Novel carbon nitride/hydroxyapatite composite material for effective CO2 electroreduction into C1-based product

The reduction of CO2 using different techniques represents a promising technology in the ongoing efforts to manage carbon dioxide emissions. One of the most intriguing technologies is CO2 electrochemical reduction (CO2-ER) into carbon-based products, which is currently undergoing deep investigation. Although CO2-ER has great potential for environmental applications, many holdbacks are present regarding energy efficiency, reaction selectivity, and overall conversion rate. Electrocatalytic materials must be developed to address the aforementioned drawbacks in the CO2 electrochemical process. Here, we present an innovative composite material composed of graphitic carbon nitride (CN) properly functionalized with metal nanoparticles, such as copper and hydroxyapatite nanorods (HAP). Carbon nitride is a conductive and supportive material, and thanks to its porous structure, it can be easily functionalized. HAP, a calcium phosphate mineral with highly versatile properties, has recently gained attention in material science and environmental applications both as an adsorbent and as an acid-base catalyst. An innovative and effective electrocatalyst can be obtained by combining different materials with different properties and features. It may be employed in CO2-ER in order to decrease the total overpotential and to increase faradic efficiency towards added value molecules, particularly C1 molecules such as methane and formic acid. Cu@CN and HAP_Cu@CN are materials that exploit the singular features of their components to promote a more efficient reduction of carbon dioxide towards methane and formic acid, compared with other copper-carbon based catalysts such as multi-walled carbon nanotubes (MWCNT) or copper-zinc alumina material (CZA). Electrochemical investigation were performed using a three-electrode cell, in a 0.1 M KHCO3 aqueous solution, comprising a working electrode (carbon paper polished with the catalyst), a reference electrode (saturated Ag ׀AgCl) and a counter electrode (Pt wire) (Fig.1, a). Linear sweep voltammetry (LSV) curves show a net increase in the current density slope at potentials lower than −1.5 V (vs Ag ׀AgCl) when HAP is present in the catalyst, indicating that admixing with HAP results in a slight increase in the activity of the systems. To highlight the products formed in the presence of catalysts, a 1-hour chronoamperometric (CA) test was performed. Despite the inevitable presence of the parasitic hydrogen evolution reaction (HER), the catalysts showed CO2ER activity. The relative faradic efficiencies (FE) of HER and CO2ER are consistent with LSV data, showing a predominant HER at a lower cathodic potential than -1.5 V (vs Ag׀AgCl). For Cu@CN and HAP_Cu@CN, H2 FE decreased significantly by increasing cathodic potential, while other copper-based catalysts showed low catalytic activity and hydrogen production. Focusing on the performance of the composite with HAP and CN, the catalyst showed significant FE towards formic acid (Fig.1, b): at cathodic potential such as -1.8 V (vs Ag׀AgCl) selectivity reached 55%, proving that the apatite may influence the reaction mechanism through the presence of different acidic and basic sites. The composite material showed promising results in CO2RR towards methane or formic acid, important C1 added-value molecules, that have applications in the energy field or in chemical transformations