METAL-PEPTOIDS ELECTROCATALYSTS

Abstract

The invention provides metal-peptoid complexes for use as electrocatalyst in water oxidation processes.

Claims

1. An oxidative aqueous medium comprising a metal-peptoid complex comprising at least one metal ion associated to a peptoid oligomer incorporating at least one metal-binding ligand and at least one proton acceptor group; and a borate species.

2. The medium according to claim 1, wherein the metal-peptoid complex has an organic chain and at least one metal ion associated therewith, wherein the organic chain is of a structure A-B, wherein A is at least one metal-binding ligand and B is at least one proton acceptor group, wherein A and B are associated via an amide bond, or a linker spacer comprising one or more organic functionality.

3. The complex according to claim 1, wherein the metal-peptoid complex is a N-substituted glycine oligomer comprising between 2 and 15 glycine units.

4. The complex according to claim 1, wherein the at least one metal binding ligand is a N-containing heteroaryl, biaryl or fused heterocyclic.

5. The medium according to claim 1, wherein the at least one binding group is selected from Bipy, Terpy, Dipi and Pico, respectively being of formula ##STR00011## wherein the wiggly line designates a point of connectivity to N atom of a glycine unit.

6. The medium according to claim 1, wherein the at least one proton acceptor group comprises an atom selected from oxygen, nitrogen and sulfur.

7. The medium according to claim 6, wherein the at least one proton acceptor group is selected amongst groups containing hydroxyl functionalities and amine functionalities.

8. The medium according to claim 1, wherein the at least one proton acceptor group is selected from ethanol-yl, PA1, PA2 and PA3, respectively having structure, ##STR00012##

9. The medium according to claim 1, wherein the peptoid comprises at least one metal binding ligand and at least one proton acceptor group, wherein the at least one metal binding ligand is selected amongst pyridinyl, Bipy, Terpy, DiPi and Pico, and the at least one proton acceptor group is selected from ethanol-yl, PA1, PA2 and PA3.

10. The medium according to claim 1, wherein the peptoid comprises one or two metal binding ligands and one or two proton acceptor groups.

11. The medium according to claim 1, wherein the metal ion is selected from copper ions, cobalt ions, manganese ions, nickel ions, iron ions, ruthenium ions, rhodium ions, and iridium ions.

12. The medium according to claim 1, wherein the metal peptoid complex comprising a metal ion and a peptoid, selected from: ##STR00013## ##STR00014## ##STR00015## ##STR00016## ##STR00017## ##STR00018## ##STR00019##

13. The medium according to claim 12, being a Cu-peptoid or a Co-Peptoid or a Mn-Peptoid.

14. The medium according to claim 1, wherein the peptoid is BEE having structure: ##STR00020## optionally associated to a copper metal ion, a manganese metal ion or a cobalt metal ion.

15. An electrocatalyst comprising or consisting a metal peptoid complex comprising at least one metal ion associated to a peptoid oligomer comprising at least one metal-binding ligand and at least one proton acceptor group, wherein the electrocatalyst is for use in a method of electrocatalysis in an aqueous medium comprising a borate species.

16. A process for water oxidation, the process comprising using a medium according to claim 1.

17. The process according to claim 16, wherein the process is carried out in an electrochemical cell comprising a cathode compartment including a cathode electrode; an anode compartment including an anode electrode comprising the electrocatalyst for oxidation reactions; wherein a water solution is supplied to the anode compartment to be electrochemically oxidized on the electrocatalyst.

18. A process for generating hydrogen gas, the process comprising treating an aqueous medium according to claim 1, under conditions permitting oxidation of the water and generation of hydrogen gas.

19. The process according to claim 18, wherein the electrocatalyst is a metal complex of BEE.

20. A device for electrolysis of water, the device comprises an electrode comprising an electrocatalyst according to claim 15, wherein the device is configured to receive and hold water containing a borate species.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0093] In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0094] FIG. 1 depicts a structural difference between a peptide and a peptoid.

[0095] FIG. 2 depicts exemplary metal binding groups. The wiggly lines indicate connectivity to the peptoid structure.

[0096] FIG. 3 depicts exemplary proton acceptor groups. The wiggly lines indicate connectivity to the peptoid structure.

[0097] FIG. 4 depicts the submonomer method of synthesis used for preparing peptoids according to some embodiments of the invention.

[0098] FIG. 5 depicts substitution by ethylamine via its protected amine derivative N-Boc-ethylenediamine.

[0099] FIG. 6 depicts incorporation of ethanol and propanediol via their silyl-protected amine derivatives.

[0100] FIGS. 7A-C depict exemplary peptoids and complexes thereof according to the invention.

[0101] FIGS. 8A-C shows sequence of the peptoid BEE, the molecular structure of [Cu.sub.2(BEE).sub.2(H.sub.2O)](ClO.sub.4).sub.4 and the ORTEP view (thermal ellipsoids set at 50% probability) of the cation [Cu.sub.2(BEE).sub.2(H.sub.2O)].sup.4+ crystalized from water. The counter anions and selected hydrogen atoms are omitted for clarity.

