Over 29,000 different species of animal have evolved over the millennia to breath air. Each breath they take involves a process which is miraculously complex. In step one of a two part gas exchange, ambient air is taken into the lungs and oxygen is transferred to the capillaries. In the next step the process allows carbon dioxide to be released from the bloodstream into the lungs to be exhaled to atmosphere.
It is in the alveoli, those tiny sacs at the end of the micro-sized tube like bronchioles, where the magic happens. If the oxygen were to simply diffuse through the micron thick membrane of the alveoli, air bubbles would form in the bloodstream and death would follow. The remarkably complex bodily function of respiration uses the hydroscopic properties of the alveoli membrane to repel water molecules on the bloodstream side, while attracting water molecules on the air side.
It is this ability that prevents the air from diffusing directly through the membrane, thereby preventing gas embolisms (bubbles). This gas exchange process not only provides a crucial survival benefit, but is also very effective, and highly efficient. It is one mechanism which allows the body’s muscles to perform to their limits.
The efficiency of this biological gas exchange has inspired researchers at Stanford University, near San Jose, California, to develop a process which mimics respiration in the lungs. Their goal is to improve the effectiveness of the current technologies used in fuels cells and metal-air batteries.
Emulating the alveoli membrane with a thin polyethylene film, they split water in a battery and used the resulting hydrogen for fuel. The oxygen is then used in a secondary process. In the first step, the hydrogen that is produced at the anode is transported through the membrane for collection. This step emulates the act of exhalation in the lungs. The membrane’s hydroscopic property prevents froths from developing and allowing the gas to effectively pass through it.
At the cathode, the oxygen is also transported through a membrane made of the polyethylene. In a process mimicking inhalation, the oxygen is delivered to an electrode surface to be used in electrochemical reactions to produce electricity.
Lead author of the paper published in the scientific journal Joule, Jun Li stated; “The breathing-mimicking structure could be coupled with many other state-of-the-art electro-catalysts, and further exploration of the gas-liquid-solid three-phase electrode offers exciting opportunities for catalysis,”
Although lab results look very encouraging, the technology is far from commercialization. Even though the very thin polyethylene material remains hydrophobic longer than conventional carbon based gas diffusion layers, and it is able to achieve higher current density rates, it has a low threshold for heat (it deforms and starts to melt at temperatures above 100 degrees Celsius). Comparable nanoporous hydrophobic materials are being studied, a tolerance for higher temperature is a definite asset, if the efficiency can exceed that of polyethylene.
This research moves the quest for an economically viable and environmental neutral source of hydrogen a little closer to realization.