Liquid Fuels from Carbon and Hydrogen

Most technology development of alternative fuels production focuses on photosynthetic routes due to the endless supply of light and straightforward extraction of energy from it. However, there is also a wide range of processes and products for non-photosynthetic pathways in renewable fuel production. A class of these, electrofuels, can utilize an extensive range of microorganism and energy sources to produce variety of fuel types. Hawkins et al. (2013) discussed the key process and elements of carbon fixation for electrofuel production. The current blueprint of electrofuel projects focuses on autotrophic microorganisms using carbon fixation pathways to consume CO2 directly for the production of energy-dense liquid fuels. Electrofuel production requires complementary expertise of multiple fields such as synthetic biology, metabolic engineering, and microbiology to produce the desire type of fuel from the CO2-fixing species. Detailed biochemical characterization of each autotrophic species with cycle type, target product, and enzyme varieties can help to improve the efficiency of fuel production.  Chieh-Hsin Chen

Hawkins, A.S., McTernan, P.M., Lian, H., Kelly, R.M., Adams, M.W., 2013. Biological conversion of carbon dioxide and hydrogen into liquid fuels and industrial chemicals. Current opinion in biotechnology 24, 376−384

                  Electrofuel production is the process of using non-photosynthetic microorganisms to convert CO2 directly to energy-dense fuel. There are multiple sources of the electrons needed to power electrofuel process, including H2, formate, carbon monoxide, and electricity. In the paper, the authors focus on the use of hydrogen gas as source of reducing power for CO2 fixation, with hydrogen acting as a reducing agent for electron carriers and production of pyruvate. Though both aerobic and anaerobic process the microorganisms produce storable fuel.
                  There are currently six different biological pathways occurs spontaneously for carbon fixation. Each pathway has unique features arising from its molecular and biological context, and they differ in efficiency. One of the most common types of CO2-fixation is Calvin-Benson-Bassham (CBB) cycle found in plants, algae, and many bacteria. For example, Ralstonia eutropha is a metabolically diverse autotrophic bacterium that can grow on CO2 and H2; it is able to store excess carbon as polyhydroxyalkanoates (PHA). Wood-Ljundahl (W-L), 3-hudroxypropionate (3HP), 4-hydroxybutyrate (4HB), reductive tricarboxylic acid(rTCA) and dicarboxylates (DC) are also various pathways of CO2-fixation. One important characteristic to the various CO2-fixation pathways is the tolerance of the enzyme and redox carrier molecules to oxygen. Some pathways are found in anaerobic organisms, and these pathways utilize oxygen-sensitive reduced ferredoxins as electron carriers.
                  The efficiency of CO2-fixation depends on multiple factors. On average, it is found that W-L pathway and rTCA cycle were the most energy efficient routes, while CBB comes in as least efficient. One of the analyses of carbon fixation reaction is examining the ATP requirement and the reduction potential of electron carriers. For example, ferredoxin has much lower reduction potential than NADPH, but the extra energetic contribution from the difference is insignificant compared to the large difference in ATP cost for pyruvate formation between cycles. Another analysis looked into the individual reactions in carbon fixation pathways. There are many half-reactions in the process of carbon fixation, and some of the processes like carboxylation and carboxyl reduction reaction are energetically unfavorable. To increase the efficiency and reduce ATP requirements, some pathways manage to couple unfavorable reactions to exergonic reaction other than ATP hydrolysis. One of the faster ways to speed up a reaction is the use of enzymes; combining enzymes in novel ways may improve the efficiency of the pathway or reduce the energetic cost. The conversion of CO2and hydrogen to electrofuel is also dependent on the use of enzymes, carbonic anhydrase and hydrogenase.
                  Hydrogenase catalyzes the reversible conversion of molecular hydrogen and protons in the presence of electron carrier. Based on the molecular structure, hydrogenase can be categorized: [NiFe]-hydrogenase, [FeFe]-hydrogenase, [Fe]-hydrogenase. Different microorganisms appear to utilize different type; [NiFe]-hydrogenase is widespread among bacteria and archaea, [FeFe]-hydrogenase is found in anaerobic bacteric and eukaryote, and [Fe]-hydrogenase is found only in certain archaea. In CBB cycle hydrogenase helps catalyzing the oxidation of hydrogen for the reduction of NADP+, where in 3Hp and 4HB cycle hydrogenase catalyze the oxidation of reduced ferredoxin and generates an in gradiet that is used for ATP synthase. Other than the use of hydrogenase, enzyme that enhances the concentration of CO2 also plays a huge role in carbon fixation.
                  Any fuel or organic product from CO2 that relies on carbon fixation depends on the concentration of CO2 in the environment. Thus the mechanism of increasing the concentration of CO2 has developed to compensate the low ambient CO2 concentration. CBB cycles utilizing RubisCO, 2 CO2-fixing enzyme that has low attraction for CO2 and does not differentiate well between CO2 and the competing substrate O2, especially need this distinct mechanism to avoid unfavorable lost of carbon in photorespiration. Carbon concentrating mechanisms have only been described for CBB cycle because 3HP/4HB uses bicarbonate (HCO3) instead of directly using CO2. However, exploitation of carbon fixing enzyme for microbial eletrofuels can improve the engineering of enzyme.

