Lawrence Livermore National Laboratory

Monika Biener (17-ERD-017)

Executive Summary

We are developing a new synthesis capability for all-carbon macro-assemblies with charge-storage performance similar to lithium-ion batteries and improvement of the charge-storage capability of current super-capacitor technology by a factor of ten. These materials may enable new photovoltaic solar cell technologies, and have a potentially large impact on global carbon dioxide emissions through improved electric energy storage and harvesting.

Project Description

Porous graphene (a thin layer of pure carbon) macro-assembly materials combine an exceptionally high surface area with high electrical conductivity. They are based on an Earth-abundant element, and offer deterministic control over pore morphology and porosity. The high electrical conductivity and stability of this material are a consequence of covalently cross-linking graphene oxide by utilizing its chemical functionality (e.g., epoxide and hydroxide groups) and conversion of these groups to conductive carbon–carbon bonds during pyrolysis. However, taking full advantage of the properties of graphene macro-assemblies for super-capacitor applications in long-term compact energy storage requires optimization of interfacial capacitance by increasing the density of states in the vicinity of the Fermi level (i.e., the total chemical potential for electrons) and that the material can store more electrons. Grafting with fullerenes (a series of hollow carbon molecules) as electron acceptors has the potential to enhance energy storage performance of super capacitors. We are developing fullerene–graphene macro-assemblies for the next generation of all-carbon-based electrochemical super capacitors with charge-storage performance similar to that of lithium-ion batteries and power performance similar to that of super capacitors. We intend to create the tailored assemblies by anchoring fullerene molecules to the three-dimensional graphene backbone. Fullerene-functionalized graphene materials promise to overcome the current charge-storage limitations of carbon-based super capacitors by combining the high charge-storage capacitance of fullerenes with the high electrical conductivity of three-dimensional graphene macro-assemblies. The high charge-storage capacitance of fullerene originates from its energetically low-lying, triply degenerated lowest-unoccupied molecular orbital, which is capable of storing up to six electrons (0.1 electrons per carbon). This is ten times higher than the interfacial capacitance of graphene (0.01 electrons per carbon). In fact, the charge-storage capacitance of fullerene–graphene is similar to or higher than that of today's standard lithium-ion battery electrode materials.

We intend to explore the functional capabilities of graphene macro-assemblies with fullerene derivatives via van der Waals interactions (i.e., comparatively weak distance-related bonds) and covalent bonding, as well as test energy-storage performance of the resulting carbon-60 grafted graphene macro-assembly as an electrochemical super-capacitor material. In addition, we will validate the measured interfacial capacitance via density-functional theory calculations. We expect the development of fullerene-grafted graphene macro-assemblies to result in transformational performance improvements of super capacitors. Our research will add a new synthesis capability for fullerene-grafted, nanometer-scale carbon materials to the Lawrence Livermore National Laboratory. Beyond the relevance to super capacitors, these materials may enable new photovoltaic solar-cell technologies. Both applications have a potentially large impact on global carbon dioxide emissions through improved electric energy storage and harvesting. We intend to leverage the Laboratory's expertise in the synthesis, characterization, and functional capabilities of graphene macro-assemblies, as well as high-performance computing; a collaboration with a leading specialist in the field of fullerene functional capability at the University of Texas, El Paso; and our expertise in electrochemical characterization of air-sensitive materials.

Mission Relevance

Our research is relevant to DOE's science and energy goals, and the Laboratory's energy and resource mission research challenge by potentially improving charge-storage performance of the current super-capacitor technology by a factor of ten. Efficient electrical energy storage charge is a critical component toward further expansion of renewable-energy generation. The proposed work will also strengthen the Laboratory's core competency in advanced materials and manufacturing by adding a new innovative synthesis capability for tailored fullerene and graphene composite feedstock materials for additive manufacturing.

FY17 Accomplishments and Results

In FY17 we (1) successfully synthesized five different linker molecules and attached them to the fullerenes; (2) demonstrated physical adsorption (in which the electronic structure of the atom or molecule is barely perturbed upon adsorption) for two types of functionalized fullerenes on graphene macro-assemblies, and achieved loading levels of up to 80 percent; and (3) tested these composite materials for electrochemical storage capacity, which revealed a distinct improvement. However, the capacity improvement fades during cycling of the voltage, emphasizing the need for covalent bonding.

Figure 1.
The charge storage capacity of carbon-based super capacitor materials is enhanced by adding functionalized carbon-60 fullerenes, thus creating a hybrid system (a). Fullerenes have a high density of states (DOS) close to the Fermi level (EF), as revealed by Discrete Fourier Transform calculations (b). The experimentally measured charge-storage capacitance of the hybrid system is significantly increased by adding these extra states (c), making this system much more compatible with Li-ion batteries while combining high power and high-energy performance.