Abstract
Exploration of efficient molecular water oxidation catalysts for long-term application remains a key challenge for the conversion of renewable energy sources into fuels. Cuboidal {Co4O4} complexes keep attracting interest as molecular water oxidation catalysts as they combine features of both heterogeneous and homogeneous catalysis with bio-inspired motifs. However, the application of many cluster-based catalysts for the oxygen evolution reaction still requires new stabilization strategies. Drawing inspiration from the stabilizing effects of natural polymers, we introduce a conductive polymer-hybrid approach to covalently immobilize {Co4O4} cubane oxo clusters as oxygen evolution catalysts. Polypyrrole is applied as an efficient p-type conducting polymer that promotes hole transfer during the oxygen evolution reaction, resulting in higher turnover frequency compared to the pristine {Co4O4} oxo cluster and heterogeneous Co-oxide benchmarks. The asymmetric coordination of {Co4O4} not only mitigates catalyst decomposition pathways, but also increases the catalytic efficiency by exposing a directed cofacial dihydroxide motif during catalysis.
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Introduction
The bottleneck for the generation of hydrogen via water splitting is the oxygen evolution reaction (OER)1,2,3,4, involving the concerted transfer of four electrons and four protons for each dioxygen molecule via high-energy intermediates. Nature accomplishes this using a cuboidal {CaMn4O5} cluster, the oxygen-evolving complex (OEC) of photosystem II (PSII)5. This natural system has inspired the design of various synthetic multi-metallic molecular catalysts for the OER, in search of structure-activity relationships, reaction mechanisms and stabilizing environments6,7,8,9,10,11.
While molecular OER catalysts offer more options for mechanistic insight and synthetic flexibility than heterogeneous catalysts, they still face challenges associated with the oxidative instability of many ligands at high anodic potentials12,13,14. This raises the fundamental task of pursuing strategies to stabilize molecular catalyst systems. A major impetus is the search for a conducting substrate with high surface area that is modified with a molecular catalyst15; this strategy may hold the key to the most active catalysts by offering tunable energetics and kinetics16.
Cobalt-based cubane catalysts, such as Co4O4(OAc)4(py)4, stand out as a promising molecular catalysts for OER studies due to their tunable structural properties and capability to facilitate multiple proton-electron transfer reactions17,18,19. In the following, we tap into its full potential by: (1) enhancing the stability of the molecular cubane catalyst, and (2) establishing a suitable conductive substrate that can be robustly and efficiently attached to the molecular cubane catalysts.
Proteins as polymer matrices found in natural enzymatic systems, such as in PSII, set a good example for addressing molecular stability issues as they offer tailored coordination environments for active sites and stabilize catalytic centers5,13,20,21,22,23. Making use of these principles, we set out to create a system that features an asymmetric cubane active site that is designed to prevent aggregation of the catalyst and can also further be embedded in conductive matrices to enhance the OER activity24,25,26,27,28.
Polypyrrole (Ppy) has been extensively studied as a conducting polymer due to its high conductivity, low toxicity, low cost, and ease of preparation29,30. Its organic backbone enables structure engineering to provide anchoring sites for {Co4O4} molecular catalysts. To address possible issues of Ppy electrodes in terms of overoxidation and decreased conductivity during prolonged cycling due to ion doping and dedoping, several methods have been demonstrated to enhance their cycling stability31,32,33,34. Therefore, conducting polymers have continually attracted considerable interest as conductive substrates for water splitting35,36,37,38. To covalently attach the {Co4O4} molecular catalyst to polypyrrole, a covalent linker, 1H-pyrrole-1-propionic acid (ppa), is used to replace the acetate ligands in the initial cubane. Subsequently, the pyrrole ring of ppa can be polymerized with pyrrole monomers, resulting in a conductive material with embedded active sites39,40,41,42.
Herein, we report on the characterization and enhanced OER performance of {Co4O4}(OAc)(ppa)3(py)4 cubane (Co4O4-ppa3) through asymmetric structure design and immobilization into polypyrrole. The resulting copolymerized macromolecule not only enhances the durable performance of {Co4O4}, but also facilitates charge transport during OER due to the conductive nature of polypyrrole43. Meanwhile, experimental and theoretical evidence demonstrates that the asymmetric coordination of Co4O4-ppa3 cubane leads to oriented active sites14, resulting in improved stability. We present concepts for the immobilization of stabilized cubane catalysts into conductive polymer matrices together with computational insight into the influence of intramolecular hydrogen bonding (IHB) on their OER mechanisms.
