Morphology and mechanism of highly selective Cu(II) oxide nanosheet catalysts for carbon dioxide electroreduction

Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C2+ products, selectivities (> 400 mA cm−2) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu0 dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C2H4 was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH4 and ethanol. We demonstrate evidence that adsorbed methyl species, *CH3, serve as common intermediates of both CH3H and CH3CH2OH and possibly of other CH3-R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO2 electrolysis on Cu catalysts.

V alorizing atmospheric CO 2 into C 2+ products by electrochemical method holds the promise to store renewable surplus electricity while closing the global carbon cycle [1][2][3] . Efficient CO 2 electrolysis requires advanced electrocatalyst designs [4][5][6][7][8][9][10] , an understanding of competing reaction pathways 11,12 , and a reliable device performance at industrial conditions [13][14][15][16] . Among a wide variety of catalysts tested for electrochemical CO 2 reduction reaction (CO2RR), copper-based materials, in particular Cu oxides, CuO x , have been enjoying attention thanks to their wide chemical selectivity for a variety of multi-carbon products, such as C 2+ hydrocarbons and oxygenates. It has been reported that over 16 kinds 17 of products can be formed on copper catalysts through multiple proton-coupled electron transfer processes.
The cathodic electrode potentials during the catalytic CO2RR invariably drive the chemical transformation of operating CuO x catalysts into metallic Cu phases. This prompted their designation as oxide-derived copper (OD-Cu) catalysts. As a consequence of the chemical reduction metallic Cu 0 , complex time trajectories of catalyst morphology, chemical state, and catalytic selectivity ensues. A full molecular correlation and understanding of these concomitant trajectories has remained elusive. It has been suggested that the increased local pH value [18][19][20] of OD-Cu, in direct proportion to the surface roughness, plays an important role for favorable ethylene selectivity compared to methane 21 . Other works highlighted the distinct chemisorption of CO intermediates [22][23][24] . Yet other reports related the catalytic performance to grain boundaries 25 , to undercoordinated sites 26 , to the presence of subsurface oxygen 27,28 or to residual near-surface Cu +29 . Also, in most studies, the end point of the morphological evolution of designer OD-Cu catalysts has remained in the dark.
Shape-selected cubic Cu 2 O catalysts-largely obtained through potential cycling in presence of surface-active electrolyte additives-have been receiving particular attention as active and selective CO2RR electrocatalysts 30,31 . Thanks to the structure sensitivity of the C-C bond formation on square atomic Cu 0 surface lattices and the cubic nature of the Cu 2 O crystal system, the resulting preferential Cu(100) facets offer kinetic faradaic efficiency and onset potential benefits for the formation of C 2+ products, in particular C 2 H 4 32 .
To follow the evolution in chemical state and morphology of electrocatalysts under stationary electrode potential conditions, operando X-ray-based 33 as well as in situ vibrational spectroscopies 31,[34][35][36] , and in situ electrochemical transmission electron microscopy (TEM) [37][38][39] were used to record changes in catalyst chemical state 40,41 , local structure 13 , or intermediates 42 . However, to date, real-time tracking individual CO2RR product yields and product onset potentials on OD-Cu catalysts at fast time scales under non-stationary, transient conditions has remained very challenging. This operational mode, however, is very relevant for practical CO2RR electrolyzers powered by intermittent input electricity from renewable sources.
Even though Cu-based CO2RR electrocatalysts have shown excellent C 2 H 4 efficiencies in highly alkaline pH 13-15 conditions 13,43 , practical CO2RR electrocatalysis must operate at neutral pH~7 conditions and perform at industrially relevant current densities > 200 mA cm −213, 14 . To achieve this, the catalysts must be cast in gas diffusion electrodes (GDEs) that are deployed in single zero-and non-zero gap flow electrolyzer cells. While a few recent studies have reported the use of free-standing cubic Cu 2 O nanocubes [44][45][46] inside GDE designs, the majority of OD-Cu based GDE studies relied on top-down approaches involving modified bulk Cu 43 . Compared to bulk Cu-based catalysts, nanostructured free-standing catalysts are more desirable as they exhibit higher surface-to-volume ratio and are more amenable to assembly in large-scale GDEs.
