Protonic Ceramic Cells for Hydrogen Production and Purification

The environmental issues caused by the combustion of conventional fossil fuels and the energy crisis are driving a shift towards clean and highly efficient utilization of fossil fuels and towards an increasing share of renewable energy sources. Many clean energy technologies have been developed to tackle the challenges. Among them, protonic ceramic cells (PCCs) are emerging as a potential next-generation technology for electrochemical energy conversion and storage due to its various unique advantages such as potentially low cost and lower operating temperatures (400-600 °C) thanks to its high protonic conductivity. Nowadays, there are two main obstacles to developing PCCs: 1) lack of cost-effective and scalable manufacturing process; 2) lack of appropriate electrode materials for PCC applications. This project focuses on the second obstacle.

The first part of this Ph.D. thesis is devoted to developing high-performance and durable oxygen electrodes for PCCs. First, a literature review on oxygen electrodes for PCCs is conducted (Chapter 3), where the recent research progress is comprehensively summarized and reviewed, and prospective pathways for further development are provided.

Experimental research on oxygen electrodes is then devoted to two aspects: 1) improving the oxygen electrode performance at intermediate temperatures (400-600 ℃); 2) enhancing the electrode durability, particularly under high steam content for protonic ceramic electrolysis cell (PCEC) applications. Throughout the Ph.D. work, the electrochemical performance of the developed oxygen electrodes is evaluated in a symmetrical cell configuration as a function of temperature, oxygen partial pressure (PO₂), and steam partial pressure (PH₂O). In order to determine the rate-determining step (RDS) among the various electrode reaction processes in the oxygen electrode, equivalent circuit modelling (ECM) is applied in deconvoluting the electrochemical impedance spectra. In addition, the relationship between performance degradation and microstructure changes is further investigated.

In Chapter 4, a LaCoO₃ (LC)-BaZr_0.8Y_0.2O_3-δ (BZY20) composite oxygen electrode is developed by infiltrating the LC catalyst into the porous BZY20 backbone. Symmetrical cells with a configuration of LC-BZY20//BZY20//LC-BZY20 are prepared, and the performance and durability of the LC-BZY20 electrode are investigated using electrochemical impedance spectroscopy (EIS). The electrode polarization resistance (R_P) is measured as 0.28, 0.62. 1.09 and 1.45 Ω cm² per single electrode in humidified synthetic air at 600, 550, 500, and 450 °C, respectively. Furthermore, the developed LC-BZY20 electrode displays good stability, without significant performance degradation when tested at 600 °C in humid air for 900 h. The influence of PO₂ and PH₂O on the response of the EIS is further studied, and a set of chemical and electrochemical processes involved in the steam splitting reaction in the LC-BZY20 electrode is proposed.

In Chapter 5, another type of composite oxygen electrode is prepared by infiltrating a mixed protonic-electronic conducting material, Ba_0.5Gd_0.8La_0.7Co₂O_6−δ (BGLC), into the proton-conducting BZY20 backbone. The composite oxygen electrode is studied in the symmetric cell configuration (BGLC-BZY20//BZY20//BGLC-BZY20). Three electrode reaction processes are observed from the electrochemical impedance spectra measurement, which are tentatively assigned to the diffusion of adsorbed oxygen/proton migration/hydroxyl formation, oxygen reduction, and charge transfer, from the low- to high-frequency range, respectively. The BGLC-BZY20 electrode developed in this work shows a polarization resistance of 0.22, 0.58, and 1.43 Ω cm² per single electrode in 3% humidified synthetic air at 600, 550, and 500 °C, respectively. During long-term measurement, the cell shows no degradation in the first 350 hours but degrades afterward due to the formation of BaCO₃ on the electrode surface.

The second part of this thesis (Chapter 6) focuses on the development of PCCs for electrochemical hydrogen pumping (EHP). Benefiting from the booming development of PCCs, an increasing number of advanced manufacturing techniques and membrane materials have been developed for hydrogen purification in recent years. This part of the work is in collaboration with Shanghai Institute of Ceramics and University of Science and Technology of China where the PCC cells for hydrogen pumping are being made. Our efforts are then devoted to two aspects: 1) studying electrode (anode, cathode) reaction mechanism by using the distribution of relaxation times (DRT) and ECM to deconvolute the measured impedance spectra; 2) clarifying the cell degradation mechanisms via durability testing and correlating the electrochemical performance degradation with electrode microstructure changes.

Large footprint (144 cm²) PCCs with NiO-BZCY//BZCY//NiO-BZCY for hydrogen pumping are successfully fabricated by the tape-casting, lamination and co-sintering methods. Kinetics analysis of electrode reaction processes is performed via symmetrical cell testing. According to the EIS and DRT results, the cell resistance is dominated by the resistance of the dissociative adsorption of hydrogen processes at 650-700 °C or by that of the hydrogen diffusion at 750-800 °C. Moreover, the cell shows good electrochemical performance and durability in a 90 hour EHP testing. A hydrogen flux of 8.7 mL·cm^-2·min^-1 is obtained at 1.33 A cm^-2, with a Faradaic efficiency of 90% for pumping pure hydrogen out of a 50H₂/50N₂ gas mixture at 650 °C.

In summary, the findings reported in this thesis demonstrate the potential of electrode nano-engineering via infiltration and applications of PCC cells in hydrogen purification, promoting the PCC technology for further integration into the future sustainable energy system.