Results
Ligand exchange on {Co4O4} cubanes
Co4O4(OAc)4(py)4 cubane (Co4O4−0) (Supplementary Figs. 1–5, Supplementary Tables 1–3) was synthesized and purified according to published procedures7,44. Single-crystal X-ray crystallography (Fig. 1a) showed the presence of Co4O4-0 with a symmetric structure stabilized by four acetate ligands and four pyridine ligands, which was further confirmed by 1H NMR spectroscopy (Supplementary Fig. 3). To immobilize {Co4O4} in the polypyrrole matrix, the ligand ppa was chosen due to its unsaturated pyrrole ring which can be polymerized into polypyrrole to form a conjugated structure and its carboxyl functional group, which can replace the labile acetate ligands of the cubane. The starting cubane, Co4O4-0, underwent a ligand exchange process45 resulting in acetate ligand displacement by a ppa ligand (Fig. 1a). Subsequently, Co4O4(OAc)3(ppa)(py)4 (Co4O4-ppa1) (Supplementary Figs. 6–9) was synthesized under ppa stoichiometric control, followed by purification by column chromatography. The successful synthesis of Co4O4-ppa1 was established by 1H NMR spectroscopy (Supplementary Fig. 7). The signal at δ = 2.11 ppm in both spectra was attributed to methyl protons (CH3-COO) on the acetate ligands. The protons in the pyridine ring exhibited three different chemical shifts (8.40–8.43, 7.64–7.70, and 7.17–7.21 ppm). The binding of ppa was apparent from the proton signals at 6.61–6.68, 6.05–6.11, 4.05–4.28, and 2.66–2.87 ppm. Accordingly, the ratio of ppa:acetate:pyridine was 1.02:2.95:4.00, as deduced from the integration of the corresponding proton signals, which was close to the theoretical ratio of Co4O4-ppa1 (1:3:4). The formation of Co4O4-ppa1 was further confirmed by high-resolution electrospray mass spectrometry (HR-ESI-MS). A peak at 852.9 m/z. was found, corresponding to Co4O4-0 (calculated: 852.3 g/mol) (Supplementary Fig. 4). After replacing one acetate with ppa, a peak at 931.4 m/z was then detected, suggesting the successful formation of Co4O4-ppa1 (Supplementary Fig. 8).
a Structural comparison of Co4O4-0 and Co4O4-ppa1 (red, blue, gray, and light blue represent O, Co, C, and N, respectively). b, c Non-aqueous redox and quantitative CV analysis of Co4O4-0 and Co4O4-ppa1. Voltammograms were recorded at 21 °C at a scan rate of 50 mV/s in CH3CN solutions containing 0.1 M nBu4NPF6 as the supporting electrolyte and 1 mM catalyst. All potentials are referenced to the reversible redox couple of ferrocene/ferrocenium ion (Fc/Fc+). d Cyclic voltammograms of 1 mM Co4O4-0 in 0.1 M Na2SO4 aqueous electrolyte at pH 1–12 with a scan rate of 10 mV/s. e Cyclic voltammograms of 1 mM Co4O4-ppa1 in 0.1 M Na2SO4 aqueous electrolyte at pH 1–12 with scan rate of 10 mV/s.
In addition to the mono ppa substituted Co4O4-ppa1, Co4O4(OAc)2(ppa)2(py)4 (Co4O4-ppa2), Co4O4(OAc)(ppa)3(py)4 (Co4O4-ppa3), and Co4O4(ppa)4(py)4 (Co4O4-ppa4) were synthesized as well by reacting Co4O4-0 with increasing amounts of ppa. The formation of Co4O4-ppa2, Co4O4-ppa3, and Co4O4-ppa4 was confirmed by 1H NMR and HR-ESI-MS; crystals suitable for single-crystal X-ray diffraction were grown of Co4O4-ppa4 (CCDC 2260930) in acetone (Supplementary Figs. 10–19, Supplementary Tables 4-6). The measured molecular and theoretical formula weights, respectively, are summarized in Supplementary Table 7. Notably, the isomers and mixtures with similar physical and chemical properties rendered the purification and crystallization challenging. Overall, different degrees of ligand exchange for the {Co4O4} cubane were achieved by controlled addition of ppa ligand and purification.
Influence of the asymmetric coordination on the OER activity
Cyclic voltammetry (CV) of Co4O4-0 and Co4O4-ppa1 was conducted with 1 mM catalyst in CH3CN (Fig. 2c, d). In both cases, two reversible potentials were observed, suggesting a similar redox behavior of the two samples46. For Co4O4-0 cubane, the reversible potentials E1/2 = 281 and 1456 mV vs Fc/Fc+ were attributed to the [CoIII4]/[CoIII3CoIV] and [CoIII3CoIV]/[CoIII3CoV] redox couples, respectively45; where E1/2 = (Epc + Epa)/2, with Epc and Epa being the cathodic and anodic peak potentials (Fig. 1b and Supplementary Fig. 20). From the cyclic voltammogram of Co4O4-ppa1, the E1/2 for the [CoIII4]/[CoIII3CoIV] and [CoIII3CoIV]/ [CoIII3CoV] redox couples are slightly shifted to 295 and 1511 mV (vs Fc/Fc+) after one acetate ligand was replaced by ppa (Fig. 1c and Supplementary Fig. 21)45.
a Proposed aggregation route of symmetric cubane Co4O4−0 at pH 12. b Stability tests of Co4O4-0 and Co4O4-ppa1 by chronoamperometry (1.425 V vs Ag/AgCl). c Raman spectra of FTO background, Co4O4-0 stability test after 0.5 and 3 h on FTO, Co4O4-0 reference, Co3O4 reference, and CoOOH reference. d Raman spectra of FTO background, Co4O4-ppa1 stability test after 0.5 and 3 h on FTO, Co4O4-ppa1 reference, Co3O4 reference, and CoOOH reference. e Proposed aggregation route for asymmetrically coordinated cubane Co4O4-ppa1 at pH 12.
