Electrochemical Studies
The cyclic voltammogram (CV) of catalyst 1 at 1 mM in propylene carbonate obtained using a glassy carbon electrode displayed a pronounced catalytic wave. Propylene carbonate solvent was chosen for the electrocatalysis because of it has wide potential window with an oxidative limit of >2.0 (vs. NHE), and weak coordinating ability in comparison with water.7,50-51 The onset of the catalytic wave was observed at 1.13 V versus normal hydrogen electrode (NHE, the potentials presented in this study are referenced to it). The cyclic voltammogram of catalyst 1 revealed a one-electron oxidation wave at 1.14 V vs. NHE, corresponding to the oxidation of Se-2 to Se-1,52 while a weak reverse peak was also observed (Figure 2A). A solution of bimetallic zinc catalyst 1 (1 mM) in propylene carbonate solution containing 1% water shows an increase in anodic current of 19 µA in comparison to without water (Figure 2A).
Further, the anodic current increases up to 31 µA with increasing water concentration in the electrochemical cell, which is indicative of an electrocatalytic OER process. The catalytic current remained unaltered beyond 4% of water, suggesting saturation of OER with a significant increase in the anodic current (ΔI = 16.42 µA) with kobs = 7.28 s-1 (Figure S4, SI). However, the first oxidation wave remains unaltered during the electrocatalysis, implying the stability of the ligand during water oxidation by catalyst 1. Further, a decrease in the catalytic current was observed with an increase in the scan rate (Figure S5, SI), which indicates that the current is associated with the catalytic process.
The addition of water to the solution (1 mM) of diselenide 3, which is a ligand for zinc selenolate catalyst 1, increases the current by only ΔI = 0.12 µA (Figure S28, SI). Moreover, the addition of water does not enhance the current, which implies the inability of diselenide ligand 3 to catalyze water oxidation. Similarly, ZnCl2 solution (1 mM) does not catalyze the water oxidation as no change in the anodic current was observed (Figure S29, SI), and instead, precipitation was observed in the electrochemical cell during the first cycle of electrocatalysis.
Next, we sought to study the hydrogen evolution reaction (HER) from water in propylene carbonate solution by using glassy carbon (GC) working electrode. For this, the electrochemical experiment was performed in the presence of 1 mM of catalyst 1 and 0.25% of water under cathodic (negative) potential. A successive increase of water in the cell results in an increase in the cathodic current (Figure 2C). The maximum cathodic current 12 µA (ΔI = 9 µA) was elevated at 1.5% water concentration. However, the onset potential of -0.256 V vs. NHE was observed along with rate constant (kobs) of 47.32 s-1 (Figure S10, SI). Notably, the hydrogen evolution reaction (HER) from water in the presence of catalyst 1 under acidic conditions (using aqueous acetic acid and strong trifluoracetic acid, TFA) led to low cathodic current 14.8 µA (ΔI = 5.1 µA) and 3.2 µA (ΔI = 0.8 µA), (Figure S14 and S16, SI) contrary to the earlier reported catalysts in which addition an acid increases the cathodic current. It seems that the hydrogen evolution reaction (HER) catalyzed by zinc selenolate catalyst 1 proceeds preferentially by the deprotonation from water and may not from the acid, and significantly low rate constant in a strong trifluoroacetic acid could be due to the poor stability of the catalyst 1 in TFA.
To gain a deeper insight into the catalytic activity, the kinetic activities have been studied for OER and HER. A plot of icat/ip vs. [H2O]1/2 was found to be linear, indicating bimolecular first-order reaction kinetics (Figure S4 and S10, SI). Similarly, a linear plot for icat vs. [catalyst 1] was observed upon varying the concentration of zinc selenolate catalyst 1, which suggests first-order reaction kinetics with respect to the catalyst 1 (Figures 2B and 2D).
