Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy

Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy See Wee Chee, Aram Yoon, Lichen Bai, Federico Franco, Chao Zhan, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3310807/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 24 Jan, 2025 Read the published version in Nature Materials → Version 1 posted You are reading this latest preprint version Abstract Determining the active phase of an electrocatalyst at work is key to understanding its properties. However, the operating morphology of electrocatalysts is challenging to investigate because they can restructure into different motifs under applied potential due to changes in their oxidation state. These transformations will further alter their catalytic properties. Here, we employ a multi-modal approach centered on electrochemical liquid cell transmission electron microscopy (EC-TEM) to study the evolution of cubic Cu 2 O pre-catalysts during the electrocatalytic nitrate reduction reaction and unveil how redox kinetics determine the working catalyst morphology. We found drastic differences in catalyst restructuring during operation and a strong dependency of its composition on the applied potential and the chemical environment. Moreover, by matching the timescales of morphological changes observed in EC-TEM with time-resolved chemical state information obtained from operando transmission soft X-ray microscopy, hard X-ray absorption spectroscopy and Raman spectroscopy, we reveal that Cu 2 O can be kinetically stabilized for extended durations under moderately reductive conditions due to the formation of surface hydroxides. Finally, we rationalize how the interaction between the electrolyte and the catalyst influences the ammonia selectivity. Physical sciences/Chemistry/Catalysis/Electrocatalysis Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis Physical sciences/Materials science/Techniques and instrumentation/Imaging techniques Physical sciences/Materials science/Techniques and instrumentation/Microscopy/Transmission electron microscopy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Electrocatalytic chemical conversion reactions such as carbon dioxide reduction reaction (CO 2 RR) 1 , 2 and nitrate reduction reaction (NO 3 RR) 3 , 4 are key to the advancement of various green energy solutions. However, it can be difficult to identify the active catalyst species in these reactions, even when the metallic state is supposed to be the stable phase, because the catalyst can change its oxidation state during reaction according to the external stimuli. Although the Pourbaix diagram 5 can be used to rationalize the stable oxidation state/phase at different applied potentials and pHs, they are equilibrium diagrams which do not consider the kinetics of redox transitions and their effect on the catalyst morphology. For example, they do not include information about how oxide-to-metal transformations occur, how different facets can reconstruct differently under the same reaction conditions, how interactions between the catalysts and the electrolyte can alter catalyst surface or how reaction intermediates and products may lead to further changes. The challenge here is two-fold. First, one must elucidate the working morphology of the electrocatalyst. Second, one needs to disentangle the role the observed morphological changes on the catalytic performance. There are, nonetheless, only a few methods 6 , 7 that can visualize the nanoscale restructuring dynamics of a catalyst and follow the catalyst as a function of the applied potential and electrolyte conditions. It is even more challenging to resolve the local chemical state of these features because most operando techniques for extracting chemical information, such as Raman spectroscopy and X-ray absorption spectroscopy (XAS), are “broad beam” methods, where the data is an ensemble signal derived from a large probed region. This gap between nanoscale imaging and ensemble-averaging spectroscopy limits our ability to rationalize how catalyst morphology impacts the overall performance of these complex but important reactions. NO 3 RR is foremost among various electrochemical conversion reactions in terms of its need for more insight into the electrocatalyst's phase during reaction. Notably, it is a promising strategy for mitigating freshwater pollution from agricultural fertilizer run-off and industrial waste 8 , and is also studied for its potential to produce NH 3 9–11 , which is an important chemical in industry and a candidate carrier for green hydrogen 12 , 13 . While Cu is one of the most studied electrocatalyst materials for NO 3 RR due to its optimal nitrate adsorption energy 11 , 14 , whether metallic Cu 15 – 18 , Cu oxides or a Cu-Cu oxide interface 19 are the key species for the selective NH 3 formation has remained largely unresolved. According to the Pourbaix diagram 20 , 21 , the metallic phase of Cu should be the stable phase of Cu under typical NO 3 RR conditions, but studies using in situ Raman spectroscopy had suggested that an oxide phase might exist during the reaction 14 , 19 . Cu and its oxides are also known to be susceptible to etching 22 and facet modification 23 , 24 by NH 3 . Furthermore, it has been reported that NO 3 RR can drive the dissolution and re-growth of single atom Cu catalysts 3 , and the clustering of small aggregates into larger nanoparticles (NPs) 25 . Here, we use electrochemical liquid cell transmission electron microscopy (EC-TEM) accompanied by correlated multi-modal operando investigations that include electrochemical liquid cell transmission X-ray microscopy (EC-TXM), operando XAS and operando Raman spectroscopy of the same pre-catalysts to visualize in real time how the structure and composition of Cu 2 O cubes evolve as a function of the applied potential during NO 3 RR. We found that the working electrocatalyst morphology was determined by three processes, (i) the dissolution of Cu 2 O, (ii) the re-deposition of Cu from soluble Cu complexes 26 , 27 and (iii) the reduction of Cu 2 O to metallic Cu. We also discovered a coexistence of Cu 2 O with metallic Cu for extended reaction durations, thereby providing insight into the active Cu species during NO 3 RR. Results & Discussion For the operando microscopy experiments, we prepared well-defined Cu 2 O cubes on the carbon working electrode of the EC-TEM chips via electro-deposition 28 , 29 as shown schematically in Fig. 1 a. The as-prepared Cu 2 O cubes have an average size of 250 nm and consist of six {100} facets without the exposure of other minor facets such as {110} or {111} (Suppl. Figure 1). All voltages indicated in this paper are referenced against a Ag/AgCl electrode and then converted to the reversible hydrogen electrode scale (RHE) using the Nerst equation and the bulk pH of the electrolyte. Intriguingly, the image sequences show that the cubes do not undergo significant change in the commonly employed 0.1 M Na 2 SO 4 + 8 mM NaNO 3 electrolyte for NO 3 RR (Fig. 1 b), during the initial sweep towards cathodic potentials. Cu 2 O should reduce directly to metallic Cu at the higher overpotentials of the sweep, where according to the Pourbaix diagram, the redox potential for the transformation of Cu 2 O to metallic Cu is 0 V RHE in a solution with pH 7 20,21 , and metallic Cu is the stable phase below − 0.2 V RHE onwards. The stability of the Cu 2 O cubes is remarkable because these redox transformations usually lead to morphological changes. For comparison, Fig. 1 c depicts a Cu 2 O cube under CO 2 RR conditions in CO 2 -saturated 0.1 M KHCO 3 at a potential similar to that applied in the NO 3 RR experiment. As we reported previously 29 , the latter cubes undergo fragmentation together with the re-deposition of small particles, a behavior that differs from the morphologically much more stable NO 3 RR samples at the same applied potentials. The linear sweep voltammograms acquired during these two experiments are available as Suppl. Figure 2. Next, we studied these Cu 2 O cubes systematically at different sustained potentials from − 0.2 V RHE to -0.6 V RHE (Fig. 2 a-e) to probe further their morphological stability during NO 3 RR. For these extended experiments, we adopted an intermittent imaging protocol (images captured at 15-minute intervals with the electron beam blanked the rest of the time) to minimize beam-induced dissolution of the Cu 2 O cubes (see discussion in Suppl. Note 1) and ensure that the catalyst re-structuring kinetics we extract from the collected data are as accurate as possible. The electrochemical current profiles over time at each potential measured in these EC-TEM experiments are provided as Suppl. Figure 3. At -0.2 V RHE (Fig. 2 a), the cubes were stable during our entire observation, with no significant restructuring observed. From − 0.2 to -0.5V RHE , dissolution/re-deposition is the main restructuring pathway. At -0.3 V RHE , the cubic form persisted for almost 135 min (Fig. 2 b), while the cube completely dissolved after 140 min at -0.4 V RHE (Fig. 2 c) and after 90 min at -0.5 V RHE (Fig. 2 d). The lighter contrast of the cube exterior in Fig. 2 c compared to the middle of the cube at 60 and 80 min is explained by the cube corners and edges being etched first. The weaker contrast of the dissolving Cu 2 O cubes compared to that of the growing Cu NPs also suggests that the dissolving cubes were still in oxide form. Two Cu 2 O cubes were captured in the images acquired at -0.6 V RHE (Fig. 2 e). One cube shrank and restructured into a smaller cube with a void in the center and then became rougher due to small NPs attaching to its surface, while another completely dissolved within the same time frame. We also highlight that the intensity of the cubes in the TEM images obtained from − 0.3 to -0.5 V RHE gradually decrease, whereas the intensity of the cube at -0.6 V RHE is brighter, implying that cube-like frame at -0.6 V RHE is metallic. Moreover, the interplay of dissolution/re-deposition and direct reduction at the more cathodic potentials means that the terminal catalysts morphologies of oxide pre-catalysts vary depending on the applied potential. Next, we repeated the NO 3 RR experiments in a H-type cell with Cu 2 O cubes electrodeposited on carbon paper to check for the consistency of the EC-TEM results with standard reaction geometries. Figures 2 f and 2 g show lower magnification images of samples from the EC-TEM experiments with scanning electron microscopy (SEM) images of samples extracted from H-type cell experiments after 2 hours of reaction at three different applied potentials, -0.2, -0.4 and − 0.6 V RHE . Electron diffraction patterns taken from samples extracted after reaction show that the cubes did not undergo significant restructuring at -0.2 V RHE and remain Cu 2 O whereas samples reacted at -0.6 V RHE were largely metallic (Suppl. Figure 4). Conversely, samples reacted at -0.4 V RHE show a mixture of residual Cu 2 O and metallic Cu structures (Suppl. Figure 4–6). The morphological differences between the sample after reaction in the H-type cell at -0.4 V RHE and − 0.6 V RHE further support that the catalysts indeed restructure through different pathways as described by our EC-TEM experiments. Inductively coupled plasma mass spectrometry measurements of the electrode and the electrolyte in the H-type cell after reaction further show that Cu dissolution happens at all the applied potentials (Suppl. Figure 7). Therefore, these experiments indicate that the Cu 2 O cubes undergo a gradual dissolution under NO 3 RR conditions, which in turn leads to the re-deposition of metallic particles elsewhere on the working electrode with shapes and sizes that are modulated by the applied potential. To obtain unambiguously the oxidation state of the catalyst species present during reaction and rule out the possibility that the ex situ identified Cu 2 O phase are the result of re-oxidation during the return to open circuit potential 30 , 31 (OCP), we performed operando EC-TXM measurements on the Cu 2 O cubes by transferring our EC-TEM holder into a TXM at the BESSY II synchrotron facility as illustrated in Suppl. Figure 8. This unique arrangement maintains the same reaction environment between the two experiments, while enabling time-resolved operando measurements of Cu absorption edges under applied potential without compromising the sustained electrolyte flow as X-rays are attenuated less by the electrolyte and enclosing membranes. Thus, the evolution of the electrode’s composition can be tracked during NO 3 RR. Figures 3 a-d show the time-resolved evolution of the Cu 2 O catalysts during NO 3 RR at -0.4 V RHE as observed by EC-TXM in the form of the colored maps that were reconstructed from a XAS image stack using linear combination fitting (LCF) 32 . Cu 2 O is depicted in red and metallic Cu in yellow. The maps show that Cu 2 O and Cu are the dominant phases present over the duration of the NO 3 RR and that the oxide and metallic phases coexist under specific reaction conditions, but they are spatially separated. CuO is also detected (blue color) but it is present only in small quantities and is not clearly visible from the maps. The corresponding decomposed spectra are shown in Fig. 3 e-p, where the yellow, red and blue colored lines represent the respective Cu species. The total contribution of the individual spectra in Fig. 3 e-p