Photo-catalytic or Photo-electrochemical ...

Photo-catalytic or Photo-electrochemical Decomposition of Water

National Centre for Catalysis Research Indian Institute of Technology Madras Presentation No.11 Dated 29th April 2017


Questions, suggestions and remarks these can form a habit to all of us


Photo-catalytic or Photo-electrochemical Decomposition of Water 1. One of the promising steps towards generating clean and renewable alternatives for fossil fuels. The dynamic nature of the electronic energy levels is postulated. 2. The need for knowing the positions of the energy levels of conduction band and valence band of semiconductors in situ. 3. The layer type of semiconductors and water adsorption was postulated 4. The special features of HOIP will be taken up in another presentation. 5. Question ? Layered structure – ionic nature of the bond 6. Needed key characteristics of the material considered, that is the lifetime of the photo-generated electron–hole pair, the surface area of the catalyst (that also influenced the former factor), the reproducibility and the photo-hydro stability of the employed materials. Various possibilities pointed out 4 including z scheme

• One of the most important problems facing humanity in the 21st century is building an enduring, sustainable energy economy. Although fossil fuels can supply the estimated global energy demand well into the foreseeable future, this strategy has catastrophic environmental implications due to carbon dioxide (CO2) emissions, a leading contributor to the greenhouse gas effect. • The technology of renewable energy also has economic benefits such as the reduced health and environmental restoration costs, job creation, and the intellectual property. • Even though the cost of renewable energy is dropping, it is still the major limitation for the implementation of these technologies. Continuous technological advances are needed to make renewable energy cost competitive with fossil fuels and drive the evolution of how we consume energy. Sunlight is an ideal energy source because it is, for all practical purposes, completely sustainable and delivers more energy to earth in one hour than is consumed globally per year.

• Although solar energy holds great promise, its large scale integration requires the efficient conversion of light into storable, usable forms of energy. Among them water splitting, or the direct photo-electrochemical (PEC) conversion of water into its constituent elements, H2 and O2. • Water splitting, though, will have the farthest reaching impact. The ability to generate H2 from water has vast uses in the energy sector beginning with its storage in a liquid fuel to replace gasoline. There is a need for H2 to use in fuelcells, resulting in more efficient and decentralized method of electricity production compared to combusting fossil fuel. • Annual requirement of water to supply the necessary energy in the form is H2 is only 0.01 % of annual rainfall, or 2×10–6 % of the water in the world’s oceans. Developing globally viable catalytic materials for this process is the primary focus of this presentations

Schematic illustration of research field of photo-catalysts.

In photoelectrochemical (PEC) water splitting, heterojunction electrodes consisting of two or more dissimilar semiconductors offer more advantages over those made from single semiconductors. Pairing tungsten trioxide (WO3) with bismuth vanadate (BiVO4) is a promising strategy that helps to achieve better charge separation and improved overall performance in PEC water splitting. In this work, we have demonstrated a WO3/BiVO4/TiO2 core–shell nanostructured photoanode for efficient water splitting and stability. A detailed valence band (VB) edge analysis of WO3/BiVO4 was performed with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy, and it was revealed that due to electronic equilibration between WO3 with BiVO4, Fermi energy level shifting occurs, forming a p–n junction at the interface that allows electrons from the photoexcited BiVO4 to be transferred efficiently to WO3, achieving better charge separation and higher efficiency. This electrode exhibited significantly higher photocurrent density than the individual WO3 and BiVO4 electrodes, producing an unprecedented photocurrent of 4.2 mA cm2 at 1.23 V versus a reversible hydrogen electrode under simulated sunlight without an added catalyst. A thin layer of photo-catalytically inactive amorphous TiO2 was deposited onto WO3/BiVO4 to react with surface defects, deactivating them toward surface charge-carrier recombination, and to increase the stability of photoanodes. J. Mater. Chem. A, 2017,5, 1455-1461

(a) Direct band gap plot of the UV-Vis absorption data for pristine WO3 and BiVO4 on an FTO glass substrate. (b) XPS VB edge spectra of pristine WO3 and the WO3/BiVO4 heterojunction on an FTO glass substrate. The schematic band alignment constructed by optical and VB edge analysis for (c) pristine WO3 (d) the WO3/BiVO4 heterojunction. SCR stands for the space-charge-region.

