Diamond Decorated with Copper Nanoparticles for Electrochemical

May 29, 2013 - Progress and Perspective of Electrocatalytic CO 2 Reduction for Renewable Carbonaceous Fuels and Chemicals. Wenjun Zhang , Yi Hu , Lian...
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Diamond Decorated with Copper Nanoparticles for Electrochemical Reduction of Carbon Dioxide Nianjun Yang,* Fang Gao, and Christoph E. Nebel Fraunhofer Institute for Applied Solid State Physics (IAF), Tullastrasse 72, 79108 Freiburg, Germany S Supporting Information *

ABSTRACT: Electrochemical CO2 reduction has been investigated on a planar diamond electrode in aqueous and nonaqueous solutions. On a diamond electrode decorated with copper nanoparticles, CO2 reduction starts from −0.1 V versus a normal hydrogen electrode (NHE) when a mixture of water and ionic liquid ([H2O] = 10 μM) is used. The current density reaches 5.1 ± 0.1 mA cm−2 for CO2 reduction at a potential of −1.3 V versus NHE. The main products are formic acid and formaldehyde. Moreover, the electrode system is stable and has a long lifetime. It is thus promising to be applied in the future for mass production of industrial chemicals and liquid fuels using CO2 as the source of raw material.

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the deactivation of the electrode. Furthermore, its carbon surface chemistry brings a large variety of functionalization possibility, providing tunable and versatile platforms.6 Organic metal complexes and metal oxides have been immobilized on diamond electrodes to improve the selectivity of CO 2 reduction.6 In this way, the loss of active catalytic centers will be minimized. In addition, “dirty layers” accumulated by fouling or deposited on the diamond surface can be facilely removed with different methods (e.g., plasma treatment, electrochemical cleaning process, wet-chemical boiling, etc.). Thus, it is possible to reactivate or refresh diamond surfaces.5 As for the solvent, room temperature ionic liquid (RTIL) will be the best solvent and supporting electrolyte. The potential window of a diamond electrode (defined with the absolute current density of 1.0 mA cm−2 as a critical value) in RTIL is widest and about 5.8 V, which is wider than those in aqueous solutions and in organic solutions (Supporting Information, Figure S1). In RTIL, CO2 has 6−10 times higher solubility than in aqueous solutions and also lower reduction potentials.7,8 Moreover, using RTIL reduces dramatically the amount of impurities and contaminations. Therefore, higher current density and better selectivity is expected in RTIL for electrochemical CO2 reduction. Herein, we report electrochemical CO2 reduction on a planar boron-doped diamond electrode in aqueous and nonaqueous solutions. As one of best and most efficient catalysts for electrochemical CO2 reduction, copper nanoparticles have been utilized to enhance the selectivity of products and the efficiency of the reduction.1−3 RTIL has been used as the solvent to

lectrochemical and photoelectrochemical reduction of carbon dioxide has been extensively studied on metal and semiconductor electrodes.1 Depending on the electrode material and the potential applied, carbon dioxide has been converted into different chemicals, including carbon monoxide, hydrocarbons, alcohols, and organic acids.1,2 Electrochemical CO2 reduction has been thus suggested as a potential way to synthesize useful industrial chemicals and liquid fuels and finally to decrease global warming. However, on most electrodes, the current density for electrochemical CO2 reduction is smaller than 1.0 mA cm−2 at ambient temperature and pressure. The selectivity of products is also poor. Carbon monoxide and hydrogen are the main byproducts.1,2 The conversion efficiencies, defined as the ratio of the free energy of the products obtained to that consumed, are as low as about 30− 40%. Higher conversion efficiency has been shown to be possible, but high potentials over −1.5 V versus a normal hydrogen electrode (NHE) are required.2,3 In addition, deactivation of metal electrodes and/or the catalysts has been confirmed as an unavoidable defect during electrochemical CO2 reduction.1 In order to improve the selectivity of products, to reduce potentials for CO2 reduction, to have long lifetimes of electrode systems, and eventually to increase the conversion efficiencies, novel construction of electrode systems (including electrode material, the solvent, and the catalyst) are highly required. Boron-doped diamond will be the best electrode material for electrochemical CO2 reduction.4 It has shown many advantages over metal and other semiconductor electrodes.4−6 For example, it has a high overpotential for hydrogen evolution, which is especially useful to provide high selectivity during electrochemical CO2 reduction, since hydrogen generation is often one of the unwanted competing side reactions. It is stable in different harsh media, thus minimizing electrocorrosion and © 2013 American Chemical Society

