Nafion Dispersion Research Articles

Effect of Annealing Treatment for the Morphology and Proton Transport Property of Nafion Thin-Film on Platinum Electrode

Introduction Improving oxygen reduction reaction (ORR) activity of cathode catalyst is required for wide spread of polymer electrolyte fuel cells. Because ORR occurs at the interface between cathode catalyst and ionomer (Nafion), understanding the structure of Nafion on the cathode catalyst is important to design reaction field with high activity. The morphology of the Nafion on the cathode is strongly influenced by humidity, thickness, annealing treatment and so on[1-3]. The proton conductivity of Nafion increases with humidification, and the change degree depends on the thickness of Nafion.[3] However, the correlation between morphology and proton conductivity of the Nafion on the cathode have not still been clearly understood. In this study, we examined the correlation between morphology and proton conductivity under humidity of Nafion thin-film with annealing treatment via grazing incidence small angle X-ray scattering (GISAXS) and electrochemical impedance spectroscopy. Experimental For proton conductivity measurements, Nafion thin-films were prepared on interdigitated array of gold electrodes on SiO2 substrate by soaking the electrode on Nafion dispersion. For GISAXS measurements, Nafion thin-films were prepared on Pt sputtered P doped-silicon substrates by spin casting of Nafion dispersion. Annealing treatment was conducted under 160oC under N2 atmosphere for 1 hour for each thin-film prepared by the different methods. Proton conductivity under humidity was measured by applying an alternating potential of amplitude 100 mV over a frequency 7 MHz to 0.01 Hz. GISAXS measurements were performed under humidity in beamline BL40B2 at SPring-8, Japan. Results and Discussion The proton conductivity of the Nafion thin-film without annealing treatment was larger than that of the thin-film with annealing treatment under 20-80% relative humidity. Figure 1 shows that 1 D line profiles parallel to the substrate obtained from 2D GISAXS patterns of Nafion thin-films with or without annealing treatment under various relative humidity (RH). For the non-annealed thin-film (Figure 1 (a)), a broad peak, which is attributed to water domain in Nafion [4], appeared around q = 1.5 nm-1. The peak shifted lower q and the intensity increased with humidification. In the annealed thin-film (Figure 1(b)), the peak appeared around higher q (2.0 nm-1) higher than non-annealed thin-film, and the degree of intensity change with humidification was smaller. These results mean that annealing treatment suppresses growth of water domain in Nafion with humidity, which decreases proton conductivity. Acknowledgments This research is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Reference [1] A. Kusoglu, D. Kushner, D. K. Paul, D. Karan, M. Hickner, A. Weber, Adv. Funct. Mater., 24, 4763 (2014) [2] D. K. Paul, H. K. Shim, J. B. Giorgi, K. Karan, J. Polym. Sci. B Polym. Phys., 54, 1267 (2016) [3] D. K. Paul, R. McCreery, K. Karan, J. Electrochem. Soc., 161, F1395 (2014) [4] A. Kusgglu, T. J. Dursch, A. Z. Weber, Adv. Funct. Mater., 26, 4961 (2016)

Effect of Dispersing Solvents of Ionomer Binder Solutions in Catalyst Inks on Performance and Durability of Membrane-Electrode Assembly

Proton exchange membrane fuel cells (PEMFCs) is currently considered as a potential next-generation alternative energy technology because of the high energy density and high abundance of hydrogen in nature. However, many issues still exist to be improved in terms of performance and durability for the wide use of PEMFCs as a clean and efficient power system. One of the most important parts in PEMFCs is membrane-electrode assembly (MEA) which provides the electrochemical reactions at electrode where fuel/oxidant (gas), hydrated ions (liquid) and electron (solid) co-exist (so called triple phase boundary), the permeation of fuel and/or oxidant gas and the drainage of water product. In MEA, water and IPA based Nafion dispersion is the most widely used as the ionomer binder for conduction of hydrated ions, formation of pores and adhesion of solid catalyst. The structure of electrodes in MEA could be significantly determined by characteristics of ionomer binder solutions where polymer dispersion interacts with solvent. Upon the structure of the electrodes formed by agglomerates of catalyst and ionomer binder with different size, it has reported that MEA performance and durability would be highly affected. The former could be achieved by reducing overvoltages, and the latter by reducing electrode cracking (i.e., well adhesion of catalyst by ionomer). Electrode structure of MEA, including the Pt-ionomer interface, is determined when electrodes are solidified from catalyst inks during a drying step. Thus, the properties of the solvent system in catalyst inks play an important role in determining the electrode structure in MEA. A fundamental understanding of the phenomenological outcome from the relationship between morphology and property for perfluorosulfonic acid (PFSA) membranes is crucial in obtaining high power and durable MEA. In this study ion and liquid water behavior in PFSA membranes formed from different dispersing solvents were investigated with ionomer particle sizes, solvent viscosity, solvent dielectric constants and distribution which cause various morphology. In addition, MEAs with different electrodes prepared by different ionomer binder solutions were investigated in terms of I-V polarization, cyclic voltammetry, electrochemically-active surface area (ECSA), and chemical stability. Acknowledgment This work was supported by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20173010032100).

