A Cobalt@Cucurbit[5]uril Complex as a Highly Efficient Supramolecular Catalyst for Electrochemical and Photoelectrochemical Water Splitting

Abstract A host–guest complex self‐assembled through Co2+ and cucurbit[5]uril (Co@CB[5]) is used as a supramolecular catalyst on the surface of metal oxides including porous indium tin oxide (ITO) and porous BiVO4 for efficient electrochemical and photoelectrochemical water oxidation. When immobilized on ITO, Co@CB[5] exhibited a turnover frequency (TOF) of 9.9 s−1 at overpotential η=550 mV in a pH 9.2 borate buffer. Meanwhile, when Co@CB[5] complex was immobilized onto the surface of BiVO4 semiconductor, the assembled Co@CB[5]/BiVO4 photoanode exhibited a low onset potential of 0.15 V (vs. RHE) and a high photocurrent of 4.8 mA cm−2 at 1.23 V (vs. RHE) under 100 mW cm−2 (AM 1.5) light illumination. Kinetic studies confirmed that Co@CB[5] acts as a supramolecular water oxidation catalyst, and can effectively accelerate interfacial charge transfer between BiVO4 and electrolyte. Surface charge recombination of BiVO4 can be also significantly suppressed by Co@CB[5].


Introduction
Photoelectrochemical (PEC) water splitting is apromising strategy for converting solar energy into renewable fuels,such as hydrogen, [1] however,efficient catalytic water oxidation to provide protons for hydrogen production is considered to be ak ey challenge and one of the major obstacles for overall water splitting. [2] Thes luggish multi-electron and multiproton processes involved in the water oxidation reaction made many semiconductors having al ow catalytic activity towards the oxygen evolution reaction (OER). Thus,itiswell accepted that semiconductors should be integrated with water oxidation catalysts to achieve efficient solar water splitting. [3] Recent studies have reported considerable improvements in the OER catalyzed by non-precious metal-based catalysts such as cobalt, iron and nickel in the form of oxide,hydroxide and alloy with activity and stability benchmarks surpassing those of RuO 2 and IrO 2 under alkaline conditions. [4] Bismuth vanadate (BiVO 4 )s howed great prospect as ap hotoanode material to be coupled with catalysts for OER, due to its appropriate band structure. [5] BiVO 4 has been coupled with various OER material catalysts,s uch as Co 3 O 4 , [6] FeOOH/ NiOOH, [7] FeOOH, [8] CoPi, [9] NiBi [10] and NiB alloy. [11] However,t hese heterogeneous catalysts for OER on the electrode surface are generally prepared through hydrothermal, electrodeposition or photo-electrodepositionm ethods.Insuch processes,the thickness of the catalyst film has to be carefully controlled because it considerably influences the light absorption and the charge transport, therefore,ultra-thin layers of catalysts are beneficial for efficient catalyst-semiconductor hybrid photoanodes. [2,8] Apart from heterogeneous OER catalysts coupled with semiconductors,m olecular catalysts have also attracted great attention owing to their high activity,s tructural diversity,a nd facility in mechanistic studies. [12,13] However,f ew efforts have been made to apply host-guest supramolecular complexes as molecular catalysts for semiconductor-based photoanodes.
Cucurbituril (CB[n]) is ak ind of macrocyclic host compounds,w hich can readily coordinate with metal cations as carbonyl groups directed towards the cavity of its cyclic structure.S trong complexes can be formed with cations by ion-dipole interactions. [14,15] Furthermore,C B[n]c an be anchored or physiosorbed onto the surface of metal oxides (MO x ), [16,17] which indicates that the host-guest complexes based aCB[n]and ametal cation could be immobilized onto the surface of ametal oxide-based semiconductor through the remaining carbonyl groups.I nt he CB[n]f amily,C B [5] and CB [7] are moderately soluble in water (with concentration of 20-30 mM), [18] which is favorable for the construction of water splitting devices.Inthe present work, we selected CB [5] and Co 2+ cation for the supramolecular assembly,a nd immobilized Co@CB [5] on the surface of metal oxides to study its activity for electrochemical and PEC water splitting. Fort he first time,w ec ould demonstrate ah ost-guest complex Co@CB [5],w hich is applied to ap orous ITOo rB iVO 4 surface,achieving the electrochemical and PEC water oxidation performance comparable with that of state-of-the-art heterogeneous water oxidation catalysts,b ut with am uch simpler preparation procedure.T hese new findings show the huge potential of host-guest supramolecular complexes as catalysts for efficient OER.

