Operando Synthesis of High-Curvature Copper Thin Films for CO2 Electroreduction

As the sole metal that could reduce CO2 to substantial amounts of hydrocarbons, Cu plays an important role in electrochemical CO2 reduction, despite its low energy efficiency. Surface morphology modification is an effective method to improve its reaction activity and selectivity. Different from the pretreated modification method, in which the catalysts self-reconstruction process was ignored, we present operando synthesis by simultaneous electro-dissolution and electro-redeposition of copper during the CO2 electroreduction process. Through controlling the cathodic potential and CO2 flow rate, various high-curvature morphologies including microclusters, microspheres, nanoneedles, and nanowhiskers have been obtained, for which the real-time activity and product distribution is analyzed. The best CO2 electro-reduction activity and favored C2H4 generation activity, with around 10% faradic efficiency, can be realized through extensively distributed copper nanowhiskers synthesized under 40 mL/min flow rate and −2.1 V potential.


Introduction
Excessive CO 2 emission has caused severe climate and environment problems, which is breaking the sustainability of human society. Thus, there is an urgent demand for converting CO 2 into fuels or commodity chemicals. Compared with other methods of CO 2 conversion, including thermochemical, photochemical, and biochemical methods, the electrochemical reduction method is particularly important and attractive in view of the following merits: (1) Feasible combination with renewable sources like solar, wind, and nuclear energy; (2) low cost and safe operating conditions; (3) easy control over reaction pathways via changing the potential, electrocatalysts, and electrolytes. In recent years, there has been plenty of interest absorbed into the electrochemical reduction of CO 2 , which reduces carbon dioxide to multi-hydrocarbons at ambient temperature [1][2][3][4], facilitating the carbon cycle and relieving energy problems. Amongst various metal electrocatalysts studied in aqueous systems, Cu has a unique ability to reduce carbon dioxide to substantial amounts of hydrocarbons, such as CH 4 , C 2 H 4 , and HCOOH [5][6][7]. Unfortunately, the energy efficiency of Cu for CO 2 electroreduction is low, such that high over-potential must be performed to activate inert CO 2 molecules, which gives rise to many efforts for improving activity and selectivity of the CO 2 reduction reaction (CO 2 RR) on copper.
The intrinsic complexity of multi-electron transfer reaction, CO 2 RR activity, and selectivity was highly affected by the composition and morphology of catalysts, electrolytes, and pH value. Various metal catalysts could be categorized into four groups based on their product distributions: reduction reaction of Cu. A three-electrode cell including NiTi as working electrode, Pt mesh as counter electrode, and Ag/AgCl (1M Cl − ) as reference electrode was used. All potentials reported in this paper are quoted with Ag/AgCl. The polycrystalline Cu film was deposited at room temperature by chronoamperometry at −0.34 V. After 300 s deposition, the electrode was taken out and rinsed gently with deionized water several times.

Operando Synthesis of High-Curvature Copper During Electroreduction Of CO 2
Electroreduction of CO 2 was conducted at the ambient temperature in an H-type two compartment cell, as shown in Figure 1. A standard three-electrode system was used, employing Cu/NiTi working electrode, a Pt mesh counter electrode, and an Ag/AgCl reference electrode. A CO 2 -saturated 0.1 M KHCO 3 aqueous solution was used as the electrolyte for CO 2 RR. Firstly, a linear sweep voltammetry (LSV) technique was employed to get the CO 2 RR activity of the original copper thin film, and then a chronoamperometry technique was used to perform CO 2 RR for 1 h at four specific potentials with continuous CO 2 bubbles in the solution, during which process the electro-dissolution and electro-redeposition of copper ions occurred and resulted in high-curvature morphology. In addition, the CO 2 flow rate effect on the surface morphology of operando synthesized copper was studied.
Materials 2019, 12, x FOR PEER REVIEW 3 of 10 counter electrode, and Ag/AgCl (1M Cl − ) as reference electrode was used. All potentials reported in this paper are quoted with Ag/AgCl. The polycrystalline Cu film was deposited at room temperature by chronoamperometry at −0.34 V. After 300 s deposition, the electrode was taken out and rinsed gently with deionized water several times.