Ink Degradation Phenomena and Its Impact on Crack Formation of Fuel Cell Catalyst

The catalyst layer (CL) is one of critical component for determining the performance of proton exchange membrane fuel cells (PEMFCs), and the production process of CL is important issue (Figure 1). In order to form the CL, catalyst ink mixed with platinum-supported carbon (Pt/C), ionomer solution, and a solvent is necessary. When the catalyst ink is coated on the sheet and dried, the CL is formed. By transferring the catalyst layer to the membrane, catalyst coated membrane can be made [1]. The process from the initial characteristics of the catalyst ink to the drying process determines the structure of CL and dominates the characteristics of the fuel cell. Thus, quality control of the catalyst ink is important to form a high performance and uniform CL [2]. In the previous study, we have reported that oxidation reaction of alcohol, as catalyzed by platinum, resulted in degradation of the catalyst ink [3]. However, the impact of catalyst ink degradation on the catalyst layer formation process has not been clarified. In this study, the composition change of the catalyst ink was investigated in more detail using gas chromatography mass spectrometry (GC/MS). Furthermore, the catalyst layer was prepared from the catalyst ink, and the influence of the catalyst ink degradation on the formation of the micro- to millimeter scale structure of the catalyst layer was investigated. Catalyst ink was prepared by using the mixture of water and 1-propanol (NPA) as a dispersant. We used platinum catalyst supported on carbon black and Nafion dispersion D2020 (Chemours). Pre-mixing of catalyst ink was performed using ultrasonicaton to break large Pt/C agglomerates into smaller units. As a main mixing process, a high-speed rotary type mixer Filmix (Primix) utilizing shearing force, specialized for producing nanometer slurry was used. The prepared ink was immediately transferred to a glass sample container and sealed tightly. After 20 days, the composition of the catalyst ink was analyzed using a GC/MS (JMS-700, JEOL, Tokyo, Japan). In Figure 2, result of GC/MS indicates that new 4 compounds were present in the ink composition in addition to water and NPA. Production of propionaldehyde and propionic acid was reported in the previous study [3], but present result showed production of propyl propionate and propane, 1,1-dipropoxy-. It is considered that not only oxidation reaction of the NPA but also dehydration reaction of each compound was occurred as shown in Figure 3. Both physical properties indicate low solubility in water. Since the liquid phase of the catalyst ink is composed of hydrophilic components, these hydrophobic compounds might cause phase separation and affect the production process of CL. Figure 4 shows the results of microscope observation of the CL made from the catalyst ink. Many cracks existed in the formed CL. It is considered that the hydrophobic compounds caused phase separation in the catalyst ink, and those compounds acted as rupture point in the CL formation process. As a comparison experiment, another catalyst layer was made from catalyst ink using ethanol (C2H6O) as a solvent. Even if ethanol follows the same reaction path as in Figure 3, no hydrophobic compounds will be produced. Thus, it is considered that cracks are hardly formed. Figure 5 shows the CL surface made from the catalyst ink using ethanol, and no crack was formed. These results indicate that when hydrophobic compounds are produced from solvent alcohol by the catalytic action of Pt in the catalyst ink, the formation process of the CL and the quality of the formed CL are significantly affected. Acknowledgment: This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The authors thank the Suzukakedai Materials Analysis Division for performing sample analysis using GC/MS.

Conductive Membrane Coatings for High-Rate Vanadium Redox Flow Batteries.

A conductive coating of carbon nanotubes (CNTs) and Nafion dispersion in water was deposited on a Nafion membrane via air-controlled electrospray. When the coated membrane was assembled into a large single cell of a vanadium redox flow battery (VRB) with a surface area of 35 cm2, it was found that its cycling performance was greatly enhanced at much higher current densities than was afforded by the pristine Nafion membrane. A masking technique was also applied during the electrospraying process to create alternating domains of coated and uncoated membrane surfaces, which helped to mitigate the restriction of proton transport through the membrane due to the coating, while still decreasing the surface resistivity and thus the interfacial resistance of the membrane. Our results reveal that a very small mass of CNTs (∼0.015 mg CNT/cm2) enabled large improvements in the capacity retention and voltaic efficiencies of the vanadium redox battery during charging and discharging. This method has shown to be a reasonably fast, simple, and scalable technique for improving rate capability of VRBs, with the potential for extension to other redox flow battery systems.

Membrane Electrolysis for Organic Chemical Hydride Synthesis with Water Oxidation

To reduce carbon dioxide emissions, much renewable energies that are uneven distribution with fluctuation must be introduced. Toluene-methylcyclohexane organic chemical hydride system is a promising technology of hydrogen storage and transportation. We have focused on membrane electrolysis for hydrogenation of toluene with water oxidation as an efficient energy carrier synthesis process. In this system, we are using PtRu/C, perfluorocarbon sulfuric acid membrane, and DSE®for oxygen evolution reaction as cathode electrocatalyst, electrolyte membrane, and anode, respectively. For the practical application, higher conversion without side reaction of hydrogen evolution reaction and lower cell voltage was needed (1-3). In this study, porous flow filed with dispersed hydrogenation catalyst on the porous medium and low overpotential anode have been developed to improve conversion and cell voltage. Experimental A single cell electrolyzer made of titanium with 100 cm2 of projected electrode area was used. A cathode that was a carbon paper (10BC, SGL) of 0.02 mg cm-2 of Pt loading which was coated 0.5 mg cm-2 of PtRu/C (TEC61E54, TKK) with Nafion dispersion was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 117, DuPont). PtRu/C on the carbon paper and Pt in the carbon paper were electrocatalyst and non-electrochemical catalyst, respectively. A fine mesh DSE® anode with IrO2based electrocatalyst is used for oxygen evolution. The cathode flow field was the carbon paper of the cathode itself. Backing of the anode to push it to the membrane was titanium web. 10 cm3 min-1 of 50 % toluene – methylcyclohexane or 0.53 cm3 min-1 of 100 % toluene and 10 cm3 min-1 of 1 M (=mol dm-3) of H2SO4 were supplied to the cathode and anode for hydrogenation of toluene, respectively. 0.53 cm3 min-1 of toluene correspond to 100 % of hydrogenation of toluene at 0.4 A cm-1. Cell voltage was determined with 4 mV s-1of voltage sweep and steady state measurements. Toluene current efficiency and conversion were determined with the steady state measurements of constant voltage electrolysis with hydrogen generation rate measurement and constant current electrolysis with gas chromatography of the product analysis, respectively. Results and discussion Figure 1 shows the cell voltage (a) and the current efficiency (b) as a function of the current density for 10 cm3 min-1 of 50 % toluene – methylcyclohexane and 0.53 cm3 min-1 of toluene feed at 60oC. The open symbols are for the present cell design of this study, and the close symbols are for the previous cell design with the conventional mesh DSE® and serpentine flow filed without the Pt loading in the cathode carbon paper. The circle and triangle of the cell voltage (a) are the averages of 1 – 10 s and 120 – 180 s of the measurements, respectively. The present design was much lower cell voltage of 1.83 V at 0.4 A cm-2 and higher current efficiency of 100 % than previous design for those of 2.13 V and 90 % at 0.4 A cm-2, respectively. Furthermore, the difference between the scan voltammogram and the steady state voltammogram and, also 1 – 10 s and 120 – 180 s of the steady state electrolysis for the present design were smaller than that for the previous design. The porous flow field would enhance mass transfer of toluene to the electrocatalyst layer, and Pt loading in the flow field would work as hydrogenation catalyst of by-products hydrogen and toluene to increase current efficiency. The fine mesh anode would decrease the overpotential of the anode itself and that of the cathode with uniform current distribution because of the uniform contact with the membrane. The toluene conversion to methylcyclohexane was more than 90 % at 0.4 A cm-2 of the constant current electrolysis with 0.53 cm3 min-1of 100 % toluene feed without significant cell voltage increase as shown in Fig. 1. Therefore, we successfully confirmed that this process will realize one-step continuous electrochemical energy carrier synthesis process with water oxidation. This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). The Institute of Advanced Sciences (IAS) in YNU is supported by the MEXT Program for Promoting Reform of National Universities. We appreciate the person concerned them. 1) S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 2016, 7, 238. 2) K. Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, S. Mitsushima, Electrocatalysis, 2017, 8, 164. 3) S. Mitsushima, K. Nagasawa, Y. Sawaguchi, K. Matsuzawa, A. Kato, Y. Nishiki, PRiME2016 abstract, 2016, #2511. Figure 1