Received: February 4, 2013 Accepted: May 14, 2013 Published: May 29, 2013 5764

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improve CO2 solubility and to realize electrochemical CO2 reduction at lower potentials. In the mixture of water and RTIL ([H2O] = 10 μM), CO2 reduction starts from −0.1 V versus NHE on a planar diamond electrode coated with copper nanoparticles. At a potential of −1.3 V versus NHE, this reduction current further increases into 5.1 ± 0.1 mA cm−2. Under these conditions, the main products are formic acid and formaldehyde.

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EXPERIMENTAL DETAILS Materials. Boron-doped diamond (BDD) films were grown in a microwave-enhanced chemical vapor deposition reactor, and trimethylboron (TMB) was used as the boron source.9a The boron concentration detected by secondary ion mass spectrometry is (3−5) × 1020 cm−3. The deposition of copper nanoparticles on a planar diamond electrode has been performed using a wet-chemical synthesis method.9b At first, 25 μL of 1.0 M NaBH4 in 0.1 M NaOH solution was dropped onto a clean diamond electrode for the adsorption of NaBH4. The electrode was then rinsed with water and blow dried with N2 to remove excessive NaBH4. The formation of copper nanoparticles was done by adding a 25 μL of 1.0 mM CuCl2 solution onto the electrodes. After that, the electrode was again cleaned with water and blow dried with N2. After washing copiously, a copper nanoparticle coated diamond electrode was fabricated. Methods. Scanning electron microscope (SEM) images were recorded on a SEM (Hitachi 4500, Japan) at an acceleration voltage of 30 kV. Electrochemical experiments were conducted on a Biological multi-channel potentiostat (Biological Inc., France) with a three-electrode configuration. A platinum wire acted as the counter electrode, and a Ag/AgxO electrode behaved as the reference electrode. The potential difference of a Ag/AgxO reference electrode from a NHE is −0.3 V. The working electrode was a diamond electrode, either a planar diamond electrode or a copper nanoparticle coated diamond electrode. Before saturating aqueous and nonaqueous solutions with CO2, the solutions were first purged with nitrogen for at least 15 min and then with CO2 for 30−60 min. The experiments in RTIL 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6) were conducted in a nitrogen gas protected glovebox. For product analysis experiments, a two-compartment electrolysis cell was applied. After saturating the solutions with CO2, a potential of −1.3 V versus NHE was applied for 1.5 h. The headspace of the cathodic chamber was continuously purged with CO2 into the sampling loop of a gas chromatograph (GC) to enable periodic quantification of the gas-phase products. The solution-phase products were quantified by highperformance liquid chromatography (HPLC) analysis of the electrolyte at the conclusion of the electrolysis.

Figure 1. SEM image of a planar diamond electrode decorated with copper nanoparticles.

Copper nanoparticles cover the whole diamond electrode surface. They are uniform in size and homogeneously distributed. Please note here that the homogeneous deposition of copper nanoparticles was done in a wet-chemical way.9b Electrochemical deposition led to often heterogeneously distribution of copper nanoparticles on diamond electrodes due to the heterogeneous electronic properties of a borondoped diamond electrode.9b−e Similar results have been reported about the deposition of gold nanoparticles on a diamond electrode as well.9e Systemic analysis of these particles with respect to their size and size distribution has been done by counting. These particles have sizes of 15(±5) nm, indicating a narrow size distribution. The density of these copper nanoparticles is about 4.6(±0.5) × 1010 cm−2, resulting in a surface coverage of 81(±1.0)%. Larger copper nanoparticles up to 1.0 μm in diameter were tried by electrochemical overgrowth. In these cases, the densities of copper nanoparticles decreased by 2−3 orders, resulting in lower surface coverage of copper nanoparticles on the diamond. For most of the measurements shown in this paper, we used diamond electrodes coated with copper nanoparticles having sizes of 15(±5) nm, a density of 4.6(±0.5) × 1010 cm−2, and surface coverage of 81(±1.0)%. These copper nanoparticles have been applied as the catalysts for electrochemical CO2 reduction in aqueous and nonaqueous solutions. CO2 Reduction in Aqueous Solutions. Figure 2 shows linear sweep voltammograms of a planar diamond electrode in a

