Spent tea leaves templated synthesis of highly active and durable cobalt-based trifunctional versatile electrocatalysts for hydrogen and oxygen evolution as well as oxygen reduction reactions

The rational design of high-performance trifunctional catalysts for oxygen reduction, oxygen and hydrogen evolution reactions is of vital importance for the implementation of green energy conversion technologies. Herein, trifunctional electrocatalysts comprising cobalt nanoparticles uniformly embedded in porous carbon networks were fabricated using spent tea leaves (STL) template via a one-step carbothermal-reduction strategy at different temperatures. STL played a dual synthetic function to construct the nanocatalysts acting as efficient scavengers to trap cobalt cations via electrostatic interactions and as carbon sources to generate the porous carbon matrix. Full characterization of the as-synthesized materials revealed the crucial role of temperature on the crystallinity, surface area, number of surface defects and interfacial charge distribution properties. Furthermore, the trifunctional catalytic activity of the nanoparticles can be nicely tuned by varying the carbonization temperature. Co@PC-7 displayed a superior trifunctional catalytic activity rendering an excellent performance for hydrogen production with an overpotential potential of 153 mV (vs RHE) to achieve 10 mA·cm-2, and an impressive bifunctional catalytic performance for the overall oxygen activity with an ultralow potential difference of OER and ORR (ΔE =η10- E1/2) of 0.69 V, which is one of the lowest values reported in the literature for transition metal nanocatalysts. The remarkable performance of Co@PC-7 is mainly ascribed to its unique structural properties, which give rise to highly desirable charge distributions at the metal/carbon electrochemical interfaces to perform efficient trifunctional water splitting electrocatalysis.