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All sources NASA NTRS DOE OSTI

DOE OSTI2022

Experimental Studies of Graphene-Coated Polymer Electrolyte Membranes for Direct Methanol Fuel Cells

The two main technical limitations of direct methanol fuel cells (DMFCs) are the slow kinetic reactions of the methanol oxidation reaction (MOR) in the anode and the crossing over of unreacted methanol through the proton exchange membrane (PEM). It is a common practice to use Nafion membranes as PEMs, which have high ion exchange capacity. However, Nafion-based membranes also have high fuel permeability, decreasing fuel utilization, and reducing the potential power density. This article focuses on using graphene-coated (Gr-coated) PEMs to reduce fuel crossover. Protons can permeate across graphene, and thus, it can be employed in various devices as a proton conductive membrane. Here, we report the efficiency of Gr-coated Nafions. We tested performance and crossover at three different temperatures with four different fuel concentrations and compared it to a Nafion PEM that underwent the same test conditions. We found that the adhesion of Gr on to PEMs is insufficient for prolonging fuel cell operation, resulting in Gr delamination at high temperatures and higher fuel crossover values compared to lower temperature testing. Furthermore, the results for 7.5M methanol fuel show a reduction of up to 25% in methanol crossover, translating to a peak power density that increases from 3.9 to 9.5 mW/cm 2 when using a Gr-coated PEM compared to a Nafion PEM at 30 °C.

25 ENERGY STORAGE↗

DOE OSTI2021

Fabrication and Characterization of Cross-Linked Phenyl-Acrylate-Based Ion Exchange Membranes and Performance in a Direct Urea Fuel Cell

Ion exchange membranes (IEMs) are crucial for direct fuel cells, including direct methanol and direct urea fuel cells (DUFCs). While commercially available IEMs (e.g., FAA-3-50) show decent power density in direct fuel cells, they experience considerable methanol or urea crossover, reducing device performance and motivating design of IEMs that suppress fuel crossover. Here, we prepare cross-linked IEMs with high mechanical toughness utilizing a cross-linker (methylenebis(acrylamide)), hydrophobic monomer (phenyl acrylate (PA) or phenyl methacrylate (PMA)), and charged monomer (2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) for cation exchange or methacroylcholine chloride (MACC) for anion exchange). To validate these membranes in a fuel cell application, we perform DUFC experiments utilizing a PA/MACC AEM and observe good power density compared to FAA-3-50. Furthermore, to understand the role of urea crossover in DUFC performance, permeabilities of both membranes to urea are measured by diffusion cells with in situ ATR-FTIR spectroscopy, where our PA/MACC exhibited lower urea permeability than FAA-3-50.

36 MATERIALS SCIENCE↗

NASA NTRS1995

Performance of PEM Liquid-Feed Direct Methanol-Air Fuel Cells

A direct methanol-air fuel cell operating at near atmospheric pressure, low-flow rate air, and at temperatures close to 60oC would tremendously enlarge the scope of potential applications. While earlier studies have reported performance with oxygen, the present study focuses on characterizing the performance of a PEM liquid feed direct methanol-air cell consisting of components developed in house. These cells employ Pt-Ru catalyst in the anode, Pt at the cathode and Nafion 117 as the PEM. The effect of pressure, flow rate of air and temperature on cell performance has been studied. With air, the performance level is as high as 0.437 V at 300 mA/cm2 (90oC, 20 psig, and excess air flow) has been attained. Even more significant is the performance level at 60oC, 1 atm and low flow rates of air (3-5 times stoichiometric), which is 0.4 V at 150 mA/cm2. Individual electrode potentials for the methanol and air electrode have been separated and analyzed. Fuel crossover rates and the impact of fuel crossover on the performance of the air electrode have also been measured. The study identifies issues specific to the methanol-air fuel cell and provides a basis for improvement strategies.

