A Place in the Sun for Artificial Photosynthesis? - ACS Energy Letters

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A Place in the Sun for Artificial Photosynthesis? Jinzhan Su, and Lionel Vayssieres ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00059 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 20, 2016

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A Place in the Sun for Artificial Photosynthesis? Jinzhan Su and Lionel Vayssieres* International Research Center for Renewable Energy (IRCRE), School of Energy & Power Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China. ABSTRACT: Artificial photosynthesis enables the possibility of generating clean, sustainable and large scale energy resources through water splitting and renewable chemical fuel syntheses. With several decades of research, the scientific community still encounters great academic and technological challenges to efficiently generate hydrogen from water with sunlight at large scale and low cost. Recent developments based on novel system designs have led to significant advances in the fundamental understanding of light-induced charge dynamics and related interfacial chemical reactions and efficiencies. This article provides a thorough overview of the very recent progress in new materials and photoelectrochemical system designs, surface engineering strategies and efficient new co-catalysts for hydrogen and oxygen evolution reactions (HER and OER) as well as advanced spectroscopic studies related to photogenerated charge dynamics and interfacial electronic structure. A focus is given on newly developed strategies to improve the rate-determining steps, as well as emerging approaches for surface modifications, surface reaction dynamics and optimal device structure design from a charge transfer viewpoint. Finally, a promising new system is being proposed and described derived from the overall understanding, mechanisms, and concepts reported in the literature for efficient and stable water splitting systems.

tries and continents, via naturally occurring atmospheric and oceanographic current patterns. The origin of this strong imbalance between human activities and the environment can be found in the endless growth economic system in place in major countries worldwide. Indeed, this model inherently requires the use of endless cheap energy to be sustained, hence the massive use of coal and fossil fuels as energy sources for a more profitable energy return on energy invested which might be good for the economy but clearly isn’t for our environment, health and sustainable future. Energy conservation has rightfully been pushed forward upon people but turned out not to be as effective as originally thought given the fact that when fuel/energy demand decreases so does its price which in turn leads to more consumption (of cheaper energy). Technology, through innovations has always helped boosting the economy and did it numerous times throughout the civilizations even so recently with solid state lighting technology for instance. However, technological innovations enabling the large scale implementation of a renewable, sustainable and environmentalfriendly substitute energy source into our societies is clearly in itself an entire different problem and daunting task. Moreover, the transition period time must be contained, ideally within a decade, to avoid devastating environmental pollution and health impacts as well as energy shortages, resources depletion and world economic crisis. The technological advancements must also involve large scale, clean, and cost-effective fabrication techniques at moderate to low temperature and be based on highly efficient materials that mostly contain earth abundant elements, easily extractable and fully recyclable rather than expensive, scarce and toxic metals and rare earths. In addition, given that conventional technologies which at-

In our opinion, this decade is witnessing the widest consequences of man-made activities on our planet. The environmental risk for public health is now at a record high and does represent an even greater challenge than global warming and climate change. Indeed, staggering air and water pollution worldwide –now chronic in certain major cities in Asia and in the Western world- due to toxic gases, chemicals, and ultrafine particles from industry, agriculture and transportation sectors has become one of the most, if not The most, important problem humanity is facing. Consequently, it is now crucial to transition to new societies where environmental, energy, and economic policies are no longer based on endless-growth financial models and fossil-fuel technologies to substantially decrease our ecological footprint and environmental and health impact. As a matter of fact, everyone is at risk of inflammatory reactions and lung infections to cancer and death. It affects millions every year and not only in emerging countries but in rich nations as well as air and water pollution knows no borders, easily crossing coun-

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tempt to improve the efficiency and performance of existing materials and devices by further development along the same incremental approaches are reaching their limits, it is also crucial to develop novel (multi)functional materials where bulk limitations are overcome by changing the fundamental underlying physics and chemistry, by e.g. nanoscale design and quantum confinement effects. Solar energy conversion to chemical fuels Solar energy is indeed the largest exploitable renewable energy resource, providing more energy to our blue planet per hour than the total energy consumed by human activities in one year. Direct conversion of solar energy into chemical fuels does represent an optimal approach to address the globally growing energy demand in a sustainable way. Photosynthesis is a massive production activity by nature when sunshine sheds on earth. The product is food for life. As those products being converted, stored and transformed with time, fossil fuel was generated and contained beneath earth’s crust. Natural photosynthesis, which is being conducted by cyanobacteria, algae, and plants, uses the solar energy to reduce CO2 to carbohydrates. Cyanobacteria and plants have evolved into highly organized photosynthetic systems. In this system1 (figure 1a), solar-to-chemical energy conversion was conducted with interconnected light-harvesting systems, highly efficient charge separation functions, and the reaction centers Photosystem I and II (PSI and PSII), followed by a dark reaction of the electron-transport chain of the cytochrome with coenzyme NADPH as a redox carrier and the Calvin cycle for CO2 reduction. In PSII, a central pair of chlorophylls, P680, is excited with electron for CO2 reduction and then its oxidized form, P680⋅+ to be recovered by the electron from a Mn4Ca-oxo cluster, which carries out the biological oxidation of water. Natural photosynthesis is conducted inside the body of organisms, with the help of numerous proteins, enzymes and support of living environment. This makes the natural photosynthetic machinery a complex system relevant to several different disciplines, such as biochemistry, biophysics, molecular and structural biology, quantum mechanics, as well as physiology and ecology. Artificial Photosynthesis, which aims to emulate natural processes using man-made devices, is the best way to convert and store the solar energy using chemical fuels as feedstock2. For an artificial system (figure 1b), a more practical approach is a direct and efficient way to convert abundant materials in nature like (sea)water and CO2 to useful chemicals, e.g., H2, CO, and hydrocarbons. Compared to photovoltaic solar energy conversion, artificial photosynthesis converts sunlight into chemical products. While different from electricity energy storage, chemical fuel storage is more viable for massive energy storage, a requirement for future alternative renewable and sustainable energy sources. The challenges to achieve efficient artificial photosynthesis are now the focus of many worldwide ongoing scientific efforts and will have to be solved in the near future by concomitant multidisciplinary efforts from scientists from all basic sciences as well as engineers from various fields.

