Photoelectrochemical Water Splitting

C H A P T E R

28 Photoelectrochemical Water Splitting Prabhakarn Arunachalam and Abdullah M. Al Mayouf Electrochemistry Research Group, Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia

28.1 INTRODUCTION One of the most essential concerns for human society is perceived with the increasing awareness in producing a low-cost, clean, and copious source of energy to ensure minimal impact to the environment. In fact, the solar energy with its infinite quantity is likely to be the clean and low-cost renewable energy resource. The solar power arriving the earth’s surface corresponds to that delivered by 130 million power plants of 500 MW [1 6]. Presently, solar energy-based technology is realizing great recognition owing to its numerous properties, which include its ability to operate without noise, toxicity, or greenhouse gas release [7 11]. The viability for the advancement of solar energy at the TW scale depends on discovering economically compatible solutions to the intrinsic variability of solar power supply problems [12]. Nature elucidated these problem by means of photosynthesis, the method that transforms solar energy into chemical energy stored in the form of atomic bonds of the adenosine triphosphate molecule. Likewise, water photoelectrolysis through semiconductor materials is recognized to be the most essential challenge for the transformation and storage of solar energy [13]. To accomplish this “artificial photosynthesis” an eco-friendly PEC cell must be established, comprised of stable semiconductor materials and designed in such a way that the appropriate PEC reactions take place at the semiconductor/solution interface [14]. Out of anxiety for natural source exhaustion and ecological debates, the visible-light driven electrolysis of water to generate oxygen and hydrogen fuels and the conversion to electricity, inspire the advancement of clean energy systems for producing energy.

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00028-0

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FIGURE 28.1 Scheme of a basic photoelectrochemical cell.

The simplest structure of a PEC cell that concurrently executes both oxygen evolution reaction (OER) and H2 evolution reactions (HER) is shown in Fig. 28.1. At the cathode the reduction reaction takes place (2H1 1 2e2-H2) and at the photoanode the reaction of O2 evolution is carried out (2H2O-4H1 1 O2 1 4e2). The hydrogen economy is increasingly investigated to substitute the fossil fuel resources. However, storing hydrogen therefore offers a means to store electrical energy, e.g., surplus electricity produced by power stations controlled by the weather, wind farms, or by solar farms. To achieve this, PEC water splitting driven by solar energy creates an eco-friendly methodology to store energy in the covalent bonds of H2 [15]. To effectively transform solar photons into such a chemical fuel, semiconductorbased materials skilled at absorbing a great part of the solar spectral region, with a suitable energy level position, and low overpotential to perform a water splitting reactions are essential. In this line, exhaustive research works have been carried out to advance competitive n-type semiconducting materials employed as water splitting photoanodes, as the oxygen evolution reaction (OER) is both kinetically and thermodynamically more challenging, and the photoanode materials are damaged by severe oxidizing circumstances [16]. Various kinds of semiconductor oxide photoelectrodes (TiO2 [17], Fe2O3 [18,19], WO3 [20,21], BiVO4 [22 24], etc.) have been engaged for fabricating PEC devices to oxidize water and generate molecular oxygen. In particular, titanium dioxide (TiO2) has been recognized to be the most suitable contender for PEC water splitting due to its band-edge levels, greater optical stability, great chemical inertness, photostability, and cost-efficiency [25 28]. Various efforts have been devoted to improving TiO2 light absorption behaviors and making this catalytic material sensitive to visible-light irradiation, which also offers benefits of hindering recombination rate of charge carriers and higher anatase crystallinity [29,30]. Alternatively, BiVO4 has fascinated researchers worldwide as the most appropriate water splitting photoanode materials for PEC water splitting [31 34]. BiVO4 gratifies numerous desires, like n-type semiconductor with a direct band gap size of 2.4 2 2.5 eV (have the tendency upwards of B7.5 mA/cm2 photocurrent), engrosses full visible-light region of solar spectrum, and has a friendly nature with basic and neutral

