Gas from light and water

SolarFuelCell_2362

The government is looking for techniques for producing new fuels from CO2, light and water. Late last year, the Foundation for Fundamental Research on Matter (FOM), the Netherlands Organisation for Scientific Research (NWO) and Shell allocated € 5 million to this purpose. One of the seven proposals to be funded was from TU Delft.

More and stronger earthquakes in Groningen and a majority in the Netherlands House of Representatives for reducing the gas-exploration activities in the area – these developments signal the end of an era in which natural gas was an obvious source of energy and income for the state. In the meantime, the Netherlands has acquired a unique infrastructure of gas pipelines, and it would be a shame not to use it.

The research programme entitled ‘CO2-neutral fuels’, which was launched in the spring of 2013 by FOM, NWO and Shell, has made € 5 million available for the clean production of CO2-neutral fuels from water and carbon dioxide. The objective is illustrated in a cheerful video: electricity from solar panels supplies a factory in which water and carbon dioxide (CO2) are converted into methane (CH4). The gas flows through pipes to reach the farthest corners ­of our country. As natural gas – in this case, more specifically, solar gas – is burned, carbon dioxide and water are released, thereby closing the circle. In this way, we can live happily ever after.

Storage
Storage is another matter to consider with regard to the advance of solar energy, according to
Dr Arno Smets (EEMCS). Although solar energy currently accounts for only a small share (1%), what will happen when this share reaches 10% or 20%? “Storage will become a bottleneck”, predicts Smets. “We have to be able to dump energy through a chemical conversion.” One way to do this would be to generate hydrogen through electrolysis (i.e. using electricity to split H2O into H2 and O2) and converting it into methane (CH4) using CO2. Unlike hydrogen, methane lends itself well to storage, if necessary in the gas fields of Groningen, which would then be empty.

A video* on the research conducted by Lihao Han (EEMCS) and Fatwa Abdi (Applied Sciences) on a solar cell that produces hydrogen provides a glimpse of the future: we see a simple, square plexiglass box. The box stands alone – there are no wires attached. Then, a spotlight flashes on the cell. The camera zooms in and, lo and behold, small bubbles are rising from a screen in the middle of the cell: hydrogen from light.

According to Abdi, a capacity of 10% should ultimately be feasible: “We have now reached about half of this goal. If we achieve a capacity of 10% with large-scale installations, we can bid farewell to fossil fuels.” Then we will cover our roofs with combination cells that generate hydrogen, which we will use to fill our hydrogen cars – free, and without taxes.

APPEL
With the funds they have received from FOM and other sources (€ 750 thousand), two new PhD students will start working on a revised model, for which the researchers have high expectations. In their proposal to NWO/FOM, the researchers even refer to the prospect of a total capacity (solar to hydrogen) of 15%.

“Now that we know how the cell works, we also know where the problems lie”, explains Abdi’s supervisor Prof. Dr Bernard Dam. He is thinking of improving the charge separation (e.g. by preventing the loss of electron-hole pairs due to receding of electrons), improving the absorption of light and optimising the mobility of the charge carriers (allowing greater formation on the electrodes).

The design used by the applicants Arno Smets and Dr Wilson Smith (from Dam’s group) in their proposal, entitled APPEL,2 is quite different from that developed by Abdi and Han. They actually reverse the entire design: in their design, hydrogen is formed on a semi-conducting photo-cathode with a catalytic layer.

The two PhD students will continue to develop the new design in the coming years, under the supervision of Smets and Smith. For example, they will be considering ways of protecting the semi-conductor against corrosion due to water. The idea is to develop a ‘passivation layer’ that separates the semi-conductor and the water, while allowing the electrons to pass, thus producing hydrogen on the cathode.

The long run
It will also be necessary to develop new oxygen generating catalysts to replace the usual platinum. Ideally, the anode should also continue to absorb a portion of the light. To this end, the yellowish bismuth vanadate of the cell’s surface must be replaced by a different, darker oxide, which will absorb a greater portion of the spectrum. It is hoped that this will generate a stronger current, which is expected to increase the production of hydrogen. It is obviously crucial for the solar cells in the background to be able to generate as much or more electricity with less light.

Dam acknowledges, “This is research for the long run.” Therefore Dam would like to have research support of a more structural nature. “The production of hydrogen from sunlight is a topic that is essential to a sustainable society”, asserts Dam, who would like to see funding for applied research focus more on new industries than on existing ones.

1 Fatwa F. Abdi, Lihao Han, Arno H. M. Smets, Miro Zeman, Bernard Dam & Roel van de Krol, Efficient solar water
splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode, Nature, 29 July 2013.
2 Earth Abundant Materials based Monolithic Photovoltaic-Photo Electrochemical Device toward 15% Solar-to-
Hydrogen Conversion Efficiencies – Acroniem: APPEL
3 www.fom.nl/co2

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