Performance of Ferrite‐Based Electrodes for Steam Electrolysis in Symmetrical Solid Oxide Cells

There is an increasing interest in the exploration of non‐Nickel cathode materials for steam electrolysis in solid oxide electrolysis cells (SOEC) for green hydrogen production with high Faradaic efficiencies. Ferrite‐based ceramic materials have drawn a lot of attention in this regard due to their appreciable mixed ionic electronic conductivity. This work aims to explore a ferrite‐based mixed ionic electronic conductor electrode for symmetrical SOEC that can contribute significantly to simplifying the manufacturing processes. A composite of silver (Ag) and A‐site deficient lanthanum strontium cobalt ferrite ((La0.60Sr0.40)0.95Co0.20Fe0.80O3‐x), is studied for steam electrolysis in a yttria stabilized zirconia electrolyte‐supported symmetrical tubular solid oxide cell. A considerable current density of 250 mA cm−2 is obtained at 1.5 V and 800 °C in a Helium‐Steam atmosphere (50% humidified) with a corresponding polarization resistance as low as 0.15 Ω‐cm2. The polarization resistance is comparable to a number of electrodes reported in the literature for steam electrolysis. However, a 10% drop in current density is observed during the first 20 h of electrolysis at 1.5 V and 800 °C in a Helium‐Steam atmosphere (50% humidified), but no further drop is encountered during the next 46 h of continuous operation.


Introduction
The integration of renewable energy with numerous energy conversion and storage devices is increasing worldwide for long-term energy storage.Solid oxide electrolysis cells (SOEC) are efficient devices that can convert energy to fuels such as H 2 and syngas.Hydrogen can be used in fuel cells, hydrogen generators, and in multiple value-added products such as ammonia, methane, methanol, gasoline, dimethyl ether (DME), etc. Advantageously, DOI: 10.1002/admi.202400001waste heat from the downstream of synthesis processes can be utilized in SOECs, boosting the energy efficiency of the system.The potential of this technology in terms of efficiency, scalability, and viability is attractive to industry as kW to MW scale units are already demonstrated for renewable hydrogen production. [1]rincipally, SOEC operation is the reverse of commercialized solid oxide fuel cell technology (SOFC), and enormous research has been done on the optimization of various cell and stack components such as anode, cathode, electrolyte, and interconnects.Conventional Nickel-Yttria stabilized Zirconia (Ni-YSZ) anodes of SOFCs have also been evaluated for high-temperature solid oxide water electrolysis applications. [2]The Ni-YSZ-based cathode-supported cells with a thin micron size Yttria stabilized Zirconia electrolyte layers have demonstrated high current densities (> 3A cm −2 at <1.6 V) which are in the range of the technical parameters set by the Department of Energy, USA (DOE). [3]In electrode-supported cells the cathode and the electrolyte can be co-sintered reducing fabrication time and cost.However, one of the challenges with traditional Nibased electrodes is that these materials require a continuous supply of reducing gas such as H 2 to avoid oxidation of Ni to NiO resulting in overall energy efficiency loss since at least 10% hydrogen needs to be recycled continuously. [2,4]To eliminate need for hydrogen recirculation, redox stable ceramic electrodes including a number of perovskites and fluorites have been explored, which do not require hydrogen recirculation. [5]5a-g] In our earlier studies, we evaluated Lanthanum Strontium Manganate (LSM) and Gadolinium doped Ceria (GDC) based composites for CO 2 electrolysis which showed polarization resistance ≈0.5 Ω-cm 2 at 1.5 V at 800 °C.We found an enhanced stability provided by Gadolinium doped ceria to the electrode for CO 2 electrolysis. [6]5g,7] The ferrite perovskites have also been evaluated as an anode in extreme reducing condition such as for methane-fueled SOFC where majority of host lattice phase is retained even after 400 h of cell operation. [8]Yang et al., [9] evaluated lanthanum ferrite electrodes for CO 2 electrolysis in unsymmetrical cell configuration.Polarization resistance as low as 0.26 Ω-cm 2 was recorded at 1.5 V at 800 °C.Recently, Marasi et al. [10] tested 5 mol% Ru doped lanthanum strontium ferrite (LSF) as symmetrical electrodes for CO 2 electrolysis with 300 μm La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3- (LSGM) electrolyte for reversible solid oxide cell operation at 850 °C.In H 2 -fuelled fuel cell mode, power density was 602 mW cm −2 and for CO 2 electrolysis current density was 1.39 A cm −2 at 1.5 V. Cell showed no degradation over 200 h of reversible operation in 50/50 CO 2 /CO environments.
An advantage with ferrite-based perovskites is that these electrodes can be used both as anode and cathode (symmetrical cell), however, there are limited studies available on the stability of ferrite-based perovskites as fuel electrodes for solid oxide applications.In this work, the electrocatalytic activity of A-site deficient (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x (LSCF) perovskite electrode has been evaluated for steam electrolysis in tubular solid oxide cells.The purpose of this study is to evaluate this versatile material in a symmetrical configuration, which could provide manufacturing flexibility and reduce the fabrication cost of SOEC.Low electrode polarization resistance of ≈0.15 ohm cm 2 at 800 °C was obtained with robust electrolyte-supported tubular solid oxide cell and performance is comparable with a number of cathodes reported for steam electrolysis in literature.The long-term stability of these electrodes has been evaluated in air atmosphere and for steam electrolysis on scalable tubular solid oxide cells.
There are few studies on the evaluation of ferrite-based electrolysis for steam electrolysis, and stability of electrodes have been mostly evaluated for CO 2 electrolysis without insights on the degradation mechanism.7a,11] In this work, further insights have been provided by conducting both physical and electrochemical characterization to determine the prolonged stability of such electrodes in a partially reducing environment for steam electrolysis.