[0102] FIGS. 9A-B show (A) UV-Vis spectra of 0.1 mM Cu.sub.2(BEE).sub.2 in water and in borate buffer solution; insert: 0.3 mM of Cu.sub.2(BEE).sub.2 in water and in borate buffer solution and (B) CV of 0.2 M borate buffer solution at pH 9.35 in the presence or absence of 1 mM Cu.sub.2(BEE).sub.2. Scan rate=100 mV/s, glassy carbon electrode as the working electrode. Inset: CV of the Cu.sup.IICu.sup.I/Cu.sub.2.sup.I and Cu.sub.2.sup.II/Cu.sup.IICu.sup.I couple at the range between 0.5 to 0.6 V vs. NHE.

[0103] FIGS. 10A-B show the total accumulated charge (A) and O.sub.2 evolution (B) during CPE in 0.2 M borate buffer (pH 9.35), in the absence or presence of 0.5 mM Cu.sub.2(BEE).sub.2 using glassy carbon as working electrode at applied potential of +1.35 V. Evolved O.sub.2 was measured by a fluorescence probe placed in the headspace of the electrochemical cell during the CPE.

[0104] FIGS. 11A-B show the accumulated charge (A) and evolved O.sub.2 (B) during each 30 min CPE cycle. Experiments done at 1.35 V with 0.5 mM Cu.sub.2(BEE).sub.2 at pH=9.35 in 0.2 M borate buffer solution. Every 30 min the CPE was stopped, and the pH was readjusted to 9.35.

[0105] FIGS. 12A-D show the CVs of 0.5 mM Cu.sub.2(BEE).sub.2 at different scan rates with (A) a broad scanning range (0.4 to 1.9 V) or (B) a narrow scanning range (0.3 to 0.6 V); inset shows the linear regression of i.sub.d versus v.sup.1/2. (C) Linear regression of i.sub.cat/i.sub.d versus v.sup.1/2. (D) k.sub.obs values at various scan rates and the red line represents the value of average k.sub.obs, they were done by FOWA. All experiments were performed in 0.2 M borate buffer at pH 9.35.

[0106] FIGS. 13A-B show (A) Pourbaix diagram of Cu.sub.2(BEE).sub.2 in 0.2 M borate buffer in range of pH 811; (B) CVs of Cu.sub.2(BEE).sub.2 in 0.2 M borate buffer dissolved in H.sub.2O or D.sub.2O; insert is the enlarged scale of range between 0.8 V to +0.6 V.

[0107] FIG. 14 depicts a proposed mechanism cycle of Cu.sub.2(BEE).sub.2 for water oxidation in borate buffer solution.

[0108] FIGS. 15A-D show (A) Molecular and (B) crystal structure of Cu.sub.2BE.sub.2; (C) CVs of 1 mM of Cu.sub.2BEE.sub.2 and Cu.sub.2BE.sub.2 in 0.2 M borate buffer at pH 9.35; (D) accumulated charge and (insert) oxygen during 30-minute CPE experiment with 750 mV in 0.2 M borate buffer at pH 9.35.

[0109] FIG. 16 shows a plot of TOF- relationship of Cu.sub.2BEE.sub.2 and Cu.sub.2BE.sub.2 in 0.2M borate buffer at pH 9.35; data extracted from FOWA analysis.

[0110] FIGS. 17A-B compares use of different borate buffers. (A) is the CV comparison in 0.4 M borate buffer at pH 7. The E1/2 of Cu.sub.2(BDiE).sub.2 is +1.55 V and the E1/2 of Cu.sub.2(BEE).sub.2 is +1.44 V. However, the current intensity of Cu.sub.2(BDiE).sub.2 is much higher than that of Cu.sub.2(BEE).sub.2, which is 75 vs. 14 A. In the same condition, the turnover frequency values were also calculated by foot-of-the-wave analysis (FOWA), showing that Cu.sub.2(BDiE).sub.2 is over 100-fold faster than Cu.sub.2(BEE).sub.2 for water oxidation, 470 vs. 4 s-1, respectively. (B) is the CV comparison in 0.2 M borate buffer at pH 9.35. The overpotential of both catalysts are quite similar.

DETAILED DESCRIPTION OF EMBODIMENTS

Result and Discussion

[0111] Synthesis and Characterization of BEE and its corresponding Cu(II) complex. The peptoid BEE was synthesized using a solid-phase method, cleaved from the solid support and purified by high-performance liquid chromatography (HPLC, >95% purity). The molecular weight measured by electrospray mass spectrometry (ESI-MS) was consistent with the mass expected for its sequence. This peptoid was treated with one equiv. of Cu(ClO.sub.4).sub.2.Math.6H.sub.2O in methanol, and after 4 hours of stirring, greenish-blue precipitate was obtained. The precipitate was isolated and recrystallized from water. The structure consists of one [Cu.sub.2(BEE).sub.2(H.sub.2O)].sup.4+ cation (having two Cu.sup.II ions) and four perchlorate (ClO.sub.4.sup.) anions (Cu.sub.2(BEE).sub.2. FIG. 8 shows the sequence of the peptoid BEE, the molecular structure of [Cu.sub.2(BEE).sub.2(H.sub.2O)](ClO.sub.4).sub.4 and the ORTEP view (thermal ellipsoids set at 50% probability) of the cation [Cu.sub.2(BEE).sub.2(H.sub.2O)].sup.4+ crystalized from water. The counter anions and selected hydrogen atoms are omitted for clarity.

