Pd homojunctions enable remarkable CO2 electroreduction.

3D Pd aerogels with a controllable homojunction density are synthesized using an innovative melting-casting technology. The homojunction-rich Pd aerogels selectively reduce CO2 to CO with a 92.3% faradaic efficiency and durability over 10 h, benefiting from the strong coupling between the electrons and the adsorbed intermediates at the phase-mismatch interface.