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How to obtain fuel from water on a sunny day


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To cite this article  use:  Gutiérrez, O., Méndez, M. How to obtain fuel from water on a sunny day. J. Nano Sc. Tech, 3(2015)36-43

Oliver Y. Gutiérrez*

Minerva Méndez Martínez

Technische Universität München, Department of Chemistry and Catalysis Research Center, Lichtenbergstrasse 4, D-84747 Garching, Germany

* oliver.y.gutierrez@gmail.com

 

The sun is the great hope of humanity to solve all energy and pollution problems. However, to efficiently transform solar energy into fuel using present technology is a major challenge. This text describes how nature and human technology make use of the same physicochemical principles and similar strategies to generate chemical and electric energy (photosynthesis, solar cells). We also describe the routes to produce fuel from water and carbon dioxide using photocatalysts, materials that enhance a chemical reaction driven by light. Finally, we address the potential and challenges of this new field of science called photocatalysis and its synergy with nanotechnology.

Planet earth receives an enormous amount of energy from the sun every day. All this energy is much more than what is needed to maintain all activities of modern societies. In order to put this in perspective, the solar energy absorbed by our planet in an hour is equivalent to the energy used by human beings in one year (O. Morton, Nature 443, 2006 19-22). However, most of that energy is simply dissipated in numerous natural phenomena (wind, waves, surface warming, etc.). Only a tiny fraction of that energy is currently being directly used by humans; for instance, in solar cells, wind turbines and tidal generators. Therefore, our dependence on fossil fuels, i.e., petroleum, gas, and coal still dominates the global economy and, in many aspects, the course of modern history. The extraction of these fossil sources and the generation of energy from them are inevitably associated with destruction of ecosystems and generation of green house gases. Needless to say, this has serious consequences for our environment and endangers the wellbeing of future generations.

Why are we still using fossil fuels instead of the super abundant solar energy? The answer is as simple as sad. The utilization of fossil fuels is much more convenient; at least at the present time. The utilization of solar light is more expensive than the production of fuels from fossil sources since, even though solar light is free, the technology to make use of it is expensive. The use of solar light is, currently, also not efficient enough to satisfy the world’s energy consumption. In one word, we just do not know how to take approach of this enormous source of energy provided by the sun.

At this point, many readers may be thinking, wait a minute, but we have solar cells with increasing efficiency. Yes, we have that technology. Those flat and black squares on the roof of houses are small wonders of science and technology that convert solar light directly into electricity. This technology is subject of strong debate because while its supporters claim that the only real drawback is its installation price, the truth is that our energy demand still cannot be satisfied with solar panels. But that is another story.

Nature learned to convert solar light into chemical energy millions of years ago. The process is called photosynthesis. The term now sounds trivial and yet the process is responsible of life as we know it. We learn about photosynthesis in primary school, but we rarely think about its complexity and importance. In simple terms, during photosynthesis, plants take carbon dioxide and water and create from simple CO2 complex organic molecules including sugars and polymers. In other words, CO2 is upgraded.

Can we convert solar energy into chemical energy as plants do? Yes we can, theoretically. We know how photosynthesis works. Indeed, the principle is the same as used in solar cells. To explain this similarity we have to start using chemical jargon and explain the peculiarities of conductors and semiconductors as well as introduce the band theory.

The band theory postulates that two energy levels (so called “bands”) in solid material exist and are available to be populated by the electrons of the atoms constituting the material. These energy levels are the valence band and conduction band. At sufficiently low temperatures, all electrons are located only in the valence band. The electrons in this band are responsible for the chemical bonding, the force that keeps the atoms together. On the other hand, the electrons which make it to the conduction band (after an energetic excitation) easily move from atom to another atom within the material and are, therefore, responsible for the electric conductivity of the material.

Conductor materials are easy to understand because the most typical kind of conductor materials is very visible in our daily life, metals. Valence and conduction bands in metals overlap, which means that electrons from the chemical bonds can go into the conduction band “for free” (energetically speaking) and “travel around” along the material. These wandering electrons are observable on a macroscopic scale as an electric current.

In contrast, semiconductors have valence and conduction bands separated by an “energy gap”. In this case, electrons cannot go to the conduction band and wander freely. That is why, under normal conditions, semiconductor material do not conduct electricity. Fortunately for nature and for us, all what is needed to alter the normal conditions is an external energy input, an energy kick that pushes (or excites) electrons to the conduction band. We have to thank this phenomenon for all technology based on semiconductor material. These are all our electronic devices including solar cells. Figure 1 presents a classic representation of the energy bands in semiconductor and conductor material as well as the movement of electrons between bands.