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RESULTS AND DISCUSSION Deposition of Copper Nanoparticles on Diamond. It is known that metal nanoparticles generated using electrochemical methods are not homogeneously distributed on a diamond surface due to the nonuniform electronic properties of a diamond electrode.9b−e Recently, we developed a method to synthesize uniform and homogeneous metal nanoparticles on a diamond surface by use of a wet-chemical synthesis approach.9b Figure 1 shows one SEM image of copper nanoparticles deposited on a planar diamond electrode with this approach.

Figure 2. Linear sweep voltammograms of a planar diamond electrode in 0.5 M LiClO4 solution at a scan rate of 0.01 V s−1 (a) before and (b, c) after saturating with CO2. In curve c, the electrode is coated with copper nanoparticles. 5765

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0.5 M LiClO4 solution (a) before and (b) after saturating with CO2. Due to a high overpotential for hydrogen evolution on the diamond electrode, in a wide potential range only the background current is seen. Hydrogen evolution starts when the potential is negative than −1.3 V versus NHE, similar as those reported.10 After saturating the solution with CO2, a broad and steady-state-like reduction wave appears at a slow scan rate (e.g., 0.01 V s−1) in the potential range of −0.5 to −1.5 V versus NHE. The magnitude of this wave decreases if the solution with lower CO2 concentration (via bubbling at lower speed or with shorter time) has been used. The limiting cathodic current results from electrochemical reduction of saturated CO2 near the electrode surface. This type of limit current was noticed previously for electrochemical CO2 reduction in a NaHCO3 solution on a copper electrode.11 To get deeper insight into these mass transport limited processes (cathodic currents), more experiments such as impedance measurements at different potentials in the solutions with different CO2 concentration (via different bubbling times and speeds) are required. An increase of scan rate leads to shape changes of the voltammograms, for example, from steady-statelike voltammograms into peak-shaped ones (Supporting Information, Figure S2), indicating a transition from convergent to planar diffusion as the time scale of the measurement changes. Figure 2c shows the voltammogram of CO2 reduction on a planar diamond electrode decorated with copper nanoparticles. A two-step, steady-state-like reduction process is seen. The first steady-state-like wave appears in the range of −0.1 to −0.5 V versus NHE, and the second one is in the range of −1.0 to −1.4 V versus NHE. In other words, the involvement of copper catalyst makes CO2 reduction reaction start at −0.1 V versus NHE. This potential for CO2 reduction is much lower than most of reported electrodes system,1−3,10,12 indicating high efficiency of this electrode combination. The variation of the sizes and the densities of copper nanoparticles did not alter the starting potential for CO2 reduction (−0.1 V vs NHE) if the surface coverage of copper nanoparticles is higher than 10%. Lower surface coverage of copper nanoparticles than 10% shifted the starting potential of CO2 reduction in a negative direction. For example, a surface coverage of 5% for copper nanoparticles showed a starting potential for CO2 reduction at about −0.3 V versus NHE. Moreover, once diamond electrodes were applied with higher surface coverage of copper nanoparticles, the reduction currents increased in the potential range of −0.1 to −1.4 V versus NHE. With surface coverages of 10%, 20%, and 40% for copper nanoparticles, the current densities for CO2 reduction at the potential of −1.3 V versus NHE were −0.05, −0.13, and −0.30 mA cm−2, respectively. The current densities at the potential of −0.4 V versus NHE were −0.01, −0.02, and −0.04 mA cm−2 for the surface coverages of 10%, 20%, and 40% for copper nanoparticles, respectively. As shown in Table 1, the highest current density (−0.54 mA cm−2 at the potential of −1.3 V vs NHE) was achieved when we used diamond electrodes coated with copper nanoparticles having sizes of 15(±5) nm, a density of 4.6(±0.5) × 1010 cm−2, and surface coverage of 81(±1.0)%. Therefore, in the rest of this work, this type of decorated diamond electrodes was used. Further GC-HPLC analysis of reduction products shows the presence of H2, CH4, C2H4, and ethanol after CO2 reduction process under these conditions. CO2 Reduction in Nonaqueous Solutions. We examined electrochemical CO2 reduction in nonaqueous solutions with a planar diamond electrode before and after modification with

