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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
4
Robust Implementation of Bioinspired Chemistry for Energy
During this session of the workshop, Thomas Moore, G. Tayhas Palmore, and Mark Emptage presented examples of how bioinspired chemistry for energy is being implemented within their organizations. These projects utilize some of the key fundamentals described in Chapter 3 to present the big picture of how bioinspired chemistry can improve the energy field.
MIMICKING BIOLOGICAL ENERGY TRANSDUCTION
Thomas Moore of Arizona State University began by discussing how nature has adapted for 3.4 billion years through fierce evolutionary competition. He said that researchers need to follow nature’s example, but accept the fact that in meeting human energy needs there may be ways to improve on nature. He defined the purpose of the workshop as a way to explore how to improve the natural process of photosynthesis and incorporate the kinetically and thermodynamically successful features in human-engineered constructs.
Moore compared the technological branch of solar energy conversion, essentially photovoltaics, with the biological branch. He explained how a standard fuel cell that operates on oxygen and hydrogen produces water and electromotive force. A typical human-engineered fuel cell operates at 50-60 percent power conversion efficiency and uses platinum or other noble metals as catalysts.
Moore then explained how mitochondria are biological fuel cells. The oxygen reduction taking place in a mitochondrion is exactly the same as in a standard fuel cell. Using several enzymes and only earth-abundant elements, the mitochondrion converts electrochemical potential to biochemical work with efficiency greater than 90 percent. This is a steady-state process in which protons are pumped across the membrane to maintain its electrical potential. If the membrane potential drops by a factor of two, the cell dies. The high efficiency of energy transduction by mitochondria is the fundamental difference between standard fuel cells and mitochondria (Figure 4.1).
Moore talked about the opportunity to produce electricity with photovoltaics, but noted that a really important advance would be to use sustainably produced electricity to produce fuels for the transportation sector. Nature has the catalysts that direct reactions such as the reduction of CO2 to reduced, energy-rich carbon compounds. These catalysts operate at room temperature and yield essentially pure products. The problem is that these natural catalysts do not “recognize” electromotive force and cannot use it efficiently to drive synthetic reactions. The research challenge is to focus on effectively “wiring” enzymes into sources of electromotive force. He said that discovery is a key to achieving sustainable energy production and use. Arizona State University, for example, has a large metagenomics program in which researchers travel to various locations looking for microorganisms with catalysts that can carry out useful reactions.
Moore also compared water splitting by photosynthetic systems with a human engineered system consisting of three PV cells operating in series driving a commercially available electrolyzer (Figure 4.2). Series operation is necessary for Si-based PV cells to provide the voltage necessary to oxidize water and reduce protons to hydrogen in the electrolyzer. He pointed out that photosynthesis uses two photons per electron and has a threshold for absorption of light at about 700 nm. The PV cells need three photons per electron but, because their threshold is about 1100 nm, which means they gather about twice as many of the available photons, they are probably more efficient at water oxidation than photosynthesis.
Moore highlight ed several projects at Arizona State University that are using biological and bioinspired chemistry for energy discoveries.
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
FIGURE 4.1 Comparison of mitochondria and engineered fuel cells.
SOURCE: Presented by Thomas Moore, Arizona State University.
FIGURE 4.2 Comparison of photosynthesis and a silicon-based photovoltaic system.
SOURCE: Presented by Thomas Moore, Arizona State University.
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
Biodiesel to Fuel a Large Power Plant. Researchers at ASU’s Center for Bioenergy and Photosynthesis have calculated that a 25 × 25 km field of bioreactors using cyanobacteria to fix carbon could uptake all of the carbon dioxide produced by a 1.6 GW power plant and subsequently provide the biomass as lipid to fuel the power plant. The parameters necessary to achieve this goal are a seven percent power conversion efficiency for photosynthesis, 40 percent conversion efficiency of biomass to fuel, 50 percent conversion efficiency of fuel to electricity, and 80 percent conversion efficiency of land area covered by the bioreactors. This system would then be carbon neutral in operation and produce about 1.6 GW of electrical power. The key to making this feasible is to achieve a seven percent power conversion efficiency for cyanobacteria. Moore noted that the area required to produce a specified amount of energy scales directly with the energy conversion efficiency of the system or device.
Tubes in the Desert. This project offers one approach to the design of bioreactors to produce biodiesel using the photosynthetic capabilities of microalgae or cyanobacteria. Solar energy conversion by photosynthetic organisms offers the almost inestimable advantage of self assembly, self replication and self repair of the key photoconverter. Cyanobacteria in bioreactors do not require arable land, which, Moore says, avoids the conflict of using agricultural land for energy production. The bacteria grows quickly year-round with a doubling time of 0.5-1.0 day. The reactors do not use a large amount of water, which is good for dry locations, such as Arizona. There is a high output of biomass: The practical range is 40-100 tons/ha/yr dry biomass and the optimal level is 500 tons/ha/yr. The output can be a high-value liquid fuel created with genetic and engineering controls. The process is not cellulosic or lignocellulosic, explained Moore.
