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Artificial Photosynthesis methods for splitting water

Introduction

Currently, hydrogen is mainly produced from fossil fuels such as natural gas by steam

reforming.

CH4+H2O CO + 3H2 (1)

CO + H2O CO2+H2 (2)

Fossil fuels are consumed in this process and CO2 is emitted. (kudo & Yugo, 2008)

Artificial photosynthesis (APS) is one alternative, with possible more controlled

technology for the process of converting solar energy to chemical energy, which aims to imitate

natural photosynthesis (NPS) using synthetics materials. There are several ways for solar

hydrogen production; most important is electrolysis of water using a solar cell, reforming of

biomass and photocatalytic or photoelectrochemical water splitting (artificial photosynthesis),

that will be focused in this paper.

However, construct an efficient APS device capable of producing molecular fuels such

as hydrogen at a scale and cost that can compete with fossil fuels it remains a big challenge and

significant advances in efficiency are required before such devices will be able to compete with

conventional energy sources. (Tachibana, Vayssieres, & Durrant, 2012)

Bases of photocatalytic water splitting

Three main reaction processes determine photosynthetic reactions: light-harvesting

processes; charge generation and separation processes; and catalytic reaction processes. The

kinetics and thermodynamics balances of these processes determine overall efficiency. The

photon energy is converted to chemical energy accompanied with a largely positive change in the

Gibbs free energy through water splitting as shown inFigure .

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Figure 1- Photosynthesis by green plants and photocatalytic water splitting as an artificial photosynthesis

(http://www.rsc.org/ej/CS/2009/b800489g/b800489g-f2.gif).

The main process in a photocatalytic reaction is divided in three steps as we can see in

Figure 1. The first step is photon absorption that consists in generation of e-and h

+with sufficient

potentials for water splitting, the second step is charge separation and migration to surface

reaction sites and suppression of recombination, the last step is construction of surface reaction

sites for H2 and O2 evolution.

Figure 1- Main processes in photocatalytic water splitting (http://www.rsc.org/ej/CS/2009/b800489g/b800489g-f4.gif)

In the first step, absorption of photons to form electron hole pairs, photocatalytic

reactions proceed on semiconductor materials. Thus, several families of semiconductor have

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been investigated, they include metal oxides (Cu2O, TiO2, Fe2O3, WO3, and BiVO4), metal

sulfides (CdS, CdZnS) and chalcopyrites (CuInS, CuGaS). Large band gap semiconductors

(more than 3 eV) such as TiO2and graphitic carbon nitride g-C3N4 can be suitable for driving.

The overall water-splitting process semiconductors have a band structure in which a band

gap with a suitable width separates the conduction band from the valence band, so when the

energy of incident light is larger than that of a band gap, electrons and holes are generated in the

conduction and valence bands, respectively. The photogenerated electrons and holes cause redox

reactions similarly to electrolysis. Water molecules are reduced by the electrons to form H2 and

are oxidized by the holes to form O2 for overall water splitting.

Water-splitting process has some restrictions such as the bottom level of the conduction

band has to be more negative than the redox potential of H+/H2 (0V vs. NHE), while the top

level of the valence band be more positive than the redox potential of O2/H2O (1.23 V),

therefore, the theoretical minimum band gap for water splitting is 1.23 eV that corresponds to

light of about 1100 nm, shown inFigure 2. The band structure is just a thermodynamic

requirement but not a sufficient condition. The band gap of a visible-light-driven photocatalyst

should be narrower than 3.0 eV (l4415 nm). ZrO2, KTaO3, SrTiO3 and TiO2 possess suitable

band structures for water splitting. These materials are active for water splitting when they are

suitably modified with co-catalysts.

Figure 2 Principle of water splitting using semiconductor photocatalysts

(http://www.rsc.org/ej/CS/2009/b800489g/b800489g-f5.gif)

The second step represented below inFigure 3is strongly affected by crystallinity and

particle size,the higher the crystalline quality is, the smaller the amount of defects is and if the

particle size becomes small, the distance that photogenerated electrons and holes have to migrate

to reaction sites on the surface becomes short and this results in a decrease in the recombination

probability.