The stability of zinc selenolate catalyst 1 under oxygen and hydrogen evolution reaction conditions is confirmed by constant potential electrolysis (CPE) at an applied potential of 1.34 V (vs. NHE for OER) and -0.26 V (vs. NHE for HER) for 2h at GC electrode surface in which a significant change in current was not observed (Figure 3A and 3B). Further, the stability of catalyst 1 under electrocatalysis was also confirmed by Electron Dispersive X-Ray Spectroscopy (EDXS) in which decomposition of catalyst 1 was not realized as a residue for Zn or zinc oxide/ zinc selenide was not observed in the spectra (Figure 3C, EDXS, blue part), and also scanning electron microscopy (SEM) study of Pt-electrode does not show any change in the surface morphology (Figures 3C). The UV-Visible study was also performed on the solution from electrolysis cell after 2 h of the bulk electrolysis and before the electrolysis. The characteristic absorbance at 380 nm of zinc selenolate catalyst 1 was nearly identical (red and blue lines in Figure 3D) to the one before electrolysis (black line) confirms that the structure of the catalyst 1 remains unchanged after the electrocatalysis. These results demonstrate that the bimetallic zinc selenolate complex 1 serves as a robust catalyst for water splitting in a homogeneous system. Also, the evolved oxygen from the water during electrocatalysis was then determined by CPE, reveals a Faradic efficiency of 79% for oxygen evolution (Figure S22, SI).
Heavier analog mercury selenolate 4 also catalyzed oxygen evolution reaction (Figure, S23, SI), albeit low cathodic current 14 µA, and rate constant (0.038 s-1) (Figure S24, SI), presumably attributed to monometallic nature, was observed in comparison to the bimetallic zinc selenolate catalyst 1. Further, mercury selenolate 4 failed to catalyze the hydrogen evolution reaction (HER) from water (Figure S27). Mercury selenolate 4 was found unstable under negative potential as deposition was observed on the surface of the working electrode, presumably due to reduced mercury and could be due to feasible standard reduction potential ( 0.74V vs. SHE) of mercury ion to mercury than that of hydrogen ion to H2.
Mechanistic Studies
Mechanistic insights on bimetallic zinc selenolate 1 catalyzed OER and HER from water were gained from the theoretical assessment of the likely reaction pathways supported by the mass analysis and control experiments. The free energies of intermediates along with possible HER and OER pathways, depicted in Scheme 3, were calculated at DFT/B3LYP/def2-TZVP level of theory.
Both reactions proceed via adsorption of two hydrogen-bonded water molecules, with one coordinating to Zn(a) and the other forming a hydrogen bond to bridging oxygen yielding 1a (ΔG = 54.91 kcal mol-1), the formation of which also confirmed by mass spectrometry (Figure S1, SI). In OER, diaqua species 1a undergoes proton-coupled electron transfer (PCET) to form 1b (confirmed by mass spectrometry Figure S2, SI) with Zn(a)-OH and Zn(b)-OH2 centers. Spin-density and NBO analysis (Figure S30, SI) indicates that the electron is lost primarily from Se(a), consistent with the Se-2/Se-1 oxidation peak seen in the CV. A second PCET, accompanied by intramolecular rearrangements, leads to selenenic acid 1c. Here selenium plays a crucial role in stabilizing the –OH bridging it to Zn(a), whereas the second –OH migrates to bridge the two Zn centers and bridging µ-phenolic oxygen become terminal. The next two successive PCET steps provide selenoxide species 1d and the intramolecular ZnO---H(N) hydrogen-bonded intermediate 1e, respectively. Poor stability of selenoxide (Se=O) bond attributed to weaker π-overlap of selenium with oxygen53 and better leaving group tendency of selenium in the ligand37,53 facilitate 1e to undergo an intramolecular rearrangement to 1f containing divalent selenium center and a peroxo linkage (rO-O = 1.48Å) between Zn centers and a phenolic ring. Subsequently, better ligation ability of water than oxygen to zinc would lead to oxygen evolution from 1f with the concomitant release zinc selenolate catalyst 1a.
The HER mechanism, also initiated by the formation of 1a, proceeds with the abstraction of a proton from water by Se(a) to form 1g under applied negative potential. Subsequently, H from –Se(a)H and –NH groups combine to evolve as H2 yielding selone 1h.37 An added electron then reduces 1h to the anion radical intermediate 1i where the electronic charge on Se (0.24 e-) and radical on N (0.33 e-) are in conjugation through the phenyl ring (0.39 e-) as confirmed using spin density (see inset of Scheme 3) and natural bond order (NBO) analysis. Next, the addition of water molecule and removal of OH– anion effectively adds a proton to the system resulting in the radical intermediate 1m stabilized by conjugation with the phenyl ring. Subsequently, the sequential addition of electron and proton regenerates the water adsorbed state 1a (Scheme S3, SI).