Methanol-Air Fuel↗

NASA NTRS1995

Trioxane: A Fuel For Direct-Oxidation Fuel Cells

Trioxane identified as high-energy, nontoxic, solid substitute for formaldehyde as water-soluble fuel for use in direct-oxidation fuel cells. Found to undergo facile electrochemical oxidation to water and carbon dioxide at platinum and platinum-alloy electrodes in liquid-feed-type fuel cells that contain acid electrolytes or solid proton-exchange membrane electrolytes. Exhibits less crossover than do such conventional fuels as methanol and formaldehyde. Being solid at ambient temperature, trioxane offers significant advantages in handling and transportation. Synthesized from natural gas with relative ease.

Olah, George A.↗

DOE OSTI2022

Impact of hydrophobic pendant phenyl groups on transport and co-transport of methanol and acetate in PEGDA-SPMAK cation exchange membranes

CO 2 reduction cells are innovative devices that reduce CO 2 into valuable chemicals (i.e. methanol (MeOH) and acetate (OAc – )) on the cathode. A major challenge with such devices is to develop ion exchange membranes that allow ion-selective transport (i.e., protons for cation exchange membranes, CEM) and suppress the crossover of CO 2 reduction products. To design such membranes, it is important to understand multi-solute transport behavior of these solutes. Previously, our group reported acetate diffusivities in co-transport with MeOH increase in sulfonated CEMs, where we speculated charge screening of the electrostatic interactions by co-diffusing MeOH has an impact. Here, crosslinked membranes fabricated by photopolymerization of poly(ethylene glycol) diacrylate (PEGDA), 3-sulfopropyl methacrylate potassium (SPMAK, SO 3 – -containing ionomer), and a phenyl-containing comonomer either phenoxyethyl acrylate (PEA) or poly(ethylene glycol) phenyl ether acrylate (PEGPEA)) are investigated. We observe OAc – diffusivities to both (1) PEGDA-SPMAK and (2) PEGDA-PEGPEA increase in co-diffusion with MeOH and those to (3) PEGDA-SPMAK/PEGPEA decrease. To rationalize this emergent co-transport behavior, we speculate (1) electrostatic interactions are interfered with by co-diffusing MeOH, (2) chain mobility (segmental dynamics) increases in the presence of MeOH and (3) for films with both hydrophobic (PEGPEA) and hydrophilic (SPMAK) comonomers chain mobility (segmental dynamics) decrease due to interactions between the comonomer sidechains depressing overall solute diffusivities. Finally, while further investigations are needed, this work contributes to improving our fundamental understanding of the relationships between polymer film chemistry, solute chemistry, and emergent cotransport behavior observed and described in this work (and others) towards enabling the design of improved membrane materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Facilitated Direct Liquid Fuel Cells with High Temperature Membrane Electrode Assemblies

Dimethyl ether (DME) is a liquid fuel of great potential impact due to its exceptionally high energy density. However, it has received minimal prior investigation as an alternative to either purified hydrogen or other liquid fuels, including methanol (MeOH). In the limited published literature work on direct dimethyl ether fuel cells, regardless of operating temperature, PtRu (either supported or unsupported on carbon) has been established as the standard catalyst of choice. The majority of the work in this program also utilized a Johnson Matthey (JM) HiSPEC ® 12100 PtRu/C (nominally 50% Pt, 25% Ru) while looking at electrode optimizations, beginning of life (BoL) performance, pressure- and temperature-dependent studies to look at the effect of binding affinity of DME oxidation intermediates, mass transport effects, crossover studies, and durability. However, it does also investigate some promising alternatives to PtRu/C as well, which should be investigated in more detail in further work. Those catalysts include a pair of ternary PtRuPd/C catalysts (from Los Alamos National Laboratory (LANL) and Pajarito Powder, LLC. (PP)) as well as a Pt 2 Bi Black catalyst from Professor Anastasios Angelopoulos of the University of Cincinnati (UC). This work achieved several project objectives, including an optimization of the membrane electrode assembly (MEA) process using PtRu/C anode catalyst. Additionally, these direct dimethyl ether fuel cells (DDFCs) were able to match or exceed many performance metrics for the state-of-the-art (SOA) direct methanol fuel cells (DMFCs), a primary and more evolved competitor to direct dimethyl ether fuel cells. This included peak specific power, total platinum group metal (PGM) loading, crossover current, degradation rate, start/stop cycling losses, and anode specific current.

09 BIOMASS FUELS↗