(Invited) Low Temperature Electrolysis for Hydrogen and Oxygen Generation - a Tutorial on Catalyst and Electrode Development for Proton and Anion Exchange Membrane-Based Systems

Abstract

Hydrogen generation via electrolysis is rapidly gaining international interest for energy storage applications due to the carbon-free chemical cycle and response characteristics of the technology. At the same time, the need for a sustainable source of hydrogen has been widely recognized, not just as a potential transportation fuel, but to limit CO2 production and fossil fuel consumption from existing industrial processes such as ammonia generation. Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. Significant growth has therefore occurred in recent years in water electrolysis research, especially in catalyst research for the hydrogen (HER) and oxygen (OER) evolution reactions. Commercial proton exchange membrane (PEM)-based electrolysis has reached scales of several hundred kg/day, providing a relevant pathway for industrial scale hydrogen generation. PEM electrolysis also has tremendous potential for continuing cost reduction, leveraging system and manufacturing scaling laws as well as leveraging advancements in PEM fuel cell materials, manufacturing, and analysis tools. Order of magnitude improvements in some of the highest cost elements are easily achievable. Many of the technology elements are known, but need to be refined and validated in a manufacturing environment, including modifications in materials and methods of fabrication. At the same time, alternate pathways based on anion exchange membranes (AEMs) are showing promise, as membrane chemistry has advanced and understanding of electrode processing has developed. The basic environment of the AEM electrolysis cell enables a broader range of catalyst materials and the potential to eliminate the highest cost materials such as platinum group metals and commercially pure titanium from the stack. However, in both the AEM and PEM case, there are complex interactions at the electrode level which need to be considered in catalyst and membrane development. First, the liquid electrolyte environment used for catalyst activity screening, where all of the catalyst surface is accessible to the reactant is often not comparable to a complex, 3-dimensional, ionomer-based electrode. Also, similar to automotive fuel cells, the operating environment is highly important and should be considered when claiming improvements over state of the art. For example, catalyst performance at very low current densities may indicate inherent activity but may not represent capability at typical device operating currents. Similarly, a membrane which cannot operate at differential pressure may be highly limited in utility even if more efficient than current commercial solutions. This talk will describe some of the complex interactions that need to be considered, typical operating requirements, and stages of development where relevant conditions should be introduced.