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conditions, nontoxic, and relatively cheap [35]. In particular, BiVO4 grasps the greatest of catalytic performance with 8.1% solar to hydrogen (STH) efficiency via double-junctioned GaAs/InGaAsP photovoltaic (PV) device [36]. Quite recently, Qiu et al. reported the tandem configuration with a single perovskite solar cell and achieved unaided water splitting with a STH conversion efficiency of up to 6.2% for more than 10 h [37]. Subsequently, there are few concerns on the availability of bismuth in the Earth crust [38], and various kinds of compositional alterations have been investigated to attain an inexpensive metal vanadate built on this method [39 41]. In this line, BiVO4 photoanodes must satisfy many of the requirements; their sluggish electronic properties produce low solar conversion efficiencies, inhibiting their commercial usage in PEC systems at present. In particular, the incorporation of an effective, stable, and inexpensive water oxidation catalyst on the photoactive semiconductor material is crucial to attain the directed techno-economical necessities. Recently, Prussian Blue (PB)-type electrocatalyst materials were engaged as supporting cocatalysts and a hole-storage layer to safeguard unstable BiVO4 anode against photocorrosion in PEC water splitting [42 44]. To address this issue, various approaches have been reported, including ion doping [45 47], nanostructuring [48], surface modification with passivation layers or electrocatalysts [32,49,50], and combinatorial synthesis [51]. It is highly in demand to look for nonoxide-based semiconductors for PEC water oxidation. Oxynitride-based photoanodes have the potential to substitute oxide-based anodes for light absorption to generate H2 and O2 from H2O by a stoichiometric ratio [52 54]. Scaife described that it is intrinsically tougher to progress an oxide photocatalyst which has a properly negative conduction band (CB) and a narrow band gap (i.e., ,3 eV) for light absorption owing to the greater positive valence band (VB) (at c. 13.0 V vs NHE) produced by the O 2p orbital [55 56]. In this book chapter, we describe recent progress in the growth of earth-abundant electrocatalysts for water splitting in the context of their probable use in PEC solar-tohydrogen devices. Moreover, we describe various examples of complete and functional PEC solar-to-hydrogen devices and critically evaluate the continuing disputes in this field. Finally, this is a well-timed review to investigate the numerous renewable hydrogen generation schemes and also to give greater consideration to the visible-light driven solardriven water electrolysis.

28.2 PRINCIPLES OF PEC WATER SPLITTING PROCESS The free energy change for the transformation of one molecule of H2O into H2 and 1/2O2 in standard conditions is ΔG 5 237.2 kJ/mol, which according to the Nernst equation, agrees to ΔE 5 1.23 V per transported electron. Accordingly, the usage of a semiconductor that conveys this reaction with the backing of light, means that the semiconductor must absorb photons with energy .1.23 eV (light absorption of 1008 nm) and transform this energy into H2 and O2, as schematically presented in Fig. 28.2. The actual processes involved in PEC processes are described in Fig. 28.3. To achieve this, there are four main physiochemical steps that need to be satisfied to complete PEC water splitting reaction. These processes include: (i) capture of photons;

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Water splitting (Artificial photosynthesis)

(A)

CB

H2

e–

H2O

Energy

Band gap H2O

h+

O2

H2 + O2

VB Chemical energy ΔG0 = 237 kJ/mol

H2O Powdered photocatalyst

Water

(B) –2.0 SiC

2.8eV

H+/H WO3 Fe2O3 2

2.0

2.3eV

MoS2 1.75eV

1.0

1.1eV

Si 3.0eV

1.7eV

3.6eV

TiO2

2.4eV

Cds Cdse 3.0eV

3.2eV

0

3.4eV

KTaO3 SrTiO3 5.0eV

V vs. NHE (pHD)

–1.0

GaP

2.25eV

Zns ZrO2

O2/H2O

3.0 4.0

FIGURE 28.2 Photocatalytic water splitting: (A) graphical representation of water splitting via semiconductor photocatalyst; (B) band structure of various kinds of semiconductors and its corresponding redox potentials of water splitting. Source: Reprinted with permission from K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 440 (2006) 295 295. Copyright from Royal Chemical Society.

(ii) creation/separation of electron hole pairs; (iii) separated charge carriers that need to be transported via charge transfer; which must be catalytically active. During the last two processes, induced electron/hole pairs can either recombine in the bulk, and therefore both effective charge separation and higher transportation of electron/hole pairs are desired; and (iv) surface chemical reactions. Both the potential of the induced electron/ hole pairs and appropriate water oxidation kinetics are vital for effective water splitting reaction. Usually, the ideal photoanode material must satisfy a number of criteria to carry out the water photoelectrolysis. These are: (1) to display a strong (visible) light absorption with a band gap varying between 1.8 and 2.4 eV; (2) to display great chemical inertness both in the darkness and under illumination; (3) band edge energy position that supports the water redox potentials; (4) to effectively separate/transport the charge carriers within

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FIGURE 28.3 A basic energy diagram for a photoanode (n-type semiconductor). Various stages of process are presented, namely: (i) light absorption; (ii) chargetransfer; (iii) transportation of charge carriers; and (iv) surface chemical reactions.

the semiconductor material to keep the rate of water-splitting reaction faster; (5) to show low charge transfer resistance at the liquid/semiconductor interface (low overpotentials); and (6) to have low cost.

