Cellulose Nanofiber-Carbon Nanotube Biotemplated Composite Nickel/Nickel Oxide Aerogels for Pseudocapacitor Electrodes

Electrochemical pseudocapacitors made from metal oxides are a critical technology for energy storage. While there has been a considerable body of research into pseudocapacitor synthesis, there are several challenges to its widespread implementation, namely, mechanical durability and active surface area. To address this need and to develop low-cost, transition porous metal nanomaterials for energy storage applications, one strategy is to use naturally occurring biopolymer-based templates to better tune material properties such as feature size, pore structure, conductivity, material phase, and mechanical strength. Examples of this approach to control nanostructure and functionality include biotemplated gelatin, silk fibroin, and cellulose nanofiber composite metal aerogels. Where biopolymers confer nanostructure shape control and chemical functional groups to mediate metal phase deposition, the addition of carbon allotropes such as graphene and carbon nanotubes allows for strength and electrical and thermal conductivity. In this work, we demonstrate the synthesis a porous, mechanically strong material with tunable pore size, nanowire length, diameter, and material phase which offers the possibility of electro-mechanical materials that enable device fabrication at multiple length scales with an overall decrease of systems mass and corresponding increase in performance metrics. This is achieved using a hybrid carboxymethyl cellulose nanofiber-carbon nanotube (CNF-CNT) composite hydrogel. Hydrogels equilibrated in a nickel salt solution are chemically reduced, rinsed, and then supercritically dried. Subsequent pyrolysis, and thermal annealing results in a porous NiO psuedocapacitor aerogel. Aerogels were characterized using scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry, nitrogen gas adsorption-desorption with and Brunauer-Emmett-Teller surface area analysis, vibrating-sample magnetometry, and compressive stress. Electrochemical performance was determined with electrochemical impedance spectroscopy and cyclic voltammetry. Due to the low-cost synthesis and tunable properties, these composite CNF-CNT-metal/metal oxide aerogels are expected to provide material solutions to a wide range of energy storage, catalytic, and fuel-cell applications.