Photoelectrocatalytic Water Splitting: Significance of Cocatalysts

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Photoelectrocatalytic Water Splitting: Significance of Cocatalysts, Electrolyte and Interfaces Chunmei Ding, Jingying Shi, Zhiliang Wang, and Can Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03107 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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Photoelectrocatalytic Water Splitting: Significance of Cocatalysts, Electrolyte and Interfaces Chunmei Dinga, Jingying Shia, Zhiliang Wanga, Can Lia,* a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences; Dalian National Laboratory for Clean Energy, Dalian 116023, China.

1 ACS Paragon Plus Environment

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ABSTRACT: The efficiency of photoelectrocatalytic (PEC) water splitting is limited by the serious recombination of photogenerated charges, high overpotential and sluggish kinetics of surface reaction. Herein, we describe the recent progress on engineering electrode-electrolyte and semiconductor-cocatalyst interfaces with cocatalysts, electrolyte and interfacial layers (interlayers) to increase PEC efficiency. Introducing cocatalysts has been demonstrated to be the most efficient way to lower the reaction barrier and promote the charge injection to reactants. In addition, it is found that electrolyte ions can influence the surface catalysis remarkably. Electrolyte cations on surface can influence the water splitting and backward reactions, and anions may take part in the proton transfer processes, indicating that the fine tuning of electrolyte parameters turns out to be an important strategy for enhancing the PEC efficiency. Moreover, the careful modification of the interface between cocatalysts and the semiconductor via suitable interlayers is critical for promoting the charge separation and transfer which can indirectly influence the surface catalysis. The mechanisms of surface catalysis are assumed to be in a way that photogenerated holes are supposed to be transfered to the surface active sites forming high-valent species which then oxidize the water molecules. Many key scientific issues about the generation of photovoltage, the separation, storage and transfer of carriers, the function of cocatalysts, the roles of electrolyte ions and the influences of other parameters during PEC water splitting will be discussed in detail with some perspective views.

KEYWORDS: photoelectrocatalysis, water splitting, interface, interlayer, cocatalyst, electrolyte


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1. INTRODUCTION Photoelectrocatalytic (PEC) water splitting is one of the most promising artificial photosynthesis approaches for solar fuel production. Since the work by Fujishima and Honda in 1972,1 tremendous interests in PEC water splitting were ignited.2,

3

However, its practical

application has been severely limited by many technical difficulties. In recent years, thanks to the global surge in solar energy research interests, PEC water splitting has received renewed attention. Photoelectrocatalysis mimics the Z-scheme natural photosynthesis that photogenerated holes (or electrons) migrate to the anode (or cathode) surface to participate in the water oxidation (or reduction) reaction. Since the reaction sites are separated spatially, there is no need for separating H2 and O2. Above all, the techno-economic evaluation of PEC hydrogen generation shows it is cost-competitive with fossil-based technologies, providing that the technical barriers to implement PEC water splitting on a large scale are overcome and the solar-to-hydrogen (STH) efficiency reaches 10%.4 The STH efficiency is an important metric for benchmarking and performance evaluation. It can be expressed as a production of the light absorption efficiency (ηabs), the charge separation efficiency (ηsep) and the injection efficiency (ηinj) of photogenerated carriers to the reactants.5 Thus, these key criteria must be met simultaneously for efficient PEC water splitting: (1) broad light absorption range for efficient sunlight utilization; (2) efficient charge transfer from the bulk of the photoelectrode to the surface; (3) rapid consumption of photogenerated carriers for the surface reaction with minimum overpotential; (4) excellent durability for practical applications. In the past decades, the advance of material science and electrode fabrication technologies has boomed this area greatly. The STH efficiencies of BiVO4 and Ta3N5, the leading performers of single photon-absorber photoanodes, have been increased progressively.6-11 The surface


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modification approach has enabled the efficient and stable operation of a photoanode for 1700 h.12 And the efficiencies of photovoltaic-PEC coupled systems are also encouraging.13, 14 Nevertheless, PEC water splitting is still facing the problems of low efficiency, poor stability and high cost. Much effort has been made on energy band tunning11, 15 and morphology control9, 16-19 which can enhance the ηabs and ηsep. Besides, there are systematic researches on interface and surface engineering that improves the ηsep and ηinj. Herein, we will address the recent advances on promoting PEC water splitting via loading cocatalysts, tuning electrolyte conditions, engineering interfaces and other parameters. The mechanism of surface reaction will also be discussed, and the critical scientific issues and challenges in this field will be overviewed. 2. FUNDAMENTALS OF PEC WATER SPLITTING PEC water splitting involves the photogenerated carriers on surface and reactants adsorbed on surface or in electrolyte. The reaction takes place at the electrode-electrolyte interface. Figure 1 shows the simplified scheme of the semiconductor-electrolyte double-layer interface. When the Fermi level (EF) of the semiconductor is higher than the redox potential of the electrolyte (Eredox), a space charge layer with a thickness of about 1–0.1 μm will be formed in the near surface region of the semiconductor in contact with the solution.20, 21 Under open circuit condition or a slightly positive potential, the space charge layer is a depletion layer in which mobile majority carriers are less than the bulk phase and there is an excess of immobile ionized donors.20, 21 An inversion layer with accumulated mobile minority carriers will be formed when the applied potential is extremely positive.20, 21 Thus, the depletion or inversion layer contains positive charges. The solution side of the double layer will be negatively charged, forming a Helmholtz compact layer (about 3–5Å in thickness) and a Gouy diffuse layer (the thickness can be ignored when the concentration of the


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electrolyte is high). The Helmholtz layer contains trapped electrons in surface states, adsorbed ions, solvate molecules, etc.20-22 Therefore, the potential drop across the double-layer is contributed by three parts: ∆φSC in the space charge region, ∆φH in the Helmholtz layer, and ∆φG in the Gouy layer.20,

21

The potential drop in the solution is typically much smaller than that in the

semiconductor when the ionic strength of the electrolyte is high and the density of surface states (Nss) on the semiconductor is low (< 1012 cm-2).22, 23 Therefore, the change of potential mainly changes the extent of band bending in the space charge layer which then influences the EF of the semiconductor and density of carriers on surface. However, when Nss is high (1013–1014 cm-2), ∆φH becomes dominant, resulting the so-called “Fermi level pinning effect”.22,