represents the amount (thickness) of each species, which indicates that the content of metallic Cu species increases (Fig. 3 e-h) during reaction, whereas Cu 2 O decreases (Fig. 3 i-p). The details of the data acquisition and processing are discussed in Suppl. Note 3. Most importantly, these results confirm the sluggish reduction kinetics of the large Cu 2 O cubes in the Na 2 SO 4 + NaNO 3 electrolyte, and that the metallic phase forms when the dissolved Cu species re-deposit on the working electrode due to the reductive potential employed. We further verified that the slow reduction of the Cu oxide cubes extends to larger reaction volumes with operando hard XAS measurements of samples electro-deposited on carbon paper in our home-built electrochemical XAS cell 33 . In Fig. 3 q, we plot the changes in the Cu K-edge valence states (from 8950 to 9105 eV) that were obtained from operando XAS. The weight of the Cu valence state is extracted by LCF of the X-ray absorption near-edge structure (XANES) of the oxide-derived Cu catalyst collected at a constant potential of -0.4 V RHE in 0.1 M Na 2 SO 4 + 8 mM NaNO 3 electrolyte. As seen in Fig. 3 r, the fraction of Cu 2 O decreased but not completely after more than 2 hours of electrolysis, while the fraction of metallic Cu increased correspondingly, eventually to almost a 1:1 ratio of Cu 2 O:Cu. The XANES results agree with the persistence of Cu 2 O and the continual evolution of the Cu species seen in the EC-TEM (Fig. 2 c) and EC-TXM (Cu-L 3 edges in Fig. 3 r) results at -0.4 V RHE . Minute amounts of the CuO species were also detected during the experiment. The change in the weights of the three species over time exhibit similar trends in both TXM and XAS, confirming that the results we obtain in the EC-TEM cells indeed extrapolate to a larger ensemble of catalyst particles. This overall agreement between different methods and experimental geometries means that we can use the in situ TEM image sequences to quantify the potential-dependent dissolution and re-deposition rates. Our method for fraction extraction from the EC-TEM images and additional analysis of the re-deposited particles is described in the Suppl. Note 4. As shown in Fig. 4 a, the cube fraction decreases over time at an increasing rate as the potential decreases from − 0.2 V to -0.5 V RHE . The sample at -0.6 V RHE deviates from this trend (dark purple line) because of the direct reduction of Cu 2 O to metallic Cu. In Fig. 4 b, we use the cube dissolution rate to estimate the Cu 2 O to Cu ratio at a certain potential and use it to visualize the majority phase (greater than 50%) at different times. We further compare the Cu 2 O/Cu ratio with NH 3 conversion activity (current density) and selectivity (Faradaic Efficiency, FE) obtained from our bench top electrochemistry measurements. In Fig. 4 c, we plot the linear sweep voltammogram of the Cu 2 O cubes prepared on carbon paper and in Fig. 4 d-e, their product distribution as a function of the applied potentials. The measured yield rate and the FE towards NH 3 was much higher at -0.6 V RHE as compared to -0.2 V RHE and − 0.4 V RHE , implying that the change in catalytic selectivity is related to the faster rate of oxide to metal conversion at -0.6 V RHE (Fig. 4 b). Next, we performed EC-TEM studies in various electrolyte compositions to elucidate the mechanism behind Cu 2 O stabilization. Suppl. Figure 9a and 9b describe experiments using pure 0.1 M Na 2 SO 4 and 0.1 M Na 2 SO 4 + 8 mM NaNO 2 (i.e. nitrite reduction) respectively. In both cases, the cubes behaved similarly to their behavior with NO 3 RR where they gradually reduced in size until they fragment/reduce at longer reaction duration, which means the Cu 2 O stability is related to the Na 2 SO 4 supporting electrolyte and not the reactant. Re-deposition was, however, much less in Na 2 SO 4 compared to its NaNO 3 /NaNO 2 containing counterparts. We attribute this difference to how the local pH during electro-reduction differs in the presence and absence of NO x species. Under applied cathodic potentials, the pH at the electrocatalyst surface increases as hydroxyl ions form 34 , 35 due to the reduction of e.g. H 2 O, O 2 , NO 2 − and NO 3 − . In particular, NO x RR results in higher currents and consequently a steeper rise in the local pH as compared to when only hydrogen reduction takes place. This process can bring the pH of a neutral electrolyte above 12 34 , triggering the formation of soluble Cu hydroxides. To look at the effect of electrolyte pH, we further performed experiments in 0.1 M Na 2 SO 4 where the pH was increased to 10 by adding NaOH. As shown in Suppl. Figure 9c, it altered the amount of re-deposition observed. Lastly, to probe the influence of ammonium ions, we deliberately added NH 4 OH into the 0.1 M Na 2 SO 4 carrier electrolyte, which led to rapid restructuring of the cubes as shown in Suppl. Figure 9d. To explain these results, we consider the phase stability of Cu as a function of pH and in the presence of NH 3 . A complex series of reactions encompassing different acid–base chemistries, Cu(OH) 2 precipitation, and complex ion formation are known for the Cu-NH 3 system 36 , 37 (see Suppl. Note 5). Specifically, the equilibrium between solid Cu(OH) 2 and the Cu(NH 3 ) 4 2+ complex depends on the NH 3 concentration, with Cu(OH) 2 precipitation being favored at low concentrations due to the poor solubility of Cu(OH) 2 36 . We hypothesize that the sluggish reduction observed may be the result of transient surface Cu(OH) 2 formation induced by the interfacial pH rise in the course of the NO 3 RR. Cu(OH) 2 formation may also be more favorable in Na 2 SO 4 , due to the electrolyte’s inability to buffer the local pH increase from electro-reduction 34 , as compared to the KHCO 3 electrolyte used in CO 2 RR, thereby leading to the differences in re-structuring behaviors. To validate this hypothesis, we studied the chemical changes taking place on the surface of the Cu 2 O cubes with operando Raman spectroscopy measurements. Figure 5 a shows the results of experiments at constant applied potentials performed with cubes electrodeposited on glassy carbon plates. At OCP, the Raman spectrum shows three features of bands centered at 415, 520, and 630 cm − 1 respectively, which are in good agreement with the reported values of Cu 2 O 38 , 39 . When − 0.2 V RHE was applied, the band intensity at 520 and 630 cm − 1 decreased over time but continued to persist, which is consistent with the gradual dissolution of Cu 2 O. At -0.4 V RHE , a new peak at 475 cm − 1 started to emerge, while the peak at 630 cm − 1 flattened, indicating oxide to metal transition. At -0.6 V RHE , the characteristic bands of Cu 2 O were less pronounced and additional weak bands emerged at 450, 475 and 590 cm − 1 . The band around 475 cm − 1 can be assigned to the Cu-O-H vibration 39 – 41 or Cu(OH) 2 40 , while the peak at 590 cm − 1 is often assigned to CuO 38 or adsorbed O-species on Cu 41 . These results suggest the formation of a transient intermediate oxide or hydroxide phase during electrolysis, which is also supported by the extended presence of Cu(I) signatures and the weak but persistent Cu(II) signatures in the EC-TXM and operando XAS measurements in Fig. 3 . It was, however, difficult to obtain the Raman signatures of absorbates with these samples due to the relatively low loading of the cubes that we were able to electrodeposit and the overlap of the D/G bands of the glassy carbon support with surface absorbate bands, which limits the signal-to-noise ratios at those bands. Thus, we repeated the Raman measurements with cubes electrodeposited at higher loading on carbon paper to improve the signal-to-noise ratios to identify surface adsorbed species or intermediate species in the vicinity of electrode. These results are discussed in Supplementary Note 6. Hence, we arrive at a restructuring mechanism illustrated in Fig. 5 b where surface hydroxides first form on the Cu 2 O cubes due to the pH increase induced by electrolysis, which delays the oxide reduction. The subsequent NH 3 production and added pH rise from continued NO 3 RR then destabilizes this hydroxide layer and forms soluble Cu complexes, thereby initiating catalyst evolution via re-deposition from Cu complex reduction or the aggregation of migrating NPs. We also performed more EC-TEM experiments to check that the delayed restructuring kinetics extend to other pre-catalyst geometries. Suppl. Figure 10 describes the evolution during NO 3 RR of Cu 2 O truncated octahedrons and metallic frames created by pre-reducing the Cu 2 O cubes. Both samples are stable during early-stage reaction. With more time, re-deposition similar to that seen in the cubes was observed in the Cu 2 O octahedrons whereas little re-deposition was noticeable with the metallic frames, which was likely due to less dissolution occurring when we start from metallic pre-catalysts. The correlated microscopy and spectroscopy experiments presented here therefore indicate that the morphology of the Cu catalysts during NO 3 RR at a given pH is governed by a complex, time- and potential-dependent interplay of three processes: (i) dissolution of oxide and hydroxide species, (ii) metal re-deposition, and (iii) oxide catalyst reduction. According to the Pourbaix diagram 20 , 21 , metallic Cu is the only stable species under the specific applied potentials and pH of our experiments, but we have shown that oxidic and metallic phases can co-exist over extended durations and over a broad range of applied potentials, which has significant implications in terms of determining the active species for producing NH 3 . It has been suggested previously 19 , based on operando spectroscopy measurements of CuO pre-catalysts, that Cu/Cu 2 O interfaces are responsible for NH 3 production 19 , but these methods cannot differentiate the distribution of these species on the nanoscale. As we have shown in this work, the presence of both spectroscopic signatures in ensemble-averaging measurements does not necessarily mean that the two phases are spatially connected. Furthermore, we have demonstrated that the de-coupled metallic Cu and Cu oxide phases can persist for the extended durations at mild cathodic potentials (less than − 0.5 V RHE ), and that a high residual abundance of Cu 2 O in the operando XAS measurements corresponded to a low NH 3 production efficiency in our electrolysis data of equivalent samples. The improvement of NH 3 selectivity with increasing overall metallic character of the samples, therefore, advocates that metallic Cu is the active phase for producing NH 3 compared to Cu 2 O, in agreement with recent work on the topic 15 – 18 . In this case, the strong stability of the Cu 2 O cubes and their sluggish reduction kinetics in the often-used Na 2 SO 4 carrier electrolyte, are detrimental for the NH 3 production by delaying the onset of selective NH 3 formation. By showing the diverse behaviors that can be derived in different electrolytes and reaction conditions, our work also illustrates the critical need to pay attention to how the electrolyte can influence the restructuring of catalysts and the stability of oxide, hydroxide, and metallic phases before we attempt to generalize results across different studies and reactions. So far, the description of electrolyte effects in electrolysis has been largely confined to cation adsorption effects 42 – 45 and re-structuring induced by aggressive halide anions 44 – 46 , whereas studies of pH had focused on its impact on reaction mechanisms and NH 3 selectivity 8 , 10 , and not catalyst phase stability. Significant additional microscopy work, such as the one presented here will be required to separate the impact of electrolyte-driven morphological transformation from the much better understood associated electronic and chemical changes. Furthermore, current computational models still cannot rationalize the impact of an explicit complex electrolyte on the catalyst restructuring and its associated influence on the creation of active sites. Efforts to improve these models and advance the theory describing electrocatalytic processes, will undoubtedly require more accurate representations of dynamic catalyst surfaces. The challenge here is serious, since theoretical mechanistic insight must consider two simultaneously occurring dynamic processes, namely the one that the catalyst material experiences, in parallel to that undergone by the reactants, both of which being coupled and driven by the local chemical potential. 