The use of renewable low-cost carbohydrate as a high-density hydrogen carrier would solve the largest obstacle to the hydrogen economy - hydrogen storage. The integration of on-board biotransformation for hydrogen generation, hydrogen PEM fuel cells, and an electric motor leads to an ultra-high energy efficiency power train system for personal vehicles, call sugar fuel cell vehicle (SFCV). Through it, a small fraction of the U.S. biomass resource (i.e., ~600 million metric tons of dry biomass per year) would be sufficient to meet 100% of the U.S.'s light-duty transportation fuel needs.
Before we achieve this ultimate goal, we must solve several obstacles. In short terms, we are develoing several applications, such as low-cost biohydrogenation for synthesis of chiral compounds for the pharmaceutical industry, high energy-density biodegradable sugar batteries for electronics, satellite hydrogen generation stations for hydrogen fuel cell vehicles, and distributed electricity generators.
Transportation Fuel Challenges and Needs
Currently, transportation accounts for about 20% of global energy consumption. Potential solutions for the future transportation sector are few, because of some special constraints for the transportation sector, such as high power density and high energy storage capacity in a small volume; fast fuel refilling; affordable costs of fuels; affordable vehicles; costs for rebuilding/updating infrastructure; concerns of safety, environment, scalability, and society. It is widely-believed that energy used for the transportation sector will be shifted to hydrogen/electricity because of diverse and renewable primary energy sources for generation of hydrogen/electricity, high energy conversion efficiency, and low pollutants omitted1,2.
Liquid fuels featuring high energy density along with small-size and low-cost internal combustion engines (ICE) are providing a convenient, affordable transportation solution. However, soaring fuel prices, ebbing fuel reserves, accumulating greenhouse gases, and national security needs lead us to seek alternative renewable fuels. Available or predictable biomass resources for producing liquid biofuels solve only a fraction of transportation fuel needs (e.g., 30% in year 2030) because of the low energy conversion efficiencies in ICE.
Batteries are an electricity storage medium that can serve our transport needs. However, several factors, such as very low energy storage density (e.g., only 1 to 5% of that of liquid fuels), slow recharging, high costs, concerns about safety, waste treatment, and lithium resource, raise some doubts about the potential impact of battery electric vehicle (BEV). Some experts believe that BEV may represent a small fraction of the passenger vehicle market.
Hydrogen is a clean energy carrier with much higher energy storage density than batteries, but its storage is No. 1 challenge. In order to dramatize and incentivize hydrogen research, the H-Prize has been established to competitively award cash prizes that will advance the commercial application of hydrogen energy technologies. The 2009-11 H-prize will be awarded in the area of storage materials in mobile systems for light-duty vehicles. But the hydrogen storage materials that can meet the requirements of density, pressure, temperature, charging kinetics, discharge kinetics, cycle life, and safety are not available, to our knowledge.
Here we suggest an out-of-the-box solution - use of renewable biomass carbohydrates as a high-density hydrogen carrier. This new solution can efficiently address the above challenges for the transportation sector. Here we present the recent advances in cell-free synthetic pathway biotransformation (SyPaB), the roadmap of SyPaB from high-end to low-end applications, and its potential impacts.
A Sweet Solution to the Hydrogen Economy - Sugar as High H Carrier
Cellulosic biomass is the most abundant renewable biological resource (ca. 1 x 1011 tons/year)3. Biomass is produced locally, and is more evenly distributed than are fossil fuels. Each year, the overall chemical energy stored in biomass by terrestrial plants is approximately 6-7 times the total human energy consumption. Also, renewable carbohydrates (e.g., cellulosic materials and starch) are less expensive based on energy content than are other hydrogen carriers, such as hydrocarbons, biodiesel, methanol, ethanol, and ammonia1. The use of a small fraction of low-cost renewable biomass for producing transportation fuels (e.g., cellulosic ethanol and hydrogen) provides benefits to the environment, economy, and national security3.
Cell-free SyPaB, a new synthetic biology technology, can implement complicated biochemical reaction networks by assembling a number of purified enzymes and coenzymes4,5. As compared to complexity in vivo living biological systems, SyPaB may be regarded as the Alexander Sword that can cut the Gordian knot due to its engineering flexibility.
Non-natural synthetic pathways have been designed to split water by using the chemical energy in carbohydrates and waste thermal energy from the environment6,7 (Eq. 1).
C6H10O5 (aq) + 7 H2O (l) Ë 12 H2 (g) + 6 CO2 (g) - 
These non-natural synthetic catabolic pathways are comprised of numerous enzymes in one pot (Fig. 1).
Figure 1. Non-natural synthetic enzymatic pathway for high-yield generation of hydrogen from carbohydrate.
The pathway contains four biocatalytic modules:
- a chain-shortening phosphorylation reaction for producing glucose-1-phosphate (G-1-P) catalyzed by glucan phosphorylase (Eq. 2);
- generation of glucose-6-phosphate (G-6-P) from G-1-P catalyzed by phosphoglucomutase (Eq. 3);
- generation of 12 NADPH from G-6-P through a pentose phosphate pathway (Eq. 4); and
- generation of hydrogen from NADPH catalyzed by hydrogenase (Eq. 5).