To further investigate the effect of coordination asymmetry on the electrochemical properties of Co4O4-ppa1 in aqueous solution at different pH values, CVs were recorded in water for Co4O4-0 (Fig. 1d) and Co4O4-ppa1 (Fig. 1e) over a pH range from 1 to 12. The Co4O4-0 cubane (Fig. 1d) showed similar CVs in aqueous solution from pH 5 to 10 with one reversible redox couple at E1/2 = 1.25 V (vs NHE), which corresponds to the [CoIII4]/[CoIII3CoIV] redox couple. Once the pH was adjusted below 3, the [CoIII4]/[CoIII3CoIV] redox couple shifted to higher potentials, most likely due to protonation of lattice oxygen in the {Co4O4} cubane forming a Co4O4-H+ species44,47. Upon increasing the pH to 11, water oxidation assisted by Co4O4-0 was observed. At a 10 mV/s scan rate, OH- is rapidly consumed, resulting in a CV with two peaks48. Increasing the pH to 12 resulted in a significant rise in the current density of the anodic wave and disappearance of the [CoIII4]/[CoIII3CoIV] redox couple. For Co4O4-ppa1 cubane (Fig. 1e), the ppa ligand prevented protonation of {Co4O4} and stabilized it under acidic conditions (pH 1–3) to some extent due to its inertness and hydrophobicity. Under alkaline conditions, Co4O4-ppa1 behaved similarly to Co4O4-0, showing water oxidation at pH 11 and high current density of the anodic wave at pH 12. The CV of the ligand (Supplementary Fig. 22) was found to contribute to the irreversible peak at E1/2 = 1.25 V (vs NHE). The homogeneous OER stability of Co4O4-0 and Co4O4-ppa1 was evaluated at pH 12 with fluorine-doped tin oxide (FTO) glass working electrodes (Fig. 2b), and the formation of O2 was confirmed by gas chromatography (Supplementary Fig. 23). At a constant potential of 1.622 V (vs NHE), the current of Co4O4-0 decreased gradually after 2 h, while the current of Co4O4-ppa1 remained stable for 3 h. Raman spectroscopy (Fig. 2c) showed that Co4O4-0 started to deposit on the FTO after 0.5 h of OER stability testing and was then converted to Co3O4 on the surface of the electrode after 3 h49,50. Meanwhile, Raman spectroscopy of Co4O4-ppa1 electrodes (Fig. 2d) did not show obvious signals from the Co4O4-ppa1 molecule or cobalt oxide after 0.5 h of OER testing, and the Raman signals of the Co4O4-ppa1 molecule emerged at 1026, 1217, 1563, and 1607 cm-1 after 3 h of oxygen evolution. Before and after the stability test, the CVs of Co4O4-0 and Co4O4-ppa1 (Supplementary Figs. 24 and 25) were recorded in the initial solution, which supported our observation from the Raman spectra. The SEM images (Supplementary Fig. 26) of the electrodes showed deposition of rough materials on the FTO surface after 3 h stability testing in Co4O4-0 cubane solution, while the Co4O4-ppa1 molecule formed a flocculent surface in 3 h. The energy-dispersive X-ray (EDX) mapping of the FTO electrode from the stability test of Co4O4-0 (Supplementary Fig. 27) suggests that the deposit contained only small amounts of N and mainly consisted of Co and O, while the EDX of the Co4O4-ppa1 working electrode (Supplementary Fig. 28) showed significant amounts of N. Combined Raman spectroscopy, SEM, and EDX results support the decomposition of the symmetric Co4O4-0 cubane into cobalt oxides, while the asymmetrically coordinated Co4O4-ppa1 very likely retained its molecular structure.
At high pH, hydroxide ions can displace the acetate ligands of Co4O4-0 to generate the active form of the OER catalyst, [Co4O4(OAc)3(OH)2(py)4]-, which contains a cofacial dihydroxide motif 44. The active edge sites of cobalt oxide exhibit an analogous cofacial dihydroxide motif as present in the {Co4O4} cubane51. However, [Co4O4(OAc)3(OH)2(py)4]- can further undergo acetate substitution by hydroxide ligands, resulting in a higher probability for condensation processes between cubanes (Fig. 2a)14. This is likely to reduce the number of available Co4O4-0 molecules for OER through precipitation of cobalt oxides on the electrode. The introduction of a ppa ligand into the cubane molecule changes the original symmetric cubane into an asymmetrically coordinated structure. Due to the inert character and hydrophobicity of the ppa ligand, the two Co centers linked with ppa are less prone to attack so that the ppa ligand is less likely to be replaced with hydroxide (Fig. 2e). The dissociation energy barrier (Fig. 2e) of acetate from Co4O4-ppa1 cubane was also evaluated by density functional theory (DFT) calculations and was found to be lower compared to that of the ppa ligand. Although the asymmetrical Co4O4-ppa1 cubane reduced the probability of aggregation by a lowering the probability of matched orientation, aggregation cannot be completely avoided. Therefore, further efforts are required to stabilize the {Co4O4} cubane.
