Finally, I would like to thank my friends and family for their continued support throughout my education. Implementations of these electrochemical devices, such as hydrogen fuel cells, direct methanol fuel cells (DMFCs), and vanadium redox flow batteries (VRBs), require a proton-selective separator/electrolyte. Nafion, developed in the 1960s, is a proton-conducting polymer film commonly used as a separator/electrolyte in these devices, but Nafion exhibits poor proton selectivity, is functional up to only 100◦C, and has transport properties dependent on moisture content. .
Additionally, hBN and graphene are atomically thin and can therefore improve device efficiency by minimizing ohmic loss through the separator/electrolyte.
Background .1 Nafion.1Nafion
Furthermore, ion conductance in Nafion is sensitive to the hydration of the membrane, where increasing water content widens the ion-permeable channels and increases the conductivity. Furthermore, the areal conductivity of the 2D crystals followed an Arrhenius temperature dependence, from which activation energies of ∼0.3 eV (hBN) and ∼0.8 eV (graphene) were calculated. In a subsequent paper, a 10:1 ratio of proton:deuteron conductance through graphene and hBN was reported in support of a hypothesis that zero-point fluctuations of the initial energy state of the ions due to hydrogen bonds in Nafion partially explain the discrepancy between the experimentally and theoretically determined activation energies (13).
Recently, theoretical calculations and a gas permeation study have suggested a dominant binding/flip transport mechanism in graphene, in which protons bind to the pristine graphene lattice and flip to the other side, where they are released ( 11 ).
CHAPTER TWO
Membrane Assembly Fabrication
The Nafion|2D-material|Cu assembly was passed through the laminator four times in each plane direction to smooth out any wrinkles on the surface. The Nafion|2D-material|Cu assembly was then floated Cu side down in 0.2M APS for at least four hours to remove the Cu film, followed by thorough rinsing with DI water. The Nafion|2D-material assembly was then placed 2D-material side down on a second Nafion 211 film and laminated and hot-pressed as before, resulting in the Nafion|2D-materials|Nafion assembly (Figure 2.1b).
A 2D/G peak ratio of ~2 indicates monolayer material, while a D/2D peak ratio of ~0.03 indicates low defect density.
Conversion of Nafion to Ionic Form
The membranes were characterized by cyclic voltammetry (CV), from which we isolated the linear sweep region in the resulting IV curve. The reciprocal of the slope of the linear fit to this range is the ionic resistance of the cell. The blue and left axes correspond to the resistance of the cell, while the red and right axes correspond to the conductivity of the irrigation solution.
Approximate error bars for cell resistance data representing standard deviations are included in Table 2.1. Simultaneously, the conductivity of the DI water soaking solution was measured and found to increase with soaking time, indicating the release of K+ ions into the DI water and corresponding H+ exchange in the membrane. Therefore, it is possible that contact with DI water will convert Nafion 211 to its acidic form.
However, the cell resistance with 12 hours of DI water-soaked K+ form membranes exceeded that of the acid form reference point. Therefore, it is vital for the integrity of the measurements to carefully monitor the rinsing and soaking treatment of DI water prior to measuring a membrane in non-acidic form. Such research is beyond the scope of this thesis, but may include soak/rinse time experiments similar to those presented here, but with longer time frames and a higher resolution electrochemical cell.
Four-Electrode D-S Cell .1 Design.1Design
- Modeling and Validation
Devanathan-Stachurski (D-S) cells are four-electrode electrochemical cells originally developed to measure hydrogen permeation through metals, but can also be used to measure resistance across a membrane. In a four-electrode cell, a current is applied between working and counter electrodes while potential is measured across separate working voltage sensor and counter voltage sensor electrodes. Platinum wires (Alfa Aesar, 99.9% (metal base)) were used as working/counter electrodes, and Ag/AgCl grain electrodes (A-M systems) were used as sensor electrodes (Figures 2.9, 2.8).
Pt wires are working/counter electrodes, Ag/AgCl electrodes are working/counter sense electrodes. Ideally, the sensing electrodes would be located as close to the membrane as possible and out of the current path. Given the physical layout of the cell, only one of these was possible, so the Ag/AgCl sensor electrodes were placed outside the current path (Figure 2.9.
By further modeling the individual membrane components as series resistances, the 2D material contributions can be isolated from the larger membrane assembly. 2RN af ion=Rcell, N af ion|N af ion−Rcell, without membrane (2.5) where Rcell, N af|N af is the resistance of a cell loaded with a Nafion|Nafion membrane (without 2D material) and each component of the right side of the equation can be measured directly in the D-S cell. The model in figures 2.10 and 2.11 clearly lacks the capacitive components which are often present in electrochemical systems.
If the capacitive effects are significant, hysteresis would be expected in the resulting voltammagrams and the curves would vary with scan speed. Examination of the linear sweep region of the CV curves supports the claim that there is no scan rate dependence for transport measurements (Figure 2.13).
