Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ

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Short Communication

Mitigation of the delamination of LSM anode in solid oxide electrolysis cells using manganese-modified YSZ Na Li, Michael Keane, Manoj K. Mahapatra, Prabhakar Singh* Department of Chemical, Materials and Biomolecular Engineering, Center for Clean Energy Engineering, University of Connecticut, 44 Weaver Rd., Storrs, CT 06269-5233, United States

article info

abstract

Article history:

Manganese-modified yttria-stabilized zirconia (Mn-YSZ) has been used to suppress

Received 25 January 2013

interfacial degradation and anode delamination during short term (w200 h) electro-

Received in revised form

chemical testing of solid oxide electrolysis cells (SOEC). Two fabrication methods have

28 February 2013

been used for YSZ surface modification: solid state diffusion of manganese into YSZ

Accepted 6 March 2013

substrate and solegel coating of a Mn-YSZ layer on YSZ substrate. Both the solid state

Available online 9 April 2013

diffusion and solegel coating methods stabilized the electrical performance; however,

Keywords:

dicates reduction in the formation of lanthanum zirconate on the Mn-modified electrolyte

Solid oxide electrolysis cell

surface. It is proposed that the presence of the porous solegel coating prevents high

Manganese-modified yttria-

pressure oxygen build up and results in the mitigation of the anode delamination.

stabilized zirconia

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

anode delamination was mitigated only by the solegel method. Post-test analysis in-

Solegel coating

reserved.

Strontium-doped lanthanum manganite Anode delamination Lanthanum zirconate

1.

Introduction

Hydrogen production from water using solid oxide electrolysis cell (SOEC) at 800e1000
C remains attractive due to thermodynamic system efficiency of over 50% and minimal greenhouse gas emissions [1]. In its simplest form, an SOEC is comprised of a dense oxygen ion-conducting electrolyte layer and two porous electrodes. The electrolyte is typically yttria stabilized zirconia (YSZ). The fuel electrode (cathode) is usually nickel-YSZ cermet composite whereas the commonly used air electrode (anode) is strontium doped lanthanum

manganite (LSM) or LSMeYSZ composite [2]. Anode delamination has been frequently observed and considered as one of the largest contributor to the cell performance degradation [3]. Several mechanisms responsible for anode delamination have been proposed in the literature. Virkar proposed that the anode delamination is due to the buildup of high oxygen pressure within the electrolyte and at the anodeeelectrolyte interface due to differences in ionic and electronic conductivity between the electrode and the electrolyte [4]. Rashkeev and Glazoff proposed that La and Sr substitutional defects positioned in ZrO2 near interface significantly change oxygen

* Corresponding author. E-mail address: [email protected] (P. Singh). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.036

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transport which develops pressure in the interfacial region that eventually results in the delamination [5]. Chen et al. suggested that the delamination of LSM anode results from the formation of nanoparticle clusters under high oxygen partial pressure at the interface [6]. Keane et al. related anodeside lanthanum zirconate (LZ) formation to cell performance degradation and anode delamination [7]. Though several approaches for the mitigation of electrode delamination has been suggested including modification of the electronic and ionic resistances of the device components [4] and operation under AC voltage rather than DC voltage [8], the delamination phenomena has neither

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been fully understood nor prevented for the LSM anode. In this communication, we report an approach for mitigating the delamination and increasing the longevity of the SOEC device.

2.

Experimental

An electrolyte supported symmetric cell configuration was selected for this study. The cell consists of a w190 mm thick (ZrO2)0.92(Y2O3)0.08 electrolyte, and w25 mm thick (La0.8Sr0.2)0.98 MnO3x anode and cathode. Solid state diffusion was used

Fig. 1 e (a) Changes in current density with time for U, D and SG-cells; (b) Changes in ohmic and non-ohmic resistance with time for U, D and SG-cells.

