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We herein report the development of the sputtering process for the fabrication of low-temperature thin film solid oxide fuel cells (TF-SOFCs) supported by anodized aluminum oxide (AAO). The process development has been conducted for each component, anode, electrolyte, electrolyte/cathode interlayer, and cathode to scale up the cell from 1cm x 1cm to 15 cm x 15 cm. In order to fabricate a uniform thin film regardless of the size of AAO, sputtering conditions such as power, pressure, angle, target-to-substrate distance (TSD), and time have been developed. The recipe for the anode and cathode has aimed at a porous structure with nano-grains for high reactivity. In contrast, the recipe for the electrolyte has been optimized for thickness uniformity and high density. With these sputtering recipes TF-SOFCs with the required microstructure and thickness uniformity have been successfully fabricated on 15 cm x 15 cm AAO substrates.

A novel cell design and its corresponding manufacturing processes are being evaluated to demonstrate a highly efficient, durable, and reduction-oxidation (redox) resistant solid oxide electrolysis cell (SOEC) technology for hydrogen production. The cell design includes a unique 3D hydrogen electrode configuration composed of two layers ‒ a 3D hydrogen electrode support layer (for improved cell redox resistance) and an exsolved perovskite hydrogen electrode active layer (for enhanced cell performance and increased durability). The hydrogen electrode support layer consists of specifically designed 3D networks of electrode materials; tetragonal zirconia (TZ) and Ni-yttria stabilized zirconia (YSZ). Each phase is topologically connected throughout the electrode with controlled geometry and connectivity. The TZ component plays a role as an outer frame to provide structural support, while the Ni-YSZ component inside the TZ frame imparts electrical conductivity for the hydrogen electrode support layer. The hydrogen electrode active layer, which primarily supports electrochemical reaction, is a nickel-substituted perovskite that will exsolve nickel nanoparticles on the oxide surface when exposed to a reducing environment and is redox-reversible. A fabrication scheme being developed for this cell design incorporates a printing-based additive manufacturing (AM) process for producing the 3D hydrogen electrode. This paper presents the progress to date in developing the printing fabrication process for the 3D hydrogen electrode support and evaluating exsolved nickel-substituted perovskite for the hydrogen electrode active layer.

A high-performance solid oxide fuel cell (SOFC) stack technology is being developed, based on a design concept that incorporates prime-surface metallic interconnects with the versatility of stacking different types of single cells (sintered cells or metal-supported cells). A preliminary design of the prime-surface interconnect based on an egg-carton shape configuration has been advanced for this stack concept. Laboratory-scale prime-surface interconnects have successfully been fabricated by stamping and evaluated for mechanical loading, flow distribution and current collection properties. A process based on sputtering has been developed for fabricating supported thin-film (several micrometers thick) cells for this stack technology. Fabricated thin-film cells have shown desirable structural characteristics and exceptional performance on different fuels at reduced temperatures (550o-650oC). Sputtered cells (with yttria stabilized zirconia (YSZ) for the electrolyte, gadolinia-doped ceria (GDC) for the electrolyte/cathode interlayer, nickel-YSZ for the anode and lanthanum strontium cobalt iron perovskite (LSCF)–YSZ for the cathode) have exhibited peak power densities of ~1.7 W/cm2 and ~2.1 W/cm2 with hydrogen fuel and air at operating temperatures of 600oC and 650oC, respectively.

High-temperature electrochemical cells based on solid electrolytes have been considered for synthesis and production of hydrogen and a variety of other fuels and chemicals. Solid oxide electrolysis cells (SOECs) for example, are being developed for hydrogen production (from water), syngas production (from mixtures of water and carbon dioxide) and oxygen recovery (from water or carbon dioxide for space applications). The main attractive features of the SOEC are its simple chemistry, environmental compatibility and high electrical-to-hydrogen efficiency. Solid oxide fuel cell (SOFC type cells have been investigated for production of chemicals and cogeneration of electricity and chemicals (in this case, the cell is referred to as CSOFC); examples include production of C2 hydrocarbons and syngas from methane. The key features of the CSOFC include its high selectivity and the possibility to produce valuable chemicals such as hydrocarbons using methane-rich gases from non-fossil origins. Current state-of-the-art SOECs and CSOFCs, both fundamentally derived from the more advanced conventional SOFC technology are still in their feasibility/development stage. This paper discusses certain aspects in selected materials, performance and operation of the SOEC for hydrogen production from water and the CSOFC for production of C2 hydrocarbons from methane.