Hydrogenase on a Carbon Electrode. Michael Graetzel designed a system that permits nanoparticulate material to absorb light using a dye. After it absorbs the light, the dye injects electrons into a conduction band. This creates electromotive force through an interface between a molecular system and a semiconductor. This experiment involved a Graetzel-type photo anode being sensitized by a porphyrin that upon excitation puts an electron into a wire that connects to a cathode. If the cathodic reaction is positive and there is a good catalyst, Moore explained that electromotive forces will be produced. But if the cathodic reaction is negative in electrochemical potential, such as proton reduction, energy-rich fuel is produced. After the electron injection, a radical cation of the dye is left. The cation then oxidizes a coenzyme, NADH, for which there are many different dehydrogenase enzymes that can oxidize many different bioorganic materials and reduce the NAD. NAD is thus a mediator or carrier of electrons between the bioorganic material and the photoanode. This type of electrochemical cell is nonregenerative, providing the opportunity to produce reduced material. By using a platinum carbon cathode, hydrogen can be produced. Essentially, every two electrons produce one hydrogen from this process. In collaboration with a group at the National Renewable Energy Laboratory (NREL), scientists at Arizona State University have been putting hydrogenase enzymes on a carbon electrode to successfully produce hydrogen. Hydrogenase enzymes use only iron and nickel metals to carry out the catalytic process.
Failed Water Oxidation Experiment. Although direct water oxidation using an artificial photosynthetic device is not possible because the necessary catalyst does not yet exist, Moore is developing an approach to “push the envelope”. His group is working to create a hybrid system that would use a naturally occurring photosystem II to oxidize water. Moore explained that the anticipated electron flow would be from illuminated PS II particles, oxidizing water, to the photoanode, where the process described above would produce hydrogen. Thus two photons would be used to move an electron from the redox level of water to the level of hydrogen—much in the same way that natural photosynthesis uses two photons to move an electron over roughly the same redox span. The output of photosystem II is a reduced quinone, a quinol. The quinols carrying the electrons from water would then be the input into the artificial system to produce hydrogen. Moore noted that the aim was to create a hybrid system to explore ways to couple two photosystems together and to investigate the thermodynamic and kinetic constraints involved. Unfortunately, this experiment has not worked to date.
Moore explained that the experiment failed because using the quinols as electron donors to the photoanode recombination limited the output potential to values that were not negative enough to reduce protons to hydrogen. When NADH is the electron donor, photo currents for hydrogen appear. When NADH is oxidized, a radical cation with a pKa of −4 remains. This cation immediately drops the proton off, generating a species that donates a second electron. There is no recombination reaction and no recombination current from the mediator to the anode with that mediator. When a quinone mediator is used, the first electron is removed from the quinone and the radical species formed, and various semiquinones are in the electrochemical region where recombination reactions from the electrode can occur. The rate of this recombination limits the potential the cathode can reach. Moore talked about trying to use an electrode with a blocking layer to solve the problem. He concluded that the experiment raised some important questions about efficiency, redox span, and the limitations that must be overcome in wiring enzymes to electrical circuits. While nature obviously “got it right” for nature’s purposes, there is room for improvement when using bioinspired processes to meet human energy needs.
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
BIOFUEL CELLS
G. Tayhas Palmore of Brown University presented a diagram of the four components of energy conversion: biomimetics, purified or engineered enzymes, synthetic biology, and bacterial isolates. Palmore gave a brief tutorial on how fuel cells work (Figure 4.3). She explained how they convert chemical energy into electrical energy by separating the oxidation and reduction reactions into two separate chambers using a membrane. The anode is the electrode where hydrogen is oxidized to protons. The oxidation of hydrogen releases electrons drives an electrical load, which is coupled with the movement of ions through the membrane that drives the reduction of oxygen to water. For a hydrogen-oxygen fuel cell, the open circuit voltage is 1.2 volts. The three-phase interface between a catalyst breaks the bonds. A current collector then takes the electrons that are released from those bonds away from the active site. Finally, said Palmore, an ion conductor moves the protons or ions released away from the catalytic site to repeat the process.
Palmore also explained that the biomimetic approach uses inspiration from biology to develop new chemistry to perform catalysis. Palmore focused on using biology to do the energy conversion, both in the free form as purified enzyme extract and where new organisms have been isolated and coupled with electrode surfaces (Figure 4.4).