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Over the last few years, significant academic progresses have been achieved in many aspects including new materials, more sophisticated structures, systems, and devices, better fundamental understanding of interfacial electronic structure, reaction processes and physical and chemical phenomena as well as new approaches to enhance each of these aspects.3 Several reviews on novel materials, structure design, and kinetics have been reported.3-7 This perspective highlights recent progress and focus on promising materials and future opportunities in the development of efficient and robust devices as well as new strategies to improve the rate-determining steps, as well as emerging studies on surface modifications, surface reaction dynamics and optimal design for charge transfer and surface catalytic reactions. Artificial photosynthesis with Carbon compounds production Carbon compound-based fuel is considered as a high density fuel which meets requirements for energy carrier in stationary power generation and transportation. It is also regarded as a possible alternative for petrochemicals as fuel feedstock and carbon source for renewable polymers, plastics and solvent. Direct CO2 reduction is a promising way for solar energy conversion and storage in the form of chemical fuels. It varied with different reaction pathways/products.8 For these reaction pathways, less electrons involved in the reaction required a more negative reduction potential. The one-electron reduction of CO2 to the CO2 •− radical anion (E = −1.9 V vs SHE) is a fundamentally challenging process as it requires a high activation energy. Synchronous proton-coupled twoelectron reduction of CO2 to CO, formate or methanol is less restricted in energy-cost as it uses H2O as both the electron donor and the proton source. Pioneering work on photocatalytic CO2 reduction was reported by Halmann9 in 1978, who bubbled CO2 over a ptype GaP cathode illuminated with a Hg lamp yielding formic acid, formaldehyde, and methanol. Most of the commonly reported photosynthetic systems are mimicking the natural photosynthetic process yet incorporating only the most essential steps, i.e. CO2 reduction and H2O oxidation. Significant developments in a variety of photocatalytic systems for photochemical CO2 reduction have been achieved and reviewed.10 The reported systems are hybrids of semiconductors, molecular metal-complex catalysts or coupled with co-catalysts and plasmonic photocatalysts working under sunlight irradiation. To achieve efficient CO2 reduction, various new materials are being tested such as g-C3N4 which appeared to be a multifunctional robust photocatalyst. A heterogeneous photocatalyst system that consists of a ruthenium complex and carbon nitride (C3N4) was also developed for the reduction of CO2 into formic acid, reaching a turnover number higher than 1000 and an apparent quantum yield of 5.7 % at 400 nm.11 Carbon nitride was also coupled with metal oxides as a nature-inspired organic semiconductorbased system.12 Popular systems used nowadays for CO2 reduction consist of a metal complex photocatalyst such


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ACS Energy Letters

as ruthenium complexes,13 or Mn(I) molecular catalyst14, yet a sacrificial reductant is always required as electron donor. Direct reduction of CO2 to CO using unstable ptype semiconductors as photocathodes are still under scrutiny since new surface protections are being adopted and shown successful such as for instance TiO2-passivated III-V compounds15 and Cu2O electrodes.16 Metal dotsdecorated semiconductors were also used as catalysts17 as noble metals were found to have an enhancing effect over CO2 conversion to hydrocarbons. Heterostructured compounds such as AlGaN/GaN give an efficiency of 0.13% comparable to that of natural photosynthesis in plants.18 Beside semiconductor catalysis, extensive efforts are currently being invested to re-engineer (genetically) photosynthetic organisms (e.g., green algae and cyanobacteria) for more efficient photobiological biomass and H2 conversion. An autonomous biophotovoltaic cell, which coupled two independent processes of light capturing by PS2 and PS1 in a Z-scheme structure, is able to produce both electrical and chemical energy.19 New electrocatalysts are also being developed to reduce the activating energy of CO2 reduction. Partially oxidized atomic cobalt layers show overpotential of only 240mV and high intrinsic activity towards formate production.20 As the organic chemical production system include both CO2 reduction and H2O oxidation with quite different reaction conditions efficient production of organic chemicals remains a very difficult challenge. Artificial organic chemical photosynthesis is still not feasible due to inefficient CO2 reduction process.

best feedstock for a synthetic, renewable and clean fuel? The best non-toxic, most abundant and geographically balanced and freely available resource on earth is (sea)water, hence hydrogen generation from water using photosynthesis appears as the ideal scenario. The efficient capture of solar energy to power the electrolysis of water, generating hydrogen (H2) as a fuel directly (e.g. in combustion engine or in fuel cell vehicles) is the key to a clean and sustainable future. Although numerous new strategies are reported, the fundamental obstacle for efficient solar water splitting remains unchanged. The major obstacle in pursuing an efficient and stable solar-to-hydrogen conversion device is the development of robust and long-lasting wateroxidation catalysts. The discovery of photoelectrochemical effect of TiO2 by Fujishima and Honda21 in 1972 led to decades of sustained effort to develop stable and efficient photoelectrochemical (PEC) cell and photocatalyst for solar water splitting. Various oxides, sulfides and nitrides semiconductor have been explored.

Figure 1 Conversion of solar energy to chemical fuels by natural and artificial photosynthesis approaches. (a) Natural photosynthesis is conducted by complex systems within photosystem I and II in different organisms. (b) An artificial photosynthesis system is a simplified version with dedicated components for efficient light absorption, charge transportation and surface redox reaction.

Artificial photosynthesis with water splitting approach Hydrogen production is a more favorable and competitive process than CO2 reduction as the overall CO2 reduction yields for most systems are limited. Considering large scale application using sunlight to convert atmospheric CO2 into organic fuels, CO2 feedstock is an issue comparing to hydrogen production as CO2 concentration is relatively low in the atmosphere. Thus, what represents the

Figure 2 Schematic elementary steps from light energy to chemical bond energy: a) Light absorption: the oscillating electric field of the incoming light induces an oscillating dipole within the chromophore associated with the photosyn-


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thetic system, leading an electron transition to a higher elec22 tronic energy state. b) Charge carrier transportation/separation: photoexcited electrons and holes are separated by the space charge regions established at the interfaces, the energy being carried by the electrons and holes, c) Surface reaction: The electrons and holes reduce and oxidize the water to produce hydrogen and oxygen respectively with energy stored in the chemical bonds.

As depicted in figure 2, converting light energy to chemical bond energy requires three main steps which are light absorption, charge carrier transportation/separation and surface reaction. All three steps should be efficient enough to achieve substantial overall conversion efficiencies. However to date, there is no single material that meets simultaneously all the requirements of band gap, band edge position, electronic properties and long term stability for efficient water splitting. Back in 2001, a III-V semiconductor junction device consisting of AlGaAs/Si and capable of splitting water at a record efficiency of 18.3% was reported.23 Unfortunately, stability problem in aqueous solutions prevented its application for practical solar water splitting. With the advent of nanoscience and nanotechnology, material structures and functions can now be engineered and precisely tuned to achieve enhanced electronic and surface properties as well as overcoming bulk limitations. Nanostructural design Researchers turned back to earth-abundant oxides such as TiO2, WO3, and αFe2O3. This return is promoted by the emergence of low cost nanostructure synthetic procedures such as hydrothermal and template-assisted methods24. These fabrication techniques are simple and easily accessible to researchers in various fields and draw intense attention onto novel nanostructure material development for artificial photosynthesis. In this context, earth-abundant transition metal oxides became model photoelectrodes intensively investigated for their photoelectrochemical water-splitting properties.25 For most of them, charge recombination still remains a major barrier for efficient photochemical conversion. The competition between charge transfer (separation) and recombination determines the internal quantum efficiency of the water splitting reaction. For materials with poor charge mobility, nanostructuring is a successful strategy to overcome the trade-off between light absorption and charge transport to the solid/electrolyte interface for water oxidation, as depicted in figure 3a. The natural photosystems have evolved to unique scaffolds that efficiently handle the complex photosynthesis processes such as light harvesting and charge collection. These biological structures have been investigated as versatile template for the assembly of photocatalyst.26 A tree-like structure with “stems” and “branches” was constructed using conductive Al-doped ZnO,27 which is used as conductive scaffolds for efficient electron collection. Heterojunctions