PEC cell in a (a) single and (b) double compartment configuration. Indicated are ports for the WE, CE, and for inlet circulation for gas detection. An optional port for a reference electrode (RE) is shown for 3-electrode experiments. J. Mater. Res., Vol. 25, No. 1, Jan 2010 Stirring is also optional, because it is not always desired.

(i) Benchmark efficiency (suitable for mainstream reporting) (a) solar-to-hydrogen conversion efficiency (STH) (ii) Diagnostic efficiencies (to understand material performance) (a) applied bias photon-to-current efficiency (ABPE) (b) external quantum efficiency (EQE) = incident photon-to-current efficiency (IPCE) (c) internal quantum efficiency (IQE) = absorbed photon-to-current efficiency (APCE).

Band diagram of a photoelectrochemical water-splitting cell, illustrating the various processes of photon absorption, electronhole excitation, charge transport, and interfacial reactions. Region I represents an ohmic contact. Region II is a single band gap n-type semiconductor. Region III is the aqueous electrolyte. Region IV is the counter electrode. A connection between the ohmic contact and the counter electrode completes the circuit.

Band Edge Positions of Commonly Reported Nitride Photocatalysts J. Mater. Chem. A, 2016, 4, 2801-2820

The red dotted line represents the band edge positions of InxGa1−xN with x increasing from left to right (0–1).

J. Mater. Chem. A, 2016, 4, 2801-2820

Schematic illustration of the main process steps in water splitting, including photoexcitation, carrier generation, diffusion, recombination, water oxidation, proton diffusion, and reduction reaction on the surface of nanowire photocatalysts.

Although great progress has been made to date, the fascinating properties of III-nitride on artificial photosynthesis are still yet to be fully explored. An optimum artificial photosynthesis system needs to simultaneously enhance the light harvesting, charge carrier separation and transfer, and surface redox reactions. It is expected that the effective coupling of excellent photon absorption and charge carrier transport properties of III-nitride nanowire arrays with efficient cocatalyst can boost the system efficiency. The development of advanced electrocatalyst with high turnover rates and low overpotentials for water splitting and CO2 reduction, and the engineering of interface between the cocatalyst and III-nitride semiconductor with minimum loss will maximize the overall performance, which may eventually make artificial photosynthesis devices a reality. Semiconductor and Semimetals, http://dx.doi.org/10.1016/bs.semsem.2017.02.004

PEC solar water splitting is a powerful, but complex process. For direct photoelectrochemical decomposition of water to occur efficiently and sustainably, several key criteria must be met simultaneously: the semiconductor system must generate sufficient voltage upon irradiation to split water, the bulk band gap must be small enough to absorb a significant portion of the solar spectrum, the band edge potentials at the surfaces must straddle the hydrogen and oxygen redox potentials, the system must exhibit long-term stability against corrosion in aqueous electrolytes, and finally, the charge transfer from the surface of the semiconductor to the solution must be facile to minimize energy losses due to kinetic overpotential and selective for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). To date, no cost-effective materials system satisfies all of the technical requirements listed above for practical hydrogen production. While research and development is ongoing to discover materials with bulk and interfacial characteristics that meet these criteria, advances in material science and interfacial electrochemistry are still needed. Z. Chen et al., Photoelectrochemical Water Splitting, SpringerBriefs in Energy, DOI: 10.1007/978-1-4614-8298-7_1,

SpringerBriefs in Energy, DOI: 10.1007/978-1-4614-8298-7_1

Band structure of an n-type photoanode water splitting device, (a) Illustrating the various processes of photon irradiation, electron–hole pair formation, charge transport, and interfacial reactions, (b) Illustrating the energetic requirements associated with the minimum thermodynamic energy to split water, catalytic overpotentials for the HER and OER half-reactions, and photovoltage

SpringerBriefs in Energy, DOI: 10.1007/978-1-4614-8298-7_1

Theoretical maximum solar-to-hydrogen (STH) conversion efficiency (left axis) and photocurrent (right axis) as a function of material band gap.

