Realization of Oxygen Reduction and Evolution Electrocatalysis by In Situ Stabilization of Co Nanoparticles in a Redox‐Active Donor‐Acceptor Porous Organic Polymer

Abstract

Bifunctional electrocatalysts that can catalyze both oxygen reduction and evolution reactions have high significance in the development of energy storage materials such as fuel cells and metal‐air batteries. Here, we have designed and synthesized a redox active porous organic polymer (TAPA‐PG) by the Schiff base condensation reaction between 4,4′,4′′‐triaminotriphenylamine (TAPA, donor) and 1,3,5‐triformylphloroglucinol (PG, acceptor). The TAPA‐PG possesses permanent microporosity and shows spherical nano‐morphologies with diameter of 400–500 nm. TAPA‐PG shows electrocatalytic activity towards oxygen reduction reaction (ORR) with an overpotential of 190 mV. Interestingly, TAPA‐PG is found to be efficient for in situ reduction of the Co(II) to Co(0) under ambient conditions, and results in a cobalt nanoparticles‐stabilized polymer matrix (Co@TAPA‐PG). Co@TAPA‐PG displays bifunctional electrocatalytic activity for ORR as well as oxygen evolution reaction (OER). Co@TAPA‐PG shows improved ORR catalysis as the overpotential dropped down significantly to 120 mV. The OER catalytic performance results in the maximum current density of 15.8 mA/cm2.The overpotential for OER is found to be 560 mV at the current density of 10 mA/cm2.

Bimodal Heterogeneous Functionality in Redox‐Active Conjugated Microporous Polymer toward Electrocatalytic Oxygen Reduction and Photocatalytic Hydrogen Evolution

“…Further, electron paramagnetic resonance (EPR) studies were performed for both CMPs in the solid state at room temperature. A sharp and intense signal at g =1.949 and 1.942 for TAPA‐OPE‐mix and TAPA‐OPE‐gly, respectively, is indicative of the formation of charge‐separated species at room temperature (Figure ) . Thus, both theoretical and experimental studies unambiguously support the redox‐active nature and subsequent formation of charge‐separated species for both CMPs.…”

“…We have previously reported the in situ stabilization of metal nanoparticles such as Co in a TAPA‐containing porous matrix for enhanced electrocatalytic activity . With the aim of increasing the H 2 evolution, we envisioned that the in situ stabilization of Pt nanoparticles on the CMP matrices could be advantageous, as the photogenerated electron can easily reach the surface‐attached Pt nanoparticles without recombination .…”

“…However, sluggish reaction kinetics and the subsequent use of precious‐metal‐based catalysts to catalyze electrochemical and photochemical reactions are found to be major limitations to employing these processes on a large scale . Several materials such as metal oxides, phosphides, sulfides, chalcogenides, inorganic or organic–inorganic hybrid porous and carbon‐based materials are known in the literature separately as either electrocatalysts or photocatalysts, but purely organic‐based catalysts working as electro‐ or photocatalysts have hardly been explored …”

“…Recently, the design of redox‐active CMP materials by integrating electron‐donor and ‐acceptor organic struts has drawn immense interest, as they have shown potential for catalyzing the electrochemical oxygen reduction reaction (ORR), which is a half‐cell reaction in fuel cells and metal–air batteries . Moreover, redox‐active CMPs were also found to be capable of in situ generation and stabilization of metal nanoparticles (NPs), and subsequently, such metal nanoparticle stabilized CMP (NP@CMP) materials have shown significantly improved electrocatalytic activities . Furthermore, organic porous materials have been explored recently as heterogeneous catalysts for the photochemical H 2 evolution reaction .…”

“…However,s luggish reactionk inetics and the subsequent use of precious-metal-based catalystst oc atalyze electrochemical and photochemical reactions are found to be majorl imitations to employing these processes on al arge scale. [16][17][18][19] Severalm aterials such as metal oxides, phosphides, sulfides, chalcogenides, inorganic or organic-inorganic hybrid porousa nd carbon-based materials are knowni nt he literature separately as either electrocatalysts [4,[20][21][22][23][24][25][26][27][28] or photocatalysts, [14,[29][30][31][32] but purely organic-based catalysts working as electro-or photocatalystshave hardly been explored. [33][34][35][36][37][38] Conjugated microporous polymers( CMPs) are an emerging class of porous organic materials, which have demonstrated a treasuret rove of applications.…”