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Along with nanostructuring, efforts have also been devoted to the search of charge-separation enhancing structure such as heterojunctions, as shown in figure 3b. In terms of band alignment, three types of semiconductor heterojunctions can be defined: straddling gap (type I), staggered gap (type II) and broken gap (type III) of which type II is the ideal band structure for efficient charge separation. Axial heterostructure nanowire can be obtained by conformal deposition of shell on the nanowire surface, forming a core-shell structure. Since the report of nanostructured WO3/BiVO4 heterojunctions for photoelectrochemical application,28 this system has become a popular heterostructure. An helical WO3/BiVO4 core-shell heterojunction has been reported reaching a photocurrent density of ~5.35±0.15 mA/cm2 at 1.23 V vs RHE.29 While for some heterojunctions with straddling gap (type I), such as ZnO/Fe2O330 enhanced charge separation was also observed. The energy difference between conduction band edge and Fermi level can be achieved by adjusting the doping level. Hence a preferred energy match between the two materials can be achieved with a built-in voltage established by electron flowing from one side to the other until the Fermi levels on both sides equalizes. Tree-like hierarchical structures combining the advantages of enhanced light trapping, larger surface area, and better photogenerated electron–hole separation31 do indeed exhibit superior photoelectrochemical activity. Another efficient way to reduce recombinations consists in depositing an ultrathin layer onto a nanostructure, which not only increases the photon absorption but also decreases the distance for charge carriers to travel before reaching the surface. For example, photocurrents as high as ~3 mA/cm2 at 1.23 V vs. RHE was achieved on an ultrathin hematite film deposited on a 3-D nanophotonic structure.32 Advances over the past few years have demonstrated remarkable improvements in efficiency through nanostructuring and heterojunction design. As a result, the typical photocurrent density for these materials increased29, 33 to values of 3-5 mA/cm2. However, nanostructured engineered photoelectrodes seem to have reached their limitations in terms of activity improvement within their absorption range of wavelength. Exploiting traditional and new narrow bandgap semiconductors Promising materials with unique properties which could possibly make a breakthrough are under intense scrutiny. For instance, metal-free nitride such as C3N4 have attracted great attention due to its narrow bandgap (2.7 eV) and a conduction band position sufficiently negative to drive proton reduction to hydrogen while showing relatively good stability34. It was investigated by doping (S, F) or incorporated with other semiconductors, even graphene35 showing high performance for overall water splitting. Oxynitrides and nitrides composites are emerging as popular materials given their narrow band gaps and appropriate band levels for both water oxidation and reduction. For instance, GaN nanowire with an internal quantum efficiency of ~51% has been reported.36 Tantalum oxynitride (TaON) and nitride (Ta3N5) were also re-


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ported as good overall water splitting materials with nearideal bandgaps and band edge positions. External quantum efficiency of up to 45% at 400nm was reported.37 Typical metal oxynitrides with d10 electronic configuration emerged again as they are capable of driving overall water splitting under light with wavelengths below 480 nm. A more complex “perovskite-type” oxynitride, LaMgxTa138 was developed yielding overall water xO1+3xN2-3x (x≥1/3), splitting at wavelengths of up to 600 nm. Silicon and III-V semiconductors which have optimum bandgap, long carrier lifetimes, high charge carrier mobility and proper band edge positions for unbiased water splitting are getting substantial interest as new strategies are being developed to protect them against (photo)corrosion in aqueous solutions. This approach appears somehow more promising compared to those aiming at overcoming improper intrinsic electronic properties of known stable materials. Surface passivation A passivation layer is applied to improve the (photo)chemical stability of semiconductor when operating in an electrolyte.39 The most effective material, so far, for passivation is TiO2, which has been tested on Si, GaAs, and InP.40 Other materials such as TiO2 or ultrathin carbon sheath were deposited on Cu2O-based heterojunctions as a surface protection layer against photocorrosion to form buried semiconductor junctions for improved performance in photocurrent density and onset potential.41 Dual-Layer Thin TiO2/Ni coatings was also utilized to stabilize polycrystalline BiVO4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte.42 SrTiO3 can also act as a transparent protection layer for silicon photocathodes, with lattice matched, preventing corrosion without compromising photocatalytic redox activity.43 Other narrow-bandgap semiconductors, such as sulfides and nitrides, also face stability issues. The same strategy was used to improve their stability using a protection layer consisting of amorphous TiO2 or noble metals.44,45 Since passivation layers can readily be deposited onto high surface area and high aspect ratio nanostructures by diverse fabrication techniques, (e.g. atomic layer deposition (ALD), spin coating, electrochemical deposition, sputtering, electron beam evaporation or dip casting),39 they are being adopted as a simple yet very efficient way to facilitate the use of well-known aqueousunstable semiconductors. Table 1 summarizes the most common ones along with their performances and stability specifications.

Figure 3 Common new strategies to improve overall efficiency: a) Smart nanostructures: ordered hierarchical structures improve the surface area while keeping grain boundaries at a minimum; b) Heterojunctions: promote the electron hole separation by driving them toward opposite directions due to inherent built-in electric field; c) Surface protection: a passivation layer is conformally deposited to prevent corrosion and eliminate surface state recombination centers; d) Surface modification: co-catalyst deposition and surface engineering facilitate the water reduction and oxidation reactions.

Surface Engineering In addition to protecting the semiconductor from corrosion, surface passivation layers46 are also used to remove the surface states that facilitate surface electron– hole recombination, which is of key importance for the performance of the semiconductors, as depicted in figure 3c. For instance, improved hematite-based photochemical water splitting has been reported through passivation of surface states using very thin layers, such as FexSn1−xO4, and ALD Al2O3, and TiO2.47 On the other hand, surface states may have positive effects on the charge transport and surface reactions. For example, the so-called “black TiO2” has attracted enormous attention due to its hydrogenation-induced surface disorder48 and unusual photocatalytic activity. It was found that hydrogen doping eliminates the recombination centers at the surface and inhibits the recombination of light-induced electrons and holes.49 Using electrochemical impedance spectroscopy (EIS), Bisquert and co-workers50 suggested that water oxidation took place predominantly from surface trapped holes, not directly from valence band holes with help of surface trap-mediated charge transfer process at the hematite/electrolyte interface. Surface modification enables the adjustment of the energetic position of the band edges, demonstrating that it may be possible to engineer the energetics at the semiconductor/electrolyte interface allowing unbiased water splitting with a single photoelectrode with a bandgap of less than 2 eV. Indeed, improving the band edge energetics and the photocurrent onset of GaInP2 by modifying the surface energetics via surface modification by self-assembling with phosphonic acid58


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was reported. Titanium dioxide (TiO2) coating on p-type gallium-indium phosphide was also found effective at suppressing the reverse process of unproductive recombination by an enhanced charge separation of the excited electrons from the holes. 59 Interfaces other than surface between active layer and electrolyte were also engineered for enhanced charge separation and transportation. For instance, underlayers working as barrier layers for electron migration from the transparent conducting substrate (typically, fluorinedoped tin oxide, FTO) to the active layer was used to prevent the recombination of electrons in FTO with photoexcited holes in the valence band of the active layer, leading to better charge separation.60 Interface engineering enhancing the contacts between semiconductor and FTO substrate was achieved by synergistically employing flame reduction and TiCl3 solution reduction.61 It was also reported that sub-10 nm rutile TiO2 nanoparticles with the presence of Ti(III),62 with surface/sub-surface defects not only shifted the top of the valence band upwards for band-gap narrowing but also promoted better chargecarrier separation reaching state-of-the-art activity for visible-light-driven water splitting. Interface engineering was also used to create homojunctions in Cd0.5Zn0.5S nanorod by twin planes parallel to each other, in which zinc-blende (ZB) and wurtzite (WZ) segments alternatively occured along the direction. These type-II staggered band alignment between WZ and ZB segments resulted in an intense concentration of homojunctions which are capable of promoting charge separation.63 As a result, the quantum efficiency for the photocatalytic hydrogen production reached a record 62%. Table 1. Performance of semiconductor electrodes with a passivation layer for PEC water-splitting Semiconductor