47 Our results revealing phase co-existence also opens the possibility that different species may be responsible for activating specific steps of the conversion reaction. Hence, we expect operando approaches that incorporate chemically resolved microscopy within multi-modal spectroscopic investigations, as we demonstrated here, to play a vital role moving forward in the understanding of electrocatalytic processes by providing a path towards mapping such complexity. In summary, operando EC-TEM and EC-TXM measurements have revealed that the morphologies of Cu 2 O pre-catalysts during NO 3 RR and their evolutionary pathways are sensitive to the reaction time, applied potential and the nature of the electrolyte. As expected, the rate of oxide reduction accelerates with increasing negative applied potentials, but spatially-separated oxide and metallic phases can co-exist over extended reaction times under moderately reductive potentials. More importantly, the kinetics of the different re-structuring processes, which have been unveiled here, determine that final morphology of the catalysts. Our results also indicate that the nature of the electrolyte can introduce time-dependent selectivity changes in the early stages of the catalyst restructuring, which will help resolve on-going controversies regarding the active state of Cu for selective NH 3 production. Finally, this work impacts our understanding of how electrocatalysts evolve under reaction condition through the discovery of Cu oxide and hydroxide stability. In addition, we unveiled local structural and chemical heterogeneities that develop under electrochemical working conditions, even on a pre-catalyst sample initially characterized by a narrow size, shape and compositional distribution. Thus, our findings emphasize the need of operando characterization methods to establish connections between materials’ structural and compositional characteristics under specific reaction environments and external stimuli and their electrocatalytic performance. Methods Specimen preparation The Cu 2 O cubes (250 nm) were synthesized on the polished glassy carbon (vitreous, SPI) plates, carbon paper and the carbon electrode of the Hummingbird Scientific EC-TEM chips using an electro-deposition protocol we previously developed. 28 , 29 The deposition solution consists of a mixture of 5 mM copper sulfate-pentahydrate (CuSO 4 ·5H 2 O, Sigma Aldrich) and 12.5 mM of potassium chloride (KCl, Sigma Aldrich). After the synthesis, the samples were rinsed with ultrapure water and then used for the subsequent NO 3 RR experiments. Electrolyte preparation for nitrate reduction The electrolyte used for nitrate reduction experiments is an aqueous solution of 0.1 M Na 2 SO 4 (anhydrous, 99.99%, Suprapur)) + 8 mM NaNO 3 (Sigma-Aldrich, ReagentPlus®, ≥ 99.0%). Operando EC-TEM The EC-TEM experiments were performed in a Thermo Fisher Scientific 300 kV Titan TEM operated in STEM mode and a Hummingbird Scientific Bulk Liquid Electrochemistry TEM holder with Pt counter and Ag/AgCl (3 M KCl) reference electrodes. The EC-TEM top chips have a 50 nm thick silicon nitride membrane windows and were also produced by Hummingbird Scientific. The bottom chips have a 50 nm silicon window and 250 nm spacers. The electrochemistry experiments were performed using a Biologic SP-200 potentiostat. The potentials were measured against the built-in Ag/AgCl reference. The TEM holder was pre-filled with 0.1 M Na 3 SO 4 + 8mM NaNO 3 solution during cell assembly to fill the entire fluid path with electrolyte. After loading into the TEM, the syringe was filled with 0.1 M Na 3 SO 4 + 8mM NaNO 3 and introduced at a flow rate of 1.25 mL min − 1 . Linear sweep voltammetry from − 0.5 V to − 1.1 V AgAgCl (repeated twice) was first used to determine the onset potential for the NO 3 RR using a scan rate of 15 mV s − 1 and to ensure that the applied potential was consistent between experiments, followed by chronoamperometry for up to 2 hours at − 0.2, -0.3, -0.4, -0.5.1 and − 0.6 V RHE (converted from Ag/AgCl scale using the Nerst equation) with the new catalyst specimen and fresh electrolyte at each condition. In situ imaging was always performed under conditions with electrolyte in the cell, as determined from the image contrast. We stayed under an electron flux of 1.75 e − Å −2 s − 1 at all times to minimize electron beam-induced artifacts with an electron probe current of approximately 220 pA. The acquired images have a resolution of 1024 × 1024 pixels. For continuous imaging, the images were acquired at 1 frame per second. For intermittent imaging, the images were acquired every 15 minutes with the electron beam blanked-in between. The image segmentation for the EC-TEM movies was performed using built-in functions and scripting in MATLAB (See Suppl. Note 4 for details). NO 3 RR product analysis and detection The electrochemistry experiments for product analysis and ex situ imaging were conducted by using an Autolab potentiostat (PGSTAT 302N) and a custom-made H-type electrochemical cell, whereby the cathodic and anodic compartments were separated by a piece of anion exchange membrane (Selemion AMV, AGC Inc.). A platinum gauze (MaTecK, 3600 mesh cm − 2 ) and a leak-free Ag/AgCl electrode (LF-1, Alvatek, potential 0.198 V vs. standard hydrogen electrode) were used as the counter and the reference electrodes, respectively. The anodic compartment (with counter electrode) was filled with 18 mL 0.1 M Na 2 SO 4 electrolyte while the cathodic compartment (with the working electrode) was filled with 18 mL 0.1 M Na 2 SO 4 electrolyte + 8 mM NaNO 3 . Before the electrochemical tests, the anodic and cathodic solutions were de-aerated by continuously bubbling Ar (grade 6.0, 99.9999%) with a 20 mL min − 1 flow rate (Bronkhorst). During the chronoamperometric measurements, a constant Ar flow (10 mL min − 1 ) was used to maintain the inert atmosphere. Linear sweep voltammetry was performed with a scan rate of 5 mV s − 1 , and chronoamperometry (2 hours reaction time) was used for product distribution measurements. UV-Vis spectroscopy (Agilent Cary 60) was used to detect and quantify ammonia and nitrite in the electrolyte following the procedures established in previous literature 48 , 49 . The liquid samples were diluted to match the suitable detection range for spectrophotometric analysis of each analyte and the sample absorbance was measured in the range 400–800 nm. The indophenol-blue method was used for the determination of ammonia 48 , 49 . For nitrite (NO 2 – ) quantification, 3 mL of the diluted sample were added to a glass vial containing 35 mg of white powder from a commercial nitrite test kit (photometric 0.002–1.00 mg L − 1 NO 2 -N, 0.007–3.28 mg L − 1 NO 2 − , Spectroquant, Merck). Details on quantification can be found in the Suppl. Note 2. Ex situ TEM and SEM measurement The ex situ TEM imaging was also performed with the Thermo Fisher Scientific 300 kV Titan TEM. The EC-TEM chips were loaded using a Hummingbird Scientific Tomography holder for before and after reaction comparisons. The EC-TEM chips were rinsed in ultrapure water after they were disassembled from the EC-TEM holder and immediately transferred into the TEM. The ex situ SEM imaging of the bulk samples was performed using a Thermo Fisher Scientific Apreo SEM. Operando EC-TXM measurement Operando EC-TXM experiments were conducted at the U41-TXM beamline in BESSY II (Berlin, Germany). The beam-size was 26 µm × 26 µm with a nominal resolution of 20 nm. The image stacks were collected using a charge coupled detector (CCD) at 1340 pixel × 1300 pixel and the exposure time of 1 second per energy. 10 nm monochromator slit was used. The intensity of the incident radiation was monitored and adjusted to have a photon count constant (approximately 15000 count per pixel) at the background area (no specimen) when liquid is fully filled. Image stacks were acquired as the beam energies were scanned from 926 to 965 eV, which encompassed both Cu-L 3 and Cu-L 2 edges. A Hummingbird Scientific electrochemistry holder was used for the operando measurements. The applied potential was controlled with a Biologic potentiostat. The reference electrode was a Pt-pseudo reference on the chip and the counter was Pt. The reference potential was calibrated against an external Ag/AgCl electrode to ensure a potential comparable to the EC-TEM experiments were applied. Details regarding the data processing, including accurate alignment, atomistic background subtraction, data normalization, spectra averaging and linear combination fitting (LCF) of the spectra images can be found in Suppl. Note 2. Operando XAS Operando time-resolved X-ray absorption fine-structure spectroscopy (XAFS) experiments were performed at P64 beamline of the PETRA III synchrotron (Hamburg,Germany) in quick XAFS (QXAFS) mode. Measurements were performed at the Cu K-edge (8979 eV). The intensity of the incident radiation was monitored by a gas ionization chamber filled with pure N 2 . Additional ionization chambers were used to acquire spectra of a Cu foil in transmission mode for calibration purposes at the beginning of each QXAFS scan. The beam-size was less than 2 × 2 mm. The XAS data were collected in fluorescence mode using a PIPS detector at one spectrum per second and one spectrum per 5 seconds. For these operando XAS experiments, we used a home-made single-compartment electrochemical cell 33 . The applied potential was controlled with a Biologic potentiostat. Argon was flowed into gas compartment at 10 ml/min. The sample was Cu 2 O cubes prepared on the carbon paper electrode and the 0.1 M Na 2 SO 4 + 8 mM NaNO 3 electrolyte was continuously circulated through the cell using a double-channel peristaltic pump. Data extraction and calibration were performed using the JAQ software of the P64 beamline. Further data processing and analysis of the XANES spectra were performed according to the procedures described previously in reference 50 . Operando Raman measurement The operando Raman spectra were obtained using a Renishaw (InVia Reflex) confocal Raman microscope and a water immersion objective with a long working distance (Leica microsystems, 63x, numerical aperture of 0.9) was chosen. The glassy carbon experiments used a 785 nm laser with 0.1% laser power (0.36 mW). The carbon paper experiments used a 633 nm laser with laser power of about 5%. The objective was protected from the electrolyte by a Teflon film (DuPont, film thickness of 0.013 mm). Then, a drop of water was used to drive away the air between the film and the objective to match the refractive index to ensure efficient excitation and collection of the Raman signal. The electrochemical measurements were performed in a home-built spectro-electrochemical cell made of Teflon and controlled by a Biologic SP-240 potentiostat. The cell was equipped with a reference electrode (leak-free Ag/AgCl, Alvatek), a counter electrode (Pt ring), and a working electrode with the catalyst electrodeposited on glassy carbon. A 15 ml Ar-purged 0.1 M Na 2 SO 4 + 8mM NaNO 3 solution was used as an electrolyte. During the experiment, the Raman spectra were acquired every 5 minutes over 1 hour of reaction. Declarations Contributions AY, SWC, and BRC conceived the project, planned the experiments, wrote the manuscript. AY prepared specimens, conducted the operando EC-TEM studies and operando EC-TXM studies, ex situ TEM and ex situ SEM analysis, and analyzed the data. LB measured NO 3 RR selectivity and analyzed the electrochemical data. FY assisted with the sample preparation and performed some of the operando EC-TEM experiments. FF provided the initial inspiration for the project and developed the experimental protocol for H-type cell measurements and product analysis. CZ and LB conducted operando Raman measurements and analyzed the results. MR and JT planned and collected the operando XAS data and performed the analysis and interpretation. CP and SW planned the operando EC-TXM measurements with AY and SWC , operated the TXM and helped with analysis of the TXM data. MM and HS were involved in the XAS measurements and helped with analysis and interpretation of the results. Acknowledgements AY and CZ thank the Alexander von Humboldt Foundation (AvH) for supporting them with an AvH postdoctoral research grant. L. Bai acknowledges the support from the Early Postdoc Mobility Fellowship (P2ELP2_199800) of Swiss National Science Foundation. FY acknowledges the Chinese Scholarship Council for sponsoring her PhD. This work was partially funded by the German Federal Ministry for Education and Research (BMBF) under the grant Catlab (03EW0015B), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under project no. 406944504 – SPP 2080 and Germany´s Excellence Strategy – EXC 2008 – 390540038 – UniSysCat. AY also thanks the SPP 2080 “DynaKat” Early Career Research Scholarships for Female Scientists for partial funding. We are very grateful to Dr. Antonia Herzog, Dr. Uta Hejral, Dr. Arno Bergmann, and Shih-Yu Fu for their time and help with the XAS and TXM beamtime measurements. We further acknowledge Dr Andrea Martini and Dr. Christoph Scheurer for their helpful discussions with regards to analysis of the TXM data, Dr Eduardo Ortega for his help with some of the TEM data analysis and Mr. Walter Wachsmann for the ICP-MS measurements. 