Electrochemical Performance of LSCF-Ag Electrode Based Symmetrical SOEC
Solid oxide electrolysis was performed in mixture of Helium-Steam (50% relative humidity) at the cathode side and air at the anode side at 800 °C.As shown in Figure 1a, current density up to 338 mA cm −2 was measured at an applied potential of 1.6 V at 800 °C.Further increasing the SOEC operation temperature to 875 °C, a similar current density was obtained only at 1.4 V due to accelerated kinetics of the electrodes and increased ionic conductivity of the YSZ electrolyte at high temperature.Notably high current densities have been obtained with this ferrite-based symmetrical electrodes specifically on electrolyte-supported SOECs (500 μm thick).The electrolyte-supported structure provides better mechanical strength especially, in case of SOEC integration with renewable energy resources which is intermittent in nature.
Recently, Deka et al. [12] evaluated Ni and Co-doped La 0.7 Sr 0.2 FeO 3 catalysts for CO 2 and H 2 O co-electrolysis in an unsymmetrical SOEC.They reported electronic conductivity of doped ferrites from 80 to 163 S cm −1 at 800 °C in 40%CO 2 +3%H 2 O/He environment.Specifically, for La 0.7 Sr 0.2 Co 0.2 Fe 0.8 O 3 , it was measured to be 163 S cm −1 in the same conditions.However, long-term stability of such electrodes still needs to be evaluated.In air, electronic conductivity of ferrites is reported higher >200S cm −1 at 800 °C. [13]s fuel environment in the present study is almost similar to that reported by Deka et al. [12] high current densities in the present SOEC were obtained due to 1) mixed ionic and electronic conductivity provided by LSCF and Ag phase and 2) catalytic activity by LSCF phase only.Previous studies using Ag [5c,7c,14] electrodes have shown that Ag has negligible catalytic activity for steam or CO 2 splitting and it mainly provides electronic conductivity to the electrodes.
Figure 1b shows AC impedance spectra of SOEC exposed to humidified Helium-Steam (50% relative humidity) under different potentials at 800 °C.The ohmic resistance as seen from high frequency x-axis intercept is 1.70 Ω-cm 2 which is slightly higher than a calculated value of 1.22 Ω-cm 2 in accordance with thickness of electrolyte considering electrolyte conductivity of 0.045 S cm −1 at 800 °C.The difference may be attributed to contribution from contact resistance between electrodes, electrolyte, and current collectors.
The polarization resistance determined from difference between low and high frequency intercept decreased to 0.15 Ω-cm 2 with increase in applied potential from OCV to 1.5 V.This is in accordance with previous studies that higher applied potential facilities the kinetics of the electrodes and reduces charge transfer resistance as per Butler-Volmer relationship.The impedance spectra consisted of two distinct arcs.The first one is the high frequency (HF) arc that relates to charge transfer processes and the second one is the low-frequency arc (LF) that involves mass transfer process like diffusion and adsorption/desorption of reactant/product species.Both the arcs got depressed with increasing applied potential, indicating enhanced cell kinetics.As can be seen, the LF arc at OCV dominated overall polarization resistance and depicted a typical Warburg type impedance, indicating very high resistance to mass transfer processes.Interestingly, the contribution of the LF arc to overall polarization resistance decreased with potential (from 85% at OCV to 20% at 1.5 V), reflecting improved mass transfer kinetics.LSCF being an excellent mixed ionic electronic conductor develops a high concentration of oxygen vacancies at higher potentials and elevated temperatures in a complete or partially reducing atmosphere. [15]The synergistic effect helps reduce the resistance under higher applied voltage.
In terms of overall area-specific resistance (ohmic + polarization), it is to be noted that ohmic resistance was the dominating parameter, contributing more than 90% of the overall resistance.This agrees well with the monotonically increasing nature of the V-I curve with a single slope throughout the entire potential range (Figure 1a).5a-c,7b,16]

