Understanding the Mechanical Challenges in Proton Conducting Ceramics

Proton-conducting ceramic cells are emerging as a promising technology for the electrification of fundamental sectors of the energy and chemical industry due to their flexibility and wide applicability, ranging from electric grid balancing to the production of fundamental chemical building blocks such as hydrogen and ammonia.
As for other high-temperature electrochemical devices, the development and large-scale deployment of proton-conducting ceramic components are strongly dependent on the fabrication of devices with satisfactory mechanical properties that can ensure their long-term reliability.
During fabrication and operation, these devices are affected by mechanical stresses arising from a large variety of factors, including thermal gradients, thermal expansion coefficient mismatch between components, chemical expansion due to dissociative adsorption of water and pressure gradients. Generation of such mechanical stresses in brittle components as the ones composing proton-conducting ceramic cells can not only cause fracture of the component in the short term but, coupled with chemical and microstructural degradation of the device, can lead to delayed failure of the latter.
Extensive characterization of the mechanical properties of the materials both at room temperature and upon short and long-term exposure to relevant operating conditions is required to formulate appropriate development strategies and design adequate operation procedures in order to retain the structural integrity of the device over its operational lifetime.
The aim of the research activity described in this dissertation is to contribute to the body of knowledge in regard to the mechanical properties of proton-conducting ceramics.
The research activity here described can be divided into two main topics: the characterization of the fracture toughness and slow crack growth behaviour of metal proton-conducting ceramic composites adopted as load-bearing components in proton-conducting ceramic cells, and the evaluation of the characteristic strength of proton-conducting ceramic tubular half-cells at different operating conditions and upon long-term exposure to environments simulating the operating conditions of the device.
The double torsion technique was adopted for the characterization of Ni-BaCe_0.2Zr_0.7Y_0.1O_3-δ and Ni-BaCe_0.4Zr_0.4Y_0.2O_3-δ supports in terms of their fracture toughness and subcritical crack growth behaviour. The two metal ceramic composites showed a fracture toughness equal to 3.04 MPa*m^1/2 and 1.74 MPa*m^1/2, respectively. The difference in the latter property between the two composites was associated mainly to their different porosity although the role of grain size and ceramic phase composition deserves further analysis.
Furthermore, both composites appeared to be affected by slow crack growth due to the interaction of water molecules with the crack tip, ensuring fracture propagation at stress intensity factors equal to approximately 81% and 86% for Ni-BaCe_0.2Zr_0.7Y_0.1O_3-δ and Ni-BaCe_0.4Zr_0.4Y_0.2O_3-δ, respectively.
During this investigation, it was noticed that Ba volatilisation and consequent partial decomposition of the ceramic phase can strongly hinder the mechanical stability of the composite due to fast fracture propagation at relatively low stress intensity factors.
It was concluded that, although acceptable, the fracture toughness of protonic metal-ceramic components is generally lower than the fracture toughness of metal-ceramic composites adopted for Solid Oxide Cells and that further optimization of the microstructural parameters and fabrication conditions is required to improve their resistance towards fracture propagation, especially in order to avoid the undesired volatilisation of Ba. On the other hand, it was also concluded that protonic metal-ceramic composites appear to be less prone towards subcritical fracture propagation compared to the aforementioned solid oxide cell cermets.
In two subsequent studies, the characteristic strength of tubular proton-conducting ceramic half-cells was investigated via a high-throughput four-point bending technique. The objective was to assess how exposure to simulated operational conditions affects the morphological, chemical, and mechanical properties of components, thereby elucidating their impact on device mechanical integrity.
In the first of these two studies, tests were carried out at room temperature in ambient air and at 650°C in reducing environments. Additionally, four sets of samples underwent different ageing protocols for 1000 hours, with increasing steam partial pressure in reducing atmospheres before testing at high temperature.
A decline in mechanical performance was observed after exposure to elevated temperatures, attributed to reduced fracture resistance of the ceramic phase and increased ductility of Ni in the fuel electrode. Prolonged exposure to anhydrous reducing atmospheres did not significantly degrade specimen strength. However, changes in the hydration state of the protonic lattice under steam-rich atmospheres led to reduced Weibull parameters, possibly due to stress generation upon dehydration of the perovskite lattice and thermal cycling. The extent of degradation appeared proportional to steam partial pressure. Furthermore, the formation and agglomeration of exsolved Ni nanoparticles under reducing conditions are expected to hinder the device's mechanical reliability.
Similar observations were made also from the analysis of the data obtained in the following investigation where the impact of ammonia-rich atmospheres on characteristic strength and Weibull moduli of tubular half-cells was characterized via exposure to anhydrous and humidified ammonia for 100 hours at 650°C followed by four-point bending tests at the same temperature in anhydrous reducing conditions.
It was concluded that the two principal degradation phenomena affecting the aforementioned samples in all the environmental conditions considered in these two investigations can be attributed to variations in the hydration state of the BCZY phase and the exsolution of Ni nanoparticles.
The data obtained from the experimental activity allowed the development of a finite element model simulating a tubular proton-conducting ceramic cell in electrolysis operation. In light of the satisfactory mechanical strength of the components analysed via four-point bending and of the stress field obtained from the finite element model simulation, the devices are expected to ensure good mechanical reliability in the investigated conditions and the main risk factor is represented by large internal pressurization. However, further work should focus on the impact of creep and thermal cycling on the long-term reliability of the components.
The characterization work described in this dissertation allowed the definition of suitable development strategies to improve the mechanical performance of proton-conducting ceramic composites, including microstructural and chemical optimization of the perovskite phase. Furthermore, the adoption of non-protonic supports has been proposed as a promising fabrication strategy to manufacture high-strength proton-conducting ceramic composites, and it is further discussed.