Shape-Controlled CO ₂ Electrochemical Reduction on Nanosized Pd Hydride Cubes and Octahedra

Electrochemical CO ₂ reduction reaction (CO ₂ RR) provides a potential pathway to mitigate challenges related to CO ₂ emissions. Pd nanoparticles have shown interesting properties as CO ₂ RR electrocatalysts, while how different facets of Pd affect its performance in CO ₂ reduction to synthesis gas with controlled H ₂ to CO ratios has not been understood. Herein, nanosized Pd cubes and octahedra particles dominated by Pd(100) and Pd(111) facets are, respectively, synthesized. The Pd octahedra particles show higher CO selectivity (up to 95%) and better activity than Pd cubes and commercial particles. For both Pd octahedra and cubes, the ratio of H ₂ /CO products is tunable between 1 and 2, a desirable ratio for methanol synthesis and the Fischer–Tropsch processes. Further studies of Pd octahedra in a 25 cm ² flow cell show that a total CO current of 5.47 A is achieved at a potential of 3.4 V, corresponding to a CO partial current density of 220 mA cm ⁻² . In situ X-ray absorption spectroscopy studies show that regardless of facet Pd is transformed into Pd hydride (PdH) under reaction conditions. Density functional theory calculations show that the reduced binding energies of CO and HOCO intermediates on PdH(111) are key parameters to the high current density and Faradaic efficiency in CO ₂ to CO conversion.