In this contribution, we report on a new family of free-standing 2-dimensional Cu(II) oxide electrocatalysts for the CO2RR under neutral pH conditions. Owing to their 2D nature, Cu(II)O nanosheets (referred to as "2D CuO NS") feature a highly preferred (001) facet orientation. We report a facile one-step synthesis of 2D CuO NS and then track their unique catalytic reactivity and concomitant morphological evolution under applied electrode potentials using in situ electrochemical liquid TEM and operando X-ray absorption spectroscopy (XAS). Next, using a new differential electrochemical mass spectrometry (DEMS) technique with millisecond resolution, we discuss previously unreported time evolutions of kinetic onset potentials of a range of different products over the course of hours. Conclusions from our DEMS data challenge and refine our current mechanistic understanding of the mechanistic link between CH 4 and ethanol (EtOH). Finally, we document the favorable CO2RR performance of 2D CuO NS inside GDEs of commercial electrolyzers under industrially relevant neutral pH conditions. Owing to their emerging stable dendritic structure, their high catalytic reactivity, and the resulting performance stability, oriented 2D CuO NS precursor catalysts constitute an interesting alternative to conventional cubic Cu 2 O catalysts for the electrochemical conversion of CO 2 into C 2 H 4 .

Results
Synthesis and characterization of 2-dimensional CuO nanosheets. Free-standing CuO nanosheet catalysts (CuO NS) were prepared by solvothermal formation and processing of Cu (OH) 2 intermediates in alkaline condition ( Supplementary  Fig. S2). The wrinkled thin Cu(OH) 2 layers showed high aspect ratios prior to decomposing into CuO and H 2 O. Figure 1a,b presents scanning electron microscopy (SEM) image and transmission electron microscopy (TEM) image of as-prepared CuO NS. The 2D character of the material was retained during the thermal decomposition of Cu(OH) 2 , while larger sheets split into small ones. The final rectangular CuO NS exhibited serrated edges on the short edges. Supplementary Fig. S3a indicates a stacked structure of individual free-standing CuO NS of 3−4 nm thickness. High-resolution transmission electron microscopy (HR-TEM) (Fig. 1c) of CuO NS revealed well-defined lattice fringes with an interplanar spacing of 2.8 Å, corresponding to {110} planes of monoclinic CuO. Selected area electron diffraction (SAED) images ( Fig. 1d) showed rhombic diffraction spots along the [001] zone axis. This can be seen by indexed (020), (-110), (-200), (-1-10), (0-20) and (1-1;0) planes, indicating that the CuO NS had {001} exposed surfaces. The relative intensity of the diffraction spots is presented in Supplementary Fig. S3b. The crystal structure is provided in Supplementary Fig. S4 with indexed (001), (110) and (11-1) planes.
A 2D grazing-incidence wide-angle X-ray scattering (GI-WAXS) pattern of CuO NS on glassy carbon electrode is shown in Fig. 1e. The pattern is commensurate with a crystalline CuO phase. Minor contributions of a residual Cu 2 O phase are seen at low angles, yet all higher order reflections were absent evidencing their trace character. Importantly, the (002) and (11-1) reflections of CuO exhibited a stronger intensity in the meridional direction, evidencing that the CuO NS are stacked along the <00 l> direction, supporting the HR-TEM results. A partial contour plot and integrated CuO NS line profiles are shown in Supplementary  Fig. S5, indicating the coexistence of Cu 2 O traces and major CuO phase.
CO2RR reactivity, stability, and ex situ morphology in H-cells. The catalytic CO2RR activity of the CuO NS in pH neutral conditions and without any further pretreatment was recorded for up to 60 h. Product yields as function of iR-corrected electrode potential (E RHE ) after 1 h electrolysis are plotted in Fig. 2a. C 2 H 4 started to evolve at significantly more anodic electrode potentials compared to CH 4 , which offered a roughly 300 mV-wide potential window of exclusive C 2 H 4 production (their detailed timeresolved onset potentials see DEMS section below). CH 4 formation showed a rapid growth beyond −0.9 V RHE and exceeded C 2 H 4 production at −0.97 V RHE . The observed partial current densities under the chosen conditions ( Fig. 2b) are testament to previously unachieved hydrocarbons selectivities under pH neutral conditions. The partial current density of C 2 H 4 reached 6.2 mA cm −2 at −0.97 V RHE , while CO remained at very low levels during the entire given overpotential range. The Faradaic efficiencies (FEs) within the potential window of exclusive C 2 H 4 production and beyond are compared in Supplementary Fig. S6. The suppression of the HER and concomitant increase in C 2 H 4 are noteworthy for H-cell studies.