28.3 PHOTOANODE MATERIALS During last decades, several kinds of photocatalyst materials for PEC water splitting reaction have been demonstrated, and huge research expenses have been devoted in this area. This section reviews the most favorable economical and efficient anodes materials engaged so far, and puts forward prospective approaches to enhance their PEC behaviors.

28.3.1 TiO2-Based Photocatalysts TiO2-based photocatalysts have been recognized to be the most suitable applicant for visible-light driven water splitting, and have been extensively considered since 1972, owing to its suitable band-energy levels, its nontoxicity, as well as its photostability [57]. However, owing to its larger band-gap (B3.2 eV: anatase; B3.1 eV: rutile phase), merely 5% of the solar spectrum can be absorbed, which reduces its widespread use and results in a very low maximal theoretical STH efficiency. In recent years, several research works have been undertaken to incorporate TiO2 with various kinds of anions or cations to lengthen its operating range into the visible-light region in order to advance the whole absorption as well as preserving its good photostability and low cost [29,58]. Until now, sensitization of TiO2 electrode surface with a smaller band-gap semiconductor/dyes [59,60], nonmetal, and metal nanoparticles doping [61,62] has been mostly employed to improve the PEC performance of TiO2 materials. Though, in most of these doping strategies because there is no significant band gap change, no considerable improvement in PEC performances has been reported. While the doping approach can spread the

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absorption range into the visible light region, the optical absorption is still very restricted beyond 450 nm. Despite these facts, Mao et al. advanced black-colored hydrogenated TiO2 nanocrystals, relating to a band gap energy of 1.0 eV rather than the 3.2 eV usually observed for pure TiO2, favoring a much superior PEC efficiency [63]. In recent years, surface plasmon resonance (SPR) has been used in PEC water electrolysis process with extended light absorption in the whole UV visible region of the solar spectrum [64,65]. Additionally, SPR is an inherent stuff of metal nanoparticles, where the collective oscillation frequency is extremely sensitive to the size and shape of the metals. For example, Auincorporated TiO2 nanowire electrodes showed superior photocurrent generation at 710 nm and improved photoactivity, which is credited to the excitation of SPR of Au [66]. Similarly, significant research works have been undertaken on Ag doping on TiO2 electrode; the Ag nanoparticles act as an electron sink in the role of Ag SPR effect for the photoinduced electron hole pairs and thus results in improved PEC performance [67 69]. Quite recently, we reported the incorporation of Ag onto mesoporous TiO2 (meso-TiO2), fabricated by the evaporation-induced self-assembly method and the improved PEC over various ranges of Ag nanoparticles over mesoporous TiO2 is shown in Fig. 28.4. Moreover, the PEC results demonstrated that the maximal photocurrent density of Ag/meso-TiO2 nanospheres photoanodes reaches 1.0 mA/cm2 (for [AgNO3] 5 1 mM) which is nearly a two-fold enhancement over that of meso-TiO2 photoanodes. Under illumination condition, the enhanced photocurrent at lower potential shows that the incorporation of Ag particles reduced the recombination of electron/hole pairs. Loading TiO2 photoanode materials with other lower band gap semiconductors to create a heterojunction are alternative favorable methods to produce visible light, whereas the loaded semiconductors assist as a photosensitizer and builder for internal electric field through the interface. The heterojunctioned materials tends to have internal potential bias, which considerably encourages the excited hole and electrons pairs separation and transport via the interface, resulting in a reduced recombination. Recently, Choi et al. developed a heterojunction CdTe/TiO2 photoelectrodes, enhancement in PEC performance credited to the optimization of Fermi level, band positions, and the conductivity of CdTe layer [70]. Similarly, TiO2 nanotube arrays were modified with Cu2O semiconductors [71,72].

