Sub-Nanometer Nanoclusters of Copper Atop Single-Atom Copper Moieties toward Electrochemical CO ₂ Hydrogenation to Methane

The electrochemical CO2 reduction (eCO2R) offers a compelling route for converting CO2 into value-added fuels and chemicals. Among CO2-derived products, methane (CH4) occupies a distinct position, serving both as a key intermediate for emerging cascade electro-oxidation to oxygenates and as a strategically important extraterrestrial fuel that can be generated in situ from off-planet CO2 resources. Although Cu-based catalysts capable of selectively producing CH4 have been reported, they seldom sustain high selectivity at practically relevant current densities. Here, we created a single-step co-pyrolysis strategy toward generating and anchoring Cu sub-nanometer clusters (CuSNC) atop Cu-Nx single-atom (SA) motifs embedded within N-doped carbon (NC), with controllable nanostructures through tuning of the synthesis parameters. Complementary spectroscopic analyses and density functional theory (DFT) calculations help reveal a structure−activity correlation that could guide the catalyst design. The CuSNC@NC sample synthesized at 550 °C pyrolysis temperature (best described and modeled as Cu3-CuN4 domains) represents the most effective combination of cluster size, metal-nitrogen coordination, and adsorption energetics needed to selectively promote CH4 generation versus other eCO2R products. Incorporating pulsed electrolysis and hydrophobicity-modulated transport tuning at the triple-phase boundary (TPB) further enhanced CH4 production achieving a partial CH4 current density of ∼321 mA cm−2, 53% Faradaic efficiency (FECH4), and less than 4% combined FE for other eCO2R products, simplifying downstream CH4 purification or upgrading. This work establishes generalizable principles for controlling Cu cluster atomicity and metal−nitrogen coordination, both of which are recognized determinants of CH4-efficient eCO2R.