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J Am Chem Soc 143:7578–7587 Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryMovie1.avi Supplementary Movie 1 SupplementaryMovie2.avi Supplementary Movie 2 SupplementaryMovie3.avi Supplementary Movie 3 SupplementaryMovie4.avi Supplementary Movie 4 NO3RRSIResubmissionClear.pdf Cite Share Download PDF Status: Published Journal Publication published 24 Jan, 2025 Read the published version in Nature Materials → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3310807","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":359210540,"identity":"90a80cbf-3d77-416d-a465-4fea3fa7aba2","order_by":0,"name":"See Wee 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15:41:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3310807/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3310807/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41563-024-02084-8","type":"published","date":"2025-01-24T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":65629925,"identity":"e6217eb2-395a-4c5d-8fd0-89cc66801d4b","added_by":"auto","created_at":"2024-09-30 16:25:35","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":180973,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eDifferences in the restructuring of Cu\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eO catalysts between NO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eRR and CO\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eRR during initial linear sweep voltammetry scan:\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e (a) Schematic of EC-TEM experimental configuration where the Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO pre-catalyst was electrodeposited on the working electrode of an EC-TEM chip prior to the experiment. Snapshots showing the restructuring of Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO cubes as observed by operando EC-TEM during linear sweep voltammetry under NO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR conditions in (b) 0.1 M Na\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e + 8 mM NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e (Suppl. Movie 1) and (c) an image sequence describing the behavior of identically synthesized cubes under CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR conditions in CO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e-saturated 0.1 M KHCO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e at a similar applied potential range (Suppl. Movie 2). The electron flux used in these experiments is 1.75 e\u003c/em\u003e\u003csup\u003e\u003cem\u003e- \u003c/em\u003e\u003c/sup\u003e\u003cem\u003eA\u003c/em\u003e\u003csup\u003e\u003cem\u003e-2 \u003c/em\u003e\u003c/sup\u003e\u003cem\u003es\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e. All experiments are referenced to a Ag/AgCl reference electrode and then converted to RHE using the Nernst equation.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/777f4f425ffa0a17c88ca67a.png"},{"id":65629927,"identity":"3cd88d60-243a-4ed2-ad89-a15dc401b883","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":152813,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003ePotential- and time-dependent restructuring of Cu\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eO cubes observed with EC-TEM:\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e Operando EC-TEM time series of Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO cubes restructuring acquired with intermittent imaging at (a) -0.2, (b) -0.3, (c) -0.4, (d) -0.5 and (e) -0.6 V\u003c/em\u003e\u003csub\u003eRHE\u003c/sub\u003e\u003cem\u003e during the reaction at the indicated times in 0.1 M Na\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e + 8 mM NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. A new sample was used at each applied potential. The electron flux used in these experiments is 1.75 e\u003c/em\u003e\u003csup\u003e\u003cem\u003e- \u003c/em\u003e\u003c/sup\u003e\u003cem\u003eA\u003c/em\u003e\u003csup\u003e\u003cem\u003e-2 \u003c/em\u003e\u003c/sup\u003e\u003cem\u003es\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e. (f) and (g) compare after reaction images of in situ and ex situ experiments. Images in (f) are obtained from the EC-TEM experiments after 2 hours at each designated potential.\u0026nbsp; (g) SEM images of Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO cubes electrodeposited on carbon paper and reacted on the bench top for two hours at the same applied potentials.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/da49eb893342acfe32656614.png"},{"id":65629928,"identity":"5428044d-80f4-4e8c-b6c0-3062f1a71005","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":199659,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCorrelation of the evolution of the morphology and chemical state of\u003c/strong\u003e \u003cstrong\u003eNO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eRR Cu\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003eO catalysts\u003c/strong\u003e \u003cstrong\u003emeasured by operando TXM and XANES\u003c/strong\u003e. (a-p) Spatially and temporally evolving Cu catalysts and their valence states observed by TXM. Colored map of Cu\u003csub\u003e2\u003c/sub\u003eO cubes and redeposited Cu catalysts (a) before reaction in the dry state, (b) after 25 min, (c) 50 min and (d) 75 min of TXM acquisition at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e in 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e + 8 mM NaNO\u003csub\u003e3\u003c/sub\u003e. (e-p) XAS intensity integrated over the colored area of images in (a-d) from the TXM image stacks where the colored spectra correspond to (e-h) Cu (yellow), (i-l) Cu\u003csub\u003e2\u003c/sub\u003eO (red), and (m-p) CuO (blue), respectively. (q) Cu K-edge X-ray absorption near-edge structure (XANES) measured at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e in 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e + 8 mM NaNO\u003csub\u003e3.\u003c/sub\u003e (r) Temporal evolution of the weight of the Cu, Cu\u003csub\u003e2\u003c/sub\u003eO and CuO species measured by Cu L\u003csub\u003e3\u003c/sub\u003e and K edge evolution by operando TXM and XANES, respectively.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/c6605582e4b89d526e1b3bf9.png"},{"id":65629926,"identity":"794205c9-7542-449e-9aa7-e6d8aedded78","added_by":"auto","created_at":"2024-09-30 16:25:35","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":34180,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eTemporal evolution of oxidic and metallic Cu phases and its impact on ammonia selectivity\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e. (a) The fractions of the cubes within the EC-TEM images are calculated by dividing the area of particles (cubes or NPs) at time t by the initial projected area of Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO catalysts at time 0. (b) Plot depicting the majority Cu phase at different applied potentials and reaction times. The solid line indicates the estimated time to reduce the size of a Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO cube by 50 %. The dotted line denotes 2 hours of NO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR. (c) Linear sweep voltammetry of electrodeposited Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO NCs on carbon paper measured from 0.1 to -0.6 V\u003c/em\u003e\u003csub\u003e\u003cem\u003eRHE\u003c/em\u003e\u003c/sub\u003e. \u003cem\u003e\u0026nbsp;(d) the yield rate of NO\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003csup\u003e\u003cem\u003e-\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e and NH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, and (e) the Faradaic Efficiency (FE) of NO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR products at -0.2, -0.4 V and -0.6 V\u003c/em\u003e\u003csub\u003e\u003cem\u003eRHE\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/a99228b99df6d341ba4991a0.png"},{"id":65629931,"identity":"b4e0438c-996d-476b-b5e6-fced8d678553","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":145415,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eProbing surface chemistry changes in Cu\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cem\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e\u003cstrong\u003eO cubes using operando Raman spectroscopy and schematic detailing catalyst restructuring mediated by oxide/hydroxide formation. \u003c/strong\u003e\u003c/em\u003e\u003cem\u003e(a) Operando Raman spectra of Cu\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eO cubes electrodeposited on glassy carbon measured during NO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eRR in 0.1 M Na\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003eSO\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e + 8 mM NaNO\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e at OCP, -0.2 V\u003c/em\u003e\u003csub\u003e\u003cem\u003eRHE\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, -0.4 V\u003c/em\u003e\u003csub\u003e\u003cem\u003eRHE\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, and -0.6 V\u003c/em\u003e\u003csub\u003e\u003cem\u003eRHE\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e. The average of the 12 measurements is plotted. The measurements were conducted with a fresh specimen for each potential. (b) Schematic describing the possible restructuring mechanisms depending on applied cathodic potential and how the Cu(OH)\u003c/em\u003e\u003csub\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e ⇌ Cu(OH\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e)\u003c/em\u003e\u003csup\u003e\u003cem\u003e 2-\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e/Cu(NH\u003c/em\u003e\u003csub\u003e\u003cem\u003e3\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e)\u003c/em\u003e\u003csub\u003e\u003cem\u003e4\u003c/em\u003e\u003c/sub\u003e\u003csup\u003e\u003cem\u003e2+\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e equilibrium may be controlling the dissolution/re-deposition process .\u003c/em\u003e\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/e67b09f54bf4ef400b0ab656.png"},{"id":74719663,"identity":"d9bb92a8-22b6-4e49-bd3d-f68b392db442","added_by":"auto","created_at":"2025-01-25 08:05:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1710982,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/29303bc9-eb06-433a-a12e-bd1534f5dd84.pdf"},{"id":65629929,"identity":"846bc3a1-16d7-470b-a9ce-d04024f5780e","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"avi","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1400476,"visible":true,"origin":"","legend":"Supplementary Movie 1","description":"","filename":"SupplementaryMovie1.avi","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/0cd91f8789af5051ae75162c.avi"},{"id":65630445,"identity":"ca0440b9-b89c-458a-8f0b-6d3522915d69","added_by":"auto","created_at":"2024-09-30 16:33:36","extension":"avi","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1751328,"visible":true,"origin":"","legend":"Supplementary Movie 2","description":"","filename":"SupplementaryMovie2.avi","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/e76d4612f7b928a774ab535f.avi"},{"id":65629939,"identity":"b0d0b54b-052d-4a82-aa21-7045c75b6c86","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"avi","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":19508834,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Movie 3\u003c/p\u003e","description":"","filename":"SupplementaryMovie3.avi","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/27a9c57036db515ffe079f59.avi"},{"id":65629933,"identity":"b21324ca-e18f-4604-8d3a-fc9a5f402b91","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"avi","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":15056918,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Movie 4\u003c/p\u003e","description":"","filename":"SupplementaryMovie4.avi","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/5ad589dd56b9933b21d51e20.avi"},{"id":65629935,"identity":"c51097ff-472f-4544-b18a-446c0a228dc5","added_by":"auto","created_at":"2024-09-30 16:25:36","extension":"pdf","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":4699576,"visible":true,"origin":"","legend":"","description":"","filename":"NO3RRSIResubmissionClear.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3310807/v1/4abd9bd9494ba4df147c0785.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy","fulltext":[{"header":"Introduction","content":"\u003cp\u003eElectrocatalytic chemical conversion reactions such as carbon dioxide reduction reaction (CO\u003csub\u003e2\u003c/sub\u003eRR)\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and nitrate reduction reaction (NO\u003csub\u003e3\u003c/sub\u003eRR)\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e are key to the advancement of various green energy solutions. However, it can be difficult to identify the active catalyst species in these reactions, even when the metallic state is supposed to be the stable phase, because the catalyst can change its oxidation state during reaction according to the external stimuli. Although the Pourbaix diagram\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e can be used to rationalize the stable oxidation state/phase at different applied potentials and pHs, they are equilibrium diagrams which do not consider the kinetics of redox transitions and their effect on the catalyst morphology. For example, they do not include information about how oxide-to-metal transformations occur, how different facets can reconstruct differently under the same reaction conditions, how interactions between the catalysts and the electrolyte can alter catalyst surface or how reaction intermediates and products may lead to further changes.\u003c/p\u003e \u003cp\u003eThe challenge here is two-fold. First, one must elucidate the working morphology of the electrocatalyst. Second, one needs to disentangle the role the observed morphological changes on the catalytic performance. There are, nonetheless, only a few methods\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e that can visualize the nanoscale restructuring dynamics of a catalyst and follow the catalyst as a function of the applied potential and electrolyte conditions. It is even more challenging to resolve the local chemical state of these features because most \u003cem\u003eoperando\u003c/em\u003e techniques for extracting chemical information, such as Raman spectroscopy and X-ray absorption spectroscopy (XAS), are \u0026ldquo;broad beam\u0026rdquo; methods, where the data is an ensemble signal derived from a large probed region. This gap between nanoscale imaging and ensemble-averaging spectroscopy limits our ability to rationalize how catalyst morphology impacts the overall performance of these complex but important reactions.