Figura 1 (2)

Figure 1. Representation of the two energy levels (bands) for electrons (e-) in two kinds of materials. The valence band is represented by the blue blocks and the conduction band is represented by the orange blocks. To the left, the situation of a semiconductor material where the bands are separated; the electrons are in the valence band but can be exited towards the conduction band by a energy excitation (light or heat). To the right, the situation of a conductor material where the bands overlap (there is no band gap in between); the electrons move freely between bands.

For the topic that we are discussing, the energy input that moves the electron from the valence band to the conduction band is a photon (hν), the elemental particle of light. We hope that with the description of semiconductor materials given above, the understanding of the principle of solar cells becomes clear. That is, photons (light) are absorbed in the semiconductor of the cell (silicon) and converted into the energy needed to put electrons into the conduction band. When the electrons jump, they leave “holes” behind in the valence band. These holes are simply the absence of electrons but the not-so-demanding readers may imagine that the holes are particles with positive charge (remember that electrons have negative charges). Being in the conduction band, the electrons may travel in an external circuit before recombining with the holes; thus, producing the desired electric current (Figure 2 shows a schematic view of the process). In the following, let us simply refer to electrons and holes as charges.

Figura 2 (1)

Figure 2. Representation of the movement and use of charges, electrons (e-) and holes (h+) in a photovoltaic cell and the photosynthesis process. In the photovoltaic cell (left) the photons (hν) excite electrons from the valence band (blue block) towards the conduction band (orange block) generating mobile electrons and holes; the electrons travel in an external circuit to produce electricity and recombine with the holes closing the cycle. In photosynthesis (right), the photons generate charges, which are efficiently separated; the holes react with water to produce oxygen and other species, whereas the electrons produce NADPH (nicotinamide adenine dinucleotide phosphate), which is the compound that starts the conversion of carbon dioxide.

Coming back to photosynthesis, the principle of the phenomena in a plant is similar to that in a solar cell. Photons are absorbed and converted into the energy that creates charges (this does not happen in solid semiconductor materials but in natural dyes). The beauty of photosynthesis is that the electrons and holes do not recombine but are efficiently separated and used to perform chemical reactions (after all, the origin of chemical bonding and reactivity is, precisely, the transfer of electrons). In fact, the charges are used to unchain a series of chemical reactions, which are responsible for the “upgrading” of CO2 and water to complex molecules. Figure 3 shows schemes of the creation and charge utilization in solar cells and photosynthesis.

Figura 3 (1)

Figure 3. Representation of the photocatalytic degradation of organic compounds on semiconductor materials; photons (hν) generate charges that travel to the surface of the semiconductor. The holes (h+) react with the organic compounds and water producing species with electric charge (P+, H+) and free radicals (OH.). The electrons (e-) also generate free radicals. All these chemical species are highly reactive and can produce the complete degradation of organic compounds into carbon dioxide (CO2) and water.

Let us ask again, Can we convert solar energy into chemical energy as plants do? Yes we can, theoretically. One of the most trendy and cutting edge research topics in chemistry and physics is the conversion of CO2 to higher compounds in the presence of water, that is, “artificial photosynthesis”.

The strategy to achieve artificial photosynthesis is to use semiconductor materials able to separate the charges created from the absorption of photons to different locations and perform chemical reactions, just like it happens in a plant. Such materials are called photocatalysts because they can accelerate chemical reactions involving the charges created by light. The design of these materials require a deep understanding of the nature of charge generation and transfer to the surface, as well as of the nature of the active site where the charges react with adsorbed reactants.

Figura 4 (1)

Figure 4. Representation of the photocatalytic degradation of organic compounds on semiconductors decorated with metal nanoparticles. Photons (hν) generate charges that travel to the surface of the semiconductor. The holes (h+) react with organic compounds to start its degradation towards carbon dioxide (CO2) and protons (H+). The electrons (e) are collected by the metal nanoparticles and react with protons to produce hydrogen (H2).

Unfortunately, we are still far from that understanding. Artificial photosynthesis is one of the holy grails of modern science however the challenge of mimicking nature is at this point too big to overcome. The most reliable studies have reported the production of just a few part per million of CO and small molecules (formaldehyde, methanol) from CO2 and water under ultraviolet (UV) light. We understand that for the average reader these results are not promising at all and actually, they are not promising for the specialized reader either. However, the photochemical conversion of CO2 must be investigated at a fundamental level because only the understanding of the chemistry and physics involved would make it a feasible solution to the energy problems of our society.