Electrode Preparation and Characterization
TheC o@CB [5] supramolecular complex was assembled on the porous indium tin oxide (ITO) and porous BiVO 4 substrates to evaluate the electrochemical-driven and PEC oxygen evolution activity of the proposed host-guest assembled devices,r espectively.T he porous ITOs ubstrate was prepared by ad octor-blade method (see the Supporting Information for details). TheBiVO 4 thin film was prepared by modifying amethod reported by Choi and co-workers. [19] This porous BiVO 4 film has the advantages of improving electronhole separation and suppressing bulk carrier recombination, [12] while also providing al arge specific surface area for loading the Co@CB [5] onto the electrode surface.The porous ITOsubstrate was composed of octahedron shape particles as indicated by scanning electron microscope (SEM) images ( Figure S1a). Ac ross-sectional view of the film showed that the porous ITOl ayer had an average thickness of approximately 1.3 mm( Figure S1b). Thep orous BiVO 4 film was composed of worm-like particles as indicated by SEM images ( Figure S2a). Ac ross-sectional view of the film showed that the BiVO 4 layer had an average thickness of approximate 1 mm( Figure S2b). Energy dispersive spectroscopy (EDS) mapping analysis was performed on aselected cross-sectional area to determine the elemental distribution ( Figure S2c), which demonstrated ahomogeneous distribution of Bi, V, and Oe lements in the film of the porous BiVO 4 .
At ime-of-flight secondary ion mass spectrometry (TOF-SIMS) with Bi 3 + as the primary ion source was further employed to detect the presence of Co@CB [5] on the surface of ITOa nd BiVO 4 .A ss hown in the SIMS images of Co@CB [5]/ITOa nd Co@CB [5]/BiVO 4 ( Figures S6 and S7), some related secondary ions assigned to cobalt combined with the fragments of CB [5] could be observed uniformly distributing on the surface of electrodes,i ndicating that Co@CB [5] was successfully immobilized on the surface of ITOa nd BiVO 4 .
Them orphological features of Co@CB [5] functionalized electrodes were then investigated by SEM. As shown in Figures S8 and S9, no obvious morphological changes were observed after the immobilization of CB [5] and Co@CB [5] on the porous substrates,b ecause the assembly process is amolecular level surface modification.
X-ray photoelectron spectroscopy (XPS) measurements were performed to analyze the molecular level surface composition and electronic states of the Co@CB [5] complex functionalized MO x substrates.T he XPS survey spectra of Co@CB [5]/ITO, Co@CB [5]/BiVO 4 and reference electrodes are shown in Figures S10 and S11. In XPS high-resolution spectra of CB [5]/ITOa nd Co@CB [5]/BiVO 4 (Figures 1a,b,  Figures 2a,b), when CB [5] was immobilized on the surface of MO x ,t he N1 ss ignal corresponding to the C-N of CB [5] at ab inding energy of 400.15 eV can be observed, which indicated that CB [5] can be successfully immobilized on the surface of porous MO x substrates by the simple soaking method. When Co@CB [5] was immobilized on ITOo r BiVO 4 ,c lear signal of Co 2p were observed at the binding energy around 781 eV,w hich did not appear for the bare substrates and neither for the CB [5]/MO x electrodes.M eanwhile,s mall offsets from the main signal of N1 sc ould be observed. Thes ignal of Co 2p for Co@CB [5]/ITOw as fitted by four peaks,namely Co 2p 3/2 at 781.0 eV,Co2p 3/2 satellite at   786.3 eV,C o2 p 1/2 at 796.7 eV,a nd aC o2 p 1/2 satellite at 802.6 eV.The signal of Co 2p for Co@CB [5]/BiVO 4 was fitted into three peaks,C o2 p 3/2 at 780.7 eV,aCo 2p 3/2 satellite at 785.9 eV,and Co 2p 1/2 at 796.6 eV,(the Co 2p 1/2 satellite signal is not assigned here owing to its overlap with the BiVO 4 signal). Please note that the binding energies of Co 2p for Co@CB [5] complex were different from those of Co-based oxides or hydroxides, [21,22] indicating amolecular nature of the Co@CB [5] on MO x surfaces.
