Elucidating the role of cobalt nanoparticles and Mn-phosphate in etched ZIF-67/phthalimide-NC and phthalimene oxide for supercapacitor and electrochemical oxygen evolution reaction applications

Abstract

Electrochemical supercapacitors and the electrochemical oxidation of biomass-derived oxygenates have great significance for long-term high-performance devices. However, appropriate sites with redox features remain a bottleneck for electrochemical oxidation and capacitance retention. Herein, N-doped carbon sheets with Mn-phosphate-doping and Co-metal nanoparticles were synthesized via a facile one-pot activation and calcination of the layered potassium phthalimide salt without inclusion of any additional activators or template. The unique 2D-structure of the obtained microporous carbon flakes with a layered structure provides a sturdy N-C matrix for prolonged charging/discharging with abundant active adsorption sites and an effective route for rapid electrolyte ion transport with a shorter diffusion distance for the adsorption/desorption of ions. Through these merits, K-Ph-NC offers high capacitance and outstanding rate performance with an incredible energy density in capacitor devices, and the specific capacitance of the as-prepared K-Ph-NC is proportional to the number of micropores. K-Ph-NC was further transformed to a K-Ph-Oxide, a graphene oxide version of K-phthalimide, by using an improved Hummer's method by using Mn-salt and phosphoric acid, which resulted in a phthalimene oxide doped with Mn-phosphate. In addition, a composite of K-Ph-NC with ZIF-67 was thermally calcined at 700 °C under an Ar atmosphere, which resulted in e-ZIF-67/K-Ph-NC with an etched surface. A comparative electronic and structural analysis followed by a capacitance retention and electrochemical oxygen evolution reaction study revealed the role of Co-nanoparticles as compared to the Mn-phosphate doping in the resulting materials. A symmetric supercapacitor device exhibited a maximum SE value of 22.7 W h kg⁻¹ with a maximum SP of 10 416.7 W kg⁻¹, which is mainly due to the favorable microporous pore architecture in e-ZIF-67/K-Ph-NC as compared to K-Ph-NC and K-Ph-Oxide. This highlights the role of cobalt nanoparticles in e-ZIF-67/K-Ph-NC with an etched outer surface. A promising overpotential of 450 mV at 10 mA cm⁻² in the OER by e-ZIF-67/K-Ph-NC can be correlated to the charge transfer resistance across the electrode–electrolyte interface.