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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4165

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Supplementary Information

Eliminating degradation in solid oxide electrochemical cells by reversible operation

Christopher Graves*, Sune Dalgaard Ebbesen, Søren Højgaard Jensen,

Søren Bredmose Simonsen, Mogens Bjerg Mogensen

Department of Energy Conversion and Storage, Technical University of Denmark,

Risø Campus, Frederiksborgvej 399, DK-4000 Roskilde, Denmark

*Correspondence to: [email protected]

Energy storage efficiency

Although reversible fuel cells are typically perceived as having a lower round-trip electricity storage

efficiency than conventional batteries, there is no inherent reason that this should be so, and there are

a number of redox storage chemistries that can be used in reversible fuel cells which match the near-

100% theoretical (thermodynamic) round-trip efficiency of conventional batteries. One such redox

chemistry is CH4 + 2 O2 ↔ CO2 + 2 H2O (see also the references in the main article), for which the

maximum theoretical round-trip efficiency assuming irreversible heat losses (∆G/∆H) is 90-100%.

Another high-efficiency redox chemistry is 2NH3 + 1.5 O2 ↔ N2 + 3 H2O, for which the maximum

roundtrip efficiency is 90-96%. In both cases the maximum efficiency is given as a range which

depends on the temperatures of operating and storage of chemicals (e.g. whether the H2O is stored as

steam or liquid water). Practical efficiencies are somewhat lower and depend on the overpotentials at

which the reversible fuel cell is operated and the system design. Also, even for lower-efficiency redox

chemistries such as H2O ↔ H2 + O2, in solid oxide electrochemical cells the inefficiency yields high

temperature (high value) heat, which can either be exported from the system and used, or stored and

re-utilized in the system to increase the efficiency.

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Capacity factors corresponding to experimental operating profiles

Charge-discharge cycles with 5 h in fuel-cell mode and 1.5 h in electrolysis mode and operating

points as in Fig. 2 would provide energy balancing with approximately 2 Wh cm-2 of storage (–1 A

cm-2 x 1.33 V x 1.5 h) and retrieval (0.5 A cm-2 x 0.84 V x 5 h) of electrical energy. The reversible

operating profiles are therefore relevant for energy balancing of intermittent power supplies such as

wind and solar power which have capacity factors of 20-40%. Note that the round-trip electric-to-

electric energy storage efficiency would be only 63% (0.84 V / 1.33 V) for these charge-discharge

cycles, assuming no energy was needed to obtain steam for electrolysis and neglecting inefficiencies

that would be present in a commercial system. However, as mentioned earlier, (i) the specific

operating profiles were used in this study to demonstrate the principle; other redox chemistries

besides H2O ↔ H2 + O2 can provide higher efficiency and can be stored with higher energy density

than hydrogen; and (ii) heat utilization/storage can improve efficiency.

Location of microstructural damage near the oxygen-electrode/electrolyte interface

The precise location near the oxygen-electrode/electrolyte interface where the microstructural damage

occurs seems to depend on many parameters of cell fabrication and operating conditions. For

example, whereas in prior studies 7,13 (see the main text for the references list) we observed porosity

within the solid electrolyte close to the interface, and in another case we observed deterioration both

within the electrolyte and right at the interface (Fig. S4) for a cell tested at harsh conditions, in the

present study the damage was only found right at the interface even though the same type of cell was

employed, which is likely due to the different operating conditions that were used in the long-term

electrolysis tests. Both locations have been reported in prior studies, which have used various

operating conditions, cell fabrication methods, and electrode and electrolyte materials (including the

same materials as used here). Clearly, differences in electrode and interface composition,

microstructure, sintering temperature, electrode geometry, operating temperature, oxidant gas

composition, and overpotential, lead to different local internal pressure and transport pathways for

oxygen evolution and thereby cause microstructure deterioration to occur within the electrolyte 7,13–





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15,18,21,22,27,31 and/or at the interface 22–32 (see the main text for the references list). Regardless of the

precise location, it is well established that high overpotential, and corresponding high pO2,int, drives

detrimental microstructural changes near the electrode/electrolyte interface during electrolysis

operation.