Immobilization through copolymerization processes
The above electrochemical results showed that the ppa ligand provided both stability and catalytic efficiency to the {Co4O4} cubane. To further exploit these advantages, attempts for a higher degree of ppa substitution of the title {Co4O4} cubane were made. Among the available Co4O4-ppax complexes, Co4O4-ppa3 was selected based on two criteria: its asymmetric coordination and the increased ratio of ppa to cubane molecules. To prepare the polymeric material, Co4O4-ppa3 was first oxidized with addition of NH4PF6 to form [Co4O4-ppa3][PF6] (Supplementary Figs. 29–32)44. The oxidized cubane served as both oxidizing agent and monomer to form Co4O4-ppa3-Ppy (Supplementary Figs. 33 and 34), in which the Ppy and ppa ligands were linked by covalent bonding (Fig. 3a-d). Powder X-ray diffraction (PXRD) was used to characterize the products (Fig. 3e). The sharp diffraction peaks of the starting material Co4O4-0 appeared at lower angles (2θ = 3–7°), while the range of 7-30° also showed crystallinity of the material. After polymerization, Co4O4-ppa3-Ppy exhibited amorphous features similar to Ppy-ppa52. The composition of Co4O4-ppa3-Ppy was further investigated by SEM-EDX and it was found to contain C, N, O, P, F, S, and Co. The UV-vis spectrum of Co4O4-ppa3 exhibited a slight bathochromic shift compared with the absorption of Co4O4-0 (Supplementary Fig. 35). Raman spectra (Fig. 3f) showed that Co4O4-0 and Co4O4-ppa3 cubane exhibited similar characteristic features, while some weak peaks of Co4O4-ppa3-Ppy in the range of 570-770 cm-1 are from cubane as the pristine Ppy-ppa copolymer did not show these features. The peak at 1722 cm-1 in the IR spectrum (Fig. 3g) of the Ppy-ppa sample was assigned to the -COOH group of the ppa ligand and the corresponding IR peak of -COO- was shifted to 1550 cm-1 in Co4O4-ppa3-Ppy53. The solubilities of Co4O4-0, Co4O4-ppa3, and Co4O4-ppa3-Ppy in water are 0.203, 7.15·10-4, and 4.03·10-7 mol/L, respectively, as measured by flame atomic absorption spectroscopy. The electrical conductivity of Co4O4-ppa3-Ppy (5.25·10-3 S cm-1) was measured by four-point probe method, attributable to the formation of SO42- doped polypyrrole.
a–d Synthesis of Co4O4-ppa3-Ppy starting from Co4O4-0. e Powder XRD patterns of Co4O4−0, Co4O4-ppa3, Co4O4-ppa3-Ppy, and Ppy-ppa. f Raman spectra of Co4O4−0, Co4O4-ppa3, Co4O4-ppa3-Ppy, and Ppy-ppa. g FT-IR spectra of Co4O4−0, Co4O4-ppa3, Co4O4-ppa3-Ppy, and Ppy-ppa. h High-resolution Co XPS spectra of Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-PF6, and Co4O4-ppa3-Ppy.
High-resolution X-ray photoelectron (XP) spectra of Co4O4-0, Co4O4-ppa1, Co4O4-ppa3, [Co4O4-ppa3]PF6, and Co4O4-ppa3-Ppy are presented in Fig. 3h and Supplementary Figs. 5, 9, 15, 31, and 33. The Co 2p spectra of Co4O4-0, Co4O4-ppa3, and [Co4O4-ppa3][PF6], and Co4O4-ppa3-Ppy in Fig. 3h showed doublet peaks of Co 2p3/2 and 2p1/2 with satellite features. Since all the cobalt atoms in the cubanes exhibit analogous octahedral coordination geometries, the binding energy of Co rises with increasing oxidation state, other than in Co3O4 with both tetrahedrally and octahedrally coordinated Co centers54. The Co 2p3/2 peak displayed a slightly positive shift after replacement of three acetate ligands by ppa ligands (Fig. 3h). The pyrrole ring is an electron-donating group, which pushes electron density towards the carboxyl side, resulting in an increase of the basicity of ppa, rendering a high oxidation state of cobalt more likely55. After the oxidization by ceric ammonium nitrate in the presence of NH4PF6, both Co 2p3/2 and 2p1/2 peaks underwent an obvious shift toward higher binding energy, confirming oxidation of the {Co4O4} cubane in this step. The peaks of Co 2p3/2 and 2p1/2 were shifted from 780.1 and 795.0 – 780.5 and 797.1 eV, respectively, in Co4O4-ppa3-Ppy (Fig. 3h), which could be caused by the use of Na2S2O8 during the copolymerization process. Co4O4-ppa3, [Co4O4-ppa3][PF6], and Co4O4-ppa3-Ppy showed similar signals to those of the starting material Co4O4-0 in the O 1 s and C 1 s energy regions (Supplementary Figs. 5, 9, 15, 31, and 33). After the introduction of ppa ligands into the structure, the N 1 s energy regions of Co4O4-ppa3, [Co4O4-ppa3]PF6, and Co4O4-ppa3-Ppy showed an additional peak at 400.2 eV, corresponding to pyrrolic-N56. The atomic compositions and atomic ratios derived from XP spectra are summarized in Supplementary Table 8. According to the atomic ratios, the composition of Co4O4-ppa3-Ppy can be summarized as (C43H47Co4O12N7)(PF6)1.96(C4H5N)17.2(SO42-)2.75.
