The biggest obstacle to the widespread implementation of polymer electrolyte membrane fuel cells (PEMFCs) is their cost, primarily due to the use of platinum catalysts. The high intrinsic catalyst activity exhibited on a rotating disk electrode (RDE) is rarely realized in a membrane electrode assembly (MEA), which is a long-standing challenge for PEMFCs and a cause of low catalyst utilization. To translate the high RDE performance of a catalyst into a MEA, the design of an ideal ionomer/catalyst interface is proposed: a thin, conformal ionomer film covers the maximum surface of a Pt nanoparticle and thus simultaneously maximizes catalyst utilization, (i.e., high mass activity and electrochemically active surface area) and O₂ diffusion rate (i.e., high current density performance) without compromising proton conduction. Building such an interface is a long-standing challenge due to the lack of interaction between the ionomer and catalyst particles, resulting in large ionomer agglomerates and inhomogeneous ionomer coverage over the catalyst nanoparticle, with consequent poor fuel cell performance. In this work, this ionomer/catalyst interface has been engineered, utilizing the electrostatic attraction between positively charged catalyst and negatively charged ionomer particles in a catalyst ink and preserved in a solid catalyst layer. As a result, this interface leads to previously unachieved proton exchange membrane fuel cell performance in terms of both catalyst utilization (75% vs. 45%) and peak/rated power density (i.e., 1.430/0.930 W cm⁻², H₂/air, cathode Pt loading: 0.1 mg_Pt cm⁻²) for pure Pt catalysts, even better than those of Pt alloy catalysts. This work demonstrates the formation of an interface in the liquid phase (using ultra-small-angle X-ray scattering in combination with cryo-TEM, isothermal–titration–calorimetry) and the preservation of the interface in the solid catalyst layer (using TEM) and estimates the effective coverage and thickness of the ionomer film (using limiting current density, RDE and fuel cell performance).

Ion-conducting polymer, or ionomer, membranes play a key role as the ion-conducting electrolyte in electrochemical devices, including fuel cells and electrolyzers. The performance of ionomers, for example, its proton conductivity, is defined by its unique molecular structure and multiscale morphology. Perfluorinated sulfonic acid ionomers are the most used ionomers, and they are cast into membranes and thin films from alcohol-rich dispersions. In this work, we examine the connections between the dispersion solvent used for casting and resulting membrane properties. Understanding and controlling the hierarchical phase separated morphology of ionomer membranes and elucidating its relationship to transport properties is crucial for improving device performance. Synchrotron-based x-ray scattering is well-suited to probe the morphology in polymers that lack a high degree of long-range order, including ionomer membranes. However, it is still challenging for conventional high energy hard x-ray scattering to resolve all the morphological details in chemically heterogeneous ionomers. Here, we use emerging synchrotron tender resonant x-ray scattering (TReXS) near the sulfur K-edge to improve scattering contrast and enhance chemical specificity to the sulfonate groups in perfluorinated sulfonic acid ionomers. We find that TReXS reveals unique energy-dependent mesoscale morphological features not apparent in hard x-ray scattering, and these features depend on dispersion composition and ionomer chemistry. These structure-property relationships can be used to guide design of next-generation ionomers for energy applications. Furthermore, this work highlights synchrotron resonant x-ray scattering as an emerging tool for structural characterization of ionomers and electrochemically active polymers in general.