CHAPTER THREE
RESULTS AND DISCUSSION
Coverage model
For the assumption to be valid, the 2D material and the Nafion components must be intact (no defects or holes). In the case of CVD graphene, the pristine 2D material component is difficult to grow and not trivial to transfer to a membrane assembly. In fact, the etching study in (15) shows that defects are present in the graphene film before transfer.
As such, a parallel circuit model is proposed here that includes potential defects in a 2D material film (Figure 3.2). Define coverage, C, as the ratio of pristine 2D material area (Apristine) to total 2D material area (Atotal). Assuming that the active area measured in the D-S cell, Aactive is representative of the entire membrane,.
Since coverage is environment independent, it can be a valuable figure to quantify the quality of the 2D material after membrane fabrication. The data from Table 3.1 are not used to demonstrate the model due to the IR drop variability in the measurements, where Nafion|2D material|Nafion containing membranes cannot be reliably distinguished from Nafion|Nafion membranes. The active area for each membrane was defined by a 5/8" diameter disc, and each electrolyte solution was 0.1M xCl, where x is the neutral form of the cation.
Integrating the coverage model results in a σlattice of 1.5 S/cm2 - a nominal decrease over σ2D− material due to the high (97%) coverage of pristine graphene over the membrane. For comparison, σN af ion calculated from the same data is 2.1 S/cm2 - the graphene lattice is similarly conductive to protons as Nafion 211.
CHAPTER FOUR
FLOW CELL DEVELOPMENT
Background
A primary challenge to flow battery development is the concern about cross-species between the positive half-cell solution (catholyte) and the negative half-cell solution (anolyte). Unlike other flow battery chemistries, where the electrolytes contain different elements and therefore crossover leads to contamination, the VRB has only vanadium in solution in each electrolyte.
Design
The flow cell's customization options are made possible by the exclusive use of off-the-shelf and two-dimensional parts (figure 4.2). Tubes, nuts and bolts are off-the-shelf items readily available in various dimensions from distributors such as McMaster-Carr (MMC). The 2D design of each part allows for rapid geometric changes in the flow field (by replacing flow plates) as well as scaling up the overall dimensions of the cell, including changing the active area by modifying the inner (masking) gaskets.
Each change is expected to take less than 30 minutes from change in CAD model to finished production on a laser cutter. A version of the cell was built to demonstrate a vanadium redox (flow) battery (VRB) with an active area of 2 cm x 2 cm, and the materials were chosen accordingly for compatibility with an acidic electrolyte (Table 4.1). The masking gasket is only needed if a reduction of the active area is desired and is not shown in figures 4.4 and 4.5.
VRB Validation
First, the anolyte should change color from blue to purple as V4+ is converted to V2+, while the catholyte should change from blue to yellow/orange as V4+ is converted to V5+. Second, the current passed between the two electrodes at a constant potential should reach a lower asymptote (i.e., leakage current) (20). The catholyte (framed in green) remains blue, while the anolyte (framed in white) changed color from blue to purple.
During the charging process, the anolyte quickly (visible after ∼15 min) made the expected color change from blue to purple; however, the catholyte remained blue (Figure 4.7). Stream points are single data points attributed to user interactions with the system during operation. The charging process was not completed after 1 hour, as seen by the lack of color change of the catholyte and the continuous decrease of the current (Figures 4.7 and 4.8).
Integrating the current over time indicates a charge of 190 mA·h, compared to an expected capacity of almost 1600 mA·h (Equation 4.4, assuming 100% coulombic efficiency). 3600s) = 1.6 Ah (4.4) Although a charge/discharge cycle was not completed, the flow cell design was validated by the charging experiment. To characterize the performance of 2D material membranes in a VRB, future experiments could charge/discharge the battery with different membranes (e.g.
CHAPTER FIVE CONCLUSION
Summary
Future Work
Such a mechanism could be a hollowed-out tube with an outer diameter equivalent to the inner diameter of the existing cell ports and an inner diameter equivalent to the outer diameter of the pellet reference electrodes. Alternatively, a custom D-S cell with fixed-position Luggin capillaries has been ordered, which can be used in place of the existing cell (Appendix B). Then, the data from the experiments can be processed with the proposed parallel circuit model to compare ion transport mechanisms between CVD-graphene and CVD-hBN.
The same experimental approach can be combined with variation in growth parameters to tailor selectivity and conductance in 2D materials. Additionally, the coverage figure can be validated against electron microscopy and Raman spectroscopy as a method to determine 2D material quality. Unit-level testing of 2D material membranes is enabled by the flow cell presented in Section 4.
COVID-19 prevented the device from fully functioning as a VRB, but such operation remains worthwhile. Battery performance, as measured by energy efficiency and OCV drop, for example, can be compared between different membrane configurations. Furthermore, the long-term integrity of 2D material-containing membranes can be investigated, with the expectation that 2D materials can strengthen polymer membranes such as Nafion, leading to greater durability.
The same flow cell can be configured as a hydrogen fuel cell or direct methanol fuel cell (DMFC) for testing the same membranes in a difference device application.
APPENDIX A
2D MATERIAL GROWTH
APPENDIX B
CUSTOM D-S CELL
