Atomic-Level Valence-State Engineering Redirects CO₂ Electroreduction on Cu Nanoclusters

Atomically precise thiolate-protected Cu nanoclusters (NCs) typically suffer from an intrinsic bias toward the two-electron formate pathway in CO₂ electroreduction, limiting access to more deeply reduced products. Breaking this selectivity within a structurally well-defined system remains a significant challenge due to the difficulty of precisely tuning the Cu valence states. Here, we address this limitation through atomic-level valence-state engineering by introducing [S@Cu₅₀S₁₂(S ^t Bu)₂₀(CF₃COO)₁₂] (S@Cu₅₀) NC, featuring a controlled Cu-(I)/Cu-(II) ratio within a conserved structural framework. Single-crystal analysis reveals a core-shell S@Cu₁₄S₁₂@Cu₃₆ architecture, while XPS confirms an increased Cu-(II) population compared to the reference [Cu₅₀S₁₂(S ^t Bu)₂₀(CF₃COO)₁₂] (Cu₅₀) analogue. Despite similar overall catalytic activity, S@Cu₅₀ exhibits a striking shift in product selectivity during CO₂ electroreduction, suppressing formate formation (Faradaic efficiency of <11% vs 38% in Cu₅₀) and enabling CH₃OH production with a Faradaic efficiency of ∼19% at -1.0 V vs RHEabsent in the Cu₅₀ system. Density functional theory calculations attribute this mechanistic switching to valence-induced electronic modulation, which stabilizes CO-derived intermediates and promotes sequential hydrogenation toward CH₃OH, in contrast to HCOO stabilization in the reference NC. This work establishes that subtle modulation of the Cu-(I)/Cu-(II) balance can fundamentally redirect reaction pathways, providing a molecular-level strategy to overcome intrinsic selectivity limitations in Cu NC catalysis.