SPECS Researchers Use Resonant X-ray Diffraction to Better Understand Ion Location in Doped Polymers
Electrochemical systems underpin many modern energy devices and have undergone extensive refinement and evolution. Central to this development is the role of conductive polymers, polymers that can conduct electricity. In the context of a world increasingly gravitating towards sustainable energy and net-zero carbon goals, conductive polymers stand out as one beacon of promise towards low-cost energy systems. When implemented as electrodes, they offer a fusion of flexibility, light-weight characteristics, and the ability to be tuned for specific applications, a combination hard to achieve with traditional inorganic materials. Moreover, their synthesis and processing can be more environmentally benign, aligning with green chemistry principles. By replacing more energy-intensive materials and facilitating efficient-energy storage and conversion, conductive polymers can play a pivotal role in reducing carbon emissions. Within this backdrop, the Center for Soft PhotoElectrochemical Systems (SPECS) embarks on a quest to decode the nuanced interactions of ions within these polymers, a key step in unlocking their full potential as fuel-producing electrodes and propelling us closer to a net-zero carbon future.
SPECS is looking to use conductive polymer electrodes to drive chemical reactions that produce energy-storing molecules. While conductive polymers have been around for decades, their use as photoelectrode materials is relatively recent. Effective electrodes, regardless of their makeup, should be able to transport charges in the form of electrons to the electrode–electrolyte interface and transfer that charge to drive a reaction, all while physically withstanding long-term usage. With these criteria in mind, SPECS is tackling this challenge as it relates to conductive polymer electrodes using three thrusts: (1) Understanding the transport of charges through the polymer, (2) understanding how charges are transferred to drive chemical reactions, and (3) controlling the durability of the polymer material for repeated use as an electrode.
Where exactly are the ions? This simple question is at the heart of one of SPECS’ early fundamental questions. The ions in the electrolyte play a crucial role in a polymer’s ability to transport and transfer charges. In inorganic semiconductors, generated charge carriers can freely move through the material. This differs in conductive polymers, where charge carriers need to be stabilized and escorted across the polymer backbone by a counter ion from the electrolyte. Polymers can act like sponges—absorbing solvent molecules and ions from the electrolyte. When trying to understand the fundamental movement of charges (and therefore counter ions) through a polymer, this “sponge” effect can make pinpointing the exact location of the supporting counterion hard to determine.
Researchers working for SPECS have begun to answer the question of where ions reside in the polymer structure by using a combination of experimental and computational results in a recently published article1. The polymer system chosen for this study was a thiophene-based polymer, referred to as P3MEEMT, a polymer with side chains that allow water molecules and ions that make up the electrolyte to be taken up by the polymer matrix easily, shown in Figure 1. To unveil exactly where the ions go when charges are on the polymer backbone, SPECS researchers used resonant X-ray diffraction, abbreviated RXRD.
RXRD can be cleverly employed to monitor the exact position of an atom through its resonant response to X-ray energy. This resonant response changes based on the surrounding crystallographic environment of the atom being monitored. In this case, the Cl atom present on the electrolyte anion, ClO4-, can be monitored in the polymer lattice environment, especially with respect to the S atom present on the polymer backbone. Changes in the diffraction of Cl atoms based on the applied potential to the polymer film were observed, indicating that the position of the anion changes in response to charge carriers on the polymer, as illustrated in Figure 2. Computational models were paired with the experimental results for an added dimension of understanding. Using computationally derived models of the polymer/counterion and simulating the experimental conditions, computational results mirrored the experimental results.
Overall, SPECS researchers found that the ions from the electrolyte can reside far away from the formed charges on the polymer backbone. The agreement between the experimental results and the simulated results lends credence to these findings. The major implication of such findings is that ions may not need to be as physically “close” to a charge to compensate for it, providing insights into how charges are transported and supported on a polymer electrode. A fundamental understanding of how charges and their escorting ions are transported through a polymer is crucial to developing guiding principles for polymer electrode design, and ultimately their widespread use as electrodes for renewable fuel production.
(1) Flagg, L. Q.; Onorato, J. W.; Luscombe, C. K.; Bhat, V.; Risko, C.; Levy-Wendt, B.; Toney, M. F.; McNeill, C. R.; Freychet, G.; Zhernenkov, M.; Li, R.; Richter, L. J. Resonant X-Ray Diffraction Reveals the Location of Counterions in Doped Organic Mixed Ionic Conductors. Chem. Mater. 2023, 35 (10), 3960–3967. https://doi.org/10.1021/acs.chemmater.3c00180.
Aspects of the X-ray analysis (MFT) and DFT (CR) were supported by the Center for Soft PhotoElectroChemical Systems (SPECS), an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, within the U.S. Department of Energy’s Office of Science (DE-SC0023411).