Longer-term 20 h and 60 h electrolysis was carried out to track the CuO NS selectivity and stability at selected potentials ( Fig. 2c−e). CuO NS catalysts maintained a stable absolute C 2 H 4 production rate during the 60 h electrolysis, even past replacements of the electrolyte (Fig. 2d, e). By contrast, the CH 4 production decreased and dropped to very low values after 25 h, making CuO NS voltage efficient (low overpotentials), C 2 H 4 -selective and very performance-stable electrocatalysts.
The initial catalyst activation period over the first few hundred minutes merits a closer time resolution of the reactivity coupled to a correlation to the sheet morphology (Supplementary Figs. S7−S10). The time trajectories of CO, CH 4 , and C 2 H 4 production ( Supplementary Fig. S7) exhibited similar patterns over the first 2−3 h of CuO NS catalyst activation. The hydrocarbon production rate peaked earlier with increasing applied overpotential. Consistent with data from Fig. 2a, exclusive and sustained catalytic C 2 H 4 production was detected at −0.84 V RHE at 1.6 nmol cm −2 s −1 , exceeding previous reports 47 . Near −1.0 V RHE , hydrocarbon production peaked already after 2 h, followed by a steady drop in CH 4 generation (cf. Fig. 2d).
Morphological changes of CuO NS ( Supplementary Fig. S8) after the first hour electrolysis near −1.0 V RHE involved rounding of the nanosheet with agglomerations. After prolonged electrolysis at −0.76 V RHE and −0.84 V RHE (Supplementary Figs. S9, S10), the rounding led to a sheet fragmentation of the initial CuO NS appeared to be more obvious. Finally, after a 60-h electrolysis at −1.0 V RHE , the sheet fragments re-assembled into larger agglomerates with rough surfaces (Supplementary Fig. S11). In a comparative study of the evolution of the catalyst morphology as a function of electrode potential under identical conditions, the CuO NS catalyst was drop cast on a rough carbon fiber paper rather than on smooth glassy carbon. Thanks to better dispersed NS, the catalytic current density increased ( Supplementary  Fig. S12). Now, ex situ SEM studies after 1 h reaction ( Supplementary Fig. S13) evidenced that rough supports slowed down the morphological transformations of the CuO NS. Unlike shown in Supplementary Fig. S8, part of the sheet morphology appears still intact ( Supplementary Fig. S13c  Real-time morphology using in situ electrochemical TEMfrom sheets to dendrites. The ex situ microscopic studies revealed rapid (< 1 h) fragmentation of CuO NS during CO2RR associated with rapid kinetic activation and monotonically rising product selectivity (Supplementary Figs. S7, S8 and S13). Of particular interest was the applied bias of −0.84 V RHE , where hydrocarbon generation was limited exclusively to C 2 H 4 . To learn more about the NS catalyst morphology that enabled exclusive C 2 H 4 -selectivity, we followed the transformations of the CuO NS at that same bias using in situ electrochemical liquid TEM experiments. These studies were carried out in a Protochips Poseidon holder that hosted a flow-through electrochemical chip cell ("in situ TEM E-chip cell", Supplementary To check the influence of beam and liquid layer effects, we conducted a prolonged 40 min OCP experiment in the IL TEM Echip cell. In agreement with the in situ results, the CuO NS with 100 s of nanometer size fractured into fragments of few nanometer size ( Supplementary Fig. S16b, c) without disintegration.
Next, we used the in situ TEM E-chip cell to conducted realtime imaging of the CuO NS in the pH = 6.9 buffer solution, still  We conclude that facet-orientated CuO NS rapidly transform into nanoscale reactive sheet fragments that slowly reassemble into stable large agglomerations and tree-like Cu dendrites at rates depending on applied potential/reaction time. Dendritic structures are known to emerge in diffusion-limited growth regimes 48,49 , typically present under convective flow conditions of electrochemical flow cells. We therefore hypothesize dendritic structures-characterized by their unique tree-like arborization with self-similar branch structure across several length scales-constitute the kinetically preferred morphological end point of our Cu catalysts in flow conditions; 50 a hypothesis that will be corroborated further below in our electrolyzer flow cell studies below.
