Application of High-Temperature Raman Spectroscopy (RS) for Studies of Electrochemical Processes in Solid Oxide Fuel Cells (SOFCs) and Functional Properties of their Components

Abstract

SOFC efficiency is directly determined by the electrodes, optimization of which is related to the study of the mechanisms of electrochemical current-generating reactions taking place. Such studies are tricky due to the high operating temperature of SOFCs, high current loads, and corrosive gas media. A promising research method is Raman spectroscopy (RS) under operating conditions of SOFC. At ISSP RAS, a combined experimental technique and setup have been created that combines the capabilities of traditional electrochemical methods, as well as high-temperature Raman spectroscopy [1]. In order to study the processes in the electrochemically active zone, a special geometry of samples was developed on basis of optically transparent single crystal membranes of an anionic conductor with a counter-electrode of a toroidal shape. With the use of this combined technique and special geometry of samples in previous works, studies of the kinetics of reduction and morphological changes of nickel in composite SOFC anodes were carried out. It was shown that the first reduction cycle differs significantly from the subsequent ones both in the initial delay and in the total time of the process. SEM studies have shown that these changes are associated with a morphological rearrangement that occurs during the first reduction cycle: the grain size of NiO is significantly reduced compared to the initial one. For model SOFCs investigated in the OCV mode [2] after the initial microstructural rearrangement in the redox cycle, the behavior of standard cermet anodes in a H2–N2 flowing atmosphere can be described using Avrami model. The influence of the composition of fuel on Raman spectra obtained from the internal interface in the current load mode was also investigated [3], and the correlations of obtained data with the cell voltage were studied. It was shown that the corresponding mechanisms that determine the reaction kinetics can be associated with the transfer of ions through the GDC|YSZ. Subsequent rate limiting steps such as electrochemical oxidation of hydrogen in the Ni-GDC layer are undetectable for the geometry under test due to the limited penetration depth of the laser beam. Comparative studies of model SOFCs with a supporting anode substrate (ASC) and a supporting solid electrolyte (ESC) were carried out using the in-situ RS technique [4]. It was shown that the addition of the GDC indicator layer makes it possible to carry out an in-situ study of the chemical potential of oxygen at the internal interface of the anode|electrolyte depending on current load or fuel mixture, but the GDC/YSZ two-layer thin-film electrolyte exhibits electronic or gas leakage due to poor stability in a reducing atmosphere. Despite the significantly lower internal resistance compared to the ESC, the quality of the ASC thin film membrane severely limits research depending on the operating temperature, current load and fuel composition. In this case, the use of thin-film membranes significantly reduces the effect of 8YSZ on the Raman spectra of the internal interface. After post-processing and normalization, the obtained Raman spectra show a similar effect of the current load or the hydrogen content in the fuel gas mixture on the GDC peak area (460 cm-1). The linear dependence of the OCV on the peak area of ~460 cm-1 makes it possible to assess the relationship between changes in the peak area and the evolution of the chemical potential of oxygen at the anode under a current load. Comparative analysis of the anodic impedance for different fuel mixtures suggests that spectroscopic measurements of the internal interfaces provide direct information on the contribution of the fuel oxidation reaction to the total losses in SOFC. In addition to studies of complete fuel cells, high-temperature Raman spectroscopy is used to study the structure of single-crystal samples of anionic conductors [5], including at the operating temperature of SOFCs. High-temperature RS allows one to obtain additional information on the microstructure of samples both at room temperature and at working temperature, where most other research methods do not allow research. New combined technique was used to study other components of high-temperature electrochemical devices: sealing glasses [6] for SOFCs and optical glasses [7]. This work was supported by Russian Scientific Foundation, grant no. 17-79-30071.[1] D.A.Agarkov et al. ECS Trans., vol.68, iss.1, pp.2093-2103 (2015).[2] D.A.Agarkov et al. Solid State Ionics, vol.302, pp.133-137 (2017).[3] D.A.Agarkov et al. Solid State Ionics, vol.319C, pp.125-129 (2018).[4] D.A.Agarkov et al. Solid State Ionics, vol.344, p.115091 (2020).[5] D.A.Agarkov et al. Solid State Ionics, vol.346, p.115218 (2020).[6] A.Allu et al. ACS Omega, vol.2, pp.6233-6243 (2017).[7] M.K.Kokila et al. Solid State Sciences, vol.107, p.106360 (2020). Figure 1