23

Surface states are

mainly caused by the crystal lattice defects and the adsorbed species on the surface. They are closely related to the PEC activity and may perform as recombination centers or charge transfer mediators.20,

24

In a word, the semiconductor property and the electrolyte condition can both

influence the interface property and the charge transfer process. In dark and open-circuit condition, the EF of a semiconductor with low Nss will equilibrates with the Eredox in the electrolyte, as shown in Figure 2a, resulting an upward band bending and a built-in field. The barrier height (Vbarrier) determines the theoretical maximum energy that can be generated at the semiconductor-liquid junction. Under irradiation, the quasi-Fermi level is used to describe the non-equilibrium condition. As Figure 2b shows, the quasi-Fermi level of electrons (EF,n) is close to the flat band potential (Efb), and the quasi-Fermi level of holes (EF,p) shifts downwards, generating an open-circuit photovoltage (Vph).25-27 The maximum Vph is determined by the Efb of the semiconductor and the Eredox in the electrolyte (Equation 1). And there will be a significant potential drop due to the Helmholtz layer in the case of “Fermi level pinning”, resulting a reduced


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Vph.23, 26 The Vph is usually smaller than the band gap of the material, and the photoelectrolysis of water is possible only if the Vph exceeds 1.23 V. Experimentally, Vph can be measured by the difference of the open-circuit potential (Eoc) in dark and under irradiation versus the reference electrode (Equation 2).26 In addition, Vph can be estimated by the onset potential (Eonset) shift of the anodic current in dark and under illumination, during which the kinetic overpotential of the reaction (ηk) should also be considered (Equation 3). It can be seen that the Eonset under irradiation is influenced by two factors (Vph and ηk) together.23 In addition, many kinetic processes govern the electron and hole concentrations at the interface under quasi-equilibrium conditions. Along with the targeted charge transfer from the valence band to the redox reagent (Jredox), there are several recombination pathways including the bulk recombination (Rbulk), space charge layer recombination (RSC) and surface state recombination (RSS).27 The key challenges of photoelectrocatalysis are to reduce charge recombination, increase Vph and reduce ηk, which we will discuss in detail in the following parts. Vph-max = Vbarrier = Efb – Eredox

(1)

Vph = Eoc-dark – Eoc-light

(2)

Vph = Eonset-dark – Eonset-light = Eredox + ηk – Eonset-light

(3)

The water splitting reaction shows low activity as it is an uphill reaction thermodynamically with high activation energy or overpotential, and the kinetics of multiple proton coupled electron-transfer is slow, especially for water oxidation. The water oxidation reaction is accompany with the formation of oxo-intermediates and involves four steps, as shown by Equation (4)–(7):28, 29 H2O (l) + * ↔ HO* + H+ + e-

(4)

HO* ↔ O* + H+ + e-

(5)


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O* + H2O (l) ↔ HOO* + H+ + e-

(6)

HOO* ↔ * + O2 (g) + H+ + e-

(7)

The symbol * represents an adsorption site which is usually the O vacancy in the topmost layer. The symbols *OH, *O and *OOH represent the chemisorbed species. H2O first adsorbs onto the surface and then undergoes the oxidation reactions (4) and (5), forming *O species which will react with H2O molecules to form the *OOH intermediate. Lastly, O2 is released from the *OOH.28, 29 Theoretical calculations reveal that there is a volcano relationship between the water oxidation overpotential and the value of the standard free energy difference (∆G0HO* – ∆G0O*) on various catalysts, and the overpotential is determined by the O* adsorption energy,29 which means either step (5) or (6) is potential determining.29 Another result shows that the water oxidation activities of perovskites exhibit a volcano-shaped dependence on the occupancy of the eg-symmetry electron of the surface transition metal cations which can greatly influence the binding of intermediates to the oxide surface and thus the water oxidation reaction.30 The peak activity is obtained at a medium eg occupancy which means high covalency of transition metal–oxygen bonds.30 Therefore, we can see that the interaction between the surface and oxo-species is a critical factor for the water oxidation reaction. The activity depends greatly on the structure, electronic and adsorption properties of the catalysts. Applying catalysts with medium O* adsorption energy and high covalency of transition metal–oxygen bonds maybe an efficient strategy. 3. ENGINEERING ELECTRODE-ELECTROLYTE INTERFACE WITH COCATALYSTS Recently, many novel materials have emerged as efficient water oxidation electrocatalysts, including (Fe, Co, Ni)-based hydroxides,31-34 Mn-based oxides,35-37 Cu-based hydroxides,38, carbon materials,40-42 metal phosphides or phosphates,43,

44

perovskite oxides,30,


45, 46

39

etc. The

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discovery of robust electrocatalysts has promoted the progress of photoelectrocatalysis greatly. Actually, introducing electrocatalysts as cocatalysts has been widely used in photocatalysis to reduce the activation energy.47 In PEC water oxidation, the pioneering works are the successful deposition of CoPi as water oxidation cocatalysts (WOCs) on photoanodes,48, 49 resulting in a great reduction of overpotential and enhancement of photocurrent. We applied Co-borate (CoBi) on BiVO4 photoanodes and found that the Eonset is negatively shifted by 320 mV, and the activity and stability are obviously improved, as shown in Figure 3a, b.50 The current density vs. potential (J-E) curve changes from a concave shape with low fill factor to a convex curve with much higher current and fill factor, indicating the recombination is suppressed. Evidenced by the electrochemical impedance spectra (EIS), the CoBi cocatalyst can not only accelerate the water oxidation reaction but also promote the charge transfer across the semiconductor-electrolyte interface.50 Overall water splitting is realized with an applied bias of less than 0.3 V which is nearly the theoretical minimum of BiVO4 (Figure 3c, d).50 In addition, it is found that the photocurrent of TiO2 can be increased after electrochemical reduction which produces a surface disordered layer acting as an efficient WOC (Figure 4a), because the charge transfer resistance at the electrode-electrolyte interface is decreased (Figure 4b), and the ηinj and ηsep are both increased (Figure 4c, d).51 For hydrogen evolution reaction (HER), many photocathodes such as p-Si and Cu-based materials suffer from (photo)chemical corrosion and the activity decays quickly when the extraction and utilization of electrons are not efficient enough. After depositing cocatalysts such as Mo2C52, MoSx53, 54 and transition metal phosphide55, these photocathodes display high activity and stability. Recently, the coupling of molecular cocatalysts with photoelectrodes has come into sight. The charge transfer between semiconductors and molecular catalysts has been proved feasible in