Two-pronged strategy of Pr3+-CTYS/Pt for visible-light-driven overall water splitting. https://doi.org/10.1016/j.apcatb.2017.04.063

Visible-light-driven overall splitting water is a potential and sustainable approach for hydrogen generation. Although many photocatalysts have been reported to be active for this reaction, the efficiency of overall splitting water is still quite low. In this work, a two-pronged strategy is adopted to overcome two key restrictions on visible-light-driven photocatalytic overall water splitting by taking advantages of visible-to-ultraviolet upconversion (UC) effect as well as inhibiting hydrogen-oxygen recombination reaction over the photocatalyst. In order to realize that purpose, a composite photocatalyst with high stability was designed by assembling three components consisting of visible-to-ultraviolet UC Pr3+-Y2SiO5, UV-responsive semiconductor photocatalyst CaTiO3 and perfluorodecalin as an oxygen transfer regent. By the first strategy, the visible-to-ultraviolet UC unit is capable of converting visible irradiation to UV light emission, which effectively excites UV-responsive photocatalyst CaTiO3. The photocatalytic activity has been raised up to 200% by regulating the amount of visible-to-ultraviolet UC Pr3+-Y2SiO5in the designed composite photocatalyst Pr3+-Y2SiO5/CaTiO3. The photocatalyst exhibited high photochemical stability and catalytic stability in four recycle reactions. By the second strategy, hydrogen and oxygen recombination on photocatalyst surface has been effectively inhibited by an oxygen transfer reagent FDC to capture and take away newly generated oxygen from catalyst surface. This two-pronged strategy is not only convenient and efficient, but exhibits potential versatility for the most stable UV-responsive semiconductor photocatalysts to realize overall split water by visible light irradiation. https://doi.org/10.1016/j.apcatb.2017.04.063

Ind. Eng. Chem. Res. 2017, 56, 4611−4626

Schematic illustration of photocatalytic H2 evolution and organics degradation over a semiconductor photocatalyst loaded with cocatalysts.

Crystal structures of TMDs with a typical formula of MX2. (a) Three-dimensional model of the MoS2 crystal structure. (b) Unit cell structures of 2H-MX2 and 1T-MX2. Acc. Chem. Res., 2015, 48 (1), pp 56–64

Nano Research, January 2015, Volume 8, Issue 1, pp 175–183

Schematic illustrating charge-transfer behavior and H2 evolution active sites for 1T-MoS2 and 2H-MoS2.

Schematic illustration of the charge transfer in TiO2/MG composites. J. Am. Chem. Soc., 2012, 134 (15), pp 6575–6578

Ind. Eng. Chem. Res. 2017, 56, 4611−4626

This short review focuses on recent developments in the synthesis of TMDs based composites for photocatalytic H2 evolution and pollutants degradation. 2D layered TMDs can be used as effective co-catalysts for the modification of kinds of materials, such as oxides, sulfides, and salts, via different methods including the in situ reduction and post-combination. Due to the superior cocatalytic properties, 2D TMDs nanosheets are postulated to have great potential to replace conventional noble-metal co-catalysts.

Schematic illustration of research field of photo-catalysts.

Charge Separation Mechanism in Natural Photosynthesis

Nature Photonics 6, 511–518 (2012)

Nature Photonics 6, 511–518 (2012)

Forward and backward reactions in a Z-scheme system with shuttle redox mediators

J. Phys. Chem. B, 2005, 109 (33), pp 16052–16061

Adv. Funct. Mater. 2015, 25, 998.

