Reading into our research

Perspectives from fundamental materials to electrochemical energy systems

Our research activities relate to several energy conversion technologies, which have been reviewed and discussed in a few articles, such as low-temperature water-splitting electrolysis, polymer electrolyte fuel cells (for heavy-duty applications), and CO2 electrolysis. 

In terms of specific ionomer-related research topics, a comprehensive review of perfluorinated ionomers can be found in our seminal PFSA Chemical Reviews article. 

In addition, the following reviews and perspectives provide insights into particular topics and technologies related to ionomers:

• Flow battery membranes with a focus on transport phenomena, including the performance tradeoffs
• Ionomers for bipolar membranes covering their chemistry, structure, and physics related to the performance of electrochemical processes (in collaboration with Boettcher Group)
• Ionomer-coated catalysts in CO2 electrolysis modulating the microenvironments for enhanced product selectivity (in collaboration with Weber and Bell Groups)
• Cation selectivity and partitioning in ionomers with a theoretical framework for Donnan equilibrium and selective partitioning
• Mechanochemistry in ion-conducting soft matter with a focus on chemical-mechanical coupling effects on degradation

Ionomers: PFSAs and beyond

For anyone interested in learning the basics of PFSA ionomers and proton-exchange membranes, sections 1-2 of the PFSA Chemical Reviews are a good start. Then, the following papers could be studied to learn more about…

…hydration and transport phenomena, including water uptake behavior, structure-hydration relationships of PFSA membranes and thin films with equivalent-weight and side-chain chemistry effect, electroosmosis and water transport, interfacial transport, water diffusion, conductivity-hydration relationships, transport phenomena with/in electrolytes and organic solvents.

In addition, some of our papers (in collaboration with Weber Group) on continuum modeling of transport phenomena present a thermodynamic modeling framework for water uptake in ionomers, a transport model for their conductivity, and demonstrate how the transport properties could be linked to the nanoscale and mesoscale structures.

We have expanded these structure-transport characterizations to include ionomer systems in organic sorbates and alcohols (PFSA-PEM), electrolytes (AEMs), and electrolytes (PEMs). 

…for a deeper dive into cation effects in ionomers, one can start with the Cation selectivity and partitioning in ionomers with a theoretical framework and literature review of cation selectivity values for Nafion.  In addition, we have reported hydration, conductivity, nanostructural, and mechanical properties of Nafion membrane fully exchanged with different cations. More recently, we focused on water uptake and ion transport of Nafion doped with low to high concentrations of Cerium cations and Cobalt cations, which provided fundamental insight into the cation transport mechanisms in mixed proton-cation systems. A companion letter compares the ion-partitioning (curves) of Cerium-proton in ionomer membrane and thin films to elucidate the interplay between confinement effect and ion interactions. Another paper reports the effect of cations on stress-strain curves of Nafion membranes from a polymer physics perspective. 

…exploration of new ionomer chemistries - For investigations focusing on sulfonated proton-exchange ionomers (beyond Nafion), we have studied new chemistries of multi-acid side chain ionomers and their structure-transport relationships through extensive chemical-structural investigations and structure-hydration characterization with X-rays, including Sulfur-edge resonant X-ray scattering. More recently, in collaboration with 3M Company, new ionomers with varying side-chain and end-group chemistry were investigated in thin film motifs for use as catalyst-ionomers. This study was followed by another systematic investigation focusing on the interplay between the chemistry and solvent composition to guide solution-processing and ink optimization efforts for fuel cell catalyst layers. 

In addition to the above-listed comprehensive coverage of cation/proton-exchange ionomers, we have examined anion-exchange ionomers in terms of AEM structure-properties in electrolytes, AEM conductivity-permselectivity in water, electrochemical water transport in AEMs, anion-exchange thin films, and their interfaces in CO2 electrolysis.  

We have also investigated the effect of reinforcements on membranes, including the structure-function relationship and mechanical durability of PFSA-based reinforced-PEMs, which elucidates how reinforcement leads to anisotropy in swelling and transport properties. In addition, the properties of reinforced AEMs in water and liquid electrolytes are discussed for electrolysis.

The table below shows an example of how to navigate our research papers based on the ionomer membrane type and form:

^{* in collaboration with Weber & Boettcher Groups; ** in collaboration with Weber & Bell Groups}

Morphological Characterization, including X-ray studies, has been an essential component of many of our papers. Specifically, we have used small/-wide-angle X-ray scattering (SAXS/WAXS) of Nafion membranes by investigating the effect of 

• Humidity/hydration
• Surface wetting interactions
• Thermal annealing and temperature
• Cation-exchange
• Organic solvents
• Dispersion solvent composition
• Compression
• Thickness
• Reinforcement
• Hygrothermal ageing
• Chemical degradation

We have also published works on more advanced energy-tunable X-ray scattering techniques (Resonant X-ray scattering) to resolve the chemical specificity of the ionomer structures with varying chemistries and analysis of X-ray scattering for thin films using electric-field intensity.

