Benchmarking Water Electrolysis Catalysts: Making the Right Comparisons

Wednesday, 31 May 2017: 11:45
Grand Salon C - Section 13 (Hilton New Orleans Riverside)
K. E. Ayers, N. Danilovic, W. L. Gellett, and C. Capuano (Proton OnSite)
Significant growth has occurred in recent years in water electrolysis research, especially in catalyst research for the hydrogen (HER) and oxygen (OER) evolution reactions. Materials research is still essential for continuing cost reduction in electrolysis, in order to enable higher penetration of renewable technologies for hydrogen production. Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. However, many claims made about catalyst activity for new materials relative to state of the art are misleading at best, due to lack of understanding of device operation and the multiple factors that contribute to catalyst selection. This talk will describe some of the complex interactions that need to be considered, and the importance of working with industry in vetting the true potential of new materials and working on the right scientific challenges.

First, there are misconceptions in the literature around the highest activity catalysts today. While platinum is a clear benchmark for HER in both acidic and basic electrolyte, iridium/iridium oxide is not the most active catalyst in either solution. Ruthenium oxide has lower overpotential than iridium oxide in acid, but is not as stable as iridium at the typical cell voltages for industrial electrolyzer systems. Similarly, in basic solutions, nickel-based catalysts have been used in commercial alkaline electrolyzers for decades. Many of these catalysts have lower activation potential than iridium oxide for OER in concentrated potassium hydroxide, which is used as the electrolyte in these legacy systems. However, translating this activity to ionomer-based systems with no supporting electrolyte has been a major challenge. Therefore, comparing rotating disk electrode (RDE) results for catalyst powders in concentrated acid or base to iridium oxide is not sufficient for determining the value of new compositions, or for claiming activity or stability improvements over the best existing catalysts.

A related challenge is that to be a viable catalyst in an ion exchange membrane-based device, electrode interactions need to be considered. Catalyst materials have to be manufacturable, with sufficiently high surface area and conductivity to sustain the current densities required for commercial devices (ie at current densities well in excess of RDE). In addition, particularly for the anion exchange membrane based systems, catalyst-ionomer interactions can reduce the catalyst activity significantly vs. flooded electrolyte results. Using pH 14 electrolyte as the circulating fluid in these devices eliminates many of the advantages of translating from legacy systems to membrane-based systems. Understanding ionomer effects or leveraging more benign electrolytes such as carbonate are of higher interest to industry than introducing potassium hydroxide back into these systems. Finally, testing at realistic cell voltages and current densities is essential for credible comparisons to existing commercial catalysts.