Table 1. Absolute Values of Current Densities for CO2 Reduction at a Potential of −1.3 V versus NHE on Two Electrodes in Different Solutions current density (mA cm−2)

electrode diamond diamond decorated with copper nanoparticles

aqueous solution (0.5 M LiClO4)

nonaqueous solution (BMIM-PF6)

mixture of aqueous and nonaqueous solutions (10 μM H2O in BMIM-PF6)

0.15 0.54

0.24 1.7

0.75 5.1

copper nanoparticles. Instead of normal organic solvents like acetonitrile or methanol, RTIL has been used as the solvent and the supporting electrolyte as well. Figure 3 shows the results of electrochemical CO2 reduction on a planar diamond electrode in BMIM-PF6 at a scan rate of

Figure 3. Linear sweep voltammograms of a planar diamond electrode in BMIM-PF6 at a scan rate of 0.01 V s−1 (a) before and (b, c) after saturating with CO2. In curve c, the electrode is coated with copper nanoparticles.

0.01 V s−1. Before saturating BMIM-PF6 with CO2, up to −2.2 V versus NHE only background current is seen (Figure 3a). The increase of cathodic current at potentials negative than −2.2 V versus NHE is probably due to the decomposition of BMIM-PF6. After saturating BMIM-PF6 with CO2, a clear broad cathodic shoulder is seen at around −2.0 V versus NHE (Figure 3b), which is similar with those reported in other RTILs.8,13 The cathodic wave at −2.0 V versus NHE indicates the formation of CO2•− anion radicals in BMIM-PF6. These radicals are further reduced with adsorbed CO2, resulting in producing of C2O42− related products in solutions.8,13 The potential for CO2 reduction in BMIM-PF6 is noticed to be about 1.5 V higher than that in aqueous solutions. Although CO2 has 6−10 times higher solubility in RTIL, the reduction current at the potential of −1.3 V versus NHE (see Table 1) is quite similar with that obtained in aqueous solutions. This is due to a lower diffusion coefficient of CO2 in this highly viscose solvent. On a planar diamond electrode coated with copper nanoparticles (Figure 3c), CO2 reduction starts at about −0.7 V versus NHE, namely, the potential for CO2 reduction in BMIM-PF6 reduces from −2.0 to −0.7 V versus NHE after adding copper catalyst. A steady-state-like wave is seen in the potential range of −1.0 to −1.5 V versus NHE. As seen in Table 1, the current density at −1.3 V versus NHE increases from 0.24 mA cm−2 on a planar diamond electrode to 1.7 mA cm−2 after decorating the diamond electrode with copper nanoparticles. These results suggest RTIL (e.g., BMIM-PF6) is 5766

