PASADENA – For decades, the development of renewable energy – and the policy debates that surround it – has focused largely on electricity generation. But more than 60% of the world’s energy is provided directly by chemical (mainly fossil) fuels, with no intermediate conversion to electricity. No realistic effort to combat global warming by cutting carbon emissions can ignore this fundamental constraint.
Indeed, in the United States and other industrialized countries, many applications that rely on fossil fuels (such as air transport or aluminum production) cannot be reconfigured to use electrical power. Moreover, fossil fuels are required to produce electricity as well, both to meet demand and to compensate for the intermittency of renewable energy systems such as wind or solar power. Is there really a scalable, low-carbon alternative?
One promising approach is artificial photosynthesis, which uses non-biological materials to produce fuels directly from sunlight. The sun is a nearly inexhaustible energy source, while energy stored in the form of chemical bonds – like those found in fossil fuels – is accessible, efficient, and convenient. Artificial photosynthesis combines these features in a viable technology that promises energy security, environmental sustainability, and economic stability.
While natural photosynthesis provides a complex, elegant blueprint for the production of chemical fuels from sunlight, it has significant performance limitations. Only about one-tenth of the sun’s peak energy is used; annualized net energy-conversion efficiencies are less than 1%; significant amounts of energy are expended internally to regenerate and maintain the exquisite molecular machinery of photosynthesis; and the energy is stored in chemical fuels that are incompatible with existing energy systems.
However, artificial photosynthesis, inspired by its natural variant, has demonstrated a potential for far superior performance, and provides energy in a form that can be used in our current energy infrastructure. Moreover, a fully artificial system would not require arable land or potable water, nor would it force a choice between food and fuel production in land use.
Existing energy technologies already can be combined to generate chemical fuels efficiently, though indirectly, from sunlight, but not yet in a configuration that is simultaneously practical, scalable, and economically feasible. Likewise, the overall efficiency of a fully integrated sunlight-to-fuel energy-conversion system can be more than ten times greater than the most energy-efficient biological systems, but the capital costs are too high for commercial deployment. Researchers’ top priority must therefore be to develop a solar-fuel generator that combines cost-effective scalability with robustness and efficiency.
The key to creating such a system lies in using earth-abundant materials that can perform the essential functions of absorbing light and facilitating fuel-forming chemical reactions. Just as chlorophyll serves to absorb light in natural photosynthesis, suitable materials are needed to capture and convert sunlight in artificial systems. Although silicon’s light-absorbing properties are suitable for photovoltaic devices, the near-0.5 volts that it generates is too weak to split water in a solar-fuel generator.
An artificial system also requires catalysts to facilitate the efficient production of chemical fuels. These catalysts must be highly active, stable, and, for global scalability, composed of earth-abundant elements, such as iron, nickel, or cobalt, not the scarce metals now used, such as ruthenium or iridium.
In addition, the system components must be integrated in a manner that ensures that they all function optimally under a common set of operating conditions. A deployable system must also incorporate cost-effective architectures, manufacturing processes, and installation methods.
Most important, such systems must work safely. In most implementations of artificial photosynthesis, energy-rich fuels are co-produced with oxygen, resulting in dangerous explosive mixtures. Membranes, or other physical and chemical barriers, must be developed in order to isolate the products from one another in a reliable fashion. Such partitions would also eliminate the need for complex peripheral processing equipment that would be necessary to separate the products prior to use in most applications.
So, what would an artificial photosynthetic system look like? The template is not a solar panel connected to an electrolysis unit, but rather a thin roll of sandwiched plastic-like layers, much like the high-performance fabrics found in rain jackets, that can be unfurled as needed. The top material would absorb the water and carbon dioxide from the air, and the next, light-absorbing layer would harness the sun’s energy to produce the fuel. Separated by the membrane, the fuel would not be vented to the air but instead would wick out through the bottom of the material into a collector tank for use on demand in our existing energy-supply infrastructure.
Ideally, solar-fuel generation should offer flexibility in the types of chemical fuels that can be produced from sunlight. In its simplest incarnation, water is split into hydrogen and oxygen gases. The hydrogen could be converted into a liquid fuel by upgrading biofuels, for example, or it could be reacted with carbon dioxide from flue gas or otherwise processed to produce liquid fuels for use in transportation applications. Alternatively, catalysts, as in natural photosynthetic systems, could directly reduce carbon dioxide, in this case to methanol or methane. The most effective systems would be able to offer either gaseous or liquid fuels.
Recent advances in nanoscience, materials science, chemistry, and physics have provided the tools needed to make rapid progress in this field. The ultimate prize is a clean-energy technology that is within reach and that could provide the basis for a safe, secure, and sustainable energy future.