Catalytic performance of embedded {Co4O4} cubane catalysts
The OER activities of Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, and Ppy-ppa as electrocatalysts were evaluated with a standard three-electrode system in 1.0 M KOH and benchmarked against synthesized CoOOH and Co3O4. The amount of loaded Co on the electrodes was determined from ICP results (Supplementary Table 9). The synthetic details and XRD characterizations of the benchmarks can be found in the Supplementary Information (Supplementary Figures). As shown in Fig. 4a, linear sweep voltammetry (LSV) of the investigated catalysts was recorded without any iR compensation. Co4O4-0 showed an onset potential of ~1.62 V vs RHE, and a Tafel slope of 71.16 mV/dec (Fig. 4b, Supplementary Table 10). After replacement of three acetate ligands by ppa ligands, Co4O4-ppa3 displayed enhanced OER catalytic activity compared with Co4O4-0, with a lower onset at ~1.57 V vs RHE and smaller Tafel slope (66.38 mV/dec). This suggests that the inert ppa ligand did not obstruct the OER and had a favorable influence on the activity. This phenomenon corroborates the previous conclusion that the asymmetrically coordinated structure leads to non-equivalent metal centers facilitating the formation of a cofacial dihydroxide motif.
a Linear sweep voltammograms of OER for Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, Ppy-ppa, CoOOH, and Co3O4 at a scan rate of 10 mV/s in 1 M KOH. b Tafel plots of Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, Ppy-ppa, CoOOH, and Co3O4. c The double-layer capacitance (Cdl) of Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, Ppy-ppa, CoOOH, and Co3O4, determined from CV in a non-Faradaic region (0.86–0.98 V vs. RHE) at scan rates of 10, 20, 30, 40, and 50 mV s-1. d Nyquist plots of the OER for Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, CoOOH, and Co3O4 under an applied potential of 1.7 V vs. RHE. e OER stability test of Co4O4-0, Co4O4-ppa3, Co4O4-ppa3-Ppy, CoOOH, and Co3O4 at 20 mA cm-2. f Co K-edge EXAFS spectra of Co4O4-0, Co4O4-ppa1, Co4O4-ppa3, Co4O4-ppa3-Ppy, and post-Co4O4-ppa3-Ppy (phase uncorrected). g Proposed function of p-type Ppy during OER.
Polymerization of Co4O4-ppa3 with Ppy results in the formation of Co4O4-ppa3-Ppy, which demonstrates further enhanced OER activity, with the lowest onset potential among the investigated compounds of ~ 1.55 V vs RHE and a slightly higher Tafel slope (69. 08 mV/dec). Both of the benchmarks showed onset potentials at ~ 1.59 V vs RHE while CoOOH had the lowest Tafel slope (54.42 mV/dec). This illustrates that the rate determining step for CoOOH and {Co4O4} cubane is different57. To check for catalytic activity of the Ppy, control experiments were conducted with pristine Ppy-ppa copolymer (Fig. 4a), which exhibited almost no activity for OER compared with the blank electrode. Therefore, the higher OER performance of Co4O4-ppa3-Ppy was not arising from catalytic activity of the Ppy matrix. The electrochemical double-layer capacitance (Fig. 4c and Supplementary Fig. 38, Supplementary Table 10) of Co4O4-ppa3-Ppy (Cdl = 116.1 mF cm-2), which is directly correlated to the electrochemically active surface area (ECSA), is smaller compared to that of Co4O4-ppa3 (446.2 mF cm-2) or Co4O4-0 (250.1 mF cm-2). To better understand the mechanism behind the enhanced catalytic activity of Co4O4-ppa3-Ppy, the turnover frequencies (TOFs) of all catalysts based on the ECSA (Supplementary Tables 11) were determined at an overpotential (η) of 420 mV, where all samples had a relatively low current58. The TOF of Co4O4-ppa3-Ppy (0.379 s-1) was 11.8 and 34.5 times higher than that of Co4O4-ppa3 (0.032 s-1) and Co4O4-0 (0.011 s-1), respectively. This result indicates that the single cubane on the Ppy polymer exhibits faster reaction kinetics and higher activity.