Is there any hope for photochemistry to contribute converting light into chemical energy (in a reasonable period of time)? Yes, there is hope. An example is the worldwide benchmark photocatalyst: TiO2 (titanium dioxide). This is a semiconductor that exhibits very high photocalytic activity (as reported in uncountable studies), which adsorbs UV light. The most explored application of TiO2 as a photocatalyst is the degradation of organic compounds with UV-light, which may be a useful approach to clean polluted water streams trough photooxidation.

In this case, degradation means that organic compounds polluting water are converted in small molecules by the action of radicals and other chemical species with charge that is created when light is adsorbed by TiO2. The mechanisms of the interactions among radicals, charged species and reactants can be very complex depending on the structure of the molecules being converted, but the objective is to have only CO2 and water as products from the photo-oxidation. Figure 5 shows a visualization of this phenomenon.

Figura 5 (1)

Figure 5. Representation of the overall water splitting on complex photocatalytic materials. Photons (hν) generate charges that travel to the surface of the semiconductor. The holes (h+) react with water to produce oxygen and protons (H+) while the electrons (e) are collected by the metal nanoparticles (left). The protons diffuse trough the oxide layer surrounding the metal nanoparticles and react with the electrons to produce free hydrogen (which diffuses back). On the contrary, produced oxygen cannot cross the oxide layer and reach the metal nanoparticle.

How could the treatment of polluted water produce chemical energy? Well, if TiO2 is modified, decorated with, let’s say Pt nanoparticles (in general any noble metal would do the job), the electrons generated by the adsorption of light would be efficiently used to generate hydrogen. Let us explain the phenomena. A TiO2 crystal adsorbs a UV photon generating charges. The holes travel towards the surface of the semiconductor and “steal” electrons from adsorbed molecules. This initiates degradation processes leading to CO2. On the other hand, the electrons reaching the Pt nanoparticles would “jump” into the conduction band of the noble metal (much more attractive for the electrons than the bands of the semiconductor). The excess of electrons in the metal reacts with protons and produces molecular hydrogen (Figure 6 illustrates this process). And so we find another important concept to produce fuels with photocatalysis: “reforming” of organic compounds. The idea is simple, mixtures of CO2 and H2 could be produced from aqueous solutions of organic compounds using efficient photocatalysts. H2 is per se a valuable compound for many industrial processes and a fuel by itself. The H2-CO2 mixture may be combined with air and fed into a fuel cell to generate more green energy (but that is also another story).

Figura 6 (1)

Figure 6.Representative image of the Rh@Cr2O3/GaN:ZnO system. This image from electron microscopy shows how the solid solution of gallium nitride and zinc oxide (GaN:ZnO) supports rhodium (Rh) nanoparticles covered by a layer of chromium oxide. Image adapted from K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue, K. Domen: Noble-Metal/Cr2O3 Core/Shell Nanoparticles as a Cocatalyst for Photocatalytic Overall Water Splitting. Angewandte Chemie International Edition, 2006, 45, 7806-7809. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

Is it possible to split water in the absence of other reagents with visible light? Yes, we can in theory. The so-called overall water splitting is as challenging as CO2 photoconversion but is energetically less demanding and exhibits larger conversion rates. Furthermore, there are some relatively well studied photocatalysts able to split water which are good examples of the application of nanotechnology. These photocatalysts are worth describing because surely, they are the precursors of more efficient materials in the future.

Scientist from Japan clearly lead the field of photocatalysis. Domen and coworkers have synthesized the most efficient material for overall water splitting reported so far. This photocatalyst is a solid solution of gallium nitride and zinc oxide (GaN:ZnO) “decorated” with particles of a co-catalyst based on Rh. A co-catalyst is an additive that helps the catalyst to produce H2 without producing charges itself. The synthesis of the GaN:ZnO semiconductor itself is a triumph of “band engineering”. Whereas the pure white compounds GaN and ZnO have a very high band gap, the combination (a yellow powder) absorbs visible light creating charges. The co-catalyst is a complex system whose nature is still under debate. The most accepted picture of it is an Rh nanoparticle (below 5 nm) surrounded by a thin layer of chromium oxide (Cr2O3). The way that all these components work together in order to split water in solution is fascinating and offers a perfect example of synergy between compounds of different nature.