[0112] Electrochemical Studies. The electrochemical and electrocatalytic properties of Cu.sub.2(BEE).sub.2 were evaluated in 0.2 M borate buffer at pH 9.35. The cyclic voltammetry (CV) was obtained in air using a glassy carbon (GC) working electrode and Ag/AgCl reference electrode. All the potentials are reported vs. the normal hydrogen electrode (NHE) by adding 0.197 V to the measured potential. As shown in FIG. 9B, two irreversible anodic waves at E.sub.pa=+0.05 V and +0.24 V were observed, and these correspond to the Cu.sup.IICu.sup.I/Cu.sub.2.sup.I and Cu.sub.2.sup.II/Cu.sup.IICu.sup.I redox couples. In addition, a high catalytic wave at E.sub.pa=+1.9 V was obtained, with an onset near +1.15 V, which corresponds to an onset overpotential of about 470 mV. To confirm that the catalytic wave represents water oxidation, the scan was revered and scanning was continued to a second scan, which showed a new reduction peak at 0.3 V assigned to oxygen reduction. This observation indicates that the oxygen is generated during or after the catalytic process, thus the catalytic process is water oxidation. Notably, Cu(ClO.sub.4).sub.2.Math.6H.sub.2O is not stable in basic conditions as it immediately forms a blue precipitate. It may be therefore concluded that the use of BEE enables to obtain high catalytic peak intensity, and leads to significant stabilization of the Cu center in borate buffer in basic conditions.

[0113] The distance between Cu1 and Cu2 is 4.492 and they are bridged by one H.sub.2O molecule. The two CuOH bonds, Cu1-O2 and Cu2-O11 are 2.491(7) and 2.415(6) respectively, representing asymmetry. The Cu1-OCu2 angle is 132.9(3). Each Cu is coordinated to three N atoms (two from bipyridine of one peptoid and one from the secondary amine of the other peptoid) and three O atoms (one from the H.sub.2O bridge, one from the OH group and one from a backbone carbonyl group).

[0114] FIG. 9 shows the (a) UV-Vis spectra of 0.1 mM Cu.sub.2(BEE).sub.2 in water and in borate buffer solution; insert: 0.3 mM of Cu.sub.2(BEE).sub.2 in water and in borate buffer solution and (b) CV of 0.2 M borate buffer solution at pH 9.35 in the presence or absence of 1 mM Cu.sub.2(BEE).sub.2. Scan rate=100 mV/s, glassy carbon electrode as the working electrode. Inset: CV of the Cu.sup.IICu.sup.I/Cu.sub.2.sup.I and Cu.sub.2.sup.II/Cu.sup.IICu.sup.I couple at the range between 0.5 to 0.6 V vs. NHE.

[0115] Complex characterization in solution was done in water and in 0.2 M borate buffer at pH 9.35, which is slightly above the pKa of B(OH).sub.3, where B(OH).sub.3 could react with water to form B(OH).sub.4.sup.. Thus, this is the minimum pH in which B(OH).sub.4.sup. is the dominant species and can potentially promote the water oxidation process. It was found that the UV-Vis spectra of Cu.sub.2(BEE).sub.2 in water (from where the crystal was obtained) and in borate buffer are identical (FIG. 9A), where 245 nm is assigned to n-* transition of the amide backbone and BPy, 320 nm to Cu-BPy species and 680 nm to the d-d* transition of Cu.sup.II. Further studies in borate buffer showed that (i) the absorption is linearly dependent on the catalyst concentration, indicating that the complex exists as a single species in the dinuclear form in the buffer solution, and (ii) that the spectrum does not change after 24 hours, demonstrating that the complex is stable in the buffer conditions. Electron spin resonance spectra (EPR) were performed in borate buffer at three different pH conditions, namely pH 7.50, 9.35 and 11.70. The spectra taken from the solutions at pH<9.4 revealed a signal at half-field region (M.sub.S=2) at pH 7.50 and 9.35, indicating a triplet state species. Interestingly, this signal disappeared at pH 11.7, and the spectrum showed well-resolved four hyperfine structure at M.sub.S=1 typical for a mononuclear Cu complex, indicating that at high pH Cu.sub.2(BEE).sub.2 decomposes to the mononuclear Cu species (n=1, 1=3/2). This comparison clearly demonstrates the existence of the dinuclear Cu.sub.2(BEE).sub.2 at pH 9.35. The broad hyperfine structure at lower pH suggests the presence of an extended magnetic exchange between the two Cu.sup.2+ centers, probably via hydrogen bonding. Interestingly, the ESI-MS spectrum of the complex in water shows a peak at m/z=1371.1326 consistent with [Cu.sub.2(BEE).sub.2].sup.4++3ClO.sub.4.sup. (z=1) as well as a peak at 1491.1327, which is consistent with [Cu.sub.2(BEE).sub.2(H.sub.2O)] and has a much lower intensity. Therefore, it was suggested that the complex exists in solution in the dicopper form, and that the bridging H.sub.2O is labile. The structure of Cu.sub.2(BEE).sub.2 was further supported by FT-IR and Raman spectroscopies. The FT-IR transmittance of the peptoid BEE at 1653 cm.sup.1 represents the (CO of) amide bond stretching, and broad peaks at 3200 and 3390 cm.sup.1 are assigned to the combination of NH and OH stretching. Following CO coordination, a new peak arose at 1682 cm.sup.1, assigned to the stretching of (CO(Cu)). In addition, transmittance signals at 3267 and 3408 cm.sup.1 represent the stretching of (Cu)NH and (Cu)OH, respectively. Upon dissolving the complex in borate buffer, a similar spectrum to the one acquired in water, showing signals at the same wavenumbers, was obtained. To characterize the CuO and CuN bonds at low wavenumber region we measured the Raman spectrum of the complex, and this showed no shifts neither in water nor borate buffer at the range of 200 to 1500 cm.sup.1, demonstrating that the CuO and CuN stretching remain the same in both conditions. Overall, these results indicate that Cu.sub.2(BEE).sub.2, which is most likely in equilibrium with Cu.sub.2 (BEE).sub.2 (H.sub.2O), exists both in water and in 0.2 M borate buffer at pH 9.35 and is stable in these conditions.

