Scientists from the California Institute of Technology (Caltech), partnering with the Lawrence Berkeley National Lab (LBL) and with additional participation of SLAC National Accelerator Laboratory at Stanford University and the University of California campuses at Berkeley, Irvine, Santa Barbara, and San Diego have combined efforts to tackle global warming. The partnership, the Joint Center for Artificial Photosynthesis (JCAP), has recently won the US Department of Energy's (DOE) Solar Fuels Hub competition.
We often tell audiences that the amount of solar energy reaching the Earth in one hour would be adequate to displace the use of fossil fuels globally for a full year. The sun is a powerful source of energy, but mankind has continued to use its radiation almost uniquely for growing crops. Now, in an effort to forestall global warming and produce a secure supply of transportation fuels, researchers are working to develop processes similar to photosynthesis that harness sunlight to make fuels from carbon dioxide (CO2) and water. In fact, since 1999, transportation-related CO2 emissions have been the largest end-use source of energy-related CO2 in the US.1 Thus, carbon-neutral solar fuels would have a large impact on net carbon emissions.
Is such a fuel feasible? Twenty years ago, the answer would likely have been “no.” But science has advanced far since then, yielding new nanoscale materials and technologies, new understanding of chemical reactions, new diagnostic equipment, and new computational techniques. All this, together with recent developments at the institutions comprising the JCAP team, suggest that today the answer to the feasibility question might be “yes.”
The distinct element in the natural photosynthetic process is a light-harvesting apparatus that absorbs light and uses the energy to separate electric charge, creating currents and voltages that drive chemical reactions. These reactions involve catalysts that use electrical voltages to create the chemical conditions needed to crack ambient water molecules and form hydrogen and oxygen. Other catalysts reduce CO2 to form sugars. This is the chemical process that JCAP scientists will use as a model. The idea is to reduce CO2 to produce fuels instead of sugars. Components made of inorganic materials will be arranged in a membrane that allows the passage of protons and that separates the products to maintain charge neutrality and prevent recombination. This process is in the very early stages of development. Few inorganic light harvesters or catalysts, made of abundant and inexpensive materials, have been developed and they have certainly not been optimized to work together efficiently in a single system.
However, in the last two years, researchers at Caltech and LBL have demonstrated that several types of nanomaterials can act as light absorbers. Similarly, only a few man-made catalysts were known to facilitate water splitting, and those were not yet efficient or stable enough to be fielded in such a system. But in the past year, researchers at LBL discovered improved catalysts for water-splitting reactions that produce oxygen2,3 and hydrogen.4 These absorbers and catalysts are not yet optimized, and the necessary proton-conducting membrane is not yet available. However, research groups throughout the world, as well as in JCAP, are now working to develop these pieces of the system.
In January 2010, the DOE issued a funding-opportunity announcement for a research and development center—an Energy Innovation Hub—to act as accelerator and integrator in the community of solar-fuels researchers. In JCAP, the award of up to $122 million over five years will help to undertake a large, science-focused effort to discover and optimize catalysts and light absorbers using, among other tools, high-throughput technologies. For photocatalysis discovery, for example, JCAP plans to perform 1 million experiments per day. Similar high-throughput rates are planned to explore semiconductor-based nanophotovoltaic objects that will perform light-absorber and charge-separator roles. Other research will focus on membranes and methods of assembling components into a device.
Figure 1. This image of an artificial photosynthetic fuel farm is compatible with the Joint Center for Artificial Photosynthesis (JCAP) vision. Current research is performed at the nanometer (nm) scale. The JCAP vision is to build prototypes on the centimeter (cm) scale within five years.
The aim of these efforts is to yield a nanoscaled, artificial photosynthetic device. JCAP will then develop cost-effective methods for orienting, assembling, and interconnecting nanoscale functional assemblies into macroscale, fully functional materials and systems. Over the course of the program, JCAP plans to scale from nanometer to meter in under 10 years (see Figure 1). This will involve development of full hardware-based, macroscale prototypes that capture all critical length scales and phenomena of importance for device operation. Among these are fluid flows, feedstock input and output streams, optical input paths, mechanical system properties, and physical form factors, all of which will help to make a set of prototype solar-fuel generators.
Figure 2. JCAP sees its role as a hub both as providing a service to the community and representing a unique force for accelerating the development of solar fuels. Blue squares indicate roles that JCAP will play. PEC: Photo-electrochemical cell. SERC: Sustainable Energy Research Center. EFRC: Energy Frontier Research Center.
JCAP intends to play a significant role in bringing the existing research community together. It will preferentially undertake projects that can only be done in a large research organization or that greatly accelerate the rate of progress in bridging knowledge gaps relative to what could be done in an individual research group or by a small consortium of research groups. The great body of research and large pool of ideas that the smaller research groups produce will be brought to JCAP for benchmarking and fed into its high-throughput systems to produce optimized results (see Figure 2). The Center will perform other community roles, such as producing annual reviews and arranging courses for graduate students, to foster communications with and between researchers, acting as a force to move this research to market as fast as possible.
California Institute of Technology
Nathan Lewis is the principal investigator of JCAP. He received his PhD in chemistry from the Massachusetts Institute of Technology and is currently the George L. Argyros Professor of Chemistry.