Nitrogen-Rich Conjugated Microporous Polymers with Improved Cobalt(II) Density for Highly Efficient Electrocatalytic Oxygen Evolution.

ABSTRACT

Developing efficient oxygen evolution catalysts (OECs) made from earth-abundant elements is extremely important since the oxygen evolution reaction (OER) with sluggish kinetics hinders the development of many energy-related electrochemical devices. Herein, an efficient strategy is developed to prepare conjugated microporous polymers (CMPs) with abundant and uniform coordination sites by coupling the N-rich organic monomer 2,4,6-tris(5-bromopyrimidin-2-yl)-1,3,5-triazine (TBPT) with Co(II) porphyrin. The resulting CMP-Py(Co) is further metallized with Co2+ ions to obtain CMP-Py(Co)@Co. Structural characterization results reveal that CMP-Py(Co)@Co has higher Co2+ content (12.20 wt %) and affinity toward water compared with CMP-Py(Co). Moreover, CMP-Py(Co)@Co exhibits an excellent OER activity with a low overpotential of 285 mV vs RHE at 10 mA cm-2 and a Tafel slope of 80.1 mV dec-1, which are significantly lower than those of CMP-Py(Co) (335 mV vs RHE and 96.8 mV dec-1). More interestingly, CMP-Py(Co)@Co outperforms most reported porous organic polymer-based OECs and the benchmark RuO2 catalyst (320 mV vs RHE and 87.6 mV dec-1). Additionally, Co2+-free CMP-Py(2H) has negligible OER activity. Thereby, the enhanced OER activity of CMP-Py(Co)@Co is attributed to the incorporation of Co2+ ions leading to rich active sites and enlarged electrochemical surface areas. Density functional theory (DFT) calculations reveal that Co2+-TBPT sites have higher activity than Co2+-porphyrin sites for the OER. These results indicate that the introduction of rich active metal sites in stable and conductive CMPs could provide novel guidance for designing efficient OECs.