\u003c/p\u003e \u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003eRR is foremost among various electrochemical conversion reactions in terms of its need for more insight into the electrocatalyst's phase during reaction. Notably, it is a promising strategy for mitigating freshwater pollution from agricultural fertilizer run-off and industrial waste\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, and is also studied for its potential to produce NH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e9\u0026ndash;11\u003c/sup\u003e, which is an important chemical in industry and a candidate carrier for green hydrogen\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. While Cu is one of the most studied electrocatalyst materials for NO\u003csub\u003e3\u003c/sub\u003eRR due to its optimal nitrate adsorption energy\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, whether metallic Cu\u003csup\u003e\u003cspan additionalcitationids=\"CR16 CR17\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, Cu oxides or a Cu-Cu oxide interface\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e are the key species for the selective NH\u003csub\u003e3\u003c/sub\u003e formation has remained largely unresolved. According to the Pourbaix diagram\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, the metallic phase of Cu should be the stable phase of Cu under typical NO\u003csub\u003e3\u003c/sub\u003eRR conditions, but studies using \u003cem\u003ein situ\u003c/em\u003e Raman spectroscopy had suggested that an oxide phase might exist during the reaction\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Cu and its oxides are also known to be susceptible to etching\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e and facet modification\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e by NH\u003csub\u003e3\u003c/sub\u003e. Furthermore, it has been reported that NO\u003csub\u003e3\u003c/sub\u003eRR can drive the dissolution and re-growth of single atom Cu catalysts\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, and the clustering of small aggregates into larger nanoparticles (NPs)\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHere, we use electrochemical liquid cell transmission electron microscopy (EC-TEM) accompanied by correlated multi-modal \u003cem\u003eoperando\u003c/em\u003e investigations that include electrochemical liquid cell transmission X-ray microscopy (EC-TXM), \u003cem\u003eoperando\u003c/em\u003e XAS and \u003cem\u003eoperando\u003c/em\u003e Raman spectroscopy of the same pre-catalysts to visualize in real time how the structure and composition of Cu\u003csub\u003e2\u003c/sub\u003eO cubes evolve as a function of the applied potential during NO\u003csub\u003e3\u003c/sub\u003eRR. We found that the working electrocatalyst morphology was determined by three processes, (i) the dissolution of Cu\u003csub\u003e2\u003c/sub\u003eO, (ii) the re-deposition of Cu from soluble Cu complexes\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e and (iii) the reduction of Cu\u003csub\u003e2\u003c/sub\u003eO to metallic Cu. We also discovered a coexistence of Cu\u003csub\u003e2\u003c/sub\u003eO with metallic Cu for extended reaction durations, thereby providing insight into the active Cu species during NO\u003csub\u003e3\u003c/sub\u003eRR.\u003c/p\u003e"},{"header":"Results \u0026 Discussion","content":"\u003cp\u003eFor the \u003cem\u003eoperando\u003c/em\u003e microscopy experiments, we prepared well-defined Cu\u003csub\u003e2\u003c/sub\u003eO cubes on the carbon working electrode of the EC-TEM chips via electro-deposition\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e as shown schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The as-prepared Cu\u003csub\u003e2\u003c/sub\u003eO cubes have an average size of 250 nm and consist of six {100} facets without the exposure of other minor facets such as {110} or {111} (Suppl. Figure\u0026nbsp;1). All voltages indicated in this paper are referenced against a Ag/AgCl electrode and then converted to the reversible hydrogen electrode scale (RHE) using the Nerst equation and the bulk pH of the electrolyte. Intriguingly, the image sequences show that the cubes do not undergo significant change in the commonly employed 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e3\u003c/sub\u003e electrolyte for NO\u003csub\u003e3\u003c/sub\u003eRR (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), during the initial sweep towards cathodic potentials. Cu\u003csub\u003e2\u003c/sub\u003eO should reduce directly to metallic Cu at the higher overpotentials of the sweep, where according to the Pourbaix diagram, the redox potential for the transformation of Cu\u003csub\u003e2\u003c/sub\u003eO to metallic Cu is 0 V\u003csub\u003eRHE\u003c/sub\u003e in a solution with pH 7\u003csup\u003e20,21\u003c/sup\u003e, and metallic Cu is the stable phase below \u0026minus;\u0026thinsp;0.2 V\u003csub\u003eRHE\u003c/sub\u003e onwards. The stability of the Cu\u003csub\u003e2\u003c/sub\u003eO cubes is remarkable because these redox transformations usually lead to morphological changes. For comparison, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec depicts a Cu\u003csub\u003e2\u003c/sub\u003eO cube under CO\u003csub\u003e2\u003c/sub\u003eRR conditions in CO\u003csub\u003e2\u003c/sub\u003e-saturated 0.1 M KHCO\u003csub\u003e3\u003c/sub\u003e at a potential similar to that applied in the NO\u003csub\u003e3\u003c/sub\u003eRR experiment. As we reported previously\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e, the latter cubes undergo fragmentation together with the re-deposition of small particles, a behavior that differs from the morphologically much more stable NO\u003csub\u003e3\u003c/sub\u003eRR samples at the same applied potentials. The linear sweep voltammograms acquired during these two experiments are available as Suppl. Figure\u0026nbsp;2.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNext, we studied these Cu\u003csub\u003e2\u003c/sub\u003eO cubes systematically at different sustained potentials from \u0026minus;\u0026thinsp;0.2 V\u003csub\u003eRHE\u003c/sub\u003e to -0.6 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-e) to probe further their morphological stability during NO\u003csub\u003e3\u003c/sub\u003eRR. For these extended experiments, we adopted an intermittent imaging protocol (images captured at 15-minute intervals with the electron beam blanked the rest of the time) to minimize beam-induced dissolution of the Cu\u003csub\u003e2\u003c/sub\u003eO cubes (see discussion in Suppl. Note 1) and ensure that the catalyst re-structuring kinetics we extract from the collected data are as accurate as possible. The electrochemical current profiles over time at each potential measured in these EC-TEM experiments are provided as Suppl. Figure\u0026nbsp;3. At -0.2 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), the cubes were stable during our entire observation, with no significant restructuring observed. From \u0026minus;\u0026thinsp;0.2 to -0.5V\u003csub\u003eRHE\u003c/sub\u003e, dissolution/re-deposition is the main restructuring pathway. At -0.3 V\u003csub\u003eRHE\u003c/sub\u003e, the cubic form persisted for almost 135 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), while the cube completely dissolved after 140 min at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) and after 90 min at -0.5 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed). The lighter contrast of the cube exterior in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec compared to the middle of the cube at 60 and 80 min is explained by the cube corners and edges being etched first. The weaker contrast of the dissolving Cu\u003csub\u003e2\u003c/sub\u003eO cubes compared to that of the growing Cu NPs also suggests that the dissolving cubes were still in oxide form.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTwo Cu\u003csub\u003e2\u003c/sub\u003eO cubes were captured in the images acquired at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). One cube shrank and restructured into a smaller cube with a void in the center and then became rougher due to small NPs attaching to its surface, while another completely dissolved within the same time frame. We also highlight that the intensity of the cubes in the TEM images obtained from \u0026minus;\u0026thinsp;0.3 to -0.5 V\u003csub\u003eRHE\u003c/sub\u003e gradually decrease, whereas the intensity of the cube at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e is brighter, implying that cube-like frame at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e is metallic. Moreover, the interplay of dissolution/re-deposition and direct reduction at the more cathodic potentials means that the terminal catalysts morphologies of oxide pre-catalysts vary depending on the applied potential.\u003c/p\u003e \u003cp\u003eNext, we repeated the NO\u003csub\u003e3\u003c/sub\u003eRR experiments in a H-type cell with Cu\u003csub\u003e2\u003c/sub\u003eO cubes electrodeposited on carbon paper to check for the consistency of the EC-TEM results with standard reaction geometries. Figures\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg show lower magnification images of samples from the EC-TEM experiments with scanning electron microscopy (SEM) images of samples extracted from H-type cell experiments after 2 hours of reaction at three different applied potentials, -0.2, -0.4 and \u0026minus;\u0026thinsp;0.6 V\u003csub\u003eRHE\u003c/sub\u003e. Electron diffraction patterns taken from samples extracted after reaction show that the cubes did not undergo significant restructuring at -0.2 V\u003csub\u003eRHE\u003c/sub\u003e and remain Cu\u003csub\u003e2\u003c/sub\u003eO whereas samples reacted at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e were largely metallic (Suppl. Figure\u0026nbsp;4). Conversely, samples reacted at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e show a mixture of residual Cu\u003csub\u003e2\u003c/sub\u003eO and metallic Cu structures (Suppl. Figure\u0026nbsp;4\u0026ndash;6). The morphological differences between the sample after reaction in the H-type cell at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e and \u0026minus;\u0026thinsp;0.6 V\u003csub\u003eRHE\u003c/sub\u003e further support that the catalysts indeed restructure through different pathways as described by our EC-TEM experiments. Inductively coupled plasma mass spectrometry measurements of the electrode and the electrolyte in the H-type cell after reaction further show that Cu dissolution happens at all the applied potentials (Suppl. Figure\u0026nbsp;7). Therefore, these experiments indicate that the Cu\u003csub\u003e2\u003c/sub\u003eO cubes undergo a gradual dissolution under NO\u003csub\u003e3\u003c/sub\u003eRR conditions, which in turn leads to the re-deposition of metallic particles elsewhere on the working electrode with shapes and sizes that are modulated by the applied potential.