|In a conventional PEM fuel cell, protons migrate through a steam membrane (left). In the SuperProtonic solid-acid fuel cell (right), protons hop through a dry crystal net. (Click to enlarge)|
Hydro Technology Ventures, the investment company of oil, energy and aluminum giant Hydro, has invested in SuperProtonic, a development-stage company formed by CalTech scientists working on Solid-Acid Fuel Cells (SAFC).
The goal of the solid-acid fuel cell work is to develop a fuel cell that is equivalent on a $/kW basis to internal combustion engines based on reducing the use of precious metal catalysts, lowering the cost of the electrolyte and the electrodes, and enabling a simpler design than in PEM cells.
Fuel cells all basically work from the same principle. A catalyst at the anode (positively charged electrode) strips hydrogen of electrons. The ions left behind flow through an electrolyte toward the cathode (negatively charged electrode). The electrolyte is impermeable to electrons, which travel to the cathode via an external circuit, creating the power that drives the motor. At the cathode, the electrons and hydrogen ions combine with oxygen from air, creating water.
Different electrolytes transport ions with varying efficiencies as a function of temperature, so that each of these types operates in a different temperature range. The electrolytes in PEM cells are relatively highly conductive, generating abundant power quickly—definitely attractive in an automotive application.
Because they use liquid water to help move the ions through the electrolyte, PEMs need to operate below 100º C, or else require auxiliary systems to keep the polymer electrolyte hydrated. But at such lower temperatures, precious-metal catalysts work slowly and can become inactivated by binding to carbon monoxide.
Because solid-acid cells can operate at higher temperatures, thereby increasing catalytic efficiency, and because their basic materials are less expensive than those of PEM cells, the SAFCs could prove to be a more economical and efficient alternative for widescale application.
|Types of Fuel Cells|
|Solid-Acid (SAFC)||100–300º C||Solid acids, eg CsHSO4|
|Polymer Electrolyte Membrane (PEM)||70–100º C||Sulfonated polymers, eg Nafion|
|Alkali (AFC)||100–250º C||Aqueous KOH|
|Phosphoric Acid (PAFC)||150–220º C||H3PO4|
|Molten carbonate (MCFC)||500–700° C||H3PO4|
|Phosphoric Acid (PAFC)||150–220º C||(NA,K)2CO3|
|Solid oxide (SOFC)||700–1,000° C||(Zr,Y)O2-5|
Solid acids are chemical intermediates between normal salts and normal acids. Physically, the materials are similar to salts. At low temperatures, they have ordered structures. At higher temperatures, however, some solid acids undergo transitions to highly disordered structures that can increase ion conductivity dramatically.
SuperProtonic uses cesium hydrogen sulfate (CsHSO4), a solid acid formed by the reaction between sulfuric acid and cesium sulfate (a salt), as an example.
The bisulfate (HSO4-) group forms a tetrahedron with an oxygen atom at each corner and a hydrogen atom sitting on one of the oxygens. At room temperature, all the sulfate groups have a fixed orientation. When the temperature is raised, disorder sets in and the sulfate groups reorient, changing the positions of the hydrogen atoms as they do so.
Essentially, these sulfate groups rotate almost freely—and every 100 reorientations or so, they’re in exactly the right position for a proton transfer to happen. Conductivity values for the acid salts are comparable to the conductivity of Nafion and other polymer electrolytes, but at slightly higher temperatures. A number of different solid acid compounds with such behavior have been discovered.
A longstanding worry with solid-acid fuel cells has been that the solid acids would dissolve in the water formed at the cathode. SuperProtonic deals with this by operating the fuel cell above 100º C—the steam is harmless to the otherwise water-soluble electrolyte.
A second, more recently recognized problem is the reaction between hydrogen in the fuel and the sulfur in the solid acid, breaking down the electrolyte and generating hydrogen sulfide.
To counter this, Superprotonic tired replacing the sulfur with phosphorus by using cesium dihydrogen phosphate (CsH2PO4). At the higher temperatures of operation, however, the H2 reacts with oxygen to form water, causing the remaining solid to crumble.
The research team countered that by adding a small amount of water vapor to the electrolyte, which prevents the hydrogen molecules from leaching out of the solid to form more water.
More work clearly needs to be done, but successful commercialization of the SAFC technology would be a big boost, cost-wise, to the development of fuel cell vehicles.
Hydro Technology Ventures is one of six investors in the new technology and has led the work to establish SuperProtonic. The other investors are: OnPoint; the U.S. Army’s venture fund; Nth Power; CMEA; Innovation Valley Partners and Batelle Ventures.