According to Palmore, the following parameters are important for both bio- and conventional fuel cells:
Fuels, such as methanol, ethanol, glucose, and higher-level organics, can also be considered since they all have potentials similar to hydrogen. The fuel will depend on
FIGURE 4.3 A fuel cell converts chemical energy into electrical energy. H2/O2: open circuit voltage ~1.2 V.
SOURCE: Image courtesy of G. Tayhas Palmore, Brown University.
what is used in the anode and what is used in the cathode, which is oxygen.
The current density is also important and depends on the rate of catalysis and the movement of ions across the membrane.
It is important to consider pH, temperature, and pressure since conventional fuel cells use catalysts that function at very low pH and high temperatures.
Palmore continued her talk with some examples of research that has been done in the field.
Itamar Willner and Eugenii Katz’s Glucose Fuel. Wilner and Katz (Hebrew University of Jerusalem) used an enzyme-based biofuel cell that had direct contact (i.e., tethered to an electrode) to produce glucose as a fuel.1 The experiment showed the power of chemical control in organizing the assembly on the surface. Wilner and Katz took advantage of modified thiol-gold chemistry, a PQQ mediator with the thiol, and added a reactive carboxylic acid that could be reacted with the FAD cofactor enzyme. Using a cathode reaction, they also demonstrated a truncated peroxidase that was tethered to the surface. They demonstrated that open-circuit voltage could reach approximately 140 microamps per centimeter squared. The maximum power was at about 32 microwatts. The voltage was low because peroxide was used as the oxidant instead of oxygen.
Adam Heller’s Glucose Fuel. Another example of enzyme-based glucose oxidation was done by Heller’s group (University of Texas at Austin).2 They worked with redoxactive polymers, which are molecular wires. Heller’s team used the osmium-bisbipyridine complexes and tuned them with their ligands to adjust the potential relative to an anodic fuel like glucose or a cathodic fuel like oxygen. The complexes were then immobilized with the enzymes to catalyze the conversion of glucose to gluconolactone and oxygen to water. The system demonstrated the following:
It is a membrane system, physically separating the anodic and cathodic reactions.
It survived about seven days before the current began to diminish.
Palmore pointed out how difficult it is to reduce oxygen to water, and explained that it can be done using a number of enzymes in the copper oxidase family. She has used laccase because it is a monomeric protein but is unlike dehydrogenases that depend on a cofactor and a binding site. Laccase has a redox center of four copper ions that are reduced from
1
I. Willner, E. Katz, F. Patolsky, and A. F. Buckmann. 1998. Journal of the Chemical Society, Perkin Transactions 2, 1817-1822.
2
N. Mano, F. Mao, and A. Heller. 2003. Journal of the American Chemical Society 125: 6588.
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
FIGURE 4.4 Microbial and Enzymatic Biofuel Cells.
SOURCE: Image courtesy of G. Tayhas Palmore, Brown University.
copper (II) to copper (I) with subsequent binding of oxygen to reduce it to water.
Palmore presented some recent examples of simple chemistry done on the surface of proteins that can have a tremendous effect on their overall stability. Jungbae Kim and Jay Grate of Pacific Northwest National Laboratory used simple chemistry to modify lysine residues on enzyme substrates and then tethered them to siloxane, allowing it to gel and harden. This example combines chemistry, nanotechnology, and biotechnology, and shows how chemists can affect how we look at catalysts.
Palmore also talked about examples of microbial systems. Derek Lovley’s laboratory at University of Massachusetts, Amherst, is looking at doing electrochemistry in ocean or bay sediments. New organisms are being discovered through their research and the use of a microbial fuel cell in which carbohydrates are taken to carbon dioxide using Rhodoferax ferrireducens have been demonstrated.
Palmore concluded by highlighting her wish list for future research, which is detailed in chapter 6.
BIOFUELS FROM BIOMASS INITIATIVES AT DUPONT
Mark Emptage of DuPont stated that biomass is the only carbon-based renewable resource for fuel and that there really needs to be a focus on biofuels. However, he believes there is a limit to grain ethanol in the United States, and to produce more ethanol, cellulosics need to be used. Emptage explained that there are many ways of converting biomass to a variety of energy needs. He focused his discussion on hydrolysis of biomass to fermentable sugars and converting the sugars into biofuels. Emptage focused on ethanol, but explained that the technology developed for ethanol can be applied to other fuels as well. He believes biorefining infrastructure should be an add-on to current infrastructure. Next, Emptage discussed the by-products of producing ethanol: The lignin and biomass that result from separating out ethanol can be used as fuel for the entire facility, and the glucose can be used for products other than fuel.
