(Invited) Challenges and Limitations of Materials for Membrane-Based Water Electrolysis at Megawatt Scale
In water electrolyzers, the overvoltage of the oxygen evolution catalyst is a key efficiency loss, typically contributing over 300 mV of overpotential in proton exchange membrane systems. In addition, the catalyst loading is very high, in order to maintain activity throughout operating lifetimes surpassing 50,000 hrs, due to the lack of stability of most catalyst supports in acidic environments at electrolysis potentials. While catalyst cost is not currently a key driver in the overall system cost, as other costs are decreased through system scale up and improvement in other processes, catalyst utilization must be improved in order to meet overall cost targets. Therefore, research must focus not only on composition of the catalyst but also electrode structure and application method. Proton has shown that catalyst composition, process conditions, and electrode formulation can all improve performance vs. current commercial baselines. Catalyst loadings also have the potential to be significantly decreased without loss in performance.
The membrane is also a large contributor to the efficiency losses, especially for the much thicker membranes typically used currently vs. state of the art fuel cells, as electrolysis membranes must withstand substantial differential pressure, while fully hydrated. However, they do not typically undergo freeze-thaw cycles, or changes in hydration. Conductivity under hot, dry conditions is also not a primary concern. Therefore, the material challenges and solutions are specific to electrolysis, although much of the understanding from fundamental PEM fuel cell research can be applied. With the right combination of material properties tailored for electrolysis, Proton has demonstrated that membrane thicknesses of 50-75 microns are very feasible even at 400 psi differential pressure operation.
Finally, the bipolar assembly is the biggest cost driver, exceeding the cost of the membrane electrode assembly, primarily due to the aggressive conditions it is designed to withstand. On the hydrogen side of the cell, the bipolar plate has to be resistant to hydrogen embrittlement, while on the oxygen side of the cell, the plate has to be corrosion resistant at potentials of 2V, in the locally acidic environment of the PEM electrode. These constraints therefore severely limit the selection of suitable materials for this component. Titanium is a common choice, but adds expense not only in the base material but also in the manufacturing, since titanium is typically very difficult to work. Alternate methods of fabrication and coatings have enabled reduction of over half of the metal in the stack over the last several years, in addition to reduction in scrap.
This talk will discuss progress in each of these areas and implications in development of megawatt scale electrolysis. Progress in system development will also be discussed.