Supramolecular Catalyst with [FeCl4] Unit Boosting Photoelectrochemical Seawater Splitting via Water Nucleophilic Attack Pathway

doi:10.21203/rs.3.rs-3429457/v1

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Supramolecular Catalyst with [FeCl₄] Unit Boosting Photoelectrochemical Seawater Splitting via Water Nucleophilic Attack Pathway

https://doi.org/10.21203/rs.3.rs-3429457/v1

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published 06 Mar, 2024

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Propelled by the structure of water oxidation co-catalysts in natural photosynthesis, molecular co-catalysts have been believed for a long time to possess the developable potential in artificial photosynthesis. However, the interfacial complexity between a light absorber and the molecular co-catalyst limits its structure stability against pH value and charge transfer efficiency. To overcome the challenge, a supramolecular scaffold with the catalytic unit instead of the molecular structure is, for the first time, reported. It is found that the [FeCl₄] catalytic sites in the supramolecular matrix (polythiophene, polyaniline, or polypyrrole) undergo a water-nucleophilic attack defining the rate-limiting step of the water oxidation reaction, while the supramolecular matrix can be in situ grown on the surface of photoelectrode to enable a strongly coupled interface. A well-defined BiVO₄ photoanode surface hybridized with [FeCl₄] catalytic sites in polythiophene matrix reaches 4.72 mA cm^− 2 at 1.23 V vs reversible hydrogen electrode under AM 1.5 G illumination, which also exhibits great stability for photoelectrochemical seawater splitting due to the restraint on chlorine evolution reaction by [FeCl₄] unit, the anti-corrosion ability of polythiophene as well as the strongly coupled interface. This work provides a novel solution to the challenge of the interface charge transfer of molecular co-catalyst hybridized photoelectrode.

Physical sciences/Energy science and technology/Renewable energy/Solar energy/Artificial photosynthesis

Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis

Natural photosynthesis inspires us to utilize sunlight to address the energy and environmental crisis. In natural photosynthesis, the oxygen evolution reaction (OER) is the core reaction, in which the light absorber of chlorophyll is connected to the water oxidation catalyst of metal center-ligand molecular with ideal charge transfer efficiency.^1,2 Enlightened by such delicate structures in nature, many efforts have focused on simulating those molecular structures to achieve the same complexity and precision of natural photosynthetic processes, such as metal-centric ligands or complexes, cubane complexes, polyoxometalate (POM), etc., which strive to reproduce the coordination environment, combined geometrical and electronic properties of natural water oxidation catalysts.^3–5 Until now, molecular catalysts can achieve a high turnover frequency (TOF) of thousands for water oxidation that is much higher than inorganic cocatalysts.^6–8 Triggering considerable efforts devoted to anchoring the molecular catalysts onto light absorbers for building highly efficient artificial photosynthesis systems.^8,9 However, the solar-driven water oxidation efficiency in whatever artificial photosynthesis system is profoundly restricted by the interface between the light absorber and molecular catalyst, as the photo-induced charges always need across the interface formed by a molecular anchoring that is sensitive to the surrounding chemical environment.^10–13 Therefore, solar-driven water oxidation based on molecular catalysts still faces a formidable challenge in their interface engineering.

Supramolecular catalysts, which are non-covalent crosslinks of linear polymers or macromolecular groups connected by non-covalent bonds, exhibit high and precise catalytic activity due to their free delocalized electrons and the presence of host-guest active coordination bonds.^14–16 More importantly, it can be further developed to the polymeric system mediated by π-stacking interaction and cross-links, thus reinforcing structure rigidity.^17–19 However, the designed supramolecular catalysts have a complex structure and require meticulous synthesis procedures. Furthermore, although there are a few reports of supramolecular catalysts as OER active catalysts in photoelectrochemical (PEC) water oxidation, the strongly coupled interface between supramolecular catalysts and light absorber has always been ignored and remained a great challenge to prolong its long-term stability.^5,20,21

In this work, aided by a FeCl₃ initiator, an [FeCl₄] active unit in the supramolecular (polythiophene, (PTh), polyaniline (PANI) or polypyrrole (PPy)) matrix can be naturally formed the polymerization process, which acts as an excellent OER catalyst to promote the charge carriers transfer, prolongs the charge lifetime and optimizes the surface reactions kinetics of a BiVO₄ (BVO) photoanode for PEC water splitting through a simple in situ chemical polymerization strategy. While, the [FeCl₄] active unit in the PTh presents the best stability and anti-corrosion even in seawater, boosting photocurrent density of BiVO₄ photoanode from original 1.61 to 4.72 mA cm^− 2 with long stability of 40 hours for PEC seawater splitting.

