renewable electricity into chemical energy, in form of hydrogen or other high energy-density fuels. Most established SOEC configurations are electrolyte- or fuel-electrode-supported. Recently, Metal-Supported SOECs (MS-SOECs) have attracted great attention thanks to their cheaper materials and superior mechanical properties compared to traditional ceramic configurations.

The European E-TANDEM project aims to enable the production of a carbon-neutral diesel-like e-fuel for the marine and heavy-duty transport sectors. Said e-fuel is produced from H2O, CO2, and renewable power in a once-through hybrid-catalytic process that integrates three sub-processes: electrocatalysis for e-syngas (H2+CO) production, thermocatalysis for hydrocarbon synthesis from syngas, molecular chemocatalysis to oxo-functionalize the hydrocarbons. In this study, the focus is on the first step of the conversion. High-temperature co-electrolysis of H2O and CO2 through MS-SOECs is investigated.

Three SOECs were tested, denoted as cells A, B, C.

State-of-the-art (SoA) ceramic Ni/YSZ (Nickel/Yttria Stabilized Zirconia) fuel-electrode-supported cell, used as reference.

One type of MS-SOEC fabricated at DTU Energy, with metal/ScYSZ (Scandia-doped YSZ) fuel electrode infiltrated with Ni/GDC (Gadolinium-doped Ceria) (percolating nickel content).

Another type of MS-SOEC fabricated at DTU Energy, with metal/LSFNT (lanthanum-strontium-iron-nickel titanate) fuel electrode, infiltrated with Ni/GDC (low nickel content, not percolating).

The durability of these cells in co-electrolysis mode was investigated. The cells were tested under constant current density (0.5 A/cm2) for 500 hours. Mass spectroscopy was used (with cells B and C) to measure the outlet fuel composition throughout the tests.

The tests were carried out at 650 °C and 1 atm, with a fuel electrode inlet gas mixture of 10:47:43 H2:H2O:CO2, and H2O-CO2 conversion of 37% (by Faraday equation). Under thermodynamic equilibrium conditions, the mixture gives a syngas H2/CO ratio of ca. 2 and negligible methane content, which is suitable for downstream e-fuel synthesis in the E-TANDEM framework.

During test of cell A, the measurement of the lambda sensor placed downstream of the cell confirms that the outlet fuel composition corresponds to equilibrium, meaning that the desired syngas was obtained. With cells B and C, the outlet gas composition was measured by mass spectroscopy. The outlet H2/CO ratio was 2.17 with cell B, meaning that the outlet syngas composition was close to equilibrium, and 1.32 with cell C, meaning that it deviated from equilibrium. These findings demonstrate that the SoA fuel electrode, rich in nickel, promotes RWGS (Reversed Water Gas Shift) reaction, which leads to the equilibrium distribution of H2, H2O, CO, CO2. On the other hand, the much lower amount of nickel present in the fuel electrode of cell C is not sufficient to catalyse the RWGS to the extent of reaching the equilibrium composition.

Cell A started the 500h durability test from 1237 mV and showed a degradation of 332 mV/1000h or 26.8%/1000h. Cell B started from 1322 mV and degraded with an average rate of 238 mV/1000h or 18%/1000h. Cell C showed a worse initial performance (1608 mV) and the degradation rate was significantly higher, 948 mV/1000h or 59%/1000h.

Electrochemical Impedance Spectroscopy (EIS) was periodically carried out to follow the trends of resistance contributions of the cells. It revealed that for all cells the increase in total resistance was mainly attributed to polarization contributions. Cell A showed a small increase in ohmic resistance, which instead slightly decreased in cell B, i.e. there was an activation, and remained stable in cell C until an increased degradation after ca. 300 h of operation.

Scanning Electron Microscopy (SEM) characterization was carried out to assess the long-term operation impact on the cells’ microstructure. SEM showed that, both in cells B and C, a dense chromia scale protected from corrosion the metal in the support and the fuel electrode. Signs of chromium migration via evaporation and precipitation in the fuel electrode were observed in cell C as agglomeration of chromia at the interface between electrolyte and fuel electrode.

This work has been funded by the European Union under grant number 101083700 (E-TANDEM). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.