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| Fig. 1: A typical plant cell. (Source: Wikimedia Commons) |
Fossil fuels are, almost by definition, limited in quantity. With the increase in population and in energy demand, we will need sooner or later a replacement for that kind of energy source. A solution could be producing more electricity using renewable sources and developing efficient ways to store that energy (e.g. batteries). This will certainly be necessary, but for some applications, like airplanes, a large energy storage density is required, which is not yet satisfied by current batteries. Moreover, by 2030 only 22% of the total energy consumption will be on the form of electricity, which means we will need to find ways to satisfy the 78% demand with other sources, for example, fuels. [1] Finding a good replacement for fossil fuels is, then, crucial for future energy sustainability. Ideally, that replacement fuel would need to be abundant and easily produced. Since the Sun is the ultimate source of the vast majority of Earth's energy at the surface, a natural expectation is that the energy for the synthesis of this ideal fuel comes from the Sun.
Plants also had a similar problem to solve. They needed a way to store the abundant energy from the Sun to survive at night, to grow, and to reproduce. Millions of years of evolution and natural selection resulted in the process of photosynthesis, which produces glucose and oxygen from carbon dioxide, water, and photons. Inspired by this process, some have tried to find processes that artificially recreate and adapt photosynthesis to create fuels, which are then called solar fuels. If solar fuels prove to be cost efficient, they will be a great replacement for fossil fuels, at the same time contributing to a carbon neutral future.
In this report, we start by briefly describe natural photosynthesis. Subsequently, we go over today's main different artificial photosynthesis processes. Finally, we present the most promising forms to leverage these new technologies.
Photosynthesis is a process happening in the chloroplasts of vegetable cells (see Fig. 1). As was stated in the Introduction, it produces glucose and oxygen from carbon dioxide, water, and photons coming from the Sun. Basically, the photon oxidizes the water, and the free electron and proton are used to reduce a carbon molecule, making glucose and liberating oxygen. The first part of the process is light dependent, whereas the second part, also called the Calvin cycle, is light independent (no photons involved). For every six molecules of water and carbon dioxide, one glucose molecule is produced, along with six dioxygen molecules.
It is interesting to analyze the efficiency of the process. We define efficiency as the energy content of the biomass produced in a given area divided by the total solar irradiance in that same area. Using this definition, the standard efficiency for crop plants is of the order of 1%, reaching 3% for some microalgae. [2] As we shall see, it is not a high efficiency compared to the photoelectrochemical cell efficiency. However, photosynthesis has some crucial advantages. For example, it can absorb carbon dioxide even at very low concentrations, the quantum efficiency is nearly 100%, the cell can repair itself if it gets damaged, and the final product, glucose, is highly stable and transportable.
The goal for artificial photosynthesis is to keep the advantages of natural photosynthesis while improving the disadvantages, in particular the efficiency. Like natural photosynthesis, it consists in oxidizing an electron donor (usually water) with light, and then reduce some other chemical to synthetize the solar fuel.
There are three main categories of systems for artificial photosynthesis using water to get hydrogen (which has a high energy density). [3] They are:
Photochemical system (PC)
Photoelectrochemical cell (PEC)
Photovoltaic-electrolysis (PV-E)
A PC system produces directly the hydrogen and oxygen through chemical reactions in the same "bath". It is very simple and cheap, but also inefficient. In fact, the efficiency is usually less than 1%. Moreover, it produces a mixture of hydrogen and oxygen that must be separated to avoid reverse reactions (high cost).
The PV-E system is the intuitive use of photovoltaic panels to generate electricity to electrolyze water. It is fairly efficient, with typical values over 10% (higher than natural photosynthesis), but also costly (especially compared with fossil fuels). Since PV-E already uses mature technology, the efficiency will likely not increase very much in the next years.
Finally, the PEC system offers a balanced solution. It has a fairly high efficiency and it is not very costly. It integrates the PV-E compound system into one single unit, that is, it uses a semiconductor-electrolyte interface. At the same time, the hydrogen and oxygen are produced in different electrodes, so there is no risk of reverse reactions or explosions. This process can even compete with fossil fuel derived hydrogen, provided that the efficiency becomes higher than 10% and the lifetime longer than 5 years. Since this is today the most balanced solution, we are going to describe it in a little more detail.
Basically, the simplest PEC system consists on having one semiconductor photoelectrode (light-sensitive electrode) and one standard metallic electrode. The perfect semiconductor photoelectrode would absorb a large portion of the solar light, easily transport charge (electrons and holes), and be stable. The perfect material is yet to be found. Some techniques used to improve the efficiency are, for example, using separated p-type photocathode and n-type photoanode in a tandem configuration. In a tandem configuration, the semiconductor with the higher band stays on top of the other, maximizing the absorption spectrum range. Such a tandem configuration may yield an efficiency of over 25%, much higher than the typical PV-E.
Although there is still much progress to be made, especially in developing better semiconductor materials for the photoelectrodes, PEC-type artificial photosynthesis seems very promising, already delivering high efficiencies at a reasonable cost. There were already reported benchmark efficiencies of the order of 19%, and the theoretical limit has not been attained. This technology seems therefore a very appealing solution to the fossil fuel problem.
© Diogo Pinto Leite de Bragança. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] S. Styring, "Artificial Photosynthesis For Solar Fuels," Faraday Discuss. 155, 357 (2012).
[2] R. E. Blankenship et al., "Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement," Science 332, 805 (2011).
[3] S. Chu et al., "Roadmap on Solar Water Splitting: Current Status and Future Prospects," Nano Futures 1, 022001 (2017).
