Electrochemistry offers a clean pathway to reduce greenhouse gas emissions in manufacturing, chemical production, transportation, and to store excess energy from intermittent renewables like wind and solar. My research is focused on improving electrochemical energy storage and conversion technologies through rational material design. I develop new, higher performance electrodes and advanced techniques to study material structure-property relationships during operation. I am particularly interested in the functionality of transition metal oxides and polymeric electrodes in aqueous systems for use as battery electrodes and electrocatalysts for hydrogen production, oxygen reduction/evolution, CO2 reduction, and ammonia production.
Sr Res Scientist-Physical, Materials Science and Engineering
Honors & Awards
Best Presentation: In Situ/Operando Characterization of Energy Materials, Fall MRS Meeting, Materials Research Society (2020)
Best Nano Portfolio Presentation, Center for Nano and Molecular Science, The University of Texas at Austin (2016)
Certification In Nanoscience and Nanotechnology, Center for Nano and Molecular Science, The University of Texas at Austin (2016)
First Prize in Research Excellence in Renewable & Clean Energy, The University of Texas at Austin (2016)
Ph.D., The University of Texas at Austin, Chemistry (2016)
B.S., Stanford University, Chemistry, Materials Science & Engineering (Minor) (2012)
Selectivity of Electrochemical Ion Insertion into Manganese Dioxide Polymorphs.
ACS applied materials & interfaces
The ion insertion redox chemistry of manganese dioxide has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and nonprotonic cations in interstitial sites. Despite many reports on individual ion-polymorph couples, much less is known about the selectivity of electrochemical ion insertion in MnO2. In this work, we use density functional theory to holistically compare the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+, Al3+) in aqueous and nonaqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that for nonprotonic cations, ion selectivity depends on the oxygen coordination environments inside a polymorph. When H+ is present, however, the driving force to form hydroxyl bonds is usually stronger. In aqueous electrolytes, only three ion-polymorph pairs are thermodynamically stable within water's voltage stability window (Na+ and K+ in alpha-MnO2, and Li+ in lambda-MnO2), with all other ion insertion being metastable. We find Al3+ may insert into the delta, R, and lambda polymorphs across the full 2-electron redox of MnO2 at high voltage; however, electrolytes for multivalent ions must be designed to impede the formation of insoluble precipitates and facilitate cation desolvation. We also show that small ions coinsert with water in alpha-MnO2 to achieve greater coordination by oxygen, while solvation energies and kinetic effects dictate water coinsertion in delta-MnO2. Taken together, these findings explain reports of mixed ion insertion mechanisms in aqueous electrolytes and highlight promising design strategies for safe, high energy density electrochemical energy storage, desalination batteries, and electrocatalysts.
View details for DOI 10.1021/acsami.2c16589
View details for PubMedID 36546548
- Electrochemical ion insertion from the atomic to the device scale NATURE REVIEWS MATERIALS 2021
Correlative operando microscopy of oxygen evolution electrocatalysts.
2021; 593 (7857): 67–73
Transition metal (oxy)hydroxides are promising electrocatalysts for the oxygen evolution reaction1-3. The properties of these materials evolve dynamically and heterogeneously4 with applied voltage through ion insertion redox reactions, converting materials that are inactive under open circuit conditions into active electrocatalysts during operation5. The catalytic state is thus inherently far from equilibrium, which complicates its direct observation. Here, using a suite of correlative operando scanning probe and X-ray microscopy techniques, we establish a link between the oxygen evolution activity and the local operational chemical, physical and electronic nanoscale structure of single-crystalline beta-Co(OH)2 platelet particles. At pre-catalytic voltages, the particles swell to form an alpha-CoO2H1.5·0.5H2O-like structure-produced through hydroxide intercalation-in which the oxidation state of cobalt is +2.5. Upon increasing the voltage to drive oxygen evolution, interlayer water and protons de-intercalate to form contracted beta-CoOOH particles that contain Co3+ species. Although these transformations manifest heterogeneously through the bulk of the particles, the electrochemical current is primarily restricted to their edge facets. The observed Tafel behaviour is correlated with the local concentration of Co3+ at these reactive edge sites, demonstrating the link between bulk ion-insertion and surface catalytic activity.