Operando Synthesis of High-Curvature Copper During Electroreduction Of CO2.
Electroreduction of CO2 was conducted at the ambient temperature in an H-type two compartment cell, as shown in Figure 1. A standard three-electrode system was used, employing Cu/NiTi working electrode, a Pt mesh counter electrode, and an Ag/AgCl reference electrode. A CO2-saturated 0.1 M KHCO3 aqueous solution was used as the electrolyte for CO2RR. Firstly, a linear sweep voltammetry (LSV) technique was employed to get the CO2 RR activity of the original copper thin film, and then a chronoamperometry technique was used to perform CO2RR for 1 h at four specific potentials with continuous CO2 bubbles in the solution, during which process the electro-dissolution and electro-redeposition of copper ions occurred and resulted in high-curvature morphology. In addition, the CO2 flow rate effect on the surface morphology of operando synthesized copper was studied.

Characterization of CO2RR Activity and Products Distribution of High-Curvature Copper.
I-t curves at different bias potentials during operando synthesis of copper were shown to evaluate the activity of CO2RR, and a gas chromatograph (GC) was connected to the vent of H-type electrochemical cell for gaseous products real-time analysis. The GC was equipped with a thermal conductivity detector (TCD) to quantify hydrogen and a hydrogen flame ionization detector (FID), equipped with a methanizer to quantify carbon monoxide, methane, and ethylene, was connected to the vent of the H-type electrochemical cell for real-time gaseous products analysis. The parameters of the GC were set as follows: Oven temperature was 360 °C, TCD temperature was 120 °C, FID temperature was 150 °C, and the column temperature was 70 °C.

Characterization of Surface Morphology and Composition of Copper Thin Films.
Samples were rinsed gently with deionized water after a 1 h reaction. The surface morphology and composition of the Cu film, synthesized in situ under different potentials and different CO2 flow rates, were analyzed through a scanning electron microscope (SEM) and energy spectrum analysis (EDS).

Similar Morphology and Catalytic Activity of Original Electrodeposited Films
Original morphology of electrodeposited copper thin film is shown in Figure 2a, which indicates the typical granular morphology arising from island growth in eletrodeposition. It is noted that four samples swept to different end potentials had a similar granular morphology with the similar roughness and sub-micro particle size. Linear sweep voltammetry (LSV), at a scan rate of 50

Characterization of CO 2 RR Activity and Products Distribution of High-Curvature Copper
I-t curves at different bias potentials during operando synthesis of copper were shown to evaluate the activity of CO 2 RR, and a gas chromatograph (GC) was connected to the vent of H-type electrochemical cell for gaseous products real-time analysis. The GC was equipped with a thermal conductivity detector (TCD) to quantify hydrogen and a hydrogen flame ionization detector (FID), equipped with a methanizer to quantify carbon monoxide, methane, and ethylene, was connected to the vent of the H-type electrochemical cell for real-time gaseous products analysis. The parameters of the GC were set as follows: Oven temperature was 360 • C, TCD temperature was 120 • C, FID temperature was 150 • C, and the column temperature was 70 • C.

Characterization of Surface Morphology and Composition of Copper Thin Films
Samples were rinsed gently with deionized water after a 1 h reaction. The surface morphology and composition of the Cu film, synthesized in situ under different potentials and different CO 2 flow rates, were analyzed through a scanning electron microscope (SEM) and energy spectrum analysis (EDS).