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Figure 3- Effects of particle size and boundary on photocatalytic activity

(http://www.rsc.org/ej/CS/2009/b800489g/b800489g-f7.gif)

The important points for the final step involves the surface chemical reactions,analysis of

active sites and surface area. Even if the photogenerated electrons and holes possess

thermodynamically sufficient potentials for water splitting, they will have to recombine with

each other if the active sites for redox reactions do not exist on the surface. Usually Pt, NiO and

RuO2 are loaded as co-catalysts to introduce active sites for H2 evolution. (kudo & Yugo, 2008)

Products and Viability

The photoelectrochemical cell shown inFigure 4(a) is a great move toward solar fuel

production via artificial photosynthesis, which uses sunlight to consume a biofuel such as

glucose, ethanol, or methanol and generate hydrogen gas.

Hydrogen can be used directly as a fuel but also used to reduce CO2 to formic acid or

carbon monoxide as precursors for higher molecular weight carbon compounds as discussed by

Benson et al. (Benson, Kubiak, Sathurn, & Smieja, 2009)

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Figure 4- Schematic diagrams of a photoelectrochemical biofuel cell (a) and an artificial photosynthetic water splitting cell (b)

(http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2009/achre4.2009.42.issue-

12/ar900209b/production/pdfimages_v02/normal.img-004.jpg)

The photoactive component is the anode, which is related to those of dyesensitized

nanoparticulate wide band gap semiconductor cells for electricity production. The anode is glass

covered with a transparent conductor such as indium tin oxide (ITO) or fluorinated tin oxide

(FTO). A thin layer of nanoparticulate SnO2 or TiO2 is sintered onto the conductor. (Gust,

Moore, & Moore, 2009)

Economically saying storage of energy in chemical bonds through electrolysis, in which

water is split into H2 and O2 in an electrolyzer, it`s a challenge once Pt-based electrolysis in

acidic or neutral media is expensive and unlikely to be scalable to the levels that would be

required for this process to be material in global primary energy production. One cheaper

alternative could be Ni-based electrolysis in basic aqueous solutions, however requires scrubbing

the input stream to remove the CO2 ; additionally, even the best fuel cells are only 50 to 60%

energy-efficient and the best electrolysis units are 50 to 70% energy efficient , so the full-cycle

energy storage/discharge efficiency of such a system is currently only 25 to 30%. Clearly, bettercatalysts for the multielectron transformations involved in fuel formation are needed.

However, no human made catalyst systems, , have yet been identified that show

performance even close to that of the natural enzymatic systems. Development of such catalysts

would provide a key enabling technology for a full solar energy conversion and storage system.

Whether the fuel-forming system is separate, as in a PV-electrolysis combination, or integrated,

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as in a fully artificial photosynthetic system that uses the incipient charge-separated electron-hole

pairs to directly produce fuels with no wires and with only water and sunlight as the inputs, is an

interesting point of discussion from both cost and engineering perspectives.

Nevertheless, cost-effective, efficient capture, conversion, and storage of sunlight have to

be considered because each of these functions has its own challenges, and integration of them

into a fully functioning, synergistic, globally scalable system will require further advances in

both basic science and engineering. Such advances, together with advances in existing

technologies, will be required if the full potential of solar energy is to be realized.(Lewis, 2007)

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Works Cited

Benson, E., Kubiak, C., Sathurn, A., & Smieja, J. (2009). Electrocatalytic and homogeneous approaches to

conversion of CO2 to liquid fuels. Chem.Soc.Rev, 89-99.

Gust, D., Moore, T. A., & Moore, A. L. (2009). Solar Fuels via Artificial Photosynthesis.ACCOUNTS OF

CHEMICAL RESEARCH, 1890-1898.

kudo, A., & Yugo, M. (2008). Heterogeneous photocatalyst materials for water splitting. The RoyalSociety of Chemistry, 253278.

Lewis, N. S. (2007). Toward Cost-Effective Solar Energy Use. Science, 798-800.

Tachibana, Y., Vayssieres, L., & Durrant, J. R. (2012, August). Artificial photosynthesis for solar water-

splitting. NATURE PHOTONICS , 511-518. Retrieved from www.nature.com.