Passivation layer Material

Condition

Performance

Stability

Ref

9.4% efficient Si PV

~10% decreased after 100h

40

15.6% @ −0.32 VRHE

2 h @ 27 mA/cm

Thickness (nm)

Si, GaP, GaAs

TiO2

4 –143

1 M KOH, 1.25 Sun

Si

TiO2

15

0.5M H2SO4 100 mW/cm

Si

TiO2

100

1 M HClO4, AM 1.5G, λ> 635 nm

Si

NiO

50

1 M KOH, 38.6 mW/cm AM 1.5G, λ > 635 nm

>10%

300h @ 14.5 mA/cm

InP

TiO2

3

CO2 saturated 0.5 M KCl

Faraday eff. 83.6%

12h

54

CH3NH3PbI3

Au/Ni

80/8

AM 1.5G, 100 mW/cm

12%

14% current remaining for 2 16min @ 2mA/cm

55

rutile TiO2

TiO2

2

1 M KOH, AM 1.5G, 100 mW/cm

50% @ 370nm

3h @ 1.3mA/cm

BiVO4

TiO2

1

0.1 M KOH pH=13, 135 mW/cm

Faraday eff. 100%

3h @ 1.4mA/cm

Cu2O/TaON

C

3

0.5 M NaOH pH=13.6

59% @ 400nm

1h @ 2.7mA/cm

10.5%

15% decrease after 80h

TiO2

62.5

1.0 M KOH AM 1.5G

2

2

2

51

2

52

60h @ 22mA/cm

2

AM 1.5G 100 mW/cm GaAs/InGaP

2

2

2

53

2

56

2

42

2

41

2


57

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Table 2 Summary of the performance of OER and HER catalysts for solar water splitting. Electrode

Electrolyte

Performance (bias vs RHE)

Stability

Ref.

2

∼15% decay in 600s

64

2

No significant decay in 48 h

65

OER Co3O4/ BiVO4

0.5 M buffer solution, pH= 7

2.71 mA/cm , 1.23 V

FeOOH/NiOOH/ BiVO4

0.5 M phosphate buffer, pH=7

2.73 mA/cm ; 0.6 V 2

CoOx/ NiO/ BiVO4

0.1 M KPi solution, pH =7

2.5 mA/cm , 0.6 V

15% decay in 16h

66

Pt/MnOx/BiVO4

0.02 M NaIO3

650 µmol /h/g

-

67

Co–Pi /a-Fe2O3

1 M NaOH (pH=13.6)

2.8 mA/cm , 1.23 V

20% decay in 75 h

68

Ta2O5/Rh2O3/SrTiO3:Sc

H2O (250 mL)

H2:4030 μmol/h/g,

No significant decay in 10h

69

No significant decay in 2h

70

6% decay in 6 h

71

2

no significant decay for 170s

72

2


73

2


74

-

75

76

2

O2:2030 μmol/h/g 2

Co-Pi/W:BiVO4

0.1 M KPi buffer (pH= 8)

0.7 mA/cm , 0.6 V

Co3O4/Rh/Ta3N5

1M NaOH, pH=13.6

5 mA/cm , 1.23 V

Ni(OH)2/Fe2O3

1M KOH, pH =14

0.4 mA /cm , 1.23 V

Pi-Fe2O3

0.1 M(K-Pi) buffer, pH =7

1.26mA /cm , 1.23 V

Co-Pi/Pt:Fe2O3

1M KOH

4.32 mA/cm , 1.23 V

Ni(OH)2/IrO2/Ti-Fe2O3

1 M NaOH, pH=13.6

1.7 mA/cm , 1.23 V

2

2

HER MoS2/ZnIn2S4

0.43 M Na2S and 0.5 M Na2SO3

153 µmol / h/0.05g


NiS/g-C3N4–CdS

10 mL triethanolamine; 90 mL distilled water

2563 µmol /g / h

20% decay in the 2

WS2/mpg-CN

lactic acid (10 vol.%)

12 µmol/0.05g/ h

70% decay in the 2

MoS2/Cu2O

25% (v/v) methanol mixed with 0.1M Na2SO4, pH = 7

0.175mA/cm , −0.1 V vs. SCE

7% decay after 9h

79

MoS2/Graphene -CdS

300 mL of lactic acid aqueous solution (20%)

1.8 mmol/0.2g/h

no significant loss of activity for 5 recycles

80

rGO/TiO2

20% v/v methanol aqueous solution

13996 μmol g h

-

81

Ni/RGO

2.0 × 10 M Eosin Y (EY); 7.7 × −2 10 M trimethylamine (TMA), pH=10

94.3 μmol/0.01g/h

70% decay in the 2

−4

Surface reactions One of the major obstacles for achieving efficient and stable overall water splitting is directly related to the uncontrolled surface charge properties. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are obviously crucial for the development of efficient solar water-splitting devices.83 So further improvement must include reducing the bias required to reach high current densities, that is to substantially lower the onset potentials while enhancing the kinetics (Tafel slope). Table 2 summarizes the performance of recently reported OER and HER catalysts for solar water splitting. OER is the process of generating molecular oxygen through electrochemical oxidation of water. This reaction

2

−1

−1

nd

recycle

77

nd

recycle

78

nd

recycle

82

proceeds through multistep proton-coupled electron transfers, which are kinetically sluggish. Furthermore, in most metal oxides, the localized oxygen p nature of the valence band contribute to a low bandwidth (flat band) which leads to large effective masses and low mobility of holes84 and thus, inefficient interfacial hole transfer. A co-catalyst (or electrocatalyst) is usually needed to reduce the activation energies for the rate-limiting step in water photo-oxidation, as depicted in figure 3d. Without such catalysts, photogenerated holes reaching the surface may not accept electrons from water, but may instead recombine with electrons in the conduction band of the semiconductor. Traditional effective OER catalysts are typically IrO2 and RuO2 but serious limitations would


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occur for their large scale implementation due to their limited availability in nature and very high cost.

hole traps that trigger the oxidation of neighboring water molecules.99

Earth abundant co-catalysts have received great attention as they represent the only viable solution to replace scarce and expensive noble metals.85 For instance, Cobased thin films formed by oxidative electrodeposition from Co(II) solutions in the presence of inorganic phosphate, borate, and other buffered electrolytes showing enhanced catalytic reaction have been proposed by Nocera and co-workers.86-87 Indeed, Co-Pi co-catalysts were found to facilitate the water oxidation reaction by suppressing surface recombination on materials such as Fe2O3,88 BiVO4,70 Si,89 TaN,90 FeOOH, and (Ni, Fe)OH.9192 Ni-Fe layered double hydroxides (LDH) were also used as efficient co-catalyst for water oxidation on Ta3N5,93 which not only improved its efficiency but also dramatically reduced photocorrosion (maintaining 80% of the photocurrent for 2h), and reaching 6.3 mA at 1.23V vs RHE. Nanoporous BiVO4 photoanode modified with FeOOH/NiOOH achieved a photocurrent density of 2.73 mA/cm2 at 0.6 V vs. RHE.65 CoFeOx is another promising OER electrocatalyst working in alkaline solution with a significant stability94 and can also be used for HER as a bifunctional catalyst.