Porous BVO photoanode as a light absorber was synthesized based on Choi's method without further modification.²² The supramolecular catalyst consisted of a PTh scaffold with [FeCl₄] active units can be in-situ grown on the surface of BVO by one-pot polymerization of thiophene monomers using FeCl₃ as an initiator (Fig. 1a), the synthesis procedure in detail was given in supporting information. The reduction of FeCl₃ to FeCl₂ occurs during the polymerization process, and a portion of FeCl₃ can trap chloride ions to form coordination bonds due to its high electron affinity and Lewis acid nature, resulting in a stable [FeCl₄] tetrahedral structure formed in the PTh matrix.^23–25

The [FeCl₄] unit is determined by ⁵⁷Fe Mössbauer spectra. As revealed in Fig. 1b, the spectra can be well-fitted with two doublets whose Mössbauer parameters are collected in Table 1, corresponding to two coordination and doping modes that are similar to those of polythiophene and its derivatives.^26–28 Both principal doublets have the low isomer shift (IS) and quadrupole splitting (QS), which indicates that Fe is trivalent to form [FeCl₄] units and the coordination number is six.²⁹ The lower quadrupole splitting represents an asymmetric distortion in the tetrahedral structure of [FeCl₄] units, which provides the possibility of weak coordination with the S atom of polythiophene to form the S-[FeCl₄]-S, and this slight distortion will further improve the catalytic performance.³⁰ The structure of [FeCl₄] is chemically stable, and its non-covalent coordination and interaction with polythiophene constitute a homogeneous iron-based supramolecular polymer. Furthermore, the Fe^III in polythiophene has a larger density of 3d electrons and it performs a high spin state, which would be beneficial for the OER process.^{31, 32} The [FeCl₄] unit in the supramolecular structure of polythiophene is further demonstrated by time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurement (Fig. 1c and Supplementary Fig. 1).³³ Different kinds of secondary ions fly at different speeds due to specific mass charge ratios and thus can be detected sensitively. The typical secondary negative-ion [FeCl₄]⁻ is detected at 197.8 m/z, additionally, typical ion fragment signals attributed to BVO electrodes (VO₃⁻, 98.9 m/z) and thiophene (C₄HS⁻, 81.0 m/z) are also detected, which further demonstrate the existence of a Fe-based coordination structure.

Table 1

Mössbauer Parameters of PTh/BVO
	IS (mm/s)	QS (mm/s)	Area (%)
Doublet 1	0.30	0.87	94.1
Doublet 2	0.10	0.71	5.9

The composition and electronic states of BVO and PTh/BVO photoelectrodes are compared by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) spectra (Supplementary Figs. 2, 3, 4, 5 and 6). As shown in Supplementary Fig. 2, accessorial signals of Fe, S and Cl can be found after in-situ grown polythiophene. In high-resolution XPS spectra of PTh/BVO, the Cl 2p signal attributed to the [FeCl₄] in the polythiophene can be observed at a binding energy of 198.0 eV and 199.7 eV (Supplementary Fig. 3a). In S 2p XPS spectra of PTh and PTh/BVO (Supplementary Fig. 3b), in addition to verify the existence of [FeCl₄] unit in the polythiophene, a new S 2p peak appears at 163.5 eV for PTh/BVO, and all of S peaks shift towards high energies, which is attributed to the nucleophilic S atom bonding to the empty orbital formed by the unpaired lone pair electrons of Bi atoms at the BiVO₄ surface. The formed chemical bonding is believed to facilitate the charge transfer and reinforce the interface stabilization.^{34, 35} Similarly, high energies shift of peak is also observed in Bi 4f and V 2p XPS spectra (Supplementary Fig. 4), suggesting that the electronic structure change is attributed to the interaction between Bi atoms and the polythiophene layer in the interface region, and polythiophene acted as electron acceptor.^{36, 37} Fe 2p XPS spectra in Supplementary Fig. 5 clearly show Fe signals in both PTh and PTh/BVO, where four peaks can be identified. The binding energy and signal parameters of Fe 2p XPS spectra are not the same as those of previously reported Fe-based oxides and hydroxide cocatalysts, which indicates a Fe-Cl coordination compound nature in polythiophene without other impurities.^38–40