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photocatalysis.56 In photoelectrocatalysis, various molecular WOCs have been successfully assembled on dye sensitized photoanodes.57 And it is reported that the Ru-based molecular catalyst covalently anchored on hematite brings an obvious enhancement of photocurrent, and the high efficiency can last for about 3 h,58 but the stability of the molecular catalyst itself for longer time is worrying because it may be photodegraded. In addition, bio-inspired cubane-like Mo3S4 molecular clusters with hydrophobic ligands can efficiently catalyze the HER when coupled to a p-Si photocathode, leading to a shift of Eonset by 550 mV and a good stability.59 These results hint us that molecular catalysts may have great potential in PEC water splitting. The most compelling point of employing molecular catalysts in photoelectrocatalysis lies in that it may help us to get insight into catalytic mechanisms and catalyst structure at molecular level, and further provide a guideline for exploring and designing robust heterogeneous catalysts. In a word, loading cocatalysts can increase the photocurrent and reduce the onset overpotential via promoting the transfer and consumption of carriers for surface reactions. This usually means a J-E curve with high fill factor which is also helpful for further constructing dual-photoelectrode systems. In addition, the existence of a cocatalyst layer can not only protect the photoelectrode physically, but also prevent the electrode from photocorrosion. One thing to note is that the amount of the cocatalyst must be optimized, because the cocatalysts may block the light reaching the semiconductor and the resistance of the cocatalyst may increases when it is too thick. As is turns out, cocatalysts candidates that are highly active for the surface reaction, highly pervious to light, inexpensive, and readily fabricated on photoelectrodes are more attractive for the application in photoelectrocatalysis. The cocatalyst can be either amorphous structure with high density of active sites and fine mass diffusion ability or a conformal layer with high crystallinity, but should be


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highly conductive to the carriers such as metallic materials or metal oxides with alterable valence state. Recently, a biphasic catalyst composed of a compact and continuous nanocrystalline Co3O4 layer that is impervious to phase transformation and impermeable to ions and thus provides effective protection of the underlying substrate, and a secondary phase of structurally disordered and chemically labile Co(OH)2 which ensures a high concentration of catalytically active sites is introduced on p+n-Si photoanodes, achieving a strikingly high activity.60 4. ENGINEERING ELECTRODE-ELECTROLYTE INTERFACE WITH ELECTROLYTE Besides of cocatalysts, the electrolyte is another important factor affecting surface catalysis. A slight change of the electrolyte condition may bring a significant change of the interface catalysis. Figure 5a displays that the initial photocurrent of Co3O4/TiO2 shows slight difference in LiOH, KOH and NaOH electrolyte, but the photocurrents after 11 h operation demonstrate a ratio of 1.5: 1.3: 1. We investigated various materials including TiO2, (Co3O4, Ferrihydrite, MnOx)/TiO2, BiVO4 and (CoBi, NiBi, Ferrihydrite, Co3O4)/BiVO4, and found that the PEC and electrocatalytic water oxidation activity in basic electrolytes with different cations show an unexpected trend of Li+ > K+ > Na+, especially for long time reaction (Figure 5b).61 Such an “abnormal” order is the balance effect of two factors: the distinct extents of the weakening of O−H bond on electrode surface after interacting with the hydrated cations, and the different rates of backward reaction (oxygen reduction) in various electrolytes (Figure 5c, d). Li+ not only brings the most significant extent of O−H activation but also is most effective for avoiding back reaction.61 However, the effect of cations in acid environment is still elusive. One report shows that the water oxidation activity on WO3 photoanodes is the highest with K+ in acidic electrolytes and the lowest with Li+ owing to the strongest adsorption of Li+ on the electrode blocking the oxygen evolution active sites,62 which is


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inconsistent with our result in basic condition. Moreover, there is an alternative view on the effects of cations that larger cations may serve as buffering agents which can lower the pH near the cathode and thus influence the CO2 reduction activity, because the pKa for cation hydrolysis decreases with increasing cation size.63 Possibly, there are similar effects during water oxidation reaction that the smallest Li+ results the highest local pH near the anode and brings an increase of water oxidation activity, but this is still an open research question. Therefore, the intrinsic reason behind these phenomenon needs further research, and the answer may be similar and related to the functions of different cations in nature photosynthesis, during which the cationic environment of surface groups can affect the electron transfer steps in photosystem II.64 As for the anions, some reports show that the anions in the electrolyte may influence the phase 65

or structure66 of cocatalysts and undergo side reactions.67 We found that that the activity and

stability of bare and CoBi modified BiVO4 in borate electrolyte are both much higher than that in Na2SO4, as shown in Figure 3a, b.50 That’s partially because borate ions are the composition of CoBi and there may be coordination equilibrium between Co and borate ions, affecting the structure stability of cocatalyst itself. More importantly, borate electrolyte is more conducive to the charge transfer across the electrode-electrolyte interface, as the interface charge transfer resistance is much smaller in borate electrolyte.50 In addition, borate ions are more favorable for proton transfer because of the strong proton accepting property, which can inhibit the buildup of protons on the electrode surface and suppress the corrosion of the electrode.50, 68 The foregoing discussion shows that the electrolyte may influence the proton transfer mechanism which is another tough and important issue in PEC water splitting. Although every hole transfer step involves a proton transfer in principle, the sequence of the electron−proton transfer in


