Using Ion Exchange Capacity to Control Water Uptake in Electrodes of Low-Temperature Anion Exchange Membrane Electrolyzers

Abstract

Hydrogen today is mainly produced by steam reforming natural gas or other fossil fuels. However, this method produces low purity hydrogen with a high concentration of carbonaceous species and ultimately does not relieve dependencies on fossil fuels or reduce harmful greenhouse gasses. Hydrogen production from water electrolysis is a low-carbon or carbon-free alternative to steam reforming and can be utilized as a chemical means of storing excess energy from intermittent sources such as wind and solar.Solid polymer electrolyte electrolysis based on anion exchange membranes (AEM) seeks to combine the benefits of more established water electrolysis technologies such as alkaline electrolysis (AEL) and proton exchange membrane electrolysis (PEMEL). The high pH environment in AEM electrolysis (AEMEL) promotes facile oxygen evolution reaction kinetics and enables the use of electrodes with little to no platinum group metals. AEMEL systems also have the advantage of a compact design without the need for constant circulation of a concentrated KOH electrolyte.The polymer membranes used in AEM devices have seen significant advances in recent years, with the development of highly conductive, stable and low-cost membranes for AEM fuel cells (AEMFC). Less attention has been paid to the ionomers in the gas diffusion electrodes. Proper water management has been a key factor in obtaining maximum performance in AEMFCs and it is reasonable to infer that water management will also play an important role in achieving high performance in AEMEL.In this study, a series of high performance polynorbornene tetrablock copolymer ionomers were synthesized and used to understand the effects of hydrophobicity and ionic conductivity on the performance of low-temperature AEMEL. The ion exchange capacity (IEC) of the ionomers was used to control the amount of swelling and water uptake within the catalyst layer. Teflon was also used to add hydrophobicity to the catalyst layer without sacrificing ionic conductivity. It was found that low IEC at the oxygen evolving electrode and high IEC at the hydrogen evolving electrode enables more efficient water electrolysis.Financial support for the authors provided by the EERE H2@Scale program (U.S. Department of Energy) is gratefully acknowledged.