Phase Transition Mapping by Means of Neutron Imaging in SOFC Anode Supports during Reduction under Applied Stress

Planar solid oxide electrochemical cells (SOC) are composed of at least three layers: a anode, a electrolyte and a cathode [1]. Cermet Ni-YSZ (yttria stabilized zirconia) is a material widely used for production of anodes in solid oxide fuel cells (SOFC) and cathodes in solid oxide electrolysis cells (SOEC), but also for supports. Mechanical and electrochemical performance of these layers, and thus whole cells, depends on their microstructure. One of the commonly used deposition techniques of the Ni-YSZ supports is tape-casting from a NiO-YSZ slurry, followed by sintering and reduction of NiO during the initial operation of the cell. Conditions, i.e. temperature, atmosphere and stress, applied during this procedure, determine the microstructure of the support and thereby the cell performance, therefore in-situ observation of these processes under different conditions can provide knowledge needed to optimize the SOC performance. 

Neutron imaging [2] is a nondestructive measurement method holding a great potential for in-situ characterization of SOC. In contrast to X-ray imaging, due to high material penetration by neutrons, it is possible not only to image real size SOC stacks, but also to perform experiments in-situ, using sample environment providing the SOC operation conditions. Here we present the results of energy resolved neutron imaging measurements applied to in-situ investigations of Ni-YSZ anode supports for SOFCs.

The energy resolved neutron imaging technique is based on the measurement of neutron transmission through the sample depending on neutron energy/wavelength. In the achieved patterns, sudden changes in transmission/ attenuation – so called “Bragg edges”- occur at certain neutron wavelengths, which correspond to d-spacing values present in the investigated polycrystalline material. Thus, such Bragg edge patterns (fig.1a) contain information about elements, crystal structure, crystalline phases in a sample, texture or strain field.

Bragg edge patterns can be measured both at continuous and pulsed neutron sources. In the first case, where a polychromatic continuous neutron beam is generated, Bragg edge patterns are obtained by selecting specific neutron wavelengths using monochromators and subsequent measurements of transmission for different wavelengths. In the second case, where pulses of white neutron radiation are created, neutrons with different wavelength (and velocity) reach the sample at different times. Using detectors able to record simultaneously intensity of the transmitted beam and corresponding neutron arrival time, whole Bragg edge patterns are acquired at once. This approach is called time of flight (TOF) method.

We present the results of in-situ investigation of NiO reduction and re-oxidation in anode supports performed by means of energy resolved neutron imaging at both continuous (SINQ, PSI ,fig.1b) and pulsed neutron source ( ISIS, fig.1a). Figure 1a presents Bragg edge patterns recorded at ISIS around an edge corresponding to the d-spacing of NiO(200). Different patterns were measured at different times during the reduction. It is apparent, how the height of the NiO edge decreases with time. As the edge height depends on the amount of the NiO phase, it is possible to compare the reaction rate in different regions of the sample by evaluating the Bragg edge patterns for these regions. In our previous work [3] we have demonstrated by ex-situ Bragg edge neutron imaging  feasibility to detect and distinguish Ni and NiO phases within the Ni-YSZ composite. Figure 1b shows the Ni and NiO phase distribution in Ni-YSZ bars after consecutive reduction and re-oxidation processes performed at a continuous neutron source.

 In-situ neutron imaging was performed using our custom built furnace [4]. Its design allows to meet the requirements of the equipment used at neutron imaging instruments, to achieve the required spatial resolution of images, and at the same time, to provide conditions required for conducting NiO reduction i.e. high temperature and reactive atmosphere. Since in a real SOFC, anode supports are always exposed to high thermal and mechanical stresses, progress of NiO-YSZ reduction should be investigated in samples under applied stress [5]. Therefore, our furnace is equipped with a loading system for applying stress to the sample during the reduction process. In this work, we present results of in-situ observation of the phase transition during the NiO-YSZ reduction and re-oxidation performed under different conditions: different temperatures, with and without applied stress.

[1]   S. Singhal,  Solid State Ionics, vol. 135, no. 1–4, pp. 305–313, Nov. 2000.

[2]   M. Strobl et al., J. Phys.D.Appl.Phys., vol. 42, no. 24, p. 243001, Dec. 2009.

[3]  M. G. Makowska et al.; J. Appl. Cryst. 48, doi:10.1107/S1600576715002794, 2015.

[4]  M. G. Makowska et al.; submitted to RSI , 2015.

 [5]   H. L. Frandsen et al., Conf. Proc. Eur. Fuel Cell Forum, Lucerne, Switz., 2014.