[0116] Water Oxidation Experiments. Evolution of molecular oxygen was investigated by controlled potential electrolysis (CPE) experiment at +1.35 V with a porous glassy carbon as working electrode in a 0.2 M borate buffer solution at pH=9.35 (FIG. 10).

[0117] FIG. 10 shows is the total accumulated charge (a) and O.sub.2 evolution (b) during CPE in 0.2 M borate buffer (pH 9.35), in the absence or presence of 0.5 mM Cu.sub.2(BEE).sub.2 using glassy carbon as working electrode at applied potential of +1.35 V. Evolved O.sub.2 was measured by a fluorescence probe placed in the headspace of the electrochemical cell during the CPE.

[0118] Within 30 minutes, the evolved oxygen concentration was increased by 35.5 mol with 0.5 mM Cu.sub.2(BEE).sub.2 and by 1.5 mol without it. This electrolysis experiment afforded a charge accumulation of about 15.2 C in the presence of Cu.sub.2(BEE).sub.2, and of only 1.4 C in the absence of Cu.sub.2(BEE).sub.2. Based on a 4e.sup. process, the 34 mol of evolved oxygen and the total charge of 13.8 C accumulated in this process, and the initial amount of catalyst in solution, the Faradaic efficiency (FE %) and the catalytic turnover number (TON) were calculated to be 95% and 13.6 respectively in only 30 min. The current measured during the CPE increased dramatically in the first 4 min of the reaction up to a value of about 3.2 mA/cm.sup.2, and was maintained steady in the next 26 min. This suggests that at the beginning of the reaction, the active species is formed via drastic conformational changes. This active catalyst can be either an insoluble species formed on the surface of the working electrode or an in-situ soluble molecular species formed in the solution. To probe this point, we repeated the first 500 seconds of the CPE using ITO working electrode 5 times in the same solution. After each experiment, the potential was turned off and the electrode was left in the solution for 5 min. The current increase during the first reaction was identical to the one observed in the 30 min CPE experiment, and each repetition showed a recovery from high to low current density, followed by a drastic increase during the electrolysis. Importantly, the current response of the electrode, after the CPE experiment, which was rinsed but not polished, in buffer solution that did not contain the catalyst, was identical to the current response obtained from a clean electrode in the same buffer solution, supporting the homogeneity of the CPE process. These results eliminate the attribution of an insoluble (adsorbed) species toward the increased current. Knowing that borate buffer can coordinate to Cu during the catalysis, it may be suggested that Cu.sub.2(BEE).sub.2 is the pre-catalyst that forms the active catalyst while reacting with borate species from the buffer solution upon applying a potential. To confirm the participation of the buffer in the catalytic process, we first compared the CV scans of Cu.sub.2(BEE).sub.2 in phosphate buffer and in borate buffer in the same conditions (0.2 M ion strength and pH 9.35). It was found that the onset potential of the catalytic event is about 250 mV lower in borate buffer than in phosphate buffer solution, suggesting that borate is a non-innocent buffer that can enhance the catalytic activity. Indeed, when quantitative amounts of borate species were added into a phosphate buffer solution containing the catalyst Cu.sub.2(BEE).sub.2 the onset and the catalytic events shifted to lower potential, and the intensity of the catalytic peak increased with an increasing amount of added borate species. To further support the participation of the borate buffer in the electrocatalytic process, a series of CV scans in different buffer concentration was conducted. The results showed that while no catalytic activity was observed in un-buffered water solution (0 mM buffer concentration, i.sub.(H2O)), the catalytic peak current (i.sub.(cat)) was gradually increasing as the buffer concentration increased from 30 to 250 mM. The (i.sub.(cat)/i.sub.(H2O)).sup.2 is proportional to the buffer concentrations, signifying the participation of the buffer ions in the catalytic event. These results also imply that the structural change of Cu.sub.2(BEE).sub.2 does not stem from a catalyst decomposition but might be attributed to the coordination of borate species to Cu.sub.2(BEE).sub.2. On the other hand, when the buffer concentration increases from 250 to 350 mM, the intensity of the catalytic wave plummets, and almost does not change once the buffer concentration is increasing. This might indicate that at high buffer concentrations polyborate ions are being formed, rather than the active catalyst, leading to an inhibition of the catalytic process.