Optimized performance of a fuel cell membrane electrode assembly (MEA) depends on several factors. Key among them are the electrolyte density and the interface between the electrolyte and electrodes (anode and cathode). In this work, SOFC half cells were fabricated by tape casting, lamination, and co-sintering with constraint. These half-cells were then printed with cathode material and re-fired to form the MEA. The effect of both constraint and firing profile were investigated. Optimization of firing profile and constraint load facilitated production of MEA’s with minimal camber. Study of these cells, fired in different profiles, displayed that interface issues easily overshadow the effect of electrolyte density with regard to cell performance and stability in use. In addition, infiltration was performed on the as-fabricated single cell samples. The performance of the infiltrated cell conducted in 100% hydrogen as well as in 7.3% ethanol in helium will be presented.

Direct fuel utilization, pressurization and reversibility are three operating characteristics enabling solid oxide fuel cells (SOFCs) for a broad spectrum of applications. Direct uitilization of practical fuels for the SOFC has been demonstrated. For example, high electrochemical performance (a peak power density of about 400 mW/cm2) without evidence of anode coking was obtained at 800oC in short-term tests with a SOFC operating directly on 7.3% ethanol balance He fuel. Operation of the SOFC under pressure has been shown to improve cell performance. For example, when the pressure was increased from 1 to 3 atm, the cell voltage at 0.4 A/cm2 and 800oC increased from 0.68 to 0.72V with 64%H2-36% N2, 75% fuel utilization. Pressurization, on the other hand, might accelerate cell performance degradation. Operation reversibility allows the SOFC to be operated in fuel cell, electrolysis and cyclic modes. For example, a 10-cell reversible SOFC stack (active area of 142 cm2) was successfully operated over 1,000 hours alternating between fuel cell (with hydrogen and methane as fuel for power generation) and electrolysis (with steam for hydrogen production) modes. This paper presents and discusses selected results on direct fuel utilization capability, pressurization and operation reversibility of the SOFC.

An anode supported solid oxide fuel cell (SOFC)with a dual layer anode consisting of a Cu-ceria impregnated Ni-yttria stabilized zirconia (YSZ) support outer layer and a Ni-YSZ electroactive inner layer was investigated for the direct operation on ethanol. High electrochemical performance without evidence of anode coking was demonstrated at 800C in short term tests. A peak power density of about 400 mW/cm2 was obtained with 7.3% ethanol balance He fuel and air oxidant.

A reversible solid oxide fuel cell (RSOFC) is a SOFC that can function efficiently under both power generation mode for electricity production and electrolysis mode for hydrogen/chemical production. The RSOFC thus can be used as a fuel cell or an electrolysis cell or both depending on the application. Multi-cell RSOFC stacks have been built and reversible operation has been demonstrated. Experiments on RSOFCs (yttria-stabilized zirconia or YSZ electrolytes, Ni/YSZ hydrogen electrodes, and perovskite oxygen electrodes) showed certain performance irreversibility of the perovskite electrode. Significant performance degradation was observed under electrolysis mode for predominantly electronic-conducting oxygen electrodes such as Sr-doped lanthanum manganite (LSM). Mixed-conducting oxygen electrodes such as lanthanum strontium cobalt iron oxides (LSCF) showed better performance and performance stability. Long-term operation of RSOFC stacks using stainless steel interconnects at 800oC showed elemental (Sr and Cr) migration and interactions at oxygen electrode/interconnect interfaces.

Solid oxide fuel cells (SOFCs) have been developed for power generation applications and in the reverse operating mode for hydrogen and oxygen production. In both operating modes, stable performance and high efficiency are very much dependent on operating characteristics of the stack within the system. Several of the critical parameters include temperature gradient across the cell, voltage and temperature variations between cells, cell-to-cell contacts, and fuel and oxidant utilizations. This paper discusses certain factors important to the operation of SOFC stacks and systems including cyclic operation between power generation and hydrogen production modes.