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each step and its role in the whole water oxidation process are quite complicated. Recently, a proton-electron transfer sequence in the rate-determining step during PEC water oxidation on hematite is revealed.69 As shown in Figure 6a, a transition regime is observed in the J-E curves at pH 10−12 and there is a photocurrent plateau which shifts positively at higher pH, because the dominant hole acceptor switches from OH- to H2O in this regime and a more positive potential is required for significant OH- depletion at higher pH. With sufficient OH- (pH > 12), oxidation of OH- dominates the photocurrent in the whole potential region. Moreover, pronounced kinetic isotope effects (KIE, the ratio of photocurrent in H2O and D2O) are observed at pH < 12 and high applied potential, whereas the KIE value drops at high pH and low potential and becomes nearly unity at pH > 13 (Figure 6b). A higher KIE value indicates the oxidation of H2O molecules is the dominant reaction and the rate determining step. In other words, the PEC water oxidation reaction proceeds through a concerted proton-electron transfer pathway where the electron transfer from water molecular to the surface-trapped holes is accompanied by proton transfer to the solvent.69 A lower KIE value means OH- becomes the dominant acceptor of holes and the mechanism changes to electron transfer. This is further confirmed by the EIS at pH/pD 10 (Figure 6c). Only the low-frequency semicircle reflecting the resistance of interfacial charge transfer to water shows an obvious isotope effect with a KIE value of 3.5, consistent with that obtained from the photocurrent at 1.23 VRHE. In addition, as shown in Figure 6d, there is an obvious increase of the PEC water oxidation activity with appropriate proton acceptors (buffer bases), which is consistent with our result on BiVO4. And the dependence of the J-E profile on the buffer concentration is similar to that observed in Figure 6a as the pH increases. This further confirms that anions can act as proton acceptors to accelerate the proton transfer and maintain the local OH- concentration.69 It can be


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expected that anions with higher proton accepting and buffering properties can provide an expressway for proton transfer. The discussions above are about the effects of ions in electrolyte on PEC water oxidation, but little attention is paid to the electrolyte effects on HER. Generally, the first and rate-determining step is H3O+ + e- + * ↔ *H + H2O(l) during HER in acid, but it is H2O(l) + e- + * ↔ *H + OH- in basic condition. Therefore, a high reduction current and low overpotential are expected at low pH values, and many photocathodes operate at pH < 7 and even pH 0 as long as the materials are acid proof. What’s more, the electrolyte ions may also interfere with the reduction process. It was reported that the HER activity on Pt-Ni(OH)x electrode can be enhanced via Li+ addition which enhances the generation of hydrogen intermediates and the destabilization of the HO–H bond.70 In a word, further insight into the roles of electrolyte ions is critical for surface catalysis, and may provide a new strategy of promoting surface catalysis via tailoring the electrolyte conditions, such as the concentration, ion species and pH value. 5.

ENGINEERING

SEMICONDUCTOR-COCATALYST

INTERFACE

WITH

INTERLAYERS The results above clearly demonstrate that robust cocatalysts and suitable electrolyte condition can promote the surface reaction. Furthermore, to match with the fast surface reaction on cocatalysts, sufficient photogenerated carriers must be extracted from the bulk electrode to the surface. The poor charge carrier separation and transport may result in the accumulation and recombination of carriers and corrosion of the electrode. Due to the mismatch between cocatalysts and semiconductors, many defects and interfacial energy levels may exist at the interface, which brings serious recombination during the charge separation and transport process. And the


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electrocatalyst may not perform well as expected after being integrated onto photoelectrodes. Therefore, much attention should be paid to the engineering of semiconductor-cocatalyst interface. Recently, the undesired donor-like interfacial defects and their detrimental effects on charge transfer are well recognized and diminished via constructing a Schottky heterojunction n-Si/TiOx/ITO/NiOOH photoanode which shows a remarkably low Eonset (0.9 VRHE), high ηinj (> 90%) and ηsep (up to 100%) in a wide potential range.71 From Figure 7a, the charge transport property of n-Si/ITO is notorious due to the existence of donor-like interfacial defects acting as undesirable recombination centers located below the conduction band of n-Si, but a high charge transfer current is observed after introducing a thin layer of TiOx.71 The interaction between Si species and TiOx may result in the formation of Si–O–Ti interfacial species acting as a charge transfer pathway.71 The n-Si/TiOx/ITO Schottky structure shows a relatively high ηsep, but its water oxidation photocurrent is still low (Figure 7b, c). After further loading NiOOH, the water oxidation activity is highly improved (Figure 7b, c), because NiOOH not only plays a role of efficient WOC, but also can influence the energetic alignment of the heterojunction and the charge separation. The work function of the surface is elevated with NiOOH, thus the built-in electric field and the barrier height of the n-Si/TiOx/ITO junction are enhanced, resulting a larger driving force of charge separation (Figure 7d).71 This work demonstrates the critical role of rationally regulating the interfacial and surface energetics via loading cocatalysts and interface engineering between the semiconductor and cocatalysts. Many attractive materials of photoanodes such as Ta3N5 have high theoretical STH efficiency but suffer from poor charge separation and severe photocorrosion. As depicted in Figure 8a, a layer of ferrihydrite (Fh) was introduced between the semiconductor and cocatalyst for more efficient


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charge transfer and separation.72 The coexistence of Fh and Co3O4 permits the improvement of stability from few minutes for bare Ta3N5 photoanodes to 6 h at above 5 mA cm-2 (Figure 8b). Co3O4 plays a role of cocatalyst which enhances the ηinj and contributes greatly to the high photocurrent (Figure 8c), and more importantly, an obvious increase of charge storage amount is observed (Figure 8d). Thus, the remarkable improved stability is mainly due to the hole-storage ability of Fh, named the hole-storage layer (HSL) which can capture holes from Ta3N5, store them temporarily and then transfer them to the surface cocatalyst for efficient water oxidation.72 In addition, the stability of Ta3N5 can be further prolonged to 24 h when using Ni(OH)x/MoO3 bilayer as more efficient HSL.73 It is found that MoOx works as a hole-transport layer extracting holes from Ta3N5 to the HSL, indicating the importance of the photoanode-HSL interface engineering. More strikingly, the photoresponse of Ta3N5 approaches the theoretical limit (about 12.3 mA cm-2 at 1.23 VRHE) achieving a STH of 2.5% via rationally integrating it with TiOx, Fh/Ni(OH)x composite HSL and efficient molecular WOCs on the surface,6 as shown in Figure 9. TiOx functions as electron-blocking layer which inhibits the flow of electrons to the surface and reduce the recombination greatly.6 In addition, it is notable that the deliberately chosen molecular WOCs appear to be more efficient to utilize the stored holes in the HSL for water oxidation than the heterogeneous cocatalyst Co3O4, suggesting the match between HSL and cocatalyst is also critical.6 Interestingly, combining Co-based and Ir-based complex together results in a synergistic effect, and Co-complex is supposed to bridge the pathway of holes from the HSL to Ir-complex for water oxidation.6 The systematic results on Ta3N5 demonstrate that introducing a HSL between the semiconductor and cocatalysts can efficiently promote the charge separation and transfer, and thus