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of BVO and PTh/BVO photoelectrodes are shown in Fig. 1d-e. The pristine BVO exhibits a three-dimensional 2nanoporous structure with an average grain size of more than 100 nm (Fig. 1d) and the thickness of the film is about 1.2µm (Supplementary Fig. 7). After the in situ polymerization of PTh on the surface of BVO, the surface of BVO becomes rough (Fig. 2e). X-ray diffraction (XRD) patterns of BVO and PTh/BVO photoelectrodes do not show a significant difference (Supplementary Fig. 8), All diffraction peaks can be indexed to the monoclinal BVO (JCPDS: 14–0688). High-resolution TEM (HR-TEM) image reveals that an amorphous PTh layer with a thickness of around 10 nm is tightly and uniformly coated on the BVO surface (Fig. 2f). Moreover, the lattice fringes of BVO are clearly discernible with a spacing of 0.308 nm, which corresponds to the (121) plane of monoclinal BVO, while the lattice spacing of 0.194 nm can be indexed to the (240) plane of monoclinal BVO. Energy-dispersive X-ray spectroscopy (EDS) mapping of PTh/BVO shows that the signals of Bi, V, O, C, S, Fe and Cl elements are uniformly distributed in the entire region (Fig. 1g).

Due to the chemically bonded interface formed between the polythiophene and BVO, the transfer of photogenerated charge carriers at the interface would be significantly promoted. The kinetics of surface charge transfer and recombination characteristics are investigated by intensity-modulated photocurrent spectroscopy (IMPS) spectra (Fig. 2a-d), where the IMPS spectra were conducted by a high-frequency semicircle located in quadrant IV and a low-frequency semicircle located in quadrant I, corresponding to bulk charge diffusion and surface-associated process, respectively.⁴¹ The rate of charge transfer and recombination can be represented by two constants k_trans and k_rec. In the normalized IMPS spectrum, k_tr / (k_trans + k_rec) can be obtained from the intercept of the low-frequency semicircle, and k_trans+k_rec can be determined from the frequency at the maximum imaginary current.^{42, 43} According to Fig. 2a-b, in the bias range of 0.4-1.2V vs. RHE, the IMPS plots of PTh/BVO have a smaller radius of high-frequency semicircle and a larger intercept with x-axis than those of BVO. This corroborates that the PTh/BVO photoanode has higher charge transfer efficiency and favorable carrier kinetics. In addition, the pseudo-first-order rate constants k_trans and k_rec can quantitatively describe the rates of charge transfer and recombination. As shown in Fig. 2c, The PTh/BVO photoanode exhibits lower k_rec in the bias range, implying that surface charge recombination is well suppressed. Moreover, the PTh/BVO photoanode also demonstrates a higher transfer rate constant k_trans than the BVO photoanode, which plays a determining role in the surface reaction kinetics (Fig. 2d).⁴⁴ These characteristics are consistent with those molecular catalysts loaded cases,^{20, 45} which confirm that the supramolecular PTh layer with [FeCl₄] unit can act as an excellent OER catalyst to not only accelerate the kinetics of water oxidation but also alleviate surface charge recombination.