Tracking the chemical state and local coordination of CuO NS using operando XAS. To examine the changes in the chemical state and local coordination environment of the CuO NS concomitant to the morphological evolution, operando XAS experiments were conducted (Fig. 4). Visual inspection of X-ray absorption near edge spectra (XANES) of reference materials and CuO NS catalyst under OCP conditions (Supplementary Fig. S22) corroborated earlier findings as to the Cu(II) state of the CuO NS catalyst. Extended X-ray absorption fine structure (EXAFS) simulations proved that the CuO reference and the CuO NS catalyst shared the same atomic structure (Supplementary Fig. S23 and Supplementary  Table S1). There were no significant contributions from Cu(OH) 2 and Cu 2 O in the CuO NS film detected, which is owed to the volume sensitivity of XAS. Hence, the catalyst structure at rest conditions was indeed a highly oriented CuO sheet.
After voltammetric scanning to −0.84 V RHE , the complete chemical reduction to metallic Cu 0 was observed after 130 min during CO2RR. Interestingly, the reduction from cupric oxide, CuO, to Cu 0 occurred directly, i.e. without detectable formation of Cu 2 O intermediates (Fig. 4a). Linear combination fittings revealed that after the voltammetry, 35% of the bulk volume was already Cu 0 (Fig. 4b, c). At the end of the chronoamperometry, metallic Cu accounted for 99%, with the remaining 1% within the uncertainty of the data quality. Possible effects of residual traces of subsurface oxygen or higher valent Cu species can neither be excluded nor confirmed 33 , yet appear unlikely in absence of stabilizing agents such as halogenides. The gradual change from CuO to Cu 0 is also indicated by the intensity decrease of the Cu-O bond (1.94 Å) and the intensity increase of the Cu-Cu (2.53 Å) bond (Fig. 4d, e). Coordination numbers (CN) of Cu-O decreased with the reduction process, while CN of Cu-Cu increased, and stabilized at 10 after 120 min (Supplementary Fig. S25). There is neither variation in the Cu-O bond length (first coordination sphere), nor in that of Cu-Cu. The obtained CN of Cu 0 is smaller than bulk fcc Cu 0 , indicating bulk and/or surface lattice defects associated with undercoordinated Cu atoms. This is in agreement with the in situ TEM results that suggested the formation of nano-sized Cu sheet fragments. Undercoordinated Cu sites formed during the CuO NS fragmentation and visualized in Fig. 3, typically exhibit strong chemisorption that has been associated with outstanding C-C coupling 23,25 .
Mechanistic implications from DEMS-based catalytic rates and product onset potentials. A differential electrochemical mass spectrometry (DEMS) set-up equipped with an electrochemical capillary cell 12 with milli-second time resolution enabled tracking kinetic product formation rates and yields under transient catalytic CO2RR. Thanks to its time resolution, the DEMS technique accurately captured the electrode potentials where product generation sets in (so-called onset potentials). Onset potentials are important mechanistic information that can be associated to DFT-computational mechanistic predictions. Reliable onset potential evolutions of a variety of CO2RR products have not been reported before.
We performed a continuous 9-h DEMS measurement while cycling the potential between −0.2 V RHE and −1.0 V RHE . The anodic turning potential was set deliberately as to prevent Cu from re-oxidation during the cycling. Each cycle took about 320 s. Figure 5a compares the transient mass ion currents of CH 4 , C 2 H 4 and ethanol (EtOH) at m/z = 15, 26, and 31, respectively, as function of cycle number and time. C 2 H 4 and EtOH production rates doubled during the first hour, peaked at the 10 th cycle (45 min), while CH 4 production showed continues increase.