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enhance the activity and stability of photoanodes. What’s more, the photoanode-HSL and HSL-cocatalyst interfaces need careful engineering. Similar results have been observed on other materials. After modifying Fe2O3 with conjunct IrOx/Ni(OH)2, much higher photocurrent is observed compared with that with mono-cocatalyst (Figure 10).74 It is inferred that Ni(OH)2 functions as a HSL capturing the photogenerated holes from hematite across the hematite/IrOx interface.74 In addition, dual-cocatalyst modification that consecutively applying two different WOCs, FeOOH and NiOOH, on BiVO4 photoanodes can reduce the recombination at the BiVO4-WOC interface and create a more favorable Helmholtz layer potential drop at the WOC-electrolyte junction.8 It is found that the BiVO4-WOC interface recombination in BiVO4/NiOOH is more substantial than BiVO4/FeOOH and bare BiVO4 because of the mismatch between BiVO4 and NiOOH, and BiVO4/FeOOH shows more serious WOC-electrolyte interface recombination than BiVO4/NiOOH due to the poor water oxidation property of FeOOH.8 However, for the BiVO4/FeOOH/NiOOH electrode, the BiVO4-WOC and WOC-electrolyte junctions are simultaneously optimized, because FeOOH interlayer can reduce the BiVO4-WOC interface recombination and the NiOOH cocatalyst will realize faster water oxidation kinetics and bring a more negative Helmholtz layer potential drop to achieve a more negative Efb for the photoanode.8 For photocathodes, it is also reported that the performance of Cu2O modified by cocatalyst/TiO2/Al:ZnO protective layers is closely related to the crystallinity of the TiO2/Al:ZnO interlayers.75-77 In short, enhancing the charge separation and reducing charge recombination via the rational integration of efficient cocatalysts and careful semiconductor-cocatalyst interface modification is critical for improving the activity and stability of photoelectrodes. The interface engineering between the interlayers and the cocatalysts on surface are critical for electron transfer


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ACS Catalysis

and the final performance of the integrated photoelectrode. Cocatalysts can perform admirably if and only if the semiconductor and interlayers are exactly adaptive to the cocatalyst on surface. Thus, there is much work to do before loading cocatalyst to a photoelectrode. The semiconductor, interlayers and cocatalysts should well-match with each other, both in the energy level and structure properties. In addition, the roles of the interlayers between the semiconductor and cocatalysts somewhat mimic the natural photosynthesis which involves various electron transfer intermediates such as the tyrosine groups between the light absorber and water oxidation center, the series of electron transport chains between the two photosystems, etc. Moreover, engineering the interfaces in photoelectrodes via introducing carefully selected interlayers is actually a universal tactic. Introducing an electron transport layer between the semiconductor and the substrate can enhance the charge separation and suppress the recombination.78 Besides, the commonly used strategy of junction construction to enhance the charge separation is also realized by introducing an interlayer which can form hetero-junction or phase-junction with the semiconductor underneath.79-81 As shown in Figure 11a, the surface phase-junction shows a negative effect in a particulate photoelectrode due to severe interfacial recombination at the particle-particle interface,82 and similar result was observed that the photocurrent of a particulate Ta3N5 photoanode is enhanced apparently after refined necking treatment (Figure 11b).83 In contrast, the photocurrent of a planar TiO2 photoanode can be enhanced obviously via constructing phase junction with tailored phase alignment and interface structure.84 As shown in Figure 12, the TiO2-AR photoanode with anatase (A) and rutile (R) phase-junction shows much higher photocurrent than TiO2-dAR, TiO2-RA and mono-phase TiO2 electrodes, because the phase alignment of TiO2-AR is more favorable for the forward electron migration to the 17 ACS Paragon Plus Environment

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substrate, and the gradient phase structure obtained by carefully adjusting the fabrication condition can inhibit the undesirable interface trapping/recombination processes between two phases.84 The final performance of a photoelectrode is determined by many factors, and the champion belongs to those with efficient light absorption, good charge separation and transfer, robust surface reaction and high durability. Thanks to the aforementioned strategies for enhancing the PEC water splitting efficiency, the STH value has been increased remarkably in these years,6-9, 11, 12, 60, 71, 72, 85-95 as shown in Figure 13. It can be seen that most of the outstanding photoanodes are obtained with efficient WOCs and fine interface modification, indicating us the future working direction to further enhance the efficiency. To gain an overview of interface engineering with interlayers and cocatalyst, Table 1 summarizes the species of interlayers and cocatalysts that have been applied on various semiconductor photoelectrodes in literatures, providing a database for the researchers. 6. MECHANISM STUDIES ON SURFACE CATALYSIS Along with developing materials and strategies to enhance the efficiency of PEC water splitting, some researchers concentrate on the mechanism of surface catalysis. In natural photosynthesis, the water oxidation center CaMn4O5 advances from the ground state S0 to S4 state via accepting four photogenerated holes stepwise, and then the high-valent Mn oxidizes water molecules to O2.96 Analogously, extensive studies about CoOx-based WOCs indicate that its electrocatalytic function is associated with the oxidation of Co(II, III) species and the formation of Co(IV)-O intermediates97, 98. Recently, the direct coupling between oxygens has been observed on the dicobalt edge sites of CoOx.99 In PEC water oxidation, the rate-determining step may also involve O−O bond formation which requires the multistep oxidation of active sites similar with that in nature. Hematite photoanodes are the most studied prototypical systems. For bare photoanodes, photogenerated holes


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are supposed to oxidize the metal centers on surface which turn out to be catalytic centers. The oxidation of surface bounded Fe(III)−OH to higher-valent Fe species (such as Fe(IV)=O) has been observed.100, 101 In addition, it has been revealed that the rate order of water oxidation on hematite transits from a first order (slow) reaction at low density of holes on surface to a third order (faster) mechanism once the holes are sufficient to enable the oxidation of the neighbour metal atoms.102 In the case of WOCs modified photoanodes, it is widely observed that the addition of cocatalysts on photoanodes can increase ηinj greatly.89 And it is found that the transient photocurrent of CoPi/hematite shows a substantial anodic charging current when turning the light on and a cathodic discharge current when the light is off, as shown in Figure 14.103 The transient current depends on the thickness of CoPi and is attributed to the oxidation of Co(III) by holes to Co(IV) which is then reduced by water.103 It is inferred that CoPi plays a role of collecting holes from illuminated hematite, which can improve the interface charge separation and increase water oxidation efficiency.103 In other words, photogenerated holes are firstly injected to the catalytic center, and then to the reactants. However, there is still a debate whether any enhanced activity after loading cocatalysts results from specific catalytic function of the cocatalysts or by retardation of recombination kinetics.104 The latter voice mainly comes from James R. Durrant et al. who probed the carrier dynamics in CoPi/hematite by transient absorption spectroscopy and found that CoPi increases the lifetime of holes in the photoelectrode.105, 106 They argued that the enhanced PEC activity results from the enhanced electron depletion and the reduced recombination, implying that CoPi plays a non-catalytic role in the process. A similar conclusion that the treatment of hematite with Co(II) suppresses surface recombination instead of promoting hole transfer has also been obtained by intensity-modulated photocurrent spectroscopy.107 In particular, there is still a lack of