To further disclose the charge carrier kinetics process, time-resolved photoluminescence (TRPL) and transient-state surface photovoltage (TS-SPV) spectra were measured (Supplementary Fig. 9). As shown in Fig. 2e, after fitting the TRPL spectra as a bi-exponential function of time, the charge carrier lifetime consists of two-time constants, namely the fast decay component (τ₁) and the slow decay component (τ₂), which correspond to the intrinsic defect trapping of photo-generated charges and electron-hole recombination, respectively.^46–48 All parameters of the TRPL measurement are summarized in Table S1 in the supporting information. It can be seen that both τ₁ and τ₂ of PTh/BVO are longer than that of BVO. In particular, τ₂ of PTh/BVO elevates to 7.30 ns compared to pristine BVO (4.65 ns), which proves that the enhanced separation and diffusion of electron-hole pairs due to the inhibited surface recombination by the PTh layer.⁴⁹ Compared to TRPL spectra, the TS-SPV spectra can reflect the generation, separation, migration and recombination of charge carriers on the electrode surface, and directly exhibit the photogenerated charge concentration and lifetime. The TS-SPV measurements were carried out under 355nm laser radiation, and the results are presented on a normal timescale (Supplementary Fig. 10) and a logarithmic time scale (Fig. 2f), respectively. It can be seen that the recombination time of the PTh/BVO photoanode is approximately 10 times longer than that of the BVO photoanode. Moreover, the signal intensity of PTh/BVO is more than 42 times stronger than that of BVO. Interestingly, the TS-SPV spectrum of PTh/BVO exhibits distinguishable time domain, from which behaviour of charge carrier kinetics can be divided: 1) when the laser radiates, photogenerated holes of BVO are injected into the PTh layer because of the built-in electric field at the interface and the holes extraction by [FeCl₄] unit, this process is quite rapid to reach the maximum and is manifested by a sharp signal response peak (1.2×10^− 7 s, Peak 1);⁵⁰ while, the photoexcited electrons of supramolecular catalyst in PTh layer are also injected into the BVO, whereas the latter process is negligible; 2) a minority of the surface holes are trapped, so the photovoltage signal has a small period of sharp drop (1.2×10^− 7 to 4.0×10^− 7 s);⁵¹ 3) the PTh layer stores the injected holes so that these holes have long lifetime and avoid fast charge recombination, which is displayed as a flattening of the photovoltage signal and reaching a stable intensity (2.0×10^− 6 s, Peak 2); 4) eventually, the photogenerated holes and electrons are completely recombined. The results illustrate that the PTh layer does not cause a negative effect on charge transport, but might enable additional charge extraction and storage capacity to improve the interface effect between the light absorber and molecular catalyst.

To demonstrate the [FeCl₄] unit on improving the PEC performance, the PTh layers on the BVO surface were polymerized by other two initiators, (NH₄)₂S₂O₈ (APS) and HAuCl₄. PEC performances of BVO and three kinds of PTh/BVO (noted PTh-APS/BVO and PTh-Au/BVO) photoanodes were measured in 0.5 M borate buffer (pH = 9.0) electrolyte under AM 1.5G simulated sunlight illumination (100 mWcm^− 2). It can be seen that the photocurrent densities of three kinds of PTh/BVO photoanodes are all higher than that of BVO, indicating the positive effect of the PTh layer on improving charge carrier kinetics. Remarkably, the PTh/BVO with FeCl₃ as initiator has much higher photocurrent density than those with APS and HAuCl₄ as initiators. By regulating the concentration of FeCl₃, polymerization time and electrolyte, the precise utilization yields of FeCl₃ and the optimal experimental parameters were obtained (Supplementary Fig. 11). The photocurrent density increases from 1.61 mA cm^− 2 for BVO to 4.72 mA cm^− 2 for PTh/BVO with FeCl₃ as initiator at 1.23V vs. RHE, which is approximately 2.9 times higher than the pristine BiVO₄ photoanode and also stands out among the previous counterparts (Fig. 3a, Table S2). Furthermore, an evident cathodic onset potential shift (~ 220 mV) is observed, indicating robust charge separation efficiency and optimized reaction kinetics.^{36, 52} Moreover, the OER activities of the PTh/BVO with different initiators were also elucidated in Supplementary Fig. 12. A great contribution from [FeCl₄] unit to OER activity enables the cathodically shifted overpotential by ~ 297mV compared with that of BVO (Supplementary Fig. 13a), and the Tafel slope is decreased from 386 mV/decade of BVO to 264 mV/decade of PTh/BVO (Supplementary Fig. 13b). The results demonstrate that the PTh supramolecular with [FeCl₄] unit could serve as a co-catalyst to improve OER reaction kinetics as well as facilitate charge transfer process.^{53, 54} Additionally, chopped transient photocurrent density at 1.23 V vs. RHE were conducted in Supplementary Fig. 13. The presence of severe electron-hole recombination at the interface will cause a steep spike of the photocurrent, however, the PTh/BVO shows a negligible spike, ascribing to fast water oxidation kinetics.⁵⁵ Correspondingly, the charge transfer efficiency (ƞ_transfer) and charge transport efficiency (η_transport) are calculated by using sodium sulphite as a hole scavenger (Supplementary Fig. 15, S16 and S17), which shows a higher η_transfer value (81.3%) of PTh/BVO than that of BVO photoanode (36.6%) at 1.23V vs. RHE, but a little change in η_transport due to the main co-catalyst effect on OER reaction kinetics. Applied bias photon-to-current efficiency (ABPE) of BVO and PTh/BVO were calculated in Fig. 3b. The ABPE for PTh/BVO is 1.34% at 0.75V vs. RHE, which is much higher than that of pristine BVO (0.27%, 0.94V vs. RHE). The favorable photoconversion efficiency at low potentials will be the most significant feature for achieving unbiased solar water splitting. Incident photon to current conversion efficiencies (IPCE) of BVO and PTh/BVO photoanodes are shown in Fig. 3c. The IPCE value after light harvesting efficiency correction of PTh/BVO reaches above 80% at wavelengths from 350–450 nm at 1.23 V_RHE, which is much higher compared with pristine BVO. The UV spectra (Supplementary Fig. 18) show that there is negligible enhancement of light absorption, so the role of charge transfer is the most decisive in the elevation of IPCE.

