Development of solid oxide electrochemical cells using highly active perovskite-based electrodes

Abstract

Solid oxide cells (SOCs) are powerful electrochemical devices for sufficient and simultaneous production and storage of eco-friendly energy. The SOCs can operate reversibly in a solid oxide fuel cell (SOFC) mode to generate electricity from chemical fuels, and a solid oxide electrolysis cell (SOEC) mode to convert electrical power into chemical fuels. An SOC is composed of high-density electrolyte, oxygen electrode, and fuel electrode. During operation, the oxygen electrode provides an active site for oxygen reduction or evolution reaction (ORR / OER), while the fuel electrode oxidizes fuel (e.g. H$_2$, CH$_4$ and CO) or produces fuel (e.g., electrolysis of H$_2$O and CO$_2$). Both electrodes must provide a sufficient number of active sites for electrode reactions as well as suitable routes for rapidly transporting species (ions, electrons, and gas molecules, etc.) involved in the continuous electrode reaction. Thus, both electrodes are essential to improve SOC performance and stability. Currently, Cobalt-containing perovskite materials have been intensively investigated as bifunctional oxygen electrodes for solid oxide cells because of their mixed ionic-electronic conducting nature. However, the direct application of these materials to the most widely used zirconia-based electrolytes has been limited because of their thermo-chemical incompatibility. To address this issue, we introduce a cobalt-free manganite-perovskite with significantly improved electrocatalytic activity by doping with Bi which possesses stereochemically active lone pair electrons. Replacing Bi with Pr in Pr$_{0.8}$Sr$_{0.2}$MnO$_{3−δ}$ increases the oxygen vacancy concentration and promotes oxygen ion incorporation and charge transfer processes at the electrode. (Chapter III) Meanwhile, nickel/yttria-stabilized zirconia (Ni–YSZ) has been the most widely used fuel electrode material for SOCs due to its high catalytic activity, operational reliability, and acceptable cost. However, the long- term durability of Ni–YSZ is inadequate due to inherent redox instability and severe carbon deposition during operation. To address this, alternatives to Ni-YSZ electrode materials, including various perovskite oxides, have been developed and investigated, many of which have shown significant catalytic activity and stability. Recently, a more cost-effective, single-step, and efficient ‘exsolution’ process by which the nano-scale metallic catalysts are spontaneously formed directly from the perovskite oxide lattice in reducing conditions has been proposed. Compared to the conventional infiltration method, the exsolved nanoparticles are strongly anchored and exhibit inherently improved distribution with superior size uniformity on the perovskite oxide support, resulting in greatly enhanced catalytic activity and durability. we develop a highly catalytically active and durable perovskite-based fuel electrode material— La$_{0.6}$Sr$_{0.4}$Co$_{0.15}$Fe$_{0.8}$Pd$_{0.05}$O$_{3-𝛿}$ (LSCFP)—for reversible SOCs. The LSCFP material under the fuel electrode condition is fully transformed into a stable Ruddlesden-Popper phase decorated by bimetallic Co-Fe nanocatalysts. The SOC with LSCFP fuel electrode yielded outstanding performances in both fuel cell (2.00 W/cm$^2$) and electrolysis cell (2.23 A/cm$^2$ at 1.3 V) modes at 850 °C, with remarkable reversible- cyclic stability. (Chapter IV) This heterointerfaces of fuel electrode also effective for electrolysis of CO2. To achieve highly robust and active catalyst for CO2 reduction reaction, we tailored double perovskite oxide (PrBaCo$_2$O$_{5+δ}$) by doping Fe and Mn in the B-site of perovskite lattice Pr$_{4/3}$Ba$_{2/3}$Co$_{2/3}Fe$_{2/3}$Mn$_{2/3}$O$_{5+δ}$ (PBCFM21). (Chapter V) This systematic study suggests a method for rational design of advanced alternative electrodes for application in SOC.