Magnetic-Field Assisted Co-Reduction Synthesis of High Surface Area FeNi Nanowire Aerogels Electrocatalysts

Transition metal aerogels are an interesting multifunctional nanomaterial for many electrocatalytic reactions due to their high porosities and surface areas, conformability, and lightweight properties. However, an ongoing challenge is their synthesis.^1,2 Traditional methods to prepare transition metal aerogels, such as carbothermal synthesis, suffer drawbacks such as aggregation, slow reactant diffusion times, a need for templates which can block catalytic active sites, and formation of a brittle final product which limits conformability.² We present the magnetic-field assisted synthesis as a template-free alternative method that allows the rapid formation of transition metal nanowires (NWs) that self-assemble into an intertwined network of NWs to form an aerogel. In a typical reaction, high concentrations of metal salts³ are chemically reduced with NaBH₄ inside a 150 mT magnetic field. The reduced metals nucleate into magnetic nanoparticles that are aligned inside the magnetic field. As additional metal ions are chemically reduced, they grow along the already existing particles and allow coalescence of the previously formed nanoparticles and growth into NWs. Crowding effects and gas evolution cause a turbulent solution which promotes the intertwining of the NWs into the aerogel which can allow for improved mechanical properties compared to samples prepared in the absence of a magnetic field. Control of the aerogel nanowire properties can be achieved through tuning of the synthesis parameters such as metal ion ratios and concentrations, counterions, pH, and the applied magnetic field strength. Post-treatments such as thermal treatment, or galvanic displacement can allow tuning of the nanowire crystal structure and elemental composition. Building on previous studies involving reduction of CoCl₂ and FeCl₃ to form CoNW and FeNW aerogels with controlled lengths, aspect ratios, surface areas, and magnetic and electrocatalytic properties, we have co-reduced Fe²⁺ and Ni²⁺ salts to form bimetallic FeNiNW aerogels. Tuning of the synthesis parameters such as metal ion ratios and concentrations, counterions, pH of the reaction solution, and the applied magnetic field strength allowed control of the FeNiNW properties including composition, surface area, nanowire aspect ratios, crystallinity, and magneto-responsiveness. Post-treatment such as heating enables the thermal oxidation of the FeNiNW aerogels and improves the crystallinity of the material. Finally, electrochemical testing (CV/EIS) demonstrates the potential for electrocatalysis of the oxygen evolution reaction.

References:
1. Magnetic – Field Assisted Synthesis of Cobalt Nanowire Aerogels for Tunable Structural, Magnetic, and Electrochemical Properties. ChemRxiv July 19, 2024. https://doi.org/10.26434/chemrxiv-2024-gsms1.
2. Jiang, X.; Du, R.; Hübner, R.; Hu, Y.; Eychmüller, A. A Roadmap for 3D Metal Aerogels: Materials Design and Application Attempts. Matter 2021, 4 (1), 54–94. https://doi.org/10.1016/j.matt.2020.10.001.
3. Burpo, F. J.; Nagelli, E. A.; Morris, L. A.; McClure, J. P.; Ryu, M. Y.; Palmer, J. L. Direct Solution-Based Reduction Synthesis of Au, Pd, and Pt Aerogels. Journal of Materials Research 2017, 32 (22), 4153–4165. https://doi.org/10.1557/jmr.2017.412.