To further understand how the PTh matrix with [FeCl₄] unit serves as a high co-catalyst for OER, Mott-Schottky (M-S) and electrochemical impedance spectroscopy (EIS) measurements were investigated. As displayed in Supplementary Fig. 19, the flat band potential (E_fb) of BiVO₄, which is determined by extrapolating the Mott–Schottky plot to 1/C² = 0, positively shifts from 0.38 to 0.49 V after PTh coating. This change in E_fb indicates the formation of a p-n junction.⁵⁶ In addition, according to the effective surface area of the nanoporous BVO electrode, the charge carrier densities (N_D) of PTh/BVO are calculated as 3.96×10¹⁹ cm^− 3, which was 5.8 times higher than that of BVO (6.87×10¹⁸ cm^− 3), corroborating that [FeCl₄] unit as electron donor robust the electronic conductivity.⁵² Furthermore, the equivalent circuit for interpretation of the EIS results is illustrated in Supplementary Fig. 20 and the electrochemically active surface areas (ECSA) are shown in Supplementary Fig. 21. The higher slope of PTh/BVO illustrates that the redox-active Fe sites have increased the OER active surface, which further implies the enhancive double-layer capacitance. The fitted capacitance of the surface state (C_trap) as well as the photocurrent density for BVO and PTh/BVO are illustrated in Fig. 3d.⁵⁷ The C_trap values of both electrodes reach the highest at the photocurrent onset potential, while the overall smaller C_trap value of PTh/BVO indicates a rapid depletion of surface holes during water oxidation. Further on, the PTh/BVO exhibits a significant surface charge release rate, and the accumulated charges are completely depleted within a narrow voltage range, which proves the faster water oxidation process on the surface of PTh/BVO accounting for the enhanced photocurrent density.⁵⁸ Supplementary Fig. 22 exhibits the illumination intensity modulated EIS spectra of BVO and PTh/BVO. The reaction order can be calculated from log/log plots of photocurrent density and hole density to further explain the OER dynamics and mechanism on the surface. As shown in Fig. 3e, the reaction orders of PTh/BVO and BVO are 1.27 and 1.94, respectively, indicating that PTh/BVO has a faster kinetics of water oxidation.⁵⁹ According to previous reports, the results suggest that the OER on the surface of PTh/BVO is mainly the water nucleophilic attack (WNA) mechanism since [FeCl₄] units act as the active sites of OER like the coordination metal centers in molecular catalysts,^{11, 60} and this one-hole transfer pathway is easier to be realized, thus allowing the surface charge to participate in the water oxidation reaction more rapidly.⁶¹ Subsequently, kinetic isotope effects (KIE) are used to deeply explicate the mechanism of water oxidation, which contains information about proton transfer kinetics, so as to reflect the rate-determining step (RDS) in the whole reaction process. The KIE values are defined as the ratio of the photocurrent densities measured in water and deuteroxide electrolytes (Supplementary Fig. 23). It can be observed from Supplementary Fig. 23b that the KIE values of BVO are all higher than 2, indicating that the proton transfer process is the RDS of the water oxidation reaction, corresponding to the O-H bond rupture.20 The KIE values of PTh/BVO are less than that of BVO, which means that the proton transfer process is well promoted, thus the water oxidation reaction kinetics can be improved more effectively and holes can be fast transferred to the water.^{62, 63} This can be elucidated by the good hydrophilicity of PTh (Supplementary Fig. 24), as well as the reduced energy barrier of water oxidation intermediates formation.

