Fuel Cell and Electrolysis Operation of Solid Oxide Cells Containing 3D-Printed Electrode Supports in H2/H2o and CO/CO2 Gas Mixtures
Authors: Geisendorfer Nicholas R. , Barnett Scott A 
Published: May 2020

Electrolysis of CO2 using solid oxide cells is important both for renewable energy storage technologies as well as for upcoming, flagship space missions. Nevertheless, the performance of solid oxide cells operating for CO2 electrolysis is generally diminished compared to an identical cell performing H2O electrolysis. The reduced performance when operating in CO2 results both from slower reaction kinetics as well as slower gas transport, which manifest in the performance as large activation and concentration polarizations, respectively. The finite diffusivity of both H2/H2O and CO/CO2 gas mixtures leads to concentration losses which limit overall fuel cell or electrolysis performance, particularly at high current densities. Performance can be improved by diminishing the thickness of the porous electrode support layers, hence decreasing the gas diffusion distance between the flow channels and electrochemically active sites, but this also limits the mechanical support provided to the cell. Structured support layers which cover only some of the electrode functional layer potentially allow substantial areas of the cell to be very thin, hence easing gas transport from the flow channels to the electrochemically active sites, while largely maintaining the mechanical support for the cell. However, fabricating these layers out of SOC-relevant materials (such as Ni-YSZ) and incorporating them with the functional layers of the cell can be challenging.

Here, we present an extrusion-based 3D-printing technique capable of fabricating these layers, which are then integrated with tape-cast layers using traditional lamination and a multi-step firing procedure. This strategy has recently been shown to improve H2O electrolysis performance in cells with a Ni-YSZ fuel electrode, YSZ electrolyte, and LSCF oxygen electrode, operating at 800°C in low-steam conditions (3% H2O/97% H2) by a factor of approximately 2.5 compared to cells possessing traditional, monolithic support layers (the cell containing 3D-printed support generated 258 mA/cm2
at 1.3 V and 800°C). Microstructure of the 3D-printed support layers is investigated using scanning electron microscopy. The cells are characterized using current-voltage measurements and impedance spectroscopy between 600 and 800°C with various fuel gas compositions. All tests are performed with air at the oxygen electrode. Mechanical considerations governing the design of 3D-printed electrode supports will also be discussed.

Although Ni-based electrodes have long demonstrated excellent performance for fuel cell and electrolysis operation using H2/H2O gas mixtures, their low stability when operating in CO/CO2, in part due to the poor reoxidation stability and coking resistance of Ni, requires the development of more-stable, alternative electrode materials. Besides these considerations, reaction kinetics for CO2 electrolysis are further slowed by the relative weak adsorption of CO2 molecules. Perovskite fuel electrodes with exsolved catalytic nanoparticles have been shown to possess competitive performance compared to conventional Ni-based electrodes, while nevertheless exhibiting higher stability to reoxidation and resistance to coking. Additionally, the vacancies produced in the perovskite as a result of the exsolution of electron donor B-site cations provide additional, stronger adsorption sites for CO2 molecules, making them strong candidates for fuel electrodes for CO2 electrolysis. Here, we utilize two recently reported, A-site deficient perovskite fuel electrode materials Sr0.95(Ti0.3Fe0.63Ni0.07)O3-δ (STFN) and Sr0.95(Ti0.3Fe0.63Ru0.07)O3-δ (STFRu) for electrolysis of CO2. The electrochemical performance of these electrode materials for CO2 electrolysis, will be presented for the first time. Current voltage measurements and impedance spectroscopy are carried out across a range of fuel gas compositions and operating temperatures. Finally, initial results for combining these electrode materials with 3D-printed electrode supports to counteract both activation and concentration losses will be presented.