Long-Term Stability Testing of LSCF-Ag During Steam Electrolysis
To evaluate the stability of LSCF-Ag electrode for steam electrolysis, SOEC was operated at 1.5 V in Helium-Steam (50% relative humidity) at 800 °C as shown in Figure 2a.Current density slightly dropped from 250 to 225 mA cm −2 during the first 20 h of operation, but then remained reasonably stable at 225 mA cm −2 for the next 46 h.
In some studies, delamination at anode/electrolyte interface, due to increased oxygen partial pressure, is reported as a cause of deterioration in performance of SOEC at low current densities.This issue is well reported in literature but mostly at high current densities exceeding 1.5 A cm −2 . [17]To investigate the stability of our anode (LSCF-Ag), we operated a similar symmetrical SOEC in air at 500 mA cm −2 at 800 °C to determine the impact of high current densities on anode/electrolyte interface under the current cell configuration (Figure 2b).The initial voltage was measured to be 0.78 V that dropped down to 0.73 V after 66 h of cell operation during electrolysis.This remarkable stability of LSCF-Ag in air at such a high current density confirmed that there was no air-side detrimental contribution to the performance of our cell for steam electrolysis.It also indicated that electrode coarsening under these operating conditions in air had minimal effect on the performance of our cell.
For Helium-Steam electrolysis at 1.5 V and 800 0 C, no significant changes in the ohmic resistance were seen during 66 h of operation.However, polarization resistance increased from 0.75 Ω-cm 2 after 30 min of continuous operation to 1.35 Ω-cm 2 after 66 h of testing (Figure 2c).Based on the excellent stability results of LSCF-Ag in air (Figure 2b), the observed increase in polarization resistance during electrolysisunder steam environment can be attributed to the cathode.Both the HF and LF arcs increased significantly during the 66 h of operation (Figure 2c). at 800 °C at different magnifications.The electrode depicts appreciable micro-porosity with pores above 0.2 μm in size (Figure 3a-c).LSCF particles were mostly spherical and elliptical in shape with size ranging from 0.2 to 0.6 μm.Ag particles appeared in different shapes, varying in size between 1.0 and 1.5 μm.As seen from the EDS maps (Figure 3d), LSCF phase was uniformly distributed throughout the electrode, whereas Ag particles showed a sporadic dispersion.Figure 4 shows SEM and EDS of tested samples in steam atmosphere.Ag agglomeration was observed in the tested samples in steam electrolysis.However, no significant increase in the ohmic resistance was recorded during the cell operation since Ag content is relatively high in the LSCF-Ag composite (70 wt.%, which is ≈50 vol%), and the electronic conductivity of Ag (6.3 × 10 7 S m −1 ) is several orders of magnitude higher than LSCF (2.5 × 10 4 S m −1 ). [18]In metal-ceramic composites we have observed that electronic and ionic conductivity of mixed oxide needs to be high to compensate the conductivity loss due to agglomeration of metal particles.

