Two-dimensional materials for energy conversion and storage

Two-dimensional (2D) materials with varied structured features are showing promise for diverse processes. We focus on their energy applications in electrocatalysis of the oxygen reduction reaction, the oxygen evolution reaction, the hydrogen evolution reaction, CO2 reduction reactions, photocatalytic water splitting and CO2 reduction, electrical double layer capacitors, pseudocapacitors, and batteries. Effects of synthesis parameters and surface modification are examined as a means to tune conductivity, catalytic activity, and other performance-related properties. Activity parameters of leading 2D materials and their hybrids are discussed and compared with more classical benchmark materials to provide an evolutionary perspective of performance progress. Doped graphenes are currently producing about half their theoretical electrostatic maximum energy storage in electrical double layer capacitors at about 260 F g−1. Nanosheet pseudocapacitors have yielded significant early advances in hybrids of graphene with layered double hydroxides and with metal oxide nanosheets to store energy at about 3000 F g−1. These pseudocapacitor results also have enabled promising early developments in using similar electrodes in batteries. Nanosheet hybrid structures are also yielding improved electrodes for lithium and sodium ion batteries. High electrical conductivity, robustly porous nanosheet assemblies, and facile ionic and molecular diffusion pathways are design criteria important for nanosheet-based energy conversion and storage materials. Development opportunities and challenges are summarized.