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experiments about product analysis and conversion efficiencies at different reduction potentials are undergoing. However, the efficiencies at the potential of −1.3 V versus NHE are higher than those reported in aqueous solutions1,10−12 and quite similar (slightly higher) with those reported in nonaqueous solutions.8,12,13 Higher efficiency than 80% is expected once the surface of copper nanoparticles, the interaction of copper with diamond, the surface structure of the diamond electrode, and the composition of the solvents are optimized. Stability Test. To test stability of the copper nanoparticles as well as the diamond electrode, we applied electrolysis experiments in the mixture of water and BMIM-PF6 ([H2O] = 10 μM) for electrochemical CO2 reduction at different potentials. As shown in Figure 5, the current remains stable

a better solvent for electrochemical CO2 reduction. Under these conditions, the main reduction products found in GCHPLC analysis were formaldehyde. Please note that the reduction products in nonaqueous solutions vary much when different solvents and supporting electrolytes are used. The coexistence of impurities (such as water in RTIL) also affects dramatically the final products during the CO2 reduction process. CO2 Reduction in Mixture of Aqueous and Nonaqueous Solutions. It is known that water is important to get useful chemicals in nonaqueous solutions by electrochemical CO2 reduction. When the water concentration is higher than that of CO2 in solutions, CO2 reduction is highly suppressed.4,14 By adding a tiny amount of water into organic solvents, the molecular ratio of CO2 to H2O varies. Resulting from the reactions of reduction radicals (e.g., CO2•− anion in RTIL) with water, this ratio determines reduction products in nonaqueous solutions and finally affects the reduction efficiency. We then investigated electrochemical CO2 reduction in the mixture of water and BMIM-PF6 in which the amount of water is little ([H2O] = 10 μM). Figure 4a shows the

Figure 5. Electrolysis curves for electrochemical CO2 reduction in the mixture of BMIM-PF6 and water ([H2O] = 10 μM) on a planar diamond electrode coated with copper nanoparticles at the potentials of (a) −0.1, (b) −0.3, (c) −0.6, (d) −0.9, (e) −1.2, and (f) −1.6 V vs NHE.

at each potential applied. At potentials more negative than −1.6 V versus NHE, noticeable decline in the current is seen over time. The curve becomes noisy as bubbles begin to form on the electrode surface, which cover the areas of the electrode until they grow large enough to be released, causing fluctuations in the current. We have compared the currents during electrolysis at different potentials with those obtained at the same potentials from voltammetric measurements. The variation tendency of the currents matches with each other while the magnitude of the currents from electrolysis is slightly smaller than those from voltammetric experiments. This is probably due to bubble formation and/or deactivation of catalysts/ electrodes. A better solution for more stable catalysts is to form carbide-based, core−shell structures (e.g., a nickel−carbide core and a copper shell) or to bond catalysts covalently (e.g., organic metal complexes) on the diamond surface. Comparison. Figure 6 compares the results obtained on a planar diamond electrode before (Figure 6A) and after (Figure 6B) decorating with copper nanoparticles in different solutions, namely, (a) 0.5 M LiClO4 solution, (b) BMIM-PF6, and (c) the mixture of water and BMIM-PF6 ([H2O] = 10 μM). The linear sweep voltammograms of a planar diamond electrode in BMIM-PF6 (Figure 6d) and in the mixture of water and BMIMPF6 ([H2O] = 10 μM) (Figure 6e) are shown as well. To draw a better and clearer picture, the current densities at −1.3 V versus NHE are listed as examples in Table 1 for a planar diamond electrode before and after decorating with copper nanoparticles. From these voltammograms and the values shown in Table 1, one can see clearly the best electrode/ solution system for CO2 reduction is using a planar diamond electrode coated with copper nanoparticles and using a mixture of water and BMIM-PF6.

Figure 4. Linear sweep voltammograms of a planar diamond electrode in the mixture of water and BMIM-PF6 ([H2O] = 10 μM) at a scan rate of 0.01 V s−1 (a) before and (b, c) after saturating with CO2. In curve c, the electrode is coated with copper nanoparticles.

