Measurement of deuterium isotope separation by polymer electrolyte fuel cell stack

Processes for separating hydrogen isotopes are important for future energy applications. Several separation methods are based on electrolytic process; however, electrolysis consumes large amounts of electric energy. In this study, we demonstrate deuterium isotope separation from a mixture of H₂ and D₂ gases using a polymer electrolyte fuel cell stack. To identify the most efficient process, we investigated two flow patterns for the fuel gas, namely, parallel and serial flow. The electrical power of the stacks depended on the flow pattern when a high current was generated. We attribute this dependence on membrane dehydration and water droplet formation in the serial flow, which passed through the single cells in a straight path. However, the stack with the serial path showed a high separation factor (α = 6.6) indicating enrichment of deuterium water during the operation. The long reaction path of the fuel gas contributed to effective separation. The fuel utilization in individual cells suggested the potential for even more effective separation processes by a serial flow path.     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

Influence of doping level in Yttria-Stabilised-Zirconia (YSZ) based-fillers as degradation inhibitors for proton exchange membranes fuel cells (PEMFCs) in drastic conditions

Yttria-Stabilized-Zirconia fillers with different Y₂O₃ loadings are used to prepare composite Nafion membranes for PEMFCs. XRD and BET demonstrate the formation of a c-ZrO₂ mesoporous structure. SEM reveals a size reduction of the agglomerates increasing the ZrO₂ doping level. A good mechanical resistance, no variation into the water retention, swelling restraint and an increased Ion Exchange Capacity (IEC) of the membranes are found respect to reference membrane, above all for highly doped membranes, indicating an acidic properties enhancement. Proton conductivity (PC) at 100%RH (80–100 °C) is unchanged for composite membranes compared to reference. At 75%RH, PC is positively affected by the highest YSZ loadings. Fenton's test on membranes evidences a higher oxidative chemical stability for composite membranes. This improved stability is confirmed by accelerated stress test in drastic conditions: composite highly doped membranes work for more than 110 cycles with a good performance and lower H₂-crossover against 95 cycles and higher H₂-crossover than reference membrane.     

Inorganic–organic membranes based on Nafion, [(ZrO2)·(HfO2)0.25] and [(SiO2)·(HfO2)0.28]. Part I: Synthesis,thermal stability and performance in a single PEMFC

This work reports the preparation, characterization and test in a single fuel cell of two families of hybrid inorganic-organic proton-conducting membranes, each based on Nafion and a different “core-shell” nanofiller. Nanofillers, based on either a ZrO₂ “core” covered with a HfO₂ “shell” (ZrHf) or a HfO₂ “core” solvated by a “shell” of SiO₂ nanoparticles (SiHf), are considered. The two families of membranes are labelled [Nafion/(ZrHf)_x] and [Nafion/(SiHf)_x], respectively. The morphology of the nanofillers is investigated with high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray spectroscopy (EDX) and electron diffraction (ED) measurements. The mass fractions of nanofiller x used for both families are 0.05, 0.10 or 0.15. The proton exchange capacity (PEC) and the water uptake (WU) of the hybrid membranes are determined. The thermal stability is investigated by high-resolution thermogravimetric measurements (TGA). Each membrane is used in the fabrication of a membrane-electrode assembly (MEA) that is tested in single-cell configuration under operating conditions. The polarization curves are determined by varying the activity of the water vapour (a_H2O) and the back pressure of the reagent streams. A coherent model is proposed to correlate the water uptake and proton conduction of the hybrid membranes with the microscopic interactions between the Nafion host polymer and the particles of the different “core–shell” nanofillers.     

Effect of the Nafion content in the MPL on the catalytic activity of the Pt/C-Nafion electrode prepared by pulsed electrophoresis deposition

The aim of this work is to study the effect of Nafion content in the microporous layer (MPL) on the electrophoretically deposited Pt/C-Nafion electrode performance. First, the MPLs are prepared with different Nafion ionomer contents (5, 10, 20, 30 and 40 wt%) on the carbon paper substrates. Next, Pt/C-Nafion electrodes are prepared by pulsed electrophoresis deposition (pulsed EPD) from a Pt colloidal solution as a plating bath. The catalytic activities of the prepared Pt/C-Nafion electrodes are evaluated using the cyclic voltammetry (CV) technique for hydrogen oxidation reaction (HOR). Also, a PEMFC single cell test is carried out using the Pt/C-Nafion electrodes prepared with the pulsed EPD method as a cathode. The mass-specific power density for the Pt/C-30wt% Nafion electrode is $1578.8\text{mW}{\text{mg}}_{\text{Pt}}^{?1}$, which is higher than the rest of the Pt/C-Nafion electrodes. The Nafion content in the cathode MPL should affect the performance of the PEMFC, especially at a high current density.     

