3D Nanoporous Graphene Based Single-Atom Electrocatalysts for Energy Conversion and Storage

ConspectusThis Account will provide an overview and analysis on recent research of 3D nanoporous graphene based single-atom electrocatalysts for energy conversion and storage applications. In order to meet the increasing energy demands and assist in the transition from a global economy that relies heavily on fossil fuels to one that utilizes more renewable energy sources, there is urgent need to develop high-performing electrocatalysts toward renewable energy related reactions. These catalysts are expected to have low overpotentials, high reaction selectivity, long cycling stability, and, importantly, lower materials costs to address the challenges of traditional nanoparticulate noble metal catalysts. One approach to conquering a transition of such scale is the use of single-atom electrocatalysts which can bring benefits of lowered materials cost, increased reaction selectivity, and improved catalytic activity. However, despite some success in recent years there remain challenges such as the stability of single-atom catalysts over time, especially at high operating temperatures and potentials, and the limited loading of single-atom catalysts on a support matrix. This Account will discuss the use of graphene as the support matrix with heavy focus on the progress in developing 3D nanoporous graphene, synthesized by chemical vapor deposition. We will cover how 3D nanoporous graphene possesses a large specific surface area, open porosity, high electric conductivity, and bound geometric and topological defects and analyze how it is emerging as an excellent support material for developing chemically stable and catalytically active single-atom catalysts with a large loading amount and high electrocatalytic efficiency. Additionally, we will highlight the benefits of the presence of topological and geometric defects, from curved graphene in the 3D structure, and how they can bring more chemically active sites, and how, together with chemical dopants, they can improve the catalytic performance, stability, and loading amount of single-atom catalysts via their strong chemical affinity with transition metals and the formation of active M–Nx–Cy (M = transition metals, N-nitrogen, C-carbon) moieties. Furthermore, we will discuss two common methods for fabrication of single-atom catalysts on nanoporous graphene: (1) controlled etching for non-noble and noble metal catalysts and (2) atomic layer deposition for noble metal catalysts. The electrochemical performances of these single-atom catalysts will be reviewed with the applications toward hydrogen evolution reaction, oxygen reduction reaction and oxygen evolution reaction. This Account highlights advances made in solving the challenges of developing 3D nanoporous graphene based single-atom catalysts and the benefits that the new single-atom catalysts can bring to the applications in energy storage and conversion.