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direct observation of nascent catalytic intermediates like metal oxide radicals and the formation of O–O bond during the water oxidation cycle. Therefore, further researches regarding the mechanism that underpin the observed phenomena are imperative. 7. OPTIMIZING PARAMETERS OF DEVICE DESIGN AND OPERATION CONDITIONS Besides of the fundamental research on the above issues, optimizing other technical parameters about device design, mass transport, operation conditions, and the safe and effective hydrogen gas collection over large areas for storage and end-use are necessary but often underestimated so far. As for the device configuration, a two-electrode cell constructed by two compartments separated by a membrane is more feasible from a practical perspective. Therefore, the device design should consider the membrane area for minimum cost and efficient charge transport, the distance between two electrodes for minimum solution resistance, the illuminated area and direction of photoelectrodes for maximum photoabsorption, and the pressure equilibrium between two components. And special structures that can promote the mass transport of reactants, intermediates and products throughout the device will be necessary during designing the electrode and the cell for long term operation.108 In addition, the allowable electrolyte thickness will be a crucial design parameter for scale-up of PEC systems, because photoelectrodes are commonly illuminated through electrolyte of up to a few cm and this will influence the STH efficiency greatly, especially for photons of λ > 700 nm and tandem PEC systems.109 To solve these issues, an alternative concept called “electrolyte-less PEC cell” has been proposed, but it requires photoelectrodes with specific nanostructure.110 What’s more, considering the temperature increase during exposure to sunlight, analyzing the performance at temperatures greater than room temperature is also critical. Otherwise, a cooling system should be introduced, which will highly increase the cost. The enhancement of


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PEC performance with elevated temperature has been observed on hematite and BiVO4 photoanodes,111, 112 but the stability of photoelectrodes, sealing and construction of the cell, cell pressure in the presence of evolving gases, kinetics of transport and the reaction, as well as the mobility of carriers may be significantly altered by temperature.110, 113 In a word, appropriate cell design and operation conditions are necessary to obtain a highly efficient PEC system. 8. SUMMARY In closing, much progress has been made on promoting PEC water splitting efficiency via cocatalysts, electrode-electrolyte and semiconductor-cocatalyst interface engineering, etc. Firstly, introducing cocatalysts on photoelectrodes is validated to be the most efficient strategy to lower the reaction barrier and promote the charge transfer to reactants. Secondly, more attention should be paid to the electrolyte which can influence the surface catalysis remarkably. Electrolyte cations on surface may influence the water splitting and back reactions, and anions can take part in the proton transfer processes during surface catalytic reaction. In addition, the interface engineering between cocatalysts and the semiconductor is critical for the charge transfer from semiconductor to cocatalysts. Coupling cocatalysts with other interlayers such as hole-transport, hole-storage and electron-blocking layers can greatly enhance the efficiency and stability of photoelectrodes. Lastly, a possible catalytic mechanism during PEC water oxidation has been signified that photogenerated holes are supposed to be injected to the surface active site generating high-valent species which then oxidize the water molecules. To the end, how far can PEC water splitting goes depends on further innovations on the critical issues of this area: enhancing the light absorption, improving the charge separation and transfer, and most importantly, promoting the surface catalysis by more efficient cocatalysts.


ACS Catalysis

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ϕ semiconductor І

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electrolyte IІI

ІI

IHP OHP

+

ϕbulk ∆ϕsc

-+ +

+

-

Solvate molecules

-

+ + + + + + - ∆ϕH + + ∆ϕG + + +

-

Surface trapped electrons + Ionized donors

-

Anions + Cations

1–0.1μm 3–5Å Figure 1. A model of the double-layer structure of an n-type semiconductor electrode in contact with the electrolyte under equilibrium condition, and the potential (φ) profile across the double-layer region (blue line). ∆φSC, ∆φH, ∆φG are the potential drop in the space charge layer (I), Helmholtz layer (II) and Gouy layer (III), respectively. The Helmholtz layer contains electrons trapped in surface states, specially adsorbed ions, electrostatically adsorbed ions, solvent molecules, etc. IHP designates the inner Helmholtz plane indicating the distance of specifically adsorbed anions (OH- on Lewis acid sites) and few cations (H+ on Lewis basic sites) at the semiconductor surface. OHP is the outer Helmholtz plane marking the closest distance of the ions electrostatically adsorbed on the electrode surface.


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ACS Catalysis

E

dark

(a)

E

irradiated e-

e-

e-

(b)

e-

EF,n Vbarrier

e- eEC

Eredox

EF

RSS

Vph Rbulk

hv

Eredox

RSC

EF,p

Jredox EV

+h h+ h

h+

h+

h+

h+

+ h+ h

+

Figure 2. (a) The electron energy (E) profile of an n-type semiconductor in equilibrium with the electrolyte in dark condition. EC is the conduction band, EV is the valence band, and Vbarrier is the barrier height caused by band bending. (b) The quasi-static energy profile and the charge transfer and recombination pathway of an n-type semiconductor under continuous illumination in contact with the electrolyte. Jredox is the target charge transfer from the valence band to the redox reagent, Rbulk is the bulk recombination, RSC is the space charge layer recombination, RSS is the surface state recombination, and Vph is the open circuit photovoltage. EF,n and EF,p is the quasi Fermi level of electrons and holes under illumination, respectively.


