Reports: UNI1050094-UNI10: Order vs. Disorder: Does Pore Geometry Affect the Electrochemical Performance of Carbon Electrode Nanoarchitectures?

Justin C. Lytle, PhD , Pacific Lutheran University

Generous funding from the American Chemical Society Petroleum Research Fund (ACS PRF) during the 2010-2011 academic year has continued to be essential to my professional development as a young scientist at Pacific Lutheran University, a primarily undergraduate institution. This funding enabled eight undergraduate students in my research group to study porous carbon nanomaterials that store electrochemical energy as electrochemical capacitance. We hypothesized in February 2009 that the pore geometry of porous carbon nanoarchitectures is irrelevant to the magnitude of an electrode's specific capacitance, because double-layer capacitance is directly proportional to an electrode's electrifiable surface area alone. To address this hypothesis, my research group used ACS PRF funding throughout the 2010-2011 academic year to develop and optimize methods that fabricate and characterize two types of carbon nanomaterials: carbon inverse opal papers (CIOPs) and carbon nanofoams (CNFs). Both materials were designed to have comparable physico-chemical properties except for intentional differences in the periodicity of their pore networks.In the 2009-2010 funding cycle, we developed a multi-step fabrication method that assembles CIOP electrodes. To the best of our knowledge, this is the first report of an inverse opal fabric. This discovery is significant to the scientific community because it introduces mechanical flexibility into inverse opals, which have until now been formed as powders, thin films, and monoliths. Carbon inverse opal papers also conduct more than two orders of magnitude greater electronic conductivity than conventional carbon inverse opal monoliths (CIOMs), which is a clear advantage for applications in electronics and electrochemistry. In March 2010, though, we realized that individual CIOP samples discharge widely variable magnitudes of electrochemical capacitance. This observation complicated our hypothetical comparisons to CNFs, as stated above.With ACS PRF funding in 2010-2011, eight undergraduate students worked part-time and full-time to fabricate and evaluate the source of variance in our capacitance measurements. We were surprised to discover that our CIOP electrodes have electrochemically accessible surface areas that range from 1-430 m2 g-1-which correlates directly to the wide range of capacitance that we measured. We observed via scanning electron microscopy that electrodes with crusts on their exterior surfaces also discharge relatively small capacitances, and that the highest surface area CIOPs have aperiodic pore structures that resembled those of CNFs. In response, we modified our fabrication protocol to minimize the amount of crusts that form on the exterior of each paper, and we assess the electrochemical surface area of each electrode in order to remove materials with atypically small or large surface areas. As a result, we find that CIOPs typically have electrochemical surface areas of 175-200 m2 g-1, which is consistent with the notion that the ~300 m2 g-1 of surface area that CIOMs have is reduced by ~33% as carbon fibers are added as conductive dead weight to make the inverse opal fabric.This toolkit of quality control measurements that we developed for CIOPs in 2010-2011 has also become essential to make our hypothesized comparison to CNF materials. In March 2010, nitrogen porosimetry data indicated that our CNFs had far too much surface area (~550 m2 g-1) to make a valid comparison in our hypothesis: that carbon nanoarchitectures with either ordered or disordered pore networks will discharge similar capacitances when all other physico-chemical properties are similar. In summer 2011, we successfully decreased the surface area of CNFs to be ~200 m2 g-1 by decreasing the ratio of the resorcinol-formaldehyde monomer in the precursor solution. Future Hg porosimetry tests will elucidate the resulting pore size distribution in these CNF materials, and if the pore sizes of both CIOPs and CNFs are similar, we will compare the discharge capacitance of both materials as hypothesized above.Financial support from ACS PRF also enabled us to create two new avenues of research from our existing work. We first hypothesized in 2010-2011 that CIOPs would discharge greater magnitudes of electrochemical capacitance at higher discharge rates than comparable CIOMs because CIOPs have electronic conductivity values that are >150 times greater than those of CIOMs. My research group conducted this comparison during summer 2011, and our initial data shows that both CIOPs and CIOMs discharge similar capacitances with increasing current densities. We need to reproduce these experiments before arriving at further conclusions, but at present, the data suggests that ionic charge transport through the pore network of CIOPs and CIOMs-and not the electronic conductivity-limits the magnitude of discharged capacitance.Secondly, we hypothesized to Research Corporation in fall 2010 that the rate of Zn corrosion in Zn-air batteries will be accelerated on the high-surface area of CNF materials. This hypothesis was funded with a Cottrell College Science Award in spring 2011, which I attribute to the initial research support that I received from ACS PRF. My research group's initial Zn electrodeposition strategies deposit Zn on the exterior of CNF electrodes, but we are crafting future deposition duty cycles that we believe will more gradually deposit metallic Zn throughout the porous architecture.I attribute the majority of our research progress to the generous funding that I have received from ACS PRF because it allowed me to employ eight student researchers at different times of the 2010-2011 academic year. This financial support allowed my undergraduate students to continue their summer research projects into the fall and January terms on our campus, and to present our findings at the 241st National Meeting of the American Chemical Society. I am currently writing a manuscript to publish our comparison of CIOPs and CIOMs, and we anticipate a future manuscript to discuss the effect of pore geometry on capacitance.Undergraduate students in my research group benefited from the hands-on experience with state-of-the-art nanomaterials, electron microscopy, and electrochemistry that one would more likely encounter in a graduate school environment. Without question, these experiences have made them better scientists and will open professional doors for them. Finally, ACS PRF funds made it possible for my students and me to give three oral presentations at the 2011 Spring National Meeting of the ACS in Anaheim, CA.
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