View details for DOI 10.1038/s41586-021-03454-x
View details for PubMedID 33953412
Tuning electrochemially driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis.
Structure-activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.
View details for DOI 10.1038/s41563-020-00877-1
View details for PubMedID 33432142
Energetic Control of Redox-Active Polymers toward Safe Organic Bioelectronic Materials.
Advanced materials (Deerfield Beach, Fla.)
Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side-products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox-active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side-reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high-performance, state-of-the-art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2 O2 ), a reactive side-product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox-active organic semiconductors based on donor-acceptor copolymers that prevents the formation of H2 O2 during device operation. This study elucidates the previously overlooked side-reactions between redox-active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte-gated devices in application-relevant environments.
View details for DOI 10.1002/adma.201908047
View details for PubMedID 32125736
- Interpreting Tafel behavior of consecutive electrochemical reactions through combined thermodynamic and steady state microkinetic approaches ENERGY & ENVIRONMENTAL SCIENCE 2020; 13 (2): 622–34
- Activation of ultrathin SrTiO3 with subsurface SrRuO3 for the oxygen evolution reaction ENERGY & ENVIRONMENTAL SCIENCE 2018; 11 (7): 1762–69
Water electrolysis on La1-xSrxCoO3-delta perovskite electrocatalysts
2016; 7: 11053
Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B-O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co-O bond and the concentration of oxygen vacancies are controlled through Sr(2+) substitution into La(1-x)Sr(x)CoO(3-δ) . We attempt to rationalize the high activities of La(1-x)Sr(x)CoO(3-δ) through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.
View details for DOI 10.1038/ncomms11053
View details for Web of Science ID 000372721700001
View details for PubMedID 27006166
View details for PubMedCentralID PMC4814573
Anion charge storage through oxygen intercalation in LaMnO3 perovskite pseudocapacitor electrodes
2014; 13 (7): 726–32
Perovskite oxides have attracted significant attention as energy conversion materials for metal-air battery and solid-oxide fuel-cell electrodes owing to their unique physical and electronic properties. Amongst these unique properties is the structural stability of the cation array in perovskites that can accommodate mobile oxygen ions under electrical polarization. Despite oxygen ion mobility and vacancies having been shown to play an important role in catalysis, their role in charge storage has yet to be explored. Herein we investigate the mechanism of oxygen-vacancy-mediated redox pseudocapacitance for a nanostructured lanthanum-based perovskite, LaMnO3. This is the first example of anion-based intercalation pseudocapacitance as well as the first time oxygen intercalation has been exploited for fast energy storage. Whereas previous pseudocapacitor and rechargeable battery charge storage studies have focused on cation intercalation, the anion-based mechanism presented here offers a new paradigm for electrochemical energy storage.
View details for DOI 10.1038/NMAT4000
View details for Web of Science ID 000338482300021
View details for PubMedID 24880729
Reversible Electrochemical Charging of n-Type Conjugated Polymer Electrodes in Aqueous Electrolytes.
Journal of the American Chemical Society
Conjugated polymers achieve redox activity in electrochemical devices by combining redox-active, electronically conducting backbones with ion-transporting side chains that can be tuned for different electrolytes. In aqueous electrolytes, redox activity can be accomplished by attaching hydrophilic side chains to the polymer backbone, which enables ionic transport and allows volumetric charging of polymer electrodes. While this approach has been beneficial for achieving fast electrochemical charging in aqueous solutions, little is known about the relationship between water uptake by the polymers during electrochemical charging and the stability and redox potentials of the electrodes, particularly for electron-transporting conjugated polymers. We find that excessive water uptake during the electrochemical charging of polymer electrodes harms the reversibility of electrochemical processes and results in irreversible swelling of the polymer. We show that small changes of the side chain composition can significantly increase the reversibility of the redox behavior of the materials in aqueous electrolytes, improving the capacity of the polymer by more than one order of magnitude. Finally, we show that tuning the local environment of the redox-active polymer by attaching hydrophilic side chains can help to reach high fractions of the theoretical capacity for single-phase electrodes in aqueous electrolytes. Our work shows the importance of chemical design strategies for achieving high electrochemical stability for conjugated polymers in aqueous electrolytes.