For the desired clean energy transition, hydrogen technologies are sought to play a critical role from electrolyzers to produce clean hydrogen from water to fuel cells to decarbonize heavy-duty transportation. Centerpiece to the successful operation of these technologies is the durable performance of the membrane-electrode assembly systems, which consists of an ion-conducting membrane between the electrodes where electrochemical reactions take place. Ion-conducting polymers, ionomers, play a key function in these systems as the separator solid-electrolyte, which requires typically a well-hydrated, phase-separated morphology that can block reactant species while transporting water and ions (protons). In addition, ionomers are also present in the electrodes as a nanometer thick film with a dual function of conducting medium and catalyst binder. In such heterogeneous structures, ionomer forms many interfaces with catalyst particles (e.g., Platinum in fuel cells) and interactions with the other species thereby making it challenging, yet critically important, to characterize their multi-scale structural features. What emerges from these distinct roles of ionomers in fuel cells and electrolyzers is the need for characterizing their structure in relation to the expected properties and desired performance for the operation. For example, for membrane, while structure-property relationships have been commonly investigated via x-ray scattering techniques, with almost three decade of work on interpretation of the so-called ionomer peak associated with the water nano-domains. However, the need for understanding and mitigating membrane degradation requires an expansion to structure-property-durability relationship. Similarly, for catalyst ionomers, their confined structure interfacing metal substrates have been idealized and examined by grazing-incidence x-ray scattering or reflectometry techniques. Yet, understanding the formation of catalyst layers from the mix of catalyst inks and ionomers require a shift to characterize their structure-processing-property relationships. This presentation will provide an overview of the current state of research on ionomer characterization with synchrotron x-rays and the ongoing an expansion of modalities that aim to image not only ionomer structure, but also their evolution under various applied stimulus via in-situ and operando x-ray techniques. Selected examples will be highlighted as case studies, demonstrating, first, how small/wide-angle x-ray scattering (SAXS/WAXS) can be repurposed with various environmental controls to understand structural changes due to degradation as well as the presence of cationic particles added to mitigate degradation. We will also show how energy-tunable x-rays at targeted absorption energies can be used to enhance the scattering signal and the acquired structural information. Then, we will show how grazing-incidence (GI) SAXS/WAXS can be employed to track the real-time evolution of the ionomer film nanostructure and crystallinity on a substrate during casting from a solution. These examples will be discussed together to provide a broader summary the current status and the future directions of both ionomer research for fuel cells and electrolyzer and the required characterization needs using synchrotron x-rays.

Microalgae are an important feedstock in aquaculture with significant economic potential in generating a diversity of bioproducts. To facilitate expansion of microalgal cultivation, a continuous automated bioreactor that uses waste acid to increase carbon bioavailability in seawater for enhanced biomass production was designed and tested with Tetraselmis suecica UTEX2286 . Carbon bioavailability was inferred from culture pH and bioreactor headspace CO ₂ concentration measurements and controlled via acidification and seawater dilution. Operating over a period of several days, the culture exhibited greater biomass productivity at a pH setpoint of 7-7.5. Outside of this range, algal activity slowed, accompanied by greater CO ₂ released to the headspace and lower pH during incubation. Increasing the carbon introduced to the bioreactor by increasing the dilution factor did not significantly increase the algal productivity. Importantly, acidification led to statistically significant gains in biomass productivity. Preliminary cost analysis showed while seawater is inexpensive, the acid cost drives the overall cost of the designed bioreactor system. Thus, the designed bioreactor and control scheme supports algal cultivation but requires low-cost acid to be economical, which may be achieved by strategically integrating microalgae cultivation with other coastal industries.

Ocean alkalinity enhancement (OAE) is a promising marine carbon dioxide removal strategy, yet its effectiveness depends on avoiding secondary abiotic precipitation that reduces alkalinity and carbon uptake. We conducted controlled microcosm experiments across temperature (7.5–29.5°C) and salinity (6.5–32.5) gradients to quantify NaOH-driven precipitation thresholds relevant to coastal OAE deployments. Higher temperatures accelerated precipitation and lowered the Ω _ar threshold for precipitation, while lower salinities led to shifts in mineralogy mediated by pH, DIC, and Mg/Ca stoichiometry. Ω _ar thresholds for CaCO₃ formation were substantially higher than previously reported, and runaway precipitation occurred only at temperatures ≥ 16°C under high alkalinity additions. Integrating experimentally-derived, temperature- and salinity-dependent saturation thresholds into a regional biogeochemical model produced substantial, spatially variable reductions in added alkalinity and atmospheric CO₂ uptake. These results provide operational bounds for safe and efficient OAE implementation and highlight the necessity of explicit precipitation parameterizations in predictive modeling frameworks.