Thin-Film Solid Oxide Fuel Cells

Introduction

Solid oxide fuel cells (SOFCs) are efficient electrochemical devices that directly convert the chemical energy into electrical energy, and continuous electricity can be produced as long as fuel is consistently fed. The fuel is supplied into the anode side where it undergoes oxidation process, and the released electrons are transferred to an external circuit. Most SOFC systems do not require an external reformer and can directly utilize hydrocarbon fuels because of their high operating temperature, typically close to 1,000°C.

Fuel cell researches have been active since the first invention of hydrogen fuel cell by William Grove using sulfuric acid solution and zinc and platinum electrodes in 1839. Following Grove, Mond and Langer in 1889 reported fuel cell performance enhancement by considering a porous electrode structure which became the electrode structure of modern fuel cell (Mond and Langer 1890). After that, the use of fuel cell continued to create new milestones, and most scientists attempted to develop fuel cells with various fuels and electrolytes throughout the remainder of the century.

Of many types of fuel cells, SOFC was originally invented to replace a commercial light source by using solid ion conductor w'hich is made of 85% zirconia and 15% yttria. But it disappeared after tungsten lamp was introduced in 1905. In 1937, Baurand Preis first operated SOFCs at 1,000°C using coke as the fuel and magnetite as the oxidant with the knowledge of previous research works regarding SOFCs (Baur and Preis 1937). Until the 1960s, fundamentals of SOFC in empirical phase were developed, and after 1960, the number of patents regarding SOFC technology rapidly increased. The first paper “A solid electrolyte fuel cell” was published in English by Weissbart and Ruka in 1962 (Weissbart and Ruka 1962). They used Zr0g5Ca0,50,85 as an oxygen ion conductor and revealed that the cell output is essentially limited by the resistance of the electrolyte and incompact electrolyte structure. A remarkable step forward from 1970 was the development of the EVD (electrochemical vapor deposition) method for perfect closing of pores of the solid electrolyte (Feduska and Isenberg 1983). Conventional SOFC systems have been researched for more than a couple of decades. In 1999, the Solid State Energy Conversion Alliance (SECA) by US Department of Energy initiated the development of SOFC stacks and systems for quick commercialization at low cost. From 2003, US government started the hydrogen fuel initiative program to develop hydrogen fuel cell vehicles and infrastructures for commercialization.

Yttria-stabilized zirconia (YSZ), which is an oxygen ion conductor, is the most typical electrolyte material for SOFCs. Typical operating temperatures for current SOFCs are between 800°C and 1,000°C. A high operating temperature is necessary in order to activate the ion transportation process across the electrolyte, as well as for higher electrochemical reaction kinetics on electrodes. Therefore, researches on how to lower operating temperatures without sacrificing high fuel cell performance have been active. Reducing operating temperature has a great potential in minimizing interfacial diffusion between electrode and electrolyte, simplifying integration of components, alleviating material degradation, and improving thermal cycling capabilities. Most importantly, it offers an opportunity for SOFCs to be a mobile power source rather than only a stationary power plant. However, lowering operating temperature results in inevitable performance drop from both the slow ionic transportation and sluggish electrode reaction processes. Since the ionic transportation across the electrolyte is a process following Ohm’s law, the resistance is proportional to the distance the ion travels. It is therefore intuitive to minimize the electrolyte thickness in order to have lower ohmic resistance.

As such, using a thinner electrolyte for lower ohmic resistance and therefore lower operating temperature has been a trend for the past two decades in SOFC research. The thickness of the typical YSZ electrolyte ranges from micrometers using powder-based processes to only 10 nm using vacuum-based thin-film deposition (Baek, Liu, and Su 2017). The cell configuration also varies from the “electrolyte support,” having a thick YSZ as the main mechanical support of the entire cell, to the “free-standing configuration without support,” having a completely free-hanging membrane. Instead of using conventional powder or slurry followed by sintering process, the thin membranes are fabricated using thin-film deposition techniques, so that a very thin film can be fabricated. The challenge will be to ensure the membrane is completely gas-tight to prevent gas leakage and pinhole-free to prevent current leakage. In addition, scaling up the cells with nanoscale thin-film membranes is another major challenge, and several solutions suggesting new' cell configurations instead of conventional SOFCs have been published.

This chapter will discuss the configurations that have been used for TF-SOFCs and review the relevant literature to explore their pros and cons. The context concerns only SOFCs using thin-film electrolytes of sub-micrometer thickness and the supporting structure specifically designed for it. The typical “thin-film electrolyte” referred to in many literatures that has thickness in the range of tens of micrometers is not within the scope of the discussion here. Specifically, this chapter will review the following:

  • 1. Overview of TF-SOFCs operating below 500°C including their fabrication methods, cell configurations, electrochemical performance, and technical issues.
  • 2. State-of-the-art cell configurations to improve fuel cell performances and membrane stability against thermo-mechanical stresses.
  • 3. Methodology of scaling up the membrane electrode assembly (MEA) of TF-SOFCs to achieve higher total power output within a confined reaction area.
 
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