Peidong Yang: We would like to outperform nature in terms of energy conversion efficiency
Peidong Yang is photographed in his lab, office and on campus at UC Berkeley in Berkeley, Cali., Monday, Sept. 14, 2015.

Photos by Alison Yin/AP Images for John D. & Catherine T. MacArthur Foundation
Natural photosynthesis is one of the most widespread, yet one of the most complex processes of transforming solar energy into chemical energy.

Scientists have been trying for a decade to reproduce it in laboratory conditions in order to produce inexpensive energy from the most common substances on Earth: water and air.

            And mass photosynthesis could help resolve one of the most serious problems of contemporary society – air pollution by carbon dioxide emissions.

           The team under Global Energy Prize winner Peidong Yang, Director of the Kavli Energy Nanoscience Institute (ENSI), Professor of University of California, Berkeley, succeeded, with the help of artificial photosynthesis, in creating a new form of chemical energy – transformed from solar energy and kept in chemical bonds.

          Just what is this “liquid sunlight” and how will this new technology change the development of modern energy? Peidong Yang tells us in an interview with Global Energy.

It is now popular in Europe – and throughout the world – to consider the idea of abandoning carbon-based energy and embark on a transition to clean sources. A large number of varied technologies has appeared on producing energy without CO2 – hydrogen energy, thermal cells, renewable energy sources, CO2 traps, and others. How does your system work and what is the advantage of the technology of artificial photosynthesis and its use in creating solar batteries?

– Indeed, renewable energy sources are more and more important in the current and future society in order to combat the climate changes and global warming by significantly mitigating CO2 emissions. It is important to keep in mind that as of today traditional fossil fuel still dominates our overall global energy portfolio, it continues to emit massive amounts of CO2 into the environment while humankind exacts the energy out of the fossil fuel. It is good to see now many countries are starting to aggressively reduce carbon emissions and hope to achieve net zero carbon emissions in the next few decades.

In order to achieve these ambitious net zero emission goals, we will need tap into more renewable energy sources. This naturally would include many renewable energy technologies, solar cells being one of the most important ones. There are many others such as renewable hydrogen technology, wind, geothermal, carbon capture and utilisation.

Solar cells, or photovoltaics, are indeed extremely important in terms of capturing unlimited solar energy and converting it into electricity. However, because of the intermittent nature of solar irradiation, one will need to worry about energy storage or ways to integrate with the existing power grids. In this respect, what we have been working on, artificial photosynthesis, offers the energy conversion and storage solution in one single step. Unlike the solar cell, where solar energy is captured and converted into electricity, our artificial photosynthetic system converts the solar energy directly into chemical energy. It is also this artificial photosynthetic process that one can use to transform CO2 into useful chemicals,     e. g. liquid fuels like butanol. Converting solar energy and storing it in  high energy density chemical bonds using CO2 as the feedstock is the powerful feature of the artificial photosynthesis. It would fundamentally accomplish the two important goals with one single technology: converting unlimited solar energy into chemical energy, and converting CO2 into value-added chemicals that are compatible with the existing energy, chemical and pharmaceutical industries.

–        For the average person without specialised knowledge, your technology more or less amounts to a repetition of a process that occurs in plants. Is this tantamount to trying to reinvent the wheel? What are the main differences that we can see in this technology?

– This is the very reason the technology we are developing is called “artificial photosynthesis”. It is true that we are mimicking nature for its powerful photosynthetic process in green leaves and plants. In natural photosynthesis, the net reaction starts from carbon dioxide, water with solar energy input, the final products are carbohydrates and oxygen, which is released back into the environment by the green leaf and plants. However it is important to keep in mind that energy conversion efficiency of natural photosynthesis is relatively low, typically less than 1%.

For our artificial photosynthesis, the net chemical reaction is similar, you start with CO2, water and sunlight, and the final products are oxygen, and other value added chemicals such as methane, acetate (a common chemical intermediate towards many other important chemicals), liquid fuel like butanol, biodegradable polymers, and even some chemical precursors for certain drugs.

While the general molecular process, the bond breaking and formation process shares great similarity, artificial photosynthesis uses laboratory developed semiconductors for the purpose of solar energy capture; and designed catalysts for the chemical transformation. Essentially we would like to learn from nature and in the meantime, we would also like to outperform nature in terms of energy conversion efficiency by designing better semiconductor light absorbers and better catalysts in the overall system.

–        Many experts have noted that biological solar panels like these have a low energy conversion efficiency ratio – figures of 0.1 % to 1 % have been cited. In contrast, the ratio achieved by current solar panels already exceeds 20 %. Are such comparisons appropriate? How efficient is your technology?

– We are talking about solar-to-chemical conversion efficiency here. Indeed, it is relatively low compared to the efficiency of a typical silicon solar panel. However, that may not be a fair comparison as solar cell panel is for solar-to-electricity conversion only.

We recently reported solar-to-chemical efficiency of 3.6% in the journal Joule. This value, however, is already much better than that of natural photosynthesis in green leaves. Still, it is true that there is plenty of room for improvement in term of overall efficiency.

On the other hand, my group has been largely working on a fully integrated version of such artificial photosynthetic systems. If one broadens the discussion for the purpose of carbon capture and conversion, a direct combination of a solar panel, a water electrolyser (to produce hydrogen), and possibly a bioreactor could in principle give us better efficiency. However, I would not call this artificial photosynthesis any more as this is a simple combination of three well-established processes: photovoltaics, water electrolysis and biocatalysis.

