Victor Luca, 17-Jan-21.
Unpublished.
In one way or other it is the sun that powers the Earth. Without the sun, the only source of energy on Earth would be that actually bound up in the chemical bonds of the constituent materials and the energy left-over from the Earth’s formation in the form of a molten core and solidified rocks that get cooler nearer the surface. The sun is essentially a nuclear fusion reactor where the principal reaction involves the fusion of hydrogen nuclei to form helium.
It is the sun and other stars that created hydrogen and all the other heavier elements. The first element created was hydrogen and it remains by far the most abundant element in the universe.
The Sun has been in constant operation for eons and as far as we humans are concerned its remaining mileage is infinite. The sun showers the globe with ∼120,000 TW (Terra Watts) of power which is thousands of times current global energy needs.
It is the Sun’s energy that drives the climate system and photosynthesis. This energy comes to us in the UV, visible and infrared regions of the electromagnetic spectrum. Energy also comes to us via cosmic rays and other energetic particles which we can disregard for now.
In an article in The Beacon of 8-Jul-20 I briefly described what has come to be known as The Hydrogen Economy. Here I wish to explore one aspect of that theme in greater detail. Namely, the direct conversion of sunlight into hydrogen.
Being the first element in the periodic table of elements, hydrogen is therefore the simplest. Hydrogen gas (H2) is often called an energy carrier because we do not usually use the energy directly by burning it. However, when it is burned, all you get is water. Now let that sink in for a moment.
It appears that hydrogen can be sent down reticulated natural gas networks without too much trouble, and used as we normally use natural gas today. Other uses for hydrogen include the storage of renewable energy and for powering Fuel Cell Electric Vehicles (FCEV). The first FCEV to be sold in NZ will be the Hydundai Nexo. All of these ways of using hydrogen are being trialed in places such as Denmark.
While hydrogen is the most abundant element in the universe it is not abundant on Earth. You can't just drill for it as we do with natural gas. Hydrogen gas is usually generated industrially from hydrocarbon feed stocks (e.g. methane) using thermal energy through a chemical processes known as steam reforming. This is the method most commonly used to produce the lion's share of the hydrogen that the chemical industry uses as a feed stock to make other chemicals. Alternatively hydrogen can be produced from coal using the process of gasification.
Depending on the exact feed stock used in the steam reforming process, one tonne of hydrogen gas will produce about 9 - 12 tonnes of CO2. Aside from emitting CO2, steam reforming and other processes require temperatures in the range 700-1000 oC.
So steam reforming is a dirty way of making a clean fuel but it is the process used to make more than 90% of the hydrogen presently synthesized on the planet.
Another method of generating hydrogen is to split water using thermal, electrical, photonic (light) and biochemical energy. These methods would generate no carbon emissions since there is no carbon in water. Another option for producing hydrogen gas on a mass scale is to use nuclear fission heat. I will leave this for another time.
That big nuclear reactor in the center of our solar system (aka the Sun) produces photonic energy that can be readily converted into electricity using photovoltaic panels and the electricity then used to split water H2O into H2 and O2 via electrolysis. This is a stock- standard technique and you could do it in your back yard using a battery and some bits of graphite to function as electrodes. The trick however, is to obtain maximum efficiency using catalytically active electrodes. You could use almost any metal as an electrode but is best to use materials that are chemically stable and that have low cost.
Nowadays, water electrolysis accounts for only 4% of the world’s hydrogen production. I pointed out in my previous Beacon article that recently companies like Toshiba have started generating solar hydrogen on a commercial scale. They use solar PV and wind farms to generate the renewable electricity which they then use to produce hydrogen via electrolysis. The hydrogen can then be stored in high pressure tanks and converted back into electricity at will using a device called a Fuel Cell.
Other global companies such are AREVA are also getting involved in this space. Whilst the process is relatively inefficient even with the best electrodes, we get the energy for nothing. So who cares?
Another, perhaps even more elegant and direct method of making hydrogen using the sun’s energy, would be to irradiate a semi-conductor photo-electrode with sunlight to split water directly. In this process little or no electricity would need to be supplied to the system, rather the sun does all the work. Although this seems more elegant, the process suffers from even lower efficiency than electrolysis, typically just a few percent.