SEM and XRD Characterization of Fresh and Tested Cell
Figure 5a shows X-ray diffraction pattern of fresh (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x )-Ag composite electrode after heat treatment in air at 850 °C.A cubic perovskite phase based on Sr 0.60 La 0.40 Co 0.20 Fe 0.80 O 2.9 (lattice parameter 0.3884 nm) was the major phase along with a cubic silver (Ag) phase (lattice parameter 0.4087 nm).Traces of Sr 1 . 1 La n .9 FeO 4 and La 2 O 3 (<0.5%)were also present in A-site deficient (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x phase.The X-ray diffractograms of the top surface of LSCF-Ag cathode as well as cathode-electrolyte interface of the cell tested at 800 and 875 °C in solid oxide electrolysis mode is given in Figure 5b,c, respectively.The cathode-electrolyte interface reflected peaks from cubic Ag, rhombohedral Sr 0.4 La 0.6 FeO 2.8 with traces of cubic Fe 3.5 Co 0.5 phase.Similarly, the cathode top surface depicted peaks corresponded to cubic Ag and rhombohedral Sr 0.4 La 0.6 FeO 2.8 phases with traces of cubic Fe 3.5 Co 0.5 and CoO phases, confirming that partial decomposition of LSCF takes place in Helium-steam under current operating conditions.Unlike fresh Ag-LSCF electrode, the X-ray diffractogram of the surface of tested LSCF-Ag cathode showed evidence of trace amount of CoO phase.Formation of such non-conducting and catalytically inactive CoO phase can limit charge-transfer processes over time, thus posing higher resistance to steam splitting reaction.This could be a reason for the increased polarization resistance observed after 66 h long-term testing of steam electrolysis, especially the inflation of the HF arc pertaining to charge-transfer processes.
There are reports on LSCF phase change in strong reducing conditions.For example, Wang et al. [19] reported increase in lattice parameters of rhombohedral LSCF with decreasing oxygen partial pressure.Thus, for a better understanding, LSCF-Ag (1:1 wt.%) composite was externally reduced in 5%H 2 -Ar at 800 °C. Figure 5d shows the XRD spectra of LSCF-Ag after reduction in 5%H 2 -Ar for 8 h.The increase in lattice parameter of LSCF from 0.3884 (±0.0001) nm in air to 0.3918 (±0.0001) nm after reduction in 5%H 2 -Ar can be attributed to the partial reduction of Fe 4+ (0.0585 nm) to Fe 3+ (0.0645 nm) and Co 2+ (0.07 nm) to Co (0.125 nm).This is also visible from the peak shift of LSCF phase to a lower Bragg's angle in case of the sample reduced in 5%H 2 -Ar at 800 °C (Figure 5a).Additional peaks of Fe 0.9 Co 0.1 (1.6%±0.3%)and Sr 1.1 La 0.9 FeO 4 (7.3%±0.4%)were observed in the spectra.The exsolution of Fe 3.5 Co 0.5 phase has been previously reported to be active for electrochemical performance in solid oxide cell. [20]Though LaSrFeO 4 has lower electronic conductivity that could affect the performance of SOEC.Table 1 provides the summary of the XRD characterization of fresh and tested LSCF-Ag cathode.
Furthermore, surface cation segregation has been realized in most of the perovskites at elevated temperatures>600 °C which can deteriorate the SOE performance.On the other hand, it has been realized that A site cation deficient perovskite show higher chemical stability upon thermal annealing and reduce the extent of cation segregation at the surface and hence reduce the formation of secondary phases. [21]No performance deterioration has been noticed in oxygen environment, though the formation of strontium-rich secondary phases such as Sr 1.1 La 0.9 FeO 4 can be present in reducing atmosphere which can lead to performance degradation over the period.A possibility of cation surface segregation has also been reported even in A site deficient ferrite-based perovskites (La 0.6 Sr 0.4 ) 1-x Co 0.2 Fe 0.6 Nb 0.2 O 3- (x = 0, 0.05, 0.10) evaluated for CO 2 electrolysis by Niu et al. [22] On the other hand high catalytic activity and surface oxygen exchange of LSF phase in the presence of LaSrFeO 4 has been observed for N 2 O decomposition, methane combustion and ammonia oxidation reaction. [23]20a] This could be the reason for prolonged stability of LSCF-Ag electrodes specially in the presence of Ag conductive phase.In addition, partially reducing conditions in the presence of steam could have further avoided the complete decomposition of this phase.