Microscopy characterization

After testing, the cells were examined by microscopy. Cross-sections for scanning electron

microscopy (SEM) were prepared by vacuum embedding pieces of the cells in epoxy followed by

grinding and polishing. A Zeiss Supra 35 FE-SEM was used for imaging. The samples were prepared

for transmission electron microscopy (TEM) by casting the samples in epoxy for stability and

successively thinning to a thickness of ca. 100 nm with a focused ion beam (FIB) by using a Zeiss

Crossbeam 1540xb. Transmission and dark field scanning transmission electron microscopy (STEM)

was carried out by using a JEOL 3000F equipped with a STEM unit and a high angle annular dark

field (HAADF) detector. For compositional analysis, energy dispersive spectroscopy (EDS) was

carried out by using an Oxford Instruments EDS detector.

Fig. S1: The same impedance spectra shown in Fig. 2b in the main article, now additionally showing the data measured at 418 h divided by 2.8 (both the real and imaginary impedance values) and with data below 20 Hz not shown. The number labels (n=0-5) above each impedance spectrum give the frequency in 0.9685 x 10n Hz.





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Fig. S2: Overview of one reversible cycling test, of which the segment starting at 2835 h is shown in Fig. 2a, showing current density i, cell voltage U, cell temperature T, and ohmic resistance R. The first and last measurements of ohmic resistance at 800 ºC and 750 ºC are circled to indicate the total change in ohmic resistance during the test. During open-circuit, fuel cell mode operation, and electrolysis mode operation at –0.5 A cm-2, ~25 L/h of a pH2/pH2O≈50/50 gas mixture was supplied to the fuel-electrode. During electrolysis mode operation at –1.0 A cm-2, ~13 L/h of a pH2/pH2O≈10/90 gas mixture was supplied. In all modes, 50 L/h O2 was supplied to the oxygen-electrode. During fuel-cell mode operation before ~400 h, gasses with a higher pH2/pH2O ratio were supplied to the fuel-electrode and air was supplied to the oxygen-electrode. Time starts (0 h, 20 February 2012 at 16:50) when the temperature reached 850 ºC after spending 5 h at 996 ºC during which NiO was reduced to Ni in the Ni-YSZ fuel-electrode. At the top are two zoomed views of current density and voltage measurements during the cycles in the final two segments.





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Fig. S3: Scanning electron micrographs imaged (a,c) with an in-lens detector at low-voltage (0.9 kV) and (b,d) with a secondary electron detector at high voltage (10 kV), of the two cells shown in Fig. 2: (a,b) the cell which was operated in constant current electrolysis mode and (c,d) the cell which was operated in reversible cycling mode. In (a) and (c) the light colored particles are percolating Ni and LSM. Comparing (a) and (c) does not show a large difference in percolation of Ni or LSM particles (light colored) in the electrodes, indicating no major loss of electronic pathways or reaction sites during the long-term electrolysis operation. Comparing (b) and (d) shows no significant microstructural differences. In (b), the slightly shaded area, which indicates a lower area fraction of solid phase in the LSM-YSZ electrode near the interface with the electrolyte, may be a weakening and separation/delamination of the interface (supported by the transmission electron micrographs shown in Fig. 3).





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Fig. S4: Scanning electron micrographs, which were imaged with a secondary electron detector at 15 kV, of polished cross-sections of different regions of the oxygen-electrode/electrolyte interface of a cell tested in electrolysis mode for a longer time period (>1000 h), at higher cell potential (up to 1.93 V), at higher current densities (up to 1.5 A/cm2), and at higher temperature (850 °C) than the cell described in Figs 2, 3, S1 and S3. These micrographs clearly illustrate both of the microstructural deterioration effects reported in literature that are caused by the high internal oxygen pressure degradation mechanism: (i) In all of the regions (a, b, c, and d), lateral porosity and cracks can be observed in the dense electrolyte ~2 μm from the interface; (ii) Delamination at the interface can also be observed, with ~0.5 to 1 μm sized voids in some regions (b and d) and ~4 μm void space in other regions (c). The black void space is epoxy that has been used for microscopy sample preparation. Note that if the electrode/electrolyte interface was completely delaminated like that shown in (c) across the entire cell, the cell could not be operated because no current can flow across such voids. Also, because weight was applied to the cell during operation, it is likely that the large voids formed during cell dismounting and microscopy sample preparation – during operation the interface region was likely only weakened, not delaminated to the extent shown in (c) and in some literature reports. All scale bars are 10 μm.

Fig. S5: Comparison of the change in the ohmic resistance during consecutive cycles comprised of 1 h electrolysis mode and either () 5 h in fuel-cell mode or () 5 h at open circuit. For comparison, the cycles with 5 h in electrolysis mode and 5 h in fuel-cell mode, as shown in Fig. 4, are also shown ().



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