The integrated mass ion charges of CO, CH 4 , C 2 H 4 and EtOH represent the product yields over cycle and time (Fig. 5b) and were correlated with the chemical state of the Cu catalyst. Parallel  Supplementary Fig. S15. The whole movie is shown as Supplementary Movie 1. d Current/potential profiles over time with marked time points, t 1 -t 4 (vertical lines), corresponding to the images in e-h. Linear sweep voltammetry (LSV) is performed after 10-second OCP measurement in a pH = 6.9 buffer solution flow (the OCP profile is given in Supplementary Fig. S17a) with scan rate of 50 mV/s. The following Chronoamperometry (CA) are acquired at −0.84 V RHE (the first potential). The whole movie is shown as Supplementary Movie 4. The corresponding images at i t 0 1 = 2.5 s and j t 0 4 = 460 s with the second LSV + CA (the 10-second OCP profile before LSV is given in Supplementary  Fig. S17b) at −1.23 V RHE . Current/potential profiles over time with marked time points, t 0 1 -t 0 4 (vertical lines) and the images snapshots are shown in Supplementary Fig. S18. The whole movie is shown as Supplementary Movie 5. The corresponding images at k t 0 1 0 = 2.5 s and l t 0 4 0 =1000 s with the third LSV + CA (the 10-second OCP profile before LSV is given in Supplementary Fig. S17c)  to the emerging defective metallic Cu 0 catalyst, CH 4 , C 2 H 4 and EtOH showed increasing rates and yields over the first 10 cylces (~45 min). Thereafter, C 2 H 4 remained partly stable, while CH 4 and EtOH showed opposite trends. This was in contrast to CO, the yield of which showed a complex response to the C 1 and C 2 pathways. The electrode onset potentials of these four key reaction products over 100 potential cycles are displayed in the spider/radar plot of Fig. 5c. This diagram represents-to our knowledge-the first such time-resolved evaluation of the evolution of product onset potentials. This experiment offers novel and noteworthy mechanistic implications and hypotheses. The vanishing oxidic character of CuO NS (Fig. 4c) coupled to sheet fragmentation and re-aggregation during the first hour (~10 th cycle) shifted the onset potentials of both C 2 H 4 (green) and CO (red) from −0.57 ± 0.07 V RHE and −0.38 ± 0.08 V RHE to −0.63 ± 0.09 V RHE and −0.48 ± 0.05V RHE , and after 9 h (100 th cycle) further to −0.77 ± 0.02 V RHE and −0.48 ± 0.03 V RHE , respectively. Experimental onset potentials of Fig. 5c can be associated with the theoretical concept of limiting electrode potentials 51,52 of the CO and C 2 H 4 pathways, that is, the required minimal overpotentials before the last elementary kinetic step turns energetically downhill and the reaction proceeds 51 . The onset potentials of C 2 H 4 and CO shifted concertedly, yet stayed apart by 150−250 mV. We attribute the shift to a decline in the number of active Cu surface sites. The varying offset represents the electrical energy (limiting overpotential) needed to kick-start the dimerization of *CO toward C 2 H 4 53 and corroborates their well-known mechanistic link.
More important are the mechanistic implications of the similar constant onset potentials of CH 4 and EtOH (−0.8 V RHE to −0.9 V RHE over 9 h). There appears to exist a common elementary step that controls the onset potentials of both CH 4 and EtOH. Against conventional mechanistic wisdom 53,54 , this suggests a much closer mechanistic link between CH 4 and EtOH than previously thought 52,53 . We have previously observed an opposite correlation in CH 4 and EtOH faradaic production 55 , by carrying out statistical analysis on the product formations on a series of Cu facets 56 . A careful observation was that, while the stepped Cu(210), i.e. (2(100)x(110)) surface is highly selective for CH 4 , Cu (n10), i.e., (n(100)x(110), n > 2) facets are highly EtOH selective 57 . Combining this analysis with the experimental observation here of onset potentials of CH 4 and EtOH tracking each other-we put forward the new hypothesis that products with a common -CH 3 group (CH 3 -H, CH 3 -COOH, CH 3 -CHO & NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-20961-7 ARTICLE CH 3 -CH 2 OH) could share a methyl intermediate. We speculate that 3 or more atom-wide (100) terraces at a (110) step enable the stabilization of an intermediate, that couple with *CH 3 to EtOH. Figure 5e-g illustrate this new mechanistic concept. Reactive methyl *CH 3 intermediates adsorbed at the (110) step while *CO preferably adsorbed on the (100) terraces. *CO dimerization on terraces forms C 2 H 4 at lower overpotentials, while *CH 3 require larger overpotential to become catalytically reactive at −0.8 V RHE . Given the terrace is wide enough, *CH 3 combines with a carbonaceous adsorbate on Cu(100) to form EtOH. If the terrace becomes too small as on the (210) facet, it can no longer accommodate carbonaceous adsorbates. Now, *CH 3 forms preferentially methane. This mechanism is directly confirmed by Cu single crystal FE data in Fig. 5d, however the concept of *CH 3 as a common precursor for CH 4 and EtOH has completely overlooked before, and hence is not incorporate in any of today's mechanistic schemes. We note that this direct large-overpotential EtOH pathway is separate from the in-directly pathway, where acetaldehyde has been shown as a precursor for ethanol 58-60 , and we have discussed and shown that acetaldehyde can be reduced to EtOH 55 .