ACS Catalysis

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Figure 3. (a) J-E curves under chopped light illumination, (b) chronoamperometry measurement of BiVO4 and CoBi/BiVO4 photoanodes at 0.4 VSCE in 0.2 M sodium borate (pH 9) and 0.5 M Na2SO4 (pH 9) electrolyte with a scanning rate of 20 mV s-1. (c) Water splitting activities of CoBi/BiVO4 vs. Pt system in the first hour under different biases. Light source: Xe lamp (λ > 420 nm); (d) A schematic description of the two-electrode PEC water splitting system and the electrode-electrolyte interface. (Reproduced with permission from ref 50. Copyright 2013 Royal Society of Chemistry.)


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ACS Catalysis

(a)

(b)

(c)

(d) NR R-NR

NR R-NR NR-CoPi R-NR-CoPi

Figure 4. (a) Cross-section schematic of the photoanode with a disordered shell for solar water splitting. The (b) EIS in 0.5 M Na2SO4 at 0.64 VRHE (inset: equivalent circuit; solid line: simulated data), (c) charge injection, (d) charge separation efficiency curves of the TiO2, TiO2/CoPi, reduced-TiO2 and reduced-TiO2/CoPi. NR and R-NR represent pristine and reduced TiO2 nanorod arrays, respectively. (Reproduced with permission from ref 51. Copyright 2015 American Chemical Society.)


ACS Catalysis

(b)

(a)

-2

J /(mA cm )

1.2

water oxidation activity Li+ > K+ > Na+

0.6

(d)

O−H weakening

H O e-

O2

0.07

10

20

h+

back reaction +

Li

0.06

Na

+

K

O-H change/Å

M+(H2O)x−OH

-2

(c)

J /(A cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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+

Figure 5. (a) Chronoamperometry measurements of the Co3O4/TiO2 electrode at 1.23 VRHE in 1 M NaOH, LiOH or KOH electrolytes (Xe lamp, 300 mW cm-2) and (b) the corresponding water oxidation activity trend. (b) The schematic description of the interaction between hydrated cations and OH species on the electrode surface (the water oxidation photocurrent is the photocurrent of TiO2 photoelectrode at 1.23 VRHE after 10 h operation, the back reaction current is the oxygen reduction current at 0.45 VRHE on FTO, and the O−H change is the change of bond length of OH species adsorbed on TiO2 surface before and after interacting with cations). (c) The backward reaction and the weakening extent of O−H bond after interaction with different cations. (Reproduced with permission from ref 61. Copyright 2015 American Chemical Society.)


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ACS Catalysis

(a)

(b)

(c)

(d)

Figure 6. (a) J-E curves of hematite photoanodes in unbuffered electrolytes (0.5 M NaClO4 ) at different pH levels (the inset shows the photocurrent densities at 1.23 VRHE and the onset potentials). (b) KIE values calculated from the ratio of steady photocurrent in H2O and D2O for hematite photoanodes in unbuffered electrolyte at various pH levels at 1.2 VRHE and under LED illumination (λ = 470 nm, 216 mW cm-2), and the supposed electron-proton transfer pathways during interfacial hole transfer for water oxidation. (c) Nyquist plots of EIS measured in H2O and D2O for hematite photoanodes at 1.23 VRHE in an unbuffered electrolyte at pH/pD 10. (d) J-E curves of hematite in different concentrations of borate buffer solution at pH 10. (Reproduced with permission from ref 69. Copyright 2016 American Chemical Society.)


ACS Catalysis

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(a)

(b)

(c)

(d)

Eonset 0.9 V

Figure 7. (a) Cyclic voltammetry curves of the n-Si/ITO and n-Si/TiOx/ITO photoelectrodes in 5 mM ferri/ferrocyanide solution under illumination. (b) The charge injection and separation efficiencies of the n-Si/TiOx/ITO (black), n-Si/TiOx/ITO/NiOOH (blue) photoanodes under illumination. (c) J-E curves of n-Si/TiOx/ITO (black), n-Si/TiOx/ITO/NiOOH (blue) and p++-Si/TiOx/ITO/NiOOH (red) photoelectrodes in 1 M LiOH (Solid line: under irradiation; dash line: in the dark). Light source: M 1.5G sunlight simulator (100 mW cm-2); Scan rate: 20 mV s-1. (d) The energy

band

diagram

under

thermodynamic

equilibrium

in

dark

condition

of

the

n-Si/TiOx/ITO/NiOOH photoanode and the cross-section High-Resolution TEM image. (Reproduced with permission from ref 71. Copyright 2016 American Chemical Society.)


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ACS Catalysis

(b)

(a)

O2

H2O

h+ h+ h+ h+

e- e-

e-

e-

Cocatalyst

Hole-Storage Layer Semiconductor

Conductive substrate

(c)

(d)

Figure 8. (a) The representation of the hole-storage approach. (b) Chronoamperometry measurements at 1.23 VRHE, (c) curves of charge injection efficiencies and (d) charge-storage amount vs. potential of Ta3N5, Fh/Ta3N5, Co3O4/Ta3N5 and Co3O4/Fh/Ta3N5 photoanodes under AM 1.5G simulated sunlight (100 mW cm-2) in 1 M NaOH aqueous solution. (Modified with permission from ref 72. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA.)


ACS Catalysis

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(a)

(b)

Figure 9. (a) The schematic presentation of the integrated Ta3N5 photoanode. (b) The J-E curves of the complex 2/complex 1/Ni(OH)x/Fh/TiOx/Ta3N5(P) photoanode under AM 1.5G simulated sunlight (100 mW cm-2) in 1 M NaOH (pH 13.6) aqueous solution (inset: the enlarged view of saturated photocurrent). Complex 1: Co cubane cluster (Co4O4(O2CMe)4(CNpy)4); Complex 2: [Cp*Ir(L1)Cl]Cl (L1 = 2, 2’-bi-2-imidazoline). (Reproduced with permission from ref 6. Copyright 2016 The Royal Society of Chemistry.)


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ACS Catalysis

(a)

(b)

Figure 10. (a) The scheme of the charge transfer from hematite to H2O through Ni(OH)2 and/or IrO2. (b) Chronoamperometry measurement of Ti-Fe2O3 (blue), Ti-Fe2O3/Ni(OH)2 (black), and Ti-Fe2O3/Ni(OH)2/IrO2 (black) under stepped potential (green, dash curve). (Reproduced with permission from ref 74. Copyright 2015 American Chemical Society.)