View details for DOI 10.1021/jacs.1c06713
View details for PubMedID 34469688
- Strong Catalyst-Support Interactions in Electrochemical Oxygen Evolution on Ni-Fe Layered Double Hydroxide ACS ENERGY LETTERS 2020; 5 (10): 3185–94
- Electrochemical Reactivity of Faceted beta-Co(OH)(2) Single Crystal Platelet Particles in Alkaline Electrolytes JOURNAL OF PHYSICAL CHEMISTRY C 2019; 123 (31): 18783–94
Decoupling the roles of carbon and metal oxides on the electrocatalytic reduction of oxygen on La1-xSrxCoO3-d perovskite composite electrodes
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
2019; 21 (6): 3327–38
Perovskite oxides are active room-temperature bifunctional oxygen electrocatalysts in alkaline media, capable of performing the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with lower combined overpotentials relative to their precious metal counterparts. However, their semiconducting nature necessitates the use of activated carbons as conductive supports to generate applicably relevant current densities. In efforts to advance the performance and theory of oxide electrocatalysts, the chemical and physical properties of the oxide material often take precedence over contributions from the conductive additive. In this work, we find that carbon plays an important synergistic role in improving the performance of La1-xSrxCoO3-δ (0 ≤ x ≤ 1) electrocatalysts through the activation of O2 and spillover of radical oxygen intermediates, HO2- and O2-, which is further reduced through chemical decomposition of HO2- on the perovskite surface. Through a combination of thin-film rotating disk electrochemical characterization of the hydrogen peroxide intermediate reactions (hydrogen peroxide reduction reaction (HPRR), hydrogen peroxide oxidation reaction (HPOR)) and oxygen reduction reaction (ORR), surface chemical analysis, HR-TEM, and microkinetic modeling on La1-xSrxCoO3-δ (0 ≤ x ≤ 1)/carbon (with nitrogen and non-nitrogen doped carbons) composite electrocatalysts, we deconvolute the mechanistic aspects and contributions to reactivity of the oxide and carbon support.
View details for DOI 10.1039/c8cp06268d
View details for Web of Science ID 000459584900049
View details for PubMedID 30688319
Anion-Based Pseudocapacitance of the Perovskite Library La1-xSrxBO3-delta (B = Fe, Mn, Co)
ACS APPLIED MATERIALS & INTERFACES
2019; 11 (5): 5084–94
We have synthesized a library of perovskite oxides with the composition La1- xSr xBO3-δ ( x = 0-1; B = Fe, Mn, Co) to systematically study anion-based pseudocapacitance. The electrochemical capacitance of these materials was evaluated by cyclic voltammetry and galvanostatic charging/discharging in 1 M KOH. We find that greater oxygen vacancy content (δ) upon systematic incorporation of Sr2+ linearly increases the surface-normalized capacity with a slope controlled by the B-site element. La0.2Sr0.8MnO2.7 exhibited the highest specific capacitance of 492 F g-1 at 5 mV s-1 relative to the Fe and Co oxides. In addition, the first all-perovskite asymmetric pseudocapacitor has been successfully constructed and characterized in neutral and alkaline aqueous electrolytes. We demonstrate that the asymmetric pseudocapacitor cell voltage can be increased by widening the difference between the B-site transition metal redox potentials in each electrode resulting in a maximum voltage window of 2.0 V in 1 M KOH. Among the three pairs of asymmetric pseudocapacitors constructed from SrCoO2.7, La0.2Sr0.8MnO2.7, and brownmillerite (BM)-Sr2Fe2O5, the BM-Sr2Fe2O5//SrCoO2.7 combination performed the best with a high energy density of 31 Wh kg-1 at 450 W kg-1 and power density of 10 000 W kg-1 at 28 Wh kg-1.
View details for DOI 10.1021/acsami.8b19592
View details for Web of Science ID 000458347900040
View details for PubMedID 30640433
- Bifunctional OER/ORR catalytic activity in the tetrahedral YBaCo4O7.3 oxide JOURNAL OF MATERIALS CHEMISTRY A 2019; 7 (1): 330–41
Exceptional electrocatalytic oxygen evolution via tunable charge transfer interactions in La0.5Sr1.5Ni1-xFexO4±δ Ruddlesden-Popper oxides.
