High Performance Impurity and Carbon Tolerant and Microstructurally Durable Fuel Electrodes for Solid Oxide Electrolysis Cells

The Solid Oxide Electrolysis Cell (SOEC) technology provides a method of energy storage, that is required in the transition to relying on renewable energy sources. Energy storage is needed because electricity production when using solar-panels and wind-turbine is dependent on weather-phenomena, which can not be adjusted to match the energy consumption. The energy transition is motivated by the need to reduce the anthropogenic CO₂ emissions from combustion of fossil fuels to reduce global warming. SOECs have the ability to play a double role in the reduction of CO₂ emissions since they, in addition to storing energy, can use CO₂ as the feedstock, where after the splitting the energy is stored in the CO. The CO can then be combined with H₂ to create create hydrocarbons, alcohols or other useful molecules thtn can be used as feed stocks in chemical industry or as “synthetic” fuel. These molecules can replace compounds normally sourced from the petrochemical industry thus reducing the use of fossil resources.
A major issue impeding widespread implementation of SOECs for CO production insufficient lifetime. In this project, the fuel electrode degradation of state-of-the-art anode supported (ASC) Ni/YSZ SOECs has been studied. The degradation mechanisms addressed include: Inhibition of the reaction sites through carbon deposition or poisoning by impurities in the gas feed, and microstructural damage through coarsening or migration of Ni.
The degradation of ASC Ni/YSZ SOECs was studied through a series of long-term (≈1000 h) durability test under CO₂ electrolysis operations, during which their performance were monitored by electrochemical impedance spectroscopy (EIS). The microstructural changes of the SOECs were characterised post-operation using scanning electron microscopy, and Raman spectroscopy for detection of carbon in the fuel electrodes. In all tests in this project the cells were operated at a temperature of 750 °C with a fuel inlet composition of 10/90 CO/CO₂ and with O₂ supplied to the oxygen electrode.
The impact of the current density in the cell and tje fuel utilisation was studied by testing SOECs at various combinations of these parameters (0.375-0.75 A cm^-2 and 29/58 % CO₂ utilisation). The study has revealed that carbon deposition occurs when the TPB-related overpotential of the fuel electrode η_TPB reaches 190-200 mV. η_TPB is the total fuel electrode overpotential minus voltage losses due to conversion and diffusion. It furthermore found that the best durability and performance is observed when current density is 0.375 A cm^-2, where the low production rate is compendated by a strongly extended lifetime, such that overall lifetime yield is maximised. One of the tested cells had it gas cleaned upstream by passing it through crushed Ni/YSZ fuel electrode. This was expected to remove gas phase impurities e.g. sulphur containing species that would otherwise be reduced at the TPBs in the active fuel electrode leading to degradation. This test displayed improved performance in comparison to the tests without, indicating the presence and negative effect of gas phase impurities in the used CO₂.
The impact of infiltration of CGO in the fuel electrode for performance and lifetime was investigated. Two SOECs where the fuel electrodes had been infiltrated with different loadings of Ce_0.9Gd_0.1O_1.9 (CGO) were tested at 0.625 A cm^-2 with 29% CO₂ utilisation. The tests revealed that, while the addition of CGO improves electrochemical performance and durability, it did not affect the threshold for carbon formation. However, the infiltration by CGO clearly mitigates Ni migration. Attempts were made to detect carbon deposition in-operando coupling EIS and Raman spectroscopy. While carbon was successfully observed through the electrolyte, its presence was determined to stem from another source than operation. Based on the findings in this project optimal operation of SOECs for CO₂ electrolysis should be carried out with CGO infiltrated fuel electrodes, cells should be operated at low current density (i ≈ 0.375 A cm^-2 at 750 °C), effectively a current density that keeps η_TPB below 200 mV, and the supplied CO₂ should be cleaned upstream to reduce the level of sulphur species below 5 ppb.