Significance Statement
In a recent article done by Park et al. and published in Journal of Power Sources, investigations showed that electrochemical properties of thin film solid oxide fuel cells (TF-SOFCs) at low temperature is also a function of grain size of nanostructured Ni-yttria-stabilized zirconia (Ni-YSZ) composite anode.
This was achieved by analyzing the output power and impedance spectra at various cells by operating conditions in order to compare electrode reaction mechanism due to amount of grain size of anode in full-cell test.
Despite high efficiency of solid oxide fuel cell (SOFC), its high operating temperature of (≥ 700-800 °C) has been a major setback due to its fast degradation and poor reliability. Lowering the operating temperature of SOFC has been a major case of study because unwanted chemical reactions are prevented and use of less expensive material such as stainless steel are allowed. However at lower operating temperatures, electrode activity of SOFC worsens hence its performance is determined by polarization losses induced by electrode reaction at low temperature.
Although extensive research has been made on nanostructured cathodes due to its polarization loss in low temperature performance of SOFCs, it has also been predicted that loss of anode at temperature (≤ 600 °C) also has significant effect as that of cathode. Since metals are used as catalyst in nanostructured anode, fabrication has been difficult due to agglomeration of metal-phase at SOFC operating temperature leading to reduction in triple-phase boundary (TPB) length, loss of conductivity and cell stability degradation. However, fabrication of a uniform nanostructured Ni-YSZ was successful using PLD. Its layer was designed to suppress Ni agglomeration and effectively support thin-film electrolytes with thickness approximately 1µm or less which has a higher TPB density to that of conventional Ni-YSZ anodes.
Discerning the impact of nanostructured anode functional layer (nano-AFL) has proved difficult experimentally due to the fact that nano-AFL is so thin that catalyst amount in layer is minute in compares to that of whole anode support and it has been extremely difficult to build thin electrolytes directly over supports without nano-AFL due to the surface condition of the supports. Nevertheless, Park et al. has been successful in building TF SOFCs with and without nano-AFL and effect of nano-scale grain-size of AFL has been studied effectively.
In experiment set-up, two different unit cells; one with nano-scale Ni-YSZ AFL was denoted as NS-cell and that without nano-AFL is denoted by MS-cell. Cell operating temperature varied from 650 °C to 500 °C at intervals of 50 °C while electrochemical impedance spectra (EIS) and current-voltage-power (I-V-P) curves were obtained at each temperature. From the results, the peak power density of NS-cell at 650 °C reached 1775 mWcm-2 which is better than MS-cell with 1650 mWcm-2.
It was also noticed that performance ratio (PNS-cell/PMS-cell) increases with decrease in temperature at 0.7 V as performance of NS-cell was twice higher than MS-cell at 500 °C and I-V drop of MS-cell was faster at lower current density. These two results show that the influence of nano-AFL on the full cell performance increases with decreasing temperature and electrode activation loss is significant in MS- cell.
With each EIS observed over a frequency of 105 Hz to 10-1 Hz at AC amplitude impedance of 50 mV, impedance in frequency range of 10 Hz ≤ f ≤ 104 Hz was considered to be a major contributor to total impedance of both cells and exhibits a significant temperature dependency, however an overlay in impedance was noticed in two stages; (102 Hz ≤ f ≤ 103 Hz) and (103 Hz ≤ f ≤ 104 Hz).
From the Bode plots, first frequency impedance was mainly related to the cathode while at second frequency impedance, temperature was reduced during anode half-cell test and impedance at this stage increased. It was also seen from the Bode plots of NS and MS cells that the resistance of anode reaction is much smaller in the NS-cell than in MS-cell due to small particles size of nano-AFL.
These results prove that performance improvement of NS-cell with respect to MS-cell at lower temperature is due to facilitation of charge transfer at the anode. This shows that anode performance can significantly affect cell performance at low temperature.
Park et al. study proves that anode reaction also affects cell performance at low temperature significantly. Substantial conclusion was also made that it’s important to minimize grain size of Ni-YSZ despite its possible fabrication in order to obtain reasonable cell performance at low temperatures which also aids the lowering of SOFCs operating temperature.
Journal Reference
Jung Hoon Park1,2, Seung Min Han2, Kyung Joong Yoon1,3, Hyoungchul Kim1,3, Jongsup Hong1, Byung-Kook Kim1, Jong-Ho Lee1,3, Ji-Won Son1,3 . Impact of nanostructured anode on low-temperature performance of thin-film-based anode-supported solid oxide fuel cells. Journal of Power Sources, Volume 315, 31 May 2016, Pages 324–330.
Show Affiliations- High-temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, South Korea
- Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea
- Nanomaterials Science and Engineering, Korea University of Science and Technology, KIST Campus, Seoul 02792, South Korea
Go To Journal of Power Sources
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