Research

The availability of low-cost but intermittent renewable electricity (e.g., derived from solar and wind) underscores the grand challenge to store and dispatch energy so that it is available when and where it is needed. Redox-active materials promise the efficient transformation between electrical, chemical, and thermal energy, and are at the heart of carbon-neutral energy cycles.

Understanding design rules that govern materials chemistry and architecture holds the key towards rationally optimizing technologies such as batteries, fuel cells, electrolyzers, and novel thermodynamic cycles. Electrochemical and chemical reactions involved in these technologies span diverse length and time scales, ranging from Ångströms to meters and from picoseconds to years.

As such, establishing a unified, predictive framework has been a major challenge. The central question unifying our research is: “can we understand and engineer redox reactions at the levels of electrons, ions, molecules, particles and devices using a bottom-up approach?” Our approach integrates novel synthesis, fabrication, characterization, modeling and analytics to understand molecular pathways and interfacial structure, and to bridge fundamentals to energy storage and conversion technologies by establishing new design rules.

We specifically work on reactions and devices based on the migration of Li⁺, H/OH^-, Na⁺/K⁺ and O^2-.

Research Themes

1. X-ray characterization “tool kit” for probing redox (electro)chemistry in-situ

Properties of redox-active materials vary exponentially with (electro)chemical potential and are heterogeneous both at the atomic length scale and at the macroscale. The Chueh Group is at the forefront of developing new in-situ X-ray spectroscopy and microscopy approaches to probe electrochemically-active materials while reactions take place.

To “see” the gas/solid interface, such as those in solid-oxide fuel cells, we combined microfabricated solid-state electrochemical cells with ambient-pressure, surface-sensitive X-ray spectroscopies (Feng et al., Nature Commun. 2014; Mueller, et al., Nature Commun. 2015; Gopal et al., Nature Commun. 2017). This characterization platform has enabled us to probe electrochemical reactions at the atomistic and molecular level, providing a previously unavailable window into ion-insertion reactions. To probe the liquid/solid interface, such as those in lithium-ion batteries and aqueous electrolyzers, we combined microfluidic cells and X-ray microscopy to track single-particle electrochemistry in real time (Li et al., Adv. Func. Mater. 2015; Lim et al., Science 2016), providing a nanoscale, in-situ view of ion-insertion reactions.

In collaboration with SLAC National Accelerator Laboratory, we are also pushing the limits of temporal resolution by utilizing X-ray free-electron lasers to monitor a single ion hop on the order of picoseconds, which is the most fundamental process that governs ionic transport in solids.

2. Bridging the enormous span of length scales in electrochemistry

Lithium-ion batteries, for example, are transforming mobility through electric vehicles and electricity grid through the storage of intermittent renewables. Metrics such as energy density, lifetime and safety are controlled by phenomena that span enormous length scales, from individual atoms to full systems. Despite the significant progress over the past three decades, we still lack a complete understanding of how each length scale connects to one another, and most importantly, controls the behavior of the device. The Chueh Group, in collaboration with its partners, is bridging this enormous span through integration of theory, advanced characterization, and data analytics.

For lithium-ion batteries, we are focusing on three electrode materials: lithium iron phosphate, lithium layered oxides, and graphite. At the atomistic length scale, we discovered that cation migration within the layered oxide crystal structure is coupled to the redox activity of oxygen anions (Gent et al., Nature Commun. 2017). Such a discovery holds the key towards exceeding the theoretical capacity limit. At the single-particle length scale (sub-microns), we directly quantified the local Li-insertion rate in real time, providing a complete nanoscale picture of local current density, overpotential, and phase propagation (Li et al., Adv. Func. Mater. 2015; Lim et al., Science 2016). This has been a grand challenge in electrochemistry. Such observations have led to new, unified models for explaining unusual electrochemistry behaviors (Li et al., Nature Mater. 2015; Li et al., Adv. Mater. 2015) as well as kinetics phase diagram for eliminating electrochemical heterogeneity in battery electrodes. At the many-particle, agglomerate length scale (~a few microns), we developed a new understanding of how electrochemistry, mechanics, and heterogeneity are connected (Gent et al., Adv. Mater. 2016). Mechanical coupling is crucial in battery materials because most expand anisotropically when ions are inserted. Finally, at the device level, we are taking a top-down approach, analyzing large set of battery cycling data (tens of billions of data points) to discover new physics as well as to optimize battery charging, formation and prediction.