                  The key to carbon fixation is the electrofuel-processing microorganism. The host development requires genetic engineer as well as knowledge of gene regulation and metabolism in the target host cell. Electrofuel is promising relatively to poor efficient photosynthetic biofuels, but only a handful of electrofuel organism are developed or reported.

Improving Biofuel Cells

Biofuel cells transforming biological fuels such as ethanol or sugar into electricity are safe and ecofriendly source of power. However, biofuel cells are often limited to low voltage and are insufficient to provide necessary power for daily use. Like other traditional electrolyte batteries, stacking up the biofuel cells may boost their single cell voltage to an applicable level. Miyake et al. (2013) performed experiments on multiple ways to improve the voltage of biofuel cells utilizing fructose as the energy source. When stacking cells, each cell has to be isolated with proper wrapping to prevent short-circuits. The authors layered the biofuel cells with enzyme-modified carbon fabric strips and hydrogel sheets to ensure the ion-conduction between the anode and cathode fabric layers; the hydrogel sheet also served as a fuel tank. The modification effectively improved the performance of both bioanodes and biocathodes and maximizes cell power to 0.64 mW at 1.21V.  Chieh-Hsin Chen
Miyake, T., Haneda, K., Yoshino, S., Nishizawa, M., 2013. Flexible, layered biofuel cells. Biosensors and Bioelectronics 40, 45-49.

                  To increase the efficiency and the power of biofuel cells, the authors made three modifications to the cells; to prevent short-circuits due to ion-conduction. The first modification was the preparation carbon fabric anodes, where are multi-walled with carbon nanotubes to increase the reaction surface area; they are heated in 400 °C and immersed in multiple solutions such as D-fructose dehydrogenase to increase the efficiencies. The result of this modification is significant: it almost doubles the current density. In the first modification, the authors also found that the performance of cells is significantly affected by buffer concentration; buffer is added to stabilize the local pH level change caused by the oxidation process. With a stabilized pH level, enzymes in the carbon fabrics perform with the highest efficiency with a 0.5M buffer with maximum current produced at 0.6V of 15.8 mA.
                  The second modification is the gas-diffusion of carbon fabric cathodes. This process followed the process used for Biliruben oxidase (BOD) cathodes. BOD can catalyze the reaction of O2 to H2O without election transfer mediators. The cathode is also treated with heat and multiple solutions including BOD and a surface coat of carbon nanotube solution to make it hydrophobic. To test the effect, the electrode strip was put in an oxygenic pH 5 buffer testing the electric potential versus the current capacity. The performance of a BOD-modified strip reaches to about 1.9 mA cm-2; the additional carbon nanotube coating onto the BOD-modified cathode strip was enhanced to 2.9mA cm-2. The hydrophobic carbon nanotube coating controls the penetration of excess solution into the carbon fabric electrodes allowing the conduction to optimize. The authors also conclude that control of the buffer concentration may optimize the performance with maximum current of 4.6 mA cm-2 at 0V using a .25M buffer solution utilizing an oxygen supply from the ambient air through the carbon fiber.