\u003c/p\u003e \u003cp\u003eTo obtain unambiguously the oxidation state of the catalyst species present during reaction and rule out the possibility that the \u003cem\u003eex situ\u003c/em\u003e identified Cu\u003csub\u003e2\u003c/sub\u003eO phase are the result of re-oxidation during the return to open circuit potential\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e (OCP), we performed \u003cem\u003eoperando\u003c/em\u003e EC-TXM measurements on the Cu\u003csub\u003e2\u003c/sub\u003eO cubes by transferring our EC-TEM holder into a TXM at the BESSY II synchrotron facility as illustrated in Suppl. Figure\u0026nbsp;8. This unique arrangement maintains the same reaction environment between the two experiments, while enabling time-resolved \u003cem\u003eoperando\u003c/em\u003e measurements of Cu absorption edges under applied potential without compromising the sustained electrolyte flow as X-rays are attenuated less by the electrolyte and enclosing membranes. Thus, the evolution of the electrode\u0026rsquo;s composition can be tracked during NO\u003csub\u003e3\u003c/sub\u003eRR. Figures\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-d show the time-resolved evolution of the Cu\u003csub\u003e2\u003c/sub\u003eO catalysts during NO\u003csub\u003e3\u003c/sub\u003eRR at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e as observed by EC-TXM in the form of the colored maps that were reconstructed from a XAS image stack using linear combination fitting (LCF)\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Cu\u003csub\u003e2\u003c/sub\u003eO is depicted in red and metallic Cu in yellow. The maps show that Cu\u003csub\u003e2\u003c/sub\u003eO and Cu are the dominant phases present over the duration of the NO\u003csub\u003e3\u003c/sub\u003eRR and that the oxide and metallic phases coexist under specific reaction conditions, but they are spatially separated. CuO is also detected (blue color) but it is present only in small quantities and is not clearly visible from the maps. The corresponding decomposed spectra are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee-p, where the yellow, red and blue colored lines represent the respective Cu species. The total contribution of the individual spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee-p represents the amount (thickness) of each species, which indicates that the content of metallic Cu species increases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee-h) during reaction, whereas Cu\u003csub\u003e2\u003c/sub\u003eO decreases (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei-p). The details of the data acquisition and processing are discussed in Suppl. Note 3. Most importantly, these results confirm the sluggish reduction kinetics of the large Cu\u003csub\u003e2\u003c/sub\u003eO cubes in the Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;NaNO\u003csub\u003e3\u003c/sub\u003e electrolyte, and that the metallic phase forms when the dissolved Cu species re-deposit on the working electrode due to the reductive potential employed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWe further verified that the slow reduction of the Cu oxide cubes extends to larger reaction volumes with \u003cem\u003eoperando\u003c/em\u003e hard XAS measurements of samples electro-deposited on carbon paper in our home-built electrochemical XAS cell\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eq, we plot the changes in the Cu K-edge valence states (from 8950 to 9105 eV) that were obtained from \u003cem\u003eoperando\u003c/em\u003e XAS. The weight of the Cu valence state is extracted by LCF of the X-ray absorption near-edge structure (XANES) of the oxide-derived Cu catalyst collected at a constant potential of -0.4 V\u003csub\u003eRHE\u003c/sub\u003e in 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e3\u003c/sub\u003e electrolyte. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003er, the fraction of Cu\u003csub\u003e2\u003c/sub\u003eO decreased but not completely after more than 2 hours of electrolysis, while the fraction of metallic Cu increased correspondingly, eventually to almost a 1:1 ratio of Cu\u003csub\u003e2\u003c/sub\u003eO:Cu. The XANES results agree with the persistence of Cu\u003csub\u003e2\u003c/sub\u003eO and the continual evolution of the Cu species seen in the EC-TEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) and EC-TXM (Cu-L\u003csub\u003e3\u003c/sub\u003e edges in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003er) results at -0.4 V\u003csub\u003eRHE\u003c/sub\u003e. Minute amounts of the CuO species were also detected during the experiment. The change in the weights of the three species over time exhibit similar trends in both TXM and XAS, confirming that the results we obtain in the EC-TEM cells indeed extrapolate to a larger ensemble of catalyst particles.\u003c/p\u003e \u003cp\u003eThis overall agreement between different methods and experimental geometries means that we can use the \u003cem\u003ein situ\u003c/em\u003e TEM image sequences to quantify the potential-dependent dissolution and re-deposition rates. Our method for fraction extraction from the EC-TEM images and additional analysis of the re-deposited particles is described in the Suppl. Note 4. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, the cube fraction decreases over time at an increasing rate as the potential decreases from \u0026minus;\u0026thinsp;0.2 V to -0.5 V\u003csub\u003eRHE\u003c/sub\u003e. The sample at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e deviates from this trend (dark purple line) because of the direct reduction of Cu\u003csub\u003e2\u003c/sub\u003eO to metallic Cu. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, we use the cube dissolution rate to estimate the Cu\u003csub\u003e2\u003c/sub\u003eO to Cu ratio at a certain potential and use it to visualize the majority phase (greater than 50%) at different times. We further compare the Cu\u003csub\u003e2\u003c/sub\u003eO/Cu ratio with NH\u003csub\u003e3\u003c/sub\u003e conversion activity (current density) and selectivity (Faradaic Efficiency, FE) obtained from our bench top electrochemistry measurements. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, we plot the linear sweep voltammogram of the Cu\u003csub\u003e2\u003c/sub\u003eO cubes prepared on carbon paper and in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed-e, their product distribution as a function of the applied potentials. The measured yield rate and the FE towards NH\u003csub\u003e3\u003c/sub\u003e was much higher at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e as compared to -0.2 V\u003csub\u003eRHE\u003c/sub\u003e and \u0026minus;\u0026thinsp;0.4 V\u003csub\u003eRHE\u003c/sub\u003e, implying that the change in catalytic selectivity is related to the faster rate of oxide to metal conversion at -0.6 V\u003csub\u003eRHE\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNext, we performed EC-TEM studies in various electrolyte compositions to elucidate the mechanism behind Cu\u003csub\u003e2\u003c/sub\u003eO stabilization. Suppl. Figure\u0026nbsp;9a and 9b describe experiments using pure 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e2\u003c/sub\u003e (i.e. nitrite reduction) respectively. In both cases, the cubes behaved similarly to their behavior with NO\u003csub\u003e3\u003c/sub\u003eRR where they gradually reduced in size until they fragment/reduce at longer reaction duration, which means the Cu\u003csub\u003e2\u003c/sub\u003eO stability is related to the Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e supporting electrolyte and not the reactant. Re-deposition was, however, much less in Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e compared to its NaNO\u003csub\u003e3\u003c/sub\u003e/NaNO\u003csub\u003e2\u003c/sub\u003e containing counterparts. We attribute this difference to how the local pH during electro-reduction differs in the presence and absence of NO\u003csub\u003ex\u003c/sub\u003e species. Under applied cathodic potentials, the pH at the electrocatalyst surface increases as hydroxyl ions form\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e due to the reduction of e.g. H\u003csub\u003e2\u003c/sub\u003eO, O\u003csub\u003e2\u003c/sub\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. In particular, NO\u003csub\u003ex\u003c/sub\u003eRR results in higher currents and consequently a steeper rise in the local pH as compared to when only hydrogen reduction takes place. This process can bring the pH of a neutral electrolyte above 12\u003csup\u003e34\u003c/sup\u003e, triggering the formation of soluble Cu hydroxides. To look at the effect of electrolyte pH, we further performed experiments in 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e where the pH was increased to 10 by adding NaOH. As shown in Suppl. Figure\u0026nbsp;9c, it altered the amount of re-deposition observed. Lastly, to probe the influence of ammonium ions, we deliberately added NH\u003csub\u003e4\u003c/sub\u003eOH into the 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e carrier electrolyte, which led to rapid restructuring of the cubes as shown in Suppl. Figure\u0026nbsp;9d.\u003c/p\u003e \u003cp\u003eTo explain these results, we consider the phase stability of Cu as a function of pH and in the presence of NH\u003csub\u003e3\u003c/sub\u003e. A complex series of reactions encompassing different acid\u0026ndash;base chemistries, Cu(OH)\u003csub\u003e2\u003c/sub\u003e precipitation, and complex ion formation are known for the Cu-NH\u003csub\u003e3\u003c/sub\u003e system\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e (see Suppl. Note 5). Specifically, the equilibrium between solid Cu(OH)\u003csub\u003e2\u003c/sub\u003e and the Cu(NH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2+\u003c/sup\u003e complex depends on the NH\u003csub\u003e3\u003c/sub\u003e concentration, with Cu(OH)\u003csub\u003e2\u003c/sub\u003e precipitation being favored at low concentrations due to the poor solubility of Cu(OH)\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e36\u003c/sup\u003e. We hypothesize that the sluggish reduction observed may be the result of transient surface Cu(OH)\u003csub\u003e2\u003c/sub\u003e formation induced by the interfacial pH rise in the course of the NO\u003csub\u003e3\u003c/sub\u003eRR. Cu(OH)\u003csub\u003e2\u003c/sub\u003e formation may also be more favorable in Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, due to the electrolyte\u0026rsquo;s inability to buffer the local pH increase from electro-reduction\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, as compared to the KHCO\u003csub\u003e3\u003c/sub\u003e electrolyte used in CO\u003csub\u003e2\u003c/sub\u003eRR, thereby leading to the differences in re-structuring behaviors. To validate this hypothesis, we studied the chemical changes taking place on the surface of the Cu\u003csub\u003e2\u003c/sub\u003eO cubes with \u003cem\u003eoperando\u003c/em\u003e Raman spectroscopy measurements.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the results of experiments at constant applied potentials performed with cubes electrodeposited on glassy carbon plates. At OCP, the Raman spectrum shows three features of bands centered at 415, 520, and 630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e respectively, which are in good agreement with the reported values of Cu\u003csub\u003e2\u003c/sub\u003eO\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e. When \u0026minus;\u0026thinsp;0.2 V\u003csub\u003eRHE\u003c/sub\u003e was applied, the band intensity at 520 and 630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e decreased over time but continued to persist, which is consistent with the gradual dissolution of Cu\u003csub\u003e2\u003c/sub\u003eO. At -0.4 V\u003csub\u003eRHE\u003c/sub\u003e, a new peak at 475 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e started to emerge, while the peak at 630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e flattened, indicating oxide to metal transition. At -0.6 V\u003csub\u003eRHE\u003c/sub\u003e, the characteristic bands of Cu\u003csub\u003e2\u003c/sub\u003eO were less pronounced and additional weak bands emerged at 450, 475 and 590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The band around 475 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be assigned to the Cu-O-H vibration\u003csup\u003e\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e or Cu(OH)\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e40\u003c/sup\u003e, while the peak at 590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is often assigned to CuO\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e or adsorbed O-species on Cu\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. These results suggest the formation of a transient intermediate oxide or hydroxide phase during electrolysis, which is also supported by the extended presence of Cu(I) signatures and the weak but persistent Cu(II) signatures in the EC-TXM and \u003cem\u003eoperando\u003c/em\u003e XAS measurements in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It was, however, difficult to obtain the Raman signatures of absorbates with these samples due to the relatively low loading of the cubes that we were able to electrodeposit and the overlap of the D/G bands of the glassy carbon support with surface absorbate bands, which limits the signal-to-noise ratios at those bands. Thus, we repeated the Raman measurements with cubes electrodeposited at higher loading on carbon paper to improve the signal-to-noise ratios to identify surface adsorbed species or intermediate species in the vicinity of electrode. These results are discussed in Supplementary Note 6.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHence, we arrive at a restructuring mechanism illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb where surface hydroxides first form on the Cu\u003csub\u003e2\u003c/sub\u003eO cubes due to the pH increase induced by electrolysis, which delays the oxide reduction. The subsequent NH\u003csub\u003e3\u003c/sub\u003e production and added pH rise from continued NO\u003csub\u003e3\u003c/sub\u003eRR then destabilizes this hydroxide layer and forms soluble Cu complexes, thereby initiating catalyst evolution via re-deposition from Cu complex reduction or the aggregation of migrating NPs. We also performed more EC-TEM experiments to check that the delayed restructuring kinetics extend to other pre-catalyst geometries. Suppl. Figure\u0026nbsp;10 describes the evolution during NO\u003csub\u003e3\u003c/sub\u003eRR of Cu\u003csub\u003e2\u003c/sub\u003eO truncated octahedrons and metallic frames created by pre-reducing the Cu\u003csub\u003e2\u003c/sub\u003eO cubes. Both samples are stable during early-stage reaction. With more time, re-deposition similar to that seen in the cubes was observed in the Cu\u003csub\u003e2\u003c/sub\u003eO octahedrons whereas little re-deposition was noticeable with the metallic frames, which was likely due to less dissolution occurring when we start from metallic pre-catalysts.