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Abstract

The direct electrochemical conversion of carbon dioxide (CO₂) into multi-carbon (C₂₊) products still faces
fundamental and technological challenges. While facet-controlled and oxide-derived Cu materials have
been touted as promising catalysts, their stability has remained problematic and poorly understood. The
present work uncovers changes in the chemical and morphological state of supported and unsupported Cu₂O
nanocubes during operation in low-current H-Cells and in high-current Gas Diffusion Electrodes (GDEs)
using neutral pH buffer conditions. While unsupported nanocubes achieved a sustained C₂₊ faradaic
efficiency of around 60% for 40 h, the dispersion on a carbon support sharply shifted the selectivity pattern
towards C₁ products. Operando XAS and time-resolved electron microscopy revealed the degradation of
the cubic shape and, in the presence of a carbon support, the formation of small Cu-seeds during the
surprisingly slow reduction of bulk Cu₂O. Here, the initially (100)-rich facet structure has presumably no
controlling role on the catalytic selectivity, whereas the oxide-derived generation of under-coordinated
lattice defects, as revealed by the operando Cu-Cu coordination numbers, can support the high C₂₊ product
yields.