XAS measurements performed at synchrotron facilities in the soft x-ray range (below 1-2 keV) is also an efficient way to investigate the electronic structure of doped impurity incorporation in a host material. Ti doping is a known correlate to efficiency enhancement of hematite photoanodes due to increased carrier concentrations and carrier diffusion lengths of both electrons and holes. XAS spectra at the O K-edge revealed that Ti incorporation creates new oxygen 2p-hybridized states, overlapping with and distorting the unoccupied Fe 3d–O 2p band of αFe2O3. Ti L2,3-edge XAS spectra also showed that titanium is indeed incorporated as Ti(IV), regardless of the fabrication techniques and so, with no long-range titania order detected as well as without the formation of any significant concentrations of Fe(II).100

X-ray spectroscopy techniques Interestingly, x-ray absorption spectroscopy (XAS) emerged as a very efficient tool to investigate the activity origin of OERs. The active sites within (Ni,Fe)OOH can be identified by XAS at Fe-edge by using high energy resolution fluorescence detection (HERFD).92 The active species in NiFe and NiMn DHs are iron and manganese, while in NiCr DH, nickel is the active species.95 Combining the results of X-ray absorption fine structure (XAFS) measurements with the predictive power of density functional theory (DFT) suggest that in Ni-Fe DH the Fe−O bond length contraction enables optimal adsorption energies of OER intermediates (i.e. −OH, −O, and −OOH) over Fe sites, which account for the increased OER activity of FexNi1−xOOH. The synergistic effect of Ni and Fe make the FexNi1−xOOH OER catalysts energetically more favorable than pure NiOOH or FeOOH.96 In-situ X-ray absorption near-edge structure (XANES) spectroscopy showed an increase in the oxidation state of Ni from Ni3+ to Ni3.6+ together with a highly covalent Fe(IV)−O bond under OER process, which confirmed that the charge transfer between Ni and Fe through a “Ni−O−Fe” bond is responsible for the high catalytic activities of NiFe(OH)x.97 In-situ O K-edge XAFS was used to directly probe the active species in Ni−Bi electrocatalyst systems during the OER and found that the water oxidation catalysis proceeds at the domain edge of NiO6 octahedra.98 The information gained from XAFS measurements and DFT calculations indicated the presence of under coordinated Mn(III)O5 units located at the boundary of the amorphous network is essential for the catalytic activity. Under external positive bias, the Mn(III)O5 units act as

Polarization-dependent XAS also enabled the study of the orbital anisotropy of hematite nanorod-arrays compared to bulk crystals by probing O K- and Fe L2,3-edges.101 The orbitals in the lowest energy region are found to be strongly-hybridized Fe 3d (a1g) orbitals and O2- ligand 2p orbitals, oriented along the c-axis, which is parallel to the substrate surface, that is perpendicular to the direction of electron conduction and light propagation in operating electrodes for [110]-oriented αFe2O3 nanocrystals. The study also demonstrated that the Fe L3-edge line shape and aspects of polarization dependence can be reproduced by crystal field atomic multiplet calculations of 2pto-3d transitions for Fe3+ in the D3d point group symmetry of metal ions in the corundum structure. Both the O K-edge and Fe L3-edge spectra possess features that may be related to the high density of surface atoms in this nanoscale system, associated with partial coordination and therefore reduced symmetry compared to that for Fe3+ in bulk crystals. Optimizing OER/HER performance With growing numbers of co-catalysts being reported, the next logical step is to further improve their activities. In this context, porous structure engineering has been applied to selectively etching NiGa LDH nanoplate to topotactically convert them into porous metal chalcogenides, e.g. β-Ni(OH)2 and NiSe2, by selective etching.102 Piezotronic effect was also reported able to improve the performance of OER catalysts on semiconductor photoelectrodes103. Combining different co-catalysts as conjugated ones with synergetic effect to further improve their activities has also been proved successful. Indeed, NiOH catalyst is capable of efficiently capturing the photogenerated holes from host catalyst as a hole-storage layer (HSL) and feed the efficient IrO2 electrocatalyst with holes for water oxidation.75 For HER, noble metals such as Pt and Rh are excellent promoters for H2 evolution but can also catalyze the (undesirable) back reaction, which limits the solar-tohydrogen (STH) efficiency in artificial photosynthesis. To avoid such a drawback reaction, transition-metal oxides such as NiOx and RuO2 that do not exhibit activity for


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ACS Energy Letters

water formation from H2 and O2 are usually used as cocatalysts for overall water splitting. Recently, an oxidized platinum cluster104 was also reported as an efficient HER catalyst while suppressing the undesirable hydrogen back-oxidation. NiO/Ni on carbon nanotube sidewalls also acts as highly effective HER electrocatalysts with activity similar to platinum.105 New non-precious metal electrocatalysts were tested for hydrogen evolution.106 For example, MoS2 has been studied as an electrocatalyst for HER. Layered MoS2/C107 showed high-performance. Hydrogen production rate was improved by 28 times after loading Mo3S4 onto NaTaO3.108 Other new compounds such as molybdenum carbides also exhibit remarkable electrocatalytic performance for hydrogen production.109 It is also been found that metal free co-catalyst such as carbon dots are capable of suppressing back reactions. For instance, C3N4 photocatalyst does split water into hydrogen and peroxide, a second material carbon nanodots breaks the peroxide down before it can damage the generated hydrogen.110 Besides protection, graphene quantum sheets were used as a non-precious catalyst on Si111 nanowire photocathodes for solar hydrogen generation. Most noble metal-free catalysts are highly desirable for cost effectiveness but still underperform compared to Pt. An alternative strategy is to reduce the use of Pt content while achieving high exchange current density with small Tafel slope to increase its mass activity. A successful example is the ternary Pt−TiO2−N-rGO nanocomposite which exhibits a superior HER activity with a small Tafel slope.112 Charge dynamics studies Although the phenomenological advantages of material and structural engineering for improving efficiency are now widely confirmed, their atomic scale/microscopic origins are far from understood. Understanding the water splitting mechanism and electron transport/separation kinetics is essential to develop an efficient and durable solar water splitting technology in a more rational manner. A complete mechanistic understanding of the ratedetermining steps would provide insight into structurecomposition-reaction rates relationships to increase the overall kinetics of the reactions. Aiming at the significant improvement of solar water splitting/artificial photosynthesis overall efficiency, attention should be focused to better fundamental knowledge of the thermodynamics and kinetics of photoelectrodes. Several techniques have been applied to probe the dynamics of photogenerated carriers at the millisecond to second time scales. Indeed, interfacial, surface and back electron transfer recombination at these time scales have been considered to be at the origin of the transient currents observed under chopped light illumination 113 and can be identified by several transient techniques. A simple method is the transient behavior of the photocurrent or photovoltage decay. These behaviors provide insight into surface states, trap charging and recombination timescale.

