Nanoscale surface engineering for ceramic fuel cells [electronic resource]
Keywords: Mechanical Engineering

Ceramic fuel cell (CFC) is an all-solid-state energy conversion device and usually refers to fuel cells employing solid ceramic electrolytes. The present generation of ceramic fuel cells can be classified into two types according to the electrolytes they use: oxygen ion conducting fuel cells, or solid oxide fuel cells (SOFCs) and proton conducting fuel cells (PCFC or PCOFC). CFCs usually have the highest operating temperature of all fuel cells at about 600~1000oC for reasonably active charge transfer reactions at the electrode-electrolyte interface and ion transport through the electrolyte. This high CFC's operating temperature has limited practical applications. The goal of my Ph.D. research is to minimize the activation losses at the electrode/electrolyte interface by nanoscale engineering to achieve decent performance of ceramic fuel cells at lower operating temperatures (300~500oC). This dissertation has three main nanoscale surface engineering approaches according to the fuel cell components: electrode structure, composite electrolyte structures with thin interlayers, and the fabrication of three-dimensional fuel cell membrane-electrode assemblies (MEAs). We would call the first part of the dissertation as nanoscale electrode structure engineering for ceramic fuel cells. It describes the fabrication and investigation of morphologically stable model electrode structures with well-defined and sharp platinum/yttria stabilized zirconia (YSZ) interfaces to study geometric effects at triple phase boundaries (TPB), which is known as the actual electrochemical reaction site. A nanosphere lithography (NSL) technique using monodispersed silica nanoparticles is employed to deposit nonporous platinum electrodes containing close-packed arrays of circular openings through the underlying YSZ surface. These nano-structured dense Pt array cathodes exhibited better structural integrity and thermal stability at the fuel cell operating temperature of 450~500oC when compared to porous sputtered Pt electrodes. More importantly, electrochemical studies on geometrically well-defined Pt/YSZ sharp interfaces demonstrated that the cathode impedance and cell performance both scale almost linearly with aerial density of TPB length. These controlled experiments also allowed for the estimation of the area of the electrochemical reaction zone. This information can be used as a platform for designing the electrode structure to maximize the performance of ceramic fuel cells. The second part of the experiment is about electrolyte surface structure engineering by fabricating composite electrolyte structures. This study describes, both theoretically and experimentally, the role of doped ceria cathodic interlayers and their surface grain boundaries in enhancing oxygen incorporation kinetics. Quantum mechanical simulations of oxygen incorporation energetics support the experimental results and indicate a low activation energy of only 0.07eV for yttria-doped ceria (YDC), while the incorporation reaction on YSZ is activated by a significantly higher energy barrier of 0.38eV. For experiments, epitaxial and polycrystalline YDC, gadolinia-doped ceria (GDC) thin films were grown by pulsed laser deposition (PLD) on the cathode side of 300[Mu]m-thick single crystalline (100) and 100[Mu]m-thick polycrystalline YSZ substrates, respectively. For the composite electrolyte sample with YDC interlayer, the Oxygen isotope exchange experiment was conducted employing secondary ion mass spectrometry (SIMS) with high spatial resolution (50nm). The surface mapping result of 18O/16O shows high activity at surface grain boundary regions indicating that the grain boundary regions are electrochemically active for oxygen incorporation reaction. Fuel cell current-voltage behavior and electrochemical impedance spectroscopy measurements were carried out in the temperature range of 350oC-450oC on both single crystalline and polycrystalline interlayered cells. Results of dc and ac measurements confirm that cathodic resistances of cells with epitaxial doped-cerium oxides (GDC, YDC) layers are lower than that for the YSZ-only control cell. This is attributed to the higher surface exchange coefficient for doped-cerium oxides than for YSZ. Moreover, the role of grain boundary density at the cathode side external surface was investigated on surface-engineered electrode-membrane assemblies (MEA) having different doped-ceria surface grain sizes. MEAs having smaller surface grain size show better cell performance and correspondingly lower electrode interfacial resistance. Electrochemical measurements suggest that doped-ceria grain boundaries at the cathode side contribute to the enhancement of oxygen surface kinetics. These results provide an opportunity and a microstructure design pathway to improve performance of LT-SOFCs by surface engineering with nano-granular, catalytically superior thin doped-ceria cathodic interlayers. Thirdly, as a reaction surface engineering for SOFC, we investigated a novel method for creating a three-dimensional (3-D) fuel cell architecture to enhance fuel cell performance by increasing the area of the electrolyte membrane. The research describes the fabrication and operation of a low temperature 3-D protonically conducting ceramic fuel cell featuring a close packed and free standing crater patterned architecture achieved by nanospherical patterning (NSP) and dry etching techniques. The cell employed conformal layers of yttria-doped barium zirconate (BYZ) anhydrous electrolyte membrane (~120nm) sandwiched between thin (~70nm) sputtered porous Pt electrode layers. The fuel cell structure achieved the highest reported peak power densities up to 186 mW/cm2 at 450oC using hydrogen as fuel. To further investigate the proton conductivity of the electrolyte, which is BYZ, we studied the effect of crystalline structures on proton conductivity of BYZ thin films. The results showed that the grain boundaries impede the proton transport through the grain boundary and cause extremely high resistance for ionic transport in the film. This experimental result also can provide significant implications in designing proton conducting ceramic fuel cells. All these efforts and investigations were intended to enhance the ceramic fuel cell performance at low operating temperatures (300--500oC) by improving electrode/electrolyte interface electrochemical reactions. We expect to achieve further enhancement when we combine the approaches each other. For example, fabrication of three-dimensional fuel cells with doped-ceria interlayers and composite electrolyte structures with optimized electrode nano-structures. Investigations are on-going in our laboratory as a future work.