Rheological Characterization of Interparticle Interactions and Polymer Entanglements in Fuel Cell Catalyst Dispersions

Rheometry is a powerful tool to characterize the structure and interparticle interactions in colloidal dispersions, like those used to cast fuel cell catalyst layers. The rheological properties of colloidal dispersions are dependent on many factors including concentration of the dispersed particles, polymer entanglements, particle porosity, and aggregation. There are many publications examining the rheological properties of Nafion dispersions or carbon black dispersions, however, there is little published research examining the rheological properties of fuel cell catalyst inks containing both Nafion and carbon-black-supported catalysts.1-4 This may stem from the fact that the dispersions often used for lab-scale fabrication of fuel cell electrodes tend to be dilute, and thus the rheological properties are dominated by the dispersion medium (solvent), not the dispersed catalyst and ionomer. However, for roll-to-roll production of electrodes, the inks are an order of magnitude more concentrated, and the rheological properties of these inks are significantly influenced by the properties of and interactions between the catalyst and ionomer. In this work we used steady-shear and oscillatory rheometry to quantify the influences of carbon type, presence of catalyst particles, and ionomer-to-carbon ratio on the rheological properties of catalyst inks. We found that, for neat carbon black dispersions, the rheological properties were non-Newtonian and highly dependent on the pore volume of the particles. The addition of Pt to the particles or Nafion to the dispersion decreased the viscosity. The decrease in viscosity with the addition of Nafion was due to Nafion acting as a surfactant and stabilizing the carbon black particles, consistent with studies of aggregation kinetics.5 Interestingly, we found that in dispersions with Nafion, when neat carbon black was replaced with Pt-on-carbon, the change in rheological properties was dependent on carbon type: we observed an increase in viscosity for high-surface carbon but a decrease for Vulcan XC-72 (Figure 1). This suggests a difference in interparticle interactions and polymer entanglements that is dependent on carbon type and Pt location (i.e. on the surface or inside carbon pores), which may have implications for fuel cell performance. We explored these differences using oscillatory shear rheometry, which probes the viscoelastic properties of the dispersions that are dependent on interparticle interactions, including polymer entanglements. Figure 1 Steady-shear rheology measurements of the relative viscosity (viscosity of dispersion/viscosity of dispersion media) of carbon black/Nafion dispersions with and without Pt for Vulcan XC-72 (left) and high surface carbon (right). Acknowledgements: This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with the National Renewable Energy Laboratory. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell Technology Office, program manager Nancy Garland. 1. Moore, R. B. & Martin, C. R. Chemical and morphological properties of solution-cast perfluorosulfonate ionomers. Macromolecules 21, 1334–1339 (1988). 2. Aldebert, P., Gebel, G., Loppinet, B. & Nakamura, N. Polyelectrolyte effect in perfluorosulfonated ionomer solutions. Polymer 36, 431–434 (1995). 3. Gibaud, T., Frelat, D. & Manneville, S. Heterogeneous yielding dynamics in a colloidal gel. Soft Matter 6, 3482 (2010). 4. Negi, A. S. & Osuji, C. O. New insights on fumed colloidal rheology—shear thickening and vorticity-aligned structures in flocculating dispersions. Rheol. Acta 48, 871–881 (2008). 5. Shukla, S., Bhattacharjee, S. & Secanell, M. Rationalizing Catalyst Inks for PEMFC Electrodes Based on Colloidal Interactions. ECS Trans. 58, 1409–1428 (2013). Figure 1