[0119] The onset overpotential was determined by a series of CPE experiments with different applied potentials (+1.25, +1.20, +1.15, +1.10 V vs. NHE). Within 900 seconds (15 minutes), CPE at +1.10 V did not result in oxygen generation until the applied potential was increased to +1.15 V. Thus, we can conclude that +1.15 V is the minimal applied potential for the catalysis process with Cu.sub.2(BEE).sub.2, which corresponds to an onset overpotential of 470 mV. This value is considerably low compared to other reported Cu-based complexes at mild pH conditions.

[0120] Catalyst Stability and Re-Use. Importantly, under these reaction conditions, no precipitation was observed, and the catalyst remained intact as evident from the UV-Vis spectrum and CV taken before and after 30 min of electrolysis. A pH decrease to 9.05 was measured, and is consistent with proton formation during electrolysis. As the catalyst remained intact after CPE experiment, we envisioned that the catalytic solution could be recycled by adjusting its pH back to 9.35 after 30 min of reaction towards a significant increase in the overall TON. To explore this possibility, 0.5 mM Cu.sub.2(BEE).sub.2 solution was subjected to a CPE experiment with an applied potential of +1.35 V, in which the reaction was stopped every 30 min. and the pH was adjusted to 9.35 (FIG. 11). Keeping the amount of Cu.sub.2(BEE).sub.2 constant in the solution, we have obtained a TON as high as 51.8 in only 2 hours (4 repeating cycles).

[0121] FIG. 11 shows the accumulated charge (a) and evolved O.sub.2 (b) during each 30 min CPE cycle. Experiments done at 1.35 V with 0.5 mM Cu.sub.2(BEE).sub.2 at pH=9.35 in 0.2 M borate buffer solution. Every 30 min the CPE was stopped, and the pH was readjusted to 9.35.

[0122] Homogeneity Studies. At this point we wished to explore whether the catalyst Cu.sub.2(BEE).sub.2 is operating solely in solution or if an active catalyst is being formed and deposited on the electrode surface leading to a heterogeneous process. To this aim, we removed the glassy carbon electrode from a solution of Cu.sub.2(BEE).sub.2 (0.5 mM in 0.2 M borate buffer, pH 9.35) after carrying out 25 continuous CV scans from 0.4 to 2.1 V, rinsed it with water (the electrode was not polished), and placed it in fresh 0.2 M borate buffer solution at pH 9.35 without Cu.sub.2(BEE).sub.2. The CV scan of this solution showed only the buffer response. Scanning electron microscope (SEM) images of the working electrode surface before and after 25 continuous CV scans, showed no particle deposited on the electrode surface. In addition, another 30-min CPE experiment was conducted with ITO as the working electrode, which was analyzed by HR-SEM and EDS before and after the reaction. No particles on the electrode's surface were found in HR-SEM, and no Cu element was found attached during EDS spectrum after electrolysis. Moreover, the CV scans at different scan rates (FIG. 12) show that the peak current (i.sub.d, corresponding to the reduction peak current of Cu.sub.2.sup.II/Cu.sup.IICu.sup.I) varies linearly with the root of scan rate, v.sup.1/2. Considering the irreversibility, i.sub.d and v.sup.1/2 follow the relation as below:

[00001]$\begin{array}{cc}{i}_d=0.446{\left(n_{d}F\right)}^{3/2}A\left[\mathrm{Cu}\right]{\left(\frac{D_{\mathrm{Cu}}v}{\mathrm{RT}}\right)}^{1/2}& \left(\mathrm{eq}.1\right)\end{array}$

[0123] The value of the diffusion coefficient D.sub.Cu was calculated to be 1.2810.sup.5 cm.sup.2/s, which is consistent with the diffusion-controlled process (10.sup.610.sup.5 cm.sup.2/s). Collectively, these results indicate that the catalytic process is truly homogeneous.

[0124] FIG. 12 shows the CVs of 0.5 mM Cu.sub.2(BEE).sub.2 at different scan rates with (a) a broad scanning range (0.4 to 1.9 V) or (b) a narrow scanning range (0.3 to 0.6 V); inset shows the linear regression of i.sub.d versus v.sup.1/2. (c) Linear regression of i.sub.cat/i.sub.d versus v.sup.1/2. (d) k.sub.obs values at various scan rates and the red line represents the value of average k.sub.obs, they were done by FOWA. All experiments were performed in 0.2 M borate buffer at pH 9.35.

