SOFC Performance

As mentioned earher, with hydrogen as the fuel, the OCV of SOFCs is lower than that of MCFCs and PAFCs (see discussion in Section 7.2). However, the higher operating temperature of SOFCs reduces polarisation at the cathode. So, the voltage losses in SOFCs are governed mainly by ohmic losses in the cell components, including those associated with current collection. The contribution to ohmic polarisation in a tubular cell is typically some 45% from cathode, 18% from the anode, 12% from the electrolyte, and 25% from the interconnect, when these components have thickness of 2.2, 0.1, 0.04 and 0.085 mm, respectively, and resistivities at 1000°C of 0.013, 3 x 10 10, and 1 ohm cm, respec- 

like all fuel cell types, show an enhanced performance with increasing ceU pressure. Unlike low- and medium-temperature cells, the improvement is mainly due to the increase in the Nemst potential. We showed in Section 2.5.4 that the voltage change for an increase in pressure from Pi to P2 follows very closely the theoretical equation 

The temperature of an SOFC has a very marked effect on its performance, though the details will vary greatly between cell types and materials used. The predominant effect is that higher temperatures increase the conductivity of the materials, and this reduces the ohmic losses within the cell. As we saw in Chapter 3, ohmic losses are the most important type of loss in the SOFC. 

As was mentioned in the section on cell interconnects, one of the main advantages of operating at lower temperatures is the possibility of using cheaper construction materials and methods. Making electrolytes and electrodes that work well at lower temperatures is a major focus of current SOFC research. 

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Figure 8-14 SOFC Performance at 1000°C and 350 mA/cm, 85% Fuel Utilizatiou aud 25% Air Utilization (Fuel = Simulated Air-Blown Coal Gas Containing 5000 ppm NH3,...

D.B. Stauffer, R.R. Engleman Jr., J.S. White, J.H. Hirschenhofer, "An ASPEN/SP SOFC Performance User Block," DOE Contract DE-AC21-88-FE-61684, Task 14, September 1993. 

The catalytic activity of the anode toward oxidation reactions is a dominant factor in determining SOFC performance, particularly with hydrocarbon fuels. In Cu—cermet anodes, the only role played by Cu is that of electronic conductor. The Cu does not appear to have any catalytic function, and the oxidation reaction in the TPB relies on the addition of other components, primarily ceria. The evidence for this is as follows. First, Cu—YSZ anodes that do not contain ceria exhibit very low performance, even though they are stable in hydrocarbon fuels. Second, substitution of Cu with Au has essentially no effect on anode performance. Since Au is usually thought to be catalytically inert, it seems unlikely that Cu and Au would perform in a similar manner if Cu had a catalytic function. 

SOFC performance largely depends on the choice of the electrode material. Selection... 

The material and structural design of the electrolyte, anode, and cathode is still the primary challenge for improving SOFC performance. The ultimate goal is to achieve high efficiency and a long performance lifetime, while operating at lower... 

Preparation of mixed metal oxide precursors for SOFCs represents a very complex chemical process in which a metal may form oxides, hydroxides, and various complex basic salts as intermediates. Understanding of the relationship between the calcination process, the final composition, particle sizes, sinterability, and SOFC performance for nickel, copper and cobalt-based anode materials is a necessity [26]. 

This is followed in Section 26.4 by a discussion of mesoscale modehng of the SOFC electrodes in which the SOFC electrodes are explicitly resolved and the detailed reactive transport and electrochemistry is modeled. Section 26.5 briefly describes nanoscale approaches for modeling the transport and reactions of species in the SOFCs, which are suitable for elucidating kinetic and mechanistic issues relevant to SOFC performance. 

The inclusion of Knudsen diffusion in simulations of SOFCs can have a significant effect on the transport in the pore space of the electrodes [17] and should be included in macroscale models to ensure an accurate simulation of the SOFC performance. 

