Fuel Cells and the Self Ignition Temperature of Hydrogen
Authors: Bossel Ulf 
Published: February 2020

According to common wisdom the open circuit voltage of 1.23 Volt is obtained when hydrogen and oxygen are electrochemically combined under standard conditions in a fuel cell. However, for low temperature fuel cells this theoretical values have never been experimentally confirmed, while SOFC experiments performed above the temperature of self-ignition (585°C) verify the theoretical OCV of about 1.1 Volt (lower than 1.23 V because of higher temperature). Apparently, hydrogen and oxygen cannot react directly according to (H2 + ½ O2 => H2O) at temperatures typical for fuel cells with polymer, acidic or alkaline electrolytes. At low temperature the electrochemical combination of the two gases appears to follow a more complex path of chemical reactions.

 

1. Hydrogen ions migrate from anode to cathode through the polymer electrolyte.

2. At the cathode these ions meet oxygen ions and combine to hydroxyl compounds (H+ + O2- => OH-). This reaction is possible at low temperatures (photosynthesis!). It occurs on the electrolyte-cathode interface. The electrochemical exchange between anode and cathode starts with a positively charged hydrogen ion and ends with one negatively charge hydroxyl ion. Thus the charge transfer number is two.

3. Within the cathode diffusion layer (not necessarily on the cathode-electrolyte interface) two hydroxyl ions combine to peroxide that then releases one oxygen ion to form water (OH- + OH- => H2O2 => H2O + O2-). This chemical reaction does not contribute to the OCV of the fuel cell.

4. The oxygen ion released is available to “welcome” another hydrogen ion. A second oxygen ion is supplied from the cathode air. The process can continue with step 2.

In balance, the heat of H2O-formation of -286 kJ/mol is obtained for the complete process in accordance with the law of energy conservation. However, for the steady state reaction (H+ + O2- => OH-) an OCV of only 0.876 Volt is obtained. This is in excellent agreement with experimental observations. Apparently, below the temperature of self-ignition the dominant steady reaction in a low temperature fuel cell is the formation of hydroxyl, while the initial OCV (about 1.1 V) is evidence of a start-up process not yet identified.

 

There are no chemical processes at the anode that could be assisted by catalytic action of platinum. As the catalytic material (platinum) is finely dispersed in the diffusion layers, but not deposited on the electrode-electrolyte interfaces, it may play a role in the dissociation process of hydrogen and oxygen. No catalysts are needed for the two chemical reactions on the cathode (OH-
+ OH- => H2O2 and H2O2 => H2O + O2-).

 

Fuel cells reactions are of spontaneous nature. However, hydrogen and oxygen can react spontaneously (H2 + ½ O2 => H2O) only in the SOFC regime above the temperature of self-ignition, but not at lower temperatures. Below the temperature of self-ignition hydrogen and oxygen are combined to hydroxyl. The resulting OCV is 0.876 Volt. Catalysts like platinum improve the conversion rates, but cannot lower the temperature of self-ignition. It seems that the accepted fuel cell theory needs to be revised.