Experimental Investigation of the Effect of Operating Pressure on the Performance of SOFC and SOEC

Abstract

The effect of operating pressure on the performance of SOFC and SOEC was investigated electrochemically under pressurized and depressurized conditions (0.01-1.0 MPa) using practical size single cells. It was found that the diameter of low frequency semi-circle appears in impedance spectra was not so much affected by operating pressure with electrolyte supported cell as was expected theoretically for gas conversion impedance. On the other hand, the diameter significantly depends on the operating pressure with anode supported cell especially when the operating pressure becomes low, indicating the contribution of diffusion to the semicircle. Numerical simulation in which diffusion processes in electrodes were considered was proposed to explain the observed behavior.

Pressurized Operation of a Planar Solid Oxide Cell Stack

“…[16] This decrease in cell resistance is caused by the increase with pressure of the frequency of reactants hitting the triple phase boundary and subsequently converting to products, [22,23] as well as decreased diffusion resistance. [17,18] The open circuit voltage was found to increase with increasing pressure as predicted by the Nernst equation. The same gain in performance is observed in electrolysis mode as compared to fuel cell mode (equal decrease in ASR as a function of pressure), but this is obscured by the observed increase in OCV.…”

“…When air is supplied to the air-electrode both binary gas diffusion and Knudsen gas diffusion will contribute to the impedance, although on hydrogen electrode supported single cells the contribution from air-electrode is expected to be small compared to the contribution from the fuel-electrode. [18,34] On single cells, switching from air to oxygen have been shown to affect distribution of relaxation-time (DRT) deconvolutions of impedance spectra at around 10 Hz. [35] Due to the difference in geometry between single cells and stacks this could possibly be the same process that affects the stack impedance at a characteristic frequency around 1-2 Hz.…”

“…The obtained exponent of -0.90 deviates from the a pressure dependency of -1 which would be expected from simple gas conversion CSRT considerations. [30] Momma et al investigated button cells at pressures ranging from 0.1 to 10 bar [18] and noted that "The time constant calculated from low frequency arc is exactly proportional to pressure at high pressure region, although it became rather saturated at lowest pressure presumably because of the effect of diffusion in porous media", and later that "Under de-pressurized conditions, Knudsen diffusion became predominant as compared to molecular diffusion". A coupling between gas conversion in the gas channels and Knudsen gas diffusion in the porous support layer and electrode could explain why the exponent derived in this work deviates from -1.…”

“…[14,15] Although several research institutes have shown the effect of pressurisation when operating solid oxide cells as fuel cells, examination of the performance of SOECs at increased pressure has only recently been initiated. [16][17][18][19][20][21] A significant decrease of approximately 20% in the internal resistance in both fuel cell and electrolysis cell mode of a Ni-YSZ based cell (Ni-YSZ|YSZ|LSM-YSZ) was found by increasing the pressure from 1 to 10 atm, when operated with 50% H2O -50% H2 supplied to the Ni-YSZelectrode and O2 supplied to the LSM-YSZ-electrode. [16] This decrease in cell resistance is caused by the increase with pressure of the frequency of reactants hitting the triple phase boundary and subsequently converting to products, [22,23] as well as decreased diffusion resistance.…”

Characterization of a Planar Solid Oxide Cell Stack Operated at Elevated Pressure

“…12,13 An increased frequency of reactants hitting the electrochemical reaction sites, 19,20 and a decrease of the diffusion resistance 13,14 are the main reasons for the decreased resistance with increasing pressure.…”