\u003c/p\u003e \u003cp\u003eThe correlated microscopy and spectroscopy experiments presented here therefore indicate that the morphology of the Cu catalysts during NO\u003csub\u003e3\u003c/sub\u003eRR at a given pH is governed by a complex, time- and potential-dependent interplay of three processes: (i) dissolution of oxide and hydroxide species, (ii) metal re-deposition, and (iii) oxide catalyst reduction. According to the Pourbaix diagram\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, metallic Cu is the only stable species under the specific applied potentials and pH of our experiments, but we have shown that oxidic and metallic phases can co-exist over extended durations and over a broad range of applied potentials, which has significant implications in terms of determining the active species for producing NH\u003csub\u003e3\u003c/sub\u003e. It has been suggested previously\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, based on \u003cem\u003eoperando\u003c/em\u003e spectroscopy measurements of CuO pre-catalysts, that Cu/Cu\u003csub\u003e2\u003c/sub\u003eO interfaces are responsible for NH\u003csub\u003e3\u003c/sub\u003e production\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e, but these methods cannot differentiate the distribution of these species on the nanoscale. As we have shown in this work, the presence of both spectroscopic signatures in ensemble-averaging measurements does not necessarily mean that the two phases are spatially connected. Furthermore, we have demonstrated that the de-coupled metallic Cu and Cu oxide phases can persist for the extended durations at mild cathodic potentials (less than \u0026minus;\u0026thinsp;0.5 V\u003csub\u003eRHE\u003c/sub\u003e), and that a high residual abundance of Cu\u003csub\u003e2\u003c/sub\u003eO in the \u003cem\u003eoperando\u003c/em\u003e XAS measurements corresponded to a low NH\u003csub\u003e3\u003c/sub\u003e production efficiency in our electrolysis data of equivalent samples. The improvement of NH\u003csub\u003e3\u003c/sub\u003e selectivity with increasing overall metallic character of the samples, therefore, advocates that metallic Cu is the active phase for producing NH\u003csub\u003e3\u003c/sub\u003e compared to Cu\u003csub\u003e2\u003c/sub\u003eO, in agreement with recent work on the topic\u003csup\u003e\u003cspan additionalcitationids=\"CR16 CR17\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. In this case, the strong stability of the Cu\u003csub\u003e2\u003c/sub\u003eO cubes and their sluggish reduction kinetics in the often-used Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e carrier electrolyte, are detrimental for the NH\u003csub\u003e3\u003c/sub\u003e production by delaying the onset of selective NH\u003csub\u003e3\u003c/sub\u003e formation.\u003c/p\u003e \u003cp\u003eBy showing the diverse behaviors that can be derived in different electrolytes and reaction conditions, our work also illustrates the critical need to pay attention to how the electrolyte can influence the restructuring of catalysts and the stability of oxide, hydroxide, and metallic phases before we attempt to generalize results across different studies and reactions. So far, the description of electrolyte effects in electrolysis has been largely confined to cation adsorption effects\u003csup\u003e\u003cspan additionalcitationids=\"CR43 CR44\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e and re-structuring induced by aggressive halide anions\u003csup\u003e\u003cspan additionalcitationids=\"CR45\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e, whereas studies of pH had focused on its impact on reaction mechanisms and NH\u003csub\u003e3\u003c/sub\u003e selectivity\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, and not catalyst phase stability. Significant additional microscopy work, such as the one presented here will be required to separate the impact of electrolyte-driven morphological transformation from the much better understood associated electronic and chemical changes. Furthermore, current computational models still cannot rationalize the impact of an explicit complex electrolyte on the catalyst restructuring and its associated influence on the creation of active sites. Efforts to improve these models and advance the theory describing electrocatalytic processes, will undoubtedly require more accurate representations of dynamic catalyst surfaces. The challenge here is serious, since theoretical mechanistic insight must consider two simultaneously occurring dynamic processes, namely the one that the catalyst material experiences, in parallel to that undergone by the reactants, both of which being coupled and driven by the local chemical potential.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e Our results revealing phase co-existence also opens the possibility that different species may be responsible for activating specific steps of the conversion reaction. Hence, we expect \u003cem\u003eoperando\u003c/em\u003e approaches that incorporate chemically resolved microscopy within multi-modal spectroscopic investigations, as we demonstrated here, to play a vital role moving forward in the understanding of electrocatalytic processes by providing a path towards mapping such complexity.\u003c/p\u003e \u003cp\u003eIn summary, \u003cem\u003eoperando\u003c/em\u003e EC-TEM and EC-TXM measurements have revealed that the morphologies of Cu\u003csub\u003e2\u003c/sub\u003eO pre-catalysts during NO\u003csub\u003e3\u003c/sub\u003eRR and their evolutionary pathways are sensitive to the reaction time, applied potential and the nature of the electrolyte. As expected, the rate of oxide reduction accelerates with increasing negative applied potentials, but spatially-separated oxide and metallic phases can co-exist over extended reaction times under moderately reductive potentials. More importantly, the kinetics of the different re-structuring processes, which have been unveiled here, determine that final morphology of the catalysts. Our results also indicate that the nature of the electrolyte can introduce time-dependent selectivity changes in the early stages of the catalyst restructuring, which will help resolve on-going controversies regarding the active state of Cu for selective NH\u003csub\u003e3\u003c/sub\u003e production. Finally, this work impacts our understanding of how electrocatalysts evolve under reaction condition through the discovery of Cu oxide and hydroxide stability. In addition, we unveiled local structural and chemical heterogeneities that develop under electrochemical working conditions, even on a pre-catalyst sample initially characterized by a narrow size, shape and compositional distribution. Thus, our findings emphasize the need of \u003cem\u003eoperando\u003c/em\u003e characterization methods to establish connections between materials\u0026rsquo; structural and compositional characteristics under specific reaction environments and external stimuli and their electrocatalytic performance.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e \u003cstrong\u003eSpecimen preparation\u003c/strong\u003e \u003cp\u003eThe Cu\u003csub\u003e2\u003c/sub\u003eO cubes (250 nm) were synthesized on the polished glassy carbon (vitreous, SPI) plates, carbon paper and the carbon electrode of the Hummingbird Scientific EC-TEM chips using an electro-deposition protocol we previously developed.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e The deposition solution consists of a mixture of 5 mM copper sulfate-pentahydrate (CuSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;5H\u003csub\u003e2\u003c/sub\u003eO, Sigma Aldrich) and 12.5 mM of potassium chloride (KCl, Sigma Aldrich). After the synthesis, the samples were rinsed with ultrapure water and then used for the subsequent NO\u003csub\u003e3\u003c/sub\u003eRR experiments.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eElectrolyte preparation for nitrate reduction\u003c/strong\u003e \u003cp\u003eThe electrolyte used for nitrate reduction experiments is an aqueous solution of 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (anhydrous, 99.99%, Suprapur))\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e3\u003c/sub\u003e (Sigma-Aldrich, ReagentPlus\u0026reg;, \u0026ge;\u0026thinsp;99.0%).\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003e\u003cem\u003eOperando\u003c/em\u003e EC-TEM\u003c/strong\u003e \u003cp\u003eThe EC-TEM experiments were performed in a Thermo Fisher Scientific 300 kV Titan TEM operated in STEM mode and a Hummingbird Scientific Bulk Liquid Electrochemistry TEM holder with Pt counter and Ag/AgCl (3 M KCl) reference electrodes. The EC-TEM top chips have a 50 nm thick silicon nitride membrane windows and were also produced by Hummingbird Scientific. The bottom chips have a 50 nm silicon window and 250 nm spacers. The electrochemistry experiments were performed using a Biologic SP-200 potentiostat. The potentials were measured against the built-in Ag/AgCl reference.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe TEM holder was pre-filled with 0.1 M Na\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8mM NaNO\u003csub\u003e3\u003c/sub\u003e solution during cell assembly to fill the entire fluid path with electrolyte. After loading into the TEM, the syringe was filled with 0.1 M Na\u003csub\u003e3\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8mM NaNO\u003csub\u003e3\u003c/sub\u003e and introduced at a flow rate of 1.25 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Linear sweep voltammetry from \u0026minus;\u0026thinsp;0.5 V to \u0026minus;\u0026thinsp;1.1 V\u003csub\u003eAgAgCl\u003c/sub\u003e (repeated twice) was first used to determine the onset potential for the NO\u003csub\u003e3\u003c/sub\u003eRR using a scan rate of 15 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and to ensure that the applied potential was consistent between experiments, followed by chronoamperometry for up to 2 hours at \u0026minus;\u0026thinsp;0.2, -0.3, -0.4, -0.5.1 and \u0026minus;\u0026thinsp;0.6 V\u003csub\u003eRHE\u003c/sub\u003e (converted from Ag/AgCl scale using the Nerst equation) with the new catalyst specimen and fresh electrolyte at each condition.\u003c/p\u003e \u003cp\u003e \u003cem\u003eIn situ\u003c/em\u003e imaging was always performed under conditions with electrolyte in the cell, as determined from the image contrast. We stayed under an electron flux of 1.75 e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026Aring;\u003csup\u003e\u0026minus;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at all times to minimize electron beam-induced artifacts with an electron probe current of approximately 220 pA. The acquired images have a resolution of 1024 \u0026times; 1024 pixels. For continuous imaging, the images were acquired at 1 frame per second. For intermittent imaging, the images were acquired every 15 minutes with the electron beam blanked-in between. The image segmentation for the EC-TEM movies was performed using built-in functions and scripting in MATLAB (See Suppl. Note 4 for details).\u003c/p\u003e \u003cp\u003e \u003cstrong\u003eNO\u003csub\u003e3\u003c/sub\u003eRR product analysis and detection\u003c/strong\u003e \u003cp\u003eThe electrochemistry experiments for product analysis and \u003cem\u003eex situ\u003c/em\u003e imaging were conducted by using an Autolab potentiostat (PGSTAT 302N) and a custom-made H-type electrochemical cell, whereby the cathodic and anodic compartments were separated by a piece of anion exchange membrane (Selemion AMV, AGC Inc.). A platinum gauze (MaTecK, 3600 mesh cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and a leak-free Ag/AgCl electrode (LF-1, Alvatek, potential 0.198 V vs. standard hydrogen electrode) were used as the counter and the reference electrodes, respectively. The anodic compartment (with counter electrode) was filled with 18 mL 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte while the cathodic compartment (with the working electrode) was filled with 18 mL 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e3\u003c/sub\u003e. Before the electrochemical tests, the anodic and cathodic solutions were de-aerated by continuously bubbling Ar (grade 6.0, 99.9999%) with a 20 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e flow rate (Bronkhorst). During the chronoamperometric measurements, a constant Ar flow (10 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was used to maintain the inert atmosphere. Linear sweep voltammetry was performed with a scan rate of 5 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and chronoamperometry (2 hours reaction time) was used for product distribution measurements.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eUV-Vis spectroscopy (Agilent Cary 60) was used to detect and quantify ammonia and nitrite in the electrolyte following the procedures established in previous literature \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. The liquid samples were diluted to match the suitable detection range for spectrophotometric analysis of each analyte and the sample absorbance was measured in the range 400\u0026ndash;800 nm.\u003c/p\u003e \u003cp\u003eThe indophenol-blue method was used for the determination of ammonia\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e,\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. For nitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026ndash;\u003c/sup\u003e) quantification, 3 mL of the diluted sample were added to a glass vial containing 35 mg of white powder from a commercial nitrite test kit (photometric 0.002\u0026ndash;1.00 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NO\u003csub\u003e2\u003c/sub\u003e-N, 0.007\u0026ndash;3.28 mg L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, Spectroquant, Merck). Details on quantification can be found in the Suppl. Note 2.