ACS Catalysis

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(b)

(a)

Figure 11. (a) A scheme of the particulate Ga2O3 photoanode with surface α/β phase junction. (Reproduced with permission from ref

82

. Copyright 2015 American Chemical Society.) (b) The

effects of refined necking treatment on the photocurrent of particulate Ta3N5 photoanode and the scheme of the particle-particle interface charge transportation. (Reproduced with permission from ref 83. Copyright 2016 Royal Society of Chemistry.)


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ACS Catalysis

(a)

(b)

(d)

(c)

Figure 12. (a) Schematic diagrams showing the phase junction effects on charge separation and transfer in photocatalytic (PC) system and PEC system. (b) Schematic diagrams showing the possible charge transfer processes across the phase junction (dotted arrows represent undesirable interface trapping/recombination processes). (c) J-E curves of TiO2 with type A (TiO2 deposited at 0.3% O2), type B (TiO2-dAR obtained by rapid thermal annealing of the FTO-(amorphous)TiO2-Ti precursor deposited at 12% O2 partial pressure followed by depositing a Ti layer) and type C (TiO2-RA electrode) phase alignments. (d) Comparison of the PEC performance of various TiO2 electrodes. FTO-TiO2-AR is fabricated by rapid thermal annealing of the FTO-(amorphous)TiO2-Ti precursor deposited by gradually adjusting the O2 partial pressure from 12% to 0%. (Reproduced with permission from ref 84. Copyright 2016 Royal Society of Chemistry.)


ACS Catalysis

2.5

Ta3N5

2.0

[6]

Ir,Co-complex/Ni(OH)x/Fh/TiOx/Ta3N5

BiVO4

STH/ %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NiOOH/FeOOH/N:nanoporous-BiVO4 + NiOx/p n-Si [95]

n-Si Fe2O3

[11]

[12]

NiOx/CoOx/SiOx/n-Si [7] NiFeOx-Bi/porous-BiVO4 NiOOH/FeOOH/nanoporous-BiVO4 [8] CoPi/Ba:Ta3N5 [9]

1.5

NiO/CoOx/BiVO4 [90]

CoPi/gradient-W:BiVO4 [14]

[60] +

Co(OH)2/Co3O4/p n-Si

Co3O4/Fh/Ta3N5 [72]

1.0 FeOOH/porous-BiVO4

NiOOH/TiOx/ITO/n-Si

[88]

FeOOH/BiVO4 [92]

0.5

[85]

CoPi/W:BiVO4 [89]

Ru:Fe2O3 [93]

[94]

CoPi/Ag/Fe2O3

+

CoOx/p n-Si

IrOx/Ta3N5[87]

[91]

CoPi/Pt:Fe2O3

[86]

[71]

IrOx/Fe2O3

0.0 2010

2011

2012

2013 2014 Year

2015

2016

2017

Figure 13. The summary of the outstanding photoanodes based on single photon-absorber and their maximum

STH

values

in

these

years.

The

STH

of

nanoporous-BiVO4

was

the

applied-bias-photon-to-current efficiency (ABPE) measured in a two-electrode system, and the STH of others are estimated according to the equation: STH (estimated) = J(mA cm-2)·(1.23 – E) (VRHE) /100 (mW cm-2), in which J is the photocurrent density obtained from the J-E curves measured in a three-electrode system in literatures6-9, 11, 12, 60, 71, 72, 85-95. The corresponding references are indicated in the brackets. The colon symbol means doping, and NiFeOx-Bi means the NiFeOx catalyst deposited in borate buffered electrolyte.


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ACS Catalysis

(c)

(a)

(b)

Figure 14. (a) J-E curves of a bare hematite electrode (red solid line) and the same electrode after depositing 1 (orange dotted line), 2 (yellow short dashed line), 15 (green dashed double dotted line), 45 (teal long dashed line) and 90 (blue dashed single dotted line) mC cm-2 CoPi catalyst at 5 mV s-1. (b) The proposed mechanism of the catalytic process. (c) The anodic and cathodic transient current at an applied bias of 1.05 VRHE. Condition: pH 6.9 buffered aqueous solution, 1 sun illumination. (Reproduced with permission from ref 103. Copyright 2012 American Chemical Society.)


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Table 1. Summary of the cocatalysts and interlayers employed on various photoelectrodes cocatalysts/interlayers

electrode

CoPi,51, 86, 93 CoPi/Ti:SiOx,114 Co3O4,115 Co(OH)2/Co3O4,116 IrOx,91 Fe2O3 74

IrOx/Ni(OH)2, FeOOH,

117, 118

32

NiFeOx, Ru complex,

58, 119

Ir complex

120

Co(OH)2/Co3O4,60 CoOx,94, 121 NiOx,95 NiOx/CoOx/SiOx,12 NiOx/Ni/SiOx,65

(p+) n-Si

NiOOH/ITO/TiOx,71 Ir/TiOx/SiOx,122 NiCrOx/TiO2,123 NiRuOx,124 MnOx125 CoPi,14, 89 CoBi,50 CoOx/NiO,90 NiBi,126 FeOOH,88, 92 NiOOH/FeOOH,8, 11 BiVO4 NiFeOx-Bi,

7

La0.3Co0.1Ce0.6Ox127

CoPi,9 Co3O4,128 Co3O4/Fh,72 IrO2,87, 129 Ir, Co complex/Ni(OH)2/Fh/TiOx6

Ta3N5

RhOx/CoOx130

BaTaO2N

Pt/TiO2,131 CoP,132 Wo2C,133 Mo2C,52 MoSx,134 Mo3S4 complex,59 Ni complex135

p-Si

(Pt,75, 76 RuOx,136 MoSx,53, 54 Ni-Mo54)/TiO2/AZO, RGO,137 NiOx138

p-Cu2O


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ACS Catalysis

ACKNOWLEDGMENT

This work was financially supported by the National Natural Science Foundation of China (No. 21603225), 973 National Basic Research Program of the Ministry of Science and Technology of China (No. 2014CB239400) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB01020300).

AUTHOR INFORMATION * Corresponding author: E-mail: [email protected] NOTES The authors declare no competing financial interest. REFERENCES (1)

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Table of Contents Interface

ee-

h+ storage

O2

h+ transport

H2

e- blocking

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ACS Catalysis

Cocatalyst Electrolyte

h+

cations ---OH---- + H+ O2 anions

h+


Photoelectrocatalytic Water Splitting: Significance of Cocatalysts

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