Electrochemical impedance spectroscopy (EIS) was used to study the charge transfer behavior across the electrode/electrolyte interface during OER at η of 470 mV. Figure 4d and Supplementary Fig. 39 show the Nyquist plots of the catalysts, and the most common Randles equivalent circuit was used to fit the impedance data and model catalyst interfaces, as shown in Supplementary Fig. 40. The Nyquist plot consists of three regions, the first appearing at high frequency before the semicircle associated with the electrode/electrolyte interface resistance (Rs), the second part appearing at low frequency and representing diffusion resistance, and the third part covered by a semicircle associated with the charge transfer resistance (Rct) of redox active Faradaic OER by the adsorption/desorption of OH−/O2 at the electrode surface59,60. Supplementary Table 12 summarizes the measured EIS parameters of catalysts, while Supplementary Fig. 41 shows the Bode plots of the catalysts. Co4O4-ppa3 had a much lower electron/charge transfer resistance (Rct) value of 0.95 Ω compared with that of 3.16 Ω for Co4O4-0; while the electrode/electrolyte interaction induced resistance (Rs) was similar for both complexes. This reveals that ligand induced asymmetric coordination is beneficial for charge transfer during the OER process. After polymerization with Ppy, the Co4O4-ppa3-Ppy showed an obvious reduction of Rs (1.33 Ω) and a slight increase of Rct (0.96 Ω). The blank copolymer Ppy-pya without embedded {Co4O4} cubane showed high values of Rs (2.57 Ω) and Rct (105.48 Ω). Therefore, we can conclude that the Ppy in the system accelerated the OER on {Co4O4} active centers by the reduction of both electrode/electrolyte interaction resistance and charge transfer resistance. To further explore the role of Ppy in the catalyst, Bode plots of Co4O4-ppa3-Ppy as a function of applied potentials are presented (Supplementary Fig. 42). At the low frequency of 0.1 Hz, the impedance reflects the diffusion speed of electrolyte to the electrode surface61. With increasing potential beyond the onset, higher OER current is generated, promoting mass transfer and resulting in lower impedance. At a high frequency of 10000 Hz, the impedance reflects the charging/discharging of the electric double layer61, which decreases continually from 1.8 Ω – 1.4 Ω over the applied potential range. Although Ppy itself did not show obvious catalytic activity, the oxidized Ppy served as a hole-generator to further enhance the OER activity (Fig. 4g)62. Ppy may offer extra channels for OH− transport to the cubane active center, thereby reducing the resistance of OH−/electrode.
Stability of Co4O4-ppa3-Ppy during OER was studied through chronopotentiometry at a current density of 20 mA cm−2 for >60 h, as shown in Fig. 4e. Notably, stable catalytic performance of Co4O4-ppa3-Ppy was observed over the measured period with negligible loss of potential over time. In the early stage of the stability test, the Faradaic efficiencies (Supplementary Table 13) were ~98.6%, 99.6% and 97.5% for Co4O4-ppa3-Ppy, Co4O4-ppa3, and Co4O4-0, respectively. The CVs of Co4O4-0, Co4O4-ppa3, and Co4O4-ppa3-Ppy after stability tests (Supplementary Fig. 43) showed two small reversible peaks arising from the conversion of Co(OH)2/Co3O4 and Co3O4/CoOOH, respectively63. The Fourier Transform FT|k3χ(k)| of the extended X-ray absorption fine structure (EXAFS) spectra (Fig. 4f and Supplementary Fig. 44), and the normalized Co K-edge X-ray absorption near-edge structure (XANES) spectra (Supplementary Fig. 45) were used to study the structure of the embedded cubane moieties before and after the stability test. Interatomic distances, atomic coordination numbers (N), and Debye-Waller factors (σ2) calculated by fitting the FT|k3χ(k)| spectra are given in Supplementary Table 14. The FT|k3χ(k)| spectra of Co4O4-0, Co4O4-ppa1, Co4O4-ppa3, and Co4O4-ppa3-Ppy (Fig. 4f) are similar, which further confirms the preservation of the cubane structure in the soft polymer. After a 64 h stability test, the FT|k3χ(k)| spectrum of post-Co4O4-ppa3-Ppy showed the main two Co-O = 1.45 Å and Co-Co = 2.42 Å peaks characteristic of the local structure of the cubane Co4O4 moiety, thus indicating the structural integrity of the catalyst after OER. The small intensity Co-Co scattering peak appearing around 5.21 Å, could be due to the formation of a CoOOH phase. Raman spectra (Supplementary Figs. 46 and 47) were recorded to further investigate the post-catalysts. After 6 h of OER tests, the Raman signal of Co4O4-ppa3-Ppy (Supplementary Fig. 41) showed similar features to the starting catalyst. After a longer 60 h catalytic performance, the signal of the cubane was reduced, and the signal of CoOOH or CoxOy was not clearly detected. To further determine the composition, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Supplementary Fig. 48), together with energy dispersive X-ray (EDX) spectroscopy, and selected area electron diffraction (SAED) were used to characterize the post-catalytic material. The initial catalyst (Supplementary Figs. 48a, b and 49) exhibited cluster or particle functionalization for both the surface and interior of Ppy. Considering the cubane with dimensions around a = 13, b = 14, and c = 28 Å (Supplementary Table 4), it can be roughly regarded as a 2 × 3 nm cluster, and a few close cubanes can already result in the obvious graininess in the STEM micrographs, confirmed by EDX data. The post catalyst Co4O4-ppa3-Ppy after 6 h (Supplementary Fig. 48c, d and 50) showed similar features without distinct nanoparticles. The SAED patterns of these two samples (Supplementary Fig. 53a, b) showed amorphous phases, indicating the structural stability of the cubanes. Upon extension to 18 h, both HAADF-STEM images (Supplementary Fig. 48e, f) and EDX mapping Supplementary Fig. 51) revealed no discernible differences compared to the initial catalyst. Despite the SAED patterns, (Supplementary Fig. 53c) for the majority of the sample remained amorphous, a few areas exhibited crystalline SAED patterns with hexagonal symmetry (Supplementary Fig. 53d). After 60 h of OER tests, crystalline nano-platelets formed within the sample (Supplementary Figs. 48h and 52), and their SAED patterns could be assigned to (Supplementary Fig. 53e, f) a mixture of CoOOH and Co(OH)2. This suggests that a certain fraction of the cubanes underwent decomposition, releasing reducible CoIII. Given the presence of CoOOH deduced from EXAFS spectra, we assume that this decomposition of cubanes resulted in the formation of CoOOH under OER conditions.