3. Point defect chemistry of redox-active interfaces

Point defects such as vacancies and localized electrons dominate the electrochemical functionalities of gas/solid and liquid/solid interfaces, yet material design rules concerning these defects are not well established. Our guiding question is: how does the broken symmetry at electrochemical interfaces alter the defect chemistry? Using in-situ, interface-sensitive X-ray probes, we are carrying out quantitative investigations of point defect chemistry by carefully controlling the atomic nature of the interface and quantifying the defect concentrations at well-defined chemical potentials.

Our efforts are focused on model systems including rare-earth-doped CeO_2-x fluorites and alkaline-earth-doped LaTMO_3-x perovskites (TM = 3d transition metal cations). These materials were selected for their relevance to high-temperature electrocatalysis, such as the oxygen-reduction reaction in solid-oxide fuel cells. We prepare atomically-defined surfaces and interfaces (Shi et al., ACS Nano 2016), and utilize this high degree of control of interfacial properties to deconvolve competing effects.

In our study of oxygen-vacancy-rich CeO_2-x, we observed that oxygen vacancies at the gas/solid interface are stabilized relative to the bulk due to a decreased Ce-O coordination at the surface and greater vibrational degrees of freedom (Feng et al., Nature Commun. 2014; Gopal et al., Adv. Mater. 2016), and are moreover tuned by misfit strain (Gopal et al., Nature Commun. 2017). Similar studies on (La,Sr)FeO_3-x-(La,Sr)CoO_3-xsolid solution perovskites revealed a strongly covalent TM-O bonding near the surface, which alters the nature of the electronic defect (Mueller et al., Nature Commun. 2015). We directly observed that localized electron holes on oxygen form in place of localized electrons on the TM, the energetics and localization of which are extremely sensitive to the interfacial crystallography and dopant concentration.

4. Molecular insights into interfacial redox reactions

How do electrochemical reactions such as water splitting and oxygen reduction proceed kinetically in these materials, and what are their rate-determining steps? Answering these questions is a crucial step toward establishing rational design rules for improving electrochemical efficiencies.

Utilizing the in-situ X-ray characterization platforms described earlier, we are investigating how the reaction rates scale with concentration of reaction intermediates, such as adsorbates, surface point defects, and electrons. In the case of the water-splitting reaction at the CeO_2-x/gas interface, we unambiguously determined that the one-electron reduction of hydroxyl adsorbate is rate determining, in contrast with the common belief that oxygen-ion-incorporation is limiting (Feng et al., Nature Commun. 2015). The study was also extended to CO₂ dissociation on the same material, which proceeds via a two-electron reduction of the carbonate^-adsorbate. There, the rate-limiting step was identified as the first-electron reduction of CO₂ (Feng et al., Phys. Chem. Chem. Phys. 2015). The improved molecular insights into these reactions on CeO_2-x point to the surface electrons, rather than the surface oxygen vacancies, as the main participants in the rate-determining process. We are also carrying out similar studies for aqueous electrochemical reactions such as the oxygen evolution reactions.

5. Exploring new modes of energy storage and conversion

Drawing from the fundamental insights and rational design rules on redox-active materials summarized above, the Chueh Group has developed several new materials and device architectures with substantially enhanced energy conversion efficiencies and reversibility. For renewable hydrogen production from water, we have demonstrated a new class of elevated temperature photo-electrochemical cells based on oxygen-ion conductors and metal-oxide light absorbers with intermediate band gaps such as α-Fe₂O₃ and BiVO₄. In these cells, thermal energy enhances the transport of photoexcited electron carriers (Ye et al., J. Mater. Chem. A 2015; Zhang et al. Energy Environ. Sci. 2016), and the gas/solid interface provides a new pathway to separate photoexcited electrons and holes (Ye et al., Phys. Chem. Chem. Phys. 2013). We are also investigating liquid-metal-based electrochemistry, which does not require the usual triple-phase boundaries for reactions to take place. Liquid-metal-based flow batteries, for example, promise high reversibility and is suitable for grid-level energy storage.