Many of our investigations also employ these x-ray scattering techniques to ionomers as membranes (bulk form), dispersions (liquid solution form), and nanometer-thick spin-cast thin films on support, including the effect of substrate metallic support, chemistry, casting, and solvent. Furthermore, we have also used time-resolved x-rays to monitor the evolution of an ionomer structure during casting (solvent evaporation): from solution to a thin film. 

Ionomer Thin Films and Interfaces

Ionomer thin films exist as nanometer-thick electrolyte “thin films” binding the catalytic particles in the electrodes of electrochemical energy conversion systems (See section 6: Thin Films and Interfaces of PFSA Chemical Reviews article). These ionomer thin films transport ionic and gaseous species to/from the catalyst sites and help facilitate the electrochemical reactions occurring in the electrodes. For example, in fuel cells, ionomers and their interaction with the Pt and C catalyst particles contribute to the transport resistances observed therein, as discussed in a perspective article on the topic. Thus, any improvement in the electrode performance and/or catalyst utilization requires an understanding of the role of ionomer in electrodes, how they contribute to the transport (resistances) in catalyst layers, and how their structure-properties could be investigated in thin-film form at interfaces to develop a clearer picture of catalyst ionomers. 

When confined to nanoscales, ionomer behavior deviates from its bulk analogue (membranes), resulting in a change in substrate interactions, structural orientation, thermal transition, water uptake, swelling, swelling kinetics, ion conductivity, ion partitioning, and mechanical properties. The origins of these property changes have been linked to the nanostructural changes inferred from Grazing-incidence X-ray Scattering (GIXS/GISAXS) experiments we conducted at the ALS. These studies encompass investigation of a larger number of chemical and environmental factors controlling the nanostructure, from the effects of casting and reducing/oxidizing environment to the effects of substrates, cations, thickness, and equivalent weight. In terms of chemistry, we have studied the effect of side-chain chemistry (PFSAs) as well as multi-acid side-chain chemistries beyond Nafion. and how they influence the swelling-conductivity and can be used to modulate substrate-ionomer interactions. 

The table below lists some of our papers on ionomer thin films & interfaces and their focus areas:

Bridging ionomer thin film properties to catalyst-layer performance is an important step toward translating the fundamental understanding of ionomer interfaces in applied electrochemical systems, whether they are fuel-cells, water electrolysis, or CO2 electrolysis. We have examined this topic in collaboration with various research groups and industry. These studies encompass:

Linking PFSA thin film structure-properties to fuel-cell electrode performance at high current density, with a focus in side-chain chemistry. A catalyst ionomer study (in collaboration with GM) examined how equivalent weight (EW) and ionomer chemistry influence ionomer thin film properties measured ex-situ and in-situ electrochemical diagnostics of fuel cells. Engineering the ionomer-catalyst interface for improved mass activity and performance by using strategies such as controlling the distribution and coverage of PFSAs on Pt catalysts with additives (in collaboration with Case Western), and carbon surface functionalization for improved catalyst utilization (in collaboration with Purdue). Such electrode ionomer studies have more recently been expanded to hydrocarbon catalyst ionomers with a discussion on the origins and mitigation of the transport resistances with the aid of thin film properties (in collaboration with IMTEK and Simon Fraser University).

We have recently applied this research approach to investigate the water-splitted electrolyzers and the role of ionomer interactions with Iridium catalysts in controlling the thin-film structure-function and the electrode performance. 

Mechanical Characterization and Mechanochemistry

In terms of mechanical response and properties of ion-exchange membranes, the tensile stress-strain behavior of Nafion-based proton-exchange membranes was investigated in water vapor and liquid water environments. The effect of cations on stress-strain response and fracture behavior was elucidated in a paper elucidating the interplay between the deformation mechanisms and ionic interactions. 

Our recent studies examined the compressive stress-strain relationship of electrolyzer cell components in liquid water and creep of hydrated electrolyzer membranes in water. We have also examined the effect of compression on chemical degradation, water uptake, and conductivity-structure of PFSA membranes, as well as their compression creep response in water. 

In terms of chemical-mechanical coupling in ionomers, we have studied how the water uptake of an ionomer membrane changes under compression (due to additional pressure and water loss), which was examined further in a subsequent study in terms of its impact on nanostructure (change in domain spacing and orientation under compression) and proton conductivity changes under compressive loads (and its anisotrpopy due to deformation and domain-orientation).

Lastly, we extended our investigations to the stiffness of thin films under confinememt (<100 nm) and also discussed the chemical-mechanical aspects of ionomer deformation and degradation in a perspective article on mechanochemistry.