Preparation and Nc/T plots of un-crystallized Nafion 1100 and semi-crystalline Nafion 1000

The last advances on the application of INCA (Ionomer Nc Analysis) methodology for a better understanding of perfluorinated ionomers are reported and discussed. It was found that INCA is a very suitable technique for the determination of the melting temperature (Tm) of un-crystallized and semi-crystalline perfluorinated ionomers. Furthermore, for these determinations, it is even more precise than dynamic mechanical analysis, a method essentially developed for polymers in which the water-uptake is negligible. Of interest, it is the information that INCA methodology gives on the phenomenon of the water-uptake memory of perfluorinated ionomers.     

New modified Nafion-bisphosphonic acid composite membranes for enhanced proton conductivity and PEMFC performance

Proton exchange membranes remain a crucial material and a key challenge to fuel cell science and technology. In this work, new Nafion membranes are prepared by a casting method using aryl- or azaheteroaromatic bisphosphonate compounds as dopants. The incorporation of the dopant, considered at 1 wt% loading after previous selection, produces enhanced proton conductivity properties in the new membranes, at different temperature and relative humidity conditions, in comparison with values obtained with commercial Nafion. Water uptake and ionic exchange capacity (IEC) are also assessed due to their associated impact on transport properties, resulting in superior values than Nafion when tested in the same experimental conditions. These improvements by doped membranes prompted the evaluation of their potential application in fuel cells, at different temperatures. The new membranes, in membrane-electrode assemblies (MEAs), show an increased fuel cell maximum power output with temperature until 60 °C or 70 °C, followed by a decrease above these temperatures, a Nafion-like behaviour when measured in the same conditions. The membrane doped with [1,4-phenylenebis(hydroxymethanetriyl)]tetrakis(phosphonic acid) (BP2) presents better results than Nafion N-115 membrane at all studied temperatures, with a maximum power output performance of ～383 mW cm^?2 at 70 °C. Open circuit potentials of the fuel cell were always higher than values obtained for Nafion MEAs in all studied conditions, indicating the possibility of advantageous restrain to gas crossover in the new doped membranes.     

Operando infrared spectroscopy of the fuel cell membrane electrode assembly Nafion–platinum interface

The Nafion–Pt interfaces in membrane electrode assemblies of operating fuel cells were studied by operando infrared spectroscopy. The potential dependence of atop adsorbed CO peak frequencies were measured over the potential range of 0–600 mV (vs. NHE) at 60 °C. Complex Stark tuning of peak frequencies arise from a combination of potential dependent coverage effects, and changes in the extent of back-donation from the metal d-band to the renormalized 2π^∗ MO of CO_ads. The Nafion–Pt interface was studied at higher potentials (initiating at open circuit) by examining platinum reflectivity as a function of electrode potential. The oxygen reduction onset-current is coincident with the observance of a 2% step-increase in Pt reflectivity and emergence of Nafion–Pt interface spectra.     

Gas-diffusion layer's structural anisotropy induced localized instability of nafion membrane in polymer electrolyte fuel cell

The design of robust polymer electrolyte fuel cell requires a thorough understanding of the materials' response of the cell components to the operational conditions such as temperature and hydration. As the electrolyte membrane's mechanical properties are temperature, hydration and rate dependent, its response under cyclic loading is of significant importance to predict the damage onset and thus the membrane lifetime. This article reports on the variation in stress levels in the membrane induced due to the gas-diffusion layer's (GDL) anisotropic mechanical properties while accurately capturing the membrane's mechanical response under time dependent hygrothermomechanical conditions. An observation is made on the evolution of negative strain in the membrane under the bipolar plate channel area, which is an indication of membrane thinning, and the magnitude of this strain found to depend upon the GDL's in-plane mechanical properties. In order to come up with a strategy that reduces the magnitude of tensile stresses evolved in the membrane during the hygrothermal unloading and to increase the membrane's lifetime, we numerically show that by employing a fast hygrothermal loading rate and unloading rate strategy, significant reduction (in this study, nearly 100%) in the magnitude of tensile stresses is achievable. The present study assists in understanding the relation between materials compatibility and durability of fuel cell components.     

Stabilizing phosphotungstic acid in Nafion membrane via targeted silica fixation for high-temperature fuel cell application

With PWA as proton transfer and silica as water retainer, stable phosphotungstic acid/silica/Nafion (PWA/Si–N) composite membrane is non-destructively fabricated and exhibits excellent stability and high temperature proton conductivity. Compared with pristine Nafion, high temperature proton conductivity is significantly enhanced due to the collaboration between –SO₃H ionic clusters and the in-situ filled silica embedded PWA nanoparticles. PWA is stabilized in the ionic clusters via in-situ catalyzing the hydrolysis silica precursor targeted filled into the –SO₃H ionic clusters. Stable proton conductivity of the PWA/Si–N membrane at 110 °C and 60% RH is high to 0.058 S/cm, which is 2.4 folds of that of Nafion. At the same time, the composite membrane still maintains good mechanical and thermal stability. As a result, high temperature fuel cell performance of the composite membrane is improved by 41% compared with the pristine Nafion membrane. The in-situ coating method proved to be an effective method to solve the stability of PWA in Nafion membrane, especially the inorganic oxide with good hygroscopicity as the modifier.