Similar Morphology and Catalytic Activity of Original Electrodeposited Films
Original morphology of electrodeposited copper thin film is shown in Figure 2a, which indicates the typical granular morphology arising from island growth in eletrodeposition. It is noted that four samples swept to different end potentials had a similar granular morphology with the similar roughness and sub-micro particle size. Linear sweep voltammetry (LSV), at a scan rate of 50 mV/s, is used to provide a quantitative assessment of the catalytic performance of four studied electrodes. We note that they have the same onset potential of about −0.9 V and almost the same CO 2 RR kinetics (indicated by the slope of LSV curves in the Figure 2b). mV/s, is used to provide a quantitative assessment of the catalytic performance of four studied electrodes. We note that they have the same onset potential of about −0.9 V and almost the same CO2RR kinetics (indicated by the slope of LSV curves in the Figure 2b).

Effect of Cathodic Potential on the Morphology and Product Selectivity of Copper Films
It is known that cathodic potential, as an important parameter in CO2RR, not only determines the pathway of reaction but also affects the copper reconstruction process (shown schematically in Figure 3), e.g., more negative potentials could facilitate the nucleation rate of copper [27][28][29][30]. Therefore, it would be informative to probe the correlations between the as-formed morphology and the resultant CO2RR activity. Depending on the applied potential, different structural morphologies emerged. Various morphologies of operando-synthesized copper could be observed, shown in Figure 4. At −1.7 V, the surface is mainly covered with microspheres, with a diameter of about 4~6 μm, and scattered nanometer needles, with a length of about 1~3 μm, which changed to a flower-like nanoneedle, with a length of about 5~7 μm, at −1.9 V. As the overpotential increased to −2.1 V, sharper nanowhiskers, with the length of about 6~10 μm and a length-to-diameter ratio of 30~70, appeared rapidly and then turned into a uniformly dispersed short length of about 3 μm nanowhiskers, with a length-to-diameter ratio of 5~10, at −2.3 V. This morphology evolution was due to tip effect and hydrogen bubble generation (which served as dynamic template for redeposition) during the simultaneous dissolution and redeposition of Cu ions. We also analyzed the element composition of the high-curvature morphologies (shown in Figure 5 and Table 1). The high-curvature morphology is composed of a mixture of Cu and its oxidation state. Such copper oxides would contribute to relatively higher CO2RR activity than pure Cu [31].

Effect of Cathodic Potential on the Morphology and Product Selectivity of Copper Films
It is known that cathodic potential, as an important parameter in CO 2 RR, not only determines the pathway of reaction but also affects the copper reconstruction process (shown schematically in Figure 3), e.g., more negative potentials could facilitate the nucleation rate of copper [27][28][29][30]. Therefore, it would be informative to probe the correlations between the as-formed morphology and the resultant CO 2 RR activity. Depending on the applied potential, different structural morphologies emerged. Various morphologies of operando-synthesized copper could be observed, shown in Figure 4. At −1.7 V, the surface is mainly covered with microspheres, with a diameter of about 4~6 µm, and scattered nanometer needles, with a length of about 1~3 µm, which changed to a flower-like nanoneedle, with a length of about 5~7 µm, at −1.9 V. As the overpotential increased to −2.1 V, sharper nanowhiskers, with the length of about 6~10 µm and a length-to-diameter ratio of 30~70, appeared rapidly and then turned into a uniformly dispersed short length of about 3 µm nanowhiskers, with a length-to-diameter ratio of 5~10, at −2.3 V. This morphology evolution was due to tip effect and hydrogen bubble generation (which served as dynamic template for redeposition) during the simultaneous dissolution and redeposition of Cu ions. We also analyzed the element composition of the high-curvature morphologies (shown in Figure 5 and Table 1). The high-curvature morphology is composed of a mixture of Cu and its oxidation state. Such copper oxides would contribute to relatively higher CO 2 RR activity than pure Cu [31].
Materials 2019, 12, x FOR PEER REVIEW 4 of 10 mV/s, is used to provide a quantitative assessment of the catalytic performance of four studied electrodes. We note that they have the same onset potential of about −0.9 V and almost the same CO2RR kinetics (indicated by the slope of LSV curves in the Figure 2b).

