|Where the PEM fuel cell sends protons (H+) across the membrane, the Daihatsu cell sends anions (OH-) across its membrane. Click to enlarge.|
Daihatsu Motor, working with Japan’s National Institute of Advanced Industrial Science and Technology (AIST), has developed a new fundamental fuel cell technology: a platinum-free, direct hydrazine fuel cell (DHFC), which uses an anion-exchange polymer electrolyte.
Conventional PEM (proton exchange membrane) fuel cells for vehicles use strongly acidic electrolyte membranes and therefore must possess high corrosion resistance. The use of expensive platinum in the electrode catalyst material has been a common approach. Daihatsu essentially reversed the PEM model to create an alkaline anion exchange fuel cell. The new technology uses hydrazine hydrate (N2H4·H2O) as the fuel.
Where the PEM fuel cell sends protons (H+) across the membrane, the Daihatsu cell sends anions (OH-) across its membrane.
The Daihatsu technology uses alkaline electrolyte membranes, allowing the use of less expensive metals such as cobalt and nickel as an electrode catalyst (instead of platinum) and other inexpensive materials to be used in the separator and other component parts.
|Output and current densities from the new fuel cell. Click to enlarge.|
Using hydrazine hydrate as the fuel and the newly developed electrode catalyst, the new fuel cell can produce a high output of 0.5 W/cm2 (as measured by Daihatsu), which is comparable to the output obtained from a hydrogen fuel cell using platinum.
Since hydrazine hydrate is a liquid fuel, it is easy to handle during filling and its energy density is also high. However, high-concentration hydrazine hydrate is designated as a poisonous substance under Japan’s Poisonous and Deleterious Substances Control Law, and it must be handled under the same safety standards applicable to gasoline and most industrial chemicals.
|The hydrazine hydrate fueling system. Click to enlarge.|
Daihatsu developed a technology that fixes the hydrazine hydrate inside the fuel tank through the use of a polymer, minimizing the adverse effects that any dispersed fuel could have on humans or the environment should the fuel tank be damaged during a collision, but that makes the required amount of liquid hydrazine hydrate available in a timely manner for electricity generation in the fuel cell.
The fuel tank is filled with a granulized polymer embedded with carbonyl group (>C=O) for capturing the hydrazine hydrate. When the hydrazine hydrate enters the tank, it reacts with the carbonyl group (dehydration-condensation reaction) and bonds with the polymer, becoming a solid called hydrazone (>C= N2H2), which can be safely stored.
To release hydrazine hydrate from hydrazone, warm water circulates through the hydrazone (>C=N2H2), causing hydrolysis. This reaction returns the hydrazone to the original carbonyl group (>C=O), and releases liquid hydrazine hydrate (N2H4·H2O), which is then supplied to the fuel cell.
The new fuel cell system offers numerous benefits, according to Daihatsu, including resource conservation, low cost, high output, and safe and easy fuel handling. The company plans to accelerate further research and development of the technology.
Given the number of issues that must be addressed—including improvements in the polymer for fixing the fuel, enhancement of both the performance and durability of the fuel cell, and establishment of the necessary infrastructure—Daihatsu hopes to establish wide-ranging partnerships with relevant parties and proceed with further R&D.
A paper describing the technology is published in the journal Angewandte Chemie International Edition. The journal labelled the study a “Hot Paper.”
Daihatsu is the mini-vehicle unit of Toyota Motor.
Koichiro Asazawa, Koji Yamada, Hirohisa Tanaka, Akinori Oka, Masatoshi Taniguchi, Tetsuhiko Kobayashi; “A Platinum-Free Zero-Carbon-Emission Easy Fuelling Direct Hydrazine Fuel Cell for Vehicles”; Angewandte Chemie International Edition Published online, DOI: 10.1002/anie.200701334
Daihatsu technology backgrounder