Plants, during photosynthesis, use sunlight as an energy source to convert water and carbon dioxide into carbohydrates (glucose) and oxygen. Animals, on the other hand, obtain the energy they need by taking in oxygen through respiration and consuming carbohydrates from food. They emit (produce) carbon dioxide and water. Glucose has an extremely high energy density. For example, a 150g serving of rice, which includes large amounts of glucose, contains 240kcal of energy. This is equivalent to 96 AA batteries. As an extremely stable substance, glucose is also easy to handle, in the sense that there is no risk of injury to living organisms resulting from spontaneous collapse accompanied by a catastrophic release of energy. This is a vital requirement for an energy source to be used to support life.

How do living organisms extract energy from glucose? Biological catalysts known as "enzymes" play a key role in this process. Enzymes are known to accelerate extremely specialized reactions in the body. We also know that several types of enzymes can work together to break down stubborn substances, such as glucose, into carbon dioxide. The processes involved include the glycolytic pathway and the citric acid cycle. Flows of electrons and protons (positive ions of hydrogen) released through the breakdown of these substances inside living things are used to convert energy into various forms, including heat and chemical energy, which are used to maintain biological activity. Bio Battery extracts energy directly from a carbohydrate (glucose) using the power of enzymes, which play a central role in systems used by many living organisms to obtain energy (*2). Sony pioneered the development of cells based on this technology, and (in August 2007) was able to operate a Walkman using four cells arranged in series producing 50mW/40cc (1.25mW/cc) per unit (*3).

Like a conventional fuel cell battery, Bio Battery basically consists of an anode, cathode, electrolyte and separator. However, Bio Battery has certain specific characteristics. First, biological enzymes are used as catalysts for the anode and cathode. Second, enzymes and electronic mediators (which transfer electrons between enzymes, and between enzymes and electrodes) are fixed on the anode and cathode.

Glucose is broken down on the anode side of the battery, producing protons (H⁺) and electrons (e^-). The protons (H⁺) are transferred to the cathode side through the separator, while the electrons (e^-) are transported to the cathode side through the mediator, which transfers them to the external circuit. The cathode uses the enzymes to drive an oxygen-reduction reaction which ultimately produces water using both the protons (H⁺) and the electrons (e^-) transferred from the anode. These reactions at the anode and cathode generate electric energy by creating proton (H⁺) and electron (e^-) flow in the cell system.

As described above, enzymes and electron mediators are immobilized on the anode and cathode of Bio Battery. The enzymes that need to be immobilized on the anode are glucose dehydrogenase and diaphorase, and the mediators are 2-methyl-1,4-naphthoquinone (vitamin K3, VK3) and nicotinamide adenine dinucleotides (NAD(H)). The immobilizing method selected by Sony to meet these requirements was a poly-ion complex method based on an anionic polymer, polyacrylate, and a cationic polymer, poly-L-lysine (PLL) (*4, 5). PLL is also used to immobilize the enzyme, bilirubin oxidase and the electron mediator (potassium ferricyanide) onto the cathode. The cathode is exposed to the air to create a three-phase interface among oxygen, water and the enzymes (*4). A highly-concentrated electrolyte (phosphate buffer solution, pH7, 1M) was chosen to ensure the efficient transfer of protons into the porous carbon electrodes. As a result, Sony was able to achieve an extremely high output of 1.5mW/cm²@0.3V by 2007 (*3).

Sony has since improved output to 3mW/cm²@0.5V (*5) by changing the anode electron mediator from VK³ to 2- amino-1, 4-naphthoquinone (ANQ), and to 5mW/cm²@0.5V by using a new electrolyte, an imidazole buffer solution (2M, pH7) (*6). In April 2010, the power density was doubled to 10mW/cm²@0.5V (*7, 8) by using a water-repellent cathode, which can still function even when immersed in the electrolyte, instead of the exposed cathode that had been used previously. Over the past few years, Sony has also begun to achieve positive results with a number of other initiatives. These include the addition of another enzyme on the anode to carry out a secondary decomposition of gluconic acid produced by the breakdown of glucose, and the use of biotechnology to develop highly-durable artificial enzymes (*7,9,10).

Sony is progressively creating and testing new prototypes based on these technologies. By the time of the International Hydrogen & Fuel Cell Expo (FC EXPO 2009) in February 2009, the technology had progressed to the stage where power density reached 5mW/cm², and Sony was able to demonstrate a Walkman powered by a battery that was half the size of earlier units. At Toy Forum 2010 on January 18 and 19, Sony was able to demonstrate a 10mW/cm² cell mounted in a compact remote-controlled car prototype manufactured by TOMY Company, Ltd. This product attracted keen interest as an eco toy car.

While many technological challenges still remain, Bio Battery has great potential as a next-generation energy device. Advantages include its excellent harmony with the environment as a product fueled by a carbohydrate (glucose) having high energy density. Sony will continue to work toward the commercialization of this technology in the near future, initially for use in toys and other low-power products. The longer-term goal for R&D in this area is to further enhance performance to ultimately develop batteries suitable for notebook computers and other mobile devices.

References

1. Sony and the Environment
        (Click "Indoors." Then, click "bio battery" (lower left))


2. Ikeda, T., Bio Denki Kagaku no Jissai [Bio Battery Chemistry Today], CMC Publishing Co., Ltd. (2007)


3. http://www.sony.net/SonyInfo/News/Press/200708/07-074E/index.html


4. H. Sakai, et al, Energy Environ. Sci., 2, 133 (2009).


5. Y. Tokita, et al, ECS Trans. 13 (21), 89 (2008).


6. H. Sakai, et al, ECS Trans.16 (38) 9-15 (2009).


7. H. Sakai, et al, Meet. Abstr. - Electrochem. Soc. (2010), in press.


8. T. Nakagawa, H. Mita, et al, Meet. Abstr. - Electrochem. Soc. (2010), in press.


9. D. Yamaguchi, et al, Meet. Abstr. - Electrochem. Soc. (2010), in press.


10. H. Kumita, et al, Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem., 54 (2), (2009).