To further demonstrate that the Co@CB [5] is amolecular catalyst on MO x ,t he surface and morphology properties of the Co@CB [5]/ITOa nd Co@CB [5]/BiVO 4 samples were studied at atomic level by spherical aberration corrected transmission electron microscope (ACTEM). As revealed by high-angle annular dark-field (HAADF) images in Figure S12, the Co@CB[5]/ITOn ano-particle exhibited octahedron shape with regular and clear edge.A ss hown in Figure 1c,Energy-dispersive X-ray spectroscopy (EDS) mapping images of Co@CB [5]/ITOshowed the presence of Co,N and Co nt he surface of ITOn ano-particle.A tomic-scale HAADF images in the Figure 1d revealed that there is no structural decomposition observed after the attachment of Co@CB [5] complex, meanwhile,noC ooxides or hydroxides could be observed on the terminations of ITOn ano-particle after the immobilization of Co@CB [5] complex. This is in consistence with the phenomena of molecular anchoring of Co@CB [5] complex on ITOs urface.A st he element atomic mass of tin and indium in bulk ITOnanoparticle is far larger than that of cobalt, therefore,itishard to pinpoint the single cobalt molecular complex absorbed on the surface of ITOby comparing the difference in atomic resolution HAADF images.F igures S13 and S14 have shown atomic-resolution TEM images and corresponding EDS mapping images of Co@CB [5]/ITOi nd ifferent scales,w hich suggested that the cobalt components were uniformly distributed on the surface of ITO. In addition, according to the results of EDS analysis in selected area, the metal composition ratio of Co:I n: Sn was estimated to be 1: 92:7,this result was in accordance with the magnitude of surface mono-molecular layer distribution for Co components. [23] Thei nterface morphology of Co@CB [5]/BiVO 4 was characterized by high-resolution transmission electron microscope (HRTEM), as shown in Figure 2c,t here were no obvious metal oxide films or nanoparticles covered on the surface of Co@CB [5]/BiVO 4 nanoparticle.U nfortunately, when high energy electron beam was focused on BiVO 4 ,t he surface of BiVO 4 was damaged, detailed structure information for the surface of Co@CB [5]/BiVO 4 could not be obtained by atomic resolution AC-STEM. HAADF-STEM and corresponding EDS mapping images of Co@CB [5]/ BiVO 4 showed that elements of Co,Nand Cc ontents in the Co@CB [5] complex were uniformly distributed on the surface of the BiVO 4 nanoparticles (Figures 2d and S15). All above characterizations suggested that the Co@CB [5] on the surface of MO x is amolecular level immobilization.

Electrochemically Driven OER Catalysis by the Co@CB[5] Complex
TheO ER performance of Co@CB [5]/ITOw as first evaluated by linear sweep voltammetry (LSV). LSV measurements were performed at 25 8 8Cinastandard three-electrode cell with 1.0 Mborate buffer solution (pH 9.2) as the electrolyte.Asshown in Figure 3a,the ITOsubstrate and CB [5]/ITO electrode were silent for OER. In contrast, the anodic current density curve of the Co@CB [5]/ITOe lectrode rose sharply beyond al ow onset potential of 1.05 Vv s. normal hydrogen electrode (NHE), followed by ad ramatic increase at more positive potentials.Acurrent density of 1.0 mA cm À2 was achieved at h = 485 mV.F or comparison, Co/ITO electrode was also prepared by immersing ITOin1.0 mM Co 2+ solution for 30 mins (the same immersion time as for Co@CB [5]/ITO). TheCo/ITO electrode exhibited amuch lower anodic current density than that of the Co@CB [5]/ITOe lectrode.L inear fitting of the Co@CB [5]/ITOe lectrode gives aT afel slope of 59.5 mV dec À1 (Figure 3b). Thes teady-state catalytic activities of Co@CB [5]/ITOw ere determined by chronopotentiometric measurements during electrolysis at constant current densities over 5h (Figure 3c). TheCo@CB [5]/ITOelectrode required only 1.23 Va nd 1.29 Vv s. NHE to achieve the current density of 1.0 mA cm À2 and 2.5 mA cm À2 ,respectively. Theo verpotential requirement kept stable during the test, which indicated that the Co@CB [5]/ITOi ss table for OER. Theamount of electrochemically generated oxygen from the Co@CB [5]/ITOs ystem was confirmed by gas chromatography (GC). When at otal charge of 3.6 Cp assed through the electrode,8 .62 mmol of O 2 was detected by GC,l eading to aF araday efficiency of 92.6 %( Figure S16).
