Non-noble-metal-based porphyrin covalent organic polymers as additive-/annealing-free electrocatalysts for water splitting and biomass oxidation

Abstract

Though metalloporphyrins have proved their efficiency as a class of efficient electrocatalysts, their practical use is often restricted due to poor stability and lack of an effective electrochemical surface. Combining metalloporphyrins in the skeleton of covalent organic polymers, though, is an effective strategy for developing efficient electrocatalysts for OER and HER; due to the restricted conductivity of such materials, addition of external additives or annealing is a must, which increases the cost of the material development. Here, we report two metalloporphyrin-based covalent organic polymers, COP-POR-Ni and COP-POR-Co, which work as excellent annealing- and additive-free electrocatalysts for overall water splitting in an alkaline medium. The polymers achieved a current density of 1 mA/cm2 at 300 and 370 mV with turnover frequencies (TOFs) of 9.1 × 10–3 and 2.68 × 10–3 s–1 at 2 mA/cm2 for COP-POR-Ni and COP-POR-Co, respectively, for alkaline water oxidation. The catalysts are also active for the selective conversion of 5-hydroxymethylfurfural (HMF), a plant biomass carbohydrate, oxidation to a platform chemical 2,5-furandicarboxylic acid (FDCA) with the coproduction of hydrogen at the cathode. HER activity was also achieved with overpotentials of 468 and 348 mV at a current density of 1 mA cm–2 for COP-POR-Ni and COP-POR-Co in an acidic medium, respectively.