Solid oxide electrolyzer cells (SOEC), working typically at greater than 800^oC with current research efforts toward lowering it down to 600^oC, has been a promising solution to generate hydrogen from steam due to its high conversion efficiency and relatively low required energy input as compared with near room temperature electrolysis of H₂O. Over the past few years, degradation/stability of SOEC has been a major concern for long term applications, and delamination of oxygen electrode was reported as the main reason for this degradation or failure issue [1-4]. Theoretical analyses [5-7] has shown that the actual oxygen chemical potential can be well above or below the corresponding values at the two electrodes in a SOEC using predominantly oxygen ionic conducting membrane, leading to either delamination at oxygen electrode or electroreduction at fuel electrode. Meanwhile, the same theoretical analyses also indicate that such an issue may be overcome by introducing a finite and small level of electronic conduction in the electrolyte membrane. The objective of this work is to experimentally verify some of the concepts revealed by the above theories.

 The chemical potential of oxygen inside the electrolyte close to the oxygen electrode, μ^c_O2 , is given by Ref.[6]

 μ^c_O2 = μ^I_O2 - 4F [ E_AR_e(c)/R_e - (E_A-E_N)R_i(c) / R_i ] (1)

and the oxygen chemical potential close to the fuel electrode, μ^a_O2 , is given by

 μ^a_O2 = μ^II_O2 + 4F [ E_AR_e(a)/R_e - (E_A-E_N)R_i(a) / R_i ] (2)

where E_N is cell Nernst potential governed by the oxygen partial pressures at the two electrodes and E_A is the applied voltage. μ^I_O2 and ^II_O2 are the oxygen chemical potentials at the oxygen and fuel electrodes, respectively. R_i(c) and R_i(a) are the polarization resistances of the oxygen and fuel electrodes, respectively. R_e(c) and R_e(a) are the electronic resistances at the oxygen electrode/electrolyte and fuel electrode/electrolyte interfaces, respectively. R_e and R_i are the net electronic and ionic area specific resistances (ASR) of the cell, respectively.

Based on the electronic and ionic resistance corresponding to the electrolyte and the electrodes and the applied voltage, it is shown that for a purely ionic conductor such as 8YSZ,

 μ^c_O2 - μ^a_O2 ≈ 4FE_N + 4F(E_A-E_N) [ (R_i(c)+R_i(a))/Ri ]  (3)

and it changes to

 μ^c_O2 - μ^a_O2 ≈ 4F(E_N-E_A)R_i(el)/Ri (4)

if the electrolyte is a MIEC such as CeO₂ doped YSZ.

Depending on the magnitude of the applied voltage, the above equations predict that the oxygen chemical potential difference across the electrolyte may be either out of bonds for a purely ionic conductor or within the bonds for a MIEC under at relatively small voltages, as indicated by Figs. 1a and 1b.

Experiments were thus designed and performed in this study following the concepts revealed by the above theories. 8YSZ or CeO₂ doped 8YSZ discs were die pressed and sintered at 1500^oC. Conventional LSM/YSZ and/or Pt electrodes were applied symmetrically on the two sides of the electrolyte discs, and fired at typical temperatures for both types of electrodes. Such cells were allowed to pass a DC current of smaller than 0.35 A/cm² at 700^oC in air. Voltage between the two electrodes was monitored using a digital multimeter. EIS spectra were collected before and after a period of current application for 24 h. After that the cells were cooled to room temperature for EDS analysis of oxygen concentration across the electrolyte layer.

Under similar testing conditions, the preliminary results clearly showed that the electrode polarization resistances were either significantly reduced or increased in 8YSZ supported cells, but could remain at a fairly stable level in CeO₂doped 8YSZ cells after 24 h current application, as shown in Figs. 1c to 1f. Ongoing work is on the measurements of oxygen concentration across the electrolyte layer and possible microstructural changes near the electrode/electrolyte interfaces before and after current application.

Acknowledgements: This work was supported by the National Science Foundation under Grant Number NSF-CBET-1604008 and by the US Department of Energy under Grant Number FG02-06ER46086.

References:

[1] Herring JS, Int J Hydrogen Energy, 32, 440-50 (2007).

[2] Momma A, Kato T, Kaga Y, Nagata S. J Ceram Soc Japan, 105, 369-73 (1997).

[3] Mawdsley JR, Carter JD, Kropf AJ, Yildiz B, Maroni, VA. Int J Hydrogen Energy, 34,4198-207 (2009).

[4] Laguna-Bercero, MA, Campana R, Larrea A, Kilner JA, Orera VM. J Power Sources, 196,8942-7 (2011).

[5] Virkar AV. J. Power Sources, 147, 8-31 (2005).

[6] Virkar AV. Int J Hydrogen Energy, 35, 9527-43 (2010).

[7] Virkar AV. Tao G, Int J Hydrogen Energy, 40, 5561-77 (2015).