Fikile R. Brushett

Transition of lithium growth mechanisms in liquid electrolytes

Next-generation high-energy batteries will require a rechargeable lithium metal anode, but lithium dendrites tend to form during recharging, causing short-circuit risk and capacity loss, by mechanisms that still remain elusive. Here, we visualize lithium growth in a glass capillary cell and demonstrate a change of mechanism from root-growing mossy lithium to tip-growing dendritic lithium at the onset of electrolyte diffusion limitation. In sandwich cells, we further demonstrate that mossy lithium can be blocked by nanoporous ceramic separators, while dendritic lithium can easily penetrate nanopores and short the cell. Our results imply a fundamental design constraint for metal batteries (“Sands capacity”), which can be increased by using concentrated electrolytes with stiff, permeable, nanoporous separators for improved safety.

A Carbon-Supported Copper Complex of 3,5-Diamino-1,2,4-triazole as a Cathode Catalyst for Alkaline Fuel Cell Applications

The performance of a novel carbon-supported copper complex of 3,5-diamino-1,2,4-triazole (Cu-tri/C) is investigated as a cathode material using an alkaline microfluidic H(2)/O(2) fuel cell. The absolute Cu-tri/C cathode performance is comparable to that of a Pt/C cathode. Furthermore, at a commercially relevant potential, the measured mass activity of an unoptimized Cu-tri/C-based cathode was significantly greater than that of similar Pt/C- and Ag/C-based cathodes. Accelerated cathode durability studies suggested multiple degradation regimes at various time scales. Further enhancements in performance and durability may be realized by optimizing catalyst and electrode preparation procedures.

Alkaline Microfluidic Hydrogen-Oxygen Fuel Cell as a Cathode Characterization Platform

We report on an alkaline microfluidic fuel cell for catalyst and electrode characterization. Its constantly refreshing alkaline electrolyte stream enables autonomous control over the flow rate and electrolyte composition, as well as independent analysis of the individual electrodes, rendering this platform a powerful analytical tool. Here, polytetrafluoroethylene PTFE-bonded Ag/C and Pt/C cathodes are investigated and optimized using several characterization techniques, including chronoamperometry and electrochemical impedance spectroscopy. A loading of 40 wt % PTFE and hot pressing of the electrodes were found to lead to the best performance. Moreover, improvements in cell performance as a function of increasing KOH were investigated and the dual effects of enhanced oxygen reduction reaction activity and improved ionic conductivity were decoupled. Peak power densities as high as 110 mW/cm 2 were obtained, suggesting that the current alkaline fuel cell configuration may also hold promise as a

A subtractive approach to molecular engineering of dimethoxybenzene-based redox materials for non-aqueous flow batteries

The development of new high capacity redox active materials is key to realizing the potential of non-aqueous redox flow batteries (RFBs). In this paper, a series of substituted 1,4-dimethoxybenzene based redox active molecules have been developed via a subtractive design approach. Five molecules have been proposed and developed by removing or reducing the bulky substituent groups of DBBB (2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene), a successful overcharge protection material for lithium-ion batteries. Of these derivatives, 2,3-dimethyl-1,4-dimethoxybenzene (23DDB) and 2,5-dimethyl-1,4-dimethoxybenzene (25DDB) are particularly promising as they demonstrate favorable electrochemical characteristics at gravimetric capacities (161 mA h g−1) that approach the stability limit of chemically reversible dimethoxybenzene based structures. Diffusivity, solubility, and galvanostatic cycling results indicate that both 23DDB and 25DDB molecules have promise for non-aqueous RFBs.

High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

Non-aqueous redox flow batteries (NAqRFBs) employing redox-active organic molecules show promise to meet requirements for grid energy storage. Here, we combine the rational design of organic molecules with flow cell engineering to boost NAqRFB performance. We synthesize two highly soluble phenothiazine derivatives, N-(2-methoxyethyl)phenothiazine (MEPT) and N-[2-(2-methoxyethoxy)ethyl]phenothiazine (MEEPT), via a one-step synthesis from inexpensive precursors. Synthesis and isolation of the radical-cation salts permit UV-vis decay studies that illustrate the high stability of these open-shell species. Cyclic voltammetry and bulk electrolysis experiments reveal the promising electrochemical properties of MEPT and MEEPT under dilute conditions. A high performance non-aqueous flow cell, employing interdigitated flow fields and carbon paper electrodes, is engineered and demonstrated; polarization and impedance studies quantify the cells low area-specific resistance (3.2–3.3 Ω cm2). We combine the most soluble derivative, MEEPT, and its tetrafluoroborate radical-cation salt in the flow cell for symmetric cycling, evincing a current density of 100 mA cm−2 with undetectable capacity fade over 100 cycles. This coincident high current density and capacity retention is unprecedented in NAqRFB literature.