[0125] Kinetic Studies. CV scans of solutions containing different concentrations of the catalyst were performed to gain some insight regarding its kinetics. The linear dependence of the catalytic peak current on the catalyst's concentration, points out that the water oxidation is performed by a single molecular catalysis reaction with first-order kinetics. Therefore the catalytic process obeys the relationship displayed in eq. 2:

i.sub.cat=n.sub.catFA[Cu](k.sub.catD.sub.cu).sup.1/2(eq. 2)

[0126] The correlation between i.sub.cat/i.sub.d and v.sup.1/2 could then be obtained and the value of the rate constant could be calculated by the linear slope of i.sub.cat/i.sub.d and v.sup.1/2 (FIG. 13) as shown in eq. 3:

[00002]$\begin{array}{cc}\frac{i_{\mathrm{cat}}}{i_d}=1.827k_{\mathrm{cat}}^{1/2}{v}^{-1/2}& \left(\mathrm{eq}.3\right)\end{array}$

[0127] From these equations, the slope value (FIG. 13A) was calculated to be 20.766 with the correlation coefficient (R) being 0.98, and k.sub.cat was calculated to be 129 s.sup.1. To further conform the fast kinetics by Cu.sub.2(BEE).sub.2, FOWA was performed at various scan rates and the average k.sub.obs was determined as 5503 s.sup.1 (FIG. 13B). To the best of our knowledge, the TOF value calculated by either method is the highest reported for copper-based homogeneous electrocatalysts, calculated by one (or both) of these methods.

[0128] FIG. 13 shows (a) Pourbaix diagram of Cu.sub.2(BEE).sub.2 in 0.2 M borate buffer in range of pH 811; (b) CVs of Cu.sub.2(BEE).sub.2 in 0.2 M borate buffer dissolved in H.sub.2O or D.sub.2O; insert is the enlarged scale of range between 0.8 V to +0.6 V.

[0129] Mechanism. It may be suggested that Cu.sub.2(BEE).sub.2 exists in solution in equilibrium with Cu.sub.2(BEE).sub.2(H.sub.2O). Based on the behavior of the current during our CPE experiments, the current intensity dependence on the buffer concentration and the relevant literature, it is proposed that in the first step of the reaction, the borate buffer coordinates to the Cu center upon applying a potential. To further support the structural change associate with borate coordination to Cu.sub.2(BEE).sub.2(H.sub.2O), and to explore what is the potential required for borate coordination to take place, a series of spectroelectrochemistry experiments at different applied potentials were performed. At applied potential +1.10 V, which is lower than the onset potential in borate buffer (+1.15 V), the UV-Vis spectrum does not change after 400 s of electrolysis, indicating that no structural change has occurred. In contrast, once the applied potential is higher than the onset potential (e.g. +1.35 V and +1.55 V), substantial changes in the UV-Vis absorbance were observed during electrolysis: the absorbance band at 245 nm increased, a new shoulder band appeared near 309 nm (associated with the CuB(OH).sub.4.sup. complex) and the band near 320 nm (associated with Cu-BPy) decreased. In addition, an isosbestic point was observed near 312 nm, implying the structural change is taking place within the Cu-BPy species, and suggesting that the borate species coordinate to Cu to form a new Cu-BPy-borate complex represented by the shoulder band near 309 nm. As control, the same experiment was conducted in phosphate buffer at an applied potential of +1.6 V and this resulted in no change in this region (about 309 nm). The results from the spectroelectrochemistry experiments support our proposal that borate species coordinates to the Cu center of the catalyst and indicates that the borate coordination happens only when the applied potential is higher than the onset potential. These results signify that borate coordination to Cu occurs together with the electron(s) and proton(s) transfer(s) that initiate water oxidation.

[0130] To quantify the electron(s) and proton(s) transfer(s) that occur in the first step of the reaction and propose the structure of this borate-bound complex, DPV scans in different pH conditions were performed and plotted vs. the catalytic potentials to form a Pourbaix diagram (FIG. 14). The catalytic wave obtained from the DPV scans consists of two oxidation waves when pH<10 and three waves when pH10. The slope value of the first wave <pH 10 is 0.053, which is close to 0.059, the nH.sup.+/ne.sup. transfer process. However, in pH10, the oxidation event splits into two events: a slope of 0.136 representing 2H.sup.+/1e.sup. process and a slope of 0, indicating only 1e.sup. transfer process. Thus, the overall oxidation event at pH<10 can be rationally concluded as 2H.sup.+/2e.sup. process because the value of its slope is 0.053 which is similar to 0.059 determined by the Nernst equation. Combining these results and the asymmetrical bond length between the H.sub.2O molecule and the Cu centers in Cu.sub.2(BEE).sub.2(H.sub.2O), we propose that the first step of the catalytic reaction proceeds via two consecutive PCET processes. In the first process, the shorter Cu.sup.IIH.sub.2O is oxidized to Cu.sup.IIIOH and the longer Cu.sup.IIH.sub.2O bond cleaves, enabling the coordination of the non-innocent buffer species, which occurs during the second PCET process.