MRI and 1H/19F NMR Investigation of Dispersion State of PEFC Catalyst Ink

The catalyst layer (CL) of polymer electrolyte fuel cell (PEFC) comprises catalyst particles, catalyst support (e.g., carbon particles), and proton-conductive ionomers. The CL plays a critical role in PEFC operation, and also the production process of CL is of significant importance in terms of the cell performance as well as the system cost. The decal method has been used in the commercial production of CL in membrane electrode assemblies, in which the catalyst ink is prepared by mixing catalyst particles and ionomer in the solvent, coated on a decal substrate, dried to form a particulate layer with a specified thickness, and transferred to a membrane via hot pressing [1,2]. Since the three-phase interface (catalyst-void-ionomer) is essential for the mass transport during the electrochemical reaction, it is desirable to control the microstructure of CL [3]. Therefore, the catalyst ink and its use in the decal process should be studied in detail. We have developed a novel approach to investigate the characteristics of catalyst ink by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy [4]. Although traditional NMR studies of ionomers have been reported [5,6], the detailed discussion regarding the spatial inhomogeneity of particles in catalyst ink and their dynamics during the drying process has been lacking. MRI enables us to visualize the spatial variations in the sample characteristics which cannot be accessed through other spectroscopy techniques [7]. In this study, we applied MRI and 1H/19F NMR to the catalyst ink for the decal process for gaining an insight on the dispersion state of catalyst and ionomer. Using the mixture of water and NPA as a dispersant, we prepared catalyst ink samples. We used platinum catalyst supported on carbon black (Pt/C) and Nafion dispersion D2020 (Chemours). Pre-mixing of catalyst ink was performed using ultrasonicaton to break large Pt/C agglomerates into smaller units. We then used a thin-film spin system high-speed mixer Filmix (Primix), which is designed for the production of slurries of nanometer-sized particles, in the main mixing process. Because Nafion works as a dispersant of Pt/C particles, the dispersion state of Nafion has an impact on the dispersibility of the Pt/C particles and eventually on the stability of catalyst ink. We conducted MRI/NMR measurements for the catalyst ink samples at each stage of preparation process to investigate changes in the dispersion state of Pt/C particles and Nafion. We found a fully-mixed sample exhibited a MRI image with submillimeter fine contrast because air bubbles were mixed in the sample during the main mixing process. It was difficult to remove air bubbles from the sample only by leaving it at rest for several hours. This observation was supported by the rheology measurement of catalyst ink. Hence we needed to use a magnetic stirrer to effectively perform the degassing of catalyst ink after the main mixing process. The degassed state of catalyst ink was confirmed by MRI image and 1H NMR spectrum. 19F NMR spectra showed remarkable changes concerning the peaks associated with the main and side chains of Nafion, which indicated the dispersion state of Nafion varied during the mixing process. In the present work, we have demonstrated that our experimental approach can provide a unique capability to investigate the dispersion state of catalyst ink. It is expected that the catalyst ink and its drying process will be investigated to optimize the fabrication process of PEFC catalyst layer. Acknowledgment: This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Time Lapse Visualization of Catalyst Ink Degradation by X-ray CT

The catalyst layer (CL) is one of critical component for determining the performance of proton exchange membrane fuel cells (PEMFCs), and the production process of CL is important issue. In order to form the CL, catalyst ink mixed with platinum-supported carbon (Pt/C), ionomer solution, and a solvent is necessary. When the catalyst ink is coated on the sheet and dried, the CL is formed. By transferring the catalyst layer to the membrane, catalyst coated membrane can be made [1]. The process from the initial characteristics of the catalyst ink to the drying process determines the structure of CL and dominates the characteristics of the fuel cell. Thus, quality control of the catalyst ink is important to form a high performance and uniform CL [2], but it is difficult to observe how the characteristics of the catalyst ink change with time from the appearance. In this study, time lapse visualization of inside the catalyst ink was performed by X-ray CT. Catalyst ink was prepared by using the mixture of water and NPA as a dispersant. We used platinum catalyst supported on carbon black and Nafion dispersion D2020 (Chemours). Pre-mixing of catalyst ink was performed using ultrasonicaton to break large Pt/C agglomerates into smaller units. As a main mixing process, a high-speed rotary type mixer Filmix (Primix) utilizing shearing force, specialized for producing nanometer slurry was used. The prepared ink was immediately transferred to a glass sample container and sealed tightly. Thereafter, it was placed in the X-ray CT (Shimadzu) immediately and time lapse three-dimensional visualization was performed. Figure 1 shows sliced image of catalyst ink. The contrast of the image corresponds to the amount of absorption of X-rays, and it is possible to determine the state of the substance. In the black region, X-rays are easy to transmit, which is a light substance. In the initial state, micro to millimeter scale bubbles (several black dots) were dispersed, however, the other part was uniform light gray, which shows that the catalyst ink was homogeneously mixed. On the other hand, drastic state change inside the catalyst ink with the passage of time was observed. It was found that the volume of bubbles increased with the passage of time. Different color regions (dark gray) were also emerged between the bubbles. Changes with time of the volume of the homogeneous part of the ink, bubbles, and the emerged phase are shown in the graph (Figure 2). Observed changes in the catalyst ink are thought to be due to alcohol decomposition by the catalyst. For comparison, pseudo catalyst ink using carbon black not carrying platinum was visualized, but no change was observed. Analysis of the gas phase in the upper part of the glass sample container showed that the CO2 concentration was increased. From these results, NPA was decomposed by the platinum catalyst, and it is considered that generation of gas and formation of different liquid components were occurred. These experimental results suggest the importance of management of catalyst ink preparation, storage, and coating processes. Acknowledgment: This study is based on the results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Conductive Membrane Coatings for High Rate Vanadium Redox Flow Batteries

Global electricity production from renewable sources has grown dramatically in recent years. The intermittent nature of renewable forms of electricity generation has proven difficult to incorporate into regional electrical grids. Grid-scale electricity storage methods are a promising solution to the inefficiencies of unpredictable electricity generation. Redox flow batteries have demonstrated the ability to be used as reasonably low-cost, long-term, grid-scale electrical storage methods. Specifically, vanadium redox flow batteries (VRBs) have been of special interest due to their chemical stability, long life cyclability, and potential for high electrical capacity compared to other redox chemistries. Accompanying these advantages are some research challenges. There have been a number of efforts to improve the electrical conductivity of VRBs by physically or chemically altering the electrodes of the cell. Most VRBs use a carbon or graphite felt as a porous cathode and anode of the battery – the large surface area from the felt is used to increase the number of reaction sites at which the redox reaction can occur. Many modifications to the electrodes have been investigated, including using various highly conductive carbons, metal catalysts, and covering the carbon felt with hydrophilic surface groups. Less attention has been spent improving the membrane surface properties to improve electrical conductivity in the cell. A new approach to improve the rate capability of VRBs involves improving the electrical conductivity of the surface of the membrane itself using air-assisted electrospray deposition of conductive carbons mixed with a binder. A conductive coating of carbon nanotubes (CNTs) and Nafion dispersion in water was used to coat a Nafion 117 membrane via air-assisted electrospraying to improve the rate capability and cycling performance of VRBs. It was found that electrospraying a highly conductive coating directly onto the surface of the membrane allowed for stable cycling performance at nearly double the current density that was afforded by the pristine Nafion membrane. A templating technique was used during the electrospraying process to allow for alternating domains of coated and uncoated membrane surface, which helped reduce the restriction of proton transport across the membrane, further improving rate capability and capacity retention of the VRB. The interfacial resistance between the membrane and the electrode was greatly diminished with the addition of a very small mass (<0.05 mg/cm2) of CNTs. This method has shown to be a fast, simple, and scalable technique for improving the rate capability of vanadium redox flow batteries, with the potential for extension to other redox flow battery systems.

