Researchers Develop Lithium-Water Electrochemical Cell for the Controlled Generation of H2 and Electricity
|Schematic representation and operating principles of the lithium–water electrochemical cell used for hydrogen generation: (1) external circuit and (2) inside of lithium–water electrochemical cell. Source: Wang et al. Click to enlarge.|
Scientists from the Energy Technology Research Institute, AIST in Tsukuba, Japan, have developed a lithium-water electrochemical cell for the controlled generation of hydrogen and electricity. The researchers, headed by Haoshen Zhou, foresee the use of this process in fuel cells for mobile applications. A paper on their work was published in the journal ChemSusChem.
Although direct chemical reactions between water and certain metals—alkali metals including lithium, sodium and others—can produce a large amount of hydrogen in a short time, these reactions are too intense to be controlled. (E.g., the high-school chemistry demonstration of the violent reaction between sodium and water.)
In the present work, the intense direct chemical reaction between lithium and water is transformed into a controllable electrochemical reaction in a lithium–water electrochemical cell, through which the controllable generation of hydrogen is achieved easily.
—Wang et al.
The closed fuel cell system developed by Dr. Zhou and his team has two compartments separated by a water stable lithium super-ionic conductor glass film (LISICON). One compartment contains a metallic lithium electrode (anode) in an organic solvent (1m LiClO4 in ethylene carbonate/dimethyl carbonate), while the other contains an aqueous electrolyte solution (LiNO3/H2O) with a carbon-based hydrogen generation electrode (cathode).
On discharge, the metallic lithium (anode) is converted into lithium ions and the generated lithium ions diffuse from the organic solution across the LISICON film to the aqueous solution. Simultaneously, hydrogen gas is generated on the cathode. During this process, electrons pass around the external circuit, forming the current.
Only lithium ions can pass across the LISICON film. The rate of both half reactions within the lithium–water electrochemical cell can be controlled by the current, indicating a controllable hydrogen generation.
Another attractive aspect of this technology is that lithium metal can be produced from salt solutions (e.g., sea water) by using sunlight. In other words, energy from the sun can be “stored” in the metal, and then be used on demand by reacting the lithium in the fuel cell. Recharging the battery would be a matter of replacing the lithium metal cell.
Hydrogen generation from the lithium–water cell suffers from several weaknesses at this early stage, according to the researchers:
- The hydrogen production rate is still limited by the internal resistance of the lithium–water cell. The conductivity of the solid-state electrolyte film (LISICON) needs to be improved in future studies.
- Good mechanical strength is also demanded from the LISICON film.
- The LiOH concentration in the lithium–water cell is increased gradually during the hydrogen generation process. The inherent low solubility of LiOH and the reaction of LiOH with traces of CO2 may also limit the hydrogen production and the performance of the lithium–water electrochemical cell; this issue requires further studies.
Nevertheless, they note that hydrogen generation by the lithium–water electrochemical cell is controllable, and may be directly applied for energy conversion devices. The researchers suggest the establishment of a new possible route for hydrogen generation: lithium–metal production using solar energy and controllable hydrogen generation from lithium–water electrochemical cells.
Lithium, which is already widely used in various lithium ion batteries and will also be applied in the lithium-air fuel cell and this lithium-water/hydrogen/fuel cell system in the future, may lead humanity to enter a new sustainable lithium society, based on smart grid systems of lithium energy networks.
Yonggang Wang, Huiqiao Li, Ping He, Haoshen Zhou (2010) Controllable Hydrogen Generation from Water. ChemSusChem 2010, 3, No. 5, 571-574 doi: 10.1002/cssc.201000049