Theeffective coverage of Co 2+ on the surface of ITO was estimated as 4.54 10 À9 mol cm À2 according to the linear relationship between the peak current of Co 2+/3+ and the scan rate ( Figure S17, Eq. S1 and Eq. S2), and the order of magnitude for such catalyst loading is consistent with that of molecules immobilized on the surface of porous metal oxide substrates. [24] This total loading amount of Co 2+ was further confirmed by inductively coupled plasma optical emission spectrometer (ICP-OES). Thet otal Co 2+ loading amount is 4.98 10 À9 mol cm À2 ,which was slightly larger than the redox active Co 2+ .The turn over frequencies (TOFs) of Co@CB [5]/ ITOb ased on the Co 2+ loading (redox active) current densities vs.v arious overpotentials were calculated based on LSV data (operated in al ow scan rate of 10 mV s À1 )a nd Eq. S3. [25] Thel ogarithm of the TOFs varied linearly on the applied overpotentials from 460 to 550 mV,a ss hown in Figure 3d.ATOFo f0 .3 s À1 was achieved at h = 460 mV, which reached 9.9 s À1 at h = 550 mV.T able S1 lists the performance of Co@CB [5]/ITOi nt he current work and previously reported anodes for electrocatalytic water oxidation, including molecular and inorganic material catalysts immobilized on conducting glass.This comparison shows that the Co@CB [5]/ITOelectrode is one of the best OER anodes in terms of overpotential, current density,a nd TOF.

Photo-electrochemical Water Oxidation by Co@CB[5]/BiVO 4
TheP EC water oxidation performance of Co@CB [5]/ BiVO 4 photoanode was measured by linear sweep voltammetry (LSV) with as can rate of 10 mV s À1 .T he absorption spectrum of BiVO 4 did not change after Co@CB [5] was immobilized on the surface of BiVO 4 ( Figure S18), hence,the band gap of BiVO 4 and Co@CB [5]/BiVO 4 were the same.
As shown in Figure 4a,the photocurrent density of the asprepared BiVO 4 at 1.23 Vv s. RHE was approximate 1.8 mA cm À2 .I nt he presence of sulfite as hole scavenger, the photocurrent density of BiVO 4 increased to 5.4 mA cm À2 . These results are comparable with previously reported values, [26] which suggested that the quality of BiVO 4 film in our work is reliable.When Co@CB [5] was immobilized on the surface of BiVO 4 ,the photocurrent density of the Co@CB[5]/ BiVO 4 electrode was measured to be 4.8 mA cm À2 at 1.23 V vs.R HE without the hole scavenger sulfite,w hich is much greater than that of unmodified BiVO 4 ,CB [5]/BiVO 4 and Co/ BiVO 4 electrodes.T he CB [5]/BiVO 4 electrode exhibited an similar anodic photocurrent density curve than that of the pristine BiVO 4 electrode ( Figure S19), indicating CB [5] itself can not affect the water oxidation of BiVO 4 .After immersing BiVO 4 in Co 2+ solution for 30 minutes,t he photocurrent density of Co/BiVO 4 electrode at 1.23 Vvs. RHE increased to 2.3 mA cm À2 ,which is much lower than that of the Co@CB[5]/ BiVO 4 electrode (4.8 mA cm À2 ). Thea pplied bias photon to current efficiency( ABPE) was calculated from the corresponding LSV curve in Figure 4a   Under chopped-light illumination, the bare BiVO 4 only generated alow photocurrent density less than 0.25 mA cm À2 at aconstant applied potential of 0.6 Vvs. RHE;however, the Co@CB[5]/BiVO 4 photoanode generated ap hotocurrent density of ca. 2.4 mA cm À2 ,underlining the indispensable role of Co@CB [5] in water splitting (Figure 4d). During long-term photolysis at 0.6 Vv s. RHE under continued visible-light irradiation, the magnitude of the photocurrent generated by Co@CB [5]/BiVO 4 slowly decreased from 2.5 to 2.3 mA cm À2 over 30 min ( Figure S21). Thep hotogenerated O 2 in the headspace was quantified by GC.Assuming a4e À process for O 2 evolution, the number of electrons passing through the electrode agreed well with the amount of O 2 detected, representing aF aradaic efficiency of 90.3 %f or Co@CB [5]/ BiVO 4 photoanode ( Figure S22).