Solvent effect on the Nafion agglomerate morphology in the catalyst layer of the proton exchange membrane fuel cells

In this work, we investigated the influence of the solvents used in the catalyst ink on performance of the proton exchange membrane fuel cells (PEMFCs). The mobility of the polymeric chains in 2.5 wt% Nafion dispersion is measured by fluorine-19 nuclear magnetic resonance after the use of different organic solvents including isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO) and N-Methyl-2-pyrrolidone (NMP). Subsequently, the electrochemical properties, e.g., direct-current polarization and electrochemical impedance spectra, are characterized. As a result, the catalyst layer fabricated from the solvents with high main-chain mobility created more intimate contact at triple-phase boundary, enlarging the electrochemically available surface area and reducing the charge-transfer resistance, mainly due to a strong interaction between solvent molecule and Nafion ionomer. It is demonstrated that the use of the membrane electrode assemblies (MEAs) fabricated from NMP- and DMSO-based catalyst inks enable a H2/O2 PEMFC to yield 100% and 33% higher power densities at 0.6 V, respectively, as compared to the MEA fabricated from IPA-based catalyst ink.

Improvement of Current Efficiency of a Membrane Electrolyzer for Electrohydrogenation of Toluene As Hydrogen Carrier Synthesis

Introduction In order to reduce carbon dioxide emissions, a significant number of renewable energies that are uneven distribution with fluctuation must be introduced. Therefore, to increase renewable energies, energy carrier technology is needed for storage and transportation. Toluene-methylcyclohexane organic chemical hydride system is one of promising technologies as hydrogen storage and transportation. Electrohydrogenation of toluene with water splitting has higher theoretical energy conversion efficiency compare to a series process of water electrolysis and hydrogenation. Cathode side is a cathode membrane assembly with PtRu/C, which is applied PEFC technology. Anode is a dimensionally stable electrode for oxygen evolution reaction in acidic electrolyte using industrial electrolysis technology. In our previous study, we demonstrated good performance of the electrolyzer with hydrophilized membrane; however, hydrogen generated with low concentration of toluene feed, which should be improved (1). In this study, the effect of the design of toluene feed flow field on the cell voltage and the current efficiency has been investigated to increase conversion ratio from toluene to methylcyclohexane without hydrogen generation. Experimental A single cell electrolyzer made of titanium with 100 cm2of projected electrode area was used to determine the performance. Figure 1 shows the schematic drawing of flow field for the cathode. A parallel flow, which is usual for industrial electrolysis and liquid electrolyte fuel cells, a serpentine flow, which is conventional for polymer electrolyte fuel cells, and an interdigitated flow have been investigated to improve the performance of the electrolyzer. The anode flow field was parallel. A cathode was a carbon paper (35BC, SGL) coated 0.5 mgcm-2 of PtRu (TEC61E54, TKK) with Nafion dispersion. The cathode was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 117, DuPont) for a cathode membrane assembly. The membrane of the cathode side was mechanically hydrophilized. A DSE® anode with IrO2 based electrocatalyst is used for oxygen evolution. Backing of the anode was titanium felt. The anode was uniformlypressed on the membrane by elastic force of the titanium felt. 10 cm3 min-1 of toluene or 50% toluene-methylcyclohexane mixture and 1M (=moldm-3) of H2SO4were supplied to the cathode and anode for hydrogenation of toluene, respectively. Cell voltage was determined with 4 mVs-1 of voltage sweep from 1 V for toluene hydrogenation up to 0.5 A cm-2of the current density. Current efficiency was determined with constant cell voltage electrolysis with the volume measurement of gas evolution from cathode during the electrolysis.Internal resistance (iR) was determined with higher frequency intercept of AC impedance method. Results and discussion Figure 1 shows the cell voltage and the current efficiency as a function of the current density for electrohydrogenation of toluene at 60oC with various flow fields for the cathode. 100% of toluene or 50% toluene – methylcyclohexane mixture was fed to the cathode chamber. The internal resistances by AC impedance method were 0.25 Ω cm2 for all electrolyzers, which is almost same as the membrane area resistivity (2). The cell voltages for 50 and 100% toluene feed as a function of current density were almost the same for each flow design of the cathode. The cell voltage of parallel and interdigitated flow was 2.0 V at 0.4 Acm-2, and the cell voltage of the serpentine was a little larger than the other flow patterns. The current efficiency of hydrogenation decreased with the increase of current density, and the order from high current efficiency was the serpentine, interdigitated, and parallel flows. The difference among flow patterns was significantly for 100 % toluene feed, but it was very small for 50 % toluene feed. Turbulence flow would be better for mixing than laminar flow to feed toluene to the catalyst layer, and the order seems to be same to the flow velocity. Flow field design is important to increase toluene conversion. In addition, improvement of catalyst layer should also be important, because current efficiency of 50 % toluene feed seems to be controlled by mass transfer in the catalyst layer. Acknowledgment This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). The Institute of Advanced Sciences (IAS) in YNU is supported by the MEXT Program for Promoting Reform of National Universities. We appreciate the person concerned them. References 1) S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiki, Electrocatalysis, 2016, 7, 238.2) S. Slade, S. Campbell, T. Ralph, F. Walsh, J. Electrochem. Soc., 2002, 149, A1556. Figure 1