To further clarify the effect of the [FeCl₄] active site presented in the PTh layer on the OER process, cyclic voltammetry (CV) curves are shown in Fig. 3e. From the CV curves, it is clear that there are two oxidation peaks before the onset potential of steady-state water oxidation. Compared with the CV curve of BVO, the peak at 0.81V may be attributed to the oxidation of species V, and this oxidation can be analyzed to be inhibited by the PTh layer.^{64, 65} A slight peak located at 1.54 V can be ascribed to the oxidation of the Fe site, indicating that a small amount of Fe in the PTh layer undergoes a Fe^III to Fe^IV transition, and this valence change will cause a part of the holes to be stored.⁶⁶ The hole storage capacity of PTh can facilitate the hole to the electrode/electrolyte interface to efficiently participate in the water oxidation process.⁶⁷ On the other hand, these Fe species in the PTh matrix acted as redox-active sites, and the turnover frequencies (TOF) versus overpotential were calculated in Supplementary Fig. 25.

Stability and reproducibility are two important properties that determine whether photoanodes can be generalized to practical applications. J-T curve in Fig. 4a indicates that the PTh/BVO has stability at least for 50 hours. To our surprise, PTh/BVO also has high photocurrent density (Supplementary Fig. 26) and great stability in seawater electrolytes. As shown in Fig. 4b, the photocurrent density of the PTh/BVO can remain stable for about 5h in seawater, while in sharp contrast, the BVO was rapidly corroded due to faster chlorine evolution oxidation (CER). The ICP-MS measurements further reveal that the containment of V element dissolution was much less than the bare BVO photoanode after J-T measurement in water electrolyte for 40h (Supplementary Fig. 27), and the morphology and chemical structure of the photoanode did not change significantly (Supplementary Fig. 28–31). Different from the previous surface functionalization and passivation strategies, this phenomenon can be attributed to the Cl⁻ storage properties of polythiophene.⁶⁸ As shown in Supplementary Fig. 32, In the CV curve, in addition to the oxidation peak of V species, the PTh/BVO electrode showed an oxidation peak and a reduction peak at 0.66 and 0.36 V vs. RHE, respectively. The corresponding reactions for 1 and 2 are as follows:

PTh + Cl⁻ → PThCl + e⁻ (1)

PThCl + e⁻ → PTh + Cl⁻ (2)

This ox-red cycling of Cl ions could avoid direct contact of Cl⁻ ions with the BVO surface. Moreover, the inhibitory effect on chlorine oxidation by Fe-Cl coordination bonding exists in the PTh matrix.⁶⁹ Furthermore, we fabricated 100 PTh/BVO photoanode (Fig. 4c), their photocurrent densities and stability present a fluctuation of only 2.55% and 4.40%, respectively, indicating good reproducibility.

The schematic diagrams of the PTh/BVO for PEC water splitting are shown in Fig. 4d. Profited by supramolecular like PTh/[FeCl₄] catalyst modification, the sturdy “S-Bi” bonding enables the interface stable enough to optimize the charge transfer and the lifetime of the photogenerated hole. The Cl⁻ stored in the PTh matrix can effectively avoid the corrosion of CER to the electrode, enabling molecular catalyst-like behaviors but superior stability to molecular catalysts. Moreover, due to the single active site and valence transition nature of [FeCl₄], the WNA mechanism occurs more tendentiously during the water oxidation, which OH⁻ continuously nucleophilic attacks to complete the proton-electron coupling process (Fig. 4e). In addition to the PTh, this supramolecular co-catalyst can also be extended to other conductive polymers, such as polyaniline (PANI) and polypyrrole (PPy).⁵⁶ By using FeCl₃ as an initiator, the [FeCl₄] active sites in PANI and PPy are still similar to PTh (Supplementary Fig. 33), by which both PANI/BVO and PPy/BVO exhibit similar morphology (Supplementary Fig. 34) and positive effect on improving PEC performance (Supplementary Fig. 35).