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003e\u003cem\u003eEx situ\u003c/em\u003e TEM and SEM measurement\u003c/strong\u003e \u003cp\u003eThe \u003cem\u003eex situ\u003c/em\u003e TEM imaging was also performed with the Thermo Fisher Scientific 300 kV Titan TEM. The EC-TEM chips were loaded using a Hummingbird Scientific Tomography holder for before and after reaction comparisons. The EC-TEM chips were rinsed in ultrapure water after they were disassembled from the EC-TEM holder and immediately transferred into the TEM. The \u003cem\u003eex situ\u003c/em\u003e SEM imaging of the bulk samples was performed using a Thermo Fisher Scientific Apreo SEM.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003e\u003cem\u003eOperando\u003c/em\u003e EC-TXM measurement\u003c/strong\u003e \u003cp\u003e \u003cem\u003eOperando\u003c/em\u003e EC-TXM experiments were conducted at the U41-TXM beamline in BESSY II (Berlin, Germany). The beam-size was 26 \u0026micro;m \u0026times; 26 \u0026micro;m with a nominal resolution of 20 nm. The image stacks were collected using a charge coupled detector (CCD) at 1340 pixel \u0026times; 1300 pixel and the exposure time of 1 second per energy. 10 nm monochromator slit was used. The intensity of the incident radiation was monitored and adjusted to have a photon count constant (approximately 15000 count per pixel) at the background area (no specimen) when liquid is fully filled. Image stacks were acquired as the beam energies were scanned from 926 to 965 eV, which encompassed both Cu-L\u003csub\u003e3\u003c/sub\u003e and Cu-L\u003csub\u003e2\u003c/sub\u003e edges.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eA Hummingbird Scientific electrochemistry holder was used for the \u003cem\u003eoperando\u003c/em\u003e measurements. The applied potential was controlled with a Biologic potentiostat. The reference electrode was a Pt-pseudo reference on the chip and the counter was Pt. The reference potential was calibrated against an external Ag/AgCl electrode to ensure a potential comparable to the EC-TEM experiments were applied.\u003c/p\u003e \u003cp\u003eDetails regarding the data processing, including accurate alignment, atomistic background subtraction, data normalization, spectra averaging and linear combination fitting (LCF) of the spectra images can be found in Suppl. Note 2.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003e\u003cem\u003eOperando\u003c/em\u003e XAS\u003c/strong\u003e \u003cp\u003e \u003cem\u003eOperando\u003c/em\u003e time-resolved X-ray absorption fine-structure spectroscopy (XAFS) experiments were performed at P64 beamline of the PETRA III synchrotron (Hamburg,Germany) in quick XAFS (QXAFS) mode. Measurements were performed at the Cu K-edge (8979 eV). The intensity of the incident radiation was monitored by a gas ionization chamber filled with pure N\u003csub\u003e2\u003c/sub\u003e. Additional ionization chambers were used to acquire spectra of a Cu foil in transmission mode for calibration purposes at the beginning of each QXAFS scan. The beam-size was less than 2 \u0026times; 2 mm. The XAS data were collected in fluorescence mode using a PIPS detector at one spectrum per second and one spectrum per 5 seconds.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eFor these \u003cem\u003eoperando\u003c/em\u003e XAS experiments, we used a home-made single-compartment electrochemical cell\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. The applied potential was controlled with a Biologic potentiostat. Argon was flowed into gas compartment at 10 ml/min. The sample was Cu\u003csub\u003e2\u003c/sub\u003eO cubes prepared on the carbon paper electrode and the 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8 mM NaNO\u003csub\u003e3\u003c/sub\u003e electrolyte was continuously circulated through the cell using a double-channel peristaltic pump.\u003c/p\u003e \u003cp\u003eData extraction and calibration were performed using the JAQ software of the P64 beamline. Further data processing and analysis of the XANES spectra were performed according to the procedures described previously in reference \u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cstrong\u003e\u003cem\u003eOperando\u003c/em\u003e Raman measurement\u003c/strong\u003e \u003cp\u003eThe \u003cem\u003eoperando\u003c/em\u003e Raman spectra were obtained using a Renishaw (InVia Reflex) confocal Raman microscope and a water immersion objective with a long working distance (Leica microsystems, 63x, numerical aperture of 0.9) was chosen. The glassy carbon experiments used a 785 nm laser with 0.1% laser power (0.36 mW). The carbon paper experiments used a 633 nm laser with laser power of about 5%. The objective was protected from the electrolyte by a Teflon film (DuPont, film thickness of 0.013 mm). Then, a drop of water was used to drive away the air between the film and the objective to match the refractive index to ensure efficient excitation and collection of the Raman signal.\u003c/p\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical measurements were performed in a home-built spectro-electrochemical cell made of Teflon and controlled by a Biologic SP-240 potentiostat. The cell was equipped with a reference electrode (leak-free Ag/AgCl, Alvatek), a counter electrode (Pt ring), and a working electrode with the catalyst electrodeposited on glassy carbon. A 15 ml Ar-purged 0.1 M Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;8mM NaNO\u003csub\u003e3\u003c/sub\u003e solution was used as an electrolyte. During the experiment, the Raman spectra were acquired every 5 minutes over 1 hour of reaction.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eContributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAY, SWC, and BRC\u003c/strong\u003e conceived the project, planned the experiments, wrote the manuscript. \u003cstrong\u003eAY\u003c/strong\u003e prepared specimens, conducted the \u003cem\u003eoperando\u0026nbsp;\u003c/em\u003eEC-TEM studies and \u003cem\u003eoperando\u0026nbsp;\u003c/em\u003eEC-TXM studies, \u003cem\u003eex situ\u003c/em\u003e TEM and \u003cem\u003eex situ\u003c/em\u003e SEM analysis, and analyzed the data. \u003cstrong\u003eLB\u003c/strong\u003e measured NO\u003csub\u003e3\u003c/sub\u003eRR selectivity and analyzed the electrochemical data. \u003cstrong\u003eFY\u0026nbsp;\u003c/strong\u003eassisted with the sample preparation and performed some of the \u003cem\u003eoperando\u0026nbsp;\u003c/em\u003eEC-TEM experiments.\u003cstrong\u003e\u0026nbsp;FF\u0026nbsp;\u003c/strong\u003eprovided the initial inspiration for the project and\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003edeveloped the experimental protocol for H-type cell measurements and product analysis. \u003cstrong\u003eCZ\u0026nbsp;\u003c/strong\u003eand \u003cstrong\u003eLB\u0026nbsp;\u003c/strong\u003econducted \u003cem\u003eoperando\u003c/em\u003e Raman measurements and analyzed the results. \u003cstrong\u003eMR and JT\u003c/strong\u003e planned and collected the \u003cem\u003eoperando\u0026nbsp;\u003c/em\u003eXAS data and performed the analysis and interpretation. \u003cstrong\u003eCP\u0026nbsp;\u003c/strong\u003eand \u003cstrong\u003eSW\u003c/strong\u003e planned the \u003cem\u003eoperando\u0026nbsp;\u003c/em\u003eEC-TXM measurements with \u003cstrong\u003eAY\u003c/strong\u003e and \u003cstrong\u003eSWC\u003c/strong\u003e, operated the TXM and helped with analysis of the TXM data. \u003cstrong\u003eMM and HS\u003c/strong\u003e were involved in the XAS measurements and helped with analysis and interpretation of the results.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAY and CZ thank the Alexander von Humboldt Foundation (AvH) for supporting them with an AvH postdoctoral research grant. L. Bai acknowledges the support from the Early Postdoc Mobility Fellowship (P2ELP2_199800) of Swiss National Science Foundation. FY acknowledges the Chinese Scholarship Council for sponsoring her PhD. This work was partially funded by the German Federal Ministry for Education and Research (BMBF) under the grant Catlab (03EW0015B), the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under project no. 406944504 \u0026ndash; SPP 2080 and Germany\u0026acute;s Excellence Strategy \u0026ndash; EXC 2008 \u0026ndash; 390540038 \u0026ndash; UniSysCat. AY also thanks the SPP 2080 \u0026ldquo;DynaKat\u0026rdquo; Early Career Research Scholarships for Female Scientists for partial funding. We are very grateful to Dr. Antonia Herzog, Dr. Uta Hejral, Dr. Arno Bergmann, and Shih-Yu Fu for their time and help with the XAS and TXM beamtime measurements. We further acknowledge Dr Andrea Martini and Dr. Christoph Scheurer for their helpful discussions with regards to analysis of the TXM data, Dr Eduardo Ortega for his help with some of the TEM data analysis and Mr. Walter Wachsmann for the ICP-MS measurements. Finally, we thank the Helmholtz-Zentrum Berlin f\u0026uuml;r Materialien und Energie (Berlin, Germany), and DESY (Hamburg, Germany), members of the Helmholtz Association HGF, for the allocation of dedicated synchrotron radiation beamtime.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGao D, Ar\u0026aacute;n-Ais RM, Jeon HS (2019) Roldan Cuenya, B. Rational catalyst and electrolyte design for CO2 electroreduction towards multicarbon products. Nat Catal 2:198\u0026ndash;210\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePopović S et al (2020) Stability and Degradation Mechanisms of Copper-Based Catalysts for Electrochemical CO2 Reduction. 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Nature 570:504\u0026ndash;508\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Wang C, Li M, Yu Y, Zhang B (2021) Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges. Chem Soc Rev 50:6720\u0026ndash;6733\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJeon HS et al (2021) Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO2 Pulsed Electroreduction. J Am Chem Soc 143:7578\u0026ndash;7587\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3310807/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3310807/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDetermining the active phase of an electrocatalyst at work is key to understanding its properties. However, the operating morphology of electrocatalysts is challenging to investigate because they can restructure into different motifs under applied potential due to changes in their oxidation state. These transformations will further alter their catalytic properties. Here, we employ a multi-modal approach centered on electrochemical liquid cell transmission electron microscopy (EC-TEM) to study the evolution of cubic Cu\u003csub\u003e2\u003c/sub\u003eO pre-catalysts during the electrocatalytic nitrate reduction reaction and unveil how redox kinetics determine the working catalyst morphology. We found drastic differences in catalyst restructuring during operation and a strong dependency of its composition on the applied potential and the chemical environment. Moreover, by matching the timescales of morphological changes observed in EC-TEM with time-resolved chemical state information obtained from \u003cem\u003eoperando\u003c/em\u003e transmission soft X-ray microscopy, hard X-ray absorption spectroscopy and Raman spectroscopy, we reveal that Cu\u003csub\u003e2\u003c/sub\u003eO can be kinetically stabilized for extended durations under moderately reductive conditions due to the formation of surface hydroxides. Finally, we rationalize how the interaction between the electrolyte and the catalyst influences the ammonia selectivity.\u003c/p\u003e","manuscriptTitle":"Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-30 16:25:31","doi":"10.21203/rs.3.rs-3310807/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-materials","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nmat","sideBox":"Learn more about [Nature Materials](http://www.nature.com/nmat/)","snPcode":"","submissionUrl":"","title":"Nature Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"eed3d629-cca0-4868-a97e-e77407f1e375","owner":[],"postedDate":"September 30th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":38222043,"name":"Physical sciences/Chemistry/Catalysis/Electrocatalysis"},{"id":38222044,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis"},{"id":38222045,"name":"Physical sciences/Materials science/Techniques and instrumentation/Imaging techniques"},{"id":38222046,"name":"Physical sciences/Materials science/Techniques and instrumentation/Microscopy/Transmission electron microscopy"}],"tags":[],"updatedAt":"2025-01-25T08:05:08+00:00","versionOfRecord":{"articleIdentity":"rs-3310807","link":"https://doi.org/10.1038/s41563-024-02084-8","journal":{"identity":"nature-materials","isVorOnly":false,"title":"Nature Materials"},"publishedOn":"2025-01-24 05:00:00","publishedOnDateReadable":"January 24th, 2025"},"versionCreatedAt":"2024-09-30 16:25:31","video":"","vorDoi":"10.1038/s41563-024-02084-8","vorDoiUrl":"https://doi.org/10.1038/s41563-024-02084-8","workflowStages":[]},"version":"v1","identity":"rs-3310807","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3310807","identity":"rs-3310807","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}