Synthesis and Characterization of Electrospun Ni-BZY Nano-Fiber Anodes

Abstract

Solid oxide fuel cell (SOFC) and related Protonic Ceramic Fuel Cell (PCFC) technologies have demonstrated significant promise for the efficient and clean electrochemical conversion of stored energy held in a wide variety of renewable fuels, decreasing dependence on traditional fossil fuels. The high efficiencies of both devices arise from the direct chemical-to-electrical energy transformation, achieved without moving parts and the losses inherent to combustion technologies. As in all fuel cells, electrons are generated via oxidation of fuel at the anode to power an external load before returning to the cathode to reduce an oxidant, usually O2. Separating the two electrodes is a dense, gas-tight, electronically insulating electrolyte that permits ion diffusion to complete the circuit. The SOFC’s solid oxide electrolyte passes oxides from the cathode to the anode, but the large activation barrier associated with this process requires high operating temperatures (≥700 ˚C), exacerbating degradative processes and adding challenges for thermal management. In contrast, the PCFC’s perovskite electrolyte permits protonic diffusion from the anode to the cathode, with a smaller activation barrier leading to lower operating temperatures (450 – 700 ˚C). Despite recent advancements in emerging PCFC materials, the overall efficiency and long-term stability of the device is still impacted in part by the anode’s ability to catalyze fuel oxidation, generate protons, and shuttle electrons to an attached current collector. These processes in turn are strongly linked to the morphology and stability of the anode—often a cermet composed of a Ni catalyst/electronic conductor and protonic ceramic such as yttrium-doped barium zirconate (BZY). In particular, continuity is needed for the Ni, BZY, and pore networks while maximizing boundaries between each phase (three-phase boundary). Traditionally, anodes are fabricated by mixing appropriate powders together without much control over the morphology over the micron-sized components. In this study, we explore an alternative approach to systematically engineer Ni-BZY nanofibers within the anode to enhance overall device performance through improving the underlying electrochemical processes at the anode. Low nickel-content BZY anodes (<40 wt. % Ni) for PCFCs are fabricated through electrospinning, with two distinct methods evaluated for nickel loading. First, uniform BZY nanofibers are synthesized by electrospinning a gel containing BZY metal cations and 5 wt. % Polyvinylpyrrolidone (PVP) as dispersant in ethanol/acetic acid solution followed by sintering at 900°C for 3 h in air, and subsequent electroless deposition of nickel on the fiber surfaces. Second, Ni-BZY is prepared by electrospinning a polymer containing a nickel salt and the same BZY precursors followed by co-sintering at 900°C for 3 h. The Ni-BZY fibers underwent morphological, crystallinity and elemental characterization via SEM, XRD, TEM and Raman microscopy. Initial comparison of the fibers fabricated by both methods has shown that uniform nickel loading of ~35 wt. % on the fibers was achieved in the second method. Slurries containing fibers made from both methods are deposited as anodes onto BZY electrolytes, and the complete cells are tested electrochemically under hydrogen at elevated temperature and compared with a control PCFC constructed using conventional powder materials with comparable nickel loading. In summary, electrospun nanofibers, known for their excellent permeability and faster charge transfer due to their porous structure, present an intriguing alternative for high-performance PCFC anode materials. Figure 1