Investigation of Pt, Pt3Co , and Pt3Co / Mo Cathodes for the ORR in a Microfluidic H2 / O2 Fuel Cell

We report on the performance and durability of four Pt-based cathode catalysts in a microfluidic H 2 /O 2 fuel cell: commercial unsupported Pt and Pt 3 Co as well as in-house acid-treated Pt 3 Co (Pt 3 Co-at) and Pt 3 Co/Mo. Commercial Pt 3 Co was used as the starting material for both Pt 3 Co-at and Pt 3 Co/Mo. The composition of the resulting catalysts was confirmed via X-ray photoelectron spectroscopy analysis. In situ cathode studies were performed using an acidic microfluidic H 2 /O 2 fuel cell with an analytical platform. The electrolyte flow rate was optimized to minimize the effects of water management such that fuel cell performance is kinetically limited by the oxygen reduction reaction (ORR). In addition, electrolyte concentration was separately varied to determine cathode performance as a function of acidic pH. All four catalysts demonstrated good short-term activity and stability under fuel cell operating conditions in harsh acidic environments, with the Pt 3 Co/Mo alloy exhibiting the highest activity. Furthermore, both modified catalysts, Pt 3 Co/Mo and Pt 3 Co-at, exhibited superior durability compared to commercially available Pt 3 Co and Pt in the accelerated cathode aging studies performed within the microfluidic fuel cell via potential cycling. In situ impedance analysis of the Pt 3 Co/Mo cathode revealed enhanced catalyst stability and electrode durability as the cause of the dramatic improvements in long-term performance.

Reduction potential predictions of some aromatic nitrogen-containing molecules

Accurate quantum chemical methods offer a reliable alternative to time-consuming experimental evaluations for obtaining a priori electrochemical knowledge of a large number of redox active molecules. In this contribution, quantum chemical calculations are performed to investigate the redox behavior of quinoxalines, a promising family of active materials for non-aqueous flow batteries, as a function of substituent group. The reduction potentials of 40 quinoxaline derivatives with a range of electron-donating and electron-withdrawing groups are computed. Calculations indicate the addition of electron-donating groups, particularly alkyl groups, can significantly lower the reduction potential albeit with a concomitant decrease in oxidative stability. A simple descriptor is derived for computing reduction potentials of quinoxaline derivatives from the LUMO energies of the neutral molecules without time-consuming free energy calculations. The relationship was validated for a broader set of aromatic nitrogen-containing molecules including pyrazine, phenazine, bipyridine, pyridine, pyrimidine, pyridazine, and quinoline, suggesting that it is a good starting point for large high-throughput computations to screen reduction windows of novel molecules.

BF3-promoted electrochemical properties of quinoxaline in propylene carbonate

Electrochemical and density functional studies demonstrate that coordination of electrolyte constituents to quinoxalines modulates their electrochemical properties. Quinoxalines are shown to be electrochemically inactive in most electrolytes in propylene carbonate, yet the predicted reduction potential is shown to match computational estimates in acetonitrile. We find that in the presence of LiBF4 and trace water, an adduct is formed between quinoxaline and the Lewis acid BF3, which then displays electrochemical activity at 1–1.5 V higher than prior observations of quinoxaline electrochemistry in non-aqueous media. Direct synthesis and testing of a bis-BF3 quinoxaline complex further validates the assignment of the electrochemically active species, presenting up to a ∼26-fold improvement in charging capacity, demonstrating the advantages of this adduct over unmodified quinoxaline in LiBF4-based electrolyte. The use of Lewis acids to effectively “turn on” the electrochemical activity of organic molecules may lead to the development of new active material classes for energy storage applications.

Dendrite Suppression by Shock Electrodeposition in Charged Porous Media

It is shown that surface conduction can stabilize electrodeposition in random, charged porous media at high rates, above the diffusion-limited current. After linear sweep voltammetry and impedance spectroscopy, copper electrodeposits are visualized by scanning electron microscopy and energy dispersive spectroscopy in two different porous separators (cellulose nitrate, polyethylene), whose surfaces are modified by layer-by-layer deposition of positive or negative charged polyelectrolytes. Above the limiting current, surface conduction inhibits growth in the positive separators and produces irregular dendrites, while it enhances growth and suppresses dendrites behind a deionization shock in the negative separators, also leading to improved cycle life. The discovery of stable uniform growth in the random media differs from the non-uniform growth observed in parallel nanopores and cannot be explained by classic quasi-steady “leaky membrane” models, which always predict instability and dendritic growth. Instead, the experimental results suggest that transient electro-diffusion in random porous media imparts the stability of a deionization shock to the growing metal interface behind it. Shock electrodeposition could be exploited to enhance the cycle life and recharging rate of metal batteries or to accelerate the fabrication of metal matrix composite coatings.

Full-field synchrotron tomography of nongraphitic foam and laminate anodes for lithium-ion batteries.

Nondestructive methods that allow researchers to gather high-resolution quantitative information on a materials physical properties from inside a working device are increasingly in demand from the scientific community. Synchrotron-based microcomputed X-ray tomography, which enables the fast, full-field interrogation of materials in functional, real-world environments, was used to observe the physical changes of next-generation lithium-ion battery anode materials and architectures. High capacity, nongraphitic anodes were chosen for study because they represent the future direction of the field and one of their recognized limitations is their large volume expansion and contraction upon cycling, which is responsible for their generally poor electrochemical performance. In this work, Cu6Sn5 coated on a three-dimensional copper foam was used to model a high power electrode while laminated silicon particles were used to model a high energy electrode. The electrodes were illuminated in situ and ex situ, respectively, at Sector 2-BM of the Advanced Photon Source. The changes in electrode porosity and surface area were measured and show large differences based on the electrode architecture. This work is one of the first reports of full-field synchrotron tomography on high-capacity battery materials under operating conditions.