Nanopore Patterned Pt Array Electrodes for Triple Phase Boundary Study in Low Temperature SOFC

Abstract

This paper describes the fabrication and investigation of morphologically stable model electrode structures with well-defined and sharp platinum/yttria-stabilized zirconia ͑YSZ͒ interfaces to study geometric effects at triple phase boundaries ͑TPBs͒. A nanosphere patterning technique using monodispersed silica nanoparticles, which are applied to the YSZ surface by the LangmuirBlodgett method, is employed to deposit nonporous platinum electrodes containing close-packed arrays of circular openings 300-400 nm in diameter through which the underlying YSZ surface is exposed to the gas phase. These nanostructured dense Pt array cathodes exhibited better structural integrity and thermal stability at the solid oxide fuel cell ͑SOFC͒ operating temperature of 450-500°C 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 the aerial density of TPB length. These controlled experiments also demonstrated that when normalized with respect to TPB length, the performance of different cells with different TBP densities agree well each other, indicating that TPB length governs cell performance especially in the activation polarization regime, as expected. Cells with a higher TPB density achieved better fuel cell performance in terms of higher power density and lower electrode impedance.Solid oxide fuel cells ͑SOFCs͒ are efficient energy conversion devices that are being developed for practical applications. Due to large activation energies ͑ϳ1 eV͒ for oxide ion transport in solid oxide electrolytes and relatively sluggish oxygen reduction reaction at the cathode, SOFCs are usually operated at elevated temperatures ͑800-1000°C͒ to obtain practically meaningful fluxes and fuel cell performance. Typically, an SOFC element is made of an yttriastabilized zirconia ͑YSZ͒ electrolyte layer, a mixed conducting ceramic cathode such as La 1−x Sr x Co 1−y Fe y O 3 ͑LSCF͒ and La 1−x Sr x MnO 3−␦ ͑LSM͒, and a cermet anode such as Ni/YSZ.The operation of SOFCs at elevated temperatures may be desirable for enhanced kinetics and transport purposes but poses serious challenges in microstructural and thermal stability, seal integrity, aging and degradation, thermal cycling, and cost of materials and fabrication. To mitigate some of these problems, recent efforts have been aimed toward lowering the operating temperature of SOFCs to an intermediate temperature ͑IT͒ regime of 600-800°C, i.e., IT-SOFCs. 1-4 Although oxygen chemical diffusion in mixed conducting electrodes such as LSCF and LSM is relatively fast at elevated temperatures, 5-9 their catalytic activities and transport rates decrease precipitously with temperature that leads to increased activation losses at ITs. 1,[10][11][12][13] In recent years, the thrust of our research effort has been aimed to further lower the operating temperature of SOFCs to a regime between 300 and 500°C by employing thin-film structu...

Engineering of the electrode structure of thin film solid oxide fuel cells

“…Many studies to lower the electrolyte resistance of SOFCs have been conducted by exploring alternative electrolyte materials and by developing cell structures with thin film technologies [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. Recently, ceria-based electrolyte materials have received attention due to their high ionic conductivity and reactivity at lower temperatures.…”

“…Some research groups have fabricated nano-scale thin film SOFCs with freestanding membranes by applying microelectromehanical system techniques [9][10][11][12][13][14][15]. They have also demonstrated that the electrode/electrolyte interfacial area of the freestanding thin film SOFC can be drastically increased by various nano-scale fabrications [11][12][13]. However, the freestanding structures with only tens of nanometer thick membranes will inevitably suffer from thermomechanical issues.…”

Silver Nanomesh as a Cathode for Solid Oxide Fuel Cells

“…The porous silver nano-mesh was created by a recipe specified in our previous publication (11). As a result, well-defined 500~600 nm diameter pores were patterned on the silver thin film in a close-packed hexagonal array.…”

Enhanced performance and endurance of nano-porous platinum solid oxide fuel cell electrodes by oxygen partial pressure cycling

“…Kim et al reported a 0.7 mW/cm 2 peak power density at 450°C for a conventionally tested SOFC device with a commercial 8-YSZ substrate and 80-nm-thick sputtered Pt electrodes. [1] It performed similarly to this study's "no pretreatment" device tested at the same temperature. This study's FG/air-cycled device, on the other hand, had a 10× higher peak power density than the literature device at 450°C.…”

“…However, a major difficulty associated with nanoporous Pt electrodes, even at low temperatures, is the tendency for grain coarsening and film break-up on the surface of electrolytes such as yttria-stabilized zirconia (YSZ). [1][2][3][4][5][6] These morphological changes can result in severe loss of triple-phase boundaries (TPBs -regions where gas, electrolyte, and electrode coexist), which in turn increases the activation loss and overpotential required to run the fuel cell. At low-to-moderate temperature, slow cathode kinetics tend to limit the performance of SOFCs, and high TPB area is a key structural feature for efficient O 2 reduction and oxygen ion incorporation into the electrolyte at the cathode side of the cell.…”