Effect of Cathodic Potential on the Morphology and Product Selectivity of Copper Films
It is known that cathodic potential, as an important parameter in CO2RR, not only determines the pathway of reaction but also affects the copper reconstruction process (shown schematically in Figure 3), e.g., more negative potentials could facilitate the nucleation rate of copper [27][28][29][30]. Therefore, it would be informative to probe the correlations between the as-formed morphology and the resultant CO2RR activity. Depending on the applied potential, different structural morphologies emerged. Various morphologies of operando-synthesized copper could be observed, shown in Figure 4. At −1.7 V, the surface is mainly covered with microspheres, with a diameter of about 4~6 μm, and scattered nanometer needles, with a length of about 1~3 μm, which changed to a flower-like nanoneedle, with a length of about 5~7 μm, at −1.9 V. As the overpotential increased to −2.1 V, sharper nanowhiskers, with the length of about 6~10 μm and a length-to-diameter ratio of 30~70, appeared rapidly and then turned into a uniformly dispersed short length of about 3 μm nanowhiskers, with a length-to-diameter ratio of 5~10, at −2.3 V. This morphology evolution was due to tip effect and hydrogen bubble generation (which served as dynamic template for redeposition) during the simultaneous dissolution and redeposition of Cu ions. We also analyzed the element composition of the high-curvature morphologies (shown in Figure 5 and Table 1). The high-curvature morphology is composed of a mixture of Cu and its oxidation state. Such copper oxides would contribute to relatively higher CO2RR activity than pure Cu [31].     CO2RR activity of various high-curvature coppers was revealed from chronoamperometric i-t curves. With continuous CO2 bubbled at a flow rate of 40 mL/min, the chronoamperometry technique was adopted to perform CO2RR for 1 h at four specific potentials. As shown in Figure 6c, the reduction current increased with more negative potentials. Meanwhile, the total reduction current of different reaction potential was stable. Though the total reaction current was as high as about 45 mA/cm 2 at −2.3 V, it was revealed by gas chromatography that the major reduction current owed to the electroreduction of water (2H2O* + 2e − →H2 + 2OH − ) instead of CO2. For CO2RR, the main gas products were CO, CH4 and C2H4, which were monitored every 15 min. As shown in Figure 6a, the percentage of the three main CO2RR products changed with bias potentials. At −1.7 V and −1.9 V, CO was the dominant product with percentages of 59% and 73%. When the more negative potential was applied for operando synthesis, like −2.1 V and −2.3 V, C2H4 proportion increased to 65% and 60% of the three main gas products as a preferred product. It was speculated that higher local electric field around high-curvature nanowhiskers of copper could improve bubble nucleation rates and cation stabilization, which favored C2H4 formation during electroreduction of CO2 [15,27,32]. As was shown by the energy spectrum, high-curvature morphology contained the oxidation state of Cu, which also made the electrode exhibit good ethylene selectivity [33,34]. Additionally, we also compared C2/C1 product ratios of various high-curvature copper (Figure 6b), and found that widely distributed nanowhiskers of copper had a high C2/C1 product ratio (about 1:6), which was more than five times than that synthesized under −1.7 V and −1.9 V. Moreover, C2/C1 product ratio of different morphological copper fluctuated slightly within 1 h of the operando synthesis process, as shown in Figure 6b, and it was surprising to see an increasing C2/C1 product ratio of nanorod copper, synthesized under −2.3 V, from 1.1-at the first quarter to 1.8 at the fourth quarter of the hour. This be    CO2RR activity of various high-curvature coppers was revealed from chronoamperometric i-t curves. With continuous CO2 bubbled at a flow rate of 40 mL/min, the chronoamperometry technique was adopted to perform CO2RR for 1 h at four specific potentials. As shown in Figure 6c, the reduction current increased with more negative potentials. Meanwhile, the total reduction current of different reaction potential was stable. Though the total reaction current was as high as about 45 mA/cm 2 at −2.3 V, it was revealed by gas chromatography that the major reduction current owed to the electroreduction of water (2H2O* + 2e − →H2 + 2OH − ) instead of CO2. For CO2RR, the main gas products were CO, CH4 and C2H4, which were monitored every 15 min. As shown in Figure 6a, the percentage of the three main CO2RR products changed with bias potentials. At −1.7 V and −1.9 V, CO was the dominant product with percentages of 59% and 73%. When the more negative potential was applied for operando synthesis, like −2.1 V and −2.3 V, C2H4 proportion increased to 65% and 60% of the three main gas products as a preferred product. It was speculated that higher local electric field around high-curvature nanowhiskers of copper could improve bubble nucleation rates and cation stabilization, which favored C2H4 formation during electroreduction of CO2 [15,27,32]. As was shown by the energy spectrum, high-curvature morphology contained the oxidation state of Cu, which also made the electrode exhibit good ethylene selectivity [33,34]. Additionally, we also compared C2/C1 product ratios of various high-curvature copper (Figure 6b), and found that widely distributed nanowhiskers of copper had a high C2/C1 product ratio (about 1:6), which was more than five times than that synthesized under −1.7 V and −1.9 V. Moreover, C2/C1 product ratio of different morphological copper fluctuated slightly within 1 h of the operando synthesis process, as shown in Figure 6b, and it was surprising to see an increasing C2/C1 product ratio of nanorod copper, synthesized under −2.3 V, from 1.1-at the first quarter to 1.8 at the fourth quarter of the hour. This be  CO 2 RR activity of various high-curvature coppers was revealed from chronoamperometric i-t curves. With continuous CO 2 bubbled at a flow rate of 40 mL/min, the chronoamperometry technique was adopted to perform CO 2 RR for 1 h at four specific potentials. As shown in Figure 6c, the reduction current increased with more negative potentials. Meanwhile, the total reduction current of different reaction potential was stable. Though the total reaction current was as high as about 45 mA/cm 2 at −2.3 V, it was revealed by gas chromatography that the major reduction current owed to the electroreduction of water (2H 2 O* + 2e − →H 2 + 2OH − ) instead of CO 2 . For CO 2 RR, the main gas products were CO, CH 4 and C 2 H 4 , which were monitored every 15 min. As shown in Figure 6a, the percentage of the three main CO 2 RR products changed with bias potentials. At −1.7 V and −1.9 V, CO was the dominant product with percentages of 59% and 73%. When the more negative potential was applied for operando synthesis, like −2.1 V and −2.3 V, C 2 H 4 proportion increased to 65% and 60% of the three main gas products as a preferred product. It was speculated that higher local electric field around high-curvature nanowhiskers of copper could improve bubble nucleation rates and cation stabilization, which favored C 2 H 4 formation during electroreduction of CO 2 [15,27,32]. As was shown by the energy spectrum, high-curvature morphology contained the oxidation state of Cu, which also made the electrode exhibit good ethylene selectivity [33,34]. Additionally, we also compared C 2 /C 1 product ratios of various high-curvature copper (Figure 6b), and found that widely distributed nanowhiskers of copper had a high C 2 /C 1 product ratio (about 1:6), which was more than five times than that synthesized under −1.7 V and −1.9 V. Moreover, C 2 /C 1 product ratio of different morphological copper fluctuated slightly within 1 h of the operando synthesis process, as shown in Figure 6b, and it was surprising to see an increasing C 2 /C 1 product ratio of nanorod copper, synthesized under −2.3 V, from 1.1-at the first quarter to 1.8 at the fourth quarter of the hour. This be caused by the increase of the nanowhisker distribution area in the copper electro-dissolution and electro-redeposition process during 1 h CO 2 RR.