[0131] FIG. 14 depicts a proposed mechanism cycle of Cu.sub.2(BEE).sub.2 for water oxidation in borate buffer solution.

[0132] First, in the crystal structure of Cu.sub.2(BEE).sub.2(H.sub.2O), the distance between the oxygen atom of the OH group in the unbound ethanolic side chains and the proton atom of the bridging H.sub.2O molecule is 3.247 , suggesting weak electrostatic interaction between these atoms and a possible H-bonding that should stabilize the formed Cu.sup.IIIOH center. Second, due to the equilibrium of boric acid, B(OH).sub.3+H.sub.2OB(OH).sub.4.sup.+H.sup.+ (pKa=9.2), it is reasonable that in our reaction conditions, i.e. pH 9.35, the coordinating borate species is B(OH).sub.4.sup. anion. Thus, in the second PCET process B(OH).sub.4.sup. coordinates to Cu forming a Cu.sup.IIIOB(OH).sub.3 while a proton is being abstracted into the solution. This is in accordance with our observation that borate coordination to Cu center only occurs when the applied potential is above the onset potential, i.e., the potential that initiates catalysis. Third, in this oxidation event, the linear slope (0.016) obtained from the plot of E vs. ln[B(OH).sub.4.sup.] matches the stoichiometry ratio of 1:1 Cu.sub.2(BEE).sub.2(H.sub.2O):[B(OH).sub.4.sup.]. Together, both PCET processes shown below lead to the formation of I (step 1, FIG. 15):

[CO.sup.II.sub.2(BEE).sub.2(H.sub.2O)].sup.4++[B(OH).sub.4].fwdarw.{Cu.sup.III.sub.2(BEE).sub.2(OH)[B(O)(OH).sub.3]}.sup.3++2H.sup.++2e.sup. (Step 1)

[0133] According to the Pourbaix diagram, the second oxidation event is also 1H.sup.+/1e.sup. transfer process as its slope value is 0.075 (close to 0.059). The H-bond between the Cu.sup.IIIOH center and the OH group of the ethanolic side chain might accelerate the proton transfer in this process, to form II (Step 2, FIG. 15), in which the CuOB(OH).sub.3 center is stabilized by a H-bonding between the O atom from the OH group of the unbound ethanolic side chain and one of the H atoms of B(OH).sub.3:

{Cu.sup.III.sub.2(BEE).sub.2(OH)[B(O)(OH).sub.3]}.sup.3+.fwdarw.{Cu.sup.III.sub.2(BEE).sub.2(O.Math.)(B(O)(OH).sub.3]}.sup.3++H.sup.++e.sup. (Step 2)

[0134] FIG. 15 shows (a) Molecular and (b) crystal structure of Cu.sub.2BE.sub.2; (c) CVs of 1 mM of Cu.sub.2BEE.sub.2 and Cu.sub.2BE.sub.2 in 0.2 M borate buffer at pH 9.35; (d) accumulated charge and (insert) oxygen during 30-minute CPE experiment with 750 mV in 0.2 M borate buffer at pH 9.35.

[0135] Next, the kinetic isotope effect (ME) of complex Cu.sub.2(BEE).sub.2 was evaluated based on the difference of the catalytic currents acquired in H.sub.2O and D.sub.2O (FIG. 6b). According to the equation: k.sub.H2O/k.sub.D2O=(i.sub.H2O/i.sub.D2O).sup.2, the H.sub.2O/D.sub.2O KIE value was calculated to be 3.0. This value is consistent with a possible atom-proton transfer (APT) process in the OO bond formation rate-determining step (RDS). Knowing that the OH group can stabilize the metal center via H-bonding, we suggest that it acts as an intramolecular proton acceptor in the OO bond formation via APT process. In addition, a previous study has provided evidence that B(OH).sub.4.sup. could be a co-catalyst in this reaction, acting as an oxygen donor that provides one of its OH groups to attack the Cu.sup.IIIO.Math. species, forming the OO bond. Together, the APT and OO bond formation processes lead to III (step 3, FIG. 15):

{Cu.sup.III.sub.2(BEE).sub.2(O.Math.)[B(O)(OH).sub.3]}.sup.3+.fwdarw.{Cu.sup.IIICu.sup.II(BEE)(BEEH)[(OO)B(O)(OH).sub.2]}.sup.3+ (Step 3)

[0136] To support the role of the side chain OH groups as proton acceptors, a control peptoid dimer BE, was prepared, which lacks one of the ethanolic side chains and generates the corresponding Cu complex via the same synthetic procedure used for the preparation of Cu.sub.2(BEE).sub.2. The crystal structure obtained from a single crystal analysis of this complex suggests the formation of the dinuclear complex Cu.sub.2(BE).sub.2 (FIG. 16A-B). Notably, in this crystal structure, there is no bridging H.sub.2O molecule between the two copper centers, probably due to its shorter CuCu distance (4.270 vs. 4.492 of Cu.sub.2(BEE).sub.2); and the only OH side chain of BE is bound to the Cu center, illustrating no dangling OH groups in this complex. Following the solid-state characterization, the complex was characterized in solution both in 0.2 M borate buffer at pH 9.35 and in water, applying a variety of techniques including UV-Vis, FT-IR, EPR and Raman spectroscopies as well as ESI-MS.

