Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy

Article https://doi.org/10.1038/s41467-024-50379-w Reversible metal cluster formation on Nitrogen-doped carbon controlling electrocatalyst particle size with subnanometer accuracy Received: 4 March 2024 1234567890():,; 1234567890():,; Accepted: 9 July 2024 Janis Timoshenko 1 , Clara Rettenmaier 1, Dorottya Hursán 1, Martina Rüscher1, Eduardo Ortega 1, Antonia Herzog 1, Timon Wagner 1, Arno Bergmann 1, Uta Hejral 1, Aram Yoon1, Andrea Martini1, Eric Liberra1, Mariana Cecilio de Oliveira Monteiro 1 & Beatriz Roldan Cuenya 1 Check for updates Copper and nitrogen co-doped carbon catalysts exhibit a remarkable behavior during the electrocatalytic CO2 reduction (CO2RR), namely, the formation of metal nanoparticles from Cu single atoms, and their subsequent reversible redispersion. Here we show that the switchable nature of these species holds the key for the on-demand control over the distribution of CO2RR products, a lack of which has thus far hindered the wide-spread practical adoption of CO2RR. By intermitting pulses of a working cathodic potential with pulses of anodic potential, we were able to achieve a controlled fragmentation of the Cu particles and partial regeneration of single atom sites. By tuning the pulse durations, and by tracking the catalyst’s evolution using operando quick X-ray absorption spectroscopy, the speciation of the catalyst can be steered toward single atom sites, ultrasmall metal clusters or large metal nanoparticles, each exhibiting unique CO2RR functionalities. Copper-based materials exhibit a unique ability to convert CO2 into hydrocarbons and other valuable chemicals and fuels1–3 through the electrocatalytic CO2 reduction reaction (CO2RR). The selectivity of copper toward a certain chemical from a broad distribution of possible reaction products, however, is hard to control. One strategy is to use Cu and nitrogen co-doped carbon materials (Cu-N-C) as catalysts, featuring singly dispersed cationic Cu species. Analogous materials based on other transition metals have been used as electrocatalysts for various processes4–9, including the CO2 conversion to CO9–11. Nonetheless, in CuN-C, the cationic Cu sites were found to be unstable under CO2RR, forming metallic particles12,13. Surprisingly, this process is reversible, with particles redispersing upon lifting the reducing conditions. Such reversible behavior holds for a broad range of Cu-N-C catalysts, including those based on Cu phthalocyanine (CuPc)12,14–16, or those employing different metal/covalent organic frameworks as precursors12,13,17–19. 1 Here we show that the switchable nature of Cu sites provides an opportunity to form in-situ unique catalytic structures with distinct functionality. We rely on a pulsed CO2RR protocol alternating between the working (cathodic) potential (Ec) and an anodic potential (Ea)20. By varying the durations of the cathodic and anodic pulses, and by tracking the catalyst’s evolution using operando quick X-ray absorption fine structure (QXAFS) spectroscopy20–22, we were able to control the average size of the Cu particles with subnanometer accuracy. Unlike previous works where the particle size was fixed by catalyst preparation16, our approach enables steering the catalyst structure on the fly and reversibly switching between different catalytic functionalities. Thus, it allows us to explore structure-property relationships in the challenging regime of sub-nanometer particle sizes. In particular, singly dispersed cationic Cu species were shown to favor hydrogen production, ultrasmall Cu clusters yielded methane, while larger Cu Department of Interface Science, Fritz-Haber Institute of the Max-Planck Society, Berlin, Germany. Nature Communications | (2024)15:6111 e-mail: ; 1 Article nanoparticles - CO and multicarbon products. Our findings reconcile previous reports on the high selectivity of Cu-N-C catalysts to hydrocarbons15,23 with those on particle size effect in CO2RR24,25. They underscore the challenge of extrapolating the structure-properties relationship derived from larger nanoparticles to ultradispersed clusters featuring just a few atoms, emphasizing that for clusters that are less than 2 nm in size, even slight variations in sizes can result in strong, non-monotonic changes in their physico-chemical properties26–28. Results Copper particles under static and pulsed CO2RR Cu-N-C catalysts were prepared using an impregnation-calcination method from a ZIF-8 precursor29,30. Ex-situ characterization using X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), and high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM) confirmed the incorporation of Cu into the nitrogen-doped carbon support and lack of clusters in the as-prepared samples. See ref. 29. for the results of ex-situ characterization, and Supplementary Fig. 1 for additional HAADF-STEM images. Static CO2RR experiments were conducted in a CO2-saturated 0.1 M KHCO3 electrolyte at − 1.35 V (bulk pH ≈ 6.8). All potential values are given with respect to the Fig. 1 | Evolution of operando XANES and EXAFS spectra for Cu-N-C electrocatalysts under static and pulsed CO2RR. Cu K-edge XANES (a) and Fouriertransformed (FT) EXAFS spectra (b) for Cu-N-C during the first 400 s of CO2RR in 0.1 M KHCO3 under static − 1.35 V potential. The inset in (a) shows a structure model of the Cu single sites in the as-prepared catalyst30, visualized with VESTA software52. Evolution of FT-EXAFS (c, e) and XANES (d) spectra during pulsed CO2RR with Ec = − 1.35 V, Ea = 0.44 V and Δta = Δtc = 30 s. c, d Changes in the catalyst during the Nature Communications | (2024)15:6111 https://doi.org/10.1038/s41467-024-50379-w reversible hydrogen electrode (RHE). The formation of Cu particles in Cu-N-C was monitored by QXAFS spectroscopy with a time resolution of up to 2 s per spectrum (Figs. 1, 2 and Supplementary Figs. 2 –8). X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses (Supplementary Note 1) agree with our previous reports29. Briefly, in the as-prepared Cu-N-Cs the singly dispersed Cu2+ sites have distorted octahedral coordination (planar Cu-N4 unit with two axial O or OH groups29,30, Fig. 1a). Under applied potential, these cationic species transformed rapidly (within 100 s, Fig. 2a, d, f) into metallic particles, with an average effective diameter of ca. 1.3 ± 0.1 nm (Supplementary Note 1, Supplementary Fig. 4 and Supplementary Table 1). The metallic Cu coexists with the remaining singly dispersed Cu. The concentration of the latter after the quick initial drop changes slowly, and stabilizes at ca. 12% (Supplementary Fig. 7). Next, in order to tune the working structure of the Cu electrocatalyst, we investigated its evolution under pulsed CO2RR, with cathodic potential Ec = − 1.35 V, and anodic potential Ea = 0.44 V, and different durations of the cathodic and anodic (...truncated)