Cobalt Oxide Incorporated Nitrogen Doped Carbon Containing Electrocatalysts for Oxygen Reduction Reaction

Abstract

In this study, a comprehensive exploration of nitrogen-doped tubular carbon (NC) with embedded cobalt oxide (CoOx) as a versatile support for electrocatalysts in the oxygen reduction reaction (ORR) is presented. The straightforward one-step synthesis, involving the pyrolysis of ZIF-12 at 850 °C under an inert atmosphere, yields CoOx/NC. The synthesized CoOx/NC demonstrates promising properties as a support material for developing diverse electrocatalysts. A Pd@CoOx/NC1 catalyst, featuring low Pd loading (~ 5%), exhibits superior ORR activity in reversible hydrogen electrode (RHE) scale in terms of onset potential (Eonset ~ 1.07 VRHE, 1.02 VRHE) and half-wave potential (E1/2 ~ 0.95 VRHE, 0.91 VRHE) and current density (j~ 6.65 mA cm-2, 6.39 mA cm-2) in 0.1 M KOH and 0.1 M HClO4 respectively. Further advancements are made by introducing iron oxide (FeOx) nanoparticles on the CoOx/NC support material to synthesize a cost-effective and stable electrocatalyst consisting of non-precious metal. The exploration extends to non-noble metal catalysts, resulting in the development of CoOx-CoP/NC, where CoOx/NC is phosphidized to obtain the optimized electrocatalyst i.e., CoOx-CoP/NC. This catalyst demonstrates comparable ORR activity (Eonset ~ 0.96 VRHE, E1/2 ~ 0.81 VRHE and j ~ 5.65 mA cm-2) to 20 wt% Pt/C in an alkaline environment, along with enhanced durability (20 mV decay after 10,000 cycles). In addressing challenges associated with oxygenated radicals produced during incomplete ORR, CoOx and CoP encapsulated NC supported over ceria nanorods (CeO2) are synthesized in this study. The resulting catalyst (CoOx-CoP/NC@CeO22) exhibits outstanding electrocatalytic ORR performance (Eonset ~ 0.98 VRHE, E1/2 ~ 0.85 VRHE and j ~ 5.74 mA cm-2), surpassing 20 wt% Pt/C in alkaline environment. The porous structure of CeO2 contributes to enhanced durability and radical scavenging ability. The anti-scavenging ability of CoOx-CoP/NC@CeO22 enables the material to retain 99.9% current in challenging environmental conditions. Finally, a Pt@CoOx/NC@CeO21 electrocatalyst with low platinum loading (approximately 5%) demonstrates superior ORR performance Eonset ~ 1.11 VRHE, 1.10 VRHE, E1/2 ~ 0.89 VRHE, 0.86 VRHE and j ~ 6.96 mA cm-2 and 6.67 mA cm-2 in both 0.1 M KOH and 0.1 M HClO4, outperforming 20 wt% Pt/C. The optimized electrocatalysts retained significant amount of current in both alkaline (99.9%) and acidic environment (99.8%).