28.3.2 BiVO4-Based Photocatalysts The functional properties of BiVO4 photoanodes have been advanced by heterostructuring strategies with Fe2O3. For example, the PEC behavior of BiVO4 has been greatly enhanced when loaded on nanostructured WO3 layers, credited to the synergistic interface between the BiVO4 (offering good light gathering behaviors) and WO3 (enhanced charge carrier transport) [73 75]. Moreover, various semiconductor combinations have been effectively employed and established with significant effects for water splitting reactions, like Si/TiO2/BiVO4 [76], SnO2/BiVO4 [77], WO3/Fe2O3 [78], and Ag3PO4/BiVO4 [79]. Cai et al. reported the decoration of BiVO4 with Fe2O3 nanoparticles, where it is advised that Fe2O3 behaves as a proficient supportive catalyst for the photocatalytic degradation of organic pollutants [80]. Similarly, various kinds of Fe-based cocatalysts have been effectively engaged to decorate BiVO4 photoanodes, such as FeOOH [81] and Ni FeOx [82].

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FIGURE 28.4 (A) Linear sweep voltammograms (LSV) during in situ 1.5 AM light pulsing for mesoporous TiO2 photoanodes incorporated with various loading of Ag nanoparticles deposited in 0.5 M Na2SO4 solution at pH 13. (B) Expanded zone of LSV in (A) with various loading of Ag that photodeposited via numerous concentration of AgNO3 solution (from 0 to10.0 mM). (C) Comparison for the LSV at 50 mV/s for meso-TiO2 and Ag/mesoTiO2 nanospheres photoanodes in in 0.5 M Na2SO4 solution at pH 13 and in visible-light illumination condition, (D) and consistent relative investigation in UV-light illumination. Source: Reprinted with permission from P. Arunachalam, M.S. Amer, M.A. Ghanem, A.M. AlMayouf, D. Zhao, Int. J. Hydrogen Energy 42 (2017) 11346 11355. Copyright 2017 Elsevier.

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Recent years, Co-based WOCs have received plentiful consideration owing to the greater efficiency under near neutral pH conditions, and production cost of Co [83]. Moreover, the transport of photoinduced holes from the valence states of BiVO4 to the Co31/21 species through the water oxidation reaction significantly lower the overpotential [84 87]. There are several works carried out on Co-based catalysts on several kinds of photoanodes containing WO3, α-Fe2O3, and ZnO that have shown improved PEC water-splitting efficiency [88 90]. Moreover, significant enhancements were noticed predominantly for BiVO4 photoanodes upon the incorporation of cocatalysts. Long et al. fabricated powdered composite of BiVO4/Co3O4 on conducting substrates with four-fold enhanced photon to current efficiency related to bare BiVO4 [91]. Zhong et al. attained a photocurrent of 1.4 mA/cm2 at 1.23 VRHE with Co Pi catalyzed W-loaded BiVO4 [92]. They revealed that the cobalt phosphate (CoPi)OECs enhanced the PEC efficiency of the water electrolysis to approximately 100% and that the total PEC behaviors of Co Pi catalyzed W:BiVO4 photoelectrodes is restricted by its bulk recombination processes. However, bulk-recombination process limitations need to be rectified to achieve the efficient BiVO4-based photoelectrodes. Recently, Abdi et al. investigated the CoPi-catalyzed BiVO4 photoanodes and attained external quantum efficiencies of 90%. Moreover, they rule out the possibility of bulk recombination process and illustrated that the sluggish electron transport is vital factor to be investigated in BiVO4 materials. In addition, Abdi et al. attained the photocurrent of 1.7 mA/cm2 at 1.23 VRHE for co-catalyzed undoped BiVO4 [93]. In recent years, there have been two conflicting views to understanding the mechanism of the photocurrent enhancement in the Co Pi-loaded photoanodes. Barroso et al. described improved photoinduced charge carriers separation and carrier lifetime of semiconductor by hosting Co Pi, which was credited to the improved band bending (Fig. 28.5) [94]. On the other hand, Klahr et al. showed that there is no alteration in the band bending, and in its place advised that Co Pi quickly collects photoinduced holes from the semiconductors, thereby decreasing the carrier recombination at the surface [95]. Zacha¨us et al. examined the surface carrier dynamics of Co Pi-loaded BiVO4 photoanodes by the intensity modulated photocurrent spectroscopy (IMPS) method. The detailed investigation illustrated that the loading with Co Pi OECs reduced the recombination behavior of BiVO4 with a factor of 10 20, without considerably affecting the charge transfer kinetics. In addition, the major part of the Co Pi is passivation of the surface of BiVO4 and the photocurrent of BiVO4 photoanodes is restricted by surface recombination instead of charge transfer [96]. Lichterman and coworkers employed CoOx/BiVO4 photoelectrodes to achieve a photocurrent of 1.49 mA/cm2 at 1.23 VRHE under AM 1.5G [97]. Wang et al. revealed 17-fold enhanced PEC water electrolysis performance upon the deposition of Co3O4 for BiVO4 [98]. Recently, Maged and coworkers reported a combination of ZrO2 and α-Fe2O3 nanoparticles deposited on the surface of a BiVO4 photoanodes films. Subsequently, an incredible fivefold enhancement of the photocurrent for optimized BiVO4 Zr Fe photoanodes was reported, which can be credited to the supportive catalytic part of monoclinic ZrO2 and a-Fe2O3 nanoparticles dispersed on the surface of BiVO4 [99]. Very recently, Hegnaer et al. integrated the photoactive BiVO4 materials and electrocatalytic materials of Co Fe Prussian blue (CoFe PB) electrocatalysts. Moreover, this mixture cathodically shifts the onset potential of BiVO4 by 0.8 V and raises the photovoltage

