Water electrolyzers, technology development

Much like metal recovery from dilute streams, the use of three-dimensional electrodes will enhance the oxidation rates of low-concentration organics by increasing the surface area available for reaction. Moreover, porous electrodes which could be incorporated into cells using solid electrolytes can draw upon technologies developed for other electrochemical systems such as PEM-based water electrolyzers and fuel cells. 

The technological development of electrolyzers started with a mono polar cell consisting of a cathode part and an anode part separated by a diaphragm, hi multi-cell systems, bipolar plates are used carrying the cathode material for one cell and on its backside the anode material for the neighbor cell. The functions of the bipolar plate are the continuous supply of the membrane electrode with H2 on one side and with O2 or air on the other side and the regulation of the water balance by providing moisture for the membrane on the H2 side and remove the product water on the O2 side. 

Most current technology development is focused on the use of platinum (or platinum and ruthenium) supported on carbon particles, in order to reduce the amount of platinum required on the anode down to 0.05-0.45 mg cm", as described in [10]. These types of anodes are prone to degradation during fuel starvation due to the reaction in Equation 17.6, the oxidation of carbon, which is catalyzed by the presence of platinum [11]. This reaction proceeds at an appreciable rate at the electrode potentials required to electrolyze water in the presence of platinum (greater than approximately 1.4 V [24). This is shown schematically in Figure 17.4. The catalyst support is converted to CO2, and Pt and/or Ru particles may be lost from the electrode, resulting in loss of performance. 

In conclusion, it is now over 120 years since the alkaline electrolyzer emerged, and over 50 years since PEM and SO electrolyzers were developed. While exact data are hard to come by, almost all (99%) of the crurerrt market for electrolyzers is dominated by alkaline technology. However, in recent years, PEM electrolyzers have noticeably gained ground. As was slated in section 2.1, water electrolysis accormts for a very marginal proportion (less than 1%) of worldwide hydrogen production. 

PEM technology was originally developed as part of the Gemini space program.16 In a PEM electrolyzer, the electrolyte is contained in a thin, solid ion-conducting membrane rather than the aqueous solution in the alkaline electrolyzers. This allows the H+ ion (proton) or hydrated water molecule (HsO+) to transfer from the anode side of the membrane to the cathode side, and separates the hydrogen and oxygen... 

With the other two technologies, reversible prototypes have been developed -particularly for PEM technology [MIT 98 STR 08] - but overall the performances remain fairly limited, for reasons of compromise on the choice of materials. For instance, with PEM technology, the optimal catalysts are not the same for the positive electrode. In another example, the carbon electrodes of a PEM fuel cell have very poor resistance to the conditions highly favorable to the corrosion of carbon (and therefore to their destruction) imposed in electrolyzer mode. One as-yet-unexplored avenue for PEM technology would be to switch to using water vapor, which would facilitate reversible operation [MIT 11]. 

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