HTE, a highly promising method for producing hydrogen -Part 3-

over 1 year ago (Edited)

HTE electrolyser's works in extremely difficult circumstances, including its high temperature, hostile gaseous atmosphere, and chemical and mechanical interactions between materials. Here are some problems that can hinder the effectiveness of this technique with the proposed solutions

Chromium poisoning, a phenomenon linked to interconnectors, constitutes a problem which hampers the efficiency of the technique of hydrogen production by electrolysis of water vapour. This phenomenon leads to the deterioration of the electrochemical properties of the cell due to contamination by volatile chromium compounds, emitted by interconnectors made of metal alloys such as stainless steel or a nickel-based alloy. These alloys are used because of their resistance to high temperature oxidation, and they also form relatively electrically conductive, chromium-rich oxide layers.

Piece of chromium metal.

Noble metals like gold and platinum can be a good definitive solution to these problems, but their industrial cost is very high. Even alloys enriched with silicon or aluminium form non-toxic oxides, but their electrical conductivity is poor. Some other materials are non-toxic and form relatively conductive layers, but they resist oxidation poorly.

Ferritic alloy steel could have been a choice as a material for making interconnectors, its advantages are that it is weldable and castable, and inexpensive, and relatively resistant to high temperatures when 'it is exposed for a long time. On the contrary side, given the high pressures of an HTE electrolyser, their surface qualities are not entirely suitable because of their poor resistance to oxidation under water vapor, specifically, the electrodes became contaminated by the oxide layers that formed. In order to improve the properties and structure of this steel, some manufacturers and some researchers seek to control the nature of the oxide layers formed, for instance, in an iron/chromium alloy like the Crofer22 APU, created by Thyssen Krupp and having 22 percent chromium, adding manganese encourages the production of a mixed oxide of manganese and chromium at the extreme surface. In addition to generating fewer chromium vapors than chromium oxide alone, this oxide also performs significantly better as a conductor. In order to reduce the chromium oxide's resistance, titanium is added last and in order to slow the rate of development of the oxide, rare earths are added.