The U.S. Department of Energy has awarded an interdisciplinary team of Cornell researchers $2 million to study the combination of inorganic semiconductor nanoparticles and bacterial cells for more efficient bioenergy conversion.
For years, scientists have been trying to discover the size at which solid materials could change their internal structure in a single, swift step, like molecules do during isomerization. This unanswered question has been the missing link in scientists’ quest to map and understand the crossover from molecular isomerization, such as those that make eyesight possible, to bulk phase transitions, like the transition of graphite into diamonds. If understood, these processes could be useful for applications such as energy harvesting or quantum computing. In their recent paper published in Science (DOI: 10.1126/science.aau9464), Professors Tobias Hanrath and Richard Robinson finally reveal that a “magic size cluster” bridges this divide between how matter rearranges in the small scale of molecular isomerization and in large, solid bulk matter phase transitions.
Rileigh Casebolt and Jesus Miguel Lopez Baltazar join the group. Rileigh will be working on electrochemical CO2 reduction and Jesus will work on programmable assembly (in collaboration with Prof. Alabi)
Congratulations, Dr. Richards! for successfully defending your PhD thesis. Pictures from the defense and abstract of the presentation are below.
Bulk-nucleated vapor-solid-solid Silicon and Germanium Nanowires: Synthesis, Scaling, and Applications
Benjamin T. Richards
Cornell University, 2018
Nanowires are essential building blocks for many next-generation devices. This includes applications in solar cells, field effect transistors, thermoelectric generators, chemical sensors, and electrochemical cells. One application of intense promise is using silicon and germanium nanowires as anodes materials in lithium ion batteries, as these materials form stable compounds with superior lithium capacity that increases the anode charge capacity compared to current Li-ion batteries by a factor of 4 – 10. Unlike the bulk material, nanowires have been demonstrated to withstand the ~400% volume dilation due to the intercalation of lithium. To bring the acclaimed potential of nanowires to fruition, challenges in production, scaling, and electrochemical performance must be resolved. Conventional nanowire synthesis methods produce small yields of nanowires that require many processes (i.e., slurry mixing, coating, baking) to introduce into batteries. To meet the growing energy storage demands for personal electronics and electric vehicles, an improved method is required to produce nanowires.
In this work, I will introduce a growth mechanism that is simple, robust, and adaptable to high throughput processing of nucleating and growing nanowires from bulk metal films, known as bulk nucleated vapor-solid-solid (BN-VSS). This method prepares nanowires that are epitaxially connected to the metal substrate, allowing direct integration into batteries. I identify the kinetic conditions required to grow nanowires, the processes that occur during nanowire growth, and copper as a promising growth substrate material. Then, I will present a kinetic model of germanium nanowire growth from copper films using diphenylgermane as a precursor. This model was developed by performing rapid syntheses of nanowires using an inductive heating apparatus followed by ex-situ characterization of these films. Finally, I introduce a diagnostic technique of measuring the electrical resistance of the growth substrate to monitor the solid-state transformation that proceeds to germanium nanowire growth and demonstrate how this method can be used to inform kinetic conditions in high-throughput roll-to-roll reactors.
Kevin’s recent paper on electrochemical CO2 reduction showed that pulsing the applied potential provides a rich parameter space of previously under appreciated ‘knobs’ to tailor the selectivity of reduced products. More information can be found in the Full Paper by Kimura et al. (https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.201801130).
Electrochemical CO2 reduction reaction (CO2RR) has garnered strong interest as a promising pathway to convert CO2 emissions into higher value chemicals including fuels and hydrocarbon feedstocks. In particular, copper has been shown to produce a range of useful hydrocarbons at room temperature, which could be powered renewably. The main challenges involved with CO2RR is the low selectivity to one product, competition with the hydrogen evolution reaction (HER) and the overall high overpotentials required. Applying the electrochemical potential in a time-programmed pulse (instead of a constant potential) has interesting implications on both fundamental and applied aspects of CO2RR. From a fundamental perspective, the timing of the square wave pulsed potential provides insights into the coupled dynamics of mass transport and surface reactions. From an applied perspective, pulsed potentials significantly mitigate electrode fouling in electrolytic cells. Intrigued by the prospects of pulsed potentials applied to CO2RR we sought out to understand the underlying mechanism responsible for pulse dependent product selectivity.
We found that the application of a pulsed potential allows us to substantially suppress the HER while shifting selectivity to CH4 and CO. We attribute the improved CO2RR selectivity to a re-arrangement of surface hydrogen coverage during the pulsing. We also establish that the size and geometry of the electrode matters; when using a small copper electrode, HER was suppressed to less than 5% and methane or CO could be selectivity produced even at fairly low potentials. To decouple the interplay of surface reactions and mass transport to and from the electrode we performed rotating disk electrode experiments and compared the results to an analytical model.
On a fundamental level, our findings provide new insights into the timescales of competing pathways in CO2RR. From an applied perspective, our results present an opportunity for the product selectivity to be tuned by adjusting the temporal profile of the electrochemical potential. We anticipate our findings may help others to further understand the CO2RR pathways and improve performance for other electrocatalysts.