Abstract
Restructuring-induced catalytic activity is an intriguing phenomenon of fundamental importance to rational design of high-performance catalyst materials. We study three copper-complex materials for electrocatalytic carbon dioxide reduction. Among them, the copper(II) phthalocyanine exhibits by far the highest activity for yielding methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm−2 at the potential of – 1.06 V versus the reversible hydrogen electrode. Utilizing in-situ and operando X-ray absorption spectroscopy, we find that under the working conditions copper(II) phthalocyanine undergoes reversible structural and oxidation state changes to form ~ 2 nm metallic copper clusters, which catalyzes the carbon dioxide-to-methane conversion. Density functional calculations rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion–ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions.
Highlights
Cu2O HKUST-1 After gHKUST-1 Intensity (a.u.) 2 (°) b e h Fresh
HKUST-1 is a MOF with Cu(II) nodes coordinated by negatively charged btc linkers (Fig. 1b). [Cu(cyclam)]Cl2 features a Cu2+ ion coordinated by a non-conjugated chargeneutral ligand (Fig. 1c)
As the electrode potential returns to 0.64 V, the metallic Cu originated from HKUST-1 is oxidized to Cu2O, which is supported by the similarity to the extended X-ray absorption fine structure (EXAFS) spectrum of the Cu2O standard (Fig. 2f and Supplementary Fig. 5)
Summary
[Cu(cyclam)]Cl2 features a Cu2+ ion coordinated by a non-conjugated chargeneutral ligand (Fig. 1c). The major CO2 reduction products over the CuPc catalyst are formic acid (HCOOH), C2H4, and CO (Fig. 1d), with the Faradaic efficiencies being 25, 13, and 6% at – 0.86 V, respectively. The Faradaic efficiencies and partial current densities of the gas-phase products over the three catalyst electrodes at – 1.06 V are compared in. It can be clearly discerned that CuPc is a much more active and selective electrocatalyst than HKUST-1 and [Cu(cyclam)]Cl2 for CO2 reduction to CH4. To probe the structural and oxidation state changes of these Cu-complex electrocatalysts as they perform CO2 reduction, we carried out insitu XAS measurements (Supplementary Fig. 3) under the same electrochemical conditions.
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