Perovskite electrodes

Fig. 43. Full-cell performance with hot-pressed membrane, perovskite electrodes. Cathode removal and anode generation as a function of applied current. Lines calculated from stoichiometry, 1 mol/2 F.

For perovskite electrodes, the earliest kinetic study of hysteretic effects appears to come from Ham-mouche and co-workers, who showed that the i—rj characteristics of porous LSM/YSZ in air at 960 °C exhibit a potentiodynamic hysteresis when scanned slowly (1 mV/s) between 0 and —1200 mV cathodic polarization. " A clearer demonstration of this effect, more recently provided by Jiang and co-workers, is shown in Figure 41.232,233 Hammouche and co-workers attributed this hysteresis to the formation of oxygen vacancies in LSM at high overpotential, which (as discussed in sections 5.2 and 5.3) appears to open a parallel bulk-transport-mediated reaction pathway. However, if this was the only explanation. 

Attempts to quantify electrocatalytic activities on the basis of correlation dependences that take into account the volume properties [79-81,97,98] made it necessary to obtain well-characterized experimental data, and thus stimulated the improvement of perovskite electrode construction. Methods were developed for determining the real surface area of disperse perovskites in the electrodes with polymer binders [99], and also for measurements on quasi-smooth, highly conductive ceramics [81, 82,100]. This experience was applied successfully later in the studies of HTSC electrodes. 

The outputs of some mixed potential-type chemical sensors correlate with the type of electronic defect (i.e., n-type versus p-type), so the response has been attributed to the semiconducting behavior of the electrode material [314]. LaFeO3, which has been used as a semiconductor-type gas sensor ] 315, 316], has also been used as an electrode with YSZ [255, 263, 317] or NASICON ]317, 318] electrolytes for potentiometric NO, sensors. Strontium (i.e., (La,Sr)FeO3 ]255, 256, 284]) or strontium and cobalt (i.e., (La, Sr)(Co,Fe)O3 ]275, 280, 309]) have been added to LaFeO3 to improve electrode performance. (La,Ca)MnO3 doped with either cobalt or nickel on the manganese site has been used as the electrode for N O, sensors ]319]. The outputs of some NO, sensors with perovskite electrodes are shown in Figure 13.26 ]255, 256, 264, 275, 309, 312]. 

Figure 13.25 Outputs of CO [229, 303-306] and propylene [307, 308] sensors with YSZ electrolytes and perovskite electrodes.

Figure 13.26 Outputs of NO, sensors with perovskite electrodes [255, 256, 264, 275, 309, 312].

Models of Mixed Ionic and Electronic Conducting (MIEC) Electrodes These specialised electrode models usually consider the MIEC electrode in combination with the electrolyte and focus on correlating performance with the semiconductor characteristics of the electrode (and sometimes electrolyte) [70-72]. Recent modelling of oxygen reduction and oxygen permeation at perovskite electrodes includes both MIEC effects and classical diffusion-type analysis [73-75]. 

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