Rex Hjelm presents at BP/KRF Fellows’ Conference

Rex P. Hjelm, a Los Alamos National Laboratory and New Mexico Consortium scientist, presented the invited talk, Ionomer Physics and Fuel Cell Engineering December 4, 2019 at the BP/KRF Fellows’ Global Conference in Seoul, South Korea.

The goal of the BP program is to enhance the R&D level of Korea by attracting outstanding overseas scientists through various joint research activities in R&D fields in Korea, and to establish a long term international cooperation network. The Korea Research Fellowship (KRF) program invites outstanding postdoctoral researchers to achieve a excellent performance in their early-mid stages of their professional careers. The program provides high-potential young & overseas researchers with a chance to carry out creative and emerging fields of projects ; and supports them to grow as research leaders of tomorrow.

The objective of this research under the KRF/PB program was to probe the basic science of ionomer polymer physics to provide rules to guide engineering of advanced fuel cell devices with greatly improved performance including transport, durability, H2 crossover and voltage cyclic hysteresis properties. The work used the synergism between applied basic science and engineering design, leveraging on complementary capabilities and approaches of the Korean and US institutions.

Membrane electrode assemblies (MEAs) make up fuel cell anodes and cathodes. As such MEAs support the energy producing reaction of hydrogen and oxygen to water. They consist of an ionomer electrolyte and Pt catalyst. Commercial MEA ionomers’ have amphiphilic character from hydrophobic, Teflon, main chains and hydrophilic sulphonyl group terminated, fluoroether pendant chains. Hjelm tested the hypothesis, using structure probes, small-angle neutron scattering and neutron reflectometry, with measurements of MEA properties and performance, that the amphiphilic balance of different ionomers can be used as a tool, to control of MEA morphology, properties and performance. The work resulted in a series of rules:

• Solvent interactions with ionomers having higher amphiphilic balance, given by Flory- Huggins interaction parameters, govern main chain swelling, promoting chain entanglements, leading to improved MEA durability.
• Ionomers form rod-like micelles during MEA fabrication, the length of which is determined by the length along the main chain between the pendant groups.
• Charge effects in ionomers with lower amphiphilic balance template MEA structure, leading to the hypothesis that this, and rod-like particle aspect ratios, are associated with formation of MEA transport channels, providing a means to improve transport properties.
• The hysteresis associated with the voltage cycle-induced Pt hydrophobic (Pt metal) to hydrophilic (Pt-oxide) transition is due to pendant group movement, which is less in ionomers with lower amphiphilic balance, leading to improved fuel cell efficiency.

Hjelm’s current work is focused on implementing these rules in the design of fuel cells.

Even more recently, Hjelm’s paper titled “Unusually High Concentration of Alkyl Ammonium Hydroxide in the Cation-Hydroxide-Water Co-adsorbed Layer on Pt”, has just been accepted for publication in ACS Applied Materials & Interfaces. Authors of this paper include: Dumont, J.H., Spears, A.J., Hjelm, R.P., Hawley, M.E., Maurya, S., Li, D., Yuan, G., Watkins, E.B. and Kim, Y.S.

The NMC congratulates Rex Hjelm on his outstanding work.

Article by Carrie Talus