Combing the OER stability test and the post-catalytic analysis, the conducting polymer-stabilized cubanes maintained structural integrity and OER stability at a current density of 20 mA cm−2 for at least 6 h, with limited degradation observed after 18 h. The duration of our catalyst exceeds previous leading OER stability tests of {Co4O4} cubanes, which were conducted at a current density of 10 mA cm−2 for 5 h14.
Computational studies
Based on reported evidence supporting the presence of a cofacial dihydroxide motif under alkaline conditions14, we assigned this intermediate as the starting motif (intermediate I; Fig. 5a and Supplementary Fig. 54; atomic coordinate data are available in Supplementary Data 1). Interestingly, in our study, intramolecular hydrogen bonding (IHB) between the two hydroxide groups is always present during the calculations. As shown in Fig. 5a, O-O bond formation via the directed cofacial dihydroxide motif was computed via three different pathways: single-site (SS), double-site (DS), and synergistic route (Sy).
a Calculated energy profiles of SS (green), DS (orange), and Sy (black) pathways was modeled using the Perdew-Burke-Ernzerhof functional within the framework of the generalized gradient approximation. Relative electronic energies are given in eV. b Proposed OER mechanisms according to the calculated Sy pathway.
The transition from OHOH (intermediate I) to O*-H-O (intermediate II) or O*-O-H (intermediate II#) is the rate-determining step (RDS) of the entire water oxidation process. The free energy difference (∆E) is 2.22 eV for Sy/DS pathways. The SS pathway (Fig. 5a and Supplementary Fig. 55) has also been considered, but the ∆E is higher by 2.50 eV compared to the Sy pathway and thus unlikely.
During the substitution of acetate ligands by two hydroxide ions, several studies have obtained evidence for a proton-coupled electron transfer (PCET) process14,64,65, corresponding to the oxidation of [CoIIICoIIIOAc] to [CoIIICoIV(OH)2]. High-valent CoIV has stronger electron-withdrawing ability66,67, resulting in a lower oxygen electron density compared with the oxygen on CoIII. Together with our calculations, this suggests that IHB is present between the hydrogen on the CoIV site and oxygen on the CoIII site (Fig. 5b). In the hypothetical Sy pathway, the hydroxide on a CoIV site is first deprotonated with the formation of [O = CoVCoIIIOH], which is rapidly converted to an O-O bridged intermediate. The lower ∆E of this step for the Sy pathway can be ascribed to the low electron density of the oxygen on the CoIV site, which facilitates the deprotonation. Unlike the DS pathway (Supplementary Fig. 55), which involves a two-electron process for oxygen desorption, the Sy pathway, which separates the two-electron process into two, single-electron transfer steps is more reasonable in this bivalent [CoIIICoIV(OH)2] system.
Discussion
First, strategic asymmetric ligand modification of a cobalt cubane catalyst with a molecular linker (Co4O4-ppa1) gave rise to better OER stability compared with pristine Co4O4-0, suggesting that the asymmetric design prevents aggregation by minimizing the alignment of cubane molecules in intermolecular condensation processes. In light of the enhanced performance of Co4O4-ppa1, a further substituted {Co4O4} cubane with three ppa ligands Co4O4-ppa3, was synthesized next. This modified cubane was then clicked into the p-type conducting polymer Ppy. The resulting Co4O4-ppa3-Ppy was highly efficient for OER catalysis under alkaline conditions with a much higher TOF than its congeners and benchmarks. Experimental and theoretical results based on this model catalyst demonstrated that asymmetrically coordinated cubanes offer well-defined active sites for the OER and reduced electron/charge transfer resistance (Rct), thereby accelerating the O-O bond formation. Moreover, p-type Ppy generates holes in its conjugated structure under oxidative potential, and these holes are in favor of the OER on the embedded {Co4O4} cubane due to the robust covalent attachment. Stability tests and post-catalytic studies showed that our strategy prolonged OER applications and notably mitigated large-scale aggregation processes within the composite catalyst.
Methods
Materials and characterization methods can be found in the Supplementary Information.