(C6H10O5)n + Pi Ì (C6H10O5)n-1 + G-1-P - 
G-1-P Ì G-6-P - 
G-6-P + 12 NADP+ + 7 H2O Ì 12 NADPH + 12 H+ + 6 CO2 + Pi - 
12 NADPH + 12 H+ Ì 12 H2 +12 NADP+ - 
We propose the use of carbohydrate as a high-density hydrogen carrier2. Gravimetric density of polysaccharides is 14.8 H2 mass% where water can be recycled from PEM fuel cells or 8.33% H2 mass% without water recycling; volumetric densities of polysaccharides are >100 kg of H2/m3. Biotransformation of carbohydrates to hydrogen by cell-free synthetic (enzymatic) pathway biotransformation (SyPaB) has numerous advantages, such as high product yield (12 H2/glucose unit), 100% selectivity, high energy conversion efficiency (122%, based on combustion energy), high-purity hydrogen generated, mild reaction conditions, low-cost of bioreactor, few safety concerns, and nearly no toxicity hazards. Therefore, we suggest constructing the hydrogen economy based on renewable carbohydrate, which can be produced from natural solar cells - plants (Fig. 2).
Figure 2. Scheme of the hydrogen economy based on a renewable high-density hydrogen carrier - carbohydrate.
Applications and Roadmap of SyPaB
These enzymatic sugar-to-hydrogen reactions have numerous potential applications from biohydrogenation for the synthesis of chiral compounds, to local hydrogen generation stations, to electricity generators, to sugar batteries, as well as sugar-fuel cell vehicles, as shown in Fig. 32.
Figure 3. Applications and roadmap for SyPaB.
For the synthesis of chiral compounds by using reducing coenzymes (NAD(P)H) and oxidoreductases, this technology is the lowest-cost technology for providing reduced coenzymes (NAD(P)H) for the pharmaceutical industry (Fig. 3). As compared to available commercial enzymatic technologies, this new technology has the lowest substrate costs, has a simple reactant removal (CO2), and is based on renewable feedstock - sugars.
The production of hydrogen based on local renewable resources is a valuable alternative for supplying hydrogen to local end users - hydrogen fuel cell vehicles or other applications. Local satellite hydrogen generation stations would produce hydrogen based on this sugar-to-hydrogen approach, store the hydrogen in tanks, and refill hydrogen-fuel cell vehicles. The solid sugar powders produced locally will be easily collected and distributed, based on available solid goods delivery systems.
Figure 4a shows that an integration of this sugar-to-hydrogen system with PEM fuel cells could produce low-cost electricity from carbohydrate, especially for electricity generators and battery rechargers. The products (hydrogen and carbon dioxide) bubbles up from the aqueous reactants at low or atmospheric pressures; electricity can be generated by fuel cell stacks by using hydrogen and oxygen from the air. The reaction product water of fuel cells can be recycled for sugar dissolution.
The whole system would have very high electricity conversion efficiencies since the conversion of carbohydrate to hydrogen is endothermic, i.e., 22% of the combustion enthalpy of hydrogen comes from ambient thermal energy or waste heat from fuel cells. If phosphoric acid fuel cells are chosen, hot water can be co-generated. The whole energy (electricity and heat) conversion efficiency may be very close to 100%. After technology improvements, the proposed enzymatic hydrogen production systems would compete with diesel-to-electricity generators.
Figure 4b shows a sugar fuel cell vehicle (SFCV) based on a hybrid of an on-board bioreformer, PEM fuel cells, rechargeable batteries, and an electric motor2. This system can be regarded as a consolidation of BEV, fuel cell vehicle (FCV), and a plug-in hybrid electric vehicle (PHEV). This combination will have both high energy storage density and power density.
Solid carbohydrate powder will be refilled rapidly into the sugar container in the car at local sugar stations; the on-board bioreformer will convert the sugar solution to hydrogen and carbon dioxide by the stabilized enzyme cocktail; a small-size buffer hydrogen storage container will balance hydrogen production/consumption; feeding of a mixture of CO2/H2 or pure hydrogen in the PEM fuel cells will dramatically decrease system complexity and greatly increase system operation performances.
The waste heat release from PEM fuel cells will be coupled to the heat needed by the bioreformer. The electrical energy from the fuel cells will be sent to the motor controller/motor/gear to generate kinetic energy. When extra energy is needed for acceleration or start-up, electrical energy stored in the rechargeable peak battery will be released.
Figure 4. Electricity generator (A) and sugar fuel cell vehicle (B).
A typical compact five-person SFCV requires ~20-kW for constant-speed running on highways. Giving the efficiencies of PEM fuel cells of 50-60%, SFCV would consume ca. 1 kg of hydrogen per hour. Assuming that the on-board bioreformer has a hydrogen generation rate of 23.5 g H2/L/h, similar to the highest biological hydrogen production rate, the bioreformer tank size could be 42.8 liters, possible to put in a small car.