In summary, we have reported a strong interface coupled polymeric PTh, PANI and PPy/BVO photoanode via a simple in situ FeCl₃ initiator-induced polymerization strategy, where the FeCl₃ initiator was converted to [FeCl₄] unit having OER activity in PTh, PANI and PPy matrix. These conductive polymers have hole extraction and storage capacity to improve the behavior of charge carriers’ separation and further accelerate OER kinetics by the presence of [FeCl₄] unit, which presented as functions of supramolecular catalyst. Among them, the PTh/BVO reaches the highest photocurrent density of 4.72 mA cm^− 2 at 1.23 V vs. RHE under AM 1.5 G illumination with a better ABPE of 1.34% at 0.75V vs. RHE. Moreover, the PTh layer with good anti-corrosion of seawater and the [FeCl₄] unit suppressing Cl⁻ oxidation makes BiVO₄ great stability for PEC seawater splitting. We believe that these new findings can provide a proof-of-concept for ligand design engineering in OECs.

Preparation of BiVO ₄ photoanodes.

BiVO₄ photoanodes were fabricated by referring to Choi’s method.²² 50 mL of the precursor solution containing 0.4 M KI and 0.04 M Bi (NO₃)₃•5H₂O was adjusted pH to 1.6 ~ 1.8 by adding HNO₃. This solution was mixed with 20 mL of ethanol containing 0.23 M p-benzoquinone and was vigorously stirred for a few minutes. A typical three-electrode cell was used for electrodeposition where a piece of F-doped SnO₂ coated glass (FTO, 1×2 cm) served as the working electrode, a Pt electrode served as the counter electrode and an Ag/AgCl electrode served as the reference electrode. The cathodic deposition was conducted potentiostatically at -0.1 V vs. Ag/AgCl for 300 s at room temperature. Then, 0.2 mL DMSO solution containing 0.2 M VO(acac)₂ was placed on the reddish brown BiOI electrode, and annealed in air at 450°C for 2 h with ramping rate of 2°C min^− 1. The BiVO₄ electrode was immersed in 1 M NaOH solution for 30 minutes to remove excess V₂O₅. The resulting bare BiVO₄ electrodes were rinsed with deionized water and dried in air.

Preparation of PTh/BiVO ₄ photoanodes.

The BiVO₄ photoelectrode rinsed with deionized water was immersed in 50ml acetonitrile solution containing 50 mM thiophene monomer for 2h. Subsequently, 50ml of FeCl₃ solution (0.12 M) was added and in situ polymerized for 24h. After that, the PTh/BiVO₄ electrode was removed, rinsed with methanol, and dried at room temperature.

Photoelectrochemical measurements

The photoelectrochemical (PEC) performances of all photoanodes were evaluated in a standard three-electrode configuration cell with a platinum wire as the counter electrode, an Ag/AgCl reference electrode as a reference electrode, and 0.5 M potassium borate (pH 9.0) used as the electrolyte for all PEC measurements. Data were collected on a CHI 660E electrochemical workstation. The illumination source was a 300 W Xe arc lamp with AM 1.5G filter (100mW/cm², FX 300, Beijing PerfectLight Co. Ltd). Linear sweep voltammetry (LSV) was performed by sweeping the potential in the positive direction at a scan rate of 20 mV/s. All the potentials versus reversible hydrogen potential (vs. RHE) were converted from the potentials versus Ag/AgCl according to the Nernst equation:

E _RHE = E_Ag/AgCl + 0.059pH + 0.197 (S1)

The ABPE can be calculated by the photocurrent-potential curves under chopped AM 1.5G sunlight using the equation:

ABPE (%) = [(1.23 - V_app) × (J_light - J_dark)/P_light] ×100% (S2)

where V_app is the applied potential, J_dark and J_light are the photocurrent density (mA cm^− 2) in darkness and under AM 1.5G light, and P_light is the power density of AM 1.5G (100 mW cm^− 2).

