High electrocatalytic activity for oxygen reduction reaction (ORR) and slow degradation rate are two extremely important issues to be faced in cathodes for intermediate temperature 8 solid oxide fuel cells (IT8 SOFC). The main question still without a satisfactory answer is if it might be possible to meet these two fundamental requirements without losing other properties. Many studies have been developed concerning new materials, and in particular perovskites have received huge attention for their good properties. They present an ABO38type structure, where A and B are two cations with a different ionic radius, and they are attractive for high ionic and electronic conductivity, as well as for their good oxygen surface exchange. La0.6Sr0.4Co0.2Fe0.8O38? (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O38? (BSCF) are among the most studied perovskites investigated for electrochemical application. In detail, LSCF is attractive for its high electronic and ionic conductivity. On the other hand, BSCF compensates its lower electronic conductivity with really high oxygen vacancy concentration and diffusion rate, larger than 2 to 200 times the values of other perovskites. Despite these materials responding well to the requirements for a good ORR electroactivity, an acceptable stability has not yet been reached. Especially, barium8based electrodes (BSCF) are penalised by a strong phase transition below 900°C, which leads to a structural instability. Operating in the characteristic IT8SOFC temperature range (650°C), BSCF transforms from a cubic to a hexagonal structure less active for oxygen reduction. Besides, it is affected by carbonate formation in the presence of CO2, resulting in electrode deactivation. The presence of CO2 reduces the ability of BSCF for oxygen transport, and such a high susceptibility is attributed to the presence of Ba bond as reported by Zhang. Considering lanthanum based perovskites (LSCF), they suffer from cation interdiffusion (La, Sr), responsible for a worsening in performance with the formation of insulating phase such as La2Zr2O7 or SrZrO3 when Yttria8stabilized zirconia is used as electrolyte. A possible solution to limit the conductivity decrease due to the formation of these phases is the employment of a ceria-based electrolyte doped with a rare earth, such as gadolinium (Ce1-xGdxO2-?, GDC), as interlayer between the LSCF-based electrode and YSZ electrolyte. However, it was found that La and Sr still diffuse within this thin buffer layer. Other approaches applied to reduce detrimental cation diffusion are: (i) surface modification by means of infiltration by a second phase rich in strontium (ii) introduction of A8site deficiency (iii) use a composite material to stabilise different phases. Starting from these considerations and with the aim of reaching a mutual stabilisation between LSCF and BSCF materials, a first composite cathode with a 1:1 volume ratio was investigated. The results obtained were promising in terms of degradation rate; the composite cathode maintained a good electrocatalytic activity and showed only 5% degradation under an applied current of 200 mA cm82 for 200 hours. In the same operating conditions, pure BSCF had a degradation of 38%. In this year of project, performance and stability of LSCF-BSCF composite cathodes were examined, considering two new different compositions, one richer in LSCF and the other richer in BSCF. The investigation was carried out through X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS): measurements at different temperatures and cathodic overpotentials were performed to study the kinetic mechanism, while ageing tests were carried out to compare stability over time.
Accordo CNR/BAS (Bulgaria) - Rendiconto 2° anno Progetto
Sabrina Presto
2018
Abstract
High electrocatalytic activity for oxygen reduction reaction (ORR) and slow degradation rate are two extremely important issues to be faced in cathodes for intermediate temperature 8 solid oxide fuel cells (IT8 SOFC). The main question still without a satisfactory answer is if it might be possible to meet these two fundamental requirements without losing other properties. Many studies have been developed concerning new materials, and in particular perovskites have received huge attention for their good properties. They present an ABO38type structure, where A and B are two cations with a different ionic radius, and they are attractive for high ionic and electronic conductivity, as well as for their good oxygen surface exchange. La0.6Sr0.4Co0.2Fe0.8O38? (LSCF) and Ba0.5Sr0.5Co0.8Fe0.2O38? (BSCF) are among the most studied perovskites investigated for electrochemical application. In detail, LSCF is attractive for its high electronic and ionic conductivity. On the other hand, BSCF compensates its lower electronic conductivity with really high oxygen vacancy concentration and diffusion rate, larger than 2 to 200 times the values of other perovskites. Despite these materials responding well to the requirements for a good ORR electroactivity, an acceptable stability has not yet been reached. Especially, barium8based electrodes (BSCF) are penalised by a strong phase transition below 900°C, which leads to a structural instability. Operating in the characteristic IT8SOFC temperature range (650°C), BSCF transforms from a cubic to a hexagonal structure less active for oxygen reduction. Besides, it is affected by carbonate formation in the presence of CO2, resulting in electrode deactivation. The presence of CO2 reduces the ability of BSCF for oxygen transport, and such a high susceptibility is attributed to the presence of Ba bond as reported by Zhang. Considering lanthanum based perovskites (LSCF), they suffer from cation interdiffusion (La, Sr), responsible for a worsening in performance with the formation of insulating phase such as La2Zr2O7 or SrZrO3 when Yttria8stabilized zirconia is used as electrolyte. A possible solution to limit the conductivity decrease due to the formation of these phases is the employment of a ceria-based electrolyte doped with a rare earth, such as gadolinium (Ce1-xGdxO2-?, GDC), as interlayer between the LSCF-based electrode and YSZ electrolyte. However, it was found that La and Sr still diffuse within this thin buffer layer. Other approaches applied to reduce detrimental cation diffusion are: (i) surface modification by means of infiltration by a second phase rich in strontium (ii) introduction of A8site deficiency (iii) use a composite material to stabilise different phases. Starting from these considerations and with the aim of reaching a mutual stabilisation between LSCF and BSCF materials, a first composite cathode with a 1:1 volume ratio was investigated. The results obtained were promising in terms of degradation rate; the composite cathode maintained a good electrocatalytic activity and showed only 5% degradation under an applied current of 200 mA cm82 for 200 hours. In the same operating conditions, pure BSCF had a degradation of 38%. In this year of project, performance and stability of LSCF-BSCF composite cathodes were examined, considering two new different compositions, one richer in LSCF and the other richer in BSCF. The investigation was carried out through X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS): measurements at different temperatures and cathodic overpotentials were performed to study the kinetic mechanism, while ageing tests were carried out to compare stability over time.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