Electronic intraband gap surface states promoting charge recombination at the interface can be detected by Photo-Electrochemical Impedance Spectroscopy (P-EIS) .114 Photogenerated charge recombination can be probed by Intensity Modulated Photocurrent/Photovoltage Spectroscopy (IMPS/IMVS). Electron transport lifetime can be determined by the complex plane plots of IMPS response.115 Transient photocurrent spectroscopy (TPC), such as transient absorption spectroscopy (TAS) and EIS can also be used to investigate the kinetic competition between electron-hole recombination and water oxidation.116 Using a first rate law analysis of photoinduced water oxidation on a metal oxide semiconductor surface by photoinduced absorption spectroscopy (PIA), it was found that the accumulation of multiple holes or “oxidizing equivalents” is actually required for efficient water oxidation catalysis.117 TAS is used to measure the electron or hole transfer usually in the presence of a hole or electron scavenger. TAS is essentially a laser flash spectroscopy that tracks transient changes in absorption after an excitation pulse. It has allowed researchers to understand the dynamics of charge transfer in many photocatalytic processes by tracking transient absorbance changes at specific wavelengths. For example, it has been found that TiO2 rutile exhibited up to 10-time slower recombination kinetics than anatase. However, the photocatalytic activity of rutile was still lower due to the deficiency of its holes to drive efficient and irreversible alcohol oxidation.118 Energy and spatial distribution of intra-gap trap states in TiO2 photoanodes and their effects on dynamics of electron transport can be investigated using a time-resolved charge extraction experiments (TRCE)119. Another useful time domain method is photoinduced charge extraction by linearly increasing voltage (photo-CELIV). In this method, (photo)capacitance and (photo)conductance of photoelectrodes are simultaneously measured using a linear increasing voltage. A relaxation time τrel for the photogenerated charge carriers can thus be determined by the recorded capacitance and conductance.100 This technique provides useful information on the number of extractable carriers and their dynamics and helps with the understanding and suppression of loss mechanisms. Timeresolved microwave conductivity (TRMC) measurements120 were also used to reveal the origin of the poor carrier transport properties of undoped BiVO4 due to its low carrier mobility (ca ∼4 × 10–2 cm2V–1s–1). Tungsten doping was found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps but the increased carrier density improves the overall photoresponse. Non-contact atomic force microscopy (AFM) can also be used to identify defect charge states on catalytic surfaces as well as mapping the charge transfer within individual molecules.121 Time-resolved spectroscopic techniques do offer valuable information about the kinetics of the photoreactions. However, concerted efforts are still needed to better combine these experimental studies with theoretical modeling of the chemical reactions and interfacial elec-


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tronic structure to gain a more complete understanding of the electron transfer and surface reactions involved in the process. New systems for water splitting Structural design is an important issue for the fabrication of efficient functional materials as the entire light to chemical production conversion process occurred in several distinct steps. New systems have been designed to address the rate limiting issues in these steps, as depicted in figure 4. Novel artificial photosynthesis device configuration could further improve the STH efficiency and may thus provide a highly anticipated breakthrough.

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ble redox couples such as iodate ion (IO3-)/iodide ion (I-), ferric ion (Fe3+)/ferrous ion (Fe2+) or 3+/2+ 3+/2+ [Co(bpy)3] /[Co(phen)3] are commonly used as electron mediators.122. Unfortunately, the efficiency is usually limited by the competitive oxidation of the redox couples. Recently, a solid state electron mediator was used to replace the redox couple123. In this system, a ptype metal sulfide photocatalyst is used as the H2-evolving photocatalyst and was combined with TiO2 as the O2evolving photocatalyst and reduced graphene oxide as the solid-state electron mediator to split water into H2 and O2 in stoichiometric amounts. With such a spatial isolation, the metal sulfide in this system was also protected from photocorrosion by its cathodic condition. Systems with other solid electron mediator such as Ag,124 Au125 or even without electron mediator126 were also reported. As no redox couples were used, the stability as well as the charge transfer were simultaneously improved. Natural-artificial hybrid systems

Figure 4. Schemes of the most common structural designs of artificial photosynthesis systems: a) Z-scheme, with a redox couple as electron relay to drive complete water splitting; water reduction and oxidation reactions are occurring at two separated electrodes. b) Tandem cell, with two stacked semiconductors absorbing light at different wavelength region to further improve light harvesting capabilities. c) Particle system for overall water splitting, with a particle shaped heterojunction, loaded with HER and OER co-catalysts. d) Dyesensitized photoelectrochemical cells, with water oxidation catalysts providing electron for sensitizer regeneration. e) PV-electrolyzer, with state-of-the-art photovoltaic device providing electrical power to drive direct overall water splitting.

Z-scheme Z-scheme is a system which consists of two narrowbandgap semiconductors for photo-oxidation and photoreduction and an electron mediator (figure 4a). Zschematic water splitting have been reported as a promising approach to overcome the limitation of charge carriers recombination and back reactions of products by spatial isolation of photogenerated electron and holes. Suita-

A natural-artificial hybrid system, also known as photobiocatalyst, is a system combining a semiconductor as the light harvester and an enzyme activated by the semiconductor. This system generally involves a semiconductor enzyme/bacteria electron transfer process. The most widely studied photobiocatalyst systems so far make use of conduction band electrons of excited semiconductors to promote enzymatic reductions mediated by NAD(+) /NADH and an electron relay.127 Natural PSII based photoelectrochemical cell was also reported by re-engineering of natural photosynthetic pathways by direct coupling of the water oxidation enzyme, PS II, to the H2 evolving enzyme, hydrogenase. This is achieved by integrating the isolated enzymes into the artificial circuit of a photoelectrochemical cell.128 Natural-artificial hybrid Z-scheme system was reported which integrates plant PSII and inorganic photocatalysts129 (e.g. Ru/SrTiO3:Rh), coupled with [Fe(CN)63-/Fe(CN)64-] as an inorganic electron shuttle. Very recently, a hybrid approach combining inorganic semiconductors with a non-photosynthetic bacterium was reported. Cadmium sulfide nanoparticles anchored to bacteria served as the light harvester, enabling continuously photosynthesis of acetic acid from carbon dioxide from this “self-augmented” biological system.130 Tandem Cells Another promising photoelectrochemical configuration is the tandem structure (figure 4b). This configuration consists of two stacked semiconductors to address the band-edges mismatch or a photoelectrode biased with a solar cell that absorbs longer wavelengths transmitted through the photoelectrode. Co-Pi/BiVO4:W/Si system was found to reach a STH efficiency of 4.9%.131 Another type of tandem cell consists of a separated photoanode and photocathode which utilized different part of the solar spectrum to achieve overall water splitting without external voltage.132 For instance, a tandem structure allowing sunlight to pass through the top of a NiOOH/FeOOH/Mo:BiVO4 photoanode and reach the bottom of a Pt/CdS/CuGa3Se5/(Ag,Cu)GaSe2 photocath-