Sodium Cobalt Oxide As a Non-Platinum Cathode Catalyst for Microbial Fuel Cells

Microbial fuel cell is a device that can achieve wastewater treatment and power generation simultaneously. For the practical application of microbial fuel cell, development of alternative cathode catalyst for platinum is necessary. In this study, a kind of sodium cobalt oxide, NaCo2O4was investigated, because estimated cobalt reserve is 100 times higher than platinum, and its price is about one thousandth. Catalyst ink was prepared by mixing NaCo2O4powder with carbon black powder and Nafion dispersion. Then cathode was fabricated by applying the ink to PTFE treated carbon paper. Cathode with Pt catalyst and that with activated carbon (NORIT SX PLUS) were also prepared for comparison.ORR activity of the cathodes was measured by linear sweep voltammetry (LSV). Oxygen reduction reaction (ORR) activity of NaCo2O4 cathode increased 15-25 times by the addition of carbon black as conductive assistant material in catalyst ink. Two kinds of carbon black were tested, and a cathode with Ketjen black showed better ORR activity than that with Vulcan XC-72. By increasing coating amount of catalyst ink from 2mg/cm2 (NaCo2O4) to 4mg/cm2, ORR activity increased 1.6 - 1.7 times, which was equivalent to 50-60% of Pt catalyst (Fig) . Gram based ORR activity of this catalyst was higher than that of activated carbon (NORIT SX PLUS) catalyst (Fig). The NaCo2O4 cathode having highest ORR activity was built into a pre-acclimated MFC, and then used for the treatment of synthetic wastewater. The current density was about 3A/m2, which was a little lower than the value during the operation equipped with a Pt cathode (about 4A/m2). However, any problems such as a decrease in electricity generation were not observed in several days of operation. Power density curve of the MFC equipped with the NaCo2O4 cathode was obtained by the LSV measurement , and the maximum current density was 0.58 W/m2, which is about a half of the density achieved by a Pt cathode. Further research on the preparation condition of the cathode, such as the amount of conductive assistant material, the amount of binder, and the coating amounts of the catalyst ink, would probably contribute to further improve the cathode ORR activity, which would improve electricity generation performance of a MFC. Figure 1

MRI Investigation of Local Diffusion Phenomena in PEFC Catalyst Ink

The catalyst layer (CL) of polymer electrolyte fuel cells plays a critical role in their operation and also the production process of CL is of significant importance in terms of the cell performance as well as the system cost. Herein, we propose a novel approach to investigate the characteristics of catalyst inks by using magnetic resonance imaging (MRI) combined with nuclear magnetic resonance (NMR) spectroscopy. We developed a technique to acquire NMR spectra from a selected volume of a liquid sample and determine the local diffusion coefficients of component molecules. It was found that the diffusion coefficients of both water and n-propyl alcohol (NPA) remarkably changed depending on the mixuture composition, which indicates the change in molecular microstructure. In addition, we obtained MRI images of catalyst inks and the precipitated bed of carbon particles was visualized. We found the dependence of the local diffusion coefficient of water in the particle bed region on the diffusion time, which suggested that the molecular diffusion of solvent molecules were distracted by the closely-packed carbon particles. Furthermore, 19F-NMR spectroscopy of Nafion dispersions suggested the change in the dispersion state of Nafion molecuels. We demonstrated that our approach provided a unique capability to investigate the catalyst ink and its decal process. It will be proved that this technique is useful in investigating many samples prepared at various conditions and better understand the decal process.

Spray deposition of Nafion membranes: Electrode-supported fuel cells

Fuel cells are a key technology for the successful transition towards a hydrogen society. In order to accelerate fuel cell commercialization, improvements in performance are required. Generally, polymer electrolyte membrane fuel cells (PEFCs) are membrane-supported; the electrocatalyst layer is sprayed onto both sides of the membrane, and sandwiched between carbon-based gas diffusion layers (GDLs). In this work we redesign the membrane electrode assembly (MEA) and fabricate an electrode-supported PEFC. First the electrocatalyst layer is sprayed onto the GDL, and then Nafion dispersion is sprayed over the top of this to form a thin membrane. This method has the advantage of simplifying the fabrication process, allowing the fabrication of extremely thin electrolyte layers (down to ∼10 μm in this case), and reducing the amount of ionomer required in the cell. Electrode-supported PEFCs operate at significantly increased power density compared to conventional membrane-supported PEFCs, with a maximum of 581 mW/cm2 at 80 °C (atmospheric pressure, air at the cathode). Impedance spectroscopy confirmed that the origin of the improved performance was an 80% reduction in the membrane resistance due the thinner Nafion layer. This novel fabrication method is a step towards cheaper, thinner, fully printable PEFCs with high power density and efficiency.

Immobilization of imidazole moieties in polymer electrolyte composite membrane for elevated temperature fuel cells

Development of membrane electrolyte with reasonable proton conductivity at elevated temperature without external humidification is essential for practical applications of elevated temperature proton exchange membrane fuel cells. Herein, a novel polymer electrolyte composite membrane using imidazole as anhydrous proton carriers for elevated temperature fuel cells is investigated. The imidazole moieties are immobilized inside the Nafion/poly(tetrafluoroethylene) (PTFE) composite membrane through in situ formation of imidazole functionalized silica nanoparticles in Nafion dispersion. The thus-formed membrane exhibits strong Coulombic interaction between negatively charged sulfonic acid groups of Nafion and protonated imidazole moieties, leading to an anhydrous proton conductivity of 0.018 S cm−1 at 180 °C. With the introduction of PTFE matrix, the mechanical strength of the membrane is greatly improved. The peak power density of a single cell assembled from the hybrid membrane is observed to be 130 mW cm−2 under 350 mA cm−2 at 110 °C without external humidification and it remains stable for 20 h continuous operation. The obtained results demonstrate that the developed composite membranes could be utilized as promising membrane electrolytes for elevated temperature fuel cells.

