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Hickner, Michael A.

Publications and source records attributed to Hickner, Michael A..

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Enhanced Anhydrous Proton Conductivity in Azole Phosphonic Acid Mixtures

Azole molecules are investigated as potential candidates for proton conductors under anhydrous conditions. Since 1,2,3-triazole has the lowest melting point (T m = 17 °C), it was blended with three phosphonic acid-containing molecules (small molecules with one and two phosphonic acids per molecule and a phosphonic acid polymer) to provide a source of excess protons to enhance the proton conductivity of the blends. Here we study a wide range of compositions in each system to find that these three mixtures show a maximum proton conductivity at moderate doping compositions, approximately 5–10 azole molecules per phosphonic acid group. Using NMR diffusometry, we show that the protons bonded to nitrogen move faster than the protons bonded to carbons of 1,2,3-triazole, suggesting proton hopping between azole proton carriers. Given the high proton conductivity at 90 °C of the best mixtures, in the range of 20–60 mS/cm, this work provides a path forward for future work in anhydrous proton-conducting polymer membranes. Additionally, Raman spectroscopy was used to accurately determine the molar percentage of protonated 1,2,3-triazole. Combining that with the proton diffusion results, we find that the phosphonic acid polymer shows the most proton hopping at low acid content.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Hydrocarbon-based membranes cost-effectively manage species transport and increase performance in thermally regenerative batteries

Low-temperature heat (T<130°C) can be utilized by thermally regenerative batteries (TRBs) for power production, allowing the thermal energy to be converted to storable chemical potential energy. However, TRBs suffer from high ohmic losses and ammonia crossover, which has slowed their development. In this study, we examined how the use of six different membranes influenced TRB performance, determined the most influential membrane parameters, and identified promising membrane candidates that cost-effectively increase TRB performance. Of the six membranes examined, an inexpensive, hydrocarbon CEM (Selemion CMVN) had low ammonia crossover without compromising resistance, resulting in good performance across all metrics studied. A thin anion exchange membrane (Sustainion, 50 microns) showed a high peak power density of 82 mW cm –2 due to low resistance, but the average power density and energy density were low due to high ammonia flux. Full discharge curves using Selemion CMVN provided an average power density of 26 ± 7 mW cm –2 with an energy density of 2.9 Wh L –1 , which were large improvements on previous TRBs. A techno-economic analysis showed that Selemion CMVN had the lowest levelized cost of storage ($410 per MWh) at an applied current density of 50 mA cm –2 .

25 ENERGY STORAGE↗
DOE OSTI2019

Balancing Water Dissociation and Current Densities To Enable Sustainable Hydrogen Production with Bipolar Membranes in Microbial Electrolysis Cells

Hydrogen production using two-chamber microbial electrolysis cells (MECs) is usually adversely impacted by a rapid rise in catholyte pH because of proton consumption for the hydrogen evolution reaction. While using a bipolar membrane (BPM) will maintain a more constant electrolyte pH, the large voltage loss across this membrane reduces performance. To overcome these limitations, we used an acidic catholyte to compensate for the potential loss incurred by using a BPM. A hydrogen production rate of 1.2 ± 0.7 L-H 2 /L/d (j max = 10 ± 0.4 A/m 2 ) was obtained using a Pt cathode and BPM with a pH difference (ΔpH = 6.1) between the two chambers. This production rate was 2.8 times greater than that of a conventional MEC with an anion exchange membrane (AEM, 0.43 ± 0.1 L-H 2 /L/d, j max = 6.5 ± 0.3 A/m 2 ). The catholyte pH gradually increased to 11 ± 0.3 over 9 days using the BPM and Pt/C, which decreased current production (j max = 2.5 ± 0.3 A/m 2 ). However, this performance was much better than that obtained using an AEM as the catholyte pH increased to 10 ± 0.4 after just one day. The use of an activated carbon cathode with the BPM enabled stable performance over a longer period of 12 days, although it reduced the hydrogen production rate (0.45 ± 0.1 L-H 2 /L/d).