Effect of CO2 Flow Rate on the Morphology and Product Selectivity of Copper Films
Flow rate was another important parameter that affected the mass transfer of CO2, the copper reconstruction process, and CO2 electroreduction [33][34][35]. In the following, the effects of CO2 flow rate on the morphology of operando synthesis copper and the resulting CO2RR activity and selectivity were studied. Here, various morphologies of copper were synthesized in situ under different flow rates of CO2, 20 mL/min, 40 mL/min, 60 mL/min, and 80 mL/min, with the same bias potential at −2.1 V for 1 h in the solution. Under the lower flow rate of 20 mL/min, the morphology of copper surface became rougher and only short and thick microclusters (about 1 μm thick) dispersed on the granular copper. Cu nanowhiskers morphology emerged under 40 mL/min flow rate, a combination of micro polygon particles and rod-like copper dispersed sparsely under 60 mL/min flow rate, and a relative flat surface without any high-curvature structures exhibited when CO2 was bubbled at a flow rate of 80 mL/min (Figure 7). We also employed the chronoamperometry technique to perform CO2RR for 1 h at four different flow rates. As expected, less active sites on the flat surface of copper rendered to the lower activity of CO2RR (Figure 8d), and overall current density was about 28 mA/cm 2 under an 80 mL/min flow rate (compared to about 40 mA/cm 2 under a lower flow rate). Nevertheless, the total reduction current under each CO2 flow rate still remained stable. More distinct structure-property correlation was revealed by faradaic efficiency (FE) of gaseous products of various morphologies of copper, shown in Figure 8a-c. Optimal faradic efficiency for CO2RR was obtained on copper nanowhiskers at the third quarter of an hour under 40 mL/min flow rate, which was 1.8% ± 0.3% for CO, 1.6% ± 0.24% for CH4, and 9.9% ± 1.3% for C2H4, respectively, and the total current density was about 43 mA/cm 2 . When comparing the overall faradaic efficiency during an hour of CO2 electroreduction, nanowhiskers exhibited higher faradic efficiency for C2H4 than the other morphologies and microcluster copper synthesized under 20 mL/min flow rate performed at a higher faradic efficiency for CH4, about 5%. Combinational morphology of micro polygon and rod-like particles presented the lowest faradic efficiency for CO and CH4 production.