Recently, several groups have taken cell-level macroscale models a step further to investigate the electrochemistry through the thickness of the electrodes using the mesoscale electrochemistry approach [19, 27, 31]. In these models, no assumptions are made about a reactive zone for the electrochemical reactions instead, the electrochemistry is modeled through the thickness of the electrodes based on a mesoscale electrochemistry approach (Section 26.2.4.2) in which the explicit charge-transfer reactions [27] or a modified Butler-Volmer approach [19, 31] are modeled. This extends the effects of the electrochemical reactions away from the electrolyte interface into the electrodes. In these cell-level models, the electrochemistry is coupled to the local species concentrations, pressures, and temperatures, and provides a more detailed view into the local conditions within the fuel cell and how these local conditions affect the overall SOFC performance. 

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 replacement of traditional ceramic intercoimects with metallic components to create SOFC stacks offers significant improvements in SOFC performance along with reduced cost. However, understanding the degradation processes in terms of corrosion and chromium migration is critical for developing cost effective SOFCs. Raman spectroscopy is particularly sensitive to the relevant oxides and corrosion processes. 

Almost all studies regarding direct hydrocarbon SOFCs show comparatively poor performance (lower OCP and higher polarization resistance) with hydrocarbon fuels when compared to H2 fuel, Fig. 3.2. Since most of these tests are performed by switching fuel on the same cell, the drop in performance must be linked to the anode. It is possible that the increased polarization resistance may be due to lower diffusivity of the hydrocarbon fuels, but the electrodes are typically highly porous and the current density per unit area is relatively low. In addition, the oxidation of 1 mole of hydrocarbon fuel yields a significantly greater number of electrons than 1 mole of H2 fuel (H2, CH4, and C4H10 total oxidation yield 2, 8, and 26 moles of electrons, respectively). Furthermore, the cell OCP is an equilibrium, zero current, measurement and is therefore not directly influenced by gas diffusivity. Therefore, it is unlikely that gas diffusivity limits the performance for pure fuels at low conversion. The conclusion must then be that the anode electrocatalytic activity toward hydrocarbon oxidation is the primary factor in reduced SOFC performance. 

Fig. 3.8 SOFC performance curves for cells with Cu-ceria composite anode. The cell had a 60-pm electrolyte, and data are shown for the following fuels filled circles, n-butane at 700°C open circles, n-butane at 800°C filled triangles, H2 at 700°C and open triangles, H2 at 800°C (Reprinted by permission from Macmillan Publishers, Ltd Nature [43], copyright 2000)...

While Cu can be utilized as a catalyst, it does not contribute significantly to the overall catalytic activity of the anode. This was verified by the low performance of Cu-only anodes, particularly in hydrocarbon fuels [22], and the identical performance achieved when Cu is replaced with catalytically inert bulk Au [41]. The role of Ce02 as electrocatalyst for fuel oxidation was confirmed by replacing Ce02 with other lanthanide oxides and comparing SOFC performance with the activity of the lanthanide toward fuel oxidation [22]. The ceU performance tracked well with the M-C4H10 light-off temperature of the lanthanide. 

Using a similar method. Kin et al. obtained a power output of 500 mW/cm in humidified H2 at 700°C utilizing YSZ/LSCM anodes with 0.5-1 wt% of either Pd, Rh or Ni added as a separate phase. Fig. 3.15. These values are to be compared to the power output of 100 mW/cm for YSZ/LSCM without metals added. The addition of 1 wt% Fe to YSZ/LSCM also increased power output to 400 mW/cm [34]. An enhancement in SOFC performance with the addition of Pd was fotmd by other researchers as well, but not to the extent of a fivefold increase [53]. The smaller impact may be due to other limiting processes, such as low ionic conductivity. 

Simner SP, Anderson MD, Xia G-G, Yang Z, Pederson LR, Stevenson JW (2005) SOFC performance with Fe-Cr-Mn alloy interconnect. J Electrochem Soc 152(4) A740-A745... 

A further study [81] examined the development of internal stress as a function of temperature for an anode-supported cell based on a Sc stabilized zirconia (ScSZ) electrolyte with a NiO-3 mol%-YSZ anode, finding that there were significant changes in the internal stress in both NiO and ScSZ that were not fully reversible, Fig. 19.10. Residual stress in the electrolyte was measured as a function of the reduction cycle and found to be 400 MPa in air, but decreased to 200 MPa on reduction of the NiO, and concluded that this redox cycle would be detrimental to SOFC performance. 

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