Conclusion
The present study advances the understanding of ferritebased electrodes for steam electrolysis.A-site deficient (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x electrode was successfully evaluated for steam electrolysis in an electrolyte-supported symmetrical tubularLSCF-Ag/YSZ/LSCF-Ag solid oxide cell with GDC interlayer.A considerable current density of 250 mA cm −2 was obtained at 1.5 V and 800 °C in a Helium-Steam atmosphere (50% humidified) that gradually dropped to 225 over the first 20 h of operation and then remained stable for the next 46 h.Simultaneous analysis of the V-I curves and impedance spectra revealed that the contribution of ohmic resistance was >90% of the total area-specific resistance, which limited the overall performance of the cell.A polarization resistance as low as 0.15 Ω-cm 2 was observed for fresh cell operated at 1.5 V and 800 °C, which is comparable to most of the perovskites available in the literature.However, it increased to 1.35 Ω-cm 2 after 66 h of steam electrolysis at 1.5 V and 800 °C.An identical cell tested at 800 °C in dry air was found to be stable for 66 h even at 500 mA cm −2 .This confirmed that the increase in polarization resistance envisaged for steam electrolysis came from the cathode side, which could be due to structural changes of LSCF perovskite phase to LSF phase as seen from XRD.Thus, the stability of ferrite-based electrodes in scalable cells or at high current densities or in a more reducing atmosphere needs to be further evaluated.

