Area-Specific Resistance ASR

A fuel cell stack can be regarded as a black box into which hydrogen (gas) and oxygen (air) are inputs, and electricity and exhaust gases are outputs. For such a stack, ASR is defined as  

The cell voltage, U, should be measured independently of the leads carrying the current, i.e. separate potential probes should be used. This ASR is in most cases not very sensitive to small variations in cell voltage and fuel utilisation. By determining the ASR at a few different temperatures, an apparent activation energy, Ea, may be derived. Thus, in a voltage interval (from say 0.5 to 0.7 V) and a temperature Interval (say from 650 to 1050 C), the cell may be characterised, with fair approximation, by only two characteristic numbers, namely ASR at one temperature and Ea. 

In case the i-V curve is concave, it may be tempting to use a differential ASR (i.e. the tangent) at high current densities as this gives a nice low value. Such a number has the drawback that it does not reflect the cell performance over the full polarisation range as does the quantity defined by Eq. (1). 

ASR may be divided into ohmic resistance, R, and electrode polarisation resistance, Rp. The ohmic resistance originates from the electrolyte, the electrodes materials and the current collection arrangement. This is very much dependent on geometric factors such as thickness of the cell components and the detailed geometry of the contact between current collection and electrodes, and between electrodes and electrolyte as current constrictions may be important [41]. The electrode polarisation resistance is further divided into contributions from the various rate-limiting steps. Thus, ASR can be broken down in five terms  

It is seen that the contribution from the concentration polarisation, Rp.diff + Rp.canrer IS dominating. In an electrode-supported cell, the limitation of gas diffusion through the support is a cell-relevant resistance, whereas Rp,comvr 

---

The electrocatalytic activity of MIEC cathodes also depends strongly on the properties of the electrolyte, as shown by Liu and Wu [109], The electrode polarization resistances, RE, or area specific resistance (ASR) measured by the electrochemical... 

As a result of the transferred species, loss mechanisms occur. In terms of the first law of thermodynamics these losses are well known as polarisation losses. Polarisation losses are sensitively influenced by numerous mechanisms, which are strongly non-linear with respect to a change of the operational parameters like the current density, electrical potentials, temperature, pressure, gas compositions and material properties. These parameters are assumed to be constant in case of a differential cell area. Thus, the loss mechanisms are summarised in a constant area specific resistance ASR [ 2cm2]. A change of the local overpotential (EN(Uf) — Vceii) at constant ASR complies with a proportional change in the local current density. 

The impedance is dependent on temperature, as can be seen in Figure 4, which shows the area specific resistance (ASR) of a cell as a function of cell temperature for different gas flow rates. For the same cell temperatures, lower ASR was observed for increasing gas flow rates due to the increased gas diffusion near the electrodes that effectively reduced the overpotential resistances [4], Because the anode and cathode are often conductive, the impedance of the cell is dependent largely on the thickness of the electrolyte. Using an anode supported cell structure, a YSZ electrolyte can be used as thin as 10-20 pm or even 1-2 pm [32, 33] as compared to 0.5 mm for a typical electrolyte supported cell [26],... 

Table 1 Area specific resistances (ASR) measured on the curves presented in Figure 3 for the two stacks of three cells...

As a first approximation, these slopes can be associated to the concept of Area Specific Resistance (ASR). 

During this period, FZJ also started to develop its own standardization and QA in SOFC testing with the objective of obtaining performance measurement data of good accuracy and precision in a consistent, reliable, and repeatable manner. All aspects were systematically examined, including the physical experimental set-up, SOFC cell performance, and SOFC testing procedure. Parts of this standardization and QA are described in Section 9.2.3. AU these aspects of R D, standardization and QA finally led to excellent performance data with one type of cell resulting with a theoretical output of more than 4.4 A cm at 800 °C and 700 mV, hence more than 3 W cm . All experimental data from the tested cells were compared with each other, with the main key parameters to characterize the measured performance, the current density, and/or area specific resistance (ASR) at a defined cell output... 

For potentiostats that do not include a specific method for Ohmic drop determination, a full EIS scan over a range of frequencies from high to low can be used. In the resulting Nyquist plot, the first intercept on the X-axis is the Ohmic resistance. In case this is not directly apparent, an equivalent circuit including an Ohmic resistor can be used to fit the data and obtain the Ohmic resistance. Once the resistance is determined, it should be reported in the form of area specific resistance (ASR), as Ohm square centimeter. Thus, the area of the membrane used should be multiplied with the subtracted value from the measurement with and without membrane. 

In recent years, many CFD models for SOFC performance have been developed. Some of these models rely on the empirical notion of area-specific resistivity (ASR), not detailing the kinetics of electrochemical reactions (Yakabe et ah, 2001 Xue et ah, 2005). The others utilize the Butler-Volmer equation for the calculation of activation losses (Iwata et ah, 2000 Larrain et ah, 2003 Aguiar et ah, 2004 Yuan and Liu, 2007 Wang et ah, 2007 Ho et ah, 2008 Zhu and Kee, 2008). However, all these models are numerical and they do not give an irrefutable answer to the questions above. 

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

The ohmic resistance normalized by the active cell area is the Area Specific Resistance (ASR). ASR has the units Qcm. The ASR is a function of the cell design, material choice, manufacturing technique, and, because material properties change with temperature, operating conditions. The ASR is a key performance parameter, especially in high-temperature fuel cells, where the ohmic losses often dominate the overall polarization of the cell. 

Here a is the conductivity or the reciprocal of the resistivity, having a unit of ohm cm or more commonly S cm In electrochemical devices, the area-specific resistance (ASR, ohm cm ) of a flat sheet membrane electrolyte is of engineering importance and can be expressed as the product of the resistance and the surface area, or as the ratio between the thickness and the conductivity. The ASR is directly proportional to the voltage loss (V) of the electrolyte at the current density i (A cm ), because V = ASR x i. The expression of the thickness to conductivity ratio indicates that high conductivity (electrolyte thickness (L) lead to a low cell resistance. 

Equation (2.28) is the second central equation for fnel cell performance since it is an equation that can be nsed in the assessment of degradation, generally defined as the change of area-specific resistance (ASR) with time [41]. 

In case of symmetrical cell, the electrode response corresponds to twice the polarization resistance Rp since there are two electrodes. When normalized to the surfece (S), one can extract the area-specific resistance (ASR) (i2 cm ) ... 

Figure 7.16. Area specific resistance (ASR) at 600°C for the impregnated electrode and the conventional LSC-SDC electrode upon thermal treatment (Zhao et aL, 2008a).

In order to improve the performance of SOFC, a thirmer yttria-stabilized zirconia (YSZ) electrolyte is considered for lower ohmic resistance and for operation in the intermediate temperature range of 500°C-800°C. Ionic conductivity decreases with decrease in temperature and hence the area-specific resistance (ASR) of an electrolyte increases with lower operating temperature. Fabricating the electrolyte in a dense and thinner film reduces the ASR or the resistance to ionic transport, allowing a lower operating temperature. For this purpose, efforts are being made in fabricating SOFC cell on the basis of either a thicker anode-supported or a thicker cathode-supported SOFC... 

Ohmic resistance is often written in terms of current density and area-specific resistance (ASR) as... 

---