Brief Introduction to SOFCs

Figure 7.1 schematically shows the basic working principles of SOFCs with different ion conducting electrolytes. The three main components of an SOFC include a cathode, an electrolyte as an oxygen ion (O^2-) conductor (Figure 7.1a) or a proton (H⁺) conductor (Figure 7.1b), and an anode. Electrochemical reaction occurs between reactants (gaseous fuels such as H₂ and hydrocarbons) and oxidants (air). The electrolyte layer should be dense and gas-tight without pinholes to avoid electrical short circuit and with high fracture toughness to maintain a robust solid electrolyte membrane. The electrode layers need to be highly porous structures to facilitate efficient gas diffusion.

In an oxygen-ion-conducting SOFC, oxygen is reduced at the cathode to oxygen ions and then transported through the electrolyte, reacts with the hydrogen at the anode, and finally forms water and releases electricity. Oppositely, for a

FIGURE 7.1 Schematic diagrams of SOFCs with the associated electrochemical reactions: (a) oxygen-ion conductor, (b) Proton conductor.

proton-conducting SOFC, protons migrate through the electrolyte from the anode to the cathode side, where they react with oxygen ions to form water (H₂0). A fully dense and solid-oxide electrolyte layer is spatially sandwiched between the nanopo- rous cathode and anode, and electrons are directly released to the external circuit by electrochemical reaction from fuels.

Fuel Cell Losses

The performance of an SOFC can be characterized by its current-voltage (I-V) curve in Figure 17.2, showing the voltage output of the fuel cell with respect to a given output. The fuel cell performance is typically evaluated with this curve, and the maximum ideal voltage can be directly determined by thermodynamics and can be obtained when the fuel cell is operated under the thermodynamically reversible condition. However, the actual voltage of a real fuel cell is always less than the thermodynamically estimated voltage. As the current is drawn from the fuel cell, the output voltage is immediately dropped from the reversible cell voltage. This voltage drop characterizes the irreversible losses in a fuel cell operation, and the more the current drawn, the greater these losses.

As mentioned above, three major types of fuel cell losses, which give a fuel cell I-V curve its characteristic shape can be defined as follows:

• • Activation losses: (losses due to electrochemical reaction)
• • Ohmic losses: >/_0hmic (losses due to ionic and electronic conduction)
• • Concentration losses: //_{c (losses due to mass transport).}

Therefore, the actual voltage output for a fuel cell is expressed as the ideal voltage subtracted by the three main losses:

FIGURE 7.2 Schematic of current-voltage characteristics of an SOFC. The voltage drop results from three major losses, which are activation loss, ohmic loss, and concentration loss.

where V_themio represents open circuit voltage (OCV) determined by thermodynamics. In terms of electrochemical reaction in SOFCs, cathode kinetics for 0₂ reduction is significantly slower than anode kinetics for H₂ oxidation (Adler 2004). Therefore, the activation loss and ohmic loss are critical losses that affect performance of low- temperature solid oxide fuel cells (LT-SOFCs), and especially, ohmic loss is the most obvious source of loss in TF-SOFCs.

The ohmic loss is mainly due to the resistance of ionic charge transport through the electrolyte layer. Since the electric conductivity is significantly higher than ionic conductivity, ohmic loss mostly results from ionic transportation inside electrolyte membrane. They are simply governed by Ohm’s law:

where A is the electrochemically active area, L is the length of the ionic transport path (namely electrolyte thickness), and a is the ionic conductivity of the electrolyte material. The voltage V represents the voltage, which must be applied to transport a charge at a rate given by i.

Since fuel cells are generally compared on a per-unit-area basis using current density instead of current, area-normalized fuel cell resistance, which is area-specific resistance (ASR) with units of £2 cm² is reasonable. By using current density,у = i-A~^l and ASR, ohmic losses are expressed as

Based on Eq. 7.3, ohmic losses can be decreased either by reducing the electrolyte thickness (a dimensional property) or by using an electrolyte with higher ionic conductivity (a material property). With the help of MEMS-fabrication processes and vacuum-based thin-film deposition methods, the electrolyte thickness can be minimized to sub-micrometer scale. As shown in Figure 7.3, the electrolyte thickness of a typical TF-SOFC fabricated using powder process (Figure 7.3a) is around 50 pm which is rather a thick film, while the MEMS-based TF-SOFC using electrolyte deposited by atomic layer deposition (ALD) is only 70nm in thickness (Figure 7.3b). This drastic decrease in thickness allows the operating temperature of the cell in Figure 7.3b to be below 500°C with decent output power density.