Copper−Hydroxyapatite−Carbon Nitride Composite from Covalent Organic Framework (COF) Precursors for Efficient Electrochemical CO2 Reduction to Formate

The development of efficient and selective electrocatalysts is crucial for the electrochemical reduction of CO2 into value-added products. We report a carbon nitride-based composite, synthesized via a one-pot assembly of an imine-linked covalent organic framework (CIN-1) with embedded hydroxyapatite (HAP) and a copper-hydroxyquinone complex, followed by thermal annealing (HAP_Cu@CN). Structural and compositional analyses (XRF, XPS, TEM-EDX, Raman, and XRD) revealed intimate contact among copper nanoparticles, carbon nitride, and HAP nanorods. Electrocatalytic tests showed that only copper on carbon nitride (Cu@CN) favored methane formation (FE approximate to 60% at -0.9 V vs RHE, reaching -110 mA cm(-2) as current density), while HAP_Cu@CN selectively produced formate with 60% FE at -1.0 V vs RHE, with a current density of -55 mA cm(-2). The presence of HAP enhanced the availability of CO2 at the electrode surface via bicarbonate adsorption and suppressed hydrogen evolution, improving product selectivity without compromising current density. These findings highlight the synergistic role of HAP in steering the CO2ER toward formate production.