Experimental Section
Tubular Solid Oxide Cell Fabrication: Closed-ended yttria stabilized zirconia tubes were fabricated using isostatic pressing of 8 mol% yttria stabilized zirconia powder (purchased from Tosoh) at 170 MPa followed by sintering at 1500 °C.The tubes were roughened using 600 Grit sandpaper and cleaned thoroughly with ethanol.The tubes were then leak tested up to 5 bar using compressed air.The wall thickness and inner diameter of the tube were measured to be 0.55 and 9.37 mm.
As LSCF has a tendency to react with zirconia at high temperatures ≥ 900 °C or under applied potential, ceria nanolayers are used to reduce the probability of La 2 Zr 2 O 7 zirconate phase formation and extend the cell stability for a longer period of time. [24]In the present work, gadolinia doped ceria (Gd 0.1 Ce 0.9 O 1.9 ) interlayer was coated inside and outside of the YSZ electrolyte tube using commercial Fiaxel ink.The heat treatment of the interlayer was performed at 1200 °C for 2 h using heating and cooling ramp rates of 180 0 C h −1 .
The slurry for the cathode (fuel electrode) and anode (oxygen electrode) was prepared by mixing (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x (purchased from Fuel Cell Materials having surface area 10-14 m 2 gm −1 ) and Ag powder (purchased from Fisher Scientific having surface area 0.8-1.2m 2 gm −1 ) in a 35:65 wt.ratio with terpinol (Fuel Cell Materials) based ink vehicle and ball milled for 2 h in a Planetary Ball Mill.The symmetrical cell was prepared by coating the cathode and anode inside and outside of the tube, followed by heat treatment at 825 °C for 2 h in air.The active area of both the electrodes was measured to be ≈3 cm 2 .Silver wires of ≈1 mm diameter were used for current collection inside and outside the tube.
Physical Characterization: The perovskite phase of the as-prepared and tested samples was identified with the ICDD-JCPDS powder diffraction database using the Bruker XRD search match program EVAv5.The spectra were measured from 20-80 °with scan rate of 1°min −1 .Rietveld analysis was performed on the data using the Bruker TOPAS V5 program to calculate the lattice parameters.Lattice parameters, vertical sample displacement, peak full width at half maximum, and scale factor were all refined.Error ranges were calculated on the basis of three estimated standard deviations as calculated by TOPAS.Error values in the peaks table were calculated on the basis of a diffractometer resolution of 0.005.
The electrodes microstructure was analyzed using Merlin Benchtop scanning electron microscope (SEM) and elemental dispersive X-ray (EDX) from Zeiss, USA.
8a] The fixture was mounted inside a horizontal furnace and heated in 99.9% purity He (British Oxygen Company, BOC) to 800 °C at a heating rate of 180 °C h −1 .Steam at 50 °C (50% relative humidity) was then injected into the cathode chamber.
The current-voltage characteristics of the tubular solid oxide cell were recorded using a scan rate of 2.5 mV s −1 with a Zahner IM6e Electrochemical station (Zahner Inc., Germany).Electrochemical impedance spectra (EIS) were recorded from the frequency range of 100 KHz to 10 mHz with an amplitude of 20 mV with and without bias.
The long-term stability of SOEC was determined by recording chronoamperometry at 1.5 V. EIS was recorded to determine the changes in ohmic and electrode polarization resistance of the SOEC.evaluate the contribution of air electrode on SOEC performance during electrolysis, the stability of the cell was studied for the same amount of time while operating the cell in air on both anode and cathode at 500 mA cm −2 at 800 °C.

Figure 1 .
Figure 1.Current-voltage characteristics of LSCF-Ag electrode in symmetrical SOEC recorded in Helium-steam (50% relative humidity) fuel environment at 800 °C a) and corresponding electrochemical impedance spectroscopy recorded at OCV and 1.5 V b).Inset in (b) shows a magnified impedance spectra at 1.5 V.

Figure 2 .
Figure 2. a) Chronoamperometry of symmetrical SOEC with LSCF-Ag electrodes at 1.5 V in Helium-steam (50% humidified) environment at 800 °C.b)Chronoamperometry data of an identical SOEC operated under a current density of 500 mA cm −2 in dry air at 800 °C c) and electrochemical impedance spectra of a symmetric SOEC with LSCF-Ag electrodes tested at 1.5 V in Helium-steam at 800 °C after 30 min and 66 h of continuous operation.
Figure 3 shows SEM images of as-prepared (La 0.60 Sr 0.40 ) 0.95 Co 0.20 Fe 0.80 O 3-x -Ag electrode after heat treatment

Figure 4 .
Figure 4. Scanning electron microscopy LSCF-Ag cathode tested at 800 °C for steam electrolysis at a) 1000X resolution, b) 10 000X resolution, and c) 20 000X resolution.d) EDS maps of the same cathode surface scanned at 5000X resolution.

Figure 5 .
Figure 5. X-ray diffraction pattern of a) fresh LSCF-Ag composite electrode after heat in air at 850 °C b) top surface of LSCF-Ag cathode, c) cathode-electrolyte interface of the cell tested in solid electrolysis mode in Helium-Steam (50% humidified) atmosphere at 1.5 V and 800 °C and d) fresh LSCF-Ag composite electrode after reduction in 5%H 2 -Ar at 800 °C for 8 h.

Table 1 .
XRD characterization of fresh and tested LSCF-Ag cathode.