size distributions of ceramic powders, and was used to study the effect of the synthesis conditions of LSM, on the particle size distribution of the product powder [97].
1.5 Reversible SOFC performance
The performance of reversible SOFC may be measured in a similar way to that of SOFC or SOEC. As these are electrochemical cells, one of the chief methods used to understand their performance is electrochemical impedance spectroscopy, which is also called ac im- pedance spectroscopy, or impedance measurements. In order to understand the processes occurring in a single electrode, symmetrical cells are used. Testing of the whole cell is also essential, and long term tests of the cell in fuel cell or electrolysis mode, reveal the electrical performance of the cell over time. Degradation mechanisms in the cell can then be explored, and optimisation of the cell to produce a high, durable performance.
1.5.1 Symmetrical cell testing
Symmetrical cells are often used to examine the LSM-YSZ oxygen electrode. A typical sym- metrical cell geometry is shown in fig. 1.5.1. The cells usually have a YSZ electrolyte sup- port of 1 mm - 2 mm thickness, and LSM-YSZ and LSM layers between 5µm - 30µm thick.
Figure 1.5.1: Symmetrical cell schematic - not to scale. Ac impedance is a method that applies a small ac signal,
of magnitude in the range 10 mV - 50 mV or mA root mean square (r.m.s) to a cell which is at equilibrium. This is suffi- cient to gather data without perturbing the cell equilibrium very much. If a sinusoidal voltage is applied, the current re- sponse is measured, and vice versa. From this, the complex impedance can be calculated. A very wide range of frequen- cies can be scanned stepwise, though for measurements on symmetrical cells and fuel cells, typically frequencies in the range: 10 mHz - 1 MHz are scanned. Below this range,
measurements take a long time, and above it, no useful data can generally be obtained because of inductive effects arising from the cell contact wires, or the frequency response analyser instrument used to record measurements. The impedance of a resistor (ZR) is
equation 1.5.1, and the impedance of a capacitor ZC is equation 1.5.2, where ω is the
angular frequency.
ZR=R (1.5.1)
ZC(ω) = 1
jωC (1.5.2)
Therefore, the impedance an RC circuit: i.e. a resistor in parallel with a capacitor, is equation 1.5.3. This gives a perfect semicircle on a Nyquist plot (−Zimg,Ω vs Zreal,Ω.)
Z(ω) =R− j
For an RC circuit, there is a characteristic time constant τ, defined by equation 1.5.4,
which shows how fast the process is occurring. Different electrochemical processes will have different time constants.
τ =RC (1.5.4)
These RC elements may be combined to make an electrical equivalent circuit, which is a representation of the electrochemical processes occurring in the cell, by electrical circuit elements. This circuit should also include resistor elements to account for the ohmic resistances in the cell electrodes and electrolyte. It is important that the elements in the electrical equivalent circuit are justified by comparison with experimental data, because many different equivalent circuits can be used to fit the same impedance data. In one study, two different impedance circuits were compared, and an analysis of the microstructure of the electrode showed that one was preferred to model the electrode [98].
Measurements can be made on cells such the one in fig. 1.5.1, by connecting a wire to each current collector and doing an impedance sweep, by doing measurements at several frequencies in each logarithmic frequency decade. However, in this case, the resistance of the wires leading to the cell will also be measured. Therefore, in order to examine the resistance of the cell alone, it is better to use four wires - two for applying the ac current signal, and a separate two for monitoring the voltage. The current and voltage wires on each side of the cell should be connected together as close as possible to the cell. The measured resistance will then only be that between the connection point on one side of the cell to the connection point on the other side.
Ac impedance testing on symmetrical cells with LSM-YSZ electrodes has been used to distinguish the limiting electrochemical processes in the cell. One study examined many impedance spectra from literature, and also performed ac impedance measurements on cells, and divided the arcs seen into five different types, named A-E. These were charac- terised by their capacitance, frequency at peak −Zimg, activation energy, and dependence
of polarisation resistance on PO2 [29]. Diffusion processes are often seen have have larger time constants, and so ac impedance may be used to investigate gas diffusion through porous support tubes [99]. The relationship between oxygen partial pressure and polar- isation resistance Rp is usually defined according to the equation 1.5.5, where x is an
experimentally determined coefficient.
logRp = logPO2−x (1.5.5)
For an LSM-YSZ electrode, a value of 0.5 was found forx, in thePO2 range of 1x10−4 atm.
to 0.2 atm. Two limiting processes were found for this electrode, a dominant process that occurred at low frequency, which was ascribed to a diffusion polarisation resistance related to oxygen absorption on the electrode surface. The other process was only significant when the electrode was under polarisation, and is not definitely assigned to a physical process, but is often assigned as an activation step of the oxygen reaction [100]. The effect of pressure on the impedance of LSM-YSZ electrodes at higher oxygen pressures has also been determined, for samples with a Sm doped CeO2interlayer between the LSM-YSZ and
1.5. REVERSIBLE SOFC PERFORMANCE 49 the YSZ electrolyte. The dominant limiting process had a dependence of the polarisation resistance on oxygen pressure of x = 0.25 according to equation 1.5.5, in the range 1 bar to nearly 100 bar. It was ascribed to charge transfer in the oxygen reduction reaction. The other limiting process was invariant withPO2, and was attributed to transport of oxygen
in the electrolyte [101]. Therefore, ac impedance measurements on symmetrical cells may be used to measure the magnitude, and possibly understand the physical nature, of the fundamental processes occurring in the electrode [102].
