Iodide-mediated Cu catalyst restructuring during CO2 electroreduction

Abstract

Catalyst restructuring during electrochemical reactions is a critical but poorly understood process that determines the underlying structure-property relationships during catalysis. In the electrocatalytic reduction of CO₂ (CO₂RR), it is known that Cu, the most favorable catalyst for hydrocarbon generation, is highly susceptible to restructuring in the presence of halides. Iodide ions, in particular, greatly improved the catalyst performance of Cu foils, although a detailed understanding of the morphological evolution induced by iodide remains lacking. It is also unclear if a similar enhancement transfers to catalyst particles. Here, we first demonstrate that iodide pre-treatment improves the selectivity of hexagonally ordered Cu-island arrays towards ethylene and oxygenate products. Then, the morphological changes in these arrays caused by iodide treatment and during CO₂RR are visualized using electrochemical transmission electron microscopy. Our observations reveal that the Cu islands evolve into tetrahedral CuI, which then become 3-dimensional chains of copper nanoparticles under CO₂RR conditions. Furthermore, CuI and Cu₂O particles re-precipitated when the samples are returned to open circuit potential, implying that iodide and Cu⁺ species are present within these chains. This work provides detailed insight into the role of iodide, and its impact on the prevailing morphologies that exist during CO₂RR.

Fig. 1. Synthesis of Cu island arrays used to monitor Cu restructuring. (a) Cartoon showing the preparation of a Cu electrode for CO₂RR: 1 μm-large polystyrene spheres were self-assembled and monodispersed on the desired support. Then, a 400 nm-thick Cu layer was deposited using an e-beam evaporator. The Cu islands are exposed after removal of the polystyrene layer. The Cu array transforms into CuI after immersion and anodization in a KI aqueous solution. The CuI electrode then restructures into porous Cu filaments under CO₂RR conditions. (b) SEM images of the Cu island structure after (i) the removal of the polystyrene spheres, (ii) immersion in 0.1 M KI solution, (iii) anodization in 0.1 M KI solution and (iv) CO₂RR in iodide-free CO₂ saturated 0.1 M KHCO₃.

Fig. 2. Changes in the CO₂RR activity and product selectivity as a function of the KI pre-treatment. (a) Geometrical current density and ECSA-normalized current density of the non-treated and KI pre-treated copper arrays. (b) FE of H₂ and CO₂RR products including CO, CH₄, C₂H₄, HCOO⁻ and oxygenates. The reaction was conducted in iodide-free and CO₂-saturated 0.1 M KHCO₃ at −1.0 V_RHE for 1 hour. The error bar is the standard deviation of the repeated measurements on three different samples that were prepared with identical procedures.

Fig. 3. Copper restructuring before and after CO₂RR. The Cu-island arrays were pre-treated with different concentrations of KI solutions and then subjected to CO₂RR in iodide-free CO₂ saturated 0.1 M KHCO₃ electrolyte at −1.0 V_RHE for 1 hour. SEM images taken after pre-treatment are shown on the left, and the same samples after CO₂RR are shown on the right: (a) control (no KI exposure), (b) 0.01 M KI, (c) 0.05 M KI, and (d) 0.1 M KI. The samples were prepared on Au-coated silicon wafers.

Fig. 4. Structural transformation of lithographically-deposited Cu islands monitored via in situ EC-TEM in STEM mode using a high angle annular dark field detector. STEM images of the Cu arrays immersed in different electrolytes are shown: (a) in ultrapure water, (b) 0.01 M KI after anodization, and in (c) iodide free-CO₂ saturated 0.1 M KHCO₃ after 30 minutes of CO₂RR at −1.0 V_RHE. (d–g) CuI transformation into Cu filaments under CO₂RR conditions (−1.0 V_RHE, iodide free-CO₂ saturated-0.1 M KHCO₃). (h–k) After 30 minutes of CO₂RR, the potential was removed, and triangular CuI structures re-precipitated at OCP over time. The images are averaged by 5 frames to enhance the signal-to-noise ratio (d–k).

Fig. 5. Morphology and crystallographic structure of iodide pretreated-Cu after CO₂RR. (a) TEM image of the after-reaction structures showing Cu filaments, sporadic Cu₂O octahedra, and CuI tetrahedra. (b) Cu, (c) Cu₂O, and (d) CuI structures are shown with their respective selected area electron diffraction patterns. (e) STEM-EDX images of filament-like structures after CO₂RR, featuring three distinctive elements, Cu, O, and I.

Fig. 6. Schematic describing the proposed evolution of the iodine-pretreated Cu islands after I-anodization and under CO₂RR condition. The morphological, volumetric, and chemical changes are featured. The different chemical states of Cu are represented by the different colours, metallic Cu in brown, Cu₂O in dark yellow, and CuI in cyan.