Emptage listed DuPont’s guiding principles for cellulosic ethanol process development:
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Bioinspired Chemistry for Energy: A Workshop Summary to the Chemical Sciences Roundtable
Keep it simple with minimal unit operations and separations and minimal capital investment.
Target a single agricultural residue at the beginning (corn stover is the largest source of agricultural residues in the United States).
Maximize titer.
Life-cycle assessment—need to understand overall process to make it sustainable.
Process integration—need to have high efficiency to fit all the pieces together.
Risk mitigation—take small steps within the current infrastructure.
Emptage explained the steps involved in converting sugar to ethanol: milling, pretreatments, saccharification, conversion of biomass to fermentable sugars, fermentation, and separation. He said that the key technology is fermentation since it can be applied to butanol or other products. He also explained exactly how corn is harvested and that the two main sugars that can be extracted from biomass are glucan and xylene.
Emptage explained that Dupont is taking a new approach using Zymomonas mobilis, a natural ethanol producer found in the sap of agave plants in the tropics. It has a higher yield and productivity than yeast and has the potential to be a better organism than yeast. Emptage announced a collaboration between DuPont and POET, the largest dry-grind producer of ethanol in the United States with over a billion-gallon ethanol capacity. They are working together to develop a pilot plant in South Dakota using the new technology. He said, “This isn’t a revolutionary program. This is really an evolutionary program, just adding on to the current infrastructure.”
Emptage believes that the key remaining challenges are solids handling, having an infrastructure to collect, transport, and store biomass effectively and efficiently with its low-bulk density. Another challenge is the cost of enzymes. The goal is to make the handling of the Z. mobilis derived biomass cost competitive with grain ethanol.
DISCUSSION
Michael Wasielewski of Northwestern University asked Thomas Moore about the type of light fluxes being used to investigate the solar flux. He also asked, “Since we all know that photosynthesis has control mechanisms that actually modify electron flow, based on light flux, what kind of prospectus or perspective do we have for control mechanisms in such systems?” Moore explained that one of the factors that seems to limit natural photosynthesis is the diffusion of carbon dioxide into the system for fixing, so it is important in photosynthesis to throttle back the powerful oxidant when carbon dioxide is limiting. There is a control mechanism called nonphotochemical quenching that is related to the xanthophyl. Moore’s team is working on a model system that responds to high light and quenches excited states releasing the energy as heat, and then as the light intensity comes back down again, the system shuts itself off. He also explained that the system needs to respond to the membrane potential and pH gradient across membranes. This can be done with a potential sensitive sensor.
R. David Britt of the University of California, Davis, asked about the limits to purely biological approaches. Thomas Moore said he thinks natural photosynthesis needs to be reengineered to double or triple its power of conversion efficiency. He said that solar will ultimately solve the problem. Moore then called for research focused on fuels by photosynthesis created by cyanobacteria grown on nonarable land and photovoltaics for electricity.
Douglas Ray from Pacific Northwest Laboratories asked G. Tayhas Palmore whether the process of engineering enzymes needs to be improved. Palmore said that not much is known about engineering proteins but she hopes that it can be solved using a Brown University database. Mark Emptage said that DuPont has worked with Diversa (non Verenium Corp.), which has put together a set of technologies to allow high-throughput screening and enzyme evolution to be done. He said that there is still a need for more basic understanding about how the enzymes operate on the complex structures.
Charles Dismukes of Princeton asked Mark Emptage how DuPont plans on solving the two major problems that he said need to be addressed: costs for removal of the ethanol distillation and acetic acid inhibition. To solve the first problem, Emptage explained that consolidated bioprocessing will be necessary. That technology has not yet been developed, so it is important to figure out what to do in the near term. DuPont has looked into thermopiles, but they have very low ethanol yields. DuPont is now seeking an organism that maximizes yield, which is why they chose to work with Zymomonas. Emptage explained that one way to solve the acetic acid problem is to adjust fermentation conditions to the highest pH level tolerable. DuPont has developed more acetate-tolerant strains. In DuPont’s process with ammonia, acetamide is a by-product with ammonolysis competing with hydrolysis of the acetyl groups, which lowers the total concentration of acetic acid in the process.
Douglas Ray asked whether biobased approaches are going to scale. Emptage said that scaling in fermentation is straightforward, and that tanks can be scaled almost as large as sugar and fermentative organisms. Daniel Nocera of Massachusetts Institute of Technology stated that he is worried about the long-term scaling issue for energy, which is why he supports solar. However, advances in solar energy involve discovery research that is 50 years out. Nocera went on to say that the energy problem is a basic science problem, not an engineering problem, and people should stop focusing on the complex engineering to find a solution.