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ode, showed a STH conversion efficiency of 0.67 % over 2h without degradation. III–V tandem devices such as GaAs/InGaP tandem-junction photoanode with efficient unassisted, solar-driven water splitting was also reported.57 This structure exhibited a ηSTH of 10.5% under 1 sun illumination. An in situ surface functionalization procedure was tested on a III–V photovoltaic tandem absorber, which achieved a direct solar water splitting efficiency of 14%126 breaking the 17-year old record of 12.4%133 for a monolithic water splitting system. Nanoparticle systems for overall water splitting For photocatalysis based on (nano)particle systems, a sacrificial agent is always used as a hole scavenger to protect the photocatalyst from “self-oxidation” by photogenerated holes. Overall water splitting without any sacrificial agents is of particular interest due to its simplicity and potential low cost of operation. One successful example is Co3O4134 quantum dots photocatalyst capable of splitting pure water into O2 and H2 stoichiometrically under visible-light irradiation. Cobalt(II) oxide (CoO) nanoparticles showed an overall water splitting efficiency of around 5%.135 This particle system can be also regarded as a wireless version of the photoelectrochemical cell. The co-catalyst, representing the counter electrode, is placed on the surface of the photocatalyst, representing the working electrode (figure 4c). These systems showed unexpectedly good stability in aqueous solutions. For example, GaN:ZnO modified with Rh2−yCryO3 showed stable activity for overall water splitting for longer than 3 months.136 Surface modification were also used to prevent back reaction in this compact system. A Cr2O3 shell O2 diffusion barrier was deposited to prevent water forming back reaction over Rh in a Rh/Cr2O3 incorporated GaN nanowires.137 The co-catalyst Ni metal serves as an electron trap (site for water reduction) and NiO as a hole trap (site for water oxidation) were used to improve the overall water splitting activity of SrTiO3138 under UV light. A metal-based photocatalytic system for harvesting photons for photosynthesis has also been reported. In this system, charge carriers involved in both the oxidation and reduction steps come from the excitation of surface plasmons in an Au nanorod.139 The total efficiency is low (0.1%) but the operational lifetime of the plasmonic device is relatively long and can harvest photons over a wide solar spectrum by nanostructuring design. Dye-sensitized photoelectrochemical cells A dye-sensitized photoelectrochemical cell is the extension of the concept of dye-sensitization to the field of photochemical cells (figure 4d). In this system, water oxidation catalysts (WOCs) are coupled to the dye molecules to serve as electron donors to regenerate the oxidized dye molecules. Water molecules are oxidized to oxygen and proton by the WOCs, while the protons are reduced to hydrogen at the counter electrode. For example, a dye-sensitized photoelectrochemical cells composed of a dye, a water oxidation catalyst and a mesoporous anatase TiO2 film was synthesized by attaching chromophore-catalyst assemblies onto an electrode sur-

face. 140 During the reaction, the photoexcited dye injects an electron into the conduction band of TiO2 and is subsequently reduced by the catalyst, which oxidizes water to generate oxygen and protons. The electron in TiO2 will transport to the counter electrode to reduce protons to hydrogen.141 Another dye-sensitized photoelectrochemical cell with mesoporous TiO2 photoanode sensitized with an organic dye L0 and loaded with a molecular complex Ru1 as water oxidation catalyst, while for the nanostructured NiO photocathode, an organic dye P1 as photoabsorber and a molecular complex Co1 as hydrogen generation catalyst were used.142 However, these cells show low overall solar energy conversion efficiencies as well as poor longterm stability.143 A practical dye-sensitized solar fuel production requires long-term aqueous photocathode stability. New photosensitizers were used to prevents both dye desorption and semiconductor degradation to achieve an excellent stability.144 Photovoltaic (PV) electrolysis A recent trend for affordable solar water splitting is PVelectrolysis, which is an artificial photosynthesis system combining a photovoltaic device with a standard water electrolyzer for efficient solar hydrogen production. In this system, water is decomposed by the electrical current produced between the electrodes by the connected photovoltaic cell (figure 4e). Commercial water electrolysis is typically operated at high current densities (1-10 A/cm2). Such high current densities require a higher operating voltage (due to kinetic losses). In such operating conditions, the electrolyte conductivity is an important factor. Because H3O+ and OH- are two of the most conductive ions in aqueous solutions, electrolysis processes need to be carried out in either highly acidic or highly basic conditions. State-of-the-art electrolysis systems give a remarkable electrical-to-chemical energy efficiency of 83.3%.5 Given that organic-inorganic perovskite solar cells have emerged as very promising candidates due to high efficiency, low-cost and low manufacturing energy consumption, Earth-abundant-based electrodes were connected to a perovskite solar cell, which yielded a water-splitting photocurrent density of ~10 mA/cm2 and an overall STH efficiency of 12.3%.145 However, the stability of the system remains an issue. Another example is the Perovskite−Hematite tandem cell, in which the two semiconductors act as dual bandgap absorbers with complementary absorption spectra covering a broad part of the solar spectrum thus delivering a high efficiency.146 Perovskite/BiVO4/Co-Pi was also found to exhibit a STH of 2.5% at neutral pH.147 This system is a photovoltaic device rather than photoelectrochemical (PEC) device. A PEC device with a similar structure is the "PV-biased PEC cells", whose core component is a direct semiconductor/water junction. This structure could be a more straightforward candidate and thus a less expensive approach for solar H2 production. Future ideal systems?


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With so much attention currently devoted on the discovery of novel photocatalysts and optimal architectures for efficient solar fuel applications, these studies keep reminding us that, to the great disappointment of many, there is still no artificial photosynthetic system meeting all the requirements for a large scale, stable and efficient solar to fuel conversion. Sustained research studies into material properties engineering and reaction kinetics using sophisticated characterization and smart and original approaches are needed to create the ideal compound and architecture. By in-depth concerted analysis of recent results from the literature and by combining the existing, commonly accepted knowledge, one may derive a possible ideal structure. Except for p-n junction-based solar cells, two unique solar energy conversion devices, i.e. perovskite solar cells (PSC) and dye-sensitized solar cells (DSSC) are showing good solar-to-electrical power conversion efficiency. Both of which rely on remarkable high and broad light absorbance148. Efficient light absorption is indeed a prerequisite to harvest energy proficiently. In a typical DSSC device, electrons are injected from the dye within hundreds of picoseconds149 which result in an effective spatial charge separation. This efficient electron injection along with the large interface area from the porous structure ensures a high electron collection and utilization. In PSCs, the low recombination rate of photoexcitation is accompanied by electron and hole separations into distinct regions of the CH3NH3PbI3.150 This was explained by calculations which showed that the conduction band minimum is mainly composed of Pb 6p orbitals, while the valence band maximum consists primarily of I 5p orbitals. A photoexcitation with anisotropic charge separation due to a spontaneous photoelectric fieldenhancement was also reported to not only create highly ordered Co-Pi/BiVO4 nanopyramid-arrays151, but also enhance its water oxidation activity in neutral aqueous solutions. However, due to the inherent low hole carrier mobility of BiVO4 and without strong and permanent inner polarization (which exists in perovskite structures), only average performance where obtained. Another example of the effect of photo-induced electric field has been reported152 where transient electric fields that form upon photoexcitation within various p-GaInP2 heterostructures were recorded. The results show that the interfacial field drives charge separation and thus significantly reduce charge recombination. Inspired by the aforementioned studies, an efficient light harvesting and utilization system could be constructed by building an inner field or polarization environment. It has been reported that ferroelectric layers with strong selfpolarization do significantly improve the charge separation with polarization from an ultrathin BiFeO3 ferroelectric layer between the BiVO4/FTO interface.153 Ferroelectric materials have been investigated for photovoltaic applications due to their unique ferroelectric–photovoltaic (FE-PV) effect which arises from the photogenerated carrier separation by a built-in polarization field. However most ferroelectric materials reported to date are perovskite oxides with large bandgaps and relatively poor