                  The third modification is through double-network hydrogel films that contain fructose. This modification is prepared through a three-step process: first, the formation of one layer hydrogel film, then another layer of film, lastly with loading of 500 mM fructose. The hydrogel film is later treated with three stock solutions to secure the fructose solution in the film. Both cathodes and anodes went through the process of lamination with double-network hydrogel sheet; the lamination provides the cells with moisture, fuel sources, and buffering for the reaction. The cells are tested with 0.74V, which is about the electric potential difference between fructose oxidation and oxygen reduction. The performance of the biofuel cell is fairly good; it reached a maximum power density of 0.95 mW cm-2 at 0.36 V. However, the stability of the cell decreases drastically after a few hours due to drying of the hydrogel. More importantly, the authors found that bending the cell sheet into a cylinder effectively increases the performance of the cell. The laminated bent cell produced a maximum power of 0.64 mW at 1.21V, which is sufficient to light an LED unit. With these types of modification, we may expect a more powerful biofuel cell in the future.

Tracking Reusable Biofuel From Crops: Carbon and Solar Energy

Technology for production of biofuels has been a popular research area for many biochemists in the past ten years because finding alternatives to the use of fossil fuels can have important effects on the future of human life. Although the technology is not able to produce low cost, efficient biofuels yet, Borak et al. (2013) promote the possibilities and efficiency of biofuel production through crops. Their “PETRO approach” is used to evaluate the new crops, not only on the capture of solar energy but also the capture of carbon in atmosphere. The reverse of the carbon combustion cycle naturally occurs within plants, which use photosynthesis to convert carbon dioxide from the air to usable fuels. The energy source products from various crops are similar, but the efficiency of conversion of sunlight into energy varies. The authors summed up the total loss of energy during each process and organized data on the final energy content for each crop. They showed that using the PETRO approach for evaluating potential crops as biofuels can lead to more detail-based discussion in the scientific community.  Chieh-Hsin Chen
Borak, B., Ort, DR., Barbaum, JJ., 2013. Energy and carbon accounting to compare bioenergy crops. Current opinion in biotechnology 24, 369-375.

                  Because of the depletion of fossil fuels and the atmospheric increase of atmospheric carbon dioxide from combusting the fossil fuels, scientists are eager to find alternative fuels. The two main barriers to production of alternative fuels are the costs and shortage of potential stocks; thus the production of liquid fuels from crops has become one of the top environmental goals for future research. On a fundamental level, the concept of biofuel is replacing the process of mining with the process of agriculture; the process shift is significant because biomass has significantly higher carbon oxidation state than fossil fuels. The approach of biofuel essentially reversed the combustion of carbon-based fuels capturing the byproduct CO2 converting and storing as usable energy via carbon fixation utilizing plants and other terrestrial plant matter.
                  To further enhance the ability of the biofuel production, a detailed evaluation of efficiency of the crops and productivity of conversion of energy will be useful; however a systematic methodology for evaluation is currently lacking in the field. Different research groups studying in different geographical areas use a variety of non-comparative assumptions and approaches to calculation yields. The data consistency became one of the difficulties for further biofuel research. The authors introduced the Plants Engineered To Replace Oil (PETRO) approach that included the input of raw materials (sunlight, carbon dioxide and water), trace process of conversion by plants, and the output of liquid fuels. Photosynthesis reverses the combustion of fuels and stores the carbon energy along with solar energy in the plant; the production of biofuel extracts and concentrates carbon energy from plants, usually as ethanol, converting it into usable fuel.
                  Although plants have evolved effective photosynthetic pathway to capture light, they are not as efficient as we would like; the result of evolution does not aim for maximizing the benefits of wither for producing food or fuels. That C3 plants only utilize 4.6% of the solar energy and C4 plants utilize 6.0% suggests substantial room for improvement but even with these low capture levels of photons much of the captured energy is subsequently lost.
                  The authors looked into the difference in loss of carbon energy between C3and C4 plants in four levels: captured, harvested, purified, and processed. C3 plants lose half of their usable carbon energy in photorespiration and more in respiration. C4 plants lose less in respiration, but C4 plants lose more than half seasonally. Overall the final usable carbon energy in C3 plants is about 0.69% and 3.0% in C4 plants. But there are also some losses of carbon energy in the processing steps.

                  Although energy loss in the processing steps is small compared to the loss from photorespiration or seasonality, it is the first step that can be improved through conventional engineering. With careful genetic selection and engineering of crops, we will be ale to control the seasonality and other growth process of crops. The authors introduce data that show the differences in final fuel product of four common crops used in biofuels: maize, soybean, sugarcane, and switchgrass. The final energy content of the four different crops is very similar, but they have significant differences in overall fuel yield. Sugarcane contains the most overall fuel yield and soybean the least.