The IPCE was obtained using a monochromator coupled with a 300 W Xe lamp as the simulated light source. An applied potential of 1.23 V vs. RHE was supplied by a CHI660e electrochemical workstation and the power density at a specific wavelength was measured by a Newport power meter. IPCE values were calculated using the following equation:

IPCE (%) = [1240×J_light /λ × P_light] ×100% (S3)

where J_light is the photocurrent density (mA cm^− 2) obtained from the electrochemical workstation, λ is the wavelength (nm) of incident radiation, and P_light is the power density obtained at a specific wavelength (mW cm^− 2). The wavelength dependence of IPCE was measured under monochromatic irradiation from a 300 W Xe lamp equipped with bandpass filters.

Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sinusoidal AC perturbation of 5 mV applied over the frequency range of 10^− 2 Hz ~ 10⁶ Hz. Mott–Schottky (MS) curves were measured with a frequency of 500 Hz and an amplitude of 10 mV. From the slope and intercept of the Mott-Schottky plots, the charge carrier densities (N_D) were estimated using the following equation:

$$\frac{{A}^{2}}{{C}^{2}}=\frac{2}{{\epsilon }_{r}{\epsilon }_{0}q{N}_{D}}V-\frac{2\left({E}_{fb}+{k}_{B}T/q\right)}{{\epsilon }_{r}{\epsilon }_{0}q{N}_{D}}$$

where C is the capacitance, A is the effective surface area, V is the voltage applied to the electrode, E_fb is the flat band potential, q is the charge of an electron (1.60×10^− 19 C), ε₀ is the permittivity of vacuum (8.854×10^− 14 F cm^− 1), ε_r is the dielectric constant of semiconductor (68 for BiVO₄).

Intensity-modulated photocurrent spectroscopy (IMPS) was measured by Zahner electrochemical workstation, the intensity of the light source used is 30 W m^− 2.

Materials characterization

The ⁵⁷Fe Mössbauer spectra of polythiophene were recorded on a SEE Co W304 Mössbauer spectrometer, using a ⁵⁷Co/Rh source in transmission geometry. The data were fitted by using the MossWinn 4.0 software. TOF-SIMS was performed on the TOF-SIMS 5-100 (ION-TOF GmbH) equipped with a reflection-type TOF analyzer. A pulsed Bi primary ion beam source at an acceleration voltage of 30 keV, 100 µm×100 µm raster size. SEM images were taken on a FEI Quanta 250FEG scanning electron microscope. HR-TEM images were collected on a JEOL JEM-AFM 200F (Japan) electron microscope with (Cs-corrected/energy-dispersive X-ray spectroscopy (EDS)/EELS). X-ray diffraction (XRD) patterns were acquired on a Bruker-AXS D8 Advance diffraction system equipped with a CuKα source (λ = 1.54056 Å). UV-vis absorbance spectra were collected on a Shimadzu UV-3600 spectrometer equipped with an integrating sphere. XPS was recorded on a XPS instrument (Thermo Scientific K-Alpha+) equipped with an aluminium anode (Al Kα, λ = 1486.6 eV), and all the peaks were calibrated with a C 1s spectrum at a binding energy of 284.8 eV. PL and TR-PL were taken on Edinburgh FLS1000 equipped with an Oxford cryostat designed for optical measurement. The transient-state surface photovoltage (TS-SPV) measurements were carried out on a home-made system. The samples were excited with a laser radiation pulse (wavelength of 355nm and pulse width of 5 ns) from a third-harmonic Nd:YAG laser (Polaris II, New Wave Research, Inc.). The TS-SPV signal was recorded by using a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix). The effective surface areas of the as-prepared samples were measured by nitrogen adsorption-desorption isotherms on the Micrometrics ASAP 2020 system, employing a fitting analysis based on the Brunauer-Emmett-Teller (BET) equation.

Acknowledgements

This work was supported by NSFC (22172077, T2322013), the Natural Science Foundation of Jiangsu Province of China (BK20220932, BK20211573), the Fundamental Research Funds for the Central Universities (30921011216).

Author contributions

Z.S.L and K.Z. conceived and designed the experiments. J.M.M. carried out materials synthesis and electrochemical characterization. Y.Y. and C.L. participated in part of the synthesis. K.Z., J.M.M. and Z.S.L co-wrote and supervised the paper. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Materials & Correspondence.

Correspondence and requests for materials should be addressed to Zhaosheng Li or Kan Zhang.

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Supramolecular Catalyst with [FeCl₄] Unit Boosting Photoelectrochemical Seawater Splitting via Water Nucleophilic Attack Pathway

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