Synthesis and electrochemical characterization of porous cathode materials for intermediate temperature solid oxide fuel cells
Electrochemical characteristics of the half-cells Ce0.8Gd0.2O1.9 | La0.6Sr0.4CoO3‑, Ce0.8Gd0.2O1.9 | Pr0.6Sr0.4CoO3‑ and Ce0.8Gd0.2O1.9 | Gd0.6Sr0.4CoO3- and single cells La0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O1.9 | NiO + Ce0.9Gd0.1O2-δ, Pr0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O1.9 | NiO + Ce0.9Gd0.1O2-δ and Gd0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O1.9 | NiO + Ce0.9Gd0.1O2-δ have been studied using electrochemical methods (impedance spectroscopy, cyclic voltammetry and chronoamperometry) at various electrode potentials E and temperatures T. Systematic analysis of electrochemical results demonstrates that kinetics of oxygen electroreduction depends noticeably on porosity and chemical composition of the cathodes used. Detailed analysis of the impedance complex plane (Z’’,Z’) plots indicate that at T  773 K the mixed kinetics behavior prevails for most half-cells studied (i.e. slow mass transfer and charge transfer limiting steps have been established). At higher negative potentials and temperatures the processes tend toward purely charge transfer limited oxygen electroreduction mechanism (phase angle   –5). The values of total activation energy, calculated for various cathodes, decrease with increasing the cathode negative polarisation. In addition to the electron transfer process (reduction of oxygen), the mass transfer process of electrochemically active pieces inside micro-, mesoporous solid cathode material can probably be the rate- determining step in agreement with the fitting results of the Nyquist plots. Käesolevas töös uuriti süstemaatiliselt poolelementide Ce0.8Gd0.2O2-δ | La0.6Sr0.4CoO3‑, Ce0.8Gd0.2O2-δ | Pr0.6Sr0.4CoO3‑ ja Ce0.8Gd0.2O2-δ | Gd0.6Sr0.4CoO3- ja ühikrakkude La0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O2-δ | NiO + Ce0.9Gd0.1O2-δ, Pr0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O2-δ | NiO + Ce0.9Gd0.1O2-δ ja Gd0.6Sr0.4CoO3‑ | Ce0.8Gd0.2O2-δ | NiO + Ce0.9Gd0.1O2-δ omadusi elektrokeemiliste meetoditega (impedantsspektroskoopia, kronokulonomeetria, tsükliline voltamperomeetria).. Elektrokeemilised uuringud näitasid, et materjalide elektrokatalüütiline aktiivsus sõltub väga oluliselt nende poorsusest ja keemilisest koostisest. Impedantsspektroskoopia andmete analüüsimisel saadud tulemustest selgub, et madalamatel tempera­tuuridel (T  773 K) on valdavalt tegemist segakineetiliste protsessidega (limi­teeri­vaks on nii laenguülekande- kui ka massiülekandeprotsessid), kuid kõrgematel temperatuuridel ja elektroodi polarisatsioonidel viitavad faasinurga suhteliselt väikesed negatiivsed väärtused (  –5) põhiliselt laenguülekande poolt limiteeritud protsessile. Faasinurga väärtuste oluline sõltuvus elektroodi polarisatsioonist madalamatel temperatuuridel viitab komplitseeritud laetud hapnikuosakeste massiülekandeprotsessile poorses katoodis. Leiti, et katoodse polarisatsiooni suurenedes aktivatsioonienergia väärtused vähenevad. Lisaks aeglase laenguülekandeprotsessile on elektrokeemilise polarisatsioonilise takistuse allikaks veel difusioonilised raskused.
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