Synthesis of Co4O4-0 and Co4O4(OAc)x(ppa)4-x(py)4 (x = 0,1,2,3)
Co4O4(OAc)4(py)4 cubane was synthesized and purified by our modified method according to published procedures7,44. Cobalt(II) acetate tetrahydrate (8.54 g, 34.3 mmol) was dissolved in 100 mL of methanol, and then pyridine (2.8 mL, 34 mmol) was added under stirring. Hydrogen peroxide (30% w/w in water, 20 mL) was added dropwise to the above solution, and then the reaction mixture was refluxed for 2 h. The dark green solution was dried by rotary evaporator, and the solid was redissolved in 100 mL dichloromethane. Then 20 mL water was used to extract the residual cobalt acetate in the product. The product was collected from organic layer by adding 500 mL of hexane. The solid was dry-loaded onto a silica column and eluted with 5% methanol in acetone. The collected product was dried and further purified with Na4EDTA to remove trace Co(II). The final yield of Co4O4-0 was 2.66 g, 37%.
The purified Co4O4-0 (2 g, 2.344 mmol) was dissolved in 80 mL of ethanol. After stirring for 10 min, ppa in ethanol was added into the above solution. The mixture was heated to 75 °C for 4 h. The solution was left to stand for 24 h. A black solid was collected by centrifugation and washed with water (20 mL) three times. The dried solid was dissolved in 10 mL of dichloromethane. Hexane (40 mL) was added to the dichloromethane solution to precipitate black Co4O4(OAc)x(ppa)4-x(py)4 (x = 0,1,2,3) which were collected by decantation. The amount of ppa was adjusted for the respective target molecule. The collected product was then purified using silica column chromatography with acetone: methanol at 95: 5. For the synthesis of Co4O4-ppa3, the yield was 1.63 g, 64%.
Synthesis of Co4O4(OAc)1(ppa)3(py)4PF6
Purified Co4O4(OAc)1(ppa)3(py)4 (0.545 g, 0.5 mmol) was dissolved in 3 mL of dichloromethane and then dispersed in 30 mL ethanol. Then, ceric ammonium nitrate (0.274 g, 0.5 mmol) in 10 mL of ethanol was added to the above solution. After 5 min stirring, sodium hexafluorophosphate (0.084 g, 0.5 mmol) in 5 mL of ethanol was added to the reaction mixture, resulting in a black solid. The solid was collected and washed with water (20 mL) three times. The Co4O4(OAc)1(ppa)3(py)4PF6 was obtained by drying the solid under vacuum (0.385 g, 62%). The 1H NMR spectra are consistent with those previously reported44.
Synthesis of Co4O4(OAc)1(ppa)3(py)4-Ppy
Co4O4(OAc)1(ppa)3(py)4PF6 (0.1235 g, 0.1 mmol) was dissolved in 25 mL of ethanol. Then pyrrole (0.14 mL, 2 mmol) was added at room temperature. The solution was stirred for 30 min. Sodium persulfate (0.238 g, 1 mmol) was dissolved in 15 mL water. The solution of Co4O4(OAc)1(ppa)3(py)4PF6 and pyrrole was dropped into the sodium persulfate aqueous solution under stirring in a water/ice bath, producing a black solid. After centrifugation and washing with 3 × 40 mL water, a black solid (0.244 g) was collected.
Synthesis of Ppy-ppa copolymer
1H-Pyrrole-1-propionic acid (0.0417 g, 0.3 mmol) was dissolved in 10 mL of ethanol at room temperature. Pyrrole (0.14 mL, 2 mmol) was added into the above solution. Then the solution was added into a 15 mL aqueous sodium persulfate (0.238 g, 1 mmol) solution. After stirring for 30 min, a black solid was obtained and collected by centrifugation. The Ppy polymer (0.146 g) was obtained after 3 × 40 mL washing with water and drying at 60 °C in an oven.
Electrochemical characterization methods and and details about DFT calculations can be found in the Supplementary Information.
Data availability
The authors declare that all data supporting the findings of this study are available within the paper and its Supplementary Information files. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under the deposition number 2260930. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The atomic coordinates of the optimized computational models generated in this study are provided in the Supplementary Data 1.
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Acknowledgements
This work was funded by the Swiss National Science Foundation (Grant No. 200021_200989/1) and by the University of Zurich. S. Li thanks Chaochen Dong and Han Zhao (Department of Chemistry, University of Zurich) for their help with column chromatography and XPS, respectively. Z. Zhang and S. Li thank Prof. Jürg Hutter and Sertcan Beliz (Department of Chemistry, University of Zurich) for the discussion on computational studies. The Swiss Norwegian beamlines (SNBL) at ESRF are acknowledged for provision of beamtime and the staff for helpful support.
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S.L. conceived the project design and performed the syntheses, as well as most of the structural characterizations and electrochemical measurements. Z.Z. performed DFT calculations. W.R.M., X.H., H.C., D.C.S., and R.E. provided support with characterizations. C.A.T. performed XAS analyses. The manuscript was written by S.L. with input from the other authors, and all the authors reviewed the paper. G.R.P. supervised the project, participated in writing and revising the manuscript, and supplied financial support for the entire process.
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Li, S., Zhang, Z., Marks, W.R. et al. {Co4O4} Cubanes in a conducting polymer matrix as bio-inspired molecular oxygen evolution catalysts. Nat Commun 15, 8432 (2024). https://doi.org/10.1038/s41467-024-52514-z
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DOI: https://doi.org/10.1038/s41467-024-52514-z
