Unveiling the Structural Effects in Hybrid Copper Phosphonate Frameworks for Selective Electrocatalytic CO₂ Reduction...

Electrochemical CO₂ reduction holds tremendous promise for transforming carbon dioxide into several value-added energy feedstocks and utilizing renewable energy sources. Herein, we have developed two novel copper-based organophosphonates for selective electrocatalytic conversion of CO₂ to CH₃OH conversion. The two-dimensional layer structure of Cu₃[(Hhedp)₂(C₄H₄N₂)].2H₂O (I) and the three-dimensional Cu₃[(H₃hedp)₂(C₄H₄N₂)₄(SO₄)].2H₂O (II) have been isolated as single crystals via a hydrothermal strategy. Compound I consists of Cu²⁺ oxidation states exclusively, while compound II has Cu¹⁺ oxidation states in a network wherein a Cu²⁺-phosphonate template is embedded inside the framework. Depending on mixed valent oxidation states, compound II exhibits high selectivity compared to compound I for the electrocatalytic reduction of CO₂ to CH₃OH (C1) as the primary product and CH₃COOH (C2) as the secondary product. Notably, product selectivity is enhanced as the Faradaic efficiency (FE) of the competing hydrogen evolution reaction (HER) is significantly reduced in compound II relative to that of I, particularly at higher applied reduction potentials. The optimal ratio of Cu¹⁺ active sites in compound II plays a pivotal role in enhancing methanol selectivity, stabilizing critical intermediates, and maintaining ideal reduction potentials as a noble-metal free electrocatalyst. Moreover, the optical band gap and the Mott-Schottky measurements further suggest the title Cu-phosphonate materials could be promising and effective photocatalysts.