Oxygen reduction at porous and dense cathodes for solid oxide fuel cells

Solid-State Electrochemistry and Solid Oxide Fuel Cells: Status and Future Prospects

Solid-state electrochemistry (SSE) is an interdisciplinary field bridging electrochemistry and solid-state ionics and deals primarily with the properties of solids that conduct ions in the case of ionic conducting solid electrolytes and electrons and/or electron holes in the case of mixed ionic and electronic conducting materials. However, in solid-state devices such as solid oxide fuel cells (SOFCs), there are unique electrochemical features due to the high operating temperature (600–1 000 °C) and solid electrolytes and electrodes. The solid-to-solid contact at the electrode/electrolyte interface is one of the most distinguished features of SOFCs and is one of the fundamental reasons for the occurance of most importance phenomena such as shift of the equipotential lines, the constriction effect, polarization-induced interface formation, etc. in SOFCs. The restriction in placing the reference electrode in solid electrolyte cells further complicates the SSE in SOFCs. In addition, the migration species at the solid electrode/electrolyte interface is oxygen ions, while in the case of the liquid electrolyte system, the migration species is electrons. The increased knowledge and understanding of SSE phenomena have guided the development of SOFC technologies in the last 30–40 years, but thus far, no up-to-date reviews on this important topic have appeared. The purpose of the current article is to review and update the progress and achievements in the SSE in SOFCs, largely based on the author’s past few decades of research and understanding in the field, and to serve as an introduction to the basics of the SSE in solid electrolyte devices such as SOFCs.

Graphical abstract

Inkjet-Printed Porous Silver Thin Film as a Cathode for a Low-Temperature Solid Oxide Fuel Cell

In this work we report a porous silver thin film cathode that was fabricated by a simple inkjet printing process for low-temperature solid oxide fuel cell applications. The electrochemical performance of the inkjet-printed silver cathode was studied at 300-450 °C and was compared with that of silver cathodes that were fabricated by the typical sputtering method. Inkjet-printed silver cathodes showed lower electrochemical impedance due to their porous structure, which facilitated oxygen gaseous diffusion and oxygen surface adsorption-dissociation reactions. A typical sputtered nanoporous silver cathode became essentially dense after the operation and showed high impedance due to a lack of oxygen supply. The results of long-term fuel cell operation show that the cell with an inkjet-printed cathode had a more stable current output for more than 45 h at 400 °C. A porous silver cathode is required for high fuel cell performance, and the simple inkjet printing technique offers an alternative method of fabrication for such a desirable porous structure with the required thermal-morphological stability.