Effect of CO 2 Flow Rate on the Morphology and Product Selectivity of Copper Films
Flow rate was another important parameter that affected the mass transfer of CO 2 , the copper reconstruction process, and CO 2 electroreduction [33][34][35]. In the following, the effects of CO 2 flow rate on the morphology of operando synthesis copper and the resulting CO 2 RR activity and selectivity were studied. Here, various morphologies of copper were synthesized in situ under different flow rates of CO 2 , 20 mL/min, 40 mL/min, 60 mL/min, and 80 mL/min, with the same bias potential at −2.1 V for 1 h in the solution. Under the lower flow rate of 20 mL/min, the morphology of copper surface became rougher and only short and thick microclusters (about 1 µm thick) dispersed on the granular copper. Cu nanowhiskers morphology emerged under 40 mL/min flow rate, a combination of micro polygon particles and rod-like copper dispersed sparsely under 60 mL/min flow rate, and a relative flat surface without any high-curvature structures exhibited when CO 2 was bubbled at a flow rate of 80 mL/min (Figure 7). We also employed the chronoamperometry technique to perform CO 2 RR for 1 h at four different flow rates. As expected, less active sites on the flat surface of copper rendered to the lower activity of CO 2 RR (Figure 8d), and overall current density was about 28 mA/cm 2 under an 80 mL/min flow rate (compared to about 40 mA/cm 2 under a lower flow rate). Nevertheless, the total reduction current under each CO 2 flow rate still remained stable. More distinct structure-property correlation was revealed by faradaic efficiency (FE) of gaseous products of various morphologies of copper, shown in Figure 8a-c. Optimal faradic efficiency for CO 2 RR was obtained on copper nanowhiskers at the third quarter of an hour under 40 mL/min flow rate, which was 1.8% ± 0.3% for CO, 1.6% ± 0.24% for CH 4 , and 9.9% ± 1.3% for C 2 H 4 , respectively, and the total current density was about 43 mA/cm 2 . When comparing the overall faradaic efficiency during an hour of CO 2 electroreduction, nanowhiskers exhibited higher faradic efficiency for C 2 H 4 than the other morphologies and microcluster copper synthesized under 20 mL/min flow rate performed at a higher faradic efficiency for CH 4 , about 5%. Combinational morphology of micro polygon and rod-like particles presented the lowest faradic efficiency for CO and CH 4 production.