CO2RR electrolysis using gas diffusion electrodes. To assess the potential of CO2RR on CuO NS under industrial pH neutral conditions (1 M KHCO 3 ), we deposited the catalyst onto gas diffusion electrodes (GDE) separating CO 2 and liquid catholyte flows in single cell electrolyzer set ups (Fig. 6a). Constant current densities between 50 and 700 mA cm −2 were applied for 2 h. Figure 6b shows the product FEs versus currents at neutral pH. The initial large H 2 FE at 50 mA cm -2 is owed to relatively low electrode overpotentials at the metallic Cu 0 sheet fragments. With increasing currents, the Cu catalyst developed its excellent CO 2 RR FE of 65% at 700 mA cm −2 , with mere 13% H 2 . As the C 2 H 4 FE reached 33% at neutral pH, the partial C 2 H 4 current maxed at 229 mA cm −2 (Fig. 6c). Previously unachieved is the C 2+ product partial current density of 410 mA cm −2 (Supplementary Fig. S26). The fact that CH 4 formation is essentially  Supplementary Table S2). Product molecules considered are: m/z = 28 CO, m/z = 15 corresponding to methane, m/z = 26 corresponding to ethylene, m/z = 31 corresponding to ethanol. d Faradaic efficiencies of CH 4 , EtOH and C 2 H 4 on Cu(100) terraces of (from left to right) shrinking width and increasing density of Cu(110) steps. While Cu (100) shows little EtOH, EtOH increases with larger (110) step density and narrower (100) terrace width, peaking at Cu(310). The narrow (100) terraces on Cu(210) prevent EtOH formation in favor of CH 4 (data from Ref. 56 ). e-g Illustration of *CH 3 as common intermediate of CH 4 and EtOH: Side views of the Cu(210), Cu(310) and Cu(510) single crystal facets. Cu(210) features exclusive *CH 3 and *H adsorption toward CH 4 , while Cu(310) allows for C-C coupling between *CH 3 and carbonaceous adsorbates on the (100) terraces toward EtOH. The wider (100) terraces of Cu(510) also enables C-C coupling among carbonaceous adsorbates to C 2 H 4 . Grey: C, white: H, red:O.
suppressed at all currents (e.g., 0.58% at 700 mA cm −2 ) making C 2 H 4 the sole gaseous hydrocarbon product, merits further attention. Stability tests at 300 mA cm −2 for 24 h (Fig. 6d) revealed only negligible FE C2H4 drops over 20 h. Consistent with in situ TEM data, agglomeration resulted in long-term dendritic morphologies (Supplementary Figs. S27 and S28). To put the excellent performance of CuO NS precursors and its morphological derivatives in perspective, Fig. 6e compares the C 2 H 4 partial currents under comparable conditions. Our CuO NS meet and exceed today's top performing Cu catalysts under neutral condition in electrolyzers.