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28.3 PHOTOANODE MATERIALS

593 FIGURE 28.5 Basic model of elementary process in a BiVO4 photoanodes (A) without cocatalyst; (B) with a surface passivating cocatalyst (e.g., Co Pi); (C) with a nonpassivating cocatalyst (e.g., RuOx). Source: Reprinted with permission from F. Abdi, R. van de Krol, J. Phys. Chem. C 116 (2012) 9398 copyright from Royal Chemical Society; C. Zacha¨us, F. Abdi, L.M.P. Peter, R. van de Krol, Chem. Sci. 8 (2017) 3712 3719 published by The Royal Society of Chemistry.

by 0.45 V. In addition, an astonishing sixfold enhancement of the photocurrent at 1.23 V vs RHE is attained for BiVO4/CoFe PB photoelectrodes [98] (Table 28.1).

28.3.3 Fe2O3 Oxide Photocatalysts Another auspicious photocatalyst for PEC water electrolysis and visible-light responsivity is hematite (α-Fe2O3), which has been established to challenge the 4 electron oxidation reaction of water [100 102]. In particular, α-Fe2O3 provides more advantages over other material owing to its greater chemical inertness, low toxicity, and also due to its high natural abundance. Furthermore, it has a band gap value between 1.9 and 2.3 eV, permitting visible-light absorption which transforms to a maximal theoretical STH efficiency [103]. Though, α-Fe2O3 holds a CB position considerably more positive proton reduction potential and has a capability to be utilized for PEC water oxidation with an external bias. Additionally, α-Fe2O3 has other shortcomings, including: (1) minor charge carrier lifetime (in the order of 10212), resulting in a rapid charge carriers combination in the bulk; (2) a comparatively low absorption coefficient demanding thick film (B400 500 nm) for optimal light absorption; (3) a sluggish minority charge carrier (hole) mobility; and (4) poor water oxidation kinetics, which result in a higher recombination rate at the surface.

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TABLE 28.1 Review of Deposition Metal Oxide/Phosphate Electrocatalyst into BiVO4 Photocatalyst for Water Splitting Reaction

Catalysts

Electrolyte, Conc., pH

Nature of the Structure

Photocurrent Density (mA/cm2) at 1.23 VRHE

Ref.

Co Pi 0.1 M KPi buffer at pH 8 catalyzed W: BiVO4 photoelectrodes

Polycrystalline with primary 1.4 features having dimensions of B100 200 nm

[92]

BiVO4/Co Pi

0.5 M K2SO4 (99%, Alfa Aesar) solution buffered to pH B5.6 with 0.09 M KH2PO4 (99.5%, Fluka)/ 0.01 M K2HPO4 (99%, J.T. Baker).

