Magnetic-Field Assisted Synthesis of Transition Metal Aerogels Allows Formation of High Surface Area Electrocatalysts

Transition metal aerogels (TMAs) have emerged as promising electrocatalysts for a variety of reactions due to their lightweight properties, conformability, porous natures, and high surface areas allowing many available sites for charge transfer and catalytic activity. The catalytic properties of the TMAs can be tuned based on the elemental composition, surface environment, and structural properties, allowing opportunities to design a wide range of TMAs that can catalyze a variety of reactions. An ongoing challenge is that traditional methods to prepare TMAs 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. We have developed a magnetic-field assisted synthesis strategy to produce iron aerogels that addresses these limitations. In a typical synthesis, a ferric chloride metal salt solution was placed inside a 150 mT magnetic field. Sodium borohydride was then added as a reducing agent which resulted in immediate formation of a gel. This gel was rinsed and then either supercritically dried into an aerogel or pressed into a thin film. Scanning electron microscopy (SEM) shows that the aerogels consist of iron nanowires that are intertwined with each other, and nitrogen porosimetry confirms that they have high surface areas and porosities. The lack of templates or surfactants in synthesis allows exposed surfaces available for electrocatalysis. The iron nanowire network within the aerogel allows improved mechanical properties relative to TMAs prepared through alternate methods. The aerogels were then used as a sacrificial template for galvanic displacement with platinum which allowed dissolution of the iron and formation of high surface area platinum nanotubes. SEM shows the platinum tubes retain the nanowire network of the iron template, and electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) indicate high electrochemical accessible surface areas for electrocatalytic applications. This strategy was also applied to cobalt nanowire synthesis with subsequent thermal annealing to Co₃O₄ for pseudocapacitor applications, and for formation of bimetallic iron-nickel aerogels which are promising as electrocatalysts for water splitting.