Discussion
We have studied the synthesis and CO 2 electrocatalysis of a (001)-oriented CuO nanosheet catalyst. Evaluated in electrolyzers under industrially relevant pH neutral conditions, the CuO NS catalysts featured unprecedented performance metrics for C 2 H 4 and C 2+ . In situ and ex situ TEM studies revealed that over 10-20 min under potential control the CuO NS fractured andpresumably limited by diffusion of Cu species-aggregated into branched, dendritic Cu structures that represented the final stable catalyst morphology under flow conditions. Operando XAS studies confirmed the chemical reduction of CuO and concomitant formation of disordered and coordinatively under-saturated Cu 0 over about 2 h under reductive CO2RR conditions. These undercoordinated sites are held responsible for the high catalytic CO 2 -to-C 2+ product reaction rates. Time-resolved DEMS provided access to the evolution of product onset potentials. The concerted shift in onset potentials of CO and C 2 H 4 evidenced their close mechanistic link. Those of CH 4 and EtOH, by contrast, remained unaffected and were nearly identical. Counter to our current mechanistic understanding, this points to the existence of a common mechanistic intermediate, likely *CH 3 , feeding into competing pathways toward EtOH and CH 4 , confirming previously overlooked data trends on metallic stepped Cu(n10) single crystal facets. Based on this, Tetrahexahedrally shaped 61,62 particles exposing stepped (310) facets are predicted be optimal nanocatalysts to support the EtOH-selective pathway.

Methods
Synthesis. Nanosized CuO catalyst with sheet-like morphology was synthesized by a surfactant-free wet-chemistry route. No purification was performed for chemicals before use. CuO nanosheets were obtained by thermal decomposition of presynthesized Cu(OH) 2 intermediate. To prepare Cu(OH) 2 intermediate, 100 mg Cu (SO 4 ) 2 was dissolved in 4 mL DI-water, 2.5 mL 1 M KOH solution was then added dropwise into the solution. After 15-min stirring, 1 mL ammonium hydroxide was added. The resulting homogeneous light blue solution was then transferred to a glass pressure vessel. The sealed vessel was then heated from room temperature to 80°C in 30 min and stayed at 80°C for 12 h before it was cooled to room temperature. The products were precipitated by ethanol, separated via centrifugation at 12,649.63 g and further purified twice by ethanol and water. Afterwards they were freeze dried and stored as powders under inert atmosphere until use.
Electron microscopy (TEM, HRTEM and SAED). TEM and SAED were performed using a FEI Tecnai G 2 Microscope 20 S-Twin with a LaB6-cathode at 200 kV accelerating voltage (ZELMI Centrum, Technical University Berlin). The samples were dispersed in ethanol, ultrasonicated and drop-dried onto Cu-grids with a holey carbon film. Analysis was done using software from ImageJ. The   HR-TEM was conducted with FEI TITAN 80-300n with high brightness FEG (ZELMI Centrum, Technical University Berlin).
Electrochemical Measurements in H-cell. The electrochemical tests were firstly carried out in a custom-made two-compartment cell (H-cell), separated by an anion exchange membrane (Selemion AMV, AGC Engineering Co., LTD). All the glassware accessories used in this study were first cleaned by soaking in a "nochromix" bath and afterward in concentrated HNO 3 for at least 12 h, rinsed and sonicated with ultrapure water several times. We have worked with a constant concentration of 0.1 M CO 2 -saturated KHCO 3 solution (Honeywell). The working/ counter compartment was filled with 40 mL of electrolyte respectively. Before and during the electrochemical reaction the working compartment was purged continuously with CO 2 (30 sccm in total) from the bottom of the cell and the gas atmosphere was controlled with an in situ mass flow controller. Polished glassy carbon or carbon paper are used as working electrodes and measured with a Biologic SP 300 potentiostat. A platinum mesh 100 (Sigma-Aldrich 99.9%) was used as counter electrode (CE) and a leak-free Ag/AgCl electrode as reference electrode (Multi Channel Systems MCS GmbH). Every measurement was started with linear sweep voltammetry (LSV), performed with a scan rate of −5 mV/s starting at E = + 0.05 V RHE and ending at the working potential (between −0.78 V RHE and −1.0 V RHE ) followed by a chronoamperometric step for a certain time.