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transport properties, thus not very efficient. However, ferroelectric materials can be used for providing a built-in electric field rather than for current generation and transport. Additionally, a hole conductor is typically used in DSSCs and PSCs to extract holes. Commonly used hole conductors are polymeric hole conductor such as P3HT154 and spiro-OMeTAD155 or copper-based p-type semiconductors such as CuSCN156 and CuI157. Delafossite CuAlO2 has also been reported as an efficient hole conductor158 for p-type TCOs. This unusually high hole mobility arises from a large hybridization of the oxygen orbitals with 3d10 electrons in the Cu1+ closed shell, lowering the oxygen character and leading to dispersive valence band and low effective mass159. Cu-based materials make good hole conductors and are earth abundant and non-toxic. We proposed two new concept systems based on a ferroelectric material/semiconductor coupled structure for highly efficient solar energy conversion devices. As depicted in figure 5a, a chromophore layer using a narrow bandgap semiconductor with high light absorption coefficient is attached to electron and hole conductors on each side.

Figure 5. Proposed new systems with built-in polarization field (a) Sandwiched structure with electron and hole conductors as “by-pass channels” and electric inner field generated by polarized ferroelectric layers. (b) Dispersed ferroelectric polarization islands driving charge separation of photogenerated electron-hole pairs.


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Upon excitation of the electron-hole pairs in the chromophore layer, electrons and holes can be efficiently extracted from each side. This structure acts as a “by-pass channel structure”, with photogenerated charges taking two different and dedicated channels and flowing to different surfaces with minimum possibility of recombination. A local electric field can be built within the chromophore layer using a p-n junction or by self-polarized ferroelectric structure. Local built-in electric field can also be introduced to a semiconductor with a disordered manner. As illustrated in figure 5b, ferroelectric regions are dispersed within the semiconductor forming polarization islands. The polarization field drives the separation of the photogenerated charges around the islands. The separated electrons and holes are then collected by another type of “bypass channel structure”, such as graphene for instance, dispersed in the semiconductor. With this purpose-built structure, electron and hole pairs are generated in the semiconductor, separated by the built-in ferroelectric polarization field and collected through the “by-pass channel structures”. After the electrons and holes are reaching their respective surfaces, selected HER and OER co-catalysts (table 2) would drive and boost the water reduction and oxidation reactions highly efficiently, reaching breakthrough record STH efficiency. Summary and Future Outlook Despite great academic advances within the last decade, artificial photosynthesis for solar water splitting remains in its infancy and is frankly in critical need of efficiency standardization and certification (similar to the PV industry). This could easily be achieved through intensive industry-driven academic research (or academic-driven industrial research-whichever is the easiest and fastest) to unfold as a viable and striving technology. As important are new global energy and environmental policies, not necessarily in agreement with existing techno-economic analysis and highest profit/endless growth-driven financial systems, from governments worldwide to help implementing solar water splitting (along with all other renewable energy sources for that matter) as the cleanest sustainable energy technology for our societies. Moreover, substantial financial supports to scientists, engineers, and entrepreneurs should become available to actively spinoff academic research studies and develop businesses and companies for the large scale implementation of solar hydrogen generation. Furthermore, future incentive such as tax return and government subsidies for the public to massively adopt this new technology will also be highly beneficial. It is worth noted that, to date, there are still no solar hydrogen generation companies compared to many for the related photovoltaic sector. Despite these facts, we sincerely believe that with worldwide continuous, concerted and creative efforts from scientists and engineers at academic, national and industrial laboratories and research institutes as well as with new policies from governmental agencies, the development and implementation of efficient and stable water-splitting technologies for solar production of H2 at low cost and large scale for a clean and environmentally-friendly hydrogen-powered

societies should ultimately materialize. Without a doubt, using the two most abundant, free and geographicallybalanced resources available on our blue planet, that is the sun and seawater, to make unlimited amount of clean H2 along with O2 and Cl2 while preventing ocean acidification (due to NaOH generation as a side product) is The way forward. Indeed, it will provide clean and sustainable energy resource while releasing only H2O in the atmosphere as emission product from combustion and/or fuel cell use. This definitely makes perfect sense and would finally give artificial photosynthesis A Place in the Sun160.

AUTHOR INFORMATION Jinzhan Su got a BSc in Physics (2005) and a PhD in Thermal Engineering (2011) at Xi’an Jiaotong University and was a visiting scholar at Penn State University. He is now an assistant professor at the School of Energy & Power Engineering and a group leader on Nanophotocatalysts at the International Research Center for Renewable Energy. Lionel Vayssieres got a PhD in Chemistry (Université Pierre & Marie Curie, Paris, 1995) and was a researcher at Uppsala University, LBNL, UT Austin, Stellenbosch University, iThemba LABS, EPFL, and NIMS. Since 2012, he’s a full-time 1000-talent scholar professor at Xi’an Jiaotong University, co-founder/co-director of the International Research Center for Renewable Energy. http://ircre.xjtu.edu.cn/ Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT Financial support from the Thousand Talents plan and the National Natural Science Foundation of China (No.51202186, 51236007) is gratefully acknowledged.

REFERENCES (1) Demmig-Adams, B.; Stewart, J. J.; Burch, T. A.; Adams III, W. W. Insights from placing photosynthetic light harvesting into context. J. Phys. Chem. Lett. 2014, 5, 2880-2889. (2) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial photosynthesis for solar water-splitting. Nat. Photon. 2012, 6, 511-518; Nocera DG. The artificial leaf. Acc. Chem. Res. 2012, 45, 767-776. (3) Yang, X.; Liu, R.; He, Y.; Thorne, J.; Zheng, Z.; Wang, D. Enabling practical electrocatalyst-assisted photoelectron-chemical water splitting with earth abundant Materials. Nano Res. 2015, 8, 56-81. (4) Zhang, P.; Gao, L.; Song, X.; Sun, J. Micro- and nanostructures of photoelectrodes for solar-driven water splitting. Adv. Mater. 2015, 27, 562-568. (5) Peter, L. M. Photoelectrochemical water splitting. A status assessment. Electroanalysis 2015, 27, 864-871. (6) Dutta, S. K.; Mehetor, S. K.; Pradhan, N. Metal semiconductor heterostructures for photocatalytic


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It is now crucial to transition to new societies where environmental, energy, and economic policies are no longer based on endless-growth financial models and fossil-fuel technologies to substantially decrease our ecological footprint and environmental and health impact. Although numerous new strategies are reported, the fundamental obstacle for efficient solar water splitting remains unchanged. Aiming at the improvement of future solar water splitting/artificial photosynthesis efficiency, attention should be focused to a better fundamental understanding of thermodynamics and kinetics of the photoelectrodes.

Without a doubt, solar water splitting, using the two most abundant, geographically-balanced and free resources available on our blue planet, that is the Sun and seawater, to make unlimited amount of clean H2 along with O2 and Cl2 while preventing ocean acidification (due to NaOH generation as a side product) is the way forward.


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A Place in the Sun for Artificial Photosynthesis? - ACS Energy Letters

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