08 HYDROGEN↗
DOE OSTI2019

Substrate‐Dependent Molecular and Nanostructural Orientation of Nafion Thin Films

Abstract The effects of film thickness and substrate composition on the ionomer structure in porous electrodes are critical in understanding pathways toward developing higher performance electrochemical devices, including fuel cells and batteries. Insights are gained into the molecular and nanostructural orientation dependence for thin Nafion films (12–300 nm thick) on gold, platinum, and SiO 2 model substrates. Molecular orientation is determined from the birefringence measured using spectroscopic ellipsometry, while the nanostructural orientation of the ionic domains is measured using grazing‐incidence small‐angle X‐ray scattering. Density functional theory calculations for the molecular polarizability of the Nafion backbone and side chain show complimentary contributions to the measured birefringence values for the material. Nafion films prepared on SiO 2 substrates exhibit a nearly isotropic molecular and nanostructural orientation. Films on gold and platinum display parallel backbone orientations, relative to the substrate, with decreasing film thickness. However, a birefringence transition toward molecular isotropy is observed for 30 nm thick films on Au and Pt; while the ionic nanostructured domains continuously align parallel to the substrate. This apparent isotropic molecular orientation with increasing domain orientation highlights the difference between the backbone and side chain orientation, a key finding for elucidating transport in confined films at the interfaces.

Kushner, Douglas I.↗
DOE OSTI2019

Poly(olefin)-Based Anion Exchange Membranes Prepared Using Ziegler–Natta Polymerization

Bromoalkyl-functionalized poly(olefin)s were synthesized by copolymerization of 4-(4-methylphenyl)-1-butene with 11-bromo-1-undecene using Ziegler–Natta polymerization. The resulting bromoalkyl-functionalized poly(olefin)s were converted to quaternary ammonium-containing anion-conductive copolymers by reacting the pendant bromoalkyl group with trimethylamine or a custom-synthesized tertiary amine containing pendant quaternary ammonium moieties. Poly(olefin)-based AEMs with three cations per side chain showed considerably higher hydroxide conductivities, up to 201 mS/cm at 80 °C in liquid water, compared to that of samples with only one cation per bromoalkyl site (68 mS/cm, 80 °C, liquid water), likely due to phase separation in the triple-cation structure. More importantly, triple-cation side-chain poly(olefin) AEMs exhibited higher hydroxide conductivity under relative humidity conditions (50%–100%) than typical AEMs based on benzyltrimethylammonium cations. Here, the triple-cation the triple-cation side-chain poly(olefin)-based AEM exhibited an ionic conductivity as high as 115 mS/cm under 95% RH at 80 °C and 11 mS/cm under 50% RH at 80 °C. In addition to high ionic conductivity, the triple-cation side-chain poly(olefin) AEMs exhibited good chemical and dimensional stability. High retention of ionic conductivity (>85%) was observed for the samples in 1 M NaOH at 80 °C over 1000 h. Based on these high-performance poly(olefin) AEMs, a fuel cell with a peak power density of 0.94 W cm –2 (1.28 W cm –2 after iR correction) was achieved under H 2 /O 2 at 70 °C. The results of this study suggest a new, low-cost, and scalable route for preparation of poly(olefin)-based AEMs for anion exchange membrane applications.

25 ENERGY STORAGE↗
DOE OSTI2019

High Performance Anion Exchange Membrane Fuel Cells Enabled by Fluoropoly(olefin) Membranes

Abstract Although the peak power density of anion exchange membrane fuel cells (AEMFCs) has been raised from ≈0.1 to ≈1.4 W cm −2 over the last decade, a majority of AEMFCs reported in the literature have not been demonstrated to achieve consistently high performance and steady‐state operation. Poly(olefin)‐based AEMs with fluorine substitution on the aromatic comonomer show considerably higher dimensional stability compared to samples that do not contain fluorine. More importantly, fluorinated poly(olefin)‐based AEMs exhibit high hydroxide conductivity without excessive hydration due to a new proposed mechanism where the fluorinated dipolar monomer facilitates increased hydroxide dissociation and transport. Using this new generation of AEMs, a stable, high‐performance AEMFC is operated for 120 h. When the fuel cell configuration is subjected to a constant current density of 600 mA cm −2 under H 2 /O 2 flow, the cell voltage declines only 11% (from 0.75 to 0.67 V) for the first 20 h during break‐in and the cell voltage loss is low (0.2 mV h −1 ) over the subsequent 100 h of cell testing. The ease of synthesis, potential for low‐cost commercialization, and remarkable ex situ properties and in situ performance of fluoropoly(olefin)‐based AEM renders this material a benchmark membrane for practical AEMFC applications.

Zhu, Liang↗