Unveiling\nthe Structural Effects in Hybrid Copper\nPhosphonate Frameworks for Selective Electrocatalytic CO<sub>2</sub> Reduction Reaction

Electrochemical CO<sub>2</sub> reduction holds tremendous\npromise\nfor transforming carbon dioxide into several value-added energy feedstocks\nand utilizing renewable energy sources. Herein, we have developed\ntwo novel copper-based organophosphonates for selective electrocatalytic\nconversion of CO<sub>2</sub> to CH<sub>3</sub>OH conversion. The two-dimensional\nlayer structure of Cu<sub>3</sub>[(Hhedp)<sub>2</sub>(C<sub>4</sub>H<sub>4</sub>N<sub>2</sub>)].2H<sub>2</sub>O (<b>I</b>) and\nthe three-dimensional Cu<sub>3</sub>[(H<sub>3</sub>hedp)<sub>2</sub>(C<sub>4</sub>H<sub>4</sub>N<sub>2</sub>)<sub>4</sub>(SO<sub>4</sub>)].2H<sub>2</sub>O (<b>II</b>) have been isolated as single\ncrystals via a hydrothermal strategy. Compound <b>I</b> consists\nof Cu<sup>2+</sup> oxidation states exclusively, while compound <b>II</b> has Cu<sup>1+</sup> oxidation states in a network wherein\na Cu<sup>2+</sup>-phosphonate template is embedded inside the framework.\nDepending on mixed valent oxidation states, compound <b>II</b> exhibits high selectivity compared to compound <b>I</b> for\nthe electrocatalytic reduction of CO<sub>2</sub> to CH<sub>3</sub>OH (C1) as the primary product and CH<sub>3</sub>COOH (C2) as the\nsecondary product. Notably, product selectivity is enhanced as the\nFaradaic efficiency (FE) of the competing hydrogen evolution reaction\n(HER) is significantly reduced in compound <b>II</b> relative\nto that of <b>I</b>, particularly at higher applied reduction\npotentials. The optimal ratio of Cu<sup>1+</sup> active sites in compound <b>II</b> plays a pivotal role in enhancing methanol selectivity,\nstabilizing critical intermediates, and maintaining ideal reduction\npotentials as a noble-metal free electrocatalyst. Moreover, the optical\nband gap and the Mott–Schottky measurements further suggest\nthe title Cu-phosphonate materials could be promising and effective\nphotocatalysts.