Synthesis, Characterization and Performances of SOFC with Nanostructured NiO-GDC Anodes

As high-efficiency electrochemical devices that directly convert chemical energy into electrical energy via electrochemical reactions, Solid Oxide Fuel Cells (SOFCs) offer the possibility of an environmentally friendly approach to energy conversion. They have gained great interest due to their potential for operating at intermediate temperatures (less than 800 °C), which is essential for expanding their practical use. SOFCs have attracted increased attention as sustainable energy sources, since when supplied with hydrogen (H 2 ) and oxygen (from air), they produce electricity through the following electrochemical reactions: (1) H 2 + 1/2 O 2- <--> H 2 O + 2 e - (2) O 2 + 4 e - <--> 2 O 2- with the only by-product resulting in water (H 2 O). The energy conversion efficiency of SOFCs can theoretically reach 60-80%, as they are not constrained by the Carnot cycle, given that combustion is not involved in the SOFC process. One crucial point which attracts interest research in the field is that the SOFC power output is closely related to the electrochemical reactions at the electrodes [1, 2]. Therefore, the nanostructuration of the anode, which increases the surface-to-volume ratio of the three-phase boundary (TPB) conditions where the reactions occur, should in principle improve the SOFC performance. In this work, the preparation by Vapor-Liquid-Solid (VLS) growth technique and systematic morphological and structural characterization of nanostructured anodes based on Nikel Oxide and Gadolinium doped Ceria (NiO-GDC) nanowires (NWs) are proposed. The NiO-GDC NWs were synthesized using the VLS growth method and thoroughly characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. Additionally, the performance of the fuel cell, including power output and Electrochemical Impedance Spectroscopy (EIS) analysis, was assessed at different temperatures and hydrogen/air flow rates. Starting from the results obtained in previous works where the synthesis of NiO-GDC NWs was investigated [3, 4], in the present study the performance of the NWs-based fuel cell was compared with a commercial SOFC (Next Cell 2.0, from fuelcellmaterials company) and a hybrid one prepared by depositing NiO-GDC NWs on top of a commercial NC. In Figure 1a-b, the scheme and multilayer structure of the commercial SOFC (Next Cell) and the nanostructured SOFC with NiO-GDC NWs. After a careful preparation of the NiO-GDC NWs via VLS growth technique (shown schematically in Figure 1c), the SOFC performance was tested with the following parameters: T = 600, 700, 800° C; H 2 /AIR flux = 100/200, 150/300, 200/400. NiO-GDC NWs successfully grown by VLS technique on top of the anode of NC (commercial SOFC) and on top of the HC electrolyte with Au catalyst. The following samples were obtained and then analyzed: NC: commercial SOFC (Next Cell 2.0) NC+NWs: commercial SOFC (Next Cell 2.0) with NiO-GDC NWs deposited on the anode HC+NWs: Half Cell (NC without anode) with NiO-GDC NWs deposited as nanostructured anode Advantages of SOFCs include low emissions, long-term stability, and relatively low cost, making them a promising sustainable energy source. By enhancing anode performance through nanostructuring, SOFC electrochemical performance can be improved. The electrochemical reactions at the anode occur at the TPB, where the electrolyte, electron-conducting metal, and gas phases meet. Extending the TPB length enhances the electrochemical reaction, thereby improving anode performance. Researchers are currently investigating the role of nanomaterials in SOFC technology to enhance performance. It has been shown that reducing electrode grain size increases TPB length, particularly when the grain size is below 2 μm. In NiO-GDC NWs, the reduction of NiO and GDC grain sizes to the nanoscale significantly increases the interfacial area between NiO and GDC, thereby extending the TPB and improving the electrical performance of the anode. After systematically investigating the growth parameters that optimize NiO-GDC NWs morphology and composition using SEM, XRD, and Raman spectroscopy, the NiO-GDC NWs were grown on a commercial single-electrode button cell (with a cathode only) to assess the electrochemical behavior of the cell. A thorough analysis of the results indicates that the NiO-GDC NWs show considerable promise as an anode material for SOFCs. This study presents the synthesis and performance of NiO-GDC NWs-based SOFCs, with an in-depth investigation of the morphology and composition of the nanowires using SEM, XRD, and Raman techniques. Ultimately, NiO-GDC NWs were grown directly on a commercial single-electrode button cell to evaluate electrochemical performance, with preliminary results suggesting significant potential for NiO-GDC NWs as an anode material in SOFCs. Structural characterization by Raman and X-ray Diffraction carried out before and after electrical tests. In Figure 2, the SEM images (a), Raman spectra (b) and electrical I-V characterization of the samples: NC, NC+NWs, HC+NWs. From SEM images (in Fig. 2a, associated with samples sketches), NWs grown on the commercial anode surface (Fig. 2a middle) and on top of the electrolyte (Fig. 2a right) is shown. Raman signal (in Fig. 2b) from the anode presents NiO peaks and Scandia stabilized Zirconia (ScSZ) from the underlying electrolyte. The I-V characteristics and EIS of the fuel cell were measured using a SOFC test setup from Fiaxell SOFC Technologies TM Fiaxell. During the tests, the single-cell was placed in a KITTEC® SquadroKitted Squadro muffle furnace, with the cell sandwiched between the anode and cathode collector discs. The setup was connected to gas-tight fittings for the delivery of air and hydrogen to the cell. The tests were conducted at temperatures of 600°C, 700°C, and 800°C. The I-V characteristics were obtained using slow-scan galvanostatic linear sweep voltammetry (LSV). After the electrical tests, cracks were observed on the anode surface of NC which probably make the performance decrease. NC+NWs sample shows less cracks and a more stable power output. Unfortunately, it was observed that after the electrical tests most of the NiO-GDC NWs are destroyed, probably for the high operating temperatures or for the mechanical stress (data not shown here). At 800 °C, with hydrogen and air flows of 100 sccm and 200 sccm respectively, an Open Circuit Voltage (OCV) of 1.06 V and a power density of 373 mW/cm² were achieved. In conclusion, NiO-GDC NWs were successfully grown on top of the electrolyte and on top of the anode of a commercial SOFC, by VLS technique; a systematic morphological (by SEM) and structural (by Raman and X-ray Diffraction) characterization carried out before and after V-I electrical test; NiO-GDC NWs do not really improve the SOFC performance, however they can preserve the anode layer to be damaged during the electrical testing; nanostructured SOFC with NiO-GDC NWs grown by VLS on the electrolyte showed comparable performance with respect to the commercial multi-layered anode SOFC. Finally, the electrical I-V characterization associated with the impedance spectroscopy analysis allows us to evaluate and compare the performances of the different anodes morphology, to model the electrical equivalent circuit and extract the best deposition parameters and the SOFC nanoarchitecture, in order to further enhance the final performance. References [1] Helal, H.; Ahrouch, M.; Rabehi, A.; Zappa, D.; Comini, E. Nanostructured Materials for Enhanced Performance of Solid Oxide Fuel Cells: A Comprehensive Review. Crystals 2024, 14, 306. [2] M. Singh, D. Zappa, E. Comini, Solid oxide fuel cell: Decade of progress, future perspectives and challenges, International Journal of Hydrogen Energy. International Journal of Hydrogen Energy 46, 2021, 27643. [3] M. Singh, D. Zappa, E. Comini, NiO-GDC nanowire anodes for SOFCs: novel growth, characterization and cell performance. Mater. Adv., 2022, 3, 5922. [4] Helal H, Botticini S, Rigoni F, Zappa D, Hakkoum H, Chua D, Lee P-S, Comini E (2024). Synthesis and Charac