Schematic illustrations of (a) direct Z-scheme system and (b) type-II heterojunction

a) PL decay traces of Ti0.91O2 hollow spheres (blue), CdS hollow spheres (green) and Ti0.91O2/CdS hollow spheres (red). The inset is the PL emission spectra of Ti0.91O2 hollow spheres (blue) and Ti0.91O2/CdS hollow spheres (red). b) Schematic illustration of a traditional TiO2-CdS system (route 1) and an artificial Zscheme system (route 2).. Chem. Commun. 2015, 51, 13354.

Electrochemical generation of sulfur vacancies in the basal plane of MoS2 for hydrogen evolution

Nature Communications 8, Article Number: 15113 (2017) doi:10.1038/ncomms15113

Electrochemical generation of sulfur vacancies in the basal plane of MoS2 for hydrogen evolution

Comparison of potentials needed to reach a TOF of 2 H2 atoms per second

(i) Benchmark efficiency (suitable for mainstream reporting) (a) solar-to-hydrogen conversion efficiency (STH) (ii) Diagnostic efficiencies (to understand material performance) (a) applied bias photon-to-current efficiency (ABPE) (b) external quantum efficiency (EQE) = incident photon-to-current efficiency (IPCE) (c) internal quantum efficiency (IQE) = absorbed photon-to-current efficiency (APCE).

Please describe how we can calculate the rate of H2 production. The primary concern is the method of evaluation of photocatalytic performance. If the charge separation efficiency is the parameters to be compared, the rate must not be reported per gram of photo-catalyst. The photocatalytic rate should be compared under the conditions where the incident photons are sufficiently absorbed by the reactor (with sufficient amount of photo-catalyst).

Perovskite Semiconductors for Photoelectrochemical Water Splitting Applications Perovskite materials are adequate electrochemical catalysts using external applied bias. Recently, the design rules that connect chemical composition and electronic properties have been studied for lead halide perovskites. These design rules seem to be applicable for more robust metal oxide perovskites enabling their use in the photoelectrochemical configuration. For more reactive perovskite materials alternative routes to provide photoelectrochemical applications are discussed. These include the use of protective layers, relatively unexplored for perovskites, or the combination of a photovoltaic device and an electrolyser. The lead based perovskites delivers the maximum efficiency today

DOI: 10.1016/j.coelec.2017.04.003

To appear in: Current Opinion in Electrochemistry

Representative device configurations to convert solar energy into a fuel. a) Photoelectrochemical configuration. b) b) Semiconductor or photovoltaic device with a buried junction. c) Photovoltaic device coupled with an electrolyzer.

10.1016/j.coelec.2017.04.005


J. Am. Chem. Soc., 2015, 137 (2), pp 604–607

Perovskite oxide semiconductors have been used as efficient electro-catalysts for electrochemical reactions using an external applied bias. The review describes the different alternatives to use perovskite materials as photo-electro-catalysts for the conversion of solar energy into chemical bonds. The main requirements for a photo-electrochemical configuration are considered. Recent development in lead halide perovskite for photovoltaic applications has improved the understanding on the design rules that link chemical compositions with electrical and optical properties. It is shown that some recent literature examples using metal oxide perovskites support the validity of the design rules. The different alternatives in device configurations are considered DOI: 10.1016/j.coelec.2017.04.003


Angew. Chem. Int. Ed., 54 (2015), pp. 2955–2959

Reaction mechanism for water splitting on a surfacecoated photocatalyst

J. Am. Chem. Soc., 2016, 138 (7), pp 2082–2085

Science 09 Dec 2011: Vol. 334, Issue 6061, pp. 1383-1385 Exemplary OER currents of La1–xCaxCoO3 and LaCoO3 thin films on GCE in O2saturated 0.1 M KOH at 10 mV s−1 scan rate at 1600 rpm, capacitance-corrected by taking an average of the positive and negative scans. The contributions from AB and binder (Nafion) in the thin film and GCE are shown for comparison. The inset shows the structure of perovskite ABO3 (where A is an alkali or a rare earth, yellow; B is a transition metal, blue; and O is oxygen, red).

Proposed band structures of Bi4NbO8Cl and BiOCl at pH = 2.0.

The next presentation will be on Wednesday 3rd May, 2017 @4pm