Research

In comparison with the emerging protonic ceramic fuel cells, the solid oxide fuel cells (SOFCs) are nowadays commercially available with applications including stationary power supply and advanced hybrid fuel cell and engine systems that have the potential of achieving ultra-high efficiency of greater than 70 %. Nevertheless, there is an urgent need for further improvement of the SOFC performance in terms of power density and long-term stability to increase their market competences. The typical power density of a yttria-stabilized zirconia (YSZ)-based commercial SOFC is currently reported to be in the range of ~0.2-0.8 W/cm² depending on the cell configuration (either electrolyte supported, or anode supported), cathode materials, and the cell operating conditions. Regardless the intense research effort during the past decade, the high activation energy for oxygen reduction reaction (ORR) in the cathode is still the major cause hindering the power density the state-of-the-art SOFCs. Infiltration processes have thus been developed for deposition of nanoscale catalysts into the well-developed porous lanthanum strontium manganite (LSM) or lanthanum strontium cobalt ferrite (LSCF) based composite cathodes, to enhance the surface electro-catalytic activity and stability and accelerate the ORR.

​

• Highlight one: Chen, Y., Liang, L., Paredes Navia, S., Hinerman, A., Gerdes, K., & Song, X. (2019). Synergetic Interaction of Additive Dual Nano Catalysts to Accelerate Oxygen Reduction Reaction in Fuel Cells Cathode. Acs Catalysis.

​

Amongst the various electrocatalysts, precious metal Pt remains to be one of the most efficient oxygen reduction catalysts employed for various fuel cells operated at different temperatures, while the high cost of Pt prevents its large-scale applications. In recent years, chemical vapor based Atomic Layer Deposition (ALD) is demonstrated to be able to create a conformal and uniform surface coating layer with thickness down to the atomic scale. Such an approach could lead to the minimum loading of catalyst into the cathode of as-fabricated cells to further improve the SOFC performance. For example, when the ALD layer is ~ 5 nm in thickness and consisting discrete ~ 3 nm Pt grains, the loading of Pt is estimated to be minute of ~1.5×10⁻³ mg/cm², which is significantly lower than the target loading of < 0.1 mg/cm² that needs to be achieved for proton exchange membrane fuel cells in automotive applications. However, for SOFC operated at high temperatures of 750 °C or higher, once the ALD mono-layer of unary Pt is applied to the LSM/YSZ cathode of cells, the power density enhancement induced by ALD coating is limited to be ~140 %. Pt in the ALD layer undergoes immediate agglomeration from ~ 3 nm to ~ 70 nm in dimensions and loss of catalytic surface area due to the electrochemical operation. Pinning the Pt catalyst to be nano-sized and with uniform distribution on the ORR active sites is very much desired to further boost the cell performance while minimizing the Pt loading in the ALD layer. Here we report one simple approach of applying an ultra-thin CoOx layer to stabilize the nano-Pt particles on the LSM/YSZ cathode backbone of as-made SOFCs. Through synergetic electrochemical interactions, the Pt/(MnCo)Ox couplings were established and uniformly distributed on the entire YSZ surface, the original TPBs and at LSM/LSM surface grain boundaries. Such nanoscale Pt/(MnCo)Ox couplings are stable and dramatically accelerate the ORR. ALD coating has significantly enhanced the cell peak power density by 200 % over electrochemical operation of 504 h at 750 °C.

​

• Highlight two: Chen, Y., Gerdes, K., & Song, X. (2016). Nanoionics and Nanocatalysts: Conformal Mesoporous Surface Scaffold for Cathode of Solid Oxide Fuel Cells. Scientific Reports, 6, 32997. doi:10.1038/srep32997

​

Nanoionics has become increasingly important in devices and systems related to energy conversion and storage. Nevertheless, nanoionics and nanostructured electrodes development have been challenging for solid oxide fuel cells (SOFCs) owing to many reasons including poor stability of the nanocrystals during fabrication of SOFCs at elevated temperatures. In this study, a conformal mesoporous ZrO2 nanoionic network was formed on the surface of La1−xSrxMnO3/yttria-stabilized zirconia (LSM/YSZ) cathode backbone using Atomic Layer Deposition (ALD) and thermal treatment. The surface layer nanoionic network possesses open mesopores for gas penetration, and features a high density of grain boundaries for enhanced ion-transport. The mesoporous nanoionic network is remarkably stable and retains the same morphology after electrochemical operation at high temperatures of 650–800 °C for 400 hours. The stable mesoporous ZrO2 nanoionic network is further utilized to anchor catalytic Pt nanocrystals and create a nanocomposite that is stable at elevated temperatures. The power density of the ALD modified and inherently functional commercial cells exhibited enhancement by a factor of 1.5–1.7 operated at 0.8 V at 750 °C. Nanoionics and nanostructured electrode, which are extraordinary important for ion transport in the other electrochemical systems, have been very challenging to develop for SOFCs. To the authors’ knowledge, the present work from Prof. Song's group is the first demonstration of establishment of nanoionic network for applications of high temperature SOFCs. These experiments contribute to validation of the nanoionic mechanisms reported previously by independent researchers and offer justification for tailoring surface structure of the electrode, via nanostructural engineering with ALD and thermo-treatment. 

​

• Highlight three: Chen, Y., Gerdes, K., Paredes Navia, S. A., Liang, L., Hinerman A., Song, X. (2019). Conformal Electrocatalytic Surface Nanoionics for Accelerating High-Temperature Electrochemical Reactions in Solid Oxide Fuel Cells, Nano Lett.

​

Additive implantation of electrocatalysts onto the internal surface of porous cathodes holds great promise to accelerate the electrochemical reactions within solid oxide fuel cells (SOFCs). Here we utilize atomic layer deposition (ALD) to apply dual catalysts with (Mn0.8Co0.2)3O4 and a minute amount of Pt on the cathode consisting of lanthanum strontium manganite (LSM) and yttria-stabilized zirconia (YSZ). Coating this material with optimum ALD layer thickness resulted in a 53% reduction of polarization resistance and a 350% SOFC peak power density enhancement at 750 °C. During the electrochemical operations, the dual catalysts interact synergistically and evolve into superjacent conformal electrocatalytic (Mn0.8Co0.2)3O4 nanoionics with high-density grain boundaries and subjacent discrete nano Pt particles evenly distributed on both the LSM and YSZ. The configuration consequently extends the active electrochemical reaction sites to the entire internal surface of the cathode. For the first time in the field of SOFCs, the present work demonstrates the formation of the electrocatalytic surface nanoionics and its resultant accelerated mass and charge transfer to dramatically boost the cell performance.

The latest work from Prof. Song’s group and her collaborators on solid oxide fuel cells lead to series peer-reviewed journal paper publications, and two patents including one granted patent and one pending patent since year 2015.