Efficient and Economically Favorable Co-Free Air Electrodes for Solid Oxide Cells

Abstract

Cobalt-based oxides are considered as state-of-the-art air electrode materials for solid oxide cells (SOC). It is due to their extraordinary catalytic activity and high mixed ionic-electronic conductivity. However, geo-politically unstable regions which supply cobalt (approx. 70% global mining production in 2018 took place in the Democratic Republic of the Congo), depleting reserves and the forecasted continuously growing demand for the element in next decades (related to the Li-ion batteries market) may result in risks related to the security of supply. Due to mentioned considerations, in 2011 cobalt was classified by EU as a critical raw material [1]. For that reason, European Commission provide incentives for the development of Co-free air electrodes which will eliminate the aforementioned risks and also address the toxicity of cobalt. Moreover, special attention is paid to the socio-economic aspects related to its mining which often does not meet global standards related to the ethical supply of the element.Among proposed in literature air electrodes dedicated both solid oxide fuel cells (SOFC) and solid oxide electrolyzers (SOE) the La(Ni,Cu)O3-δ-based materials seem to be a promising and economically viable alternative. Alike cobalt, nickel and copper are of great economic importance, however they are not considered as critical raw materials and, what is worth to emphasize, can be effectively recycled without compromising the quality. Parent LaNiO3 and LaCuO3 perovskites are characterized by a set of beneficial physicochemical properties, including high electric conductivity, which in the case of LaCuO3 reaches 106 S cm-1 in an oxidizing atmosphere, large range of oxygen nonstoichiometry (up to 0.5 mol per mol of oxide) as well as high catalytic activity [2,3]. As we presented in [4] mixed Ni-Cu perovskites are stable in air while exposed to the temperature up to 900 °C, and only in the case of Cu-rich composition, 0.75 mol Cu per mol of La(Ni,Cu)O3-δ, phase-transition from P4/m to P4/mbm symmetry was recorded. Comparing to the Co-based materials La(Ni,Cu)O3-δ oxides demonstrate moderate TEC in the range from 10 to 14·10-6 K-1. This feature should provide a good thermomechanical compatibility with CGD buffer layer deposited on the 8YSZ electrolyte. LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ oxides are characterized by a large variation of oxygen nonstoichiometry (up to 0.15 mol of oxygen per oxide at 850 °C) and relatively high electrical conductivity (order of several dozen). The electrodes based on these oxides show low polarization resistance in air at 800 °C equal to 0.056 Ω cm2 and 0.054 Ω cm2 for LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ, respectively. Preliminary studies of the LSGM-supported full cell with LaNi0.5Cu0.5O3-δ as air electrode (0.87 W cm-2 in SOFC mode and ca. 1 A cm-1 at 1.3 V in SOE mode at 900 °C) revealed potential of (Ni,Cu)-based perovskite in application as an electrode for reversible solid oxide cells (rSOC).In this work the assessment of (Ni-Cu)-based perovskites in terms of the availability of critical raw materials, sustainability and economic aspect in relation to the Co-based oxides is presented. Moreover, the electrochemical studies of fuel electrode-supported cells with selected La(Ni,Cu)O3-δ materials making the air electrodes are presented. LaNi0.5Cu0.5O3-δ and LaNi0.25Cu0.75O3-δ made using sol-gel method were used to fabricate air electrodes layers sintered at relatively low temperature (below 1,000 °C) and characterized by fine microstructure allowing sufficient gas diffusion. The electrochemical measurements included current density-voltage curves, registered in SOFC and SOE modes, and EIS studies performed at different condition (temperature in the 700-800 °C range and various inlet gases composition).