Thep hotocurrent density at 1.23 Vv s. RHE and the maximum ABPE obtained from Co@CB [5]/BiVO 4 are compared to those previously reported systems and the results are shown in Table S2. Thep erformance of Co@CB [5]/BiVO 4 exceeded those of many catalyst-modified undoped-BiVO 4 photoanodes in both parameters.T he host-guest complex assembled Co@CB [5]/BiVO 4 photoanode can avoid complicated organic synthesis,a nd the high activity of Co@CB [5]/ BiVO 4 holds potential for supramolecular catalysts capable of replacing state-of-the-art metal oxides and molecular catalysts.

Post-characterization of the Electrode after the OER Test
Post-characterization of the electrode after the OER test is necessary for further proof of reaction mechanism and the stability of the as-fabricated hybrid electrodes.A ss hown in ATR-IR spectra ( Figure S23), Co@CB [5] maintained on the surface of electrodes after the OER test. As shown in Figures S24 and S25 (Figures S26-S29). Theb inding energies of both N1 sa nd Co 2p for Co@CB [5]/ITOa nd Co@CB [5]/ BiVO 4 were consistent with that of before OER test, indicating that Co@CB [5] was stable for OER. TheE DS elemental mapping images with different magnitudes in Figures S30a, S31 and S32 confirmed the uniform distribution of Co,Nand Ci nC o@CB [5]/ITOf ollowing the OER tests. Figure S30b showed the atomic-resolution TEM images of tested Co@CB [5]/ITOparticle.Byfurther observing the edge of sample under greater magnification, ac lear and wellordered surface could be observed of tested Co@CB [5]/ITO, indicating that there was no other amorphous layer or heterojunction film formed to covered ITO nano-particle surface during the OER process ( Figure S30c,d). In addition, according to the results of EDS analysis in selected area, the metal composition ratio of Co:I n: Sn for tested Co@CB [5]/ ITOp article was calculated to be 1: 91:8 ,w hich was almost the same as the ratio of pristine Co@CB [5]/ITO. TheE DS elemental mapping images with different magnitudes in Figure S33 and S34 confirmed the uniform distribution of Co,Na nd Ci nC o@CB [5]/BiVO 4 after the OER tests. Figure S35 showed the HRTEM images of the tested Co@CB- [5]/BiVO 4 particles,t here are no nanoparticles generated on the surface of Co@CB [5]/BiVO 4 during the OER process, indicating that Co@CB [5] was stable on the surface of BiVO 4 during the OER. By integrating the information obtained from the above characterizations,i tc an be convincingly concluded that Co@CB [5] maintains its supramolecular nature and is stable during the OER.

Research Articles
Kinetic Studies of Co@CB [5] for Water Oxidation To investigate the transfer of carriers in the PEC system constructed by this novel Co@CB [5] supramolecular catalyst, kinetic studies were carried out. With sodium sulfite (Na 2 SO 3 ) as ah ole scavenger, extremely fast oxidation kinetics and negligible surface recombination were expected, as shown in Figure S36, the charge transfer efficiency (h trans )a tt he electrode-electrolyte interface was calculated according to Eq. S6 and the data from Figure 4a.U sing this method, as uperior h trans of Co@CB [5]/BiVO 4 was achieved (88 %), compared with that of bare BiVO 4 (33 %) at 1.23 Vvs. RHE. This result suggests that the Co@CB [5]/BiVO 4 electrode has more rapid water oxidation kinetics than the bare BiVO 4 . [27] Electrochemical impedance spectroscopy (EIS) was performed to examine the charge transport phenomena in the bulk and at the surface of the electrodes.A ss hown in Figure 5a,s emicircles for Co@CB [5]/BiVO 4 and BiVO 4 electrodes fitted well with Randles equivalent circuit model. Charge transport at the photoanode-electrolyte interface was estimated with the values for the charge-transfer resistance (R ct )obtained from the diameter of the semicircles.Asmaller semicircle indicates better charge transfer of the corresponding photoanode at the interface.N otably,t he Co@CB[5]/ BiVO 4 electrode had amarkedly smaller R ct (96 W)value than that of bare BiVO 4 (303 W)u nder illumination at 0.6 Vv s. RHE, illustrating ab etter ability to transfer holes at the electrode-electrolyte interface of Co@CB [5]/BiVO 4 ,w hich was consistent with its high charge transfer efficiency (h trans ).
