Mengmeng Cui

Improved cathode materials for microbial electrosynthesis

Microbial electrosynthesis is a promising strategy for the microbial conversion of carbon dioxide to transportation fuels and other organic commodities, but optimization of this process is required for commercialization. Cathodes which enhance electrode–microbe electron transfer might improve rates of product formation. To evaluate this possibility, biofilms of Sporomusa ovata, which are effective in acetate electrosynthesis, were grown on a range of cathode materials and acetate production was monitored over time. Modifications of carbon cloth that resulted in a positive-charge enhanced microbial electrosynthesis. Functionalization with chitosan or cyanuric chloride increased acetate production rates 6–7 fold and modification with 3-aminopropyltriethoxysilane gave rates 3-fold higher than untreated controls. A 3-fold increase in electrosynthesis over untreated carbon cloth cathodes was also achieved with polyaniline cathodes. However, not all strategies to provide positively charged surfaces were successful, as treatment of carbon cloth with melamine or ammonia gas did not stimulate acetate electrosynthesis. Treating carbon cloth with metal, in particular gold, palladium, or nickel nanoparticles, also promoted electrosynthesis, yielding electrosynthesis rates that were 6-, 4.7- or 4.5-fold faster than the untreated control, respectively. Cathodes comprised of cotton or polyester fabric treated with carbon nanotubes yielded cathodes that supported acetate electrosynthesis rates that were ∼3-fold higher than carbon cloth controls. Recovery of electrons consumed in acetate was ∼80% for all materials. The results demonstrate that one approach to increase rates of carbon dioxide reduction in microbial electrosynthesis is to modify cathode surfaces to improve microbe-electrode interactions.

Stabilizing liquid drops in nonequilibrium shapes by the interfacial jamming of nanoparticles.

Dynamic Surfactants Surfactants are used to form a stable interface between two nonmiscible liquids, like oil and water, so that droplets of one fluid can be entrained in the other. Cui et al. (p. 460) designed a surfactant based on the association of a hydrophilic nanoparticle with a functionalized oleophilic molecule that self-assembles at a water-oil interface to produce a composite surfactant. Once adsorbed, the nanoparticles tend to remain in place causing them to accumulate and “jam” at the interface. When a drop was deformed, more surfactant could assemble at the surface, allowing droplets of various shapes to be produced. A self-assembling composite surfactant can be used to make stable droplets with complex shapes. Nanoparticles assemble at the interface between two fluids into disordered, liquid-like arrays where the nanoparticles can diffuse laterally at the interface. Using nanoparticles dispersed in water and amine end-capped polymers in oil, nanoparticle surfactants are generated in situ at the interface overcoming the inherent weak forces governing the interfacial adsorption of nanoparticles. When the shape of the liquid domain is deformed by an external field, the surface area increases and more nanoparticles adsorb to the interface. Upon releasing the field, the interfacial area decreases, jamming the nanoparticle surfactants and arresting further shape change. The jammed nanoparticles remain disordered and liquid-like, enabling multiple, consecutive deformation and jamming events. Further stabilization is realized by replacing monofunctional ligands with difunctional versions that cross-link the assemblies. The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage.

Demonstration of Feasibility of X-Ray Free Electron Laser Studies of Dynamics of Nanoparticles in Entangled Polymer Melts

The recent advent of hard x-ray free electron lasers (XFELs) opens new areas of science due to their exceptional brightness, coherence, and time structure. In principle, such sources enable studies of dynamics of condensed matter systems over times ranging from femtoseconds to seconds. However, the studies of “slow” dynamics in polymeric materials still remain in question due to the characteristics of the XFEL beam and concerns about sample damage. Here we demonstrate the feasibility of measuring the relaxation dynamics of gold nanoparticles suspended in polymer melts using X-ray photon correlation spectroscopy (XPCS), while also monitoring eventual X-ray induced damage. In spite of inherently large pulse-to-pulse intensity and position variations of the XFEL beam, measurements can be realized at slow time scales. The X-ray induced damage and heating are less than initially expected for soft matter materials.

A route to rapid carbon nanotube growth

CNTs were synthesized in 15-20 seconds by a single carbon fiber initiating the pyrolysis of ferrocene under microwave field without any other chemicals. The growth of CNTs can also be triggered by other carbonaceous materials and influenced by their configurations.

Structured Liquids with pH‐Triggered Reconfigurability

Through pH-tuning of electrostatic interactions between polymer ligands and nanoparticles at structured-liquid interfaces, liquid droplets can be directed between a jammed nonequilibrium state and a dynamic reconfigurable state. The nanoparticle-surfactant dynamics highly depend on the pH, so that the liquids can be structured using an external field and under variation of pH, or alternatively being realized by remote photo-triggering.

Three-dimensional hierarchical metal oxide–carbon electrode materials for highly efficient microbial electrosynthesis

The production of hierarchical hybrid conductive materials that are mesoporous, with pores spanning from sub-microns to microns in size, is important for large-area electrode applications. Here, a simple one-step, low-cost method to fabricate metal oxide–carbon hybrid materials with a hierarchical pore structure in a microwave oven is demonstrated. The microwave pyrolysis of ferrocene using carbon felt as a microwave absorber is a method that is rapid (takes of seconds), requires neither harsh conditions nor the use of costly equipment, and can be readily scaled up. The produced material has a high specific surface area, a multi-length scale porous structure and a high conductivity, and is quite stable, making it promising for many practical applications. As an electrode in microbial electrosynthesis, its performance is improved by a factor of five and an optimal biofilm of the microorganism is formed on the surface.

Transition in Dynamics as Nanoparticles Jam at the Liquid/Liquid Interface

Nanoparticles (NPs) segregated to the liquid/liquid interface form disordered or liquid-like assemblies that show diffusive motions in the plane of the interface. As the areal density of NPs at the interface increases, the available interfacial area decreases, and the interfacial dynamics of the NP assemblies change when the NPs jam. Dynamics associated with jamming was investigated by X-ray photon correlation spectroscopy. Water-in-toluene emulsions, formed by a self-emulsification at the liquid/liquid interface and stabilized by ligand-capped CdSe-ZnS NPs, provided a simple, yet powerful platform, to investigate NP dynamics. In contrast to a single planar interface, these emulsions increased the number of NPs in the incident beam and decreased the absorption of X-rays in comparison to the same path length in pure water. A transition from diffusive to confined dynamics was manifested by intermittent dynamics, indicating a transition from a liquid-like to a jammed state.