At first glance, the system you are presenting based on nanowires as a photo cell and bacteria as chemical catalysts is quite complex and fragile. To what extent is the system receptive to external conditions and for how long a period can it operate? How often do the batteries need to be changed and can the system operate by self-regulation?

– Yes, indeed these proof-of-concept devices typically require a great  deal of engineering improvement for future scaling up and/or commercialisation purposes. Our current laboratory prototype devices typically can operate continuously over days, and some up to a month, depending on the bacteria strains we use. An important requirement for these photosynthetic biohybrid systems developed in our lab is to maintain a healthy interface between the semiconductor and bacteria community. Another key feature here is the self-reproducing and self-regeneration capabilities of these bacteria catalysts.

–        Am I correct in understanding that artificial photosynthesis in operation will not produce energy but, rather, fuel, including carbon monoxide? How can the system be used in future? And does the system in fact contradict the main task at hand – making use of artificial photosynthesis to do away with carbon in principle? Are there other options for producing fuel (for instance, butanol or hydrogen)? Could electricity be produced?

–        First of all, energy is always conserved, we are not going to produce extra net energy by developing certain technologies.

The basic principle of artificial photosynthesis is as follows: Carbon dioxide + water + sunlight à  liquid fuels + oxygen

Very often, I call this  “liquid sunlight”. Liquid sunlight is a new form of chemical energy converted and stored in chemical bonds from solar energy. In the future, we will be able to develop the necessary artificial photosynthetic systems to convert and store solar energy in chemical bonds at GW and TW level. This is in exact alignment of our ultimate goal of mitigating CO2 emission as carbon is completely recycled back into useful fuels like butanol using such artificial photosynthetic process. It is the ultimate zero-emission solution to address our energy and environmental challenges.

–        What will be the cost of such fuel? How efficient will it be? Will this require a change in the overall energy system?

–        This is a relatively young technology and it is still a bit early to talk about the cost of such “liquid sunlight”.

As I stated above, we can now use well-developed nanoscience to guide us to design efficient light capture semiconductor nanostructures and catalysts to fix and activate CO2 and convert them into many useful chemicals such as liquid fuel, polymer, and drug intermediates, with the energy input purely from sunlight.

It is important to note these chemicals are something that we are using on a daily basis and therefore they are already an integral part of existing infrastructures. However, it is also important to keep in mind that all the fuels, polymers and drugs we are using nowadays are derived directly from fossil fuels. By developing this artificial photosynthetic technology, we are offering the exciting possibility of producing the same chemicals only with CO2 and sunlight! One could imagine in the future our chemical industry, energy industry and pharmaceutical industry can be completely powered from renewable solar energy, rather than largely relying on traditional fossil fuel.

–        Is it true that, for the moment, we are talking about laboratory experiments, still to be used in practice? How successful have these been? Are you moving towards industrial pilot projects? What is still needed for this system to be put to mass use in the future? What level of investment is required to proceed with a project of industrial production of such panels?

–        Indeed, this is a relatively young technology comparing for example with the solar cell technology. Much of it is still in the laboratory as proof-of-concept demonstration. In order to think about potential scale-up and commercialization, there are many issues still need to be addressed, such as energy conversion efficiency, long term stability, selectivity of chemicals etc. So, a significant amount of fundamental research and technology optimisation need to be supported.

However, again as I stated above, if one broadens the discussion for the purpose of carbon capture and conversion, a direct combination of a solar panel, a water electrolyser (to produce hydrogen), and possibly a bioreactor could in principle give us better efficiency, and will, at least at this stage, probably have a better opportunity for large-scale implementation.

–        Is it true the technology of artificial photosynthesis can be used on space stations and for what specific uses? Have experiments to this effect been conducted? To what extent could the use of this technology be considered a serious option for travel to Mars, for instance? In what other spheres of activity could it be used effectively?

–        Yes, indeed, this new technology will also be important for future deep space exploration including Mars, since liquid sunlight could provide the fundamental chemical, energy or even food for people living in outer space in the future. For example, Mars’s atmosphere is comprised primarily of CO2 and frozen water exists there in abundance as well. As our net reaction is CO2 + water + sunlight = chemicals/fuels + oxygen– it is quite natural for us to think about CO2 mitigation on Earth as well as chemicals/fuels production in space. These photosynthetic biohybrids could potentially produce chemical building blocks for astronauts to make critical supplies and equipment for survival on deep-space missions.

–        How could the use of nanowires be expanded in the energy sector? Are there other sectors where they could be used?

–        Semiconductor nanowires, by definition, typically have nanoscale cross-sectional dimensions, with lengths spanning from hundreds of nanometers to millimeters. These subwavelength structures represent a new class of semiconductor materials for investigating light generation, propagation, detection, amplification, modulation as well as energy conversion and storage. After more than two decades of research, nanowires can now be synthesised and assembled with specific compositions, heterojunctions, and architectures. This has led to a host of nanowire photonic and electronic devices. Nanowires also represents an important class of nanostructure building blocks for photovoltaics as well as direct solar-to-fuel conversion because of their high surface area, tunable bandgap, and efficient charge transport and collection. For example, back in 2008, we discovered that silicon nanowires could have significantly improved thermoelectric performance compared to their bulk counterpart. These silicon nanowire arrays have shown great promise as high-performance, scalable thermoelectric materials for waste heat recovery in power plants, refineries and automobiles.

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