30 nm thick Co Pi catalyzed BiVO4 photoelectrode

1.7

[93]

BiVO4/ Fe2O3/Zr

Phosphate buffer solution at pH 7.6

500 nm for pristine BiVO4; 200 nm for the optimized BiVO4 Zr electrode

1.2

[88]

BiVO4/ CoFe PB

0.1 M KPi buffer (solid lines) and after addition of the hole scavenger Na2SO3

BiVO4 photoanode, with a thickness of about 200 250 nm

1.4

[99]

p n BiVO4/Co3O4 heterojunction

0.5 M KPi buffer/1 M Na2SO3, pH 7 Discrete Co3O4 particles B10 nm

2.7

[100]

BiVO4/ ZnO/Co Pi

0.2 M Na2SO4 solution (pH 6.5)

3

[101]

BiVO4 on 1D ZnO rods with Co Pi on surface

To overcome these shortcomings, α-Fe2O3 photoanodes have been effectively doped with heteroatoms like Sn [104], Ti [105], Zr [47,106], Si [107], and Nb [46]. In particular, Si incorporated α-Fe2O3 films can reveal a superior PEC behavior, with photocurrent densities of nearly 2.7 mA cm22 at 1.23 VRHE under AM 1.5G [49]. Furthermore, cocatalyst modification, for instance, with Co Pi or FeOOH, has been revealed to speed up the surface water oxidation kinetics [108]. Lastly, thin metal oxide under/overlayers have been incorporated onto α-Fe2O3 resulting in substantial enhancement in PEC performances; these layers influence surface state passivation and result in an upsurge in charge carrier concentration and mobility [109].

28.3.4 Oxynitride-Based Photocatalysts Among the oxide-based photoanodes, the top edge of the VB involves O 2p orbitals and is positioned at about 13.0 V versus NHE at pH 0. Additionally, the CB position is more negative than the water reduction potential, resulting in a larger band gap of nearby 3.0 eV, meaning the material is inactive under visible-light region [55]. In recent years, certain oxynitrides-based photocatalysts have been demonstrated as replacements to oxide

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photocatalysts to absorb visible-light photons to photogenerate H2 and O2 from water at the stoichiometric ratio [49,56,109]. In recent years, various kinds of oxynitride-based photoanodes have been established for PEC water oxidation, with minimal externally applied forces [53,54]. Domen et al. reported the TaON/cocatalyst photoanodes with an IPCE of 76% at 400 nm with a minimal external applied force [110]. Afterwards, oxynitrides, such as LaTiO2N [111,112], SrNbO2N [110], ZnTaO2N [113], BaNbO2N [114], among other materials, have been recently advanced as photoanodes [115 118]. Perovskite-based oxynitride and related semiconductor photoanodes modified with appropriate cocatalysts have been shown to be an new pathway in enhancing PEC water splitting reaction [119,120]. In recent years, extensive research works have been proceeding to develop nonoxide-based photoanodes incorporated with low-cost cobalt and/or nickel oxides. Low-cost nickel and cobalt-based oxides have been applied in various kinds of applications [6,121,122]. In PEC applications, photoanodes can be fabricated via electrophoretic deposition [109], squeegee [53], spraying [54], and particle transfer methods [123]. Despite its capacity to absorb visible-light and chemical stability, the PEC activity of oxynitride is remarkably limited by its poor photon absorption, high recombination rate of photoexcited charge carriers, and poor OER kinetics. To overcome these limitations, oxynitride photoanodes may be incorporated with cocatalyst materials to advance the visible light photons absorption. In recent years, oxynitride photoanode materials such as LaTiO2N/CoOx, BaTaO2N/BaZrO3, and BaNbO2N [52,124–126], have been demonstrated to employ the light photons absorption with the assistance of appropriate sacrificial reagents. In particular, more consideration has been devoted to developing visible light-active low band gap semiconductor photoanodes (,2 eV). Quite recently, Maeda et al. demonstrated SrNbO2N-based photoanodes with a band gap of 1.8 eV and these showed IPCE efficiency of 10% at 400 nm at 1.23 V vs RHE [127]. Similarly, Zhang et al. demonstrated LaTaO2N photocatalysts materials fabricated by a one-step flux method and it also showed enhanced PEC performance for water oxidation reaction [128]. Very recently, we described the PEC nature of CoPi/La(Ta, Nb)O2N and CoPi/ZnTaO2N photoanodes for water oxidation in alkaline media [4,129], which succeeded with up to a three- to fourfold enhancement in the PEC performances at a lower oxidation potential. The incorporation of the CoPi OER cocatalyst could magnify the charge separation and carrier collection produced at the electrode surface, consequently enhancing PEC performance of the photoanodes (Fig. 28.6).