(Invited) Impact of Nafion Dispersion Morphology on Fuel Cell Performance and Durability

Los Alamos National Laboratory fuel cell team has discovered that Nafion dispersion morphology in dilute liquid media ca. solid content: 2.5 wt.% has distinctive characteristics, depending on dispersing fluid:[ref. 1] Small angle neutron scattering (SANS) and 19F NMR analysis indicated (i) a well-defined cylindrical dispersion in polyhydric alcohols, (ii) a gel particles that have as open network structure with sufficiently sparse physical network connections in some aprotic dispersing fluids, and (iii) a less-defined highly solvated large particle in water-monohydric alcohol mixtures. In this talk, we present the impact of dilute Nafion dispersion morphology on film/electrode formation, fuel cell performance and durability. Following subjects will be discussed with some details: 1. Change of particle morphology and gelation behavior of Nafion dispersion during evaporation process (Fig.1).[ref. 2] 2. Characterization of fuel cell electrode by microscopy and electrochemical methods. 3. Impact on fuel cell initial and long-term performance during potential cycles.[ref. 3] Other critical effects including mechanical toughness of dispersion-cast membranes and electrode reaction uniformity [ref. 4] will be also discussed at the meeting. This presentation will give some insights on recent progress of engineering production and processing of perfluorinated sulfonic acid ionomers and their application for fuel cell membrane and electrode processing. Acknowledgment U.S. Department of Energy Fuel Cell Technologies Program (Dr. Dimitrios Papageorgopoulos)

Internal Resistance Reduction of a Membrane Electrolyzer for Electrohydrogenation of Toluene As Hydrogen Carrier Synthesis

Introduction Potential of renewable energy is uneven distribution with fluctuation; therefore, to increase renewable energies, energy carrier technology is needed for storage and transportation. Hydrogen can be produced with various energies, but its volumetric energy density is too low to storage and transport. Toluene-methylcyclohexane organic chemical hydride system is one of promising technologies as hydrogen storage and transportation by invention of the dehydrogenation catalyst (1). Electrohydrogenation of toluene with water splitting has higher theoretical energy conversion efficiency compare to a series process of water electrolysis and hydrogenation. Direct and indirect electrohydrogenation processes had been proposed (2, 3). However, further challenge is needed to improve efficiency, durability, and increasing of the scale. We focused on a membrane electrolysis as a direct process with water splitting. Cathode side is a cathode membrane assembly, which is applied PEFC technology with precious metal loaded carbon electrocatalyst. Anode is a dimensionally stable electrode for oxygen evolution reaction in acidic electrolyte using industrial electrolysis technology. In our previous study, cell voltage of electrohydrogenation of toluene was almost as same as water electrolysis, but internal resistance of the electrolyzer was not small enough and conversion was very low (4). In this study, the membrane of anode side was hydrophilized to release oxygen from anode to reduce internal resistance, and characteristics of low concentration of toluene has been evaluated to reach high conversion to methylcyclohexane. Experimental A cathode was a carbon paper (35BC, SGL) coated 0.5 mgcm-2 of PtRu (TEC61E54, TKK) with Nafion dispersion. The cathode was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 212CS, DuPont) for a cathode membrane assembly. The membrane of the cathode side was hydrophilized by ZrO2 power coating or mechanically roughing. A non-treated membrane was also used for comparison. A DSE® anode with IrO2 based electrocatalyst is used for oxygen evolution. Toluene/methylcyclohexane mixture and 1M (=moldm-3) of H2SO4 were supplied to the cathode and anode for hydrogenation of toluene, respectively. The toluene concentration was in the range from 1 to 100 %. Cell voltage was determined with 4 mVs-1 of voltage sweep from 0.6 for toluene hydrogenation. Current efficiency was determined with constant cell voltage electrolysis with the volume measurement of gas evolution from cathode during the electrolysis. Internal resistance (iR) was determined with higher frequency intercept of AC impedance method in the cell voltage range from 1.0 to 2.5 V. Results and discussion Figure 1 shows the internal resistance and cell voltage as a function of current density for 100 % TL fed with various membrane treatments of anode side. The output of H2SO4 pulsed especially in high current density region, and current fluctuated above 2.2 V of the cell voltage with hydrogen evolution from cathode. This behavior would be affected by oxygen release between the membrane and the anode. The cell voltage and resistance of the hydrophilized membranes was smaller than those of non-treated membrane. At this moment, the output of H2SO4 and oxygen mixture was smooth, and there was no gas evolution reaction from cathode. Figure 2 shows the cell voltage (a) and current efficiency of hydrogenation as a function of current density for 1 to 100 % of TL-MCH fed with the rough surface membrane cell. The side reaction is only hydrogen evolution for this system. Error bars were current variation during constant cell voltage electrolysis for current and current efficiency measurements. The lines show the polarization curves with 4 mVs-1 of cell voltage sweep. For 100 and 50 % of TL fed, the current efficiency of electrohydrogenation was above 97 % up to the limit of current source of 0.44 Acm-2. At this moment, electrolysis was stable, and the cell voltage was around 2.0 V at 0.4 Acm-2. Below 10 % of TL fed, simultaneously hydrogen evolution proceeded at higher current density region with current variation. Therefore, this process should be feasible, and improvement of mass transfer in the cathode catalyst layer is important issue for real application. Acknowledgment This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We applicate the person concerned them. References 1) Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima, J. Hydrogen Energy, 2006, 31, 1348. 2) T. Hirashige, T. Ishikawa, M. Sugimasa, WO 2011/122155 Al (PCT/JP2011/053478) 2011. 3) C. Iwakura, Y. Yoshida, S. Ogata, H. Inoue, J. Mater. Res., 2011, 13, 821. 4) S. Mitsushima, Y. Takakuwa, Y. Kohno, K. Matsuzawa, A. Kato, Y. Nishiki, of 226th meeting of the ECS, #619 (2014). Figure 1

Origin of Toughness in Dispersion-Cast Nafion Membranes

The gelation behavior of Nafion dispersions was investigated using small-angle neutron scattering to better understand the mechanical toughness of dispersion-cast Nafion membranes. Three types of gelation were observed, depending on dispersing fluids: (i) homogeneous, thermally reversible gelation that was present in most aprotic polar dispersing fluids; (ii) inhomogeneous, thermally irreversible gelation as films, found in alcohols; and (iii) inhomogeneous, thermally irreversible gelation which precipitates in water/monohydric alcohol mixtures. The mechanical toughness of dispersion-cast Nafion membranes depends on the dispersing fluid, varying by more than 4 orders of magnitude. Excellent correlation between the critical gelation concentration and mechanical toughness was demonstrated with the Nafion membranes cast at 140 °C. Additional thermal effects among Nafion membranes cast at 190 °C were qualitatively related to the boiling point of dispersing fluids. Little correlation between mechanical toughne...