All reported potentials are corrected for Ohmic drop determined by electrochemical impedance spectroscopy (EIS). EC-Lab software was used to automatically correct 50% of the Ohmic drop, the remaining 50% was corrected manually. For each measurement, fresh electrolyte was used to ensure that adsorbates from previous experiments did not influence the result. Operando X-ray absorption spectroscopy (XAS). Operando X-ray absorption spectroscopy was measured at the Cu K-edges at the BESSY-II synchrotron facility at KMC-3 beamline (Helmholtz-Zentrum Berlin, Germany). CuO NS catalysts were prepared on 2×2.5 cm glassy carbon sheets (250 μm thickness, Sigradur K) and mounted in an in-house made electrochemical Teflon cell. Operando spectra of CuO NS samples were collected in fluorescence geometry from the backside of the glassy carbon electrode. We used a 13 element Si-drift energy resolving detector (RaySpec) which we equipped with an Al-shielding having a 25×25 mm Ni foil (0.00125 mm, 99.999%, Goodfellow) in front to suppress scattered light. The electrochemical cell was controlled by a SP-300 potentiostat (Biologic). We used a Ag/AgCl reference electrode and a Pt coil as counter electrode. The CuO area exposed to the electrode was 1.96 cm². The 0.1 M KHCO 3 electrolyte was purged throughout the experiment with ≈20 ml/min. All potentials were compensated for 85% Ohmic drop (R ≈ 40 Ω).
Operando differential electrochemical mass spectrometry (DEMS). Operando Differential electrochemical mass spectrometry (DEMS) was done using a custommade TU Berlin electrochemical capillary DEMS flow cell. DEMS capillary flow cell is characterized by having a well-defined electrolyte profile flow over the working electrode. The reaction products are transported into the interface liquid vacuum throughout a 0.15 mm glass capillary. The collection of high concentrated aliquot near catalyst surface and enhanced liquid-vacuum interface allows a fast and at highintensity detection of gaseous products. The high performance with DEMS capillary flow cell results in the distributed flow over the hydrophobic membrane compartment also known as the cyclonic flow. The interface liquid/vacuum promoted by PTFE hydrophobic membrane with a pore size of 20 and thickness of 50 μm (Cobetter®, Cat. No. PF-002HS) are commercially available at Hangzhou Cobetter Filtration Equipment Co., Ltd. The reaction products after been vaporized into the vacuum chamber from the flow cell were detected using a PrismaTM quadrupole mass spectrometer (QMS 200, Pfeiffer-Vacuum). The vacuum chamber was composed of two turbomolecular pumps (HiPace 80) that perform an ultimate pressure 10 −6 mbar at MS detectors. Each turbomolecular pump has an independent baking system helped a by membrane and oil pump (coupled with a molecular sieves oil trap). CO 2 reduction in flow cell electrolyzer. The flow cell electrolyzer is used for measuring the samples at high current densities, which were performed in a commercial cell supplied by ElectroCell. In all flow-cell experiments a commercial Ir-MMO plate (ElectroCell) was used as anode. The catalyst-inks were spray-coated on the microporous layer (MPL) of a Freudenberg C2 gas diffusion layer (GDL) on an area of 3 cm 2 to achieve a catalyst loading of 1 mg cm −2 . Nafion (Sigma-Aldrich, 5 wt% resin solution) was used as binder and for ionic conductivity of the catalyst-layer. 1 M KHCO 3 (500 mL, Sigma-Aldrich, BioUltra, ≥99.5%) was used as anolyte and catholyte, which were separated by an anion exchange membrane (Selemion AMV, AGC Engineering Co., LTD.). Both electrolytes were cycled through each respective compartment at 100 mL min −1 by using a peristaltic pump (PMP Ecoline, Cole-Parmer). The CO 2 (4.5 N) was supplied at a rate of 50 mL min −1 to the cathode and was flown from the back of the GDL through the catalyst layer. Measurements were performed galvanostatically for 2 h at each respective current during the catalytic tests, changing the current from low to high values. Each galvanostatic step was followed by a Potentiostatic Electrochemical Impedance Spectroscopy (PEIS) measurement to account for the ohmic drop in the calculation of RHE potentials.

Data availability
All data that support the findings of this study are present in the paper and the supplementary materials, and additional data are available from the corresponding author upon reasonable request. The Supplementary Information contains descriptions of methods, discussions on physicochemical characterization of CuO NS, in situ TEM, operando XAS, DEMS and flow cell test. The XAS data for Cu, and Cu 2 O reference materials were taken from ref. 20 .