28.3.5 Cocatalyst Selection Water electrolysis demands the requirements of potential .1.23 V vs RHE between the electrodes owing to its kinetic barriers that are generally observed in executing multielectron oxidation/reduction reactions. For instance, OER from water demands four electrons and produces four intermediate species [130]. On the bare photoanodes surfaces, the production of intermediate species generates a huge energy barrier to OER/HER and thus an overpotential is highly essential to overcome the kinetically rate-limiting multistep reactions [131]. In most of these investigated photoanodes, the electrocatalytic performances

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(A)

(B) 40 Current density (mA/cm2)

Current density (mA/cm2)

40 w/o CoPi w CoPi

30

20

10

0

w/o H2O2 w H2O2

30

20

10

0 0.0

0.4 0.8 Applied potential (V vs SCE)

1.2

0.0

0.3 0.6 0.9 1.2 Applied potential (V vs SCE)

FIGURE 28.6 Cyclic voltammogram spectra of ITO/La(Ta, Nb)O2N photoanodes (A) with and without incorporation of CoPi under illumination and dark condition and (B) in the presence and absence of sacrificial electron donor of H2O2. Dashed light-under dark; solid line-illumination conditions. Source: Reprinted with permission from P. Arunachalam, A. Al-Mayouf, M.A. Ghanem, M.N. Shaddad, M.T. Weller, Int. J. Hydrogen Energy 41 (2016) 11644 11652. Copyright 2016 Elsevier.

happen at the semiconductor liquid surface and appropriate cocatalysts need to be incorporated to decrease the overpotential (activation energy) and reduce recombination of charge carriers at the photoanode surface by performing as electron hole acceptors. The application of PEC water oxidation reaction of bare photoanodes is severely restricted owing to its inappropriate CB edge level, shorter carrier lifetimes, and sluggish oxygen evolution kinetics [132]. To overcome these setbacks, i.e., to enhance water oxidation kinetics with related to surface recombination, a usual approach is to incorporate the photoanodes surface with a suitable water oxidation cocatalyst (WOC) [133]. Therefore, the efficiency and kinetics of the overall PEC water oxidation reaction will be absolutely improved. Without a rapid and robust water oxidation process, solar fuels will certainly not have commercial value. The Ir (or Ru)-based oxide cocatalysts are amongst the maximum acting OER catalysts, but are highly expensive and scarce owing to their low earth-abundance, impeding large technological impact. During the last decades, various kinds of research efforts have been focused on creating low-cost first-row transition metal oxide OER cocatalysts. More research efforts have been carried out on examining Co-based electrocatalysts for the electrooxidation of alcohols or other related applications [134 137]. Among these, cobalt oxide has been recognized as a robust, effective WOCs that can work in neutral conditions. Nocera et al. fabricated cobalt oxide films from cobalt salts in phosphate buffer (Co Pi) solutions at applied potentials .1.1 V vs NHE [138]. In recent years, the incorporation of Co Pi cocatalyst enhanced the charge separation and carrier collection at the photoanodes surface, which resulted in enhancement in PEC efficiency. In particular, the thin Co Pi layer on the surface of photoanodes surface is the most appropriate way to capture the photoholes to generate the catalytic species of cobalt (Co41), which is vital process for water oxidation reaction [83]. However, when the Co Pi layer turn out to be thicker, the photoinduced holes have to transport between many Co Pi molecules and

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597

CoPi/electrolyte interface, which results in sluggish hole transfer and leads to a poor PEC water oxidation behavior. Anyhow, the simple grouping of cocatalyst with photoanodes does not assure success. Indeed, in-depth investigations on the mechanism of WOCs on photoanodes have pointed out that most “catalysts,” even though enhancing PEC behavior, do not behave as true catalysts, i.e., do not offer an efficient hole transport path to enhance the rate of water oxidation. In most of the cases, the creation of the semiconductor/catalyst interface does “only” improve the lifetime of surface recombination either by performing as a capacitive layer or by passivation surface states [139 141]. Very recently, Hegner et al. reported the cobalt hexacyanoferrate water oxidation catalyst, it performed as a genuine cocatalyst on top of photoactive BiVO4 photoanodes [142].