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 6 2 9 8 e6 3 0 3

to modify the electrolyte and solegel coating was used to prepare a Mn-YSZ interlayer. In the solid state diffusion method, the YSZ substrates were embedded in MnO2 powder and thermally treated at 1250
C for 2 h. In solegel method, Zr(OPr)4 was used as precursor, while 8 mol% yttria and 2 mol % manganese were introduced by using Y(NO3)3$6H2O and Mn(NO3)2$6H2O. Acetylacetone was used as a chelating agent, and the precursor was hydrolyzed with the aid of catalyst HNO3. Films were deposited by spin coating (Chemat Technology spin-coater KW-4A) at 1500 rpm. The coated sample was dried in air overnight, followed by curing at 250
C for 30 min. The coating, drying, and heating steps were repeated three times followed by sintering at 1250
C for 2 h. The LSM electrode was screen printed on both sides of the specimen and sintered at 1200
C for 2 h. Silver screen and connecting wires were subsequently attached to each electrode using silverepalladium paste as current collector and sintered at 850
C for 1 h. Two cell configurations were prepared for electrical testing (a) D-cell with the Mn-YSZ diffusion layer on both anode and cathode side, and (b) SG-cell with Mn-YSZ solegel coating at anode side only. U-cell with undoped YSZ electrolyte served as reference. The fabricated cells were electrically tested at 840
C in flowing air (300 sccm) for up to 200 h. A constant voltage of 0.8 V was applied using a potentiostat (VMP2, Bio-Logic) in SOEC mode. After completion of the test, electrode adherence was evaluated by peeling of the electrode from the electrolyte. Residual LSM present at the electrolyte surface was dissolved in hydrochloric acid to expose the electrolyte surface for interfacial analysis. An FEI Quanta 250 FEG scanning electron microscope (SEM) and the attached energy-dispersive X-ray spectroscopy (EDS) was used for the morphological and elemental distribution study. A Bruker AXS D-8 X-ray diffractometer (XRD) was used to identify the interfacial compounds.

3.

Results and discussion

Fig. 1(a) shows changes in the current density with time for various cells. For U-cell, the current density reaches a maximum after an initial improvement followed by a steep drop. Each Mn-doped cell exhibits a lower peak, takes longer time to reach the peak, and shows slower performance decay. Fig. 1(b) shows the ohmic and non-ohmic resistance trends for various cells. All cells show an increase in ohmic resistance after an initial decrease (4 h). Subsequent testing shows a rapid ohmic increase for the undoped cell while the Mn-doped cells show only slight increase of ohmic resistance. The doped cells show lower ohmic resistance than the undoped cell at the end of tests. The U-cell experiences fast non-ohmic increase after approximately 40 h, whereas the D-cell experiences similar increase after approximately 160 h, but the SGcell did not show any significant increase in 200 h. Post-test observations showed no delamination between cathode and electrolyte for any of the tested cells. Observations on the anode adherence, however, varied. For the U-cell, the anode delaminated completely from the substrate, leaving behind a dark-colored imprint on the electrolyte. For the Dcell, similar delamination was observed. For the SG-cell, the anode layer remained firmly attached to the substrate. Fig. 2 shows X-ray diffraction patterns for the electrolyte surfaces after testing for 100 h and removing LSM. The spectrum was normalized to the highest YSZ peak (111) and the yscale was magnified to compare the lower-intensity LZ peaks. The lanthanum zirconate (La2Zr2O7) diffraction pattern (JCPDS 00-050-0450) is identified at anode side only. The relative quantities of LZ to YSZ based on peak intensities are Ucell > D-cell > SG-cell. The as-fabricated electrolyte surface morphologies of the U-cell, D-cell, and SG-cell are shown in Fig. 3(a)e(c),

Fig. 2 e XRD pattern of anode-side electrolyte surface after removing LSM.

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Fig. 3 e Surface morphology of the electrolyteeanode interface for as-fabricated cells (a) U-cell, (b) D-cell, and (c) SG-cell.

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Fig. 4 e Surface morphology of the electrolyteeanode interface after testing and dissolving LSM (a) U-cell, (b) D-cell, and (c) SG-cell.