As expected from a semiconductor material, GaN:ZnO adsorbs photons creating charges. The electrons diffuse along the crystal structure of the semiconductor. Some of them reach the surface where they are “trapped” by the Rh particles (remember that the conduction band of noble metals is much more attractive for the electrons than that of the semiconductor). The excess of electrons in the metal combines with protons in solution to produce molecular hydrogen. In turn, the holes that reach the surface of the semiconductor react with water producing molecular oxygen. In normal conditions, the noble metal nanoparticle (Rh in this case) would immediately catalyze the recombination of H2 and O2 to water and the net reaction rate of water splitting would be zero. In the photocatalyst being described, the layer of chromium oxide works as a selective membrane. It allows hydrogen produced on the surface of the Rh core to diffuse towards the bulk of the reaction but it does not allow O2 (a much bulkier molecule than H2) to reach the Rh and react with water. Figure 5 shows a schematic description of the process.

In brief, the so-called Rh@Cr2O3/GaN:ZnO system consists of a semiconductor that generates charges, a co-catalyst (Rh) that traps the electrons and offers the surface needed for H2 formation, and a ceramic membrane (Cr2O3), a barrier, that avoids that the generated H2 and O2 find each other at the reactive Rh surface (Figure 6 shows an emblematic image of the material obtained using high resolution electron microscopy).Here is where we want to emphasize the role of nanotechnology for the design and preparation of better photocatalysts. There are many state-of-the-art methods in nanotechnology that allow the synthesis of well defined nanostructures just like the core-shell Rh@Cr2O3 geometry described above (Rh core surrounded by Cr2O3). Also, the porosity of ceramic material can be tuned and nicely controlled (see for instance the article about zeolites in the second number of this magazine).

Figura 7 (1)

Figure 7. Picture of a photoreactor used in a research laboratory. The components that can be observed are the container (covered with aluminum foil in this case) (A); water filter for infra red radiation (B); lamp (C); condenser (D); stirring plate (E); and diverse gas conducting lines.

If nanotechnology offers the tools to create photocatalysts, why is there such a small advance in the performance of the photocatalyst? This is because, in contrast to other applications of catalysis, photocatalysis is in early development stage. There are so many things that we do not know about the phenomena involved in the transformation of light into chemical energy. For instance, what are the processes for the transference of charges from the bulk of the crystal towards the surface? Where are and what are the active sites for oxygen evolution? What is the chemical state of the surface during illumination? How must the interphase between semiconductor and co-catalyst be in order to maximize electron transfer? We need to know all this before starting to make advances towards the design and application of photocatalytic cells.

In the lab, the experimental procedure to test catalysts in, for instance, water splitting simply consists in dropping the photocatalyst (usually powder) in water at room temperature and atmospheric pressure in a transparent container (nicely called photoreactor). Then a lamp shines on top of the container and the atmosphere over the liquid, containing the product, is moved with a pump for analysis. This is a relatively simple configuration and allows testing many materials quickly. Figure 7 shows a possible variation of a photoreactor in the laboratory highlighting the most important parts in addition to other necessary items; for example, a filter to eliminate infra red radiation (which may warm the suspension), a condenser to keep the water in the container, and a stirring plate. There are many other variations where the lamp is placed on one side of the photoreactor or immersed in the container to make the best use of the radiation.

However, this design is just for research because the production of oxygen and hydrogen in a single compartment would bring separation problems. There are other designs that might be applicable to a larger scale like a configuration in “electrodes” where semiconductor and cocatalyst are separated by a circuit, which conducts the electrons. With this configuration, oxygen and hydrogen would be produced in different compartments easing its separation. Figure 8 compares the two described configurations for photoreactors. Having these pictures in mind, and considering that such a system is easy to scale up, can we imagine a future where one day solar plants are built to produce hydrogen day by day with only oxygen as a waste product? Yes we can.

Screen shot 2016-01-12 at 6.33.46 PM

Figure 8. Representation of two possible configurations for the photocatalytic water splitting. To the left, a suspension of the photocatalyst in water where the reactions that produce hydrogen and oxygen occur in different sites of the same particle (and concomitantly in the same container). To the right, a photocatalytic cell where the adsorption of photons (hν) and the charge generation occur in a compartment where the holes (h+) are used to produce oxygen and protons (H+). The electrons (e) are conduced trough an external circuit (generating electricity) towards a second compartment where they react with protons to produce hydrogen. The protons are conducted form one compartment to another trough a conductive bridge.

References

[1] O. Morton, Nature 443, 19(2006).

[2] K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue, K. Domen, Angewandte Chemie International Edition, 45, 7806 (2006).

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