Andrew S. Westover

Surface engineered porous silicon for stable, high performance electrochemical supercapacitors

Silicon materials remain unused for supercapacitors due to extreme reactivity of silicon with electrolytes. However, doped silicon materials boast a low mass density, excellent conductivity, a controllably etched nanoporous structure, and combined earth abundance and technological presence appealing to diverse energy storage frameworks. Here, we demonstrate a universal route to transform porous silicon (P-Si) into stable electrodes for electrochemical devices through growth of an ultra-thin, conformal graphene coating on the P-Si surface. This graphene coating simultaneously passivates surface charge traps and provides an ideal electrode-electrolyte electrochemical interface. This leads to 10–40X improvement in energy density, and a 2X wider electrochemical window compared to identically-structured unpassivated P-Si. This work demonstrates a technique generalizable to mesoporous and nanoporous materials that decouples the engineering of electrode structure and electrochemical surface stability to engineer performance in electrochemical environments. Specifically, we demonstrate P-Si as a promising new platform for grid-scale and integrated electrochemical energy storage.

All Silicon Electrode Photocapacitor for Integrated Energy Storage and Conversion

We demonstrate a simple wafer-scale process by which an individual silicon wafer can be processed into a multifunctional platform where one side is adapted to replace platinum and enable triiodide reduction in a dye-sensitized solar cell and the other side provides on-board charge storage as an electrochemical supercapacitor. This builds upon electrochemical fabrication of dual-sided porous silicon and subsequent carbon surface passivation for silicon electrochemical stability. The utilization of this silicon multifunctional platform as a combined energy storage and conversion system yields a total device efficiency of 2.1%, where the high frequency discharge capability of the integrated supercapacitor gives promise for dynamic load-leveling operations to overcome current and voltage fluctuations during solar energy harvesting.

A multifunctional load-bearing solid-state supercapacitor.

A load-bearing, multifunctional material with the simultaneous capability to store energy and withstand static and dynamic mechanical stresses is demonstrated. This is produced using ion-conducting polymers infiltrated into nanoporous silicon that is etched directly into bulk conductive silicon. This device platform maintains energy densities near 10 W h/kg with Coulombic efficiency of 98% under exposure to over 300 kPa tensile stresses and 80 g vibratory accelerations, along with excellent performance in other shear, compression, and impact tests. This demonstrates performance feasibility as a structurally integrated energy storage material broadly applicable across renewable energy systems, transportation systems, and mobile electronics, among others.

Direct integration of a supercapacitor into the backside of a silicon photovoltaic device

We demonstrate a route to integrate active material for energy storage directly into a silicon photovoltaic (PV) device, and the synergistic operation of the PV and storage systems for load leveling. Porous silicon supercapacitors with 84% Coulombic efficiency are etched directly into the excess absorbing layer material in a commercially available polycrystalline silicon PV device and coupled with solid-state polymer electrolytes. Our work demonstrates the simple idea both that the PV device can charge the supercapacitor under an external load and that a constant current load can be maintained through periods of intermittent illumination, demonstrating the concept of an all-silicon integrated solar supercapacitor.

Uniform, homogenous coatings of carbon nanohorns on arbitrary substrates from common solvents.

We demonstrate a facile technique to electrophoretically deposit homogenous assemblies of single-walled carbon nanohorns (CNHs) from common solvents such as acetone and water onto nearly any substrate including insulators, dielectrics, and three-dimensional metal foams, in many cases without the aid of surfactants. This enables the generation of pristine film-coatings formed on time scales as short as a few seconds and on three-dimensional templates that enable the formation of freestanding polymer-CNH supported materials. As electrophoretic deposition is usually only practical on conductive electrodes, we emphasize our observation of efficient deposition on nearly any material, including nonconductive substrates. The one-step versatility of deposition on these materials provides the capability to directly assemble CNH materials onto functional surfaces for a broad range of applications. In this manner, we utilized as-deposited CNH films as conductometric gas sensors exhibiting better sensitivity in comparison to equivalent single-walled carbon nanotube sensors. This gives a route toward scalable and inexpensive solution-based processing routes to manufacture functional nanocarbon materials for catalysis, energy, and sensing applications, among others.

Multifunctional high strength and high energy epoxy composite structural supercapacitors with wet-dry operational stability

We demonstrate the fabrication of multifunctional structural supercapacitors that maintain energy storage capability under both mechanical stresses and water immersion. This is based on the infiltration of bisphenol A ionic liquid epoxy resin electrolytes infiltrated into nanoporous silicon interfaces that play the dual role of charge storage and mechanical reinforcement of the energy storage composite material. These structural composites maintain full energy storage capability (5–8 W h kg−1) under tensile stresses over 1 MPa, with nearly 100% energy retention after 4000 cycles. We observe this mechanical and charge storage performance to be preserved through extreme water immersion conditions in contrast to conventional polymer-based solid-state electrolytes that spontaneously lose mechanical integrity under water immersion conditions. As structural energy storage is required to simultaneously maintain mechanical integrity, store charge, and operate in unpackaged environments exposed to humidity and wet-dry conditions, we demonstrate the first device architecture capable of all these conditions while demonstrating energy capability near current packaged commercial supercapacitor devices.

