Development of the Anion Exchange Membrane Water Electrolysis

Water electrolysis is an established method to produce hydrogen from the surplus power of renewable energy sources¹. There are two technologies considered to be the most suitable when connected to intermittent power sources: polymer electrolyte membrane (PEM) water electrolysis and alkaline electrolysis with a liquid electrolyte². Both technologies are linked to certain advantages and limitations. High current densities for example are achieved with PEM electrolysis but the acidic regime requires scarce and expensive platinum-group-metal (PGM) catalysts and Ti-based stack components³. On the other hand alkaline electrolysis  is regarded as a mature and less expensive technology but is limited to low current densities⁴. In order to combine the benefits of both technologies while simultaneously overcoming their drawbacks, a thin anion exchange membrane (AEM) is used as the electrolyte, allowing the use of low-cost transition-metal catalysts and stack material⁵.

In this study we developed membrane electrode assemblies (MEAs) for the AEM water electrolysis that closely match the performance of state-of-the-art PEM water electrolysis. Furthermore we have assessed commercial AEMs to determine the ionic conductivity and ion-exchange capacity. To precisely measure the hydrogen permeability of the membranes at different temperatures, an electrochemical monitoring technique was developed. Best results were achieved with Tokuyama´s A201 AEM as the ionic conductivity reached 0.4 S/cm and a hydrogen permeability of only 10^-16 mol/(cm·s·Pa) at 70 °C. A screening method was also developed to identify and characterize the most active set of electrocatalysts. Nickel nanoparticles were found to have an overpotential of about 0.4 V towards HER and OER at 10 mA/cm² and was thus chosen for the development of MEAs. Single cells exhibited a current density of 220 mA/cm² at 2.2 V and 50 °C in deionized-water. We suppose that the low current density originates from the poor ionic conductivity of the AS4 ionomer used in the electrode layer. Nevertheless the use of 1 M KOH solution as an additional electrolyte led to an improved ionic conductivity. This optimization resulted in single cells reaching current densities of 1.5 A/cm² at 2.2 V and 50 °C. Long-term experiments were additionally conducted to gain information on the impact of both operational modes on the ionomer stability.

Our results clearly show the potential of AEM water electrolysis to meet the performance of state-of-the-art PEM electrolyzers without using scarce and expensive electrocatalysts and stack components.

Figure 1: Polarization curves of a MEA containing Nickel nanoparticles on anode and cathode electrode. Tokuyama AS4 was used as ionomer and A201 as AEM. A 25 cm² single cell was measured at 50 °C in deionized water and 1M KOH solution. Current density ranges after [1].

References:

1. D. Stolten, Ed., Transition to Renewable Energy Systems, John Wiley & Sons, Weinheim (2013)

2. D. Stolten, Hydrogen and Fuel Cells: Fundamentals, Technologies and Applications, Wiley-VCH, Weinheim (2010)

3. M. Carmo, D. L. Fritz, J. Mergel, and D. Stolten, Int. J. Hydrogen Energy, 38, 4901–4934 (2013)

4. K. Zeng and D. Zhang, Prog. Energy Combust. Sci., 36, 307–326 (2010)

5. J. R. Varcoe et al., Energy Environ. Sci. (2014) DOI: 10.1039/C4EE01303D