Thec harge transfer efficiencyd epends on the rate of water oxidization catalytic processes on the surface of BiVO 4 and the rate of surface charge recombination. Thedeuterium kinetic isotope effects (KIEs) can reflect the proton transfer kinetic information on water oxidation reactions and help us to interpret the rate determine step (RDS) of the catalytic process. [28] TheK IEs(H/D) can be defined as Eq. S7, the current densities in water and deuterated water electrolytes were contrasted at the same overpotential according to Eq. S8-Eq. S10 for KIEs(H/D) measurements in this work. [29] As shown in Figure 5b,t he value of KIEs of Co@CB[5]/ ITOe lectrode was larger than 2, indicating ap rimary KIEs for which O À Hb ond cleavage is involved in the RDS of electro-driven water oxidation for Co@CB [5],corresponding to the water nucleophilic attack mechanisms. [28] When the Co@CB [5] was anchored on the surface of BiVO 4 ,t he value of KIEs for Co@CB [5]/BiVO 4 photoanode was around 1.0 with the applied bias increasing,i ndicating the O À Hb ond cleavage is not involved in the RDS for light driven water oxidation by Co@CB [5]/BiVO 4 photoanode (Figure 5c). In contrast, the value of KIEs for bare BiVO 4 photoanode was around 1.5 (Figure 5d), suggesting that the proton transfer process was involved in the RDS of the light-driven water oxidation process for bare BiVO 4 .T hereby,t he immobilization of Co@CB [5] on the surface of BiVO 4 could greatly accelerate the proton transfer processes (water oxidation) and shift the RDS of BiVO 4 photoanode to an on proton transfer involved process.T hence,C o@CB [5] indeed played an important role in the light-driven catalytic water oxidation processes in this Co@CB [5]/BiVO 4 supramolecular assembly photoanode.
It has been known that ac o-catalyst can serve as passivation layer to improve charge-separation and charge transfer processes across semiconductor-liquid interfaces. [30] Fore xample,t he modification of CoPi on the surface of semiconductors can improve the performances of photoanodes,w hich primarily caused by reducing the surface charge recombination (passivation layer), rather than improving the catalytic ability (charge transfer kinetics). [31,32] Similar phenomena have also been observed in other systems, such as athin layer of CoFeO x modified hematite and BiFeO 3 passivated BiVO 4 photoanodes. [33,34] Converse systems have also been reported, for instance,t hin layers of FeOOH can enhance the performance of BiVO 4 for water oxidation, for which the performance was improved primarily owing to the improvement of charge transfer, rather than suppressing the surface charge recombination. 11 Molecular-based catalysts can also improve the light-driven water oxidation ability of ntype semiconductors,s uch as Co-salophen complexes and Ni 4 O 4 cubane have been reported as the co-catalyst for BiVO 4 , [35,36] in these cases,molecular catalysts can accelerate the water oxidation reaction as OER catalysts,m eanwhile, reduce the surface charge recombination.
Our results confirmed that Co@CB [5] can effectively promote BiVO 4 for light-driven water oxidation by serving as ag ood OER catalyst, however, it is necessary to further confirm whether Co@CB [5] on the semiconductor inhibited surface charge recombination by acting as as urface passivation layer. Thec harge transfer and surface recombination kinetics were quantified by intensity modulated photocurrent spectroscopy (IMPS) to understand the real role of Co@CB- [5] on the BiVO 4 .F igure 6a and 6b show typical IMPS responses of bare BiVO 4 and Co@CB [5]/BiVO 4 photoanodes in the complex plane.T he IMPS spectrum consists of two semicircles in the 4th and 1st quadrants,which correspond to the resistance-capacitance attenuation and the competition between charge transfer and recombination, respectively. [34] Accordingly,t he frequencyo ft he maximum imaginary corresponds to the sum of the charge transfer (k trans )a nd charge recombination (k rec )r ate constants (k trans + k rec = 2pf max ). Thelow frequencyintercept in the normalized form, at which the imaginary part is equal to 0, corresponds to ac harge transfer efficiencyo fk trans /(k trans + k rec ). [37] Thek ey parameters k rec and k trans are therefore readily accessible.