Ultra high-yield one-step synthesis of conductive and superhydrophobic three-dimensional mats of carbon nanofibers via full catalysis of unconstrained thin films

We directly synthesized large conductive and superhydrophobic three-dimensional mats of entangled carbon nanofibers (CNFs) using thermal chemical vapor deposition (CVD). We show that the yield obtained from the catalysis of an unconstrained thin Ni–Pd film is over an order of magnitude higher compared to that of the same thin film when bound to a substrate. The growth mechanism differs from substrate-bound growth, where catalysis occurs only on the top surface of the catalytic film, as the full Ni–Pd catalyst layer participates in the reaction and is totally consumed to bi-directionally grow CNFs. Therefore, the yield further increased with the thin film thickness, in contrast to substrate-bound growth. The unconstrained growth occurred thanks to a weak adhesion layer that delaminated during the thermal process. Additionally, we showed that the supporting substrate material strongly affected the nanostructure morphology obtained. The as-grown CNF mats were used as a three-dimensional electrode for lithium-ion batteries. We envisage these CNF mats to be an ideal platform to be functionalized for multiple applications including high-performance electrodes, sensors, electromagnetic shields, and conductive polymer-coated composites.

Particulate-free porous silicon networks for efficient capacitive deionization water desalination

Energy efficient water desalination processes employing low-cost and earth-abundant materials is a critical step to sustainably manage future human needs for clean water resources. Here we demonstrate that porous silicon – a material harnessing earth abundance, cost, and environmental/biological compatibility is a candidate material for water desalination. With appropriate surface passivation of the porous silicon material to prevent surface corrosion in aqueous environments, we show that porous silicon templates can enable salt removal in capacitive deionization (CDI) ranging from 0.36% by mass at the onset from fresh to brackish water (10 mM, or 0.06% salinity) to 0.52% in ocean water salt concentrations (500 mM, or ~0.3% salinity). This is on par with reports of most carbon nanomaterial based CDI systems based on particulate electrodes and covers the full salinity range required of a CDI system with a total ocean-to-fresh water required energy input of ~1.45 Wh/L. The use of porous silicon for CDI enables new routes to directly couple water desalination technology with microfluidic systems and photovoltaics that natively use silicon materials, while mitigating adverse effects of water contamination occurring from nanoparticulate-based CDI electrodes.

Noncovalent Pi-Pi Stacking at the Carbon-Electrolyte Interface: Controlling the Voltage Window of Electrochemical Supercapacitors.

A key parameter in the operation of an electrochemical double-layer capacitor is the voltage window, which dictates the device energy density and power density. Here we demonstrate experimental evidence that π-π stacking at a carbon-ionic liquid interface can modify the operation voltage of a supercapacitor device by up to 30%, and this can be recovered by steric hindrance at the electrode-electrolyte interface introduced by poly(ethylene oxide) polymer electrolyte additives. This observation is supported by Raman spectroscopy, electrochemical impedance spectroscopy, and differential scanning calorimetry that each independently elucidates the signature of π-π stacking between imidazole groups in the ionic liquid and the carbon surface and the role this plays to lower the energy barrier for charge transfer at the electrode-electrolyte interface. This effect is further observed universally across two separate ionic liquid electrolyte systems and is validated by control experiments showing an invariant electrochemical window in the absence of a carbon-ionic liquid electrode-electrolyte interface. As interfacial or noncovalent interactions are usually neglected in the mechanistic picture of double-layer capacitors, this work highlights the importance of understanding chemical properties at supercapacitor interfaces to engineer voltage and energy capability.

Cure Monitoring and Characterization of Epoxy/Amine Networks, Modified with 1-Butyl-3-Bethylimidazolium Tetrafluoroborate Ionic Liquid

In the present work, electrochemical impedance spectroscopy (EIS) was employed for cure monitoring of bisphenol A/F based epoxy resin- polyoxypropylenediamine hardener systems modified with various concentrations of 1-butyl-3- methylimidazolium tetrafluoroborate ([BMIM]BF4) ionic liquid (IL). These systems exhibited phase separation induced by an increase in the molar mass through the curing of the resin. The progress of the curing process was accompanied by a steady increase of impedance for all types of samples, as expected, due to the increasing viscosity of the medium for increased degree of cure. The results were compared to those of unmodified resin-hardener systems. The sensitivity of EIS as a cure monitoring technique increased dramatically with the addition of the IL. The cure kinetics of the epoxy resin was characterized by differential scanning calorimetry and the data were used to create a model for comparison with the EIS results. The change of the ionic conductivity during curing as a function of IL concentration and the effect of IL loading on mechanical properties of the resin were also studied. We demonstrate that [BMIM]BF4 loading of 5wt.% increased the room temperature ionic conductivity of the fully cured product by an order of magnitude with only a 2.5% reduction in compression strength from that of pure epoxy.