Operando-Synthesized Mechanism and Structure-Property Correlation
As shown above, proper cathodic potential and CO2 flow rate should be chosen in order to obtain the proper ratio of nucleation rate to the directional growth rate of copper, which ensures operando synthesis of various high-curvature coppers. Presumably, high-curvature morphologies originated from constricted lateral growth due to hydrogen bubbles and the concentrated electric field around tips for copper redeposition. The observed change in morphology correlated directly with CO2RR activity and selectivity, that is, copper nanowhiskers with larger length-to-diameter ratio exhibited higher faradic efficiency and favor ethylene production. As studied previously, sharp nanowhiskers could improve bubble nucleation, concentrate stabilizing cations and exhibit high field locally [27], all of which, along with higher local pH, boost the reaction, limiting the protonation of bound CO* intermediate that leads to ethylene formation.

Conclusion
In summary, various high-curvature morphologies of copper were operando synthesized during a CO2 electroreduction process by controlling cathodic potential and CO2 flow rate. The electrodeposited granular copper evolved to microspheres, nanoneedles, and nanowhiskers, respectively, when biased at −1.7 V~ −2.3 V under 40 mL/min flow rate. When biased at −2.1 V with different flow rates of CO2, rough surfaces with scattered microcluster morphology emerged under 20 mL/min flow rate, and extensive nanowhisker morphology emerged under 40 mL/min. Higher flow rates of 60 and 80 mL/min failed to form sharp whisker-like morphology but scattered distributed particle-rod-combination morphology and non-cluster surfaces instead. Benefiting from such in-situ synthesized morphologies, real-time CO2RR activity and products selectivity were studied, and the correlation between morphology and activity was also discussed. The reduction product was mainly CO at lower overpotential, while high-valued ethylene became the main reduction product when increasing the cathodic potential and the synthesis of high-curvature morphology. Morphologies synthesized under 20 mL/min and 40 mL/min flow rate showed relatively high CO2RR activity, that is, rough surfaces exhibited with microcluster morphology

Operando-Synthesized Mechanism and Structure-Property Correlation
As shown above, proper cathodic potential and CO2 flow rate should be chosen in order to obtain the proper ratio of nucleation rate to the directional growth rate of copper, which ensures operando synthesis of various high-curvature coppers. Presumably, high-curvature morphologies originated from constricted lateral growth due to hydrogen bubbles and the concentrated electric field around tips for copper redeposition. The observed change in morphology correlated directly with CO2RR activity and selectivity, that is, copper nanowhiskers with larger length-to-diameter ratio exhibited higher faradic efficiency and favor ethylene production. As studied previously, sharp nanowhiskers could improve bubble nucleation, concentrate stabilizing cations and exhibit high field locally [27], all of which, along with higher local pH, boost the reaction, limiting the protonation of bound CO* intermediate that leads to ethylene formation.