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Table 1 e Ohmic (RU) and non-ohmic resistance (Rnon-U) change and average LZ formation rates. Cell

U-cell D-cell SG-cell

RU change rate (U cm2/100 h)

Rnon-U change rate (U cm2/100 h)

4e100 h

100e200 h

4e100 h

100e200 h

0.56 0.09 0.09

e 0.17 0.07

0.38 0.14 0.13

e 0.10 0.01

respectively. The YSZ and the Mn modified YSZ diffusion layer show smooth surface with minimal porosity. The solegel coating resulted in the formation of a discontinuous porous layer. The anodeeelectrolyte interface morphologies after testing and dissolving LSM are shown in Fig. 4. For U-cell (a), the electrolyte surface shows discrete formation of LZ particles (identified by EDS analysis) and higher surface coverage than D-cell (b) and SG-cell (c). LZ formation is attributed to the evolution of oxygen at the interface according to [7]: LaMnO3 þ ZrO2 þ 0:25O2 40:5La2 Zr2 O7 þ MnO2

(1)

Table 1 shows a correlation between LZ formation rates and resistance change. The relative average LZ formation

Average LZ formation rates High Moderate Low

rates are calculated from LZ content (relative to YSZ) and total testing time. The Mn-doped cell shows slower LZ formation and corresponding smaller ohmic and non-ohmic resistance change compared to the undoped cells. The Dcell and SG-cell show comparable trend for the first hundred hours. Subsequent electrical testing beyond 100 h duration, however, showed that the resistance of the SG cell increased at much slower rate when compared to that of the D-cell. The ohmic resistance increase is explained by the loss of contact among the electrode, electrolyte and current collection layers. Similarly, the non-ohmic resistance increase is explained by the loss of TPB length for oxygen reduction and evolution. The LZ coverage of the interface reduces the LSMeYSZ contact area, decreases the TPB length, and increases both the ohmic and non-ohmic resistance. The MnYSZ interlayer alleviates the diffusion of Mn from LSM to YSZ thus delaying LZ formation during the short term tests, as confirmed by reduced LZ formation for the D-cell and SGcell compared to U-cell. Fig. 5 shows schematic representations of the proposed degradation processes at the anodeeelectrolyte interfaces of the U-cell (a), D-cell (b), and SG-cell (c). High oxygen pressure buildup at the anodeeelectrolyte interface results in grain boundary (GB) porosity in the YSZ and anode delamination as shown above [7]. The adherence of the anode with the electrolyte in the SG-cell is hypothesized to be due to the presence of porous interlayer as previously shown in Fig. 3(c). The presence of a discontinuous layer and submicron pores within the bulk provides an escape path for oxygen generated during electrochemical testing. The release of oxygen mitigates pressure buildup and subsequent delamination. Due to the continued lanthanum zirconate formation at the electrolyte surface, the improved anode adherence does not completely prevent performance degradation.

4.

Conclusions

Mn-modified YSZ electrolyte inhibits LZ formation by reducing the diffusion of Mn from the LSM electrode to the YSZ electrolyte. It is hypothesized that the porous solegel coating prevents high pressure oxygen build up and results in the mitigation of the anode delamination.

Acknowledgment Fig. 5 e Schematic representations of the degradation at the anodeeelectrolyte interfaces for (a) U-cell, (b) D-cell, (c) SG-cell.

This work was financially supported by Clean Energy Fund, University of Connecticut.

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references

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[4] Virkar AV. Mechanism of oxygen electrode delamination in solid oxide electrolyzer cells. Int J Hydrogen Energy 2010;35:9527. [5] Rashkeev SN, Glazoff MV. Atomic-scale mechanisms of oxygen electrode delamination in solid oxide electrolyzer cells. Int J Hydrogen Energy 2012;37:1280. [6] Chen K, Jiang SP. Failure mechanism of (La, Sr)MnO3 oxygen electrodes of solid oxide electrolysis cells. Int J Hydrogen Energy 2011;36:10541. [7] Keane M, Mahapatra MK, Verma A, Singh P. LSM-YSZ interactions and anode delamination in solid oxide electrolysis cells. LSM-YSZ interactions and anode delamination in solid oxide electrolysis cells. Int J Hydrogen Energy 2012;37:16776. [8] Rashkeev SN, Glazoff MV. Control of oxygen delamination in solid oxide electrolyzer cells via modifying operational regime. Appl Phys Lett 2011;99:173506.