voltammogram of a planar diamond electrode coated with copper nanoparticles in this mixture before purging CO2. Only background current is recorded up to −1.8 V versus NHE. As a control experiment, electrochemical CO2 reduction on a planar electrode without copper nanoparticles has been conducted, and the voltammogram is shown in Figure 4b. An increase of cathodic current is seen from the potential of −0.5 V versus NHE. This potential is similar with what we have observed for electrochemical CO2 reduction in aqueous solutions (Figure 2b). In this case, only a steady-steady-like wave is seen in the range of −0.7 to −1.3 V versus NHE. No steady-state-like wave is seen in the range of −0.1 to −0.5 V versus NHE. At the potential of −1.3 V versus NHE, the current density is only 0.75 mA cm−2 (see Table 1). After modifying the diamond electrode with copper nanoparticles, the cathodic current increases dramatically from the potential of about −0.1 V versus NHE. A steady-steady-like wave is clearly seen in the range of −1.1 to −1.5 V versus NHE as well as a broad wave in the potential range of −0.4 to −0.9 V versus NHE. At a potential of −1.3 V versus NHE, the current density increases to 5.1 mA cm−2 (see Table 1). The product analysis using GCHPLC shows that the main products are formic acid and formaldehyde. Other produces such as H2, CH4, and C2H4 were found as well. The product of formic acid might result from the reaction of CO2•− anion radicals with water. More detailed 5767

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composition of the mixture), and the catalyst (surface chemistry, shape, size, density, and stability) are optimized, higher conversion efficiencies than 80% are expected. In the future, the combination of the proposed electrode system with a renewable energy source from sun or wind is needed to realize electrochemical CO2 reduction in a beneficial way for the production of industrial chemicals and useful liquid fuels.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Comparison of linear sweep voltammograms of CO2 reduction on a planar diamond electrode (A) before and (B) after coating with copper nanoparticles in (a) 0.5 M LiClO4 solution, (b) BMIM-PF6, and (c) the mixture of water and BMIM-PF6 ([H2O] = 10 μM). Curves d and e are the linear sweep voltammograms of a planar diamond electrode in BMIM-PF6 and in the mixture of water and BMIM-PF6 ([H2O] = 10 μM), respectively. The scan rate is 0.01 V s−1. The solutions were saturated with CO2.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 49-761 5159647; fax: 49-761 515971647; e-mail: [email protected]. Notes

We further compared our result (Figure 7a) (e.g., one shown in this paper with a planar diamond electrode coated with

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Waldemar Smirnov for helping the preparation of diamond electrodes, Dr. Guohua Zhao for assisting with GC-HPLC measurements, and the financial support from the European Union Council under the project “MACTON” (238201).

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REFERENCES

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Figure 7. Voltammograms for electrochemical CO2 reduction on (a) a diamond electrode coated with copper nanoparticles in the mixture of BMIM-PF6 and water ([H2O] = 10 μM), (b) a copper electrode in 0.1 M KHCO3, (c) a glassy carbon electrode in 1-butyl-3-methyl imidazolium tetrafluoroborate, and (d) a platinum electrode in the mixture of acetonitrile and water.

copper nanoparticles in the mixture of water and BMIM-PF6 ([H2O] = 10 μM)) with those on a copper electrode in 0.1 M KHCO3 (Figure 7b),10 on a glassy carbon electrode in 1-butyl3-methyl imidazolium tetrafluoroborate (Figure 7c),12b and on a platinum electrode in a mixture of acetonitrile and water (Figure 7d).11 As seen in Figure 7, a 2−50 times higher reduction current and a 0.7−1.5 V lower potential have been realized for electrochemical CO2 reduction on a planar diamond electrode coated with copper nanoparticles in the mixture of BMIM-PF6 and water ([H2O] = 10 μM). Therefore, the combination we proposed is an efficient electrode system for electrochemical CO2 reduction.

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CONCLUSIONS In summary, by coating diamond electrodes with copper catalyst and mixing RTIL with tiny amount of water, the potential for electrochemical CO2 reduction reduces to −0.1 V versus NHE. At the potential of −1.3 V versus NHE, the current density reaches 5.1 ± 0.1 mA cm−2. In our RTIL, the main products are formic acid and formaldehyde. Further experiments are required regarding detailed product analysis and the evaluation of conversion efficiency at each potential. Once the electrode (nanotextured with catalysts on the top of nanostructures), the solvent (structure of ionic liquid and 5768

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