[0137] FIG. 16 shows a plot of TOF- relationship of Cu.sub.2BEE.sub.2 (red line) and Cu.sub.2BE.sub.2 (blue line) in 0.2M borate buffer at pH 9.35; data extracted from FOWA analysis.

[0138] The results obtained from all these measurements confirmed the existence of Cu.sub.2(BE).sub.2 without the bridging-water molecule also in solution. Interestingly, the CV of Cu.sub.2(BE).sub.2 exhibited a much lower catalytic wave than the one observed with Cu.sub.2(BEE).sub.2 and the onset potential was higher compared to that of Cu.sub.2(BEE).sub.2 in the same conditions (FIG. 16C). A kinetic study conducted with the peak current of Cu.sub.2(BE).sub.2 used to evaluate the k.sub.cat resulted in only 19 s.sup.1. Moreover, a CPE experiment (FIG. 16D) demonstrated that within 30 min, Cu.sub.2(BE).sub.2 performed with Faradaic efficiency of 70% and TON of only 1.2, with an 11 higher than this of Cu.sub.2(BEE).sub.2 (750 vs. 650 mV). Moreover, the TOF- relationships were also analyzed using the method of Costentin and Saveant and the calculated TOF values by FOWA. As seen in FIG. 16, prior to reaching a maximum, the TOF of Cu.sub.2(BEE).sub.2 is exponentially higher than that of Cu.sub.2(BE).sub.2 at the same II Also, to reach the TOF.sub.max, Cu.sub.2(BEE).sub.2 requires lower 11 than Cu.sub.2(BE).sub.2 (0.90 V vs. 1.05 V). To further understand the role of the OH group from the ethanolic side chain at the 2.sup.nd position of the trimer BEE, in the catalytic activity, another trimer peptoid, BPE, having a non functional propyl side chain in the 2.sup.nd position rather than this specific ethanolic group, was prepared as a control. The corresponding Cu complex, Cu.sub.2(BPE).sub.2, was prepared and characterized as described above, and its crystal structure revealed a H.sub.2O bridge between the two Cu centers as well as a CuCu distance of 4.550 , similar to the crystal structure of Cu.sub.2(BEE).sub.2. However, the CV of this complex in the same reaction conditions as before was similar to this of Cu.sub.2(BE).sub.2, showing lower catalytic activity compared to this of Cu.sub.2(BEE).sub.2. Collectively, the results obtained from the electrocatalytic and kinetic studies support our assumption that the dangling OH groups within Cu.sub.2(BEE).sub.2 play an important role in the water oxidation process (Table 1).

TABLE-US-00001 TABLE 1 Comparison between the electrochemical and catalytic properties of Cu.sub.2(BEE).sub.2 and Cu.sub.2(BE).sub.2. .sup.a FE %.sup.b TON.sup.b k.sub.cat.sup.c k.sub.obs.sup.d Cu.sub.2(BEE).sub.2 650 mV 95% 13.6 129 s.sup.1 5503 s.sup.1 Cu.sub.2(BE).sub.2 750 mV 70% 1.2 19 s.sup.1 51 s.sup.1 .sup.a: overpotential for CPE experiments; .sup.bdata from 30-min electrolysis experiments; .sup.ccalculated from eq. 3; .sup.dcalculated from FOWA (see detail in ESI).

[0139] Following the RDS, we propose that the BO bond cleaves with losing one electron coupled with the proton accepted by the dangling OH groups to form the transition state IV (Step 4, FIG. 6):

{Cu.sup.IIICu.sup.II(BEE)(BEEH)[(OO)B(O)(OH).sub.2]}.sup.3+.fwdarw.Cu.sup.IIICu.sup.II(BEE).sub.2[(OO.Math.)B(O)(OH).sub.2]}.sup.3++H.sup.++e.sup.(Step 4)

[0140] Due to the pKa of boric acid (pKa=9.2), a water molecule reacts with the coordinated B(O)(OH).sub.2 to form [B(OH).sub.4].sup., therefore, it dissociates from Cu.sup.III, where the (OO.Math.) radical can coordinate and be stabilized. Finally, dioxygen is released to complete the catalytic cycle, as Cu.sub.2(BEE).sub.2 complex is re-generated (Step 5, FIG. 15):

{Cu.sup.IIICu.sup.II(BEE).sub.2[(OO.Math.)B(O)(OH).sub.2]}.sup.3++H.sub.2O.fwdarw.[Cu.sup.II.sub.2(BEE).sub.2(H.sub.2O)].sup.4++[B(OH).sub.4].sup.+O.sub.2(Step 5)

[0141] The nature of the borate buffer seems to have no weight on the process and products of the invention. As FIG. 17A demonstrates, the overpotential of the different borate buffers are quite similar. It is rational due to the additional dangling OH groups in Cu.sub.2(BDiE).sub.2 minorly assisting the proton transfer process in the rate-determining step as an extra proton acceptor.