Engineering Hybrid Nanostructures: Novel Aqueous Routes to Integrate Electrochemically Active Metal Clusters into Carbon Nanomaterial or Polymeric Matrices for Functional Energy Storage and Conversion Materials"

Electronic conductivity of porous electrodes by Faradaic and non-Faradaic charge transfer processes impacts the energy and power densities and overall performance of electrochemical energy storage and conversion devices. Carbon nanomaterials constitute an ideal material composite platform for integration with other solid-state electrode materials in power devices. Self-assembly has been recognized as an effective strategy for the bottom–up synthesis of 3D macrostructures using graphene and carbon nanotube as building blocks. The need for chemical precursor vaporization and subsequent reaction makes techniques such as chemical vapor deposition an energy-intensive process limiting the choice of material substrates and catalysts. Aqueous and scalable materials design processes offer a cost-effective alternative with a broader range of materials selection and a more simplified control of film composition and morphology. There is a need for the development of scalable and feasible methods for uniform thin film electrode fabrication from both aqueous and non-aqueous suspensions compared to current utilized techniques such as casting, doctor blading, and spin coating techniques. We demonstrate the use of air-controlled electrospraying as a platform technology for thin film nanocomposite electrode development regardless of the type of active materials in suspension. Following air-controlled electrospraying, noble or transition metal nanoparticles are integrated with carbon nanomaterials and polymers films to serve as high surface area and lightweight electrodes for high performance batteries, supercapacitors, and fuel cells with synergistic electrochemical and mechanical properties for effective composite electrodes design. An example of a solution-based method for the integration of conductive nanoparticles is the electroless deposition of noble or transition metal clusters through a spontaneous galvanic displacement reaction with an underlying metal support. Nanoparticle clusters form due to electron transfer from thermodynamic reduction potential differences between the metal ions and the supporting metal substrate in a rapid and tunable process without the use of power sources or harmful chemical reducing agents and surfactants to produce hybrid nanocomposites. Furthermore, this resulting platform materials design methodology renders composite materials as a single electrode entity which can eliminate the challenge in electrochemical device assembly regarding material migration and inhomogeneity in the material distribution.
Moreover, single-step aqueous synthesis techniques offer the design of versatile combinations of materials with surface chemical functional groups and metal precursors to tailor electrode energy storage materials to specific catalytic or electrochemical applications. For example, storing hydrogen in atomic and nanoscale metal hydrides has been regarded as a promising transformative technology. However, the underlying mechanism of metal hydride formation in aqueous solutions is still being explored. We utilize a single-step synthesis route to combine surface-engineered carbon nanomaterials with noble metal hydride cluster particle composites. Chemical functional groups on the surface of carbon nanomaterials serve as sites for electrostatic coordination of positively charged transition or noble metal complex cations from aqueous solutions. This enables the incorporation of carbon nanomaterials with any transition or noble metal nanostructures with controlled integration of each of the individual components. Therefore, this scalable methodology offers a versatile electrode material-selection for solid-state hydrogen storage with shorter diffusion paths and overall higher reaction specific area for hydrogen storage and evolution. This scalable single-step synthesis approach can also be applied to create high surface area metal aerogels with tunable pore sizes and fuzed nanoparticle structure as monolith electrodes for wide ranging electrochemical applications.
The overall insights gained from the role of surface chemistry of carbon nanomaterials for the design of stationary electrodes materials are critical to understanding the electron transfer mechanism and the microstructure evolution in carbon based colloidal particle systems for a variety of other electrochemical applications from flowable batteries to conductive coatings. To achieve effective flowable electrode design, developing a fundamental understanding of the properties that determine the microstructure of conductive suspensions such as primary particle size and morphology, surface chemistry, aggregate morphology and network structure are imperative for specific energy storage and conversion applications. Understanding the influence of the intrinsic physicochemical properties of particles on overall conductivity can help predict and design flowable particle electrodes for specific electrochemical applications.

Prof. Enoch Nagelli is the Director of Chemical Engineering in the Department of Chemical and Biological Science and Engineering at West Point.

The Nixon Seminar Series in the Department of Human Centered Design was established through a generous gift by John W. and Lea P. Nixon, Class of 1953. Designed to provide students with exposure to leading scholars and industry professionals, the Seminar Series is an expansion of the Nixon Distinguished Speaker Series and builds upon Mr. and Mrs. Nixon’s shared vision of creating a more engaged community on campus and providing a bridge to professional knowledge and opportunities.