2018; 9 (1): 3150
The electrolysis of water is of global importance to store renewable energy and the methodical design of next-generation oxygen evolution catalysts requires a greater understanding of the structural and electronic contributions that give rise to increased activities. Herein, we report a series of Ruddlesden-Popper La0.5Sr1.5Ni1-xFexO4±δ oxides that promote charge transfer via cross-gap hybridization to enhance electrocatalytic water splitting. Using selective substitution of lanthanum with strontium and nickel with iron to tune the extent to which transition metal and oxygen valence bands hybridize, we demonstrate remarkable catalytic activity of 10 mA cm-2 at a 360 mV overpotential and mass activity of 1930 mA mg-1ox at 1.63 V via a mechanism that utilizes lattice oxygen. This work demonstrates that Ruddlesden-Popper materials can be utilized as active catalysts for oxygen evolution through rational design of structural and electronic configurations that are unattainable in many other crystalline metal oxide phases.
View details for DOI 10.1038/s41467-018-05600-y
View details for PubMedID 30089833
View details for PubMedCentralID PMC6082882
Synthesis and charge storage properties of templated LaMnO3-SiO2 composite materials
2017; 46 (3): 977–84
Mesoporous LaMnO3 with bulk surface areas in the range 225-300 m2 g-1 were prepared by direct overgrowth around the short-channel version of SBA-15 silica. The extent of LaMnO3 growth was found to be affected by the polarity of solvent system used to impregnate the SBA-15 with La3+ and Mn2+ precursors. The resulting LaMnO3-SiO2 composites were stable in refluxing NaOH, suggesting that the SiO2 was fully encapsulated. The composites were structurally characterized using a range of techniques including 2-D elemental mapping and Raman spectroscopy. The electrochemical behavior of the composites was tested for pseudocapacitance, which revealed normalized specific capacitances over 200 F g-1.
View details for DOI 10.1039/c6dt04665g
View details for Web of Science ID 000393979300041
View details for PubMedID 28009889
- Nanostructured LaNiO3 Perovskite Electrocatalyst for Enhanced Urea Oxidation ACS CATALYSIS 2016; 6 (8): 5044–51
- Tuning the Electrocatalytic Activity of Perovskites through Active Site Variation and Support Interactions CHEMISTRY OF MATERIALS 2014; 26 (11): 3368–76
An ultrafast nickel-iron battery from strongly coupled inorganic nanoparticle/nanocarbon hybrid materials
Ultrafast rechargeable batteries made from low-cost and abundant electrode materials operating in safe aqueous electrolytes could be attractive for electrochemical energy storage. If both high specific power and energy are achieved, such batteries would be useful for power quality applications such as to assist propelling electric vehicles that require fast acceleration and intense braking. Here we develop a new type of Ni-Fe battery by employing novel inorganic nanoparticle/graphitic nanocarbon (carbon nanotubes and graphene) hybrid materials as electrode materials. We successfully increase the charging and discharging rates by nearly 1,000-fold over traditional Ni-Fe batteries while attaining high energy density. The ultrafast Ni-Fe battery can be charged in ~2 min and discharged within 30 s to deliver a specific energy of 120 Wh kg(-1) and a specific power of 15 kW kg(-1). These features suggest a new generation of Ni-Fe batteries as novel devices for electrochemical energy storage.
View details for DOI 10.1038/ncomms1921
View details for PubMedID 22735445
Improved Diabetes Control and Pancreatic Function in a Type 2 Diabetic after Omeprazole Administration
CASE REPORTS IN ENDOCRINOLOGY
2012; 2012: 468609
A 43-year-old man with type 2 diabetes, opposed to insulin use and poorly responsive to oral agents added sequentially over 6 years, was placed on 40 mg omeprazole twice daily. A linear decline in daily fasting blood glucose was observed over the first two-month treatment, and his hemoglobin A1c was reduced from 11.9% to 8.2%, then sustained at 8.1% after four months. Glucose, insulin, and C-peptide response to a 2-hour glucose tolerance test were consistently improved across this time period, and calculated beta-cell mass increased by 67%. We believe these responses are consistent with activation or neogenesis of pancreatic beta cells, possibly through a gastrin-mediated mechanism.
View details for DOI 10.1155/2012/468609
View details for Web of Science ID 000215160700019
View details for PubMedID 22937295
View details for PubMedCentralID PMC3420592