Matthew Merrill

Ultralow Density, Monolithic WS2, MoS2, and MoS2/Graphene Aerogels

We describe the synthesis and characterization of monolithic, ultralow density WS2 and MoS2 aerogels, as well as a high surface area MoS2/graphene hybrid aerogel. The monolithic WS2 and MoS2 aerogels are prepared via thermal decomposition of freeze-dried ammonium thio-molybdate (ATM) and ammonium thio-tungstate (ATT) solutions, respectively. The densities of the pure dichalcogenide aerogels represent 0.4% and 0.5% of full density MoS2 and WS2, respectively, and can be tailored by simply changing the initial ATM or ATT concentrations. Similar processing in the presence of the graphene aerogel results in a hybrid structure with MoS2 sheets conformally coating the graphene scaffold. This layered motif produces a ∼50 wt % MoS2 aerogel with BET surface area of ∼700 m(2)/g and an electrical conductivity of 112 S/m. The MoS2/graphene aerogel shows promising results as a hydrogen evolution reaction catalyst with low onset potential (∼100 mV) and high current density (100 mA/cm(2) at 260 mV).

Battery/supercapacitor hybrid via non-covalent functionalization of graphene macro-assemblies

Binder-free, monolithic, high surface area graphene macro-assemblies (GMAs) are promising materials for supercapacitor electrodes, but, like all graphitic carbon based supercapacitor electrodes, still lack sufficient energy density for demanding practical applications. Here, we demonstrate that the energy storage capacity of GMAs can be increased nearly 3-fold (up to 23 W h kg−1) by facile, non-covalent surface modification with anthraquinone (AQ). AQ provides battery-like redox charge storage (927 C g−1) without affecting the conductivity and capacitance of the GMA support. The resulting AQ-GMA battery/supercapacitor hybrid electrodes demonstrate excellent power performance, show remarkable long-term cycling stability and, by virtue of their excellent mechanical properties, allow for further increases in volumetric energy density by mechanical compression of the treated electrode. Our measured capacity is very close to the theoretical maximum obtained using detailed density functional theory calculations, suggesting nearly all incorporated AQ is made available for charge storage.

Optimizing supercapacitor electrode density: achieving the energy of organic electrolytes with the power of aqueous electrolytes

The value of electrode density is often overlooked in the pursuit of impressive supercapacitor metrics. Low-density electrodes deliver the best performance in terms of gravimetric energy and power densities when only the mass of the electrodes is considered. However, energy and power values with respect to the total system mass (electrode + electrolyte) or volume are more meaningful for practical application. Low-density electrodes are impractical due to both large mass contributions by the electrolyte and large system volumes. Here, we use highly compressible graphene aerogel electrodes (up to 87.5% volumetric compression) to systematically characterize the effects of electrode density on energy and power metrics. The results reveal that electrode density is similar to electrode thickness in that both parameters have a squared effect on power. Accounting for the aqueous electrolytes mass lowered the gravimetric energy and power by almost an order of magnitude for 0.144 g cm−3 dense carbon electrodes but only by a factor of 1.5 when the electrode density was increased to 1.15 g cm−3 through compression. The high-density electrodes achieve 8 W h kg−1, 70 000 W kg−1, and 144 F cm−3 in a symmetric electrode setup after factoring in the aqueous electrolytes mass. Therefore, in the pursuit of high energy per mass, it can be just as effective to lower the systems mass with smaller electrolyte fractions as it is to use electrolytes with larger voltage ranges. High electrode densities allow aqueous electrolyte supercapacitors to attain energy densities per the system mass comparable to those of commercially-available organic electrolyte supercapacitors while maintaining 10–100× greater power.

Potential-Induced Electronic Structure Changes in Supercapacitor Electrodes Observed by In Operando Soft X-Ray Spectroscopy

The dynamic physiochemical response of a functioning graphene-based aerogel supercapacitor is monitored in operando by soft X-ray spectroscopy and interpreted through ab initio atomistic simulations. Unanticipated changes in the electronic structure of the electrode as a function of applied voltage bias indicate structural modifications across multiple length scales via independent pseudocapacitive and electric double layer charge storage channels.