Copper nanoclusters derived from copper phthalocyanine as real active sites for CO2 electroreduction: Exploring size dependency on selectivity - A mini review

The electrochemical reduction reaction of CO₂ (CO₂RR) holds promise for converting CO₂ into valuable fuels and chemicals, particularly when powered by renewable electricity, thereby aiding in reducing atmospheric CO₂ levels and addressing climate change. Copper phthalocyanine and its derivatives (Cu-Pcs) have attracted significant attention as versatile electrocatalytic materials with high selectivity toward various hydrocarbon products. However, the real active sites of Cu-Pcs for different products vary, and there is a lack of comprehensive summary. To address this gap, we analyze and summarize previous research, yielding the following insights: Cu-Pcs undergo reconstruction and demetallization during CO₂RR, with Cu_(II) converting to Cu₍₀₎, forming transient copper nanoclusters (Cu NCs). The selectivity for CO₂RR products closely correlates with the size of those derived Cu NCs. Specifically, reversible Cu NCs with ultrasmall sizes (≤2 nm), which revert to Cu-Pcs after electrolysis, exhibit high selectivity toward CH₄. As Cu NCs increase in size, there is a higher CO coverage, promoting CO generation. When Cu NCs exceed a critical threshold size (approximately 15 nm), C-C coupling can occur, facilitating the formation of multicarbon (C₂₊) products. Furthermore, the structure of macrocycles, types of functional groups, and properties of carbon substrates influence the size and electron density of Cu NCs, thereby impacting the selectivity of CO₂RR products.