dc.description.abstract | Despite the significant advantages of solid oxide fuel cells (SOFCs) such as high efficiency, environment friendliness, and fuel flexibility their high operation temperatures (>800 ⁰C) often result in fast thermal degradation, which limits their commercialization. SOFCs based on proton-conducting electrolyte (termed P-SOFC) has drawn much attention in the last few decades, because proton conduction has a substantially lower activation energy compared to that of oxygen ion conduction, rendering the possibility of a lowered operation temperature (400-800 ⁰C). However, the performance of P-SOFC is still limited, because at lower operation temperatures polarization resistance become the rate-limiting step due to the sluggish nature of oxygen reduction reaction (ORR) at the cathode side. Application of a mixed ionic-electronic conductor (MIEC) perovskite oxide La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode in P-SOFCs can restrict the water formation site to three-phase boundaries adjacent to the electrolyte/cathode interface, thereby limiting the current density. Thus, the development of cathode materials with higher charge transport and transfer rates is urgently needed for raising the power density of a fuel cell operated at intermediate temperatures.
The triple-conducting (e-/H+/O2-) oxides have been extensively studied as the most promising cathode materials for protonic solid oxide fuel cells (P-SOFCs) because of their excellent catalytic activity at lower operating temperatures. However, direct application of these cathode materials by brush-painting or screen-printing provides limited contact area with the underlying electrolyte layer, resulting in high cathode ohmic and polarization resistances. In the first section of this dissertation, it is demonstrated that a bulk heterojunction Gd0.3Ca2.7Co3.82Cu0.18O9-δ (GCCCO)-BaCe0.6Zr0.2Y0.2O3-δ (BCZY) layer with a domain width of ~5 nm can be grown by pulsed laser deposition (PLD) via spontaneous phase separation. Such a nanocomposite interlayer between spin-coated BCZY electrolyte and brush-painted GCCCO cathode can effectively increases the interfacial area between the two distinct phases and facilitates proton transport across the interface. This electrode design reduces the ohmic resistance by 0.35 Ω cm2 and the polarization resistance by a factor of three, thus significantly boosting the cell performance. Later it is identified that the limited performance enhancement after the implementation of bulk heterojunction interlayer, is attributed to issues in the underlying half-cell. Therefore, a fuel cell with same BCZY buffer layer, GCCCO-BCZY nanocomposite interlayer, and GCCCO cathode layer is tested on a commercial BCZYYb-based half-cell. Results show a more than twofold increase in power density, reaching 863 mW/cm2 at 700 ⁰C, and a dramatic 2.5-fold decrease in ohmic resistance to 0.7 Ω cm2 at 700 ⁰C. This confirms that the bottle neck in cell resistance and peak power density, with the GCCCO cathode, shifts to the anode and/or electrolyte when using a PLD-deposited GCCCO-BCZY nanocomposite cathode interlayer, highlighting the high performance of this cathode structure.
In the second section of this dissertation, the improvement of in-house substrate is accomplished by optimizing the microstructure of the fuel electrode. Since the anode support is the thickest (~1mm) part in the fuel cell, its microstructure can influence the electrochemical performance of an anode supported protonic ceramic fuel cell (PSOFCs) to a certain extent, due to the fact that the pore size and its distribution in the fuel electrode are closely linked to the mass transport resistance associated with the fuel, impacting its ability to reach the electrochemically active sites available for the hydrogen oxidation reaction. Therefore, we report our findings about the feasibility of using ashless paper-fibers as a pore former for the hydrogen electrode of P-SOFCs, demonstrating its unique microstructure with large interconnected cylindrical pores that enhance fuel transport efficiency. A Ni-BaCe0.7Zr0.1Y0.1Yb0.1 anode substrate with 20 wt % paper fibers achieve a porosity of 37.97 vol% with a lower content of paper fibers, as compared to using 30 wt% potato starch porogen (29.12 vol%). Subsequently, the thin film deposition of ceramic functional layers and interlayers on a macro-porous anode substrate is achieved using pulsed laser deposition (PLD), facilitated by its surface modification through electrophoretic deposition of a pore filler layer. An anode supported single cell with 20 wt % paper fibers exhibit the best electrochemical performance at 600 ⁰C with a peak power density of 550 mW/cm2, ohmic resistance of 0.678 Ωcm2, and a polarization resistance of 0.172 Ωcm2 as compared to the cell with 30 wt % starch porogen (389 mW/cm2, 0.802 Ωcm2, and 0.252 Ωcm2). These findings underscores the efficacy of paper fibers as a pore former for tailoring the microstructure of the hydrogen electrode, highlighting the potential for improved PSOFC performance at intermediate temperatures. | en_US |