Enhanced CO Tolerance of PEFC Anode Catalyst by Optimized Degree of Activation of Resorcinol-Formaldehyde Carbon Support

In this study, a series of resorcinol-formaldehyde carbons (RFCs) were prepared by varying the degree of activation such as 0, 42, and 58, in order to control their surface areas and porosity. Then, these RFCs were used as supports to prepare Pt2Ru3 anode catalysts. These catalysts were characterized by many techniques including BET, DH calculation, XRD, STEM and CO tolerant by polymer electrolyte fuel cell (PEFC) single cell analysis. It was found that higher degree of activation of RFCs (i.e. rc1400ac42 and rc1400ac58) have larger micropore and mesopore volume. In case of CO tolerant by PEFC single cell unit, Pt2Ru3/rc1400ac58 showed the same performance as the commercial Pt2Ru3/C in pure H2 (0.76 V at 0.2 A/cm2), but Pt2Ru3/rc1400ac58 showed superior performance at 2000 ppm CO contamination. Nafion dispersion of Pt2Ru3/rc1400ac58 was also found to be better than that of the Pt2Ru3/C, since pore structure was optimized by degree of activation.

Electrolyzer for Hydrogenation of Organic Hydride with Oxygen Evolution Reaction from Water

INTRODUCTION In order to increase renewable energy, less expensive large scale energy storage and transportation technology must be developed. Hydrogen related technologies are suitable for these applications. Energy carriers, such as organic hydrides, ammonia, and liquid hydrogen, have been investigated for large scale energy storage and transportation (1). Toluene/methylcyclohexane system is one of promising organic hydride because high energy density, easy to handle, and low toxicity. Recently, a new catalyst solved difficulty of dehydrogenation of methylcyclohexane, and practical use of the system is starting (2)The total reaction of hydrogenation of toluene with renewable energy is Total: C6H5CH3 + 3H2O → C6H11CH3 + 3/2 O2 [1] Conventional process of eq. [1] is the series reaction of water electrolysis and hydrogenation, and hydrogenation is exothermic, which is theoretical heat loss. One step electrochemical process with following reactions should improve energy conversion efficiency of hydrogenation.Cathode: C6H5CH3 + 6H+ + 6e- → C6H11CH3 [2]Anode: 3H2O → + 6H+ + 3/2 O2 + 6e- [3]In our pervious study, the eq. [2] of electrohydrogenation of toluene was catalyzed by PtRu/C on a membrane electrode assembly for polymer electrolyte fuel cells (3). However, a complete electrolyzer had not been developed.In this study, the complete electrolyzer with IrO2based electrocatalyst coated Ti has been developed as a feasibility study of electrohydrogenation system. EXPERIMENTAL 0.5 mgcm-2 of PtRu (TEC61E54, Tanaka Kikinzoku Kogyo) was coated on a carbon paper (35BC, SGL) with Nafion dispersion (DE521, DuPont) as a cathode. The cathode was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 212CS, DuPont) for a cathode membrane assembly. A DSE® anode with IrO2based electrocatalyst is used for oxygen evolution.Toluene and 1M (=moldm-3) of H2SO4were supplied to the cathode and anode for hydrogenation of toluene, respectively. Water was supplied to the cathode for water electrolysis as a comparison.Cell voltage was determined with 4 mVs-1of voltage sweep from 0.6 to 2.0 V for toluene hydrogenation. Upper limit cell voltage for water electrolysis was 2.3 V.Internal resistance (iR) was determined with higher frequency intercept of AC impedance method in the cell voltage range from 1.0 to 2.5 V. Here, direct current was close to zero during iRmeasurement. RESULTS AND DISCUSSION Figure 1 shows internal resistances and cell voltages as a function of current density fed toluene or water to cathode at 60oC with 4 mVs-1 of voltage sweep. Bold and thin lines for cell voltage are iR-free and measured, respectively. The internal resistances were around 3 Wcm2for both toluene and water fed to cathode; however, the resistance for toluene fed decreased with the increase of current density. In the most of industrial electrolysis, evaluated gas bubble between the membrane and the anode increases internal resistance. In this case, absorbed water should increase with current for toluene fed, and the resistance decreased.The cell voltages as a function of current were almost the same for toluene and water fed; however, gas evolution was not observed for toluene fed electrolysis. Therefore, hydrogenation of toluene would proceed, instead of hydrogen evolution reaction for water fed electrolysis.Theoretical cell voltage for toluene fed is smaller than that for water fed and the onset potential of toluene hydrogenation was higher than hydrogen evolution reaction (3). Some toluene would permeate through the membrane, and poison oxygen evolution reaction on the anode.Electrohydrogenation of toluene was stable and efficient electrolysis. Therefore, this electrolysis will become one of the best energy carrier technologies. ACKNOWLEDGMENT This research has been supported by the advanced low carbon technology research and development program of Japan science and technology agency. We appreciate person concerned. REFERENCES (1) J. Gretz, J. P. Baselt, O. Ullmann, H. Wendt, Int. J. Hydrogen Energy, 15, 419 (1990).(2) Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima, J. Hydrogen Energy, 31, 1348 (2006).(3) K. Miyoshi, Y. Sato, S. Mitsushima, 224thECS meeting abstracts, #759 (2013).