In order to meet requirements such as accelerating and start-up, rechargeable batteries are needed and the maximum output of the motor may be approximately 80 kW. In order to have long driving distance for SFCV per refilling, SFCVcan store ca. 5 hours of fuel, i.e., ca. 5 kg of hydrogen or 33.8 kg of sugar, costing $~6.00 for 34 killograms of carbohydrate2,3. Such an amount of sugar occupies a tank of ~ 48 liters or 12.8 gallons, similar gas tank size in a compact car.
Current gasoline/internal combustion cars require maintenance every 3,000 miles (e.g., 4,800 km) or 3 months, i.e., 50-100 driving hours. Discovery of thermophilic enzymes that are stable at 80‹C for more than 100 working hours is doable, for example, T. maritima 6-phosphogluconate dehydrogenase8. It is expected that enzyme deactivation in the bioreactor could be solved through frequent maintenance, similar to the oil/air filter change for gasoline/ICE vehicles. Fast refilling dry sugar powder from local sugar stations can be added into sugar vehicles by nitrogen blowing.
Our analysis suggests a potential SyPaB reaction rate increase of 640,000- to 32,000,000-fold by combining a number of technologies:
- increasing reaction temperatures from 30 to 80‹C or higher,
- increasing the use of enzymes responsible for rate-limited reactions,
- increasing substrate concentrations by 50-fold or higher,
- increasing overall enzyme concentrations by 10-fold or higher,
- accelerating the reaction rates by metabolite (product) channeling, and
- increasing the catalytic efficiency of enzymes2.
In practice, a conservative estimation is that hydrogen production rates would increase to 23.5 g H2/L/h by 3000-fold. In support of the feasibility of this estimation, the highest biological hydrogen production rate (i.e., 23.6 g H2/L/h) has been reported by high-cell density microbial fermentation9. Since enzymatic reactions are usually faster than microbial fermentations, increasing reaction rates by three or four orders of magnitude would be highly achievable5.
The current biohydrogen production experiments are conducted using off-the-shelf enzymes with little optimization6. The reaction rates would be accelerated greatly through intensive R&D efforts. For example, the power densities (i.e., reaction rates) of microbial fuel cells have been improved by greater than 100,000-10,000,000 fold during the past 10 plus years10.
Transportation Fuel Independence Promises
Three representative biofuels (ethanol, hydrogen, and electricity) can be produced from biomass through their respective power train systems (ICE, SFCV (Fig. 3b), and BEV). To compare different scenarios, here we calculate the biomass-to-kinetic energy (BTK) efficiency, which is energy conversion efficiency from chemical energy in biomass to kinetic energy for driving,
BTK efficiency = conversion efficiency X distribution efficiency X TTK efficiency
where conversion efficiency is energy efficiency of biomass to biofuel through biorefineries or power stations without any other energy inputs, distribution efficiency is one minus normalized energy loss during biofuel distribution processes from manufacturers to tanks, and TTK efficiency is tank-to-kinetic energy efficiency from the biofuel to kinetic energy for driving.
Figure 5 clearly suggests that the SFCV would have ~70% higher BTK efficiencies than the battery electric vehicle (BEV) and nearly four times higher efficiency than ethanol-internal combustion engine (ethanol-ICE)11. In addition to high energy efficiency, sugars have high energy storage densities (e.g., >10 MJ electricity output/kg carbohydrate) and SFCV have benefits in operations, safety, infrastructure costs, environmental impacts, and so on, as compared to BEV.
If SFCVs are widely implemented, approximately 5% of the annual U.S. net biomass production (i.e., ~600 million metric tons of dry biomass per year) would be sufficient to meet 100% of the USA's light-duty transportation fuel needs in the year 2030.
Figure 5. Comparison of biomass-to-kinetic energy efficiency based on ethanol-ICE, SFCV, and BEV.
SyPaB technology is a disruptive technology as compared to microbial fermentation 5, similar to digital camera vs optical camera. Due to its unique advantages, such as engineering flexibility, high product yield, fast reaction rate, easy process control, and broad reaction conditions5, this technology would have great applications from high-end (e.g., synthesis of chiral compounds, sugar batteries) to low-end (e.g., hydrogen, electricity, and transportation).
Developments in thermostable enzymes as standardized building blocks for cell-free SyPaB projects, use of stable and low-cost biomimetic NAD cofactors, and accelerating reaction rates are among the top research and development priorities. These obstacles are being addressed within several years5,12. Future hydrogen production costs from biomass carbohydrate would be as low as $1.20 kg of hydrogen, where carbohydrate accounts for 80% of the final price5. We envision that the sugar fuel cell vehicles will come true as in the movie "Back to the Future Part II".
AFOSR Young Investigator Award and MURI FA9550-08-1-0145.
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