Recently the push to produce hydrogen by these clean methods has heated up as folk cotton on to the fact that climate change is serious and we urgently need to wean ourselves off hydrocarbons.
The possibility to generate hydrogen by irradiating a semiconductor electrode was first demonstrated by Fujishima and Honda in 1972 [1]. A lot of water has gone under the bridge since then and still we have no high efficiency working device. Myself and a small part of my Australian research team got into this area of research around 2006 as part of a collaboration with CSIRO. At the time CSIRO had set up an entity called the National Hydrogen Materials Alliance that operated from 2006-2009 and then fizzled as folk lost interest in the hydrogen economy. It was put into the too hard basket.
The setup for such photoelectrochemical generation of hydrogen is to irradiate a transparent electrode coated with a stable transparent semiconductor material such as titanium oxide (TiO2). The electrode is glass and is coated with a transparent layer of conducting glass similar to the glass used on your cell phone’s display. A wire is connected from this working electrode to a counter electrode in order to complete the circuit (Figure 1). The electrodes are immersed in water and a multi-meter is used to measure the photovoltage and current that is generated by the two chemical reactions occurring on the surface of the electrodes when light is shinned on the working electrode. On the negative counter electrode oxygen is formed and on the positive electrode hydrogen. The amount of voltage and current is proportional to the amount of reaction and therefore the amount of hydrogen produced.
Figure 1. Typical setup for photoelectrochemical water splitting using semiconductor photoelectrodes.
The efficiency of this process depends on the electronic characteristics of the semiconductors used, most importantly the band gap and the match to the solar spectrum. Also important is the atomic structure of the photoactive coating (TiO2 shown).
Figure 2. The power that is received from the sun per m2 per nm. The rainbow colored region is known as the visible range of wavelengths because this is the rainbow we can see when we pass the suns light through a prism. The UV region of wavelengths causes sun burn and can’t be seen and longer wavelengths (infrared) is basically heat which we also can’t see by the naked eye.
Figure 2 shows that the solar irradiance in W/m2/nm is maximum at a wavelength of about 500 nm, or in other words the visible region of the spectrum. This is the same whether you are located in the upper atmosphere or at sea level. In order for an electrode to receive optimum energy from the sun it should operate around this peak wavelength, or around 500 nm. In other words, it should have a semiconductor band gap tuned to this energy. The bangap of a semiconductor is the energy required to promote an electron from the valence to conduction band (Figure 3).
Figure 3. Photocatalytic water splitting. (a) Schematic of water splitting using semiconductor photocatalyst. (b) Band structure of semiconductors and redox potentials of water splitting. Source: A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253.
Starting from about 2004 myself and my research group undertook quite a bit of work in this area using materials such as tungsten oxide (WO3) and mixtures of WO3 coupled to TiO2 [3]. Unfortunately, at the time we could not produce efficiencies that would lead to practical devises.
Almost two decades after my work in this area, and almost half a century after the initial revelations of Fujishima and Honda, there is still no commercial application of this technology because until recently only relatively low photocatalytic conversion efficiencies have been achieved.
CdSe photocatalyst with a high effiiency of >36% at excitonic peak (520 nm) with the assistance of sacrifiial reagent [4] was reported in 2012 but these were made from expensive elements. Most recently SrTiO3-based photocatalytic materials with a bandgap energy of 3.2 eV were reported which should not be expensive to manufacture. Such photocatalytic materials can lead to extremely high quantum efficiencies for photoelectrocatalytic water splitting [5].
Perhaps these recent discoveries will propel this field of research toward the full-scale and efficient practical direct solar hydrogen generation using nothing more than the Sun. Incidentally, strontium titanate is the bases of a new generation of high efficiency thin-film solar cells that could revolutionize the PV market.
References
[1] Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37.
[3] Luca et al., Sol-Gel Tungsten Oxide/Titanium Oxide Multilayer Nanoheterostructured Thin Films: Structural and Photoelectrochemical Properties. J. Phys. Chem. C 2007, 111, 18479.
[4] Han, Z., Qiu, F., Eisenberg, R., Holland, P. L., Krauss, T. D. Robust Photogeneration of H2 in Water Using Semiconductor Nanocrystals and a Nickel Catalyst. Science 2012, 338, 1321.
[5] Takata et al., Photocatalytic water splitting with a quantum efficiency of almost unity. Nature, 2020, 581, 411-414.