Impedance measurements may also be used to find the best fabrication parameters for SOFC: measurements on a range of symmetrical cells sintered at different temperatures showed that there was an optimum sintering temperature for LSM-YSZ electrodes. If the sintering temperature is too high, the TPB length and electrochemical performance are reduced. If the sintering temperature is too low, there is insufficient sintering of the LSM and YSZ particles to each other, and the ohmic resistance of the electrode increases, limiting the performance [103]. Ac impedance is able to detect small changes in microstructure, and a review of impedance data gathered on cells with different microstructure, shows completely different impedance spectra [104].
1.5.2 Whole cell testing
Testing of whole SOFC/SOEC is very important, to ascertain their electrical performance. This is accomplished with ac impedance measurements, and I/V curves. These can be measured at a series of fixed currents, or a series of fixed voltages. If the voltage is set, the current is measured, and vice versa. A check for hysteresis is often made to ensure that the cell is in a steady state, e.g. by incrementally increasing the voltage up to the maximum level, then decreasing it, and seeing if the currents are the same in both cases. The application of DC current across the cell has been observed to activate the LSM- YSZ electrode, reducing the manganese ions in the LSM, and improving cell performance [105, 106].
There are a variety of cell degradation mechanisms, which are different for the NiO/YSZ electrode, and the LSM-YSZ electrode. Firstly, the NiO/YSZ electrode, sintered when the cell is fabricated, must be reduced to the active fuel electrode Ni-YSZ, by the flow of hydrogen on the fuel side of the cell. This electrode is susceptible to degradation from a variety of mechanisms, as in table 1.5.1. Coarsening of Ni during testing was observed by two authors [22, 107], which leads to reduced TPB length, and also increased ohmic resistance in the cell. Nickel also oxidises at high steam concentrations, as the PO2 can
increase by 2-3 orders of magnitude as the steam concentration increases. During fuel cell operation, hydrogen is consumed, and steam is produced, so the local concentration of steam at the electrode surface may be very high. Ni is partially oxidised, and it was found that at a steam concentration of 40% in the system during electrolysis, irreversible damage occurs to the Ni/YSZ electrode. Redox cycling tests on Ni-YSZ electrodes showed that there was an irreversible linear expansion of the Ni-YSZ electrode, which caused cracking in the cell. This can be mitigated by optimising the Ni-YSZ microstructure. The best temperature to reduce the Ni was found to be 850 ◦C [108]. Redox cycling can cause
cracking that destroys the cell, if the effect is severe [109].
Table 1.5.1 also shows some degradation mechanisms of the LSM-YSZ electrode. Thermal cycling from 800 ◦C to room temperature, and then back to 800◦C, of an LSM-YSZ elec-
trode on a YSZ electrolyte, showed a significant increase in resistance from before and after the thermal cycle. This was assigned to a reduction in TPB length during the thermal cycle [110]. Another degradation mechanism in LSM-YSZ electrodes, is delamination from the YSZ electrolyte. This was observed to be a problem for a cell during a 200 h test at 800 ◦C, and was worse in electrolysis mode than fuel cell mode. It was thought to
be a result of oxygen being produced at the LSM-YSZ|YSZ interface, caused by the in- creased production of lanthanum and strontium zirconate phases, which lead to an oxygen build-up at the interface. The high pressure of the gas then causes delamination. An LSM- YSZ|YSZ|Ni-YSZ cell switched several times between electrolyser mode and fuel cell mode showed some irreversible increase in ohmic resistance, which was attributed to reduction of the YSZ electrolyte at the LSM-YSZ|YSZ interface during cell operation [111]. The key to preventing these degradation mechanisms is optimisation of the microstructure of the electrodes and the cell. However, new materials are also being investigated which may also help to prevent reversible SOFC degradation.
Degradation mechanism, Test conditions/ Performance electrode affected temperature (◦C) change/effects
Ni coarsening, Ni-YSZ [22] -0.3 A cm−2, 43% H
2O/H2/800 +3.2% RΩ/1000 h
Oxidation of Ni, Ni-YSZ [112] OCV, 10-40% H2O/H2/800 larger RΩ
Redox cycling, Ni-YSZ [108] 9% H2/Ar/700,850,1000 +5% expansion
Thermal cycling, LSM-YSZ [110] 0.1 V, electrolysis/800 larger Rp Delamination, LSM-YSZ|YSZ [113] 0. 25 A cm−2/800 larger R
Ω
Table 1.5.1: Degradation mechanisms of the Ni-YSZ and LSM-YSZ electrodes. RΩ =
ohmic resistance, Rp = polarisation resistance.