Moreover,the upper semicircle of the BiVO 4 photoanode did not change notably as the applied potential was increased, the value of k rec remained nearly constant with increasing potential for bare BiVO 4 (Figure 6c), indicating that ah igh charge recombination exists for the BiVO 4 photoanode over aw ide potential range.B yc ontrast, the upper semicircle of the Co@CB [5]/BiVO 4 photoanode became smaller as the applied potential increased. Notably, k rec was suppressed by af actor of 6.5-30 over the entire potential range after decoration of Co@CB [5].F or instance,at0.6 Vvs. RHE (the max point of ABPEs), the value of k rec was 3.5 s À1 for bare BiVO 4, which is 6.5 times as large as that of Co@CB [5]/BiVO 4 (0.54 s À1 ). At higher potentials,t his factor increased to 30 at 0.8 Vv s. RHE. These results suggest that the charge recombination was markedly suppressed by the Co@CB [5]. Figure 6d shows that the k trans of Co@CB [5]/BiVO 4 surpassed that of BiVO 4 at all potentials.P articularly,a tapotential of 0.6 Vv s. RHE (the max point of ABPEs), the k trans of Co@CB [5]/BiVO 4 was more than 4t imes as great as that of BiVO 4 .The charge transfer efficiency, defined as k trans /(k trans + k rec ), showed the same trend as the value determined by J water / J sulfite ,i ndicating the reliability of our measurements (Figure S37). Theratio of k rec /k trans was positively proportional to the semicircle in 1st quadrant and as mall value indicated faster charge transfer than charge recombination. [34] These results demonstrate that Co@CB [5] behaves as am olecular catalyst, which is in contrast to the mere passivation action of heterogeneous catalysts as reported, [31][32][33][34] Co@CB [5] not only accelerates the water oxidation reaction as ag ood OER catalyst for BiVO 4 ,b ut also reduces the surface charge recombination.
Based on the OER kinetics analyzed above,t he function of Co@CB [5] on BiVO 4 was summarized and shown in Scheme 2. Forthe bare BiVO 4 ,the photo generated holes can be directly consumed for water oxidation (pathway 1), or recombined with electrons that trapped by surface state (pathway 2). Due to the low intrinsic OER catalytic activity of BiVO 4 ,the injection of holes to electrolyte was limited by the proton transfer involved water oxidation reaction, leading to ap rimary KIEs for bare BiVO 4 .W hen Co@CB [5] was immobilized on the surface of BiVO 4 ,owing to the high OER catalytic activity of Co@CB [5],t he photo-generated holes could be effectively transferred and consumed for water oxidation (pathway 3);m eanwhile,d ue to the faster consumption of holes,t he photo-generated electrons will have less chance to recombine with photo-generated holes at the surface of BiVO 4 ,l eading to the higher charge transfer efficiency of Co@CB [5]/BiVO 4 in comparison to that of the bare BiVO 4 .

Conclusion
Ah ost-guest supramolecular complex Co@CB [5] as am olecular water oxidation catalyst was successfully immobilized on porous ITOa nd BiVO 4 substrates.T his supramolecular catalyst Co@CB [5] showed ah igh activity for electrochemical water oxidation when immobilized on ap orous ITO substrate,t he fabricated Co@CB [5]/ITOe lectrode exhibited aT OF of 9.9 s À1 at an overpotential of 550 mV in ap H9.2 borate buffer with good stability.W hen Co@CB [5] was immobilized on the surface of the porous BiVO 4 n-type semiconductor,the integrated photoanode Co@CB [5]/BiVO 4 showed an excellent PEC performance with ah igh photocurrent density of 4.8 mA cm À2 at 1.23 V( vs.R HE) under 100 mW cm À2 (AM 1.5) light illumination. KIEs and IMPS results confirmed that Co@CB [5] serves as asupramolecular water oxidation catalyst which can effectively accelerate surface charge transfer. Furthermore,t he surface charge recombination of BiVO 4 can also be suppressed by this hostguest supramolecular complex. Theh ost-guest complex Co@CB [5] shows ar emarkable performance for electrochemical and PEC water oxidation, which can open new opportunities for the development of supramolecular complexes as catalysts for not only efficient water oxidation, but may also for other catalytic reactions.