Flash Bottom-Up Arc Synthesis of Single Atom Catalysts for Highly Active Alkaline Fuel Cell Electrodes

Single atom electrocatalysts (SACs) mark the beginning of a new era in the field of sustainable energy conversion technologies. The atomically dispersed transition metal-nitrogen-carbon (M-N_x-C) materials emerge as a promising SACs due to their cost effectiveness, extraordinary catalytic activity and selectivity. For a decade, a variety of synthesis strategies have been developed to realize highly active M-N_x-C catalysts. However, the most widely applied synthesis methods need general requirements for either a multistep processes or the use of typically selected metals and carbon supports. It is highly desirable to develop more efficient methods for the fabrication of M-N_x-C catalysts, realizing both simple and simultaneously applicable to various metals and supports.<br/>The construction of hierarchical pore structure is equally important in the field of M-N_x-C catalysts for maximizing electrochemical accessibility to internal/external active area. Unfortunately, the recent studies have focused on maximizing the number of active sites to improve catalytic performance. Simply increasing active site density is not enough solution for more practical energy and environmental applications due to the limited access to the active sites. Therefore, developing the hierarchical pore structure and desirable morphology in M-N_x-C catalysts are crucial for effective mass transport in catalyst layer and utilization of active sites.<br/>In this work, we propose a general strategy for fabricating M-N_x-C catalysts using a flash bottom-up arc discharge method. Our strategy is applicable to a broad type of metals and nanocarbons. This study revealed that nitrogen atoms in buffer gases and ligand atoms in precursor molecules played crucial roles in dispersing and stabilizing metal atoms during nanocarbon synthesis. This work provides a new synthesis protocol and novel design for efficient M-N_x-C catalysts, which may have an impact on the industry of fuel cells. Furthermore, we demonstrate an efficient strategy for developing a hierarchical pore structure in arc nanocarbons with Co-N_x-C sites and improving their oxygen reduction activity. Low temperature thermal annealing in air was highly efficient to generate hierarchical pores by opening internal and interstitial nano-channels in carbon nanohorns. Subsequent ammonia annealing modifies coordination environments and relieves local strain around cobalt atom to form more ideal Co-N₄-C sites. The combined computational and experimental studies proved that the improved kinetic activity of arc-derived Co-N₄-C catalyst is attributed to the ligand-push effect of water molecules on the other side of Co-N₄ sites. In a single-cell experiment, a power density of 742 mW cm^-2 was achieved, which is the remarkably high value among atomic M-N_x-C catalysts using commercialized membrane electrode assemblies (MEAs).