1.5.3 Reported reversible SOFC performance
In order to illustrate the key goals for commercialisation of fuel cells, table 1.5.2 summarises the goals of the U.S. SECA (Solid-State Energy Conversion Alliance), a U.S. government 10 year partner program with industry to develop various technologies, including fuel cells. The key factors for commercialisation of fuel cells are cost, performance, and durability, as was highlighted by a fuel cell company, Versa Power Systems in 2009 at the SECA annual meeting.[62]. Reversible SOFC have been shown to be capable of a high performance - one study showed a peak of -3.6 A cm−2 at a cell voltage of 1.48 V in electrolysis mode for a
cell tested at 950◦C [15]. Factors affecting the cell performance include the porosity of the
support - a performance improvement on one cell was seen when the porosity was graded in the support [114]. However, the performance in this case is less than that measured by other authors [115], which suggests that the cell microstructure is not optimised.
1.5. REVERSIBLE SOFC PERFORMANCE 51
Parameter Phase I Phase II Phase III Power rating (kW) 3-10 3-10 3-10
Cost per kW $800 $600 $400
Efficiency (stationary) 35-55% 40-60% 40-60% Availability over 1500 h 80% 85% 90% Power drop per 500 h, constant V ≤2% ≤1% ≤0.1%
Table 1.5.2: Solid State Energy Conversion Alliance (SECA) selected targets (US D.O.E. program) [116]. Some of the solid oxide fuel cell manufacturers taking part in the SECA program are; Delphi, Rolls-Royce Fuel Cell Systems, Siemens, United Technologies and Versa Power Systems [16].
A key performance metric for reversible SOFC is their ability to reversibly store energy at high efficiency. This was discussed in section 1.2. Several different systems have been investigated to this end. One early attempt used planar SOFC, and hydrogen gas as an energy storage medium. A 0.075 m2 cell was demonstrated, which had a resistance of
0.48Ω cm−2 [117]. Another system based on SOFC made by Risoe National Laboratory, Denmark, showed a high performance in fuel cell and electrolysis mode. The improvement of electrical efficiency in electrolysis mode by utilising waste heat from nuclear power stations, or geothermal sources was discussed [15]. Another intermediate temperature reversible SOFC system, which was lanthanum gallate electrolyte supported, showed an ohmic resistance of 1 Ωcm2 at 800 ◦C, in both fuel cell and electrolyser modes. In fuel electrode supported cells, low porosity in the support limited the cell performance [118].
A larger scale system used LSM-YSZ electrodes on YSZ, and found delamination at current densities of 0.5 A cm−2 after a few days operation. This problem was not found for
LSCoF perovskites. It was also seen that the oxygen electrode polarisation was dominated by the activation overpotential. A cost model of a system running alternately in fuel cell mode and electrolysis mode, to use cheap electricity at night for electrolysis and produce electricity during the day, showed that this was only economical if there was a differential of at least 0.15-0.20 $ kW hr−1 between the daytime and nighttime prices. The storage of
heat from the fuel cell cycle to the electrolysis cell cycle was not considered [113].
Another proposed reversible SOFC system which aims to improve electrical-chemical- electrical round trip energy storage efficiency, examined doing so by replacing some of the hydrogen with methane. Thermodynamic calculations showed that little heat is generated or consumed when the cell is cycled between CO2-H2O rich and CH4-H2 rich gases. There-
fore, only a little energy is lost as heat in the electricity-methane-electricity conversion. In order to avoid coking, the system should be operated at increased pressure (∼10 bar) and
a reduced temperature of 600◦C [119]. In order to do this, different materials should be
used, as the resistivity of YSZ and LSM-YSZ are too high at lower temperatures.
A reversible SOFC cell made of LSM-YSZ|YSZ|Ni-YSZ running on hydrogen and car- bon monoxide has also been demonstrated, and the reaction mechanisms in the cell were
examined. Performance was limited by the electrolyte resistance, and charge transfer in the oxygen electrode [120]. A reversible microtubular SOFC has been demonstrated achieves power densities of 0.8 W cm−2 in fuel cell and 1.3 W cm−2 in electrolyser mode. Very high
current densities were achieved, of 6 A cm−2 in electrolysis mode, which were attributed
to electroreduction of the YSZ [121].
Proton conducting reversible fuel cells have also been investigated, and much better per- formance was obtained in electrolysis mode, comparable to some YSZ electrolyte systems, than in fuel cell mode [122]. However, the PEM-FC systems suffer from large activation voltages, which reduces their efficiency. Finally, previous research at the University of St Andrews demonstrated the concept of a tubular reversible SOFC, but power output was limited to the low mW range due to oxidation of the steel current collector during sealing [123].
While there has been some work on pressurised SOFC, notably by Siemens-Westinghouse [18], and Rolls-Royce Fuel Cell Systems are proposing a pressurised SOFC-hybrid gas tur- bine system, there has been little or no research on pressurised reversible SOFC.