Conclusion
In summary, various high-curvature morphologies of copper were operando synthesized during a CO2 electroreduction process by controlling cathodic potential and CO2 flow rate. The electrodeposited granular copper evolved to microspheres, nanoneedles, and nanowhiskers, respectively, when biased at −1.7 V~ −2.3 V under 40 mL/min flow rate. When biased at −2.1 V with different flow rates of CO2, rough surfaces with scattered microcluster morphology emerged under 20 mL/min flow rate, and extensive nanowhisker morphology emerged under 40 mL/min. Higher flow rates of 60 and 80 mL/min failed to form sharp whisker-like morphology but scattered distributed particle-rod-combination morphology and non-cluster surfaces instead. Benefiting from such in-situ synthesized morphologies, real-time CO2RR activity and products selectivity were studied, and the correlation between morphology and activity was also discussed. The reduction product was mainly CO at lower overpotential, while high-valued ethylene became the main reduction product when increasing the cathodic potential and the synthesis of high-curvature morphology. Morphologies synthesized under 20 mL/min and 40 mL/min flow rate showed relatively high CO2RR activity, that is, rough surfaces exhibited with microcluster morphology

Operando-Synthesized Mechanism and Structure-Property Correlation
As shown above, proper cathodic potential and CO 2 flow rate should be chosen in order to obtain the proper ratio of nucleation rate to the directional growth rate of copper, which ensures operando synthesis of various high-curvature coppers. Presumably, high-curvature morphologies originated from constricted lateral growth due to hydrogen bubbles and the concentrated electric field around tips for copper redeposition. The observed change in morphology correlated directly with CO 2 RR activity and selectivity, that is, copper nanowhiskers with larger length-to-diameter ratio exhibited higher faradic efficiency and favor ethylene production. As studied previously, sharp nanowhiskers could improve bubble nucleation, concentrate stabilizing cations and exhibit high field locally [27], all of which, along with higher local pH, boost the reaction, limiting the protonation of bound CO* intermediate that leads to ethylene formation.

Conclusions
In summary, various high-curvature morphologies of copper were operando synthesized during a CO 2 electroreduction process by controlling cathodic potential and CO 2 flow rate. The electrodeposited granular copper evolved to microspheres, nanoneedles, and nanowhiskers, respectively, when biased at −1.7 V~−2.3 V under 40 mL/min flow rate. When biased at −2.1 V with different flow rates of CO 2 , rough surfaces with scattered microcluster morphology emerged under 20 mL/min flow rate, and extensive nanowhisker morphology emerged under 40 mL/min. Higher flow rates of 60 and 80 mL/min failed to form sharp whisker-like morphology but scattered distributed particle-rod-combination morphology and non-cluster surfaces instead. Benefiting from such in-situ synthesized morphologies, real-time CO 2 RR activity and products selectivity were studied, and the correlation between morphology and activity was also discussed. The reduction product was mainly CO at lower overpotential, while high-valued ethylene became the main reduction product when increasing the cathodic potential and the synthesis of high-curvature morphology. Morphologies synthesized under 20 mL/min and 40 mL/min flow rate showed relatively high CO 2 RR activity, that is, rough surfaces exhibited with microcluster morphology favored methane and ethylene selectivity with C 2 /C 1 ratio about 1, and exhibited nanowhisker morphology favored ethylene selectivity (around 10% faradaic efficiency) with C 2 /C 1 ratio of 1.8. According to previous mechanistic studies, the reaction pathways of CH 4 and C 2 H 4 differ in the bound CO* intermediates [36][37][38][39]. Both morphologies synthesized under 20 mL/min and 40 mL/min flow rate stabilized the bound CO* and prevented its desorption, which resulted in not only high reaction activity but also the further reaction pathways by hydrogenation of CO* or dimerization of two bound CO*. For rough Cu with microcluster morphology, the two aforementioned pathways were coexisting and equi-favored, thus FE for methane and ethylene were both around 5%. For nanowhisker morphology, CO* hydrogenation to COH* could be suppressed by the existence of copper oxide and high local pH [37,38], thus shifting the reaction towards C 2 H 4 by stabilizing the OCCOH* intermediates (CO* + CO* + H + + e − → OCCOH* → C 2 H 4 ).
Hopefully, such operando synthesized copper and its direct effect of morphology on product distribution could inspire more attention to catalyst self-reconstruction in the design of advanced electrocatalysts and analysis of reaction mechanisms.