Elsevier

Solid State Ionics

Volume 301, March 2017, Pages 106-115
Solid State Ionics

Understanding the electrochemical behaviour of LSM-based SOFC cathodes. Part I — Experimental and electrochemical

https://doi.org/10.1016/j.ssi.2017.01.007Get rights and content

Highlights

  • Studied the effects of cathodic overpotential and microstructure in LSM cathodes

  • The microstructure matters at low bias while impedances level off at high bias

  • Identified charge-transfer at high frequency and transport process at low frequency

  • Transition from surface path to bulk path at 0.2 V identified with equivalent circuits

  • Verified the geometric requirements to correctly measure impedance under bias

Abstract

The enhancement of electroactivity and stability of solid oxide fuel cell (SOFC) cathodes is an open issue for the scientific community. Only a careful experimental approach, coupled with advanced modelling and interpretation, allows for a comprehensive understanding and optimization of materials not completely exploited, such as strontium-doped lanthanum manganite (LSM). This paper, as first part of a couple of articles, presents experimental measurements of porous LSM cathodes and gives an interpretation of the oxygen reduction reaction (ORR) mechanism. The experimental section takes into account the effect of cell geometry and different electrode microstructures resulting from sintering the cathodes at different temperatures. Electrodes are tested with electrochemical impedance spectroscopy (EIS) in three-electrode configuration and the influence of cathodic dc bias, oxygen partial pressure and temperature is analysed. The effects of dc overpotential on the processes are remarkable and a particular attention is dedicated to this parameter. The analysis of experimental data with equivalent circuits identifies a critical overpotential between 0.15 and 0.2 V. Around this value the system undergoes a modification in the ORR mechanism and a new reaction path is identified. Analogous conclusions are achieved in the second paper by using a detailed mechanistic modelling approach.

Introduction

Solid oxide fuel cells (SOFCs) constitute one of the pillars in the future strategy to build a sustainable society based on low carbon energy technologies [1]. Despite big efforts and very promising results, the SOFC technology is not yet mature to be transferred into real system applications, or the performance makes the system not fully exploitable. One of the main issues is related to the stability of materials [2], [3].

Lowering the operating temperature below typical 1150 K has been recognized as the first key parameter to reduce the cell degradation kinetics [4], thus, very active materials have been proposed in order to counterbalance the unfavourable new condition for thermally activated processes [5]. Consequently, since about ten years ago, traditional La1−xSrxMnO3 (LSM) based cathodes have been replaced by more active mixed ionic electronic conductors materials such as LSCF and BSCF [6], [7], [8], [9], [10], [11], [12], [13], which exhibit excellent oxygen electroactivity at temperatures in the order of 900–1050 K [1], [14].

The SOFC specific power has thus reached acceptable values [9], [10], [15], but degradation still remains an open issue [2], [16]. The enhancement of the cathodic performance (i.e. both stability and electroactivity) requires a detailed knowledge of chemical and electrochemical phenomena [17], [18]. This can be achieved through a careful experimental approach coupled with an advanced interpretation at high detailed level, such as provided by modelling at different length scales.

In this context, a renewed interest in LSM has recently been highlighted in the literature [19], [20], [21], [22], essentially due to an attempt to fruitfully exploit properties and advantages of LSM in SOFC systems running at lower temperature. This perovskite is perhaps inadequate if used as cathodic electrocatalytic backbone, but its proved redox stability and compatibility with other cell components are non-negligible points that account for its use in harsh conditions such as in SOEC (solid oxide electrolysis cell) oxygen electrodes.

Rather than the fabrication of electrodes based on LSM backbone, there are many examples of applications of LSM as infiltrating compound within the porous cathode structure [23], in composite electrodes [24], [25], [26], [27] or applied as thin layer [28], [29]. In all these cases the cathode shows improved performance and stability. However results clearly indicate that, in order to exploit the potential features of LSM, its behaviour must be totally understood.

In particular the understanding of the oxygen reduction reaction (ORR) remains one of the key factors to decrease voltage losses [30], [31], [32], [33], [34]. Many different interpretations have been proposed for the reaction kinetics in LSM-based systems but a comprehensive understanding has not been reached yet. In the literature, different steps have been considered as limiting factors: adsorption and/or dissociative adsorption of oxygen on LSM surface [35], first and second electron transfer [36], oxygen ion transfer through the electrode/electrolyte interface [37]. In addition, several types of oxygen transport phenomena occur within this main sequence. In fact, the possible oxygen species involved in the process can be neutral or charged. Finally, it has to be considered that the main processes occurring in the ORR mechanism could also depend on the cell/electrode preparation, testing conditions and fabrication history [38]. An example of the possible modifications in the ORR kinetic mechanism is the shift of the electroactive species from a surface path to a bulk path under specific operating conditions [21].

This study is motivated by the need to get further insight into LSM peculiarities, which could open new promising solutions for the next generation of SOFC cathodes. For this purpose, porous LSM electrodes with two different electrode morphologies are analysed, having different porosity and particle size, as a result of different sintering temperatures. By using pure LSM electrodes, the kinetic contribution in the ORR is not affected by the participation of any other phases constituting the electrode. This simplifies the microstructural characterization and model calibration performed in the second part of this series of papers. The cathodes are tested in an electrolyte supported half-cell configuration and the influence of applied overpotential, operating temperature and oxygen partial pressure is investigated to understand their effects on the kinetic properties. Experimental results are analysed with equivalent circuits, which highlight a change in the ORR controlling regime depending on the electrochemical working conditions and operating temperature. The magnitude of this change is related to the structural features of the electrodes as well as to LSM stoichiometry changes, resulting in a shift of the oxygen ionic transport from a surface to a bulk path [32], [39], [40], [41].

Particular attention has been paid to the cell geometry and the electrode alignment to ensure reliable electrochemical measurements. In addition, the experimental results constitute the supporting information for the development of a mechanistic model of the ORR, which is presented in the second part of this series of papers.

Section snippets

Experimental

The supporting electrolytes were produced by uniaxially pressing the YSZ powders (TZ-8Y Tosoh) followed by sintering at 1773 K for 1 h. The shaping of the YSZ powder was carefully designed in order to obtain pellets with diameter equal to 35 mm and thickness equal to 3 mm after sintering. A so thick electrolyte (about 3 mm) was used to meet the geometric requirements to avoid any artefacts due to possible electrode misalignment in the impedance measurements, in agreement with indications reported by

Effects of cell geometry

Some preliminary measurements were performed on the symmetrical cells prepared with the two geometric configurations, namely Cell-A and Cell-B, in order to quantify the effect of cell geometry on the impedance results at different overpotentials. Cell-A was made according to the geometric requirements suggested in the literature [50]: the working electrode (WE), with a diameter of 3 mm, was accurately aligned to the counter electrode (CE), while the reference electrode (RE) was placed co-planar

Conclusion

Although LSM has been replaced by more active electrocatalytic materials, its stability in harsh conditions for SOFC/SOEC application has driven a renewed interest of the scientific community in this cathode material. This study shows that, in order to exploit its properties, a fundamental understanding of LSM behaviour in a wide range of microstructural and operating conditions is still necessary. In order to do so, LSM-based half-cells were fabricated, characterised and tested to get an

Acknowledgements

The financial support of the Italian Minister of Education, University and Research (MIUR) National PRIN project (BIOITSOFC) protocol number 2010KHLKFC_005 is acknowledged. All the authors wish to thank the two unknown reviewers whose insightful comments have improved the quality of the paper.

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