28.4 NOBLE METAL METAL OXIDE NANOHYBRIDS-BASED PHOTOANODE In the case of noble metal TiO2 nanohybrids, by acting as an antenna that localizes the optical energy, noble metal nanoparticles have been suggested to sensitize nano-TiO2 to light with energy below the bandgap, thus generating additional charge carriers for water oxidation. Liu et al. [143] reported that the presence of Au NP in TiO2 films, under visible light illumination, had 66-fold enhanced PEC water splitting. The authors explained this enhancement by the fact that SPR could induce the electric field amplification near the TiO2 surface, which enhanced the photon absorption rate of TiO2. Zhang et al. [141] also argued that there was a matching of Au (nanocrystals) SPR wavelength (under visiblelight illumination) with the photonic band gap of TiO2, which significantly increased the SPR intensity to boost hot electron injection. Similarity, the plasmon-enhanced photocatalytic activity of iron oxide on gold nanopillars was reported by Gao et al. [142]. Table 28.2 presents the summary of noble metal hybrid strategies used to enhance the photocurrent density of oxide photoanodes. As can be seen in this table, the hybridization with noble metals (AuNP, AgNP) enhanced significantly the photocurrent density of TiO2 (and ZnO)-based photoanodes.

28.5 CONCLUSION This book chapter reviews the fundamental principles of PEC water splitting, overviews the various kinds of photoanodes materials and the scientific challenges of visible lightdriven water splitting. In principle, for water splitting via a bare semiconductor to succeed independently, the band gap energy levels need to overlap the reduction and oxidation potentials of water, and subsequently photoinduced charge carriers have appropriate overpotential for the HER and OER, respectively. Among the reviewed semiconductors, TiO2 photoanodes are low-cost and stable, but not suitable for visible-light absorption. However, Fe2O3 and BiVO4 are recognized to have relatively broader absorption, but failed to achieve theoretical maximum photocurrent, owing to its rapid recombination of photoinduced charge carriers and sluggish water oxidation kinetics. To enhance the

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28. PHOTOELECTROCHEMICAL WATER SPLITTING

TABLE 28.2 Review of Photoanodes Based on Noble Metal Semiconducting Oxide Nanohybrids Photocurrent Density (µA/cm2) Photoanode Electrode

Counter Reference Electrode Electrolyte Electrode

Applied Potential (V)

Control Hybrid Light Electrode Electrode Illumination Ref.

AgNps ZnO nanorods hybrids

Pt

0.5 M Na2SO4

Ag/AgCl

0.28 and 0.34 (Voc)shortcircuit

89

616

UV

[144]

Ag ZnO nanocomposite

Graphite

0.5 M Na2SO4

SCE

0.22 and 0.5 (Voc) shortcircuit

20

249

[145]

59

303

Visible light UV

AuNps ZnO nanopencil

Pt

0.5 M Na2SO4

Ag/AgCl

1

700

1500

AM 1.5G

[146]

AuNps ZnO nanocomposites

Pt

0.1 M NaOH

SCE

0.5

1500

2600

Visible light

[147]

AgNps ZnO nanocomposites

Pt

0.1 M NaOH

SCE

0.5

1500

2100

Visible light

[147]

AuNPs ZnO nanorods hybrids

Pt

0.1 M Na2SO4

Ag/AgCl

1

330

9110

AM 1.5G

[148]

0.5

200

350

0.5 M Na2SO4

Ag/AgCl

1/RHE

700

1450

AM 1.5G

[63]

Arrays hybrids

AuNPs ZnO nanowires hybrids Ag TiO2 nanocomposite

Pt

0.1 M KNO3

Non

Non

0.005

0.015

Visible light

[149]

Au TiO2 nanocomposite

Pt

0.05 M NaOH

Non

Non

120

280

UV

[150]

AuNPs TiO2 nanocomposite

Pt

0.05 M NaOH

SCE

0.75

0.04

0.15

UV

[151]

Au NPs TiO2 nanowires

Pt

1M NaOH

Ag/AgCl

0

820

1490

AM 1.5G

[64]

AuNps TiO2 nanotube hybrids

Pt

1 M KOH

Ag/AgCl

1.23/RHE

3

150

AM 1.5G

[141]

0.5 M Na2SO4

Ag/AgCl

1.23/RHE

800 1000 2500

Visible light

[152]

AuNP TiO2 Pt nanorod hybrids

photocurrent density of these oxide photoanodes in visible light, a new approach was developed by hybridization with noble metal nanoparticles. It is also a foremost mission in the cast of semiconductor photoanodes to regulate the morphology and semiconducting nature to permit sufficient light absorption and charge

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separation. This progress of developing highly active photoanode materials will definitely continue, and confident band gap design will soon be a certainty.

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